Emulsion Polymerization

Laboratory of Industrial Chemistry, The University of Trondheim,. The Norwegian ... Hexadecanol (HD) (Hyfatol 16, Aarhus Oliefabrik) and octadecanol (...
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1 Emulsification and Emulsion Polymerization of Styrene Using Mixtures of Cationic Surfactant and Long Chain Fatty Alcohols or Alkanes as Emulsifiers A. R. M. AZAD, J. UGELSTAD, R. M. FITCH, and F. K. HANSEN

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Laboratory of Industrial Chemistry, The University of Trondheim, The Norwegian Institute of Technology, N-7034 Trondheim-NTH, Norway

In some recent papers Ugelstad et al.(1,2,3) have reported results on the emulsion polymerization of styrene with a mixed emulsifier system consisting of Na hexadecyl sulphate and hexadecanol. Under given condi­ tions for emulsification and polymerization it was found that the monomer droplets became the main loci for initiation of polymerization. The first part of the present work describes emulsification experiments using a mixture of a cationic emulsifier, octadecyl pyridinium bromide (OPB) with n-fatty alcohols of different chain lengths, applying ordinary stirring equipment. In order to get a comparison of the monomer droplet size with that of the particles in the final latex, it was necessary to develop a method for obtaining electron microscope pictures of the monomer emulsions (4). Polymerizations of the monomer emulsions were carried out with o i l - s o l u b l e initiators. Oil-soluble i n i t i a t o r s have often been employed in emulsion poly­ merization recipes and are generally used in suspension polymerization. Whereas in the latter case the initi­ ation naturally takes place in the monomer droplets, the locus of initiation and growth of particles in emulsion polymerization with o i l - s o l u b l e i n i t i a t o r s has been open to some doubt. However, the fact that the particle size and size distribution is not very d i f f e ­ rent from the results with water-soluble i n i t i a t o r s and that the particles are generally much smaller than the droplets in the monomer emulsions indicates that with T h i s w o r k i s p a r t o f t h e t h e s i s b y A.R.M.A. a t t h e Norwegian I n s t i t u t e o f T e c h n o l o g y , Trondheim, Norway 1

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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ordinary emulsifier recipes initiation o f particles takes p l a c e mainly i nt h e aqueous phase. An a d d i t i o n a l p a r t d e s c r i b e s some r e s u l t s i n w h i c h t h e e m u l s i f i c a t i o n of t h e monomer i s c a r r i e d o u t w i t h a M a n t o n G a u l i n homogenizer. S o m e e x p e r i m e n t s we're a l s o c a r r i e d o u t where t h e hexadecanol was r e p l a c e d by hexadecane.

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Experimental

part

M a t e r i a l s : T h e s t y r e n e monomer was d i s t i l l e d t w i c e , the second time immediately p r i o r t o e m u l s i f i ­ cation. A s m a l l amount o f i n h i b i t o r (p-benzoquinone, 500 mg/kg monomer) w a s added t o a v o i d p o l y m e r i z a t i o n i n the e m u l s i f i c a t i o n experiments. OPB w a s s y n t h e s i z e d from o c t a d e c y l bromide ( p u r e , Koch L i g h t L a b . L t d . , England) and p y r i d i n e (pure, Merck, Germany); t h e p r o d u c t was washed f o u r times w i t h d r y e t h e r a n d r e c r y s t a l 1 i z e d t w i c e f r o m a c e t o n e , m.p. 7 5 ° C . Hexadecanol (HD)( H y f a t o l 1 6 , Aarhus O l i e f a b r i k ) and octadecanol (0D) ( p u r e , F l u k a , S w i t z e r l a n d ) were d i s ­ t i l l e d t w i c e i nvacuum. 2,2'-azobisisobutyronitrile ( A I B N ) ( p u r e , F l u k a ) , w a s c r y s t a l l i z e d t w i c e f r o m 96% ethanol. E i c o s a n o l , ES ( A r a c h i d i c a l c o h o l , p u r e Koch L i g h t L a b . L t d . , E n g l a n d ) , t e t r a d e c a n o l , TD ( p u r e , S c h u c h a r d t , M u n i c h ) , h e x a d e c a n e ( p u r i s , K o c h L i g h t Lab.) b e n z o y l p e r o x i d e , B P ('97% M e r c k ) , c u m e n e h y d r o p e r o x i d e , CHP ( 7 0 % i n c u m e n e , M e r c k - S c h u c h a r d t ) , o s m i u m t e t r a oxide (Merck) and d i o c t y l sodium s u l f o s u c c i n a t e (pure, Merck) were used w i t h o u t f u r t h e r p u r i f i c a t i o n . R e d i s t i l l e d water was used. Osmium t e t r o x i d e s t a i n i n g o f monomer e m u l s i o n . 0.01 c m o f t h e e m u l s i o n w a s d i l u t e d w i t h 0 . 5 c m w a t e r s a t u r a t e d w i t h s t y r e n e . To t h i s was added g r a d u a l l y w i t h m i x i n g a s a t u r a t e d s o l u t i o n o f OSO4 i n w a t e r p r e p a r e d i m m e d i a t e l y b e f o r e u s e . T h e a m o u n t o f OSO4 s o l u t i o n added was 0.13 c m , c o r r e s p o n d i n g t o a s t y r e n e :0sÛ4 m o l a r r a t i o o f 1 : 1 . 5 . A n i m m e d i a t e b l a c k e n i n g o f the emulsion took p l a c e . Samples (2-3 μΐ) were w i t h ­ drawn a f t e r 5 a n d 10 m i n u t e s p l a c e d o n a f o r m v a r a n d carbon-coated g r i d and a l l o w e d t o d r y . The g r i d s were examined i na Siemens e l e c t r o n microscope. Some experiments were c a r r i e d o u t i n which t h e 0s04:styrene molar r a t i o and time o f r e a c t i o n were v a r i e d . Inferior e l e c t r o n m i c r o s c o p e p i c t u r e s w e r e o b t a i n e d when t h e m o l a r r a t i o o f 0 s Û 4 : s t y r e n e w a s l e s s t h a n 1:1 o r h i g h e r than 2:1. P r o l o n g e d r e a c t i o n t i m e s g r e a t e r t h a n 30 minutes a l s o i n v a r i a b l y gave i n f e r i o r r e s u l t s . After d r y i n g on t h e g r i d s t h e p a r t i c l e s were s t a b l e and 3

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In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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ET AL.

