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Polymer Diffusion in Latex Films by YUANQIN LIU A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Chemistry University of Toronto © Copyright by Yuanqin Liu 2009

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Page 1: Polymer Diffusion in Latex Films€¦ · Polymer Diffusion in Latex Films Yuanqin Liu Doctor of Philosophy Department of Chemistry University of Toronto 2009 Abstract In this thesis,

Polymer Diffusion in Latex Films

by

YUANQIN LIU

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Graduate Department of Chemistry University of Toronto

© Copyright by Yuanqin Liu 2009

Page 2: Polymer Diffusion in Latex Films€¦ · Polymer Diffusion in Latex Films Yuanqin Liu Doctor of Philosophy Department of Chemistry University of Toronto 2009 Abstract In this thesis,

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Polymer Diffusion in Latex Films

Yuanqin Liu

Doctor of Philosophy

Department of Chemistry University of Toronto

2009

Abstract

In this thesis, I describe experiments that provide a new and deeper understanding of

factors that affect polymer diffusion in acrylic latex films. This is the step that leads to the

growth of mechanical properties of these films. Polymer diffusion was studied by fluorescence

resonance energy transfer (FRET) in films prepared from dye-labeled latex particles.

Poly(n-butyl acrylate-co-methyl methacrylate) [P(BA-MMA)] was chosen for the study of

copolymer composition on the polymer diffusion rate. Four sets of P(BA-MMA) copolymers

were prepared from various weight ratios of BA/MMA. Polymer diffusion was monitored as a

function of annealing temperature, and apparent diffusion coefficients (Dapp) were calculated

from the FRET data, using a simple diffusion model. The temperature dependence of polymer

dynamics (G’, G”) obtained by linear rheology measurements is in good agreement with the

temperature dependence of Dapp. Increasing the BA content of the copolymer led to an apparent

increase in long-chain branching, which is reflected in both the time dependence of Dapp and in

the dynamic moduli measurements.

To study the effect of branching on polymer diffusion rates, latex particles comprised of

branched poly(n-butyl methacrylate) (PBMA) were prepared. The degree of branching was

controlled by adding various amounts of bisphenol A dimethacrylate as a branching agent, plus

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1-dodecanethiol as a chain transfer agent to prevent gel formation and to control the polymer

molecular weight. The results of rheology (G’, G”) measurements are consistent with the

absence of entanglement in these polymers. After correcting for the effects of Tg, by comparing

results at a constant T- Tg, ET data show that the PBMA with the highest degree of branching had

the highest diffusivity.

In a separate set of experiments I tested the effect of incorporating the highly branched

PBMA (HB-PBMA) into P(BA-MMA) dispersions to examine its influence on polymer diffusion

in the latex films. Three different approaches were taken to combine these different polymers:

latex blends, using HB-PBMA seeds in the synthesis of P(BA-MMA) by semicontinuous

emulsion polymerization, and dissolving HB-PBMA in the mixture of BA and MMA for latex

particles prepared by miniemulsion polymerization. ET studies indicate that HB-PBMA

significantly enhances polymer diffusion rate, comparable with TexanolTM, a volatile organic

coalescing agent. Tensile tests show that the films containing HB-PBMA have significant higher

mechanical properties than the films containing TexanolTM.

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Acknowledgments First, I would like to express my greatest appreciation to Professor Mitchell A. Winnik for

his guidance, encouragement, and support during my doctoral study. His insight, kindness,

humor, and knowledge truly impress and affect me. From him I have learned not only how to

work, but also how to live. He is far beyond a research supervisor.

Second, I want to thank my industry supervisor, Dr. Willie Lau, who gave me this great

opportunity to work with Rohm and Haas Company, a world’s leading company in coatings

industry. Without his continuous help my research could not be carried out smoothly.

Third, my appreciation will go to my research committee members, Professor Michael

Georges, Professor Eugenia Kumacheva, Professor Gilbert Walker, and Professor Tim Bender,

for their advices and help on my research.

Fourth, I give my appreciation to my colleagues, especially Dr. Jeffrey C. Haley and Dr.

Walter F. Schroeder. Working with Jeff was very enjoyable since he is smart, creative and

humorous. Walter is an excellent research partner with highly reliance. I would also thank Dr.

Zhihui Yin, Dr. Fugang Li, Dr. Xudong Lou, Dr. Jingshe Song, Dr. Jun Wu, Dr. Jung Kwon Oh,

Mr. Kangqing Deng, Dr. Baohang Han, Dr. Sheng Dai, Mr. Mohsen Soleimani, Mrs. Neda

Felorzabihi, Mr. Robert Roller, Dr. Gerald Guerin and all other group members for their kind

help. I like to thank Rohm and Haas, Rohm and Haas Canada, and NSERC Canada for their

support of this research. My appreciation will also go to University of Toronto, Ontario Centres

of Excellence, Ontario Graduate Scholarship in Science and Technology (OGSST) for

scholarships.

Finally, I will express my deepest gratefulness to my wife Ying (Sunny) Sun and the other

family members for their love, company, and support.

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Table of Contents

Chapter 1

1 Introduction 1 1.1 Research background 1

1.1.1 Water-based coatings and environment concern 1 1.1.2 In-situ seeding emulsion polymerization 2 1.1.3 Acrylic latex 3 1.1.4 Latex film formation and polymer diffusion 3

1.2 Research objectives 5 1.3 Thesis outline 5 1.4 References 6

Chapter 2

2 Experimental 8 2.1 Materials 8 2.2 Synthesis of latices 8

2.2.1 Synthesis of poly(n-butyl acrylate-co-methyl methacrylate) P(BA-MMA) 8 2.2.2 Synthesis of branched poly(n-butyl methacrylate) PBMA 10 2.2.3 Preparation of P(BA-MMA)/HB-PBMA latex blends 11 2.2.4 Preparation of P(BA-MMA) latices containing TexanolTM 12 2.2.5 Synthesis of P(BA-MMA) latices containing hyperbranched poly(n-butyl

methacrylate) (HB-PBMA) 12 2.2.6 Synthesis of P(BA-MMA) latices containing hyperbranched poly(n-butyl

methacrylate) (HB-PBMA) seed particles 13 2.3 Characterizations of latex particles 15 2.4 Characterizations of latex polymers 15

2.4.1 Molecular weight and molecular weight distribution 15 2.4.2 Copolymer composition 15 2.4.3 Glass transition temperature 15 2.4.4 Acceptor dye concentration measurements 16 2.4.5 Gel content measurements 16

2.5 Mechanical property measurements. 16 2.5.1 Rheology measurements. 16 2.5.2 Tensile testing 17 2.5.3 Dynamic mechanical analysis 17

2.6 Film preparation 17

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2.7 Film annealing 18 2.8 Fluorescence decay measurements and data analysis 18 2.9 Calculation of apparent diffusion coefficients Dapp 20 2.10 Fujita-Doolittle fitting 20 2.11 References 21

Chapter 3

3 Effect of Polymer Composition on Polymer Diffusion in Poly(n-butyl acrylate-co-methyl methacrylate) Latex Films 23

3.1 Introduction 23 3.2 Results 24

3.2.1 Preparation and Characterization of the Latex Samples 24 3.2.2 Energy Transfer Studies of Polymer Diffusion 29 3.2.3 Polymer Diffusion in P(BA60-MMA39) Films at Different Temperatures 31 3.2.4 Polymer Diffusion in Different Composition P(BA-MMA) Films 35 3.2.5 Temperature Dependence of the Viscoelastic Properties of P(BA-MMA) Films38

3.3 Discussion 40 3.3.1 Comparison between Different Experiments 40 3.3.2 Effect of long chain branching on the time-dependence of Dapp 41

3.4 Summary 44 3.5 References 45

Chapter 4

4 Synthesis of Branched Poly(n-butyl methacrylate) via Semi-Continuous Emulsion Polymerization 47

4.1 Introduction 47 4.2 Experimental Section 49

4.2.1 Latex Preparation 49 4.2.2 Synthesis of High Molecular Weight Linear PBMA 50 4.2.3 Characterization of Latex Polymers 50

4.3 Results 52 4.3.1 Synthesis of Branched PBMAs 52 4.3.2 Architectures of Branched PBMAs 54 4.3.3 Branched PBMA Latex Particles 57 4.3.4 Rheology Measurements 57

4.4 Discussion 58 4.4.1 Control of Molecular Weight 58 4.4.2 Entanglement Considerations in branched PBMA 59

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4.5 Summary 61 4.6 Reference 62

Chapter 5

5 Effect of Branching on Polymer Diffusion in Branched Poly(n-butyl methacrylate) Latex Films 64

5.1 Introduction 64 5.2 Results and Discussion 68

5.2.1 Synthesis of Dye-Labeled Branched PBMA Latex Particles 68 5.2.2 Polymer Diffusion in Branched PBMA Latex Films at Same Temperature 73 5.2.3 Polymer Diffusion in Branched PBMA Latex Films at (Tg+20) oC 79

5.3 Summary 82 5.4 Reference 83

Chapter 6

6 Effect of Hyper-Branched Poly(n-butyl methacrylate) on Polymer Diffusion in Poly(n-butyl acrylate-co-methyl methacrylate) Latex Films 85

6.1 Introduction 85 6.2 Blending Approach 87

6.2.1 Effect of Blended HB-PBMA Particles on Polymer Diffusion in P(BA-MMA) Latex Films 87

6.2.2 Comparing the Plasticization Effect of Blended HB-PBMA Particles and TexanolTM 91

6.2.3 Effect of Blended HB-PBMA Particles on Mechanical Properties of the P(BA-MMA) Latex Films 93

6.2.4 Conclusion 95 6.3 Miniemulsion Polymerization Approach 95

6.3.1 Miscibility of HB-PBMA and P(BA-MMA) 95 6.3.2 Synthesis of Miniemulsion P(BA55-MMA44) Latex Particles 97 6.3.3 Kinetic Study of the Miniemulsion Polymerization 98 6.3.4 Effect of the HB-PBMA on Polymer Diffusion in P(BA55-MMA44) Latex Films

100 6.3.5 Comparing the Plasticization Effect of HB-PBMA and TexanolTM 103 6.3.6 Effect of HB-PBMA on Mechanical Properties of the P(BA55-MMA44) Latex

Films 105 6.3.7 Conclusion 107

6.4 Seeded Emulsion Polymerization Approach 108 6.4.1 Synthesis of Seeded P(BA55-MMA44) Latex Particles 108

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6.4.2 Effect of HB-PBMA Seed Particles on Polymer Diffusion in Seeded P(BA55-MMA44) Latex Films 109

6.4.3 Effect of HB-PBMA Seed Particles on Mechanical Properties of the Seeded P(BA55-MMA44) Latex Films 112

6.4.4 Conclusion 114 6.5 Summary 115 6.6 References 115 Appendix 6.1 The 1H NMR spectrum of P(BA55-MMA44)A

1%ME final product 117 Appendix 6.2 Tensile stress-strain curves for P(BA-MMA) latex films by miniemulsion

polymerization 118 Appendix 6.3 Values of ФET(0) and ФET(∞) of the three approaches 119 Appendix 6.4 Chemical structure of Me-β-CD 120 Appendix 6.5 Chemical structure of TexanolTM 121

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List of Tables Table 2-1. Typical semi-continuous emulsion polymerization recipe for the synthesis of non-

labeled P(BA60-MMA39) latex. Table 2-2. Typical semi-continuous emulsion polymerization recipe for the synthesis of

branched PBMA latex.

Table 2-3. Typical miniemulsion polymerization recipe for the synthesis of donor labeled

P(BA-MMA) latex containing 1 wt% HB-PBMA.

Table 2-4. Typical seeded emulsion polymerization recipe for the synthesis of A-labeled

P(BA-MMA) latex containing 5 wt % HB-PBMA seed particles.

Table 3-1. Characteristics of the P(BA-MMA) latex polymers and particles.

Table 3-2. Ea values of the latex polymers.

Table 4-1. Characteristics of unlabeled latex polymers and particles

Table 4-2. Estimate of branching.

Table 5-1. Typical recipe for the synthesis of D-labeled branched PBMA latex.

Table 5-2. Characteristics of the latex polymers and particles.

Table 5-3. Estimation of the chain structure of the different branched latices.

Table 5-4. Limiting values of energy transfer efficiency for the different labeled latex mixtures. Table 6-1. Characteristics of the P(BA-MMA) latex polymers and particles.

Table 6-2. Tg of P(BA-MMA) polymers containing HB-PBMA and TexanolTM.

Table 6-3. Tensile properties of P(BA55-MMA44) latex films.

Table 6-4. Characteristics of P(BA-MMA) latex polymers made by miniemulsion

polymerization.

Table 6-5. Miscibility of HB-PBMA and P(BA-MMA).

Table 6-6. Fitting parameters for the Fujita-Doolittle equation.

Table 6-7. Tensile testing results of P(BA55-MMA44) films.

Table 6-8. Characteristics of the P(BA55-MMA44) latex polymers and particles.

Table 6-9. Tensile testing results of P(BA55-MMA44) films.

Table A1. Comparison of ФET(0) and ФET(∞) of the three approaches.

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List of Figures Figure 1-1. Scheme of in-situ emulsion polymerization reaction.

Figure 1-2. Mechanism of latex film formation.

Figure 1-3. (A) The chemical structures of D and A are shown; (B) Polymer diffusion can only

occur after the polymer in adjacent cells comes into intimate contact. Polar material

trapped between cells can interfere with polymer diffusion.

Figure 3-1. Plot of 1/Mn against concentration of C12-SH of P(BA60-MMA39) latex samples.

Figure 3-2. 1H NMR spectra of (A) P(BA60-MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49)

and (D) P(BA40-MMA59). CDCl3 was used as solvent. Peaks a and peak b

correspond to protons at positions a and b respectively.

Figure 3-3. Phenanthrene (donor) fluorescence decay curves [ID(t)] measured at 23 oC for Phe-

P(BA60-MMA39) latex films. (1) Phe-labeled latex only, (2) a newly formed film

dried at 4 oC, consisting of a 1:1 ratio of Phe-P(BA60-MMA39) and NBen-P(BA60-

MMA39), (3) the same film as in (2) aged for 47 min at 23 oC, and (4) a solvent-cast

film from a 1:1 mixture of the two freeze-dried polymers dissolved in THF and then

annealed at 120 oC for 2 h. Note that curves (1) and (2) overlap. The inset shows

curves (1) and (2) at short times on a linear scale.

Figure 3-4. Plot of P vs [NBenMA, mM] for fully mixed solvent-cast films prepared from Phe-

P(BA60-MMA39) plus varying amounts of free monomer MBenMA. The P values

were obtained by fitting individual Phe decay curves to equation (2-5) with τD fixed

at 44.3 ns. From the slope of the plot, I calculate R0 = 2.51 nm.

Figure 3-5. Plots of ФET (A) and fm (B) vs annealing time for the P(BA60-MMA39) latex films

annealed at 23, 45, 70, and 90 oC.

Figure 3-6. Plots of ФET and fm vs annealing time for the P(BA60-MMA39) latex films annealed

at 23 oC.

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Figure 3-7. Plots of the apparent diffusion coefficient Dapp as a function of fm for (A) P(BA60-

MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49) and (D) P(BA40-MMA59)

latex films annealed at various temperatures.

Figure 3-8. Plot of ln Dapp against 1/T over the temperature range from 23 to 60 oC at fm values

of 0.59 for P(BA60-MMA39) latex.

Figure 3-9. Plot of the ΦET for films formed from D/A labeled latex mixtures annealed for

various periods of time at (A) 23 oC, (B) 70 oC and (C) 90 oC. (▲)P(BA60-MMA39),

(◆)P(BA55-MMA44), (●) P(BA50-MMA49) and (■) P(BA40-MMA59).

Figure 3-10. The master curves of Dapp values for (A) P(BA60-MMA39) at 23 oC (calculated

using Ea = 33.4 kcal/mol as a shift factor); (B) P(BA55-MMA44) at 23 oC

(calculated using Ea = 39.1 kcal/mol as a shift factor); (C) P(BA50-MMA49) at 23 oC (calculated using Ea = 45.2 kcal/mol as a shift factor) and (D) P(BA40-MMA59)

at 70 oC (calculated using Ea = 64.1 kcal/mol as a shift factor).

Figure 3-11. Plots of master curves of G' and G'' for (A) P(BA60-MMA39), (B) P(BA55-MMA44),

(C) P(BA50-MMA49) and (D) P(BA40-MMA59) latex films at T0 = 25, 50, 80 and 90 oC respectively.

Figure 3-12. Plots of shifted Dapp and log(aT) against the inverse of the absolute temperatures for

(A) P(BA60-MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49) and (D) P(BA40-

MMA59) latex films.

Figure 4-1. 1H NMR spectra of PBMAs with different branching levels. CD2Cl2 was used as

solvent. Peaks a and peaks b correspond to protons of BPDM and BMA

respectively.

Figure 4-2. UV and RI traces in the GPC analysis of branched-PBMA2.

Figure 4-3. Polymer architecture for (A) linear-PBMA1, (B) branched-PBMA2, (C) branched-

PBMA5 and (D) branched-PBMA7. These drawings assume a uniform distribution

of branch points in the polymer molecules.

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Figure 4-4. GPC traces (A) RI signal and (B) log Mn vs retention volume for linear-PBMA1 (L1),

branched-PBMA2 (B2), branched-PBMA5 (B5) and branched-PBMA7 (B7). The

vertical line in (B) indicates that the polymer with Mn = 34,000 has a retention

volume of 17.3 mL. The vertical line in (A) indicates that ca. 30% of the L1 sample

has Mn lower than 34,000.

Figure 4-5. Plots of master curves of G' and G'' for (A) linear-PBMA1, (B) branched-PBMA2,

(C) branched-PBMA5 and (D) branched-PBMA7.

Figure 4-6. Plots of master curves of G' and G'' for the high molecular weight linear PBMA

sample. The vertical dotted line corresponds to the minimum value of G”.

Figure 5-1. GPC traces (RI) for (A) A- and (B) D-labeled PBMA samples.

Figure 5-2. GPC traces of (A) LB-PBMAD and (B) LB-PBMAA. The UV and RI traces overlap,

which indicates a nearly random distribution of dye comonomers in the polymer

chains.

Figure 5-3. UV calibration curve for NBenMA in THF solution. The extinction coefficient is ε =

(2.47 ± 0.03)×104 M-1cm-1.

Figure 5-4. Plot of the ΦET for films formed from D/A labeled latex mixtures annealed for

various periods of time at (A) 23 oC, (B) 45 oC and (C) 70 oC. (О) HB-PBMAD/A,

(△) MB-PBMAD/A, (□) LB-PBMAD/A and (◇) LR-PBMAD/A.

Figure 5-5. Plot of the fm for films formed from D/A labeled latex mixtures annealed for various

periods of time at (A) 23 oC, (B) 45 oC and (C) 70 oC. (О) HB-PBMAD/A, (△) MB-

PBMAD/A, (□) LB-PBMAD/A and (◇) LR-PBMAD/A.

Figure 5-6. Plots of Dapp as a function of fm for (A) LR-PBMAD/A, (B) LB-PBMAD/A, (C) MB-

PBMAD/A, and (D) HB-PBMAD/A. As fm 1, the ET experiment loses sensitivity

because the incremental changes in ΦET become small. Thus the calculated Dapp

values for fm > 0.9 may not be meaningful.

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Figure 5-7. Master curves of Dapp for (A) LR-PBMAD/A, (B) LB-PBMAD/A, (C) MB-PBMAD/A,

and (D) HB-PBMAD/A. 23 oC was used as the reference temperature. The apparent

precipitous decrease in Dapp values at values of fm → 1 may not be real, since the

energy transfer methodology loses its sensitivity as polymers approach the fully

mixed state, and the acceptor concentrations in the films become uniform.

Figure 5-8. (A) Plot of fm for films annealed at (Tg +20) oC. (B) Plot of Dapp over fm at (Tg +20) oC. (О) HB-PBMAD/A, (△) MB-PBMAD/A , and (□) LB-PBMAD/A.

Figure 5-9. Plots of master curves of G' and G'' for (A) LR-PBMAD/A, (B) LB-PBMAD/A, (C)

MB-PBMAD/A, and (D) HB-PBMAD/A. 40 oC was used as reference temperature.

Figure 6-1. Plots of the fm versus annealing time for (A,B) P(BA55-MMA44), (C,D) P(BA50-

MMA49), and (E,F) P(BA45-MMA54) latex films containing (●)0 wt % of additives,

(▲ )5 wt % blended HB-PBMA particles, (■ )10 wt % blended HB-PBMA

particles at 23 °C.

Figure 6-2. Comparison of the plots of the the fm versus annealing time for (A,B) P(BA55-

MMA44), (C,D) P(BA50-MMA49), and (E,F) P(BA45-MMA54) latex films containing

HB-PBMA and TexanolTM at 23 °C. (●) 0 wt % of additives, (▲) 5 wt % blended

HB-PBMA particles, (■) 10 wt % blended HB-PBMA particles, (△) 5 wt %

TexanolTM, (□) 10 wt % of TexanolTM.

Figure 6-3. Comparison of the plots of the Dapp versus fm for P(BA55-MMA44) latex films

containing (●)0 wt % of additives, (▲)5 wt % of blended HB-PBMA particles,

and (△) 5 wt % of TexanolTM at 23 °C.

Figure 6-4. Tensile stress-strain curves for P(BA55-MMA44) latex films with 0 wt % additive, 5

wt % blended HB-PBMA particles, 10 wt % blended HB-PBMA particles, 5 wt %

TexanolTM, and 10 wt % TexanolTM.

Figure 6-5. Total conversion as a function of reaction time for the miniemulsion polymerization

of P(BA-MMA)A1%ME.

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Figure 6-6. Experimentally determined (points) and modeling (lines) of the partial conversion of

(■) MMA and (▲) BA vs the overall conversion for P(BA-MMA)A1%ME.

Figure 6-7. (A) Comparison of the plots of fm vs annealing time for P(BA-MMA) at 45 °C with

(*)0 wt %, (■)1 wt %, (◆)3 wt %, (▲)5 wt %, and (●)6 wt % of HB-PBMA; (B)

Plot of Dapp as a function of fm for P(BA-MMA) at 45 °C with (*)0 wt %, (■)1 wt

%, (◆)3 wt %, (▲)5 wt %, and (●)6 wt % of HB-PBMA; (C) Master curve of

Dapp for HB-PBMA in P(BA-MMA) films; (D) Comparison of the plots of fm vs

annealing time for P(BA-MMA) at 45 °C with (■)0 wt %, (□)2 wt %, (●)5 wt %,

and (△)7 wt % of TexanolTM; (E) Plots of Dapp vs fm for P(BA-MMA) at 45 °C

with (△)0 wt %, (□)2 wt %, (◇)5 wt %, and (○)7 wt % of TexanolTM; (F)Master

curve of Dapp for TexanolTM in P(BA-MMA) films.

Figure 6-8. Plot of 1/ln[Dp(T,Φa)/Dp(T,0)] vs 1/Φa at 45 °C for the P(BA-MMA) films

containing HB-PBMA

Figure 6-9. Plot of 1/ln[Dp(T,Φa)/Dp(T,0)] vs 1/Φa at 45 °C for the P(BA-MMA) films

containing TexanolTM.

Figure 6-10. (A)Tensile stress-strain curves for P(BA-MMA) latex films with various

concentration of HB-PBMA and TexanolTM. (B)Early stages of the stress-strain

curves in (A).

Figure 6-11. Comparison of the plots of fm versus aging time for P(BA55-MMA44) at 23 °C with

(●)0 wt % additives, (▲)5 wt % HB-PBMA seeds, (■)10 wt % HB-PBMA seeds,

(△)5 wt % TexanolTM, and (□)10 wt % TexanolTM. (C) and (D) show the early

stages of the fm plots in (A) and (B), respectively.

Figure 6-12. (A)Comparison of the plots of the Dapp versus fm for P(BA55-MMA44) latex films

containing (●)0 wt % additives, (▲)5 wt % HB-PBMA seeds, (■)10 wt % HB-

PBMA seeds, (△)5 wt % TexanolTM, and (□)10 wt % TexanolTM at 23 °C.

(B)Plot of 1/ln[Dp(T,Φa)/Dp(T,0)] vs 1/Φa at 23 °C for the P(BA55-MMA44) films

containing different amounts of (○)TexanolTM and (▲)HB-PBMA seeds.

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Figure 6-13. (A)Tensile stress-strain curves for P(BA-MMA) latex films with various

concentration of HB-PBMA seed particles and TexanolTM. (B)Early stages of

the stress-strain curves in (A).

Figure A1. 1H NMR spectrum of P(BA55-MMA44)A1%ME, which was synthesized by

miniemulsion polymerization and contains 1 wt % HB-PBMA. CDCl3 was used

as solvent.

Figure A2. Tensile stress-strain curves for P(BA55-MMA44) latex films with various

concentration of HB-PBMA and TexanolTM, which were synthesized by

miniemulsion polymerization.

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List of Abbreviation

AIBN 2,2′-Azobis(2-methylpropionitrile)

BA n-butyl acrylate

BMA n-butyl methacrylate

BPDM bisphenol A dimethacrylate

C12-SH 1-dodecanethiol

Dapp apparent diffusion coefficient

E’ storage modulus

E” loss modulus

G' shear storage modulus

G” shear loss modulus

ET energy transfer

fm fraction of mixing

FRET fluorescence resonance energy transfer

HB-PBMA hyper branched poly(n-butyl methacrylate)

HD hexadecane

ID donor fluorescence intensity decay

KPS Potassium persulfate

MAA methacrylic acid

Me-β-CD methyl-β-cyclodextrin

MFT minimum film formation temperature

MMA Methyl methacrylate

NBenMA 4’-Dimethylamino-2-methacryloxy-5-methyl benzophenone

P(BA-MMA) poly(n-butyl acrylate-co-methyl methacrylate)

PBMA poly(n-butyl methacrylate)

PDI polydispersity index

PheMMA Phenanthrylmethyl methacrylate

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PMMA poly(methyl methacrylate)

PTFE polytetrafluoroethylene

R0 critical Förster radius for energy transfer

SDS sodium dodecyl sulfate

TexanolTM 2, 2, 4-Trimethyl-1, 3-pentanediol monoisobutyrate

THF tetrahydrofuran

Tg glass transition temperature

VOC volatile organic compounds

ΦET quantum efficiency of energy transfer

ω shear frequency

τD fluorescent donor lifetime

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Chapter 1

1 Introduction My research work focuses on the fundamental aspects of latex film formation of acrylic

latices including poly(n-butyl acrylate-methyl methacrylate) [P(BA-MMA)] copolymer and

poly(n-butyl methacrylate) (PBMA) homopolymer. P(BA-MMA) copolymers of different

compositions were chosen as the base latex polymer because the P(BA-MMA) latex is widely

used in commercial architecture coatings. This choice makes these fundamental research

experiments also of practical importance. Since P(BA-MMA) copolymers are randomly

branched polymers, it is impossible to quantitatively study the effect of branching on polymer

diffusion. To achieve this objective, I synthesized a series of latex particles containing branched

PBMA polymer and explored their diffusion behavior.

