Colloidal Dispersions and Micellar Behavior

9 "True" Emulsion Polymerization of Acrylate Esters Using Mixed Emulsifiers

Downloaded by UNIV OF NEW ENGLAND on February 12, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0009.ch009

A. R. M . AZAD, R. M . FITCH* and J. UGELSTAD Institute for Industrial Chemistry, Norwegian Institute of Technology, 7034 Trondheim—NTH, Norway

Introduction The term "true" emulsion polymerization s i g n i f i e s the f o r mation o f a synthetic l a t e x by p o l y m e r i z a t i o n o f a monomer i n the form o f a f i n e emulsion. It i s d i s t i n g u i s h e d from "Suspen sion" or "bead" p o l y m e r i z a t i o n in t h a t the monomer d r o p l e t s are sufficiently s m a l l , so that the p o l y m e r i z a t i o n may f o l l o w classical Smith-Ewart k i n e t i c s (1) r a t h e r than bulk k i n e t i c s . Ugelstad and coworkers have shown that when vinyl c h l o r i d e i s a g i t a t e d in water with a mixed e m u l s i f i e r system o f a n i o n i c s u r f a c t a n t and a normal long chain f a t t y a l c o h o l a very f i n e emulsion is produced, and that the p r i n c i p a l locus o f the sub sequent p o l y m e r i z a t i o n appears t o be w i t h i n the monomer drop lets (2). T h i s was f u r t h e r supported by s i m i l a r studies on styrene (3). The evidence f o r the mechanism proposed has been i n d i r e c t but c o m p e l l i n g , based on two kinds o f i n f o r m a t i o n : (a) 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 o f the product l a t e x e s and (b) the p o l y m e r i z a t i o n k i n e t i c s . I d e a l l y one would like to compare 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 o f the monomer emulsion with those o f the polymer colloid formed, but the problem has been that no s a t i s f a c t o r y method was p r e v i o u s l y a v a i l a b l e to mea sure the liquid monomer droplet s i z e s , as many are below the r e s o l u t i o n o f o p t i c a l microscopes. In t h i s paper the work has been extended to a c r y l a t e e s t e r s . Furthermore, some p r e l i m i n a r y r e s u l t s are given on a new t e c h nique f o r the e l e c t r o n microscopic determination o f styrene monomer droplet s i z e s . *

Permanent address:

Department o f Chemistry and I n s t i t u t e of M a t e r i a l s S c i e n c e , U n i v e r s i t y o f Connecticut, S t o r r s , Conn. 06268, USA

135

Mittal; Colloidal Dispersions and Micellar Behavior ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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COLLOIDAL DISPERSIONS AND MICELLAR BEHAVIOR