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p r e p a r a t i o n s c o u l d be l e f t f o r a week w i t h o u t a n y n o t i c e a b l e change i n t h ee l e c t r o n m i c r o s c o p e p i c t u r e s . In some e x p e r i m e n t s t h e s t a i n e d p a r t i c l e s w e r e s h a d o w e d w i t h a 80/20 Pt/Pd a l l o y a t an a n g l e o f 30° t o a s c e r t a i n that t h ep a r t i c l e s were s p h e r i c a l . Apparatus

and procedure

The e m u l s i f i c a t i o n e x p e r i m e n t s w e r e c a r r i e d o u t i n a 500 cm glass vessel w i t h a paddle s t i r r e r f i t t e d w i t h thermometer, manometer and equipment f o r c h a r g i n g and s a m p l i n g . H o t w a t e r was p a s s e d t h r o u g h t h e o u t e r jacket o f t h ereactor. 0 Ρ Β , f a t t y a l c o h o l a n d H2O w e r e f i r s t mixed w i t h s t i r r i n g a t 70-80 °C, t h e t e m p e r a t u r e depending upon t h ec h a i n l e n g t h o f t h ea l c o h o l . After c o o l i n g t o 60 ° C , t h e s t y r e n e was added a n d t h e s t i r ­ r i n g continued a t 600 rpm. Samples were withdrawn a t i n t e r v a l s through a bottom stopcock and analysed f o r e m u l s i f i e r i n t h e aqueous phase a f t e r c e n t r i f u g a t i o n as d e s c r i b e d i na p r e v i o u s p a p e r (2) w i t h t h e e x c e p t i o n t h a t 0 . 0 0 2 M d i o c t y l sodium s u l f o s u c c i n a t e was used f o r t h e t i t r a ­ tion. From some o f t h e s a m p l e s o f t h e monomer emulsions e l e c t r o n m i c r o g r a p h s were o b t a i n e d i n the manner d e ­ s c r i b e d above. E m u l s i o n s f r o m t h e M a n t o n G a u l i n homog e n i z e r w e r e s u b s e q u e n t l y s t i r r e d a t 6 0 °C a n d a n a l y s e d i n t h e *ame m a n n e r . The p o l y m e r i z a t i o n e x p e r i m e n t s w e r e c a r r i e d o u t a s d e s c r i b e d p r e v i o u s l y {2). The i n i t i a t o r was d i s s o l v e d i n t h e monomer p r i o r t o e m u l s i f i c a t i o n . Samples were w i t h d r a w n t h r o u g h t h e bottom v a l v e i n t o 25 c m o f a short stop s o l u t i o n o f p-benzoquinone i nmethanol. The c o n v e r s i o n was d e t e r m i n e d by e v a p o r a t i o n o f t h e v o l a ­ t i l e s a t 60 °C. E l e c t r o n micrographs o f t h el a t e x p a r t i c l e s were obtained i n t h eusual way.

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Results

and discussion

E f f e c t o f η-fatty a l c o h o l a s a d d i t i v e : As found p r e v i o u s l y i n t h ea n i o n i c mixed e m u l s i f i e r systems w i t h f a t t y a l c o h o l s , t h e method o f p r e p a r i n g t h e emul­ s i o n s i s c r u c i a l when moderate s t i r r i n g i s employed. T h u s , i f t h ef a t t y a l c o h o l was added t o t h e monomer p r i o r t o m i x i n g w i t h an aqueous s o l u t i o n o f t h e e m u l s i ­ f i e r , only very coarse emulsions r e s u l t e d , which sepa­ rated within a few minutes. With t h ep r e s e n t method there took place a rapid e m u l s i f i c a t i o n , with t h e r e s u l t that a t moderate i n i t i a l e m u l s i f i e r c o n c e n t r a ­ t i o n s more than 9 7 . 5 % o f t h ee m u l s i f i e r was a d s o r b e d