1.1 Research background

1.1.1 Water-based coatings and environment concern

For many decades, industrial coatings have relied on solvent-based formulations. Today,

increasingly strict environmental regulations are the impetus for a change in technology. The

most important substitutes for solvent-based coatings are water-based coatings. Solvent-free

water-based coatings should be environment friendly. However, most commercial water-based

coatings still contain significant amounts of volatile organic compounds (VOC) as additives. For

example, solvents added as coalescing agents promote film formation, but later evaporate and

contribute to air pollution. A key challenge for the coating industry is to develop new technology

that is environmentally compliant and meets current performance specifications. To develop this

new technology, a deeper fundamental understanding of how small changes in formulation or

latex composition affect the development of useful coating properties is necessary.

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Figure 1-1. Scheme of in-situ emulsion polymerization reaction.

1.1.2 In-situ seeding emulsion polymerization

Good model latices are the essential feature of my research. The model latices should be

able to approximate the performance properties of the commercial coating products and also be

sufficiently well-defined for scientific research. Previous research in our laboratory always

employed latex particles synthesized using seed latex samples, which were prepared in advance.1, 2 The advantage of this approach is the easy control of particles size. The downside is that the

polymer composition of the final particles may not be unique, since the seeds normally contain

high molar mass polymers. Their presence may affect the polymer diffusion studies that are a big

part of my thesis research. In this research, I used an in-situ seeding process (Figure 1-1), in

which the seeds and final particles are formed from the same monomer mixture as the

subsequent monomer feed. In this way, the whole particle will have a homogeneous polymer

composition. The problem for in-situ seeding is that particles sizes are difficult to match from

batch to batch, especially for the small scales in our reations. At the beginning of my doctoral

work I tried very hard to scale down the in-situ seeding recipe from our industrial partner and

eventually managed to synthesize uniform latex particles with controlled size. All emulsion

polymerization reactions carried out in this thesis used this in-situ seeding process.

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1.1.3 Acrylic latex

In my research acrylic copolymer latices were chosen as model latices. One attractive

feature of acrylic monomers is their ability to copolymerize with each other. In this way one can

obtain copolymers with a wide range of compositions and physical properties. These various

properties make acrylic latex copolymers interesting to both industrial and academic researchers.

For example, one series of base latices for my experiments are n-butyl acrylate-methyl

methacrylate (BA-MMA) copolymers, which are also the base latices of commercial architecture

paints. With a weight ratio ranging from 60/40 to 40/60, their glass transition temperatures range

from 3 oC to 28 oC. Another series of acrylic latices are n-butyl methacrylate-bisphenol A

dimethacrylate (BMA-BPDM) copolymers, which are interesting to me because they offer

control over polymer chain branching.

1.1.4 Latex film formation and polymer diffusion

A number of recent publications have reviewed the mechanism of latex film formation3, 4

Our research group has made a significant contribution to this field.5, 6 When a latex dispersion is

cast onto a substrate and allowed to dry, an initial film will form as the particles deform and pack

into space-filling polyhedral cells. These newly formed latex films have poor mechanical

properties because there is only weak adhesion at the boundary between adjacent cells. If the

minimum film forming temperature (MFT) of the latex polymer is lower than the drying

temperature, polymer molecules will diffuse across the intercellular boundary to produce

mechanically coherent films. The film formation process is depicted in Figure 1-2.

Figure 1-2. Mechanism of latex film formation

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Due to technology limitations, polymer diffusion during latex film formation was not

investigated until 1980’s. Oberthür and coworkers studied polymer diffusion in latex films using

neutron scattering (SANS).7, 8 Our research group contributed to the field by introducing the

fluorescent resonance energy transfer (FRET) technique, which is based upon fluorescence decay

measurements, to study polymer diffusion rates.9 A typical approach includes the synthesis of

two virtually identical samples of latex particles, one of which was labeled with a fluorescent

donor dye (phenanthrene, D), while the other was labeled with an acceptor dye (anthracene or a

benzophenone derivative NBen, A). The chemical structures of D and A are shown in Figure 1-

3(A). Latex films were cast from a mixture of the two dispersions. As shown in Figure 1-3(B), D

and A are initially separated from each other in a newly formed film. FRET does not occur at

this stage and no fluorescence decay can be observed. As polymer chains diffuse, D and A are

brought together. FRET take places and leads to a strong decrease in the measured fluorescence

decay. In the past 20 years, extensive studies on polymer diffusion have been carried out using

this technique in our laboratory. A number of factors that affect the rate of polymer diffusion in

latex films were explored, which include temperature, composition of the latex, and the presence

of various additives such as coalescing agents and common surfactants.

Figure 1-3. (A) The chemical structures of D and A are shown; (B) Polymer diffusion can only occur after

the polymer in adjacent cells comes into intimate contact. Polar material trapped between

cells can interfere with polymer diffusion.

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1.2 Research objectives The objective of my research is to develop molecular-level knowledge of coalescence and

film formation from acrylic latex particles used for interior architectural coatings. When these

coatings dry, the latex particles deform and coalesce, and the useful performance properties of

the coatings arise from the diffusion of polymer molecules across the boundaries between cells

formed by the latex particles. Here my focus is on the factors that connect events at the

molecular level to the development of final film properties. Scientists in the coating industry

anticipate that this new knowledge will help to explain a series of in-house observations about

how small changes in formulation or latex composition affect the development of useful coating

properties. In addition, I hope that the new knowledge developed here can be used to enhance the

development of a new generation of high-performance latex coatings with substantially reduced

VOC content.

1.3 Thesis outline The research work in this thesis mainly focuses on the synthesis of acrylic latices, FRET

measurements of latex films, and mechanical property tests of latex polymers. I present this

thesis in six chapters.

In Chapter 2, I describe experimental methodologies for latex synthesis, characterizations

of latex particles and latex polymers, and details of the FRET technique including data analysis.

In Chapter 3, I describe polymer diffusion measurements in P(BA-MMA) copolymer latex

films by FRET. The effect of copolymer composition on polymer diffusion rates is illuminated. I

also show that the temperature dependence of polymer dynamics extracted by the rheology

experiments is in good agreement with the temperature dependence of apparent diffusion

coefficients (Dapp), which were calculated from the FRET data.

In Chapter 4, I describe the preparation of latex particles comprised of branched PBMA

via semicontinuous emulsion polymerization. The extent of branching was controlled by adding

various amounts of BPDM as a branching agent, and 1-dodecanethiol (C12-SH) was used as a

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chain transfer agent to prevent cross-linking and to control molecular weight. The degrees of

branching were determined using 1H NMR. Rheology measurements indicated no significant

entanglement contributions to the rheological properties.

In Chapter 5, as a continuation from Chapter 4, I describe the FRET studies on the effect

of different degree of branching on polymer diffusion rates in the PBMA latex films. I found that

after correcting for the effects of Tg, by comparing results at a constant T- Tg, the PBMA with the

highest degree of branching had the highest diffusivity.

In Chapter 6, I describe the use of the HB-PBMA as a polymeric coalescing agent in

P(BA-MMA) latex film formation. These approaches were taken to incorporate HB-PBMA into

P(BA-MMA) latex samples: a blending approach, a miniemulsion polymerization approach, and

a seeded emulsion polymerization approach. FRET studies show that the HB-PBMA enhances

polymer diffusion rates in films of latex prepared using all three approaches. HB-PBMA has a

similar effectiveness to a traditional coalescing agent, TexanolTM, in promoting polymer

diffusion rates. Unlike TexanolTM, HB-PBMA does not lower significantly the temperature at

which the latex form films upon drying. Mechanical measurements indicate that the HB-PBMA

has negligible or limited influence on film mechanical properties, whereas the presence of

TexanolTM causes a loss of tensile strength and toughness.

1.4 References

1 Oh, J. K.; Wu, J.; Winnik, M. A.; Craun, G. P.; Rademacher, J.; Farwaha, R. J. Polym. Sci.,

Part A: Polym. Chem. 2002, 40, 1594-1607.

2 Wu, J.; Oh, J. K.; Yang, J.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules 2003,

36, 8139-8147.

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3 Winnik, M. A. in Emulsion Polymerization and Emulsion Polymers; Lovell, P. A.; El-Aasser,

M. S. Eds. Wiley: New York, 1997.

4 Winnik, M. A. Curr. Opinion Coll. Polym. Sci. 1997, 2, 192-199; Keddie, J. L. Mat. Sci. Eng.

1997, 21, 101-170.

5 Wang, Y.; Zhao, C. L.; Winnik, M. A. J. Phys. Chem. 1991, 95, 2143-2153.

6 Winnik, M. A.; Wang, Y.; Haley, F. J. Coatings Technol. 1992, 64, 51-61.

7 Hahn, K.; Ley, G.; Schuller, H.; Oberthur, R. Colloid & Polym Sci. 1986, 264, 1092-1096.

8 Hahn, K.; Ley, G.; Oberthur, R. Colloid & Polym Sci. 1988, 266, 631-639.

9 Zhao, C. L.; Wang, Y.; Hruska, Z.; Winnik, M. A. Macromolecules 1990, 23, 4082-4087.

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Chapter 2

2 Experimental

2.1 Materials Potassium persulfate (KPS), bisphenol A dimethacrylate (BPDM), sodium carbonate

(Na2CO3), sodium dodecyl sulfate (SDS), 1-dodecanethiol (C12-SH), hexadecane (HD),

methacrylic acid (MAA), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (TexanolTM) and 2,2′-

azobis(2-methylpropionitrile) (AIBN) were used as received from Aldrich. Polystep A-16 (22%

solution of dodecylbenzene and tridecylbenzene sulfonates,1 Stepan Co., Maywood NJ) and

methyl-β-cyclodextrin (Me-β-CD) were kindly supplied by Rohm and Haas Co. and used as

received. Methyl methacrylate (MMA, Aldrich), n-butyl acrylate (BA, Aldrich),and n-butyl

methacrylate (BMA, Aldrich) were distilled at reduced pressure, and the purified monomers

were stored at 4 oC until use. Water was purified by a Milli-Q ion-exchange filtration system.

Phenanthrylmethyl methacrylate (PheMMA) was used as received from Toronto Research

Chemicals Inc. 4’-dimethylamino-2-methacryloxy-5-methyl benzophenone (NBenMA) was

synthesized as described elsewhere.1, 2

2.2 Synthesis of latices

2.2.1 Synthesis of poly(n-butyl acrylate-co-methyl methacrylate) P(BA-MMA)

A typical recipe for the semi-continuous emulsion polymerization of P(BA-MMA) is

shown in Table 2-1. In the first stage, a dispersion of seed particles was prepared by batch

emulsion polymerization with 3 wt % of a pre-emulsion of monomers, surfactant, chain transfer

agent, and water. Water (3.0 g), Polystep A-16 (0.13 g) and Me-β-CD (0.13 g) were added in a

100 mL 3-neck flask equipped with a mechanical stirrer, nitrogen inlet and condenser. The flask

was immersed in an oil bath. The system was thoroughly purged with nitrogen while the reaction

mixture was heated to 80 oC. After the reactor temperature stabilized at 80 oC, the KPS solution

(0.06 g in water 0.5 g) as an initiator and the Na2CO3 solution (0.05 g in water 0.5 g) as a pH

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buffer were added into the reactor followed by the addition of 3 wt % of monomer pre-emulsion

(0.44 g). The mixture was stirred for 20 min at 80 oC.

In the second stage of polymerization, the remaining monomer pre-emulsion was fed into

the seed latex dispersion together with an initiator aqueous solution (0.01 g in water 2.0 g). The

monomer feeding rate was kept identical (0.1 mL/min), controlled by Fluid Metering QG50

pumps, with a total feeding time of 3 h. After the addition was completed, the system was

maintained at 80 oC for 0.5 h. Then the reaction was cooled to room temperature. A latex

dispersion with ca. 50 wt % solids content was produced. The particle size is about 150 nm in

diameter with a narrow size distribution.

Table 2-1. Typical semi-continuous emulsion polymerization recipe for the synthesis of non-

labeled P(BA60-MMA39)a latex

ingredients (g) first stage second stage

H2O 3.0

Polystep A-16b 0.13

Me-β-CD 0.13

Na2CO3 0.05

KPS 0.06 0.01

monomer pre-emulsion 0.44 14.25

H2O 4.5

Polystep A-16 0.16

BA 6.0 / 47 mmol / 60 wt %

MMA 3.9 / 39 mmol / 39 wt %

MAA 0.1 / 1 mmol / 1 wt %

C12-SHc 0.025 / 0.25 wt %

a The subscripts refer to the wt % of each monomer. All latex samples contain 1 wt % MAA. b. 22 wt % surfactant solution, primarily sodium dodecylbenzene sulfonate. c. 1-Dodecanethiol, chain transfer agent, used at 0.25 wt % of total monomers.

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Fluorescence dye labeled latex samples were synthesized in a similar fashion. For the

donor (D) labeled particles, 1.0 mol % (2.4 wt %) PheMMA (based on total monomer) was

added into the monomer pre-emulsion. For the acceptor (A) labeled particles, 0.3 mol % (0.8 wt

%) NBenMA (based on total monomer) was added into the monomer pre-emulsion.

2.2.2 Synthesis of branched poly(n-butyl methacrylate) PBMA

Branched PBMA latex samples were synthesized by semi-continuous emulsion

polymerization reactions. A typical recipe for the synthesis of branched PBMA is shown in

Table 2-2. A monomer pre-emulsion was prepared by shaking a mixture of monomer, branching

agent, surfactant, chain transfer agent, and water for 30 min. In the first stage, a dispersion of

seed particles was prepared by batch emulsion polymerization. Water (3.0 g), Me-β-CD (0.02 g)

and SDS (0.03 g) were added in a 100 mL 3-neck flask equipped with a mechanical stirrer,

nitrogen inlet and condenser. The flask was immersed in an oil bath. The system was thoroughly

purged with nitrogen while the reaction mixture was heated to 80 oC. After the reactor

temperature stabilized at 80 oC, the KPS solution (0.06 g in water 0.5 g) as an initiator and the

Na2CO3 solution (0.05 g in water 0.5 g) as a pH buffer were added into the reactor followed by

the addition of 3 wt % of the monomer pre-emulsion. The mixture was stirred for 20 min at 80 oC. In the second stage of polymerization, the remaining monomer pre-emulsion was fed into the

seed latex dispersion together with an initiator aqueous solution (0.01 g in water 2.0 g) in 4 h.

The feeding rates were kept identical, controlled by Fluid Metering QG50 pumps. After the

addition was completed, the system was maintained at 80 oC for 0.5 h. Then the reaction was

cooled to room temperature.

Fluorescence donor and acceptor labeled PBMA latices were synthesized in a similar

fashion. For the donor labeled particles, 1 mol % (2.4 wt %) PheMMA (based on total monomer)

was added into the monomer pre-emulsion. For the acceptor labeled particles, 0.3 mol % (0.8 wt

%) NBenMA (based on total monomer) was added into the monomer pre-emulsion.

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Table 2-2. Typical semi-continuous emulsion polymerization recipe for the synthesis of

branched PBMA latex

ingredients (g) first stage second stage

H2O 3.0

SDS 0.030

Me-β-CD 0.02

Na2CO3 0.05

KPS 0.06 0.01

monomer pre-emulsion 0.44 14.28

H2O 10.0

SDS 0.045

BMA 4.60 / 32 mmol / 65 wt %

BPDM 1.18 / 3.2 mmol / 17 wt %

C12-SH 1.31 / 6.4 mmol / 18 wt %

2.2.3 Preparation of P(BA-MMA)/HB-PBMA latex blends

P(BA-MMA)/HB-PBMA latex blends were prepared by mixing HB-PBMA latex with the

mixtures of donor (D)- and acceptor(A)- labeled P(BA-MMA) latices. The weight ratio of donor

and acceptor labeled P(BA-MMA) is 1 : 1. And the weight ratio of D to A in this mixture is 2.9 :

1.0. The HB-PBMA content is in the range of 0 wt % to 10 wt % (based on total P(BA-MMA)

polymer).

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2.2.4 Preparation of P(BA-MMA) latices containing TexanolTM

P(BA-MMA) latices containing TexanolTM were prepared by adding 0 wt % to 10 wt %

TexanolTM (based on total P(BA-MMA) polymer) into the mixtures of D- and A-labeled P(BA-

MMA) latices. The latices were stirred for 48 h at ambient condition before use.

2.2.5 Synthesis of P(BA-MMA) latices containing hyperbranched poly(n-butyl methacrylate) (HB-PBMA)

P(BA-MMA) latices containing hyperbranched PBMA were synthesized using a

miniemulsion polymerization method. A typical recipe for the miniemulsion polymerization

reaction is shown in Table 2-3. A monomer emulsion was prepared by sonicating a mixture of all

of the ingredients in an ice bath for 20 minutes using a Branson Model 450 Digital Sonifier

(400w, microtip 40% maximum power, pulse: 1.0 s on/1.0 s off). The monomer emulsion was

then transferred into a reactor. The system was thoroughly purged with nitrogen before the

reactor was immersed in an oil bath which was pre-heated to 80 oC. The system was maintained

at 80 oC for 5 h. The reaction was then cooled to room temperature.

Fluorescence dye comonomers were used to label the latex polymers. To synthesize the

donor and acceptor labeled particles, 1 mol % (2.4 wt %) PheMMA (based on total monomer)

and 0.3 mol % (0.8 wt %) NBenMA (based on total monomer) were added into the monomer

pre-emulsion, respectively.

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Table 2-3. Typical miniemulsion polymerization recipe for the synthesis of donor labeled P(BA-

MMA) latex containing 1 wt% HB-PBMA.

ingredients (g)

H2O 13.0

SDS 0.06

HD 0.36 / 1.6 mmol

BA 5.5 / 43 mmol / 55 wt %

MMA 4.4 / 44 mmol / 44 wt %

MAA 0.1 / 1 mmol / 1 wt %

C12-SH 0.025 / 0.25 wt %

PheMMA 0.246 / 1 mol % / 2.4 wt %

HB-PBMA 0.10

AIBN 0.062

2.2.6 Synthesis of P(BA-MMA) latices containing hyperbranched poly(n-butyl methacrylate) (HB-PBMA) seed particles

HB-PBMA seeded P(BA-MMA) latex samples were synthesized by seeded emulsion

polymerization reactions. A typical recipe for the seeded polymerization reaction is shown in

Table 2-4. In the first stage, the HB-PBMA latex dispersion (2.87 g, containing 1.0 g polymer),

water (3.0 g), and methyl-β-cyclodextrin (0.12 g) were added in a 100 mL 3-neck flask equipped

with a mechanical stirrer, nitrogen inlet and condenser. The flask was immersed in an oil bath.

The system was thoroughly purged with nitrogen while the reaction mixture was heated to 80 oC.

The KPS solution (0.14 g in water 1.0 g) as an initiator and the Na2CO3 solution (0.12 g in water

1.0 g) as a pH buffer were added into the reactor

In the second stage of polymerization, a pre-emulsion (39.3 g) of monomers, surfactant,

chain transfer agent, and water was fed into the seed latex dispersion together with an initiator

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aqueous solution (0.01 g in water 2.0 g). The monomer feeding rate was kept identical (0.1

mL/min), controlled by Fluid Metering QG50 pumps, with a total feeding time of 5 h. After the

addition was completed, the system was maintained at 80 oC for 0.5 h. Then the reaction was

cooled to room temperature.

For the donor (D) labeled particles, 1.0 mol % (2.4 wt %) PheMMA (based on total

monomer) was added into the monomer pre-emulsion. For the acceptor (A) labeled particles, 0.3

mol % (0.8 wt %) NBenMA (based on total monomer) was added into the monomer pre-

emulsion.

Table 2-4. Typical seeded emulsion polymerization recipe for the synthesis of A-labeled P(BA-

MMA) latex containing 5 wt % HB-PBMA seed particles

ingredients (g) first stage second stage

H2O 3.0

HB-PBMA latex 2.87

methyl-β-cyclodextrin 0.12

Na2CO3 0.12

KPS 0.14 0.01

monomer pre-emulsion 39.30

H2O 20.0

SDS 0.084

BA 10.45 / 82 mmol / 55 wt %

MMA 8.36 / 83 mmol / 44 wt %

MAA 0.19 / 2 mmol / 1 wt %

C12-SH a 0.047 / 0.25 wt %

NBenMA 0.168 / 0.3 mol %

a 1-Dodecanethiol, chain transfer agent, used at 0.25 wt % of total monomers.

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2.3 Characterizations of latex particles The solids content of each latex dispersion was determined by gravimetry. Particle

diameters were measured by dynamic light scattering at a fixed scattering angle of 90º at 23 oC

with a Brookhaven Instruments model BI-90 particle sizer equipped with a 10 mW He-Ne laser.

Particle sizes and size distributions were also measured by capillary hydrodynamic fractionation

chromatography using a MATEC model 2000 CHDF.

2.4 Characterizations of latex polymers

2.4.1 Molecular weight and molecular weight distribution

Polymer molecular weight and polydispersity index (PDI) were measured by gel

permeation chromatography (GPC) using a Viscotek liquid chromatograph equipped with a

Viscotek model 2501 UV detector and a Viscotek TDA302 triple detector. Two Viscotek

GMHHR Mixed Bed columns were used with tetrahydrofuran (THF) as the elution solvent at a

flow rate of 0.6 mL/min. Polystyrene standards were used for calibration.

2.4.2 Copolymer composition

Copolymer compositions in P(BA-MMA) and branched PBMA latices were determined

by 1H NMR spectroscopy. 1H NMR spectra were recorded on a Varian Mercury 300 MHz NMR

spectrometer using CD2Cl2 or CDCl3 as the solvent in 5 mm NMR tubes. The residual signal of

the solvent was used as a reference in all the spectra (CD2Cl2δ 5.32, CDCl3 δ 7.24).

2.4.3 Glass transition temperature

The glass transition temperature (Tg) of copolymers was measured with a TA Instruments

DSC Q100 V7.3 Build 249 differential scanning calorimeter over a temperature range of -50 to

150 oC at a heating rate of 10 oC/min. Each sample was taken through two runs. Tg values were

calculated from the second heating run.

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2.4.4 Acceptor dye concentration measurements

The UV spectra of NBen-labeled latex samples were measured by a Lambda 25 UV/VIS

spectrometer (PerkinElmer Instruments). A calibration curve was made based on the absorbance

at 341 nm of NBenMA solutions in THF. Then the dye concentration in the NBen-labeled

polymer was computed based on this calibration curve.

2.4.5 Gel content measurements

Gel content was measured by the centrifugation method developed in our laboratory:3 A

latex sample (1.0 g) was dried to a constant weight W0. The dried polymer was subsequently

immersed in tetrahydrofuran (THF, 10 mL). The mixture was agitated gently at room

temperature for 24 h. The resulting solution was then centrifuged at 20,000 rpm for 30 min, and

the top transparent layer was poured off. When gel was present, a precipitate remained. The

precipitate was washed three more times with excess THF to remove residual sols from the gel.

The remaining sample (the gel fraction) was dried and weighed (W1). The gel content (%) was

calculated from the equation

gel content (%) = (W1/ W0) × 100 (2-1)

2.5 Mechanical property measurements.

2.5.1 Rheology measurements.

The viscoelastic response of P(BA-MMA) and PBMA samples was studied at several

temperatures above Tg with a Rheometrics RAA instrument in the oscillatory shear mode. These

experiments employed a pair of parallel plates (25 mm diameter). The frequency was scanned

between 0.01 and 100 rad/s at a constant temperature. Strain sweeps were employed to insure

that all measurements were made in the linear viscoelastic regime. The range of temperatures

studied was selected to be as close as possible to the range of temperature used in the energy

transfer experiments performed on these materials. However, the lowest temperatures used are

limited by the sample modulus at temperatures close to Tg, and by the fact that samples that are

too stiff can damage the transducer of the rheometer.

The following procedure was used to prepare the samples for the measurements of

viscoelastic properties. First, the samples were dried under vacuum at 40 oC for 12 h to eliminate

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any trace of volatiles. Then, the samples were pressed between cleaned polytetrafluoroethylene

(PTFE) sheets in a Carver Press at 100 oC to eliminate air bubbles. The thickness of the samples

was controlled using separators between the plates of the press. In this way, samples free of

bubbles, approximately 25 mm in diameter and 1 mm thick were obtained.

2.5.2 Tensile testing

Tensile testing was performed at a strain rate of 50 mm/min with an Instron 5543 tensile

tester at ambient temperature. The averaged values of tensile strength, elongation, and Young's

modulus were obtained from at least five specimens for each sample. The following procedure

was used to prepare the specimen for the stress-strain measurements. First, the samples were

dried under vacuum at 80 oC for 12 h to eliminate any trace of volatiles. Then, the samples were

pressed in a stainless steel mold between cleaned polytetrafluoroethylene (PTFE) sheets with a

Carver Press at 120 oC to eliminate air bubbles. The thickness of the samples was controlled by

the mold (22 mm × 4.8 mm× 1 mm). In this way, dumbbell shape specimens free of bubbles,

approximately 1 mm thick, were obtained.

