46975329 clay containing polymeric nano composites volume 1 l a utracki

L.A. Utracki

Clay-Containing Polymeric NanocompositesVolume 1

Rapra Technology Limited

Shawbury, Shrewsbury, Shropshire, SY4 4NR, United KingdomTelephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118

http://www.rapra.net

Clay-ContainingPolymeric

Nanocomposites

Volume 1

L.A. Utracki

C. Vasile

First Published in 2004 by

Rapra Technology LimitedShawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2004, Rapra Technology Limited

All rights reserved. Except as permitted under current legislation no partof this publication may be photocopied, reproduced or distributed in anyform or by any means or stored in a database or retrieval system, without

the prior permission from the copyright holder.

A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any materialreproduced within the text and the authors and publishers apologise if any

have been overlooked.

Typeset, printed and bound by Rapra Technology LimitedCover printed by The Printing House, Crewe, UK

ISBN: 1-85957-437-8

i

Preamble

Preamble

During the last few years terms like nanomaterials, nanocomposites andnanosystems have become fashionable. It seems that anything with ‘nano’ attachedto it has nearly a magical effect – not so much on performance as on expectations.There is an extensive worldwide effort to introduce nanotechnology for theproduction of materials with specific functional characteristics, e.g.,semiconducting, electromagnetic, optical, etc. New magneto-resistance materialswith nanometre-scale spin-flip mean free path of electrons have beencommercialised. The National Science Foundation (NSF) has solicitedcollaborative research proposals in the area of nanoscale science and engineering,including: nanoscale biosystems; nanoscale structures; novel phenomena andcontrol; nanoscale devices and system architecture; nanosystems-specific software;nanoscale processes; multi-phenomena modelling and simulation at the nanoscalelevel; studies on societal implications of nanoscale science and engineering, etc.

Nanostructures are of interest to many technologies. The potential of precisecontrol of impurities and defects in a crystal and the ability to integrate perfectinorganic and organic nanostructures may lead to a new generation of advancedmaterials. To electronics, they offer quantum devices (resonant tunnellingtransistors; single electron transistors; cellular automata based on quantum dots)and new processor architectures. To catalysis, they form the templates for catalyticactivity, zeolite pores, etc. In biology, nanostructures are components of themitochondrion, the chloroplast, and the ribosome.

The advances in the synthesis and fabrication of isolated nanostructures rangefrom colloidal synthesis of nanocrystals to the growth of epitaxial quantum dots.The techniques of molecular biology have made a wide range of biologicalnanostructures readily available through cloning and overexpression in bacterialproduction systems. Furthermore, work has begun on the use of self-assemblytechniques to prepare complex and designed spatial arrangements ofnanostructures. Techniques derived from microlithography in microelectronics(viz. photo, X-ray, and e-beam lithography) offer the potential to economicallygenerate new types of 3D-structures. In short, there is a great potential for thewide use of nanotechnology for functional materials and devices.

The central theme of this book is the use of nanotechnology for thedevelopment of new structural polymeric systems, the polymeric nanocomposites(PNCs), and particularly the clay-containing polymeric nanocomposites (CPNCs).These mass produced materials are dispersions of inorganic, nanoscale plateletsin a polymeric matrix. Economics preclude the use of most of the manufacturingmethods developed for ceramic and metallic nanocomposites. The key to thesuccess of the CPNC industry is to provide new materials significantlyoutperforming the old ones at a marginal incremental cost increase per unitvolume. Considering that at present the nanoclay is a natural mineral with well-

L.A. Utracki

ii

Clay-Containing Polymeric Nanocomposites

recognised variability of composition, the secondary concerns focus on theconsistency of performance, not only at the batch-to-batch level, but also on along-term basis.

Nanostructures are intermediate in size between molecular and micron-sizesystems, such as blends and composites. There is no doubt that structures witharchitecture controlled on the molecular level may lead to refined properties oreven new sets of performance characteristics. Chemists have known this forcenturies, viz. more recent developments in nanosize structures such as fullerenes,buckytubes, dendrimers and complex block copolymers. The unexpectedbehaviour of adsorbed monolayers of organic molecules on a high-energy crystalsurface was discovered many decades ago. In the meantime the advances ofmicroscopy reached atomic-scale, providing images of crystalline unit cells andbioactive macromolecules. Thus, it is legitimate to ask what, if anything, is sodifferent about the nanomaterials that warrants the distinction.

It is known that within the nanometre scale such properties as the meltingtemperature, the remanence of a magnet, and the band gap of a semiconductordepend upon the size of the component crystals. Furthermore, it has been shownthat the mechanical properties of metallic alloys hyperbolically increase with thereduction of domain size.

It is customary to define nanocomposites in terms of the size of the dispersedparticles and the specific behaviour they engender. Thus, at least one dimensionof these particles must be less than 10 nm. Since these particles are usuallycrystalline, the size and high surface energy leads to high surface area to volumeratio and strong orientational forces that may lead to high packing densities andquantum behaviour (explored as electronic, magnetic or optic elements inmicroelectronics technology). In the most popular nanofiller in the CPNC industry,montmorillonite, over 40% of atoms rest on the surface – the clay lamellae shouldbe treated either as giant inorganic molecules or at least as hybrids occupying thegrey zone between molecules and particles. This is not mere semantics, but hasprofound consequences as far as the fundamentals of CPNC are concerned, viz.miscibility or flow behaviour.

This book summarises the pertinent developments in the area of the scienceand technology of clay-reinforced polymeric nanocomposites. There are severalreasons for using clays, viz. availability, cost, and aspect ratio. The theory andexperiments show that to maximise the benefits of nanotechnology the clay mustbe fully decomposed into individual crystalline lamellae (exfoliated) and thesemust be uniformly dispersed in a given matrix material. Furthermore, consideringthe large aspect ratio of clay platelets, their orientation must be controlled – forsome applications perfect alignment is desirable, whereas in others isotropicityof reinforcement is essential. For example, aligning clay lamellae perpendicularto the flux direction may increase barrier properties by a factor of 100, whereasorienting them in the flux direction will hardly change the barrier propertiesover those of the matrix. Considering that the relaxation time of standard clayplatelets (aspect ratio of 200 to 300) is of the order of one hour and that theirdimensions are of the nanometre scale, the dispersion and orientation of clayduring polymer processing is challenging.

The main difficulties for CPNC technology rest in the hygroscopic characterof clay and strong solid-solid interactions. It is a relatively simple task to disperseclay platelets or lamellae (i.e., to exfoliate them) in water or in water-soluble,

iii

Preamble

polar monomers or oligomers (e.g., amino acids or glycols). However, preparationof CPNC in a hydrophobic, non-polar high molecular weight polymer, e.g., apolyolefin or polystyrene, is difficult. The most sensible way to approach theproblem is to consider the process of preparation of CPNCs as blending twohighly immiscible ingredients, i.e., from the perspective of polymer blending andcompatibilisation. As in polymer blends here also one is obliged to ensure goodinteraction between the two antagonistic components: hygroscopic clay andhydrophobic polymer.

The clay of preference is montmorillonite (MMT) with micron-sized particlesformed by stacks of three-layer sandwiches: a layer of Mg and Al oxides inbetweenthe silicate layers. These sandwiches of 0.96 nm thickness and an average diameterof about 100 to 500 nm are the desired reinforcing entities for CPNC. The chemicalconstitution of the MMT unit cell offers three types of reactive sites: anions onthe silicate surface, hydroxyl (–OH) groups, and (few) cations on the narrowedges. Historically, compatibilisation of clay involved forming an ionic bondbetween the clay surface and organophilic onium cations, especially quaternaryammonium ones. The advantage of this is that the chemical reaction not onlychanges the hydrophilic clay character into hydrophobic, but also it causes theclay particles to expand, i.e., to intercalate as a first step to the total dispersion ofthe clay platelets, i.e., to exfoliation. The disadvantage is that this chemicalequilibrium process is diffusion controlled hence it may require an excess ofintercalatant and it may take a long time to complete!

‘Compatibilisation’ with at least partial utilisation of the –OH groups hasbeen carried out using their ability to react with epoxy or acid anhydride groups.However, since the –OH groups are mostly located on the peripheries of clayparticles, there are few of them readily accessible and the reaction does notnecessarily lead to intercalation/exfoliation. Since the solid-solid interactions are100 times stronger than liquid-liquid ones, it is imperative that there is goodmiscibility between the pre-intercalated clay and the polymeric matrix – if not,even the initially exfoliated clay platelets may reassemble during processing.

The most reasonable strategy is to prepare exfoliated CPNC using multiplesteps, for example:

1. Swelling sodium montmorillonite in warm water which causes the interlayerspacing to expand from the initially dry state of 0.96 to about 1.3 nm.

2. Intercalation with cations suitable for the envisaged CPNC organophilicmolecules, onium or Lewis-base types that increase the interlayer spacing toabout 3 to 4 nm and will improve miscibility between the clay and the matrix.

3. Reactive compatibilisation of the organoclay/matrix polymer system, whichresults in stable exfoliation of the clay platelets (interlayer spacing largerthan 8.8 nm). This step may not be necessary for highly polar polymers, suchas water-soluble polymers (e.g., polyvinyl pyrrolidone or polyvinyl alcohol),or even for polyamides, but it is crucial for polyolefins or styrenics.

4. Melt compounding the pre-intercalated clay with matrix polymer. Analternative strategy is to disperse the product of step (2) in a monomer andpolymerise it. This method has been particularly successful when theintercalatant used in step (2) can be incorporated into the macromolecularchain, thus forming what has became known as ‘hairy clay platelets’ withover one thousand macromolecules end-tethered to the clay surface.

iv


To provide condensed information on the essential elements of CPNC technology,the book is divided into parts:Part 1. Introduction – presents a general overview of nanocomposites withpolymeric as well as non-polymeric matrices.Part 2. Basic elements of PNC technology – focuses on the general methods andprinciples of polymeric nanocomposites. It starts with a brief description of PNCcomprising non-clay nanoparticles (e.g., carbon nanotubes, polyhedral oligomericsilsesquioxanes (POSS), etc.), and then focuses on the clay-containing polymericnanocomposites. The individual elements of CPNC technology are discussed,namely the general characteristics of clays, methods of purification, and the diversemethods used for intercalation and exfoliation.Part 3. Fundamental aspects – discusses the pertinent aspects of thethermodynamics, thermal stability, rheology, crystallisation and mechanicalbehaviour.Part 4. Technology of CPNC – reviews the evolution of CPNC technology forspecific polymeric matrices, primarily using the patent literature. Thus, CPNCswith individual polymer matrices are reviewed, starting in historical order withpolyamide (PA), polyolefin (PO) and other thermoplastics, then epoxies,polyurethanes and other thermosets.Part 5. Performance – discusses selected properties of CPNCs, viz. mechanical,flame retardancy, and barrier.Part 6. Closing remarks – summarises the information.Part 7. Appendices – provides explanations of abbreviations, symbols, andconcepts used in the book.Part 8. References – contains well over 1,000 references to open and patentliterature up to the beginning of 2004.

Polymeric nanotechnology is in statu nascendi. In consequence, there is a bitof confusion and uncertainty about its value and the most suitable applications.Hopefully, this book will help answer some of these questions and in a small wayaccelerate wider introduction of this technology.

In 1953, after getting a chemical engineering degree and spending theobligatory six-month stage as plant engineer, I started graduate studies in thefield of phase equilibria and flow of polymer solutions. To emphasise theobjectiveness of scientific writings it is expected to use the impersonal form.However, after 50 years in the profession I wish to revert to a more personalstyle, dedicating this latest creation (and, as Hermann Mark used to say, thedearest) to Czeslawa, my Wife, Friend and Supporter of nearly as many years.Curiously, this book was not planned, but rather it evolved in response toquestions, comments and stories told to me by my colleagues from the Americas,Asia and Europe. There are too many people to whom I owe my thanks to listthem all, but I wish to express my thanks to a very special trio: to Robert Simhawho has been my mentor and brilliant star to follow for all these 40-odd years ofmy post-doc’ing with him, to Osami Kamigaito for introducing me to thefascinating world of clay-containing polymeric nanocomposites, and to mycolleague and friend of many years, Jørgen Lyngaae-Jørgensen.

Leszek UtrackiMontreal, March 2004

v

Contents

Contents Volume 1

Preamble .................................................................................................... i

Part 1 Introduction

1.1 General ............................................................................................... 1

1.2 NCs with Ceramic or Metallic Matrix .............................................. 21.2.1 Metallic Nanoparticles in Amorphous Matrix ................................ 2

1.2.2 Magnetic Oxides in Silica Nanocomposites .................................... 2

1.2.3 Optoelectronics ............................................................................... 3

1.2.4 Summary on Non-Polymeric NC .................................................... 3

1.3 NCs with Polymeric Matrix .............................................................. 31.3.1 PNC Definitions .............................................................................. 6

1.3.2 Methods of Characterisation of CPNCs .......................................... 81.3.2.1 X-Ray Diffraction (XRD) .................................................. 81.3.2.2 Small Angle Neutron Scattering (SANS).......................... 111.3.2.3 Transmission and Atomic Force Electron Microscopy

(TEM and AFM) ............................................................. 141.3.2.4 Fourier Transform Infrared Spectroscopy (FTIR) ............ 151.3.2.5 Nuclear Magnetic Resonance Spectroscopy (NMR)........ 161.3.2.6 Other Methods ................................................................ 17

1.3.3 Determination of PNC Properties ................................................. 18

1.3.4 PNC Types and Methods of their Preparation .............................. 18

1.3.5 PNCs of Commercial Interest ........................................................ 18

1.3.6 Journals and Research Groups ...................................................... 29

1.3.7 Historical Perspective .................................................................... 30

Part 2 Basic Elements of Polymeric Nanocomposite Technology

2.1 Nanoparticles of Interest to PNC Technology ................................. 352.1.1 General ..........................................................................................35

2.1.2 Layered Nanoparticles .................................................................. 35

2.1.3 Fibrillar Nanoparticles .................................................................. 382.1.3.1 Carbon Nanotubes (CNTs) ............................................. 38

2.1.3.1.1 Origin, Characteristics and Structure .................. 382.1.3.1.2 Computation of Potential CNT Properties .......... 412.1.3.1.3 Non-Polymeric Applications of CNTs ................. 442.1.3.1.4 Sources ................................................................. 462.1.3.1.5 PNC with CNTs for Electrical Conductivity ....... 462.1.3.1.6 Graphite .............................................................. 47

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2.1.3.1.7 PNC with CNTs – Thermoset Matrix ................. 482.1.3.1.8 PNC with CNTs – Thermoplastic Matrix ............ 50

2.1.3.2 Rod-Like CdSe Nanocrystals ........................................... 542.1.3.3 Imogolite ......................................................................... 542.1.3.4 Vanadium Pentoxide, V2O5............................................. 542.1.3.5 Inorganic Nanotubes ....................................................... 55

2.1.4 Other Nanoparticles ......................................................................562.1.4.1 Spherical or Nearly-Spherical Particles ............................ 562.1.4.2 Sol-Gel Hybrids ............................................................... 562.1.4.3 Polyhedral Oligomeric Silsesquioxanes (POSS) ............... 58

2.1.4.3.1 Origin and Structure ............................................ 582.1.4.3.2 Properties ............................................................. 602.1.4.3.3 Sources ................................................................. 662.1.4.3.4 Applications ......................................................... 67

2.2 Clays ............................................................................................... 732.2.1 General Characteristics .................................................................73

2.2.2 Crystalline Clays ...........................................................................742.2.2.1 Kaolins ............................................................................742.2.2.2 Serpentines ......................................................................742.2.2.3 Illite Group (Micas) ......................................................... 742.2.2.4 Chlorites and Vermiculites .............................................. 762.2.2.5 Other Clays ..................................................................... 76

2.2.2.5.1 Glauconite ........................................................... 762.2.2.5.2 Sepiolite, Palygorskite and Attapulgite ................ 762.2.2.5.3 Mixed-Layer Clay Minerals ................................. 76

2.2.2.6 Smectites or Phyllosilicates .............................................. 762.2.2.6.1 Bentonite ............................................................. 792.2.2.6.2 Montmorillonite (MMT) ..................................... 80

2.2.3 Purification of Clay ....................................................................... 84

2.2.4 Reactions of Clays with Organic Substances ................................. 852.2.4.1 Clay in Aqueous Medium................................................ 90

2.2.4.1.1 General ................................................................ 902.2.4.1.2 Reactions with Edge Cations ............................... 912.2.4.1.3 Reactions with –OH Groups ............................... 912.2.4.1.4 Reaction with the Silicilic Surface Anions ........... 912.2.4.1.5 Stabilisation by Polyelectrolytes .......................... 92

2.2.4.2 Clay Dispersion in Polar Organic Liquids ....................... 932.2.4.3 Absorption of Organic Molecules by Organoclay ........... 93

2.3 Intercalation of Clay ....................................................................... 972.3.1 Introduction ..................................................................................97

2.3.2 Intercalation by Solvents and Solutions ....................................... 100

2.3.3 Intercalation by Organic Cations ................................................102

2.3.4 Intercalation by Organic Liquids .................................................124

2.3.5 Intercalation by Monomers, Oligomers or Polymers ................... 126

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Contents

2.3.5.1 Intercalation of Purified Clay by HydrophobicCompounds ...................................................................126

2.3.5.2 Intercalation of Purified Clay by HydrophilicCompounds ...................................................................127

2.3.6 Two-Step Intercalation ................................................................1352.3.6.1 Intercalation by Silylation .............................................1362.3.6.2 Intercalation Utilising Epoxy Compounds .................... 1382.3.6.3 Intercalation Utilising Organic Anions ..........................1392.3.6.4 Intercalation Utilising Macrocyclic Oligomers

(Cyclomers) ...................................................................139

2.3.7 Intercalation by Inorganic Intercalants ........................................140

2.3.8 Melt Intercalation .......................................................................1422.3.8.1 Quiescent (or Static) Melt Intercalation ........................ 1432.3.8.2 Dynamic Melt Intercalation ..........................................149

2.3.8.2.1 Melt Mixing ...................................................... 1492.3.8.2.2 Mixing Equipment ............................................. 1502.3.8.2.3 Mixing in an Extensional Flow Field ................. 1582.3.8.2.4 Melt Intercalation in a PA Matrix ..................... 1602.3.8.2.5 Melt Intercalation in PEG Matrix ..................... 1652.3.8.2.6 Melt Intercalation in PO Matrix ....................... 165

2.3.9 Temperature and Pressure Effects on Interlamellar Spacing ........ 183

2.3.10 Layered Nanofillers, other than Montmorillonite ....................... 1852.3.10.1 Kaolinite ........................................................................1862.3.10.2 Micas and Synthetic Micas ............................................189

2.3.11 Summary of the Intercalation Methods ....................................... 198

2.4 Exfoliation of Clays ...................................................................... 2012.4.1 Principles .....................................................................................202

2.4.2 Polymerisation in the Presence of Organoclay............................. 2042.4.2.1 Monomer Intercalation – PA-6 Nanocomposites .......... 2042.4.2.2 Monomer Modification – Acrylic-Based

Nanocomposites ............................................................2062.4.2.3 Non-Reactive Intercalated Clays ...................................2092.4.2.4 Co-Vulcanisation ...........................................................2102.4.2.5 Common Solvent Method – Polyimide Based

Nanocomposites ............................................................2102.4.2.6 Other Methods – Epoxy-Based Nanocomposites .......... 2182.4.2.7 Other Methods – PU-Based Nanocomposites ................224

2.4.2.7.1 Metal Particles ................................................... 2242.4.2.7.2 Silica .................................................................. 2252.4.2.7.3 Cadmium Sulfide Particles (CdS) ....................... 2252.4.2.7.4 Organoclays ....................................................... 225

2.4.3 Melt Exfoliation ..........................................................................2322.4.3.1 PA-Based CPNCs ...........................................................2332.4.3.2 PO-Based CPNCs ..........................................................237

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2.4.3.3 PCL-Based CPNCs ........................................................2422.4.3.4 Other Systems ...............................................................245

2.4.4 Functional CPNC ........................................................................2452.4.4.1 Liquid Crystal/Clay Composite (LCC) ..........................2452.4.4.2 Biodegradable CPNC with Polylactic Acid (PLA) ......... 2462.4.4.3 Poly(N-Vinyl Carbazole)/MMT ....................................2522.4.4.4 Polydiacetylene ..............................................................2542.4.4.5 Clay-Functional Organic Molecules ..............................2542.4.4.6 Super-Absorbent CPNC ................................................2552.4.4.7 Emulsion Polymerisation of CPNC ...............................255

Part 3 Fundamental Aspects

3.1 Thermodynamics ............................................................................ 2573.1.1 Glass Transition in Thin Films ....................................................257

3.1.2 Nanothermodynamics .................................................................260

3.1.3 Vaia’s Lattice Model for Organoclay Intercalation byMolten Polymer ...........................................................................2633.1.3.1 Introduction ..................................................................2633.1.3.2 Entropic Contributions .................................................2643.1.3.3 Interactions ...................................................................2663.1.3.4 Consequences of the Model ...........................................2673.1.3.5 Model Prediction versus Static Intercalation Results ..... 270

3.1.4 Computations of Polymeric Brushes ............................................271

3.1.5 Balazs Self-Consistent Field Approach ........................................2723.1.5.1 Numerical Simulation ...................................................2733.1.5.2 Analytical Self-Consistent-Field Theory for

Compatibilised Systems .................................................2783.1.5.3 Phase Behaviour ............................................................2803.1.5.4 Contribution and Potential of the SCF Method ............ 285

3.1.6 Scaling Theory for Telechelic Polymer/Clay Systems ................... 287

3.1.7 Solid Surface Effects on Molecular Mobility ...............................2913.1.7.1 Surface Energy of Solids ................................................2913.1.7.2 Polymer Adsorption on Solid Particles ..........................2933.1.7.3 Nanoscale Rheology ......................................................2943.1.7.4 Molecular Modelling of Nanoconfined Molecules

(Intercalation) ................................................................298

3.1.8 Kinetics of Polymer Intercalation ................................................3023.1.8.1 Macromolecular Diffusion ............................................3023.1.8.2 Stationary Intercalation .................................................3043.1.8.3 Simulation of Melt Intercalation Kinetics ...................... 306

3.1.9 Pressure-Volume-Temperature Dependence for CPNC ................3093.1.9.1 Equations of State (eos) .................................................309

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Contents

3.1.9.2 Simha-Somcynsky (S-S) Equation of State .....................3103.1.9.3 Extension of S-S eos to Binary Miscible Systems ...........3153.1.9.4 Extension of S-S eos to Suspensions ..............................3173.1.9.5 Extension of S-S eos to Nanocomposites ....................... 318

3.1.9.5.1 Diluted, Exfoliated CPNC– Simplified Approach ....................................... 319

3.1.9.5.2 Dilute, Exfoliated CPNC– Gradient Mobility Approach .......................... 324

3.1.9.5.3 Intercalated CPNC – Concentration Gradient ... 3273.1.9.5.4 PVT – Concluding Notes ................................... 331

3.2 Thermal Stability ........................................................................... 3333.2.1 Thermal Stability During Processing ...........................................333

3.2.2 Flame Retardancy and High Temperature Stability .....................339

3.2.3 Photo-Oxidative Stability ............................................................340

3.3 Rheology ....................................................................................... 3413.3.1 Introduction ................................................................................341

3.3.2 Multi-Phase Flow Behaviour – An Overview ..............................342

3.3.3 Rheology and Microrheology of Disc Suspensions ...................... 344

3.3.4 Similarity Between CPNC and Liquid Crystal Flow .................... 347

3.3.5 End-Tethered versus Non-Tethered CPNC .................................. 350

3.3.6 Fourier-Transform Rheology of CPNC .......................................356

3.3.7 Rheology of CPNC with PA Matrix ............................................3563.3.7.1 Effects of Moisture ........................................................3603.3.7.2 Strain Effects .................................................................3633.3.7.3 Dynamic Flow Curves ...................................................3633.3.7.4 Apparent Yield Stress ....................................................3683.3.7.5 Zero-Shear Viscosity and the Clay Aspect Ratio ...........3693.3.7.6 Flow-Induced Orientation .............................................3703.3.7.7 Steady-State Flow Curves – Shear History Effects ......... 3723.3.7.8 Fourier Transform Analysis of CPNC ........................... 376

3.3.8 Rheology of CPNC with PO Matrix ...........................................376

3.3.9 Foaming of CPNC .......................................................................384

3.3.10 Rheology of CPNC with PS and Styrenics Matrix ...................... 387

3.3.11 Rheology of CPNC with Other Polymer Matrix Types ............... 390

3.3.12 Rheology of CPNC – A Summary ...............................................392

3.4 Nucleation and Crystallisation ...................................................... 3953.4.1 Introduction ................................................................................395

3.4.2 Fundamentals of Crystallisation ..................................................396

3.4.3 Effects of Clay on Crystallisation of PA-6 Matrix ....................... 399

3.4.4 Clay Effect on Crystallisation of Other Polyamides .................... 408

3.4.5 Crystallisation of PO Matrix .......................................................409

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3.4.6 Crystallisation of PEST Matrix ...................................................414

3.4.7 Crystallisation of Syndiotactic PS Matrix ....................................416

3.5 Mechanical Behaviour ................................................................... 4173.5.1 Micromechanics of CPNC...........................................................417

3.5.2 Prediction of Tensile Strength ......................................................426

3.5.3 Fatigue Resistance of CPNC........................................................428

Contents Volume 2

Part 4 Technology of Clay-Containing Polymeric Nanocomposites

4.1 Thermoplastic CPNC .................................................................... 4354.1.1 Polyamides (PA) ..........................................................................435

4.1.1.1 PA-Type Nanocomposites from Toyota .........................4364.1.1.2 PA-Type Nanocomposites from AlliedSignal Inc. .......... 4414.1.1.3 AMCOL Technology for PA .......................................... 4454.1.1.4 Other Technologies for the Production of CPNC

with PA Matrix .............................................................4524.1.1.5 Mechanical Exfoliation of PA-Type CPNC ................... 4624.1.1.6 PA-6/Kaolinite Nanocomposites ....................................469

4.1.2 Polyolefins (PO) ..........................................................................4704.1.2.1 Toyota Patents on PO-Based CPNC ..............................4724.1.2.2 Dow Patents on CPNC Technology for PO ................... 4764.1.2.3 Sekisui Chemical Patent on PO-Based CPNC ................4814.1.2.4 Diverse Technologies for the Preparation of

CPNC with PO-Matrix .................................................483

4.1.3 General Methods of CPNC Preparation ...................................... 4984.1.3.1 Hudson’s Clay Grafting Method ...................................4994.1.3.2 Hasegawa et al. Method with Functionalised

Compatibilisers .............................................................5004.1.3.3 CPNC with Amino-Aryl Lactam Clays .........................5034.1.3.4 Ishida’s Method .............................................................5034.1.3.5 Edge Reactions of Clay Platelets ...................................505

4.1.4 Vinyl Polymers and Copolymers .................................................5064.1.4.1 Polymerisation in the Presence of Clay ..........................507

4.1.4.1.1 Bulk Polymerisation by the Free Radicaland Coordination Methods ............................... 508

4.1.4.1.2 Emulsion and Suspension Methods ................... 5164.1.4.1.3 Solution Polymerisation Methods ...................... 526

4.1.4.2 Other CPNC Prepared by Solution Method .................. 530

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Contents

4.1.4.3 Vinyl-Type CPNC Prepared by Melt Compounding ......5334.1.4.4 Vinyl Polymer Matrix – A Summary ............................. 542

4.1.5 CPNC in Water-Soluble Polymeric Matrix .................................. 543

4.1.6 Thermoplastic Polyesters (PEST) .................................................553

4.1.7 Polycarbonate (PC) .....................................................................565

4.1.8 Liquid Crystal Polymers (LCP) ....................................................567

4.1.9 Fluoropolymers ...........................................................................569

4.1.10 CPNC with High Temperature Polymers ....................................573

4.1.11 Electroconductive CPNC.............................................................576

4.2 Thermoset CPNC .......................................................................... 5794.2.1 Epoxy Resins ...............................................................................579

4.2.2 Unsaturated Polyester Resin ........................................................588

4.2.3 Polyurethanes ..............................................................................590

4.2.4 Other CPNC with Thermoset Matrix .........................................599

4.3 Elastomeric CPNC ........................................................................ 601

Part 5 Performance

5.1 Mechanical Properties ................................................................... 611

5.2 Flame Retardancy of CPNC .......................................................... 611

5.3 Permeability Control ..................................................................... 618

Part 6 Closing Remarks

6.1 Summary ....................................................................................... 625

6.2 The Future .................................................................................... 6276.2.1 Composition ................................................................................627

6.2.2 Method of Preparation ................................................................628

6.2.3 Characterisation and Testing .......................................................629

Part 7 Appendices

7.1 General and Chemical Abbreviations ............................................ 631

7.2 International Abbreviations for Polymers ...................................... 640

7.3 Abbreviations for Organic Cations Used as Clay Intercalants ....... 646

7.4 Notations ...................................................................................... 6497.4.1 Notation Roman Letters .............................................................6497.4.2 Notation – Greek Letters .............................................................6527.4.3 Subscripts ....................................................................................6557.4.4 Superscripts .................................................................................655

7.4.5 Mathematical Symbols ................................................................655

xii


7.5 Dictionary ..................................................................................... 656 Dictionary References ..........................................................................692

7.6 Companies Active in Organoclay, and/or CPNC Technology ....... 694

Part 8 References

References ..................................................................................... 697

Part 9 Index Index ....................................................................................................765

Introduction

Part 1

Introduction

1

Introduction

1 Introduction

1.1 GeneralNanocomposites (NCs) are materials that comprise a dispersion of nanometre-size particles in a matrix. The matrix may be single or multicomponent. It maycontain additional materials that add other functionalities to the system (e.g.,reinforcement, conductivity, toughness, etc.). The matrix may be either metallic,ceramic or polymeric - only the latter type is of interest at present.

Depending on the matrix nature, NCs may be assigned into one of the threecategories:

• Polymeric (PNC),• Ceramic (CNC),• Metallic (MNC).The nanoparticles are classified as:

1. Lamellar,2. Fibrillar,3. Tubular,4. Spherical, and5. Others.For the enhancement of mechanical and barrier properties anisometric particles,especially lamellae are preferred. However, for rigidity and strength fibrillar arepreferred, while for functional NCs (e.g., optical, electrical conductivity) sphericalor other particles have also been used.

Since the aspect ratio of exfoliated, mineral MMT is p = 50 to 2000, the specificsurface is in the order of 750 to 800 m2/g. The reinforcing effect of nanoparticles isrelated to the aspect ratio (p) (ratio of the length or thickness to that of the diameter)and to the particle-matrix interactions. Independent of the actual dimensions, forp > 500 the reinforcing effects are the same as those of an infinitely large particle.Furthermore, the anisometric particles start overlapping when the volume fractionexceeds the ‘maximum packing volume fraction (φm)’ [Utracki, 1995]:

for discs: 1/φm = 1.55 + 0.0598pfor rods: 1/φm = 1.38 + 0.0376p1.4 (1)

For example, Equation 1 predicts that for discs with an aspect ratio ofp = 500 the overlapping volume fraction is φm = 0.00032 (for rods φm = 0.00004).The overlapping generates a 3D network that in melts is responsible for the yieldstress and in solid state for significant reinforcing effects. Hence a small amountof anisometric particles leads to large effects.

2


Because of the small size, the nanoparticles are invisible to the naked eye;hence, they may be used to engender reinforced, but transparent composites(polymeric or ceramic). On a molecular level, the surface energy of clay particlesis high. As a result, adsorbed molecules have a tendency to be strongly bonded inthe layer adjacent to the clay surface [Horn and Israelachvili, 1998]. This resultsin a solid-like behaviour of the 5 to 6 nm thick surface layer and progressivereduction of viscosity with distance to bulk liquid viscosity at about 100 to120 nm.

In the absence of antagonistic interactions (such as hydrophilic nanoparticlesin a hydrophobic matrix) the clay/organic liquid system may not require interfacemodification. PNCs can be used as a matrix for traditional multiphase systems(viz. blends, composites or foams), replacing neat polymers.

1.2 NCs with Ceramic or Metallic Matrix

Despotakis [2001] published a review of nanotechnology, where he discussedthe importance of the technology to diverse applications, types and sources ofnanoparticles and commercial developments.

1.2.1 Metallic Nanoparticles in Amorphous Matrix

Iron, cobalt and nickel nanoparticles are obtained either by thermal reductionunder hydrogen of silica-based matrices containing 0.1-20 wt% of metal cationsor by ionic implantation. Their morphologies and properties (electric or magnetic)depend on the processing conditions.

In the case of hydrogen reduction, two kinds of matrices have been used,dense sodalime silicate glasses and porous silica gels. In the first case, reductionis controlled by diffusion of hydrogen inside the glass and yields a broad sizedistribution. In the second case, much smaller size distributions occur with particlesexhibiting either a super-paramagnetic or -ferromagnetic behaviour according tothe reduction temperature. Differences are observed between nickel on one partand iron and cobalt on the other. The last two have a greater tendency to formsilicates and are more sensitive to re-oxidation.

In the case of ionic implantation, the implanted element appears to be both inmetallic and ionised states. The metallic fraction gives rise to nanoparticles witha super-paramagnetic behaviour. The ratio of metal:oxide depends on theimplantation parameters (dose and energy). Due to the small implantation depth,further thermal treatments can easily lead to total reduction or to a re-oxidation.

1.2.2 Magnetic Oxides in Silica NanocompositesIn the area of thin layer magneto-optical recording, apart from metallic multilayerdevices, barium hexaferrite and yttrium garnet are good candidates. An alternativeis to disperse particles inside a transparent matrix. Preliminary studies have beenperformed on the system of iron oxide/silica with 20 wt% Fe2O3.

Iron containing gels were prepared from Fe(NO3)3, tetraethyl orthosilicate(TEOS), ethanol, HNO3, H2O and formamide. After gelation and drying, themagnetic properties were measured. Below 700 °C, paramagnetic behaviour wasobserved, while at higher temperature, ferromagnetic γ-Fe2O3 particles wereformed. Near 1000 °C a new phase, closely related to ε-Fe2O3, was observedwith a coercive field of ca. 7000 Oe.

3

Introduction

1.2.3 OptoelectronicsSemiconductors with carrier lifetime of the order of one picosecond (ps) are necessaryfor ultrafast optoelectronic applications. NC semiconductors of GaAs grown bymolecular-beam-epitaxy at low temperatures (LT-GaAs) show a unique combinationof electronic and optoelectronic properties. Implantation of GaAs can result inproperties that are similar to those of LT-GaAs: sub-ps carrier lifetime, high resistivity,and high electron mobility. Carrier lifetimes as short as 30 fs were determined for thesamples annealed at 500 °C. The short carrier lifetime in high energy ion implantedGaAs seems to be controlled by the nanoscale morphology, i.e., by the carrier capture/recombination at the intrinsic point and/or by extended defects.

Another nanocomposite optoelectronic material system comprises Cu-diffusedInP crystals. These contain nanometre-size metallic precipitates that radicallymodify the dynamics of photoconductivity and photoluminescence decay.Furthermore, they significantly reduce the electron and hole trapping time. Spectralphotoresponse measurements evidenced the presence of a well-exposed Urbach-tail in the sub-band gap radiation absorption, whereas a Z-scan type experimenthas shown an enhancement in both the intensity dependent IR absorption andthe nonlinear refractive index.

1.2.4 Summary on Non-Polymeric NCThe principles of reinforcement and the strategies discussed for the polymer basedNC are valid for the other matrices. Non-polymeric NC systems studied arelisted in Table 1.

1.3 NCs with Polymeric MatrixIn consequence of the outlined fundamentals, PNCs normally require 1-3 vol% ofnanoparticles. They behave as a single phase and single component material. PNCsexhibit transparency, low density, reduced flammability, low permeability and enhancedmechanical properties. Furthermore, they may be easily modified by additives andused replacing neat polymers in polymer blends, traditional composites or foams.

There are several methods of classification of polymeric nanocomposites. Forexample, one may consider how many dimensions of the dispersed particles arein the nanometre range:

1. One dimension. The nanoparticles are in the form of sheets of one to a fewnanometres thick to hundreds to thousands of nanometres long and wide,hence they can be named as polymer-layered crystal nanocomposites[Pinnavaia and Lan, 2000a]. These systems are of principal industrial interestand the main object of this publication. There are a wide variety of bothsynthetic and natural crystalline materials that can be used.

2. Two dimensions. These nanoparticles are elongated, viz. fibres, nanotubes orwhiskers, e.g., carbon nanotubes [Ebbesen, 1997] or cellulose whiskers [Favieret al., 1997, Chazeau et al., 1999]. The polymeric nanocomposites containingsingle-walled or multi-walled carbon nanotubes (CNTs) have been extensivelystudied. At low loading, they show low density, high mechanical properties,and electrical conductivity.

3. Three dimensions. These are mainly iso-dimensional spherical particles, forexample, obtained by the sol¯gel methods [Reynaud et al., 1999], and bypolymerisation promoted directly from their surface [von Werne and Patten, 1999].

4


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5

Introduction

The materials are either structural or functional. In this book the focus is on theformer. To this date, it is primarily the structural nanocomposites based on layeredsilicates, i.e., clays (CPNC), which have been commercially produced. Clays are easilyavailable and their intercalation methods have been known since the 1930s [Theng,1974]. Owing to the nanometre-size particles, CPNC show markedly improvedmechanical, thermal, optical, and physicochemical properties when compared withneat polymers or their composites [Kojima et al., 1993a]. The improvements includemoduli, strength, heat resistance, barrier properties, flammability, etc.

Nanometre-scale structures are frequently found in biological materials withimpressive performance [Mark, 1996]. For example, bone has a structure of 4 nmthick hydroxyapatite crystals dispersed within a collagen matrix. In the desire tosynthesise analogues to biological systems several methods for constructingsynthetic composites with a degree of nanometre-scale organisation have beentried. Thus, elongated ceramic particles have been precipitated within polymermatrices by drawing the polymer during the precipitation reaction, silica andCdS have been precipitated in liquid crystal polymer (LCP) [Kovar and Lusignea,1988; Nelson and Samulski, 1995], metals have been electrodeposited inside thepores of commercial nanopore membranes [Martin, 1996], and polymers havebeen grown within the cavities of layered inorganic structures and zeolites [Okadaand Usuki, 1995; Frisch and Mark, 1996]. Furthermore, an organic-inorganicnanocomposite was formed by dissolving the inorganic polymer (LiMo3Se3)n ina conventional monomer acting as the solvent, then polymerising the matrix insitu [Golden et al., 1996].

The above mentioned methods are well suited for the preparation of aparticular composition, but they are not versatile enough to offer good controlover nanometre-scale architecture and composition in all resins. Several strategieshave been developed for constructing ordered nanocomposites with well-defined,tunable nanoarchitecture and the ability to be used in a wide variety of polymers:

1. Addition of nanoparticles, especially anisometric ones, viz. nanofibres,nanotubes or nanoplatelets. Owing to the expanding industrial interest, thepresent text will focus on this type of PNC, in particular the ones with theclay platelets.

2. Copolymerisation or grafting polymeric chains with monomers having bulkygroups, viz. the polyhedral oligomeric silsesquioxanes (POSS) [Lichtenhan

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6


et al., 1995; Lichtenhan, 1996; Haddad and Lichtenhan, 1996; Lichtenhanand Schwab, 2001; Les niak, 2001].

3. Preparation of ordered nanoscale structures using the LCP technology [Ginet al., 1998].

4. Sol-gel methods [Mauritz et al., 1995; Wen and Wilkes, 1996; Deng et al.,1998].

5. Hydrothermal method [Quian et al., 2000].

6. Others.

Polymer nanocomposites are emerging as a new class of industrially importantmaterials. At loading levels of 2-3-vol%, they offer similar performance toconventional polymeric composites with 30-50 wt% of reinforcing material. Notethat high filler loading in the latter materials causes an undesirable increase ofdensity; hence heavy parts, decreased melt flow, and increased brittleness.Furthermore, the classical composites are opaque with often a poor surface finish– these problems are absent in PNCs.

The clay-containing PNC, a CPNC, offers several advantages over the matrixpolymer or classical composites. The main improvements are in: modulus, impactstrength, heat resistance, dimensional stability, barrier properties (for gases andliquids), flame retardance, optical properties, ion conductivity, thermal stability,etc. Since these advantages are achieved at low clay loading the density is virtuallyunaffected and the CPNC may be used to replace the neat polymer in blends,composites or foams. Synergistic effects in these applications have been reported.

Current consumption of PNC is only a few kilotons per annum, projected toincrease by 2009 to 500 kton/y. The cost differential between the neat matrixand its PNC is about 10 to 15%. CPNC’s main market is in the transportindustries, with growing demand for packaging, appliances, building andconstruction, electrical and electronic, horticulture, power tools, etc.

Several reviews on nanoparticles, clays and polymeric nanocomposites areavailable, viz. on the chemistry of clays [van Olphen, 1977; Wittingham andJacobson, 1982; Newman, 1987], a short one on clay-containing PNC [Giannelis,1998], on flammability [Gilman et al., 1998; Gilman et al., 1998], a large andwell written one [Alexandre and Dubois, 2000], on clay-containing PNC [Rubanet al., 2000], on synthetic clays and resulting PNC [Carrado, 2000], onnanoparticles [Shipway et al., 2000; Ajayan et al., 2003], and more recent oneson CPNC [Utracki and Kamal, 2002; Okamoto, 2003], etc. Several editedproceedings of nanocomposite symposia are also available, viz. [Utracki and Cole,2002; Krishnamoorti and Vaia, 2002; Hahn, 2002; Komarneni, 2002; Laine,2001; Lyon, 2001; Nakatani, 2001; Benedek et al., 2001; Komarneni et al., 1997;Komarneni et al, 2000; Pinnavaia and Beall, 2000], etc.

1.3.1 PNC DefinitionsClay-containing polymeric nanocomposite (CPNC): a polymer or copolymerhaving dispersed exfoliated individual platelets obtained from an intercalatedlayered material.

Compatibilisation: Process of modification of the interfacial properties inCPNC, resulting in formation of the interphase, formation and stabilisation ofthe desired morphology.

7

Introduction

Exfoliated layered material: individual platelets (of an intercalated layeredmaterial) dispersed in a carrier material or a matrix polymer with the distancebetween them > 8.8 nm. The platelets can be oriented, forming short stacks ortactoids or they can be randomly dispersed in the medium.

Exfoliation: converting intercalate into exfoliate.Intercalant: material sorbed between platelets (of the layered material) that

binds with their surfaces to form an intercalate. Often the intercalant is an oniumsalt, viz. C18H37-NH3

+Cl-, that bonds ionically with platelet anion. Intercalationmay involve organic or inorganic salts, monomers, polymers, etc.

Intercalated material: layered material with organic or inorganic moleculesinserted between platelets, thus increasing the interlayer spacing between themto at least 1.5 nm.

Intercalating carrier: a carrier comprising water with or without an organicsolvent used to form an intercalating composition capable of achievingintercalation of the layered material.

Intercalating composition: a composition comprising an intercalant, anintercalating carrier for the intercalant polymer and layered material.

Intercalation: forming an intercalate.Layered material: synthetic or mineral inorganic compound, such as smectite

clay, formed of adjacent layers with a thickness, for each layer, of 0.3-1 nm.Interlayer spacing: also known as d-spacing, d001 or basal spacing is the mineral

thickness (in MMT = 0.96 nm) and the galley thickness, i.e., the thickness of therepeating layers as seen by XRD.

Interlayer thickness = d-spacing less the mineral layer thickness.Li-MMT: lithium montmorillonite.Miscibility: polymer system, homogeneous down to the molecular level,

associated with the negative value of the free energy and heat of mixing, ΔGm ≈ΔHm ≤ 0, and ∂2ΔGm/∂φ2>0. Operationally, it is a thermodynamically stable CPNC,where the clay platelets are well dispersed into a homogeneous polymeric matrix.

MMT: montmorillonite.Na-MMT: sodium montmorillonite.H-MMT: protonated MMT, etc.Matrix polymer: thermoplastic, thermoset, or elastomeric polymer in which

the intercalate or exfoliate is dispersed to form a clay-based polymericnanocomposite (CPNC).

Nanocomposite (NC): a matrix material (metallic, ceramic or polymeric innature) having dispersed particles, with at least one dimension that does notexceed 10 nm.

Platelets: individual layers of the layered material.Polymeric nanocomposite (PNC): a polymer or copolymer having dispersed

in it nanosized particles, viz. platelets, fibres, spheroids, etc.Short stack or tactoid: intercalated or exfoliated clay platelets aligned parallel

to each other.Spacing: Two measures of spacing are used: interlayer spacing, (also known

as d-spacing, d001 or basal spacing) and interlamellar spacing. The former

8


comprises the latter plus the platelet thickness. For example for MMT: d001 =interlamellar spacing + 0.96 (nm).

1.3.2 Methods of Characterisation of CPNCs

1.3.2.1 X-Ray Diffraction (XRD)

The key to CPNC performance is the extent of intercalation and exfoliation, XRDis the principal method that has been used to examine this. One example of thewide angle X-ray scattering data obtained for polystyrene (PS)/clay systems[Tanoue et al., 2003] is shown in Figure 1 as the scattering intensity versus 2θ,where θ is the angle of diffraction. Within the range of 2θ ≤ 10° the XRD spectrumof PS is featureless. Incorporation of 4.8 wt% of intercalated organoclay showsa distinct peak and a shoulder. Their positions and shapes provide informationon the structure of the diffracting species, the organoclay. The presence of multiplepeaks in XRD spectra is quite common - it often originates from differentorganoclay structures and its incomplete change during incorporation in apolymeric matrix [Polzsgay et al., 2004].

The instruments that measure X-ray scattering are divided into the morecommon wide angle and newer small-angle X-ray scattering machines, WAXSand SAXS, respectively. It is common to consider the scattering angle 2θ = 2° asa boundary between these two, but newer WAXS instruments frequently areable to provide reliable scattering profile down to 2θ = 1°. The interlayer spacing,d001, is commonly determined from the XRD spectrum as arbitrary intensityversus 2θ. The spacing is then calculated from Bragg’s law:

d00n = nλ/(2sinθ) (2)

where n is an integer, θ is the angle of incidence (or reflection) of the X-ray beam,and λ is the X-ray wavelength – most X-ray machines use Cu-Kα1 radiation withλ = 0.1540562 nm. For the principal reflection, n = 1, the dependence given byEquation 2 is shown in Figure 2. It is worth noting that, within the range ofinterest for CPNC (2θ = 1-12°), there is a straight-line relation between d001 and1/(2θ):

1/2θ = –0.00012773 + 0.11331d001or: d001 = 0.0011273 + 8.8253/2θ; R = 1.0000 (3)

where R is the correlation coefficient.The peak position and the interlayer spacing related to it is one part of the

information provided by XRD measurements. The intensity of the diffractionpeak and its dependence on the concentration of scattering particles yields another.Cullity and Stock [2001] in their monograph on X-ray diffraction derived thefollowing relation for the intensity (I) of the diffraction peak of α-substancemixed with β-substance:

Iα = Kφα/[φα(μα - μβ) + μβ] (4)

where φα is the volume fraction of the diffracting substance α, and μ is the massabsorption coefficient. Depending on the relative magnitude of μα and μβ withinthe full range of concentration Equation 4 predicts additivity, as well as positiveor negative deviation from it. However, within the limited range of clayconcentrations used in CPNC, the relation may be simplified to read:

9

Introduction

Iα = K´wα Ineat α (5)

where wα is the weight fraction of substance α.It has been frequently observed that during exfoliation (especially during the

mechanical exfoliation of intercalated clay in a PO matrix) the position of theXRD peak remains at the same angular position 2θ, but it broadens and its

Figure 1 XRD of PS and PS with 4.8 wt% of Cloisite® 10A (clay treated withdimethyl benzyl hydrogenated tallow ammonium chloride (2MBHTA)) [Tanoue et

al. 2003].

Figure 2 Bragg’s law dependencies for Cu-Kα1 radiation with λ = 0.1540562 nm.The straight line is given by Equation 3; correlation coefficient R = 1.0000.

10


intensity decreases. Parallel with these changes there is an enhancement of theCPNC performance. It can be postulated that in this case the exfoliation processinvolves breakage of intercalated stacks and/or peeling of individual platelets orshort stacks. On this basis Ishida and co-workers [2000] used XRD to calculatethe degree of exfoliation XE:

XE = 100 × [1 – A/A0] (6)

where A and A0 are the area under the XRD peak for the PNC and for themixture with intercalated clay, respectively. Replacing in Equation 6 the intensityratio of Equation 5 by the area under the peak ratio, is motivated by an additionalassumption that during the progressive dispersion process many stacks slightlychange the interlayer spacing hence the observed broadened peak is an envelopeover a family of peaks having similar interlayer spacings.

However, there are several possible sources for the XRD peak broadening ina CPNC – one being the assumed above existence of a variety of clay stacks witha range of similar d001 spacing. Another mechanism of peak broadening is basedon the imperfections in the crystalline lattice of m-layers of clay platelets forminga stack t = (m – 1) × d001 thick. Another mechanism of peak broadening is basedon the imperfections in the crystalline lattice of m-layers of clay platelets forminga stack t = (m – 1) × d001 thick and scattering the X-rays at angles θ1 and θ2.Because of the small angle difference between θ1 and θ2 the destructive interferenceof reflected beams is incomplete [Cullity and Stock, 2001]. Calculation leads tothe following formula credited to Scherrer:

t = kλ/(B1/2cosθ) ; k ≅ 0.9 (7)

where θ ≅ (θ1 + θ2)/2 is the angle of X-ray beam incidence corresponding to thepeak position, λ is the X-ray wavelength, and B1/2 ≅ θ1 - θ2 is peak width (inradians) at half peak height (Imax/2). From Equation 7 the number of clay plateletsper average stack with the interlayer spacing d001 is:

m = 1 + t/d001 (8)

Note that in this interpretation the peak broadening is caused by crystalline defectsin individual platelets within the stack having about constant interlayer spacing,and not by overlapping peaks that correspond to stacks with different interlayerspacing.

The development of technology often requires more precise information onthe interlayer spacing than that provided by WAXS. With growing frequencySAXS and small angle neutron scattering (SANS) are being used within the effectivescattering angle down to 2θ = 0.05 or the characteristic diffracting distance ofabout 180 nm.

Thus, for example, Bafna et al. [2003] used SAXS and WAXS to study theeffects of addition of maleated polyethylene (PE-MA) on PE/MMT structuralparameters. By performing scattering experiments on specimens oriented in threeorthogonal directions, the authors managed to determine not only the interlayerspacing, but three-dimensional (3D) orientations of six structural features, viz.size of tactoids (ca. 120 nm), organoclay (d002 ≅ 2.4 to 3.1 nm), spacing inundispersed clay (d002 ≅ 1.3 nm), clay (110) and (020) planes, thickness of PEcrystalline lamellae (d001 ≅ 19 to 26 nm), and polymer unit cell (110) and (200)planes.

11

Introduction

1.3.2.2 Small Angle Neutron Scattering (SANS)

Small-angle neutron scattering (SANS) may also be used to determine d001 spacingin PNC. The method is more sensitive, permitting the range of measurements tobe extended to small angles, thus to large spacings. Furthermore, it is readilyadapted to different specimens and provides additional structural informationnot available from XRD.

As far as nanocomposites are concerned, SANS was initially used to studystructure and interparticle interactions in aqueous dispersions of MMT, hectoriteand kaolinite [Ramsay and Lindner, 1993; Brown et al., 1998]. Measurementswere carried out on a water dispersion of MMT under static conditions andshear flow in a Couette-type cell. At concentration w = 5 to 65 g/L and low ionicstrength, the suspensions were thixotropic. The self-organised structures were moreextensive when the MMT particles were more anisotropic. Here, under no-flowcondition, SANS detected preferential platelet alignment at distances larger thand001 = 10.3 nm. Under low shear stress, MMT platelets showed preferentialalignment in the direction of flow. This orientation occurred over a large rangeof deformation rates, giving rise to anisotropic scattering; furthermore, spatialcorrelations persisted. At shear rates exceeding 104 s-1 the ordered plateletstructure was destroyed and only preferential alignment was observed. Shearalignment was also observed in dispersions of kaolinite. As before, particlealignment increases with flow rate.

More recently, SANS has been used to study the structural aspects of claydispersions in water-soluble polymers. Jinnai and co-workers [1996] investigateda four-component clay-polymer-salt-water system, consisting of polyvinyl methylether (PVME; Mn = 18 kg/mol, radius of gyration

rg

2 1 2/≅5 nm), Na-vermiculite

[Si6.13Mg5.44Al1.65Fe0.50Ti0.13Ca0.13Cr0.01K0.01O20(OH)4Na1.29], n-butyl-ammoniumchloride, and heavy water. A single vermiculite crystal was placed in the cell andthe concentration of PVME changed. It has been known [Walker, 1960] thatdepending on the temperature the system has two structures, separated by Tc = 14 °C:below Tc vermiculite shows uniform swelling (gel phase) with d001 of about 12 nmwhile above this temperature the gel structure degenerates into local tactoidswith d001 of about 2 nm (tactoid phase).

The neutron diffraction experiments were carried out at the Japan AtomicEnergy Research Institute using the SANS-J instrument. The study showed thataddition of PVME had no effect on the phase transition temperature between thetactoid and gel phases. Furthermore, in the tactoid phase the spacing of 1.94 nmindicated absence of polymer diffusion into the interlamellar galleries. However,in the gel phase the clay plates were found to be better aligned and more regularlyspaced than in the system without the polymer. The diffraction pattern from thepolymer-containing sample was sharper. It showed pronounced first-order andstrong second-order diffraction peaks, which is rare for an aqueous sample. Inthe gel phase, d001 decreased with polymer content, from 12 to 8 nm.

The conformation and location of the polymer chains in these mixtures werenot unequivocally determined. In the gel phase PVME macromolecules can fit inthe d001 = 12 nm interlayer space. They are either: (A) adsorbed onto the surfaceof a single plate, in a flattened configuration, (B) in the supernatant fluidsurrounding the gel, or (C) they form bridges between two adjacent clay platelets.In the tactoid phase it is not possible for a polymer with

rg

2 1 2/≅ 5 nm to exist as

free chains inside the tactoids.

12


Similar results were obtained when PVME was replaced by polyethylene glycol(PEG; Mn = 18 kg/mol, radius of gyration

rg

2 1 2/≅ 5 nm) [Hatharasinghe, 1998].

Thus, the new system consisted of n-butyl ammonium vermiculite, PEG, n-butylammonium chloride, and heavy water. In analogy to the system with PVME,here also as PEG volume fraction increased from 0 to 0.04 the d001 spacing(obtained by SANS) decreased from 12 to 6.5 nm (see Figure 3). However, theaddition of polymer had no effect on the phase transition temperature betweenthe tactoid and gel phases of the clay system, Tc = 13 ± 1 °C. As for the othersystem, the PEG presence in the gel phase made the clay plates more parallel andmore regularly spaced.

The d001 spacing versus PEG volume fraction, φ, was fitted to the hyperbolicrelation with three empirical parameters ai:

d001 = a0 + a1/(a2 + φ) (9)

The least squares fit to PEG data yielded the following parameter values:ao = 3.896 ± 1.793; a1 = 0.1363 ± 0.0995; and a2 = 0.0163 ± 0.0091 with thestandard deviation σ = 0.571 and the correlation coefficient squared, r2 = 0.9983. InFigure 3 the data from the previously studied system with PVME are also shown.The dependence is similar, indicating that the two chemically different polymersbring about a similar contraction of the gel phase.

It is noteworthy that the authors generalised their d001 vs. φ data using anexponential, instead of hyperbolic dependence:

d001 = a0 + a1 exp{-a2φ} (10)

Figure 3 Interlayer spacing of n-butyl ammonium vermiculite vs. volume fractionof the added water-soluble polymer: circles - PEG, squares - PVME. The solid line

was calculated by the least-squares fit of PEG data to Equation 9.[Data: Jinnai et al., 1996; Hatharasinghe et al., 1998].

13

Introduction

The least squares fit to PEG data yielded the following parameter values:ao = 6.123 ± 0.743; a1 = 6.148 ± 0.737; and a2 = 74.825 ± 24.304 with the standarddeviation σ = 0.513 and the correlation coefficient squared, r2 = 0.9986. Thus, Equations9 and 10 seem to provide similarly good descriptions for the observed phenomenon.However, they give quite different limiting values for infinite dilution of clay plateletsat φ → 0, viz. d001 = 12.26 and 80.97 nm, respectively. As can be judged from the datain Figure 3, the limiting value predicted by Equation 10 is not acceptable.

The explanation for the mechanism responsible for this reduction ofinterlamellar spacing should be consistent with the structure of macromoleculesin suspension as well as with the rapid phase transition from gel to tactoid. Inaqueous medium, n-butyl ammonium vermiculite is expected to form hydrogenbonds with surrounding water. In the absence of polymer, the interlamellar spacingis about 11 to 12 nm. Since water monolayer thickness is about 0.28 nm, thelarge interlamellar spacing most likely originates from the electrostatic repulsionof butyl ammonium chloride ions associated head-to-tail with butyl ammoniumvermiculite salt. Addition of a polymer seems to cause progressive extraction ofthe ionic species from the interlamellar species. It has been assumed that, becausea large number of solvent molecules must be desorbed to accommodate a singlepolymer molecule, the translational entropy so gained by the system provides astrong driving force for polymeric adsorption. However, this assumption maynot be correct in the aqueous media, where the hydrogen bonding of watermolecules is energetically preferred.

SANS has been used by Carrado and co-workers [1996] to monitor thestructural changes in synthetic hectorite upon hydrothermal crystallisation inthe presence of polyvinyl alcohol (PVAl). It was found that the PVAl coats thesmall initially formed silicate particles, hindering their further growth. However,upon removal of the polymer no change has been observed in the extendedinorganic network. Similarly, Muzny and co-workers [1996] monitored thedispersion of synthetic hectorite in a polymer matrix. The organically modifiedclay platelets were dispersed in polyacrylamide (PAA). The studies showed that ahomogeneous dispersion was achievable only with a large excess (equivalent tofive times the cationic exchange capacity (CEC) of the silicate or higher) of theorganic cationic intercalant.

Krishnamoorti and Giannelis [1997] studied the linear viscoelastic behaviourof end-tethered poly-ε-caprolactone (PCL) with MMT intercalated with ω-aminododecyl acid (ADA). The dynamic shear moduli, storage (G´) and loss (G´´),were determined at small amplitude on either freshly loaded specimens, orpre-sheared at large amplitude. The large-amplitude oscillatory shear significantlyreduced the linear viscoelastic response, decreasing the low frequency value ofG´´ by one and G´ by two decades. Such a sharp decrease of the signal means thatin the sheared specimens the polymeric matrix controls the CPNC flow behaviour;hence the MMT platelets are oriented. The alignment was confirmed by SANS.Similar effects have been reported in several rheological studies (see Section 3.2)as well as for injection moulded CPNCs [Kojima et al., 1994; 1995].

Ho and co-workers [2001] studied SANS of clay suspensions in non-aqueousmedia. The stated aim was to understand and optimise the potential processingconditions for clay dispersed in a polymeric matrix. As a model system the authorsused Na-MMT (Cloisite® Na+) and MMT intercalated with 2-methyl2-hydrogenated tallow ammonium chloride (MMT-2M2HT; Cloisite® 15A orC15A) from Southern Clay Products (SCP). Prior to experiments, Na-MMT was

14


fractionated and from Cloisite® 15A excess of the intercalant was extracted withhot ethanol. The purified samples were dried in a vacuum oven at roomtemperature for 3 days. The clays were then dispersed in chloroform, benzene,toluene, p-xylene, cyclohexane and octane.

Both as-received and purified samples were studied. The scattering profilesand dispersion behaviour in organic solvents of the as-received and purified C15Awere significantly different, confirming that the organic modifier was present inexcess. However, in both cases the clay platelets were fully exfoliated in chloroformwhile they were only swollen in benzene, toluene and p-xylene. The scatteringprofiles indicated that the swollen tactoids of purified clay were thinner, andtherefore more numerous. Neither the purified nor the as-received clay showedany temperature effect on scattering. According to atomic force microscopy (AFM)the average diameter of C15A platelets was in the range of 0.4-1.0 μm, giving theaspect ratio: p = 400 to 1000.

The SANS experiments at the wave vector q ≡ (4π/λ) sin(θ/2) = 0.004 to 0.517 Å-1

(θ is the scattering angle) were carried out at the National Institute of Standardsand Technology (NIST). The measurements were conducted using either dry claypowders or dispersions of C15A in deuterated organic solvents, with Na-MMT ina solution of deuterated/protonated water (D2O/H2O). For well-dispersed, individualthin circular disks, theory predicts that isotropic, total coherent scattered intensitydepends mainly on the platelet thickness, diameter, orientation, concentration, andthe scattering contrast between the solvent and the clay. The presence of the intercalantas well as formation of stacks could also be accounted for.

In agreement with the WAXS data the interlayer spacing of purified C15A wasdetermined by SANS as d001 = 2.43 nm. The d001 spacing in dry Na-MMT was foundto be d001 = 0.99 nm. Both SANS and WAXS data indicated that at a concentration ofca. 0.5 to 4 wt% in deionised water, Na-MMT platelets were fully exfoliated. Thescattering intensity varied with 1/qn, with n ≅ 2.2. This value is consistent withcalculations assuming total exfoliation of platelets, which are 1 nm thick andhave diameter of about 600 nm. For C15A in an organic solvent the thickness ofthe tallow layer depended on the solvent, increasing with the solvent solubilityparameter from 1.61 nm (for toluene or xylene) to 1.86 nm (for chloroform).Correspondingly, the calculated average number of platelets in the stack variedfrom 3 to 1, hence in chloroform the organoclay was fully exfoliated.

1.3.2.3 Transmission and Atomic Force Electron Microscopy (TEM and AFM)

At limiting low scattering angle, 2θ ≅ 2°, the XRD/WAXS scattering intensity andresolution decrease, i.e., the method is not useful for spacing: d > 8.8 nm. Withinthis range TEM may be used to determine the extent of intercalation/exfoliation.However, TEM also offers a direct method for confirming the XRD data(micrograph in Figure 4(a)) and with growing frequency it is being used at lowmagnification (micrograph in Figure 4(b)) to check on the uniformity (or lack) ofclay stack dispersion in polymeric matrix. The low magnifications are also usefulto check the purity of clay, e.g., the presence of non-layered particles such asquartz.

Dennis and co-workers [2000; 2001] proposed a method for quantificationof TEM images. Micrographs with magnification of 130,500 were cover with amask in which twelve squares of 1 × 1 inch (2.5 cm × 2.5 cm) were cut out. Thedegree of dispersion was expressed as the number of clay platelets per squareinch. This dispersion measure was found to correlate well with the tensile modulus.

15

Introduction

Atomic force microscopy (AFM) has been used mainly in a tapping mode atthe cantilever’s frequency of 300 kHz and amplitude of 50-100 nm. The differencebetween the clay modulus and that of a polymer results in good image resolution.

Starting in 1990, high resolution TEM (HRTEM) became the preferred toolfor the determination of structure, in particular of crystalline nanoparticles, viz.carbon (CNT) or boron nitride nanotubes (BN-NT). The magnification inHRTEM is x106 or better, with resolution of 0.1 nm (JEOL 4000EX, 400 kV,Cs = 1 mm, focus spread = 8 nm, divergence angle = 0.7 mrad) [Gavillet, 2001].

1.3.2.4 Fourier Transform Infrared Spectroscopy (FTIR)Since 1964 FTIR has been used to study the hydration of bentonite, and theformation of an electrical double layer between the platelets. Significant differencesin the silicate stretching region, νSi-O = 1150 to 950 cm-1, are related to water(H2O or D2O) and hydrated cation (Na+, K+,Ca+2, etc.) content, which in turn isrelated to the interlayer spacing between the clay platelets [Shrewring et al., 1995].Yan and co-workers [1996] have shown that in hydrated MMT νSi-O exponentiallydecreases with the clay-to-water ratio, all the way to exfoliation. Oxidation orreduction of the metallic ions within the octahedral clay layers introduces changesto CEC clay hydration and swellability, hence the FTIR spectrum [Yan and Stucki,1999].

The use of FTIR for characterisation of CPNCs is more recent. Initially, it hasbeen used in studies of the polymer matrix morphology, e.g., conformation andcrystallisation behaviour of sPS [Wu et al., 2001] or α- to γ-crystalline formtransition of PA-6 [Wu et al., 2002]. However, the Si-O stretching vibration hasbeen found to be very sensitive to long range interactions caused either by imposed

(a)

(b)

Figure 4 TEM of intercalated organoclay in PS matrix: (a) magnification 150k, (b)4.1k [Tanoue et al., 2003a].

16


stress [Loo and Gleason, 2003], or expansion of the interlayer spacing [Cole,2003]. The advantage of the spectroscopic methods, FTIR and Raman, is thatalong with XRD they are applicable to the intercalated system, and stretch toexfoliated CPNC that do not scatter X-rays.

To examine the interactions between the clay platelets, the intercalating agentand polymer, FTIR provides important information. By comparing theexperimental and calculated spectra the type and intensity of interactions can beidentified [Aranda and Ruiz-Hitzky, 1999]. The method has also been used toanalyse the thermal decomposition of ammonium intercalants during the meltcompounding method of CPNC preparation [Tanoue et al., 2003]. More detailsare provided in Section 3.2.

1.3.2.5 Nuclear Magnetic Resonance Spectroscopy (NMR)

Solid-state 1H, 13C, 15N and 29Si magic angle spinning (MAS) NMR spectroscopyat frequencies 100.40, 30.41 and 79.30 MHz, respectively has been used. Thetensile modulus (E) was found [Usuki et al., 1995] to be proportional to the chemicalshift (Cs; in respect to tetramethyl silane) of 15N in ammonium-clay complex withthe slope: dE/d(Cs) = 0.1097 (GPa/ppm). The chemical shifts provide informationabout the degree of clay hydration, the interactions engendered by intercalationand the structure of clay-organic matrix complexes.

NMR has also been used to determine PEG chain dynamics within the interlayerspacing of synthetic mica/MMT [Schmidt-Rohr and Spiess, 1994]. According toAranda and Ruiz-Hitzky [1992] the intercalation of MMT by PEG increased theinterlayer spacing by Δd001 = 0.8 nm. Thus, the polymer conformation within theseinterlamellar galleries may be either a 0.8 nm diameter helix, or two chains in aplanar zigzag conformation. The former was deemed to be more probable.

Measurements of the 1H NMR line widths and relaxation times across a largetemperature range were used to determine the effect of bulk thermal transitions[Kwiatkowski and Whittaker, 2001]. The 13C cross-polarity/magic angle spinningNMR spectra of PEG within the nanocomposite showed that the type of motionbeing experienced by these chains is the helical jump motion of the α-transition,thus the same as within the crystalline phase of neat PEG. The short proton spin-lattice relaxation time in the rotating frame, 1H T1ρ, measured across a widerange of temperatures by 1H NMR provided additional evidence that these chainsundergo helical jump motion. Measurements of 1H NMR spectra across a widetemperature range have confirmed that large amplitude motion of the PEG chainswithin the montmorillonite nanocomposite persist below the Tg of neat PEG.There was no observed change in the rates of relaxation at the transitiontemperatures expected for neat PEG.

Solid-state NMR, both proton and 13C, was used to study CPNC with PA-6 asmatrix [VanderHart et al., 2001]. The nanocomposites with 5 wt% organoclaywere generated either by blending or by in situ polymerisation. The systemscontained mineral MMT having non-stoichiometric amounts of Mg2+ and Fe3+

ions substituted into the octahedral, central layer of the clay platelet. The presenceof Fe3+ ion contaminants in the MMT induced paramagnetic properties. Theparamagnetic contribution to the proton longitudinal relaxation time (T1

H) is afunction of the field and Fe3+ concentration in the clay. These paramagneticproperties can be used to determine the hard-to-get information on CPNCs, viz.the degree of dispersion, the stability of intercalant, etc. Evidently, NMR can also

17

Introduction

provide information on the preponderance of α- and γ-crystalline phases of PA-6in CPNCs. The α-crystallites are characteristic of the neat PA-6, while γ-crystallitesare formed in the presence of clay platelets.

The Fe3+ induced paramagnetism of MMT and the resulting spin-diffusion-moderated reduction in longitudinal proton relaxation time, T1

H, may be used torank the degree of clay dispersion in CPNCs, and to investigate morphologicalstratification of the PA-6 α- and γ-crystallites with respect to the clay surface. Itwas found that variations in T1

H correlate well with TEM measurements of theclay dispersion. The chemical stability of dimethyl dihydrogenated tallowammonium ion (2M2HTA) used as MMT intercalant was also investigated.During the organoclay compounding with PA-6 at 240 °C most of the intercalantdecomposed, releasing a free amine with one methyl and two hydrogenated tallowsubstituents. According to the authors, the combination of temperature and shearstress in blending caused decomposition. However, judging by T1

H, the CPNCswith the best dispersion of clay also had the most extensively degraded intercalant.The polarity of PA-6 macromolecules well compensated for the loss of the2M2HTA intercalant.

Solid state NMR has also been used to quantitatively determine the degree ofclay dispersion in PS/MMT nanocomposites [Bourbigot et al., 2003]. A newmethod, similar to the one described above, was developed. In both, paramagneticFe3+ within the octahedral layer of MMT has been used. The results correlatewith XRD and TEM data. The new method is significantly faster than TEM, butwith the difference that the information pertains to the bulk of the specimen, notto its surface. Evidently, both these methods are applicable only to clays containingparamagnetic Fe3+.

1.3.2.6 Other Methods

MAS-NMR is particularly valuable for the reactive systems, i.e., those PNC thatare prepared by dispersing intercalated clay into a monomer, which is thenpolymerised. Direct evidence that the reaction involves Si atoms has been reported[Sellinger et al., 1998; Komori et al., 1999a,b].

Diverse calorimetric methods have also been used. For example, a differentialscanning calorimetry (DSC)/dynamic thermal analysis (DTA) method was usedto characterise the layered material, e.g., to study clay hydration [Lagaly et al.,1975]. Thermogravimetric analysis (TGA) has been used for examining interlayerpacking density [Vaia et al., 1994]. Chromatography was used to determine theheat of reaction involved in the formation of intercalate complex [Bruno et al.,1999].

The cone calorimeter is an indispensable tool for flammability studies. Themeasurements are conducted according to ASTM E 1354-92 at an incident fluxof 50 kW/m2 using a cone shaped heater. Exhaust flow is set at 24 L/s and thespark continues until the sample ignites. All samples should be run in duplicateand the average value reported. The results are reproducible to within about10% [Gilman et al, 1999; Gilman et al., 2000a,b; Zhu et al., 2001a,b].

Electrophoretic mobility provides an important insight into the surface chargeof clay particles. As expected, large changes have been reported as a function ofpH. This information is crucial for the optimisation of the intercalation process[Wilson et al., 1999].

18


1.3.3 Determination of PNC PropertiesStandard test methods have been used to determine PNC properties. CPNC usuallycontains low clay concentration, which results in increased melt viscosity,significant improvement of barrier properties, reduction of flammability, improvedoptical properties and increased rigidity.

Thermodynamic and flow properties and the diverse effects of nanoparticleson the nucleation and crystallinity of polymeric matrices are discussed later inthis book. For example, it has been reported that the impact strength of crystallinepolymer is reduced by incorporation of nanoparticles. For example, addition of2.2 wt% of organoclay to PA-6 reduced the impact strength by a factor of 4.3.However, recent data suggest that the problem is related to the influence of clayon the crystallisation kinetics. By proper selection of process variables, the sameimpact strength of PNC as of the matrix resin was obtained [Graff, 1999]. Thefundamentals of the mechanical behaviour, details on the measurements of themechanical and barrier properties of CPNCs, as well as on the flame retardancyare included in later sections of this book.

1.3.4 PNC Types and Methods of their PreparationAs shown in Table 2, during recent years many different polymers have beenused as a matrix for CPNC with a diversity of nanoparticles and for a variety ofapplications, from enhancement of mechanical performance, reduction ofpermeability, good flame retardancy, improved optical properties, engenderingmagnetic, electric or light-transmitting performance, biocompatibility,thermochromic effects, etc.

1.3.5 PNCs of Commercial InterestAs shown in Table 3, the commercially important PNCs all contain exfoliatedclays, in particular montmorillonite (MMT). The dimensions of the latter particlesare typically: thickness 0.96 nm, with width and length from ca. 50 to about1500 nm.

Several large plastics producing companies have heavily invested in thedevelopment and production of CPNC (Toyota Central Research andDevelopment Laboratories (Toyota – for short), Unitika, Ube, AlliedSignal-Honeywell, RTP Co., Basell, GM, Showa Denko, Bayer, BASF, Solutia, DowPlastics, Magna International, Mitsubishi, Eastman, etc.). At least one company(Nanocor) is totally dedicated to the manufacture of organoclays and polymerconcentrates for use in CPNC. Ube Industries, RTP Co. and Bayer have startedproduction of the experimental PA-based PNCs with clay nanoparticles formoulding or film blowing. GE Plastics has developed conductive grades of PCand PPE/PA experimental alloy with carbon nanotubes. Additional informationis included in Table 4.

After 2.5 years of development, recently, General Motors Corporation (GMC)announced that in collaboration with Basell Polyolefins (50-50 BASF-Shell jointventure created in 1990 by merging Elenac, Basell and Targor), Southern ClayProducts, and Blackhawk Automotive Plastics Inc., it has developed thermoplasticolefin nanocomposite (or TPO-based CPNC) for step-assist use in 2002 GMCSafari and Chevrolet Astro vans. The key to success is the exfoliation of MMTachieved during polymerisation.

Copper sulfide

19

Introduction

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25

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27

Introduction

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28


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29

Introduction

A TPO with 2.5 wt% clay is as stiff as PP with 10 times the amount of talc. ThePNC is much lighter; the weight savings can reach 20%, depending on the partand the material that is being replaced by the TPO nanocomposite. Thenanocomposites are stiffer, less brittle at low temperatures, more durable andmore recyclable than currently used materials. Parts made of nanocompositescost about as much as conventional TPO, but it takes less material to manufacturethem. GMC expects the price to improve as the volume of CPNC used by thetransport industry will increase. However, for use in body panels the TPO CPNCrequires additional research.

1.3.6 Journals and Research GroupsDuring the last few years there has been an explosion of scientific journals relatedto nanotechnology:

1. Carbon Nanotubes: Synthesis, Structure, Properties and Applications, Eds.,M.S. Dresselhaus, G. Dresselhaus and P. Avouris, Applied Physics Series,Springer, Berlin; 2001.

2. Fullerenes, Nanotubes and Carbon Nanostructures, Marcel Dekker, Inc., NewYork, 2002.

3. International Journal of Nanoscience, World Scientific, Singapore, 2002.4. Journal of Nanoparticle Research, Kluwer Academic, Dordrecht, 1999.5. Journal of Vacuum Science & Technology. B, Microelectronics and Nanometer

Structures Processing, Measurement and Phenomena, Published for theAmerican Vacuum Society by the American Institute of Physics, 1991.

6. Lab on a Chip, Royal Society of Chemistry, Cambridge, UK, 2001.7. Materials Science & Engineering. C, Biomimetic and Supramolecular Systems,

Elsevier, Amsterdam, 1999.8. Microscale Thermophysical Engineering, Taylor & Francis, London, 1997.9. Nano Letters, American Chemical Society, 200110. Nanostructured Materials, Pergamon Press, c1992-c1999; absorbed by Acta

Materialia.11. Nanotechnology, Pergamon Press, 1990.12. Physica E. Low-Dimensional Systems & Nanostructures, North-Holland,

Amsterdam, 1997.13. Virtual Journal of Nanoscale Science & Technology [electronic], American

Institute of Physics, 2000.While there is no monograph on PNC, there are several volumes of proceedingspublished [Komarneni, 1996; Komarneni et al., 2000]. Several industrial consortiahave been established for the sole purpose of advancing R&D on PNC, viz. Dow-Magna International, Toyota-Ube-Mitsui-Mitsubishi, Basell-GM-Southern Clay, etc.In Canada and the USA, there are also research consortia on CPNC at, e.g., theNational Research Council Canada (NRCC), the National Institute for Science andTechnology (NIST) and Edison Polymer Innovation Corporation (EPIC). NRCCactivities focus on CPNC development, NIST concentrates on the inflammability ofCPNCs. To the EPIC consortium five Ohio-based universities have submitted projectproposals on development of PNC for thermoplastics, coatings, elastomers, advancedcomposites, and for core technology (rheological behaviour, nanoparticle/polymer

30


interface, and particle surface chemistry) where fundamental issues will be investigatedthat apply to all of the application technologies. Activities focus on coatings,electro-optical behaviour, barrier properties, elastomers, synthesis and processing,advanced composites and core technologies. NSF announced the Nanoscale Scienceand Engineering (NSE) program for the USA universities with US$74 million budgetfor FY 2001. The number of international conferences on PNC is growingexponentially – in 2001 there were at least six major conferences in Europe andNorth America. Guided by the evident industrial interests, this monograph will focuson the PNCs developed as structural materials comprising exfoliated clay particles.

1.3.7 Historical PerspectiveIntercalation of clays started in the 1930s. For hydrophilic applications in papercoating, H2O + Na4P2O7 + compounds with -OH groups (e.g., PEG) were used athigh stresses [Maloney, 1939]. Dry smectite galley spacing of Δd001 = 0.35 nm,increased upon swelling with 3 molecular layers of H2O to 1.2-1.4 nm. In theearly 1950s, for hydrophobic applications clay started to be intercalated withquaternary alkonium chlorides. For example, fatty quatenary alkonium chlorideswere used, viz. methyl and/or benzyl hydrogenated tallow ammonium chloride(containing 2.0% C14, 0.5% C15, 29.0% C16, 1.5% C17, 66.0% C18 and 1.0%C20 alkyl groups). The organoclays, commercialised by NL Industries asBentone™, were found useful for thickening lubricating greases, oil-based muds,packer fluids and paint-varnish-lacquer removers [Hauser, 1950; Jordan, 1950].Soon, the process was described in the textbooks [Grim, 1968].

The first use of an organoclay in polymers was to reinforce elastomers [Carteret al., 1950]. The patent describes MMT intercalation using diverse oniumcompounds, viz. ammonium, phosphonium, arsonium, stannonium, etc. Theorganoclay was combined with elastomeric latex, and then processed by standardmethods. In 1961, Blumstein reported polymerisation of vinyl monomers (e.g.,methylmethacrylate) in the presence of intercalated MMT. In 1963 Nahin andBacklund patented low density polyethylene (LDPE)-organoclay 1:1 compositions.In the organoclay the amount of onium salt (e.g., hexamethylene diamine) variedfrom 4 to 16 wt%. The aim was to produce rigid, γ-ray crosslinkable compound.The patent describes methods of intercalation, hot mill blending, moulding andirradiation. Nanocomposites with PVC and PS were also discussed. In the lattercase, the authors stress the difference in behaviour of PS melts compounded withorganoclay and polymerised in its presence.

In 1976 Fujiwara and Sakamoto (from Unitika) filed a patent application forthe use of ammonium-salt intercalated clays in a hydrophobic matrix. In particular,the organoclay was added to monomer prior to its polymerisation into a polyamide– this led to the first CPNC. A few years later, Toyota obtained the first US patentfor the polymerisation of several vinyl monomers, e.g., styrene, in the presence ofclay [Kamigaito et al., 1984]. The composition contained 85 wt% of clay, henceexfoliation was impossible, but in the presence of styrene the interlayer spacing ofMMT expanded from d001 = 1.25 to 1.5 nm. Upon addition of dichloro-dimethylsilane rapid polymerisation resulted in good mechanical performance. In thefollowing years attention shifted to dispersion of small quantities of MMT inPA-6 – the work resulted in several US patents [Okada et al., 1988; Kawasumi etal., 1989]. The process consisted of caprolactam polymerisation in the presenceof organoclay.

31

Introduction

The melt exfoliation process for PA-based PNCs was developed in the early1990s at AlliedSignal [Maxfield et al., 1995, 1996; Christiani and Maxfield,1998]. The later patents describe a three step process:

1. Treating a suspension of clay with peptising Na6P6O18 in aqueous solutionat 50-90 °C with organosilanes, organotitanates or organozirconates thatincrease the interlayer spacing to at least d001 = 5 nm.

2. Saturation of the dried clay with precursor monomer then polymerising it.3. Compounding the modified clay with matrix polymer until the desired level

of exfoliation is reached.The complexes of organosilanes, organotitanates and/or organozirconates (withor without onium salt) showed prolonged thermal stability at temperatures above300 °C.

Owing to the high hydrophobicity and non-polar nature of polyolefins, preparationof PO-based CPNC is more difficult; hence historically it followed that of PA-basedones. The first patents from Toyota were for elastomers reinforced with clay and carbonblack (CB) [Usuki et al., 1989]. To prepare a PP-based CPNC Kawasumi andco-workers [1997] intercalated NaMMT using an oligomeric PO with polar telechelichydroxyl groups. Next, a general method for the preparation of PNC with diversepolymers was disclosed [Usuki et al., 1999]. The process involves three steps:

(i) Intercalation of clay with an onium ion rendering it compatible with a ‘guestmolecule’.

(ii) Contacting the organoclay with the ‘main guest molecule’ at T ≤ 250 °C.(iii) Transferring the modified clay to a reactor for PO polymerisation (preferred)

or blending with a synthetic resin.Since the guest molecule and the polymer added in the 3rd step may not be thesame, it is possible to form CPNC with a miscible blend as matrix. The patentlisted vinyl-based polymers, thermosetting resins and rubbers (e.g., PE, PP, PS,polyisobutylene (PIB), acrylics, thermoplastic polyurethane (TPU), styrene-butadiene-styrene terpolymer (SBS), liquid butadiene rubber (BR), polybutadiene(PB), etc.) as the matrix. For example, maleated-PP (PP-MA) with high acid valuehence low MW) was used as the ‘guest molecule’ that resulted in good exfoliation[Kato et al., 1997].

In a patent from Dow, PO-based CPNC was prepared by Nichols and Chou[1999]:

(i) Intercalation with an organic, polymeric or inorganic intercalant (e.g., Si(OC2H5)4,Si(OCH3)4, Ge(OC3H7)4, Ge(OC2H5)4) that resulted in Δd001 = 0.5-60 nm.

(ii) After drying at 50-80 °C or calcination at 450-550 °C, the intercalated materialwas dispersed in a monomer or melt-blended until at least 80 wt% of the layerswere exfoliated (aspect ratio p = 10-2000). The claims mention LDPE andlinear low density PE (LLDPE) copolymers with density ρ = 850-920 kg/m3 anda melt index (MI) = 0.1-10 g/min.

Another method for the preparation of PO-type PNCs followed similar steps[Hudson, 1999]:

(i) MMT was functionalised in H2O/EtOH with aminosilane, e.g., aminoethyl-dimethyl ethoxysilane;

32


(ii) Carboxylated or maleated PO was grafted to the functionalised MMT byamine-carboxyl bond; and

(iii) 0.1-50 wt% of the modified particles was dispersed in PE or PP.The key is the physical bonding through co-crystallisation between the graftedPO and the main PO resin.

One of the drawbacks of the ammonium salt intercalated NC is the lowtemperature stability. To solve the problem, Ellsworth [1999] used 10-80 phrorganophosphonium R1P+(R2)3 cations (where R1 is a C8-C24 alkyl or arylalkylgroup and each R2 is an aryl, arylalkyl, or a C1-C6 alkyl group) with a meltprocessable fluoroplastic. The dispersion was carried out by melt blending in atwin-screw extruder (TSE). The resulting interlayer spacing was d001 ≥ 3.5 nm.While organoclays intercalated with conventional, quaternary ammonium cationsare stable only up to about 250 °C, the one intercalated with tributyl-hexadecyl-phosphonium bromide (3BHDP) was reported stable to about 370 °C, making itpossible to prepare CPNC with high temperature engineering or speciality polymers.

Another method for PNC preparation uses bentonite intercalated with primaryC12- or C16-ammonium chloride and treated with an epoxy [Ishida et al., 2000].The authors consider the method applicable to any matrix polymer. The keyfactor has been the addition of 2 wt% epoxy (e.g., Epon 825; see Figure 5). Inthe first step of the process, MMT was treated with C12- or C16-ammoniumchloride, filtered then dried overnight at 100 °C. The modified clay and epoxywere added to molten polymer and melt mixed for 10 to 120 min.

The epoxy was capable of swelling the ammonium-treated clay, allowingvirtually any monomer or polymer to interdiffuse. Intercalated or exfoliatedcomposites have been prepared with 24 different polymers, using a clay loadingof 10 wt%. Comparing the XRD spectra of samples containing epoxy with thosefor samples without epoxy led to the conclusion that there was an increaseddispersion of clay in the former systems. Several factors affect the efficiency ofnanocomposite formation as well as its structure. Therefore, while intercalationand/or exfoliation are not complete for all the polymers tested, lower clay loading,longer mixing time, and higher swelling agent concentrations might have resultedin a better exfoliation. Table 5 shows that for several polymers (with 10 wt%clay (MMT-ADA) and 2 wt% epoxy (Epon 825)) mixing for 30 min resulted ina partial nanocomposite structure.

Figure 5 Chemical composition of Epon-series of epoxy resins from Shell. Epon828 contains three species: 88% with n = 0, 10% with n = 1 and 2% with n = 2(hence an average <n> = 0.14). In Epon 825 n = 0; in Epon 826 <n> = 0.07; inEpon 1001 <n> = 2.3 and in Epon 1004 <n> = 4.8. Similar resins are available

from other manufacturers, e.g., Dow (DER-series) or Ciba (Araldite-series).

33

Introduction

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)RC(enerporolhC 18 881 detailofxe

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0 071 detalacretni

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34


In principle, owing to the polarity of thermosetting monomers or pre-polymers,the exfoliation of clays in these resins is simpler. The low molecular weightmonomeric components may diffuse more easily to the interlayer galleries.For example, Pinnavaia and co-workers [Pinnavaia and Lan, 1998; Wang andPinnavaia, 1998] prepared epoxy nanocomposites with layered silicilic acid(magadiite). The CPNC showed great improvement in tensile modulus andstrength. Using alkylammonium, dimethyl di-octadecyl amine (MMT-2M2ODA)the clay layers were found spaced at about 8 nm. In another study Wang andPinnavaia [1998b] used MMT intercalated with protonated octadecylamine(MMT-ODA) and PEG, then crosslinked it with diisocyanate. Thus preparedelastomeric polyurethane nanocomposites had the clay dispersed in the form oftactoids. Detailed discussion on these systems may be found in Part 4.2.

Introduction

Part 2

Basic Elements of PolymericNanocomposite Technology

35

Nanoparticles of Interest to PNC Technology

2.1.1 GeneralNanoparticles used in polymeric nanocomposites have been divided into threecategories defined in terms of the number of dimensions of their nanometre size,viz. one dimension (platelets), two dimensions (fibres and whiskers), and threedimensions (nearly spherical particles). Layered nanoparticles that can beexfoliated into a dispersion of individual platelets are of main interest. Out ofthese, in industry the mineral or synthetic clays have a dominant position, hencethese will be discussed in greater detail.

2.1.2 Layered NanoparticlesThe layered materials of interest to CPNC technology have an average plateletthickness from 0.7 to about 2.5 nm. A partial listing is given in Table 6. The average

2.1 Nanoparticles of Interest toPNC Technology

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36


interparticle spacing between layers of the layered material or fibrils of the fibrillarmaterial may depend on concentration – the higher the loading the smaller the spacing.

Since PNCs are mainly used as structural materials, the preferred layeredmaterials are phyllosilicate clays of the 2:1 type, more precisely smectites, and inparticular montmorillonite (MMT). The layer surface has 0.25 to 0.9 negativecharges per unit cell and a commensurate number of exchangeable cations in theinterlamellar galleries. The amount of this high aspect ratio nanomaterial thatneeds to be added to a polymeric matrix to engender CPNC with improvedperformance can be as little as 5 ppm. The aim is to totally exfoliate the platelets,but frequently doublets and short stacks (tactoids) may also be present.Considering the importance of MMT a more detailed description of its structureand properties is given below.

While MMT is abundant and inexpensive its main drawback is that it is amineral with variable composition, which is impossible to totally purify. Variabilityof CPNC has frequently been blamed on structural (particle size distribution andaspect ratio) as well as chemical (surface reactivity) variability. Consequently,there is a growing interest in synthetic or at least semi-synthetic layered materialswith well-controlled physical and chemical properties. While experiments withthese model systems are essential for development of a basic knowledge ofintercalation and exfoliation, the hope is that the technology can be developed toproduce synthetic layered materials on a large-scale for the manufacture of CPNCwith consistent performance characteristics.

To prepare functional PNCs other layered materials (than MMT) as well asnon-layered materials have been used, e.g., metallic particles, metal oxides, metalsulfides, etc. The structure and size of these particles depends on the method ofpreparation, which in turn is dictated by the principal application of thesematerials. There is a growing interest in synthetic layered nanofillers (see Table 7).Hectorite has a significantly smaller aspect ratio (p) than MMT, viz. p < 100, buttheir synthetic homologues have even smaller diameter flakes, viz. p = 10 to 30.By contrast with hectorite, synthetic fluoromica (FM) can be prepared with highaspect ratio, e.g., p < 2,000 [Yano et al., 1993]. Their advantages/disadvantagesare summarised in Table 8.A simple method for producing synthetic fluoromica (FM) at a relatively lowtemperature was described by Tateyama et al. [1993]. Accordingly, a powderymixture of 10 to 35 wt% of an alkali silicofluoride (e.g., Na2SiF6) as the maincomponent (optionally with an alkali fluoride) and natural talc is heated forabout one hour at 700 to 900 °C. The resulting material has the general formula:αMFxβ(aMgF2xbMgO)xγSiO2 where M is an alkali metal (Li, Na, K), and0.1 ≤ α ≤ 2: 2 ≤ β ≤ 3; 3 ≤ γ ≤ 4; a + b = 1 are coefficients. For example,swellable FM may have the composition: talc/LiF/Na2SiF6 = 80:10:10; ortalc/Na2SiF6/Al2O3 = 70:20:10. The heating temperature greatly affectsswellability and the interlayer spacing. For example, the fluoromica producedat 700-750 °C shows the XRD peak at d001 = 0.91 nm, while that produced at780-900 °C has the XRD peak at d001 = 1.61 nm.

A one-step method for the preparation of synthetic grafted smectite clays hasbeen described [Carrado et al., 2001]. By contrast with the older methods that startwith a mineral precursor, e.g., talc, to produce Li-hectorite (CEC ≅ 0.8 meq/g), thenew process involves sol-gel hydrothermal transformation of an organotetraethoxysilane (TEOS) or organotrialkoxy silane, viz. phenyltriethoxy silane (PTES). Aqueousslurries of LiF, magnesium hydroxide, and the silane are refluxed for 2-5 days.

37


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38


The organic content in the resulting PTES-hectorite was ca. 20-25 wt%. Asevidenced by FTIR (peak at 1430 cm-1) the organics comprise phenyl groupsattached to Si via direct Si-C bonds. Furthermore, 29Si NMR peaks at -79 and-66 ppm evidenced the presence of RSi(OMg)(OSi)2 and RSi(OMg)(OSi)(OH)species, respectively. XRD of the synthetic hectorite (FH) showed the interlayerspacing, d001 = 1.3 and 1.39 nm for, respectively, PTES- and TEOS-based clays.The organoclays were found to be stable up to at least 400 °C.

2.1.3 Fibrillar NanoparticlesThe diameter (d) of the fibres of interest is d = 1 to ≤ 20 nm, and the averagelength, l = 30 to about ≤ 200 nm. Useful fibrillar materials are: carbon nanotubes,imogolite, vanadium oxide, inorganic nanotubes, cellulose whiskers, etc.

2.1.3.1 Carbon Nanotubes (CNTs)

2.1.3.1.1 Origin, Characteristics and StructureIijima from the NEC discovered carbon nanotubes (CNTs) in 1991. These arehollow tubular structures with wall thickness of 0.07 nm, and interlayer spacingof 0.34 nm. CNTs are classified as either single- or multi-wall nanotubes (SWNTor MWNT, respectively). While a SWNT consists of only a single cylinder, aMWNT consists of 2 to ≤ 30 concentric tubes.

A SWNT has an average diameter of 1.2-1.4 nm, and density (ρ) = 1.33 to1.40 g/ml. These are hollow graphene tubes with a wall thickness of 0.07 nm,readily aggregating into ropes or bundles with an interlayer spacing of 0.34 nm.The C-C bond length is aC-C = 0.14 nm, hence shorter than that in diamond,indicating greater strength. The theoretical and experimental range of values ofSWNT tensile modulus is E = 1 to 1.5 TPa (that of diamond is 1.2 TPa), tensilestrength is 11 to 63 GPa (10¯100 times higher than the strongest steel at a fraction

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39


of the weight). The electrical resistivity is about 10-4 W-cm, thermal conductivityca. 2 kW/m/K, thermal stability in vacuum up to 2800 °C, maximum currentdensity = 1013 A/m2 (electric-current-carrying capacity 1000 times higher thancopper wires) [Tans et al., 1997].

A MWNT has inner diameter (ID) = 1.2 to 5 and outside diameter (OD) = 10 toover 50 nm. MWNT are described as 2 to 10 μm long, but the limit to how longthey may grow is unknown. A MWNT looks like a rope made of bundles ofconcentric single-wall CNTs. In a bundle each tube is very thin, and the couplingbetween them is weak. As a result, even if one nanotube breaks, it has almost noeffect on the others – the crack is blocked, and the fracture stops. Practically, onemay expect MWNT to have the strength of 130 GPa, i.e., nearly 100 times thatof steel at 6-fold lower density [Ijima et al., 1996; Treacy et al., 1996; Yakobsonet al., 1996; 1997]. CNT are 200 times stronger than carbon fibres (CF) andtheir diameter is more than 1,000 times smaller.

There are a few books on CNTs [e.g., Saito et al., 1998, Harris, 1999 andDresselhaus et al., 2001] and some reviews, viz. Ajayan, 1997, Dai, 1999, Sánchez-Portal et al., 1999, Nalwa, 2000, Fisher and Brinson, 2001, Dai and Mau, 2001,Iijima, 2002, Burstein, 2003, and Mamalis et al., 2004. Less information isavailable on the use of CNTs in polymer nanocomposites, e.g., Lahr and Sandler,2000, Thostenson, et al., 2001, Maruyama and Alam, 2002 and Wang et al.,2003a. There are several journals dedicated to nanoscience and technology, andsince 2002 a specialised journal has been published: Fullerenes, Nanotubes, andCarbon Nanostructures.

If the buckyball is designated C60 then the zero-length ‘carbon nanoball’ isC70 and the carbon nanotube (see Figure 6) might be C1,000,000. From the physicspoint of view a CNT is a unidirectional single crystal, with a unit cell of about1.7 nm that keeps on propagating and repeating. The properties of nanotubesdepend on the atomic arrangement, the tube diameter and length. CNTs arelinear fullerenes that, by definition, contain only hexagonal and pentagonal faces,satisfying the Euler’s theorem. As in a buckyball, the defectless enclosed nanotubemust have 12 pentagons [Dresselhaus et al., 1996; Chico et al., 1996].

CNTs can be described in terms of the tube chirality, defined by the chiralvector and the chiral angle as shown in Figure 7. A CNT can be visualised as astrip cut and rolled from an infinitely large hexagonal sheet of graphite. Thecutting can be done at diverse angles and the strip can have a range of widths.

These can be expressed by the chiral angle (θ) and the chiral vector ( r

C n). Its unitvectors a1 and a2 are also shown [Thostenson et al., 2001]. In terms of chirality,armchair (n, m) = (n, n) and zigzag configurations (n, m) = (n, 0) are distinguished.Tubes with the chiral vector (n, m) = (5, 5), (10, 10), (9, 0) and (10, 5) areknown. Depending on chirality, CNT conductivity can be metallic orsemiconducting.

Figure 6 Carbon nanotube with one end closed.

40


Iijima and Ichlhashi [1993] discovered SWNTs two years after reporting onMWNTs. Since then, several methods for the preparation of SWNT or MWNThave been developed. These include: (1) arc-discharge, (2) laser ablation, (3) gas-phase catalytic growth from carbon monoxide, and (4) chemical vapour deposition(CVD) from hydrocarbons. For use in composites, large quantities of CNT arerequired making the first two methods too expensive. Furthermore, the CNTsare contaminated by catalyst particles, amorphous carbon, and non-tubularfullerenes, thus costly purification is required. The two latter gas-phase processes(3) and (4) are more suitable for large-scale production and tend to yield CNTswith fewer impurities [Huang et al., 1998].

Sarangi et al. [2001] have prepared CNTs starting either with diamond-likecarbon (DLC), with PE, or with CH4 + H2 gases as precursors. Microwave at2.45 GHz with RF bias of -600 V was used. The samples were annealed under vacuumat 1000 °C. Long MWNT were obtained from DLC (ID = 1.8, OD = 3-50 nm).Starting with PE, short MWNT were obtained growing from metal particles.The laboratory production rate at the National Tsing Hua University is ca. 5 gevery 40 min. The CVD method gives great purity MWNTs with high aspectratio [Tai, 2002]. Tangled CNTs with outer diameter of ca. 10-50 nm, can beproduced in large quantity and low cost, but with less control over the aspectratio and structure.

A new and simple method for CNT production was published recently [Panand Bao, 2002]. A common laboratory ethanol burner was used with a flame ofabout 25 mm in diameter in the middle and 80 mm in height with 15 mm wick.The metal substrate was austenitic stainless steel (SS) 20 x 10 x 10 mm withmechanically polished down-facing surface that was pre-etched in HNO3. TheSS block was inserted into the central core of the flame where the temperaturewas 610 to 780 °C. The flame was not controlled. After ca. 20 min a thick CNTfilm was formed. High density, vertically aligned tubes were observed. These

Figure 7 A CNT as a strip cut and rolled from a large sheet of graphite at chiralangle, θ, and different width, expressed by the chiral vector,

rC n, composed of unit

vectors ra1 and

ra2 with their magnitude expressed by the integers (n, m). The latter

are the number of steps along the zigzag carbon bonds of the hexagonal lattice.

Reproduced from Thostensen et al., copyright 2001, with permission from Elsevier.

41


grew uniformly (by the root-growth mechanism) from nickel oxide or iron oxideparticles on the stainless steel substrate. The MWNTs were 20 to 80 nm in diameterand 60 to 100 μm in length.

Another publication suggests that CNT may be quite preponderant – they havebeen found in the exhaust gases of most (if not all) trucks or cars with high mileagethat use either diesel oil or gasoline [Lee et al., 2002a]. The authors collectedsamples from silencers of unleaded gasoline cars with mileage from 1600 to 120,500.Samples were dispersed in acetone for TEM. All showed the presence ofinterconnected spherical-like carbon particles, ca. 20-40 nm. However, highresolution TEM revealed the presence of hollow polyhedral, multilayered (2 to 6)structures with d-spacing of 0.34 nm. The structure was similar to that generatedin the carbon-arc discharge nanotube process (at 2500-3000 °C).

Windle and his collaborators studied aqueous suspensions of CNTs [Shafferand Windle, 1999a]. Stable suspensions were engendered by either acid-oxidationand/or by bromination. The process reduced the tube length by a factor of abouttwo and formed oxygen-containing groups. Plots of suspension viscosity versusCNT concentration indicated the presence of an asymptote at about 3 to 4 vol%(depending on the CNT average length). The authors fitted the dispersion datato the Schltz-Blaschke equation, derived for polymer solutions. Thus an analogywas proposed between entangled polymer macromolecules in solution andentangled CNTs in aqueous suspension. At concentrations exceeding theasymptotic value (the maximum packing volume fraction, φm), e.g., at 5 vol%,the dispersion formed a viscoelastic gel. The data could be fitted to a relationdescribing suspension viscosity, e.g., to that derived by Simha [1952], providedthat the intrinsic viscosity [η] and φm, which depend on the CNT size and aspectratio, are treated as adjustable.

A more thorough rheological study of these suspensions was published byKinloch et al. [2002]. The experimental value of strain for the linear viscoelasticresponse was reported to be 1%. The steady shear flow was highly shear thinning.In another publication from this laboratory, Shaffer and Windle [1999b] usedaqueous suspension for the fabrication of CNT nanocomposites with PVAl asthe matrix. PNCs with up to 50 wt% loading were prepared. The percolationthreshold for the electrical conductivity was found to be at a CNT loading of5 to 10 wt%, indicating that CNT do not behave as rigid rods, but are coiledwith a rather low value of the average aspect ratio.

Song et al. [2003] reported that dispersion of MWNT (diameter, d ≅ 25 nmand length, l ≅ 735 nm) in aqueous medium engendered a liquid crystalline (LC)behaviour. The transition from isotropic to nematic LC took place at CNTconcentration φ > 0.043.

2.1.3.1.2 Computation of Potential CNT PropertiesTo explore the possibilities of CNTs the theoretical computation of properties ofindividual SWNTs, their aligned crystalline bundles and interactions with polymericmatrix have been carried out. These lead to sometimes surprising results that requireexperimental confirmation.

Structural properties of a CNT crystal were computed by Tersoff and Ruoff[1994]. The calculations used a valence force model for computing atomicinteractions within each tube, and the Lennard-Jones 6-12 potential to calculateinteractions between tubes in the bundle. The computations were carried out for

42


bundles of CNTs of uniform diameter, ranging from d = 1 to 6 nm. A localmaximum on the elastic modulus (M) versus d was found for dmax ≅ 1.5 nm.Similarly, the cohesive energy, Ea, versus d was found to go through a localmaximum. The computed value of the equivalent bulk modulus in the radialdirection, Mmax ≅ 0.3 eV/Å3 = 0.69 TPa, is comparable to the tensile modulus inthe axial direction, E = 1 to 1.5 TPa. The computations also indicated that whilethe small diameter tubes, d < 2 nm, are perfectly cylindrical, those with largerdiameters are flattened against each other by van der Waals forces.

Using molecular dynamics (MD) Yakobson et al. [1996] studied deformationof CNTs under load in axial compression, bending and torsion. Depending on thestress, the nanotubes reversibly transformed into different morphologies, with eachshape change associated with an abrupt release of energy and a singularity in thestress-strain curve. The authors also noted that the MD computations resulted inbehaviour predicted by the classical continuum shell model with properly chosenparameters (tensile modulus, E = 5.5 TPa, Poisson ratio, ν = 0.19, and SWNT wallthickness h = 0.066 nm). The model accurately predicted the SWNT behaviourbeyond the linear response. The simulations showed that CNTs sustain largedeformations without signs of brittleness, plasticity, or atomic rearrangements.

Lordi et al. [1999] studied the distribution of pentagons on CNT tips. Fivepentagons are located at the tube end. From high-resolution TEM and computer-image simulation, the authors identified the pentagon distribution, which, as itwas shown, controls the oxidation.

However, the applicability of the continuum model to CNT deformation isnot universal [Harik, 2001]. The author derived two non-dimensional parametersthat control SWNT buckling:

1. The SWNT aspect ratio, pCNT ≡ dCNT/LCNT <<1;2. The axial strain, ε11 = (LCNT - LCNTo)/LCNT <<1.Furthermore, it has been shown that the properties of a small diameter CNTwith d ≅ 2a1 (a1 is the width of the hexagonal C-C cell) may be calculated usinga beam model. The latter model was advanced by Odegard et al. [2002]. Theauthors linked the computational chemistry with solid mechanics by substitutingdiscrete molecular structures with equivalent-continuum models. The methodequates the molecular potential energy of a SWNT with the strain energy of arepresentative truss in the continuum model. The model was used to determinethe effective thickness and bending rigidity of a graphene sheet forming the CNT.The computations gave significantly higher values for the wall thickness thanthat used by Yakobson et al. [1996] (h = 0.066 nm) or measured as the interlayerspacing between graphene sheets (h = 0.34 nm) – for extensional modes ofdeformation h = 0.69 and for shear h = 0.57 nm were found. Similarly, thecomputed flexural modulus was higher (E = 1.22 eV) than that calculated fromRobertson et al. [1992] data (E = 0.85 eV).

In these calculations, a uniform hexagonal structure of long SWNTs wasassumed, while the synthetic CNTs do contain defects. Furthermore, the defectsmay be beneficial for the CNT-matrix interactions. For these reasons, there is acertain dichotomy between the computed and measured parameters. The greatbenefit of the computations is to show the general functional dependencies betweenan idealised structure and performance. While the absolute values of the computedparameters may not be achievable, the tendency is real, which is important forthe practitioners.

43


Computations involving PNCs with CNTs dispersed in a matrix are scarce.For example, Lordi and Yao [2000] investigated the molecular mechanics innanotube-based PNCs using a force-field molecular-model. The binding energiesand frictional forces between CNTs and polymeric matrices were found to play aminor role in determining the interfacial strength. The key factor was the helicalconformation of polymeric macromolecules around the nanotube.

Odegard et al. [2001] proposed a method for computing the reinforcing effectsof SWNTs in a PNC. The main premise has been that the size of macromolecules andnanotubes is similar, thus the basic assumptions of continuum mechanics (that thedensities of mass, momentum and energy do not depend on the length scale) are notvalid. In PNC containing SWNTs the polymer/nanotube interactions depend on thelocal atomic structure. Consequently, the bulk mechanical properties of the PNCcannot be determined through traditional approaches based on continuum. Theadopted approach involved determining: (1) a representative volume element (RVE)of the molecular structure of the nanotube and adjacent polymer chains by usingMD simulations, and then (2) an equivalent-continuum model of the RVE.Accordingly, first the vibrational potential energy was computed as a sum of theenergies associated with bond stretching, angle variation and torsion, as well as thatassociated with the energy of the non-bonded interactions (van der Waals andelectrostatic effects). These energy contributions were summed over the total numberof bonds in the RVE. MD simulations lead to an equilibrium molecular structure ofa SWNT surrounded by macromolecules. The next stage, the equivalent-continuummodelling of the RVE, consisted of two main steps: development of an equivalenttruss model and formulation of the equivalent-continuum model. The constitutiveproperties of a PE/SWNT system were computed, but the results were not presented.It is worth noting that the authors ‘concluded’ that for the computation of Young’smodulus the SWNT wall thickness h = 0.28 nm should be used.

Several reports in the literature, based on direct HRTEM or on Raman spectra,indicate slippage between the concentric shells of MWNT, between SWNT in abundle, and between CNT and the polymeric matrix. Evidently, slippage takesplace at higher strains hence it affects the tensile strength of PNC but not theinitial modulus. The basic question is whether covalent bonding of the matrixpolymer to CNTs may improve the situation. Here the serious danger is that thechange of CNT structure may significantly reduce the mechanical properties ofthe tube itself.

Frankland et al. [2002] attempted to simulate the effect of covalent bondingbetween (10,10) SWNT and either a crystalline or amorphous PE matrix. Thissystem was used as a model for the initially non-bonded interface. The simulationsindicated that grafting the matrix to SWNT could have a large influence on shearproperties. The effects of grafting on modulus were not reported.

Bulk properties of CNT/polyimide and CNT/PE systems were also computedfor various CNT composite geometries using the tree-step approach, alreadydescribed [Odegard, 2003]. The bulk mechanical properties for various CNTlengths, volume fractions, orientation, and CNT/polymer interfacial characteristicswere computed. The results indicated that modification of SWNT provides betteradhesion to the polymeric matrix, but might result in poorer overall mechanicalperformance than that of non–modified ones, where only van der Waalsinteractions between CNT and the matrix are considered.

Frankland et al. [2003] computed the mechanical properties of SWNTdispersed in an amorphous PE matrix .The main goal of this work was to compare

44


the behaviour of PNC containing continuous and short nanotubes. In both casesmechanical loading in the axial and transverse direction to the SWNT was applied.The system with continuous SWNT was highly anisotropic, with greatly increasedmodulus in the draw direction. The system with short SWNT did not showincreased modulus relative to the polymer. The MD stress–strain curves weresimulated assuming uniform strain on the entire model.

2.1.3.1.3 Non-Polymeric Applications of CNTsCNTs have a broad range of electrical, thermal, and structural properties thatdepend on the nanotube diameter, length, and chirality, or twist. They come ininsulating, semiconducting and conducting form. The use of CNTs is beingintensely explored in a wide variety of applications, e.g.:

• One of the first applications of CNTs has been in scientific instruments to probeor move atoms, e.g., for the fabrication of sharp, strong and functionalised AFMprobe tips. Thanks to CNTs the resolution of AFM has been greatly improved.

• Another application of CNTs is as conductive molecular wires. CNTs havebeen used for electrostatic charge dissipation (this topic is part of thediscussion on polymeric nanocomposites with CNTs; see Section 2.3.1.5).

• CNTs have been used in vacuum-tube field emission devices (FEDs). Forexample, lamps with CNT are twice as bright as traditional, are more energy-efficient and last ten times longer. Flat panel FEDs are being developed forcomputers and TV-sets.

• Since the electrical conductivity of CNTs varies in the presence of diversechemical gases, their application as chemical sensors is being explored.

• There are CNT structures that cause the tubes to bend when exposed tovoltage. This opens wide the field of applications as nanoscale actuators,tweezers, switches as well as mechanical binary memory (on-off) devices.

• A nanotube-based random access memory (RAM) device with a memorydensity around 1 GB/mm2 and an operation frequency around 100 GHz hasbeen reported [Rueckes et al., 2000]. It is expected that CNTs will findapplication in smaller and faster computing machines.

• In addition to their high aspect ratio and electronic capabilities, carbonnanotubes can also function as durable bearings and springs. Induced fillingof the nanotubes further extends the diversity of CNT applications.

• The outstanding thermal conductivity suggests application as heat sinks formicroelectronics, e.g., eliminating the annoying fans in laptops.

• Potential applications of CNTs in structural materials include nanocompositesfor lightweight vehicles for space, air and ground, extra strong fibres andtechnical textiles for superstrong bulletproof vests and ropes. SWNTs withthe ability to collapse under compression and then to recover are ideal forheavy-duty shock absorbers.

• CNTs have also been used for: hydrogen storage, radar-absorbing coatings,to extend battery lifetime, etc.

The main problem with the use of CNTs is the difficulty in manufacturing themin pure, reproducible form (hence the high cost, e.g., of purified SWNT ca. US$500to 1,500/g – see Table 9). The tubes are ‘sticky’ and tend to aggregate, thus whenused as actuators or tweezers they are sluggish, and in mechanical memory devices

45


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46


they are slow. Furthermore, there are serious difficulties in developing interactionsat the nanotube/matrix interface. Thus, the use of CNTs is limited to high valueadded applications and niche markets, for example, in electronics and space.

2.1.3.1.4 SourcesAs shown in Table 9, numerous organisations on three continents offer CNTs.Depending on purity, method of synthesis and geographical location, the pricevaries from US$1.5 to 1,500 per gram. There is rapid progress in the preparativemethods, which results in increasing purity of the ‘as received’ CNTs. If thepresence of residual catalyst is not detrimental to the desired performance ofnanocomposites, purification may not be necessary and the cost immediatelydrops by a factor ranging from 2 to 20 (for MWNT and SWNT, respectively).Nanocyl offers functionalised CNTs for an additional €50/g. The functionalitiesinclude: -H, -OH, -Cl, -CO, -COOH, -NH2, -SH, -SCH3, etc.

Hyperion Catalysis commercialised unpurified CNTs at US$44/kg (15-20%purity). These are also available in masterbatches with PC, PBT, PET, PS andPEEK. GE Co. has developed a polyphenylene ether (PPE)/PA alloy (NorylGTX-990EP) with ca. 2-wt% of CNT for electrostatic painting [Scobbo, 1998;Scobbo et al., 1998]. The alloy has low density (1080 kg/m3), HDT 158 °C,notched Izod impact strength 600 J/m2, flexural modulus of about 2.4 GMPa,tensile strength 60 MPa, elongation at break 22%, bulk resistivity of ca. 30 Ωmand MI = 4.8. The alloy is being used for automotive mirror housings, doorhandles, gas tank caps, fuel lines and plastic fenders. The technology is beingadapted to acrylonitrile-butadiene-styrene terpolymer (ABS), PC, thermoplasticpolyester (PEST) and PA automotive compounds.

Toray Industries has developed a catalytic CVD method of producing dual-wall carbon nanotubes (DWNT), which when commercially produced in 2004will be available at US10¢/g. According to the Mitsubishi Research Institute, by2020 the market for fullerenes and CNT will expand to US$3.6 billion. Theprojected value of nanostructured materials in 2005 and 2010 is US$96 and208 billion, respectively [Despotakis, 2003].

2.1.3.1.5 PNC with CNTs for Electrical ConductivityThe earliest use of CNTs in polymeric nanocomposites was for engenderingfunctional properties, e.g., a semi-conducting, non-linear, light-sensitive currentinjection (light-sensitive electrical conductivity) [Romero et al., 1996]; formolecular optoelectronics consisting of CNT dispersed poly(m-phenylene-vinylene-co-2,5-di-octoxy-p-phenylene-vinylene) (PmPV) with the electricalconductivity increased by up to eight orders of magnitude [Curran et al., 1998];carbon nanotubes helically wrapped by poly(phenyl acetylene) chains for reducedphotodegradation [Tang and Xu, 1999]; polymerisation of pyrrole onto CNTfor enhanced electrical, thermal and magnetic properties [Fan et al., 1999].

MWNTs were dispersed in polyphenylene vinylene (PPV) by spin coating forhigh quantum photovoltaic efficiency (ca. twice as large as that of indium-tin-oxide) [Ago et al., 1999]. A novl PNC of an electroluminescent, conjugated PmPVand MWNTs was prepared by sonication of the two constituents in toluene[McCarthy et al., 2000]. As observed under TEM, the polymer crystallised onthe CNT, coating it uniformly and growing out in the form of tree branches.MWNTs have the ends normally closed, which requires structural defects, viz.dislocations, sp3-hybridised carbon, or the presence of either pentagons or

47


heptagons. Defects may also occur along the tube body. These nucleate thecrystalline growth of the semiconjugated PmPV.

Sandler et al. [1999] reported that a CNT loading of 0.1 vol% in epoxyresulted in an increase in electrical conductivity of four orders of magnitude (to0.01 S/m). A nonionic surfactant has been used to investigate the effects of thedegree of CNT dispersion on conductivity [Gong et al., 2000].

PNCs of MWNTs (d = 80-90 nm) with polypyrrole (PPY) have been preparedusing in situ polymerisation [Chang et al., 2000]. The PNCs were characterisedusing SEM, TEM, XRD, Raman scattering, thermogravimetric analysis (TGA),conductivity, and magnetic susceptibility measuring devices. The PPY adhered tothe CNT exterior, without chemically bonding. The process affected polarondensity and the orientation of PPY macromolecules. The magnetisation of thecomposite was found to be the sum of the magnetisations of the components.The semiconductor-like conductivity of the PNC was larger than that of PPY.The thermal stability resembled that of PPY.

CNT modification, their conversion into carbides: SiC, WC, etc., coating withmetals or organic electrically conductive polymers has been carried out for adecade. Recently, one-dimensional PPY/CNT nanocomposites were produced byelectrochemical deposition of PPY onto CNT in well-aligned large arrays [Chenet al., 2001a]. The coating thickness was controlled by the film-formation charge.Uniform thickness from 10 to 93 nm was obtained, changing the morphologyfrom coated individual tubes to filling the gap between all CNT in an array,forming uniform conductive material.

Unpurified SWNTs with a broad distribution of diameters (dav ≅ 1.1 nm) andlengths was dispersed in Epon 862 to increase its thermal conductivity [Biercuk etal., 2002]. At 1 wt% loading the PNC showed a 70% increase in thermal conductivityat 40 K, rising to 125% at room temperature. The percolation threshold for electricalconductivity was between 0.1 and 0.2 wt% of the SWNT loading. These resultssuggest that the thermal and electrical properties of SWNT-epoxy nanocompositesmay be improved without the need to purify or chemically functionalise the nanotubes.

2.1.3.1.6 GraphiteWhen electrical conductivity is desired, CNT can be replaced with exfoliatedgraphite. Chen et al. [2001b] applied this strategy. The authors used naturalflake graphite with diameters ranging from 50 to 1000 μm. The graphite wasfirst expanded by treating it with a mixture of concentrated sulfuric acid andnitric acid for about 16 h. After washing and drying at 100 °C the material washeat treated at 1050 °C for 15 s, thereby expanding graphite particles in thec-axis (or orthogonal) direction by a factor of about 350 (compared to the originalgraphite). The expanded graphite was mixed with styrene (St) and methylmethacrylate (MMA) mixture (St/MMA = 70/30), in the presence of benzoylperoxide, then heated at 150 °C for 30 min and cooled to room temperature(RT). A black solid was crushed, rolled on a twin-roll mixer for 5 min, andmoulded into 4 mm thick rectangular plates.

TEM showed that the graphite was dispersed in the form of exfoliated sheetsforming 10-40 nm thick stacks. The transition from electrical insulator tosemiconductor occurred when the expanded graphite content was 1.8 wt% whileat 3.0 wt% loading the electrical conductivity increased from 10-14 to 10-2 S/cmhence by 12-decades. This enhancement may be attributed to the high aspectratio (p), of the dispersed graphite – from the percolation threshold concentration

48


its value, p ≅ 99, was calculated. To preserve the high value of p, hence theelectrical conductivity, extensive roll-milling should be avoided. Addition of upto 5 wt% of exfoliated graphite linearly increased the tensile strength from 24 to29 MPa and reduced the notched Izod impact strength from 29 to 19 J/m.

Similarly, Xiao et al. [2001] prepared PNC based on exfoliated graphite withPS as matrix. Benzoyl peroxide was dissolved in styrene then exfoliated graphitewas added to the reaction vessel. The reaction mixture was stirred for 4 h at85 °C then for 2 h at 150 °C. Increasing the amount of exfoliated graphite causedthe molecular weight (MW) and molecular weight distribution (MWD) of PS toincrease. The glass transition temperature (Tg) increased to ca. 124 °C. As acontrol, the same compositions were prepared by compounding dry blends at170 °C in a two-roll mill. The in situ prepared PNCs had a higher thermal stabilitythan either PS or PS/exfoliated graphite prepared by melt blending. The volumeresistivity versus exfoliated graphite content is shown in Figure 8. It is noteworthythat incorporation of 5 wt% exfoliated graphite into PS, reduced the volumeresistivity by about 3 or 17 decades, respectively for the PNC prepared by meltblending or by polymerisation. The difference in conductivities originates in thestructural differences between these two types of PNC.

2.1.3.1.7 PNC with CNTs – Thermoset MatrixThe more recent applications of CNT are for enhancement of mechanicalperformance. For the optimum effect CNTs should be dispersed within a polymericmatrix and bonded to it. Furthermore, since the enhancement is related to theeffective aspect ratio, the process must minimise the CNT attrition. The nanotubesare dispersed by mechanical means or by ultrasonication (1) in a monomer(s),which is then polymerised, or (2) in a polymer solution.

Figure 8 Reduction of volume resistivity of PS as a function of incorporation ofexfoliated graphite. The upper dependence is for melt compounded PNC, whereasthe lower one is for nanocomposites prepared by polymerisation of styrene in the

presence of graphite [Xiao et al., 2001].

49


The former, reactive approach is preferred for PNC with thermoset matrix.Most of the early work has been done on epoxy systems. For example, Schadleret al. [1998] dispersed 5 wt% of MWNT in Epon 828, and then cured it withtriethylene tetraamine. In standard tension and compression tests only relativelysmall increases of moduli were found, viz. tension increased from 3.10 (for neatepoxy) to 3.71 GPa, while compression rose from 3.63 to 4.50 GPa. The Ramanpeak position that indicates strain in the C-C bond under load, significantly shiftedunder compression, but not under tension. Lourie and Wagner [1998a,b,c; 1999]reported similar results for SWNT/epoxy PNC. These results indicate a basicproblem with load transfer in tension, particularly severe for PNCs containingMWNT. Only the outermost layer of each MWNT may be bonded to the matrix.The interactions between individual layers in MWNT are relatively weak, a vander Waals-type, and the shear strength between layers is small, similar to that ingraphite (σ12 ≈ 0.48 MPa), confirmed by pullout experiments in AFM [Yu et al.,2000a]. These authors measured the tensile strengths of MWNT using a‘nanostressing stage’. The outermost layer showed a Young’s modulus of E = 270to 950 GPa, and a tensile strength of 11 to 63 GPa. As the straining continued,the outer layer broke first via the ‘sword-in-sheath’ failure.

Another reason for the relatively low enhancement of epoxy properties byincorporation of CNT is the tendency of nanotubes to aggregate into bundles. Anearly constant value of the Raman peak in tension was related to tube slidingwithin the nanotube bundles and, hence poor interfacial load transfer betweennanotubes and the matrix [Ajayan et al., 2000]. It is the low modulus of the bundlesthat controls the PNC performance, and not the axial modulus of individual tubes.To enhance the reinforcing effects the authors suggested three methods:

1. Breaking the bundles into individual tube fragments and randomly dispersingthem in the matrix;

2. Radiation or chemical crosslinking of the tubes within bundles to increasethe bundle rigidity and eliminate the inner tube slippage; and

3. Obtaining strong carbon CNT/matrix interfacial interactions.Cooper et al. [2001] dispersed SWNTs or MWNTs in epoxy and studied themicromechanical properties using Raman spectroscopy. SWNTs have beendeformed in a diamond anvil pressure cell. Upon hydrostatic compression thedisordered-related peak at wavenumber 2640 cm-1 up-shifted at an initial rate of23 cm-1/GPa. However, the band downshifted upon application of a tensile stress.These Raman peak displacements provided evidence of the stress transfer fromthe matrix to CNT, thus reinforcement effects. The authors calculated the effectivemodulus of SWNT and MWNT dispersed in epoxy as > 1 TPa and about 0.3 TPa,respectively.

Raman spectroscopy combined with mechanical testing was used to probethe alignment of CNT in PNC [Wood et al., 2001]. SWNT (0.1 wt%) wasdispersed in a UV curable urethane acrylate oligomer by ultrasonication, mixedwith the curing agent and then sheared to induce flow orientation. The UV curedfilms (~150 μm thick) were evaluated, recording the Raman spectral shifts withstrain in the longitudinal and transverse direction. The shifts obtained, in paralleland perpendicular to the flow direction, were significantly different. The adhesionbetween CNT and the polymer exceeded the shear yield strength of the matrix.

Purified MWNTs were dispersed by sonication in a 1,2-dichloroethane solutionof epoxy, polyethersulfone (PES) and 4,4-diaminodiphenylsulfone at 84 °C [Qiao

50


et al., 2002]. Then, the solvent was evaporated and the film crosslinked. The rangeof MWNT loadings explored was from 0 to 40 wt%. The bending strength increasedby ca. 136% to ca. 42 MPa. The volume resistivity decreased by six orders ofmagnitude from 8.59 × 105 to 0.1415 MΩcm. For the purified MWNT thepercolation threshold of electrical conductivity was high, ca. 20 wt%, indicatingthat the CNT were isolated from each other by a layer of polymer.

2.1.3.1.8 PNC with CNTs – Thermoplastic MatrixAs in the case of the thermoset matrices, here there are also two main routes toformation: (1) by dispersing CNT in a polymer solution, and then evaporatingthe solvent, or (2) dispersing CNT in monomer(s) and then polymerising it (them).However, for thermoplastics a third method, melt compounding has also beentried. Evidently, the melt compounding method would be preferred in an industrialenvironment, thus it is being explored with growing frequency.

1. The Solution Method

Whereas the reactive approach is preferred for thermoset matrices, the solutionmethod has been favoured for thermoplastics. The method involves preparationof a polymer solution and mixing it with a dispersion of CNT in the same solvent.For example, Jin et al. [1998] first ground MWNT, then dispersed it in chloroform,and sonicated. The suspension was then added to a chloroform solution ofpolyhydroxyamino ether (PHAE; a thermoplastic reaction product of an aminewith diglycidyl ether and epoxy; BLOX from Dow). To align the nanotubes, castfilms (with up to 50 wt% CNT) were stretched at T = 95 to 100 °C up to 500%.The alignment was confirmed by XRD.

Shaffer and Windle [1999b] dispersed CNT in aqueous PVAl solutions(MW = 85-146 kg/mol), with CNT. Stirring at 480 rpm was required to preventaggregation, and then the adsorbed polymer sterically stabilised the system. Thefilms were cast under controlled water evaporation conditions. The tensile elasticmodulus and damping versus CNT concentration and temperature were examined.The data could be fitted to the theoretical expression if E = 150 MPa was assumedfor the MWNT modulus and the effective length of 35 nm. A threshold for theelectrical conductivity was obtained at about 20 vol%.

Unpurified SWNTs (d = 5 to 20 nm, l ≈ 1 μm for >70%), produced by the arcdischarge method were used in another study. The CNT and PMMA were mixedtogether in toluene in an ultrasonic bath for 24 h. Films (about 200 nm thick)were obtained by spin casting [Stéphan et al., 2000]. The Raman spectra suggestedthat PMMA intercalated into CNT bundles. At low concentration, the quantityof intercalated PMMA may lead to a destruction of bundles, causing a uniformdispersion of CNT in the solution. The films were prepared for use in multilayerdiodes.

PS (MW = 48 or 280 kg/mol) was dissolved in toluene, and MWNT wasdispersed in it using high energy ultrasonication for 0.5 to 120 min [Qian et al.,2000]. Next, the PS solution and MWNT suspension were mixed in an ultrasonicbath for 30 min. The mixture was cast producing uniform, ca. 0.4 mm thick filmfor tensile tests. The optimum sonication time increased with the nanotube length,viz. 30 and 60 min for 15 and 50 μm long tubes (d = 33.6 nm), respectively. Ahomogeneous dispersion was obtained without attrition. Tensile tests of the filmsshowed that addition of 1 wt% MWNT increases the modulus by 36-42% and

51


stress at break by 25%, indicating significant load transfer across the nanotube-matrix interface. The observed enhancement of stiffness was found to be in goodnumerical agreement with the values calculated from the relation [Hill et al., 2002]:

E /EpK

K

K

K

p l /d ; KE /E

E /E pK

E /E

E /E

c p

1

2

2

f f 1

f p

f p

f p

f p

=+

−+

+−

≡ ≡−

+≡

−

+

⎡

⎣⎢

⎤

⎦⎥

( )( )

( )( )

38

1 21

58

1 21

1

2

1

2

1

2

x xφ

φφ

φ

:

(11)

where E is tensile modulus of a composite, polymer or fibre (subscript c, p or f,respectively), p is the fibre aspect ratio and φ is the fibre volume fraction (calculatedas 0.00487). The modulus of MWNTs depends on diameter; here Ep = 450 GPawas used. Substituting these parameters into Equation 11 yields the PNC modulus(1 wt% of MWNT) for p = 446 or 1167 as Ec/Ep = 1.48 or 1.62, respectively.These values are 10% higher than the experimental data of 1.36 and 1.42,respectively. As in conventional fibre composites, the crack propagation showsMWNT pull-out, as well as crack bridging by the nanotubes.

Hill et al. [2002] produced SWNTs and MWNTs using the arc discharge andCVD methods, respectively. Dispersing into HNO3, refluxing for 48 h,centrifuging, washing and drying under vacuum purified the CNTs. Purifiedsamples were refluxed in thionyl chloride and then treated with poly(styrene-co-p-(4-(4´-vinylphenyl)-3-oxabutanol)). The functionalised CNTs were soluble inorganic solvents (e.g., toluene, THF, or chloroform), making it possible tointimately mix them with PS. A transparent cast film ca. 50 μm thick, was preparedwith 5 vol% CNT. Its transparency indicated that miscibility of functionalisedCNT with PS resulted in homogeneous dispersion.

A similar approach was used by [Mitchel et al., 2002], who dissolved anionicPS in toluene at room temperature, and then added appropriate quantities ofeither pristine SWNT or its functionalised version. The latter contained 4-(10-hydroxy decyl) benzoate groups attached to 1 in 66 carbon atoms. The solutionswere dried at room temperature, and then annealed at 180 °C under vacuum andtested. Nonlinear viscoelastic dynamic melt flow behaviour was absent for PNCscontaining 0.5 wt% of non-functionalised SWNT, but present for PNCs containing0.35 wt% of the functionalised CNT. Furthermore, the functionalised PNC hadthe percolation threshold at 1 vol% SWNT, while that for the pristine SWNT-based composites was twice this amount. Functionalisation resulted in betterdispersion and significant enhancement of performance.

Poly(p-phenylene benzobisoxazole) (PBO) has been synthesised in the presenceof 0, 5 and 10 wt% SWNT (diameter of ca. 0.95 nm) [Kumar et al., 2002]. Thereaction was conducted in polyphosphoric acid (PPA) at 100, 160 and 190 °Cfor 36 h, and then the product was spun into fibres using dry-jet wet spinning.Dried fibres were heat-treated at 40 MPa tension in N2 at 400 °C for 2 min. NoCNT aggregates were observed under a polarised microscope. The tensileproperties of the PBO fibre containing 10 wt% SWNT indicated that the modulusincreased by 21%, strain at break by 40%, and the tensile strength by 61%. Thecompressive strength was also higher by 43%. The PBO/SWNT systems

52


demonstrated less thermal shrinkage and less creep under stress. The morphologyof these fibres suggests a co-alignment of the SWNT and PBO fibrillae within theoriented composite fibre.

2. The Reactive Method

Jia et al. [1999] reported on the reactive preparation of PNC with MWNT dispersedin a PMMA matrix. Two CNTs were used: (1) as prepared (purity > 98%), and (2)the former CNT ground in a ball mill for 20 min, boiled for 0.5 h in concentratedHNO3 then washed and dried. TEM of the latter sample showed significantfragmentation of the original CNTs. The CNTs were dispersed in methylmethacrylate (MMA), the free radical initiator (2,2´-azobisisobutyronitrile (AIBN))was added and the reaction was conducted at T = 358 to 363 K. For the untreatedCNT the required amount of AIBN was three times greater than what was neededto polymerise MMA alone. The performance of the resulting PNC was poor. Forthe ball milled CNT there was no need for extra initiator and reasonableenhancement of performance was obtained (at 5% CNT tensile strength increased30%, toughness 11%, hardness 42% and HDT by 39 °C (compared to PMMA)).The authors postulated that about 24% of the untreated CNTs were attacked byfree-radicals that open their π-bonds. A C-C bonding may be generated betweenthe nanotube and the matrix. When the π-bonds are at the curved points of thenanotube, the free radical attack may result in breaking the CNTs, which becomeshorter and with opened ends.

The emulsion polymerisation method was used for the preparation of PNCwith polyaniline (PANI) as a matrix in which 0.2 to 10 wt% of CNT was dispersed[Deng et al., 2002]. TEM and XRD showed a network made of nanotubes andPANI fibres. However, according to FTIR there was no direct interaction betweenthese two components. The conductivity and thermal properties depended onCNT content, viz. PANI conductivity increased 25-fold by incorporation of10 wt% CNT.

