Polymer Association Structures - American Chemical Society

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Chapter 6

Microemulsion Polymerization of Styrene J. S. Guo, M. S. El-Aasser, and J. W. Vanderhoff Emulsion Polymers Institute and Departments of Chemical Engineering and Chemistry, Lehigh University, Bethlehem, PA 18015 The polymerization of styrene microemulsions prepared from water, sodium dodecyl sulfate, and 1-pentanol was carried out using water-soluble potassium persulfate or oil-soluble 2,2'-azobis(2-methyl butyronitrile) initiator at 70°C. The latexes were stable, bluish, and less trans lucent than the microemulsions. The polymerization rates measured by dilatometry increased to a maximum and then decreased (only two intervals). The maximum polymeriza tion rate and number of particles varied with the 0.47 and 0.40 powers of potassium persulfate concentration, and the 0.39 and 0.38 powers of 2,2'-azobis(2-methyl bu tyronitrile) concentration, respectively. The small aver age latex particle sizes (20-30 nm) and high polymer mo lecular weights (1-2x106) showed that each latex particle comprised only 2-3 polystyrene molecules. The number of particles remained unchanged when the styrene was diluted with toluene at constant oil-phase volume. The mechanism proposed for both water-soluble and oil-soluble initia tors comprised nucleation in the microemulsion droplets by radical entry from the aqueous phase, with the drop lets which did not capture radicals serving as reservoirs to supply monomer to the polymer particles (homogeneous nucleation was not ruled out, however). This mechanism was compared with those proposed for conventional emul sion polymerization and miniemulsion polymerization. Many workers have studied microemulsions since the concept was i n t r o duced i n 1943 by Hoar and Schulman (χ), who showed that mixtures of o i l , water, and a l k a l i - m e t a l soaps with c e r t a i n alcohols or amines formed transparent dispersions, which they c a l l e d "oleopathic hydrom i c e l l e s . The research then continued d e s u l t o r i l y f o r 30 years, but accelerated when microemulsions were found to be e f f e c t i v e i n enhan ced o i l recovery (2-4). The l i t e r a t u r e on microemulsions i s now ex tensive and includes several books (5-12) published since 1977· n

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0097-6156/89/0384-0086$06.00/0 1989 American Chemical Society

In Polymer Association Structures; El-Nokaly, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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In contrast to the opaque, milky conventional emulsions and miniemulsions, microemulsions are i s o t r o p i c , transparent or t r a n s l u cent, and thermodynamically s t a b l e . They form spontaneously when o i l and water are mixed with surfactant and cosurfactant (usually 1-pentanol or 1-hexanol). Vigorous a g i t a t i o n , homogenization, or u l t r a s o n i f i c a t i o n are not needed. Microemulsions are postulated to comprise dispersions of droplets of s i z e smaller than 100 nm or bicontinuous lamellar l a y e r s . Both structures are consistent with t h e i r transparency o r translucency. Which structure i s more applicable i s the subj e c t o f some controversy, a discussion o f which i s beyond the scope of t h i s paper. Microemulsion polymerization, developed about 1980, i s new compared to conventional emulsion polymerization. Atik and Thomas (13) polymerized styrene o i l - i n - w a t e r microemulsions using 2,2'-azobis( i s o b u t y r o n i t r i l e ) i n i t i a t o r or gamma-ray i n i t i a t i o n . Johnson and G u l a r i (t4) polymerized d i l u t e d styrene o i l - i n - w a t e r microemulsions using potassium persulfate or 2 , 2 - a z o b i s ( i s o b u t y r o n i t r i l e ) i n i t i a t o r s , and measured the s i z e of the microemulsion droplets and latex p a r t i c l e s by photon c o r r e l a t i o n spectroscopy. Jayakrishnan and Shah (15) polymerized styrene and methyl methacrylate o i l - i n - w a t e r microemulsions using 2 , 2 - a z o b i s ( i s o b u t y r o n i t r i l e ) or benzoyl peroxide i n i t i a t o r s . Kuo et a l . (1J5) polymerized styrene-toluene o i l - i n - w a t e r microemulsions photochemically using dibenzylketone p h o t o i n i t i a t o r and an u l t r a v i o l e t l i g h t source. Although detailed and extensive, these works have not presented a d e f i n i t i v e mechanism f o r p a r t i c l e nucleation and growth i n styrene microemulsion polymerization. This paper describes a k i n e t i c i n v e s t i gation o f t h i s system using water-soluble and o i l - s o l u b l e i n i t i a t o r s and a comparison of microemulsion polymerization with conventional emulsion polymerization and miniemulsion polymerization, to determine the nucleation and p a r t i c l e growth mechanism. v

