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Physical Characterisation of Latex Film Formation and Film Properties
by
Tecla G Weerakkody
Submitted for the degree of Doctor of Philosophy
Department o f Physics Faculty o f Engineering and Physical Sciences
University o f Surrey
August 2009
Abstract
In this work, physical characterisation of film formation and film properties of coating and adhesive formulations was performed. Organic/inorganic nanocomposite coatings draw remarkable academic and industrial interest, due to their expected enhancement of combined properties. Film formation and film properties, such as drying, transparency, and final film quality of coating systems containing Laponite clay were studied, in particular the influence of excess surfactant on the drying process. It was found that excess surfactant in the system does affect the physical characteristics of the film formation process. Experiments using magnetic resonance profiling and photographs found lateral flow of liquid from the edges to the centre. Reducing the amount of surfactant in the system improves film quality by reducing lateral flow during film formation.
In pressure sensitive adhesive (PSA) applications, such as labels or tapes, it is vital that the films remain optically transparent, regardless of the exposure to high humidity or water. Core/shell PSAs were investigated to determine their drying, water whitening, moisture absorption from high humidity, and adhesive properties. The dependence of these properties on pH and poly (acrylic acid), PAA, was studied. It was found that the hydrophilic pathways created by PAA shells do not contribute to faster drying. In addition, this investigation disproves the idea that a “hairy layer” of PAA keeps the film more open and leads to faster drying. It was found that films with hydrophilic boundaries absorbed more water. The water is evenly distributed along the boundaries, so that films still retain their transparency. It was proved that transparency or water whitening is not necessarily a reliable measurement of water uptake, at least for this system.
The findings from this study define guiding principles for organic/inorganic nanocomposite coatings and core/shell PSAs, to have better film formation characteristics and film properties.
Acknowledgements
It has been a great privilege to be a postgraduate student at the Department of Physics, where I have spent memorable few years and met so many wonderful people. I would like to thank many of whom encouraged me and were involved in the preparation of this thesis.
First and foremost I would like to express my sincere gratitude to my supervisor, Professor Joseph Keddie for his support, guidance and understanding. I am grateful to Joe for introducing me to the materials science and polymer physics, and that I was given this opportunity to work in this project. His passionate interest in science with enthusiasm and hard working always influenced me. The constant encouragement, even from the very first email I received from Joe, genuinely motivated me. I am profoundly thankful for his valuable discussions, stimulating advice and patience throughout my PhD.
I would also like to thank my second supervisor, Professor Peter McDonald, for his energetic advice, dedication and invaluable assistance (particularly when we needed practical help with the GARField magnet) throughout this research. At many stages, this work has benefltted from his expertise, which would otherwise have been impossible to obtain. My special thanks go for Dr Jamie Cleaver for his strong support, insightful discussions and especially, for the water sorption experiments which are presented in Chapter Five.
I would like to extend my appreciation to my NAPOLEON collaborators throughout this research. Prof. Jose M Asua, Prof. Maria J Barandiaran, Dr Maria Paulis (POLYMAT The University of the Basque Counfry, Spain) for their insightful discussions and comments on my work and especially the immense support and guidance I received during Emulsion Polymerisation Course also during my experiments in their laboratories. I am grateful to Ms Aitziber Lopez, Ms Mihaela Manea and the rest of the POLYMAT group for their support in the university, as well as out in the city, to make that short visit so memorable. I would like to extend my appreciation to Dr Elodie Bourgeat-Lami (CNRS-LCPP, France) for her fascinating insight into the work which is presented in Chapter Four and Dr Veronique Mellon (from the same laboratory) for preparing the samples and helpful discussions. My special thanks go to Prof. Diethelm Johannsmann (Clausthal University of Technology, Germany) and his team, especially Mr Alexander IConig for developing the software programme to process the NMR
data and training us, at University of Surrey, to use the programme, and for their invaluable, energetic discussions and thoughtful ideas. My special thanks also go to Dr Alex Routh (Cambridge University, UK) and his team for ongoing insightful discussions, comments and guidance which I received throughout my PhD. I would like to express my gratitude to Prof. Yves Holl (CNRS-ICS, France) and Dr Celine Arnold for their professional comments, suggestions and discussions and preparing some of the dialysed latex sample for this work.
In addition, I would like to express profound gratitude to all the partners of Designed Nanoscale Heterogeneities for Controlling Water-Borne Pressure-Sensitive Adhesive Performance (NsHAPe) Project, especially Prof. Peter Lovell (University of Manchester, UK) for his invaluable discussions, stimulating advice and thoughtful ideas during the six months of my work to the project and providing the samples for the work presented in Chapter Five. Also, many thanks for Dr Chunhong Lei for water whitening measurements presented in Chapter Five.
Within the Department of Physics, special thanks go to all the technical staff present and past, especially, Mrs Violeta Doulcova, Mr John William-Brown, Mr David Munro and Mr Jeff Dahllce, Dr David Hemsley, Mr Tom Gibbons, Mr Bob Derham, Mrs Liz Griffiths, Mi' Keith Proctor. All of whom, at all times, provided essential support for equipment and facilities used throughout my studies. My special thanks also go to Ms Shirley Hankers, Mrs Cristobel Soares- Smith, Mrs Noelle Hartley, Dr Annika Lohstroh and Miss Clare Harvey for spending their valuable time on proof reading my thesis. Thanks also go to present and past members of Soft Condensed Matter (SCM) Group, especially Dr Piyasiri Elcanayake, Dr Peter Doughty and Mr Simon Pitts for introducing me to NMR and immense practical help during experiments. Dr Tao Wang, Dr Carolina de Las Heras Alarcon, Ms Dan Liu, Dr Victor Rodin, Dr Eisabetta Canetta, Dr Ibraheem Bushnak, Dr Fiona Frehill for their help, advice, feedback, good company and invaluable friendship during my PhD experience. I also like to thank my office mates Piyapong Asanithi, Patnarin Woijittiphon, Akarin Intaniwet, Alice King, Eric Brunner for all the help, advice and support. Special thanks goes for Dr Izabela Jurewicz for being there for me from day one, Ms Ann Henderson for all her help and advice and all the other friends on and off the university, for all their support.
I also acknowledge the financial support for my studies from EPSRC and European Commission under NAPOLEON and NsHAPe Projects. Thanks for UK Polymer Colloid Forum for best poster award for 2007 and 2008.
I am profoundly thankful to my parents for their unconditional love and support, without whom this work would not have been possible. Special thanks also go to my brother and his family, my mother-in-law, late father-in-law and all the in-laws. Most importantly, to my husband, whose encouragement, understanding, patience and emotional support throughout the duration of my PhD, especially the last very stressful months has my undying gratitude and love.
And finally, I would like to dedicate this thesis to my lovely children Thilini and Akila for their love and the happiness they bring to my life. I love you both very much!
v
Glossary
AA acrylic acid
ACPA 4’-azobis (4-cyaiiopentanoic acid)
AFM atomic force microscopy
ABBA 2 ,2 -azobis (2 -methylpropionamidine) hydrochloride
BuA butyl acrylate
BylA behenyl acrylate
CCC critical coagulation concentration
CEC cation exchange capacity
CMC critical micelle concentration
CTAB cetyltrimethylammonimn bromide
DDAB didodecyldimethylammonium bromide
EA ethyl acrylate
FCC face-centred cubic
GARField gradient at right-angles to the field
MFFT minimum film-forming temperature
MMA methyl methacrylate
MMT montmorillonite
MRI magnetic resonance imaging
MRP magnetic resonance spectroscopy
NMR nuclear magnetic resonance
OHP Helmholtz plane
OTAB octadecyltrimethylammonium bromide
PAA poly acrylic acid
PI DA poly isodecyl acrylate
PSA pressure-sensitive adhesive
RF radio frequency
SEM scanning electron microscopy
TEM transmission electron microscopy
VOCs volatile organic compounds
y-MPTMS y~ methacryloxypropyl-trimethoxysilane
Table of nomenclature
A surface area of a sphere (nm2)
Bo external magnetic field (T)
Bi field due to RF coil (T)
D distance between two particles (nm)
Do diffusion coefficient (m2/s)
e electronic charge (C)
E evaporation rate (m/s)
Ea energy dissipated during the debonding process (J/pm2)
F force (N)
G t shear modulus (Pa)
G y magnetic field gradient (T/m)
H film thickness (pm)
h Plank’s constant (Js)
I spin quantum number
I n value of the distribution at Xn
k Boltzmann constant (J/K)
m0 integral, zeroth moment
M0 initial mass (mg)
M o o mass at the equilibrium or plateau value (mg)
MFFT minimum film formation temperature (K)
Mp mass of the pan (mg)
Mt total mass at a given time t (mg)
n number of echos
N number of the points of a distribution
no ionic concentration (mol/L)
NS number of scans
P c a p reduced capillary pressure (Pa)
Pe Peclet number
R radius (nm)
RD repetition delay (s)
SF spectrometer frequency (MHz)
SI number of points
T absolute temperature (K)
Ti spin-lattice relaxation time (s)
t 2 spin-spin relaxation time (s)
Tg glass transition temperature (°C)
V volume of a sphere (nm3)
w interaction energy (J)
WA the sum of the attractive energies (J)
W r the sum of the repulsive energies (J)
W t total interaction energy (J)
Xn the value of the independent variable
z valency of the ions in solution
AX pixel spacing (pm)
Az field of view (pm)
AQ bandwidth of the pulse (Hz)
ft zero shear rate viscosity of the polymer (Pa s)
¥ Potential (mV)
CO Lannor resonant frequency (MHz)
6 stress (MPa)
h reduced Plank’s constant (Js)
CD volume fraction
1C1 Debye screening length (nm)
8 dielectric constant or permittivity of the continuous phase (F/m)
eo dielectric constant or permittivity of vacuum (F/m)
Y charge on the particle (C)
X I'def/ T'dry
[X viscosity (Pa.s)
poo ionic concentration of ions in the bulk (mol/L)
T pulse gap (ps)
Tdef characteristic time for particle deformation (s)
T'dry characteristic time for drying (s)
? Zeta potential (mV)
Contents
List of Figures .................................................................................................................... xiii
List of Tables .................................................................................................................. xxvi
Chapter 1 - Overview...................................................... 1
1.1. References. .................................................................................................. 5
Chapter 2 - Introduction.................................................... 6
2.1. Introduction to colloidal dispersions............................................................ 6
2.2. Classification of colloidal dispersions................................................ 9
2.3. Synthesis of latex........................................................................................... 11
2.3.1. Emulsion polymerisation............................................................. 1 1
2.3.2. Miniemulsion polymerisation...........................................................
2.4. Charges in the colloidal dispersions lg .
2.4.1. Helmholtz m odel........................................................................... 18
2.4.2. Gouy - Chapman m odel.............................................................. 19
2.4.3. Stern m o d e l.................................................................. 20
2-5. Stability in the colloidal dispersions........................................................... 23
2.5.1. The DLVO Theory...................................................................... 23
2.5.2. The factors affecting the stability in the colloidal dispersions. . 26
2.5.2.1. Particle s ize ................................................................... 27
2.5.2.2. Zeta - potential............................................................... 27
2.6. Fihn formation of colloidal dispersions.................................................. 29
2.6.1. Stages of film formation................................................................ 29
2 .6 .1.1. Stage I - Evaporation of water and particle ordering 30
2.6.1.2. Stage II - Particle deformation.................................... 31Stage III - Coalescence/Interdiffusion
2.6 .1.3. 32across particle - particle boundaries.........................
2.6.2. Particle deformation and coalescence.............................. 33
2.6.2.1. Wet sintering................................................................. 33
ix
2.62.2. Dry sintering............................................... 33
2.6.2.3. Capillary deformation................................ 35
2.6.2.4. Receding water front.................................. 35
2.6 .2.5. Sheetz deformation..................................... 36
2.6.3. Recent models for drying dispersions......................................... 36
2.6.3.1. Peclet number (P e).................................... 36
2.6 .3.2. The Routh and Russel model for normal drying . . . . 38
2.6.3.3. The Routh and Russel model for lateral drying . . . . . 40
2.7. References....................................................................................................... 42
Chapter 3 - Magnetic Resonance Profiling......................................... 46
3.1. Historical Background of MRP..................................................................... 46
3.2. The Principles of NMR.................................................................................. 48
3.3. MR Profiling using GARField magnet........................................................ 52
3.4. a Typical GARField Profile......................................................................... 5 7
3.5. References.................................................................................................... 61
Chapter 4 - Physical Characterisation of Film Formation and FilmProperties of Organic/Inorganic Nanocomposite Coatings................... 64
4.1. Introduction....................................................................................................... 64
4.1.1. Surface tension, surfactant concentration and CMC.................... 85
4.1.2. Marangoni Flow............................................................................. 71
4.1.3. Overview of Nanocomposite Materials....................................... 724.2. Materials......................................................................................................... 77
4.2.1. Latexes by Route 1......................................................................... 77
4.2.2. Latexes by Route I I ...................................................... 78
4.3. Techniques..................................................................................................... 80
4.3.1. Magnetic Resonance Profiling (MRP)........................................... gO
4.3.2. Optical Transmission Measurements.............................. 904.3.3. Profilometry..................................................................................... 814.3.4. Methods for obtaining photos of the films..................................... 81
4.4. Results and Discussion........................................................................... 83
x
4.4.1. Original Route I and Route I I ....................... .............................. 83
4.4.1.1. MR Profiling and Images from sid e ......................... 83
4.4.1.2, Optical Transmission................................................... 90
4.4.2. Effect of the DDAB content on Route II latexes.......................... 9 4
2 Comparison of drying properties of ‘original’ and newpolymer/Laponite latexes............................................................... 98
MRP of ‘original’ and new polymer/Laponitelatexes........................................................................ 98
4 4 2 2 Visual appearance of the ‘original’ and newpolymer/Laponite latexes........................................... 103
4 4 3 3 Optical transmission of ‘original’ and newpolymer/Laponite latexes...................................... 103
, „ „ , Thickness variations of the films of ‘original’ and4.4.3.4. new formulation polymer/Laponite latexes............. 105
4 • 5. Conclusions..................................................................................................... 110
4.6. References....................................................................................................... 112
Chapter 5 - The Effects of Acrylic Acid and pH on the PhysicalCharacteristics of Pressure Sensitive Adhesives....................................
5.1. Introduction........................ 117
5.1.1. Water uptake.................................................... 120
5.1.2. pH and PAA dependence................................................ 125
5.1.3. Properties.................................................................................... 128
5.2. Aims of present research w ork .................................................................... 132
5.3. M aterials......................................................................................................... 133
5.4. Methods............................................................................................................ 135
5.4.1. Magnetic Resonance Profiling (M RP)........................................ 1355 4 2 Measurements of Water Whitening by Optical ^
Transmission.................................................................................5.4.3. Probe-Tack Adhesion Measurements.......................................... 136
5.4.4. Moisture Soiption Measurements.................................................. 142
5.5. Results and Discussion ................................................................... 146
5.5.1. Drying measurements..................................................................... 146
5.5.1.1. Effect of PAA shell at a lower pH (pH of 3 ) ........... 147
5 5 12 Effect of PAA shell at a higher pH adjusted with NaOH (pH of 8) ...............................................
xi
5.5.1.3. Effect of pH on latex with PAA shell........................ 149
5.5.2. Water whitening measurements.................................................... 150
5.5.3. Moisture Sorption Measurements................................ 154
5.5.3.1. Effect o f PAA at a lower pH (pH of 3 ) .................... 154
Effect of PAA at a higher pH adjusted with5 5 3 2
NaOH (pH of 8) ......................................................... 155
5.5.3.3. Effect o f pH on latex with PAA shell........................ 155
Correlation of Drying, Water Whitening and Moisture5.5.4.
Sorption Measurements................................................................ 158
5.5.4.1. Correlation of Drying and Water Whitening............
5.5.4.2. Correlation of Drying and Moisture Sorption ^
Correlation of Moisture Sorption and Water5.5.4.3.
Whitening.................................................................... 163
5.5.5. Probe-Tack Adhesion Measurements........................................... - , ~
5.6. Conclusions,
5.7. References. ,
Chapter 6 - Conclusions and Future W ork............................. 175
6.1. Conclusions.................................................. .................................................. 1 7 5
^ ̂ j Conclusions about Organic/Inorganic NanocompositeCoating Formulations............................................................. 175
f\ 19 Conclusions about Pressure Sensitive Adhesive Formulations . 1776.2. Future Work.................................................................................. I 7 9
6 2 1 Future work for Organic/Inorganic NanocompositeCoating Formulations............................................................. 179
f\ 0 0 Future work for Pressure Sensitive Adhesive Formulations. . . 1796.3. References •, Qn
Figure 2.1.
Figure 2.2.
Figure 2.3.
Figure 2.4.
Figure 2.5.
Figure 2.6.
Figure 2,7.
List of Figures
Schematic diagram illustrating the three intervals of an emulsion
polymerisation process [11]. Interval I: Particle formation stage,
represented by the increase in both the number of particles and the
polymerisation rate. Interval II: Particle growth: during which, both the
number of particles and the polymerisation rate remains relatively
constant. Interval III: The number of particles remains unchanged, while
polymerisation rate and the monomer concentration decrease[ll],. . . 13
Polymerisation rate as a function of time for three intervals of emulsion
polymerisation process [8]. During interval I, the polymerisation rate
increases, during Interval II it stays relatively unchanged and finally, it
decreases during interval III.....................................................................14
Principle of miniemulsion polymerisation. In the first step of
miniemulsion polymerisation, submicron size monomer droplets are
produced by shearing and in the second step, these droplets act as
individual batch reactors that undergo the polymerisation reaction [5,
14, 15].......................................................................................................... 16
Schematic of the Helmholtz model, which failed to explain the
experimental results [4]............................................................................ 19
Schematic of the Gouy - Chapman model [4]..........................................20
Schematic of the Stem model which consist of Stem layer, where
potential drops linearly and the diffuse layer, where the potential drops
exponentially with the distance [4]......................................................... .21
Schematic of the energy versus the distance between particles profiles of
DLVO interactions. This figure is taken from Jacob Israelachvili,
Intermolecular and Surface Forces 1992: Academic Press Limited,
London; page 248 [23]. Highly charged surfaces repel strongly and
remain stable which illustrated in lower inset ‘a’. ‘b ’ where surfaces
come into stable equilibrium at secondary minimum and colloidal
particles remain kinetically stable, ‘c’ when surfaces come into
secondary minimum and particles coagulate slowly, ‘d’ Surfaces may
remain in secondary minimum or adhere and colloids coagulate rapidly,
at the ‘critical coagulation concentration’, ‘e’ Surfaces and colloidal
particles coalesce rapidly [23]................................................................... 25
Figure 2.8. The effect of £ potential on the shape of the total interaction potential
curve for polystyrene latex. From top to bottom the £ potentials are -80
mV, -50 mV, -25 mV and -20 mV [4].................................................... 27
Figure 2.9. Illustration of the stages of film formation process [6]. In stage one,
water evaporates from the polymer film and particles re-order to a close
packed array. In stage two, particles deform, when the drying
temperature is greater than the MFFT. In the final stage, particles
coalesce and molecules diffuse, when the drying temperature is higher
than the Tg of the latex, and a mechanically coherent homogenous film is
formed..........................................................................................................29
Figure 2.10. Distance between different size particles as a function of volume
fraction of polym er.................................................................................... 31
Figure 2.11. Schematic illustrations of particle deformation theories for film
formation. (A) Wet sintering - when particles are deformed before water
has evaporated and the reduction of the polymer water interfacial energy
is the driving force. (B) Dry sintering - when water recedes before
particles are deformed and the reduction of the polymer-air interfacial
energy is the driving force. (C) Capillary deformation - when the air-
water interfacial energy is the driving force for particle deformation. (X)
concave meniscus of air-water interface. (D) Sheetz deformation -
“skin” formation by wet sintering and capillary forces due to rapid
evaporation, before the dispersion reaches close packing below . . . . 34
Figure 2.12. Schematic illustrations of P e « l and P e » l . Initially stable polymer
colloid dispersion (on left) could show vertical water uniformity (right
top) if diffusion is stronger than evaporation (A) during drying and
P e « l . Alternatively, if evaporation is stronger than diffusion (B)
during drying the result is non-uniform water distribution (right bottom)
and P e » l .................................................................................................... 37
xiv
Figure 2.13.
Figure 2.14
Figure 3.1.
Figure 3.2.
Figure 3.3.
Figure 3.4.
Figure 3.5.
Figure 3.6.
Schematic diagram of the drying regimes according to the Routh and
Russel model for normal drying and the influencing factors [37]..........40
Schematic diagram shows a lateral drying in a colloidal film with a
central wet region and dry edges. Between these regions, water fills the
void space between packed particles. The inner boundary is the particle
packing front, and the outer boundary is the drying front [39]..............41
Schematic illustration of randomly oriented magnetic dipole moments
where no external magnetic field (A), and when external magnetic field,
Bo, is applied, the orientations align with the field (B)...........................49
Schematic illustration of the splitting of the Zeeman energy levels due to
the application of a static magnetic field Bo. The lower energy level has
m = + Y z where magnetic moment is parallel to the external magnetic
field, Bo and higher energy level has m = - Y z where magnetic moment is
anti-parallel to B0 [25]................................................................................50
Cross section through the centre of the magnet which shows the shape of
the pole pieces and the sample location in relation to the magnets and
the pole pieces. The RF coil is located directly below the sample
location. Image taken from [27]...............................................................53
Schematic diagram of the GARField magnet. The magnetic field
gradient, Gy, is perpendicular to the direction of the magnetic field, Bo
[1 1 ]. Bj is the magnetic field generated by the planar radio frequency
(RF) coil. The profiles o f the sample are a measure of the intensity of
the magnetisation signal as a function of the height of the sample, in the
direction of the gradient which is shown on the right hand side 54
Typical GARField profiles of a model acrylic latex (coating), where
each profile was taken in 5 minute intervals. The arrow shows the
increase of the drying time. On right is the film-air surface (top of the
film) and on left is the film-substrate surface (bottom of the film). . . . 57
Typical GARField profiling of a latex with low Tg (adhesive) using 3D
waterfall type of plotting............................................................................58
xv
Figure 4.1.
Figure 4.2.
Figure 4.3.
Figure 4.4.
Figure 4.5.
Figure 4.6.
Figure 4.7.
(a) The structure of a single tetrahedral shaped unit of a central four-
coordinated silicon atom surrounded by four oxygen atoms and (b) the
resulting tetrahedral sheet, (c) is the structure of an octahedral unit with
a central six-coordinated magnesium atom surrounded by six oxygen
groups and (d) the resulting octahedral sheet. Image taken from [14]. 6 6
A schematic illustration of the ‘T-O-T’ stacking pattern, where one
octahedral sheet is sandwiched between two tetrahedral sheets (on the
left). The same stacking pattern can be found in Laponite structure. On
the right is the crystalline structure of a Laponite clay disk. Image taken
from [7, 9]....................................................................................................67
Acido-basic equilibrium equations: The top equation is for acidic media,
where positive species are predominant and the equilibrium shifts to the
right, and as a result, clay particles are positively charged. In the basic
media, negatively charged ions are predominant and according to the
second equation, the resulting clay particles will be negatively charged.
68
TEM image of polystyrene latex particles containing Laponite clay.
Image taken from [22]................................................................................69
TEM images of polystyrene saponite composite films; on left with
unmodified saponite and on right with saponite modified with OTAB.
Image taken from [23]................................................................................70
Schematic illustration of surface tension as a function of the logarithm
of the surfactant concentration. Surface tension of pure water or at 0 log
concentration is 72 mN/m and the inflection point where surface tension
no longer decreases with the increase of surfactant concentration is the
CMC of a given surfactant........................................................................ 71
Schematic illustration of Route I and II. In Route I (on the left),
Laponite plates were functionalised with the cationic initiator, AIBA
and dispersed into the water phase. In Route II (on the right) double
functionalised Laponite by MPTMS and DDAB, was dispersed in the
monomer phase. After the miniemulsion polymerisation in Route I it is
xvi
Figure 4.8.
Figure 4.9.
Figure 4.10.
Figure 4.11.
Figure 4.12.
Figure 4.13.
Figure 4.14.
expected that the clay will be at the surface of the latex particles,
whereas in Route II, the clay will be encapsulated within the latex
particles. Image taken from [41]...............................................................74
(a) the chemical structure of y-MPTMS molecule, (b) the chemical
structure of the DDAB molecule and (c) a schematic diagram of a
double functionalised Laponite disc with y-MPTMS molecules grafted
on its edges and DDAB molecules on its surface.................................... 75
Cryo-TEM images of polystyrene/Laponite nanocomposite particles by
Route I (a) and Route II (b). Images were taken from [43]. (c) and (d)
are the schematic representations of the polymer particles by Route I
and Route II, respectively...........................................................................76
The Easy Drop Standard setup, which was used to take the photos from
the side of a drying film. The image was taken and modified from [47] .
82
MR profiles of (a) 0 wt.% Laponite latex, synthesised by Route I and (b)
3 wt.% Laponite latex, synthesised by Route I over time. The profiles
were obtained every five minutes. In both cases the thickness decreases
at a constant rate, and non-uniformities in water concentration in the
vertical direction do not develop until the later stages of drying 83
Series of photographs taken from the side of a drying film of 0 wt.%
Laponite - Route II latex. The drying times are indicated on the figure.
................................................................................................................... 84
The GARField profiles of the latex with 3 wt.% Laponite by Route II.
The Film thickness initially decreases with no non-uniformities or
gradient in water concentration in the vertical direction. At a later stage,
the film thickness increases over time and simultaneously develops
gradient in water concentration................................................................. 85
Series of photographs taken from a side view of the drying film of 3
wt.% Laponite - Route II latex on a 2 cm x 2 cm glass substrate. The
drying times are indicated on the figure. Only the left-half of the film is
sh o w n ......................................................................................................... 86
Figure 4.15.
Figure 4.16.
Figure 4.17.
Figure 4.18.
Figure 4.19.
Figure 4.20.
Figure 4.21.
A series of MR profiles obtained at five minute intervals, from 3 wt.%
Laponite sample by Route II, showing the four stages of drying; Stage 1
- film thickness constantly decreases, no water concentration gradient,
Stage 2 - film thickness decrease slows down to a constant thickness,
Stage 3 - film thickness increases in the centre and water concentration
gradient develops, Stage 4 - film thickness decreases in the central
region. Arrows show the direction of increasing drying time................ 87
Comparison of normalised film thickness as a function of normalised
drying time for latexes with various Laponite concentrations by Route
II. The thickening effect is stronger with 3 wt.% Laponite, but it is still
noticeable with 5 wt.% and 7 wt.% Laponite concentrations................ 88
Comparison of (a) GARField profiles for the as-received latex of 3 wt.%
Laponite by Route II to profiles for (b) the same latex when its solids
content was raised to 30 wt.%, by evaporating water............................. 89
The normalized thickness as a function of the normalized drying time of
the latex with 3 wt.% Laponite by Route II. Results are shown for as-
received latex (black), latex with a higher solids content of 30 wt.%
(red), as received latex with added Servoxyl (blue), and for latex with a
30 wt.% solids content plus added Servoxyl (green).............................. 90
Optical transmissions (at a wavelength of 600 mn) as a function of
Laponite content for the films by Route I (red) and Route II (blue). . .92
Optical transmission (at a wavelength of 600 nm) as a function of
Laponite concentration for Route II latex as-received (red) and after
raising the solids to 30 wt.% and adding Servoxyl wetting agent (blue).
....................................................................................................................93
Schematic illustration of double functionalisation of a Laponite disc by
200% CEC of DDAB and MPTMS molecules, (a) A formation of a
double layer of DDAB molecules on the surface of Laponite disc, (b)
migration of excess DDAB molecules to the monomer droplet/water
interface and (c) migrated DDAB molecules diffuse to the water phase
xviii
and form surfactant aggregates when water is evaporated, 95
Figure 4.22.
Figure 4.23.
Figure 4.24.
Figure 4.25.
Figure 4.26.
Explanation of drying behaviour of the latexes by Route II. (a) Initial
polymer/Laponite particle, where excess DDAB molecules stick out of
the particle, (b) Due to water evaporation polymer particles are closely
packed near the edges of the drying film, (c) Desoiption of excess
DDAB molecules to the surrounding serum, (d) Development of a
DDAB concentration gradient, which causes a surface tension gradient
between the edges and the centre of the film, (e) Marangoni flow of
liquid from low to high surface tension regions. This process could
continue as long as the drying continues................................................ 9 7
Drying profiles of: (a) new formulation with less DDAB and low solids
content (19.2 wt.%), the film thickness decreases steadily over time and
the water distribution was non-uniform towards the end of drying, (b)
New formulation with less DDAB and high solids content (29.9 wt.%),
the film thickness decreases steadily over time and the water distribution
is uniform in the depth of the film...........................................................1 0 0
Normalised film thickness as a function of Normalised Drying Time for
original Route II (blue), new formulation with less DDAB and low
solids (red) and less DDAB with high solids (black). All three samples
contained around 3 wt.% Laponite.........................................................101
(a) Normalised zeroth moment and (b) Skewness of original Route II
nanocomposite (blue triangles), new formulation low solids (red circles)
and high solids (black squares). All three samples contained around 3
wt.% Laponite...........................................................................................102
Comparison of the photographs of the films made by (a) original Route
II, (b) new formulation with a low solids and (c) new formulation with a
high solids dispersions. The original Route II sample illustrates the most
irregularities on the film/air interface compared with the other two
xix
samples 103
Figure 4.27.
Figure 4.28.
Figure 4.29.
Figure 4.30.
Figure 4.31.
Figure 5.1.
Figure 5.2.
Comparison of optical transmission of the films obtained from original
Route II (red), new formulation with low solids (green) and new
formulation with high solids (blue) nanocomposites at a wavelength of
600 n m ..................................................................................................... 104
Thickness variations (film thickness as a function of normalised
distance, i.e. distance divided by total lateral distance) of the films by (a)
original Route II, (b) new formulation with low solids and (c) new
formulation with high solids using profilometry. It is significant that the
original Route II sample (a) has a ‘Mexican Hat’ shape, in comparison
to the other two film s..............................................................................106
Schematic illustration of a (a) concave wet film, (b) low surface tension
at the centre and high surface tension at the edges of the drying film, (c)
Marangoni flow from centre to the edges, (d) dry film with relatively
thin centre................................................................................................. 107
Illustration of the Perspex mould with the 20 mm diameter and 0.2 mm
deep cylindrical reservoir to obtain a concave wet film ..................... 107
Surface variations (vertical distance as a function of normalised, i.e.
distance divided by total lateral distance) of the dry films obtained by
initially concave wet films of (a) original Route II, (b) new formulation
with low solids and (c) new formulation with high solids using
profilometry............................................................................................. 108
Schematic illustration of (a) optically transparent film and (b) loss of
transparency due to larger pockets of water between particles 118
Schematic illustration of ‘skin formation’ or particle coalescence near
the air interface during drying of a latex film...................................... 119
xx
Figure 5,3.
Figure 5.4.
Figure 5.5.
Figure 5.6.
Figure 5.7.
Figure 5.8.
Figure 5.9.
SEM images of freeze-fractured surfaces of blend films of (a) 70% and
(b) 50% soft particles. Images taken from Reference [16]....................120
Three different types of absorption curves of relative mass uptake as a
function of square root if time; (a) typical sorption curve, (b) the ‘Two-
stage’ sorption curve and (c) the ‘Sigmoidal’ sorption curve, where the
curve is ‘S’ shaped. Drawn after [17].................................................... 122
Visible transmission spectra for films: (1) a newly formed film, (2) a
recently formed film exposed to 100 % RH for two days, (3) a film
heated in low humidity at 60 °C for two hours, (4) the well-heated film
exposed once again to 100 % RH for two days. Figure taken from [19].
.............................................................................................................124
Schematic representation of polymer particle with ‘hairy layer’, (a) At
high pH the charge repulsion by PAA chain increases and the chains are
widely spread; (b) as pH decreases, the hairy layer collapses Drawn after
[26] 126
Comparison of the water uptake of films by latex dispersions of
core/shell particles (poly styrene and poly butyl acrylate hydrophobic
core surrounded by a thin layer of hydrophilic poly acrylic acid and poly
butyl acrylate shell): % weight gain as a function of time. Films from
latex kept in the acidic form at pH 2 (filled circles), films from latex
partly neutralised at pH7 by NaOH (unfilled diamonds), films from latex
fully neutralised at pH 10 by NaOH (filled triangles), films from latex
neutralised by Ba(OH)2 at pH 7 (+). Figure taken from [2 1 ]............... 128
Predicted optical transmission of 100 pm thick films of a continuous
medium with n = 1.5 and containing spherical air voids (nv =1.0). The
volume fraction of voids (fv) is taken to be 0.25 (—); 0.025 (—) and
0.0025 (••■). The figure is taken from [25].............................................. 131
A schematic diagram of a latex particle with PIDA core and a PAA
xxi
shell, 133
Figure 5.10. (a) Photograph of the probe-tack analyser, (b) The appearance of fibrils
between the PSA and the probe during the debonding of the probe from
the PSA. Image (b) taken from [34].................................................137
Figure 5.11. Schematic illustration of cavitation and fibrillation development in PSA
debonding process, (a) Initial stress is imposed on the bulk of the film,
(b) at critical stress cavities will form in the bulk or at the interface of
the probe and the film, (c) formation of new cavities and expansion of
existing cavities, (d) the inter-cavity distance reach the initial film
thickness, (e) fibrils starting to appear before detaching. Image taken
from [7]...............................................................................................138
Figure 5.12. Typical force ( ------------ ) and displacement ( ----------- ) curves as a
function of time. Image was taken from [31] and modified. Details of
the process is given in the main text................................................ 139
Figure 5.13. The stress as a function of strain for PIDA without PAA at a pH of 3.
hi this probe-tack curve (Figure 5.13), the point when most cavities are
initiated is indicated by the maximum stress, a max. The plateau stress, ctp,
is related to the stress required to draw the fibrils. The maximum strain,
emaxor failure strain ef is the end of the deformation............................. 141
Figure 5.14. Stress-strain curves corresponding to different adhesion energies of
PSAs. Curve I is for low, Curve II is for intermediate and Curve III is
for high work of adhesion. Image taken from [7 ] .................................142
Figure 5.15. Water sorption kinetics for PIDA with 0 wt% PAA at a pH of 8 adjusted
by NaOH (blue), undergoing a humidity step change from 0% to 70%
RH at 25°C, (% mass change of the sample as a function of time in
minutes). The red line show the data fitted using the Equation 5.8 .. 144
Figure 5.16. (a) MR profiles of PIDA , acquired every five minute intervals. When
xxii
Figure 5.17.