3. The Melt Compounding Method

Considering the high cost of CNT and the need to work with small quantities ofmaterial, the early publications revealed quite unorthodox melt processing methods[Haggenmueller et al., 2000]. The authors prepared SWNT/PMMA nanocompositeswith enhanced mechanical and electrical properties by engendering high CNTalignment. First, PMMA was dissolved in DMF and combined with the SWNTdispersion in this solvent (DMF was used for purification of the CNT). The castfilm was subsequently folded and broken into pieces, then hot pressed at 180 °C.The procedure was repeated 30 times. The final dispersions (containing 1 to 8 wt%of SWNT) were melt spun to draw down ratio: DR = 20 to 3600. The elasticmodulus and yield strength of the PNC fibres increased with CNT content andDR. Raman spectroscopy indicated that the nanotubes were well aligned. Theelectrical conductivity increased with CNT content, e.g., for 1.3 to 6.6 wt% SWNT,from 0.118 to 11.5 S/m in the flow direction and from 0.078 to 7.0 S/m in theperpendicular direction.

Lozano and Barrera [2001] used an internal mixer for dispersing 2 to 60 wt%of CNT in PP. The CNT was MWNT-type prepared by CVD using large diametercatalyst particles, which resulted in large nanotubes, having diameter of ca. 80 to200 nm and length of 30 to 100 μm. Melt compounding eliminated CNT

53


agglomerates yielding isotropic, non-porous PNC. Incorporation of the nanofillerincreased the rate of crystallisation as well as the total crystallinity (from 49 forPP to 69 for PP with 60 wt% CNT). The tensile yield strength reached maximumat 5 wt% loading – for higher CNT content the strength was reduced below theneat PP value. By contrast, the dynamic tensile modulus at room temperatureincreased from ca. 5 (for PP) to ca. 18 GPa (for PP with 60 wt% CNT), i.e., by350%. The authors associated the lack of reinforcement by the CNT with theincreased brittleness of the PP, caused by its inability to further crystallise ondeformation, ‘a property brought on by the molecular restrictions caused by thefiber dispersion’.

High purity MWNT (purity >95%, diameter 20 to 30 nm, length 20 to100 μm) was uniformly dispersed in PP, ABS, PS or HIPS in an internal mixer[Andrews et al., 2002]. The samples from the mixer were crushed, and thencompression moulded into films containing v = 0.5 to 5 vol% of CNT. Thepercolation threshold for surface resistivity varied with the matrix; for PP, PSand ABS it was 0.05, 0.2 and about 9 vol%, respectively. The mechanicalproperties showed small improvements. Addition of 12.5 vol% of MWNT to PPresulted in a doubling of the modulus and a decrease of tensile strength by afactor of 1.7. For PS systems the modulus increased with 25 vol% loading from1.9 to 4.5 GPa, along with a linear increase of the glass transition temperature:Tg (°C) = 96 + 0.48v (vol%). For the other two systems there was little change inthe mechanical properties. The observed failure (nanotubes pull-out) suggestedthat surface treatment might improve interfacial bonding and increase tensilestrength.

Pötschke et al. [2002] used a commercial PC compound from Hyperion, with15 wt% of MWNT (d = 10 to 15 nm; L = 1 to 10 μm). The masterbatch wasprepared by melt compounding in a Buss Kneader. The authors diluted it with PCin a short TSE (L/D = 10) at 240 °C, to obtain concentrations of 0.5, 1, 2 and5 wt%. The percolation threshold for electrical conductivity was somewherebetween 1 and 2 wt% – the volume resistivity decreased from 1014 to about102 Ωcm. The dynamic melt flow of compression moulded PNC was studied at260 °C and the frequency range from 0.1 to 100 rad/s. Nonlinear viscoelasticbehaviour was evident for a MWNT loading above 1 wt%, coinciding with theelectrical percolation threshold. The increase of melt viscosity with nanotubecomposition was higher than that reported for either nanofibres with largerdiameters or for carbon black. This difference is caused by the high aspect ratioof the MWNT, p > 67.

Melt dispersion of SWNTs and MWNTs in PC has also been described [Sennettet al., 2003]. The MWNT prepared by CVD had d = 20 to 50 nm and L ≅ 20 μm;hence a respectable aspect ratio of about 1000. A conical micro-TSE was used at250 to 266 °C, 80 rpm under N2, and the compounding time was varied from1 to 120 min. To induce orientation, the extrudate was spun at a fibre drawspeed of 10 to 70 m/min. Good dispersion was obtained even at short residencetime (1 to 5 min), but longer mixing further improved the dispersion. Similarly,the CNT orientation improved as the fibre draw rate increased, but to obtaingood alignment draw rates > 70 m/min were required. SWNT were found to bemore difficult to disperse than MWNT. The use of lower molecular weight resinwas reported to facilitate the dispersion. Unfortunately, this short and interestingnote does not contain any data on the electrical conductivity or mechanicalperformance.

54


2.1.3.2 Rod-Like CdSe Nanocrystals

These nanocrystals show size- and shape-dependent optical and electricalproperties. Quantum rods of CdSe were prepared by pyrolysis of dimethylcadmium and selenium tributyl phosphine solution in phosphonic acid. The rodshad 3.0 to 6.5 nm diameter and were 7.5 to 40 nm long. As expected, the bandgap shifted to lower energies with increase of both the diameter and the length ofthese crystals [Li et al., 2001a].

2.1.3.3 Imogolite

Imogolite (from imogo = volcanic ash in Japanese) is weathered pumice, discoveredin Murakasami, Japan and described in 1962 by Yoshinaga and Aomine. Thispara-crystalline phyllosilicate has a composition written either as: Al2SiO3×(OH)4;SiAl2O5 · 2.5 H2O or Al2O3×SiO2×nH2O, with molecular weight per crystallineunit cell of 4754. However, while the nominal Si:Al ratio is 2, the measured onevaries from 1.05:1 to 1.15:1. Imogolite occurs as tubes of several micrometers inlength, having inside diameters of 1.0 and outside diameters of 2.0 – 2.52 nm;occasionally the tubes may branch out. The nanotubes have good rheological,adsorptive, and surface properties due to their unique structure and functionalgroups on the surface. For example, the tubes may form raft-like structures withhoneycomb-shaped cross-section [Kajiwara et al., 1986; Donkai et al., 1993].

Imogolite may also be synthesised at T = 25-100 °C from a dilute solution ofAlCl3 and Na4SiO4 via precipitation reactions. The imogolite structure has Si intetrahedral coordination and Al in octahedral coordination. Imogolite has beenused for reinforcing water-soluble polymers, viz. polyvinyl alcohol (PVAl)[Hoshino et al., 1992a] and hydroxy propyl cellulose (HPC) [Hoshino et al.,1992b]. Imogolite has several rather broad, low XRD peaks. Conclusiveidentification can be made by FTIR and TEM. Its CEC is 1.7 mol/g at pH 7.0, thecalculated specific surface is 1000, while its measured value is 900 to 1100 m2/g.The density is 2.7 g/ml, hardness = 2 to 3, refractive index (nD) = 1.47 to 1.51.The crystals are transparent and fragile [Gabriel and Davidson, 2000].

2.1.3.4 Vanadium Pentoxide, V2O5

Since CNTs are expensive and inhomogeneous (because of a wide range ofchirality), V2O5 nanofibres have been proposed as an alternative. These may beformed by polycondensation of vanadic acid (HVO3) in water. The individualflat fibres are ca. 10 nm wide, ca. 1.5 nm thick and up to several microns long. Asingle fibre has a double layer structure, each consisting of two V2O5 sheets.Consequently, each fibre consists of only four layers of vanadium atoms andrepresents a wire of molecular dimensions. Owing to the disassociation of surfaceV-OH groups (V-OH + H2O ↔ V-O– + H3O+) the fibre surface is negativelycharged. The V2O5 fibres are being investigated for electronic applications [Sinn,1999]. Furthermore, V2O5 can also be formed into ribbons, sheets or nanotubes.

V2O5 is available in a diversity of morphologies. Commonly, it is an orange-yellow to rust-brown orthorhombic crystalline solid with molecular weight of304.15, density of 3,350 kg/m3, melting point of 670-685 °C and boiling pointof ca. 1800 °C (it starts losing oxygen at 700 °C) [Macintyre, 1992; Cotton etal., 1999]. V2O5 is used as a catalyst, e.g., in the oxidation of SO to SO2, ofalcohol to acetaldehyde, of aniline to aniline black, for the manufacture of yellowleaded glasses with inhibited UV light transmission, for depolarisers, etc. Its main

55


commercial use is as colorant, with colour strengthened by tin and zirconia.Although yellows can be prepared with antimony, vanadium is stable at highertemperatures.

Wider V2O5 fibres, e.g., 1×25×(200 to 600) nm, are known as molecularribbons. These may also be synthesised at pH2 from VO(OH)3x2H2O. The ribbonthickness is determined by the molecular structure. A magnetic field aligns the ribbonsin the field direction. The ribbons are promising new materials that may be used as animportant catalyst, a building block for electronic devices and reinforcing materialwith a strongly anisotropic structure [Gabriel and Davidson, 2000].

During the hydrothermal synthesis, organic templates control the crystallisationmode. This method has been used to prepare layered structures of V2O5 in thepresence of coordination compounds, viz. 2,2´-bipyridine and ethylene diamine[Nesper et al., 2001; Ollivier et al., 1998]. Muhr et al. [2000] prepared V2O5 andmixed vanadium oxide nanotubes (VONT), reacting either primary n-alkyl amines(CnH2n+1NH2; 4 ≤ n ≤ 22) or α,ω-diamines (H2N-CnH2n-NH2; 14 ≤ n ≤ 20) withvanadium (V) alkoxide, followed by hydrothermal treatment. The monoaminetemplates tend to form wide tube openings and tube walls consisting of 2 to10 layers, whereas the diamines lead to tubes with comparatively thick walls with>10 layers. The amines get intercalated into VONT structures that have 2 to30 layers, resulting in an outer diameter (OD) = 15 to 100 nm and tube lengthL = 500 to 15,000 nm. The interlayer distances (1.7-3.8 nm) increase with thelength of the alkyl chain of the amines. The VONT collapse into amorphous format about 250 °C.

In another publication from the same laboratory [Krumeich et al., 2000]VONT was similarly prepared from vanadium alkoxides and amines by a sol-gelreaction and a subsequent hydrothermal treatment. The product showed bentVOx layers rolled into tubes. The layer structure inside the tube walls is frequentlydisordered, and several types of defects were identified. Vanadium oxide in theform of scroll-like nanorolls was prepared for use as a cathode material forrechargeable lithium batteries [Edström et al., 2001]. The rolls consisted of severallayers, separated by templates, viz. hexadecyl amine (C16H33NH2) or dodecylamine (C12H25NH2).

2.1.3.5 Inorganic Nanotubes

Nanotubes are not limited to carbon and V2O5. In 1992 inorganic fullerene-likenanotubes of tungsten disulfide were described [Tenne et al., 1992]. It seems thatany compound that forms stable two-dimensional sheets can be rolled into ananotube. Different synthetic methods may lead to nanotubes having differenttype, size and yield. The methods range from substitution of atoms in an alreadyfabricated tube [Hang et al., 1998], laser heating [Laude et al., 2000] and arcdischarge [Cumings and Zettl, 2000] to template-assisted synthesis [Shenton etal., 1999; Zhang et al., 2000a].

For example, 2-D tungsten disulfide (WS2) sheets transformed into nanotubesare extremely inert and durable. They show potential for novel scanning probemicroscope (SPM) tips. A standard AFM tip will rarely remain sharp for morethan a few hours under normal operating conditions. WS2 nanotube tips are sorigid and inert that they have been used for months with no sign of wear.

Boron nitride (BN) provides another example. These inorganic nanotubesmay be synthesised in a discharge using a tungsten electrode hollowed and filledwith boron nitride powder [Chopra et al., 1995]. In contrast to CNTs, the ones

56


from BN should all be insulating with a band gap of about 5 volts. Anothermethod of preparation is by means of a solid-state process. Graphite andhexagonal boron nitride powders were ball milled at room temperature, thenannealed below the melting points for both graphite and boron nitride, atT ≤ 1300 °C. The latter stage leads to the nucleation, re-ordering and crystalgrowth of hexagonal C and BN nanotubes of both cylindrical and bamboo-likemorphology. High energy ball milling was found to result in solid-state phasetransformations and chemical reactions [Chen et al., 1999a].

Gole et al. [2003] prepared and characterised silica-based nanospheres(monodispersed, with d ≤ 30 nm diameter), nanotubes, nanofibre arrays, andnanowires. The agglomeration of small nanospheres (d ≤ 10 nm) into wire-likegroupings has suggested a possibility of growing silica nanotubes (SiNT) by amore efficient agglomeration process. The SiNT were generated from siliconparticles at 1400 oC. Their outer diameter was, OD = 70 to 80 nm, with wallthickness of ca. 20 nm. Biaxially structured SiC-SiOx nanowires have now alsobeen generated from C/Si/SiO2 mixtures. Depending on the size, the Young’smodulus of these structures is 50 to 100 GPa.

2.1.4 Other Nanoparticles

2.1.4.1 Spherical or Nearly-Spherical Particles

For several decades carbon black and fumed silica with particle size below 50 nmhave been used in tyres, tubings, sealants, etc. More recently, functionalised silicananoparticles were copolymerised with methyl methacrylate to generate transparentfilms with good surface finish and controllable (by silica content, x) refractive index:nD = 1.509 – 0.00035x [Yu et al., 2003].

Newer, nanosized particles are metal, metal oxide, nitride, sulfide or carbidepowders. They find use in the preparation of materials with specific electricaland/or magnetic properties. CdSe, ZnS, CdS, PbS, and many others have beenused for the preparation of functional PNC, for example showing non-linearoptical quantum-size or other semiconductive properties. For example, nanosizeddispersions of ZnS and CdS in PVAl were prepared by a hydrothermal method.Thus, ZnCl2 or CdCl2 were dissolved in an aqueous solution of PVAl, then CS2was added and the mixture was heated in an autoclave for 8 h at 120 °C. Uniformparticles of ZnS and CdS had a diameter of ca. 60 to 100 and 80 to 120 nm,respectively [Qian et al., 2000]. The same solvo-thermal method was used forthe preparation of nanocrystalline particles of tin chalcogenide or chromiumnitride [Qian et al., 1999a].

Electrolytic precipitation of Cu in pores of solvent crazed polymers (e.g., PVCor PP) resulted in the formation of ca. 15 to 20 nm diameter particles [Arzhakowaet al., 2003]. The size and concentration of metal particles depended on theextent of the crazing.

2.1.4.2 Sol-Gel HybridsSol-gel hybrids are produced by methods which are based on the reaction ofprecursors (e.g., metal alkoxides M(OR)n) that result in the formation of ananometric dispersion of an inorganic phase [Mauritz et al., 1995; Wen andWilkes, 1996; Deng et al., 1998; Hand et al., 1998]. Hydrolysis and condensationleads to the formation of metal oxopolymers. The sol-gel process takes place

57


under relatively mild, well controlled conditions, hence inorganic and organiccomponents can be formed at the nanometre scale, at a wide range of composition.The hybrids usually have ≥ 10% dispersed phase. Two types of hybrids arerecognised:

1. Those where only weak interactions between the organic and inorganic speciesexist (viz. van der Waals, hydrogen bonding or electrostatic interactions).

2. Where the inorganic and organic components are chemically bonded by eithercovalent or ionic-covalent bonds. Examples of PNCs prepared by the sol-gelmethods are provided in Table 2.

Several methods of PNC preparation by the sol-gel method have been developed[Brinker and Scherer, 1990]:

The principal sol-gel process involves the formation of a colloidal suspension(sol) and gelation of the sol to form a network in a continuous liquid phase (gel).Often the hydrolysis is catalysed using either acids (acetic acid, HCl, HF, etc.),bases (ammonia, amines, KOH, etc.) or salts (e.g., KF). The rate and extent ofthe hydrolysis reaction depend on the strength and concentration of the catalyst.From a homogeneous solution of an oligomer or polymer with inorganic precursor,chemical reactions lead to dispersion of the inorganic phase in the polymericmatrix. The precursors consist of a metal or metalloid compound with reactiveligands. Metal alkoxides are most popular because they readily react with water.However, other compounds, viz. aluminates, titanates, and borates are also used,often mixed with silicon alkoxides. After the reaction, the solvent must be removedbefore the dispersed phase has time to aggregate. Control of the interface is crucial.The use of electrostatic charge or hydrogen bonding interactions may be required.For example, silica nanoparticles can be generated by hydrolysis and condensationof silicon tetraalkoxides, Si(OR)4, in polyoxazoline ethanol solution. Hydrogenbonding between Si-OH silanol groups and the carbonyl and amide functionalitiesof the polymer ascertained homogeneous and stable dispersion. It is noteworthythat these advantageous properties could be enhanced by grafting the polymerwith -Si(OR)3 groups, which after hydrolysis increase the chemical affinity betweenorganic and inorganic components.

From a homogeneous solution of an inorganic precursor and a monomer,chemical reactions lead to the formation of inorganic gels, followed bypolymerisation. For example, hydrolysis and polycondensation of silicon alkoxidesin the presence of methyl methacrylate (MMA) leads to PNC with superior opticaland mechanical properties.

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The pre-formatted building blocks (e.g., oxometallic clusters, CdS or CdSenanoparticles, metallic or oxides colloids) are functionalised, saturated with anoligomer or monomer, and polymerised. A review of this strategy has beenpresented [Anonymous, 1999a].

The sol-gel methods lead to the preparation (often at room temperature) ofPNC with inorganic nanoparticles that engender improved hardness, opticaltransparency, chemical durability, tailored porosity, and thermal resistance. Thematerials are used in optics, protective and porous films, optical coatings, windowinsulators, dielectric and electronic coatings, high temperature superconductors,reinforcement fibres, fillers, and catalysts [Zeigler and Fearon, 1990].

The phase separated morphologies of certain copolymers and ionomers canact as 3-dimensional templates during the sol-gel polymerisation of silicon alkoxideand organo-alkoxysilane monomers. In the presence of these templates theinorganic oxide or organically modified silicate nanophases grow within specificnanoscopic domains. For example, Mauritz et al. [2001] prepared a variety oforganic/inorganic nanocomposites via in situ sol-gel polymerisation of metalalkoxide and organo-alkoxysilane monomers as well as copolymerisation of theirmixtures. Thus, poly(styrene-b-isobutylene-b-styrene) was first sulfonated andneutralised, then tetraethoxy silicate was dissolved into the ionic domains.

Nafion® perfluorosulfonic acid membrane and Surlyn® ionomer have beenused as a templates for several silicate and titanate compounds. After hydrolysisand condensation of the ≡Si-OH groups, PNC with highly regular structure ofspherical dispersions, d = 2 to 5 nm, was obtained. The polymer ionic groups areentrapped within the silicate or titanate nanoparticles.

2.1.4.3 Polyhedral Oligomeric Silsesquioxanes (POSS)

2.1.4.3.1 Origin and StructureLadenburg first described the synthesis of silsesquioxanes in 1875. In 1915 Meadsand Kipping studied the hydrolysis and condensation reactions of trifunctionalsilanes, concluding that polycondensation of ‘siliconic acids’ leads to complexmixtures of little synthetic value. This delayed investigation on silsesquioxanesuntil 1955, when Barry et al. [1955] described crystallisableorganosilsesquioxanes, defining the atomic ratio as: O:Si = 3:2, as for example,in H8Si8O12 – the siloxy analog of cubane (C8H8) in which C-C bonds are replacedby Si-O-Si. However, silsesquioxane does not have to be cubic – amorphous andladder-like structures are also known. In 1965 Brown and Vogt conducted acontrollable synthesis of silsesquioxanes.

According to Marcolli and Calzaferri [1999] three synthetic routes have beenused: cohydrolysis of trifunctional organo- or hydro-silanes, substitution reactionswith retention of the siloxane cage, and corner-capping reactions. Murugavel etal. [1999] discussed the chemistry of silanetriols and triaminosilanes as usefulsynthons for the generation of three-dimensional metallosiloxanes. Starting fromstable N-bonded silanetriols and triaminosilanes, metallosiloxanes andiminosilicates with aluminium, gallium, indium, titanium, zirconium, tantalum,tin and rhenium incorporated, a heterosiloxane framework could be prepared.Since some of these contain hydrolysable functionalities they may be used asstarting materials for the preparation of supramolecular cage structures.

Functionalised POSS may be synthesised by polycondensation of trifunctionalRSiY3, where R is a hydrocarbon and Y is a hydrolysable functionality such as

59


chloride, alkoxide or silanol. However, this route does not control the placementof the functionality on the cage. Functionalisation of fully substituted silanecompounds by hydrosilylation or chlorination also does not lead to controlledsubstitution. Silylation of anionic species has been reported to producefunctionalised species, e.g., via: (a) the addition of hydrogen atoms bonded directlyto silicon atoms onto aliphatic unsaturated compounds, or (b) the addition ofsulfur onto aliphatic unsaturated compounds. The type of organic functionalitythat can be incorporated on the cage is limited. These methods have low yieldand a large percentage of impurities from side reactions that must be removed.The reaction between trisilanol with a variety of compounds of the type R´´MX3(R´´ = alkyl, alkenyl, aryl, H; M = Si, Ge, or Sn; X = halogen or alkoxide) leads toa variety of monomeric Σ8 silsesquioxanes.

In the early 1990s, Lichtenhan reacted trisilanol with R´SiCl3 orR´Si(OCnH2n+1)3 starting a new class of monomers called polyhedral oligomericsilsesquioxanes, or POSS (see Figure 9). In this case the R´ group was polymerisableor useful for grafting reactions, or sol-gel processing, e.g., acidic, acrylic, alcohol,amine, α-olefin, epoxy, ester, halides, isocyanate, organo-halides, phenol, silanes,silanols, styrenic, vinyls, etc. The process that leads to functional POSS waspatented [Lichtenhan et al., 1999]. In the presence of a metathesis catalyst aneffective amount of olefins of alkyls, cyclics, aryls, siloxyls or their isomersis ableto provide POSS with 1-8 reactive functionalities. The process has been used inthe US Air Force Laboratory to synthesise dozens of types of POSS. The productsare soluble in common solvents, e.g., chloroform, hexane or THF.

The chemical structure of POSS is: (RSiO1.5)Σn, thus as the name indicates(sesqui = ‘one and a half’) intermediate between silicas (SiO2)n and silicones(R2SiO)n. For most POSS: Σn = 8, but Σn = 6, 10 or 12 are also available. The sizeof the cage varies from about 0.7 to 3 nm. Polyhedral siloxanes or silsesquioxanessensu stricto are topologically equivalent to a sphere and are also called sphero-siloxanes. Most POSS compounds are crystalline, but changing the R-group mayresult in liquid crystal or liquid-like behaviour.

The octasilsesquioxanes (Σn = 8) have cube-shaped molecules that consist ofa Si8O12 core and eight reactive sites that may be differently functionalised. Fromthe point of view of polymer technology the mono-substituted species, R´R7Si8O12

Figure 9 Reaction between trisilanol and a compound of the type R´SiX3 (R´ =reactive group; X = halogen or alkoxide) leads to a variety of monomeric Σ8

silsesquioxanes, or POSS monomers [Lichtenhan et al., 1999].

60


(where R´ ≠ R are substituents) are the most interesting – the group R´ may providean ability to the octasilsesquioxane to enter polymerisation, copolymerisation orgrafting reaction, whereas the other functionalities, R, make the system miscible.It should be noted that incorporation of POSS into crystallisable macromoleculesreduces the crystallinity, which in turn tends to lower the chemical and solventresistance.

A new type of PNC formation was recently proposed by Ricardo et al. [2001].The authors started with octakis(hydrido dimethyl siloxy) octasilsesquioxaneQ8M8

H, converting it to octa-ethylbenzyl chloride. Using bromoester as an initiatorand CuCl as a catalyst, atom transfer radical polymerisation (ATRP) was employedto produce star-type PMMA with arms having controlled molecular weights.The concentration of catalyst and initiator control MW, MWD and the star-starcoupling reactions. The arms had Mn = 4 to 15 kg/mol (varying with conversion),and the polydispersity MW/Mn = 1.2 to 1.5. The number of arms per Q8M8

H-unitwas found to vary from 6.2 to 7, hence less than the maximum of 8. The lowernumber of arms may result from star-star coupling reactions. The new PNCsshould have unique properties, combining those of nanocomposites with thoseof the controlled star-branched polymers [Roovers, 1985]. Mechanical propertiesof these PNCs have not yet been reported.

2.1.4.3.2 PropertiesPOSS with reactive groups may be incorporated into virtually any polymer eitherby compounding (as a nanofiller), copolymerisation or blending [Lichtenhan etal., 1993; Lichtenhan, 1995; Haddad and Lichtenhan, 1996; Shockey et al., 1999;Feher et al., 1999].

POSS may provide a variety of property enhancements to existing resin systems.Owing to its chemical nature, POSS can be used to upgrade the thermal andphysical properties of most plastics. The following isotropic enhancements havebeen reported for POSS-copolymers and blends: higher decompositiontemperature, Tg increased by 100-200 °C, lower flammability (delayed combustionand reductions of heat evolution), bulk density reductions of up to 10%, increasedO2 permeability, reduced thermal conductivity, improved resistance to oxidation,increased modulus and hardness while maintaining the stress and straincharacteristics of the base resin, good processability and mouldability (viscosityreduction of up to 24% was reported) [Lichtenhan, 1996; Tsuchita et al., 1997].

Substituted POSS have been used as Wittig reagents, precursors to SiC powders,low dielectric constant materials, alumino-and gallio-silicates, silica-reinforcedcomposites, a variety of microporous materials, etc. A variety of functional groupshave been attached to POSS, viz. acrylates, silanes, silanols, olefins, epoxies,amines, esters, phenols, styrenics and thiols. Functionalised POSS was polymerisedto yield hybrid inorganic-organic homopolymers or copolymers [Tsuchita et al.,1997].The free radical reaction of propyl-methacryl-POSS gives POSS macromers,which can undergo hydrosilylation into oligomers and polymers with improvedmechanical properties, increased thermal stability to oxidation and resistance todegradation by UV [Lichtenhan, 1995]. Properties and performance of POSS-based polymers continue to be explored [Zheng et al., 2001; 2002a,b; Li et al.,2002]. The modern POSS is only about 10 years old, but between 1993 and2002 over 160 articles on POSS were published.

POSS have been successfully incorporated into a number of thermoplasticmatrices such as styrenics, acrylics, LCP, siloxanes, polyamides and more recently

61


PO. The resins are inherently reinforced by the presence of the inorganic cages ofsize between a molecule and a macromolecule. By controlling the nature of thesubstituted groups, R and R´, POSS has controllable reactivity, miscibility, lowdensity, neutral pH and low VOC. Their polymers or copolymers are isotropic,free of metals, and transparent.

In Figure 10, the compositional variation of Tg and the decompositiontemperature for 10 wt% mass loss (Td) is shown for a series of copolymers. Thesewere prepared by free-radical polymerisation of α-methyl styrene with styryl-basedPOSS. The latter contained one styryl-ethyl polymerisable group and seven inertR-groups, either cyclohexyl (-c-C6H11) or cyclopentyl (-c-C5H9). Thus, themacromer had a spherical (Si8O12) core, surrounded by seven inert groups forsolubility and one reactive. It was found that the cyclohexyl derivative is abouttwice as soluble as cyclopentyl [Haddad and Lichtenhan, 1996]. The differencein behaviour between these two types was small, but cyclohexyl-substituted POSSshowed better performance.

Figure 11 shows the variation of Tg for two series of isobutyl-styryl-POSScopolymers with either vinylpyrrolidone (P4VP-POSS) or acetoxylstyrene (PAS-POSS). The Figure demonstrates that POSS may increase the Tg, but at a relativelyhigh POSS content – for both systems incorporation of POSS initially leads to areduction in the HDT. Since the ratio of molecular weight of POSS to P4VP is8.3 the observed minimum at about 2 mol% is equivalent to 14 wt% of POSS.The authors [Xu et al., 2002] rationalised this behaviour by noting that the POSS-containing copolymer had reduced molecular weight and narrower molecularweight distribution. Furthermore, the Tg depends on:

Figure 10 Composition dependence of the glass transition (Tg) and decomposition(Td) temperatures for copolymers of 4-methyl styrene with styryl-based POSS. Inthe latter C5 and C6 stand, respectively, for R = -C5H9 and -C6H11 [1addad and

Lichtenhan, 1996].

62


1. A diluent role of POSS that reduces the dipole-dipole interaction of the matrixmonomer,

2. The dipole-dipole interaction between POSS siloxane and the polar carbonylof organic polymer, and

3. The POSS-POSS intermolecular interaction.At a relatively low POSS content, the diluent role dominates, thus Tg decreases.At a high POSS concentration, the POSS-PAS, POSS-P4VP and POSS-POSSinteractions cause the Tg to increase.

Figure 12 illustrates the variation of Tg for copolymers of butyl methacrylate(BM) with up to 50 mol% of cyclopentyl-POSS-methacrylate – a significantincrease from 50 to 245 °C was observed. The experimental data may be describedby the dependence:

T K w w w T w Tg g g= + ⋅ +[ ]( * ) / /1 1 2 1 1

3 22 2

3 2 (12)

where wi and Tgi are, respectively, the weight fraction and glass transition temperatureof component i, and K* is the binary interaction parameter between them.

The relation was derived for strongly interacting miscible blends [Utracki,1989], thus the agreement implies that there are strong thermodynamicinteractions between the two types of mers expressed by the high value of thebinary interaction parameter: K* = 6.39 ± 0.52. Since such an effect has not beenobserved in copolymers of BM with methyl methacrylate (MM), it is evident thatstiffening of the copolymer macromolecules involves the POSS units. Thecorrelation coefficient squared for the dependence in Figure 12 is r2 = 0.999. Thedependence predicts that Tg should decrease at POSS loadings over 50 wt%.

Figure 11 Variation in glass transition temperature for two series ofvinylpyrrolidone random copolymers with POSS (P4VP- POSS) and

acetoxylstyrene with POSS (PAS-POSS). Data [Xu et al., 2002].

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Figure 13 illustrates the effect of melt compounding PP with 10 wt% of POSS =[CH3SiO1.5]S8. At T < Tg ≈ 0 °C the stiffening effect is negligible, whereas at Tm >T > Tg the complex tensile modulus (E*) increased by a factor of up to two, orthe usage temperature window increased by 45 °C, i.e., the POSS compound at145 °C has the same value of E* as neat PP at 100 °C [Schwab et al., 2001]. Inother words, POSS is not effective in the glassy state, but is able to ‘reinforce’ PPat T > Tg. (Similar effects were observed for norbornyl elastomer with cyclopentylor cyclohexyl POSS [Bharadwaj et al., 2000]). Schwab et al. [2001] also providednumerical values characterising the mechanical performance of thesenanocomposites (see Table 10). Improvement of the tensile and flexural propertieswas modest, but of impact strength and HDT were significant. It is noteworthythat POSS is expected to reside in the amorphous PP phase, which constitutes ca.40% of the polymer. If so, the local POSS concentration is higher by a factor ofabout 2.5.

Using a metallocene catalyst, Zheng et al. [2001] copolymerised ethylene orpropylene with up to 10.4 mol% of norbornylethyl cyclopentyl-POSS macromer(see Figure 14). As shown in Figure 15, incorporation of the POSS sharply reducedthe crystallinity, while the decomposition temperature and the temperature for5 wt% mass loss in TGA increased to a plateaux by ca. 20 and 100 °C, respectively.The latter two properties reached their respective plateaux at POSS ≈ 20 wt%[Zheng et al., 2001].

Zheng et al. [2002b] carried out copolymerisation of styrene with the POSS-styryl macromer 1-(4-vinylphenyl)-3,5,7,9,11,13,15-heptacyclopentylpentacyclooctasiloxane. The catalyst CpTiCl3 with methylaluminoxane (MAO) was used.Random syndiotactic copolymers have been obtained with POSS content up to24 wt% or 3.2 mol % (see Figure 16).

Figure 12 Glass transition temperature of polybutyl methacrylate and itscopolymers with up to 50 mol% of heptacyclopentyl methacrylate (CpPOSS-MA)

[Lichtenhan, 1995].Points – experimental, line calculated from Equation 12.

64


Incorporation of 24 wt% POSS increased the Tg by 4 °C, destroyed sPScrystallinity, did not affect the decomposition temperature under N2, but increasedthat in air by 37 °C (reduced oxidative degradation).Li et al. [2002] copolymerised vinyl ester (VE) and 50% styrene (St) with 0, 5 or10 wt% of POSS (MW = 1305: (C6H5CHCHO)4(Si8O12)(CH=CHC6H5)4). At5 wt% of POSS the system was homogeneous, but at 10 wt% silicon-rich crosslinkeddomains of irregular shape were observed – these ranged in size from a few toabout 75 nm. Incorporating POSS into the VE/St resin had almost no influence onTg or on the width of the loss peak in the glass transition range.

Figure 13 Complex tensile modulus versus temperature for PP and its copolymerwith 10 wt% of [CH3SiO1.5]Σ8 [Schwab et al., 2001].

setisopmocstidnaPPfoTDHdnaseitreporplacinahceM01elbaTHC[htiw 3 OiS 5.1 ]Σ8 bawhcS[ .late ]1002,

ytreporP)tnemevorpmi(

MTSA HC[ 3 OiS .1 ]5 ΣΣΣΣΣ8 )%tw(tnetnoc

0 2 5 01

suludomlaruxelF)aPG(

A097D 556.1 037.1)%5.4(

757.1)%2.6(

08.1)%8.8(

htgnertselisneT)aPM(

836D 5.43 5.43)%0(

1.53)%7.1(

8.53)%8.3(

tcapmidozI)m/Jk(htgnerts

A652D 7.62 3.92)%7.9(

1.33)%42(

0.04)%05(

(TDH ° )C 864D 99 5016( °)

51161( °)

42152( °)

65


Figure 14 Norbornenylethyl cyclopentyl-POSS, C44H76O12Si8, with MW = 1021.77g/mole; soluble in THF, chloroform or hexane [Zheng et al., 2001].

Figure 15 Thermal properties of (norbornenyl-ethyl-cyclopentyl)-POSS-co-ethylene. Incorporation of POSS sharply reduced crystallinity. The decompositiontemperature, Td, and the temperature for 5% wt loss during TGA, T5%, reached a

plateaux at POSS content of about 20 wt% [Zheng et al., 2001].

66


Similar absence of effect on Tg was also observed in specimens prepared by blendingin non-reactive POSS units into VE resin. Copolymerisation with 10 wt% POSSresulted in a two-phase, crosslinked system. There was no measurable effect ofPOSS incorporation on Tg ≅ 131 ± 1 °C. At 40 °C the bending storage modulus, E´,of the VE/styrene resin and its copolymer with 10 wt% POSS was E´ = 1.24 and1.58 GPa, respectively. The flexural modulus of these specimens was 1.89 to 2.14GPa, respectively. However, addition of POSS reduced the flexural strengths byabout 21%. The VE and composite samples showed poor solvent resistance toTHF.

The following property improvements have been cited for POSS inthermoplastic or thermoset systems:

• Controlled miscibility for mixing and blending.• Maintained processability and mouldability of neat polymer.• Viscosity reductions of up to 24% (relative to silica-filled composites!).• Low density, by eliminating the need to use common fillers. For example, by

replacing silica as filler the bulk density may be reduced by up to 10%.• Extended temperature range, and resistance to oxidation and wear.• Reduced flammability by delayed combustion and low heat evolution rate.• Increased modulus and hardness while maintaining the stress and strain

characteristics of the base resin.• Increased O2 permeability and low thermal conductivity.• Simple disposal as of silica.

2.1.4.3.3 SourcesPOSS is available from Sigma-Aldrich Chemical Co. (www.sigma-aldrich.com),Gelest Inc. (www.gelest.com), The Mather Group at UCONN (University ofConnecticut), Hybrid Plastics (www.hybridplastics.com), etc.

Figure 16 Coordination copolymerisation of styrene with the POSS-styryl macromer1-(4-vinylphenyl)-3,5,7,9,11,13,15-heptacyclopentylpentacyclo octasiloxane

Reproduced from Zheng et al. [2002b], copyright 2002, with permission fromJohn Wiley & Sons, Inc.

67


Sigma-Aldrich Fine Chemicals is a global supplier. The company offers68 POSS compounds (monomers, polymers, silanols, reagent and precursors) inquantities of 1 to 10 g. Gelest was founded in 1990 to serve the advancedtechnology applications market. The company offers a dozen POSS compoundsat a cost of 100 to 450 US$/kg. The main source of POSS is Hybrid Plastics, aspin-off from the US Air Force Research Labs (AFRL). Lichtenhan and Schwablaunched the company in 1998, on a license from AFRL and an AdvancedTechnology Program (ATP) startup grant from the US National Institute ofStandards and Technology (NIST). The continuing cooperation with AFRL is forthe application of POSS-technology to rocket, aero, and space vehicle systems.Starting with six products, in 2003 Hybrid Plastics has a list of nearly 300(guaranteed 97% pure) products, grouped into four categories:

1. POSS® Molecular Silicas™ with Si–O core surrounded by non-reactive organicgroups providing miscibility with an organic matrix. These may be added(up to 50 wt%) to polymers, yielding nanocomposites with nanoscalereinforcements.

2. POSS® Silanols have 1 to 4 ≡Si–OH silanol groups. These may react withmetal or glass surfaces (or with inorganic fillers) rendering them hydrophobic.Silanols containing epoxide, methacrylate, and olefinic groups are availablefor copolymerisation or grafting.

3. POSS® Functionalised Monomers have 1 to 8 reactive groups, such as amines,esters, epoxies, methacrylates, olefins, silanes, styryls and thiols. In mostcases they are available as: 1-R-3,5,7,9,11,13,15-heptacyclopentyl cyclooctasiloxane (R-POSS).

4. POSS® Polymers possess a hybrid inorganic-organic composition and can beeither thermoplastic or thermoset. They are either (1) co-polymers withstandard monomers, or (2) neat POSS resins. The types available include:silicones, styrenics, acrylics and norbornenes.

The price depends on quantity: viz. for R&D gram quantity the price is US$200 to2,000/kg, for semi-bulk it is US$60 to 200/kg, and for production level it is> US$20/kg. ‘Sampler Kits’ contain 5 to 7 types of a specific type of POSS (e.g., POSS-methacrylate, POSS-epoxy, or POSS-silanol, 20 g each) at US$650 to US$1200 per set.Multi-ton quantities can be produced using a continuous process. In 2002 POSS costwas ca. US$400/kg. New plant for making POSS may push the price of some POSS toabout US$33/kg. Hybrid Plastics’ sales and marketing arm is Divex, Inc.

2.1.4.3.4 ApplicationsThe method of POSS incorporation depends on the category (see above) and theexpected application. Evidently, it is advisable that the molecular silicas are dissolvedin a monomeric or polymeric liquid. The dissolution should be carried out in asuitable mixer, e.g., a twin-screw extruder (TSE), as dispersing [CH3SiO1.5]Σ8 inmolten PP [Schwab et al., 2001]. The functionalised POSS monomers are mainlycopolymerised, e.g., as butyl methacrylate with heptacyclopentyl methacrylate(CpPOSS-MA) [Murugavel et al., 1999], but homopolymerisation, grafting andcrosslinking may also be carried out. However, since the cost of POSS is high (evenin comparison to engineering resins) the recommended use is < 50% incorporationinto high performance resins. In the case of POSS® Polymers, blending with otherresins in a TSE may provide a suitable solution.

68


POSS can be used in many guises as:• Additives – for heat/abrasion resistant paints and coatings. Used as

crosslinking agents, viscosity reducers, fire retardants, also to increasemechanical properties, HDT and gas permeability, to decrease dielectricproperties (in photoresists, interlayer dielectrics), etc. Thus, for example,Weidner et al., from Wacker-Chemie used POSS as a crosslinking agent.

• Plastics – in aerospace, electronic, optical, medical, biomedical andpharmaceutical applications, for catalysis, packaging, coatings, liquid crystaldisplay elements, magnetic recording media, as coupling agents, fireretardants, nanofillers in speciality polymers, dendrimers, etc. Optical disks,microelectronics, and medical products are target niches. Takamuki et al., ofKonica Corporation used POSS for transparent printing plates; Zank andSuto of Dow Corning Asia for electronic materials; Nguyen of AmericanDye Source as hot melt inks; Canon for LED, etc.

• Pre-ceramic coatings that, depending on the oxidising conditions, convert tosilicon dioxide, silicon oxy-carbide or silicon carbide. These POSS may be usedas ablative materials (for nozzles, insulations, etc.), for cladding and coatings inthe electronics industry, etc.

• Reagents, POSS have been used for catalysis supports, as monomers, crosslinkers,biological scaffolds, reactive ion-etch resistant layers; they find application inmicroelectronics, for drug delivery, etc. For example, POSS silanols are formulatedat room temperature. At T > 40°C the resin becomes tacky, and then a viscousliquid at 120 °C. The cure is catalysed by dibutyl tin diacetate, zinc acetate orzinc-2-ethylhexanoate. After condensation the resin becomes tough binder or film.Shell Oil Co. and Solvay used POSS as an epoxidation catalyst (US Patents5,750,741 and 6,127,557), etc.

• Surface Modifiers, as silane replacement in corrosion and abrasion resistantcoatings, lubricants and compatibilisers, for controlled drug release, andoptical fibre coatings.

Table 11 summarises the relative merits of three types of PNC systems: CPNCswith organoclay, those with POSS, and other PNCs with other nanofillers.

By the end of 2002, Pentron Clinical Technologies had introduced the Nano-Bond Universal Bonding System, a POSS-reinforced adhesive. The resin infiltratesetched surfaces and provides a strong bond between the tooth and dental restorativematerial. The kit includes a primer, the adhesive and a dual cure activator.

Finally, it is worth mentioning that not only cage-type POSS are known. Aladder-like variety (L-POSS):

shows superior heat, radiation, water and fireproof resistance, outstandingelectrical properties as well as the formation of high-strength film (for coatings,electronic and optical devices). Changing the side and/or end groups may modifythe molecular structure of L-POSS.The compound is used in photoresists,interlayer dielectrics and protective coating films for semiconductor devices.

69


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70


Applications for liquid crystal display elements, magnetic recording media andoptical fibre coatings have also been disclosed. L-POSS may also be used for gasseparation membranes, binders for ceramics and controlled release drugs as wellas additives in cosmetics and resins.

The ladder-like poly(methyl silsesquioxane)s (PMSQ) are used for coatings,particularly in electronics and optical devices. To improve the barrier, mechanicaland thermal properties Ma et al. [2002] tried to disperse MMT in PMSQ. Theaim was to develop high heat-resistant coating. The Na-MMT was pre-intercalatedwith trimethyl hexadecyl ammonium bromide (3MHDA), dried and ground.Dispersing the organoclay in chloroform, then adding methyl trimethoxysilane,hydrolysing it, neutralising and polymerising, resulted in the PMSQ/MMTnanocomposite:

XRD and TEM showed that MMT was only intercalated with d001 = 3.42 nm.Annealing the nanocomposite at 150 °C for 3 h did not change the spacing, butannealing at 300 °C for 3 h did reduce d001 to 3.0 nm. Furthermore the relativeintensity of the peak significantly increased. The stability of interlayer spacing atT ≤ 150 °C was explained by the molecular structure of the in situ polymerisedPMSQ. The decrease of spacing at 300 °C is most likely due to decomposition ofthe quaternary ammonium ion complex on MMT. Unfortunately, the authorsdid not report on the performance of these interesting materials.

The commercial success of a material depends on delivery of enhancedperformance at low cost. Currently, the PNCs available on the market contain2 to 5 wt% of organoclay. The market price for PA-6 and PP is about US$3,000and 1,000/ton, respectively. Assuming that a 10% increase of cost would beacceptable (if performance warrants it), the cost of compounding and organoclaymust not exceed US$300 and 100/ton, respectively. At the current cost ofcompounding (at least US$50/ton) and clay the incremental cost is ca. US$300to 400, leaving little room for profit. Thus, in the case of CPNC the commercialsuccess of PNC hinges on the intercalation and compounding costs.

Applying a similar algebra to POSS with the lowest projected price ofUS$33,000/ton the situation is difficult. Furthermore, to achieve an interestingenhancement of properties relatively large amounts of POSS must be added (from10 to 50 wt%). For applications as a structural material the cost would beprohibitive. Obviously, the economics look better when more expensive

71


engineering or speciality resins are modified by POSS. However, by the sametoken POSS use is automatically diverted from structural (large productionvolume) to functional speciality products.

From the point of view of structural performance, incorporation of POSSresulted in relatively minor gains, viz. mechanical performance about 10 timessmaller than that obtained by incorporation of the same quantity of organoclay.The effect on the transition temperatures, Tg or Tm, was found to depend on thetype of POSS, the polymeric matrix and the method of its incorporation –variations from a decrease by 100 to an increase by 200 °C were reported. Finally,the ‘lightweight’ of POSS systems is in comparison with mineral-filled compositesand it does not seems to offer much above the virtually ‘neutral’ effect on matrixdensity of organoclay.

In consequence, CPNC and POSS find applications in different domains, theformer as structural material, e.g., for the automotive or packaging industries,the latter as a functional material in electronic, space or bio-applications. Vive ladifférence!

72


73

Clays

2.2.1 General CharacteristicsClays originate from the hydrothermal alteration of alkaline volcanic ash androcks of the Cretaceous period (85-125 million years ago). The airborne ashcarried by winds formed deposits characterised by high volume bedding of ash,deposited in seas and alkaline lakes. Different opinions have been expressedregarding the mechanism of the ash to clay transformation. Probably the changebegan in marine water in reactions involving sufficient amounts of Mg+2 andNa+. Several geological processes may have lead to the formation of clays duringmillions of years [Keller, 1979; Giese and van Oss, 2002; Drits, 2003; Lagalyand Zismer 2003].

Clays are distinctive from rocks in several aspects:

1. Wet clays can be formed by application of light force and after release of thepressure they retain the imposed shape.

2. Clays are composed of extremely fine crystals, usually plate-shaped, less than2 μm in diameter and less that 10 nm thick. They are mostly phyllosilicates,i.e., hydrous silicates of Al, Mg, Fe, and other elements. Having at least onesmall dimension and large aspect ratio they have large specific surface areas.This in turn makes clays physically sorptive and chemically surface active.Several clay types carry an excess negative electric charge owing to internalsubstitution by lower valency cations, viz. Mg2+ substituted for Al3+, whichmakes clay slightly acidic. A clay deposit usually contains non-clay mineralsas impurities, viz. quartz, sand, silt, feldspar, mica, chlorite, opal, volcanicdust, fossil fragments, heavy minerals, sulfates, sulfides, carbonate minerals,zeolites, and many other rock and mineral particles ranging in size fromcolloidal to pebbles.

Clays are classified on the basis of their crystal structure and the amount andlocations of charge (deficit or excess) per basic cell. In the context of PNCs, theamorphous clays are a great nuisance as they are difficult to remove from thecrystalline ones. The crystalline clays range from kaolins, which are relativelyuniform in chemical composition, to smectites, which widely vary in theircomposition, cation exchange properties, and the ability to expand. The ease ofseparation of the individual layers is related to the interlamellar charge, x. Thelatter parameter changes from zero (talc) to x = 0.2 to 0.6 for smectites, to x = 0.6to 0.9 for vermiculites, and to x = 1 to 2 for micas [Giese and van Oss, 2002].

Clay particles are usually plate-shaped, less often tubular or scroll-like.Individual clay particles are nanometre-sized at least in one dimension. Aqueoussuspensions of clays are thixotropic and sensitive to ion concentration.

2.2 Clays

74


The synthesis of clays has been extensively studied [de Kimpe et al., 1961;Roy, 1962; Weaver and Pollard, 1973; Velde, 1977]. For example, organiccompounds facilitate the synthesis of kaolin at low temperature by condensingaluminum hydroxide into octahedrally coordinated sheets [Linares and Huertas,1971]. More recently, Carrado [2000] published an excellent review on syntheticclays and the resulting CPNCs.

2.2.2 Crystalline ClaysMost clays are crystalline, composed of fine, usually plate-shaped crystals about1 nm thick with high aspect ratio, and have large specific surface areas. Theyabsorb up to a 30-fold amount of water, and when wet, can be easily shaped –pottery is as old as human civilisation.

2.2.2.1 KaolinsThese include kaolinite, dickite, nacrite and halloysite-endellite. The structuralformulae for kaolinite and endellite are A14Si4Ol0(OH)8 andA14Si4Ol0(OH)8

.4H20, respectively. The kaolinite lattice consists of one sheet oftetrahedrally coordinated Si (with O) and one sheet of octahedrally coordinatedAl (with O and OH), hence a 1:1, or a two-layer structure. A layer of OHcompletes the charge requirements of the octahedral sheet. Adjacent cells arespaced about 0.71 nm across the (001) plane. When solvated in ethylene glycol,endellite expands to 1.0 nm in the c-direction. Halloysites are usually tubular orscroll-shaped; they may be differentiated from kaolinite and dickite by treatmentwith potassium acetate and ethylene glycol.

For example, kaolin (KGa-1) from Georgia [van Olphen and Fripiat, 1979]:

• Contains (wt%): SiO2 = 44.2, Al2O3 = 39.7, TiO2 = 1.39, Fe2O3 = 0.13,FeO = 0.08, MnO = 0.002, MgO = 0.03, Na2O = 0.013, K2O = 0.05,F = 0.013, P2O5 = 0.034.

• Loss on heating to 550 °C is 12.6 wt%.• CEC = 0.02 meq/g, the specific surface area = 10.05 ± 0.02 m2/g.• DTA: endotherm at 630 °C, exotherm at 1015 °C, dehydroxylation weight

loss 13.11% (theory 14%).• The unit cell composition is:

(Mg0.02 Ca0.01 Na0.01 K0.01)[Al3.86 Fe(III)0.02 Mntr Ti0.11][Si3.83l0.17]O10(OH)8,octahedral charge: 0.11, tetrahedral charge: -0.17, interlayer charge: -0.06.

2.2.2.2 Serpentines

Substituting Mg for Al in the kaolin structure results in the serpentine,Mg3Si2O5(OH)4. Here all three possible octahedral cation sites are filled, yieldinga tri-octahedral group carrying a charge of +6. In kaolinite only 2/3 of the sitesare occupied by Al, yielding a di-octahedral group, also with a charge of +6.Most serpentines are tubular or fibrous. Chrysotile occurs in both clino- andortho-structures.

2.2.2.3 Illite Group (Micas)‘Mica’ is a generic term applied to a group of complex aluminosilicates having asheet or plate like structure with a wide range of chemical compositions and

75

Clays

physical properties. All micas form flat six-sided monoclinic crystals with aremarkable cleavage in the direction of the large surfaces, which permits them tosplit easily into optically flat films, as thin as one micron. When split into thinfilms, they remain tough and elastic even at high temperature. The dictionarydefines mica as ‘a class of silicates having a prismatic angle 120o, eminently perfectbasal cleavage, affording thin tough laminae or scales, colorless to jet black,transparent to translucent, of widely varying chemical composition, andcrystallising in the monoclinic system’.

Illites or micas are not pure minerals. The mica structure consists of a pair oftetrahedral sheets enclosing an octahedral sheet. Between each such sandwichthere are interlayer sites, which can contain large cations. Considerable variationexists in the composition and polymorphism of the illites. A basal spacing exhibitedin XRD, d001 ≅ 1.0 nm, is somewhat broad and skewed toward wider spacings.Muscovite derivatives are typically dioctahedral; phlogopite derivatives aretrioctahedral. The cation-exchange capacity of illite is CEC = 0.2-0.3 meq/g ofdry clay. The interlayer potassium exerts a strong bond between adjacent claystructures. Thus, mica possess a 2:1 sheet structure, similar to MMT, except thatthe maximum charge deficit in mica is typically in the tetrahedral layers andcontains potassium held tenaciously in the interlayer space. As a result, micas aredifficult to exfoliate. However, once exfoliated they form dispersions of plateletswith the highest aspect ratio, thus they are particularly useful for the control ofgas or liquid permeability. Several examples of CPNC with mica are provided inthis book. Pironon et al. [2003] proposed the use of FTIR of clay-NH4+ todistinguish illite from smectite clay.

The coordination of the octahedral sheet is completed by OH anions. Thegeneral formula of mica group minerals is XY2-3Z4O10(OH)2, where X representsthe interlayer site, Y the octahedral sites and Z the tetrahedral sites. The octahedralsheet can be made up in two ways: either dominantly of divalent cations such asMg2+ or Fe2+, in which case all three sites are filled (trioctahedral mica), or elsedominantly trivalent cations such as Al3+, in which case one of the three sites isleft vacant (dioctahedral mica). If solely Si occupies the tetrahedra, the sandwichis charge-balanced and there is no need for interlayer cations – the resultingminerals are talc (trioctahedral) or pyrophyllite (dioctahedral). In true micas Alsubstitutes for Si in the tetrahedra, and charge balance is maintained by K, Na orCa, in the interlayer site. The important rock-forming micas are the trioctahedralphlogopite and the dioctahedral ‘white’ micas:

• Phlogopite: KMg3[AlSi3]O10(OH)2

• Wonesite (or sodium phlogopite): NaMg3[AlSi3]O10(OH)2

• Annite: KFe3[AlSi3]O10(OH)2

• Eastonite: K[Mg2Al][Al2Si2]O10(OH)2

• Muscovite: KAl2[AlSi3]O10(OH)2

• Paragonite: NaAl2[AlSi3]O10(OH)2

• Margarite: CaAl2[Al2Si2]O10(OH)2

• Mg-Al-celadonite: K[MgAl][Si4]O10(OH)2

• Fe-Al-celadonite: K[FeAl][Si4]O10(OH)2

The average composition of mineral mica is (wt%): SiO2 45.57; Al2O3 33.10;K2O 9.87; Fe2O3 2.48; Na2O 0.62; TiO2 trace; CaO 0.21; MgO 0.38; moisture

76


at 100 °C 0.25; P 0.03; S 0.01; graphite C 0.44; loss on ignition (H2O) 2.74. Thephysical properties of a typical phlogopite and muscovite are listed in Table 12.

2.2.2.4 Chlorites and Vermiculites

Chlorite was identified as a mineral yielding a 1.4 nm basal spacing in clays.Chlorite is a three-layer phyllosilicate separated by a Mg(OH)2 interlayer. Chlorite-like structures have been synthesised by precipitating Mg and Al between MMTsheets [Slaughter and Milne, 1960].

The interlayer sheet in vermiculite is octahedrally coordinated, 6H2O aboutMg2+. The basal spacing of vermiculite varies from 1.4-1.5 nm with the nature ofthe interlayer cation and its hydration. The cation-exchange capacity of vermiculiteis relatively high and it may even exceed that of MMT. Vermiculites are knownto have high aspect ratio (p ≤ 2,500), exceeding that of MMT by nearly oneorder of magnitude.

2.2.2.5 Other Clays

2.2.2.5.1 GlauconiteGlauconite is green, dioctahedral, micaceous, rich in Fe3+ and K+ ions. It hasmany characteristics common to illite. Glauconite may contain randomly placedexpandable layers of the montmorillonite-type. The glauconitic green sands ofNew Jersey have been used in ion exchange, water-softening installations, and asa source of slowly released potassium in soil.

2.2.2.5.2 Sepiolite, Palygorskite and AttapulgiteSepiolite and palygorskite contain a continuous two-dimensional tetrahedral sheetand thus differ from the other layer silicates by absence of the octahedral sheet.Details of the structures were described by Jones and Galan [1988]. The attapulgitestructure is similar to palygorskite minerals resembling cardboard, paper, leather,cork, or even fossil skin. These clays have distinctive properties, not shown byplaty clays. Attapulgite and palygorskite sorb both cations and neutral molecules.Typical CEC is about 0.2 meq/g of dry clay. Sepiolite and attapulgite are bestidentified by their 110 reflections in XRD, 1.21 and 1.05 nm, respectively.

2.2.2.5.3 Mixed-Layer Clay MineralsIn mixed-layer clay sheets illite may be interspersed with MMT or chlorite.Corrensite has regular alternation of chlorite and vermiculite layers.

2.2.2.6 Smectites or Phyllosilicates

Smectites are the most frequently used clays for a variety of non-ceramicapplications. These 2:1 phyllosilicates have a triple layer sandwich structure thatconsists of a central octahedral sheet dominated by alumina, bonded to twosilica tetrahedral sheets by oxygen ions that belong to both sheets (see Figure 17).Smectites are structurally derived from pyrophyllite [Si8Al4O20(OH)4], or talc[Si8Mg6O20(OH)4] by substitutions mainly in the octahedral layers, viz. Al canbe substituted by Mg, Fe, Cr, Mn or Li. When substitutions occur between ionsof unlike charge, deficit or excess charge develops on corresponding parts of thestructure. The charge imbalance is compensated by the presence of cations (usually

77

Clays

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78


Na+, Ca2+, K+) sorbed between the three-layers. The cations are stoichiometric,but held relatively loosely and are readily exchanged by other cations. The triple-sheet layers form stacks with the interlamellar gallery between them. Their cation-exchange capacity (CEC) is high, 0.8-1.2 meq/g of air-dried clay, and may beused as a diagnostic criterion of the group.

Characteristically, smectites expand in H2O or alcohol. The size andcomposition of the interlamellar gallery is highly variable – its minimum thicknessis 0.26 nm, which corresponds to a monolayer of water molecules. With each ofthe three sheets 0.22 nm thick, the minimum thickness of the phyllosilicateinterlayer spacing as read by XRD is d001 = 0.92 nm. The flat thin sheets ofsmectite crystals have irregular shape and can be up to 1,500 nm in the largestdimension. Thus, the aspect ratio of smectites is: p ≤ 1500. Owing tocounterbalancing ions present, the nominal value for d001 is taken as 0.96 nm. Asshown in Figure 18, the van der Waals interaction energy (EA) that holds thestacks together depends very much on the distance between the platelets (h), oron the interlamellar spacing.

In the early literature, the term montmorillonite was used for this group. Inthe natural state these minerals are partially hydrated, with XRD basal spacingd001 = 1.2-1.4 nm. Solvating them in ethylene glycol expands d001 to 1.7 nm,while heating to 550 °C collapses it to 0.96 nm. The DTA curves for smectitesshow three endothermic and one exothermic peak, within the ranges 150-320,

Figure 17 Idealised structure of dry phyllosilicate with the unit cell:[Al2(OH)2(Si2O5)2]2 + 5 wt% H2O. The unit cell molecular weight is 720 + waterand counterions (e.g., Na+); the minimum d001 spacing for dry MMT is 0.96 nm

(the interlamellar gallery height 0.30 nm), the surface area of the unit cell is0.458 nm2. After van Olphen and Fripiat, 1979.

79

Clays

695-730, 870-920, and 925-1050 °C, respectively. The crystal lattice is weaklybonded. The smectite lattice is expandable between the silicate layers, hence whensoaked in water it may swell to several times its dry volume (e.g., bentonites ofBadlands). A broken surface of these clays typically shows a ‘corn flake’ or ‘oakleaf’ texture.

There are several species of smectite clay, but the two of greatest commercialimportance and value are montmorillonite (MMT) and hectorite (HT). MMTtends to have sheet morphology whereas hectorite has a lath or strip morphology.Commercial availability of hectorite is limited whereas MMT deposits are largeand widely spread around the globe, e.g., 7 in Canada, 6 in the USA, two inSouth America, 15 in Europe, 7 in Africa, 3 in Australia, and 8 in Asia. Typicalchemical formulae of the smectite clays are listed in Table 13.

2.2.2.6.1 BentoniteBentonite (named after Ford Benton, Wyoming) is rich in MMT (usually > 80%).Its colour varies from white to yellow, to olive green, to brown to blue. Grades ofthis mineral show a broad spectrum of properties and consequently find a varietyof applications and uses. Its origin is a hydrothermal alteration of volcanic ashdeposited in a variety of freshwater (e.g., alkaline lakes) and marine basins(abundant marine fossils and limestone), characterised by low energy depositionalenvironments and temperate climatic conditions. The deposits date from Jurassicto as recent as the Pleistocene epoch, but most are from the Cretaceous period(85-125 million years ago).

Bentonite beds range in thickness from several centimetres to tens of metres(most 0.3 to 1.5 m) and can extend for hundreds of kilometres. Bentonite iswidely distributed on all continents. In Canada large deposits are located in British

Figure 18 Van der Waals attraction energy between two phyllosilicate layersseparated by a distance h (nm) (after van Olphen and Fripiat, 1979).

80


Columbia (French Bar, Hat Creek, Princeton, Quilchena, etc.), Alberta (Rosalind),Saskatchewan (three ca. 1 m thick beds of Na-MMT near Truax), and Manitoba(Morden). In the USA, it occurs mainly in Wyoming (overburden: 12 m thick;1997 price: US$25 to 40 a short ton; estimated deposits: ≥ 1 billion ton), Georgia,Florida, Mississippi, South Dakota, Montana, Utah, Nevada, and California.Other known deposits are located in Algeria, Tasmania, Mildura and Scone(Australia; estimated deposit of 70 million tons), San Juan (Argentina), Sarigyugh(Armenia), Campina Grande (Brazil), Guangxi and Hangzhou Linan (P. R. China),Hamza (Egypt), Gewane, Mille and Ounda Hadar (Ethiopia), Landshut(Germany), Milos (Greece), Kutch (Gujarat, India, estimated deposits: 15 millionton of good quality sodium bentonite), Java - (Indonesia; estimated deposits8 million ton), Sardinia (Italy), Annaka and Kuroishi (Japan), Pershwar (Pakistan),Holy Cross Mountains, Upper Silesia Coal Basin and Sudetes (Poland; deposits:3 million ton), Eskisehir and Biga Peninsula (Turkey), Tam Bo (Vietnam; deposits:3 million ton), etc.

Main uses for bentonite are in foundry sands, drilling muds, iron ore pelletising,absorbents, as a variety of composite liners, food additive for poultry and domesticanimals, in filtration, foods, cosmetics and pharmaceuticals. Bentonite has beenused for clarification of liquids (especially white wine and juice). Bentonite is partof most adsorbent, bleaching and catalyst clays. About 6 million tons of bentoniteis produced annually [Harben and Bates, 1990; Carr, 1994].

2.2.2.6.2 Montmorillonite (MMT)Montmorillonite (MMT) is the name given to clay found near Montmorillonitein France, where MMT was identified by Knight in 1896. It is the most common

syalcetitcemslacipyT31elbaT

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81

Clays

phyllosilicate used for the production of commercial CPNC. MMT has beenknown under several names, viz., smectite; sodium montmorillonite; sodiumbentonite or swelling bentonite (Wyoming bentonite (US)); sodium-activatedbentonite (UK); sodium-exchanged bentonite, etc.); the non-swelling (in water)bentonite is calcium montmorillonite or bentonite (Mississippi bentonite (US));Sub-bentonite (Texas bentonite, US)). Also known are magnesium montmorillonite(Saponite & Armargosite), potassium montmorillonite (Metabentonite), lithiummontmorillonite (Hectorite), etc.

The idealised structure of Na-MMT is shown in Figure 17. The unit cell isusually written as:

Triple layer sandwich of two silica tetrahedronsheets and a central octahedral sheet with 0.67negative charge per unit cellAqueous interlamellar layer containing 0.67 Na+

cations per unit cell

Thus, an idealised MMT has 0.67 units of negative charge per unit cell, in otherwords, it behaves as a weak silicilic acid. Since the molecular weight of a unit cellis Mu = 734 + water, the CEC of idealised MMT is: CEC = 0.915 meq/g (one ionper 1.36 nm2), i.e., the anionic groups are spaced about 1.2 nm apart. The chargeis located on the flat surface of the platelets. As Thiessen demonstrated in 1947,a small positive charge is also present at the platelet edges.

MMT composition varies across a relatively wide range not only with thegeographic location but also with the deposit strata [Ross and Hendrics, 1945;Ross, 1960]. The data for 100 samples gave the following ranges of composition:

1. The octahedral layer: Al3.0 - 4.0 Mg0 - 1.4 Fe3+0 - 1.0

2. The tetrahedral layers: Al0 - 0.8 Si7.2 - 8.0

3. Exchangeable cation in the aqueous layer: Na0.67 - 0.8

Chemical analysis of a typical MMT yields: SiO2 = 51.14, Al2O3 = 19.76, Fe2O3= 0.83, ZnO = 0.1, MgO = 3.22, CaO = 1.62, K2O = 0.11, Na2O = 0.42 andwater 22.8 wt%. Drying at 150 °C eliminates 14.81 wt% of water, with 7.99 wt%remaining. Another chemical analysis of MMT gave: Al 9.98 wt%, Si 20.78 wt%,H 4.10 wt%, and O 65.12 wt%. Depending on composition, MMT colour variesfrom brick red (due to Fe+3) to pale yellow or blue-grey. The CEC ranges from0.8 to 1.2 meq/g. In the montmorillonite-nontronite series, as the Fe3+ contentincreases from 0 to 28%, the refractive index ranges from 1.523 to 1.590. XRDpatterns of a hydrated MMT yield typically d001 = 1.2-1.4 nm basal spacing.These values, along with expansion to ca. 1.8 nm in glycerol, have been used forMMT identification.

The specific surface area of MMT is Asp = 750-800 m2/g (theoretical value is834 m2/g). From the cited values it follows that the density of the triple sandwichis 4.03 g/ml and that the interlamellar gallery thickness is 0.79 nm, hence theinterlayer thickness of hydrated MMT should be d001 = 1.45 nm and the averagedensity ρ = 2.385 g/ml. Drying MMT at 150 °C reduces the gallery height to0.28 nm (which corresponds to a water monolayer), hence the interlayer spacingdecreases to d001 = 0.94 nm and the average density increases to ρ = 3.138 g/ml.Assuming that MMT platelets are fully exfoliated and that locally they are parallel

Al Mg Si O OH

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82


to each other, the interlayer spacing should be inversely proportional to the clayvolume fraction, φ,

d001 = h/φ

where h ≅ 0.96 nm is the thickness of a single (exfoliated) clay platelet. Thissimple relation predicts that the interlayer thickness, e.g., of PA-6 containing,respectively, 4 and 25-wt% of clay should be 32 and 4.6 nm, which is not farfrom the measured values (see Figure 3).

Commercially, MMT is supplied in the form of powder with about an 8 μmparticle size, each containing about 3000 platelets with a moderate aspect ratio p= 10 to 300. Typical properties and applications are listed in Table 14.

For example [van Olphen and Fripiat, 1979] Na-MMT from Wyoming, SWy-1,has the composition (wt%): SiO2 = 62.9, Al2O3 = 19.6, TiO2 = 0.090, Fe2O3 = 3.35,FeO = 0.32, MnO = 0.006, MgO = 3.05,CaO = 1.68, Na2O = 1.53, K2O = 0.53,F = 0.111, P2O5 = 0.049, S = 0.05. Weight loss on heating to 550 °C = 1.59 and to

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To slow down water flowthrough soil

To produce nanocomposites

To de-colour & purifyliquids, viz. wines, juices, etc.

As filler for paper or rubber

In drilling muds to give thewater greater viscosity

As a base for cosmetics anddrugs

As an absorbent

As a base for pesticides andherbicides

As food additive for poultryand pets

For thickening of lubricatingoils and greases

For binding foundry sands

To generate thixotropy

Absorption of ammonia,proteins, dyes and other polar,aromatic or ionic compounds

83

Clays

1000 °C = 4.47 wt%. CEC = 76.4 meq/100 g, principal exchange cations are Na+

and Ca2+. DTA endotherms occur at 185 °C (shoulder at 235 °C), desorption ofwater occurs at 755 °C, dehydroxylation; shoulder at 810 °C, exotherm at 980 °C.Weight loss in dehydroxylation = 5.53 wt% (theory = 5%). Cell structure is:(Ca0.12 Na0.32 K0.05)[Al3.01 Fe(III)0.41 Mn0.01 Mg0.54 Ti0.02][Si7.98 Al0.02]O20(OH)4;octahedral charge = -0.53, tetrahedral charge = -0.02, interlayer charge = -0.55,unbalanced charge = 0.05.

The early treatments of MMT involved acid-treatment and preparation oforganoclays. The aim of the former was purification, replacement of Ca2+ for H+

and dissolution of some Fe, Al and Mg ions from the octahedral layers.Manufacture of organoclays started in the 1940s (NL Industries) for use asthixotropic additives to control flow behaviour of oils, greases, suspensions, paints,printing inks, cosmetics, etc.

As described earlier in this text, PNC literature recognises four types ofdispersion of layered silicates in a polymer matrix (see Figure 19):(A) conventional dispersion of non-intercalated clay particles with the basic dry

structure,(B) intercalated form where the interlayer spacing d001 < 8.8 nm, and(C or D) exfoliated structures where d001 > 8.8 nm with the individual platelets

either ordered (because of stress field or concentration effects) or not,respectively.

In aqueous suspension the clay platelets may adopt more complex structures (seeFigure 20). The platelet association (flocculation) may take place by face-to-face(FF), face-to-edge (FE) or edge-to-edge (EE) interactions [Qian et al., 2000]. Evidently,each clay structure results in a different set of suspension properties. It suffice to notethat from the hydrodynamic point of view the encompassed volume of suspendedparticles dramatically increases from structure (a) to (g), the increase that is reflectedin rheological properties. The EE and EF structures are especially significant.

Figure 19 Four types of clay platelet dispersions in a polymeric matrix

84


Recently, Okamoto et al. [2001a] reported formation of a ‘house of cards’ structurein PP/clay nanocomposite melt under extensional flow. The authors consideredthat high strain hardening and rheopectic effects originate from the perpendicularalignment of the silicate layers to the stretching direction. When the stress vanishes,the platelets form the complex structures.

2.2.3 Purification of ClayThe names bentonite, smectite and montmorillonite are often usedinterchangeably. However, industrially these terms represent different mineralswith different degrees of purity. Bentonite is the ore that comprises smectite clayand impurities, such as gravel, shale, limestone, etc. Purification of the ore thatresults in extraction of MMT is a complex and expensive process.

Clay is usually mined in open pits. The depth of a deposit can be from a fewcentimetres to several metres with a length of up to hundreds of metres. After theoverburden is removed, layers of clay are disked and allowed to sun dry. Theclay is removed from the pit in layers, and stockpiled in multiple layers. Next,the dry clay is transported to processing facilities in trucks that are loaded fromthe stockpile in such a way that each ‘swipe’ of the front-end loader goes throughevery layer of the stockpile, insuring homogenisation.

Polymer-grade clay should have < 5 wt% non-smectite impurities. Typicalcontaminants include silica, feldspar, gypsum, albite, anorthite, orthoclase, apatite,halite, calcite, dolomite, sodium carbonate, siderite, biotite, muscovite, chlorite,stilbite, pyrite, kaolinite, hematite and many others. MMT usually has > 50 wt%calcium montmorillonite, Ca-MMT. Since Ca-MMT (as well as H-MMT) shownon-uniform expansion in water, purification of clay often involves cationexchange into Na-MMT [Norrish, 1954]. The purification often involvesreduction of clay particle size either by mechanical means (milling, grinding,comminuting, etc.) or by application of hydrodynamic means [Cohn, 1967].

Figure 20 Structures encountered in clay suspensions [1Qian et al., 2000]

85

Clays

Clocker et al. [1976] provided a detailed description of the clay purificationprocess. Thus, clay is mixed with water, then heated by steam to pressurise(T ≤ 243 °C; P ≤ 3.5 MPa) and hydrate the clay. Next, the slurry is rapidlyexpanded to cause some delamination, non-clay particles removed and large clayparticles recycled through the steaming and expansion cycle. The slurry is left ina holding zone for up to 2 h, at optimised temperature (e.g., T = 80 to 130 °C)and pressure (P = 0 to 0.2 MPa) for the intercalation, and again separated (e.g.,by hydrocyclone) from the non-colloidal particles. Finally, the purified clay withexpanded interlayer spacing is treated with an onium intercalant (e.g., octadecylammonium chloride (ODA)), filtered out, washed and dried. The resultingorganoclay has been used for thickening solutions in various solvents and paints.Organoclay suitable for use in CPNC must be purified and uniformly intercalatedwith great care. For example, one of the principal advantages of CPNC is tocontrol diffusion of fluids through a polymeric membrane – if the non-colloidalmineral content exceeds ca. 0.5 wt%, holes can be formed around such a foreignparticle (e.g., quartz) dramatically increasing the penetrant flux.

A patent from AMCOL [Clarey et al., 2000] describes modern purificationtechnology. The clay (see Figure 21) is ground to > 90 wt% particles havingdiameter d < 200 μm. Dry clay is fed from loader (12) into storage tanks (22) and(24). Using a blower (36) and air injectors (32 and 34) the clay is transportedinto a receiving vessel (38), and through an auger (48) into a blunger (50) whereit is mixed with water. The slurry, containing 5 to 50 wt% clay + impurities, issedimented to remove stones to a waste hopper (56), while the rest is conveyedto an attrition scrubber (60). The washed clay is pumped by pump (80) into afeed tank (84) for a series of hydrocyclones (86, 98, 110, 142, 170 and 181) thatremove impurities of a size > 50 μm.

The purified slurry containing 3 to 7 wt% clay is fed to the ion-exchangecolumn feed tank (180). At this stage > 90 vol% clay particles have diameterbelow 40 μm, with mean particle size of < 7 μm. The suspension is directedupward through cation exchange columns (212 to 222) that replace Ca2+ byNa+, giving > 95 wt% Na-MMT. After the ion exchange, the clay is conveyedthrough a feed tank (240) to a high speed centrifuge (250) operating at centrifugalforce of 2.5 to 3.5 kG. The product is conveyed to a tank (258) and then througha vibrating dewatering unit (266) which produces the clay slurry containing about12 wt% solids. The slurry is transported to a spray dryer (272), where it is driedto about 9 wt% water. Finally, the material is conveyed to an air filtering baghouse(276), holding tanks (282 to 286), and then to bagging apparatus (292 and 294)[Harben and Bates, 1990]. Statistical process control is used to maintain theproduct within the accepted limits. The important process control parametersare: concentration, counterion level, purity, particle size, moisture ratio, dispersivecharacteristics, etc.

2.2.4 Reactions of Clays with Organic SubstancesMMT crystals are made of flat sheets (individual clay platelets), 0.92 nm thickand up to 1,500 nm in the largest dimension. Their specific surface area is 750 to800 m2/g. The crystals form large particles or aggregates, even after purificationthe Na-MMT particles are ca. 8 μm in diameter, each containing about 3000platelets with an aspect ratio of p = 50 to 300. To be incorporated into a polymericmatrix these particles must be dispersed into individual stacks of platelets, thendelaminated into a uniform dispersion of individual platelets in the matrix. The

86


Figure 21 Process for clay purification (see text)

Reproduced from Clarey et al. [2000], with permission from AMCOL.

87

Clays

process of delamination usually goes through two stages: intercalation andexfoliation. As evident from the definition of these terms, they differ not incharacter, but in the magnitude of the achieved platelet separation.

The attraction energy (Uattraction) of two platelets of equal thickness is givenby Equation 13 [Stokes and Evans, 1997]:

UA

h h hattraction = − +

+−

+( ) ( )⎡

⎣⎢⎢

⎤

⎦⎥⎥

11

2 2 212

1 1

2

2

π δ δ(13)

where, A11 is the Hamaker constant, h is the separation distance between plates,and δ is the platelet thickness. It is noteworthy that the surface energy of inorganicsolids is about 100 times higher than that of organic liquids. According to Equation13 the interactions between two crystalline planes decrease with the inversedsquare of the separation distance (see Figure 18). Thus, the logical approach tothe process of delamination is to increase the gallery space in several steps, insertingprogressively larger molecules – this is indeed the most common intercalation/exfoliation strategy.

However, before discussing the methods developed for the intercalation andexfoliation that lead to CPNC it is advisable to summarise the chemistry of clays.Several books and reviews are available on the topic, viz. the ever popular earlytext by Weiss [1969], Mortland’s review [1970] and other such publications[Weaver and Pollard, 1973; van Olphen and Fripiat, 1979; Grimshaw, 1980;Theng, 1974; Raussell-Colom and Serratosa, 1987; Mark et al., 1995], a reviewon synthetic clays [Carrado, 2000], etc.

Several active sites on the MMT crystal surface have been identified. In theabsence of water, for molecules not fully coordinated Al constitutes an electron-pair accepting site. Ions of Fe+3 and Fe+2 provide, respectively, oxidising (electron-accepting) and reducing (electron-donor) sites (similar activity in the presence ofother metallic impurities is expected). Interactions between the clay surface andorganic molecules by van der Waals forces and by entropic rearrangement effectshave also been observed.

From the point of view of intercalation/exfoliation that may lead to CPNCthe following three active sites are the most important [Brown et al., 1952]:

1. The platelet edges have a few positive charges that attract negatively chargedions or molecules. These sites have been used to attach organic moleculesmaking MMT organophilic. However, the reactions involving edge ions donot increase the interlayer spacing, hence they are not useful for intercalation.Evidently, once MMT is intercalated, these sites can be used, e.g., to enhanceplatelet miscibility with polymeric matrix or mechanical dispersability duringmelt compounding. For example, Na-MMT is able to react with weak organicacids, viz. tannin or ligno-sulfonates. This method has been used for dispersingclay particles in drilling mud. The reaction does not increase the interlayerd001 spacing.

2. The –OH groups (four per unit cell) may participate in hydrogen bondingand chemical reactions. These are mainly located at the crystal edges, boundto Si, Al or other octahedral ions. The face surface location of –OH groupshas also been postulated [Deuel, 1952; Uytterhoeven, 1960; 1962]. Theconcentration of –OH groups can also be enhanced by digesting MMT withNaOH solution.

88


Hydrogen bonding to the surface oxygen atoms provides the mechanismresponsible for adsorption of organic molecules by clay particles. The reactiondepends on the suspending medium. In water, hydrogen bonding by a water-soluble polymer is much less likely to take place than in an organic medium.H-bonding is significantly weaker than that with exchangeable ions. It dependson the polarity of the interacting group, hence its strength decreases from–NH3

+, to –OCH2- to –CH3 [Gonzales-Carreño et al., 1977] as well when thecharge is tetrahedrally located [Farmer and Russel, 1971]. The H-bondingdepends on the pH of the suspension, thus H-MMT is more likely to involveH-bonding than Na-MMT [Mortland, 1966]. Often H-bonding is a competitiveprocess, e.g., H-bonding between the alcohol molecules themselves moderatesadsorption of alcohol by MMT [Annabi-Bergaya et al., 1981].It has been reported that (after cation exchange) clay containing either UO2

2+

or Fe3+ can be made hydrophobic by acid-catalysed reaction in n-heptanewith octadecyl-trimethoxy silane [Giaquinta et al., 1997; Wasserman et al.,1998]. The silane film inhibited free exchange of water in and out of theinterlamellar galleries, thus after exposure to water the interlayer spacingremained stable, similar in magnitude to the dry state. The authors did notelaborate on the reaction mechanism.

3. The anionic group on the MMT face surface; MMT in aqueous medium behavesas weak silicilic acid. These sites are of principal interest for diverse intercalationmethods. It has been observed that organic compounds may diffuse into galleriesand coordinate to alkaline cations. The strength of the complex as well asexpansion of the d001 spacing depends on the cation and organic compound.For example, for a series of crown ethers absorbed by MMT, expansion byΔd001 = 0.4 to 0.8 nm was reported [Ruiz-Hitzky and Casal, 1978].

The cation exchange reactions in aqueous medium have been extensively studied.It has been known for 70-odd years that the surface cations can be exchanged forother cations. A measure of this capacity is given by the cation exchange capacity(CEC). For example, the CEC of MMT is 0.8-1.2 meq/g hence one negative ionper 1.36 nm2 is located on the flat platelet surface. These values are consideredto be optimal – sufficient concentration of ions to obtain a good level of chemicalactivity, and at the same time not too much of them to engender too strong solid-solid interactions. To facilitate the ion exchange, an aqueous slurry of sodium

montmorillonite is mechanically mixed either in the shear field (e.g., in a colloid mill)

or subjected to ultrasonics [Pérez-Maqueda et al., 2003]. It is noteworthy that in aqueous

medium clay interlayer spacing expands and platelets interact with each other, increasing

the suspension viscosity. The increase depends on the clay counterion, concentration,

and flow field [Malfoy et al., 2003]. Thus, for practical reasons the clay suspension

during the purification and ion-exchange steps is usually kept below ca. 7 wt%.

The cation exchange reaction is reversible, hence to obtain a high conversion ofNa-MMT into organoclay, an excess of intercalating organic cation RH+ is used:

RH+ + Na+ – MMT RH+ – MMT + Na+

The reaction rate depends on the type of clay, the medium, the type of cation tobe exchanged, the reaction conditions, viz. temperature (T), pH, concentrationsand geometry of clay particles, etc. For example, recently, natural smectite (Na-MMT) was intercalated with 3-methyl hexadecyl ammonium bromide (3MHDA)with a molar excess ranging from 0 to 3 CEC. The interlayer spacing increased

89

Clays

from the original value of d001 ≅ 1.4 nm to 1.9, 3.2 and 3.9 at a 3MHDA loadingof 1, 2, and 3 CEC, respectively. When heated at a rate of 2 oC/min the interlayerspacing started to collapse at T ≥ 200 oC [Lee and Kim, 2003].

To ensure that free cations are in the system, the pH is usually adjusted to atleast one unit lower than pK; a too acidic medium may cause cations to leach outfrom the clay and interfere with the desired exchange reaction.

The type of cation present within the interlamellar galleries affects the magnitudeand uniformity of interlayer spacing. For example, H+ and Ca2+ show a non-uniformexpansion in water, whereas Na+ and Li+ provide easy expansion up to totalexfoliation. Under ambient humidity the d001 depends on the cation, viz. for Na+ itis 1.4 nm, for Ca2+ it is 1.5 nm and for UO2

2+ it is 1.48 [Giaquinta, 1997].The fastest rates of exchange are reported for Na-MMT. The presence of

less hydrated monovalent ions (e.g., K+, Rb+, Cs+) and of multivalent ions (e.g.,Ca2+, Mg2+, etc.) reduces the rate. It seems that multivalent cations are capableof simultaneous interaction with anions on adjacent MMT platelets, makingintercalation more difficult. However, since the thickness of the triple sandwichis 0.66 nm (see Figure 17) compounds having the smallest dimension below0.6 nm may diffuse into the interlamellar gallery when d001 > 1.4 nm.

The reaction rates in water are faster than those in aqueous solutions of organicliquids, viz. alcohols, but there are exceptions to this rule, e.g., during intercalationwith organic cations. Increased T accelerates the process, hence the recommendedrange is T = 60-80 °C. Similarly, an increase of pressure (P) has been also reportedto speed up the reaction. The particle shape and size also affect the kinetics. Thereaction is diffusion controlled. It starts at the rim and the rate of the lineardiffusion down the slit of diameter d, is proportional to the diffusion time, td,viz. δd/δtd∝td , hence the diffusion time is proportional to the square root of d.This prediction is almost confirmed by data from Mackintosh et al. [1971]. Asshown in Figure 22 the experimental exponent a = 0.45, instead of 1/2 was found.

Figure 22 Diffusion time of K+ in exchange reaction for dodecylammoniumcation as a function of the biotite clay diameter. Data [Mackintosh et al., 1971].The broken line follows the empirical relation: td = aod

a, with ao =15.63, a = 0.45and the correlation coefficient, r = 0.9967.

90


Thus, modification of clay involves mainly a reaction with the anionic silicilicgroups on the platelet surface. In the presence of strong, but small in size cationsthe interaction is mainly ionic, e.g., silicilic acid with Na+. However, when thelatter ion is replaced with a quarternary ammonium one, the van der Waalsinteractions become more important. Their importance increases with thehydrocarbon chain length [Theng, 1964]. Another type of interaction involvesthe disruptive effects large cations have on the hydration shell around the interlayercation – the entropic effects. It has been found that the effect is more importantfor Na+ than for Ca2+ or Mg2+ [Theng, 1967].

2.2.4.1 Clay in Aqueous Medium

2.2.4.1.1 GeneralClays are hydrophilic hence the first step in their modification involves reactions inthe aqueous medium. The small and highly mobile water molecules easily diffuseinto interlamellar species, causing a lateral expansion of the clay crystals. Theprocess is diffusion controlled, hence to swell platelets that are twice as large requiresfour times longer. The rate depends on many factors and the time required to reachequilibrium swelling varies from minutes to days. Since water monolayer thicknessis about 0.28 nm, the first step involves expansion of d001 from 0.96 to about1.25 nm. Further expansion up to complete exfoliation can be accomplished byjudicious control of conditions, especially the ionic strength and counterion type.

MMT exposed to water vapour expands its basal spacing in steps (whichcorrespond to one to four molecular layers of water) to d001 = 1.25 to 2.0 nm.The origin of the water absorption by MMT is on the one hand hydration of thecounterions in the interlamellar galleries and on the other hydrogen bonding tothe clay surface. The work to remove the last monolayer of H2O is about 0.1 J/m2

at the equivalent pressure of 400 MPa.Dispersing Na-MMT in distilled water causes the ions that are associated

with the clay surface to diffuse out. The osmotic pressure pulls the ions awayfrom the clay surface, whereas the electrostatic charge tends to hold them nearthe surface. Eventually a steady state is achieved and an electrostatic doublelayer is formed that keeps the clay platelets apart. The counterions in the doublelayer are fairly mobile and can readily be exchanged. The reaction constantdepends on the ionic concentration in the double layer and in the supernatantsolution.

The double layer is formed by adsorption of solvated ions. Their concentrationand the associated electric potential decrease with the distance from the plateletsurface, following the Nernst equation:

Φ = (kBT/νe)ln(c/co) (14)

where: kB is the Boltzmann constant, v is the valence, e is the electronic charge, cand co are the ion concentration in the solution and where the potential Φ = 0,respectively.

Thus, the thickness of the double layer depends on the counterionconcentration and valency. Clay platelets can be fully exfoliated in aqueous media,but on standing, since the surface and edges have different charge, the EFinteractions may lead to formation of a ‘house of cards’ structure. If enough clayis present, all the water will be tied resulting in a gel formation. Shearing may

91

Clays

disrupt the structure, dramatically reducing the viscosity. The gelation is reversible,as evidenced by the thixotropic effects [van Olphen and Fripiat, 1979].

Another type of structure that may be formed is a tactoid – a parallel alignmentof clay platelets, up to 10 nm distant from each other, forming high clayconcentration regions in suspensions. In some systems there is a sharp transitionbetween the gel and tactoid phase. For example, an aqueous suspension of n-butyl ammonium vermiculite, n-butyl ammonium chloride and either PVME orPEG has a phase transition temperature between the tactoid and gel phases ofthe clay system, Tc = 13 ± 1 °C [Jinnai et al., 1996; Hatharasingle et al., 1998].Tactoid formation is caused on the one hand by the presence of a repulsive doublelayer and on the other by the attractive van der Waals forces. Tactoids havesmaller effects on flow behaviour than the ‘house of cards’ structures.

2.2.4.1.2 Reactions with Edge CationsTannates have been used as dispersing agents in drilling fluids. The most populartannate is quebracho tannin, a red-coloured tannin extracted from the SouthAmerican quebracho tree. Since the tannin is a weak acid, the sodium salt solutionis alkaline. Clay suspensions are dispersed by the addition of a small amount of atannate. The tannate anions are adsorbed at the edge surfaces of the clay particlesby complexing with the exposed octahedral aluminium ions. Consequently, theedge charge is reversed and a negative double layer is created by which EF and EEassociation is prevented. The anion adsorption on the clay edges is absent on thefaces of the clay plates. As a result, the basal spacing does not increase after tannateadsorption. To be effective, a phenol must contain at least three phenolic groups inthe molecule, two of which should be adjacent. Thus, 1,2,3-hydroxy benzene iseffective for dispersing MMT, but 1,3,5-hydroxy benzene is not. Similarly alizarindyes react with the platelet edges, causing dispersion [Schott, 1968; Freudenbergand Maitland, 1934].

2.2.4.1.3 Reactions with –OH GroupsThe vibration stretching frequency of structural –OH groups in MMT are slightlylower than those of unperturbed –OH groups, but easily distinguishable fromthe broadband of hydrated minerals [Farmer, 1971]. Thus, in aqueous mediumthese groups form hydrogen bonds with water molecules. The strength of theseinteractions is expected to be higher than those between water molecules and=Si=O surface groups.

2.2.4.1.4 Reaction with the Silicilic Surface AnionsAs stated before, anions are adsorbed on the edges of the clay particles and organiccations on the anionic inner surfaces of the clay platelets. This is evident fromthe high adsorption capacity of the clay as well as the resulting expansion of thed001 spacing.

The exchange reactions between Na-MMT and ammonium ions, R-NH3+Cl–,

or R4N+Cl– were studied in the early 1930s [Smith, 1934; Gieseking 1939]. Whenonium salt is added to aqueous clay suspension, the organic cations replace thecations present on the clay surfaces. There is a strong preference for the lessmobile organic cations. Often they are adsorbed quantitatively until all theexchange positions are exhausted. The ammonium groups become ionicallyattached to the clay surface while the hydrocarbon chains interact with the clay

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surface and displace the adsorbed water molecules [Hendricks, 1941; Jordan,1949; Jordan et al., 1950]. When the chains are too long to lie flat in the availablespace they may tilt, crowding within the interlamellar space. Depending on theconditions, the hydrocarbons may crystallise, hence the spatial organisation ofthe hydrocarbon segments is given by the appropriate crystalline cell unit andthe performance of such organoclay depends on temperature – whether below orabove the hydrocarbon melting point. When the hydrocarbon chains cover theclay surface, it precipitates from the aqueous suspensions. However, the formedhydrophobic clay can be homogeneously dispersed in organic medium.

Often the concentration of the organic cations exceeds the amount equivalentto the clay CEC. For example, commercial organoclays contain from 98 to 153%of intercalant per nominal CEC of MMT. The reason for the excess is to forcethe reversible reaction of ion exchange towards the organocomplex and to increasethe interlayer spacing to maximum.

The orientation of the interlayer cations, hence the d001 spacing, is usuallydetermined by the projected surface area of the cation (in a given orientation)provided that it does not exceed the available surface area per surface anion.Aromatic cations assume either a parallel or an upright position between thelayers, depending on the available space. Weiss and Kantner [1960] proposed amethod of estimating the surface charge density from the d001 spacing of complexeswith mono- and di-alkylammonium cations of different chain lengths. When themodifier molecules are large and bulky the steric effect precludes total cationexchange. The average distance between the silicilic anions on the MMT surfaceis about 1.2 nm.

However, when the concentration of the organic cations is high, theiradsorption may exceed the amount equivalent to the clay CEC. For example,quaternary ammonium compound having long hydrocarbon chains can beabsorbed in the amount equivalent of two-and-half times the CEC. The excessenters the interlamellar galleries in a head-to-tail configuration, profiting fromthe hydrocarbon/hydrocarbon chain solubility and the tendency to crystallise.Under these circumstances, the excess ammonium compounds form a layer withthe cationic groups facing the water phase, forming a diffused electric doublelayer that prevents the particles from precipitation. Thus the resulting organoclayis hydrophilic, hence unsuitable for use in organic medium, e.g., for the preparationof CPNC [Cowan and White, 1958; McAtee, 1962; Diamond and Kinter, 1963].Often, the excess modifier can be removed by washing with hot water or alcoholsolution, provided that the cations are not too large [Furukawa and Brindley,1973]. To avoid formation of the ionic double layer the ionic intercalant is usedto stoichiometry, and then its amine is added to form the head-to-tail, non-ioniccomplex.

The perfidy of Nature also provides for an opposite effect – adsorption of lessthan the stoichiometric number of organic cations. This situation has beenobserved for large molecules (e.g., codeine on MMT) adsorbed from dilutesolutions to form a monolayer on the clay surface, hence shielding adjacent anionicsites on the clay surface by steric hindrance [Weiss, 1963].

2.2.4.1.5 Stabilisation by Polyelectrolytes

Addition of water-soluble polyelectrolytes is a powerful method for controllingclay dispersion [Hesselink, 1971; Hesselink et al., 1971]. Polyelectrolyte consists

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Clays

of long-chain molecules with ionic groups usually located along the entire lengthof the chain. The polyion may be a polycation with amino groups, or a polyanionwith carboxyl, sulfate, sulfonate, or other negative groups. A single polymermolecule may contain both positive and negative groups, as is the case for proteinswith amino and carboxyl groups [Hauser, 1950].

Bio-polyelectrolytes (e.g., gum arabic, gelatin, alginates, pectin), modifiedbiopolymers (e.g., oxidised starch, carboxymethylcellulose) and syntheticpolyelectrolytes are becoming available, viz. polyacrylic acid [Warkentin andMiller, 1948], polyacrylonitrile [Mortensen, 1962], polyvinyl alcohol [Greenland,1963], etc. Addition of polyelectrolytes improves the stability of clay dispersionin the aqueous medium.

2.2.4.2 Clay Dispersion in Polar Organic LiquidsLike water molecules, polar organic compounds may be adsorbed on the claysurface. The adsorption energy of many of these compounds is comparable withthat of water. Depending on the concentration, they can displace adsorbed waterfrom clays, and be removed from the clay by washing with water. When MMT isdispersed in a polar organic liquid (e.g., alcohol, glycol or amine) the suspendingliquid molecules penetrate into the interlamellar galleries and displace water.The basal spacing of the complex depends on the size of the organic moleculesand on their orientation and packing geometry. The extent of d001 expansion inpolar organic liquids is so well defined that it can be used to identify MMT, e.g.,ethylene glycol that gives d001 = 1.7 nm has been used to identify MMT [MacEwan,1948; Bradley, 1945].

The exact mechanism of association between clay and polar molecules is notknown (similarly for water!). They may interact through the ionic groups of clayand/or through hydrogen bonding. However, there is little spectroscopic evidencefor the interaction between -CH2- or -CH3 and =SiO groups. FTIR indicates thathydrogen bonding is less important [Greene-Kelly, 1955; Brindley and Rustom,1958]. For adsorption of ammonium ions it was shown that entropic effectsprovide the driving force [Vansant and Uytterhoeven, 1972].

When the clay surface becomes covered with polar molecules containing asubstantial proportion of hydrocarbon groups, the surface becomes oleophilic,and under these conditions the organoclay can be used as an oil or grease thickener.However, since the exchange cations are still present, the complexes are usuallysensitive to water. For example, as shown by XRD, the MMT-pyridine complexat low water concentration hydrates stepwise and then exfoliates when a largeamount of water is added [van Olphen and Deeds, 1962]. Similarly low molecularweight alkane ammonium complexes (e.g., n-butyl ammonium) show a spectacularinterlayer swelling in water [Garrett and Walker, 1962]. However, pyridiniumand other large organic cation exchange complexes are not sensitive to H2O.Apparently here the steric shielding of unused silicilic anions provides sufficientprotection.

Polar, water-soluble macromolecules such as polysaccharides or polyethyleneglycol are readily sorbed by MMT, which leads to expansion of the interlamellarspace, d001. Edge sorption may also take place [La Mer and Healy, 1963].

2.2.4.3 Absorption of Organic Molecules by OrganoclayThe initial adsorption of organic molecules (as described above) may lead to changeof the clay character from hydrophilic to hydrophobic and to an expansion of the

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interlayer d001 spacing. Both factors tend to facilitate further absorption of organicsubstances by the interlamellar organic phase. The absorption may take place fromthe vapour phase, from solution or from melt. It constitutes a vital element in thestrategy of CPNC manufacture.

The mechanism of the secondary absorption is based on the miscibilityprinciple – only the substances that either can chemically react or are misciblewith the interlamellar organic phase can be absorbed. Furthermore, thethermodynamic free energy of mixing must be able to compensate for the energyof increasing separation of the clay platelets. Thus, for example, absorption of abase requires that the interlamellar phase comprises active acidic functionalities,while absorption of nonionic compounds requires that their solubility parameteris comparable to that of the interlamellar phase [Utracki, 1989; 2002b; Utrackiand Kamal, 2002a; Utracki, 2004]. The absorption leads to further expansion ofthe interlamellar space, proportional to the total number of -CH2- groups withinthe space. The process is diffusion controlled, hence a long residence time andintensive mixing may be required.

Maximum swelling is facilitated by the organophilic nature of treated clayand by the high polarity of the organic penetrant. Thus, the initial modification(in the aqueous phase) should result in the exchange of at least 50% of Na+ byorganic cations and the interlamellar space should be at least 0.8 nm. The bestsecondary intercalant should have high polarity and be organophilic, e.g.,nitrobenzene or benzonitrile. When the secondary intercalant is hydrocarbon,unsaturation is a definite asset otherwise swelling may have to be aided by additionof a well miscible (with it) polar organic liquid [Grim, 1968].

Certain organic molecules may also penetrate the interlamellar space to formcoordinated compounds by not directly interacting with the clay surface, butrather with the cations ionically bound to it. Two types of molecules are suitable,with either polar or aromatic functionalities. Since this secondary intercalationis usually conducted using air-dried organoclay (often suspended in an alcohol),the process must (1) replace the adsorbed water, and (2) form the coordinationshell around the cation. The presence of residual moisture enlarges theinterlamellar space, hence easier diffusion, but at the same time the energetics ofstep (2) must be sufficient to compensate for it. When the secondary intercalantwas either ethanol or acetone the number of molecules in a complex dependedon the interlamellar cation. Thus there were 2 for each K+, 3 for each Na+, 8-10for each Ca2+ or Ba2+, etc. The interlamellar spacing also varied, respectivelyfrom 1.3-1.4 to 1.7 nm [Bruque et al., 1982]. The complexes of Cu-, Ni-, Zn-,Cd-, Hg- or Ag-MMT with PA or thiourea are highly stable [Peigneur et al.,1978].

Complexes with water molecules forming bridges between the metal cationand the organic molecules are known for, e.g., pyridine, nitrobenzene, benzoicacid, aniline, nitriles, ketones or PEG. The latter compound forms complexeswith M+-MMT even in aqueous suspensions [Parfitt and Greenland, 1970]. Thepresence of H2O on the one hand causes expansion of the interlamellar space,but on the other it makes the system less stable at higher temperatures.

Complex formation of M+-MMT with neutral organic molecules results intheir protonation and stronger bonding. Protonation may originate directly fromthe cations present in the system, viz. H+, NH4

+ or in general M+, or from thewater molecules that are coordinated to a cation, viz. [M(H2O)x]n+. FTIR or

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Clays

NMR readily detects protonation. The molecules that undergo these reactionsare amines, azoles or phenols [Raussell-Colom and Serratosa, 1987].

In non-aqueous systems hydrogen bonding of organic molecules to the claysurface takes place. The positive charge deficiency in the tetrahedral layer resultsin negative charge that is spread out on several surface oxygen atoms coordinatedto Al3+ or Si4+ ions [Farmer and Russel, 1971]. The molecules capable of formingthe hydrogen bonds are mainly alcohols, amines, ketones, etc. Identification ofthese reactions by spectroscopic means is difficult.

In MMT at the platelet edges there is a limited number of silanol groups thatcan be reacted with a diversity of organic compounds, viz. alcohols,organochlorosilanes, isocyanates, epoxies, diazomethane:

≡ − +

− → ≡ − +

− → ≡ −

= = − → ≡ − − − −

− = → ≡ − − − −

⎧

⎨⎪⎪

⎩⎪⎪

Si OH

R OH Si OR H O

Cl Si R Si OSi R

O C N R Si O CO NH R

R CH O CH Si O CH CH OH R

( ) ( )

( ) ( )

2

3 3

2 2

Since the number of silanol groups in MMT is small, only a limited number oforganic molecules can be grafted. The grafting does not affect the interlayerspacing, but may provide for better miscibility with the organic matrix. However,the silanol groups can also be formed by treating a suspension of MMT in alcoholwith HCl [Lentz, 1964; Zapata et al., 1972]. The treatment removes surfacecations from octahedral layers:

≡ − − − − ≡ + → ≡ − ++ +Si O Mg O Si H Si OH Mg ( )2 2 2

The silanol groups in turn can be reacted with a diversity of compounds, e.g.,having groups susceptible for subsequent polymerisation: polycondensation orpolyaddition.

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97

Intercalation of Clay

2.3.1 IntroductionThe first step in the preparation of clays for use in PNC is purification of themineral (see Section 2.2.3). Depending on the ultimate use of the resulting Na-MMT, before intercalation the powder may be subjected to further preparatorysteps, either to reduce the particle size and/or to reduce the particle size distribution.Since the time (t) that intercalant needs to diffuse a distance (l) is given by theproportionality: t ∝ l2, a decrease of clay particle diameter by 30% results inreduction of the intercalation time by half. Thus, as described in patents fromSouthern Clay [Knudson and Jones, 1986; 1992a,b], the interdiffusion ofintercalating molecules is facilitated by reduction of the clay particle size – highstresses not only reduce the diameter but may also cleave the stack, reducing therequired force to bend the middle layers during intercalation. Earlier patents similarlyfocused on the reduction of clay particle size either by mechanical grinding,comminuting or by hydrodynamic forces [Cohn, 1967; Clocker et al., 1976].

Since the exchange reaction of the inorganic cation (such as Na+) for theorganic one proceeds from the clay platelet edge toward the centre as a regularfront, reduction of platelet size reduces the time required for the intercalation(increasing the reaction temperature to ca. 70 oC also helps) [Newman, 1987].Ion exchange strongly depends on pH – the optimum is about one unit below thepK-value of the organic salt.

It is noteworthy that reduction of particle diameter is detrimental for thecontrol of barrier properties, but it may not be essential for the other performancecriteria. Intercalation of clay that has a wide distribution of platelet size oftenresults in an uneven degree of intercalation, evidenced by broad XRD diffractionpeaks [Ho et al., 2001]. The peak is much sharper when more uniform size Na-MMT particles are used.

The aims of intercalation are to:

1. Expand the interlayer spacing,2. Reduce solid-solid interaction between the clay platelets and3. Improve interactions between the clay and the matrix.The first goal has been traditionally achieved by making use of the anionic chargewithin the interlamellar galleries. Since the van der Waals interactions betweensolid surfaces decrease with the square of the separating distance (see Figure 18),insertion of organic or inorganic molecules into the interlamellar space greatlyhelps to achieve the second aim. To reach the third goal, to compatibilise thesystem, the principles developed for compatibilisation of polymer blends shouldbe used [Utracki, 1994; 1998; 2000; 2002a; Ajji and Utracki, 1997; Utracki andKamal, 2002b; Ajji, 2002; Brown, 2002].

2.3 Intercalation of Clay

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For successful intercalation the selected clay should have a cation-exchangecapacity: CEC = 0.5-2.0 meq/g, as for CEC < 0.5 the ion exchange is insufficient,while for CEC > 2.0 meq/g, the interlayer bonding is too strong for easyintercalation, thus smectites and vermiculites have the optimum CEC –theoretically 1.39, experimentally 0.8 to 1.2 meq/g. By contrast, kaolin has acation-exchange capacity < 0.1 meq/g, while mica, illites, attapulgite and sepioliteare about 0.2 meq/g. As a consequence, MMT, saponite and hectorite are thepreferred clays for CPNCs, but since MMT is more abundant and it has a fairlylarge aspect ratio, p ≅ 300, (natural hectorite has the smallest) it became themain nanofiller for PNC technology. Owing to the large aspect ratio, p ≤ 2,000,of synthetic micas, natural vermiculites and natural micas, several attempts havebeen made to use them in CPNC as barrier material against permeation of gases,vapours and liquids.

Purification and intercalation can be quite abrasive, significantly reducingthe clay aspect ratio [Ferreiro et al., 2001; Jeon et al., 2004]. According to theformer authors, synthetic hectorite (Ilaponite LRD) had p ≅ 30, whereas MMThad p ≅ 100. Jeon et al. estimated the aspect ratio of MMT-based organoclay tobe ca. 80. Both teams reported that in solution organoclay tends to form sphericalparticles of diameter varying from about 20 to 240 nm.

Traditionally, the main use of the intercalated clays has been to producethixotropic effects in aqueous or non-aqueous systems, e.g., to improve papercoating, lubricant thickening or to prevent sedimentation of dispersed solids.During the last 20 years or so additional uses for clay for CPNC technology haveemerged. Intercalation and exfoliation for fine chemical delivery systems is themost recent. Thus, intercalated organoclays have had numerous uses:

• As water-based or organic liquid-based (e.g., ethanol, acetone) thickener forinsecticide paste [Hamilton, 1936].

• ‘Bentonite’ intercalated with either ammonium (e.g., with dodecyl ammonium,octadecyl dienyl ammonium, dimethyl dicetyl) or phosphonium (e.g., withtriphenyl lauryl) cation has been used, e.g., for thickening lubricating oilsand greases, oil-based muds, packer fluids, paint-varnish-lacquer removers[Jordan et al., 1950].

• Application of onium intercalated layered silicates as organophilic thickenersof organic liquids [Hauser, 1950]. A wide diversity of onium salts have beendescribed in the patent literature, viz. ammonium, phosphonium, sulfonium,etc.

• Use of MMT intercalated with quaternary ammonium (commercially availableas ‘Bentone®’ from the National Lead Co.) as an inherently thixotropicadditive in nail enamel applications. The organoclay provided suitablerheological behaviour and prevented settling of nacreous pigments [Kuritzkes,1969].

• In CPNC to facilitate diffusion of monomers and/or macromolecules intothe interlamellar space, that eventually would lead to exfoliation [Fujiwaraand Sakamoto, 1976; Okada et al., 1988].

• For delivery of cosmetic, medical or agro-active compounds, e.g., sunscreen,antibacterial, antifungal, and many others [Beall et al., 1998].

• For delivery of fungicides, pesticides, insecticides, acaricides and otheragriculturally active compounds [Beall et al., 1999a].

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Intercalation of the clay particles is diffusion controlled. Water is a ‘natural’intercalant for clays, but it can hardly be considered unique. Its efficiency is mostlikely related to the good balance between the dipole moment that drives theprocess and the molecular size that restricts its motion. In aqueous systems, watermolecules can easily diffuse in and the electrostatic double layer can push theindividual platelets apart and keep them away from each other – at low clayconcentration total exfoliation can be achieved.

On a molecular scale, intercalation can be visualised as inserting peas betweeneach pair of play-cards in a stack. Evidently, to be able to do that the cards mustbe pre-spaced and there must be a driving force for the peas to enter the narrowspace bending the cards from the edges. Thus, the way to intercalate clay is touse progressively larger molecular species. To prevent re-assembly of layers it isdesirable that at least some intercalant is bound to the clay surface.

For the application of clays in CPNC technology intercalation should increasethe interlamellar spacing to about 3-4 nm and make the clay organophilic. Thegoal is usually reached in stages.1. In dry clay the solid-solid interactions keep the interlamellar gallery at a

water monolayer level, of about 0.26 nm. The sheets are strongly bound toeach other (see Figure 18). The traditional method for reducing the solid-solid interactions, hence to reduce the resistance to intercalant diffusion intointerlamellar space, is to disperse the clay in water or an aqueous solution ofwater-soluble organic solvents, e.g., alcohol or glycol.

2. The second step usually involves exchange of Na+ for an organic cation.Since the pK of onium salts increases with the degree of substitution, in mostcases the quaternary onium is used. However, for the use of organoclay as areactive component (e.g., in thermosets) a primary or secondary onium saltmay be preferred. Furthermore, depending on the expected application theonium salt may have a functional group (e.g., a vinyl functionality).

3. In the third step the organoclay may be further treated with reactive compoundto compatibilise the clay/polymer systems. Three classes of reactivecompounds have been used: known glass-fibre sizing agents (e.g., silanes,titanates, zirconates), known reactive compatibilisers (e.g., oligomeric orpolymeric compounds with glycidyl, maleic, isocyanate and other reactivegroups), and organometallic compounds. This third step may be a part ofthe last step in the preparation of CPNC, the exfoliation of organoclay anddispersion of individual platelets in the polymeric matrix.

Several routes have been used to intercalate clay particles. They can be classifiedas follows:

1. Use of solvents or low MW solutions, such as water, alcohols, glycols, crownether or monomer solutions.

2. Use of organic cations, viz. ammonium, phosphonium or sulfonium.3. Silylation of clay platelets.4. Incorporation of inorganic compounds that form interlamellar pillars.5. Use of organic liquids, viz. monomers, macromers, oligomers, polymers (PEG,

PVAl, PDMS, PVP), copolymers, and their solutions.6. Melt intercalation.7. Others.

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2.3.2 Intercalation by Solvents and SolutionsAs was shown in Figure 17, the basal spacing of MMT is 0.96 nm. In the presenceof ambient humidity it expands to about 1.25 nm and upon addition of ethyleneglycol to 1.7 nm. Intercalation using aqueous solution of glycols, glycerol orsorbitol was reported to be facilitated by the addition of Mg(OH)2 in the amountthat changes pH to about 9.4 [Buffett, 1965; Burns, 1974].

Vaia et al. [1995a] directly intercalated Na-MMT or Li-MMT with PEG byheating the suspension to 80 °C for 2 to 6 h. The PEG displaced water molecules,expanding the interlayer spacing to d001 = 1.77 nm, but the clay could not havebeen exfoliated.

Thixotropic thermoset polyester resin was prepared for hand lay-up mouldingsby incorporating clay intercalated with polyols. Similarly, Kornmann et al. [1998]produced CPNC based on MMT and unsaturated polyester (UP). Na-MMT wasintercalated at 50 °C in MeOH with methacrylate then dried. The treated MMTwas dispersed in UP (containing 42 wt% styrene and co-octanoate). The mixture,after curing at room temperature and post curing at 70 °C, was found to beexfoliated.

In 1998 Katahira et al. [1998a,b,c,d] published a series of articles on the use ofmica for the production of PA-6-based CPNC. First, Na-mica flakes were cleavedand dispersed into hydrolysed and protonated ε-caprolactam (ε-CL). Phosphoricacid has been used as a catalyst for the protonation of ε-CL. The intercalation wasfound to occur in two steps: a rapid (even at 20 °C) exchange of Na+ in mica gallerieswith protonated ε-CL, then a significantly slower exchange of water solvated Na+

ions – the latter step could be accelerated by heating to T > 60 °C. The intercalatedmica had the interlayer spacing of d001 = 1.47 nm.

Serrano et al. [1998a] patented exfoliated CPNCs containing ≤ 99.95 wt%EVAl. No onium ion or silane coupling agent was needed for the intercalation.The intercalant was selected from between water-soluble oligomers (degree ofpolymerisation (DP) = 2-15) or polymers (DP > 15) having dipole moment greaterthan that of H2O (1.85 D). For example, P4VP, PVAl, copolymers of vinyl acetateand vinyl pyrrolidone, or their mixtures could be used. The intercalant wasabsorbed between the silicate platelets increasing the interlamellar galleries byΔd001 = 0.5 to 10 nm. The authors speculated that on the clay surface theintercalant molecules were ionically complexed with interlayer cations. Dependingon the intercalant concentration the adsorbed molecules formed from 1 to 5 layerson the clay surface.

In the following patent from the same laboratory [Serrano et al., 1998b] it wasreported that the best results were obtained using an oligomer with DP = 2-15 or apolymer containing 50-80 wt% of an oligomer. The optimum performance wasachieved by expanding the interlayer spacing to d001 = 3.0-4.5 nm. The preferredintercalants were P4VP, PVAl, or their mixtures. Intercalation was conducted ina twin-screw extruder (TSE) using intercalant solution at T = Tm + 50 °C. Amasterbatch containing 20-80 wt% of clay was prepared. The recommendedpresence of oligomers and relatively high mixing T indicate recognition of thecritical role diffusion has in the process. The method was extended to non-aqueoussystems, but using Na-MMT that contained 10-15 wt% H2O (exfoliation wasnot obtained when the water content was below 8 wt%). For example, when80 wt% PA was compounded under N2 with Na-MMT at T = Tm + 50 ≈ 230 °C,the clay platelets were exfoliated. Similarly, mixing PET with 10 wt% ofNa-MMT or PC with 50 wt% of Na-MMT resulted in exfoliation.

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N-Vinylcarbazole (NVC) was cationically polymerised by Biswas and Ray [1998;1999], in the presence of well-dried MMT, either directly at T > Tm = 64 °C, or inbenzene at 50 °C. The resulting CPNC of polyvinylcarbazole (PVK) wasintercalated, but during 100 min of polymerisation the interlayer spacing increasedfrom d001 = 0.98 to 1.46 nm. However, intercalation was not the aim of thiswork. The authors reported two important discoveries of PVK grafting to theMMT surface and of the possibility of achieving clay intercalation underanhydrous conditions. Evidently, the dipole moment and aromatic character ofNVC provided a large driving force.

Srikhirin et al. [1998] intercalated two polydiacetylenes: 14-amino-10,12-tetradiynoic acid (a diacetylenic mono-amino acid, MADA) and 10,12-docosadiyndiamine (diacetylenic diamine, DADA) into layered silicates: MMTfrom Wyoming (CEC = 0.33 meq/g) or from Arizona (CEC = 0.57 meq/g) andvermiculite (VMT). The intercalation was conducted in an aqueous ethanolsuspension for MMT and VMT over 1 and 7 days, respectively. The intercalationwas confirmed by XRD and FTIR. The d001 spacing and polymerisability of thediacetylenes depended on the length of the diacetylene molecule, the layer chargedensity of the clay, and the solvent treatment. The DADA dichloride wasintercalated into both MMT and VMT. The d001 = 1.41 and 3.1 nm was obtainedfor MMT and VMT, respectively. Intercalation with MADA resulted in d001 =1.91 and 3.68 nm for the two MMT types, and d001 = 4.5 nm for VMT. Irradiationproduced polymer only in the latter case. In the final product the interlayer spacingchanged but a little: d001 = 4.2 nm was obtained. The authors concluded thatwhen the intercalated diacetylene in MMT lays flat on the clay surface it is unablenot be polymerised owing to the lack of proper packing density. In VMT theintercalated MADA is tilted with respect to the clay surface and the diacetylenehas proper packing for propagation of radical polymerisation.

As was already mentioned, organic compounds may diffuse into galleries andcoordinate to alkaline cations [Ruiz-Hitzky and Casal, 1978]. The strength of thecomplex as well as expansion of the d001 spacing depends on the nature of thecation and the organic compound. Recently, on separate occasions, Gilman et al.[2001] and Yao et al. [2001] used crown ethers and cryptands to intercalate claysthat were subsequently dispersed in polymers (PA-6 and PS, respectively). Similarly,MMT has been intercalated with C60-fullerene [Ishikawa et al., 1997]. The processwas conducted in a phenol-formaldehyde-N-vinyl-2-pyrrolidone mixture at 60 °C,in the presence of HCl, subjecting the system to ultrasonics. The resulting thermosetswith good electrical conductivity were used in Li-batteries.

In 2001 Yao et al. used Na-MMT and K-MMT swollen overnight in water,then added a 1~3 % solution of crown ether in acetone. The complexation wasconducted for 24 h at 20 to 45 °C, then the clay complex was filtered, washed andfinally dried in a vacuum oven. The crown ethers were found to increase the d001spacing by Δd001 = 0.4 to 0.7 nm, hence they complexed with either K+ or Na+. PS-type CPNCs were prepared by dispersing ca. 1 wt% of crown ether-modified clayin styrene (St). The radical polymerisation of St in the presence of these clays wasconducted overnight at 80-120 °C. For Na-MMT-based systems XRD showed nosignificant change in the d-spacing, but CPNCs were formed from K-MMTintercalated with cis-di-cyclohexano-18-crown-6 and the interlayer spacingincreased to d001 = 7.7 nm. It is noteworthy that the complexation constant ofcrown ethers with K+ is higher than that with Na+. The larger is the constant, thestronger is the complex and the more organophilic the clay.

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2.3.3 Intercalation by Organic CationsThis method of intercalation by organic cations has been dominant for 70 yearsor so. As mentioned, the fastest exchange rates have been reported for Na-MMT.The reactions in water are faster than those in aqueous solutions of organicliquids, especially at higher temperatures, viz. T = 60 to 80 °C. Increased pressure(P) also speeds up the reaction. The diffusion-controlled, reversible reaction startsat the rim and the distance it travels is proportional to the square root of time(see Figure 22).

As shown in Table 17, there is a great diversity of intercalants for layeredclays. Functionally, these compounds are similar to compatibilisers or emulsifiers– they must diffuse into interlayer space and ascertain good thermodynamicinteractions between the modified clay and polymeric matrix. Furthermore,similarly like compatibilisers, intercalants must provide good stress transferbetween the two principal CPNC components – clay and polymer. Thus, thethree criteria for selecting intercalant are: (1) the kinetics for diffusion into theinterlamellar galleries, (2) high bonding strength with clay platelets, and (3) stronginteraction with the polymer matrix.

To facilitate selection, Tanaka and Goettler [2002] used molecular dynamics(MD) computation. The authors built a model comprising a MMT platelet, anintercalant (12 were considered), and a matrix (PA-66). Next, using MD methodsthe binding energies were computed for 600 K within each of the three modelcomponents, as well as the binary interaction energies between them. Finally, thefracture toughness parameter, Gc, was equated with the computed bindingenergies. The simulation showed that the positive charge initially placed on R4N+

spreads over the intercalant molecule; the charge is not localised on the nitrogenatom, but primarily on the substituent groups bonded to it. The chargeequilibration method showed that the binding energy between PA-66 and theclay platelet decreases almost linearly with the intercalant volume. Thus, pristineclay/PA-66 showed the highest binding strength (hence toughness). The secondbest performance was found for MMT intercalated with ω-amino dodecyl acid(ADA), and the worst for MMT with dimethyl di-octadecyl ammonium (2M2OD).Similar MD simulations were carried out for CPNC comprising MMT, intercalant,and PA-6 [Fermeglia et al., 2003]. The results on the one hand paralleled thepreviously data for PA-66, and on the other indicated that the intercalantsoriginally selected in Toyota R&D laboratories were indeed the best.

Clay reactions with ammonium ions were studied in the early 1930s [Smith,1934; Gieseking, 1939]. Later on attention shifted to the whole class of oniumsalts defined as organic compounds of the type RXHv. The element X (in itshighest valency) may be pentavalent, as in ammonium, phosphonium, arsoniumand stibonium; tetravalent, as in oxonium, sulfonium, selenonium andstannonium; or trivalent, as in iodoium [Hauser, 1950]. Clays readily react withless mobile organic cations, ionically bonding them to the surface. The organicpart is selected considering three aspects of intercalation:

1. Ability to non-ionically interact with the clay surface,2. Ability to expand the interlamellar space, and3. Miscibility with the polymeric matrix.In 2003, Lagaly and Ziesmer published an excellent review on clay chemistry[Lagaly and Ziesmer, 2003].

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Traditionally, for the preparation of organophilic thixotropic additives usedin lubricants, quaternary ammonium chlorides with aliphatic hydrocarbons wereintroduced. Later, for paints and varnishes, mixed aliphatic/aromatic ammoniumsalts were used. For latex systems, it has been found advantageous to usehydrocarbons with -OH groups, viz. hydroxyethyl, carboxyl, etc. Owing to lowcost and suitable hydrocarbon lengths, tallow oil derivatives have frequently beenused. Commercial hydrogenated tallow oil typically contains (in wt%): 2.0 C14,0.5 C15, 29.0 C16, 1.5 C17, 66.0 C18 and 1.0 C20 alkyl groups. It is obtained fromnatural oils, e.g., corn, soybean, cottonseed, castor, or animal oils and fats. Otheroils, e.g., coco or coconut, are also being used. The commercial organoclays arestill intercalated with ammonium salts having these three types of organic chains.

Unfortunately, as discussed in Part 3.2, quaternary ammonium ions decomposeat the processing temperatures of most thermoplastics, viz. at T ≥ 200 °C. Thisdecomposition leads to reduction of the interlayer spacing (reversed intercalation)and it may discolour the product, reduce mechanical properties, decrease impactstrength, etc. Nevertheless, these quaternary cations are the principal intercalantsused in the industry [Hamilton, 1936; Maloney, 1939]. It has been found thatthe d001 spacing of layered clays intercalated with alkylammonium salts dependson the length of the alkyl radical as well as on the CEC. As shown in Figure 23the spacing changes in steps [Hackett et al., 1998]. Molecular modelling and theFTIR measurements led to the model shown in Figure 24 [Vaia et al., 1994].

One of the early applications of organophilic clays was to ‘thicken’ lowviscosity liquids. Such gellants had to have good dispersability in aqueous ororganic systems. Owing to the hydrophilic character of clays, development ofgellants for organic solvents and solutions was more challenging. To minimise

Figure 23 The interlayer d001 spacing of clays with CEC = 80, 100 and 150 as afunction of the alkyl chain length in a primary ammonium salt: RNH3

+Cl-.After Hackett et al. [1998].

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the amount of clay that was necessary for creating the desired effects twoconditions had to be met:

1. The clay had to have the largest possible aspect ratio; and2. The organoclay had to be miscible in the matrix liquid.In other words, the clay had to be exfoliated. Knowing the aspect ratio the requiredconcentration could be calculated from Equation 1 as φ > φm. The conditions formiscibility could be calculated considering the molecular structure of the matrixliquid and the intercalating radical using, e.g., Hansen’s solubility parameterapproach [van Krevelen, 1993], or the equation of state approach [Utracki, 2004].

Figure 24 FTIR-based alkyl ammonium chain ordering model as a function ofalkyl length: (a) isolated, short chains in a monolayer; (b) intermediate chainsdisordered, forming a quasi-bilayer; (c) long chains with increased order and

multi-layer spacing.

Reproduced from Vaia et al. [1994], copyright 1994, with permission from theAmerican Chemical Society.

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For example, Finlayson and Jordan [1978] prepared organophilic smectite(CEC = 0.75-1.20 meq/g) by reacting it with methyl benzyl dihydrogenated tallowammonium chloride (MB2HTA). The clays were hectorite and Wyoming bentonite(CEC = 1.0-1.2 meq/g), purified and converted to sodium form. Other quaternaryammonium intercalants, viz. dimethyl dihydrogenated tallow (2M2HTA), methyltrihydrogenated tallow (M3HTA), benzyl trihydrogenated tallow (B3HTA) anddimethyl benzyl hydrogenated tallow (2MBHTA), were described as suitable aswell – selection of one or the other depends on the character of the liquid to bethickened. The intercalation was carried out at T = 66-77 °C by mixing an aqueoussuspension of 3-7 wt% Na-MMT and the quaternary ammonium compound forsufficient time to achieve adequate intercalation. The product was filtered andwashed at 60 °C, then dried and ground. When the organophilic clays are to beused in emulsions, the drying and grinding steps may be eliminated. Theseorganophilic clay complexes were used as additives to lubricating greases, oil-basedmuds, oil-based packer fluids, paint-varnish-lacquer removers, paints, foundrymoulding sand binders, etc.

A simple test was devised to determine the thickening characteristics of the neworganoclays. Thus, low viscosity oil was mixed with 4.5 wt% of organophilic clayfor 0.5 min at 1800 rpm, then 0.12 wt% of water was added and mixing continued.After 6-9 min the viscosity η > 200 Poises was obtained.

MMT reacted with dimethyl-dioctadecyl ammonium ion (2M2ODA) has beenindustrially produced as a thickener for coatings in solvents with low polarity(e.g., toluene and xylene). For polar solvents (e.g., DMF, methanol and ethanol),clay with a dimethyl-benzyl-octadecyl ammonium ion (2MBODA) has been used,but the gelling efficiency of this system was low. As a solution to the latter problem,Iwasaki et al. [1994] developed organophilic clay with hydroxy-polyoxy-ethylene-alkyl ammonium ions. The authors used clay with CEC = 0.85-1.30 meq/g andammonium salt of mono-hydroxy-polyoxy-ethylene-tri-alkyl-, mono-hydroxy-polyoxy-ethylene-di-alkyl-, di-hydroxy-polyoxy-ethylene-di-alkyl-, di-hydroxy-polyoxy-ethylene-alkyl-, tri-hydroxy-polyoxy-ethylene-alkyl- or tri-hydroxy-polyoxy-ethylene-. The number of statistical segments (n) in polyethylene glycol,-(CH2CH2O)n- was n = 2-20. A three step method of preparation was used:

1. 1 to 15 wt% clay was dispersed in water,2. The quaternary ammonium salt solution was added 0.5 to 1.5-fold on the

clay CEC,3. The organophilic clay was washed with water, dried and pulverised.According to XRD, the dehydrated smectite had the basal spacing d001 = 0.96 nm.Under ambient temperature and humidity the spacing increased to 1.2-1.6 nm.The organoclay intercalated with quaternary hydroxy-poly(oxy-ethylene)-alkylammonium ions had d001 ≥ 1.8 nm – the actual value depended on the degree ofpolymerisation of the PEG segment, -(CH2CH2O)n-. Since these groups have ahigh affinity to molecules of a polar solvent, the solvent expanded the interlayersand exfoliated the clay. The organoclay was used as gellant in the production ofcoatings, plastic products, films and adhesives containing highly polar organicsolvents such as DMF or ethanol. Good film-forming ability was achieved. In acited example 20 g of hectorite (d001 = 1.25 nm, CEC = 1.1 meq/g) was dispersedin 1 L of tap water, and 21.4 g of a quaternary ammonium salt. The mixture wasstirred at room temperature for 2 hours. The product was separated, washed,dried and pulverised. XRD confirmed formation of organoclay with d001 = 2.1 nm.

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The product formed a transparent dispersion in N,N-dimethylformamide showinggood affinity to highly polar organic solvents.

In another patent from NL Industries [Finlayson, 1980], several quaternaryammonium salts were used, viz. methyl benzyl or dibenzyl dialkyl ammoniumchloride, methyl benzyl dihydrogenated tallow ammonium chloride or methylbenzyl dicoconut fatty acid ammonium chloride. The intercalation was conductedas described in the previous patent. On an industrial scale the organoclay couldbe prepared in colloid mills or other high speed dispersers. The non-aqueous,self-activating organophilic clays were useful in paints, varnishes, enamels, waxes,epoxies, mastics, adhesives, cosmetics and the like. The use of benzyl or dibenzylradicals in the intercalant indicates that the paints that needed thickening weredispersed in at least partially aromatic solvents.

In a later patent [Finlayson and Mardis, 1983], the intercalating cation could beany quaternary onium, viz. ammonium, phosphonium, sulfonium or their mixtures.The intercalant should contain at least one alkyl group having 12-22 carbon atoms,derived from natural oils, e.g., tallow. Additional onium radicals may includemethyl, ethyl, decyl, lauryl, stearyl, benzyl and substituted benzyl, phenyl (as inN-alkyl and N,N-dialkyl anilines), alkyl phenyl, naphthalene, anthracene andphenanthrene. The clay-onium complex was reacted with organic anion. Thelatter should have molecular weight < 1 kg/mol and be derived from an organichaving a pKA < 11.0. Suitable acids include:

• Carboxylic acids (benzoic, phthalic acid, benzene tri-, tetra- and hexa-carboxylicacid); alkyl carboxylic acids: H-(CH2)n-COOH, wherein n is a number from1-20; alkyl dicarboxylic acids: HOOC-(CH2)n-COOH, wherein n = 1-8;hydroxy alkyl carboxylic acids (citric, tartaric, or 12-hydroxystearic acid);unsaturated alkyl carboxylic acids (maleic, fumaric or cinnamic acid); fusedring aromatic carboxylic acids (naphthalenic or anthracene carboxylic acid);cycloaliphatic acids (cyclohexane-, cyclopentane-, or furan carboxylic acids).

• Organic sulfuric acids: (a) sulfonic (benzene-, phenol-, dodecyl-, benzene di-or tri-sulfonic, p-toluene sulfonic acid); alkyl sulfonic or di-sulfonic acids(e.g., of butane); and (b) half-esters of sulfuric acid with lauryl or octadecylalcohol.

• Organophosphorus acids including: phosphonic; phosphinic (e.g., dicyclohexylphosphinic), thiophosphinic acids, phosphites, phosphates, e.g., dioctadecylphosphate.

• Phenols, viz. phenol, hydroquinone, t-butyl catechol, p-methoxy phenol, andnaphthols.

• Thio-acids (thio-salicylic, -benzoic, -acetic, -lauric, -stearic, etc.).• Amino acids, e.g., 6-aminohexanoic or 12-aminododecanoic.• Polymeric acids, e.g., low molecular weight acrylic acid polymers and

copolymers; styrene-maleic anhydride copolymers.• Miscellaneous acids and acid salts, which form a water insoluble precipitate

with an organic cation.The intercalation was carried out in aqueous medium by mixing the ingredientsfor 1 to 60 min, at T = 60 to 77 °C, and then filtering, washing, drying andgrinding. The amount of anion should be 0.1 to 0.5 meq/g of clay, that of organiccation 1.0 to 1.6 meq/g.

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The preferred process involves:

(a) Preparing water slurry with 1-80 wt% smectite and heating to T = 20-100 °C(b) While agitating the suspension adding organic anion and organic cation in a

sufficient amount to satisfy the clay CEC and the cationic activity of theanion

(c) Continuing the reaction for a sufficient time to form a reaction productcomprising an organic cation-organic anion complex, which is intercalatedinto clay, and then recovering the product.

In 1986, Knudson and Jones from Southern Clay Products (SCP) [Knudson andJones, 1986] reported further improvements of the intercalation method. Priorto the reaction with the ammonium intercalant the authors subjected clay tohigh-energy pugmilling in a machine. The device had a short barrel (L/D = 4-10),a motor driven screw, and perforated die plate. The reasons why the extrusionimproved the intercalation were not discussed. The authors observed thatintercalation was improved if prior to the reaction with onium salt the clay issubjected to high energy milling, dispersing or grinding. However, pugmillingwith energy of 30, 51 and 108 HP-hr/ton reduced the smectite clay particle sizefrom 475 nm to, respectively, 391, 277 and 276 nm. Since, according to thediffusion mechanism the process depends on the diffusion path length, this wouldconsiderably reduce the diffusion time. The high shear stress may not only reducethe stack diameter, but also its height.

In a typical procedure, the crude clay, e.g., bentonite, was wetted with 25 to40 wt% water, and then passed through the pugmill under conditions which impartat least 40-50 HP-hr per ton of dry clay. When the crude clay is not Na-clay,Na2CO3 may be added during the pugmilling. The clay is then dispersed in waterat a concentration below 10 wt%, screened and centrifuged to remove non-claycontaminants, such as quartz. The fine fraction from the centrifuge (d < 4 μm at4-5 wt% solids), was reacted with a quaternary amine chloride, e.g., 2M2HTA,2MBHTA, M3HTA, etc. The organoclays had high gelling efficiency so only asmall amount was needed to achieve the desired result.

Knudson and Jones [1992a] disclosed further improvements of this technology.The organoclay manufacture followed the standard steps: clay purification,conversion to Na-smectite, reduction of the platelet size by high stress shearing,and reacting it with quaternary ammonium salt. The key improvement was theuse of a Manton-Gaulin colloid mill (also known as a ‘Gaulin homogeniser’). Inthe mill the clay suspension that entered the valve area at high pressure and lowvelocity was subjected to high acceleration, turbulence and cavitation. As it passedthrough a narrow orifice the velocity increased to about sonic level. Directingthis high velocity stream to impact on a ring enhanced the milling effectiveness.As a result, the particle size was reduced from 756 to 438 or 352 nm for anenergy input of 210 or 700 HP-hours per ton, respectively. Another patentdescribes the use of Cowles disperser and Greer mill for subjecting the clay slurryto high shear flow [Knudson and Jones, 1992b]. In the Cowles disperser the clayparticles are subjected to laminar flow, while in Greer mill the clay slurry isforced to flow at high speed through a 0.254 mm gap and to impact on a deflectorplate. The gelling capacity of such treated clays was improved.