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Experimental The styrene (Polysciences) was washed with 10Î aqueous sodium hydroxide t o remove the i n h i b i t o r and vacuum-distilled under dry nitrogen; the 1-pentanol (Fisher S c i e n t i f i c ) was dried over potassium carbonate and vacuum-distilled; the potassium persulfate (Fisher S c i e n t i f i c ) was r e c r y s t a l l i z e d twice from water; the 2,2 -azobis(2-methyl butyron i t r i l e ) (Β. I . du Pont de Nemours) was r e c r y s t a l l i z e d twice from me thanol; the sodium dodecyl s u l f a t e (Henkel) was used as received; i t s c r i t i c a l m i c e l l e concentration measured by surface tension was 5*2 mM. D i s t i l l e d - d e i o n i z e d water was used i n a l l experiments. The microemulsion polymerization recipe comprised 82.25$ water, 9.05? sodium dodecyl s u l f a t e , 3*85? 1-pentanol, and 4.85$ styrene by weight. The rates o f polymerization at 70°C were measured dilatomet r i c a l l y i n a 25 ml Erlenmeyer f l a s k equipped with a 45-cm long 1-mm ID c a p i l l a r y . The microemulsions containing i n i t i a t o r were degassed, loaded i n t o the dilatometer, and polymerized i n a thermostated water bath. This polymerization procedure was described i n more d e t a i l ear l i e r (VjO. The latex p a r t i c l e s i z e d i s t r i b u t i o n s were determined us ing the P h i l i p s 400 transmission electron microscope with phosphotungstic acid negative-staining. A f t e r polymerization, the latex was poured i n t o methanol; the p r e c i p i t a t e d polymer was f i l t e r e d , washed with methanol and water, and d r i e d . The polymer molecular weight d i s ,


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t r i b u t i o n s were determined using the Waters Model 440 g e l permeation chromotograph with tetrahydrofuran as the eluant solvent.


