An Approach to High Performance Zero VOC Waterborne

Coatings :

Polymer Interdiffusion vs. Crosslin king

in

Carboxylic acid-Carbodiimide Latex Films

Hung Hoang Pham

A thesis submitted in confonnity with the requirements for the degree of Doctor of

Philosophy, Graduate Department of Chemistry, University of Toronto

6 Copyright by Hung Hoang Pham (2000)

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Abstract

This thesis examines the cornpetition between polymer interdiffusion and

crosslinking reaction in latex films. These latex films were prepared fiom a blend of two

sirnilar sofi latexes based on poly(2-ethylhexyl methacrylate) (PEHMA). P o l p e r s of the

first latex contain a srna11 amount of carbodiimide (-N=C=N-) groups plus 1 mol% 9-

anthryi methacrylate (An-MA, fluorescent acceptor), and polymers of the second latex

contain a srnail amount of carboxylic acid (-COOH) groups plus 1 mol% 9-

phenanthylrnethyl methacrylate (PheMMA; fluorescent donor). The -N=C=N- group

reacts w-ith the -COOH goup to f o m an N-acyl urea, and the Phe donor, when excited,

can transfer its energy to the An acceptor. In the initially formed film, these latex

particles are adjacent to each other, but the reactive groups and the fluorescent molecules

are still confined within their own cells. Polymer interdiffusion will bring the reactive

groups into close proximity for them to react, and will bring the fluorescent

chromophores close enough for energy transfer (ET) to occur. We use this energy

transfer to follow polyrner interdiffkion, and at the sarne time use Fourier transform

infrared (FTIR) to monitor the extent of the chemical reaction.

We find that polyrner interdiffiision is necessary for the reaction to proceed, but

the reaction retards the rate of polymer diffusion. Once the reaction occurs, long branches

and high molecular weight polymers are formed. The difision rate of the branched

polymers is much siower than that of the original linear polymers. Because the reaction

is coupled to diffusion, the reaction rate also drops. We also found that polyrners

containing -COOH groups and polymers containing -N=C=N- groups have limited

miscibili ty, but the crosslinking reaction promotes mixing between the two polymers.

We aIso find that rapid polymer interdiffusion at the early stages is important for

films to develop to 100% of gel formation. Factors that promote formation of branched

polymers at the inter-particle interface, e.g. a decrease in anneaiing temperature, or an

increase in the reactive comonomers, or an increase in the reactivity of the reaction, lead

to a decrease in polyrner interdifision. Here films with less than 100% gel content are

obtained.

We also find that these films exhibit the maximum toughness at early stages of

annealing. The toughness decreases as the extent of both crosslinking and polymer

diffùsion increases. The rnodulus increases, while the extension-at-break decreases as the

extent of both crosslinking and polymer diffirsion increases.

Acknowledgements

I am very gratefiil to my research supervisor, Professor blitcheli A. Winnik,

whose enthusiasm and wise knowledge about polyrner science attracted me to his group.

rhrough the years, he always encouraged me to work and think independently, yet

provided me with valuable discussions when needed. He has always offered me

opportunities to become involved in a number of research projects, as well as in other

group activities. 1 learned from him. not only scientific information, but aiso writing and C

comrnunicating skills. 1 would like to thank him for his supervision, guidance and

support.

My special thanks go to Dr. Jiandong Tong, Dr. Jose Paulo Farinha and Dr.

Yahya Rharbi for their h i t h i l discussions, Dr. John Spnro for his software program to

calculate the area under tensile curves, and Dr. Matthew Moffitt for his proof reading the

thssis. It \vas very enjoyable working with them.

1 would also like to thank dl members in Professor Winnik's laboratory for the

friendly environment they have created.

1 owe rny deep thanks to my parents, sisters and brother for their support,

encouragement and unconditional love.

Table of Contents

Introduction Background Objectives of the project The system to be studied Ouen-iew of the thesis References

Synthesis and Characteriuition of CCEhU and t-BCEMA Introduction Experimental 2.2.1 General preparation 2 - 7 2 PreparationofCCEMA 2.2.3 Preparation of t-BCEMA 1.2.3 Deterrnination of the IR extinction coefficient (E) of CCEMA

and t-BCEMA Results and Discussion 2.3.1 Purification and characterization of CCEMA and t-BCEMA

2.3.2 Extinction coefficient (E) of CCEMA and t-BCEMA Conchsions Re ferences

Synthesis, Characterization and Stability of Carbodiimide Functionality in Latex Dispersions and in Latex Films Introduction Experimental 3 -2.1 Preparation of seed particles (PBMA or PEHMA) 3 -2.2 Preparation of carbodiimide-containing latex particles 3.2.3 Determination of the carbodiimide (-N=C=N-) content of

carbodiirnide-containing latex particles 3 -2.4 Thermal stability of the -N=C=N- groups in carbodiimide-

containing Latex dispersions 3 -2.5 Preparation of latex films 3 -2.6 Determination of the percentage carbodiimide remaining in latex

films Results 3 -3.1 Latex synthesis and characterization 4 1

3 . Stability of the -N=C=N- moiety during emulsion copolymenzation

3 3 3 Stability of the -N=C=N- functionality in latex dispersions 3.34 Stability of the -N=C=N- functionality in latex films Discussion Conclusions Re ferences

Miscibility of PEHMA Latex Copoiymers in Films Introduction 3.1.1 Principles of non-radiative energy transfer (ET) 3.1.2 Application of ET to study miscibility in blend of polymers Experimental 4.2.1 Preparation of labeled particles 1.2.2 Preparation of dispersion- and solvent-cast films 4.2.3 Energy transfer measurements and data analysis Results and Discussion 4.3.1 Characterization of latex polymers 4.3 -2 Miscibility of PEHMA copoIIvrners

4.3 -3.1 PEHlMA-DIPEHMA-A blend 4.3 -2.2 A-tBCEMA-SIPEHMA-D blend 4.3.2.3 D-MAA-Il/PEKMA-A blend 4.3 -2.4 D-MAA- 1 l!A-tBCEMA-5 blend 4.3 -2.5 D-MAA-20IA-tBCEMA- 1 1 blend 4.3 -2.6 D-MAA- 1 1 /A-tBUEMA-5 blend

Conclusions References

Polymer Interdiffusion vs. Crosslinking in Latex Films of PEHMA Copolymers Introduction 5.1.1 Polyrner diffusion across interfaces 5.1 2 Crosslinking reaction Experimental 5.2.1 ET data analysis 5 .L? FTIR measurements and data analysis 5.2.3 Gel content and swell ratio rneasurements Resul ts 5.3.1 PEHMA-D/PEHMA-A blend 5.3 2 D-MAA- 1 1/A-tBCE1MA-5 blend

5.3 -2.1 Temperature effects 5.3 -2.2 Pre-annealing effects

5.3 -3 D-MAA-20/A-ti3CEMA- 1 1 blend 5 -3 -3.1 Temperature effects 5 -3 -3 2 Pre-annealing effects

5.3.4 D-MAA- 1 1/A-CCEMA-4.6 5 3 5 D-MAA-S/A-CCEMA-3 -2 blend

5 -3.5.1 Temperature e ffects 5 -3 -5.2 Pre-annealing effects

Discussion 5.4.1 D-MAA and A-tBCEMA system 5.4.2 D-MAA and A-CCEMA system Conclusions References

6 Evolution of Tensile Properties in Latex Films of PEHiMA Copolymers

6. 1 Introduction 6.2 Experimental 6.3 Results

6.3.1 PEHMA-DREHMA-A blend 6.32 D-MAA- 1 UA-tBCEMA-5 blend 6.3 -3 D-MAA-2O/A-tBCEMA- 1 1 blend 6.3 -4 D-MAA- 1 1 /A-CCEMA-4.6 blend 6.3 -5 D-MAA-S/A-CCEMA-3 -2 blend

6-1 Discussion 6.4.1 Effects of reactant concentrations 6.4.2 Effects of annealing time 6.4.3 Effects of annealhg temperature 6.4.4 Effects of type of crossluiking

6.5 Conclusions 6.6 References

List of Tables

Table 3. 1: Recipes to prepare PBMA and PEHMA seed particles.

Table 3.2: Recipes used in the feed stage to prepare cabodiimide- containing PBMA and PEHMA latex particles.

Ta blr 3 -3 : C haracteristics of carbodiimide-containing PBMA and PEHMA latex particles.

Table 3.4: n e percentage of carbodiimide remaining after carbodiimide- containing latexes were prepared.

Table 3.5: The percentage of carbodiimide remaining obtained from the latex dispersions afier storage at room temperature for 1 year. The percentage of carbodiimide remaining afier polymerization is aiso included for cornparison.

- - - - - - - - - a - - - - - - - - - - - -

Table 4.1 : Representative recipes in the second stage of emulsion polymerization to prepare labeled latexes.

Table 4.2: Summary of the composition of each tvpe of labeled latex polymers, dong with the âbbreviated name.

Table 4.3 : Characteristics of the labeled latex polymers. - - - - - - - - - - - - - - - - - - - * - -

Table 5.1 : Surnmary of the time it takes for individual films to reach 80% gel formation, plus values off, and the extent of - N=C=N- reaction that correspond to this time.

Table 5.2: Summary of the time it takes for individual films to reach fm = 0.6. The arnount of tBCEMA available for the crosslinking in each film is also included. Al1 films were anneaied at 60°C.

Table 5.3: Summary of the time it takes for individuai films to reach fm =

0.6. The amount of CCEMA availabie for the crosslinking in sach film is also included. Al1 films were annealed at 60°C.

- - - - - - - - - - - - - - - - - - - - a -

Table 6.1 :

Table 6.2:

Results of the tensile, ET, gel-content and swell-ratio measurements on the D-MAA- 1 L /A-tBCEMA-5 thic k film samples annealed at 60°C for various amounts of tirne.

Results of the tensile, ET, gel-content and swell-ratio measurements on the D-MAA-20/A-tBCEMA-11 thick film samples annealed at 60°C for various arnounts of tirne.

Table 6.3: Results of the tensile, ET, gel-content and swell-ratio measurements on the D-MAA-1 IIA-CCEMA-4.6 thick film samples annealed at 60°C for various amounts of tirne.

Table 6.4: Results of the tende, ET, gel-content and swell-ratio measurernents on the D-MAA- 1 UA-CCEMA-4.6 thick film samples annealed at 40°C for various arnounts of time.

Table 6.3: Results of the tensile, ET, gel-content and swell-ratio measurements on the D-MAA-S/A-CCEMA-3.2 thick film samples annealed at 60°C for various amounts of time.

Table 6.6: Results of the tensile. ET, gel-content and swelt-ratio measurements on the D-MAA-S/A-CUEMA-3.2 thick film samples annealeci at 60°C for various amounts of time.

Table 6.7: Results of the tensile, ET, gel-content and swell-ratio measurements on the D-MAA-S/A-CCEM-3.2 thick film samples annealed at 22OC for various amounts of time.

List of Figures

Figure 2.1 :

Figure 2.2:

Figure 2.3:

Figure 2.4:

- - - - - - - - - Figure 3.1 :

Figure 3.2:

Figure 3.3:

Figure 3.4:

Figure 3.5:

Figure 2.6:

Figure 3.7:

Figure 3 -8:

1 H NMR spectra of neat CCEMA, dong with the assignrnent of the chemical shift values of the important hydrogen atoms.

I3c NMR spectra of neat CCEMA. dong with the assignment of the chemical shifi values of the important carbon atoms.

FTIR spectrum of neat CCEMA.

Plots of FTLR absorbante vs. concentration of CCEMA and t- BCEMA in CHCI;. - - - - - - - - - - - - - Schematic representation of the change of the polymerization rate with time in emulsion polyrnerization.

FTIR spectra of PEHMA and P(EHMA-CO-tBCEMA) copolymers.

Plots of the % -N=C=N- remaining vs. heating time at 80°C for P(BM4-CO-CCEMA) latex dispersions at pH 4? 8 and 12- The solids content is 5 wtY0. 30

Plots of the % -N=C=N- remaining vs. time at 80°C for P(BMA- CO-CCEMA) and P(EHMA-CO-tBCEMA) latex dispersions at pH 8. The solids content is 5 wt%. 32

Plots of the % -N=C=N- remaining vs. heating time at 60°C and 8O0C for P(EHMA-CO-tBCEM-4) latex dispersions at pH 8. The solids content is 5 wt%. 33

Plots of the % -N=C=N- remaining vs. heating time at 80°C for a dispersion of P(BMA-CO-CCEMA) in water at pH 8 and in a dry latex film prepared from this dispersion. 35

Plots of the % -N=C=N- remaining vs. annealing time of P(BMA-CO-CCEMA), P(EHMA-CO-CCEMA) and P(EHMA-CO- tBCEMA) latex films at 80°C. These films were prepared from dispersions at pH 8.

Plots of the % -N=C=N- rernaining vs. annealing time of P(EM4-CO-tBCEMA) latex films in the presence and absence of P(EHMA-CO-MAA) latex copolymers. The P(EHMA-CO- MAA) dispersion was adjusted to pH 8 before it was mixed with the P(EHMA-CO-tEKEMA) dispersion. The film was dried at 22OC. Under these conditions, ammonia evaporates from the dry

film to reform the -COOH group. The films were then annealed at 60°C.

Figure 4.1 :

Figure 4.2:

Figure 4.3:

Figure 3.4:

Figure 4.5:

Figure 4.6:

Fisure 4.7:

Figure 3.8:

Figure 3.9:

Donor fluorescence decay profiles of a PEHMA copdymer latex blend film consisting of a 1 : 1 mixture Phe-labeled latex containing 1 1 mol % MAA comonomer and an An-labeled latex containing 5 mol % tBCE1MA comonomer. This film was formed at room temperature and annealed at 60°C for different periods of time.

GPC chromatograms of PEHiMA-D latex, The sample was prepared by drying the dispersion into a solid film at room temperatwe and then dissolving it in THF solvent.

Donor fluorescence decay profiles of latex films consisting of D- MAA-1 l polyrner ody, and of D-kW-11 polymer in the presence of unlabeled latex containing tBCEMA. These films were formed at room temperature and annealed at 60°C for 10 h.

Plots of Qrn VS. time of films prepared fiom the PEHMA- DIPEHMA-A blend and annealed at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast tilm was dried at Z ° C .

Plots of vs. time of films prepared from a PEHMA-D/A-t- BCEMA-5 blend and annealed at 60°C. The dispersion-cast film was prepared at 4*C while the THF-cast film was dried at 22OC.

Plots of @ET VS. time of films prepared fiom a D-MAA- I 1 /P EHMA-A blend and annealed at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast film was dried at 22°C.

Plots of OET VS. time of films prepared fiom a D-MAA- îO/PEHMA-A blend and annealed at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast film was dried at nOc.

Plots of OR. vs. time of dispersion- and THF-cast films prepared fiom the D-MAA- 1 1 /A-tBCEMA-5 blend and annealed at 60°C. Both dispersion-cast and THF-cast films were dried at 22OC.

Plots of aET and percentage of -N=C=N- remaining vs. tirne for THF-cast films prepared fiom a D-MAA- 1 1 /A-tBCEMA-5 blend and annealed at 60°C. The film was formed at 22°C.

Figure 4.1 0: Plots of OET and percentage of -N=C=N- remaining vs. time for THF-cast films prepared fiom the D-PuL4A-20IA-tBCEMA-1 t blend and annealed at 60°C. The film \vas fonned at 22OC.

F r 4 1 1 : A plot of -N=C=N- remaining (%) vs. time for latex particles containing tBCEMA when they are treated with propionic acid at room temperature.

Figure 3.12: Plots o f a m VS. time of THF-cast films prepared from the D- MAA-1 UA-BUEMA-5 blend and annealed at 60°C and then at 100°C (in dotted iine).

- - - - - - - - - - - - - - - - - - - - - - - Figure 5.1 :

Figure 5.2:

Figure 5.3 :

Figure 5.4:

Figure 5.5:

Figure 5.6:

Figure 5.7:

Figure 5.8:

FTIR spectra of a Iatex film prepared fiom a 1 : I mixture of the D-WU-1 UA-tBCEMA-5 blend. The film was fonned at room temperature and annealed at 60°C for various amounts of time t,.

Plots off, vs. time of latex films annealed at 2Z°C (room temperature), 30°C, 40°C, and 60°C. f, values were calculated fiom the aETdata. Fiims were castat4OC fiom a 1:I mixture of P EHMA-DPEHMA-A particles. 79

Plots of f, vs. tirne of latex films annealed at 2Z0C (room temperature), 30°C, 40°C, and 60°C. f, values were calculated from the area data. Films were cast at J°C fiom a 1 : 1 mixture of PEHMA-DPEHMA-A particles. 80

Plots of Dapp vs. fm at different temperature. 82

A plot of In Dwp vs. lm, in which Dqp values were calculated at fm = 0.55. 83

A master curve of Dwp vs. f,. It is constructed using Eawp = 37 kcal/mol, and the reference temperature is 2Z°C (295 K). 84

Piots of f, (O) and the arnount of -N=C=N- remaining (a) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (a). These f i h s were prepared at 2Z0C kom a 1 : 1 mixture of D- MAA- 1 1 and A-tBCEMA-5 fatex dispersion, and annealed at 60°C. 86

Plots of gel content and swell ratio for latex films prepared Erom D-MAA-1 l/A-tBCEMA-5 particles. These films were fonned at 22°C and annealed at 60°C.

Figure 5 -9: Plots of f, and the percentage of -N=C=N- remaining vs. time for dispersion-cast films prepared from a blend of D-MAA- 1 1lA-tBCEMA-5 latex. These films were formed at 22"C, annealed at 40°C (f, (O); percentage of -N=C=N- remaining (a)). and then annealed at 60°C (f, (O); percentage of -N=C=N- remaining (a)).

Figure 5.10: Plots of gel content and swell ratio vs. time at 40°C for latex films prepared fiom D-MAA- 1 UA-tBCEMA-5 particles. These films were prepared at E°C.

Figure 5.1 1 : Plots of fm (O) and the arnount of -N=C=N- remaining (a) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (O). These films were prepared at 22°C fkom a 1 : 1 mixture of D- MAA-20 and A-tBCEMA-I l latexes, and examined at 60°C.

Figure 5.12: Plots of the amount of -N=C=N- group rernaining monitored at 60°C for THF-cast films. These films were prepared at 22OC fiom the D-MAA- 1 1 /A-tBCEMA-5 and the D-MAA-20IA- tBCEMA- 1 1 polymers.

Figure 5.13: Plots of gel content and swell ratio vs. time of latex fiims prepared frorn D-MAA-20/A-tBCEMA- 1 1 particles. These films were fomed at 22°C and anneded ar 60°C.

Figure 5.1 4: Plot of f, (O) and the arnoi.int of -N=C=N- remaining (O) vs. time for the D-MAA-20/A-tBCEMA- 1 1 films monitored at 40°C. During the experiment at 40°C, some films were brought to 60°C. Plots f, vs. time at 60°C for latex films that have been annealed for various penods of time at 40°C (19 h, 44 h and 80 h) are plotted as filled squares (m). The point (O) refer to a film annealed directly at 60°C without pnor aging at IO0C.

Figure 5.15: Plots of f, (O) and the amount of -N=C=N- remaining (o) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (O). These films were prepared at 22°C from a 1 : I mixture of D- MAA- l L and A-CCEMA-4.6 latexes, and anneded at 60°C.

Figure 5-16: Plots of the amount of -N=C=N- remaining W. time at 60°C for THF-cast films prepared from the D-MAA- 1 LIA-CCEMA-4.6 sample (m) and for the D-MAA-I UA-tBCEMA-5 sarnple (O).

Figure 5.17: Plots of gel content and swell ratio vs. time for latex films prepared from D-MAA- 1 1 /A-CCEMA-4.6 particles. These films were fonned at 22OC and annealed at 60°C.

Figure 5.18: Plots off, (O) and the amount of -N=C=N- remaining (O) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (a). These films were prepared at 22'C from a 1 : I mixture of D- MAA-5 and A-CCEMA-3.2 latexes, and annealed at 60°C.

Figure 5.19: Plots of gel content and sweH ratio vs. time for latex films prepared fiom D-MAA-YA-CCEMA-3.2 particles. These films were forrned at 2S°C and anneaied at 60°C.

Figure 5.20: Plots off, (O) and the amount of -N=C=N- remaining (O) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (a). These films were prepared at 22OC from a 1 : 1 mixture of D- M M - 5 and A-CCEMA-3.2 Iatexes, and annealed at 40°C.

Figure 5.2 1 : Plots off, and the percentage of -N=C=N- remaining vs. time for latex films prepared b r n a mixture of D-MAA-5 + A- CCEMA-3.2 latex dispersions. These films were formed at Z0C, annealed at 40°C (f, (O); -N=C=N- remaining (%) (O)), and then annealed at 60°C (f, (a); -N=C=N- rernaining (m)).

Figure 5.22: Plots of gel content and swell ratio vs. tirne for films prepared from a mixture of D-MAA-5 + A-CCEMA-3.2 latex dispersions. These films were cast at 22OC, anneaied at 40°C and then at 60°C. The gel content and swell ratio data at 40°C are plotted as open triangles (A) and open diamonds (O). The gel content and swell ratio data at 60°C are plotted as filled triangles (A) and filled diamonds (+), respectively.

Figure 5.23: A plot of log(time to 0.60 f,) vs. amount of tBCEMA remaining for the MAA-tBCEMA system.

Figure 5.24: Plots off, vs. time for films prepared at 22°C Born the D-MAA- 5IA-CCEMA-3.2 latex. These films were then aged at 22OC for lh, 15 h and 5 days before they were anneaied at 60°C.

Figure 5.25: A plot of log(tirne to 0.60 f,) vs. the -N=C=N- groups content for the carbodiimide containing latex for both the MAA- CCEMA system and the MAA-tBCEPUIA system.

. . . . . . . . . . . . . . . . . . . . . . Figure 6.1 : A schematic drawing of a ~ample for tende testing.

Figure 6.2: A hypothetical stress-strain curve for a ductile sample.

Figure 6.3: Generd lcinds of stress-strain curves for various types of coatings. The scales indicate order-of-magnitude values.

Figure 6.4: Stress-strain plots of PEHMA hopmopolymer (A) engineering stress-strain curve, (B) true stress-strain curve.

Figure 6.5: Stress-strain plots of the D-MAA-11/A-tBCEMA-5 films examined at 60°C for various amounts of tirne; (A) engineering stress-strain curves, (B) true stress-strain curves.

Figure 6.6: True stress-strain curves of the D-MAA-20/A-tBCEMA-11 films anneded for different times at 60°C.

Figure 6.7: True stress-straïn cuves of the D-MAA-SIA-CCEMA-3.2 films annealed for different times at 60°C.

Figure 6.8: True stress-strain curves of the D-MAA-YA-CUEMA-3.2 films annealed for different times at 60°C.

Figure 6.9: True stress-strain curves of the D-MAA-YA-CCEMA-3 -2 films annealed for different times at 22°C.

Figure 6.10: True stress-strain curves at different anneaiing times for films prepared fiom core/shell latex particles at 23°C. The core consists of polystyrene homopolyrner, whereas the shell consists of random copolymer of styrenehutyl acrylate/acrylic acid (50:49: 1). These particles were prepared by a semi-continuous emulsion polyrnerization in the absence of chain transfer reagent. The latex films were prepared at Z°C, and then annealed at 32OC for different times.

1 Introduction

1.1 Background

A solvent-based coating formulation typically contains about 60 to 70 wt% volatile

organic solvents. These volatile organic solvents, sometimes referred to as volatile

organic compounds (VOCs), are employed in the coating formulation to dissolve resins

and to control the coating viscosity at various stages of drying after the coating has been

applied to a substrate. M e r the coating has been applied to a substrate, a solid film

forms as the VOCs evaporate into the atmosphere.

in the presence of sudight, VOCs and nitrogen oxides (NO,) react in the

atrnosphere to create ground-IeveI ozone. This ground-level ozone is a major component

of urban smog. Exposure to ground-level ozone is associated with a wide variety of

human health effects, agricultural crop loss, and damage to forests and ecosystems.

Human health effects include respiratory problems (e.g. cough, chest pain, shortness of

breath, throat irritation), decreased h g fûnction, inflammation of the lung, and possible

long-term damage to the lungs. Such adverse affects of VOCs led to the first regulation

known as Rule 442, ' introduced by the California South Coast Air Quality Management

District in 1976 to reduce VOC emission fiom coatings. Today, regdations of VOC

emission from coatings formulations has reached US federal and international levels. For

example, production and use of l , l , 1-tnchloroethane, a solvent widely used in coatings,

was to be reduced by 50% for the penod 1996-1999, and by an additional 20% for the

period 2000-2001, after which the use of the material will be prohibited. ' These regulations have forced the coatings industry to consider alternatives to

solvent-based coatings. These alternatives include reactive solvents, powder coatings,

high-solids coatings, radiation-cure coatings, and water-based coatings. 1

1.2 Objectives of the project

Among the above alternatives, coatings that used water as the main solvent are

particularly attractive. They offer several advantages such as low odor, low

combusti bi 1 itv, quick drying, and inexpensive manufacturing costs.

In water-based coatings, latex particles are one of the major components. They

form the binder in the coatings. The function of the binder is to support the pigments in

the coatings and to provide adhesion of the coating to the substrates. Latex particles are

ofien defined as polymer particles prepared by emulsion polyrnerization~ colloidally

dispersed in water. These particles are stabilized against flocculation by electrostatic or

steric repulsion provided by surfactants or polar groups at their surface. The size of these

particles is typically 30 - 500 nrn.

stage I -

stage II T 2 MFT

close contact

defomed particles

stage III

T > Tg 1

caherent filrn

Scheme 1.1 : Film formation of latex dispersion

When a latex dispersion is cast on a substrate and water is allowed to evaporate, a

continuous film is formed under appropriate conditions. This process is known as filrn

formation. Latex film formation is a complex process and will not be examined in detail

here. Interested readers may refer to paperç in the li teran~e with recent reviews by

Keddie ' and by Winnik. Latex film formation can be generaily divided into three

stages. as shown in Scheme 1.1. Ln the first stage, water evaporates, bringing particIes

into close contact. When the drying temperature exceeds the minimum film foming

temperature (MFT), the particles deform into space-filling polyhedral cells. The MFT is

defined as the minimum temperature at which a latex film becomes continuous and clear-

The MFT is usually close to the g las transition temperature (Tg) of the latex polymer-

At the end of stage II, a continuous film is farmed. However, this newly forrned film is

weak due to weak adhesion at the particle-particle boundary, provided only by van der

Waals attraction. Mechanical properties of the film develop in stage III, where polymer

molecules from one ce11 diffuse across the particle-particle boundary and form

entanglements with polymers in the adjacent cells.

WhiIe latex-based coatings are currently available on the market place, most

formulations still contain significant arnounts of VOCs (typically 1 5 - 30 wt%). The latex

particles in these formulations typically have a Tg above 30°C, and will not form films at

room temperature. The organic solvents are added to the formuiations to plasticize the

latex particles, lowering their Tg. Once the coating is on the wall, the solvents evaporate,

leaving behind a hard coating. If one simply removes VOCs fiom the formulation. films

will not be forrned, and the latex particles wilL fail off the walI as a powder. These

problems could be overcome if the formulations are based on low-Tg latex (Tg < 22OC).

The l o ~ ~ - T g latex forms films in the absence of VOCs. but the f ihs are weak and tacky.

A method of improving the mechanical performance of low Tg latex films is to crosslink 3 the coatings after they have formed a dense film on the substrate. Excessive

crosdinking, however, can cause brittleness in the coatings. 5

There are several strategies to introduce crosslinking into latex-based coatings. 6

One strategy is to copolymerize reactive hctionalities into latex particles and then mix

the latex with an extemal crosslinking reagent before the mixture is drawn down to form

a film. In this way, the pot life of the reactive groups is limited, and a competing side

reaction is intraparticle crosslinking. Another strategy involves in blending of latex

containing complementary reactive pair in separate particles. We examined the latter

stratekg.

1.3 The system to be studied

In this project, we employ a mixture of two types of latex particles. in one type of

particle, we copolymerize ethylhexyl methacrylate (EHMA) with a small amount of

methacrylic acid and 9-phenanthryimethyl methacrylate (Phe, the fluoresecent donor). In

the other type of particle, we copolymerize EHiMA with a small arnount of carbodiimide-

con taining methacrylate and 9-anthryl methacrylate (An, the fluorescent acceptor). These

latex polymers have a Tg of about -lO°C, ' well below room temperature. When these

particles are mixed, and water is allowed to evaporate at ambient conditions, the

individual latex particles defom into space-filling polyhedra to form a void-fiee film. 2.3

In this freshly formed tilm, before difision has occurred, the reactive groups are still

confined within their own cells.

stage 1 , II * newîy formed film

rate rate

crosslinking ' polymer diffusion /

heterogeneouly crosslinked film

rate polymer diffusion ' rate crosslinking

homogeneously crosslinked film

Scheme 1.2: Representative of final film morphology as a result of the competition between polymer diffusion and crosslinking.

Our objective in this project is to examine stage iII of the film formation process, in

which the development of fuial film properties depends on the competition between the

polymer interdiffusion rate and the crosslinking reaction rate. This competition is

depicted in Scherne 1.2. If the rate of polymer diffusion is faster than the rate of

crosslinking, a homogeneously mixed film is formed, followed by crosslinking

throughout the film. Under these circumstances, the final film will be entirely and

uniformly crosslinked. If, however, the rate of polymer d i fb ion is slower than the rate

of crosslinking, a crosslinked network will be created only at the particle-particle

boundary. Here a film with locaily-crosslinked menbranes will be formed. One might

expect that since the two Iimiting film morphologies are different, the mechanical

properties of these two films wouid be different.

We employ direct non-radiative energy transfer (ET) kom Phe to An to follow the

extent of polymer diffusion. Polymer diffusion brings into proxirnity the Phe and the An

groups attached to the polymers in adjacent cells. ET occurs when these groups approach

their characteristic (Forster) distance of R,, = 23 A. Polymer diffusion is necessary to

also bring the -COOH and -N=C=N- groups in close proximity for them to react. Here

the characteristics reaction distance is only a few A. We c m use the characteristic band of

the -N=C=N- fbnctionality at 2128 cm-' in the infrared specvum to follow the reaction

the -COOH and the -N=C=N- groups. To follow the mechanical strength evolved fiom

these films, we employ tensile strength measurements.

1.4 Ovewiew of the thesis

Chapter 2 describes the synthesis and chmcterization of the difunctional

mono mers cyclohexylcarbodiimidoethyl me thacrylate (CCEMA) and t-

butylcarbodiimidoethyl methacrylate (tBCEMA).

The use of these carbodiimide-methacrylate monomers to prepare carbodiimide-

containing latex particles by emulsion polymerization is described in Chapter 3.

Emulsion pol-merization is carried out in water, and the -N=C=N- groups react with 10 water. In this chapter, we describe the search for conditions to optimize the survival of

the -N=C=N- fimctionality, both during the emulsion polymerization and during storage

of the dispersion.