In Jordan’s patent [Jordan, 1994] another approach to intercalation wasadopted. The organoclay was to be used as a thickener or a gelling agent, especially

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for paints. The author noted that intercalation in aqueous slurry is advantageousas ‘solvation relaxes the clay’s structure in order to permit penetration of theorganic cations’. However, it is preferable to avoid too dilute a reaction becausethe slurry preparation takes time and space and dewatering of product is energy-intensive. A new, waterless method was developed, based on reacting preblendedclay with quaternary cations by mixing the dry materials in a high pressure reactionvessel. The process can be batch or continuous. MMT, hectorite, saponite,attapulgite, sepiolite and their combinations can be used. The onium cation hasa formula R4M+ X-, wherein M is nitrogen or phosphorous, R is alkyl, aryl, oralkyl-aryl and X is halogen or methyl sulfate. To facilitate the process 3-15 wt%of a dispersant (neopentyl glycol, pentaerythritol, hydrogenated castor oil,sulfonated castor oil, a plasticiser, toluene sulfonamide, trialkoxyphosphate, etc.)might be added. It is preferred that the dispersant has beneficial effects on thefinal product.

For example, a mixture of clay, quaternary salt and dispersant passes from aribbon blender to a high pressure mixing auger-extruder with a perforated die.At the downstream end a mixing blade assembly stirs the highly compressedmixture at pressures of 20-55 MPa. The temperature should be near, but notabove 80 °C. The mixture may be either cycled through the same or anothermixer/reactor, or it may be transferred for further processing or milled to suitableparticle size. Since the process obviates the need for slurring the clay, sand andother particulates may be present in the product. The material (10 wt% oforganoclay with the remaining constituents) was successfully used in a cablefilling application as an oil gelling agent. The dielectric constant was almostinvariant at T = 21-148 °C.

Two years later, Jordan [1996] published a refinement of the above method.Thus, a mixture of two quaternary cations was found to induce superiorperformance in a variety of applications. The mixture comprised at least 5% ofeach: X2R2N+ and XYR2N+ where X is methyl, Y is benzyl and R is an alkyl,derived from saturated tallow oil. The clay may include 3-15 wt% of a dispersant,e.g., hydrogenated castor oil.

The organoclay comprised 50-90 wt% of a smectite (MMT, hectorite, saponite,attapulgite, sepiolite, or their mixture) having exchangeable, inorganic cationsat least in part substituted by a mixture of 10-50 wt% of two organic X2R2N+

and XYR2N+ cations. The composition may include 3-15% (based on clay) of adispersant (e.g., neopentyl glycol, pentaerythritol, hydrogenated castor oil,sulfonated castor oil, toluene sulfonamide, trialkoxyphosphate). As in the previouspatent, the pre-blended mixture was processed using a blender and an auger-extruder with perforated die. The product was milled to usable particle size. Theinterlaminar spacing was not determined.

The modified clay was evaluated for paint and grease thickening applications.In the first case, the thixotropic index and the number of oversized particles weremeasured – the new organoclay was found to equal or exceed the bestcommercially available materials made using a high-cost slurry process. Thegreases were tested for consistency by utilising a Helipath instrument. Again, theresults showed that the new organoclay offered significantly better performancethan the standard materials.

Eilliott and Beall [1989] patented an organoclay composition for use as arheological additive in a variety of non-aqueous liquids, such as paints, varnishes,enamels, waxes, adhesives, inks, laminating resins, gel coats, etc. To obtain

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maximum dispersability and thickening (or gelling) efficiency, it was necessaryto add a low molecular weight polar organic to the clay. Such compounds havebeen called polar activators, dispersants, dispersion aids, solvating agents, etc.An organosilane of the type RnSiX4-n was a new alternative. In this formula, n = 1-3,R is an organic radical having C-Si link and X is a hydrolysable alkoxy, acryloxy,amino or halogen group. The new organoclay of Na-MMT (CEC ≥ 0.75 meq/g)with a quaternary ammonium ion and 0.5 to 5 wt% organosilane has been usedfor preparing thixotropic thermoset compositions from unsaturated polyestersand unsaturated aromatic monomers, e.g., styrene. The composition wascrosslinked by peroxide, and used in the preparation of glass fibre (GF) laminates.

The smectite-type clays of CEC ≥ 0.75 meq/g, e.g., Na-MMT or hectoritewere preferred. The onium was R1R2R3R4N+M-, where M is an anion, R1 is analkyl containing 12-22 C-atoms and R2-R4 are alkyls containing 1-22 C-atoms,arylalkyl groups containing 7-22 C-atoms, aryl groups containing 6-22 C-atomsand their mixtures. The long chain alkyls can be derived, e.g., from hydrogenatedtallow oil. Aryl groups include phenyl and substituted phenyl. Arylalkyl groupsinclude benzyl and substituted benzyl groups. Examples of useful quaternaryammonium compounds are 2M2HTA (preferred), M3HTA, 2MBHTA, MB2HTA,etc. The organosilanes used in this patent are the same coupling agents as used inplastics composites [Plueddemann, 1982], viz. methyl-, ethyl-, or propyl-trimethoxy silane, di-methyl- or diethyl- di-methoxy silane, tri-methyl- or tri-ethyl- methoxy silane, phenyl triethoxy silane (preferred), etc. Furthermore, thethixotropic properties of the organophilic clays can be improved by the additionof ≤ 25 wt% of hydrogenated castor oil.

Intercalation took place in a slurry process. A purified aqueous suspension of1.5-5 wt% clay was heated to 60-75 °C, then quaternary ammonium salt and theorganosilane (as an emulsion in water or alcohol) were added. To complete thereaction, agitation and heating continued for 15 to 120 min, then the organophilicclay was filtered, washed and dried at 90 °C for 24 hours. The dried product,ground and screened through a 170 mesh screen, was used as a rheological additivein a wide variety of non-aqueous liquids, viz. paints, varnishes, enamels, waxes,adhesives, inks, laminating resins, gel coats and the like. The new clays wereparticularly useful for preparing thixotropic crosslinkable compositions fromunsaturated polyesters and styrene. The developed compositions were used inthe ‘pre-gel’ and ‘directly add’ processes.

MMT intercalated with 12-amino-dodecanoic acid (ADA) was used to prepareCPNC based on PA-6, which in turn could be blended with polypropylene/ethylene-propylene rubber (PP/EPR) [Fukui et al., 1992]. For example, 100 g ofMMT (d ≈ 100 nm) were dispersed in 10 l of water, then 51.2 g of ADA and24 ml of HCl were added. The mixture was stirred for 5 min, filtered, washedand vacuum dried.

Some degree of dissatisfaction with onium ion as the principal intercalant isevident from patents deposited by Toyota Central Research and DevelopmentLaboratories (Toyota – for short). For example, [Usuki et al., 1996] described thepreparation of CPNC in three steps:

(i) Intercalation of clay with an onium ion (having ≥ 6 carbons) rendering itcompatible with a ‘guest molecule’ with a polar group in its chain and lengthequal to or larger than that of the onium alkyl;

(ii) Contacting the organo-clay with the ‘guest molecule’ at T ≤ 250 °C; and

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(iii) Transferring the modified clay to a reactor (the preferred reactive exfoliationmethod) or compounding it with a resin.

The patent listed thermoplastic and thermosetting resins as well as elastomers(e.g., PE, PP, PS, PIB, acrylics, TPU, SBS, liquid BR, PB or IR, etc.) as potentialmatrix polymers. However, the main interest was to produce CPNC in low polaritypolymers, such as PO or rubbers. The guest molecule should have a polar groupat the chain-end. The following groups might be used: hydroxyl (-OH), halogen(-F, -Cl, -Br, or -I), carboxyl (-COOH), anhydrous-carboxylic acid (maleic), thiol(-SH), epoxy radical, or primary, secondary or tertiary amine (-NH2, -NH, -N).In the described examples the authors used as onium salts 2M2ODA, 2M2TDA,or TDA; as a ‘guest molecule’ hydrogenated, low molecular weight polybutadienewith an -OH group at the chain end (HTBR), stearic acid or alcohol alone orwith HTBR, maleated PP (PP-MA with high acid value hence low MW), etc. Toascertain good miscibility of organoclay with the matrix polymer a second ‘guestmolecule’, e.g., low molecular weight polyisoprene (IR) or oligopropylene, couldbe used. The process resulted in a high degree of clay dispersion [Kato et al.,1997].

The patent on CPNC with PA as matrix [Okada et al., 1988] described thefollowing method for uniformly dispersing clay platelets with interlayer distanced001 ≥ 2.0 nm. The manufacturing process comprised three steps:

1. Intercalation of a clay (CEC = 0.5 to 2.0 meq/g, e.g., smectite, vermiculite orhalloysite);

2. Mixing the organoclay with a PA-monomer and3. Polymerisation.Only the first step is of interest in the present discussion.

The intercalation was accomplished by dispersing clay in an aqueous, acidifiedintercalant solution, followed by washing the organoclay with water to removeexcess ions, or by mixing an aqueous suspension of clay with a cation-exchangeresin previously treated with an intercalant. The easiest to treat were the sodium-substituted clays, viz. Na-MMT (CEC = 0.8-1.0) or Na-vermiculite (CEC = 1.8).Inorganic (viz. Cu2+, Al3+), or organic cations were used. In the latter case α,ω-alkyl acids H3N+ CnH2n COOH were used, with n = 12 for the preferred dodecyl(ADA), n = 14 for tetradecyl (ATDA), n = 16 for hexadecyl (AHDA), or n = 18for octadecyl (AODA). For example, to a suspension of 100 g Na-MMT in 10 Lof water, 51.4 g of ADA and 24 ml HCl were added, and the mixture was stirredfor 5 min. After filtration, the organoclay was thoroughly washed and vacuum-dried. After execution of steps 2 and 3 (dispersion in a monomer, thenpolymerisation) the resulting CPNC contained fully exfoliated clay platelets andshowed significant improvement of the mechanical and thermal properties. Asdisplayed in Figure 25 the intercalation effect on the interlayer spacing dependedvery much on the paraffin chain length and the presence of monomer; only afterpolymerisation was full exfoliation obtained.

A patent from Toyota [Kawasumi et al., 1989] reported that clays may beintercalated in an aqueous medium with ammonium salt of a primary, secondaryor tertiary linear organic amine. However, owing to the envisaged use of theorganoclay in polymerisation, the preferred intercalant has remained ADA.

Several patents on the intercalation of clays with onium ions originate fromSCP. Thus, Dennis [1998] described the preparation of organoclay compositions

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useful in grease and ink formulations. The organoclay was the reaction product ofa smectite-type clay having CEC ≥ 0.50 meq/g and a branched chain alkyl quaternaryammonium compound. Useful clays include bentonite, hectorite, as well as, syntheticsmectite-type clays, such as MMT, beidellite, hectorite, saponite and stevensite.The branched quaternary ammonium intercalant has the formula: R1R2R3C(R4R5R6)N+M–, where M– is an anion (preferably methyl sulfate, chloride or bromide),R1 = R2 = -CH3, R3 is a linear or branched saturated or unsaturated alkyl grouphaving 12-22 carbon atoms, R4 is hydrogen or a saturated lower alkyl group of1-6 carbon atoms; R5 is hydrogen or a linear or branched saturated alkyl group of1-22 carbon atoms; R6 is a linear or branched saturated alkyl group of 5-22 carbonatoms; and M is the salt anion. Especially preferred are the quaternary ammoniumsalts wherein R3 is hydrogenated tallow and R4, R5 and R6 are together a2-ethylhexyl.

Organo-MMT was prepared using the following steps:

1. Aqueous bentonite slurry (2 wt% solids) was passed three times through ahomogenising Manton-Gaulin mill (MG-mill).

2. The slurry was then heated to 66 °C and reacted for 30 min with dimethyl-hydrogenated tallow-2-ethyl-hexyl ammonium methyl sulfate (2MHTL8, acommercially available quaternary ammonium salt as Arquad HTL8), a blendof Arquad HTL8 with dimethyl dihydrogenated tallow ammonium chloride(2M2HTA, a commercially available quaternary ammonium salt as Arquad2HT), or other quaternary amines.

3. The reacted slurry was sheared again through an MG-mill at 29 MPa.4. The organoclays were vacuum filtered, fluid bed dried at 80 °C, and milled

through a 0.2 mm screen. Several commercial organoclays from SCP, e.g.,Cloisite®-25A, -6A, -15A and -20A comprise these compounds.

Figure 25 Interlayer spacing after intercalation with ω-amino acid (AA) havingn CH2 segments in the molecule. The lower curve is for intercalation with AA only,the upper curve is for AA with ε-caprolactam at 100 °C (2 g of lactam per 0.5 g of

Na-MMT + AA). Data Okada and Usuki [1995].

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The following patent [Gonzales et al., 1998a], specifically describes the preparationof MMT organoclays to be used for the production of CPNC. The organoclay is thereaction product of smectite-type clay having a CEC ≥ 0.50 meq/g and a mixture ofa first quaternary ammonium salt (e.g., 2M2HTA) with either a second quaternaryammonium salt containing a -C=C- double bond, or a chain transfer agent. Thesecond ammonium salt has the formula: R1R2R3R4N+M–, where M– is an anion(preferably methyl sulfate, chloride or bromide), and Ri are independently selectedfrom the group consisting of (a) linear or branched, saturated or unsaturatedalkyl groups having 1 to 22 carbon atoms, (b) aralkyl groups which are benzyland substituted benzyl moieties, (c) aryl groups, (d) β,γ-unsaturated groups having≤ 6 carbon atoms or hydroxy-alkyl groups having 2-6 carbon atoms, and (e)hydrogen, with the proviso that at least one of the substituents is a linear or branchedunsaturated alkyl group. The chain transfer agent may be a thiol, DL-cysteine,α-methylketone, or a halogen compound.

The intercalation process comprises the following steps:

1. Dispersing a clay in an aqueous medium,2. Heating the dispersion to T ≥ 30 °C,3. Adding a first quaternary ammonium salt, then the second quaternary

ammonium compound or the chain transfer agent, and4. Agitating the mixture to complete the reaction.For example, MMT was intercalated with 2M2HTA and either DL-cysteine, orN,N-dimethyl-amino-methacrylate. For the first system d001 spacing was determinedas 2.3 nm, whereas for the second two peaks were found at d001 = 1.4 and 2.6 nm– evidently in the latter case not all MMT tactoids were intercalated.

Another patent from SCP describes the preparation of organoclays for use asa rheological additive in an unsaturated polyester resin/styrene system [Farrowet al., 1998]. Thus, a mixture of two clays was treated with 0.35 to 0.65 meq ofthe alkyl quaternary ammonium salt per 1 g of the clay mixture. The mixturecontained 70 to 90 wt% of clay (a) selected from between sepiolite, palygorskiteand their mixtures; and 30 to 10 wt% of clay (b) selected from a smectite groupconsisting of hectorite, MMT, bentonite, beidellite, saponite, stevensite and theirmixtures. The alkyl quaternary ammonium cation was either 2M2HTA,MB2HTA, 2MBHTA or 2MHTL8, with the counterion being chloride, bromide,methyl sulfate, nitrate, hydroxide, acetate, phosphate, etc.

Production of organoclay followed five steps:

1. The clays (sepiolite, palygorskite, hectorite) were crushed, ground, slurriedin water and screened to remove grit and other impurities.

2. Dilute (1 to 6% solids) aqueous slurry of the clays was then subjected tohigh shearing in a homogenising Manton-Gaulin mill, operated with a pressuredifferential across the gap from 14 to ≥ 55 MPa.

3. The clay slurries may be mixed together, e.g., 80 wt% sepiolite with 20 wt%hectorite or 70 wt% palygorskite with 30 wt% MMT.

4. The mixture was treated with a quaternary ammonium salt.5. The resulting organoclay was dewatered, dried and ground.The products were found to display highly desirable properties when used as athixotrope in the gelling of an unsaturated polyester resin. Thus, the organoclay(0.1 to 4 wt%) may be directly dispersed in an unsaturated polyester

113


resin/monomer solution. The organoclays also showed excellent performance inhigh temperature drilling fluids, offering high gel strength at T ≤ 230 °C. Whenused with PA-66, they improved the mechanical properties, e.g., tensile strengthby 20%, tensile modulus by 31% and flex modulus by 28%.

In a patent from Ciba [Zilg et al., 1999] phyllosilicates were intercalatedwith a salt of a cyclic tertiary amidine or amidine mixture. In Figure 26 (a), thebasic structure of the now patented intercalant family is presented. In the suitablecompounds: R1 is the alkyl radical (C2 to C8), R2 is hydrogen or an aliphaticradical containing an unsaturated bond, which may be substituted by a carboxylor carbonyl group, R3 is hydrogen or alkyl; each of A and B is –CH2–, or A andB together are the radical –(CH=CH)–, and X– is an anion. In Figure 26 (b) theradical R´´ = -CnH2n+1, where n = 0, 1, 2, 3 or 4. Owing to the presence of areactive moiety in the second radical (–OH or –CH=CH–), the intercalant maybe modified to bond with a matrix polymer. By contrast with ammonium-intercalated clays those with amidine have greater thermal stability duringprocessing, higher interfacial adhesion and better dispersability. Anotheradvantage of the amidine intercalants is that they do not change the stoichiometryof the thermoset matrix reaction.

According to the patent, the phyllosilicate may be synthetic or mineral, preferablyMMT and/or hectorite, with d001 = 0.7 to 1.2 nm and CEC = 0.5-2.0 meq/g. Thecyclic tertiary amidine should have two substituted groups, one aliphatic withunsaturated bonds and/or functional groups, and the other either a hydrogen ora linear or branched aliphatic radical with one or more unsaturated and/orfunctional groups.

The following amidine compounds have been synthesised:

• ricinyl-4,5-dihydro-1-H-imidazole hydrochloride (RDI, from castor oil),• hydroxyethyloleyl-4,5-dihydro-1-H-imidazolinium hydrochloride (HDI),• aminoethyloleyl-4,5-dihydro-1-H-imidazolinium hydrochloride (ADI), and• 1-methyl-2-nortallowalkyl-3-tallow-fatty acid amidoethylimidazolinium

methosulfate (MNTFA).

Figure 26 (a) The basic structure of the cyclic amidine intercalant family;(b) Derivatives of (a) particularly suitable as intercalants (see text).

Reproduced from Zilg et al. [1999], with permission.

(a) (b)

114


Synthetic MMT was intercalated and XRD showed that the interlayer spacingincreased from d001 = 0.94 to 2.67 (for RDI), 3.27 (for HDI and ADI), and 4.01 nm(for MNTFA). The intercalation started with dissolution of the intercalant inacidified, hot water, followed by addition (while stirring) of a synthetic claysuspension. As a result of the cation exchange, flocculated clay precipitated, itwas filtered, washed (until Cl– could not be detected with AgNO3 solution), andthen dried under vacuum at 80 °C for 72 h.

According to the Ciba patent, the amidine-intercalated clays may be used forthe production of CPNC with virtually any polymeric matrix (thermoplastic,thermosetting or elastomeric), viz. PO, vinyls, styrenics, acrylics, PA, thermoplasticpolyesters (PEST), PC, polyphenylene sulfone (PSF), polyarylethers, diversepolycondensates or polyadducts, rubbers, etc. Preferred polymers are PEST, PU,thermosetting epoxy and polyurethanes.

A recent invention for polymeric (primarily PEST) composition with high barrierproperties [Barbee and Matayabas, 2000; Matayabas et al., 2000] again useslayered clay that has been cation exchanged with onium salt represented by theformula [R1R2R3R4M]+ X-, where M is N or P; X- is a halide, hydroxide, or acetateanion; R1 is a straight or branched alkyl group having at least 8 carbon atoms; R2,R3 and R4 are independently hydrogen or a straight or branched alkyl group having1 to 22 carbon atoms. The polymer barrier properties were improved by the additionof up to 30 wt% of a mixture of onium-intercalated clay (e.g., MMT-MT2EtOH)with an agent, which expands the interlayer spacing to d001 ≥ 3 nm and is misciblewith the polymer. The ratio of clay to the ‘expanding agent’ varied from 1:4 to 4:1.Suitable ‘expanding agents’ are mainly low molecular weight compounds (frommonomers to Mn = 25 kg/mol), viz. PCL, PDMS, polyepoxides, PS, polyacrylates,PC, PU, PSF, PA, PEST, polyethers, polyketones and vitamin E. The ‘expandingagents’ used in examples were: PETG 6763, PEG (MW = 3.35 kg/mol), PCL (MW= 2 kg/mol), carbinol-terminated PDMS, etc.

The process recommended by these authors for manufacturing PETnanocomposites comprised:

1. Preparing the organoclay2. Pre-swelling the organoclay with an ‘expanding agent’ and3. Incorporating the expanded organoclay in a polyester.For example, an aqueous suspension of Na-MMT (CEC = 0. 95 meq/g) wasmixed at 50 to 80 °C in a blender to form a 2 wt% slurry. The methyl tallow bis-2-hydroxyethyl quaternary ammonium (MT2EtOH) (Ethoquad T/12) was addedin sufficient amount to exchange most of the cations present in the galleries. Theprecipitate was filtered out, washed and dried. XRD of the product showed thatd001 = 2.0 nm. Next, the expanding agent (PET modified with 30 mol% of 1,4-cyclohexane dimethanol available as PETG 6763) was dissolved in methylenechloride, the organoclay was added and the mixture was blended at high speed.Next, the suspension was poured over PET pellets, followed by evaporation ofthe methylene chloride, and drying the coated pellets in a vacuum oven at 110 °C.The coated pellets were finally compounded in a TSE at 275 °C and 200 rpm.

For the manufacture of polycarbonates, polyesters and polyphenylene ethers(PPEs) from low viscosity macrocyclic oligomers, layered minerals wereintercalated with novel cations [Takekoshi et al., 1996]. The preferred type cationswere guanadinium and amidinium, viz. hexa-butyl-guanidinium. However, moretraditional onium cations were also acceptable, especially of pyrrolidine,

115


piperidine, piperazine and morpholine as well as heterocations derived fromcyclododecanes. Intercalation was performed by dispersing Na-MMT (10% H2O;CEC = 1.19 meq/g) into an aqueous methanol solution. To the resultinghomogeneous dispersion a solution of either dodecylammonium (DDA) orhexadecylpyridinium (HDP) chloride was added. A white precipitate wasrecovered by filtration and subsequently washed with water and dried. There isno data on interlayer spacing, but upon incorporation of 5% of MMT intercalatedwith DDA or HDP the modulus of CPNC with PPE as the matrix increased by15% or 29% (in respect to neat PPE), respectively. Quaternary ammonium ionswith heterocyclic rings have also been used for rubbers [Weber and Mukamal,1984].

It has been frequently shown that the size of the intercalating onium iondetermines the resulting interlayer spacing. The most exhaustive list of theseeffects can be found in Miyanaga et al. [1999] patent. Four different clays wereused, but for illustrating the effects only data for MMT (Kunipia F; CEC =1.19 meq/g) will be cited in Table 15. Intercalation was performed by dispersing15 g of clay in 1 l of deionised water at 70 °C. To the suspension a water/ethanolsolution of onium salt was added in the amount of 1.05 times the equivalent ofthe clay CEC with vigorous stirring for 30 min, and then allowed to stand. Theorganoclay was filtered, washed, and then dried under vacuum at 80 °C for72 hours. The d001 spacings as determined by XRD are listed in Table 15. Theresults indicate that the best correlation between d001 and the size of the oniumsalt involves the three largest radicals.

To prepare CPNC with high temperature processable fluoroplastics as thematrix, MMT was intercalated with phosphonium cations of the general formula:R1P+(R2)3 [Ellsworth, 1999]. The fluoroplastics of interest have either meltingpoint or glass transition temperature Tm, Tg > 220 °C. Thus, for example, 10 g ofsodium hectorite (10 g) was dispersed in alcohol/water 1:1 mixture (200 ml) at90 °C. To the suspension 50 ml hexadecyl-tributyl-phosphonium bromide inisopropyl alcohol was added. Next, the reaction mixture was heated at 90 °C for8 h with stirring. The organoclay was filtered, washed, dried at 120 °C for 24 h,milled and screened through a 325 mesh (40 micron) sieve.

To produce rubber compositions with enhanced mechanical performanceintercalated clay was incorporated [Weber and Mukamal, 1984]. As the patentclaim specifies, the intercalant can be a quaternary onium ion of the traditionalaliphatic-substituted type or selected from between: imidazolium, pyridinium,pyrrolidinium, pyrrolium, pyrrolinium, pyrazolium, triazolium, pyrimidinium,pyridazinium, pyrazinium, triazinium, indolium, indazolium, benzimidazolium,quinolinium, isoquinolinium, cinnolinium, phthalazinium, quinazolinium,quinoxalinium, naphthyridinium, quinolizinium, carbazolium, acridinium,phenazinium, phenanthridinium, phenanthrolinium, benzo[H]isoquinolinium,purinium, porphinium and pteridinium. In these compounds the heterocyclic ringscould be substituted by alkyl(s) or not, and they could be partially hydrogenated.

Preparation of CPNC in PEST matrix requires high temperature thus thermallystable organoclay. Furthermore, the organoclay usually induces a brownish color thatmust be compensated for by addition of suitable dyes. A patent from Eastman elegantlysolves these problems by using cationic dyes (e.g., optical, fluorescent brighteners)[Barbee et al., 2000c]. The preparation of these CPNCs is by melt compounding of thepre-intercalated clay with molten polymer. The clay is pre-intercalated with a dye,having the cation group (a quaternary ammonium) separated from the chromophore

116


detalacretni)g/qem91.1=CEC(TMMfognicapsreyalretnI51elbaTR,htiw 1R2R3R4N

+ aganayiM[ataD. .late ]9991,

R1 R2 R3 R4 d 100)mn(

— — — — 32.1

C 01 H 12 C 01 H 12 C 01 H 12 C 01 H 12 08.2

HC 3 C 81 H 73 C 81 H 73 C 81 H 73 11.4

HC 3 C4H9 C 81 H 73 C 81 H 73 58.2

H HC 2 hP- C 81 H 73 C 81 H 73 56.2

HC 3 HC 3 C 81 H 73 C 81 H 73 18.2

HC 3 HC 3 HC 3 C 81 H 73 31.2

HC 3 HC 3 C 01 H 12 C 01 H 12 89.1

HC 3 C8H 71 C8H 71 C8H 71 38.1

C4H9 C4H9 C4H9 C4H9 55.1

HC 3 HC 3 HC 3 HC[ 2 HC(HC 3 ]O) 03 H 85.4

HC 3 HC 3 C 21 H 52 HC[ 2 HC(HC 3 ]O) 03 H 78.3

HC 3 HC[ 2 HC(HC 3 ]O) 01 H HC[ 2 HC(HC 3 ]O) 01 H HC[ 2 HC(HC 3 ]O) 01 H 91.5

HC 3 HC 2 HC(HC 3 CO) 8H 71 HC 2 HC(HC 3 CO) 8H 71 HC 2 HC(HC 3 CO) 8H 71 96.3

HC 3 HC 3 HC( 2 HC 2 )O 2 C)OC( 71

H 53

HC( 2 HC 2 )O 2 C)OC( 71 H 53 89.2

HC 3 HC[ 2 HC(HC 3 ]O) 01 H HC[ 2 HC(HC 3 ]O) 01 H HC 2 HC 2 C)OOC( 21 H 52 64.3

HC 3 ])2/1(OP/OE[ 01 H ])2/1(OP/OE[ 01 H ])2/1(OP/OE[ 01 H 26.3

HC 3 HC 3 C[ 3H7 HC(HC 3 C) 4H8]2

HCHC 2

C[ 3H7 HC(HC 3 C) 4H8]2

HCHC 2

37.2

HC 3 HC 3 C9H 91 HC(HC 3 HC) 2 C9H 91 HC(HC 3 HC) 2 65.2

HC 3 HC 3 HC 3 C 41 H 92 C(HC 21 H 52 HC) 2 98.2

HC 3 HC 3 HC 3 C 81 H 73 31.2

HC 3 HC 3 C 81 H 73 C 81 H 73 18.2

HC 3 C 81 H 73 C 81 H 73 C9H 91 HC(HC 3 HC) 2 56.2

HC 3 HC 3 HC 3 C[ 3H7 HC(HC 3 C) 4H8]2

HCHC 2

54.2

(OP/OE:etoN 1/2 ehthtiw)locylgenelyporp-oc-locylgenelyhte(ylopstneserper)2:1oitarremonom

117


by at least two C-groups. Colourless CPNC was prepared by dry blending a mixtureof clay pigments with Claytone‚ APA and PET. The well-dried mixture was extruded at275 °C. Unfortunately, there is no information on the extent of intercalation/exfoliationin, or the properties of the CPNC. Similarly for PET, Imai et al. [2002] developedintercalant able to react with PET, with a cationic group to bind to clay and stable atleast up to 275 °C. The selected compound was a dimethyl isophthalate substitutedwith a triphenyl-phosphonium group (dimethyl isophthalate triphenyl phosphonium,DIP). In spite of lack of exfoliation, 8 wt% of the pre-intercalated clay increased thematrix modulus by 80%.

TNO patented the use of dyes as intercalants [Fischer et al., 2001], such as,e.g., methylene blue:

N S

N

CH3

H3CN+

CH3

CH3

CΓ

Virtually any polymer can be used as a matrix, but for the coating applicationsthe preference goes to PU, acrylics, siloxanes, polyesters and polyethers. Thenew intercalants offer better thermal stability, controllable amount of intercalation,ability to introduce functional groups for end-tethering the matrixmacromolecules, etc. The technology was commercialised for the production ofPlanomers®, PlanoCoatings® and PlanoColors®. The Planomer range is anorganoclay tested in diverse polymeric matrices, viz. PO, PMMA, PA, PU, PS,PC, phenolics, and biopolymers. The second material is used as a transparentbarrier material in packaging or for protection. The PlanoColors® are offered ashighly UV stable, clay-based, metal-free nanopigments in a variety of colours.

Conroy et al. [2002] prepared high temperature CPNC by dispersing up to10-wt% of clay in molten phthalonitrile monomer or oligomer that in turn waspolymerised into CPNC, stable to T ≥ 450 °C. The authors replaced the customary(and thermally unstable) ammonium intercalants by compounds with nitrilegroups. This new type of organoclay may be used with a variety of polymers,such as, PA, PC, PO, PEI, PI, TPU, PVP, PVAl, PEG, epoxy, etc. Unfortunately,this patent application does not provide examples of CPNC preparation orperformance characteristics.

In summary, the standard method of clay intercalation with organic cationsusually starts with purified bentonite or MMT. The clay has a layer of Al and Mghydroxides between two layers of silica that are negatively charged and ionicallybalanced by Na+, K+, Ca2+and Mg2+. The repeating layers are 0.96 nm thick andunder ambient conditions the basal or d-space is d001 = 1.2-1.5 nm, hence theinterlamellar space in the gallery is about 0.2 to 0.5 nm. At this stage the clay isunsuitable for use in PNC technology - the interlamellar gap is too narrow forthe diffusion of macromolecules, the solid-solid interactions between the clayplatelets are too high. The oldest and still most frequently used method has beento replace the inorganic cations with onium ones. This increases the interlayerdistance to d001 ≥ 2.5 nm, reduces the interlayer forces, makes the clay morehydrophobic and makes it amenable to dispersion in a monomer or polymer.

However, the most common intercalant, quaternary ammonium has limitedthermal stability. Ammonium-modified clay may be used to prepare CPNC by firstdispersing it in a monomer that in turn is polymerised by UV, acid, base or heat.

118


This approach is suitable, provided that the polymerisation is conducted at T <250 °C. Alternatively, CPNC may be prepared by melt blending ammonium-modified clay with a polymer that can preferably be processed at T < 200 °C. Withthe exception of elastomers, there are few industrially interesting polymers thatcan be processed at these low temperatures. It is customary to process a polymer attemperatures about 40-50 °C above its transition temperature, in the case of anamorphous polymer the glass transition, Tg, or in the case of a semicrystallineresin, the melting point, Tm. In several patents besides quaternary ammonium,phosphonium and/or sulfonium salts are also listed. It is noteworthy that claymodified with organophosphonium cations may be thermally stable up to 370 °Chence it is suitable to prepare CPNCs with a high temperature matrix polymer.

The most common cation-exchange method uses onium salt, the quaternaryammonium being the most common (the primary or secondary have been used inspecific cases to ascertain chemical reactivity, e.g., with PA-6 or thermosets). Thereason for using the quaternary ammonium salts originates from consideration ofthe binding strength to MMT. The strength increases with the number of substituentsin the ammonium cation [Maes et al., 1980], viz.

NH RNH R NH R NH R N4 3 2 2 3 4+ + + + +< < < <

The authors observed that the exchange reaction between either Na-MMT orCa-MMT and ammonium salt is thermodynamically reversible, and that the ioniccharge (usually assigned to the N-atom) is delocalised over the R-alkyl substituents.The delocalisation increases with the number of substituents (from 0 to 4) andtheir molecular weight.

Most commercially available organoclays are intercalated with quaternaryammonium – examples are listed in Table 16. The typical dry particle size ofthese materials is ca. 5 μm, with the sieve analysis: 10 vol% less than 2 μm; 50%less than 6 μm; and 90% less than 13 μm. The quaternary onium ions canquantitatively replace Na+ in Na-MMT. It can be calculated that stoichiometricreplacement by means of 2M2HTA would result in organoclay containing 34 wt%of organic phase. As the tabulated data indicate, there is up to 50% excess of theintercalant present. Elimination of this excess improves thermal stability.

Industrially Na-MMT undergoes ion exchange mainly with a quaternaryammonium chloride: R1R2R3R4N+ Cl-, to produce hydrophilic or hydrophobicintercalated MMT. The character and the interlayer spacing depend on the Ri- radicals.In several commercial organoclays R1 = R2 = -CH3 and R3 = R4 = hydrogenatedtallow oil. Other radicals used to control interactions with the matrix are benzyl,hydroxyethyl, 2-ethylhexyl, etc. Intercalation involves a slow chemical reaction:

MMT- Na+ + R4N+ Cl- ↔ MMT- R4N+ + Na+ Cl-

Owing to low mobility of the MMT- R4N+ ionic pairs, the equilibrium is shiftedto the right hand side (RHS). Nevertheless, intercalation in aqueous medium atT = 25-80 °C takes from t = 10 to 480 min, while the second stage intercalationin non-aqueous solvent may take up to 15 days. It results in a complex of MMTwith two ammonium ions and two amines: MMT2- 2RN+H3 + 2RNH2. Typicalcharacteristics of commercial organoclays are shown in Table 16 withabbreviations for intercalants listed in Table 17 and Appendix 6.3. The fourtypes of Cloisite® organoclay produced by Southern Clay Products, Inc. are shownin Figure 27. Typically, their dry particle size is ca. 5 μm, with the sieve analysis:10-vol% less than 2 μm; 50% less than 6 μm; and 90% less than 13 μm.

119


syalconagrolaicremmocfoseitreporplacisyhplacipyT61elbaT

yalconagrO tnalacretnI nossolthgieWnoitsubmoc

)%(

H2O)%(

,gnicapSd 100 )mn(

reifidoMg/qem(.cnoc)*g/lomm&

fisamoS ™ .dtL,.oClacimehCPO-OCmorf

001-EMacimcitehtnys

enon 52.1 2.1=CEC()g/qem

EAM AT2M2 0.3

ETM O3M 5.2

EEM CHOtE2M 3.2

EPM HOPPE2M 0.5

etisiolC ® .cnI,stcudorPyalCnrehtuoSmorf

TMM-aN enon 7 4 32.1 0

A6 ATH2M2 74 2 95.3 68.0&04.1

A01 ATHBM2 93 2 39.1 52.1

A51 ATH2M2 34 2 69.2 87.0&52.1

A02 ATH2M2 83 2 74.2 38.0&59.0

A52 8LTHM2 43 2 20.2 96.0&59.0

B03 HOtE2TM 23 2 68.1 58.0&09.0

A39 ATH2M 04 2< 63.2 09.0

remonaN ®

morf,roconaN

.cnI

tnalacretnI ytisneD)lm/g(

H2O)%(

/xirtaM.lppA

-acretnItnal

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TMM enon 6.2 21 )g/qem54.1=CEC( lin

LT42.I ADA ≤ 3 noitasiremylop6-AP

E82.I ASM3 7.1 ≤ 3 enahteru,yxopE 03-52

E03.I ADO 7.1 ≤ 3 enahteru,yxopE 03-52

napsoehRSA

TH2M2 2 detarutasnUlyniv,retseylop

sretse

NCT43.I HOtE2DOM 7.1 3 tlemUPT,TBP,APgnidnuopmoc

P03.I ADO 7.1 2 ,setartnecnocOPnI,EP03.C,P03.C.ziv

AVE03.C

03-52

AP44.I TH2M2 2 ,setartnecnocOPnI,EP03.C,P03.C.ziv

AVE03.C

120


Table 16 Continued...

Figure 27 Four types of Cloisite® intercalated clays

Reproduced with permission from Southern Clay Products, Inc., Gonzales, TX

TMM-aNnodesabGAeimehC-düSmorfsyalconagrO

lifonaN tnalacretnI ssol.tW)%(

ytisnedkluB)l/g(

xirtaM d 100)mn(

51 M2S2 53 044 PP,cAVE 8.2

849 M2S2 54 044 PP,cAVE 5.3

23 M2BS 03 003 TBP,TEP 8.1

919 M2BS 53 064 TBP,TEP 0.2

848 AS 52 081 AP,PP,cAVE 8.1

408 HOtE2S 03 031 UPT,TBP,TEP 8.1

487 A21CA 02 081 21-AP,66-AP,6-AP 7.1

TMM-aNnodesab.cnI,xoehRmorfsyalconagrO

43B AT2M2 74.2

C0.2:)%twni(sniatnocyllacipytliowollatdetanegordyhlaicremmoC:etoN 41 ,C0.92,51C5.0 61 C5.1, 71 C0.66, 81 C0.1dna, 02 spuorglykla

121


stnalacretniyalcsadesusnoitaccinagrO71elbaT

hB2M2HC(;edirolhcmuinommalynehebidlyhtemiD 3)2 C(- 22 H 74 )2-

N+ lC –

AD2M2 edirolhcmuinommalycedodidlyhtemiD

ADD2M2lycedodidlyhtemidromuinommalyrualidlyhtemiD

edirolhcmuinomma

ATH2M2edirolhcmuinommawollatdetanegordyhidlyhtemiD

)TH2dauqrA(

ADO2M2N,N -lyhtemiD- N,N roedimorbmuinommalycedatcoid-

edirolhc

AS2M2 edirolhcmuinommalyraetsidlyhtemiD

AT2M2 edirolhcmuinommawollatidlyhtemiD

ATHBM2 edirolhcmuinommawollatdetanegordyhlyzneblyhtemiD

ADOBM2-lyraetslyzneblyhtemidro-lycedatcolyzneblyhtemiD

edirolhcmuinomma

ASBM2 edirolhcmuinomma-lyraetslyzneblyhtemiD

IDHM2 -3-lyhtemiD-2,1 N muilozadimilycedaxeh-

ADODHM2 enimalycedatco-lycedaxehlyhtemiD

8LTHM2muinommalyxehlyhte-2wollatdetanegordyhlyhtemiD

etafluslyhtem

ADOM2 enimalycedatcolyhtemiD

ADDBVM2 enimalycedodlyzneb-lynivlyhtemiD

M2S2 ADO2M2=enimalyhtemidlyraetsiD

PDHB3 edimorbmuinohpsohp-lycedaxeh-lytubirT

ADM3 enimalycedodlyhtemirT

ADDM3 edimorbmuinommalycedodlyhtemirT

ADHM3 edimorbmuinommalycedaxehlyhtemirT

ADOM3 edirolhcmuinommalycedatcolyhtemirT

ASM3 edirolhcmuinommalyraetslyhtemirT

PDDP3 muinohpsohplycedodlynehpirT

AM4 edirolhcmuinommalyhtemarteT

A21CA α,ω )ADAosla(dicaonimalycedoD-

ADA ω lycedodonima-21ro,dica)cirualro(lycedodonimA-dica

AHA ω dicacionaxeh-onimA-

ADHA ω dicacionacedaxeh-onimA-

61-lyllA N,N -lyhtemiD- n edirolhcmuinommalyllalycedaxeh-

122


...deunitnoC71elbaT

ADOA ω dicacionacedatco-onimA-

BPA enezneb)yxonehponima-3(siB-3,1

STPA γ enalisyxohteirt-lyporponimA-

ADTA ω dicacionacedartet-onimA-

ATH3B edirolhcmuinommawollatdetanegordyhirtlyzneB

61-zBN,N -lyzneblyhtemiD- n edirolhcmuinommalycedaxeh-

)ADHBM2osla(

CPC edirolhcmuinidiryplyteC

AD enimalycedoD

ADD edirolhcmuinommalycedoD

PDD enodilorryp2-lycedoD-1

PID muinohpsohp-lynehpirtetalahthposilyhtemiD

DAH enimalycedaxeH

ADH edirolhcmuinommalycedaxeH

PDH N edirolhcmuinidiryplycedaxeh-

HOPPE2M edirolhcmuinommalocylgenelyporpyloplyhteidlyhteM

ATH2M muinomma]wollatdetanegordyh[idlyhteM

HOtE2S2M muinomma]lyhteyxordyh-2[siblyraetslyhteM

ATH3M edirolhcmuinommawollatdetanegordyhirtlyhteM

O3M edirolhcmuinommalytcoirtlyhteM

HOtE2CM edirolhcmuinommalyhteyxordyh-2-idlykla-ococlyhteM

HOtE2DDM enimalyhteyxordyh-2-idlycedodlyhteM

AHMmuinommalyhtemidlycedaxehlyhteyxo-lyolyrcahteM-2

edimorb

HOtE2DOM enimalyhteyxordyh-2-idlycedatcolyhteM

ASM edirolhcmuinommaeneryts-lyhteM

HOtE2TMedirolhcmuinommalyhteyxordyh-2-idwollatlyhteM

)21TdauqohtE(

AO edirolhcmuinommaroenimmalytcO

ADO )enimalyraetsosla;81C(enimalycedatcO

61-HON,N -lyhtemiD- n )lyzneblyhtemyxordyh-4(-lycedaxeh-

muinomma

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123


The production of CPNC ammonium intercalated clays poses several problems.Alkylamines are not benign – their permissible level is 5 ppm and lethal dose,LD50 = 0.2-0.4 g/kg. The toxicology of clay complexes with the great diversity ofoniums has not been determined, but as a precautionary measure so far CPNCshave not been used in direct contact with consumable products.

The second and well-recognised disadvantage of ammonium intercalant,especially the quaternary one, is the thermal stability. These substances startdecomposing below 200 °C, which may reduce the mechanical properties andcauses discolouration. The ammonium-modified clay may be used to prepareCPNC at low temperature, e.g., in low temperature emulsion, suspensionreactions, or by dispersing intercalated clay in a monomer that in turn ispolymerised by UV, acid, base or heat. This approach has been found suitable,provided that the polymerisation is conducted at T < 250 °C. Alternatively, CPNCmay be prepared by melt blending ammonium-modified clay with a polymerthat can be processed at T < 200 °C. However, with the exception of elastomers,there are few industrially interesting polymers that can be processed at thesetemperatures. The thermal stability of ammonium-modified clays may besomewhat improved by extracting an excess of the intercalating ammonium saltand/or by reacting it with, e.g., epoxies [Ton-That, 2000]. There are alsoindications that secondary ammonium salt has a better thermal stability than thequaternary one. Some quaternary ammonium complexes are more stable thanothers, for example, phyllosilicates intercalated with a salt of a cyclic tertiaryamidine or amidines mixture showed greater thermal stability during processing

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[Zilg et al., 1999]. Furthermore, it is expected that: (1) ammonium cations withstrong bonding to clay anions will show greater thermal stability, (2) the presenceof unbounded amines or ammonium salts will have deleterious effects on thermaldiscolouration, (3) the ammonium ion can be shielded by more stable secondaryintercalants, e.g., epoxies.

2.3.4 Intercalation by Organic LiquidsIntercalation methods described in this part are effectively the next generation ofthe methods discussed in the preceding one: intercalation by organic cation. Ithas already been mentioned that addition of organosilane or a polar organicliquid (labelled as polar activator, dispersant, solvating agent, etc.) has a beneficialeffect on the dispersion of clay platelets and their performance in diverseapplications. The present part will focus on the use of macromolecules as theintercalation aids.

Amcol has published several patents in this domain, for example [Beall et al.,1996a,b] intercalated clay using 30 to 80 wt% (per weight of the aqueous phase) ofa monomer, oligomer (degree of polymerisation, DP = 2-15) or polymer (DP > 15).Regardless of the concentration, the amount of the intercalating compositionshould be > 4 times that of clay. The intercalant should be sorbed between andpermanently bonded to the clay platelets, increasing the interlayer spacing of thephyllosilicate from 0.5-10 nm. Useful intercalants should be water-soluble andmust have a functional group (e.g., carbonyl, hydroxyl, carboxyl, amine, amide,ether, ester, sulfate, sulfonate, sulfinate, sulfamate, phosphate, phosphonate,phosphinate functionalities) or aromatic rings that provide metal-cationcomplexing to the inner surfaces of clay platelets. Binding to the platelet surfacesis by metal cation either electrostatically bonding or chelating. Another mechanisminvolves bonding of the interlayer cations with intercalant aromatic rings. Theintercalants should have sufficient affinity for the clay platelets to provide adequateinterlayer spacing and to bind to the surfaces of the platelets, without additionalcoupling agents.

An addition of metal cations (during intercalation and/or exfoliation) increasesthe suspension viscosity, most likely by complexing with the polar moieties ofthe intercalant molecules. The complexed metal salt-derived cations carry theirdissociated anions along with the cations, in the interlayer space. Such a doubleintercalant complexing occurs on the opposed platelet surfaces, resulting inrepulsion between closely spaced dissociated anions carried by the added cations,which results in increased interlayer spacing and more complete exfoliation. Theconcentration of metal salt ranges from 0.01 to 1 wt%, based on the dry weightof the phyllosilicate.

The intercalated phyllosilicates can be exfoliated before or during mixing withsolvents (e.g., alcohols, glycerols, glycols, aldehydes, ketones, carboxylic acids, amines,amides, etc.). Such a suspension may be used in a thixotropic composition, or fordelivery of any active hydrophobic or hydrophilic organic compound, such as anactive pharmaceutical, dissolved or dispersed in the carrier or solvent, in a thixotropiccomposition. Depending on the intercalation and exfoliation conditions (viz. T, pH,and ingredient concentration), the system viscosity can be adjusted in the range of0.1-5,000 Pas. The compositions are thixotropic.

The preferred water-soluble polymer intercalants are: P4VP, PVAl, and theirmixtures. The weight ratio of intercalant to Na-MMT should be 1:5 to 1:3. The

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concentration of intercalant should be 20-90 wt%, increasing the interlayer spacingto d001 = 3.0-4.5 nm. The intercalating composition should include 25-50 wt%water. The water-soluble polymer can be added as a solid along with the claywhen extruding or pug milling. Metal salt is dissolved in a suitable solvent (wateror organic solvent) and added to the intercalating composition in the amount of0.005 to 0.5 wt%.

The intercalated clay was combined with various organic liquids (with and withoutwater) to determine the effects of intercalate loading as well as temperature, pH andwater content of the intercalating composition on viscosity. For example, mixing10 wt% clay/P4VP intercalate into 84% glycerol, and 6% water resulted in a viscosityof 2-3 Pas. Heating the composition to gelation (100 °C) increased the viscosity toabout 8 Pas, and when heated to 145 °C (then cooled to room temperature) it increasedto 200-600 Pas. Addition of ethanol had a more dramatic effect. The composition:20 wt% water, 70 wt% ethanol, and 10 wt% clay/P4VP complex showed viscosityin the range from 0.4-1 kPas, without heating.

The development of intercalation technology as described by Beall et al. [1998]is a part of the technology developed over the years as evidenced by a series ofpatents [Beall et al., 1999a,b; Serrano et al., 1998a,b; Beall et al., 1996a,b]. Inanother invention, intercalates were prepared by contacting a clay either withwater, or with an aqueous solution of a water-soluble polymer and/or a water-miscible organic solvent, e.g., alcohol, followed by contact with a monomericorganic pesticide or its solution. The latter would have a polar moiety, e.g.,carboxylic acid, ester, amide, aldehyde, ketone, sulfur-oxygen or phosphorus-oxygen moiety, cyano, or a nitro moiety.

Best results were achieved using an aqueous solution of water-soluble polymer,to first intercalate the clay [Beall et al., 1999b]. Using > 10 wt% of an organicpesticide gave better sorption. Probably the organic pesticide displaces water andwater-soluble polymer and bonds to the platelets by chelation with the exchangeablecation or via electrostatic or dipole/dipole interactions. Extrusion accelerates theintercalation of pesticide between activated clay platelets. The sorbed pesticidemay increase the interlayer spacing from 0.5 to 10 nm. The intercalated claycontaining a pesticide can be used directly as a pesticide.

To intercalate Na-MMT with a herbicide (e.g., 2,4-dichlorophenoxyaceticacid butyl ester (2,4-D)) three processes were employed:

1. Mixing a dispersion of 2 wt% 2,4-D with a 2 wt% clay suspension in waterfor 4 h at room temperature.

2. Dry clay powder (about 8% by weight moisture) was gradually added to the2 wt% aqueous dispersion of 2,4-D in water and agitated at room temperaturefor 4 hours.

3. 2,4-D was dry-blended with clay, the mixture hydrated with 35-38 wt% ofwater and extruded.

All three methods resulted in clay-2,4-D intercalates. The spacing depended on thequantity of 2,4-D sorbed between clay platelets. Similar results were obtained withinsecticides, e.g., chlorpyrifos, or the pesticide trifluralin. The intercalated Na-MMTshowed d001 = 1.88 nm, an increase from 1.24 nm of untreated clay. When the wetsample was dried in a vacuum oven (10-3 torr at 60 °C for 48-60 h) causing thetrifluralin to sublimate the spacing decreased to 1.237 nm. The experiment indicatedthat trifluralin might be fully released from the interlamellar space.

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2.3.5 Intercalation by Monomers, Oligomers or PolymersHistorically, the intercalation methods described in this part were developed laterthan those by organic cation, discussed before. Cation-exchange technology alsoprofited from the addition of either: (i) an organosilane, (ii) a polar or (iii) anaromatic component. Thus, elements of this technology were used within thecategory of the cation-exchange methods in the two-step intercalation procedures.

The present section will focus on the use of monomers, oligomers, cyclomersor macromolecules to enhance the intercalation and eventually will lead to totalexfoliation of clay. The many variations of this method can be divided into twoprincipal groups:

1. Intercalation of purified clay, e.g., Na-MMT.2. Intercalation of organoclay (a two-step process).Each of these two can be further subdivided depending on whether monomer,oligomer, cyclomer or polymer was used, and whether the second step ofintercalation was necessary.

2.3.5.1 Intercalation of Purified Clay by Hydrophobic CompoundsHere intercalation of purified inorganic layered compound is of interest. Followingthe analysis of clay chemistry, two types of interactions can be explored: complexformation between an aromatic molecule and a clay cation that takes place inorganic medium, and hydrogen bonding with water soluble monomers, oligomersor polymers.

In one of the first patents on CPNC, a vinyl monomer (e.g., styrene) was usedto intercalate the clay [Kamigaito et al., 1984]. Partly due to the high concentrationof clay (85 wt% of MMT, vermiculite or Na-taeniolite) and partly to the smallthermodynamic potential for styrene diffusion into interlamellar spacing, theeffect was small. Thus, the cited values of the interlayer spacing were: beforeintercalation, d001 = 1.2-1.3 nm, and after expansion by sorption of styrene: d001= 1.5 nm. These values indicate that initially the clay contained one to twomonolayers of water. It is doubtful that styrene was able to expel this surface-bonded water, thus the expansion of d001 by about 0.2 nm may indicate theformation of a monolayer of styrene coordinated to the inorganic counterionswithin the interlayer space. According to the authors, the initial sorption processtook ‘a very short period of time’. After the free radical polymerisation of sorbedmonomer, the polymers showed a broad molecular weight distribution (MWD),viz. Mw/Mn ≥ 6.

Some vinyl monomers may be converted into polymers with narrow MWDby anionic or group-transfer polymerisation, but these reactions are not applicableto condensation polymers (e.g., PA). The Kamigaito et al., patent [1984] alsodescribes intercalation of MMT by ε-caprolactam in an aqueous medium. Theprocess for the manufacture of CPNC is a bit convoluted, as it requires additionof dichlorodimethyl silane, filtering, drying, then addition of PA-66 dissolved informic acid, heating the mixture to 200 °C (to effect copolymerisation ofcaprolactam with PA-66) and finally extrusion of a dry blend of concentratedCPNC with PA-66 pellets. The interlayer spacing was not given, but the finalpolymer mouldings showed significant improvement of modulus, strength andheat distortion temperature (HDT) in comparison to neat polymer.

However, in the following patent the authors stated that the achieved degreeof intercalation was low, dispersion of the silicate layers was not uniform, the

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clay aspect ratio was seriously reduced (thus reducing MMT effects onperformances), the bonding between the clay and the matrix was not strongenough and the polyamide matrix had a broad molecular weight distribution,viz. Mw/Mn ≥ 6 [Okada et al., 1988]. Even if the monomer molecules diffuse intothe interlamellar space they rarely bond strongly enough (by ionic or covalentbonds) to the clay surface, and the CPNC may be exfoliated, but not end-tethered.

Several publications from the Toyota group indicate that even highly polarand relatively small monomers, such as ε-caprolactam, do not provide sufficientintercalation and bonding to the clay surface – Na-MMT must be pre-intercalated[Usuki et al., 1990; 1993a,b; 1995]. The latter process was carried out with12-aminolauric acid (ADA) in aqueous medium. Obviously, instead of ADAanother highly polar, water-soluble compound may be used, but as in any othercation-exchange intercalation process, the initial step of intercalation seems torequire the presence of water. The intercalation with ADA results in nearlyquantitative cation exchange; hence after polycondensation the PA-6 matrix isend-tethered and densely packed near the clay platelet surface.

Prior to the emulsion or suspension polymerisation of styrene, the clay plateletsdispersed in aqueous media (viz. Na-MMT) may be fully exfoliated. However, inthe recovered polymer the interlayer spacing is quite small, d001 ≤ 1.7 nm,indicating re-aggregation. Thus, as a rule, to achieve a reasonable degree ofintercalation in CPNC the intercalant must bind to the clay surface. The bindingcan be achieved by ionic bonding, covalent bonding (e.g., through ≡Si-OH groups)or at least through hydrogen bonding with hydrophilic compounds.

2.3.5.2 Intercalation of Purified Clay by Hydrophilic Compounds

Owing to the hydrophilic nature of clay, intercalation by water-soluble monomers,oligomers or polymers is expected to be relatively simple. However, early attemptsto intercalate MMT with PVAl, P4VP or PEG, were not successful. Greenland[1963] reported that PVAl containing 12% residual acetyl groups increased thed001 spacing only by ca. 1.0 nm. As the concentration of polymer increased from0.25 to 4 wt%, the amount of polymer sorbed was reduced, indicating thatsorption might only be effective at polymer concentrations of 1 wt% or less.Such a dilute process would be too costly, hence no further work was carried outtoward commercialisation.

Intercalation of Na-MMT by P4VP (MW = 40 kg/mol) was carried out in1 wt% P4VP dissolved in ethanol/water solution. The clay was first repeatedlywashed with ethanol. After dispersing it in the solution, filtering out and dryingthe basal spacing expanded to 2.6-3.2 nm [Levy and Francis, 1975]. It wasconcluded that:1. The ethanol was needed to initially increase the basal spacing for the later

sorption of P4VP to take place;2. Water did not directly affect sorption of P4VP between the clay platelets;

and3. P4VP sorption was time consuming and difficult.Since 1995 Nanocor (a subsidiary of the AMCOL International Corp.; knownsince 1927 as American Colloid Co.) has patented several methods of chemicalmodification of clays. In an early patent, clay was intercalated using 30-80 wt%of a water-soluble monomer, oligomer (degree of polymerisation, DP = 2-15) orpolymer (DP > 15) [Beall et al., 1996a]. The aim was to have the intercalant

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bonded to the clay platelets, thus increasing the interlayer spacing to d001 = 3-4.5 nm(the most desirable spacing!). To bond, the intercalant needed either a strongpolar group (e.g., carbonyl, hydroxyl, carboxyl, amine, amide, ether, ester, sulfate,sulfonate, sulfinate, sulfamate, phosphate, phosphonate, phosphinate) or anaromatic ring able to form a metal-cation complex. Several types of interactionwere envisaged, viz. by metal cation electrostatic bonding, chelation of the claymetal cations, bonding between clay inorganic cations and intercalant aromaticrings, by hydrogen bonding to the surface -OH groups, etc.

The following water-soluble polymers or oligomers were reported useful asintercalants:

• P4VP (MW = 1-40 kg/mol). Its solubility can be adjusted by hydrolysis or byforming a sodium or potassium salt. Copolymers of vinylpyrrolidone andvinyl amide or γ-amine butyric acid, can be similarly treated.

• Fully hydrolysed PVAl (MW = 2 to 10 kg/mol).• Fully or partially neutralised salts of a polyacrylic acid (PAA) or a

polymethacrylic acid (PMAA, MW = 0.2 to 10 kg/mol).• Polymethacrylamide, poly(N,N-dimethyl acrylamide), poly(N-isopropyl

acrylamide), poly(N-acetamido acrylamide), poly(N-acetamidomethacrylamide), their copolymers or interpolymers containing PAA, orPMAA.

• Polyvinyloxazolidone (PVO) and polyvinyl methyl-oxazolidone (PVMO),PEG, polypropylene glycol (PPG) and their copolymers.

• Polymeric quaternary ammonium salts.• Sodium salts of acrylate/vinyl alcohol, olefin/maleic acid, polymethacrylate,

polystyrene sulfonate, styrene/acrylate/PEG dimaleate, styrene/PEG maleate/nonoxynol, etc.

• Styrene copolymers with acrylamide, acrylate-ammonium methacrylate,maleic anhydride, PVO, etc.

• Diverse copolymers, viz. cornstarch-acrylamide-sodium acrylate, diethyleneglycol-amine-epichlorohydrin-piperazine; dodecanedioic acid-cetearyl alcohol-glycol, ethylene-vinyl alcohol, hydroxy ethyl-polyethyleneimine, polyvinylmethacrylate-methacrylic acid, melamine-formaldehyde resin, phthalic anhydride-glycerin-glycidyl decanoate, metal salts of acrylic and polyacrylic acid, sucrosebenzoate-sucrose acetate, isobutyrate-butyl benzyl phthalate, sucrose benzoate-sucrose acetate isobutyrate-butyl benzylphthalate-methyl methacrylate, vinylacetate-crotonic acid, and polysaccharide copolymers.

• Prepolymers: urea-formaldehyde, urea-melamine-formaldehyde, etc.In the cited patent [Beall et al., 1996a] the preferred intercalants are P4VP,

PVAl, and their mixtures at the weight ratio of intercalant-to-Na-MMT = 1:5-1:3. By contrast with the prior publication that indicated significant difficultiesand achieved minor expansion of the interlayer space, the new intercalation inthe presence of at least 10 wt% of water proceeded quite readily using any of thethree methods. For example, to intercalate Na-MMT with P4VP (MW = 10 and40 kg/mol) or PVAl (75-99% hydrolysed) the following processes were used:

1. Mix for 4 h aqueous 2 wt% solution of P4VP (MW = 10-40 kg/mol) with2 wt% suspension of Na-MMT clay in water, at a ratio sufficient to vary theweight ratio of clay to P4VP from 4:1 to 1:4, based on dry mass.

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2. Add the clay (ca. 8 wt% of moisture) to aqueous 2 wt% solution of P4VP ina ratio as in method (1) and mix for 4 h.

3. Dry blend P4VP with Na-MMT, and then add 35-38 wt% H2O (based ondry clay) and extrude the paste.

When a dry blend of Na-MMT and powdered P4VP was mixed with 75 wt%water an exothermic reaction was observed. Apparently, the bonding reaction ofa polymer to the internal face of the clay platelets is sufficient to engender anexothermic exfoliation.

All of the three methods resulted in intercalation of MMT. The final spacingdid not depend either on the method of preparation or MW of P4VP, but on thequantity of polymer sorbed between platelets. Exfoliation did not take placeunless the clay contained at least 10 wt% water. The polymer-engenderedexpansion of the interlayer was demonstrated by drying the intercalated samplesfor 4 h at 120 °C – only a small change in the interlayer spacing was detected.The generality of the developed technology was demonstrated in later patents(see below), where Na-MMT was directly intercalated with insecticide or pesticide(e.g., 2,4-dichloro phenoxy acetic acid butyl ester), provided that their moleculeswere water-soluble and had strong polar groups.

As shown in Figure 28 the interlayer spacing increased with the concentrationof P4VP or PVAl in steps. When the intercalating composition contains < 16 wt%of intercalating polymer a monolayer is sorbed between the platelets, increasingthe interlayer spacing by < 1 nm. When the concentration is in the range from16 to 35 wt% the interlayer spacing increases by 1 to 1.6 nm. At loadings of35 to 55 wt%, the interlayer distance is increased to about 2.0-2.5 nm, whichcorresponds to three layers of intercalant sorbed between adjacent clay platelets.At loadings of 55 to 80 wt% the interlayer distance is increased to 3.0-3.5 nm,

Figure 28 Interlayer spacing as a function of PVP or PVAl concentration(calculated on dry weight of Na-MMT). Data Beal et al. [1998].

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which corresponds to 4 and 5 layers of sorbed intercalant – this is considered theoptimum interlayer distance for organoclays. When these are incorporated intopolymeric matrix, the platelets with the intercalant molecules attached to thesurface exfoliate.

Such interlayer spacing has not been achieved by direct intercalation of amonomer, an oligomer or a polymer molecule, without prior sorption of an oniumor silane coupling agent. The new method leads to easier and more completeexfoliation. The exfoliated compositions, especially the high viscosity gels, maybe used for delivery of active compounds, such as an oxidising agent for hairlotions, drugs, cosmetics, oil well drilling fluids, paints, lubricants, food gradelubricants, etc.

An important feature of the AMCOL invention is that the intercalatecompositions can be manufactured in a concentrated form, e.g., as a master gel;having 20-80 wt% intercalated or exfoliated clay. Exfoliation should providedelamination of at least about 80 wt% of the intercalated material. For suchrelatively thorough exfoliation a shear rate > 10 sec-1 may be required. Thepreferred shear rate is 100-10,000 sec-1. The shearing may be imposed bymechanical means (in a Banbury, Brabender, continuous mixer or extruder), bythermal shocks (alternatively raising and lowering the temperature), by pressurealteration (0.05-6.0 MPa sudden pressure changes) or by ultrasonics (vibrationscause portions of the composition to be excited at different phases).

Development of the intercalation technology described above has continuedin the following patents [Beall et al., 1996a,b; 1999a; Serrano et al., 1998a,b;Ferraro et al., 1998]. In another document [Beall et al., 1999b], intercalates wereprepared by contacting clay either with water, or with an aqueous solution of awater-soluble polymer and/or a water-miscible organic solvent, e.g., alcohol,followed by contact with a pesticide or its solution. The pesticide should have apolar group, e.g., carboxylic acid, ester, amide, aldehyde, ketone, sulfur-oxygenor phosphorus-oxygen moiety, cyano, or a nitro. As for P4VP, here also the resultsdid not depend upon the method, but on the quantity of active ingredient sorbedbetween clay platelets. The intercalated Na-MMT showed d-spacing = 1.88 nm,an increase from 1.24 nm for untreated clay.

In the following patent [Tsipursky et al., 1999] intercalation was also carriedout in the presence of 25 to 50 wt% of H2O using a water-soluble monomer, anoligomer and/or a polymer. The shearing could be provided by mechanical means,thermal shock, pressure alteration, or by ultrasonics. The amount of intercalantin contact with Na-MMT was ca. 20 to 60 g per 100 g of clay. As before, theintercalant should have an aromatic ring and/or a polar group: carbonyl, carboxyl,hydroxyl, amine, etc. The preferred intercalants were P4VP, PVAl, polyacrylic orpolymethacrylic acid polymers and copolymers. A wide variety of topically activecompounds (e.g., cosmetics, industrial, medicinal) could be incorporated into astable composition and evidently, so could the monomers. As before, the preferredinterlayer spacing was stated as d001 = 3.0-4.5 nm. The viscosity, η = 0.02 to5,000 Pas, could be adjusted by pH and addition of metal salts. A very long listof suitable salts was cited, from Al-acetate oxide to Zr-trifluoroacetyl-acetonate.For example, addition of 0.1 wt% of either AlCl2OH or Mg-acetate increased ηby a factor of 5.

Another disclosure [Lan et al., 1998; Lan et al., 2000] brings AMCOL’sintercalation methods closer to the mainstream of CPNC technology. Thus, claywas treated with 30 to 80 wt% of an organic intercalant, having paraffinic Cn-

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chain with n ≥ 6. The compound must also have a polar group, viz. hydroxyl,polyhydroxyl, carbonyl, polycarboxylic acids and salts, aldehydes, ketones,amines, amides, ethers, esters, lactams, lactones, anhydrides, nitrites, halides,pyridines and their mixtures. The intercalating composition should have a weightratio of organic intercalant to clay ≈ 1:4.

According to the authors, bonding by the intercalant polar ends causes themolecules to stretch in the direction normal to the platelet surface. This increasesthe interlayer spacing, while consuming little of the intercalant. As a result, thereis sufficient expansion of d001 and sufficient concentration of the remaining freecations to ascertain sorption of polymerisable monomer, oligomer, and/or polymermolecules, e.g., of an epoxy resin. The process does not require either oniumions or silane coupling agents. It can be applied to all resins, but it has beenspecifically designed for the epoxy resins. Diluting the Na-MMT/intercalantconcentrate with a monomer or oligomer, and then curing may lead to exfoliation.The presence of polymerisable monomer or oligomer in the clay galleries improvesmiscibility with the matrix polymer, hence a masterbatch can be mixed withadditional polymer.

Alternatively, the intercalant may be dispersed into a melt processable thermoplasticor thermosetting matrix oligomer or polymer, then clay and water added. The matrixpolymer may have DP = 10-100, and a melt index MI = 0.01-12 g/10 min at theprocessing temperature. For example, Na-MMT (8 wt% water; CEC = 1.2 meq/g)was mixed at room temperature with epoxy resin and 1-dodecyl-2 pyrrolidone(DDP) in a 1:1 molar ratio to the Na+. Then, water was gradually added to themixture. The solid-like mixture was extruded using a single screw extruder (SSE)and dried at 90-95 °C obtaining uniform powdered materials. XRD determinedthat incorporation of DDP and epoxy increased the interlayer spacings ofNa-MMT, d001 = 1.23 to 3.4 nm. Similar spacing was obtained using differentmixing sequences and methods, viz. by adding DDP and epoxy to clay slurry andthen drying the mixture; by adding DDP/epoxy/water emulsion to clay, extrudingusing a twin screw extruder and drying. However, when DDP was eliminatedfrom the formula the maximum interlayer spacing achieved by adding water andepoxy was d001 = 1.9 nm. In conclusion, water is the essential element for theco-intercalation by the epoxy and DDP molecules. Owing to the co-intercalation,the free liquid phase of DDP/epoxy/water disappears and the mixture becomessolid-like. DDP molecules bind to the interlayer Na+ and epoxy molecules diffuseinto the interlayer spacing. The co-intercalate has the molar/weight ratio:DDP/epoxy/MMT = 1:1:0.75.

Several types of polar organic compounds can be used as intercalants, includinglong chain ethers, esters or alcohols, having a polar end group that provides themolecule with a dipole moment greater than that of water. The listed examplesinclude aliphatic; aromatic; aryl-substituted aliphatic; alkyl-substituted aromaticalcohols and phenols, e.g., manufactured from coconut, tallow and/or palm oils.Straight-chain acids can also be used, for example, stearic, oleic, linoleic, ricinoleic,canola, castor, tallow-based, soybean or coconut. Representative aldehydes includeC6 to C28, phenyl acetaldehyde, etc. Amines (primary, secondary and tertiary) oramides are also suitable, viz. oleylamine, soya alkylamines, hydrogenated or nottallow or ditallow alkylamines, palmitamide, hexadecyl amide, stearamide,oleamide, linoleamide, etc. Suitable nitrites include hexanonitrile,n-nonadecanitrile and others. Lactones, lactams, pyridines and surface-active esterscan also be used.

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These clay-intercalate complexes can be dispersed into thermosetting,thermoplastic matrix or elastomeric oligomers or polymers. For example, theindicated thermosets are: epoxides, polyamides, polyalkylamides, polyesters,polyurethanes, polycarbonates, and their mixtures. The cited thermoplastics are:polyamides (e.g., PA-4, PA-6, PA-66, PA-6,10), polyesters (e.g., PET, PBT,polycyclohexylene terephthalate (PCT)), and polyolefins (e.g., PE, PP) as well asmany others, viz. PCL, PU, PSF, PPS, polyetherimide (PEI), polyketones, vinyls,acrylics, polyacrylonitrile, ionomers, and their blends. The elastomers may beselected from between chlorinated or brominated butyl rubber, TPU, polyesterelastomers, PVC/NBR, polybutadiene (PBD), polyisoprene (IR), EPR, EPDM,chloroprene (CR), chlorosulfonated polyethylene (CSR), poly(2,3-dimethyl-butadiene), fluoro-, silicone or polysulfide elastomers, etc.

Exfoliation of the intercalated layered material should affect > 90 wt% of thelayers, which for some systems may require a shear rate: 10 < γ (sec-1) < 10,000,for others heating, or cycling the pressure between 0.05 and 6 MPa, with orwithout heating. The shearing can be either mechanical (e.g., during extrusion,injection moulding, compounding in a TSE or an internal mixer), or by thermalshock, pressure alteration, ultrasonics, etc. The amount of intercalated/exfoliatedclay in a liquid matrix depends on the intended use and desired viscosity of thecomposition. A relatively high amount, i.e., from 10-30 wt%, may be used insolvent gels with viscosity η = 5-5,000 Pas. This may be achieved with 0.1-5 wt%organoclay, by adjusting the pH and/or by heating the composition to T = 75 to100 °C. A masterbatch with 20-80% organoclay may be prepared, but the finalcomposition should not exceed 10 wt% organoclay.

The patent applications are quite broad, addressing:

1. Preparation of a wide variety of topically active compounds (e.g., cosmetic,medicinal, and industrial viz. antiperspirants, deodorants, antidandruff orantibacterial compounds, antifungal or anti-inflammatory compounds,anaesthetics, sunscreens, analgesics, antiseptics, antiparasitics, antibacterials,steroids, herpes treatment drugs, pruritic medications, etc.).

2. Thermoplastic or thermosetting moulding CPNC for the production of sheetsand panels, formable by conventional processes, viz. vacuum or hot pressing,lamination of, for example, wood, glass, ceramic, metal or other plastics.The sheets and panels can also be produced by coextrusion.

3. Extrusion of 25-75 μm thick films and film laminates for food packaging,fabricated by conventional means, followed by biaxial stretching to orientthe clay platelets parallel to the film surface. These films exhibit increasedmodulus, wet strength, and dimensional stability as well as decreased moistureadsorption, permeability to gases and liquids, etc.

From the fundamental point of view, MMT being a weak silicilic acid has anegatively charged surface that should interact with a cationic monomer orpolymer. For example, as the discussion above demonstrated, poly-4-vinylpyridine(PVP) may easily be protonated to form water soluble, positively chargedpolyelectrolyte, which intercalates the clay. Similarly, such monomers asprotonated vinyl pyridines and their N-alkylated derivatives (4-VPH+X–) areexpected to intercalate by ion exchange. The inserted monomer can be polymerisedwithin the MMT galleys. Depending on the monomer concentration and acidity4-VPH+X– may either form 1,2- or the quaternary 1,6-polyelectrolyte with thepyridinium units in the side or in the main chain, respectively.

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In polymer technology it is often advantageous to replace one componentby a mixture of similar functional ingredients. For example, compatibilisationof two immiscible polymers is often more efficient by a mixture ofcompatibilisers than by either one. Similarly, blending one polymer with twofractions often results in better material. This philosophy has also been appliedto clay intercalation [Lan et al., 2003]. Evidently, it is easier to mix known andavailable organic intercalants than to design and synthesise new ones. Toillustrate the advantage of the mixing approach, consider the following situation.For the best performance of CPNC it is essential that the matrix is misciblewith the organic tails of the intercalant attached to the clay surface. A goodmiscibility candidate is ethoxylated or polyalkoxylated ammonium salt (e.g.,ETHOQUAD 18/25 or JEFFAMINE506). However, this compound has a highmolecular weight, and if used alone it would reduce the clay content to lessthan 50 wt%. It was found that its 50:50 combination with ODA is easy tomanufacture and use for CPNC.

The new mixed-onium organoclays have a broader range of application indiverse polymeric matrices (e.g., PVP, PVAl, PEG, PTHF, PS, PCL, PA-6), butthey have been especially designed for thermoplastic polyesters (PEST) such asPET. The CPNC may be used alone or as one component of the multi-layeredstructures with PEST, aliphatic or aromatic PA, EVAl, etc. For example, the patentdescribes preparation of CPNC by melt compounding, in a solvent or by reactivemeans, using an organoclay that was pre-intercalated with a mixture of at leasttwo organic cations. In a given example, the PET-based CPNC (with 75 to 90%of exfoliated clay platelets) showed low permeability and good mechanicalproperties, thus it was suitable for bottle blow moulding.

A similar approach was taken by Han et al. [2004]. The authors pre-intercalated Ca-MMT with trimethyl cetyl ammonium bromide (3MCA), amino-undecanoic acid (AUA), and a 1:1 mixture of these two. The organoclays wereused to prepare CPNC with PU as the matrix. XRD and TEM showed that PUcontaining MMT-AUA was exfoliated, PU with MMT-3MCA was intercalated(d001 = 3.5 nm) and the one with mixed intercalant had an intermediate behaviour(small peak of MMT-3MCA remaining). The tensile properties of the CPNCscontaining 5 wt% organoclay demonstrated an advantage of the mixed intercalantapproach – the tensile modulus and strength were the best, with the elongationat break 35% higher than that of neat PU.

Na-MMT (CEC = 0.83 meq/g) was intercalated in an aqueous solution of2 wt% of poly-4-vinyl pyridinium bromide (at a level of 5×CEC) [Fournaris etal., 1999]. Polymerisation of 4-VPH+Br– in the clay galleries was fast and it resultedin the formation of quaternised ionene, independently of the polymerisationconditions. The quaternised ionene polymer and partially protonated poly-4-vinylpyridine could also be dissolved in water and used to intercalate Na-MMTat a level of 4×CEC. The isotherms for the 1,2-PVPH+Br-, 1,6-PVP+Br–, and P4VPadsorption by Na-MMT were obtained by adding the appropriate amount ofeach intercalant to a suspension of Na-MMT and stirring it for 2 h. The productswere isolated by centrifugation, washed, and then air-dried on glass plates. Theamount adsorbed by the clay was calculated from analysis of the supernatantsolution. Independently of the amount of polymer available for intercalation,only one macromolecular sheet of P4VP or the quaternised ionene was found toenter the interlayer space causing the interlayer space to expand by Δd001 = 0.4 to0.54 nm. Thus, this amount of polymer suffices to coat the clay surfaces, making

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the clay anions not available for additional macromolecules. The surface saturationcoverage increased in the order:

partially protonated P4VP > quaternised ionene form> completely protonated P4VP.

By contrast with neutral P4VP, the partially protonated P4VP could be adsorbedat different levels. XRD showed that the d001 spacing increases with P4VP loading(see Figure 29). At a concentration of P4VP > 70 wt% the XRD peak disappeareddue to delamination. The FTIR spectra provided additional evidence of P4VPinteraction with the clay surfaces.

Fischer [1999] patented the use of block or graft copolymers for the preparationof CPNCs. By analogy to compatibiliser in immiscible polymer blends, thecopolymer must have two types of structural units, one (A) to interact with claythe other (B) with polymer. The molecular weight of part A should be MW = 0.1 to5 kg/mol (DP = 5 to 20), while for part B MW = 0.1 to 20 kg/mol. To interact withclay the structural units of part A should be hydrogen-bonding, e.g., vinylpyrrolidone, vinyl alcohol, ethylene glycol, ethylene imine, vinyl pyridine, acrylicacid, acrylamide. To interact with the matrix the structural units of part B shouldbe miscible with the matrix polymer and able to entangle. In the organoclay theinorganic clay content ranged from 5 to 600%, while in CPNC it constituted from2 to 55 wt%. Judging by the examples, preferably the intercalant is a blockcopolymer, e.g., PS-b-PEG or PS-b-PVP. The intercalation is performed in THF,followed by solvent evaporation and melt compounding with the matrix polymer.

Fischer et al. [1999; 2001] used this strategy to prepare CPNCs with eithernatural or synthetic clay and either PS or PMMA as matrix. The authors starteddirectly with Na-MMT, Na-saponite, or Na- fluorohectorite (CEC = 0.85 to

Figure 29 XRD peak position(s) for Na-MMT intercalated with partiallyprotonated poly-4-vinylpyridine. The peaks disappeared for loadings above 65

wt%. Data [Fournaris et al., 1999].

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3.2 meq/g). Block or graft copolymers were used (viz. PS-b-PEG, PMMA-b-PEG,PMMA-co-polymethacrylic acid, PS-b-P2VP; MWblock = 1 to 27 kg/mol) havingone part either identical to or miscible with the matrix polymer and the othercapable of intercalating with the clay. The interaction between the compatibiliserand clay was preferentially ionic or hydrogen bonding. The intercalation wasperformed either in the melt (copolymers with PEG) or in solution. It resulted inindividual clay platelets and/or short stacks containing 2-10 layers homogeneouslydistributed in the matrix. As expected, better mechanical performance wasobtained for block copolymers having the miscible block with molecular weightabove the entanglement molecular weight value: MW > Me.

Several clays (e.g., sodium montmorillonite, hectorite, and laponite) were melt-intercalated with PEG using microwave irradiation [Aranda et al., 2003]. Theprocess was found to depend on irradiation time and power, amount and relativeratio of the reagents, and relative humidity. The process resulted in d001 ≅ 1.8 nmindicating formation of a hydrated double layer of PEG within the interlamellargalleries. Thus, 10 minutes of microwave irradiation of moderate power was ableto accomplish a similar task as shear compounding in an internal mixer or TSE.

2.3.6 Two-Step IntercalationThis process usually involves an organoclay that has been pre-intercalated with asuitable onium cation, then treated with low molecular weight polymerisable liquid(monomer, oligomer or cyclomer), which in turn may be polymerised. This reactivelast step is essential as it leads to the ultimate level of intercalation – the exfoliation.

An early example of this process has already been discussed in relation to thepreparation of PA-6 nanocomposites [Okada et al., 1988]. The process comprisedthree steps:1. Intercalation of Na-MMT (CEC = 1.8 meq/g) with ω-amino-dodecanoic acid

(ADA) in the presence of inorganic cation, e.g., Cu2+ or Al3+;2. Mixing the intercalate with ε-caprolactam, and3. Polycondensation.As shown in Figure 25, each of the three steps contributed to the expansion ofthe interlayer spacing. The clay was first dispersed in aqueous, acidified intercalantsolution, reacted with ADA and the precipitate was washed with water to removeexcess ions. The organoclay paste was then mixed in a mortar with monomerand the mixture heated at 80 °C for 3 h to ensure dehydration and melting of thecaprolactam. The homogeneous complex was then transferred to a closed stainlesssteel vessel and heated at 250 °C for 5 h. The resulting CPNC contained exfoliatedclay platelets and showed significant improvement of the mechanical and thermalproperties.

Over the years several modifications of this basic process have been made.For example, it was found that the original process resulted in end tethering ofall macromolecules, having the end amino groups of PA firmly attached to theclay surface. As a result, the CPNC had insufficient dyeability, as well as poorcoating and printing properties. The modified method [Deguchi et al., 1992]comprised the following steps:1. Dispersing >1 wt% of Na-MMT (CEC = 1.19 meq/g) in water solution of

ADA and HCl, followed by stirring at 80°C for one hour. After washing, themixture was filtered to obtain organoclay complex (abbreviated as 12MMT;d001 = 1.25 to 1.8 nm) containing 88 wt% H2O.

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2. To the 12MMT-paste H2O and ε-caprolactam were added in a ratio:12MMT/H2O/ε-caprolactam = 1:9:9, followed by mixing.

3. The produced organoclay/water/ε-caprolactam mixture was compounded with72 wt% of either PA-6 (MW = 15 kg/mol) or PA-66 (MW = 20 kg/mol) in aTSE. The extruded strands were pelletised, and then caprolactam was extractedfrom the pellets using hot water. Excellent clay layer dispersion was observedunder TEM in the moulded CPNC, with only single platelets or doublets visible.XRD and swelling tests indicated exfoliation.

The new method resulted in CPNC in which layers of silicate are uniformly dispersedand not all amino end-groups are tethered. The material showed good mechanicalproperties, heat resistance, improved dye-affinity and whitening resistance duringstretching. This process has been used for the commercial scale production ofPA-6 nanocomposites.

Two-step intercalation is particularly important for preparation of CPNCwith non-polar polymers, viz. PO, regardless of whether the selected method isreactive or by melt compounding. Since these systems will be discussed in greaterdetail later, here only two examples are given.

Heinemann et al. [1999] have prepared PO-based PNC starting with hectoriteintercalated with dimethyl benzyl stearyl ammonium chloride (2MBSA), havingd001 = 1.96 nm. When HDPE was polymerised in the presence of the intercalatedclay, the XRD peak shifted to d001 = 1.40 nm. Best results were obtained forLLDPE-type ethylene-octene copolymers. The authors reported that performanceof the reactively prepared CPNC was better than that prepared by meltcompounding. However, the intercalated clay was found to interfere withmetallocene catalysts.

An early patent on the preparation of PO-based CPNC may serve as anillustration of the two-step melt intercalation method [Kato et al., 1997; Usuki etal., 1999]. Thus, MMT was intercalated with ODA and a guest molecule (MW =0.100-100 kg/mol) having a polar group, e.g., a maleated PO. Alternatively, anoligomeric guest molecule with a polar group could be added to the onium-intercalated clay. Next, the complex was compounded with a non-polar PO matrix.The most important feature of the invention is the use of a polar guest moleculethat can bond to the pre-intercalated clay layers and provide entangling possibilityfor the main PO macromolecules. The process is sensitive, as insufficient amountsof maleic anhydride (MAH) groups do not provide sufficient compatibilisation,and too high concentration may lead to phase separation of the guest moleculeswithin the PO matrix, which in turn results in poor CPNC performance. Evidently,as in any other phase separation process, this also depends on other factors thanMAH content, viz. molecular weight, temperature, pressure, stress field, etc.

2.3.6.1 Intercalation by Silylation

In the patent applications of 1992 MacRae Maxfield et al. [1995] introducedtraditional ‘sizing agents’ into nanotechnology. The aim was to obtain PA-basedCPNC by melt compounding. It was found that cation-exchanged MMT couldnot be exfoliated during extrusion in a twin-screw extruder. However, the situationimproved when the organoclay was treated with a silane coupling agent. In theintervening years better organoclays have been developed and today meltcompounding PA with cation-exchanged MMT can result in exfoliated CPNC.

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However, the observed enhancement of dispersion and resulting CPNCperformance by incorporation the sizing agents are valuable lessons for the manydifficult systems that remain.

The following procedure was described. A suspension of 5-15 wt% MMT inwater was heated to 80 °C in a high-speed mixer with (NaPO3)6 then anammonium salt (e.g., octadecyl-ammonium chloride) is added. The amine-complexed clay was centrifuged, washed, dried, and ball milled to 100-meshpowder. The powder was re-dried at 100-160 °C in vacuum for 8-24 h in thepresence of phosphorous pentoxide. Next, the intercalated clay was treated withamino-ethyl amino-propyl trimethoxy silane in an organic solvent (e.g., dioxane,DMSO, MEK). The modified clay (60 mmole of ammonium and 20 mmole ofsilane per 100 g of clay) was compounded into a masterbatch using a TSE. Themasterbatch was finally diluted with PA-6 to mineral concentrations of 0.1, 0.05,and 0.01 wt%.

The process was revised in the next patent [MacRae Maxfield et al., 1996]:

1. MMT (aspect ratio 15 ≤ p ≤ 300) was reacted with an organosilane,organotitanate or organozirconate capable of forming covalent bonds withthe particles and either reacting with a polymer or at least miscible with thepolymerisation product. For example, a suspension of 5-15 wt% MMT in anaqueous solution of sodium hexametaphosphate was heated at 50-90 °C. Thedispersion was combined with a solution of caprolactam-blocked isocyanato-propyl triethoxy silane and 1-trimethoxysilyl-2-(m,p-di-chloro methyl)-phenyl-ethane. The organosilane intercalated MMT was filtered, dried (at60-160 °C for 8-24 h) and ground to about 100 mesh.

2. The intercalated MMT was combined with caprolactam and aminocaproicacid having cation-exchange capabilities. The mixture was polymerised. Inthe CPNC the amount of MMT varied from 0.1 to 10 wt% and the interlayerspacing d001 ≥ 5 nm. The platelets were uniformly dispersed.

In 1998 Ogawa et al. [1998] described a general method of intercalation of layeredsilicate (Na-magadiite). The process involved silylation of interlayer silanol groupswith organochlorosilanes with different functionality. The reaction createdorganically modified surfaces where organic groups are covalently attached tothe clay surface. However, to start with, Na-magadiite was first ion exchangedwith dodecyl-trimethyl-ammonium (3MDDA). The resulting intercalated clay(d001 = 2.79 nm) was dispersed in a toluene solution of eitheroctyldimethylchlorosilane (C8H17(CH3)2SiCl (1)) or octyltrichlorosilane(C8H17SiCl3 (2)), under a blanket of N2 for 48 h. The product was separated bycentrifugation, then washed with an acetone/water mixture and dried. Duringthe reaction de-intercalation of 3MDDA took place (evidenced by the absence ofN in elemental analysis). The basal spacing decreased to d001 = 2.33 and 2.18nm, for (1) and (2) respectively. The amounts of the attached organosilyl groupswere determined to be 1.84 and 1.94 per 14SiO2 (molar ratio), respectively. Thetwo differently modified magadiites showed different adsorption behaviour. Thedifference in the composition affected the reactivity, which in turn led to a differentlevel of the basal spacing (e.g., after absorption of n-octyl alcohol: d001 = 3.20and 3.18 nm, respectively). After silylation the organosilyl groups formedmonolayers. When n-alkyl alcohols were introduced, they rearranged themicrostructure, forming a bilayer and expanding the interlamellar spaces.

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Jeong et al. [1998] and Hudson [1999] described preparing PO-based PNCin three steps:

1. Functionalising MMT with an aminosilane.2. Reacting the free amine with maleated PO (MW ≅ 20 kg/mol; ca. 1 wt%

MAH groups) either in solution (reaction time ca. 30 min) or in melt.3. Dispersing the organomodified clay in a semicrystalline PO.Cocrystallisation between the maleated and the semicrystalline PO is essential.For example, the coupling reaction between MMT and amino ethyl-dimethylethoxysilane was carried out for 2 to 4 h at T = 25-90 °C, in either ethanol andwater, or dimethyl acetamide. The monoalkoxy silane may either react with theclay surface, self-condense to form dimer, or it may remain unreacted. Graftingmay lead to precipitation. The reaction produces ethanol, which has been detected.Detection of the dimer by NMR is facile (resonance at 9.3 and 13.9 ppm). XRDof the silane treated MMT demonstrated increased interlayer separation withd001 = 2.4 to 7.3 nm. TEM showed the presence of many individual MMT-layers,but in CPNC containing 15% clay, most were found in stacks of 2-4 layers ca.1 μm long.

2.3.6.2 Intercalation Utilising Epoxy Compounds

Pinnavaia and his coworkers explored the use of epoxy compound as a secondintercalant for layered materials. As early as in 1994 [Wang and Pinnavaia, 1994;1998a,b; Wang et al., 1996], delamination of organoclays in epoxy resin wasreported. The process involved heating an onium ion intercalated smectite withepoxy at T = 200-300 °C. The work resulted in several patents [Pinnavaia andLan, 1998a,b,c; 2000a,b] and publications [Lan and Pinnavaia, 1994; Lan et al.,1995; Massam et al., 1998] from the group. While most of this work was directedtowards production of thermoset CPNC, the possibility of using the two-stepintercalation method (involving cation-exchange and epoxidation) for themanufacture of thermoplastic CPNC has also been suggested.

Giannelis and Messersmith [1996] used a similar approach. An organicallymodified smectite (MMT-MT2EtOH; Cloisite® 30B) was dispersed in an epoxyresin together with diglycidyl ether of bisphenol-A (DGEBA). The system was curedin the presence of nadic methyl anhydride (NMA), and/or benzyl-dimethyl amine(BDMA), and/or boron trifluoride monoethylamine (BTFA) at T = 100-200 °C.Exfoliation of smectite was reported with interlayer spacings d001 ≥ 10 nm andgood wetting of the silicate surface by the epoxy. The curing involved reactionbetween epoxy and the alkyl ammonium ions located in the galleries of theorganically modified clay. In other words, the reaction resulted in direct attachmentof the epoxy network to the silicate layers. The nanocomposite showed higherdynamic storage modulus in the glassy region and very much higher in the rubberyplateau region, when compared to the epoxy resin without clay. The patent focusedon the preparation of clay-epoxy nanocomposites.

In the year 2000, Ishida et al. [2000] proposed a more general approach tointercalation of smectite clays. It involves the use of either epoxy or PDMS as a‘swelling agent’ that increases the interlayer spacing of the cation-exchangedsmectites. Thus, Na-MMT was first intercalated by cation exchange reactionwith either ADA or with HDA. The products were labelled B12 and B16,respectively. The epoxy derived from the DGEBA (Epon 825) is presented in

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Figure 5. Polymer (88 wt%), intercalated clay (10 wt%), and epoxy or PDMS(2 wt%) were manually mixed for 30 min above either the Tg or Tm of the matrixpolymer. The XRD of B12 (no epoxy) showed the interlayer spacing,d001 = 1.6 nm. Upon addition of epoxy the peak became wider and located at asmaller diffraction angle, i.e., d001 increased to 2.3-1.9 nm. Thus, epoxy alonedoes not exfoliate the onium-intercalated clay. For the CPNCs the percentage ofintercalation and/or exfoliation was calculated from XRD patterns as:

Exfoliation (%)= 100×[1 - (clay peak area with epoxy)/(clay peak area without epoxy)]

The extent of exfoliation was found to increase with the mixing time, but the timerequired for the completion of the process (from 4 to 120 min) depended on the typeof system. The extent of exfoliation may be related to the polymer solubility parameter,δ, for epoxy and PDMS this is respectively, δ ≅ 9 and 8. Unfortunately, the authorsdo not indicate the mechanism involved in the clay ‘swelling’ by either of these twoagents. The method was patented [Ishida, 2000].

2.3.6.3 Intercalation Utilising Organic Anions

As discussed in Section 2.2.4, there are three types of possible interactions thatmay be used when preparing CPNC: exchangeable cations in the interlayers,-OH groups on the silicate surface, and exchangeable anionic groups at the clayplatelet edges. Only recently have the latter groups been targeted for the secondstep of smectite intercalation. Thus, natural or synthetic clay was first intercalatedwith a quaternary ammonium ion, and then treated with negatively chargedorganic molecules able to react with the edge cations. In recent patents from SCP0.1 to 1.0 wt% (on dry weight of clay) of a high charge density anionic polymersuch as a polyacrylate has been used [Powell, 2001a,b]. The clay edge treatmentwith polyacrylate was carried out before that with a quaternary ammonium ion.In a continuation of the SCP technology (see Section 4.3), polyacrylate was addedto an aqueous slurry of MMT, which then was subjected to high shear in a Manton-Gaulin mill. After shearing the clay was treated with a branched chain quaternaryammonium ion. According to the author, the polymer became strongly attachedto the clay edges, making them strongly anionic. In the subsequent treatment analkyl quaternary ammonium cation not only reacted with the clay-surface anioniccharges, but also with the edges, which resulted in uniform, hydrophobic coating,improved dispersability in the plastic matrix of the nanocomposite, and improvedproperties.

In the patent example, PA-66 was melt compounded in a TSE with dry, doublyintercalated MMT. The latter was prepared as described above, with dimethylhydrogenated tallow -2-ethyl hexyl ammonium methyl-sulfate (2MHTL8),described in a prior patent from SCP [Dennis, 1998]. Polyacrylate (e.g., AlcogumSL-76 or SL-78) was applied at a concentration of about 0.5 wt% by weight ofthe dry clay. The slurry was dewatered, dried and ground. The extrudates weresubjected to wide angle XRD – total exfoliation was observed in CPNC withclay content of ≤5 wt%.

2.3.6.4 Intercalation Utilising Macrocyclic Oligomers (Cyclomers)

The two-step intercalation process was used to produce CPNC withpolycarbonates, polyesters or polyphenylene ethers as the matrix polymer. The

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first step was the customary onium ion exchange, the second incorporation ofmacrocyclic oligomers of these polymers [Takekoshi et al., 1996]. As in manyprior patents, the onium ion could be either ammonium, phosphonium orsulfonium type. For example, MMT (CEC = 1.19 meq/g) was first intercalatedwith dodecylammonium chloride or N-hexadecyl pyridinium chloride. Next, theorganoclay was mixed with a macrocyclic oligomer of terephthalic acid-co-ethylene-co-butylene glycols, dried under a vacuum and then placed in an oilbath at 190 °C. Finally, dioctyl tin dioctoxide was added to polymerise theoligomer. The melt became viscous in about 15 s and the resulting solid polymercomposition comprised exfoliated MMT.

2.3.7 Intercalation by Inorganic IntercalantsThere is a large body of literature dedicated to the physicochemical modificationof clays for use as catalysts or catalyst supports [see for example Pullukat andShinomoto, 2001; Wypart et al., 2002; Sun et al., 2002]. During the last fewyears there has been an effort to use the insight gained into the chemistry of claysfor the production of inorganically intercalated clays. It is expected that thesenew intercalated clays will have much greater thermal stability than the currentlyused ammonium-intercalated systems.

During intercalation with inorganic intercalants pillared structures are formedbetween clay layers. Cations of Al, Zr, Be, Cr, Fe, Ni, Nb, Ta, Ho, and othershave been used individually or in mixtures. Since the intercalated clays aresubjected to calcination at T ≅ 600 °C and are mainly used in catalysis, hightemperature stability has been essential [Mitchell, 1990]. However, even incatalysis this technology is new and (the starting material being a natural product)the reproducibility of created structures is poor. The use of inorganically-intercalated clays for PNC is in statu nascendi.

Hydrothermally stable porous structures were prepared by intercalating asmectite clay (e.g., MMT) with aluminium and rare earth salts, then calcinationat T < 760 °C [McCauley, 1989]. The pillars were formed of Al and rare earthelement oxides. They expanded the silica plates to an interlayer spacing of1-5 nm. In 1979 Vaughan et al. [1979] dispersed smectite clay in an aqueoussolution of aluminium chlorohydroxide (AlCl2OH) and mixed the suspensionfor about one hour at about 70 °C. The reacted clay was recovered and heated atT = 200-700 °C to form solid pillars between the clay layers of 0.9-1.8 nm height.Addition of ZrOCl2 increased the interlayer spacing to 2.2 nm. The pillarscollapsed when heated to T > 650 °C. Katdare et al. [2000] reported thatNa-MMT dispersed in water was mixed with AlCl2OH and subjected to ultrasonicagitation at 50 kHz for 20 min. After washing, the intercalated clay was calcinedat 500 °C. Surprisingly, the XRD scan did not show any peak within the expectedrange of 2θ, which may indicate total exfoliation. This is promising news –hopefully, after some ‘compatibilisation’ step, these exfoliated individual clayplatelets may be incorporated into a polymeric matrix.

A Dow patent [Nichols and Chou, 1999] is the only one that professes toproduce inorganically intercalated clays for the manufacture of CPNC. Theintercalation involves organic and inorganic intercalants:

1. The clays of interest include MMT, nontronite, beidellite, hectorite, saponite,magadiite and vermiculite, double or mixed hydroxides, viz.Mg6Al3.4(OH)18.8(CO3)1.7H2O, chlorides, viz. ReCl3 and FeOCl,

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chalcogenides: TiS2, MoS2, and MoS3; cyanides, e.g., Ni(CN)2; and oxidesH2Si2O5, V5O13, HTiNbO5, Cr0.5V0.5S2, W0.2V2.807, Cr3O8,MoO3(OH)2,VOPO4-2H2O, CaPO4CH3-H2O, MnHAsO4-H2O, Ag6Mo10O33, etc.

2. The organic intercalant may be a water soluble polymer (e.g., PVAl, PEG,carboxymethyl cellulose, PAA, P4VP); a reactive organosilane compound, acationic surface active agent such as quaternary ammonium salt having C12to C18 moieties. The preferred quaternary ammonium cations may have suchgroups as: octadecyl trimethyl, dioctadecyl dimethyl, hexadecyl trimethyl,dihexadecyl dimethyl, tetradecyl trimethyl and ditetradecyl dimethyl.

3. The inorganic intercalant can be an inorganic polymer obtained byhydrolysing a metallic alcoholate: Si(OR)4, Al(OR)3, Ge(OR)4, Si(OC2H5)4,Si(OCH3)4, Ge(OC3H7), Ge(OC2H5)4, etc. The colloidal particles include thehydrolysed and dehydrated forms, e.g., Si(OH)4 or SiO2, Sb2O3, Fe2O3, Al2O3,TiO2, ZrO2 and SnO2, etc. The size of the colloidal particle should be about12 nm. It is preferable to modify the multilayered material with a metallicalcoholate, e.g., Ti(OR)4, Zr(OR)4, PO(OR)3, B(OR)3 and the like alone orin combination, with Ti(OC3H7)4, Zr(OC3H7)4, PO(OCH3)3, PO(OC2H5)3,B(OCH3)3, B(OC2H5)3. Metallic chlorides such as TiCl4, metallic oxychloridessuch as ZrCOCl2, and nitrate chloride can also be used.

4. The polymeric matrix can be thermoplastic, thermoset or elastomeric. Thespecifically named polymers include PO (e.g., PP, HDPE, LLDPE, ultra lowdensity PE (ULDPE), EPR, EPDM, ethylene-acrylate acid copolymer (EAA),EVAc), PEST, PA, PC, acrylics (viz. PMMA, methylmethacrylate-butadiene-styrene terpolymer (MBS)), styrenics (e.g., PS, styrene-butadiene rubber (SBR),high impact PS (HIPS), styrene-acrylonitrile copolymer (SAN), styrene-butadiene-styrene terpolymer (SBS)), TPU, etc.

Prior to intercalating the clay is swollen in an aqueous (e.g., H2O with methanolor butanol) or an organic liquid (e.g., DMF, DMSO, hydrocarbons, halogenatedhydrocarbons, benzene, xylene, cyclohexane, toluene, mineral liquids and oils).For example, a mixture of swollen clay and the polymerisable inorganic intercalantcan be contacted with a hydrolysing agent for the polymerisable intercalant toform the inorganic polymer. In general, hydrolysis is conducted at T > 70 °C.Following intercalation, the clay is centrifuged and dried at 50-80 °C. Theintercalant may optionally be calcined at T = 450-550 °C. Intercalation increasesthe original interlamellar gallery height of ≤ 0.4 to about 0.5 to 60 nm. Next, theintercalated, layered material can be dispersed in a monomer, which whenpolymerised would form the polymer matrix. Alternatively, it can be incorporatedin the molten or dissolved polymer.

Melt blending is the preferred method for preparing CPNC of a thermoplasticpolymer. Typically, the polymer is melted and combined with the desired amountof the intercalated clay using an extruder, a Banbury or Brabender mixer, acontinuous mixer, etc. Melt blending is carried out in the presence of an inertgas, such as argon, nitrogen or neon. Alternatively, the polymer may be dry mixedwith the intercalant, then heated in a mixer and subjected to a shear sufficient toform the desired composite. Sufficient exfoliation is defined as when ≥ 80 wt%of the clay platelets (aspect ratio p = 10-2000) are individually and uniformlydispersed in the polymeric matrix.

The patent does not provide any example of CPNC preparation with a specificpolymer and there are no numerical values of performance. The claims are very

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broad, naming virtually any polymer, viz. a thermoset (e.g., phenolics, epoxy,urethane or urea resin), or thermoplastic polymer (including PO, PC, TPU,styrenics) or a vulcanisable or thermoplastic rubber (e.g., EPR, PU). Similarly,the description of the performance is quite generous. The unnamed CPNCs showedexcellent balance of properties, viz. superior heat, chemical or ignition resistance,barrier to diffusion of polar liquids and gases, yield strength in the presence ofpolar solvents such as water, methanol, ethanol and the like, stiffness anddimensional stability. They are useful in diverse applications viz. in businessequipment, computer housings, transport (automotive and aircraft), electronic,packaging, building and construction industry, etc.

Bora et al. [2000] developed another method for the manufacture ofintercalated, thermally stable organoclay. Since the metal complexes with organicligands show good thermal stability, the authors expected that MMT intercalatedwith an appropriate metal complex might show superior thermal stability to thatof onium cations. The aim of the work was to intercalate MMT with bulky,three-dimensional cationic metal complexes with ligand X. Two were selected:

• Ni-CX1 ≡ [Ni{di-2-aminoethyl amine}2]Cl2 and• Ni-CX2 ≡ [Ni{2,2´:6´,2´´-ter-pyridine}2](ClO4)2.Na-MMT was converted to Ni-MMT by adding NiCl2 to an aqueous suspension ofNa-MMT (CEC = 1.14 meq/g). The Ni-MMT could also be complexed withCX1 or CX2, but the structure was different from that obtained by ion exchangebetween Na-MMT and either Ni-CX1 or Ni-CX2. This reaction was carried outby adding Ni-complexes to an aqueous suspension of Na-MMT and allowingthe reaction to proceed for 0.5 h, e.g.:

Na-MMT + Ni-CX12+ → [Ni-CX1]-MMT + Na+

The product was separated, washed and then dried at 50 ± 5 °C in an air oven.Analysis of the dried samples indicated that complexes were adsorbed by MMTup to about 0.57 meq/g, i.e., up to stoichiometry. Oriented samples on glassslides were dried at room temperature, 100, 150, 200, 250, 300, 350 and 400 °Cfor about 1 h. XRD of [Ni-CX1]-MMT showed that on heating d001 decreasedfrom 1.45 at 50 °C to 1.35 nm at 250 °C. However, XRD of [Ni-CX2]-MMTshowed d001 = 1.94 at 50 °C, decreasing to 1.90 nm at about 350 °C. Accordingly,the FTIR spectra showed characteristic bands of these complexes up to about250 and 350 °C, respectively. Thus, the thermal stability of MMT intercalatedwith Ni-CX1 and Ni-CX2 were found to be about 50 and 150 °C higher comparedto that of their metal complex salts.

The difference between Ni-CX1 and Ni-CX2 is noteworthy. In the former theligand is aliphatic, in the other it is aromatic. Ligands with aromatic rings in themain chain show higher thermal stability than the aliphatic. This generalobservation was confirmed by results for the free complexes and their MMTsalts. Furthermore, the interlayer spacing of [Ni-CX1]-MMT was significantlysmaller than that for [Ni-CX2]-MMT. This may be due to π-interactions betweenthe oxygen on MMT and the aromatic rings of Ni-CX2.

2.3.8 Melt IntercalationIn a sense, melt intercalation is a misnomer – in reality the goal of the process isnot so much intercalation as exfoliation of layered inorganic material in a molten

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polymer. However, since melt exfoliation is rarely achieved, there is some rationalefor the term.

Melt intercalation is the preferred method for the preparation of CPNC withthermoplastic matrix polymers. Usually, the polymer is melted and compoundedwith intercalated clay using an extruder, an internal mixer, a kinetic-energy mixer,etc. Compounding is carried out in the presence of an inert gas, e.g., N2.Alternatively, the polymer is first mixed with a compatibiliser (e.g., itsfunctionalised homologue) then compounded with intercalated clay. Industrially,CPNC is considered to be exfoliated when ≥ 80 wt% of clay layers (aspect ratiop = 10-2000) are uniformly dispersed in the polymeric matrix in the form ofstacks comprising not more than two platelets.

It is remarkable that macromolecules with significantly larger radius ofgyration than the interlamellar gallery height:

r dg

2 1 2

001 0 96/

.> −

are able to diffuse into the galleries. Evidently, the process will take place only ifit leads to a decrease of the free energy (ΔG), i.e.:

ΔGintercalation = ΔH - TΔS < 0 (15)

The enthalpic (ΔH) contribution usually comes from the chemical interactionbetween clay and the intercalating compound, whereas the entropic one (ΔS)usually comes from the ‘randomisation’ of the macromolecular segmentsplacement, e.g., diffusion into spaces devoid of macromolecular presence.However, the conformational energy loss caused by chain stretching from therandom coil configuration into elongated structures inside the galleys, andassociated with this the topographical and energetic constraints (e.g., caused byadsorption of the macromolecules on the clay surface) hinders the diffusion.Thus, it is expected that for successful melt intercalation there must be an enthalpicdriving force for the functionalised macromolecules. Furthermore, the interlayerspacing should be large, at least the same order of magnitude as the diameter ofthe macromolecular Gaussian coil, and sufficient time must be provided for thediffusion process to reach the centre of the stacked platelets. Two relations canbe used as guides:

1. The self-diffusion coefficient, Ds ∝ 1/Mn2.

2. The distance travelled by the diffusing macromolecules is proportional tothe square root of the diffusion time, viz. ldiff ∝ t1/2.

In practice numerous factors influence the process, affecting the validity of thesesimple relations.

The melt intercalation process can readily be divided into static and dynamic.Usually, the static process takes place under vacuum at temperatures at least 50 °Cabove the transition temperature (Tg or Tm) in the absence of mixing, thus oftenit is also called melt annealing. Dynamic melt intercalation is more-or-less astandard compounding operation, performed in an extruder, internal mixer andsimilar processing equipment.

2.3.8.1 Quiescent (or Static) Melt Intercalation

There is no commercially viable process that would start with dry, unaltered claypowder and thermoplastics melt and produce exfoliated CPNC. For example,

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attempts at a solventless intercalation of Na-MMT or Li-MMT with PEG or PSfailed [Vaia et al., 1993]. However, PS was intercalated with an organoclay(Na-MMT ion-exchanged with alkylammonium chloride) by mixing the twopowders, pressing the mixture into a pellet, and heating the pellet in a vacuum at165 °C. XRD showed that PS increased the interlayer spacing by about 0.7 nm,which corresponds to a monolayer of stretched PS macromolecule. Interestingly,the intercalated PS had no Tg within the temperature range from 50 to 150 °C.The lack of chemical bonding was demonstrated by dissolving/dispersing theCPNC in toluene and quantitatively extracting the PS.

For PEG the static intercalation (heating the polymer/Na-MMT mixture at 80 °C)was slow, requiring from 2 to 6 h. The achieved final interlayer spacing was modest,d001 = 1.77 nm [Vaia et al., 1995a]. The method failed for PS – it was found that priorto melt intercalation the MMT had to be modified by treatment with dimethyl talloworganosilicate that expanded the interlamellar gallery to at least 0.7 nm. Furthermore,the process led to intercalation, but not to exfoliation. PDMS-base CPNC was preparedby ultrasonication of organoclay (MMT-2M2HTA) with silanol-terminated-PDMS.The system was crosslinked using a mixture of tetraethyl orthosilicate (TEOS) and tin2-ethyl hexanoate. Exfoliation was facilitated by the presence of water at a concentrationcorresponding to about a monolayer on the clay surface [Burnside and Giannelis,1995; 2000]. In the latter publication exfoliation in the crosslinked PDMS/dimethyl-ditallow-MMT system was confirmed, but only intercalation (d001 increased from 2.5to 3.9 nm) was obtained when a copolymer, poly(dimethyl siloxane-co-diphenylsiloxane), with 14-18 mol% of diphenyl, was used instead of PDMS. Furthermore, noteven intercalation was achieved for either polymer with Na-MMT.

In another study, organoclay was prepared by reacting, e.g., Na-MMT with2M2ODA [Vaia et al., 1994] or Li fluorohectorite (FH) with ODA [Vaia et al.,1995b]. In the latter study, 25 mg of organoclay was mixed with 75 mg of PS andformed into pellet, which in turn was heated at T > Tg(PS) ≅ 100 °C. Meltintercalation took place in a quiescent system as a function of the annealingtemperature (Ta = 155-180 °C), annealing time (ta ≤ 400 min), and molecular weightof the intercalating PS (MW = 30-300 kg/mol). The extent of intercalation wasdetermined from XRD. Thus the interlayer spacing of organoclay (d001 = 2.13 nm)during annealing with PS (having MW = 30 kg/mol) progressively increased tod001 = 3.13 nm. The amount of polymer (Q) that diffused into the interlamellargalleries over time (t) followed the dependence:

Q t Q D a tm m

m

( ) / / exp /∞−

∞

= − −( ) ( ){ }∑1 4 2

1

α α (16)

where: αm is the m-th positive root of the Bessel zeroth-order function, and D/a2 is theeffective diffusion rate. Its temperature dependence was found to follow the Arrheniusequation with the activation energy ∂ln(D/a2)/∂(1/T) = Ea ≅ 166±12 kJ/mol. Themolecular weight dependence was found to follow the dependence: (D/a2) ∝ 1/MW1.6.The authors observed that intercalation by PS of commercial interest(MW = 300 kg/mol) at T = 250 °C should take about 10 min. The melt annealingwould not result in exfoliation. There is no information about how the terminalinterlayer spacing depends on the molecular weight of the intercalating polymer.

In the following contribution by the same authors [Vaia et al., 1996] thequiescent melt intercalation was conducted not only with PS (PS30 and PS400with MW = 30 and 400 kg/mol, respectively) but also with poly(3-bromo styrene)

145


(PS3Br; MW = 55 kg/mol). The microstructure of the organoclay/polymer systemswas examined by XRD and high-resolution transmission electron microscopy(HRTEM). In contrast to the averaging of XRD, HRTEM provides informationon local structure, spatial distribution of structural elements and defects.

When PS30 was used, the same interlayer expansion was observed as thatreported earlier, viz. d001 = 2.13 increasing to 3.13 nm. The HRTEM showed thepresence of ordered clay microstructures similar to that observed for non-intercalated (by PS) organoclays, but slightly expanded. The publication in partanswers the earlier question as to whether the final interlayer spacing dependson the polymer molecular weight. Partial intercalation by PS400 resulted insimilarly ordered structure as that observed for PS30, but the intercalated regionsexpanded more, viz. the measured interlamellar spacing of 2-3 nm, compared to2.17 determined for PS30. Since the unperturbed radius of gyration for PS30 andPS400 coils is: <r2

g>1/2 ≅ 5 and 18 nm, respectively, in neither case would the

interlayer spacing accommodate the Gaussian coil in 3D.Intercalation by PS400 was only partial. The HRTEM micrographs showed

domains of intercalated and of non-intercalated (by PS) clay platelets with diameterranging from 0.05 to 0.5 μm formed by stacks of 50 to 100 parallel platelets. Inthe intercalated and non-intercalated domains the number of layers in the stackwas similar – intercalation by PS just pushed the layers further apart. On thebasis of the HRTEM information it was concluded that polymer intercalationoccurs as a front that diffuses into organoclay interlamellar galleries from theexternal edges. The non-intercalated (by PS) domains were more prevalent towardsthe centre of the stack. Structural defects in the clay crystalline structure andirregularities in the layer stacking have been observed.

In contrast with the PS-intercalated layered stacks, intercalation with PS3Brresulted in exfoliation, evidenced by the lack of a well-defined XRD peak as wellas by the HRTEM micrographs. Evidently, the presence of -Br brings aboutstronger interaction between the intercalated clay and the polymer than thatexisting between clay and PS. Since the clay was intercalated with the primaryoctadecyl ammonium, interaction between -Br and either C18-NH3

+ or with -OHgroups at the platelets edges is possible. As observed under HRTEM, individualplatelets were intermixed with small stacks of 10 to 20 platelets with the interlayerspacing ranging from 2.1 to 6.0 nm. The heterogeneous microstructure indicatesthat the formation of these systems occurs by a more complex process than simplesequential intercalation of individual layers starting from the surface of the primaryparticle. The clay crystalline structure may contain defects or chemicalinhomogeneities that facilitate polymer transport or result in enhanced polymer-silicate interaction. Long-range electrostatic forces tend to maintain layer stacks.However, exfoliation most likely takes place layer-by-layer from the limits of theoriginal organoclay stack.

The earlier studies of PS/organoclay [Vaia et al., 1993] led to the conclusionthat PS macromolecules form monolayers inside the organoclay galleries. Thissignificantly changes the chain dynamics, e.g., resulting in absence of Tg for PSwithin the expected range of 50-150 °C. Evidently, the presence of the oniumcompound in the galleries must have influenced the macromolecular behaviour,but the effects of 2D steric restriction must have also affected the chain dynamics.

Krishnamoorti et al. [1996] melt intercalated lightly deuterated PEG(Mw = 180 kg/mol, Mw/Mn = 1.2) into Li-FH. The system was probed usingthermally stimulated current (TSC), XRD, DSC, SANS and solid-state NMR.

146


PEG chains were confined to 0.8 nm galleries. The 2H-NMR spectra were obtainedat the same temperatures for the bulk and the intercalated d-PEG. As thetemperature increased from 220 toward 250-270 K, a central peak of theintercalated d-PEG developed earlier and became narrow. Thus, the enhancedlocal chain dynamics of the intercalated d-PEG at low T may have resulted fromthe absence of chain entanglements. However, within the high T-range(T = 320-340 K), the intercalated d-PEG showed a broad base structure, whereasthe bulk d-PEG had a single narrow signal, indicating that the silicate layersrestrict macromolecular chain motion. Intercalation into the narrow interlamellarspace with height of 0.8 nm must have engendered interactions (hydrogenbonding?) between the PEG segments and the platelet surface. Furthermore, thenarrow space between the clay layers topologically restricted the d-PEG chains.

The DSC experiments on an intercalated CPNC indicated the absence of anythermal transitions of PEG; Tg or Tm. However, TSC suggested that the transition,which in bulk PEG takes place at about Tg = -55 °C, upon intercalation increasedto Tg ≅ 60 °C. The TSC peak for the intercalated system was broad and shallow,indicating a low level of molecular cooperativity near Tg.

The most comprehensive studies of the quiescent intercalation of organoclayby molten polymers included: several intercalating organic cations, synthetic andmineral clay, polymers (different chemically and with different molecular weight)and annealing temperature [Vaia, 1995; Vaia and Giannelis, 1997a,b]. Thematerial characteristics are given in Table 18.

noitalacretnitlemtnecseiuqrofscitsiretcarahclairetaM81elbaT]b7991,silennaiGdnaaiaV[

tnenopmoC edoC scitsiretcarahC

syalC FSM

iL + .cnIgninroC,93.0=a,5.1=CEC;etirotcehoroulf-iL + yalCnrehtuoS,85.0=a,0.1=CEC;etinopas-

stcudorPaN + ,iruossiM.U,1-ywS,27.0=a,8.0=CEC;TMM-

yrotisopeRyalCecruoS

muinommAstlas

81C281Q

Cn

)ADO2M2(edimorbmuinommalycedatcoidlyhtemiD)ADOM3(edimorbmuinommalycedatcolyhtem-irT

n C(sedirolhcmuinomma-lyklA- nH 1+n2 HN 3+ lC - =nhtiw,

)81dna,61-9,6

sremyloP 03SP09SP004SPrB3SPHCVP

PVP

M;enerytsyloP w M,03= w M/ n T,60.1= g 69= °CM;enerytsyloP w M,09= w M/ n T,60.1= g 001= °CM;enerytsyloP w M,004= w M/ n T,60.1= g 001= °C

M;enerytsomorb-3-yloP w M,55= w M/ n T,2= g 311= °CM;)enaxeholcyclyniv(yloP w T,79= g 021= °C

M;)enidiryplyniv-2(yloP w M,051= w M/ n ,1.2=Tg 401= °C

)g/qem(yticapacegnahcxenoitac=CEC:setoNmn(noitacrepaeraecafrusyalc=a 2)

Mw )lom/gk(thgiewralucelomegareva-thgiew=Tg (erutarepmetnoitisnartssalg= ° )C

147


As before, dry organosilicate (25 mg) and polymer powder (75 mg) weremechanically mixed and formed into a pellet using a hydraulic press at a pressureof 70 MPa. Melt intercalation was carried out by annealing the pellet in vacuumat T > Tg. Usually the samples were annealed to equilibrium, indicated by nofurther changes in the X-ray diffraction pattern. The work brought out severalimportant observations:1. Of the three clays the largest value of d001 was systematically observed for F

and the smallest for M, having, respectively, the highest and the lowest CECvalue. Note also that for the three clays there is a difference in the distributionof the anionic charge on the silicate surface. The increased localisation ofsurface charge for tetrahedrally-substituted S does not appear to affect CPNCformation when compared to the more dispersed surface charge ofoctahedrally-substituted M.

2. Li+-fluorohectorite (FH, clay F) was intercalated with the primary ODA (F18)and subsequently annealed with each of the three PS. XRD showed intercalationwith d001 = 3.11 nm. However, while the intercalation for PS30 was achievedafter annealing for less than 6 h at 160 °C, to intercalate with PS90 took 24 h,and PS400 more than 48 h. Since the XRD peak was quite similar for the threepolymers it was concluded that the kinetics of intercalation are strongly affectedby the polymer molecular weight, but that the structure of CPNC preparedunder quiescent conditions is independent of it.

3. The ammonium salt used in the cation exchange may have an importanteffect on intercalation by molten polymer. Two aspects are worth stressing:a. At T = 160 °C PS30 was unable to diffuse into the interlamellar galleries

of the F-clay intercalated with alkyl-ammonium chlorides having n ≤ 12.Intercalation with alkyl ammonium with n = 6 to 12 engendered thesame initial expansion of the interlayer spacing, viz. d001 ≅ 1.8 nm (seealso Figure 23), but apparently alkyls with n < 12 do not sufficientlyshield the hydrophilic clay surface to assure PS interpenetration.Annealing with PS30 at T = 180 °C expanded even F8 to the maximumspacing d001 ≅ 3.1 nm, i.e., increasing the gallery height by about 1.1 nm.This maximum value was found to be independent of the alkyl cation,of the annealing temperature, and of the PS molecular weight.

b. The difference between primary and quaternary ammonium cationscan be evaluated by comparing the intercalation of PS30 into FC18(primary) with that into FQ18 (secondary) and F2C18 (quaternary).For the three compositions the interlayer spacing was found to be,respectively, d001 ≅ 3.11, 3.64 and 3.80 nm. Thus, all three cationsfacilitated the secondary intercalation by PS, but different types ofammonium cation did influence the final gallery height. The differencecould originate in the difference in the ionic interaction strength for theprimary, secondary and quaternary onium ions as well as in the size ofthe quaternary group.

4. Melt intercalation also depends on the chemical nature of the intercalatingmacromolecules. Thus, in this series of styrene-derivative polymers withM2C18 the interlayer spacing increased with the relative acid/base characterand the side-group polarisability in the order: poly(vinyl cyclohexane) (PVCH)<< PS < PS3Br < PVP, giving d001 = 2.38, 3.20, 3.34 and 3.38 nm, respectively.Thus, secondary intercalation was observed for all polymers, except PVCH.

148


On the other hand, none of the polymers intercalated F2C18, leaving theinitial (after cation exchange) spacing, d001 = 3.80 nm, unchanged. It isnoteworthy that the latter spacing is larger than that observed for doublyintercalated MMT.

5. The kinetics of secondary intercalation under quiescent conditions inverselydepend on the molecular weight:Mm

w , where the exponent m = 1.6 to 2.0(exponent m = 2 is expected for pure chain reptation at T - Tg = constant).Such dependence suggests that secondary intercalation is related to the self-diffusion process. In the series of polymers: PS, PS3Br and PVP, even aftertaking into account the differences in MW, intercalation for PS requiredannealing for about 4 h, whereas that for the other two polymers for morethan 24 h. This again agrees with the self-diffusion behaviour theory, viz. thestronger the interchain interactions the smaller the self-diffusion coefficient[Kausch and Tirrell, 1989].

The question that remained unanswered is whether exfoliation in a quiescentmelt intercalation process is possible at all. Via and Giannelis [1997b] consideredexfoliation to be kinetically-limited, i.e., that transport of the clay platelets toeffect exfoliation would take a prohibitively long time, hence external forces areneeded (shear flow, ultrasonication, etc.). This idea found support in the workpublished three years later [Huang et al., 2000] which focused on the preparationof CPNC with PC as the matrix. The organoclay was MMT cation exchangedwith 2M2TA. The secondary intercalation was performed using either a polymer(PC; Mw = 25 kg/mol) or a cyclic carbonate oligomer (OCC; Mw = 2 kg/mol).The quiescent samples were prepared either in solution or in the melt. Thus,0.5 g of organoclay was added to a solution of 10 g of either OCC or PC in50 ml of CH2Cl2. After 5 min at room temperature, the solvent was removedunder vacuum and the residue was dried in the vacuum oven at 50 °C overnight.As before, 10 g of either OCC or PC was mixed with 0.5 g of organoclay, pressedinto pellets with a hydraulic press and heated to 180 °C in a vacuum oven for 1 hfollowed by heating at 240 °C for 15 min.

XRD showed that solution or melt intercalation with PC produced the basalspacing d001 = 2.47 nm, virtually identical to that of organoclay. Several hours ofmixing would be required to increase the interlayer to d001 = 3.27 nm. Intercalationwith OCC was more successful as it caused the interlayer spacing to rapidlyincrease to d001 = 3.62 nm. Thus, as was the case with styrene-derivative polymers,here also the difference between the oligo- and poly-carbonate amounted to thekinetics of intercalation. The difference in chain structure (cyclic versus linear)may have also contributed to the intercalate formation. Next, the melt intercalated(with OCC) organoclay was added to an internal mixer and processed at 100 rpmat 180 °C for 1 h, then at 240 °C for 10 min, causing ring-opening polymerisation.XRD of the material showed no peak and TEM revealed that a partially exfoliatedstructure was obtained. However, when PC was similarly mixed, only anintercalated system was obtained.

In conclusion, there is still no clear answer to the question as to whetherexfoliation in a quiescent melt intercalation process is possible. In the case of aPC/organoclay system the initial spacing was small, which indicated an absenceof PC in the interlamellar galleries, hence exfoliation would not be expected. Forthe OCC/organoclay system two elements worked in favour: (1) to start with,much larger interlayer spacing, and (2) polymerisation that engendered

149


macromolecular coil formation that expanded the interlayer space the same wayas reported for the reactive intercalation of PA-6 (see Figure 25).

As will be shown in Section 3.1, the free energy of mixing as a function of theinterlamellar separation height may have a local minimum, thus intercalationbut not exfoliation is the thermodynamically favoured state. Fundamentally, flow(energy) should help, but the magnitude of the imposed stresses must becomparable to the attraction forces between clay platelets which hyperbolicallydecrease with the interlamellar gallery height (see Figure 18). Thus, the shearstress of about 1 kPa would not be sufficient to significantly affect the interlamellarspacing from 2 nm, but conceivably so from 4 nm.

2.3.8.2 Dynamic Melt Intercalation

The terms ‘intercalation’ and ‘exfoliation’ refer to the degrees of dispersion oflayered silicates in a matrix. For maximisation of the filler effects on CPNCperformance (e.g., such as the barrier properties or the tensile/compressivemodulus) the maximum degree of dispersion is desired. Thus, intercalation isonly an intermediate step that is expected eventually to result in exfoliation – themain goal of CPNC manufacturing. Hence, melt intercalation is a process thateither was designed to lead to intermediate products (for example, to study theeffects of different process parameters) or that failed to achieve the principalobjective – exfoliation.

It was pointed out by Lan et al. [2000] that melt exfoliation by polar, water-soluble monomers, oligomers or polymers may take place in the presence of atleast 10 wt% H2O. The process may increase the interlamellar spacing all theway to total exfoliation once a minimum interlamellar gallery space is created byan exchange reaction of Na-MMT with organic cations. Similarly, melt exfoliationof a neutral P4VP was found feasible if an acidic clay or suitably modified claywas used as the inorganic component.

Dynamic intercalation and exfoliation of hydrophilic polymers (e.g., PEG,EVAc, PVP, etc.) was mentioned earlier in Section 2.3.5.2. Further details will bediscussed in Section 4. Here only melt intercalation in hydrophobic systems willbe considered.

2.3.8.2.1 Melt Mixing

Melt intercalation is part of polymer mixing and compounding operations. Thislaminar flow, in which the streamlines are smooth, comprises two types of flow:dispersive (or intensive), which involves the application of stresses that breakdomains of the dispersed phase to the desired size, and distributive (or extensive)that involves homogenisation of a fluid, accomplished by the application of strain.Melt intercalation is mainly concerned with dispersive aspects, but homogeneousdistribution of intercalated platelets or tactoids must also be achieved.

Virtually all the dispersive mixers (see the following Section) employ the shearflow field that contains the vorticity stress components causing rotation of theflow elements. This causes the aggregates to be repetitively stretched andcompressed. The process is inefficient, requiring the use of large amounts ofenergy for little effect. Better efficiency can be achieved in the extensional flowfield, but generation of these flows is more difficult [Utracki and Shi, 2002; Utrackiand Luciani, 2000]. There is no theory capable of describing the dispersiveprocesses in viscoelastic media, or even one that provides explanation for the

150


divergent observations. The complexity of the problem involves three-dimensionality, free surface, and non-stationary flow leading to complexconstitutive equations. The role of melt elasticity is not well understood and theaspect of nonlinear viscoelastic behaviour in complex flow is mostly unknown.

An elongational flow field exists anywhere where the streamlines are notparallel. This type of deformation is quite common during processing, viz. infibre spinning, film blowing, blow moulding, biaxial and uniaxial stretching andsimilar operations carried out downstream from the die. However, these operationshave little to do with mixing. Nevertheless, the extensional flow field has beenused to generate orientation either in homopolymers or in filled or reinforcedsystems. Convergent flow results in high fibre alignment in the flow direction,whereas diverging flow causes the fibres to align 90° to the major flow direction.Shearing reduces the alignment. Progress in the fundamental understanding ofmicrorheology led to designing a fully adjustable extensional flow mixer (EFM)and its dynamic version, the DEFM, now available on the market [Song, 2000].The use of an EFM for dispersing nanoparticles in polymeric matrix has beensuccessful in several research laboratories.

Mixing provides enhanced spatial homogeneity of a system, but assessmentof the quality of a mixture is difficult and time consuming. In a binary mixture ofA and B, the local concentration (in weight fraction) can be expressed as: a(x) +b(x) = 1 (where x denotes vectorial location). Similarly, the sum of the averagecompositions: aav + bav = 1. For such a system, the mixedness can be described bythe binary frequency function. For fine dispersions, variance of the composition(σ2

a), and the intensity of segregation (Is), may be defined as [Tucker, 1991]:

σ σa av s a av ava x a and I a b2 2 2= − =[ ]( ) / (17)

2.3.8.2.2 Mixing Equipment

As summarised in Table 19, the melt mixers are either batch or continuous type.The former require lower investment cost, but are more labour-intensive, havelow output and poor batch-to-batch reproducibility. Recent developments inprocess control and automation have eliminated some of these disadvantages.Continuous melt mixers comprise: extruders, continuous shaft mixers andspeciality machines. Several overviews of melt mixing are available, viz. [Utrackiand Shi, 2002; Tucker, 1991; Manas-Zloczower and Tadmor, 1994; Rauwendaal,1998], etc.

Batch mixing is not efficient for handling large capacities, but it is well suitedfor short-runs, e.g., for the manufacture of colour-concentrate masterbatches, orproducts that have to have tailored identity. Because of the high stresses, aninternal mixer can complete a cycle in minutes. The residence time, shear,temperature and, frequently, pressure may be controlled. These processingparameters may be of critical importance while compounding heat-sensitivematerials or dispersing nanofillers. The most popular batch-type mixers are thelaboratory type manufactured by, e.g., Brabender or Haake, and their homologueson the larger scale manufactured by, e.g., Banbury or Moriyama as dispersionmixers.

The most serious drawback of the laboratory internal mixers is the heatconduction by the mixing shafts. The mixer chamber is only heated from theoutside and the shafts conduct the heat away from the molten polymer.

151


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During mixing at relatively low temperatures and relatively high rotational speedof the mixing shafts, a dynamic equilibrium may be reached, i.e., the heat fromthe outside heaters is transferred to the melt, then to shafts that conduct it to theinternal drive system of the mixer. The dynamic equilibrium means that the highertemperature is at the chamber wall and lower on the shafts’ surface. The difference,ΔTchamber, of these two temperatures depends on the temperature difference betweenthat desired within the mixer and the ambient, the heat conductivity of the mixedcompound and shaft material, on screw speed, etc. Under the conditions securinga rapid exchange of material within the mixer chamber, good mixing may reduceΔTchamber. However, while compounding the engineering or speciality polymerswith high Tg or Tm, the temperature drop across the chamber was determined tobe as large as ΔTchamber ≈ 100 °C.

Continuous mixing involves the continuous loading and unloading ofcomponents. When properly performed, mixing decreases the compositionalvariations to the desired level. Continuous operations have the advantage ofproviding a stable process. The power consumption is usually lower than in batchmixing. The stresses are imposed systematically, either in the shear or in theshear-and-elongation mode of deformation. In spite of high capital costs andcomplex mixing, continuous mixing is easy to justify on the basis of the productionvolume (mixers with throughputs of up to 80 ton/h are available) and quality.The continuous mixers make it possible to control: the feed rate, screw speedand temperature, as well as the discharge orifice setting and temperature, but theaccessible range of residence times is small.

Extrusion is one of the most important forming methods in polymer processing.Virtually all polymers go through an extruder at least once, viz. compoundingand pelletising reactor powders. The extruders are classified according to theprincipal element of their construction, as:

• Single-, twin-, and multi-screw extruders.• Single-, twin-, and multi-shaft compounders.• Gear or disk extruders, e.g., Maxwell Melt-elasticity Extruder, Tadmor’s Disk

Extruder.• Special extruders, e.g., Gelimat, Patfoort, etc.Single-screw extruders (SSE) are relatively inexpensive machines for small ormedium size production lines. They are difficult to scale-up, notoriously poormixers with broad residence time distribution (excepting the case of plug flow ofcompositions with the yield stress) and relatively long residence time. Over theyears, SSEs have been made more versatile by the introduction of special mixingscrews, by using add-on mixing devices, by utilisation of two or more extrudersoperating in tandem, etc. The SSE operates under fully flooded conditions, thusthe quality and throughput depend on the screw speed, and to a lesser extent, onfeed. A summary of performance criteria of SSE and TSE is presented in Table 20.