Results and Discussion The latexes prepared using the water-soluble potassium persulfate or o i l - s o l u b l e 2,2'-azobis(2-methyl b u t y r o n i t r i l e ) i n i t i a t o r s were s t a b l e , b l u i s h , and l e s s translucent than the o r i g i n a l microemulsions. This change i n appearance was a t t r i b u t e d t o the larger s i z e o f the latex p a r t i c l e s and the greater r e f r a c t i v e index r a t i o . The e f f e c t s of i n i t i a t o r and monomer concentrations are described below. E f f e c t o f Potassium Persulfate Concentration. Figure 1 shows the v a r i a t i o n o f conversion and rate o f polymerization with time at 70°C f o r 0.14-0.69 mM potassium persulfate based on water. The polymerizat i o n rate increased with increasing i n i t i a t o r concentration, and the polymerization rate-conversion curves showed only two i n t e r v a l s ; n e i ther showed a constant rate or a g e l e f f e c t . I n t e r v a l I o f the polyme r i z a t i o n rate-conversion curve was characterized by an increase to a maximum, I n t e r v a l I I , by a decrease from t h i s maximum. The maximum polymerization rate was reached at 20-25$ conversion, i n contrast to the 2-15$ u s u a l l y observed i n conventional emulsion polymerization, which i n d i c a t e s that the p a r t i c l e nucleation stage was long, s i m i l a r to that o f miniemulsion polymerization (18,19). The long p a r t i c l e nuc l e a t i o n stage o f miniemulsion polymerization was a t t r i b u t e d to a slower rate o f r a d i c a l entry i n t o the monomer droplets, which i n turn was a t t r i b u t e d to a higher concentration of adsorbed e m u l s i f i e r on the miniemulsion droplet surface, or adsorbed mixed e m u l s i f i e r l i q u i d c r y s t a l s on the droplet surface, or the larger s i z e o f miniemulsion droplets compared to m i c e l l e s . In t h i s microemulsion polymerization, the long p a r t i c l e nucleation stage was a t t r i b u t e d t o the adsorbed layer o f surfactant-cosurfactant complex on the microemulsion droplet surface, which hindered the entry o f r a d i c a l s and thus gave a low r a d i c a l capture e f f i c i e n c y . The shorter nucleation stage and f a s t e r polymerization rate o f the microemulsion system r e l a t i v e to the miniemulsion system (18) was a t t r i b u t e d t o the smaller microemulsion droplet s i z e and higher r a d i c a l capture e f f i c i e n c y . The slower rate of the microemulsion polymerization r e l a t i v e t o that of the convent i o n a l emulsion polymerization was a t t r i b u t e d to the slower rate of free r a d i c a l entry i n t o the microemulsion droplets and the d i l u t i o n of the styrene i n s i d e the droplets by the 1-pentanol. The increasing rate o f polymerization i n I n t e r v a l I was a t t r i b u ted to an increasing number o f polymerizing p a r t i c l e s formed by nuc l e a t i o n i n the microemulsion d r o p l e t s , s i m i l a r to the mechanism proposed f o r miniemulsion polymerization by Chamberlain et a l . (20). I n t e r v a l I ends when a l l microemulsion droplets have disappeared, e i ther by capturing r a d i c a l s to become polymer p a r t i c l e s or l o s i n g monomer by d i f f u s i o n to the polymer p a r t i c l e s which have captured r a d i c a l s . In I n t e r v a l I I , the polymerization rate decreases because of the decrease i n monomer concentration i n the polymer p a r t i c l e s . No g e l e f f e c t was observed; termination occurred immediately upon entry of the second r a d i c a l i n t o the small l a t e x p a r t i c l e s which contained a growing r a d i c a l . Figure 2 shows that the maximum polymerization rate and number of p a r t i c l e s (calculated from the volume-average diameter D ) varied v



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Figure 1. Conversion-time curves and polymerization rate-time curves f o r : 1. 0.69 (+); 2. 0.48 (*); 3. 0.27 (©); 4. 0.14 (Π) mM potassium persulfate based on water.




Figure 2. Dependence o f maximum rate o f polymerization and p a r t i c l e number on potassium persulfate concentration.