Since this project involves blends of two latexes containing different copolymers,

the extent miscibility of these copolymers is important issue. In Chapter 1, we describe

ssperirnents, which exainine this point.

Chapter 5 describes the competition between the rates of polymer diffusion and

crosslinking in latex films as a fünction of the annealing temperature, reactant

concentration and substituent on the carbodiimide moiety. We also descnbe the

preparation and characterization of fluorescently labeled latex polymers.

Chapter 6 describes the results of the tensile measurements on various latex film

sarnples as a function of the extent of the reaction.

ALI the latex particles examined in this project are similar in size and size

distribution. They also have similar molecular weights and molecular weight

distributions. We assume that when a mixture of latex particles having sirnilar sizes

dries. the resdting film contains a random distribution of the particles. I l

1.5 References

' Bezinski J. J., "Replation of Volatile Organic Compound Emissions fiom Pains and

Coatings" in "Paint and Coating Testing Manual: 1 4 ~ Edition of the Gardner-Sward

Handbook", Koleske J- V. ed., 1995, chapter 1.

Keddie J. L., Morerials Science & Engineering R-Report, 1997,2 1, 10 1. ' a) Winnik M. A.? Curr. Opin. Colloid In., 1997, 2(2) 1 92. b) Winnik M. A., 'The

Formation and Properties of Latex Films" in "Emulsion Polyrnerization and Emulsion

Polqmers", El-Aasser M. and Love1 P. ed., John Wiley & Son, Chapter 14. 4 B u f i n B. G.; Grawe J. R., J. Coat. Technol., 1978, SO(64 1 ), 4 1. 5 Zosel A.; Ley G., MacromolemZes, 1993,26,2222.

Wicks 2. W., Jr., Film Formation, Federation Series on Coatings Technology, Blue

Bell. PA, 1986.

7 Brandrup J. and lmmergnt E. H. eds. "Polymer Handbook" 3rd Edition, John Wiley &

Sons, 1989. 8 Wang Y.; Zhao C.-L.; Winnik M. A., J Phys. Chem., 1991,95,2 143.

Dolphin D.; Wick A., TobuIa~ion of Infiored Spectral Data A , Wiley-Interscience

Publication, 1977.

' O Williams A. and Ibrahim T. I., Chem. Rev., 1981, 81, 589. I I (a) Winnik M. A.; Feng J., J Coarings Tech., 1996, 68(854), 39. (b) Eckersley S. T.;

Helrner B. J., J. Coafings Tech., 1997. 69(864). 97.

2 Synthesis and Characterization of CCEMA and t-BCEMA

2.1 Introduction

Carbodiimide-containing molecules have been widely used in various apptications.

These applications include organic ~~nthesis,~. ' peptide and nucleotide sy~thesis.~-' 7 photography,6 microlithiography,8 adhesives and c ~ a t i n ~ s . ~ ' The carbodiimide

fùncrionality can be prepared by several synthetic routes. Useful reviews on the synthesis

'.' and properties ' of carbodiimides have been published. A few simple sû-ategies for

sqnthesizing carbodiimides will be described briefly here.

A cIassicai method to prepare carbodiimides is by treating thioureas with lead,

silver or rnercury oxide to abstract H2S (Scheme 2.1) . '~+" Desulfiuization of thioureas

using NaOCI aiso yields carbodiimides plus sodium sdfate, sodium chloride, water and

elernentai sulfur. " However, elernental sulhir had been reported to be the major product.9

thiourea carbodiirnide

Scheme 2.1 : Classical method to prepare a carbodiimide from a thiourea.

Another way to prepare carbodiimides is to treat isocyanate with pentavalent

phosphorous ~om~ounds . '~*" ' In this reaction, two molecules of isocyanate are consumed

to yield one carbodiirnide with the extrusion of COz (Scheme 2.2). As a result, only

symmetrical carbodiimides can be prepared.

Scheme 2 -2 : Generai method to prepare carbodiimide fkom isocyante.

Carbodiimides c m also be prepared via dehydration of N,N'-disubstituted ureas

with various dehydrating agents, among which are PPh3B2-NEt3 (Scheme 2.3) 15.16

POCI;. PClj, and tosyl chioride in pyridine.

urea carbodiimide

Scheme 2 . 3 Synthesis of carbodiimide from urea.

In this project, we prepare carbodiimide monomers, namely CCEMA and t-

BCEMA? in a two-step synthesis fiom the same isocyanates. The chemical structures of

CCEiMA and t-BCEMA are shown in Scheme 2.4. ï h e synthesis of these carbodiimide

monomers has been reported in a patent by Taylor, Collins and i3assett.16

cyclohexylcarbodiimidaethyl methacrylate (CCEMA)

t-butylcarbodiimidoethyl rnethacrylate (tBCEMA)

Scheme 2.4: Chernical structure of carbodiimide-methacylate rnonomers.

2.2 Experimental

2.2.1 General preparation

'H and 13c nuclear magnetic resonance (NMR) measurements were carried out in

CDC13 using a Varian 200 MHz spectrometer. in dl 'H NMR measurements, a singlet at

7.24 pprn was selected as the reference proton. in al1 ')c NMR measurements. the center

of the CDC13 triplet at 77.00 ppm was selected to be the reference signal.

Fourier transfonn i-ed (FTIR) measurements were carried out with a Perkin

Elmer FTTR Spectrometer Spectrum 1000. High performance liquid chromatography

(HPLC) was carried out using a ~ u ~ e l c o s i $ LC 18 column and a UV detector (240 nrn).

Acetonitrile was used as an eluent at a rate of 1 .O ml/min.

2-isocyanato ethyl rnethacrylate, cyctohexylamine (Aldrich), t-butylamine

(Aldrich), triphenylphosphine (Aldrich), bromine (Aldrich), calcium hydride (CaHz,

Aidrich)' ~lorasil'" (Aldrich), and molecular sieve (4 A, Aldrich) were used as received.

Dichloromethane (CH2Cl2, Aicirich) was refluxed in the presence of C a 2 , and then

distilled prior to use.

2.2.2 Preparation of CCEMA

To a septurn-capped, flame-dried 250 mL one neck round-bottom flask containing

a stir bar, 150 mi. of fieshly distilled CH2C12 (refluxed with CaH2) and 2-isocyanatoethyl

methacrylate (29.6 g, 191 mmol) were added. The flask was cooled in an ice bath under a

gent le Stream of nitmgen. Then, a solution of cyclohexyl amine (1 8.9 g, 19 1 rnmol) in 25

mL of dry CH2C12 was slowly injected (over 20 min) to the stirring isocyanatoethyl

methacrytate solution by a syringe. When the addition was complete, the reaction was

allowed to stir in the ice bath for 1 h. M e r 1 h, the ice bath was removed, and the

reaction solution was stirred overnight at room temperature. The next day, the reaction

mixture was transferred under nitrogen atmosphere to a solution of triphenyl phosphine

dibromide, by means of a cannula.

ïhe triphenyl phosphine dibromide was prepared by the slow addition, via a

dropping funnel, of a bromine solution (36.6 g, 229 rnmol) in 10 mL of dry CH2C12 to a

magnetically-stirring solution of triphenyl phosphine (60.0 g, 229 rnmol) in 250 rnL of

CHzC12. During addition of the brornine solution, the mixture was maintained at 2OC

under a nitrogen atmosphere. The addition took about 1 h. During the addition, a white

prec i pi tate appeared and accurnulated. When the addition was complete, a solution of

triethyl amine (46.3 g, 438 mmol) in 40 mL of CH2C12 was slowly added via a syringe.

The reaction was then ailowed to stir in the ice bath for another 2 h before the cyciohexyl

amine-isocyanatoethyl methacqlate solution was added.

Dluing the slow addition of the cyclohexyl amine-isocyanatoethyl methacryIate

solution. over about 2 h, the temperature of the reaction flask was maintained between

O°C and j°C. The white precipitate gradually disappeared. The reaction mixture was then

stirred overnight at room temperature.

The next day, the reaction mixture was extracted with distilled water (2 x 200 mL).

dried over rnolecular sieves, and then concentrated by a roto-evaporator at room

temperature to give a crude waxy solid. A cmde yellow iiquid, the CCEMA monomer,

was extracted into hexme (3 x 400 mL), which was subsequently removed by means of a

roto-evaporator. Purification of cmde CCEMA kvas carried out in two ways. By one

method, crude CCEMA (27.6 g) was distilled using a Kontes' Failing Film Molecular

Still at 90°C under 0.2 mmHg. Afier the distillation, pure CCEMA (19.9 g, 72%) was

obtained as a clear liquid. By the other method. crude CCEMA (5.0 g) was passed

through a column containing lorisi il^ as the immobile phase and hexane as an eluent.

The Rf of the carbodiimide-methacrylate monomer is 0.98. After the crude product was

passed through the column and the hexane solvent was removed, pure CCEMA (3.65 g,

73%) was obtained.

CCEMA: 'H NMR (CDC13, pprn) : 1.00-1.86 (m, CH2 of cyclohexane), 1.91 (S.

CH;). 3. IO-LX (m, CH of cyclohexane), 3.40-3.50 (t, CH2), 4.124.23 (t, CH2), 5.55 (s,

CH2 geminai), 6.11 (s, CH2 geminal). "CNMR (CDC13, pprn): 18-12, 24.39, 25.30,

34-62, 45.52, 55.53, 64.38, 125.77, 136.01, 139.70 (-NCN-), 167.04 (-CO@). FTIR

(IiBr, neat): 2932,2856,2 126 (-NCN-), 1722, 1638, 145 1, 1347, 13 18, 1296, 1240, 1 163,

1051. 943, 891, 847, 815 cm". HPLC (acetonitrile, 240 MI): 3.81 min (98.6 % pure).

High resolution mass spectromeûy (HRMS) (MH+) calcd for C13HZONZ02: m/e

237.160303 g/mol, found mie 237.160452 dm01 .

2.2.3 Preparation of t-BCEMA

tBCEMA was prepared in a way similar to that of CCEMA, except that t-butyl

amine was used instead of cyclohexyl amine. tBCEMA was obtained at 75% yield after

distillation or column chromatography.

tBCEMA: 'H NMR (CDCI;, ppm) : 1.22 (s, CHj), 1.91 (s, CH3). 3.10-3.50 (t,

CH?)_ 4.121.23 (t. CH2), 5-55 (s, CH2 geminal). 6.1 1 (s, CH2 geminal). "CNMR

(CDCI;. ppm): 18.12, 31.22. 45.62, 55.21, 64.48, 125.85, 133.88, 138.03 (-NCN-),

167.12 (-COO-). FTIR W r , neat): 2972, 2128 (-NCN-), 1722, 1638, 1455, 1366, 1352,

13 18, 1296, 1238, 1163, 1036, 945, 888, 816 cm-'. HPLC (acetonitnle, 240 nm): 3.46

min (97.6 % pure). HRiMS (MH+) calcd for Ci&IlsNz02: d e 209.129003 g/mol. found

d e 209.129043 g h o l .

2.2.4 Determination of the IR extinction coeff~cient (E) of CCEMA and t-BCEMA

To determine E for the -N=C=N- band of CCEMA and BCEMA, two series of

solutions at different concentrations of each substance in CHCI; were prepared. The

solutions were transferred to a 1.0 mm NaC1 IR ce11 (International Lab) via a syringe for

FTIR measurements. The FTIR spectrurn of the monomer solution and that of the pure

solvent were compared. By subtracting the -N=C=N- absorbance (CCEMA at 2126 cm-',

tBCEM.4 at 2128 cm-') fiom the base line (2070 cm"), the -N=C=N- absorbances in

CCEMA and tBCEMA were obtained. The extinction coefficient of the -N=C=N- bond

was obtained fiom the dope of the plots of the absorbance at various concentrations

2.3 Results and Discussion

2.3.1 Purification and Characterization of CCEMA and t-BCEMA

The hvo-step synthesis of carbodiimides fiom isocyanates is presented in Scheme

2.5, where R is the cyclohexyl or t-bu@ moiety.

isocyanate amine urea

carbodiirnide

CCEMA when R = cyclohexyl t-BCEMA when R = t-butyi

Scheme 2.5: The MO-step synthesis of carbodiimide-methacrylate monomers.

In the first step, an isocyanato ethyl methacrylate reacts with an amine' either t-

butyl amine or cyclohexyl amine, to give an N.NT-disubstituted urea, which is

subsequently treated with PPH3Br2-NEtj to give a crude carbodiimide.

Purification of cnide carbodiimide can be done by either distillation, using Konte's

Falling Film Molecular Still equipment at high temperature under reduced pressure, or

column chromatography under neutral pH using ~lorasip at room temperanire. We

preferred to use column chromatopphy since it can be done at room temperature and

takes less time, reducing the risk of carbodiimide methacrylate monomers polymerizing

or rearranging at high temperanire. However, purification by either method gives

carbodiimides with a moderate yield (72 - 75 %).

CCEMA and t-BCEMA were characterized by 'H NMR, "C NMR, FTK H R M S ,

and HPLC. Analysis of 'H and I3c NMR specûa, including chernical shifi and

integration ratio, showed that CCEMA and t-BCEMA were obtained, with strong

Figure 2.1 : ' H NMR spectra of neat CCEiMA, along with the assignment of the chemicai shifi values of the important hydrogen atoms.

Figure 2.2: ')c NMR spectra of neat CCEMA, along with the assignment of the chernical shift values of the important carbon atoms.

evidence being the peak with low intensity at 140 ppm (-N=C=N-) in the "C NMR

spectra.',16 Typical 'H and 13c C R spectra of CCEMA, dong with the assignment of

chemical shifts of the important atoms, are shown in Figure 2.1 and Figure 2.2. The

chemical shifts of protons and carbons are comparable to those fiom literature l6 and fiom

simulated spectra obtained using software fkom CD Labs.

FTIR data also provided strong evidence to support the formation of CCEMA and

t-BCEMA products. Figure 2.3 shows FTIR spectrum of neat CCEMA. The -N=C=N-

moiety of CCEMA shows a strong absorption at 2126 ~ m - l . ' ~ F l IR spectrum of t-

BCEM4: not shown here, also has a strong -N=C=N- band at 2128 cm-'. In thé

remaining chapters, we will take advantage of carbodiimide having an absorption in a

distinct region of FTIR spectra to determine the carbodiimide content in various latex

particles, and also to determine the extent of crosslinking. Besides the strong absorption

of the carbodiimide moiety, the carbonyl ( G O ) ester and the carbon-carbon double

(C=C) bond also show absorption at 1723 and 1638 cm-'. HPLC traces using acetonitrile

as the eluent show t-BCElUIA at 3.46 min with 97.6 % purity, and CCEMA at 3.84 min

with 98.6 %. Since t-BCEMA is eluted at shorter time, it seems that t-BCEMA is more

hydrophobie than CCEMA.

2.3.2 Extinction coefficient (E) of CCEMA and t-BCEMA

We determine the extinction coefficient (&) of CCEMA at 2126 cm-' and t-

BCEMA at 2128 cm-' in CHCll using an FTIR spectrometer. Plots of the absorbance at

various concentrations of CCEMA and t-BCEMA in CHCI3 are shown in Figure 2.4. The

extinction coefficient of CCEMA and t-BCEMA determined from the dope of the plots is

2134 and 1342 L mol-' cm". Even though CCEMA and t-BCEMA differ only in the

aliq 1 substituent, the extinction coefficient of t-BCEMA is about 1.6 times smaller than

that of CCEblA in the sarne solvent.

0 0 1 - 0 JOI 10 lm

4-1 IODO SQ m o

Figure 2.3: FTIR spectnim of neat CCEMA.

Figure 2.4: Plots of FTïR absorbance vs. concentration of CCEMA and t-BCEMA in CHC13.

2.4 Conclusion

We successfully synthesized CCEMA and t-BCEMA with a moderate yield,

fbllowed by purification of the crude products employing distillation or column

chromatography. Both techniques Iead to pure products. These carbodiimide products

were characterized by 'H, I3c WR, FTR and HRMS.

2.5 References

L Sandler S. R. and Karo W., "Organic Functionai Group Preparations", Academic Press,

New York. 1972, Vol. 3, 188.

Williams A. and Ibrahim T. I., Chem. Rev., 1981, 81, 589. 2 Khorana H. G., Chem, Rev., 1953,53, 145. 4 Srinivasan A.; Stephenson R. W.; Olsen R. K., J. Org- Chem., 1977,42,2253. 5 Mathias L. J.; Fuller W. D.; Nissen D.; Goodrnm M., Macromolecules, 1978, 1 1, 534. 6 Przezdziecki W. M., Res. Discl., 1974, 128, 20. 7 Beardsley J. L.; Zollinger J. L., Chem. Abstr., 1978, 88, 30404.

13 Taylor J. W.; Collins M. J.; Bassett D. R., J. Coating Technol., 1995,67,43.

Taylor J. W.; Collins M. J.; Bassett D. R., ACSSymposiurn Series, 1997,663, 137. 1 O Hiatt R. R.; Shaio MA.; Goerges F., J. Org. Chem., 1979,44, 3265. 1 1 Tornaschewski G.; Breitfeld B.; Zanke D., Tetrahedron Lett., 1969, 3 19 1.

" Schmidt E.; Seefelder M., Ann., 1951, 83, 871. 13 Ostrogovich G.; Kerek F.; B u a s A.; Doca N., Tetrahedron, 1969,25, 1875.

l 4 Monagle J. J.; Campbell T. W.; McShane A.F., J. Am. Chem. Soc., 1962, 84,4288. 15 Palorno C.; Mestres R., Synthesis, 1981, 373. 16 J. W. Taylor, US Patent 5 371 148, 1994.

3 Synthesis, Characterization and Stability of Carbodiimide Functionality in Latex Dispersions and in Latex Films

3.1 Introduction

Emulsion polymerization is defined as a heterogeneous process, in which a

monomer (or a mixture of monomers) is dispersed in water with the aid of emusifier

molecules and polymerized by a water-soluble fiee-radical initiator. 1-1 Emulsion

polymerization can be carried out either in batch or semi-batch modes. The semi-batch

mode is sometimes referred to as semi-continuous. in the batch process, ail the

ingredients (monomer, initiator, emulsifier, water) are mixed into a reactor and then

poIperized to produce latex particles. In a semi-batch process, a smdl arnount of the

ingredients is first polymenzed to produce particles of small size, and then the rest of the

ingredients are fed into reactor in a controlled manner while the polymerization proceeds.

The semi-batch process was reported to offer better control of polyrner composition,

particle concentration, particle size and particle size distribution, and finai latex solids

concentration. ' The batch process can be divided into three distinct intervais (or stages)

according to theories described qualitatively fint by Harkins in 1946. and then

quantitatively by Smith and Edwart in 1949, ' based on Harkins's model. According to

the Harkins-Smith-Edwart theory, intervai I involves particle nucleation. Particle

nucleation occurs by two mechanisrns: homogeneous and heterogeneous. Nucleation

ceases when the surface area of the monomer-swollen particles becomes adequate to

accommodate a11 the emulsifier. Interval 1 is characterized by an increase in the rate of

polymerization. Interval 1 ends when al1 micelles disappear. Interval II begins with a

large population of polymer particles, but with most of the monomer still located in the

monomer droplets. During Interval II, polymerization continues in the monomer-swollen

polpner particles, with monomer being supplied by d i f i i o n fiom the rnonomer droplets

through the water phase to the particles. Interval II is characterized by a constant rate of

polymerization since the monomer concentration within each particle remains constant,

and the number of particles does not change. uitervai II ceases when the monomer

droplets disappear. intewal III begins with polymer particles saturated with monomer.

Polyrnerization within particles continues at a decreasing rate until al1 monomers within

the particles are consumed. A schematic representation of the polymerization rate in the

three intervals of the batch process of emulsion polyrnerization is iliustrated in Fig. 3.1.

Figure 3.1 : Schematic representation of the change of the polymerization rate with tirne in the batch process. 1

In the serni-continuous process, interval 1 is normally not important because

przformed seed particles determine the number of particles in the reaction. ' The serni-

continuous process can be operated under two different limiting conditions, depending on

the relative rates of the monomer feed and polymerization. If the rate of the monomer

feed exceeds the rate of polymerization, the process condition is referred to as "flooding".

In this case the rate of polymerization is the rate-determining step in the system. These

conditions are similar to the batch process. If, however, the rate of polyrnerization

e'tceeds the rate of the monomer feed, the reaction conditions are now similar to those of

Interval III in the batch process; the monomer concentration is below the equilibrim

saturation level and the polyrnerization rate is found to depend linearly on the rate of

monomer addition. The instantaneous conversion is usually high (90 - 95%). These

conditions are referred to as monomer-starved conditions. Semi-continuous

copolyrnerization w.der monomer-swed conditions fiequently produces uniform

copolymer composition ' s 5 since the system is operated under conditions such that the

polymerization rate is detennined by the rate of monomer addition, and the monomer feed

has a defined ratio of mixed monomers.

In our experiments, we employ monomer-starved conditions to prepare

poly(ethyhexy1 methacrylate) [PEHMA] homopolymer latex and to prepare latex particles

consisting of copolymers of EHMA with CCElM.4 or tBCEMA. in addition, we

investigate extensively the stability of the carbodiimide moiety during the

copolymerization reaction, and upon heating the carbodiimide-containing latex particles

afier they are prepared. We also investigate the stability of these groups after the

carbodiimide-containing latex particles have formed solid fiims.

3.2 Experimental

1 -dodecyl mercaptan (DM, Aldrich), potassium persulfate (KPS, Aldnch), sodium

dodecyl sulfate (SDS, Fisher Scientific), aqueous ammonia (NH3(*, Fisher Scientific),

sodium bicarbonate (NaHC03, Aldrich) and mixed-bed ion-exchange resins (Bio-Rad)

were used as received. n-Butyl methacrylate @MA, Aldrich) and (*)î-ethylhexyl

methacxylate (EHMA, Aldrich) were distilled before use. Water was collected fiom a

Milli-Q Water System. CCEMA and t-BCEMA rnonomers were prepared in the Chapter

II.

Particle size and particle size distribution were determined by dynamic light

scattering at an angie of 90' and at ZPC, ernploying a Brookhaven BI-90 Particle Sizer.

'vfolecular weight and molecular weight distribution were measured by gel permeation

chrornatography (GPC) at 30°C, using an instrument equipped with two styragela

CO lurnns (HR 3 and 4) and a rehctive index detector. Reagent grade THF was used as an

eluent at a rate of 0.4 mumin. Linear poly(methy1 methacrylate) standards were used to

calibrate the columns. FTIR rneasurements were made using a Perkin Elmer FTIR

Spectrometer Spectrum 1000 with a resolution of 4.0 cm-'.

3.2.1 Preparation of seed particles (PBMA o r PEHMA)

To a 1 L, 3-neck round-bottom flask, BMA or EHMA, SDS, NaHC03, and Hz0

were added. One neck of the flask was connected to a mechanical stirrer, one neck was

connected to a condenser, which was then connected to a nitrogen outlet, and the

remaining neck was comected to a nitrogen inlet. The flask was then heated to 80°C in

an oil bath while the solution was bubbled with nitrogen gas under agitation. When the

temperature of the solution reached 80°C, and the solution was allowed to stabilize for

half an hour, a solution of potassium persulfate was then injected via a syringe. The

solution becarne milky white in a few minutes afier the solution of initiator was added.

The reaction mixture was then allowed to stir at 80°C for another 2 h. The flask was then

cooled to room temperature, and the aqueous solution was transferred to a 1 L bottle for

Future use. Typical recipes preparing PBMA and PEHMA seed particles are listed in

Table 3.1.

PBMA seed particles PEHMA seed particies

BMA (g) EHMA (g) K 2 W s (g ) SDS" (g) NaHCO; (g) H 2 0 (,a)

Table 3.1 : Recipes to prepare PBMA and PEHMA seed particles.

-

-

-

temperature (OC) time (h)

a sodium dodecyl sulfate

3.2.2 Preparation of carbodümide-containing latex particles

To a 250 rnL 3-neck round bottom flask, PBMA (or PEHMA) seed dispersion

(60.0 g) was added. One neck of the flask was comected to a mechanical stirrer, one

neck was c o ~ e c t e d to a condenser and a nitrogen outlet, and the remaining neck was

connected to a nitrogen inlet. The flask was then heated in an oil bath while the seed

dispersion was bubbled with nitrogen gas. When the desired temperature was reached,

the seed dispersion was dlowed to stir for 10 min. Then an aqueous solution containing

water-soluble reagents and an organic solution containing a monomer or a mixture of

monomers and a chah transfer reagent were fed into the flask at a controlled rate via feed

purnps (The FMI Lab Pump, Mode1 RP-Go). Both lines were fed in at the same time, but

the aqueous line was adjusted so that the addition finished about 15 - 30 min after the

organic line. When the addition of the aqueous solution was complete, the flask was

allowed to stir for an additional hour at the reaction temperature before it was cooled to

room temperature. Table 3.2 summarizes the recipes used in the preparation of latex

particles bearing carbodiirnide fiinctiondity. Note that the amount of SDS uscd to

prepare carbodiimide-containing PEHMA latex particles is 0.1 g larger than that used to

prepare carbodiimide-containing PBMA latex.

Table 3.2: Recipes used in the feed stage to prepare cabodiimide-containhg PBMA and PEHMA latex particle

latex copolymers

temperature (OC) feeding time fi)

-

a sodium dodecyl sulfate 1 -dodecyi mercarptan

3.2.3 Determination of the carbodiimide (-N=C=N-) content of carbodiimide- containing latex particles

A srnall amount of a carbodiimide-containing latex dispersion, for example

P(E3MA-CO-CCEMA) latex dispersion, was first fieeze-dried, weighed in a test tube,

dissolved in a known amount of CHC13, and then transferred to a NaCl ce11 for FTIR

measurements. The difference between the FTIR spectra of the polymer in CHC13 and

that of the pure solvent resulted in the FTiR spectnim of the P(BMA-CO-CCEM-4)

copolymers. The absorbance of the -N=C=N- moiety was obtained afier subtracting fiom

the absorbance at 2126 cm-' the base line at 2070 cm-'. By assurning that the extinction

coefficient of the -N=C=N- functionality does not change fiom the CCEMA monomer to

its copolc~ner in the same solvent, we deterrnined the -N=C=N- content of the latex

polymer using the Beer-Lambert equation (A = E 1 c, where A is the -N=C=N- absorbance

of the polymer solution at 2126 cm-', E is the extinction coefficient of the -N=C=N- of

CCEMA or tBCEMA monorner, 1 is the length of the NaCl ceIl (1 .O mm), and c ( m o n )

is the concentration of the copoiyrner in CHCI; solution).

3.2.4 Thermal stability of the -N=C=N- groups in carbodiimide-containing latex dispersions

Most of the carbodiimide-fùnctionalized latex dispersions prepared had a pH of 8

and a solids content of 30 wt%. We investigated the thermal stability of latex dispersions

at 5 W~ solids content. The dispersions were first diluted with deionized water,

followed by adjustment of the pH, either by stirring the dispersion with a mixed-bed ion

exchange resin or by adding aqueous ammonia. The dispersions were then heated at the

desired temperature. During the heating, portions of the latex dispersion were removed

periodically, freeze-dried and then dissolved in CHC13 for FTIR measurements. These

solutions were analyzed by FTIR for their -N=C=N- content.

3.2.5 Preparation of latex films

Carbodiimide-containing latex films were prepared by casting a small portion of a

30 wt% carbodiimide-containing latex dispersion ont0 CaFz disks, and then allowing

them to dry under an inverted petri dish, either at 22OC (for carbodiirnide-containing

PEHMA copolyrners) or at 32OC (for carbodiimide-containing PBMA copolymers). The

drying time ranged fiom 5 - 7 h. Al1 newly formed films were transparent and crack-fiee.

The film thickness was typicalty 1 q. The CaFz disks were then mounted in holders for

FTlR rneasurements. The disks in their holders were then annealed in a forced-air oven at

the desired temperature, and penodicaliy removed for FTIR measurements.

3.2.6 Determination of the percentage carbodiimide remaining in latex films

The procedure to determine the percentage of -N=C=N- remaining in films

sarnples was different fiom that used for latex dispersions, since film thickness varied

from one sample to another, and also varied for different locations within the film.

The percentage of carbodiimide remaining in the sample was calculated as

-N=C=iUW- remaining (%) =

where (1-, =,=, v- / I,,,), and (1-,. =,+ /I,,,), are the ratios of the -N=C=N- absorbance

intensity to the absorbance intensity at 1380 cm" of a newly formed film (to) and of the

same film annealed at a particular time t. in this analysis we employed the reference

absorption at 1380 cm-' as an internai standard. This band corresponds to a C-H bending

vibration of the polymer.

To determine the amount of -N=C=N- groups lost during the drying, a portion of

the carbodiimide-containing latex was fieeze-dried for 1 h. mixed with KBr powder, and

then pressed into thin disk. Assuming there was no -N=C=N- groups lost during the

freeze-drying, we could determine the amount of the -N=C=N- lost during the drying by

cornparhg the FTIR spectrum of the KBr-pressed sample to that of the dispersion-cast

sample measured at to.

3.3 Results

3.3.1 Latex Synthesis and Characterization

Al1 carbodiimide-containing latex particies were synthesized by seeded emdsion

polyrnerization, with the carbodiimide-methacrylate cornonomers being introduced only

in the second stage. The seed particles, prepared by batch emulsion polyrnerization,

consist of either PBMA or PEHMA homopolyrner. Both the PBMA and PEHMA seed

particles have a mean diarneter of about 50 nm and high molecular weight values

(PBMA: MW = 650 000, MJMn = 2.45; PEHMA: MW = 700 000, M A " ~2.65) .

In the feed stage, we found that it was necessary to use a greater arnount of SDS to

prepare PEHMA latex particles than to prepare PBMA particles of similar sizes. Recipes

are given in Table 3.2. Preparation of PBMA dispersions using 0.6 g of SDS per 35.0 g

of BMA at 80°C result in stable latex particles, similar to results reported previously. !O, I I

We also obtain stable PBMA latex particles, with and without the carbodiimide-

methacrylate cornonomers, using the same amount of SDS under identicai polymerization

conditions. Under these conditions, we do not obtain stable PEHMA latex particles. If

we increase the amount of SDS in the recipe by 17% fiom 0.6 to 0.7 g per 35 g monomer,

we obtain stable PEHMA latex particles, and stable carbodiimide-containing PEHMA

particles. For al1 reactions, the weight ratio of carbodiimide monomer to total monomers

used in the feed stage was fixed at 2 wt'30.

Once we found conditions for preparing stable Iatex particles, we next sought

conditions to ensure that the emulsion homopolymerization of EHMA occurs under

monomer-starved conditions. The monomer starved-feed conditions are necessary to

produce a random composition of the -N=C=N- functionality dong each copolymer

chain. " v ' ~ We chose a polymerization temperature of 80°C and a rate of monomer

feeding of 0.062 &min. These conditions were previousiy employed to prepare PBMA

latex particles under monomer-starved additions. 'O * ' ' To ensure the monomer-starved

conditions are achieved in PEHMA system, we prepare PEHMA latex particles u ing

these conditions and foIlow the monomer conversion, by weight We find that the

monorner conversion at any time during feeding EHMA is always 89% or greater.