Mechanically, a major difference between SSEs and TSEs is the type oftransport that takes place within the extruder; in the former it is drag-induced(frictional drag in the solid conveying zone and viscous drag in the melt conveyingzone). There are many materials with unfavourable frictional properties thatpose severe feeding problems. In intermeshing TSEs the transport is by positivedisplacement. Its degree depends on how well the flight of one screw closes theopposing channel of the other screw. The most positive displacement is obtained

153


in a closely intermeshing, counter-rotating geometry. In SSEs the velocity profilesare well defined and fairly easy to describe, while in a TSE the flow is considerablymore complex, which results in several advantages, viz. good mixing and heattransfer, large melting capacity, good devolatilisation capacity and control overthe stock temperatures. However, the theory of TSE is not nearly as well developedas that of SSE.

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By contrast with the SSE, the TSE operates with partially filled screws. Thepolymer from a feeder is transported toward the pressure zone, usually createdby a flow restrictor, e.g., a left-handed or reverse screw element. Under the pressurethe resin pellets are compressed into a solid plug, which fractures and meltsmainly by friction between its fragments and the wall. Thus, melting in a TSE ismore efficient than that in a SSE – its length can be as short as less than onescrew diameter, L ≤ D. There is evidence that melting in a TSE affects thegeneration of polymer blend morphology differently to that in a SSE. The modulardesign of screws and barrel is universally accepted. Many TSE manufacturersstill insist that to control the dispersive-to-distributive mixing ratio simply thewidth of the mixing block should be used – a set of narrower blocks results inmore distributive, and less dispersive mixing and vice versa.

As shown in Table 21, seven types of TSE are possible; of these three (markedin bold below) are the most popular in the plastics industry:

I. 1A, screw: L-open & C-closed, discs: L& C-open (CORI - the most popular).II. 1B, L & C-open (rare)III. 1C, L & C-open (rare)IV. 2A, L & C-closed (ICRR)V. 2B, L & C-openVI. 2B, L-open & C-closed (rare)VII.2C, L & C-open (CRNI)For many years, CORI has been the compounder of choice. Its advantage arisesfrom the movement of the intermeshing surfaces in opposite directions, thus themelt free surface is continuously renewed and the screws clean each other. Inaddition, since at the intermeshing the material passes from one screw to theother (change of the drag direction) there is low probability that the materialwould go through the gap. Thus, there is no calendering pressure that may causethe screws to bend. As a consequence, CORI has been able to operate at higherscrew speeds with longer barrel than ICRR, which is considered advantageousespecially for reactive processing. However, during the last ten years or so, severalmanufacturers of these two types of extruders have demonstrated that in unbiasedtests ICRR may equal or even outperform CORI as far as the compound qualityand the throughputs are concerned. Even at lower screw speed ICRR frequentlyshowed better performance than CORI. This resulted in renewed interest inredesigning ICRR, e.g., providing it with higher screw speeds and longer barrels.

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Originally ICRR was developed as a positive displacement screw pump for viscousfluids. This is the only TSE type that is fully, axially and radially, closed. Theextrusion speed depends on the intermeshing geometry and the screw speed. Thefully intermeshing ICRR has a narrower distribution of residence times and betterprecision in controlling rapid reactions between liquid reagent and molten polymerthan a CORI. The low speed ICRRs have been used for PVC compounding andforming. At higher screw separations and speeds, the machines can be used forthe incorporation of high viscosity toughening elastomer. To make ICRR fullycompetitive with the more popular CORI the calendering gap between screwswas widened, reducing the high stresses and increasing the intensity of shearingbetween the screws and the barrel. For some applications, ICRR offers the uniqueadvantage of a strong extensional flow field, able to disperse high viscosityingredients in a low viscosity matrix. A summary of the relative merits of CORIand ICRR is given in Table 22.

In CRNI the material flow is based on a drag, not positive pumping. There is alow shear stress field, responsible for the absence of dispersive mixing. However, theinterchange of material between the screws provides good distributive mixing. Thechemical reaction proceeds on the continuously renewed surfaces, related toreorientation of the laminar flow patterns and the total strain. CRNI is well suitedfor the polymerisation of miscible, low viscosity systems. The main advantage ofCRNI is a long enough residence time, sufficient to complete slow processes. CRNIcan offer larger outputs, and more interchange of material between the two screwsthan the other types of TSEs. These machines are frequently used for the preparationof composites with fibrous fillers. It is noteworthy that the closer the clearances andintermeshing the more rapid the build-up of pressure, the narrower the lands and thelarger the clearances between screws, the greater the longitudinal mixing.

Within a TSE the shear and extensional stresses are generated by interactionsbetween two screws. The stress magnitude depends on the relative direction of thescrew rotation (co- or counter-rotating), shapes of the screw elements, the degreeto which they intermesh, and the rotational speed of the screws. The modulardesign of a TSE makes it possible to adjust the relative magnitude of the distributiveand dispersive mixing. The TSE performs a series of different functions. There is acontinuous pressure toward bigger machines with higher ‘free volume’, with longerbarrels and higher screw speeds. CORI with L/D = 100, D ≤ 455 mm and screwspeeds of up to 2500 rpm are commercially available [Sakai, 2002].

Recently Nanocor published a CORI screw design for the manufacturing ofCPNC with PO as the matrix [Qian et al., 2001]. A Leistritz TSE with D = 27 mmand L/D = 36 was used at 300-500 rpm and T = 170 to 190 °C (see Figure 30 andTable 23) [Nanocor Technical Services Group, 2001]. Two vents at 13D and24D were used. The melt exfoliation involved a two-step process:

1. Preparation of masterbatch containing 50-60 wt% of organoclay acompatibiliser and matrix polymer (e.g., PP-MA and PP), and

2. Dilution to the desired clay content (e.g., 2-7 wt%).Alternatively, a single-pass procedure can be used replacing the first vent with aside stuffer, through which the diluting quantity of matrix polymer is fed [NanocorTechnical Services Group, 2001].

Besides the ‘classical’ extruders (SSE and TSE) there are several less well-known compounders. Of these the planetary roller extruder is the oldest. Here,six or more evenly spaced planetary screws revolve around the circumference of

156


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the central or so-called ‘sun’ screw. The planetary screws intermesh with the sunscrew and the barrel. The planetary barrel section has helical groovescorresponding to the helical flights on the planetary screws. This section is usuallya separate barrel with a flange-type connection to the feed barrel section. Theplanetary roller extruders are mostly used for processing heat-sensitivecompounds, viz. rigid or plasticised PVC formulations.

More recent are the disk or screw-less extruders, based on the viscous dragtransport principle. To this category belong: stepped disk, drum, spiral Diskpackextruders and many others. Diskpack has the inherent capability of performingthe elementary steps of plastics processing by combining differently shapedrotating disks in a drum-like housing. Stationary channel blocks that cause thematerial to transfer from one disk-gap to another provide the wiping action.Melting, laminar mixing, venting, and the pumping functions are all separated.Diskpack has been used for reactive processing, blending, compounding, mixingand devolatilising [Tadmor et al., 1983].

In the early 1980s Carlew Chemicals developed Gelimat, later re-namedK-mixer (K- for the kinetic energy). This mixer uses a high-speed rotor withstaggered blades mounted on a horizontal shaft at different angles. The numberand position of the blades vary with the mixer size. The mixing is carried out atthe blade tip velocity of 30-45 m/s. There is no external heating – the kineticenergy generated by the particles impacting on each other and the mixer elementsproduces sufficient heat to flux the material within 8 to 150 s. Since polymershave low thermal conductivity, by varying the initial particle size it is possible tomelt only the skin of a particle or engender uniform melting. Once the materialreaches the desired temperature, the infrared sensor activates the bottom doorsof the chamber. The discharged dough can either be fed to a short screw extruderand pelletised, or be passed between rolls and diced. High mechanical propertieswere obtained by compounding in Gelimat PP with non-intercalated mica flakes.

To improve the distributive mixing often a static mixer (SM) is used (seeTable 24). It operates on the principle of repetitive dividing of a flow channelinto at least two new channels, reorienting them by 90°, and dividing again duringlaminar flow. Mixing by SM is related to the number of striations (Ns) generatedby a number of SM elements (ne) and the number of divisions (new channels)engendered by each element (nc):

N ns cne= (18)

The efficiency of a SM is determined by comparing:

1. The length-to-diameter ratio (L/D proportional to ne) required to producethe same degree of homogeneity;

2. The associated pressure drop, ΔPrel;3. The holdup volume, ΔVrel;

4. The relative dimensions of the device, Drel and Lrel.

The efficiency of a SM also depends on the type of liquid [La Mantia, 1996].

2.3.8.2.3 Mixing in an Extensional Flow FieldThe mixers discussed so far employ mainly the shear stress field. However, it hasbeen shown that the extensional flow field is more efficient as far as the energyconsumption per unit strain is concerned, as well as the rate of dispersive and

159


distributive mixing [Utracki and Shi, 2002; Luciani and Utracki, 1996]. Inprocessing equipment extensional flow requires non-parallel flow lines. In a SSEextensional flow is only present in specially designed mixing elements, in add-ontorpedoes or in the convergent flow into the die. In a TSE extensional flow takesplace within the kneading disks. However, even here only about 8% of mixingenergy is used to generate an extensional flow field – the rest being consumed byshear. The extensional flow mixer (EFM) is the only device that deliberately utilisesthe elongation flow for mixing.

The EFM is a fully adjustable, general-purpose, motionless mixer. The mixingaction is provided by the development of an extensional flow field through aconvergent-divergent (C-D) flow geometry. It is primarily a dispersive mixer thatshould be attached to a pressure-generating device. It efficiently homogenisesdifferent liquid systems, even those where the components’ viscosity ratios arelarge. The mixer attached to a SSE provided comparable or better mixing thantwo TSE machines from different manufacturers, equipped with high mixingscrew geometry. The C-D channels are of progressively increasing intensity, withflow in the radial not axial direction. To reduce the pressure drop, and to preventblockage, slit restrictions are used [Nguyen and Utracki, 1995]. A more recentpatent describes modifications of the original EFM design as well as its dynamicversion, the Dynamic Extensional Flow Mixer (DEFM) [Utracki and Luciani,1997]. EFM has been used for: polymer blending, incorporation of elastomersinto resins, and dispersion of high viscosity resins or ‘gel particles’ [Utracki andLuciani, 2000; Luciani and Utracki, 1996] as well as for dispersing organoclays.

Recently, preliminary tests were conducted to evaluate the suitability of EFMfor CPNC manufacture. The tests were conducted using the available device,

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160


optimised for melt blending not for dispersing nanosized solid particles.Nevertheless the results in three different laboratories have been encouraging.The tests were conducted attaching an EFM to a CORI by a gear pump [Garcia-Rejon and Simard, 2002]. The modulus of the resulting CPNC (PET with 5 wt%organoclay) was twice as large in the presence of an EFM than without it. CPNCcontaining PP with maleated-PP and organoclay were compounded using eithera TSE + EFM or a SSE + EFM. In both cases the presence of an EFM resulted insignificantly better dispersion of the clay platelets, as shown in high resolutionTEM [Song, 2002].

2.3.8.2.4 Melt Intercalation in a PA MatrixMost processes that are labelled as ‘melt intercalation’ start with pre-intercalatedclay. For example, Maxfield [1996] conducted melt intercalation in a TSE startingwith PA-6 pellets and MMT whose interlayer cations were replaced by 2M2ODA.The process was facilitated by addition of a silane coupling agent that partiallyreplaced the onium cations. The use of organosilanes of the type: RnSiX4-n (n = 1-3,with R = organic radical having C-Si link and X is hydrolysable alkoxy, acryloxy,amino or halogen group) had been proposed more than a decade earlier [Eilliottand Beall, 1989]. For example, Na-MMT (CEC ≥ 0.75 meq/g) was intercalatedwith quaternary ammonium ion and treated with a ‘sizing agent’, used inreinforced plastic manufacture [Plueddemann, 1982; Petrarch Systems, 1987],e.g., methyl trimethoxy silane.

The ammonium-modified clay may be used to prepare PNC at T < 250 °C.With the exception of some elastomers, there are few polymers that can beprocessed at these temperatures. The thermal stability of ammonium-modifiedclay systems may be improved by extracting an excess of the intercalatingammonium salt and/or by reacting it with, e.g., epoxies [Ton-That, 2000]. Inseveral patents, ammonium, sulfonium and phosphonium salts are listed [Hauser,1950; Finlayson and Mardis, 1983; Ellsworth, 1999]. Phosphonium-clay saltswere reported stable to 370 °C, but this increased thermal stability does notseem to translate into better (mechanical) performance.

CPNCs formed by melt blending have been described by Christiani andMaxfield [Christiani and Maxfield, 1998]. First MMT was intercalated usingdipentyl ammonium chloride. The authors claim that the MMT-ammoniumcomplex with secondary amine was more thermally stable (onset of decompositionat T = 275 °C) than that with either tertiary or quaternary ones. The meltintercalation of the onium complex with PA-6 was conducted using a LeistritzTSE at T = 200-250 °C and a screw speed of 250 rpm. The extruder was fed bya dry-blend of PA-6 pellets with w = 2.95 wt% of onium-clay dried powder.There is no information on the degree of dispersion achieved, but the CPNCshowed improvement (values for neat PA-6 are given in parentheses) of tensilemodulus: E = 3.4 GPa (2.4); tensile strength: σ = 80 kPa (66); ultimate elongation:ε = 44 % (26); and deflection temperature under load: DTUL = 72 °C (55).Judging by these numbers, MMT was probably well intercalated, but notexfoliated.

Liu et al. [1999] prepared CPNC by melt intercalating PA-6 with up to 10.5 wt%of organoclay (MMT-ODA) in a slowly rotating (30 rpm) TSE at T = 180 to220 °C. XRD of injection-moulded specimens indicated that during processingthe interlayer spacing increased from d001 = 1.55 to 3.68 nm. Furthermore, thearea under the XRD peak decreased after mixing hence some exfoliation probably

161


took place. The incorporation of clay affected the crystalline form of PA-6 –whereas in neat polymer only the α-form was present, after compounding withorganoclay the γ-form was also found. A good set of properties was reported.

Dennis et al. [2000; 2001] carried out fundamental studies of melt intercalationin a PA-6 (Mn = 30 kg/mol; MI = 12) matrix. In the study 5 wt% of Cloisite® 15A(C15A; MMT-2M2HTA) or Cloisite® 30B (C30B; MMT-MT2EtOH) (organoclayproperties are listed in Table 16) were used. A SSE and several TSE machineswith different screw combinations were employed. The organoclay powder(particle size d ≅ 8 μm) was dry-blended with the PA-6 pellets. The mineral contentin the CPNC generated from C30B and C15A was 3.7 and 3.1%, respectively.However, since melt blending of PA-6 with C30B resulted in easy exfoliation, thestudies were conducted with C15A.

Experimental studies of melt intercalation were carried out using four extrudertypes: SSE, CORI (low and medium shear stress screw configurations), ICRR(low medium and high shear stress screw configurations), and CRNI (low mediumand high shear stress screw configurations). The effects of mixing were evaluatedby XRD, TEM and tensile tests. It was found that the XRD peak position did notchange much (d001 = 3.2-3.8 nm), but its intensity varied, indicating diversity ofthe dispersions. TEM micrographs were used to quantify the degree of dispersion(DD) by counting the number of clay platelets or their stacks in twelve, 25 ×25 mm squares. Evidently, the higher the number the more dispersed is the CPNC.The results of this evaluation are shown in Figure 31.

Clearly the data from different mixing devices can be approximated to r2 = 0.86by a simple two parameter equation:

DD t R= × × ={ }4 37 0 00972 0 86. exp . ; . (19)

Figure 31 Degree of dispersion as a function of the average residence time, t , inseveral types of TSE with screws: corotating intermeshed (CORI), intermeshed

counter-rotating (ICRR), counter-rotating non-intermeshed (CRNI). Data [Denniset al., 2000]. The broken line follows the dependence: DD = 4.37 exp{0.0097 t}

with the correlation coefficient, r = 0.86.

162


Better correlation may be found when not only t , but also the variance of theresidence time distribution (σ 2a ; see Equation 17) is considered [Dennis et al.,2000]:

DD t t Ra a= − − + + − =4 2 0 21 564 0 0017 4100 0 982 2 2 2. . . ( ) ; .σ σ (20)

Thus, on face value, these results indicate that not the stress, but the residencetime and its distribution determine the degree of intercalation/exfoliation.However, the experiments were conducted at a single throughput hence a singlevolumetric flow rate. Thus, during these tests the residence time should be ameasure of relative strains. Furthermore, there is a correlation between the TSEconfiguration, operational parameters and the residence time and its distribution– the low-shear stress screw configuration is associated with shorter residencetime and narrower distribution. The authors remarked that in each of the TSEsthe best dispersion was obtained using medium intensity screws, and that thebest overall dispersion was obtained using low stress CRNI that provided longresidence time. It is noteworthy that for a given TSE type, the highest shearintensity configuration did not give the best dispersion.

These observations support earlier remarks by Vaia [1995] regarding diffusion-controlled intercalation. However, the shear stress during extrusion significantlyaccelerated the intercalation processes. The shear field seems to remove individualhighly intercalated platelets from the top and bottom surface of the stacks, thusfacilitating macromolecular diffusion into the remaining galleries. The proposedmechanisms present during the dynamic melt intercalation are:

1. Diffusion-controlled, sequential intercalation and peeling of platelets,2. Shear-related breaking of organoclay particles, and3. Peeling off of organoclay platelets, one at a time, from the top and bottom

layers of individual stacks.The data on PA-6/Cloisite® do not support the particle breaking mechanism (2),but at high enough stresses the clay aggregates or stacks must be subjected toattrition. Such a mechanical dispersion by means of high energy shearing (e.g., in aGaulin mill) was described in several patents (mainly from SCP) [Knudson andJones, 1986; 1992a,b; Jordan, 1994; 1996; Dennis, 1998; Gonzales, 1998; Farrow,1998; Powell, 2001a,b]. Since intercalation is diffusion controlled, reduction ofplatelet diameter shortens the diffusion time.

The mechanical properties versus DD are shown in Figure 32. Judging by thevalue of the correlation coefficient, r, a correlation exists for tensile modulus andstress (r = 0.82 and 0.75, respectively), but not for the impact strength orelongation at break (r = 0.16 and 0.07, respectively). Thus, modulus increasedlinearly with DD from E = 2.7 for PA-6 to 4.0 GPa for CPNC. Note that the fullyexfoliated PA-6/Cloisite® 30B, system showed E = 4.4 GPa. The yield stress, σ,depends on dispersion to a lesser degree, but clearly shows a positive effect,while the remaining two parameters (elongation at break and notched Izod impactstrength at room temperature) are independent.

In a related paper, molecular weight effects on melt intercalation inPA-6/organoclay systems were studied [Fornes et al., 2001]. Three PA-6 resinswere used with two levels of clay loading (see Table 25). The clay was Na-MMT(CEC = 0.92 meq/g) intercalated by SCP with methyl-rapeseed-bis(hydroxy-ethyl)ammonium chloride (MR2EtOH). Rapeseed is a natural product containing

163


Figure 32 Mechanical properties of PA-6/Cloisite® 15A as a function of thedegree of dispersion. Data [Dennis et al., 2001].

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0.22 1.3 7.1 4.76 8.16 112 371.0

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saturated and unsaturated C18 to C22 hydrocarbons. Melt intercalation wasperformed using a CORI (D = 30.5 mm, L/D = 10). The screws were assembledto achieve ‘an optimum level of shearing and residence time’ at T = 240 to 280 °C.The CPNC specimens were injection moulded, and then characterised by XRDand TEM, as well as tested for mechanical and rheological performance. The

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degree of dispersion, DD, was determined in a similar way as described by Denniset al. [2001], but the specimen thickness was smaller, the electron beam voltagewas higher and the method of calculation was different, thus the new set of datacannot be directly related to the old one. Furthermore, the organoclay used inthese studies was similar to Cloisite® 30B not Cloisite® 15A explored before,thus the degree of exfoliation was much higher, especially for the high and mediummolecular weight PA-6.

Fornes et al. [2001] calculated the specific particle dispersion per squaremicrometre of the surface of a CPNC specimen and 1 wt% of the inorganiccomponent: Sp = DD/C, where DD is expressed as the number of individualplatelets or short stacks (2-3 platelets) per 1 μm2, and C (wt%) is the concentrationof the inorganic part of the organoclay in the specimen. To extract the effect ofmolecular weight on the melt intercalation process, the Sp data in Table 25 werefitted to:

S DD C a a C a Mp n≡ = + +/ log ( )0 1 2 10 (21)

The linear regression gave the correlation coefficient squared, r2 = 0.991, hencethe postulated dependence represents the reported data reasonably well. Theanalysis gave the following parameter values: a0 = -231 ± 48, a1 = 9.5 ± 5.7, anda2 = 196 ± 33. Note that since the equation parameters, a1 and a2 are positive thespecific degree of dispersion increases with clay content and the molecular weightof the matrix, thus with the matrix viscosity. If the melt intercalation were diffusioncontrolled, the value of the a2 parameter should be negative! Thus, the dataindicate that contrary to expectation, the higher the molecular weight of PA-6the easier the dispersion. This conclusion is supported by the TEM micrographsshown in Figure 4 of the original paper [Fornes et al., 2001]. The CPNC witheither high or medium MW PA-6 have homogeneously exfoliated structures with,on average, 1.3 or 1.5 platelets per stack, whereas that with low MW polymerhas large residual stacks and 2.4 platelets per stack.

The positive effect of MW on the mechanical dispersion process is even morestriking when the differences in the residence time and the residence timedistribution are taken into account. Using the values listed in Table 25, DD wascomputed from Equations 19 and 20. The results plotted versus MW showed astrong negative effect of MW on DD:DD∝Mn

-1.6, in agreement with the diffusion-controlled mechanism. The only conclusion from these seemingly contradictoryobservations is that during melt intercalation in a compounding machine twosimultaneous processes take place: thermodynamically-driven diffusion ofmacromolecules into the interlamellar galleries, and mechanical peeling off ofplatelets or short stacks in which the diffusion-controlled intercalation sufficientlyreduced the solid-solid interactions between the adjacent clay platelets. It isnoteworthy that the zero-shear viscosity of the three polyamides varies nearly bya factor of 8, thus higher shear stresses are acting on weakened stacks in highmolecular weight matrix. Since the organoclay value, d001 = 1.8 nm is small, thediffusion of macromolecules must rapidly expand it for the shear stresses to beeffective.

Cho and Paul [2001] used SSEs and TSEs to prepare CPNC with 5 wt% ofMMT-M2REtOH in PA-6 as a matrix. The residence time distributions for theSSE and TSE were quite different, narrow with a peak at about 2.5 min for theformer and very broad with a peak at about 5 min for the latter, just opposite towhat one could expect for flow of polymer melt through SSE and TSE. This may

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be caused by the plug flow of CPNC through SSE. XRD of the organoclay showeda strong peak at d001 = 1.8 nm – compounding in a SSE reduced the peak to abarely discernible shoulder, while that in a TSE pushed it to low angle values,2θ < 2°, i.e., d001 > 4.4 nm. The presence of short stacks in both products indicatedthat exfoliation was not complete. Improved mechanical properties were reported,e.g., addition of 3.16 wt% of inorganic clay resulted in 38% higher modulus and30% higher strength (than PA-6) keeping the Izod impact strength about thesame.

Intercalated and exfoliated CPNC of PA-6 with organoclay were prepared in aTSE at T = 250 °C [Varlot et al., 2001]. The organoclay was Na-MMT cation-exchanged with methyl octadecyl bis-2-hydroxyethyl ammonium methyl-sulfate(MODA2EtOH) and dimethyl dioctadecyl ammonium chloride (2M2ODA), whichresulted in the interlayer spacing of d001 = 1.96 and 3.68 nm, respectively. Aftercompounding, the CPNC specimens were found, respectively, exfoliated andintercalated into short stacks containing 2-4 MMT platelets. The nanocompositeswere anisotropic. The preferential orientation of the MMT platelets as determinedby SAXS was parallel to the flow direction during injection moulding. The orientationof the MMT plates and the PA-6 lamellae is expected to play a major role in themechanical properties. In the CPNC the matrix crystallised preferentially in theγ-form.

2.3.8.2.5 Melt Intercalation in PEG MatrixIn his doctoral thesis of 1995 Vaia [1995] described the preparation of polymerelectrolyte nanocomposites for batteries. Thus, PEG was melt intercalated intoNa-MMT or Li-MMT with CEC = 0.8 meq/g. The process was carried out bygrinding a mixture of, e.g., 28 wt% of PEG (MW = 100 kg/mol) with 72 wt%Na-MMT, then compressing it into a sheet at 80 °C for up to 6 h. The final d001spacing was 1.77 nm, the same as that obtained in the solution intercalationmethod. Considering the high solid loading larger interlayer spacing could notbe expected. Judging by the cited patents from AMCOL, the intercalation processis facilitated by the presence of water (from 6 to 10 wt%), and increasing thePEG content may increase the interlayer spacing.

2.3.8.2.6 Melt Intercalation in PO MatrixDevelopment of CPNC with a PA as a matrix has been carried out since the late1960s. Owing to the polar nature of these polymers and advances in the chemistryof the ammonium ion, at present melt exfoliation in the PA-matrix is conductedon an industrial scale. By contrast, the melt intercalation of clays in a PO-matrixis at a relatively early stage of development. The information from the patentliterature will be discussed later. Here the focus will be on the more basic aspectsof PO-melt intercalation technology.

By contrast with PA or PEST, POs are nonpolar, nonreactive and sensitive tothermomechanical oxidation on the tertiary carbon by the free radical mechanism.The fundamental question remains whether it is at all possible for POmacromolecules to diffuse into the hydrophilic, repulsive environment of theclay interlamellar galleries.

As with PA, here also most of the work involves MMT modified with organiccompounds. Because of the difficulties encountered during polymerisation in thepresence of clay [Heinemann et al., 1999], melt intercalation is the preferred

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technique. The large resin manufacturers are pursuing the development of thepolymerisation-intercalation method with moderate success.

During the last decade, PP technology underwent a dramatic evolution thatresulted in a rapid increase of PP application in various industries, includingtransportation. Thus, it is of no surprise that PP-based CPNCs have been of interestto researchers around the globe.

The early work on PO-based CPNC was published by the Toyota group. Itsstrategy was based on a multistep intercalation:

1. Primary intercalation of clay by cation exchange with onium salt;2. Secondary intercalation with a compatibiliser, e.g., PO macromolecules grafted

with a polar compound (such as MAH); and3. Melt compounding with the matrix PO.The two critical steps are the selection of the organic cation (able to react withclay anions and interact with the compatibiliser), and selection of the polarcompatibiliser, which will react with organoclay and remain miscible with thebasic resin. Some success was also obtained by combining steps 2 and 3 into one,by modifying the PO only slightly, and compounding it directly with organoclayas a polar matrix.

In studies by Usuki et al. [1997] (which provided the basis for the first patentsin this field [Usuki et al., 1996; 1999]) Na-MMT (CEC = 1.19 meq/g) wasintercalated with 45.8 wt% of dimethyl distearyl ammonium chloride (2M2ODA),which increased the MMT interlayer spacing to d001 ≅ 3.28 nm. Next, theorganoclay was suspended in toluene and blended at a weight ratio of 1:1 withthe compatibilising ‘main guest molecule’ (hydroxylatedoligo-olefin: OO-OH; e.g., hydrogenated polybutadiene with telechelic -OHgroups, Mw ≅ 3 kg/mol, Polytail H from Mitsubishi) then dried. The interlayerspacing increased to d001 ≅ 3.87 nm, but when the ratio of OO-OH-to-organoclayincreased to 10 the XRD peak disappeared altogether, indicating that d001 ≥ 8.8 nm.Still better results were obtained by either reducing the molecular weight of theOO-OH compound, or by replacing the -OH groups by either -COOH, acidanhydride or epoxy (Polytail EP from Mitsubishi Kagaku). Next, PP (Mw = 30 kg/mol)was melt-compounded in an internal mixer at 220 °C with either: Na-MMT, theMMT-2M2ODA organoclay or the exfoliated OO-OH/organoclay material. TheTEM images taken of these three compounds showed, respectively, a micron-leveldispersion, the presence of large stacks, and exfoliation with only short stacks.Thus, good dispersion of clay platelets in molten PP was achieved when prior tocompounding the clay platelets were exfoliated by the combined effects of oniumion and the OO-OH compatibiliser. The authors stressed good miscibility of theMMT-2M2ODA organoclay with PO, e.g., PP, butyl rubber (BR), polyisoprene(IR), polybutadiene, etc.

In another publication [Kato et al., 1997] two-step exfoliation was carriedout. Thus, Na-MMT was intercalated with octadecyl ammonium chloride (ODA),which increased the MMT interlayer spacing to d001 = 2.17 nm. Melt blending itwith either maleated PP (PP-MA; Mw = 30 kg/mol, acid value = 52 mg KOH/g) orhydroxylated PP (PP-OH; Mw = 20 kg/mol, OH value = 54 mg KOH/g) was carriedout in an internal mixer at 200 °C for 15 min, with the ingredient ratio changingfrom 1:3 to 3:1. The interlayer spacing of CPNC containing 50 wt% of organoclaywas d001 = 3.82 and 4.40 nm for the PP-MA and PP-OH matrices, respectively.However, for an organoclay concentration of 25 wt% d001 = 7.22 nm was

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obtained. Note, that the PP-MA matrix had lower molecular weight thancommercially desirable, but still above the entanglement molecular weight:(Mw > Me = 2.8 kg/mol [Porter, 1995]). Furthermore, under the same conditionsthe same resin with lower acid value (PP-MA; Mw = 12 kg/mol, acid value = 7 mgKOH/g) was unable to expand the organoclay spacing. The authors concludedthat to obtain intercalation one polar group per 25 PP mers is required.Unfortunately, there is no explanation of the MAH group reaction(s) with theorganoclay – is it with the primary ammonium ion, with clay surface -OH groups,or the surface cations at the platelet edges?

In the following publications from Toyota [Kawasumi et al., 1997; Hasegawaet al., 1998] again a three-step melt intercalation was used. Thus, Na-MMT wasfirst intercalated with ca. 32 wt% ODA, which increased the interlayer spacingfrom d001 = 1.2 to 2.2 nm. The same procedure was repeated with syntheticfluoromica (FM) (Somasif ME100 from CO-OP Chem.). As the ‘main guestmolecule’ two grades of PP-MA were used (Yumex 1001 and 1010 from Sanyo,with Mw = 40 and 30 kg/mol, acid number = 26 and 52 mg KOH/g, Tm = 154 and145 °C, respectively). The powders of organoclay (7.3 wt%), PP (melt flow rateMI = 16 g/min; 70.8 wt%) and Yumex (21.9 wt%) were dry-blended, thencompounded in a TSE at 210 °C. About 5 wt% of the clay (MMT or FM) wasincorporated. The miscibility of PP with PP-MA was examined at 200 °C bymeans of an optical microscope – Yumex 1001 was found miscible while Yumex1010 was immiscible. The degree of clay dispersion in injection-mouldedspecimens was determined by XRD and TEM. It was found that compoundingorganoclay with PP caused a reduction of the interlayer spacing, while that witheither PP-MA increased it to d001 = 5.9 and 6.4 nm for MMT and FM, respectively.In three-component systems (organoclay/PP-MA/PP) a significant difference inthe degree of dispersion was observed for systems with Yumex 1001 and 1010.In the first case, the miscibility of the PP/PP-MA blend resulted in a high degreeof dispersion (displayed in TEM micrographs), but with residual short stacksdetected as a shoulder on the XRD (d001 ≅ 3.3 nm). In the case of the immisciblePP/Yumex 1010 system the XRD peaks indicated reduced interlayer spacing tod001 ≅ 5.5 and 5.9 nm for MMT and FM, respectively. The authors stated thatthere is ‘strong hydrogen bonding between the maleic anhydride groups and theoxygen groups of the silicates’, suggesting that MAH groups react with the claysurface -OH functionalities.

Evidently, preparation of CPNC with such non-polar polymers as PP requiresthermodynamic compatibilisation. The ‘compatibilising’ molecules (e.g., PP-MA)must be selected considering several aspects:

1. Interaction with pre-intercalated clay;2. Miscibility with the matrix polymer;3. Ability to entangle with the matrix, and4. The effects on the crystallinity of the matrix.As Kawasumi et al. have shown [1997] the miscibility of PP-MA with PP is ofgreat importance. As expected, when the MAH content exceeds a critical acidvalue (≤ 52 mg KOH/g) the immiscibility with PP results in a micron-size dispersionof PP-MA/organoclay phase in PP. Thus, a high degree of maleation leads toimmiscibility and poor results. Similarly [Kato et al., 1997], when the degree ofmaleation is too small, there is insufficient interaction with the organoclay and

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again platelet dispersion is poor. However, miscibility depends on concentration,temperature and pressure [Utracki and Kamal, 2002b]. Furthermore, the structureof the copolymer is important. When maleation is conducted in the presence of arelatively high concentration of free radicals, random grafting of individual MAHgroups onto PP short chains takes place. Grafting PP with a small amount of freeradicals in the presence of styrene may lead to high molecular weight PP withfew SMA-chains attached to it. Significant differences in the compatibilisingcapabilities of these two types of PP-MAH compounds are to be expected. It isnoteworthy that the commercially available PP-MA compounds widely differ inpurity, polydispersity and type of grafting hence their performance in CPNCsvary widely.

Hasegawa et al. [2000a] explored the earlier observation from the Toyotalaboratory that melt compounding of lightly maleated PO (compatibiliser not needed)with pre-intercalated clay may lead to a high degree of dispersion. Thus, ca. 5 wt%of organoclay (MMT-ODA) was melt-incorporated into each of the three, slightlymaleated POs: PP (Mw = 210 kg/mol; MI = 150 g/10 min; MAH = 0.2 wt%), PE(MI = 2 g/10 min; MAH = 0.62 wt%), and EPR (Mw = 270 kg/mol; MI = 0.06 g/10 min;MAH = 0.55 wt%). The blending was carried out at 200 °C in a TSE (D = 30 mm,L/D = 45.5). Excellent dispersion of clay platelets was shown by XRD and TEM(in decreasing order) for EPR, PP and PE. In the latter system residual short stackswith d001 ≅ 3 nm were detected. The authors also showed that under the samecompounding conditions, non-maleated PO gave a coarse, ca. 1 μm in size,dispersion of organoclay aggregates with d001 ≅ 2.2 nm.

Next, the effects of organoclay concentration on melt intercalation werestudied [Hasegawa et al., 2000b]. Again, Na-MMT (CEC = 1.19 meq/g) wasintercalated with ODA, which increased the interlayer spacing from d001 = 1.2 to2.2 nm. The organoclay (2.1 to 5.3 wt%) was melt compounded with PP-MA(Mw = 209 kg/mol; acid number = 2.1 mg KOH/g; MAH = 0.2 wt%) in a TSE at200 °C. Good dispersion was obtained – the XRD spectra were free of d001 peaks,but a small shoulder for the highest clay content indicated the presence of shortstacks. The tensile modulus increased linearly with clay content (by nearly afactor of two), whereas the increase of tensile strength was more modest (from21 to 25 MPa), stabilising at about 4 wt% clay loading. Compounding oforganoclay with non-maleated PP gave a coarse dispersion of organoclayaggregates with unchanged interlayer spacing (d001 = 2.2 nm), higher modulus,but lower elongation at break and strength than the neat resin.

A three-step procedure was successfully used for melt intercalation of PP (MI =1.5 g/10 min) with PP-MA (MAH = 1 wt%, MI = 1.2 g/min) and MMT intercalatedwith 2M2ODA [Zhang et al., 2000b]. Mixing PP/PP-MA (ratio 50:1) withorganoclay was carried out in a TSE at 200 °C. As a result, the interlayer spacingof the organoclay (d001 = 1.74 to 3.6 nm) progressed towards exfoliation, leavingonly a small shoulder in the XRD spectrum (short stacks with d001 ≅ 3.4 nm).Tensile and impact strength at T = 5 °C were determined (see Figure 33). Thus,incorporation of organoclay increased the tensile strength from about 29 to31 MPa. The effect of organoclay on impact strength is more difficult to judge,as addition of 0.1 wt% organoclay increased it from 9 to 26 kJ/m2, whereasfurther addition of organoclay erratically reduced it to 6 kJ/m2. One may suspectthat PP-crystallinity and/or PP/PP-MA miscibility plays a role. More recently, theeffects of additional shearing were investigated [Zhang et al., 2003]. During thepacking stage of injection moulding an oscillatory flow through the mould cavity

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was generated. The CPNC was partly intercalated – shearing improved the degreeand homogeneity of dispersion. However, this improvement was at a cost ofplatelet orientation in the flow direction. Some bent MMT platelets were observedunder TEM. As a result the improvement of tensile strength was modest,decreasing with clay loading from positive 19% to negative 19% for 1 and 10 wt%of organoclay, respectively.

Several modifications of these procedures have been proposed. For example,the interlayer spacing of an ammonium-intercalated MMT was expanded byswelling with an organic solvent with boiling point: BP = 100-200 °C, viz. ethyleneglycol, naphtha or heptane [Wolf et al., 1999]. The swollen organoclay was thencompounded with PP in a TSE at 250 °C, replacing the solvent molecules by PPsegments and evacuating the solvent. The extent of dispersion is unknown, butthe authors reported absence of an XRD peak in the 2.0-4.0 nm range.

The aspect of the thermal stability of the organic cation used to intercalateNa-MMT was also analysed [Lee et al., 2000a]. PP (Mw =278, Mn = 72 kg/mol),PP-MA (Mn = 23 kg/mol, 2 wt% MAH) and organoclay (Cloisite® 20A or 30B,i.e., MMT with 2M2HTA or MT2EtOH, respectively) were melt-mixed for 10 minat 210 °C in an internal mixer at the weight ratio of about 70:22:8, respectively.The XRD of the mixtures with C20A showed that the interlayer spacing increasedfrom that of organoclay (d001 = 2.47 nm) to 3.4 nm for C20A with PP-MA, andto full exfoliation for the three-component CPNC. The opposite trend was seenfor mixtures with C30B. After compounding with PP-MA and/or with PP/PP-MAthe C30B interlayer spacing (d001 = 1.86 nm) was reduced to about 1.4 nm.Furthermore, it was found that compounding under a blanket of N2 or in airgave different results. The effect was also observed for binary PP mixtures withorganoclay (without PP-MA) where a change of the interlayer spacing was not

Figure 33 Tensile and impact strength (at 5 °C) versus organoclay content forPP/PP-MA/organoclay [Zhang et al., 2000b].

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expected. The FTIR measurements indicated thermal desorption anddecomposition of organic ions in C30B. By contrast, C20A heated with PP retainedits relatively large spacing, which in the presence of PP-MA expanded all the wayto exfoliation. The C30B spacing is small to start with and upon heating it readilycollapses, preventing PP-MA diffusion into interlamellar space. This observationis particularly interesting since C30B melt-exfoliated during melt compoundingwith PA-6 in a TSE at T = 200-250 °C [Dennis et al., 2000]. Evidently PA chainends were able to react with the -OH groups of the intercalant either preventingtheir decomposition or forming their own PA-clay bonds through amine or amidegroups.

Cloisite® C6A, C20A, C25A, and C30A (see Table 16) were also used with threePP resins of different melt index (PP-L, MI = 820; PP-M, MI = 200; and PP-H,MI = 60) and four PP-MA resins (unknown MW) containing 0.55, 1.1, 3 and5 wt% MAH [Kim et al., 2000a]. First, PP-L was mixed with 5 wt% of organoclayin an internal mixer at 200 °C and 20 rpm for 10 to 30 min. In systems containingC20A (MMT-2M2HTA) the interlayer spacing increased by 0.73 nm, while forthe three other organoclays it decreased by 0.1 to 0.2 nm. However, since themagnitude of the XRD peak decreased on mixing, some exfoliation may occur.Next, three component mixtures: PP:PP-MA: Cloisite® = 80:15:5 wt%, wereprepared. The best results were obtained using PP-L and PP-MA containing thelowest concentration (0.55 wt%) of MAH. Apparently, the phase separation athigher MAH content interfered with exfoliation. In another report [Ryul et al.,2001] PP-M with Cloisite® C10A (2MBHTA) or C20A (2M2HTA) wascompounded in an internal mixer (T = 180 °C, 75 rpm, t = 20 min) equippedwith an ultrasonic wave generator. In this series of experiments the PP-MA wasreplaced by styrene. The role of ultrasonics was to generate free radicals as wellas to help in breaking the organoclay stacks, thus facilitating intercalation. A highdegree of dispersion was obtained in the case of PP-M:styrene:C10A = 80:15:5 wt%;for C20A the d001 peak shifted from 2.39 to 3.65 nm. Ultrasonics caused chainscission of PP followed by copolymerisation with styrene.

The superiority of C20A (2M2HTA) over the other organoclays indicates thatthe most favourable conditions for achieving a high degree of dispersion are:

1. A relatively large initial interlayer spacing of organoclay that makes diffusionof the compatibilising molecules possible,

2. A relatively small area of the clay surface blocked by the intercalating ion,3. Availability of sufficient concentration of -OH groups on the clay surface,4. Chemical bonding between the compatibiliser reactive group (e.g., MAH)

and ≡Si-OH group,5. Miscibility of the compatibiliser with the base resin, and6. High enough molecular weight of the compatibiliser to obtain entanglement

with the matrix molecules.The above observations are also valid for CPNC prepared with synthetic hectorite(FM, Somasif ME100 from CO-OP Chem.) [Reichert et al., 2000]. First, thesynthetic clay (FM) was treated with protonated alkyl amines, viz. butyl (C4),hexyl (C6), octyl (C8), dodecyl (C12), hexadecyl (C16) and octadecyl (ODA). Theprimary ammonium ion intercalation resulted in expansion of the interlayerspacing from 0.95 to 1.98 nm (see Figure 34). PP (Tm = 163 °C; MI = 3.2 g/10 min)

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was compounded with 20 wt% of PP-MA (type E: Mw = 22.9 kg/mol, Mw/Mn = 2.6,MAH = 2.9 wt% or type H: Mw = 32 kg/mol, Mw/Mn = 8, MAH = 4.2 wt%) and0, 5 or 10 wt% of the organoclay in a CORI at 300 rpm and T = 190 to 230 °C.Only C12, C16 and C18 amines produced organoclays (d001 ≥ 1.68 nm) that couldbe dispersed in the compatibilised PO matrix. The interlayer spacing was foundto increase with MAH content, viz. 10 wt% of FM intercalated with either C16-or C18-amine, when blended with 20 wt% PP-g-MA (type-H) and 70 wt% PPgave the best performance, viz., increase of Young’s modulus by 129%, yieldstress increase by 32%, notched Izod impact strength decrease by 18% andelongation at break decrease by 99%. XRD and TEM indicated that fullexfoliation was not achieved. The miscibility of PP-MA with PP was not examined.

Manias et al. [2001] reacted Na-MMT with either 2M2ODA or ODAobtaining two organoclays. PP was grafted with ca. 0.5 wt% of p-methylstyrene(PP-MS; Mw = 200 kg/mol), then the p-methyl group was converted into either-(CH2)3-OH (PP-OH) or -CH2-maleic anhydride (PP-MA). A block copolymerof PP with PMMA was also prepared (PP-b-PMMA; Mw = 15 kg/mol). CPNCswere:

(i) Extruded at 180 °C using a SSE with residence time up to 10 min,(ii) Compounded in an internal mixer at 170 to 180 °C for up to 20 min, or(iii) Ultrasonicated in trichlorobenzene.

Figure 34 Synthetic clay (Somasif ME10) intercalated with salts of primaryamines, CnH2n+1NH2. The interlayer spacing (d001) and amine content (w) are

plotted as functions of the number of carbon atoms in the amine (n). The amountof adsorbed amine and the stepwise increase of d001 may be approximated with the

linear dependence with the correlation coefficient, r = 0.980 and 0.996,respectively. Data [Reichert et al., 2000].

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According to XRD and TEM all three methods resulted in similarly structuredCPNC: intercalated tactoids and exfoliated platelets. Thus, reacting MMT-2M2ODA with PP-MA resulted in expanded interlayer spacing from d001 = 2.4 to3.0 nm, whereas mixing it with either PP-MS or PP-OH gave d001 = 3.4 nm. TheMMT-ODA was found to expand when blended with PP-b-PMMA from d001 = 2.2to 3.2 nm. The short note does not contain either the postulated mechanism ofreaction between the functionalised PP and the organoclay or information abouthow the chemical structure and blending time affected the exfoliation. It seemsthat in the series of MMT-2M2ODA with grafted PP the interlayer expansionwas controlled by the presence of the styrenic group – the presence of MAH gavelower expansion than that obtained from the non-polar -CH3 group. MMT-ODAwith PP-b-PMMA also showed a similarly modest expansion of the interlayerspace by Δd001 ≅ 1 nm.

Oya et al. [2000] examined the role of clay as well as a secondary intercalanton the degree of dispersion and mechanical properties of CPNC with PP. Thus,synthetic hectorite (FM), MMT and mica (MC) were intercalated with aquaternary ammonium ion (2MHDODA) of the formula:(CH3)2N+(C16H33)(C18H37). Next, the organoclay was dispersed in a toluenesolution of diacetone acrylamide (CH2=CHCONHC(CH3)2CH2COCH3; DAAM)with a free radical AIBN initiator. After polymerisation of DAAM a solution ofPP-MA was added, the product was precipitated, washed, dried, and finally meltcompounded with PP. In parallel, CPNC was similarly prepared without DAAM.In all cases only intercalation was obtained – the largest interlayer spacing (andthe overall best dispersion) was obtained for doubly intercalated FM preparedwith poly-DAAM. However, the best mechanical properties at 3 wt% of clayloading were these of CPNC with MC. The authors speculated that the differenceoriginates in the relatively high modulus of MC pristine platelets. In conclusion,mica may be a better nanofiller than FM, but the performance of CPNCs withMMT was nearly as good. The additional polymerisation step did not result inthe expected improvement of dispersion and/or mechanical performance.

Intercalated nanocomposites of PP-MA with 2, 4 and 7.5 wt% clay were preparedvia melt extrusion at 200 °C in a TSE [Nam et al., 2001]. The organoclay wassynthesised by an ion exchange reaction between Na-MMT (CEC = 1.10 meq/g;d001 = 2.31 nm after ion exchange) and ODA. The PP was modified by graftingwith 0.2% MAH (PP-MA; Mw = 195 kg/mol and Mw/Mn = 2.98). It was foundthat the interlayer spacing (as measured by XRD) decreased with the clayconcentration (from d001 = 3.24 to 2.89 nm), while the thickness of the short claystacks increased (see Figure 35). The XRD and TEM data indicated that theextent of exfoliation and/or stacking is controlled by the amount of clay. Thegrafted MAH-groups in the PP-MA chains promoted interactions with the clayparticles by diffusion of PP chain into the space between the silicate galleries.

The authors focused on the effect of the organoclay on PP-MA crystallinityand hierarchical structure formation. The hierarchical structure starts with theintercalated clay spacing of 2¯3 nm, to crystalline lamellae thickness of 7-15 nmand spherulitic texture of 10 μm diameter. Two important observations weremade. After crystallisation at 80 °C, the PP-MA formed a rod-like crystallinetexture of about 10 μm length consisting of an interfibrillar structure. The totalvolume fraction of the crystalline phase was about constant, but with the increasingclay content the γ-phase crystalline content increased from zero to 10.5%. Theformation of the γ-phase originates in the reduction of the PP-MA chain mobility

173


affected by the intercalation of PP chains into the interlamellar gallery space.The orthorhombic unit cell of the γ-phase consisted of two parallel helices with aninclination of 40° to the lamellar surface. The diffraction peak of the 130-plane ofthe γ-phase appears at 2θ = 19.3°. The clay particles are located inbetween thecrystalline fibrils made of assembly of lamellae. Thus, even when the lamellarparameters (thickness and spacing) remain about constant, the crystalline fibrilsize and interstack correlation distance decrease with clay concentration. TheCPNC showed enhanced moduli (compared to the matrix) related to the claycontent and aspect ratio by the Halpin-Tsai relation.

The system: PP (Mw = 280 kg/mol) with PP-MA (Mw = 47 kg/mol; 2.5 wt%MAH) and organoclay (ODA and n-decyl amine, treated with silane; d001 = 2.3 nm)was melt-mixed in an internal mixer at 180 °C and 150 rpm [Marchand andJayaraman, 2001]. The aim was to maximise the degree of clay dispersion byvarying the PP-MA content (0 to 14 vol%) at a constant organoclay content of1.8 vol%. The optimum concentration of PP-MA was found to be ca. 9-10 vol%.Furthermore, it was reported that a higher degree of exfoliation was obtainedusing ODA than its silane-treated homologue.

In a recent publication from NCL in Pune, a SSE (T = 160 to 200 °C, at45 rpm) was used to melt-blend PP/PP-MA into organoclays [Kodgire et al., 2001;Hambir et al., 2001; 2002]. The organoclays used in this study were Cloisite®

6A (2M2HTA; see Table 16), and Nanoclay (N1; Na-MMT with CEC = 1.35 meq/gion exchanged with ODA) from Nanocor Inc. The material characteristics areshown in Table 26. The composition of the CPNC was: 84 wt% PP, 12 wt% PP-MA,and 4 wt% organoclay. XRD of the Na-MMT gave the interlayer spacing d001 = 1.2 nm.For the intercalated clays, Cloisite® 6A (C6A) and Nanoclay® ODA (N1),respectively, three peaks: d001 = 1.2, 1.8 and 3.3 nm, and a single: d001 = 2.45 nm,

Figure 35 Interlayer spacing, d001, stack thickness, t, and its aspect ratio, p, inPP-MA/stearyl ammonium-MMT. Data [Nam et al., 2001].

174


were observed. In the presence of C6A the XRD showed the presence of multiplepeaks, indicating non-uniform structure.

Melt-blending the two organoclays with PP and PP-MA (Polybond 3150 fromUniroyal Chemical) resulted in a reduction of the XRD peaks’ intensity. This wasparticularly evident with C6A; for N1 the interlayer spacing shifted to a highervalue (d001 = 2.7 nm), but the reduction of intensity was not as pronounced.Thus, both systems did not exfoliate, but the degree of dispersion was better in thepresence of C6A. However, as data in Figure 36 and Table 26 indicate, it is N1 thatyields higher rigidity. The effect may originate in the different type and/differentcontent of the crystalline phase or the platelets’ aspect ratio. The performance ofPP was enhanced by incorporation of PP-MA and organoclay. About a 35%increase in the tensile modulus and about a 10% increase in the tensile strengthwere observed. The thermal decomposition temperature increased from 270 toabout 400 °C (for C6A). The most interesting observation is the dramaticallydifferent large-scale morphology for PP and CPNC based on it; while PP3 crystallisesin well-known spherulites, the presence of PP-MA/organoclay (PP3/PB/6A) changedit into a fibrillar structure. After prolonged crystallisation the fibres grow in lengthand diameter, but do not revert to the spherulites. Furthermore, the PP/clay systemcrystallises at temperatures higher by ca. 10 to 20 °C than neat PP, suggestingstrong nucleating capabilities of the organoclays.

In another study from the same laboratory [Hambir et al., 2002], three gradesof PP and two of PP-MA were used (see Table 26). Judging by XRD spectra, meltintercalation with PB was superior to that of VB – evidently the acidity of thelatter graft copolymer was insufficient. The data also indicated that the use oflower MW PP resulted in a higher degree of dispersion (judged by the reductionof the peak areas).

Tethered PP/organoclay CPNCs were prepared via melt compounding in aTSE at T = 180, 190, 200 and 190 °C (from hopper to die), at a screw speed of

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175


180 rpm. For testing, dried pellets of the nanocomposite were injection-mouldedat 200 °C with a mould temperature of 30 °C [Liu and Wu, 2001]. The organoclaywas prepared from Na-MMT (CEC = 0.80 meq/g) by ion exchange reaction withtrimethyl hexadecyl ammonium bromide (3MHDA). In the presence of dibenzoylperoxide the product was compounded in an internal mixer for 60 min with glycidylmethacrylate, at a weight ratio of 13:2. This epoxidised product (E-MMT) wasthen compounded with PP. XRD of Na-MMT, 3MHDA and E-MMT gave thefollowing interlayer spacings: d001 = 1.24, 1.96 and 2.98 nm, respectively. XRDof PP with 5 wt% of 3MHDA indicates that the d001 peak of the organoclayremains at the same position, i.e., PP does not intercalate into the organoclay. Bycontrast, XRD of PP with different loadings of E-MMT (1, 3, 5, and 7 wt%)showed expansion of the interlayer spacing, viz. 4.92, 5.09, 4.61 and 4.75 nm,respectively. Thus, E-MMT can lead to intercalation but not to exfoliation. Themechanical properties of the CPNC improved with addition of organoclay,increasing the storage modulus (by 40% at 7 wt% loading), not affecting theimpact strength and slightly reducing Tg. The addition of clay did not change thecrystal structure of PP, however silicate layers acted as nucleating agents, increasingthe crystallisation peak temperature of PP from Tc = 110.5 °C to about 120 °C.

Nanocor recently recommended CORI screw design for the manufacture ofCPNC with PO as a matrix [Qian et al., 2001]. A Leistritz TSE with D = 27 mmand L/D = 36 was used at 300 to 500 rpm and T = 170 to 190 °C. Two vents at13D and 24D were used. The recommended configuration is given in Table 23.The process involves two-steps:

1. Preparation of masterbatch containing about 50 wt% of organoclay, acompatibiliser (ca. 25 wt%), and a matrix polymer (e.g., PP-MA and PP).

2. Dilution to the desired clay content (e.g., 2 to 7 wt%).

Figure 36 Temperature dependence of the tensile storage modulus, E´, for PPand PP/PP-MA/organoclay for: Cloisite® 6A and Nanocor N1. Data [Kodgire et

al., 2001].

176


The latter process may be carried out using either a TSE or a SSE [Cho et al., 2002].For example, first, organoclay/PP/PP-MA masterbatch was prepared. It contained50 wt% Nanomer® I.30P (MMT-ODA) with 25 wt% PP-MA and 25 wt% PP.Next, dilution of the masterbatch with PP (to 6 wt% organoclay in the finalCPNC) was carried out in either a SSE (containing different mixing screws oradd-on mixers) or in a CORI. For comparison, PP nanocomposites were preparedfollowing a single-step procedure. Thus, PP, PP-MA and 6 wt% I.30P werecompounded in a CORI. The interlayer spacing of all six CPNC batches preparedby different methods was small, viz. d001 = 2.7 to 2.8 nm. Thus, intercalation notexfoliation was obtained. The mechanical properties of the neat resin, CPNCprepared by dilution of the masterbatch either in a SSE or in a TSE, as well as ofCPNC prepared by direct compounding are presented in Table 27. CPNC preparedby the two-step method showed slightly better performance than that preparedby direct compounding in a TSE, but dilution in a TSE results in a marginallybetter performance than that in any SSE configuration (independent of whichmixing unit was installed). The difference between the two-step and single-stepis most likely related to the difference in d001. The origin of the increased interlayerspacing may be related to the total residence time of the CPNC subjected to meltcompounding. Since the stated aim of the process is exfoliation, more detaileddiscussion can be found in Section 2.4 and Part 4.

By contrast with the customary use of ammonium-intercalated clays, Pozsgayet al. [2001] treated Na-MMT with N-cetyl pyridinium chloride. Meltcompounding of this organoclay with PP in an internal mixer produced ratherdisappointing results as the tensile strength of PP was found to decrease with theorganoclay loading (from 1 up to 10 wt% clay). The decrease superimposed ondata obtained for a PP mixture with neat Na-MMT, indicated a lack of miscibilitybetween PP and the cetyl C16H33-paraffin group of the intercalant.As reported by Wang et al. [2001a], organoclay may also act as a compatibiliser inpolymer blends. The authors dispersed 10 wt% of Nanomer I.30 TC (MMT-ODA).Comparing the binary blend PA-6/PP = 9/1 with the three-component systemPA-6/PP/I30 = 9/1/1 it was found that addition of organoclay significantly improvedthe tensile properties, viz. modulus increased from 2.5 to 4.7 GPa, tensile strength

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177


from 65 to 73 MPa, but at a cost of impact strength, which decreased from 70 to13 J/m. Compatibilisation of the blend by addition of PP-MA reduced the modulusto 3.4 GPa, but slightly improved the strength and impact strength (to 75 MPa and17 J/m, respectively). Evidently, in the four-component system PP-MA migrated tothe clay-PA-6 interface, thus its role as the blend compatibiliser was reduced.

The fundamental principle of additive incorporation into a multiphase polymericsystem is that it must be inserted into a specific phase – if in the blend one phase isrigid and the other ductile, increasing the volume of each of these with the sameadditive will have opposite effects. For example, it is known that incorporation ofPS into the rubbery phase of HIPS increases the ductility of the system, whileincorporation of glass fibre (GF) into PA-66 in its blend with ABS increases themodulus [Utracki, 2002]. Obviously, the same principles are valid for CPNC withpolymer blend as a matrix. Recently two blends, PC/ABS and PA-6/ABS wereprepared by melt compounding with MMT-3MHDA [Wang et al., 2004]. TheTEM micrographs showed that in the former CPNC clay platelets dispersed in theABS phase, whereas in the latter system, they were dispersed mainly in the PA-6phase. The dynamic self-organisation of organoclay was studied, but unfortunatelythere is no information regarding the effect on performance.

Extensive studies on melt intercalation of PP were carried out by Ton-That et al.[2002]. The authors focused on CPNC containing PP melt compounded in a TSE at200 °C with 2 wt% of Cloisite® 15A and 4 wt% of a compatibiliser. Several commercialPP-MA compounds, a PP-AA and an onium-terminated PP have been used ascompatibilisers. XRD spectra have shown that the interlayer spacing d001 ≤ 3.9 nmwas achieved. TEM confirmed this by displaying a number of short stacks along withindividual platelets dispersed in the matrix. The highest values of tensile modulus wereobtained for PP-MA with 0.5 wt% MAH and with PP-AA with 6 wt% acrylic acid.

To close this discussion on the melt intercalation of PP, it is necessary to stressthat in many reports all the observed differences in the performance of CPNCand neat PP matrix resin are assigned to the more-or-less exfoliated clay. However,the behaviour of this semicrystalline polymer strongly depends on crystallinity:crystallographic form, type and size of crystal, total crystallinity, etc. Rare arethe publications that even consider these effects.

In 2001 Saujanya and Radhakrishnan published interesting work on the effectsof calcium phosphate (CaPO3) particles on the crystallinity of PP. The authorsprepared nanosized CaPO3 using an in situ deposition technique in the presenceof PEG or PEG/PVAc. The composite was prepared by grinding together 2, 5 or10 wt% of CaPO3 with PP in an agate mortar and pestle. A small quantity of thepowder was isothermally crystallised on the microscope hot stage. The growthof spherulites as well as the intensity (grey scale) of transmitted light in the cross-polar mode was recorded.

The results were compared with those for PP containing conventionallyprepared CaPO3. Both filled PP as well as neat PP crystallised in the monoclinicα-crystalline phase. However, as the size of CaPO3 particles (d) decreased, theisothermal crystallisation rate increased dramatically – this is illustrated inFigure 37 as a decrease of the crystallisation half-time (t1/2) versus 1/d. The overallcrystallisation rate (G) was found to follow the dependence: log G ∝ (1/d).Similarly, the nucleation efficiency of CaPO3 particles was found to increase withtheir inverse diameter, resulting in a decrease of the ultimate spherulite size andenhanced optical transparency of the PP/nanoparticle composites. These resultswere further confirmed by DSC analysis.

178


Figure 37 Crystallisation half-time versus reciprocal diameter of calciumphosphate powder in PP. Data [Saujanya and Radhakrishnan, 2001].

The dependence of t1/2 versus 1/d presented in Figure 37 confirms that at the samevolume fraction the efficiency of a nucleating agent depends on its surface area.The dependence is not linear, suggesting that as the particle size is reduced tonanosize, additional factors start to play a role, viz. particle shape, aspect ratio,curvature, increased proportion of the high energy surface atoms at a cost of theshielded atoms in the interior, etc. The smallest CaPO3 particle with d = 7 nm (i.e.,specific area, Asp = 69 m2/g) has a surface area one order of magnitude smaller thanMMT (Asp = 700-800 m2/g). However, it is organoclay not neat mineral that getsincorporated into the PP matrix hence the shielding effect of the organic intercalantplays an important role – often the clay surface is fully shielded from the matrix.

Another point that often escapes attention is the actual location of thenanoreinforcing platelets in the semicrystalline polymer matrix. Thethermodynamics of crystallisation require that any foreign material should beexpelled from the growing crystal. Hence, the organoclay must be located mainlyin the non-crystalline part of the CPNC, in the liquid phase of PP at T > Tg ≅ 0 °Cor in the glassy part at lower temperatures. Since the crystallinity of PP in CPNCmay be as high as 68%, the clay concentration in the non-crystalline phase maybe three times higher than average. Owing to the crowding effect, the interlayerspacing in this case may be reduced by a factor of up to 3. Thus, on the one handlarge effects of organoclay addition into a semi-crystalline matrix may not be assignificant as those in an amorphous one and on the other the mechanism of thenanoclay nucleating activity should be taken into account.

The key information from the relatively large number of reports on meltintercalation of PP/organoclay systems is summarised in Table 28. As far as thetensile modulus is concerned, the data indicate that at low clay loading it increaseslinearly with clay content. For exfoliated CPNC with PA-6 as the matrix thedependence (up to 10 wt% of clay) may be expressed as:

E E E a w aR matrix≡ + =/ ; .1 0 20 0 (22)

where w is clay (inorganic part only) content.

179


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As discussed in Section 3.5.1 (Micromechanics of CPNC), well-dispersed CPNCwith PP as the matrix follow the same dependence. This is quite remarkablesince the parameter ao is a measure of the hydrodynamic volume which dependson the aspect ratio – for hard spheres the Einstein prediction is ao ≈ 0.005 (forpoorly dispersed organoclays the constant ao ≈ 0.07). Thus, the observeddependence indicates that in these solid CPNCs the aspect ratio of clay plateletsis p ≈ 277. In short, at least as far as tensile modulus is concerned, one can getsimilarly high performance form the PP/clay system as that of the renownedPA-6/clay.

As evident from this discussion, PP is the principal polyolefin of interest.Numerous patents and research publications on CPNC with PO as the matrixusually mention other polymers, but the cited examples invariably focus on PP.One of the exceptions is a publication by Wang et al. [2001b], who preparedLLDPE-based CPNC using MAH grafted LLDPE (PE-MA) and one of fourorganoclays, viz. Cloisite® 20A (2M2HTA), and synthesised alkyl ammonium-MMT (CEC = 1.19 meq/g) with alkyls: dodecyl, hexadecyl or octadecyl(respectively C12M, C16M, ODAM). The interlayer spacings of C20A, C12M,C16M, and ODAM were determined by XRD as d001 = 2.47, 1.36, 1.79 and1.85 nm, respectively. Two types of PE-MA were used, one a direct product ofMAH grafting on LLDPE, and the second obtained by mixing neat LLDPE withPE-MA (0.85 wt% MAH). The CPNC were prepared by melt compounding inan internal mixer at 140 °C and 60 rpm.

The XRD scans at 2θ = 2 to 10°, for CPNCs with 5 wt% of either C20A orODAM did not show a d001 peak indicating exfoliation – confirmed by TEM.For the two other organoclays there was a significant reduction of the peak area– only a trace of a peak was observed for C16M and a weak one for C12M.Thus, for exfoliation of alkyl ammonium-MMT in an LLDPE matrix at leastC16-alkyl chain in the organoclay is required. The authors also determined thatthe critical level of MAH grafted onto LLDPE is 0.1 wt% – below this level evenC20A or ODAM could not be exfoliated.

Several publications report that full exfoliation in PE/clay CPNC has beenachieved. However, these were prepared by polymerisation of ethylene in thepresence of organoclay. The encountered problem is that the system is notthermodynamically stable, and during the subsequent processing the plateletsreaggregate, reducing the interlayer spacing and the performance.

2.3.9 Temperature and Pressure Effects on Interlamellar SpacingThe ‘standard’ intercalation procedure (e.g., by ammonium salts) is carried outunder mild conditions, viz. ambient pressure, P = 0.1 MPa, and T = 20-80 °C.However, melt exfoliation of CPNC may involve much more severe treatment.There is little information in the literature as to how T and P affect the interlayerspacing. What is available, seems to be system-specific.

For example, the temperature effects on the interlayer spacing were found todepend on the onium ion and (to a lesser extent) on the type of clay. In Figure 38variation of the interlayer spacing, d001, as a function of T is presented forammonium salt intercalated magadiite (a) and beidellite (b); circles representtetradecyl ammonium-tetradecyl amine complex (C14H29N+H3× C14H29NH2),while triangles represent dimethyl ditetradecyl ammonium [(CH3)2N+(C14H29)2].Magadiite is a layered sodium silicate: Na2Si14O29

.9H2O. Evidently, the interlayerspacing variations with temperature depend on the ammonium radical, and to a

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Figure 38 Interlayer spacing, d001, as a function of temperature for (a)magadiite and (b) beidellite intercalated with salts of tetradecyl ammonium-

tetradecyl amine (C14; circles) or dimethyl ditetradecyl ammonium (2(C1C14);squares; open squares show values after cooling). Data [de Siquira et al., 1999].

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lesser extend on the type of clay [de Siquira et al., 1999]. The authors explainedthe observed changes in the interlayer spacing as caused by molecularrearrangement of the ammonium radicals, melting of the n-paraffin groups, orchanges of hydration.

Information on the effects of pressure (P) is even more difficult to find.However, there are indications that with increasing P the interlamellar galleryheight decreases. High-resolution data were obtained using time-of-flight neutrondiffraction of hydrated Ca- and Na-smectite and vermiculite [de Siquira et al.,1999]. The measurements were conducted in a special Ti/Zr cell that couldwithstand simultaneously T ≤ 350 °C and P ≤ 200 MPa.

The data are presented in Figure 39. One of the more interesting conclusionsfrom that work was that the interlayer water is denser than that in the bulk: ρinterlayer= 1.06 versus ρ bulk = 0.874 g/ml. In other words, there is a reduction of freevolume in the vicinity of the clay particle surface. If so, the compressibility shouldalso be reduced. The transitionless reduction of the interlayer spacing of Ca-smectitegives the isothermal volumetric compressibility of the hydrated clay: κ = 4.8-10-5

MPa-1, to be compared with bulk water compressibility of κ = 4.5-10-4 MPa-1 – areduction by a factor of 9.

2.3.10 Layered Nanofillers, other than MontmorilloniteExcepting the patents that list a plethora of diverse layered nanofillers, the majorityof published research on CPNC and all the industrial activities focus on swellablesmectite clays, such as natural montmorillonite or synthetic hectorite (quite similarto MMT). However, the properties of polymer/clay nanocomposites do dependon the kind of layered material that has been used. Thus, it is advisable to considerthe possibilities of CPNC formation with clays other than MMT.

Figure 39 Pressure dependence of d-spacing for calcium- and sodium-smectiteand vermiculite. Data [de Sequira et al., 1999].

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2.3.10.1 Kaolinite

The structural formula for kaolinite is A14Si4Ol0(OH)8, and it is a 1:1 type clay,hence different from the commonly used 2:1 type smectites. Its lattice consists ofone sheet of tetrahedrally coordinated SiO4 and one sheet of octahedrallycoordinated AlO2(OH)4. Kaolinite has a low value of CEC = 0.02-0.04 meq/gand large aspect ratio. In neat kaolinite the adjacent cells are spaced about 0.71 nmacross the (001) plane. A layer of -OH covers the octahedral sheet forming stronghydrogen bonds between the layers, hence only a limited number of polar guestspecies, viz. N-methyl formamide (NMF) or dimethyl sulfoxide (DMSO), can beintroduced. However, once the layers are separated, the -OH functionality maybe used for hydrogen bonding with some polymers. Thus, kaolinite has quite adifferent structure than MMT, viz. one side of the interlayer space is coveredwith hydroxyl groups of the AlO2(OH)4 octahedral sheets and the other side iscovered by oxygen atoms of the SiO4 tetrahedra. As a result, CPNC with kaoliniteare expected to exhibit different behaviour from those with MMT. In 1996 Tunneyand Detellier provided an overview of the earlier work.

The first incorporation of a polymer into kaolinite was reported in the early1990s. In a series of publications Sugahara and his colleagues [Sugahara et al.,1992] intercalated kaolinite with monomeric acrylonitrile, acrylamide orvinylpyrrolidone then induced thermal polymerisation. However, this methoddid not provide a means for controlling the molecular weight and its distribution.The first direct intercalation of kaolinite with macromolecules took place morerecently [Tunney and Detellier, 1996]. The motivation for this work was a searchfor anisotropic ionic conductivity. In other layered materials two-dimensionalconfinement of PEG provided the desirable performance. In other words, heretotal exfoliation was neither expected nor desired. The applied method consistedof two steps:

1. Intercalation of kaolinite with a solvent, and2. Compounding the intercalated kaolinite with a polymer, which progressively

displaced the solvent molecules in the interlamellar galleries.The authors have shown that it is possible to prepare two distinct ethylene glycolphases of kaolinite, with d001 = 0.94 and 1.08 nm. The former intercalate had theethylene glycol unit covalently bound to the interlayer aluminol surface, whilethe other had a more weakly bound intercalated phase.

The logical next step was to intercalate larger oxyethylene-based moleculesinto kaolinite. Thus, first DMSO or NMF was used to intercalate kaolinite, andthen PEG-3400 or PEG-1000 displaced the low molecular weight intercalant.This was accomplished by heating PEG with ca. 17 wt% DMSO-intercalatedkaolinite in a round-bottom flask for 9 days at T = 155 °C. The product waspurified and dried in an oven at 100 °C for 3 days. Elemental analysis: carbon7.90%; hydrogen 2.63%. XRD gave d001 = 1.112 ± 0.004 nm. The calcinationweight loss was 29.1%.

Several samples were prepared from combinations of clay, intercalatingsolvents and melt-intercalating PEG resins. Their interlayer expansion wascomparable, ranging from d001 = 1.085 to 1.119 nm, i.e., providing an interlamellarspace of Δd001 ≅ 0.37-0.41 nm. This indicates that the intercalated oxyethyleneunits are arranged in a flattened monolayer conformation. It is noteworthy thatintercalation, using PEG-1000 dissolved in either water or 1,4-dioxane, was

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unsuccessful. The success of the intercalation by the melt method could be due toa strong concentration effect of the polymer and lower stabilisation energy ofpolymer segments.

Larger interlayer expansion was obtained by reacting kaolinite with phenyl-phosphonic acid (PPA) in a water/acetone (1:1) solution at 95 ± 5 °C for up to19 days [Guimarães et al., 1998]. The topotactic reaction was stoichiometric,viz.:

Al2Si2O5(OH)4 + 4 H2O3PPH → Al2Si2O5(OH)(HO3PPH)3×2H2O + 3H2O

As a result of reaction the interlayer spacing increased from that of kaolinite(d001 = 0.716 nm) to the spacing of the kaolinite phenyl-phosphonate (KPP):d001 = 1.502 to 1.645 nm, i.e., providing an interlamellar space of Δd001 ≅ 0.786-0.929 nm. The analysis suggested that phenyl-phosphonate groups were graftedto the kaolinite platelets. The materials were stable up to ca. 450 °C.

In another report [Guimarães et al., 1999] hydrated kaolinite phenyl-phosphonate (KPP-hyd) was reacted with hexylamine. The reaction produced astable light-yellow compound (KPP-hex). XRD gave d001 = 1.636 nm, consistentwith the size of the intercalating hexylamine. The thermal analyses showed thatat T ≅ 230 °C hexylamine molecules decomposed, but the resulting KPP remainedstable up to 498 °C, at which temperature the grafted phenyl phosphonate startedto decompose and dehydroxylation of kaolinite takes place.

In 1999 Gardolinski et al. [1999] intercalated kaolinite in two steps, firstreacting it at 60 °C with an aqueous solution of DMSO. After washing andcharacterisation, the resulting complex: Al2Si2O5(OH)4(DMSO)0.4 wasstoichiometrically reacted at RT with N-methyl-2-pyrrolidone (NMP). The productwas Al2Si2O5(OH)4(NMP)n (where n = 0.39 ± 0.02). XRD gave d001 = 1.231 nm,i.e., an expansion of 0.11 nm over the DSMO complex. The presence of NMPmolecules in the interlamellar space resulted in a notable enhancement of thethermal stability – while the DMSO-complex decomposes at 175 °C, the NMPcomplex remains stable up to 431 °C.

In the next publication from this group, first kaolinite was reacted with DMSO,producing a kaolinite-DMSO complex. The complex was subsequently reacted witheither PEG or bacterial polyhydroxybutyrate (PHB) in the molten state at 130 or180 °C, respectively [Gardolinski et al., 2000]. The displacement of DMSO moleculestook several days – highly ordered polymer/kaolinite CPNC was obtained. In theinterlamellar galleries of kaolinite, PEG macromolecules formed a polymericmonolayer. The characteristics of the kaolinite-complexes are listed in Table 29.

As mentioned above, Sugahara and his colleagues synthesised several kaolinite-polymer compounds (viz. kaolinite-PAN, kaolinite-PAAM, or kaolinite-P4VP)by first intercalating kaolinite with the appropriate monomers and thenpolymerised by thermal treatment. The serious drawback of this procedure is thelack of control of the degree of polymerisation of the resulting polymers.Recognising the problem, the authors developed an alternative intercalationmethod [Komori et al., 1999b]. The publication is particularly valuable as thatthe authors also reported the unsuccessful attempts. The process resembles the‘common solvent method’ discussed in the next Chapter. Thus, pre-intercalatedkaolinite at room temperature is dispersed in a polymer solution. For examplewhen kaolinite-ammonium acetate was used as the intermediate for theintercalation of P4VP (Mw = 10 kg/mol) dissolved in water, the displacement was

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not successful, leading to de-intercalation of ammonium acetate. However, whenkaolinite-methanol (K-MeOH) was used as the intermediate and methanol asthe solvent for P4VP, the 24 h long polymer infusion increased the interlayerspacing. The product was centrifuged and yellowish white powders were obtainedwithout washing. K-MeOH was also used to intercalate such organic species asalkylamines, p-nitroaniline, and ε-caprolactam.

XRD of kaolinite, K-MeOH and kaolinite-P4VP determined that d001 = 0.72,1.11 and 1.24 nm, respectively. Elementary analysis showed that the number ofP4VP mers per kaolinite unit was 1.9, hence the formula:[Al2Si2O5(OH)4]×(C6H9NO)1.9. The complex kaolinite-P4VP can easily bedestroyed by washing with ethanol or water – P4VP could be removed after afew minutes of washing. The authors determined that out of the 1.9 mers perunit cell, about 1.1 are adsorbed on the stack surface with only 0.8 mers in theinterlayer space, i.e., twice as much as is obtained by in situ polymerisation(0.4 P4VP mers per kaolinite-unit cell).

Komori et al. [2000] reported a partial methoxylation of -OH groups in akaolinite. Kaolinite-NMF complex (d001 = 1.08 nm) was dispersed in methanoland mixed for a day. After centrifugation, the complex was redispersed and theprocedure repeated seven times. The final, wet K-MeOH complex (d001 = 1.11 nm)was separated and dried in air to convert it into methoxylated compound(K-OMe; d001 = 0.86 nm). The amount of methoxy groups was determined byanalysis. The results can be written as Al2Si2O5(OH)4-x(OCH3)x with x ≅ 0.36.The authors stressed that even when methanol can readily be removed from theK-MeOH complex, methoxy groups are more stable, e.g., they and watermolecules remain in the interlayer space of kaolinite after air drying.

The next publication from the group focused on the usefulness of K-OMe as anintermediate for the preparation of CPNC with PA-6 as the matrix [Itagaki et al.,2001]. A two-stage process was used, first preparing kaolinite-PA-6 intercalationcompound, using a kaolinite-6-amino-hexanoic acid (AHA) as a precursor, thenkaolinite-PA-6 was melt compounded with PA-6 (commercial 1015B grade fromUbe Industries Co.). The compounding was carried out using a TSE at 240 °C and300 rpm. XRD gave d001 = 1.15 nm indicating that the layered structure of kaolinitewas not exfoliated. For comparison, PA-6 was also compounded without clay(PA-6(extruded)) and with unmodified kaolinite (PA-6/kaolinite). The PA-6/MMTcommercial 1015C2 grade from Ube was used as a reference. The properties ofthese CPNCs are listed in Table 30. The relative properties of the last two

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compositions in the table are of interest. Taking into account different clay loadings,the performance ratio of PA-6/kaolinite-PA-6 to that of PA-6/MMT (per 1 wt% ofclay) is: 1.18, 1.28, and 1.2, for the tensile strength, tensile modulus and impactstrength, respectively. Evidently, the properties are not proportional to the claycontent, thus the performance ratios showing about 20% better performance forCPNC with kaolinite than with MMT may not be correct. However, it is intriguingthat kaolinite with only slightly expanded interlayer spacing offers at leastcomparable performance to that of commercial-grade CPNC with exfoliatedMMT. Since kaolinite in the PA-6/kaolinite-PA-6 material is not exfoliated, theenhancement of properties must be related to the surface effects. The surface ofkaolinite particles is covered by -OH groups, which interact with PA-6 viahydrogen bonding. The particle size of kaolinite is larger than that of MMT,which might also contribute to the enhanced properties.

2.3.10.2 Micas and Synthetic MicasThe composition and polymorphism of micas vary considerably (see Section 2.2.2).There are 34 phyllosilicate minerals with a layered or platy texture that areclassified as micas. The commercially important micas are muscovite andphlogopite. Mica sheets are transparent to opaque, resilient, reflective, refractive,dielectric, chemically inert, insulating, lightweight and hydrophilic. Mica is alsostable when exposed to electricity, light, moisture and extreme temperatures. Ithas been fabricated into parts for electronic and electrical equipment.

Micas have a 2:1 sheet structure, similar to MMT, but the maximum chargedeficit is in the tetrahedral layers and contains K+ that is held tenaciously in theinterlayer space. The XRD basal spacing d001 ≅ 1.0 nm is broad and skewedtoward wider spacings. The illite’s CEC = 0.2-0.3 meq/g of dry clay. Since thenatural mica sheet diameter can be almost as large as one metre (e.g., in Calcutta– nowadays Kolkata – museum), the numerical value of the aspect ratio isridiculously large. The intercalation strategies for micas and smectites are thesame. Exfoliated mica sheets in polymeric matrix with aspect ratio of p > 1,200 havebeen reported [Yano et al., 1993; 1997].

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190


The early work on mica/polymer systems was aimed at improving theperformance of mica for electrical applications. For example, in 1981 Ishizakaand Fujii patented composites comprising an organosiloxane resin, mica andacid phosphates, suitable for use in the manufacture of mica products with superiormechanical strength, electrical characteristics, water resistance and heat resistance.

Synthetic micas are also available, e.g., Barasym SMM-100, a muscovite-type; mica-montmorillonite with CEC = 0.7 meq/g or Somasif ME100, afluoromica (or fluorohectorite) with CEC = 0.7-0.8 meq/g and d001 = 0.95 nm.By contrast with natural micas the synthetic ones have been reported as havinglow aspect ratio, thus inefficient for improving the barrier or mechanicalproperties. However, it may be that the low aspect ratio is not inherent, butrather caused by mechanical attrition during compounding in molten polymer.Micas: ‘DM clean A’ from Topy Ind. Co. have CEC = 1.19 meq/g and a highaspect ratio of p = 1230. The chemistry of mica was discussed in Section 3.4.

Yano et al. [1993] prepared CPNC with polyimide (PI) as matrix in which2 wt% of one of the four clays (hectorite, saponite, MMT and synthetic mica)was dispersed. The clays had CEC = 0.55, 1.00, 1.19 and 1.19 meq/g, and aspectratio: p ≅ 46, 165, 218, and 1,230, respectively. They were intercalated withDDA, filtered, washed and freeze-dried. The interlayer spacing was: d001 ≅ 1.5(hectorite), and 1.8 nm for the three other clays. To prepare PI/clay films, 2.49 wt %of organoclay was dispersed in dimethyl-acetamide and vigorously stirred for3 h at 90 °C, diamino diphenyl ether was added and the mixture was stirred at30 °C for 30 min, then pyromellitic dianhydride was introduced and the mixturestirred for an additional 6 h. The resulting solution was spread on a glass plate,solvent was evaporated off for 2 days then the film was heated at 100 °C for 1 h,at 150 °C for 1 h, and at 300 °C for 2 h under N2. A 60 μm thick film ofpolyimide with 2 wt% of clay was obtained. The XRD spectra showed thatCPNC with MMT and synthetic mica had no peaks indicating exfoliation, thiswas confirmed by TEM. XRD of PI with either hectorite or saponite showedpeaks at d001 = 1.5 nm, but smaller in intensity than those in the respectiveorganoclays. This implies that roughly one layer of organic molecules exists inthe interlamellar space. TEM of CPNC with saponite showed dispersion ofindividual platelets with a small number of tactoids. In the PI/hectorite systemmost clay formed aggregates. This difference in dispersability goes in the reversedirection to the aspect ratio, hence it is either the CEC or, more probably, thechemical reactivity of these clays that leads to these effects.

It is to be expected that the higher the aspect ratio and degree of dispersion,the higher the PI properties. This indeed is the case as far as water vapourpermeability is concerned (see Figure 40). Two elements account for the reductionof permeability (see also Part 5.3): dispersion of high aspect ratio of orientedplatelets and reduction of the free volume. The relative permeability coefficient(PR) (see Figure 41) is given by:

PR = P/P0 = d/d´ = [1 + pφ/2]-1 (23)

where φ is the volume fraction of the platelets, P is the permeability coefficient atplatelet content φ, P0 is that at φ = 0 and p = L/W is the aspect ratio. The relationwas derived assuming perfect alignment of individual clay platelets – if for agiven system this assumption is not obeyed, the aspect ratio necessary to fit suchdata will be a projection of the true aspect ratio on a plane perpendicular to the

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flux direction. As shown in Figure 40, the permeability reduction (at constantconcentration of clay in PI matrix) is fully accounted for by the clay platelets’aspect ratio. Furthermore, addition of 2 wt% of synthetic mica reduced the PIthermal expansion coefficient at 100 °C by 40%. Smaller factors were obtainedfor the other clays with smaller aspect ratio.

The same intercalation method was successfully used for Na-MMT andsynthetic fluoromica (FM, Somasif ME-110 from CO-OP Chem. Co.) [Kawasumiet al., 1997; Hasegawa et al., 1998]. First the clays were intercalated with ODA,then organoclay powder (7.3 wt%), PP (melt flow rate = 16 g/min; 70.8 wt%)

Figure 40 Relative permeability of polyimide containing 2 wt% of clays versusaspect ratio [Yano et al., 1997]. The line was calculated from Equation 23.

Figure 41 Tortuosity model: d = film thickness; d´ = tortuous path of diffusingmolecules; L and W are platelet diameter and thickness, respectively, hence the

aspect ratio p = L/W.

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and PP-MA (21.9 wt%) were dry-blended, then compounded in a TSE at 210 °C,incorporating about 5 wt% of clay (MMT or fluoromica). It was shown thatcompounding organoclay with PP caused a reduction of the interlayer spacing,while in the presence of PP-MA it increased to d001 = 5.9 and 6.4 nm for MMTand FM, respectively. In other words, neither one of these two clays was exfoliated,but the degree of dispersion was higher for the system with FM. The authorspresented a strong case for the critical importance of PP/PP-MA blend miscibility– well dispersed clay platelets were evident in TEM micrographs for misciblesystems, with a shoulder on the XRD spectrum indicating the presence of tightlyspaced short stacks.

In 1998 Katahira et al. [1998a,b,c,d] published a series of articles on the useof mica for the production of PA-6-based PNC. Thus, purified Na-mica flakeswere dispersed in hexanoic acid-ω-ammonium phosphate, prepared by treatingε-caprolactam with phosphoric acid. The intercalation was rapid even at 20 °C,but the exchange of Na+ was accelerated by heating to T > 60 °C. The monomer-intercalated mica showed d001 = 1.47 nm. During polymerisation (under low orhigh pressure) the mica exfoliated. Owing to the high dispersion of high aspectratio clay platelets excellent amelioration of the bending strength and moduluswere obtained for the CPNC. Thus, the formation of CPNC consisted of threesteps:

1. Protonation of ε-caprolactam by phosphoric acid,2. Intercalation of mica by ion exchange of Na+ with protonated lactam,3. Polymerisation at T ≥ 260 °C which expanded the interlamellar spacing all

the way to exfoliation.Badesha et al. [1998] prepared CPNC with fluoroelastomer matrix and dispersedin it mica-type clay (SCPX-984 from SCP). The aspect ratio of mica plateletsranged from p = 50 to 1000. The clay was first intercalated with 2M2ODA. Thefluoroelastomers of interest were copolymers and terpolymers of vinylidenefluoride, hexafluoropropylene, and tetrafluoroethylene, known commercially asViton, Fluorel, Aflas, Tecnoflon, etc. These may be cured with, e.g., a bisphenoland organophosphonium salt (accelerator). Other additives, such as colouringagents, processing aids, conductive fillers, initiators and accelerators may also beadded. The intercalated mica could be dispersed in the matrix by compounding,e.g., milling prior to curing. During compounding the fluoroelastomer chainspenetrate the organophilic clay, causing each platelet to be surrounded by polymer,hence exfoliating. The exfoliated nanocomposite may be formed with or withoutgoing through the intermediate stage of intercalation. An example describesaddition of 10 phr of organoclay to 100 parts of Fluorel, and then milling thecompound for 15 min on a two-roll rubber mill with a tight nip at 27 to 38 °C.XRD indicated that no intercalation had taken place. However, when the millingtemperature was increased to 50 °C, the interlayer spacing increased to d001 = 3.3 nm– hence intercalation. However, at still higher milling temperature, T = 66 °C,fully exfoliated CPNC was produced.

Zilg et al. [1999a] investigated the correlations between the polymercomposition, type of clay, its concentration and mechanical properties. The matrixwas based on diglycidyl ether of bisphenol-A (DGEBA) cured withhexahydrophthalic anhydride. Three types of clay were used:1. Synthetic fluoromica (FM; Somasif ME 100 from CO-OP Chem.),

193


2. Purified Na-MMT (particle diameter 15 μm, BET surface area 25 m2/g fromSüdchemie AG) and

3. Synthetic hectorite (FH; Optigel SH from Südchemie AG).

The clays were made organophilic by ion exchange with various alkyl ammoniumions: dodecylamine (DDA), N,N-dimethyl didodecyl amine (2M2DDA),N,N,N-trimethyl dodecyl ammonium chloride (3MDDA), N-methyl dodecylN,N-bis(2-hydroxyethyl)-ammonium chloride (MD2EtOH; AKZO EthoquadC12), 12-aminododecanoic acid (ADA), a,x-bis(aminopropyl)-terminatedoligo(propylene oxide), known as Jeffamine of the D series (JAD 130, 300, 500,800), N,N,N,N-dimethyl dioctadecyl ammonium chloride (2M2ODA) andN,N,N,N-dimethyl benzyl-octadecyl ammonium chloride (2MBODA). Uponintercalation the interlayer spacing of FM increased in the expected sequence,e.g., after ion exchange with MD2EtOH the neat clay spacing of d001 = 0.94increased to 1.74 nm, and then upon matrix polymerisation to d001 = 6.79 nm.The authors reported that (at 5 wt% of clay content) FM was difficult to dispersebeyond short stacks. Easier to disperse (but still in form of short, intercalatedstacks) was MMT, while FH exfoliated quite readily.

Enhanced toughness was associated with the formation of dispersedanisotropic laminated nanoparticles consisting of intercalated silicates. Theauthors confirmed the earlier report from Pinnavaia’s laboratory that d001 spacingincreases with the onium salt alkyl chain length, and that primary amines increasethe interlayer spacing more than the quaternary ones. During curing, the formermost likely reacted with epoxy which the latter were unable to do. With 10 wt%of the ammonium salt used on the silicates, not all were properly intercalated,i.e., XRD showed a diffraction peak indicating intercalation. On the other hand(e.g., for FH intercalated with 2MBODA) TEM micrographs showed randomlydispersed individual platelets, suggesting exfoliation. An interesting observationcame from comparing the spacings calculated from XRD and AFM micrographs,viz. 6.8 and 11 nm, respectively. The explanation offered is that the individualclay platelets are quite flexible and bend under the force of the AFM test tip.

The mechanical properties of four series of epoxy-based CPNCs aresummarised in Figure 42. Surprisingly, there is little improvement of propertiesby 2MBODA intercalation – the tensile strength of neat MMT or FH is higher(mid-concentrations) than that after intercalation. The best fracture toughnesswas reported for FM and MMT, intercalated or not.

In another publication from the same laboratory [Zilg et al., 1999b], CPNCsbased on a PU matrix were prepared by dispersing synthetic fluoromica (FM;Somasif ME100 from CO-OP Chem. Co.). The clay was manufactured by heatingNa2SiF6 with talc. The FM lamellae consist of a sheet of octahedral aluminasandwiched in between two sheets of tetrahedral fluorosilica. Because in theoctahedral sheet Mg2+ ions substituted for some Al3+ ones, the lamellae areanionically charged. Similarly, like MMT, FM behaves as a weak silicilic acid, withNa+ as counterions in the interlamellar gallery. During dispersion, the fairly largeFM particles of average diameter around 5,000 nm (aspect ratio p ≤ 5,000) werebroken down into anisotropic nanoparticles resembling short fibres of 500 nmlength and 20 to 50 nm diameter, thus the effective aspect ratio (peff) for the claywas only peff ≅ 20 to 50. Two series of CPNC samples were prepared, with non-intercalated and with intercalated FM. In the latter case, FM was intercalated with2MBODA (Ethoquad C12) in aqueous medium. Dry, intercalated FM was dispersed

194


in trihydroxy-terminated oligo(propylene glycol) with Mn = 3.8 kg/mol, which,after high shear mixing was cured with diisocyanatophenyl methane, acceleratedwith 0.6 wt% N,N-dimethyl-benzylamine at 80 °C. According to XRD, theinterlayer spacing of neat FM (d001 = 0.9 nm) increased in the CPNC to 8.8 nm.

Figure 42 Mechanical properties of four series of epoxy-based CPNC: MMT - notintercalated purified bentonite; MMT/B - MMT intercalated with N,N,N,N-dimethylbenzyl octadecyl ammonium chloride (2MBODA); FM/B - fluoromica intercalated

with 2MBODA; H - not intercalated hectorite; H/B - hectorite intercalated with2MBODA. Data [Zilg et al., 1999a].

195


The clay lamellae were well dispersed within the matrix, but without fullexfoliation. The mechanical behaviour of the two series of PU/FM nanocompositesis presented in Figure 43. Intercalation significantly improved the tensile strength(by 60-240%) and elongation at break (by 130-400%), but it had a negative effecton modulus. The CPNC seems to have clay platelets bonded to PU. In conclusion,dispersion of intercalated synthetic fluoromica in a polyurethane matrix results inthe formation of intercalated (not exfoliated) structures that offer significantimprovement of mechanical properties (in comparison to neat PU).

An interesting use of PA-6-based CPNC was reported by Cheng et al. [2000].The authors found significant differences in the suitability of PA-6 and its CPNC forthe preparation of microporous membranes. While neat PA-6 generated asymmetricmembranes with tight skin and a porous sublayer, the intercalated mica/PA-6 system(PA-6/mica nanocomposites, M1030D from Unitika; Mn = 15 kg/mol, MI = 19)produced skinless microporous membrane with an open, bicontinuous structurethat could easily be adjusted. Hence, while PA-6 produced membrane useless formicrofiltration, the mica-filled CPNC engendered perfectly adjustable, usefulproducts.

To close this discussion on CPNCs with non-MMT layered nanofillers, theuse of layered double hydroxides (LDH) should be mentioned. A recent reviewdescribed the synthesis and characterisation of these materials [Leroux and Besse,2001]. The LDH ideal structural formula is:

[MIIxMIII

1-x(OH)2]intra[Am-x/m·nH2O]inter

where MII and MIII are metal cations, A is the anion, and intra and inter denotethe intralayer domain and the interlayer space, respectively. The structure consistsof edge-sharing M(OH)6 octahedra. Partial MII to MIII substitution induces a positivecharge for the layers, balanced with the presence of the interlayer anions. LDH are

Figure 43 Mechanical behaviour of PU/FM and PU/FM-2MBODAnanocomposites. (Full symbols – PU/FM, i.e., with non-intercalated clay; opensymbols – PU with FM-2MBODA (Ethoquad C12)). Data [Zilg et al., 1999b].

196


often prepared via coprecipitation using MII and MIII salts at constant pH, mostlybasic conditions. Their charge density is significantly higher than that of Na-MMT,viz. 0.25 to 0.40 nm2/charge versus 0.70 nm2/charge for the latter. Correspondingly,the CEC for the LDH material ranges from 0.6 to 4.8 meq/g, whereas that forNa-MMT is about 1 meq/g. A lower CEC makes exfoliation easier.

There are three principal methods of LDH intercalation:

(a) Using such monomers as aniline, pyrrole, ω-aminoacid, methyl methacrylate,vinylbenzene sulfonate, vinylpyrrolidone, vinyl acetate, etc.;

(b) Direct intercalation of extended polymer chains in the host lattice, forexample, PEG/MoO3, poly(p-phenylene)/molybdenum bronze, PANI/MoS2,or PEG/NbSe2. Sometimes, a two-step intercalation (as for MMT) may haveto be used;

(c) Transformation of the host material into a colloid and precipitation in thepresence of the polymer.

Thus, numerous methods for the preparation of LDH/polymer systems have beenused, viz. coprecipitation, exchange, in situ polymerisation, two-step surfactant-mediated incorporation, hydrothermal treatment, reconstruction, or restacking.The latter method, effective via the exfoliation of the LDH layers, appears to bemore appropriate for the capture of monomers. Examples of polymer/LDHsystems are given in Table 31.

These multicomponent systems are thermally more stable than the pristineinorganic compounds, leading, for example, to potential applications in flameretardant composites. A large variety of LDH/polymer systems may be tailoredconsidering the highly tunable interlayer composition coupled to the choice of theorganic moiety. DNA may be stabilised in the interlayer space of (Mg2AlNO3) as agene reservoir. Some of the intercalated polymers present excellent physicalproperties such as conductive properties (PANI), insulation (PS), or ion-gateproperties (polypyrrole (PPY)), but are difficult to process because of a lack ofmechanical strength. Numerous studies have focused on the use of conductivepolymers as capacitor, rechargeable battery materials or in electrochromic windows.

Hsueh and Chen [2003] prepared LDH (by co-precipitation in NaOH solutionof Mg(NO3)2·with Al(NO3)3), with polyimide (PI) and epoxy matrix, respectively.According to TEM and XRD, in both nanocomposites the LDH were exfoliated.In parallel, the mechanical performance of these systems was enhanced. In CPNCwith a PI matrix the elongation at break (εb) and maximum tensile strength (σy)were obtained at ca. 4 and 5 wt% organoclay loading, respectively. Similarly, forCPNC with epoxy matrix εb reached maximum at about 3 wt%, while σycontinuously increased up to the highest organoclay concentration of 7 wt%.For both systems the tensile modulus and thermal decomposition temperaturealso increased, while the thermal expansion coefficient decreased with LDHloading. The fine dispersion of the inorganic component was illustrated byphotographs, which showed that in the whole range of LDH concentration theepoxy nanocomposites remain transparent.

While exfoliation of any layered nanofiller is easier in a matrix with polargroups, dispersion of clay or LDH in non-polar polymers is difficult. For thesereason two publication from B. Qu laboratories are of particular interest [Chenand Qu, 2003; Chen et al., 2004]. The first describes exfoliation of Mg3Al(OH)8-(C12H25SO4), or MgAl-LDH for short, by intercalation with PE-g-MAH in asolution of xylene under reflux.

197


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lAVP aC 66.0 lA 33.0 )HO( 2 )HO( -33.0 H· 2O a 8.1

(yloP α,β )etatrapsa-

gM 47.0 lA 62.0 )HO( 2 OC( 3)-2

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poc 80.2

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71.0 Hn·71.0· 2O poc ,02.113.1

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)locylgenelyhte(ylop=GEPdna,)etanoflusgnitalpmetasetacidnipocelihw,txetehtnidebircsederac,b,asdohteM*

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198


The platelets of LDH were about 0.48 nm thick and ca. 70 nm in diameter. Thenanocomposites containing 5 wt% LDH had a higher decomposition temperature(by ca. 60 °C), and greater thermal stability than PE-g-MA. In the secondpublication Zn3Al(OH)8(C12H25SO4), or ZnAl-LDH for short, was prepared byspontaneous self-assembly in an aqueous solution of Zn(NO3)2, Al(NO3)3, andC12H25SO4Na at pH ≅ 10. As before, the exfoliation of ZnAl-LDH wasaccomplished by refluxing its suspension in a xylene solution of (unmodified!)LLDPE for 24 h. The exfoliated nanocomposites contained up to 20 wt% ofZnAl-LDH. Better thermal stability was observed. The method is expected to beapplicable to other polymers, viz. PP, PS, rubbers, and the polar ones.

Another interesting layered material is Zr(HPO4)2·H2O, α-zirconiumphosphate (α-ZrP). Clearfield et al., synthesised it in 1964 and for years studiedits ion exchange ability [Clearfield et al., 1969; 1972]. In comparison to MMTthe α-ZrP has higher charge (CEC = 2), smaller interlayer spacing (d001 = 0.76 nm),and it can be prepared with a desired aspect ratio (at least 100) and particle sizedistribution. Crystalline α-ZrP has the layers formed by Zr and O atoms of thephosphate groups, with one -OH group pointing into the interlamellar galleries.More recently, to study the interrelations between CPNC structure andperformance, the team prepared the first nanocomposites with exfoliated α-ZrPin epoxy [Sue et al., 2004]. Thus, α-ZrP was synthesised, intercalated withmonoamine-terminated polyether (Jeffamine M715), and then 1.9 vol% of theintercalated α-ZrP was incorporated into DGEBA. According to TEM, XRDand the optical transparency of the cured nanocomposites, full exfoliation wasachieved. The influence of hydrogen-bonded α-ZrP intercalate on the mechanicalproperties of epoxy was investigated. The volume percentages of the specimencomponents were: α-ZrP = 1.9, Jeffamine = 18.7 and DGEBA + curing agent =79.4 vol%. Thus the cured specimen contained a large amount of the plastifyingintercalant: CH3O(CH2CH2O)14CH2CH2NH2. The rubbery plateau modulus ofthe nanocomposite was about 4.5 times higher than that of the matrix. Upon theaddition of 1.9 vol% α-ZrP, the tensile modulus increased by 50%, and the yieldstrength improved by 10%, but the elongation at break was drastically reduced.However, the mode-I critical stress intensity factor, KIC, indicated that the fracturetoughness is not significantly affected by the addition of the intercalated nanofiller– the epoxy resins are inherently brittle and/or notch-sensitive.

Nanocomposites of PA with up to 65 wt% of hydroxyapatite have beenprepared for use as a load-bearing bioactive material for bone repair or substitution[Jie et al., 2003].

2.3.11 Summary of the Intercalation MethodsClay intercalation for use as rheological additives, catalyst supports ornanoreinforcements has been practiced for nearly 70 years. The initial applicationswere for aqueous systems, and then for organic ones (e.g., thickening of oils orgreases). Intercalation for the preparation of CPNC is more recent, but the oldtechnology (developed for the greases) dominates the market. The developmentsare summarised in Table 32.

199


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201

Exfoliation of Clays

The high surface-to-volume ratio of nanoparticles leads to a high reinforcementefficiency. Thus, CPNCs with well dispersed platelets at a low clay loading of 2to 5 wt% show highly increased modulus, yield strength, DTUL as well as reducedflame propagation and permeability. In the crystallisable polymer matrix, theclay platelets (if not totally covered by the intercalant organic tails) promotefaster crystallisation and higher levels of crystallinity, which results in improvedsolvent and moisture resistance, but reduced impact strength. The presence ofclay may result in modification of the crystalline structure of the matrix polymer(e.g., α to γ transformation in PA-6), which may promote enhancement of theperformance characteristics.

Owing to the nature of the nanoscale reinforcement, the CPNC may be treatedas an improved grade of a homopolymer, hence it may be used as a replacementfor its matrix polymer in diverse multicomponent polymeric systems, viz. blends,composites or foams. For example, Akkapeddi in 2000 reported using CPNCfor making either short or long glass fibre (GF) reinforced composites, gettinggood processability (e.g., fast moulding cycle), low density, further improvementsof modulus, strength, moisture resistance, permeation barrier, etc. The materialswere aimed at automotive parts (viz. fuel system components, fuel tanks, doorand rear quarter panels, consoles, door panels, pillars, under hood components),packaging (containers, films for food and electronics packaging), appliances,building & construction, electrical & electronic, lawn & garden, power toolapplications, etc.

There are four basic structures for polymer/clay mixtures:

1. Conventional clay-filled composite with micron-sized aggregates of clayparticles.

2. Nanocomposites with intercalated clay.

3. Exfoliated nanocomposites with locally ordered structure, where the orderingis imposed by flow and concentration, φ > φmax.

4. Exfoliated nanocomposites with disordered structure, φ < φmax.

Not all performance characteristics depend to the same degree on exfoliation.However, as Kojima et al. [1993] have shown (see Table 33), the benefits increasewith the degree of dispersion. Thus, the goal of CPNC technology is to achievethe highest possible degree of clay exfoliation, i.e., the best dispersion anddistribution in a polymeric matrix.

Exfoliation is the last step in the preparation of CPNC. The methods forachieving it can be discussed under three titles:

2.4 Exfoliation of Clays

202


1. Polymerisation in the presence of organoclay.2. Melt compounding a polymer with a suitable organoclay complex.3. Other exfoliation methods:

• Combining the organoclays with latex.• Ultrasonic exfoliating of organoclay particles in a low MW polar liquid.• Others, viz. sol¯gel templating, co-precipitation, etc.

In Part 4 of this book the patent literature on CPNC is summarised, first withthermoplastic polymer matrices, then with those of thermosets and elastomers.Since the maximum performance is achieved when clay is exfoliated, theexfoliation methods are discussed there in depth. For this reason, this chapter onthe basic elements of CPNC technology will focus on the general aspects ofexfoliation technology.

2.4.1 PrinciplesThe layered inorganic materials that are to be used as nanofillers, e.g., natural orsynthetic clays, have strong ionic and van der Waals interactions and smallinterlamellar spacing, much too small for allowing organic molecules, monomers,oligomers or polymers, to diffuse into. Expansion of the interlayer spacing tototal exfoliation is accomplished in several steps.

Since clays are strongly hydrophilic, the first step is suspending them in anaqueous medium. Few types of hydrophilic, highly polar, low MW polymers(viz. PEG or P2VP) may be used to exfoliate directly such a swollen clay. For thegreat majority of CPNCs with hydrophobic polymeric matrices the swollen claymust be intercalated with one or two intercalants (e.g., onium salt and silane oran epoxy compound). The hydrophobic monomers and polymers require thatthe nanofiller is pre-intercalated, and often compatibilised.

The following four pathways have been distinguished [Usuki, 2001]:1. Hydrophilic matrix with strong polar groups, e.g., P2VP or PEG:

clay swollen clay CPNCwater hydrophilic polymer

clay swollen clay CPNCwater polar organic

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203


2a. Hydrophobic matrix with strong polar groups, e.g., PA or PEST:

clayintercalant(s)

intercalated claymonomer

expanded claypolymerisation

CPNC

2b. Hydrophobic matrix with strong polar groups, e.g., PA:

clayintercalant(s)

intercalated clay

+ polar polymercompounding

CPNC

3. Hydrophobic non-polar matrix, e.g., PP:

clayintercalant(s)

intercalated clay + compatibiliser

+ non-polar polymercompounding

CPNC

The difficulty in achieving exfoliation increases in the sequence from pathways 1to 3. The key to good performance is on the one hand obtaining initial expansionof the interlayer galleries for penetration by monomer(s) or polymer, and on theother good thermodynamic miscibility between the preintercalated clay plateletsand the polymeric matrix. In the case of the reactive exfoliation processes(pathways 1 and 2a) an important additional condition is that the polymerisationinside the gallery is at least as fast as that outside it. When this condition is notmet, the polymer formed outside the interlamellar galleries may hinder expansionof the interlayer space, leading to intercalation but not exfoliation. It is noteworthythat the clay and the intercalating onium salt often affect the polymerisation rate– one must ascertain that these effects are positive, i.e., that these catalyse thereaction. Another often forgotten requirement is the stability of the clay-matrixbond. The CPNC must survive the forming stage! There are very few polymersthat can be processed below 200 °C. Above this temperature the ammoniumintercalants (especially quaternary) are not thermally stable (especially in thepresence of oxygen and shear field). Reaggregation of clay platelets (that wereexfoliated during in situ polymerisation) during melt processing of CPNC hasbeen reported.

As the listed pathways indicate, either a single one- or a two-step sequentialintercalation-cum-compatibilisation may be required. Thus, the pathways focuson the chemical aspects of intercalation. Unspecified in these are the contributionsof physics: the thermodynamics that controls the phase behaviour as well as Pand T effects on the interlayer spacing, molecular diffusion, flow which introducesdispersive forces, radiation-absorption which may accelerate the exfoliationprocess under the influence of, e.g., ultrasonics, etc.

Pathway 1 has been presented in sufficient detail in Section 2.3.5.2, hencethere is no need to discuss it again. Pathway 2a is pertinent to virtually anypolymeric nanocomposite, with thermoplastic, thermoset or elastomeric matrixhence a brief description of these systems will be given. Finally, pathways 2b and3 belong to the most important mechanical exfoliation methods and need moredetailed analysis.

204


2.4.2 Polymerisation in the Presence of OrganoclaySeveral different methods have been explored to prepare CPNC by chemical reactions.Following Usuki’s suggestions [Usuki, 2001], one may categorise them as:

1. Monomer intercalation – viz. preparation of PA-6 nanocomposites.2. Monomer modification – viz. preparation of acrylic-based nanocomposites.3. Non-reactive intercalated clays – viz. preparation of styrenic-type CPNC.4. Co-vulcanisation – viz. preparation of NBR-based nanocomposites.5. Common solvent method, frequently used to prepare PI-based CPNC.6. Others.In the methods 1 and 2 the clay is intercalated with a compound that subsequentlyenters the polymerisation reaction – polycondensation in method 1, and radicalpolymerisation in method 2. Thus, as a result of either of these two processesend-tethered CPNC is obtained.

2.4.2.1 Monomer Intercalation – PA-6 Nanocomposites

Preparation of CPNC via in situ polymerisation was used at the Toyota Institutefor the first nanocomposites with PA-6 as matrix. The process involved threesteps [Deguchi et al., 1992]:1. Intercalation of Na-MMT with ω-amino dodecanoic acid chloride in water

– the interlayer spacing increased from d001 = 0.96 (dry) to 1.3 (wet) to 1.8nm (intercalated).

2. Mixing the intercalated clay with ε-caprolactam and water at the ratio of1:9:9 - the interlayer distance further increased to d001 = 3.87 nm. At thisstage, a catalyst and an activator were added.

3. Polymerisation at 100 and 250 °C for 48 h followed by TSE extrusion. Ahigh level of exfoliation was obtained, with rare short stacks containing two-to-three clay platelets evident in TEM micrographs.

The following year, Usuki et al. [1993a] found that clay, intercalated withω-amino dodecanoic acid can be swollen by molten ε-caprolactam (30 to 98 wt%).Polymerisation under nitrogen at 250 to 270 °C for 48 h resulted in well-dispersedsilicate layers in a PA-6 matrix. The TEM images showed single platelets even inCPNC containing 31 wt% MMT. At 2 wt% of organoclay loading, the modulusincreased to 1.5 times that of PA-6, the heat distortion temperature increasedfrom 75 to 140 °C, and the moisture permeability was reduced by 50%. At thesame time the density increased from 1140 to 1150 kg/m3, i.e., by a theoreticallypredictable 0.88% [Ube Industries, Ltd., 2000]. The method was reportedeconomic, efficiently producing a PNC with any PA matrix, having a wide viscosityrange for all processing operations. The material had good mechanical properties,heat resistance, improved dye affinity and whitening resistance during stretching.

When discussing exfoliation it is important to pay attention to clayconcentration and the aspect ratio, p. During shear flow platelets rotate and themaximum packing volume fraction of such ‘encompassed volume’ spheres isφmax = 0.62. It can be shown that for such platelets:

φmax = 0.93/p (24)

When: φ > φmax the platelets cannot rotate and have to locally align. In this casethe distance between them can be calculated from geometry as:

205


d h w h a a wclay clay polymer local clay001 0 1100 1= − + ≈ + +( / )( / ) /ρ ρ (25)

where hclay is the platelet thickness, ρclay and ρpolymer are densities, and wlocal is thelocal clay concentration (in wt%), a0 and a1 are corresponding equationparameters.

Owing to the tendency of the clay platelets to align parallel to each other inlocal stacks, the clay concentration within the stack is not known, hence theparameters in Equation 25 cannot be calculated from first principles. However,since it can be assumed that the concentration within stacks is proportional to theaverage, global concentration: wlocal ∝ w (wt%), d001 should be inverselyproportional to the clay content. Thus, for organoclays with large aspect ratiothe formation of local stacks takes place at low concentration, and the interlayerspacing may be small, not because of a lack of exfoliation, but due to the crowdingeffects. It is worth noting that at φ > φmax the interlayer thickness is independentof the aspect ratio – p only dictates the concentration at which the local stackingstarts.

Okada and Usuki [1995] published data on the interlayer spacing in PA-6/clay,measured by XRD and TEM. Using Equation 25 the maximum packing for freeplatelet rotation is predicted for organoclay content below 1.13 wt%. From therelation, at a clay content of 2 wt% the interlayer spacing d001 is 44.2 nm (seeFigure 44). Evidently, starting with ω-aminoacid end-tethered CPNC wereproduced.

Emulsion polymerisation was used to prepare CPNCs with PMMA as a matrix[Choo and Jang, 1996]. Thus, MMA was polymerised in the presence of Na-

Figure 44 Basal spacing in CPNC versus organoclay content as measured by XRDand TEM. The organoclay was MMT intercalated with ω-dodecyl acid:

NH2(CH2)11COOH and ε-caprolactam . Data [Okada and Usuki, 1995]. The lineindicates Equation 25 dependence with a0 = 1.37; a1 = 0.856; and the correlation

coefficient r = 0.991.

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MMT. The intercalating PMMA macromolecules were found oriented parallelto the clay lamellae. The interlayer distance decreased with MMT loading fromd001 =1.73 nm at 10 phr to 1.28 at 50 phr (33 wt%). Both the thermal stability(char formation on decomposition) and tensile properties were enhanced. Thesystem was not end-tethered, but the ion-dipole interactions were strong enoughto be the driving force for the introduction of MMA monomer into theinterlamellar galleries and bonding of the PMMA molecules to clay surfaces.

This work was extended to the emulsion copolymerisation of ABS in thepresence of clay [Jang et al., 2001]. The clay content varied from 10 to 50 phrand correspondingly the interlayer spacing changed from d001 = 1.75 to 1.56 nm.Thus, it seems that the process results in expansion of Na-MMT particles (ca.500 nm diameter) by monomer intercalation. In either case clay was not exfoliated.Similar results were obtained by adding an aqueous dispersion of Na-MMT toSBR latex, then coagulating the emulsion, washing, drying and curing [Zhang etal., 2000b]. Clay ‘bundles’ 4 to 10 nm thick and about 200 to 300 nm long wereobserved, thus intercalation not exfoliation was obtained.

Better results for MMT exfoliation in PMMA matrix were obtained using themonomer modification method, described in Section 4.2.2.2. Exfoliation wasalso obtained using a polar intercalant-compatibiliser, e.g., PCL [Kim et al.,2001a]. The other approach is based on clay intercalation with a compound thatchemically participates in the subsequent polymerisation.

Full exfoliation was reported for a PE/MMT system prepared by Ziegler-Nattapolymerisation of ethylene in the presence of organoclay [Jin et al., 2002]. Thelatter was Cloisite® 30B (MMT with MT2EtOH) treated with TiCl4 to affix thecatalyst to the intercalant –OH group. As was expected, polymerisation in thepresence of Na-MMT had little effect on the clay interlayer spacing. The authorsprocessed the exfoliated polymerisation product by compression moulding or TSEextrusion. The processing resulted in a partial re-aggregation of MMT plateletswith d001 ≅ 1.4 nm. The extent of re-aggregation depended on the processingconditions – it was slight for compression moulding, but quite pronounced forextrusion.

The polymerisation route was also selected by Alexandre et al. [2002a,b].Thus PE was polymerised in the presence of either MMT or hectorite. The clayswere first treated with trimethyl aluminium-depleted methyl aluminoxane, andthen a Ti-based constrained geometry catalyst and monomer were incorporated.The tensile properties of the resulting CPNC were poor and essentially independentof the nature and content of the silicate. When hydrogen was used to control themolecular weight of PE the tensile modulus was significantly increased (from0.69 to 2.48 GPa for PE and PE with 11.4 wt% clay), but at a cost of a dramaticreduction of the strain at break (from 244 to 3%, respectively). Clay exfoliationin the reaction products was confirmed by XRD and TEM. However, as it hasbeen reported before, during melt processing of the CPNC the interlayer spacingpartially collapsed. Evidently, the system is thermodynamically unstable and thestrong solid-solid interaction between clay platelets drives the phase separation.

2.4.2.2 Monomer Modification – Acrylic-Based Nanocomposites

Usuki et al. [1995] introduced the term ‘monomer modification’ to indicate thatthe organoclay was intercalated with a reactive compound, which subsequentlyparticipates in polymerisation. One may presuppose that preparation of PA-6/clayCPNC (which starts with ω-amino dodecanoic acid) also belongs to this category.

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However, while PA polycondensation results in a single attachment of amacromolecule to a clay platelet, the monomer modification strategy results in acopolymerisation of the reactive monomers, one type of these being ionicallybonded to clay. Thus, the resulting copolymer may have numerous molecularunits bonded to the clay surface. Monomer modification will usually start withintercalation of clay using molecules that have at least one group capable ofreacting with other monomer(s), forming a multitethered, exfoliated CPNC.

This approach has been used for the preparation of PMMA/organoclay [Biasciet al., 1994]. The organoclay was MMT intercalated with quaternary ammonium:either 2-(N-methyl-N,N-diethyl ammonium iodide)-ethylacrylate (QD1) or with2-(N-butyl-N,N-diethyl ammonium bromide)-ethylacrylate (QD4) – owing tothe bulkiness of the intercalating molecules only up to 58% of the clay activesites were exchanged. The radical copolymerisation of MMA with the intercalatedclay was carried out either in bulk or in acetonitrile at 60 °C. As a result ofintercalation and subsequent polymerisation the interlayer spacing increased tod001 = 1.45 to 1.78 nm, respectively. The length of MMA sequence in the formedcopolymers was 3.5 and 1.1 to 8.2, after bulk and solution reaction, respectively.Apparently, bridging the interlamellar gallery by the macromolecular chainprevented exfoliation. When the sequence was reversed (intercalating Na-MMTwith MMA/QD1 or MMA/QD4 copolymers) the interlayer spacing increased tod001 = 2.96 nm and thermal stability was enhanced.

Usuki et al. [1995] prepared CPNC by first copolymerising ethyl acrylate(90 mol% of EA: CH2=CHCOOC2H5), acrylic acid (10 mol% of AA:CH2=CHCOOH), and a quaternary ammonium salt of dimethyl aminopropylacrylamide: Q = (CH2=CHCOOCH2CH2CH2N+(CH3)2C3H7)Cl– (0.12, 36, 56, and0.89 mol%). Next, the copolymer was mixed with an aqueous suspension ofNa-MMT to induce intercalation. Four CPNCs were prepared with a clay contentof 1, 3, 5, and 8 wt%. CPNCs with MMT ≥ 3 wt% showed high viscosity andyield stress. In comparison to the neat copolymer, incorporation of the end-tetheredclay increased viscosity by 4 to 6 orders of magnitude. The CPNCs were used forthe preparation of transparent films (crosslinked with melamine). Gas permeabilitythrough the film followed Equation 23, with p ≅ 100 (see Figure 45).

Heinemann et al. [1999] prepared PO-based PNCs starting with hectoriteintercalated with dimethyl benzyl stearyl ammonium chloride (2MBODA), havingd001 = 1.96 nm. When HDPE was polymerised in the presence of the intercalatedclay, the XRD peak shifted to d001 = 1.40 nm. Best results were obtained forLLDPE-type ethylene-octene copolymers. The authors reported superiorperformance of the reactively prepared CPNCs over that prepared by meltcompounding. However, the intercalated clay was found to interfere with themetallocene catalysts and the processing of these CPNCs resulted in re-aggregationof the clay platelets.

A bulk polymerisation method was used to prepare PS-based PNC [Fu andQutubuddin, 2000; 2001], starting with MMT intercalated by dimethylvinylbenzyl dodecyl ammonium chloride (2MVBDDA). Owing to good miscibilityof 2MVBDDA with styrene, a uniform dispersion of platelets was obtained.Polymerisation for 48 h at 60 °C resulted in exfoliation. At 7.6 wt% of clay, themodulus of the PNC was about 70% higher than that of PS.

MMT was intercalated with either oligo (n = 25)-ethylene-glycol diethyl methylammonium chloride (SPN) or with methyl-trioctyl ammonium chloride (STN).The organoclays were dispersed in either MMA or styrene by ultrasonication for

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7 h then radically polymerised. For comparison, PMMA and PS were alsosynthesised in the presence of the quaternary amines [Okamoto et al., 2000;2001b,c]. In the earlier publication, polymerisation of the CPNCs systematicallyresulted in decreased interlayer spacing, viz.:

1. MMA/STN; before polymerisation d001 = 2.96 - after 2.66 nm.2. MMA/SPN; before polymerisation fully exfoliated - after a shoulder at

4.55 nm.3. Styrene/SPN; before polymerisation nearly totally exfoliated with only a small

shoulder at d001 = 3.65 - after a large peak at 3.65 nm.This behaviour suggests a thermodynamically driven phase separation. Thedynamic mechanical temperature scans indicated only a minor change in thestorage modulus. In the latter publication only the SPN organoclay was used.Polymerisation of the MMA/SPN system was carried out as in the earlierpublication, but in the presence of small amount of polar comonomer, viz.N,N-dimethyl aminopropyl acrylamide (PAA), N,N-dimethyl aminoethylacrylamide (AEA) or acrylamide (AA). The radical polymerisation resulted inexfoliation. The storage modulus versus T scan indicated a significant increase,viz. from 1 to 3 GPa.

Similarly good exfoliation was reported for PS/organoclay systems [Zhang etal., 2003]. The authors used one non-reactive intercalant (3MHDA) and threereactive ones, each containing one methacryloyloxy and two methyl groups, andin addition either a benzyl, octyl or hexadecyl group. Na-MMT was pre-intercalated with each of these, and then dispersed in styrene. The suspensionwas transformed into oil-phase for γ-ray initiated suspension polymerisation. At5 wt% organoclay loading, exfoliation was obtained for all three reactiveorganoclays, while only intercalation occurred with the MMT-3MHDA.

Figure 45 Oxygen permeability through crosslinked acrylic film versus claycontent. The line was calculated from Equation 23 with p = 100 . Data [Usuki et

al., 1995].

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Soap-free emulsion polymerisation of MMA was used to prepare exfoliatedCPNC with PMMA as a matrix [Choi et al., 2001]. Thus, the reaction was carriedout in two steps. In the initial step, Na-MMT was dispersed in water with 25%(of the total amount) of MMA and 2-acryl-amido-2-methyl-1-propane sulfonicacid (AMPS). The ratio of MMT/AMPS varied from 8 to 67. AMPS played atriple role as a surfactant, intercalating agent and coreactant. In the second step,the remaining monomer was used to complete the reaction. XRD and TEMshowed that the PNCs containing up to 10 wt% of Na-MMT were exfoliated.Surprisingly at 10 wt% of Na-MMT the exfoliation occurred after 10 min ofreaction. The molecular weight of PMMA in PNC slightly decreased with theAMPS content, while the glass transition temperature (Tg) and storage modulus(E´) significantly increased.

2.4.2.3 Non-Reactive Intercalated ClaysTranslucent acrylic nanocomposites were recently described [Dietsche et al., 1999;2000]. The CPNC was prepared by polymerisation of methyl methacrylate-dodecyl methacrylate copolymer in the presence of 2-10 wt% bentoniteintercalated with N,N,N,N-dimethyl dioctadecyl ammonium (2M2ODA).Addition of n-dodecyl methacrylate improved interactions and accounted for theimproved stiffness-to-toughness balance, higher Tg and thermal stability, incomparison to the corresponding copolymer. Evidently, this polymerisationstrategy does not create covalent bonding between clay and the acrylic matrix.The only bonding that could take place is by cocrystallisation of the C12 paraffinicchains.

Recent patents from Eastman [Barbee and Matayabas, 2000; Matayabas etal., 2000] described PET-based CPNC with enhanced barrier properties. Thus,MMT was intercalated (with a quaternary onium salt) and then treated with asecondary intercalant, e.g., PEG, PCL or vitamin E. The organoclay (d001 = 2.2 to4.2 nm) was incorporated into PET either by polycondensation or meltcompounding, followed by solid-state polymerisation. TEM showed mostlyindividual platelets with only few tactoids and aggregates.

PC-based CPNC was prepared starting with PC-cyclomer and MMTintercalated with dimethyl ditallow ammonium cation (2M2TA; B34 from Rheox,Inc.; d001 = 2.47 nm) well washed with water/ethanol to remove excess quaternaryammonium salt [Huang et al., 2000]. Cyclic oligomers of PC have lower solutionand melt viscosities (compared to the corresponding polymer), thus theintercalation process is significantly easier. For example, the organoclay wasdispersed in CH2Cl2 and mixed with either PC or PC-cyclomer. After 5 min ofmixing with PC, the basal reflection was identical to that of B34. By contrast,intercalation with PC-cyclomer was quick, increasing d001 to 3.62 nm. Theenhanced intercalation rate may be related not only to the lower viscosity, henceenhanced diffusivity of the cyclomer, but possibly to the difference in moleculararchitecture as well, the absence of end groups and the intermolecular interactionsbetween the cyclomer and the clay surface.

Similar differences between the intercalation processes of PC and PC-cyclomerwere observed during melt processing. After 24 h of melt annealing with PC d001= 3.27 nm was obtained – the same as obtained by intercalation from solution.By contrast, melt annealing with PC-cyclomer readily resulted in d001 = 3.8 nm.Furthermore, when a mixture was compounded in an internal mixer for 1 h at180 °C, an exfoliation was achieved. Subsequently, raising the temperature to

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240 °C for 10 min, caused ring-opening polymerisation, converting the cyclomerto linear PC (MW = 40 kg/mol). TEM showed the presence of individual layersas well as tactoids consisting of 3 to 5 platelets. Thus, again partially exfoliatedCPNC was obtained without covalent bonding between the clay and the matrix.

2.4.2.4 Co-Vulcanisation

Toyota was the first to develop a process for producing polyolefin (PO)-basedCPNC [Usuki et al., 1989]. However, the patent claims extend to ‘vinyl-basedpolymeric compound, a thermosetting resin and a rubber’. Ammonium salt havinga terminal vinyl group was used to intercalate MMT. The product was mixedwith a vinyl-based monomer and/or oligomer, e.g., ethylene, propylene, butadiene,methylmethacrylate, styrene, etc. The mixture could be polymerised either inbulk, suspension or in solution reaction, by a radical, cationic, anionic,coordination or condensation mechanism. Polymerisation took place within theinterlayer space, expanding the interlayer distance to d001 > 3.0 nm.

However, to prepare exfoliated CPNC with a rubber, first MMT was reactedwith low molecular weight liquid rubber. For example, in an aqueous solution ofDMSO a liquid acrylic rubber (amino-terminated butadiene-acrylonitrile rubber(ATBN), MW = 3400, 16.5% AN), HCl, and Na-MMT (clay platelet thickness= 1.0 nm, p = 1000, CEC = 1.19 meq/g) were dispersed. The reaction productwas filtered and dried. Pulse NMR indicated that a strong bond was formedbetween MMT and ATBN, with ca. 20% of the rubber molecules restricted nearthe interface. XRD indicated that the (001) peak of MMT had disappeared,d001 > 8.8 nm, and the MMT platelets were uniformly dispersed in the matrix.Next, the complex (containing 32.5 wt% of MMT) was cooled with liquid N2,crushed by a hammer mill and mixed with NBR (AN = 33%). The blends(containing 5 or 10 wt% MMT) were vulcanised with sulfur. Superiorperformance in tensile, dynamic viscoelastic, and swelling tests were observed.

A similar method has been used to prepare ATBN-intercalated MMT,subsequently mixed with NBR and vulcanised [Kojima et al., 1993a]. TEM ofthe resulting CPNC showed intercalated, well-dispersed short stacks of MMTdispersed in the matrix. The permeability reduction for H2 and H2O through theCPNC (containing 3.6 wt% of MMT) when compared to a standard rubberwith 10 vol% of carbon black was reduced by 37 and 26%, respectively.

2.4.2.5 Common Solvent Method – Polyimide Based NanocompositesYano et al. [1993; 1997] discovered that organoclay intercalated by ion exchangewith dodecyl ammonium chloride (DDA), could be homogeneously dispersedin N,N-dimethyl acetamide (DMAc). Thus, to prepare PI/clay films, organoclaywas dispersed in DMAc, then diamino diphenyl ether, pyromellitic dianhydridewas added and the solution stirred for 6 h. The film was cast from ahomogeneous mixture of organoclay and polyamic acid, and was heated at300 °C to polymerise. With 2 wt% of clay the water and CO2 permeabilitydecreased to 50%. XRD and TEM showed that the CPNC was exfoliated [Lanand Pinnavaia, 1994].

As an extension of this procedure, nematic liquid crystal (LC) with (up to2 wt%) MMT was prepared [Kawasumi et al., 1998]. Thus, MMT wasintercalated with a variety of ammonium cations, including 4-cyano-(4´-biphenyl-oxy)-undecyl ammonium salt, which showed enhanced miscibility to the LC.

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The intercalated clay was than dispersed in dimethyl formamide (DMF) and theLC was added. The solvent was slowly evaporated at 50 °C under vacuum, whilestirring. When the clay had good affinity for LC, the system was homogeneouslydispersed. The CPNC exhibited a bi-stable and reversible electrooptical effectbetween a light scattering state and transparent state, which could be selected bychanging the frequency and voltage of the applied electric field.

In 1999 Yang et al., further examined the common solvent method for thepreparation of CPNC with PI as matrix. The work focused on the influence ofthe intercalation agents upon dispersability of organoclay in a selected solvent(DMAc) as well as that in PI. First, Na-MMT was intercalated with either aminoacids, primary aliphatic amines or quaternary ammonium salt, viz.: (1)p-aminobenzoic acid (ArNCO), (2) ethanolamine (HONH), (3) N,N-dimethylaminoethyl methacrylate (DMAEM), (4) dodecylamine (12CNH), (5) 1-hexadecylamine (16CNH), (6) hexadecyl-trimethyl amine (3MHDA) and (7)6-aminohexanoic acid (6NCO). The interlayer spacing of the organoclaysincreased from d001 = 1.26 nm (Na-MMT) to, respectively, 1.27, 1.29, 2.16,2.87, 3.70, 4.05, and 4.96 nm. However, the dispersability did not follow thesame order – the best dispersability in DMAc and PI was observed for theintercalants 1, 2, 5 and 7, the worst for 3 and 6. Evidently, the dispersabilitydepends on the type of functional groups present in the solvent or PI and thebulky group of the intercalation agent. Thus, one can disperse clay aggregates(or tactoids) well without exfoliation, but the CPNC properties depend on thehomogeneous dispersion of MMT platelets.

The organosoluble PI was based on pyromellitic dianhydride (PMDA) and4,4´-diamino-3,3´-dimethyl-diphenyl methane (MMDA). The organoclay wasadded to DMAc and agitated for 3 h at 90 °C before adding it to MMDA solutionin DMA, which was followed by addition of PMDA. The mixture reacted atroom temperature for 6 h then it was cast on a glass and heated at 100 °C for6 h, 150 °C for 4 h and 270 °C for 2 h under N2 to obtain MMT/PI hybrids.

The properties of the final product greatly depended on the degree ofdispersion. Only when the MMT was well dispersed did the CPNC show goodperformance, e.g., simultaneous high strength and toughness, improved thermalstability, decreased thermal expansion coefficient, retention of the solubility ofthe polyimide matrix and high optical transparency. The chemical structure ofan intercalation agent imposes great influence on the dispersion of MMT. MMTmodified with 1-hexadecyl amine (HDA) showed the best dispersion behaviourand the best set of properties.

Tyan et al. [1999a,b] observed that imidisation of polyamic acid (PAmA) isaccelerated by the presence of organoclay. The organoclay was prepared byintercalating Na-MMT (CEC = 0.764 meq/g) with p-phenylene diamine. Duringthe intercalation only one amine group was converted into a cation (–NH3

+).The other -NH2 group was able to react with the dianhydride end group ofPAA when these molecules diffused into interlayer galleries. PAA was synthesisedby dissolving 4,4´-oxydianiline (ODAn) and pyromellitic dianhydride (PMDA),in DMAc at 25 °C under N2. To a viscous PAmA solution a suspension oforganoclay in DMAc was added then mixed to obtain PAA/organoclay/DMAc.During the reaction, the intercalant became an integral part of the PI molecules,making these nanocomposite more thermally stable and mechanically stronger.The spin-coated films containing 0, 2, 5 or 7 phr of organoclay were dried thenheated at 150, 200, 230 and 250 °C. XRD indicated that the interlayer spacing

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of organoclay was d001 = 1.549 nm, but all PAA/organoclay systems wereexfoliated. In the presence of organoclay the imidisation temperature and theimidisation time were reduced. For example, the imidisation temperature wasreduced from 300 to 250 °C, while at 250 °C the reaction time was reduced to15 min (for 7 phr of organoclay). Initially, the reaction followed the first-orderkinetics:

ln(1-p) = -kt (26)

where p is the extent of reaction at a reaction temperature T, k is the rate constantand t is the reaction time. The temperature effects were well-described by theArrhenius dependence:

k = A exp{-Ea/RT} (27)

where A is the pre-exponential factor, R is the gas constant, and Ea is the activationenergy (see Figure 46).

In the next paper 4,4´-oxydianiline (ODAn) was used as an intercalant [Tyanet al., 2000]. After drying in a vacuum oven at 80 °C TGA indicated the presenceof 5.41 wt% structural water and 29.0% of the intercalant. The interlayerspacing of the organoclay was d001 = 1.5 nm. PAmA was prepared by dissolvingODAn in DMAc under N2 then adding 3,3´-4,4´-benzophenone tetracarboxylicdianhydride (BTDA). The system was stirred for 1 h, which produced a viscousPAmA solution to which different quantities of organoclay suspension in DMAcwere added (to yield 0, 1, 2, 3, 5, and 7 wt% of organoclay in the product),and then mixed for 12 h. The organoclay-PAmA suspension was cast, and thensolvent was removed under vacuum at 30 °C over 48 h. Imidisation was carriedout in an air circulation oven at 100, 150, 200, and 300 °C for 1 h and then at400 °C for 5 min. In the final product full exfoliation was obtained. The

Figure 46 Arrhenius plot of the imidisation rate constant for 0, 2, 5 and 7 phr oforganoclay in the CPNC. Parameters of the Arrhenius equation are listed. Data

[Tyan et al., 1999a,b].

