EFFECT OF INORGANIC PIGMENTS
ON POLYMER INTERDIFFUSION
IN LOW-Tg LATEX FILMS
Mitsuru Kobayashi
A thesis submitted in conformity with the requirements
for the degree of Master of Science
Graduate Department of Chemistry
University of Toronto
O Copyright by Mitsuru Kobayashi (2000)
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ABSTRACT
Effect of Inorganic Pigments on Polymer Interdiffusion
in Low-Tg Latex Films
Master of Science (2000)
Mitsuru Kobayashi
Department of Chemistry, University of Toronto
This thesis examines the influence of inorganic pigments on latex film formation.
We carried out fluorescence resonance energy transfer measurements on latex films for
following the extent of inter-particle polymer diffusion. We found that large calcium
carbonate particles (300 nm x 1,000 nm) had little effect on the polymer diffusion rate for
films conidning up to 80 wt % filler. whereas 25 nm silica particles had a significant
influence in poly(methy1 methacrylate-CO-2-ethylhexyl acrylate) latex films. We also
examined the influence of silica particle size on this rate in poly(buty1 methacrylate) latex
films. We found that the 12 and 25 nm SiOi particles significantly affect the extent of
mixing with increasing amounts of filler. The rate decreases as the filler size decreases.
We propose that there is a low mobility polymer layer near the filler surface than the bulk
polyrner, and that the smdl silica particles can also act as obstacles.
ACKNOWLEDGEMENTS
First 1 would like to express my deep appreciation to my research supervisor,
Professor Mitchell A. Winnik. He encouraged me to work independently, yet gave me a
lot of suggestions when needed. He also encouraged me to join a lot of meetings with
industry people, which 1 believe improved my communication and presentation skills.
I would also like to thank Professor Winnik and Professor Douglas Reeve,
Director at the Pulp and Paper Centre. University of Toronto. for inviting me to the
Surface Science Consortium and encouraging me to give presentations to those who work
in the pulp and paper industry. I also appreciate Prof. Reeve for reviewing my MSc
Thesis and giving me valuable comments.
1 would also like to thank Professor Eugenia Kumacheva for reviewing my MSc
Thesis and for giving me valuable suggestions. I also had good opportunities to discuss
my research work with her at the Surface Science Consortium.
Special thanks go to Oji Paper Co., Ltd.. for allowing me to study polymer
chemistry at the University of Toronto and for financial support.
My special thanks also go to Dr. Yahya Rharbi. Dr. Ewa Odrobina, Dr. Hung H.
Pham, Dr. Ronghua Liu, Dr. Jiangdong Tong, and Mr. Lan Cao, for valuable discussions
on my research project, and Dr. Matthew Moffitt for proofreading the thesis. it was
wonderful for me to interact with them.
I would ûlso like to thank dl other members in Prof. Winnik goup, for usehl
discussions and suggestions. and the friendly atmospheres.
Finally, I would like to thank my wife Hanimi, my son Yoshiki, my parents. sister
and brother, for supporting and encounging my graduate study and research for two
years.
TABLE OF CONTENTS
ABSTRACT
ACKNOWLEDGEMENTS
LIST OF TABLES
LIST OF FIGURES
INTRODUCTION
Background
Emulsion Polymerization
Latex Film Formation
Polymer interdiffusion in Latex Films
Fluorescence Resonance Eneqy Transfer (FRET)
References
EFFECT OF PIGMENT TYPES ON POLYMER
INTERDIFFUSION
Introduction
Experimentai
2-2- 1. Materials for this study
2-2- la. Materials for Emulsion Polymerization
2-2- 1 b. Inorganic Pigments
2-2-2. Synthesis of Dye-labeled Monorner
2-2-3. Synthesis of Poly(MMA-CO-EHA) Copolymer Latex
2 - 2 4 Characterization of Latex
2-2-4a. Particle Size Measurement
2-2-4b. Gel Permeation Chromatography (GPC)
2-2-4c. Solid Content of latex dispersions
. . 11
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viii
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1
1
1
4
5
6
7
9
9
IO
10
10
11
12
13
14
14
15
15
2-2-5. Film Preparation
2-2-6. Energy Transfer Measurement
2-2-7. Data and Data Analysis
Results
2-3- 1. Dye Distribution Analysis by GPC
2-3-2. Polymer Interdiffusion in Latex Films Using Fluorescence
Resonance Energy Tnnsfer (FRET) Technique
2-3-3. Efficiency of Energy Tnnsfer, @&O) in Newly Formed Films
2 - 3 4 Maximum Efficiency of Eneqy Transfer a&=) in Latex
Films
2-3-5. Effect of Pigment on the Rate of Polymer Interdiffusion
Discussion
Conclusions
References
EFFECT OF SEICA PARTICLE
POLYMER INTERDIFFUSION
Introduction
Expenmen ta1
3-2- 1. Materiais for this study
3-2- la. Materials for Emulsion Polymerhtion
3-2- 1 b. Synthesis of Dye-Labeled Latex Particles
3-2- lc. Colloidal Silica
3-2-3. Characterization of Latex and Colloidal Silica
3-2-2a. Particle Size Measurements
3-2-2b. Gel Permeation Chromatography (GPC)
3-2-2c. Solid Content of latex dispersions
3-2-3. Film Prepmtion
3-2-4. Energy Transfer Measurement
3-2-5. Data and Data Analysis
3.3 Results
3-3-1. Dye Distribution of PBMA
3-3-2. Determination of Silica Particle Sizes by DLS and TEM
3-3-3. Fluorescence Resonance Energy Transfer Technique for Latex
Film Formation
3 - 3 4 Initial Efficiency of Energy Transfer, @ d o ) in Newly Formed
PBMA Latex Films.
3-3-5. Maximum Efficiency of Energy Transfer, ad-) in PBMA
Latex Films
3-3-6. Effect of Silica Particle Size on the Rate of Polymer
Interdiffusion
3-3-7. Analysis of die Diffusion Process
3-3-8. Effect of Silica Particle Size on Diffusion Coefficients
3-3-9. Dependence of the Polymer Diffusion Rate on the Volume
Fraction of Silica
3-3- 10. Fundamental Mechanism of Polymer Diffusion
3.4 Discussion
3-41 . @ d o ) in Newly Formed PBMA Latex Films in the
Presence/Absence of Silica
3-42 Effect of Silicri on Gd200 h)
3-4-3. Effect of Silica on the Polymer Diffusion Rate
3.5 Conclusions
3.6 References
4 FUTURE WORK
4.1 Effect of Modification on the Polymer Diffusion Rate of Latex
Polymers in the Presence of Minerai Fillers
4.2 Effect of Other Constituents (Water-soluble Polymen, Thickeners) on
the Polymer Difision Rate of Latex Polymers in the Presence of
Mineral Fillers
4.3 Application of FRET Technique to Styrene-butaddiene and Styrene- 105
acrylate Latex systems
4.4 References
vii
LIST OF TABLES
Table 2.1 Recipe for preparing 1 10 nm P(MMA-CO-EHA) latex.
Table 2.2 Characteristics of the latex and inorganic pigments. The particle size.
Tg, and molecular weight for both Phe-P(MMA-CO-EHA) and MeAn-
P(MMA-CO-EHA) are very sirnilar to those for MeAn-P(MMA-CU-
EHA).
Table 2.3 Final area and maximum efficiency of eneqy transfer values.
------..-----
Table 3- 1 Recipe for preparing LOO nm PBMA latex.
Table 3-2 Charactenstics of the latex and colloidal silica. The particle size. Tg.
and molecular weight for Phe-PBMA are very similar to those for An-
PBMA.
Table 3-3 The mean particle sizes and the particle size distributions for silica
used for this study.
Table 3-4 Values for M O ) and @&200 h) at 60 OC.
TabIe 3-5 QE7(2 h) at difkrent annealing temperatures.
Table 3-6 D,, and the corresponding f,(t) values for films containing different
types and amounts of silica filler.
Table 3-7 Volume content of PBMA polymers (vol %) near the filler surface,
relative to the total polymer volume in latex films. The filler content is
40 wt% for d l the calculations.
LIST OF FIGURES
Figure 1.1 Schematic representation of the initial stage of an emulsion
polyrnerization system.
Figure 1.2 Schematic representation of the latex film formation process.
Figure2.1 Chernicalsuucturesofmonomersusedforthisstudy.
Figure 2.2 Scanning electron microscopy (SEM) images for precipitated
calcium carbonate (CaC03, Le ft-hand side) and colloidal silica
(SiO2. right-hand side).
Figure 2.3 Chemical structures of dye-labeled monomers.
Figure 2.4 GPC chromatognms for (a) Phe-P(MMA-CO-EHA) and (b)
MeAn-P(bfMA-CO-EHA) latex polymers. The samples were
prepared by drying a latex dispersion. followed by dissolution of
polymer into THF solvent. Cume (1): the fluorescence signal for
two-stage polymer; curve (2): the refractive index signal for two-
stage polymer; and curve (3): the refractive index signal for seed
polymer.
Figure 2.5 Schematic representation of polymer interdifision for a latex
film. When a latex film is annealed well above its glass transition
temperature (Tg), the film has a zone of interdifision which
contains mixtures of D- and A-labeled polymer molecules.
Figure 2.6 Schematic representation of polymer interdifision at the
molecular level. When a latex film is annealed above irs Tg, D-
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2. L O
Figure 2.1 1
and A-labeled polymer molecules interdiffuse across the ce11
boundary and the distances between D and A groups become
smaller.
Donor fluorescence decay profiles (a) in a P(MMA-CO-EHA) latex
film, (b) in a P(MMA-CO-EHA) latex film containing 80 wt %
CnCOj, md (c) in s P(MM.4-CO-EH.4) latex film containing 20 wt
% SiO?. Each film was annealed at 50 f 1 "C for (1) O min, (2) 60
min, and (3) 990 min. The decay profiles are integrated to obtain
the area under the curves.
Effect of CaC03 content on the initial efficiency of rnergy
transfer. We plot the initial energy efficiency of energy transfer
OET(0) vs CaC03 content in latex films (wt %).
Effect of SiO2 content on the initial efficiency of energy transfer.
We plot the initial efficiency of energy uansfer &-(O) vs Si02
content in latex films (wt %).
Comparison of P(MMA-CO-EHA) extent of mixing, f,(r) vs
annealing time for films containing different arnounts of calcium
carbonate: O wt % (O), 30 wt % (e), 80 wt % (o), and 90 wt % (a).
The weight-average molecular weight (MW) of the P(MMA-CO-
EHA) latex was ca. 240,000. Films were annealed simultaneously
at 50 f I OC.
Comparison of P(MMA-CO-EHA) extent of mixing, f,(r) vs
annealing time for films containing different amounts of silica: O
wt % (O), 20 wt % (a), 40 wt % (a), and 50 wt % (i). The weight-
average molecular weieht ( M d of the PMMA-CO-EHA) latex was
ca. 240,000. Films were anneaied simultaneously at 50 f 1 OC.
Figure 2.12 Schematic representation of the mnealing process for a latex film
containing silica particles (25 nm). The silica particles can act as
obstacles and retard polymer diffusion andor reduce the mobility
of polymer molecules near the pigment surface.
Figure 2.13 Schematic representation of the annealing process for a latex film
containing precipitated calcium carbonate. The latex particles
spread locally dong the pigment surface.
Figure 3.1 Chernical structure of butyl methacrylate used for this study.
Figure 3.7 Chemical structures of dye-labeled monomers used for this study.
Figure 3.3 GPC chrornatograms for (a) Phe-PBMA and (b) An-PBMA latex
polymers. The samples were prepared by drying a latex dispersion.
followed by dissolution of polymer into THF solvent. Curve (1):
the fluorescence signai for two-stage polymer; curve (2): the
refractive index signal for two-stage polymer; and curve (3): the
refractive index signal for seed polymer.
Figure 3.4 Transmission elecuon rnicroscopy (TEM) images for various
particle sizes of silica. (a) K-25, (b) K-50, (c) S-12. (d) S-45.
Figure 3.5 Particle size distributions of silica particles used for this study. (a)
K-25, (b) K-50, (c) S-12, (d) S-45. The particle sizes were
determined by transmission electron microscopy (TEM).
Figure 3.6 Donor fluorescence decay profiles in a PBMA latex film after
annealed for (1) 0 min, (2) 60 min, (3) 330min, (4) 12,000 min.
Figure 3.7 Donor fluorescence decay profiles in a latex film. (a) PBMA with 59
40 wt % of 25 nrn SiO-, (K-25). (b) PBMA with 40 wt % of 50 nm
Si@ (K-50), (c) PBMA with 40 wt % of 12 nm SiO- (S-13, (d)
PBMA with 40 wt % of 45 nrn SiO2 (S-45) after annealed for (1) O
min. !2) 60 min. (3) 330min. (4) 12.000 min. respectively.
Figure 3.8 Plots of the initial efficiency of enegy transfer, OET (O), vs Si02 6 1
contents in newly formed films.
Figure 3.9 Plots of the maximum efficiency of energy transfer. (200 h), 64
vs SiO- contents. Films were annealed for 200 h at 60 OC.
Figure 3.10 Plots of the maximum efficiency of rnergy uansfer, Gn(2 h). vs 65
annealing temperature. Films contain 40 wt 8 of SiO2 and were
annealed for 2 h.
Figure 3.11 Plots of the maximum efficiency of energy transfer, Qm (200 h), 66
vs the surface to volume ratio (lldsiot). Films contain 40 wt % of
SiO2, and were annealed for 200 h at 60 O C . The diarneters of S a l
were determined by transmission electron microscopy (TEM).
Figure 3.12 Plots of the extent of mixing f,(r), as a function of annealing time. 70
Films contain (a) 25 nm of SiO2 (K-25), (b) 50 nm of SiO? (K-SO),
and were annealed simultaneously for each series of sample films
at 60 OC.
Figure 3.13 Plots of the extent of mixing f,(t), as a function of annealing time. 72
Films contain (a) 12 nm of Si& (5-12), (b) 45 nm of SiOz (S-43,
and were anneaied simultaneously for each series of sample films
xii
at 60 O C .
Figure 3.14 Plots of the extent of mixing f,(t), vs the square root of annealing
time. Si02 content for al1 Si02 contained films is 40 wt %. Those
films were annealed at 60 OC.
Figure 3.15 Plors of the dope vdues in Figure 3.13. as a functinn of the
surface to volume ratio, l/dsio2. The diameters of Si02 were
determined by transmission electron microscopy (TEM).
Figure 3.16 Mean apparent diffusion coefficients, Da,,, as a function of the
extent of mixing f,(t). Films contain (a) 25 nm of SiO2 (K-25), (b)
50 nm of SiO2 (K-50). and were annealed simultaneously for each
series of sample films at 60 OC.
Figure 3.17 Mean apparent diffusion coefficients. Dapp. as a function of the
extent of mixing fm(f). Films contain (a) 12 nm of S i 0 (S-12). (b)
45 nm of Si02 (S-45). and were annealed simultaneously for each
series of sample films at 60 O C .
Figure 3.18 Plots of the extent of mixing fm(t) vs the square root of annealing
time for latex films containing different arnounts of (a) S-12 and
(b) K-25.
Figure 3.19 Plots of the dope values in Figure 3.17, as a function of the silica
volume fmction.
Figure 3.20 Plots of 4, vs fm(t) for PBMA latex films containing O wt % ( O ) ,
10 wt % (a), 20 ~t % (CI), 30 wt % (a), a d 40 ~t % (A) of S- 12.
Figure 3.21 Master curve of Dvp vs f,(t) for PBMA latex films containing O
wt % (O), 10 wt % (a), 20 wt % (o), 30 wt % (a), and 40 wt % (A)
of S-12.
Figure 3.22 Plot of {~~[D,(T,~~)/D~(T.o)]}- ' vs Qf " for PBMA latex films
containing various fractions of S- 12.
Figure 3.23 Plots of DZF, vs ,(t) for PBMA latex films containing O wt % (01.
10 wt % (a), 20 wt % (a), 30 wt % (i), and 40 wt % (A) of K-25.
Figure 3.24 Master curve of D,, vs f,(r) for PBMA latex films containing O
wt % (O), I O wt % (m), 20 wt % (o), 30 wt % (i), and 40 wt % (A)
of K-25.
Figure 3.25 Plot of ( I ~ [ D ~ ( T , ~ ~ ) / D ~ ( T , O ) ] } " vs <Dr " for PBMA latex films
containing various fractions of K-25.
Figure 3.26 Plots of ATg vs the silica volume fraction for PBMA latex films
containing S-12 (O) and K-25 (a).
Figure 3.27 Plots of ATg vs the total volume of the polymer near the silica
surface for PBMA latex films containing S-12 (O) and K-25 (a).
The diameters of silica were determined by transmission electron
rnicroscopy (TEM).
Figure 3.28 Plots of ATg vs the total volume of the polymer near the silica
surface for PBMA latex films contûining S-12 (O), K-25 (a), S-45
(a), and K-50 (i). The diameters of silica were determined by
transmission ekchon microscopy (TEM).
Figure 3.29 Plot of the g l a s transition temperature Tg2 vs thickness of the
polymer layer tipi rigidified by the silica filler for the polymer
near the silica filler surface. The thickness is expressed based on
the radius of gyration RG for the PBMA polymer with a MW =
50,000.
Figure 3.30 Plots of ( In[D,(T, ar)/Dp(T. 0.233)] }-1 vs { i-6(1.7R&D~io2/d)-'
for PBMA latex films containing S-12 (O) or K-25 (a).
Figure 3.3 1 Schematic representation of rnorphology difference for latex films
containing either smaller (left-hand side) or larger (right-hand
side) size of SiO?. The silica fillers can act as obstacles and retard
polymer interdiffusion ancilor reduce the mobility of polymer
molecules near the filler surface.
Figure 3.32 Schematic representations for pol ymer immobilization near the
filler surface and obstacle effect.
Figure 3.33 Schematic representation for polymer molecules immobilized near
the filler sul-face. We calculated the thickness of the immobilized
polymer layer based on the radius of gyration, RG, and 2RG using
RG (PBMA, MW = 50.000) = 4.7 nm.
1. INTRODUCTION
1-1. Background
Synthetic binders play a key role in the process of paper and b oard coatings. 1
These products have become more and more important since the use of latex as a coating
binder was first examined in 1946. The sheet and print quaiity of coüted papérs Ilas bem
improved for many years due to their supenor characteristics as pigment binders.
The reason that latex is widely used as a pigment binder in paper coatings is that it
has overcome a lot of problems such as binder migration, print mottle, backtnp mottle.
blister, warer retention. different coating rheology. It has also irnproved smoothness,
gloss, pnnt gloss, pnnt fidelity. press performance. reduced n w material waste. and
convenience in handling.
