Morphology of Two-Stage Latex Particles - ACS Symposium Series

25 Morphology of Two-Stage Latex Particles Polystyrene and Styrene-Butadiene Copolymer Pair Systems

Downloaded by PENNSYLVANIA STATE UNIV on June 15, 2013 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch025

D. I. LEE Designed Latexes & Resins, Michigan Division, Dow Chemical Company, Midland, MI 48640 Staged emulsion polymerization produces heterogeneous structured latexes which exhibit a wide variety of properties depending on their particle morphology. It is thus extremely important for the design of structured latex products to understand the factors controlling the particle morphology. In addition to this technological importance, a better understanding on the morphological aspect of latex particles may shed some light on the mechanism of emulsion polymerization as well as polymer mixing on a microscopic scale. Williams and his co-workers (1-9) proposed a core-shell morphology for two-stage emulsion polymerization, based on a monomer-rich shell model. Gardon (10, 11) argued against a monomer-rich shell model, and suggested the possibility of surface polymerization due to the fast diffusion of monomer molecules. Napper (12) also argued against a monomer-rich shell on the basis of diffusion theory, but explained the core-shell formation by means of the surface-active oligomeric radicals in the absence of chain transfer activity. They all agreed on the possibility of surface polymerization, but the question remained whether surface polymerization alone would be sufficient for a core-shell morphology in two-stage emulsion polymerization over a wide range of variations in polymerization conditions, polymer compatibility, etc. In this study, the morphology of two-stage (styrene//styrene-butadiene) latex particles was investigated as a function of polymer phase ratio (stage ratio), molecular weights (chain transfer agent level), polymer compatibility (butadiene level in S-B copolymers), polymerization sequence (polystyrene first and S-B copolymer second or vice versa), and polymerization conditions (flooded or starved, polymerization temperature, etc.). This paper describes the preparation of two-stage latex samples and the morphological characterization of their particles by electron microscopy.

