development of free standing interference films for paper …17803/fulltext01.pdf · paper as well...

Institutionen för Fysik, Kemi och Biologi

MASTER OF SCIENCE THESIS

DEVELOPMENT OF FREE-STANDING INTERFERENCE FILMS FOR PAPER AND PACKAGING APPLICATIONS

Johan Holmqvist

Executed at STFI-Packforsk AB, Stockholm - Sweden

2008-03-03

LITH-IFM-EX--08/1920—SE

Linköpings universitet Institutionen för Fysik, Kemi och Biologi 581 83 Linköping

Rapporttyp Report category Licentiatavhandling x Examensarbete C-uppsats D-uppsats Övrig rapport _______________

Språk Language Svenska/Swedish Engelska/English ________________

Titel Title UTVECKLING AV FRISTÅENDE INTERFERENSTUNNFILMER FÖR PAPPERS- OCH PAKETERINGSTILLÄMPNINGAR DEVELOPMENT OF FREE-STANDING INTERFERENCE FILMS FOR PAPER AND PACKAGING APPLICATIONS Författare Author

Johan Holmqvist

Sammanfattning Abstract The newfound capability of creating moisture sensitive interference multilayered thin films (MLTFs) comprising microfibrillated cellulose and polymers has not previously been possible to implement on surfaces other than silicon wafer strips. Being able to incorporate interference MLTFs on fibre-based materials would introduce the possibility for new applications within authentication, sensing and customer attraction for the paper and packaging industry. By using trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane we were able to hydrophobically modify silicon substrates, enabling interference MLTF lift-off and thus the creation of free-standing MLTFs of approximately 400 nm thickness. Contact dried MLTFs approximately 250 nm thick, were successfully transferred to copy- and filter paper as well as to cellulose-based dialysis membranes. We can also report on the successful synthesis of interference MLTFs directly on a fibre composite material and on aluminium. Initial tests of a method to quantify the pull-off conditions of the MLTFs from the fluorinated surfaces using the Micro Adhesion Measurement Apparatus showed promising results.

ISRN: LiTH-IFM-EX-08/1920-SE _____________________________________________ Serietitel och serienummer ISSN Title of series, numbering

Nyckelord Keyword Layer-by-layer, interference thin film, free-standing, moisture sensor, polyelectrolyte, surface self-assembly, silanization

Datum Date 2008-03-03

URL för elektronisk version http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-11398

Avdelning, Institution Division, Department

Applied Physics Department of Physics, Chemistry and Biology Linköping University

Institutionen för Fysik, Kemi och Biologi

EXAMENSARBETE

UTVECKLING AV FRISTÅENDE INTERFERENSTUNNFILMER FÖR

PAPPERS- OCH PAKETERINGSTILLÄMPNINGAR

Johan Holmqvist

Examensarbete utfört vid STFI-Packforsk AB, Stockholm - Sverige

2008-03-03

Handledare Sven Forsberg – STFI-Packforsk AB

Hjalmar Granberg – STFI-Packforsk AB Lars Wågberg – Kungliga Tekniska Högskolan

Examinator

Kajsa Uvdal – Linköpings Universitet

Opponent Ida Hederström – Teknisk Biologi, Linköpings Universitet

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© Johan Holmqvist

A report from STFI-Packforsk

DEVELOPMENT OF FREE-STANDING INTERFERENCE FILMS FOR PAPER AND

PACKAGING APPLICATIONS

MASTER OF SCIENCE THESIS

Johan Holmqvist

March 2008

Cluster: Distribution restricted to:

Dedicated to and in memory of Arne and Astrid Bråten

tenker ofte på dere mormor og morfar

ABSTRACT

The newfound capability of creating moisture sensitive interference multilayered thin films

(MLTFs) comprising microfibrillated cellulose and polymers has not previously been

possible to implement on surfaces other than silicon wafer strips. Being able to incorporate

interference MLTFs on fibre-based materials would introduce the possibility for new

applications within authentication, sensing and customer attraction for the paper and

packaging industry. By using trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane we were able

to hydrophobically modify silicon substrates, enabling interference MLTF lift-off and thus

the creation of free-standing MLTFs of approximately 400 nm thickness. Contact dried

MLTFs approximately 250 nm thick, were successfully transferred to copy- and filter paper

as well as to cellulose-based dialysis membranes. We can also report on the successful

synthesis of interference MLTFs directly on a fibre composite material and on aluminium.

Initial tests of a method to quantify the pull-off conditions of the MLTFs from the

fluorinated surfaces using the Micro Adhesion Measurement Apparatus showed promising

results.

ACKNOWLEDGEMENTS

During the entire process of this thesis Hjalmar Granberg and Sven Forsberg of STFI-

PACKFORSK AB as well as Lars Wågberg of the Royal Institute of Technology have all

continuously supervised and challenged me towards becoming more proficient in my areas

of competence. I am truly grateful for having had such a competent and supporting team

of supervisors.

Thank you Kajsa Uvdal, Linköpings Institute of Technology, for being the examiner of this

thesis, and thank you Ida Hederström for providing the opposition.

Furthermore I would like to thank STFI-PACKFORSK AB for financial support and for

trusting me with this task.

Thank you Mikael Ankerfors and colleagues, STFI-Packforsk AB, for providing the

microfibrillated cellulose needed, and for the support on this subject.

I would also like to thank everyone at the department of fibre- and polymer technology for

sharing numerous tips, laughs and discussions with me, making me feel as a member of the

team, starting day one. Lars-Erik, thanks for all of your support regarding just about

everything, invaluable!! Erik, thanks for all of your help regarding the MAMA. Oskar, thank

you for the help regarding the contact-angle measurements. Christian, your advice

regarding the MFC/PEI system has been of great value. Caroline, thanks for being KTH-

Caroline .

Mom, Dad, Camilla, Tom-Kjetil: Thank you for everything…

Helena - I Love You

Gruk!

LIST OF ABBREVIATIONS

DLS – Dynamic Lights Scattering, (apparatus/method)

JKR – Johnson, Kendall, Roberts, (adhesion model/MAMA)

LbL – Layer by Layer, (method)

MAMA – Micro Adhesion Measurement Apparatus, (apparatus/method)

MFC – Microfibrillated Cellulose, (film constituent)

MLTF – Multilayered Thin Film

PDADMAC – poly(diallyl-dimethyl-ammoniumchloride), (film constituent)

PDMS – poly(dimethyl siloxane), (MAMA-probe constituent)

PEI – poly(ethyleneimine), (film constituent)

PFOS – trichloro(1H,1H,2H,2H perfluorooctyl)silane, (SAM constituent)

PSS – poly(sodium 4-styrenesulfonate), (film constituent)

SAM – self assembled monolayer

SPI – steps per increment, (MAMA terminology)

TABLE OF CONTENTS

1 INTRODUCTION .................................................................................................... - 1 -

1.1 Problem Statement ...................................................................................... - 2 - 1.2 Objectives .................................................................................................. - 3 - 1.3 Outline and Structure ................................................................................... - 3 -

2 THEORY ............................................................................................................... - 5 - 2.1 The Layer-by-layer Technique ....................................................................... - 5 - 2.2 Interference in Thin Films ............................................................................ - 6 - 2.3 Dynamic Light Scattering.............................................................................. - 8 - 2.4 Release Techniques ...................................................................................... - 9 - 2.4.1 Dissolving a Sacrificial Layer by an Organic Solvent...................................................... - 9 - 2.4.2 The use of Fluorinated Surfaces................................................................................ - 10 - 2.4.3 Release through pH Sensitive Disintegration of a Sacrificial Layer................................. - 12 - 2.4.4 Dissolving the Substrate: ......................................................................................... - 13 - 2.4.5 Electrochemical Manipulation of Surface Charge (hypothesis) ...................................... - 14 - 2.4.6 Our Selected Strategy .............................................................................................. - 15 - 2.5 Fluoro-Silanization..................................................................................... - 16 - 2.6 Micro Adhesion Measurement Apparatus, (MAMA) ....................................... - 19 - 2.6.1 The Instrument ...................................................................................................... - 20 - 2.6.2 MAMA Procedure .................................................................................................. - 21 -

3 EXPERIMENTAL .................................................................................................. - 25 - 3.1 Materials................................................................................................... - 25 - 3.1.1 Miscellaneous:........................................................................................................ - 25 - 3.1.2 Substrates: ............................................................................................................. - 25 - 3.1.3 Polyelectrolytes: ..................................................................................................... - 25 - 3.1.4 Microfibrillated Cellulose: ........................................................................................ - 26 - 3.1.5 Silanisation: ........................................................................................................... - 26 - 3.1.6 Micro Adhesion Measurement Apparatus, (MAMA) ................................................... - 26 - 3.2 Instruments .............................................................................................. - 26 - 3.3 Methods & Laborative Setups ..................................................................... - 27 - 3.3.1 Preparation of Polyelectrolyte Solutions .................................................................... - 27 - 3.3.2 Preparation of the Microfibrillated Cellulose .............................................................. - 27 - 3.3.3 Preparation of Silicon Wafer Slides ........................................................................... - 28 - 3.3.4 Fluoro-silanisation .................................................................................................. - 28 - 3.3.5 Preparation of Spray-painted Fibre-Composite (Kofes-demonstrator) ........................... - 28 - 3.3.6 Preparation of Aluminium strips............................................................................... - 29 - 3.3.7 Manual Layer-by-Layer Procedure ............................................................................ - 29 - 3.3.8 Automated Layer-by-Layer using the Nanostrata Stratosequence .................................. - 29 - 3.3.9 The Prepared Films ................................................................................................ - 30 - 3.4 Micro Adhesion Measurement Apparatus (MAMA)........................................ - 31 - 3.4.1 Preparative ............................................................................................................ - 31 - 3.4.2 Experimental ......................................................................................................... - 31 -

4 RESULTS AND DISCUSSION................................................................................... - 33 - 4.1 Preparative ............................................................................................... - 33 - 4.1.1 Material Characterisation ......................................................................................... - 33 - 4.1.2 Contact Angle Verification of Hydrophobicity of Fluorinated Substrates ....................... - 36 - 4.2 Free-standing Films ................................................................................... - 37 - 4.3 Transfer to New Carrier Materials ................................................................ - 41 - 4.4 Interference Film Synthesis directly on Aluminium and Fibre-based Substrates .. - 44 - 4.5 Micro Adhesion Measurement Apparatus (MAMA)........................................ - 47 - 4.5.1 Successful Release Using MAMA Pull-off.................................................................. - 47 - 4.5.2 Evaluation of the MAMA Experiments regarding Pull-off. .......................................... - 53 - 4.6 Observations............................................................................................. - 54 - 4.6.1 (Microfibrillated Cellulose | poly-Ethyleneimine) – Gels ............................................. - 54 - 4.6.2 The Colour Gradient at the Edge of a MLTF ............................................................. - 54 -

5 SUGGESTIONS FOR FURTHER WORK AND APPLICATIONS ....................................... - 57 -

6 CONCLUSIONS .................................................................................................... - 59 -

7 REFERENCES ...................................................................................................... - 61 -

8 LIST OF FIGURES AND TABLES............................................................................. - 65 -

9 STFI-PACKFORSK DATABASE INFORMATION......................................................... - 75 -

Master of Science Thesis Johan Holmqvist

- 1 -

1 INTRODUCTION

Recent technological advances in the nano-material field have widely broadened the

horizon on the development of interesting new materials. With either novel or improved

performance, these new materials can be implemented in the production of new products

and new product-applications. Numerous factors influence the success or the failure of a

product or feature comprising a novel material or function. Key aspects currently in focus

of the industry, when designing new materials are the increased cost-effectiveness in the

production, the added value of a product by the application or material and the

environmental consequences of the production of the material or product.

Three main fields of current industrial interest and importance, with which this thesis is

intimately linked are: customer attraction-, authenticity verification- and sensing-applications. One

could argue that a new ability to attract a customer’s attention would be of great value to

the advertisement and packaging industries. Further on, not being able to authenticate the

contents of a parcel, package or other product, is a growing concern, especially as the

global trade of merchandise via the internet is increasing, sometimes making it hard to

know and validate that one has bought and received the same product as was ordered.

Further adding to this problematic development is product piracy of bootleg copies with

forged security devices. The ability to easily maintain a reliable authentication device, that is

relatively hard and expensive to make bootleg copies of, could be of large value to

numerous industries.

To exemplify possible future applications, one could postulate a carton of milk changing

colour upon a customer touching it(customer attraction), or a pharmaceutical container, which

contents can be authenticated by a specific interactive identity tag on the container, as the

tag is exposed to a specific stimuli(authenticity/sensing application).

In order to meet future market demands, a new technology platform enabling prototype

manufacturing of stimuli sensitive, opto-active, nano-scale, interference-films has been

developed at STFI-Packforsk AB. The key feature of these films is their stimuli-induced

change of colour, which makes the films, in themselves, sensors. The possibility for a film


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to work as a sensor is limited to the types of stimuli that can be made to produce a signal,

in this case the change of colour.

The research of these films has been focused on, but not limited to, using exhaled breath as

stimuli. When exposed to an increased humidity, the films change colour. Film-uptake of

water causing the film to swell is one suggested explanation for the colour shift. Other

stimuli, such as mechanical pressure and heat etc, remain to be investigated, as do the

possibilities to couple adsorption of specific molecules to receptors on the film surface.

Antibody-antigen bonding could be one possible solution to the latter.

The observed colour and change thereof, is dependent upon the refractive indices of the

chosen film materials and the surrounding media, as well as on the film thickness. These

parameters can be controlled in the manufacturing process by carefully selecting the film

constituents and by controlling to which extent and thickness, the film is allowed to be

synthesised. Due to the fact that the films can function at an incredibly low thickness it is

possible to manufacture these films maintaining a low unit-cost, using renewable sources of

material in order to maintain sustainability.

1.1 Problem Statement

At present, cut strips of silicon-wafers, silicon being a well studied and readily available

model surface, are used as substrate for the film synthesis. The use of silicon as substrate is

advantageous in several ways. Silicon-wafers have both flat and well defined surfaces

needed in order to synthesize smooth interference-films of uniform colour.

The use of silicon however also has one disadvantage. The hydrophilic nature of the silicon

makes adhesion between the used film constituents (also hydrophilic) and the substrate

very strong. The currently used film constituents within the frames of our research-project

are micro-fibrillated cellulose and polyelectrolytes. In order to implement the suggested

sensor application, the thin film needs to be easily transferable between the substrate upon

which it was synthesised, and the target product or material. One possibility to circumvent

this problem would be synthesising the film directly on the target product.