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modulus, the maximum stress and the elongation at break of these CPNCs areincreased with organoclay content – see Figure 47.

CPNC with PI as a matrix has also been produced starting with MMT(CEC = 1.15 meq/g) intercalated with N-hexadecane pyridinium chloride (HP-MMT)[Gu and Chang, 2001]. PAA was prepared in a THF/MeOH solution, dissolvingin it at room temperature first ODAn then pyromellitic dianhydride (PMDA)and mixing the resulting solution for 3 h. Then a solution of triethylamine (Et3A)was added and the process took another 3 h of stirring to complete. Next, to astirred viscous pale-yellow solution a suspension of HP-MMT was added at 30 °C.After 6 h of stirring, the solution was coated on a glass plate to a thickness of250 μm. The film was dried at 25 °C for 30 min, 40 °C for 30 min, and 80 °C for2 h, and then cured at 150, 200, and 300 °C under N2 to produce a transparentPI-based CPNC film with a thickness of 26 ± 2 μm. Results showed that theorganoclay was fully exfoliated in the polymer matrix, had high modulus(2.38 GPa), tensile strength (110 MPa), elongation at break (11%), and lowcoefficient of thermal expansion (35%, compared to 100% for neat PI).

In the next paper from the group, the same HP-MMT was used to prepareCPNC with PI as a matrix using two methods [Gu et al., 2001]:

1. Blending a DMAc solution of ODAn with the organoclay dispersion in DMAcbefore adding PMDA; and

2. Blending a DMAc solution of PAmA with the organoclay dispersion in DMAc.Compositions containing 1, 3, 5 and 10 wt% of organoclay were prepared – theinterlayer spacing was: d001 = 2.2 to 1.4 nm, respectively. Tensile, thermal, dielectricand water absorption properties of these CPNCs were studied. The propertiesdepended on the clay type, clay concentration, and the method of preparation.The best performance was obtained using 3 wt% HP-MMT prepared by the

Figure 47 Mechanical properties (tensile modulus, E; tensile strength, σ; andelongation at break, ε) of PI with MMT-ODA. Data [Tyan et al., 2000]. The three

functions are well approximated by straight lines – their parameters are given.

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second method: tensile strength = 110 MPa, tensile modulus = 2.35 GPa,elongation at break = 14.5%.

Delozier et al. [2002] used several methods to prepare PI-based CPNC, withMMT that was intercalated with a long chain aliphatic quaternary ammoniumcation. The methods included:

1. Mixing the organoclay into a high molecular weight poly(amide acid) solution;2. Mixing as in (1), followed by sonication; and3. In situ preparation, starting with dispersion of organoclay in N-methyl-2-

pyrrolidinone (NMP), then addition of 4,4´-oxydianiline (ODAn) and3,3´,4,4´-benzophenone tetracarboxylic dianhydride (BTDA) to prepare highmolecular weight poly(amide acid). The imidisation step involved a high-temperature treatment (i.e., 1 h each at 100, 200 and 300 °C in air or N2).

The best results were obtained using the latter approach - the specimen obtainedusing the other methods had aggregated clay particles that caused film failure intensile tests (stress concentration). However, high shear mixing using ahomogeniser improved intercalation and exfoliation, but the films were extremelybrittle making mechanical properties impossible to measure.

The CPNCs that contained 3-8 wt% of organoclay, were characterised by DSC,TGA, TEM, XRD and thin film tensile properties. After thermal treatment of amideacid films that caused imidisation, the films with clay were darker than those withoutand the interlayer spacing was reduced. During imidisation at T = 200 to 300 °Cthermal degradation of the quaternary ammonium took place. The effect was lesspronounced when imidisation was carried out under N2. The degree of dispersionwas assessed from TEM micrographs. There is evidence that in the in situ preparedCPNC exfoliation dominates. The exfoliated particles were 200¯700 nm long and1¯10 nm thick. However, XRD analysis indicated that the interlayer spacing wasabout constant, d001 = 1.34 nm - the peak intensity increased with clay content. Itis noteworthy that this spacing is closer to that known for Na-MMT than for theorganoclay (d001 = 2.37 nm). The polyimide/organoclay hybrid films exhibitedhigher room temperature tensile moduli, but lower strength and elongation tobreak than the control films. The Tg for the neat ODAn¯BTDA system was 283 °Cand remained constant within 4 °C for the hybrid films. At 5 to 8 wt% of clay thetemperature for 5% weight loss was ca. 517 °C.

Chang et al. [2001b,c] studied the preparation of CPNC in polybenzoxazole(PBO) matrix. The common solvent method was chosen with DMAc as the solvent.MMT (CEC = 1.19 meq/g) was intercalated with primary hexadecyl ammoniumsalt (MMT-HDA). Polyamic acid (PAmA) was synthesised in DMAc, then thesuspension of MMT-HDA in DMAc was added. The solution was cast, solventwas evaporated at 50 °C and the films, 10-15 μm thick, were thermally treated.The conversion to PI was carried out at 300 °C for 1 h under N2. The final conversionto PBO was accomplished at 550 °C. CPNCs with 0, 1, 2, 4 and 8 wt% of organoclaywere prepared. Good exfoliation in systems containing up to 4 wt% MMT-HDAwas found, although short stacks were visible in TEM micrographs. The specimenswere thermally stable up to 611 °C, retaining about 75% of weight at 900 °C– clay presence hardly affected these properties. However, the tensile strength wasdoubled for 4 wt% loading and modulus increased by 37%.

A one-step method for the solution preparation of PI nanocomposites wasalso proposed [Huang et al., 2001a,b]. First, MMT was intercalated with primary(dodecyl or hexadecyl, DDA or HDA, respectively) or quaternary (trimethyl

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hexadecyl, 3MHDA) ammonium ions. The organoclay was then dispersed inm-cresol by mixing and ultrasonication for 1 h at 100 °C. To the suspension thereactants (diphenyl ether-tetracarboxylic anhydride and diamino dimethyldiphenyl methane) were added. The reaction was conducted at room temperaturefor 2 h, and at 180 °C for 3 h. The solution was cast, then heated stepwise at 70,120, 200 and 270 °C for a total of 22 h. The interlayer spacing of MMT(d001 = 1.3 nm) increased upon intercalation to 1.75, 3.37 and 4.01, for DDA,HDA and 3MHDA, respectively. Nanocomposites comprising: 0, 2, 3.2, 5, 10and 20 wt% of MMT were prepared with MMT-3MHDA. Fully exfoliatedCPNCs were obtained only for 2 and 3.2 wt% of clay. The mechanical propertieslinearly increased with clay content up to 5 wt%. In the full range of investigatedcompositions the decomposition temperature increased from 518 to 524 °C, whileTg(PI) = 286 °C increased by 4 °C and the coefficient of thermal expansion (CTE)hyperbolically decreased following the dependence:

CTE = a0 + a1/(a2 + w) (28)

The PI data for MMT content (w) versus CTE followed the dependence witha0 = 30.0 ± 4.0; a1 = 203.9 ± 78.5; a2 = 5.19 ± 1.59; and standard deviationσ = 1.85, correlation coefficient squared r2 = 0.9995 and coefficient ofdetermination (CD) = 0.988 (see Figure 48).

Delozier et al. [2003] investigated the influence of the cation exchange capacity(CEC = 0.63 to 1.11 meq/g) on the degree of exfoliation in a PI matrix. To adjust theCEC the Na-MMT was ion-exchanged with 1N LiCl, then centrifuged and placed inan air circulating oven at T = 120 to 170 °C for 24 h – the higher the temperature,the lower the resulting CEC. Next, the treated clay was intercalated with an aromaticprimary di-ammonium compound: 1, 5-di(3-amino-phenoxy)-3-oxapentane (BAOD).CPNC with polyamic acid was prepared in NMP (16% solids) at room temperature,under N2 in 24 h. The solution was cast, dried, then cured at T ≤ 300 °C. The firstseries of specimens was prepared with clays having different CEC, but at a constant

Figure 48 Coefficient of thermal dispersion of PI containing 0-20 wt% of MMT.Data [Huang et al., 2001a,b]. The broken line follows Equation 28.

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clay loading of 3 wt%. XRD showed the presence of a peak for all specimen withcorresponding values of d001 = 1.20 to 1.31 nm, with the largest value for CEC≅ 0.7 meq/g. However, TEM showed a random dispersion of individual clay plateletsand short stacks. Using these selected clays, the second series of specimens wasprepared with clay loading of up to 8 wt%. A small enhancement of the tensilemodulus was accompanied by reduction of strength and elongation at break. TGAshowed a significant increase of thermal stability.

Better dispersion was obtained by dispersing MMT-DDA in a NPM solution ofphotosensitive polyamic acid (PAA) [Hsu et al., 2003]. The latter was prepared bypolymerising pyromellitic dianhydride, oxydiphthalic anhydride, and oxydianiline.The photosensitive formulations contained 2,3,4-tris(1-oxo-2-diazonaphthoquinone-5-sulfonyloxy)-benzophenone as the photosensitiser and 3 wt% organoclay. Thefilms were transparent and tough. XRD and TEM indicated exfoliation beforeand after thermal imidization. The thermal expansion coefficient was 23% lowerthan that of film that did not contain the organoclay. The tensile modulus increasedby 14%, while the strength and elongation at break decreased by 15 and 33%,respectively.

An intrinsically photosensitive PI/MMT system was prepared by solutionpolymerisation at 180 °C of 4,40-diamino-3,30-dimethyldiphenylmethane(MMDA) with benzophenone-3,30,4,40-tetracarboxylic dianhydride (BTDA) andisoquinoline in the presence of MMT-HDA [Liang et al., 2004]. The solutionwas either cast or spin coated, devolatilised and imidized at T ≤ 280 °C. XRDand TEM indicated exfoliation in the full range of MMT concentrations. Excellentmechanical properties were obtained – in the full range of MMT content (0 to3 wt%) the modulus, tensile strength and elongation at break increased by 211,48 and 11%, respectively. At the same time the thermal expansion coefficientwas reduced by up to ca. 32%, and Tg increased by 6 °C. While the addition oforganoclay did not affect the inherent PI solubility, the presence of MMTsignificantly reduced the rate of solvent absorption. Finally, the photolithographicproperties of PI remained unaffected by the clay presence, but only forconcentrations up to 2 wt%.

Polybenzoxazines are thermoset phenolic resins developed to overcome theshortcomings of the novolacs and resoles. They show excellent properties, viz.heat resistance, good electrical properties, flame retardance, dimensional stability,toughness, stable dielectric constant, and low moisture absorption - all these atrelatively low cost. They can be synthesised from inexpensive raw materials,cured without strong acid or base catalyst, and do not release by-products duringpolymerisation, thus they are attractive candidates for many applications [Riesset al., 1985].

Agag and Takeichi [2000] prepared CPNC of polybenzoxazine (PBOa) byfirst intercalating Na-MMT with octyl (OA), dodecyl (DDA) or stearyl (ODA)ammonium chloride, then mechanically mixing with different amounts of aPBOa precursor, bifunctional bis(3-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl)isopropane (B-a). The ring opening polymerisation of pristine B-a started at223 °C, while in the presence of organoclay at T = 177 to 190 °C. According toXRD, the MMT intercalated with either DDA or ODA became totally exfoliatedin the product. Viscoelastic measurements showed that the Tgs of the CPNCswere higher than that of neat PBOa. Increased storage modulus, decompositiontemperatures and thermal stability were also noted. However, as the authors

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remarked, the method was difficult because of the high viscosity of the monomerand the small difference between its gelation temperature and the melting point.

More recently, several types of CPNCs with PBOa as a matrix have beenprepared [Takeichi et al., 2002]. The authors described the effects of thepreparation method and that of type and content of organoclay on the CPNCproperties. Two monomers were used: B-a and monofunctional 3-phenyl-3,4-dihydro-2H-1,3-benzoxazine (P-a). These produced crosslinked PB-a and linearPP-a matrix, respectively. Na-MMT (CEC = 1.19 meq/g) was intercalated withammonium salts of such amines as tyramine, 2-phenyl-ethyl amine, aminolauricacid, and DDA.

The nanocomposites were prepared either by melt or solution method. Themelt method involved incorporating organoclay into B-a or P-a above its meltingpoint (Tm = 100 and 60 °C, respectively). The solution method involved dispersingthe organoclay in a solvent NMP or tetrahydrofuran (THF) at 80 °C for 1¯2 hand then blending with either B-a or P-a at 80 °C for 3 h. The blends were caston glass plate, dried at 60 °C for 16 h, then cured at 100, 150, 200 and 240 °Cfor 1 h each. Good dispersion was obtained in NMP, but poor in THF. TheCPNCs were transparent, red-wine coloured with thickness ranging from 0.2 to0.4 mm.

The intercalation increased the interlayer spacing, from d001 = 1.24 nm ofNa-MMT only to ≤ 1.83 nm. Thus, it is remarkable that using either method ahigh degree of exfoliation in the cured nanocomposites was obtained. Evidently,changing the solvent, concentration and procedure affected the result, viz. whenTHF was used, a small XRD peak remained indicating intercalated clay, butwhen NMP was used total exfoliation was achieved. As before, increasing theorganoclay concentration reduced the level of platelet dispersion. The effect ofclay addition on P-a was much larger than on B-a. The smaller P-a moleculespenetrated into the clay galleries more easily and the number in contact with thecatalytic clay surface should be larger than with the bigger B-a molecules. Themelt method also resulted in exfoliated structures [Agag and Takeichi, 2000].

DSC showed that the incorporation of any type of organoclay lowered thecuring temperature. The effect was particularly large for a small amount oforganoclay - addition of 5 wt% reduced the onset curing temperature by ca.50 °C, suggesting a catalytic effect of the organoclay on ring openingpolymerisation. A plot of Tg versus clay content showed a maximum at about4 wt%, with all the CPNC values higher than that of neat resins. As evidencedby TGA, an addition of organoclay improved the thermal stability – the bestresults were obtained for MMT intercalated with tyramine (p-ethyl-amine phenol).

In summary, the work on PBOa-type CPNCs illustrates the importance ofmiscibility and chemistry on the exfoliation process. Even when the organoclayhad small expansion of the interlamellar galleries (Δd001 ≤ 0.59 nm), but was welldispersed in a solvent and/or monomer, good exfoliation was achieved after curing.As in thermoplastic systems, here also the best results were obtained when thematrix polymer was chemically bound to clay by reactive intercalant (tyramine).High energy shearing may help, but it does not replace good thermodynamicsand chemistry of the system.

The common solvent method has also been used to prepare POSS-typenanocomposites in PI matrix [Tsai and Whang, 2001a]. The polyimide/polysilsesquioxane-like (PI/PSSQ-like) films had 3D structure with linear PI blocks

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and a crosslinked PSSQ-like structure. The system showed higher thermal stabilityand char yields than pure PI from 4,4´-diamino-diphenyl ether and3,3´-oxydiphthalic anhydride (ODPA). In a series of X-PIS films (PI modifiedwith p-amino-phenyl tri-methoxysilane (APTS), where X is the molecular weightof each PI block), decreasing the PI block length enhanced the storage modulus,tensile modulus, and Tg but reduced the α-relaxation damping peak intensity,density and elongation. The change in the moduli and Tg may be caused by anincrease in the crosslinking density and rigidity of the network. The changes inthe peak intensity and density may be caused by increased free volume or the PIinterblock separation and decreases in the interblock PI chain interaction. Thedecrease in elongation is related to increased rigidity. The activation energy ofthe α-transition depended on the length of the PI block. A maximum value wasreached for the chain length of 10 kg/mol because of two opposing factors:crosslinking density and free volume.

In a series of X-PIS-y-PTS (X-PIS modified with phenyl trimethoxysilane (PTS),where y is the weight-ratio percentage of PTS to APTS-polyamic acid) films witha constant PI block length, the storage modulus, tensile modulus, Tg, density,and α-relaxation damping peak intensity decrease with the PTS content. Thiscould be because of the increase of the PSSQ-like domain size with the PTS content,which leads to the introduction of more free volume or interblock separationand to a decrease in the interblock chain interaction force.

2.4.2.6 Other Methods – Epoxy-Based NanocompositesCPNCs with thermoset matrix were already considered in the preceding sectionwhere the common solvent methods were discussed. However, owing to thespecificity of the epoxy systems it seems desirable to summarise specifically thework carried out on their synthesis. A short review on these systems was recentlypublished [Wang et al., 2000a]. As these authors remarked, the strategy for thepreparation of epoxy-based CPNC depends on whether the cured system is glassy(with Epon 826) or elastomeric (with Epon 828 – see Figure 5). In the first case,it was advantageous to disperse organoclay in epoxy overnight at 50 °C beforeadding the curing agent. In the second case, organoclay should be added directlyto a mixture of epoxy with a curing agent. Kornmann, in his doctoral thesis of2001 offers an overview of nanocomposites technology with a special emphasison the preparation of CPNC with epoxy resin matrix.

Pinnavaia et al., explored the use of epoxy compounds as the second intercalant oflayered materials. As early as in 1994 [Wang and Pinnavaia, 1994; 1998a,b], exfoliationof organoclays in a glassy epoxy resin was reported. The process involved heating pre-intercalated smectite with epoxy resin at T = 200-300 °C. Synthetic Na+-magadiitewas intercalated with a series of ammonium ions, viz. CH3(CH2)17NH3-n

+(CH3)n,where n = 0-3, by ion-exchange reaction in the presence of neutral amine [Wang etal., 1996]. For primary amine (ODA or C18; n = 0) the interlayer spacing, d001 =3.82 nm, was found consistent with the structure of onium ions and neutral amineinclined at an angle of about 65°. As the number of methyl groups increased from n= 1 to 2 and 3, d001 decreased to 3.74, 3.20 and 3.41 nm, respectively. Epon 828 wasused along with Jeffamine (PPG-bis(2-aminopropyl ether)) as the curing agent. Thedesired amount of organomagadiite was added to the mixture and stirred for 60min, degassed and cured at 75 °C for 3 h and at 125 °C for a different length of time.When that time exceeded 5 min, even for systems with 15 wt% organoclay, fullexfoliation was recorded. In the cured epoxy matrix good exfoliation was obtained

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for magadiite intercalated with either the primary or secondary onium ions (n = 0 or1). In the case of higher amines (n = 2 or 3) XRD indicated an interlayer spacing ofd001 = 7.82 and 4.10 nm, respectively. The observed strong effects of the intercalantstructure was interpreted as due to the high acidity of ODA, which is reduced by thepresence of methyl groups, especially when n = 2 or 3. The acidity is responsible forthe intercalant’s catalytic effects on the curing reaction within the interlamellar galleries,which lead to exfoliation. Good enhancement of the tensile strength was reported.The enhancement was shown to depend on the degree of exfoliation, e.g., at 10 wt%of clay the tensile strength increased by a factor of 4.8, 3.5, and 2.3 for the exfoliated,ordered exfoliated and intercalated systems, respectively (n = 1-3). The transparencyand high barrier properties makes these CPNCs attractive as packaging materialsand protective films.

The early work resulted in several patents [Pinnavaia and Lan, 1998a,b]. Theauthors observed that the extent of epoxy resin diffusion into the interlamellargalleries depends on the hydrophobicity controlled by the chain length of thealkylammonium cations. Thus, to start with Na-MMT was cation exchangedwith alkylammonium cations with different alkyl chain length (CnH2n+1, wheren = 4, 8, 10, 12, 16 or 18). Equivalent amounts of the epoxy resin (Epon 82827.5 wt %) and the polyether-amine (72.5 wt %) were mixed at 75 °C for 30 min,then 10 wt% of the organoclay was added, mixed and cured. The XRD resultsindicated that the clays with alkyl chain length n ≥ 10 were exfoliated. Themechanical performance was significantly enhanced by exfoliation.

In later patents [Pinnavaia and Lan, 1998c; 2000a,b], Na-MMT was treatedwith HCl to prepare protonated clay, H-MMT (d001 = 1.05 nm). Next, polyether-amine was added to the aqueous suspension and stirred at room temperature for6 h (d001 = 4.6 nm). After evaporation of the suspending medium the gel ofintercalated clay was mixed with Epon 828. The XRD pattern showed absenceof the clay diffraction peaks, suggesting exfoliation. The tensile strength andmodulus were superior in comparison to CPNC prepared with alkylammoniumcation intercalated clay (see Figure 49). However, it is worth stressing that thepresence of ≥ 5 wt% MMT reduced the heat of reaction [Butzloff et al., 2001].Exfoliation was observed only at a MMT concentration below 2.5 wt% – abovethis limit the system was progressively less exfoliated and more intercalated.

Intercalated CPNCs in glassy epoxy resin were obtained by dispersingNanomer I.28E (MMT-3MODA) into Epon 825 at 65 °C, degassing and curingwith a stoichiometric amount of Jeffamine D400 at 65 °C for 15 min [Zerda andLesser, 2001]. The XRD data indicated that the interlayer spacing of organoclay,d001 = 2.28 nm, increased during pre-swelling in Epon to 3.42 nm, then shrankduring curing to 3.21 nm. The tensile modulus increased from 2.8 GPa for neatresin to 3.2 GPa at 12.5 wt% of clay. However, the tensile stress was reducedfrom 70 to 46 MPa. For the middle range of organoclay concentration (2.5 to 10wt%) the fracture energy nearly doubled. The results are understandableconsidering the presence of large scale (10-30 μm) aggregates, evident in SEMmicrographs. The measured fracture energy release rate (G1C) was shown todepend on the generated surface area, as measured by AFM.

Lü et al. [2001] studied the dispersability of MMT intercalated with eitherCH3(CH2)17NH3

+- (ODA) or CH3(CH2)17N(CH3)3+ (3MODA) in epoxy resin,

which was a reaction product of the diglycidyl ether of bisphenol-A (DGEBA;MW = 380 g/mol) with p,p´diaminodiphenylmethane (DDM) or methyl-tetrahydrophthalic anhydride (MeTHPA). The CPNC was prepared in two steps:

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1. Dispersing organoclay in DGEBA (either directly at 70-80 °C for 20-60 min,or in chloroform at room temperature for 60 min), and

2. Incorporation of the curing agent followed by degassing, casting and curing.The products have been studied using XRD and DSC.

Dispersing the organoclays into epoxy increased the interlayer spacing fromd001 = 2.4 to 3.7 nm, indicating enhanced intercalation. The two mixing proceduresgave equivalent results. Using DDM as a curing agent, full exfoliation was rapidlyachieved for MMT intercalated with the primary amine (ODA), but not whenMMT was intercalated with the quaternary onium (3MODA) - evidently, thequaternary ammonium ion does not have the catalytic activity observed for ODA.However, anhydride curing with MeTHPA resulted in exfoliation of eitherorganoclay. As the authors remarked, it is important that the exfoliation time isshorter than the gel time – in the studied systems both these processes requirednearly the same time, viz. at 80, 100, 120 and 150 °C the exfoliation time (asobserved by XRD) was 60, 15, 8 and 5 min, respectively. Furthermore, theexfoliation must take place before the extragallery epoxy matrix reaches its gelpoint. The factors facilitating the curing inside the interlamellar gallery spaceenhance the exfoliation.

Nanomer 1.30E (MMT-ODA) was dispersed in a matrix obtained by curingEpon 828 with m-phenylene diamine (MPDA) by stirring and sonication forabout 30 min and degassing under vacuum [Chin et al., 2001]. To the mixture 5,14.5 or 25 phr of MPDA was added at 60 °C, the mixture was heated for 2 h at80 °C and for 2 h at 135 °C. The structural evolution in the CPNC was examinedby time-dependent small-angle X-ray scattering (SAXS) using synchrotronradiation. A conventional XRD, AFM and DSC were used to probe the staticstructure and the reaction kinetics. The interlayer spacing of the Nanomer 1.30E

Figure 49 Tensile strength and tensile modulus of epoxy-based CPNC. Thesubscript ‘H’ indicates a new method of intercalation that starts with protonatedH-MMT; ‘18’ indicates systems where clay was intercalated with C18 H37 NH3

+

cation. Data from [Pinnavaia and Lan, 1998c; 2000a,b].

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was d001 = 2.3 nm, increasing upon dispersion in Epon to 3.9 nm. Addition of thecuring agent and heating for 2 h at 80 °C did not change the latter spacing.Heating the mixture at 135 °C for additional time caused progressive expansionof the interlamellar space toward exfoliation: d001 ≥ 20 nm. The latter spacingwas observed for less than an equimolar amount of the curing agent and clayconcentration up to 5 wt% (for 20 wt% clay d001 = 8.3 nm). When Epon wascured with more than equimolar MPDA, exfoliation was suppressed – high curingagent concentrations seem to favour extragallery crosslinking. Exfoliation wasobtained for the amount of curing agent ranging from zero to less than itsstoichiometric amount. The time-dependent SAXS measurements indicated thatexfoliation proceeded by progressive delamination of the stacks.

While most authors prepare CPNC with epoxy (DGEBA cured with acidanhydride) matrix using either MMT-ODA or MMT-3MODA, recently MMTpre-intercalated with an ‘alkyl ammonium salt with hydroxyethyl groups’ wasused [Zhang et al., 2004]. According to TEM, at 3 wt% of organoclay loadingonly intercalated stacks with d001 ≅ 5.5 nm were obtained. However, in spite of alack of exfoliation, excellent performance was obtained: impact strength increasedby around 85%, tensile strength by 22%, storage tensile modulus (at T < Tg) by33% and Tg increased by ca. 14 °C.

A new approach to clay exfoliation in epoxy matrix involves the use ofaromatic amine hardener as the clay intercalant, dispersing such organoclay inepoxy, and then curing the system [Ma et al., 2004]. For example, m-xylylene-diamine (m-C6H4(CH2-NH2)2 or MXD for short) was acidified and used topre-intercalate Na-MMT. The epoxy (DGEBA) was added to the desired quantityof purified organoclay suspension, and after stirring the mixture was dried. Finally,CPNC was obtained by curing the mixture with a stoichiometric amount of4-aminophenyl sulfone (DDS). Within 2θ = 1 to 9o XRD did not show any peakthat may have indicated exfoliation. However, TEM showed the presence ofrandomly dispersed, ca. 3 nm thick clay platelets, possibly bridged by acidifiedMXD. The authors speculated that the disorderly exfoliated structure originatedfrom the large crosslinked epoxy molecules connected to the clay platelet via theMXD hardener-intercalant. No information on performance of this CPNC wasgiven.

Kornmann et al. [2001] studied the effect of the cation-exchange capacities(CEC) on the formation of CPNC in epoxy matrix. Two MMTs with CEC = 0.94and 1.4 meq/g (MMT09 and MMT14, respectively) were intercalated with ODA.Epon 828 and a polyoxyalkylene diamine curing agent (Jeffamine D-230) wereused. First, the epoxy was mixed with the desired amount of organoclay at 75 °Cfor several hours, and then a stoichiometric amount of Jeffamine was added. Themixture was outgassed under vacuum, poured into a mould, then cured for 3 h at75 °C and post-cured for 12 h at 110 °C. The dispersion was studied using XRD,SEM and TEM. The interlamellar spacing of the two clays increased fromd001 = 0.97 for MMT09 to 1.72 nm for organophilic MMT09-ODA, and fromd001 = 1.21 for MMT14 to 2.14 nm for the MMT14-ODA. Surprisingly, swellingthe former organoclay in Epon resulted in exfoliation (d001 > 8.8 nm), whereas thelatter slowly intercalated to reach d001 = 3.47 nm after 24 h. The high surfaceenergy of MMT is known to attract polar species such as epoxy molecules hencethey diffuse into the interlamellar galleries. If these are able to polymerise, the nextmolecules are able to diffuse into the interlamellar galleries causing exfoliation,hence self-polymerisation of epoxy in MMT09-ODA may be postulated. Within

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MMT14-ODA there is a significantly larger amount of the intercalant present,leaving less space for the Epon molecules to diffuse into, and probably lessopportunity to contact the ammonium group. It is expected that curing the alreadyexfoliated mixtures of Epon with MMT09-ODA will result in exfoliated CPNC.However, exfoliation (as defined by the absence of a 001 peak in XRD spectra)was also achieved in systems comprising MMT14-ODA.

Although XRD showed exfoliation, TEM indicated that clay platelets formedstacks with interlamellar spacing of 11 and 9 nm for MMT09 and MMT14,respectively. Furthermore, a XRD peak at 2θ = 20° that corresponds tocrystallographic planes 110 and 020 of the clay, was observed and its intensityincreased with the clay content. Its presence demonstrates that the XRD analysisis sufficiently sensitive to detect the presence of the clay in the nanocompositeand to quantify it. SEM was also used to investigate dispersion of MMT14 in thenano and micro-composites. The latter system was prepared by dispersing thenon-intercalated clay in the matrix. Here XRD showed two peaks: a broad oneat d001 = 1.4 nm and a sharp one at 0.97 nm. For both systems clay aggregateswere observed, albeit significantly smaller in the (supposedly) exfoliated nano -than in the microcomposite (ca. 10 μm particles).

Recently, high performance epoxy-nanocomposites were prepared by dispersingsynthetic sodium fluorohectorite (FH; Somasif ME-100; CEC = 1.0 meq/g) in amatrix composed of tetraglycidyl 4,4´-diaminodiphenyl methane (TGDDM) curedwith 4,4´-diamino diphenyl sulfone (DDS) [Kornmann et al., 2002]. The FH wasintercalated using one of the following compounds: ODA, 1-methyl-2-norstearyl-3-stearinoacid-amidoethyl-dihydro-imidazolinium metho-sulfate (W75),hydroxyethyldihydro-imidazolinium chloride (HEODI), and ricinyl-dihydro-imidazolinium chloride (RDI). ODA is known to produce organoclays that can beexfoliated during curing of epoxy systems, while the dihydro-imidazolines are knownto impart good thermal stability. The matrix was N,N,N´,N´-tetraglycidyl 4,4´-diamino-diphenyl methane (TGDDM) cured with 4,4'-diamino-diphenyl sulfone(DDS). The influence of the intercalants on curing reactions, the morphology, matrixTg and the mechanical properties was investigated. TGDDM epoxy resin was heatedto 100 °C under vacuum in a high shear mixer then an organoclay was added andthe mixture was mixed for 3.5 h. Next, the temperature was increased to 140 °Cand a stoichiometric amount of the DDS was added, mixed for 30 min and thenpoured into a mould. Curing was performed in several steps, e.g., 2 h at 140 °C +2 h at 177 °C + 7 h at 200 °C.

DSC studies of the curing reaction indicate that FH has no influence onTGDDM polymerisation – the reaction with or without the clay had theexothermic peak at Tpeak = 316 °C. However, 10 wt% of FH-W75, FH-HEODIor FH-RDI reduced the exothermal peak by 6 ºC and with 10 wt% of FH-ODA,by 11 °C. Thus, the intercalants do catalyse curing, but the dihydro imidazolinesare not as effective as the ODA ions. This suggests that FH-ODA would producethe best exfoliation, if not for the thermal decomposition of ammonium ionswhich starts near 200 °C.

The interlayer spacing of intercalated FH and that of cured epoxynanocomposites is presented in Table 34. Intercalation expands the interlamellargallery by Δd001 = 0.96 to 2.26 nm. Once the organoclays are dispersed in theepoxy matrix, the gallery increases for FH-ODA, FH-HEODI and FH-RDIwhereas it decreases for FH-W75. In the latter case the TGDDM and the DDSwere not able to diffuse between the layers, the polymerisation took place outside

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the interlamellar galleries, compressing them by Δd001 = 0.7 nm. Apparently, FH-W75 is immiscible with some components of the epoxy matrix. At least part ofthe reason for immiscibility is that only partial ion exchange took place duringintercalation. None of the nanocomposites produced contained fully exfoliatedclay. TEM confirmed the interlayer spacing deduced from the XRD data – indeed,the largest average spacing was observed for FH-RDI, but some nonintercalatedplatelets were also visible. The SEM micrographs also indicated the presence ofclay aggregates, particularly bad (some as large as 20 to 30 μm) for FH-W75 andFH-ODA. This is an important observation! Without the evidence of SEM onewould concentrate the interpretation of data on the nanometre-scale morphology,whereas for a number of performance characteristics (for example the impactproperties) the micron-scale aggregates may be more important.

The tensile properties of the nanocomposites were examined. The clayconcentration effect on the tensile modulus is shown in Figure 50. The results forFH-W75 are clearly the worst, falling even below the microcomposites withnonintercalated FH. Of the remaining three systems, the best is the one with FH-RDI and the worst with FH-ODA – here at last the expected correlation with thedegree of platelet dispersion is observed. The fracture energy for all the intercalatedsystems was found to be higher than that for the microcomposite with FH, butagain the data for FH-W75 were the lowest and these of FH-RDI the highest. At5 vol% clay loading the fracture energy was nearly twice as large as that for theneat matrix resin.

Epoxy resins are inherently brittle and several methods have been used toimprove toughness without loss of stiffness. Since CPNC with an epoxy matrixfrequently show improved modulus and reduced ductility, a growing number ofpublications have appeared, trying to solve this problem. Thus, for example,Ratna et al., 2003 used a combination of linear (DGEBA) and 11-arm starbranched epoxy to prepare CPNC with 5 wt% of MMT-ODA. A high degree ofintercalation was obtained. In spite of the phase separation between the twoepoxies, good performance was reported, viz. enhancement (in comparison to

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cured DGEBA) of flexural strength and modulus by 21 and 26%, respectively,and impact strength by 114%.

Fröhlich et al. [2003] toughened their epoxy (hexahydrophthalic acidanhydride-cured DGEBA) with PPG-PEG liquid rubber treated with methylstearate. Up to 15 wt% organoclay (synthetic fluorohectorite pre-intercalatedwith MDD2EtOH) was dispersed as short stacks with d001 = 2.94 nm. The thermal,and mechanical properties were determined. Addition of the additives reducedthe modulus and improved the toughness. In parallel, the yield strength, and Tgalso decreased. At a later date the same epoxy system was reinforced by a diversityof layered silicates intercalated with phenolic alkyl imidazoline amide cations[Fröhlich et al., 2004]. Natural and synthetic clays (bentonite and fluorohectorite,respectively) were pre-intercalated and used at loadings of up to 10 wt%. Forcomparison, the same epoxy was prepared with talc as well. Intercalationexpanded the interlayer spacing to about d001 = 3.3 to 3.7 nm, but dispersing it inepoxy slightly reduced the gallery height (by ca. 0.3 nm). The mechanicalperformance of the samples with organoclay was quite similar to that of the talc-filled composite. The poor performance of organoclay-filled CPNC seems to havebeen caused by immiscibility between the organoclay tails and the matrix.

2.4.2.7 Other Methods – PU-Based Nanocomposites

Polyurethanes (PU) have been mixed with nanosize particles for a number ofdecades. Carbon black, metal and silica particles have been used. Three examplesselected from between many will be mentioned.

2.4.2.7.1 Metal ParticlesPU was dissolved in dimethyl acetamide and a solution of metal ions (iron, cobalt,nickel and copper) was added [Chen et al., 1996]. The metal ions were reduced

Figure 50 Tensile modulus versus fluorohectorite (ME-100) content in hightemperature epoxy systems. Data [Kornmann et al., 2002]. For explanation of the

Figure legend see text.

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by sodium borohydride under mild conditions into amorphous fine powders.Polar polymer and low metal concentration favour smaller particles. The polymerchains prohibited excessive aggregation of the metal atoms and had a protectiveeffect on the fine metal powders. An energy disperse X-ray spectrometer connectedwith TEM proved that the dispersed particles were metal clusters with the sizesranging from 10 to 150 nm.

2.4.2.7.2 SilicaA series of PU nanocomposites was prepared with 0-50 wt% of silica, having aparticle size of about 12 nm [Havni and Petrovic, 1998]. SEM showed regulardistribution and small spacing between neighbouring particles at allconcentrations. For all samples XRD showed a single broad maximum at about6° and a shoulder at 20°. The Tg of PU increased with the silica concentration. Aparallel series of PU filled with micron size silica had higher density, hardnessand modulus than nanosilica filled systems, but the tensile strength and elongationat break were dramatically better for the latter systems. Composites with nanosilicawere clear and transparent while those with micron size silica were opaque.Petroviç et al. [2000] prepared PU nanocomposites by dispersing amorphousSiO2 particles (average diameter 12 nm) in MEK. Next, PPG was added, MEKwas removed, MDI added and the system was cured at 100 °C for 16 h. The finalconcentration of SiO2 was up to 50 wt%. Properties of these nanocompositeswere compared to those of microcomposites containing crystalline SiO2 with anaverage particle diameter of 1.4 μm. The nanocomposites showed 3-fold highertensile strength and elongation at break, but lower modulus and hardness.

The sol-gel process has also been used to generate nanosized silica particleswithin a PU matrix [Goda and Frank, 1997]. At lower concentration the silicapenetrated the hard segment domains of PU, at higher it disrupted its orderedstructure.

2.4.2.7.3 Cadmium Sulfide Particles (CdS)Nano-CdS particles were prepared by reverse micellisation [Hirai et al., 1999].They were dispersed into PU, by surface-modification with 4-hydroxythiophenolor 2-mercaptoethanol, followed by polyaddition of ethylene glycol with toluene-2,4-diisocyanate. The resulting CdS-PU powder could be dissolved in organicsolvents such as DMF to make nanoCdS/PU transparent films that showedquantum-size effects. These were used for photocatalytic generation of hydrogen.

2.4.2.7.4 OrganoclaysSeveral already cited broad patents on CPNCs have stated that the inventedtechnology is also applicable to PU [Usuki et al., 1989; Nichols and Chou, 1999,Beall et al., 1996a; 1999b]. One of these general patents was granted to Pinnavaiaand Lan [2000a,b]. However, the cited examples are only for epoxy-based CPNC.The patent information will be discussed later on in this book.

PUs are prepared by the reaction of isocyanate and polyols:

HO-R-OH + OCN-R´-NCO → -(-CO-NH-R´-NH-CO-O-R-O-)n-

The polyol component -R- usually contains a base, such as a secondary or tertiaryamine. Therefore, these may react with the protons within the clay interlamellargallery enhancing the intercalation.

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Wang and Pinnavaia [1998b] prepared thermoset PU-based CPNC by curinga PU network in the presence of MMT intercalated with C12H25NH3

+ orC18H37NH3

+ (Nanocor C12A and C18A, respectively). The organoclays weresolvated in one of several polyols commonly used in PU synthesis, viz. ethyleneglycol (EG), polyethylene glycol (PEG), polypropylene glycol (PPG), and glycerolpropoxylates (Voranols). The solvation could be carried out at room temperature,but it was more rapid at higher temperatures, say T = 50 °C for 12 h. The d001-spacing depended mainly on the chain length of the onium ion. As shown inTable 35, the observed values of interlayer spacing agree with calculations (dcalc).Initially, the onium ion paraffin tails in the non-solvated clay are oriented parallelto the clay platelets, as judged by the values d001 = 2.22 and 2.30 nm, then theyreorient from this position to optimise solvation by polyol. PU-based PNCs wereformed by adding a methylene diphenyl diisocyanate prepolymer (MDDP; MW= 1050; functionality = 2.0) to a mixture of organoclay and Voranol V230-238.Before use, the polyol was degassed under vacuum at 100 °C for 6 h. Thediisocyanate was added and the mixture was stirred at 70 °C for 15 min beforedegassing at 95 °C under vacuum. The mixture that contained ≤ 10 wt% oforganoclay was pourable. The bubble-free mixture was poured into a mould forcuring at 95 °C for 10 h under N2.

The alkylammonium-exchanged ions of the intercalated clay were consideredto react with isocyanate and were counted as contributors to the polymerisationstoichiometry; hence the amount of V230-238 was reduced accordingly. Nocatalyst was used. During curing at 95 °C the interlayer spacing increased withtime from d001 = 3.74 to 5.08 nm. The presence of the Braggs reflections indicatedthat the clay layers formed large tactoids. However, the distance between thenanolayers was in the range where matrix reinforcement could be expected.

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of elastic properties. By contrast, PU-based CPNC exhibits improvement oftoughness, as well as of modulus. An illustration of the mechanical behaviour isshown in Figure 51. The tensile strength, tensile modulus, and strain-at-breakversus organoclay concentration are plotted for PU prepared from V230-238,MDDP and MMT intercalated with ODA (C18A). Evidently, incorporation ofnanoclay both strengthened and toughened the elastomeric matrix in comparisonwith the neat polymer. Even when the clay platelets are in the form of intercalatedtactoids, they strengthen, stiffen and toughen the matrix. At an organoclay loadingof 10 wt% the values of all three functions at least doubled. It is noteworthy thatthe PU-based nanocomposites retained high optical transparency.

Chen et al. [1999b] prepared CPNC with PU as the matrix going through astage of polycaprolactone-based nanocomposite as an intermediary. Thus, firstNa-MMT in aqueous solution was intercalated with 12-aminolauric acid. Theadduct was collected, washed, dried, ground and screened through a 325-meshsieve. Next, the organoclay was dispersed in ε-caprolactone which subsequentlywas polymerised for 3 h at 170 °C while stirring. PU was prepared in two steps:

1. Mixing 4,4´-diphenyl-methane diisocyanate (MDI), polycaprolactone dioland DMF for 2 h at 70 °C, then reacting the mixture with 1,4-butanediol for30 min.

2. The PCL-based PNC was added and the combined mixture was allowed toreact for 30 min.

The prepolymer solution was spread, dried under vacuum and cured at 80 °C for10 h. XRD showed that the clay was exfoliated and that the reaction productcomprised crystalline PCL.

Thus, the prepared CPNC was a complex system with segmented PU formingthe matrix chemically bonded to the randomly dispersed PCL-based exfoliatednanocomposites. Size exclusion chromatography (SEC) or gel permeationchromatography (GPC) indicated that the Mn of PCL was only 3.62 kg/mol, and

Figure 51 Mechanical behaviour versus organoclay content of PU-based CPNC.Data [Wang and Pinnavaia, 1998b].

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that in PU, Mn decreased with the content of PCL-based nanocomposite, fromMn = 52 to 13 kg/mol. The miscibility of the PU/PCL-clay blend was not studied.The mechanical properties versus clay content plot showed a surprising behaviour(see Figure 52). For example, in tensile tests the tensile strength increased linearlywith clay content, but the elongation at break went through a sharp maximum at0.74 wt% clay. In the lap-joint shear strength tests (ASTM D1002) the modulusincreased linearly with clay content while the strength went through a localmaximum at a clay content of 1.3 wt%.

CPNC were prepared by dispersing synthetic fluoromica (FM; Somasif ME100;aspect ratio p ≤ 5,000) in a PU matrix [Zilg et al., 1999b]. The ME-100 behavesas a weak silicilic acid, with Na+ as counterions in the interlamellar gallery. FMwas intercalated in aqueous solution with methyl-dodecyl bis(hydroxyethyl)-ammonium chloride (MDD2EtOH; Ethoquad C12). After drying it was dispersedin trihydroxy-terminated oligo(propylene glycol) having Mn = 3.8 kg/mol, thenreacted with diisocyanato-phenyl methane catalysed with N,N-dimethyl-benzylamine (NNDB) at 80 °C. The reaction rate was slightly slowed by thepresence of organoclay. At 10 wt% clay loading the CPNC was not exfoliated,but d001 increased from 0.9 (dry ME-100) to 8.8 nm. Evidently, the intercalantand polyol diffused into ME-100 interlamellar galleries. TEM showed that theintercalated clay formed stacks, 500 nm in length and 20 to 50 nm in thickness.The stacks were well dispersed within the PU matrix. As shown in Figure 53, thetensile strength and elongation at break increased with the organoclay content.However, a less satisfactory correlation was reported for the tensile modulus andthe Shore-A hardness – the data were scattered.

PU-based CPNC were prepared starting with Na-MMT (CEC = 0.76 meq/g)which was intercalated (3 h stirring in aqueous suspension at 60 °C) with either12-aminolauric acid (ADA) or benzidine (BZD), forming the complexes: MMT-

Figure 52 Tensile (ASTM D412) and lap-joint strength (ASTM D1002) test datafor PU containing dispersed PCL-based CPNC. Data [Chen et al., 1999b].

229


ADA and MMT-BZD, respectively [Chen et al., 2000]. The intercalated clay waswashed, dried under vacuum at 80 °C for 12 h, then ground and screened througha 325-mesh sieve. In a parallel operation, MDI and polytetramethylene glycol(PTMG) at a molar ratio of 2:1 were dissolved in DMF and heated to 90 °C for 2.5h to form a prepolymer. To the prepolymer 1,4-butanediol was added with rapidmixing at 90 °C for 10 min. Next, an appropriate amount of organoclay (to give 1,3 or 5 wt% of organoclay in CPNC) was dispersed in 10 ml of DMF, added to theprepolymer mixture and the reaction was allowed to proceed to completion in 3 hat room temperature under mixing. The final concentration of PU in DMF was 30wt%. After removing DMF at 80 °C and degassing the mixture an elastic film wasobtained.

XRD of MMT-ADA and MMT-BZD gave d001 = 1.7 and 1.54 nm, respectively.The difference was attributed to the size of the intercalant molecule. The PU-basedPNCs prepared with 1, 3 and 5 wt% MMT-ADA, as well as with 1 and 3 wt%MMT-BZD were all exfoliated. This conclusion was supported by TEMmicrographs – randomly placed, about 1 nm thick layers of organoclay wereobserved. However, the PNC with 5 wt% MMT-BZD had a broad peak ofd001 = 2.47 nm indicating intercalation.

The molecular weight of PU in neat resin and in CPNC prepared in the presenceof MMT-ADA or MMT-BZD was Mn ≅ 11 kg/mol. Similarly, for all seven samplesthe glass transition temperature was -58 ≤ Tg ≤ -59 °C. Also, as expected, thewater absorption was found to vary from 1.3 to 1.7 wt%. Surprisingly, TGAanalysis of PU-based CPNC containing MMT-ADA showed that it started todegrade faster than neat PU. This may be due to the presence of extra ADAunattached either to clay or to PU. However, at higher temperatures (T > 350 °C)the PU/clay nanocomposite displayed higher thermal resistance than that of PU.The stability of the MMT-BZD/PU system was better than that with MMT-ADA.

Figure 53 PU-based CPNC – tensile strength and elongation at break versusfluoromica clay content. Data [Zilg et al., 1999b].

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The mechanical properties of the PU-based PNC are shown in Figure 54. Additionof MMT-ADA only slightly improved the tensile strength and elongation at breakover that of neat PU. However, the effect of MMT-BZD was quite significant –improvement of the tensile strength by a factor of 2.0 and the elongation at break bya factor of 2.8. The large contrast between the effects of MMT-ADA and MMT-BZD was explained by the interactions between the intercalant, clay and PU matrix.ADA has one terminal -NH2 group that might form a complex with Na-MMT andstill be able to react with a -NCO to form urea. BZD had two terminal -NH2 groupsthat can participate in these interactions/reactions. Therefore, ADA can form linearchains while BZD a crosslinked one. Schematics of these interactions/reactions arepresented in Figure 55, which illustrates the molecular structure in PU-based PNCformed in the presence of (a) MMT-ADA and (b) MMT-BZD.

Ma et al. [2001] prepared elastomeric PU based nanocomposite starting withMMT (CEC = 0.9-1.0 meq/g, with particle size of about 40-70 μm) intercalatedwith trimethyl hexadecyl-ammonium chloride (3MHDA). Three types of polyolof different MW and functionalities were used: PPG1, and PPG2 having Mn =1and 2 kg/mol, respectively, and glycerol propoxylate (GPO3, with Mn = 3 kg/mol). Dry organoclay was dispersed in GPO3 for 2 h. The degassed dispersionwas mixed with the PU prepolymer, prepared by reacting PPG1 and PPG2 withtoluene diisocyanate (TDI) at 80 °C. The mixture was cured at 80-90 °C for 2 h.CPNCs containing either PPG1 or PPG2 were also prepared. The XRD dataindicated that MMT was intercalated with the interlayer spacing d001 ≤ 4.6 nm.

The tensile strength and elongation at break reached maximum values at ca.8 wt% clay. The maximum improvement of the tensile strength and elongationat break was by a factor of 2 and 5, respectively (based on neat PU). TGA showedthat the temperature for the maximum thermal decomposition increased from Td= 377 (PU) to 385 °C. The best properties were observed for CPNC comprisingGPO3. Nanocomposites prepared by first dispersing the organoclay in low MWpolyol had better mechanical properties than those prepared by dispersingorganoclay in high MW polyol.

In 2001 Hu et al., reported a similar degree of dispersion. The authors firstexpanded the interlayer spacing of MMT-3MHDA in PEG and then added TDIand caused polymerisation. The reported interlayer spacing was: for MMTd001 = 1.26; for organoclay 1.96; and for CPNC with (10 wt% organoclay)d001 ≤ 4.85 nm. In accord with these observations, only intercalation could beexpected in systems containing clay intercalated with quaternary ammonium ions.

An interesting strategy has been used by Yao et al. [2002]. Differentamounts of Na-MMT (from Nanocor) were mixed with modified polyetherpolyol (MPP) at 50 °C for 72 h. The mixture was then blended with a knownamount of modified 4,4´-diphenylmethylate diisocyanate (M-MDI) and moulded,curing it at 78 °C for 168 h. Thus, MPP was used as an intercalating agent thatexpanded the interlamellar galleries from Δd001 = 0.14 to 0.64 nm – evidently,only a moderate level of intercalation was achieved. It was observed that with anincreasing amount of clay the heat of reaction was reduced. The reduction wasascribed to immobilisation of MPP that intercalated the clay, being stronglybonded to the solid surface.

In spite of the low degree of dispersion, the CPNC showed significant improvementof the tensile strength and strain at break, increasing with the amount of clay. At a clayloading of 21.5 wt%, tensile strength increased by ca. 44% and the strain at break by20%. The storage modulus at T < Tg of the soft segments increased by 350%.

231


Figure 54 Mechanical behaviour of PU-based CPNC containing MMT intercalatedeither with 12-aminolauric acid (ADA) or with benzidine (BZD) [Chen et al.,

2000].

Figure 55 Schematic representation of the molecular structure in PU-based CPNCformed in the presence of (a) ADA-MMT and (b) BZD-MMT.

Reproduced from Chen et al., 2000, with permission from Elsevier.

232


The thermal conductivity initially decreased with clay loading (by ca. 10% for17% clay) then returned to the original level.

MMT is known to be hygroscopic and, as TGA experiments indicate, it maycontain a significant amount of adsorbed and chemically bound water [Ogawaet al., 1992; Menéndez et al., 1993]. The -NCO groups readily react with water,for example, and 1.0 g of water will consume 18.0 g of toluene-diisocyanate(TDI). Cloisite® 10A (MMT-2MBHT) was de-watered by dispersing it in toluenefollowed by azeotropic distillation for 6 h [Zhang et al., 2003]. Next, thedehydrated organoclay was dispersed in PEG, reacted with TDI at 80 °C for 4 h,and then cured at 90 °C for 15 h under vacuum – the clay concentration rangedfrom 0 to 7 wt%. The authors reported that only about 4.1% of the organoclay-OH groups were able to react with -NCO groups of TDI, hence grafting the PUmatrix directly to the clay surface. The reaction was detected by FTIR. XRD ofCloisite® 10A and the CPNC gave d001 values of about 2.3 and 3.5 nm, respectively.In spite of the lack of exfoliation the mechanical properties were found to begreatly improved, viz. the dynamic tensile modulus, E, increased by a factor of10, the tensile strength and the elongation at break by about 160%. Theseimprovements in the intercalated systems are clear evidence of the benefits ofdirect matrix-filler bonding.

2.4.3 Melt ExfoliationMelt blending is the preferred method for preparing CPNC with a thermoplasticpolymer matrix. Typically, the polymer is melted and combined with the desiredamount of the intercalated clay in an extruder, internal, kinetic energy or acontinuous mixer. Melt blending is carried out in the presence of an inert gas,such as argon, neon or nitrogen. Alternatively, the polymer may be dry mixedwith the intercalant, then heated in a mixer and subjected to a shear sufficient toform the desired degree of dispersion. While total exfoliation can be achieved forsome diluted systems, the industrial standard requires that ≥ 80 wt% of the clayplatelets (aspect ratio p = 10 to 1500) are individually and uniformly dispersedin the polymeric matrix, with the rest forming short stacks.

There are several advantages of the melt versus the reactive exfoliation method.The process is more economic, it is closer to the ultimate product manufacturer,hence it is better suited for rapid changes of formulation, it is more versatileoffering ready adaptability, it involves more participants with the technical know-how (hence it offers more rapid penetration of the industry), and finally it doesnot require that a big polymer production line is dedicated to developing a materialwith uncertain application and hesitant market.

As discussed in Section 2.3.8, the beginning of melt exfoliation started with thequiescent, diffusion-controlled melt intercalation process. For example, Vaia et al.[Vaia et al., 1993; Vaia and Giannelis, 1997b] prepared PS-based CPNC by sucha static melt processing method.

Melt exfoliation of clay in PDMS matrix was pioneered by Burnside andGiannelis [1995; 2000]. Na-MMT was first intercalated with dimethyl ditallowammonium bromide (2M2TA), and then dispersed in the matrix by sonication.Similarly, silicone rubber-type CPNCs were prepared by melt intercalation [Wanget al., 1998]. In this work Na-MMT was intercalated with trimethyl hexadecyl-ammonium bromide (3MHDA). The CPNCs with ≤ 20 phr of organoclay (≤ 9wt% of clay) were prepared by mechanical mixing. The resulting nanocomposites,

233


characterised by XRD, TEM and TGA, were reported intercalated with d001 =3.71 nm. Thus, the ordered organosilicate structure of clay was still present inthe CPNC, forming stacks that were ca. 50 nm thick and uniformly dispersed inthe silicone rubber matrix. The mechanical properties and thermal stability ofthe hybrids were very close to those of aerosilica-filled silicone rubber. Meltintercalation/exfoliation of hydrophilic polymers, such as P4VP, PEG or PVAl isrelatively easy. The strategies used have been described in Section 2.3.5.2.

More recently, Zanetti et al. [2000a,b] described the melt exfoliation offluorohectorite (fluoromica, FM) in a PVAc matrix. The melt-produced CPNCsshow a similar set of properties as those obtained by the reactive route, viz.superior heat, chemical or ignition resistance, barrier to diffusion of gases andpolar liquids such as water, methanol, ethanol etc., yield strength, stiffness anddimensional stability. These CPNCs may be useful in diverse applications viz. inbusiness equipment, computer housings, transport (automotive and aircraft, viz.in under the hood and exterior body trim applications), electronic, packaging,building and construction industry, etc.

However, this method is at an early stage of development with numerousunanswered questions still remaining. A consensus emerges that the clay platelets,e.g., individual lamellae of MMT, should be treated as any other structural plates,with their own value of mechanical performance characteristics, e.g., modulus,maximum strain at break, etc. There is TEM evidence that in shear the clayplatelets tend to bend. The work on FM showed extensive reduction of the aspectratio, but it is not known whether the phenomenon is general and whichmechanism controls the attrition.

2.4.3.1 PA-Based CPNCsMaxfield et al., from AlliedSignal (now Honeywell) [1995; 1996; Christiani andMaxfield, 1998] were the first to propose melt exfoliation in a PA-6 matrix. Theaim was to produce PA-based CPNC with advantageous performance, withoutinfringing on the Toyota reactive exfoliation method. The method comprisesthree steps:

1. Treating an aqueous suspension of clay (the preferred platelet diameter is15 ≤ D ≤ 300 nm) with peptising Na6P6O18 at 50-90 °C, and then withorganosilanes (e.g., aminoethyl aminopropyl trimethoxysilane),organotitanates or organozirconates. The process increases the interlayerspacing to d001 ≥ 5 nm. The authors suggested that to facilitate intercalationthe size of clay particles may be reduced, or the mixture may be exposed toheat, ultrasonic cavitation, microwaves, etc. The complexes of organosilanes,organotitanates and/or organozirconates (with or without onium salt)improved the thermal stability at T > 300 °C.

2. Saturating the dried clay (interlayer spacing d001 ≥ 1.5 nm) with a monomer,e.g., ε-caprolactam, (the patent claims are broad, stretching to ‘one or morethermosetting and/or thermoplastic polymers or rubbers’) then polymerisingit, which increases the interlayer spacing to d001 ≥ 5 nm.

3. Compounding the modified clay at T ≥ Tm with matrix polymer until thedesired level of exfoliation is reached. The authors emphasised the formationof γ-phase crystalline PA-6 by compounding with clay.

234


Melt compounding was also used by Liu et al. [1999] to prepare PA-6/organoclayPNC. First, Na-MMT was intercalated with octadecyl ammonium chloride(ODA), which increased d001 from 0.98 to 1.55 nm. Next, PA-6 was dry blendedwith ≤ 17 wt% of the organoclay, then melt-compounded in a TSE and injectionmoulded. Melt blending increased the interlayer spacing to d001 = 3.68 nm andreduced XRD peak intensity, indicating a partial exfoliation. Similar results wereclaimed by the RTP Co. for melt compounded PA-6 [Dahman, 2000]. In thelatter study various organoclays were used at different concentrations. Meltexfoliation was carried out in a TSE.

As described in Section 2.3.8.2.4, Dennis et al. [2000; 2001] reported that meltblending of PA-6 with Cloisite® 30B (2M2EtOH) resulted in exfoliation, thus thefundamental studies on the intercalation processes were conducted with PA-6/Cloisite®

15A (2M2HTA) as a model system. The work has shown that at least for thisorganoclay/polymer system, intercalation depends solely on the residence time andthe residence time distribution inside the processing equipment.

The observed difference in the ability of Cloisite® 30B and Cloisite® 15Aorganoclays from SCP to exfoliate demonstrated the importance of the chemicaltreatment of the clay, which controls the clay/polymer interactions. As shown inTable 16 the exfoliating Cloisite®-30B (methyl-tallow-bis-2-hydroxyethyl) differsfrom the intercalating Cloisite® 15A (bis-methyl-bis-hydrogenated tallow) on threeaccounts:

1. It contains one (non-hydrogenated) tallow group versus two hydrogenatedin C15A,

2. It contains two hydroxyethyl groups versus two methyl groups of C15A and3. It has lower substitution of organic ions (0.9 for C30B versus 1.25 meq/g for

C15A).In short, C30B has a lower density of the organic intercalant within the interlamellargalleries than C15A and at the same time it has higher polarity, which originatesfrom two -OH groups and unsaturation of the tallow group. Thus, Cloisite® 30Bhas stronger attraction for PA molecules causing them to diffuse into galleries, andmore space for them to do so.

In another publication from the same laboratory an experimental organoclaySCPX-2004 was used [Cho et al., 2000; Cho and Paul, 2001]. This new organoclayis a close cousin of Cloisite® 30B – in SCPX-2004 the tallow is replaced withrapeseed, which contains ca. 95% of C22 alkyl chain with single unsaturation atC13 (erucyl). In both cases the same Na-MMT was used with CEC = 0.91 meq/g.SCPX-2004 has been successfully used for exfoliation via direct melt compoundingof PA-6 (Mn = 29.3 kg/mol) with 0-20 wt% of the organoclay, using either a TSEor a SSE at 240 °C. The extruded pellets were injection moulded into standardtest mould. For the SCPX organoclay, XRD gave the interlayer spacingd001 = 1.8 nm. CPNC (with 5 wt% clay) was prepared either in a TSE or a SSE;the former process engendered full exfoliation, while the latter only a partialexfoliation, but large domains of unaltered organoclay remained. TEM confirmedthese observations. The individual layers were found aligned along the flow axis.The average clay platelet thickness and length were determined as approximately3 and 120 nm, respectively. At a clay content of 3.16 wt% it produced a respectablemodulus (3.7 GPa versus 2.7 GPa for neat PA-6 and 4.3 GPa for PA-6/C30B)and excellent elongation at break (38% versus 40% for neat PA-6 and 10% forPA-6/C30B).

235


TGA showed that due to the degradation of the quaternary alkylammonium,the nanocomposites had lower stability than neat PA-6. A series of CPNC with5 wt% clay were prepared varying temperature (T = 230-280 °C) and screwspeed (N = 80-280 rpm) in the TSE. Their mechanical properties were also given.To detect tendencies, the published data were fitted to a linear relation:

Y = a0 + a1 T + a2 N (29)

where Y is any of the four variables listed in Table 36, N is screw speed, and a1and a2 are equation parameters. Evidently, there is no correlation between thetwo compounding variables and impact strength as well as elongation at break.These properties may be more related to the crystallinity of the specimens thanthe process variables. However, there is strong correlation for the modulus(r2 = 0.9999) and the yield stress (r2 = 0.9998). Increasing the compoundingtemperature and screw speed results in higher modulus. One may speculate thatthe degree of platelet dispersion also increases. For yield strength the coefficienta1 < 0, hence an increase of T reduces the yield strength. Most likely, thermaldecomposition of the SCPX quaternary alkylammonium weakens the bondingbetween dispersed clay platelets and the matrix (see also Figure 32).

Cho and Paul also plotted the relative strength, modulus and elongation atbreak versus the mineral clay content using data from four sources: their ownlaboratory and from the literature [Christiani and Maxfield, 1998; Liu et al.,1999; Okada et al., 1988; Okada and Usuki, 1995]. Three of the CPNCs wereprepared by melt compounding, whereas the fourth one was prepared by the insitu reactive exfoliation method. Cloisite® 30B gave slightly higher modulus andstrength than SCPX, but the latter was far superior as far as the elongation atbreak is concerned. The plots indicated a good linear increase of the relative

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modulus with clay content, a similar increase of the tensile strength and poorlycorrelating decrease of the elongation at break. The authors also reported asynergistic effect on tensile strength and modulus when the exfoliatednanocomposite was used as the matrix for a glass fibre reinforced composite.

Work from the same laboratory by Fornes et al. [2001] indicated that duringmelt intercalation two processes simultaneously take place: thermodynamically-driven diffusion of macromolecules into the interlamellar galleries, and mechanicalpeeling off of platelets or short stacks. If during the former process the diffusion-controlled intercalation sufficiently reduced the solid-solid interactions betweenthe adjacent platelets, the latter process may lead to exfoliation. It is noteworthythat the minimum interlayer thickness of the organoclay needed for the PA-6macromolecules to diffuse into is relatively small, d001 = 1.8 nm. However, thechemical character of the intercalated onium cation, hence miscibility with PA-6,is important as the diffusion of PA-6 molecules expands the interlamellar galleryheight at least by a factor of two. The stress field present during the compoundingalso seems essential for efficient exfoliation.

The next contribution from this group was analysis of the behaviour of aseries of CPNCs with PA-6 as the matrix and containing up to 4.5 wt% oforganoclay [Fornes et al., 2004]. Three types of organoclays with quaternaryammonium ions were tested. These were MMT-type, intercalated with: tetramethyl(4MA), trimethyl hydrogenated-tallow (3MHTA), or dimethyl di(hydrogenated-tallow) (2M2HTA). The nanocomposites were prepared by melt compoundingin a TSE. XRD and TEM demonstrated that the degree of dispersion increasedfrom 4MA (the worst) to 3MHTA. The modulus and yield strength increased inthe same order, while the elongation at break showed the opposite trend. Theseresults indicate that for melt intercalation by PA-6, a minimum interlayer spacingis required – the minimum being located between d001 = 1.36 and 1.80 nm,measured for MMT-4MA and MMT-3MHTA, respectively. The second importantinformation concerns the interaction between the PA-6 macromolecules and theorganoclay – in spite of larger initial interlayer spacing (d001 = 2.42 nm) MMT-2M2HT did not disperse as well as MMT-3MHTA. Evidently, a more thoroughcovering of clay platelet in MMT-2M2HTA by immiscible (with PA-6) long alkyltails than that produced by MMT-3MHTA is to blame. Partial access of the claysurface to PA-6 chain-ends provides compatibilisation, similar to that providedby PO-MAH in CPNC with PO as the matrix.

Exfoliated CPNC was obtained by melt compounding PA-6 with doubly-intercalated MMT [Liu et al., 2003]. Thus, MMT was first intercalated with3MHDA, and then with the diglycidyl ether of bisphenol A. A high level of claydispersion was achieved up to 5.6 wt% of inorganic content. At this loading themechanical properties (with reference to neat PA-6) increased by: tensile modulus112%, tensile strength at yield 71%, flexural modulus 89%, flexural strength54%, impact strength 34%, etc. In parallel, the HDT of PA-6 of 62 °C increasedto 157 °C.

Hasegawa et al. [2003] described quite a revolutionary method for theproduction of PA-based nanocomposites. Thus, PA-6 was compounded in a TSEwith an aqueous suspension of Na-MMT (2 wt% solids). During compoundingthe water was removed by vacuum-aided devolatilisation. The PA-matrix wasnot hydrolysed. According to OM and TEM the clay platelets were exfoliatedand dispersed homogeneously. The tensile and flexural moduli of the CPNC were28 and 14% higher than those of neat PA-6, respectively. Similarly, the tensile

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and flexural strengths were higher by 28 and 12%, respectively. The Izod impactstrength of the new nanocomposites was 12% lower compared to PA-6 and HDTincreased from 75 °C for the neat resin to 102 °C. Gas permeability was 31%lower than that of neat PA-6. In short, the properties of the new PA-6/MMTnanocomposites were comparable to those of commercial CPNC from Ube (seeTable 62), prepared by compounding PA-6 with 2 wt% MMT-ADA organoclay.

CPNCs with PA-66 as a matrix were prepared in a high strain rate TSE [Nairet al., 2002]. The final composition contained 1 to 15 wt% of unidentifiedorganoclay from SCP. While no details on the compounding protocol were given,the strategy adopted by the group involves:

1. The use of organoclay suitable for melt compounding.

2. Preparation of a concentrate by compounding the organoclay with the matrixresin in a TSE.

3. Using a continuous or batch mixer, dilute the concentrate to the finalcomposition and pelletise.

4. Adjust the MW of PA by solid-state polycondensation.

5. Form final object by injection moulding, extrusion, etc.

The processing conditions are not critical, but moderation is recommended – theprocess must provide sufficient residence time at sufficient shear stress at atemperature near to the Tm of the matrix resin. According to the authors, thevariables that mostly affect the dispersion are: screw design, rotational speed,throughput rate and the feeding method.

The cited publication focused on the fracture mechanics study of PA-66with different types of layered silicate having nanoscale (fully dispersed) ormultiscale (mixed nanoscale/microscale) structure. Independently of the structureand concentration, the fracture toughness was proportional to the size of thecrack-tip plastic zone at fracture. At low clay concentration, the tougheningeffects in CPNC increased with exfoliation and strong clay/matrix interactions.However, at high clay concentrations, where the initiation toughness was low,no significant effect of these microstructural factors was observed – thecomposites with mixed nanoscale/microscale particulates exhibited the besttoughness. In the latter systems high initiation toughening from the nanoclaywas combined with high propagation resistance from the micron sized particles.The organoclays seem to influence shear flow at the crack tip, thus playing animportant role in the toughening mechanism.

Incarnato et al. [2003] melt compounded a commercial ADS copolyamide withup to 9 wt% Cloisite® 30B. The specimens showed mixed intercalated/exfoliatedclay dispersion with altered matrix crystalline morphology, e.g., high content ofthe γ-crystalline phase, and reduced crystallisation temperature.

2.4.3.2 PO-Based CPNCs

Plastics comprise over 15 wt% of an automobile weight. Owing to its low priceand good performance the share of PP is expanding – it already constitutes 68%in an Opel Astra. Incorporation of 4 wt% of exfoliated clay should improve theflexural strength of PP by ≥ 50%, the modulus by at least 35%, and HDT by≥ 32 °C, while maintaining the ductility to 10 °C. This makes the developmentof PP-based nanocomposites commercially important. Serious efforts toward melt

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exfoliation of layered materials in PP have been carried out in Japan, NorthAmerica and Europe. Since most of these efforts lead to intercalation, in partthey have already been discussed in Section 2.3.8.2.

Usuki et al. [1997] described an early, simple route to PO-based CPNC. First,Na-MMT was cation-exchanged with dimethyl distearyl ammonium chloride(2M2ODA, clay content = 52.8 wt%, d001 ≅ 3.3 nm). Next, an olefin oligomer withtelechelic -OH groups (a diol, carbon number = 150 to 200) was dissolved in toluene,and different quantities of 2M2ODA were added to it. After evaporation of toluene,the d001 of the doubly intercalated MMT-2M2ODA/PO was found to depend on thediol-to-2M2ODA ratio: for the ratios 1:1, 3:1 and 5:1 the XRD peak position didnot change, giving d001 ≅ 4.4 nm, indicative of the presence of short stacks, but whenthe ratio was 10:1 a full exfoliation was obtained. Finally, 5 wt% of MMT-2M2ODA/PO (diol-to-2M2ODA = 1:1) was compounded with PP in an internalmixer at 220 °C until the torque reached a plateau (ca. 5 min). XRD and TEMshowed that in the resulting CPNC the clay was exfoliated and uniformly dispersed.It is noteworthy that in the absence of diol, organoclay dispersed only into micron-size aggregates. The authors concluded that the oligomeric or telechelic diol hasgood chemical affinity to 2M2ODA, so it can be inserted into the interlamellar spacein the presence of toluene. After evaporation of toluene, the -OH groups hydrogenbond with MMT. The presence of oligomer increased the interlayer spacing enoughfor the PP macromolecule to diffuse into the predominantly hydrocarbon filledinterlamellar galleries, eventually exfoliating the organoclay. The short note doesnot report on the performance of these systems, but since the hydrophobic organoclayis dispersed in PP without covalent bonding between these two or efficiententanglement formation, one would not expect strength to be improved.

Another report from the Toyota laboratory also describes melt-intercalation ofdoubly intercalated clay in PP [Usuki et al., 1999]. MMT was intercalated with aprimary ammonium salt (ODA), and then dispersed in a toluene solution of end-hydroxylated PBD or maleated-PP. The complex was well-dried, added to moltenPP at 200 °C and mixed for 30 min. The interlayer spacing of the product was d001= 3.82 nm. However, as reported by Kato et al. [1997], adequate exfoliation canonly be obtained by melt compounding when the acid value of the MA-PP isreasonably high. Thus, MMT-ODA (d001 = 2.17 nm) was melt blended with PP-MA in an internal mixer at 200 °C for 15 min. For the PP-to-organoclay ratio= 3:1, d001 = 7.22 nm was found, indicating advanced intercalation. The authorsconcluded that one polar group per 25 PP-mers is required to achieve exfoliation.

In 1997 Kawasumi et al. [1997] described another version of the process.Thus, PP-based CPNC was prepared by compounding PP, PP-MA, and eitherMMT (CEC = 1.19 meq/g) or fluoromica (FM) intercalated with stearylammonium chloride (ODA). Two types of PP-MA were used: Yumex 1001 andYumex 1010 from Sanyo Chem., with Mw = 40 and 30 kg/mol, acid number 26and 52 mg KOH/g, Tm = 154 and 145 °C, respectively. Intercalation of MMTincreased the interlayer spacing from d001 = 1.2 to 2.2 nm (inorganic content67.4 to 68.4 wt%). Similarly, nearly quantitative intercalation of ODA in FMwas achieved (d001 = 2.2 nm; inorganic content 68.8 to 70.8 wt%. Exfoliationwas carried out by first dry blending the three ingredients, and then compoundingthem in a TSE at T = 210 °C. The dried pellets were injection-moulded.

Miscibility of the PP/PP-MA blends was evaluated using an optical microscope.Phase separation was observed in PP/ Yumex 1010 blends, while PP/ Yumex 1001mixtures were miscible. Evidently, miscibility of PP/PP-MA depends on the relative

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concentration of ingredients and temperature, but mainly on the concentration ofMAH in PP-MA. Yumex 1010 contains 52 mmole of KOH/g of polar MAH groupsthat tend to form their own phase. In consequence, a relatively strong XRD peakwas observed for the PP/Yumex 1010 systems (d001 = 5.9 nm), indicating lack ofexfoliation. By contrast, the XRD of PP/Yumex 1001 exhibited a relatively smalland highly reduced peak indicating a high degree of exfoliation with only fewshort stacks present. TEM confirmed these results.

In compositions containing PP with either MMT-ODA- or FM-ODA, butwithout PP-MA compatibiliser, the clay formed particles several hundredmicrometres in size. Thus, these materials should be considered to be conventionalmineral-filled composites. Evidently, addition of compatibilising PP-MA isessential if exfoliation is to take place. Thus, the polar molecules, viz. telechelicdiol or PP-MA, must first intercalate into the interlamellar space. The drivingforce of this secondary intercalation originates from the hydrogen bondingbetween the polar groups of the compatibiliser (-OH, MAH or -COOH) and theclay surface =Si=O, ≡Si-OH or the intercalant =NH groups. As the interlayerspacing of the clay increases, the solid-solid interactions between the layers arereduced. Under the influence of a strong shear field the doubly intercalated claydisperses in the PP matrix. However, when the doubly intercalated clay isimmiscible with the matrix, phase separation occurs and the clay concentrates indomains where exfoliation is unable to take place.

Another modification of these methods was published by Hasegawa et al. [2000a].Thus, MMT-ODA was compounded with three slightly maleated polyolefins, viz.PP (Mw = 210 kg/mol; MI = 150 g/10 min; MAH = 0.2 wt%), PE (MI = 2 g/10 min;MAH = 0.62 wt%), and EPR (Mw = 270 kg/mol; MI = 0.06 g/10 min;MAH = 0.55 wt%). The blending was carried out at 200 °C in a TSE (D = 30 mm,L/D = 45.5). Excellent dispersion of clay platelets was shown by XRD and TEM (indecreasing order) for EPR, PP and PE. In the latter system short stacks withd001 ≅ 3 nm were visible.

Similarly, Okamoto et al. [2001a] prepared CPNC using organoclay (MMT-ODA from Nanocor Inc.) and PP-MA (from Exxon Chem.; Mw = 195 kg/mol,Mw/Mn = 2.98; 0.2 wt% of MAH; Tm = 141 °C). Introduction of such a smallamount of MAH was enough to obtain sufficient interaction between the PPmatrix and the organoclay. Three CPNCs with MMT contents of 2, 4 and 7.5 wt%were prepared at 200 °C in a TSE (TEX30α-45.5BW from JSW). The extrudedand pelletised strands were moulded into ca. 2 mm sheets. The interlayer spacingof the organoclay, d001 = 2.31 nm increased to 3.24, 3.03 and 2.89 at the threeclay concentrations, respectively. At the same time TEM microscopy showed afine dispersion of the clay platelets, with about 150 nm in length and about 5 nmin thickness. The XRD diffraction peaks most likely resulted from short stackscomprising 2-3 platelets.

The effects of organoclay concentration on melt intercalation were alsoinvestigated [2000b]. Thus, MMT-ODA was melt compounded with PP-MA. Gooddispersion was obtained - XRD spectra were free of d001 peaks, but a small shoulderfor the highest clay content indicated the presence of short stacks. Apparently, thechemical similarity of the paraffinic intercalant to PP matrix and the PP-MAcompatibiliser improved the degree of exfoliation, albeit by a small amount.

Another approach to the double intercalation strategy for the preparation ofPO-based CPNC was proposed by Hudson [1999] and by Jeong et al. [1998].The three steps are:

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1. Functionalising MMT with an aminosilane.2. Reacting the free amine with maleated PO (MW ≅ 20 kg/mol; about 1 wt%

MAH) either in solution or in melt.3. Dispersing the organomodified clay in a semicrystalline PO.XRD of the silane treated MMT demonstrated an increase in interlayer separationwith d001 = 2.4 to 7.3 nm. TEM showed the presence of many individual MMT-platelets, but in CPNC containing 15% clay most were in stacks of 2-4 layers,ca. 1 μm long. Co-crystallisation between the maleated and the semi-crystallinePO is essential to provide a mechanism of stress transfer between the matrix andclay particles.

Serrano et al. [1998] patented CPNC that comprised 40-99.95 wt% EVAland more than 2 wt% of exfoliated MMT. No onium ion or silane couplingagent was required, provided that water-soluble intercalants were used, e.g., P2VPor PVAl. These hydrogen-bonded to the platelet surface, covering the Na+ andthus shielding the EVAl from its degrading influence. For example, MMT wascompounded in a TSE with an intercalant at T = Tm + 50 °C. The CPNCs couldbe used as external, heat-resistant body parts for the automotive industry; fortyre cords; food wrapping with enhanced gas barrier properties; electricalcomponents; food-grade drink containers, etc.

The process for preparation of exfoliated PO nanocomposites developed inNanocor laboratories involves the use of proprietary organoclays, I.30P and I.31PS(containing a silane coupling agent) [Lan and Quian, 2000; Nanocor’s TechnicalService Group, 2001, Nanocor Technical data sheets, 2001]. The process involvesaddition of 2-5 wt% of PP-MA which acts as a compatibiliser between 2-6 wt%of organoclay and PP. The compounding was carried out in a Leistritz CORITSE with L/D = 36. The screw configuration is shown in Figure 30. Thecompounding temperature (including the devolatilisation zone) was 180 °C,increasing progressively to 190 °C at the die. Two approaches were examined. Inthe first a masterbatch that containing 50-60 wt% of organoclay was prepared,then diluted to the final composition of 2-6 wt% organoclay. In the second, thefinal composition was compounded directly. In both cases a high degree ofexfoliation was obtained, although some short stacks were evident on the TEMmicrographs. Good properties were reported, viz. tensile strength increased by 8or 15%, tensile modulus increased by 29 or 28%, flexural strength increased by18 or 22%, etc., for direct (single pass) compounding, and the two-step process,respectively. These values were calculated from the authors’ data for CPNC with4 wt% organoclay. They seem to be short of the expected benchmark set by theautomotive industry. There is quite a bit of data scattering. Within the experimentalerror the single pass provides as good CPNC as the one involving masterbatch.One of the reasons may be matrix degradation – since PP is prone tothermomechanical chain scission, two passes through an extruder are expectedto engender more damage.

Polypropylene-based CPNCs were also prepared by melt intercalation withtwo organoclays, three PP-MA containing 1.5 to 5.8 wt% MAH and a PA-6resin [Wang et al., 2001a]. The two organoclays were MMT-ODA (NanomerI.30 TC) and Cloisite® 20A (C20A; MMT-2M2HTA from SCP). Thecompounding was carried out in an intermeshing, CORI TSE (Leistritz ZSE-27;L/D = 40) operating at T = 180 to 230 °C and at screw speed N = 100 to 200 rpm.To achieve better dispersion, PP-MA was mixed with C20A, then PP was addedand compounded to make PP/PP-MA/C20A nanocomposites.

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Since PA-6 has Tm ≅ 219 °C, I.30 with better thermal stability than C20Awas chosen for the preparation of CPNC with PP/PA-6 blend. The compoundingsequence was altered, first blending PP with PP-MA, and then adding I.30 at 100rpm. PA-6 was incorporated using two procedures:

1. Two-step process where first PA-6, PP-MA, and I.30 were mixed, then PPwas added;

2. Three-step process, where first PP-MA was mixed with I.30, then with PA-6,and finally with PP.

As summarised in Table 37 the compounding resulted in intercalation. Thepresence of PA-6 did not increase the d001 value, but reduced the intensity of theXRD diffraction peak, indicating a higher degree of exfoliation.

The molecular weight and MAH content of the compatibilising PP-MA stronglyaffect the nanostructure and the properties of PP nanocomposites. PP-MA with alower molecular weight and a high MA content leads to better dispersion, butreduces the mechanical performance. Similarly, addition of PA-6 increases theinterlayer spacing, but reduces the tensile strength. Addition of a high molecularweight PP-MA with lower MAH content may improve both stiffness andtoughness, but improvements are limited viz. 14% for tensile strength, 49% in

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tensile modulus and 30% in Izod impact strength. The best overall performancewas obtained from the system: PP + 15 wt% PP-MA (1.5 wt% MAH) + 5 wt%Cloisite® 20A.

Following the successful direct melt exfoliation of Na-MMT in a PA-6 matrixby Hasegawa et al. [2003] the method was extended to a Na-MMT/PP system[Kato et al., 2003]. A TSE with D = 32 mm, and L/D = 77 was used at T = 200to 220 °C and screw speed of 300 rpm, with residence time of ca. 6 min. Thebarrel was divided into three functional sections: (1). mixing, (2). exfoliation,and (3). devolatilisation. Into section (1), first PP and PP-MAH were fed fromthe main hopper to be melt compounded with Na-MMT (fed a bit downstream).Into section (2) ca. 10% of water was injected (at the processing temperaturesthe saturated vapour pressure is 1.5 to 2.3 MPa) to expand the interlayer spacingof Na-MMT, and facilitate reactions between MAH and the clay surface. Insection (3) a two-stage devolatilisation was carried out. As a result, the clay wasuniformly dispersed as either exfoliated platelets or intercalated short stacks.The mechanical properties of these nanocomposites were nearly the same as thoseof conventional CPNC prepared by compounding with organoclay, for example,the flexural modulus of the new CPNC was 2.76 versus 2.88 GPa.

2.4.3.3 PCL-Based CPNCsWith the growing concern for recycling industrial and municipal waste,biodegradable polymers and their nanocomposites are becoming increasinglyattractive [Akovali et al., 1998]. As discussed in Section 5.4.2, polylactic acid(PLA) has been selected by Toyota for progressive replacement of PP compositesin automotive applications. Poly-ε-caprolactam (PCL) is another biodegradablealiphatic polyester with good potential for the consumer market. However,performance of both these resins, PLA and PCL, should be enhanced byincorporation of exfoliated clay platelets.

In a series of papers from Mons-Hainaut the focus has been placed on thedevelopment of a melt compounding process for the preparation of CPNC withPCL as the matrix, which would show significant improvement of mechanicalproperties at low clay loading [Pantoustier et al., 2001; Lepoittevin et al., 2002a].Thus, commercial PCL (Mn = 49, Mw = 69 kg/mol) was mixed with several clays:

(C1) Na+-MMT;(C2) MMT¯DDA;(C3) MMT-ODA;(C4) MMT-2M2ODA (Cloisite®25A); and(C5) MMT-MHT2EtOH (Cloisite®30B).

The mixing was carried out in a two-roll mill at 130 °C for 10 min, followed bycompression moulding into 3 mm thick specimens. Compositions containing 1,3, 5 and 10 wt% of MMT were prepared.

The interlayer spacing of the neat organoclays (after dispersing 3 wt% inPLC) was: d001 = 1.21/1.23; 1.38/1.37; 1.87/2.56; 2.9/3.6; and 1.84/3.1 nm, forC1 to C5, respectively. Evidently, Na+-MMT and MMT¯DDA did not intercalate,whereas modest intercalation was obtained for the remaining organoclays, wherethe diffusion of PCL macromolecules increased the interlamellar galleries by Δd001= 0.69, 0.7 and 1.26 nm, respectively. A summary of the mechanical performanceis displayed in Figure 56. Except for the modulus, the best performance from betweenthe six materials is that of neat matrix, PCL.

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A partial explanation for the poor mechanical performance can be found in thereduction of crystallinity. To compensate for it a higher clay loading is required.Thus, CPNC with up to 10 wt% MMT were prepared with C4 and C5 organoclays[Lepoittevin et al., 2002a,b]. The highest tensile modulus was obtained for 10 wt%C5 (increase by a factor of 1.85), but the material became brittle (7% elongationat break). A better balance of properties was obtained for a 5 wt% C5 loading,viz. modulus increased by 45% and the elongation at break, εb = 560%. TheCPNC also showed improved thermal stability and flame resistance.

Lepoittevin et al. [2002b] also prepared CPNC with PCL as the matrix via insitu intercalative ring-opening polymerisation of ε-caprolactone catalysed bydibutyl-tin dimethoxide, Bu2Sn(MeO)2. For comparison, 1, 3, 5, and 10 wt % ofC1, C4 and C5 organoclays were incorporated. The C1 and C4 content had noeffect on the PCL molecular weight (Mn = 14 to 24 kg/mol), but as C5 contentincreased, the Mn was found to decrease, probably due to chain transfer by theintercalant’s –OH groups. The polymerisation resulted in a modest intercalationof C1 (d001 increased from 1.2 to 1.63 nm for the 3 wt% in PCL matrix); inintercalation of C4 (d001 increased from 1.86 to 2.68 nm); and in nearly fullexfoliation of C5, confirmed by TEM. TGA showed better thermal stability forthe exfoliated than for the intercalated CPNC, both exceeding the degradationtemperature of neat PCL.

Tortora et al. [2002] also used the polycondensation route, but starting withMMT pre-intercalated with ω-amino dodecyl acid, 0 to 44 wt% of MMT-ADA.According to XRD exfoliation was achieved for CPNC with up to 16 wt% clay.

Figure 56 Tensile properties of polycaprolactam (PCL) and its mixtures containing3 wt% of MMT in the form of organoclays C1 to C5 – see text. The vertical

broken lines are drawn to help judge the magnitude of the property change inmaximum stress at break, tensile modulus and the maximum strain at break. Data

[Pantoustier et al., 2001].

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Incorporation of clay slightly reduced the Tm (from 62 to 57 °C), but significantlyreduced the matrix crystallinity as well as the onset of degradation temperature:Td = 382 (for PCL) to 188 °C (for CPNC with 44 wt% clay). The reductioncould in part be explained by the decrease of the matrix molecular weight withincreasing organoclay content (from Mn = 11.6 to 6.5 kg/mol). The permeabilityof dichloromethane or water vapours through these CPNCs was also significantlyreduced.

The same method was used to prepare CPNCs of PCL polymerised in thepresence of up to 44 wt% of MMT-ADA [Pucciariello et al., 2004]. As theorganoclay content increased the molecular weight decreased, viz. Mn from 11.6to 6.5 and Mw from 14.9 to 7.8 kg/mol. According to XRD the nanocompositeswith low organoclay content (6 and 18 wt%) were exfoliated, whereas thosecontaining 22, 30 or 44 wt% were only intercalated. In parallel with the interlayerspacing there was a significant difference in thermal behaviour – the CPNC withexfoliated structures showed an increase of Tc, Tm and the heats of transition,whereas the intercalated ones showed the reverse trend – these thermal propertiesdecreased with organoclay content. Surprisingly, as the clay content increasedthe crystallisation rate decreased, but the PCL morphology remained spherulitic.Since clay platelets act as nucleating agents, the slower crystallisation most likelyresulted from hindrance of molecular motion in the progressively more and morecrowded interlamellar galleries.

In related work, PCL was polymerised in the presence of 3 wt% ofco-intercalated MMT [Gorrasi et al., 2004]. The co-intercalants were:(CH3)2(CH2CH2OH)(C16H33)N+ Cl– or 2MEtOHHDA, and (CH3)3(C16H33)N+

Cl– or 3MHDA. The co-intercalation was carried out with the ratio of these twoammonium salts ranging from 0 to 25, 50, 75, and 100%. Similarly, as in thepreceding communication, the PCL molecular weight decreased with2MEtOHHDA (i.e., -OH group) content (the clay loading being constant) fromMn = 56 to 28 kg/mol. According to XRD, only the CPNC with more than 50%2MEtOHHDA were exfoliated, confirming the importance of good bondingbetween clay and matrix. The tensile modulus increased with the organoclayhydroxyl group content to about 9-fold of the value for neat PCL. Furthermore,as the –OH group content increased, so did the grafting density of PCL to clay,and as a consequence the gas and vapour diffusion (e.g., dichloromethane orwater) were reduced.

Considering on the one hand the relative ease of producing exfoliated CPNCwith PCL, and on the other hand its miscibility with several commerciallyimportant resins (e.g., SAN, ABS, PVC, CPE, PEST, PEMA, etc.), it is only naturalthat this CPNC would be used as a vehicle for nanoreinforcing PCL-misciblepolymer blends [Choe et al., 2002]. To ascertain system miscibility, 20 to 40 wt%of PCL (MW = 10 to 100 kg/mol) with well dispersed organoclay has been used.The preferred clay is MMT pre-intercalated withCH3(CH2)17N+(HOCH2CH2)2CH3 Cl- , acidified CH3(CH2)n-NH2 orCH3(CH2)nNHR ( where n = 8 to 18, and R is a hydrocarbon). As the citedexamples show, melt compounding at T ≤ 140 °C for the required time (e.g.,20 min!) results in exfoliation, whereas melt compounding at T > 140 °C resultsin intercalation with d001 < 4 nm. Compounding in a single step involvessimultaneously mixing organoclay with PCL and a miscible resin. Compoundingin two steps involves first, the preparation of organoclay concentrate (≤ 30 wt%)in PCL, and subsequently diluting it with the selected blend component (e.g.,

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PVC, SAN or ABS). It was found that incorporation of organoclay improvedmechanical properties (tensile modulus increased by 350%) and heat deflectiontemperature (by 50 °C). With these advantages, CPNC may replace theconventional ABS resin and its blends with PVC, for example, in electronic housingapplications.

To prepare CPNC of SAN, Lee and Kim [2004] dispersed Na-MMT incaprolactam, polymerised the suspension, and then melt compounded the productwith SAN. Surprisingly, XRD indicated full exfoliation within the PCL domains,while Tg showed good miscibility between PCL and SAN. The stress-strain curvesfrom the tensile tests at room temperature indicated significant improvement ofthe stress and elongation at break.

2.4.3.4 Other Systems

Melt compounding is becoming a method of choice for CPNCs with a variety ofpolymeric matrices. The method has not been successful in the case of a PS matrix[Tanoue et al., 2003]. The authors compounded three grades of PS with Cloisite®

10A under diverse processing conditions. The interlayer spacing versus residencetime in the TSE initially slightly increased to decrease at longer times toward thevalue of non-intercalated MMT. The reason for this behaviour was a rapidthermomechanical decomposition of organoclay by the Hofmann eliminationmechanism, followed by chain scission of PS. Better results could be obtainedusing MMT pre-intercalated with either imidazolium cations [Wang et al., 2003],or with ammonium-ion terminated oligostyrene [Su et al., 2004]. Owing to thehigher decomposition temperature, and good miscibility with the matrix, asatisfactory dispersion of clay platelets has been obtained.

Intercalated CPNC with PMMA as the matrix were prepared by melt mixingwith 10 wt% of commercial organoclays, viz. Cloisite® -10A, -20A, -25A and–30B [Kumar et al., 2003]. The interlayer spacing of the organoclays was foundto increase by 0.7 to 1.4 nm. The glass-transition temperature of these CPNCwas lower than that of neat PMMA by ca. 10 °C.

Melt compounding was also used to disperse 1 to 10 wt% of Cloisite® 30B inan epoxy matrix [Yasmin et al., 2003]. Thus, a three-roll mill was used at 60 °Cto exfoliate the clay. The tensile stress-strain curves indicated that as the organoclayloading increased the modulus increased as well (by 67% at 10% loading), butthe strain at break decreased (by a factor of ca. 5.7). Incomplete degassing, leavingvoids in the specimen was most likely responsible for these modest improvements.However, since the method does not require solvent it is environmentally-friendlyand worth pursuing.

2.4.4 Functional CPNCThe previous examples described CPNC prepared as structural materials, withimproved strength, modulus, and/or high barrier properties. However, the methodsare quite general, applicable to other materials, designed to perform specificfunctions. A few examples will be mentioned.

2.4.4.1 Liquid Crystal/Clay Composite (LCC)Kawasumi et al. [1998] prepared nematic liquid crystal (LC) with up to 2 wt%of MMT. First, the clay was intercalated with a variety of ammonium cations,

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including primary and quaternary alkyl (C8, C12, C18, etc.) as well as primaryand tertiary amines with biphenyl groups, e.g., 4-cyano-(4´-biphenyl-oxy)-undecylammonium salt, that showed good miscibility with LC. MMT intercalation wasconducted in aqueous medium at 50 °C stirring the suspension for 3 h, washingthe precipitate with ethanol and freeze-drying it. The interlayer spacing variedwith the intercalant from d001 = 1.37 to 3.28 nm. Next, the organoclay (0.2 to 2wt% in the final composition) was dispersed in DMF or dimethyl acetamide(DMAc) and the LC was added. The solution was spin-coated on slide glass,DMF slowly evaporated at 50 °C, and then under vacuum. When the clay hadgood affinity for LC, the system was homogeneous. The CPNC exhibited bistableand reversible electrooptical effects between a light scattering and transparentstate that could be selected by changing the frequency and voltage of appliedelectric fields. To activate transparency a 50 ms electrical field (60 Hz, 100 V)application was required. To induce opacity a 50 ms high frequency field (1.5kHz, 100 V) was needed. When the field was switched off the transparency (inthe first case) or opacity (in the second) remained. The memory effect is relatedto the orientation of clay particles caused by application of the low frequencyfield and randomisation by the high frequency one. This new CPNC is a potentialcandidate for light controlling glasses, high information display devices (that donot require active addressing devices), erasable optical storage devices, etc.

2.4.4.2 Biodegradable CPNC with Polylactic Acid (PLA)

Lactic acid (LA), a colourless liquid with chemical formula CH3CHOHCOOHwas identified by Carl W. Scheele in 1780. It exists in two optically active forms,dextro- and laevo-, often designated as D-lactic acid and L-lactic acid. Ordinary(or racemic) lactic acid is optically inactive. Pharmaceutical and food industriesprefer the L-lactic acid and lactates since the human body does not metabolisethe D-form. Lactic acid has been produced by fermentation controlled by specificbacteria, using sugar as the starting raw material.

While LA can be synthesised, its commercial production is based onfermentation. The starting material is usually starch from any plant (e.g., corn,potatoes or sweet potatoes). In the first step, starch is recovered from plant materialand enzymatically converted to glucose, which in turn is fermented bymicroorganisms (e.g., Lactobacillus amylovorus) to PLA [Yan et al., 2001]. Forhigh productivity the fermentation requires immobilisation of the bacteria and acomplex mixture of nutrients.

Carothers in 1932 produced a low molecular weight polylactic acid (PLA) byheating lactic acid under vacuum. Following this discovery DuPont manufacturedmedical grade sutures, implants and packaging for controlled drug releaseapplications. Today the two principal routes to PLA are:

1. Direct condensation followed by chain coupling, and

2. Ring-opening polymerisation of lactide(s).

In the first process the polycondensation water is removed by azeotropicdistillation and high vacuum – the approach used by Carothers (nowadays byMitsui Toatsu Chem.) to produce a low to intermediate MW polymer. This productmay be used directly, or if high MW is required, it can be chemically coupledusing isocyanates, epoxies, anhydrides, carbodiimides, ortho-esters, etc. Thesecond process also starts with polycondensation of LA under mild conditions,

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to produce a low MW intermediary PLA, which in turn can be catalyticallyconverted to cyclic dimer – the lactide. The latter has three forms that can beseparated by distillation: L-, D-, and DL- or meso- [Kõhn et al., 2003]. The ringopening polymerisation of the lactide leads to a wide range of molecular weights.Several manufacturers supply PLA, viz. Galactic, Mitsui Toatsu Chem., MitsubishiPlastics, Inc. (‘Ecoloju’ Biodegradable Plastic Films and Sheets), Shimadzu,Trespaphan-Celanese AG (biaxially oriented PLA, BOPLA), and Cargill DowPolymers. The latter company announced a major expansion aimed primarily atthe fibre and nonwovens market.

Typical properties of PLA are listed in Table 38. L-PLA shows better mechanicalproperties than meso-PLA. The former properties are comparable to these of typicalcommodity resins, viz. PP, PS or PVC. The main problem is the relatively low Tgand crystallinity. In consequence, PLA has a tendency to permanently deform understress at T > 50 °C. It is expected that incorporating nanoclays may solve bothproblems. Solution crystallised PLA was shown to reach over 80% crystallinity[Fischer et al., 1973]. Another problem – the relatively high density of PLA – maybe eliminated by microfoaming a properly formulated CPNC.

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In 1998 Rhone-Poulenc obtained a US patent for nanocomposites preparedby dissolving polyethylene glycol- and/or polypropylene glycol-polylactic acidcopolymers in an organic solvent followed by adding it to an aqueous suspensionof clay [Spenleuhauer et al., 1998]. CPNC could be recovered by precipitation(without additional colloidal protective agents) or by microfluidisation and solventevaporation.

Also in 1998, Toyota established the Biotechnology & Afforestation BusinessDepartment. The group, elevated to the level of an independent business division,actively promotes business activities in forestation, agriculture and other fieldscontributing to the solution of such problems as food shortages and environmentaldegradation. In 2001, Toyota jointly with Mitsui & Co., Ltd started a joint venturein Indonesia. The aim is to produce ca. 100 kton/year of sweet potatoes from6,000 ha dedicated farmland for the production of PLA and animal feed.Production will start in early 2004. The PLA plant will be built next to the animalfeed processing plant. In the meantime there is serious ongoing research andengineering design effort for the manufacture of lactic acid, its polymerisationinto PLA, and upgrading the performance of the latter by means of CPNCtechnology. The main concerns are heat resistance, mechanical performance anddurability. Toyota believes that upgraded PLA can be used to replace PP fabricsand mouldings in automobile interior parts, such as seat mouldings, interiordoor finish, pillar garnish, etc. Some of these parts were already introduced in aconcept car presented at the 2002 motor shows.

CPNCs with PLA as a matrix can be prepared either by polymerisation in thepresence of organoclay (e.g., by the monomer intercalation method) or by meltcompounding. Ray et al. [2002a] selected the latter route. PLA (D-enantiomercontent of 1.1-1.7 wt%) was vacuum dried, and dry blended with MMT-ODA.The inorganic content varied from 2 to 4.8 wt%. The effect of a compatibiliser,a low molecular weight o-PCL (α,ω-hydroxy-terminated with Mw = 2 kg/mol),was also studied. The mixtures were melt-compounded in a TSE at 190 °C.

XRD and TEM provided evidence of intercalation but not exfoliation. As aresult of melt-compounding the interlayer spacing of MMT-ODA, d001 = 2.31 nm,increased to higher values, dependent on the clay content, viz. d001 = 3.10 (for2 wt%) and 2.89 nm (for 4.8 wt%). Incorporation of the compatibiliser did notchange the XRD peak position, but it increased its intensity. The number of clayplatelets per stack was calculated as 10 to 13. Good bonding between the clayplatelets and the matrix was evidenced by melt rheology – the low frequencystorage modulus strongly increased with the organoclay content. The degree ofcrystallinity of PLA was determined as 36% – incorporation of either organoclayor PCL increased it to ca. 50%. However, the latter additive simultaneouslydecreased the Tg by about 5 °C. The two additives also had opposite effects onthe storage modulus – the clay increased it, the compatibiliser decreased its value.

Interesting and unexpected results were obtained by burning the CPNC. Theresulting cinder was a highly porous, ‘house of cards’, a sort of ceramic foam,with density, ρ = 0.187 g/ml. Since the BET surface area of MMT is 780 m2/g andthat of the ceramic foam is 31 m2/g, the reduction is about 25-fold [Ray et al.,2002b]. XRD indicated that the sintered material had amorphous structure.

A more detailed study of the effect of clay on PLA crystallinity was publishedseparately [Ray et al., 2002c]. In this case, instead of MMT-ODA + PCLcompatibiliser, a fluoromica (FM; CEC = 1.2 meq/g; p ≈ 250) pre-intercalatedwith methyl coco-alkyl bis(2-hydroxyethyl)-ammonium ion (MC2EtOH) was

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used. Melt compounding of PLA with 4 wt% of this organoclay was carried outin a TSE at 210 °C. Compounding increased d001 from 2.1 to 3.1 nm, but itreduced the PLA Mw from 177 to 150 kg/mol. The effect of the organoclay oncrystallinity was quite modest, viz. the degree of crystallinity increased by ca.4%, Tg decreased by 4 °C, Tm was unchanged and Tc increased by 28 °C. However,at the same time the improvement of the mechanical properties of PLA wassignificant – flex modulus, storage modulus and flex strength at 25 °C all increasedby: 26, 25 and 9%, respectively. Surprisingly, incorporation of the organoclayaccelerated the rate of PLA biodegradability by ca. 60%.

In the fourth paper of this series [Ray et al., 2003], PLA was melt compoundedin a TSE at 210 °C with 4, 5 or 7 wt% of organoclay. The latter was MMT (CEC= 0.9 meq/g; p ≈ 100), pre-intercalated with trimethyl octadecyl ammonium(3MODA). Melt compounding only slightly reduced the Mw of the PLA matrix(from 177 to 165 kg/mol), and virtually did not affect Tg and Tm. The strongestincrease of the PLA degree of crystallinity was for 4 wt% clay loading – an increasefrom 36 to 65%. At the higher organoclay contents the crystallinity decreased to56 and 54%, respectively.

Again, these nanocomposites were only intercalated. While the interlayerspacing of organoclay was d001 = 1.93 nm, incorporation into PLA melt at thelevel of 4, 5 and 7 wt% organoclay increased this value to 3.05, 2.7 and 2.8 nm,respectively. The data also indicate that here the number of platelets in a shortstack was about 4. Nevertheless, the improvement of the mechanical properties(see Figure 57) was quite impressive – note that the tensile strength and theelongation at break go through a local maximum at about 4 wt%. On the otherhand, the Young’s modulus and the heat distortion temperature linearly increasewith the clay content in the whole range of compositions. In Figure 58 the relativeoxygen permeability is shown versus organoclay content. The experimental data

Figure 57 Mechanical properties of a PLA/MMT-3MODA system as functions ofthe organoclay content. The data points are experimental [Ray et al., 2003], the

lines have been included to guide the eye.

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follow Equation 23 with the aspect ratio, p = 24. Since the average number ofplatelets in the stack is about 4, and the interlayer spacing is about 3 nm, theeffective length of the stack is 288 nm – to be compared with the estimate by theauthors of approximate platelet diameter, D ≈ 200 nm.

As has been noted before, incorporation of MMT enhanced thebiodegradability of PLA. It is interesting that the enhancement is not observedduring an initial period (30 < ti < 50 days). Afterwards, without the organoclaythe biodegradation proceeds at a slow rate, whereas in its presence it isdramatically increased – all specimens decomposed in 60 days. Since thebiodegradation proceeds by hydrolysis of the ester linkages, one may speculatethat the difference is related to the size of the PLA crystals, as well as the presenceof ≡Si-OH hydroxyl groups. The melt flow behaviour of these systems stronglysuggests that these groups react with PLA, causing end tethering. However, sucha non-catalysed reaction is slow and numerous ≡Si-OH groups are most likelyleft behind.

Paper 5 of this series examines the influence of different organoclays on theperformance of PLA nanocomposites [Ray et al., 2003b]. At a constant loadingof 4 wt%, the best performance was obtained using synthetic fluoromica (FM)pre-intercalated with methyl di-polyethylene glycol coco ammonium ion: theflexural modulus increased by 26%, strength by 9%, HDT increase by 17 °C,and O2 permeability was reduced by a factor of 2.8. Furthermore, biodegradabilityof this system was twice as rapid as that of neat PLA. The final contribution inthis series [Ray and Okamoto, 2003] focused on the flow and foamability ofPLA containing up to 7 wt% of MMT-ODA. Non-linear viscoelastic behaviourwas observed in dynamic, steady state and extensional flow fields. High qualityfoams with bubble size of 2.59 ± 0.55 μm were produced.

Figure 58 Relative oxygen permeability of a PLA/MMT-3MODA system as afunction of organoclay content. The calculated dependence suggests low a clayplatelet aspect ratio of p = 24 or their misalignment. Data [Ray et al., 2003].

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Recently, fundamental studies on the crystalline and supermolecular structuresof CPNC with PLA as matrix were carried out [Pluta et al., 2002]. The specimenswere prepared by melt compounding in an internal mixer (at 60 rpm and 180 °Cfor 10 min). Thus PLA (MW = 166 kg/mol; 4.1 mol% D-lactide) was mixed with0.3 wt % Ultranox 626 stabiliser and 3 wt% (inorganic content) of a nanofiller.As the latter, either Na-MMT (CEC = 0.92 meq/g; p = 250 to 500; d001 = 1.22 nm)or MMT intercalated with MHT2EtOH from SCP were used. The compoundswere compression moulded at 185 °C into 0.5 mm thick sheets. Three solidificationprocedures were used: quenching, annealing of the quenched sample at 120 °C,and crystallisation from the melt at 120 °C for 3 h. XRD showed that melt-compounding MHT2EtOH with PLA resulted in intercalated CPNC – theinterlayer spacing increased from 2.1 to 3.14 nm. By contrast, Na-MMT onlyformed a microdispersion in PLA matrix. Microscopic observations of thespecimen structures are summarised in Table 39. The authors noted that in thenanocomposites the clay short stacks were incorporated into the interlamellarand/or intralamellar regions. The data are a bit disappointing as the nanofillerhad negligible effects on the Tg as well as on crystallinity. However, it was foundthat during melt compounding with clay or organoclay PLA degraded less thanwithout it, viz.: decrease of Mn for PLA, NC and MC was 41.2, 19.6 and 22.1%,

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respectively. The processability of these compositions was comparable to that ofthe neat PLA. An improvement of the CPNC thermal stability under oxidativeconditions was also noted.

CPNC based on polycaprolactone (PCL) were prepared by melt mixing [Di etal., 2003]. The exfoliation of Cloisite® 30B was achieved for an organoclay loadingbelow 10 wt%. In parallel with the degree of dispersion was the increase of thePCL crystallisation temperature. A significant enhancement in mechanicalproperties and thermal stability was reported.

For packaging applications, Paul et al. [Paul et al., 2003] studied thepreparation and properties of CPNC with plasticised PLA. To make these systemscompetitive with the standard packaging resins (viz. PE or PP) such PLA propertiesas thermal stability and gas barrier properties should be enhanced. Thus PLA(Mw = 155 kg/mol) was plasticised with PEG (Mw = 1 kg/mol; 5, 10 or 20 wt%)and melt-compounded with a nanofiller (1 to 10 wt% of clay) in the presence of0.3 wt% of Ultranox® 626 stabiliser. Four Cloisite‚s from SCP were used: Na-MMT,C25A, C20A and C30B (see Table 16). The compounding was done in an internalmixer at 180 °C and 20 rpm for 4 min, then at 60 rpm for 3 min. The compoundwas pressed into 3 mm thick sheets. According to XRD, at a loading of 3 wt% ofthese four nanofillers, only intercalation was obtained. The interlayer spacingsincreased from d001(Na+) = 1.21 to 1.77 nm, d001(C25A) = 2.04 to 3.24 nm,d001(C20A) = 2.36 to 3.65 nm, and d001(C30B) = 1.84 to 3.80 nm. The intercalationprocess was complicated by two secondary intercalants, PLA and PEG. Intercalationof Na-MMT was most likely done by PEG, but in the case of organoclays bothpartake in the process. Plasticisation reduced the Tg from 55 to 15 ± 1 °C, andslightly increased the Tm from 167 to 170 ± 1 °C, i.e., the nanofillers have beenfound unable to affect the PLA transition temperatures. On the other hand, thedecomposition temperature was significantly increased, e.g., addition of 3 wt% ofC30B increased it by 40 °C. The flammability was also suppressed. The mechanicalproperties were not reported.

Bionolle, a commercial biodegradable polybutylenesuccinate (PBS) was usedas CPNC matrix for a series of organoclays [Someya et al., 2004]. The latter werebased on MMT pre-intercalated with primary and tertiary ammonium cations(e.g., DDA, ODA, ADA, N-lauryldiethanolamine, and 1-[N,N-bis(2-hydroxyethyl)amino]-2-propanol). MMT-ODA provided CPNC with the largest interlayerspacing, d001 = 3.27 nm, but even in this case the improvement of mechanicalperformance was relatively minor, for example, at 3 wt% clay loading tensilemodulus increased by 40% while strength decreased by 9%. A similar enhancementof rigidity and reduction of strength was reported by Okamoto et al. [2003] forCPNC with PBS as matrix – only intercalation was achieved.

Several other water-soluble polymers have been proposed for biodegradableapplications of CPNC, viz. PVAl, EVAl, PEG or PCL. While melt blending resultedin intercalation, the reactive route leads to exfoliated CPNC [Pramanik, et al.,2001; 2002; Kwiatkowski and Whittaker, 2001; Pantoustier et al., 2002].However, none of these materials has been as intensely promoted as the CPNCwith MMT and PLA.

2.4.4.3 Poly(N-Vinyl Carbazole)/MMTPolymerisation of purified N-vinyl carbazole (NVC) was conducted in the presenceof dehydrated MMT either in bulk, just above Tm = 64 °C, or in benzene solution at

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50 °C [Biswas and Ray, 1998]. Both methods resulted in a similar conversion versustime plot. The cationic polymerisation at 50 °C was catalysed by MMT. First, theNVC was adsorbed by the MMT surface, forming a complex either with thecounterion or with metal oxide impurities in MMT (e.g., Fe2O3), which resulted inelectron transfer that initiated polymerisation. Up to 10 wt% of poly-N-vinyl carbazole(same as poly-9-vinyl carbazole, PVK) was retained by MMT after extraction bybenzene, indicating the presence of end-tethered macromolecules. XRD showed thatas a result of polymerisation the characteristic MMT diffraction peak (d001 = 0.99 nm)was significantly reduced in intensity and a new one (d001 = 1.46 nm) appeared. Thelatter indicated intercalation, but judging by its low intensity, probably someexfoliation took place. Considering the high concentration of MMT in these systems(≤ 43 wt%) exfoliation was not to be expected. Incorporation of MMT significantlyimproved the thermal stability; whereas PVK totally decomposes at 700 °C, the CPNClost only 12.2 wt% at 683 °C. However, the most astonishing result of the processwas the increase of DC conductivity by 10 orders of magnitude: from 10-16 to 10-6

S/cm, and doping may increase it even further.In the subsequent paper the authors discussed polymerisation of NVC in the

presence of FeCl3-impregnated MMT [Ray and Biswas, 1999]. XRD displayedonly one d001 = 0.98 nm peak, thus the MMT-FeCl3 complex was not intercalatedduring polymerisation. TEM showed the clay particles to be in the range 30-40 nm. However, the DC conductivity of the CPNC was in the range 3.1 x 10-5 S/cmat 5.9 wt% of FeCl3, dependent on the FeCl3 loading. In this context it may beinteresting to note that polymerisation on the surface of nanoparticles can beused for the preparation of self-assembled monolayers. For example, brush-typecore-shell macromolecular structures were polymerised on the surface of Auparticles with diameter d = 2-5 nm by means of surface-initiated living cationicpolymerisation of 2-oxazolines [Jordan et al., 2001].

In 2002 Suh and Park prepared CPNC with MMT-ODA dispersed in a PVK(Mw = 100 kg/mol) matrix. Three organoclays were prepared, containing 0, 50,75 and 100% of intercalant per 100% of the clay’s CEC. Dissolving PVK inchlorobenzene, and then adding a suspension of 1 wt% organoclay resulted inCPNC. XRD of the organoclay indicated that the interlayer spacing increasedwith ODA content from d001 = 1.25, 1.72, 1.77 and 1.89 nm, for 0, 50, 75 and100% CEC, respectively. The spacing in dry, cast films of PVK/MMT-ODA alsodepended on the ODA content. Evidently, for 0% (neat MMT) a non-intercalated,simple, filled system was obtained. Intercalated CPNC were obtained forcompositions containing 75 and 100% CEC of ODA. However, the systemcontaining a low density of the intercalant in organoclay, PVK/MMT-50%ODAwas found exfoliated. This, seemingly surprising observation is in excellentagreement with thermodynamic theories discussed in Section 3.15.

Yu et al. [2004] prepared a series of CPNCs by dispersing an organoclay inNVC, and then UV-photoinitiating in situ polymerisation with triarylsulfoniumsalt as the initiator. The organoclay was Na-MMT pre-intercalated with acidifieddiallyl amine: (CH2=CH-CH2)2NH2

+ Cl–. The final CPNC contained 0 to 1 wt%of clay. Within this range of composition, Mn decreased from 2273 to 1438 g/mol,while Tg increased from ca. 48 to 78 °C. This Tg increase is intriguing, consideringthe low clay loading, poor exfoliation, and the low and decreasing (with claycontent) molecular weight of the matrix (note that Tg decreases with Mn as:Tg = Tg∞ - A/Mn).

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2.4.4.4 Polydiacetylene

An attempt was made to intercalate MMT, vermiculite or mica with diamine-diacetylene (DADA), monoamine-diacetylene (MADA), or ω-aminoacid-diacetylene(AADA), then polymerise these using 60Co irradiation [Srikhirin et al., 1998]. Forthis purpose, 14-amino-10,12-tetradiynoic acid and 10,12-docosadiyndiamine(diacetylenic diamine), were synthesised. The approach was to intercalate the clayswith a cationically terminated diacetylene monomer by cation exchange. Theintercalation was carried out by mixing the clay with diacetylene ammonium saltsolution in 10% EtOH/water for up to seven days at 70 °C. XRD and FTIRconfirmed the intercalation. The interlayer spacing of the intercalate decreasedwith the number of washings, e.g., from d001 = 4.5 to 3.7 nm. The interlayer spacingand polymerisability of the intercalated diacetylene were found to depend on themolecular length, the CEC of the clay, the type of diacetylene molecule, and thesolvent treatment.

Polymerisation of a diacetylene may occur when the monomer has properpacking in the solid state. The polymerisation results in a conjugated backbone,providing π-electron delocalisation along the backbone which gives polydiacetylenestheir unique electrical and optical properties. The polymerisation of the intercalateddiacetylene should result in growth of the polydiacetylene chain along the interlayergallery. However, packing of the intercalated monomers controls the reactivity ofthe diacetylene/clay complex. Since the intercalated DADA/MMT, AADA/MMTand AADA/vermiculite lie flat on the clay surface they lack the proper geometry tobe polymerised. The intercalated MADA/vermiculite and MADA/MMT are tiltedwith respect to the clay surface, but polymerisation was only observed inMADA/vermiculite where the intercalated diacetylene has proper packing geometryand monomer density (smaller area per intercalated molecule).

The conductivity of I2-doped polyaniline (PANI) approaches that of Cu, butthe polymer is brittle and sensitive to humidity and oxygen. Recently MMT wasintercalated with aniline hydrochloride, and then aniline was electropolymerised,increasing the interlayer spacing to d001 = 1.47 nm [Feng et al., 2001].Unfortunately, the note did not report data on the stability and conductivity ofthis potentially interesting material.

2.4.4.5 Clay-Functional Organic Molecules

The intercalation of indigo blue into clay is credited for the preservation of vividcolours in Maya’s frescos.

Several patents from AMCOL (e.g., [Beall et al., 1999b]) describe the directintercalation of clay with organic molecules having functional properties, e.g.,fungicides, pesticides, insecticides, acaricides and other agricultural or medicallyactive compounds. These systems were prepared by dispersing clay in an aqueousmedium, followed by adding a solution of a functional compound. The lattermust have a polar moiety, e.g., carboxylic acid, ester, amide, aldehyde, ketone,sulfur-oxygen or phosphorus-oxygen moiety, cyano, or a nitro moiety. Judgingby XRD diffraction the compounds intercalated into the interlayer galleries. Insome cases strong ionic bonding was obtained, in others (viz. the pesticidetrifluralin) the bonding was of the physical adsorption type, allowing thecompound to be fully released from the interlamellar space.

There is another group of compounds of industrial interest: molecules withspecific functionalities, e.g., catalysis, photoluminescence, photochromism, optical

255


nonlinearity, etc. Using such molecules as intercalants may impose molecularordering that enhances the performance. Furthermore, intercalation often resultsin improved thermal and oxidative stability caused by reduced thermal motionand/or reduced oxygen diffusivity.

Recently [Kim et al., 2001] nanocomposites of organic laser dye-hectoritecomplexes were prepared. Thus, nanocomposites of hectorite (CEC = 0.44 meq/g)with 7-diethylamino-4-methyl coumarin dye were produced. By alternateadsorption of positively charged polyelectrolyte and a negatively charged hectorite/coumarin complex, a multilayered film was fabricated. The coumarin was acidifiedand then intercalated into hectorite by the ion exchange reaction. The originalinterlayer spacing of hectorite (d001 = 0.98 nm) slowly increased with coumarincontent up to the total exchange of inorganic cations (from 1.34 to 1.39 nm),then more rapidly up to 2.22 nm at a coumarin loading of 20.4 wt% (252% ofthe CEC). This suggested different packing of coumarin molecules within theinterlayer galleries – a flat monolayer below 100% CEC and a tilted monolayerabove. As a consequence, CPNC films with high molecular order and thermalstability were prepared for photonic applications. A multilayered composite filmshowed a linear increase in the absorption and in the fluorescence intensity withthe number of deposited layers. Other photofunctional chromophores andfluorophores with a high degree of molecular orientation order and enhancedelectrooptical properties can be prepared. By adjusting the intercalant-clayinteractions the quantum efficiency of fluorescence can be adjusted. Moleculardynamic simulation of the adsorption of methylene blue by MMT, beidellite andmuscovite has been carried out, predicting d001 = 1.23 to 1.57 nm [Yu et al.,2000b].

CPNC have been prepared by polymerising 2-ethynyl pyridine within MMT[Liu et al., 2001]. Intercalation was carried out in benzene, by first dispersing init dehydrated Ca2+-MMT (CEC = 1.20 meq/g), adding 2-ethynylpyridine (2-EPy)and then stirring the mixture at 65 ± 3 °C for 24 h. Initially, the suspension wasred. The colour darkened gradually to reach dark brown colour when the reactionwas terminated. Thus, 2-EPy polymerised spontaneously within the interlayergalleries. The complex was centrifuged and washed several times with benzenethen dried. Up to 21.3 wt% of 2-EPy was adsorbed, increasing the interlayerspacing up to d001 = 1.64 nm, consistent with the van der Waals dimension of apyridine ring (0.66 nm). Thus, the polyacetylene macromolecules are flat on theMMT surface with pyridine rings tilted with respect to the surface.

2.4.4.6 Super-Absorbent CPNCMMT was dispersed in an aqueous solution of Na-acrylic acid, N,N´-methylene-bis-acrylamide (crosslinker) and K2S2O8 (free radical initiator) [Lin et al., 2001].The reaction (4 h at 60-70 °C) resulted in a slightly yellow powder. The waterabsorbency (Q) strongly depended on the crosslinker concentration (e.g., 1.1 at0.05% to 0.7 kg/g at 0.13%), MMT content (maximum Q was found at 30 wt%MMT), and the degree of neutralisation. Interlayer spacing has not been measured.

2.4.4.7 Emulsion Polymerisation of CPNC

The preferred methods for the preparation of PS-based CPNCs are emulsion,solution or bulk (co-) polymerisation that start with organoclay dispersed in amonomer phase. Since clay intercalation is usually in water, emulsion

256


polymerisation is an obvious choice. However, when the hydrophobicity inducedby intercalation is insufficient, the clay will not be transferred to the monomerphase, leading to a limited improvement of the resulting polymer properties. Forexample, SBR-based PNC was prepared by emulsion polymerisation in thepresence of onium-intercalated MMT. The PNCs showed superior barrier andmechanical properties [Elspass et al., 1997; 1999]. SAN-based PNCs [Noh andLee, 1999] were prepared by emulsion or solution polymerisation.Thermogravimetric analysis showed that at 500 °C the amount of remainingresidue was: 5 wt% of SAN, 20 wt% of solution-type PNC and 70 wt% ofemulsion-type. There is a detailed discussion on the dispersion of organoclaysduring emulsion or suspension polymerisation in Section 4.1.4.1.

Introduction

Part 3

Fundamental Aspects

257

Thermodynamics

3.1 Thermodynamics

Nanostructures are intermediate in size between molecular and micron-sizestructures. They contain a countable number of atoms and thus resemblemolecules. Their small size and structure result in strong interactions and highsurface-to-volume ratio. Nanostructures may show specific electronic andmagnetic characteristics, often dominated by the quantum effects. Their propertiesdepend upon the size, the shape and arrangement of the atoms; for example,substitution of Mg for Al in the octahedral layer of smectites is essential forintercalation and exfoliation in a polymeric matrix. Thus, there is a need todevelop new fundamental tools capable of describing and predicting propertiesof the nanoscale structures.

3.1.1 Glass Transition in Thin FilmsAs the size of the particles decreases, the transition temperatures change as well.For example, the melting point of bulk PE is about Tm ≅ 400 K, but the Tm of itsnanosized particles with diameter of 5 and 13 nm is 218 and 266 K, respectively.Similar reductions were reported for the glass transition temperatures (Tg) ofthin film. In the latter case, three types of behaviour were distinguished:

(1) for thin films grafted to a solid substrate,(2) for supported films, and(3) for unsupported films [Forrest and Dalnoki-Veress, 2001].The two latter cases are under active investigation at present.

1. For type (1) the Tg depends on the degree of molecular bonding and it mayeither decrease or increase with decrease of film thickness.

2. For type (2) there is a reduction of Tg with reduction of film thickness thatseems to be independent of molecular weight. An empirical relation wasderived:

T T a hg gb= − ( )⎡

⎣⎢⎤⎦⎥

1 /δ

(30)

where Tgb is the glass transition of the bulk phase, h is the film thickness,and a and δ are empirical parameters which depend on the system, viz. forPS a = 3.2 nm and δ = 1.8; for PMMA a = 0.35 nm and δ = 0.8. It wasreported that the behaviour was independent (or nearly so) of the substrate.A mechanism that may explain such Tg behaviour is the presence of a thinsurface layer having high molecular mobility and higher free volume fractionthan that in the bulk [Forrest and Dalnoki-Veress, 2001]. The thermodynamicanalysis of the surface/interface energy indicated that there are two basicmechanisms responsible for the minimisation of the surface energy: high

258


concentration of highly mobile chain ends and migration toward the surfaceof low molecular weight components (low MW fractions and additives)[Helfand and Tagami, 1971; 1972].Recently, Monte-Carlo (MC) simulation was used to study the behaviour ofthin polymeric films. As shown in Figure 59, the computations gave resultsclosely resembling the experimental results. Furthermore, it was found thatthe thin films have a fluid-like interfacial region where mobility is considerablyhigher than in the bulk film [Jain and de Pablo, 2002]. The computed resultsindicated that it is not an effect of different skin composition, but rather thechain conformation. In agreement with Helfand and Tagami calculations,near the surface MC simulations predicted high concentration of chain endsand reduced density. In this region the linear macromolecules have non-Gaussian coil configuration.

3. For type (3) the Tg depends on both film thickness and weight-averagemolecular weight (Mw). The reduction of Tg is significantly more pronouncedthan in type (2). For high Mw the following dependence was found to describethe behaviour:

T T b M M h hg g w w− = × −* * *ln( / ) ( ) (31)

where symbols with asterisk (*) are reference variables.For PS: b = 0.70 ± 0.02 K/nm, Mw

* = 69 ± 4 kg/mol, h* = 10.3 ± 0.1 nm, and

Tg* = 423 ± 2 K. The dependence is illustrated in Figure 60.

Chow [2002] proposed the glass transition theory for unsupported, nanoscalepolymeric films with thickness < 100 nm. The theory is based on the Gibbs andDiMarzio time-independent theory that Tg is related to the loss of configurationalentropy of a liquid during the cooling experiment. The entropy in turn can berelated to the free volume fluctuations, which depend on the temperature andfilm thickness. The Langevin equation was used to derive the followingdependence:

ln , / / exp / /T h N T k N N C h h N Ng g B o p o o( )[ ] = −( ) ⋅ − ( ) ⋅[ ]⎧

⎨⎩

⎫⎬⎭

∞ 22

Δθ

(32)

where Tg and Tg∞ are the polymer glass transition temperature in the film and inthe bulk, respectively; h and ho are the film thickness and its critical value equalto the radius of gyration; kB is the Boltzmann constant; N and No are the numberof statistical segments in a polymer and in a reference state; ΔCp is heat capacity.The exponent θ = 0.7 was determined for polystyrene. The relation predicts thatthe Tg of low molecular weight polymer thin films has higher value than that ofhigh molecular weight. This unexpected result is confirmed by experiments andthe empirical dependence is illustrated in Figure 60. The author also derived anexpression predicting variation of Tg with the strength of the polymer-substrateinteractions. For low (or absent) interactions the dependence for free film wasrecovered; hence Tg(film) < Tg(bulk). For strongly interacting systems the Tg inthin film was predicted to have a higher value than that observed for the bulkpolymer: Tg(film) > Tg(bulk).

Another type of nanosized particles with unexpected Tg behaviour was recentlydescribed [Mi et al., 2002]. The authors prepared single-macromolecular coilsize globules of polyacrylamide (PAAM; Mw = 5,000 to 6,000 kg/mol) byatomisation spraying of a dilute polymer solution, followed by drying. Thermalanalysis showed that the globules had a higher Tg than that of the same bulk

259

Thermodynamics

Figure 59 Glass transition temperature for thin, supported PS films. Linerepresents experimental data, points computed as T k Tg B g

* ≡ −ε 1 (where kB is theBoltzmann constant and e is the nearest-neighbour interaction energy – see text).

Figure 60 Glass transition temperature for thin, freestanding PS films. Linescomputed from Equation 31 for the indicated molecular weights.

260


polymer, viz. 205 versus 193 °C, respectively. The explanation offered is basedon the configurational entropy of a single macromolecule in the bulk and in theglobular state. The analysis indicated that polymer chains in the latter state shouldhave Tg higher by a few percent (absolute T-scale), e.g., by ca. 10 °C, whichindeed was observed. However, this analysis neglects the surface energy difference(which acts in the opposite direction), as well as the interaction of the globulewith the substrate (which tends to increase Tg).

In conclusion, three factors have been identified as affecting the glass transition:molecular weight (MW), film thickness (h), and the interactions between filmand the substrate (χ). The theory predicts that high molecular weight polymerdiffusing into narrow interlamellar galleries will have high mobility under thecondition of poor interaction with the clay, but poor interactions means absenceof the driving force for diffusion. On the other hand, strong interactions meansgood ‘motivation’ for the macromolecules to diffuse, bind with the clay substrateand ‘solidify’ into a layer of adsorbed macromolecules with low segmentalmobility.

3.1.2 NanothermodynamicsIn the early 1960s Hill noted that the equilibrium thermodynamic properties ofsufficiently small systems differ from those predicted by the classicalthermodynamics of large systems [Hill, 1994]. More recently, the field wasrenamed nanothermodynamics [Hill, 2001a; 2001b]. Originally, the treatmentconsidered homogeneous, small systems, such as individual macromolecules orcolloidal particles, each composed of n-elements. For n → ∞ the macroscopicproperties are recovered. However, when n is small there are at least two significantcontributions to the energetic state of such a system, the first depends on thesurface energies, and the second on the entropy related to the dynamic state ofthe particles. For example, a single platelet of MMT, owing to its thickness(0.96 nm) and large aspect ratio, has a specific surface area of ca. 800 m2/g, andabout 36% of its atoms are on the surface. Considering the high surface energyof solids (e.g., surface energy of freshly cleaved mica is γ = 4,500 mN/m) thesurface effects are expected to dominate the thermodynamic state of nanoparticles.Since in most systems the miscibility and phase equilibria are determined by afine balance of small energetic and configurational contributions, the surfaceeffects are expected to influence them, and thus to be critical for the performanceof CPNC.

For the principle of nanothermodynamics to apply to a macroscopic system,it must be visualised as an ensemble of small subsystems (or aggregates)independent of each other. The number of elements n in each aggregate does nothave to be the same. A good example is a semicrystalline substance havingnumerous small crystals of various sizes that contain n molecules. The substancemay show size-related effects, e.g., of the melting point, width of the meltingtransition, heat capacity, glass transition temperature, etc. Other examples areprovided in recent publications on statistical thermodynamics of metastabledroplets [Hill and Chamberlin, 1998], on ferromagnetism [Chamberlin, 2000]or on phonon scattering by the boundary of nanocrystals [Shrivastava, 2002]. Inthe latter paper it was shown that the specific heat of crystalline materials dependson particle size.

One of Gibbs contributions to the equilibrium thermodynamics was additionof the chemical potential to the basic energy-heat-work equation. This

261

Thermodynamics

concentration-dependent contribution is essential for understanding andcomputing diverse thermodynamic properties, viz. chemical reactions, phaseseparation, gas solubility, osmotic pressure, etc. In a sense, incorporation of thenanoscale effects into the thermodynamics is the next step in generalisation ofthe thermodynamic behaviour. Thus, Hill formally wrote the total (subscript ‘t’)energy of a macrosystem as:

dE TdS PdV dN edn

e E n

t t t i i t

i

t S V Nt t i t

= − + +

≡

∑ μ ,

, ,( / ),

∂ ∂(33)

where T is temperature, P is pressure, V is volume, S is the entropy, μi is thechemical potential and Ni is the number of molecules of species ‘i’. The last termin this dependence, edn originates from the extra contribution of the small system.In Hill’s concept, any macroscopic system may be described in terms ofcontribution from all elements, viz.: Xt º nX, where X = E, S, V, Ni, with n beingan extensive variable. The variable e is a sort of chemical potential of the wholeensemble, i.e., a ‘subdivision potential’ that varies with T, P and μi:

de SdT VdP N d

S e T V e P N e

i i

i

P T i i T Pi i j

= − + −

− = ( ) = ( ) = ( )∑ μ

μ∂ ∂ ∂ ∂ ∂ ∂μ μ μ

/ ; / ; /, , , ,

(34)

The nanothermodynamic properties usually differ in different ‘environments’.For example, a rigid incompressible aggregate of n = 102 or 103 spherical moleculesin a constant temperature bath may be considered as having two ‘environmentalvariables’, n and T. The aggregate entropy can be computed considering thetranslational, rotational and vibrational motions of the molecules. However, onemay ‘construct’ such an aggregate considering that the constant-temperature bathcontains individual molecules with chemical potential, μ. Depending on the valueof μ the molecules will form an aggregate containing n´ molecules, hence nowthe ‘environmental variables’ are μ and T. One may choose a value of μ that willresult in n = n´. However, the calculated entropy for the first case (n, T) will besmaller than that for the second (μ, T). The reason for the discrepancy is thepresence of fluctuations in the second case that cannot be directly incorporatedin the first case. However, as the aggregate approaches macroscopic size, i.e.,when n → ∞, this entropy difference will disappear or in other words, when anensemble contains only one aggregate, the ‘classical’ macroscopic equilibriumthermodynamics is recovered.

Hill considered four sets of ‘environments’: (N, V, T); (N, P, T); (μ, V, T); and(μ, P, T). For example, the ‘subdivision potential – e’ of a one-component smallsystem in the four sets of variables was computed.

1. Environmental Variables N, V, T:From the partition function:

Q N V T E N V kTii

, , exp , /( ) = − ( ){ }∑ (35)

the following relationships are derived:

− = − = − + +− ( ) = − − +

kT Q E TS PV N e

d kT Q SdT PdV dN

ln

ln

μμ

(36)

262


2. Environmental Variables N, P, T:From the partition function:

Δ N P T E N V kT PV kTjj V

, , exp , / exp /,

( ) = − ( ){ } −{ }∑ (37)


− = − + = +− ( ) = − − +

kT E TS PV N e

d kT SdT VdP dN

ln

ln

Δ μΔ μ

(38)

3. Environmental Variables μ, V, T:From the partition function:

Ξ μ μ, , exp , / exp /

,V T E N V kT N kTjj N

( ) = − ( ){ } −{ }∑ (39)


− = − − = − +− ( ) = − − −

kT E TS N PV e

d kT SdT PdV Nd

ln

ln

Ξ μΞ μ

(40)

4. Environmental Variables μ, P, T:From the partition function:

ϒ

μ μ, P, T( ) = − ( ){ } { } −{ }∑ exp , / exp / exp /, ,

E N V kT N kT PV kTjj N V(41)


− = − + − =− ( ) = − + − =

kT E TS PV N e

d kT SdT VdP Nd de

ln

ln

μμ

(42)

The latter case is particularly important since here the system is ‘completely open’and all environmental variables are intensive. Note that the three variables: μ, Pand T cannot all be independent in a macroscopic system but they can in a smallsystem. In nanosystems these intensive variables determine the mean size N (anextensive variable). This type of system is found in biology, in bulk magneticmaterials, a Gibbs surface excess resulting from an adsorbent molecule at theend of a one-dimensional lattice gas, or metastable supersaturated gaseous statesnear the gas-liquid transition point.