Figure 5.18.
Figure 5.19.
Figure 5.20.
Figure 5.21.
the film reached its final thickness, the intensity of the NMR signal is
from the mobile polymer, (b) the zeroth moment (the area under each
profile), which is proportional to the water content of the sample at that
time, as a function of drying time, (c) the film thickness as a function of
time. Both the zeroth moment and the film thickness have a final value
due to the signal from the mobile polymer............................................. 146
(a) Solids fraction as a function of drying time and (b) as a function of
drying time normalised by the initial film thickness, for as receive,
therefore pH = 3, for PIDA with 3 wt.% PAA (filled squares) and for
PIDA with no PAA (empty squares). Pure PIDA reach the maximum
solids fraction earlier then PIDA with PAA sample, therefore, pure
PIDA completed the drying before PIDA with PAA............................ 147
Solids fraction as a function of drying time normalised by the initial film
thickness for pure PIDA and PIDA with PAA at a pH of 8 adjusted by
NaOH. The effect of PAA shell at a higher pH adjusted by NaOH is
insignificant............................................................................................. 149
Solids fraction as a function of drying time normalised by the initial film
thickness for PIDA with PAA (pH = 3), a pH of 8 adjusted by NaOH
and NH4OH..............................................................................................150
Photograph to compare the change in the optical transparency over time:
(a) PET substrate with the latex film as soon as it was submerged in
water, (b) after 1 hour, (c) after 7 hours and (d) after 24 hours. Image
from C.-H. Lei, University of Surrey...................................................151
Optical transmission at a wavelength of 600 nm as a function of the time
the film immersion in water. Open symbols are for the PSA films by
pure PIDA latexes and the filled symbols are for the PIDA with PAA
samples. The squares for low pH latexes (pH of 3), diamonds are for a
pH of 8 adjusted with NH4OH and the circles are for a pH of 8 adjusted
with NaOH. PIDA with PAA (pH of 3; filled squares) least lost its
xxiii
transparency and pure PIDA with a pH of 8 adjusted with NaOH the
fastest to lose its transparency................................................................. 152
Figure 5.22.
Figure 5.23.
Figure 5.24.
Figure 5.25.
Figure 5.26.
Figure 5.27.
% mass change as a function of square root of time for 0 wt.% PAA
sample (empty squares) and 3 wt.% PAA sample (filled squares).
Sample with PAA in its shell shows higher % mass change, compared to
no PAA sample........................................................................................ 154
% mass change as a function of square root of time for 0 wt.% PAA
sample (empty squares) and 3 wt.% PAA sample (filled squares) at a pH
of 8 adjusted by NaOH. Sample with PAA in its shell shows higher %
mass change, compared to no PAA sample, in a basic form................ 155
% mass change as a function of square root of time for the samples with
PAA in their shells. Red squares for a pH of 3, blue circles for a pH of 8
adjusted by NaOH and black diamond for a pH of 8 adjusted by
NH4OH...................................................................................................... 156
M m as a function of calculated characteristic time, t, for all six samples.
The samples without PAA showed lower compared with the
samples which contained PAA in their shells.......................................157
Optical transmissions of the films after 200 minutes in water as a
function of normalised time to lose all the mobile water within the films.
The drying properties get desirable towards the direction of arrow 1 and
the water resistance properties get desirable towards the direction of
arrow 2 ....................................................................................................... 160
(a) M n as a function of normalised time to lose all mobile water with in
the films. The drying properties get better towards the arrow 1 and the
dry films adsorb less moisture from high humidity towards the arrow
head two. (b) t as a function of normalised time to lose all the mobile
water with in the films. Towards the direction of arrow head two, the
xxiv
dry films adsorb moisture faster from high humidity. 162
Figure 5.28.
Figure 5.29.
(a) and (b) x as a function of optical transmissions (at 600 mn) of
the films after 200 minutes in water........................................................164
(a) The adhesion properties of PSA film with 3 wt.% PAA in the shell,
at a pH of 3, performed at 90 minutes, 24 hours and 6 days after it was
cast, (b) The adhesion properties of PSA film with 3 wt.% PAA, at a pH
of 8 by NH4OH for different drying times and (c) summarises the
adhesion energy of PSA film with 3 wt.% PAA in shell at a pH of 3 and
at a pH of 8 NH4OH for different drying times..................................166
xxv
List of Tables
Table. 2.1.
Table. 2.2.
Table. 3.1.
Table. 4.1.
Table. 4.2.
Table. 4.3.
Table. 5.1.
Table. 5.2.
Table. 5.3.
Colloids as products and processes in everyday life [4 ] ............................. 8
Classification of colloids depending on the type of dispersed and continuous
media, with some common examples [2 ] ...........................................................9
The meanings of the first five moments [16].............................................59
Characteristics of Polymer/Laponite nanocomposite latexes synthesised
through miniemulsion polymerisation with AIBA functionalised Laponite
for Route I ..................................................................................................... 77
Characteristics of Polymer/Laponite nanocomposite latexes synthesised
through miniemulsion polymerisation with MPTMS/DDAB
functionalised Laponite for Route II............................................................78
Components and characteristics of the ‘original Route II and new
formulations (less DDAB/low solids and less DDAB/high solids) 98
Summary of the compositions and characteristics of the latex samples
....................................................................................................................... 134
Calculated and characteristic time, t for different samples 157
A summary of the results for all six samples of PSAs............................. 169
xxvi
Chapter 1
Overview
Film formation of latex has been studied since the early 1950s, due to the enormous
practical interest in these types of products. [1-3]. Application of modem
instrumental methods and the fundamental understanding of the process were greatly
developed during the last couple of decades. Latexes are being used in a broad range
of fields from adhesives, inks, paints, coatings, drug delivery systems, medical assay
kits, gloves, paper coatings, floor polish, films, carpet backing, foam mattresses,
cosmetics and many more [4]. The increased use of latexes as key replacement
materials for many solvent-based systems has largely been driven by tightening
legislation worldwide [5].
Contributions by scientists to this field during recent years have revolutionised the
development of latex synthesis [6 ]. Additionally, the interest in film formation
mechanisms was renewed thanks to the introduction of new techniques such as
atomic force microscopy (AFM), fluorescent labelling, environmental scanning
electron microscopy, direct nonradiative energy transfer, small angle neutron
scattering and other optical techniques [2 ]. hi the literature, leading scientists have
Chapter 1
developed theoretical models to understand the mechanisms of film formation [7].
Even so, due to the complexity of the film formation process, there is still room for
development.
This thesis is written in the frame of NAPOLEON - NAnostmctured waterborne
POLymeEr films with OutstaNding properties - project [8] which began in June
2005. The main objective of this project is to develop a technology platform to
produce waterborne nanocomposite colloidal dispersions to overcome the
environmental issues associated with coatings and adhesives dissolved in organic
solvents. These dispersions are to be of high solids, which offer energy savings in
production and transport. The breakthrough idea is to use waterborne nanocomposite
nanoparticles with well controlled structure as building blocks to produce films with
outstanding properties. Among these properties are good adhesion to substrates,
increased hardness, strength and wear-resistance with low dirt pick-up, higher
impermeability to liquids and gases, and greater fire resistance.
To achieve these objectives, the NAPOLEON Project integrated 9 EC companies and
12 academic research centres. Nanoparticle production and study of their process
and properties were mainly expected from the academic partners, while industries
ensured the proper scale up and product development. All 21 partners of this project
were divided into different key scientific aspects, called ‘work packages’,
considering their expertise, and assigned one or several tasks. The University of
Surrey group is part of work package four, which focuses on the film formation
process of the waterborne nanocomposite colloidal dispersions developed by some of
the other NAPOLEON partners.
The overall task of this thesis was to investigate the film formation process of the
latexes developed by the NAPOLEON project through the physical characterisation
of the process and film properties. A series of organic/inorganic waterborne
nanocomposites for coating application were investigated. In addition, the influence
of pH and acrylic acid (AA) on the physical characteristics of film formation and
film properties of waterborne pressure sensitive adhesive films were studied.
2
Chapter 1
The contents of the thesis are organised as follow.
Chapter Two provides an introduction, which is to present the different scientific
fields used throughout this work. It starts with a general introduction to polymer
colloids and their classification, followed by an introduction to latex dispersions and
their synthesis by different polymerisation methods. Then, the charges in the
colloidal dispersions, their stability and the factors affecting their stability are
discussed. This is followed by a detailed insight into film formation of colloidal
dispersions, stages of film formation, driving forces of particle deformation and the
supporting models for drying dispersions.
Chapter Three is devoted to magnetic resonance profiling (MRP). Out of all the
techniques used in this study, such as UV/visible spectroscopy for optical
transmission measurements, gravimetric analysis for moisture sorption
measurements, probe tack tests for adhesion measurements, profilometry for surface
variation analysis and the methods used for obtaining photos, MRP was used to
collect the majority of data. For that reason, Chapter Three is dedicated to historical
background, the fundamental principles of MRP followed by a detailed description
o f MRP using the GARField magnet, together with MRP measurements and further
calculations on these measurements.
Chapter Four explores the physical characterisation of the film formation process and
the film properties of organic/inorganic nanocomposite coating formulations. Methyl
methacrylate (MMA) and butyl acrylate (BuA) were used as the monomers to obtain
the organic phase, and clay (Laponite) was used as the inorganic phase. These
coating formulations were synthesised by miniemulsion polymerisation. Two
systems referred to as “Route I” and “Route II”, were developed. The main
difference between the two approaches lies in where the clay - Laponite - was at the
beginning of the polymerisation process. In Route I, Laponite was in the water phase
and in Route II, it was dispersed in the monomer phase. In Route I, the clay plates
are expected to be encapsulated in the latex particles. Whereas, in Route II, the latex
particles are expected to be surrounded by the clay plates. At the beginning of this
3
Chapter 1
chapter, an introduction is given to Laponite clay particles and synthesis of
nanocomposite formulations by Route I and II. The film formation and film
properties of Route I and II latexes were investigated. Greater insights into latexes by
Route II are given, as they were significantly different to that of Route I, followed by
discussions on the modifications which were made to Route II latexes to improve the
drying properties.
Chapter Five presents studies on the influence of pH and acrylic acid (AA) on drying
and water whitening of latex pressure-sensitive adhesive (PSA) films. “Water
whitening” is the loss of optical transparency when colloidal films are exposed to
high humidity or soaked in water, which is a common problem. To minimize the
water whitening, a film should achieve complete particle coalescence during drying.
However, early stage good coalescence could lead to “skin formation” and slow film
drying - which is not wanted in most applications. The coalescence of particles near
the air interface during drying is referred to as “skin formation”. The skin can lead to
slower transport of water and hence retard water loss during drying and cause water
entrapment within the film. Trapped water is also associated with inhibited
interdiffusion and hence weaker films. Preventing coalescence could avoid skin
formation, but the resulting film could be subject to water whitening and poor barrier
properties. In this chapter, the effects of AA and pH on drying, “water whitening”,
water sorption from high humidity, and adhesion properties o f waterborne adhesive
films and correlation between them are reported.
Chapter Six presents the final reviews of this research work and proposes potential
future work.
4
Chapter 1
1.1. References:
1. Guigner, D., Fischer, C., and Holl, Y., Film formation from concentrated
reactive silicone emulsions. 1. Drying mechanism. Langmuir, 2001.17(12):
p. 3598-3606.
2. Keddie, J.L., Film formation o f latex. Materials Science & Engineering R~
Reports, 1997. 21(3): p. 101-170.
3. Brown, G.L., Formation o f Films from Polymer Dispersions. Journal of
Polymer Science, 1956. 22(102): p. 423-434.
4. Hunter, R.J., Introduction to Modern Colloid Science. 1993: Oxford
University Press.
5. Jotischlcy, H., Coatings, regulations and the environment reviewed. Surface
Coatings International Part B-Coatings Transactions, 2001. 84(1): p. 11-20.
6 . Qiu, J., Charleux, B., and Matyjaszewski, K., Controlled/living radical
polymerization in aqueous media: homogeneous and heterogeneous systems.
Progress in Polymer Science, 2001. 26(10): p. 2083-2134.
7. Routh, A.F. and Russel, W.B., Deformation mechanisms during latex film
formation: Experimental evidence. Industrial & Engineering Chemistry
Research, 2001. 40(20): p. 4302-4308.
8 . http ://www. ehu. es/nanoleon/.
5
Chapter 2
Introduction
2.1. Introduction to colloidal dispersions
Evidence of humans’ use o f colloids dates back to the earliest records of civilisation.
Written records o f Egyptian pharaohs and Stone Age paintings in the Lascaux caves
of France were found to be produced by colloidal pigments. Colloidal systems were
used in many of our earliest technological processes, such as papermaking, pottery
making and cosmetic and soap fabrication. In the literature one can trace that
Francesco Selmi, in 1845 described the first examples of colloidal particles and
defined their common properties. In the 1850s Michael Faraday made extensive
studies of solid colloidal gold particles in water. But, it was Thomas Graham, in
1861, who coined the term colloid, meaning “glue” in Greek. Over the years, the
fundamentals have developed in parallel to industrial usage of colloidal systems [1 ].
Colloidal dispersions are defined as at least a two phase system, where one phase,
usually sub micrometer particles (the dispersed phase) is dispersed in the second
phase (continuous phase). The dimension of the dispersed phase is within the range
Chapter 2
from 1 nm to 1 jam. The continuous phase may be gas, liquid or solid whilst the
dispersed phase also can be gas, liquid or solid [2 ].
The large surface area and high surface area-to-volume ratios, due to their very small
dimensions, forms an extremely important and a basic characteristic of all colloidal
systems that involves directly with its ability to interact with its environment. Larger
particle surface area allows more functionalised groups to be on its surface. As
reactions take place on the surface, this leads to faster reactions between colloidal
particles and their environment.
As an example, the surface area of a sphere (A) given by 4nR2, and its volume (V) by
(4/3) ttR where R is the radius. The surface area-to-volume ratio for a spherical
particle is therefore given by:
Furthermore, 1 litre of a dispersion of 50% solid content with particles of 100 nm
surface area.
In addition, colloidal particles are in constant motion in the dispersion, and
demonstrate Brownian motion, which results from a random number of impacts of
random strength from random directions in any short period of time. Brownian
motion was first recorded by the botanist Brown while studying a suspension of
pollen grains using an optical microscope [3].
The understanding of colloidal phenomena has advanced in recent years at a greater
speed due to their importance as nanomaterials and nanotechnology in everyday life.
The products and processes listed in Table 2.1 show some cases where colloid
technology is directly involved [4].
A 4 ttR1 3(2 .1)
radius would have a particle surface area of 5 xlO4 m2. In comparison, the area of a
football pitch is around 1 0 4 m2, which shows that colloidal particles have a large
7
Chapter 2
Table 2.1. Colloids as products and processes in everyday life [4]
ProductsH Surface coatings : paints, photographic films, video tapes
■ Cosmetics and personal care : creams, toothpaste, hair shampoo
■ Household products : liquid detergents, polishes, fabric conditioners
H Agrochemicals : pesticides, insecticides, fungicides
■ Pharmaceuticals : drug delivery systems, aerosol sprays
■ Food : butter, chocolate ice cream, mayonnaise
■ Pigmented plastics
B Fire-fighting foams
Processes■ Clarification of liquids : water, wine beer
■ Mineral processing : flotation, selective flocculation
■ Detergency : ‘soil’ detachment, solubilisation
H Oil recovery : drilling fluids, oil slick dispersal
■ Engine and lube oils : dispersion of carbon particles
H Silting of river estuaries
H Ceramic processing
■ Road surfacing : bitumen surfacing
In natural systems■ Biological cells
■ Mists and fogs
8
Chapter 2
2.2. Classification of colloidal dispersions
Colloids can be classified depending 011 the type of dispersed and continuous phase.
Table 2.2 gives a summary of various possible types of dispersions, their technical
name with some common examples.
Only the gas/gas dispersion is not listed in Table 2.2, as those systems could last only
for a very short time [2 ].
Table 2.2. Classification of colloids depending on the type of dispersed and continuous media, with some common examples [2 ].
Dispersed phase Continuous phase Technical name Examples
Solid Gas Aerosol Smoke, dust
Liquid Gas Aerosol Fog, mist
Solid Liquid Sol or colloidal solLatex paints, adhesives, pigmented ink
Liquid Liquid Emulsion Milk, mayonnaise, hand cream
Gas Liquid Foam Foam, froth, whipped cream
Solid Solid Solid sol Ruby glass, cranberry glass
Liquid Solid Solid emulsion [5] Cheese, gelatine
Gas Solid Solid foam Aerogel,Styrofoam
9
Chapter 2
Colloidal dispersions may be classified as lyophilic (‘solvent-loving’) or lyophobic,
(‘solvent hating’), depending on the ease with which the system can be re-dispersed
once it is allowed to dry [2], Furthermore, when the particles of the dispersed media
are nearly of the same size (only vary by one or two percent), the colloidal system is
called monodisperse, and in the opposite cases they are heterodisperse or
polydisperse.
When solid particles are dispersed in a liquid, it is called a sol. If these solid particles
are polymer particles and the liquid is water, then the resulting dispersion is latex [6].
Increased interests in latexes were initiated in the beginning of the 1980s. The
underpinning reasons were a combination of increased public awareness on
environment and increased knowledge of the negative environmental and health
effects of solvent-based products. These products emit volatile organic compounds
(VOCs) to the atmosphere. The introduction of legislation reducing and regulating
the emission of VOCs has driven industry to convert from traditional solvent-based
products to more environmentally friendly products [7]. hi response, the demand for
waterborne systems was drastically increased. Currently latexes are undergoing
extensive research and development as key replacement materials for many solvent-
based products. One should appreciate that there is still much to understand and
develop, so it is still a developing science.
10
Chapter 2
2.3. Synthesis of latex
Latexes are commonly produced by emulsion and miniemulsion polymerisation
processes. Both processes are performed in water and produce stable latex
dispersions.
2.3.1. Emulsion polymerisation
Emulsion polymerisation process allows the production of submicron size polymer
particles, finely dispersed in the aqueous medium, thus forming latexes [8], that have
many advantages. One of them is the low viscosity of the latex it produces. Due to
the low viscosity, heat removal is easy and good temperature control can be obtained.
This allows a higher polymerisation rate (i.e. shorter production time). Low viscosity
of the system allows an easy removal of unreacted monomers and VOCs. Emulsion
polymerisation is environmentally friendly as water is used as the reaction medium.
There is no need for further treatment of the latex obtained by emulsion
polymerisation before their applications (Post-polymerisation additives, e.g. surface-
active agents, antioxidants, fungicides etc. can be added to the latex if needed). More
importantly, new products can be produced by emulsion polymerisation to meet
today’s market needs. The noticeable disadvantage is that there could be some
impurities in the polymer, like emulsifier and the rest of the initiator, which gives
water sensitivity to the polymer. In modern days up to a certain degree this is tackled
by using reactive surfactants [9]. Surface active molecules with an active vinyl group
are known as reactive surfactants. They are used to bind the surfactant chemically to
the surface of the particles. The advantage of using reactive surfactants is to reduce
desoiption of water during film formation and also to reduce the water sensitivity of
the latex film [1 0 ].
Emulsion polymerisation process itself is a complex heterogeneous free-radical-
initiated chain polymerisation process. A typical emulsion polymerisation starts with
11
Chapter 2
water, hydrophobic monomer (or mixture of monomers), surfactant and a water-
soluble initiator. The most important aspect of a surfactant (also referred to as
emulsifier) is its structure, which gives it its amphiphilic nature. It consists of at least
two parts: a polar group (hydrophilic, usually a water solubility enhancing functional
group) and a nonpolar group (hydrophobic or lyophilic, usually a long alkyl chain).
The surfactant molecules perform the dual function of providing sites for particle
nucleation, as well as providing colloidal stability to the growing particles [9].
In principle the reaction is initiated using a water-soluble initiator(s), and the most
commonly used is the inorganic salt of persulfuric acid (e.g. potassium persulfate).
The initiator dissociates into two sulphate radical anions which can initiate the
polymerisation reaction [10]. The stability of the reaction mixture is normally
ensured by the utilization of stabilizer(s). Other ingredients such as buffers and chain
transfer agents are also commonly present in a reaction mixture [9]. Usually, the
monomer(s) is dispersed in an aqueous surfactant solution with a concentration
exceeding the critical micelle concentration (CMC). At or above CMC, surfactants
form aggregates. The aggregates formed in an emulsion polymerisation are often
spherical and refer to as micelles [9], the core of which is swollen by the monomer.
Emulsion polymerisation is usually characterised by three distinctive intervals,
named interval I, interval II and interval III. Figure 2.1 illustrates these three
intervals.
12
Chapter 2
interval I *T
monomer T droplet &C
S
R\free radical surfactant initiator latex particle micelle
Interval If. j /-f-x
y . 9 * < $ ■
Interval Iff.
ft
ET
,
Figure 2.1. Schematic diagram illustrating the three intervals of an emulsion polymerisation process [11]. Interval I: Particle formation stage, represented by the increase in both the number of particles and the polymerisation rate. Interval II: Particle growth: during which, both the number of particles and the polymerisation rate remains relatively constant. Interval III: The number of particles remains unchanged, while polymerisation rate and the monomer concentration decrease[l 1 ].
Upon mixing the monomer(s), water and surfactant(s), the latter molecules cluster
into micelles with their hydrophobic cores being swollen with the monomer. This
stage is illustrated in the interval I - Particle formation stage - in Figure 2.1. The
interval I represented by an increase in the number of particles. The polymerisation
rate also increases, as shown in Figure 2.2. However, the bulk of the monomer exists
in the form of large-size droplets with surfactant molecules absorbed on to their
surfaces. When the initiator is added radicals are formed in the aqueous phase and as
these radicals are too hydrophilic to enter into the organic phase of the system, they
react with the monomer dissolved in the aqueous phase, forming oligoradicals. After
13
Chapter 2
adding a few monomer units, the oligoradicals become hydrophobic enough to be
able to enter into the micelles. Due to the high concentration of monomer in the
micelle, the oligoradicals that entered into the micelle grow fast and form a polymer
chain (or particle) and this is known as nucleation [1 0 ] .
As the nucleation progresses, the number of micelles decreases, because they become
polymer particles, and after some time all the micelles disappear. This corresponds to
the end of interval I. The duration of interval I vary within the range of 2 - 10 %
conversion. This depends on a few factors: the type and the concentration of
surfactants, the initiation rate, the degree of water solubility of monomers, etc.
Interval II extends from 5 - 1 0 % to 3 0 -7 0 % conversion. At the end of interval II,
monomer droplets disappear [8 ]. hi interval III the monomer, only present in the
particles, continues to polymerise, resulting in both decreasing monomer
concentration and polymerisation rate, as shown in the Figure 2.2. The number of
particles remains the same [8 , 1 2 ].
Figure 2.2. Polymerisation rate as a function of time for three intervals of emulsion polymerisation process [8]. During interval I, the polymerisation rate increases, during Interval II it stays relatively unchanged and finally, it decreases during interval III.
In emulsion polymerisation three types of processes are commonly used, they are:
batch, semi-continuous (or semi-batch) and continuous, hi batch polymerisation all
14
Chapter 2
the ingredients are added at the beginning of the processes and the polymerisation
starts as soon as the initiator is added to the system. The temperature rises with the
simultaneous formation and growth of the particles. Removal of heat generated by
polymerisation is a daunting task in batch polymerisation [9]. hi a semi-continuous
polymerisation process, one or more ingredients are added continuously or in
batches. The advantage of this process compared to batch polymerisation is the
ability to have more control over the process. The rate of polymerisation and the rate
o f generation and removal of heat can be controlled more easily. Greater control over
the particle number, colloidal stability, copolymer consumption and particle
morphology are other aspects of the semi-continuous polymerisation process [10]. In
the continuous process, the ingredients are fed continuously into a stirred tank (or
more than one connected in series). The latex product is also simultaneously
removed at the same rate. The main advantages are high production rate, more
controlled heat removal and higher quality of latexes [1 0 ].
2.3.2. Miniemulsion polymerisation
Miniemulsion polymerisation has been identified as a special case of conventional
emulsion polymerisation. It is a heterogeneous process. Miniemulsions are aqueous
dispersions of stable nanodroplets of a diameter within a range o f 50 - 500 mn with a
narrow size distribution. Monomer droplet size and the size distribution are the most
important parameters of the miniemulsion polymerisation as they affect both
miniemulsion stability and droplet nucleation directly [13]. The main differences
between miniemulsion polymerisation and emulsion polymerisation are the addition
of a hydrophobic agent (also called a co-stabilizer or an osmotic pressure agent) and
high shearing of the system such as with ultrasonication [12]. These so-called ‘co-
surfactant’ hydrophobic agents enhance the stabilisation of the submicron size
monomer droplets produced by the high shearing [1 2 ].
The first step of the miniemulsion polymerisation process is to form submicron size
stable monomer droplets through the shearing of the system containing the dispersed
15
Chapter 2
phase, the continuous phase, a surfactant and a hydrophobic agent. This step is
shown in Figure 2.3, left and the middle panels. In the second step, these droplets
undergo the polymerisation reaction (Figure 2.3, middle and right panels). The
absence of a diffusion process makes this process hugely useful for industrial scale
for continuous reaction processes [14].
r r
ultrasoundm Phase I • •
• • • 3 ^ 3 ^ reaction
- v i 1© - o
* N s
^ o ^^ Phase II
stable nanodroplets I : I copyas small compartments (nanoreactor)
Figure 2.3. Principle of miniemulsion polymerisation. In the first step of miniemulsion polymerisation, submicron size monomer droplets are produced by shearing and in the second step, these droplets act as individual batch reactors that undergo the polymerisation reaction [5, 14, 15].
In emulsion polymerisation the particle size is determined by the kinetic parameters
like temperature and initial initiator concentration. Where as, in miniemulsion
polymerisation it is essentially the amounts of surfactant and the monomer present
and the intensity of the shearing used to prepare the droplets that determine their size.
In the miniemulsion polymerisation, all or most of the stable submicron size droplets
which were formed are polymerised into polymer particles. The key issue for a
successful miniemulsion polymerisation is for these droplets to remain stable for the
entire reaction time. Therefore, the choice of a suitable hydrophobic agent is vital
[8].
There are some unique advantages of miniemulsion polymerisation over an emulsion
polymerisation. Undoubtedly, the monomer droplets are the main focus of the
16
Chapter 2
process. Therefore, the nucleation step is not as complex as in emulsion
polymerisation. Furthermore, the system is simplified to two phases, a water phase
and monomer/polymer particles throughout the process. Miniemulsion
polymerisation is a far better process for the very water-insoluble monomers, as they
would transport slowly through the aqueous phase in an emulsion polymerisation
[16]. It is also very useful process for conducting polymerisations of monomers with
significantly different water solubilities and for various encapsulating systems [17,
18].
17
Chapter 2
2.4. Charges in the colloidal dispersions
According to fundamental thermodynamics, all systems strive to decrease their total
free energy in a system kept at a constant temperature [19]. hi addition, between
neutral particles in any suspension, due to the induced dipoles, there is an attractive
van der Waals interaction. Due to this attractive force, the particles tend to aggregate.
To prepare stable colloidal dispersions it is necessary to introduce interactions
between particles that are opposite to the van der Waals attractions. One way of
achieving this is to have a charge on the particles. This surface charge would result in
a repulsive inter-particle force [2 0 ].
During synthesis, many polymer particles gain a surface charge, due to the ionisable
groups at the end of the polymer chains. In addition, the surface charge can be further
modified by changing the environment of continuous phase, as an example by
changing the pH, or by adding a differently charge surfactant [4].
In an electrolyte, the charged particles are surrounded by ions that shield their
surface charge. The boundary layer of ions is defined as the electrical double layer.
In the literature, there are a few models to describe how the potential changes as a
function of the distance from the centre of the particle, into the surrounding medium.
During 1860’s Helmholtz presented the first model [21], Then Gouy and Chapman
worked independently to develop their models. Later Stern combined these models
and further developed them [4].
2.4.1. Helmholtz model
Helmholtz suggested a simple model of a charged metal interface immersed in
electrolyte. In his model, he suggested that the surface charge is counterbalanced by
a layer of oppositely charged ions. Figure 2.4 shows, in the proposed model the
potential is dropped across the single layer of ions [4].
18
Chapter 2
Figure 2.4. Schematic of the Helmholtz model, which failed to explain the experimental results [4].
2.4.2. Gouy - Chapman model
In early 1910 Gouy and Chapman independently developed a model, where the
surface charge of the solid is balanced by a diffuse layer of ions. This diffuse layer
contains an excess of ions of opposite charge on the charge of the surface as shown
in Figure 2.5 [4].
19
Chapter 2
© © ©© © ©
© ° ©© © © © ©
Diffuse layer
© ©©
©
©
©©©
Figure 2.5. Schematic of the Gouy - Chapman model [4].
2.4.3. Stern model
Later, Stem combined both the Helmholtz and Gouy-Chapman models and further
developed them. In his model, the counter charge exists in two distinctive regions:
( 1 ) the inner layer of ions of opposite charge to the surface, ‘stuck’ to the surface and
(2) a diffuse layer, as shown in Figure 2.6. The plane between the electrode surface
and the outer Helmholtz plane (OHP) is defined as the Stem layer, which is the
closest approach of the centre of a hydrated ion to the electrode. At the electrode
surface the potential is \\is and at the OHP the potential is mu. The potential drop
across the Stem layer (\jjs - MU ) is assumed to be linear. However, in the diffuse
layer the potential drops exponentially [4].
20
Chapter 2
Outer Helmholtz plane
Stem layerY*̂ Diffuse layer
Figure 2.6. Schematic of the Stem model which consist of Stem layer, where potential drops linearly and the diffuse layer, where the potential drops exponentially with the distance [4].
The electrostatic potential can be positive or negative and depends on the chemical
nature of the surface group and on the surface charge density. Boltzmann’s
distribution law says that if the energy associated with some state or condition of a
system is s then the frequency with which that state or condition occurs, or the
~ s/ k T
probability of its occurrence, is proportional to e where T is the system’s
absolute temperature and where k is the Boltzmann Constant = 1.38 x 10'23 J/K. For a
plane surface, if the ions are distributed according to the Boltzmann’s distribution
law in the diffuse layer, the potential energy ( ®x) at a distance x from the surface
can be determined by [2 2 ]:
0 , = <J>se (-fa) (2.2)
21
Chapter 2
For a spherical particle with a radius of R, the potential energy (O,.) at a distance rx
from the particle is given by:
the double-layer “thickness” [22]. The Debye screening length is a characteristic
length for a given aqueous solution. The magnitude of the Debye screening length
depends solely on the properties of the liquid and not on any other property of the
surface, e.g. its charge or potential [23].
K can be calculated by the following equation.
where e, electronic charge = 1.6 x 10' 19 C; no, ionic concentration; Z , valency of the
ions in solution; s ; the dielectric constant of the continuous phase; So, the dielectric
constant of vacuiun. At 298 K the Debye length of 1:1 electrolytes such as NaCl
298 K, for NaCl solution, the Debye length, UK is equal to 30.4 nm for 10'4 M
solution, 9.6 nm at 1 mM solution, 0.96 nm at 0.1M solution and 0.3 nm at 1 M
solution. The Debye length for totally pure water at pH 7 is 960 nm [23].
(2.3)
The parameter K is important, as K 1 is the Debye screening length or more usually
(2.4)
aqueous solutions is equal to 0.304 / in mn< For 2:1 and 2:1 electrolytes/ sjlNaCl]
(e.g., CaCL and Na2S0 4 ) it is 0.176 / in nm and for 2 :2 electrolytes (e.g.,/ *v \CctCl2 J
M gS04) the Debye length is equal to 0.152 / ----------- in nm. As an example, at/ f [ M g S Q 4]
22
Chapter 2
2.5. Stability in the colloidal dispersions
One of the important aspects of colloidal dispersions is the stability of the colloidal
state. This phenomena, which fascinated Faraday 140 years ago, still remains
interesting to today’s scientists, although with more sophisticated implications. The
competition between attractive van der Waals and repulsive double-layer forces
determines the stability or instability of many colloidal dispersions [24]. The stability
in colloidal dispersions is defined either in terms of their tendency to aggregate or
their tendency to sediment under the action of gravity [4]. Reversible aggregation is
often called flocculation while irreversible aggregation is called coagulation [24].
2.5.1. The DLVO Theory
During the 1940s a major advance in colloidal science occurred when two groups of
scientists - Boris Deijaguin and Lev Landau from the Soviet Union and Evert
Verwey and Theo Overbeek from the Netherlands - independently published their
analysis of the stability of colloidal systems. It is now called the DLVO theory in
honour of them [24].
The DLVO theory assumes that the interparticle interactions control the stability of
colloidal system. It considers two types of forces: the attractive van der Waals force
and the repulsive double layer force. The van der Waals forces operate irrespective
of the chemical nature of the particles or the medium, and if the particles are similar,
this force is attractive. On the other hand, the double layer forces are acquired due to
a charge on the particles; either from surface charge groups or by specific ions
adsorbed to the particles from the solution and are sensitive to variations in
electrolyte concentration and pH. The double layer forces are repulsive and may be
considered as fixed in a first approximation, hi addition, the van der Waals attraction
must always be greater than the double layer repulsion at small distances, D, between
particles because it is a power-law interaction, that is interaction energy,
23
Chapter 2
W oc ~ y'j^n , whereas the double-layer interaction energy remains finite or rises
much more slowly as D decreases to 0 [23].