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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w i t h i n 15-20 m i n u t e s . T a b l e I g i v e s some r e s u l t s o f t h e amount o f e m u l s i f i e r a d s o r b e d o n t h e d r o p l e t s a f t e r 15 m i n u t e s s t i r r i n g w i t h 2 g / d m H^O o f O P B a n d w i t h v a r i o u s m o l a r r a t i o s o f h e x a d e c a n o l (HD)t o OPB. I t a p p e a r s t h a t a HD:0PB r a t i o o f 2:1 i s s u f f i c i e n t t o bring about p r a c t i c a l l y t o t a l adsorption o f e m u l s i f i e r on t h e d r o p l e t s . F i g . 1 g i v e s t h e r e s u l t s o f some e x p e r i m e n t s w i t h OPB a n d h e x a d e c a n o l i n w h i c h t h e a m o u n t o f O P B a d s o r b e d on t h e d r o p l e t s w a s f o l l o w e d a s a f u n c t i o n o f t i m e a t 60 °C a n d w i t h s t i r r i n g a t 6 0 0 r p m . I t a p p e a r s t h a t i n a l l c a s e s t h e a m o u n t o f OPB a d s o r b e d o n t h e d r o p l e t s r a p i d l y reaches an optimal value. As t h e s t i r r i n g i s c o n t i n u e d t h e OPB i s g r a d u a l l y t r a n s p o r t e d b a c k t o t h e aqueous phase, i n d i c a t i n g a gradual degradation o f t h e emulsion. This d e g r a d a t i o n c o u l d be f o l l o w e d by e l e c ­ tron micrographs o f the emulsion a f t e r OSO4-Staining as d e s c r i b e d a b o v e . Fig. 2 gives an e l e c t r o n micrograph o f one o f t h e e m u l s i o n s a f t e r 15 m i n u t e s s t i r r i n g . The droplet s i z e s a r e i n t h e r a n g e o f 0.4 t o 1.5 ym. F i g . 3 s h o w s a n e l e c t r o n m i c r o g r a p h o f t h e same e m u l s i o n a f t e r 21 h s t i r r i n g a t 6 0 ° C . T h e d r a s t i c i n c r e a s e i nd r o p l e t s i z e i s c l e a r l y apparent. F i g . 4 g i v e s some r e s u l t s w i t h f a t t y a l c o h o l s o f different chain length. I t a p p e a r s t h a t w i t h t h e C-j^ f a t t y a l c o h o l t h e e m u l s i f i c a t i o n i spoor a n d t h e emulsion i s r e l a t i v e l y unstable. As t h e chain l e n g t h of t h e f a t t y a l c o h o l i si n c r e a s e d the s t a b i l i t y o f t h e emulsion gradually increases. With t h e Cpo a l c o h o l t h e amount o f e m u l s i f i e r a d s o r b e d on t h e d r o p l e t s a f t e r 20 h s t i r r i n g a t 6 0 ° C i s o n l y r e d u c e d f r o m 9 5 t o 9 0 %. F o r c o m p a r i s o n some e x p e r i m e n t s w e r e c a r r i e d o u t i n w h i c h t h e f a t t y a l c o h o l was d i s s o l v e d i n t h e monomer phase. The r e s u l t s o f these experiments are given i n T a b l e I I . I t appears t h a t i n t h i s case t h e amount o f OPB a d s o r b e d o n t h e d r o p l e t s a f t e r 1 5 m i n u t e s i s v e r y low. A f t e r 20 h s t i r r i n g t h e amount a d s o r b e d i s s t i l l o n l y c a . 50 % w h i c h , when compared t o t h e r e s u l t s i n F i g . 1, seems t o c o r r e s p o n d t o an " e q u i l i b r i u m " v a l u e at the given temperature and s t i r r i n g r a t e . The p r o n o u n c e d d e p e n d e n c e o f t h e d e g r e e o f e m u l s i ­ f i c a t i o n upon t h e order o f mixing w i t h f a t t y a l c o h o l s is not y e t s a t i s f a c t o r i l y explained. Several p o s s i b i l i t i e s may be a d v a n c e d . a ) D u r i n g t h e s t i r r i n g , i n r e g i o n s of high shearrate i nthe neighbourhood o f the s t i r r e r , t h e r e w i l l be formed c o n t i n u o u s l y small d r o p l e t s w h i c h , without s t a b i l i z a t i o n , will continuously coalesce with the l a r g e r d r o p l e t s inthe bulk o f the mixture. In the p r e s e n c e o f t h e m i x e d e m u l s i f i e r t h e s e d r o p l e t s may b e

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In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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AZAD E T AL.

Emulsion

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of Styrene

Total OPB added

ο CM

X

1.5

CO

ε •opietsi

en

1.0 A

Ό

3:1

1:1

>rbed

0.5

(Λ Ο

OPB

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s

0

200

A00

600 800 Time, minutes

1000

1200

U00

1800

1600

Figure 1. OPB adsorbed on the monomer droplets as a function of time at different molar ratios HD:OPB. Styrene — 83.3 g, H 0 — 250 g, OPB — 2.0 g/dm H 0. Temp. = 60° C. Stirring — 600 rpm. 2

Figure 2. Electron micrograph of monomer emulsion after 15 min stirring for HD:OPB. Molar ratio 6:1 (Figure I, curve E).

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Figure 3. Electron micrograph of mono­ mer emubion of Figure 2, after 21 hr stirring at 600 rpm and 60°C

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

EMULSION POLYMERIZATION

total OPB added

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Eicosanc il _^Octa decanol

Hexac leca nol

Tetradi canol

Time, minutes Figure 4. OPB adsorbed on the monomer droplets as a function of time for different long chain fatty alcohols. Styrene = 83.3 g, H O = 250 g, OPB = 2.0 g/dm H O. Temp. = 60°C. Stirrings600 rpm. Molar ratio fatty alcohoWPB = 3:1. s

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In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Table

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I . OPB a d s o r b e d o n t h e monomer d r o p l e t s a f t e r 1 5 minutes o f s t i r r i n g . HD d i s s o l v e d i n t h e water phase p r i o r t o e m u l s i f i c a t i o n . Temp. = 6 0 ° C . S t i r r i n g = 6 0 0 r p m . OPB = 2 . 0 g / d m H 0 , H 0 = 2 5 0 g . Styrene = 83.3 g. 3

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2

2

M o l a r r a t i o Amount o f OPB i n t h e aqueous p h a s e , g/dm^ w a t e r HD:0PB 1.88 0.21 0.06 0.05 0.06

0:1 1:1 2:1 3:1 4:1

Amount o f OPB a d s o r b e d on monomer d r o p l e t s . q/dm3 w a t e r 0.12 1.79 1.94 1.95 1.94

T a b l e I I . OPB a d s o r b e d o n s t y r e n e d r o p l e t s a f t e r d i f f e r e n t s t i r r i n g t i m e s w h e r e HD i s d i s s o l v e d i n t h e monomer Temp. = 6 0 ° C , S t i r r i n g = 600 rpm. OPB = 2.0 g/dnv H 0 . H 0 = 250 g . S t y r e n e = 8 3 . 3 g . 2

Molar r a t i o HD:0PD

2

Time o f Amount o f OPB i n s t i r r i n g t h e aqueous phase (min) g/dm w a t e r 15 1.82 1230 1.06 15 1.76 1330 0.95 3