There is a continuing need to lower costs of coated paper production while at the
sarne time maintaining or improving paper quality. To reach this goal, one would like to
have the knowledge necessary to design the structure of the coating in a way that
optirnizes the use of latex as a pigment binder. For this purpose. one needs a deeper
understanding of the role of the latex binder in the coating. We approach this problem by
trying to understand how a large amount of pigment in the coating formulation affects the
coalescence of latex particles and the subsequent polymer diffusion that enhances the
mechanical properties.
1-2. Emulsion Polymerization
Emulsion polyrnenzation has developed into a widely used process for the
production of synthetic latexes since its fint introduction on an industrial scale in the
mid-1930s. ' Today, various kinds of synthetic polymers are prepared by emulsion
polyrnerization. Examples include synthetic rubber, high-impact polymers, latex foam,
latex paints, paper coatings, carpet backing, adhesives, binders for non-woven fabrics.
barrier coatings additives for consuuction materials such as Portland cement, mortar and
concrete, and sealants and additives. Latex particles are also used for a growing number
of specialty applications such as: diagnostic tests. immunoassays, biological cell-labeling.
drug delivery systems, chrornatographic separations, and as size calibration standards for
many instruments such as blood counters and electron microscopes.
Emulsion polymerization is defined as a free-radical-initiated chain
polymerization, in which a monomer or a mixture of monomers is dispened in an
aqueous medium, with the aid of ernulsifier molecules and polymerized by a water-
soluble kce radical initiator. ' Water is the main ingrdient in ernulsion polymerizstion. II
maintains a low viscosity and provides good heat transfer. In addition, it serves to isolate
the polymerization loci. This is one of the advantages in emulsion polymenzation, termed
compartmentalization. The water also acts as the medium through which monomer passes
from monomer droplets to the growing particles. It is the locus of water-soluble initiator
decomposition and oligomer formation. and the medium of dynamic exchange of
surfactant between the phases.
Surfactants are surface-active molecules. In emulsion polymerization, they
perform two major functions. One is to provide sites for panicle nucleation. and the other
is to provide colloidal stability to the growing particles as a result of their adsorption at
the particle-water interface. Anionic surfactants are the surfactants most commonly used
in emulsion polymenzation. Cationic surfactants are also used for special applications in
paper coatings and asphalt additives. Nonionic surfactants are also used to control the
latex particle morphology. and to enhance the colloidal stability against mechanical shear,
freezing, and added electrolytes. Reactive surfactants. which are surface active molecules
with an active vinyl group, are also used in order to bind surfactants chernically to the
latex surface. In this case, they have an advantage of reduced desorption during film
formation and reduced water sensitivity of the latex films. Water-soluble polymers such
as poly(viny1 alcohol) and hydroxyethyl cellulose can be used as non-surface-active
emulsifiers,
The most commonly used water-soluble initiaton are salts of persulfuric acid,
such as potassium persulfate and ammonium persulfate. On heating, persulfate ions
dissociate into two sulfate radical anions, which c m initiate emulsion polymerization.
Redox initiaton, typically a mixture of an oxidizing agent and a reducing agent, are used
for emulsion polymerization at a low temperature.
A variety of chah transfer agents, such as mercaptans, are used to control the
molecular weight of the emulsion polymer.
Two major mechanisms for particle formation have been proposed for emulsion
polymerization. These are micellar nucleation. and homogeneous nucleation. Both
processes are considered to proceed simultaneousiy. In micellar nucleation. initiator
radicds genented in the aqueous phase enter the monomer-swollen surfactant micelles,
as single radicals or ~lipndicals, =d initiatc potymcnzation to fom monorner-swollen
polymer particles, which grow by propagation reactions. In homogeneous nucleation
radicals genented in the aqueous phase propagate by adding monomer units to form
water-soluble oligomers until they reach the lirnit of their solubility in the aqueous phase
and precipitate out of solution. A schematic representation of the initial stage
emulsion polymenzation system is shown in Figure 1.1.
Micelle d = 5-10 nm
Monomer is solubilized.
Monomer droplet d = 1,000-10,000 nrn
Initiator + 2 Re (Radicals) , Monomer-swollen polyrner particle
Aqueous phase
Figure 1.1: Schematic representation of the initial stage of an emulsion polymerization
system.
1-3. Latex Film Formation
One of the major applications for emulsion polymers is in the area of waterbome 4 coatings. The latex onginally consists of a colloidal dispersion of particles in an aqueous
medium. Upon drying the particles are transformed into a void-free and mechanicalty
coherent polymer film. In this process. the latex particles have to be deformed in order to
fil1 space between the particles, which results in polyhednl structure. For this process to
sccur, the temperature should be above the minimum film-forming temperature (MFQ of
the system. The M F ï often corresponds to the glass transition temperature of the latex
polymer in the presence of water. Here. one must have sufficient adhesion between the
latex cells so that one can improve the mechanical and barrier properties of the film.
According to Winnik. the process of Iatex film formation can be divided into
three stages. In the first stage, water evapontes until the particles corne into close contact
to each other. In the second stage, the latex particles deform and begin to fil1 the
interstitial spaces between the individual particles at temperatures above the MFFT.
Finally, in the third stage, the particles begin to coalesce and interdiffuse, which results in
the formation of a mechanically sound film. The ease of latex film formation has a strong
influence on the properties of paper coatings. as well as paints and coatings. A schematic
representation of the latex filrn formation process is shown in Figure 1.2.
Aqueous Iatex solid content 20-50 wt %
Stage 1 b
water evaporates
Stage II b
particle deforms T2MFT Packing of deformed partides
Stage III / 1 1 1 ~ 1 / 1 / 1 / 1 / / / / / 1 / / I / l / / / l l l / l / / / / / / / l / / / / / aging and further coalesceEce t 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
T 2 T g Mechanically coherent füm
Figure 1.2: Schematic representation of the latex film formation process.
1-4. Polymer Interdiffusion in Latex Fiims
Scientists studied the surface morphology of latex films by electron microscopy
many years ago, and found that the cellular structure in newiy-formed latex films
disappeared upon aging or annealing. It seemed that this process was related to the
growth in mechanical properties of the films. Bradford and Vanderhoff referred to this
process as gradualfurther coalescence, and Voyutski ' used the term autoliesion. In his
publication, autohesion referred to the diffusion of polymer chains across the interparticle
or intercellular boundaries. Unfortunately, they did not have any procedures that allowed
one to measure this diffusion directly. Over the past 15 years, severd techniques have
allowed one to measure this difision quantitatively, and a broad body of theory has been
created to describe the diffusion process.
Polymer diffusion across interfaces, which is also termed polymer inrerdifùsion,
is an important feature in many processes such as heding of the interfaces in latex films,
polymer weiding, crack healing, powder coatings, and compression-molded polymer
powders. The tenn interdifision is used for processes of mixing, intermingling, and
homogenization at the molecular and macroscopic levels. Interdiffusion implies diffusion
among distinguishable molecules.
The theory of polyrner diffusion across interfaces has been a fascinating subject
for many years. Polyrner chains adjacent to an interface have a different distribution of
conformations to those in bulk. As they diffuse across the interface. the conformations are
iandonized md thercfore one cbtains an extra entropic dnving force for the interdiffusion
of surface polymer chains. If the polymer chains are sufficiently short, the diffusion will
follow Fick's laws. In latex films, polymer interdiffusion is recognized as an important
process in development of mechanical strength.
There are sevenl techniques to charactenze aspects of the polymer diffusion
process in latex films. Those are transmission electron microscopy (TEM), Y atornic force
microscopy (AFM). freeze-fracture TEM. 5s 10 A few other techniques allow one to
measure the rates of polymer interdiffusion. Small angle neutron scattering (SANS) and
fluorescence resonance energy transfer (FRET) are the most activety used to study the
rate of polymer interdiffusion. In this study, we use fluorescence resonance energy
vansfer (FRET) technique.
1-5. Fluorescence Resonance Energy Transfer (FRET)
Ruorescence resonance energy transfer (FRET) involves the transfer of the
excited state energy from a fluorescence donor (D) to a fluorescence acceptor (A). 11
FRET takes place without emission of a photon. and it occurs as a result of a resonant
dipole-dipole interaction between the donor and acceptor molecules. The rate of energy
transfer w(r) is expressed by the following equation:
where .rD0 is the donor fluorescence lifetime in the absence of acceptors, and Ro is the
characteristic distance (the Forster distance) over which energy transfer takes place. Sb
This rate is deterrnined by several factors. such as the extent of overlap of the emission
spectnim of the donor with the absorption spectrum of the acceptor, the relative
orientation of the donor and acceptor transition dipoles, and the distance between the
donor and acceptor molecules.
FRET was fint applied to biochernical research, '' and it has also been extensively 13-15 employed in the field of polymer science. One of the most interesting features is that
this technique can be applied to the study of polymer diffusion across the inter-particle
boundary in latex films, Sb. 16. 17 which is also important for paper coatings that contain a
large amount of minenl filtex-s. In this study Ive examine how rhose fillen affect the rate
of polymer interdiffusion in latex films.
1-6. References
Macnair, A. K., Task Group Chaiman, Syithetic Coaring Adhesives, TAPPI PRESS,
Atlanta, 1998.
Lovell, P. A.: El- Aasser. M. S ., Emiilsion Polymen'zation and Emtilsion Polymers,
Wiley, Chichester, 1997.
30' Annual Short Course "Advances in Ernulsion Polymerization and Latex
Technology" El-Aasser, M. S. ed., Lehigh University, PA, Vol. 1, 1999.
(a) Paton, T. C., Paint Flow and Pigment Technology, Wiley, New York. 1979. (b)
Turner, G. P. A., lntroduction to Paint Chemistry and Principles of Paint
Technology, 3" ed., Chapman and Hall, London. 1985.
(a) Wang, Y.; Kats, A.; Juhué, D.; Winnik, M. A. Langmoir 1992, 8, i435. (b)
Winnik, M. A. The Formation and Properties of Latex Films in Emulsion
Polymerization and Emulsion Polymers, Lovell, P. A., El-Aasser, M. S., Eds., Wiley,
New York, 1997, p 467.
Bradford, E. B.; Vanderhoff, J. W., J. Macromol. Chem., 1966,1,335.
(a) Voyutski, S. S., J. Polym. Sci.. Part A. 1958, 32, 528. (b) Voyutski, S. S.,
Autohesion and Adhesion of High Polymers, Wiley-Interscience, New York, 1963.
(a) Joanicot, M.; Wong, K.; Maquet, J.; Chevalier, Y.; Pichot, C.; Graillat, C.;
Lindner, P.; Rios, L.; Cabane, B. Prog. ColIoid Polym. Sci., 1990, 81, 175. (b)
Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Lindner, P.; Cabane,
B. Colluid Polym. Sci., 1992,270,806.
9. (a) Wang, Y.; Juhue, D.; Winnik, M. A.; Leung, O.; Goh, M. C. Lcingmzrir, 1992, 8,
760. (b) Goh, M. C.; Juhue, D.; Leung, O.; Wang, Y.; Winnik, M. A. Langmuir,
1993,9, 1319.
10. (a) Roulstone, B. J.: Wilkinson, M.; Hearn, J.; Wilson, A. J. Polyrn. Int., 1991, 24,
87. (b) Roulstone, B. J.; Wilkinson, M.; Hearn, J. J. Polym. Inr.. 1992, 27.43.
1 I . Lakowicz. J. R. Principles o f Flitorescence Spectroscop~. Plenum Press, New York,
1983.
12. Herman, B . Fluorescence Microscopy and fluorescent Probes, edited by Slavik. J..
Plenum Press. New York, 1996, p 1.
13. Rager, T.; Wegner, G.; Winnik. M. A. ~facroniolecules, 1997,30,49 1 1 .
14. Marowetz, H. and Amrani, F. Macmmolecrrles, 1978, 11,28 1 .
15. Qui, X. P.; Jiang, M. Polymer. 1995. 36,360 1 .
16. Winnik, M. A. Cwr. Opin. Colloid In., 1997.2. 192.
17. Boczar, E. M.; Dionne, B. C.: Kirk, A. B.; Lesko. P. M.; Koller, A. D.
Macronzolecrrles, 1993.26, 5772.
2. EFFECT OF PIGMENT TYPES ON POLYMER
INTERDIFFUSION
2-1. Introduction
Latex polymer is widely used as a pigment binder for various types of coated 1 papers. It is of significant importance to understand the factors that affect the structure
of the coating, especially the spatial arrangement of pigment particles and binder. These
factors are important for the control of product quality and the design of new products. ' Understanding latex particle deformation and the evolution of film properties is also
important because these are the steps that promote adhesion to the inorganic pigment and
adhesion to the cellulose fiben. The amount of binder in a paper coating is small, due to
the relatively high cost compared to the cost of conventional pigments such as clay and
calcium carbonate. Thecefore. it is essential to optirnize the characteristics of the latex,
both for maintaining or improving the quality of the paper, and for lowering the paper
cost.
It is well known that, during the drying process, latex particles deform above the
glass transition temperature (Tg) of the po1ymer to form a void-free solid cornprised of
polyhedrd cells. Over time, this cellular structure is lost as polymer diffuses across the
intercellular boundaries and it creates a continuous polymer matnx. There have been a
number of studies on wetting and adhesion of polymer latex particles to inorganic 4-6 pigments in paper coatings. These include the scanning electron microscope studies by
Scnven et al., and the recent neutron scattering study by Joanicot et ai. In spite of this
effort, it is still not well understood how the latex particles deform in the presence of
large arnounts of inoqanic pigments dunng film formation.
Our research group has developed a quantitative technique based on fluorescent
spectroscopy for studying polymer interdiffusion in latex films. In this technique, one
mixes two essentially identical types of latex particles in dispersion pnor to film
formation. Both types of latex particles are covalently labeled with about I mol % of a
fluorescent dye, one with a dye (D) that can act as a donor in a fluorescence resonance
energy transfer (FRET) experiment, and the other with a corresponding acceptor dye (A).'
In a FRET experiment, the donor dye is excited selectively with light at a wavelength
where it has a strong absorption. This excited dye can fluoresce, or, if there is a nearby
acceptor dye, the enegy can be transferred to the acceptor dye via a resonant coupling of
the transition dipoles. The energy transfer process occurs as described in eq 2-1. When a
dispersion containing donor- and accepter-labeled particles dries to form a film, some of
the boundaries separate donor- and acceptor-labeled polymer. As these polymen diffuse
across the boundaries. they bring donor and acceptor dyes into proxirnity, allowing the
extent of energy transfer to increase.
D * + A + D + A * (3- 1)
Over the past 15 years. we have used this technique to examine a varie ty of hcton
that affect the rate of polymer diffusion in latex films. Most of these experiments were
carried out on pigment-free dispersions, with the intent of understanding how the
properties of the binder phase itself evolves. "ecently we began to examine the
influence of pigments on the polymer diffusion rate in latex films. In this work, we have
been interested in modeling the effect of pigments in latex paints. which have a higher
binder content than that of paper coatings. In one study, Feng et al. found that hard
polymer filler particles such as poly(rnethy1 methacrylate) (PMMA) and RopaqueB
(Rohm & Haas) significantly retard the rate of polymer interdiffusion. In this paper, we
describe the application of FRET measurements to the study of the polymer interdiffusion
process in latex films that contain large amounts of inorganic pigments, typical of those
used in paper coatings. We show that calcium carbonate pigment and silica have very
different effects on the rate of polymer diffusion in these films.
2-2. Experimental
2-2-1. Materials for this study
2-2-la. Materials for Emulsion Polymerization
Monomers, such as methyl methacrylate (MMA, Aldrich, 99%), and (2-
ethylhexyl) acrylate (EHA, Ruka, 99%) were distilled under vacuum under a N2
aunosphere, and stored in refngerator pnor to use. Potassium persulfate (K2S2O8, KPS,
Aldrich, 99%), sodium bicarbonate (NaHC03, Caledon, 99%), sodium dodecyl sulfate
(Ci2H250S03'Na+, SDS, Aldrich. 98%) and 1-dodecanethiol (Ci2HXSH, DM, Aldrich,
98%) were used as supplied. Distilled water was further punfied through a Millipore
~ i l l i - ~ ~ system. Chernical structures of monomers used for this study are shown in
Figure 2.1.
MMA EHA
Figure 2.1: Chernical structures of monomers used for this study.
2-29 1 b. Inorganic Pigments
Precipitated caicium carbonate (TP-221GS). dispersed with sodium polyacrylate.
was supplied as a suspension by Okutûma Kogyo Co., Ltd. This pigment has an
ellipsoidal shape, termed scalenohedrd calcite, with a length of 0.7 to 1.0 Pm, and a
diameter of 0.25 to 0.3 Pm. This pigment is cornmonly used for high gloss coated paper.
ColIoidal silica (Klebosol 30R25) was supplied as a suspension by Clariant Corporation.
The filler particles are amorphous sphencd silica beads with a diameter of around 25 nm
and a narrow size distribution. 'O The particles show good dispersibility in waier. Both
pigments were used as supplied. Scanning electron microscopy (SEM) images for CaC03
and SiOl are shown in Figure 2.2. Further pigment characteristics are shown in Table 2.2.
SWc 'IL x,iTticr :-:: 4 :i . P x : c i r rue 2 5 '.'- . - 6)
- -. . -- -- - - . 'r"
Figure 2.2: Scanning electron microscopy (SEM) images for precipitated calcium
carbonate (CaC03, left-hand side) and colloidal silica (SiO?, right-hand side).
2-2-2. Synthesis of Dye-labeled Monomer
9-Phenanthrylmethyl methacrylate (Phe-MMA) were synthesized previously. ' l 9-
Methacryloxyrnethyl- 10-rnethylanthracene (MeAn-MMA) has recently been synthesized
by Liu et al., using anthraquinone as a srarting material. " We use MeAn-labeled
monomer io carry out emulsion polymerization of acrylate monomers. The chernical
structures of dye-Iabeled monomen are shown in Figure 2.3.
Phe-MMA MeAn-MIMA
Figure 2.3: Chernical structures of dye-labeled monomen.
2-2-3. Synthesis of Poly(iMMA-CO-EHA) Copolymer Latex
Poly(MMA-CO-EHA) copolymer latex samples (monomer weight ratio of 1: l),
labeled with I mol % of a fluorescent dye [either a donor, phenanthrene (Phe) or an
acceptor, anthracene (An)], were prepared by semi-continuous emulsion
copolymerization at 80°C, using KPS as the initiator, SDS as the surfactant. and L -
dodecanethiol as the chain transfer agent. The reaction conditions were similar to that
described previously. l 3 AS a donor-labeled monorner. we used (9-phenanthry1)methyl
methacrylate (Phe-MMA) to introduce the donor dye. The synthesis of this monomer is
described in ref 1 1. As an acceptor-labeled monomer. we used 9-rnethacryloxymethyl- 10-
methylanthracene (MeAn-MMA) descnbed in ref 12. The recipe for L 10 nm P(MMA-CO-
EHA) latex is shown in Table 2.1. We used the sarne seed latex particles obtained in the
first stage for preparing both the Phe- and MeAn-labeled latex particles. Their
characteristics are shown in Table 2.2. These latex particles, referred to as P(MMA-CO-
EHA), have a diameter of L 10 nm, and a Tg of 7 OC.