Experimental Materials. Methyl methacrylate monomer vas vacuum d i s t i l l e d twice before u s i n g . F a t t y a l c o h o l s were a l s o twice d i s t i l l e d i n vacuo. Other m a t e r i a l s have been described p r e v i o u s l y (jjTT Procedures. A n a l y t i c a l methods and p o l y m e r i z a t i o n t e c h nique have been r e p o r t e d e a r l i e r (k). Samples o f the monomer emulsions were prepared by a s t a i n ing technique i n which a measured q u a n t i t y o f concentrated aqueous OsO^ s o l u t i o n was added t o the emulsion. A f t e r 10 and 15 minutes samples were d r i e d on e l e c t r o n microscope g r i d s and observed under the microscope. Results and D i s c u s s i o n E m u l s i f i e r A d s o r p t i o n : Methyl methacrylate (MMA) i s much more p o l a r than v i n y l c h l o r i d e or s t y r e n e , so that e m u l s i f i e r adsorption was expected to be l e s s f a v o r a b l e . This was con firmed i n experiments on the adsorption o f sodium hexadecyl s u l f a t e (SHS) i n the presence o f hexadencanol (HD) as shown i n F i g u r e 1. Only about 50% o f the i o n i c e m u l s i f i e r i s adsorbed i n i t i a l l y upon s t i r r i n g with monomer, and t h i s decreases w i t h time. V o i d and Groot showed that the a d d i t i o n o f s a l t l e d to enhanced adsorption of sodium dodecyl s u l f a t e onto N u j o l emul sions along with greater s t a b i l i t y (5.). This i s presumably due to the r e d u c t i o n i n l a t e r a l e l e c t r o s t a t i c r e p u l s i o n s among a d sorbate molecules as a r e s u l t o f the increase i n i o n i c s t r e n g t h . With mixtures o f i o n i c e m u l s i f i e r s and s t r a i g h t c h a i n f a t t y a l c o h o l s Goddard (6) showed that increased adsorption r e s u l t e d s o l e l y from hydrophobic i n t e r a c t i o n s o f the long c h a i n s . Thus we expected that an increase i n i o n i c strength i n the mixed e m u l s i f i e r system would act independently o f the hydrophobic i n t e r a c t i o n t o f u r t h e r enhance adsorption o f the i o n i c emul sifier. The dramatic experimental confirmation o f t h i s i s a l s o shown i n F i g u r e 1. Thus the a d d i t i o n o f sodium c h l o r i d e to a concentration o f 10" M was s u f f i c i e n t to cause 91% o f the sodium hexadecyl s u l f a t e to be adsorbed, although subsequent desorption upon continued s t i r r i n g occurred r a t h e r r a p i d l y . When the s a l t concentration was increased f u r t h e r to 1(H-M, 9&% o f the i o n i c surfactant was adsorbed, and t h i s was accompanied by a marked r e t a r d a t i o n i n the k i n e t i c s o f both the adsorption and subse quent desorption (Figure l ) . We should a n t i c i p a t e that a s i m i l a r s a l t e f f e c t would be experienced upon the a d d i t i o n o f an i o n i c i n i t i a t o r such as potassium p e r s u l f a t e . 2



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Emulsion Polymerization

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The maximum s i z e o f the monomer emulsion d r o p l e t s vas r e duced upon the a d d i t i o n o f s a l t from ^ 3 ]xm t o *1 μιη diameter, as determined by o i l - i m m e r s i o n l i g h t microscopy. The minimum s i z e c o u l d not be observed as i t was below the r e s o l u t i o n of these o p t i c s , and because we have been unable to develop an e l e c t r o n microscopic s t a i n i n g technique f o r a c r y l a t e e s t e r s . I f the amount o f organic phase i s increased while keeping the amount o f mixed e m u l s i f i e r and water constant, more emul s i f i e r i s adsorbed. Thus as the phase r a t i o o f MMA: H2O i s i n creased from 1:3 t o 1:1 t o 2 : 1 , a s i x - f o l d change, the maximum amount o f e m u l s i f i e r adsorbed i n c r e a s e s from 75% t o 99% i n the absence o f added s a l t . These r e s u l t s are f o r the system sodium hexadecyl s u l f a t e (SHS): octadecanol (OD) (l:k mole r a t i o ) and are shown i n F i g u r e 2. The r e s u l t s with MMA. may be compared with those obtained e a r l i e r with styrene (3 h). The SHS/HD e m u l s i f i e r / a l c o h o l system i s much more s t r o n g l y adsorbed onto t h i s more hydrophobic l i q u i d , such t h a t at a 1:3 mole r a t i o o f SHS:HD, 9&% o f the i o n i c s u r f a c t a n t i s adsorbed without added e l e c t r o l y t e . These and other r e s u l t s are shown i n F i g u r e 3. In the presence o f i o n i c i n i t i a t o r we may expect even higher amounts o f e m u l s i f i e r adsorbed and slower r a t e s of adsorption and d e s o r p t i o n . I t must be s t r e s s e d that the processes observed here are kinetically controlled. The e m u l s i f i e r and f a t t y a l c o h o l are both precent i n the aqueous phase at the s t a r t o f the e m u l s i f i c a t i o n process. Small d r o p l e t s formed i n the r e g i o n o f h i g h shear r a t e near the moving paddles are s t a b i l i z e d by the formation o f a complex l a y e r o f e m u l s i f i e r and f a t t y a l c o h o l at the surface o f the d r o p l e t s . The r a t i o o f f a t t y a l c o h o l to e m u l s i f i e r i s thus i n i t i a l l y r e l a t i v e l y high and gives a high degree o f s t a b i l i z a t i o n towards coalescence. With t i m e , the f a t t y a l c o h o l i s p a r t l y t r a n s f e r r e d to the i n t e r i o r o f the d r o p l e t s so that the amount remaining at the i n t e r f a c e i s apparently i n s u f f i c i e n t t o maintain s t a b i l i t y towards coalescence and consequent desorption of ionic emulsifier. The q u a l i t a t i v e aspects o f the i n t e r a c t i o n s between i o n i c e m u l s i f i e r and a l c o h o l at the i n t e r f a c e have been described by V o i d and M i t t a l ( £ ) . 9