4:1 10:1

Amount o f OPB a d s o r b e d o n monomer d r o p l e t s g/dm w a t e r 0.18 0.94 0.24 1.05 3

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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r a p i d l y covered with 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 , which w i l l prevent the coalescence f o r the time n e c e s s a r y f o r d i s p e r s i o n o f a l l t h e monomer to take p l a c e . b)During s t i r r i n g , f r e s h o i l - w a t e r i n t e r ­ faces a r e created by t h e paddle s t i r r e r . The alcohol and e m u l s i f i e r p r e s e n t i n t h e w a t e r p h a s e d i f f u s e r a p i d l y t o t h i s f r e s h l y formed i n t e r f a c e , r e s u l t i n gi n a momentary high c o n c e n t r a t i o n o f a l c o h o l a t t h e interphase. T h i s may c a u s e a local lowering o f t h e i n t e r f a c i a l t e n s i o n w h i c h may d r a s t i c a l l y f a c i l i t a t e t h e e m u l s i f i c a t i o n . As shown b y D a v i e s a n d Haydon ( 6 ) , a high c o n c e n t r a t i o n o f f a t t y a l c o h o l i n a d d i t i o n t o e m u l s i f i e r a t the i n t e r f a c e w i l l lower γ t o a p p r o x i m a t e l y z e r o v a l u e w h i c h may l e a d t o s p o n ­ taneous e m u l s i f i c a t i o n . D a v i e s a n d Haydon have added the f a t t y a l c o h o l t o t h e o i l phase p r i o r t o mixing with the water solution o f the e m u l s i f i e r . In this case, t h e r e f o r e , they had t o apply a r e l a t i v e l y very high concentration o f f a t t y a l c o h o l . c ) I t i s also possible that the development o f t r a n s i e n t i n t e r f a c i a l t e n s i o n g r a d i e n t s o n t h e monomer d r o p l e t s f o r m e d may f a c i l i t a t e emulsification with f a t t y alcohol present in t h e aqueous phase. Immediately a f t e r a droplet i s separated i n t o two d r o p l e t s , the i n t e r f a c i a l tension, γ, w i l l t e n d t o be h i g h e r a t t h e p o i n t s o f c l o s e s t a p p r o a c h than a t t h e more d i s t a n t p a r t s o f t h e i n t e r ­ faces. The ensuing gradient i n γ tends t o suck aqueous s o l u t i o n between t h e newly formed d r o p l e t s f o r c i n g them a p a r t a n d hence p r o v i d i n g them w i t h time to s t a b i l i z e themselves against coalescence a f t e rt h e i n t e r f a c i a l tension gradient has vanished ( 5 J . Fig. 5 gives electron micrographs o f latexes prepared w i t h AIBN a s i n i t i a t o r from e m u l s i o n s w i t h a c o n s t a n t styrene:HoO w e i g h t r a t i o = 1:3, a c o n s t a n t a m o u n t o f OPB a n d w i t h HD:0PB m o l a r r a t i o s o f 0, 1 : 1 , 3:1 a n d 4:1 r e s p e c t i v e l y . I n t h e f i r s t c a s e t h e l a t e x contains only small p a r t i c l e s with a r e l a t i v e l y 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 . W i t h a HD:0PB r a t i o o f 1:1 t h e p a r t i c l e s a r e s o m e w h a t l a r g e r a n d m o r e m o n o disperse. I n both cases t h e p a r t i c l e n u c l e a t i o n apparently takes place completely i nt h e aqueous phase. The l a r g e r p a r t i c l e s i n t h e s e c o n d c a s e p r o b a b l y s t e m from t h e f a c t t h a t t h e amount o f e m u l s i f i e r i n t h e water phase i slower, due t o t h e f a c t that a r e l a t i v e l y l a r g e p a r t i s a d s o r b e d o n t h e monomer d r o p l e t s . However there i s s t i l l enough e m u l s i f i e r present i nt h e aqueous phase t o b r i n g about p r a c t i c a l l y complete p a r t i c l e n u c l e a t i o n i n t h a t p h a s e . I n t h e c a s e s o f H D : 0 P B = 3:1 a n d 4 : 1 t h e s i t u a t i o n i s c o m p l e t e l y d i f f e r e n t . We h a v e in both cases a bimodal d i s t r i b u t i o n . The major part

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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AZAD E T AL.

Figure 5. Electron micrographs of final latexes with varying amounts of hexa­ decanol (HD). Styrene = 166.7 g, H 0 = 500 g, OPB = 2.0 g/dm H 0. Temp. = 60°C. A1BN — 1.0 g in 166.7 g styrene. Molar ratios ΕΌ.ΟΡΒ = (A): 0, (B): 1:1, (C): 3:1, and (D): 4:1. OPB remaining in the water phase after emulsification of monomer (A) —1.78, (B) = 0.35, (C) = 0.08 g/dm H 0. 3