Table 2.1: Recipe for preparing 110 nm P(MMA-CO-EHA) latex.
MMA (g) -
EHA ($1
MeAn-MMA (g)
Phe-MMA (g)
a. 1 mole % relative to the total monomer fed in the second stage.
b. Reaction temperature.
c. Reaction time.
First stage -
(batch process)
1.75
Water (g)
KPs (g)
2-2-4. Characterization of Latex
2-2-Sa. Particle Size Measurement
The particle size and size distributions were determined by dynamic light
scattering employing a Brookhaven BI-90 particle sizer. This measurement is based oii
quasi-elastic light scattering (QELS), dso referred to as dynamic light scattering (DLS).
1.75
Second stage
(under monomer starved condition)
58.8
0.06
Phe-Iabeled
17.5
17.5
0.75 a
MeAn-labeled
17.5 1
17.5
0.78 "
27.0
0.06
27 .O
0.06
2-2-4b. Gel Permeation Chromatogaphy (GPC)
Molecular weights and molecular weight distributions were measured by gel
permeation chromatognphy (GPC), using two Ultrastyngel columns (500 + 10'' A, or 10"
A + lo5 A for very high molecular weights) with tetrahydrofuran ( T m as the eluent and
a flow rate of 0.4 mUmin. A srnall portion of a latex dispersion was dned in the oven for
2 h at 120 OC. Then the polymer was dissolved in tetrahydrofuran (THF) to give a
solution of c3. 0.5 :vt 9 polynier concentration. The solution was filtered before it was
injected into the column. Colurnns were calibrated with poly(methy1 methacrylate)
(PMMA) standards. Dual detectors (WATERS Series R-400 Differential Refnctometer
as a refractive index detector and KRATOS FS 970 Spectrofluoro Monitor as a
fluorescence detector) were used to detect the presence of the donor and acceptor dyes
and to ensure that these fluorescent dyes are randomly disuibuted in the polymer
backbone.
2-2-4c. Solid Content of latex dispersions
The solids content of latex dispersions was measured gravimeincally by
measuring the weight of a small portion of a latex dispersion (WL) and the weight of solid
after dned (Ws) as:
They were found to be ca. 30 wt %.
Table 2.2: Charactenstics of the latex and inorganic pigments. The particle size, Tg, and
molecular weight for both Phe-P(MMA-CO-EHA) and MeAn-P(MMA-CO-EHA) are very
similar to those for MeAn-P(MMA-CO-EHA).
a. The length of the pigment is 700 - 1000 nm.
' P(MMA-EH*)
Phe- + MeAn-labeled
b. Nominal molecular weights based upon linear methyl methacrylate standards.
2-2-5. Film Preparation
Latex films were prepared From dispersion mixtures with a 1: 1 number ratio of
Phe- and MeAn-labeled P(MMA-CO-EHA) latex particles and different amounts of
inorganic pigment. The final dispersions, with total solid contents from ca. 30 wt % to 60
wt %, were adjusted to pH 9, using diluted potassium hydroxide solution for the
dispersions with Si02 and those without pigment, and with diluted hydrochlonc acid
solution for the dispersions with &CO3. Film formation was carried out by the following
procedure. For each film, we took three drops from each dispersion using a Pasteur
pipette and spread them ont0 a quartz plate. Then we leveled off those dispersions using a
stainless steel blade, and the film was allowed to dry for 10 minutes in air at 23 O C ,
followed by storage in the cold room at 4 OC to minimize the amount of polymer
interdiffusion in the film. A typicd film thickness was 100 km. Films formed from latex
aione were crack-free and transparent. The films became more turbid as one increased the
diameter (nm) 110 250 - 300 "
Precipitated
Calcium carbonate Colloidd silica
amount of precipitated calcium carbonate. All films containing colloidal silica were
transparent and crack-free up to 40 wt %. but small cracks were observed in the film
containing 50 wt % of silica.
2-2-6. Energy Transfer Measurement
Al1 films were annealed at 50 i 1 O C for polymer diffusion measurements. For
each senes of samples ro be compared, the films were anneaied sirnuitanrously.
Fluorescence decay profiles were measured by the single photon-timing
technique.'" Samples were excited at 300 nm. and emission was detected at 350 nm. A
bandpass filter (350 f 5 nm) was used to minimize the scattered light and interference due
to fluorescence from directly excited acceptors. For each measurement. it took about 10
to 15 minutes to collect 5000 counts in the maximum channel. Prim to each measurement,
a film sample was placed in a quartz tube. and the tube was degassed with flowing
nitrogen gas.
Due to the high pigment content. many of the films we examine are turbid or even
opaque. For these films, the extent of light penetration into the film is limited. Due to a
relatively large extent of light scattering in opaque samples. the depth of light peneuation
may be as small as the wavelength of the excitation light (here 300 nm). One therefore
needs to dign the optics carefully to minimize the amount of scattered light reaching the
detector. Because the scattered light is at shorter wavelengths than the ernitted light,
proper use of filters can rninimize its contribution to the measured decay. We excite the
donor at 300 nm and detect the emission at 350 nm with a bandpûss filter (350 f 5 nrn).
Because scattered light has the sarne time profile as the excitation pulse, one can correct
the measured decay profile for any residual contribution due to scattered light. 15
2-2-7. Data and Data Analysis
The latex films we examined were prepared from a 1:1 mixture of donor- and
acceptor-labeled latex particles. We monitor the polymer diffusion process by meauring
changes in the extent of energy m s f e r between the donor and acceptor dyes attached to
these latex polymers. When a donor dye D is excited, it can transfer its energy to any
nearby acceptor dyes A. The important feature of this process is that the rate of energy
transfer w(r) depends sensitively on the distance r between the donor and acceptor
molecules:
where roo is the donor fluorescence lifetime in the absence of acceptors, and is the
characteristic distance (the Forster distance) over which energy transfer takes piace. ' For energy transfer for donon and acceptors randomly distributed in a three
dimensional Euclidean space, the donor decay function will have a stretched exponentiai
fom ' :
where Io is the intensity at zero time. and P is a panmeter proportional to the local
concentration of acceptors CA:
where NAv is Avogadro's number and # describes the averaged relative orientation of the
donor and acceptor di pole moments.
In our expeximents. we measure donor fluorescence decay profiles b ( t ) . To fit
each decay curve, we use the following phenomenological equation:
While this equation is sirnilar in form to eq 2-41. no meaning is ascribed to the
individual fitting panmeters. The panmeters Al , A?, and p are obtained from the fit of
each decay profile, and we use these fitting parameters to in t ep te ID ( t ) andytically,
from decay time t = zero to t = infinity.
The efficiency of energy transfer, @n(t) is defined as shown in eq 2-6:
nrrmbrr of ET events = 1 - 1; I D (4
@, (4 = nimber of photons absorbed (2-6)
J ' " I ; O dt
where J œ ID ( t)dt is the area under the fluorescence donor decay profile obtained from O
area (t) Qm(r) = 1 -
While the extent of eneqy transfer can in principle be determined by measunng
the intensities of donor and acceptor fluorescence, this experiment suffen from several
artifacts, particularly the absorption by A of light ernitted by the excited D. In Our
expenments, we avoid this problem by carrying out fluorescence decay experiments. We
measure the influence of the acceptor dye on the decay rate of the donor dye following
pulsed excitation. In the absence of acceptor, the phenanthrene chromophore we employ
as the donor dye undergoes an exponential decay with a lifetirne roo. In Our analysis of
the donor fluorescence decay data, we assume that al1 deviations from a stnctly
exponential donor decay profile are due to FRET. For the films we examine here, roo =
45 ns.
Another useful parameter is the extent of mixing,&(t), expressed in terms of the
growth in energy transfer efficiency, normalized by the maximum change associated with
complete mixing. 16
#, (r) - @, (O) - area(0) - area(t) f, (0 = -
@ ( ) - @ (O) area(0) - ares(=)
where [O&) - #do) ] represents the change in the efficiency of energy transfer between
the initially prepared film and a film annealed for time t.
2-3. Results
2-3-1. Dye Distribution Analysis by GPC
In the next section, we describe how polymer chains behave in latex films before
and during annealing. In order to carry out the FRET measurements, one needs to know if
the dyeç are randomly distributed in the polymer backbones, which affects physical
behavior of the labeled polymer. We assume that the dyes are randomly distributed dong
the polymer chnins, 3nd &si the nurnbrr of dyes is proportional to the polymer chain
length. It is important to chuacterize not only the overall content of the dye-labeled
monomer in the polymer backbones. but also the distribution of the dye attached to the
polymer.
In order to examine this. Sosnowski suggested that one determine the fluorescent
dye distribution in the polymer chains by employing Gel Penneation Chromatognphy t 7 (GPC) analysis. In this section, we describe how we analyzed the dye distribution of
different kinds of labeled polyrner latexes in a simple, qualitative way. using GPC. We
use tandem detectors: one is based upon the difference of refractive index between eluent
(here, THF) and the polymer (Ri detector), and the other is based upon the fluorescent
intensity (fluorescence or UV detector). The data are collected simultaneously, so that one
can distinguish where the fluorescence intensity cornes from.
Figure 2.4 shows a GPC chromatogram for (a) Phe-labeled P(MMA-CO-EHA) and
(b) MeAn-labeled P(MMA-CO-EHA) latex polymers. We show three curves: curve (1) for
the fluorescence signal for the two-stage polymer, curve (2) for the refractive index (RI)
signal for the two-stage polymer, and curve (3) for the Ri signai for the seed polymer. In
curve (1) and (2), the signai derived from seed polyrner is not visible because total
content of seed polymer is only 10 wt 8, and the distribution of the molecular weight is
partly overlapped with that of polymer fonned in the second stage. One can also see that
curves (1) and (2) have the sarne trend, which indicates that dyes are uniformly labeled
dong polymer backbones. The fluorescence signai is shifted slightiy to the left side of the
RI signal because the fluorescence detector is placed just in front of the Ri detector. In
addition to that, there is no fluorescence signal at long elution time, which indicates that
there is no fluorescent comonomer left unattached to the polymer backbones, and no
fluorescent oiigorner formed during the preparation of the labeled latex particles. We
prepared the labeled latex under monomer-starved condition, which enables us to obtain
polymer in which fluorescence dyes are randornly distributed dong the polymer chahs.
In Figure 2 . k one notices that this polymer has a broad molecular weight
distribution. We obtain the weight average molecular weight of ca. 240,000, and its
polydispersity index (PD1 = M W / Mn) of 4.6. The same trend can be seen in Figure 2.4b,
indicating that the broad molecular distribution is derived from the nature of the
cmnonomers (hsrc, MMX and EHX).
d 1
6 8 l O 12 1.1 16 18 20 22 2.1 16 6 8 10 17 14 16 18 10 21 2.1 26
Retention Tirne (min) Retention Tirnr (min)
Figure 2.4: GPC chromatognms For (a) Phe-P(MMA-CO-EHA) and (b) MeAn-P(MMA-
CO-EHA) latex polymers. The samples were prepared by drying a latex dispersion,
followed by dissolution of polymer into THF solvent. Curve (1): the fluorescence signal
for the two-stage polymer; curve (2): the refractive index signal for the two-stage
polymer; and curve (3): the refractive index signal for the seed polymer.
2-3-2. Polymer Interdiffusion in Latex Films Using Fluorescence Resonance Energy
Transfer (FRET) Technique
In Figure 2.5. we present a drawing of a plana. section of an idedized latex film
prepared from a mixture of D- and A-labeled latex particles in the absence of pigment.
This type of ordered structure is obtained if the particles are uniform in size and
oganized into a face-centered cubic array at a high solid content as the dispersion dries."
When the latex film is annealed or allowed to age, polyrner diffusion across the
intercellular boundmies brings the donor- and accepter-labeled polymers into proximity.
A schematic representation of polymer interdiffusion across the intercellular boundary in
a latex film is shown in Figure 2.6. When two adjacent cells are labeled, respectively.
with D and A chromophores, polymer interdiffusion will bring the D and A dyes closer
together. This process leads to a measurable increase in energy transfer.
D A Annealing A D D -+
D A T > T g
: zone of interdiffusion
Figure 2.5: Schematic representation of polymer interdiffusion for a latex film. When a
latex film is annealed well above its glass transition temperature (Tg), the film has a zone
of interdifision which contains mixtures of D- and A-labeled polymer molecules.
1 Annealing T > Tg
Figure 2.6: Schematic representation of polymer interdiffusion at the molecular level.
When a latex film is annealed above its Tg, D- and A-labeled polymer molecules
interdiffuse across the ce11 boundary and the distances between D and A groups become
smaller.
In this chapter. we examine the influence of inorganic pigments on the polyrner
diffusion rate. We use two types of inorganic pigments. One is precipitated calcium
carbonate (CaC03) with a much larger diameter than the latex panicles. The other
pigment is colloidal silica (SiO?) with a smaller diameter than the latex particles. Those
pigments may be expected to play different roles dunng the film formation process. In the
second stage of film formation, the latex puticles are likely to be located between the
CaC03 pigments, because the size of the CaC03 particles is much larger than that of the
latex particles. On the other hand, SiOl may be located between the latex paaicles,
because the size of SiOl particles is much smaller than that of the latex particles.
19 Feng et al. studied the effect of poly(methy1 rnethacrylate) (PMMA) filler
particles on the rate of polymer diffusion in films prepared from poly(buty1 methacrylate)
(PBMA). In their experiments, they examined films containing a constant fraction (35 vol
%) of PMMA particles of different sizes. They found that the difision rate decreased in
proportion to the increase in the surface area of the hard filler particles. i.e., with a
decrease in the hard particle size at constant filler volume. Based on their results, we
expect to see a larger effect on the polymer interdiffusion rate in the films containing
SiO2, due to its much larger surface area, than those containing CaC03.
Typical donor fluorescence decay profiles in a latex film are shown in Figure 2.7.
Figure ?.?a shows data obtained for a latex film containing no pigment. Figure 2.7b
shows the corresponding decay traces obtained for û film containing 80 wt % CaCO,.
Figure 2 . 7 ~ shows the corresponding decay traces obtained for a film containing 20 wt %
SiO2. When the latex film is annealed at temperatures well above Tg, polyrner
interdifision occurs. One c m see the evolution of polymer interdiffusion in the films by
Iooking at the extent of curvature of the decay profiles. As one anneals the Film for longer
times, the curvature becomes more pronounced, which indicates that polymer
interdiffusion is promoted by heat and annealing time. One can see that polymer diffusion
rate is fast for the first 60 minutes. and then slows down in Figure 2.7b. One can also see
that the extent of polymer interdiffusion for the films containing pigment is smaller than
that for the film containing no pigment.
(a) P(h1hIA-cwEHA) iI
O 50 100 150 200 250
Time (ns)
lm
* mo *œ VI e U CI
c - 100
1 O
O 50 100 150 200 250
Time (ns)
O 50 LOO 150 200 250
Time (ns)
Figure 2.7: Donor fluorescence decay profiles (a) in a P(MMA-CO-EHA) latex film. (b)
in a P(MMA-CO-EHA) latex film containhg 80 wt % CaC03, and (c) in a P(MMA-CO-
EHA) latex film cont-ng 20 wt % SiO2. Each film was annealed at 50 t 1 OC for (1) 0
min. (2) 60 min. and (3) 990 min. The decay profiles are integrated to obtain the area
under the curves.
2-33, Efficiency of Energy Transfer, M O ) in Newly Formed Films
In this section we examine the influence of pigment on the extent of energy
transfer in newly formed films. If these films are prepared at low enough temperature;
M e or no polymer interdiffusion will take place. Energy transfer will occur only across
the interface between cells formed by the D- and A-labeled latex particles. Under rhese
circumstances @&O) is a measure of the interfacial area between D- and A-labeled celis 19 in the film. The ncwly fomcd films we rxarrinsd were dlorved !O dry at room
temperature (23 OC, i.e., above the minimum film forming tempenture). But as soon as
the film appeared to be dry, it was transferred to the cold room at 4 O C for storage until
the decay profile of the cold film could be measured. Since the Tg of the matrix polyrner
is 7 OC, we imagine that minimal polymer diffusion occurs when the films are prepared in
this way. We use experimental values of area(0) and @do) to examine the effect of
pigment on the contact between D- and A-labeled latex particles in the newly formed
films.
Effect of CaCOl
In Figure 2.8 we plot @&O) vs &CO3 content (wt 8) for a series of freshly
prepared films. The values of #&O) obtained range from 0.082 to 0.095. In other
experiments on nascent films prepared from sirnilar-sized latex particles at temperatures
close to the minimum film forming temperature, @do) values on the order of 0.05 to
0.07 were obtained. 'O These results suggest that Little polymer diffusion has occurred in
the samples we have examined. Note also that meaningfui data are obtained for films
containing 80 wt % and 90 wt % CaC03. These films contain large amounts of air voids
and, as a consequence, are opaque. We obtained a flat line through al1 the data points.
Measurements were repeated several times, and the error bars were attached to each set of
data in order to examine how the data were dispened. We took the average vdue anci
showed both the minimum and the maximum vaiues as error bars. The results indicate
chat the measurements are reproducible. By carefblly aligning the optics for the
measurement, the extent of light scattering reaching the detector is minimized. Some
scattered light at the excitation wavelength is detected. but we correct its contribution to
the fluorescence signal as described in ref 15, by the software used for data analysis.
One can see in Figure 2.8 thar the magnitude of @do) is essentidly independent
of the arnount of CaC03 in the film. This result indicates that even when large amounts of
CaC03 is present, there is a comrnon extent of interfacial contact in the film between cells
formed from D- and A-labeled latex particles. One could imagine that wetting of the
pigmcnt by ille htcx pdymer rnight eïen promcte the extent of mixing in the newly
formed films. This does not occur. We conclude that CaC03 pigment does not promote
coalescence of the latex particles.
Effect of SiO2.
In Figure 2.9 we plot values of # d o ) vs Si02 (wt %) for newly fomed latex
films. In the case of latex films containing CaC03, useful films could be prepared
containing as much as 90 wt % pigment. Here we were limited to films containing up to
50 wt 8 Sioz. When we attempted to prepare films containing larger amounts of silica.
those films were so brittle after they dned that they could not be hnndled. In the films
shown in Figure 2.9, we obtained @do) values ranging from 0.060 to 0.079. The initial
values of the energy vansfer quantum efficiency are slightly lower than in the case of
CaCO,. In addition, one cm see bat, unlike CaC03, @do) values decrease slightly with
increasing amounts of Si02. As we mentioned above, @ d o ) is prirnarily a measure of the
interfacial area between D- and A-labeled cells in the film. The data in Figure 2.9 indicate
that the SiOl pigment with a diarneter of 25 nm appears to have a srnall effect either on
reducing the interfacial area between D- and A-labeled cells in the system, or on
suppressing the lirnited extent of polymer difision that occurs during film formation.