Ernulsion polymerization with the monomer conversion ranging from 90 to 95% is

considered to be characteristic of monomer-starved conditions. ' The final monomer

conversion of EHMA, measured 1 h after the monomer feed is complete, is 100%. We

are, therefore, convinced that emulsion homopolymerization of EWMA is carried out

under monomer-starved conditions at 80°C with 0.062 &min of monomer addition.

We also assume that the reaction remains monorner-starved when EHMA is

copolymerized with CCEMA or t-BCEMA at an identical temperature and feed rate.

We also carried out emulsion polymerization of EHMA at 60°C using a feed rate

of 0.56 &min for monomer addition. We wanted to examine whether lowering the

polperization temperature would increase the carbodiimide remaining during

polymerization. In the polymerization of EHMA iaelf under these conditions. we find

again that the monomer conversion ranges from 90 - 93% at al1 feed times. Thus we

assume that both homo- and copolyrnerization of EHMA with carbodiimide monomers at

60°C and 0.56 mumin feed rate also occur under rnonomer-stamed conditions.

Utilizing these conditions, we prepared four different types of latex particles:

P(BMA-CO-CCEMA), P(BMA-CO-tBCEMA), P(EHMA-CO-CCEMA) and P(EHMA-CO-

tBCEMA). Each latex dispersion contains 2 wt?40 of carbodiimide comonomers. The

charactenstics of these carbodiimide-containing PBMA and PEHMA waterbome

particles are listed in Table 3.3. Since they are prepared using an almost identical recipe

and under the same conditions, they have similar characteristics, including particle

diameters, solids contents, dispersion pHs, molecular weights and molecular weight

distributions. Except, P(BMA-CO-CCEMA) has a higher MW value.

Table 3.3: Characteristics of carbodiimide-containing PBMA and PEHMA latex particles.

diameter (nm)

solids content (%)

Figure 3.2 shows FTIR spectra of solvent-cast films of PEHMA homopolymer

and PEHMA copolyrner containing tBCEMA. To prepare these films, the latex sarnples

were freeze-dried and then dissolved in THF. The P(EHMA-CO-tBCEM4) was

precipitated in hexane to remove possible traces of tBCEMA that did not polyerize.

Since only a small amount of ti3CEM.A (2 wt%) was used, the FTIR spectmm of the

P(EHMA-CO-tBCEMA) copolymer is very similar to that of the PEHMA homopolymer.

However, the characteristic absorption of the -N=C=N- hinctionality is easily observed at

2 128 cm". This result indicates that the tBCEMA monomer has become incorporated

into the polymer backbone.

21 28 cm" I 1380 cm-'

I PEHMA

wavenumber (an-')

Figure 3.2: FTIR spectra of PEHMA and P(EHMAso-tBCEMA) copolymers.

3.3.2 Stability of the -N=C=N- moiety during emulsion copolymerization

We use the IR band at 2126 cm-' (CCEiMA) and 2128 cm-' (BCEMA) to monitor

the survival of the -N=C=N- group during emulsion copolymerization. Table 3.4 shows

the amount of carbodiimide moiety remaining for various polymerization conditions.

These conditions include the pH of the dispersions, the polymerization temperature, the

choice of the base monomer, and the substituents on the carbodiimide functionality.

We first consider the effect of the pH of the aqueous serum on the amount of

carbodiirnide remaining in P(BMA-CO-CCEMA) latex copolyrners durùig emulsion

polymerization. For preparation of P(BMA-CO-CCEMA) latex particles in an unbuffered

reaction (both stages) at 80°C, we find that no carbodiimide moiety remained. The

aqueous s e m becomes acidic because the initiator, potassium persulfate, produces HO-

S 0 3 K as a by product. When some base is present, for example NaHC03 lefi over fiom

preparation of the seed, the carbodiimide content increases, and increases fûrther to 68%

when sufficient Nd-iC03 is included in the feed stage.

The choice of the base polymer is also important. We find that when the base

monomer, BMA, is replaced by EHMA during emulsion polymerization at 80°C, the

fraction of carbodiimide successfùlly incorporated into the latex increases. For example,

we obtained 86% carbodiimide survival when EHMA was copolymenzed with CCEMA,

an increase fiom 68% obtained for the P(BMA-CO-CCEMA) copolymer. The choice of

the base polymer becomes less significant when tBCEMA is ùie comonorner. We

observed aimost identical carbodiimide content, 91 % for P(BMA-CO-tBCEMA), and 92%

for P(EHMA-CO-tBCEMA) latex copolymers.

The substituents on the -N=C=N- hctionality aIso make a significant

contribution to its stability. Maintaining BMA as the base monomer, we observed an

increase of 23% in the carbodiimide content when CCEMA was repIaced by tBCEMA as

the comonomer, keeping the polymerization conditions at 80°C and pH 8. When EHMA

was used as the base monomer, we found only a small increase (6%) in the -N=C=N-

content when comparing P(EHMA-CO-CCEMA) with P(EHMA-CO-tBCEMA) latex

copolymers. Under optimum conditions of NaHC03 as a buffer, EHMA as the base

monomer and tBCEMA as the fünctional cornonomer, we can prepare functional latex at

80°C with an incorporation of 92% of the -N=C=N- groups from the feed. When we

decreased the polymerization temperature from 80°C to 60°C, the tBCEMA content

increased from 92% to 98%.

Table 3.3: The percentage of carbodiimide remaining afier carbodiimide-containing latexes were prepared.

latex copolymers pH of dispersion

temperature (OC)

feed rate (mLlmin)

arnount of carbodiimide

remaining (%)

3.3.3 Stability of the -N=C=N- functionality in latex dispersions

The stability of the reactive functionality at long storage times is referred to as the

pot life (or shelf life) of the system. Here we investigate swvival of the -N=C=N-moiety

at 80°C, conditions more severe than common storage conditions for latex dispersions.

We Vary the pH of the latex dispersions, the base polyrner, the substituents on the - N=C=N- group, and the heating temperature. in al1 expenments examined here. we use

latex dispersions containing 5 wt?h solids content.

We first examine P(BMA-CO-CCEMA) latex at pH 4, 8 and 12. Here the latex

dispersions were first diluted, adjusted to the desired pH, and then heated to 80°C. Some

sarnples of latex dispersions were also treated with a small amount of mixed-bed ion

exchange resin at room temperature for 5 min. This procedure lowered the pH of the

solution to 4. These dispersions were then heated to 80°C. To adjust the pH to 12.

aqueous NH; was added to the dispersion. Figure 3.3 shows the percentage of

carbodiimide remaining in P(BMA-CO-CCEMA) latex dispersions rnonitored at different

times as a h c t i o n of the pH of the dispersions. From this Figure, we observe two

aspects of carbodiimide stability, the rate of the hydrolysis and the limiting extent of the

hydrolysis. There appears to be a distribution of rates of hydrolysis, a fast rate shown by

the steep dope at short times. This is followed by a slorver rate (smdler slope) at longer

times.

heating time (h)

Figure 3.3 : Plots of the % -N=C=N- remaining vs. heating time at 80°C for P(BMA-CO- CCEMA) latex dispersions at pH 4, 8 and 12. The solids content is 5 wt%.

Over short times, the rates of hydrolysis of dl dispersions are fast, and faster when

the aqueous medium is acidic than when the pH is 8 or 12. Over long times, these rates

decrease significantly, especially when the pH of the dispersions is 8. At this pH, the

rates appear to drop to zero afier 6 h. When the pH of the aqueous medium is either 4 or

12. the rates of hydrolysis at longer times are reduced, but these rates are similar under

acidic and basic conditions.

The lirniting extent of hydrolysis also differs depending on the pH of the

dispersions. A significant arnount of the -N=C=N- groups are hydrolyzed at short tirnes.

Over this times (c 7 h), the extent of hydrolysis is greater in the acidic dispersion than in

either the buffered or basic dispersion. For example, 37% of the -N=C=N- groups are

hydrolyzed in the fmt 5 h when the dispersion is acidic, during which only 23% of these

groups are hydrolyzed at pH 8 and 17% at pH 12. At long times (30 h), we observe no

îurther loss of the -N=C=N- moiety at pH 8, whereas at pH 12 and 4 the amount of the - N=C=N- group loss increases to 25% and 45%, respectively.

in Figure 3.4, we plot the percentage of carbodiimide remaining vs. heating time

at 80°C for P(BMA-CO-CCEMA) and P(EHMA-CO-tBCEMA) latex dispersions at pH 8.

Unlike the P(BMA-CO-CCEMA) dispersion, for wkich the rates of hydrolysis are fast at

short times and drop to zero at long times, the hydrolysis of the P(EHMA-CO-tE3CEM.A)

latex dispersion is slower at short times but continues for times at which the hydrolysis of

P(EHMA-CO-CCEMA) appears to have stopped. For example, 23% of the -N=C=N-

rzroups of the P(BMA-CO-CCEMA) dispersion are hydrolyzed over the first 6 h, during L

which only 7% the -N=C=N- groups are lost fiom the P(EHMA-CO-tBCEMA) latex

dispersion. Afier 3 1 h, 17% of the -N=C=N- groups of the P(EHMA-CO-tBCEMA) latex

were hydrolyzed, compared to 23% of the -N=C=N- groups of the P(BMA-CO-CCEMA)

dispersion.

heating time (h)

Figure 3.4: Plots of the % -N=C=N- remaining vs. time at 80°C for P(BMA-CO-CCEMA) and P(EHMA-CO-tBCEMA) latex dispersions at pH 8. The solids content is 5 \a%.

As an example of the temperature effect on the stability of carbodiimide moiety,

we examine the P(EHMA-CO-tBCEMA) latex dispersion at 60°C and 80°C. We plot the

percentage of tBCEMA remaining vs heating time as a function of temperature in Figure

3.5. An obvious observation here is that at every heating time, the rates and the extent of

hydrolysis at 60°C are reduced to about half of that at 80°C. For example, 10% of

tBCEk1A are hydrolyzed at 60°C over a penod of 40 h, compared to 20% of tBCEMA at

80°C over the sarne heating time. This observation is consistent with our finding that a

decrease in the reaction temperature is accompanied by an increase in the carbodiimide

content of the latex during emulsion copolymerization.

O 10 20 30 40 50 60

heating time (h)

Figure 3.5 : Plots of the % -N=C=N- remaining vs heating time at 60°C and 80°C for P(EHMA-CO-tBCEMA) latex dispersions at pH 8. The solids content is 5 wt%.

We also examine long-time effects of latex storage at room temperature. Table

3.5 Iists the percentage of carbodiimide monomers used in the original recipe, after

carbodiimide-containing latex particles are just prepared, and after these latex dispersions

have been stored for 1 year at room temperature. From the Table, we observe that during

the preparation of carbodiimide-containing latexes, the arnount of carbodiimide

remaining increases when BMA is replaced by EHMA, and as well as when CCEMA is

replaced by tBCEMA. We can not distinguish whether the base monomer or the

carbodiimide-methacrylate comonomer makes a stronger contribution to the carbodiimide

stability. When these carbodiimide-containing latex dispersions are stored at room

temperature over a period of 1 year, we observe that the survival of the carbodiimide

hctionality depends more strongly on the type of the carbodiimide-methacrylate

comonomer than on the choice of the base polyrner. For example, we found that a larger

arnount of carbodiimide survives when the latex polymers (both PBMA and PEHMA)

contain tBCEPvlA (77 - 78%) than when the same polymers contain CCEMA (41 - 44%)

as the cornonomer.

Table 3 3: The percentage of carbodiimide remaining obtained fiom the latex dispersions afrer storage at room temperature for 1 year. The percentage of carbodiimide remaining after pol yrnerization is aiso included for cornparison.

latex copolymers carbodiimide monomer in the original recipe

(%)

carbodiimide remaining after polymerization

(%)

carbodiimide remaining after 1

year storage (%)

3.3.1 Stability of the -N=C=N- functionality in latex films

Before examining the stability of the -N=C=N- fûnctionality in latex films, we

first examine the stability of this group during the preparation of films. To determine the

extent of the hydrolysis during the drying process, we first determine the ratio of

1-, =, = ,. - / I , , , from fieeze-dried samples, assuming that there is no hydrolysis taking

place during the sampte preparation. We then use this ratio as a reference for the analysis

of nascent films fieshly prepared fiom dispersions. Freeze-dried films are prepared as

follows: a smalI arnount of carbodiimide-containing latex dispersion was first freeze-

dried over I h. The dried polymer was then mixed with KBr powder, and then pressed

into thin disks for FTR measurements. We find that the arnount of carbodiimide Iost

during film formation is 5% for P(BMA-CO-CCEMA), 3% for P(EHMA-CO-CCEMA)

and 2% for P(EHMA-CO-tBCEMA).

This analysis relies on equation 1 to calculate the percentage of carbodiirnide

remaining in a series of latex films. To follow the timedependence of the

1-,=,.,-II,,, ratio and to compare the rates of hydrolysis of different latex films, we

need to define the onset time b. We found it convenient to make the first measurements

on nascent latex films 1 h after the last visible wet spot of each film disappeared. We

de fine this time as b.

+ P(BMA-co-CCEMA) latex dispersion + P(BMA-co-CCEMA) latex film

O 10 20 30 40 50

time (h)

Figure 3.6: Plots of the % -N=C=N- remaining vs. heating time at 80°C for a dispersion of P(BMA-CO-CCEMA) in water at pH 8 and in a dry latex film prepared fiom this dispersion.

We first look at the stability of the -N=C=N- groups of the P(BMA-CO-CCEMA)

latex films, and compare this to the stability of the same latex polymer heated in the

dispersed state at pH 8. As seen in Figure 3.6, the rate of -N=C=N- group Ioss is much

smaller in the latex film than in the dispersion. In the dispersion, 23 % of the

Functionality is hydrolyzed in the first 6 h, compared to only 7% -N=C=N- group loss in

the latex films. To reach 23% CCEMA loss in the latex film, more than 40 h of

annealing time is required-

Figure 3.7 shows the percentage of carbodiimide remaining in P(BMA-CO-

CCEMA), P(EHMA-CO-CCEW) and P(EHMA-CO-tBCEMA) latex films anneaied at

80°C. We observe that the base polymers and the carbodiimide substituent have similar

effects on the stability of the -N=C=N- functionality in films and in aqueous dispersions.

Films of the P(Ef-EMA-CO-tBCEMA) latex were the most stable, with only 4 % of the -

N=C=N- groups loss over 36 h at 80°C.

tirne (h)

Figure 3 -7: Plots of the % -N=C=N- remaining vs annealing time of P(B1MA-CO- CCEMA), P(EHMA-CO-CCEMA) and P(EHMA-CO-tBCEMA) latex films at 80°C. These films were prepared fiom dispersions at pH 8.

When a film of a 1:l mixture P(EHMA-CO-tBCEMA) and P(EHMA-CO-MAA)

polymers are formed, a rapid decay occurs, as shown in Figure 3.8. This result indicates

that a reaction between the carboxylic acid and the carbodiimide has occurred.

Preparation of these films and analysis of the extent of the crosslinking reaction will be

esplained fully in the next chapter.

+ ~(EHMA-co-MAA) + P(EHMA-CO-tBCEMA) il- P(EHMA-CO-tBCEMA) only

annealing tirne (h)

Fiame 3 -8: Plots of the % -N=C=N- remaining vs. annealing time of P(EHMA-CO- tBCEMA) latex films in the presence and absence of P(EHMA-CO-MAA) latex copolymers. The P(EHMA-CO-W) dispersion was adjusted to pH 8 before it was mixed with the P(EHMA-CO-tBCEMA) dispersion. The film was dried at 22OC. Under these conditions, amrnonia evaporates fiom the dry film to reform the -COOH group. The films were then annealed at 60°C.

3.4 Discussion

Carbodihide groups are sensitive to hydrolysis. During emulsion

copolymerization with carbodiirnide-methacrylate comonomers, no carbodiimide groups

rernain when the pH of the aqueous medium is acidic. When some NaHC03 is left over

fiom preparation of the seed, the carbodiimide content increases, and increases further to

68% when sufficient NaHCOl is included in the feed stage. We explain these resuits in

terms of a rapid acid-induced hydrolysis, which occurs as the monomer is transferred

fiom monomer droplets to growing particles, where polymerization takes place. To reach

the particles, the carbodiimide monomers must first corne in contact with water. Water is

knowm to react with the carbodiimide moiety to give urea, and the rate of the reaction is

promoted by acid. '' Under buffered conditions, the hydrolysis is much slower, such that

carbodiimide monomers diffuse into the particles faster than the carbodiimide moiev can

react with water. As a consequence, we prepare the remaining carbodiirnide-

functionalized latex particles at pH 8, usine NaHCO; as a buffer in both the batch and

feed stage.

When we compare EKMA with BMA as the base monomer with CCEMA as the

cornonorner, we fmd an increase in the -N=C=N- content fiom 68 to 86% for

polymerization at 80°C and a monomer addition rate of 0.062 mWmin. At pH 8, most of

the CCEMA is successfûlly transferred to growing particles and polyrnerized. The

hydrolysis reaction c m take place at two different locations in the system. Hydrolysis can

occur in the water phase during transport of the CCEMA from monomer droplets to

particles. Hydrolysis can also occur at the surface of the particles after CCEMA has

successfully been transferred or incorporated into the latex particles. EHMA is more

hydrophobic than B M A and, as a consequence, the PEHMA latex polymer is more

hydrophobic than the PBMA latex polymer. An increase in hydrophobicity fiom PBMA

to PEHIMA results in a substantial increase of the CCEMA survival during the reaction.

The effect of hydrophobicity of the base polymers on the incorporation of carbodiimide

groups becornes less significant when t-BCEMA is used as a comonomer. Here we

obsenre only a tiny increase (1%) in the -N=C=N- content for P(BMA-co-~BCELMA) VS.

P(EHMA-CO-tSCEMA) latex copolyrners. Within experimental error, the two values are

identical.

When we compare tBCEMA with CCEMA as the cornonomer with BMA as the

base monomer, we find an increase in the -N=C=N- survival during emulsion

polyrnerization at 80°C fiom 68 to 91%. The t-butyl group of tBCEMA provides greater

stenc hindrance to hydrolysis of the -N=C=N- groups than does the cyclohexyl group of

CCEMA. When EHMA is used as the base monomer, the steric hindrance of the

carbodiimide comonomers plays a less important role. We infer that the water solubility

inside the PEHMA phase is lower than that in PBIMA. ïhere are fewer demands placed

on the t-butyl group to protect the carbodiimide. We found oniy a 6% increase in the - N=C=N- group content in going fiom the P(EHMA-CO-CCEMA) (86%) to P(EHMA-CO-

tBCEMA) (92%).

An interesting question is whether the steric hindrance of the t-butyl group is more

effective than the increased hydrophobicity of PEHMA in protecting the -N=C=N- group

during emuIsion polymerization. To address this question, we choose the P(BMA-CO-

CCEMA) latex copolymer with 68% carbodiimide remaining as our reference, and

compare this reaction to those used to prepare P(BMA-CO-tBCEMA) and P(EHMA-CO-

CCEMA) latex copolymers, keeping the polyrnerization temperature at 80°C and the

monomer addition rate at 0.062 W m i n . When the base monomer BMA is replaced by

EKMA- we find that the carbodiimide content increases by 18%. When the carbodiimide

comonomer CCEMA is replaced by tBCEMA, we find that the carbodiimide content

increases by 23%. In this case, the t-butyl of tBCEMA has a more pronounced efYect.

To compare the shelf life of these carbodiimide-containing latex particles, we set

sarnples of the dispersions aside to age for one year at room temperature (22 f 1°C). Here

we find that the steric hindrance of the carbodiimide comonomer plays a more important

role in promoting -N=C=N- stability, as shown in Table 5 . irrespective of the base

polymer, the latex containing tBCEMA has a much higher -N=C=N- group fraction f i e r

1 ysar than those containing CCEMA.

Temperature also contributes significantly to the -N=C=N- stability. During

emulsion polymerization, the lower the reaction temperature the higher the carbodiimide

content of the dispersion. From Table 3.4, when the polymerization temperature is

decreased fiorn 80°C to 60°C, alrnost ai1 of the carbodiimide comonomer groups (98%) of

the tBCEMA are incorporated into polyrner chahs. High temperature also has a

deleterious e ffect on the carbodiimide content during heating the carbodiimide-containing

latex particies dispersed in water. For P(EHMA-CO-tBCEMA) latex dispersion, the rate

and the extent of hydroiysis at 60°C are about half of those at 80°C.

We observe that al1 the factors that affect the stability of the -N=C=N- groups

during emulsion copolymerization aiso affect in similar ways the stabiliv of the

fünctionality during aging (storage) of the carbodiimide-containing latex dispersions.

Thcse factors include the pH of the dispersions, the hydrophobicity of the base polymers,

the steric hindrance of the substituent on the carbodiimide, and the temperature.

3.5 Conclusions

We describe monomer-starved conditions appropriate for preparing PEHMA latex

particles, and the use of these conditions to prepare carbodiimide-fiuictionalized PEHMA

latex particies. The carbodiimide hctionality is sensitive to its environment. We

examined the sensitivity of the carbodiimide to the pH of the dispersions, the

hydrophobicity of the base polymer, the steric hindrance of the substituent carbodiirnide

moiety, and the temperature. When the pH of the dispersions is acidic, we found a rapid

acid-induced hydrolysis. In the presence of NaHC03 (at pH 8), the carbodiimide groups

are stable. Both the hydrophobicity of the base polymer and the steric hindrance on the

carbodiimide moiety enhance the stability of -N=C=N- group. Steric hindrance (a t-butyl

group) on the carbodiimide moiety is more effective in providing stability in aqueous

dispersion over long storage times at roorn temperature. Both during emulsion

polymerization and during storage of the dispersion, increasing the temperature from

60°C to 80°C led to more rapid hydrolysis of the carbodiimide.

3.6 References

1 Gilbert R. G., "Emulsion Polymerization: A -Mechanistic Approach", Academic Press,

Piirma I., "Emulsion Polyrnerization", Academic Press, New York, 1982. 3 27" Annual Short Course "Advances in Ernulsion Polyrnerization and Latex

Technology" El-Aasser M. ed., Lehigh University, PA, Vol. 1, 1 996. 4 Mark H. F.; Bikaies N.; Overberger C.; Menges G., "Encyc1opedia of Polymer. Sci.

Eng." Vol. 6, 2nd Ed., John Wiley & Sons, New York, 1986. 5 Chatte rjee S.; Bane rjee M.; Konar R. J. Polym. Sci. .- Polym. Chem., 1979, 1 7 , 2 193.

6 (a) Harkins W. D., J. Chern. Phys., 1945, 13, 381. (b) Harkins W . D., J. Chem. Phys.,

1946, 14,47. (c) H a r h s W. D., J. Am. Chem. Soc., 1947,69, 1428. 7 (a) Smith W. V.; Ewart R. H., J. Chern. Phys., 1948, 16, 592. (b) Smith W. V.; Ewart

R. H.) J. Am- Chem. Soc., 1948,70,3695.

XU Z.; Lu G.; Cheng S.; Li L.; J. Appl. Pol'. Sci., 1995,56, 575.

9 Verstegen J. M. G., Ph.D thesis, University of Eindhoven, 1998.

'O Zhao C.-L.; Wang Y.; Hniska 2.; Wimik M. A., ~b~acrornolecules, 1990,23,4082.

1 1 Feng J., Ph.D thesis, University of Toronto, 1997.

" Wall F.; Delbecq C.; Florin R. E.J Polym. Sci., 1952,9, 177.

l 3 Doremaele G. V., Ph.D. thesis, Eindhoven University of Technology, The Netherlands,

1990. 14 WiIIiarns A. and Ibrahim T. I., Chem. Rev., 1981,8 1,589.

Miscibility of PEHMA Latex Copolymers in Films

4.1 Introduction

Direct non-radiative energy transfer (ET) has been widely used in biology,

biochemistry and polymer science as a tool to study a variety of problems related to

structure and dynarnics. '" In p o l p e r science, ET has been used to midy molecular

cluster formation in solutions, '" the miscibility of polymers in polymer blends, 7-10

diffusion of small molecules in rubber polymers, " and the boundary-layer interphase in 12 block copolyrners. ET has d s o been utilized to study the surface morphology of latex

particles in the dispersed state, 13-" the intemal structure of core-shell latex particles, 15

16 the particle-particle boundary of latex blends, and polymer diffusion across the particle-

particle boundary in latex film. 17-19

4.1.1 Principle of non-radiative energy transfer

ET is a bimofecular reaction between an excited donor (D*) and a second dye that

serve as an acceptor (A). The principle of ET is shown in Scheme 4.1: where D and D*

are the ground state and the excited state of the donor. A and A* are the ground state and

the excited state, respectively, of the acceptor. Energy transfer occurs h m the excited

donor (D*) to the ground-state acceptor (A) chromophore through resonant coupling.

This process is governed by a long range dipole-dipole interaction.

Scheme 4.1 : An illustration of the energy transfer reaction.

The rate k&) of energy transfer by this resonant coupling mechanism is given

by the Forster expression, 20

where K' is the orientation factor for the dipole-dipole interaction of the donor and the

acceptor. It equds to 2 3 for mobile chromophores and 0.475 for inunobile, randomly

distrïbuted D and A molecules. "' is the quantum yield of the donor fluorescence in

the absence of the acceptor; n is the refiactive index of the medium; N , is Avogadro's

number; and s, is the fluorescent lifetirne of the donor in the absence of acceptor. R is

the distance between the centers of the donor and the acceptor, and J is the normaiized

spectral overlap integral, given by

where F,(A) is the fluorescence intensity of the donor in the absence of the acceptor at

wavelength A, and €*(A) is the molar absorption coefficient of the acceptor at 1. Equation

(4.3) represents the overlap of the emission spectnun of the donor with the absorption

spectnim of the acceptor, modified by the factor A".

The rate of energy transfer kET(R) depends on (1) the extent of overlap of the

emission spectnrm of the donor and the absorption spectnim of the acceptor, (2) the

relative orientation of the donor and acceptor transition dipoles, and (3) the distance

behveen the excited donor and acceptor. kE~(R) can be expressed in another way,

where & is defined as the distance between the donor and acceptor chromophores at

which the rate of ET equals the rate of donor decay in the absence of the acceptor. & is

ofien referred to as the Forster critical ET distance

In the phenanthrene-anthracene pair examined in this projec~ Ro is 2.3 m."

DET is a powerfuI technique because km, as s h o w in equation 4.3. is uiversely

proportional to the sixth power of the donor-acceptor distance (R).

A u s e h l parameter is the quantum yieId of energy transfer (OR), which is defined

where kf is the rate of fluorescence decay (Scheme 4. l), and k, represents the sum of the

rates of al1 other deexcitation processes.

If dl the D-A pairs are separated by the same distance R, the donor decay will be

exponential with a life time rm. Under these circumstances, O, can be written in terms

of lifetimes of the donor determined in the presence (rDA) and absence of acceptor (TD)

4.1.2 Application of DET to study miscibüity in blend of polymers

In this chapter, we employ the parameter am to charactenze the miscibility of the

various PEHMA copolyrners. in the latex system examined here, we label one type of

lares particles with Phe, and the other latex particles, with An. When these labeled

particles are mixed in dispersion, deposited on a substrate, and the water is allowed to

evaporate, a continuous film is fonned. In the newly formed film, before diffusion has

occurredg the Phe- and An-labeled polymers are still confined within their own cells. OET

is small because only cross-boundary energy transfer can occur. If the polymers fiom

these latex particles are miscible, the polymers will diffuse from one ce11 to the adjacent

cell. bringing Phe and An groups into proxirnity. (PET increases and reaches a maximum

value. This maximum value should correspond to that obtained fiom films prepared fiom

a solvent-cast solution of the sarne mixture of polymers. in solution in a good solvent for

the polymers, the Phe- and An-labeled polymers are homogeneously mixed. We smploy

THF as a good solvent for al1 of the PEHMA polymers examined here. In films prepared

fiom THF solution, polymers which are immiscible in the bulk will demix as the solvent

evaporates. Kinetic factors will limit the extent of demixing if solvent evaporation is

rapid. For miscible polymers, aET values obtained fiom the freshly formed THF-cast

film should not change when the same film is annealed. For polymers that are not

miscible, fiirther polymer demixing may occur when the film is annealed, ieading to

segregation of Phe and An. ui this case, Orr will decrease firom its initial value.

in this and subsequent chapters of this thesis, we examine only those blends in

rvhich PEHMA is the base polymer. For the experiments we carry out, PEHMA-based

latex po lymers offer two advantages over the PBMA-based counterparts. The fim

advantage is the low Tg of this polymer, which has a Tg value of -lO°C for the 23 hornopolymer. Because of the low Tg of PEHMA, it foms clear and continuous latex

films at room temperature (22OC) and at 4°C without the aid of VOCs. The second

advantage is that the -N=C=N- groups are more stable to hydrolysis than they are with the

PBMA as the base polymer (Chapter 3).

4.2 Experimental

9-Phenanthrylmethyl methacrylate (PheMMA) and 9-anthryl methacrylate

(AnMA) were synthesized previously. 2425 Methacrylic acid (MAA, Aldrich) was distilled

prior to use. t-BCEMA, and CCEMA were prepared as described in Chapter 2. The gel

pemeation chrornatograph (GPC) was eguipped with hvo styragelm columns (HR 3 and

3), a fluorescence detector and a rehctive index detector. The excitation wavelength was

set at 300 rn for Phe- and 350 nm for An-labeled polymers. Reagent grade THF was

used as an eluent at a rate of 0.4 &min at 30°C. Linear poly(methy1 methacrylate)

standards were used to caiibrate the colwruis. UV-visible spectroscopy was performed

using a Hewlett Package 8452A Diode Array Spectrophotometer to determine the dye

content in latex particles. The extinction coefficients (in THF solvent) used to calculate

the dye content in individual labeled Iatex is 1.15 x 10%lM''cm-' at 298 nm for Phe, and

8-10 x 104 M-'cm-' at 364 nm for An.

4.2.1 Preparation of labeled latex particles

We prepared another batch of PEKMA seed particles, similar

Chapter 3, and then used these to prepare labeled latexes. These

prepared using the recipe and conditions tabulated in Table 3.1.

to those prepared in

seed particles were

Table 4.1 lists the

representative recipes used in the second stage of emulsion polymerization to prepare

three types of labeled latex particles. Each labeled latex is given an abbreviated name.

The two labeled latex samples containing no reactive groups, P E K U - D and PEHMA-A,

were prepared using the recipe in the second colurnn of Table 4.1. PheMMA was used in

the preparation of PEHMA-D; and AnMA., in the synthesis of PEHMA-A. These latexes

were prepared at 80°C without NaHC03 buffer in the second stage. The recipe listed in

the third column was used to prepare a donor-labeled latex containing 1 1 mol% of MAA.