2.5.3 Dynamic mechanical analysis

The storage modulus (E’) and loss modulus (E”) of latex polymers were measured by

dynamic mechanical tests using a TA Instruments DMA Q800 dynamic mechanical thermal

analyzer in the single cantilever mode. The frequency used was 1.0 Hz, and the heating rate was

2.0 oC/min. The rectangular specimens were prepared using the procedure described above with

a stainless steel mold (20 mm × 5.3 mm× 0.5 mm). The thickness is ca. 0.5 mm. The

experiments were carried out from -80 oC to 100 oC.

2.6 Film preparation Latex films for fluorescence decay measurements were prepared from 1:1 particle

mixtures of the D- and A-labeled dispersions. Several drops of a latex dispersion (about 20 wt %

solids content) were spread on a small quartz plate (20 × 8 mm). The film was allowed to dry

uncovered at 4 oC until visually transparent. It took about 2 h for a film to dry. The films

prepared in this way have a thickness of ca. 60 μm.

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Solvent-cast films were prepared from the same latex mixture. A latex film was allowed to

dry at 23 oC overnight, and the dry film was dissolved in a minimum amount of THF. The

solution was re-cast onto a small quartz plate and allowed to dry at room temperature for 24

hours.

2.7 Film annealing Latex films on quartz plates were placed directly on a high mass (2 cm thick) aluminum

plate in an oven preheated to the annealing temperature and then annealed for various periods of

time. Under these conditions, I estimate that it takes less than 1 minute for the film to reach the

preset oven temperature. The annealed films were taken out of the oven and placed directly on

another high mass aluminum plate at 4 oC for 2 minutes before carrying out fluorescence decay

measurements.

2.8 Fluorescence decay measurements and data analysis Fluorescence decay profiles of the films at 23 oC were recorded using a nanosecond Time-

Correlated Single Photon Counting System from IBH.4 Each film was placed in a quartz tube and

excited with a NanoLED (λex = 296 nm). An emission monochromator (350 ± 16 nm) was used

to minimize the amount of scattered light from the sample entering the detector. Data were

collected until 5000 counts were accumulated in the maximum channel. The instrumental

response function was obtained by using a degassed p-terphenyl solution (0.96 ns lifetime) as a

mimic standard.5

For all the latex polymers examined in this work, the donor (Phe) decay profile in films

free of acceptors was exponential with a lifetime τD = 44.3 ns. For films containing both donor

and acceptor chromophores, the fluorescence-decay profiles became non-exponential. The shape

of the curve depends upon the details of the donor-acceptor pair distribution. In a system

containing uniformly distributed donor and acceptors in three dimensions in the absence of

diffusion, the donor fluorescence intensity decay ID( t ′ ) following instantaneous excitation is

described by the Förster equation,6

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( )[ ]2/1//exp)( DDD tPtAtI ττ ′−′−=′ (2-2)

where

[ ]A23

34 3

0

2/122/3 RNP A⎟

⎠⎞

⎜⎝⎛= κπ (2-3)

Here, P is proportional to the acceptor (quencher) concentration [A]. R0 is the critical Förster

radius for energy transfer, which for the Phe/NBen pair takes a value of 2.51 ± 0.04 nm.7 NA is

Avogadro’s number. The orientation factor 2κ describes the average orientation of dipoles of

donor and acceptor molecules. For a random distribution of immobile chromophores in three-

dimensions, 2κ is replaced by 2κ =0.476, a situation typical of dyes in polymer matrices.5, 8

The quantum efficiency of energy transfer ΦET(t) is defined by the middle term in the

following expression

DD

DET

tarea

ttI

tttIt

τ)(1

d)(

d),(1)(

0

0

0 −=′′

′′−=Φ

∫∫

(2-4)

where )(0 tI D ′ is the decay curve of donor fluorescence intensity in the donor-only film. Because

the unquenched donor decay profile was exponential in each sample, its integral is equal to the

unquenched donor lifetime τD. Here t is the annealing time after film preparation; t ′ is the

fluorescence decay time; and area(t) refers to the normalized area under the fluorescence decay

curve of a film annealed for time t. To obtain the area for each decay profile, I fitted each decay

curve to the empirical equation (2-5) and then evaluate the integral analytically from the

magnitude of the fitting parameters, A1, A2, and p.1, 2, 9

[ ] )/exp()/(/exp),( 22/1

1 DDDD tAtptAttI τττ ′−+′−′−=′ (2-5)

The fraction of mixing fm is an important parameter measuring the extent of growth of ФET

due to polymer diffusion, and is defined in such a way that it corrects for the energy transfer

efficiency in the nascent films. Values of fm are calculated from fluorescence decay data using

the following equation

( ) ( ) ( )( ) ( )

00

ET ETm

ET ET

tf t

Φ − Φ=

Φ ∞ − Φ (2-6)

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where, in principle, the numerator represents the change in energy transfer efficiency between

the freshly prepared film and that annealed for time t, and the denominator describes the

difference in energy transfer efficiency between the initial and the fully mixed films. Because

some polymer diffusion can occur during sample drying, I fitted data to equation (2-6) using a

value of ФET(0) = 0.07, which corresponds to films formed from latex particles of the size

employed here, which achieve intimate contact upon drying,10, 11 but for which no polymer

diffusion takes place.12

2.9 Calculation of apparent diffusion coefficients Dapp If the diffusing polymer is initially distributed uniformly through a sphere of radius R, by

assuming a Fickian diffusion model for spherical geometry a concentration profile C(r, t) of the

polymer corresponding to an apparent diffusion coefficient Dapp can be calculated. A numerical

method can be then used to obtain the best Dapp matching an experimental fm value to the

theoretical expression:

∫−=≈R

sm drrtrCV

ff0

24),(11 π (2-7)

where C(r, t) is the concentration of polymer at radius r and time t, and V is the volume of a

particle of radius R. In equation (2-7) it has been assumed that the “quantum fraction” of mixing

fm is equal to the mass fraction of mixing fs. Our group has analyzed this assumption in the past

finding that fm and fs values are proportional for all values of fm ≤ 0.7, except at the very end of

the experiment, and that the assumption fm = fs overestimates the diffusion coefficient by a factor

of 2-3.

2.10 Fujita-Doolittle fitting The diffusion coefficients were fitted into the Fujita-Doolittle equation (FDE), which is a

one-parameter model used to compare the diffusion coefficients in the presence of a plasticizer to

that in its absence.13 The FDE is shown as following:

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( )( ) ( ) ( )

( )TTf

TfTD

TD

a

pp

p

ap

βφφ 0,

0,0,

,ln

21

+=⎟⎟⎠

⎞⎜⎜⎝

⎛−

(2-8)

where Dp represents the polymer diffusion coefficient, Φa refers to the volume fraction of the

coalescing agent, fp(T,0) is the fractional free volume of the polymer with no added coalescing

agent, and β(T) is the difference in fractional free volume between the coalescing agent and the

polymer at temperature T. {ln[Dp(T, Φa)/Dp(T, 0)]}-1 was plotted against 1/Φa to give a linear line.

From the intercept and the slop I calculated fp(T,0) andβ(T).

2.11 References

1 Oh, J. K.; Wu, J.; Winnik, M. A.; Craun, G. P.; Rademacher, J.; Farwaha, R. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1594-1607.

2 Oh, J. K.; Wu, J.; Winnik, M. A.; Craun, Gary P.; Rademacher, J.; Farwaha, R. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3001-3011.

3 Tronc, F.; Liu, R.; Winnik, M. A.; Eckersley, S. T.; Rose, G. D.; Weishuhn, J. M.; Meunier, D. M. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 2609-2625.

4 O’Connor, D. V.; Phillips, D. Time-Correlated Single Photon Counting. Academic Press: New York, 1984.

5 James, D. R.; Demmer, D. R. M.; Verrall, R. E.; Steer, R. P. Rev. Sci. Instrum. 1983, 54, 1121-1130.

6 Bartels, C. R.; Buckley, C.; Graessley, W. W. Macromolecules 1984, 17, 2702-2708.

7 Liu, Y.; Haley, J. C.; Deng, K.; Lau, W.; Winnik, M. A. Macromolecules 2007, 40, 6422-6431.

8 Baumann, J.; Fayer, M. D. J. Chem. Phys. 1986, 85, 4087-4107

9 Liu, Y.; Haley, J. C.; Deng, K.; Lau, W.; Winnik, M. A. Macromolecules 2008, 41, 4220-4225.

10 Wool, R.P.; O’Connor, K.M., J. Appl. Phys., 1981, 52, 5953-5963.

11 Kim, Y.H.; Wool, R.P. Macromolecules 1983, 16, 1115-1120.

12 Farinha, J.P.S.; Martinho, J.M.G.; Yekta, A.; Winnik, M.A. Macromolecules 1995, 28, 6084-6088.

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13 Juhué, D.; Wang, Y.; Winnik, M. A. Makromol. Chem. Rapid Commun. 1993, 14, 345-349.

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Chapter 3

3 Effect of Polymer Composition on Polymer Diffusion in Poly(n-butyl acrylate-co-methyl methacrylate) Latex Films

3.1 Introduction Latex paints contain significant amounts of volatile organic solvents (VOCs).1 These

solvents serve as fugitive plasticizers. They soften the particles so that the forces associated with

drying are sufficient to deform the spherical latex particles into polyhedral cells that form a

continuous and void-free film. They also enhance the rate of the diffusion of polymer molecules

across the boundaries between these cells. This is the step that leads to the development of good

mechanical properties of the latex film. At this time the polymer film is often soft and tacky.

Over time, the VOCs evaporate from the film, increasing its glass transition temperature and its

hardness at room temperature. The disadvantage of VOCs is that they contribute to air pollution.

New knowledge is needed, however, to develop latex coatings that do not require VOCs and

have similar or enhanced performance properties to current technology. My thesis research has

the objective of providing some of this new knowledge.

As a step in this direction, I carried out a study of polymer diffusion in films formed from

a series of latex consisting of n-butyl acrylate-methyl methacrylate-methacrylic acid (BA-MMA-

MAA) copolymers of different compositions. These acrylic latices are widely used in

architectural coatings (house paints). Typical coatings in current use consist of BA/MMA weight

ratios of 50/49 with 1 wt % MAA. Increasing the BA content in the latex reduces the glass

transition temperature (Tg) of the latex polymer, however, the other effects are kept unknown. In

this chapter, I described experiments that explore how the composition of the latex polymer

affects the rate of polymer diffusion (and its temperature dependence), for a series of BA/MMA

latex containing 1 wt % MAA, all with polymers of very similar molar mass (Mn ≈ 40,000

g/mol, Mw/Mn ≈ 3.0). Their glass transition temperatures range from 4 oC to 28 oC. These

polymer diffusion rates were studied using the fluorescence resonance energy transfer (FRET)

methods developed in our laboratory.2 To perform the FRET experiments, fluorescent donor (D)-

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and acceptor (A)-labeled latex particles were synthesized using semi-continuous emulsion

polymerization. Latex films were cast from a 1:1 mixture of D- and A-labeled latex samples.

Polymer diffusion was monitored as a function of annealing temperature, and apparent diffusion

coefficients (Dapp) were calculated from the energy transfer data using a simple diffusion model.

These values increased with annealing temperature and decreased with Tg. Rheology

measurements recorded the response of the dynamic moduli (G', G”) with respect to oscillatory

shear frequency (ω) over a range of temperature close to that of the diffusion experiments. It was

found that the temperature dependence of polymer dynamics extracted by the rheology

experiments is in good agreement with the temperature dependence of Dapp. Increasing the BA

copolymer content leads to an apparent increase in long-chain branching, which is reflected in

both the time dependence of Dapp and in the dynamic moduli measurements. I can concluded that

a greater degree of branching leads to a broader distribution of polymer diffusion coefficients,

and a stronger time dependence of Dapp.

3.2 Results

3.2.1 Preparation and Characterization of the Latex Samples

Poly(n-butyl acrylate-co-methyl methacrylate) [P(BA-MMA)] was used as the base

copolymer in my diffusion study. All latexes were prepared by semi-continuous emulsion

polymerization. In previous work in our laboratory, in order to obtain similar size D- and A-

labeled particles, unlabeled seed particles were used for the synthesis of both the donor and

acceptor labeled latex, and the polymerizable dye derivative was added only in the second stage.3, 4 By using the same unlabeled seed particles it is easy and efficient to control particle size. These

seeds represent ca. 8 - 10 wt % of the final latex particles. If the seed particles are prepared by

batch emulsion polymerization, they normally have a higher molar mass and broader PDI than

the second stage polymer. To some extent, this may lead to a non-uniform dye distribution in the

particles. Our laboratory has always treated this as a minor problem.

Nevertheless, in order to overcome the disadvantage of preformed non-labeled seeds, I

used an in-situ seeding process in the experiments described here. While this is common practice

in industry for large-scale reactions, this is a skill-testing challenge for emulsion polymerization

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reactions run on the small scale (10 g of total monomers) I employ to synthesize labeled latex.

On a small scale, slight variations in particle nucleation can have large consequences for particle

size and size distribution. One can also experience problems in matching the polymer molar

mass and PDI between samples. The challenge for me, which in the past led to the use of

common unlabeled seeds, was the need for the donor and acceptor labeled particles to have

similar diameters and contain polymers of similar Mn and PDI. For the latex samples described

here, I employed the same monomer pre-emulsion containing: monomers, dye comonomers,

surfactant, chain transfer agent and water for making the seed particles in the first stage and for

particle growth in the second stage as well. These reactions worked well, and I was able to

achieve reasonable control over particle size and polymer molar mass, not only for D- and A-

labeled particles of a given composition, but for the entire series of latex examined here.

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Table 3-1. Characteristics of the P(BA-MMA) latex polymers and particles.

Latex sample a Mn PDI Tg

(oC)

d b

(nm)

dnc

(nm)

dw/dnc solids

content

(%)

P(BA60-MMA39) 31,000 3.7 4 151 156 1.1 48.2

P(BA60-MMA39)D 31,000 3.1 156 166 1.1 44.2

P(BA60-MMA39)A 29,000 2.1 3 141 159 1.3 54.1

P(BA55-MMA44) 48,000 2.9 7 152 159 1.2 44.3

P(BA55-MMA44)D 47,000 3.0 151 163 1.0 43.1

P(BA55-MMA44)A 46,000 2.8 7 157 142 1.1 41.1

P(BA50-MMA49) 50,000 2.0 170 150 1.2 42.3

P(BA50-MMA49)D 45,000 2.7 12 159 146 1.1 40.2

P(BA50-MMA49)A 43,000 2.2 191 203 1.1 45.7

P(BA40-MMA59) 51,000 2.0 148 152 1.3 47.6

P(BA40-MMA59)D 44,000 2.1 28 147 155 1.1 29.4

P(BA40-MMA59)A 41,000 3.7 27 152 151 1.2 52.0

a Superscript “D” and “A” refers to D- and A-labeled latices, respectively.

b Data for the particle diameter d from the BI-90 Particle Sizer.

c Number average dn and weight average dw diameter data from the CHDF 2000 (MATEC).

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Figure 3-1. Plot of 1/Mn against concentration of C12-SH of P(BA60-MMA39) latex samples.

All emulsion polymerizations contained 1 wt % of MAA. In commercial latex, small

amounts of methacrylic acid are normally employed to enhance the colloidal stability of the latex.

I follow this practice here. The four pairs of samples I synthesized had monomer weight ratios of

BA:MMA:MAA of 60:39:1, 55:44:1, 50:49:1 and 40:59:1. These copolymers are named

according to their BA:MMA compositions as P(BA60-MMA39), P(BA55-MMA44), P(BA50-

MMA49) and P(BA40-MMA59). The glass transition temperatures (Tg) of the copolymers were

measured by DSC, giving Tg ca. 4 oC for P(BA60-MMA39), 7 oC for P(BA55-MMA44), 12 oC for

P(BA50-MMA49) and 27 oC for P(BA40-MMA59). All values are similar to those estimated values

from the Fox equation using Tg(PBA) = -47 oC and Tg(PMMA) = 105 oC.5

Dodecyl mercaptan (C12-SH) was added in the emulsion polymerization reactions as a

chain-transfer agent both to control the molecular weight and to limit or suppress gel formation.

My target in these studies was to obtain high molar mass with very low gel content. This will

serve as a baseline for future experiments with latex comprised of lower molar mass polymer. In

order to optimize the reaction conditions, a series of P(BA60-MMA39) latex samples were

prepared in the presence of various amounts of C12-SH. The latex polymer samples obtained

were analyzed by a triple detector GPC. Molecular weights were determined using polystyrene

standards. As shown in Figure 3-1, the plot of 1/Mn against [C12-SH] was linear, indicating that

in the presence of β-cyclodextrin, C12-SH provides good control over polymer molar mass.

When the amount of C12-SH in the reaction was 0.25 wt % based on monomer in the pre-

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Figure 3-2. 1H NMR spectra of (A) P(BA60-MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49) and (D) P(BA40-MMA59). CDCl3 was used as solvent. Peaks a and peak b correspond to protons at positions a and b respectively.

emulsion, the polymers obtained had less than 5% gel; with lower amounts of chain transfer

agent, the latex formed had a significant gel content. For example, 0.10 wt % C12-SH led to a

product containing 46% gel content. Thus all the latex samples used in the diffusion experiments

were prepared in the presence of 0.25 wt % C12-SH. The characteristics of all of the latex

particles synthesized are summarized in Table 3-1. The Mn values were in the range of 30,000 to

50,000 with a PDI between 2 and 3.7. By comparing Mn of the dye labeled and non-labeled latex

polymers (see Table 3-1), I infer that the dye monomer did not significantly affect the

polymerization reaction.

The 1H NMR spectra of the four different latex polymer compositions are compared in

Figure 3-2. From the ratio of integrals of peaks a and b, which correspond to protons at position a

and b respectively, I calculated the mole ratios of BA:MMA were 1.5:1.0 for P(BA60-MMA39),

1.1:1.0 for P(BA55-MMA44), 0.8:1.0 for P(BA50-MMA49) and 0.5:1.0 for P(BA40-MMA59), the

weight ratios of BA:MMA were 65:35 for P(BA60-MMA39), 58:42 for P(BA55-MMA44), 51:49

for P(BA50-MMA49) and 41:59 for P(BA40-MMA59). These results indicate that the composition

of the polymers closely resembled the monomer feed composition in my emulsion

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polymerization reactions under monomer-starved conditions. The particle size and size

distribution were characterized by both right-angle dynamic light scattering and by CHDF. As

shown in Table 3-1, all samples have particle diameters of ca. 150 nm. For all latex samples the

dw/dn values obtained by CHDF are less than 1.3, which indicates a narrow size distribution.

3.2.2 Energy Transfer Studies of Polymer Diffusion

Films for FRET experiments were prepared from a 1:1 mixture D- and A-labeled latex

particles. Several drops of the latex mixture was cast onto a small quartz plate (20 × 8 mm) and

allowed to dry in a refrigerator at 4 oC over 1 h. The film thicknesses were ca. 60 μm. The films

obtained from P(BA60-MMA39), P(BA55-MMA44) and P(BA50-MMA49) D/A mixtures were

transparent and free of cracks. However, the films prepared from P(BA40-MMA59) latex were

turbid and showed some cracks even if they were dried at 23 oC. I attribute this behavior to the

high minimum film formation temperature (MFT) of this sample caused by the high Tg of the

latex polymers. Freshly formed latex films were transfer to the sample chamber of the

fluorescence decay instrument in a cold quartz tube, and fluorescence decays were measured

immediately. This whole process took less than 2 min. The films were then annealed in a pre-

heated oven for various periods of time at constant temperatures. The fluorescence decays were

monitored as a function of annealing time for samples annealed at different temperatures.

Figure 3-3 shows representative donor fluorescence decays for a D-labeled P(BA60-

MMA39) latex film [curve (1)] and a D/A mixed P(BA60-MMA39) latex film aged for various

periods of time at room temperature (23 oC) [curves (2-3)]. The decay curve (1) in Figure 3-3 is

exponential with a lifetime of 44.3 ns. Films of the D-labeled P(BA60-MMA39) latex with the

other three BA-MMA compositions gave the same lifetimes. For these polymers, I see that the

donor lifetime is independent of polymer composition. Curve (2) in Figure 3-3 is the decay

profile of a newly formed film from a 1:1 D/A mixture. It is not exponential, but the deviation at

early times is small (shown in Figure 3-3 inset) because very little polymer diffusion has

occurred. I assume that the major contribution to the curvature of this plot is energy transfer

across the particle boundaries. From the decays of a series of similar films, I calculate quantum

efficiencies of energy transfer (ФET) of 0.06-0.07 in newly formed films using equation (2-4). I

take 0.06 as the value of ФET(0). Curve (3) in Figure 3-3 depicts the decay profile of the film in

curve (2) after 47 min aging at 23 oC. The increased curvature at early decay times in an

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indication that some polymer diffusion has taken place, resulting in an increase in ФET.

Curve (4) in Figure 3-3 shows much more pronounced curvature. It represents the decay

profile for a film cast from a THF solution. In solvent cast films, the donor and acceptor labeled

polymers may be thought of as randomly mixed in solution, but can undergo some demixing

upon drying, due to correlation hole effects. I refer to the ФET value obtained from solvent-cast

films as ФET(lim). It represents the limiting maximum value of ФET that could be obtained from

diffusive mixing. This value can be smaller than, but is often equal to ФET(∞), the value

corresponding to complete randomization of the dyes in the system. For latex films formed from

linear polymers, ФET(lim) and ФET(∞) are commonly very similar in magnitude, but in latex

films consisting of highly branched polymers or polymer with a significant gel content, ФET(lim)

< ФET(∞).

Values of ФET(∞) were determined in a series of model experiments in which samples of

each of the Phe-labeled polymers were mixed with different amounts of NBenMA as a low

molar mass acceptor. Films were prepared by solvent casting and ID(t) decay profiles were

measured. Individual decays were fitted to equation (2-5), and values of the fitting parameter P

were plotted against [NBenMA] (Figure 3-4). The plot were linear and for each polymer led to a

Figure 3-3. Phenanthrene (donor) fluorescence decay curves [ID(t)] measured at 23 oC for Phe-P(BA60-MMA39) latex films. (1) Phe-labeled latex only, (2) a newly formed film dried at 4 oC, consisting of a 1:1 ratio of Phe-P(BA60-MMA39) and NBen-P(BA60-MMA39), (3) the same film as in (2) aged for 47 min at 23 oC, and (4) a solvent-cast film from a 1:1 mixture of the two freeze-dried polymers dissolved in THF and then annealed at 120 oC for 2 h. Note that curves (1) and (2) overlap. The inset shows curves (1) and (2) at short times on a linear scale.

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0.00.20.40.60.81.01.21.41.61.8

0 5 10 15 20 25 30

[NBenMA, mM]

P

Figure 3-4. Plot of P vs [NBenMA, mM] for fully mixed solvent-cast films prepared from Phe-P(BA60-MMA39) plus varying amounts of free monomer MBenMA. The P values were obtained by fitting individual Phe decay curves to equation (2-5) with τD fixed at 44.3 ns. From the slope of the plot, I calculate R0 = 2.51 nm.

value of Ro = 2.51 nm, consistent with the value reported previously.5 From this value and the

composition of the 1:1 Phe/NBen latex films, I calculated values of ФET(∞) = 0.50. I used this

value in all of my calculations of fm [(equation (2-6)].

3.2.3 Polymer Diffusion in P(BA60-MMA39) Films at Different Temperatures

A series of Phe- and NBen-labeled P(BA60-MMA39) latex films were cast at 4 oC and

dried for 1 h. The films were annealed at various temperatures, and their fluorescence decay

curves were measured at different periods of annealing time. From the newly formed films I

found the quantum efficiency before annealing, ФET(0) = 0.06-0.07. The maximum ФET value

was obtained from fully mixed D/A films which were cast from a THF solution of 1:1 Phe- and

NBen-labeled P(BA60-MMA39) polymers. The ФET value for this newly formed solvent cast film

was 0.42. Annealing this film at 120 oC for 2 h lead to a decrease of the ФET value to 0.38, and

there was no additional decrease with further annealing. I take ФET(lim) = 0.38.

The calculated ФET values are plotted against annealing time in Figure 3-5(A) for

experiments at different temperatures ranging from 23 oC to 90 oC. The curves show a large

increase in ФET values at early stages and a smaller increase at longer times. The plot shows that

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Figure 3-5. Plots of ФET (A) and fm (B) vs annealing time for the P(BA60-MMA39) latex films annealed at 23, 45, 70, and 90 oC.

Figure 3-6. Plots of ФET and fm vs annealing time for the P(BA60-MMA39) latex films annealed at 23 oC.

ФET has a strong temperature dependence. From 23 oC to 90 oC, not only the plateau ФET values,

but also the growth rate of ФET increased significantly. Since I know that ФET(0) = 0.06 and

ФET(∞) = 0.5, fraction of mixing fm values were calculated from the corresponding areas under

the donor decay profiles using equation (2-6). In Figure 3-5(B), I plot fm as a function of

annealing time. The curves have a similar shape to those in Figure 3-5(A). At 90 oC, maximum

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Figure 3-7. Plots of the apparent diffusion coefficient Dapp as a function of fm for (A) P(BA60-MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49) and (D) P(BA40-MMA59) latex films annealed at various temperatures.

mixing of donor and acceptor was achieved in 90 min with ФET(lim) = 0.38 and fm = 0.75.

For architectural coatings, one of the most important considerations is the time scale for

polymer diffusion at room temperature. In our laboratory, this is 23 °C. To emphasize that the

polymer molecules in films of the P(BA60-MMA39) latex, with Tg = 4 °C undergo substantial

diffusion at room temperature, I plot the evolution of ФET and fm in Figure 3-6.