Recently, this treatment was extended to one-dimensional adsorption of gas[Hill, 2001a; 2002b]. Consider a surface of B adsorbent, with moleculesindependent of each other, from each of which a chain of M binding sites extendsvertically. B and M are both very large. The B molecules, at T and μ, go on andoff the sites. There is an interaction energy (w) between any two adsorbedmolecules on nearest-neighbour sites of a chain. Also, there is an interactionenergy w′ between an adsorbent molecule and an adsorbed molecule that occupiesthe first site of the chain. The mean number of adsorbed molecules per chain isdenoted Nex and typically its magnitude is of the order of 1 to 10. For B surfaceexcesses (small system) free energy is:

E TS N B eB

dE TdS d N B edB

ex

ex

= + +

= + ( ) +

μμ

(43)

So far, nanothermodynamics have not been applied to PNC.

ϒϒ

263

Thermodynamics

3.1.3 Vaia’s Lattice Model for Organoclay Intercalation by MoltenPolymer

3.1.3.1 Introduction

In his PhD thesis of 1995 Vaia used a lattice model to describe the thermodynamicbehaviour of CPNC comprising organically modified layered silicates, e.g., micaor MMT (the work was published two years later [Vaia and Giannelis, 1997a;1997b]). Initially, the interlayer galleries are occupied by hydrated alkali metalcations, which during intercalation are exchanged for organic ones – most oftenquaternary alkylammonium ones. Because of the negative charge on the silicatesurface and crowding of intercalant groups, the alkyl tail of the alkylammoniummolecule projects away from the surface. However, as the data in Figure 23demonstrate, the expansion of the interlamellar gallery proceeds step-wise, atlower efficiency than would be expected from fully stretched hydrocarbon chains.

At the processing temperature of CPNC, Tprocess ≥ 150 °C, the interlayerstructure is expected to be disordered with a density comparable to that of liquidalkanes. Preparation of CPNC involves the formation of nanostructures wherethe matrix molecules (monomers, macromers or cyclomers introduced duringthe reactive process or macromolecules incorporated during melt compounding)diffuse into the interlayer galleries, either as a result of static annealing, dynamicmixing or both. Polymer diffusion into galleries expands the silicate layers.Depending on the degree of penetration and resulting interlayer spacing, eitherintercalated or exfoliated nanocomposites are obtained. Their structure dependson the length, density and type of intercalant, on the type, molecular weight andstructure of the polymer, on the concentration of the pre-intercalated clay, aswell as the polymer-clay and polymer-intercalant interactions.

Vaia modelled CPNC as two parallel platelets pre-intercalated with shortchains tethered to the clay surface, and then dispersed in an infinite sea of moltenpolymer (infinitely diluted system). During the second stage of the intercalationthe macromolecules diffuse into the interlamellar galleries. Within these galleriesthe polymer forms a dilute solution with intercalant molecules. The basicassumptions are that the system is incompressible, has constant polymer density,and the intercalant is end-tethered. Furthermore, it was assumed that theinteractions that account for the miscibility/immiscibility of polymer/organoclaysystems are polymer-intercalant and polymer-clay type – the clay-clay interactionsand their variation with the gallery height were neglected. The final state of theintercalation is given by the equilibrium thermodynamics. The author assumedthat the interlamellar gallery height does not exceed the full extension of theintercalant chains on both gallery surfaces, thus the model accounts only for theinitial intercalation of organoclay by molten polymer. A thermodynamicdescription takes into account the entropy-related configurations of theconstituents and the interactions between them. The external forces (e.g., shearing)are not considered.

Derivation starts with the assumption that the macroscopic thermodynamicdescription of the system is obtained considering a single sandwich of the organoclayclay (having infinite breadth and width and the initial separation h0) embedded inmolten polymer. As the macromolecules diffuse into the gallery, the interlamellarspacing increases to h. The Helmholtz free energy change, ΔF, associated with theinterlayer expansion from ho to h, is given by the internal energy change (associated

264


with intermolecular interactions), ΔE, and the combinatorial entropy change(associated with configurational changes of the constituents), ΔS:

Δ Δ ΔF F h F h E T So≡ − = −( ) ( ) (44)

where T is the absolute temperature. Accordingly, the negative value, ΔF < 0,would indicate that, from the equilibrium thermodynamics point of view, theinterlayer expansion is favourable. The average interlayer volume fraction, ϕ i ,of the end-tethered intercalant chains ( ˆ / )ϕ2 = h ho and the intercalated polymer

( ˆ ˆ )ϕ ϕ1 21= − are expressed in terms of gallery height, h. The end-tetheredintercalant molecules can only reach the distance h∞ of fully extended alkyl chainlength, thus uniform mixing will not be possible when the interlayer height exceedsits double size, h > 2h∞. Therefore, the mean-field approximation is only validfor ( / )ϕ1 1 2≤ − ∞h ho .

3.1.3.2 Entropic Contributions

There are three contributions to the configurational entropy, ΔS: (1) from changesassociated with the intercalated clay, (2) from the diffusing polymer and (3) fromthe intercalant molecules. During polymer diffusion into the interlamellar galleriesthe clay platelets are pushed apart, but their translational entropy is relativelysmall and Vaia considered this contribution negligible. For the polymer, theconfigurational entropy arises from changes of the macromolecular confinement,from its unperturbed random coil structure in the melt to a solution with theintercalant molecules within the gallery; hence there is a loss of the macromolecularconfigurational entropy. By contrast, the tethered intercalant chains gainconfigurational freedom (thus entropy) as the gallery height increases. Thus, ΔShas an entropy loss associated with confining the macromolecules and the entropygain of the tethered intercalant during the gallery height increase from h0 to h:

Δ Δ ΔS S Spolymer i ercalant= + nt (45)

For thermodynamically favourable intercalation of organoclay by polymer theHelmholtz free energy should be negative hence the favourable entropiccontribution, ΔS, should be positive.

The entropic contribution per interlayer volume (superscript ‘v’) of amacromolecule diffusing into the interlayer gallery was expressed in terms of theDolan and Edward’s theory for confined random-flight polymer chains withexcluded volume:

Δ ϕ νS R h h u mpolymer

v / ˆ / / /= −( ) +( )1 1 12

1 16 3a aπ (46)

where v1, al, and m1 are the molar volume per segment, the segment length, anddegree of polymerisation of the polymer, respectively. Excluded volume is includedin the dimensionless parameter, u. Since all the parameters in Equation 46 arepositive, the entropic contribution of the polymer diffusing into clay gallery isnegative hence it hinders the intercalation process. Using estimated numericalvalues of parameters (see Table 40) the entropic effect per surface area, ΔSA/R,was computed.

To express the second contribution, ΔSintercalant, the authors used the Huggins-Flory lattice model for polymer solutions. The original relations were modifiedto account for the restricted freedom of the end-tethered alkyls and the influenceof the silicate surface on their conformation. The entropy change per interlayervolume was expressed in terms of the increased layer separation from h0 to h:

265

Thermodynamics

cimanydomrehtehtgnitupmocrofdesusretemaraP04elbaT]5991,aiaV[ataD.seitreporp

retemaraP lobmyS eulaV

htgneltnemgesremyloP a1 5.2 a2

emulovralomremyloP v1 3v2

emulovdedulcxE mu 12/1- 8.0

)ADO(tnalacretnI C 81 H 73 HN- 3+ m2 5.9=

)mn(htgneltnemgestnalacretnI a2 52.0

)lm(emulovralomtnalacretnI v2 33

)mn(thgiehyrellaglaitinI h0 3.1

)mn(thgiehyrellaglaniF h2 ∞ a2= 2m2 57.4

Δ ϕ ψ ψ ϕ ϕ ϕ ψ ψ

ψ

S R v c m v v c

h h h

i ercalant s so s so

s

ntv / ˆ / ln ˆ / ln ˆ ˆ / ln

/ cos /

= ( ) −( ) − ( ) ≅ ( ) −( )≅ ( ) ( )∞

2 2 1 1 1 1 2 2

22 2with a π

where νi, mi, and ϕi are the molar volume per segment, the number of segments perchain, and the interlayer volume fraction, respectively. The statistical surfacefractions of interlayer sites near the surface that are occupied by the tethered chainsare expressed as: ψs ≡ ψs (h); ψso ≡ ψsh0 ≥ ψs h ≥ h0). The statistical surface factor, c,accounts for the inaccessibility of sites on the far side of the surface (c = 0.75 for z= 4, decreasing to 0.5 for z → ∞, where z is the lattice coordination number).Finally, a2 is the intercalant segment length of the tethered chain and R is the gasconstant. Since ln c < 0, Equation 47 predicts that the entropic contribution ofthe intercalant is positive, thus favourable for intercalation.

The parameters of the configurational entropy that originates from thepolymer, intercalant and their sum are listed in Table 40; the computeddependencies are presented in Figure 61. Evidently, the loss of polymer entropyduring its diffusion into a gallery is partially compensated by the configurationalgains of the intercalant molecules. While initially there is a possibility of a smallincrease of the interlayer spacing (by 0 < h - ho < 0.8 nm sufficient to accommodate1 to 2 macromolecules) any larger expansion is forbidden by a rapid decrease ofthe total entropic change. Thus, in the absence of favourable energetic interactionsany further polymer diffusion into the interlamellar galleries is not possible.

As it will be evident from the experimental data presented in Section 4, manyCPNC prepared by the melt or reactive intercalation method (e.g., in PS or POmatrix), show increased interlayer spacing, Δd001, by about 0.35 to 0.75 nm,which corresponds to single or two macromolecular layers inside the interlamellargallery. On the basis of Vaia’s entropic calculation one may postulate that inthese systems the energetic contribution to the thermodynamics is negligibly small.

(47)

266


3.1.3.3 Interactions

Vaia assumed that the energetic contributions originate in interactions betweenthe three components of the system: the silicate, s, the tethered intercalant, a,and the polymer, p. Furthermore, the total change of the internal energy is takenas a sum of the energy change associated with each pairwise interaction, εjk, withthe number of contacts expressed by the contact area. The pairwise interactionenergy per contact was expressed in terms of the cohesive or interfacial energyper lattice site. Thus, the internal energy change of the system upon polymerintercalation is:

Δ ε ε εE A A A Aspf

sp apf

ap saf

sai

sa= + + −( )where Ajk

f and Ajki is the total area of contact between components j-k for the final

(f) and the initial (i) system, respectively. The dependence was simplified assumingthat initially all the lattice sites within the interlayer gallery are occupied bytethered intercalant chain segments, thus

A A Asp saf

sai= −

The next step in the derivation was a bit arbitrary, using van Opstal et al. [1991]expression for the internal energy change per interlayer volume that takes placeduring intercalation:

Δ ϕ ϕ Δϕ ϕ ε ε

ϕe

h r

r r r h

sp sa o ap

o

ν ε= =+( )

− − ( ) − ( )[ ]ˆ ˆˆ ˆ / /

ˆ / /

,1 2

1 2 2

2 1 2 1

2

1 1(48)

Figure 61 Changes of entropy during diffusion of macromolecules intointerlamellar galleries. Initially, the positive contribution from the intercalant

compensates for the negative one from polymer. After expansion by ca. 0.8 nm, theentropic contribution becomes negative and, in the absence of negative enthalpic

contribution, the diffusion is expected to stop. After [Vaia, 1995].

267

Thermodynamics

where Δε is the effective interaction energy per interlayer volume and ri is theradius of the interaction surface, related to the molar volume and segment lengthby the simple dependence: r N ai i A i

2 = ν / π (where NA is Avogadro’s number). Thechange in interactions at the interlayer surface was written as εsp, sa = εsp – εsa. Toencourage intercalation by diffusion of macromolecules the energetic contribution,Δev, thus the numerator in Equation 48, should be negative. Evidently, this canbe expected when the interaction energies of polymer: εap < 0 and/or εsp < εsa –when polymer ‘likes’ to cohabit the gallery, interacting with intercalant chainsand/or the clay surfaces.

The total free energy change associated with polymer diffusion intointerlamellar galleries and increased interlamellar spacing may be calculated fromEquations 44-48. In Figure 62 the total free energy change during polymerdiffusion into the gallery space is presented for εap ≅ 0 (van der Waals interactionsbetween intercalant chains and macromolecules) and for four values of theinteraction parameter, εsp, sa. Other parameters were taken from Table 40.

As the author noted, the values for the interaction parameters, εij, are notreadily available. Thus, he proposed that these might be replaced by interfacialenergies between interacting species i-j, viz. εap ~ vap and εsp, sa ~ vsp – vsa. However,since the interfacial energies are also inaccessible, Vaia adopted the procedureproposed by van Oss [1994] of calculating the interfacial energy from the surfaceenergies of the interacting species. The procedure considers that the interactionsare composed of polar/associative (ν i

p ) and dispersive/dipolar (ν id ) forces:

ν ν ν

ν ν ν ν

ν ν ν

ij ijp

ijd

ijp

i i j

ijd

id

jd

= +

= − −⎛⎝

⎞⎠

= −⎛⎝

⎞⎠

+ − −2

2

(49)

where ν i+ and ν i

− are respectively, the electron acceptor and donor contributions.Luciani et al. [1996; 1997; 1998] showed that the interfacial properties are

dominated by the polar and hydrogen bonding interactions, whereas the bulkproperties are dominated by the dispersive forces. In consequence, another methodfor calculating the interfacial energy has been proposed. The method involvescomputation of the interfacial tension coefficient from the difference betweenthe dipolar, polar and hydrogen bonding components of the two interacting speciesi-j, viz.:

ν Θ δ δ δ δ δ δij i d j d i p j p i h j h

n

k T= ( ) −( ) + −( ) + −( )⎧⎨⎩⎫⎬⎭1

2 2 2

, , , , , ,(50)

where k1(T) and n are equation parameters and Θ ≅ 0.4 is the relative measure ofthe importance of the dispersive contribution (its value for bulk interactions is4). Since the solubility parameter can be calculated from the group contributionmethod [Hansen, 1967; 1994; 1995; 2000], Equation 50 is general.

3.1.3.4 Consequences of the Model

Vaia’s model predicts that the only source of favourable entropic contributiontoward intercalation originates in a greater number of possible configurations ofthe tethered intercalant molecules. This contribution reaches maximum when inthe organoclay the intercalant has a highly ordered, pseudocrystalline structure

268


and it ‘dissolves’ into polymer diffusing into the interlamellar gallery. However,reduction of entropy by such a ‘melting’ process is relatively small. Thus, thetheory predicts that the entropic effects are small and mainly detrimental tointercalation by diffusion of macromolecules; hence the only method for achievingintercalation is by ascertaining favourable energetic contributions.

Adopting the derivation by van Opstal et al. [1991], Vaia simplified the energeticcontribution given by Equation 48. The magnitude and sign of the effectiveinteraction parameters is related to pairwise interactions between the threeconstituents, the accessibility of interaction sites, the interlayer packing density,and the size of the intercalant chain. In most organoclays the intercalants haveapolar, paraffinic moieties, thus van der Waals dispersion forces dominate polymer-intercalant interactions. In most cases, these interactions are characterised by asmall positive value of εap. By contrast, the clay surface is polar, thus if the diffusingpolymer has groups able to form polar or hydrogen bonds, the polymer-surfaceinteractions may be more favourable to exfoliation, thus εsp,sa < 0. In the case ofpolar polymers, the favourable interactions may also be engendered by suitablymodifying the intercalant molecules. However, such a modification must becarefully designed, as too strong interactions during an early stage of intercalationmay form a tightly packed polymer layer around the clay particles, slowing downfurther polymer diffusion into the galleries.

The enthalpic contributions in Equation 48, εsp,sa and εap, are scaled with hoand r2, respectively. Since, as discussed above, the former may be negative andthe latter positive, contrary to the expectations, better chances for exfoliationmay be found starting with organoclay having small (but adequate to polymer

Figure 62 Changes of the free energy during intercalation/exfoliation by polymerfor four indicated values of the interaction parameter between clay and polymer.

With strong ‘specific’ energy of interaction the system is expected to exfoliate. Formiddle values it may intercalate, while in the absence of energetic contribution

little change of Δd001 is expected. After [Vaia, 1995].

269

Thermodynamics

diffusion) value of the interlayer spacing, d001 ≅ h0 + 0.96. On the other hand, forreducing the small, parasitic contribution of εap the value of r2 should be maximisedby selecting intercalant with large molar volume moieties. Thus, not only themagnitude of the energetic interactions, but the intercalant structure is important.When the interaction parameter, εsp, sa = εsp – εsa < 0 an intercalation, then athigher values, an exfoliation is possible. According to the adopted mechanism, itis not a binary interaction parameter, but the relative magnitude of two binaryinteraction parameters that counts. If exfoliation is to take place: εsp < εsa, i.e.,clay must interact more strongly with the macromolecules than with the intercalantchains. Accordingly, the role of the latter seems to be to sufficiently open theinterlamellar galleries facilitating the diffusion of macromolecules. Thus, againthe thermodynamic argument favours organoclays with small, but sufficientinterlayer spacing.

Serious difficulties are expected, and indeed found, when trying to exfoliateorganoclay with nonpolar olefinic macromolecules, e.g., PP. For these systems acompatibiliser strategy has been used. Thus, polar groups are introduced (e.g.,by maleation) into macromolecules having the same molecular configuration asthe nonpolar polymer. The key strategy is for these compatibilisingmacromolecules to engender strong polymer-clay interactions. The nonpolar partof these modified macromolecules must remain miscible with the nonpolarpolymer, ascertaining a single-phase structure of the matrix. Since the magnitudeof the Helmholtz free energy depends on the fine balance between small entropicand enthalpic contributions, these ‘compatibilising’ strategies may be suitablefor the polymer-induced exfoliation.

In summary, the mean-field statistical lattice model is a first approximationof the polymer melt intercalation. It is a simplified equilibrium thermodynamicsmodel, which considers only diffusion of molten polymer into a single galleryformed by two organoclay platelets immersed in an infinite sea of polymer melt.Of the three components: clay (s), polymer (p) and intercalant (a), the binary s-sinteractions have not been considered. Under normal circumstances theseinteractions are about 100 times stronger than those between organic segments,i.e., a-a or p-p. The s-s interactions are responsible for the difficulties in breakingthe clay particles into short stacks, doublets and finally into individual exfoliatedplatelets. The model does not incorporate the ‘small system’ contributionsintroduced by Hill. One must also ponder whether for the nanoscale system theassumption of random mixing (e.g., absence of molecular adsorption on thesurface of crystalline solid) is valid. Furthermore, the model assumes absence ofexternal forces during melt exfoliation.

In spite of the simplifications and omissions the model helps in understandingthe mechanisms responsible for polymer intercalation. The insight gained leadsto recommended procedures for enhanced intercalation hence improved CPNCperformance. The recommendations are based on considerations of the entropicand enthalpic contributions to equilibrium thermodynamics. The free energy ofthe process is a fine balance of these two. The entropic penalty of macromolecularconfinement within the interlamellar galleries may be, at least partially,compensated by the increased conformational freedom of the intercalant chainas the gallery expands. The model indicates that to be successful in formingexfoliated CPNC strong energetic interactions between the diffusing polymerand clay are required. Vaia is less enthusiastic about inducing interactions betweenintercalant and polymer, expecting hindered kinetics of polymer diffusion.

270


3.1.3.5 Model Prediction versus Static Intercalation Results

Vaia [1995] carried out static melt intercalation of several organoclays. Thus,Li+-fluorohectorite (FH, CEC = 1.5 meq/g), Li+-saponite (S, CEC = 1.0 meq/g),and Na+-MMT (MMT, CEC = 0.8 meq/g) were intercalated with excess ofdimethyl-dioctadecyl ammonium bromide (2M2ODA), trimethyl-octadecylammonium bromide (3MODA), or a primary alkylammonium chlorideCnH2n+1NH3

+Cl-, where n = 6, 9-16, and 18. To prepare the specimens, powdersof dry organoclay (25 mg) and polymer (75 mg) were mixed by hand andcompressed into a pellet, which was then statically annealed for up to 48 h undervacuum at T > Tg. XRD was employed to detect the equilibrium intercalation. PS(Mw = 30, 90 and 400 kg/mol), P4VP, poly-3-Br-styrene (PS3Br), and polyvinylcyclohexane (PVCH) were used as the matrix polymers.

Melt intercalation at 170 °C in PS30 of FH-ODA, FH-3MODA, MMT-2M2ODA,and S-2MODA resulted in intercalation with gallery height expanded from ca.1.5 to 2.3 nm; hence Δd001 = 0.8 nm. It is noteworthy that FH-ODA having highCEC value was intercalated whereas its two analogues with lower charge density,MMT-ODA and S-ODA, were not. Replacing the primary octadecyl chain (inFH-ODA) by one in a quaternary onium (in FH-3MODA) resulted in a similarbehaviour, but replacing it by two octadecyl chains in quaternary onium (inFH-2M2ODA) hindered diffusion of PS into galleries. Evidently, for meltintercalation the packing density within the galleries as well as the thermodynamicmiscibility is important.

Pellets of PS30/FHn were annealed at 120, 140 and 160 °C (FHn = FHintercalated with n–CH2 groups). No intercalation was observed for n ≤ 12 whereho ≅ 0.8 nm, whereas for n > 12 (ho ≥ 1.0 nm) the final gallery height was independentof T. However, at 180 °C intercalation takes place for all n ≥ 6 organoclays. Apossible explanation for such behaviour is the formation of a pseudocrystallinealkyl phase near the clay surface, preferentially for shorter alkyls at T < 180 °C.The structure may be distorted by too long alkyls or by high temperature.

Next, the influence of the matrix polymer was studied. Vaia observed thatthe MW of PS affected the FH-ODA intercalation kinetics, but not the finalexpansion of the gallery height. This could not be expected from the derivedmodel, as the entropic contribution, ΔSv

polymer, contains the term dependent onthe degree of polymerisation, m1. For PS30, PS90, and PS400 full intercalationat 160 °C was reached after ca. 6, 24 and 48 h, respectively. As shown in Figure 63,there is a strong correlation between the Mw and the time required to reachequilibrium intercalation, teq.

Incorporation of MMT-2M2ODA into the styrene-derivative polymers resultedin a logical sequence of increasing gallery heights. Thus, Δh ≡ h – ho = 0, 0.82,0.96 and 1.0 were obtained for PVCH, PS, PSBr and P4VP, respectively. Suchordering is to be expected when the critically important interactions between theintercalating polymer and the silicate substrate take place. The pendant ringpolarisability increases in the order: PVCH << PS < PS3Br, P4VP, while theintercalation rate decreases in reverse order: PS >> PS3Br, P4VP. The intercalationkinetics depend not only on the Mw, but also on the degree of interaction betweenthe polymer and clay. At low clay loading, the large polarity of PS3Br and/orP4VP may lead to exfoliation.

Thus, in accord with the thermodynamics calculations, formation of CPNCdepends on the sign and magnitude of the interaction parameters (εjk) and on theinitial interlayer structure of the organoclay. For the polymer to interact favourably

271

Thermodynamics

with the clay, εsp,sa < 0 and the polar contribution to the interfacialenergy,

ν ν νij

pijp

ijd< >0; and . According to Equation 48, the internal energy is

zero when:

ε εsp sa ap sp sa ap ch r, ,/ / /= − =2 or when ε ε ξ (51)

where εjk are the interaction parameters (the subscripts indicate: the silicate, s,the tethered intercalant, a, and the polymer, p), h is the gallery height, r2 is radiusof the interacting species 2, ξc is a characteristic parameter of the system.Substituting the corresponding surface energy contributions into this relation a‘product map’ was constructed. The map (see Figure 64) defines the area wherethe favourable (negative) values of the energetic contribution are expected.

3.1.4 Computations of Polymeric BrushesSince the early 1990s the computer simulation of end-tethered molecules hasbeen gaining attention. While in the context of this book, the end tethering to aplane is of central interest, the approach is more general – tethering to a point(star molecules), line (graft copolymers) and self-tethering (network formation)find applications in many fields [Grest and Murat, 1995]. The result of endtethering to a plane has often been labelled in the literature as a ‘polymeric brush’,‘grafted layer’ or ‘hairy clay platelet (HCP)’.

Two simulation methods have been used: Monte Carlo (MC) and moleculardynamics (MD). The MC method stochastically generates molecularconfigurations either on a lattice or off-lattice (in continuum). On the lattice thechain molecules are modelled using self-avoiding random walk (SAW). Adoptingthe non-reversal random walk reproduces the Rouse dynamics. However, the

Figure 63 Empirical correlation between the PS matrix molecular weight and thetime required for reaching equilibrium intercalation under static melt annealing at

160 °C. Data [Vaia and Giannelis, 1997b].

272


lattice (e.g., diamond or cubic) model limits the possible molecular configurations.It works well for low segment density, but it is difficult to use when the density ishigh, as for example in planes with end-tethered linear macromolecules. Thesimulations show that as the grafting density increases the macromolecules, toavoid overcrowding, stretch in the vertical or z-direction. In the absence of asolvent (‘dry brush’) the incompressibility of the polymer chain is responsible forthe stretching.

The off-lattice continuum models, MC or MD, perform better at high segmentdensity. Here the bond angle and/or length are allowed to vary, i.e., in the ‘pearl-necklace’ model bond length is fixed while in the ‘bead-and-spring’ model it isnot. MD solves equations of motions for each statistical segment, i:

m d r dt U m dr dt W ti i i i

2 2/ /( ) = −∇ − ( ) + ( )Γ (52)

where Ui is the total potential for segment i, Γ is the bead friction and Wi is arandom force acting on each bead. The interactions are assumed to follow theLennard-Jones potential within specified radial distances for the repulsion andattraction.

The numerical method that is important in describing the configuration ofpolymeric brushes is based on the self-consistent mean-field approach.

3.1.5 Balazs Self-Consistent Field ApproachThe self-consistent field (SCF) lattice modelling of polymer adsorption on a solidsurface is relatively recent [Fleer et al., 1993]. The 3D lattice facilitates countingof the possible number of conformations, for example by the step-weighed Markovprocess. This modern approach to thermodynamics heavily depends on

Figure 64 A ‘product map’ showing that favourable energetic interactions (Δev < 0)are expected for polymers with dominating acid or base character. After [Vaia and

Giannelis, 1997b], see text.

273

Thermodynamics

computational techniques. The method is well described by Fleer et al., in thecited reference.

The SCF approach, developed for describing polymers at interfaces, isparticularly useful for modelling the thermodynamic behaviour of systemscomprising nanoparticles. For example, Nowicki [2002] described theconformational properties of long polymer chains in the presence of nanoparticles.The author used a random self-avoiding walk (SAW) model on a 3D lattice. Thecomputed properties included the conformational entropy, segment distributionsin the not necessarily Gaussian coils, and dimensions of the coiled chain attachedto a particle.

Since 1997 Balazs and her colleagues have published several articles exploringthe SCF model capabilities for the description of the equilibrium thermodynamicproperties in CPNC. The authors modelled the systems by combining Markovchain statistics with a mean field approximation for free energy. The aim was notto obtain quantitative predictions, but rather to show how the system may bedesigned for the best performance.

3.1.5.1 Numerical Simulation

To study the effects of interactions, the authors adopted a model system thatresembles the one used by Vaia, i.e., a sandwich of two infinitely large clay plateletsimmersed in a ‘bath’ of molten polymer [Balazs et al., 1998]. For the computationsa 3D lattice was divided into z = 1 to M layers. The lattice was assumedincompressible, with all sites occupied by statistical segments, without voids orsmall molecules. The intercalant chains were assumed to be linear alkyls, end-tethered to the platelet surface. Initially they formed a melt pool inside thesandwich, i.e., between two confining walls, into which the polymer may diffuseunder the influence of favourable thermodynamic interactions. The equationsfor such a model can be solved self-consistently. The self-consistent potential is afunction of the polymer segment density distribution and the Huggins-Flory typebinary interaction parameters between all components.

Properties of the system are averaged over the x and y directions, but theychange perpendicular to the clay platelet z-direction (hence ho ≤ z ≤ h). As in theearlier lattice models, here also the excess free energy is expressed by entropicand enthalpic terms, the first one computed from the configurational probability,Gi(z), the other expressing the energetic change caused by two interacting elements:

F z z G z z z z z dzi i ij i j

iji

( ) = ( ) ( ) + ( ) −( ) ( ) ( )∫∑∑φ φ φln / ' ' '1 2 χ η (53)

where φi is the polymer concentration, χij is the Huggins-Flory type binaryinteraction parameter between species ‘i’ and ‘j’ and η(z - z′) is the short-rangeinteraction function. The connectivity of segments belonging to the samemacromolecule was incorporated by means of Green’s function. Assuming theinitial composition of the system, number of segments per species and themagnitude of the binary interaction parameters leads to determination of theself-consistent concentration profile as a function of the gallery height, z.

The authors considered the binary interaction parameters between threecomponents: clay platelets (s), polymer (p) and intercalant (a), identified as,χsa = χsp = 0; χap = χ (variable). As the polymer diffuses from the surrounding‘bath’ into the interlamellar gallery the distance between the two platelets increases

274


from the initial value ho to h. The free energy change during intercalation iscomputed as: ΔF = F(h) – F(ho).

It is noteworthy that in these computations the interactions between the clayplatelet and either intercalant or polymer were assumed neutral, non-perturbing.The only type of interaction of interest was the one between intercalant andpolymer. This focused interest contrasts with the opinion expressed by Vaia. Balazset al., admit that the intercalant-polymer interactions may unfavourably affectthe kinetics of polymer diffusion into the galleries. However, there is nothing inthe SCF methodology that would preclude systematic studies of the propertychanges when varying other interaction parameters.

For the case of a non-intercalated clay sandwich immersed in a molten polymer(see Figure 65) computations of the free energy per unit area as a function of theinterlamellar gallery height, ΔF/A = f(h), show that even for χsp = 0 the freeenergy is positive, and thus intercalation is unfavourable. The effect is entropic –the macromolecules in contact with the surface have reduced conformationalprobability. When to start with the clay is intercalated and the macromoleculesare able to interact with the intercalant chains the enthalpic contribution maycompensate for the confinement effects. The computations for χap = -0.01 to0.02 show a systematic increase of the free energy, ΔF/A = f(h). For polymer withNp = 100 statistical segments, intercalant with Ni = 25 and the intercalant graftingdensity ρ = 0.04, the polymeric intercalation was predicted only for χap ≤ 0.005.However, when Np = 300 was assumed, no intercalation was predicted for χap = 0.In other words, the miscibility between intercalant and macromolecules is reducedwith increased polymer molecular weight. To compensate for this effect one maywant to increase the Ni value from 25 to 50 or 100 – indeed, the computationsshow that for χap = 0, this strategy leads to exfoliation, for χap = 0.01 it allows forthe polymer to intercalate, but for χap = 0.02 it makes the matter worse. Theseobservations based on the free energy changes are supported by the computedsegmental density profiles φ = φ(z).

While some results of the numerical calculations could be reasoned out fromthe thermodynamic principles of polymeric systems, the effects of changes on theclay platelets’ grafting density, ρ = 1/s (where s is the surface area per oneintercalant molecule), would be difficult to predict. Note that this parameter isrelated to the intercalant concentration in the system, intercalant structure andto the CEC of the clay (e.g., for MMT with CEC = 1 the surface area per oneionic group is 1.244 nm2). The CEC varies (see Section 2.2) from 0.02 (kaolin) to1 (MMT) to 2.5 (hectorite). SCF computations for ρ = 0.04 to 0.12 show that forthe latter value ΔF is high, thus as the packing density within the gallery increasesit becomes harder for the macromolecules to diffuse into it and mix withintercalant chains. In short, on the one hand it is hard for macromolecules todiffuse in between non-intercalated clay platelets, but on the other diffusion isdifficult when the intercalation density is high, i.e., there is an optimum in graftingdensity for forming polymer/clay nanocomposites.

According to these computations the optimum value of CEC (or ρ) dependson the interaction between the polymers and the intercalant, χap. In practice, theintercalant ions may be primary, secondary, tertiary or quaternary ammoniumor phosphonium; they may have shorter or longer hydrocarbon chains, and (inthe case of the most popular quaternary type) they may have one to four longchain groups. In the latter case, the results of SCF computations predict that theintercalant with a single long tail should be better than the one with two or

275

Thermodynamics

Figure 65 Free energy per unit area as a function of surface separation, h, for fivedifferent values of the polymer-intercalant interaction parameter, χap. Other

parameters are: N = 100; Ni = 25 and χsa = χsp = 0. The grafting density ρ = 0.04and 0.12 for Figure (a) and Figure (b), respectively. The cartoons (c), left and rightshow, respectively, the initial and final state, where the surfaces are separated by

macromolecules [Balazs et al., 1998].Reprinted with permission from [Balazs et al., 1998]. Copyright 1998 American

Chemical Society.

276


three. This may explain the behaviour observed for the melt exfoliation of PA-6[Dennis et al., 2000; 2001]. The authors melt blended PA-6 with 5 wt% of eitherCloisite® 15A or Cloisite® 30B. TSE compounding with the former organoclayresulted in intercalation, whereas that with the latter in exfoliation. Both arebased on montmorillonite intercalated with quaternary ammonium ions: dimethyldihydrogenated tallow quaternary ammonium chloride and methyl tallowbis-2-hydroxy ethyl quaternary ammonium chloride (MMT-2M2HTA andMMT-MT2EtOH, respectively). Thus a higher packing density is expected forCloisite® 15A than for Cloisite® 30B. Evidently, the ability of macromolecules todiffuse into interlamellar galleries of pre-intercalated clay depends not only onthe type of radicals (e.g., aliphatic versus aromatic) but also on the intercalantstructure, viz. the number of long aliphatic chains attached to onium ion. Vaia[1995] observed that the kinetics of intercalation for FH clay intercalated with2M2ODA is slower than that with either ODA or 3MODA.

The SCF numerical computations were also carried out for CPNC systemswith a compatibiliser. Two methods of compatibilisation were explored: (1) byaddition of compatibiliser between the intercalant chains and the polymer, and(2) by addition of a compatibiliser, which is miscible with the polymer and ableto bind directly to the clay surface.1. To examine the efficiency of the first approach, the computations were carried

out for the system with Na = 25, Np = 100, and χap = 0.01. Different amountsof a polymeric compatibiliser (subscript c; Nc = 100) were added. Theinteractions of compatibiliser with intercalant (subscript a) and polymer(subscript p) were assumed to be: χac = χpc < χap. The simulation showed, asexpected, that addition of a compatibiliser improves the ability of polymerto intercalate. However, the process was found inefficient, requiring at least10% of the compatibiliser that forms an interphase between the tetheredintercalant layer and molten polymer. Evidently, the need for a large amountof compatibiliser would have a negative effect on CPNC cost and performance.Furthermore, enhanced interaction between the intercalant and polymer mayhinder the intercalation/exfoliation process not only for kinetics reasons,but also owing to the loss of the conformational entropy.

2. To examine the efficiency of the second approach, computations were carriedout for the system where bare clay platelets are dispersed in a mixture of apolymer with its homologue containing reactive group (called a ‘sticker’) atone chain end. Aside from the sticker, the functionalised chains werechemically identical to the matrix polymer. For the computations, the polymerand compatibiliser chain lengths were assumed to be: Np = 100 and Nf = 75,respectively. Strong interactions between the clay and the sticker group wereassumed (χsf = -75), while the other interaction parameters were set equal to0. Figure 66 indicates that this approach is highly promising. There is asubstantial effect upon addition of 5% of the functionalised polymer, butlittle further gain upon addition of up to 70%. The dependence shows thatthe method leads to exfoliation, creating stable polymer/clay dispersions.Evidently, in practice one may start with pre-intercalated clay, but one havinglow grafting density, ρ.

The key to the second approach is the presence of the ‘sticker’ groups at the chainends able to strongly interact with the clay surface. In principle, such a functionalisedcompatibiliser could be either a homopolymer with a strong polar group or an AB

277

Thermodynamics

diblock copolymer with, e.g., a short hydrophilic A-block andB-block chemically identical to the matrix polymer. The large organophilic B-blockwould extend away from the surface, mix with the matrix and cause separation ofthe clay platelets. For the best performance the molecular weight of the B-blockshould be higher than the entanglement molecular weight of the polymer;Mn(B) > Me(B). Such a system would be sterically stabilised against re-aggregation.

Other numerical calculations (SCF with Markov-chain statistics) focused onthe effect of macromolecular architecture on the miscibility of the polymer-claysystem [Singh and Balazs, 2000]. Again, two infinite, parallel, organoclay plateswere immersed in a molten polymer. The value of the binary Huggins-Floryinteraction parameters was assumed χij = 0, intercalant chain length, Na = 25,and its grafting density ρ = 0.04. The miscibility of the polymer/clay mixture wasstudied by increasing the number of branches for polymers of fixed molecularweight. The analysis showed that starting with linear macromolecules the freeenergy per unit area ΔF/A < 0, thus the system is miscible. As the number ofbranches increases the value of ΔF/A became more negative. The plot of ΔF/Aversus h indicates that changes in the macromolecular architecture affect theCPNC morphology, viz. for a linear polymer there is a local minimum suggestingpreference for the intercalated structures, while for a ten-armed star there is apreference for exfoliation. When the value of χap was assumed to be 0.01, ΔF/Ashifted to higher values – only for the ten-armed star there was a local, negativeminimum, indicating an intercalated structure. The enhanced miscibility betweenthe organoclay and the polymers with higher number of branches is mainly dueto the compactness of the macromolecules that can more easily interact with andinterpenetrate the short, grafted layer.

Figure 66 Free energy per surface area versus gallery height for clay-polymer-compatibiliser (functionalised polymer) system. The latter polymer content (0 to

70 wt%) is indicated.

278


3.1.5.2 Analytical Self-Consistent-Field Theory for Compatibilised Systems

Since numerical analysis indicated that dispersion of clay in a mixture offunctionalised polymer and its non-functionalised homologue shows greatpromise, the authors developed an analytical SCF theory [Balazs et al., 1998].The adopted model is analogous to that evaluated by numerical means. Thus,the polymeric phase comprises a volume fraction, φ, of monodispersed,functionalised chains and (1 - φ) of polydispersed, non-functionalised chains.The functionalised and non-functionalised chains are chemically identical, differingby the presence of one terminal group in the functionalised chain. However, theymay also vary by the number of segments, N, each of diameter a. The end-groupis highly attracted to the clay surface, and reacting with it forms a HCP structure,effectively pushing the sheets apart. However, these functional groups do notinteract between themselves or with the other components of the system. Thebare clay platelets are modelled as planar surfaces, each having an area A. Thedegree to which the functionalised polymer binds to the surface is related to thegallery height, h.

Within the gallery, the functionalised macromolecules (each of length N) are eitherattached to the surface (na) or free (nf). Similarly, the number of non-functionalisedchains of length Pi is ni. Thus, the total number of segments between two clayplatelets is given by:

N N n n n P Ah atotal a f i i

i

= +( ) + =∑ / 3 (54)

and the total free energy of the system is:

Δ Δ μ

Δ Δ Δ Δ

F F n n n

F F F E

brush a f i i

i

brush stretch mix a

= − +( ) −

= + +

∑ μ(55)

where μ is the chemical potential of the end-functionalised chain, μi is that of thenon-functionalised, polydispersed polymer, ΔFstretch accounts for the stretchingof the attached macromolecules, ΔFmix accounts for the free energy of mixing offunctionalised and non-functionalised chains, and ΔEa is the energy of attachingthe functional group to the surface. By substituting the appropriate terms thefollowing expression for the total free energy of the system was derived:

Δ

φ

φ

F RT N N

Nk h h Nk h Nk x dx h

N P Pk h Pk x dx h

total total

h

i i i i

h

i

/ /

/ exp exp /

/ exp exp /

max max

max

( )( ) =

−( ) + − −{ } −{ }⎡

⎣⎢⎢

⎤

⎦⎥⎥

+ ( ) − −{ } −{ }⎡

⎣⎢⎢

⎤

⎦⎥⎥

∫

∫∑

2 2 2 2 2 2 2

0

2 2 2 2

0

3 1

1

(56)

where the maximum platelet separation before a homogeneous matrix polymerlayer is formed is given by:

h a N kTmax / / ln /= ( ) + +( )2 2 1 3π ε φ (57)

In these relations: k2 ≡ 3π2/8a2N2, while ε = –ΔEa/na is the energy gain per onereactive group.

Note that for h > hmax, a layer of bulk polymer appears between the outeredges of the two brushes. This however does not change the free energy of the

279

Thermodynamics

system. Thus, at h = hmax, the gain in free energy due to intercalation/exfoliationby the polymeric matrix is at maximum. This limit depends only on the chainlength of the functionalised polymer, the reaction energy and concentration – thelatter in a logarithmic form, hence sensitive to small changes at low concentration,but insensitive at high.

The adsorbed amount of functionalised polymer follows the dependence:

Θ φ

φ

h h Nk h Nk x dx

N Pk h Pk x dx

h

i i

h

i

i

( ) = − −{ } −{ }

− −{ } −{ }

∫

∫∑

exp exp

exp exp

max

max

2 2 2 2

0

2 2

0

2 2

(58)

Evidently, the amount of functionalised polymer that is bound to the surfacedepends on the gallery height, h, with the slope increasing with molecular weightof the functionalised (N) and non-functionalised (Np = Pi) chains.

In Figure 67 the function Θ = Θ (h) is presented for various values of thelength of non-functionalised chains, Np = 1 to 100. The length of the functionalisedchains, Nf = 75 and its volume fraction φ = 0.05 were assumed constant. Theinteraction energy was set as ε = 12.5 kBT. Increasing Np reduces the miscibilityof the functionalised with non-functionalised chains, what causes thefunctionalised chains to migrate to the strongly interacting clay surface. This inturn leads to increased adsorption of the functionalised chains on the clay surface.Up to ca. h = 20 the dependence Θ = Θ (h) is about linear. It is noteworthy thatfor Nf = 75 the dependence is common for Np = 50 and Np = 100, suggesting thatas a rule of thumb Nf ≈ Np may offer the optimum condition for adsorption.

Figure 67 The amount of adsorbed functionalised polymer with Nf = 75 andconcentration φ = 0.05 for four chain lengths of the matrix polymer, Np = 1, 10, 50

and 100. After [Balazs et al., 1998].

280


It was found that the derived analytical expressions agreed well with thenumerical predictions by SCF. For example, the dependencies shown in Figures 66and 67 could be generated by either of these two models. Similarly, the numericalSCF data on the thickness of the unperturbed brush, hmax, also show satisfactoryagreement with the analytical prediction.

Recently, SCF was used to determine a range of independent variables thatguarantee thermodynamic miscibility in a realistic model system, which comprisesfour components: solid clay platelets, low molecular weight intercalant, polymericmatrix, and an end-functionalised compatibiliser [Kim et al., 2004]. In thesimulation, realistic values of the binary interaction parameters were used. Theresults showed that intercalation and exfoliation is expected within limited rangesof independent variables. Furthermore, it was found that the presence of bareclay surface (e.g., generated by thermal decomposition of intercalant) stronglyhinders the clay dispersion. The 2D simulation successfully identified the mostinfluential factors (e.g., compatibiliser type and concentration) and establishedtheir optimum ranges.

3.1.5.3 Phase Behaviour

SCF methods have been used to compute the phase behaviour for CPNC modelsystems [Balazs et al., 1999]. The initial model considered a mixture of individualclay platelets dispersed in polymer melt. The platelets were rigid disks of diameterD and thickness L, while the polymer was made of flexible chains of length N.The volume fraction of the platelets and the polymer in the incompressible mixturewas φd, and φp = 1 - φd, respectively. The interaction between the polymer statisticalsegment and a platelet site was expressed by the Huggins-Flory interactionparameter, χ.

Two miscible systems were distinguished: isotropic with clay platelets randomlyoriented in respect to each other, and a nematic with mutually aligned platelets.Depending on the CPNC applicability, e.g., for the mechanical or barrierproperties, either one of these two structures may be preferred.

The formation of the isotropic and nematic structures was predicted using amodified Onsager model for the nematic ordering of rigid rods. The followingexpression was derived for the free energy:

F n K b nd d d d d d p p= + + − −( )( ) −( ) −[ ] +ln ln / lnφ σ φ ν ρ ν φ φ1 1 χ (59)

where K is a constant, np is the number of polymer molecules, nd is the number ofplatelets per unit volume, and vd = (πD2L)/4 is the volume of one disk. Theparameters: σ and ρ describe the orientation of the disks with respect to,respectively, the nematic direction and each other. In the isotropic phase, σ = 0and ρ = 1, while in the nematic phase:

σ ν φ

ρ γ ν φ

n d d

n d d

b

b

= −( ) −( )[ ] −

≡ ( ) = − ( ) −( )

−2 2 1 1

4 2 1

1 2ln / ln

/ sin / / ln

/π

π(60)

where γ is the angle between two disks. The analysis for disks with an aspectratio p ≡ D/L = 30 predicts the existence of a phase diagram with three regions:immiscible, isotropic and nematic. The region of miscibility decreases withincreasing N and χ. Furthermore; increasing p promotes nematic ordering andreduces the isotropic phase.

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Thermodynamics

This initial investigation was significantly expanded during the following years[Ginzburg and Balazs 1999; 2000; Ginzburg et al., 2000; 2001]. The new modelpredicts phase diagrams that include: isotropic, nematic, smectic, columnar, plastic(or house-of-cards), and crystalline structures. For these calculations, the authorscombined the SCF model with the Somoza-Tarazona formalism of the densityfunctional theory (DFT). The resulting free energy functional was minimisedwith respect to both the orientational and positional single-particle distributionfunction, potentially determining all phases and the coexistence regions.

The free energy of a system was written as a functional of a single-particledistribution function,

γ ρr r r rr n r f n,( ) = ( ) ( ), where

rr and

rn are the coordinate and

the nematic director, respectively, while ρ

rr( ) is disk number density, and

f n

r( ) isthe Onsager orientational distribution function. The fluid was assumedincompressible; i.e., the sum of the volume fractions of polymer, φp, of intercalant,φi, and clay, φc, equals 1: thus φp + φi + φc = 1.

The free energy consisted of three terms, Fid, Fster and Fint:

1. Fid – the free energy of an ‘ideal gas’ of polymer

This term can be written as a sum of the translational and orientationalcontributions coming from the clay particles, and that originating from thepolymer, Fid = Fc + Fp, respectively, the latter calculated from the Huggins-Florytheory:

F kT r r dr r f n f n dndr Nid m p p/ ln ln / ln= ( ) ( ) + ( ) ( ) ( )( ) + ( ) ( )∫∫ ρ ρ ρ ν ν φ φ

r r r r r r r r4π (61)

where N is the chain length of the polymer, ρ

rr( ) is the positional (or local) density

of clay platelets, n is the total volume of the system, vm is the monomer volumeand φp is the volume fraction of the polymer. For the intercalated organoclayplatelets the effective thickness is: Leff = L + 2ρNi, and the effective volume ofsuch a particle: Veff = (π/6)D2Leff.

2. Fster – the contribution due to the excluded-volume effects for clay platelets

A semi-empirical steric interaction expression was used:

F kT r V f V r drster excl phe hs/ /= ( ) ( )[ ] ( )( )∫ ρ Ψ Φ

r r r(62)

where Ψhs is the Carnahan-Starling function that describes the excess free energydensity for hard spheres as a function of their packing fraction;

Φ

rr( ) is the

smoothed, local volume fraction of clay; Vexcl(f) is the average excluded volumeper particle for a given orientational distribution; and Vphe is the excluded volumeper particle for perfectly aligned ellipsoids.

The sum of the free energy terms, Fid + Fster, describes an athermal dispersionof hard ellipsoids in a polymer matrix, capable of forming liquid crystalline(nematic) or crystalline phases. To generate smectic or columnar phases, stronganisotropic long-range interactions are required. These are provided by the Fintcontribution.

3. Fint – the enthalpic interactions between clay plateletsThis term was derived from the pair correlation function for the particles, g(1,2);hence it is mostly determined by the excluded-volume effects:

F kT r f n r f n n n g V r r dr dr dn dnint / / ,= ( ) ( ) ( ) ( ) ( ) × −( ) ( ) −( )∫1 2 1 1 21 1 2 2 1 2 1 2 1 2 1 2ρ ρ δ

r r r r r r r r r r r r(63)

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where for overlapping or not-overlapping particles the mean-field pair correlationfunction g(1,2) = 0, or g(1,2) = 1, respectively. The δ-function allows only paralleldisk configurations. The potential function V(r1 – r2) is expressed as:

V r D r D U z

r( ) = ( ) − ( )⎡⎣⎢

⎤⎦⎥ ( )⊥π / /4 12 2

(64)

where rr = (x, y, z), and r⊥ = (x2 + y2)1/2. The interaction potential per unit area,

U(z), has two components: U1(z) originating from electrostatic and van der Waalsinteractions between ‘bare’ clay particles, and U2(z) originating from theintercalant and polymer chain contributions within the interlamellar gallery. Theattractive and short-ranged U1(z) term depends on the chemical structure of theclay, and a priori was written as:

U z E z Lo eff1 1( ) = − − −( ){ }[ ]exp (65)

where Eo is the interaction energy between clay platelets and Leff is an effectivethickness of clay platelet. The second component, U2(z), was computed using theSCF method. The shape of this potential depends on N, Ni, ρ and the binaryinteraction parameters, χ.

The equilibrium morphology was computed for the mixtures of a polymer(N = 300) with intercalated clay platelets, modelled as oblate disks having D = 30,L = 1, thus p = 30. Two values for the clay-clay interaction strength were used,E0 = 0 (no long-range attraction), and E0 = 0.1 kBT/a2 (strong attraction). All thebinary interactions, except that between polymer and intercalant were assumed= 0; the latter χap = χ varied from –0.05 to +0.05. As far as the intercalation wasconcerned, the following systems were considered: (a) ρ = 0.2, Ni = 5, (b) ρ = 0.04,Ni = 25, (c) ρ = 0.02, Ni = 50, (d) ρ = 0.04, Ni = 50, and (e) ρ = 0.02, Ni = 100.Note that the total amount of intercalant in systems (a)-(e) was: θ = ρNi = 1, 1, 1,2, and 2, respectively.

The calculations of the phase diagram were carried out in two steps:

1. Using SCF the free energy profile U2(z) was computed for the five systems,varying the binary interaction parameter within the indicated range, first forE0 = 0, then for E0 = 0.1 kBT/a2.

2. The phase diagram was constructed as a map: χ versus φ (see Figure 68). Thetotal volume fraction of the intercalated clay: φc ≤ 0.601. Schematicrepresentation of the phases is shown in Figure 69.

The phase diagrams for the considered systems are complex. For χ > 0, the systemsare immiscible, showing coexistence between the polymer-rich isotropic phaseand the clay-rich crystalline phase. For χ close to 0, the two-phase region becomesnarrower and splits into isotropic-nematic and nematic-crystal phases. The triplepoint (I-N-Cr) and the width of the nematic phase depend on the system. Thecomputed smectic phases were always metastable. For χ < 0, the isotropic-nematicand nematic-crystal coexistence regions become narrower and shift toward highervalues of φ. When χ is strongly negative, the new ‘plastic solid’ (or house-of-cards) and columnar phases appear. Evidently, at low clay loadings, the strongrepulsion between neighbouring disks forces them to adopt more energeticallyfavourable ‘edge-to-face’ or ‘house-of-cards’ configurations. This leads to eithergelation or crystallisation. At higher clay volume fractions, the steric excludedvolume effects dominate the long range disk-disk repulsion, forcing the formationof columnar and crystal phases.

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Thermodynamics

Figure 68 The map of phase behaviour for polymer-clay mixtures, obtained byvarying the binary interaction parameter between polymer and intercalant, from χ= -0.05 to +0.05 and the inorganic clay content from φ = 0 to 0.2. The 5 systems

are: (a) ρ = 0.2, Ni = 5, (b) ρ = 0.04, Ni = 25, (c) ρ = 0.02, Ni = 50, (d) ρ = 0.04, Ni

= 50, and (e) ρ = 0.02, Ni = 100 (see text). The interaction potential between clayplatelets, e0 = 0, was assumed. The phases are: I, isotropic; N, nematic; Cr, crystal;PS, plastic solid; Col, columnar. Points represent calculated coexistence densities;lines serve as a guide to the eye. Dashed lines represent approximate locations ofphase transition boundaries (exact calculation was not possible) [Ginzburg et al.,

2000].Reprinted with permission from [Ginzburg et al., 2000]. Copyright 2000

American Chemical Society.

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Figure 69 Schematics of possible phase structures for dispersed platelets in apolymer: (a) isotropic, (b) nematic, (c) smectic A, (d) columnar, (e) plastic solid

(house-of-cards), and (f) crystal. The nematic director rn is aligned in the Z

direction, while the platelets lie in the XY plane.Reprinted with permission from [Ginzburg et al., 2000]. Copyright 2000


285

Thermodynamics

The phase maps in Figure 68 show similarity for the same total intercalantloading, θ = 1 and θ = 2, viz. systems (a)-(c) and (d)-(e), respectively. However,for platelets intercalated with short densely grafted chains (system a), theimmiscibility between the clay disks and polymer dominates the phase behaviourof the system. This observation confirms the previous SCF computations, whichsuggested that the present strategy of intercalation leads to an unfavourablethermodynamic environment. Instead of high saturation of clay surfaces withC12 to C18 alkyls an intercalation with fewer, strongly bound to the surface, longchain molecules should be carried out. Note that increasing Ni from 25 (systemb) to 50 (system d) moves the isotropic-nematic-crystal triple point upward,extends the stability region of the nematic phase to higher χ values and narrowsthe two-phase isotropic-nematic region, indicating thermodynamically stableexfoliated composites. Comparing systems c and e leads to similar conclusions.

One may object to the above computations on the ground that assumption ofE0 = 0 for the clay particles is not realistic. It is known that the surface energy ofa crystalline solid is high and that there is a strong van der Waals interactionbetween the clay platelets. Thus, it is interesting to see how the results are affectedby the imposition of the strong clay-clay attraction, viz. E0 = 0.1 kBT/a2.

For systems with θ = 1, the clay-clay attractions dominate the entropiccontribution of the surfactant molecules. The phase diagrams show theimmiscibility between polymer and clay within the full range of χ. For systemswith θ = 2 (see Figure 70) the miscibility is significantly diminished in comparisonto the corresponding E0 = 0 cases (compare with systems (d) and (e) in Figure 68).The isotropic-nematic-crystal triple point is shifted downward, the areas occupiedby isotropic and nematic phases is reduced, and the nematic-crystal transitionoccurs at lower values of φ.

3.1.5.4 Contribution and Potential of the SCF Method

The discussed SCF calculations start with a model of a two-platelet (intercalatedor not) sandwich in a molten polymer and consider the influence of diverseparameters on the equilibrium thermodynamic properties, viz. miscibility andphase diagrams. The kinetic aspects of the polymer intercalation or exfoliationprocess are not being addressed. Furthermore, the effects introduced by energyinput from external sources have not been considered. The analysis of kinetics isbeyond the scope of the model, but the effects of the mechanical energy input(e.g., during compounding in an extruder) are not.

Balazs and her colleagues applied the SCF model to study the influences ofsuch parameters as the molecular weight of the polymeric chain, size and graftingdensity of the intercalant, magnitude of the binary interaction parameter for thepolymer, intercalant and clay, influence of the clay-clay interaction,compatibilisation strategies using compatibiliser for the intercalant-polymer orfor clay-polymer interfaces. One may argue whether the magnitude of the selectedparameters has been optimal, but the numerical comparison of model predictionswith experimental data has not been the goal. However, these are the mostthorough fundamental studies of CPNC, offering guidance toward better methodsof exfoliation in thermodynamically stable systems.

One of the important findings of the SCF calculations demonstrated thatincreasing the length of the intercalant chain (beyond the typically used C12-C20)to oligomeric of polymeric values should enhance the thermodynamic stabilityof exfoliated systems. Another highly practical finding is that there is little to be

286


gained from compatibilisation of the polymeric matrix with the intercalant chains– a large quantity of the compatibiliser would be required. By contrast, directcompatibilisation of the clay/polymer shows greater potential. For this purpose,the end-terminated macromolecular chains with single strongly bonding group(a ‘sticker’) are preferred. Their chain length should be smaller, but of the sameorder of magnitude as that of the matrix polymer. Evidently, for the practicalreasons (kinetics) one would start with pre-intercalated clay, and use the functionalcompatibiliser with molecular weight just above the critical value forentanglement.

More recently complex phase diagrams were computed. This was accomplishedby combining the SCF-generated free energy profiles with the Somoza-Tarazonafree energy functional. The computations demonstrated how variations of theprincipal parameters affect the formation of isotropic, nematic, smectic, columnar,crystal, and plastic solid (house-of-cards) phases. The calculated phase diagrams

Figure 70 Phase diagrams of polymer-clay mixtures:(a) ρ = 0.04, Ni = 50, and (b)ρ = 0.02, Ni = 100. The interaction potential betweenclay platelets, e0 = 0.1, was assumed. The phase diagrams for the other three cases

shown in Figure 68 indicate immiscibility.Reprinted with permission from [Ginzburg et al., 2000]. Copyright 2000


287

Thermodynamics

are in qualitative agreement with the predictions of the SCF model. As the lengthof the grafted chains and/or their density increases, the miscibility between theclay sheets and the polymer is improved, and the resulting mixture can exhibitexfoliated (isotropic or nematic) structures for a range of clay volume fractions.For short intercalant chains, the polymer is unable to penetrate the galleries andthe system becomes immiscible. The computations demonstrated that the phaseequilibrium of CPNC is sensitive to the specific features of clay-clay, clay-polymer,intercalant-polymer and clay-compatibiliser interactions. For quantitativeprediction of the phase behaviour, one must correctly describe all theseinteractions, e.g., by using molecular simulation.

There is a large difference between the idealised mathematical model of CPNCand the physical reality. For obvious reasons the ingredients used in the simulationare homogeneous, monodispersed in size and molecular properties. Furthermore,the simulation is based on equilibrium thermodynamics – the forces responsiblefor the evolution of idealised CPNC structures are exclusively thermodynamicand the final morphology is at thermodynamic equilibrium in the molten state.The reality is quite different as the ingredients are far from homogeneous, thereare numerous process additives in each commercial resin, the clays are mineralproducts with a host of compositional and structural defaults, the system ismechanically dispersed which may either disturb the shape of clay platelets(attrition or bending) and/or induce platelet orientation, then usually cooled downto room temperature which induces vitrification or crystallisation that causeinternal stresses, etc. These are some of the reasons that make direct comparisonbetween the model and reality problematic. The model can and does serve as aguide – with future developments the guide will become progressively morerealistic, able to account for the numerous contributions mentioned in thepreceding sentences. However, considering the complexity of the CPNC systems,molecular modelling is the only method to examine the contributions of eachelement, the only way to understand the mechanisms involved, and the onlylogical approach to maximise nanocomposite performance.

3.1.6 Scaling Theory for Telechelic Polymer/Clay SystemsUsing numerical and analytical SCF calculations Zhulina et al. [1999] investigatedthe interactions in a system composed of polymer mixture with a sandwich ofclay platelets dispersed in it. The mixture consisted of molten polymer with itsend-functionalised, telechelic homologue, containing two reactive ‘stickers’, oneat each end of the chain. These reactive groups were highly attracted to theplatelets’ surfaces. The computed free energy profiles showed distinctive minima,even at low concentration of the end-functionalised chains. The studies showedthat the telechelic chains could react with both clay platelets of the modelsandwich, thus bridging the interlamellar gallery. The SCF computations showedthat telechelic polymers may promote the formation of thermodynamically stableintercalated systems, but bridging the interlamellar gallery could preventexfoliation. However, it was not clear whether the presence of telechelic chainsnecessarily prohibits exfoliation.

To search for possible conditions under which thermodynamically stableexfoliated CPNC might be formed in the presence of telechelic polymers Kuznetsovand Balazs [2000a; 2000b] applied the scaling theory. The computations againstarted with a model of two clay platelets immersed in a polymer melt. Eachplatelet had the surface area A and it was pre-intercalated with short alkyl chains

288


that reduced the clay-clay interactions to zero. The melt was a mixture of telechelicflexible N-polymers (their volume fraction = φ) and non-functionalised P-polymer(volume fraction = 1-φ). These polymers had P ≥ N >> 1 statistical segments,each of diameter a. The N-polymer had two ‘sticker’ terminal groups, one ateach end of the chain. The groups could interact with the clay surface with theenergy per sticker-clay contact of ε.

Thus, the gallery between the platelets was filled by N- and P-polymers. A fractionof the N-chains was anchored to the platelets with the rest (as well as P-chains) beingfree (subscript f). The anchored chains could be attached to the surface by one(subscript t for ‘tails’) or two ends (subscripts l or b for ‘loops’ or ‘bridges’,respectively). The following values of the Huggins-Flory interaction parameterswere assumed: χNN = χPP = 0, and χ = χNP. The free energy of the chains within thegallery was expressed as a sum of the individual contributions:

F = Fads + Fint + Fcomp + Fel + Fent + Fdem (66)

where Fads is the sticker-surface adsorption energy for N-chains, Fint is theinteraction energy between the N and P chains, Fcomp

is the compression energyfor chains at relatively low interlamellar gallery height, h, and Fel is the elongationenergy for loops, tails, and bridges, Fent is the entropic energy for mixing thedifferent types of chain configurations within the brush layers, and Fdem is thedemixing energy associated with extracting N and P chains from the surroundingmelt and localising these chains in the confined layers.

The free energy of the system, F, was written deriving the six components ofEquation 66 and casting them in terms of the reduced gallery height: a = h/Na,αp= h/Pa, αtot = htot/Na and αl = hl /Na, where htot stands for the total height of thebrush, htot ≤ h/2 (with h being the gallery height), and hl is the average distancethat a loop molecule reaches from the clay surface. Its value depends on theenergy per contact between the clay platelet and a functionalised chain end (ε/T),the volume fraction of functionalised chains in the bulk melt, N and P, the numbersof segments in each functionalised and non-functionalised chain, respectively,and the Huggins-Flory interaction energy parameter, χNP. Next, F was numericallydifferentiated to obtain the conditions for local minima in respect to individualvolume fractions: φP, φl, φt, φb, and φf, calculated from the respective number ofstatistical segments:

φ φl t b f l t b f p p i

i

n Na Ah n Pa Ah, , , , , , / ; / ; = = =∑3 3 1φ (67)

The free energy, F, was computed in its reduced form (in respect to temperatureand surface area) as a function of the reduced gallery height:

Fa AT Na Ah F T versus h Na2 3/ / / /= ( )( ) ≡α α (68)

The initial computations were carried out for the dispersion of intercalated clayin a telechelic, bi-functionalised melt (φ = 1). The results are presented in Figure 71as the reduced free energy and volume fractions of different conformations versusthe reduced gallery height, α. Four regions are indicated:

(I) The chains t, l, and b are highly compressed. The reduced free energy rapidlydecreases with α to negative values (miscibility).

(II) A shallow, local minimum of free energy, which originates in weak stretchingof all the chains within the brushes. The number of bridges decreases andtends to zero on the border of this region.

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Thermodynamics

(III)This is the ‘relaxation’ region, where the free chains appear. The free energyis further reduced with α.

(IV)Separate brush layers are formed with unperturbed Gaussian free chains fillingthe space between them, thus exfoliation.

The minimum in region (II) is thermodynamically metastable hence there is nooptimal spacing (intercalation) other than exfoliation. This is surprising as itcould be expected that the bridging chains give rise to a thermodynamic barrierto separating clay platelets beyond the point where the bridges are disrupted.Evidently, this energy cost is compensated by that gained by adsorption of freechains, which can penetrate the gap at larger α. The results imply that clay sheetscan be exfoliated in a melt of telechelic chains. However, the metastable minimumin region (II) can lead to a kinetically trapped degree of dispersion.

Additional computations show that the magnitude of the local minimum canbe controlled by the sticker adsorption energy, ε/T, and the molecular weight ofthe N-chain. Evidently, increasing the former and decreasing the latter forces thereduced free energy deeper into the negative values, increasing the miscibility.However, for practical reasons this may not be the best solution. For sufficientlysmall values of the barrier height, an imposed stress or increased temperature canhelp overcoming the kinetic barrier and lead to exfoliated CPNC.

In a system comprising intercalated clay dispersed in a mixture offunctionalised (N) and non-functionalised (P) polymers the plots of the free energyper area (F/T)(a2/A) versus the reduced gallery height, α, are similar to that shownin Figure 71. However, at low volume fraction of N-chains, φ < 0.05, and highe/T-values, the minimum in region (II) is a global one, forcing the system into a

Figure 71 The reduced free energy for different conformations versus the reducedgallery height, α. Four regions of behaviour are indicated. After [Kuznetsov and

Balazs, 2000a]. See text.

290


thermodynamically stable intercalation. One solution is to use a higherconcentration of the N-chains, another to increase interaction between N- andP-chains, i.e., to have χNP < 0.

The phase diagram in Figure 72 summarises these findings. The data are plottedin terms of ε/T versus the volume fraction of polymer-N, φ, for the case of N = 1000and P = 100. The curves (calculated by equating the free energies) between theintercalated, exfoliated and immiscible phases correspond to the first-order phasetransitions. At low contact energies between the sticker group and clay surface,the system is immiscible, while at higher values of ε/T, it can either be intercalatedor exfoliated (at high φ). In the intercalated state, loops, tails and bridges fill thegallery. In the exfoliated state loops and tails form the brushes. The transitionfrom an intercalated to an exfoliated system is related to the increased number oftails, which in turn is facilitated by a higher value of φ and the formation of a freepolymer layer between the brushes. The relative magnitude of the intercalated-to-exfoliated region very much depends on the N/P ratio. Figure 72 illustrates aperverted case with the functionalised chains being ten times longer than thematrix polymer. In practice, N ≤ P and under these conditions the intercalatedarea is quite small, relegated to ε/T < 5 and φ > 0.15 – the miscible clay/polymersystems are mainly exfoliated.

To summarise, the scaling theory was shown to be appropriate for the analysisof free energy in three-component systems composed of pre-intercalated clay,bifunctionalised (telechelic) compatibiliser and its non-functionalised homologuewithin a wide range of parameters, viz. ε, N, P, φ and χ. The computations indicatethat in the melt of only N-chains there are no thermodynamically stableintercalated states – the system is either immiscible or it exfoliates. It is easier to

Figure 72 Phase diagram for the polymer-clay mixture containing φ volumefraction of functionalised polymer (N = 1000) and non-functionalised polymer

(P = 100). After [Kuznetsov and Balazs, 2000a]. See text.

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Thermodynamics

expand the gallery height in a mixture of telechelic and non-functionalisedpolymers.

The scaling-theory shows a significant influence of the chain length ratio N/Pon the location of the phase boundaries between the immiscible, intercalated andexfoliated states of the polymer-clay mixtures. Increasing N at fixed P with N ≤ P,causes the immiscible phase to transform into an intercalated or exfoliated one.At the same time, the intercalated-exfoliated transitional volume fractions weaklydepend on N. When changing the value of P at constant value of N with N ≥ P,the intercalated phase expands at a cost of the exfoliated one. Theoretical analysisof the clay/polymer/functional polymer indicates that the most promising strategyfor exfoliation is by using long-chain functionalised compatibiliser of smallerbut comparable in magnitude molecular weight to the matrix polymer. Promisingresults were obtained for functionalised linear polymers having either one reactivechain-end, or two reactive chain-ends. Experimental confirmation of thisconclusion can be found, for example, in the work by Hoffmann et al. [2000a].

Since multi-block copolymers form morphologies that may be controlled bythe composition and molecular weight of the blocks, there is a concerted effortto explore the potential structure formation of nanocomposites with blockcopolymer as a matrix. Thus, for example, SCF was used to analyse the structureof nanocomposite thin films (of di-block copolymer with nanoparticles) confinedbetween two walls [Lee et al., 2003]. In such restricted geometry, particles aredriven to the wall, effectively modifying their chemical nature and the filmstructure. The changes are robust, relying on the entropic effects. They can beexploited to form nanoscale devices with particles assembled into nanowires.Another suggested application of the phenomenon is the use of such systems forcoatings – the high concentration of solid particles near the surface makes such acoating more fracture and abrasion resistant.

3.1.7 Solid Surface Effects on Molecular MobilityThe effects of a solid phase on organic chain mobility are important forunderstanding nanoparticle behaviour in a polymeric matrix. Crystalline solidsshow high surface energy and a tendency for aggregation. The key to the successof CPNC is the stable and uniform dispersion of exfoliated clay platelets or othernanoparticles (e.g., carbon nanotubes).

3.1.7.1 Surface Energy of Solids

The surface energy depends on the nature and structure of materials. Its value isdefined in terms of the thermodynamic equilibrium of the solid with its saturatedvapour. Evidently, measurements of the surface energy of a liquid are significantlymore straightforward than those of the solids.

The surface energy, σs, is defined as:

σ ν μs s s

i is

i

U TS s n= − = + ∑1 (69)

where s is the surface area, the superscript ‘s’ indicates the surface quantities oftotal energy (U), entropy (S) and the number of moles (n) of species ‘i’ withchemical potential, μi, adsorbed on the surface. Thus, the equilibriumthermodynamics predicts that the total surface energy may change due totemperature and composition. The variable, ν1, is the surface tension (subscript1 indicates a single component in equilibrium with its vapours). In the absence of

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adsorbed foreign substance (ni = 0) the surface tension coefficient, ν1, is given bythe ratio of surface energy to surface area; hence it may also be called the specificsurface energy.

The specific surface energy of solids, ν1, is difficult to measure as these areoften subjected to 3D residual stresses, surface roughness all the way to nanoscale,readily adsorb environmental gas, vapour or liquid molecules and may becontaminated by compounds that are difficult to remove. The highest measuredsurface energy is for diamond: ν1(C-diamond) = 9820 mN/m. A high value wasalso determined for orthoclase, KAlSi3O8, viz. ν1 = 7770 mN/m [Brace and Walsh,1962]. The specific surface energy of a metal in equilibrium with its own vapouris also high, viz. ν1(Cu) = 1,430, ν1(Au) = 1,510 mN/m, similarly for crystallineoxides, viz. ν1(CaO) = 1,310, ν1(MgO) = 1,090 mN/m. By contrast, amorphousbodies have significantly lower ν1 than crystalline. For example SiO2:ν1(amorphous) = 260, ν1(hydrated, amorphous) = 130, while ν1(quartz) = 1030and ν1(hydrated quartz) = 422 mN/m [Condon and Odishaw, 1967; Stokes andEvans, 1997].

These values for crystalline solids should be compared with those of organiccompounds. For example, the polymer surface tension coefficients at 20 °C rangefrom 10 to 49 mN/m (for fluorinated acrylic to polyesters or polyamides,respectively) [Brandrup et al., 1999]. Similar values have been measured fororganic liquids – the highest is ν1(glycerol) = 63 mN/m, with –OH groups on thesurface. In short, the surface energy of a crystalline solid is about two orders ofmagnitude greater than that of organic liquids.

Owing to morphological nanorugosity the surface of crystalline solids is usuallyhighly complex. This is particularly true for mineral silicates where the surfacemay be defined only in statistical terms. The clays, due to variation of composition,show great variation of the surface heterogeneity [Papirer and Barard, 1998].The latter also varies with the type of adsorbed compound. Evidently, adsorptionaffects the surface energy of crystalline solids. For example, the specific surfaceenergy of freshly cleaved mica in vacuum is ν1 = 4,500 mN/m whereas thatmeasured in air is 375 mN/m.

One of the consequences of the surface energy contribution to the energeticsof a body is the dependence of the transition temperature on the degree ofdispersion. For example, the melting temperature of bulk PE is Tm ≅ 400 K, butPE-particles with diameter varying from 13 to 5 nm have Tm = 266 to 218 K.Another consequence of the high surface energy is the aggregation of solidpowders. According to Johnson et al. [1971], the force between two interactingspheres (of radii R1 and R2) is given by:

ε = ν11πR1R2/(R1 + R2) (70)

For R1 = R2 the relation simplifies to read ε = ν11πR/2. Equation 70 is general andindependent of the elastic modulus. The attraction force is then proportional tothe radius – the smaller the particle, the smaller is the force of individual contact.However, dividing the same mass of crystalline powder into the number of particlesrequires that NiRi

3 = NjRj3, thus a change of the total aggregation energy, E = Nε,

i.e.:

Niεi/Njεj ≡ Ei/Ej = (Rj/Ri)2 (71)

For example, to disperse the same volume of CaCO3 particles of about 10 nm diameterwould require 62,500-fold more energy than that needed for a standard filler powder

293

Thermodynamics

with diameter of about 2.5 μm. Furthermore, since the dispersing force duringcompounding is given by the product of radius and stress, Rσij, to achieve the sameefficiency the stress should be increased by a factor of 200.

3.1.7.2 Polymer Adsorption on Solid ParticlesAs discussed by Fleer et al. [1993], any interface between two phases (solid,liquid or gas) induces changes to properties. For example, solid readily adsorbsmacromolecules from a solution. The thickness of the adsorbed layer usuallyextends to the magnitude of the radius of gyration of the macromolecular coil,

sθ

2 1 2/= 5 to 35 nm. The adsorption depends on the polymer (its chemistry,

conformation, molecular weight, polydispersity, etc.), on the solvent, temperatureand the substrate surface energy. The model computations indicate that in theabsence of direct chemical bonding, macromolecules interact with a solid surfaceby physical contact of several statistical segments labelled as ‘trains’ (seeFigure 73). The trains create loops and tails. The relative proportion of thesestructures depends on the molecular weight, e.g., loops are absent for short chains,but dominant for long ones.

Recently van der Gucht et al. [2002] proposed a lattice model for describingthe behaviour of an ideal polymer solution at a surface. The ratio of the surfaceexcess volume fraction, φex, of polymer molecules with N-statistical segments, tothe bulk volume fraction of polymer, φb, was found to follow a simple dependence:

φ φ χ χex b

s scA B/ = + −( )[ ]∞ (72)

where A ≅ 5/6 and B ≅ 1/5 are constants, χs is the Silberberg adsorption parameter,and χ sc

∞ is the critical adsorption energy for infinite chain lengths. With decreasingchain length the adsorption/depletion transition shifts to lower χs values. Thiseffect is strongly enhanced if the end segments of the chain adsorb preferentially.

Cosgrove et al. [1987; 1991] used neutron scattering to study polymeradsorption from dilute solutions on solids (mica or PS latex particles). The resultspresented in Figure 74 indicated that homopolymer concentration exponentiallydeclines from the surface in the z-direction, while the copolymer concentrationprofile follows a parabolic dependence. The thickness of the adsorbed layer variedfrom one system to the next, but within a relatively narrow range of values. Theroot-mean-square (rms) thickness was slightly larger than the unperturbed radiusof gyration, viz. 3.6 versus 4.8 nm for copolymer and 6.8 versus 9.0 nm for PS,respectively. After evaporation of solvent the thickness of the adsorbed layer wasreduced by about 50% (viz. to 3.3 and 4.4 nm, respectively) indicating high

Figure 73 Schematic representation of adsorbed macromolecule. Loop, train andtail are shown.

294


packing density in the original layer adsorbed from dilute solution. The rmsthickness of the adsorbed layer is one measure of the phenomenon, another beingthe distance in the z-direction, zmax, over which the polymer concentration ismeasurably higher than the bulk concentration, φ > φb. For the systems studiedby Cosgrove et al., zmax = 24 nm was reported. As shown in Figure 74, all otherparameters being the same, the magnitude of zmax very much depends on thepolymer molecular weight.

Israelachvili et al. [1984] reported that when two saturated layers of PS onmica approach each other, starting at a characteristic distance, zc, they attracteach other. The experiment was conducted in a cyclohexane solution of PS, nearthe Θ-condition using the surface force analyser (SFA; see Figure 75(a)) designedand originally built by Israelachvili in 1978. The distance zc = 60 to 120 nmdepended on the PS molecular weight and the thermodynamic miscibility. Theresults were well reproduced in two different laboratories.

3.1.7.3 Nanoscale Rheology

In the layer adjacent to the clay surface the adsorbed molecules have a tendencyto be strongly adsorbed [Israelachvili, 1985; Horn and Israelachvili, 1998]. Thissolid-like behaviour of the first z = 2-9 nm thick surface layer is followed byanother one, in which the viscosity progressively decreases in the z-direction,finally to reach the level of the bulk solution viscosity at z ≅ 120 nm. The forcemeasurements for PDMS squeezed between two mica plates showed a solid-likebehaviour at a distance of about 4 nm, and exponentially decreasing viscositytoward the bulk value at a distance of about 17 nm. For the measurements theIsraelachvili’s surface force balance was modified to impose either a steady-stateor dynamic shearing (see Figure 75).

Figure 74 Concentration profiles of adsorbed PEG on deuterated PS latexparticles. Data [Cosgrove et al., 1987].

295

Thermodynamics

Figure 75(a) The surface force analyser (SFA)(a) Reprinted from Intermolecular and Surface Forces, J. Israelachvili.

Copyright 1992, with permission from Elsevier.

Figure 75(b) The SFA was used to measure the viscosity of adsorbed PS-X from atoluene solution. The results indicate a solid-like behaviour for the first adsorbed

layer: 2z < 220 ± 10 nm [Klein et al., 1993]. See text.(b) Reprinted with permission from [Klein et al., 1993].

Copyright 1993 American Chemical Society.

296


Confirming the earlier observations, Klein et al. [1993] reported that non-functionalised anionic PS poorly adsorbs from dilute toluene solution onto amica surface. The measurements gave the correct magnitude of the solutionviscosity within a few nanometres from the surface. The situation was dramaticallydifferent when the PS chains were terminated with the zwitterionic group,-(CH3)2N+-(CH2)3SO3

–, giving ammonium-terminated PS-X. The molecular weightof PS-X was Mw = 375 kg/mol, Mw/Mn = 1.03 and the radius of gyration intoluene, Rg = 57.5 nm. Figure 75(b) also shows variation of the ‘effective mobility’parameter, G, in the z-direction, thus orthogonal to the mica surface. The ‘effectivemobility’ parameter was defined as:

G R K A A z zo H o≡ ( ) − = −( )6 1 22 2

π ω η/ / / (73)

where R is the mean radius of curvature of the mica sheets, K is the spring constant,ω is the frequency, A and Ao are amplitudes of oscillation, z is the distance fromthe mica surface and zH is its value (known as the hydrodynamic thickness) atwhich the extrapolated value G = 0, and ηo is the zero-shear viscosity of the bulkliquid.

With the low grafting density (mean spacing between attached chains of s =14.5 ± 1.5 nm) the hydrodynamic thickness nearly doubles the radius of gyration.Furthermore, within the first layer from the mica surface (110 nm or so thick)the segmental mobility of PS macromolecules is practically zero, indicating asolid-like behaviour of the adsorbed chains.

More recently, the surface force apparatus was redesigned and thenanorheological measurements were carried out on undiluted polybutadiene (PBD;Mw = 6.95 kg/mol; the entanglement molecular weight, Me = 1.85) [Luengo etal., 1997]. The initial measurements of the normal forces at very slow approach(2-3 min per point) indicated the presence of two regions: (1) a steeply repulsiveforce with an incompressible ‘hard wall’ at zhw ≅ 5 nm of solid-like PBD adsorbedon the mica surface, and a roughly exponential decay at larger distances. Theexponentially decaying region had a decay length of about 3.5 nm, which is closeto the unperturbed radius of gyration, Rg = 3 nm. These repulsive forces indicatestrong binding of polymer to the surface, at least over the time scales of the forceruns (1-2 h), and an immobilised layer of thickness of about 1.5 Rg per surface.The chain conformations in such confined surface layers may be in the glassy orrubbery state.

Next, the dynamic oscillatory measurements were carried out at strains γ < 30%,frequency, ν (Hz) = 0.03 to 80 and with the separation distance, 2z = 10 to250 nm. As the distance between two mica sheets decreased, three regions of thedynamic behaviour could be distinguished: (1) bulk, (2) intermediate, and (3)tribological. Figure 76 and Figure 77 present results of these measurements.

(1) Bulk: For 2z > 200 nm the phase angle (between strain and stress signals) δ ≈ π/2,and the measured storage and loss shear moduli (G´ and G´´, respectively)followed the bulk behaviour.

(2) Intermediate: At smaller z-gaps, δ < π/2 while the values of the shear moduliwere higher, showing an ‘elastomeric-like’ plateau indicating a 3D structure.

(3) Tribological: At the smallest gaps, 2z ≤ 12 nm, δ = 0, or a Hookean-typeresponse of an elastic body was obtained. On the flow curve (log viscosityversus log deformation rate) the data followed a straight line with the slope= –1, i.e., η γ ν ν∝ ′ ∝ ′′ ∝1 2/ ˙ ; : or andG G where ν is the radial frequency.

297

Thermodynamics

Figure 76 Effect of surface separation (distance D = 2z) on the storage (G´) andloss (G´´) moduli of PBD at T = 25 °C and frequency n = 13 Hz. Data [Luengo et

al., 1997].

Figure 77 Effect of frequency on the shear moduli for PBD at T = 25 °C,D = 2z = 10, 30, 180 and 250 nm [Luengo et al., 1997]

Copyright 1997 American Chemical Society.

298


The authors also reported the presence of normal stresses during the steady-state shearing. Caused by them, initially the gap between mica sheets wouldincrease toward an equilibrium separation, to return to the original separation inabout one minute after stopping. The effect depended on the initial separationand the shear rate. Since a similar behaviour has been observed for ‘brush’ layersof zwitterion-terminated PS end-grafted to mica in toluene, the inference is thatPBD coils are effectively bound to the surfaces not by ionic but van der Waalsforces.

It should be stressed that such behaviour is not limited to macromolecules.Low molecular weight organic solvents having either spherical or short-chainmolecules (e.g., alkanes, octamethyl-cyclotetrasiloxane, oligomers, etc.) behavedsimilarly under confinement [Gee and Israelachvili, 1990; Granick, 1991; Huand Granick, 1992]. For example, when dodecane was confined to a diminishinggap from ca. 5 to 2.8 nm it underwent an abrupt transition (similar tocrystallisation) to a solid-like state. Virtually all confined liquids solidified on themica surface. Since the process is rate-dependent (time scale ranges from minutesto hours), the thickness of the solidified layer varied from two molecular layersup. In some cases, with increasing confinement some liquids showed a smoothincrease of viscosity by up to 7 orders of magnitude. For either of these two typesof low molecular weight liquids the layer thickness within which the viscositywas above that in the bulk was quite large.

3.1.7.4 Molecular Modelling of Nanoconfined Molecules (Intercalation)The behaviour of molecules in the direct proximity of a smooth solid surface hasbeen studied for a number of years. The research has been motivated not only byintellectual curiosity, but also by practical concerns in geology, biology, pharmacy,etc. Surface force measurements, surface force microscopy, surface tunnellingmicroscopy, X-ray or neutron scattering and other modern methods have replacedthe classical measurements of the adsorption isotherms. However, the biggestprogress in understanding the solid/liquid interaction has been gained fromcomputer simulation techniques, using Monte Carlo (MC) or molecular dynamic(MD) procedures. This work is of particular interest to the CPNC technology asin these materials the specific surface area, A(MMT) ≅ 800 m2/g, has a strongmultiplying effect, even at low clay loading. As it was discussed in the precedingparts, molecules (short chains as well as macromolecules) strongly adsorb on thecrystalline surface either from solution or from melt. While the solid-like behaviourextends only to 2 to 9 nm, the reduced mobility may extend to over 100 nm.Considering that the thickness of a MMT platelet is 0.96 nm, even the minimumlevel of immobilisation has a large multiplying effect on the reinforcing solidcontent.

Two excellent overviews on the molecular modelling of adsorption andordering at solid interfaces are available [Hentschke, 1997; Binder et al., 2004].The former author discusses both aspects of adsorption, chemosorption involvingdirect chemical bonding to substrate and physicsorption, involving repulsive,dispersive, multipole and induced interactions. Several examples of goodagreement between the computational and experimental data are shown. Bycontrast the approach of Binder et al. is more pedagogical – a tutorial introductionto the techniques and the basic algorithms of MD. As an example, the authorscomputed self-diffusion constants, viscosity, and thermal conductivity of moltenSiO2, as well as vitrification – a non-equilibrium behaviour.

299

Thermodynamics

Computations using MC or MD methods, as well as various models haveshown that macromolecules near solid surfaces are greatly affected – theirmolecular arrangements, conformations and dynamics are strongly perturbedwith respect to the isotropic bulk [Brown et al., 2003]. Thus, the surface layersin contact with a solid are densely packed with chain segments. The orderedlayers extend into the liquid phase for two to three times the transverse diameterof the macromolecular chain. The surface also distorts the Gaussian distributionof chain segments to a length of about the radius of gyration of the polymer. Inconsequence, the chains whose centre of mass is close to the surface are flattenedwith a substantial fraction of segments forming the first densely packed layers.Furthermore, the diffusivity is non-isotropic – hindered in the vertical z-directionand enhanced in the planar xy-direction.

MC simulations are mostly carried out on a lattice. While the model greatlyreduces the computing time, it is not totally realistic. The simulations have beenlimited to systems with polymeric chains represented as random, self-avoidingwalks on simple lattices. The computations have been used for clarification ofspecific aspects, for indicating a solution to specific problems. However, the latticemodels are not suitable for studies of packing and ordering. For this purposeout-of-lattice models are more realistic [Binder, 1995].

MD simulations were used to study the static and dynamic properties of 2:1layered silicates (CEC = 0.8, 1.0 and 1.5 meq/g for two MMT clays and a hectorite,respectively) intercalated with alkyl-ammonium ion, having 6 to 18 -CH2- groups[Hackett et al., 1998]. The results indicated that within the gallery, the alkylchains show strong layering tendencies. Shorter alkyl chains form stable‘monolayers’ with d001 = 1.32 nm. The space between the clay surface oxygenatoms is about equal to the diameter of a single -CH2- group. As the alkyl chainlength increases, a bilayer (d001 = 1.8 nm) and trilayer (d001 = 2.27 nm) disordered,intertwined structure was predicted. For most bilayer configurations, nearly halfof the -CH2- groups are in the layer opposite the grafted ammonium group ofthat chain. In a trilayer configuration, -CH2- groups tend to jump to the middlelayer, but few to the layer opposite to the grafted chain end. The trans-gaucheconformer ratios as well as the trans-gauche transition rates were computed. Thedata were found to agree with FTIR observations by Vaia et al. [1994]. Thus,within the interlamellar galleries of alkyl ammonium intercalated clay the alkylsshow dual structure – crystal-like for the surface-bonded trans-trans part anddisordered, liquid-like away from the surface.

According to a terse announcement, Amcol International developed a computermodel that predicts the type of organoclay structure that will form with a givenintercalant [Beall, 1999]. The model is reported to quantitatively predict thed001-spacings. It may also be used to compute the energetics of exfoliation by apolymer to form a CPNC with ionic or ion-dipole bonding. As an example, thepredicted versus experimental d001 spacing of MMT was given. The valuesreproduced in Table 41 have a standard deviation of ± 0.06 nm.

Molecular modelling has also been used to examine the validity of severaltheoretical assumptions, confronting them with directly performed numerical‘experiments’. In 2000 Sumpter et al. [2000] used MD to simulate polymernanoparticles comprising up to 300,000 atoms, with a variety of chain lengths.The results showed that the ratio of surface atoms to the total number of atomsfor the nanoparticles is very large and the surface effects are responsible for theproperties of CPNC being so different from those of the bulk polymer.

300


The dynamics of the organic phase were studied using NMR surface-sensitivecross polarisation and bulk-sensitive spin-echo experiments. Synthetic, Fe3+-freefluorohectorite (FH) was intercalated with ODA, and then melt-annealed withPS under static conditions [Zax et al., 2000]. Within the gallery the polymerconstituted 68 wt% (ODA the rest). The molecular dynamics simulation suggestedthe presence of a symmetrical five-layer structure of the organic phase. Thus,near the surface there was a preponderance of phenyl groups, next the backbonecarbons intermixed with the alkyl groups. The highest probability of finding theintercalant carbon was within the central layer. The NMR results showed thatthe chain segments (both phenyl and -CH2- groups) that interacted with the surfacewere dynamically inhibited. By contrast, within the central layers the segmentswere more mobile than in the bulk at comparable temperature. The transitionfrom the glassy to molten state took place over a wide temperature range – startingat lower T and ending higher than for the bulk PS. Furthermore, by contrastwith the bulk behaviour, the PS inside the interlamellar gallery space did notshow the customary, isotropic molten phase behaviour.

Anastasiadis et al. [2000] used dielectric spectroscopy to study the segmentaldynamics of 1.5 to 2.0 nm thick polymer film confined between clay platelets ina nanocomposite. For the study hectorite and MMT were intercalated withdimethyl-dioctadecyl-ammonium bromide (2M2ODA). Mixing the dryorganosilicate with up to 30 wt% of poly(methyl-phenyl siloxane) (PMPS,Mw = 2.6 kg/mol, Mw/Mn = 1.2) resulted in formation of h = 1.5 to 2 nm thickpolymeric films inbetween the organically modified silicates. The PMPS segmentalmotion is dielectrically active. Three processes were detected: (1) slow, resemblingthe one for bulk-PMPS; (2) intermediate, possibly related to orientational motionsof the intercalant; and (3) fast due to the confined PMPS.

derusaem-DRXdnadetupmoc-DMneewtebnosirapmoC14elbaT,gnicapsreyalretni d 100 ± smetsystnalacretni-TMMrof,mn60.0

]9991,llaeB[

tnalacretnI detupmoC d 100)mn(

derusaeM d 100)mn(

ΔΔΔΔΔd 100 )mn(

)enoN( 19.0 69.0 60.0

retaW 22.1 62.1 40.0

matcalorpaC 16.1 86.1 70.0

matcalorytuB 28.1 09.1 80.0

enodilorryP-2 09.1 09.1 00.0

N enodilorryp-yxordyhlyhtE- 00.2 00.2 00.0

enodilorryplycedoD 05.4 04.4 01.0-

lyraetsonoM 37.1 97.1 60.0

lorecylG 48.3 49.3 01.0

301

Thermodynamics

Measuring the dielectric spectra of PMPS with 2M2ODA eliminated thepossibility that the fast relaxation was due to the presence of intercalant. Theother possible mechanisms include the restriction placed by the interlayer spacingon the cooperative volume. The effective confinement of the polymer chain wasof the order of a few statistical segment lengths of PMPS. The effect of confinementleads to the local reorientation of macromolecules which in turn changes thelocal dynamics, evidenced by the presence of a new faster mode than that in thebulk polymer with weaker temperature dependence. Simulations showed thatchains adopt a preferentially parallel configuration near a wall with oscillationsin the monomer density profile. These lead to a dynamic anisotropy with enhancedparallel and reduced perpendicular monomeric mobility extending over distances.Under severe confinement this interphase is anticipated to extend over the wholefilm thus leading to fast relaxation.

Interesting MD studies were carried out by Yu et al. [2000b]. For over 60 yearsmethylene blue (MB) has been used to measure clay surface area and CEC. Theauthors computed adsorption of MB on a model beidellite, MMT and muscovitemica, in the presence or absence of water. A variety of configurations, includingsingle and double layers, parallel or inclined to the basal surface were found inagreement with the XRD results. The MD computations indicated that thistraditional method of clay characterisation has hidden dangers. As far as theCPNC technology is concerned these MD simulations demonstrated thepossibilities of formation of diverse structures, dependent on the type of clay,intercalant loading, and possible contamination by other substances (e.g.,moisture).

Vacatello [2001a] carried out MC simulations for dense polymer melts withsolid, spherical nanoparticles. The model incorporated off-lattice approximationand conformational distribution of the simulated chains, similar to that of realpolymers. The results showed that at the interface the polymer segments aredensely packed in the form of ordered shells around the nanoparticles, analogousto the layer formation near a planar solid surface. The thickness of the shells wasapproximately twice the transverse diameter of the polymer isodiametrical (tooccupy a single lattice site) segments. The size of these (adopted after Flory et al.[1984]) was taken to be equal to about 3.5 -CH2- units hence having a diameterof ca. 0.45 nm. The nanoparticles behaved as highly functional physical crosslinks,reducing mobility of the polymer chains. Since the effect was related to specificsurface area, it increased with 1/R2, where R is the radius of the particle.Furthermore, the conformational distribution of the polymer was perturbed bythe presence of these solid nanoparticles. Thus, for example, the average dimensionof chain segments was reduced, the polymer chains were either totally containedwithin the interface shell of a single particle or they formed bridges connectingdifferent particles. Some macromolecules visited several nanoparticle shells, andeach particle was in contact with many different polymer chains.

These calculations simulate the structure and dynamics of a polymer/solidsurface system well. The results, determined by topology and entropy, areapplicable to diverse situations, including clay intercalation. The presence ofpreferential interactions between polymer and solid should not significantly changethe computed structure, the order of the interface shells and the conformationsof the polymer chains. Obviously, since so far computations have been carriedout for binary clay/polymer systems, the presence of a third component will affectthe structure. This would be expected for clays modified with quaternary onium

302


ions having more than one type of substituent groups. However, the overall imagethat emerges from these models is expected to remain intact: reduced segmentalmobility near the high-energy clay surface, increasing towards the characteristicbulk melt mobility at a distance of ca. 100 nm.

Manias and Kuppa [2002] provided a short topical review, focusing on thesimulation of structure and dynamics of PS macromolecules in slits. The MD resultswere compared to experimental results for CPNC containing onium-modifiedfluorohectorite dispersed in a PS matrix. Reasonable agreement was found.

3.1.8 Kinetics of Polymer IntercalationThe term ‘kinetics of polymer intercalation’ has several meanings. To theoreticiansconsidering a model sandwich in molten polymer ‘bath’ it describes how fast thetwo platelets will move apart under the influence of the matrix polymer diffusinginto the interlamellar gallery. To an academic researcher investigating diverseaspects of intercalation/exfoliation (viz. effects of intercalant, type of clay,molecular parameters of the matrix polymer, processing conditions, etc.) understatic conditions, it may mean the rate with which the position of the XRD peakis displaced. To most industrial researchers the term immediately conjures imageof a compounding line – will the diffusion be fast enough to engender exfoliationduring the available residence time?

Evidently the three cases are related, but there is a level of increasingcomplications. The model sandwich is the simplest case. The static intercalationis complicated by the presence of stacks of organoclay platelets. The stacks havedifferent number of platelets, up to 300 or so. While intercalation of the externallayers may resemble that of the model sandwich, the internal layers will see ahindered process all the way to the centre. The simplest analogy would be astack of cards subjected to simultaneous separation from the edges by balls havinglarger diameter than the thickness of the card. The process would lead toprogressive bending of the cards from the centre toward the margin, requiringextra energy. Melt compounding in an extruder is further complicated by thepresence of intricate stress fields that are expected to accelerate the staticintercalation process, but at the same time may cause attrition and reaggregationunder compressive stresses.

3.1.8.1 Macromolecular DiffusionAs was mentioned in Section 3.1.3, diffusion of a polymer into the interlamellargallery very much depends on the magnitude of the driving thermodynamic forces.Considering the results of the equilibrium thermodynamic computations one mustrealise that owing to the loss of entropy the diffusion must involve enthalpicinteractions, hence it is not of the entropy-driven self-diffusion type. The requiredenergetic interactions may take place between the polymer and either intercalant(χap), or clay (χsp) or both. For χap < 0 the polymer diffuses into the energeticallyfavourable gallery and it interacts with the two intercalant layers, which couldlead to a kinetically trapped intercalated state. If χap < 0 leads to the prediction ofkinetically trapped intercalation, then a solution may be that reverse magnitude,χap ≥ 0, is needed. However, the SCF calculations indicated that when polymerand organoclay are immiscible the expansion of galleries by polymer diffusionwould not take place. An elegant solution to the problem is to use a mixture offunctionalised and non-functionalised polymers, where the end-functionalised

303

Thermodynamics

macromolecules strongly interact with clay, i.e., χsp << 0. The tails of thesecompatibilising macromolecules should not favourably interact with intercalantchains, i.e., χap ≥ 0, but be miscible with the matrix polymer. The kinetics ofintercalation for this case depend on the system (magnitude of interacting energies,intercalant density and structure) as well as on the molecular weight of thediffusing macromolecules [Balazs et al., 1998].

Diffusion is defined as a change of the centre-of-mass position of amacromolecule (rCM) over time that is significantly longer than that required forthe configurational rearrangements. The simplest case is entropy-driven diffusionof a molecule in a background of identical molecules – self-diffusion, Ds. Whenthe background molecules are not identical the process is called tracer diffusion,D*. When the diffusion takes place in a mixture of two species, the mutualdiffusion coefficient is given by Fick’s law, as a ratio of the flux (J) to theconcentration gradient (∇c): DM = –J / ∇c. In the entangled melts the diffusion isslow, with Ds = 10-10 to 10-16 m2/s, thus 1 h diffusion results in the displacement:

Δr tDCM s= 6 = 1.5 mm to 1.5 μm, respectively [Kausch and Tirrell, 1989].

The MW-dependence of the diffusion coefficient can be calculated from theRouse beads-and-springs model of polymeric chains as DRouse = (kBT/ζ)/N, whereζ is the friction coefficient and N is the number of beads (proportional to MW),and the longest relaxation time, τRouse ∝ N2. The Rouse model is adequate for Dsand D* of low molecular weight species. For high molecular weight melts Doi-Edwards reptation model still uses Rouse kinetics within the confining tube, butto escape from the tube it must move over its primitive path:

D M M D

D M for M M

D M for M Mreptation e Rouses e

s e

=∝ ≤

∝ >⎧⎨⎩

( / )( / )/

/ 4 15

1

1 2 (74)

The prediction of the Doi-Edwards theory was experimentally confirmed, e.g.,by Klein et al. [1983] who showed that for linear PBD for logDs versus logMw,the slope: n = –1.95 ± 0.1. Similar values were reported for other polymers, viz.for PE and PS [Fleischer, 1987]. A wider range of the slope values, n = –1.66 to –2.25was reported by von Meerwall [1983].

In the case of mutual diffusion of polymer M in a matrix of polymer P withenergetic interaction between the two species, the chemical potential and theconcentration gradient control the process. Thus, DM contains terms related tothe binary interaction coefficient, χ12 ≤ 0, and the volume fraction of thecomponents, φ [Brochard et al., 1983]:

D M M DM e Rouse= −( )( )φ φ χ1 2 12 / (75)

The dependence indicates that while DM is a function of other variables, theMW-dependence is stronger than in the self-diffusion case. The mutual diffusionis related to the molecular weight of the diffusing molecules, but not of thesurrounding matrix, P. This conclusion originates from the assumption that theprocess is reptation-controlled and at large values of matrix molecular weights:P > 2Pe the reptation tube is independent of P. The relation is limited to misciblesystems, χ12 ≤ 0, since when the interaction coefficient is positive the systemphase separates and mutual diffusion does not take place.

When the molecular weight P < Pe the tube undergoes continuous renewal –the surrounding molecules are releasing the confinement and rebuilding it. Within

304


this region there is a strong dependence of DM on P-a, with a = 2.5 or 3.0, dependenton the assumption. At a still lower range of non-entangled matrix, P << Pe, a = 1.

The derived relationships are based on the mean-field approach, where eachstatistical element within either the diffusing or surrounding chain has the same,average character. In consequence, these relationships are well applicable tointercalation where the polymeric chain diffusing into the gallery is composed ofstatistical segments favourably interacting with either the intercalant segmentsor the clay. Diffusion of the end-functionalised compatibiliser macromoleculeswill require further considerations.

3.1.8.2 Stationary IntercalationVaia et al. [1995b] studied the kinetics of stationary melt intercalation. Theorganoclay was synthetic fluorohectorite (FH), ion exchanged with octadecylammonium (ODA) or MMT intercalated with dimethyl-dihydrogenated tallowammonium (2M2HTA; Claytone-40). Four narrow molecular weight PS resins(Mw = 30, 68, 90 and 152 kg/mol) and one polydispersed commercial PS (Styron685, Mw = 300, Mw/Mn = 2.6) were used. The organoclay and PS were ground tothe nominal size of 175 ± 25 μm. Melt intercalation was carried out using a pellet(containing 25 mg of organoclay and 75 mg of PS) at T > Tg = 100 °C, undervacuum. The kinetics of PS diffusion into the galleries was followed by XRD (insitu and ex situ). For FH-ODA with PS-30 the measurements showed progressivedisappearance of the d001 = 2.13 nm peak and growth of another at d001 = 3.13 nm.

The XRD peak intensity, Iij,(of the i-th peak and the j-th substance) isproportional to its weight fraction, wj:

w t I t K

w I Kt w t w I t Ij ij j T ij

j ij j T ijj j ij ij

( ) ( ) /

( ) ( ) /( ) ( ) / ( ) ( ) / ( )

*

*

=∞ = ∞

⎫⎬⎪

⎭⎪⇒ ≡ ∞ = ∞

ρ μρ μ

ζ (76)

where ρj, μT*

and Kij are, respectively, density, mass absorption coefficient and aconstant. The time-dependent quantity ζ(t) is a fraction of the melt-intercalatedorganoclay. The kinetic curves, ζ(t) versus t, depend on composition (organoclay,polymer, its Mw), temperature (T), pressure (P), etc. The kinetic of intercalationcurves were fitted to the relation derived by Breen et al., [1987] for vapour sorptionof MMT:

ζ( ) / expt Da tm m

m

= − ( ) −{ }−

=

∞

∑1 4 2 2 2

1

α α (77)

The latter authors modelled MMT as a cylinder of stacked circular disks, each ofthe radius α. The diffusion was assumed Fickian, with the diffusivity constant,D. Integration of the diffusion equation yields the above relation, where am is them-th positive root of the zero order Bassel function. The predicted dependence isshown in Figure 78.

Fitting the kinetic data to Equations 76 and 77 Vaia et al., were able to determinethe effective diffusion rate parameter, D/a2. For the PS30/FH-ODA system thetemperature dependence was of the Fulcher-type: log(D/a2) = ao – a1/(T – T∞),where a1 = 604 ± 40 K and T∞ = 322 K. For the series of narrow molecularweight distribution (MWD) PS the data followed Equation 74. In Figure 79 theself-diffusion data extracted from these measurements (FH-ODA withmonodispersed PS resins) as well as for PS fractions [Fleischer, 1987; Antonietti

305

Thermodynamics

Figure 78 Kinetics of melt intercalation under static conditions displayed as achange of the fraction of melt-intercalated organoclay, ζ(t), with diffusion time.

The Breen et al., function for D/a2 values from 0.002 to 0.03.

Figure 79 Molecular weight dependence of the effective diffusion coefficient, D/a2,for PS at 180 °C and for the self-diffusion coefficient of monodispersed PS resinswith FH-ODA. After [Fleischer, 1987; Antonietti and Sillescu, 1985] – see text.

306


and Sillescu, 1985] are plotted versus Mw. The solid lines are drawn with thetheoretical slope of –2, predicted by the reptation theory. The coefficient governingthe kinetic diffusion into galleries, Da-2, is ten decades larger than Ds for PS.Assuming that the diffusion into the galleries is the same as in the case of self-diffusion, one obtains for the apparent radius of the FH platelets a = 214 nm.This seems to be the right order of magnitude. The reasonableness of this quantityimplies that diffusion into a gallery with, h = 1.2 to 2.2 nm, is not particularlyaffected by the narrow slit geometry. This is striking, since the unperturbed radius

of gyration of the PS samples used was

rΘ2 1 2/

= 5 to 15 nm (for Mw = 30 to

300 kg/mol) hence significantly larger than h. Thus PS macromolecules do notdiffuse as a Gaussian coil but as extended macromolecules with chain diametercontrolled by the segmental motion. According to Flory, the average diameter ofa paraffinic chain is equal to about 3.5 -CH2- units, thus having the diameter ofca. 0.45 nm, for PS this magnitude is about twice as large, ca. 0.90 to 0.95 nm.

In another publication [Vaia et al., 1996] melt intercalation was carried outon synthetic FH pre-intercalated with either dodecyl (DDA) or octadecyl (ODA)ammonium ion. Two narrow molecular weight PS resins, with Mw = 30 and400 kg/mol, and one poly-3-bromo-styrene (PS3Br) PS with Mw = 55 and Mw/Mn = 2were used. XRD studies showed that in contrast with intercalated PS30/FH-DDA,the PS3Br/FH-DDA system was exfoliated. Clearly, the polar Br-group in thepara position increased the polymer-silicate interactions to the extent thatexfoliation was possible. The exfoliation was partially confirmed by TEM.Stacking of 5 to 20 layers with interlayer spacings of between 2 and 4 nm and thepresence of the original crystallites or the primary particle were discernible.Individual silicate layers were observed near the edge whereas small coherentlayer packets separated by polymer-filled gaps are prevalent toward the interiorof the primary particle. The larger expansion of the gallery height and disorderedstructure were apparently responsible for the disappearance of the XRD peak.

The heterogeneity of the CPNC structure, stacking in the primary particleimplies that the process is more complex than simple sequential separation ofindividual layers starting from the surface of the primary particle. Long-rangeforces (e.g., stresses associated with gallery expansion and layer bending) willmaintain stacking of an optimal size. Layer-by-layer delamination may take placenear the edge of the primary particle but it is unlikely in the central part. Theincreased polarity of PS3Br (with respect to PS) will interact more with the polarFH surface. As a result, the frictional coefficient, associated with polymer diffusioninto the gallery, should increase slowing the melt intercalation kinetics.Qualitatively, these effects were observed.

Chen et al. [2003] followed the procedure used by Vaia et al. However, in thiscase poly(styrene-b-isoprene) block copolymers (PS-b-PI) diffused intointerlamellar galleries of synthetic fluorohectorite, pre-intercalated with ODA.Intercalation at T = 100 to 170 °C was followed by XRD for up to 70 h. Thecopolymer diffused similarly on either side of the order/disorder transitiontemperature. The intercalation rate decreased as the PS block increased.

3.1.8.3 Simulation of Melt Intercalation KineticsMelt intercalation can be visualised as a flow of molten polymer into narrowslits. As discussed before, the driving force for the process is not the configurationalentropy, but enthalpic interactions with either low molecular weight intercalant,

307

Thermodynamics

or (preferably) with clay. In other words, it is the gradient of chemical potentialthat drives the diffusion. In the presence of flow, there is also frictional resistanceretarding the process. Thus, the flux of melt from the molten matrix reservoirinto the gallery in the x-direction has diffusional and viscous components [Masonand Viehland, 1978]:

− = +J

c x D

k T x

c x B p

xo

B

o( ) ( )∂∂

∂∂

μη

(78)

where c(x) is molecular concentration in x-direction of flow, Do is the diffusioncoefficient, μ is the chemical potential, Bo ≅ h2/12 is a geometric factor, η isviscosity and p is mean value of the driving pressure in the x-direction. From theGibbs-Duhem equation and Fick’s law this dependence can be used to expressthe effective transport diffusion coefficient as [Nicholson et al., 1996]:

D

c x

D

k T

c x B

c x

D

k T

c x h k T

Dtranso

B

o o

B

B

o

= ( ) +⎡

⎣⎢

⎤

⎦⎥

⎛⎝⎜

⎞⎠⎟⎛⎝⎜

⎞⎠⎟

+⎡

⎣⎢⎢

⎤

⎦⎥⎥

∂∂

∂∂

μη

μηln

( )

ln ( )( )

112

2

(79)

For the non-entangled melts, the Stokes equation holds, hence Doη/kBT = 1/3πa(where a is segmental diameter). Thus, the diffusional component dominates forsmall slit height, h, and the viscous one when the gap is wide.

The kinetics of melt intercalation by polymer diffusing into a clay slit understationary (no mixing) conditions was modelled using MD [Lee et al., 1998a;1999; 2000; Baljon, 1999]. The simulation model was a face centred cubic latticeoriented with planes parallel to the xy plane of the clay platelet. On each side ofthe lattice stack there was a reservoir of molten polymer, equilibrated at constantT and P. To start the process, two central lattice layers were removed to form arectangular slit into which the polymer could diffuse. The size of the slit wasfixed, non-expanding by the diffusion process. The system was strictly binary,without a low molecular weight intercalant. The polymer molecules wererepresented by anharmonic bead-spring chains. Polymer beads, separated by adistance r, interacted with the interaction energy ε according to the truncated,repulsive Lennard-Jones potential for r ≤ rc ≡ σ21/6. The finite extendable non-linear elastic (FENE) model was used to express the bonding potential betweennearest-neighbour beads along the chain, separated by distance r. The bead-latticepotential with the interaction energy εbl was given by truncated, attractive Lennard-Jones potential at rc = 2.2σ. Thus, the bead-bead interactions were either zero orrepulsive at short distance, while the bead lattice was attractive. The simulationswere performed at constant T* =kBT/ε = 1, assuming that the ratio of theinteraction parameters, ε/εbl = 1, 2 or 3 (in latter publications the ratio went upto 10), and the random force in y and z directions obeys the Gaussian statistics.

An analytical model for diffusion of polymer into a slit gallery of length L inthe x-direction was derived in the form of Laplace transforms:

ˆ ( )sinh

cosh ( / ) sinh

/

/ζ s

s d D

sL D

in out

in

=+[ ]

≡

ΨΨ Ψ Ψ

Ψ

1 2

2 4

(80)

where Din and Dout are diffusion coefficients inside and outside the slit. Since theLaplace transform is a function of sL2 divided by s, the intercalation kinetics,

308


ζ(t), depend on tL-2. The superposability of the kinetic curves computed fordifferent values of L would provide evidence that melt intercalation is a diffusion-controlled process.

The computations were performed for L = 43σ to 341σ. The results ζ(t) versust/L2 indeed superposed onto a single curve. Assuming that Dout

= ∞ the effectivediffusion coefficient, Din = Deff was calculated from Equation 80 by fitting thedependence to the simulated kinetic curve. Excellent agreement between thesimulated data and the analytical prediction with the fitted value of Deff wasobtained.

It is important to note that as the interaction ratio, ε/εbl, increases the computedkinetics slow down and the diffusion coefficient decreases. Since the diffusiondriving force is proportional to this ratio, crowding at the entry to the slit isevidently slowing down the intercalation – the macromolecules want immediatelyto interact with the slit walls instead of diffusing further into the gallery in amore civil way. Crowding at the gap by macromolecules eager to interact withslit surfaces also slows the intercalation process. As a result, the polymerconcentration decreases linearly in the x-direction and shows two local maximain the z-direction. While the x-gradient is a measure of the non-equilibrium aspectof the process, the enhanced concentration of the interacting polymer segmentsnear the silicate wall is not.

Simulation was also carried out to determine the centre of mass diffusion ofpolymeric chain within the reservoir. The computations determined that theequilibrium, bulk self-diffusion coefficient, Ds = 0.006σ2/τ, but the effectivediffusion coefficient governing the intercalation Deff = 3σ2/τ, hence 500 timeslarger. The authors argued that once the slit is open the process is no longergoverned by the equilibrium properties. Computations of the centre of massdisplacement of the 1st chain to enter the slit show that its diffusion coefficient iscomparable to Deff, while the macromolecules that follow diffuse more slowly.Simulation of diffusion inside the gallery, in the y-direction gave very small values– about 1/10 of the equilibrium Ds.

The amphiphilic intercalant may enhance the intercalation kinetics (relativeto the case of homopolymer intercalants) and form novel structures. Thus, in thelast publication of this series melt intercalation with symmetrical diblockcopolymer was simulated [Lee et al., 2000]. The model slit surfaces were assumedgrafted with low molecular weight intercalant. The system was maintained atconstant pressure to permit the slit to open as polymer intercalates. The kineticsof intercalation were simulated for different values of surface-block-A and surface-block-B interaction parameters. It was concluded that suitable diblock copolymersmight be used to intercalate clay.

Starting with the Landau and Lifshitz one-dimensional equation of motion,Ginzburg et al. [2001] derived a simple relation that describes the dynamics ofmelt intercalation. The derivation is based on a model, which assumes the presenceof ‘kinks’ – a sudden, local expansion of the interlayer spacing between two clayplatelets. Accordingly, intercalation is a process of localised excitations that movethe kink, opening the interlamellar space. The model stipulates that the kinksoriginate in the interplay between the double-well potential for the clay-claylong-range interaction, bending elasticity of the platelets, and sufficiently strongexternal shear force.

309

Thermodynamics

3.1.9 Pressure-Volume-Temperature Dependence for CPNCThere are several reasons for the study of the pressure-volume-temperature (PVT)properties of a condensed system. These are pertinent for materials processing –the compressibility and the thermal expansion coefficients are required for themodelling of flow through virtually any processing machine. However, there is astronger reason for these measurements – describing the experimental data bymeans of an adequate theory, provides an insight into intermolecular attractionsand repulsions in a given system. Furthermore, a free volume parameter may beextracted from the PVT data, and then used to predict dynamic and kineticproperties, viz. flow, ageing, viscoelasticity. Finally, it is important to establishthe effects of additives (gaseous, liquid or solid!) on all these properties.

3.1.9.1 Equations of State (eos)

In his PhD thesis of 1873, van der Waals proposed the first equation of state (or eos).The relation is usually written in terms of reduced variables (indicated by tilde):

P a V V b T R P V V T

V V V T T T P P P

V b V T a Rb T P a b P

P V RTc c c

c c c

+( ) −( ) = +( ) −( ) =

= = =

= = = = = ==

/ / ˜ / ˜ ˜ ˜

˜ / *; ˜ / *; ˜ / *

* ; * / ; * /

/ /

2 2

2

3 3 1 8

3 8 27 27

3 8

or

(81)

Of interest to van der Waals were low molecular weight liquids with a well-defined critical point – the coordinates of this point, Pc, Vc and Tc, were used asthe reducing parameters. For T = 0 the hard-core volume, Vo = b. van der Waalsconsidered that molecules move in ‘cells’ made by the surrounding moleculeswith uniform potential. The volume, within which the centre of a molecule canfreely move, is what defines the free volume. The free volume fraction is usuallydefined as: f = Vf /V = V – Vo/V, where Vf = V – Vo. Detailed methods ofcomputation of Vo from the chemical structure have been developed, viz. [Bondi,1968; van Krevelen, 1993; Porter, 1995].

Several comprehensive reviews of the eos’s used for polymeric liquids havebeen published. For example, Rodgers [1993] collected PVT data for 56 polymersat P ≤ 200 MPa and melting T-range from 50 to 170 °C. The review presentsfundamentals of the theories and it evaluates fit to experimental data. Fiveprominent eos were examined. These are listed in chronological order.

1. Flory-Orwoll-Vrij (FOV) [1964]:

˜ ˜ / ˜ ( ˜ ) / ˜ ˜

˜ / *; ˜ / *; ˜ / *

* ; * /( ); * * / *

/

* * * * *

PV T V VT

V V V T T T P P P

V T s c k P c k T VB B

= − −= = =

= = =

− −1 11 3 1

3ρ η(82)

where: ρ*, s*, η*, c* are respectively: the ‘hard-sphere’ radius, number ofcontacts per segment, the segment-segment interaction energy, and thecoordination number (kB is the Boltzmann constant).

2. Simha-Somcynsky [1969] (S-S) – will be discussed in the next section.3. Sanchez-Lacombe [1978] (S-L):

310


˜ ˜ / ˜ ˜ ln ˜ / ˜ / ˜ ˜

˜ / ˜ * / ; ˜ / *; ˜ / *

* / * *; * * /

PV T V r TV

V V V T T T P P P

r M P RT T kw B

= − −( ) + −( )[ ] −

= = = == =

1 1 1 1

1

ρ ρ

ρρwhere ε

(83)

where Mw is the weight-average molecular weight, while ρ* is thecharacteristic density parameter. The parameter r represents the number oflattice sites occupied by the r-mer. Evidently, its presence in the eos negatesthe principles of corresponding states – it can be recovered only for r → ∞.

4. Hartmann and Haque [1985] (H-H):

˜ ˜ ˜ ln ˜

˜ / ; ˜ / ; ˜ /

/PV T V

V V V T T T P P B

5 3 2

0 0 0

= −= = =

(84)

The characteristic pressure reducing parameter, B0, has been identified as theisothermal bulk modulus extrapolated to T = 0 and P = 0.

5. Dee and Walsh [1988] (D-W):

˜ ˜ / ˜ . ˜ / ˜ . ˜ . ˜

. ; ˜ / *; ˜ / *; ˜ / *

/PV T qV T V V

q V V V T T T P P P

= −( ) − ( ) −( )≅ = = =

− − − −1 0 8909 2 1 2045 1 011

1 07

1 3 1 2 4

(85)

Within the low-pressure region Rodgers found that all these five eos are adequate– the worst performance was of S-L and the best of S-S theories. However, as thepressure increased, FOV and S-L dependencies started to perform poorly, whileS-S and D-W continued to provide good description. Rodgers cited the followingdeviations: for FOV ΔV × 104 = ± 22 (7), for S-L ΔV × 104 = ± 33 (10), for H-HΔV × 104 = ± 9 (6), for D-W ΔV × 104 = ± 6 (5), and for S-S ΔV × 104 = ± 7 (4).These differences were computed for the full pressure range; the values inparentheses are for the low pressure range, P ≤ 50 MPa.

3.1.9.2 Simha-Somcynsky (S-S) Equation of State

The Simha and Somcynsky [1969] (S-S) theory is based on the lattice-hole modelpresented in Figure 80 for a binary mixture of two liquids with linear chainmolecules (A and B) and with holes or vacancies (V). The model assumes thatmolecular segments occupy y-fraction of the lattice sites. The remaining part, h=1 – y, is occupied by holes simulating the molecular disorder. The fraction h maybe viewed as a particular measure of free volume content. The placement of theoccupied and vacant sites is random. For a simple liquid (A = B) the configurationalthermodynamic properties, such as the PVT (eos) relations, or the cohesive energydensity, are characterised by three quantities: the maximum attraction energy,ε*, between a pair of chain segments, the corresponding segmental repulsionvolume, v*, and the number 3c of volume dependent, external degrees of freedom.In terms of these quantities and the number of segments per chain, s, thecharacteristic pressure, temperature and volume parameters can be defined, viz.:

P* = qzε*/(sv*); T* = qzε*/Rc; V* = v*/Ms hence: P*V*/RT* = c/sMs(86)

with Ms the molar mass of the statistical segment, qz = s(z-2) + 2, the number ofinterchain contacts in a lattice of coordination number z = 12, and R the gasconstant. The variables of state, P, T, V, are then scaled in terms of these. Sincethe theory is based on the corresponding-states principles, these reduced variables

311

Thermodynamics

define a universal, reduced ˜ ˜P T V− − surface for all liquids. In other words, theygenerate master isotherms and isobars. Provided the theory is quantitativelysuccessful, a superposition of experimental and theoretical lines will yield thescaling parameters and thus the material characteristic interaction parameters,ε* and v*.

To derive the eos, S-S first calculated the partition function, Z, for all possiblenumbers of arrangements of occupied sites and empty holes in a lattice with zcoordination number. The Helmholtz free energy is directly given as: F = -RTlnZ. In reduced variables the obtained free energy function F has theform: ˜[ ˜ , ˜ , ( ˜ , ˜ )]F F V T h V T= . Thus, in addition to the usual volume and temperaturedependence, F contains the hole fraction h, which in turn must vary as: h h V T= ( ˜ , ˜ ) .This variation is obtained by minimising the free energy at a specified volumeand temperature, i.e.:

( ˜ / ) ˜, ˜∂ ∂F V V T = 0 (87)

From the Helmholtz free energy the pressure is obtained as:

˜ ( ˜ / ˜ ) ˜P F V T= − ∂ ∂ (88)

From the above relations the S-S eos is derived in the form of coupled equations:

˜ / ˜ ( ) ( . . ) / ˜PV T yQ Q T= − + −−1 2 1 011 1 20451 2 2η (89)

3 1 3 1 3 033 2 409 6 1 1 02 2c yQ Q T s s n y y[( / ) /( ) ( . . ) / ˜ ] ( ) [( ) / ]η η− − − − + − − − =l (90)

with Q V= 1 /( ˜ )y and η = 2-1/6yQ1/3. The two equations not only predict how thespecific volume varies with pressure and temperature, but at the same time howthe free volume parameter, h, changes with these. Of all the eos, only the onederived by S-S explicitly gives the hole fraction, h = 1 – y, which is directly relatedto the free volume fraction, f. Equations 89 and 90 provide a correspondingstates description of the PVT behaviour of any liquid. Once the four characteristic

Figure 80 Simha-Somcynsky lattice-hole model for two constituent liquidmolecules (A and B) with vacancies (V). The energetic binary interaction

parameters are also shown [Simha et al., 2001].

312


parameters: P*, V*, T*, and 3c/s are known, the specific volume and all itsderivatives are known in the full range of P and T. For linear molecules the externaldegrees of freedom are proportional to the number of segments: 3c = s +3. Thus,for linear polymers, where s >> 3, the external degree of freedom: 3c/s ≅ 1; hencefor polymers, only three parameters: P*, V*, and T*, are required.

The S-S theory also provides a simple expression for the reduced cohesiveenergy density (CED):

CED U V y V yV yV˜ ˜ / ˜ ( / ˜ )( ˜ ) . . ( ˜ )= = − −[ ]− −2 2 409 1 0012 2 (91)

where U is the internal energy of the system – one of the intensive thermodynamicquantities. From CED the solubility parameter can be calculated as

δ = = ×CED CED P˜ * . Accordingly, δ is also an intensive function ofindependent variables: δ = δ(T, P), independent of other variables, viz. the typeof solvent used to dissolve a given polymer. Recent analysis of δ for 38 differentmolten polymers shows that, in accord with van der Waals prediction, the product

δV UV∝ ( ˜ ˜ ) /1 2 is approximately constant, thus δ is proportional to polymer density,

ρ = 1/V, which in turn depends on T and P [Utracki and Simha, 2004; Utracki,2004]. Comparison with the values listed in handbooks for δ indicate that theseare significantly lower than computed from the polymer melt properties. Theorigin of this discrepancy has been traced to the free volume contribution – sincethe experimental values for δ are determined in solution, the macromolecularenvironment there (i.e., free volume) is equivalent to that found in molten polymerat high temperature, viz. T ≅ Tg + 300. Considering that different polymers havedifferent compressibility and thermal expansion coefficients, the differencebetween the solubility parameters of two liquids (supposedly related to theirmiscibility) will change with T and P. Users beware!

The full range of the reduced independent variables of polymer melts wascalculated as [Utracki and Simha, 2001a]:

1 6 100 7 1 0 100 35. ˜ . ˜< × < < × <T Pand (92)

Owing to the overlapping properties of polymers, the range of variables in theseinequalities is much wider than that experienced by a single resin. Within theselimits the coupled eos in Equations 89 and 90 can be approximated bypolynomials. The goodness of fit was judged by considering the values of thestandard deviation (σ), the correlation coefficient squared (r2) and the coefficientof determination (Cd). The reduced specific volume follows the dependence:

ln ˜ ˜ ˜ ˜ ˜ ˜/V a a T P a a a P a P To= + + + + +( )[ ]1

3 22 3 4 5

2 2 (93)

The values of ai parameters and the statistics of fit are listed in Table 42.Considering the value of the standard deviation (σ = ± 0.18%) the fit is consideredsatisfactory.

The following expression well approximates the hole fraction:

h V T a a V a T a V a To

˜ , ˜ / ˜ ˜ / ˜ ˜/( ) = + + + +1 23 2

32

43 (94)

Again, the goodness of the fit can be judged by the parameter values listed inTable 42. However, even with only three parameters (i.e., assuming that a3 = a4 = 0)the standard deviation of σ = 0.55% was computed.

313

Thermodynamics

Furthermore, the scaled CED˜ -values computed from the S-S eos wereapproximated by:

CED a a V a T a V a VTo˜ / ˜ ˜ / ˜ ˜ ˜= + + + +1

22 3 4 (95)

The reduced solubility parameter, ˜ ˜( ˜ , ˜ )δ = δ T P was well approximated by:

˜ ˜ / ˜ ˜ ˜ ˜δ = + + + +( )a a P T a a P a P To 1 2 3 4

2 (96)

The numerical values of the fitting parameters for equations 95 and 96 are listedin Table 43. Evidently, the polynomial expressions well approximate the eosprediction, even with three parameters.

There are several practical advantages of these polynomial relationships. Inspite of their simple form they provide good approximation of thermodynamicproperties in the molten state within the full range of P and T. Their derivatives,e.g., the thermal expansion coefficient and the compressibility factor:

α β≡ ( ) ≡ ( )− =

∂ ∂ ∂ ∂ln / ln /V T V PP const T const

and

respectively, can easily be calculated for any temperature and pressure.Furthermore, fitting Equation 93 to experimental data results in rapiddetermination of the reducing parameters, P*, V* and T*. When highly accuratevalues of these parameters is required (viz. for determining the binary interactionparameters, ε* and v*), the latter values may be used as the initial estimates forthe iterative fitting of the data to the coupled Equations 89 and 90. This approachgreatly shortens the iteration process and provides highly reliable solutions.

T S-SottifatadlaimonylopehtfoscitsitatS24elba soe :.ylevitcepser,49dna39noitauqE;dna

]a1002,ahmiSdnaikcartU[ataD

sretemaraP )*V/V(golrofseulaV hrofseulaV

a0 64301.0- ± 43000.0 302.1 ± 020.0

a1 458.32 ± 230.0 929.1- ± 740.0

a2 0231.0- ± 2100.0 930.01 ± 942.0

a3 7.333- ± 5.2 927.0 ± 620.0

a4 5.2301 ± 6.32 24.812- ± 92.01

a5 9.9231- ± 8.25 —

_ 38100.0 35200.0

r2 37999.0 56999.0

Cd 76999.0 99899.0

stnemmoC tasrorretsoM

˜ ˜ ˜V V(T, P)=

h h(V,T)= ˜ ˜

.V ≤ 0 93

314


It is evident that the reducing parameters of the S-S eos, P*, V* and T* (or theinteraction parameters incorporated there) must depend on the polymer molecularcharacteristics (its conformation, configuration, molecular weight, etc.) as wellas on the additives, present in all commercial polymers [Utracki, 2002b].

As an example Table 44 lists the values of the reducing parameters for severalPS resins. The molecular weight of resins marked Z (for Zoller) is Mw = 110 to0.9 kg/mol. Nova PS1301 (Mw = 270 kg/mol) is a ‘crystal’ (transparent) PS forextrusion, thus probably relatively free of additives. Dow 667 is an injectionmoulding grade containing 2.5 wt% mineral oil. The other commercial resins,BASF 1424 and Monsanto HH105, have unknown additives. The indicatedauthors computed the last four sets of parameters from the same set ofexperimental data (measured by Quach and Simha in 1971), but using differentprocedures. Evidently, the characteristic parameters depend not only on polymerMW and diverse additives, but also on the routine used to compute them, viz.sequential or simultaneous, the criteria used for minimisation of deviations, etc.

In many cases, only a relative change of polymer specific volume is of interestand the use of listed in the literature values of P*, V* and T* is acceptable. Inthis case it is possible to compute the PVT surface and obtain all the derivativeproperties (viz. thermal expansion coefficient, compressibility, etc.) using tabulatedparameters. When more fundamental information is required, repetitive tests ofa given resin must be conducted with precision ΔV < 0.2%.

δδ

CED˜ δδ

.V ≤ 0 95

CED˜ygreneevisehocdecuderehtfonoitatneserperciarbeglA34elbaT

.,retemarapytilibulosdecuderdna,,ytisned]a1002,ahmiSdnaikcartU[ataD

retemaraP decudeR decudeR

1#noisreV 2#noisreV 1#noisreV 2#noisreV

a0 3231.0 ± 4400.0 350.1– ± 160.0 5059.0 ±3100.0

88759.0 ±88000.0

a1 9565.0 ± 3300.0 295.0– ± 650.0 za 58000.0– ±50000.0

a2 2988.0– ±0430.0

79.4– ± 62.0 287.4– ± 130.0 801.5– ±020.0

a3 za 63.2 ± 21.0 190.6 ± 090.0 37.01 ± 61.0

a4 za 47.3 ± 42.0 za 02.31– ± 94.0

σ 29300.0 98100.0 37600.0 98200.0

r2 69999.0 199999.0 39999.0 99999.0

Cd 67899.0 417999.0 35199.0 44899.0

stnemmoC tiftnellecxEstnatsnoc3htiw

tasrorretsoM 3htiwtifdooGstnatsnoc

tasrorretsoMP ≅ 0

orezdemussa=za

315

Thermodynamics

3.1.9.3 Extension of S-S eos to Binary Miscible SystemsExtension of S-S theory to CPNC systems can be carried out in three logicalsteps:

(1) Extension of the original theory for one-component liquids to two-componentmiscible systems,

(2) Extension of the latter theory to suspensions of solid particles in polymericliquid and

(3) Development of a specific model for CPNC.During the last 30-odd years S-S eos has been used to describe a variety ofequilibrium and non-equilibrium thermodynamic problems in single andmulticomponent systems, viz. solutions, blends, foams and composites. In thissection the use of S-S eos for bicomponent, miscible systems will be outlined. Amajor assumption of these derivations is the mean-field approach – the misciblecomponents (e.g., polymer segments or solvent molecules) are randomly placedon the lattice.

sniserSPtnereffidrofdetupmocsretemarapnoitcaretnI44elbaT

niseRSP *P)aPM(

T*)K(

V*)g/lm(

Mo)lom/g(

v*)lom/lm(

εεεεε*)lom/Jk(

011-Z 7.837 02911 01369.0 94.64 87.44 70.33

43-Z 1.687 63811 03569.0 24.34 29.14 49.23

9-Z 5.497 07611 02969.0 47.24 24.14 78.23

90-Z 1.977 40501 05789.0 14.44 58.34 57.33

1031SPavoN 5.347 32711 95259.0 78.54 96.34 94.23

4241FSAB 4.318 97311 07749.0 89.04 38.83 35.13

501HH 1.118 56211 09339.0 12.14 94.83 22.13

766-woD 0.477 82711 12059.0 91.44 99.14 05.23

dnahcauQ]1791[ahmiS

3.547 08621 08959.0 21.94 51.74 41.53

dnaikcartU]a1002[ahmiS

5.996 48621 03269.0 22.25 52.05 51.53

nnamtraH te.la ]9891[,

7.517 19721 06269.0 54.15 35.94 54.53

]3991[sregdoR 9.517 04821 04369.0 95.15 07.94 85.53

rohtuaehtybdetupmocerewelbatsihtnisretemarapfostes8tsrifehT:etoN)UAL(

316


Jain and Simha [1980; 1984] demonstrated that for such binary mixtures theS-S theory is directly applicable provided that the reducing and interactionparameters are treated as averages. Thus, fitting Equations 89 and 90 toexperimental data yields the composition-dependent averages indicated by theangular brackets, viz. X . Extension of the S-S theory has to account for thepresence of component-1 (mole fraction x1) and component-2 (x2 = 1 – x1):

P qz s T qz R c V M

P V R T c s M

c c x c x s s x s x

M M s x M s x s x s x

o

o

o o

* * / * ; * * / ; * * /

* * / * / /

;

/

= ( ) = =

= ( )( )= + = +

= +( ) +( )

ε ν ε ν

1

1 1 2 2 1 1 2 2

1 1 1 02 2 2 1 1 2 2

(97)

Similarly, the interaction parameters ε* and v* are becoming compositionalaverages over 11, 22 and 12 interactions. The interacting site fractions, X1 andX2 = 1 – X1, are determined by the lattice interchain contact, zqi, and the molefractions xi, thus:

X q zx q zxi i i i i= ∑/ (98)

where: zqi = si(z – 2) + 2 with z = 12. Then the averages <ε*> and <v*> arerelated to the individual interactions as follows:

ε ν ε ν ε ν ε* * ; ,* * * * *p p p pX X X X p= + + =12

11 11 1 2 12 12 22

222 2 4 (99)

The two values of p reflect the assumed Lennard-Jones 6-12 pair potential. Thetwo relations in Equation 99 have six parameters, four of which are accessiblefrom the PVT measurements of the neat components (i.e., polymer-1 and polymer-2),thus the two cross-interaction parameters, ε12

* and ν12* can be determined. Their

value may indicate attractive or repulsive cross-interactions, but as experiencewith liquid mixtures showed, the strong attractive energetic interactions usuallylead to a smaller value of the volumetric interaction parameter. To facilitatecomputations, Equation 99 can be transformed into:

ν ν ε ε

ν νν

ν

ε

ε

ν νν

ν

* ; *

*

*

* *

*

*

*

*

2 4

211

2 22

411

2 12

1 2 12 122

22

22 222

11

2

11

4 12

1 2 12 124

22

22 224

11

2

2

= × = ×

≡ + + =⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜⎜

⎞

⎠⎟⎟

≡ + + =⎛

⎝⎜

ΞΞ

ΞΞ

Ξ

Ξ

and

X X X e X e

X X X e X e⎞⎞

⎠⎟

⎛

⎝⎜⎜

⎞

⎠⎟⎟

4

11

ε

ε

*

*

(100)

with the following definitions:

e e12 12 11 22 22 11 12 12 11 22 22 11≡ ≡ ≡ ≡ε ε ε ε ν ν ν ν ν ν* * * * * * * */ ; / ; / ; / ;.The outlined extension of the S-S theory to binary systems has been successful

in describing thermodynamics, e.g., phase equilibria, and solubility of gases [Xieand Simha, 1997; Simha and Moulinié, 2000]. As shown in Figure 81 the

317

Thermodynamics

assumption of random mixing is also valid for miscible polymer blends ofpolyphenylene ether (PPE) with PS [Utracki and Simha, 2001a].