"True" Emulsion P o l y m e r i z a t i o n : Ugelstad and coworkers have p r o v i d e d evidence that when f r e e r a d i c a l s are formed i n the presence o f these very f i n e p a r t i c l e s i z e monomer emulsions, p o l y m e r i z a t i o n occurs almost e x c l u s i v e l y i n the e m u l s i f i e d monomer d r o p l e t s (2_,3_,h). In favorable cases few i f any new p a r t i c l e s are nucleated i n the aqueous phase. T h i s behavior can be understood i n terms o f the theory o f homogeneous n u c l e a t i o n



C O L L O I D A L DISPERSIONS A N D M I C E L L A R

BEHAVIOR

Figure 1. Kinetics of adsorption of sodium hexadecyl sulfate (SHS) (2g Γ of H 0) in the presence of twice as many moles of hexadecanol (WD) in methyl methacrylate emulsions containing various amounts of salt at 60° C; stirring rate, 600 rpm 1

2

T o t a l SHS Added

100

200

300

Figure 2. Kinetics of adsorption of sodium hexadecyl sulfate (SHS) (2g Γ of H 0) in the presence of four times as many moles of n\-octadecanol (OD) in methyl methacrylate emulsions, at various phase ratios of MMA:H 0 as shown at 60°C; stirring rate, 600 rpm 1

2

2



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Emuhion Polymerization

of polymer c o l l o i d s i n which i t i s p o s t u l a t e d that new polymer p a r t i c l e s are formed by the s e l f n u c l e a t i o n o f growing primary oligomeric f r e e r a d i c a l s i n the continuous phase. Particle n u c l e a t i o n only occurs where a primary r a d i c a l i s not p r e v i o u s l y captured by a monomer d r o p l e t . The p r o b a b i l i t y o f capture i s p r o p o r t i o n a l t o the product o f the number c o n c e n t r a t i o n and the r a d i u s (N-r) o f the monomer d r o p l e t s (8). In conventional emulsion p o l y m e r i z a t i o n the value o f N«r i s so low that the number o f primary r a d i c a l s captured by e m u l s i f i e d monomer d r o p l e t s i s n e g l i g i b l e ( £ ) . In "true" emulsion p o l y m e r i z a t i o n , on the c o n t r a r y , the value o f N*r i s so l a r g e t h a t most, i f not a l l , the primary oligomeric r a d i c a l s are captured by monomer d r o p l e t s and no or v e r y l i t t l e aqueous phase n u c l e a t i o n o f new polymer p a r t i c l e s o c c u r s . T h i s p i c t u r e i s complicated by a c o n s i d e r a t i o n o f the c o l l o i d a l s t a b i l i t y o f the primary p a r t i c l e s (10). Even i f some new p a r t i c l e s are formed, they may not be observed because they are unstable and coagulate i n t o the l a r g e r p a r t i c l e s or drop lets ( l l ) . Thus the presence o f even small amounts o f e m u l s i f i e r i n the aqueous phase i s important i n s t a b i l i z i n g new polymer particles. Other f a c t o r s , such as monomer s o l u b i l i t y , the r a t e of i n i t i a t i o n , i o n i c s t r e n g t h , and d i f f u s i o n c o e f f i c i e n t s a l s o a f f e c t the r a t e at which r a d i c a l s are captured, and are d i s c u s s e d more f u l l y elsewhere {8 12). 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 o f the l i q u i d styrene mono mer emulsions has f o r the f i r s t time been obtained by OsOlj. s t a i n i n g p r i o r t o e l e c t r o n microscopy. An example i s shown i n F i g u r e k* The monomer i s s t a i n e d by a very r a p i d r e a c t i o n which probably i n v o l v e s the formation o f s t a b l e osmate monoesters and (polymeric) d i e s t e r s (13,14): 9