2

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by w e i g h t c o n s i s t s o f p a r t i c l e s o f d i a m e t e r 0.4 t o 1.5 urn. By c o m p a r i n g w i t h t h e e l e c t r o n m i c r o g r a p h s o f t h e c o r r e s p o n d i n g m o n o m e r e m u l s i o n s i n F i g . 6, i t i s apparent t h a t these p a r t i c l e s stem from i n i t i a t i o n i n monomer d r o p l e t s . In addition, there are a consider­ a b l e n u m b e r o f p a r t i c l e s o f a b o u t 0 . 2 ym n o t p r e s e n t i n the monomer e m u l s i o n . These p a r t i c l e s t h e r e f o r e most probably stem from n u c l e a t i o n i n t h e aqueous phase. In F i g . 7 a r e g i v e n t h e r e s u l t s o f k i n e t i c m e a s u r e m e n t s o f t h e l a t e x e s g i v e n i n F i g . 5. I t a p p e a r s that i nthe case o f pure e m u l s i f i e r the k i n e t i c r e s u l t s a r e i n a g r e e m e n t w i t h common e x p e r i e n c e i n e m u l s i o n polymerization o f styrene with a water soluble i n i t i ­ ator. The r a t e i s a p p r o x i m a t e l y c o n s t a n t up t o high conversion. I n c a s e C a n d D, h o w e v e r , t h e s i t u a t i o n i s completely d i f f e r e n t . The i n i t i a l rate i s r e l a t i v e l y high a n d d e c r e a s e s s i g n i f i c a n t l y up t o about 30 % c o n v e r s i o n when i t s t a r t s i n c r e a s i n g s l o w l y . W i t h t h e r e a c t i o n t a k i n g p l a c e i n t h e monomer d r o p l e t s t h e r a t e should be g i v e n by r = k [M] p

M

(D

(

w h e r e LM]M i s t h e c o n c e n t r a t i o n o f m o n o m e r i n t h e d r o p l e t s , \X\\\ t h e c o n c e n t r a t i o n o f i n i t i a t o r i n t h e d r o p l e t s , Vjv| t h e t o t a l v o l u m e o f m o n o m e r d r o p l e t s p e r 1 H2O a n d r i s g i v e n i n m o l s e c " p e r 1 H 0 . W i t h l i t e r a t u r e values o f k = 300 1/mole-sec, k t = 1 0 1 / m o l e - s e c , k j = 1 . 0 x 1 0 " s e c " ' a n d [M]M = 8 . 4 m o l e / 1 , V = 0 . 3 8 3 1/1 H 0 , t h e c a l c u l a t e d i n i t i a l r a t e i s approximately 20 g PS/1 H 0 / h . T h e e x p e r i m e n t a l i n i t i a l r a t e i s f o u n d t o be 20 g PS/1 H 0 / h , i n good agreement with the c a l c u l a t e d value. In t h e b e g i n n i n g t h e r a t e d e c r e a s e s w i t h ^ i n c r e a s i n g conversion i naccordancewith e q u a t i o n ( 1 ) ([MJm d e ­ creases). Beyond 30 % c o n v e r s i o n t h e r a t e i n c r e a s e s s l i g h t l y , probably due t o a continuous decrease i n k . 1

2

8

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5

M

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t

peroxide gave r e s u l t s comparable t o those with AIBN: i n i t i a t i o n both i n monomer d r o p l e t s a n d i n t h e aqueous phase, a l t h o u g h t o v a r y i n g d e g r e e s , was o b s e r v e d , a s s h o w n i n F i g s . 8 a n d 9. U n d e r t h e s a m e c o n d i t i o n s 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 , benzoyl peroxide gave t h e h i g h e s t d e g r e e o f monomer d r o p l e t i n i t i a t i o n , w h i l e cumene h y d r o p e r o x i d e l e d t o more n u c l e a t i o n i n t h e aqueous phase. A more f i n e l y d i s p e r s e d monomer d r o p l e t emulsion could be achieved by i n c r e a s i n g t h e c o n c e n t r a t i o n o f

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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1.

Figure 7. Polymer formed as a function of time with varying amounts of HD for the experiments given in Figure 5. (Letters (A), (B), (C), and (D) refer to Figure 5.)

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Figure 8. Electron micrographs of monomer emulsion (A) and final latex (B) when benzoyl peroxide was used as the oil soluble initiator. Styrene = 166.7 g, H 0 = 500 g, HD:OPB — 4:1, OPB = 2.0 g/dm H 0. Temp. = 60°C. BP — 2.0 g in 166.7 g styrene. 2

3

2

Figure 9. Electron micrographs of monomer emulsion (A) and final latex (B) when cumene hydroperoxide was used as the oil soluble initiator. Styrene = 166.7 g, H 0 = 500 g, HD:OPB = 4:1, OPB = 2.0 g/dm H 0. Temp. = 60°C. CHP = 2.5 g in 166.7 g styrene. 2