To quantitatively compare polymer diffusion rates at different temperatures, one needs to

be able to compute diffusion coefficients D. Because there is no proper way to calculate absolute

values of D for mixtures of polymers of different lengths and extents and distribution of branches,

I resort to a prescription that has served our research group well in the past: I calculate apparent

diffusion coefficients Dapp by fitting fm data to a Fickian diffusion model. 4,6, 7

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In Figure 3-7(A), values of Dapp calculated in this way for P(BA60-MMA39) films are

plotted against fm values for various annealing temperatures. At each temperature, these Dapp

values decrease with increasing annealing time as slower diffusing species make their

contribution to the growth in ФET. The plot also shows that the diffusion rate is faster at higher

temperature for the same fm value. For example, at fm = 0.59, the annealed film sample gave a

value of Dapp = 0.007 nm2/s at 23 oC, 0.16 nm2/s at 45 oC, and 4.4 nm2/s at 60 oC.

Arrhenius-type plots (ln Dapp vs 1/T) of the data in Figure 3-7(A) are linear for Dapp values

at fm = 0.59. These plots are shown in Figure 3-8. From the slopes of these plots, I obtained an

apparent activation energy Ea = 33.4 kcal/mol over the temperature range 23-60 oC. Since

temperature affects the rate of diffusion by a change in the monomeric friction factor, the

magnitude of Ea should be independent of fm. Therefore, I used this value of Ea as a shift factor to

create a master curve of Dapp values at 23 oC. The shifted values calculated in this way are shown

in Figure 3-10 below. The success in generating the master curve serves as strong support for the

validity of my analysis to obtain Dapp values.

-6.0

-4.0

-2.0

0.0

2.0

2.9 3.1 3.3 3.5

(1000/T) [1/K]

ln D

app

(nm

2 /s)

E a = 33.4 kcal/mol

Figure 3-8. Plot of ln Dapp against 1/T over the temperature range from 23 to 60 oC at fm values of 0.59 for P(BA60-MMA39) latex.

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3.2.4 Polymer Diffusion in Different Composition P(BA-MMA) Films

To compare the polymer diffusion in P(BA-MMA) latex films with various polymer

compositions, I monitored the increase in ФET for a series of latex films of each composition,

each annealed at a series of temperatures. In Figure 3-9, these ФET values are plotted as a

function of annealing time at their corresponding temperatures. Figure 3-9(A) shows that at 23 oC, three of the four latex films of different composition undergo a significant extent of diffusion

on the time scale of tens of hours. The Tg values of these polymer compositions range from 3 to

12 °C (Table 3-1), and the rate of diffusion at room temperature increases with decreasing

sample Tg.

Only the P(BA40-MMA59) film showed no detectable diffusion at this temperature. It has a

Tg slightly above room temperature, 28 °C. There are several noteworthy features of this

particular set of films. When these films were cast and dried at 4 °C, well below the MFT, they

were turbid and cracked. Better films for polymer diffusion studies were obtained by casting and

drying at room temperature although they were still not clear and crack free due to their high

MFT. Upon annealing at higher temperature, the films became more transparent, but the film

became fully transparent only after it was annealed for ca. 1 h at 90 oC. Thus dry sintering plays

a role in particle coalescence in these films.8 An increase in ФET in these films due to polymer

diffusion could be measured at 70 °C [Figure 3-9(B)]. This occurred over the first hour and then

appeared to cease. I speculate that this increase in ФET is due to the contribution of diffusion of

low molar mass chains in the sample. For the remainder of the experiment, polymer diffusion

was very slow at this temperature, but became more pronounced at 90 °C [Figure 3-9(C)].

The other samples underwent rapid polymer diffusion at 70 °C. For P(BA60-MMA39) film,

ФET approached ФET(lim) (0.38) in a few minutes. The P(BA55-MMA44) and P(BA50-MMA49)

latex film samples exhibited a behavior analogous to that of the high Tg sample: rapid diffusion

at early times, which appeared to level off at a ФET value less than ФET(lim) [ФET(lim) = 0.50 for

P(BA55-MMA44) and 0.52 for P(BA50-MMA49)]. These two films reached somewhat higher

values of ФET values when annealed for several hours at 90 °C.

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Figure 3-9. Plot of the ΦET for films formed from D/A labeled latex mixtures annealed for various periods of time at (A) 23 oC, (B) 70 oC and (C) 90 oC. ( )P(BA60-MMA39), ( )P(BA55-MMA44), ( ) P(BA50-MMA49) and ( ) P(BA40-MMA59).

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It is noteworthy that ФET(lim) values for P(BA55-MMA44) and for P(BA50-MMA49) are

higher than that for P(BA60-MMA39), and indistinguishable from ФET(∞). This result suggests

that there is a higher degree of branching or some undetected microgel in this P(BA60-MMA39)

sample that limits the extent to which donor and acceptor labeled polymers can interpenetrate.

Dapp values for these films were calculated as a function of fm at each temperature. These

plots are presented in Figure 3-7(A)-(D). From Arrhenius plots of Dapp values as described above,

apparent activation energies were computed. The values of Ea and the temperature ranges for

which they were obtained are listed in Table 3-2. These were then used as shift factors to create

Master Curves of the diffusion data as shown in Figure 3-10.

Figure 3-10. The master curves of Dapp values for (A) P(BA60-MMA39) at 23 oC (calculated using Ea = 33.4 kcal/mol as a shift factor); (B) P(BA55-MMA44) at 23 oC (calculated using Ea = 39.1 kcal/mol as a shift factor); (C) P(BA50-MMA49) at 23 oC (calculated using Ea = 45.2 kcal/mol as a shift factor) and (D) P(BA40-MMA59) at 70 oC (calculated using Ea = 64.1 kcal/mol as a shift factor).

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Table 3-2. Ea values of the latex polymers.

energy transfer

experiments Rheology Measurements

latex Ea

(kcal/mol)

Temperature

range

Ea

(kcal/mol)

Temperature

range

T0

(oC)

C1

C2

(K)

P(BA60-MMA39) 33.4 23-60 oC 28.7 25-70 oC 25 10.0 114.9

P(BA55-MMA44) 39.1 23-60 oC 34.2 50-70 oC 50 9.1 112.6

P(BA50-MMA49) 45.2 23-90 oC 50.3 80-180 oC 80 14.2 107.1

P(BA40-MMA59) 64.1 90-120 oC 65.6 90-130 oC 90 16.3 120.4

3.2.5 Temperature Dependence of the Viscoelastic Properties of P(BA-MMA) Films

The Williams-Landel-Ferry (WLF) equation 9 is widely employed to describe the

temperature dependence of polymer diffusion using parameters obtained from viscoelastic

relaxation measurement.4,7, 10 To describe polymer diffusion, the WLF equation takes the

following form

( ) ( )02

01

0

0loglogTTC

TTCTD

DTaT −+−

−== (3-1)

where D0 is the diffusion coefficient at an arbitrary chosen reference temperature T0. C1 and C2

are parameters that depend on the choice of the T0, and they are easily transferred to other

reference temperatures.

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For each composition, I carried out oscillatory shear measurements as a function of

frequency over a range of temperature close to that of the energy transfer experiments. Non-

labeled samples, which have similar molecular weight and PDI to dye labeled samples (see Table

3-1), were used for viscoelastic measurements.

I measured the storage modulus (G’) and loss modulus (G”) as a function of frequency (ω)

at a series of temperatures ranging from 25 to 100 oC for P(BA60-MMA39), from 50 to 120 oC for

P(BA55-MMA44), from 80 to 180 oC for P(BA50-MMA49) and from 130 to 200 oC for P(BA40-

MMA59) (not shown). I used the time-temperature superposition principle (TTS) to obtain the

shift factors (aT) of the temperature dependence. Strictly speaking, the TTS principle can be only

applied to a system in which the various relaxation times belonging to a given relaxation process

have the same temperature dependence, such as linear amorphous polymers above Tg. P(BA-

MMA) copolymer is composed of polydisperse chains with various degrees of branching.11 It is

well-known that branching may affect slightly the temperature sensitivity of the viscoelastic

response, but TTS basically holds.12, 13 In Figure 3-11(A)-(D), I show the G' and G'' master

curves after applying TTS, by choosing T0 = 25 oC for P(BA60-MMA39), T0 = 50 oC for P(BA55-

MMA44), T0 = 80 oC for P(BA50-MMA49) and T0 = 90 oC for P(BA40-MMA59) as the reference

temperatures. Only shifts in the horizontal scale were applied. Shift factors at each temperature

were extracted using the generally accepted procedure of overlaying plots of tan(δ) (G''/ G') for

data at different temperatures. The rheological response of all four samples is consistent with

what is normally found in entangled polymer melts, in particular, that G' > G'' over the portion of

the relaxation spectrum that is usually associated with the plateau regime. As observed in this

figure, good matching between curves was obtained.

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3.3 Discussion

3.3.1 Comparison between Different Experiments

Values of the apparent activation energy in the range of temperatures studied can be

obtained by plotting ln(aT) in Arrhenius fashion against the inverse of the absolute temperature,

as empty squares shown in Figure 3-12. Normally the ln(aT) vs 1/T plot is curved, but when the

data are limited over a relatively narrow range of temperatures, the plot appears linear. The

activation energy Ea for viscoelastic relaxation can be calculated from the slope of this linear

fraction of the plot. The magnitude of Ea value will increase as the measurement temperature

approaches Tg. From Figure 3-12 average values of Ea for each latex sample were calculated over

the temperature range close to energy transfer experiments. I compare these Ea values as well as

their corresponding temperature ranges with those obtained from energy transfer experiments in

Table 3-2. It can be clearly observed that the Ea values from the two different methods are in

good agreement when a similar temperature range was chosen.

Figure 3-11. Plots of master curves of G' and G'' for (A) P(BA60-MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49) and (D) P(BA40-MMA59) latex films at T0 = 25, 50, 80 and 90 oC

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In Figure 3-12, I make a direct comparison between data obtained from rheology

measurements and from diffusion experiments. I plotted both data sets in the same graph, where

shift factors obtained from rheology are shown as empty squares and those from the diffusion

experiments are shown as filled squares. The diffusion data used in Figure 3-12(A) were taken

from Figure 3-8, and then shifted vertically to compensate for the reference temperature. The full

line represents the WLF fitting, calculated using the C1 and C2 listed in Table 3-2. Figure 3-

12(B), 11(C), and 11(D) were plotted using the same process. The diffusion data and rheology

data appear to track together with changing temperature.

3.3.2 Effect of long chain branching on the time-dependence of Dapp

One of the striking features of my data is the time-dependence of Dapp. This effect is best

appreciated by examining the master curves of the shifted Dapp vs. fm in Figure 3-10. Dapp

Figure 3-12. Plots of shifted Dapp and log(aT) against the inverse of the absolute temperatures for (A) P(BA60-MMA39), (B) P(BA55-MMA44), (C) P(BA50-MMA49) and (D) P(BA40-MMA59) latex

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decreases by several orders of magnitude over the course of the experiment for each of the four

copolymers. In Figure 3-10(B)-(D), there is a sharp drop in Dapp for values of fm less than about

0.2; the high initial rate of diffusion made it impossible for me to capture the short time behavior

in the P(BA60-MMA39) sample [Figure 3-10(A)], as I could not make measurements at fm values

less than 0.2 in this sample.

My focus here is on the change in Dapp over the range in fm values from 0.2 to 0.8. For

P(BA60-MMA39), Dapp drops by about a factor of 100. For P(BA55-MMA44), there is roughly a

factor of 50-100 reduction in Dapp. There is a significant amount of scatter in the data for the

P(BA50-MMA49) master curve data at fm = 0.2; taking this into account, I find that Dapp decreases

over the relevant fm range by a factor of 10 to 50. Again, there is scatter in the data in the

P(BA40-MMA59) master curve at fm = 0.2, but I estimate an overall drop in Dapp of a factor of 5 to

10.

I rationalize the decrease of Dapp with increasing time in terms of the distribution of

diffusion coefficients of various species in the system. My colleague Dr. Jeffery Haley proposed

a simple example to illustrate how this might occur. Imagine a system consisting of two species,

one with D = 1 nm2/s and a second with D = 0.01 nm2/s. Calculations indicate the effect of a

diffusing species on the time dependence ΦET is greatly diminished once that species has

diffused over some characteristic distance in the sample. The actual distance involved for a

particular experiment will depend on details such as the latex particle sizes and the ratio of donor

labeled particles to acceptor labeled particles. Assigning a number to this distance is not so

important. The important fact is that for both species in this hypothetical sample, this distance is

the same. This means that in this example, the slower moving species will take 100 times longer

to diffuse over this characteristic distance. In an energy transfer experiment, one measures the

change in ΦET vs. time, and then convert this information into Dapp with a diffusion model.14

Over the course of an experiment in this hypothetical system, the Dapp value extracted will be

some sort of weighted average of the apparent diffusion coefficients of the two species in the

system. Early in the experiment, Dapp will be weighted more heavily towards the diffusion of the

faster moving species, as the motion of the faster moving species is largely what causes the

change in ΦET. Once the fast moving species has diffused over a characteristic distance, its

contribution to the increase in energy transfer has reached its maximum value. The experiment is

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no longer sensitive to the faster species diffusion, and the rate of ΦET increase will drop

significantly. The result of this is that the extracted Dapp value will now be weighted more

heavily towards the diffusion coefficient of the slower moving species. Of course, this drop in

Dapp is not expected to be sudden for the hypothetical system, but instead occurs gradually.

Based on this argument, I suspect that the time-dependence of Dapp between fm = 0.2 and

fm = 0.8 is mostly due to a broad distribution of diffusion coefficients for polymer chains present

in the sample. This breadth is apparently larger in P(BA60-MMA39) and P(BA55-MMA44) than it

is in P(BA50-MMA49) and P(BA40-MMA59), based on the relative magnitudes in the change of

Dapp with fm. I cannot rationalize these differences in terms of sample PDI, as no trend in the PDI

values in Table 3-1 exists. Instead, I suspect that differences in the breadth of diffusion

coefficients are due to differences in the details of molecular architecture between samples. In

particular, the presence of long chain branching can dramatically decrease the diffusion

coefficient of a polymer;15 a sample consisting of a range of branching architectures would be

expected to have a broad distribution of diffusion coefficients, and a time-dependent Dapp.

It is difficult to make quantitative determinations about the relative degree of long chain

branching for polymers studied here, but I can make some general qualitative comments based

on the rheology data in Figure 3-11. The rheological responses of P(BA60-MMA39) and P(BA55-

MMA44) are clearly quite different from P(BA50-MMA49) and P(BA40-MMA59). The most

obvious difference is the lack of any terminal regime in the P(BA60-MMA39) and P(BA55-

MMA44) data, despite the fact that the data is over a similar range of reduced frequencies. I

believe that the differences in mechanical response between samples are due to differences in

molecular architecture. Dr. Jeffery Haley, who has a lot of experience with polymer rheology,

noticed that the rheological responses of P(BA60-MMA39) and P(BA55-MMA44) are strikingly

similar to what is reported for a crosslinking polymer in the vicinity of the gel point.16 This is not

to say that P(BA60-MMA39) and P(BA55-MMA44) are in fact gels, but they likely share some

limited structural similarities. In particular, the very broad distribution of mechanical relaxation

times suggests a high degree of branching with a wide range of branch lengths. I believe that a

substantial fraction of the P(BA60-MMA39) and P(BA55-MMA44) samples are made up of

branched polymer. The data for P(BA50-MMA49) and P(BA40-MMA59) are quite different

[Figure 3-11 (C) and (D)], and show crossovers between G’ and G’’ in the interval between the

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rubbery zone and the terminal zone of the master curve. This response is typical for linear

polymers of unimodal molecular weight distributions of moderate PDI, and is anticipated by

theory.17 Based solely on the rheological response of P(BA50-MMA49) and P(BA40-MMA59), I

suspect that chains with significant degrees of long chain branching make up a relatively small

amount of the overall population of chains in the system.

It is well established that chain transfer to polymer often produces highly branched

structures in the emulsion polymerization of n-butyl acrylate.18, 19 This is consistent with what I

infer from rheology experiments, where evidence suggests that a greater degree of long chain

branching is present in P(BA60-MMA39) and P(BA55-MMA44). This accounts for the broader

distribution of diffusion coefficients in these samples that is inferred from the time dependence

of Dapp.

3.4 Summary I synthesized donor and acceptor labeled P(BA-MMA) copolymer latex particles by semi-

continuous emulsion polymerization in the presence of 0.25 wt % C12-SH. Four sets of

copolymers were prepared from various weight ratios of BA and MMA. Weight ratios of

BA:MMA:MAA for these latexes are 60:39:1, 55:44:1, 50:49:1 and 40:59:1. Their glass

transition temperatures (Tg) are 4 oC, 7 oC, 12 oC and 28 oC respectively. Donor labeled latex

samples were prepared in the presence of 1 mol % of PheMMA as the dye-containing

comonomer. Acceptor labeled latex samples were prepared in the presence of 0.3 mol % of

NBenMA. The latex particles had diameters of ca. 150 nm with narrow size distribution. FRET

experiments were used to determine the apparent polymer diffusion coefficients as a function of

temperature for each of the latex samples.

Analysis of the diffusion data gave apparent activation energy Ea values of ca. 33 kcal/mol

for P(BA60-MMA39), 39 kcal/mol for P(BA55-MMA44), 45 kcal/mol for P(BA50-MMA49) and 64

kcal/mol for P(BA40-MMA59). The temperature dependence of the polymer diffusion coefficients

closely matches the temperature dependence extracted from a master curve analysis of the

rheology data for each latex. The rheology data lead me to conclude that latex polymers with

increasing BA content have a greater degree of long-chain branching. Differences in long-chain

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branching also show up in the diffusion measurements, which indicate a larger distribution of

polymer diffusion coefficients in samples with higher BA content. These results will guide the

development of the next generation of low VOC coatings.

3.5 References

1 Paton, T. C. Paint Flow and Pigment Technology Wiley: New York, 1979. 2 Zhao, C. L.; Wang, Y.; Hruska; Z.; Winnik, M. A. Macromolecules 1990, 23, 4082-4087. 3 Oh, J. K.; Wu, J.; Winnik, M. A.; Craun, G. P.; Rademacher, J.; Farwaha, R. J. Polym. Sci.,

Part A: Polym. Chem. 2002, 40, 1594-1607. 4 Wu, J.; Tomba, J. P.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules 2004, 37,

2299-2306. 5 Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook 4th ed.; John Wiley & Sons:

1999; 2336. 6 Oh, J. K.; Wu, J.; Winnik, M. A.; Craun, Gary P.; Rademacher, J.; Farwaha, R. J. Polym. Sci.,

Part A: Polym. Chem. 2002, 40, 3001-3011. 7 Oh, J. K.; Tomba, J. P.; Ye, X.; Eley, R.; Rademacher, J.; Farwaha, R.; Winnik, M. A.

Macromolecules 2003, 36, 5804-5814. 8 Sperry, P. R.; Snyder, B. S.; O'Dowd, M. L.; Lesko, P. M. Langmuir 1994, 10, 2619-2628. 9 Ferry, J. D. Viscoelastic Properties of Polymers Wiley: New York, 1980. 10 Nemoto, N.; Landry, M. R.; Noh, I.; Yu, H. Polym. Commun. 1984, 25, 141-143. 11 Wu, J.; Oh, J. K.; Yang, J.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules

2003, 36, 8139-8147.

12 Bartels, C. R.; Buckley, C.; Graessley, W. W. Macromolecules 1984, 17, 2702-2708. 13 Carella, J. M.; Gotro, J. T.; Graessley, W. W. Macromolecules 1986, 19, 659-667. 14 Wang, Y.; Zhao, C-L.; Winnik, M. A. J. Chem. Phys. 1991, 95, 2143-2153. 15 McLeish, T. C. B. Advances In Physics 2002, 51, 1379-1527. 16 Winter, H. H.; Chambon, F. J. Rheol. 1986 30, 367-382. 17 Wasserman, S. H.; Graessley, W. W. J. Rheol. 1992 36, 543-572.

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18 Former, C.; Castro, J.; Fellows, C. M.; Tanner, R. I.; Gilbert, R. G. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 3335 - 3349.

19 Gonzalez, I.; Leiza, J. R.; Asua, J. M. Macromolecules 2006, 39, 5015 -5020.

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Chapter 4

4 Synthesis of Branched Poly(n-butyl methacrylate) via Semi-Continuous Emulsion Polymerization

4.1 Introduction Latex paints, particularly house paints, are formulated with latex particles consisting of

branched polymer. Branching occurs naturally and not intentionally through the choice of

monomers used in the synthesis by emulsion polymerization. Coatings referred to as “all-

acrylate” are typically copolymers of n-butyl acrylate (BA) and methyl methacrylate (MMA),

whereas the remainder of the market is dominated by latex consisting of copolymers of n-butyl

acrylate and vinyl acetate (VAc). In the emulsion copolymerization of these monomers, the n-

butyl acrylate and vinyl acetate propagating radicals have a strong propensity to abstract

hydrogens from the polymer backbone, both intramolecularly and intermolecularly, leading

respectively to short- and long-chain branches. Lovell’s group1-3 in particular has presented

careful studies of these chain-transfer-to-polymer reactions, quantifying the mean number of

branch sites per polymer. From the understanding that has been developed, one would predict

that the extent of branching in P(BA-MMA) copolymers would increase as the fraction of BA in

the monomer mixture was increased. Nevertheless, it remains very difficult to determine the

number of long chain branches. With the addition of a chain transfer reagent to the reaction

mixture, some control is possible over the number average molecular weight (Mn). Even with our

current understanding of the factors that affect the formation4-6 and properties7, 8 of latex films,

we have a little knowledge of the influence of chain branching on these properties. When

grafting competes with polymerization, as in the reactions described above, even characterizing

the polymer and distinguishing long-chain from short-chain branches becomes a challenge. 9-13

Long chain branches, particularly if they participate in entanglements, would be expected to have

a significant influence on polymer diffusion in latex films and in the polymer rheology.

Extensive branching should lead to structures resembling hyperbranched polymers, which have

some properties similar to dendrimers. This chapter is based on the hypothesis that one can begin

to learn about the importance of branching in latex polymers through the study of polymers in

which there is greater control over the molecular weight and extent of branching.

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Many methods have been developed for the synthesis of branched polymers. Gauthier

used successive steps of anionic polymerization and chloromethylation to synthesize a series of

hyperbranched polystyrenes. 14 - 17 D. M. Knauss developed a convergent living anionic

polymerization method to produce polystyrene (PS) with dendritic branching.18 A series of long-

branched PS of various architectures were prepared by anionic polymerization using a

multifunctional initiator. 19 Fréchet and his co-workers synthesized hyperbranched polymers

using self-condensing vinyl polymerization (SCVP).20, 21 , 22 Others have used group transfer

polymerization 23 and controlled radical polymerization (e.g., atom transfer radical

polymerization (ATRP)) 24 , 25 for the preparation of hyperbranched polymers. For example,

Steven P. Armes prepared branched polymers via ATRP and reversible addition fragmentation

chain transfer (RAFT) polymerization.26, 27 Even though well-defined hyperbranched structures

could be produced, none of these methods can be applied generally.

Recently Sherrington’s group reported a facile and broadly applicable method for

producing branched polymers from vinyl monomers. 28 - 33 In this approach, they carry out

conventional free radical polymerization of the vinyl monomer in the presence of both a cross-

linking agent (to generate branches) and a chain transfer agent (to prevent gel formation). With

the correct balance of these two reagents, soluble branched polymer can be obtained. For

example, they polymerized methyl methacrylate (MMA) in the presence of but-2-ene-1,4-

diacrylate (BDA, a bifunctional comonomer). To inhibit gelation, they added 1-dodecanethiol

(C12-SH) as a chain transfer agent. They found that a high yield (77–97%) of soluble polymer

was produced when relatively low levels (<2 mol %) of BDA were employed together with a

similar or a higher level of C12-SH.28 The effects of various branching agents on polymer

architecture and properties were also studied. In this way, they synthesized a variety of soluble,

branched copolymer architectures using polyacrylate branching agents containing between two

and six acrylate functional groups.31

More interestingly, the approach was applied to the synthesis of highly branched PMMA

in aqueous emulsion polymerization.28 Different ratios of cross-linker and chain transfer agent

were added into a series of batch emulsion polymerization reactions. When proper amount of

chain transfer agent was used, highly branched PMMAs were produced without cross-linking.

The polymers obtained had relatively low molecular weights (Mn < 3,300 g/mol) and broad

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molecular weight distributions (Mw/Mn > 10). Despite these limitations, this new strategy

provides a facile one-step method for making highly branched polymers confined to colloidal

nanoparticles.

I was interested in exploring Sherrington’s emulsion polymerization approach. In this

chapter I describe experiments in which I applied this method to the polymerization of n-butyl

methacrylate (BMA). Instead of batch emulsion polymerization, where problems can arise

because of unfavorable reactivity ratios or chain transfer constants, I used semi-continuous

emulsion polymerization, which normally provides better control over molecular weight and

molecular weight distribution. My primary goal in this first set of experiments was to see if semi-

continuous emulsion polymerization extended the range of accessible molecular weights, and

offered better control over molecular weight and its distribution. Then I was interested in how

polymer properties (such as the glass transition temperature, and linear rheological properties)

varied with the extent of branching. I found that the molecular weights of the branched PBMA

that I obtained were dramatically higher than the PMMAs made in batch emulsion by the

Sherrington group, while the molecular weight distributions were much narrower.

4.2 Experimental Section

4.2.1 Latex Preparation

Latex samples were synthesized by semi-continuous emulsion polymerization reactions. A

typical recipe for the synthesis of branched PBMA is shown in Table 2-2. A monomer pre-

emulsion was prepared by shaking a mixture of monomer, branching agent, surfactant, chain

transfer agent, and water for 30 min. In the first stage, a dispersion of seed particles was

prepared by batch emulsion polymerization. Water (3.0 g), Me-β-CD (0.02 g) and SDS (0.03 g)

were added in a 100 mL 3-neck flask equipped with a mechanical stirrer, nitrogen inlet and

condenser. The flask was immersed in an oil bath. The system was thoroughly purged with

nitrogen while the reaction mixture was heated to 80 oC. After the reactor temperature stabilized

at 80 oC, the KPS solution (0.06 g in water 0.5 g) as an initiator and the Na2CO3 solution (0.05 g

in water 0.5 g) as a pH buffer were added into the reactor followed by the addition of 3 wt % of

the monomer pre-emulsion. The mixture was stirred for 20 min at 80 oC. In the second stage of

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polymerization, the remaining monomer pre-emulsion was fed into the seed latex dispersion

together with an initiator aqueous solution (0.01 g in water 2.0 g) in 4 h. The feeding rates were

kept identical, controlled by Fluid Metering QG50 pumps. After the addition was completed, the

system was maintained at 80 oC for 0.5 h. Then the reaction was cooled to room temperature.