0097-6156/81/0165-0405$05.00/0 © 1981 American Chemical Society

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

406

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Experimental


Materials. S t y r e n e a n d b u t a d i e n e monomers w e r e p o l y m e r i z a t i o n g r a d e , a v a i l a b l e f r o m Dow C h e m i c a l Company. A c r y l i c a c i d was a t e c h n i c a l g r a d e monomer f r o m Dow B a d i s c h e . The p o l y m e r i z a t i o n s u r f a c t a n t was s o d i u m d o d e c y l d i p h e n y l o x i d e s u l f o n a t e a v a i l a b l e f r o m Dow. The p o l y m e r i z a t i o n i n i t i a t o r was s o d i u m p e r s u l f a t e , a n d bromoform a n d c a r b o n t e t r a c h l o r i d e t h e c h a i n transfer agents. P r e p a r a t i o n o f L a t e x Samples. Two-stage l a t e x samples were p r e p a r e d by e m u l s i o n p o l y m e r i z a t i o n o f t h e s e c o n d - s t a g e monomer mix i n t h e p r e s e n c e o f t h e f i r s t - s t a g e polymer l a t e x . The f i r s t s t a g e l a t e x e s w e r e e i t h e r i n - s i t u o r s e p a r a t e l y made u s i n g a n e x t e r n a l l y prepared p o l y s t y r e n e l a t e x seed. The mode o f p o l y m e r i z a t i o n was a s e m i - c o n t i n u o u s p r o c e s s f o r b o t h s t a g e s . A s e r i e s of l a t e x samples w i t h v a r y i n g stage r a t i o s (LS-1 t o LS-4) was p r e p a r e d a t 90°C by e m u l s i o n p o l y m e r i z i n g s t y r e n e a n d b u t a d i e n e monomer m i x e s (S/B/AA/CHBr3: 59/40/1/0.5) i n t h e p r e s e n c e o f p o l y s t y r e n e l a t e x (S/AA: 9 9 / 1 ) . A c r y l i c a c i d (AA) was used f o r l a t e x s t a b i l i t y . The s t a g e r a t i o s v a r i e d f r o m 40/60 t o 10/90. B o t h s t a g e monomer m i x e s were c o n t i n u o u s l y added o v e r 4 h o u r s , r e s p e c t i v e l y , b a s e d o n 100 p a r t s monomer m i x . One-hour cook-down was p r o v i d e d b e t w e e n t h e s t a g e s , r e g a r d l e s s o f t h e s t a g e r a t i o v a r i a t i o n . The aqueous m i x c o n t a i n i n g s o d i u m p e r s u l f a t e (0.7 p a r t s ) , s o d i u m d o d e c y l d i p h e n y l o x i d e s u l f o n a t e ( 1 p a r t ) , and s o d i u m h y d r o x i d e ( 0 . 0 5 p a r t ) was c o n t i n u o u s l y added o v e r 6 1/2 h o u r s w i t h o u t a n y i n t e r r u p t i o n between t h e s t a g e s . The f i n a l p a r t i c l e s i z e s were 1500 A and t h e f i n a l p o l y m e r s o l i d s were about 47%. Two l a t e x s a m p l e s ( L S - 5 and L S - 6 ) were p r e p a r e d i n t h e same manner a s d e s c r i b e d a b o v e e x c e p t t h a t 5 p a r t s o f c a r b o n t e t r a c h l o r i d e were added i n b o t h t h e f i r s t a n d second s t a g e monomer m i x e s , b a s e d o n 100 p a r t s monomer m i x . The s t a g e r a t i o s w e r e 50/50 and 20/80, r e s p e c t i v e l y . A s e r i e s o f l a t e x s a m p l e s w i t h v a r y i n g S/B r a t i o s i n t h e second s t a g e ( L S - 7 , LS-8 a n d L S - 9 ) was p r e p a r e d a t 90°C, u s i n g an e x t e r n a l l y p r e p a r e d p o l y s t y r e n e l a t e x o f 1350 A. The s e c o n d - s t a g e S/B r a t i o s w e r e 70/30, 90/10, and 9 5 / 5 , r e s p e c t i v e l y . The s t a g e r a t i o was 20/80. N e i t h e r v i n y l a c i d n o r c h a i n t r a n s f e r a g e n t was u s e d . Two l a t e x s a m p l e s ( L S - 1 0 and LS-11) were p r e p a r e d by s w i t c h i n g t h e o r d e r o f monomer a d d i t i o n b e t w e e n a s t y r e n e m i x (S/AA: 98/2) and a h i g h b u t a d i e n e m i x (S/B/AA: 28/70/2) a t t h e s t a g e r a t i o o f 50/50. The p o l y m e r i z a t i o n t e m p e r a t u r e was 80°C d u r i n g t h e c o n t i n u o u s a d d i t i o n o f monomer m i x e s and t h e n r a i s e d t o 100°C f o r o n e - h o u r cook-down. A l l t h e l a t e x samples a r e l i s t e d i n Table I .


25. LEE

Morphology

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Latex

407

Particles

Table I .


The L i s t o f L a t e x S a m p l e s Latex Samples

Stage R a t i o s 1 s t Stage/2nd Stage

Compositions 1st Stage 2nd S t a g e

LS-1

99S/1AA

59S/40B/1AA/0.5

CHBr3

40/60

LS-2

99S/1AA

59S/40B/1AA/0.5

CHBr3

30/70

LS-3

99S/1AA

59S/40B/1AA/0.5 CHBr3

20/80

LS-4

99S/1AA

59S/40B/1AA/0.5

10/90

CHBr3

LS-5

100S/5 C C U

60S/40B/5 C C U

50/50

LS-6

100S/5 C C U

60S/40B/5 C C U

20/80

LS-7

100S

70S/30B

20/80

LS-8

100S

90S/10B

20/80

LS-9

100S

95S/5B

20/80

LS-10

98S/2AA

28S/70B/2AA

50/50

LS-11

28S/70B/2AA

98S/2AA

50/50

S = Styrene,

Β = B u t a d i e n e , AA = A c r y l i c

Acid


408

E M U L S I O N P O L Y M E R S AND

EMULSION

POLYMERIZATION

E l e c t r o n Microscopy. For m o r p h o l o g i c a l c h a r a c t e r i z a t i o n e l e c t r o n m i c r o s c o p y was e x t e n s i v e l y u s e d i n c o n j u n c t i o n w i t h t h e osmium t e t r a o x i d e s t a i n i n g m e t h o d .