We cal1 this latex polymer D-MAA-I 1, where D (donor) represents the fluorescent dye,

MAA refers to methacrylic acid, and the number 11 is the amount (in mol%) of the MAA

used in the original recipe. The other carboxylic acid-containing latexes, D-MAA-5 and

D-MAA-20, were also prepared fiom the recipe in the third column of Table 4.1, but with

5 and 20 mol% of MAA, respectively. These W-conta in ing particles were ais0

prepared at 80°C in the absence of NaHC03 buffer. Note that the arnount of seed particles

and ingredients used to prepare these MAA-containing latexes are doubled compared to

that used to prepare the PEHMA-D or the A-tBCEMA-5 particles. The recipe shown in

the last column was used to prepare two acceptor-labeled latexes containing 5 and 11

mol% of tBCEMA, A-tBCEMA-5 and A-tBCEMA-11. Here A (acceptor) refers to the

fluorescent dye, BCEMA (t-butylcarbodiimidoethyl rnathacrylate) refen to the reactive

comonomer, and the nurnbers 5 aad 11 refer to the amount (in mol%) of the tBCEMA

used in the original recipe. These tBCEMA-containing particles were prepared at 60°C in

the presence of NaHCOj buffer. Two other accepter-labeled latexes containhg CCEM4,

A-CCEMA-3.2 and A-CCEMA-4.6, were also prepared. using the recipe and conditions

in the last column, except that tBCEMA was replaced by CCEMA. In a11 recipes. the

amount of chain transfer reagent and the fluorescent dye was kept at 1 WYO and 1 mol %,

respectively.

Table 4.1 : Representative recipes in the second stage of emulsion polymerization to

PEHMA-D

temperature ( O C ) feeding rate (mL/min)

- ~p

D-MAA- 1 1

" 5.5% sohds content

4.2.2 Preparation of dispersion- and solvent-cast films

Films were prepared on quartz plates for energy transfer measurements, and on

CaFt disks for FTIR measurements. These films were nomally prepared fiom the same

mixture of labeled latexes.

A 1 : 1 blend of Phe- and An-labeied latex was prepared by mixing the two aqueous

dispersions, which was then divided into two portions. One portion was freeze-dried and

used to prepare solvent-cast films. The other was used to prepare latex films, which we

shall refer to as "dispersion-cast". To prepare a mixture of latex containing the

carbodiimids-containing particies, the carboxylic acid-containing particles and the

PEHMA-D latex were first neutralized with aqueous amrnonia to pH 8. One portion of

the mixture of latex dispersions was cast directly onto quartz plates and CaF2 disks. The

CaF2 disks and quartz plates were then placed under an inverted petri dish at either 22°C

or 4'C. The drying time ranged fiom 5 - 7 h for films dried at 22OC and 1 - 2 days for

films dried at 4OC. Al1 films were transparent and crack free. Typical thickness of films

for ET and FTTR measurements were about O. 1 - 0.3 mm and 1 - 2 pm, respectively. We

began the first ET and FTIR measurements about 1 h afier the Iast visible trace of the wet

spot in the center of the films disappeared. We arbitrarily refer to this tirne as to.

To prepare solvent-cast films, a small portion of the fieeze-dried sample was

dissolved in THF solvent and cast ont0 a CaFz disk for FTIR measurements and ont0 a

quartz plate for ET measurements. The drying time for these solvent-cast films ranged

from 5 - 15 min at 22OC with low relative humidity (10 - 30%). These films are

transparent with typical thickness from 1 - 2 m. Another small portion of the fieeze-

dried polyrners was mixed with dried KBr powder and then pressed into a thin KBr pellet

for FTIR measurements. The IR spectnim of this KBr pellet was used to monitor the

arnount of -N=C=N- groups Iost for each sample during the drying.

1.2.3 Eoerg-y transfer measurements and data analysis

Al1 fluorescence decay measurements employed the time-correIated single photon

counting technique, 26 using a pulsed deuterium lamp as the excitation source. In the Phe-

An system employed here, we set the excitation wavelength at 300 nm and the emission

wavelength at 350 nm. An interference filter for the emission at 350 * 5 nm was placed in

front of the ernission monochromator.

For each fluorescent decay measurement, the quartz plate supporting a polyrner

film was first placed in quartz tube and flushed with Nz gas. After each measurement, the

quartz plate was removed fiom the tube and placed directly on an aluminurn plate under a

petri dish in a forced-air oven at 60°C. The quartz plates were removed from the oven

periodically, cooled to room temperature and re-measured under N2 atmosphere.

Representative fluorescence donor decays, measured as a function of annealing

time (t, ns)

Figure 4.1 : Donor fluorescence decay profiles of a PEHMA copolymer latex blend film consisting of a 1 : 1 mixture Phe-labeled latex containing 1 1 mol % MAA comonomer and an An-labeled latex containing 5 mol % tBCEMA comonomer. This film was formed at room temperature and a ~ e a i e d at 60°C for different periods of time.

time at 60°C, of a D-MAA- 1 Ut-BCEMA-5 latex film are shown in Figure 4.1 . In the

newly formed film, the ID(t) profile is nearly exponential, consistent with little ET across

the particle-particle boundary. But as the films are annealed for various periods of times,

the areas under the decay curves decrease, indicating a growth in ET due to polyrner

diffusion.

The quantum efficiency of energy transfer (t) can be evaluated fiom the

expression

where areaoA(b) is the area under the normalized decay curves of donor fluorescence in

the presence of acceptor afier an annealing tirne tn, and a r e a ~ is the area under the

normalized decay curves of the donor fluorescence in the absence of acceptor- The areao

does not change with time.

To obtain the area under a donor fluorescence decay c w e , we fit each decay

c u v e to an empincal equation (4.8) and then evaluate the integrai from the magnitude of

the fitting parameters.

I , ( t ) = A, exp

From a global perspective, the first term of the equation 4.8 is denved from the

regions of the tilm, where the donor and acceptor are mixed. The second term originates

primarily from the regions containing the donor only. The fitting parameters Ai, Az and P obtained fiom each profile are usehl for area intergration, but their physical meaning is

not important here. These integrated areas have dimension of tirne, and define an average

decay time Cr+ for the sarnple.

4.3 Results and Discussion

4.3.1 Characterization of latex polymers

Al1 latex particles were synthesized by seeded emulsion polymerization, with the

fluorescent and functional cornonomers being introduced only in the second stage under

monomer-starved conditions. The seed particles, prepared by batch emulsion

polymerization, consist of PEHMA homopolymer. These PEHMA seed particles, similar

in characteristics to those prepared in Chapter 3, have a mean diameter of about 48 nm

and a high molecular weight (PEHMA: M w = 700 000, MJMn =2-5 1).

From these seed particles, we prepared 10 batches of labeled latexes. Only some

of these Iabeled latexes will be examined in this Chapter. The polymer compositions of

individual latex are listed in Table 4.2, dong with an abbreviated narne. In each

abbreviated name, the base polymer is sometimes not included, but it implies the PEHMA

polymer. Two batches of latex dispersion, namely PEHMA-D and PEHMA-A, contain

Table 4.2: Surnrnaxy of the composition of each type of labeled latex polymers, dong with the abbreviated name.

latex polyrner

--- -

abbreviated name

PEHMA-D PEHMA-A

no reactive comonomer. They were prepared using the recipe and conditions in the

second column of Table 4.1 - and differ only in the fluorescent comonomer. Three batches

of latex dispersion containing MAA (D-MAA-5, D-MA& 1 1, D-MAA-20) were prepared

according to the recipe in the third column of Table 4.1, each of which differs only in the

arnounts of MAA being used. Two batches of latex dispersion containing CCEMA and

three batches of latex dispersion containing ti3CEM.A were prepared using the recipe and

conditions in the last column of Table 4.1. Note that the carbodiimide-containing latex

particles are labeled with An, while the carboxylic acid-containing latex are labeled with

Phe.

The characteristics of these labeled latex polyrners are swnmarized in Table 4.3.

Since they were prepared fiom similar recipes, they have similar particle sires and size

Table 4.3: Characteristics of the labeled late

PEHMA-D PEHMA-A

partic le diameter (nm)

solids content

(%)

carbodiimide content

(%)

distributions, solids content, molecular weights and molecular weight distributions. UV-

visible analysis of solutions of the latex polymers Ui THF shows that individual latex

dispersion contains at least 99% of the fluorescent comonomer fiom the original recipe.

From the Table 4.3, we note that the carbodiimide content in the A-tBCEMA-3.7

Iatex polymers afier emulsion polymerization is found to be 66%' a value lower than what

we expected. This low value may be the result of some hydrolysis of the carbodiimide

oroups that occurs either during the emulsion polymerization or storage of the monomers. - We employ GPC with tandem fluorescence detector and a rehctive index

detectors to analyze the fluorescent dye distribution in the latex polymers.'7 An exarnple

of a GPC chromatogram of a PEHMA-D polymer i s illustrated in Figure 4.2. The

fluorescence trace (dotted line) shows only one signal whereas the refractive index (solid

line) shows two signals. From the refractive index chromatograrn, the signal at the

shorter retention time (about 13 min) belongs to the seed polymers. These sced polymers

were prepared in the absence of CTA and fluorescent comonomer, and hence they shouId

have a high MW value, and be detected only by the rehctive index detector. The second

signal at longer retention tirne (about 17 min) belongs to the polymers prepared in the

second stage of the emulsion polymerization. Since these polymers were formed in the

presence of the CTA and PheMMA, they show signals in both the fluorescence and

refractive index chromatograms at longer elution time. The fluorescence signal is shified

slightly to the lefi of the rehctive index signal because the fluorescence detector is

placed in front of the refractive index detector. However, the intensity and width of both

signals, afier being normalized, overlap, indicating that the polymers are uniformly

labeled. In addition, there is no fluorescence signal at about 24 min, an indication that

there is no small fluorescent comonomer left unattached to polymer c h a h and no

fluorescent oligomer formed during the preparation of the labeled particles. We prepare

our labeled latexes under monomer-starved conditions, and under these conditions the

Phe chromophores are randomly distributed but fully incorporated into the polymer

chains. From this result, we infer that the reactive cornonomers are also randomly

distri buted.

elution time (min)

Fi-me 4.2: GPC chrornatograms of PEHMA-D latex. The sample was prepared by drying the dispersion înto a solid film at room temperature and then dissolving it in THF solvent

4.3.2 Miscibility of PEKMA copolymers

At an early stage of this project, we want ed to establish that the presence of

bc t iona l groups in these Iabeled latex polymers, e.g. the reactive cornonomers and the

reaction products (N-acylurea, anhydride, urea groups) being produced in the system. do

not interfere with the ET process from the Phe to the An. We first examine films in

which only phenanthrene (no anthracene) is present. Figure 4.3 shows fluorescence donor

decays of the D-MAA-5 latex films annealed at 60°C for 10 h. These films were prepared

in the presence and absence of unlabeled PEHMA latex containing 3 mol% tBCEMA

(Chapter 3). When annealed, the film containing the tBCEMA-functionalized latex leads

to 50% gel formation. Frorn the Figure, both decay curves are exponential with a lifetime

equal to 46.0 f 0.3 ns, sirnilar to the typical unquennched lifetime of the excited Phe

chromophore (45 ns) in acrylate latex films. These results indicate that there are no

species in the D-MAA-5 copolymers that interfere with the fluorescence of the Phe

chromophore or which quench its emission. In addition, we l e m that there are no species

in the tBCEMA-containing latex particles or species that results fiom the crosslinking

reaction with WU that interfere with the fluorescence of the Phe chmmophores. We

corne to the same conciusion when fluorescence donor decays are measured for films

containing CCEMA-functionalized latex particles. We are convinced that in dl labeled

Iatex examined here, when a D-labeled latex is mixed with a A-labeled latex, the only

process leading to an increase in the Phe* fluorescence decay rate is ET fiom Phe* to the

An chromophore.

D-MAA-5 only

time (ns)

Figure 4.3: Donor fluorescence decay profiles of latex films consisting of D-MAA-11 polymer only, and of D-MAA-I 1 polyrner in the presence of uniabeled latex containing tBCEMA. These films were formed at room temperature and annealed at 60°C for 10 h.

4.3.2.1 PEHMA-D/PEHMA-A blend

We first examine the rniscibility of PEHMA homopolymers and its copolymer in

the absence of crosslinking. These experiments were designed as control experiments.

We examine the miscibility of PEHMA-D i- PEHMA-A latex polymers. Here the

polymers are chemically identical. One would expect that a fully rnixed film should

result. We prepare two films £iom a 1 :1 mixture of these Iabeled latex polymers. One

film is prepared by casting fkorn an aqueous latex dispersion, and the other, fiorn THF

solution. In the newly formed THF-cast film, we obtain = 0.52. When the same film

is annealed at 60°C for various amounts of time, aET remains at 0.52 + 0.01. This result

indicates that the Phe- and An-labeled polymers are fûlly rnixed in the fkeshly prepared

film. and remain miscible when the same film is annealed at 60°C. For films prepared at

4°C fiom the latex dispersion, we obtain On= 0.06. We believe that IittLe polymer

annealing time (min)

Figure 4.4: Plots of VS. t h e of films prepared fiorn the PEHMA-D/PEHMA-A blend and annealed at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast film was dried at 22°C.

diffusion occurs at this temperature and that cross-boundary ET is the major contribution

to this value. When the same film is annealed at 60°C, aET increases to 0.38 over 2 min,

and then to 0.49 over 12 min. This result indicates that difision leading to polymer

mixing has occurred. The value of aET at 0.49 signifies that the mixing between

PEHMA-D and PEHMA-A polymers is nearly complete and will reach completion if the

film is annealed for a longer time. The @ET values obtained from the dispersion-cast and

THF-cast films at different times are plotted in Figure 4.4. We take the value = 0.52

as that for fully mixed films, and use this value as a reference to compare to those of other

blends.

4.3.2.2 A-tBCEMA-S/PEHMA-D blend

We repeat the same experiments on the dispersion- and THE-cast films prepared

annealing time (min)

Figure 4.5: Plots of On VS. tirne of films prepared from a PEHMA-D/A-t-BCEMA-5 blend and anneaied at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast film was dried at Z°C.

fiom these blends, and plot (DR. as a function of annealing tirne in Figure 4.5. Here we

also see that fiom both the dispersion-cast and THF-cast films behave in a similar

way to the blend of PEHMA-D and PEHMA-A. We infer fiom this result that polyrners

A-tBCEMA-5 are fùlly miscible with PEHMA-D.

4.3.2.3 D-MAA-1 l/PEHMA-A blend

We note an interesting behavior of a, when films are prepared from blends

involving a latex polymer containing -COOH groups. The first example of such blend

consists of an equal amount of D-MAA-11 and PEHMA-A latex polymers. We

neutraIized the mixture of latex dispersions to pH 8 with aqueous ammoni- and then

prepared a dispersion- and a THF-cast film fiom this mixture. ET measurements were

carried out on these films; values were calculated fiom the data and are plotted in

Figure 4.6. In the fieshly dispersion-cast film formed at 4OC, we obtain @, = 0.03.

When the same films are m e a l e d at 60°C, a, values increase to 0.21 in 10 min, and

then slowly increase to 0.24 over 1 h- The increase in a, signifies that polymer rnixing

occurs. However, this increase ceases at 0.24, a value that represents a maximum for the

system. When the sample is annealed for an additional 3 h, an essentially identical value

is found, a, = 0.23. Limited miscibility of the latex polyrners prevents 0, fiom

reaching the value expected for a fiilly mixed sample ( a, = 0.52).

Further evidence to support the limited miscibility of these latex polymers is

obtained from the solvent-cast film. When a fieshly formed film prepared fkom a THF

solution of the same blend is examined, we obtain (D- = 0.40.> a value smaller than that

expected (0.52) for the fiilly mixed film. This result indicates that D-MAA-11

copolymers and PEHMA-A polyrners segregate during THF evaporation. When the sarne

film is anneded at 60°C, 0E7. decreases to 0.25 over 20 min, indicating that further

demixing of the polymer molecules takes place, causing M e r separation of the Phe and

An groups. At longer annealing time (eg. 4 h), decreases to 0.23, a value similar to

that obtained fiom the annealed dispersion-cast film.

annealing time (h)

Figure 4.6: Plots of VS. time of films prepared fiom a D-MAA- 1 1 /PEHMA-A blend and annealed at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast film was dried at 23OC-

We next examine what happens when we increase the arnount of MAA in the

copoIymer from 11 to 20 mol%. In Figure 4.7, we plot values vs. time for films

prepared from a D-MAA-2OPEHMA-A blend. In the fieshly formed film prepared at

4OC from the latex dispersion, we obtain = 0.03. When this film is annealed, On

increases to 0.14 over 1 h, and then to a maximum value of 0.16 over 7 h. This value is

lower than that obtained fiom the D-MAA-1 l/PEHMA-A blend (0.24), which indicates

that when the MAA content of the copolymer is doubled, the miscibility between D-

MA4-20 and PEHMA-A polymers is further reduced. With longer annealing time, 20 h.

a small decrease in OR. is observed (Figure 3). When a film of D-MAA-20PEHM-A-A

blend is freshly prepared fiom THF solution, we obtain On = 0.23, smaller than that

(0.40) obtained for the film prepared fiom D-MAA-11 + PEHMA-A in THF solution.

This result indicates that poiymers segregate substantially as the solvent evaporates and

that the segregation is stronger in the presence of the copolymer containing the larger

arnount of MAA. When this film is annealed, decreases and approaches 0.16 in 15 h,

a value close to that obtained fiom the dispersion-cast film.

1 dispersion-cast

annealing tirne (h)

Figure 4.7: Plots of QEf VS. time of films prepared fiom a D-MU-20PEHMA-A blend and annealed at 60°C. The dispersion-cast film was prepared at 4OC while the THF-cast film was dried at 22OC.

4.3.2.4 D-MAA-11/A-tBCEMA-5 blend

We repeat the same experiments on films formed fiom D-MAA-1 VA-tE3CEMA-5

blend, and plot values obtained at 60°C for different tirnes in Figure 4.8. When a

film is fieshly prepared from the latex blend dispersion, OET = 0.1 1. It is likely that both

cross-boundary ET and local interdihion contribute to this value, since this film is

formed at room temperature. When the film is annealed at 60°C, OET increases to 0.36 in

1 h, and then to 0.52 over 12 h. This is, for us, a very satisfjhg result, because it

indicates that a fülly mixed film is obtained. We note, however, that it takes hours for the

polyrners of this blend to mix completely whereas for the polymers of the PEHMA-

DPEHMA-A blend, fidl mixing occurs on the tirne scale of minutes.

It is known that the rate of polymer diffusion varies with chain length and is

sensitive to the presence of branching. l8 For linear polymers shorter than the

entanglement molecular weight, the diffusion coefficient Di characterizing the difision 1 28.29 of a given chah of length Ni, decreases as Ni- . For linear polyrners longer than the

entanglement molecular weight, the diffusion coefficient D, decreases as ~i-'. in the

D-MAA-Il + A-tBCEMA-5 blend examined here, polymer difision and the crosslinking

reaction are coupled. The polymer chah length and the extent of branching increase as

the reaction proceeds. It, therefore, takes a longer tirne for the polyrners to mix.

We examine the an. curve obtained fiom the THF-cas film. This curve is plotted

as the filled triangles in Figure 4.8. This curve shows several interesting aspects. in the

annealing time (h)

Figure 4.8: Plots of On VS. tirne of dispersion- and THF-cast films prepared fkom the D- MAA- l UA-tBCEMA-5 blend and aanealed at 60°C. Both dispersion-cast and THF-cast films were dried at 2Z0C.

newly forrned film, = 0.42, a value lower than 0.52. This result informs us that

segregation between D-MAA-11 and A-tBCEMA-5 polymers occur as the THF

evaporates. When this film is annealed at 60°C, On decreases to 0.40 in 20 min. Some

demixing of the polymers contributes to a decrease in Upon longer annealing, QR-

increases. To rationalize this result, we imagine that the reaction between the -N=C=N-

groups and the -COOH groups promotes miscibility, both by creating graft copolymer and

by converting half of the -COOH groups to an N-acyl urea. At long times, an. reaches

0.52, indicating that the film is fdly mixed.

We can compare the state of mixing as indicated by Q E ~ , with the extent of

carbodiimide reaction, examined by the decrease in the FTIR intensity at 2 128 cm-'. The

results for the reaction obtained fiom the THF-cast film is plotted as the open diarnonds

in Figure 4.9, dong with the corresponding <On. value for cornparison. During film

annealing time (h)

Fi,gre 4.9: Plots of OR. and percentage of -N=C=N- remaining vs. time for THF-cast films prepared fiom a D-MAA- I UA-tBCEMA-5 blend and annealed at 60°C. The film was fonned at 22OC.

preparation (THF evaporation), we detect that 14% of the -N=C=N- groups are lost.

When the same film is annealed for 0.5 h, the loss of -N=C=N- groups increases to 22%.

Over this tirne, polymer demixing occurs. When the same film is annealed for 15 h, the

conversion of -N=C=N- groups increases to 83%. During this tirne, polymer mixing

begins to dominate and is complete in 12 h at 60°C.

4.3.2.5 D-MAA-20/A-tBCEMA-11 blend

Next we consider what happens to the polymer miscibility when we double the

amount of the reactive groups, fiom 11 to 20 mol% for MAA and from 5 to 11 mol% for

tBCEMA. in Figure 4.10, we plot values vs. time for data obtained fiom the THF-

cast films. On the right hand axis of the plot, we also indicate the extent of the reaction of

the -N=C=N- groups. Both films were prepared from the same THF solution of D-MAA-

2O/A-tBCEMA-I 1 blend. in the newly formed film, we obtain OET = 0.38, a value lower

annealing tirne (h)

Figure 4.10: Plots of and percentage of -N=C=N- remaining vs. time for THF-cast films prepared fiom the D-MAA-20lA-tBCEMA- 1 1 blend and annealed at 60°C. The film was formed at 22°C.

than that of the blend of the polymers containing half of this amount of reactants ((DR. =

0.42). This result indicates that an hcrease in the reactive group content increases the

degree of polymer segregation in the solvent-cast film, even though 16% of the -N=C=N-

eroups react during the drying process. When these films are annealed for short penods CI

of time (25 min) at 60°C, polymer demixing occurs as shown by a decrease in from

0.38 to 0.36. Over this time, the extent of the carbodiimide reaction increases to 25 %.

When the same film is annealed for longer times, (PR. increases to 0.43 over 1 h, and then

reaches a limiting value of 0.51 over 12 h. This result indicates that upon prolonged

annealing, the polymers in the film become completely mixed. During this t h e , the

extent of the carbodiimide reaction increases to about 60 %. In spite of the crosslinking

reaction, the increase in the reactant content does not prevent complete mixing of the

polymer molecules in the system.

4.3.2.6 D-MAA-11/A-tBUEMA-5 blend

In the presence of acid, -N=C=N- groups undergo rapid hydrolysis to form the

corresponding urea. We take advantage of this system to create a mode1 system for

comparing the rniscibility of two of our copoIymers. There are, in principle, hvo methods

one can use to prepare the urea-containing latex particles. In one method, the urea-

containing latex polymer can be prepared directly fiom the urea-methacrylate monomer.

This urea-methacrylate monomer can be prepared by treating the isocyanate-methacwlate

with t-butylamine. Unfortunately, the urea-methacrylate monomer prepared does not

dissolve in the EHMA monomer during the preparation of the urea-containing latex

particles. This approach is not suitable. in addition, molecular weight and molecular

weight distribution, particle size and size distribution of the urea-containing latex

particles prepared fiom this way wili not be identical to that of the carbodiirnide-

containing waterborne particles. In the other method, the -N=C=N- groups in

carbodiimide-containing latex particles can be deactivated by treating the carbodiimide-

containing latex particles with an organic acid. This method replaces the carbodiirnide

with the urea fiuictionality while the characteristics of the carbodiimide-containhg latex

particles remain the same. To proceed we took a sarnple of A-tBCEMA-5 dispersion and

treated it with propionic acid to conven al1 of the t-butyl carbodiimide groups to the t-

butyl urea. We refer to this modified polymer as A-tBUEMA ("U" is urea).

O 10 20 30 40 50 60 70

time (h)

Figure 4.1 1 : A plot of -N=C=N- remaining (%) vs. time for latex particles containing tBCEMA when they are treated with propionic acid at room temperature.

In Figure 4.1 1, we plot the arnount of the -N=C=N- remaining, monitored by

FTIR, at various arnounts of time when the A-tBCEMA-5 latex dispersion is treated with

propionic acid in a stoichiometric amount at room temperature. From the plot, we see

that it takes 60 h to cornpiete the conversion process.

in Figure 4.12, we compare the behavior of a 1 : 1 of D-MAA- I 1 /A-tBUEMA-5

cast fiom solution in THF with that of a latex film. h the newly fomed film prepared

from the latex dispersion at 22'C7 we obtain = 0.13. Upon annealing at 60°C, @ET

increases rapidly to 0.28 over 0.5 h, and then to 0.35 over 7 h. In the newly formed film

cast fiom THF solution, we obtain am = 0.44, a value indicating that some segregation of

the polymers occurs during evaporation of the soivent When the same film is annealed at

60°C for 4 h, OET remains unchanged at 0.44. With further annealing over 14 h, @ET

decreases slightly to 0.42. This indicates that little polymer segregation has occurred.

The On value obtained for this blend (0.42) is larger than that obtained for the D-

blAA- 1 l ff EHMA-A blend. We take this difference to indicate that while D-MAA- l l +

A-BUEMA-5 are not fidly miscible, they are more miscible that D-MAA- 1 1 + PEHMA-

A. Dialhl ureas are both hydrogen bond donors through their N-H bonds, and hydrogen

bond acceptors. We anticipated a favorable thermodynarnic interaction between the -

COOH groups on one copolymer and the dialkyl urea groups on the second copolymer

which would enhance their rniscibility compared to P(EHMA-CO-MAA) + PEHMA. Our

results confirm this expectation. We also examined the effect of increasing the annealing

temperature from 60°C to 100°C for 20 h. Here we see a decrease in O E ~ h m 0.42 to

0.37, an indication that the increase in temperature promotes m e r polymer segregation.

0.0 I 1 . I I I <

O 10 20 30 40

time (h)

Figure 4.12: Plots of VS. t h e of THF-cast films prepared from the D-MAA-1 UA- tBUEMA-5 blend and annealed at 60°C and then at 100°C (in doned line).

4.4 Conclusions

We describe the miscibility of blends involving poly(ethylhexy1 methacrylate)

[PEHMA] latex copolymers using the direct non-radiative energy transfer @ET)

technique. When the polymers in both components of a blend are PEHMA

homopolymers, we obtain a fully mixed film. When one of the components in the blend

is replaced with a PEHMA copolyner containing 5 mol% t-butylcarbodiimidoethy1

methacrylate (tBCEMA), we also obtain a fûlly mixed film. However, if a PEHMA

copolymer containing I l mol% methacrylic acid @MA) is mixed with PEHMA

homopolymer, the miscibility between the polymers is substantially reduced. and reduced

M e r when the amount of MAA is increased to 20 mol%. A Ereshly fonned THF-cast

film prepared from a blend of the PEHMA copolyrner containing 1 1 mol% MAA and the

PEHMA copolymers containing 5 mol% tBCEMA exhibits some segregation of the

polymers. This segregation persists when the same film is annealed over short times (20

min) at 60°C. Over longer times, full mixing of the copolymers occurs, accompanying

the reaction between the -COOH and -N=C=N- groups to form N-acyl ureas. When a

film of the same copolymers is prepared fiom a latex dispersion and annealed, a fully

mixed film also results. When latex films are formed fiom particles containing twice as

much reactive fimctional group ( I l mol% -N=C=N-; 20 mol% -COOH), polymer

diffusion is faster than the reaction, but the crosslinking reaction promotes complete

mixing.

4.5 References

1 Herman B., Fluorescence Microscopy and Fluorescent Probes, Edited by Slavik J.

Plenum Press, New York, 1996, 1.

Lakowicz J. R., Principles of Fluorescence Spectroscopy, Plenum Press, New York,

1953. 3 Morawetz H., J. Polym. SciiPart A. PoZyrner Chernisoy, 1999,37, 1725. 4 Yamamoto H.; Mizusaki M.; Yoda K.; Morishima Y., Macromolecules, 1998,3 l(1 l),

3588.

5 Rager T.; Wegner G.; Winnik M. A., Macromolecules, 1997,30(17), 49 1 1. 6 Hu Y . S., Smith G. L-, Richardson M. F., McCormick C . L., Macromolecules, 1997,

30(12), 3526. 7 Marowetz H. and Amrani F. Macromolecules 1978, 11,28 1. 8 Zhao H. Y.; Tang T.; Wang Z. G.; Huang B. T., J. AppZ. PoZym. Sci., 1999,71(6), 967. 9 Qui X. P.; Jiang M., Polymer 1995,36(1 S), 360 1. 1 O Sankarapandian M.; Kishore K. Macromolecules 1993,27(25), 7278.

' ' Deppe D. D.; Miller R. D.; Torkelson J. M., Journal of Polymer Science Part B:

Polymer Physics 1996, 34(l7), 2987. 12 Ni S. R.; Zhang P.; Wang Y. C.; Winnik M. A., ~MacromoZecuZes, 1994,27(20), 5742.

l3 Nakashima K.; Duhamel J.; Winnik M. A., J. Phys- Chem., 1993,97, 10702. 14 Nakashima K.; Zhang YS.; Duhamel J.; Feng J.; Winnik M. A., Langmuir. 1993, 9,

2825. 15 Marion P.; Beinert G.; Juhue D.; Lang J., Macromolecules, 1997,30, 123. 16 Feng J.; Yekta A.; Winnik M. A., Chem. Phys. Letr.. 1996, 260, 296. 17 Winnik M. A.? " n e Formation and Properties of Latex FiIms" in "Emulsion

Polyrnerization and Emulsion Polymers", El-Aasser M. and Love1 P. edited. John Wiley

& Sont Chapter 14.

'' Winnik, M. A., Curr. Opin. Colloidln, 1997,2(2) 192. 19 Boczar E. M.; D ~ O M ~ B. C.; Kirk A. B.; Lesko P. M.; Koller A. D., Macromolecules,

1993, 26(21), 5772. 20 Forster T., Comprehensive Biochemistry, Florkin M. and Statz E. H. edited, 1967,

Elsevier, New York, V. 22,6 1.

" Berlman 1. B., Energy Transfer Parameters of Aromatic Compounds, Academic Press,

New York, 1973. 22 Wang Y.; Zhao C.-L.; Winnik M. A., J. Phys. Chem., 1991,95,2 143. 23 Brandmp J. and Immergnt E. H. eds., "Polymer Handbook" 3rd Edition. John Wiley &

Sons, 1989.

Zhao Ca-L.; Wang Y.; H w k a 2.; Winnik M. A.' Macrornolecules, 1990,23,4082.

1s Feng J., Ph.D. thesis, University of Toronto, 1 996. 76 O'Connor D.; PhilIips D., Time-Resolved Single-Photon Counting, Academic Press,

New York, 1984.

" Sosnowski S.; Feng I.; Winnik M. A., J. Poiym. Sci. : Polym. Chern., 1994,32, 1497.