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The problem with which this thesis will wrestle concerns the need for a developed

methodology of film transfer, in order to be able to fully implement the capacities of our

thin interference films.

1.2 Objectives

To start solving the problem, four areas of main focus were decided upon, as presented

below.

Survey of the possibility to evaluate different substrates or methods, regarding film

releaseability.

Development of a method to produce free-standing interference thin films. (Free-

standing either completely or suspended in solution)

Investigation of the possibility to transfer interference thin films between the

substrate used for synthesis and fibre-based materials.

Research the feasibility of synthesis of multilayered thin films directly on fibre-

based materials.

1.3 Outline and Structure

To get a better understanding of the current situation regarding the research progress in the

field of thin film synthesis and the release of such films from their original substrates of

synthesis, we decided on carrying out a literature study. The outcome of this study,

intended to enlighten us on the possibilities and difficulties accompanied with the release

of thin films with thicknesses on the nano-scale, proved valuable as it suggested a plural of

previously tested methods. These methods were however not directly applicable to our

purpose, mainly due to two reasons. Firstly, the fact that our work focuses on the release

and synthesis of interference thin films, dependent upon not exceeding a certain thickness

(section2.2). Secondly that the films whilst thin enough to show interference behaviour

regarding colouring, must also be thick enough to enable intact release or transfer of the

film. Thus, we set out to modify and further develop a method for thin film release.


- 4 -

This thesis continues with a description of key theoretical areas needed for explaining the

contents of this thesis, after which the results from the above mentioned literature study

and the concluding strategies we were able to obtain from them are presented. The

experimental section of this thesis is then documented and this is followed by the results

section and a discussion thereof. The terminating part of this thesis is devoted to

suggestions for future research areas, linked to our results, and this part is in turn followed

by a concluding section.


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2 THEORY

This section will describe the key theoretical elements used throughout the thesis.

2.1 The Layer-by-layer Technique Although initially discovered by Iler1 in the 1960s, the use of the Layer-by-layer (LbL)

technique was not widely developed until the re-discovery of the technique by Decher2, 3 in

the 1990s. Since then numerous of research groups have embraced this technique, and

quite a substantial base of knowledge has been established4.

The principle of LbL depicted by Figure 2-1 is based on the adsorption of polyions or

particles to an oppositely charged substrate by alternated immersion of the substrate in

solutions of these polyions. By dipping a substrate (often negatively charged silicon) that

carries surface charge into a solution of oppositely charged polyions, one initial layer of

polyion(A) is adsorbed. This adsorption inverts the surface charge, enabling the adsorption

of a layer of oppositely charged polyion(B) to the already adsorbed layer. This again inverts

the surface charge, making adsorption of polyion(A) possible yet another time. By cycled

immersion, multilayered thin films (MLTFs) can be synthesized1-4.

Figure 2-1 The LbL-assembly is illustrated. Two types of oppositely charged molecules, A and B, are alternately adsorbed onto the substrate. By repeating the procedure, MLTFs can be synthesised.

A simple way to describe the pathway used for synthesis of a specific MLTF is labelling as

(A|B)x, where the MLTF has undergone x number of dippings in polyion(A) and x number

dippings in polyion(B). This type of annotation is used throughout this thesis.


- 6 -

The LbL-technique is not limited to the use of polyelectrolytes for assembly. Magnetite

particles5, gold nanoparticles6, clay platelets7 and microfibrillated cellulose (MFC) 8-11 are

examples of other materials that have been used in the creation of MLTFs. By carefully

selecting the constituents, precise control of the growth-rate of the assembled multilayers is

possible, as is the synthesis of smooth MLTFs.

This thesis focuses on the multilayered thin films made from microfibrillated cellulose and

oppositely charged, positive poly(ethylene-imine) (PEI) illustrated by Figure 2-2.

Figure 2-2 showing the structural formula of the PEI-molecule. As is illustrated, the molecule is branched.

Furthermore, LbL-multilayering is not dependent upon the use of oppositely charged

particles for the assembly. Hydrogen bonding is one alternative, that has been recently

examined12, 13.

2.2 Interference in Thin Films

Rays of incident reflected at various interfaces of a thin coating undergo interference that is

either constructive or destructive. This gives rise to the colouring of thin film coatings

given certain conditions.

The observed interference colour is dependent upon the refractive indices of the thin film

and the surrounding media. It is also dependent upon the thickness of the thin film and by

analogy to this, on the smoothness. The case of a substrate-bound (Figure 2-3) MLTF

differs slightly from that of a free-standing (Figure 2-4) one, in that the surrounding media


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in the case of a free-standing film, is the same on both sides compared to the different

media observed for each side of a substrate bound MLTF.

n2(MLTF)

n1(air)

n3(substrate)

d

n1(air)

n3(substrate)

d

Figure 2-3 illustrating a substrate bound MLTF. The surrounding media are air and substrate, (indicated by their refractive indices (n1) and (n3)). The MLTF-parameters are the thickness (d) and the refractive index (n2). (inspired by illustration in14)

n1(air)

n1(air)

n2(free-standing MLTF)d

n1(air)

n1(air)

n2(free-standing MLTF)d

Figure 2-4 illustrating the somewhat different properties of a free-standing MLTF. The medium on both sides of the MLTF is the same and has refractive index (n1). The MLTF has a thickness of (d) and a refractive index (n2). (inspired by illustration in14)

A deeper look into the interference phenomenon is given by Halliday et al.15, where the

mathematics of interference (not explained here) is described.

By observing the interference colour of our synthesized MLTFs we were able to keep track

of their approximate thickness, using a MLTF interference model, previously developed at

STFI-Packforsk8. Determination of the thickness of a MLTF is enabled given the refractive

indices of the MLTF-constituents, the surrounding media and the observed colour of the


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MLTF under daylight illumination. The use of the model is not specifically reported on in

this thesis since verification of the interference phenomena is possible by ocular inspection.

One important result from the modelling of the interference behaviour of both free-

standing and silicon-attached MLTFs is however utilized. By employing the model to

systems similar to those in this thesis with respect to refractive indices and surrounding

media, interference caused colouring behaviour was predicted (verified by laborative work

not included in this thesis) for MLTFs of thicknesses up to approximately 1μm. The

reasons for the transparent and colourless behaviour of thicker films include that the

interference of the films give rise to relatively few interference peaks when they are thin

and additional peaks when they get thicker. When enough peaks are introduced they smear

the ocular possibility to detect different colour resulting in the loss of apparent colour

(based on the model developed by Anderson8).

2.3 Dynamic Light Scattering

To characterize the solutions used for the LbL assembly with respect to molecular size and

surface charge (estimated by zeta-potential), a Dynamic Light Scattering (DLS) equipment

was used. The theory behind these measurements lies beyond the scope of this thesis,

however the zeta-potential and DLS will be explained briefly.

The DLS equipment measures the diffusion of light-scattering particles or ions and the

particle size is estimated through the obtained hydrodynamic radius.

Macromolecules can become charged when in an aqueous solution through ionization.

This ability and the amount of induced charge depend on the functional groups

incorporated in the macromolecule. Macromolecular ions attract oppositely charged

counter ions present in the solution. The attracted counter ions can be divided into two

types; namely the ones that are most attracted by the molecule and follow it through its

motion, and the ones loosely attached, that do not stick to the molecule. This in turns gives

rise to a slipping plane, defined as the border between these types of ions. At the slipping

plane the electric-potential is different from that at the macromolecular surface itself. The


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potential at the slipping plane is called zeta-potential, and we have used it to verify that our

polyelectrolytes behave as expected in solution regarding charge16.

2.4 Release Techniques

We decided to conduct a literature-study in order to get an understanding of what had

already been researched in the field of free-standing multilayered thin films. The key

outcome of this study is presented below in a condensed form. The results enabled us to

construct our strategy which is explained in the continuation of this section.

In order to facilitate the release of multilayered thin films, synthesized through alternated

dipping of substrates in solutions of oppositely charged adsorbates, several different

techniques have been used previously. For our purpose, these established techniques offer

different possible experimental designs, however all are not directly possible to implement

for our cause. Our choice of method, described towards the end of this section, was

influenced by some of these techniques, but offers modifications in the experimental

design, largely due to the fact that our choice of material is different from those already

mentioned. Another reason being that our aim not only includes the successful release of

thin films, but also consists of the investigation of the possibility to transfer the thin films

from the substrate to a new carrier material. Furthermore, in order to be subjects of the

desired interference phenomena, the films must not exceed a certain thickness.

2.4.1 Dissolving a Sacrificial Layer by an Organic Solvent

One way to create a free-standing MLTF is to synthesize the target-film onto a substrate

that has been previously coated with a sacrificial layer, i.e. a layer which in its turn is readily

dissolvable in a solvent that does not affect the target film, illustrated by Figure 2-5 . After

the LbL-deposition of the target film on top of the sacrificial layer, the sample is immersed

in a solvent which dissolves the sacrificial layer and thereby renders the target-film released

from its substrate, freely suspended in the solvent.

An already well-established application of this idea has been successfully carried out by

Mamedov et al 5. and Tsukruk et al 6, who both report the use of an assemble-dissolve

technique based on the coating of a substrate with cellulose-acetate, using its solubility in


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acetone to create a free-standing film from the target film synthesized on top of the

sacrificial layer.

Key features: Dissolvable sacrificial layer. Limitations:

Solvent must not affect or react with target-film. Sacrificial layer must be readily dissolvable in a solvent that preserves an intact target-

film. The use of organic solvents such as acetone is not environmentally advantageous.

Advantages: Preparation of very thin (~30nm) free-standing MLTF suspended in solution is possible.5

Sacrificiallayer Target-film

Solvent -addition

Dissolvedsacrificial

layerTarget-film

(free-standing)

Figure 2-5 The sample (left) is submerged into the solvent. The sacrificial layer then starts to dissolve (right) due to its solubility in the solvent, thus leaving a free-standing target film.

2.4.2 The use of Fluorinated Surfaces.

In order to achieve minimal adhesion of contaminants to a surface, the use of Teflon or

other fluorinated materials has been industrially implemented. The frequent use of Teflon

coating in the manufacture of frying pans exemplifies this. A fluorinated surface is

hydrophobic and this is different from other popular substrates for MLTF-deposition,

which mainly consist of glass slides and cut strips of silicon wafers, which are both mainly

hydrophilic.


- 11 -

By using Teflon substrates or by fluoro-silanizing a hydrophilic substrate, (often a silicon

wafer-strip), one creates a possible approach in the development of synthesis of free-

standing MLTFs. The idea is that while the LbL adsorption of the MLTF constituents

might not proceed as efficiently as it would with a hydrophilic substrate, the synthesized

MLTF will not, when finally adsorbed, adhere to the same extent to the substrate and thus,

it will become easier to peel-off mechanically with for instance tweezers.

Successful implementation of this technique in order to achieve free-standing MLTFs has

been reported by Lutkenhaus et al. 12 as well as by Jaber et al. 17 Both approaches take

advantage of a Teflon-coated surface. An illustration is provided in Figure 2-6.

Key features: Switch from hydrophilic to hydrophobic substrate to lower the attractive force between the MLTF and the substrate. Limitations:

Use of fluorinating agents such as fluoro-silanes needs to be evaluated from a sustainability perspective. Though the environment needs to be considered, the use of fluoro-silanes to coat silicon-wafers is not largely material consuming, since only substrate-modification is needed.

Successful implementations of this technique have been limited to the release of relatively thick MLTFs (about 8-9μm) 12, 17. Advantages: This technique offers the possibility of dry-release. Except from the use of fluorinating agents, this procedure demands no further modification to standard LbL protocol.


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CF3 CF3CF3 CF3CF3CF3CF3CF3 CF3CF3 CF3CF3CF3CF3

CF3 CF3CF3 CF3CF3CF3CF3

Fluorinated surface

MLTF- weakly adhered to the hydrophobic surface

MLTF-peeled off usingtweezers

Silica surface

Figure 2-6 By introducing CF2 and CF3 groups to the silicon surface, the adhesion between the MLTF and the substrate is decreased. This makes removal possible, in this figure exemplified by peeling the MLTF of using tweezers. For illustrating purposes the films are illustrated as a A-B-A pattern. The films of his thesis are multilayered i.e. (A|B)20 (not shown here).

2.4.3 Release through pH Sensitive Disintegration of a Sacrificial Layer

Yet another previously shown method to obtain a free-standing MLTF is the use of a pH-

sensitive sacrificial layer, depicted by Figure 2-7. Standard LbL procedure describes the

alternate use of positively and negatively charged polyelectrolytes in the MLTF synthesis,

with the electrostatic interaction between these layers as main contributing factor of these

forming4. It is however possible to generate MLTFs by other means than that of

electrostatic interaction of the constituents.

By choosing materials that together have hydrogen bonding capability, one can generate a

layered film that is sensitive to the pH-conditions of the surrounding environment. This is

due to the fact that the hydrogen-bearing functional group of a hydrogen bonding pair, can

be protonated/de-protonated by alteration of the pH of the solution as is for the use of a

carboxylic-acid functionality, bonded to an ether functionality13. The hydrogen bonding and

thereby disintegration of the sacrificial layer can thus be controlled since de-protonating the

acid functionality will deprive it of its hydrogen bonding capability.

To obtain a free-standing MLTF, one can then use a pH-sensitive film as sacrificial layer,

and onto it continue the LBL-synthesis with their constituents, creating a film that is


- 13 -

insensitive to pH-alterations, at least in the same pH interval that is intended for use in

disintegrating the sacrificial layer.

Use of such a system has been previously researched by Decher and colleages13. Their

choice of poly(acrylic acid) and poly(ethylene oxide) as constituents of the sacrificial layer,

exemplifies the above reasoning regarding pH-sensitivity.

Key feature: pH-responsive sacrificial layer. Assemble at one pH, and disintegrate the sacrificial layer at another. The target film must be stable and not influenced by the pH alteration. Limitations: Find a target MLTF that resists disintegration at the target pH. Construct a sacrificial layer that disintegrates upon reaching a specific pH. Advantages: This wet-release method should facilitate the release of relatively thin MLTFs (~200nm reported) 13.

Target-film(free-standing)(A/B)n (C/D)m

+pH-change

Figure 2-7 As the pH of the surrounding medium is changed, the sacrificial layer which is held together by hydrogen bonds between constituents A and B dissolves, due to the induced loss of hydrogen bonding capability. The target film is unaffected by the treatment and is left free-standing in the solution.