Results F i g u r e s 1A and 1A* a r e t h e t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s a t 0s0i+-stained two-stage ( s t y r e n e / / s t y r e n e - b u t a d i e n e ) l a t e x p a r t i c l e s a t t h e s t a g e r a t i o o f 40/60 ( L S - 1 ) and t h e i r u l t r a t h i n c r o s s - s e c t i o n s , r e s p e c t i v e l y . I t i s c l e a r from the m i c r o g r a p h s t h a t when s t y r e n e and b u t a d i e n e monomers w e r e p o l y m e r i z e d i n the presence of p o l y s t y r e n e l a t e x p a r t i c l e s , the second-stage S-B c o p o l y m e r p h a s e - s e p a r a t e d a s m i c r o d o m a i n s w i t h i n t h e f i r s t s t a g e p o l y s t y r e n e p h a s e . F i g u r e s I B and I B show 0 s 0 i * - s t a i n e d t w o - s t a g e l a t e x p a r t i c l e s a t t h e s t a g e r a t i o o f 30/70 ( L S - 2 ) and t h e i r u l t r a - t h i n c r o s s - s e c t i o n s , r e s p e c t i v e l y . At t h i s stage r a t i o , t h e s e c o n d - s t a g e S-B c o p o l y m e r s a r e s t i l l d i s p e r s e d i n t h e p o l y s t y r e n e p h a s e , however, some p a r t i c l e s a p p e a r t o be made up o f two c o n t i n u o u s p o l y s t y r e n e and S-B c o p o l y m e r p h a s e s . F i g u r e 1C shows OsOi+-stained t w o - s t a g e l a t e x p a r t i c l e s a t t h e s t a g e r a t i o o f 20/80 ( L S - 3 ) . A t t h i s s t a g e r a t i o , t h e s e c o n d - s t a g e S-B c o p o l y m e r became a c o n t i n u o u s p h a s e . T h i s s u g g e s t s t h a t p h a s e i n v e r s i o n t o o k p l a c e between t h e s t a g e r a t i o s o f 30/70 and 20/80. I t i s i n t e r e s t i n g t o n o t e t h a t t h i s p h a s e i n v e r s i o n may h a v e been a r e s u l t of t h e c l o s e p a c k i n g o f S-B c o p o l y m e r m i c r o d o m a i n s . F i g u r e ID i s the t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h of OsOtt-stained two-stage l a t e x p a r t i c l e s a t t h e s t a g e r a t i o o f 10/90 ( L S - 4 ) . T h i s m i c r o g r a p h c l e a r l y shows an o f f - c e n t e r e d p o l y s t y r e n e p h a s e e n c a p s u l a t e d w i t h t h e S-B c o p o l y m e r . F i g u r e 2A shows t h e u l t r a - t h i n c r o s s - s e c t i o n s o f O s O i t - s t a i n e d two-stage (styrene//styrene-butadiene) l a t e x p a r t i c l e s at the s t a g e r a t i o o f 50/50 ( L S - 5 ) . F i g u r e 2B i s t h e u l t r a - t h i n c r o s s s e c t i o n o f O s O ^ - s t a i n e d l a t e x f i l m made f r o m LS-6 ( s t a g e r a t i o = 20/80). U n l i k e t h e m i c r o - p h a s e s e p a r a t i o n s t r u c t u r e shown i n F i g u r e s 1A and 1 A , t h e s e t w o - s t a g e l a t e x p a r t i c l e s p r e p a r e d w i t h h i g h l e v e l s o f c a r b o n t e t r a c h l o r i d e a r e composed o f two d i s t i n c t p o l y m e r p h a s e s , s u g g e s t i n g a c o m p l e t e p h a s e s e p a r a t i o n due t o t h e h i g h e r m o b i l i t i e s o f p o l y m e r m o l e c u l e s i n v o l v e d . F i g u r e 2A shows a hemispherical s t r u c t u r e at the equal stage r a t i o . F i g u r e 2B s u g g e s t s t h a t as t h e amount o f t h e s e c o n d - s t a g e S-B c o p o l y m e r i n c r e a s e d a b o v e t h e e q u a l s t a g e r a t i o , t h e S-B c o p o l y m e r began t o e n c a p s u l a t e t h e f i r s t - s t a g e p o l y s t y r e n e , r e s u l t i n g i n an o f f c e n t e r e d p o l y s t y r e n e p h a s e i n t h e c o n t i n u o u s S-B c o p o l y m e r p h a s e . F i g u r e s 3A, 3B, and 3C show t h e u l t r a - t h i n c r o s s - s e c t i o n s o f 0s0i+-stained two-stage ( s t y r e n e / / s t y r e n e - b u t a d i e n e ) l a t e x p a r t i c l e s a t t h e s t a g e r a t i o o f 20/80, whose S/B r a t i o s i n t h e s e c o n d s t a g e a r e 70/30 ( L S - 7 ) , 90/10 ( L S - 8 ) , and 95/5 ( L S - 9 ) , r e s p e c t i v e l y . I t c a n be s e e n f r o m t h e m i c r o g r a p h s t h a t t h e s i z e o f p o l y s t y r e n e p h a s e domains d e c r e a s e s w i t h d e c r e a s i n g b u t a d i e n e l e v e l i n t h e s e c o n d - s t a g e S-B c o p o l y m e r s and becomes so s m a l l a t t h e S/B r a t i o 1