One can see that the error bars for each data point are srnail, which indicates that the
measurements are reproducible. Like CaCOi, SiOz does not pmmote coalescence of the
latex particles dunng film formation.
0.00 I I t t I 1 I I I 1
O 20 40 60 80
CaCO, content (wt %)
Figure 2.8: Effect of CaC03 content on the initial efficiency of energy transfer. We plot
the initial energy efficiency of energy transfer @do) vs CaCOj content in latex films (wt
%).
SiO, content (wt %)
Figure 2.9: EfTect of SiOl content on the initial efficiency of energy transfer. We plot the
initial efficiency of energy transfer M O ) vs SiO? content in latex films (wt %).
2-3-4. Maximum Effciency of Energy Transfer Q'nim) in Latex F i
In order to evaluate the extent of mixing. f,(t) defined by eq 2-8, we need to know
the value of #da), which corresponds to full mixing of the polymer. If the film is fully
mixed, one should have a random distribution of donors and acceptors, and the decay
profile should be descnbed by eq 2 4 . Under these circumstances, the rate and efficiency
of eneqy transfer will be determined only by &, k, and the concentration of acceptors in
tliz filrii. In order ro obtain ihis value experimcntally, one necds a mode! comsponding to
a "fully mixed sample. In the energy transfer experiments we carry out, On reaches its
maximum value when the polymers in the film diffuse a distance on the order of the
original particle radius. There are several ways to obtain a sample which will serve as a
model for a&=). First, one takes a film and anneals it at a higher temperature. Since the
polymer rate is strongly accelented by increasing temperature, (PET will norrnally
increase npidly to its maximum value. Altematively, one can dissolve a dry film sample
in an organic solvent. In solution, one expects full mixing of the polymer molecules. A
film cast from this solution is then a good model for the determination of O&=). In
pigment-Free films, both approaches give similar values of ares(-) values from which the
corresponding O&-) are caiculated. In this section. we examine the effect of pigment on
the magnitude of On(=).
For pigment-free latex films, we obtained areu(=) values from a solvent-cast film.
This film was prepared from a dry P(MMA-CO-EHA) film prepared from a 1: I mixture of
D- and A-labeled particles, which was subsequently dissolved in tetrahydrofuran o. The solution was cast onto a quartz plate and allowed to dry at room temperature for 12 h.
For these films, we obtained an ares(=) value of 18.2 ns, and a @E~(Q)) value of 0.60, and
these values did not change when the film was annealed at 80 OC for 1 h. In contrast,
when a sarnple of the latex film itself was annealed at 50 OC for 112 h, we obtained an
ares(=) value of 22.6 ns, and a @Ei(w) value of 0.50. We infer from these results, that for
prolonged anneding at 50 OC, lu11 mixing is not achieved after 112 h. As one cm see in
Table 2.3, a latex film heated at 60 O C for this length of time gave values closer to that
found for the solvent-cast film, ares(=) = 19.7 ns, and a @da) value of 0.57.
-
a. Annealing time.
b. Annealing tempenture.
c. Integnted area under the donor fluorescence decay profile.
Table 2.3: Final area and maximum efficiency of energy transfer values.
Effect of CaCOI,
When films
values of ares(=)
were prepared in the presence of pigment, we obtained different
and M m ) . This result was unexpected. As a consequence. we
i
2
3 iI
4
5
6
7
8
9
10
1 I
examined this effect in some detail, particularly for the films containing large amounts of
CaC03. As seen in Table 2.3, a film containing 90 wt % CaC03, mnealed for 112 h at 50
OC had am(=) = 30.5 ns, and @R(-) = 0.33. M e n we repeated this expenment at 60
OC, @&O) increased to 0.36, still well below the value expected for complete
interdifision. Even anneding a film containing 90 wt % CaC03 for 30 min at 120 OC
increased @dm) to only 0.48. In order to force mixing, we added THF to a latex film
containing 90 wt % CaCO,. The polymer was kept in the presence of the solvent for
t m ~ i "
min
60
6750
6750
6750
6750
30
30
30
-
6750
6750
CaC03
wt %
O
O
90
O
90
90
90
90
90
80
80
Tanneai
OC
80
50
50
60
60
100
120
140
50
60
@ET
0.60
0.50
area
ns
18.2
22.6
30.5
19.7
29.3
26.1
23.8
24.5
20.6
29.4
28 .O
solvent cast
0.33
0.57
0.36
0.42
0.48
0.46
0.55
0.36
0.39
6 +THF
several minutes, and then the solvent was allowed to evaporate. This film, after drying 12
h at room temperature. had a significantly higher extent of energy transfer, Gd-) = 0.55,
Effect of SiO?.
We also determined ares(-) values for films containing up to 50 wt % silica. For
samples annealed 112 h at 50 OC. these values ranged from 22.2 ns (no silica) to 28.7 ns
(50 wt % of SiO3, corresponding @dm) values from 0.51 to 0.37. As in the case of
calcium carbonate, the presence of pigment appears to lower the maximum efficiency of
energy transfer, @da), accessible by anneaiing the latex films. This effect is likely to be
a consequence of the pigment aff'ecting the extent of diffusion that occurs in the system.
To examine the influence of pigment on the rate of polymer diffusion, we need to
introduce values of fm(t). Since f,(t) is a measure of the extent of mixing in the system. we
need to choose values for ares(=) and that describe only the latex polymer in the
system, and are not influenced by any effect of the pigment on limiting the total extent of
interdiffusion. For this reason. we use values obtained for the pigment-free polymer
obtained by solvent casting: ares(=) = 18.2 ns. and #da) = 0.60.
2-3-5. Effect of Pigment on the Rate of Polymer Interdiffusion
In Figure 2.10, we plot values of f,(t) as a function of annealing time at 50 OC for
films containing different amounts of CaC03. In carrying out these experiments, we
assume thnt the annealing temperature of 50 O C does not affect the characteristics of
CaC03 pigment. The f,(r) values were calculated with eq 2-8 from the areas under the
fluorescent decay curves, using a common area(m) value of 18.2 ns. The most important
result in Figure 2.10 is that polyrner interdiffusion is not significantly retarded by the
presence of the CaC03 pigment. The diffusion rate is unchanged for amounts of pigment
up to 30 wt 96. Even at 80 wt %, the effect is srnail. Various experiments were repeated a
number of times, and experirnents were carried out at different tempentures. The trends
s h o w in Figure 2.10 are consistent. To cary out meaningful expenments, a set of
samples must be annealed simultaneously. It is difficult to reproduce the exact profile of
f,(r) vs t for experiments carried out at different rimes. The problem is one of reproducing
the oven temperature. The polymer diffusion rate is so sensitive to tempenture (with
apparent activation energies ranging frorn 160 ?' to 400 kllmol ") that even small
temperature differences between expenments have a noticeable effect on the rates of
polymer diffusion.
In Figure 2.1 1 we plot values of f,(t) as a function of annealing time at 50 O C for
films containing different arnounts of Sioz. Here we see that the presence of SiO2 in the
film lias a pronounseci e f k t on reducing the rate of polyner interdiffusion. Therc is
greater retardation when one increases the amount of SiO2 in the latex films.
O 60 120 180 240 300 1000
T h e (min)
Figure 2.10: Comparison of P(MMA-CO-EHA) extent of rnixing, f;.(t) vs annealing tirne
for films containing different amounts of calcium carbonate: O wt % (O), 30 wt % (a), 80
wt % (O), and 90 wt % (i). The weight-average molecular weight ( M W ) of the P(MMA-
CO-EHA) latex was ca. 240,000. Films were annealed sirnuItaneously at 50 2 1 OC.
O 60 120 180 240 300 1000 T h e (min)
Figure 2.11: Comparison of P(MMA-CO-EHA) extent of mixing. f,(t) vs anneding time
for films containing different arnounts of silica: O wt % (O), 20 wt % (a), 40 wt % (a),
and 50 wt 8 (a). The weight-average molecular weight ( M W ) of the P(MMA-CO-EHA)
latex was ca. 240,000. Films were ûnnealed simultaneously at 50 t 1 OC.
2-4. Discussion
The polymer we examine as the binder for our model paper coatings is prepared
from equal parts by weight of MMA and EHA using serni-continuous emulsion
polymerization to maintain a uniform composition throughout the polymerization. The
mole ratio of MMA / EHA components is 65 1 35. and the polymer has a Tg of 7 OC. The
seed particle used in the synthesis was prepared by batch emulsion polymerization in the
absence of iuiy shah tramfer agciit, whercas a small m o u n t of 1-dcdecxxthiol :vas
added in the second stage to limit the molecular weight. A broad molecular weight
distribution was found for these latex polyrners by gel permeation chromatography
(GPC). We know little about branching in the polymer we produce. Giving the reactivity
of the polymerizing acrylate radical and the presence of the teniary hydrogen on the EHA
side chain. it is likely that the polymer we examine contains substantial branching.
Paralle! analysis of the latex polyrner by GPC, using tandem fluorescence and refractive
index detectors. establishes that the fluorescent dyes are covalently bound to the polymer
and uniformly incorponted.
In pigment-free latex films, polymer diffusion is slow at room temperature. but
faster at 50 OC. Under these conditions. 43 O C above Tg, the initial stage of polymer
diffusion (to f,(t) = 0.4) is rapid, followed by a slower rate of increase. As mentioned
above, the polymer molecular weight is broad, and there is likely to be a distribution of
bnnching as well. The smaller and more compact structures are likely to have faster
invinsic diffusion rates. In latex films containing a mixture of species with a distribution
of diffusivities, the fastest diffusing species make the largest contribution to fm(t) at early
times. One can in principle obtain a deeper insight into the nature of the diffusion process
by assuming a model and calculating apparent diffusion coefficients. We will defer this
type of analysis for the next chapter. The tirne necessary for Full mixing is much longer
than those shown in Figure 2.10 and Figure 2.1 1. As seen in Table 2.3, even 6750 min
(1 12 h) anneaiing at Tg + 43 OC does not lead to full mixing. Here Q>rr = 0.50 whereas the
fully mixed sample, prepared by solvent casting, has @= = 0.60. It rnay be that some of
the polymers in the sample, as a consequence of long-chah bnnching, have a star-like
structure. Such polyrnen have a very slow rate of diffusion in polymer melts. "
In the presence of pigment, the polymer diffusion rate is retarded. The effect is
large for silica, which has a large surface to volume ratio. and rnuch smailer for calcium
carbonate, where the surface to volume ratio is ca. 10 tirnes smaller. In addition, the
surfaces of the pigment are very different. The Si-OH groups at the surface of the silica
are likely to be strong hydrogen bond donors toward the ester groups in the latex polymer.
The results for silica p d l e l those found by Feng et al. for PMMA particles added as
hard organic fillen to PBMA latex films.
There are two types of explanallons for the effect of filler on a reduced polyrner
diffusion rate. In the first model, the hard pigment surface serves to make the adjacent
polymer matrix more rigid. It is well known that polymer chains adjacent to a @id
surface have decreased mobility. " Tsagaropoulos and Eisenberg '5 proposed a three-
Iayer model in terms of polymer inobility. They studied changes in the glass transition
temperature associated with adding small silica particles as fillen to various bulk
polymers. As increasing amount of filler was added. a new high glass transition
tempenture (Tg) was found in addition to the Tg for the bulk polymer. As more filler was
added. they found a decrease in the magnitude of the signal from which Tg was
determined. In their model. the surface Iayer of polymer adjacent to the pigment is
suongly immobilized, but in addition, nearby polymer also has its mobility restricted. It is
this nearby polymer that contributes to the elevated Tg. From this perspective, one reason
for the decreased rate of polymer diffusion found here is that the polymer molecules near
to a pigment surface have decreased mobility.
In the case of small silica particles as the pigment, another effect can become
important. As shown in Figure 2.12, the silica particles are rnuch smaller ihan the latex
particles. When the film dnes. one c m imagine that the silica particles deconte the
interface between adjacent cells. In this way they c m serve as obstacles to the diffusion of
latex polymer. It is important to recognize that obstacles slow down diffusion without
affecting the intrinsic mobility of the difising species. The obstacles at the interface
opente in a different way to force molecules to difise along more tortuous pathways to
reach the same extent of interdiffusion. As a consequence, the time necessary for mixing
is longer.
As shown in Figure 2.13, the CaC03 particles are much larger than the cells
formed from the individual latex particles. It would be very unlikely that these large
particles could act as an obstacle to intercellular polyrner diffusion. The retardation that is
observed at very high pigment levels is likely to the ngidifying effect of the nearby
pigment surface.
Figure 2.13 also illustrates an important feature of the binder-pigment rnorphology
whan srnall amounrs of binder are used with C C O j as the pigment.
Joanicot et al. examined the binder-pigment morphology of such systems. They
spread a carboxyl functionalized styrene-butylacrylate latex dispersion on a calcium
carbonate crystal plate and observed the structure by atomic force microscopy (AFM).
Here, they took the same surface coverage as for mineral pigments in typical matte paint.
which has a pigment volume concentration (PVC) of 70 8. They have shown that the
binder dispersion spreads completely dong the crystal surface, but those latex polymer
particles in the dispersion do not form a continuous film. The latex polymer forms
patches on the surface unless they are annealed at very high temperatures. Consequently,
Joanicot et al. concluded that. in an actual coating, the CaC03 pigments can be fixed by
isolated latex particles. i.e. stuck together at many discrete points by droplets of latex
polymer. Each droplet originated from the localized coalescence of many latex particles.
In Our case, the latex particles are not acid-functionalized, but we can provide a
few further insights into this model. We know from Our measurements on newly formed
films that @ d o ) does not change as the fraction of pigment in the system increases. From
this result we infer that the interfacial area between donor- and acceptor-labeled cells
does not change significantly. even when there is 90 wt % pigment. The pigment, of
course, is denser than the latex particles, so 90 wt b pigment corresponds to about 25 vol
% binder. When these samples are mnealed, polymer difision leads to an increase in
C P ~ , but the last stages of mixing are strongly retarded. As one sees in Table 2.3. 112 h of
annealing for 90 wt % CaC03, even at 60 OC, leads to Q'n( 1 12 h) = 0.36, f,(t) = 0.53. We
interpret this result to indicate that upon annealing, the blob of latex film does not behave
exactly as shown in Figure 2.13, but rather spreads Locally dong the pigment surface. In
this way, we imagine that a significant fraction of the polymer molecuies adsorbs to the
polymer surface in such a way that they do not participate in the polymer interdifhision
process.
It is aiways possible that the chromophores in the polyrner interact differently with
the pigment surface than the polymer backbone. This effect would be very difficult to
measure, but selective adsorption would be analogous to processes involved in adsorption
chromatography. If this were the case, one could rationalize the result seen at long
annealing times in ~ h e film containhg 90 :vt 8 &CO;. If polymer diffusion were
accompanied by flow dong the pigment surface, chromophore adsorption might lower the
fraction of donor and acceptor groups able to participate in energy transfer. Another
aspect of our measurements that one should keep in mind is that our films are opaque.
Our experiments measure fluorescence in the reflectance mode from the samples. The
excitation light may penetrate into the sample to a depth not much larger than the
wavelength of light. Thus we may be observing selectively processes that occur in the
fint micrometer of ihese 100 pm thick films.
: zone of interdiffusion
: SIIica
Figure 2.12: Schematic representation of the annealing process for a latex film
containing silica particles (25 nm). The silica particles can act as obstacles and retard
polymer diffusion andor reduce the mobility of polymer molecules near the pigment
surface.
I Annealing ' D A \
: zone of interdiffusion : Calcium carbonate
Figure 2.13: Schematic representation of the annealing process for a latex film containing
precipitated calcium carbonate. The latex particles spread locally dong the pigment
surface.
2-5. Conclusions
We employed the fluorescence energy transfer technique to measure the rate of
polymer interdiffusion in the presence of large amounts of inorganic pigment. We
obtained sirnilx values of the initial efficiency of energy transfer in the presence of
different amounts of CaC03 and silica pigment. This result indicates that neither pigment
pemirbs the interfacial area between donor- and accepter-labeled particles in the newly
fomcd film, nar do they pmmote coûlescence of the tntex pxticles. CaCO; has little
effect on the polymer diffusion rate in the binder phase, but at very high solids content
(80, 90 wt %). the diffusion rate is slowed. In contrast, 25 nm diameter silica particles
have a much more pronounced effect on slowing the rate of polymer diffusion. Because
of sample brittleness, we could study films containing up to 50 wt % (3 1.2 vol %) silica.
In these films. the binder is present as the continuous phase. At 90 wt % CaC03 content.
the volume fraction of binder is about 0.25. Here the binder serves prirnarily to glue the
pigment particles together. Because of the air voids in the matrix, the films are essentially
opaque.
2-6. References
Macnair, A. K., Task Group Chairman, Synthetic Couring Adhesives, TAPPI PRESS.
Atlanta, 1998.
LePoutre. P., Prog. in Org. Coat.. 1989, 17,89.
(a) Wang, Y.; Kats, A.; Juhué, D.; Winnik, M. A., Langmuir, 1992, 8. 1435. (b)
Winnik, M. A., The Formation and Properties of Latex Films in Emdsion
Polymerization and Emiiision Polymers, Lovell, P. A., El-Aasser, M. S., Eds., Wiley,
New York, 1997, p 467. (c) Keddie, J . L., Mat. Sci Eng., 1997,21, 10 1.
(a) Granier, V.; Sartre, A., Langmuir, 1995, i l , 2 179. (b) Butt, H. J.; Gerhm, B .,
Langmuir, 1995,11,4735. (c) Unenl, W. N., Langmuir, 1998, 14,2201. (d) Sheehan,
I. G.; Whalen-Shaw, M.. Tappi J., 1990, 73, L 7 1.
5. (a) Sheehan, J. G.; Takamura, K.; Davis, H. T.; Scnven, L. E., Tappi J., 1993, 76, 93.
(b) Ming, Y .; Davis, H. T.; Scnven, L. E.; Takamura. K.; Vodnick, J. L., TAPPI 1995
Coating Conference Proceedings. TAPPI PRESS, p 39 1.
6. Joanicot. M.: Granier. V.; Wong, K., Prog. in Org. Coat., 1997,32. 109.
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F!i<orescence Specrroscopy, Plenum Press, New York. 1983.
8. (a) Farinha, J. P. S.; Matinho, J. M. G.; Kawaguchi, S.; Yekta, A.; Winnik, M. A.,
Macromolecules, 1995. 38, 6084. (b) Kim, H.; Winnik. M. A., Macramolecdes,
1995, 28,2033. (c) Winnik. M. A.; Pinenq, P.: Krüger, C.; Zhang, J.: Yaneff. P. V.. J.
Coat. Technol., 1999, 7I,47.