3

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In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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the mixed e m u l s i f i e r system ( F i g . 1 0 ) . However, t h e monomer d r o p l e t s u r f a c e does n o t i n c r e a s e p r o p o r t i o n a ­ t e l y t o t h eamount o f e m u l s i f i e r added s o t h a t t h e con­ c e n t r a t i o n o f e m u l s i f i e r l e f t i n t h e aqueous phase a f t e r e m u l s i f i c a t i o n i s i n t h i s case i n c r e a s e d . I t turns o u tthat this f a c t leads t o that one gets a pre­ dominant n u c l e a t i o n i n t h e aqueous phase ( F i g . 1 1 ) . The r e s u l t s a r e i n a c c o r d a n c e w i t h p r e v i o u s r e s u l t s w i t h m i x e d e m u l s i f i e r s y s t e m s o f Na h e x a d e c y l sulphate a n d h e x a d e c a n o l w i t h I ^ S p O g a s i n i t i a t o r (2). Also i n the present case with o i l s o l u b l e i n i t i a t o r and with OPB i t i s t h e r e f o r e a n e c e s s a r y c o n d i t i o n f o r o b t a i n i n g a h i g h d e g r e e o f monomer d r o p l e t i n i t i a t i o n t h a t n o t o n l y a f i n e d i s p e r s i o n o f monomer i s a c h i e v e d b u t t h a t a t t h e same t i m e t h e c o n c e n t r a t i o n o f e m u l s i f i e r l e f t in t h e aqueous phase i svery low. E f f e c t o f hexadecane as a d d i t i v e : In a s e r i e s o f papers H a l l w o r t h and C a r l e s s (7,8,9,10) have i n v e s t i ­ gated t h e e f f e c t o f t h enature oT t h e i n t e r n a l phase on t h e s t a b i l i t y o f o i l i n w a t e r e m u l s i o n s a s w e l l a s the e f f e c t o f a d d i t i o n o f long chain f a t t y a l c o h o l s with sodium dodecyl sulphate o r sodium hexadecyl s u l p h a t e as t h ei o n i c e m u l s i f i e r . They found t h a t l i g h t petroleum and chlorobenzene emulsions prepared o n l y w i t h s o d i u m h e x a d e c y l s u l p h a t e w e r e much l e s s s t a b l e than those produced using t h elonger chain paraffins, white s p i r i t and light l i q u i d p a r a f f i n s . Most i n t e r e s t i n g l y ,however, they found t h a t a d d i t i o n of smal1 amounts o f l i g h t l i q u i d p a r a f f i n s t o t h e 1 i g h t petroleum o r chlorobenzene l e d t o an increase i n s t a b i 1 i t y which even surpassed t h a t which was o b t a i n e d w i t h 1 o n gc h a i n f a t t y a l c o h o l s . I t s h o u l d be noted , h o w e v e r , t h a t Hal 1 w o r t h e t a l . i n p r e p a r i ng t h e i r emulsions a p p l i e d t h eusual method o f a d d i t i o n o f t h e a d d i t i v e s t o t h e main component o f t h ei n t e r n a l phase before mixing with the water s o l u t i o n o f t h e anionic emulsifier. As shown above a n d d i s c u s s e d i n p r e v i o u s papers, t h i s procedure does n o t lead t o r a p i d e m u l s i ­ f i c a t i o n w i t h f a t t y a l c o h o l s when o r d i n a r y , m o d e r a t e s t i r r i n g i sapplied f o r preparing t h e emulsions. In order t o i n v e s t i g a t e t h ep o s s i b l e a p p l i c a t i o n s o f t h e r e s u l t s o f Hal 1 w o r t h a n d C a r l ess on e m u l s i o n polymeriz a t i o n we h a v e c a r r i e d o u t s o m e e x p e r i m e n t s w i t h h e x a ­ decane as a d d i t i v e . I tturned o u tthat with o r d i nary s t i r r i ng e q u i p m e n t , a d d i t i o n o f h e x a d e c a n e d i d n o t g i v e the r a p i d e m u l s i f i c a t i o n which c o u l d be o b t a i ned w i t h the 1ongchai η f a t t y a l c o h o l s . Therefore a s e r i e s o f e x p e r i m e n t s were c a r r i ed o u t i nwhich t h e e m u l s i o n s a f t e r p r e m i x i ng w e r e h o m o g e n i z e d w i t h a M a n t o n G a u l i η

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EMULSION

Figure 10. Electron micrographs of monomer emulsions prepared with varying concentrations of OPB. Styrene = 166.7 g, H 0 = 500 g. Temp. = 60°C. Molar ratio ΗΌ.ΟΡΒ = 3:1. OPB, = (A) 2.0, (B) 4.0, (C) 6.0, and (D) 8.0 g/dm H 0. OPB remaining in the water phase after emulsification of monomer, (A) = 0.10, (B) = 0.53, (C) = 1.12, and (D) — 3.14 g/dm H 0. 2

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In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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AZAD E T A L .

Figure 11. Electron micrographs of final latexes from the experiments given in Figure 10 when polymerized using AIBN as initiator. A1BN = 4.0 g in 166.7 g styrene. (Letters (A),(B),(C), and (D) refer to Figure 10.)

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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laboratory homogenizer. As expected* the order o f m i x i n g h a d no d e t e c t a b l e i n f l u e n c e o n the r e s u l t i n g e m u l s i o n s even w i t h l o n g c h a i n f a t t y a l c o h o l s , when such an e f f e c t i v e homogenizing d e v i c e was a p p l i e d . In F i g . 12 a r e g i v e n some r e s u l t s w h e r e t h e e f f e c t of hexadecane i s compared with that o f hexadecanol on the s t a b i l i t y o f the emulsions u s i n g the Manton Gaul i n h o m o g e n i z e r a n d w i t h t h e c a t i o n i c OPB e m u l s i f i e r . T h e f i g u r e a l s o i n c l u d e s a r e s u l t o f a n e x p e r i m e n t w i t h OPB without anya d d i t i v e , which as expected led t o a very unstable emulsion. As s h o w n , t h e a p p l i c a t i o n o f t h e h o m o g e n i z e r f o r p r e p a r i n g the emulsions d i d not lead t o a n y i n c r e a s e i n the s t a b i l i t y o f the emulsion w i t h hexadecanol as a d d i ­ t i v e as determined by m e a s u r i n g the amount o f adsorbed OPB a s a f u n c t i o n o f t i m e w i t h s t i r r i n g a t 6 0 ° C . A d d i t i o n o f hexadecane leads t o an extremely s t a b l e emulsion even a t the r e l a t i v e l y severe c o n d i t i o n s o f s t i r r i n g a t 60 °C. I n f a c t , the emulsion w i t h hexa­ decane a s a d d i t i v e i s even more s t a b l e than the o n e obtained with n-eicosanol. In F i g s . 1 3 , 1 4 , 15 a n d 16 a r e g i v e n e l e c t r o n micrographs o f the emulsions with hexadecanol and hexa­ decane i m m e d i a t e l y a f t e r p r e p a r a t i o n a n d a f t e r a b o u t 20 h r s s t i r r i n g a t 6 0 ° C . When c o m p a r i n g F i g . 1 3 a n d F i g . 2, b o t h w i t h h e x a d e c a n o l , i t appears that the a p p l i c a ­ t i o n o f the homogenizer has r e s u l t e d i nthe formation o f a l a r g e r number o f s m a l l d r o p l e t s i n the range o f 0 . 2 - 0 . 3 urn. T h i s d o e s n o t , h o w e v e r , l e a d t o a m o r e stable emulsion. A f t e r 20 h s t i r r i n g a t 6 0 °C a l l t h e small d r o p l e t s have disappeared and the e l e c t r o n micro­ graph o f the emulsion, F i g . 14, i s r a t h e r s i m i l a r t o t h e o n e shown i n F i g . 3. T h i s i s i n a g r e e m e n t w i t h t h e r e s u l t s o f t h e m e a s u r e m e n t s o f a d s o r p t i o n o f OPB ( F i g . 12). With hexadecane the e l e c t r o n micrographs of the e m u l s i o n i m m e d i a t e l y a f t e r p r e p a r a t i o n ( F i g . 1 5 ) show a p p r o x i m a t e l y t h e same s i z e a n d s i z e d i s t r i b u t i o n a s obtained with hexadecanol. With hexadecane, however, the e l e c t r o n micrograph taken a f t e r 23 h s t i r r i n g a t 60 °C ( F i g . 1 6 ) , r e v e a l s t h a t t h e s m a l l d r o p l e t s t o a large extent are s t i l l present and that only a r e l a ­ t i v e l y smal1 number o f 1 a r g e r d r o p l e t s i n the range o f 1 ym h a v e b e e n f o r m e d . F i g . 17 g i v e s r e s u l t s o f a p o l y m e r i z a t i o n e x p e r i ment w h e r e A I B N w a s a d d e d t o t h e monomer b e f o r e homo­ g e n i z i n g the mixture. The e l e c t r o n micrograph o f the l a t e x s h o u l d be compared w i t h t h a t o f t h e monomer ( F i g . 15). I t a p p e a r s t h a t we a g a i n h a v e i n i t i a t i o n b o t h i n the monomer d r o p l e t s a n d i n t h e aqueous p h a s e .