4.2.2 Synthesis of High Molecular Weight Linear PBMA

High molecular weight linear PBMA was synthesized by miniemulsion polymerization

using SDS/hexadecane as surfactant/co-stabilizer and 2,2′-Azobis(2-methylpropionitrile) (AIBN)

as initiator. A mixture of BMA, SDS, hexadecane, AIBN, and water was emulsified using a

Branson Models250 digital sonifier (40% amplitude) at 0 oC for 30 min. The reaction was carried

out at 80 °C for 5 h. Water was evaporated and the polymer was dried in a vacuum oven for 24 h

at 40 °C. The dry polymer was fractionated using hexane/ethanol (v:v = 3:1). Then the high

molecular weight fraction was collected. GPC and rheology measurement were carried out on

this sample.

4.2.3 Characterization of Latex Polymers

Differential Refractometer (dn/dc). The refractive index increment (dn/dc) was measured

using a Brookhaven Instruments model BI-DNDC differential refractometer at 35 oC. For each

polymer sample, five concentrations were used. △n was plotted against concentration to give

dn/dc. The obtained dn/dc values were listed in Table 4-1.

Triple Detector Array Gel Permeation Chromatography (TDA/GPC). Polymer molecular

weights and molecular weight distributions were measured by gel permeation chromatography

(GPC) using a Viscotek liquid chromatograph equipped with a Viscotek model 2501 UV

detector and a Viscotek TDA302 triple detector array (TDA). Two Viscotek GMHHR Mixed

Bed columns were used with tetrahydrofuran (THF) as the elution solvent at a flow rate of 0.6

mL/min. The GPC column oven temperature was 35 oC and the injection volume was 0.1 mL.

The absolute molecular weight was calculated using the appropriate dn/dc value. All molecular

weight data are listed in Table 4-1.

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4.3 Results

4.3.1 Synthesis of Branched PBMAs

There were three objectives in this work. First, I wanted to prepare “high molecular

weight” branched polymers, which here refers to a number-average molecular weight (Mn) of at

least several tens of thousands g/mol. Second, I wished to control the extent of branching in these

high Mn branched polymers, which means the extent of branching in the final product could be

tuned by adjusting the recipe for latex synthesis. Finally, I hoped to achieve the above two

objectives in a straight-forward one-pot reaction.

With the three goals in mind, I carried out the synthesis of n-butyl methacrylate (BMA)

copolymers by semi-continuous emulsion polymerization using bisphenol A dimethacrylate

(BPDM) as the branching agent and 1-dodecanethiol (C12-SH) as the chain transfer agent. A

typical recipe is presented in Table 2-2. I chose BPDM as the branching agent. While the

reactivity ratios for the reaction with BMA are not known, and these will be influenced by the

relative solubility of the two monomers in the aqueous medium, I assume that I can overcome

any problems with the reactivity ratio differences by running the reactions under monomer-

starved conditions. The aromatic rings of BPDM provide UV and NMR signatures for analyzing

its incorporation into the polymer. C12-SH is an efficient chain transfer agent to suppress gel

formation in semi-continuous emulsion polymerizations of acrylate and methacrylate

monomers. 34 To ensure good transport of these reactants, particularly C12-SH, through the

aqueous phase during the synthesis, methyl-β-cyclodextrin (Me-β-CD) was included in the

reaction mixture.35 While many semi-continuous emulsion polymerization reactions are carried

out in the presence of pre-formed seed particles, I wished to avoid the contribution of even small

amounts of high molecular weight linear polymer typical of PBMA seed particles. Therefore, I

generated seed particles in situ using a small fraction (typically 3 wt %) of the total feed of

monomer pre-emulsion in a batch reaction. The pre-emulsion contained the monomer mixture,

water and additional surfactant. The remaining pre-emulsion was then fed continuously into the

reaction over 5 h. All of the potassium persulfate (KPS) initiator was added in the first stage.36

Samples of latex produced from the above process were dried in an oven overnight. The

dry polymers were then used for molecular weight measurements using a TDA/GPC. Values of

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Figure 4-1. 1H NMR spectra of PBMAs with different branching levels. CD2Cl2 was used as solvent. Peaks a and peaks b correspond to protons of BPDM and BMA respectively.

dn/dc were determined independently using a differential refractometer. As shown in Table 4-1,

the dn/dc values increase as the fraction of BPDM in the polymer increases. Without BPDM,

linear-PBMA1 has a dn/dc value of 0.067. The dn/dc value becomes 0.079 for branched-PBMA2

which contains 1 mol % of BPDM (relative to BMA). However, there is only a small increase of

dn/dc when the BPDM fraction was increased to 10 mol % (for branched-PBMA7, dn/dc =

0.084). The calculated molecular weights are presented in Table 4-1. For most samples shown in

Table 4-1, Mn is greater than 40,000 g/mol, and Mw is greater than 100,000 g/mol. Among all the

branched polymers, there is one sample with a much larger Mn than all the others (for branched-

PBMA1, Mn = 160,000 g/mol). Thus the first goal, synthesis of “high molecular weight”

branched polymers, was achieved. Moreover Mw/Mn is less than 2.8 for most cases, which

indicates that the molecular weight distribution is relatively narrow. Another feature can also be

observed from the molecular weight data, which is that the molecular weights are comparable for

the linear PBMA1 sample and the branched-PBMA2–8 samples. This feature demonstrates the

feasibility of controlling the molecular weight of branched polymers by tuning the ratio of

branching agent and chain transfer agent.

In addition to obtaining “high molecular weight” branched polymers, controlling the

extent of branching and preventing gel formation at the same time was also a challenge.

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Branching control was realized by adjusting the feed ratio of BMA/BPDM, while gel formation

was avoided by feeding appropriate amount of C12-SH. As presented in Table 4-1, four molar

feed ratios of BMA/BPDM were investigated, in which the BPDM content ranged from 0 mol %

to 10 mol % (relative to BMA). After polymerization, the BPDM content in each polymer was

determined from the 1H NMR spectrum (Figure 4-1) and the results are listed in Table 4-1. One

can observe that all experimental ratios are similar to the corresponding feed ratios, even for

samples with the highest BPDM concentration (branched-PBMA6–8). Thus the extent of

branching could be varied by modifying the feed ratio of BMA/BPDM. No gelation was detected

in any of the latex. Gelation could be suppressed when an appropriate amount of C12-SH was fed

together with the branching agent.

4.3.2 Architectures of Branched PBMAs

I deduced the architectures of the branched PBMA chains from a combination of

TDA/GPC traces and 1H NMR spectra. From the TDA/GPC data and dn/dc values, I calculated

Mn values and polydispersities of the samples. A GPC trace for branched-PBMA2 is shown in

Figure 4-2. Here one sees that the RI trace and UV trace (due to BPDM groups) overlap very

well. This result suggests that the BPDM groups are relatively uniformly distributed over the

polymer chains of different molecular weight. The absence of a UV peak at low mass indicates

that all of the BPDM monomer was incorporated into the polymer. I used the Mn values and the

BMA/BPDM ratios to calculate the average number of BMA units (NBMA) and BPDM units

(NBPDM) per polymer chain. The BPDM units divide the polymer chain into a number of “parts”.

The network functionality (f ) of BPDM is 4; thus (f -1)=3 “parts” are added to the chain for each

BPDM unit. The average number of “parts” per chain (X) was calculated with the equation

BPDMNX ×+= 31 (4-1)

and the average number of BMA units per “part” (nBMA) is given by

XNn BMABMA /= (4-2)

The calculated results of four representative samples containing different BMA/BPDM

ratios are shown in Table 4-2. All samples have a similar average number of BMA units per

chain, whereas X values increase significantly with NBPDM. As a consequence, nBMA drops as X

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Figure 4-2. UV and RI traces in the GPC analysis of branched-PBMA2

increases. For example, the most highly branched sample branched-PBMA7 contains an average

of only 3 BMA units between branch points. In contrast, linear-PBMA1 has no branch points.

The information in Table 4-2 was used to generate the idealized graphical depiction of the chain

architectures shown in Figure 4-3. As a linear chain, linear-PBMA1 adopts a random coil

structure (Figure 4-3A). Containing only four branch points per chain, branched-PBMA2

maintains a loose random-coil-like as depicted in Figure 4-3B. This description of the polymer

shape in solution is consistent with the hydrodynamic Rh values acquired by dynamic light

scattering (see Table 4-1). For the more branched PBMAs of similar molecular weight, the chain

dimensions in solution become more compact. Branched-PBMA7, the most highly branched

polymer chain, is divided by an average of 34 branch points per chain into more than 100 parts,

which makes the chain in solution act like a dense sphere (Rh = 2.1 nm, see Table 4-1 and Figure

4-3D). The decreasing Rh with increasing degree of branching was also consistent the GPC

elution sequence which can be observed from the GPC traces and a plot of log Mn versus

retention volume of the four PBMAs presented in Figure 4-4.

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Figure 4-4. GPC traces (A) RI signal and (B) log Mn vs retention volume for linear-PBMA1 (L1), branched-PBMA2 (B2), branched-PBMA5 (B5) and branched-PBMA7 (B7). The vertical line in (B) indicates that the polymer with Mn = 34,000 has a retention volume of 17.3 mL. The vertical line in (A) indicates that ca. 30% of the L1 sample has Mn lower than 34 000

Figure 4-3. Polymer architecture for (A) linear-PBMA1, (B) branched-PBMA2, (C) branched-PBMA5 and (D) branched-PBMA7. These drawings assume a uniform distribution of branch points in the polymer molecules.

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Table 4-2. Estimate of branching

a the average number of BMA units per chain b the average number of BPDM units per chain c the average number of “parts” divided by BPDM per chain d the average number of BMA units between two branching points

4.3.3 Branched PBMA Latex Particles

The synthesis methodology reported here offers control over branching without loss of

control over particle size and size distribution. Particle size was measured by dynamic light

scattering using a BI-90 particle sizer, and the results are shown in Table 4-1. The average

diameters for all samples were in the range of 100 to 150 nm. These are typical values for semi-

continuous emulsion polymerization.

4.3.4 Rheology Measurements

Viscoelastic measurements were carried out on the four representative samples listed in

Table 4-2. The storage modulus (G’) and loss modulus (G”) were measured as a function of

frequency (ω) at a series of temperatures ranging from 40 to 140 oC. In previous experiments, I

showed that the time-temperature superposition (TTS) principle could be used to construct

master curves for poly(n-butyl acrylate-co-methyl methacrylate) polymers with various degrees

of branching.34 Here I used the TTS principle to obtain shift factors (aT) of the temperature

dependence of individual log G’ and log G” vs. log ω plots. The G' and G'' master curves

obtained, for T0 = 40 oC as the reference temperature, are shown in Figure 4-5. The rheological

Latex samples NBMAa NBPDM

b Xc nBMA d Tg (°C)

L1, linear-PBMA1 392 0 1 392 26

B2, branched-PBMA2 401 4 13 31 21

B5, branched-PBMA5 393 27 82 5 8

B7, branched-PBMA7 344 34 103 3 2

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log

G' o

r log

G"

(Pa) D

logω (rad/s)

log

G' &

log

G"(

Pa)

C

logω (rad/s)

0

2

4

6

-5 -4 -3 -2 -1 0

log

G' &

log

G"

(Pa)

G''

G'

A

logω (rad/s)

1

3

5

7

-6 -5 -4 -3 -2 -1

log

G' &

log

G"

(Pa)

G''

G'

B

logω (rad/s)

1

2

3

4

5

6

7

-3 -2 -1 0 1 2

G''

G'

1234567

-5 -4 -3 -2 -1 0 1

G''

G'

log

G' o

r log

G"

(Pa) D

logω (rad/s)

log

G' &

log

G"(

Pa)

C

logω (rad/s)

0

2

4

6

-5 -4 -3 -2 -1 0

log

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log

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A

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2

3

4

5

6

7

-3 -2 -1 0 1 2

G''

G'

1234567

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G''

G'

Figure 4-5. Plots of master curves of G' and G'' for (A) linear-PBMA1, (B) branched-PBMA2, (C) branched-PBMA5 and (D) branched-PBMA7.

response of the four samples shows that G'' > G' over the entire frequency range. These plots do

not show any crossovers of the G' and G'' curves in the interval between the high frequency end

of the rubbery zone and the terminal zone. For comparison, I examined the corresponding

behavior of a high molecular weight linear PBMA sample (Mn = 790,000, Mw/Mn = 1.2). These

data are presented below in the Discussion section.

4.4 Discussion

4.4.1 Control of Molecular Weight

Controlling molecular weight and polydispersity are an essential objectives in polymer

synthesis. In recent years, there have been many reports describing the synthesis of branched

polyacrylates and polyacetates by radical polymerization using the combination of a branching

agent and a chain transfer agent. To date, none of them showed the ability to control the

molecular weight of branched polymers. In most cases, the branched polymers have low Mn and

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very broad molecular weight distributions.28-33 Our study shows that in semi-continuous

emulsion polymerization, one can achieve reasonable control over Mn and narrower

polydispersities than reported previously. The key factors that provide molecular weight control

in the synthesis of branched PBMA are summarized in the following paragraph.

Literature results indicate that if the chain transfer agent fraction is less than a critical

level, gel formation will occur. 28-33 It is also well known that too much CTA will result in

oligomers as well as broad polydispersities. Thus it is necessary to determine the appropriate

concentration of CTA. Because each BPDM will add two more chain ends to the backbone, at

most two C12-SH molecules are needed to cap the chain ends to prevent cross-linking. Hence, the

C12-SH concentrations used here are in the range of 1.5 to 2.2 equivalents relative to BPDM, and

these compositions gave fairly high molecular weight products without any detectable gel.

Maintaining the monomer-starved condition in the second stage of all semi-continuous

emulsion polymerizations is the second consideration. The key feature of monomer-starved

condition is that almost all of the monomers are consumed within a short time after being added

into the reactor. In our reactions, the result is that monomer and branching agent always

polymerized at the feed ratio. It was impossible for BPDM to accumulate. The presence of a

suitable amount of C12-SH, which was fed with the monomers, prevented cross-linking. The

resulting polymer chains had a similar composition and length.

The last point is the presence of the phase transfer agent, Me-β-CD, which guaranteed

transport of monomers and C12-SH from the monomer droplets to reaction loci despite the

difference in their water solubility. In fact, effective transport of reactants is a prerequisite to

maintaining monomer-starved conditions, since monomers with poor water solubility will

become concentrated in the monomer droplets as the reaction proceeds instead of reaching the

growing polymer particles.

4.4.2 Entanglement Considerations in branched PBMA

In this section, I consider the potential role of entanglements in the rheology behavior of

the linear and branched PBMA samples. The G’ and G” master curves shown in Figure 4-5A for

the linear PBMA sample do not indicate the influence of entanglements. To understand this

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Figure 4-6. Plots of master curves of G' and G'' for the high molecular weight linear PBMA sample. The vertical dotted line corresponds to the minimum value of G”.

result, I begin by estimating the critical molecular weight for entanglement, Me, of PBMA. I

synthesized a high molecular weight sample of linear PBMA by miniemulsion polymerization.

As a form of bulk polymerization, miniemulsion polymerization commonly leads to high

molecular weight polymer. I then fractionated the polymer to obtain a sample with Mn = 790,000,

Mw/Mn = 1.2. Master curves Figure 4-6) were constructed from obtained G' and G'' values

measured as a function of frequency at a series of temperatures. These have the properties

expected for entangled chains: G' is greater than G'' in the rubbery zone, and I expect that the

two curves will cross over in the terminal zone if I were to extend the measurement time and

temperature. Since there is not a clear plateau on the G' master curve, I followed a suggestion by

my colleague Dr. Jeffery Haley and estimated the plateau modulus GNo as the value of G’

corresponding to the frequency (log ω) of the minimum in G”. From the expression

e o

N

RTMGρ

= (4-3)

where ρ is the polymer density, T is the absolute temperature, and R is the gas constant, I

calculate that Me for PBMA is approximately 34,000 g/mol.

Using this value, I can begin to understand the extent to which entanglements play a role

in the rheology of the samples. Taking linear-PBMA1 as an example, the TDA/GPC curve in

Figures 3-4(A) and 3-4(B) indicates that ca. 30% of the linear-PBMA1 sample by mass has a

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molecular weight that is less than 34,000 g/mol. This 30% value represents a lower bound of the

amount of material in the sample that is below 34,000 g/mol, as the TDA/GPC analysis is known

to underestimate the polydispersity by undercounting the lower molecular weight components of

a sample.

Given that 30% of the sample is unentangled, I can treat this 30% as a viscous theta

solvent for the remaining 70% of the sample that has a molecular weight greater than 34,000

g/mol. Now, I need to consider how this “solvent” dilutes the entanglement network of the

remaining high molecular weight polymer. The spacing between polymer entanglements in

polymer melts and solutions follows a well established scaling relation37

αφ −~2a (4-4)

where a is the entanglement spacing (tube diameter), φ is the volume fraction of polymer with a

molecular weight greater than Mc, and α is a scaling exponent that ranges between 1.0 and 1.3.37

If I assume that the number of entanglement lengths required to produce an entangled polymer is

fixed for PBMA (i.e., it is not a function of φ ), then I can derive a scaling law for the effective

critical molecular weight for entanglement in a polydisperse melt, Mc,eff. This scaling law is

αφ −~,effcM (4-5)

From this relation, I estimate Mc,eff ≈ 150,000 g/mol by assuming α = 1. In other words, I

believe that only polymers that are above 150,000 g/mol in the polydisperse sample are fully

entangled with each other. Only about 10% by weight of the linear-PBMA1 sample has a

molecular weight greater than 150,000 g/mol. This relatively small fraction of the sample is

probably very hard to detect in our G’ measurements. For such a loosely entangled network, the

terminal relaxation time of the entangled fraction may not be well enough separated from the

terminal relaxation time of the unentangled fraction to show up in the experiment. For the same

reason, the branched PBMA samples are unentangled as well.

4.5 Summary I reported the synthesis of latex particles comprised of high molecular weight PBMA with

various degree of branching using semi-continuous emulsion polymerization method. The most

important contribution of this work is the successful control of molecular weight and molecular

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weight distribution. Also, all branched polymers are free of gel. Latex particles composed of the

branched PBMAs are monodisperse in size. Polymer chain structures were estimated based on

the molecular weight and the 1H NMR data. Rheology measurements showed that the polymers

were unentangled despite they are linear or branched. The effect of branching on polymer

diffusion rate will be discussed in Chapter 5.

4.6 Reference

1 Ahmad, N. M.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2822-2827. 2 Britton, D.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2828-2837. 3 Heatley, F.; Lovell, P. A.; Yamashita, T. Macromolecules 2001, 34, 7636-7641. 4 Wu, J.; Tomba, J. P.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules 2004, 37,

4247-4253. 5 Farinha, J. P. S.; Wu, J.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules 2005,

38, 4393-4402. 6 Oh, J. K.; Yang, J.; Rademacher, J.; Farwaha, R.; Winnnik, M. A. Macomolecules 2003, 36,

8836-8845. 7 Kim, H.-B.; Wang, Y.; Winnik, M. A. Polymer 1994, 35, 1779-1786. 8 M. A. Winnik, “The Formation and Properties of Latex Films,” in Emulsion Polymeriza-tion

and Emulsion Polymers, M. El-Aasser, P. Lovell, eds., 1997, Chp.14, 468-517. 9 Ahmad, N. M.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2822-2827. 10 Britton, D.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2828-2837. 11 Plessis, C.; Arzamendi, G.; Leiza, J. R.; Schoonbrood, H. A. S.; Charmot, D.; Asua, J. M.

Macromolecules 2000, 33, 5041-5047. 12 Plessis, C.; Arzamendi, G.; Leiza, J. R.; Schoonbrood, H. A. S.; Charmot, D.; Asua, J. M.

Macromolecules 2001, 34, 5147-5157. 13 Plessis, C.; Arzamendi, G.; Alberdi, J.M.; Van Herk, A.M.; Leiza, J.R.; Asua, J.M.

Macromolecular Rapid Communications 2003, 24, 173-177. 14 Gauthier, M.; Moller, M. Macromolecules 1991, 24, 4548-4553. 15 Gauthier, M.; Li, W. ; Tichagwa, L. Polymer 1997, 38, 6363-6370. 16 Li, J.; Gauthier, M. Macromolecules 2001, 34, 8918-8924. 17 Yuan, Z.; Gauthier, M. Macromolecules 2006, 39, 2063-2071.

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18 Knauss, D. M.; Al-Muallem, H. A.; Huang, T.; Wu, D. T. Macromolecules 2000, 33, 3557-3568.

19 Lee, J. S.; Quirk, R. P.; Foster, M. D. Macromolecules 2005, 38, 5381-5392. 20 Fréchet JMJ, Henmi M, Gitsov I, Aoshima S, Leduc MR, Grubbs RB. Science 1995, 269,

1080-1083. 21 Hawker CJ, Fréchet JMJ, Grubbs RB, Dao J. J. Am. Chem. Soc. 1995, 117, 10763-10764. 22 Weimer MW, Fréchet JMJ, Gitsov I. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 955-970. 23 Simon PFW, Radke W, Muller AHE. Macromol Rapid Chem 1997, 18, 865-873. 24 Matyjasewski K, Gaynor SG, Muller AHE. Macromolecules 1997, 30, 7034-7041. 25 Matyjasewski K, Gaynor SG. Macromolecules 1997, 30, 7042-7049. 26 Bannister, I.; Billingham, N. C.; Armes, S. P.; Rannard, S. P.; Findlay, P. Macromolecules

2006, 39, 7483-7492. 27 Vo, C.-D.; Rosselgong, J.; Armes, S. P.; Billingham, N. C. Macromolecules 2007, 40, 7119-

7125. 28 O’Brien, N.; McKee, A.; Sherrington, D. C. Polym. Commun. 2000, 41, 6027-6031. 29 Costello, P. A.; Martin, I. K.; Slark, A. T.; Sherrington, D. C.; Titterton, A. Polymer 2002, 43,

245-254. 30 Isaure, F.; Cormack, P. A. G.; Sherrington, D. C. J. Mater. Chem. 2003, 13, 2701-2710. 31 Slark, A. T.; Sherrington, D. C.; Titterton, A.; Martin, I. K. J. Mater. Chem. 2003, 13, 2711-

2720. 32 Isaure, F.; Cormack, P. A. G.; Sherrington, D. C. Macromolecules 2004, 37, 2096-2105. 33 Baudry, R.; Sherrington, D. C. Macromolecules 2006, 39, 1455-1460. 34 Liu, Y.; Haley, J. C.; Deng, K.; Lau, W.; Winnik, M. A. Macromolecules 2007, 40, 6422-6431. 35 Lau, W. “Method for forming polymers,” U. S. Patent 5,760,129, June 2, 1998. 36 Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics Oxford Science Publications:

Oxford, 1986. 37 Watanabe, H. Prog. Polym. Sci. 2000, 24, 1253-1403.

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Chapter 5

5 Effect of Branching on Polymer Diffusion in Branched Poly(n-butyl methacrylate) Latex Films

5.1 Introduction When a dispersion of soft latex particles is applied to a substrate and allowed to dry, it

forms a transparent film. This process occurs through three major steps: (i) drying, which brings

the particles into contact; (ii) deformation, in which the particles are transformed into space-

filling polyhedral cells; and (iii) polymer diffusion. The diffusion of polymer molecules across

the interfaces between adjacent cells is the step that builds the mechanical properties of the film.

1-13 Since branching in general, and long chain branches in particular, should have an important

influence on the polymer diffusion step, it is important that I develop a better understanding of

the role of branching in the formation of mechanically coherent films. In a previous study,14 I

examined the rates of polymer diffusion in a series of P(BA-MMA) copolymer latex films and

compared the diffusion rates with the linear viscoelastic properties of the film. The results

pointed to an increase of branching with increasing BA fraction in the copolymer affecting both

the diffusion and rheological properties of the polymer. For example, I found that a higher BA

content led to a broader distribution of apparent diffusion coefficients. Since it was not possible

to quantify the extent of long-chain branching in those polymers, I began to think about ways in

which one could vary the extent of branching in a systematic way in a model latex polymer to

study the influence of branching on the polymer diffusion.

As a step in this direction, I have modified the methodology developed by Sherrington and

co-workers 15-20 (the “Strathclyde approach”) in Chapter 4 to synthesize high molar mass gel-free

branched latex polymers by semi-continuous emulsion polymerization under monomer starved

conditions. In this approach, one initiates the reaction under batch conditions using a small part

of the monomer mixture, and then adds the remaining monomer at a sufficiently slow rate that

the monomer is consumed essentially as fast as it is added. As our monomer, I employed n-butyl

methacrylate (BMA). PBMA is a film-forming latex. The polymer produced under these reaction

conditions is linear, and the high molecular weight polymer has a glass transition temperature

close to 30 °C. My experiments employed bis-phenol A dimethacrylate (BPDM) as the cross-

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65

linking agent and C12-SH as the chain transfer agent. I obtained latex particles uniform in size

consisting of polymer with Mn values on the order of 50,000 and Mw/Mn = 2. Depending upon

the amounts of BPDM and C12-SH used, the polymers obtained ranged from linear or lightly

branched to highly branched.21

In this chapter I show that this methodology is tolerant of dye-containing comonomers. I

was able to synthesize D- and A-labeled PBMA latex particles in which the degree of branching

could be controlled independently, while maintaining similar average molecular weights.