9. Feng, J.; Odrobina, E.; Winnik, M. A., Macrornolecrdes, 1998,31,5290.
10. Rharbi, Y.; Cabane, B.; Vacher. A.: Joanicot, M.; Boue, F.. Ertrophys. Lett.. 1999.46.
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I l . Zhao, C. -L.; Wang, Y.; Hruska. 2.; Winnik. M. A., Macromoiecrries. 1990. 23.4082.
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14. O'Connor, D. V.: Phillips, D., Tirne-correlated Single Photon Cotmting, Academic.
New York, 1984.
15. Martinho, I. M. G.; Egan, L. S.; Winnik, M. A.. Anal. Chern., 1987,59,86 1.
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Y.; Winnik, M. A.; Haley, F., J. Coat. Technol., 1992.64, 5 1. (c) Kim, H.; Wang, Y.;
Winnik, M. A., Polymer, 1994, 35, 1779. (d) Kim, H.; Winnik, M. A.,
Macromolecules, 1994,27. 1007.
17. Sosnowski, S.; Feng, J.; Winnik, M. A., J. Polyni. Sci.: Polym. Chem., 1994. 32,
1497.
18. (a) Joanicot, M.; Wong, K.; Maquet, 1.; Chevalier, Y.; Pichot, C.; Graillat, C.;
Lindner, P.; Rios, L.; Cabane, B., Prog. Colloid Polym. Sci., 1990, 81, 175. (b)
Chevalier, Y.; Pichot, C.; Graillût, C.; Joanicot. M.; Wong, K.; Lindner, P.; Cabane,
B., Colloid Polym. Sci., 1992,270, 806.
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21. Wang, Y.; Winnik, M. A., J. Phys. Chem., 1993, 97,2507.
22. Wang, Y.; Winnik, M. A., Macronzolecicles, 1993.26.3 147.
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Fetters, L. J.; Graessley, W. W., Macromolecriles, 1986, 19, 785. (c) Shull, K. R.;
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3. EFFECT OF SILICA PARTICLE SIZE ON POLYMER
INTERDIFFUSION
3-1. Introduction
Many polymer coatings used in a wide variety of applications such as paints.
paper coatings, contain a large amount of inorganidorganic pigments. One of the major
reasons for inuoducing these pigments is that soft polymeric matrices c m be reinforced
by hard fillers. For example. naturai or synthetic rubber is commonly reinforced by
carbon black particles. and silicone elastomers (PDMS) can be reinforced by silica. 1
Since silica is chernically inactive and optically transparent, it can be used in a wide range
of applications. nnging from paints and magnetic fluids to high-quality paper coatings. Z
In the paints and adhesives industries, vinyl acetate, acrylic ester, synthetic rubber and
other polymer latex have been used in coatings and adhesives for fribric and paper. J
Colloidd silica is used with these polymer emulsions in order to improve adhesion.
durability, and abrasion resistance. The silica also serves to prevent stickiness and
improves the washing resistance of the coatings. When colloidal silicû is used as a
constituent for paints, a superior stain resistant film can be formed due to its anti-static
characteristics. In paper coatings. this property provides better quality on sheet feeding,
and thus silica is used as an anti-static agent.
Colloidal silica is also widely used for paper coatings. For instance, high-quality
coated paper for graphics printing has a smooth layer on the top of the coating in order to
enhance gloss. ' Other inorganic pigments such as clay, calcium carbonate, talc, alurnina,
titanium dioxide. are also used with latex polymers to avoid stickiness to the stainless
steel drum. For this type of paper, one would like to make the surface of the paper more
rigid, in other words, prevent it from sticking to the hot stainless steel cirum.
Another application of (colloidal) silica for high-quality paper coatings is for
photognphic quality ink jet printing paper. ' In this case, the coating contains both silica
and a latex polymer with a relatively high glas transition temperature (Tg), as well as
water-soluble polymers such as poly(viny1 alcohol), casein, and starch. The ink receiving
Iayer is required to have a large pore volume to keep the ink jet printing ink at the surface
of the coated paper, and dry the ink dispersion as quickly as possible. Silica also serves to
prevent the ink dispersion from spreading honzontally and penetrating vertically. In this
application, the latex particles act as silica binder, but are also required to keep their
shape, which rneans that less coalescence is preferred in this system. These different
applications of silica-plus-latex in coatings require a broad understanding of the nature of
their interaction. Here we examine the influence of silica nanospheres on the rate and
extent of polymer diffusion in an ücrylate latex film.
In this chapter we examine the influence of silica as a filler on polymer
interdiffusion in poly(buty1 rnethacrylate) latex films. We carry out fluorescence
resonance energy transfer (FRET) rneasurements on latex films that allow us to follow the
extent of polymer diffusion as a function of time after the latexfpigment dispersion dries.
In this study we compare four different types of colloidal silica, and discuss how silica
fillers affect the rate of polymer interdiffusion in latex films. We focus on the effect of
silica particle size on polymer interdiffusion rate in poly(buty1 methacrylate) latex films.
3-2. Experimental
3-2-1. Materials for this study
3-2-la. Materials for Emulsion Polymerization
Butyl rnethacrylate (BMA. Aldrich. 99%) was distilled under vacuum under a N2
atmosphere, and stored in refrigerator prior to use. Potassium persulfate (K2S20s, KPS,
Aldrich. 99%), sodium bicarbonate (NaHCO,. Caiedon, 998) , sodium dodecyl sulfate
(Ci2H30S03*Na', SDS, Aldrich. 98%) and 1-dodecanethiol (Ci2HaSH, DM, Aldrich,
988) were used as supplied. Distilled water was further purified through a Millipore
~ i l l i - ~ ~ ~ system. The structure of butyl methacrylate used for this study are show in
Figure 3.1.
Figure 3.1: Chemicd structure of butyl methacrylate used for this study.
9-Phenanthrylmethyl methacrylate (Phe-MMA) were synthesized previously. 6
The synthesis of (9-anthry1)methacrylate (An-MA) was also described in ref 6b. The
structures of these dye-labeled monomers are shown in Figure 3.2.
Figure 3.2: Chernical structures of dye-labeled monomers used for this study.
3-2-1 b. Synthesis of Dye-Labeled Latex Particles
Butyl methacrylate (BMA) was distilled under vacuum pnor to use. Potassium
persulfate (KPS), sodium bicarbonate (NaHC03), sodium dodecyl sulfate (SDS) and 1-
dodecanethiol were used as supplied. Distilled water was further punfied through a
Millipore ~ i l l i - ~ ~ system. Poly(buty1 methacrylate) (PBMA) latex samples, labeled
with 1 mol 8 of a fluorescent dye [either a donor, phenanthrene (Phe) or an accepter,
anthncene (An)], were prepared by semi-continuous emulsion copolymenzation at 80 OC,
using KPS as the initiator, SDS as the surfactant, and 1-dodecanethiol as the chain 7 transfer agent. The reaction conditions were similar to those described previously. As a
donor-labeled monomer, we used (9-phenanthry1)rnethyl methacrylate (Phe-MMA) to
intmduce the donor dye. As an acceptor-labeled monomer, we used 9-mthryl
methacrylate (An-MA). The recipe for 100 nm PBMA latex is shown in Table 3.1. We
used the same seed latex particles obtained in the first stage for preparing both the Phe-
and An-labeled latex particles. Their chmcteristics are shown in Table 3.2. These latex
particles, referred to as PBMA, have a diameter of 100 nm. and a Tg of ca. 20 OC. The
solid content of the dispersion was measured gravimetrically and was found to be ca. 30
wt %.
3-2-lc, Colloidal Silica
Colloidal silica dispersions (30 wt % solids). Klebosol 30W5 (K-25) and
Klebosol 30R50 (K-50), were supplied by Clariant Corporation. The filler particles are
amorphous spherical silica beads with diameters of 25 nm and 50 nm, respectively.
Colloidal silica dispersions (20 wt % solids). Snowtex-O (S-12) and Snowtex-OL
(S-49, were supplied by Nissan Chernical Industries. These silica particles are also
amorphous sphencal beads with diameters of around 12 nrn and 45 nm, respectively.
Both samples contain acidic small particle silicasol. with a free anion content less than
300 ppm. ' The conductivity values for these dispersions are ca. 300 pS/crn, much lower
than Klebosol dispersions. Further charactenstics of these particles are shown in Table
3.2.
Table 3.1: Recipe for preparing LOO nm PBMA latex.
An-MA (g)
Phe-MMA (g)
Water (g)
l -Dodecanethiol (g)
First stage
a. 1 mole % relative to the total monomer fed in the second stage.
Second stage
(under monomer starved condition) (batch process)
3.5
57 .O
0.06
0.2 1
0.08
80
1.5
b. Reaction tempenture.
c. Reaction time.
Phe-Iabeled
35.0
0.68 "
27 .O
0.06
0.60
0.30
80
18
An-labeled
35.0
0.65 "
27 .O
0.06 ,
0.60
0.30
80
18
Table 3.2: Characteristics of the latex and colloidal silica The particle size, Tg, and
molecular weight for Phe-PBMA are very similar to those for An-PBMA.
1 1 1 Colloidal siIica 1 PBMA Klebosol 1 Klebosol
Phe- + An- / 1 30R25 30R50 labeled
Diameter (nm) i i û 0 ' i 25" i 50"
DLS 1 90.8 1 25.6 1 73.1
M w / M n 2.5 - - pH of dispersion 7 . W .4 9 .O13 .O 9.013.0
(S- 12) (S-45)
a. Determined by quasielastic light scattering (Brookhaven BI-90).
b. Obtained from the brochures.
c. Determined by dynamic light scattering (DLS).
d. Determined by transmission electron microscopy (TEM).
e. The data beforelafter ion-exchange are shown for PBMA latex and Klebosol.
Conductivity of
dispersion (pS/cm)'
3-2-2. Characterization of Latex and Colloidal Silica
3-2-Sa. Particle Size Measurements
Dynamic light scattering (DLS) experiments were carried out on a variable angle
laser light scattenng photometer from Broo khaven Instruments Corporation. A 5 m W
vertically polarized He-Ne laser from Spectra physics was the Iight source. The silica
dispersion in water was filtered through disposabie 0.45 pm Gelman filters into g las
scattenng cells. The cells were placed into the BI-ZOOSM goniometer and sat in a vat of
8001 160 2,0001300 1,2001 150
themostated toluene which matched the index of refraction of the glass cells. The angular
range of the goniorneter was 7'462'. The scattered light was detected by a
photomultiplier interfaced to the BI-2030AT digital correlator with 136 channels and
measured the correlation Function in real time. Dynamic light scattering data were 9 analyzed following the method of cumulants. The logarithm of the nonnalized intensity
autocorrelation function, g"' r, cm be fitted into a power series in terms of the delay time
(1 Ing r=-Tl?+
where rl is the first cumulant, r2 the second cumulant, and so forth. Once ri is
deterrnined, the concentration and angular dependence can be expressed as
where D, is the z-average diffusion coefficient. C is a parameter that is chancteristic of
the molecular architecture. kD is the effective interaction parameter, and q is the scattering
vector, with ifs magnitude given by
where 0 is the scattering angle, no is the refractive index of solvent, and is the
wavelength of the laser beam in a vacuum. From the difision coefficient, the z-average
hydrodynamic radius, Rh, CM be calculated frorn the Stokes-Einstein relation
where q is the solvent viscosity. For the determination of the z-average diffusion
coefficient for each sarnple, 90' scattering angle and one concentration were used.
Dynarnic light scanering data were also anaiyzed using the CONTIN 'O method to
determine the distribution of hydrodynamic size.
Transmission electron microscopy (TEM) measurements were carried out on a
Hitachi mode1 600 electron microscope. The sarnples were prepared as follows. Thin
carbon films (ca. 5 A) were grown on mica as a support. Then 25 pL of the silica
dispersion in water was added onto the carbon-coated mica surface. After the water was
evaporaied, cach carbon film :vas flonted off the micz support in water and deposited onto
a 300 mesh Gilder copper grid. The sample was air-dried before introduction into the
electron microscope. Staining of the sample was unnecessary. The diameter of the silica
was measured directly from the TEM micrograph. using "Photoshop" software. For each
simple, around 100 to 200 particles were measured and the average was taken as the final
diane ter.
3-2-2b. Ge1 Permeation Chromatography (GPC)
Molecular weight and molecular weight distributions were measured by gel
permeation chrornatography (GPC). in the s m e manner with that descnbed in Section 2-
3-4b.
3-2-2c. Soüd Content of latex dispersions
The solids content of latex dispersions was measured in the sarne manner with that
described in Section 2-2-4c.
3-2-3. Film Preparation
Before we prepared films from the PBMA latex and colloidal silica dispersions,
the latex and Klebosol dispersions were cleaned by treating the individual dispersions
with a prepurified ion-exchange resin (AG-508-X8 rnixed-bed resin, Bio-Rad) to rernove
the ionic surfactant and other ionic species. For each sample, the pH decreased from 8-9
to 2-3 after the ion-exchange. The Snowtex dispersions were used as supplied. Latex
films were prepared from dispersion mixtures with a 1:I number ratio of Phe- and An-
labeled PBMA latex particies and different amounts of silica. nie final dispersions, with
total solids content ranging from Ca. 8 wt % to 19 wt %, have pH = 3-4. Film formation
was carried out by the following procedure. For each film, using a Pasteur pipette. we
spread eight drops from each dispersion mixture onto a quartz plate. The film was
allowed to dry over 40 minutes in an oven at 3 1 f 1 OC, followed by storage in the cold
room at 4 OC to minimize the amount of polymer interdiffision in the film. A typical film
thickness was 150 Fm. Films formed from latex alone were crack-free and transparent.
-411 of the cher films were transparent and crack-free up to 31) wt % silica content. but
small cracks were observed in the films containing JO wt % filler. It was hard to work
with films containing more than 40 wt % filler.
3-2-4. Energy Transfer Measurement
Al1 films were annealed at 60 f I OC for polymer diffusion measurements. For
each series of samples to be cornpared. the films were annealed simultaneously.
Fluorescence decay profiles were measured by the single photon-timing
technique. " Samples were excited at 300 nm, and the emission was detected at 350 nm.
A bandpass filter (350 i 5 nm) was used to minimize the scattered light and interference
due to fluorescence from excited accepton. For each measurernent, it took about 10 to 15
minutes to collect 5000 counts in the maximum channel. frior to each measurement, a
film sample was placed in a quartz tube, and the tube was degassed with flowing nitrogen
gas.
3-2-5. Data and Data Analysis
Data analysis was carried out according to Section 2-3-7.
3-3. Results
3-3-1. Dye Distribution of PBMA
As we have shown in Section 2-34, it is important to know if the fluorescent dyes
are randomly distributed in the polymer backbones. In order to examine this, we canied
out Gel Permeation Chromatography (GPC) andysis.
In the case of labeled PBMA latex polymers, their molecular weight distributions
are much narrower than those of labeled P(h4MA-CO-EHA) polymers, as seen in Figure
2.4. The polydispersity index (PDI) for PBMA latex polyrners is ca. 2.5. One cm clearly
see the peak derived from seed polymer because the weight-avenged molecular weight of
seed polymer is ca. 800.000, whereas thût of second stage polymer is ca. 50,000. GPC
chromatograms for Phe-PBMA are illustrated in Figure 3.3a. and those for An-PBMA are
illustnted in Figure 3.3b.
Retention Time (min) Retention Time (min)
Figure 3.3: GPC chromatograms for (a) Phe-PBMA and (b) An-PBMA latex polyrnen.
The samples were prepared by drying a latex dispersion, followed by dissolution of
polymer into THF solvent. Curve (1): the fluorescence signal for the two-stage polyrner;
curve (2): the refnctive index signal for the two-stage polymer: and curve (3): the
refnctive index signal for the seed polymer.
3-3-2. Determination of Silica Particle Sizes by DLS and TEM
In this chapter, we examine the effect of silica panicle size on the rate of polymer
interdifision. Therefore, it is important to know the exact silica particle size. We
employed both dynamic light scattering (DLS) and transmission electron microscopy
(TEM) measurements to determine the particle size, and compared the data with those
descri bed in the product brochures. 'J' TEM images for each silica panicle are shown in
Figure 3.4. The particle size disuibutions for each siiica panicie decermined by Lhe TEM
experiments are shown in Figure 3.5. The data are summarized in Table 3.2.
When we employed DLS. the particle size data for K-25 and S-12 are very similar
to those described in the product brochures. However. the DLS diameters for K-50 and S-
45 are much larger than those described in the product brochures. DLS data correspond to
the volume average particle size (hydrodynamic radius), so the larger particle size data
are more emphasized in the calculation.
We also employed TEM measurements, and determined the silica particle size.
Approximately 100 to 200 particles were measured, and we determined the number
average particle size for each silica filler. The particle size distributions for K-25. K-50.
and S-12 are quite narrow, but that for S-45 is relatively broad, as seen in Figure 3.5d.
Consequently, the average particle sizes obtained by TEM for K-25, K-50, and S-12 are
very close to those obtained by DLS. On the other hand, the average particle size obtained
by TEM for S-45 is much smaller than that obtained by DLS. due to the relatively broad
particle size distribution.
In order to examine the silica particle size distributions in DLS measurements, we
introduce polydispersity. The first two moments of the distribution G(T) is as follows:
where q is the scattering vector and D is the average difision coefficient. Eq 3-6 shows
that p? is proportional to the variance of the "intensity" weighted diffusion coefficient
distribution. Thus it carries information on the width of the size distribution. The
magnitude and units of pz are not immediately useful for characterizing a size
distribution. In addition, distributions with the same relative width (sarne shape) may
have very different means and variances. For these reasons a relative width (reduced
second moment) is defined as follows:
Polydispenity has no units. It is close to zero for monodisperse samples. If this
value exceeds 0.080, it is considered to be a broad size distribution. The mean particle
sizes and polydispersities obtained from DLS analysis are summarized in Table 3.3. One
c m see that the polydispersity for S-12 is very large (0.283), which means that it has a
broad particle size distribution in DLS measurement. On the other hand. polydispersity of
K-50 is 0.005, indicating that it is almost monodisperse.
In the TEM analysis, we calculared the standard deviations and used those values
as a measure of particle size distributions. For S-45, the standard deviation is 13.7 nm.
indicating that it has a broad particle size distribution. The mean particle sites and the
standard deviations obtained from TEM analysis are summarized in Table 3.3.
Figure 3.4: Transmission electron rnicroscopy (TEM) images for various particle sizes of
silica. (a) K-25, (b) K-50, (c) S-12, (d) S-45.
Diameter (nm)
"9
O 20 JO 60 80 100 120 O 20 JO 60 80 100 120 Diarneter (nm) Diameter (nm)
Figure 3.5: Pûaicle size distributions of silica particles used for this study. (a) K-25. (b)
K-50, ( c ) S-12, (d) S-45. The particle sizes were determined by transmission electron
microscopy (TEM), and ûndyzed with the image analysis software "Photoshop."