Furthermore, the hole fraction, h, computed from PVT data was found tocorrelate with the dynamic properties, e.g., flow of low molecular weight liquidsand their solutions, that of polymers, their blends and foamable compositions[Utracki, 1986; Utracki and Simha, 2001b].

Thus, the random mixing assumption was proved adequate for describingthe compositional variation of PVT surfaces in a wide range of miscible, binaryliquids. Several sets of binary interaction parameters, ε ν12 12

* *, , were extractedfrom the experimental data. Analysis indicates that they may be well approximatedby simple geometric (Berthelot’s) and algebraic (in radius) averages, respectively:

ε ε ε

ν ν ν

12 11 22

1 2

12 11

1 2

22

1 3 3

8

* * * /

* * / * / /

= ×( )= ( ) + ( )⎡

⎣⎢⎤⎦⎥

δ

δν

e

(101)

In these dependencies the two adjustable parameters are close to unity, δe, δv ~ 1(maximum departure from unity was reported by Simha and Moulinié [2000] tobe ca. 13%). Note that the Berthelot’s geometric mean rule with δe = 1 has beenshown to be valid in many applications.

The next step is extension of this binary treatment to suspensions.

3.1.9.4 Extension of S-S eos to Suspensions

The key in extending the S-S multicomponent theory to suspensions is theassumption that the solid particles can be treated as giant, rigid molecules, differing

Figure 81 Scaling volume, V*(ml/g) T*(k), and temperature parameters asfunctions of composition (mol fraction) in PPE/PS blends: the correlation

coefficient, r, is also given [Utracki and Simha, 2001a].Reproduced with permission, Copyright 2001 Wiley-VCH-Macromol.

318


from the matrix macromolecules by size and the interaction parameters. Bothcomponents, the matrix and the solid particles, are placed on a lattice, assumingthat hard-core specific volumes of the components are about equal: ν ν11 22

* * .For the flexible polymeric chain the external degree of freedom, 3c, is assumed tobe proportional to the number of statistical segments, s, and for macromolecules3c1/s1 → 1. By contrast, for solid particles this parameter tends to zero, 3c2/s2 → 0.Furthermore, since now the PVT behaviour of component 2 (solid particles!) isnot accessible, additional assumptions are needed to solve Equation 99.

The validity of these assumptions was demonstrated by Simha et al. [1984;1986] and by Papazoglou et al. [1989]. The aim of these studies was to examinethe suitability of the S-S eos theory for describing the observed changes in thesystem modulus, thermal expansion coefficient and the compressibility engenderedby the addition of solid particles. The authors computed these functions for specificratios of the interaction parameters, viz.:

ν ν ν ν ν ν

ε ε ε ε

22 11 12 11 22 11

1 33

22 11 12 11

1 1 1 2 1 0492

5 1 2 10

* * * * * * /

* * * *

/ : ; / / / .

/ ; / .

= = + ( )⎡⎣⎢

⎤⎦⎥

⎧⎨⎩

⎫⎬⎭

=

= = −

(102)

It has been shown that the extended S-S theory was able to account for differentexperimental sets of data and demonstrated that for an optimised ‘adhesion ratio’,ε /ε12

∗11∗ , good agreement between the S-S eos and several continuum-based theories

of the system modulus was achieved.It is noteworthy that the relationships in the first line of Equation 102 are

consistent with the fundamental assumption of the lattice theories that the cellsize for any component in the system should not differ by more than 10% fromthe average. The Berthelot’s geometric mean rule was also assumed valid, thus:

ε ε ε ε12 11 22 11 2 24* * * */ / .= ≅ .

3.1.9.5 Extension of S-S eos to NanocompositesThe effect of nanoparticles on CPNC behaviour seems to be disproportionate tothe clay content. For example, addition of 0.64 vol% of MMT increased theHDT of a PA-6 matrix by 70 °C, tensile modulus by 70%, and flexural modulusby 130%, but reduced oxygen permeability to 50%, etc.

While XRD, TEM and SEM provide an assessment of the inorganic phasedispersion in a polymeric matrix they do not provide information on theinteractions involved. Rheology (discussed in the following part of this book) isa sensitive tool for observation of the interaction and structural effects on thebehaviour of multiphase systems, but owing to its sensitivity to structure andorientation it cannot be used for the direct determination of interactions [Utrackiand Kamal, 2002a]. In this part the utility of the PVT measurements for thispurpose will be examined.

In the simplest case, dilute CPNC may be visualised as a three-componentmixture: polymer, clay particles (stacks of platelets or aggregates), and exfoliatedclay platelets with attached polymer chains. However, in many systems, lowmolecular weight intercalants as well as standard additives (stabilisers, lubricants,etc.) are also present.

~

319

Thermodynamics

The PVT behaviour represents a volume-average response hence the computedparameters, P* , V * , T * , ε * and ν * , are volume averages.Furthermore, the data fitting to eos makes it possible to compute the hole fractionwithin the full range of independent variables. This function is also an averageresponse of the matrix and dispersed phases. Up to this point the procedure isstraightforward, identical to that applied for a single-component system. Thus,for each composition one obtains predictive behaviour for V = V(P, T), h = h(P,T) as well as the average values for the reducing and interaction parameters. Aplot of these parameters versus composition offers an opportunity to predict thePVT behaviour of intermediate compositions as well.

However, to calculate the magnitude of the individual binary interactionparameters, ε νij ij i* *, , , j = 1, 2, the system structure must be modelled. Themodel is an idealisation of the perceived CPNC structure, formulated assuming abinary system. The assumptions introduced ought to be simple but realistic. Thereis a multitude of CPNC types (intercalated and exfoliated; end tethered and withan intermediate layer of low molecular weight intercalant molecules forming acloud around clay platelets; systems with compatibiliser or without; etc.).Furthermore, the structure may vary with concentration and process variables,viz. exfoliated in diluted systems under large strains, forming short stacks athigher concentration and/or high stress and temperature. For these reasons,different models may be required to interpret different CPNC behaviour. Thesimplest would be the one for randomly dispersed clay platelets in a molten matrix– expected to be valid for diluted, exfoliated nanocomposites.

3.1.9.5.1 Diluted, Exfoliated CPNC – Simplified ApproachThe PVT behaviour of PA-6 and a CPNC containing this matrix polymer mayserve as an illustration of eos applicability to CPNC. Two commercial resinsfrom Ube Ind., PA-1015B and PA-1015C2 were used. According to themanufacturer’s information, the former is a neat PA-6 and the latter is ananocomposite based on PA-6 containing 2 wt% organoclay, i.e., 1.6 wt% (or0.64 vol%) of clay. In the following text these resins will be labelled PA and PANC,respectively. Their PVT behaviour was measured within the ranges T = 300-580 Kand P = 0.1 to 190 MPa. Prior to testing the material was dried for 48 h at 80 °C.Since the instrument used measures only the incremental changes of the specificvolume as a function of P and T, first an absolute value of the specific volume atambient condition was measured in a glove box with an accuracy of Δ ≤ ± 0.001 ml/g[Simha et al., 2001].

For each resin at least six runs were carried out, two isobaric the othersisothermal. Furthermore, some runs were repeated up to three times. Owing tothermal decomposition at higher temperatures, the reproducibility between thesetwo types of measurements was poor, worse for PANC than for PA. For thisreason only single sweep, isothermal data were considered reliable. The specificvolumes of molten PA and PANC at ambient pressure and T = 240 °C weredetermined as: V240 = 1.0182 ± 0.0154 and 1.0009 ± 0.0150, respectively. Theobserved reduction of the specific volume, V, (by 1.7%) is greater than thatexpected on the basis of additivity (1.2%). Examples of the experimental dataand the ability of S-S eos to describe the behaviour are displayed in Figure 82.Evidently, eos describes the observed behaviour for molten PA and PANC well.

320


Figure 82 Comparison of experimental (solid circles) and computed data fromSimha-Somcynsky eos (open circles) isotherms in the molten state for two Uberesins; PA-6 and PANC within the range of pressure from ambient to 200 MPa

(every 10 MPa).Reprinted with permission from [Utracki et al., 2003]. Copyright 2003 American

Chemical Society.

321

Thermodynamics

The deviations from the theoretical dependence at ambient P and the highest Tare caused by thermal decomposition and degassing.

The reducing parameters (P*, V*, T*) were obtained by the simultaneous leastsquares fit of all data points to the S-S eos (Equations 89 and 90). Their values, thestatistics of fit, and interaction parameters computed from Equation 86 are listedin Table 45. To directly compare the specific volume variations, V = V(P, T), forPA and PANC the predicted dependencies were computed by substituting thereducing parameters from Table 45 into Equations 89 and 90. To avoid crowdingthe figure, only five pressures are shown. Computations simultaneously providedh = h(P, T). The results are presented in Figure 83.

In Figure 84 the ratios V(PA)/V(PANC) and h(PA)/h(PANC) at the same Tand P are shown. While the addition of organoclay to PA reduced the specificvolume by about 1% (insensitive to P and T) the effect on the hole fraction ismore pronounced (by ca. 15%) and more dependent on the independent variables.

Up to this point the treatment of the PVT data for PA and PANC were identical– for the former it resulted in the material-characteristic reducing and interactionparameters, whereas for the latter only average values were obtained. To calculatethe individual binary interaction parameters of Equation 99, the site fractions,Xi, must be calculated from a CPNC model.

The CPNC from Ube is known to be exfoliated, free of interfering lowmolecular weight intercalant and the clay concentration is low. Thus, the firstproposed model is that of randomly dispersed clay platelets in a molten PA matrix.Accordingly, the clay platelets may be approximated by disks of diameterd = 200 nm and thickness t = 1 nm, density, ρ = 2.5 g/ml, with ‘molecular’ mass,M = NAρπd2h/4 = 47,296 kg/mol and ‘molecular’ volume Vp = M/ρ = 18.92 × 106

ml/mol. This volume is assumed to occupy s2 lattice sites. In the lattice model ofa mixture, the hard-core volumes of the constituents should not differ too much.For the assumed 6-12 potential the factor 21/6 relates the positions of potential

54elbaT soE ataD.ebUmorfCNAPdna6-AProfatadTVPottifhamiS[ late ]1002,.

retemaraP 6-AP CNAP

,derauqstneiciffeocnoitalerroC r2 999999.0 899999.0

,atadfonoitaiveddradnatS σ 21100.0 172100.0

,noitanimretedfotneiciffeoC DC 206899.0 055899.0

P )aPM(* 7521 ± 8 4611 ± 31

T )K(* 43111 ± 23 70311 ± 45

01 4 V )g/lm(* 3.9198 ± 6.9 6.8888 ± 2.61

Ms )lom/g( 215.72 882.03

ε 11 )lom/Jk( ε 11 32.13=* ± 90.0 <ε 17.13=>* ± 51.0

v 11 )lom/lm( v 11 45.42=* ± 30.0 <v 29.62=>* ± 50.0

322


Figure 83 Specific volume (a) and hole fraction (b) for PA-6 and PANC versustemperature at five pressures: ambient, 50, 110, 150 and 190 MPa. Note that a

small difference in density translates to a substantial loss of the free volumeparameter, h.

Reprinted with permission from [Utracki et al., 2003]. Copyright 2003 AmericanChemical Society.

minimum and onset of repulsion. For PA νhard PA* ( ) = v*/21/6 = 21.86 ml/mol. Thechain length of PA-6, s1 = 801, and that of clay, s Vp hard2 = / *ν = 7.8677 ×105.With these numbers and the inorganic clay content of 1.6 wt% and the molefractions, xi, then the site fractions X1 = 0.99263, X2 = 1 – X1 are calculated.

323

Thermodynamics

Figure 84 The V and h data from the preceding figure are plotted as ratios at thesame T and P (for each point). Note the reverse P-dependence – while the ratios of

h increase with P, those of V decrease.Reprinted with permission from [Utracki et al., 2003]. Copyright 2003 American

Chemical Society.

As stated before, Equation 99 contains six interaction parameters, viz. twoexperimentally determinable quantities, ε ν11 11

* * and and four unknown:

ε ν ε ν12 12 22 22* * * *, , and that must be determined. However, the lattice model

requires that ν ν11 22* * , thus as before ν ν22 111 1* *.= may be assumed. The use of

Berthelot’s geometric mean rule, ε ε ε12 11 22

1 2* * * /= ( ) , reduces the problem to two

unknowns ν ε12 22* * and .

The adopted model leads to the following results (‘11’ represents polymer-polymer, ‘22’ represents clay-clay and ‘12’ represents polymer-clay interactions)[Simah et al., 2001]:

ε ε εν ν ν

11 12 22

11 12 22

32 09 313 54 3 063

24 89 33 53 24 89

* * *

* * *

. ; . , ( / )

. ; . . ( / )

= = =

= = =

and

and

kJ mol

ml mol

In summary, the model of bare clay platelets randomly dispersed in the PA-6matrix resulted in determining that ε ε∗

22 ≅ 95 11* , i.e., that ε22

* , is about two ordersof magnitude larger than the liquid-liquid interaction parameter, ε11

∗ . This largedifference is consistent with the values of the specific surface energy discussed inSection 3.1.7.1. However, the model resulted in inconsistency as far as therepulsion volume parameter is concerned. Even assuming near identity of theinteracting volumes, ν ν11 22

* *= , the computed value for the 12 interaction,

ν ν12 111 35* *.≅ × , is much larger than the theoretical requirement of ν νij* *.≤ ×1 1 11 .

To examine the validity of the additive model an intermediate compositionwas prepared by mixing equal portions of PANC and PA. The PVT behaviourwas determined and the values of the average parameters, ε * and ν * , wereextracted. The use of the simple model failed to yield consistent values for thebinary interaction parameters – again ν ν12 111 1* *.> × was obtained.

~

324


Besides the unexplained large value of the volume repulsion parameter, ν12* ,

the disproportionately large reduction of the matrix free volume by addition of0.64 vol% of clay is inconsistent with the model – the model simply does notoffer any mechanism that could justify these results. The assumption of nakedclay platelets randomly floating in molten PA-6 contradicts the high energy ofcrystalline solids discussed in Section 3.1.7.1, the results computed from MDadsorption of organic molecules on the clay surface, and the direct measurementsof solidification of macromolecules (by means of surface force apparatus andneutron scattering) on the high-energy solid surface (see Section 3.1.7.2). Thus,the simple, additive model had to be modified.

3.1.9.5.2 Dilute, Exfoliated CPNC – Gradient Mobility ApproachAgain, two commercial resins from Ube, PA and PANC, were used [Utracki et al.2003]. An intermediate composition of PA:PANC = 1:1 was prepared. Since thediluted composition was extruder-blended in a TSE, the original resins were alsore-extruded under the same processing conditions. As shown in Table 46 thePVT dependence of the three compositions was well described by S-S eos. Thenew scrupulous measurements confirmed the previously reported large reductionof the free volume parameter, h = h(P, T).

The new CPNC model used for the interpretation of the experimental valuesof the average binary interaction parameters: ε * and ν * , is based on threesets of information:

S-SgnittiffoscitsitatS64elbaT soe ,APfoatadlatnemirepxeehtotikcartU[erutxim1:1riehtdnaCNAP late ]3002,.

retemaraP AP 1:1=CNAP/AP CNP

noitalerroC,tneiciffeoc r2

999999.0 899999.0 899999.0

noitaiveddradnatS,atadfo σ

21100.0 54100.0 72100.0

fotneiciffeoC,noitanimreted DC

206899.0 105899.0 055899.0

P )aPM(* 7521 ± 8 6811 ± 11 4611 ± 31

T )K(* 43111 ± 23 46311 ± 94 70311 ± 45

01 4 V )g/lm(* 3.9198 ± 6.9 1.3498 ± 3.41 6.8888 ± 2.61

Ms )lom/g( 215.72 007.92 882.03

ε 11 )lom/Jk( ε 11 32.13=*± 90.0

<ε 94.13=>*± 92.0

<ε 17.13=>*± 51.0

v 11 )lom/lm( v 11 45.42=*± 30.0

<v 65.62=>*± 63.0

<v 29.62=>*± 50.0

325

Thermodynamics

(1) The reduction of molecular mobility near the crystalline surface computedfrom MD and measured experimentally. This has been discussed in Section3.1.7. Several researchers reported an increase of viscosity as the distancefrom the clay platelet decreased toward the last 4 to 6 nm – at that stage theviscosity reached an immeasurably high value, taken as an indication ofsolidification. For example, the dynamic oscillatory measurements identifiedthree regions: bulk behaviour for z > 100 nm, intermediate, and solid-likefor z > 6 nm where Hookean-type stress-strain behaviour was observed[Luengo et al., 1997].

(2) The molecular structure of Ube PANC. According to an early patent, thepreparation of exfoliated PNC involved three steps [Deguchi et al., 1992]:(a) intercalation of MMT with ω-dodecanoic acid ammonium chloride (ADA),(b) mixing the intercalated clay with ε-caprolactam and water, and (c)compounding the adduct in a TSE with ca. 70% of PA-6. Chain-ends titrationindicated that about 1/3 of the macromolecules were end-tethered to theclay surface.

(3) The observed PVT behaviour of PA, PANC and their 1:1 mixture. The PVTdata were collected with painstaking care, investigating all possible influences,sources of error and ad nauseam verifying the reproducibility. The new setsof data for PA and PANC confirmed the accuracy of the preliminary findings,and lead to similar values of the interaction parameters. However, calculationof the binary interaction parameters using different pairs of the three polymericcompositions (PA, PNC and their 1:1 mixture) resulted in different values ofthe binary interaction parameters. The assumption of random mixing isprobably not correct for the CPNC system studied, and a different modelmust be proposed.

The new model considers that in these diluted and exfoliated PNC:

1. Clay platelets are exfoliated and at the concentration of 0.64 vol% (hence φ= 0.0064) they are randomly distributed in the PA-6 matrix.

2. 1/3 of clay cations are used for end-tethering the PA-6 macromolecules.3. Contrary to the previous assumptions, the clay platelets are not bare, but

rather they are enrobed in two PA-6 layers of different structure andproperties.

4. Directly on a clay platelet there is the first or inner layer of solidified PA-6,zsw ca.6 nm thick. As the distance from the clay platelet surface exceeds thislimit, an intermediate layer starts. This layer, where PA-6 chain mobilityprogressively increases, extends from ca. 6 to ca. 100 nm from the plateletsurface, thus 6 ≤ zin (nm) ≤ 100. At still larger distance, z > 100 nm, thesegmental mobility of this exterior layer is the same as that in the bulkpolymeric matrix.

This model of ‘hairy clay platelets’ (HCP) [Utracki and Lyngaae-Jørgensen, 2002]implies that only at φ < 0.005, where the clay platelets are more than 200 nm apart(the assumed diameter of the model disk is 200 nm), are the values of the interactionparameters expected to be constant. At higher concentrations, φ > 0.005 or w > 1.2wt%, the HCPs overlap, PA-6 macromolecules with bulk properties are absent,and the interactions depend on composition. Thus, the adopted model considersindividual clay platelets to be covered by solidified polymer and dispersed in amatrix composed of PA macromolecules of the intermediate and exterior layers. It

326


is important to note that in this new model the random placement of binarycomponent has been assured by virtue of redefining the dispersed particles as HCP.

In the calculations it was assumed that the ‘12’ parameters characterising thehetero-contacts are approximated by averages – the repulsive volume by thealgebraic average of the radii, the attractive interactions by Berthelot’s geometricmean rule, i.e., that in Equation 101: δe, δv = 1. Applying the model to Equation99 resulted in the sets of binary interaction parameters listed in Table 47. Themodel considers the clay platelets to be covered by two layers of PA-6 with thesegmental mobility increasing with the distance z from the platelet surface. Inconsequence it is expected that the interaction parameters will be similar

ε ε ε11 12 22* * *≈ ≈ . This is indeed shown by data listed in Table 47.

Thus, for the simplest diluted and exfoliated CPNC systems, the PVTmeasurements provide well-defined functions of V = V(P, T) and h = h(P, T).Furthermore, from the reducing parameters the average values of the energeticand volumetric parameters can be unambiguously calculated. However, thecalculations of the individual interaction parameters ‘11’, ‘12’, and ‘22’ verymuch depend on the adopted model – on the definition of what physically are theinteracting species ‘1’ and ‘2’. Thus, when ‘1’ is taken as an average statisticalsegment of the PA macromolecule and ‘2’ an inorganic ‘bare’ clay platelet theenergetic interactions: ε ε22 1195* *≅ are found. On the other hand, when the modelspecifies that species ‘1’ are statistical segments of PA in the melt and ‘2’ areinorganic clay platelets embedded in layers of PA with reduced chain mobility,the energetic interactions change dramatically to ε ε22 11

* *≅ .On face value both approaches are correct and the relative magnitudes of the

energetic interactions have physical sense. However, while the former model wasunable to explain the significant reduction of free volume, the latter model providesa direct and unequivocal response: it is precisely the strong interaction betweenthe clay surface and PA-6 segments that causes the reduction of segmental mobilityand the reduction of free volume. The latter model also agrees with the well-documented adsorption of organic molecules of inorganic, high surface energysubstances. It is consistent with our understanding of this type of CPNC.

.erutxim1:1riehtdnaCNAP,APnisretemarapnoitcaretnI74elbaTikcartU[ataD .late ]3002,

metsySstcatnoC

6-AP 1:1=CNAP/AP CNAP

εεεεε*ji

)lom/Jk(v*

ji)lom/lm(

εεεεε*ji

)lom/Jk(v*

ji)lom/lm(

εεεεε*ji

)lom/Jk(v*

ji)lom/lm(

11 32.13 45.42 12.13 45.42 )96.62( 45.42

21 — — 35.13 57.52 )61.92( 57.52

22 — — 68.13 99.62 68.13 99.62

fosnoitairavehtCNAPnidnaerutximCNAP/APehtnI:setoN v*ji saera

foseulaV.txetehtnidetalutsop ε*ji fosecirtamrofdetupmocerewsesehtnerapni

rofrorrelatnemirepxeehT.segarevagnittiferayehtecnehytilibomtnereffid ε*ji

dna v*ji si ± dna5.1 ± .ylevitcepser,%0.1

327

Thermodynamics

3.1.9.5.3 Intercalated CPNC – Concentration GradientPolystyrene-based CPNC serve as an example of the third type of PVT behaviour[Tanoue et al., 2003a]. Commercial polystyrene (PS1301; Mw = 270 kg/mol,melt flow rate MFR = 3.5 g/10 min) was compounded with organoclay (Cloisite®

10A = MMT intercalated with 1.25 meq/g of 2MBHTA d001 = 1.92 nm). Thecompounding was carried out in a Leistritz co-rotating TSE-34 (L/D = 40) atuniform barrel temperature of 200 °C, screw speed of 200 rpm, and feed rate of5 kg/h (the average residence time t = 240 s). The organoclay (according tosubsequent TGA: 0, 1.4, 2.8, 5.7, 10.6 and 17.1 wt%) was added to moltenpolymer using a side-feeder.

XRD analysis showed that the melt compounding resulted in a partialintercalation (interlayer spacing increased to d001 = 4.2 to 4.8 nm) accompaniedby a simultaneous collapse of the interlamellar gallery spacing (d001 decreasedfrom 1.92 to ≤ 1.7 nm) caused by the thermo-oxidative decomposition of theorganoclay. TEM showed the presence of short stacks with few exfoliated platelets.Enhancement of the mechanical properties by addition of the organoclay waspoor.

The PVT data were collected within the range of temperatures (T = 300-520 K)and pressures (P = 0.1 to 190 MPa). For each composition two to five isothermalruns were carried out. A high degree of reproducibility was obtained. As before,the data were fitted to S-S eos, Equations 89 and 90. The non-linear least squarefit yielded the reducing parameters, P*, V* and T*, as well as the free volumefunction, h. The parameters and the statistics of the fitting procedure are listed inTable 48. Using Equation 86 the average interaction parameters, ε * and ν * ,were calculated from the computed values of P*, V*, and T*. As an example, theh = h(P, T) dependence for neat PS 1301 is shown in Figure 85 – the ones forother compositions were similar. As expected, h increases with T and decreaseswith P.

Figure 85 Hole fraction as a function of temperature and pressure for neat PS 1301.

328


yksnyc

moS-a

hmiS

eht

gnittif

fo

scitsitatS84

elbaT

soe

,SPr

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329

Thermodynamics

The most interesting question was how the addition of different amounts oforganoclay affected the free volume fraction, i.e., the h-function. A plot of therelative hole fraction, h(PNC)/h(PS) at constant pressure P = 1 MPa and threetemperatures, T = 360, 460 and 560 K, is shown in Figure 86. A similar plot ofthe relative specific volume, V(PNC)/V(PS), versus w linearly decreased in thefull range of composition, consistently with the constant ratio of densities in themolten state: ρ(organoclay)/ρ(PS) = 1.49.

The free volume in PANC with 2 wt% organoclay dispersed in PA-6 atT = 300-590 K and P = 0.1 to 200 MPa was lower by 12 to 17% than that of thematrix (see Figure 84). Since during the reactive exfoliation the intercalant(ω-amino dodecyl acid (ADA)) became the first mer that end-tethered PA-6macromolecules to the clay surface, there was nothing in these nanocompositesto prevent adsorption of PA-chains on the high surface energy clay platelets. Theadsorption and resulting reduction of chain mobility caused such a large loss offree volume. The dependencies displayed in Figure 86 are most interesting sincein these PS-based CPNC the degree of intercalation/exfoliation is incomparablylower than that in PA-6/MMT. Furthermore, here the 2MBHTA intercalant is instoichiometric excess (over MMT CEC), thus it forms an intermediate layerbetween the inorganic, high-energy clay surface and PS matrix. As a consequence,one would not expect PS macromolecules to be adsorbed and loose mobility.Apparently, in spite of these obstacles, the clay is able to reduce PS segmentalmobility. Furthermore, the data show that initially the relative loss of free volumeis proportional to the clay content – more clay, less free volume. Figure 86 indicatesthat h(PNC) < h(PS) for organoclay content from 0 to ca. 16 wt%. However, thedependence reaches a local minimum at ca. 4 wt% organoclay (4 to 6% loss ofthe free volume) then starts to increase with the organoclay loading. The lattereffect is related to progressively poorer dispersion of clay platelets.

Figure 86 The relative hole fraction of PS-based CPNC versus organoclayconcentration at P = 1 MPa and T = 360, 460 and 560 K.

330


The dependencies in Figure 86 seem to suggest that should the concentrationincrease still further the hole fraction of CPNC would exceed the value of the PSmatrix. Olson et al. [1997] used positron annihilation lifetime spectroscopy (PALS)for CPNC containing 75 wt% organoclay (FH-ODA) within the range oftemperature from –60 to 140 °C. The CPNC was prepared by dry blending thecomponent powders, compressing and annealing it for 48 h at 170 °C. Annealingincreased the organoclay interlayer spacing from d001 = 2.253 to 3.031 nm.

From the PALS data the free volume fraction, f, can be calculated asproportional to the product of the positron lifetime, τ3, and intensity, I3.Accordingly, the ratio: f(CPNC)/f(PS) was calculated from the cited data for T= –60and 140 °C – the ratio was found equal to 0.535 and 0.544, respectively. Thus,in these non-degraded, highly loaded CPNC the free volume fraction is about46% smaller than that in PS. Even correcting for the reduction of the organicphase content (in these CPNCs there is about 40 vol% clay) these measurementsstill indicate a reduction of free volume by about 10%, both in the glassy as wellas in the molten state of PS.

Using Equation 86 the average interaction parameters, ε * and ν * , werecalculated (see the last two rows of Table 48). Their parallel variation withcomposition is noteworthy (they are linked through the Lennard-Johnsonpotential). In Figure 87 the relative free volume at three temperatures (the samedata as shown in Figure 86) is plotted versus ε * . The dependence is linear witha high value of the correlation coefficient r2 ≥ 0.99992. In the lower part of thefigure the organoclay loading is indicated – evidently the reduction of h is relatedto the average interaction parameters, thus indirectly to the clay loading.

Figure 87 Relative hole fraction of PS1301 with 0 to 17.1 wt% of Cloisite® 10A atT = 360, 460 and 560 K as a function of the average energetic interaction

parameter, ε * . In the upper part the linear regression results are shown; in thelower part the organoclay concentration for each data point is given. Data

[Tanoue et al., 2003a; 2004].

331

Thermodynamics

3.1.9.5.4 PVT – Concluding NotesPVT measurements offer unique insight into the structure and interactions ofCPNC. The experimental data can be well described in terms of the Simha-Somcynsky equation of state (S-S eos). The relation provides two sets of valuableinformation; the functional dependence of the free volume parameter, h, and theaverage interaction parameters, ε * and ν * .

From the admittedly limited information on the PVT behaviour of CPNC itseems that h provides a relatively simple and accurate measure of the averagedegree of exfoliation – as the degree of exfoliation increases the free volumecontent in CPNC decreases. Since the mechanism is based on the degree ofmacromolecular adsorption on the clay surface, the decrease must be related tothe magnitude of the clay-polymer interactions and composition. The lattervariable has two influences as initially its increase provides a larger surface forthe polymer to adsorb onto, and at higher concentration it reduces the interlayerspacing, thus reducing the total surface available for adsorption. Recently, fromanalysis of the PVT and PALS data for PP the authors determined that at ambientpressure, within the range of T = 300 to 400 K the hole volume increases linearlyfrom vf = 0.12 to 0.22 nm3, while the number of holes per gram remains constantNh ≅ 0.38 (±0.02) × 1021, or ca. 0.34 holes per nm3 [Kilburn et al., 2003].

Analysis of the concentration dependence of the average interactionparameters, ε * and ν * offers invaluable insight into the interactions insideCPNC. The calculated values of the individual binary interaction parametersvery much depend on the adopted model. So far the adopted models have beensimple. The insight gained suggests that for the interpretation of CPNC data thepresent version of the S-S theory (derived for binary, randomly mixed systems)should be extended to non-random mixtures. Xie et al. [1992] have madepreliminary efforts in this direction.

332


333

Thermal Stability

3.2 Thermal Stability

3.2.1 Thermal Stability During ProcessingThe stability of the clay-intercalant complex is essential for the melt exfoliationprocess to succeed. Experience shows that the lack of thermal stability complicatesthe production and forming processes of CPNC not only for engineering andspeciality resin (which require high processing temperature), but also for CPNCbased on PO or PS. Owing to the lack of polar groups these polymers need well-intercalated clay and a compatibiliser (a functionalised polymer or copolymer).The stability aspect is also important for the characterisation of CPNC, viz. PVTor rheological measurements where CPNC may be exposed to high temperaturefor long periods. Several researchers reported that MMT intercalated with a longchain aliphatic quaternary ammonium cation is unstable at temperatures above180 or 200 °C. A brief summary of the work on the thermal stability oforganoclays is provided in Table 49 and Table 50.

Early studies on the thermal decomposition of n-alkyl amines were publishedby Ogawa et al. [1992] and by Menéndez et al. [1993]. The former teamintercalated MMT with quaternary ammonium chloride, (CH3)3CnH2n+1N+Cl-

(where n = 8, 12, 14, 16, and 18), while the latter used primary amine,CnH2n+1NH2 (n = 1 to 6) to intercalate γ-titanium phosphate in either vapour oraqueous phase: γ-Ti(H2PO4)(PO4)×2H2O, (TiP) to obtain TiP with 1.3 mol ofalkyl amine. In both cases the interlayer spacing increased linearly with n:

d001 = ao + a1n (nm) (103)

For the MMT with quaternary ammonium ions the parameter values were: a0 = 0.7311and a1 = 0.0851 with the correlation coefficient squared r2 = 0.9947, while forthe TiP systems a0 = 1.12 and a1 = 0.230. The n-alkyl amines formed bimolecularlayers.

To follow the thermal degradation processes, the intercalated TiP was heatedup to 600 °C. Two weight loss processes were identified: (1) at T = 80-110 °C lossof crystallisation water; and (2) a complicated thermal decomposition occurring indiscreet steps, evidenced as peaks in DSC thermograms. The first step at T = 130 to195 °C was caused by a loss of ca. 30% of n-alkyl amine and a reduction of d001;now a0 = 1.12 and a1 = 0.189. The second step at T = 190-270 °C involved loss ofanother 30% of n-alkyl amine, reduction of d001 to a0 = 1.12 and a1 = 0.157, but thebimolecular structure survived up to this point. The third step at T = 335-375 °Cwas caused by a loss of another 40% and a reduction of d001 that involved bothparameters of Equation 74; now a0 = 0.89 and a1 = 0.044. Other peaks followed.

Maxfield et al. [1995; 1996] reported that incorporation of a traditional ‘sizingagent’, e.g., silane or titanate, enhances the thermal stability. More recent workby Yu et al. [2003] places some doubts on the generality of such a statement.

334


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335

Thermal Stability

The authors did not observe any significant changes to the thermal decompositionof organoclay treated with t-butyl dimethyl chlorosilane. In air, the decompositionstarted with a small TGA peak at about 150 °C and a major one centred at250 °C. Under nitrogen the decomposition started at ca. 180 °C and proceededin three steps with peaks at 254, 320 and 438 °C.

In their patent of 1998, Christiani and Maxfield noted that a MMT-ammoniumcomplex with secondary amine was more thermally stable (onset of decompositionat T = 275 °C) than that with either tertiary or quaternary ones. Thus, for meltexfoliation in a PA matrix, they preferred MMT pre-intercalated with dipentylammonium chloride.

Gilman et al. [2000a; 2000b] reported that during extrusion compoundingof PS with MMT-2M2ODA (dimethyl dioctadecyl ammonium chloride) at 185 °Cthe intercalant was thermally degraded. The authors pointed out two factorscausing this unwanted effect: increased temperatures and the presence of air. Theoxidative degradation also affected the matrix PS, broadening its polydispersity,Mw/Mn from 1.8 to 2.40. The degradation was found detrimental to the flameretardancy of CPNC, as the re-aggregated clay offered less protection to thepolymer. Since extrusion of PS with inorganic NaMMT, under the same processingconditions, did not affect the PS, it was concluded that the free radicals formedduring the intercalant degradation were to blame. The authors proposed themechanism, where the byproducts of the Hofmann elimination mechanism[Hofmann, 1851] of quaternary ammonium reacted with oxygen to give peroxyradicals, which in turn attacked the PS macromolecules.

Tanoue et al. [2004a; 2004b] reported similar observations for CPNC preparedby melt compounding PS with MMT-2MBHTA (Cloisite® 10A; C10A) at 200 °C.FTIR transmission spectra were measured on a Thermo Nicolet spectrometer ata resolution of 4 cm-1 and with an accumulation of 128 scans. To enhancedetectability of the chemical changes, difference spectra were calculated for theextruded samples by subtracting out the spectrum of neat PS and comparing theresults with the spectrum of C10A. The analysis identified characteristic peaksof the dimethyl benzyl amine (2MBA) - a byproduct of the Hofmanndecomposition [Aldrich Co., 1997; Bellamy, 1975]. In addition to these peaks,there are strong carbonyl peaks at 1705 and 1736 cm-1 in most of the extrudedsamples containing organoclay, which corresponds to =C=O groups formed byoxidation of the double bond in the long-chain olefin - another byproduct of theHofmann decomposition. Finally, there is evidence of a ketone carbonyl grouppresent in PS. The rheological measurements of the re-extruded CPNC showedthat the zero-shear viscosity, ηo, was reduced by about 30% each time. This isparticularly noteworthy since increasing the residence time in a TSE by a factorof up to 10 did not further reduce the CPNC matrix viscosity. Similarly, extrusionof neat PS had only a small (ca. 2%) effect on ηo. Thus, the combination ofoxygen, temperature and organoclay was responsible for the degradation of PS.

CPNC of PP (Mw = 278, Mn = 72 kg/mol), PP-MA (Mn = 23 kg/mol; 2 wt%MAH) and an organoclay (Cloisite® 20A or 30B; C20A = MMT-2M2HTA orC30B = MMT-MT2EtOH) were prepared by melt blending to have about 5 wt%of clay [Bellamy, 1975]. These dry blends were compounded at 210 °C for 10 minin an internal mixer at 50 rpm. During the process both organoclays lost aproportional amount (to the initial) of the constituent elements (e.g., lost of about10% of C), which indicates that thermal decomposition took place even underthese relatively mild conditions. This confirmed the suspicions that thermal

336


desorption of intercalant takes place during thermal treatment, leading to adecrease of the interlayer spacing, viz. for C20A from d001 = 2.49 to 2.40 nm andfor C30B from d001 = 1.84 to 1.50 nm, respectively.

Bora et al. [2000] studied the thermal decomposition of MMT intercalatedwith either [Ni{di(2-aminoethyl)amine}2 (Ni1) or [Ni(2,2':6',2'-terpyridine)2](Ni2). The thermal stability of these Ni-containing amines was 50 and 150 °Chigher compared to their metal-free compounds. On heating, the interlayer spacingof the first compound decreased from d001 = 1.45 (at 50 °C) to 1.35 (at 250 °C)while the second decreased from d001 = 1.94 (at 50 °C) to 1.90 (at 350 °C). DTAof Ni1 had 5 peaks indicating loss of water at 140 °C, and then a stepwiseexothermic (oxidation) decomposition of the amine at 273 °C (5.5% mass loss),at 354 °C, 456 °C and 557 °C. Ni2 also showed five DTA peaks, first at about100 °C (loss of water), and then a series of exothermic (oxidation) peaks at ca.340 °C, 400 °C (3.5% loss), 505 °C (8.5%), and 645 °C. XRD showed a constantposition for the interlayer spacing up to about 350 °C, starting to decrease at ca.400 °C.

Zhu et al. [2001a] described the properties of PS-based CPNC prepared bybulk polymerisation in the presence of onium salts: N,N-dimethyl-n-hexadecyl-(4-vinyl benzyl) ammonium-Cl (VB-16), N,N-dimethyl-n-hexadecyl-(4-hydroxymethyl benzyl) ammonium-Cl (OH-16) and n-hexadecyl triphenyl phosphonium-Cl (P-16). XRD showed that the interlayer spacing of Na-MMT (d001 = 0.96 nm)increased after intercalation by VB-16, OH-16 and P-16 to, respectively, 2.87,1.96 and 3.72 nm. After polymerisation the first system was exfoliated whereasthe latter two intercalated with d001 = 3.53 and 4.06 nm, respectively. Accordingly,TEM showed discrete clay layers for CPNC with VB-16, only intercalated shortstacks for CPNC containing OH-16, and a mixture of both intercalated andexfoliated structures for CPNC with P-16. TGA/FTIR studies indicated that bothammonium and phosphonium compounds degrade by a Hoffmann eliminationmechanism, schematically shown in Figure 88.

As shown in Table 50, the MMT-P-16 system was the most thermally stable- the onset of degradation took place at a temperature ca. 30 to 50 °C higherthan that of the MMT-ammonium. This may be useful when CPNC must beprocessed at temperatures up to ca. 240 °C. TGA also showed that CPNCs startdegrading at higher temperatures than the matrix polymer. Cone calorimetrydata revealed a significantly reduced rate of heat release for the nanocomposites.

X-ray photoelectron spectroscopy (XPS) was used at T ≤ 500 °C to examinethese PS-based CPNCs [Wang, et al., 2002a]. Degradation of the organiccomponents was detected at T = 100 to 200 °C, while at T = 200 to 250 °CMMT started to decompose into SiO2 and Al2O3. Similarly, Du et al. [2002] usedXPS to investigate the thermal degradation of PMMA nanocomposites containing

Figure 88 Hofmann elimination mechanism for the thermal decomposition of anammonium cation. After [Saunders, 1965].

337

Thermal Stability

MMT pre-intercalated with VB16, hexadecyl-allyl dimethyl ammonium chloride(Allyl16), and hexadecyl vinyl-benzyl dimethyl ammonium chloride (Bz16). Thethree CPNCs were prepared by bulk polymerisation. For both series of CPNCwith PS or PMMA as the matrix, the authors demonstrated that during the thermaldegradation, the clay accumulates on the surface. Its composition changed atT = 200 to 300 °C. The change (presented as changes of the Si/Al ratio) wasfound to depend on the type of intercalant, viz. 2MBHd, 2M-allyl-Hd, or2M-styryl-Hd (Hd = hexadecyl).

Zhu et al. [2002] also studied the thermal degradation and flame retardancy (byTGA and Cone Calorimetry, respectively) of the latter systems (PMMA/MMT-VB16,-Allyl16 or -Bz16). Using XRD and TEM the authors found that thenanocomposites obtained by bulk polymerisation of MMA in the presence oforganoclays having reactive groups (C=C double bonds) showed better dispersionof platelets than those obtained with MMT-Bz16, but all these systems were onlyintercalated. The thermal stability (by TGA) was found to be improved byincorporation of 3 wt% of clay; the temperature for 10 wt% loss for neat PMMA,and nanocomposites with MMT-Bz16, MMT-VB16 and MMT-Allyl16 wererecorded as: 246, 235, 271, and 295 °C, respectively. Recent publications fromthe same laboratory describe the thermal stability and flame retardancy (by TGAand Cone Calorimetry, respectively) of nanocomposites in a PE [Zhang and Wilkie,2003] and PP [Wang and Wilkie, 2003] matrix. These intercalated nanocompositeshave been prepared by melt blending in an internal mixer. The PO-organoclaysystems showed mixed immiscible-intercalated structures that improved when

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338


PO-MA graft copolymer was incorporated. The results from cone calorimetrysuggest that nanocomposite formation has occurred, since there is a significantreduction (by 30 to 40%) in the peak heat release rate (for more information onflame retardancy see Section 5.2). However, these data do not differentiate betweenthe stability of the clay-intercalant complex and that of the matrix.

Delozier et al. [2002] performed the TGA analysis of their organoclay (MMTwith a quaternary ammonium containing tallow group) in air and nitrogen. Theorganoclay started to degrade in air at 150 °C, lost approximately 1% weight at200 °C, 5.5% at 250 °C, and 12% at 300 °C. Under nitrogen the organoclay wasmore thermally stable, viz. at 250 °C the weight loss was 4% and at 300 °C itwas 9%. The effects were specifically blamed on the degradation of the longhydrocarbon tails. The process is known to begin near 200 °C and it acceleratesas the temperature increase [Xie, et al., 2000].

Degradability of CPNC, comprising PA-6 and 5 wt% of MMT pre-intercalatedwith ω-amino dodecyl acid (ADA) was studied [Davis et al., 2003]. Compoundedand dried specimens of neat PA-6 and CPNC were injection moulded at 300 °C.Under identical conditions, CPNC degraded (Mn decreased by ca. 40%), whilePA-6 did not. ε-Caprolactam and terminal vinyl groups were detected in theproduct.

The Hofmann elimination mechanism was used to explain the discoloration,molecular weight decrease, and reduction of mechanical performance of CPNCwith PA-6 as the matrix [Fornes et al., 2003; 2004]. The authors compoundedthe CPNC in a TSE at 240 °C and then injection moulded at 260 °C. The relative(to extruded PA-6) colour index decreased (i.e., colour intensity increased) withorganoclay loading. However, the value for CPNC (proportional to that of neatorganoclay) decreased with the reduction of matrix molecular weight. Intercalantswith more unsaturation, -C=C-, and/or hydroxyl-ethyl groups, -C2H4-OH,developed stronger discoloration. The authors concluded that more thermallystable intercalants are needed. Similar observations were reported for CPNCwith PC as the matrix [Yoon et al.; 2003].

Evidently, thermal degradation of the organic cation poses a serious problemwhen preparing and processing CPNC. The experience shows that whenblanketing by nitrogen is not used, the degradation mainly depends on the numberof reprocessing steps and to a minor extent on the total residence time at highertemperature. The organoclays containing quaternary ammonium intercalants arethermally unstable at temperatures above 150 to 165 °C. However, the onset ofdegradation can be pushed up to above 200 °C by ascertaining the absence of O2and controlling the composition, the method of treatment (e.g., extent of shearing)and time of heating. Furthermore, the decomposition of intercalant in case ofhighly polar macromolecules that may directly interact with clay surface, e.g.,PA, is not as critical as that in the case of hydrophobic polymers with low polarity,such as PO or styrenics.

The use by Inoue et al. [2002] of melamine as an intercalant is an importantdevelopment. This the first time a compound known as a flame retardant hasbeen used as intercalant, replacing the notoriously unstable quaternaryammonium. This promising route is very much worth pursuing.

From the chemical point of view, the least stable are the quaternary ammoniumions with a structure: Ri4N+, where Ri is a straight paraffinic chain. Reduction ofthe degree of substitution is expected to improve the thermal stability, i.e.,RiH3N+ > Ri2H2N+ > Ri3HN+ > Ri4N+ (but at the same time the ionic strength

339

Thermal Stability

may decrease). Another method for improving the thermal stability involves doublealkylation of the β-carbon(s), e.g., replacing straight paraffinic chain, viz.RCH2CH2-N+ by RC(CH3)2CH2-N+ [Starnes, 2002]. The enhancement of MMT-ammonium stability by the use of ammonium-Ni complexes has been reported;the decomposition temperature was 50 to 150 °C higher than that without theNi-complex. Another method of enhancing thermal stability is by the use of the‘sizing agents’, e.g., silanes or titanates [Maxfield, 1996].

Much less information is available on the stability of phosphonium systems.Stability up to 370 °C was claimed by Ellsworth [1999], but more recent work byZhu et al. [2001a] indicates a more modest effect, viz. stability of MMT-phosphoniumhigher by ca. 30 to 50 °C than that of MMT-ammonium. One possible source forsuch discrepancy is the method used for defining the onset of decomposition.When the degradation products have low vapour pressure and high solubility inthe degrading specimen the TGA analysis may be misleading - degradation mayhave taken place, but the byproducts did not evaporate. For this reason, XRD/FTIRfollowed by evaluation of the key properties after heating is essential.

During the last 10 years or so, the search for more thermally stable organoclayshas intensified (see Sections 2.3.2 to 2.3.7). The research on inorganic intercalantswas a significant step in this direction, but owing to the complexity and associatedcosts so far this approach has not gained industrial approval. Observations thatclay readily interact with complex, aromatic salts resulted in repeated attemptsto incorporate these, more thermally stable molecules as intercalants. Thus,replacing ammonium salt by that of a cyclic tertiary amidine leads to greaterthermal stability and more efficient intercalation of clay [Zilg et al., 1999]. Sincecomplex, aromatic molecules are often either functional or coloured (conjugateddouble bonds), Eastman and TNO patented the use of dyes as intercalants [Barbeeet al., 2000c; Fischer et al., 2001]. For similar purposes, Imai et al. [2002]developed intercalant (dimethyl isophthalate substituted with a triphenyl-phosphonium group) able to react with PET, bind to clay and be stable up to atleast 275 °C.

Evidently, other stable organic compounds may be used for this purpose -limitations are only imposed by availability and costs.

3.2.2 Flame Retardancy and High Temperature StabilityThe advantageous properties of CPNC include increased thermal stability andreduced flammability [Gilman et al., 1998; 2001]. In 1998 NIST organised anindustrial consortium to study the flammability of polymers and their CPNCs.The results of the first year of studies were summarised as follows. (1) In CPNCwith 2% to 8% MMT, flammability was reduced by 50 to 80%, without anincrease of smoke. (2) The reduced flammability was achieved along withimproved physical properties. (3) Incorporation of MMT enhanced charformation, which acts as an insulator and a mass transport barrier. (4) Imidazoliumtreated MMT showed greater thermal stability and formed finely dispersednanocomposites with PS and PA-6.

The flame retardancy is often accompanied by increased high temperaturestability as measured by thermogravimetric analysis (TGA). One of the reasonsfor such a correlation is the amount of volatiles - the higher their volume, thegreater is the weight loss and the greater the flame intensity. However, in the caseof CPNC, this relationship very much depends on the relative flammability, the amountof the matrix polymer and the organoclay intercalant [Zhu et al., 2001a, b].

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The TGA results are usually presented in the form of three parameters: T10(°C), T50 (°C), and char content (wt%); the first two indicating the temperatureat which 10 or 50% weight loss was recorded, and the last represents the fractionof material which remains at 600 °C. It is evident that even the initial 10%weight loss takes place at much higher temperatures than the thermal degradationof organoclay (see Section 3.2.1). Recently, MMT was intercalated with oligomericmolecules, bringing the organic content in the organoclay to 65-73 wt% [Su etal., 2004]. In this case, both flammability, and T10 decreased with organoclayloading.

3.2.3 Photo-Oxidative StabilityTo study the effect of organoclay on the photo-oxidative degradation of PP, fourfilm specimens were extruded, and then irradiated in the presence of oxygen atλ > 300 nm, 60 °C for up to 120 h [Morlat et al., 2004]. Besides neat PP, thespecimens comprised organoclay (MMT with dimethyl ditallow ammonium), ormaleated-PP, and both of these additives. The authors reported that while themechanism of PP photo-oxidation did not change, the induction period and therate of oxidation were modified, resulting in a decrease in material stability, butthe same degradation products were formed, in the same relative concentrations.While the reason for the reduced photo-oxidative stability was not identified,several contributing mechanisms have been postulated. (1) Hofmann eliminationprocess during extrusion that engenders unsaturated end groups, which couldcombine with oxygen during irradiation. (2) MMT may catalyse polymerdegradation, especially in the presence of Fe2+ and Fe3+ ions, detected in theorganoclay. (3) Maleated-PP readily photo-oxidises, forming volatile byproducts.

The catalytic and sometimes inhibiting (catalyst poisoning) activities of clayshave been known for a long time, e.g., as suspected agent of life on Earth [Ferris,2002], in the petroleum industry [Hettinger, 1991], and in general chemicalreactions [Newman, 1987]. The catalytic activities of MMT and its derivativeshave been reported frequently [see for example: Okada et al., 1988; Usuki et al.,1989; Lan and Pinnavaia, 1994; Biswas and Ray, 1998; Heinemann et al., 1999;Agag and Takeichi, 2000; Pullukat and Shinomoto, 2001; Lü et al., 2001; Wypartet al., 2002; Sun et al., 2002]. Stackhouse et al. [2001] investigated self-catalysedin situ intercalative polymerisation within Na-MMT. The authors have shownthat catalysis occurs at the clay lattice-edge where -OH groups and exposed Al+3

ions act as strong Brønsted and Lewis acid sites, respectively.

341

Rheology

3.3 Rheology

Modern rheology is expanding into new domains of research, which includeparallel observations of flow behaviour along with other, flow-imposed physicalproperties, viz. orientation, electrical conductivity, magnetic properties,birefringence, etc. For example, small-angle neutron scattering (SANS) has beenused to characterise clay dispersion in non-aqueous media [Ho et al., 2001].More recently the focus has been on rheology performed on a progressively smallerscale [Mukhopadhyay and Granick, 2001]. The use of surface force apparatusmakes it possible to study the steady-state and dynamic shear behaviour of liquidfilms 0.3 nm to 1 μm thick.

However, the credit for the ultimate reduction of scale goes to Sakai et al.[2002]. The authors carried out steady-state and dynamic rheologicalmeasurements on a single telechelic, HS-terminated PS macromolecule (Mw = 39,Mn = 22 kg/mol). The force-extension data followed the worm-like chain model,giving the ultimate extension length, lmax = 20 to 60 nm, which agrees with thetheoretical value of the macromolecular contour length. The dynamicmeasurements performed at frequency ν = 0.3 to 30 Hz and different extensionsof the macromolecule offered a unique method for determining the inter- andintra-molecular interactions and their effects on segmental mobility.

3.3.1 IntroductionBefore discussing the flow behaviour of CPNC it is worth summarisinginformation on the flow of clay suspensions.

• The early application of clay suspensions was for increasing the viscosity ofaqueous or organic liquids, to ‘give body’ and facilitate application, e.g., inpaints or greases. This has been efficiently accomplished by the ability ofclay to form face-edge interactions that leads to ‘house-of-cards’ 3D (gelled)structures. The suspensions showed strong thixotropic effects, rapidlyincreasing with concentration up to full gelation of the system at about5 vol%.

• The computational and experimental data show that molecules form layeredstructures on the face of crystalline solids. The first 2-3 layers show solid-like behaviour and the solid surface effect stretches a long distance in thez-direction. The z-distance where the viscosity is equivalent to that of neatliquid depends on the molecular mobility; on average it stretches to 10-12 layers, but in the case of macromolecules the distance may be as large as100 to 120 nm.

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• The crystalline solid-induced low mobility is reflected in the reduced freevolume (in comparison to the neat matrix polymer at the same T and Pconditions) that leads to increased viscosity, Tg, stiffness, etc.

• In CPNC there are at least three material components: polymeric matrix,clay and intercalant. The degree of dispersion and the type of structure verymuch depend on the interactions between the components (equilibriumthermodynamics), the intercalation kinetics, and imposed stresses/orientation(non-equilibrium effects).

• Owing to anisometry of the clay platelets six types of morphology have beenpredicted: isotropic, nematic, smectic, columnar, house-of-cards, andcrystalline. The phase diagram at thermodynamic equilibrium depends onthe concentration and relative strength of interactions between the threecomponents. Evidently, the external stress field (imposed by flow, sonification,pressure cycling, etc.) affects these structures. Once disturbed, the systemtakes a long time (from minutes to hours) to return to equilibrium.

• Depending on the method of preparation, the CPNC is either end-tetheredor non-tethered. The former resemble highly branched, ‘hairy clay platelet’structures with ca. a thousand macromolecules attached to the clay plateletthrough the initial intercalant. The latter systems resemble a composite:polymer reinforced with plate-like solids whose inorganic-phase dimensionsare enlarged by the intercalation and adsorption of organic molecules.

• Evidently, the degree of dispersion (intercalated versus exfoliated) and claycontent affects CPNC performance.

• Owing to the thermo-oxidative degradability of the ever-popular quaternaryammonium intercalants at T > 200 °C, the time the material spends at thesetemperatures may seriously affect its composition and structure, and hencethe flow behaviour.

It is known that the flow of multiphase polymeric systems is sensitive to thedispersed phase size, shape, and surface characteristics as well as to the transientgeometric structures and interactions between the components, i.e., the matrixpolymer, clay and intercalant [Utracki, 1989; 1995; Utracki and Kamal 2002b].Thus, rheology not only complements the traditional methods of CPNCcharacterisation (such as XRD and TEM), PVT measurements, permeability andmechanical testing, but in addition it provides information on the dynamicbehaviour of these complex materials.

3.3.2 Multi-Phase Flow Behaviour – An OverviewRheology is a part of continuum mechanics hence the principles of continuity,homogeneity and isotropy are normally assumed to hold. The continuity principlerequires that there is no discontinuity of material properties from one mathematicalpoint to another; homogeneity demands that there is no concentration gradient,and isotropy implies that the flow does not impose orientation on the flowelements. In molten multiphase systems (i.e., in polymer alloys, blends, composites,foams and nanocomposites) these three principles are rarely valid. Thus, therheology of the multiphase systems follows its own principles, extending the useof the general rheological dependencies. Obviously, the basic definitions ofrheological functions, e.g., viscosity, η, dynamic shear moduli, G´ and G´´, dynamicshear compliance, J´ and J´´, etc., are identical. However, owing to the numerous

343

Rheology

influences, viz., concentration, morphology, flow geometry, timescale, type offlow field, thermodynamic interactions between the phases, and many others, itis difficult to relate the measured rheological functions to the intrinsic physicalproperties of the CPNC.

The major distinction between single-phase and multiphase rheometry is theeffect of the flow field on the rheological response. Depending on the type andintensity (strain) of the flow field the morphology of the tested fluid may bemodified. Since the structure modification is related to strain, it is to be expectedthat responses at high and low strains will differ. For this reason, the selected testmethod should reflect the final use of the data. Because of the sensitivity ofmorphology to the test conditions, there is a serious disagreement betweenpredictions of the continuum-based theories and experiments, summarised inTable 51. Since morphology is the characteristic property of a given material(e.g., it affects the performance), testing the same multiphase system under differentflow conditions is equivalent to testing different morphologies, thus differentmaterials. When simulation of flow through a die is attempted, large straincapillary flow is useful, but if the material characterisation is important, lowstrain dynamic testing should be used [Utracki, 1988].

Flow fields affect the morphology of individual dispersed particles (orientation,dispersion, coagulation, etc.) as well as the structure of the whole deformed body(e.g., skin-core effect, weld-lines, flow encapsulation). The most efficientorientation fields are extensional. Using convergent and divergent flow one maycontrol orientation of anisometric particles, e.g., in fibre-filled materials, insemicrystalline polymer melts, in liquid crystal polymers or in CPNC. There isless information on the flow-induced orientation of clay platelets, since due totheir size these particles are less susceptible to orientation than fibres. Furthermore,their orientation seems to depend on whether they are end-tethered or not. In astrong extensional flow field the former may orient perpendicularly to thestretching direction (entangled matrix stretching), whereas the latter will normallyorient parallel to the stresses. A two-stage orientation mechanism was observedin converging flow, but studies of the effect of flow on the orientation of nanofillerhave just begun.

elpmisrofsnoitciderpdesab-muunitnocfonosirapmoC15elbaT.sdiulfesahpitlumrofsnoitavresbolatnemirepxehtiwdiulf

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lacigoloehRnoitcnuf

diulfelpmiS diulfesahpitluM

taytisocsiVgnihsinav

etarnoitamrofed

lanoisnetxEytisocsiv

ηE = ηE )erusserpecnartne(]2791,llewsgoC[

ηE ≠ ηE )erusserpecnartne(

ssertslamrontsriFecnereffid

N1 = N1 )llewsetadurtxe(]0791,rennaT[

N1 ≠ N1 )llewsetadurtxe(

lim ( ˙ ) lim ( ) lim ( ˙) /˙ ˙γ ω ε

η γ η ω η ε→ → →

= ′ =0 0 0

3E lim ( ˙ ) lim ( ) lim ( ˙) /˙ ˙γ ω ε

η γ η ω η ε→ → →

≠ ′ ≠0 0 0

3E

344


The time-temperature superposition principle, t-T, has been a cornerstone ofviscoelastometry. It has been used to extend the customary 2-4 decades offrequency measurements to the whole range of variability of at least 15 decades.The t-T principle breaks down in any mixture with more than one type ofrelaxation time distribution, viz. in polymer blends (miscible or not), in LCPover the range of transition temperatures, or in polymer-filler system withtemperature-activated association modes. In CPNC the t-T principle is expectedto hold for the end-tethered systems over a limited (by the transition temperature)range of conditions. Evidently, any change of miscibility between matrix polymer,intercalant and/or compatibiliser will affect the t-T behaviour.

3.3.3 Rheology and Microrheology of Disc SuspensionsThe simplest rheological dependence for Newtonian suspensions is that of relativeviscosity ηr, versus volume fraction of the suspended particles, φ:

η η φ η φ η φr n

nk k= + [ ] + [ ]( ) + + [ ]( )−1 1

2

1K (104)

where the intrinsic viscosity, [η], depends on the rigidity and shape of thesuspending particles. The first of the equation parameters, k1 = kH is known asthe Huggins constant, indicating the interaction between the dispersed phaseand the matrix. In shear fields the particles rotate with the period dependent onthe rate of shear, γ , and the aspect ratio, p:

t p p= ( ) +( )2 1π γ/ ˙ / (105)

There are two conventions for defining the aspect ratio of ellipsoids. Accordingto one school, there is a continuity of transition from one type of ellipsoid ofrotation to the other and the aspect ratio is defined as a ratio of the major tominor axes; p´ = a1/a2. Thus, for prolate ellipsoids p´ > 1 and for oblate p´ < 1 withspheres being the intermediate step with p´ = 1. In this convention t-dependencepredicted by Equation 105 is perfectly symmetrical, giving the same period ofrotation for the rodlike ellipsoid with p´ = pr and for the disk-like ellipsoid withp´ = pd = 1/pr [Goldsmith and Mason, 1967]. However, this symmetry is notobserved for the intrinsic viscosity ([η] is a measure of hydrodynamic volume).According to the second convention p is defined as the ratio of the largest to thesmallest dimension, thus for rods: p = length/diameter > 1, whereas for discs,p = diameter/thickness > 1. The latter system is used in this book, thus p = p´ forfibres and spheres, whereas p = 1/p´ for discs.

Owing to the rotation, discs generate the maximum resistance to flow in oneposition (perpendicular to the flow direction) and the minimum in another. Forthis reason [η] is a periodic function of two angles of orientation in flow[Goldsmith and Mason, 1967], from which one can compute the upper and lowerbound as well as the time-averaged value. In their excellent chapter onmicrorheology the authors wrote the time-averaged intrinsic viscosity as:

η[ ] = ( ) ( ) −[ ]p p/ . ln .0 74 3 2 5 4 (106)

The experimental data of [η] versus p for anisometric particles gave empiricalrelationships for rods, discs or hard spheres [Utracki, 1989]. For discs with p ≤300 the following relation was found:

η[ ] = + −( )2 5 1. a pb (107)

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Rheology

where a = 0.025 ± 0.004 and b = 1.47 ± 0.03 with the correlation coefficientsquared, r2 = 0.9998 and standard deviation, σ = 0.622. Brodnyan [1959] adoptedMooney’s relation between relative viscosity and volume fraction of hard spheresand proposed an experimentally modified formula for prolate ellipsoids. At infinitedilution this formula has the same form as the relation between [η] and p givenin Equation 107 for discs, but with a = 0.399 and b = 1.48.

For freely rotating prolate ellipsoids (rods) with p > 20 the followingdependence was derived [Simha, 1940; Simha, 1952; Frisch and Simha, 1956]:

η[ ] = + ( ) −+ ( ) −

⎡

⎣⎢⎢

⎤

⎦⎥⎥

1415 5

13 2 4 5

12 0 5

2p

p pln . ln .(108)

while for the oblate ellipsoids (discs) with p >> 1 Simha [1940] derived:

η[ ] = ( )( )16 15/ / arctan p p (109)

Figure 89 shows that [η] very much depends on the shape and the aspect ratio ofanisometric particles. As is to be expected, the strongest enhancement of [η] withp is observed for rods (prolate ellipsoids or fibres). Oblate ellipsoids (disks) showintermediate behaviour between that of prolate ellipsoids and hard spheres. Fordisks the prediction by Simha virtually superimposes on that by Kuhn and Kuhn[1945] – they both form the upper bound for [η]. Goldsmith and Mason [1967]proposed a relation that gives a lower bound, while the experimental values arelocated in-between. The [η] is sensitive to polydispersity as well as to the regularityof shape.

Figure 89 Intrinsic viscosity for rods and discs versus aspect ratio. In the lattercase the theoretical, (time-averaged) and the experimental dependencies are shown.The Einstein value for monodispersed hard spheres, 2.5 for p = 1 is illustrated by

the horizontal line.

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The plotted dependencies are averages – when initially the particles are allaligned (e.g., in an electric field) and upon imposition of flow they start to rotate,the rheological response (e.g., shear viscosity or [η]) will vary with the periodgiven by Equation 105. Ivanov et al. [1982] reported on the periodic oscillatorynature of suspension viscosity in shear flow.

The above relations are valid within the high dilution region where the freerotation of anisometric particles is feasible. Detailed analysis of flow within thisregion is available [Goldsmith and Mason, 1967; van de Ven, 1989]. As theconcentration increases the particle-particle interaction becomes stronger andthe higher terms in Equation 104 have to be included. The relation breaks downonce the concentration of particles exceeds the limit of possible free rotation.This limit, φ > φmax, can be calculated assuming that the discs are circular and thevolume they require for free rotation is that of the encompassed sphere. For themonodispersed hard spheres and oblate ellipsoids the (experimental) maximumpacking volume fraction is φm ≅ 0.62 and φmax = 0.62/p. Empirically, for p < 50 aweaker dependence was found:

φmax . .= +( )−1 55 0 0598

1p (110)

As shown in Figure 90, the empirical dependence for φmax predicts higher values.The most probable reason for the disparity is the tendency of discs to align parallelto each other, even at low concentration (the same reason why, in suspensions,the spheres readily make doublets), thus prematurely forming the structuresexpected within the φ > φmax region, where the discs must adopt locally parallelorientation with the spacing dependent on concentration, d001 = a0 + a1/φ (seeFigure 44 in Section 2.4).

These simple relations are valid for suspensions without strong interactions.Dispersion of clays in aqueous media is complicated by the presence of electrostatic

Figure 90 Maximum packing volume fraction for freely rotating discs versusaspect ratio. Lower solid line – theoretical, assuming monodispersed particles,

upper broken line – empirical, Equation 1.

347

Rheology

forces, which results in the formation of 3D structures (e.g., house-of-cards) oraggregation [Jogun and Zukoski, 1996].

3.3.4 Similarity Between CPNC and Liquid Crystal FlowAs discussed in Section 3.1.5.3, the equilibrium thermodynamics predicts thatdepending on the interactions and concentration in CPNC six phase structuresare expected: isotropic, nematic, smectic-A, columnar, house-of-cards, and crystal.A similar list of phase structures is known for liquid crystal polymers (LCP):isotropic, nematic, cholesteric, smectic-A and smectic-C. However, while in CPNCplatelet orientation creates these diverse structures, in LCP the focus is on therod-like molecules or side groups with the transition between the phases dependenton temperature.

Liquid crystals are defined as systems ‘that are molecularly ordered yet possessmechanical properties resembling those of fluids’ [Porter and Johnson, 1967].Considering that it is more difficult to build disc-shaped molecules than rigid-rod type, the focus of LCP has been on the latter systems. Furthermore, there aresignificant difficulties in developing theoretical treatment for LCP with discmoieties [Ciferri, 1991].

To simplify interpretation of the flow behaviour, rheological tests are usuallycarried out either within a specific phase or at the phase boundary. There arenumerous publications describing behaviour of nematic LCP, modelled as a systemof rods, which during the shear flow may undergo periodic rotation. Rheologyof the smectic (layered grease structure) and cholesteric phases is not as welldocumented. However, it seems that there are similarities between the flowbehaviour of nematic LCP and CPNC, especially these containing end-tetheredmacromolecules.

Figure 91 Three regions of flow of nematic LCP. The viscosity versus shear ratedependence as generalised by Onogi and Asada [1980].

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In nematic LCP, the morphology is characterised by local orientation withineach domain, evident in rheo-optical studies. As shown in Figure 91, there arethree regions of flow for nematic LCP: (I) low deformation rate shear-thinningregion; (II) the (Newtonian?) plateau region; and (III) the power law shear thinningregion [Onogi and Asada, 1980]. Within region I, the polydomain structuredominates the flow. The authors remarked that here the rheological response ishighly variable, depending on the history of the sample, method of preparation,wall temperature, etc. Depending on these, the size of the domains and the extentof the interdomain interaction varies. Rheo-optics indicates that as the rate ofshear increases the intensity of transmitted polarised light also increases. However,only within region II does the polarised light show systematic oscillations ofintensity, indicating rotation of the nematic domains. Thus, while in region Ithere is strong interaction between nematic domains that rheologically mimicthe yield stress, within region II the domains are dispersed in a monodomaincontinuous matrix, which also dominates region III. In this region, initially domainflow proceeds by the tumbling motion, but as the deformation rate increases thetumbling is replaced by flow alignment.

One of the most intriguing characteristics of nematic LCP is the behaviour ofthe first normal stress difference, N1. Experiment and theory indicate that for theflow of nematic LCP there is a region of the deformation rates where N1 < 0 [Kissand Porter, 1980; Marrucci, 1991]. The change from the tumbling to flow-alignedstationary monodomain flow takes place at shear rates in the middle of the negativeN1 range. Theory also predicted negative values of N1 for the transitory responseafter start-up at low shear rates.

Owing to the diversity of CPNC structures one must be careful in comparingtheir flow to that of nematic LCP. In LCP the structure is defined by the molecularconstitution and temperature, while in CPNC it depends on composition, degreeof tethering and exfoliation. Thus, similarity of flow behaviour is expected forthe nematic LCP and diluted end-tethered CPNC, but not for the CPNC whereplatelets enrobed in intercalant are dispersed in a polymeric matrix. The threeregions of flow observed in LCP have also been observed in end-tethered PA-6/MMTsystems, but the negative values of N1 were not found [Utracki and Lyngaae-Jørgensen, 2002]. The reason may be greater difficulty in tumbling flow for thehairy clay platelets, HCP, the heterogeneity of sizes and structures responsiblefor a broad transition range, inherently weaker effects of the aspect ratio forplatelets than that for rods, unsuitable timescale of the experiment, experimentaldifficulties, etc.

Another similarity in the rheological response between CPNC and LCP is thetransient behaviour at start-up [Metzner and Prilutski, 1986]. During the steady-state shearing of hydroxypropyl cellulose solution in a cone-and-plate geometrythe shear stress, σ12, overshoot reached a value of 20 Pa then decreased to 12.5and after 250 s of shearing it increased to a steady-state value of 22 Pa. Evenstronger and slower responses were reported for N1. Over the years many studiesof stress overshoot have been reported. It was noted that the magnitude dependson the shear history – the longer the specimen is undisturbed the larger is thestress overshoot (up to a limit).

Two mechanisms could account for this: (1) orientation of anisometric particlesand (2) interaction between the domains. However, the consensus seemed toemerge that the latter process, a disruption of the polydomain structure duringflow, is the correct one [Viola and Baird, 1986]. The latter authors carried out in

349

Rheology

parallel rheological and WAXS studies of stress growth, interrupted stress growth,stress growth after flow reversal and stress relaxation. In spite of large rheologicaleffects, WAXS provided strong evidence that orientation effects were absent duringthe shear flow. The extensional flow produced orientation, but its changes occurredon a different timescale to those in the stress field. Thus, the disruption of theinteractive domain structure was proposed as the stress overshoot mechanism.

Pitches are notoriously complex comprising at least 461 identified chemicalcomponents, with molecular weights ranging up to 2 kg/mol [McNeil, 1983].Owing to unsaturation, some of the molecules have a rigid rod structure, whilesome others (formed by fusing 4 to 10 aromatic rings) are discotic, hence theirstructure may be considered intermediate between the rod-like LCP and CPNC.The presence of these anisometric molecules is responsible for the nematic pitchbehaviour. The anisometric compound content may be adjusted by extraction orby addition of synthetic compounds.

Fleurot and Edie [1998] studied the rheology of three mesophase pitches usedfor production of carbon fibres. The steady shear data placed them within regionsI and II of the Onogi and Asada classification. The transient shear response wasdetermined by pre-shearing the samples at γ = 0.5 s-1 for 2 min then letting thesample rest for 8 to 4000 s, and then restarting the flow. The results are shown inFigure 92. For all three samples the stress growth depended on composition. Thenormalised peak height increased with the rest time after pre-shearing, but fordifferent pitches the form of the dependence was different and a master curvecould not be generated. The determined domain size was large, decreasing withshear rate ( γ = 0.5 to 10 s-1) from a = 16 to 6 μm. The authors applied thedomain theory of LCP with a qualitative success.

Figure 92 Normalised stress growth peak for three mesophase pitches versus resttime before re-shearing at the rate of shear of 0.5 s-1. After [Fleurot and Edie,

1998].

350


Marrucci [1984] formulated the domain flow theory assuming a balance betweenthe alignment tendency of the velocity field and the elastic resistance to deformationof the director field. The Eriksen distortion stress, σE, was taken as proportional tothe elastic constant, K, and inversely proportional to the domain size. Noting thedomain size within the polydomain region I as ao and during the flow as a, thestress associated with the recoverable (elastic) energy was calculated as:

σE oK a a − −−( )2 2 (111)

While the average stress is given by this dependence, the flow depends on thelocal orientation – high velocity is expected in the region where the orientationdirector and velocity vector are parallel to each other, and low velocity for theopposite case. As a result, the rheological relation between the shear stress andthe deformation rate was scaled down to read:

σ η γ= −( )equil o oa a a˙ / (112)

Equating these two equations gives the relation between the reduced viscosity,η/ηequil, and the reduced deformation rate,

˙ ˙ /γ γ ηredu o equila K= ( )2 (see Figure 93a).

Figure 93b shows the complementary dependence of the relative domain sizereduction versus the relative deformation rate. The derived relation has twoasymptotic limits. At low deformation rates the initial slope is predicted to be –1/2(in contrast to the value of –1 predicted for the yield stress). This slower decreaseof the reduced viscosity with shear rate is in good agreement with manyobservations of LCP flow. The second asymptote is obtained at higher rates,where the plateau value of region II is recovered. Thus, as simple as the derivationis, it provides sound explanations for the shape of the flow curves of LCPs. Sincethe basic assumption was the presence of large domains with aligned particles,this treatment may be useful for interpreting the flow behaviour of CPNC systems.

3.3.5 End-Tethered versus Non-Tethered CPNCThe rheological studies on CPNC are quite recent. A relatively early publicationreported on the steady-shear flow of MMT-2M2T dispersed in either diphenyldimethyl siloxane or in polydimethylsiloxane (PDMS) [Krishnamoorti et al.,1996]. In the former system clay was intercalated, whereas in the latter it wasexfoliated. Characteristically, the flow curves for both systems showed a largeNewtonian plateau. As expected, the zero-shear viscosity, ηo, increased withorganoclay loading, but at high deformation rates matrix viscosity was recovered.The plot of ηo versus concentration is presented in Figure 94 and that of relativeviscosity ηr versus φ in Figure 95. The presence of a Newtonian plateau indicatesthat there is no 3D structure in these systems; hence the interactions betweenorganoclay particles are weak. However, the ηr versus φ dependence in the latterFigure shows that while for the exfoliated system (up to 13 wt% clay) thedependence is typical for diluted suspensions with high value of the intrinsicviscosity, [η] = 9.3, the intercalated system gives evidence of interactions at aloading of ca. 4 wt%. Furthermore, its value of [η] is lower than that calculatedfor the exfoliated system, indicating lower apparent aspect ratio, hence stackformation. Such behaviour is typical for suspensions. The nanoparticles, exfoliatedor not, behave as a filler – no surprise here.Krishnamoorti et al. also carried out preliminary studies on the flow behaviour ofCPNC with PA-6 as matrix. More information on the topic was given in the

~

351

Rheology

following publication [Krishnamoorti and Giannelis 1997]. Flow of three resinsfrom Ube was studied. These contained 0, 2 and 5 wt% of organoclay. The authorsalso measured the dynamic flow behaviour for CPNC of poly-ε-caprolactone (PCL)with up to 10 wt% organoclay. For the latter system validity of the time-temperaturesuperposition was reported. The Arrhenius plot of the frequency shift factor, aTversus 1/T did not depend on the clay loading. Thus, excepting the apparent yieldbehaviour at low frequencies, the matrix polymer dominated the flow.

Both these systems with PA-6 or PCL as a matrix were prepared bypolymerisation in the presence of intercalated clay, which engendered directbonding between the MMT surface and the macromolecules – a hairy clay platelet(HCP) structure. The authors labelled these systems as ‘end-tethered polymerlayered silicate nanocomposites’. The G´ and G´´ moduli were found to increasewith clay loading. Their power law dependence in the terminal zone was differentfrom that observed for homopolymers. At low frequencies (ω < 10 rad/s) the

Figure 93 Predicted by the Marrucci’s domain flow theory (a) apparent reducedviscosity and (b) relative domain size versus shear rate. See text.

352


Figure 94 Zero-shear viscosity of CPNC of dimethyl ditallow ammonium MMTdispersed in diphenyl dimethyl siloxane (intercalated) and in PDMS (exfoliated).

Data [Krishnamoorti et al. 1996].

Figure 95 The data from Figure 94 re-plotted as ηr versus φ. The exfoliatedclay has higher aspect ratio hence higher [η] and less particle-particle interaction

than the intercalated one. Data [Krishnamoorti et al., 1996].

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Rheology

initial slope of G´ and G´´ was found to decrease with increasing concentrationof organoclay. Thus for 0, 2 and 5 wt% loading:

g´´ ≡ (d log G´´/d log ω)ω < 10 = 0.93, 0.80 and 0.70, respectively.

These values should be compared to the expectations: for neat polymer g´ = 2and g′′ = 1, for the LCP domain flow g´´ = 1/2, while for systems with yield stressg´´ = 0. Thus, these CPNC behaved like a solution of LCP in a molten polymer,and not like filled melts with yield stress. At high strains the platelets may bealigned, which would greatly reduce the rheological response. In short, theseend-tethered systems behaved differently than could be expected from dilutedsuspensions (see also: Section 3.2.7).

A slightly different approach was used by Wagener and Reisinger [2003].Instead of using the initial slope of G´ and G´´ the authors calculated the ‘shearthinning exponent’ from the initial slope of the complex viscosity versus frequencydependence: n ≡ d logη*/d log ω. Evidently, the parameter is a reflection of the3D structure. There are many mechanisms responsible for such effects (e.g.,compatibilisation of immiscible mixture of polymers [Utracki, 2002a]), but inCPNC, when other factors are constant, the exponent n may reflect the degree ofplatelet dispersion. Indeed, for PBT with 4 wt% organoclay, empirically ncorrelated with Young’s modulus: Y(GPa) ≅ 2.5 – 1.2n.

It is to be expected that the rheological behaviour of the end-tethered systemswill differ from that where such direct bonding is missing [Hoffmann et al.,2000a]. The authors prepared two types of CPNC based on PS. In the first, theclay was intercalated with phenyl-ethyl amine, and then dispersed in PS, while inthe second the clay was exfoliated with amine-terminated PS. A plot of the storageshear modulus, G´ versus reduced frequency ωaT for the neat PS and PS withintercalated clay nearly superimposed one on top of the other – addition oforganoclay did not change the basic flow properties of the PS matrix. By contrast,the exfoliated, end-tethered CPNC (with amine-terminated PS) showed largeincreases of G´ at low ω, indicating a network formation.

In the next publication, the melt flow behaviour of two exfoliated CPNCswas studied. Two types of clay: synthetic fluoromica, FM, and mineral MMT,were used [Hoffmann et al., 2000b]. First, the clays were intercalated using eitherprotonated ω-amino dodecanoic acid (ADA) or water, then dispersed in ADA,which in turn was polymerised into PA-12. The use of ADA-intercalated clayresulted in the formation of end-tethered CPNC with platelets chemically bondedto the matrix, but with surprisingly small interlayer spacing, d001 ≤ 2 nm. The useof water as a swelling agent resulted in exfoliated nanocomposites, but withoutthe chemical bonds between clay and the polymer chains.

A stress-controlled rheometer was used in the parallel plate geometry atω = 1 to 25 rad/s assuring linear viscoelastic behaviour. Comparison of the neatPA-12 matrix flow with those for the two CPNC showed significant differences.The presence of a superstructure was deduced from the low frequency behaviour.The slopes of G´´ in the terminal region were: g´´ = 0.4 to 0.6 for the end-tetheredand 0.9 for the not-tethered CPNC, thus lower than those of PA-12 with similarmolecular characteristics. Furthermore, flow of CPNC with exfoliated but non-tethered clay platelets, was dominated (at least up to 4 wt% organoclay loading)by the matrix behaviour, with only a minor contribution from clay. Tetheringdramatically enhanced the storage and loss shear moduli, by one and by one-halfdecade, respectively. This observation is particularly important since XRD showed

354


that the end-tethered CPNC reached only a moderate level of intercalation.Evidently, the end-tethering has stronger influence on flow than exfoliation. Duringinjection moulding of these CPNC the clay platelets became oriented in theinjection direction [Kim et al., 2001b].

There is also evidence that significant modification of the flow behaviour (incomparison to classical filled systems) is possible without end-tethering of thematrix macromolecules. One of the examples comes from Ren et al. [2000]. Theauthors dispersed up to 9.5 wt% MMT (intercalated with 2M2ODA) in a matrixof polystyrene-b-polyisoprene (PSIR, Mw = 17.7 kg/mol). Linear viscoelastic flowwas studied at T = 80-105 °C. The t-T superposition required simultaneoushorizontal and vertical shifting of the shear moduli: bTG´ and bTG´´ versus aTω.While aT for all compositions followed the same WLF dependence, bT did not –there was a dichotomy between the matrix copolymer and CPNC sets of data,indicating different macromolecular structure engendered by the organoclay. XRDindicated intercalation with d001 expanded from 2.25 nm (in organoclay) to3.45 nm. Thus, these nanocomposites are neither end-tethered nor exfoliated.The stress relaxation data in the terminal zone (see Figure 96) showed a solid-like behaviour of these CPNC samples. The effect was particularly pronouncedat ≥ 6.7 wt% organoclay, resembling that observed for the exfoliated end-tetherednanocomposites. The solid-like behaviour was evident plotting G´ versus ω – atlow frequencies the storage modulus was nearly constant with g´ ≅ 0. The authorsattributed this behaviour to the presence of stacks of intercalated clay platelets,each stack randomly oriented, but forming a 3D network. A large-amplitudeoscillatory shear was able to orient these structures and reduce the solid-likebehaviour. However, that explanation may be partial. Dispersion of the sameorganoclay (MMT-2M2ODA) in polyisoprene (IR) did not change d001; hencethere was no diffusion of IR macromolecules into the interlamellar galleries.Evidently, during the dispersion of MMT-2M2ODA in PSIR it was the PS blockthat diffused into the interlamellar galleries, expanding them by Δd001 = 1.2 nm.This intercalation may suggest that there is a certain degree of interaction between

Figure 96 Stress relaxation, G(t), for PSIR and its CPNC. The solid lines werecomputed from dynamic moduli, G´ and G´´.

Reprinted with permission from [Ren et al., 2000]. Copyright 2000 AmericanChemical Society.

355

Rheology

MMT and the aromatic rings of PS. As discussed above, the non-tethered clayplatelets in the PS matrix showed limited influence on the CPNC flow. Thedramatic modification of the rheological behaviour shown in Figure 96 suggeststhat in the copolymer clay is preferentially dispersed in PS-block domains, phaseseparated from those of IR-blocks.

The data in Figure 96 also illustrate that the simple relation developed byFerry [1980] for a single component melt is also valid in these rheologicallycomplex systems:

G t G G Gt( ) = ′( ) − ′′( ) + ′′( )=1 0 4 0 4 0 014 10/ . . .ω ω ω ω (113)

Sometimes, the end-tethering may be replaced by strong interactions between polargroups and the clay surface. Schmidt et al. [2000, 2002a] measured birefringenceand small angle neutron scattering (SANS) during Couette shear flow of claysuspensions in aqueous polymer solution. Thus, 3 wt% of synthetic hectorite (FH;platelets with diameter d = 30 nm and thickness h ≅ 1 nm) was dispersed in anaqueous solution of 2 wt% polyethylene glycol (PEG, Mw = 103 kg/mol) at pH = 10and a NaCl concentration of 1 mmol/L. The birefringence indicated mechanicalcoupling between clay platelets and PEG. At low rates of shear, ˙ γ γ< −

critical s30 1,birefringence was dominated by the clay platelets, but at high by the polymerchains stretched in the flow direction. SANS data indicated that at ˙γ γ> critical theflow was strong enough to induce chain orientation. The clay platelets (withinaggregates with diameter d = 32-233 nm) were oriented in the flow direction withthe surface normal in the neutral (not radial) direction. In a later publication theauthors used SANS to study the effects of shear on clay platelet orientation [Schmidtet al., 2002b]. As the rate of shear increased the clay platelets progressively oriented.At the highest flow rates the macromolecules became stretched in the flow direction.

Lim and Park [2000; 2001] prepared CPNC by dispersing up to 10 wt% ofMMT (intercalated with 2M2HT; Cloisite® 6A) in either PS or in PS-co-MA(containing 7 wt% MAH). Compounding in an internal mixer increased d001from 2.94 nm (Cloisite) to 3.45 and 3.37 nm for PS and PS-co-MA, respectively.In spite of the small difference of the interlayer spacing the low frequency dynamicmoduli showed large differences, viz. G´(10% clay, ω = 0.02) ≅ 0.1 and 6 kPa,with the corresponding differences in the initial slope of g´ = 0.59 and 0.24,respectively. Shearing these highly loaded samples at γ = 120%, ω = 1 rad/s for30 min reduced the effects of association, but did not eliminate them entirely.

The cited examples demonstrate that instead of a sharp distinction betweenthe classical, filler-like influence of organoclays and the effects assigned to theend-tethered CPNCs, there is a continuous spectrum of rheological behaviour.The rheological response is associated with the ability of dispersed particles oraggregates to interact with each other. In the presence of electrostatic interactionsin aqueous media the platelets readily form edge-face ‘house-of-cards’ structures,even at low loading of ca. 5 wt%. However, in organic media there is a doublelayer of semi-solid organic molecules that shields the silicate surfaces. Here, theformation of 3D structures is possible by two mechanisms – crowding, as inclassical composites, or by entanglements. In the latter case the strongest effectsare to be expected when the terminal groups directly bond macromolecules tothe clay surface. However, entanglements may also be promoted by the use ofblock copolymers or by forming associations between the clay surface and polargroups of the polymer chain. Further examples of the effects of tethering will befound in the following parts.

~

356


3.3.6 Fourier-Transform Rheology of CPNCApplication of the Fourier transform methods in rheology is at least 30 years old[Wapner, 1971; Wapner and Forsman, 1971]. While the authors were interestedin extracting the true linear viscoelastic response from vibrating reed experiments,their approach was general, incorporating all possible modes. During the lastfew years Fourier-transform rheology (FTR) has gained more prominence mainlyas a tool for the analysis of non-linear viscoelastic response in polymeric systemssubjected to large amplitude oscillatory shear (LAOS). FTR is capable of detectingand measuring the higher harmonics that in the past were only qualitativelycharacterised by, e.g., the Lissajou stress-strain loops [Wilhelm, 2002].

In view of the complex rheological behaviour of CPNC it seems thatapplication of this method may provide a suitable tool for the characterisation ofthe non-linear viscoelastic response in CPNC as well as for measurements of thekinetics of orientation effects.

FTR analysis starts with writing the absolute magnitude of the shear rate inthe dynamic flow field, with frequency ω and strain amplitude Ao, as:

˙ cosγ ω ωt A to( ) = (114)

The Fourier transform of this dependence yields an expression with only evenharmonics:

˙ cos cosγ ω ωt a a t a to( ) ∝ + + +1 22 4 K (115)

Thus, the stress being a product of rate of shear and shear-dependent viscosity isexpressed as a sum of odd harmonics:

σ ω ω ω∝ + + +A t A t A t1 3 53 5cos cos cos K (116)

The simplest method for the analysis of the FTR signal is to plot the relativemagnitude of the odd harmonic peaks divided by the first: Rn(ω) = I(nω)/I1(ω),with n = 3, 5, 7, … For the linear viscoelastic fluids Rn(ω) = 0, whereas for non-linear materials Ri(ω) = 1/n, thus the strongest third harmonic, R3(ω), containsall the important information.

Wilhelm demonstrated that the strain dependence of R3(ω) follows two simplerelations:

R R kL L3 1ω γ γ γ γ( ) = − − −( ){ }[ ] >exp / ; (117)

R RB

C L3 11

1ω

γγ γ( ) = −

+ ( )⎡

⎣

⎢⎢

⎤

⎦

⎥⎥

>; (118)

In these relations R is a measure of the maximum intensity, γ is the applied strain,γL is the maximum strain for the linear viscoelastic response, while k, B and Care parameters characterising the relative intensity change of the 3rd harmonicpeak. The former relation, Equation 117, has been used to determine the limit ofthe viscoelastic linearity, whereas the latter provides a better fit to data withinthe full range of strains.

3.3.7 Rheology of CPNC with PA MatrixKrishnamoorti et al. [1996] studied the flow of two end-tethered CPNCs preparedby in situ polymerisation (in the presence of pre-intercalated MMT) of

357

Rheology

ε-caprolactone (PCL) and ε-caprolactam. In these systems the polymer chainswere end-tethered to the silicate surface via cationic intercalants. The linearviscoelastic properties, the large amplitude oscillations and orientation wereexamined. Unfortunately, the molecular weight varied with silicate loadings whichmade evaluation of the data a bit difficult. The t-T superposition principle wasfound to be valid and the master curves could be prepared in the whole range ofconcentration (up to 10 wt% clay). The authors reported that the nanocompositeswere readily aligned at large amplitude oscillatory shear. The alignment resultedin a change of slope within the terminal zone as well as in reduction of the dynamicmoduli by about one order of magnitude. Furthermore, a steep increase of thecomplex viscosity, η*, was reported during strain sweeps at low frequencies, ω = 1or 3 rad/s. Since the tanδ = G´´/G´ was reduced, the effect was related to increasesof G´, caused by enhanced interactions between the flow domains.

The industrial production of CPNC started with commercialisation of PA-6nanocomposites by Ube (on a license from Toyota). In spite of thirty-odd yearsof technology developments there have been relatively few publications dedicatedto melt flow studies of these systems. By contrast, several processes for themanufacture of PA-6 nanocomposites have been published [Okada et al., 1988;Deguchi et al., 1992; Okada and Usuki, 1995; Ube Ind., 2000]. The Toyota/Ubeprocess involves high temperature ring-opening polymerisation of ε-caprolactamin the presence of MMT pre-intercalated with ω-amino dodecanoic (lauric) acid(ADA). The molecular weight of the PA-6 matrix is about Mn = 22 kg/mol andthe organoclay content 2 wt%. The composition and properties of natural MMTmay vary with geographical location and local strata. However, an idealised Na-MMTunit cell can be written as:

Thus, the molecular weight of a MMT unit cell is Mu = 734 + water. The cationexchange capacity of the idealised MMT is CEC = 0.915 meq/g, with Na+ spacedca. 1.2 nm apart.

Assuming that every clay plate and every cation on its surface is available forthe exchange reaction of Na+ for ADA and that each lauric acid group starts thepolycondensation, leads to the conclusion that near the clay platelets the end-tethered PA-6 chains are densely packed. Since the molecular weight of PA-6 isMn = 22 kg/mol, the resulting clay content of the fully grafted hairy clay platelets(HCP) should be 4.74 wt%. As the clay platelets are 0.96 nm thick and theiraspect ratio is p = 287 (see below), each platelet would have about 140,000 end-tethered macromolecules, i.e., Mn ≅ 3×106 kg/mol of a single HCP. Thus,polycondensation that involves every Na+ on the Na-MMT surface results information of an entity that resembles a star-branched or dendritic macromolecule

Triple layer sandwich of two silicatetrahedron sheets and a centraloctahedral sheet with 0.67 negativecharge per unit cell

Aqueous interlamellar layer containing0.67 Na+ cations per unit cell

Al Mg Si O OH

n H O Na

3 33 0 670 67

8 20 4

2 0 67

. ..

.

[ ] ( )

×( )

−( )

+

c

358


of a very high molecular weight. Near the clay surface the macromolecules arecrowded and immobilised.

Polycondensation results in CPNC containing exfoliated clay particles withPA-6 macromolecules tethered to their surface. For the reason of dyeability theseCPNC need to be diluted with about twice the amount of neat PA-6. Alternatively,the grafting efficiency of the clay platelets may be reduced and HCP with Mn ≅ 1to 3×106 kg/mol are produced. To form these materials, standard processingmethods, e.g., extrusion or injection moulding, may be used. The improvementof the rigidity, tensile and flexural strength, heat distortion temperature and gasbarrier properties over neat PA-6 have been rationalised on the basis of the surfaceeffects, change in crystallinity as well as by the orientation effects imposed by theprocessing flow field.

Kojima et al. [1994] studied the orientation and crystallinity of extruded PA-6CPNC calendered films containing 0.18, 0.46 and 0.74 vol% of clay. XRD, TEMand DSC measurements were carried out. It was found that clay platelets as wellas the γ-crystals of PA-6 had planar orientation. The orientation direction wasindependent of the clay content, but the degree of orientation increased with it.It seems that during the flow through a T-die followed by subsequent stretchingbetween chilled rolls the clay platelets became in-plane oriented, then the matrixpolymer crystallised with the chain axes parallel to the clay surface. The γ-formof PA-6 has four monomeric units in the monoclinic unit cell with hydrogenbonding between parallel chains. This form is often associated with the formationof extended chain crystals during processing involving elongational flow.

In the following paper the authors reported on the orientation engendered byinjection moulding of CPNC into an end-gated, 3 mm thick mould cavity [Kojimaet al., 1995]. Here the MMT content was 2.2 vol%. XRD and TEM measurementswere carried out. In Figure 97 the 002-reflection peak intensity of γ-PA-6 versusinjected bar thickness, z, is shown. Three regions of orientations are evident:skin with in-plane orientation for the PA-6 macromolecules as well as for theclay platelets, intermediate with chains perpendicularly oriented to the clayplatelets and the flow direction, and the centre layer where the macromolecularchain orientation is perpendicular to the clay surface, and clay platelets areprimarily oriented perpendicular to the flow direction.

It is interesting that during the flow through rolls that engenders film stretchingor during fountain flow in the mould cavity the macromolecular chains of PA-6get oriented in-plane and crystallise with the chain axis parallel to the clay surface.However, at lower stresses the lamellae are oriented along the clay surface withchains oriented perpendicular to it. Kojima et al. speculated that this is the naturalbehaviour of PA-6 chains, highly crowded near the clay surface caused by thehigh grafting density.

More recently, Medellin-Rodriguez et al. [2001] studied SAXS/WAXSorientation of the same Ube CPNC during steady-state shearing between parallelplates at γ = 60 s-1, and T = 240 °C for up to 20 min. Compression moulded filmspecimens, 0.25 mm thick and containing 0, 2, and 5 wt% of MMT were used.At this relatively low shear rate and at a temperature near the melting point therewas a gradual change of clay platelet alignment (see Figure 98). Owing to thevorticity component in the shear stress matrix, the end-tethered clay platelets areexpected to tumble with the period given by Equation 105. The increasingscattering intensity (in the direction perpendicular to the shear field) indicates

359

Rheology

Figure 97 XRD peak intensity of γ-PA-6 versus injected bar thickness, z. Threeregions of lamellar orientation are shown: skin with in-plane orientation,

intermediate with perpendicular orientation and central (see text).Data [Kojima et al., 1995].

Figure 98 MMT platelet orientation in PA-6/MMT-ADA nanocomposites with 0, 2and 5 wt% of MMT-ADA as a function of the steady-state shearing time at

γ = 60 s -1, and T = 240 °C. Data [Medellin-Rodriguez et al., 2001].

360


progressive orientation in the shear vector. Evidently, at φ < φmax the tumblingmotion of the clay platelets continues, but since the motion is periodic with longresidence time in the preferred direction the overall platelet orientation is in theflow direction (e.g., see Goldsmith and Mason [1967]). The authors also reportedthat randomisation of the clay orientation after cessation of shear is slow,substantially slower than the relaxation of polymer chains. Thus, at T = 240 °Cit takes at least 12 min to randomise the platelets, whereas the PA-6 relaxationtime is about 0.4 s. In consequence, crystallisation during normal processingconditions is bound to preserve the flow-induced clay orientation in the product.

3.3.7.1 Effects of MoistureThe Ube PA-6 and PANC containing 0 and 2 wt% organoclay, respectively, werethoroughly studied in dynamic and steady-state shear flow [Utracki and Lyngaae-Jørgensen, 2002]. Prior to compounding or testing the material was dried for48 h at 80 °C under vacuum. As shown in Figure 99, these conditions weresufficient to achieve about 97% of the equilibrium complex modulus, G*. Thetwo commercial resins were blended in proportions of: 0, 25, 50, 75 and 100 wt%PANC. The studied samples (two ‘as received’ and five extruded) are listed inTable 52.

The rheological properties of polyamides are known to depend on themeasurement time, e.g., see [Khanna et al. 1996] and references cited therein.This reversible change is related to the variation of the moisture content andchanges of molecular weight associated with it, i.e., to reversibility of thepolycondensation-hydrolysis reaction. For these reasons, first the time sweepswere measured under a blanket of N2 at T = 240 °C, frequency ω = 6.28 red/s andstrains γ = 10 and 40% for 1 h. After testing, the specimens did not show signs ofoxidative reactions – they remained off-white, indicating that the increase of the

Figure 99 Complex shear modulus as a measure of the PA-6 drying time undervacuum at 80 °C. Data [Utracki and Lyngaae-Jørgensen, 2002]. The line follows

the exponential dependence: log G* = ao – a1 exp{-a2/t} with ao = 2.87 ± 0.01;a1 = 0.43 ± 0.01; a2 = 1.4 ± 0.1 and the standard deviation σ = 0.02; r2 = 0.99995.

361

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362


shear moduli was caused by polycondensation. The response was found to beindependent of frequency (ω = 0.1 to 100 rad/s) and strain (γ = 10 or 40%). Thedata were fitted to a second order polynomial, with t being the sweep time (in sec):

′ = ′ + ′ + ′ ′ ≡ ′ = ′ + ′

′′ = ′′+ ′′ + ′′ ′′ ≡ ′′ = ′′+ ′′

G G G t G t R dG dt G G t

G G G t G t R dG dt G G t

o

o

1 22

1 2

1 22

1 2

2

2

; /

; /(119)

In spite of the fact that before loading into a rheometer the specimens were well-dried,during the 1 h in the rheometer the shear moduli, G´ and G´´, increased by a factor of2 to 5 and 1.4 to 1.8, respectively. From these changes the rates of G´ and G´´ increase,R´ and R´´, were calculated (see Equation 119, Table 52 and Figure 100).

As shown in Figure 100, the rates R´ and R´´ are both positive hence the twomoduli increase with time, but increasing the organoclay content caused an increaseof R´ and a decrease of R´´. In other words, addition of organoclay accelerates thetime-induced increase of G´, but it slows down that of G´´. This dual effect isindicative of two parallel mechanisms: (1) polycondensation and (2) interactionbetween the hairy clay platelets (HCP). Owing to the presence of the residual lowmolecular weight amines (introduced during the intercalation and/or polymerisationsteps) an addition of organoclay slows down polycondensation. As the concentrationof clay in CPNC increases, the interactions between the HCPs increase as well.Since these time effects were independent of the test conditions, it was concludedthat the explored variables had negligible effects on orientation – the effects werechemical and thermodynamic in nature. As a consequence, the parameters listed inTable 52 were used to recalculate all the subsequent rheological data to the initialtime. In consequence, the strain sweep, frequency sweep and other data reportedbelow are freed of these time effects.

Figure 100 Concentration dependence of the initial rate (data extrapolated tot = 0) of G´ and G´´ changes. Lines represent the least-squares fit. Data [Utracki

and Lyngaae-Jørgensen, 2002].

363

Rheology

3.3.7.2 Strain Effects

The strain sweeps were conducted in the range γ = 0 to 100% at ω = 6.28 rad/sfor about t ≤ 10 min. At this relatively high frequency the compositions containing100, 75 and 50% of CPNC show viscoelastic non-linearity at strains of γ ≥ 12 to20%. The strain effects on G´ and G´´ were well approximated by the KBKZ-typenon-linearity expression:

′′( ) = ′′ + ′′ − ′′[ ] +G G G Go f f fγ γ γ γ γ/ ; : /1 1001

22

3 strain fraction (120)

Knowing the parameters of this relation permits calculation of the straindependence of G´ and G″ for any composition (at ω = 6.28 rad/s). Evidently, thelargest strain effects are expected for the sample with the highest concentrationof clay, i.e., for PANC. However, even here at γ = 50% the reduction of G´ andG″ is relatively small: 15 and 7.5%, respectively.

3.3.7.3 Dynamic Flow CurvesTo determine whether CPNC obeys the t-T superposition principle, the sampleswere first pre-sheared then scanned from ω = 100 to 0.1 rad/s at T = 230 to260 °C. As shown in Figure 101, good superposition was achieved. Neglectingthe lowest frequency data for G´ (at the limit of the equipment sensitivity) theslopes for G´ and G´´ are 1.18 and 0.90, respectively, hence already within thepower law region.

According to Ferry [1980] the frequency shift factor, aT, depends on the freevolume fraction, f:

log ; a B

f ff hT

o

= −⎡

⎣⎢

⎤

⎦⎥ ≈1 1

(121)

Figure 101 Time-temperature superposition for Ube PANC at the four indicatedtemperatures

364


where B is the equation constant, fo is the free volume fraction at a referencetemperature, To. It has been shown that f is well approximated by the Simha-Somcynsky hole fraction, h. Under ambient pressure, the S-S coupled equationsyield the following dependence [Utracki and Simha, 2001a]:

h T T T T T r= − + + ≡ =0 0921 4 89 12 56 0 999992 2. . ˜ . ˜ ; ˜ / *; . (122)

The value of the reducing parameter for CPNC in Table 46 is: <T*> = 11307 ± 54.Substituting Equation 122 into Equation 121 gives:

log. . ˜ . ˜ . . ˜ . ˜

log /

a BT T T T

or a a a a T a T

To o

T o

=− + +

−− + +

⎡

⎣⎢⎢

⎤

⎦⎥⎥

= + + +( )

1

0 0921 4 89 12 56

1

0 0921 4 89 12 56

1

2 2

1 2 32

(123)

It is noteworthy that for a2T >> 1 (i.e., well above the glass transition temperature),the dependence leads to the Arrhenius equation.

Substituting numerical values into the Equation 123 (B = 0.3817 was used)gives the prediction shown as a dotted curve in Figure 102. Both dependencies(the experimental and calculated) seem to follow the Arrhenius dependence withthe activation energy of flow:

ΔHη = R[d log aT/d (1/T)] = 18 kJ/mol.

Figure 102 Temperature dependence of the frequency shift factor for PANC (seeFigure 101). Comparison between the experimental and computed (from Equation

123) dependencies.

365

Rheology

The highest value of log aT corresponds to T = 230 °C, i.e., about 10 °C aboveTm(PA-6) = 220 °C, thus some pre-crystallisation and/or stress-induced structuralchange is to be expected.

The frequency dependence of the dynamic viscosity (η´ = G´´/ω) is shown inFigure 103. The frequency sweeps were conducted at T = 240 °C, strains γ = 40%,from ω = 0.1 to 100 or from 100 to 0.1 rad/s. For PA and the mixture containing25 wt% PANC, the shear moduli did not depend on the scan directions. Thus,scanning by either increasing or decreasing frequency, or scanning in the samedirection but starting from different frequency, produced different rheologicalresponses and the observed differences increased with clay loading. These shearhistory effects were well reproduced within ± 2% by two operators who usedtwo rheometers and a variety of specimens. A Newtonian plateau was observedonly for PA and the mixture containing 25% PANC – the higher is the clayconcentration the higher is the negative slope at ω < ωc. However, even for neatPANC the slope is small, ca. –0.1, smaller than what could be expected for thedomain flow. Evidently there are interactions between HCP in this region, but ata clay loading of 0.64 vol% their strength and/or intensity are relatively low.

To analyse the frequency dependence of G´´ the data at γ = 10% scanned inboth directions and these at 40% scanned from 100 to 0.1 rad/s were fitted to theKrieger-Daugherty type dependence, rewritten for dynamic flow [Utracki, 1988]:

′ ≡ ′′ = + ′′[ ] ≡ ′ = + ′′( )⎡⎣⎢

⎤⎦⎥

− −

η ω η ψ ω ψη ψG G G G G Gcorr o

m

corr o

m

/ / ; / /1 112

2 2 (124)

The dependence was derived for linear viscoelastic, pseudoplastic systems; henceit is unable to describe the yield stress. The least-squares fit of Equation 124 todata is shown in Figure 103. Excepting PA all compositions showed some ‘solid-like’ behaviour at ω < ωc ≅ 1.4 ± 0.2 rad/s. Thus, only data above ωc could beused to determine the parameters of Equation 124 (see Table 53).

Figure 103 Frequency scans for extruded PANC/PA mixtures at 240 °C at γ = 40%from 100 to 0.1 and from 10 to 0.1 rad/s (different specimens); points –

experimental, lines – Equation 124 [Utracki and Lyngaae-Jørgensen, 2002].Reproduced with permission, copyright Springer 2002.

366


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367

Rheology

The presence of the critical frequency, ωc, at which the HCP associationsvanish, is worth commenting. The transition is independent of the clay content.Thus, it is related to the relaxation time of the aggregates, or by analogy to theLCP-type flow, to the domain size. The effect is associated with the formation ofa structure that, at the selected level of T and γ, breaks at ω = ωc. This correspondsto the relaxation time of τaggr = 4.5 s, i.e., significantly longer than that for PA-6:τPA = 0.2 s. Identifying ωc with the transition from Region I to Region II of theLCP-type behaviour and using the Marrucci [1984] relation:

˙ /γ η≅ K ao equil

2 (125)

with the elastic constant K ≅ 10-11 (N), leads to the estimated value of the domainsize for the three compositions (PANC, 75 and 50%) of ao = 310 nm. This maybe a coincidental agreement, but as will be shown below the aspect ratio, p =296, was calculated from the barrier properties and suspension viscosity – thevalue gives the bare platelet diameter d = 275 ± 9 nm, quite close to the domainsize in CPNC flow.

It can be shown that for incompressible, linear viscoelastic liquids there is aninterrelation between G´ and G´´ through the relaxation spectrum, H(λ):

′ =( )

+ ( )′′ =

( )+ ( )

∞∞

∫∫GH d

GH d

/ ; /ωλ λ λ

ωλω

λ λ

ωλ2

2 200 1 1

(126)

Since these functions are valid in the whole range of relaxation times, they arealso valid within narrow ranges inside this interval, say from λ = t to (t + Δ), thus:

′( ) =( )

+ ( )= ′′( ) ≤ ≤ +G

HG t t/ / ; ω

ξ ξ

ωξξ ω ξ2

21

Δ Δ (127)

The mean value theorem only requires that the integrals be continuous and theinterval Δ small, hence the proportionality between the two moduli should bevalid in the full range of the relaxation time and frequency. For Maxwell fluidsthis means that:

′( ) = ′′( ) ( ) = ′( )G G or/ / ω λ ω ψ ω λη ω2 (128)

where λ is the main relaxation time. Thus, the simple Maxwell model representsthe flow behaviour for fluids in which the ratio of parameters in Equation 124:m2/m1 = 2. A similar prediction can be obtained using other fluid models proposedby, e.g., Oldroyd, Spriggs, Bird-Carreau, Bogue, Meister, and others. However,as the data in Table 53 show, this condition is approximately observed only forPA-6 and the 25% PANC mixture – for the other systems m2/m1 ≅ 5.5, hencethese systems are rheologically complex.

The deviation from linear viscoelastic behaviour at ω < ωc ≅ 1.4 ± 0.2 rad/s isdramatically evident in Figure 104, where the ratio: G´/G´´ is plotted as functionof G´´. The plot is suggested by the Doi and Edwards [1978] theory predictingthat the log G´/G´´ versus log G´´ dependence for ‘well behaving’ liquids shouldfollow a straight line with the slope of 1:

ln / ln ln /′ ′′( ) = ′′ + ( )G G G M RTe6 5ρ (129)

368


Figure 104 displays the log G´/G´´ versus log G´´ relations for all PA/PANCcompositions, frequencies and strains. The predicted dependence is close to theobserved only for neat PA-6. For the clay-containing samples the deviation fromthe expected linearity increases with the nanoparticle content, hence it most likelyoriginates from the interparticle interactions that more strongly affect the storagethan the loss modulus. In simple words, addition of tethered clay particles to aPA-6 matrix results in higher values for the stored energy than expected from asecond order fluid.

3.3.7.4 Apparent Yield Stress

The increasing value of G´´ as the frequency decreases below ωc = 1.4 ± 0.2 rad/s isrelated to the formation of a 3D structure, that may be treated as an apparentyield stress. One can extract the yield function: Y ≡ η’exp/η’lin = G´´exp/G´´lin, fromwhich the apparent yield stress can be calculated as: σy(ω) = (Y - 1)G´´lin. Someyears ago, for compatibilised polymer blends, a theory for the dynamic yieldstress was proposed [Utracki 1989]. The conceptual model assumed formationof dynamic aggregates of the dispersed drops. The strength of the drop-to-dropinteraction was σ y

o , the relaxation time of the dynamic aggregate was τy, and theexponent u accounted for the aggregate polydispersity:

σ ω σ τ ωy y

oy

u( ) = − −{ }[ ]1 exp (130)

This model may be also applied to CPNC. For end-tethered CPNC the interactingentity is the HCP. Here the polydispersity of size is related to polydispersity ofthe aspect ratio, which for CPNC is narrower than that found in polymer blendshence u ≅ 1 may be postulated.

Figure 104 Inverse loss tangent versus G´´ for PA/PANC mixtures at T = 240 °Cand ω = 0.1 to 100 rad/s. Solid symbols for γ = 10%, open symbols for g = 40%.The predicted slope = 1 is also shown. [Utracki and Lyngaae-Jørgensen, 2002].

Reproduced with permission, copyright Springer 2002.

369

Rheology

The yield stress function at ω < ωc is presented in Figure 105. The data arewell represented by Equation 130 with σ y

o = σy(φ), τy = 0.59 ± 0.03 and u ≅ 1.The solid-like structure formation starts at about 20 wt% PANC reaching amaximum value of σ y

o = 23 Pa for neat PANC. It is noteworthy that the onset ofthe yield stress takes place at a MMT concentration of about 2.5 times lowerthan that calculated for the platelet maximum packing fraction. Thus, the 3Dstructure formation originates from the presence of the end-tetheredmacromolecules – it is of the chain entanglement type.

3.3.7.5 Zero-Shear Viscosity and the Clay Aspect Ratio

The zero-shear viscosities (ηo) in Table 53 are a function of the (computed fromPVT data) hole fraction, h. The dependence (see Figure 106) follows the generalrelation [Utracki, 1983; Utracki and Simha, 2001b; Utracki, 2002a]:

ln ; /ησ = + ≡ +( )a a Y Y a ho s S1 21 (131)

where: ησ indicates constant stress viscosity, ai are parameters and h is the holefraction in the Simha-Somcynsky eos. For n-paraffins a1 = 0.79 ± 0.01 and a2 = 0.07was found. For CPNC analysis the parameter a2 (which is only needed to linearisethe dependence) was assumed to be zero. The data in Figure 106 show about thesame rate of viscosity increase with decrease of h for CPNC as that found forn-paraffins.

The values of ηo have been also used to determine the intrinsic viscosity fromEquation 104. Its value, [η] = 105.5 ± 22.5 (k1 = kH = 0.52 ± 0.058, and themeasures of fit: σ = 0.0674 and r2 = 0.9983) were then used to calculate theaspect ratio, p for the MMT from Equation 107, p = 287 ± 9. The value is ingood agreement with that calculated from the relative permeability for oxygen, p= 286 for the PANC resin.

Figure 105 Frequency dependence of the yield stress function Y ≡ η’exp/η’lin = G´´exp/G´´linfor PA/PANC mixtures at T = 240 °C. Lines computed from Equation 130.

[Utracki and Lyngaae-Jørgensen, 2002], copyright Springer 2002.

370


3.3.7.6 Flow-Induced OrientationAt frequency ω > ωc and strain γ = 10% the rheological signals were the same forscans up or down the frequency. This however was not the case for γ = 40% – atthese higher strains the orientation effects became important. The highest degreeof orientation was expected at the highest frequency where all the flow curvesfor CPNC collapsed into a single dependence.

To verify these expectations TEM was used on two PANC specimens, onedynamically sheared at ω = 100 rad/s and γ = 40% for 15 min, and another alsoinserted into a rheometer, but not sheared (see Figure 107). To determine the clayorientation, the specimens were microtomed close to the disc border (maximumshear strain) in the planar and circumferential directions of the moulded disc[Perrin, 2002]. In the first specimen the clay platelets (uniformly dispersed, about1 nm thick) were found oriented parallel to the disc thickness (the lowest resistanceto the dynamic shearing). In addition to single platelets, small stacks of 2 to 3platelets were also observed – in the micrographs these resembled tree brancheswith a single Y- or a double ψ-branching, which may suggest a crystallographicfault of the mineral clay. The slices microtomed parallel to the surface showedmainly cross-cuts of the MMT platelet and few in-plane platelets (average size:470 x 220 nm) suggesting that platelets were not perfectly aligned. The TEM ofa not-sheared specimen showed a random orientation. Characteristically, in thiscompression moulded specimen most of the MMT were bent – more so thanthose in the sheared specimen.

The orientation plays a major role at the start-up of rheological tests, e.g., ina steady-state or dynamic shearing. At low deformation rates (see Figure 108(a)), e.g., γ = 0.003 or 0.01 initially there is a rapid increase of the shear stressfollow by a moderate increase caused by polycondensation. At higher deformation

Figure 106 Constant stress dynamic viscosity of PANC mixtures with PA asfunctions of the (computed from PVT) hole fraction – see text. The zero-shearviscosity as well as dynamic viscosity at constant stress (G´´ = 50 MPa) show

similar behaviour.

371

Rheology

Figure 107 TEM of PANC from Ube shows ‘in plane’ orientation of MMTplatelets. Orientation in not sheared specimens was random, with many bent clay

platelets [Perrin, 2002].

rates (Figure 108 (b)) the signal goes through a local minimum followed by thepolycondensation effect.

As was the case for LCP, interrupted stress growth studies have also beencarried out on these CPNC systems. The work is usually performed in threestages: (1) pre-shearing; (2) allowing the system to rest for a specific time; (3)shearing either in the same or the opposite direction. The most interesting resultsare obtained within the Onogi-Asada plateau Region II.

Figure 109 shows the stress growth functions for two PANC specimens. Both

were identically pre-sheared for 300 s at γ = 0.1 s-1. Next, the shearing stoppedfor either 600 s (specimen (1)) or 1200 s (specimen (2)), and then they were

sheared for 300 s, at γ = 0.1 s-1, but in the opposite direction. The data werefitted to a distribution curve:

η η= + ( )equil ob t

a t a1 (132)

The parameter b provides a measure of the breath of distribution: tw/tn = 1 + 1/bor tz/tn = 1 + 2/b, etc. The parameter ao is a measure of the orientation effect,while a1 is a measure of the rate with which the overshoot dissipates, i.e., theplatelets re-align with the field. The dependence very well described the observedcurves – it is difficult to distinguish the experimental points from the computedlines. The maximum values from the stress overshoot experiments are plottedversus the rest time in Figure 110. The quality of fit as well as the fitting parametersare listed in Table 54.

Differentiating Equation 132 provides the conditions for the local extremumof the function at tmax, from which a simple relation for the incremental increaseof shear viscosity is obtained:

ln ln ln ln ; / lnmax maxΔη η η≡ −( ) = + −( ) = −equil oa b t t b a1 1 (133)

Thus, for rest times of trest = 0, 600 and 1200 s, values of Δη = 0, 124 and 171were obtained. As shown in Figure 110, the data follow a single exponentialdependence. As the peak height increases it becomes narrower, thus the value ofb increases and the dispersion parameter, tw/tn decreases.

372


3.3.7.7 Steady-State Flow Curves – Shear History EffectsDuring the dynamic flow studies it was noted that the rheological responses variednot only with strain and frequency, but also with the frequency scanning direction.Since the effects of polycondensation were extracted prior to data plotting, theseadditional variations reflected changes in orientation and domain size.

Figure 108 Stress growth function of PANC at T = 240 °C and at γ = 0.01 (a) or0.1 (b). In the first case shearing in the cone-and-plate tooling was for 10 s only, in

the second for t = 1000 s.

373

Rheology

The steady-state shearing at γ ≤ 10 s-1 was carried out in a cone-and-plategeometry, increasing the shear rate from the initial value with ‘wait time’ betweenthe consecutive data acquisition of Δt = 360 s. Starting at γ = 0.001 s-1, initiallythe viscosity increased, then it decreased to the plateau value before reaching thepower law dependence. When the sweep started at γ = 0.01 s-1 the viscosity values

Figure 109 Two PANC specimens pre-sheared at γ = 0.1 s-1 for 300 s: (1) relaxedfor 600 s then sheared for 300 s, at the same rate, but in the opposite direction; (2)

had the rest time twice as long. The curves computed from Equation 132 arevirtually indistinguishable from the experimental dependence.

Figure 110 The incremental increase of shear viscosity for PANC versus the resttime after pre-shearing. As shown, the increase follows a single exponential.

374


immediately started to decrease toward the plateau then power law regions.However, it is important to note that in these two cases the plateau levels weredifferent by about 10%. Apparently, the initial shearing of the specimen in thefirst experiment for 1975 s more than in the second one was responsible for thedevelopment of larger domains yielding a higher plateau value. This effect wasadditional to that of the time-dependent polycondensation.

Next, the flow curves were determined starting at the same γ = 0.01 s-1, butchanging the wait time between data points, Δt from 0 to 540 s (see Figure 111).The flow behaviour systematically varied with Δt. The slower was the sweeptime, the better defined and the higher was the Region II plateau. As the figureillustrates, similarly well-defined plateaux were found by plotting N1/ γ versus γfor these five rate sweeps. It is noteworthy that proportionality between N1 and

γ was observed for LCP at low deformation rates for a concentrated LCP solutionin cresol [Kiss and Porter, 1980; Moldenaers and Mewis, 1992]. Proportionalitybetween N1 and γ was also observed for colloidal suspensions, block copolymers(especially those with higher molecular weight blocks) and multi-branched starpolymers [Kotaka and Watanabe, 1987; Masuda et al., 1987]. The Larson andDoi [1991] theory for polydomain flows predicts that:

N p pd d1 12

2 1 2 2 22 1 1 1/ ; //

σ λ λ= −( ) = +( ) −( )−(134)

where λ is a characteristic ‘tumbling’ parameter given by the domain aspect ratio,pd. For thermotropic LCP the value of this parameter is λ = 1.01 to 1.05 indicatingthat N1 > σ12 [Ugaz et al., 2001]. This is not the case for CPNC in Region IIwhere the ratio N1/σ12 < 1 and consequently λ = 4.4, hence the equivalent ellipsoidaspect ration pd ≅ 1.6. At the higher shear rates, in the beginning of the power

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a1 3669.0 ± 1000.0 1469.0 ± 5000.0 8079.0 ± 2000.0 4959.0 ± 2000.0

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tw t/ n b/1+1= 331.3 645.2 844.3 750.2

375

Rheology

law Region III, for LCP negative first normal stress was observed; this behaviourwas not found in CPNC.

It has also been reported that in solutions of hydrophobically modified alkali-soluble emulsions G´ is nearly a linear function of ω, and in concentratedsuspensions of non-colloidal spheres N1 is proportional to γ [Brady and Bossis,1985; English et al., 1997]. Proportionality of N1 to σ12 (with the proportionality

Figure 111 (a) Shear viscosity and (b) a ratio of the first normal stressdifference and the deformation rate, N1/ γ , for PANC at 240 °C. A series of theindicated wait time between the data points, Δt = 0 to 540, was used. See text.

b

376


factor being characteristic of the system) was also reported for the latter system[Zarraga et al., 2000].

3.3.7.8 Fourier Transform Analysis of CPNC

The FTR method was used to analyse the rheological signals from an ARESrheometer for the two resins from Ube, PA-6 and PANC, at 240 °C. The followingindependent variables were used: strain, γ = 20-70%; frequency, ν = 0.1, 1.0 and10 Hz; and the shearing time, t ≤ 1100 s. For the analysis the computer programdeveloped by Manfred Wilhelm was used. Within this range of variables PA-6was found to follow linear viscoelastic principles, thus the tests focused on thePANC resin. Figure 112 shows the raw data for PANC as a function of frequency.The imposed test frequency was ν = 10 Hz. As evident from the figure, the intensityof the first peak, I1, is about 100 times stronger than that of the third harmonic,I3. From such plots the relative intensity, R3(ω) = I(3ω)/I1(ω) was calculated. Theresulting map is shown in Figure 113.

The dependence shown in Figure 113 will not be a surprise to rheologists – ithas been known that non-linearity increases with strain and frequency (note thatfor CPNC at 0.1 Hz the non-linearity was so small that I3 was difficult to measure).However, the value of FTR is in quantification of these influences, as well as inshowing the influence of the shearing time on the evolution of 3D structure. Innanocomposites as in LCP, the term ‘structure’ encompasses two types ofcontribution: orientation and association. The time dependence of R3 shown inFigure 113, most likely originates from the latter effects.

3.3.8 Rheology of CPNC with PO MatrixLim and Park [2001] studied the dynamic flow behaviour of exfoliated (accordingto XRD) CPNC with PE as the matrix. The system was prepared by mixing in aninternal mixer poly(ethylene-g-maleic anhydride) with up to 10 wt% of Cloisite®

6A (MMT-2M2HTA) for 10 min at 210 °C. The PE-MA resin had MW = 212 kg/moland contained 0.8 wt% MA. The dynamic flow data showed systematic increasesof the dynamic moduli as well as of their slopes within the terminal region. Asshown in Figure 114, the ratio G´(CPNC)/G´(matrix) strongly increases withclay loading and decreases with test frequency.

The authors also oriented the CPNC (with 10 wt% organoclay), shearing it atγ = 120% (LAOS). The tests were conducted until a plateau was achieved. Theresults for the PE-MA system were compared with those obtained for PS and PS-MAwith the same amount of Cloisite® 6A. The principal difference between thesethree CPNC systems was the degree of exfoliation – at 5 wt% organoclay loadingPE-MA was exfoliated whereas the other systems were intercalated with d001 =3.45 and 3.37 nm, respectively.

The time to reach the plateau in the LAOS experiments with PE-MA was tplat= 7200 s, whereas for PS and PS-MA system it was four times shorter. Furthermore,the frequency scan after alignment showed that G´ for PS-system approached thematrix dependence, whereas for PS-MA the G´ versus ω dependence wasintermediate between that for non-aligned CPNC and that of the matrix. Theauthors also reported that G´ in aligned or non-aligned CPNC with PE-MA asthe matrix showed a similar dependence, especially at low frequency. As thefrequency increased, the dependence slowly drifted toward that for the matrix.Judging by the relatively fast alignment of the two styrenic CPNCs, it seems that

377

Rheology

Figure 112 Fourier transform rheology, FTR, data for PANC at 240 °C, after 600s of shearing at 70% strain with imposed frequency of 10 Hz. The harmonic peak

(I3) at 30 Hz is evident.

Figure 113 Relative intensity of 3rd harmonic peak, I3/I1, for PANC at 240 °C.The plot illustrates how the non-linearity varies with strain, frequency and

shearing time.

378


in spite of high clay content the dispersed assemblies were relatively large andhad low aspect ratio. The high value of tplat agrees with the image of delaminatedplatelets hindered in their rotation by crowding of the encompassed volumes.Furthermore the observed drift of G´ versus ω dependence after alignment towardthat of the matrix indicates that alignment is easier at higher frequencies than atω = 1 rad/s, which was used in LAOS. Considering the long period of plateletrotation (see Equation 105) the frequency most likely affected the extent ofinteraction between platelets without forcing them to rotate.

Similar CPNCs, containing PE-MA and silicates of different aspect ratios,were studied by Wang et al. [2002]. The PE-MA was LLDPE grafted with0.85 wt% MAH. The silicates were two organoclays (Cloisite® 20A, C20A andLaponite® SCPX2231, SCPX) and a synthetic SiO2. The organoclays were bothintercalated with 2M2HTA and had the nominal aspect ratio, p = 100 to 200and 20 to 30, respectively. The silica had spherical particles with diameter of ca.1.8 μm. XRD did not show diffraction peaks down to 2θ = 2°; hence d001 > 4.4 nm.As it is to be expected, the initial slope, g´ = d lnG´/d lnω, a measure of deviationfrom linear viscoelasticity, decreases with nanofiller content and aspect ratio.Note that the matrix alone showed a lower value than that expected for a linearviscoelastic liquid (g´ = 2), indicating that the system is phase segregated, withMA domains acting as local crosslinks.

The data in Figure 115 indicate that g´ decreases with clay aspect ratio andconcentration. Other factors, such as the degree of exfoliation and potentialassociation of the intercalated clay platelets have also been identified [Lepoittevinet al., 2002a,b]. Association is suspected in the system containing MMTintercalated with MT2EtOH (Cloisite® 30B).

Figure 114 Relative storage moduli of PE-MA with Cloisite® 6A at T = 210 °C.Curves for frequency ω = 0.12 and 119 rad/s are shown. Data [Lim and Park,

2001].

379

Rheology

While the end-tethered CPNC with PA-6 as a matrix showed rheological behavioursimilar to LCP, other CPNC may show behaviour resembling that of filledpolymers with yield stress. The review by Giannelis et al. [1999] provided severalexamples of such behaviour. For example, the steady-state shearing of apoly(dimethyl0.95 diphenyl0.05 siloxane) containing up to 35 wt% of MMTintercalated with 2M2HTA (Cloisite® 6A) showed a steep increase of low-shearviscosity with clay loading [Krishnamoorti et al., 1996]. In this case exfoliation(suppressed at high loadings) seems only to intensify the known filler effects.However, the authors also studied end-tethered PCL and PA-6 nanocompositeswith exfoliated MMT. Judging by the reported log G´ versus log ω plots, at aMMT loading of 1 to 3 wt% the behaviour resembled that described in thepreceding parts for PA-6 based CPNC (LCP-type), but at a clay loading of 5 and10 wt% the CPNC behaved as a filled-system. Note that at these high loadingsthe platelets are crowded, unable to tumble along the flow lines.

Galgali et al. [2001] prepared CPNC by melt blending PP (82-97 wt%;MFI = 3 and Mw = 300 kg/mol) with maleic anhydride grafted PP (PP-MA,Polybond 3200) as compatibiliser and Cloisite® 6A (MMT-2M2HTA). The ratioof PP-MA to organoclay was either 0:1 or 1:1. The mixtures were characterisedby TEM and XRD at T = 200 °C. Dynamic flow measurements were performedat T = 200 °C and ω = 0.06 to 100 rad/s on samples annealed in a controlledstress rheometer for t = 0 to 3 h. The ‘terminal’ slope of log G´ versus log ω at thelowest frequencies, ω = 0.06 to 1 rad/s, depended on the compatibiliser and claycontent as well as on annealing time (see Figure 116). The data were analysedusing the linear function:

′ ≡ ′( ) = + + +

→ −g d G d a a w a w a to o PP MA claylim log / logω

ω0

1 2 3 (135)

Figure 115 The initial slope of the storage modulus versus frequency dependence,g′≡ (∂lnG′/∂lnω)T for LLDPE-MA with two organoclays C20A and SCPX (withaspect ratio: p ≅ 100-200, and 20-30, respectively) and with silica (p = 1). Data

[Wang et al., 2002].

380


The data fit generated the following set of parameters:ao = 1.48 ± 0.09; a1 = -0.085 ± 0.013; a2 = -0.043 ± 0.015; and a3 = -0.028 ± 0.027 withthe standard deviation σ = 0.15 and the correlation coefficient squared, r2 = 0.986.Evidently, the dependence is dominated by PP-MA then by the organoclay content,but (within the statistical error) not by the annealing time. This strong variationof the terminal slope is noteworthy considering that, according to XRD, thesystem was only intercalated with d001 = 3.3 nm. On the other hand, TEM showedthe presence of large clay aggregates, probably along with a few exfoliatedplatelets. Since addition of PP-MA did not change the interlayer spacing, thecompatibiliser most likely coated the exterior of the intercalated stacks of MMTplatelets. Thus, the reduction of the terminal G´ slope is most likely related to theformation of interactive 3D structures, but not to LCP-like behaviour.

Next, creep experiments were carried out at T = 200 °C. For CPNC with9 wt% MMT, changing the shear stress from σ12 = 10 to 2000 Pa reduced theviscosity by nearly four decades – in the vicinity of σ12 = 1 kPa. The latter valuewas identified as an apparent yield stress – apparent, since even at the lowestimposed stress the viscosity was measurable (η ≅ 107 Pas). This behaviour isconsistent with a 3D structure formed with interacting domains, formingaggregates with their own relaxation times (see Equation 130).

Creep measurements were also carried out at σ12 = 10 Pa (for 3% and 6%clay) and 50 Pa (for 9% clay). The data were collected every 15 min for a periodof 3 h during which the sample was annealed inside the rheometer. The creepcompliance was significantly lower for CPNC specimens containing MA-PP. Forthe specimens containing 6 and 9 wt% of PP-MA and organoclay the effectincreased with the annealing time. Evidently some structure build up was takingplace under the low stress creep. It is noteworthy that dynamic yield stress has

Figure 116 The initial slope of the storage modulus versus frequencydependence, g′≡ (∂lnG′/∂lnω)T for PP/PP-MA/ Cloisite® 6A. Upper line for CPNC

without compatibiliser (PP-MA), lower for PP-MA content the same as that ofCloisite® 6A. Data from [Galgali et al., 2001].

381

Rheology

been observed for compatibilised blends that, prior to compatibilisation, showeda regular pseudoplastic behaviour with an upper Newtonian plateau. Themechanism responsible for the CPNC dynamic yield stress may be similar to thatfor blends – enhanced interactions between the dispersed domains that formed adynamic, percolating 3D network.