1



25.

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Morphology

of Two-Stage

Latex

Particles

409

Figure 1. Transmission electron micrographs of OsO -stained two-stage (S//S-B) latex particles (A, B, C, and D) and their ultrathin cross sections (A' and B') showing the effect of stage ratio on morphology ((A and A' 40/60 (LS-1); (B and B') 30/70 (LS-2); (C) 20/80 (LS-3; and (D) 10/90 (LS-4)) k



410

EMULSION POLYMERS

AND EMULSION POLYMERIZATION

Figure 2. Ultrathin cross sections of OsO stained two-stage (S//S-B) latex par ticles at the stage ratios of 50/50 and 20/80, respectively, snowing the effect of molecular weight ((A ) LS-5, (B) LS-6) r

Figure 3. Ultrathin cross sections of ΟsO^-stained two-stage (S//S-B) latex particles at the stage ratio of 20/80 showing the effect of polymer compatibility ((A) 30 parts butadiene (LS-7); (B) 10 parts butadiene (LS-8); and (C) 5 parts butadiene (LS-9))



25.

LEE

Morphology

of Two-Stage

Latex

Particles

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o f 95/5 t h a t p h a s e s e p a r a t i o n i s no l o n g e r d i s c e r n a b l e by t h e p r e s e n t e l e c t r o n m i c r o s c o p y method. F i g u r e s 4A a n d 4B a r e t h e u l t r a - t h i n c r o s s - s e c t i o n s o f OsOi+s t a i n e d two-stage (styrene//styrene-butadiene) and ( s t y r e n e butadiene//styrene) l a t e x p a r t i c l e s a t t h e s t a g e r a t i o o f 50/50 (LS-10 and L S - 1 1 ) , r e s p e c t i v e l y . L a t e x s a m p l e s w e r e m i x e d w i t h a p o l y m e r i z a b l e monomer m i x o f b u t y l and m e t h y l m e t h a c r y l a t e s , c u r e d , and m i c r o t o m e d f o r e x a m i n a t i o n . F i g u r e 4A shows p a r t i c l e c r o s s - s e c t i o n s much s m a l l e r t h a n t h e a c t u a l p a r t i c l e s i z e o f L S - 1 0 . I t a p p e a r s t h a t s i n c e t h e embedding monomer s o l u t i o n was a s o l v e n t f o r p o l y s t y r e n e , t h e c o n t i n u o u s p o l y s t y r e n e p h a s e was d i s s o l v e d and s m a l l S/B c o p o l y m e r m i c r o d o m a i n s w e r e l e f t b e h i n d . This i s f u r t h e r e v i d e n c e t h a t t h e s e c o n d - s t a g e S-B c o p o l y m e r s p h a s e separated a s microdomains w i t h i n the f i r s t - s t a g e p o l y s t y r e n e p h a s e , a s shown i n F i g u r e s 1A a n d 1 A . F i g u r e 4B shows somewhat s w o l l e n and d e f o r m e d p a r t i c l e c r o s s - s e c t i o n s , s u g g e s t i n g t h a t t h e f i r s t - s t a g e c r o s s - l i n k e d S-B c o p o l y m e r s were a c o n t i n u o u s p h a s e . I n d e e d , t h e f o r m e r (LS-10) behaved l i k e a h