Table 3.3: The mean particle sizes and the particle size distributions for silica used for
this study.
Mean diameter Polydispersity
I TEM
Mean diameter
DLS "
Determined by dynamic light scattenng (DLS).
Determined by transmission electron rnicroscopy (TEM).
Each standard deviation (SD) was calculated as follows,
1 SD = - Z ( x i - F)' , where n is the sample size and 1 is the mean. (. - I
Fluorescence Resonance Energy Transfer Technique for Latex Film
Formation
In this study. we examine the influence of colloidal silica (SiO:) as a filler on the
rate of polymer interdiffusion. We compare four different types of Si02 with dianeters
ranging from 12 nm to 50 nm.
Feng et al. l 3 studied the effect of poly(methy1 methacrylate) (PMMA) filler
particles on the rate of polymer interdiffusion in films prepared from poly(buty1
methacrylate) (PBMA). In their experimenrs, they examined films containing a constant
fnction (35 vol %) of PMMA particles of different sizes. They found that the diffusion
rate of the polymer decreased in proportion to the increase in the surface area of the hard
filler particles, i.e., with a decrease in the hard particle size at constant filler volume.
Based on their results, we expected to see a larger effect on the polymer interdifision
rate in the films containing smaller Si&, due to its much larger surface area.
In Chapter 2, we examined the effect of two inorganic pigments on polymer
interdiffusion in a low-Tg latex film. Cdcium carbonate and colloidal silica were used as
mode1 inorganic pigments, and poly(methy1 methaclylate-CO-2-ethylhexyl acrylate)
copolymer latex was used as the latex binder. We found in that system that, due to its
large pigment size, calcium carbonate did not significantly retard the rate of polymer
interdiffusion, even if the pigment content is above the critical pigment volume
concentration (CPVC). But the limiting extent of polymer interdiffusion upon long
anneaiing was significantly reduced: we found that the maximum efficiency of energy
tramfer, QC?i-) for the film containing large amounts of CaCOl (e.g 90 wt %) was much
smaller than that for the film containing no CaC03. In contrast. the presence of silica
significantly retarded the rate of polymer interdiffusion, even when the filler content is
below CPVC. In addition, the value for the film containing large amounts of SiOz
(e.g. 40 wt %) is much srnaller than that for the film containing no Sior.
Typical donor fluorescence decay profiles for latex film samples at different
stages of annealing are shown in Figure 3.6 and Figure 3.7. Figure 3.6 shows data
obtained for a latex film containing no silica filler. Figure 3.73 shows the corresponding
decay traces obtained for a film containing 40 wt % of 25 nrn diameter SiO? (Klebosol
30R25, K-25). Figure 3.7b shows the corresponding decay traces obtained for a film
containing 40 wt % of 50 nm SiO2 (Klebosol 30R50, K-50). Figure 3 . 7 ~ shows the
corresponding decay traces obtained for a film containing 40 wt O/c of 12 nm SiO2
(Snowtex-O, S-12). Figure 3.7d shows the corresponding decay traces obtained for n film
containing 40 wt % of 45 nm Si02 (Snowtex-OL, S-45). When these latex films were
annealed at 60 OC, well above the polymer Tg (20 OC), polymer interdiffusion takes place.
One can see the evolution of polymer interdiffusion in the films by looking at the extent
of curvature of the decay profiles. As one anneals the film for longer times, the curvature
becomes more pronounced, which indicates that polymer interdiffusion is promoted by
heat and annealing time.
If one compares the curves in Figure 3.7a and Figure 3 . 7 ~ with those in Figure
3.6, one sees less curvature in the decay curves at comparable annealing times (at 60 O C )
when silica is present in the sample. This result tells us that polymer diffusion is retarded
over the entire annealing time. In the decay curves themselves, one c m see that there is
greater retardation of the diKusion rate in the samples containing smaller silica particles.
For exampie, in Figure 3.7a and Figure 3 . 7 ~ ~ one can see that the extent of energy transfer
is strongly suppressed due to smailer size of SiO?. One can also see, in Figure 3.7b and
Figure 3.7d, that the rate of polymer interdiffusion is suppressed for the first several
hours, but finally reaches a similar extent of energy transfer after 200 h of annealing at 60
OC, as that shown in Figure 3.6.
PBMA latex film
O 50 100 150 200 250 Time (ns)
Figure 3.6: Donor fluorescence decay profiles in a PBMA latex film after annealed for
( 1 ) O min, (2) 60 min, (3) 330min. (4) 12,000 min.
O 50 100 150 200 250 T i e (ns)
O 50 100 150 200 250 Time (ns)
10000 10000 (d) PBMA + 40 wt % S-15
1000 1 O00 h h u U
œ .I
V1 g 100 œ
U 100 r L
I 3 CI
10 10
I 1 O 50 LOO 150 200 250 O 50 100 150 200 250
Time (ns) Time (ns )
Figure 3.7: Donor fluorescence decay profiles in a latex film. (a) PBMA with 40 wt % of
25 nm SiOr (K-25), (b) PBMA with JO wt 8 of 50 m SiO2 ( M O ) , (c) PBMA with 40
wt % of 12 nrn Si02 (S-12). (d) PBMA with 40 wt % of 45 nm Si02 (S-45) after annealed
for (1) O min, (2) 60 min. (3) 330min. (4) 12,000 min. respectively.
3-3-4. Initial Efficiency of Energy Transfer, M O ) in Newly Formed PBMA Latex
Films
In this section we examine the influence of filler on the extent of energy transfer
in newly formed films. If these films are prepared at low enough temperature, little or no
polymer interdiffusion wil1 take place. Energy transfer will occur only across the interface
between cells formed by the D- and A-labeled latex particles. Under these circumstances,
M O ) is a mesure of the interfacial area between D- and A-labeled cells in the film. 13
The newly formed films we exarnined were allowed to dry uncovered in an oven at 31 k 1
OC over 40 min, but as soon as each film appeared to be dry, it was transferred to the cold
room at 4 O C for storage until the decay profile of the cold film could be measured. Since
the Tg of the matrix polymer is Ca. 20 OC, we imagine that minimal polymer difision
occurs when the films are prepared in this way. We use expenmental values of @ d o ) to
examine the effect of filler on the contact between D- and A-labeled latex particles in the
newly formed films.
We examined four types of colloidal silica dispersions. The diameters for the
various filler particles are 25 nrn for Klebosol 30R25 (K-25), 50 nm for Klebosol 30R50
(K-5Gj. 12 nm for Snowtrx O (S-12). ancl 45 iim for Snowîex OL (S-45). notc that
two of the samples have sirnilar mean diameters (Klebosol30R50 and Snowtex OL), and
that S-45 has a particularly broad size distribution.
Effect of K-25 and K-50 on #-fi
In Figure 3.8 we plot @ d o ) vs Si02 content (wt %) for a series of freshly
prepared films containing either K-25 or K-50. Usefui films could be prepared containing
as much as 40 wt % (23.3 vol %) filler content. When we attempted to prepare films
containing larger amounts of silica. those films were so brittle after they dried that they
could not be handled. For K-25. the values of #&O) obtained nnge from 0.036 to 0.085.
For K-50. on the other hand, @do) values nnge from 0.084 to 0.093. In other
experiments on nascent films prepared from sirnila-sized latex particles at temperatures
close to the minimum film forming temperature, @ d o ) values on the order of 0.05 to
0.07 were obtained. lJ These resuits suggest that little polymer diffusion has occurred in
the samples we have examined. However, the extent of energy transfer for newly formed
films is different for films containing two different sizes of SiO2. One can clearly see that,
as one increases the amount of K-25 in the latex film, @ d o ) vaiues decrease, whereas the
magnitude of @ d o ) is essentially independent of the mount of K-50 in the film. This
result indicates that even when large amounts of K-50 are present, there is a common
extent of interfacial contact in the film between cells fonned from D- and A-labeled latex
particles. On the other hand K-25 filler appears to have a larger effect either on reducing
the interfacial area benveen D- and A-Iabeled cells in the system, or on suppressing the
limited extent of polymer diffusion that occurs during film formation than K-50 filler.
O 10 20 30 40 SiO, content (wt %)
Figure 3.8: Plots of the initial efficiency of energy transfer, Om(0), vs SiO, contents in
newly formed films.
Effect of S-12 and S-45 on @do).
In Figure 3.8 we also plot # d o ) vs SiOZ content (wt %) for a series of freshly
prepared films containing either S-12 or S-45. For latex films containing either S- 12 or S-
45, useful films could be prepared containing as much as 10 wt % filler content. For films
containing S-12, the values of #do ) obtained range from 0.020 to 0.085. For films
containing S-45. on the other hand, values range from 0.079 CO 0.086. Here we see
the same trend as the case of K-25 and K-50. These results also suggest that little polymer
diffusion has occurred in the samples we have exarnined. However, Iike K-25 and K-50,
the extent of energy transfer for newly formed films is different for films containing the
two different sizes of SiOz particles. When the films contain the smaller size of %O2, the
filler appears to reduce the interfaciai area between D- and A-labeled latex particles. As
one increases the amount of S-12 in the latex film, @ d o ) values decrease, whereas those
of S-45 are constant. This result also indicates that, even when large amounts of S-45 is
present, there is a common extent of interfacial contact in the film between cells formed
from D- and A-labeled latex particles. The magnitude of the change in M O ) is even
larger for S-12 filler than for K-25 filler. Detailed @&O) values for films containhg
different types and mounts of silica filler are surnrnarized in Table 3.4.
3-34. Maximum Efficiency of Energy Transfer, Q>n(-) in PBMA Latex Films
In order to evaluate the extent of mixing, f,(t) defined by eq 2-8, we need to know
the value of @da), which corresponds to full mixing of the polymer. If the film is fully
rnixed. one should have a random distribution of donors and acceptors, and the decay
profile should be described by eq 2 Ja . Under these circumstances. the rate and efficiency
of energy transfer will be determined only by %, 2, and the concentration of acceptors in
the film.
There are three ways to obtain a sample which will serve as a model for #d=).
First. one takes a film and anneals it for sufficiently long times. Second. one takes a film
and anneals it at higher temperatures. Since the polymer diffusion rate is strongly
accelented by increasing temperature, will norrndly increase rapidly to its maximum
value. Finally, one can dissolve a dry film sample in an organic solvent. In solution. one
expects full mixing of the polymer molecules. A film cast from this solution is then a
p o d model for the determination of # d m ) .
We have found in the past, for filler-free films. that dl these approaches give
similar vdues of ares(=) from which the corresponding @ET(=) values are calculated. '3
In this section, we examine the effect of filier on the magnitude of @d-).
In the experiments reported here, we obtained ares(=) values for filler-free latex
films, from a solvent cast film. This film was prepared from a dry PBMA film prepared
from a 1: 1 mixture of D- and A-labeled particles, which was subsequently dissolved in
tetrahydrofuran (THF). The solution was cast ont0 a quartz plate and allowed to dry at
room temperature for 12 h. For these films, we obtained an ares(=) value of 14.5 ns, and
a @da) value of 0.68, and these values did not change when the film was annealed at 80
O C for 1 h. In contrat, when a sample of the latex film itself was anneaied at 60 "C for
200 h, we obtained an are+) value of 16.1 ns, and a @dm) value of 0.65. One can see
that a latex film heated at 60 O C for this length of time gave values close to those obtained
from the solvent-cast film.
Effect of silica particle size on for a long annealing time at 60 O C .
When films were prepared in the presence of filler and annealed for 200 h at 60
OC, we obtained values of area(200 h) and #&O0 h) that depended on the amount of
filler present. We determined area(200 h) and #EI(200 h) values for films containing up
to 40 wt 8 K-25. For samples annealed for 200 h at 60 OC, these values ranged from 16.6
os (no silica) to 21.9 ns (40 wt% K-25), corresponding @A200 h) values from 0.64 to
0.52. The SiO2 filler has its most pronounced effect when it is present in amounts greater
than 20 wt %. In the case of K-25, the presence of filler appears to lower the Q>n(200 h)
values accessible by annealing the latex films.
We also determined area(î-O0 h) and #&200 h) values for films containing up to
40 wt % for K-50. For sarnple films annealed for 200 h at 60 OC. these values ranged from
16.1 ns to 17.0 ns, corresponding to #d200 h) values from 0.64 to 0.62. Unlike the case
of K-25, the presence of this filler does not Iead to a lowering of @&200 h) values.
In the case of films containing S-12 filler, we see that #E1(200 h) values decrease
with increasing amount of filler, following the same trend observed for films containing
K-25. However, the magnitude of Gd200 h) decrease for films containing S-12 is even
larger than that of S-12. We determined area(200 h) and @E.r(200 h) vaiues for films
containing up to 40 wt % for K-25. For samples annealed for 200 h at 60 OC, these values
ranged from 17.0 ns (no silica) to 31.6 ns (40 wt % S-121, corresponding to Q>n(200 h)
values from 0.63 to 0.30. Even the presence of a srnall amount of S-12 affects the
@&O0 h) values. and as the filler content is increased, it ha an even larger effect on
@d200 h) values.
We also determined area(200 h) and @d200 h) values for films containing up to
40 wt % S-45. For sarnples annealed for 200 h at 60 OC, these vdues ranged from 16.2 ns
to 16.9 ns, corresponding to QSr(2OO h) values from 0.64 to 0.63. As in the case of K-50,
the presence of filler does not lower a d 2 0 0 h) values.
In summary, the presence of 50 nm diameter silica particles (K-50 and S-45) has
almost no effect on area(200 h) values and @E7(200 h) values. The values we calculate
are almost identical to those in the filler-free film. where essentially full mixing of the
donor- and ricceptor-labeled polymer occurs. We conclude that these relatively large
particles have little effect on the extent of polymer diffusion that takes place when the
films were heated for 200 h at 60 OC. In contrat, we observe significant changes in
~ m z ( 2 O O h) and @k-l(?OO h) :.dues in latex films containing the srnaller si!ica particles
(K-25 and S- 12). The results are sumrnarized in Figure 3.9. Here the filler particles retard
polymer diffusion to such an extent that the extent of mixing after 200 h annealing is
significantly reduced. Detailed Od200 h) values for films containing different types and
amounts of silica fiIler are summarized in Table 3.4.
0e4 C l K-50 (50 nm) 0 S-12 (12 nm)
w e u
O 10 20 30 40 SiO, content (wt%)
Figure 3.9: Plots of the maximum efficiency of energy transfer, Qm (200 h), vs Si02
contents. Films were anneaIed for 200 h at 60 OC.
In Figure 3.10, we plot Od200 h) values for films containing 40 wt % of SiOz
annealed for 200 h at 60 O C , as a function of the inverse diameters of SiO2. The number
average diameten of silica particles were determined by transmission electron
microscopy (TEM). Since l/dsio2 corresponds to the surface to volume ratio of SiO2 filler,
one can see that there is a linea. relationship between Qn(200 h) values and the surface
to volume ratio.
Figure 3.10: Plots of the maximum efficiency of energy transfer. Om (200 h). vs the
surface to volume ratio (l/dsioz). Films contain 40 wt % of SiO?, and were annealed for
200 h at 60 OC. The diameters of SiOt were detennined by transmission electron
rnicroscopy (TEM).
Effect of silica oarticle size on at hioh annealine temperatures.
In the previous section, we saw that in the presence of small silica particles (S-12,
K-25) the extent of polymer diffusion that occurred in at 60 O C over 200 h was
significandy reduced. Here we examine this effect from a different perspective, in which
we mnealed films containing 40 wt % (33.3 vol I) silica for 2 h at various tempentures.
The results are shown in Figure 3.1 1. @E7(2 h) values increase with the annealing
temperature, and approach 0.6 even in the presence of S-12 and K-25. Detailed @*
values for both 10 min and 120 min are shown in Table 3.5.
These results indicate that interaction of the PBMA polymer with the surface of
the silica particles slow down the rate of polymer diffusion. On the other hand, we learn
that this interaction is not so strong as to suppress a part of the polymer rnixing process.
60 90 120 Annealing Temperature ( O C )
Figure 3.11: Plots of the maximum efficiency of eneqy transfer, am(2 h), vs annealing
temperature. Films contain 40 wt % of SiO2 and were anneded for 2 h.
Table 3.4: Values for @do) and w 2 0 0 h) at 60 OC.
PBMA + S-12
PBMA + S-45
PBMA + K-50
SiO2
content
(wt %)
Table 3.5: G d 2 h) at different annealing tempentures.
PBMA .t
40 wt % K-50
PBMA + 40 wt% $12
PBMA + 40 wt 96 S-45
10
min
10
min
120
min
10
min
Determination of areal4 and @d-).
To examine the influence of pigment on the rate of polymer interdiffusion, we
need to introduce values of ares(=) or #dm) into the calculation of f,(t). Since fm(t) is a
measure of the extent of rnixing in the PBMNsilica system, we need to determine both
are+) and ad=) values that describe only the latex polymer in the system, so that we
c m consider separately the effect of filler on the rate and the total extent of polymer
difision. For this reason, we use values obiained for the filier-free polymer obtained by
solvent casting; ares(=) = 14.5 ns, and M m ) = 0.68 in the calculation of ail f, values.
120
min
3-3-6. Effect of Silica Particle Size on the Rate of Poiymer Interdiffusion
In this section we examine the influence of silica on the rate of polymer diffusion
in PBMA latex films. To calculate f,(t) values via eq 2-8, we employ values
corresponding to each film, but #dm) values obtained as descnbed in the preceding
section.
Effect of K-25 and K-50.
In Figure 3.12a. we plot values of fm(t) as a function of annealing time at 60 OC for
films containing different amounts of K-25. The f,(t) values were caiculated with eq 2-8
from the areas under the fluorescent decay curves, using a common ares(=) value of 14.5
ns. We see that the presence of K-25 in the film has a pronounced effect on reducing the
rate of polymer interdiffusion. Even in the latex film with 10 wt % silica, it is obvious
that K-25 retards the rate of polymer interdiffusion. There is greater retardation as one
increases the amount of K-25 in latex films. When one increases the amount of K-25 in
the latex film up to 40 wt %. one can see a significant effect of retardation on the polymer
diffusion rate.
In Figure 3.12b we plot values of f,(t) as a function of annealing time at 60 OC for
films containing different arnounts of K-50. One of the interesting features in Figure
3.12b is that polymer interdiffusion is retarded by the presence of K-50 filler. but the
extent of retardation is much smaller than that of K-25. The use of K-50 filler up to 20 wt
% seems to have little or no effect on the rate of polymer interdiffusion. Even in the latex
film with 40 wt % K-50, the effect is much smdler than that of K-25.
0.8 a
0.6 e .c, w
E cw 0.4
0.2
0.0 F O 60 120 180 240 300 12000
Time (min)
(b) K-50, 60°C cwt - a18 -1
O 60 120 180 240 300 12000 Time (min)
Figure 3.12: Plots of the extent of rnixing f,(t). as a function of annealing time. Films
contain (a) 25 nrn of SiOr (K-25). (b) 50 nm of Si02 (K-50). and were annealed
simultaneously for each series of sample films at 60 OC.