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according to the 0.47 and 0.40 powers, respectively, of potassium persulfate concentration, which indicated that the number of p a r t i c l e s reached a constant value at the maximum polymerization r a t e , and no more p a r t i c l e s were nucleated a f t e r I n t e r v a l I . This dependence was consistent with the 0.40 power predicted by Smith-Ewart case 2. However, the mechanism and k i n e t i c s of microemulsion polymerization were d i f f e r e n t from those of conventional emulsion polymerization. P a r t i c l e nucleation occurred i n the microemulsion d r o p l e t s , with the f r a c t i o n becoming p a r t i c l e s determined by the i n i t i a t o r concentra t i o n . An increase i n i n i t i a t o r concentration increased the r a d i c a l f l u x to the microemulsion d r o p l e t s , which i n turn increased the f r a c t i o n of droplets converted to polymer p a r t i c l e s . The average number of r a d i c a l s per p a r t i c l e ΊΙ depended on the p a r t i t i o n i n g of 1-pentanol between the dispersed and continuous phases, which was d i f f i c u l t to determine. Nevertheless, the values of η were calculated to be i n the range 0.06-0.15 assuming that a l l of the 1-pentanol was i n the aque ous phase or the p a r t i c l e . These values were much smaller than the 0.5 value of Smith-Ewart case 2. Figure 3 shows that the latex p a r t i c l e s i z e d i s t r i b u t i o n s were broad, with p a r t i c l e s ranging from 5 to 40 nm. The droplet s i z e s of the microemulsions could not be measured by electron microscopy, but e a r l i e r measurements by photon c o r r e l a t i o n spectroscopy of a micro emulsion which was i d e n t i c a l except f o r the s u b s t i t u t i o n of brine f o r the water gave an average droplet s i z e of 3-65 nm (14). The larger p a r t i c l e sizes of the latex p a r t i c l e s were attributed to the fact that not a l l of the microemulsion droplets captured r a d i c a l s and be came polymer p a r t i c l e s during I n t e r v a l I ; some served as reservoirs to supply monomer to the polymer p a r t i c l e s by d i f f u s i o n through the continuous phase. The broad p a r t i c l e s i z e d i s t r i b u t i o n s were a t t r i b u ted to the long nucleation stage and the absence of a constant-rate interval. Figure 4 shows that the polymer weight-average molecular weights were 1-2x10^ and varied with only the -0.07 power of potassium per s u l f a t e concentration. The high molecular weights and low dependence on persulfate concentration was attributed to the slow rates of r a d i c a l entry, which segregated the r a d i c a l s i n s i d e the microemulsion droplets and reduced the p r o b a b i l i t y of bimolecular termination. S i milar r e s u l t s were obtained i n an e a r l i e r study (J£) on the e f f e c t of polymerization temperature. From the latex p a r t i c l e sizes and polymer molecular weights, each latex p a r t i c l e comprised only 2-3 polymer mo l e c u l e s . Candau et a l . (21_) obtained polyacrylamide of very high mo l e c u l a r weight (10?) i n the inverse microemulsion polymerization of acrylamide and concluded that each latex p a r t i c l e comprised only one polymer molecule. Broad p a r t i c l e s i z e d i s t r i b u t i o n s , high polymer mo l e c u l a r weights, and a low dependence of polymer molecular weight on persulfate concentration were also found f o r the miniemulsion polym e r i z a t i o n of styrene (V8). ,

Effect of 2.2 -Azobis(2-Methyl B u t y r o n i t r i l e ) Concentration. Figure 5 shows the v a r i a t i o n of conversion and rate of polymerization with time at 70°C f o r 0.66-2.15 mM 2,2 -azobis(2-methyl butyronitrile) based on water. These curves show the same k i n e t i c features as those of the potassium persulfate system. The polymerization rate increased w i t h increasing 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) concentration, and only two i n t e r v a l s were found; however, the maximum polymeriza,

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Figure 3· Transmission electron micrograph o f polymerized styrene microemulsion with phosphotungstic acid negative-staining. 6.60