The total potential energy of interaction ( W t) can be expressed as the sum of the
attractive (W A) and repulsive (Wr) energies [23]:
WT = W A + W R (2.5)
The attractive energy can be written in more detail as:
WA = -AR/ 6 D (2.6)
where A is the Hamalcer constant and typical values for A of condensed phases,
whether solid or liquid, are about 10' 19 J for interactions across vacuum [23].
The repulsive energy in more detail given by:
WR ={647tlcBTR pa>r 2 I K 2)e~KD (2.7)
where is the ionic concentration of ions in the bulk, and y is the charge on the
particle [23].
24
Chapter 2
Figure 2.7. Schematic of the energy versus the distance between particles profilesof DLVO interactions. This figure is taken from Jacob Israelachvili, Intermolecular and Surface Forces 1992: Academic Press Limited, London; page 248 [23]. Highly charged surfaces repel strongly and remain stable which is illustrated in lower inset ‘a’, ‘b* where surfaces come into stable equilibrium at secondary minimum and colloidal particles remain lcinetically stable. *c’ when surfaces come into secondary minimum and particles coagulate slowly, ‘d’ Surfaces may remain in secondary minimum or adhere and colloids coagulate rapidly, at the ‘critical coagulation concentration’, ‘e’ Surfaces and colloidal particles coalesce rapidly [23].
The total potential energy of interaction (Wj) curve has a number of important
features: The particles with highly charged surfaces in a dilute electrolyte have a
relatively long Debye length; hence they have a long-range repulsion which is quite
strong. This repulsion would peak usually around 1-4 nm at the energy barrier. This
situation is illustrated in Figure 2.7 (a). When the concentration of the electrolyte
25
Chapter 2
solution is slightly higher, the Debye length is not as long as in the previous
situation. As the distance decreases, the total potential curve goes through a
significant secondary minimum, usually beyond 3 nm, before the energy barrier,
which is illustrated in Figure 2.7 (b). The primary minimum is defined as the
potential energy minimum when the particles are at contact. If the primary minimum
is deep and the energy barrier is too high to be overcome, the particles could stay in
the secondary minimum or they could be totally dispersed in the solution. The
particles with a very low charge density have a relatively short Debye length; their
energy barrier would be much lower leading to a slower aggregation called
flocculation (Figure 2.7 (c)). When the energy barrier falls below the interaction
energy, W = 0 (Figure 2.7 (d)), the concentration of the electrolyte is called the
critical coagulation concentration (CCC). Then the particles coagulate rapidly, and
this colloidal system is referred to as being unstable. For the particles with very low
or zero surface charge, the Debye length is very small or nonexistent, hi this case, the
interaction curve approaches the pure van der Waals attraction curve and particles
attract each other strongly, which is illustrated in Figure 2.7 (e) [23].
There are two types of aggregation: Coagulation is the rapid aggregation that occurs
in the absence of an energy barrier or a primary maximum (Figure 2.7 (e)). This
leads to a strong irreversible structure. Flocculation is the reversible aggregation that
occurs in a secondary minimum (Figure 2.7 (b), (c), (d)). Flocculation is reversible
on the addition of energy to the system usually by applying shaking, stiring or other
mechanical process [4].
2.5.2. The factors affecting the stability in the colloidal
dispersions
In the literature, it has been reported that various factors could affect the stability of
colloidal system. The ion type and their concentration in the system, the size of the
particles and the value of the £ - potential of the particles are considered to be most
important [4].
26
Chapter 2
2.5.2.1. Particle size
The total potential (Vt) is directly proportional to the particle size, at small particle
sizes (<100 nm radius). However, at larger sizes, the variation of the total potential is
not that straight forward. If all other factors remain constant, a larger particle radius
leads to a higher energy barrier therefore increased electrostatic stability. For small
particle sizes the energy barrier is directly proportional to the radius [4].
2.5.2.2. Zeta - potential
The Zeta (£,) - potential is the potential at the shear plane, the effective location of
the solid/liquid interface. It is possible to measure Zeta (C,) - potential
experimentally using electrokinetic methods. [4], [25].
D (nm)
Figure 2.8. The effect of ̂ potential on the shape of the total interaction potential curve for polystyrene latex. From top to bottom the £ potentials are -80 mV, -50 mV, -25 mV and -20 mV [4].
Figure 2.8 show an example for polystyrene particles in 1 mM NaOH solution.
Since, V r is proportional to the square of surface potential; a doubling of C, potential
27
Chapter 2
should lead to a quadrupling of the value of Vmax. From the VT as a function of
distance plot in Figure 2.8, when the £ potential is -25 mV the value of Vmax is
around 40 kB. When the (^potential is doubled to -50 mV, Vmax is around 160 kB.
This shows that V r is proportional to the square of surface potential.
28
Chapter 2
2.6. Film formation of colloidal dispersions
When a latex dispersion is applied on a substrate and evaporation is allowed, a
formation of a continuous, homogeneous film under appropriate conditions is called
film formation [6 ]. Formation of a latex film arises with the coalescence of the latex
particles. Normally, in a latex dispersion, these individual latex particles are held
apart by stabilising forces resulting from the charges on polymer particles or by the
addition of emulsifiers and stabilisers.
2.6.1. Stages of film formation
In the most general sense, film formation involves three stages [6 ]: Stage I -
Evaporation of water and particle ordering, Stage II - Particle deformation and Stage
III - Coalescence/Interdiffusion across particle-particle boundaries. These three
stages are schematically illustrated in Figure 2.9.
Polymer dispersionPolymerparticles
Aqueous phase
Mechanically coherent homogenous film
Waterevaporation
> = >
T>Tg
< = >
Particles in close packing
Coalescence/Interdiffusion/
aging Honey-comb structure of deformed particles
Figure 2.9. Illustration of the stages of film formation process [6]. In stage one, water evaporates from the polymer film and particles re-order to a close packed array. In stage two, particles deform, when the drying temperature is greater than the Minimum Film Formation Temperature, MFFT. In the final stage, particles coalesce and molecules diffuse, when the drying temperature is higher than the Tg of the latex, and a mechanically coherent homogenous film is formed.
29
Chapter 2
2.6.1.1. Stage I - Evaporation of water and particle
ordering
At the beginning of the Stage I of the film formation process the particles move with
their characteristic Brownian motion. The water concentration in the polymer
dispersion is uniform. During this stage, water evaporates at a constant rate across
the water/vapour interface and water evaporation rate is close to that of pure water
[26]. According to the conventional thermodynamic theory, the main parameters
affecting the drying performances at this stage are the temperature, the relative
humidity and the vapour pressures of the water surface and the air. Differences in
size of the polymer particles and their composition have no noticeable effect on the
drying rate. Additionally, film thickness also has no effect on the drying rate [27].
The mass transport o f water vapour from the water surface to air occurs by diffusion
and convection. The evaporation predominantly occurs across the water/gas
interface, rather than across the polymer/gas interface [6]. Furthermore, within the
dispersion, the transport of water between the particles is faster than that through the
particles.
As evaporation of water proceeds the concentration of the polymer particles in the
dispersion increases and the particles come into close proximity. In addition, the
colloidal stability is affected as well. At this stage, the particle ordering in the
dispersion is a function of volume fraction of particles and ionic strength. Both of
these factors change as a result of evaporation of water.
If we consider particles are arranged in a face-centred cubic (FCC) structure, Figure
2 . 1 0 illustrates the dependency of the average distance between particles on the
volume fraction of polymer dispersion (® po/), for different sized particles. For a
given value of ® po/ ( less than 0.74), the distance between larger particles is greater
than for smaller particles. As an example, at 40wt% solids, 200 nm radius particles
are 100 nm apart compared with 90 nm radius particles that are only around 50 mn
apart.
30
Chapter 2
Volume fraction of poljmer [+]
Figure 2.10. Distance between different size particles as a function of volume fraction of polymer.
2.6.1.2. Stage II - Particle deformation
The transition from Stage I to Stage II of film formation occurs with a significant
decrease in the rate of evaporation. This is presumed to be due to decrease in
diffusivity due to tighter packing of the particles. It has been generally accepted, at
the transition point all the particles have reached close packing but not yet start to
deform. Furthermore, Stage II is associated with the development of optical clarity.
For a mixture of two phases with two different refractive indexes to be optically
clear, the size of each phase must be far below the wavelength of light. Therefore,
optically clear films could still contain nm size voids of water [6]. As evaporation
31
Chapter 2
continues further, the interfacial and capillary forces exceed the mechanical
resistance of the particles, resulting in particle deformation. A void-free,
mechanically weak honey-comb structure is formed. For this to occur, the application
temperature must be higher than the minimum film-forming temperature (MFFT) of
the system. In the literature, the MFFT is defined as the minimum temperature at
which a latex cast film becomes continuous and clear. If a latex dries below this
critical temperature, it is expected to be opaque and powdery [28]. At this stage
interfaces still exist between the particles.
2.6.I.3. Stage III - Coalescence/Interdiffusion across
particle - particle boundaries
Coalescence, which is dissolution of the cell membranes, brings the polymer particle
cores into close contact, and permits interdiffusion of the polymer molecules
between particles [29]. During this stage the film properties change significantly, as
polymer inter-diffusion across the particle boundaries imparts mechanical strength
and resistance character to the film. The mechanical strength of a given film
increases with the depth of polymer interdiffusion. The diffusion rate increases with
increasing temperature and decreasing molecular weight.
This final stage is characterised by a very slow rate of water loss until it is complete.
In this final stage, it has been suggested that water could move from the bulk of the
film to the polymer/air interface through the channels between the particles or by
diffusion through coalesced polymer particles, which is considerably slower. As
coalescence progresses, the water transport eventually slows to approach that of
diffusion alone. Now, the evaporation rate-limiting step is the transport of water to
the polymer/air surface. The diffusion rate increases with increasing temperature and
decreasing molecular weight.
32
Chapter 2
2.6.2. Particle deformation and coalescence
Particle deformation and coalescence is favourable from the thermodynamic point of
view, as they correspond to a decrease in the surface area of the particle-water or
particle-air interfaces. In the literature, there are a number of theories and models that
have been presented, each considering different driving forces for particle
deformation.
2.6.2.I. Wet sintering
For wet sintering, the driving force for the particle deformation and subsequent
coalescence is provided by the interfacial tension between the particles and water
[30]. In wet sintering, which is illustrated in Figure 2.11 A, the particles should
deform before water has evaporated. Later, it was concluded by Dobler et al. that
wet sintering can contribute to particle deformation, but the process is to be too slow,
when compared with evaporation, to be the primary driver under normal conditions
[31].
2.6.2.2. Dry sintering
When the polymer-air surface tension provides the driving force for particle
deformation and subsequent coalescence it is referred to as dry sintering (Figure 2.11
B). During dry sintering the water recedes before particles are deformed and the
temperature is above the glass transition temperature (Tg) of the polymer. Sperry and
co-workers [32] reported experimental evidence for dry sintering. They dried a film
well below the polymer Tg to prevent the possibility of particle deformation, and then
raised the temperature. The appearance of this film was compared with another film
cast wet at a higher temperature. They reported that both films reached the cloudy to
clear transition at the same temperature after a similar time. It was proved that the
33
Chapter 2
presence of water is not important at lower temperatures. Later, Lin and Meier [33]
argued, that atmospheric humidity preserves the residual water at the particle contact
and the particle deformation is caused by the capillary pressure associated with this
residual water at the particle contact, which was later named as moist sintering.
Figure 2.11. Schematic illustrations of particle deformation theories for film formation. (A) Wet sintering - when particles are deformed before water has evaporated and the reduction of the polymer water interfacial energy is the driving force. (B) Dry sintering - when water recedes before particles are deformed and the reduction of the polymer-air interfacial energy is the driving force. (C) Capillary deformation - when the air-water interfacial energy is the driving force for particle deformation. (X) concave meniscus of air-water interface. (D) Sheetz deformation - “skin” formation by wet sintering and capillary forces due to rapid evaporation, before the dispersion reaches close packing below.
34
Chapter 2
2.6.2.3. Capillary deformation
Brown [34] suggested that the air-water interfacial tension dominates the particle
deformation. As evaporation proceeds, because of the presence of particles, in the
neck region, the water creates a meniscus. There is a negative pressure on the convex
side of the air-water interface. This capillary pressure increases as the water
evaporates from the close packed spheres. According to Brown’s hypothesis, the
atmospheric pressure, pressing on the exposed particles at the surface compresses the
film [34].
According to Brown’s criterion for film formation,
when the film formation of latex occurs, where Gt is the polymer’s shear modulus
determined on the observation of the strain resulting from the application of the
stress for the time of evaporation, y is the surface tension of the air-water interface
[34]. Capillary deformation is illustrated in Figure 2.11 (C), and concave meniscus of
air-water interface is marked by X.
In 1995 Keddie and co-workers [35] first put forward this inhomogeneous regime.
According to them, the deformation initially occurs by capillary mechanisms. By the
time the capillary pressure reaches its maximum, the deformation is not yet
completed. As the evaporation continues, the water recedes through the drying film,
leaving behind the dry particles. At this stage the deformation system switches to
either a dry or moist sintering mechanism.
(2 .8)
2.6.2.4. Receding water front
35
Chapter 2
2.6.2.5. Sheetz deformation
Sheetz [36] proposed that half-way through the film formation process, wet sintering
and capillary forces create a thin membrane or “skin” layer at the air-dispersion
interface, before the dispersion reaches close packing below. Formation of the skin
layer is illustrated in Figure 2.11 (D). This skin could be permeable to water vapour
but hinder evaporation. Sheetz believed that measured evaporation rates were
certainly fast to be only by evaporation from the air-water meniscus. In addition, he
observed that a continuous latex film placed in contact with wet latex did not prevent
its film formation. One of the principles on which Sheetz’s model is based that the
energy for film formation is supplied as heat from the surroundings. This heat is
change into useful work by evaporation of water from the drying film. This water
diffuses through the ‘skin’ at the film surface. As a result it generates a compressive
force normal to the film. Sheetz compared this to a piston, which is permeable to
water vapour, compressing water. As water evaporates from the top of this piston,
creates an osmotic pressure in the dispersion below. Despite this, there is not enough
direct evidence for ‘skin’ at a drying latex surface. Therefore, the theory proposed by
Sheetz not proven valid or invalid [6 ].
2.6.3. Recent models for drying dispersions
Further to the several studies mentioned above, new ideas were further developed by
several scientists during the last few years. Out of them, Routh and Russel’s model
for normal drying is the latest and the most complete theoretical model to date.
According to the Routh and Russel model [37], different drying and particle
deformation regimes can be predicted for normal drying dispersions by determining
two dimensionless numbers Pe (Peclet number) and X[37]. X is the ratio of the
characteristic time for particle deformation to the characteristic time for drying, for
normal drying dispersions.
36
Chapter 2
2.6.3.I. Peclet number (Pe)
According to Routh and Russel, the Peclet number is a measure of the relative rate of
the recession of the water by evaporation over the rate of particle transport by
Brownian diffusion [38]. The Peclet number can be used to predict the vertical water
uniformity of a film during drying. The Peclet number, Pe, is defined as
P e = — (2.9)D 0
where, H is the film thickness; E is the evaporation rate; Do is the diffusion
coefficient of the particles.
If diffusion is stronger than evaporation, we get P e « 1, then a more uniform drying
is observed. If evaporation is stronger than diffusion, we get P e » 1, which
corresponds to non-uniformity in drying, which schematically illustrated in Figure
2 . 12.
Figure 2.12. Schematic illustrations of P e « l and P e » l . Initially stable polymer colloid dispersion (on left) could show vertical water uniformity (right top) if diffusion is stronger than evaporation (A) during drying and P e « l . Alternatively, if evaporation is stronger than diffusion (B) during drying the result is non-uniform water distribution (right bottom) and P e » l .
37
Chapter 2
For colloidal particles, the Stokes-Einstein Diffusion coefficient, Do is given by
d - k TD ° ~ 6 ^ R (210)
where, /u is the solvent viscosity and kT is the thermal energy. Substituting for Do into
Eq. 2.9, leads to
GmiRHE
P e ’ — t r ~ <2“ »
Therefore, we can conclude the solvent viscosity, the particle radius, the film
thickness, the evaporation rate and the drying temperature affect the uniformity of a
film during drying. Except the temperature, as all the other parameters increase, the
Peclet number increases, therefore non-uniform drying is predicted. According to Eq.
2.11, the temperature is inversely proportional to Peclet number and we can predict
more uniform drying with higher temperature. However, the temperature also affects
the evaporation rate, E. Therefore, Peclet number’s dependency on the temperature is
complicated.
2.6.3.2. The Routh and Russel model for normal
drying
As stated earlier, the Routh and Russel model predicts different drying and particle
deformation regimes for normal drying dispersions by determining two
dimensionless numbers Pe and X [37]. X is the ratio of the characteristic time for
particle deformation to the characteristic time for drying, for normal drying
dispersions. It can be written
38
Chapter 2
o _ T<1efA ~ — (2 .1 2 )
d i y
where, the characteristic time for particle deformation is given by
Tdef ~ (2.13)r
where, ?]0 is zero shear rate viscosity of the polymer and y is the interfacial energy
for the polymer-water or polymer-air interfaces. The characteristic time for drying
can be written as:
= Y (2.14)
Therefore, by substituting for zdcf and rdiy into Eq. 2.12,
x = 1]sf f (2-15)yH
The number X determines the type of deformation valid during the drying process
and separated into four regimes as shown in Figure 2.13.
1. X<1 -W et sintering occurs when the deformation is slow and is
driven by the surface tension between the particles and the solvent (or
water).2 * • •2. 1< X <10 - Capillary deformation occurs when the capillary pressure
is the driving force for film formation. The particles are close-packed
without undergoing deformation.
3. 1 0 2 < X < 104- The receding water front is an inhomogeneous regime
between the capillary and dry sintering mechanisms.
4. X > 104 - Dry sintering occurs when the driving force is the surface
tension between the polymer and the air.
39
Chapter 2
Softer particles are more likely to show wet sintering. A higher evaporation rate or
thicker film leads to a higher Peclet number and non-uniformity in drying. Softer
particles with a higher Peclet number in the wet sintering regime leads to skin
formation during drying [37].
S o f t n e s s o f t h e p a r t i c l e s
X
10000
100
Dry/moist sintering
F ilm
Receding waterfront \r a te
Capillary ^ ............deformation j Partial skinning
i
Wet sintering j Skinning
1 Pe
Figure 2.13. Schematic diagram of the drying regimes according to the Routh and Russel model for normal drying and the influencing factors [37].
2.6.3.3. The Routh and Russel model for lateral drying
A process referred to as lateral drying occurs when the solids fraction nearer to the
centre remains close to the initial value and a high solids region develops at the edge.
As the drying continues, a drying front or the air/water interface moves from edge to
the centre. On the other hand, when the solids fraction increases relatively uniformly
or to only a limited extent in the lateral direction, this is called vertical drying. In this
case, drying does not occur strongly from the edges, unlike lateral drying [39].
The various theories that attempt to explain the origin to lateral flow do not allow
predicting when an eventual drying front will occur. In 1998, Routh and Russel
40
Chapter 2
developed a model using a dimensionless number, the reduced capillary pressure
(Pcap), to predict the onset for the propagation of a drying front in dispersion. That
is, PCap can predict how long the edges of the drying dispersion remain wet [40]. The
reduced capillary pressure is a function of several parameters and can be written as
follows:
p 20 j 3 cap 751 E
(2.16)
where $ tl is the volume fraction of solids at close-packing of the particles, and rjod is
the low shear viscosity of the dispersion [40]. This model predicts that when Pcap <
1, water recedes from the edges of the film. Pcap is often larger than 1 at the initial
stages of drying and is enough to support regions with more densely packed
particles, usually at the edges and maintain a wet surface. However, Pcap decreases
with the time as particles become close-packed at the edges and advance towards the
centre of the film [40].
Figure 2.14 shows the suggested cross section of a lateral drying colloidal film. Two
types of boundaries can be seen: one separates a wet dispersion from a flooded close-
packed array of particles which is called a particle-packing front, and the other
separates the flooded array from fully dry, packed particles, which is drying front
[39].
V i• • • i i
s s s x s s s s s s s s w / / / / / / / / / / /
x s V s s s s s s w s s X X s \ s s s s s s s w
•Wet dispersion
Particle-packing front
n—M Close-packed U— region X
Dry region
Drying front
Figure 2.14. Schematic diagram shows a lateral drying in a colloidal film with a central wet region and dry edges. Between these regions, water fills the void space between packed particles. The inner boundary is the particle packing front, and the outer boundary is the drying front [39].
41
Chapter 2
2.7. References:
1. Evens D.F., W.H., The Colloidal Domain. 1999: Wiley-VCH, New York.
2. Hunter, R.J., Introduction to Modern Colloid Science. 1993: Oxford
University Press.
3. Renn, J., . ., . , Einstein’s invention o f Brownian motion 2005: Ann. Phys. 23
-3 7 .
4. Cosgrove, T.e.b., Colloid Science: Principles, methods and applications.
2005: Blackwel Publishing, UK.
5. Ugelstad, J., Hansen, F.K., and Lange, S., Emulsion Polymerization o f
Styrene with Sodium Hexadecyl Sulfate/Hexadecanol Mixtures as Emulsifiers
- Initiation in Monomer Droplets. Makromolekulare Chemie-Macromolecular
Chemistry and Physics, 1974.175(2): p. 507-521.
6 . Keddie, J.L., Film formation o f latex. Materials Science & Engineering R-
Reports, 1997. 21(3): p. 101-170.
7. Jotischky, H., Coatings, regulations and the environment reviewed. Surface
Coatings International Part B-Coatings Transactions, 2001. 84(1): p. 11-20.
8 . Qiu, J., Charleux, B., and Matyjaszewski, K., Controlled/living radical
polymerization in aqueous media: homogeneous and heterogeneous systems.
Progress in Polymer Science, 2001. 26(10): p. 2083-2134.
9. Asua, J.M., Miniemulsion polymerization. Progress in Polymer Science,
2002. 27(7): p. 1283-1346.
10. Lovell, P.A., El-Aasser, M.S.,, Emulsion Polymerisation and Emulsion
Polymers. 1997: John Wiley & Sons Ltd. UK.
11. Bardosova, M. and Tredgold, R.H., Ordered layers o f monodispersive
colloids. Journal of Materials Chemistry, 2002.12(10): p. 2835-2842.
12. Mellon, V., Synthesis and characterisation o f Waterborne Polymer/Laponite
Nanocomposite Latexes through Miniemulsion Polymerisation, in PhD
Thesis. 2009, CNRS-LCPP (Laboratory of Chemistry and Processes of
Polymerization), France.
42
Chapter 2
13. Bouanani, F., Bendedouch, D., Maitre, C., Teixeira, J., and Hemery, P.,
Characterization o f miniemulsion polymerization by small-angle neutron
scattering. Polymer Bulletin, 2005. 55(6): p. 429-436.
14. Landfester, K., Synthesis o f colloidal particles in miniemulsions. Annual
Review of Materials Research, 2006. 36: p. 231-279.
15. Antonietti, M. and Landfester, K., Polyreactions in miniemulsions. Progress
in Polymer Science, 2002. 27(4): p. 689-757.
16. Wu, X.Q., Schorlc, F.J., and Gooch, J.W., Hybrid miniemulsion
polymerization o f acrylic/allcyd systems and characterization o f the resulting
polymers. Journal of Polymer Science Part a-Polymer Chemistry, 1999.
37(22): p. 4159-4168.
17. Erdem, B., Sudol, E.D., Dimonie, V.L., and El-Aasser, M.S., Encapsulation
o f inorganic particles via miniemulsion polymerization. I. Dispersion o f
titanium dioxide particles in organic media using OLOA 370 as stabilizer.
Journal of Polymer Science Part a-Polymer Chemistry, 2000. 38(24): p. 4419-
4430.
18. Tiarks, F., Landfester, K., and Anonietti, M., Encapsulation o f carbon black
by miniemulsion polymerization. Macromolecular Chemistry and Physics,
2001.202(1): p. 51-60.
19. Everett, D.H., Basic Principles o f Colloid Science. 1988: The Royal Society
of Chemistry.
20. Attard, P., Recent advances in the electric double layer in colloid science.
Current Opinion in Colloid & Interface Science, 2001. 6(4): p. 366-371.
21. Erkseliu, S., Film formation from Dispersions - Preparation and
M echan ism s in PhD Thesis. 2006, Lund University, Sweden.
22. Theo, G.M. and Ven, V.D., Colloid Science. 1989: Academic Press Limited,
London.
23. Israelechvili, J., Intermolecular & Surface Forces. 1992: Academic Press
Limited, London.
24. Evans, D.F., Wennerstrom, H.„ The Colloidal Domain. Second edition ed.
1999: Wiley-VCH, New York.
43
Chapter 2
25. Bennett, G., Gorce, J.P., Keddie, J.L., McDonald, P.J., and Berglind, H.,
Magnetic resonance profiling studies o f the drying o f film-forming aqueous
dispersions and glue layers. Magnetic Resonance Imaging, 2003. 21(3-4): p.
235-241.
26. Vanderho.Jw, Bradford, E.B., and Carringt.Wk, Transport o f Water through
Latex Films. Journal of Polymer Science Part C-Polymer Symposium,
1973(41): p. 155-174.
27. Croll, S.G., Drying o f Latex Paint. Journal of Coatings Technology, 1986.
58(734): p. 41-49.
28. Eckersley, S.T. and Rudin, A., Mechanism o f Film Formation from Polymer
Latexes. Journal of Coatings Technology, 1990. 62(780): p. 89-100.
29. Kessel, N., Physical and chemical Aspects o f the film formation o f Self-
Crosslinldng Acrylic acrylic latex. 2007, University of Surrey, UK.
30. Vanderhoff, J.W., Tarkowski, H.L., Jenkins, M.C., and Bradford, E.B.,
Theoretical consideration o f the interfacial forces involved in the coalescence
o f latex particles. J. Macromol. Chem, 1966.1(2): p. 361-397
31. Dobler, F., Pith, T., Lambla, M., and Holl, Y., Coalescence Mechanisms o f
Polymer Colloids .1. Coalescence under the Influence o f Particle Water
Interfacial-Tension. Journal of Colloid and Interface Science, 1992. 152(1):
p. 1 - 1 1 .
32. Sperry, P.R., Snyder, B.S., Odowd, M.L., and Lesko, P.M., Role o f Water in
Particle Deformation and Compaction in Latex Film Formation. Langmuir,
1994.10(8): p. 2619-2628.
33. Lin, F. and Meier, D.J., A Study o f Latex Film Formation by Atomic-Force
Microscopy . l . A Comparison o f Wet and Dry Conditions. Langmuir, 1995.
11(7): p. 2726-2733.
34. Brown, G.L., Formation o f Films from Polymer Dispersions. Journal of
Polymer Science, 1956. 22(102): p. 423-434.
35. Keddie, J.L., Meredith, P., Jones, R.A.L., and Donald, A.M., Kinetics o f Film
Formation in Acrylic Latices Studied with Multiple-Angle-of-Incidence
44
Chapter 2
EIlipsometry and Environmental Sem. Macromolecules, 1995. 28(8): p. 2673-
2682.
36. Sheetz, D.P., Formation o f Films by Drying o f Latex. Journal of Applied
Polymer Science, 1965. 9(11): p. 3759-&.
37. Routh, A.F. and Russel, W.B., Deformation mechanisms during latex film
formation: Experimental evidence. Industrial & Engineering Chemistry
Research, 2001. 40(20): p. 4302-4308.
38. Routh, A.F. and Russel, W.B., A process model for latex film formation:
Limiting regimes fo r individual driving forces. Langmuir, 1999. 15(22): p.
7762-7773.
39. Salamanca, J.M., Ciampi, E., Faux, D.A., Glover, P.M., McDonald, P.J.,
Routh, A.F., Peters, A.C.I.A., Satguru, R., and Keddie, J.L., Lateral drying in
thick films o f waterborne colloidal particles. Langmuir, 2001. 17(11): p.
3202-3207.
40. Routh, F. and Russel, W.B., Horizontal drying fronts during solvent
evaporation from latex films (vol 44, pg 2088, 1998). Aiche Journal, 2002.
48(4): p. 917-918.
45
Chapter 3
Magnetic Resonance Profiling
Out of all the techniques used in this research work, magnetic resonance profiling
(MRP) was employed to collect the majority of data. Therefore this chapter will give
a description of MRP with its background, principles and theory.
3.1. Historical Background of MRP
The nuclear magnetic resonance (NMR) phenomenon was first reported by Rabi and
co-workers [1] in 1938 when they discovered it during an ion beam experiment.
They experimented with a beam of lithium chloride molecules passing through a
magnetic field and observed the resonance peaks of lithium and chlorine, hi 1940s
two groups of scientists observed the phenomenon of NMR in bulk matter. It was in
1946 when Purcell, Torrey and Pound [2, 3] succeeded in their attempts to detect the
NMR of protons in paraffin. Independently, dming the same time, Bloch’s group [4]
detected the NMR of protons in water. In 1952 Bloch and Purcell were awarded the
Nobel Prize for Physics, for their contribution to NMR. Since then NMR has become
a common and routine technique for scientists. It has been used to study solids,
liquids and gases, as well as various combinations of the three; as a few examples,
emulsions, biological tissues and gases or liquids diffusing through solid materials.
The first commercial NMR spectrometer was available in 1953.
Chapter 3
Throughout the years scientists in various disciplines have contributed to this field.
Swiss Physical Chemist Richard Ernst [5, 6] won the 1991 Nobel Prize in Chemistry
for his contribution to the development of high resolution NMR spectroscopy. Later,
the 2003 Nobel Prize in Medicine was awarded to Sir Peter Mansfield [7, 8] and
Lauterbur [9], for their independent development of NMR.
Magnetic resonance imaging (MRI) was developed from the knowledge gained in the
study of NMR. An MR profile is a one dimensional map of intensity of the NMR
signal as a function of position in one direction. An MR image is a two dimensional
picture created by several MR profiles of an object. MRI has developed most rapidly
in clinical diagnosis where the importance of non-invasive, non-ionizing, high-
resolution (tens of micrometers) imaging technique is most obvious. Since the first
commercial MRI scanner was developed in 1978, MRI has become one of the
routinely performed procedures in hospitals. Furthermore, MRI has become
increasingly popular with numerous applications, as the MRI technique is completely
non-invasive and it allows spatial resolution down to a few micrometers. NMR is
successfully used to study drug release [10], drying of polymer films [11-13], drying
of glue layers [14], drying and skin development in PSAs [15, 16], water distribution
in semicrystalline polymer layers [17], photo-initiated cross-linking in latex
dispersions [18, 19], behaviour of water in water-swollen cellophane films [20],
cement hydration [2 1 , 2 2 ] and many more.
Medical MRI is based on imaging the nuclei in mobile water, which seems to be
in all places in the human body. The image contrast comes usually from the level of
water mobility. If the water becomes less mobile, the imaging becomes more
difficult. The magnetic resonance line widths of mobile are narrow, and very high
gradients are needed to separate the resonances of less mobile nuclei [23]. hi
addition, NMR Image contrast is governed by one of several NMR parameters and
one can get information about water mobility, chemical potential, self-diffusion
coefficient, coherent flow or temperature, depending upon the exact form of the MRI
measurement [24]. Even though MRI has evolved into a complex, interdisciplinary
science over the years, it’s still young and growing.
47
Chapter 3
3.2. The Principles of NMR
NMR relies upon the fact that the nuclei of many atomic isotopes, including the
hydrogen proton, are magnetic and are extremely sensitive to the local magnetic
environment [23], Any nucleus that contains air odd number of protons and/or of
neutrons has a spin greater than zero. The most commonly used nuclei in NMR are
]H and l3C. In NMR experiments, the sample is placed in an external magnetic field
in which the nuclei can precess like a spinning top in a gravitational field. The
resonance frequencies of different nuclei are proportional to the applied magnetic
field [24].
When placed within an external magnetic field, a sub-atomic particle, such as an
atomic nucleus (e.g. 13C), with a magnetic moment adopts one of a set of allowed
configurations with respect to that field. Each allowed configuration corresponds to a
different energy level of the particle. The number of allowed energy levels is dictated
by the intrinsic (i.e. non-orbital) angular momentum or spin quantum number, /. I
may have any of the values 0, %, 1, 3/2, 2, 5/2........ The actual magnitude of the
intrinsic angular momentum, I of the particle is given by:
| I |= & V / ( / + l ) (3.1)
where, and A is Plank’s constant (h) divided by I n which is equal to 1.055 x 10"34 J.s,
and is called the reduced Planck’s constant. The reduced Planck’s constant is taken
as the unit of angular momentum [24].
When there is no external magnetic field, the magnetic dipole moments are randomly
oriented, hi this situation, the net magnetisation is zero. If an external magnetic field,
B0, is applied along the z-axis, the magnetic moments of the nuclei align with this
field, as shown in the Figure 3.1.
48
Chapter 3
A BFigure 3.1. Schematic illustration of randomly oriented magnetic dipole moments where (A) no external magnetic field, and (B) when external magnetic field, Bo, is applied, the orientations align with the field.
When an external magnetic field, Bo is applied along the z-axis, the z component of
the angular momentum or spin, 7Z, can have mtl values, where m is the magnetic
quantum number. The values for m = -7, - 7 + 1 , . . . , I - 1 , I and there are 27+1
possible values of m. This generates (27+1) number of energy levels and they are
known as the Zeeman energy levels. For instance, the hydrogen nucleus which has
only one proton with spin, 7 = Yz has two Zeeman energy levels: m = + Y z and m = - Yz.
These two orientations can be separated by the application of an external magnetic
field, Bo. This is known as the Zeeman effect, which splits each level into its
component states, each having a different value of m. Figure 3.2 illustrates the
splitting of the Zeeman energy levels occurs due to the application of a static
magnetic field Bo. The intensity of the transition which can be induced between those
two levels is proportional to the population excess of the lowest level compared with
the highest level. In a real sample, there could be around 1026 nuclei. These nuclei
split into energy levels due to the application of static magnetic field Bo, according to
Boltzmann distribution [25]:
N high - A E= e x p --------
kT (3.2)
49
Chapter 3
where Nhigh.iow are the populations of the high and low energy states respectively, AE
is the energy difference between two energy levels, k is Boltzmann constant and T is
the absolute temperature of the system [25].