2.4.4 Dissolving the Substrate:

One method similar to the other mentioned techniques describing sacrificial layers is one

using a layer of SiO2 as substrate or sacrificial layer. This technique takes advantage of the

dissolving ability of hydrofluoric-acid (HF) on a SiO2-surface. Previously demonstrated by

Kotov and coworkers7 is one technique which features a SiO2-covered glass slide as


- 14 -

substrate for the LbL synthesis. Upon completion of the MLTF-depositioning, the sample

is treated with HF to dissolve the sacrificial layer.

Key features: SiO2 as substrate or sacrificial layer, dissolved by HF. Limitations: Reactivity of HF towards target film must be prevented. HF needs careful handling. Advantage: Relatively thin films have been reported (~50-200nm) 7

LbL-assembly

Sample is treatedwith HF

+

Deposition of sacrificial -

layer

Figure 2-8 The HF assemble-dissolve technique is depicted. Initially LbL assembly onto a sacrificial layer is performed and this is followed by HF treatment. When treated with HF, the SiO2-sacrificial layer is removed, rendering the target film free-standing in the surrounding media.

2.4.5 Electrochemical Manipulation of Surface Charge (hypothesis)

It is possible that the use of metal substrates could simplify the release of MLTFs. This

hypothesis is based on the use of the metal substrates as electrodes. The metal will have a

net negatively charged outermost surface18. This could possibly be used to enable

electrostatic LbL depositioning of alternating positively and negatively charged poly-

electrolytes. When the desired film thickness has been reached, a potential is applied which

makes the substrate a positively charged electrode. This change of surface charge from

negative to positive could induce the desorbtion of intact MLTFs, as the innermost

polyelectrolyte layer would be repelled from a surface charge of the same sign (+/-)

however no such reports have been found.

Key features: polarisable substrate and the application of surface potential.

Limitations: Electrostatic interaction drives the adsorbtion. System must not be harmed by the applied potential.

Advantages: Simple methodology


- 15 -

Electro-chemicalcell

Appliedpotential

LbL-assembly

Figure 2-9 Due to the applied potential the surface of the metal-substrate undergoes a change of polarization, becoming positively charged, and thus repellent of the also positive, electrode-near first layer of the LbL-assembled MLTF.

2.4.6 Our Selected Strategy

Although all of the above mentioned techniques offer relatively easy and already

established methods towards creating free-standing MLTFs, we ended up with modifying

the ones comprising Teflon substrates, by creating our own Teflon analogues using fluoro-

silanizing of silicon substrates to fit our purpose.

The advantages of being able to handle the release of the target-film in a dry environment

outweighed the alternatives involving wet-release leading us to choose a modified version

of the above mentioned fluorinated surface approach. Although Teflon-coated glass slides

are readily available for purchase our desire to be able to monitor the interference

behaviour of the MLTFs lead us to develop a method using silicon as a substrate, enabling

us to benefit from its contrasting refractive index, compared to glass. Selecting silicon thus

simplifies the detection of interference colours in the film by providing a substrate with

substantially different refractive index, than the MLTFs.

Being able to peel-off the film using tweezers or similar, in a dry or semi-dry environment

(film could be wet although the substrate is not submerged in any solution) would if

possible, facilitate the handling, as one would not have to separate the MLTF from the

liquid media incorporated with a wet release strategy.


- 16 -

Another advantage of using this method is the fact that the transfer of MLTFs from the

substrate to a different carrier material such as paper, through contact drying could be

investigated. Contact drying meaning that the MLTF when still connected to the substrate

is brought into contact with the target ‘carrier’-material (paper-sheet), under wet or humid

conditions, and is then dried together, hypothetically allowing for the transfer of the MLTF

to the paper.

As the main scope of this thesis includes the development of release techniques for MLTF

intended for applications based on the interference properties of the MLTFs, our work was

focused on trying to release as thin MLTFs as possible. As reported by numerous research-

groups, creation of relatively large (order of square cm) free-standing MLTFs of 8-9μm

thickness is readily possible. As described by section 2.2, these thick films do not show the

interference colours of the MLTFs we seek. Consequently, if we are able to create a free-

standing segment of MLTF that shows interference colours we will have successfully

released MLTFs ten times thinner than previously reported.

Key features: Dry release of interference MLTFs through weak adhesion between MLTFs and the

hydrophobic fluoro-silanized silicon wafers. Possible transfer of MLTFs between substrate by using contact drying or other

mechanical manipulation.

Limitations: Mechanically freeing thin films can be difficult in a way that preserves structure. Our results show that the thin films are fragile (section 4.2). Advantages:Dry-release should simplify handling, as a MLTF that is suspended in solution is thought to be harder to handle.

2.5 Fluoro-Silanization

Incorporation onto surfaces of -CF2- and terminating -CF3 groups in order to achieve non-

stick, highly hydrophobic surfaces is a well known methodology somewhat pioneered by

Dupont in the creation and implementation of various Teflon coatings.19 The successful

LbL assembly of MLTFs on Teflon-coated surfaces12, 17 inspired us to research the

possibility of releasing multilayered thin films from substrates analogous to these.


- 17 -

Silicon wafers have a native SiO2 outermost layer. The introduction of hydroxyl groups to

such surfaces followed by the immersion of these silicon substrates in fluoro-chloro-silane

containing solutions (Figure 2-10), presents a possibility to create CF3-terminated self-

assembled monolayers (SAMs) on silicon.20 For the silanization to take place, surface-

bound water has been reported as a prerequisite.21

The proposed mechanism for the self-assembly of the silane onto the silicon substrate

proceeds in four steps21. Firstly, the silane molecules physisorb to the outermost adsorbed

water layer of the silicon substrate. Then the silicon atom of the silane undergoes

hydrolysis, Figure 2-11, thus changing its chlorine substituents into hydroxyl groups. The

silane molecules then undergo condensation so as to covalently attach to the silicon

substrate, Figure 2-12. The final step in the silanisation consists of the in-plane, covalent

bonding of the silane molecules amongst themselves, increasing the stability of the SAM,

which is illustrated by Figure 2-13.

ClCl ClSi

CH2

CH2

CF3

CF2

CF2

CF2

CF2

CF2

Figure 2-10 Trichloro (1H,1H,2H,2H -perfluorooctyl) silane, with its hydrophobic fluorine-containing tail indicated by a blue bar.

ClCl

Cl

Si OHOH

OH

Siwater

Figure 2-11 Surface-bound water enables hydrolysis of the silane molecules changing their three Cl groups into OH groups.


- 18 -

SiO

SiO

SiO

Si

OOH H

OHOH

OH

Si OHOH

OH

Si

+ +

SiO

SiO

SiO

Si

OO

OH Si OHSiOH OH

+H2O

Figure 2-12 The silane molecules condense onto the silicon-wafer surface forming the hydrophobic SAM. The reaction frees water.

SiO

SiO

SiO

Si

OO

OH Si OHSiOH OH

SiO

SiO

SiO

Si

OO

OH Si OHSiO + H2O

Figure 2-13 In-plane stabilizing through covalent bonding between SAM-forming silane molecules. The ability of the silane molecules to covalently attach to each other is thought to stabilize the formed

SAM.

One drawback in using di- or trichloro substituted silane molecules is the fact that the

multiple reactive hydroxyl groups of the silicon atom of the silane enables an unwanted

agglomeration of the silane molecules and the possible adsorbtion of these aggregates onto

the silicon surface. This agglomeration can be avoided by choosing a monochloro

substituted silane, as illustrated by Figure 2-14, however this prevents the above mentioned

fourth step of the silanisation mechanism (Figure 2-13) resulting in a decreased stability of

the SAM due to the lack of in-plane covalent cross-linking. Only one hydroxyl group is

hydrolysed for further reaction. A more excessive discussion on this topic is presented by

Dutoit et al. 22


- 19 -

RR

OH

Si

RR

OH

Si

RR Si

RR

O

SiOHOH

OH

Si

OHOH

OH

Si OHSi

OOH Si

OHOH

O

Si

OH

OH

OSi

a) b)

Figure 2-14 a) illustrating the possible formation of larger aggregates for the trichloro-substituted silane, which can covalently attach to the surface (unwanted), compared to a monochloro-substituted silane depicted in b) bearing two protective-groups, thus enabling it to either produce a dimer, or attach to the surface. (A produced dimer can not covalently attach to the wafer surface by the same chemistry)

We selected trichloro(1H,1H,2H,2H-perfluorooctyl)silane (PFOS), illustrated by Figure

2-10 as our SAM-forming reagent, based on its tri-substituted nature and its commercial

availability.

2.6 Micro Adhesion Measurement Apparatus, (MAMA)

Being able to measure the force needed to separate a controlled area of MLTF from its

substrate, was desired in order to evaluate different methods of surface pre-treatment, for

MLTF-release i.e. quantify the ease with which a MLTF can be lifted off from its substrate.

We therefore set out to try and further develop an already existing technique; Micro

Adhesion Measurement Apparatus, (MAMA)23, in order to possibly answer if our

modifications to the silicon substrates had in fact facilitated the release of MLTFs.

The MAMA-technique is currently used on measurements of the adhesion between

different substrates24, often a half-spherically shaped poly-dimethyl siloxane (PDMS)-probe

and a flat surface (the substrate). Similar instrumental setups are also used, exemplified by

the one used by Chaudbury and Whitesides25. While the applied load and the contact area

between the surfaces are being continuously monitored, the two surfaces are firstly brought

into contact with each other and are then separated. Generally, the surfaces will adhere to


- 20 -

each other. If they do not, then separation of the surfaces would occur as the point of zero

applied load is passed. However, since the surfaces do often adhere, a negative load is then

needed to separate the surfaces.

For our purposes, if the MLTF and the probe are bound strong enough, the separation

could possibly occur at the interface between the substrate and the MLTF, rather than

between the two surfaces that were originally brought together (MLTF and probe). This

would mean a transfer of MLTF from substrate to probe. The measurements would thus

result in a value of the load needed to pull-off the MLTF per area of MLTF , which is also

monitored by the instrument. This value can have two main contributors, namely the

adhesive force between the substrate and the MLTF (sought), and the force required to

free an internal piece of MLTF from the MLTF surrounding it, i.e. a cohesive breakage.

Our work was focused on investigating the possibility to achieve strong adhesion between

the probe and the MLTF through electrostatic attraction between a negatively charged

outermost surface (MFC outermost layer) of the MLTF, and a positively charged probe

(PEI-coated).

2.6.1 The Instrument

The MAMA instrument23, as is illustrated by Figure 2-15, consists of an analytical balance

and a microscopy-coupled camera for measurement, as well as a motorized probe holder.

By photographically monitoring the contact area between the surfaces and correlating it to

the load at which the MLTF is pulled off from the substrate one can estimate the force per

area needed for release and thus by these values compare different surface treatments to

each other. The possibility to detect the change in contact area is contributed to the elastic

and transparent nature of the PDMS probe. The PDMS probe deforms when pressed

against the MLTF, forming a circular contact area that increases with the applied load. The

probe is also transparent enough to be able to be photographed and looked through using

a microscope. This is used to obtain the measurements of the contact area. To obtain load

values the sample is mounted on an analytical balance during the experiment. This enables

measurement of the applied and the pulling loads.


- 21 -

Analysis-balance

Microscope

Camera Computer

Analysis-balance

Microscope

Camera Computer

Figure 2-15 shows (left) the schematics of the MAMA-instrument. The PDMS half sphere is mounted on the motorized sample holder and is brought into contact with the surface of the sample (between balance and microscope). To the right a successful lift-off is pictured, where a controlled amount of MLTF has been transferred to the PDMS-probe. A white indent in the sample illustrates the corresponding area of the MLTF that has been lifted off.

2.6.2 MAMA Procedure

The method includes a mounting-, loading-, unloading-, pulling- and a pull-off stage, as is

illustrated by Figure 2-16.

Pull-offPulling

UnloadingLoading

Figure 2-16 showing a sequence of a MAMA-experiment. The vertical arrows (grey) indicate applied and withdrawing load. Multiple horizontal arrows indicate a stepwise increment or decrement of the applied load.

One way to follow the experiment is by looking at the ‘load versus time’ or ‘load versus

measurement point’ plot depicted in Figure 2-17. Starting at zero, the applied load (grams)

increases to a maximum, which is held during a specified time. This is followed by a

decrease of applied load, until zero is obtained. This is the point at which the surfaces


- 22 -

would separate, were there no adhesion between them. By applying negative load until

breakage i.e. pulling the probe away from the substrate, obtaining a load that corresponds

to the adhesive force between the surfaces and their contact area could be made possible.

Load vs. Measurement Point

-8

-6

-4

-2

0

2

4

6

0 5 10 15 20 25 30 35 40 45

Measurement Point(number)

Load

(g)

Serie1

A B C

D

Figure 2-17 shows a Load (g) vs. Measurement Point plot for a MAMA-experiment. Four zones are indicated by red arrows. A-loading, B-maximum load, C-unloading (negative load => pulling), D-maximum pulling load or pull-off load.

The loading and unloading is preformed in parts. The probe holder is controlled by a step-

motor, and each increment or decrement (measurement point) of load is defined as a

number of steps. Thus, the applied load is not directly controlled for each increment, with

respect to absolute value, but is instead controlled by the maximum load allowed (for the

experiment), and the number of steps per increment. The number of steps per increment

(SPI) can be defined by the user. The time of an entire experiment is not possible to

control directly, though a substantial part of the time is due to the time set at maximum

load (at least for our measurements). This is due to the fact that the number of unloading

increments depends on the attractive force between the probe and the sample.


- 23 -

The possible controlling parameters thus contain maximum load, time at maximum load,

SPI-loading, SPI-unloading, SPI-pulling, and the corresponding times separating the

increments, one time per phase i.e. loading, unloading and pulling.

Normally, the established Johnson, Kendall and Roberts(JKR) model26 is applied to the

obtained data once the photographs have been measured for diameter in order to get the

force per area relationship. However, the explanation of this theory is beyond the scope of

this report, as our primary goal is to investigate and assess whether pull-off using the

MAMA is a possible and suitable method, fitting our future needs of surface treatment

evaluation.