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t i o n rate was attained at 10-15$ conversion, as compared to 20-25$ f o r the water-soluble persulfate i n i t i a t o r and 2-15$ f o r conventional p e r s u l f a t e - i n i t i a t e d emulsion polymerization. These differences were attributed t o the f a c t that the 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) i s soluble i n 1-pentanol, and the 1-pentanol i s s l i g h t l y soluble i n water, which r e s u l t s i n the generation of some r a d i c a l s i n the aqueous phase. Thus, two mechanisms were postulated f o r the nucleation of p a r t i c l e s : entry o f r a d i c a l s i n t o the droplets from the aqueous phase and generation o f r a d i c a l s i n s i d e the droplets. I n t h i s case, the polymerization rate would be determined by the equilibrium p a r t i t i o n i n g of r a d i c a l s between the monomer droplets and the aqueous phase. However, two r a d i c a l s generated i n an microemulsion droplet would have a high p r o b a b i l i t y o f recombining before i n i t i a t i n g polymerization. The recombination would give a slower polymerization r a t e , and the p a r t i c l e nucleation would depend mainly on the entry o f r a d i c a l s from the aqueous phase. Therefore, the r a d i c a l generation rates and polymerization rates f o r potassium persulfate and 2,2 -azobis(2-methyl butyr o n i t r i l e ) i n i t i a t o r s were compared. The calculated r a d i c a l generat i o n rate f o r 0.27 mM persulfate i n i t i a t o r at 70°C was 7.57x10 radicals/ml/sec (22). For 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) , the calculated r a d i c a l generation rates were 4.98x10 3 radicals/ml/sec based on aqueous phase (2.15 mM) and 8.45x10™ radicals/ml/sec based on styrene (36.5 mM) (22). Because o f the p a r t i t i o n i n g o f t h i s i n i t i ator between the monomer and aqueous phases, the actual r a d i c a l generation rate should l i e between these two values. Although the c a l culated r a d i c a l generation rate f o r the 2.15 mM (based on water) 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) was sevenfold greater than that f o r the 0.27 mM potassium p e r s u l f a t e , the polymerization rate was slower. Therefore, the higher calculated r a d i c a l generation rate was probably outweighed by the high p r o b a b i l i t y o f recombination i n the microemulsion droplets. The f a c t that the nucleation stage was shorter than f o r potassium persulfate was a t t r i b u t e d to the greater e f f i ciency o f r a d i c a l capture because the 1-pentanol acted as the medium. Figure 6 shows that the maximum polymerization rate and number of p a r t i c l e s varied as the 0.39 and 0.38 powers, r e s p e c t i v e l y , o f the 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) concentration; these values were s i m i l a r to the 0.47 and 0.40 powers found f o r persulfate i n i t i a t o r , and higher than the 0.21 and 0.21 powers found f o r the miniemulsion polymerization o f styrene i n i t i a t e d by 2,2 -azobis(2-methyl butyronit r i l e ) (18). Thus, f o r both the water-soluble potassium persulfate and o i l - s o l u b l e 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) i n i t i a t o r s , the primary source o f r a d i c a l s was the aqueous phase. With the larger droplet s i z e s o f miniemulsion polymerization, the source o f r a d i c a l s was both the monomer droplet phase and the aqueous phase. The p a r t i c l e s i z e s and molecular weight d i s t r i b u t i o n s were s i m i l a r to those found f o r persulfate i n i t i a t o r . Figure 7 shows that the weight-average molecular weight varied with the -0.55 power o f 2,2 -azobis(2methyl b u t y r o n i t r i l e ) concentration, which i s close t o the -0.60 power found f o r the conventional emulsion polymerization o f styrene using persulfate i n i t i a t o r . Possible reasons are: the aqueous phase i s the primary source o f r a d i c a l s i n both cases; the 1-pentanol i n the microemulsion polymerization improves the r a d i c a l capture e f f i c i e n c y by acting as the medium; and the s i z e s o f microemulsion droplets and e m u l s i f i e r micelles are s i m i l a r .