Bt
▼ ▼m,= -1/2
A E = Y ftB 0
m,= +1/2
Figure 3.2. Schematic illustration of the splitting of the Zeeman energy levels due to the application of a static magnetic field Bo. The lower energy level has m = +V2 where magnetic moment is parallel to the external magnetic field, B0 and higher energy level has m = -Vi where magnetic moment is anti-parallel to B0 [25].
Charged particles with spin / gain a magnetic moment proportional to that spin:
ju = y h l (3.3)
where y is magnetogyric ratio for the particle and it is a constant for a given isotope:
‘H, 13C, 3iP. For'H , — = 42.58 xlO 6 Hz T' 1 [26].2 K
The energy for the Zeeman interaction between an external magnetic field, Bo and
such a magnetic moment is:
E = - / ' A (3.4)
For the external magnetic field Bo in the z direction, with the magnetic moment
operator inserted, this can be written as:
50
Chapter 3
E = - y h I 2B 0 (3.5)
The energy of the low (equation 3.6) and the high (equation 3.7) levels are given by
F v =- y h B ,
2 2
E _ + Jftgp -X 2
The two energy levels of the hydrogen proton are separated by:
(3.6)
(3.7)
A £ = (3.8)
This is the energy required to induce a transition from the lower to the higher energy
state. The energy separation of the states depends linearly on the strength of applied
magnetic field B0.
The transition of the nucleus between the two energy levels is accompanied by
absorption of a photon. The energy of this photon must exactly match the energy
difference between the two states, The energy, E, of a photon is related to its
frequency, co, by Plank’s constant, % .
A E = yhB0 = hco0
a>0 = rB o (3.9)
where is the Larmor resonance frequency. The energy difference is influenced by
the chemical nature of the nucleus; therefore protons in different chemical
environments can be identified separately, because of small differences in the local
magnetic field applied to the nucleus. This is the principle of NMR spectroscopy.
51
Chapter 3
3.3. MR Profiling using GARField magnet
A simple way of explaining how MRI works is that when a sample composed of
water molecules, which each contain two hydrogen nuclei (or protons), goes inside a
powerful magnetic field, these protons align with the direction of the field. When a
radio frequency (RF) electromagnetic field (pulse) is briefly turned on with the use of
an RF coil, these protons now alter their alignment relative to this RF field. When
this field is turned off the protons return to the original magnetisation alignment. The
signal which is created by these alignment changes is detected by the same RF coil.
A magnetic field gradient across the sample allows the distribution of the resonant
frequencies of the nuclei according to their position. Applying additional magnetic
fields (pulses) during the experiment allows an image of the sample to be built up.
For this research work, the GARField magnet was used to study the mobile lH in a
drying sample. The same setup can be used to study the mobile 13C or 31P in a
sample. 13C or 31P will have its own magnetogyric ratio, y. The frequency of the RF
pulse, which used to excite the chosen nuclei, should be adjusted accordingly to the
chosen isotope.
GARField, standing for Gradient At Right-angles to the Field, is a small permanent
magnet with shaped pole pieces [11, 27]. These pole pieces which are shown in
Figure 3.3 create a strong magnetic field gradient in the vertical direction (Gy in
Figure 3.4.) with a constant magnetic field (Bo in Figure 3.4.) in the horizontal
direction. The field Bo has a curvature of less than ±5 pm over a 5 x 5 mm region in
the sample area [27]. This magnet can be used to investigate a few tens of pm thin
films to hundreds of pm larger samples. The main advantage of the GARField
magnet over conventional solid-state NMR techniques is its ability to study samples
containing both solid and liquid components. More importantly, GARField is
particularly suitable for the investigation of planar, waterborne colloidal systems
during their various stages of drying [1 1 ].
52
Chapter 3
y [cm]6 -
4 - 2 “
0 -
-2 “
-4“
-6 -
Figure 3.3. Cross section through the centre of the magnet which shows the shape of the pole pieces and the sample location in relation to the magnets and the pole pieces. The RF coil is located directly below the sample location. Image taken from [27].
Magnetic resonance profiling relies upon the fact that the magnetic nuclei of atoms
(e.g. the hydrogen proton) precesses in a magnetic field at a localised resonant
frequency directly proportional to the field strength (equation 3.9). A magnetic field
gradient, Gy, leads to the resonant frequencies within the sample depending on the
position. The Larmor resonant frequency B)(y) now encodes position along the y-
direction as: [24].
a>(y) = yBn + yGv.y (3.i0)
In an experiment, if the Larmor resonant frequency is known, one can
calculate the position along the y-direction (y) using this equation.
I 1--------------1-------------- 1
NdFeBr 0 2 6 12 m a g n e t
z[cm]
Pole Piece
Samplel o c a t i o n
53
Chapter 3
As shown in Figure 3.4, a latex sample (or other film) is horizontally placed on the
locator tape above the radio-frequency (RF) coil. This RF coil is used to excite and
detect the MR signal via a current arising from the transient response of the nuclei to
a resonant RF stimulus. The size of the RF coil is of the order of 3 mm in diameter.
The shaped pole pieces of the GARField magnet ensures the optimization of
profiling through thin, planar samples. The static magnetic field, B0, of 0.7 T, is
parallel to the sample plane. The excitation field, Bi is due to the RF coil. The
dBmagnetic field gradient, Gy = — - which is approximately 17.5 T/m, is
dy
perpendicular to the sample plane [1 1 ].
GravityBo
7Sample
Gy Bi Profile
A A A
Coverslip. \\ Locator tape
Height
Intensity
RF Coil
Figure 3.4. Schematic diagram of the GARField magnet. The magnetic field gradient, Gy, is perpendicular to the direction of the magnetic field, Bo [11]. B\ is the magnetic field generated by the planar radio frequency (RF) coil. The profiles of the sample are a measure of the intensity of the magnetisation signal as a function of the height of the sample, in the direction of the gradient which is shown on the right hand side.
For this research work, 20 mm x 20 mm and 180 pm thick glass cover-slips were
used as the substrate. The sample was cast on these cover-slips to get the desired
thickness. As a rough guide, around 70 pi of 50 wt. % solid content latex would
produce a 300-400 pm thick wet film. Access to the magnet pole piece to place the
sample is from above. Immediately after the casting, the wet sample on a glass
54
Chapter 3
cover-slip was placed inside the magnet. NMR profiles were obtained under room
temperature and humidity.
The NMR signal is obtained from an excitation using a quadrature echo sequence:
90x-x-(90y-x-echo-x-)„ [11, 23]. Consider a coordinate set rotating at a given
frequency on the x-axis. In this frame, following a 90x° pulse, all the magnetisations
lie along the y-axis. After a time (pulse gap) x, a 90 y° pulse is used. The 2x runs from
the centre of one pulse duration to the centre of the next pulse duration. After a
second x, all the magnetisations once again come along the y-axis, forming an echo.
(90y~x-echo-x-) is repeated n times to get n number of echos.
90x-x-(90y-x-echo-x-)„ is repeated NS (number of scans) times to average each
echo train. Each echo is recorded with SI number of points, and the time between
two of these points is the dwell time, dw. The intensity o f the successive echo
profiles follow an exponential decay described by the time constant 1/T2. T2 is the
spin-spin relaxation time which is sensitive to the mobility of the water. For instance,
T2 is longer for free water and shorter for polymer particles. As a result, the intensity
of each GARField profile recorded during an excitation sequence is differently
weighted to water and polymer particles [11]. Between each excitation pulse
sequence, a lapse of time, which is relaxation delay, RD is left to allow the system to
return to its equilibrium. Usually RD is of the order of three times Ti. Ti is the spin-
lattice relaxation time. For a typical latex experiments, n is 32, x is 75 ps, dw is 0.7
ps, SI is 256, NS is 32, RD is 3.5 s and the spectrometer frequency, SF, is 29.6 MHz.
These echoes are in the time domain (intensity as a function of time) and they are
Fourier transformed to get the signal in the frequency domain (intensity as a function
of frequency), hence position is determined using equation 3.10. The resulting data
are added together to obtain a profile of signal intensity as a function of position.
The resolution is calculated by [24]:
A r = ---------l- (3.11)G x y x SI xdw
55
Chapter 3
By increasing SI and dw one can increase the resolution, but it would significantly
increase the time taken to obtain an echo. This extra time is multiplied by n and NS
times for every obtained profile. Doing so, one could lose vital information on a
drying experiment, as the number of profiles that can be obtained would be reduced.
In addition, the resolution can be increased by increasing G.
An increase in gradient will decrease the field of view of the image ( Az), according
to [24]:
AQAz = — — (3.12)
G x y
where AQ is the bandwidth of the pulse. A smaller field of view has a very practical
importance. The thickness of the initial film should be smaller than the field of view
to obtain profiles which represent the whole of the film. The range of frequency -
and hence the maximum thickness - is also influenced by -7 - [27].dw
For each and every experiment, the time delay between profiles was changed as
desired. By choosing the same MR parameters, direct comparisons of profiles for
different samples are allowed. In order to correct for the decline in sensitivity over
the film thickness, the profile shapes are normalised by an elastometer standard.
56
Chapter 3
3.4. A typical GARField Profile
a>a£3</)a>*-•(0■4-*</>J33(/)
250 300
Thicknns of dry film HtiihtGim)
Initial thicknoss ofthofilm
Figure 3.5. Typical GARField profiles of a model acrylic latex (coating), where each profile was taken in 5 minute intervals. The arrow shows the increase of the drying time. On right is the film-air surface (top of the film) and on left is the film- substrate surface (bottom of the film).
A typical GARField profile is a map of the NMR signal-intensity as a function of the
height of the film, as shown in Figure 3.5. The time of the first profile was defined as
0 minutes and in this example the profiles were taken in 5 minute intervals. As the
drying time increases, the height (thickness) of the film decreases, mainly due to the
evaporation of water in the film. The film thickness at a given drying time can be
measured along the X-axis. On the other hand, as the drying time increases, the
intensity of the NMR signal decreases, and this is proportional to the density of
mobile water in the sample. When the intensity of the signal does not decrease as
drying time increases, it is assumed that all the mobile water has evaporated. The dry
film thickness can be calculated for the samples with known solids content. As an
example, if the initial film thickness of a sample is 300 pm and the solids content is
50 wt. %, then the dry film thickness is around 150 pm, assuming % voids are zero.
57
Chapter 3
More recently, a 3D waterfall type of plot has been introduced, where drying time is
plotted along the third-axis. The intensity of the NMR signal has been normalised to
the initial intensity, as shown in Figure 3.6. This ensures that the first profile is
relatively uniform. For latex with a low Tg, after the film reached its final thickness,
the NMR signal intensity is still very high as there is molecular mobility in the
polymer melt and one can obtain a NMR signal from this mobile polymer. Figure 3.5
shows GARField profiles of a coating formulation, where the intensity of the signal
drops to zero. Figure 3.6 shows GARField profiles of an adhesive formulation, where
intensity of the signal does not drop to zero, as GARField is sensitive to the mobility
of soft polymer particles. In an adhesive formulation, it is assumed that the film is
dry when the thickness and the intensity do not decrease with increasing drying time.
Figure 3.6. Typical GARField profiling of a latex with low Tg (adhesive) using 3D waterfall type of plotting.
The moments of statistics provide model-free information about a distribution and
they have been used to analyse the NMR profiles obtained from drying latex films.
The definition of the z'-th moment is given by [28]:
Height(gm)
N
(3 .1 3 )
58
Chapter 3
where N is the number of the points of a distribution, Xn is the value of the
independent variable, (which is the vertical position in an experiment), AX is the
pixel spacing, and In is the value of the distribution at Xn (NMR intensity in our case)
[29]. In the literature, the normalized moments are often used, and normalisation is
done by dividing the moment by the zero-th moment, m0. The definition of the
normalized z-th moment is given by [28]:
N
Y l „ ( X „ -ml)
where m* is the normalized first moment. To calculate the normalized first moment,
m* is set to zero in this equation [29].
The physical meanings of the first five moments are given in Table 3.1 [29].
Table 3.1. The meanings of the first five moments [29]
m0 integral
*mx average
m*2 variance
m \ / / skewness
r * / q-3 mV * kurtosis
59
Chapter 3
Software was used to calculate these moments using NMR data obtained by the
GARField magnet [29]. The zeroth moment, m0, which is the integral of a
distribution or the area under the NMR profile, is proportional to the water content of
the sample at the time when the profile was taken. The knowledge of the initial film
thickness and the solids fraction allows the zeroth moment to be converted to water
mass per unit area of the film. At the point where the zeroth moment no longer
changes over the time, it was assumed that the water content was zero. (Note that a
signal is often obtained from the polymer.) The plots of the zeroth moment and the
thickness as a function of time give an indication of any changes in drying rate [29].
When studying drying, it is important to know if the film dries homogeneously. For
homogeneous drying, the content of the mobile water within the drying film is
uniform with depth from the surface. Therefore, the NMR profile is symmetric. On
the other hand, for non-uniform drying, the NMR profile is asymmetric. Using the
same software, the skewness of each NMR profile from the second and third
normalised moments (m *2 a n d ) has been calculated. The skewness was used as a
measurement of non-homogeneity of the drying film [29].
60
Chapter 3
3.2. References:
1. Rabi, I.I., et al., A new method o f measuring nuclear magnetic moment.
Physical Review, 1938. 53(4): p. 318-318.
2. Pound, R.V., and Purcell, E.M., Measurement o f Magnetic Resonance
Absorption by Nuclear Moments in a Solid. Physical Review, 1946. 69(11-1):
p. 681-681.
3. Purcell, E.M., Torrey, H.C., and Pound, R.V., Resonance Absorption by
Nuclear Magnetic Moments in a Solid. Physical Review, 1946. 69(1-2): p.
37-38.
4. Bloch, F., Hansen, W.W., and Packard, M., The Nuclear Induction
Experiment. Physical Review, 1946. 70(7-8): p. 474-485.
5. Ernst, R.R., Nuclear Magnetic Double Resonance with an Incoherent Radio-
Frequency Field. Journal of Chemical Physics, 1966. 45(10): p. 3845.
6 . Ernst, R.R., and Anderson, W.A., Sensitivity Enhancement in Magnetic
Resonance .2. Investigation o f Intermediate Passage Conditions. Review of
Scientific Instruments, 1965. 36(12): p. 1696.
7. Mansfiel.P., and Grannell, P.K., NMR Diffraction in Solids. Journal of
Physics C-Solid State Physics, 1973. 6(22): p. L422-L426.
8 . Mansfield, P., and Grannell, P.K., Diffraction and Microscopy in Solids and
Liquids by NMR. Physical Review B, 1975.12(9): p. 3618-3634.
9. Lauterbur, P.C., Image Formation by Induced Local Interactions - Examples
Employing Nuclear Magnetic-Resonance. Nature, 1973. 242(5394): p. 190-
191.
10. Hyde, T.M., Gladden, L.F., and Payne, R., A Nuclear-Magnetic-Resonance
Imaging Study o f the Effect o f Incorporating a Macromolecular Drug in
Poly (Glycolic Acid-Co-Dl-Lactic Acid). Journal of Controlled Release, 1995.
36(3): p. 261-275.
11. Gorce, J.P., et al., Vertical water distribution during the drying o f polymer
films cast from aqueous emulsions. European Physical Journal E, 2002. 8(4):
p. 421-429.
61
Chapter 3
12. Salamanca, J.M., et al., Lateral drying in thick films o f waterborne colloidal
particles. Langmuir, 2001. 17(11): p. 3202-3207.
13. Ciampi, E., et al., Lateral transport o f water during drying o f alJcyd
emulsions. Langmuir, 2000.16(3): p. 1057-1065.
14. Bennett, G., et al., Magnetic resonance profiling studies o f the drying o f film-
forming aqueous dispersions and glue layers. Magnetic Resonance Imaging,
2003. 21(3-4): p. 235-241.
15. Mallegol, J., et al., Skin development during the film formation o f waterborne
acrylic pressure-sensitive adhesives containing tacldfying resin. Journal of
Adhesion, 2006. 82(3): p. 217-238.
16. Mallegol, J., et al., Origins and effects o f a surfactant excess near the surface
o f waterborne acrylic pressure-sensitive adhesives. Langmuir, 2002. 18(11):
p. 4478-4487.17. Ciampi, E., and McDonald, P.J., Skin formation and water distribution in
semicrystalline polymer layers cast from solution: A magnetic resonance
imaging study. Macromolecules, 2003. 36(22): p. 8398-8405.
18. Wallin, M., et al., Depth profiles o f polymer mobility during the film
formation o f a latex dispersion undergoing photoinitiated cross-lin/dng.
Macromolecules, 2000. 33(22): p. 8443-8452.
19. Hellgren, A.C., et al., New techniques fo r determining the extent o f
crosslinldng in coatings. Progress in Organic Coatings, 2001. 43(1-3): p. 85-
98.
20. Laity, P.R., et al., Structural studies and diffusion measurements o f water-
swollen cellophane by NMR imaging. Cellulose, 2000. 7(3): p. 227-246.
21. McDonald, P.J., et al., A unilateral NMR magnet fo r sub-structure analysis in
the built environment: The Surface GARField. Journal of Magnetic
Resonance, 2007.185(1): p. 1-11.
22. McDonald, P.J., et al., Two-dimensional correlation relaxometry studies o f
cement pastes performed using a new one-sided NMR magnet. Cement and
Concrete Research, 2007. 37(3): p. 303-309.
23. McDonald, P.J. and Newling, B., Stray field magnetic resonance imaging.
Reports on Progress in Physics, 1998. 61(11): p. 1441-1493.
62
Chapter 3
24. McDonald, P J ., Stray field magnetic resonance imaging. Progress in Nuclear
Magnetic Resonance Spectroscopy, 1997. 30: p. 69-99.
25. Canet, D., Nuclear Magnetic Resonance : Concepts and Methods. 1996: John
Wiley & Sons.
26. Hore P. J., Nuclear Magnetic Resonance 1995: Oxford University Press.
27. Glover, P.M., et al., A novel high-gradient permanent magnet for the
profiling o f planar films and coatings. Journal of Magnetic Resonance, 1999.
139(1): p. 90-97.
28. Konig, A.M., Weerakkody, T.G., ICeddie, J. L., Johannsmann, D.,
Heterogeneous Drying o f Colloidal Polymer Films: Dependence on Added
Salt. Langmuir, 2008. 24(14): p. 7580-7589.
29. Konig, A.M., Light Scattering and Magnetic Resonance Imaging -
Investigation o f the Film Formation Process o f Latexes. 2007, Clausthal
University of Technology: Clausthal-Zellerfeld.
63
Chapter 4
Physical Characterisation of Film Formation
and Film Properties of Organic/Inorganic
Nanocomposite Coatings
4.1. Introduction
In the earliest years of the polymer industry, inorganic fillers were used as extending
agents to reduce the cost of polymer-based products. Soon fillers were recognized to
be an integral component in many applications involving polymers, particularly for
reinforcement purposes. The reinforcement efficiency of inorganic fillers is strongly
related to their aspect ratio (diameter/thickness) therefore platelet-like fillers have
drawn significant interest. Among the different platelet-like fillers, a huge amount of
work has focused on clays [1 ].
Polymer/clay nanocomposites were first reported by Blumstein in 1965, when he
demonstrated the polymerisation of vinyl monomers with Montmorillonite (MMT)
[2, 3]. In 1992, the researchers at the Toyota Central Research laboratories reported
that the incorporation of small amounts o f MMT into nylon-6 resulted in a hugely
Chapter 4
surprising enhancement of thermal and mechanical properties of the resulting
nanocomposite material [4], Their results have drawn a remarkable renewal of
interest both academic and industrial, to this class of material; particularly due to
their enhanced thermal, chemical, mechanical and rheological properties [5-8], By
combining inorganic fillers with organic polymers, one can expect to combine the
inorganic properties with the properties of polymers, such as flexibility and
processability. Furthermore, the fact that a small amount of clay minerals could
cause a significant increase in properties makes these organic/inorganic hybrids
specially attractive to industry [9]. Advantages of clays are that they have active sites
such as hydroxyl groups and exchangeable interlayer cations [10, 11]. In addition,
the small dimensions of the individual layers and the high aspect ratio of clay
minerals render them particularly attractive in several areas of materials science [8].
In this current research work Laponite RD was chosen as the inorganic material to be
incorporated into the organic polymer nanocomposites.
Laponite RD is a synthetic disk-shaped clay which is available in very high purity
and constant quality. The discs are around 30 nm in diameter and around 1.2 nm in
thickness. It has been reported that the surface area is ca. 800 m2/g, with a density of
2.57 g/ml. The listed negative surface charge density is 0.014 e7A2, and the reported
hydroxyl group concentration is 0.36 rneq/g determined by titration with triethyl
aluminium [8 , 12, 13]. Like most of the clays, Laponite can be easily modified. Pre-
treatment is usually required in order to improve its compatibility with a polymer
matrix and to achieve a good dispersion. In addition, clay minerals also have a high
surface area to volume ratio, due to their small size. As a result, when Laponite is
incorporated into an organic matrix to form nanocomposites, the contact between the
two phases is increased and the properties of the resulting material are expected to be
improved [9].
The structure of Laponite consists of two-dimensional layers with a central sheet of
Mg306 octahedra (O) sandwiched between two external sheets of Si(0 ,OH)4
tetrahedra (T), in a stacking pattern of ‘T-O-T’. The tetrahedral shaped units (Figure
4.1 (a)) serve as the basic structural components of the tetrahedral sheets (Figure 4.1
65
Chapter 4
(b)). Each of these tetrahedral units consists of a central four-coordinated silicon
atom surrounded by four oxygen atoms. These oxygen atoms are linked with other
nearby silicon atoms, thereby serving as inter-unit linkages to hold the sheets
together. The octahedral-shaped units (Figure 4.1 (c)) serve as the basic structural
components of the octahedral sheets (Figure 4.1 (d)). Each of these octahedral units
consists of a central six-coordinated magnesium atom surrounded by six oxygen
groups, and once again, are linked with other nearby magnesium atoms, thereby
serving as inter-unit linkages to hold together the sheet [7, 9]
• and o = silicon; O and O = oxygen, hydroxyl
(c) (d)# magnesium; O and O = oxygen
Figure 4.1. (a) The structure of a single tetrahedral shaped unit of a central four-coordinated silicon atom surrounded by four oxygen atoms and (b) the resulting tetrahedral sheet, (c) is the structure of an octahedral unit with a central six- coordinated magnesium atom surrounded by six oxygen groups and (d) the resulting octahedral sheet. Image taken from [14].
Figure 4.2 shows a schematic illustration of the ‘T-O-T’ stacking pattern. Laponite
can swell in water by incorporation of water between these sheets. This swelling
capacity is reversible, meaning that Laponite can absorb or lose water depending on
the pressure and temperature of its media.
66
Chapter 4
Tetrahedral Sheet
Octahedral Sheet
Tetrahedral Sheet
Figure 4.2. A schematic illustration of the ‘T-O-T’ stacking pattern, where one octahedral sheet is sandwiched between two tetrahedral sheets (on the left). The same stacking pattern can be found in Laponite structure. On the right is the crystalline structure of a Laponite clay disk. Image taken from [7, 9].
An isomorphic substitution, which is the replacement of one atom by another of
similar size in a crystal lattice without disrupting or changing the structure, of Si4̂ by
Al3+, for instance, in the outer tetrahedral sheet will generate a negative charge on the
Laponite particle. Similarly, Al3+ in the central octahedral sheet can be substituted by
Mg2+ and also could generate a negative charge in the interlayer region. This excess
of negative charge adsorbs cations to the clay surface, in a similar way to how a
magnet attracts iron filings, and they are exchangeable. This phenomenon gives clay
minerals an important and interesting property, which is its cation exchange capacity
or CEC. The CEC is defined as the number in moles of monovalent cations that can
be substituted to counterbalance the negative charge of 100 grams of clay. Typical
CEC values are in the range of 60-120 meq/lOOg [15]. The Laponite used in this
study has a CEC of 75 meq/lOOg [7, 16]. CEC is a constant for a given clay mineral
and does not depend on pH. However, there is a second cause of charge on the
Laponite particles due to Si-OH (acid), Mg-OH and Al-OH (basic) groups at the
edge of the layers. The Laponite used in this study has a hydroxyl group
concentration of 36 meq/lOOg [16]. The hydroxyl group concentration depends on
pH. Depending on the pH of the media, the clay particles are positively or negatively
charged. In acidic media positive species are predominant, and in basic media it is
negatively charged species that are predominant. The acido-basic properties of these
sites can be explained by using the equilibrium equations shown in Figure 4.3.
10A
67
Chapter 4
X— OH + H+ T_ X— OH2+
X— OH ^ X— 0" + H+
Figure 4.3. Acido-basic equilibrium equations: The top equation is for acidic media, where positive species are predominant and the equilibrium shifts to the right, and as a result, clay particles are positively charged. In the basic media, negatively charged ions are predominant and according to the second equation, the resulting clay particles will be negatively charged.
In the literature, it was reported that several research groups proposed a variety of
clay modifications for nanocomposite synthesis. These treatments for clays are
performed in order to optimise the dispersion or to chemically link the polymer
matrix and the clay [1]. Three main clay modification systems have been reported:
cation exchange, silane grafting, and adsorption of polar polymers. In cation
exchange, the structural inorganic cations of the clay interlayer are exchanged by
organic cations that can bring, in some cases, a reactive or functional group. These
functional groups further improve chemical compatibility between the mineral and
the organic metrix. The amount of organic cations that can be exchanged depends on
the amount of the exchangeable sites, hence it depends on the CEC and the structure
of the clay [1]. In silane grafting, the hydroxyl groups on the edges of the clay plates
are used to covalently bond organosilanes onto the clay surfaces. Organosilanes are
molecules with the general formula of RnSiX4.11, where R represents the organic part
of it, and its functional group (for example acrylate, styrene etc.) will serve as an
anchor for the organic matrix to produce a covalent bond. X is a hydrolysable group
(for example halogen, allcoxy etc.). Organosilanes are known to improve the material
properties, such as chemical adhesion, wetting, rheology and mechanical resistance;
hence they are widely used in optics, coatings and catalysis [1,9, 17, 18]. It has been
reported that some polar* polymers adsorb onto the clay surfaces via hydrogen
bonding and van der Waals attraction forces [19, 20].
Clay modifications have further developed over the years. In 2006, Negrete-Herrera
and co-workers [7] reported the synthesis of clay-armoured latex particles via
emulsion polymerisation. In the process, they used Laponite which was organically
68
Chapter 4
modified through cation exchange by using 2 ,2 -azobis (2 -methylpropionamidine)
hydrochloride (AIBA) as the polymerisation initiator or 2-(methacryloyoxy) ethyl
trimethyl ammonium chloride used as a comonomer. These modified Laponite clays
were then dispersed in the water phase prior to emulsification with the monomer
phase. Cauvin, Colver and Bon [21] successfully synthesised Laponite armoured-
latex particles of polystyrene via a Pickering-stabilised miniemulsion polymerisation.
Sun and co-workers [22] developed a novel polystyrene-encapsulated Laponite
system via miniemulsion polymerisation. The Laponite particles were modified
through cation exchange using an ammonium salt, cetyltrimethylammonium bromide
(CTAB). Then the modified clay was mixed in the monomer phase prior to
emulsification with the water phase. In Figure 4.4, the Transmission electron
microscopy (TEM) image reveals the irregular coarse-edged latex particles
containing Laponite clay discs.
Figure 4.4. TEM image of polystyrene latex particles containing Laponite clay. Image taken from [22].
In 2006 Tong and Deng [23] synthesised polystyrene nano-saponite suspension via
miniemulsion polymerisation. They showed that the stability of the suspension and
the intercalation degree of the clay strongly depended on the pre-treatment of
saponite with the cationic surfactant, octadecyltrimethylammonium bromide
(OTAB). In addition, they argued that the pre-treatment process not only intercalated
69
Chapter 4
the clay layers but also converted the nanoclay particles from being hydrophilic to
hydrophobic. It is clear from the TEM image (Figure 4.5 left) that when unmodified
saponite was used in the polymerisation, the saponite particles were segregated
together in the polystyrene matrix. However, it was shown, that when saponite
particles were pre-modifid with OTAB, they were intercalated and uniformly
dispersed in the polystyrene matrix (Figure 4.5 right).
Figure 4.5. TEM images of polystyrene saponite composite films; on left with unmodified saponite and on right with saponite modified with OTAB. Image taken from [23].
4.1.1. Surface tension, surfactant concentration and CMC
It is well known that in the bulk of a condensed phase, the intermolecular forces act
between the molecules (or atoms) in a symmetric fashion. At the interface or surface,
there is an imbalance of the forces as the local chemical environment changes. This
imbalance in forces results in a surface tension, y . This surface tension acts to
minimise the surface area [24]. All surfactants possess the common property of
lowering the surface tension when added to water. Pure water has a surface tension
of 72 mN/m and gradually decreases with increasing concentration at low
concentrations and is a linear function of the logarithm of surfactant concentration
70
Chapter 4
until it reaches the critical micelle concentration, CMC of the surfactant. At
concentrations well below CMC, the hydrophobic ends of the surfactant molecules
orient with their hydrophilic tail in the water. As concentration of surfactant
increases, more and more molecules come to the surface. When the surface is
saturated with the hydrophobic ends of the surfactant, it is called a ‘monolayer’. At
the same concentration, micelles form. Micelles occur when a group of hydrophilic
ends surrounds their hydrophobic ‘tails’ and shield them from water. Now with the
increase of the surfactant concentration, the surface tension does not decrease. This
inflection point is called critical micelle concentration, CMC. Figure 4.6 illustrates a
typical plot of surface tension as a function of the logarithm of concentration of
surfactant [25-30].
Figure 4.6. Schematic illustration of surface tension as a function of the logarithm of the surfactant concentration. Surface tension of pure water or at 0 log concentration is 72 mN/m and the inflection point where surface tension no longer decreases with the increase of surfactant concentration is the CMC of a given surfactant.
4.1.2. Marangoni Flow
The Marangoni effect, first identified as ‘tears of wine’ by physicist James Thomson
in 1855, was later named after the Italian physicist Carlo Marangoni, a decade later.
Marangoni flows are those induced by a surface tension gradient generated either by
71
Chapter 4
a composition or a temperature variation along a free liquid surface [31-35], Xu and
Luo [36] reported that in an evaporation process, the nonuniform evaporation from
the liquid/vapour interface could produce a temperature gradient, which in turn, it
should generate a surface tension gradient along the liquid surface. They argued that,
this surface tension gradient usually drives a convective Marangoni flow inside the
liquid. Fanton and Cazabat [35] studied the spreading of liquid films driven by
surface tension gradients induced by evaporation from a mixture with two
components. Many researchers have studied the Marangoni effect over many years,
as it plays a key role in coating, thin film deposition, crystal growth and production
of photonic materials [37-39].
4.1.3. Overview of Nanocomposite Materials
For the current study, miniemulsion polymerisation was used to synthesise
polymer/Laponite latex dispersions. Two systems were adopted. First, using the fact
that the latex particles are the replica of the miniemulsion droplets, it was possible to
encapsulate the clay plates, if clay plates are initially dispersed inside the monomer
droplets. For this approach, the clay plates were functionalised with an organophilic
treatment in order to be compatible with the monomer phase. On the other hand,
clay-armoured latex particles were synthesised via miniemulsion polymerisation, by
anchoring a free initiator to the clay surface through cation exchange. This
modification gives the clay particles an amphiphilic status. This encouraged
subsequent clay interaction with the miniemulsion droplets. Then the clay with
attached radicals nucleated reactions in the nano-droplets, which then progressively
formed polymer particles with an inner polymer core surrounding an outer shell of
clay sheets [1]. Taking aboard these techniques, two systems of polymer/clay
(Laponite) nanocomposites were developed. They were both synthesised by
miniemulsion polymerisation at the CNRS-LCPP (Laboratory of Chemistry and
Processes of Polymerisation), Lyon, France, one of the academic partners of the
NAPOLEON Project. These two systems are hereafter referred to as Route I and
Route II [1, 16].
72
Chapter 4
In Route I, the clay was modified, using the cationic initiator AIBA, which was
incorporated in the Laponite layers through cation exchange. This modification gave
the Laponite plates a reactive ability to initiate the polymerisation process at the
surface of the Laponite plates [40]. Initially, the cation exchange was done at 100%
of CEC. These AIBA modified Laponite plates were then dispersed in water with
surfactant. The monomer, methyl methacrylate (MMA) and butyl acrylate (BuA)
were mixed with a hydrophobe, Behenyl acrylate (BylA) and poured into the clay
suspension. The polymerisation was carried out at 70 °C for three hours under
nitrogen flow. As Laponite carries the initiator, the polymerisation starts from the
clay surface and it is expected that the Laponite plates are located on the surface of
the droplets and therefore on the surface o f the polymer particles (Figure 4.7 left).
73
Chapter 4
Route I S LaPonite Powder/ ........... v
Functionalisation I exfoliation ^ 4
Route II
% ^ ^
* * •
A * *Water —
* * * * *
* * * * * *
Monomer(s) 'T T "
^ M iniemulsion Polym erisation ^
• 3 *
Polymerparticles
Laponitediscs
Water
Figure 4.7. Schematic illustration of Route I and II. In Route I (on the left), Laponite plates were functionalised with the cationic initiator, AIBA and dispersed into the water phase. In Route II (on the right) double functionalised Laponite by MPTMS and DDAB, was dispersed in the monomer phase. After the miniemulsion polymerisation in Route I it is expected that the clay will be at the surface of the latex particles, whereas in Route II, the clay will be encapsulated within the latex particles. Image taken from [41].