- 24 -


- 25 -

3 EXPERIMENTAL This section is intended to describe key experimental details of the preformed research.

3.1 Materials In this part of the section the chemicals and other materials used are listed.

3.1.1 Miscellaneous:

NaCl, analytical-grade from Merck

Ethanol, (EtOH) 96% vol, VWR international, [EC-label: 200-578-6]

Water: Milli-q, Millipore - Synergy 185 apparatus, 18,2 MΩ*cm

For different trials of film release: Adhesive tape, filter paper, copy paper, glass slides and

flat-ended tweezers were used.

3.1.2 Substrates:

Polished silicon wafers, MEMC Electronic materials, S.p.A., Novara Italy,

150mm/Cz/1-0-0/Boron/p-type, [PUR-0007 Rev.5 38647]

Aluminium foil: standard commercially available.

Kofes, fibre based composite of approximately 40% poly(lactic acid) STFI-

Packforsk AB27, 28

Grafitti-paint, colour: Copper Chrome, Montana Cans

3.1.3 Polyelectrolytes:

poly(Sodium 4-styrenesulfonate)(PSS), Mw: 70.000 Da, Sigma-Aldrich [Cas 25704-

18-1]

poly(diallyl-dimethyl-ammoniumchloride)(PDADMAC), Mw: 500.000 Da, reactant

grade, CDM Alcofix III.

poly(ethyleneimine)(PEI) Mw: 60.000 Da, Acros Organics [CAS 9002-98-6]

Gelatine, Gelatine Porcine Skin, Type A, 300 bloom, Sigma, [CAS 9000-70-8]

Carrageenan, Sigma-Aldrich, [CAS 9062-07-1]


- 26 -

3.1.4 Microfibrillated Cellulose:

The microfibrillated cellulose (MFC), Gen 2, batch 16, 2% wt., produced at STFI-

Packforsk AB. Major contributing functionality in making the fibrills anionic in

aqueous environment are carboxylic acid functionalities according to manufacturer.

3.1.5 Silanisation:

n-heptane, puriss, 99%, Riedel-de Haën, [CAS 142-82-5]

Trichloro(1H,1H,2H,2H-perfluorooctyl)silane(PFOS), 97% in heptane, Sigma-Aldrich [CAS 78560-45-9]

3.1.6 Micro Adhesion Measurement Apparatus, (MAMA)

PDMS half-spheres, approximate diameter of 1mm by ocular inspection. Prepared as by Eriksson24.

3.2 Instruments The key instruments are listed below. Provided in the list of references are links to the web-

pages of the developing companies. These describe the instruments in a comprehensive

matter, not possible here.

Plasma-cleaner, Harrick-Plasma29

Contact-angle measurements, KSV Cam 20030

Dynamic Light Scattering instrument, Malvern, Nano-zeta series16, 31

Micro Adhesion Measurement Apparatus (MAMA)23

Sonics Vibra-Cell, rod sonicator, 3mm titanium tip32

Dipping robot, Nanostrata, Strato sequence VII33


- 27 -

3.3 Methods & Laborative Setups The section describes the methods used throughout the thesis.

3.3.1 Preparation of Polyelectrolyte Solutions

All polyelectrolyte solutions were prepared using Milli-Q. A desired concentration of

1mg/ml was achieved by weighing in an appropriate mass of polyelectrolyte, and then

adding a corresponding volume of Milli-Q. Polyelectrolyte solutions were then subjected to

characterisation of pH and zeta-potential. Table 3-1 gives an overview of the prepared

polyelectrolyte solutions. The polyelectrolyte solution concentration of 1mg/ml were

prepared in order to roughly maintain a 0.01M concentration with respect to the repetitive

unit of the polyelectrolytes (monomolar concentration), the use of which has been reported

on17. All solutions were allowed to temperate before use and were thus used at room

temperature, ~22,5ºC. The pH was set by using 0.1M NaOH and 0.1M HCl respectively.

Table 3-1 The type of polyelectrolyte ion and the pH of the used solutions.

cationic5.5Gelatine

anionic9.6Carrageenan

anionic5.5PSS

cationic5.5PDADMAC

cationic10.8PEI

anionic6.8MFC

poly (+)/(-)pHPrepared Solutions:

3.3.2 Preparation of the Microfibrillated Cellulose

The MFC, produced at STFI-Packforsk was prepared similarly to the method previously

described by Wågberg11. The 2% wt MFC gel stock was diluted with Milli-Q in the ratio of

1g of gel per 12ml of Milli-Q. This diluted MFC was then dispersed, ~15ml at a time, in a

20ml glass flask, using a Sonics Vibracell rod-sonicator with a 3mm titanium probe for 10

minutes at 25% of the maximum amplitude setting. The MFC fractions were then pooled

and centrifuged at 8.000 g for 2 hours. This was preformed in order to separate larger

aggregates of MFC from the desired well-dispersed and well-separated microfibrills. The


- 28 -

resulting supernatant was carefully transferred from the centrifuge tubes into a glass

container for storage using an autopipette and the pellet was discarded. The MFC was then

characterized with respect to size, zeta-potential and pH.

3.3.3 Preparation of Silicon Wafer Slides

The silicon wafers were cut into slides of two sizes, approximately 7 cm x 1 cm for manual

LbL protocol and 7 cm x 2.5 cm for use with the Nanostrata system. These slides were

then excessively rinsed in the order of Milli-Q - EtOH - Milli-Q and were then dried under

a N2-flux. Following this, the slides were placed in the plasma cleaner for 2 minutes at

medium (10W) effect. The plasma treatment removes surface contaminants, as well as

renders the silicon substrates with a clean hydrophilic surface29.

3.3.4 Fluoro-silanisation

Immediately following the plasma cleaning step, the silicon slides were submerged in a 0.1

M NaOH solution. This was done in order to introduce surface hydroxyl groups.

Following a Milli-Q washing step and N2-flux drying the silicon strips were immersed in a 1

mM solution of PFOS in heptane for 20 minutes. The PFOS solution was freshly prepared,

adding PFOS to a heptane-containing beaker undergoing stirring. The solution was allowed

3 minutes of stirring, before it was used. This was followed by the rinsing of physisorbed

silane molecules by sonicating the wafer strips in a heptane bath for further 20 minutes

which in turn was followed by excessive rinsing in heptane followed by Milli-Q. Finally the

silicon strips were dried under a N2-flux.

The PFOS solutions were never reused and were always freshly prepared due to reports of

similar chemicals undergoing rapid unwanted agglomeration34. Due to the nature of the

involved chemicals, all handling of PFOS and heptane was carried out under a fume-hood.

3.3.5 Preparation of Spray-painted Fibre-Composite (Kofes-demonstrator)

The Kofes, supplied by STFI-Packforsk, was spray-painted with copper paint by holding

the sample vertically, and horizontally spraying it. The sample was allowed to dry overnight,

before it was cut into 7 cm x 1 cm strips and was subjected to the LbL procedure. One


- 29 -

larger piece of the Kofes demonstrator, approximately 10 cm x 10 cm was also produced,

intended to function as a larger scaled demonstrator of the LbL dipping procedure and the

resulting interference colouring behaviour of the coating.

3.3.6 Preparation of Aluminium strips

The aluminium foil was thoroughly washed in the order Milli-Q – EtOH – Milli-Q

followed by sample drying with a N2-flux. It was then folded into strips of the approximate

size 7 cm x 1 cm, after which it was treated in the plasma cleaner, at medium effect (10 W)

for 10 minutes. Following this the samples underwent the LbL procedure.

3.3.7 Manual Layer-by-Layer Procedure

The manual dipping of the substrates was performed by letting the samples stand upright

in 15mL beakers, approximately filled to two-thirds. Care was taken not to dip the entire

sample in an effort to try to minimize contamination from the tweezers used to handle the

samples. The samples were dipped with the following repetitive cycle. Initially the substrate

was dipped for 10 minutes in PEI. This was followed by two consecutive 5 minute rinses in

Milli-Q. The samples were then dried under a N2-flux before they were submerged for 20

minutes in MFC. Following this, the samples were again rinsed with Milli-Q by two 5

minute dips. Before entering the PEI solution for the second time, the samples were dried

using N2.

For the films containing substrate-near PDADMAC and PSS layers, the above method was

altered in the following way regarding the layering of these polyelectrolytes. PEI was

substituted with PDADMAC (10min dip) and MFC with PSS (10min dip). When the

desired amounts of dipping cycles in these polyelectrolytes had been preformed, additional

PEI|MFC layering was preformed as initially described.

3.3.8 Automated Layer-by-Layer using the Nanostrata Stratosequence

When the dipping-robot was utilized, three rinses in Milli-Q were used. This was because

of it being the default setup for the robot. Otherwise the general procedure was the same


- 30 -

as for the manual dipping, and is shown by Figure 3-1, however the rinsing times were

altered to 3 minutes for each beaker.

Figure 3-1 The figure illustrates the dipping cycle used with the dipping robot (cycle starts at substrate). The substrate is mounted in a sample holder and is then alternately dipped in positively and negatively charged polyelectrolyte-solutions.

3.3.9 The Prepared Films

Approximately one hundred samples were prepared in total. Due to the often destructive

nature of the release attempts i.e. manipulation with razors, knives adhesive tape. For

clarity, the selected samples featured and discussed in the results section of this thesis are

presented in Table 3-2


- 31 -

Table 3-2 The samples featured in this thesis.

Experiment Build-up Colour (substrate) Colour (released) Thickness (modeled)

MLTF free-standing (PEI|MFC)22 green violet 400nm

MLTF copy-paper (PDADMAC|PSS)4-(PEI|MFC)10 orange transparent whitish 250nm

Kofes strips (Carrageenan|Gelatin)10 transparent(glossy) - -

Kofes demonstrator (Carrageenan|Gelatin)10 transparent(glossy) - -

MAMA (PDADMAC|PSS)4-(PEI|MFC)10 orange transparent whitish 250nm

3.4 Micro Adhesion Measurement Apparatus (MAMA)

The experimental part related to the MAMA is described below.

3.4.1 Preparative

The probe holder (glass surface) was thoroughly cleaned with Milli-Q – EtOH – Milli-q

and was then dried with a N2-flux. The PDMS half-speres were mounted on a clean glass

slide and were then rinsed in the order of Milli-Q - EtOH- Milli-Q, after which they were

plasma-treated for 1 minute on medium (10 W) effect. This was followed by 10 minutes of

incubation in PEI after which the probes were thoroughly rinsed with Milli-Q before being

dried under an N2-flux and transferred to the glass surface of the probe holder. The

substrates were MLTFs prepared on fluorinated surfaces according to the above mentioned

protocol. One difference was that the MLTFs in these experiments consisted of two parts,

the substrate-near being (PDADMAC|PSS)4 and the surface-near being (PEI|MFC)10. The

release of (PDADMAC|PSS)-MLTFs demonstrates a more general method than would the

release of a pure (PEI|MFC) MLTF, due to the fact that MFC is not generally available for

purchase, whereas both PDADMAC and PSS are commercially available.

3.4.2 Experimental

We focused our investigation on the pull-off of a MLTF with an outermost negatively

charged MFC surface from a fluorinated silicon surface. The PDMS probe was coated with

positively charged PEI to complement and bond to the negatively charged MFC,


- 32 -

outermost on the MLTF. The planned and performed experiments are shown by Table 3-3.

Due to numerous mishaps and the quite rigorous and time-consuming experimental

preparations, only a couple of measurements were preformed. These however proved

successful regarding MLTF-release, as is described further in the results & discussion

section of the thesis.

The sample was mounted on the balance which was then tared (zero-load). The probe was

then pressed against the sample resulting in an increasing load on the balance. Through the

transparent probe, made from polydimethylsiloxane (PDMS), the camera registered the

contact area between the PDMS probe at the measure points and the sample making it

possible to correlate an area and a load to one another. When the maximum load had been

applied during the desired time, the unloading started. The unloading continued until the

MLTF had been transferred or until the surfaces were separated. Both dry and wet

measurements were carried out. What differentiated the two types from each other was that

a droplet of Milli-Q(wet) was applied onto the substrate prior to the PDMS probe and the

substrate being brought together, in the wet technique. This wet technique was applied to

increase the electrostatic bonding capability of the surfaces. Once the maximum load had

been applied, the excess water was blown away using a N2-flux.

Table 3-3 showing the experimental setup that was planned and preformed using the Micro Adhesion Measurement Apparatus (MAMA).

1g 2g 5g 10g steps steps steps steps

1h 20* 100 20 100 20 100 20*** 1002h 20 100* 20 100 20** 100 20 100

described in results&discussion* probe pulled off from holder (no glue used)

** probe broke (pulled in two pieces)*** caused contact area to take up entire frame (no measurent possible)

unmarked cancelled due to time shortage

Max

-Del

ay

Maximum Load

(All experiments were performed with 5s between each increment/decrement of load (SPI), and with MLTF-buildup [(PDADMAC|PSS)4-(PEI|MFC)10]


- 33 -

4 RESULTS AND DISCUSSION

In this section the results are presented and discussed.

4.1 Preparative

This section features the preparative characterisations.

4.1.1 Material Characterisation

Provided below is information regarding particle size that was obtained for PEI and MFC

using dynamic light scattering, illustrated by Figure 4-1 and Figure 4-2, respectively.

Figure 4-1 The figure gives an estimation of the diameter of the PEI molecules. Three over-layered curves are shown indicating an approximate diameter of ~5 nm. As the figure indicates, larger aggregates do exist, but are relatively few with respect to the total volume of light-scattering substance.


- 34 -

Figure 4-2 The figure indicates a value for MFC that would normally correspond to particle/aggregate diameter (~10 nm). In this case however, because the fibrills are assumed to have a somewhat cylindrical geometry, this value is thought to correspond to the diameter of the cylinder (as is discussed further in the continuation of this section).

The solutions of PEI and MFC were further characterised with respect to zeta-potential, as

is shown by Table 4-1, Figure 4-3 and Figure 4-4. These data were also retrieved using the

dynamic light scattering apparatus.

Table 4-1 The zeta-potential of MFC as well as that of PEI, as shown in the column headed ZP. Also shown is mobility and conductivity as well as sample temperature.


- 35 -

Figure 4-3 The distribution of the dynamic-light-scattering measurement data obtained for PEI regarding zeta-potential. Three over-layered peaks at ~25mV.

Figure 4-4 Three over-layered curves showing the distribution of the zeta-potential of MFC, derived from the dynamic light scattering measurements, (~-115 mV).

The results from the DLS measurements of the zeta-potentials confirm that the preparative

work of this thesis is in accordance with the previously reported research8, 10, 11 on MLTF

synthesis with PEI and MFC as positively and negatively charged constituents respectively.