Through direct non-radiative energy transfer experiments on films formed from these latices, I

was able to monitor rates of polymer diffusion as a function of the degree of branching. These

experiments lead to the conclusion that for this set of samples, the polymer with the highest

degree of branching has the highest diffusivity. I discuss the possible reasons for this result in the

sections below.

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66

Table 5-1. Typical recipe for the synthesis of D-labeled branched PBMA latex.

ingredients (g) first stage second stage

H2O 3.0

SDS 0.030

Me-β-CD 0.02

Na2CO3 0.05

KPS 0.06 0.01

monomer pre-emulsion 0.52 16.71

H2O 10.0

SDS 0.045

BMA 4.60 / 32 mmol / 64 wt %

BPDM 1.18 / 3.2 mmol / 16 wt %

PheMMA 0.090 / 1 mol %

C12-SH 1.31 / 6.4 mmol / 18 wt %

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68

5.2 Results and Discussion

5.2.1 Synthesis of Dye-Labeled Branched PBMA Latex Particles

The protocol employed here for the synthesis of dye-labeled latex particles follows the

strategy which I described in Chapter 4. BPDM served as the branching agent, and C12-SH was

used as the chain transfer agent. By generating seed particles in situ using a small fraction

(typically 3 wt %) of the monomer pre-emulsion in a batch reaction, I avoided the contribution of

small amounts of high molecular weight linear polymer typical of PBMA seed particles. The in-

situ seeding process also helped to achieve a homogeneous distribution of the dye comonomer in

latex particles. A typical recipe for the synthesis of Phe-labeled branched PBMA latex is

presented in Table 5-1. I added 1 mol % (based on total monomer) of the fluorescent donor dye

comonomer (PheMMA) into the monomer pre-emulsion. To prepare A-labeled PBMA latex, 0.3

mol % of NBenMA was used instead of PheMMA. A phase transfer agent (Me-β-CD) was also

added to enhance the transport of C12-SH and BPDM from the monomer droplets through the

aqueous phase to the growing particles during the polymerization.22

By varying the concentration of BPDM, I synthesized matched pairs of D- and A- labeled

latex particles with different degrees of branching. In our notation, “LR” refers to linear polymer,

synthesized without BPDM. The “low-branched” (LB) polymer was synthesized with 1 mol %

BPDM (based upon total monomer); the “medium-branched” MB polymer with 5 mol % BPDM;

and the “highly branched” (HB) polymer with 10 mol % BPDM. The type of fluorescent dye is

indicated by a superscript “D” or “A”, which represents “donor” or “acceptor”, respectively. For

example, the Phe-labeled middle-branched PBMA sample is written as MB-PBMAD.

To obtain meaningful absolute molecular weights, I carried out gel permeation

chromatography (GPC) measurements using an instrument equipped with triple detector array

(TDA/GPC) in conjunction with dn/dc values determined independently by differential

refractometry. I observed that the values of dn/dc increased as the fraction of BPDM in the

polymer increased (Table 5-2), but were independent of the type of labeled dye. The data in

Table 5-2 show three important features. First, all samples have relatively high molecular

weights. Mn values are greater than 39,000 g/mol, and Mw values are greater than 67,000 g/mol.

Second, the molecular weight distribution is narrower than that typically obtained by traditional

semi-continuous emulsion polymerization. Calculated values of Mw/Mn are less than 1.9. While

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69

the light scattering detector provides reliable values of Mw, it can underestimate Mn. Thus I

consider these Mw/Mn values to be indistinguishable from 2.0. Third, the molecular weights are

similar for all samples independent of the degree of branching and of the type of dye comonomer.

These results demonstrate our ability to control the molecular weight and to label the branched

polymers.

The degree of branching was controlled by tuning the feed ratio of BMA/BPDM. The

mole ratios of BMA/BPDM incorporated into the latex polymers were calculated from 1H NMR

spectra (Table 5-2) as described in Chapter 4.3.2. The experimental values are very close to the

feed ratios for all degrees of branching. Combining the BMA/BPDM ratios with the Mn values, I

estimated the chain structures of the dye-labeled branched PBMA samples (Table 5-3). NBMA and

NBPDM represent the average number of BMA units and BPDM units per polymer chain. Since

each BPDM unit introduces two more end groups into the chain, the average number of end

groups per chain (Nend) is given by the equation

Nend = 2 + 2×NBPDM (5-1)

In addition each BPDM divides the chain into three segments between branch or end

points. This average number of BMA units between two adjacent branch points (nBMA) is given

by the expression:

nBMA = NBMA/(1 + 3×NBPDM) (5-2)

The calculated average values of NBMA, NBPDM, Nend, and nBMA are collected in Table 5-3.

One can see that the Nend values increase significantly with NBPDM, as expected.

The results also show that the glass transition temperature (Tg) decreases as Nend increases.

For example, LR-PBMAD, which is linear, has a Tg value of 24 oC. In contrast, the Tg of HB-

PBMAD is only 2 oC. This decrease in Tg can be understood from the increment in free volume

associated with the higher average number of end groups per chain and from the larger content of

oligomer in this sample. There is an indication of this oligomer content in the GPC elution

profile shown in Figure 5-1. Increased branching at constant molar mass led to a more compact

polymer structure in solution and to a pronounced decrease in the glass transition temperature in

the bulk state. Information about polymer dimensions in solution is available from Rg values

determined from the GPC light scattering detectors (Table 5-2) and from the GPC elution

sequence as shown in Figure 5-1.

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70

Figure 5-1. GPC traces (RI) for (A) A- and (B) D-labeled PBMA samples.

These results show that by the semi-continuous emulsion polymerization, I was able to

control the extent of branching without loss of control over particle size and size distribution. As

shown in Table 5-2, the average diameters for all the latex particles were in the range of 100 to

140 nm. The size distributions are very narrow in all cases. The other feature of semi-continuous

emulsion polymerization under monomer starved conditions is that it ensures the random

distribution of dye comonomers in the polymer chains. Evidence for the uniform dye distribution

is provided by the GPC traces in Figure 5-2, where the RI detector monitors elution of the

polymer, whereas the UV signal responses to the presence of the chromophore. The close

overlap of the UV and RI traces indicates a close to random distribution of the chromophores in

the polymer chains.

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71

Figure 5-2. GPC traces of (A) LB-PBMAD and (B) LB-PBMAA. The UV and RI traces overlap, which indicates a nearly random distribution of dye comonomers in the polymer chains.

Page 89: Polymer Diffusion in Latex Films€¦ · Polymer Diffusion in Latex Films Yuanqin Liu Doctor of Philosophy Department of Chemistry University of Toronto 2009 Abstract In this thesis,

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Table 5-3. Estimation of the chain structure of the different branched latices.

Latex samples NBMAa NBPDM

b Nend c nBMA

d

LR-PBMAD 353 0 2 353

LR-PBMAA 371 0 2 371

LB-PBMAD 266 3 8 23

LB-PBMAA 328 5 12 20

MB-PBMAD 275 17 36 5

MB-PBMAA 253 13 28 6

HB-PBMAD 231 23 48 3

HB-PBMAA 265 25 52 3 a Average number of BMA units per chain. b Average number of BPDM units per chain. c Average number of end groups per chain. d Average number of BMA units between two branching points.

Table 5-4. Limiting values of energy transfer efficiency for the different labeled latex mixtures.

D/A (1/1)

Labeled latex mixture

CNben

[μmol/g

(NBen /polymer)]

CNben

[mmol/L

(NBen/polymer film)]

[ФET(∞)exp] a [ФET(∞)cal] b

LR-PBMAD/A 11.1 11.7 0.40 0.43

LB-PBMAD/A 10.5 11.0 0.38 0.42

MB-PBMAD/A 10.2 10.7 0.36 0.41

HB-PBMAD/A 11.5 12.1 0.39 0.44

a Limiting values of ΦET obtained from fluorescence decays for THF-cast films. b Limiting values of ΦET calculated from the Förster equation (eq. 2-2) using R0 = 2.51 nm.14

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73

5.2.2 Polymer Diffusion in Branched PBMA Latex Films at Same Temperature

In this section I analyze the effect of branching on polymer diffusion rates in branched

PBMA latex films annealed at the same temperature. In these experiments I monitored the

increase in energy transfer efficiency of branched latex films, annealed for specified times at a

given temperature. Before I compare ФET data to evaluate changes in polymer diffusion, it is

necessary to examine if samples with different branching degree have similar limiting values of

energy transfer efficiency in the state of full mixing (ФET(∞)). To obtain samples to serve as a

model for ФET(∞), I dissolved the labeled latex mixture in an organic solvent. In solution, one

expects full mixing of the D- and A-labeled latex polymers. A film cast from the solution is then

a good model for the determination of ФET(∞).

A dry latex film containing a 1:1 (w/w) mixture of D- and A-labeled linear or branched

polymers was dissolved in a minimum amount of THF. This solution was re-cast onto a quartz

plate and allowed to dry at room temperature for 24 hours. Then, from the fluorescence decay

profiles I obtained the limiting values of energy transfer efficiency corresponding to the state of

full mixing, under the assumption that the two labeled polymers mix completely when dissolved

in that solvent and that they do not demix upon drying. The values obtained for the different

samples, listed in Table 5-4 as ФET(∞)exp, are in the range of 0.36 – 0.40. Since the differences

among the ФET(∞)exp values for the different branching degree are similar to the experimental

uncertainty of quantum efficiency measurements (ca. ± 0.02), these limiting values can be

considered identical within experimental error.

To confirm that these ФET(∞)exp values truly represent fully randomized D- and A-labeled

branched polymer mixtures, I calculated the limiting values of energy transfer efficiency using

the Förster model given by eqs 2-2 and 2-3. To measure the acceptor concentration in each latex

film sample, a calibration curve was constructed by measuring the UV absorbance at 340 nm of a

series of acceptor standard solutions (Figure 5-3). The UV absorbance for each film was then

measured at the same wavelength and the acceptor concentration was calculated from the

calibration curve. The obtained acceptor concentrations in the polymer film range from 10.7 to

12.1 mmol/L (acceptor/polymer film), are listed in Table 5-4. These concentration values were

used (eq 2-3) to calculate the values of Förster fitting parameter P corresponding to each

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74

0

0.2

0.4

0.6

0.8

0 1.0 2.0 3.0

C (NBenMA) [10-5M]

Abs

. (34

1 nm

)

0

0.2

0.4

0.6

0.8

0 1.0 2.0 3.0

C (NBenMA) [10-5M]

Abs

. (34

1 nm

)

Figure 5-3. UV calibration curve for NBenMA in THF solution. The extinction coefficient is ε = (2.47 ± 0.03)×104 M-1cm-1.

branched latex film. This parameter accounts for the influence of energy transfer in a system

containing uniformly distributed donor and acceptors in three dimensions. Then, limiting values

of energy transfer efficiency were calculated (ФET(∞)cal) using eq. 2-2 and eq. 2-5, and

considering the pre-exponential factor in the Förster equation as A = 1. Table 5-4 lists the

obtained ФET(∞)cal values for the different branched latex mixtures. One can see that these results

are in good agreement with those obtained experimentally from THF-cast films, indicating that

the ФET(∞)exp values truly correspond to the state of full mixing at molecular level of D- and A-

labeled branched polymers.

To study polymer diffusion rates, latex films with different degrees of branching were cast

from 1:1 D/A mixtures. All latex films were clear and crack free. These films were annealed at

different temperatures, and ET measurements were carried out at various annealing times. To

compare the polymer diffusion rates in the branched PBMA latex films, I plotted the ФET values

as a function of annealing time at common annealing temperatures in Figure 5-4. Figure 5-4(A)

shows that at 23 oC, the HB-PBMAD/A and MB-PBMAD/A films undergo a significant extent of

diffusion on the time scale of hours. In contrast, the LB-PBMAD/A and LR-PBMAD/A films show

a much smaller increase of ФET values for the same time scale. The ФET(0) value of 0.20 for the

HB-PBMAD/A sample indicates that a significant amount of polymer diffusion occurred during

sample drying. One conclusion that can be drawn from these data is that the polymer diffusion

rate at 23°C increases as the Tg of the sample decreases.

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75

0.0

0.1

0.2

0.3

0.4

0.5

-5 0 5 10 15 20 25 30 35 40 45 50

Annealing time (h)

ФET

23 oC

LR

LB

MB

HB

0.0

0.1

0.2

0.3

0.4

0.5

-0.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

Annealing time (h)

ФET

45 oC

LB

MB

HB

LR

0.0

0.1

0.2

0.3

0.4

0.5

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

Annealing time (h)

ФET

LR70 oCLB

MB

HBC

A

B

0.0

0.1

0.2

0.3

0.4

0.5

-5 0 5 10 15 20 25 30 35 40 45 50

Annealing time (h)

ФET

23 oC

LR

LB

MB

HB

0.0

0.1

0.2

0.3

0.4

0.5

-0.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

Annealing time (h)

ФET

45 oC

LB

MB

HB

LR

0.0

0.1

0.2

0.3

0.4

0.5

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

Annealing time (h)

ФET

LR70 oCLB

MB

HBC

A

B

Figure 5-4. Plot of the ΦET for films formed from D/A labeled latex mixtures annealed for various periods of time at (A) 23 oC, (B) 45 oC and (C) 70 oC. (О) HB-PBMAD/A, (△) MB-PBMAD/A, (□) LB-PBMAD/A and (◇) LR-PBMAD/A.

Page 93: Polymer Diffusion in Latex Films€¦ · Polymer Diffusion in Latex Films Yuanqin Liu Doctor of Philosophy Department of Chemistry University of Toronto 2009 Abstract In this thesis,

76

C

A

B

0.0

0.2

0.4

0.6

0.8

1.0

-5 0 5 10 15 20 25 30 35 40 45 50

Annealing time (h)

f m

LR

23 oC

LB

MB

HB

0.0

0.2

0.4

0.6

0.8

1.0

-2 0 2 4 6 8 10 12 14 16 18 20 22 24

Annealing time (h)

f m

45 oC

LR

LB

MBHB

0.0

0.2

0.4

0.6

0.8

1.0

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Annealing time (h)

f m

LR

70 oCLB

MB

HBC

A

B

0.0

0.2

0.4

0.6

0.8

1.0

-5 0 5 10 15 20 25 30 35 40 45 50

Annealing time (h)

f m

LR

23 oC

LB

MB

HB

0.0

0.2

0.4

0.6

0.8

1.0

-2 0 2 4 6 8 10 12 14 16 18 20 22 24

Annealing time (h)

f m

45 oC

LR

LB

MBHB

0.0

0.2

0.4

0.6

0.8

1.0

-0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Annealing time (h)

f m

LR

70 oCLB

MB

HB

Figure 5-5. Plot of the fm for films formed from D/A labeled latex mixtures annealed for various periods of time at (A) 23 oC, (B) 45 oC and (C) 70 oC. (О) HB-PBMAD/A, (△) MB-PBMAD/A, (□) LB-PBMAD/A and (◇) LR-PBMAD/A.

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77

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

45 oC

23 oC

70 oC

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

45 oC

23 oC

70 oC

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

) 45 oC

23 oC

70 oC

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

) 45 oC

23 oC

70 oC

A

0 0.2 0.4 0.6 0.8 1.0 fm

0 0.2 0.4 0.6 0.8 1.0 fm

0 0.2 0.4 0.6 0.8 1.0 fm

0 0.2 0.4 0.6 0.8 1.0 fm

B

DC

100

10-2

10-4

100

10-2

10-4

102

100

10-2

10-4

102

100

10-2

10-4

102

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

45 oC

23 oC

70 oC

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

45 oC

23 oC

70 oC

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

) 45 oC

23 oC

70 oC

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

1.0E+02

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

) 45 oC

23 oC

70 oC

A

0 0.2 0.4 0.6 0.8 1.0 fm

0 0.2 0.4 0.6 0.8 1.0 fm

0 0.2 0.4 0.6 0.8 1.0 fm

0 0.2 0.4 0.6 0.8 1.0 fm

B

DC

100

10-2

10-4

100

10-2

10-4

102

100

10-2

10-4

102

100

10-2

10-4

102

Figure 5-6. Plots of Dapp as a function of fm for (A) LR-PBMAD/A, (B) LB-PBMAD/A, (C) MB-PBMAD/A, and (D) HB-PBMAD/A. As fm 1, the ET experiment loses sensitivity because the incremental changes in ΦET become small. Thus the calculated Dapp values for fm > 0.9 may not be meaningful.

The ФET values of all films are plotted as a function of the annealing time at 45 oC in

Figure 5-4(B). It can be observed that polymer diffusion rates in the LB-PBMAD/A and LR-

PBMAD/A films significantly increased with respect to the films annealed at 25 ºC (Figure 5-

4(A)). Their ФET values approached 0.30 over three hours. Upon annealing at 70 oC, all latex

film samples exhibited a behavior analogous to that of the low Tg sample: rapid diffusion at early

times, followed by a leveling off at a ФET value close to the corresponding ФET(∞)exp value. In

Figure 5-5, I plot the values of fraction of mixing fm as a function of annealing time for the

branched latex films at the different annealing temperatures. These fm values were calculated

from eq 2-6, using the corresponding ФET(∞)exp value for each labeled latex mixture given in

Table 5-4. One can see that each fm curve presents similar features to the corresponding ФET

curve in Figure 5-4.

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78

To compare polymer diffusion rates quantitatively at different temperatures, I calculated

apparent mean diffusion coefficients Dapp that characterize the diffusive transport of the labeled-

polymer chains in the latex film. I computed these values by fitting the fm data to a Fickian

diffusion model for spherical geometry. These Dapp values are not the true center-of-mass

diffusion constants for the polymers; however, our experience has shown that Dapp provides a

realistic measure of the changes in polymer diffusion rates.10-13, 20-21 Dapp values for all samples

were calculated as a function of fm at each temperature (Figure 5-6). To compare the Dapp values,

I created Dapp master curves for all samples by vertically shifting the data points at 45 oC and 70 oC to overlap the data points at 23 oC (Figure 5-7). The most impressive feature in Figure 5-7 is

that the master curves of the HB-PBMAD/A and the MB-PBMAD/A films are at least two orders of

magnitude higher than those of the LB-PBMAD/A and LR-PBMAD/A films. For the same value of

fm, the sample with a lower Tg has higher Dapp value. For example, at fm = 0.84, the plot shows a

value of Dapp = 2.5×10-1 nm2/s for the HB-PBMAD/A film, 2.8×10-2 nm2/s for the MB-PBMAD/A

film, 1.5×10-4 nm2/s for the LB-PBMAD/A film, and 1.7×10-6 nm2/s for the LR-PBMAD/A film23.

Since the Tg has an important effect upon the diffusive capability of polymer chains, a proper

analysis of the effect of branching on polymer diffusion should involve a correction for the

different Tg values of the branched samples. I develop this analysis in the next section.

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79

C

A B

D

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

C

A B

D

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

1.0E-06

1.0E-04

1.0E-02

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

100

10-2

10-4

10-6

0 0.2 0.4 0.6 0.8 1.0 fm

Figure 5-7. Master curves of Dapp for (A) LR-PBMAD/A, (B) LB-PBMAD/A, (C) MB-PBMAD/A, and (D) HB-PBMAD/A. 23 oC was used as the reference temperature. The apparent precipitous decrease in Dapp values at values of fm → 1 may not be real, since the energy transfer methodology loses its sensitivity as polymers approach the fully mixed state, and the acceptor concentrations in the films become uniform.

5.2.3 Polymer Diffusion in Branched PBMA Latex Films at (Tg+20) oC

Because of the significant Tg effect on polymer diffusion, it is difficult to quantify the

influence of branching on polymer diffusion from the previous analysis of the ET data. Models

of polymer rheology suggest that polymers of similar composition (i.e., with similar friction

coefficients) and similar molecular weights, but different Tg values, should have similar mobility

when they are annealed at the same constant temperature above their Tg’s.24 To eliminate the

effect of Tg, latex films were prepared as described previously and each annealed at (Tg + 20) °C.

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0.0

0.4

0.8

1.2

-5 5 15 25

Annealing time (h)

f m

MBHB

LB

(T g+20) oC

1.0E-02

1.0E-01

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

MB

LB

HB

(T g+20) oC

A

B

0.0

0.4

0.8

1.2

-5 5 15 25

Annealing time (h)

f m

MBHB

LB

(T g+20) oC

1.0E-02

1.0E-01

1.0E+00

0 0.2 0.4 0.6 0.8 1

f m

Dap

p (n

m2 /S

)

MB

LB

HB

(T g+20) oC

A

B

Figure 5-8. (A) Plot of fm for films annealed at (Tg +20) oC. (B) Plot of Dapp over fm at (Tg +20) oC. (О) HB-PBMAD/A, (△) MB-PBMAD/A , and (□) LB-PBMAD/A.

ET measurements were then carried out on samples cooled to room temperature to stop polymer

diffusion during the measurement.

In Figure 5-8(A), the calculated fm values are plotted as a function of annealing time for

the films annealed at (Tg + 20) °C. The plot shows that fm increases rapidly at the early stages,

followed by a slower increase at longer annealing times. In the early stage, the sample with a

higher degree of branching has a higher fm value. For example, the fm value after 1 hour

annealing is 0.65 for the HB-PBMAD/A film, 0.55 for the MB-PBMAD/A film, and 0.19 for the

LB-PBMAD/A film. After annealing for one day, the three films showed similar values of fm.

To quantify the effect of branching on the distribution of diffusion rates in the system,

apparent mean diffusion coefficients Dapp were calculated by fitting the fm data to a spherical

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diffusion model that satisfies Fick’s second law of diffusion. I then plotted Dapp as a function of

fm in Figure 5-8(B). It can be observed that the branching structure had significant influence

upon Dapp. The figure shows that at fm < 0.90 the sample with a higher degree of branching has a

greater Dapp value. It also can be observed that Dapp decreases in different manners for the three

films. For the HB-PBMAD/A film, Dapp sharply decreases by one order of magnitude over a small

range of fm. For the LB-PBMAD/A film, Dapp gradually decreases over a wide range of fm. The

decreasing rate of Dapp for the MB-PBMAD/A film is intermediate between those of the previous

two samples. From all these experiments I conclude that practically in the whole range of

fraction of mixing an increase in the branching degree of PBMA latex enhances the polymer

diffusion rate during film formation process.

In Figure 5-8(B), the Dapp plot indicates that the degree of branching has a significant

effect on polymer diffusion rate. One might normally expect that increasing the degree of

branching would retard the polymer diffusion rate. However, our experimental data show the

opposite result. I believe that the main reason for this behavior is related to the architectures of

the branched PBMA samples. As indicated in Table 5-3, the branched PBMA samples have short

branches. For example, each LB-PBMAD/A chain contains branches with an average length of ca.

20 BMA units. The more branched PBMA samples are composed of even shorter branches. The

MB-PBMAD/A sample contains ca. 6 BMA units per branch, and each HB-PBMAD/A branch has

an average length of ca. 3 BMA units. These branches are much too short to form entanglements

between polymer chains.21 The lack of entanglement in these branched PBMA samples has been

established by detailed linear rheology measurements (Figure 5-9). It is the formation of

entanglements with different polymer chains that slows down the polymer diffusion in branched

samples. Thus it is not surprising that the HB-PBMAD/A sample does not have a slower diffusion

rate than the LB-PBMAD/A sample. Furthermore, instead of forming entanglements, the short

branches constrain the polymer chains to form a compact structure. This result is reflected in

values of Rg obtained in the TDA/GPC measurements (Table 5-2). For example, the LB-

PBMAD/A sample has a Rg value of 2.8 nm while the HB-PBMAD/A sample has a Rg value of 2.1

nm. Because the magnitude of Rg is related to that of the hydrodynamic radius Rh, and molecules

with smaller Rh diffuse faster than that with greater Rh, the decrease in Rg can explain why the

HB-PBMAD/A sample diffuses faster than the other samples.

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Figure 5-9. Plots of master curves of G' and G'' for (A) LR-PBMAD/A, (B) LB-PBMAD/A, (C) MB-PBMAD/A, and (D) HB-PBMAD/A. 40 oC was used as reference temperature.

5.3 Summary I synthesized four sets of latex particles comprised of PBMA with various degree of

branching. To carry out ET study, fluorescence donor and acceptor were incorporated in the

PBMA particles respectively. All branched polymers have similar molecular weights and are free

of gel. The latex particles have similar particle size and narrow size distribution. The ET

technique was applied to study the effect of branching on the rate of polymer diffusion in the

PBMA latex films. Comparison of the diffusion data indicates that Tg has a significant effect on

polymer diffusion when the films were annealed at 23 oC. By examining films annealed at (Tg +

20) oC, I could correct for the influence of Tg. In this way I found that the diffusion rate of

branched PBMA increases as the degree of branching increases. The more highly branched

polymers have more compact structures.

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5.4 Reference

1 Dillon, R. E.; Metheson, L. A.; Bradford, E. B. J. Colloid Sci. 1951, 6, 108-117. 2 Henson, W. A.; Taber, D. A.; Bradford, E. B. Ind. Eng. Chem. 1953, 45, 735-737. 3 Brown, G. L. J. Polymer Sci. 1956, 22, 423-426. 4 Hwa, C. H. J. J. Polym. Sci., Part A 1964, 2, 785-796. 5 Myers, R. R.; Schultz, K. R. J. Polym. Sci., Part A 1964, 8, 755-764. 6 Sheetz, D. P. J. Appl. Polym. Sci. 1965, 9, 3759-3773. 7 Hahn, K.; Ley, G.; Schuller, H.; Oberthür R. Colloid & Polymer Sci. 1986, 264, 1092-1096. 8 Hahn, K.; Ley, G.; Oberthür R. Colloid & Polymer Sci. 1988, 266, 631-639. 9 Chevalier, Y.; Pichot, C.; Graillat, C.; Joanicot, M.; Wong, K.; Maquet, J.; Lindner, P.; Cabane,

B. Colloid & Polymer Sci. 1992, 270, 806-821. 10 Keddie, J. L. Materials and Science and Engineering 1997, 21, 101-170 11 Wu, J.; Oh, J. K.; Yang, J.; Winnik, M. A.; Farwaha, R.; Rademacher, J. Macromolecules

2003, 36, 8139-8147. 12 Oh, J. K.; Tomba, P.; Ye, X.; Eley, R.; Rademacher, J.; Farwaha, R.; and Mitchell M. A.