In Route II, an organosilane coupling agent, y-Methacryloxypropyl-trimethoxysilane
(y-MPTMS), was grafted to the edges of the Laponite plates. Laponite was dispersed
in the monomer phase. In addition, didodecyldimethylammonium bromide (DDAB)
[42], an alkyl ammonium surfactant, was integrated into the basal faces of the clay
through cation exchange. This double modification was carried out in order to render
a good compatibility to the Laponite plates with the monomer mixture. The double
74
Chapter 4
functionalised Laponite plates were dispersed in the monomers and hydrophobe. In
the resulting emulsion, the polymerisation was started by an injection of 4’-azobis (4-
cyanopentanoic acid) (ACPA) and was carried out at 70 C for three hours under
nitrogen flow. As the double modified Laponite plates show good compatibility with
the monomers, they are expected to remain inside the monomer droplets; thereafter
inside the polymer particles. Route II is shown in Figure 4.7 (right).
Figure 4.8 shows the chemical structure of (a) y-MPTMS and (b) DDAB molecules
and (c) a schematic of the double functionalised Laponite disc.
(a)
0 (C H 2)3-S i(0 C H 3) 3
O
MeO—
Figure 4.8. (a) the chemical structure of y-MPTMS molecule, (b) the chemicalstructure of the DDAB molecule and (c) a schematic diagram of a double functionalised Laponite disc with y-MPTMS molecules grafted on its edges and DDAB molecules on its surface.
Figure 4.9 shows cryo-TEM images of composite latex particles of (a) Route I, and
(b) Route II. In Figure 4.9 (a) dark filaments can be clearly seen at the surface of the
polymer particles and these are interpreted as being the clay plates. In (b) the
Laponite is encapsulated by the polymer phase as indicated by the dark lines within
75
Chapter 4
the particles [16]. Schematic representation of polymer particles by Route I and
Route II are shown by (c) and (d) respectively.
(d)
Figure 4.9. Cryo-TEM images of polystyrene/Laponite nanocomposite particles by Route I (a) and Route II (b). Images were taken from [43]. (c) and (d) are the schematic representations of the polymer particles by Route I and Route II, respectively.
76
Chapter 4
4.2. Materials
4.2.1. Latexes by Route I
The latexes were prepared at the CNRS-LCPP (Laboratory of Chemistry and Process
o f Polymerisation), Lyon, France, by Veronique Mellon. Methyl methacrylate
(MMA) and butyl acrylate (BuA) were used as monomers in a 50:50 weight ratio to
synthesise the nanocomposite latexes by miniemulsion polymerisation. A series of
latexes was prepared with 0 wt. % (pure latex - for comparison), 3 wt. %, 5 wt. %
and 7 wt. % Laponite based on monomers. For Route I, modified Laponite with
ABBA initiator, at 100% of the Cation Exchange Capacity (CEC) was used. In all
cases the solids content was around 20 wt. %. Table 4.1 summarises the different
characteristics of Route I latexes.
Table 4.1. Characteristics of Polymer/Laponite nanocomposite latexes synthesised through miniemulsion polymerisation with ABBA functionalised Laponite for Route I.
Monomer AIBA
(%
CEC)
Laponite
(wt.%)
Droplet
size
(nm)
Particle
size
(nm)
Conversion
(% )
MMA/BuA 0.3 0 119 83 82
MMA/BuA 10 0 3 119 1 0 1 73
MMA/BuA 1 0 0 5 116 104 75
MMA/BuA 10 0 7 149 128 96
77
Chapter 4
4.2.2. Latexes by Route II
For Route II, MPTMS-DDAB twice functionalised Laponite was used. The initiator
was ACPA (4,4’-azobis(4-cyanopentanoil acid); 0.5 wt. % based on monomers were
used. Again, the solids content for all cases was around 20 wt. %, and Table 4.2
summarises the different characteristics of Route II latexes.
Table 4.2. Characteristics of Polymer/Laponite nanocomposite latexessynthesised through miniemulsion polymerisation with MPTMS/DDABfunctionalised Laponite for Route II.
Monomer MPTMS/
DDAB
(% CEC)
Laponite
(wt.%)
Droplet
size
(nm)
Particle
size
(nm)
Conversion
(% )
MMA/BuA 0 0 152.5 97.4 96.4
MMA/BuA 2 0 0 3 133.5 97.0 96.4
MMA/BuA 2 0 0 5 116.3 105.4 80.9
MMA/BuA 2 0 0 7 167.0 129.2 51.9
In Table 4.1 and Table 4.2, the droplet size represents the size of the monomer
emulsion droplets before polymerisation and the particle size represents the size of
the polymer particle after the polymerisation. It is widely accepted that the droplet
size and the number should remain close to the size of particles during
polymerisation. A 1:1 ratio indicates there is neither monomer diffusion nor
coalescence before and during polymerisation, so that most nanodroplets keep their
identity while being converted into particles. If the droplet and particle sizes are
equal, the number of particles (Np) and the number of droplets (Nd) are also equal,
that is to say the ratio of (Np /Nd) should be close to one. It is widely accepted that the
Np /Nd ratio in the range of 0.8-1.2 is considered reasonable. If the Np /Nd ratio is
greater than one, then it indicates that the droplets are breaking up as a result o f a
secondary nucleation. On the other hand, if Np /Nd ratio is less than one, it indicates
78
Chapter 4
droplet coalescence, which results in bigger particles [9]. In the tables, the
conversion as a % represents the amount of monomer being converted into polymer.
The target normally is to reach close to 100% conversion. If the conversion is 96.4%,
it means that 3.6% of the monomer remains unpolymerised in the resulting product
after the polymerisation process.
79
Chapter 4
4.3. Techniques
4.3.1. Magnetic Resonance Profiling (MRP)
For MRP experiments, 20 mm x 20 mm and 180 pm thick glass cover-slips were
used as the substrate. The latex was applied on to the coverslips to get the desired
thickness. As a rough guide, around 70 pi of 50 wt.% solid content latex would
produce a 300-400 pm thick wet film. As soon as the latex was cast on to the glass
coverslip, it was placed inside the magnet and NMR profiles were taken at room
temperature and ambient humidity. The magnetic field, Bo, was 0.7 T when the
measurements were performed. The gradient strength, Gy, was approximately 17.5
T/m. The NMR signal is obtained from an excitation using a quadrature echo
sequence: 90x-x-(90y-x-echo-x-)n [44, 45]. For typical latex experiments, the
number of echoes, n, is 32; the pulse gap, x, 75 ps; the dwell time, DW, is 0.7 ps; the
number of points per echo, SI, is 128 and the spectrometer frequency, SF, is 29.6
MHz. For each and every experiment, the time delay between profiles and the total
drying time was changed as desired. The same MR parameters were used for all
experiments, so that direct comparisons of profiles for different samples were
allowed. The profile shapes were normalised by an elastometer standard in order to
correct for the decline in sensitivity over the film thickness.
4.3.2. Optical Transmission Measurements
Optical transmission measurements of the films were carried out with a
spectrophotometer (Campsec 350, Cambridge, UK). The resulting films from the
MRP experiments were used to measure the optical transmission at a wavelength of
600 nm.
80
Chapter 4
4.3.3. Profilometry
Thickness variations of the dried films were investigated using a stylus profilometer
(Dektalc 8 Surface Profiler by Veeco Instrument Limited, Cambridge, UK). The
Dektak Surface Profiler is an advanced surface texture measuring system that
accurately measures surface texture below the sub-micrometer scale. The instrument
has a vertical range of 5 nm to 0.262 mm. A scan length of up to 50 nun can be
scamied within 3-200 seconds and a vertical resolution of 4 nm for a 0.262 mm range
scan. Veeco Instruments Limited claims that the Dektak 8 Surface Profiler combines
high repeatability, low-force sensor technology which allows easier measurement of
soft materials and characterization of sub-micron lines and spaces. The advanced 3D
data analysis with a high aspect-ratio tip is also ideal for measuring shallow as well
as deep structures. The result of the profilometry is a map of the dry film’s thickness
as a function of lateral distance. As the maximum distance, which is from one end to
the other end of each film, varies between samples, the distance was normalised by
dividing each sample’s distance by it’s maximum distance and multiplied by 10 0
Photographs of the drying film were taken using a camera on a contact angle
analyser (Kruss Easy Drop Standard, Drop Shape Analysis System by Kruss GmBH,
Hamburg, Germany). The instrument uses a monochrome interline CCD 752 X 582
pixel camera with a 6.5x zoom. The system is capable of automatic recording of a
sequence of images at defined time intervals. The samples were prepared by using
around 70 pi of the dispersion applied on to a 20 mm x 20 mm glass coverslip, the
same way as for the GARField experiments. The automated recording system was set
up to record the images at desired time intervals. Figure 4.10 shows a photograph of
the setup.
[46].
4.3*4. Methods for obtaining photos of the films
81
Chapter 4
Sampleplatform Light
Figure 4.10. The Easy Drop Standard setup, which was used to take the photos from the side of a drying film. The image was taken and modified from [47].
82
Chapter 4
4.4. Results and Discussion
4.4.1. Original Route I and Route II
4.4.1.1. MR Profiling and Images from side
Figure 4.11 (a) compares a series of profiles obtained from 0 wt.% Laponite latex
synthesised by Route I to a series obtained with latex containing 3 wt.% Laponite,
prepared by Route I (Figure 4.11 (b)). It was found that in the pure latex (0 wt.%
Laponite), the film thickness decreased at a constant rate and there was no sign of
skin-like layer forming on the top layer of the film. Both films show non-uniformity
in the profiles at the later stages of drying. There is no significant difference in the
drying profiles for the two types of films. Photographs from the side of the films also
appear very similar for the two types of latex dispersions.
Figure 4.11. MR profiles of (a) 0 wt.% Laponite latex, synthesised by Route I and (b) 3 wt.% Laponite latex, synthesised by Route I over time. The profiles were obtained every five minutes. In both cases the thickness decreases at a constant rate, and non-uniformities in water concentration in the vertical direction do not develop until the later stages of drying.
o.o0 100 200 300 400
Height (11m)100 200 300 400
Height(tim)
Method 1 (AIBA) with 3% Laponite
1 .2 -[
0.8
Figure 4.12 shows a series of photographs of a drying film of 0 wt.% Laponite -
Route II latex. The film was cast on to a 2 cm x 2 cm x 0.18 mm glass coverslip. The
photos were taken from the side of the film and only the left half of the film is
83
Chapter 4
shown. The drying time of each photo is indicated on the left. Initially, the thickness
of the wet film decreases over the time. About 30 minutes into drying, the drying
front proceeds from the edge of the film, resulting in a thinner film near the edges.
These two processes simultaneously continue till the film is completely dried.
60 min.
70 min. — ■ ■ — i=a—
80 min. — — M
90 min. — ■■■
100 min. u
110 min. ------------
120 min. - — ,
Figure 4.12. Series of photographs taken from the side of a drying film of 0 wt.% Laponite - Route II latex. The drying times are indicated in the figure.
In comparison, the GARField profiles (Figure 4.13) and photographs taken from a
side view of a drying film (Figure 4.14) of 3 wt.% Laponite - Route II latex,
however, are noticeably different. The film thickness of the sample, determined by
MR profiling, initially decreases with no gradient in water concentration. That is, the
water concentration within the film from top to bottom is more or less the same.
After about 60 minutes into drying, the film thickness increases in the central region
of the film and simultaneously a gradient in the water concentration develops. At this
stage, images from the side (shown in Figure 4.14) confirm that the edge regions
continue to thin over time. As drying continues the film thickness decreases in the
central regions.
0 min.
10 min.
20 min.
30 min.
40 min.
50 min.
84
Chapter 4
Film thickness(iam)
Figure 4.13. The GARField profiles of the latex with 3 wt.% Laponite by Route II. The Film thickness initially decreases with no non-uniformities or gradient in water concentration in the vertical direction. At a later stage, the film thickness increases over time and simultaneously develops gradient in water concentration.
In order to describe the drying process, it was divided into four stages as follows:
Stage 1: The film thickness decreases at a constant rate. No gradient in water
concentration is found.
Stage 2: In the film centre, the change in film thickness slows and then the film
thickness is constant over time. A drying front simultaneously proceeds from the
edge of the film, and the film thickness becomes thinner near the edges.
Stage 3: The thickness in the centre of the film increases over time, as the edge
regions continue to thin over time. The contact angle between the central region and
the edge region increases over time. A gradient in water concentration develops.
Stage 4: The film appears dry near the edges, and the central region decreases in
thickness.
These four stages were shown in Figure 4.14, in a series o f photographs and in
Figure 4.15, with the GARField profiles.
85
Chapter 4
0 min.
10 min.
20 min
30 min.
40 min.
5 0 min. ___ „
60 min. __
iSta je 1
7 0 min.
80 min.
90 min.L
10 0 min.
1 10 min.
12 0 min.
Stajc 2
13 0 min.
14 0 min
St<age 3
Stqge 4
Figure 4.14. Series of photographs taken from a side view of the drying film of 3 wt.% Laponite - Route II latex on a 2 cm x 2 cm glass substrate. The drying times are indicated on the figure. Only the left-half of the film is shown.
Photographs in Figure 4.14 confirm at the end of Stage four that the centre looks
slightly thicker. One can argue that the vast amount of lateral flow is water, and with
it some polymer particles could have flowed as well. A ridge is observed near the
edge of the film.
86
Chapter 4
0 100 200 300 400 500» •*» « *•*"»> Height (^m)
Figure 4.15. A series of MR profiles obtained at five minute intervals, from 3 wt.% Laponite sample by Route II, showing the four stages of drying; Stage 1 - film thickness constantly decreases, no water concentration gradient, Stage 2 - film thickness decrease slows down to a constant thickness, Stage 3 - film thickness increases in the centre and water concentration gradient develops, Stage 4 - film thickness decreases in the central region. Arrows show the direction of increasing drying time.
A similar type of drying process was found in the other Route II nanocomposite
samples containing Laponite at higher concentrations (5 wt.% and 7 wt.%). As a
simple means to compare them, the thickness was normalised by dividing by the
initial thickness, and the drying time was normalised by dividing by the total drying
time. The resulting normalised film thickness as a function of normalised time is
shown in Figure 4.16 for dispersions of 0, 3, 5 and 7 wt.% of Laponite by Route II.
Film thickness increases are only seen in the dispersions that contain Laponite -
prepared by Route II - but not in the pure latex.
87
Chapter 4
Normalised Time
Figure 4.16. Comparison of normalised film thickness as a function of normalised drying time for latexes with various Laponite concentrations by Route II. The thickening effect is stronger with 3 wt.% Laponite, but it is still noticeable with 5 wt.% and 7 wt.% Laponite concentrations.
In order to understand the causes of this effect, experiments were conducted to
determine separately the effects of the latex solid content and the surface tension.
The solids content was increased in latex prepared by Route I and II by evaporating
the water. Known amounts of latex samples in open glass bottles were left on the
shaker, until they reached the desired solid content. The viscosity of the dispersion
was simultaneously increased by this procedure. Figure 4.17 shows the effect of
raising the solids content up to 30 wt.% in the latex with 3 wt.% Laponite by Route
II. After the solids was increased up to 30 wt.%, it was possible to see that the
thickness decreases at a relatively constant rate, although there is some non
uniformity in the water distribution.
88
Chapter 4
Figure 4.17. Comparison of (a) GARField profiles for the as-received latex of 3 wt.% Laponite by Route II to profiles for (b) the same latex when its solids content was raised to 30 wt.%, by evaporating water.
Servoxyl [29], a commercial wetting agent, containing an anionic surfactant, was
added to adjust the surface tension and the spreading of the film on the substrate.
Servoxyl was received as a 35 wt.% solution in water from Elementis Specialities.
0.75 g of Servoxyl 35 wt.% was added to every 50 g of solids in the latex.
The time-dependence of the thickness of the latex with 3 wt.% Laponite by Route II
is compared in Figure 4.18 for four formulations. The effects of solids content and
wetting agent were determined individually and together. It was seen that raising the
solids content to 30 wt.% (and hence increasing the viscosity) has a pronounced
effect in leading to a constant rate of thickness decrease. The addition of wetting
agent (Servoxyl) leads to a slightly more uniform rate, but to a lesser extent.
Film thicknesslttm)
89
Chapter 4
Normalized drying time
Figure 4.18. The normalized thickness as a function of the normalized drying time of the latex with 3 wt.% Laponite by Route II. Results are shown for as-received latex (black), latex with a higher solids content of 30 wt.% (red), as received latex with added Servoxyl (blue), and for latex with a 30 wt.% solids content plus added Servoxyl (green).
4.4.1.2. Optical Transmission
A transparent sample is one that transmits light so as to render the objects beyond it
perfectly visible. At the other extreme, an opaque sample transmits no light, and all
information about the objects lying behind it is lost. A translucent sample lies
between these two extremes. They transmit a part of light, but diffuse, scatter or
absorb some of it, so that the objects beyond it are not readily visible [48, 49]. Three
factors could affect the optical transparency of the films studied in this work: the
scattering from the second phase, the scattering by the surface due to the surface
roughness and the absorption by the material. First of all, the optical transparency
could be reduced by the effects of the light scattering when films contain phases of
differing refractive indexes, with a size of the second phase far below than the
wavelength of light (600 nm). van Tent and Nijenhuis [48] have shown that smaller
the second phase, or in their case the pore sizes, the more transparent the film. By
increasing the pore size, the transmission would drop dramatically.
90
Chapter 4
Secondly, the surface roughness could also decrease optical transparency. If the
surface of the sample is perfectly smooth, the incident beam is split into a reflected
ray, (which is transmitted back) and a refracted ray which is transmitted into the
sample, upon incidence. If the surface is not smooth, it scatters light upon incidence.
In this case, in addition to the reflected and refracted rays, a significant amount of
light now propagates in multiple directions. If the surface is perfectly smooth or
glossy, then reflected rays predominate. If the surface is not smooth, the multiple
direction propagated rays predominate. Then the sample is commonly described as
having haze, haziness, milkiness or cloudiness [49].
Finally, absorption by the material could also reduce the optical transparency. For
instance, coloured films absorb certain wavelengths of light.
The transmitted light intensity can be calculated by the following equation [49]:
I = I 0exp"'" (4.1)
where I is the transmitted light intensity and Io is the incident light intensity and x is
the film thickness, p, the absorption coefficient, is a property of the material. It is
mathematically defined as [49]:
4 71M ~ — ' k (4,2)
where k is the extinction coefficient.
Optical transmission measurements were carried out to indicate the film quality. The
optical transmission of the films made from Route I and Route II with 0, 3, 5 and 7
wt.% of Laponite concentrations were measured. The latex films on glass coverslips
from the GARField experiments were used to measure the optical transmission. The
dry film thicknesses were between 300 - 500 pm. Figure 4.19 shows the
transmission (%) as a function of Laponite content (wt.%) for both Route I and Route
II. The graph shows that the increase in the concentration of Laponite did not
91
Chapter 4
decrease the transparency for the films from Route I latex, but there was a significant
drop in transparency in Route II films with increased Laponite concentration.
0 1 2 3 4 5 6 7 8 Laponite content ( wt.%)
Figure 4.19. Optical transmissions (at a wavelength of 600 nm) as a function of Laponite content for the films by Route I (red) and Route II (blue).
Furthermore, the transmission of the films by Route II - as received and with 30
wt.% solids and added Servoxyl - was measured as a function of Laponite
concentration. The intention was to identify if by raising the solids content and
adding Servoxyl there was an effect on the film quality of Route II latexes. The
results are shown in Figure 4.20. According to the graph, raising the solids content
and adding Servoxyl appears to further reduce the transparency for Route II latexes.
92
Chapter 4
Laponite content ( wt.%)
Figure 4.20. Optical transmission (at a wavelength of 600 nm) as a function of Laponite concentration for Route II latex as-received (red) and after raising the solids to 30 wt.% and adding Servoxyl wetting agent (blue).
Out of the causes given above, it is true that the films with Laponite plates have
materials with different reflective indexes, but the sizes of the second phase are very
much smaller than the wavelength of the light. Therefore, it is reasonable to rule out
the effects of Laponite plates or clusters made with few plates on transparency
variations. On the other hand, increased concentrations of Laponite in films could
have affected the absorption coefficient, which ultimately affects the intensity of the
transmitted light. In addition, thickness variations in the films also affect the intensity
of the transmitted light. The dry film thicknesses were between 350 - 400 pm. The
most significant factor affecting the transparency should be the film/air surface
roughness. From the photographs of the films (not shown), it was clear the surfaces
of the Route II samples were more rough or had more wrinkle-like unevenness
compared with Route I films. In addition, Route II samples were cloudier compared
with Route I films. This would give a clear explanation for the results shown in
Figure 4.19. The transmission variations between the samples with different Laponite
concentrations should have been the differences in the absorption coefficients, which
depend on the material. Different absorption coefficients should have been the key to
the results shown in Figure 4.20. By raising the solids and adding Servoxyl, the
93
Chapter 4
absorption coefficients of these films should have been affected. It is difficult to
explain the reasons for transmission trends which are shown in Figure 4.20.
4.4.2. Effect of the DDAB content on Route II latexes
As stated earlier, in Route II latex, Laponite plates were doubly functionalised by
MPTMS and DDAB before being incorporated into monomer droplets and
polymerised. The amount of DDAB content was 200% CEC of Laponite. There are
several advantages of higher DDAB content in a system, such as, better stability of
the monomer/clay dispersion, consequently a more stable latex and also a higher
monomer conversion and lower secondary nucleation [9]. As the amount of DDAB
in the system was two times the CEC, one can argue that DDAB was forming a
double layer on the clay surface (Figure 4.21 (a)). Furthermore, some of the DDAB
molecules which are adsorbed via their hydrophobic tail could migrate to the
monomer droplet/water interface (Figure 4.21 (b)). Then they could diffuse through
the water phase and form surfactant aggregates (Figure 4.21 (c)). It should be
mentioned that aggregates (micelles) would only be formed at high concentrations -
above CMC. They might form when water is evaporated and concentration increases.
Laponite disc
MPTMS DDAB
(a)
94
Chapter 4
(b)
(c)Figure 4.21. Schematic illustration of double functionalisation of a Laponite disc by 200% CEC of DDAB and MPTMS molecules, (a) A formation of a double layer of DDAB molecules on the surface of Laponite disc, (b) migration of excess DDAB molecules to the monomer droplet/water interface and (c) migrated DDAB molecules diffuse to the water phase and form surfactant aggregates when water is evaporated.
95
Chapter 4
Based on the above mentioned argument, the following process was put forward to
explain the drying behaviour of Route II acrylic Laponite nanocomposite latex.
1. The initial polymer particles with encapsulated Laponite discs arguably have
excess DDAB molecules at their surface (Figure 4.22 (a)).
2. In the first stages of drying, the evaporation of water brings the particles into
close packing at the edges of the film[50] (Figure 4.22 (b)).
3. At the edges of the film, due to the desorption of the excess DDAB, free
DDAB molecules can be found in the serum (Figure 4.22 (c)).
4. As desorption of DDAB mainly occurs at the edges of the film, a surfactant
concentration gradient could develop within the drying film. Higher DDAB
concentration at the edge of the film and a relatively lower concentration in
the central regions of the film develops (Figure 4.22 (d)). This concentration
gradient causes a lateral surface tension gradient within the drying film; a
lower surface tension at the edges and a higher tension in the central regions
develops.
5. There is a driving force to reduce this surface tension gradient. This force
naturally causes a flow of liquid away from the low surface tension regions.
In the literature, this mechanism is identified as the ‘Marangoni flow’ [31-34,
36]. Due to the Marangoni flow, one could expect a flow of water within the
drying film from the edges to the central regions (Figure 4.22 (e)).
6 . As evaporation of water from the drying film continues, this process
continues.
96
Chapter 4
InitialParticle packing at theedge of the film Desorption of D D A B :
N so u rce of DDAB at the edge only*
(a) _ (b)4mh • * i * •
DDAB
/Marangoni flow of surfactant ^
M ,
ly - Surface tension gradient develops
h i g h e r ylo w y lo w y
(e) (d)
Figure 4.22. Explanation of drying behaviour of the latexes by Route II. (a) Initial polymer/Laponite particle, where excess DDAB molecules stick out of the particle, (b) Due to water evaporation polymer particles are closely packed near the edges of the drying film, (c) Desorption of excess DDAB molecules to the surrounding serum, (d) Development of a DDAB concentration gradient, which causes a surface tension gradient between the edges and the centre of the film, (e) Marangoni flow of liquid from low to high surface tension regions. This process could continue as long as the drying continues.
To test this idea, new polymer/Laponite nanocomposite samples were synthesised by
miniemulsion polymerisation at the CNRS-LCPP. For the new formulation,
compared with the ‘original' Route II formulation, only 100% CEC of DDAB with
MPTMS was used to twice functionalise the Laponite plates. Then this twice
functionalised Laponite was incorporated into the monomer phase prior to
polymerisation. Two new samples were synthesised both with around 3 wt.%
Laponite, but one sample with a lower solid content (19.2 wt.%) and the other with
97
Chapter 4
relatively higher solid content (29.6 wt.%). Table 4.3 summarises the components of
the three formulations.
Table 4.3. Components and characteristics of the ''original Route II and new formulations (less DDAB/low solids and less DDAB/high solids).
Original New formulation New formulation
Route II / low solids / high solids
Amount of DDABCEC-cation exchange
capacity
2 x CEC* CEC* CEC*
Amount of DDAB
As a % of monomer11.42 x 10'3 5.71 x 10‘3 5.71 x 10"3
Solids content (wt.%) 2 0 19.2 29.6
ACPA as initiator Yes Not known Not known
Surfactant (wt.%) 2 1.7 1
Hydrophobe wt.%) 6 Not known Not known
Droplet size (nm) 133.5 93.6 1 1 2 . 1
Particle size (nm) 97 90.4 132.6
Conversion (%) 96.4 95.4 8 8 .1
Laponite content
based on monomers3 2 .8 3.4
4.4.3. Comparison of drying properties of ‘original’
and new polymer/Laponite latexes
4.4.3.1. MRP of ‘original’ and new polymer/Laponite
latexes
Two problems were identified with the ‘original’ Route II, 3 wt.% Laponite latex.
The thickness increased in the film centre and vertical non-uniformity in water
98
Chapter 4
distribution developed ( Figure 4.13). The latexes prepared with the new formulation,
where only 100% CEC of DDAB was used to functionalise the Laponite plates,
clearly show an improvement in drying properties. With less DDAB with low solid
content (19.2 wt.%) the sample shows a steady decrease in film thickness over time,
but develops non-uniformity in the water distribution towards the end of the drying
(Figure 4.23 (a)). The high solid content (29.9 wt.%) sample showed the most
improved drying properties; during drying the thickness decreases steadily over time
and the water distribution was uniform in the depth of the film (Figure 4.23 (b)).
Dry ng time (Mn)
(Profiles were taken every 3 rrin, total ctying time 132 rrins)
100 200 300 400 500
99
Chapter 4
50 100 150 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 Height (pm)
Figure 4.23. Drying profiles of: (a) new formulation with less DDAB and low solids content (19.2 wt.%), the film thickness decreases steadily over time and the water distribution was non-uniform towards the end of drying, (b) New formulation with less DDAB and high solids content (29.9 wt.%), the film thickness decreases steadily over time and the water distribution is uniform in the depth of the film.
As a simple means to compare the drying profiles of these samples, the thicknesses
were normalised by dividing by the initial thicknesses, and the drying times were
normalised by dividing by the initial thickness. Doing so, the effects of initial
thickness variations were eliminated. The resulting normalised film thickness as a
function of normalised drying time is shown in Figure 4.24 for these three samples.
Compared to the large thickness increase with the original Route II sample, the
samples made with the new formulation showed better drying properties. While the
low solids sample still showed a slight thickness increase, the high solids sample
illustrates the best drying properties of all three samples as it showed hardly any
thickness increase.
100
Chapter 4
Normalised Drying Time (min/ ̂m)
Figure 4.24. Normalised film thickness as a function of Normalised Drying Time for original Route II (blue), new formulation with less DDAB and low solids (red) and less DDAB with high solids (black). All three samples contained around 3 wt.% Laponite.
Furthermore, moments of statistics (zeroth moment and skewness) were used to
analyse the MRP data. The zeroth moment of a drying profile is proportional to the
water content of the sample (within the area of the RF coil) at a given time. The
skewness is a measurement of non-uniformity of a drying profile. New formulation
samples show a gradual decrease in the normalised zeroth moment or the water
content of the samples. The original Route II sample was unusual as it shows a
significant increase in the zeroth moment half away through the drying process
(Figure 4.25 (a)). This can be correlated with the thickness increase in the centre of
the sample.
101
Chapter 4
a>NTJo(fl15EL .oz
3> 0.6o>c| 0.5CO
0.4
0.3
0.2
0.1
0.0
• ■■■■■■
0.00 0.05 0.10 0.15 0.20 0.25
Normalised Drying time (minI p m)
0.30
Figure 4.25. (a) Normalised zeroth moment and (b) Skewness of original Route II nanocomposite (blue triangles), new formulation low solids (red circles) and high solids (black squares). All three samples contained around 3 wt.% Laponite.
When comparing the skewness of drying profiles, note that the higher the maximum
skewness, then the more non-uniform the drying is. Out of these three samples, the
high solids sample using the new formulation showed the most uniform drying and
has the lowest skewness of all three (Figure 4.25 (b)). However, the original Route
II sample and the new formulation with low solids sample do not follow this trend.
102
Chapter 4
4.4.3.2. Visual appearance of the ‘original’ and new
polymer/Laponite latexes
The visual appearance of the films cast for the GARField experiments were further
investigated by taking photographs from above. Figure 4.26 shows the photographs
of the films made from (a) the original Route II, (b) less DDAB and low solids
content dispersion and (c) less DDAB and high solids content dispersion. By
comparison of the photographs, it is revealed that the quality of the films with the
new formulation where less DDAB was used, is higher than the original Route II
films. To the naked eye, the films made from the new formulation with a high solids
sample showed the least irregularities out of all three samples. The lateral flow of
liquid in the original Route II film could have caused the wrinkle-like irregularities.
(a) (b) (c)
Figure 4.26. Comparison of the photographs of the films made by (a) original Route II, (b) new formulation with a low solids and (c) new formulation with a high solids dispersions. The original Route II sample illustrates the most irregularities on the film/air interface compared with the other two samples.
4.4.3.3. Optical transmission of ‘original’ and new
polymer/Laponite latexes
The quality of the films obtained by the GARField experiments was studied by
comparison of optical transmission at a wavelength of 600 nm. The optical
transmission of the three samples is illustrated in Figure 4.27.
103
Chapter 4
100
80
gc.2 60 w(A
E(A
« 40t-
20
0
Figure 4.27. Comparison of optical transmission of the films obtained from original Route II (red), new formulation with low solids (green) and new formulation with high solids (blue) nanocomposites at a wavelength of 600 nm.
The film made from the original Route II shows the least transmission, whereas the
other two samples transmit significantly more light. As mentioned earlier, in section
4.4.3.2, in comparison of their visual appearances, the film made from the original
Route II showed the most irregularities. Therefore it is expected the film made from
the original Route II would show the lowest transmission, as explained in section
4.4.1.2. Between these three films, the surface roughness or irregularities should
have been the predominant reason for the transmission variations. The original Route
II film should has the most scattering from the surface in multiple directions, hence
the least transmission.
As all three films contained 3 wt.% Laponite, scattering from the different second
phases and the Laponite’s contribution to the absorption coefficient should be more-
or-less the same. It is significant that the new formulation latexes contained less
surfactant DDAB. The original Route II samples contained excess DDAB, and it is
possible that this excess surfactant clustered together, to be large enough to scatter
light. In the new formulations only one times CEC of DDAB was used, as a result,
they lack this possible effect created by excess DDAB. This could be another
104
Chapter 4
explanation for the films prepared using new formulations to have higher
transmission, compared with the lower transmission of the original Route II film.
4.4.3.4. Thickness variations of the films of ‘original’ and
new formulation polymer/Laponite latexes
To further investigate the proposed argument of the original Route II samples having
excess DDAB in the system which leads to the development of a concentration
gradient within the drying film and flow of liquid, profilometry was used to study the
thickness variations across the dry films. With the Marangoni flow of surfactant,
water and polymer particles also could flow. This polymer particle movement could
result in a thickness variation across the dry films. A Dektak 8 stylus profiler was
used to investigate surface texture and the thickness of the dry films.
The thickness variation of the film from the original Route II exhibits more of a
“Mexican Hat” shape (Figure 4.28 (a)) in comparison to the other two films (Figure
(b) and (c)). This is evidence of inward flow of latex during drying, as a result of the
surface tension gradient development. The films obtained from the new formulations
also have a lower edge, but the increased thickness in the middle of the films was
noticeably missing. These thickness variations could have caused the visual
differences and the transmission differences observed in sections 4.4.3.2 and 4.4.3.3.
105
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Figure 4.28. Thickness variations (film thickness as a function of normalised distance, i.e. distance divided by total lateral distance) of the films by (a) original Route II, (b) new formulation with low solids and (c) new formulation with high solids using profilometry. It is significant that the original Route II sample (a) has a ‘Mexican Hat’ shape, in comparison to the other two films.
The proposed process in Figure 4.22 illustrates the situation when the wet film is
convex. Due to the evaporation of water, the particles at the edge of the film pack
into closed pack array. At the edges of the film, excess surfactant desorption into the
serum results in a low surface tension. Relatively high surface tension remains in the
middle of the film. This surface tension gradient induces Marangoni flow of
surfactant, polymer particle and water inwards; hence we observed a ‘Mexican Hat’
shape thickness variation in the dry film. If some how we manage to produce a
concave wet film (Figure 4.29 (a)), particles should start to pack into close-packed
array from the centre of the film. Now, there will be excess surfactant desorption in
the centre. As a result low surface tension could develop in the centre, compared to a
relatively high surface tension at the edges of the film (Figure 4.29 (b)). As a result,
now Marangoni flow should occur from the centre to the edge of the film (Figure
4.29 (c)). If this argument is valid, the dry film should have a relatively thin centre
and be slightly thicker somewhere between the centre-to-edge region (Figure 4.29
(d)).
106
Chapter 4
(a) (b)
(c) (d)
Figure 4.29. Schematic illustration of a (a) concave wet film, (b) low surface tension at the centre and high surface tension at the edges of the drying film, (c) Marangoni flow from centre to the edges, (d) dry film with relatively thin centre.