Extracted from the creep data the zero shear viscosity, ηo, increased with thePP-MA and clay content by two to three orders of magnitude over that of thematrix resin, but the activation energy of flow, Eη = 33.4 kJ/mol, was found to beunchanged. The compatibilised CPNC also showed a solid-like response thatapparently originated from the interactions between compatibilised clayaggregates. The presence of MA-PP increased the probability of 3D structureformation (the solid-like rheological response) similar to that observed for theend-tethered chains.

Solomon et al. [2001] reported on the linear and non-linear rheology of CPNCwith PP as the matrix. The samples were prepared by melt mixing of organoclay, PP(Mw = 246 kg/mol and Mw/Mn = 6.1), and compatibiliser (PP-MA, Mw = 92 k g/mol,Mw/Mn = 2.6 MA = 0.43 wt%). The weight ratio of organoclay to compatibiliserwas 1:3. The organoclay was prepared by cation exchange ofNa-MMT (d001 = 1.1 nm) with stearyl amine (C-18), tri-decyl amine (C-13), ordi-tri-decyl amine (2C-13). The inorganic content in these CPNCs was variedfrom 1.30 to 6.17 wt%. Melt mixing was carried out in an internal mixer underN2 at T = 175 °C for 40 min. XRD measurements indicated that intercalationabout doubled the original interlayer spacing in Na-MMT (from d001 = 1.1 toabout 2.3 nm), but compounding with PP and PP-MA induced only small changes.

As shown in Table 55, at a loading of 4.8 wt% clay, depending on theintercalant the interlayer spacing changed by Δd001 = 0 to 0.8 nm. Addition ofclay significantly increased G´ and G´´, but the time-temperature superpositionprinciple was found to be obeyed with the same horizontal shift factors (aT) asthose for PP. Evidently, increasing the amount of organoclay increased themagnitude of the dynamic moduli. Furthermore, as one might have expected, therheological response depended on the type of intercalating onium salt, but it issurprising that (see Figure 117) the increase correlates so well with the interlayerspacing. It is difficult to comprehend how such a small change of the interlayerspacing (Δd001 ≤ 0.9 nm!) could be responsible for the 40-fold increase of G´,especially since these systems were only ‘mildly’ intercalated. It may be that atthe high loading (4.8 wt% inorganic clay) the interactions between clay and

serutximnidnasyalconagrotaenfognicapsreyalretnI55elbaTnomoloS[ataD.AM-PP/PPhtiw late ]1002,.

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:fotlasmuinomma yalconagro CNPC

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382


intercalant parallel those between intercalated stacks in the PP matrix – the latterare responsible for the rheological response.

To study the viscoelastic non-linearity Solomon et al. used shear flow reversals.First, a CPNC specimen was sheared for 300 s at γ = 0.005 to 1.0 s-1 recordingthe value of the shear stress (σ12), then the flow was stopped for a time, trest, andthe specimen was re-sheared at the same rate of shear, but in the opposite direction.As has been observed for LCP and CPNC with PA-6 as the matrix, the magnitudeof the stress overshoot (σmax) increases with trest. The authors constructed mastercurves by plotting: (σmax/σ∞ - 1)/c versus trest (where σ∞ is the value of σ12 at longtime and c is MMT loading). The linear scaling with c indicates that during non-linear deformation the stress response originates in the individual clay domains,thus the network is destroyed by deformation. The stress overshoot is related tothe distribution of particle orientations in CPNC and the stress-induced destructionof the network. Furthermore, the linear scale suggests that the domain size isindependent of loading. It is interesting to note that, not unexpectedly, themagnitude of the stress overshoot depended on the rate of shearing (seeFigure 118). The anisometric structure of the CPNC was considered responsiblefor the non-linear flow behaviour.

CPNC of PP with organoclay and PP-MA was prepared by melt compounding[Li et al., 2003]. The ratio of compatibiliser (containing 0.31 wt% of MAH) toorganoclay was 3. Strong non-linear viscoelastic behaviour was observed atorganoclay loadings exceeding 2 wt%, i.e., 8 wt% of organoclay + PP-MA. Theinterlayer spacing only slightly expanded from that of neat organoclay, viz. fromd001 = 3.5 to 4.2 nm. The strong non-linearity in the dynamic response indicatedthe formation of a 3D structure readily altered by pre-shearing. Similarly, as forPA-based CPNC, (see Figure 109 and Figure 110) stress overshoot was observed.

Figure 117 Storage shear modulus at constant frequency (G´ at T = 180 °C and ω= 10 rad/s) versus interlayer spacing, d001, for PP with 4.8 wt% MMT intercalated

with (from the bottom left) tri-decyl amine (C13), di-tri-decyl amine (2C13), amixture of these (C13+ 2C13), and stearyl amine (C18). Data [Solomon et al., 2001].

383

Rheology

Considering the low degree of clay dispersion, it is unlikely that the origin of theobserved non-linearity is the platelet/platelet interaction. Similarly, since the degreeof PP-MA maleation was low, it is doubtful that it phase-separated from the PPmatrix. Thus, the most probable structure responsible for the non-linearviscoelastic behaviour is the interaction between domains of organoclay tactoidsembedded in a cloud of the compatibiliser, the latter bonded to the clay platelets.

Okamoto et al. [2001a,d] studied the rheology of CPNC with PP-MA as thematrix. The system was prepared by melt compounding at T = 200 °C maleated-PP (PP-MA, 0.2 wt% MAH) with 0, 2, 4 and 7.5 wt% of MMT intercalatedwith stearyl ammonium ion (ODA). TEM showed fine dispersion of the silicatestacks ca. 193 to 127 nm long and about 5 to 10.2 nm thick, respectively.According to XRD the interlayer spacing of the organoclay increased from onlyd001 ≅ 2.31 to 3.24, 3.03 and 2.89 nm, respectively. The PP crystalline lamellaethickness and spherulite diameter did not depend on clay content. For therheological study the rotating clamps, Meissner-type elongational RME rheometerwas used at T = 150 °C and Hencky strain rates ε = 0.001 to 1.0 s-1.

The stress growth function in elongational flow, log ηE+ versus log ε for the

three CPNC compositions showed strain hardening (SH). The latter function isdefined as a logarithm of a ratio of the stress growth function in elongation tothree times that for the linear viscoelastic response in shear, with both valuestaken at the same deformation time, t, viz.:

SH E S

t≡ ( )+ +log /η η3 (136)

SH has been observed for entangled polymers (e.g., LDPE), for highlypolydispersed resins (e.g., some LLDPE where Mw/Mn = 10 to 50) as well as forthe new bimodal metallocene PO. Partial crosslinking as well as dissolution ofultrahigh molecular weight polymer into its standard resin are also known toinduce SH. The phenomenon is essential for several processing operations, viz.

Figure 118 The ratio of the maximum stress overshoot to its plateau valueversus the rate of shear, γ . Data [Solomon et al., 2001].

384


foaming, film blowing, blow moulding, wire coating, etc. As exemplified inFigure 119 by data for the branched polycarbonate of bisphenol-A, PK, for asingle-phase polymer, plot of SH versus Hencky strain, ε ε= ˙t , does not dependon the strain rate and the value of its slope is characteristic for the material.

There is a striking difference in the SH behaviour for a single-phase polymerand CPNC. In the latter case (see Figure 120) only the high strain rate resultsfollow the customary straight-line dependence. However, even in this range thereis no superposition of data taken at different Hencky strain rates, ε . For lowerrates of straining the SH is significantly higher – it almost seems that for eachstrain rate there is a specific polymeric system with its own structure andrheological response. A cross-plot of SH at constant Hencky strain versus strainrate gives a simple dependence: SHε=const = ao + a1log ε . One may speculate thatthe flow disrupts a network of interacting domains, and then it orients them.

A similar enhancement of SH was reported by Kotsilkova [2002] for CPNCof PMMA with 10 or 15 wt% of smectite pre-intercalated with methyl diethylpropylene glycol ammonium ions. The nanocomposites were prepared by radicalpolymerisation of MMA in the presence of organoclay. The elongational viscosityand birefringence were measured at 180 °C, using a rotating clamp, optorheometerdesigned by Kotaka at strain rates, ε = 0.01 to 1.0 s-1 at Hencky strains ε = 0.8 to3. The author noted a correlation between SH and birefringence – as the formerincreased so did the latter.

Owing to disturbance of the stress distribution pattern by suspended solidparticles, the extensional flow of classical composites shows not SH, but itsopposite, strain softening [Takahashi, 1996]. The SH reported by Okamoto etal. for CPNC signalled the basic difference in rheological behaviour betweenmacro- and nanocomposites. Evidently, the nanosize of clay platelets may be oneof the elements that differentiate these systems, but the end-tethering (engenderedby interactions between clay and maleic anhydride moieties) might also play arole. Furthermore, as the authors observed, there are significant structural changesduring flow. At low extensional or shear flow rates, e.g., ε = γ = 0.001 s-1, a‘house-of-cards’ structure was observed under TEM. At high extensional flowrate, ε = 1.0 s-1, the platelets were found oriented perpendicularly to the stretchdirection. Formation of either structure requires energy input; hence increasedviscosity is to be expected.

Another important observation of Okamoto et al. is that the linear viscoelasticenvelope of the stress growth function in shear was about one decade lower thanthat measured in elongation. This type of behaviour is expected from multiphasesystems with yield stress [Utracki, 1995]. Furthermore, unlike single-phasepolymer melts, the low deformation rate ( γ = ε = 0.001 s-1) stress growth functionsof CPNC (in shear or elongation) increase with test time, t ≤ 300 s, not showinga tendency to reach a steady-state. The authors concluded that flow-inducedinternal structure is different in shear than in elongation.

3.3.9 Foaming of CPNCIt has been recognised that SH stabilises flow during the processing steps thatinvolve elongational flows, particularly during extrusion foaming. Owing to thepoor SH of classical isotactic PP, the resin was difficult to foam. The CPNCs ofPP-MA (with 0, 2, 4 and 7.5 wt% organoclay) discussed above were autoclave-foamed with supercritical CO2 at P = 10 MPa and T = 130.6 °C to 143.4 °C,which was well below the melting temperature of CPNC and PP-MA [Okamoto

385

Rheology

Figure 120 Strain hardening parameter, SH, versus Hencky strain at indicated ratesof extension, ε , for CPNC of PP-MA with 4 wt% of C18-MMT (d001 = 3.03 nm).

Data [Okamoto et al., 2001]. The absence of superposition of data for constant valuesof ε indicates the presence of a 3D network of interacting organoclay particles.

Figure 119 Typical strain hardening plot for a single-phase polymer, viz. forbranched PC at 270 °C [Utracki and Sammut, 2000; unpublished].

386


et al., 2001a, d; 2002; Nam et al., 2002]. The foams of PP-MA and CPNC with2 wt% organoclay had closed cell structures with pentagonal and/or hexagonalfaces. For the higher clay concentrations, the cells had closed spherical structure.The foamed CPNCs had a high cell density of 107-108 cells/ml, the homogeneityof cell size was in the range of 20-120 μm, the cell wall thickness was 5-15 μm,and the low mass density was 0.05-0.3 g/ml. Within the cell walls clay particleswere biaxially aligned along the cell boundary. TEM showed that MMT plateletswere oriented parallel to the wall surface. In the junction of three adjacent cellsthe platelet orientation near the walls was parallel to the proximate one with arandom orientation in the centre of the three-cells junction.

However, it seems that SH is not the only mechanism responsible for enhancedfoamability of nanocomposites – it has been shown that CO2 foaming of eitherPMMA-based CPNC or neat PMMA geometrically constrained 75-100 μm thickfilms follows a similar mechanism and leads to a decrease of cell size and increaseof cell density [Siripurapu et al., 2002].

The batch foaming of PP-based CPNC (clay content 2, 4, and 7.5%) wascarried out in a high-pressure cell equipped with a microscope and high-speeddigital video recording camera [Taki et al., 2003]. Significant reduction of thebubble size (from 155 to 34 μm) and increase of cell density (from 2.5 to 220 cells/nL)has been observed. Furthermore, the foam compression modulus increased by afactor of 6.These observations were confirmed in the sequel publications. Thus,preparation of micro- and nanocellular foams in PLA-based CPNC has beenaccomplished [Fujimoto et al., 2003; Ray and Okamoto, 2003]. Foaming ofPLA/organoclay nanocomposites has been conducted using supercritical CO2.Compared to neat PLA foam, the CPNC foam showed smaller cell size and highercell density, confirming that the dispersed silicate particles acted as nucleatingsites for cell formation and the increase in SH stabilised the bubble growth. Thebiaxial flow-induced alignment of the platelets along the cell boundary also hada positive effect on the strength of the bubble wall, hindering coalescence. PC-based CPNC were also successfully foamed [Mitsunaga et al., 2003]. The relativelyeasy foamability of CPNC with supercritical CO2 to give materials with smallcell diameter (nanofoams with 200 nm diameter bubbles have been produced)may be in part due to the high solubility of CO2 in most polymers. Even in thecase of insolubility (e.g., PEG swells but does not dissolve in CO2) its presenceincreases the free volume content, thus macromolecular mobility, which on theone hand leads to the expansion of interlayer spacing [Zhao and Samulski, 2003]and on the other to higher foaming rates.

As recent events demonstrate, CPNC foaming is close to commercialisation.For example, PU-based CPNC foams with submicron size cells combiningexceptional mechanical, thermal, and barrier properties have been identified asprime candidates for high efficiency insulation in refrigerators [Domszy, 2004].

The patenting activities also reflect the high expectations from this technology.For example, a general patent for foaming CPNC was applied for [Lee et al., 2003].Its claims are quite broad, specifying a diversity of polymers (PS, PMMA, PP, PA,PU, elastomers, and their blends), of organoclays (of MMT, HT, FH, saponite,laponite, and beidellite), incorporated in amounts ranging from ≤ 0.5 to ≤ 20 wt%,and using from ≤ 1 to ≤ 7 wt% of a blowing agent (supercritical CO2). The resultingclosed or open cell foams should have cells with diameter ranging from 15 to20 μm, and cell density from 106 to 109 cells/ml. Extrusion or batch foaming qualitydepends on the CO2 content, melt temperature, and pressure drop rate.

387

Rheology

3.3.10 Rheology of CPNC with PS and Styrenics MatrixCPNC with PS or a styrene copolymer as a base have been prepared bypolymerisation (in solution, suspension or bulk) or by melt processing methods.Because of the amorphous matrix these CPNCs are advantageous as models forstudies of, e.g., mechanical or barrier properties, without the complications dueto crystallinity. However, as will be evident from the few cases discussed below,exfoliation of clay in a PS matrix is difficult and it requires careful considerationof chemistry. Furthermore, as discussed in Section 3.2, the work with PSnanocomposites is seriously complicated by the thermal decomposition of thequaternary intercalant, which in the presence of oxygen leads to formation ofperoxy radicals that in turn cause degradation of the PS matrix. Thus, in spite ofextensive academic studies these systems are not commercialised.

Hoffmann et al. [2000a] prepared two types of CPNC by intercalating syntheticfluoromica (FM; Somasif ME-100; CEC = 0.7-0.8 meq/g) with either amine-terminatedPS (ATPS; Mn = 5.8 kg/mol) or 2-phenyl-ethyl amine (PEA; Mn = 121 g/mol).The intercalation was carried out in a THF/H2O solution at 40 °C. The driedorganoclays were compounded with the PS at 200 °C in a microcompounder for5 min. According to XRD, the interlayer spacing of FM, d001 = 0.95 nm expandedafter intercalation with PEA to 1.4 nm and with ATPS to > 4 nm. TEM of theCPNC showed the presence of large clay aggregates in CPNC with PEA (C-PEA), and full exfoliation in CPNC with ATPS (C-PS). In the latter system it wasestimated that the clay platelets of ca. 1 nm thick were about 600 nm long and100 nm wide. The dynamic rheological measurements showed a sharp differencein behaviour between these two systems. The presence of 5 wt% clay in C-PEAcaused G´ = G´(ω) to parallel the dependence of the matrix, with only a slightincrease caused by the presence of organoclay acting as filler. In the exfoliated C-PS system the low frequency slope in the terminal zone was about 0.5 (instead of2, as in the former case). The authors concluded that in the presence of ATPS theclay platelets formed a network. The observed large difference in behaviour wasascribed to the length of the intercalating compound. Accordingly, one may controlthe degree of intercalation/exfoliation by varying the chain length of the intercalantchain.

A more recent publication [Meincke et al., 2003] extended this work to aseries of compositions containing 0 to 10 wt% of FM pre-intercalated with eitherPEA or ATPS. For either system good time-temperature superposition wasobtained, with virtually identical WLF c1 and c2 constants as those for the PSmatrix. Analysis of the dynamic data showed a dramatic change in the van Gurpplot (plot of arctan G´´/G´ versus G*) – the dependencies for PS and for PS withFM-PEA were virtually identical, quite different from that of FM-ATPS.Furthermore, it was observed that the plateau modulus follows the dependence:

GN PSn0 1∝ / φ , where φPS is the PS matrix volume fraction, and the exponent n = 0

when FM-PEA was used, and n = 2 when FM-ATPS was incorporated. The hairyplatelet model (HCP) was postulated.

This experimental finding confirms the theoretical conclusions (seeSection 3.1.5) by Balazs and her colleagues [1999; 2000]. Their theoretical analysisof clay dispersed in a mixture of polymer with its functionalised homologueshowed that the most promising strategy for developing an exfoliated system isby using long-chain end-functionalised compatibiliser. Its chain length should besmaller but comparable to that of the matrix polymer.

388


The rheology of CPNC with a di-block copolymer of PS and IR (PS-IR; Mw =17.7 kg/mol; 44 wt% PS) was of interest to Ren et al. [2000]. The authors dispersedorganoclay (MMT with CEC = 0.90 meq/g, intercalated with a dimethyldioctadecyl ammonium ion, 2M2ODA) in a toluene solution of PS. The fiveCPNCs had clay content ranging from 0 to 9.5 wt%. According to XRD,intercalation increased the interlayer spacing of MMT from d001 = 0.95 to 1.3 nm,whereas solution blending of PS or PS-IR increased it further to d001 = 2.1 to2.5 nm, independently of the clay content. Thus, these CPNCs were onlyintercalated. Dynamic shear tests indicated that to achieve time-temperaturesuperposition both horizontal (aT) and vertical (bT) shifting was necessary. Whilea plot of aT versus T was common for all compositions, a similar plot for bTdegenerated into two dependencies: for PS-IR and for CPNCs. The authors alsoreported significant difference in the rheological responses for specimens orientedor not.

Considering the small degree of interlayer expansion, at first it is surprisingthat the dynamic data show concentration dependence of the G´ and G´´ slopesin the terminal zone. However, the reported observations indicate that only thePS part of the copolymer can be inserted into the interlamellar galleries, with thePI block being outside. Thus, the structure of these CPNCs is of stacks of MMTplatelets intercalated with 2M2ODA and PS-blocks surrounded by IR-blocks.Formation of the 3D structures responsible for the rheological behaviour in theterminal zone is due to the interaction between IR clouds. This situation verymuch resembles that of compatibilised immiscible polymer blends. Finally, it isworth recalling that, as shown in Figure 96, the stress relaxation data computedfrom the dynamic moduli span five decades and still follow the simple Ferry’srelation.

The work was extended to systems containing up to 5 wt% of an organoclay,either a fluorohectorite (FH) modified with 3MODA, MMT intercalated with2M2ODA, or laponite intercalated with 2M2ODA. According to XRD the degreeof dispersion increased in the order of decreasing aspect ratio – from FH toMMT to laponite. Similarly, as for PA-6 systems [Utracki and Lyngaae-Jørgensen,2002], here also the critical frequency, ωc, was observed – at ω < ωc viscoelasticnon-linearity was observed.

Fu and Qutubuddin [2000, 2001] started their work by preparing apolymerisable intercalant, vinyl-benzyl-dimethyl dodecyl ammonium(2MVBDDA), which was ion-exchanged with either Na-MMT or Ca-MMT (d001= 4.62 and 4.0 nm, respectively). The purified and dried organoclay was dispersedin styrene and copolymerised. XRD and TEM of the samples indicated fullexfoliation. However, only limited data on the rheological and mechanicalbehaviour were reported. The samples had a viscous gel structure with yieldstress and shear thinning behaviour.

Okamoto et al. [2000] used Na-MMT (CEC = 0.866 meq/g) intercalatedeither with oligo(oxy-propylene) diethyl methyl-ammonium chloride,[(C2H5)2(CH3)N+(O¯iPr)25]Cl-, or methyl- trioctil-ammonium chloride,[CH3(C8H17)3N+]Cl-. The intercalated clays (SPN and STN, respectively) weredispersed in MMA or styrene (St) via ultrasonication at 25 °C for 7 h then themonomer was polymerised. The organoclay content was 10 wt%. The meaninterlayer spacing of the neat organoclays was: d001 = 4.20 and 1.81 nm for SPNand STN, respectively. The spacing expanded in monomer suspension. However,polymerisation in a MMT matrix resulted in PMMA/STN nanocomposite with

389

Rheology

d001 = 2.66 nm, smaller by about 0.3 nm from the value in monomer suspension.SPN suspended in MMT was fully exfoliated, while PMMA/SPN nanocompositeshowed a small shoulder at d001 ≅ 4.55 nm. For the St/SPN suspension, a smallshoulder was found at d001 ≅ 3.85 nm, i.e., reduction of the interlayer spacing ofneat SPN. The PS/SPN nanocomposites showed strong diffraction peaks,demonstrating that in this case polymerisation leads to ordered intercalatednanocomposites.

The frequency sweeps for the suspensions showed two types of rheologicalbehaviour. The suspensions of intercalated MMA/STN and St/SPN systemsindicated solid 3D, gel-like behaviour with frequency-independent dynamicmoduli. By contrast, the exfoliated MMA/SPN suspension showed strongfrequency dependence. The data indicate that at 10 wt% loading in the monomerthe expanded, intercalated clay stacks strongly interact with each other. However,surprisingly, the exfoliation into individual platelets seems to eliminate theinteractions. For the polymerised CPNC systems only a viscoelastic temperaturesweep was carried out.

Kim et al. [2002] emulsion polymerised PS in the presence of Na-MMT highlyswollen in water. CPNC containing 0, 2, 5 and 10 wt% of clay were prepared.Polymerisation increased d001 from 1.195 nm determined for dry Na-MMT to1.511 nm measured for the three CPNCs. Thus, in these nanocomposites theinterlamellar galleries of MMT were expanded to h = 0.55 nm hence slightlylarger than estimated from the Flory diameter of the paraffinic chain (0.45 nm),but significantly smaller than that of PS (0.83 nm). As a result, the clay waspoorly dispersed in the PS matrix. The flow curves, log η versus log γ , showed asimple pseudoplastic behaviour, following the dependence:

η η λγ= + ( )⎡

⎣⎢⎤⎦⎥

−o

n1

1˙ (137)

Here, ηo is the zero shear viscosity, λ is the longest relaxation time, and n is apower law exponent. From the values of ηo the intrinsic viscosity listed by theauthors, [η] = 112 and the aspect ratio p = 270 was calculated. Thus, the assembliesof intercalated MMT platelets are highly anisometric. In consequence, scansconducted by increasing and decreasing the rate of shear resulted in a hysteresisloop, with the former scan data forming the upper and the latter scan data formingthe lower part of the loop. The size of the loop increased with clay concentration.

Three PS grades from Nova Chem with Mw = 310, 270, and 230 kg/mol weremelt compounded in a TSE (T = 200 °C, screw speed of 200 rpm, feed rate of5 kg/h) with 1 to 20 wt% of Cloisite® 10A (MMT-2MBHTA) [Tanoue, et al.,2003b]. It was found that each re-extrusion of CPNC reduced the zero-shearviscosity, ηo, by about 30%. This is particularly interesting since increasing theresidence time in a TSE by a factor of up to 10 did not reduce further the CPNCmatrix viscosity. Similarly, extrusion of neat PS had only a small (ca. 2%) effecton ηo. Thus, the combination of oxygen, temperature and organoclay wasresponsible for the degradation of PS.

The rheological properties of these CPNCs were measured in the steady shearand dynamic mode at 160, 200 and 240 °C. The time-temperature (t-T)superposition was found valid, with the horizontal and vertical shift factors beingnearly independent of the organoclay content and PS grade. The extrapolatedzero-shear viscosity and zero-shear storage modulus slightly increase withorganoclay content. According to the results of strain sweeps (γ = 0~100%,

390


T = 200 °C, ω = 6.28 rad/s) for γ > 40%, the storage and loss moduli of allspecimens (including PS) decreased with strain. The frequency sweeps showedthat the storage and loss modulus increase with organoclay content. At lowfrequency (ω < 0.1rad/s), the initial slopes of the log G´ or log G´´ versus log ω (g´or g´´, respectively) decreased with organoclay content, e.g., for 0 to 10 wt%organoclay g´ decreased from 1.9 to 1.6, and g´´ from 0.99 to 0.95. In steady-state shear flow tests in a capillary rheometer, the power law index of the shearviscosity decreased with increasing organoclay content. The extensional flowbehaviour of the PS matrix and the CPNC were also studied. It was found thatincorporation of the degraded organoclay resulted not in strain hardening, butin a small strain softening effect, similar to that observed by Takahashi [1996]for polymeric composites with solid particles.

Sohn et al. [2003], studied PS with Cloisite® 25A (MMT-2MHTL8). A solvent(chloroform) casting method was used to prepare CPNC with 0, 2 and 10 wt%of organoclay. Upon mixing with PS the organoclay interlayer spacing increasedfrom d001 = 1.94 to 3.27 nm. The dynamic flow measurements were carried outat 200 °C. In spite of the achieved expansion of the interlamellar galleries, therheological data show small increases of both, G´ and G´´ moduli – a behaviourtypical to composites, but not nanocomposites. Thus, similarly as in the work byRen et al. [2000], here also only a monolayer of PS could be inserted into theorganoclay gallery space.

As these studies on PS-based CPNC indicate, preparation of CPNC with PSas the matrix has been largely unsuccessful. The exceptions are the systems whereamine-terminated PS was used as intercalant (as in publications from Friedrich’sor from Qutubuddin’s laboratories), to directly bond to the clay surface and toform a miscible blend with the matrix. Evidently, PS is immiscible with alkyl-intercalants. The macromolecular diffusion into interlamellar galleries most likelyoriginates in the tendency of aromatic molecules (i.e., benzene rings) to complexwith surface cations. However, once the first macromolecule is inserted, on theone hand the attractive sides are shielded and on the other the interaction betweenbenzene side groups and clay expose the paraffinic chain. As a consequence, theintercalated stacks of organoclay particles phase-separate from the matrix. Thesituation is worse when during melt compounding the intercalant decomposesand bare clay platelets re-aggregate into stacks with mineral clay spacing, e.g.,d001 ≈ 1.4 to 1.7 nm.

3.3.11 Rheology of CPNC with Other Polymer Matrix TypesOwing to solubility in water and polarity of statistical segments, polyethyleneglycol (PEG), poly-ε-caprolactone, polyacrylics, polyvinyl esters and theirhydrolysed versions, e.g., poly(ethylene-co-vinyl alcohol), have been frequentlyused for the preparation of CPNC in aqueous media. Recently, interest in thesesystems was sparked by their potential application as solid-state electrolytes.

Hyun et al. [2001] studied the flow behaviour of PEG/MMT. The authorsprepared these systems by solvent casting. Three organoclays: Cloisite® 15A(C15A; d001 = 2.96 nm), Cloisite® 20A (C20A; d001 = 2.47 nm), and Cloisite®

25A (C25A; d001 = 2.02 nm) comprised MMT (CEC = 0.95 meq/g; d001 = 1.23 nm)modified with either 2M2HTA (32% excess in 15A and no excess in 20A) or2MHTL8 (25A – stoichiometric). XRD showed that dispersion of these organoclaysin PEG resulted in expansion of the interlayer spacing by Δd001 = 0.52, 1.2 and1.03 nm, respectively. Thus the resulting CPNCs were intercalated, but not

391

Rheology

exfoliated – concentration had only a small effect on d001. TEM showed highlyanisometric aggregates, ca. 200 nm thick and several micrometres long.

Rheological properties of these systems were measured in the steady-stateand dynamic shearing modes in parallel-plate geometry at 120 °C. The steady-state data were fitted to the generalised Carreau-Yasuda equation:

η η λγ= + ( )⎡

⎣⎢⎤⎦⎥

−( )o

n

12 1 2

˙/

(138)

During steady-state shearing the systems showed a pseudoplastic behaviour.Addition of organoclay increased the shear viscosity – the strongest effects wereobserved for C15A, then C20A and the weakest for C25A. Evidently, increasingthe concentration of organoclay also increased the strength of the rheologicalsignal. From the cited values of ηo for the CPNC with C25A, the intrinsic viscosity[η] = 44 and the aspect ratio, p = 155 were calculated, confirming the TEMobservations of anisometric aggregates. Shearing while increasing and thendecreasing the deformation rate produced hysteresis loops, with the maximumloop height observed for the highest hs material near the rate of shear: γ 1/λ.The concentration dependence of the relative shear viscosity was fitted to theKrieger-Daugherty [1959] equation:

η η η φ φφ σ

η φr const

≡ = −[ ]→−[ ]

=/ / max

max

0 121 (139)

For the multiphase systems, this relation must be taken at constant stress, σ12 = const.For monodispersed particles, the maximum packing volume fraction, φmax, has aunique, theoretically predicted value. For polydispersed suspensions, its valuedepends on the polydispersity of size, shape and orientation of the dispersedparticles, thus it has to be treated as an adjustable parameter.

Dynamic measurements were carried out to analyse the CPNC structure withinthe linear viscoelastic region at strain γ = 0.03. G´ and G´´ were measured fornanocomposites containing C25A at clay content 0 to 17 wt%. The modulishowed a monotonic increase at all frequencies. The initial slope of the storagemodulus, g´ ≡ d log G´/d log ω < 2 was observed for all concentrations (includingneat PEG). A solid-like behaviour was particularly pronounced at w ≥ 9 wt% oforganoclay. This may indicate that within this region, the clay aggregates areunable to rotate hence they are prevented from relaxing. It was reported thatorganoclay enhanced thermal stability of the nanocomposites.

Gelfer et al. [2002] prepared CPNC by melt blending commercial organoclayswith either poly(ethylene-vinylacetate) (EVAc; 3.1 and 8.15 mol% VAc) orneutralised poly(ethylene-methacrylic acid) (PEMA; 3.1 mol% methacrylic acid).The reason for using these copolymers originated from the observation thatorganoclay is easier to disperse in highly polar polymers. Thus, EVAc and PEMAwere used as models for studying the structure, property and processingrelationships in CPNC. The organoclays were: Cloisite® 6A (C6A; d001 = 3.59 nm),Cloisite® 20A (C20A; d001 = 2.47 nm), and Nanomer I30 E (I30). They all containMMT intercalated with (1) 2M2HTA (6A and 20A) or (2) N-tallow alkyltrimethylene diamine chloride (?) (I30 usually comprises MMT-ODA). Theintercalant content was 45 wt% in C6A, and 30 wt% in C20A and in I30. XRDof the CPNC indicated intercalation with d001 = 2.1 to 4.1 nm. In all systems, Tmand crystallinity were not significantly affected by the presence of organoclays,

~

392


suggesting that clay particles were shielded from the matrix by the intercalantmolecules and predominantly confined to the amorphous phase.

Small strain oscillatory experiments were carried out at constant strainamplitude (γ = 0.06), frequency 0.1 < ω < 100 rad/s and T = 120-200 °C (aboveTm). Under these conditions the specimens remained stable for t < 30 min. TheCPNC with EVAc as a matrix showed solid-like behaviour at small-strainoscillatory shear, but it was able to yield and flow under a steady shear – thecharacteristic performance of physically crosslinked systems. In contrast, theCPNC with PEMA as matrix exhibited a melt-like rheological behaviour, with aminimal contribution by the organoclays. The controlled stress rheometer wasused to determine the yield stress. The authors speculated that the carbonyl groupsof VAc in EVAc interact with the clay surface, resulting in physically crosslinkedstructures. By contrast, the interactions between PEMA and the clay wereconsidered weak (due to repulsion between carboxyl anions and the negativelycharged clay surface) preventing the formation of structures in these systems.The extrapolated ηo versus T showed an upswing at T > 200 °C, indicating physicalcrosslinking. The time-temperature superposition has not been discussed, butjudging by the reported rheological data, its applicability to the studied systemsis dubious.

3.3.12 Rheology of CPNC – A SummaryIn summary, the rheological studies of CPNC in shear and elongation demonstratethat even at low clay loading the flow is frequently complex. One source ofcomplication that seldom is even considered by rheologists are the chemicalchanges within the system, caused by the decomposition of intercalant, which inturn affects the interlayer spacing and thermomechanical degradation of the matrixpolymer.

As discussed on the preceding pages, CPNCs show a range of performancethat starts with the traditional behaviour of filled systems and ends with end-tethered nanocomposites showing quite distinct flow characteristics. For the end-tethered CPNC, at low or moderate concentration of clay platelets, the shearflow may be interpreted using the LCP theories. Following Onogi and Asadaclassification, three regions of flow can be clearly identified:

(1) At low deformation rates – a solid-like yield stress behaviour, caused by a3D structure.

(2) At middle strain rates assemblies of clay platelets undergo either a tumbling(in shear) or stretching (in elongation) motion.

(3) At high rates of deformation, the platelets become oriented in the sheardirection, which causes the shear viscosity to decrease nearly to the level ofthe matrix (the effect is particularly evident in the steady-state flow).

The end-tethered systems show the formation of 3D structures at a concentrationof about 0.5 vol% clay. These structures are responsible for the non-linearviscoelastic flow behaviour, characterised by classic rheological tests or Fouriertransform rheology [Wilhelm, 2002; Debbaut and Burhin, 2002]. This method isparticularly well suited for quantification of the non-linear effects as a functionof composition, strain rate, strain, temperature, etc.

The unique character of CPNC is evident in the extensional flows. The studieson these flows lead to the conclusion that the presence of exfoliated clay platelets

393

Rheology

able to interact with the matrix (e.g., end-tethered systems) results in significantenhancement of strain hardening. This effect agrees very well with the ‘hairy clayplatelet’ (HCP) model of CPNC. Thus, in analogy to improved processability ofsome resins by blending them with branched homologues (e.g., common industrialblends of LLDPE with LDPE) one may use CPNC technology to improve filmblowing, blow moulding or foaming (and microfoaming) of difficult to processresins. At high extensional flow rates, the platelets may be oriented perpendicularto the stretch direction, which causes the transient viscosity to move into thestrain hardening region. Both effects are stronger for the end-tethered than thefree platelets systems, especially at higher clay loading.

394


395

Nucleation and Crystallisation

3.4 Nucleation andCrystallisation

3.4.1 IntroductionPolymers may crystallise when: (1) their molecules have sufficiently regularstructure and mobility, (2) the temperature is: Tg < T < Tm, (3) there are nucleipresent and (4) the rate of crystallisation is sufficiently high.

Nucleation is the initial stage of the phase separation during which a newphase is formed on a minute amount of substance that acts as a nucleus forsubsequent phase growth. In Nature, fog, rain or snow are formed through thisprocess. Nucleation of crystals requires seed crystals, viz. self-formed nuclei, dustparticles or nucleating agents. Thus, crystallisation may take place via ahomogeneous crystallisation mechanism, where the molecules self-assemble intoordered entities having critical size for the crystal growth, or via a heterogeneouscrystallisation mechanism, where the molecules assemble on the surface of aforeign body. In polymer technology, especially for injection moulding, the desiredhigh crystallisation rates often require the addition of nucleating agents.

A nucleating agent is a substance that forms nuclei for the growth of crystalsin a supercooled polymer melt. Virtually any solid body with a high energy surfacemay act as a nucleating agent. However, for efficient nucleation it is necessarythat the crystalline structure of the nucleating agent closely matches the crystallinestructure of the polymer [Vesely, 1996]. Several specific crystalline or crystallisablesubstances have been developed, viz. 4-biphenyl carboxylic acid, aluminium salts(benzoate, phenyl-acetate, tert-butyl-benzoate), antimony compounds (trioxide,phosphates), sodium salts (4-methyl-valerate, benzoate, β-naphthoate, caproate,cinnamate, succinate), pigments, etc. These substances generate from 2 to20 million nuclei per 1 mm3.

The nucleating agents may preferentially induce a specific crystallographicform of the polymer. For example, addition to isotactic-PP of either 1,2,3,4-bis-(3,4-dimethyl-benzylidene sorbitol) or N,N´-dicyclohexyl-2,6-naphthalatedicarboxamide, preferentially generate α-iPP or β-iPP, respectively. The nucleatingefficiency depends on several independent variables, such as temperature, pressure,stress, part thickness as well as the presence of other processing additives. Forthis reason, during the last few years combinatorial methods have been used tostudy polymer nucleation and crystallisation.

As stated above, an efficient nucleating agent must have a high-energy surface– the larger the specific surface, the more efficient it is expected to be. Nucleationinvolves initial adsorption of macromolecules on the surface. The process isparticularly efficient if the foreign body is able to provide an energetic matrix forthe formation of thermodynamically favourable crystalline forms. Alternatively,the crystalline cell type and size of the nucleating agent may induce a transitorycrystalline polymer form that upon annealing transforms into a stable form of

396


higher packing density. Several researchers have reported this behaviour for CPNCwith PA-6 as the matrix.

The nucleating efficiency, ϕ, may be expressed in terms of the energy ratiorequired to generate a nucleus in a heterogeneous nucleation over that in ahomogeneous one [Dobreva and Gutzow, 1993]. The authors assumed thatnucleation is the rate-determining step, while the extent of crystallinity is constant.Thus, the nucleation rate, r, depends on the degree of supercooling, ΔTp = Tm - Tc:

r A E T E V nk T S

E hetero E o

n p n m B m m

n n T Tm

= −{ } =

≡ ( ) ( ) ≈

exp / ; /

/

Δ Δ2 3 2 2ωσ

ϕ hom(140)

where En is the energy required to form a nucleus, ω is a geometrical factor, σ is thespecific surface energy, Vm is the molar volume of the crystallising substance, ΔSmis the melting entropy, n is the Avrami exponent and kB is the Boltzmann constant.According to Equation 140, the nucleating activity factor (ϕ) is given by the ratioof the slopes of ln r versus 1/ΔTp

2. Note that 0 ≤ ϕ ≤ 1, with ϕ = 0 indicates thehighest activity of the nucleating agent and ϕ = 1 indicates total lack of it.

Nanofillers such as clays may have strong nucleating effects. The wide varietyof intercalants, intercalating methods and compatibilisers may form a barrierbetween the high-energy clay surface and the semicrystalline polymer matrix. Inthis section the recorded effects of nanoclay on the crystallinity of CPNC will besummarised. Since CPNC with PA and PP matrices are of great academic andindustrial interest the focus will be on these two classes of nanocomposites.

3.4.2 Fundamentals of CrystallisationThe growth rate of lamellar crystal is controlled by the degree of undercooling,ΔT = Tm - Tg and the macromolecular diffusion rate towards the crystal growthfront. A maximum rate of crystal growth takes place near Tmax ≈ (Tg + Tm)/2. Inother words, the rate of crystal growth, G, is governed by the activation energyrequired to transport crystalline molecules across the solid-liquid interface (ΔE)and the work necessary to form a critical nucleus (ΔF*). The crystallisation growthrate is usually expressed as [Turnbull and Fisher, 1949]:

G G E R T T G k To c o B= − −( ){ } −{ }exp / exp * /Δ Δ (141)

where Go is a constant, To is the temperature at which motions necessary for thetransport of molecules through the liquid-solid boundary cease, and Tc is thetemperature of crystallisation. At low supercooling the growth rate is nucleationcontrolled, while at high supercooling it is diffusion controlled.

Inherent to this model is an assumption that ΔG* depends on the size andshape of the homogeneously formed nucleus. The crystallisation may take placeif the nucleus reaches a critical size [Hoffman et al., 1976]:

Δ ΔG T T T h T Te m m f m* /= +( ) −( )[ ]32 2

2σ σ (142)

where σ and σe are the side and end free energies of the crystal, Δhf is the free enthalpyof fusion and Tm is the equilibrium melting temperature. The critical size forhomogeneous nuclei is about 100 nm3, comparable to the macromolecular chain size.

Binsbergen [1973] considered the second type of nucleation, the heterogeneousone. The author assumed that a foreign substance, e.g., a dust particle or a nucleatingagent, facilitates formation of a polymeric nucleus, similar to that formed in

397


homogeneous nucleation excepting the presence of a high energy solid surface thatlowers the magnitude of ΔG*. As a result the critical size of the nucleus is reduced,which in turn leads to crystallisation at lower undercooling, thus:

Δ Δ ΔG T T T h T Te m m f m* /= ( ) +( ) −( )[ ]16 2

2σ σ σ (143)

where Δσ is the specific interfacial free energy difference between the nucleusand the nucleating agent. The overall crystallisation kinetics of blends is oftendescribed by the Avrami equation [Avrami, 1939]:

α t kt n( ) = − −{ }1 exp (144)

α(t) is the weight fraction of a crystalline part at time t, whereas n and k are equationparameters. The Avrami index (n) depends on the type and geometry of nucleationand the crystal growth, thus it may be written as n = nnucleation + ngrowth. The Avramirate parameter k, is often expressed in terms of the crystallisation half-time, t1/2:

k tn= ( )ln / /2 1 2 (145)

Derivation of the Avrami equation is based on several assumptions, such as theshape constancy of the growing crystal, the constant rate of radial growth, lackof induction time, the uniqueness of the nucleation mode, complete crystallinityof the sample, random distribution of nuclei, constant radial density, primarynucleation process (no secondary nucleation) and absence of an overlap betweenthe growing crystallisation fronts. Furthermore, the often-cited form of thedependence is simplified – as Ga ęski [1995] pointed out, the relation is onlyvalid for sporadic or instantaneous nucleation. The derived expressions aredifferent for crystallisation in films and in the bulk.

Pérez-Cardenas et al. [1991] developed a modified Avrami expression,separating the primary and the secondary (subscripts ‘p’ and ‘s’, respectively)crystallisation effects. Thus, the crystalline weight fraction, α, was written as:

α = αp + αs (146)

Accordingly, the crystallisation proceeds in three steps: (I) the initial primarycrystallisation, (II) mixed primary and secondary crystallisation and (III) puresecondary crystallisation. The authors expressed the weight fraction of the polymercrystallised by primary and secondary crystallisation as ζ. In consequence, thecrystallisation may be described by two equations:

1 1 11

0

− = − − ′( ) −( ) + ′( ) +⎡

⎣⎢⎢

⎤

⎦⎥⎥

≤′ ′ −∫α ς τ τ α ςexp ; kt k t kn xp kt k t dn n n n nt

(147)

1 1− = −( ) ′{ } ′{ } >′ ′α ς α ςexp exp ; *k t k tn n (148)

Now, six parameters are required to describe the process: k and n (the primarycrystallisation parameters) depend on crystallisation temperature, the nature ofprimary nucleation and the fast growth; the secondary crystallisation parameters,k´ and n´ depend on the conditions under which the slow crystallisation of theremaining amorphous regions takes place, ζ, indicates the weight fraction ofmaterial crystallised up to the moment the primary crystallisation ends and t*indicates the start of the pure secondary crystallisation.

Furthermore, Avrami theory is limited to isothermal processes. Since polymerprocessing is mostly non-isothermal, the theory has been extended by Ozawa

398


[1971] who considered the Avrami constant k to depend on temperature, k = k(T),and the crystallisation time t as dependent on the cooling rate: t = Φ–m (m is theOzawa exponent).

A more general approach to non-isothermal crystallisation was developed byKamal and Chu [1983]:

α

α

t k T nt dt

T k T nT T

R

dT

R

nt

o

n

T

T

o

( ) = − − ( )⎧⎨⎩

⎫⎬⎭

( ) = − − ( ) −⎛⎝⎜

⎞⎠⎟

⎧⎨⎪

⎩⎪

⎫⎬⎪

⎭⎪

−

−

∫

∫

1

1

1

0

1

exp

exp

(149)

where k(T) and n are Avrami’s isothermal parameters. For DSC scans the followingexpression has been successful:

′ −( )[ ] −( )[ ] = −( ) − −( )[ ] [ ]= + ′ ≡ = −( ) −( )

α β

β α α α

p p ons p p ons p

p ons pn

T T a n E T T RT

T T t d dt nkt

/ /

: /

1 1

1

2

1where and(150)

where β is the heating rate, Tons is the onset temperature, Tp is the peak temperaturereached after time t, and E is the activation energy of the crystallisation process.

In a semicrystalline homopolymer, the change in free energy of melting permole of monomer is given by:

ΔGu(T) = ΔHu - TΔSu (151)

where ΔHu and ΔSu are the enthalpy and the entropy changes on melting,respectively. For an infinitely thick crystal at equilibrium melting temperatureTm, ΔGu(Tm) = 0 and:

Tm = ΔHu/ΔSu (152)

The value of Tm can experimentally be determined from Hoffman-Weeks plots,where the experimental melting point is plotted as a function of the crystallisationtemperature, Tc. Extrapolation of experimental data to the Tm = Tc line gives theequilibrium value of Tm.

Xu et al. [2001] studied the nonisothermal crystallisation kinetics of CPNCwith POM or with PP [2002] as the matrix. Two CPNC with POM were preparedin a roller mill at T = 175 to 180 °C; the first comprised 5 wt% of Na+-MMT andthe second MMT-3MHDA. The interlayer spacing in these systems was d001 = 1.92and 3.52 nm, respectively. The nonisothermal crystallisation kinetics wereinvestigated by DSC at 5 to 40 K/min cooling rate. The difference in the values ofthe Avrami exponent n between POM and the nanocomposites suggests atri-dimensional growth with heterogeneous nucleation. At a given cooling ratethe crystallisation of neat POM was slightly slower than that of eithernanocomposite. The activation energies, 387, 330, and 329 kJ/mol weredetermined for the nonisothermal crystallisation of POM, POM/Na+-MMTnanocomposite, and POM/MMT-3MHD, respectively.

A similar process was used to prepare nanocomposites with PP with 3 wt%MMT-3MHDA. A 3 mm thick sheet was compression moulded at 180 °C. Theinterlayer spacing was d001 = 3.88 nm. The crystallisation rate increased withincreasing cooling rates for both PP and its nanocomposite. The crystallisationrate of CPNC was higher by 40% at a cooling rate of 5 K/min, but virtually

399


identical at the fastest cooling rate of 40 K/min. The activation energies wereestimated as 189 and 156 kJ/mol for PP and nanocomposite, respectively. Thus,incorporation of 3 wt% of organoclay into PP affected the matrix crystallisationmore than 5 wt% in POM.

General procedures for evaluating crystallinity in polymeric systems by XRDand DSC have been developed, e.g., by Murthy and Minor [1990] and by Khannaand Kuhn [1997], respectively. Additional information on the topic can be foundin [Mathot, 1994; Chan et al., 1995; Hammami et al., 1995], etc.

3.4.3 Effects of Clay on Crystallisation of PA-6 MatrixNucleating PA by means of standard nucleating agents (e.g., silica, talc, Na-phenylphosphate, etc.) leads to higher crystallinity that translates into higher modulus,hardness, yield strength, HDT, improved abrasion and water absorbance, butreduced elongation at break and impact strength. The characteristic propertiesof PA-6 and PA-66 are listed in Table 56. Both these polymers are polymorphs,the former crystallising mainly in α-form, γ-form and a metastable β-form.

Historically, PA-6 is more important for CPNC technology. There is a large bodyof information in the patent and open literature for these systems. As far as theeffects of clay addition on crystallisation of PA-6 are concerned, the focus has beenon nucleation, crystalline structure and total crystallinity. In PA-6 the stable monoclinic

sedimaylopfosretemarapcitsiretcarahC65elbaT

ytreporP 6-AP 66-AP

noitidnoC eulaV noitidnoC eulaV

noitisnartssalG,.pmet Tg )K(

yrDHR%05

033-023392-672

yrDHR%05

153803

tniopgnitleM,).qe( Tm )K(

yrD 394 yrD 2.245

,ytisneD ρm/gk( 3)

yrD 0311 yrD 0521-0221

lacipyT,ytinillatsyrc

α )%(

yrD 05 yrD 06-04

llectinUsnoisnemid

)mn(

α mrof-γ mrof-

659.0 × 427.1 × 108.0339.0 × 886.1 × 874.0

αI mrof-β mrof-

75.1 × 50.1 × 37.194.0 × 8.0 × 27.1

selgnallectinU)srem#(

α mrof-γ mrof-

)4(5.76)4(121

αI mrof-β mrof-

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400


α-form has a planar zigzag chain conformation, the metastable (pseudo-) hexagonalγ-form has a twisted chain, while the β-form is less well identified and often consideredto be an intermediate stage between the two others. The α-form is most common,readily obtained during melt processing. A nearly pure α- and γ-form may be obtainedby, respectively, treating the specimen in superheated water vapour at 150 °C, andby reversibly complexing it with iodine from KI aqueous solution [Penel-Pierron etal., 2001a,b]. Uniaxial drawing increases the α-form content and reduces that of theγ-form. The tensile strength is higher for the α-form, while the γ-form shows bettertoughness. The solid/solid transition of α-crystal into γ-crystal, observed during heatingor cooling, is known as the Brill transition.

At the crystallisation temperature, Tc > 450 K PA-6 crystallises in a mixtureof α- and γ-forms, whereas above this limit essentially in nearly a pure α-form[Privalko et al., 1979]. Thus, the α-form is more stable at higher temperatures.The authors reported that addition of glass beads or aerosil to PA-6 did notchange the molecular mechanism of crystallisation. PA-66 crystallises in at leastsix forms, of which the αI- and β-form dominate. Incorporation of organoclayinto PA-6 increases the nucleation density, the proportion of the γ-form in thecrystal as well as modifying the total crystallinity content. While the former twoeffects are generally accepted the latter is more controversial – some authorsreport enhancement, some others reduction while newer publications indicatethat both may be present within specific ranges of conditions, e.g., using a differentcooling rate or annealing method.

Kojima et al. [1994] used XRD and DSC to study orientation and crystallinityof the exfoliated PA-6/MMT system. The platelets of MMT and the γ-form ofPA-6 crystallites were reported parallel to each other. The α-form was not detected.Incorporation of 0.2 to 0.8 vol% MMT increased the degree of crystallinity ofPA-6 (to 31%) by 5 to 7%, but the effect did not depend on composition. In thefollowing publication it was reported that in injection moulded tensile bars theorientation depended on the depth, but the γ-form of PA-6 still dominated thecrystalline phase [Kojima et al., 1995]. In the surface layer of the moulding,MMT platelets and chain axes of crystalline PA-6 were parallel to the surface. Inthe intermediate layer the MMT platelets remained parallel to the surface oftensile bar, but the chain orientation rotated 90o (hence orienting perpendicularlyto both, bar surface and MMT). In the central layer the platelets were randomlyoriented along the flow axis with chain axes perpendicular to them.

Usuki et al. [1995] studied CPNCs of PA-6 containing ca. 5 wt% of MMT,synthetic mica, saponite or hectorite (CEC = 1.2, 1.0, 1.0 and 0.5 meq/g,respectively). Prior to reactive exfoliation in ε-caprolactam, the clays werepre-intercalated with ω-amino lauric acid (ADA). Owing to end-tethering, theconcentration of –COOH macromolecular chain ends was about twice as large asthat of –NH2. The authors established correlations between the performance (e.g.,tensile strength, modulus, HDT), the strength of the clay-polymer interaction(determined by the 15N-NMR chemical shift, CP) and the degree of crystallinity, X(wt%). The latter was calculated from the DSC-measured heat of fusion, takingthe literature value for the heat of fusion of PA-6 crystal as 188 J/g. Surprisingly, itwas found that X decreased upon incorporation of organoclay. At the same time,the mechanical performance (e.g., tensile modulus) increased with CP from 1.11for PA-6 to 2.02 GPa for CPNC with mica. These observations are summarised inFigure 121 – note that excepting the degree of crystallinity, X (wt%), the otherperformance characteristics for CPNC diverge from those of PA.

401


Liu et al. [1999] prepared CPNC by melt blending PA-6 in a TSE with 1 to18 wt% of MMT (CEC = 1.0 meq/g) intercalated with ODA. After compoundingat T = 180 to 220 °C, the CPNC with more than 10 wt% of organoclay showedintercalated structures with d001 increased from 1.55 to 3.68 nm. However, atlow MMT content the samples were exfoliated. In accord with the Usuki et al.[1995] results, the authors observed a difference in crystallisability between PA-6and its CPNC – addition of any amount of clay reduced the supercooling effectto a common level, indicating enhanced nucleation. However, by contrast withthe Usuki et al. [1995] results, here all specimens (with 0 to 6.7 wt% MMT)showed about the same crystallinity. The difference most likely originates in thedifferent internal structure of these CPNC – in the former work the reactiveexfoliation resulted in end-tethering, whereas compounding PA-6 with pre-intercalatedMMT generated exfoliated CPNC (at lower clay content), but without end-tethering. XRD of PA-6 showed a virtually pure α-crystallographic form, whereasthe polymer in CPNC crystallised mainly in the γ-form.

Wu and Liao [2000] prepared CPNCs of exfoliated synthetic saponite in aPA-6 matrix using the reactive route with a unique pre-intercalation step; syntheticclay was dispersed in water/ε-caprolactam/phosphoric acid solution, then themixture was polymerised at T = 80 to 270 °C under pressure. The authors do notprovide information about the platelet dispersion. XRD and DSC were used tostudy the effect of clay on crystallinity, which in turn was related to performance.The specimens containing 0, 2.5 and 5 wt% clay were pressed into sheets at240 °C, then either slowly cooled or quenched to room temperature. Slow cooling

Figure 121 Crystallinity, relative tensile strength, TS/TS(PA), and relative HDT(expressed in Kelvin), HDT/HDT(PA) as functions of the 15N-NMR chemical shift,

CP. Systems: PA-6 with 5 wt% of exfoliated clay – see text. Data [Usuki et al.,1995].

402


resulted in nearly pure γ-form crystals in the sample containing 2.5% clay, nearlypure α-form in the sample containing 5 wt% clay and polymorphic crystals inPA-6. By contrast, quenching yielded a nearly pure γ-form for the sample with5 wt% clay, predominantly α-form for PA-6 and intermediate polymorphouscomposition for the sample containing 2.5 wt% clay. To transform γ- into α-formrequired 2 h heating at 210 °C. Addition of synthetic saponite increased thecrystallisation rate, but the crystallinity was not determined. The tensile modulusand HDT showed significant improvements.

Akkapeddi [2000] used a melt compounding method for the manufacture ofPA-6 based CPNC. Thus, organoclay of MMT or hectorite was dispersed in PA-6at a level of 2 to 5 wt% of clay. The author reported that clay platelets hadstrong surface nucleation effects that promoted faster crystallisation and a higherlevel of crystallinity in relation to that of neat PA-6 (particularly at the surfaceand in thin-wall injection mouldings). For example, in 1 mm thick PA-6 injectionmoulded bars the crystallinity was 16 to 18%, while in CPNC (4 wt% clay) thecrystallinity was 50% throughout the specimen. It is noteworthy that whilst inPA-6 specimens the skin had lower crystallinity than the core, in CPNC thesituation was reversed.

Giza et al. [2000] reported enhanced crystallinity in spun fibres of PA-6/MMT(from Ube). While the crystallinity content in drawn fibres of PA-6 was about51%, in those containing 2 and 5 wt% clay it was 59 and 63%, respectively. Partof the reason for the difference may be the higher drawing temperature for CPNC(170 °C) compared with PA-6 (150 °C). Furthermore, the high-speed drawingthat produced the highest crystallinity was also instrumental in transforming theγ-form into predominantly α-form. Increased crystallinity translated into highermodulus, viz. the tensile modulus for CPNC with 0, 2, and 5 wt% clay wasabout 8, 9 and 10 GPa, respectively.

Intercalated or exfoliated CPNC were prepared by extrusion compoundingof PA-6 with ca. 3 wt% of pre-intercalated MMT, Cloisite® 30B (C30B);d001 = 1.8 nm or Cloisite® 6A (C6A); d001 = 3.6 nm [Varlot et al., 2001]. Thecompounds were injection moulded into 4 mm thick bars, which subsequentlywere characterised by XRD. In accord with Kojima et al. [1995], the authorsreported significant variation of crystalline morphology with depth. Thus in thethree specimens: PA-6, PA-6 + C30B, and PA-6 + C6A, the surface layer was richin γ-form. The central, bulk layer of the injection moulded bar of PA-6 showedpreponderance of α-form, whereas γ-form dominated the two other specimens.The CPNC showed MMT platelets aligned with the flow direction. The averagedistance between MMT platelets in a PA-6A sample was determined from theSAXS spectra to be ca. 35 nm. The same value was calculated from thecomposition, assuming perfect exfoliation and uniform distribution of theplatelets.

VanderHart et al. [2001] used solid-state proton and 13C NMR to study 17 CPNCcompositions based on PA-6, provided by Ube Ind. and SCP. Except three samplesthat contained laponite clay, the CPNC comprised natural, pre-intercalated MMTdispersed either by in situ polymerisation or by melt compounding. The clayswere found to promote growth of the γ-form of PA-6 in all CPNC samples, whilethe α-form characterised the neat PA-6. The total crystallinity in neat PA-6 was40% – all in α-form. In CPNC the crystallinity was ca. 33 to 45% with the α-form ranging from 2 to 75% (after injection moulding and annealing). However,most specimens annealed at Ta = 214 °C showed only a partial conversion of γ-form

403


to α-form (e.g., from 4 to 19%). The CPNC showed higher Tc and Tm than thatof PA-6. Since the main objective was to study paramagnetic effects there is littlemore information on crystallinity.

To prepare CPNC with PA-6 as a matrix, Liu and Wu [2002a,b] used a three-stepmethod. Thus, Na-MMT (CEC = 0.8 meq/g) was intercalated with trimethylhexadecyl ammonium bromide (3MHDA). The precipitate of the reaction in H2Owas dried under vacuum then compounded in an internal mixer with the diglycidylether of bisphenol-A (MW = 360 g/mol). In the last step the doubly intercalatedMMT was dispersed in molten PA-6 by means of compounding in a TSE at ca.230 °C. The product containing 5 wt% clay was used in crystallographic studiespublished in several articles, summarised below. More recently, the work wasextended to a PA-66/MMT system [Liu and Wu, 2002a; 2003]. A highercrystallisation temperature for CPNC than that for neat PA-66 (Tc = 237 insteadof 226 °C), reduction of spherulite size, the presence of a γ-phase instead of anα-phase were reported. Similar behaviour was observed for an exfoliated PA-66/MMTsystem [Zhang et al., 2003].

The DSC and XRD studies on the influence of the cooling rate on thecrystallinity in PA-6 and CPNC demonstrated that the two materials behavedifferently [Wu et al., 2001a]. Thus, as the cooling rate increases the rate ofcrystallisation of PA-6 decreases while that of CPNC increases. In Figure 122two separate sets of experiments are presented. The DSC data representexperiments with well-defined cooling rates, whereas those measured by XRDare plotted versus estimated ones. The dependencies are similar. The XRD datashow even stronger divergence of behaviour between PA and CPNC than DSC.Furthermore, WAXS data indicated that PA-6 was highly polymorphous – onlyat the slowest cooling rate was nearly pure α-form obtained. By contrast, the

Figure 122 Crystallinity versus cooling rate for PA-6 and PA-6 containing 3 wt%of doubly pre-intercalated MMT. Two methods were used: DSC with well defined

cooling rates and XRD with roughly estimated cooling rates. Data [Wu et al.,2001]. See text.

404


CPNC diffraction showed nearly pure γ-form – only at the slowest cooling ratedid the α2-peak-emerge.

A partial explanation for the diverse behaviour of PA-6 as a neat resin and asa nanocomposite matrix can be gleaned considering the results of an annealingexperiment, displayed in Figure 123 [Liu and Wu, 2002a]. Granules of PA orCPNC were placed between two glass slides, heated in an oil-bath up to 250 °Cfor 10 min, and then quenched in liquid N2. The films were annealed in an oil-bath at different Ta for 1 h, re-quenched and their crystallinity was determinedby XRD. In the Figure the α- and γ-form crystalline content is plotted versus Ta– the total crystallinity is a sum of these two forms. During the compoundingand pelletising some crystallinity in PA-6 and CPNC certainly developed. However,this crystallinity was to be eliminated by keeping the specimens for 10 min at250 °C. Thus, the subsequent quenching generated 0 and 48% crystallinity inPA and CPNC, respectively. The 1 h annealing of PA-6 at T ≥ 100 °Ccharacteristically formed g-crystals that subsequently converted to α-form. Thetotal crystallinity reached a maximum value of 36% at 180 °C. By contrast,CPNC maintained ca. 49% of crystallinity in the γ-form up to ca. 130 °C. Abovethis temperature γ- progressively converted to α-form – the latter reached itsmaximum content of ca. 19% at 180 °C. Thus, the presence of organoclayincreased the total crystallinity of the PA-6 matrix well above the level observedfor the neat resin, and it had stabilised the γ-form to higher temperatures beforethe onset of the solid-solid phase Brill transition from γ- to α-form. It is noteworthythat the DSC-determined Tm of PA-6 and its CPNC was 223 and 219 °C,respectively, in good agreement with the known melting points for the α- and γ-crystals.

FTIR studies of PA-6 and CPNC led to the conclusion that the presence oforganoclay weakens the hydrogen bonding in PA-6. The γ-form was found

Figure 123 Determined by XRD crystallinity of PA and CPNC (containing 5 wt%of doubly pre-intercalated MMT) as a function of the annealing temperature. Theα- and γ-form crystalline content is shown – see text. Data [Liu and Wu, 2002].

405


preferentially located near the clay platelets, whereas the α-one preferentially inthe bulk [Wu et al., 2002a].

Murase et al. [2002] used FTIR, XRD and DSC to study the structure of PA-6with 2 wt% of either synthetic mica or natural MMT. The PA-6/mica was preparedby melt polymerisation of ε-caprolactam while the PA-6/MMT was industrialresin from Ube Ind. The films were prepared by pressing at 250 °C for 2 minfollowed by quenching into an ice-water bath and then annealing at 110 °C for6 h. In PA-6 only the α-form was found, in the Ube nanocomposite both α- andγ-crystalline forms, and in the mica-containing CPNC only γ-form crystals weredetected. The latter composition also showed the highest degree of crystallinityof about 58% followed by that of PA/MMT and PA-6 (37%). Judging by thereported results, it seems that mica in the studied CPNC was only intercalated,whereas MMT in the Ube nanocomposite was exfoliated. Thus, it is difficult tospeculate whether the observed difference in the composition of the crystallinephase is due to the nature of these two clays or to the degree of exfoliation.

Kamal et al. [2002] studied the crystallisation kinetics of PA-6 and its CPNC(Ube PA-1015B and –1015C2, respectively) under pressure P = 50 to 200 MPa.Isobaric volume changes associated with the crystallisation process weredetermined at constant T and P, from which the crystallinity was calculated as:

X(%) = 100(V0 – Vt)/(V0 – V∞) (153)

where V0 is the initial (melt) volume, Vt is the volume at time t, and V∞ is thevolume at the end of crystallisation. Isobaric heating or cooling was used todetermine the melting or crystallisation temperatures of PA-6 in its neat stateand in CPNC. The data were fitted to a linear dependence:

Tm,c(°C) = ao + a1P(MPa) (154)

The numerical values of the parameters ai, along with the correlation coefficientsquares, r2, are listed in Table 57. It is noteworthy that the Tm for both resinsfollows the same dependence, whereas Tc of the CPNC is lower by about 6 °Cthan that of neat PA-6. It is also interesting that within the experimental error ofthese measurements, the pressure gradient of Tm and Tc, dTm,c/dP = a1, is aboutthe same for PA-6 and CPNC.

The kinetics of crystallisation were studied at constant T and P. For bothresins, the data plotted according to the linearised Avrami equation showed twointersecting linear dependencies. Apparently, the crystallisation started with theγ-form, and then proceeded to the α-form. Thus, within the first zone the γ-form

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406


was dominant while within the second zone the α-form dominated. The Avramiexponent n for the γ-form was between 1.0 and 3.2 in PA-6 and between 0.9 and2.6 in CPNC. Its value for the α-form was between 1.0 and 2.1 in PA-6 andbetween 1.2 and 2.6 in PNC.

In Figure 124 and Figure 125 the crystallisation rate of, respectively, PA-6and PANC is expressed as the crystallisation half-time, t1/2, versus the free volumeparameter,

h h P T= ( )˜ , ˜ , where , ˜P T are reduced pressure and temperature,

computed for each resin from the Simha-Somcynsky eos. Evidently, the kineticsof crystallisation not only depend on h, but on P as well. To more clearly see theeffects of the independent variables, the experimental data were fitted to a lineardependence, t1/2 = t1/2(h, P). The correlation coefficient squared, r2 = 0.74 to 0.95was obtained. At the same value of h and P, the rate of crystallisation of theα-crystals is systematically slower (t1/2 larger) than that of the γ-crystals. PA-6 inCPNC crystallises faster than in its neat form. Surprisingly, in these isothermaland isobaric experiments the pressure seems to play a secondary role to h as faras the crystallisation kinetics is concerned. This was not the case for the isobaricscans (at constant scanning rate) where high pressure slowed down either meltingor crystallisation in PA and CPNC.

Bureau et al. [2002] reported on the crystallinity and mechanical propertiesof compression moulded PA-6 and its CPNC (Ube Ind., PA-1015B and 1015C2).Specimens were moulded at 250 °C for 90 sec at P = 0.7 MPa, then either cooledto room temperature under pressure at a rate of ca. 30 °C/min, or quenched inice water. Some quenched specimens were subsequently annealed at 80 °C for24 h under vacuum. It was expected that these procedures would producespecimens containing, respectively: the α-form (cooled), amorphous PA-6(quenched), and the γ-form (annealed).

The DSC scans of PA-6 specimens showed a crystallisation peak followed byan α-form melting peak at Tm = 221 °C. The crystalline content was estimatedas: 31% (cooled), 7% (quenched) and 21% (annealed). DSC scans of cooled orquenched CPNC showed a double melting point, Tm = 213 and 221 °C, indicatinga polymorph with either γ- (in the cooled sample) or α-form (in the quenchedsample) predominating. The crystalline content in both these two samples wasca. 25%, thus lower for the cooled specimen (than that in neat PA-6) andsignificantly higher in the quenched specimen, indicating rapid nucleation byMMT. Identification of the a- or g-form was confirmed by XRD and FTIR.

The tensile test results are summarised in Table 58. There is strong correlationbetween the mechanical behaviour and morphology. All PA-6 specimens showednecking with strain at break varying between 120% and 670%, depending onthe moulding condition. For the cooled CPNC specimens, higher Young´s modulusand yield stress (with brittle fracture without necking) was observed. The tensilestrength and the Young´s modulus of the CPNC were ca. 15% higher than thoseof PA-6. 25% polymorphous crystallinity in CPNC yielded a higher tensilemodulus than PA-6 with a 31% α-form crystalline phase. The data indicate thatimprovements of rigidity and strength caused by addition of MMT are related tothe reinforcing effects of the nanofiller and not to the increased γ-crystals content.

The effect of matrix molecular weight on the kinetics of isothermal and non-isothermal crystallisation in a PA-6/MMT system was studied by Fornes andPaul [2003]. Incorporation of ca. 0.5 to 1 wt% clay resulted in the highest PA-6crystallisation rate. While for neat PA-6 the crystallisation rate decreases withmolecular weight, in the case of CPNC the maximum crystallisation rate was

407


Figure 124 Half-time for crystallisation of the γ- and α-forms in PA-6 versus thefree volume fraction parameter, h. Solid points are experimental, open are

computed from the linear fit to Equation 94. Data [Kamal et al., 2002]. See text.

Figure 125 Same dependence as in Figure 124 but for a CPNC sample of 2 wt%organoclay in a PA-6 matrix (Ube PA-1015 C2). Data [Kamal et al., 2002]. See

text.

408


hardly affected by it. Furthermore, the largest enhancement of the crystallisationkinetics was reported for systems with the highest molecular weight (the authorspostulate that this was related to the highest degree of exfoliation). However, thedegree of crystallinity decreased with the matrix molecular weight.

3.4.4 Clay Effect on Crystallisation of Other PolyamidesKuchta et al. [2000] studied crystallisation of PA-11 in its CPNCs. The authorsprepared several compositions by in situ polycondensation or melt compounding.To start with, MMT was pre-intercalated with ω-amino-undecanoic acid.Polycondensation yielded CPNC with 2.4 to 19.7 wt% MMT, while compoundingof PA-11 with organoclay (10 min in a recirculating mini-TSE) yielded CPNCwith 1.7 to 31 wt% clay. TEM and synchrotron radiation indicated uniformdispersion of MMT in the PA-11 matrix.

By contrast with PA-6 CPNC, where the γ-form has been observed already atlow clay loadings, in the PA-11 nanocomposites the α-form remained stable upto 20 wt% MMT. In the sample containing 5.6 wt% MMT the α- to γ-form Brilltransition was observed at T ≅ 100 °C, the same as in neat PA-11. Thus, theinfluence of MMT on the solid-solid transition in PA-11 is less significant thanthat in PA-6. Heating PA-11 and its CPNC resulted in different extents of lamellarthickening. While in neat PA-11 the lamella thickness at 180 °C is 72% higherthan that at room temperature, in CPNC the constraint by the MMT layersreduced the increase to 17%. Reactively prepared CPNC showed a reduction ofcrystallinity at MMT content above 20 wt%. The CPNC had enhanced thermalstability, tensile modulus and increased elastic behaviour over a broadertemperature range than the neat resin. Zhang et al. [2004] used the same methodfor the preparation of neat PA-11 and its CPNC containing 5 wt% MMT.However, the γ-form of PA-11 was induced and stabilised by MMT. Furthermore,the hydrogen bonding in neat resin and its CPNC was quite different.

The crystallisation behaviour of PA-1010 and its CPNC (containing up to10 wt% MMT) were studied using DSC and the results were interpreted withAvrami’s equation [Zhang and Yan, 2003]. Addition of 1 wt% MMT increased

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409


the crystallisation rate, but at higher loading (i.e., 6 and 10 wt% MMT) the ratedecreased. The thermograms for all specimens showed multiple melting peaks.However, such crystallisation parameters as the Avrami’s exponent, n, thecrystallisation temperature, and the heat of crystallisation were virtuallyindependent of the MMT content. In the following publication [Liu et al., 2004],CPNC of PA-1010 was prepared by melt compounding with up to 10 wt% ofMMT pre-intercalated with either methyl tallow dihydroxyethyl ammonium(MT2EtOH) or trimethyl hexadecyl ammonium (3MHDA) salts. XRD showedsignificantly better clay dispersion for the MT2EtOH series (than 3MHDA), whichin turn resulted in superior mechanical performance. At 7 wt% MMT loadingthe glass-transition temperature increased by 8.5 and 1.0 °C for the specimenswith MT2EtOH and 3MHDA, respectively. Addition of the former organoclayaccelerated the crystallisation rate of the matrix and changed its crystallisationbehaviour.

Wu et al. [2002b] studied the nucleating effect of MMT on the crystallisationof PA-1212. The exfoliated CPNC was prepared by melt compounding of pre-intercalated MMT with PA-1212. The film (ca. 0.5 mm thick) was moulded at10.0 MPa and 200 °C for a few minutes, then quenched. Non-isothermalcrystallisation and melting tests were conducted in a DSC under N2. The samplewas heated to 220 °C and kept at this temperature for 10 min, then cooled at arate of 5 to 40 °C/min to 50 °C, and then re-heated to 200 °C at a rate of 10 °C/min.

For PA-1212 and its CPNC, an increased cooling rate shifted the Tc to lowertemperatures, as well as reducing and broadening the Tc peak. Compared to PA-1212the CPNC had narrower exothermic peaks at higher temperatures. In Figure 126the peak crystallisation temperature of PA-1212 and its CPNC, Tc, is plottedversus the cooling rate, r. The overall time of crystallisation, tc ≡ (Ti-Te)/r versusr is also presented (the subscripts i and e indicate the initial and final temperature).Thus, addition of organo-MMT shifts the Tc to a higher temperature and shortensthe crystallisation time, tc. Both these functions indicate enhanced nucleation.The authors also calculated the time for the initiation of crystallisation (ti) aswell as the nucleating activity coefficient, ϕ = 0.71. As expected, these parametersalso indicate enhanced nucleation by MMT. Optical microscopy under polarisedlight, POM, confirmed this conclusion – the spherulites in neat resin were about15¯25 times larger than those in its nanocomposite.

3.4.5 Crystallisation of PO MatrixPP has a complicated crystalline microstructure, which depends on the mechanismand the rate of crystallisation. The four principal crystalline structures of PP are:monoclinic (α), hexagonal (β), triclinic (γ), and smectic or quenched polymorphic.The monoclinic α-form is usually obtained under typical industrial and laboratoryprocessing conditions. The β-form is thermodynamically less stable, and the shearstress or specific nucleating agents enhance its formation. The other forms arequite rare. The characteristic parameters of PP and HDPE are listed in Table 59.

When injection moulding thin-walled articles of PP, it is customary to add anucleating agent. The addition reduces the moulding cycle and improves opticalas well as mechanical properties. Inorganic compounds such as talc, silicates, clays,and carbon black have poor nucleating ability. The preferred nucleating agents areorganic salts, viz. Na-succinate, Na-glutarate, Na-caproate, Na-(4-methyl) valerate,Na-benzoate, Na-β-naphthoate, Al-benzoate, Al-tert-butylbenzoate, bis-benzylidene sorbitol, etc. Pigments are also known to nucleate PP. For example,

410


red pigment (Quinacridone) is an efficient a-crystal nucleating agent, while whitepigment (White PE MB) was found to nucleate the β-form.

By contrast with PP, PE crystallises rapidly, thus additives are rarely used tonucleate it. However, K-stearate has been used to reduce spherulite size.Nanocomposites with PE as matrix are still rare. Recently CPNCs, containingPE-MA and silicates of different aspect ratios, were studied by Wang et al. [2002].The PE-MA was LLDPE grafted with 0.85 wt% MAH. The –MA groups wererandomly placed along the LLDPE macromolecules. The silicates were twoorganoclays (Cloisite® 20A, C20A and Laponite®SCPX2231, SCPX) and syntheticSiO2. The organoclays were both intercalated with 2M2HTA and had the nominalaspect ratio, p = 100 to 200 and 20 to 30, respectively. The silica had sphericalparticles with diameter of ca. 1.8 μm. XRD did not show diffraction peaks downto 2θ = 2°; hence d001 > 4.4 nm. Surprisingly, addition of any of these threesilicates reduced the total crystallinity, X, of PE-MA by an amount proportionalto the silicate volume fraction, φ: X(%) ≅ 41.5 - 120φ. Probably the reaction ofthe randomly grafted –MA groups on the silicate surface disrupted formation ofthe ordered macromolecular crystals.

Young´s modulus and yield stress increased with increasing crystallinity andincreasing α-form content, while elongation at break increased with increasingβ-crystal content [Mubarak et al., 2000]. Addition to PP of 0.05 wt% 1,2,3,4-bis-(3,4-dimethyl-benzylidene sorbitol) or N,N´-di-cyclohexyl-2,6-naphthalatedi-carboxamide, generated the α- or β-form, respectively [Ellis et al., 2001].Infrared microscopy has been used to study the crystalline morphology of thesetwo crystalline forms.

CPNC with PP as a matrix are more recent than these of PA [Kurokawa et al.,1996; 1997]. The authors used an elaborate process involving pre-intercalation ofsmectite clay with ammonium ion, polymerisation of di-acetone acryl amide on the

Figure 126 Non-isothermal crystallisation of PA-1212 and its CPNC as a functionof the cooling rate from 220 °C. Data [Wu et al., 2002].

411


organoclay, and then mixing the doubly intercalated clay with maleated-PP and finallywith PP. Exfoliated and well-dispersed clay platelets were observed under the TEM.Kato et al. [1997] simplified this procedure by melt-mixing PP with oligopropylenemodified by MAH (PP-MA) and MMT intercalated with stearyl-ammonium salt(ODA). CPNC with a high degree of exfoliation was obtained when PP-MA wasable to (1) bond to the clay and (2) be miscible with the PP matrix.

There are several variants of these procedures, with different amounts of thePP-MA compatibiliser, of different structure, molecular weight and MAH-content.Since these variables control the mutual miscibility of PP with PP-MA, this typeof CPNC shows a wide range of properties. For example, pre-intercalation andcompatibilisation may results in a system where the clay particles are shieldedfrom the matrix, preventing direct PP-clay contact, hence the matrix crystallinestructure entirely depends on the behaviour of the PP/PP-MA blend. In such aCPNC the effect of clay on matrix crystallisation may be negligible.

There are two major differences between CPNC with PA and that with POmatrix. (1) ω-Amino acid may be used to intercalate clay, thus after polymerisationthe clay is in full contact with the matrix. By contrast, owing to the stronghydrophobicity of PO-macromolecules such a structure could not form in thelatter systems. (2) The crystallinity of PA rarely exceeds 50%, thus is significantly

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lower than that in isotactic PP where X = 60 to 70% is often observed. Since theintercalated clay platelets cannot enter the crystalline domains, they must beconcentrated in small pockets of the amorphous or meso-crystalline phase. Thismeans that the local concentration is about three times higher than the average,thus (excepting low clay loading of ca. w ≤ 0.5 wt%) the platelets are unable tofreely rotate and form parallel stacks with local ordering – full exfoliation isdifficult to achieve.

The National Chemical Laboratories (NCL) in Pune conducted systematicstudies of clay effects on crystallisation of PP and the resulting performance ofthe CPNC. Hambir et al. [2001] prepared PP-based CPNC with MMT (CEC =1.35 meq/g) intercalated with octadecyl amine (ODA) and compatibilised withmaleated PP (MA-PP). XRD gave the interlayer spacing of d001 ≅ 2.5 nm. Theisothermal crystallisation of the PP and PP/MMT-ODA systems was carried outby DSC. The thermograms indicated that the clay presence narrows thecrystallisation peak and reduces the crystallisation time hence it accelerates PPcrystallisation. As shown in Figure 127, crystallisation in neat PP and in its CPNCmay have different activation energy, but this conclusion hinges on the validityof two data points at low temperatures (T = 100 and 136 °C) where reducedchain mobility may already start slowing down the crystallisation rate. Opticalmicroscopy under polarised light showed a dramatic change of morphology –large, well-defined spherulites in specimens crystallised at 122 °C PP, butcrystallites in the form of a fibrous mat in those crystallised at 142 °C CPNC.

In the following publication from the same laboratory [Kodgire et al., 2001]PP was compounded in a single-screw extruder (SSE) with 12 wt% MA-PP and4 wt% of commercial organoclay, either Cloisite® 6A (C6A; MMT, CEC = 1.4 meq/gintercalated with 2M2HTA), or Nanomer I.30 (MMT, CEC = 1.35 meq/gintercalated with ODA). The XRD-determined interlayer spacing in Na-MMT wasd001 = 1.2 nm, in C6A (two peaks) d001 = 1.2 and 1.8 nm and in I.30 d001 = 2.45 nm.

Figure 127 Crystallisation time as a function of crystallisation temperature in non-isothermal DSC scans for PP and PP/MMT-ODA. Data [Hambir et al., 2001].

413


Compounding with PP increased d001-values, but the system containing C6A wasexfoliated only after compounding with MA-PP, while that with I.30 intercalated(d001 = 2.75 nm). However, the enhancement of mechanical properties was similarfor either CPNC (hence independent of the degree of clay dispersion) – ca. 35%increase in the tensile modulus and about a 10% increase in the tensile strength. Inaccord with the results reported in the preceding publication, the Tc of PP increasedafter addition of clay from 122 °C (for neat PP) to 142 °C. Furthermore, in CPNCthe crystallites formed a fibrous mat that coarsened with time, but did not transforminto spherulites. Thus, clay nucleated a very specific PP morphology.

Another publication from this laboratory provides additional information[Hambir et al., 2002]. This time the ODA intercalated clay was prepared inhouse and used along with C6A. Furthermore, three PP resins (Mw = 164, 241and 362 kg/mol) and two MA-PP resins (Mw = 188 kg/mol, MAH = 1%; and151 kg/mol, MAH = 0.64%) were used. The isothermal crystallisation of PP andPP/clay was studied by DSC at Tc = 115 to 131 °C. The PP/clay systems showeda sharper crystallisation peak than that of PP. Thus, crystallisation of PP wasaccelerated by organoclays. Furthermore, the crystallisation half-time dependedon the organoclay content, Mw of PP as well as on the presence of MA-PP.However, the change of Mw from 241 to 362 had a more pronounced effect thanaddition of up to 4 wt% of organoclay. The crystallisation activation energyseems to be more influenced by the Mw of PP than the other variables. Thus,incorporation of organoclay resulted in enhanced nucleation and fastercrystallisation of the PP matrix. The clay also changed the macromorphology ofthe crystalline phase – in CPNC instead of spherulites fibrillar structures havebeen observed. However, since the authors did not calculate the unit cell, it isunknown whether the addition of organoclay modified the crystal cell structure,e.g., from the common α-monoclinic into the more desirable β-hexagonal one.

Saujanya and Radhakrishnan [2001] studied the crystallisation of PP in thepresence of calcium phosphate nanoparticles (CaP). These were prepared fromCaCl2 and Na3PO4 in a MeOH solution of PEG. Varying the concentration ratioof PEG to CaCl2 from 0 to 32, resulted in reduction of β-ortho CaP (monoclinic)particle diameter from d = 82 to 7 nm. It is noteworthy that in the presence ofCaP PP crystallises in spherulitic form with a monoclinic α-crystalline cellstructure. At 2 wt% CaP the t1/2 and the ultimate spherulite size decreased withthe particle size from 45 to 20 μm. The strong nucleation efficiency of CaPoriginates in high surface energy and large surface area, viz.:

ln(G/Go)∝1/d (155)

where G is the crystallisation rate in the presence of CaP and Go without it. Theoptical transparency of the PP/CaP-nanoparticles system was higher than thatcontaining conventional CaP at the same concentration. The DSC data showedthat incorporation of CaP increased the crystallisation temperature of PP from110 to 114 °C and reduced the width of the crystalline peak as compared to PP.The heat of fusion of neat PP was determined as 77.9 J/g whereas that of thenanofilled PP as 132.9 J/g, indicating that in the presence of 2 wt% CaP thecrystallinity of PP nearly doubled. Such a large effect has not been observed forcompatibilised PP/organoclay systems. The most likely reason is the presence ofan organic phase around the clay particles. Since the intercalant molecules areimmiscible with PP, they form a barrier, separating the crystallisable matrix fromthe high-energy clay surface.

414


3.4.6 Crystallisation of PEST MatrixSeveral patents on the preparation of PET-nanocomposites claim that additionof clay increases the crystallisation rate. When organoclay is used, thecrystallisation temperature and the melting point increase. Again, since there aredifferent methods for the preparation of these materials as well as differentintercalants, it is difficult to predict the extent of these effects.

Tsai [2000] grouped the processing methods described in the patent literatureinto three categories: melt compounding with organoclay, polymerisation inthe presence of organoclay and polymerisation in the presence of synthetic, notpre-intercalated clay (e.g., fluoromica dispersed in ethylene glycol prior topolycondensation with p-terephthalic acid). However, in these publicationscrystallinity has been of little concern.

The crystallisation process and crystal morphology in PET/organoclay CPNCwas studied by Ke et al. [1999]. The nanocomposites were prepared by in situpolymerisation of PET in the presence of up to 5 wt% organoclay (MMT withCEC = 0.7 to 1.1 meq/g intercalated by a ‘proprietary’ method). At lowmagnification SEM and TEM showed a large number of solid particles. Theinterlayer distance, d001 = 1.4 to 3.5 nm, was determined by TEM and XRD,hence only intercalation was achieved. In consequence, while the modulusincreased with clay loading the stress at break decreased. The authors used theAvrami equation for the analysis of non-isothermal crystallisation. The pertinentinformation is summarised in Table 60. As observed for other CPNC systems,also here addition of organoclay increased the crystallisation temperature andthe crystallisation rate. However, in the studied system, the melting temperatureand the heat of melting decreased with organoclay content. The most likelyexplanation is an excess of the non-identified, intercalating agent. CPNC with5% of clay had a broad distribution of particle sizes from ca. 10 to 1,000 nm,with a maximum at about 80 nm. It was difficult to detect spherulites in thesePET/clay systems. The macromolecular chains diffused into the interlamellargalleries.

A more recent publication focuses on poly(ethylene terephthalate-co-ethylenenaphthalate), PETN [Chang and Park, 2001a,b]. First, Na-MMT (CEC = 1.19 meq/g)was intercalated with hexadecyl ammonium salt (HDA). PETN (92 mol% ethyleneterephthalate) was solution-blended with the organoclay then cast into film ofca. 10 to 15 μm thick. The interlayer spacing, as measured by XRD, increased

,erutarepmetnoitasillatsyrC06elbaT Tc ,tniopgnitlem, Tm,,emit-flahnoitasillatsyrc t 2/1 ,noisuffotaehdna, ΔHm TMM/TEProf,

eK[ataD.smetsys .late ]9991,

)%tw(TMM Tc (° )C Tm (° )C t 2/1 )s( ΔΔΔΔΔHm )g/J(

0 471 952 801 0.05

5.0 102 852 – –

5.1 602 752 34 5.34

5.2 902 452 84 0.44

0.5 802 252 63 2.54

415


from d001 = 1.199 (for Na-MMT) to 2.596 nm (for MMT-HDA). The PETNcontaining 2 to 4 wt% MMT-HDA was only intercalated (d001 = 3.104 nm).Furthermore, as the concentration of the organoclay increased to 6 wt% theinterlayer spacing decreased to d001 = 2.662 nm, indicating that at higherconcentration the organoclay aggregates (also observed in SEM). DSCmeasurements of these CPNCs have shown that the glass-transition temperature(Tg), melting temperature (Tm), and heat of fusion (ΔHm) are nearly independentof the organoclay loading (see Table 61). Thus, the total crystallinity of PETNdoes not seem to be affected by MMT-HDA. The structure of these CPNCs is aphase-separated matrix with intercalated, internally ordered organoclay domainsdispersed in it. Nevertheless, incorporation of the organoclay did enhance thethermal stabilities and mechanical properties (maximum in the tensile strengthand modulus was obtained for 4 wt% loading).

Imai and his colleagues from AIST prepared PET-based CPNC by melt, andthen solid-state polymerisation in the presence of ca. 3.7 ± 0.2 wt% of clay [Imai etal., 2003; Saujanya et al., 2003]. Fluoromica (FM), FM pre-intercalated withtriphenyl dodecyl phosphonium bromide (3PPC12), or FM pre-intercalated withtriphenyl di(methoxycarbonyl)phenoxy decyl phosphonium bromide (3PPPC10)were used. The interlayer spacings were: d001 = 1.33, 1.82, and 1.86 nm, respectively.For the crystallisation studies neat PET (PET-0), CPNC with FM (PET-1) and CPNCwith FM-3PPPC10 (PET-2) were used. The isothermal crystallisation tests showedthat the crystallisation rate, and crystallinity increase in the order from PET-0 toPET-2; the opposite tendency was observed for the activation energy. The resultsseem to indicate that, at least for the crystallisation of PET, the degree of dispersionis more important than the surface energy of the dispersed clay. This observationmay explain the small effect of organoclay on PET crystallisation, reported byWang et al. [2003b; 2004], and by Ou et al. [2003].

Recently nanocomposites with poly(trimethylene terephthalate) (PTT) havebeen prepared using the solution intercalation or melt compounding method,and their crystallisation behaviour was studied [Ou, 2003; Liu et al., 2003]. Thefirst author used MMT pre-intercalated with cetyl-pyridinium chloride (CPC),whereas the others used MMT-MT2EtOH. For both systems XRD indicated

,erutarepmetnoitisnartssalG16elbaT Tg ,tniopgnitlem, Tm dna,,noisuffotaeh ΔHm C/NTEProf, 61 .setisopmoconanTMM-

ataD ]1002,kraPdnagnahC[

)%tw(ADH-TMM Tg (° )C Tm (° )C ΔΔΔΔΔHm )g/J(

0 67 632 23

1 47 732 13

2 37 832 13

3 57 832 03

4 57 832 23

6 47 632 03

416


intercalation with short stacks dispersed in the matrix. From the DSC thermogramsthe authors concluded that organoclay behaves as a nucleating agent, enhancingthe crystallisation rate of PTT. As in other systems, here also a maximumcrystallisation rate was observed at relatively low organoclay loadings.

3.4.7 Crystallisation of Syndiotactic PS MatrixWu et al. [2001] analysed the effects of MMT on the chain conformation andcrystallisation of syndiotactic polystyrene (sPS) using FTIR, XRD, and TEM.The CPNC was prepared by solution dispersing of 5 wt% of MMT or organoclay(MMT pre-intercalated with cetyl-pyridinium chloride, CPC) in a dichlorobenzenesolution of sPS at 140 °C for 24 h. For the first system TEM showed largeaggregates (dimensions from a few tenths to 100 nm), but a high degree ofintercalation (size 1-2 nm) in the second system: sPS/MMT-CPC.

sPS has four crystalline forms: α-, β-, γ-, and δ-, which can be divided into twogroups: (i) γ- and δ- forms possessing a helical trans-trans and gauche-gauche (TTGG)chain conformation and (ii) α- and β- forms possessing a planar all-trans (TTTT)‘zigzag’ conformation. Addition of clay changes the sPS chain conformation fromTTGG to TTTT. This phenomenon leads to a change in the mechanism of molecularpacking during melt-crystallisation. The clay plays a vital role in facilitating theformation of the thermodynamically favoured all-trans β-crystal, particularly in thinfilms. Crystallising the sPS/MMT system at a cooling rate of 1 °C/min from 320 °Cresulted in the highest absolute crystallinity of β-form of 56%. This is to be comparedwith 49% obtained for sPS/organoclay and 42% for neat resin. Thus, clay maysignificantly affect the chain conformation and crystallisation of sPS, but the effectvery much depends on clay surface energy. It is noteworthy that the effect of poorlydispersed clay with significantly reduced surface contact area affected sPS more thanthe finely dispersed cetyl-pyridinium covered clay platelets.

Another team of researchers [Wu et al., 2002a] confirmed these observations.The nanocomposites were prepared by dispersing 0.5, 1 and 5 wt% organoclayin sPS/xylene solution. The organoclay was MMT pre-intercalated with 3MHDAand then treated with a mixture of monomers (styrene and methacrylate in theratio 4:1), which were then polymerised into copolymer having short PS blockson the outside. Both XRD and TEM showed exfoliation and random dispersionof clay platelets in the matrix. XRD also indicated polymorphism of sPS, whichstrongly depended on thermal history and clay content. Isothermal crystallisationat higher Tc favoured formation of β-crystals. Thus, during the process the α-crystalscould be transformed into β-form. During crystallisation, the sPS chains achievedbetter alignment at higher Tc. In the presence of MMT or at lower Tcconformational defects may preclude perfect chain alignment.

At the present state of knowledge, it is impossible to make generally validstatements about the influence of organoclay on the polymeric matrix crystallinity.There are many factors that influence the outcome. The presence of a high-energyclay surface most often leads to an enhanced crystallisation rate, which in turnresults in a different, less thermally stable crystalline form (viz. γ-form in PA-6,β-form in PVDF [Priya and Jog, 2003], etc.) crystallising at higher Tc and meltingat higher Tm. Notwithstanding the higher crystallisation rates, the total crystallinitycontent may increase or decrease with organoclay content. These effects arecontrolled by the equilibrium thermodynamics (type and intensity of interaction)as well as dynamics (orientation and annealing).

417

Mechanical Behaviour

One of the reasons for the interest in CPNC is enhancement of the mechanicalperformance of the matrix polymer at low clay loading. For example, additionof 2 wt% of organoclay to PA-6 increased the flexural modulus by 26%, andtensile strength by 14% [Ube Ind., 2000]. Similarly, dispersion of 2 and 5 wt%of vermiculite in PP increased the tensile strength by 18 and 30%, the tensilemodulus by 20 and 54%, and the storage modulus by 204 and 324%, respectively[Tjong et al., 2002]. A thorough review of the theories of mechanical behaviourin a multiphase polymeric system can be found in the PhD thesis of D. Colombini[1999].

3.5.1 Micromechanics of CPNCThe assumed model of a nanocomposite is highly simplified, containing eitherexfoliated clay platelets or intercalated short stacks, aligned with the stress tensor.The particles are assumed to perfectly adhere to the matrix. The simplest modelof perfectly aligned large reinforcing particles leads to the volumetric rule ofmixtures for the tensile modulus in the stress and transverse direction:

in stress E E f E

transverse E E E

c m m L f f

ct m m f f

:

: / / /

= += +φ φ

φ φ1(156)

where E is tensile modulus, φ is the volume fraction, φm = 1 - φf ; subscripts c, mand f stand for composite, matrix and filler, respectively; and fL is the filler particlelength correcting factor. The dependence predicted by Equation 156 is presentedin Figure 128. Note that the predictions are supposed to be valid for infinitelylarge filler particles with perfect adherence to the matrix. For the computationsthe moduli ratio was assumed to be Ef /Em = 100 or 1000.

Several modifications of Equation 156 have been published:

Nicolais Narkis E E

Faber Farris E E

Kerner E E AB B G G E E

A

B E E E E A

Neilson

c m fb

c m f

b

c m f f c m c m

m m

f m f m

& :

& :

: / ; / /

/

/ / /

= −( )= −( )

= +( ) −( ) =

= −( ) −( )= −( ) +( )

1

1

1 1

7 5 8 10

1

αφ

φ

φ φ

ν ν

:: / ; / /

/,max ,max

E E AB B G G E Ec m f f c m c m

f f f

= +( ) −( ) =

= + −( )[ ]1 1

1 1 2

φ φ

φ φ φ

Ψ

Ψ

(157)

where νm is the Poisson’s ratio.

3.5 Mechanical Behaviour

418


Halpin-Tsai (H-S) rigorously derived the tensile modulus in the stress direction,Ec:

E E p

E E p E E E

c m f f

r r r f m

/ /

/ ; /

= +( ) −( )= −( ) +( ) ≡

1 2 1

1 2

κφ κφ

κ(158)

Here p is the aspect ratio defined in this book as p = (platelet diameter)/(plateletthickness). For the modulus in transverse direction p = 1 may be assumed. For p→ ∞ Equation 158 converts into Equation 156 with fL → 1. The predictedbehaviour is illustrated in Figure 129.

According to Brune and Bicerano [2002] the above relationships may be usedonly for predicting the modulus of fully exfoliated CPNC with oriented, clayplatelets perfectly adhering to the matrix. When the clay platelets are onlyintercalated then the variables: p, Ef, and φf obtained from fitting the equationsto data must be consider empirical. On the other hand, the authors derived thefollowing relationships between these variables for exfoliated and intercalated(symbols with prime) systems:

′ = ( ) + −( )( )[ ]′ = + −( )( )[ ]′ =

+ −( )( )+ −( )( )

p p N N s t

N s t

EE N s t

N s tr

r

/ / / /

/ /

/ /

/ /

1 1 1

1 1 1

1 1

1 1 1

φ φ (159)

In this relation the aspect ratio, p´, the volume fraction, φ´, and the tensile modulus,E´, refer to the platelet stack composed of N-clay layers, each with thickness tand the interlamellar gallery spacing, s ≅ d001 – 0.96 nm. However, since the

Figure 128 Relative tensile modulus for Ef /Em = 100 or 1000 in the stress andtransverse to it directions. See Equation 156.

419


dependence does not have the correct upper limit for the s/t ratio, the parameterN in the dependence should be replaced by:

′ = + −( )( ) −

N N N s t11

/φ

φ(160)

Evidently, Equations 157 to 159 are valid for idealised, well-oriented CPNCswhere the platelets are perfectly bonded to the matrix. The important teachingof this theory is the strong influence of the degree of dispersion on the relativemodulus, Ec/Em = E´/Em. In Figure 130 variation of the relative modulus forintercalated CPNC is plotted as a function of the number of platelets in the stack,N, and the size of the interlamellar gallery expressed as: s/t = (d001/t) – 1 ≅ d001 – 1.The role of this parameter is to express the modification of the macromolecularbehaviour within the galleries. It is noteworthy that the steepest decline ofperformance is predicted for stacks containing 2 to 3 platelets, i.e., for thesemost often observed in CPNC. In other words, the modulus of a truly exfoliatedsystem is at least twice as high as that obtained for the most common ‘exfoliated’ones.

Brune and Bicerano [2002] proposed a further refinement to the abovetreatment for disk-shaped filler particles. The aim was to correctly predict theeffects of the aspect ratio. The derived equation is in a scaled form:

EE p E p

E p E p

p

E pE

E

Esc c

c c r f

rf

m

≡( ) − =( )→ ∞( ) − =( ) =

−( )−( ) −( ) + +

≡1

1

2 1

1 1 1 2φ; (161)

The relation virtually traces the prediction of the Halpin-Tsai relation (seeFigure 131). Within the customary range of the clay content, φf < 5 vol%, it isinsensitive to composition, but it strongly depends on Er and p. It is important tonote that when a high Er value is required, platelets with larger aspect ratioought to be used. Considering that E(clay) ≅ 170 GPa and the modulus ofunoriented, unfilled engineering plastic ranges from 2 to 4 GPa the clay platelets

Figure 129 Predictions of the Halpin-Tsai Equation 158 for relative modulus inthe stress direction for Ef /Em = 100 or 1000 and p = 10 or 1000.

420


Figure 130 Prediction of the Brune-Bicerano theory (see Equation 159) of therelative modulus of intercalated CPNC versus the average number of platelets in

the stack for the interlayer spacing, d001 ≅ (s/t) + 1. For the most common value ofd001 ≅ 3 nm stacks of ca. three platelets show only 40% of the modulus expected

for exfoliated CPNC.

Figure 131 Scaled tensile modulus versus aspect ratio according to Halpin-Tsai (H-T) and Brune-Bicerano (B-B) equations. Moduli ratio: Er = Ef /Em = 100 or 1000,

filler volume fraction, φf = 0.01 or 0.05. Both relations provide equivalentdependence, insensitive to φf but sensitive to Er.

421


with p ≅ 200 would reach about 90% of the theoretically possible modulus. ForCPNC with more ductile polymeric matrices clay with still higher p should beused.

Hui and Shia [1998] derived a simplified H-S theory for the tensile modulusof oriented fibre or flake reinforced composites. The simplification involved theassumption that the Poisson’s ratio, νm = 0.5, is the same for both componentsand that there is perfect adhesion between filler and matrix. For flakes orientedin the stress direction the modulus is given by:

E E

E

g gp

p

p g

pg p

c m

r

= − ++

⎡

⎣⎢

⎤

⎦⎥

⎧⎨⎪

⎩⎪

⎫⎬⎪

⎭⎪

≡ +−

+ −( )−( ) − ( )

−

≡ −( )+( ) −

−≅

−

14

1 3

11

3 11 2

1

13 1 0 25 2

12

1

2

2

2

2

φξ ξ

ξ φ φ

φ π

Λ

Λ

/

.; /

(162)

Predictions of Equation 162 are higher and are more sensitive to the aspect ratiothan those from Halpin-Tsai (H-T) Equation 158.

The Hui-Shia (H-S) theory was evaluated using the tensile modulus data forCPNC with PDMS as the matrix [Shia et al., 1998]. The experimental results didnot follow the theoretical predictions of H-T and H-S theories. To explain thediscrepancy the authors postulated that the difference originates in imperfectbonding between the matrix and clay. This introduced two ‘effective’ quantitiesinto the H-S model: the aspect ratio and clay volume fraction. Both were assumedto depend on the interfacial shear stress, σi, determined by fitting the data to theequation – the calculated σI-value was in the range of kPa. The interfacial shearstress was further decomposed into an intrinsic and a frictional shear stress, theformer decreasing with the increasing volume fraction of the inclusion and thelatter increasing linearly with strain. For the CPNC studied, the effective frictioncoefficient was calculated as 0.0932.

However, as shown in Figure 132, in the whole range of concentration thedata are well described by Brune-Bicerano Equations 159-161, with Ef = 170 GPaand Em = 1 MPa. The numerical values of the remaining variables were takenfrom Shia et al. [1998]. The least squares fit gave a reasonable value of the aspectratio, p = 239. The number of platelets in the stack increased from the initialvalue of N = 2 to 4 at the highest concentration. The standard deviation and thecorrelation coefficient squared were: σ = 0.18 and r2 = 0.995, respectively.

Ji et al. [2002] expanded Takayanagi’s two-phase model into three-phases:the matrix (m), interphase (i), and filler (f), randomly distributed in the matrix.The filler particles may be spherical, cylindrical or lamellar. The incorporationof an interphase is particularly important for the nanoparticles. According to themodel, the three phases are connected to each other in series and in parallel. Therealistic representation of a filled system is then reduced to three idealised regions,A, B, and C connected in series. The general form of the derived relation is:

1 11 1E E E E E E Ec m m Bi m Ci f

= − + −−( ) +

+−( ) + −( ) +

β β ϕα α

ϕα α λ λ

(163)

422


where α, β, ϕ and λ are idealised dimensions of the interphase and filler regionsdetermining the volume fraction of each of the three components, viz. Vf = λϕ; Vi= αβ – λϕ; and Vm = 1 – Vf - Vi. The moduli of the composite, matrix and fillerare given as: Ec, Em, and Ef. The interphase modulus is defined for the modelregions B (in series) and C (in parallel) as EBi and ECi , respectively. When particlesare relatively large the interphase contribution is negligible, Vi → 0 andTakayanagi’s equation is recovered.

The principal advantage of this theory is incorporation of the interphasecontribution. As stated before, the authors consider two types of interphasesconnected to other phases either in parallel or in series. In the case of CPNCsthese may be treated as the interphase inside the stack of clay platelets, and thatbetween the stacks and the matrix. The two types depend on the distance fromthe filler surface respectively, linearly and exponentially:

120

00 0E

E E

E EE E E E E k

Bi

i m

i mCi i m i m=

( )−

= +[ ] ≡ln /

; / : /with(164)

where E0i is the interphase modulus on the filler surface at τ = 0 and k is one ofthe model parameters. The platelet and the interphase shapes were assumed tobe square, thus:

α β τ ϕ λ= = ( ) +[ ] = =2 1/ ; t V Vf f (165)

Substituting these relations into Equation 164 yields:

Figure 132 Relative modulus versus clay content for CPNC with PDMS as thematrix. Experimental data [Shia et al., 1998] and theoretical prediction from

Brune-Bicerano (B-B) theory. See text.

423


1 1 2 1 2 1

1 2 1 2 1 1

1 2 1 2 1

E

t V

E

t V V

t V E t V k E k

V

t V E t

c

f

m

f f

f m f m

f

f m

=− ( ) +[ ]

+( ) +[ ] −

− ( ) +[ ]⎧⎨⎩⎫⎬⎭

+ ( ) +[ ] −( )

+− ( ) +[ ]⎧⎨⎩

⎫⎬⎭+ ( ) +[ ]

τ τ

τ τ

τ τ

/ /

/ / / ln

/ / VV V k E V Ef f m f f−⎧⎨⎩⎫⎬⎭

+( ) +1 2/

(166)

For CPNC the controlling parameters are: the dispersed particle size: t << ϕ = λ,thickness of the interfacial region (τ), filler-to-matrix modulus ratio (Ef/Em), anda parameter (k) describing a linear gradient change in modulus between the matrixand the surface of the particle. Since t and Ef/Em are in principle known, themodel has two adjustable parameters: τ and k; validity of the model can be judgedby the ‘reasonableness’ of these parameters computed from fitting Equation 166to the experimental data.

To test their model, Ji et al. [2002] prepared CPNCs of PA-6/MMT and measuredthe tensile properties. The experimental data were fitted to the theoreticaldependence with τ = 7 nm and k = 12. It is worth recalling (see Section 3.1.7) thatthe thickness of the solidified layer of organic compounds on the surface of clayplatelets as determined in the surface force analyser (SFA) ranges from ca. 3 to 12nm, with most of the values centred near 6 nm. For polymers the layers withreduced segmental mobility stretched to a distance of about 100 nm. However,these measurements were taken in a liquid not solid state. In the model the twoparameters, τ and k characterise the interphase: τ its thickness and k its rigidity.The computed value of τ = 7 nm well agrees with the thickness of the PA-6 layerthat is adsorbed on the clay surface and solidified in the molten state, e.g., at240 °C. Evidently, this is a highly satisfactory outcome! As shown in Figure 84,the solidification was found to significantly reduce the free volume in moltenpolymer. Reduced free volume should translate into higher modulus, thus k > 1would be expected. The experimental value of k = E0i/Em = 12 is high (especiallysince Ef/Em = 40 was assumed), but it may be reasonable (the ratio of the crystallineto the amorphous modulus for the same polymer ranges from 2 to 6 [Ferry, 1980]).

In this book there are several tables listing the tensile modulus versus clayloading (inorganic part). These values were used to generate Figure 133. The aimwas not to prove or disprove any of the listed theories, but to show how the bestCPNC performs in an amorphous polymer matrix (PS), in a semicrystalline end-tethered exfoliated matrix (PA-6), and in a semicrystalline not tethered matrixwith a compatibiliser (PP). For these systems MMT was used with an averageaspect ratio p = 200 to 300. The results indicate that independently of the matrixthe CPNC modulus doubles at clay content of about 5 wt%. CPNC comprisingfluoromica (FM) instead of montmorillonite (MMT) has lower modulus, mostlikely caused by lower aspect ratio, p. Lower modulus is also observed for systemswith poorly dispersed clay platelets.

In dilute systems the tensile modulus linearly increases with clay loadingfollowing the relation:

E E E aw w wtR m≡ = + [ ] = + ≤/ ; %1 1 8η φ (167)

424


Figure 133 Relative tensile modulus versus clay content for CPNC of PA-6, PP,PLA and PS with montmorillonite (MMT), as well as for PP with fluoromica

(FM). See text.

Here the reinforcing factor [η] is the hydrodynamic volume, for monodispersedhard spheres (p = 1) it is equal to the Einstein value of 2.5. The parametera =

η ρ ρ η[ ] ( ) [ ]/ / /100 314f m . The lowest predicted value for hard spheres is a

= 0.008. For poorly exfoliated PP, PA-6 and polylactic acid (PLA) the values ofER increase with the slope a = 0.07. Substituting the equivalent value of [η] intoEquation 107, gives the aspect ratio: p = 93. On the other hand, fully exfoliated,best performing systems follow the dependence with a = 0.20 equivalent to p = 200.The latter value is close to that expected for the commercial organoclays withMMT. It is noteworthy that the values for PP reinforced with FM can also beapproximated by Equation 167 with a = 0.13 equivalent to p = 150. This value isclose to p = 132 calculated from data by Zilg et al. [1999a].

It has been universally accepted that the values measured for CPNC followingwell-established procedures for polymeric specimens represent the inherent materialbehaviour. However, the same PS/organoclay or PP/organoclay/compatibilisersystem tested in two laboratories resulted in not only different values, but alsodifferent tendencies for increasing clay content. Part of the answer can be foundin the recent work by Uribe-Arocha et al. [2003]. The authors prepared CPNCof PA-6 (with 0 and 5 wt% clay) by injection moulding the specimens withthickness of 0.5, 0.75, 1.0 and 2.0 mm. These were subsequently tested by DMAand in a tensile tester. Both methods showed only a small effect of thickness onthe performance of PA-6. However, the effects for CPNC with 5 wt% clay weresevere. As an example, Figure 134 displays the dependence of the tensile modulus(E) on specimen thickness (t). The data indicate that increasing t from 0.5 to2 mm does not affect the tensile modulus of neat PA-6, while it reduces E(CPNC)by ca. 25%. Similarly, for PA the stress at 2% strain ranged from ca. 46 to52 MPa,whereas for CPNC specimens with thickness: t = 0.5, 0.75, 1.0 and 2.0 mm thestress at 2% strain was: 84, 84, 72 and 61 MPa, respectively. Other measuredparameters showed similar dependencies.

~

425


Explanation for this behaviour rests in the skin-core structure of injection mouldedspecimens. The data show only limited effect of skin on PA-6 behaviour – themolecular alignment near the mould surface was not too severe. In the case ofCPNC, the orientation of clay platelets near the wall causes significantimprovement of modulus. It is noteworthy that theoretically the ratio:

E(perfect orientation)/E(random orientation) = 1/2

thus the observed decrease by 50% is reasonable. Specimens with thicknesst ≤ 0.75 mm had similar stress-strain dependence, whereas there is significantreduction of yield stress, σy, for thicker specimens. SEM of these specimens testedunder uniaxial loading showed multiple voiding in the core. After this initialstage (evidenced by necking) the skin with aligned clay platelets maintained theload – the specimen failed when the skin broke.

During the last few years a new method of computation of mechanicalproperties of nanocomposites has been developed. This multiscale approach ismost advanced for systems containing carbon nanotubes [Odegard et al., 2002].The method consists of three-steps: (1) atomic/molecular dynamics (MD)modelling of the nanoparticle and its interactions; (2) construction of arepresentative volume element (RVE); and (3) computation of macroscopicbehaviour of the nanocomposite from RVE, using either classical continuumexpressions or a finite element method (FEM). The latter method was used bySheng et al. [2004]. For CPNC the authors applied a multiscale modelling strategyassuming that: (1) at a length scale of millimeters, the structure is one of highaspect ratio particles within a matrix; (2) at micron-scale, the clay particles areeither exfoliated or in the form of intercalated stacks; (3) at a nanoscale theinteractions between the matrix and nanoparticles must be computed. In addition,the matrix may be amorphous or semicrystalline. In the latter case the effects of

Figure 134 Tensile modulus versus specimen thickness for PA-6 and its CPNCwith 5 wt% clay. Data [Uribe-Arocha, 2003].

426


polymer lamellae orientation and transcrystallisation must be accounted for.Starting with the structural parameters (extracted from XRD and TEM) andplatelet modulus obtained from MD, a layer of matrix surrounding thenanoparticle was modelled. Next, numerical or analytical models based on the‘effective clay particle’ were used to calculate the CPNC elastic modulus. Thenew model correctly predicted the elastic moduli for CPNC with MXD6 or PA-6matrix. The model predicted only a moderate increase in the overall modulus forexfoliated clay as compared to that comprising stacks of intercalated clay. Shenget al. even speculated that experimentally (due to curving of clay platelets) fullexfoliation might not result in the highest modulus.

3.5.2 Prediction of Tensile StrengthPrediction of the tensile strength, σ, for composites or nanocomposites is moredifficult than that for modulus. Strength involves transmission of stresses throughthe tested specimen. Thus, strength must depend on the degree of exfoliation, thetype of bonding between clay platelets and the matrix, as well as on thecompatibiliser. The simplest case is for continuous particles that perfectly adhereto the matrix. The tensile strength in the stress direction is expected to follow thevolumetric rule of mixtures:

σ σ φ σ φ

σ σ σ φ σ

σ σ σ

c m m f f

R c m f r

r f m

= +

≡ = + −( )≡

/

/

1 1 (168)

where, as before, the subscripts c, m and f stand for composite, matrix and filler,respectively. An example of experimental findings is shown in Figure 135. In thefigure, the relative tensile strength versus volume fraction of organoclay is shown.

Figure 135 Relative tensile strength for a biodegradable polyester of glycol +butanediol + succinic + adipic acids (Mw = 60 kg/mol) with Cloisite® 30B (circles)

and Cloisite® 10A (squares). Data Lee et al. [2002b].

427


The CPNC contained a biodegradable polyester of glycol + butanediol + succinic+ adipic acids (Mw = 60 kg/mol) as a matrix in which either Cloisite® -30B or -10A(C30B or C10A, respectively) were dispersed [Lee et al., 2002]. XRD data indicatedthat the former system was well dispersed, especially at high clay concentration.By contrast, the system with C10A was intercalated with d001 = 3.25 nm. Evidently,the hydroxyl group in C30B provided superior interaction between the intercalatedorganoclay and the matrix, while the benzyl of C10A failed. As a result thetensile strength of the C30B system followed the volumetric rule of mixtures upto at least 10 vol%, while the C10A system followed it only up to ca. 4 vol%.The latter type of behaviour with a local maximum is most common. The secretof success is to find at which composition the maximum performance is reachedand optimise the process to obtain reproducible performance.

According to Equation 168, the slope of the straight line in Figure 135 providesa low value for the relative strength: σr = σf/σm = 6.39, giving σf = 6.39 × σm= 83 MPa. By analogy to well described micas, the flexural strength of clay plateletsshould also depend on the aspect ratio, p. The tensile strength of mica withp = 100 is about σf = 240 MPa [Milewski and Katz, 1987], i.e., about three timeshigher than the value calculated for the clay platelets. The reduced tensile strength(as well as moduli) of the reinforcements may originate from: stress concentrationat the edges, imperfect alignment with the stress direction, imperfect spacing inthe matrix, imperfect interaction between platelets and the matrix, interactionbetween the flakes, etc. Furthermore, the failure can take place by diversemechanisms, e.g., by fracturing the flake or by pulling it out.

The full reinforcing effect can be reached only if the width of the reinforcingplatelet, W, is [Clegg and Collyer, 1986]:

W>Wc = tσf /σfm (169)

where t is flake thickness and σfm is the interfacial shear strength between clayplatelet and matrix. These considerations lead to revision of Equation 168 into:

σ σ φ σ φ σ φ σ

φ φ

c m m R f f R f R r

R

m f f f

F F

Fu

uh u

u p G E

= + = + −( )

= −( )⎡

⎣⎢⎢

⎤

⎦⎥⎥

− ( )

= −( )[ ]

;

tanhsec

/

1 1

1 1

1

(170)

where Gm is the matrix shear modulus, Ef is the flake tensile modulus and FR isthe filler strength reducing factor. For the aliphatic polyester (APES)/clay system[Lee et al., 2002b] the calculated value of FR = 0.667 leads to a value for thetensile strength of clay platelets σf = 125 MPa, approaching the expected strength,but still smaller by a factor of about 1.8. It should be noted that the definition ofFR in Equation 170 predicts that: 0.666 ≤ FR ≤ 0.900.

Kerner’s composites theory provides the following relation for the tensilestrength:

σ σ φ φ

ν ν

σ σ σ σ

c m f f

m

f m f m

AB B

B A

= +( ) −( )( ) −( )

= −( ) +( )

1 1

8 10

1

/ ; :

/

/ / /

where

A = 7 – 5 m (171)

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The dependence is identical to that shown in Equation 157 with strength (σ)replacing the modulus (E).

Finally, an interesting relation was proposed by Turcsáyi et al. [1988]. Theauthors corrected for the decreasing load bearing cross-section of a specimenand for the effects of the interfacial interactions:

σ σ σ φ φ φ

ρ σ σR c m f f f

f i m

B

A l

≡ = −( ) +( ) { }( ) ( )

/ / . exp ; :

ln /

1 1 2 5 where

= 1+ fB(172)

In Equation 172 Af and ρf are the specific surface area and density of the reinforcingparticles, while l and σi are the thickness and strength of the interphase. It isworth noting that B < 0 for σi < σm – the other variables only scale the effect. Thedependence predicted by Equation 172 is illustrated in Figure 136. For the yieldstress of composites to be about equal to that of neat polymer the value of theB-parameter should be about 3. Substituting the largest possible values (Af =750 m2/g; ρf = 5000 kg/m3 and l = 100 nm) indicates that this condition would bemet if σi > 1.01σm. Thus, for nanocomposites able to adsorb and solidify themacromolecules one may expect that the tensile strength will increase with loadingof clay platelets.

3.5.3 Fatigue Resistance of CPNCProperties such as modulus and strength provide information on the materialbehaviour at relatively low deformation under steady-state loading. In particular,the modulus (measured as the initial slope of the stress-strain curve) reflects theinitial structure of the material. The other type of mechanical testing addressesthe ultimate properties – how the material breaks. The three principal test methods

Figure 136 Relative tensile strength of CPNC as a function of the clay plateletvolume fraction and the interaction parameter B from Equation 172 [Turcsáyi et

al., 1988].

429


used by fracture mechanics are impact tests, steady-state deformation (at stressesexceeding the strength of the material) and fatigue [van der Giessen andNeedleman, 2002].

In the latter case the fatigue crack grows when the stress intensity (cyclingbetween σmin and σmax) is much smaller than that needed for crack growth understeady-state loading. The cycling causes failure at stresses well below the tensilestrength level. Analysis of this behaviour is done by determining the stress versusnumber of cycles to failure (S-N diagram) curves, usually followed by studies ofthe mechanism of crack propagation and determination of the fracture energy.

Fatigue crack growth will take place if and only if the following three conditionsare met: (1) the stress amplitude, σa = σmax – σmin exceeds a critical value; (2) themaximum stress intensity σmax exceeds its critical value; and (3) there is an energydissipation process during the cycle (fatigue fracture cannot occur in a fully elasticsystem without memory). Above the threshold stress value, the crack growthrate, ∂l/∂N, is a function of the stress amplitude, σa. To generate an S-N diagramthe specimen may be subjected either to constant stress amplitude or to the meanstress, respectively defined as [Moet, 1986]:

σa = σmax – σmin; σmean = (σmax + σmin) / 2 (173)

Construction of the S-N curve is time consuming since for statistically valid resultsca. 10 samples have to be tested at each stress level.

In Figure 137 a schematic S-N plot for PS is shown [Sauer et al., 1976]. Thespecimens were tested at a frequency of 1600 cycles/min at constant maximumstress, σmax = 17.25 MPa, systematically varying sa from about 4.75 to 17.25 MPa.There is a substantial difference in the material behaviour when cycling is done atthe same level of stress amplitude, but either fully in tension, e.g., σmin = 0 and σmax= 17.25 MPa, or in tension and compression, e.g., σmin = -8.62 and σmax = +8.62 MPa

Figure 137 Schematic representation of the S-N plot for PS at 500 Hz. After [Saueret al., 1976].

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– the corresponding lifetime is N = 100 and 30 k-cycles, respectively. Thus, stressreversal has a deleterious effect on the specimen endurance. As shown inFigure 137, at σa ≅ 6 MPa there is a change in the slope of the S-N plot, suggestingtwo different mechanisms of fracture below and above this limit. Since the usualrequirement is that the material survives 107 to 108 cycles, here the ‘endurancelimit’ is about 4.4 MPa.

The main difference in fatigue behaviour between metals and polymers is thesensitivity to test frequency. While metals are insensitive to frequency, polymerswith high energy dissipation and low thermal conductivity are. Depending onthe material characteristics (e.g., internal friction coefficient and thermalconductivity), frequency, ν, and stress amplitude, σa, two mechanisms have beenidentified. The boundary between these two is determined by the rate of energyinput: (∂E/∂t) ∝ν × σa. At low rates (∂E/∂t) < (∂E/∂t)crit the material is able todissipate the energy of deformation, thus the temperature rises only to a specific,dynamic level. Under these conditions, the fatigue fracture proceeds by theconventional fatigue crack initiation and propagation (FCP) mechanism. However,when the rate of energy input exceeds the critical value, there is an unboundedtemperature increase. Under these circumstances FCP cannot proceed and thespecimen fails by the thermal fatigue failure (TFF) mechanism [Crawford andBenham, 1975]. These authors developed an empirical relation between the criticalvalue of the stress amplitude, σa, crit, and frequency, νcrit:

∂ ∂ ∂ ∂ σ νE t E t c c A V

crit a crit crit/ / log /,( ) = ( ) ⇒ = −( )1 2 (174)

where ci are material parameters, while A and V are specimen surface area andvolume, respectively.

The data schematically shown in Figure 138 are based on the reversed loadfatigue tests of polyoxymethylene (POM or acetal) at different levels of frequency:ν = 0.167 to 10 Hz and stress amplitude, σa. At low frequency and low σa theconventional FCP fracture mechanism was observed and the data followed theFCP-type dependence, shown in Figure 137. However, when the frequency andstress amplitude exceeded the critical values for POM, the number of cycles tofailure, N, dramatically decreases forming TFF-branches, each for a specificfrequency. Evidently, the two mechanisms lead to significantly differentmorphology of the fractured surfaces. Stress concentration caused by a notchreduces the critical value of the energy rate, thus it increases the probability ofthe TFF mechanism.

The FCP mechanism often starts by craze formation which, depending on thematerial and test conditions, leads either to cracks or shear banding, which inturn proceed to the crack propagation stage. The crack opens either in a singlecraze or shear band. The initiation is best described in terms of the fracturemechanics stress intensity factor, σth ∝ l r/ where l is the crack length and r itsradius of curvature. Growth of the crack length with the number of cycles duringthe fatigue test was found to be proportional to the amplitude of the stressconcentration factors in the tensile mode, σKI = σI, max – σI, min:

∂ ∂ σl N co lm/ = Δ (175)

where co and m are characteristic constants of this ‘Paris law’ equation. Continuumtheories that assume that the crack growth rate is proportional to the crack openingdisplacement predict the exponent value: m = 2. Nguyen et al. [2001] have

431


developed a finite element model to simulate the fatigue behaviour of a specimenunder plane strain. The material was characterised by a continuum plasticitymodel, with a combination of isotropic and kinematic hardening, a cohesive lawthat progressively softens and has loading-reloading hysteresis, which results inthe crack tip blunting. The numerical calculations of the fatigue crack growthrate predicted m ≈ 3, confirmed with the experimental data for aluminium. Thedamage accumulation models predict the exponent value, m = 4, frequentlyobserved for polymeric systems.

The effect of exfoliated clay on fracture and fatigue behaviour has seldombeen studied. For example, PA-6 and CPNC of PA-6 containing 2 wt% oforganoclay (Ube resins: PA-1015B and -1015C2, respectively) were dried andinjection moulded [Bureau et al., 2001; Gloaguen and Lefebvre, 2001]. The formerauthors measured the tensile and fracture toughness using either dried orconditioned at 50 RH specimens. The tensile properties were determined in themachine direction, while the fracture tests with a notch were carried out in thetransverse direction. The tensile modulus, E, tensile strength, σy, elongation atbreak, εb, stress intensity factor in mode I, KIc, and the fracture energy parameter(composed of two parts, elastic and plastic: Jc = Jel + Jpl) were determined. Theparameter Jc is a measure of an internal energy change per unit crack length. Itcharacterises the fracture toughness prior to the onset of stable crack extension.Its value is independent of the in-plane dimensions.

Selected results from Bureau et al. [2001] are listed in Table 62. Note that themechanical properties of semicrystalline reinforced polymers depend oncrystallinity and reinforcement. Addition of clay to PA-6 induces formation ofthe γ-crystalline form, which during annealing may convert to the more stableα-form (more information on the topic is in Section 3.4.2). Furthermore, injectionmoulded PANC specimens have skin dominated by the α-form and core by the

Figure 138 S-N plot for two mechanisms of failure in POM: thermal fatigue failure(TFF) and fatigue crack propagation (FCP). After [Crawford and Benham, 1975].

432


γ-form. In these nanocomposites the level of crystallinity is also higher than thatin neat PA-6, which in part accounts for the increased values of E and σy as wellas for the reduction of εb.

The data in Table 62 show that addition of 2 wt% of organoclay (equivalentto 0.64 vol% of MMT) has a large effect on the tensile (Young´s) modulus, E,especially when the matrix is plasticised by the moisture. There is also a significantincrease of the yield (or ultimate) strength, σy. However, the elongation at break,εb, is reduced – the dry CPNC specimens are particularly brittle. From the fracturetest data the stress intensity factor, KIc, and the fracture energy, Jc, were computed.For CPNC the values of these parameters are also below those for neat resin.Noteworthy is the difference in the moisture effect on the fracture mechanics ofPA and CPNC – while for PA the humidity changes linear-elastic behaviour intoan elastoplastic one; for CPNC linear-elastic behaviour is observed for both, dryand conditioned specimens.

These results are in good agreement with data reported by Gloaguen andLefebvre [2001], who also studied the Ube material. The authors carried outtensile tests at constant strain-rate of 10-3 s-1, recording the stress, linear strain,and volume strain (a specific video-extensometer was used). CPNC showed brittlebehaviour at room temperature (εb = 5-7%). At T = 80 °C (above the glasstransition temperature, Tg) the elongation at break for PANC was higher thanthat for PA-6 (εb = 900 versus 800%). The authors also reported large volumeexpansion during tensile tests; about twice as large for CPNC as that for thematrix polymers, PA-6 or PP. The cavitation in CPNC was large and orientation-dependent, similar to that observed in HIPS. In the authors’ opinion, the cavitationoriginates in local damage at the polymer/clay interface, and thus in an unspecifiednanoscale defect (crystalline organisation in the vicinity of the clay platelet,localised necking and fibrillation, nanoscale interfacial cavitation, etc.).

The recent publication by Bellemare et al. [2002] describes fatigue test resultsfor the same two Ube resins (PA-1015B and PA-1015C2). The measurements

stidna6-APfoerutarepmetmoortaseitreporplacinahceM26elbaT,2C5101dnaB5101-AP.dnIebU(yalconagro%tw2htiwCNPC

uaeruB[ataD.)ylevitcepser .late ]1002,

ytreporP AP CNPC AP/CNPC

lobmyS stinU yrD .dnoC 1 yrD .dnoC yrD .dnoC

E aPG 7.2 8.0 1.4 8.1 5.1 52.2

σyaPM 47 93 001 25 53.1 33.1

εb% 571 007 01< 006 60.0< 98.0

K cI maPM 2/1 9.4 ± 1.0 – 8.1 ± 2.0 0.3 ± 2.0 73.0 ± 3.0 –

Jc mJk 2- 1.8 ± 4.0 4.02 ± 52.0 57.0 ± 81.0 5.4 ± 75.0 19.0 ± 6.0 22.0 ± 2.0

:etoN 1 moortadenoitidnocerewsnemicepsesehttahtsetacidni.dnoCtnemnorivneytidimuhevitaler%05anierutarepmet

433


were conducted at constant stress ratio, σmax/σmin = const, at maximum stressσmax = 75 or 57 MPa, and at frequency ν = 5 Hz. The authors found that thefatigue resistance of both resins, PA-6 and PANC, is similar. However, in thelatter material the crack initiations occurred within the specimen, near an inorganicparticle. The authors speculated that some inorganic, micron-sized contaminantsof the clay were responsible for the fracture initiation. However, the mechanismof crack propagation in PA-6 and CPNC was quite different. The latter materialhad a higher tendency to follow the thermal fatigue fracture mechanism (TFF).After a similar number of cycles, N = 104 to 105, there was an onset of volumeexpansion in CPNC, but volume reduction in PA-6. The critical maximum cyclicstress for PA-6 and CPNC was determined as, respectively, 60 and 78 MPa. Thefractured surfaces indicated a three-step process – an initial zone of crazing,followed by fatigue crack propagation, and a rapid final fracture. The initiationstarted at or near large, micron size particles containing variable concentrationsof Si, Mg, Ca and Al – most likely mineralogical contaminants.

Kim et al. [2001b] prepared CPNC from synthetic fluoromica (FM, SomasifME-100) intercalated with ω-amino-dodecanoic acid (ADA). The organoclaywas dispersed in ADA, which was then polymerised. The injection mouldedspecimens were microtomed then inserted into a high voltage electron microscope(HVEM). The microtomed slices, ca. 500 nm thick, were deformed in situ, inHVEM. The intercalated clay platelets formed local stacks in the PA-12 matrix.The local morphology of CPNC results in remarkable differences in thedeformation processes. During the deformation of CPNC the stacks tiltperpendicular to the direction of the applied load. The localised damage at thepolymer/clay interface induced cavitation and fibrillation. The mainmicromechanical deformation consists of microvoid formation inside the stackedsilicate layers. Depending on the orientation of the stacked layers, a differentamount of energy is dissipated by splitting, sliding or opening of the stack. Thisenhances the CPNC toughness well above that of PA-12 matrix polymer. Sincethe microvoids are surrounded by bonded clay platelets that make the systemrigid, the CPNC modulus is increased. The high specific surface area of the clayand covalent bonding of the PA-12 chains to the clay surface noticeably alter thelocal chain dynamics.

Studies of the fracture mechanics of CPNC with a PO matrix are very rare[Chen et al., 2003]. The CPNC was prepared by compounding PP-MA with upto 50 wt% of MMT-ODA (Nanomer I.31PS) in a TSE at 210 °C. Re-compoundingwas used to homogenise the system, but TEM and XRD showed poor dispersionof clay platelets, especially after injection moulding. Tensile strength and modulusincreased with the clay loading, but the J-integral fracture resistance decreasedwith an increase in the clay content, viz. from ca. J1c = 1.2 to ca. 0.2 kJ/m2 at 10and 50 wt% organoclay, respectively.

Tensile stress-strain and fatigue behaviour of CPNC with either PA-6 or PPhave been measured [Mallick and Zhou, 2003]. The specimens produced by RTPcontained 3 and 5 wt% MMT, respectively. The standard S-N tests were conductedat ν = 1 Hz and T = 21.5 °C. Fatigue fracture of CPNC with PA-6 as the matrixthat required high stresses (60 to 70 MPa) was caused by thermal failure, whileat lower stresses the FCP mechanism was observed. PP-type nanocompositesfractured at σ < 40 MPa only by the FCP mechanism.

Cavitation was also observed in CPNC with a glassy epoxy matrix [Zerdaand Lesser , 2001]. XRD showed that in the post-cured samples the MMT was

434


only intercalated (d001 = 3.21 nm). With increasing clay loading the modulusslightly increased (ca. 15% at 12.5% loading), while the values of σy and εb werereduced, which is consistent with a particulate-reinforcement mechanism. Incompression tests the presence of clay had marginal effects on the stress-strainbehaviour. Results of the fracture toughness measurements were expressed interms of the stress intensity factor, KIc, and the energy release rate, GIc, which isa measure of the new surface energy, ν1. Both these parameters reached a localmaximum at a clay loading of about 3.5 wt%. At this clay concentration thevalue of KIc and GIc was higher by about 68 and 170% than those of the matrix,respectively. Atomic force microscopy in the tapping mode was used to determinethe surface area after fracture, ΔA = Ameasured/Aprojected. Plotting the energy persurface area (GIc = 2 × surface energy):

G G AIcreduced

Ic= +( )/ 1 Δ (176)

as a function of the clay content showed that GIcreduced is about constant. Therefore,

the primary mechanism for toughening in the intercalated systems is the creationof new surfaces. The authors expressed the opinion that intercalation affordsproperty improvements that are unavailable to fully exfoliated systems.

Incorporation of organoclay into rubbers leads to materials with a new set ofproperties, viz. high stiffness, tensile strength, reasonable elongation at breakand tear strength [Nah et al., 2001]. In their preliminary report the authors stressedthe unique fracture surface morphology of CPNC with NBR as the matrix. At15 phr loading of organoclay the tear strength, Gc, was nearly 4-times higherthan that of the matrix and comparable to that of carbon black (CB) filled rubber.At the same time the modulus of CPNC was at least twice as large as that ofCB-filled rubber.

In summary, it seems that in CPNC (with clay particles intercalated orexfoliated, end-tethered or not), fracture proceeds by a crazing-and-crackingmechanism, frequently with large volume expansion during the fracture. To datepure shear banding fracture without volume expansion has not been observedfor CPNC. The initiation may indeed be a mineral contaminant or a nanoscaledefect at the polymer/clay interface. According to Bureau et al. [2001] the internalenergy change per unit crack length, Jc, is significantly lower in CPNC than inthe neat polymer.

46975329 clay containing polymeric nano composites volume 1 l a utracki

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