Macromolecules 2003, 36, 5804-5814. 13 Oh, J. K.; Wu, J.; Winnik, M. A.; Craun, G. P.; Rademacher, J.; Farwaha, R. Journal of

Polymer Science, Part A: Polymer Chemistry 2002, 40, 3001-3011. 14 Liu, Y.; Haley, J. C.; Deng, K.; Lau, W.; Winnik, M. A. Macromolecules 2007, 40, 6422-

6431. 15 O’Brien, N.; McKee, A.; Sherrington, D. C. Polym. Commun. 2000, 41, 6027-6031. 16 Costello, P. A.; Martin, I. K.; Slark, A. T.; Sherrington, D. C.; Titterton, A. Polymer 2002, 43,

245-254. 17 Isaure, F.; Cormack, P. A. G.; Sherrington, D. C. J. Mater. Chem. 2003, 13, 2701-2710. 18 Slark, A. T.; Sherrington, D. C.; Titterton, A.; Martin, I. K. J. Mater. Chem. 2003, 13, 2711-

2720. 19 Isaure, F.; Cormack, P. A. G.; Sherrington, D. C. Macromolecules 2004, 37, 2096-2105. 20 Baudry, R.; Sherrington, D. C. Macromolecules 2006, 39, 1455-1460. 21 Liu, Y.; Haley, J. C.; Deng, K.; Lau, W.; Winnik, M. A. Macromolecules 2008, 41, 4220-

4225. 22 Lau, W. “Method for forming polymers,” U. S. Patent 5,760,129, June 2, 1998. 23 Note that Dapp = 0.1 nm2/s = 10-15 cm2/s

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24 Ferry, J. D. Viscoelastic Properties of Polymers. Wiley: New York, 1980.

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Chapter 6

6 Effect of Hyper-Branched Poly(n-butyl methacrylate) on Polymer Diffusion in Poly(n-butyl acrylate-co-methyl methacrylate) Latex Films

6.1 Introduction Most commercial latex paints contain significant amount of coalescing agents, which are

used to promote the formation of mechanically coherent coatings as the film dries. Traditional

coalescing agents are usually volatile organic compounds (VOC) such as 2, 2, 4-trimethyl-1, 3-

pentanediol monoisobutyrate (TexanolTM) and ethylene glycol monobutyl ether (EB). The effect

of coalescing agents on the film formation and the latex film properties has attracted a lot of

interests in the past decades. Numerous studies were carried out. For example, Zohrehvand and

Nijenhuis studied the role of coalescing agents in the film formation process by analyzing the

film turbidity.1 Previous work in our laboratory examined polymer diffusion rate in the presence

of coalescing agents 2 - 4 As a representative of traditional coalescing agents, the effect of

TexanolTM on polymer diffusion in poly(n-butyl methacrylate) (PBMA) latex films was carefully

studied by the energy transfer (ET) technique. Quantitative analysis of the ET data established

that TexanolTM is efficient at promoting polymer diffusion in PBMA films.4 By using a steady-

state fluorescence technique, Pekcan and Canpolat studied interdiffusion of poly(methyl

methacrylate) (PMMA) polymer chains in the presence of heptane during annealing of latex

above its glass transition temperature.5 Although VOCs significantly enhance polymer diffusion

rates during film formation, they ultimately escape to the atmosphere and contribute to air

pollution.

In an attempt to solve the problem, many scientists and engineers have been working on

developing environmentally compliant coatings with comparable or improved properties. Much

effort has been put into the development of low VOC containing latex paints.6-9 Novel reactive

coalescing agents were also synthesized. For example, Rissanen synthesized five glycidyl

compounds, some of which have better performance than a commercial nonreactive coalescing

agent, NexcoatTM 795. 10 Previous work in our research group showed that beta

hexamethoxymethyl melamine was an effective reactive coalescing agent in a model automotive

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base coat formulation.11 However, until now there are still no good substitutes to replace VOCs

as coalescing agents.

My research objective is to examine the efficiency of a new coalescing agent candidate, a

hyper-branched poly(n-butyl methacrylate) (HB-PBMA). The HB-PBMA was originally

synthesized for the studies of the effect of branching on polymer diffusion in latex films.12 The

experiments described in Chapter 4 and 5 show that the diffusion rate of branched PBMA

increases as the degree of branching increases. Since the HB-PBMA has a fast diffusion rate, I

has the idea that it might be able to enhance polymer diffusion rate of other polymers in latex

films. With this hypothesis in mind, I designed the following three approaches to incorporate the

HB-PBMA into poly(n-butyl acrylate-co-methyl methacrylate) [P(BA-MMA)] latex samples and

studied its effect on polymer diffusion rates in latex films.

1. The blending approach I used semi-continuous emulsion polymerization to

synthesize HB-PBMA and P(BA-MMA) latices, followed by direct mixing the two latices to

produce HB-PBMA/P(BA-MMA) latex blends.

2. The miniemulsion polymerization approach. HB-PBMA made by semi-continuous

emulsion polymerization was dried and then dissolved in a monomer mixture of BA and MMA.

The monomer mixture was sonicated together with water and other ingredients to form a

monomer miniemulsion. This miniemulsion was then polymerized to produce P(BA-MMA)

latex particles containing the HB-PBMA.

3. The seeded emulsion polymerization approach. In this approach, HB-PBMA latex

particles synthesized by semi-continuous emulsion polymerization were used as seed particles in

the seeded emulsion polymerization of BA/MMA to prepare P(BA-MMA) latices containing the

HB-PBMA seeds.

I then studied polymer diffusion in latex films cast from the above latex dispersions. The

polymer diffusion rates were compared with those of the P(BA-MMA) latex films containing

TexanolTM. The mechanical properties of this series of films as measured by tensile tests were

compared.

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6.2 Blending Approach

6.2.1 Effect of Blended HB-PBMA Particles on Polymer Diffusion in P(BA-MMA) Latex Films

Three pairs of D- and A-labeled P(BA-MMA) latices were synthesized using semi-

continuous emulsion polymerization as described in Chapter 2. All P(BA-MMA) latices

contained 1 wt % of MAA. The three pairs of samples have monomer weight ratios of

BA:MMA:MAA of 55:44:1, 50:49:1 and 45:54:1. These copolymers are named according to

their BA:MMA compositions as P(BA55-MMA44), P(BA50-MMA49) and P(BA45-MMA54). The

glass transition temperatures (Tg) of the copolymers were measured by DSC, giving Tg ca. 6 oC

for P(BA55-MMA44), 13 oC for P(BA50-MMA49) and 21 oC for P(BA45-MMA54). All values are

similar to those estimated values from the Fox equation using Tg(PBA) = -47 oC and Tg(PMMA)

= 105 oC. The characteristics of all of the latex particles synthesized are summarized in Table 6-1.

The Mn values were in the range of 81,000 to 86,000 g/mol with a PDI between 1.8 and 2.2. The

particle size and size distribution were characterized using a BI-90 particle sizer. As shown in

Table 6-1, all samples have particle diameters of ca. 110 nm with narrow particle size

distributions.

HB-PBMA latex was synthesized using semi-continuous emulsion polymerization as

described in Chapter 4. The HB-PBMA latex polymer was characterized by GPC and 1H NMR

spectroscopy. GPC data show that Mn is 40,000 g/mol and Mw/Mn is ca. 1.9. The 1H NMR

spectrum gives an average molar ratio of BMA units over branching points. From the

combination of molar mass and 1H NMR spectrum I found that the average number of BMA

units between the adjacent branching points is only 3, which shows that the polymer chain is

hyper-branched. HB-PBMA/P(BA-MMA) latex blends were then prepared by mixing the HB-

PBMA latex with 1:1 mixtures of D- and A-labeled P(BA-MMA) latex dispersions. For each

BA:MMA composition, the HB-PBMA/P(BA-MMA) latex blends have three weight fractions of

the HB-PBMA of 0 wt %, 5 wt % and 10 wt %. For ET experiments, latex films were cast from

the latex blends onto small quartz plates and allowed to dry at 4 oC. The thickness of dried films

was ca. 60 μm. Fluorescence decay measurement was immediately carried out on the freshly

formed latex film. The films were then annealed at 23 oC in a humidity chamber (54 % relative

humidity), and fluorescence decays were monitored as a function of annealing time.

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Table 6-1. Characteristics of the P(BA-MMA) latex polymers and particles

Molecular weight b (g/mol) Particle size c Latex sample a

Mn Mw Mw/Mn d (nm) Poly.d

solids content

(%)

P(BA55-MMA44)D 83,000 161,000 1.9 112 0.034 46.6

P(BA55-MMA44)A 81,000 158,000 2.0 113 0.069 47.0

P(BA50-MMA49)D 86,000 155,000 1.8 104 0.026 46.0

P(BA50-MMA49)A 84,000 156,000 1.9 105 0.022 46.0

P(BA45-MMA54)D 85,000 188,000 2.2 115 0.057 46.6

P(BA45-MMA54)A 83,000 167,000 2.0 114 0.052 45.1 a Superscript “D” and “A” refers to D- and A-labeled latices, respectively. Subscript refers to

copolymer weight composition. b Absolute molecular weights were measured by GPC using a Viscotek TDA302 triple

detector array. c Particle size and size distribution were measured by a BI-90 Particle Sizer. d Polydispersity (poly) is a measure of the width of the particle size distribution, taking values

close to zero (0.000 to 0.020) for nearly monodisperse samples, and small (0.020 to 0.080) for narrow size distributions.

From analysis of the fluorescence decays I calculated quantum efficiencies of energy

transfer, ФET, for all films using eq. 2-4. For each sample, the ФET value obtained from a freshly

formed film was defined as ФET(0). The value obtained from a solvent-cast film was defined as

ФET(∞). The fraction of mixing fm was then calculated using eq. 2-6. Values of fm for all samples

are plotted as a function of annealing time in Figure 6-1. The plot shows that the fm value

increases as the annealing time increases for all films. The films with higher concentrations of

HB-PBMA have greater fm values than those with a lower HB-PBMA concentration at the same

annealing time. For example, to reach 28% of mixing it took 46 h for the P(BA55-MMA44) film

blended with 0 wt % of HB-PBMA particles, whereas it took only 19 h for the film blended with

5 wt % HB-PBMA particles (Figure 6-1A). The copolymer compositions also affect polymer

diffusion rates as I described in Chapter 3. The films with higher BA composition have greater

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values of fm than those with lower BA composition at the same annealing time. For example, it

took 91 h to reach 20% of mixing for the P(BA45-MMA54) film blended with 10 wt % of HB-

PBMA particles, whereas for the P(BA50-MMA49) film blended with 10 wt % of HB-PBMA

particles, fm already exceeded 28% (Figure 6-1C, E).

The early stage of the fm plots are shown in Figure 6-1B, D, F. From these figures one can

observe that the enhancement of HB-PBMA on polymer diffusion takes place in the first 8 h.

The enhancement of HB-PBMA is significant for the P(BA55-MMA44) and the P(BA50-MMA49)

films, but is limited for the P(BA45-MMA54) films. For instance, the fm plot of the P(BA45-

MMA54) film blended with 5 wt % HB-PBMA particles overlaps that of the P(BA45-MMA54)

film without any additives.

Table 6-2. Tg of P(BA-MMA) polymers containing HB-PBMA and TexanolTM

Tg (oC) at different additive concentrations Latex Additive

0 wt % 5 wt % 10 wt %

P(BA55-MMA44) HB-PBMA 6.1 4.0 3.7

P(BA50-MMA49) HB-PBMA 13.2 11.3 9.0

P(BA45-MMA54) HB-PBMA 21.4 18.3 17.1

P(BA55-MMA44) TexanolTM 6.1 -4.6 -12.3

P(BA50-MMA49) TexanolTM 13.2 1.6 -6.9

P(BA45-MMA54) TexanolTM 21.4 8.7 -2.0

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Figure 6-1. Plots of the fm versus annealing time for (A,B) P(BA55-MMA44), (C,D) P(BA50-MMA49), and (E,F) P(BA45-MMA54) latex films containing (●)0 wt % of additives, (▲)5 wt % blended HB-PBMA particles, (■)10 wt % blended HB-PBMA particles at 23 °C.

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6.2.2 Comparing the Plasticization Effect of Blended HB-PBMA Particles and TexanolTM

To illustrate the plasticization efficiency of the blended HB-PBMA particles, I compare its

effectiveness with that of TexanolTM, a commonly used coalescing agent in coating industry. To

incorporate TexanolTM into latex particles, various amount of TexanolTM were added into the

three pairs of D- and A-labeled P(BA-MMA) latices. The mixtures were stirred for 24 h to

ensure that TexanolTM had completely mixed with the latex particles. Latex films were then cast

and ET experiments were carried out under the same conditions as those used for the HB-

PBMA/P(BA-MMA) blend samples.

Values of fm were calculated from the fluorescence decays, and plotted as a function of

annealing time in Figure 6-2. For comparison, the fm plots of the HB-PBMA/P(BA-MMA) blend

samples are shown in Figure 6-2 as well. The addition of TexanolTM increased polymer diffusion

rates as expected. The fm plots have the same tendency as those of the HB-PBMA/P(BA-MMA)

blend films, which is that the enhancement on polymer diffusion takes place in the early stage.

Figure 6-2B shows that in the early stage the film containing 10 wt % TexanolTM, fm has higher

values than the blend film blended with 10 wt % of HB-PBMA particles. The difference between

the two fm curves decreased as annealing time increased, and eventually disappeared after the

film was aged at 23 oC for 64 h. This results indicates that 10 wt % blended HB-PBMA particles

and 5 wt % TexanolTM have similar effect on polymer diffusion rates on the time scale of days.

For the P(BA50-MMA49) and the P(BA45-MMA54) samples, the films containing TexanolTM have

significant greater values of fm than the films blended with the HB-PBMA particles (Figure 6-2C,

E).

To quantitatively compare the plasticization effect of the blended HB-PBMA particles and

TexanolTM, I fitted the fm values of the P(BA55-MMA44) films to the Fickian diffusion model, and

calculated the apparent diffusion coefficients, Dapp. Figure 6-3 shows the Dapp vs fm plots of the

P(BA55-MMA44) films containing 5 wt % HB-PBMA particles, 5 wt % TexanolTM, and 0 wt %

additives. Both HB-PBMA and TexanolTM enhance the values of Dapp by an order of magnitude

over a broad range of fm. In the early stage (fm < 0.25), the TexanolTM-containing film has higher

Dapp than the HB-PBMA blend film. But the difference of Dapp between the two films decreased

as fm increased, which leads to a cross over of the two Dapp plots at fm = 0.40.

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Figure 6-2. Comparison of the plots of the the fm versus annealing time for (A,B) P(BA55-MMA44), (C,D) P(BA50-MMA49), and (E,F) P(BA45-MMA54) latex films containing HB-PBMA and TexanolTM at 23 °C. (●) 0 wt % of additives, (▲) 5 wt % blended HB-PBMA particles, (■) 10 wt % blended HB-PBMA particles, (△) 5 wt % TexanolTM, (□) 10 wt % of TexanolTM.

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6.2.3 Effect of Blended HB-PBMA Particles on Mechanical Properties of the P(BA-MMA) Latex Films

To examine whether the blended HB-PBMA particles will affect the mechanical

properties of the P(BA-MMA) latex films, I carried out tensile tests on five P(BA55-MMA44)

latex samples containing different concentrations of blended HB-PBMA or TexanolTM. The

P(BA55-MMA44) latex dispersions were dried in PTFE dishes at room temperature to form films

with a thickness of ca. 0.7 mm. These films were then cut into strips (35.0 mm × 7.0 mm) for

mechanical tests, which were performed at a strain rate of 50 mm/min with an Instron 5543

tensile tester under ambient conditions. At least six specimens were measured for each sample.

The representative tensile stress-strain curves are shown in Figure 6-4, which well

represents the quantitative results listed in Table 6-3. The values of Young’s modulus are the

secant moduli at 3.0 % elongation, and the values of toughness are the integrals of the area under

the tensile stress-strain curves. One can observed that values of Young’s modulus and toughness

of the two HB-PBMA-containing films are similar to those of the film without any additives. For

tensile stress, the films containing HB-PBMA have slightly smaller values than the film without

any additives. The TexanolTM–containing films have significant lower tensile properties than the

film without any additives. For example, the film containing 5 wt % TexanolTM has a Young’s

Figure 6-3. Comparison of the plots of the Dapp versus fm for P(BA55-MMA44) latex films containing (●) 0 wt % of additives, (▲) 5 wt % of blended HB-PBMA particles, and (△) 5 wt % of TexanolTM at 23 °C.

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Figure 6-4. Tensile stress-strain curves for P(BA55-MMA44) latex films with 0 wt % additive, 5 wt % blended HB-PBMA particles, 10 wt % blended HB-PBMA particles, 5 wt % TexanolTM, and 10 wt % TexanolTM.

modulus of 0.02 MPa, which is less than one third of that of the film without any additives. The

value of toughness of the film containing 10 wt % TexanolTM is only 22 % of that of the film

with 0 wt % additive. All tensile results suggest that the blended HB-PBMA particles have

negligible influence on Young’s modulus, tensile stress, and film toughness.

Table 6-3. Tensile properties of P(BA55-MMA44) latex films

Additives Ultimate tensile stress

(MPa)

Young’s modulus

(MPa)

Toughness

(MJ/m3)

0 wt % a 6.8 ± 0.5 0.07 ± 0.01 1800 ± 120

5 wt % HB-PBMA b 4.6 ± 0.3 0.05 ± 0.00 1530 ± 120

10 wt % HB-PBMA b 6.0 ± 0.6 0.06 ± 0.02 1650 ± 170

5 wt % TexanolTM 2.5 ± 0.3 0.02 ± 0.00 750 ± 160

10 wt % TexanolTM 1.4 ± 0.2 0.01 ± 0.00 420 ± 100 a 0 wt % sample refers to the P(BA55-MMA44) latex film without any additives. b The HB-PBMA particles were blended.

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6.2.4 Conclusion HB-PBMA/P(BA-MMA) latex blends were prepared by mixing HB-PBMA and P(BA-

MMA) latices, both of which were synthesized by semi-continuous emulsion polymerization. By

examining the polymer diffusion behavior in the latex films cast from the HB-PBMA/P(BA-

MMA) blends, I found that the blended HB-PBAM particles significantly enhance polymer

diffusion rates in the P(BA-MMA) latex films. Tensile studies on these latex films reveal that the

blended HB-PBMA particles have negligible effect on the film tensile properties.

6.3 Miniemulsion Polymerization Approach

6.3.1 Miscibility of HB-PBMA and P(BA-MMA)

The hypothesis that the miniemulsion polymerization approach can increase the

plasticization efficiency of the HB-PBMA depends on a good miscibility of HB-PBMA and

P(BA-MMA) at the molecular level. To examine the miscibility, one needs to compare the

experimental values of ΦET [ΦET(lim)] with the theoretical ΦET [ΦET(∞)] of a fully mixed film

composed of the two polymers. The two ΦET values should be essentially identical if the two

polymers are completely miscible.

To measure the ΦET(lim), I prepared two sets of latex blends. In the first set, HB-PBMAD

latex was mixed with P(BA-MMA)A latices at a polymer weight ratio of 1:9. In the second set,

HB-PBMAA latex was blended with P(BA-MMA)D latices at a polymer weight ratio of 1:9. For

each set, the P(BA-MMA) latices have three BA:MMA compositions, so there are in total six

latex blends. They are named as BL1 to BL6 (Table 6-5). Latex films were then cast from the six

latex blends and allowed to dry at 23 oC overnight. The dry films were dissolved in minimum

amounts of THF. The solutions were re-cast onto small quartz plates and allowed to dry at room

temperature for 24 hours. Fluorescence decays were measured. Values of ΦET(lim) were then

calculated from these decays and listed in Table 6-5. Values of ФET(∞) were determined as

described in Chapter 3. Table 6-5 shows that the ΦET(lim) values match the ΦET(∞) values for

all latex blend samples. I conclude that the HB-PBMA can be fully mixed with P(BA-MMA) at a

weight ratio of 1:9.

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Table 6-5. Miscibility of HB-PBMA and P(BA-MMA)

Latex blend composition Latex blend

10 wt % 90 wt % ΦET(∞) a ΦET(lim) b

BL1 HB-PBMAD P(BA55-MMA44)A 0.50 0.50

BL2 HB-PBMAD P(BA50-MMA49)A 0.48 0.49

BL3 HB-PBMAD P(BA45-MMA54)A 0.48 0.50

BL4 HB-PBMAA P(BA55-MMA44)D 0.12 0.13

BL5 HB-PBMAA P(BA50-MMA49)D 0.12 0.14

BL6 HB-PBMAA P(BA45-MMA54)D 0.12 0.13 a Values of ΦET(∞) were calculated using eq. 2-3 and eq. 2-4. b Values of ΦET(lim) were obtained from THF solution cast films.

6.3.2 Synthesis of Miniemulsion P(BA55-MMA44) Latex Particles

The objective of the experiments described in this section is to determine the effect of HB-

PBMA on polymer diffusion and mechanical properties in latex films of P(BA-MMA) prepared

by miniemulsion polymerization. Two steps were taken to prepare the HB-PBMA containing

latex particles. The first step was to synthesize the HB-PBMA latex using semi-continuous

emulsion polymerization (Mn = 40,000 g/mol, Mw/Mn = 1.9). The second step was to incorporate

the HB-PBMA into the P(BA-MMA) latex particles. Chapter 6.2 described a blending approach,

in which HB-PBMA and P(BA-MMA) latices were directly mixed. The disadvantage of the

approach is that the HB-PBMA is initially separated from the base latex in the newly cast film.

Here, to achieve complete incorporation of HB-PBMA into the base latex, I dissolved the HB-

PBMA in a monomer mixture (BA:MMA:MAA = 55:44:1) and used miniemulsion

polymerization to prepared P(BA55-MMA44) latices. HB-PBMA, together with dye comonomer,

co-stabilizer, initiator, and chain transfer agent, were dissolved in the monomer mixture to form

an organic phase. Surfactant was dissolved in deionized water to form an aqueous phase. The

two phases were then mixed and the mixture was sonicated for 20 min to form a miniemulsion.

The miniemulsion was transferred into a reactor. The reactor was purged with nitrogen for 30

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min before it was immersed in an 80 °C oil bath. After 5 h the reaction mixture was cooled to

room temperature.

By varying the ratio of HB-PBMA:monomers (BA:MMA:MAA ≡ 55:44:1), I synthesized

a series of D- and A-labeled latex particles with HB-PBMA weight fractions ranging from 1 wt

% to 6 wt % (based on total polymer). In my notation, the type of fluorescent dye is indicated by

a superscript “D” or “A”, which represents “donor” or “acceptor”, respectively, and a subscript

refers to the weight % of HB-PBMA. For example, P(BA-MMA)A1%ME refers to the A-labeled

P(BA-MMA) containing 1 wt % of HB-PBMA made by miniemulsion polymerization.

Particle sizes of the P(BA-MMA) latices were measured by dynamic light scattering using

a BI-90 particle sizer. The miniemulsion methodology offers good control over particle size and

size distribution. The average diameters for all samples were in the range of 117 to 136 nm

(Table 6-4). All samples have narrow particle size distributions. The molecular weights were

measured using a GPC equipped with a triple detector array. The GPC data in Table 6-4 show

that all samples have similar molecular weight and narrow molecular weight distribution. The Mn

values are in the range of 32,000 ~ 46,000 g/mol, and the values of Mw/Mn are less than 2.0. The

values of Tg (shown in Table 6-4), which were measured using a TA Instruments DSC Q100

differential scanning calorimeter, decreases as the weight fraction of HB-PBMA increases. The

solids contents were determined by gravimetric analysis, and all latex samples contain ca. 30 wt

% solids. The overall composition of the P(BA-MMA) was measured using 1H NMR, and the

results are listed in Table 6-4. All polymers have similar BA:MMA molar ratio, which is close to

the BA:MMA reactant ratio.

6.3.3 Kinetic Study of the Miniemulsion Polymerization

During the miniemulsion polymerization, aliquots (1.0 mL) were transferred to 20 mL

vials pre-cooled with liquid nitrogen. The samples were then freeze-dried for conversion,

molecular weight, and 1H NMR measurements (Figure A1). The overall conversion of monomers

is plotted as a function of time in Figure 6-5, where one can see that complete conversion is

reached in the first hour of reaction.

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0%

20%

40%

60%

80%

100%

0 20 40 60Reaction Time (min)

Con

vers

ion

Figure 6-5. Total conversion as a function of reaction time for the miniemulsion polymerization of P(BA55-MMA44)A

1%ME.

My colleague Dr. Walter Schroeder proposed a free-radical copolymerization model based

on the terminal model 13 - 15 to simulate the course of composition drift of BA and MMA

monomers within each droplet. The model considers the reaction as a bulk copolymerization and

gives the monomer reactivity ratios for BA (rBA = 0.39) and MMA (rMMA = 2.03).16 The obtained

values of reactivity ratios are in accord with those reported in the literature for the same binary

system. 17 The estimated reactivity ratios were used as kinetic parameters to model the

copolymerization behavior of BA and MMA monomers during the miniemulsion polymerization

reaction. In Figure 6-6 the partial conversions of the monomers are plotted against overall

fractional conversion of monomers. Points represent the experimental data and continuous lines

show the model predictions. It can be observed that the model fits very well the conversion of

both monomers during the whole reaction. These results, together with the proper agreement

between the estimated reactivity ratios and the values reported in the literature,17 show that the

copolymerization kinetics were not sensitively affected by the presence of the HB-PBMA in the

feed.