a r d l a t e x , b u t t h e l a t t e r (LS-11) behaved l i k e a s o f t l a t e x . The m o r p h o l o g y o f t w o - s t a g e l a t e x p a r t i c l e s was g r e a t l y a f f e c t e d by v a r i a t i o n s i n p o l y m e r i z a t i o n c o n d i t i o n s : e.g., batch v s . s e m i - c o n t i n u o u s , f l o o d e d v s . s t a r v e d , low v s . h i g h p o l y m e r i z a t i o n t e m p e r a t u r e , e t c . Two-phase s t r u c t u r e s o f t w o - s t a g e l a t e x p a r t i c l e s r e a r r a n g e t o w a r d a more s t a b l e s t a t e ( i . e . , c o a r s e n i n g or i n t e r m i x i n g ) , i f the p o l y m e r i z i n g p a r t i c l e s a r e f l u i d . Such r e a r r a n g e m e n t was f o u n d t o be a c c e l e r a t e d by a h i g h t e m p e r a t u r e , flooded polymerization. Conversely, a low temperature, starved p o l y m e r i z a t i o n i m m o b i l i z e d two-phase s t r u c t u r e s d u r i n g t h e p o l y m e r i z a t i o n process, l e a d i n g t o a c o r e - s h e l l morphology. !

Discussion A c o r e - s h e l l m o r p h o l o g y was r e p o r t e d f o r t w o - s t a g e c o m b i n a t i o n s o f m o d e r a t e l y c o m p a t i b l e p o l y m e r s : ΡMA a n d PMMA ( 1 3 ) , a n d PEA a n d PMMA ( 1 4 ) . I n a d d i t i o n , i t was f o u n d t h a t h y d r o p h o b i c l a t e x p a r t i c l e s were e a s i l y e n c a p s u l a t e d w i t h h y d r o p h i l i c p o l y m e r s (14, 1 5 ) . These o b s e r v a t i o n s , a l o n g w i t h t h e k i n e t i c e v i d e n c e (16), support the f a c t t h a t p a r t i c l e s u r f a c e i s a main l o c u s o f e m u l s i o n p o l y m e r i z a t i o n a n d t h i s s u r f a c e p o l y m e r i z a t i o n may l e a d to a c o r e - s h e l l s t r u c t u r e f o r two-stage emulsion p o l y m e r i z a t i o n , i f t h e r e i s l i t t l e o r no interpénétration. On t h e o t h e r h a n d , o u r c u r r e n t s t u d y shows t h a t i n t h e c a s e o f i n c o m p a t i b l e p o l y s t y r e n e and S-B c o p o l y m e r s , t h e s e c o n d - s t a g e p o l y m e r f o r m s a d i s p e r s e phase w i t h i n the f i r s t - s t a g e polymer p a r t i c l e , d e s p i t e s u r f a c e polymerization. Matsumoto e t . a l . (17) a l s o r e p o r t e d a m i c r o phase s e p a r a t i o n o f t h e second-stage p o l y s t y r e n e i n the f i r s t stage p o l y e t h y l a c r y l a t e p a r t i c l e . S p e r l i n g e t . a l . ( 1 8 , 19) s t u d i e d the dynamic m e c h a n i c a l b e h a v i o r s o f two-stage l a t e x i n t e r p e n e t r a t i n g networks and observed a s h e l l - c o r e phase