'S Doi M. and Edwards S. F., "The Theory of Polyrner Dynamics", Oxford University

Press. New York, 1986.

19 Kausch H. H.; Tirrell M., Annu. Rev. Morer. Sci., 1989, 19,341.

5 Polymer Interdiffusion vs. Crosslinking in Latex Films of PEHMA Copolymers

5.1 Introduction

5.1.1 Polymer diffusion across interfaces 1 Polymer difision across interfaces has been a subject of intense interest for

many years. This type of diffusion plays an important role in technological processes,

which include welding of polymer slabs, crack healing, 3" sintering of polymer powders

by compression molding, " and the formation and aging of latex films. 7.8 The feahtre

these processes have in common is that in the initiai state, polymer molecules are

confined to opposite sides of the interface, which acts as a dividing plane. Polymer

diffusion across this intedace generates entanglements, which increase the strength of

adhesion. in this process, the interfaces are said to "heal".

In latex coatings, polymer diffusion is recognized as an important process in

development of mechanical strength. This recognition was first mentioned by Voyutskii

in 1958. He emphasized that the forces acting on particle deformation are not sufficient

to produce mechanically strong films, and there m u s be polymer diffusion, which he

termed "autohesion", which leads to healing of the weak boundaries between particles

and the development of mechanical properties in the film. Only recently have the tools

become available to study polymer difision directly.

Techniques to characterize aspect of the healing process for interfaces in latex

coatings are transmission electron microscopy (TEM), 9-14 atomic force microscopy

W M ) , 15-19 fieeze-fracture TEM, attenuated total reflect Fourier transform infiared

20 21-33 dynamic mechanical analysis, 1 3 . 2 r . 3 1 ~ 4 spectroscopy, t ende measurements,

modulated-temperature differential scanning calorimetry. 35 Two techniques, small-angle

and direct non-radiative energy transfer (ET) 8 neutron scattenng (SANS)

measurements, allow one to study polymer difision across the interface.

in this project, we employ ET measurements to monitor polymer interdiffusion in

latex films. For latex films, which do not undergo crosslinking, the rate of polyrner

diffusion can be characterized by a mean diffusion coefficient D describing the average

difision rate of the polymea of various molecular weights in the sample. From SANS

experiments D values can be calculated directly fiom the data, whereas for ET

experiments oniy "apparent" values @,), which are found to be proportionai to D, c m

be calcuiated from the extent of mixing (Tm).

We use this approach, which employ the Fickian spherical diffusion rnodel, to

calculate Dvp values for some of our experiments. '' This model assumes that one of the

di f is ing species is initially distributed uniformly in a sphere of radius (R), with an initial

concentration (C,). It diffuses outward into a rnatrix of the other component. [ts

concentration C,,,, at radius r and time t, is given by

Co Dr (R - r)' ( R + r)' = [ e R + r ) + e R f ) ] - { e x - ]-ex{- ]} C,r,, 2 zfi, ZK 4Dr, 4Dt,

The fraction of the d i f i i n g species, fD, that has diffused across the original

boundary at time t, is

Dapp values are evaluated through the assumption that fD = fm. We seek the best

Dapp that satisfies the above two equations. The assurnption that fD = f, is not correct, but

simulations have shown that for conditions similar to those employed here, f, is

proportional to fo for f, 5 0.7. in addition, Dqp values cafculated fiom fm are

proportional to the true D values.

If a chemical coupling reaction occurs at the same time as difision, the polymer

molecular weight will increase and Dwp values will decrease. The evaluation of Dwp

values from this type of data will be extremely cornplex.

5.1.2 Crosslinking reaction

Thermoset solvent-based coatings have been available for many years. Water-

based coatings with post-application crosslinking have aiso been known for at least two

decades. In the most general ternis' there are three types of crosslinking reactions:

hydrogen bond formôoon, ionic-interaction with metals and covalent bond formation.

Commercial emulsion polymers employing crosslinking reactions are currentiy availabie

in the market place. The type of crosslinking employed differs for different applications.

For example, an ionically crosslinkable systern is used in floor coatings and consist of

zinc acetate plus carboxylic acid-containing polymers. This crossIinking system produces

am bient curable fiims with excellent hardness and water resistance. This ionically

crosslinkable coatings can be removed with an ammonia water solution that complexes

the 2n'- ions, thus dissolving the coating.

Melamine-formaidehyde derivatives are used as extemal crosslinking agents for

Iatex films containing OH groups. Covalent bonds result kom the acid-catalyzed reaction

between the -OH-containing latex and the -CH2-O-CH3 of the melamine molecule

present. This type of system is widely used in industrial and automobile coatings. 38.39

Other crosslinking systems employing covalent bond formation are urea-aldehyde, 40

4 1 42 43 epoxy-amine, epoxy-carboxylic acid acetoacetoxy-amine, isocyanate-amine, 4.4 aziridine-carboxylic acid, oxazoline-carboxylic acid, and carbodiimide-carboxylic

acid."

Crosslinking requires functional groups. There are a few different strategies to

introduce reactive groups into waterborne coatings formulations. 46 One approach

(.'water-reduci ble" coatings) involves dissolving pol p e r s containing reactive

functionalities in a small arnount of organic solvent, followed by the addition of excess

water to the organic solution to disperse the polymers. These dispened particles are then

mixed with an extemal crosslinking reagent before they are cast into films. Another

approach involves copolymerizing reactive functionalities into latex particles during

emulsion polymerization. in this type of system, one can introduce an external

crosslinking reagent into the formulation just before application to a substrate. The

mixture ofien has a short pot iife, and crosslinking can occur within individual particles

before the coating forms a film. In one strategy to avoid this problem, one blends two

latex dispersions containing complementary reactive pairs. Under these circumstances,

no reaction c m occur between the groups until the reactive particles corne into contact.

Zn this chapter, we examine the cornpetition between polymer diffusion and

crosslinking reaction in carboxylic acid-carbodiimide PEHMA latex system. We label

the carboxylic acid-containhg latex with a srnall amount of PheMMA (the fluorescent

donor) and the carbodiimide-containing latex with a small amount of PLnMA (the

fluorescent acceptor). We use ET from the Phe to the An to follow the extent of polymer 4 7 diffusion, because polymer diffusion brings these groups into proximity. ET occurs

when these groups approach their characteristic (Foster) distance of & = 23 A. In

addition, polyrner diffusion also brings the -COOH and the -N=C=N- groups into

proximity for them to react. We c m then use the characteristic band of the -N=C=N-

bctionality (2 125 - 2 130 cm-') in the infrared specvum 49 to follow the reaction.

Three possible products can be obtained from the reaction between a carboxylic

acid (-COOH) and a carbodiimide fùnctionality (-N=C=N-): an anhydride, a urea and an

N-acylurea, As shown in Scherne 5.1, oniy the N-acylurea product can contribute to

stable crosslinking. The anhydride will lead to transient crosslinking, but on exposure to

moisture will hydrolyze to yield to two carboxylic acid rnoieties.

Taylor and Bassett have shown that in a polar environment, at elevated

temperatures and in the presence of amines, 1,3-dicyctohexylcarbodiimide reacts with

acetic acid to form predominantly the N-acylurea product. In our system, latex films are

prepared at room temperature from a 1: 1 mixture of ammonia-neutraiized carboxylic

acid-containing latex particles and carbodiimide-containing latex particles. Upon drying,

ammonia evaporates to regenerate the carboxyl group in its protonated form. These films

are then annealed at elevated temperature. Under these conditions, we believe that the N-

acyl urea will be formed predominantly.

carboxylic acid-containing polymer carbodiimide-contai- polymer

R N-ac y lurea anhydride urea

carboxylic acid-containing polymer (2)

Scheme 5.1 : Representation of the reaction between carbodiimide- and carboxylic acid- containing polyrners. The wavy lines refer to the polymer backbone. The anhydride adduct formed in the reaction will be eventually hydrolyzed by water.

5.2 Experimental We used the labeled latexes described in Chapter 4 to study the cornpetition

between polymer diffusion and crosslinking in latex films. The films were prepared as

described in Chapter 4.

5.2.1 ET data anaiysis

We characterize the diffusion process in terms of the fraction of mixing f,, which

represents the fnctional growth in energy transfer eficiency in the system.

where [aET(tn) - aET(~)] is the change in enerm transfer efficiency of the film fkeshly

prepared and of the same film annealed for time (t,). [QET(b,) - mET(to)] is the difference

in energy transfer efficiency of the Mly mixed film and the newly formed film. Area(b)

is the area under the fluorescence decay curve of a newly fomed film. Since the

dispersions take several hours to dry, we define to as 1 h after the "wet spot" visible in the

center of the film disappeared. Area(t,) represents the corresponding quantity determined

from the same film annealed for time (t,). Area(b,) represents the area under the decay

cuwe of a fully mixed film. We ofien mode1 this process by preparing a film fiom a

solution of the polymers in THF.

A mixture of polymers containing carboxylic acid and carbodiimide fünctionality

represents a special case. To obtain the area(&) of this mixture, we prepared a film cast

fiom THF solution, and the film must be annealed for a long period of tirne until of

this film approaches to 0.52, a value similar to that obtained for films prepared fiom the

PEHMA itself. We assume that when QET reaches 0.52, polymers in this film are hlly

mixed. As shown in Chapter 4 that, polymers containing carboxylic acid and

carbodiimide functionality in the film fieshiy prepared fiom THF solution exhibits some

segregation (OR- = 0.4). When this film was annealed for a long time (12 h),

increases to 0.52, an indication that polymers containing carboxylic acid groups are

miscible with polymers containing carbodiimide groups.

5.2.2 FTIR measurements and data analysis

Al1 FTIR measurements were made with a Perkin Elmer FTIR Spectnim 1000

Spectrometer with a resolution of 4.0 cm". Before the first measurement in a series,

carried out 1 h after the last visible trace of the wet spot in the center of the films

disappeared, the Ca& disks containing polymer films were mounted on FTIR holders.

Afier each measurement, the entire FTIR sampie holder was placed directly onto an

aluminurn plate under a petri dish in a forced-air oven for different amounts of time. The

sample was periodically removed fiom the oven, cooled to room temperature and re-

measured.

The -N=C=N- functionality has a characteristic band between 2 125 - 2 1 30 cm-' in

the IR spectnim. ''' We rely on the intensity ratio of this band to that of the band at 1380

cm" ( I - ,= ,= , v - / i , , ao ) to determine the amount of -N=C=N- groups lost during the

preparation of individual THF- and dispersion-cast films. The band at 1380 cmd'

corresponds to a C-H bending vibration of the polymer. 50.5' We also rely on this intensiw

ratio, measured as a function of tirne, to follow the disappearance of -N=C=N-, which we

anribute to the crosslinking reaction between the -COOH and -N=C=N- groups.

To determine the amount of -N=C=N- groups lost during THF or water

evaporation, we compared the I-,=,=,-/II, , , ratio in the FTIR spectnim of the U r -

pressed sample to that of either the dispersion- or THF-cast sample measured at b. To

follow the reaction between the -N=C=N- groups and the -COOH groups, we employ

equation 5.3

(1-.v=c=.v- /113m),- -N=C=N- remaining (%) = ) d o 0

(1-,v=c=,v-/ln80

h e ( I V / ) d ( i C 1 , ) are the ratios of the -N=C=N- absorbance

intensity to the absorbance intensity at 1380 cm" of a newly formed film (to) and of the

same film annealed at a particular time (t,).

Representative FTTR spectra, measured as a function of time, of tilms prepared

fiom a mixture of carboxylic acid- and carbodiimide-containing latex are shown in Figure

5.1. If only the N-acyl urea were formed, we would expect the decrease in the -N=C=N- - 1 52 band (2 126 - 2128 cm") to be accompanied by an increase in intensity at 1643 cm -

This band is weak and hard to quanti@, but our spectra show a srnall decrease in intensity

at this wavenumber for a shon annealing time (30 min), followed by an increase at longer

times.

wavenumber (cm")

Figure 5.1 : FTIR spectra of a latex film prepared fiom a 1 : 1 mixture of the D-MAA- 1 UA-tBCEMA-5 blend. The film was fonned at room temperature and annealed at 60°C for various amounts of time t,.

5.2.3 Gel-content and swell-ratio measurements

Gel-content and swell-ratio measurements were determined by weight. Individuai

sarnples were removed from the oven. cooled to room temperature, weighed (Wo) and

then irnmersed in excess 1,4-dioxane for at least 24 h to allow the uncrosslinked

component to dissoIve. The films were then removed fiom the solvent, touched with a

dry filter paper to remove liquid solvent on the film surface, weighed (WI), and then

allowed to dry further in air for at Ieast another 24 h to remove the remaining solvent.

When completely clrie4 the undissolved polymers were weighed (W2). The gel content is

expressed in equation 5.3,

gel content (%) = - x100 (2) The swell ratio is calculated as

4 wel l ratio = - 4

5.3 Results

5.3.1 PEHMA-D/P EHMA-A b h d

We first examine the polymer diffusion in films prepared from a 1:l mixture of

PEIILMA-D and PEHMA-A latex particles. These particles contain no reactive groups,

and hence crosslinking does not occur in this system. To capture the early stages of

polymer dimision, these films were prepared at 4OC in a cold room, because we had prior

evidence that polymer diffusion in acetoaceroxy-containing PEHMA copolymer 1

films takes place at a significant rate at room temperature (22°C)- 53

We calculate fm values directly fiom the area under fluorescence decay curves,

atex

and

also from OR. values. Experirnents were carried out at 22°C on films annealed at 22OC,

30°C, 40°C and 60°C. We plot the obtained f, vzlues calculated fiom the data in

Figure 5.2, and those calculated from the area data in Figure 5.3. From both Figures, the

f, curves for each annealing temperature are essentially identical, as they should be, since

the same integrated areas are used. Difierence could occur because a subtraction step is

used in calcuiating aET, and the calculation of f, involves taking the ratio of two

differences. Values of f, calculated fiom QET might have a greater uncertainty. For

example, the fm plot at 22°C in Figure 5.2 is aimost identical to the fm plot in Figure 5.3 at

the same temperature. In individual films in Figure 5.2, we obtain Om (b) = 0.04 to 0.08.

We believe that cross-boundary ET contributes to this value. Upon longer annealing, am approaches 0.52, a value we assign to that of a fully mixed film (Chapter 4).

The first important observation in both Figures is that f, in each plot increases as

the annealing time increases, indicating that the PEHMA-D polymen d i h e into the

regions containing the PEHMA-A polymen, and the PEHMA-A polymen di f i se into

the regions containing PEHMA-D polyrners. The shape of these curves is also important.

time (h) time (h)

time (min)

O 2 4 6 8

time (min)

Figure 5.2: Plots of fm vs. time of latex films annealed at 22OC (room temperature), 30°C, 40°C, and 60°C. fm values were calculated fkom the aET data Films were cast at 4OC fiom a 1 : 1 mixture of PEHMA-DPEHMA-A particles.

A large increase in f, values occurs at early times, folIowed by a smaller increase as the

time increases. There is a distribution of polymer molecular weights in each latex

particle. I t is likely that the srnall polyrner molecules d i f i se faster than large polymer

molecules, and as a result, they contribute to the increase in f, at early times. Polymers

with larger molecular weight diffuse slower, and thus contribute to the increase in f, at

longer times.

time (h)

time (min)

time (h)

O 2 4 6 8

tirne (min)

Figure 5.3: Plots of f vs. time of latex films annealed at 22OC (room temperature), 30°C, 40°C, and 60°C. f, values were calculated fiorn the area data, Films were cast at 4OC fiom a 1 : 1 mixture of PEHMA-DPEHMA-A particles.

From the plots in both Figures, we can also see that f, depends strongly on the

amealing temperature. For example, fm reaches 0.67 over only 2 min at 60°C, while at

Z ° C it requires 2 days to reach a similar fm d u e . This result infonns us that polymer

difision is sensitive to the change in temperature. An increase in the anneaiing

temperature leads to an increase in polymer difision, and a decrease in the annealing

temperature leads to a decrease in polymer diffirsion.

The W i l l i a m s - L e - F e (WLF) expression j4 is used to describe viscoelastic

properties of polymers over a wide range of temperature change. This expression relates

the rate of diffusion processes, which depend upon backbone motions of the polgmers to

changes in free volume in the system. The essence of this anaiysis is that the temperature

has a larper effect on the thermal expansion of the matrix than it does on any other

intrinsic barrier to diffusion. For polymer difision, the WLF equation takes the

where To is an arbitrarily chosen reference temperature, and Do is the diffusion coefficient

determined at that temperature. D is the diffbsion coefficient at temperature T, and CI

and CI are parameters characteristic of a particular polymer.

Over a limited range of temperatures, the change in diffusion rate can be described

by an Arrhenius expression, in which the magnitude of the activation energy, E., will

depend on T-Tg, where T is the temperature in the range of the rneasurements. 5' For a

diffusion coefficient, D, the Arrhenius equation is written

where Do is the diffusion coefficient at the reference temperature To; R is the gas

constant. m d T is the annealing temperature in Kelvin.

In out expenments, we calculate an "apparent mean diffusion coefficient", Dqp, as

described above. We remind the reader that the Dqp values are not tme "mean" diffusion

coefficients and averaged over al1 of the components in the sample. Previous studies in

our laboratory have shown that Dg, values are proportional to mean D values. and that

experiments at different temperatures can be understood if Dvp values are compared for

common values of fm. These calculated Dapp values are plotted against fm in Figure 5.4.

The apparent kink in the data near fm = 0.5 for 30°C is unlikely to be real.

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 .O

fm

Figure 5.4: Plots of D,, vs. f, at different temperatures.

From the Dwp values, an apparent activation energy, EkgP, for polymer diffusion

of PEHMA polymers can be evaluated From an Arrhenius plot at equal extents of mixing

( f ) We show the Arrhenius plots in Figure 5.5 for data at f, = 0.55. Eaqp is calculated

from the dope of the plot and has a value of 37 kcal/mol. Zhao, Wang and W i ~ i k

found a similar value for PBMA diffusion in the range of 70°C to 90°C (T-Tg SO°C).

2.9 3.0 3.1 3.2 3.3 3.4 3.5

IIT x IO-^ (K)

Figure 5.5: A plot of In Dwp vs. lm, in which Dqp values were calculated at f, = 0.55.

From these results, we attempt to constmct a master c u v e of Dapp VS. fm. The

master curve is obtained by using a shift factor based on Eavp = 37 kcaUmol to shift al1

values of Da,, to 22°C (295 K). The master c w e allows one to predict the time it takes

for the system to reach a certain f, value at a certain temperature. Experimentally, one

c m obtain a f, value on the lower end of the f, scale at 60°C, but the error is large

because poIymer diffusion is very fast at early times at this temperature. At the other

extreme, it is difficult to follow the diffusion to large f, values at 2Z0C, because the

polymer diffusion is very slow at this temperature.

We plot the master curve in Figure 5.6, constmcted using equation 5.10. The

master curve establishes that polymer difision for al1 f, values is characterized by the

same apparent activation energy.

Figure 5.6: A master c w e of Da,, vs. f,. It is constructed using EqVp = 37 kcailmol. and the reference temperature is 22OC (295 K).

5.3.2 D-MAA-1 UA-tBCEMA-5 blend

As the first example of the reactive latex blend, we examine a latex film formed

from a dispersion containing a mixture of an equal amount of ammonia-neutralized D-

MAA-I l and A-tBCEMA-5. in the newly formed film, examined 1 h after the drying is

complete at 22OC, we find that = 0.1 1. We believe that both local interdiffusion and

cross-boundary ET contribute to this value, because the films are formed at 2Z0C, 20" to

30°C above the g l a s transition temperature (-1 O°C) of these latex polymers. The range in

T-Tg is due to the influence of the -COOH groups on the Tg of the copolymer (see

Section 5.3.3). In these films, the extent of ET increases as the films are anneaied at

60°C, reaching am = 0.52 for complete mixing (Chapter 4). A plot of fm as a function of

annealing time is presented in Figure 5.7. In the plot, fm increases rapidly to 0.60 in 1 h

and then to 0.83 in 4 h, while over 4 h, only 30% of the -N=C=N- groups react. This

result indicates that rnolecuIar mixing due to diffusion occurs much faster than the

crosslinking reaction.

We compare the f, plots in Figure 5.7 to that in Figure 5.3 at 60°C. In Figure 5.7.

f, values obtained fiom the D-MAA-1 UA-tE3CEMA-5 film are significantly smaller at

evew annealing tirne, compared to those obtained fiom the PEHMA-DPEHMA-A film.

For example, for the system to reach f, = 0.60, the D-MA4-1 I/A-tBCEMA-5 film

requires 1 h whereas the PEHMA-DR-EHiLIA-A film requires less than 2 min. These

results indicate that the polymer diffusion rate decreases significantly as the chernical

reaction increases the length and branching of the polymer chains.

One other aspect of the data in Figure 5.7 merits a comment. We compare in

Figure 5.7 the rate of -N=C=N- consumption in the latex film and in a film of the same

two copolymers cast fiom THF solution. We find that 14% of -N=C=N- groups are lost

during the evaporation of THF solvent to form a solid film (over 10 - 15 min), cornpared

to the loss of 18% of these groups during the preparation of the film fiom the latex

dispersion (over 5 - 7 h). At t = 0, the THF-cast film has an aET value of 0.42 (Section

4.3.2.4), a value much higher than that of the film prepared fiom the dispersion (0- 1 1).

This result informs us that the reactants in the THF-cast film are much closer to one

another. The reaction between them should require only local segmental difision of the

polyrner backbone and little translational diffusion of the polymer chains. The

instantaneous rates of reaction are given by the slopes of the time-conversion plots in

Figure 5.7. These rates are higher for the solvent-cast film for every annealing time.

There is no indication after 28 h annealing of the reaction ceasing because crossiinking

suppresses polymer diffusion.

annealing time (h)

Figure 5.7: Plots off, (O) and the amount of -N=C=N- remaining (a) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also ploned as filled circles (O). These films were prepared at 22OC fiom a 1 : 1 mixture of D-MAA- 1 1 and A-tBCEMA-5 latex dispersion, and annealed at 60°C.

We follow the extent of crosslinking by measuring the gel-content and the swell-

ratio of the films prepared fiom the sarne mixture of labeled latex particles and annealed

at 60°C. The results are plotted in Figure 5.8. As seen in Figure 5.8, no gel is formed

until 1 h annealing time, at which only 10% of the -N=C=N- groups have reacted to form

11 wT% of insoluble polymers. Over this time, f, reaches 0.60. As the annealhg time

increases to 2 h, the gel content increases significantly to 77 %, accompanied by a drastic

decrease in the swell ratio fiom 22 to 5. During this time, f, reaches 0.73, while the

extent of carbodiirnide reaction increases to 20%. Upon longer amealing (15 h), the

swell ratio decreases to 2, and the gel content increases to 92%. Theoretically. this is the

value expected for fiil1 gel formation, since the Iinear polyrner in the seed polymers

represents 8% of the polymer mass. The seed polymer does not contain reactive groups,

and hence does not becorne part of the crosslinked network. The result of the gel content

measurements is a strong indication that polyrner diffusion is complete, because in order

for the system to reach 100% gel, every polyrner chain containing -COOH groups and

evexy polymer chain containing -N=C=N- groups must be close enough for these groups

to react- We find that when the gel formation reaches its fui1 extent, f, is greater than

0.96. and the extent of -N=C=N- reaction is close to 60%.

gel content

swell ratio

anneafing time (h)

Figure 5.8: Plots of gel content and swell ratio for latex films prepared fiom D-MAA- 1 UA-tBCEMA-5 particles These films were formed at 22OC and annealed at 60°C.

5.3.2.1 Temperature effects

To examine the effect of temperature on the system, we repeated these

expenments on latex films anneded at 40°C. These films were prepared tiom the same

mixture of latex dispersions that were w d to prepare films for examination at 60°C. We

plot the fm values (O) and the extent of -N=C=N- reaction (i) monitored at 40°C in

Figure 5.9. From the plots, one sees that as the annealing temperature decreases, both the

diffusion rate and reaction rate are decreased. For example, fm reaches 0.60 over 1 h at

60°C (Figure 5.7), whereas at 40°C, it requires about 30 h for f, of the system to reach a

similar fm value. During this time, the extent of carbodiimide reaction in the film

annealed at 40°C increases to 25%. To reach 25% carbodiirnide reaction at 60°C, the film

requires only 4 h (Figure 5.7).

In Figure 5.9, one sees two distinct regions. At the early times, the rate of

polymer diffusion is faster than the rate of crosslinking. For example, f, reaches 0.57

over 21 h, while over this time only 21% of -N=C=N- groups have reacted. As the

annealing time increases, both rates are reduced, but the rate of polymer diffusion is

reduced to a greater extent than the rate of the crosslinking reaction. For example, it

takes 45 h (45 h - 90 h) for f, in the system to increase from 0.67 to 0.69, compared to an

increase in the extent of -N=C=N- reaction of about 18% over the sarne penod of time.

These results indicate that the polymer diffusion rate is more sensitive than the chemical

reaction rate to the reduction in temperature.

One way of examining the effect of temperature on the diffision process in the

presence and absence of the crosslinking is to compare the time it takes for films to reach

fm = 0.6. At 60°C, f, of the PEHMA-DIPEHMA-A film reaches 0.6 over about 1.3 min

(Figure 5.2, lower left plot), whereas it takes 1 h for the D-MAA-11IA-tBCEMA-5 film

to reach a similar fm value (Figure 5.7). Here the time dizerence is about 1 h. When the

two similar films are compared at 40°C, the t h e difference increases to 29 h. For

example, fm of the PEHMA-DPEHMA-A sample reaches 0.6 over 1 h at 40°C (Figure

5.3, lower left plot), whereas it requires 30 h for the D-MAA-11IA-tBCEMA-5 film to

have a similar f, value at the same temperature (Figure 5.9). From this cornparison, we

see that temperature plays an important role in this system.

Another way of looking at the effect of temperature on the diffusion process is to

compare the tirne difference (at f, = 0.6) for the D-MAA-11/A-tBCEMA-5 films to that

of PEHMA itself when the temperature is reduced from 60°C to 40°C. A decrease in the

temperature fkom 60°C to 40°C causes an increase in the time it takes for fm of both

systems to reach 0.6 value. In the PEHMA-D/PEHMA-A films, this time increases fiom

1.3 min to 1 h (Figure 5.3, lower right plot and lower lefi plot), an increase of about 1 h.

A much longer time (29 h) is needed in the D-MAA-11/A-tBCEMA-5 when the

temperature decreases from 60°C (Figure 5.7, f, = 0.6 over 1 h) to 40°C (Figure 5.9, f, =

0.6 over 30 h). Here one sees that a reduction in temperature does not affect polymer

d i f i i o n of the two systems in the same way. Lowenng the temperature reduces polymer

diffusion in the system that couples crosslinking to a greater extent than in the system fiee

of crosslinking.

5.3.2.2 Pre-annealing effects

In Figure 5.9, we examine the consequences on f, (e) and on the extent of - N=C=N- reaction (m) of increasing the temperature (to 60°C) afier many hours of

anneafing time (h)

Figure 5.9: Plots of fm and the percentage of -N=C=N- remaining vs. time for dispenion-cast films prepared fiom a blend of D-MAA-1 I /A-tBCEMA-5 latex. These films were formed at 22OC, annealed at 40°C (fm (O);

percentage of -N=C=N- remaining (O)), and then annealed at 60°C (Q (O);

percentage of -N=C=N- remaining (i)).

annealing at 40°C. From these plots, both f vaiues and the extent of -N=C=N- reaction

increase rapidly as the annealing temperature increases fiom 40°C to 60°C. For exarnple,

f, increases fiom 0.69 to 0.8 1, and the extent of -N=C=N- reaction increases from 48 to

55%. Both extension of branches fiom the crosslinked network and d i f i i o n of

polymers that remained unattached to the network contribute to the increase in f, values.

The results of gel content and swell ratio measurements, on films annealed at

40°C, are plotted in Figure 5.10. No gel formation can be detected until the films are

annealed to 4 h (4%). During this tirne, only 4% of -N=C=N- groups react! while f,

increases to about 0.3. Upon longer annealing, the gel content increases to 60% over 21

h, and then to 83% over 72 h. Durhg this time, f, reaches 0.68, and the extent of

carbodiimide reaction increases to 37%. With M e r annealing (to 98 h), there is no

significant increase in the measured gel content, even though the amount of -N=C=N-

groups reacted reaches 47%. During this time, there is only a very small increase in f,.

An analysis of these results is presented in the Discussion part of this Chapter.

annealing time (h)

Figure 5.10: Plots of gel content and swell ratio vs. time at 40°C for latex films prepared fiom D-MAA-11IA-tBCEMA-5 particles. These films were prepared at 22Oc.

5.3.3 D-MAA-2O/A-tBCEMA-11 blend

We next examine what happens to the polymer diffusion and the extent of

crosslinking reaction when we double the amount of the reactive comonomers in

individual latex samples, fiom 11 to 20 mol% for W and fiom 5 to 11 mol% for

tBCEMA. We plot f, data (O) for films in Figure 5.1 1, dong with the extent of -N=C=N-

reaction measured from the dispersion-cast film (a) and the THF-cast film (a). These

films were prepared at 22OC from the D-MAA-20 + A-tBCEMA-Il polymers, and

annealed at 60°C. We see in Figure 5.11 that f, reaches 0.19 over 0.5 h, and then 0.50

over 4 h. During this t h e , the extent of -N=C=N- reaction monitored fiom the

dispersion-cast film increases to 19%. From these results, we leam that during this time

the rate of polyrner diffusion is faster than the rate of crosslinking reaction. When these

films are annealed for longer times, f, reaches 0.70 over 38 h, while the extent of - N=C=N- reaction increases to 46% over 40 h. Here we see that the rate of increase in f,

values is reduced significantly, indicating that polymer diffusion is retarded substantially.

There is no indication that polymer diffusion ceases because of the reaction.

In Figure 5.1 1, we also see that the instantaneous rate of the reaction in the THF-

cast film. at every annealing tirne, is greater than that in the dispersion-cast film.

To examine the consequences of increasing the reactant concentrations on the

diffusion-reaction system, we compare the data in Figure 5.1 1 to those in Figure 5.7. In

Figure 5.1 1, we see that f, obtained fkom the film prepared from the D-MAA-2OIA-

tBCEMA-I 1 dispersion reaches 0.19 over 0.5 h, a value much Iower than that obtained

(0.45) from the film prepared fiom the D - W - 1 UA-tBCEMA-5 dispersion. This result

indicates that an increase in the concentration of reactive comonomers leads to a decrease

in the initial rate of polymer diaision.

Three factors contribute to this decrease in the f,. The first factor is that the

increase in the reactant concentrations causes an increase in polymer immiscibility. We

showed in Chapter 4 that in the newly fomed films cast from THF-solution, the extent of

copolymer segregation is greater for films containing the higher amount of the reactive

comonomers (D-MAA-20/A-tBCEMA- 1 1) than for the film (D-MAA-1 l/A-tBCEMA-5).