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E f f e c t o f Styrene Concentration. To determine the e f f e c t o f concent r a t i o n , the styrene was d i l u t e d with toluene while keeping the v o l ume o f the o i l phase constant, i n the expectation that the s i z e o f the microemulsion d r o p l e t s , and thus the competition f o r r a d i c a l s , would be s i m i l a r . The polymerizations were c a r r i e d out at 70°C using 0.55 mM potassium p e r s u l f a t e based on water. Figure 8 shows that the higher styrene concentrations gave f a s t e r polymerization r a t e s ; however, Figure 9 shows that the maximum rates o f polymerization varied with the 2.33 power o f styrene concentration, which cannot be accounted f o r by d i l u t i o n alone. Figure 10 shows that the weight-average molecular weight decreased with decreasing monomer concentration, as expected. Table I shows the p a r t i c l e s i z e d i s t r i b u t i o n data f o r the d i f f e r e n t styrene concentrations. D , D , and D are the number-avern

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age, volume-average, and weight-average diameters determined by e l e c tron microscopy, D ( i ) i s the volume-average diameter corrected f o r the volume o f toluene, which was assumed t o be l o s t during the e l e c tron microscopy, and PDI i s the p o l y d i s p e r s i t y index D /D . The v a l ues o f D ( i ) and the p o l y d i s p e r s i t y index were s i m i l a r f o r the three d i f f e r e n t styrene concentrations, which i n d i c a t e s that the constant o i l phase volume gave the same microemulsion droplet s i z e and hence the same p a r t i c l e nucleation and growth mechanisms. The foregoing mechanism proposed f o r the microemulsion polymerization o f styrene i s based on the assumption that the mimcroemulsions comprised dispersions o f small o i l droplets i n a continuous water medium. However, some workers (23,24) have shown by Fourier-transform pulse-gradient spin-echo NMR s e l f - d i f f u s i o n methods that 1-pentanol cosurfactant i n 2/1 1-pentanol/sodium dodecyl s u l f a t e r a t i o gives dynamic or bicontinuous lamellar microemulsions. Whether these s t r u c tures form a t the 0.4/1 1-pentanol/sodium dodecyl s u l f a t e r a t i o used here i s not known. I f the microemulsions comprised bicontinuous l a mellar structures with a l t e r n a t i n g monomer and water l a y e r s , the part i c l e nucleation would occur i n the monomer layer according t o the d i f f u s i o n o f r a d i c a l s formed by i n i t i a t o r decomposition, with the growing s p h e r i c a l p a r t i c l e s adsorbing surfactant to disrupt the l a mellar s t r u c t u r e . To d i s t i n g u i s h between these two s t r u c t u r e s , however, requires further study. v

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Conclusions Polymerization o f styrene o i l - i n - w a t e r microemulsions using potassium persulfate or 2,2»-azobis(2-methyl b u t y r o n i t r i l e ) i n i t i a t o r gave s t a ble latexes which were b l u i s h and l e s s translucent than the o r i g i n a l microemulsions. The mechanism and k i n e t i c s o f polymerization were




96

Figure 8. Conversion-time curves f o r 0.55 mM potassium persulfate with a constant-volume o i l phase of: 1. 100/0; 2. 75/25; 3- 50/50 styrene/toluene weight r a t i o .


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LOG [STYRENE] Figure 10. Dependence of weight-average molecular weight on weight f r a c t i o n of styrene i n the styrene/toluene mixture.


98


d i f f e r e n t from those o f conventional emulsion polymerization or mini emulsion polymerization. The conversion-time curves showed only two i n t e r v a l s , with neither a constant-rate region nor a g e l e f f e c t . The p a r t i c l e nucleation stage continued to 20-25$ conversion f o r p o t a s s i um persulfate and 15-20$ conversion f o r 2,2 -azobis(2-methyl butyro n i t r i l e ) ; t h i s long nucleation stage was a t t r i b u t e d to the slow rate of r a d i c a l entry i n t o the microemulsion droplets. The small average p a r t i c l e s i z e s (20-30 nm) and high molecular weights ( 1-2x10** ) i n dicated that the p a r t i c l e s comprised only 2-3 polymer molecules. The maximum rate o f polymerization and number o f p a r t i c l e s varied with the 0.47 and 0.39, and the 0.40 and 0.38, powers o f the potassium p e r s u l f a t e and 2,2 -azobis(2-methyl b u t y r o n i t r i l e ) concentrations, r e s p e c t i v e l y . The p a r t i c l e s i z e d i s t r i b u t i o n s f o r d i f f e r e n t styrene concentrations a t constant o i l phase volume were s i m i l a r ; thus, the droplet s i z e s and mechanisms o f p a r t i c l e nucleation and growth were also s i m i l a r . A mechanism was proposed f o r the styrene microemulsion polymerization: the p a r t i c l e s were nucleated by capture o f r a d i c a l s from the aqueous phase f o r both water-soluble and o i l - s o l u b l e i n i t i a t o r s , and the microemulsion droplets which d i d not capture r a d i c a l s served as r e s e r v o i r s to supply monomer to the polymer p a r t i c l e s . The p o s s i b i l i t y of homogeneous nucleation was not ruled out.