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6.3.4 Effect of the HB-PBMA on Polymer Diffusion in P(BA55-MMA44) Latex Films

Polymer diffusion was studied by ET experiments. Latex films for ET experiments were

prepared from 1:1 mixtures of donor- and acceptor-labeled latex particles containing the same

concentration of HB-PBMA. Two drops of the latex mixture were cast onto a small quartz plate

and allowed to dry at 4 oC. The thickness of dried films was ca. 60 μm. All films were

transparent and free of cracks. Fluorescence decay measurement was immediately carried out on

the freshly formed latex film in a cold quartz tube using a time-correlated single-photon counting

instrument. The films were then annealed at 45 oC, and fluorescence decays were recorded at

various periods of time.

From the above decays I calculated ФET for films containing different concentrations of

HB-PBMA using eq. 2-4. The fraction of mixing fm was then calculated using eq. 2-6. Values of

fm for all samples are plotted as a function of aging time in Figure 6-7A. The plot shows that the

fm value increases as the annealing time increases for all films. The films with higher

concentrations of HB-PBMA have greater fm values than those with a lower HB-PBMA

Figure 6-6. Experimentally determined (points) and modeling (lines) of the partial conversion of (■) MMA and (▲) BA vs the overall conversion for P(BA55-MMA44)A

1%ME.

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concentration at the same annealing time. For example, it took 12 h at 45 °C to reach 45% of

mixing for the P(BA-MMA) film containing 0 wt % of HB-PBMA, whereas for the film

containing 6 wt % HB-PBMA, fm already exceeded 80%.

To quantitatively compare the effect of HB-PBMA concentration on polymer diffusion

rate, I fitted the fm values to a spherical Fickian diffusion model, and calculated apparent

diffusion coefficients Dapp. The Dapp values for all films are plotted as a function of fm in Figure

6-7B. The plot shows that at the same fm value the film containing higher fraction of HB-PBMA

has a greater value of Dapp, which indicates that the addition of HB-PBMA significantly

enhanced the polymer diffusivity in the latex films. I hypothesized that the HB-PBMA acts like a

traditional coalescing agent, which enhances polymer diffusion rate by increasing the free

volume. Thus its influence on polymer diffusion rate should follow the Fujita–Doolittle equation

(eq. 2-8). I calculated the magnitude of the term {ln [Dp(T, фa)/Dp(T,0)]}-1 from Figure 6-7B for

each HB-PBMA concentration. In the calculation, I equated Dp with Dapp and assumed that the

HB-PBMA has a density close to that of P(BA-MMA). I then used the term ln[Dp(T, фa)/Dp(T,0)]

= ln Dp(T, фa) - ln Dp(T,0) as a shift factor to superimpose the Dapp data obtained from the films

containing HB-PBMA to that containing 0 wt% HB-PBMA. As shown in Figure 6-7C, all of the

data superimpose onto a single master curve. In Figure 6-8, I plot values of {ln [Dp(T,

фa)/Dp(T,0)]}-1 as a function of 1/фa. The linear plot confirms that the polymer diffusion in the

HB-PBMA containing films followed the behavior predicted by the Fujita–Doolittle model (eq.

2-8). The intercept yielded a fp(T,0) value of 0.034, which is the fractional free volume of P(BA-

MMA) without any coalescing agent at 45 oC. From the slope I obtained a β(T) value of 0.034,

which is the difference in fractional free volume between the HB-PBMA and the P(BA-MMA) at

45 oC. The fitting parameters are in a reasonable range,4 which indicates that the HB-PBMA can

enhance the P(BA-MMA) latex film formation. To demonstrate the efficiency of the HB-PBMA

as a coalescing agent, I carried out parallel experiments to obtain the β(T) and fp(T,0) values of

TexanolTM in the P(BA-MMA)0%ME sample prepared by miniemulsion polymerization. The

parameters were then compared with those of the HB-PBMA. The details are described in the

following section.

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Figure 6-7. (A) Comparison of the plots of fm vs annealing time for P(BA55-MMA44)ME latex films at

45 °C containing (*)0 wt %, (■)1 wt %, (◆)3 wt %, (▲)5 wt %, and (●)6 wt % of HB-

PBMA; (B) Plot of Dapp as a function of fm for P(BA-MMA) at 45 °C with (*)0 wt %, (■)1

wt %, (◆)3 wt %, (▲)5 wt %, and (●)6 wt % of HB-PBMA; (C) Master curve of Dapp for

HB-PBMA in P(BA-MMA) films; (D) Comparison of the plots of fm vs annealing time for

P(BA-MMA) at 45 °C with (■)0 wt %, (□)2 wt %, (●)5 wt %, and (△)7 wt % of

TexanolTM; (E) Plots of Dapp vs fm for P(BA-MMA) at 45 °C with (△)0 wt %, (□)2 wt %,

(◇)5 wt %, and (○)7 wt % of TexanolTM; (F)Master curve of Dapp for TexanolTM in

P(BA-MMA) films.

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6.3.5 Comparing the Plasticization Effect of HB-PBMA and TexanolTM

To illustrate the plasticization efficiency of the HB-PBMA, I compare its effectiveness

with that of TexanolTM. TexanolTM was incorporated into the P(BA55-MMA44)0%ME latex as

described in Chapter 6.2.2. Latex films were then cast and ET experiments were carried out

under the same conditions as those used for the HB-PBMA containing films.

Values of fm were calculated from the fluorescence decays, and were plotted as a function

of annealing time in Figure 6-7D. The addition of TexanolTM increased the polymer diffusion

rate as expected. The plot has the same tendency as that of the HB-PBMA containing P(BA-

MMA) films in Figure 6-7A. The fm values increased more rapidly as the TexanolTM

concentration increased. I then fitted these fm values to the Fickian diffusion model, and

calculated Dapp values. The Dapp values obtained are plotted as a function of fm in Figure 6-7E.

The figure shows that Dapp decreases as fm increases, and the films with higher concentrations of

TexanolTM have greater values of Dapp, which is consistent with our previous findings in

TexanolTM containing PBMA films.4

0.0

1.0

2.0

3.0

0 30 60 90

Фa-1

{ln[D

app(

T, Ф

a)/D

app(

T,0)

]}-1

Figure 6-8. Plot of 1/ln[Dp(T,Φa)/Dp(T,0)] vs 1/Φa at 45 °C for the P(BA55-MMA44)ME films containing different amounts of HB-PBMA.

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0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 20 40 60

Фa-1

{ln[D

app(T

, Фa)

/Dap

p(T

,0)]}

-1

Figure 6-9. Plot of 1/ln[Dp(T,Φa)/Dp(T,0)] vs 1/Φa at 45 °C for the P(BA55-MMA44)0%ME films containing different amounts of TexanolTM.

Using the procedure mentioned above, I calculated the shift factor ln[Dp(T, фa)/Dp(T,0)] =

ln Dp(T, фa) - ln Dp(T,0). I then used the shift factor to create a master curve of Dapp values in

Figure 6-7F and plotted values of {ln [Dp(T, фa)/Dp(T,0)]}-1 as a function of 1/фa in Figure 6-9.

The slops of the obtained straight line gave a β(T) value of 0.055, and the intercept yielded a

fp(T,0) value of 0.030. The value of fp(T,0) obtained in these experiments is very close to that

(0.034) obtained in the HB-PBMA experiments (shown in Table 6-6).

The two fp(T,0) values, which represents the fractional free volume of P(BA-MMA)

without any coalescing agent, are very close since the same P(BA-MMA) latex was used in both

experiments. This important result indicates the validity of the data analysis. The β(T) parameter

is a measure of the efficiency of the added coalescing agent, which means a greater β(T) value

represents a higher plasticization efficiency. Table 6-6 shows that the β(T) value of HB-PBMA is

ca. 62 % of that of TexanolTM. The comparison indicates that the HB-PBMA is a promising

candidate for an environment-friendly coalescing agent. In addition to enhancing polymer

diffusion rates, good coalescing agents should also maintain or improve mechanical properties of

latex films. This is also one of my concerns and will be discussed in the following section.

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Table 6-6. Fitting parameters for the Fujita-Doolittle equation.

P(BA55-MMA44)/HB-PBMA P(BA55-MMA44)/TexanolTM

fp (T,0) a 0.034 0.030

β(T) a 0.034 0.055 a T = 45 oC.

6.3.6 Effect of HB-PBMA on Mechanical Properties of the P(BA55-MMA44) Latex Films

I tested mechanical properties of the miniemulsion P(BA55-MMA44) samples used in the

previous ET experiments. Samples for tensile testing were prepared as described in section 6.2.3.

For each sample, at least five measurements were performed under ambient conditions. In Figure

6-10A I plot representative tensile stress-strain curves for each sample. The highest points

represent rupture. One can notice that the addition of 5% TexanolTM significantly lowers the

ultimate tensile stress while it increases the ultimate strain by a factor of 2. In contrast, HB-

PBMA has much less influence on the mechanical properties of the polymer films. For example,

the plot of the film containing 3 wt % HB-PBMA overlaps that of the film without any additive

(marked 0% in Figure 6-10) over a broad range of strain. Quantitative values of the tensile stress

at break are shown in Table 6-7. Here one can see that the tensile stress of the film containing 5

wt % TexanolTM is less than one third of that of the additive-free film. Note that all of the films

containing HB-PBMA have similar ultimate tensile stress values, and that these are twice that of

the 5 wt % TexanolTM film.

Young’s modulus was measured from the initial slopes at the early stage of the stress-

strain curves (Figure 6-10B). These values are presented in Table 6-7. The values of Young’s

modulus of the HB-PBMA containing films are similar to that of the film without any additives.

However, for the film obtained by adding 5 wt % TexanolTM, modulus decreased by one order of

magnitude. The toughness of a film is obtained as the integrated area under the stress-strain

curve. It is a measure of the energy a sample can absorb before it breaks. I compare the values of

toughness of the various film samples in Table 6-7. These results show that the addition of 5%

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TexanolTM results in the lowest value of toughness and the HB-PBMA has less effect on film

toughness.

From the tensile testing results it can be concluded that the HB-PBMA has negligible

influence on Young’s modulus and an insignificant effect on the ultimate tensile stress and the

film toughness. For TexanolTM, the reduced tensile strength is to some extent balanced by an

increased strain at break. Never the less the toughness is still reduced compared to the other

samples.

Table 6-7. Tensile testing results of P(BA55-MMA44) filmsa

samples Ultimate tensile stress

(MPa)

Young’s modulus

(MPa)

Toughness

(MJ/m3)

P(BA55-MMA44)0%ME 5.1±0.9 0.29±0.08 1156±209

P(BA55-MMA44)1%ME 3.0±0.4 0.28±0.02 555±33

P(BA55-MMA44)3%ME 3.5±0.3 0.42±0.03 981±128

P(BA55-MMA44)5%ME 2.8±0.4 0.23±0.04 687±103

P(BA55-MMA44)0%ME & 5% TexanolTM

1.6±0.1 0.04±0.00 519±18

a Values are the average of a minimum of 5 measurements.

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Figure 6-10. (A)Tensile stress-strain curves for P(BA55-MMA44) latex films with various concentration of HB-PBMA and TexanolTM, which were synthesized by miniemulsion polymerization. (B)Early stages of the stress-strain curves in (A).

6.3.7 Conclusion I used miniemulsion polymerization to synthesize D- and A-labeled P(BA55-MMA44) latex

particles containing various concentrations of HB-PBMA. The presence of the HB-PBMA had

no effect on the polymerization kinetics. All of the latex polymers have similar molar mass, and

the latex particles have a similar particle size and a narrow size distribution. Fluorescence decay

experiments were carried out to study the effect of the HB-PBMA on the rate of polymer

diffusion in the P(BA55-MMA44) latex films. Comparison of the diffusion data to those of the

TexanolTM-containing films indicates that HB-PBMA has similar plasticization efficiency to

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TexanolTM. Tensile testing reveals that the HB-PBMA containing films have better mechanical

properties than the TexanolTM containing films.

6.4 Seeded Emulsion Polymerization Approach

6.4.1 Synthesis of Seeded P(BA55-MMA44) Latex Particles

I synthesized P(BA55-MMA44) latex samples using the HB-PBMA latex particles as seeds.

The feed ratio of BA:MMA:MAA was 55:44:1. By varying the ratio of seeds:monomes, I

synthesized two pairs of D- and A-labeled latex particles with HB-PBMA weight fractions of 5

wt % and 10 wt % (based on total polymer). In my notation, the type of fluorescent dye is

indicated by a superscript “D” or “A”, which represents “donor” or “acceptor”, respectively, and

a subscript refers to the weight % of HB-PBMA seeds. For example, P(BA55-MMA44)A5%SD

refers to the A-labeled P(BA55-MMA44) containing 5 wt % of HB-PBMA seed particles.

The molecular weights were measured using a GPC equipped with a triple detector array.

The GPC data in Table 6-8 show that all samples have similar molecular weights and narrow

molecular weight distributions. The Mn values are in the range of 22,000 ~ 25,000 g/mol, and the

values of Mw/Mn are less than 5.0. The values of Tg (shown in Table 6-8), which were measured

using a a TA Instruments DSC Q100 differential scanning calorimeter, decrease as the weight

fractions of the HB-PBMA seeds increase. Particle sizes of the P(BA55-MMA44) latices were

measured by dynamic light scattering using a BI-90 particle sizer. The seeded emulsion

polymerization offers good control over particle size and size distribution. The average diameters

for all samples were in the range of 237 to 251 nm (Table 6-8). All samples have narrow particle

size distributions. The solids contents were determined by gravimetric analysis, and all latex

samples contain ca. 40 wt % solids.

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Table 6-8. Characteristics of the P(BA55-MMA44) latex polymers and particles

Molecular weight a (g/mol) Particle size b Latex sample

Mn Mw Mw/Mn d (nm) Poly.c

Tg

(oC)

solids content

(%)

P(BA55-MMA44)D5%SD 25,000 124,000 5.0 237 0.059 6.6 37.2

P(BA55-MMA44)A5%SD 23,000 100,000 4.3 241 0.042 5.6 41.4

P(BA55-MMA44)D10%SD 22,000 98,000 4.5 251 0.059 3.1 38.1

P(BA55-MMA44)A10%SD 22,000 104,000 4.7 239 0.04 1.9 37.6

a Absolute molecular weights were measured by GPC using a Viscotek TDA302 triple detector array.

b Particle size and size distribution were measured by a BI-90 Particle Sizer. c Polydispersity (poly) is a measure of the width of the particle size distribution, taking values

close to zero (0.000 to 0.020) for nearly monodisperse samples, and small (0.020 to 0.080) for narrow size distributions.

6.4.2 Effect of HB-PBMA Seed Particles on Polymer Diffusion in Seeded P(BA55-MMA44) Latex Films

Polymer diffusion was studied by ET experiments. Latex films for ET experiments were

prepared from 1:1 mixtures of D- and A-labeled seeded P(BA55-MMA44) latex particles

containing the same weight % of HB-PBMA seeds. Two drops of the latex mixture was cast onto

a small quartz plate and allowed to dry at 4 oC. All films were transparent and free of cracks.

Fluorescence decay measurement was immediately carried out on the freshly formed latex film

in a cold quartz tube using a time-correlated single-photon counting instrument. The films were

then aged at 23 oC in a humidity chamber (54 % relative humidity), and fluorescence decays

were monitored as a function of aging time.

I calculated ФET and fm from the obtained decays using eq. 2-4 and eq. 2-6, respectively.

Values of fm for all samples are plotted as a function of annealing time in Figure 6-11A. The

figure shows that the fm values increase as the aging time increases for all films. The films with

higher concentrations of HB-PBMA seeds have greater fm values than those with a lower HB-

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PBMA seed concentration at the same annealing time. For example, it took 64 h at 23 °C to

reach 30% of mixing for the P(BA55-MMA44) film containing 0 wt % of HB-PBMA, whereas for

the film containing 10 wt % HB-PBMA seeds, fm already exceeded 47%. For comparison, the fm

plots of the P(BA55-MMA44) films containing 5 wt % and 10 wt % TexanolTM are shown in

Figure 6-11B. The two figures indicate that the HB-PBMA seeds and TexanolTM have similar

effect on fm. I show the early stages of these fm plots in Figure 6-11C and D, which show that

TexanolTM has greater enhancement on fm in the early stages.

To quantitatively compare the effect of the HB-PBMA seeds and TexanolTM on polymer

diffusion rate, I fitted the values of fm to a spherical Fickian diffusion model, and calculated Dapp.

The Dapp values are plotted as a function of fm in Figure 6-12A for the films containing HB-

PBMA seeds, TexanolTM, and 0 wt % additives. The plots show that both HB-PBMA seeds and

TexanolTM improve the polymer diffusion rates. The films containing 5 wt % HB-PBMA seeds

have greater values of Dapp than the films containing 5 wt % TexanolTM over a broad range of fm.

With 10 wt % HB-PBMA seeds, Dapp increases by an order of magnitude. The values of Dapp of

the TexanolTM-containing films decrease more quickly than those of the seeded P(BA55-MMA44)

films.

Using the Fujita-Doolittle fitting procedure described above, I calculated the shift factor

ln[Dp(T, фa)/Dp(T,0)] and plotted values of {ln [Dp(T, фa)/Dp(T,0)]}-1 as a function of 1/фa for

TexanolTM and the HB-PBMA seeds in Figure 6-12B, in which the intercept value of fp(T = 23 oC,0) (0.033) was calculated from the previous value of fp(T = 45 oC,0) (0.034 from Table 6-6)

and the shift factor of the Dapp between the two temperatures. The slops of the obtained straight

line gave a β(T = 23 oC) value of 0.041 for TexanolTM, and a β(T = 23 oC) value of 0.060 for the

HB-PBMA seed particles. This result indicates that the HB-PBMA seed particles are efficient

agents for promoting polymer diffusion in P(BA55-MMA44) films.

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Figure 6-11. Comparison of the plots of fm versus aging time for P(BA55-MMA44) at 23 °C with (●)0 wt % additives, (▲)5 wt % HB-PBMA seeds, (■)10 wt % HB-PBMA seeds, (△)5 wt % TexanolTM, and (□ )10 wt % TexanolTM. (C) and (D) show the early stages of the fm plots in (A) and (B), respectively.

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6.4.3 Effect of HB-PBMA Seed Particles on Mechanical Properties of the Seeded P(BA55-MMA44) Latex Films

I tested tensile properties of the seeded P(BA55-MMA44) samples and compare the results

with those of the TexanolTM-containing films. The specimens were prepared as described in

Chapter 6.2.3 and tensile experiments were performed under ambient conditions. Figure 6-13A

shows representative tensile stress-strain curves for each sample. Quantitative values of tensile

Figure 6-12. (A)Comparison of the plots of the Dapp versus fm for P(BA55-MMA44) latex films containing (●)0 wt % additives, (▲)5 wt % HB-PBMA seeds, (■)10 wt % HB-PBMA seeds, (△)5 wt % TexanolTM, and (□)10 wt % TexanolTM at 23 °C. (B)Plot of 1/ln[Dp(T,Φa)/Dp(T,0)] vs 1/Φa at 23 °C for the P(BA55-MMA44) films containing different amounts of (○)TexanolTM and (▲)HB-PBMA seeds.

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properties were obtained from at least five specimens for each sample. These values are depicted

in Table 6-9.

Table 6-9 show that the HB-PBMA-seeded P(BA55-MMA44) samples have similar values

of Young’s modulus to the film without any additives, which can also be observed from Figure

6-13B. The values of toughness of the HB-PBMA-seeded P(BA55-MMA44) films are close to that

of the film without any additives as well. These values are much greater than those of the

TexanolTM-containing samples. The P(BA55-MMA44)5%SD film has an ultimate tensile stress

value of 3.78 ± 0.34 MPa. Although this value is 56% of that of the film without any additives,

it is still greater than that of the P(BA55-MMA44)5%T film.

From the tensile testing results I conclude that the incorporated HB-PBMA seed particles

have negligible influence on Young’s modulus and toughness of the films. The presence of HB-

PBMA reduces the film tensile stress, but the influence is smaller than that of TexanolTM.

Table 6-9. Tensile testing results of P(BA55-MMA44) films

Samples

Ultimate tensile stress

(MPa)

Young’s modulus

(MPa)

Toughness

(MJ/m3)

P(BA55-MMA44)0% a 6.76±0.49 0.07±0.01 1790±120

P(BA55-MMA44)5%SD 3.78±0.34 0.06±0.02 1490±110

P(BA55-MMA44)10%SD 2.39±0.18 0.05±0.01 1400±150

P(BA55-MMA44)5%T b 2.50±0.25 0.02±0.00 750±160

P(BA55-MMA44)10%T c 1.39±0.20 0.01±0.00 420±100

a P(BA-MMA) sample made by semi-continuous emulsion polymerization without any additives.

b P(BA-MMA) sample made by semi-continuous emulsion polymerization with 5 wt % post-added TexanolTM.

c P(BA-MMA) sample made by semi-continuous emulsion polymerization with 10 wt % post-added TexanolTM.

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Figure 6-13. (A)Tensile stress-strain curves for P(BA55-MMA44) latex films with various concentration of HB-PBMA seed particles and TexanolTM, which were synthesized by seeded emulsion polymerization. (B)Early stages of the stress-strain curves in (A).

6.4.4 Conclusion I used seeded emulsion polymerization to synthesize D- and A-labeled P(BA-MMA) latex

particles containing various concentrations of HB-PBMA seed particles. All of the latex

polymers have similar molar mass, and the latex particles have similar particle sizes and narrow

size distributions. Fluorescence decay experiments were carried out to study the effect of the HB-

PBMA seeds on the rate of polymer diffusion in the P(BA-MMA) latex films. Comparison of the

diffusion data to those of the TexanolTM-containing samples indicates that the HB-PBMA seeds

have a slightly greater enhancement on polymer diffusion rates. Tensile testing reveals that the

HB-PBMA-seeded P(BA-MMA) latex films have better mechanical properties than the

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TexanolTM-containing films. Their values of Young’s modulus and toughness are similar to those

of the films without any additives.

6.5 Summary I incorporated the HB-PBMA into the P(BA-MMA) latices using three approaches:

blending approach, miniemulsion polymerization approach, and seeded emulsion polymerization

approach. ET studies show that the HB-PBMA, which contains ca. 19 wt % C12-SH, enhances

polymer diffusion rates in all three cases. It has similar effectiveness to TexanolTM. Mechanical

measurements indicate that the HB-PBMA has negligible or limited influence on film

mechanical properties, while TexanolTM significantly decreases film mechanical properties. The

combination of ET and mechanical studies suggests that the HB-PBMA is a potential coalescing

agent which offers certain advantages over traditional VOCs in latex coatings.

6.6 References

1 Zohrehvand, S.; Nijenhuis, K. te Colloid Polym Sci 2005, 283, 1305-1312. 2 Wang, Y.; Winnik, M. A. Macromolecules 1990, 23, 4731-4732. 3 Winnik, M. A.; Wang, Y.; Haley, F. Journal of Coatings Technology 1992, 64, 51-61. 4 Juhué, D.; Wang, Y.; Winnik, M. A. Makromol. Chem. Rapid Commun. 1993, 14, 345-349. 5 Pekcan, Ö.; Canpolat, M. Polymers for Advanced Technologies 2003, 5, 479-484. 6 Wustmann, U.; Ardaud, P.; Perroud, E.; Jeannette, T. European Coatings Journal 1997, 11,

1022-1024. 7 Strepka, A.; Joshi, M. V.; Arendt, W. D. U.S. Pat. Appl. Publ. 2008, US 2008103237 A1

20080501 CAN 148:497774 AN 2008:529069 8 Anchor, M. J.; Drewno, G. W. U.S. Pat. Appl. Publ. 2008, US 2008119600 A1 20080522

CAN 148:563540 AN 2008:613601 9 Brandenburger, L. B.; Sicklesteel, B.; Owens, M. J. PCT Int. Appl. 2002, WO 2002068547

A1 20020906 CAN 137:218473 AN 2002:676114

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10 Lahtinen, M.; Glad, E.; Koskimies, S,; Sundholm, F.; Rissanen, K. Journal of Applied Polymer Science 2002, 87, 610-615.

11 Winnik, M. A.; Pinenq, P.; Kruger, C.; Zhang, J.; Yaneff, P. V. Journal of Coatings Technology 1999, 71, 47-60.

12 Liu, Y.; Haley, J. C.; Deng, K.; Lau, W.; Winnik, M. A. Macromolecules 2008, 41, 4220-4225. 13 Kruse, R. L. Polym Lett. 1967, 5, 437-439. 14 O’Driscoll, K. F.; Reilly, P. M. Makromol. Chem., Macromol. Symp. 1987, 10/11, 355. 15 Buback, M.; Feldermann, A.; Barner-Kowollik C. Macromolecules 2001, 34, 5439-5448.

16 Liu, Y.; Lau, W.; Winnik, M. A. “Effect of Hyper Branched Poly(n-butyl methacrylate) on

Polymer Diffusion in Miniemulsion Poly(n-butyl acrylate-co-methyl methacrylate) Latex

Films,” in preparation.

17 Bradley, M. A., Prescott S. W., Schoonbrood H. A. S., Landfester K., Grieser F. Macromolecules 2005, 38, 6346-6351.

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Figure A1. 1H NMR spectrum of P(BA55-MMA44)A1%ME, which was synthesized by miniemulsion

polymerization and contains 1 wt % HB-PBMA. CDCl3 was used as solvent.

Appendix 6.1

The 1H NMR spectrum of P(BA55-MMA44)A1%ME final product (100 %

monomer conversion).

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Figure A2. Tensile stress-strain curves for P(BA55-MMA44) latex films with various concentration of HB-PBMA and TexanolTM, which were synthesized by miniemulsion polymerization.

Appendix 6.2

Tensile stress-strain curves for P(BA-MMA) latex films by miniemulsion

polymerization

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Appendix 6.3

Values of ФET(0) and ФET(∞) of the three approaches

Table A1. Comparison of ФET(0) and ФET(∞) of the three approaches

Approach Additive ФET(0) ФET(∞)

Blending approach 0 wt % additives 0.07 0.48

Miniemulsion polymerization approach 0 wt % additives 0.10 0.51

Seeded emulsion polymerization approach 0 wt % additives 0.07 0.48

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Appendix 6.4

Chemical structure of Me-β-CD

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Appendix 6.5

Chemical structure of TexanolTM

2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (TexanolTM)