From this result, we infer that when films are fieshly prepared from latex dispersions,

particles containing the higher content of the reactive comonomers have a lower degree of

rniscibility, At least initially, polymer dimision between these pairs of particles in this

film will be slower. From this point of view, we can conchde that in the early stages,

polymer diffusion of the D-MAA-20/A-tBCEMA-11 film will be slower than that in the

D-bLAA- I UA-tBCEMA-5 film.

annealing time (h)

Figure 5.1 1 : Plots off, (O) and the amount of -N=C=N- remaining (O) vs. tirne for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also ploned as filled circles (O). These films were prepared at 22°C h m a 1 : 1 mixture of D-MAA-20 and A-tBCEMA- 1 1 latexes, and examined at 60°C.

The second factor that contributes to the decrease in the initial rate of polymer

d i f i i o n is the increase in the Tg of the latex polymer as a result of the increase in the

amount of the reactive comonomers. Polyrner diffusion is known to depend on (T-Tg),

where T is the annealing temperature. Here, the latex film contains both the D-MAA-20

copolymer and the A-tBCEMA-11 copolymer. The initial rate of polymer diffusion is

controlled by the copolymer that has the higher Tg value. We performed differential

scanning calorimetry @SC) experiments on individual latexes to find their Tg values, but

our DSC instrument was too insensitive to show the Tg transition. However, we can

estirnate the Tg of these copolymers using the Fox equation, 55

where WA is the weight fraction of EHMA and WB is the weight fraction of MAA or

1BCEMA. TL* is the Tg of PEHMA homopolyrner, which has a value of -1 O°C (263 K).

T g ~ is the Tg of PMAA or PtBCEMA homopolymer. To our knowledge, the Tg of the

Pti3CEMA homopolyrner is not known. The Tg of PMAA is reported to be +228"C (510

K). j6 From these literature Tg values, we calculate the Tg of D-MAA-11 and D-MA4-

20 copolymers to be -4OC for the former and +3"C for the latter. Even though Tg values

of PEHMA containing tBCEMA are not available. we believe that the MAA-containing

copolper has a higher Tg than the tBCEMA-containing copolymers because the

tBCEbL4 molecule contains a longer side chain, and has no groups that are capable of

forming hydrogen bonds. From these results we can conclude that the initial rate of

polymer d f i s i o n is controlled by the polymer containing -COOH groups, and as a result,

this rate is slower in the film containing higher amount of -COOH groups (D-PVIAA-

20/A-tBCEMA- 1 1 film).

The third factor is the increase in the extent of crosslinking as a result of the

increase in the reactant concentrations. The coupIing reaction increases the polymer size

and leads to a decrease in the polyrner difiion rate in the initial stage. We find that 18%

of -NZC=N- groups are lost in both the D-MAA-20/A-tBCEMA-11 and D - W - 1 1 / A -

tBCEMA-5 films during the evaporation of water. This result implies that twice the

number of the reactive groups is consumed in the D-MAA-20/A-tBCEMA-Il film. As a

result, more chah elongation has occurred and more branches have fonned in the D-

M-U-20IA-tBCEMA-11 film, which in tum decreases the rate of polyrner diffusion.

As the annealing proceeds, f, for the D-MAA-1 l/A-tBCEMA-5 film reaches

0.60 over 1 h, and then 0.96 over 12 h. By comparison, f, for the D-MAA-2O/A-

tBCEMA-I 1 film reaches 0.61 over 16 h, and then to 0.70 over 38 h. To reach 0.96, the

D-MAA-ZO/A-tBCEMA-11 film mut be annealed for a very much longer time. Here

one sees a substantial decrease in polyrner diffusion as a result of an increase in the

reactant concentration.

We compare the time necessary for films to reach f, = 0.6 for the D-MAA-2OIA-

tBCEMA-11 sample (Figure 5.1 1) and the D-MAA-1 UA-tBCEMA-5 sample (Figure

5.7). In the D-MAA-I IIA-tBCEMA-5 film at 60°C, it takes just 1 h, whereas it requires

much Iong time (16 h) for the D-MAA-20/A-tBCEMA-11 film to reach a similar value.

From this comparison, we see that polymer diffusion decreases substantially when we

increase the concentration of the reactive cornonomers in the latex.

We next compare the time-conversion plots fiom the THF-cast films in Figure

5.1 1 and Figure 5.7. We replot the data Corn these Figures in Figure 5.12, with filled

circles representing the data for the D - W - 1 UA-tBCEMA-5 sample, and open squares

reprrsenting the data for the D-MAA-ZO/A-tBCEMA-1 1 sample. One would expect that

an increase in the reactant concentration would result in an increase in the rate of the

reaction. We do not observe that here. We see that the rates of the reaction in both films

are almost identical at early times. For example, the extent of carbodiirnide reaction

reaches about 22% over 2 h in both films, as shown in Figure 5.12. Upon longer

annealing, the rate of the reaction in the film containing higher the reactant concentration

(D-MAA-20/A-tBCEMA- 1 1 film) is reduced relatively faster than the rate of the reaction

in the film containing lower the amount of the reactant @-MAA- I 1 /A-tBCEMA-5 film).

For example, in the D-MAA-1 IIA-tBCEMA-5 film, 88% of the -N=C=N groups react

over 28 h, whereas only 56% of the same groups are lost over the same time span fkom

the D-MAA-20/A-tBCEMA-11 film. We l e m fiom this result that an increase in the

reactant concentration leads to an increase in the extent of crosslinking, and as

consequence, restricts the translationai diffusion and segmental mobility that brings the

reactive groups closer for them to react.

0-MAA-11 /A-tBCEMA-5

20 -

O

D-MM-20IA-t0CEMA-11

60°C 1 1 l I : I I I L I L L I . L , , , . , I , , . , , . . L . , < , . ) , , , I I . , .

O 5 10 15 20 25 30 35 40 45 50

annealing time (h)

Figure 5.12: Plots ofthe amount of -N=C=N- group remaining monitored at 60°C for THF-cast films. niese films were prepared at 2t°C fiom the D-MAA-1 UA- tBCEMA -5 and the D-MAA-ZWA-tBCEMA- 1 1 polyrners.

We follow the extent of crosslinking in the D-MAA-20/A-tBCEMA- 1 1 blend at

60°C by measuring gel content and swell ratio, and plot these values in Figure 5-13.

There is no gel formation in the fieshiy formed film and in the film annealed for 0.5 h.

When the film was annealed for 1 h, we detected a 20% gel fraction. The gel fraction

increased rapidly to 65% over 4.5 h. During this time, f, reached 0.5 and the extent of - N=C=N- reaction increased to 19%. When the films were anneaied for longer times, the

gel content continues to build, to 79% over 19 h. As the annealing t h e is increased to 38

h, the gel content remains unchanged at 80%. During this time (19 to 38 h), f, increases

from 0.61 to 0.70, and the extent of carbodiimide reaction increases fiom 36 to 44%.

Here the gel formation does not reach 100%.

Figure 5.13: Plots of gel content and swelI ratio vs. time of latex films prepared fiom D- MAA-20/A-tBCEMA-l l particles. These films were formed at 22°C and mealed at 60°C.

100 - 25

80 - A A - 20 gel content

5.3.3.1 Temperature effects

We also repeat ET and FTiR experiments on films cast fiom the same mixture of

latex dispersions and then annealed at 40°C. The f, values are plotted as open circles,

and the amount of -N=C=N- remaining is plotted as open squares in Figure 5.14. We also

include in the Figure the f, c u v e ( 0 ) of the film annealed at 60°C for cornparison. An

obvious observation from the two f, plots (40°C and 60°C) is that the f, curve of the film

annealed at 60°C is steeper than that of the film mealed at 40°C, indicating that polyrner

diffusion at 60°C is rnuch faster than that at 40°C. For example, f, values for the film

annealed at 40°C reach 0.30 over 80 h, whereas f, for the film annealed at 60°C reaches

0.7 in less than half that amount of the time (36 h). It is also interesting to compare the f,

plot for this film at 40°C to that for the D-MAA-11/A-tBCEMA-11 sarnple annealed at

the sarne temperature (Figure 5.9). in the D-MAA-1 VA-tBCEMA-5 film, f, reaches

- l5 .O

Ci

2 - - - 1 0 *

- 5 swell ratio

A v O -

l i i r i l - i i i l i i t i O O 10 20 30 40

annealing time (h)

about 0.70 over 80 h. in contrast, f, for the D-MAA-20/A-tBCEMA- I 1 sample reaches

only 0.30 over the same amount of time. It would require an enormous amount of time

for this sample to reach f, = 0.7.

annealing time (h)

Figure 5.14: Plot of fm (O) and the amount of -N=C=N- remaining (O) vs. time for the D- MAA-20/A-tBCEMA-11 films monitored at 40°C. During the experiment at 40°C, some films were brought to 60°C. Plots fm vs. tirne at 60°C for latex films that have k e n annealed for various penods of time at 40°C (19 h, 44 h and 80 h) are plotted as filled squares (m). The point (a) refer to a film annealed directly at 60°C without pnor aging at 40°C.

5.3.3.2 Pre-annealing effects

While carrying out the experiments at 40°C described above, 1 had the idea of

investigating what would happen if I increased the annealing temperature to 60°C during

the experiment. An example of this type of experiment nom the D-MAA-11lA-

tBCEMA-5 films was presented previously in section 3 -3 -2.2. For the D-MAA-2O/A-

tBCEMA-11 sample, we plot these f, values as filled squares also in Figure 5.14. These

films were annealed for various arnounts of time at 40°C (19 h, 44 h and 80 h) before the

temperature was rapidly raised to 60°C. There are a few interesting fanires bom these

plots. We observed from these plots that fm increase rapidly at shoa times, followed by

srnall increase at longer times. We also noticed that slopes of these f, curves are

different. The slope is steeper for the film that has the least pre-annealing time at 40°C.

For example, the film with zero pre-annealing time has the steepest slope, and the film

with the longest pre-annealing time (80 h) has the shallowest slope.

5.3.4 D-MAA-1 VA-CCEMA4.6 blend

In this section we examine what happens to polymer diffusion and crosslinking

reaction processes when we replace tBCEMA with CCEMA as the reactive cornonorner.

One should note that the CCEMA and tBCEMA cornonomer differ only in the substituent

attached to the "other end" of the -N=C=N- group, the cyclohexyl group vs. the t-butyl

group, as shown in Scheme 2.4. One should also note that the CCEMA group undergoes

patial hydrolysis during emulsion polymerization. In the recipe used to prepare the A-

CCEMA-4.6 latex, 4.6 mol% CCEMA was added to the reaction mixture, but only 3.7

mol% was found to remain after the synthesis of the particles.

Like the other blends, we prepared films at t2'C fiom a 1: 1 mixture of D-MAA-

11 and A-CCEMA-4.6 latex, and carried out ET and FTIR measurements on films

annealed at 60°C. in Figure 5.1 5' we plot f, vs. tirne for this film, as well as the extent of

carbodiimide reaction. We aiso plot data on the extent of -N=C=N- reaction for a film of

the sarne copolyrners, cast fiom THF solution. During the transformation of the latex

dispersion into a solid film, we find that 32% of the -N=C=N- groups are lost, a value

much higher than that (18%) for the film prepared fiom the D-MAA-1 UA-tBCEMA-5

latex dispersion. In the newly fomed dispersion-cast film, we obtain On = 0.08, a

srnaller value compared to that (0.1 1) obtained for the film prepared fiom the D-MAA-

I VA-tBCEMA-5 sample. The increase in the loss of -N=C=N- group during the drying

suggests that sorne high molar mass molecules are fonned due to crosslinking reaction at

the particle-particle boundary. When the same film is annealed at 60°C, fm increases ta

0.44 over 0.5 h, and then to 0.63 over 2 h. During this time, the extent of the

carbodiimide reaction reaches 24%. This result clearly indicates that over this time the

rate of polymer dimision is faster than the rate of the crosslinking reaction. Upon longer

annealing, f, reaches 0.77 over 1 5 h, and the extent of the carbodiirnide loss reaches 34%

over 8 h. There is no indication that polymer diffusion ceases on this t h e scale.

There are severai interesting features, which become apparent, when we compare

the f, and extent of carbodiimide reaction plots in Figure 5.15 to those in Figure 5.7.

From the fm plots, we see that the f, values in both c w e s reaches about 0.45 in 0.5 h.

During this time, 11% of the -N=C=N- groups in the D-MAA-1 UA-CCEMA-4.6 film

react (Figure 5.1 9, but only 4% of the -N=C=N- groups react in the D-MAA- I 1 /A-

tBCEMA-5 film (Figure 5.7). ïhis result indicates that the crosslinking reaction occurs

faster in the D-MAA-ll/A-CCEMA-4.6 film than in the D-MAA- 1 1/A-tBCEMA-5 film.

It implies that the -N=C=N- group of the CCEMA is more reactive toward the -COOH

0.8

0.6

fm

a - - THF -c&

C

h - - 1 2 0

- 1 4 0 a C .- C .- E

0.4 - +

60°c : . . - . ' . * , . ' , . . . ' , , , ,

aJ - 6 0 t II O II

- 8 0 ?

100 O 2 4 6 8 10 12 14 16

annealing time (h) Figure 5.1 5 : Plots of fm (O) and the amount of -N=C=N- remaining (O) vs. time for

dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled c h i e s (O). These films were prepared at 22OC fiom a 1: 1 mixture of D-MAA-11 and A-CCEMA-4.6 latexes, and annealed at 60°C.

group of the MAA than the -N=C=N- group of the tBCEMA toward the -COOH group of

the MAA. This result is consistent with our results on the stability of the two different -

N=C=K- groups to hydrolysis, as described in Chapter 3.

To compare interdiffusion rates in the two systems, we begin by noting that

polymer molecular weights are similar (A-CCEMA-4.6, MW = 86 000; A-rBCEMA-5, M W

= 63 000; D-MAA-11, MW = 41 000). Since the same D - b u - 1 1 latex polymers are

used in both systems, we compare only MW of the A-CCEMA-4.6 to that of the A-

tBCEMA-5. At early stages of film mealing at 60°C, polymer d i h i o n is faster in the

D-.M4,4- I UA-tBCEM4-5 film than in the D-MAA-1 UA-CCEMA-4.6 sample. For

exarnple, f, of the D-MAA-1 I/A-tBCEMA-5 sample (Figure 5.7) reaches 0.60 over only

1 h, whereas fm of the D-MAA-1 I/A-CCEMA-4.6 sample (Figure 5.15) requires 2 h to

increase to a similar value. Upon longer annealing, fm of the D-MAA-1 l/A-tBCEMA-5

sample reaches 0.96 over 12 h, during this time fm of the D-MAA-11 /A-CCEMA-4.6 film

increases to only 0.7. The reduction of interdiffusion rate of the D-MAA-1 l/A-CCEMA-

4.6 sample is caused by the more rapid reaction between the -N=C=N- group of the

CCEMA and the -COOH of the MAA.

We next compare the rates of the reaction in the two samples cast from THF

solutions. We replot the extent of -N=C=N- reaction in both film examined at 60°C in

Figure 5.16. We note that the molar ratio of -COOH/-N=C=N- groups is 2: 1 for the D-

MAA- l 1 /A-tBCEMA-5 sample, and about 2:O.S for the D-MAA- l 1 /A-CCEMA-4.6

sample because some of -N=C=N- groups of the CCEMA were hydrolyzed during the

emulsion polymerization. in the D-MAA-1 UA-CCEMA-4.6 sample, we detect that 74%

of the -N=C=N- groups react over 2 h, whereas only 23% of the -N=C=N- groups react in

the D-MAA-1 l/A-tBCEMA-5 sarnple over the same time span. This rapid reaction at

early times retards the motion of the polyrner backbone and the diffusion of polymer

chahs that bring the reactive groups in close enough proximity for them to react. As a

consequence, the rate of the reaction is significantly reduced at longer times. For

esarnple, only 13% of the -N=C=N- groups in the D-MAA-1 I/A-CCEMA-4.6 film react

over 6 h (2 to 8 h), whereas 3 1% of these groups in the D-MAA-1 LIA-tBCEMA-5 film

are consumed over the same period of t h e .

annealing time (h)

Figure 5.16: Plots of the amount of -N=C=N- remaining vs. time at 60°C for THF-cast films prepared fiom the D-MAA-1 VA-CCEMA-4.6 sample ( 0 ) and for the D-MAA- 1 1 /A-BCEMA-5 sarnple (O) .

We follow the extent of crosslinking in the D-MAA- 1 1 /A-CCEMA-4.6 b l e d at

60°C by measuring gel content and swell ratio, and plot these obtained values in Figure

5 - 1 7. in the newly formed film, we obtain 20% of insoluble polyrnen. We note that 32%

of -N=C=N- groups are lost during the evaporation of water leading to film fornation-

We believe diat most of this loss is due to the crosslinking reaction, and that this reaction

occurs primarily at the particle-particle boundary. When these films are annealed Over 2

h, the gel content increases rapidly to 79% over 2 h, corresponding to a rapid increase in

fm to 0.63. Upon longer annealing, the gel content increases to 83% over 15 h. During

this time, f, increases to 0.73.

annealing time (h)

Figure 5.17: Plots of gel content and swell ratio vs. time for latex films prepared fiom D- MAA-1 UA-CCEMA-4.6 particles. These films were formed at 22OC and annealed at 60°C.

5.3.5 D-MAA-S/A-CCEMA-3.2 blend

Next we examine what happens to the system when we decrease the amount of the

reactive comonomers, from I l to 5 mol% for MAA and from 4.6 to 3.2 mol% for

CCEMA. For the carbodiimide-containing latex, 3.2 mol% CCEMA was used in the

original recipe, but only 2.5 mol% -N=C=N- groups survived the preparation of the latex

particles. In Figure 5.18, we plot fm (O) and the extent of -N=C=N- reaction (a)

monitored in latex films prepared fiom a 1:l mixture of D-MAA-5 and A-CCEMA-3.2

particles. In the freshly formed film cast fiom a mixture of latex dispersions at 22'C. we

find mET = 0.06. When this film is annealed at 60°C, f, reaches 0.68 over 0.25 h, and

then to 0.78 over 0.5 h. Over this time, only 10% of -N=C=N- groups react. Upon

annealing for 6 h, f, increases to 0.96, an indication that the film has undergone nearly

hl1 mixing. During this time, the extent of the carbodiimide reaction reaches 58%. From

these results we conclude that one method to increase the rate of polyrner diffusion in a

system, in which the fhctiond groups are very reactive, is to reduce the reactant

concentration in the polymer. For exarnple, in the latex blend of D-MAA-1 1/A-CCEMA-

4.6, it takes 1 5 h for the system to reach fm = 0.73. By reducing the amount of both MAA

and CCEMA in the polymer, we frnd that it takes less than 0.5 h for the D-MAA-YA-

CCEMA-3 -2 filrn to reach a similar fm value.

annealing tirne (h)

Figure 5.18: Plots of f, (O) and the amount of -N=C=N- remaining (a) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (a). These films were prepared at 22OC fiom a 1 : 1 mixture of D-MAA-5 and A-CCEMA-3 -2 latexes, and annealed at 60°C.

We aiso plot the extent of the reaction obtained fiom the THF-cast film (O) in

Figure 5.18. Here we see that the instantaneous rates of reaction in the solvent-cast filrn

are higher than those in the dispersion-cast film for every anneaiing time.

In Figure 5.19, we plot the results of gel content and swell ratio measurements on

films prepared from the D-MAA-SIA-CCEMA-3.2 blend. There is no gel formation at to.

When the films are annealed at 60°C, the gel content rapidly increases to 56% over 0.5 h,

and then to 77% over 2 h. Upon longer annealing (7 h), we obtain 92% gel formation.

Taking account of the linear polymer present in the seed, this represents 100% gel

fraction. This result indicates that every polymer molecule containing reactive groups

becomes attached to the crosslinked network. Over this time, f, is greater than 0.96, and

the extent of carbodiimide reaction is about 60%.

2 4 6

annealing time (h)

Figure 5.19: Plots of gel content and swell ratio vs. time for latex films prepared fiom D- MAA-YA-CCEMA-3.2 particles. These films were fomed at 22OC and annealed at 60°C.

5.3.5.1 Temperature effects

To examine the effect of temperature on these processes we repeat ET and FTIR

measurements on films formed from the same mixture of latex dispersions and annealed

at 40°C. We plot f, values (O) and the extent of the -N=C=N- reaction (O) in Figure 5.20.

We observe two different regions in the Figure. At short times, f, values increase rapidly

to 0.22 over 1 h, and then to 0.32 over 3 h. During this t h e , the extent of carbodiirnide

reaction reaches 12%. This result indicates that the rate of polymer diffision is faster

than the rate of crosslinking a< short t h e . As the annealing time increases, both rates are

reduced, but the rate of polymer d i h i o n appears to be retarded more than the rate of

crosslinking. For example, f, increases fiom 0.32 to 0.40 when the annealed time is

increased from 3 h to 19 h. During this time, the extent of carbodiimide reaction

increases from 12% to 32%.

annealing time (h)

Figure 5.20: Plots of f, (O) and the amount of -N=C=N- rernaining (O) vs. time for dispersion-cast films. The amount of -N=C=N- remaining in the THF-cast film is also plotted as filled circles (a). These films were prepared at ZZ°C from a 1 : 1 mixture of D-MAA-5 and A-CCEMA-3.2 latexes, and annealed at 40°C.

5.3.5.2 Pre-annealing effects

When the same films are subsequently annealed at 60°C, we see an increase in

both f and the extent of -N=C=N- reaction. We plot these data as filled circles and filled

squares in Figure 5.21, dong with the data obtained at 40°C for cornparison. Both f, and

the extent of carbodiimide reaction increase rapidly when the temperature is suddenly

increased to 60°C. SirniIar to the films anneaied at 40°C, the rate of polymer difision at

early tirnes at 60°C is faster than the rate of crosslinking. At longer times, the rates are

reversed, such that the rate of the reaction is faster than that of polymer diffusion. We

also observe that a large ùicrease in f, (from 0.4 to 0.8) occurs when the temperature is

raised to 60°C. Polymer diffusion of mobile polymen and the diffusion of branches

contribute to this increase in f, values.

annealing time (h)

Figure 5 -2 1 : Plots of f, and the percentage of -N=C=N- remaining vs. time for latex films prepared fiom a mixture of D-MAA-5 + A-CCEMA-3.2 latex dispersions. These films were formed at 22OC, annealed at 40°C (f, (O); -N=C=N- remaining (%) (a)), and then annealed at 60°C (f, ( O ) ; -N=C=N- rernaining (W.

The results of gel content and swell ratio measurements. on films annealed at 40°C

and on films then heated at 60°C, are plotted in Figure 5.22. The data for gel content at

40°C are plotted as open triangles (A), and the data for swell ratio are plotted as open

diamonds (O). We use similar syrnbols, but filled, to describe the gel content and swell

ratio for the films then heated at 60°C. in the newly formed film, we obtain a small

fraction (7%) of insoluble polymers. When the films are annealed at 40°C. the gel

fraction increases to 54% over 5 h, and then to 60% over 15 h. During this time, f,

reaches about 0.4, and the extent of carbodiimide reaction increases to 26%. When the

temperature is suddenly increased to 60°C, the gel fraction also increases to 77% over the

nest 8 h (15 h to 23 h).

annealing time (h)

Figure 5.22: Plots of gel content and swell ratio vs. time for films prepared from a mixture of D-MAA-5 + A-CCEMA-3.2 latex dispersions. These films were cast at 22OC, annealed at 40°C and then at 60°C. The gel content and swell ratio data at 40°C are plotted as open triangles (A) and open diamonds (O).

The gel content and swell ratio data at 60°C are plotted as filled triangles (A) and filled diamonds (+), respectively.

5.4 Discussion The competition between polymer diffusion and the crosslinking reaction in the

latex films prepared fiom PEHMA copolymers is a complex process. Polymer diffusion

is necessary to bring the functional groups into close enough proxirnity for them to react.

In r e m . the formation of branches and the increase in polymer molecular weight reduce

the rate of m e r polyrner dimision. In addition, polyrner miscibility changes as the

reaction proceeds. The presence of -COOH groups in the PEHMA polymer limits the

miscibility of these copolymers with PEHMA polymer containing -N=C=N- groups. This

Iimited miscibility hinders polymer diffusion across the particle interfaces at e d y stages

of the healing process. However, when 'Lhe reaction between the -COOH and the - N=C=N- groups occurs, generating N-acyIurea bonds, the branched polymers fomed

become compatibilizers, produced in situ by the chemicai reaction. in a simple picture of

this process, the dornains containing the branched polyrners (and even crosslinked

polymers) provide regions for the mobile W-con ta in ing polymers and tBCEMA-

containing polymers to mix. Borner and Hope have reviewed the topic of

compatibilizers produced in situ by chernicd reaction in polymer blends. 57

It is known that the rate of polymer diffusion varies with chah Iength and is

sensitive to the presence of branching. For linear polymers shorter than the entanglement

molecular weight, the diffusion coefficient Di characterizing the diffusion of a given

chain of length Ni, decreases as N;'. For linear polymers longer than the entanglement -2 58 molecular weight, the diffusion coefficient Di decreases as N, - In al1 blends examined

here, the polymer chains, before reaction, are linear, with an average chain length ( M W =

60,000). When the reaction between the -COOH groups and -N=C=N- groups occurs, the

linear polymers elongate and fonn branches. As a result, the rate of polymer diffkion

decreases substanüally. When the rate of polyrner d i f i i o n decreases, the rate of

crosslinking also decreases because both polyrner diffusion and polyrner segment

diffusion are necessary to bring the reactive groups close enough to react.

An interesting question to ask in this diffusion-reaction system is whether

crosslinking suppresses polymer diffusion? One indication of fùll stoppage of polymer

diffusion would be indicated by a plateau region on a f, plot. We find that there is no

plateau region in any of our fm plots' within the time examined. We conclude that

polymer d i f i i o n in these systems is not completely suppressed. However, we find that

crosslinking reaction retards polymer diffusion sigriificantly in al1 of our examples. The

best example to illustrate this point is found in the D-MAA-1 VA-CCEMA-4.6 latex

blend. We employ f, values to characterize the extent of mixing due to the d i f i i o n

process, and the slope of the f, plot to characterize the rate of polyrner diffusion. The

steeper the slope the faster the polymer d i f i e . When the diffusion is suppressed, the

slope of the plot will be zero. If, however. the diffision is retarded, the dope will

decrease. In Figure 5.1 1 , the slopes of the f, curve decrease as the annealing tirne

increases to 2 h. Upon longer amealing, the slope drops significantly, an indication that

the diffusion of the polymer slows down substantially due to the reaction. As f, values

increase above 0.8, the DET expenment rn out of signal and loses sensitivity to M e r

diffusion. There is no indication that the slope will reach zero. We believe fiom our data

that polymer difision will eventually cease, but on a t h e scale and diffusion length

scale that we can not measure.

One of the more interesting discoveries made in the course of these experiments is

the relationship between the extent of polymer diffusion and the extent of gel formation.

In dl samples examined here, polyrner d i f i i o n mut be complete or nearly complete for

al1 the reactive polymer can be converted to gel. We find only two systems in which

films evolve to this extent. In the D - b M - l VA-tE3CEM.A-5 film examined at 60°C, we

ses that f, of this film reaches 0.96 over 12 h (Figure 5.7), and gel fonnation reaches 92%

3 h later. Recall that when the gel content plot reaches 92% (Figure 5.8 or Figure S.I9),

the system is at full gelation because the seed polymers contain 8% linear polymen.

Similarly, fm for the D-MAA-5/A-CCEMA-3.2 films reaches 0.96 over 6 h (Figure 5.18),

whereas it requires 7 h for the system to undergo full gelation (Figure 5.19). From these

results we learn that polymer diffusion must be complete prior to the system reaching its

maximum possible gel fonnation. We believe that the seed polymers dissolve in the

solvent during the gel content rneasurements.

Three factors reduce the rate of polymer d i f k i o n at the initial stage: a decrease in

the annealing temperature, an increase in the reactant concentration, and an increase in

the reactivity of the crosslinking reaction. The D-WU-S/A-CCEMA-3.2 films provide

an example showing that a reduction in the annealing temperature causes a decrease in

the initial rate of interdimision, and its consequence on the extent of gel formation. When

this film is annealed at 60°C, f, increases rapidly to 0.68 over only 15 min (Figure 5.18).

As the anneding time increases, f, reaches close to unity over 6 h, and f i i H gel content

develops over 7 h (Figure 5.19). When the annealing temperature is 40°C, f, for this film

reaches only 0.22 over 1 h, and then 0.40 over 19 h (Figure 5.2 1)- Over this tirne, the gel

content reaches oniy about 60% (Figure 5.22). When this film is suddenly heated to

60°C, the gel content increases to 79% over 8 h. This amount of time is similar to that

needed for the same films, not pre-annealed at 4O0C, to develop to a Mly crosslinked

network (92% gel).

We find two examples which show that an increase in the reactant concentrations

leads to a crosslinked network with less than full gel formation. In the D-MAA-11IA-

tBCEMA-5 samples annealed at 60°C, f, rapidly reaches 0.6 over 1 h (Figure 5.7), and

the gel content develops to its maximum value over 15 h (Figure 5.8). Ln contrast, the D-

MAA-20/A-tBCEMA- 1 1 film which contains a higher concentration of the reactive

comonorners, f, reaches only 0.2 over 1 h (Figure 5.1 l), and over 39 h annealing ùme,

only 80% gel is detected (Figure 5.13). Comparing the data obtained from the D-MAA-

S/A-CCEMA-3.2 samples to those obtained fiom the D-MAA-11/A-CCEMA-4.6

sarnples, we see a similar result. In the film containing the lower amount of the reactive

comonomers, polymer diffusion is rapid at early stages, and films reach fùll gelation,

whereas in the film containing higher the reactant concentration, interdiffusion is slow,

and only 80% gel is obtained. From the results of the two examples, we learn that as the

concentration of the reactants increases, polymer diffusion in the initial stage decreases,

and the annealed films have a lower gel content. It is possible that the films at Iess than

full gel contents may eventuaily reach 92% gel content, because polymer segment

difision still occurs, but at a slow rate.

To see the effect of the increase in the reactivity of the reaction on the initial rate

of polyrner diffusion and its consequences on the crosslinked network, we compare the

D-MAA- 1 1 /A-tBCEMA-5 system to the D-MAA- 1 UA-CCEMA-4.6 system. The

reaction is faster in the film containing CCEMA than in the film containing tBCEMA. In

the D-MA.4-1 UA-tBCEMA-5 sample annealed at 60°C, f, increases to 0.6 over 1 h

(Figure 5-71, and the gel content reaches to 92% over 15 h (Figure 5.8). in the D - W -

I UA-CCEMA-4.6 sample annealed at the same temperature, the film requires 2 h for f,

to reach 0.6 (Figure 5.15). The gel content of this film increases to 80% over 15 h

(Figure 5-18), the same amount of time needed for the D-MAA-1 VA-tBCEMA-5 system

to reach to 92% gel. From these results, we see that as the fimctional group reactivity is

increased, and interdiffusion decreases substantially, such that the system does not

develop to a fully crosslinked network.