Furthermore the estimated size of the constituents correspond to those reported by Axnäs

and Wågberg10, 11. The fact that the real sizes of the microfibrills differs from those

obtained by the DLS experiments in this and other reports is discussed further by Axnäs 10.

In short it can be explained by the fact that the Brownian motion of particles utilized in

DLS measurements is not likely to proceed in the lengthwise orientation of the MFC.


- 36 -

Whereas the DLS apparatus assumes a globular particle conformation, the microfibrills, as

shown by atomic-force microscopy and transmission electron microscopy10, are roughly

cylindrically shaped with approximate measurements of 10 nm in diameter and up to 1 μm

in length. The DLS values thus correspond well with the diameter of the microfibrills.

4.1.2 Contact Angle Verification of Hydrophobicity of Fluorinated Substrates

To verify the success of the fluorinating step, contact-angle measurements were carried out.

The fact that spontaneous de-wetting occurred when rinsing of the fluorinated surfaces in

Milli-Q was preformed gave a hint of success in the change of surface behaviour from

hydrophilic to hydrophobic. However, we wanted to quantify this by determining the

contact angle.

Measuring at three separate points on a surface, we reached contact angles of 110.5

degrees, for Milli-Q against the fluorinated surface. As a reference the contact angle of

Milli-Q on a silicon wafer was measured to about 15 degrees. This value correlates well

with the previously reported contact angles of self-assembled monolayers of PFOS

analogues on silicon. Achieving contact angles of up to around 130-140 degrees have been

reported, although such high contact angles rather suggest the multilayering of molecules

than ordering and further packing of molecules within one monolayer20.

Photographs representing reference and substrate measurements are provided in Figure 4-5

.


- 37 -

14.92 15.07

110.38 110.53

Figure 4-5 To the left the reference silicon substrate and to the right the fluorinated substrate is shown. The measurements of left and right contact angles (in red) preformed with the KSV Cam 200 instrument

resulted in contact-angles of ~15° and ~110.5° for the reference and fluorinated surfaces respectively. This indicates a change of surface behaviour from being largely hydrophilic to hydrophobic, as is expected

for the fluoro-silanisation.

Although contact angle measurements do not directly offer a characterisation on an atomic

or molecular level with respect to the orientation of the adsorbates on the surface, as do

other forms of analysis methods, they were deemed satisfying enough for our purposes, by

clearly indicating that a surface modification had occurred.

4.2 Free-standing Films

Being able to create free-standing MLTFs is one of the main objectives of this thesis. This

section describes the successful steps towards achieving our goal, namely controlled release

of interference MLTFs rendering free-standing interference MLTFs.

While earlier researchers on this subject have either created relatively thick MLTFs, about

~8-9 microns thick, (which is too thick from an interference perspective) combined with

Teflon coated substrates to obtain films, releasable by tweezers, or have used wet-release

strategies in order to obtain free-standing MLTFs (described in section 2.4), we report the

successful release of relatively thin MLTFs using a dry method.


- 38 -

The main strategy of success incorporates the use of adhesive tape. Either a tape frame /

window was cut in the tape, thus enabling a free-standing segment of released film to be

created covering the window, as demonstrated by Figure 4-6, or the edge of a piece of tape

was used, as per Figure 4-7. For the former, the tape provided two key aspects, namely an

adhesive functionality and a supporting functionality.

Employing the adhesive tape was done because of the fact that removing the thin films

from their substrates using tweezers or razor-blades, as reported by others, was not

applicable, due to the thinness and fragility of our interference MLTFs.

The films depicted by Figure 4-6 and Figure 4-7 were released from the same substrate,

namely a (PEI|MFC)22-MLTF. As is clearly shown, the films show different colours

depending on the angle of observation, with the MLTF of Figure 4-6 being yellow and the

MLTF of Figure 4-7 showing a violet colour. The colour of the film while still attached to

the silicon substrate, pictured by Figure 4-9, was mainly green. This illustrates the

Figure 4-6 Mounting of a (PEI|MFC)22 -MLTF on black copy-paper. The released MLTF is supported by a tape-window (shown within red circle). The MLTF supported inside the window is yellow. The darkening of the piece of MLTF that is furthest away in this picture is due the MLTF having lost its support from the tape, resulting in it partially curling out of sight (downward in figure)

Figure 4-7 A (PEI|MFC)22 MLTF-segment released by the use of the end part of a piece of tape. This angle of observation shows a violet colouring of the MLTF.


- 39 -

dependency on the substrate of the observed colour, since the free-standing films released

from this substrate show a weakly violet colour (Figure 4-8) when viewed from the same

angle as in Figure 4-9 of the substrate bound MLTF. Figure 4-8 illustrates the same MLTF

as in Figure 4-7, only the angle of observation again is different. It was noted at this point

that the colour-dependency could be attributed both the nature of the surrounding medium

(air-air or silicon-air), and the angle of observation.

The tape was attached to the MLTF and allowed to stick for approximately 5 minutes. The

tape was then carefully removed by gradually lifting at one end of the tape, until the entire

strip of the tape was free.

Our desire to produce MLTFs, thin enough to show interference colours, while being

strong enough to withstand the somewhat rough handling of the lift-off or transfer

procedure, proved tedious to fulfil. Although the surface modification made film release

easy, as shown by numerous tape-releases, the produced films still thin enough to show

clear interference colours were as expected, quite fragile. Thus, the release of intact thin

films with a predetermined surface area was demonstrated to be possible, however they

rapidly broke, ending up like the pictured MLTF in Figure 4-10, with ruptures through the

middle of the window. The ruptures are though thought to have arisen due to the handling

of the samples, as the MLTF was entirely suspended directly after lift-off.

Figure 4-8 Picture of (PEI|MFC)22 that is free standing, photographed roughly perpendicular to the surface to the plane of the MLTF.

Figure 4-9 illustrates (PEI|MFC)22 while still attached to the fluorinated silicon-substrate. Photograph taken perpendicularly to the surface.


- 40 -

Figure 4-10 As can be seen, the MLTF no longer pans the entire window in the tape, (red arrows indicate areas where the film has disconnected from the supporting window and curled). Black copy-paper was used as background, onto which the windowed tape with the MLTF was been placed. To the left a red scale-bar approximately indicates 1cm.

All of the release-attempts were carried out by hand, and this is thought to have made the

release of intact films harder. The ability to manufacture intact free-standing MLTF-

segments could possibly be simplified by lifting the adhesive tape with an automated

device, capable of slow and smooth increments or movement. The strain on the film would

thus be smaller.

Summarizing the experiments preformed with adhesive tape, we can conclude that we have

managed to perform release of interference-MLTFs. Furthermore, controlling the intact

area that was managed to be made free-standing proved hard. Incorporating windows in

the tape strips proved successful. However, the film segments suspended in the window

easily ruptured introducing cracks in the film. By using the end sections of adhesive tape to

release MLTFs, sections of about 1 cm x 1 cm was readily prepared, these were however

not possible to control regarding shape. These films, lacking the tape window support also

showed a tendency to double back on themselves creating folded structures.


- 41 -

4.3 Transfer to New Carrier Materials

One key inspiring idea for the technique used to delaminate the generated MLTFs from

their substrates (fluorinated silicon) and applying them on to a new carrier material, was to

bring the surface of the film, whilst still situated on the substrate, in contact with a new

carrier material, under wet or damp conditions, and to then let the sample and target

material dry under applied pressure. This procedure would hopefully facilitate stronger

adhesion between the MLTF and the new material, than between the MLTF and the

fluorinated silicon surface, resulting in film transfer to the target material.

As new target materials, several different subjects were tried, including cellulose-based

dialysis membranes (not shown), filter paper and copy paper.

Although successful transfer was observed for all cases, no straightforward recipe could be

deduced, by which guaranteed MLTF-transfer could be accomplished. This was due to the

fact that part of the trials ended up in failures regarding transfer, without obvious reasons.

It should be noted though, that demonstrator-scale preparation was readily possible for

both copy- and filter paper.

Figure 4-11 and Figure 4-12 show successful transfer of interference MLTF to copy paper.


- 42 -

Figure 4-11 showing photograph of a (PDADMAC|PSS)4 – (PEI|MFC)10 MLTF transferred to copy paper through overnight contact drying of substrate and copy paper. As is pictured, only partial transfer has occurred. Towards the left, multiple colours are shown due to shifting thickness of the MLTF (compare with Figure 4-12). The black line is introduced as scale bar, indicating the width of silicon wafer slide, approximately 1cm.


- 43 -

Figure 4-12 showing the same (PDADMAC|PSS)4 – (PEI|MFC)10 MLTF as in Figure 4-11 as well as the substrate from which it was transferred. As indicated by the red circle different colours are apparent on the substrate. These are due to differences in thickness of the MLTF. The thickness dependent differential colouring observed in Figure 4-11 is however not captured here, due to the angle of observation being different. The red squared area illustrates the perimeter of the contact area between the substrate and the copy paper during the contact drying. The approximate measurements of the red square and hence the substrate are 1cm x 7cm.

The amount of transferred MLTF varied, sometimes consisting only of very scarcely placed

dots of interference MLTF, and other times consisting of the majority of MLTF that had

been in contact with the target material. For pergamin paper only dots were observed to be

successfully transferred.

The best results were accomplished by placing the MLTF-covered silicon strips between

sheets of the desired target-material followed by wetting with Milli-Q. The silicon strip and

the target material sheets were then in turn, placed between microscopy glass slides which

were subjected to pressure by mounting them between clamps, thus pressing the

underlying MLTF and the target surface together, as indicated by Figure 4-13.


- 44 -

The samples were left to dry together overnight and were evaluated the following morning.

The applied force was not measured; however generally, the results came out better when

more pressure was applied.

Figure 4-13 illustrating the use of clamps and glass slides to press together a sample and a piece of copy paper during contact drying. Paper and sample are pressed together while still wet from Milli-Q and are allowed to dry overnight. The process, although not foolproof yielded demonstrative transfer from fluorinated silicon surface to copy paper (illustrated here) and filter paper as well as dialysis membranes (not shown).

4.4 Interference Film Synthesis directly on Aluminium and Fibre-based

Substrates

For the purpose of familiarizing with the ease, strengths and possibilities accompanied with

the layer-by-layer technique, decision was made to test whether multilayering of

polyelectrolytes could produce interference-MLTF, directly on aluminium or fibre-based

substrates. The work of Andersson8 greatly influenced these experiments regarding

selection of polyelectrolytes. Andersson showed that MLTFs based on Carrageenan and

Gelatine possibly showed swelling behaviour upon introduction of the films to moisture

(exhaled breath was thoroughly investigated), inducing interference colour-shifts. This

system was thus tested on the kofes demonstrator as one primary intent with the creation

of the kofes itself was the need for a new type of material/functionality demonstrator, not

limiting the mindset of the spectator in any way when visualising the possible areas of use

for the material demonstrated by the kofes27, 28.


- 45 -

Aluminium foil and a fibre/poly(lactic acid)-composite were the selected substrates for the

(Carrageenan|Gelatine) multilayering procedure.

To make detection of MLTFs easier, the composite material was spray painted with a

copper-tone before the LbL MLTF synthesis. The copper toned paint was chosen due to

its refracting index being different from the one of the composite material.

Figure 4-14 The left part of the top strip of composite has been coated with a MLTF of (Carrageenan|Gelatin)10. Below an uncoated reference-strip is shown.

The figures below show the results of the successful multilayering of

(Carrageenan|Geltin)10 producing MLTFs with withheld sensitivity to humidity, by shift of

colour, as is illustrated below in Figure 4-15 and Figure 4-16.

Similarly, aluminium strips were coated according to (Carrageenan|Gelatine)10. These

aluminium strips showed interference colour-shifts to the naked eye. Photographs were

however not captured on camera, due to the relatively weak intensity of the MLTF on the

aluminium substrate.


- 46 -

Figure 4-16 The same strip as in Figure 4-15, after the evaporation of the applied humidity.

These results show the power of the layer-by-layer technique, enabling precise coating of a

wide variety of surface structures and materials. No other preparation than the spray-

painting was conducted on the composite material (Figure 4-17). It can thus be concluded

that the successful synthesis via the LbL technique of MLTFs showing interference

colouring is not always dependent upon specially prepared substrates such as thoroughly

cleaned, smooth and well defined silicon wafer strips.

Figure 4-15 Exhaled breath has been deposited onto the (Carageenan|Gelatine)10-film. The absorbed humidity, that has caused the film to swell, and thus change its colour through interference, is shown as a dark band in the middle of the picture, that is not present in Figure 4-16.


- 47 -

4.5 Micro Adhesion Measurement Apparatus (MAMA)

The results of the MAMA experiments are described below. These results show that the

release of MLTFs by the MAMA equipment is possible under certain conditions. Although

they do give some qualitative assessments of the feasibility of this kind of experiments, they

do not provide a quantitative evaluation. Such an investigation was initially planned, but

was cancelled due to shortage of time.

4.5.1 Successful Release Using MAMA Pull-off

Of the experiments performed, two resulted in successful pull-off (circled in red in Table

3-3). These are described below, starting with the 2g maximum load experiment.

Figure 4-17 showing the ~10 cm x 10 cm spray-painted kofes piece before multilayering. (the grid scale is 1 cm)


- 48 -

2g maximum load experiment: Figure 4-18 shows the Load versus Time plot of the experiment resulting in pull-off at a

load of -25 grams. This value was considered quite large since the fact that the circular

maximum area of contact between the probe and the MLTF has an approximate diameter

of 330μm and that the standard adhesion measurements using MAMA (although not

directly comparable to our experiments) report on loads in the range of hundreds of

milligrams. The normally employed JKR theory can not be implemented fully because of

the fact that we induce a cohesive breakage.

At points A and B the maximum load is applied. Variations in the readout of load in this

section could be due to adhesive interactions, as the actual position of the probe is held

constant. One large contributing factor to the differences in applied load should be the fact

that excess water is fluxed away between points A and B. Load readouts of the

measurement points are shown in Table 4-2, as are the diameters of the contact areas. C

indicates a point right before zero load is passed and D a point closely following the zero-

passage. If there were no adhesive force between the surfaces, then they would not stick

together beyond the zero-load point. E-I are points corresponding to roughly -5g,-10g,-

15g,-20g and -24g (pull-off) respectively. The J value is indicative of no load, as is expected,

as the probe (with MLTF attached) and substrate are no longer in contact, Figure 4-19J.