Effect of S- 12 and S-45.
In Figure 3.13a, we plot values of f,(t) as a function of annealing time at 60 OC for
films containing different amounts of S42. The presence of S-12 in the film has a
significant effect on reducing the rate of polymer interdiffusion. Only 10 wt % of filler
content causes a significant retardation of the polymer interdiffusion rate. There is also a
greater retardation effect when one increases the amount of S-12 in latex films. When one
increases the amouni of S-12 iii ille latex film up :O 10 wr %, the nte of polyner diffusion
is substantially reduced. This effect is even larger than that for films containing K-25
filier.
In Figure 3.13b we plot values of f,(t) as a function of annealing time at 60 O C for
films containing different amounts of S-45. Here one notices that polyrner interdiffusion
is retarded by the presence of S J 5 filler. but the extent of retardation is much srnaller
than that of S-12. The use of S-45 filler up to 20 wt % seems little effect on the rate of
polymer interdiffusion, as one can see for films containing K-50 filler. Even in the latex
film with 40 wt % S-45, the effect is much srnaller than that of S-12.
1.0
0.8
0.6
0.4 I
0.2 I
3
0.0 O 60 120 180 240 300 12000
Time (min)
(b) S-45,60°C (wt %)O
O 60 120 180 240 300 12000 Time (min)
Figure 3.13: Plots of the extent of mVting f,(t), as a function of annealing tirne. Films
contain (a) 12 nm of SiO2 (S-12), (b) 45 nm of Si02 ( S A S ) , and were anneded
simultaneously for each senes of sample films at 60 OC.
3-3-7. Analysis of the Diffusion Process
In this section we analyze the f,(t) data more deeply to try to understand how
mineral fillers affect the polymer molecules as they dif ise across the latex ceil
boundaries. If the diffusion follows Fick's laws, one expects that the extent of rnixiog
f,(t) will be proportional to the square root of time. l5 We plot fm(t) as a function of t'" in
Figure 3.14. These plots are linear for values of f,(t) up to 0.7 for the film without silica.
In addition, the films conrainine different sizes of SiO? filler also eive reasonable linear
plots. We fit each senes of data to a straight line. and note that they al1 have a small
positive intercept. In the analysis described below. we only consider the slopes obtained
by the least-squares best fits to the lines shown in Figure 3.14. and ignore the intercepts at
t = O (with f,(r) S 0.06).
Figure 3.14: Plots of the extent of rnixing f,(t), vs the square root of annealing time. Si02
content for al1 SiOl contained films is 40 wt %. Those films were annealed at 60 O C .
The dope of each line is a measure of the polymer mobility in the film. To
examine how the surface-to-volume ratio of the silica particles affects this rnobility. we
plot the value of the slope &(t) 1 tl") against l/dsio2 in Figure 3.15. The data fa11 on a
smooth curve, but not a straight line.
When Feng et al. " examined the influence of PMMA particles as fillers on the
rate of polymer diffusion in latex films. they found a linear dependence of (f,(t) I tl") on
l/dsioz. In their experiments, the filler content was kept constant at 35 vol %.
Figure 3.15: Plots of the slope values in Figure 3.14. as a function of the surface to
volume ratio, l/dsioz for 40 wt% (23.3 vol %) silica filler. The mean diameters of SiOi
were detemiined by transmission electron microscopy (TEM).
3-3-8. Effect of Silica Particle Size on Diffusion Coef'fïcients
Another measure of the polymer mobility is the apparent diffusion coefficient
Da,, which descnbes the rate of movement of the polymer molecules across the polymer-
polymer interface between adjacent cells in the film. l w 8 We calculate the diffusion
coefficient of the polymer by fitting the extent of mixing f,(t), obtained from the energy
transfer measurements, to a spherical diffusion mode1 which satisfies Fick's laws of
diffusion, where D, is the true difision coefficient. 19
This mode1 assumes that the diffusing substance is initially distributed
in a sphere of radius R with an initial concentration Co. 'O At rime r we have
75
(3-8)
uniformly
Dapp values are calculated by equating f, with the fractional mass f, which has
diffused ûcross the interface, f, = M t I Mm. where M, = ( 4 3 ) s r ~ ~ ~ o . and carry out a
numencal integration to find the best D,, value which satisfies the equation
Simulations have shown '' that for the particle size and concentration of acceptor
employed here, f,(t) increases more npidly thanf,(t). Thus values calculated for Dapp are
larger than those for D,. The simulations also show that Dapp is proportional to D, for
values of f,(t) up to 0.7, and in this range the two D values differ by a factor of 3.
Experiments on latex films involve polymers with a distribution of chain lengths
and a corresponding distribution of Ds values. The 4, values we obtained are apparent
mean diffusion coefficients avenged over d l the chain lengths in the sample latex 13
polymer and the annealing history of the film. I 3 Previous experience - has shown that
molecular weight polydispersity leads to Dapp values that decrease with increasing fm(t). l7
The short chain polymers dominate the initial polymer diffusion. At longer times, the
growth in @ d t ) and f,(t) is due to the diffusion of longer chah polymers. Because D,,
values Vary with f,(t), values of Dapp from separate expenments should be compared at
similar extents of mixing.
The data for Da,, and the corresponding fm(t) values are surnmarized in Table 3.6.
Table 3.6: D,, and the corresponding f,(t) values for films containhg different types and
arnounts of silica filler.
PBMA
+
K-25
L
PBMA
+
S-15
0.13
0.36
0.50
0.60
0.77
0.9 Z
0.093
0.081
0.083
0.064
0.050
0.040
O. 103
0.081
0.078
0.063
0.036
0.005
0.06 1
0.067
0.060
PBMA
+
0.069
0.036
0.05.)
0.043
0.033
0.033
0.23
0.37
0.47
0.60
0.76
0.93
0.073
0.084
0.082
0.70
0.25
0.38
0.50
0.67
0.9 1
0.26
0.39
053
0.64
0.79
0.93
0.20
0.36
0.17
K-50
0.104
0.086
0.08 1
0.026
0.029
0.030
0.024
0.020
0.037
0.080
0.070
0.059
0.054
0.04 1
0.003
0.056 ---
0.029
0.W
0.26
0.40
0.53
0.16
0.2 1
0.28
0.35
0.49
0.87
0.08 1
0.063
0.056
0.016
0.034
0.003
0.24
0.36
0.49
0.22
0.40
0.54
0.066 0.065 0.64 - PP
0.053 0.8 1 0.017 0.79
0.004 0.94 0.003 0.9 1
0.56
0.69
0.94
0.080
0.054
0.054
0.12
0.22
0.31
0.39
0.56
0.92 1
0.1 1
0.32
0.42
0.52
0.67
0.085
0.066
0.065
0.23
0.33
0.45
0.042
0.028
0.024
0.019
0.015
0.025
0.030
0.0 16
0.0 13
0,012
0.0 10
0.0 12
0.73
0.35
0.45
0.56
0.73
0.92
0.063 O 5 7
0.13
O. 17
0.21
0.29
0.42
0.71
0.064
0.05 1
0.038
0.038
0.027
O.Oû3
0.053
0.030
0.027
0.025
0.092
0.042
0.00s
0.19
0.25
0.3 3
0.43
0.62
0.9 I 0.92
o n 0.94
0.003
0.042
0.003
0.77
0.92
Effect of K-25 and K-50.
In Figure 3.16a, we plot values of Da,, as a function of f,(t) for films containing
different amounts of K-25. Here we see that the presence of 20 wt % K-25 in the film has
a pronounced effect on reducing the rate of polymer interdiffision. We compare the Dapp
values at f,(t) = 0.35. One sees that the Dapp value decreases from 0.08 1 (nmls') for the
film containing no silica, to 0.027 (nm/s2) for the film containing 20 wt % K-25 and to
0.01 1 (nm/s2j for the îïini cuntainiiig 10 wî B K-25. When one i n m a e s the amount of
K-25 in the latex film up to 40 wt %, one can see a significant efTect of retardation.
However, the difference in Dapp values appears to be greater between the film with O and
20 wt % filler than those between 20 and 40 wt %.
In Figure 3.16b we plot values of Dapp as a function of fm(t) for films containing
different amounts of K-50. Unlike the case of K-25. Dapp values do not change
significantly with increasing amount of K-50 filler. We compare the Da,, values at fm(t) =
0.50. The Dapp value decreases from 0.08 1 (nm/s2) for the film containing no silica. to
0.065 (nrn/s2) for the film containing 20 wt % K-50 and to 0.050 (nrn/s2) for the film
containing 40 wt % K-50.
Effect of S- 12 and S-45,
In Figure 3.17a. we plot values of Da, as a function of f,(r) for films containing
different arnounts of S- 12. As in the case of K-25, we see that the presence of S- 12 in the
film has a significant effect on reducing the rate of polymer interdiffusion. We compare
the Rpp vaiues at fm(t) = 0.3 1. The Dapp value decreases from 0.10 (nmls') for the film
containing no silica, to 0.016 (nm/s2) for the film containing 20 wt % S-12 and to 0.004
(nrn/s2) for the film containing 40 wt % S-12.
In Figure 3.17b we plot a,, values as a function of f,(t) for films containing
different amounts of S-45. Polymer interdiffusion is retarded by the presence of S-45
filler, but the extent of retardation is much sinaller than that of S-12. We compare the D,,
values at f,(t) = 0.50. The D,, value decreases from 0.078 (nmls') for the film containing
no silica, to 0.030 (nrn/s2) for the film containing 20 wt 8 S-45 and to 0.023 (nds') for
the film containing 40 wt % S 4 5 .
n nni 1 I 1 l 1 l 1 1 i 1 I
Figure 3.16: Mean apparent diffusion coefficients, D,,, as a function of the exrent of
rnixing f,(r). Films contain (a) 25 nm of SiO? (K-25), (b) 50 nm of SiO2 (K-50), and were
annealed simultaneousIy for each senes of sample films at 60 O C .
0.001 1 1 I 1 i 1 I 1 I 1
0.0 0.2 0.4 0.6 0.8 1 .O
f&)
Figure 3.17: Mean apparent diffusion coefficients, Da,,, as a function of the extent of
mixing f,(t). Films contain (a) 12 nm of SiOz (S-12), (b) 45 nm of SiO2 (S-45), and were
annealed simultaneously for each series of sample films at 60 OC.
3-3-9. Dependence of the Polymer Diffusion Rate on the Volume Fraction of Silica
In Section 3-3-7 we described the dependence of polyrner diffusion rate on the
size of silica particles. and showed that smaller sized silica had a larger effect on the rate
of polymer diffusion. In this section we describe how the amount of silica affects the
polymer diffusion nie.
- In Figure 3.18 we plot f,(t) as a function of rl" for films containing (a) S-12 and
(b) K-25. As rve described in Section 3-3-7. al! cf the data give reasonable linear plots,
indicating that polymer diffusion follows the Fick's laws even in the presence of silica
particles. The slopes decrease as one increûses the amount of silica, and the same trend
c m be seen for films containing different sizes of S Q . We plot those slope values as a
function of silica volume fraction. We assume that the density of silica is 2.2 g/crn3. One
can see in Figure 3.19 that when the slope vaiues are plotted ûgainst silica volume
fraction, we obtain a smooth curve but not a straight line.
a::: 1 /.
(a) S-12,60 O C c
Figure 3.18: Plots of the extent of mixing f,(t) vs the square root of anneaiing time for
latex films containing different amounts of (a) S- 12 and (b) K-25.
0.00 0.05 0.10 0.15 0.20 0.25
SiO, volume fraction
Figure 3.19: Plots of the siope values in Figure 3.18. as a function of the silica volume
fraction.
3-3-10. Fundamental Mechanisrn of Polymer Diffusion
Viscoelasticity in polymers is one of the most important characteristics that lead to
a strong dependence of polymer diffusion on factors such as temperature, polymer
molecular weight, and distribution, the amount of added low molecular weight diluents,
and the degree of cross-linking. '' We try to understand the effects of such factors
obtained by different techniques and elucidate the fundamental mechanism of polymer
diffusion and its correlation with polymer propenies derived from other types of
viscoelastic measurements.
Viscoelastic properties of polymers can be well described by the Williams-
Landel-Ferry (WLF) equation. '' This equation describes temperature effects above the
Tg of the polymer on dynamic properties of polymers related to backbone motions in
tenns of changes in the free volume in the system. This equation is based on the idea of
time-temperature superposition. Events which occur on one time scale at a given
temperature To occur on a faster time scaie at higher temperature. In the traditional WLF
analysis, one defines a shift factor a ~ = thr, in terms of the shift dong the time axis needed
to bring two curves, representing measurements at different T. into correspondence at a
reference tempenture T,.
log a~ =
where Ci and C2 are parameters characteristic of a particular polymer. and 4 is the
diffusion coefficient determined at the reference temperature. In a study of the creep
cornpliance of PBMA, Ferry and coworkers '3 were able to fit the data at T, = 373 K, Ci =
14.5 and Cz = 255 K. Those values were found to be independent of molecular weight for
PBMA sarnples of MW > 6.0 x IO".
In our approach to data analysis, we shift plots of the apparent diffusion
coefficient Da,, vs. the extent of mixing fm(t) dong the D-mis at constant T to bnng the
curves into correspondence at zero silica content (mf = O). Thus we can define a shift
factor bT as
This shift factor is similar in form to that employed in the Fujita-Doolittle
expression, even though the assumptions of this mode1 do not entirely fit Our system.
Through the shift factor h, we introduce free volume theory, assurning that rnolecular
uanspon as descnbed by Dqp is regulated by the availability of free volume in the
system. 2.5
The specific occupied volume of a Iiquid Vo is defined as the volume of the
equilibrium liquid at O K. Therefore the specific Free volume VF is given by
where V is the specific volume of the liquid structure at any tempenrure T. As the
temperature is increased from O K, the increase in volume is accompanied by
homogeneous expansion of the matenal due to increasing amplitude of vibrations with
temperature, and also by formation of holes which are distributed discontinuously
throughout the material at any instant.
The self-diffusion coefficient of one component system derived from free volume
theory is expressed below:
where Dl is the self-diffusion coefficient. Doi is a pre-exponential factor. Vm is
the average hole free volume per molecule in the liquid (or per gram of the polymeric
liquid), V is the cntical local hole free volume required for a molecule (or jumping unit in
case of polymers) to jump to a new position, E is the critical energy which a molecule
must obtain in order to overcome the attractive forces holding it to its neighbors. T is the
absolute temperature and k is the Boltzman constant.
In the case of two components. one assumes that the change in volume is only due
to the change in the available hole free volume. One also assumes there is no volume
change during mixing, and that the rnolecular weight of the solvent is equal to the
molecuiar weight of a jumping unit of the polyrner chain. Fujita '6 denved an expression
for the diffusion coefficient D of a polymer in the presence of small molecular species:
where f is a total free volume of the system, B is the minimum hole size or jump size
required for the diffusion of a given molecule or rnolecular segment, A is a
proportionality constant that depends on the size and shape of a jumping unit of the
polymer chain, T is the absolute temperature, and R is the gas constant.
The free volume of a two cornponent system is in fact die sum of the hctional
fkee volume contributed by individual components. In our case those are polymer and
silica filler particles, whereas in Fujita's derivation Qf is the volume fraction of
plas ticizer.
where 0, is the volume fraction of polymer with fiactional free volume fp, af is the
volume fraction of silica filler particles with fractional free volurnefi, and P =fr -Ji is the
difference between the fractional free volume of silica filler particles and polymer. When
Of refers to a small molecule miscible with the polymer, one can obtain the Fujita-
Doolittle equation from eq 3-15 and 3-16. The Fujita-Doolittle equation describes the
influence of the volume fraction of the additive on the polymer diffusion coefficient at
given temperature as expressed belocv 26 :
where D(T, Qf) is the diffusion coefficient of polymer at temperature T in the presence of
QI volume fraction of plasticizer, D(T. O) is the diffusion coefficient of polymer at
temperature T in the absence of plasticizer. The terms fp(T, 0) and P(T) are defined in eq
3-16, at given temperatures.
Silica particles act to rigidify the surrounding matrix and are not solutes in the
traditional sense. " If for the sake of argument we assume that silica particles act as
"antiplasticizer." one may obtain any mathematical relationship between the ratio of
diffusion coefficients and the volume fraction of silica fiIIers. In order to examine this
idea, we first calculate the magnitude of the term (ln[Dp(T,O)lDp(T,<Dr)]}'l for each set of
data, with Qi ranging from 0.048 to 0.233. We assume that silica fillers have a density of
2.2 @m3. We equate Dp with D,,. Since the plots of D,, vs f,(t) in Figure 3.20 are
overdl paralle1 for al1 sets of data, the value of {ln[D,(T,O)/Dp(T,~f)] 1'' is considered to
be almost constant at different f,(r) values. Consequently al1 the data can be
superimposed. and we obtain a single master curve of a,, vs f,(t) for PBMA latex films
containing different amounts of S-12 in Figure 3.2 1.
In Figure 3.22 we plot values of {ln[D,(T,O)/Dp(T,@I)l}'l vs of-' for PBMA latex
films containing various fractions of S-12. We obtain a stnight line which indicates that
the shift factor is a function of the silica volume friction. If we plot
(ln[D,(T,O)/D,(T,~f)]}" vs Or -'? as in eq 3-17, we also obtain a straight line, widi a
negative slope. The plot also has a negative intercept. Too strict adherence to the Fujita-
Doolittle mode1 would lead to the strange conclusion that fp(T,O), the free volume in the
polymer in the absence of additive. is negative.
Figure 3.20: Plots of Da,, vs f,(r) for PBMA latex films containing O wt % (O), 10 wt %
(a), 20 wt % (O), 30 wt % (a), and 40 wt % (A) of S-12.
Figure 3.21: Master curve of Da,, vs fm(r) for PBMA latex films containing the S-12
volume fraction Of = O ( O ) , 0.048 (a), 0.102 (a), 0.163 (a), and 0.233 (A).
Figure 3.22: PLot of (ln[D,(T,O)/D,(T, af)]}-' vs -' for PBMA latex films containing
various fractions of S- 12.
We also examine this behavior with a different size of silica filler K-25. The same
trends can be seen in plots of &,, vs f,(r), master curve, and a plot of
(ln[D,(T,O)ID,(T,~f)] )- ' vs aI ? Those data are shown in Figure 3.23, Figure 3.24, and
Figure 3.25.
Note that ihe intercept is 0.19 and the dope is 0.035 in Figure 3.22 for the PBMA
latex films containing S-12. For the PBMA latex films in Figure 3.25 containing K-35.
the intmept is 0.36 and thc slopc is 0.05 1.