5.4.1 D-MAA and A-tBCEMA system

in this section, we discuss the system involving blends of polymers containing

MPLA and tBCEMA. To relate polymer diffusion and the crosslinking reaction to the

development of gel formation in these blends, we constmct Table 5.1. To prepare the

Table, we choose a gel fiaction of 80% as a reference point, and then find the f, value

and the extent of carbodiimide reaction fiom various films that correspond to this gel

content. For example, it takes 3 h (Figure 5.8) to reach 80% gel formation in the D-

MAA-l UA-tBCEMA-5 latex film annealed at 60°C. The f, value and the extent of

carbodiimide reaction that corresponds to this tirne are, respectively, 0.78 and 22%

(Figure 5.7). Similar information is collected for the D-MAA-11/A-tBCEMA-5 latex

films annealed at 40°C, and for the D-PUWA-2O/A-tBCEMA-11 latex films annealed at

60°C. These results are also presented in the Table. One sees from the Table that the D-

MAA-l UA-tBCEMA-5 films annealed at 60°C reach 80% gel in the shonest time (3 h)

and have the highest f, value. These films also require the least amount of the -N=C=N-

groups to react. From these results we Ieam that rapid polymer diffusion leads to rapid

development of the crosslinked network with low consurnption of the reactive groups.

To explain these results, we examine Scheme 5.1. In the Scheme, polymers

containing MAA and @CEMA are initially confined within their own cells, and the cells

are separated. Each of these polymer chahs may contain up to severai reactive groups,

but we show only 2 groups per chah in the drawing. in the newly formed film, the cells

\ T ~ ~ = +

a MAA-containing polymer a tBCEMA-containing polymer

I 1 st groups react

inter-molecular reaction

2nd groups react /,cw

intra-molecular reaction (gel formation remains constant)

inter-rnolecular reaction (gel formation increases)

Scheme 5.2: A schematic representation of inter- vs. intra-molecular reactions in reactive grail copolymers.

are adjacent, such that the reactive groups are close enough that they can react pnor to

interdiffusion. When the fint pair reacts, it occurs in an inter-molecular fashion, joining

the MAA- and tBCEMA-containing polymers. A longer, branched polymer chain is

formed, and the other reactive pair is now on the same molecule. The mobiiity of the

elongated chain is significantly reduced. When this reactive pair reacts, the reaction can

occur within the same poiymer molecule (an intra-molecular or cyclization reaction) or

with the complementaq group attached to a different chain (an inter-molecular reaction).

The competition between inter- and intra-molecular reactions depends on the relative

rates of polymer and segment d i h i o n of the mobile chain as well as on the absolute rate

of the chemical reaction. Rapid interdihion will promote the inter-molecular reaction,

and a slow interdiffusion should favor the intra-rnolecuiar reaction. The inter- molecular

reaction builds gel in the system, whereas no additional gel is formed if the reaction

occurs intra-molecularly. We can apply this mode1 to explain the results obtained fkom

the D-MAA-1 UA-tBCEMA-5 films. In these films, annealed at 60°C, polymer difision

is rapid (f, = 0.6 over 1 h), and we imagine that the system undergoes rapid inter-

morecular reactions. As a result, it requires a relatively small arnount of -N=C=N- group

consumption to yield 80% insoluble polymers.

Table 5.1 : Summary of the time it takes for individual films to reach 80% gel formation? plus values off, and the extent of -N<=N- reaction that correspond to this tirne.

D-MAA- 1 I /A-tBCEMA-5 (60°C)

D-MAA- 1 1 /A-tBCEMA-5 (40°C)

D-MAA-2O/A-tBCEMA- 1 1 (60°C)

-N=C=N- reacted (W

22

36

time to 80% gel 0i)

3

60

*O I l

fm

0.78

0.68

To examine the effect of the reactant concentration on polymer diffusion, we

compare the f, data for the latex film containing no reactive groups to f, data for the

films containing the reactive cornonomers. Al1 films were annealed at 60°C. For the

PEHMA-D/PEHMA-A film, no crosslinking takes place, f, reaches 0.60 in less than 2

min (Figure 5 . 3 , whereas it takes 1 h for the D-MA44 UA-tBCEMA-5 film (Figure 5.7)

to reach a similar f, value. When the reactant concentrations are doubled, as in the D-

MAA-20/A-tBCEMA-l l fiIm, it requires 16 h for this film to reach f, = 0.6 (Figure

5.11). We collect these times and the arnount of tBCEMA available for the crosslinking

reaction for each biend in Table 5.2. Taking account of the amount of the tBCEMA-

containing polymers in the film and of the hydrolysis of the -N=C=N- groups during

emulsion polymerization (Table 4.3), the arnount of tBCEMA available for the

crosslinking is less than half of that used in the latex synthesis.

We plot these data as a function of the amount of tI3CEM.A in Figure 5.23. One

sees an interesting feature in the Figure in that the data fit well to a semi-log plot. The

plot suggests that as the concentration of tBCEMA increases linearly, the time it take for

the system to reach f, = 0.6 increases exponentially, and in terms of polymer diffusion,

the diffusion rate decreases exponentially. Here one sees that crosslinking has a

tremendous effect on polyrner difision.

TabIe 5 2: Surnmary of the time it takes for individual films to reach f, = 0.6. The amount of tBCEMA avaiiable for the crosslinking in each film is also included. Al1 films were annealed

PEHMA-DPEHMA-A

D-MAA- 1 1 /A-tBCEMA-5

D-MAA-20/A-tBCEMA- 1 1

tBCEMA available for crosslinking in the film

(mol%)

time to 0.60 f, (min)

Fi*pre 5.23: A plot of log(time to 0.60 f,) vs. amount of tBCEMA remaining for the MAA-tBCEMA system.

5.4.2 D-MAA and A-CCEMA system

We now examine the system involving blends of polymers containing MAA and

CCEMA. The reaction of the -N=C=N- group of the CCEMA is faster with the -COOH

group of the MAA than the -N=C=N- group of the tBCEMA with the -COOH group of

the W.

We examine the effect of aging the films at room temperature (22OC) before

annealing them at 60°C. Three films were prepared at 22OC fiom a 1:1 mixture of D-

MAA-11 and A-CCEMA-4.6 latex, and aged at 22OC for different amounts of time (1 h,

15 h, and 5 days) before they were a ~ e a i e d at 60°C. in the films aged for 1 h, 15 h and 5

days, we find On = 0.08,O.l I and 0.14, respectively. We plot f, vs. time for these films

annealed at 60°C in Figure 5.24. One sees from the Figure that three different films, aged

annealing time (h)

Figure 5.24: Plots off, vs. time for films prepared at 22OC fkom the D-MAA-5/A- CCEMA-3 -2 latex. These films were then aged at 22OC for 1 h, 1 5 h and 5 days before they were annealed at 60°C.

for diree different penods of tirne, give three different f, c w e s . The film with the least

arnount of aging (1 h) has the steepest slope. In addition, the f, values for this film' at

cornmon annealing times, is higher than those for films with longer aging time at 22°C.

For example, f, for the film aged for 1 h at 22OC reaches 0.63 over 2 h, while over this

time f, for the film aged at 22OC for 15 h and 5 days reaches 0.60 and 0.48, respectively.

These results suggest that crosslinking between the -N=C=N- and -COOH groups takes

place at room temperature, such that chah elongation and branch formation reduce the

rate and extent of polyrner diffusion. .

Since both poIymer diffusion and reaction can occur in al1 of our latex films, we

had to establish a protocol so that al1 films to be compared were treated in a cornmon

way. To minimize the extent of the reaction at room temperature before sample

annealing, we begin each of our measurements in al1 series at 1 h after the films dry.

As in the case of the MAA-tBCEMA system, we constmct Table 5.3, in which we

compare the amount of CCEMA available for crosslinking in the D-MAA-ll/A-

CCEMA-4.6 and D-MAA-5/A-CCEMA-3.2 films, and the time it takes for these films to

reach f, = 0.6 at 60°C. Taking into account the hydrolysis of the -N=C=N- groups during

the preparation of the CCEMA-containing latex (Table 4.9, and the amount of the

carbodiirnide-containing polymer used in the film, the amount of CCEMA available for

the crosslinking is less than half of that in the original recipe. We plot logttime to 0.6 fm)

vs. the amount of CCEMA in Figure 5.25. We, again, see in the Figure that the data fit

ro a straight line (r2 = 0.97). The result indicates that as the reactant concentration

(CCEMA) increases linearly, polymer d i f i i o n decreases exponentially.

In Figure 5.25, we also plot log(time to 0.6 fm) vs. the arnowit of tBCEMA of the

WU-tBCEh4.A system for cornparison. One sees from the Figure that the slope of the

MAA-CCEMA system is w-ice as large as that of the MAA-tBCCEMA system. This

result suggests that as the reactivity of the reaction increases, maintaining the amount of

the reactive cornonomer constant, polymer diffision decrease significantly.

Table 5.3: Sumrnary of the time it takes for individual films to reach f, = 0.6. The amount of CCEMA available for the crosslinking in each film is also included. Al1 films were annealed at 60°C.

PEHMA-DPEHMA-A

D-MAA-S/A-CCEMA-3 -2

( D-MAA- I l/A-CCEMA-4.6 I

CCEMA available for crosslinking

(mol%)

0.0

1 -22

1.73

time to 0.60 f, (min)

1.3

15

120

CCEMA or tBCEMA (mol%)

Figure 5.25: A plot of log(time to 0.60 f,) vs. the -N=C=N- groups content for the carbodiimide containing latex for both the MAA-CCEMA system and the MAA-tBCEMA system.

5.5 Conclusions Polymer d i f i i o n is an important process in the development of a fiilly

crosslinked film in carboxylic acid-carbodiimide latex blends. The rate of polymer

diffusion is sensitive to the extent and locus of the crosslinking reaction. As the reaction

proceeds, polymer diffusion decreases substantially. We also l e m that the initial rate of

polymer diffusion is an important factor for the films to develop fdly. A decrease in the

temperature. an increase in the reactant concentration and an increase in the reactivity of

the carbodiimide, al1 of which lead to an increase in the rate of formation of long chains

and branches at the particle-particles interface, decrease the initial rate of polymer

interdiffusion.

5.6 References

' Wool R. P., "Polymer Interfaces", Hanser Publisher, 1995.

Whïtlow J.; Wool R. P., Mc~crornoledes, 1991,24, 5926. ' Wool R. P. and O'Conner K. M., J Appl. P h p . 1981,52,5194 and 5993. 1 Kim Y. H.; Wool R. P., Macrornolecules, 1983, 16, 1 1 15. : (a) L i ~ é iM.; Klein A.; Miller G.; Sperling L. H.; Wipal G., J. Macromol. Sci.. Phys.,

1988, 27(2&3), 217. (b) Yoo J.; Sper!ing L. H.; Glinka C.; Klein A., A4acromolecules,

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24, 2868.

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(b) Hahn K.; Ley G.; Schuller H.; Obenhur R., Colloid Polymer Sci., 1988, 266, 63 1 . 7 (a) Voyutskii S., J. Polym. Sci.. Parr A, 1958' 32, 528. (b) Voyutskii S., Autohesion

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(b) Wang Y.; Zhao C.-L; Winnik M. A., J. Chem. Phys.. 1991,95, 2143. (c ) Wang Y.;

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1218.

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" Sperling L. H.; Klein A. Sambasivam M., J. Polym. Materials, 1996, 13(1), 1 . 2 S Hourton D. J.; Schafer F. U. ; Bates J. S., J. Appl. Polym. Sci., 1996,60(13), 2409.

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Chevalier Y., Trend in Polymer Science, 1996,4, 197.

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j8 Grace S. A.; Petzoldt J., "Coating Automotive Plastics With 2K Waterbome

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137.

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" Other by-products of this reaction are the urea plus the carboxylic anhydride. No bands

due to the anhydride product at 1760 and 1830 cm" could be detected, but the

anhydride may hydrolyze to the carboxylic acid, which we cannot detect because of the

intense ester band in the IR spectnim at 1740 cm-! 5 3 Feng, J. R.; Pham, H- H.; Stoeva, V.; Winnik, M. A. J. Polym. Sci Parr B: Polymer

Physics 1998, 36, 1 129. 54 Ferry, J.D. Viscoelastic Properties of Polymers, Wiley, New York, 1980. 5 5 Fox T. G., Bull. Amer. Phys. Soc., 1956, 1 , 123.

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19. 341.

6 Evolution of Tensile Properties in Latex Films of PEHMA Copolymers

6.1 Introduction

Tensile measurements have been widely used as a method to examine the

mechanicd properties of varîety materiais. The prime consideration in this test is the

matenal's deformation under stress. In a tende test, a sample (Figure 6.1) is elongated at

constant rate. The force (F) required maintaining a constant rate of elongation is

detennined.

Figure 6.1 : A schematic drawing of a sarnple for tensile testing.

A hypothetical example of a tensile plot is shown in Figure 6.2. The vertical axis,

referred to as stress, is force (F) divided by the initial cross-sectional area (A& The

horizontal axis, referred to as strain, is the fiactional increase in the sample Iength. The

stress and strain are expressed in equattion 6.1 and 6.2, respectively.

F - F stress = O = - - - A0 h x c

Young's modulus = slope of the stress-strain curve at short strain

GB

GY

stress

€Y strain

Figure 6.2: A hypothetical stress-strain curve for a ductile sample.

The t e m "tensile properties" refers to al1 properties that can be determined fiom the

tensile tests. As shown in Figure 6.2, Young's modulus (E or elastic modulus) is defined

by the dope of the stress-strain curve at short strain. If the force is removed on a sample

in this region, the elongated sample relaxes back to its original length. At the yield point,

the slope equals zero. The strain at the yield point (cY) is called the "elongation-at-yield".

The stress at the yield point (av) is called the yield strength. Elongation is continued

until the sample breaks. The stress at the break point (cB) is called the "tensile strength''

or .-stress-at-break". The strain at the break point (EB) is called the "elongation-at-break"

or "extension-at-break". "Tensile energy to break" or ''toughness" ' is determined by the

area under the stress-strain cuve divided by the sample volume. It is evident in Figure

6.1 that the initial cross-sectional area (Ao) wi11 decrease as A1 increases. For rubbery

materials, one assumes that these changes do not Iead to significant change in the volume

of the sample?

A stress-strain curve can be presented in two different ways. in an "engineering"

stress-strain plot, F/& (stress) is plotted as a function of (AMo x 100) (strain). A.

decreases and 1 increases with time during the testing. A "tnie" stress-strain plot takes

accounr of the changes in cross-section area of the sample. For elongation by a factor of

À (AVlo), the width and thickness each decrease by a factor of Xi. Thus A;. = &A, and

FÀ/AO VS. A is ploned in the tme stress-strain curve.

Schematic stress-strain curves for various types of coatings are shown in Figure

6 Scales on the graphs give an orderof-magnitude indication of property values.

stress 1

ductile

10%

strain strain

15 MPa

stress

I

500% strain

Figure 6.3 : General kinds of stress-strain curves for various types of coatings. The scales indicate order-of-magnitude values.

Brittle materials (Figure 6.3A) have hi& a Young's modulus value, a stress-at-break up

to about 70 MPa, and an elongation-at-break below 10%. Ductile materials usually have

lower a Young's modulus value, a stress-at-break in the 25 to 40 MPa range, and an

eIongation-at- break of about 100%. The lower in the plot (Figure 6.3B) corresponds to a

ductile sample that necks down at the yield point, and M e r elongation (1 10%) occurs

with an increase in length of the necked part of the sample. Elastomeric (rubbery)

samples have a much lower Young's modulus, a stress-at-break of about 15 MPa. and an

elongation-at-break in the 400 to 500% range.

Stress-strain c w e s are nonnally measured at a constant rate of strain, and the

results obtained depend on strain rate selected. In generd, a higher rate resuîts in higher

modulus. The two c w e s in Figure 6.3B could represent the same material, subjected to

strain at different rates. In fact, if the strain rate is increased to a large enough value, a

ductile sample will exhibit the tensile characteristics of a brittie material.

A sample can exhibit different stress-main curves if it is tested at different

temperatures. As the temperature increases both the modulus and yield strength

decreases while the elongation-at-break generally increases. Poly(methy1 rnethacrylate),

for example, exhibits brittle characteristic at room temperature, but at higher temperatures

near Tg. it shows the stress-strain curve of a ductile ~ a m ~ l e . ~

In this Chapter, we examine the evolution of tensile properties of films prepared

from carboxylic acid- and carbodiimide-functionalized latex particles. These films are

prepared fiom labeled latex particles. Al1 tensile rneasurements will be examined at room

temperature and at a constant strain rate.

6.2 Experimental

Al1 tensile experiments were carried out using a Sintech 1 apparatus (MTS Systems

Corp.), which was equipped with a 50 lb load ce11 and a pair of pneumatic clamps. The

pneumatic clamps were used to hold each specirnen such that the pressure applied on the

specimen remains constant throughout the test and constant fkom one sample to the next.

The tensile tests were carried out in a roorn in which temperature and relative humidity

were kept constant at 22 f 1OC and 50 k 2%, respectively. The cross-head speed was set

at 19 mdmin .

For the tensile experiments examined in this Chapter, we used the Iabeled latex

sarnples described in Chapter 4. The preparation of films for tensile tests was different

than the preparation of films for ET and FTIR measurements. First, the arnmonia-

neutralized carboxylic acid-containing latex dispersion (30 wt% solids) was weighed

(25 .O g) into a one-neck round bottom flask, foliowed by the carbodiimide-containhg

latex dispersion (25.0 g, 30 w-t0/0 solids). Milli-pore water (50 g) was then added to the

flask. The dispersion was then degassed by evacuation, followed by re-exposure back to

amosphenc pressure. This process was repeated several times. It is essentid to the

experiment for the latex dispersion to fom a film without visible air bubbles. Findly the

dispersion was fiitered through a Kimwipe paper and transfened directly into a Teflon

mold (20 cm x 12 cm x 6 mm).

The mold was then placed into a Nalgene acrylic desiccator (1 2 x 12 x 12 in., Fisher

Scientific) at 2 1°C for a period of 4 - 7 days. The desiccator door was used to control the

drying rate. Sometimes, an additional Teflon mold containing just water was placed into

the desiccator to maintain high humidity conditions during the drying process. Slow

drying is essential for obtaining films with uniform thickness. The films obtained in this

way were fiee of visible air bubbles and mud-cracks. They presented a nice and smooth

uniform surface and had thicknesses of 0.7 f 0.2 mm. Each film was then cut into 3 or 4

sections. and these sections were annealed in an oven under a petri dish for different

times. For each annealing tirne, the section was removed fiom the oven, cooled to room

temperature and then cut int? a specimen using a rnetallic die (ASTM Dl 707-84). The

specimens were then placed in the conditioned chamber (22'C, 50% humidity) for 24 h

prior to tensile measurements, allowing the samples to equilibrate to the testing

conditions. Each of the specimens has the following shape.

' Ihe tested specimens were then used for ET, gel content and swell ratio

measurements. These rneasurements were carried out as described in Chapter 4 and 5.

6.3 Results

Al1 of the f ihs used in the tende experiments were prepared fiorn a 1 : 1 mixture of

Phe- and An-labeled latexes. One could, then, measure the extent of polymer

interdiffusion, gel content and swell ratio on these tensile samples, and compare directiy

the results to those obtained from the tensile experiments. Films (0.7 + 0.2 mm) used for

tensile measurements were thicker than those used for the ET, FTIR, gel content and

swell ratio experiments described in previous Chapters. To obtain a smooth and air-void

fiee film. the latex dispersions were dried slowly at room temperature (4 - 7 days). For

cornparison, it takes only 5 - 7 h for the latex dispersion to dry to films for ET, FTIR, gel

content and swell ratio measurements. After dumbbell-shape specimens were cut fiom

the film, the samples were conditioned for 24 h in a chamber (22'C and 50%) prior to

test. Under these circumstances, some polymer diffusion across the particle-particle

boundary occurs, and crosslinking reaction takes place to form branches and to extend

polymer chains.

It is important to point out that the results obtained from the tensile samples carmot

be compared to those obtained from the films examined in Chapter 5 because these films

have a different to. We have learned fiom the ET experiments on the D-MAA-1 l/A-

CCEMA-4.6 blend (Chapter 5) that the extent of polymer diffusion, examined at 60°C, is

lower for films that age longer at room temperature. Here, the tensile samples take longer

time to dry, and when they are dry, interdifision and crosslinking reaction have already

taken place. However, we could compare the results obtained fiom the tensile samples

prepared from different latex dispersions or fiom the tensile samples prepared fiom the

sarne latex dispersion but annealed for different amounts of time. To compare the extent

of polymer diffusion arnong different blends, we prefer to use OEI. values.

strain (%)

strain

Figure 6.4: Stress-strain plots of PEHMA hopmopolymers (A) engineering stress-strain curve, (B) true stress-strain curve.

6.3.1 PEHMA-D/PEHMA-A blend

We examine first the tensile properties of films formed fiom the PEHMA

homopolymers, and use these data as the control expenments. These samples are sofl and

tac. to touch. Proper handling of these samples requires great care. In the films, before

annealing. we fmd O E ~ = 0.35, indicating that significant polymer diffusion has taken

place in these films. A representative a stress-strain curve for a PEHMA sarnpie is shown

in Figure 6.4. We present the sarne data in two different plots: an engineering stress-

strain cuve (Figure 6.4A), and a true stress-strain curve (Figure 6-48). These samples do

not break at long extension. They just reach the upper limit of the experirnental senings.

These samples, and in fact al1 samples exarnined in this Chapter, exhibit a decrease in

cross-section area with increasing strain.

For the PEHMA films, before annealing, we find On = 0.35, an indication that

polymer diffusion has occurred significantly in these films (aET = 0.52 for a hlly mixed

films. as described in Chapter 4). The mean modulus value of these films is 0.06 MPa

and the mean elongation-at-break is greater than 10. When these films are annealed at

60°C for 15 min, the modulus increases little to 0.09 MPa, while the elongation-at-break

remains greater than 10. Over this tirne, an increases to 0.48, an indication that polymer

diffusion is nearly complete.

6.3.2 D-MAA-1 UA-tBCEMA-5 biend

in this section, we examine tensile properties of films prepared frorn the 1:l

mixture of labeled D-1MAA-II and A-tBCEMA-5 latex dispersions. We show the

engineering stress-strain plot of these samples examined at 60°C for different arnounts of

times in Figure 6.5A. The same data are replotted in the m e stress-strain fashion in

Fi,pre 6-33. -4 critical difference b e ~ e e n the two Figures is the stress-at-break. In the

engineering c w e s (Figure 6.5A), the plots show that the stress-at-break appears to

increase as the annealing time increases, whereas in the hue stress-strain c w e s (Figure

6.5B), we see that the stress-at-break increases to 1 h, and then decreases as the films are

annealed over 24 h. Note that one would reach an erroneous conclusion about the effect

of annealing on the tensile strength fiom the engineering stress-strain plot.

We surnmarize in Table 6.1 the results of the tensile data obtained fiom the m e

stress-strain plots. In this Table, we also include the results fiom the ET, gel content and

swell ratio measurements carried out on these samples. It took five days at roorn

temperature for the latex dispersion to dry to a film, followed by an additional day at the

sarne temperature in the testing chamber prior to the experiments. In these films, we

obtain CDET = 0.19. Poiymer dimision bas occurred. We are not able to measure the

extent of carbodiimide reaction directly fiom this film, but we detect a significant amount

of gel (24%). This result indicates that crosslinking has occurred, and occurred primarily

at the particle-particle boundary. These films have an average elastic modulus of 0-1 7

MPa, a value higher than that of the PEKMA itself (0.06 MPa). The MAA component of

the D-MAA-Il latex raises the Tg of the polymer by about 7OC, fiom -lO°C for PEHMA

to -3OC for the copolymer with 11 mol% MAA (see Chapter 5). The crosslinking

reaction also contributes to this hcrease.

One also sees fiom the plot that these samples break, with an average extension-at-

break of 6.5 and stress-at-break of 10.7 MPa. The tensile energy to break or "toughness"

of these samples is 8.2 MPa. As the annealing time is increased to 1 h, polymer diffusion

increases (<Dm reaches 0.41), and M e r reaction occurs. We find the moduius and

stress-at-break increase, but the elongation-at-break decreases (5 . l), while the toughness

remains unchanged. The elongation-at-break continues to decrease to 3.3 over 3 h

annealing at 60°C, and then to 1.8 over 24 h, while over this time, the modulus increases

to 0.35 MPa- in contrast to the modulus, the stress-at-break and toughness decreases

during this time. Over this time, polymer d i f i i o n in these films is nearly complete, as

indicated by = 0.49, and the gel content reaches 90%. Taking account of the

uncrosslinked seed polymers, a full gel content achieves.

From these results we learn that the introduction of reactive cornonomers into the

system increases the film modulus significantly, fi-om 0.06 MPa for PEHMA itself to 0.17

MPa for the D-MAA-I UA-tBCEMA-5 films. This modulus continues to increases as

polymer d i h i o n proceeds and the extent of the reaction increases. The Young's

modulus doubles its initial value (fiom 0.17 to 0.35 MPa) when polymer diffusion is

nearIy complete, and the maximum gel formation is obtained. The extension-at-break

strain (%)

strain

Figure 6.5: Stress-strain plots of the D-MAA-1 UA-tBCEMA-5 films exarnined at 60°C for various amounts of time; (A) engineering stress-strain curves, (B) true stress-strain curves.

Table 6.1 : Results of the tende, ET, gel-content and swell-ratio measurements on the D- MAA- 1 ]/A-tBCEMA-5 thick film samples annealed at 60°C for various amounts of time.

'?'ET

Young's modulus (MPa)

stress-at-break (M'Pa)

extension-at-break

toughness (MPa)

gel content (%)

swell ratio

without anneding

0.19

0.17= f 0.02

10.7& +_ 0.9

6.ja k 0.6

8.2= f 0.4

24

32

L these resuIts were calculated f?om the average of 5 sarnples before annealing. these results were calculated from the average of 5 samples annealed for 1 h these results were calculated from the average of 3 sarnples annealed for 3 h. these resulrs were caIculated fkom the average of 4 samples annealed for 24 h. Each standard deviation was calculated as foIlows,

behaves in the opposite way. It decreases in the nascent films once we have introduced

the reactive groups into the system. These values continue to decrease to a value (1.8)

that is almost 4 times lower than its initial value (6.5) as the a ~ e a l i n g time at 60°C is

increased to 24 h. The stress-at-break remains unchanged at about 11 MPa when the

films are annealed over a short period of time (1 h), but decreases slightly to 9.1 MPa

when these films are anneaied longer (24 h). The film toughness behaves in a way

similar to the stress-at-break. It rernains unchanged over 1 h, and then decreases fiom 8.8

MPa to 4.3 MPa over 24 h. Here we see that the toughness values of a fully crosslinked

films (4.3 M'Pa ) is only half of that of the films without annealing (8.2 MPa). The

optimum value of the toughness of this system is obtained at early times.

6.3.3 D-MAA-2O/A-tBCEMA-1 1 blend

We next examine what happens to the system when we increase the reactant

concentrations, fiom 11 mol% of MAA to 20 mol% and fiom 5 mol% of tBCEh,lA to

i 1%. Representative true stress-strain plots measured at room t e m p e m e for samples

examined at 60°C are s h o w in Figure 6.6. The mean values of individual t ende

properties are collected in Table 6.2, dong with the an, gel content and swell ratio data.

Figure 6.6: True stress-strain c w e s of the D-MAA-20/A-tBCEMA-1 I films amealed for different times at 60°C.

Table 6.2: Results of the tensile, ET, gel-content and sweli-ratio measurements on the D- MAA-ZO/&tBCEMA- I I thick film samples annealed at 60°C for various

ET

Young's modulus (MPa)

stress-at-break (MPa)

extension-at-break

toughness ( m a )

gel content (%)

swell ratio

L " thrse results were calculated fiom the average of 3 samples before annealing.

without anneal ing

0.10

0.5 la 1+ 0.03

10.6" I 0.7

4.ja 0.6

2.9" I 0.3

15

26

these resulü were calculated fiom the average of 5 samples annealed for 2 h. these results were caiculated f?om the average of 5 samples annealed for 1 1.5 h. these results were calculated fiom the average of 6 samples annealed for 36 h. Each standard deviation was calculated as follows.

in the films before annealing, we obtain OR. = O. IO. In these films, we fmd 15% of

insoluble polymers, indicating that some crosslinking has occurred. The modulus value

of these films is 0.51 M P q a value that is three times higher than that of the D-MIL40

1 I/A-tBCEMA-5 films (0.17 MPa). We believe that the increase in the Tg of the film (to

+j°C), caused by the increase in the MAA content, is a major part of the increase in the

Young's modulus. While the Young's modulus increases as the concentration of the

reactant increases, the extension-at-break decreases. The extension-at-break decreases

because of weak adhesion at the interface of the MAA-containing particles and the

tBCEMA-particles, caused by an increase in the immiscibility between these polymers as

a results of the increase in the amount of the reactive cornonomers present in the latex

particles.

2 h

0.28

0.57~ + 0.05

15.0~ + 1.2

~ - 7 ~ + 0.2

9.2b k 0.7

57

6

11.5 h

0.38

0.70' f 0.04

13.5' +. 1 .O

1.4= f: 0.1

6.0' + 0.6 78

3

When films are annealed over 2 h, reaches 0.28, and the gel content also

increases to 56%. These results inforrn us that healing of particle-particle interface due to

polymer difhsion occurs, which we imagine might result in an increase in the elongation-

at-break. We fmd, instead, that it decreases fiom 4.5 to 2.7. Over this time, the Young's

rnodulus increases slightly, while the stress-at-break increases from 10.6 to 15.0 MPa.

Here, the toughness increases significantly during this t h e , ftom 2.9 to 9.2 MPa. The

stress-at-break aIso increases substantially fiom 10.6 to 15.0 MPa over this anneaiing

tirne, but upon longer annealing to 1 1.5 h, it decrease to 13.5 MPa, and then remains

constant at about 13.7 MPa over 36 h. During this time, the Young's modulus increases

to 0.70 MPa over 11.5 h, and then to 0.82 MPa over 36 h. Over this time, reaches

0.35, and the gel content increases to 8 1%. The elongation-at-break continues to decrease

to 1.3 during this the .

We learn fiom these results that as the concentration of the reactant increases, the

Young's modulus increases substantially, but the extension-at-break decreases. The

toughness of this film @-MAA-20/A-tBCEMA-11 films), before annealing, is low (2.9

MPa). But upon longer annealing to 2 h, it increases to its maximum value of 9.2 MPa, a

value comparable to that of the D-MAA- 1 1/A-tBCEMA-5 system (8.8 MPa). Here, we

find that the maximum toughness of both films upon annealing is reached at early times.