- 49 -

Load vs TimeSuccessful(2g)

-30

-25

-20

-15

-10

-5

0

5

10

0:00:00 0:14:24 0:28:48 0:43:12 0:57:36 1:12:00 1:26:24 1:40:48

Time(h:min:s)

Load

(g)

A B CD

E

F

G

H

I

J

Figure 4-18 showing the Load versus Time plot of the 2g experiment yielding MLTF-release. A one hour delay at maximum is followed by the unloading phase. This continues to a load of approximately -24 grams, at which the film is released from its substrate. A-J indicate the approximate experiment coordinates for the data of Table 4-2 and the pictures in Figure 4-19.

Table 4-2 showing data of the 2g maximum load MAMA-experiment for selected measurement coordinates. Pictures of the A-J are illustrated in Figure 4-19, and C* is illustrated by Figure 4-20C.

Figure label Measure Point Load (g) radius^3 (μm^3) diameter (μm)A 11 2,0782 7144476 385,2004533B 17 1,7756 8948069 415,2150663C 28 0,0314 8951656 415,2705412D 29 -0,1294 8922480 414,8188884E 60 -5,0891 8097306 401,6152354F 93 -10,1097 7581709 392,9033208G 129 -15,4156 6824814 379,3676278H 159 -20,1392 6102523 365,4824021I 189 -24,07 4507116 330,3666815J 212 0,0013

C* 6,2091 8218228 403,6045537


- 50 -

Figure 4-19 showing photographs taken during the 2g maximum load MAMA experiment, at the measurement points A-J. The fuzziness of picture A is due to Milli-Q still present on the sample. In B the Milli-Q has been fluxed away by N2. The whitish circular area is the contact area between the PDMS probe and the MLTF. Outside of this the yellow surrounding parts of the MLTF clearly shows. D-I shows the influence of pulling, as the contact area decreases. J is a picture taken after the MLTF has been pulled off. A clearer picture of this is provided by Figure 4-20. (Due to the instrument being located in a laboratory where other instruments and users simultaneously work, the background lighting varied, as can be seen by comparing B and H.)

Figure 4-20 A-B showing differently scaled photographs of the substrate in the 2g maximum load MAMA experiment, after successful pull-off. The whitish area is the underlying silicon wafer. As can be clearly seen, the pulled-off segment is not nearly a perfect circle. Picture C, which is referred to as C* in Table 4-2 is a photograph of the PDMS probe with MLTF attached, post-release. Noteworthy are the cracks of the PDMS probe that are clearly visible in C. Also visible are the protruding fringes of the MLTF at the edges of the probe in C. Approximate diameter of the MLTF in C (corresponding to diameter in A-B) is 404μm.


- 51 -

The cracks of the PDMS probe shown in Figure 4-20C could possibly be prevented by

lowering the maximum load. The fact that the pulling load is relatively large compared to

the maximum load applied, 2g applied load, to be compared by -24g of pulling load,

however also suggest that the deformation could be due to the pulling load, that can not be

controlled as it is dependent upon the adhesion between the substrate and the probe.

Evaluation of MAMA data by applying JKR theory is not accurate if the PDMS probe is

deformed, as the equations employed involve parameters coupled to the characteristics of

the PDMS probe. A more thorough explanation is given in 24.

5g-maximum load experiment Shown below in Figure 4-21 is the Load versus Time plot for the 5g-maximum load

MAMA-experiment. The setup for this experiment as well as for the previous is given by

Table 3-3. The 2 hour delay at maximum is followed by the unloading, that in this case

proceeded to a ~-6.87g load.

The transfer of a segment of MLTF from the substrate to the PDMS probe was successful

also in this experiment. Photos of the MLTF attached to the probe as well as on the

corresponding MLTF-free area on the substrate are shown by Figure 4-22. No cracks were

observed within the probe, although the applied load was 5g in this experiment, compared

to 2g for the experiment where cracks occurred. This suggests that the deformation of the

PDMS probe rather could be due to the intense pulling of ~24g in the 2g maximum load

experiment.

The ridges in the MLTF shown by Figure 4-22C could possibly have arisen due to

deformation of the PDMS probe during loading and unloading. Furthermore from Figure

4-22A-B cracks in the MLTF can be distinguished against the silicon substrate. This could

suggest that the cohesive break within the film does not proceed uniformly. The

approximate diameter of the pulled-off segment of the MLTF was 515μm.


- 52 -

Load vs Time

-8

-6

-4

-2

0

2

4

6

0:00:00 0:14:24 0:28:48 0:43:12 0:57:36 1:12:00 1:26:24 1:40:48 1:55:12 2:09:36

Time (h:min:s)

Load

(g)

Figure 4-21 showing the 5g MAMA-experiment with 2h delay at maximum load. This plot has fewer measurement points than does that of the 2g experiment. This is due to the setup, which specifies 100 steps per increment/decrement of load every 5 sec, compared to 20 steps for the 2g experiment. (the bump into negative time at beginning of maximum load phase is an artefact due to curve-fitting in Microsoft- excel software and should be disregarded)

Figure 4-22 shows photographs from the 5g maximum load MAMA experiment. Successful pull-off was preformed. A and B show the area where the MLTF has been pulled off. It is clear that the MLTF at the edges is somewhat shredded and that the cohesive break within the MLTF was not perfectly circular. The approximate diameter for the pulled off piece of MLTF was 515μm. C shows a photograph, where the focus lies on the outermost part of the PDMS probe where the MLTF is situated. Ridges in the MLTF are visible, probably due to the fact that the PDMS is elastic and deformes/reformes when put in and out of contact with the substrates. No cracks were observed in the probe.


- 53 -

4.5.2 Evaluation of the MAMA Experiments regarding Pull-off.

Trying to draw any sharp conclusions from only two successfully performed experiments

would not be realistic since it would need an over-interpretation of the obtained data.

We however managed to perform what we sought to investigate, and can report on

successful transfer of MLTF from a fluorinated silicon substrate to a PEI-coated PDMS

probe. These results are deemed promising although comprehensive work is needed in

order to enable an accurate use of the instrument for MLTF pull-off measurements in the

future.

Although successful transfer was accomplished, the area near the edges of the contact area

of the MLTF still attached to the substrate suggest that the separation of the part of MLTF

that is attached to the probe from the rest of the MLTF might not be a uniform process.

Since the experiments included the application of a droplet of Milli-Q on the MLTF before

contact was made with the probe, visibility was greatly inhibited as is shown in Figure

4-19A.

One type of experiment that could greatly enhance both quality and feasibility of the

experiment would be one where the pulled-off MLTF is not attached to a larger piece of

MLTF. Thus one would eliminate the need for cohesive breakage of the MLTF. These

kinds of experiments would however require some modifications to the instrument.

Visibility would have to be good enough for precise control even under wet conditions, in

order to make fitting of probe and MLTF piece possible through the microscope. This is

currently prevented by the poor visibility accompanied with introduction of Milli-Q

droplets. Also, a way to prepare the corresponding mini-segments of MLTFs of controlled

area would be advantageous.

We were able to try this type of experiment in dry-conditions but without any success.


- 54 -

4.6 Observations In this section a few unexpected observations are presented.

4.6.1 (Microfibrillated Cellulose | poly-Ethyleneimine) – Gels

When the nanostrata system was utilized without drying (N2-flux), gels started to form on

the substrates after approximately five multilayering cycles in MFC and PEI. These gels

were not investigated but pictures are provided in

Figure 4-23. The formation of gels when the drying part of the multilayering cycle was

omitted caused us to select drying times longer than initially intended. Gel-formation was

observed for MFC|PEI concentrations of both 1g/L and 0.1g/L.

Figure 4-23 illustrating a fluorinated substrate having undergone 10 multilayering cycles in PEI|MFC. The formed gel is clearly visible on top of the silicon substrate. Excess water has been removed through careful tilting of the substrate. Left and right pictures are of the same substrate through different angles of observation.

4.6.2 The Colour Gradient at the Edge of a MLTF

Due to evaporation of water from the polyelectrolyte solutions and the dipping times used

throughout this thesis, a colour gradient could be observed at one end of our MLTFs. This

was because of the fact that the loss of water from the beakers resulted in that successively

less and less of a substrate could be submerged in the solutions. The gradient can be used

to roughly estimate the thickness of the MLTF ( it can be used to estimate whether a blue

MLTF colouring is of the first or second order). Care must be taken when using the

gradient for any kind of estimation because of the fact that the gradient’s existence depends

upon the evaporation of water from the beaker. Thus, if very little water evaporates, no


- 55 -

gradient will be observed regardless of how many cycles or colour-shifts a MLTF has

undergone. Figure 4-24 shows a gradient of a MLTF.

Figure 4-24 shows a gradient of colour at the edge of the MLTF. The area of the gradient is indicated by the red arrow. From this picture, one can estimate the blue colour of the MLTF to be of second order, since the gradient suggests that the MLTF has previously been coloured in the order; blue light blue yellow orange red violet blue(second order). The gradient is read from top to bottom


- 56 -


- 57 -

5 SUGGESTIONS FOR FURTHER WORK AND APPLICATIONS

Although demonstrator preparation of MLTFs on paper was successfully accomplished,

larger pieces of films proved hard to produce given the currently applied methods. Thus, a

quantitative study of film transfer from fluorinated surfaces to different types of paper

would be very interesting to perform, e.g. continuation of the MAMA method presented in

section 4.5.1.

The incorporation of automated devices to improve precise handling of the MLTFs during

and after MLTF release could possibly enable the synthesis of free-standing pieces of

MLTFs larger than the ones managed in this thesis. Using the MAMA to evaluate different

surface treatments in a release perspective is far from accomplished, leaving lots to be

investigated in the future. MLTF release using MAMA was however accomplished,

showing that the suggested method does have a promising future. Development of the

apparatus would be needed to make wet pull-off measurements of prepared mini-segments

feasible. This however promises to become a challenge to be solved in the future.

In a broader perspective, it would be very interesting to investigate the mechanical

properties of the free-standing MLTFs in an effort to produce larger quantities of more

stable MLTFs. Understanding which parameters of the synthesis that affect the rigidity of

the films would possibly enable this.

The fact that the films can be made free-standing while still thin enough to show

interference colouring has introduced the possibility to create petite moisture sensitive

sensors. An investigation regarding the possibility to couple the moisture-sensing ability to

the detection of other parameters is one major area of great interest that remains to be

performed.


- 58 -


- 59 -

6 CONCLUSIONS

To our knowledge, we are the first to report on the success of creating free-standing

multilayered interference thin films made from nano-cellulose and polyelectrolytes. The

free-standing and transferable multilayered interference thin films (MLTFs) were prepared

by hydrophobically modifying the silicon-substrates ordinarily used for synthesis:

The free-standing films were lifted from the fluorinated silicon substrate using

adhesive tape, functioning either as a surrounding support frame or as a single-

ended support. The thinnest MLTF made free-standing was a [PEI|MFC]22-film of

approximate thinness 400 nm (according to model). This MLTF was green when

substrate bound and showed a violet colour once made free-standing. The largest

piece of free-standing MLTF created from a [PEI|MFC]22-film had an approximate

area of one square centimetre. When mounted on black copy-paper the MLTFs

withheld its moisture sensitivity.

The interference thin films successfully transferred to fibre-based materials were

(PDADMAC|PSS)4 – (PEI|MFC)10 with an approximate thickness of 250 nm.

These were orange while connected to the silicon substrate, and

transparent/whitish when transferred to copy paper.

We were also able to create interference thin films directly on a fibre composite material

surface (kofes), pre-treated with a copper-toned paint. The films were

(Carrageenan|Gelatine)10.

Furthermore we also demonstrated the possibility of using the Micro Adhesion

Measurement Apparatus (MAMA) in MLTF pull-off experiments. If further developed,

this technique could be used to evaluate substrates from a releaseability perspective.

Hopefully this newfound ability will inspire not only researchers in the field of thin film

science.

The possibility to transfer multilayered interference thin films to different types of fibre-

based materials and the possibility to create free-standing films, as well as the possibility to

synthesise these films on substrates other than smooth silicon substrates is important, as


- 60 -

future already thought of applications of this technology platform heavily rely on the

MLTF not being bound to a silicon substrate. Packaging applications as identity tags would

describe this latest example in an ‘easy to comprehend’ way.

Thank you!


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7 REFERENCES

1. Iler, R. K. Journal of Colloid and Interface Science 1966, 21, (6), 569-594.

2. Decher, G.; Hong, J. D. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 1991, 95, (11), 1430-1434.

3. Decher, G. Science 1997, 277, (5330), 1232-1237.

4. Decher, G.; Schlenoff, J. B., Multilayer thin films: sequential assembly of nanocomposite materials. Wiley-VCH: Weinheim, 2003.

5. Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, (13), 5530-5533.

6. Jiang, C. Y.; Tsukruk, V. V. Soft Matter 2005, 1, (5), 334-337.

7. Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nature Materials 2003, 2, (6), 413-U8.

8. Andersson, T. Moisture sensitive opto-active films for paper products. Master of Sciense Thesis, Chalmers, Stockholm-Gothenburg, 2007.

9. Aulin, C.; Shchukarev, A.; Lindqvist, J.; Malmstrom, E.; Wagberg, L.; Lindstrom, T. Journal of Colloid and Interface Science 2008, 317, (2), 556-567.

10. Axnäs, K. Build-up of Polyelectrolyte Multilayers with Microfibrillated Cellulose. Master of Science Thesis, KTH, Stockholm, 2006.

11. Wagberg, L.; Decher, G.; Norgren, M.; Lindstrom, T.; Ankerfors, M.; Axnas, K. Langmuir 2008, 24, (3), 784-795.

12. Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. Journal of the American Chemical Society 2005, 127, (49), 17228-17234.

13. Ono, S. S.; Decher, G. Nano Letters 2006, 6, (4), 592-598.


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14. Österman, J.; Nordling, C., Physics Handbook for Science and Engineering,p274. Studentlitteratur: Lund, 1999.

15. Halliday, D.; Resnick, R.; Walker, J., Fundamentals of Physics,p 874-878. Sixth edition ed.; Wiley Inc.: New-York, 2001.

16. Zeta-sizer; Nano-series; Instrument-Manual, Malvern Instruments. United Kingdom, 2004.

17. Jaber, J. A.; Schlenoff, J. B. Journal of the American Chemical Society 2006, 128, (9), 2940-2947.

18. Björefors, F., Correspondence per e-mail. In Electrochemistry ed.; Linköping University, IFM-Applied Physics, Correspondence via e-mail, 2007.