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Total OPB added

800 Time,

1600 minutes

3200

2400

Figure 12. OPB adsorbed on the monomer droplets as a function of time for pure OPB, OPB + hexadecanol (HD), and OPB + hexadecane as emulsifier, homogenized with the Manton Gaulin homogenizer and afterwards stirred at 60°C using the paddle stirrer (600 rpm). Styrene — 333.8 g/dm H 0, OPB = 2.0 g/dm H 0. Temp. = 60°C. Molar ratio hexadecanohOPB — 4:1, hexa­ decane :OP Β = 4:1. 3

3

2

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Figure 13. Electron micrograph of monomer emuhion of Figure 12 with OPB + hexadecanol (HD) immediately after homogenization

Figure 14. Electron micrograph of mono­ mer emulsion of Figure 13 after 21 hr stirring at 60°C and 600 rpm

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Figure 15. Electron micrograph of monomer emulsion of Figure 12 with OPB + hexadecane immediately after homogenization

POLYMERIZATION

Figure 16. Electron micrograph of monomer emulsion of Figure 15 after 23 hr of stirring at 60°C and 600 rpm

Figure 17. Electron micrograph of final latex from an emuhion prepared as the one in Figure 15 with A1BN added to the styrene before homogenization, homogenized, and polymerized. Styrene = 250 g, H 0 = 750 g, OPB — 2.0 g/dm H O. A1BN = 6.0 g in 250 g styrene. Molar ratio hexadecane:OPB = 4:1. Temp. = 60°C. 2

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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1.

AZAD E T AL.

Emulsion

Polymerization

of

Styrene

19

H a l l w o r t h and C a r l e s s (H)) d i s c u s s s e v e r a l p o s s i b i l i t i e s f o r t h ee f f e c t o f l i g h t l i q u i d p a r a f f i n on t h e s t a b i l i t y o f e m u l s i o n s w i t h l i g h t p e t r o l e u m o r chlorobenzene as the main components. They seem t o p r e f e r an e x p l a n a t i o n p r e v i o u s l y advanced by them a n d several other authors f o r the e f f e c t o f f a t t y a l c o h o l , namely t h a t t h ei n c r e a s e d s t a b i l i t y i sdue t o t h e formation o f an i n t e r f a c i a l complex between t h e a d d i ­ t i v e and sodium hexadecyl sulphate. The condenced mixed f i l m w i l l r e s i s t coalescence p r i m a r i l y by v i r t u e of i t s r h e o l o g i c a l p r o p e r t i e s . With mixed f i l m s o f t h e present t y p e , t h eimportance o f t h ef i l m v i s c o e la s t i c i ty l i e s i n i t s a b i l i t y t o maintain e l e c t r i c a l repulsion between approaching d r o p l e t s by preventing l a t e r a l d i s ­ placement o f t h eadsorbed i o n s . The e f f e c t i v e paraff i n i e o i l has chains a t l e a s t as long as those o f t h e a l k y ! s u l p h a t e a n d w i l l b e a s s o c i a t e d b y v a n d e r Waal s' f o r c e s with t h ehydrocarbon chain o f the alkyl sulphate at t h e i n t e r f a c e . Davies a n d Smith (11) measured t h es t a b i l i t y o f a s e r i e s o i l i nwater emuTsions i n c l u d i n g benzene and hexane as the main component o f t h ei n t e r n a l phase and w i t h a d d i t i o n o f small amounts o f hexadecane and long c h a i n f a t t y a l c o h o l s . They measured t h echange i n d r o p l e t s i z e with time and found a remarkable increase of t h es t a b i l i t y by a d d i t i o n o f hexadecane which was f a r more e f f e c t i v e than h e x a d e c a n o l . Davies and Smith reject t h e explanation o f Hallworth and Carless o f t h e e f f e c t o f hexadecane on t h es t a b i l i t y o f hexane emul­ sions on thermodynamic grounds. The a c t i v i t y c o e f f i ­ c i e n t o f an a l k a n o l i n a l k a n e i s i n t h er e g i o n o f 15 a t 25 ° C , a n d i s a p p r o x i m a t e l y i n d e p e n d e n t o f c h a i n length. The n o n - i d e a l i t y o f alcohol as solutes i n alkanes c a n t h e r e f o r e be a t t r i b u t e d e n t i r e l y t o t h e hydroxy! group. S i m i l a r l y , t h ea d s o r p t i o n o f a l k a n o l s f r o m t h e a l k a n e p h a s e t o t h e o/w i n t e r f a c e i s d o m i n a t e d by t h e c h a n g e i n t h e e n v i r o n m e n t o f t h e h y d r o x y ! g r o u p and n o t t o t h e m e t h y l e n e g r o u p s . On t h e o t h e r h a n d , the long c h a i n alkane (Ci5) w i l l behave almost ideally i n a s h o r t e r c h a i n o i l (C5) a n d t h e r e s h o u l d be no tendency for t h elonger chain f r a c t i o n t o concentrate at t h e i n t e r f a c e . Davies and Smith suggest t h a t the e f f e c t o f a d d i t i o n o f small amounts o f hexadecane on s t a b i l i t y may b e d u e t o a p r e v e n t i o n o f e m u l s i o n d e g r a d a t i o n b y molecular diffusion. This approach t o emulsion i n ­ s t a b i l i t y was f i r s t presented by H i g u c h i and M i s r a (12), and was b a s e d o n t h e f a c t t h a t s m a l l d r o p l e t s w i l l demonstrate deviations i nphysical properties as compared t o l a r g e r d r o p l e t s o r plane s u r f a c e s .