412

EMULSION POLYMERS

AND EMULSION POLYMERIZATION

Figure 4. Ultrathin cross sections of OsOrstained two-stage (S//S-B) and (S-B// S) latex particles at the stage ratio of 50/50, respectively, embedded in a polymerizable mix of butyl and methyl methacrylates ((A) LS-10, (B) LS-11 (reverse of LS-10))


25.

LEE

Morphology of Two-Stage Latex Particles

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s e p a r a t i o n , i n a d d i t i o n t o a micro-phase s e p a r a t i o n o f the secondstage polymer i n the c o r e . A l l these observations o b v i o u s l y sug g e s t t h a t t h e m o r p h o l o g y o f t w o - s t a g e l a t e x p a r t i c l e s depends n o t o n l y o n s u r f a c e p o l y m e r i z a t i o n , but a l s o t h e k i n e t i c a n d t h e r m o dynamic a s p e c t s o f p o l y m e r i z i n g l a t e x systems. W i t h t h e s e f i n d i n g s i n m i n d , we c a n n o t o n l y p r e d i c t a. p r i o r i t h e m o r p h o l o g y of two-stage l a t e x p a r t i c l e s from the knowledge o f p o l y m e r i z a t i o n s y s t e m s , but a l s o c o n t r o l t h e i r m o r p h o l o g y .


Summary The m o r p h o l o g y o f t w o - s t a g e ( s t y r e n e / / s t y r e n e - b u t a d i e n e ) a n d ( s t y r e n e - b u t a d i e n e / / s t y r e n e ) l a t e x p a r t i c l e s was f o u n d t o v a r y from a c o r e - s h e l l s t r u c t u r e t o a complete phase s e p a r a t i o n w i t h v a r i o u s two-phase s t r u c t u r e s i n between, depending on p o l y m e r i z a t i o n sequence, p o l y m e r i z a t i o n c o n d i t i o n s , polymer c o m p a t i b i l i t y , m o l e c u l a r w e i g h t s , polymer phase r a t i o , e t c . Acknowledgement The a u t h o r w o u l d l i k e t o t h a n k Ε. B. B r a d f o r d , R. A. W i t h e r s , and J . C o t t e r f o r t h e i r c o n t r i b u t i o n s i n e l e c t r o n m i c r o s c o p y , R. A. W i l l e n c y f o r h i s c o n t r i b u t i o n o f M i c r o g r a p h s 4A a n d 4B a n d E. F. S t e v e n s f o r h i s a s s i s t a n c e i n t h e e x p e r i m e n t a l work.

Abstract The morphology of two-stage (styrene//styrene-butadiene) latex particles was studied with respect to variations in stage ratio, molecular weights, styrene-butadiene (S-B) copolymer composition, polymerization sequence, and polymerization conditions. The morphological features observed were mainly structures resulting from phase separation rather than the core shell morphology expected from surface polymerization with no interpenetration. At intermediate chain transfer agent (CTA) levels in the second-stage polymer phase-separated as micro domains within the first-stage polymer particles, then underwent phase inversion to become a continuous phase as the amount of the second-stage polymer increased. At high CTA levels, polymers were completely separated, resulting in a hemispherical morphology at the equal stage ratio. Upon further increase in the amount of the second-stage polymer, asymmetric encapsulation occurred. Phase domain sizes decreased with decreasing butadiene in the S-B copolymers, as expected from better compatibility. This study strongly suggests that the morphology of two-stage latex particles greatly depends on the thermodynamic nature of polymeriz ing latex systems, polymerization conditions, and the order of polymerization.


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RECEIVED April 6, 1981.


Morphology of Two-Stage Latex Particles - ACS Symposium Series

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