19. Dupont. http://www2.dupont.com/Teflon_Industrial/en_US/ 2008-02-07.

20. Kulinich, S. A.; Farzaneh, M. Surface Science 2004, 573, (3), 379-390.

21. Brzoska, J. B.; Benazouz, I.; Rondelez, F. Langmuir 1994, 10, (11), 4367-4373.

22. Dutoit, B. M.; Barbieri, L.; von Kaenel, Y.; Hoffmann, P. A. H. P. In Self-assembled real monolayer coating to improve release of MEMS structures, TRANSDUCERS, Solid-State Sensors, Actuators and Microsystems, 12th International Conference on, 2003, 2003; Barbieri, L., Ed. 2003; pp 810-812 vol.1.

23. http://www.dda.se. Web-page 2008-02-23.

24. Eriksson, M.; Notley, S. M.; Wagberg, L. Biomacromolecules 2007, 8, (3), 912-919.

25. Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, (5), 1013-1025.

26. Johnson K. L.; Kendall K.; Roberts A. D. Proc. R. Soc. London 1971, A324, 301.

27. Lindström, M.; Razavi, F.; Nobell, N., Kofes Demonstrator development at STFI-Packforsk AB. In 2007.


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28. Ny-Teknik Deras varuprov liknar ingenting. http://www.nyteknik.se/nyheter/innovation/forskning_utveckling/article42753.ece (2008-02-29),

29. Harrick-plasma Plasma cleaner. http://www.harrickplasma.com/products_cleaners.php (2008-02-24)

30. KSV-Cam-200 http://www.ksvltd.com/content/index/cam200 (2008-02-24),

31. Malvern-Instruments; Zetasizer; nano-zeta http://www.malvern.com/LabEng/products/zetasizer/zetasizer.htm (2008-02-24),

32. Sonics-Vibracell-VCX-500 Sonicator. http://www.sonics.biz/lp-vibra.htm (2008-02-24),

33. Nanostrata-Inc. Stratosequence VII. http://www.nanostrata.com/ (2008-02-24),

34. Bunker, B. C.; Carpick, R. W.; Assink, R. A.; Thomas, M. L.; Hankins, M. G.; Voigt, J. A.; Sipola, D.; de Boer, M. P.; Gulley, G. L. Langmuir 2000, 16, (20), 7742-7751.


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8 LIST OF FIGURES AND TABLES

List of figures:

FIGURE 2-1 THE LBL-ASSEMBLY IS ILLUSTRATED. TWO TYPES OF OPPOSITELY

CHARGED MOLECULES, A AND B, ARE ALTERNATELY ADSORBED ONTO THE

SUBSTRATE. BY REPEATING THE PROCEDURE, MLTFS CAN BE SYNTHESISED. ....... - 5 -

FIGURE 2-2 SHOWING THE STRUCTURAL FORMULA OF THE PEI-MOLECULE. AS IS

ILLUSTRATED, THE MOLECULE IS BRANCHED. ................................................................ - 6 -

FIGURE 2-3 ILLUSTRATING A SUBSTRATE BOUND MLTF. THE SURROUNDING MEDIA

ARE AIR AND SUBSTRATE, (INDICATED BY THEIR REFRACTIVE INDICES (N1) AND

(N3)). THE MLTF-PARAMETERS ARE THE THICKNESS (D) AND THE REFRACTIVE

INDEX (N2). (INSPIRED BY ILLUSTRATION IN14) ................................................................. - 7 -

FIGURE 2-4 ILLUSTRATING THE SOMEWHAT DIFFERENT PROPERTIES OF A FREE-

STANDING MLTF. THE MEDIUM ON BOTH SIDES OF THE MLTF IS THE SAME

AND HAS REFRACTIVE INDEX (N1). THE MLTF HAS A THICKNESS OF (D) AND A

REFRACTIVE INDEX (N2). (INSPIRED BY ILLUSTRATION IN14) ........................................ - 7 -

FIGURE 2-5 THE SAMPLE (LEFT) IS SUBMERGED INTO THE SOLVENT. THE

SACRIFICIAL LAYER THEN STARTS TO DISSOLVE (RIGHT) DUE TO ITS

SOLUBILITY IN THE SOLVENT, THUS LEAVING A FREE-STANDING TARGET

FILM. ........................................................................................................................................... - 10 -

FIGURE 2-6 BY INTRODUCING CF2 AND CF3 GROUPS TO THE SILICON SURFACE, THE

ADHESION BETWEEN THE MLTF AND THE SUBSTRATE IS DECREASED. THIS

MAKES REMOVAL POSSIBLE, IN THIS FIGURE EXEMPLIFIED BY PEELING THE

MLTF OF USING TWEEZERS. FOR ILLUSTRATING PURPOSES THE FILMS ARE

ILLUSTRATED AS A A-B-A PATTERN. THE FILMS OF HIS THESIS ARE

MULTILAYERED I.E. (A|B)20 (NOT SHOWN HERE). ........................................................... - 12 -


- 66 -

FIGURE 2-7 AS THE PH OF THE SURROUNDING MEDIUM IS CHANGED, THE

SACRIFICIAL LAYER WHICH IS HELD TOGETHER BY HYDROGEN BONDS

BETWEEN CONSTITUENTS A AND B DISSOLVES, DUE TO THE INDUCED LOSS OF

HYDROGEN BONDING CAPABILITY. THE TARGET FILM IS UNAFFECTED BY THE

TREATMENT AND IS LEFT FREE-STANDING IN THE SOLUTION. ............................... - 13 -

FIGURE 2-8 THE HF ASSEMBLE-DISSOLVE TECHNIQUE IS DEPICTED. INITIALLY LBL

ASSEMBLY ONTO A SACRIFICIAL LAYER IS PERFORMED AND THIS IS FOLLOWED

BY HF TREATMENT. WHEN TREATED WITH HF, THE SIO2-SACRIFICIAL LAYER IS

REMOVED, RENDERING THE TARGET FILM FREE-STANDING IN THE

SURROUNDING MEDIA. ......................................................................................................... - 14 -

FIGURE 2-9 DUE TO THE APPLIED POTENTIAL THE SURFACE OF THE METAL-

SUBSTRATE UNDERGOES A CHANGE OF POLARIZATION, BECOMING

POSITIVELY CHARGED, AND THUS REPELLENT OF THE ALSO POSITIVE,

ELECTRODE-NEAR FIRST LAYER OF THE LBL-ASSEMBLED MLTF. ............................. - 15 -

FIGURE 2-10 TRICHLORO (1H,1H,2H,2H -PERFLUOROOCTYL) SILANE, WITH ITS

HYDROPHOBIC FLUORINE-CONTAINING TAIL INDICATED BY A BLUE BAR. ......... - 17 -

FIGURE 2-11 SURFACE-BOUND WATER ENABLES HYDROLYSIS OF THE SILANE

MOLECULES CHANGING THEIR THREE CL GROUPS INTO OH GROUPS. .................. - 17 -

FIGURE 2-12 THE SILANE MOLECULES CONDENSE ONTO THE SILICON-WAFER

SURFACE FORMING THE HYDROPHOBIC SAM. THE REACTION FREES WATER...... - 18 -

FIGURE 2-13 IN-PLANE STABILIZING THROUGH COVALENT BONDING BETWEEN

SAM-FORMING SILANE MOLECULES. THE ABILITY OF THE SILANE MOLECULES

TO COVALENTLY ATTACH TO EACH OTHER IS THOUGHT TO STABILIZE THE

FORMED SAM............................................................................................................................ - 18 -


- 67 -

FIGURE 2-14 A) ILLUSTRATING THE POSSIBLE FORMATION OF LARGER AGGREGATES

FOR THE TRICHLORO-SUBSTITUTED SILANE, WHICH CAN COVALENTLY

ATTACH TO THE SURFACE (UNWANTED), COMPARED TO A MONOCHLORO-

SUBSTITUTED SILANE DEPICTED IN B) BEARING TWO PROTECTIVE-GROUPS,

THUS ENABLING IT TO EITHER PRODUCE A DIMER, OR ATTACH TO THE

SURFACE. (A PRODUCED DIMER CAN NOT COVALENTLY ATTACH TO THE

WAFER SURFACE BY THE SAME CHEMISTRY) .................................................................. - 19 -

FIGURE 2-15 SHOWS (LEFT) THE SCHEMATICS OF THE MAMA-INSTRUMENT. THE

PDMS HALF SPHERE IS MOUNTED ON THE MOTORIZED SAMPLE HOLDER AND

IS BROUGHT INTO CONTACT WITH THE SURFACE OF THE SAMPLE (BETWEEN

BALANCE AND MICROSCOPE). TO THE RIGHT A SUCCESSFUL LIFT-OFF IS

PICTURED, WHERE A CONTROLLED AMOUNT OF MLTF HAS BEEN

TRANSFERRED TO THE PDMS-PROBE. A WHITE INDENT IN THE SAMPLE

ILLUSTRATES THE CORRESPONDING AREA OF THE MLTF THAT HAS BEEN

LIFTED OFF............................................................................................................................... - 21 -

FIGURE 2-16 SHOWING A SEQUENCE OF A MAMA-EXPERIMENT. THE VERTICAL

ARROWS (GREY) INDICATE APPLIED AND WITHDRAWING LOAD. MULTIPLE

HORIZONTAL ARROWS INDICATE A STEPWISE INCREMENT OR DECREMENT OF

THE APPLIED LOAD................................................................................................................ - 21 -

FIGURE 2-17 SHOWS A LOAD (G) VS. MEASUREMENT POINT PLOT FOR A MAMA-

EXPERIMENT. FOUR ZONES ARE INDICATED BY RED ARROWS. A-LOADING, B-

MAXIMUM LOAD, C-UNLOADING (NEGATIVE LOAD => PULLING), D-MAXIMUM

PULLING LOAD OR PULL-OFF LOAD. ................................................................................. - 22 -

FIGURE 3-1 THE FIGURE ILLUSTRATES THE DIPPING CYCLE USED WITH THE

DIPPING ROBOT (CYCLE STARTS AT SUBSTRATE). THE SUBSTRATE IS MOUNTED

IN A SAMPLE HOLDER AND IS THEN ALTERNATELY DIPPED IN POSITIVELY

AND NEGATIVELY CHARGED POLYELECTROLYTE-SOLUTIONS. .............................. - 30 -

FIGURE 4-1 THE FIGURE GIVES AN ESTIMATION OF THE DIAMETER OF THE PEI

MOLECULES. THREE OVER-LAYERED CURVES ARE SHOWN INDICATING AN

APPROXIMATE DIAMETER OF ~5

NM. AS THE FIGURE INDICATES, LARGER AGGREGATES DO EXIST, BUT ARE

RELATIVELY FEW WITH RESPECT TO THE TOTAL VOLUME OF LIGHT-

SCATTERING SUBSTANCE. .................................................................................................... - 33 -


- 68 -

FIGURE 4-2 THE FIGURE INDICATES A VALUE FOR MFC THAT WOULD NORMALLY

CORRESPOND TO PARTICLE/AGGREGATE DIAMETER (~10 NM). IN THIS CASE

HOWEVER, BECAUSE THE FIBRILLS ARE ASSUMED TO HAVE A SOMEWHAT

CYLINDRICAL GEOMETRY, THIS VALUE IS THOUGHT TO CORRESPOND TO THE

DIAMETER OF THE CYLINDER (AS IS DISCUSSED FURTHER IN THE

CONTINUATION OF THIS SECTION). .................................................................................. - 34 -

FIGURE 4-3 THE DISTRIBUTION OF THE DYNAMIC-LIGHT-SCATTERING

MEASUREMENT DATA OBTAINED FOR PEI REGARDING ZETA-POTENTIAL.

THREE OVER-LAYERED PEAKS AT ~25MV. ....................................................................... - 35 -

FIGURE 4-4 THREE OVER-LAYERED CURVES SHOWING THE DISTRIBUTION OF THE

ZETA-POTENTIAL OF MFC, DERIVED FROM THE DYNAMIC LIGHT SCATTERING

MEASUREMENTS, (~-115 MV). ................................................................................................ - 35 -

FIGURE 4-5 TO THE LEFT THE REFERENCE SILICON SUBSTRATE AND TO THE RIGHT

THE FLUORINATED SUBSTRATE IS SHOWN. THE MEASUREMENTS OF LEFT AND

RIGHT CONTACT ANGLES (IN RED) PREFORMED WITH THE KSV CAM 200

INSTRUMENT RESULTED IN CONTACT-ANGLES OF ~15° AND ~110.5° FOR THE

REFERENCE AND FLUORINATED SURFACES RESPECTIVELY. THIS INDICATES A

CHANGE OF SURFACE BEHAVIOUR FROM BEING LARGELY HYDROPHILIC TO

HYDROPHOBIC, AS IS EXPECTED FOR THE FLUORO-SILANISATION. ....................... - 37 -

FIGURE 4-6 MOUNTING OF A (PEI|MFC)22 -MLTF ON BLACK COPY-PAPER. THE

RELEASED MLTF IS SUPPORTED BY A TAPE-WINDOW (SHOWN WITHIN RED

CIRCLE). THE MLTF SUPPORTED INSIDE THE WINDOW IS YELLOW. THE

DARKENING OF THE PIECE OF MLTF THAT IS FURTHEST AWAY IN THIS

PICTURE IS DUE THE MLTF HAVING LOST ITS SUPPORT FROM THE TAPE,

RESULTING IN IT PARTIALLY CURLING OUT OF SIGHT (DOWNWARD IN FIGURE)- 38 -

FIGURE 4-7 A (PEI|MFC)22 MLTF-SEGMENT RELEASED BY THE USE OF THE END PART

OF A PIECE OF TAPE. THIS ANGLE OF OBSERVATION SHOWS A VIOLET

COLOURING OF THE MLTF. .................................................................................................. - 38 -

FIGURE 4-8 PICTURE OF (PEI|MFC)22 THAT IS FREE STANDING, PHOTOGRAPHED

ROUGHLY PERPENDICULAR TO THE SURFACE TO THE PLANE OF THE MLTF....... - 39 -


- 69 -

FIGURE 4-9 ILLUSTRATES (PEI|MFC)22 WHILE STILL ATTACHED TO THE

FLUORINATED SILICON-SUBSTRATE. PHOTOGRAPH TAKEN PERPENDICULARLY

TO THE SURFACE..................................................................................................................... - 39 -

FIGURE 4-10 AS CAN BE SEEN, THE MLTF NO LONGER PANS THE ENTIRE WINDOW IN

THE TAPE, (RED ARROWS INDICATE AREAS WHERE THE FILM HAS

DISCONNECTED FROM THE SUPPORTING WINDOW AND CURLED). BLACK

COPY-PAPER WAS USED AS BACKGROUND, ONTO WHICH THE WINDOWED

TAPE WITH THE MLTF WAS BEEN PLACED. TO THE LEFT A RED SCALE-BAR

APPROXIMATELY INDICATES 1CM...................................................................................... - 40 -

FIGURE 4-11 SHOWING PHOTOGRAPH OF A (PDADMAC|PSS)4 – (PEI|MFC)10 MLTF

TRANSFERRED TO COPY PAPER THROUGH OVERNIGHT CONTACT DRYING OF

SUBSTRATE AND COPY PAPER. AS IS PICTURED, ONLY PARTIAL TRANSFER HAS

OCCURRED. TOWARDS THE LEFT, MULTIPLE COLOURS ARE SHOWN DUE TO

SHIFTING THICKNESS OF THE MLTF (COMPARE WITH FIGURE 4-12). THE BLACK

LINE IS INTRODUCED AS SCALE BAR, INDICATING THE WIDTH OF SILICON

WAFER SLIDE, APPROXIMATELY 1CM. ............................................................................... - 42 -

FIGURE 4-12 SHOWING THE SAME (PDADMAC|PSS)4 – (PEI|MFC)10 MLTF AS IN FIGURE

4-11 AS WELL AS THE SUBSTRATE FROM WHICH IT WAS TRANSFERRED. AS

INDICATED BY THE RED CIRCLE DIFFERENT COLOURS ARE APPARENT ON THE

SUBSTRATE. THESE ARE DUE TO DIFFERENCES IN THICKNESS OF THE MLTF.