To test this idea, a mould was prepared by using poly(methyl mathacrylate)
(Perspex) as shown in Figure 4.30. The cylindrical reservoir for the wet latex is 20
mm in diameter and 0.2 mm deep. The mould was treated for 30 minutes in UV in
order to make the surface hydrophilic [51]. Into this reservoir, about two drops of
wet latex were applied, so that all the edges of the reservoir were covered with latex.
The sample was left to dry in ambient conditions and it was observed that the film
was starting to dry from the middle. Once the film was completely dry, its surface
was examined by the profilometry.
Figure 4.30. Illustration of the Perspex mould with the 20 mm diameter and 0.2 mm deep cylindrical reservoir to obtain a concave wet film.
107
Chapter 4
The surface variations of the dry concave films are presented in Figure 4.31. The
vertical axis is vertical distance and it’s somewhat different to the film thickness.
This was due to the way the profilometry was done on the samples. The scans were
done from one wall of the reservoir to the other wall across the centre of the reservoir
to obtain the hills and the valleys of the surface. It is clear from the similarity of
Figure 4.31 (b) and (c), that the new formulation films have thin centres and a
relatively smooth gradual decrease in thickness from the edge of the film. In
comparison, the original Route II film (Figure 4.31 (a)) also has a thin centre, but the
decrease in thickness is not a smooth gradual decrease. It is significant that there is a
circular ridge about 20 - 30% of the radial distance from the edge.
Figure 4.31. Surface variations (vertical distance as a function of normalised, i.e. distance divided by total lateral distance) of the dry films obtained by initially concave wet films of (a) original Route II, (b) new formulation with low solids and (c) new formulation with high solids using profilometry.
First of all, these results confirm that both of the new formulation films show a
similar pattern. The films are relatively thin in the centre, due to the concave wet
film; there is a smooth gradual thickness decrease from edge to the centre as there
was no lateral flow of any kind. In comparison, the original Method II film also has a
relatively thin centre, as one could expect from a concave wet film, but what’s
significant is the film thickness variation from the edge to the centre. One can easily
identify the thickness increase somewhere around 20 - 30% of the radial distance
from the edge the edge of the film on both sides. The other profilometry scans done
on this sample (not shown) confirm that this thickness increase is consistent all-
around the film. A possible explanation to this result is that original Method II wet
108
Chapter 4
latex had excess DDAB in the system, and as the wet film was concave, particles
first started to pack into a close-packed array at the centre. This would develop an
excess surfactant desorption in the centre which would lead a low surface tension in
the centre of the drying film compared to the edges of the film. As a result,
Marangoni flow should direct from the centre to the edge of the drying film.
Profilometry of the dry film confirms the thickness increase somewhere between
centre and the edge of the film. The consistency of the thickness increase in all
directions across the film confirms that the Marangoni flow occurs from centre to the
edges.
109
Chapter 4
4.5. Conclusions
Physical characteristics o f film formation and film properties of organic/inorganic
nanocomposite coating formulations were studied. Samples from two main systems,
original Route I and original Route II, were examined. Each system comprised four
latexes with different Laponite contents (0, 3, 5 and 7 wt.%). In Route I latexes,
Laponite plates were located on the surface of the polymer particles and in Route II
latexes, the clay particles were encapsulated within the latex particles.
The drying measurements and the photographs from the side of drying films confirm
that the original Route I, pure polymer (0 wt.% Laponite) sample and the original
Route I with Laponite samples dry with no significant differences. In comparison,
the original Route II latexes with Laponite showed thickness increases in the centre
duiing drying, and 3 wt.% Laponite sample showed the strongest effect. It was
argued that the excess DDAB (two times CEC), which was used to fimctionalise the
Laponite plates before incorporation into the monomer droplets, caused this
thickening effect. Based on this argument, two new Route II latexes were developed
with less DDAB (only 100% CEC of Laponite) in the system, one latex with lower
solid content (19.2 wt.%) and the other having a relatively higher (29.6 wt.%) solid
content. During drying, the new formulation samples were uniform vertically and
showed no lateral flow. The optical transmission and structure of the films of original
Route II were affected by the lateral flow of liquid.
As it was argued that the excess DDAB in the original Route II latexes was the cause
for the thickening effect; the excess DDAB could have created a double layer on the
clay surfaces and some of it could have diffused through to the water phase.
Desorption of DDAB was more significant when the particles were closely packed.
As a result, a wet convex film develops a surfactant concentration gradient within the
drying film, with a higher concentration at the edges of the film and a lower
concentration towards the centre of the film. This causes a lateral surface tension
gradient, with a higher surface tension in the centre and lower surface tension
towards the edges of the drying film. The gradient induces a Marangoni flow of
110
Chapter 4
liquid from low to high surface tension regions (Figure 4.22). Thickness variations of
the dry films confirm a ‘Mexican Hat’ shape figure for the original Route II, 3 wt.%
Laponite sample. Further investigation of concave wet films confirmed this
argument. In a concave wet film, particles should closely pack at the centre before
the edges of the drying film. As a result, lower surface tension now should develop in
the centre and Marangoni flow is from the centre towards the edges of the drying
film. It was confirmed by profilometry, that the thickness increased in the middle of
the radial distance in the concave wet film made from the original Route II sample.
The film obtained from the original Route II sample showed the least transmission
and most irregularities on the film/air surface compared with the films from the new
formulation dispersions. It was suggested that the surface irregularities, the internal
structure of the film, and the affected absorption coefficient could have contributed
to the low transmission of the film obtained from the original Route II sample.
It was proven that excess surfactant in a system can affect the physical characteristics
of the film formation process, and by adjusting the amount o f surfactant in the
system the film characteristics can be improved.
I l l
Chapter 4
4.6. References
1. Faucheu, J., Gauthier, C., Chazeau, L., Cavaille, J-Y., Mellon, V., Bourgeat-
Lami, E.„ Short review and recent advances in clay/polymer nanocomposites
obtained by miniemulsion polymerisation. submitted.
2. Blumstei.A, Polymerization o f Adsorbed Monolayers .2. Thermal
Degradation o f Inserted Polymer. Journal of Polymer Science Part a-General
Papers, 1965. 3(7pa): p. 2665-&.
3. Blumstei.A, Polymerization o f Adsorbed Monolayers J. Preparation o f Clay-
Polymer Complex. Journal of Polymer Science Part a-General Papers, 1965.
3(7pa): p. 2653-&.
4. Kojima, Y., Usulti, A., Kawasumi, M., Olcada, A., Fukushima, Y., Kurauchi,
T., and Kamigaito, O., Mechanical-Properties o f Nylon 6-Clay Hybrid.
Journal of Materials Research, 1993. 8(5): p. 1185-1189.
5. Kickelbiclc, G., Schubert, U, Synthesis, Functionalization and Surface
Treatment o f Nanoparticles; Baraton, M.-I., Ed. 2002: American Scientific
Publishers, CA, USA.
6 . Bourgeat-Lami, E., Organic-inorganic nanostructured colloids. Journal of
Nanoscience and Nanotechnology, 2002. 2(1): p. 1-24.
7. Negrete-Herrera, N., Putaux, J.L., and Bourgeat-Lami, E., Synthesis o f
polymer/Laponite nanocomposite latex particles via emulsion polymerization
using silylated and cation-exchanged Laponite clay platelets. Progress in
Solid State Chemistry, 2006. 34(2-4): p. 121-137.
8 . Herrera, N.N., Letoffe, J.M., Putaux, J.L., David, L., and Bourgeat-Lami, E.,
Aqueous dispersions o f silane-functionalized laponite clay platelets. A first
step toward the elaboration o f water-based polymer/clay nanocomposites.
Langmuir, 2004. 20(5): p. 1564-1571.
9. Mellon, V., Synthesis and characterisation o f Waterborne Polymer/Laponite
Nanocomposite Latexes through Miniemulsion Polymerisation, in PhD
Thesis. 2009, CNRS-LCPP (Laboratory of Chemistry and Processes of
Polymerization), France.
112
Chapter 4
10. Lee, D.C. and Jang, L.W., Preparation and characterization o f PMMA-clay
hybrid composite by emulsion polymerization. Journal of Applied Polymer
Science, 1996. 61(7): p. 1117-1122.
11. Olphen, v., An Introduction to Clay Colloid Ahemistry. 1997: Wiley, New
York.
12. Al-Mukhtar, M., Touray, J.C., and Bergaya, F., Synthetic clay fo r the study o f
swelling behaviour o f clayey soils: Na-laponite. Comptes Rendus De L
Academie Des Sciences Serie Ii Fascicule a-Sciences De La Terre Et Des
Planetes, 1999. 329(4): p. 239-242.
13. Utracld, L.A., Sepehr, M., and Boccaleri, E., Synthetic, layered nanoparticles
fo r polymeric nanocomposites WNCO. Polymers for Advanced Technologies,
2007.18(1): p. 1-37.
14. Grim, R.E., Clay mineralogy. 1968, New York: McGraw-Hill.
15. Mackenzie, R.C., The Differential thermal investigation o f clays. 1966:
London : Mineralogical Society (Clay Minerals Group),.
16. Herrera, N.N., Letoffe, J.M., Reymond, J.P., and Bourgeat-Lami, E.,
Silylation o f laponite clay particles with monofunctional and trifunctional
vinyl alkoxysilanes. Journal of Materials Chemistry, 2005.15(8): p. 863-871.
17. Yang, J.T., Fan, H., Bu, Z.Y., and Li, B.G., The preparation ofMMT-
supported initiator and its application in the emulsion polymerization o f
styrene. Acta Polymerica Sinica, 2006(6): p. 779-784.
18. Ogawa, M., Okutomo, S., and Kuroda, K., Control o f interlayer
microstructures o f a layered silicate by surface modification with
organochlorosilanes. Journal of the American Chemical Society, 1998.
120(29): p. 7361-7362.
19. Ogata, N., Kawakage, S., and Ogihara, T., Poly(vinyl alcohol)-clay and
poly(ethylene oxide)-clay blends prepared using water as solvent. Journal of
Applied Polymer Science, 1997. 66(3): p. 573-581.
20. Shen, Y.H., Sorption o f non-ionic surfactants to soil: the role o f soil mineral
composition. Chemosphere, 2000. 41(5): p. 711-716.
113
Chapter 4
21. Cauvin, S., Colver, P.J., and Bon, S.A.F., Pickering stabilized miniemulsion
polymerization: Preparation o f clay armored latexes. Macromolecules, 2005.
38(19): p. 7887-7889.
22. Sun, Q.H., Deng, Y.L., and Wang, Z.L., Synthesis and characterization o f
polystyrene-encapsulated laponite composites via miniemulsion
polymerization. Macromolecular Materials and Engineering, 2004. 289(3): p.
288-295.
23. Tong, Z.H. and Deng, Y.L., Synthesis o f water-based polystyrene-nanoclay
composite suspension via miniemulsion polymerization. Industrial &
Engineering Chemistry Research, 2006. 45(8): p. 2641-2645.
24. Goodwin, J., Colloids and interfaces with surfactants and polymers : an
introduction. 2004,: New York J. Wiley.
25. Caria, A., Regev, O., and Khan, A., Surfactant-polymer interactions: Phase
diagram and fusion o f vesicle in the didodecyldimethylammonium bromide-
poly (ethyleneoxide)-water system. Journal of Colloid and Interface Science,
1998.200(1): p. 19-30.
26. Attwood, D., Florence, A.T., Surfactant Systems: Their chemictry, pharmacy
and biology. 1983: Chapman and Hall, London, New York.
27. Segota, S., Heimer, S., and Tezak, D., New catanionic mixtures o f
dodecyldimethylammonium bromide/sodium
dodecylbenzenesulphonate/water I. Surface properties o f dispersed particles
(vol 274, pg 91, 2006). Colloids and Surfaces a-Physicochemical and
Engineering Aspects, 2006. 280(1-3): p. 245-245.
28. Yang, J.P., Xie, J.Y., Chen, G.M., and Chen, X.C., Surface, Interfacial and
Aggregation Properties o f Sulfonic Acid-Containing Gemini Surfactants with
Different Spacer Lengths. Langmuir, 2009. 25(11): p. 6100-6105.
29. Ogitani, S., Bidstrup-Allen, S.A., and Kohl, P.A., Factors influencing the
permittivity o f polymer/ceramic composites for embedded capacitors. Ieee
Transactions on Advanced Packaging, 2000. 23(2): p. 313-322.
30. Svitova, T.F., Smirnova, Y.P., Pisarev, S.A., and Berezina, N.A., Self-
Assembly in Double-Tailed Surfactants in Dilute Aqueous-Solutions. Colloids
114
Chapter 4
and Surfaces a-Physicochemical and Engineering Aspects, 1995. 98(1-2): p.
107-115.
31. Hu, H. and Larson, R.G., Analysis o f the effects o f Marangoni stresses on the
microflow in an evaporating sessile droplet. Langmuir, 2005. 21(9): p. 3972-
3980.
32. Girard, F., Antoni, M., and Sefiane, K., On the effect o f Marangoni flow on
evaporation rates o f heated water drops. Langmuir, 2008. 24(17): p. 9207-
9210.
33. Hibiya, T., Nagafuchi, K., Shiratori, S., Yamane, N., and Ozawa, S., Attempt
to study Marangoni flow o f low-Pr-numher fluids using a liquid bridge o f
silver. Advances in Space Research, 2008. 41(12): p. 2107-2111.
34. Rongy, L. and De Wit, A., Marangoni flow around chemical fronts traveling
in thin solution layers: influence o f the liquid depth. Journal of Engineering
Mathematics, 2007. 59(2): p. 221-227.
35. Fanton, X. and Cazabat, A.M., Spreading and instabilities induced by a
solutalMarangoni effect. Langmuir, 1998.14(9): p. 2554-2561.
36. Xu, X.F. and Luo, J.B., Marangoni flow in an evaporating water droplet.
Applied Physics Letters, 2007. 91(12): p. -.
37. Wang, H.T., Wang, Z.B., Huang, L.M., Mitra, A., and Yan, Y.S., Surface
patterned porous films by convection-assisted dynamic self-assembly o f
zeolite nanoparticles. Langmuir, 2001.17(9): p. 2572-2574.
38. Nguyen, V.X. and Stebe, K. J., Patterning o f small particles by a surfactant-
enhanced Marangoni-Benard instability. Physical Review Letters, 2002.
88(16): p. -.
39. Truskett, V. and Stebe, K.J., Influence o f surfactants on an evaporating drop:
Fluorescence images and particle deposition patterns. Langmuir, 2003.
19(20): p. 8271-8279.
40. Herrera, N.N., Persoz, S., Putaux, J.L., David, L., and Bourgeat-Lami, E.,
Synthesis o f polymer latex particles decorated with organically-modified
laponite clay platelets via emulsion polymerization. Journal of Nanoscience
and Nanotechnology, 2006. 6(2): p. 421-431.
41. Bourgeat-Lami, E., Evaluation Meeting 2008, NAPOLEON Project. 2008.
115
Chapter 4
42. Chidambaram, M., Sonavane, S.U., de la Zerda, J., and Sasson, Y.,
Didecyldimethylammonium bromide (DDAB): a universal, robust, and highly
potent phas e-transfer catalyst fo r diverse organic transformations.
Tetrahedron, 2007. 63(32): p. 7696-7701.
43. Bourgeat-Lami, E., Mellon, V., Pardal, F., Putaux, J., Me Kenna, T.,
Bonnefond, A., Micusilc, M., Paulis, M., Leiza, J. R., Schreiber, E.,
Landfester, K., Lohmeijer, B.„ Acrylic/Clay Nanocomposite Latexes:
Synthesis, Structure and Properties. Submitted.
44. McDonald, P J . and Newling, B., Stray field magnetic resonance imaging.
Reports on Progress in Physics, 1998. 61(11): p. 1441-1493.
45. Mallegol, J., Bennett, G., McDonald, P.J., Keddie, J.L., and Dupont, O., SJdn
development during the film formation o f waterborne acrylic pressure-
sensitive adhesives containing tacldfying resin. Journal of Adhesion, 2006.
82(3): p. 217-238.
46. http://www.veeco.com/stvlus-t>rofiler-svstems/index.aspx. [cited.
47. http:/Avww.kruss.de/en/home.html, [cited.
48. van Tent, A. and Nijenhuis, K.T., The film formation ofpolymer particles in
drying thin films o f aqueous acrylic latices - II. Coalescence, studied with
transmission spectrophotometry. Journal of Colloid and Interface Science,
2000. 232(2): p. 350-363.
49. Meeten, G.H., Optical Properties o f Polymer. 1986: Elsevier Science
Publishers Ltd, UK.
50. Routh, A.F. and Russel, W.B., Horizontal drying fronts during solvent
evaporation from latex films. Aiche Journal, 1998. 44(9): p. 2088-2098.
51. Vig, J.R. and Lebus, J.W., Uv-Ozone Cleaning o f Surfaces. Ieee Transactions
on Parts Hybrids and Packaging, 1976.12(4): p. 365-370.
116
Chapter 5
The Effects of Acrylic Acid and pH on the
Physical Characteristics of Pressure Sensitive
Adhesives
5.1. Introduction
Due to increasing public awareness and tighter environmental legislation in recent
times, the importance of waterborne polymer colloids (i.e. latexes) for adhesives and
coatings has never been so high, as they discharge minimal amounts of volatile
compounds (VOCs) into the atmosphere [1, 2 ]. Since most of the solvent-borne
coatings and adhesives are based on water-insoluble polymers, aqueous dispersions
are needed as a way forward for development of these products [2] . In preparing
polymers in a form suitable for formation into waterborne coatings and adhesives,
emulsion polymerisation is a particularly convenient method [3, 4],
Polymers with low glass transition temperatures (in the range of -40 to -50 °C) which
demonstrate tack are known as adhesives [5]. Pressure sensitive adhesives (PSAs)
are a type of adhesive that form a bond between the adhesive and the adherent
Chapter 5
instantly and firmly under the application of light pressure, without covalent bonding
or activation [6-9]. PSAs are tacky at room temperature. The bond forms as the
adhesive is soft enough to flow and to wet the adherent. The adhesive must be stiff
enough to resist the flow when stress is applied [10]. The adhesive materials are
usually applied to one of the substrates (i.e. the backing) to be bonded. The adhesive
is protected by applying a release liner to the open adhesive surface. Paper labels and
tapes are the most common backings. When the label or tape is to be used, the
release liner is removed and the backing bonded to a desired surface by applying
light pressure [4].
In applications such as labels or tapes, it is vital that the PSA film remains optically
transparent, regardless of the humidity or exposure to water. The loss of optical
transparency when adhesive films are exposed to high humidity or soaked in water,
is referred to as ‘water whitening’ [11]. Water whitening is a common problem
encountered in waterborne PSAs. It is attributed to the scattering of light by pockets
of water between polymer particles. If the size of these pockets of water is greater
than the order of 1/10th of the wavelength of light or in the region of 50 nm, the film
develops opacity (Figure 5.1 (b)). A film with no water whitening also might absorb
water but if this water is evenly distributed at the particle boundaries and not creating
water pockets greater than on order of 1 / 1 0 th of the wavelength of light, then it is
expected that the film will remain transparent (Figure 5.1 (a)). An adhesive film with
no water whitening should also be a good barrier [12] to liquid water transport. To
minimise the water whitening, it is expected that a film should achieve complete
particle coalescence during drying.
Figure 5.1. Schematic illustration of (a) optically transparent film and (b) loss of transparency due to larger pockets of water between particles.
118
Chapter 5
However, early-stage good coalescence could lead to another problem, ‘skin
formation’ [13, 14]. The particle coalescence near the air interface during early-
stages of drying is referred to as skin formation, which is illustrated in Figure 5.2.
The transport of water through a polymer skin layer is significantly slower than
transport around the particles [15]. This retarded water loss can cause water
entrapment within the drying film. Trapped water is also associated with inhibited
interdiffusion and hence weaker films.
Skin formation
water
Figure 5.2. Schematic illustration of ‘skin formation’ or particle coalescence near the air interface during drying of a latex film.
Prevention of early-stage good coalescence should avoid the skin formation and
water entrapment within the film during drying. On the other hand, good coalescence
at the later stages of drying should prevent water whitening and increase the barrier
properties. Therefore, there has been increased industrial as well as academic interest
to meet these conflicting requirements.
In recent years there has been enhanced interest and much work on water absorption
in coatings and PSAs being reported. Selections of reports are presented here under
three headings:
1 . Water uptake [16-19]
2. pH and PAA dependence [20, 21]
3. Properties [22-26]
Each of these will be considered separately here after.
119
Chapter 5
5.1.1 Water uptake
Agarwal and Farris [16] reported the water absorption by films prepared from blends
of soft (low glass transition temperatures - low Tg) and hard (high Tg) acrylic-based
latex particles. They have shown that the blends with large proportions of the soft
particles absorb much larger amounts of water compared with those with hard
particles. These films turn white/opaque upon water absorption but regain their
transparency upon redrying. SEM images of freeze-fractured surfaces of wet films by
larger proportions of soft particles show micrometer size pockets of water. Figure 5.3
shows the freeze-fractured SEM images of blend films of (a) 70% and (b) 50% soft
particles. Blend films with a higher amount of soft particles (a) show much larger
holes compared to a film made from a relatively lower amount of softer particles.
The implications are that the PSAs used in current study also should have relatively
large, presumably micrometre size, pockets of water upon water absorption.
Figure 5.3. SEM images of freeze-fractured surfaces of blend films of (a) 70% and (b) 50% soft particles. Images taken from Reference [16].
In relation to Agarwal and Farris’s observations, one can conclude that acrylic-based
latex blend films absorb a significant amount of water from liquid water. These films
were easily removed from their substrates (glass plates) indicating that they lost
significant amount of their mechanical properties and turned white/opaque or lost
transparency upon water absorption. Furthermore, the absorbed water created water
120
Chapter 5
reservoirs or pockets like clusters between particles which can be attributed to
scattering of light. This could be because the polymers they used, (poly(methyl
methacrylate-co-ethyl acrylate - P(MMA-co-EA) and poly(methyl methacrylate-co-
buthyl acrylate - P(MMA-co-BA)), were significantly hydrophobic. If the particle
boundaries were hydrophilic, the absorbed water should have been evenly distributed
along the boundaries and reservoir type water clusters should have been less obvious.
In addition, clusters of surfactant between particles also could have been contributed
to these water reservoirs, upon hydration of surfactant salts. Furthermore, the blends
with a higher percentage of softer particles create larger holes upon water absorption
which confirms that the flexibility of the softer particles allow the water reservoirs to
grow bigger compared to the less flexible harder particles. The fact that these films
easily regain their original weight and transparency upon re-drying confirms that the
water absorption is reversible and no bonds were formed between the absorbed water
and the particles.
Van der Wei and Adan [17] studied the transport and equilibrium adsoiption of
moisture in polymer films and organic coatings. Three different types of absorption
curves were discussed. Firstly, the typical sorption curve, where mass varies linearly
as a function of the square root of time. With increasing time, the sorption curve
smoothly levels off to a saturation level of mass. Secondly, there can be a ‘two-stage’
sorption curve, where the first part is a typical sorption followed by a slow
absorption before ultimately reaching the saturation level. They argued, in this case
that there is more than one contribution to the absoiption process; a diffusion part
and one or more structural parts, which result from polymer relaxations. Therefore,
the total weight gain is the sum of these contributions. Thirdly, there can be the
‘Sigmoidal’ sorption curve, where the curve is ‘S’ shaped with a point of inflection.
Due to the slow establishment of equilibrium at the surface of the film, the kinetics
appears somewhat different than the standard. Figure 5.4 illustrate these three types
of soiption curves.
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Chapter 5
Figure 5.4. Three different types of absorption curves of relative mass uptake as a function of square root if time; (a) typical sorption curve, (b) the ‘Two-stage’ sorption curve and (c) the ‘Sigmoidal’ sorption curve, where the curve is ‘S’ shaped. Drawn after [17].
hi 2006 Chen and co-workers [18] investigated the wetting-resistance, the water-
resistance and the thermal stability of coating formulations. They found that after
perfluoroalkyl groups were introduced into polymer chains, these properties were
evidently enhanced. According to their calculations, the surface free energy of the
film with fluorine was much lower than that of the film without fluorine. When
perfluoroalkyl groups with low surface tension were introduced into polymer chains,
the surface energy of the latex film was greatly lessened and the latex film could not
be wetted by water easily. Furthermore, their water absorption results suggested that
the water-resistance of the film was successfully modified by introducing fluorine
into the polymer. Hydrophobic perfluoroalkyl groups prevented water molecules
entering into the film’s inner, and thus enhanced the water resistance of the films. In
addition, they found the water absorption of the latex films increased with increasing
PAA amount, due to the hydrophilicity of PAA. However, they foimd that the water
absorption was decreased first and then increased for the latex films which contained
very hydrophilic sulfosalt groups. Overall, more sulfosalt, led to higher water
absorption of the latex films. An increasing sulfosalt amount would decrease the
latex particle sizes, which results in a dense latex film. Therefore, the water
molecules find it difficult to penetrate through the film surface. Thus, the water
absorption of the latex film decreased. Under these two contrary effects, the authors
found that the lowest water absoiption occurred at 0.95% of sulfosalt, from their
range of 0 .8% to 1 .2 %.
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Chapter 5
Implications are that the PSAs with PAA shells in current study also show higher
water absorption, in comparison to the PSAs without PAA, due to the hydrophilicity
of PAA.
Feng and Winnik [19] reported the role of water in polymer diffusion in latex films.
For a hydrophobic polymer, water has little influence on the polymer diffusion rate.
Water absorption in those films does increase the film turbidity, both for recently
formed and for well-annealed films. For hydrophilic polymer films, the presence of
water increases the diffusion coefficient by a factor of five at 60 °C. They found that,
upon neutralisation of the carboxylic acid groups with NaOH, the polymer diffusion
is much retarded in dry films but greatly enhanced in wet films. Furthermore, the
polymer diffusion coefficients for the wet films are about two orders of magnitude
larger than those for the corresponding dry films. In addition, neutralisation with
NH3 results in intermediate diffusion rates, between those of im-neutralized and
NaOH-neutralised films in both dry and wet conditions. It was reported when dense,
crack-free films were formed from latex dispersions, they are often transparent, or
sometimes semitransparent. This transparency often depends on the film micro
heterogeneity, film thickness or water content. It was reported that due to the micro-
heterogeneous nature of the newly formed films, the transparency of latex films in
many cases is lower than that of films cast from homogeneous solutions. In other
cases relatively highly transparent films were obtained, especially when the
temperature of film formation is well above the MFFT. A possible explanation could
be that, in the films formed at high temperatures, some interparticle polymer
diffusion may have occurred during the film formation process.
Furthermore, Feng and Winnik [19] found that their films which were dried close to
room conditions were fairly transparent, and transparency was higher if the films
were relatively thinner. In addition, annealing at elevated temperatures in dry
environment led to an increase in transparency. When their films were exposed to
water vapour or immersed in liquid water for some time (e.g. two days), they became
turbid. They found that the water content for their film exposed to 100% RH, was 3.5
wt.% and that for a film immersed in liquid water was as high as 15 wt.%. They
123
Chapter 5
confirmed that water becomes more concentrated in the interparticle boundary
regions richer in polar groups, in their case, -OSO3H groups. This in-homogeneity of
water distribution in the films would lead to turbidity for the films exposed either to
high humidity or to liquid water.
Feng and Winnik [19] have seen when these water-containing, turbid films were
heated in low humidity environment, that water was evaporated and the film
transparency increased and the films became clearer. When these heated, clear films
were once again placed into high RH or water, they once again became highly turbid.
These visible transmittance results for newly formed film, a film exposed to 100 %
RH, then heated for two hours at 60 °C at low humidity, and then placed into high
RH again are presented in Figure 5.5.
_ 120 PgUig 80<H
<cr
-..... 1 1 1
^ ( 3 )
—T 1
- N ( i )
^ (4 )
—
!---------- 1---------- 1 1V 2 ) -
— j_______ 1________400 500 600
WAVELENGTH (nm)700
Figure 5.5. Visible transmission spectra for films: (1) a newly formed film, (2) a recently formed film exposed to 100 % RH for two days, (3) a film heated in low humidity at 60 °C for two hours, (4) the well-heated film exposed once again to 1 00 % RH for two days. Figure taken from [19].
One would expect that as polymer molecules were well mixed by interdiffusion, the
heterogeneity in the initial latex films would gradually disappear and the
hydrophobic polymer matrix would become more resistance to water. However,
Feng and Winnik have seen that while the molecular mixing increases the overall
homogeneity of the system, some degree of nonuniform distribution of polar groups
124
Chapter 5
exists in their polymer. When these films were exposed to water, domains rich in
polar groups were found to take up more water and these regions were large enough
to scatter light. Therefore, Feng and Winnik confirmed that the water-containing
films remained turbid, no matter how much polymer diffusion has taken place.
There are couple of observations in Feng and Winnik work which could apply to the
current study. First of all, Feng and Winnik showed that the films made from the
latexes with heterogeneous particles had lower transparency than that of films made
from homogeneous solutions. As the PSAs used in this study are homogeneous
(average particle diameter between 162 mn - 164 nm, section 5.3.), the transparency
between the samples should not be affected by the particle size distribution. Then,
they found that the films which were exposed to high humidity absorbed less water
compared with the films which were immersed in liquid water. This study could also
show higher water absorption from liquid water than vapour.
5.1.2. pH and PAA dependence
In most PSA applications, several monomers can be incorporated into the polymer,
in order to impart desired properties [7]. PAA or Methacrylic Acid (MAA) is often
used in both coating and adhesive formulations to impart colloidal stability, freeze-
thaw stability, and improved film forming properties [24], The carboxylic acid
comonomer forms a major component of water-soluble chains on the surface of the
latex particle. It will provide both steric and electrostatic stabilisation of the colloid.
The surface hydrophilic chains are referred to as a ‘hairy layer5 [26]. This hairy layer
of poly PAA chains is sensitive to the pH. With increasing pH the charge repulsion
by the PAA chains increases. As a result the PAA chains are widely spread (Figure
5.6 a) and the hydrophobicity of the surface decreases [26]. As pH decreases, the
hairy layer will collapse (Figure 5.6 b) and the hydrophobicity of the surface
increases.
125
Chapter 5
A B
Figure 5.6. Schematic representation of polymer particle with ‘hairy layer’, (a) At high pH the charge repulsion by PAA chain increases and the chains are widely spread; (b) as pH decreases, the hairy layer collapses. Drawn after [26].
De Bruyn and co-workers [20] reported work on electrosterically stabilized latex
having a polystyrene core and a poly PAA hydrophilic layer as the shell. They
observed that the hairy layer thickness of PAA on particles increases when the pH is
increased. They performed small angle neutron scattering measurements over a range
of contrasts for three latexes with different high and low pH values. They reported
that the parameters obtained by fitting to standard core/shell models were consistent
with the shell being highly hydrated. Furthermore, they found that the shell was
about 89% hydrated at low pH and about 94% hydrated at high pH. The core was
found to contain about 3% acrylic acid. By doubling the proportion of acrylic acid in
the reaction, it was shown that shell thickness was increased by about 2 0 %.
In the current study, the samples with PAA shells at a higher pH should be expected
to dry slower than the samples at a lower pH, as PAA shells are more hydrated, and
release of water from the drying film should be slower.
Rharbi and co-workers [21] studied adhesive films made by core-shell particles with
hydrophobic cores made of a poly(styrene) and poly(butyl acrylate) copolymer and
thin hydrophilic shells made of poly(acrylic acid) (PAA) and poly(butyl acrylate)
126
Chapter 5
pBA) copolymer. In addition to these dispersed polymer particles, the aqueous phase
also contained soluble amphiphilic polymers, surfactant and salts. These soluble
species were eliminated through dialysis and equilibrium with ion exchange resins.
The ionisation of the PAA groups at the shell was controlled through the addition of
a base, NaOH. They found that the films made with un-dialysed latexes showed
greater water uptake, as the soluble species that were initially in the aqueous phase
has become trapped in the membranes. The films made with dialysed latexes also
showed greater water uptake, if they were neutralised by NaOH in the aqueous
phase. In these films, PAA shells are polyelectrolytes, and therefore the membranes
remained hydrophilic. They observed the water uptake by the films made with
dialysed latexes, but when kept in the acid form, the uptake was very low. They
suggested their results were due to the fact that the surface polymers remained in the
acidic form, and the membranes became hydrophobic in the dry state. Figure 5.7
summarises their results. The films adjusted to pH 10 by NaOH at aqueous stage
gained the most water compared with the acidic form at pH 2 which gained the least
amount of water. Latex neutralised by NaOH to be pH 7 was intermediate [21].
127
Chapter 5
% Weight gain
1) 500 1000 1500 2000Time (b)
Figure 5.7. Comparison of the water uptake of films by latex dispersions of core/shell particles (poly styrene and poly butyl acrylate hydrophobic core surrounded by a thin layer of hydrophilic poly acrylic acid and poly butyl acrylate shell): % weight gain as a fimction of time. Films from latex kept in the acidic form at pH 2 (filled circles), films from latex partly neutralised at pH7 by NaOH (unfilled diamonds), films from latex fully neutralised at pH 10 by NaOH (filled triangles), films from latex neutralised by Ba(OH)2 at pH 7 (+). Figure taken from [21].
Implications from Rharbi and co-workers [21] are that in current study, samples with
PAA shells at a higher pH should absorb more water compared with the samples at a
lower pH, as PAA shells in acidic form should become hydrophobic in the dry state.
5.1.3. Properties
Wang and co-workers [22] investigated the effects of pH on film drying, mechanical
and adhesive properties of waterborne poly(butyl acrylate-co-acrylic acid) films. In
films cast from acidic colloidal dispersions, hydrogen bonding between carboxylic
acid groups dominates the particle-particle interactions, whereas ionic dipolar
interactions are dominant in films cast from basic dispersions. They have shown by
force spectroscopy using atomic force spectroscopy and macroscale mechanical
measurements that those latex films with hydrogen-bonding interactions have lower
elastic moduli and are more deformable. In addition, these films yielded higher
adhesion energies. In comparison, in basic latexes, ionic dipolar interactions
128
Chapter 5
increased the moduli of the dried films. These films were stiffer and less deformable
and, consequently, exhibited lower adhesion energies. Furthermore, they have shown
that the rate of water loss from acidic latex was slower, because of hydrogen bonding
with the water. They concluded that although acidic latexes offer greater adhesion,
there is a limitation in the film formation.