6.3.4 D-MAA-111A-CCEMA-1.6 blend

We next examine what happens to the system when we replace tBCEMA with

CCEMA as the reactive comonomer. It took about 5 days for the latex dispersions to dry

to fom bubble-free films. The resuits of the tensile tests, as well as the ET, gel content

and swell ratio measurernents are swnmarized in Table 6.3. In the films, before

annealing, we obtain mET = 0.22, a value much higher than that obtained for films

prepared in 5 - 7 h fiom the same latex dispersions on a quartz plate, and then aged for 5

days at 22OC (mm = 0.12, Figure 5.24). We attribute this result to the longer exposure of

the CCEMA-containing particles to water which may have caused a greater extent of

hydrolysis of the -N=C=N- groups. This type of hydrolysis would lower the

concentration of these groups at the particle surface available for the crosslinking reaction

to occur. As a consequence, the D-MAA-1 IIA-CCEMA-4.6 films have a greater extent

of polymer d i h i o n .

We compare the tensile properties of these films to those obtained for the D-MAA-

1 IIA-tBCEMA-5 films. In the initially formed films containing CCEMA, the modulus

(0.22 MPa) is a linle higher than that obtained fiom the films containing 5 mol%

tBCEM.4 (0.17 MPa). However, the extension-at-break of these films (3.5) is half of that

obtained for the tBCEMA-containing films (6.7). The toughness of these films (6.1 MPa)

is also lower compared to that of the tBCEMA-containing films (8.2 MPa).

When the D-MAA-1 l/A-CCEMA-4.6 films are annealed at 60°C for 1 h, the elastic

modulus increases tiom 0.22 to 0.27 MPa, but the stress-at-break remains constant at

about 10 MPa, while the extension-at-break decreases fiom 3.5 to 2.6, and the toughness

Table 6.3: Results of the tensile, ET, gel-content and swell-ratio measurernents on the D- MAA-I UA-CCEMA-4.6 thick film samples anneaied at 60°C for various amounts of time.

Young's modulus (MPa)

stress-at-break (MPa)

extension-at-break

toughness (MPa)

gel content (%)

swell ratio

Q>ET

I 1

' these results were calculated fiom the average of 4 samples before annealing. these results were calculated fiom the average of 3 samples annealed for 1 h. these results were calculated fkom the average of 3 samples ameaied for 4 h. these results were calculated fiom the average of 3 samples annealed for 25 h.

without anneal ing

0.22

Each standard deviation was calculated as follows,

- S = [ ( - Y ) , where II is the m p ! e size and X ir the mean.

n-1

aIso decreases fiom 6.1 to 5.4 MPa. Over this time, OET increases from 0.22 to 0.36, and

the gel content increases fiom 33 to 63%. As the annealing time is increased, the

toughness of the CCEMA-containing films continues to decrease to 4.1 MPa over 25 h;

the extension-at-break, to 1.6; and the stress-at-break, also to 8.3 MPa; whereas the

elastic modulus remains unchanged at 0.28 MPa. D u ~ g t h i s tirne, OR. reaches 0.45.

We see that the maximum toughness of this system (6.1 MPa) is obtained fiom films

without annealing, for which = 0.22.

We next examine what happens to the system when we decrease the amealing

temperature From 60°C to 40°C. We prepared a different set of films but fiom the sarne

latex mixture. We summarize the resuits in Table 6.4. We h t compare the mults

obtained fiom the films before annealing to those of the initially formed films prepared

fiom the sarne latex mixture anneaied at 60°C (Table 6.3). One sees that the results are

reproducible since both films were prepared fiom the same latex dispersion under strictly

identical conditions.

The most important observation in Table 6.4 is that there are small changes in the

tensile properties of the system as the anneaiing tirne increases. Over 24 h at 40°C, the

modulus experiences a smail increase fkom 0.21 to 0.25 MPa, while the elongation-at-

break decreases fiom 3.2 to 2.4. Both polymer difision and the crosslinking reaction

proceed. For example, we see that when films are annealed over 24 h at 40°C, am increases fiom 0.23 to 0.35, indicating that polymer diffusion occurs to a small extent,

and the gel content increases to 63%. During this tirne, the Young's modulus increases

from 0.21 to only 0.25 MPa, but the extension-at-break decreases h m 3.2 to only 2.4,

while the toughness exhibits a small decrease, fiom 5.8 to 5.2 MPa. However, when the

same films are annealed at 60°C over the same amount of time (25 h), the Young's

modulus increases fiom 0.22 to 0.28 MPa, while the extension-at-break decreases fkom

3 -5 to 1.6, and the toughness, 60m 6.1 to 4.1 MPa. Over 25 h at 60°C, @ET increases

fiom 0.22 to 0.79, indicating that polymer d i f i i o n is more significant in these films.

Note, however, that the films anneaied at higher temperature have a somewhat lower

toughness (4.1 MPa) than those annealed at 40°C (5.2 MPa).

Table 6.4: Results of the tende, ET, gel-content and swell-ratio rneasurements on the D- L W - l IA-CCEMA-4.6 thick film sarnples m e a l e d at 40°C for various

Q'ET

Young's modulus (MPa)

stress-at-break (M'Pa)

extension-at-break

toughness (MPa)

gel content (%)

swell ratio

I

these results were calculated from the average of 3 samples before annealing. b these results were calculated kom the average of 4 samples annealed for 3 h

these results were calcuIated fYom the average of 5 samples annealed for 6 h. these resulrs were calculated Eiom the average of 3 samples annealed for 24 h. Each standard deviation was calculated as follows,

I - - S = [ - ( - ] , where * is the ,ample size and X is the rnean.

6.3.5 D-MAA-SIA-CCEMA-3.2 blend

In this section, we examine what happens to the system when we Iower the

concentration of the reactive cornonomers, fiom 1 1 to 5 mol% for MAA and fiom 4.6 to

3.3 mol% for CCEMA. Tensile, ET, gel content and swell ratio measurements were

carried out on films annealed at 60°C for various amounts of times. Representative mie

stress-smin curves are plotted in Figure 6.7, and the results are summarized in Table 6.5.

We first compare the resuits obtained at different annealing times. In the nascent

films, we obtain aET = 0.32 and 38% gel formation. The elastic modulus, stress-at-break,

extension-at-break are 0.20 MPa, 13 .O MPa and 4.1, respectively. These combined

tensile properties indicate that the films have an average toughwss of 7.5 MPa As the

annealing time is increased, the stress-at-break and extension-at-break decrease, while the

modulus exhibi ts a small increase fiom 0.1 8 to 0.22 MPa. Over 9 h at 60°C, the stress-at-

break decreases f?om 13.0 to 9.9 MPa, and the extension-at-break decreases fiom 4.1 to

2.3. During this tirne, the gel content increases to 75%, and reaches 0.48. Again, we

find the maximum toughness of the system is obtained trom films prior to annealing.

O 1 2 3 4 5

strain

Figure 6.7: True stress-strain curves of the D-MAA-SIA-CCEMA-3.2 films annealed for different times at 60°C.

We next compare these results to those obtained for the D-MAA-I UA-CCEMA-4.6

films (Table 6.3). Al1 films were examined at 60°C. in the D-MAA-5/A-CCEMA-3.2

films, without annealing, OR. = 0.20, a value that is little lower than that of the D-MAA-

1 UA-CCEMA-4.6 films (OR. = 0.23). The Young's modulus of these films (0.20 MPa)

is also slightly lower than that of the D-MAA-1 VA-CCEMA-4.6 films (0.22 MPa).

Here, we see that a decrease in the reactive group concentrations decreases the modulus.

However, we find an increase in the stress-at-break, the extension-at break, and thus the

toughness, in the films containing the lower amount of the reactive groups. The stress-at-

break increases from 9.8 MPa for the films containing the higher amount of the reactive

comonomers to 13.0 MPa for the films containing the lower amount of the reactive

groups. The extension-at break increases fiom 3.5 for the films containing the higher

arnount of the reactive comonomers to 4.1 MPa for the films containing the lower arnount

of the reactive groups. As a resuit, the toughness in the films containing the lower

arnount of the reactive groups (7.8 MPa) is higher than that of the films containing the

higher amount of the reactive comonomers (6.1 MPa).

As the annealing time increases to 25 h, the modulus of the films containing the

Table 6.5: Results of the tende, ET, gel-content and swell-ratio measurements on the D- M - Y A - C C E M A - 3 . 2 thick film samples anneded at 60°C for various amounts of time.

ET

Young's modulus (MPa)

stress-at-break (MPa)

extension-at-break

toughness @Pa)

gel content (%)

swell ratio

without annealing

15 min

I

these results were calculated 6om the average of 4 &ples before annealing. b these results were calculated fkom Le average of 4 samples annealed for I ~ m i n . ' these results were calculated fiom the average of 4 samples annealed for 1 h.

these results were caIculated 6om the average of 5 samples annealed for 9 h. Each standard deviation was calculated as follows,

I

- SD = (--!--z(x, - X!)' , vheie n is the sarrtplc rize and X is the mean.

n-1

higher amount of the reactive cornonomers increases fiom 0.22 to 0.28 MPa, whereas the

films containing the lower amount of the reactive groups have an insignificant increase in

the modulus over 9 h (fiom 0.20 to 0.22 MPa). For this film, we find @ET = 0.49, and

0.45 for the film containing higher amount of the reactive groups. The stress-at-break of

the films containing the lower arnount of the reactive comonomers decreases fiom 13.0 to

9.9 MPa) over 9 h, whereas in the fiims containing the higher amount of the reactive

comonomers, it seems to increase fiom 9.8 to 10.6 MPa over 1 h, and then decreases to

8.3 MPa upon longer annealing (25 h). The extension-at-break of both films decreases,

but the value dways remain higher for the films containing the lower arnount of the

reactive comonomers than it is for the films containing the higher amount of the reactive

comonomers.

We next examine what happens to the system when crosslinking is suppressed in

60°C -

-

-

-

-

without annealing 1 1 I 1

4

strain

Figure 6.8: True stress-main curves of the D-MAA-YA-CWEMA-3.2 films annealed for different times at 60°C.

the D-MAA-5/A-CCEMA-3.2 blend. To proceed, we treated the A-CCEMA-3.2 latex

dispersion with a stoichiometric amount of propionic acid to hydrolyze the carbodiirnide

groups to urea groups. We refer the propionic-acid-treated A-CCEMA-3.2 as A-

CUEMA-3.2 ("U" is urea). This conversion method is similar to that described in

Chapter 3. When d l the -N=C=N- groups have reacted, as indicated by Fï ïR, films of a

1 : 1 mixture of the A-CUEiMA-3.2 and D-MAA-5 latex were prepared. Tensile, ET, and

gel content measurements were carried out on these films, and the results are summarized

in Table 6.6. Representative true stress-strain curves obtained fiom the D-MAA-S/A-

CUEMA-3.2 films annealed for different times at 60°C are shown in Figure 6.8. We find

no gel formation neither in the nascent films nor in the films anneaied for 3 h at 60°C,

indicating that there is no permanent crosslink formation in this system. While there are

no -N=C=N- groups to react with the -COOH groups, hydrogen-bond formation can

Table 6.6: Results of the tensile, ET, gel-content and swell-ratio measurements on the D-MAAJ/A-CUEMA-~.~-&~~~ film samples annealed at 60°C for various amounts of time.

without annealing

0E-r

Young's modulus (MPa)

stress-at-break (MPa)

extension-at-break

toughness (MPa)

gel content (%)

I 1

' these results were calculated fiom the average o f 4 samples before annealil these results were calculated Gram the average of 4 samples annealed for 3 h. Each standard deviation was calculated as follows,

1 - -

SD = (-!-z(x, - ~ ) 2 ) ~ , where n ir the rample size and X is the rnean. n-1

occur in the system due to the presence of the urea and the carboxylic acid groups. In the

nascent films, we find (PEI- = 0.29, an indication that polymer diffusion in these films has

occurred. These films exhibit a Young's moduius of 0.20 MPa, a stress-at-break of 12.7

MPa, an extension-at-break of 6.7 and a toughness of 10.9 MPa. These films have a

higher modulus value than that of PEHMA itself. When these films are annealed for 3 h

at 60°C, the modulus increases slightly to 0.26 MPa, while the toughness remains the

same.

We compare these results to those obtained fiom the D-MAA-S/A-CCEMA-3.2

blend at the same annealing temperature (Table 6.5). In the initially formed films, we

obtain DET = 0.29 fQr the films containing CUEMA, a value higher than that obtained for

the films containing CCEMA (QET = 0.20). This result indicates polymer mixing has

occurred to a greater extent of in the films containing CUEMA than in the films

containing CCEMA. However, these films yield the same modulus values (0.20 MPa)

and similar values of the stress-at-break (12.7 - 13.0 MPa). Surprising results are found

when we compare the extension-at-break and toughness. These values are higher for the

films containing CUEMA (extension-at-break: 6.7; toughness: 10.9 MPa) than for the

film containing CCEMA (extension-at-break: 4.1 ; toughness: 7.8 MPa). We l e m fiom

these results that the presence of hydrogen-bonds in the urea-containing films leads to

higher toughness due to larger elongation-at-break values. As the annealing time is

increased, the elongation-at-break in the films containing CUEMA decreases slightly

from 6.7 to 6.3 over 3 h, whereas it decreases in the films containing CCEMA fiom 4.1 to

2.3 over 9 h. The toughness in these films decreases significantly from 7.8 to 4.8 MPa,

while it decreases slightly in the films containing CUEMA fiom 10.9 to 9.6 MPa.

We next examine the evolution of the tensile properties of the D-MAA-S/A-

CCEMA-3.2 films at arnbient temperature (2Z°C f 2). Representative true stress-strain

plots of these samples for different aging times are shown in Figure 6.9. The results are

summarized in Table 6.7. h the nascent films, we obtain On = 0.17, and the gel content

= 35%. Over 1 month, the an. value increases to only 0.27, while the gel formation

increases to 53%. The modulus increases slightly fiom 0.25 to 0.33 MPa, while the

extension-at-break decreases fiom 5.0 to 3.4, and the toughness also decreases fiom 9.5 to

7.7 MPa, Here we see that the maximum toughness (9.5 MPa) is aiso obtained in the

films without annealing.

Figure 6.9: Tme stress-strain curves of the D-MAA-5/A-CCEMA-3.2 films annealed for different times at 22OC.

5 days 20 days 30 days

I

- SD = [-$ (xt - xi)' , where n is the m p l e sizc and X is the mean.

n-

Table 6.7: Results of the tensile, ET, gel-content and swell-ratio measurements on the D- MAA-YA-CCEMA-3 -2 thick film samples annealed at 22OC for various

6.4 Discussion

Before discussing our results on crosslinking systems, we review some important

experiments reported in the literature on the tensile properties of thermoplastic latex

films. A study by Zoel and Ley describes the evolution of tensile properties for linea.

homopolymer latex films. They prepared films fiom latex particles of poly(buty1

methacrylate) PBMA], a polymer with a Tg (30°C) slightly higher than ambient

temperature. In the newly formed films, before interdiaision occurs, the stress-stran

curves show brinle characteristics (Figure 6.5). The brittle behavior rapidly changed to a

ductile characteristic after the films were annealed at 90°C. They observed that both the

amounts of time.

@ET

Young's modulus ( m a ) 1 stress-ai-break (MPa)

extension-at-break

toughness (MPa)

gel content (%)

swell ratio

, ' these results were calculated fiom the average of 5 samples before annealing. b these results were calculated fiom the average of 3 samples annealed for 5 days.

these results were calculated fiom the average of 4 samples annealed for 20 days. * these results were calculated fiom the average of 4 samples annealed for 1 rnonth.

Each standard deviation was calculated as follows,

without anneal ing

0.17

0.25" + 0.04

14.7= i 1.7

5.0a f 0.6

9Sa + 0.2

35

8

-.

&

extension-at-break and the strain-at-break increased with increased anneaiing. but they

characterized the system primarily in t ems of toughness. In their samples, the toughness

increased rapidly at the early stages of annealing and then levels off when chains has

d i h e d a distance corresponding to their radius of gyration. They pointed out that

polymer interdiffision across particle-particle interfaces leading to entanglements is

essential for latex films to develop toughness. Their pal-mer had a MW value of 5 x 10'

g/mol, which is significantly higher than the critical entanglement rnolecular weight for

PBMA (M. = 2.1 x 10" g/mol).

In the same they also showed that latex films prepared fiom PBMA

particles with a MW value (2 x 10' g/mol) lower than that of M, exhibits brittle hcture,

even for annealed films. These films exhibit brittle behavior because the polymers are

too short to generate entanglernents. They also exmined tensile properties of latex films

cast fiom pre-crossliiiked latex particles (2 mol% allyl methacrylate). In contrast to our

systems, these polymers became fully crosslinked within individual particles during the

emulsion polyrnerization reaction used to prepare these particles. Films formed fiorn

these pre-crosslinked particles showed brittle behavior, and this behavior remain

unchanged even afier the filrns were annealed. The films are brittle because the

crosslinked polymers prevent d i f i i o n across interface to f o m entanglernents. ïhese

studies illustrate the important role of polyrner interdiffusion on the evolution of

toughness of a latex film.

In a similar study but for different latex polymers, Gauthier and coworkers 7

obtained similar results for uncrosslinked polymers of high molecular weight. The newly

formed latex films exhibited bnttle behavior. With increased annealing tirne, polymer

interdifiion proceeds, leading to the healing at the particle-particle interface and

generating entanglernents. As a result, the stress-at-break, elongation-at-break and

toughness increase. In Figure 6.10, we present true stress-strain curves for the latex films

they describe, as a huiction of a ~ e d i n g time at 32OC. They observed two different types

of evolution of the tensile properties. For short annealing times, the stress-at-break

increased quickly first, and then (100 min later) the strain-at-break increased rapidly and

reached a plateau. For times longer than 500 min, the strain-at-break stabilized. In

contrast, the stress-at-break still increased slowly . They aiso recognize that the increase

in the elongation-at-break is due to the diaision of polymers across particle-particle

boundaries, which leads to an increase in entanglements. The increase in the stress-at-

break at short times shows formation of van der Waals bounds between particles,

resulting in the cohesion of the whole material. The increase in the stress-at-break at

longer times c m be related to a redistribution of the polyrner chains in the film, which

increases intermolecular van der Waal's interactions.

Figure 6.10: True stress-strain c w e s at different annealing times for films prepared fiom core/shell latex particles at 23OC. The core consists of polystyrene homopolymer, whereas the shell consists of random copolymer of styrenehutyl acrylatelaciylic acid (50:49: 1). These particles were prepared by a semi-continuous emulsion polymerization in the absence of chah transfer reagent. The latex films were prepared at 23OC, and then annealed at 32OC for different times.

In generai, crosslinking by covalent bonds increases the stress-at-break, but

decreases the elongaùon-at-break. One often finds that an increased level of crosslinking

is accompanied by an increases the toughness. However, excessive crosslinking can

cause films to become brittle and which leads to a drastic reduction in the elongation-at-

break. ' Toughness and elongation-at-break are closely related, since the area under the

stress-strain curve determines toughness. High elongation-at-break usually gives a large

area under the curve and thus a high value of toughness.

In our systems, in which interdiffusion is coupled with a crosslinking reaction, the

evolution of tensile properties from these films is a complex process. Polymer diffusion

generates entanglements and the crosslinking locks the entangled polymers in place.

There are two sources of crosslinking present in these systems: hydrogen-bonds and

covalent bonds. The hydrogen-bonds are formed by the carboxylic acid groups present in

the MAA-containing particles, whereas the covalent bonds are produced by the reaction

between the -N=C=N- groups and the -COOH groups. In the newly formed films,

hydrogen-bonds can be formed at the interface of MAA-containing particles as well as

nithin these particles. The reaction between the -N=C=N- groups and the -COOH groups

can be generated only at the interface behiveen the MAA-containing particles and the

tBCEMA-containing particles. As films are annealed, the extent of polymer diffusion

increases, such that the MAA-containing polymers can dimise into both adjacent cells of

MAA-containing polymers, and into adjacent cells of tBCEMA-containing polymers. in

both cases, the degree of hydrogen-bonding and covalent bond formation increases as the

extent of d i h i o n increases. Here the formation of chain entanglernents, covalent bonds

fiom the reaction between the -N=C=N- groups and the -COOH groups, and hydrogen-

bonds ail effect the tensile properties of the films.

One may think that once the carbodiimide consurning reaction occurs, the number

of hydmgen-bonds is reduced since a COOH group is removed fiorn the system for each

-N=C=N- group reacted. This is not the case. When the reaction between a -N=C=N-

group and a -COOH group occurs, the number of hydrogen-bonds behiveen two COOH

groups is reduced, but at the sarne time the systern generates an N-acyl urea- This moiety

contains both hydrogen-acceptor and hydrogen-donor sites. It can form hydrogen-bonds

with itself and with the -COOH groups on the other polymer. As a result of the reaction,

the system generates a covalent bond, but the extent of hydrogen-bonding remains

unc hanged.

Al1 of our latex films exhibit stress-strain characteristics of an ehstornenc material,

with low Young's modulus, high extension-at-break and no yield point. Because of the

large extension-at-break, these sarnples also exhibit a decrease in & as Al increases.

Taking these changes into account, we analyze the t ende properties of our latex films

usins the true stress-strain cuwes.

6.4.1 Effects of reactant concentrations

We first consider the influence of the reactant concentrations on the evolution of

film properties. For PEHMA itself, the films are weak and tacky, exhibithg a modulus of

less than O. 1 MPa and no elongation-at-break occurs, even for strains Iarger than 10. The

polymers exhibit flow when subjected to large strains. When PEHMA is copolymerized

with 1 1 mol% MAA or 5 mol% tBCEMA, and films are prepared from a mixture of these

particles, the film modulus increases, and the films break when stretched suficiently.

The modulus for this film (0.17 Mpa) is almost twice that of the PEHMA itself (0.09

MPa). When we double the arnount of the reactive cornonomers, fiom 1 1 to 20 mol% for

MAA and from 5 to 11 mol% for tBCEMA, the modulus increases significantly to 0.55

MPa, while the extension-at-break decreases fiom 6.5 to 4.5. We believe that the

increase in the MAA content leads to an increase in Tg of the latex films as weli as to an

increase in the extent of crossllliking due to both hydrogen-bonds and covalent bonds.

Both features contribute to the increase in the modulus. The increase in concentration of

both reactants reduces the miscibility of the MAA-containing copolymer with the

tBCEM.4-containing copolymer. This effect limits the initial rate of polymer diffusion

across the particle-particle boundary, and thus reduces the elongation-at-break.

The concentration of the reactive cornonomers present significantly efTects the

toughness of the initially fomed films significantly. We find thai the toughness of the D-

MAA-l UA-tBCEMA-5 films (8.2 MPa before annealing) is much higher than that of the

D-MAA-20/A-tBCEMA-11 films (2.9 MPa before annealhg). To interpret this result,

we consider adhesive forces that present at the particle-particle interfaces. As the reactant

concentrations increase, the extent of hydrogen-bond formation fiom the MAA-

containing particles to the adjacent MAA-containing particles increases, but the

miscibility between the MAA-containing polymers and the tBCEMA-containing

polymers decrease. As a result, in the films containing the higher amount of the reactive

comonomers has a stronger adhesion at the interface between the MAA-containing

particles, and a weaker adhesion at the interface between the W - c o n t a i n h g particles

and the ~BCELMA-containing particles. The weak adhesion at the interface between the

MAU-containhg particles and the ti3CEM.A-containing particles causes the sample to

break at shorter extension-at-break and rnay be at lower stress-at-break, and as its

consequence, leads to lower the toughness in the films containing the higher arnount of

the reactive comonomers.

In a different blend, El-Aasser and his coworkers reported that the toughness in

films prepared from a mixture of amino-fùnctionalized polybutadiene (PBD-NH2) and

isocyanate-functionalized poly(rnethacrylate/acrylate) (PMBT) decreases as the PBD-

NH2/ PMBT ratio increases. They attributed these results to the increase in the

immiscibility of the PBD and the PMBT, caused by an increase in the PBD-NH2/ PMBT

ratio.

6.4.2 Effects of annealing time

;Ne next examine the effect of annealing time on the evolution of tensile

propenies. As the annealing time increases, both polymer diffusion and the reaction

occur, generating entanglements and crosslinkings (covalent bonds and hydrogen-bonds).

The rnodulus and extension-at-break depend on these parameters. The modulus increases,

while the extension-at-break decreases as the entanglements and crosslinking increases.

In our systems, entanglements and crosslinkings are continuously generated, and thus the

modulus continues to increase, and the extension-at-break continues to decrease as the

annealing tirne increases.

in al1 samples examined here, the maximum toughness is obtained at early times.

In the W - C C E M A system, in which the reaction is fast, the maximum value is reached

in the films without annealing, whereas in the MAA-tBCEMA system, films requires

some arnounts of annealing time. For example, to yield maximum toughness, the D-

MAA-l l/A-tBCEMA-5 films were annealed for 1 h at 60°C, while in the D-MAA-20IA-

tBCEMA-11 films: turice as much annealing time (2 h) was necessary. In the MAA-

CCEMA system, annealing time is not necessary to obtain the maximum toughness in the

D-MAA- 1 1 /A-CCEMA-4.6 and D-MAA-5IA-CCEW-3.2 films.

6.4.3 Effects of annealing temperature

We next examine the influence of annealing temperatures on the film properties.

At hi& temperature, polyrner difision and the reaction occur rapidly, and as a result, the

tensile properties change substantially. At low temperature, polyrner diffusion and the

reaction are slow, the tensile properties change little. We look at the data fiom the D-

MAA- 1 l /A-CCEMA-4.6 films annealed at 60°C (Table 6.3) and 40°C (Table 6.4). When

these films are annealed at 60°C over 25 h, the modulus increases fiom 0.22 to 0.28 MPa,

while the modulus of the same films examined at 40°C over a similar amount of time

increases fiom 0.21 to only 0.25 MPa. The extension-at-break also decreases to a greater

extent for the films annealed at higher temperature. It reduces 50% (from 3.5 to 1.6) over

25 h. whereas only 23% (fiom 3.2 to 2.4) are reduced at 40°C over a similar amount of

rime. The toughness reduces also to a greater extent for the films annealed at higher

temperature. For the films anneaied at 60°C, it reduces from 6.1 to 4.1 MPa over 25 h,

during this time the same films annealed at 40°C, insignificant reduction in the toughness

is observed (from 5.8 to 5.2 MPa). As a result, the films annealed at lower temperature

have a higher toughness but a lower modulus than the films annealed at higher

temperature.

6.4.4 Effects of type of crosslinking (covalent vs. hydrogen-bond)

In this section, we look at what happens to the system when the carbodiimide

functionality is converted to the urea moiety. By converting the carbodiimide

functionality to the urea group, we prevent the reaction between the -N=C=N- and - COOH groups, but at the same time we convert the system to undergo crosslinking by

hydrogen-bond formation only.

We compare the tensile properties of the D-1%-SIA-CUEMA-3.2 films (Table

6.6) to those obtained fiom the D-MAA-5/A-CCEMA-3.2 films (Table 6.5) at the same

extent of polymer diffusion (On. = 0-29). in the initially formed films prepared fiom the

D-IMAA-YA-CUEMA-3.2 Iatex dispersions, On = 0.29, whereas in the D-MAA-SIA-

CCEMA-3.2 blend. films are annealed for 15 min for aET to reach 0.29. Both films

exhibit similar values of the modulus (18 - 20 MPa) and the stress-at-break (12.2 - 12.7

MPa). However, the extension-at-break is much higher for the films containing

hydrogen-bonds (6.7 MPa) than for the films containing covalent bonds (4.1 MPa). Thus,

the toughness is higher for the D-MAA-5/A-CUEAU-3.2 films (10.9 MPa) than it is for

the D-MAA-SIA-CCEMA-3.2 films (7.8 MPa). When both films are annealed at 60°C,

the extension-at-break in the films containing CCEMA decreases from 3.7 to 2.3, but in

the films containing CUEMA, it remains unchanged (6.7 to 6.3). The toughness of the

films containing hydrogen-bonds decreases insignificandy, but in the D-WU-YA-

CCEMA-3.2 films. it decreases fiom 6.9 to 4.8 MPa over 9 h. We see here that the films

containing hydrogen-bonds have higher values of elongation-at-break and toughness than

the films containing groups with capability of forrning covalent bonds.

6.5 Conclusions

We examined the evolution of tensile properties of latex fiims prepared from

reactive latex particles. We found that the modulus increases while the extension-at-

break decreases when the reactive groups are incorporated. When films prepared the

same reactive Iatex and are annealed for various amounts of time, the modulus increases

with times, but the extension-at-break decreases. The maximum toughness of these

systems is obtained at early times. A decrease in the annealing tempenture does lead to

films with low value of toughness.

6.6 References

1 Hill L. W. "Mechanical Properties of Coatings", Federation Senes on Coatings

Technology, Brezinsk D. and Miranda T. J., Eds., Federation of Societies for Coatings

Technology, Philadelphia, 1972. ' Akionis, J. J. and Macknight, W. J., introduction to Polqmer Viscosity, 2"* ed., Wiley

Interscience, New York, 1 983. 3 Nielson L. E., Mechanicd Properties of Polymers and Composites, Vol. 1, Marcel

Dekker, New York, 1974. 4 Hill L. W. "Dynarnic Mechanical and Tensile Properties" in "Paint and Coating Testing

Manual - 14" edition of the Gardner-Sward Handbook", Koleske J. V. ed., 1995, 534. 5 Cowie J. M. G. "Polymers: Chemistry and Physics of Modem Materials", International

Textbook Company Limited, 1973,232. 6 (a) Zosel A. and Ley G., Macromolecules, 1993,26(9), 2222. (b) Zosel A. and Ley G.,

"Role of Interdifiion in Film Formation of Polymer Latices" in '"'Film Formation in

Waterborne Coatings", ACS Symposium Series 648, Provder T.; Winnik M. A.; Urban

M. W. eds., 1996, 155. 7 Gauthier C.; Guyot A.; Perez J.; Sindt O., "Film Formation and Mechanical Behavior

of Polymer Latices" in "Film Formation in Waterborne Coatings", ACS Symposium

Series 648, Provder T.; Winnik M. A.; Urban M. W. eds., 1996, 163.

XuI. : Dimone L. V.; Sudol E. D.; Shaffer L. O.; El-Aasser S. M.J. Appl. Polym. Sci.

1998,69,977.

Glossary

An-MA : 9-anthryl methacrylate

t-BCEMA : t-butylcarbodiimidoethy1 methac y late

BMA : n-butyl methacrylate

CCEMA : cyclohexylcarbodiimidoethyl methacrylate

CTA : chah transfer reagent

DM : 1 -dodec ylmercaptan

EHMA : ?r 2-ethyhexyl methacrylate

ET : energy transfer

FTIR : Fourier transform infkared

GPC : gel permeation chrornatography

HPLC : high performance liquid chromatography

HRMS : high resolution mass spectroscopy

KPS : potassium persulfate

MAA : methacrylic acid

MFT : minimum film forming temperature

NMR : nuclear magnetic resonance

Phe-MMA : 9-phenanthrylmethyl methacrylate

SDS : sodium dodecyl sulfate

Tg : glass transition temperature

VOCs : volatile organic compounds

E : extinction coefficient

: quantum yield of energy transfer

f, : fiaction of mixing