V II ,c

s

0

+

N

- c — o

^0

^os 0

0

I

• C - 0 I ' c — ο

χ

^0 0s

%

x

o

- C I —c

0 9^0- G 0s I o' 0— c

( 3

— 0 s 0

2

0

„0s^ 0 0 monester |

J 20s0

N

ι — C )

+ OsO^

v l

diester + 20s0

|

3

The r e a c t i o n i s over i n a few minutes and i s apparently q u a n t i tative. The mole r a t i o o f OsOl* to double bonds must be c a r e f u l l y c o n t r o l l e d to ensure complete r e a c t i o n w h i l s t l e a v i n g a minimum o f "excess" OsOl* which tends t o darken the background of the micrographs.


140


BEHAVIOR


total_SH_S a d d e d

200

400

600

800 time

1000

1200

1400

(minutes)

Figure 3. Kinetics of adsorption of sodium hexadecyl sulfate (SHS) at a con centration of 2.13 gl' of H 0 in the presence of varying amounts of hexadecanol (HD) in styrene emulsions, at 60°Ç; stirring rate, 600 rpm. M oh ratios of SHS:HD are shown for each curve. 1

2

Figure 4.

Osmium-stained styrene monomer emulsion. 213g Γ H 0 SHS with 3 X HD (4640X) 1

2


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ET AL.


141

I f these osmate e s t e r s remain i n contact with the aqueous environment l o n g enough, h y d r o l y s i s presumably occurs t o form among other decomposition p r o d u c t s , 0s02. Thus we have observed that i f the f i x e d styrene d r o p l e t s are allowed to stand f o r s e v e r a l hours at room temperature without separating the aqueous phase, subsequent e l e c t r o n microscopy shows no emulsion p a r t i c l e s and only a dense gray background, presumably due t o 0s0 . In the case o f a c r y l a t e and methacrylate e s t e r s , we have been unable t o o b t a i n s a t i s f a c t o r i l y s t a i n e d EM specimens o f the e m u l s i f i e d - monomer d r o p l e t s , presumably because o f a much more r a p i d h y d r o l y s i s of the more p o l a r intermediate osmate esters. Upon p o l y m e r i z a t i o n , 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 of the r e s u l t a n t polystyrene l a t e x i s almost i d e n t i c a l t o that of the o r i g i n a l monomer emulsion. For instance the styrene monomer emulsion shown i n F i g u r e k when polymerized with potassium p e r s u l f a t e i n i t i a t o r , produced the l a t e x shown i n F i g u r e 5· In a d d i t i o n t o the l a r g e r p a r t i c l e s , there i s seen a number of new smaller p a r t i c l e s formed we b e l i e v e by aqueous phase n u c l e a t i o n and subsequently s t a b i l i z e d by small amounts o f i o n i c e m u l s i f i e r remaining i n the aqueous phase. In the case o f MMA, i n the absence of e l e c t r o n micrographs o f the monomer emulsions, the evidence f o r "true" emulsion p o l y m e r i z a t i o n , although i n d i r e c t , i s c o n v i n c i n g . For i n s t a n c e , the three emulsions shown i n F i g u r e 2, when polymerized, y i e l d e d the polymer c o l l o i d s shown i n F i g u r e s 6, 7 and 8. The p a r t i c l e s i z e s of the l a t e x e s formed from the 1:3 and 1:1 MMA:H2Û phase r a t i o s are much smaller than the droplet s i z e s of the corresponding monomer emulsions as estimated by l i g h t microscopy. In c o n t r a s t , the 2:1 phase r a t i o emulsion produced d r a m a t i c a l l y l a r g e r average s i z e polymer p a r t i c l e s . Once again evidence f o r some aqueous phase n u c l e a t i o n can be observed.