The implications are that the PSAs in the current study are also expected to show a
relation between adhesion and drying properties.
Yang, Li and Wang [23] reported difference in adhesion properties and the water
resistance of four different acrylic PSAs with the same composition of their
constituent co-polymers but stabilised by four different anionic surfactants. They
used two conventional low-molecular-weight surfactants (a sodium salt and an
ammonium salt) and two anionic monomers (a sodium salt and an ammonium salt).
Water absorption of PSA films was determined by a gravimetric method. The peel-
strength retention of PSA tapes after immersion in water was compared. They
reported that both the adhesion properties and the water resistance of the acrylic
PSAs stabilised by anionic monomers were better than the acrylic PSAs stabilised by
low-molecular-weight surfactants. Furthermore, these properties of PSAs with
ammonium surfactants were better than the sodium surfactants. These differences
were mainly caused by the different migration ability and their different hydrophilic
nature of the four surfactants in the PSA layers. They found that low-molecular
weight surfactant can easily migrate and concentrate but the migration ability and the
degree of enrichment of the surface by an ammonium surfactant is less than that of a
sodium surfactant.
In 1999 Tzitzinou and co-workers theoretically modelled the dependency of the
optical transmission on the radius of the voids of a film for different volume fractions
of voids [25]. They used Rayleigh scattering theory [27] to relate optical
transmission, T, to volume fraction of voids/, using the equation :
129
Chapter 5
In T =-32 ;r 4rv3f vd m -1
2̂
A4V ^ nt2 +1 y J(5.1)
Where rv is the radius of the voids, assuming that the voids are spherical in shape, d
is the optical path length which is the film thickness, X is the wavelength of light, and
m is the relative refractive index; which is equal to n fnp> with nv being the reflective
index of air voids (nv = 1 ) and np the reflective index of the fully dense polymer (the
value of np would depend on the polymer). The number of voids per unit volume, N,
is related to f v by
N -(5-2)
Using the equations 5.1 and 5.2, one can, in principle, determine N from a
measurement of T. They noted that equation 5.1 applies only for certain ranges of
void size and refractive index. Hence, they tested its applicability by measuring T
over a wide range of A, and reported that the dependence of ln(T) is weaker than
(-AT4), predicted by equation 5.1. Nevertheless, they proved that optical transmission
can be qualitatively related to void size and concentration in their films.
130
Chapter 5
Radius of Void (nm)
Figure 5.8. Predicted optical transmission of 100 pm thick films of a continuous medium with n - 1.5 and containing spherical air voids («v =1.0). The volume fraction of voids (fv) is taken to be 0.25 (—); 0.025 (—) and 0.0025 (•••). Figure taken from [25].
Figure 5.8 shows the dependency of the optical transmission on radius of voids
predicted by the equation 5.1, assuming that the volume fraction of voids remains
constant with increasing radius of the voids. It was predicted that transmission
decreases with increasing void radius. The loss of transmission was highest with the
largest volume fraction of voids (0.25) and was lowest with the smallest. For all three
volume fraction of voids, as the radius of void decreased the transmission was
increased. Furthermore, it is shown that, for a void radius of 30 mn, the film can be
nearly transparent, nearly opaque or partially transparent, depending on the void
volume fraction (and hence the number of voids per unit volume) [25].
131
Chapter 5
5.2. Aims of present research work
In the literature review, it was shown that much research has been done on water
uptake [16-19], pH and PAA dependence [20, 21] and properties [22-26] of latex
films. It was shown that the pH value of the wet latex influences these properties in
the dry films. Little attention was given to the drying properties [22]. However, no
extensive studies of how the pH and PAA shell of wet core/shell PSAs in a single
system affect the drying (i.e. water coming out from a wet latex film), water
whitening and water sorption (i.e. liquid water or vapour water going into a dry latex
film) and adhesion properties have been reported. A systematic study of these
relations will lead to a better understanding of the correlation between these
properties. One could expect that latex systems which achieve good coalescence
during drying would show good water resistance properties as well. The reason
being, that good coalesced particles would be expected to show better water
resistance properties, hence less liquid and vapour water absoiption from the
environment. In addition, adhesion properties also have been studied. It is expected
that the films with well coalesced particles will have greater adhesion energy. The
water uptake could interfere with coalescence of particles and decrease cohesive
strength of the films. Furthermore, attention was given to control the conditions of
the wet latex in order to achieve the desired qualities of the dry PSAs.
This study investigates the effects of PAA and pH on drying, water whitening, water
sorption and adhesion properties of PSA films. Core-shell latex particles, with a
poly(isodecyl acrylate) (PIDA) core and a PAA shell, were used as a model PSA.
Particles with only PIDA core were expected to resist water whitening and water
sorption as PIDA is hydrophobic due to its long C10H21 chain. In comparison, the
particles with hydrophilic PAA shells were expected to absorb more water. The pH
of the latex dispersions was adjusted from acidic to basic by NaOH and NH4OH. In
the basic latex dispersions adjusted by NaOH Na+ cations would remain in the
dispersion, hi comparison, the latex dispersions adjusted by NH4OH would not have
any (or less) cation effect as volatile NH3 would evaporate to the atmosphere.
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Chapter 5
5.3. Materials
The poly (isodecyl acrylate) (PIDA) latexes, with and without acrylic acid (PAA),
were prepared and dialysed at the University of Manchester, UK by Andrew Foster
and Michael Rabjohns. As a means of adjusting the hydrophilicity, 3 wt.% acrylic
acid (PAA) was co-polymerised in the shell of the PIDA particles. Two commercial
surfactants (Rhodapex AB/20 and Dowfax 2A1) in an active ratio of 1:1, 2 wt.% on
monomer has been used. The solid content of the latexes, before dialysis is in the
range from 54 -56 wt.% and the particle diameters were between 156 nm - 159 nm.
The latexes were dialysed by pouring lOOg of latex into a length of Visking tubing,
which was then sealed and immersed in a large excess of de-ionized water and left
for seven to ten days. During this time, the de-ionized water was changed to fresh
water at least once a day. The dialysed latexes were then re-concentrated by rotary
evaporation at around 35 °C until the solids contents were reached around 45 %.
After dialysis, the solids content was 42.6 - 43.1 wt.% and the average particle
diameter was between 162 nm -164 nm. A schematic diagram of a latex particle with
PIDA core and an PAA shell is shown in Figure 5.9.
PIDA core
AA shelloFigure 5.9. A schematic diagram of a latex particle with PIDA core and a PAA shell.
PIDA and PIDA with PAA core/shell (97 wt.% / 3 wt.%) latexes were at a pH of 3
and solids content was around 50 wt.%. The pH was adjusted to 8 by 1M NaOH or
1M NH4OH solutions in water. For this study, six samples were used: PIDA at a pH
of 3, PIDA at a pH of 8 adjusted with NaOH, PIDA at a pH of 8 adjusted with
NH4OH, PIDA/PAA core/shell (97 wt.% / 3 wt.%) at a pH of 3, PIDA/PAA at a pH
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Chapter 5
of 8 adjusted with NaOH, PIDA/PAA at a pH of 8 adjusted with NH4OH. Table 5.1
surnmarises the compositions and characteristics of the latexes.
Table 5.1. Summary of the compositions and characteristics of the latex samples.
SamplepHof
wet latex
Solidscontent(wt.%)
Average Particle diameter
(nm)
Overall amount of
PAA (wt.%)
OPAA 3 42.6 162 0
0 PAA, pH 8 by NaOH
8 42.6 162 0
0 PAA, pH 8 by NH4OH
8 42.6 162 0
3 PAA 3 43.1 164 3
3 PAA, pH 8 by NaOH
8 43.1 164 3
3 PAA, pH 8 by NH4OH
8 43.1 164 3
134
Chapter 5
5.4. Methods
5.4.1. Magnetic Resonance Profiling (MRP)
MRP experiments were carried out using the GARField magnet. For these
experiments 2 0 mm x 2 0 mm and 180 pm thick glass cover-slips were used as the
substrate. The latex was applied on to the coverslips to get the desired thickness. As a
rough guidance around 70 pi of 50 wt.% solid content latex would produce a 300-
400 pm thick wet film. As soon as the latex was cast on to the glass coverslip, it was
placed inside the magnet and NMR profiles were taken at the room temperature and
humidity. The magnetic field, Bo, is 0.7 T when the measurements were performed.
The gradient strength, Gy, is approximately 17.5 T/m. The NMR signal is obtained
from an excitation using a quadrature echo sequence: 90x-T-(90y-T-echo-T-)„ [28,
29]. For typical latex experiments, the number of echoes, n, is 32; the pulse gap, t,
75 ps; the dwell time, DW, is 0.7 ps; the number of points per echo, SI, is 128 and
the spectrometer frequency, SF, is 29.6 MHz. For each and every experiment the
time delay between profiles and the total drying time can be changed as desired. The
same MR parameters were used, so that direct comparisons of profiles for different
samples were allowed. The profile shapes were normalised by an elastometer
standard in order to correct for the decline in sensitivity over the film thickness.
The GARField data were further analysed as follows [30]. The solids content, at
a given drying time, t was calculated by the following equation.
where mo(t) is the zeroth moment at drying time t, mo(0) is the zeroth moment at
t = 0 , <J>0 is the solids content at t = 0 , H(t) is the thickness at time t, and Ho is the
initial thickness. The initial thickness, Ho, was determined from the MRP profiles. In
(5.3)
135
Chapter 5
doing so, it was assumed that the upper edge of the water profile coincides with the
upper edge of the film.
5.4.2. Measurements of Water Whitening by Optical
Transmission
The latex dispersions were cast onto PET substrates (30 cm x 20 cm) using a hand
held bar applicator with a defined wet thickness of 40 pm. The films were dried in
ambient conditions for 24 hours and under laminar air flow on heated plates at 110
°C for three minutes. A strip of the film was cut from the centre of the film and was
submerged in de-ionized water in a square cuvette. The film face was positioned
parallel with the cuvette walls. The optical transmission of the films, while in the
water, was obtained at various time intervals. Optical transmissions of the films were
performed with a UV/visible spectrophotometer (Campsec M350, Cambridge, UK).
During a measurement, the wavelength of light was varied from 300 to 900 nm. The
percent transmission was recorded to indicate the transparency of the film.
5.4.3. Probe-Tack Adhesion Measurements
The probe tack test usually characterises instant adhesion properties of PSAs under
light pressure [9, 31-34]. The time of contact between the adhesive and the probe can
be specifically controlled and is usually very short, around one second. The
parameters that influence adhesion are the rheological properties of the adhesive
layer and the nature of its interactions with the substrate or the probe. The shape of
the probe is also important. As spherical probe is more favourable than a flat-ended
probe, as it gives more reproducible results [6 , 35]. The reason is that the spherical
probe avoids the difficulty of a proper parallel alignment between the probe and the
film. On the other hand, a flat probe gives a much more uniform stress field and
strain rate under the probe surface.
136
Chapter 5
In 1985 Zosel reported [36] the development of probe-tack experiments. The
pulling force needed to detach a flat, solid punch from an adhesive film was recorded
during the entire separation process which was performed at a constant velocity. The
traction curves obtained showed that the force increases sharply and reaches a peak
value, then it drops suddenly and stabilizes at a plateau value and eventually
vanishes. The adhesion energy is the work done during the entire separation process
[32].
Figure 5.10 (a) Photograph of the probe-tack analyser, (b) The appearance offibrils between the PSA and the probe during the debonding of the probe from the PSA. Image (b) taken from [34].
Probe-tack adhesion measurements were followed on a commercial instrument
(Figure 5.10 (a)) (Microsystems Texture Analyser, Godalming, UK) using a high
energy spherical, stainless steel surface probe with 2.5 cm diameter. The samples
were prepared on glass as substrate using a 2 0 0 pm cube applicator; hence the wet
film thickness was 200 pm. Films were dried in the ambient conditions. The probe
was pressed in contact with the PSA film with a 4.9 N force for 1 second, before
being removed at a constant velocity of 100 pm/sec. Force and displacement
measurements were used to generate force-distance curves [37, 38].
137
Chapter 5
The debonding process can be separated into several stages which illustrated in
Figure 5.11. During the probe-tack test, when the debonding process of PSA film
begins an initial stress will appear in the bulk of the film (Figure 5.11 (a)) [32],
When the initial stress reaches a critical stress, cavities will form in the bulk of the
film or at the interface between the PSA film and the probe (Figure 5.11 (b)) [31,
39]. As internal stress continues to increase, more cavities will appear and existing
cavities will expand simultaneously (Figure 5.11(c)). Theses cavities further grow,
and the inter-cavity distance reaches the same order as the initial thickness of the
film (Figure 5.11 (d)). The thin walls between the cavities extend mainly in the
direction of traction [32] and extend into filaments known as ‘fibrils’ (Figure 5.10
(b)) [36], before detaching (Figure 5.11 (e)). This process, known as fibrillation,
enormously contributes to the energy of adhesion [33, 39]. For high performance
PSAs, it is vital to have high cavitation and fibrillation [7].
Figure 5.11 Schematic illustration of cavitation and fibrillation development in PSA debonding process, (a) Initial stress is imposed on the bulk of the film, (b) at critical stress cavities will form in the bulk or at the interface of the probe and the film, (c) formation of new cavities and expansion of existing cavities, (d) the intercavity distance reaches the initial film thickness, (e) fibrils starting to appear before detaching. Image taken from [7].
138
Chapter 5
A typical plot of force and displacement as a function of time of a probe-tack curve
is shown in Figure 5.12 [31]. At time zero, ‘start’ is when the probe is starting to
move towards the PSA film. At point 1, the probe is touching the PSA film. From
point 1 until 2 is the force applied by the probe; in this case it is 80 N. From point 2
to 3, the probe is held in contact with the PSA film; in this case it is for about five
seconds.
Figure 5.12. Typical force ( ------------) and displacement ( ------------ ) curves as afunction of time. Image was taken from [31] and modified. Details of the process is given in the main text.
At point 3, the probe is starting to pull back and the displacement starting to
progress. At point 4 the load becomes positive, which means that the compressed
layer has returned to its original value. The displacement corresponds to zero. Point
4 till 5, the film is starting to stretch and cavities will start to form (Figure 5.11 (b)
indicate this stage). Point 5 is where the most cavities are initiated (Figure 5.11 (d)
indicates this stage). Point 5 until 6 is when the cavities are continuing to expand to
create thin walls. As the internal stress continues to increase the expanded cavities
139
Chapter 5
start to open into fibrils (Figure 5.11 (e) corresponds to this stage). Around point 6 ,
fibrils are well formed. From point 6 until 7, stretching of fibrils or fibrillation
occurs. Point 7 to 8 indicates the detachment of fibrils from the probe.
The experimental data of force-distance probe - tack curve was converted to a
nominal stress (a) - nominal strain (s) curve using equations 5.4 and 5.5
F(5.4)
h - F
e = ~ T ~ (5'5)o
where F is force (N), Ao is probe contact area, around 2 mm2, h is distance travelled
by the probe above the PSA surface and ho is the initial film thickness. A stress -
strain curve of PIDA without PAA at a pH of 3 after five days of drying time is
shown in Figure 5.13 [9].
140
Chapter 5
0 1 2 3 4 5 6
Strain
Figure 5.13. The stress as a function of strain for PIDA without PAA at a pH of 3. In this probe-tack curve (Figure 5.13), the point when most cavities are initiated is
indicated by the maximum stress, a max. The plateau stress, a p, is related to the stress required to draw the fibrils. The maximum strain, smax or failure strain Sf is the end of the deformation.
The energy dissipated during the debonding process, Ea, is proportional to the area
under the probe-tack stress-strain curve and can be calculated by using equation 5 .6 .
efE a = K J a{s)ds (5 6 )
0
If the initially formed cavities coalesce and form a crack, the interfacial debonding of
the PSA will be rapid and the practical work of adhesion and the energy dissipation
will be low. On the other hand, if the coalescence of neighbouring cavities does not
occur, the walls between cavities will be extended as fibrils during the debonding
process. As a result, a relatively large work of adhesion can be achieved [7]. Figure
5.14 summarises different stress-strain curves corresponding to different works of
adhesion.
141
Chapter 5
Figure 5.14. Stress-strain curves corresponding to different adhesion energies of PSAs. Curve I is for low, Curve II is for intermediate and Curve III is for high work of adhesion. Image taken from [7].
In Figure 5.14, Curve I is for a low work of adhesion. In this case, during the
debonding, an interface crack propagates and these PSAs display very low energy
dissipation. Curve II has an intermediate work of adhesion. During the debonding of
the PSA, cavities propagate but these cavities do not transform into fibrils or else
they detach prematurely. The overall work of adhesion is relatively higher than curve
I, but still the adhesive does not perform as a high energy PSA. Curve III represents
high work of adhesion. During debonding cavities form and develop into fibrils.
These fibrils extend to very large distances before PSA detachment from the probe
and high adhesion energy can be obtained. Curve III behaviour is associated with a
more dissipative material. The type of the stress - strain curve depends on the
viscoelastic properties of a given PSA [7].
5.4.4. Moisture Sorption Measurements
Gravimetric analysis of moisture sorption measurements were performed using the
IGAsorp Moisture Sorption analyser (by Hiden Isochema Limited, Warrington, UK)
142
Chapter 5
[40]. This instrument continuously monitors the mass of a sample and is able to
control both the relative humidity and the temperature of the chamber. It is fully
automated and computer controlled. The sample container for the IGAsorp Moisture
Sorption analyser is a gas permeable micromesh stainless steel pan, in the shape of a
cone. The pan hangs from the balance with a gold chain. The balance capacity is 5 g,
but the maximum sample capacity is 4.5 g when using the sample container and
hang-down chain. The typical sample size is 10-200 mg and the resolution is 0.1 pg
for 100 mg. The instrument has a maximum flow rate of 500 ml/min and minimum
flow rate of 100 ml/min humidity. The method used for humidity is laminar flow
wet/dry gas mixing at a constant total mass flow rate with feed back control. Inlet
pressure is 45-90 psi gauge or 3-6 bar gauge. The instrument has a humidity
measurement accuracy of +/- 1% (0-90% RH) and +/- 2% (90-95% RH).
Temperature is measured by a platinum resistance thermometer and the measurement
accuracy is +/- 0 .1 °C.
For moisture soiption experiments, the latex dispersions were cast onto the silicon-
coated paper using a pipette to get the desired thickness. As a rough guide, 70 pi
latex on a 20 mm x 20 mm area would produce a film with 300-400 pm thickness.
The films were dried in ambient conditions for 24 hrs and then dried in an oven at
110 C for three minutes. The samples were stored in a sealed desiccator with silica
gel in it. For these experiments, the pans were pre-cleaned with acetone and dried.
The mass of the empty pan was measured. Then the latex film was peeled off from
the substrate and placed in the pan and was loaded into the chamber of the
instillment. Initially the sample was dried in zero humidity air at 25 °C for four hours.
The mass of the sample at this stage was taken as the reference mass for the uptake
of water. Then the humidity of the chamber was increased to 70 % and the
temperature was set to be 25 °C and the mass change was recorded until it was
deemed that equilibrium had been reached, which is when there is no mass change. If
the equilibrium camiot be predicted successfully, the instrument will continue for a
given maximum time at the given conditions. For these experiments, this given
maximum time was four hours. The instrument provides the results in a form of a
143
Chapter 5
variation of mass of the sample with time (kinetic information). Also the equilibrium
data can be presented in the form of an isotherm [40].
The % mass change at a given time, t, M was calculated by the following equation.
(M, -M „)M = j — ----- ^-4x100% (5.7)K - mJ
where Mt is the total mass at a given time t (mg), Mp is the mass of the pan (mg), M0
is the initial mass (mg).
Figure 5.15 shows the water sorption kinetics for PIDA with 0 wt% PAA at a pH of
3, plotted as % mass change of the sample as a function of time (in minutes).
Time (minutes)
Figure 5.15. Water sorption kinetics for PIDA with 0 wt% PAA at a pH of 8
adjusted by NaOH (blue), undergoing a humidity step change from 0% to 70% RH at 25 C, (% mass change of the sample as a function of time in minutes). The red line show the data fitted using the Equation 5.8.
144
Chapter 5
Using the following equation to fit to the data:
l - e x pf - t \ \
(5.8)
one can calculate x, the characteristic time, which is the time it takes to reach around
63 % ofM m, the equilibrium or plateau value. M w gives an indication how
hydrophilic the dry film is. If M m is lower, a lesser amount of water was absorbed by
the dry film, hence the film is less hydrophilic. If is higher, the dry film was
absorbed larger amount of water, hence it should be more hydrophilic, in
comparison. In addition, the characteristic time gives an indication of how slow or
fast the water uptake by the dry film is. A higher x means it takes a longer time to
reach around 63% ofM m, therefore, a slower uptake of water by the dry film, hi
comparison, a lower x indicates that it takes a shorter time to reach 63% ofM w;
therefore, the water uptake by the dry film is faster.
145
Chapter 5
5.5. Results and Discussion
First of all, the drying results (water going out from a wet film) are presented. Then,
water whitening, which is water going into a dry film from liquid water followed by
water absorption results, which is water going into a dry film from vapour, are
presented.
5.5.1. Drying measurements
Figure 5.16 (a), shows the series of MR profiles acquired at five minute intervals for
pure PIDA, latex sample. Attention is drawn to the NMR signal at the beginning of
the drying, where the intensity of the signal is contributed by the signal from the
mobile polymer and the water. When the drying is completed and the film reached its
final thickness, the intensity of the NMR signal is from the molecular mobility in the
polymer melt.
D r y in g t i m e (m in )
Figure 5.16. (a) MR profiles of PIDA , acquired every five minute intervals. When the film reached its final thickness, the intensity of the NMR signal is from the mobile polymer, (b) the zeroth moment (the area under each profile), which is proportional to the water content of the sample at that time, as a function of drying time, (c) the film thickness as a function of time. Both the zeroth moment and the film thickness have a final value due to the signal from the mobile polymer.
NMR signal from mobile polymer + water
200 300 400Film Thickness (nm)
NMR signal from mobile polymer
1.01„ „ C 0.8 «
061 0.40.2 |0.0 z
146
Chapter 5
Figure 5.16 (b), shows the zeroth moment, which is proportional to the water content
of the film at the time when the profile was taken, as a function of the drying time.
Figure 5.16 (c), shows the film thickness as a function of the drying time. Even
though the drying has completed, the zeroth moment and the film thickness have a
positive value due to the signal from the mobile polymer. When the intensity remains
unchanged with the increasing drying time, it is taken that the film is completely dry.
5.5.1.1. Effect o f PAA shell at a low er pH (pH = 3)
By comparing latexes of pure PIDA at a pH of 3 and PIDA with an PAA shell also at
a pH of 3, one can find the effect of the PAA shell in an acidic form on drying.
Figure 5.17 (a), shows the comparisons of the solids fraction as a function of drying
time and (b), as a function of drying time normalised by initial film thickness for
PIDA with 3 wt.% PAA (at a pH of 3) and for PIDA with no PAA (at a pH of 3).
100 150 200
D rying tim e (m in)
250 02 0.4 0.6 08
Drying tim e/ Initial th ick n ess (rriiVum)
Figure 5.17. (a) Solid fraction as a function of drying time and (b) as a function of drying time normalised by the initial film thickness, for as receive, therefore pH = 3, for PIDA with 3 wt.% PAA (filled squares) and for PIDA with no PAA (empty squares). Pure PIDA reach the maximum solid fraction earlier then PIDA with PAA sample, therefore, pure PIDA completed the drying before PIDA with PAA.
By normalising drying time by the initial thickness, it eliminates the effect of the
small differences in the initial thicknesses of the films on drying. As PIDA with no
147
Chapter 5
PAA sample reaches the maximum solids fraction sooner than the PIDA with 3 wt.
% PAA sample, it confirms that pure PIDA dries faster than PIDA with PAA.
However, it was expected that the hydrophilic boundaries of PAA shells would help
the water transport.
5.5.I.2. Effect of PAA shell at a higher pH adjusted with NaOH (pH = 8)
By comparing latexes of pure PIDA at a pH of 8 adjusted by NaOH and PIDA with
PAA shell also at a pH of 8 adjusted by NaOH, one could compare the effect of an
PAA shell in a basic form on drying.
Figure 5.18 compares the solids fraction as a function of drying time normalised by
the initial thickness of the films for pure PIDA and PIDA with 3 wt.% PAA, both at a
pH of 8 adjusted by NaOH. The results show that there is no significant difference
between them. This emphasises that at a higher pH adjusted by NaOH, the PAA shell
does not contribute to drying properties. This result is different from the results of the
same samples at a lower pH of 3. One explanation could be the effect of Na+ from
NaOH in the system. The repulsion between Na+ ions in the system could be keeping
the water pathways open so that the water can reach to the top of the film. If the Na+
effect is stronger, the effect of hydrophilic boundaries of PAA is less important. As a
result, both the films dry in a similar way.
148
Chapter 5
Drying time/ Initial thickness (min/nm)
Figure 5.18. Solids fraction as a function of drying time normalised by the initial film thickness for pure PIDA and PIDA with PAA at a pH of 8 adjusted by NaOH. The effect of PAA shell at a higher pH adjusted by NaOH is insignificant.
5.5.1.3. Effect o f pH on latex w ith PAA shell
To investigate how the hydrophilic channels created by PAA shells at different pH
values affect drying, the drying of PAA shell samples at a pH of 3, and at a pH of 8
adjusted by NaOH and NH4OH were compared. On one hand, the hydrophilic
channels created by PAA could led to a faster drying, as they facilitate pathways for
the water to reach to the top of the film. On the other hand, as they are hydrophilic,
they could hold on to the water and lead to a relatively slower drying process.
Figure 5.19 presents the effect of pH on drying properties of PIDA with PAA PSAs.
PIDA with PAA at a pH of 3 was compared with a pH of 8 adjusted by N aO H and
N H 4O H . In N H 4O H aqueous solution, there will be an equilibrium between N H 4+ +
O H ' and aqueous NH3 + H20 . During drying, N H 3 will evaporate. In N a O H aqueous
solution, the counterion concentration is unaffected. The results reveal that at the
149
Chapter 5
lower pH (pH = 3) the PIDA sample dries faster then the higher pH (pH=8 ) samples.
An explanation could be that, at a low pH, PAA brushes collapse on to the particle
surfaces and create hydrophilic channels or pathways for the water to reach the top of
the film.
1.00
0.96 J
2 0.92o
CO
0.88 -
3AA : pH = 33AA : pH = 8 by NaOH3A A : pH = 8 by NH4QH
0.1 0.2 0.3 0.4 0.5 0.6
Drying time/ Initial thickness (min/nm)
0.7
Figure 5.19. Solids fraction as a function of drying time normalised by the initial film thickness for PIDA with PAA (pH = 3), a pH of 8 adjusted by NaOH and NH4OH.
5.5.2. W ater w hitening m easurem ents
All the PSA films that were submerged in de-ionized water in a square cuvette
gradually turned white, as shown in Figure 5.20. Figure 5.20 is a photograph of (a)
PET substrate with a latex film as soon as it was submerged in water, followed by (b)
after 1 hour, (c) after 7 hours and (d) after 24 hours in the water.
150
Chapter 5
Figure 5.20. Photograph to compare the change in the optical transparency over time: (a) PET substrate with the latex film as soon as it was submerged in water, (b) after 1 hour, (c) after 7 hours and (d) after 24 hours. Image from C.-H. Lei, University of Surrey.
The change in the transparency was measured with a UV/visible spectrophotometer
at a wavelength of 600 nm and plotted as a function of the time duration of the film
immersion in water. The results of all six PSA samples are shown in Figure 5.21.
The PIDA with PAA sample at a pH of 3 (named as 3PAA : pH = 3 in Figure 5.21)
lost the least of its transparency. In comparison, pure PIDA at a pH of 8 adjusted
with NaOH (named as OPAA+NAOH in Figure 5.21) lost its transparency within
around 100 hours of immersion in water. One would expect, PSA samples with
higher pH (i.e. a pH of 8) adjusted by NaOH or NH4OH, to be more hydrophilic,
hence absorb more water in comparison with lower pH (i.e. a pH of 3) samples [19].
The results from current research are in line with this argument. The contradictory
result was the higher transmission shown by the acidic form PIDA with PAA sample
in comparison with the lower transmission by more hydrophobic pure PIDA film at a
pH of 3.
151
Chapter 5
Time in water (hr)
Figure 5.21. Optical transmission at a wavelength of 600 nm as a function of the time the film immersion in water. Open symbols are for the PSA films by pure PIDA latexes and the filled symbols are for the PIDA with PAA samples. The squares for low pH latexes (pH of 3), diamonds are for a pH of 8 adjusted with NH4OH and the circles are for a pH of 8 adjusted with NaOH. PIDA with PAA (pH of 3; filled squares) least lost its transparency and pure PIDA with a pH of 8 adjusted with NaOH the fastest to lose its transparency.
It is clear from the Figure 5.21 that within all three groups of different pH values,
that is pH of 3, pH of 8 adjusted by NaOH and pH of 8 adjusted by NH4OH, the
films with PAA have higher transparency compared to their counterparts. This
should be due to the fact that the PAA in the shell is hydrophilic and the water which
was absorbed stays along the boundaries. Water along the particle boundaries should
not scatter light. In comparison, in the samples with no PAA, the particle boundaries
are more hydrophobic due to the hydrophobic PIDA. When these films were
submerged in water, the absorbed water is more likely to be pushed into pockets
between hydrophobic boundaries. These pockets of water are more likely to scatter
light, hence relatively low transmission compared to the samples with PAA on their
shells would result.
152
Chapter 5
The higher pH samples adjusted by NaOH (with and without PAA) have a lower
transmission than the samples adjusted by NH4OH. It is concluded that the counter
ions are playing a part. In the higher pH samples adjusted by NH4OH, volatile NH3
leaves to the atmosphere, hi comparison, in the samples with higher pH adjusted by
NaOH, the Na+ remains within the system, preferably between polymer particles.
There are two possibilities: there could be water hydration associated with Na+ ions,
or repulsion between these Na+ ions also encourages the creation of water pockets, as
the films were immersed into the water. It is justifiable that either or both of these
possibilities or any other reason related to Na+ ions are significant, as both films with
pH adjusted by NaOH lost their transparency relatively within very short periods of
time compared to the other samples.
Even though, the transparency was used as an indicator of the amount of water
uptake, it should be noted that there is a question of accuracy of this practice, as the
relation between water uptake and transparency is not fully known. As an example, a
film could uptake a large amount of water, but if this water was evenly distributed
along the boundaries, without creating water pockets, then the film should still
transmit light [11]. Tzitzinou and co-workers [25] predicted the relation between
optical transmission and radius of air voids and volume fraction of those voids
(Figure 5.8). Nevertheless, it’s not proven that these air voids can be used as a direct
measurement of water uptake. As a step forward, the moisture sorption
measurements were performed. Now, the dry films were exposed to high humidity
and the moisture soiption was measured as a weight gain. The weight increase within
the dry film was only caused by the water vapour absorbed by the film. Therefore,
moisture soiption measurements are a direct method of measuring the water uptake
by the dry films. Transmission measurements were also carried out on the films
which were exposed to high humidity, but the changes in the transparency were too
small to detect by the UV/visible spectrophotometer.
153
Chapter 5
5.5.3. M oisture Sorption M easurem ents
Moisture from high humidity going into a dry film was measured, and the results are
presented in a similar order as the drying measurements, to look separately at the
effects of PAA and pH on moisture uptake. Figures 5.22, 5.23 and 5.24 represent the
sample mass response to a step change in humidity from 0 - 70% RH, [17]. Note that
at short times the initial mass uptake will be dominated by adsorption, and at later
times the mass increase arises due to diffusion of water into the film.
5.5.3.1. Effect o f PAA at a low er pH (pH o f 3)
Figure 5.22 shows a comparison of the % mass change as a function of square root of
time for films with and without PAA. The sample with 3 wt.% PAA in the shell
showed a higher % mass change compared with the sample with 0 wt.% PAA. It is
likely that the sample with PAA in its shell had higher hydrophilicity on its particle
surfaces compared with the sample with no PAA, hence it showed greater % mass
change at high humidity.
0 10 20 30 40TimeA1/2 (minA1/2)
Figure 5.22. % mass change as a function of square root of time for 0 wt.% PAA sample (empty squares) and 3 wt.% PAA sample (filled squares). Sample with PAA in its shell shows higher % mass change, compared to no PAA sample.
154
Chapter 5
5.5.3.2. Effect of PAA at a higher pH adjusted with NaOH (pH of 8)
Figure 5.23 compares the effect of PAA in the shell at a higher pH of 8 adjusted with
NaOH. At a higher pH of 8 , the sample with PAA in the shell showed the greater %
mass change, compared with no PAA. Figure 5.22 and 5.23 show that having PAA in
the shell makes the samples more hydrophilic in both acidic and basic form.
0 10 20 30 40Tim e1'2 ( (m in)1'2)
Figure 5.23. % mass change as a function of square root of time for 0 wt.% PAA sample (empty squares) and 3 wt.% PAA sample (filled squares) at a pH of 8
adjusted by NaOH. Sample with PAA in its shell shows higher % mass change, compared to no PAA sample, in a basic form.
5.5.3.3. Effect of pH on latex with PAA shell
Figure 5.24 compares the effect of pH on % mass change for the samples with PAA
in their shells. The lower pH film shows the highest % mass change. Furthermore,
of the pH of 8 adjusted by NaOH is very close to that of lower pH. The
155
Chapter 5
difference between these two samples is the characteristic time, x, which is 160
minutes for lower pH and 280 minutes for higher pH adjusted by NaOH. In other
words, the lower pH film shows fast uptake of moisture compared with higher pH. In
contrast, a pH of 8 adjusted by NH4OH shows the lowest % mass change; but its x is
slightly higher than that of low pH but significantly lower than the sample with a pH
of 8 adjusted by NaOH.