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Literature Cited 1. Hoar, J. P.; Schulman, J. H. Nature 1943, 152, 102. 2. Improved O i l Recovery by Surfactant and Polymer Flooding; Shah, D. O.; Schechter, R. S., Ed.; Academic: New York, 1977. 3. Surface Phenomena in Enhanced Oil Recovery; Shah, D. O., Ed.; Plenum: New York, 1981. 4. Sunder Ram, A. N.; Shah, D. O. In Emulsions and Emulsion Tech nology, Part 3; Lissant, K. J., Ed.; Marcel Dekker: New York, 1986; p 139. 5. Microemulsions - Theory and Practice; Prince, L. M., Ed.; Aca demic: New York, 1977. 6. Micellization, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977. 7. Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 2. 8. Microemulsions; Robb, I. D., Ed.; Plenum: New York, 1982. 9. Surfactants in Solution; Mittal, K. L., Ed.; Plenum: New York, 1984; Vol. 3. 10. Macro- and Microemulsions - Theory and Applications; Shah, D. O., Ed.; ACS Symposium Series No. 272; American Chemical Socie ty: Washington, D.C., 1985. 11. Microemulsion Systems; Rosano, H. L.; Clausse, M., Ed.; Marcel Dekker: New York, 1987. 12. Microemulsions: Structure and Dynamics; Friberg, S. E.; Bothorel, P., Ed.; CRC: Boca Raton, 1987. 13. Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 4279. 14. Johnson, P. L.; Gulari, E. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 3967. 15. Jayakrishnan, Α.; Shah, D. O. J . Polym. Sci., Polym. Lett. Ed. 1984, 22, 31. 16. Kuo, P. L.; Turro, N. J.; Tseng, C. M.; El-Aasser, M. S.; Van derhoff, J. W. Macromolecules 1987, 20, 1216.



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17. Guo, J. S.; El-Aasser, M. S.; Vanderhoff, J. W. J . Polym. Sci., Polym. Chem. Ed. in press. 18. Choi, Y. T.; El-Aasser, M. S.; Sudol, E. D.; Vanderhoff, J . W. J. Polym. Sci., Polym. Chem. Ed. 1985, 23, 2973. 19. Delgado, J., Ph.D. Dissertation, Lehigh University, Bethlehem, 1986. 20. Chamberlain, B. J.; Napper, D. H.; Gilbert, R. G. J. Chem. Soc. Faraday Trans. I 1982, 78, 591. 21. Candau, F.; Leong, Y. S.; Fitch, R. M. J . Polym. Sci., Polym. Chem. Ed. 1985, 23, 193. 22. Polymer Handbook, 2nd ed.; Brandrup, J.; Immergut, E. H., Ed.; John Wiley & Sons: New York, 1975. 23. Stilbs, P.; Rapacki, K.; Lindman, B. J. Colloid Interface Sci. 1983, 95, 583. 24. Ceglie, Α.; Das, K. P.; Lindman, B. J Colloid Interface Sci. 1987, 115, 115. RECEIVED August 22, 1988


Polymer Association Structures - American Chemical Society

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