Figure 3.23: Plots of a,, vs f,(t) for PBMA latex films containing O wt % (O), 10 tvt %
(a), 20 wt % (O), 30 wt % (a), and 40 wt % (A) of K-25.
Figure 3.24: Master curve of D,,, vs f,(r) for PBMA latex films containing the K-25
volume fraction <Pr = O (O), 0.048 (a), 0.102 (O), 0.163 (m), and 0.233 (A).
Figure 3.25: Plot of {In~,(T,O)ID,(T, Of)]}-' vs Of-' for PBMA latex films containing
various frictions of K-25.
One notices that our system containing both PBMA latex polymer and silica looks
very similar to that containing polymer and a plasticizer (or an antiplasticizer). However,
there is an important difference between the silica-filled polymer and one of the key
assumptions made in denving the Fujita-Doolittle equation. In a plasticized polymer, the
presence of the plasticizer adds iree volume to the system and lowers Tg. in the silica-
filled system, the filler particles rigidify the matrix surrounding each particle. and reduce
the free volume of the systern. Since the silica particles do not dissolve in the polymer.
one cannot sirnply apply our system to the mode1 proposed by Fujita and Doolittle.
WLF analvsis.
Another way of analyzing this data is that one can correlate [hem to the Williûrns-
Landel-Feny (WLF) equation, as seen in eq 3-1 1. Here we assume that PBMA latex
polyrner consists of two different regions with different glass uansition tempentures
(Tg), and chat the polymer near the silica tiller surface has a higher Tg. " Thus one can
i nrroduce
where ATg is the Tg difference between the polymer near the silica filler surface and the
bulk polymer. T,' is an arbitrary chosen reference temperature. Eq 3- 1 1 can be rewritten
as
In Our analysis, the shift factor is descnbed in eq 3-12. We equate ln a~ = ln b,
and calculate ATg. Plots of ATg vs the silica volume fraction are shown in Figure 3.26.
The data fall on a srnooth curve for both series of films, but not a stmight line.
We normalize those data to a constant surface-to-volume ratio. Here the diameters
of silica particles were determined by transmission electron microscopy (TEM). The data
can be supenmposed onto a single curve as shown in Figure 3.27.
Figure 3.26: Plots of ATg vs the silica volume fraction for PBMA latex films containing
S-12 (O) and K-25 (a).
Figure 3.27: Plots of ATg vs the total volume of the polymer near the silica surface for
PBMA latex films containing S-12 (O) and K-25 (e). The diameters o f silica were
determined by transmission electron rnicroscopy (TEM).
To pursue this analysis further, we assume that the PBMA polymer c m be divided
into two types of domains. The polymer near the silica surface is assumed to have a
higher Tg, and the bulk polymer has its normal Tg. The Tg for the polymer nea. the silica
surface can be calculated by the following equation.
where TgaPP is the apparent Tg for the totd PBMA polymer (= 293 + ATg), 0, and Q2 are
the volume fractions of the domains of bulk polymer and the polymer near the filler
surface, respectively, and Tgl and Tg2 are the glass transition temperatures for the bulk
polymer and the polymer near the filler surface. TgI of the bulk polymer is 293 K. Thus
one can simulate the Tg2 values by making assurnptions about the magnitude of 0,. We
assume (0, values based on the idea that the thickness of the rigidified layer 8pl is on the
order of the radius of gyntion Rc of the polymer in the matrix. In our system for PBMA
with a MW = 50.000, Rc = 4.7 nm. '"n Figure 3.28, we present a plot of ATg vs the totd
volume of the polymer near the silica surface for PBMA latex films coniaining various
sizes of silica particles. The arnount of rigidified polymer is calculated assurning Spi =
RG. We have a situation with one measunble (bT) and two unknowns (Tgr, tipi). Various
pairs of Tgz and 6,i values will explain Our results. For example, if 6,i = 4.7 nm (Ro),
Tgz = 344 K, whereas if 6,[ = 1.2RG, Tgz = 335 K. Both of these values are above the
annealing temperature of the polymer diffusion expenments. A plot showing the
relationship between ATg and Ôpa& is presented in Figure 3.29.
Figure 3.28: Plots of ATg vs the total volume of the polymer near the silica surface for
PBMA latex films containing S-12 (O), K-25 (a), S-45 (O), and K-50 (m). The diameters
of silica were determined by transmission electron microscopy (TEM).
Figure 3.29: Plot of the glass transition temperature Tgz vs thickness of the polymer layer
6pi rigidified by the silica filler for the polymer near the silica filier surface. The
thickness is expressed based on the radius of gyration RG for the PBMA polymer with a
A third model for understanding the influence of silica on polymer diffusion is
based on the observation that the films become brittIe when Qsio2 exceeds 0.233. In this
model we make the assumption that when QSio2 = 0.233, al1 the polymer in the film has
become rigidified. Taking into account the diameters of the silica particles, we Rnd that
this result corresponds to a layer of rigid polymer of the thickness 8.0 nm surrounding
each particle for both S-12 and K-25. Since RG for the PBMA is estimated to be 4.7 nm.
i1iè rigidificd layer corresponds to 1.7 Rû. In order to proceed. we define a new chift
factor
In Figure 3.30. we plot the reciprocal value of this shift factor against the
reciprocal of the total volume of non-rigidified polymer in the system. The volume of
rigidified polymer is calculated in the following way: the term (6ld) refers to the surface
to volume ratio, and (6/d)(L.7Rc@sio2) refers to the total volume fraction of the polyrner
rigidified by the silica particles. Both plots for films containing S-12 and K-25 give
reasonable straight lines.
Figure 3.30: Plots of ( ln[D,(T, û+)/D,(T. 0.233)] 1'' vs ( 1-6( 1 *' for PBMA
latex films containing S- 12 (O) or K-25 (a).
3-4. Discussion
3-1-1. M O ) in Newly Formed PBMA Latex Films in the PresencdAbsence oPSilica
The initial efficiency of energy transfer. WO), in latex films provides a measure
of the ratio of intedacial area between D- and A-IabeIed latex cells to the volume of the
D-labeled phase. l 3 As one can see in Figure 3.8. @ d o ) values decrease with increasing
arnount of K-25 and S-12, whereas those of K-50 and S-45 are constant. The effect of K-
50 and S-45 on reducing the DIA interfacial area is very small, or almost negligible even
if their filler content is larger. On the other hand. it seems that K-25 and S-12
significantiy reduce the DIA interfacial area.
One explanation to ihis is that the film morphology in the presence of K-25 or S-
12 is different from that in the presence of K-50 or S-45. Due to its small size, filler up to
d = 25 nrn c m form a network iocated between D- and A-labeled latex ceils. The
presence of srnaIl particles in the interface reduces the interfacial area and the subsequent
efficiency of energy transfer. Those filler particles may even surround the latex particles
and prevent the polymer molecules from dif ishg across the ce11 boundaries. On the
other hand, K-50 and S-15 may not cover the whole latex panicles, and ailow polymer
interdiffusion to occur more easily. But at this point we do not know how film
morphoiogy changes in the presence of different sizes of filler, especially those particles
smaller than the latex particles.
There is evidence in the litenture ''JO that when one adds small hard particles to a
dispersion of larger soft latex particles, the small particles form a percolation network in
the film fornid upun dqing. WC depict ihis gpe of structure in the h w i n g in Figure
3.3 1. In this two-dimensional cross-section of the newly formed film, the small particles
form a connected network in the space between adjacent cells. By occupying a significant
fraction of the area between the faces of adjacent cells. the particles act to separate D-
labeled polymer from A-labeled polymer in the newly formed film.
Figure 3.31: Schematic representation of morphology difference for latex films
containing either smaller (left-hand side) or larger (right-hand side) size of Sioz. The
silica fillers can act as obstacles and retard polymer interdiffusion andor reduce the
mobility of polymer molecules near the filler surface.
We have no specific evidence about the rnorphoiogy of the films containing the
larger (45 nm, 50 nm diameter) silica particles. We can conclude from their lack of
influence on OEI(O) values that these particles do not occupy a significant fraction of the
interfacial area between adjacent D- and A-labeled cells in the nascent film.
3-4-2. Effect of Silica on w 2 0 0 h)
In the absence of silica, when latex films are annealed for long penods of time,
polymer diffusion leads to complete mixing of D- and A-labeled polymer. In the presence
of silica, we see a pronounced reduction in the extent of mixing in films containing 20 to
40 wt % of the srnall silica particles S-12 and K-25. As one sees in Figure 3.9. after 200 h
annealing at 60 OC, substantial amounts of polymer remain unmixed, leading to decreased
values of #&Ml lij. To explain ihis efkci, we imagine that the polymer adjacent to the
particle surface is adsorbed to the silica. The Si-OH groups at the surface of the silica are
likely to be strong hydrogen bond donors toward rhe ester groups of the PBMA.
Adsorbed polymer would require either very long times or elevated temperature to
desorb. In accord with this idea, we found, Figure 3.1 1. that after 2 h anneaiing at 120 O C ,
QR. values for films containing 40 wt % S-12 or K-25 approached the value expected for
complete interdiffusion of the latex polymer. The larger particles S 4 5 and K-50 had a
much smaller influence on the magnitude of a d 2 0 0 h). The plot in Figure 3.10 suggests
that the magnitude of this effect is related almost entirely to the difference in surface-to-
volume ratio for these filler particles.
3-4-3. EfFect of Silica on the Polymer Diffusion Rate
As we have shown above, the polymer diffusion rate is retarded in the presence of
silica as a filler. The effect of retardation is larger for the smaller size silica particles,
which have a kger surface to volume ratio.
There are two types of explmations for the effect of filler particles on reducing the
polymer diffusion rate as seen in Figure 3.14 to Figure 3.17. In the first model, the hard
pigment surface serves to make the adjacent polymer matrix more rigid. This is the
traditional "filler effect" in which filler particles increase the modulus of elastomers. It is
well known that polymer chahs adjacent to a rigid surface have decreased rnobility. '' On
the left-hand side of Figure 3.32, we present a drawing indicating how each filler particle
acts to reduce the mobility of polymer near its surface. Our explanation of the behavior of
the shift factor described above is based upon this idea.
Tsagaropoulos and Eisenberg " proposed a three-layer mode1 in terms of polymer
mobility. They determined Tg values from the maximum in tan6 From dynamic
mechanical (DMA) measurements. They studied changes in the glass transition
temperatures associated with adding small silica particles as fillers to various bulk
polymers. As increasing amounts of filler were added, a new high glass transition
temperature (Tg) was found in addition to the Tg for the bulk polymer. As more filler was
added, rhey found s decreze In the magnitude of the DMA sienal from which Tg was
determined. In their model, the surface layer of polymer adjacent to the filler is strongly
imrnobilized, but in addition, nearby polymer also has its mobility restricted. It is this
nearby polymer that contributes to the elevated Tg. From this perspective, one reason for
the decreased rate of polymer diffusion found here is that the polymer molecules near the
filler surface have decreased mo bility .
Immobilized polymer Obstacle effect layer at the fffler surface
Figure 3.32: Schematic representations for polymer immobilizaiion near the filler surface
and obstacle effect.
Based upon the filler-effect model, we can try to estimate what fraction of the
polymer in the system would have i ts mobili ty reduced in the presence of 40 wt % (23.3
vol 8) siiica of each type examined here. We depict a dnwing in Figure 3.33 and
descnbe that a polymer Iayer, which has a thickness of the radius of gyration RG or 2Rû,
surrounds the whole silica surface. We begin by noting that RG = 4.7 nm for BPMA of
MW = 50,000. '* For the various silica particles with diameters of 12, 25 45 and 50 nm,
their number ratio at 40 wt % to the LOO nrn PBMA panides is 176: 19:3.3:2.4: 1. This
calculation assumes a density of 2.2 @m3 for silica. As shown in Table 3.7, when we
consider polyrners within a distance RG h m the particle surface and sum over the surface
area of al1 of the silica particles. we find that for K-50 that only 20 % of the volume of the
PBMA phase is affected by the filler. For S-12. al1 of the PBMA polymer is afTected. If
we go a step further and assume that polymer molecules within 2RG are influenced by the
presence of the silica surface, we find that al1 of the polymer would be affected in the case
of S-12 and K-25. It is important to remember that aggegation of the silica particles
would expose less area to the polymer film, and lower the influence as predicted in Table
3.7. Nevertheless, we know from the uansparency of the films that even in the presence
of 23.3 vol % silica, there is not sufficient aggregation to create voids that would scatter
light.
Figure 3.33: Schematic representation for polymer molecules immobilized near the filler
surface. We calculated the ihickness of the immobilized polymer layer based on the
radius of gyration, b, and 2RG using RG (PBMA, MW = 50,000) = 4.7 nm.
An alternative model also predicts diat the presence of particles would retard the
diffusion rate. In the obstacle model, the particies act as inert obstacles, and polymers in
the system must diffuse around thern. By increasing the tortuosity of the diffusion path,
obstacles increase the time required for the polymers to mix. A drawing depicting this
model is shown on the right-hand side of Figure 3.32.
The results we obtain on the influence of silica particles on On(200 h) values
indicate to us that polymer adsorption ont0 the surface of the silica plays an important
role in aHecting the difision rate of the polyrner in the filled systems. We suspect,
however, that the filler effect is not the whole story. When we examine how plots of Dqp
vs. f,(r) are affected by an increase in the arnount of silica present, we see a striking
difference between the behavior for the films containing S-12 or K-25, compared to that
in films contaiaining S-45 or K-50. In Figure 3.1% and Figure 7.l?b, we cee thnt the
retardation effect of the larger silica particles is ver)) sirnilar throughout the interdiffusion
process. In conuast, we find that the smaller particles exhibit a much larger influence on
the early-tirne diffusion of the polymer. Thus we suspect that the obstacle effect of the
small silica particles also contributes to the reduction of the polymer diffusion rate, and
that this effect is most pronounced at srnall extents of mixing.
Table 3.7. Volume content of PBMA polymers (vol %) near the filler surface. relative to
the total polymer volume in latex films. The filler content is 40 wt % for al1 the
calculations.
Polymers within RG
(4.7 nm thickness)
(d = S-12 12 nm) I
Polymers within ZRG
(9.3 nm thickness)
K-25
(d = 25 nm)
K-50
(d = 50 nm)
48-8
20.4
100.0
48.2
3-5. Conclusions
We employed the fluorescence resonance energy transfer (FRET) technique to
measure the rate of polymer interdifision in the presence of colloidal silica, which has a
large surface area to volume ratio. The initial efficiency of energy transfer is constant in
the presence of different amounts of 50 nm SiO?, but it decreases with increasing arnounts
of 12 nm and 25 nm SiO?. This resuIt indicates that 12 nm and 25 nm SiO2 reduces the
intedaciai ÿrea ortwren donor- arid acczptor-tabeled particles in the newly fomed film
with increasing amount of filler, and they prevent the latex particles from the initiai
codescence. The extent of retardation in latex films with 50 nm SiO? is srnall, whereas
that of 12 nm and 25 nm SiO2 is much more pronounced on slowing the rate of polymer
interdifision. The mean apparent diffusion coefficient (D,,) of the latex film with 12 nm
SiOl (40 wt%) is 10 times smaller than the latex film without filler. Because of sample
brittleness. we could study films containing up to 40 wt % SiO? content. In these films,
the binder is present as the continuous phase.
3-6. References - -
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A.; Joanicot, M.; Boue. F. Eitrophys. Lett. 1999,46,472.
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U.S. Patent 5,750,200, 1998. (c) Imabeppu, K.; Asano, S.; Ohashi, H.; Nojima, K.;
Suzuki, E.; Sakaki, M. U.S. Patent 5,741,584, 1998. (d) Asano, S.; Ohashi, H.; Kondo,
H.; Nojima, K.; habeppu, K.; Sakaki, M.; Suzuki, E. U.S. Patent 5,670,242, 1997. (e)
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4. FUTURE WORK
4-1. Effect of Modification on the Polymer Diffusion Rate of Latex Polymen in the
Presence of Mineral Fiilers
We have shown in this study that minera1 fillers affect the polymer diffusion rate
in latex films. In the actuai paper ioatings, most of the latex polyrners arc cxboxylzted
due to their high stability in dispersions. Carboxyl groups also play an important role of
binding mineral fillers. Thus it is interesting to examine the effect of carboxyl group on
the polyrner diffusion rate in the presence of various types of rnineral fillen.
Kim and Winnik ' have exarnined the effect of carboxyl group on the polymer
diffusion rate in poly(buty1 methacrylate) latex films using the FRET technique. They
found that even the carboxyl content increases up to 6 mole %. they observed that
polymer diffusion stiIl takes place near the late.u/latex interface.
The latex polymer that we used for this study is expected to have a substantial
branching as descnbed in Chapter 2. but the gel content is quite low ( l e s than 10 wt %).
The latex used for paper coatings has a higher gel content, depending on the types of
coated paper and the types of the printing method. Therefore it is important to know how
the degree of cross-linking affects the polymer diffusion rate in the presence of rnineral
fillers.
Tarnai et al. exarnined the effect of cross-linking on the polymer diffusion rate.
Surprisingly, polymer difision takes place even in the latex film with a 100 wt 76 gel
content. Based on their results, we expect that polymer diffusion occurs in the
commercial coating process.
4-2. Effect of Other Constituents (Water-soluble Polymen, Thickeners) on the
Polymer Diffusion Rate of Latex Polymers in the Presence of Mineral Fiilers
Although latex polymer is extensively used as a pigment binder for various types
of coated papers, the amount of latex used is very small. Other constituents such as water-
soluble polymen [starch, casein, poly(viny1 alcohol)] and thickeners (carboxymethyl
cellulose, associative thickeners) are also used for the actual paper coatings. It is
interesting to examine the effect of those constituents durhg latex film formation.
4-3. Application of FRET Technique to Styrene-butadiene and Styrene-acrylate
Latex systems
We have used acrylic latex polyrners throughout this study. In paper coatings.
styrene-butadiene latex is die most commoniy used syntlietic bindzr due m high
performance for commercial printing. Styrene-acrylate copolymer latex is dso used for
paper coatings. Both synthetic latex polymen have styrene in polymer backbones.
There is a problern that styrene has a tluorescence background thût affects energy
transfer from phenanthrene to anthracene molecules. It is dificuli to remove the
contribution of styrene to analyze the energy uansfer data. We hope that a new pair of
fluorescence donorhcceptor pair will be developed in the hiture,
4-4. References
1. (a) Kim, H-B.; Wmg. Y.; Winnik, M. A. Pofymer 1994, 35, 1779. (b) Kim, H-B.;
Winnik, M. A. Mucrurnolecules 1994, 27. 1007. (c) Kim, H-B.; Winnik, M . A.
Macrontalecrtles 1995,28,2033.
2. Tamai, T.; Pinenq, P.: Winnik, M. A. Macrumolecriles 1999.32. 6102.
3. Thayer, A. M . Chem. Eng. News 1993,71,28.