0.9 |-----
0.8 -
0.7 -a>» 0.6 -no 0.5 -tn| 0.4 -
SS 0.3 -
0.2 -
0.1 -
0.0 - L.
0
Figure 5.24. % mass change as a function of square root of time for the samples with PAA in their shells. Red squares for a pH of 3, blue circles for a pH of 8
adjusted by NaOH and black diamond for a pH of 8 adjusted by NH4OH.
Table 5.2 summarises the calculated M ^ a s a %), equilibrium or plateau value
obtained from the water sorption kinetics and characteristic time for the different
samples which were examined.
3 AA pH33 AA pH8 by NaOH 3 AA pH8 by NH40H
10 20 30 40
Time172 ((min)172)
156
Chapter 5
Table 5.2 Calculated M ^ and characteristic time, r for different samples.
Sample (% ) t (min)
0 PAA (pH=3) 0.35 103
0 PAA (pH=8 , by NaOH) 0.58 2 0 0
0 PAA (pH=8 , by
NH4OH)0.38 184
3 PAA (pH=3) 0.79 160
3 PAA (pH=8 , by NaOH) 0.75 280
3 PAA (pH=8 , by
NH4OH)0.53 180
■ 3AA PH s= 3□ 0AA PH 5= 3♦ 3AA PH := 8 by NH4 OH
<0 0AA PH := 8 by NH4 OH• 3AA PH s= 8 by NaOHO 0AA PH = 8 by NaOH
0.2 -
0.0 —1---------- ■ 1 * 1 * 1 *--------100 150 200 250 300
Characteristic time (min)
Figure 5.25. as a function of calculated characteristic time, x, for all sixsamples. The samples without PAA showed lower M ^ compared with the samples which contained PAA in their shells.
157
Chapter 5
M m is an indication of how hydrophilic the sample is. M m values in Table 5.2 and
Figure 5.25 show that the samples with 0 wt.% PAA have lower values
compared with higher values of 3 wt.% PAA samples. A possible explanation could
be that the hydrophilic PAA on the particle shells creates hydrophilic pathways in the
dry films, and water is easily adsorbed into the films through these pathways. The
suggestions of De Brayn and co-workers [20], which was that the hairy layer
thickness of PAA shell increase with the increase of pH, was less obvious in this
current work. It was found that was higher at a pH of 3. Further more, Rharbi
and co-workers [2 1 ] reported that their un-dialysed adhesive films by core-shell
particles with PAA shells showed greater water uptake. The films made with
dialysed latexes also showed greater water uptake, if they were neutralised by NaOH,
but water uptake was very low for the samples kept in the acidic form. The current
results are partially in an agreement with Rharbi and co-workers’ observations. All
the samples in the current work are dialysed. For 3 wt.% PAA, a pH of 8 adjusted by
NaOH shows a higher compared to 0 wt.% PAA. In contradiction to their results,
the current results show the films made with dialysed acidic form of PAA also show
a higherMro compared to basic form PAA, adjusted by NH4OH.
5.5.4. Correlation of drying, water whitening and moisture
sorption measurements
Possible correlations between water loss, liquid water uptake and vapour uptake are
investigated. The results are presented against each other. Desirable properties of a
given parameter are indicated by the direction of an arrow head, and an expected
trend of a chosen parameter is shown by a coloured, dotted line.
158
Chapter 5
5.5 .4 .I. C orrelation o f drying and w ater w hitening
From the GARField measurements, the time to lose all the water from each PSA
sample was obtained. It was assumed to be when the intensity of the signal continued
to remain constant with increasing drying time that all the mobile water was
evaporated. This time was normalised by the initial film thickness of each sample.
Using the optical transmission measurements, the transmission at 600 nm was
obtained for each sample after each was immersed in water for 200 minutes. Figure
5.26 compares the transmission after 200 minutes in water as a function of
normalised time to lose water for all six PSA samples.
If a wet film develops a skin during drying, the normalised time to lose water should
be relatively higher. If this skin acts as a barrier, then the film should show a higher
transmission after soaking in water. This correlation is represented by the green
dotted line on Figure 5.26. If we argue that hydrophilic films hold water, then the
samples with higher normalised time to lose water should have lower transmission
after 200 minutes in water. In comparison, more hydrophobic films should hold less
water, hence there will be a lower normalised time to lose water and higher
transmission. This correlation is represented by the yellow dotted line.
159
Chapter 5
u 1-05 13| 0.8<A C
E 0.6o oCM
| 0.4 re
■i 0.2wE(Ag 0.0H
0.3 0.4 0.5 0.6 0.7
Normalized time to lose water, tdry, (minI nm)
Figure 5.26. Optical transmissions of the films after 200 minutes in water as a function of normalised time to lose all the mobile water within the films. The drying properties get desirable towards the arrow 1 and the water resistance properties get desirable towards the arrow 2 .
The lower the normalised time to lose water, then the faster is the drying, which is
indicated by the direction of the arrow head one. The greater the transmission of the
film after 2 0 0 minutes in water, then the higher is the water resistance properties,
which is indicated by the direction of the arrow head two. Therefore, out of all six
samples, the two PSA latex samples with a pH of 3 show the most desirable
combined properties. These two samples have a relatively higher transmission after
2 0 0 minutes in water and a relatively lower normalized time to lose water compared
with the other remaining four samples.
From the results presented in Figure 5.26, it can be concluded that pH of the wet
latex affects the transmission of the dry film after it was immersed in water for 2 0 0
minutes. Both the lower pH samples, at a pH of 3, showed higher transmission
properties compared with the higher pH samples. In the same manner, the lower pH
samples also showed better drying properties compared with higher pH samples. No
obvious correlation of drying and water whitening is to be seen. Very roughly, the
samples with and without PAA, follow the argument that hydrophobic films hold less
■ ■ 3AA : pH = 3□ 0A A : pH = 3
.•** + 3A A : pH = 8 by NH4OH
, ■ x. „• O 0A A : pH = 8 by NH4OH’D ' '' S • 3A A : pH = 8 by NaOH
O 0A A : pH = 8 by NaOH
♦0
I •' .- % ■ y V • 0
1 . 1 . 1 ,
160
Chapter 5
water, hence lower normalised time to lose water, tdry, and higher transmission. The
basic samples form films also roughly following the argument that hydrophilic films
hold on to the water, hence higher tdry, and lower transmission.
5.5.4.2. C orrelation o f drying and m oisture sorption
Again, if we consider the skin development during drying as a barrier to lose water
(hence normalised time to lose water is higher) and a barrier for water to go in to the
film (low M x ), then this inverse correlation would be represented by the yellow
dotted line on Figure 5.27 (a). If we compare characteristic time, i, and drying
together, one could expect the skin development to hold water within the film during
drying, hence a higher normalised time to lose water would correlate with a higher x.
This positive correlation is represented by the green dotted line on Figure 5.27 (b).
0.3 0.4 0.5 0.6 0.7Norm alized tim e to lose water, t ^ , (m in/ ^m)
■ 3AA PH ; 3□ 0AA PH = 3♦ 3AA PH = 8 by n h 4o h
o 0AA PH = 8 by n h 4o h
• 3AA PH = 8 by NaOHo 0AA PH - 8 by NaOH
161
Chapter 5
(b)■ 3AA PH == 3□ OAA PH := 3♦ 3AA PH == 8 by NH4OH
o OAA PH == 8 by NH4 OH• 3AA PH 5= 8 by NaOHo OAA PH = 8 by NaOH
Normalized time to lose water, tdry) (minI ^m)
Figure 5.27. (a) M ^ as a function of normalised time to lose all mobile water with in the films. The drying properties get better towards the arrow 1 and the dry films adsorb less moisture from high humidity towards the arrow head two. (b) t as a function of normalised time to lose all the mobile water with in the films. Towards the direction of arrow head two, the dry films adsorb moisture faster from high humidity.
In addition to the drying properties getting more desirable towards the direction of
arrow 1 in Figure 5.27 (a), the dry films show less hydrophilic behaviourin the
direction towards arrow 2 with smallerM x . Out of all six samples, the PSA film
without PAA with a pH of 3 shows the best combined drying and water adsorption
properties. In addition, except for this PSA film, the rest of the films somewhat
follow the expected trend (yellow dotted line) between and normalised time to
lose water, tdry. With low values of x (Figure 5.27 (b)), the dry films show fast uptake
of moisture. One could argue that a fast drying film should show fast water uptake as
well. It is observed in Figure 5.27 (b), the PSAs with a pH of 3 are in a greater
agreement with this concept and the samples with a pH of 8 adjusted by NH4OH are
in least agreement. However, it should be noted that during drying of a wet film, the
film formation is not yet completed. As the water leaving the drying film, close
packing of particles, particle deformation, and coalescence/interdiffusion/aging of
162
Chapter 5
particles (Figure 2.9) still exist. When a dried film is exposed to high humidity or
submerged in water, the film formation is complete. Therefore, a fast drying film
does not necessarily show fast uptake of moisture/water as the nature of the films in
the two different situations is different.
S.5.4.3. C orrelation o f m oisture absorption and w ater
w hitening
Correlation between water going into a dry film from liquid water and water going
into a dry film from vapour was studied. It was expected that films with high
transparency would have lower , and the samples with higher M m would be less
transparent. This correlation is represented by the yellow dotted line in Figure 5.28
(a), furthermore, it was expected that the films with higher transmission after 2 0 0
minutes in water would have better barrier properties. The same films are to be
expected to have higher t, as the films have better barrier properties. This correlation
is represented by the green dotted line in Figure 5.28 (b).
As discussed earlier, the greater the transmission of the film after 200 minutes in
water, the higher are the water resistance properties, indicated by the direction of the
arrow head one, in Figure 5.28 (a) and (b). It is more desirable to have a l o w e r ;
then, these films should be less hydrophilic. This correlation is indicated by the
direction of the arrow head two, in Figure 5.28 (a). Even though, it is indicated
towards the direction of arrow head two, in Figure 5.28 (b), that lower t is more
desirable, it is open for discussion.
It was observed in Figure 5.28 (a), that was affected by PAA; for all three
different pH values, the samples with PAA had a higher (unbroken brown curve
in Figure 5.28 (a)) compared with the samples without PAA (dashed brown curve in
Figure 5.28 (a)). The films with PAA were more hydrophilic, hence they absorbed
163
Chapter 5
more water, and as a result, higher M ̂ . With the exception of the acidic form film
with PAA, there is a general downward trend between A/^and transmission. The
correlation between t and transmission after 2 0 0 minutes in water shows a rough
negative connection.
Transmission after 200 mins in water
■ 3AA PH == 3□ 0AA PH 1= 3♦ 3AA PH ss 8 by NH4 OH
0 0AA ■■za. = 8 by NH4 OH• 3AA PH == 8 by NaOH0 0AA PH s= 8 by NaOH
Transmission after 200 mins in water
■ 3AA : pH = 3 □ 0AA : pH = 3 + 3AA : pH = 8 by NH4OHo 0AA : pH = 8 by NH4OH• 3AA : pH = 8 by NaOH O 0AA : pH = 8 by NaOH
Figure 5.28. (a) M ^ and (b) x as a function of optical transmissions (at 600 nm) of the films after 2 0 0 minutes in water.
164
Chapter 5
There is a possibility of phase inversion occurring during drying. Phase inversion is
the phenomenon whereby the dispersed phase inverts to become the continuous
phase and vice-versa under conditions determined by the system properties, volume
ratio and energy input [41]. If, phase inversion occurs during drying in PAA samples,
there is a possibility o f having PIDA, hydrophobic pathways instead of PAA,
hydrophilic pathways. As a result PAA samples also should behave like pure PIDA
samples.
5.5.5. Probe-Tack A dhesion M easurem ents
Experiments were carried out to determine whether slow drying has any
consequences for adhesion properties. Differences in the drying times were observed.
One could argue that the presence of water in a PSA film could interfere with
coalescence and decrease the cohesive strength.
Figure 5.29 (a), reveals adhesion properties of the PSA film with 3 wt.% PAA in the
shell at a pH of 3. Tack measurements have been performed on the same film at
different points in time: at 90 minutes, 24 hours, and 6 days after it was cast. £-max and
Ea both were elevated with increasing drying time. The adhesion properties of PSA
film with 3 wt.% PAA, at a pH of 8 by NH4OH are shown in Figure 5.29 (b). As the
drying time increases, £^ax and E& both were increased. The elevation was significant
between one and five days. Figure 5.29 (c), summarises the adhesion energy of PSA
film with 3 wt.% PAA in the shell at pH 3 and at pH 8 by NH4OH for different
drying times. Earlier, using drying measurements it was shown that films prepared
by low pH were fast-drying compared with the films prepared at high pH. As a
result, the low pH film lost a significant amount of water during the first day
compared to the period between day one to day six. The adhesion energy follows the
same trend; there is a significant increase during the first day, and a lesser increase
from day one to day six. In comparison, slow drying higher pH films lost a
significant amount of water from day one to day six. It is speculated the adhesion
energy was significantly increased during the periods when most water was lost.
165
Chapter 5
90 min 24 hrs 6 days
Strain
□ 3AA: pH = 3 o 3AA: pH = 8 by NH4OH
Drying time (days)
Figure 5.29. (a) The adhesion properties of PSA film with 3 wt.% PAA in the shell, at a pH of 3, performed at 90 minutes, 24 hours and 6 days after it was cast, (b) The adhesion properties of PSA film with 3 wt.% PAA, at a pH of 8 by NH4OH for different drying times and (c) summarises the adhesion energy of PSA film with 3 wt.% PAA in shell at a pH of 3 and at a pH of 8 NH4OH for different drying times.
166
Chapter 5
5.6. Conclusions
The effects of PAA and pH on drying, water whitening, water sorption from high
humidity and adhesive properties have been investigated. The drying measurements
confirm that pure PIDA samples dry faster than the samples with PIDA with 3wt.%
PAA shells, at a lower pH of 3. An explanation could be that the films with more
hydrophobic PIDA should have hydrophobic particle boundaries and during drying
these hydrophobic boundaries could push the remaining water to the top of the film,
resulting in faster drying. Physically, there could be a de-wetting of the water from
the polymer. The sample with 3 wt.% PAA in their shells should have hydrophilic
boundaries, and during drying water molecules could hang on to these hydrophilic
boundaries, hence a slower drying is observed. On the other hand, one could argue
that these hydrophilic boundaries facilitate pathways for the water to be transported
to the top of the film. But, there is little evidence for the second argument. An
important conclusion o f the current work is, at least fo r this system, that hydrophilic
pathways do not contribute to faster drying.
Furthermore, the investigations of the effects of pH on PAA revealed that in an
acidic form, the sample dries faster than in a basic form. De Bruyn and co-workers
[20] reported that the hairy layer thickness of PAA on particles increases when the
pH is increased and the shell was more hydrated. The current research findings are in
an agreement with De Bruyn’s findings, as slower drying was observed with high
pH. When the shell was more hydrated, release of water from the drying film was
slow. On the other hand, one would argue that hairy layers could speed up drying by
keeping the boundaries “open”. Current investigations disprove, at least fo r this
system, the idea that a hairy layer o f PAA keeps the film more open and leads to
faster drying.
Water whitening experiments revealed that the samples with a higher pH of 8
adjusted by NaOH or NH4OH, lost their transparency faster and more than the
samples with a lower pH of 3. This difference in transmission could be due to the
fact that higher pH samples are more hydrophilic, hence absorbed more water and
lost their transparency. The lower pH samples are more hydrophobic and absorbed
167
Chapter 5
less water, therefore they remained transparent. These results are in accordance with
the findings of Chen and co-workers [18], who performed wetting-resistance
experiments using contact angle measurements of the films which were modified by
introducing hydrophobic perfluaroalkyl groups into the polymer. These groups
prevented water molecules into the film’s interior, and improved the water resistance
properties. The water whitening results of the films prepared using 3 wt.% PAA and
0 wt.% PAA at a pH of 3 were somewhat surprising, as 3 wt.% PAA is more
hydrophilic, but it is also the most transparent. The relation between water uptake
and transparency is not fully known. A film which uptakes a large amount of water
could still transmit light, if this water was evenly distributed along the particle
boundaries (Figure 5.1). In fact, in the current study, the PSAs with 3wt.% PAA
shells should even support the distribution of absorbed water along their hydrophilic
boundaries. In addition, the moisture absorption by high humidity experiments
shows that M m is the highest for 3 wt.% PAA in an acidic form. An important
conclusion o f the current work is that the films with hydrophilic boundaries absorb a
large amount o f moisture, but still retain their transparency, as absorbed moisture is
evenly distributed along the boundaries. Transparency or water whitening is not
necessarily a reliable measurement o f water uptake, at least fo r this system.
Water sorption curves of % mass change as a function of square root of time in the
current study shows the typical soiption curve type which was discussed by Van der
Wei and Adan [17]. The soiption curves show an initial linear dependency of mass as
a function of square root of time, and then smoothly level off to a saturation level of
mass, with increasing time. The soiption results were comparable with the rest of
these current findings and earlier literature findings alike. More hydrophilic films
(PIDA with 3 wt.% PAA shells) adsorbed more water compared to less hydrophilic
films (0 wt. % PAA).
The results from current research of drying measurements (td,y), water whitening
measurements (T), water soiption measurements (M ro and x) and the adhesion energy
after one day of drying from probe-tack adhesion measurements for all six samples
are summarised in Table 5.3.
168
Chapter 5
Table 5.3 A summary of the results for all six samples of PSAs.
Sampletdry
(min/pm)T 00
Tau
(min)
adhesion
energy
(J/m2)
OPAA
(pH=3)0.31 0.67 0.35 103 50.3
OPAA
(pH=8, by 0.55 0 .0 1 0.58 2 0 0 109.6
NaOH)
OPAA
(pH=8, by 0.65 0.26 0.38 184 74.5NH4OH)
3 PAA
(pH=3)0.38 0.99 0.79 160 64.6
3 PAA
(pH=8, by 0.5 0 .0 1 0.75 280 48.5NaOH)
3 PAA
(pH=8, by 0.65 0.33 0.53 180 50.1
NH4OH)
In the Table 5.3, the results marked with blue were the most desirable for
applications out of all the six samples, for that particular type of measurement, and
the results marked with green were second best. The PIDA sample without PAA at a
pH of 3 shows the better results in more cases than the rest of the samples. The
effects of calculated characteristic time, 1 (min), from water sorption measurements,
on performance of PSAs need to be discussed and investigated further. We are
unable to conclude whether high or low value of x would be desirable in a PSA.
From these results it can be concluded that the PIDA sample without PAA at a pH of
3 shows the best compromise with the most desirable properties out of this series.
This film showed the most desirable transparency measurement result and the second
169
best results for drying andM ^. The worst compromise are the PSAs with 3 wt.%
PAA, with higher pH adjusted by NaOH and NH40H, as these films failed to
perform well in any type of measurement.
________________________________________________________________ Chapter 5
170
Chapter 5
5.7. R eferences
1. Jotischky, H., Coatings, regulations and the environment reviewed. Surface
Coatings International Part B-Coatings Transactions, 2001. 84(1): p. 11-20.
2. Jovanovic, R. and Dube, M.A., Emulsion-based pressure-sensitive adhesives:
A review. Journal of Macromolecular Science-Polymer Reviews, 2004.
C44(l): p. 1-51.
3. Lovell, P.A., El-Aasser, M.S.,, Emulsion Polymerisation and Emulsion
Polymers. 1997: John Wiley & Sons Ltd. UK.
4. Garrett, J., Lovell, P.A., Shea, A.J., and Viney, R.D., Water-borne pressure-
sensitive adhesives: Effects o f acrylic acid and particle structure.
Macromolecular Symposia, 2000.151: p. 487-496.
5. Fitch, R.M., Oplymer Colloids: A Comprehensive Introduction. 1997:
Academic Press, New York.
6 . Satas, D.E., Handbook o f Presure-Sensitive Adhesive Technology. Vol. 2nd
edition. 1989: Van Nostrand Reinhold, New York.
7. Wang, T., Interfacial Control in Colloidal Nanocomposites fo r Presure-
Sensitive Adhesives. 2008, University of Surrey, Guildford, UK.
8 . Lindner, A., Lestriez, B., Mariot, S., Creton, C., Maevis, T., Luhmann, B.,
and Brummer, R., Adhesive and Theological properties o f lightly crosslinked
model acrylic networks. Journal of Adhesion, 2006. 82(3): p. 267-310.
9. Creton, C., Pressure-sensitive adhesives: An introductory course. Mrs
Bulletin, 2003. 28(6): p. 434-439.
10. Wang, T., Lei, C.H., Dalton, A.B., Creton, C., Lin, Y., Fernando, K.A.S.,
Sun, Y.P., Manea, M., Asua, J.M., and Keddie, J.L., Waterborne,
nanocomposite pressure-sensitive adhesives with high tack energy, optical
transparency, and electrical conductivity. Advanced Materials, 2006. 18(20):
p. 2730-+.
11. Donkus, L.J., Solvent-like performance from emulsion PSAs: Advances in
water-whitening resistance. Adhesives Age, 1997. 40(10): p. 32-+.
171
Chapter 5
12. Aramendia, E., Barandiaran, M.J., Grade, J., Blease, T., and Asua, J.M.,
Improving water sensitivity in acrylic films using surfmers. Langmuir, 2005.
21(4): p. 1428-1435.
13. Ciampi, E. and McDonald, P.J., Sian formation and water distribution in
semicrystalline polymer layers cast from solution: A magnetic resonance
imaging study. Macromolecules, 2003. 36(22): p. 8398-8405.
14. Keddie, J.L. Film Formation o f Waterborne Coatings 2006. Emulsion
Polymerisation Processes Course.
15. Narita, T., Hebraud, P., and Lequeux, F., Effects o f the rate o f evaporation
and film thickness on nonuniform drying o f film-forming concentrated
colloidal suspensions. European Physical Journal E, 2005.17(1): p. 69-76.
16. Agarwal, N. and Farris, R.J., Water absorption by acrylic-based latex blend
films and its effect on their properties. Journal of Applied Polymer Science,
1999. 72(11): p. 1407-1419.
17. van der Wei, G.K. and Adan, O.C.G., Moisture in organic coatings - a
review. Progress in Organic Coatings, 1999. 37(1-2): p. 1-14.
18. Chen, Y.J., Zhang, C.C., and Chen, X.X., Emulsifier-free latex o f fluorinated
acrylate copolymer. European Polymer Journal, 2006. 42(3): p. 694-701.
19. Feng, J.R. and Winnik, M.A., Effect o f water on polymer diffusion in latex
films. Macromolecules, 1997. 30(15): p. 4324-4331.
20. De Bruyn, H., Gilbert, R.G., White, J.W., and Schulz, J.C., Characterization
o f electrosterically stabilized polystyrene latex; implications fo r radical entiy
Idnetics. Polymer, 2003. 44(16): p. 4411-4420.
21. Rharbi, Y., Boue, F., Joanicot, M., and Cabane, B., Deformation o f cellular
polymeric films. Macromolecules, 1996. 29(12): p. 4346-4359.
22. Wang, T., Canetta, E., Weerakkody, T. G., Keddie, J.L., Rivas, U.„ pH
Dependence o f the properties o f waterborne pressure-sensitive adhesives
containing acrylic acid American Chemical Society, 2009.1(3): p. 631-639.
23. Yang, Y.K., Li, H., and Wang, F., Studies on the water resistance o f acrylic
emulsion pressure-sensitive adhesives (PSAs). Journal of Adhesion Science
and Technology, 2003.17(13): p. 1741-1750.
24. Blackley, D.C., Polymer latices/ 2, Types oflatices. Vol. 2. 1997: Chapman
& Hall, London
172
Chapter 5
25. Tzitzinou, A., Keddie, J.L., Geurts, J.M., Peters, A.C.I.A., and Satguru, R.,
Film formation o f latex blends with bimodal particle size distributions:
Consideration o f particle deformability and continuity o f the dispersed phase.
Macromolecules, 2000. 33(7): p. 2695-2708.
26. Vorwerg, L. and Gilbert, R.G., Electrosteric stabilization with poly(aciylic)
acid in emulsion polymerization: Effect on Idnetics and secondary particle
formation. Macromolecules, 2000. 33(18): p. 6693-6703.
27. Gabriel, G.J., Microscopic Theory o f Rayleigh-Scattering. Physical Review
A, 1973. 8(2): p. 963-990.
28. McDonald, P.J. and Newling, B., Stray field magnetic resonance imaging.
Reports on Progress in Physics, 1998. 61(11): p. 1441-1493.
29. Mallegol, J., Bennett, G., McDonald, P.J., Keddie, J.L., and Dupont, O., Skin
development during the film formation o f waterborne acrylic pressure-
sensitive adhesives containing tacldfying resin. Journal o f Adhesion, 2006.
82(3): p. 217-238.
30. Koenig, A.M., Weerakkody, T.G., Keddie, J.L., and Johannsmami, D.,
Heterogeneous drying o f colloidal polymer films: Dependence on added salt.
Langmuir, 2008. 24(14): p. 7580-7589.
31. Lakrout, H., Sergot, P., and Creton, C., Direct observation o f cavitation and
fibrillation in a probe tack experiment on model acrylic Pressure-Sensitive-
Adhesives. Journal of Adhesion, 1999. 69(3-4): p. 307-359.
32. Chikina, I. and Gay, C., Cavitation in adhesives. Physical Review Letters,
2000. 85(21): p. 4546-4549.
33. Gay, C. and Leibler, L., Theory o f tacldness. Physical Review Letters, 1999.
82(5): p. 936-939.
34. Wang, T., Lei, C.H., Liu, D., Manea, M., Asua, J.M., Creton, C., Dalton,
A.B., and Keddie, J.L., A molecular mechanism fo r toughening and
strengthening waterborne nanocomposites. Advanced Materials, 2008. 20(1):
p. 90-+.
35. Chuang, H.K., Chiu, C., and Paniagua, R., Avery Adhesive Test yields more
performance data than traditional probe. Adhesives Age, 1997. 40(10): p.
18-23.
173
Chapter 5
36. Zosel, A., Adhesion and Tack o f Polymers - Influence o f Mechanical-
Properties and Surface Tensions. Colloid and Polymer Science, 1985. 263(7):
p. 541-553.
37. Chiche, A., Dollhofer, J., and Creton, C., Cavity growth in soft adhesives.
European Physical Journal E, 2005.17(4): p. 389-401.
38. Creton, C., Hooker, J., and Shull, K.R., Bulk and interfacial contributions to
the debonding mechanisms o f soft adhesives: Extension to large strains.
Langmuir, 2001. 17(16): p. 4948-4954.
39. Shull, K.R. and Creton, C., Deformation behavior o f thin, compliant layers
under tensile loading conditions. Journal of Polymer Science Part B-Polymer
Physics, 2004. 42(22): p. 4023-4043.
40. Cleaver, J.A.S. and Wong, P., Humidity-induced surface modification o f
boric acid. Surface and Interface Analysis, 2004. 36(13): p. 1592-1599.
41. Yeo, L.Y., Matar, O.K., Hewitt, G.F., Ortiz, E.S., , Phase Inversion and
associated phenomena : A review. To appeal* in Multiphase Science and
Technology.
174
Chapter 6
Conclusions and Future Work
6.1. C onclusions
The overall task of this research, which was to investigate the physical characteristics
of the film formation process and film properties of latexes, was achieved. A series
of organic/inorganic waterborne nanocomposites for coating formulations and a
series of waterborne pressure sensitive adhesive formulation samples were studied.
The samples from two main systems, Original Route I and Original Route II, were
studied. In Route I latexes, Laponite plates were located on the surfaces of the
polymer particles; and in Route II latexes, the clay particles were encapsulated within
the latex particles. For each system four different latexes with different Laponite
contents (0, 3, 5 and 7 wt.%) were studied. Each of these samples was investigated to
6.1.1. C onclusions about organic/inorganic
nanocom posite coating form ulations
Chapter 6
determine their drying properties, as well as their visual appearance and the optical
transmission of dry films. It was found that the samples by Route I dried with no
significant differences in comparison to a standard acrylic latex. In contrast, latexes
by Original Route II with Laponite showed thickness increases in the centre of the
films during drying. All latexes with Laponite, showed this effect, but the 3 wt.%
sample showed the strongest effect. This was confirmed by the GARField profiles
and the images taken from the side of the drying films.
It was understood that for Route II, Laponite plates were doubly functionalised by
DDAB and y - MPTMS in order to render a good compatibility to the Laponite plates
with the monomer mixture. The amount of DDAB used was significant, which is
twice the CEC of Laponite. It was argued that the excess DDAB in the system could
cause a surface tension gradient within the drying film which would induce a
Marangoni flow of liquid [1-3]. These dried films were proven to have a ‘Mexican
Hat’ shape thickness variation. This concept was further investigated with a concave
wet film; where particles were expected to close pack from the centre of the drying
film and to induce Marangoni flow from the centre towards the edges of the film. It
was confirmed that the dry film thickness increase was in the middle of the radial
distance from the centre to the edge of the dried film.
Two new formulations for Route II, with 3 wt.% Laponite were developed with only
100 % of the CEC of Laponite in their systems; one with low solids (20 wt.%) and
the other with a relatively high solid content (30 wt.%). The drying properties of
these two new formulation Route II samples were dramatically improved. Films cast
from concave wet films showed no evidence for any lateral liquid flow, indicationg
that Marangoni effects were not active.
It was concluded that excess surfactant in a system could affect the physical
characteristics of the film formation process, and by adjusting the amount of
surfactant in the system, these properties could be improved.
176
Chapter 6
6,1.2. C onclusions about pressure sensitive
adhesive form ulations
An extensive study on water going out of a wet film (i.e. drying), liquid water going
into a dry film (i.e. water whitening), and vapour going into a dry film (i.e. vapour
absorption from high humidity) along with the adhesion properties of PSA films has
been completed.
Two sets of PSA formulations, one set with hydrophobic PIDA and the other set with
a PIDA core surrounded with a PAA shell, were investigated for the effects of PAA
and pH on drying, water whitening, vapour sorption from high humidity and
adhesive properties. Three pure PIDA samples with a pH of 3, a pH of 8 adjusted by
NaOH, and a pH of 8 adjusted by NH4OH were investigated. In parallel, three PIDA-
core PAA shell samples with the same pH values were also studied.
It was clear that pure PIDA samples at a pH of 3 showed the most desirable drying
properties. One would expect that the hydrophobic PIDA boundaries would push or
de-wet the remaining water from the drying film to the top of the film, resulting in
faster drying. There was less experimental evidence to support the idea that the
hydrophilic boundaries of PIDA/AA core/shell particles enable water transport
during drying. Among the samples with a PAA shell, the lower pH samples dried
faster than the higher pH. These results can be explained using De Bmyn’s findings
[4]. It was found that the hydrophilic pathways created by PAA shells do not
contribute to faster drying. In addition, this investigation disproves the idea that the
hairy layer of PAA keeps the film more open and leads to faster drying.
Moisture soiption experiments proved that more hydrophilic films (PIDA with PAA
shells) absorbed more moisture from high humidity compared to less hydrophilic
films with no AA (Figure 5.28 (a)). It was shown that the extent of drying affects the
adhesion properties of PSAs. Adhesion energy increases as drying proceeds.
177
Chapter 6
Water whitening experiments confirmed that the more hydrophilic, higher pH
samples lost their transparency faster and more than the low pH samples, as one
would expect if hydrophilic boundaries would allow water transport more than the
less hydrophilic boundaries. These findings can be explained using the earlier reports
of Chen and co-workers [5]. Drawing on the water vapour sorption experiments, it is
clear that films with hydrophilic boundaries (PSA film with 3 wt.% PAA at a pH of
3) absorbed more water than the film without PAA. However, the hydrophilic films
still retain their transparency. A likely explanation is that the 3 wt.% PAA film at a
pH of 3 absorbs water uniformly along the particle boundaries, so that light is not
scattered. It was shown that transparency or water whitening is not necessarily a
reliable measurement of water uptake, at least for this system.
178
Chapter 6
6.2. Future Work
6.2.1. Future work for organic/inorganic
nanocomposite coating formulations
To progress further in this study, additional insight into the excess surfactant in the
system is needed, hi the proposed process (Figure 4.22.), desorption of excess
DDAB molecules into the surrounding serum is significant. Attempts to confirm this
movement of surfactant is needed. This study would be most complete if one could
model the experimental results of Marangoni flow of fluid where surface tension
gradient develops in a system. Microscopy of film cross-section could be used to
explain the loss of optical clarity.
6.2.2. Future work for Pressure Sensitive Adhesive
formulations
In Chapter Five, the liquid water going into a dry film was investigated by optical
transmission, and the moisture sorption was investigated by a gravimetric method. In
the future, attempts could be made to verify the water whitening by a gravimetric
method and, as a comparison, moisture absorption from vapour could be supported
by optical transmission. As mentioned in Chapter Five, efforts were made to carry
out transmission measurements on the films which were exposed to high humidity.
The procedure was quite challenging as the changes in the transparency were too
small to detect by the UV/visible spectrophotometer. More sensitive equipment could
resolve the difficulties of transparency measurements of the PSA films which are
exposed to high humidity.
179
Chapter 6
6.3. References
1. Girard, F., Antoni, M., and Sefiane, K., On the effect o f Marangoni flow on
evaporation rates o f heated water drops. Langmuir, 2008. 24(17): p. 9207-
9210.
2. Hu, H., and Larson, R.G., Analysis o f the effects o f Marangoni stresses on the
microflow in an evaporating sessile droplet. Langmuir, 2005. 21(9): p. 3972-
3980.
3. Hibiya, T., et al., Attempt to study Marangoni flow o f low-Pr-number fluids
using a liquid bridge o f silver. Advances in Space Research, 2008. 41(12): p.
2107-2111.
4. De Brayn, H., et al., Characterization o f electrosterically stabilized
polystyrene latex; implications fo r radical entry Idnetics. Polymer, 2003.
44(16): p. 4411-4420.
5. Chen, Y.J., Zhang, C.C., and Chen, X.X., Emulsifier-free latex o f fluorinated
acrylate copolymer. European Polymer Journal, 2006. 42(3): p. 694-701.
180