2

9

Summary and Conclusions Mixed e m u l s i f i e r s can be used to o b t a i n w i t h low shear r a t e s extremely f i n e emulsions o f polymerizable monomers. For the f i r s t time 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 of styrene emulsions can be ob t a i n e d by OsOi| s t a i n i n g followed by e l e c t r o n microscopy. These systems e x h i b i t "true" emulsion p o l y m e r i z a t i o n i n that the emul s i f i e d monomer d r o p l e t s serve as the p r i n c i p a l l o c i o f p o l y m e r i z a tion. P a r t i c l e s i z e s o f the r e s u l t i n g l a t e x e s are g e n e r a l l y from about 250 nm t o 2000 nm i n diameter. The formation o f new, s m a l l er p a r t i c l e s by aqueous phase n c u e l a t i o n can be understood i n terms o f the theory o f homogeneous n u c l e a t i o n of polymer c o l l o i d s .




BEHAVIOR

Figure 5. Polystyrene latex formed from the monomer emul sion shown in Figure 4 (4640X)

Figure 6. PMMA latex formed from emulsion at phase ratio of MMA:H 0 = 1:3. 2g Γ H 0 SHS with 4 X OD (5300X). 1

2

2



AZAD

E T AL.


Figure 7. PMMA latex formed from emulsion at phase ratio MMA:H 0 = 1:1. 2g Γ H 0 SHS with 4 X OD (5300X). 1

2

2

Figure 8. PMMA latex formed from emulsion at phase ratio MMA:H 0 — 2:1. 2g Γ H 0 SHS with 4 X OD (5300X). 1

2

2


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Acknowledgement s T h i s work was supported i n p a r t by a F e l l o w s h i p t o A . R. M. Azad by the Norwegian Agency f o r I n t e r n a t i o n a l Development (NORAD), i n p a r t by a " F u l b r i g h t " Research Grant from the U . S . E d u c a t i o n a l Foundation i n Norway (RMF) and i n p a r t by the Royal Norwegian C o u n c i l f o r S c i e n t i f i c and I n d u s t r i a l Research (NTNF) f o r which the authors are most g r a t e f u l .


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J., F l a g s t a d , Η., Hansen, F . Κ., and Ellingsen, Sci., Symposium No. 42, (1973), 473. J. E l - A a s e r , M. S., and Vanderhoff, J. W . , Sci. Polym. L e t t . Ed., (1973), 11, 503. J., Hansen, F . Κ., Lange, S., Makromol. Chem.

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V o l d , R. D . , G r o o t , R. C., P r o c . 4 t h . Int · Congr. Surface A c t i v e Substance, V o l . 2, p. 1237, Gordon and Breach New York, 1967. Goddard, E . D . , Kung, H. C. Chem. S p e c i a l t i e s M f r . A s s o c . , P r o c . Ann. Meeting, (1965), 52, 124. V o l d , R. D . , Mittal, K. L., J. C o l l o i d I n t e r f a c e Sci. (1972), 38, 451. F i t c h , R. Μ., Brit. Polym. J., (1973), 5, 467. Smith, W. V . and Ewart, R. H., J. Chem. Phys. (1948),

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F i t c h , R. M . , Tsai, C. Η., in "Polymer C o l l o i d s , " R. M. Fitch, Ed p. 73 Plenum P r e s s , New York, 1971. Dunn, A . S., Chong, L. C.-H., Brit. Polym. J., (1970), 2 , 49. F i t c h , R. M . , S h i h , L . B., Kolloid-Z. Z. P o l y m . , i n p r e s s . Riemersa, J. C . , B i o c h i m . Biophys. A c t a , (1968), 152, 718. K o r n , E . D. J. Cell. Biol., (1967), 34, 627.


Colloidal Dispersions and Micellar Behavior

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