THE THICKNESS DEPENDENT DIFFERENTIAL COLOURING OBSERVED IN

FIGURE 4-11 IS HOWEVER NOT CAPTURED HERE, DUE TO THE ANGLE OF

OBSERVATION BEING DIFFERENT. THE RED SQUARED AREA ILLUSTRATES THE

PERIMETER OF THE CONTACT AREA BETWEEN THE SUBSTRATE AND THE COPY

PAPER DURING THE CONTACT DRYING. THE APPROXIMATE MEASUREMENTS

OF THE RED SQUARE AND HENCE THE SUBSTRATE ARE 1CM X 7CM. ...................... - 43 -

FIGURE 4-13 ILLUSTRATING THE USE OF CLAMPS AND GLASS SLIDES TO PRESS

TOGETHER A SAMPLE AND A PIECE OF COPY PAPER DURING CONTACT

DRYING. PAPER AND SAMPLE ARE PRESSED TOGETHER WHILE STILL WET

FROM MILLI-Q AND ARE ALLOWED TO DRY OVERNIGHT. THE PROCESS,

ALTHOUGH NOT FOOLPROOF YIELDED DEMONSTRATIVE TRANSFER FROM

FLUORINATED SILICON SURFACE TO COPY PAPER (ILLUSTRATED HERE) AND

FILTER PAPER AS WELL AS DIALYSIS MEMBRANES (NOT SHOWN)............................. - 44 -


- 70 -

FIGURE 4-14 THE LEFT PART OF THE TOP STRIP OF COMPOSITE HAS BEEN COATED

WITH A MLTF OF (CARRAGEENAN|GELATIN)10. BELOW AN UNCOATED

REFERENCE-STRIP IS SHOWN............................................................................................... - 45 -

FIGURE 4-15 EXHALED BREATH HAS BEEN DEPOSITED ONTO THE

(CARAGEENAN|GELATINE)10-FILM. THE ABSORBED HUMIDITY, THAT HAS

CAUSED THE FILM TO SWELL, AND THUS CHANGE ITS COLOUR THROUGH

INTERFERENCE, IS SHOWN AS A DARK BAND IN THE MIDDLE OF THE PICTURE,

THAT IS NOT PRESENT IN FIGURE 4-16............................................................................. - 46 -

FIGURE 4-16 THE SAME STRIP AS IN FIGURE 4-15, AFTER THE EVAPORATION OF THE

APPLIED HUMIDITY. ............................................................................................................... - 46 -

FIGURE 4-17 SHOWING THE ~10 CM X 10 CM SPRAY-PAINTED KOFES PIECE BEFORE

MULTILAYERING. (THE GRID SCALE IS 1 CM)................................................................... - 47 -

FIGURE 4-18 SHOWING THE LOAD VERSUS TIME PLOT OF THE 2G EXPERIMENT

YIELDING MLTF-RELEASE. A ONE HOUR DELAY AT MAXIMUM IS FOLLOWED BY

THE UNLOADING PHASE. THIS CONTINUES TO A LOAD OF APPROXIMATELY -24

GRAMS, AT WHICH THE FILM IS RELEASED FROM ITS SUBSTRATE. A-J INDICATE

THE APPROXIMATE EXPERIMENT COORDINATES FOR THE DATA OF TABLE 4-2

AND THE PICTURES IN FIGURE 4-19. .................................................................................. - 49 -

FIGURE 4-19 SHOWING PHOTOGRAPHS TAKEN DURING THE 2G MAXIMUM LOAD

MAMA EXPERIMENT, AT THE MEASUREMENT POINTS A-J. THE FUZZINESS OF

PICTURE A IS DUE TO MILLI-Q STILL PRESENT ON THE SAMPLE. IN B THE MILLI-

Q HAS BEEN FLUXED AWAY BY N2. THE WHITISH CIRCULAR AREA IS THE

CONTACT AREA BETWEEN THE PDMS PROBE AND THE MLTF. OUTSIDE OF THIS

THE YELLOW SURROUNDING PARTS OF THE MLTF CLEARLY SHOWS. D-I SHOWS

THE INFLUENCE OF PULLING, AS THE CONTACT AREA DECREASES. J IS A

PICTURE TAKEN AFTER THE MLTF HAS BEEN PULLED OFF. A CLEARER PICTURE

OF THIS IS PROVIDED BY FIGURE 4-20. (DUE TO THE INSTRUMENT BEING

LOCATED IN A LABORATORY WHERE OTHER INSTRUMENTS AND USERS

SIMULTANEOUSLY WORK, THE BACKGROUND LIGHTING VARIED, AS CAN BE

SEEN BY COMPARING B AND H.) ......................................................................................... - 50 -


- 71 -

FIGURE 4-20 A-B SHOWING DIFFERENTLY SCALED PHOTOGRAPHS OF THE

SUBSTRATE IN THE 2G MAXIMUM LOAD MAMA EXPERIMENT, AFTER

SUCCESSFUL PULL-OFF. THE WHITISH AREA IS THE UNDERLYING SILICON

WAFER. AS CAN BE CLEARLY SEEN, THE PULLED-OFF SEGMENT IS NOT NEARLY

A PERFECT CIRCLE. PICTURE C, WHICH IS REFERRED TO AS C* IN TABLE 4-2 IS A

PHOTOGRAPH OF THE PDMS PROBE WITH MLTF ATTACHED, POST-RELEASE.

NOTEWORTHY ARE THE CRACKS OF THE PDMS PROBE THAT ARE CLEARLY

VISIBLE IN C. ALSO VISIBLE ARE THE PROTRUDING FRINGES OF THE MLTF AT

THE EDGES OF THE PROBE IN C. APPROXIMATE DIAMETER OF THE MLTF IN C

(CORRESPONDING TO DIAMETER IN A-B) IS 404ΜM....................................................... - 50 -

FIGURE 4-21 SHOWING THE 5G MAMA-EXPERIMENT WITH 2H DELAY AT MAXIMUM

LOAD. THIS PLOT HAS FEWER MEASUREMENT POINTS THAN DOES THAT OF

THE 2G EXPERIMENT. THIS IS DUE TO THE SETUP, WHICH SPECIFIES 100 STEPS

PER INCREMENT/DECREMENT OF LOAD EVERY 5 SEC, COMPARED TO 20 STEPS

FOR THE 2G EXPERIMENT. (THE BUMP INTO NEGATIVE TIME AT BEGINNING

OF MAXIMUM LOAD PHASE IS AN ARTEFACT DUE TO CURVE-FITTING IN

MICROSOFT- EXCEL SOFTWARE AND SHOULD BE DISREGARDED) .......................... - 52 -

FIGURE 4-22 SHOWS PHOTOGRAPHS FROM THE 5G MAXIMUM LOAD MAMA

EXPERIMENT. SUCCESSFUL PULL-OFF WAS PREFORMED. A AND B SHOW THE

AREA WHERE THE MLTF HAS BEEN PULLED OFF. IT IS CLEAR THAT THE MLTF

AT THE EDGES IS SOMEWHAT SHREDDED AND THAT THE COHESIVE BREAK

WITHIN THE MLTF WAS NOT PERFECTLY CIRCULAR. THE APPROXIMATE

DIAMETER FOR THE PULLED OFF PIECE OF MLTF WAS 515ΜM. C SHOWS A

PHOTOGRAPH, WHERE THE FOCUS LIES ON THE OUTERMOST PART OF THE

PDMS PROBE WHERE THE MLTF IS SITUATED. RIDGES IN THE MLTF ARE

VISIBLE, PROBABLY DUE TO THE FACT THAT THE PDMS IS ELASTIC AND

DEFORMES/REFORMES WHEN PUT IN AND OUT OF CONTACT WITH THE

SUBSTRATES. NO CRACKS WERE OBSERVED IN THE PROBE. ...................................... - 52 -

FIGURE 4-23 ILLUSTRATING A FLUORINATED SUBSTRATE HAVING UNDERGONE 10

MULTILAYERING CYCLES IN PEI|MFC. THE FORMED GEL IS CLEARLY VISIBLE

ON TOP OF THE SILICON SUBSTRATE. EXCESS WATER HAS BEEN REMOVED

THROUGH CAREFUL TILTING OF THE SUBSTRATE. LEFT AND RIGHT PICTURES

ARE OF THE SAME SUBSTRATE THROUGH DIFFERENT ANGLES OF

OBSERVATION. ........................................................................................................................ - 54 -


- 72 -

FIGURE 4-24 SHOWS A GRADIENT OF COLOUR AT THE EDGE OF THE MLTF. THE

AREA OF THE GRADIENT IS INDICATED BY THE RED ARROW. FROM THIS

PICTURE, ONE CAN ESTIMATE THE BLUE COLOUR OF THE MLTF TO BE OF

SECOND ORDER, SINCE THE GRADIENT SUGGESTS THAT THE MLTF HAS

PREVIOUSLY BEEN COLOURED IN THE ORDER; BLUE LIGHT BLUE

YELLOW ORANGE RED VIOLET BLUE(SECOND ORDER). THE

GRADIENT IS READ FROM TOP TO BOTTOM ................................................................... - 55 -


- 73 -

List of tables:

TABLE 3-1 THE TYPE OF POLYELECTROLYTE ION AND THE PH OF THE USED SOLUTIONS. .............................................................................................................................. - 27 -

TABLE 3-2 SHOWING THE SAMPLES FEATURED IN THIS THESIS. ....................................... - 31 -

TABLE 3-3 SHOWING THE EXPERIMENTAL SETUP THAT WAS PLANNED AND PREFORMED USING THE MICRO ADHESION MEASUREMENT APPARATUS (MAMA). ...................................................................................................................................... - 32 -

TABLE 4-1 THE ZETA-POTENTIAL OF MFC AS WELL AS THAT OF PEI, AS SHOWN IN THE COLUMN HEADED ZP. ALSO SHOWN IS MOBILITY AND CONDUCTIVITY AS WELL AS SAMPLE TEMPERATURE. ...................................................................................... - 34 -

TABLE 4-2 SHOWING DATA OF THE 2G MAXIMUM LOAD MAMA-EXPERIMENT FOR SELECTED MEASUREMENT COORDINATES. PICTURES OF THE A-J ARE ILLUSTRATED IN FIGURE 4-18, AND C* IS ILLUSTRATED BY FIGURE 4-19C............... - 49 -


- 74 -

According to STFI-Packforsk's Confidentiality Policy this report is confidential until 2008-03-03

Development of free-standing interference films for paper and packaging applications STFI-Packforsk report no.

- 75 -

9 STFI-PACKFORSK DATABASE INFORMATION

Title Development of free-standing interference films for paper and packaging applications

Author Johan Holmqvist

Abstract The newfound capability of creating moisture sensitive interference multilayered thin films (MLTFs) comprising microfibrillated cellulose and polymers has not previously been possible to implement on surfaces other than silicon wafer strips. Being able to incorporate interference MLTFs on fibre-based materials would introduce the possibility for new applications within authentication, sensing and customer attraction for the paper and packaging industry. By using trichloro (1H, 1H, 2H, 2H-perfluorooctyl) silane we were able to hydrophobically modify silicon substrates, enabling interference MLTF lift-off and thus the creation of free-standing MLTFs of approximately 400 nm thickness. Contact dried MLTFs approximately 250 nm thick, were successfully transferred to copy- and filter paper as well as to cellulose-based dialysis membranes. We can also report on the successful synthesis of interference MLTFs directly on a fibre composite material and on aluminium. Initial tests of a method to quantify the pull-off conditions of the MLTFs from the fluorinated surfaces using the Micro Adhesion Measurement Apparatus showed promising results.

Keywords Layer-by-layer, interference thin film, free-standing, moisture sensor, polyelectrolyte, surface self-assembly, silanization

Classification

Type of publication Master of Science Thesis

Report number

Publication year 2008

Language English

STFI-Packforsk AB Visit Drottning Kristinas väg 61 | Mail Box 5604, 114 86 Stockholm, Sweden

Tel +46 8676 70 00 | Fax +46 8411 55 18 | www.stfi-packforsk.se | [email protected] VAT no. SE556603110901

STFI-Packforsk is one of the world’s leading R&D companies in the fields of pulp, paper, graphic media, packaging and logistics. The activities range from basic research to direct assignments along three value chains: packaging, graphic media, and bio-based energy and chemicals. The combined competence, from material science to consumer value, is utilised to find solutions applicable at the customers. STFI-Packforsk is located in Stockholm and Örnsköldsvik, and in Trondheim, Norway through its subsidiary PFI AS.

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