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

EMULSION

20

for

For the case o f an o i l d r o p l e t low o i l solubilities:

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C

- C

r

i n water one has

exp (2 M)/(r RT)

œ

Y

POLYMERIZATION

(2)

P

where C i s the s o l u b i 1 i t y o f d r o p l e t s o f radius r , C the s o l u b i 1 ity o f an i n f ini t e l y 1arge d r o p l e t , M is the molecular weight o f the o i l , γ i s the i n t e r f a c i a l t e n s i o n , ρ the d e n s i t y o f the o i l , R the g a s constant and Τ t h e a b s o l u t e t e m p e r a t u r e . The increase i n solu b i l i t y w i t h d e c r e a s i n g r wi11 make t h e s m a l 1 d r o p l e t s t h e r m o d y n a m i c a l l y more u n s t a b l e wi t h r e s p e c t t o t h e 1arger ones. The r a t e o f d i s s o l u t i o n o r g r o w t h o f a d r o p l e t can be e x p r e s s e d a s (3)

G = 47rDr(C -C ) s

0

w h e r e Ce i s t h e c o n c e n t r a t i o n o f o i 1 i n t h e w a t e r p h a s e i n e q u i l i b r i urn w i t h t h e d r o p l e t , C i s t h e concentra­ t i o n a t some d i s t a n c e f r o m t h e d r o p l e t , 1 a r g e a s com­ pared t o r ,and D i s the d i f f u s i o n c o e f f i c i e n t o f t h e oi1 in water. Because the r a t e o f growth o r d i s s o l u ­ t i o n o f a d r o p l e t must be equal t o i t s r a t e o f change o f m a s s , o n e a l s o may w r i t e 0

6

= 4wr p 2

(4)

dr/dt

Higuchi andMisra consider a bimodal emulsion w i t h n d r o p l e t s o f r a d i us r a n d n ] d r o p l e t s o f r a d i u s r]. From the above e q u a t i o n s a n d mass b a l a n c e conside­ r a t i o n s the r a t e o f change o f the smal1 s i z e d d r o p l e t s ( r-| ) w a s f o u n d t o b e : 2

2

(5) where Κ = (2yM)/(pRT). The r a t e o f c h a n g e w i 1 1 b e d i r e c t l y p r o p o r t i o n a l to the s o l u b i 1 i t y o f the o i l . M o r e o v e r , i t s h o u l d be noted t h a t i f the r a d i u s o f the d r o p l e t i s decreased ten f o l d the r a t e o f change w i l l occur a thousand times f a s t e r . H i g u c h i a n d M i s r a have a l s o t r e a t e d the more complex s i t u a t i o n o f a polydi sperse emulsion. Davi s a n d Smith f i n d that the equation developed f o r degradation by t h e m o l e c u l a r d i f f u s i o n r o u t e a s e x p r e s s e d b y E q . (5) may e x p l a i n t h e r a t e o f d e g r a d a t i o n o f d i f f e r e n t emulsions and the e f f e c t o f the s o l u b i 1 i t y o f t h e

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

1.

AZAD E T A L .

Emulsion

Polymerization

of

21

Styrene

i n t e r n a i phase on t h es t a b i l i t y o f the emulsion. Higuchi and Misra a l s o c o n s i d e r the e f f e c t o f a d d i t i o n o f a s m a l 1 q u a n t i t y o f a n a d d i t i ve w h i c h i s c o n s i d e r ­ a b l y 1 ess water s o l u b l e than the main c o n s t i t u e n t o f the i n t e r n a l phase. F o r a t w o c o m p o n e n t c a s e o n e may w r i t e : C

xW

" x xL

k

C

e

x

P ( V >

C

zW

» z zL

k

C

e

x

P ( V

r

6

7

>



where C i s t h e e q u i 1 i b r i urn c o n c e n t r a t i o n o f t h e m a i η component! χ i nw a t e r wi t h a d r o p l e t s i z e r , C l i s the c o n c e n t r a t i o n i n the d r o p l e t , C w and C l a r e t h e c o r r e s p o n d i ng v a l u e s f o r t h e mi n o r c o m p o n e n t ζ,

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w

x

z

z

Κ

χ

= 2 V /RT, Y

X

K

z

= 2 V /RT Y

Z

(8)

k and k a r e t h er e s p e c t i v e di s t r i b u t i o n c o e f f i c i e n t s , and V a n d V a r e t h e r e s p e c t i v e p a r t i a l m o l a l v o l u m e s of t h ecomponents χ and ζ i n the d r o p l e t . I f k z