Chapter 3
Morphology in Miscible and Immiscible Polymer Blends Interplay of Polymer Photophysics with Polymer Physics
Downloaded by UNIV OF ARIZONA on January 22, 2013 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch003
Curtis W. Frank and Wang-cheol Zin Department of Chemical Engineering, Stanford University, Stanford, CA 94305
Applications of excimer fluorescence as a molec ular level probe of chain configuration for isolated polymer chains and as a morphological tool for the study of phase separation are reviewed. In the f i r s t section, previous studies on the use of a strictly one-dimensional random walk model to describe energy migration in miscible blends are critically examined for blends of polystyrene (PS) with poly(vinyl methyl ether) (PVME) and poly(2-vinyl naphthalene) (P2VN) with either poly(cyclohexyl methacrylate) (PCMA) or polystyrene. The apparent energy migration efficiency is two orders of magnitude higher than expected, lead ing to the conclusion that energy migration in dilute blends is at best quasi-one-dimensional with a non -negligible amount of cross-loop hopping. In the second section, the PS/PVME phase diagrams for mono disperse PS of molecular weights 100,000 and 1,800,000 blended with PVME of number average molecular weight 44,600 and M /M =1.4 are determined. A temperature and concentration dependent binary interaction param eter is evaluated from the cloud point data and used to generate spinodal curves. Previous fluorescence results on spinodal decomposition in PS/PVME are re interpreted using this new phase diagram to yield good agreement with the deGennes-Pincus theory. In the final section, annealing experiments for blends cast from tetrahydrofuran, chlorobenzene and toluene are used to demonstrate that toluene casting leads to the equilibrium binary blend morphology. w
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I t has been e s t i m a t e d t h a t 15 t o 20 p e r c e n t o f a l l e n g i n e e r i n g p l a s t i c s produced i n the U n i t e d S t a t e s today a r e polymer b l e n d s o r a l l o y s , which are p h y s i c a l m i x t u r e s o f homopolymers and/or c o p o l y mers. A s e r i o u s problem e x i s t s , however, i n t h a t the b u l k prope r t i e s o f the b l e n d w i l l depend on the degree o f m i x i n g a c h i e v e d . Such m i x i n g i s d i f f i c u l t t o a c c o m p l i s h on the m o l e c u l a r l e v e l and 0097-6156/87/0358-0018S06.00/0 © 1987 American Chemical Society
In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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3. FRANK AND ZIN
Morphology in Polymer Blends
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e x i s t i n g experimental techniques e x h i b i t widely v a r y i n g s e n s i t i v i t i e s t o t h e d i s t a n c e s c a l e over w h i c h i n h o m o g e n e i t i e s a r e d e t e c t i b l e . A major o b j e c t i v e o f r e s e a r c h i n o u r l a b o r a t o r y has been t o e s t a b l i s h excimer f l u o r e s c e n c e as a q u a n t i t a t i v e photophysi c a l t o o l f o r t h e e l u c i d a t i o n o f morphology and dynamics i n polymer b l e n d s . An e x t e n s i v e r e v i e w o f the p h o t o p h y s i c s o f excimer format i o n i n the a r y l v i n y l polymers has been p u b l i s h e d . [1] The a p p l i c a t i o n o f t h i s t e c h n i q u e as a m o r p h o l o g i c a l t o o l r e q u i r e s t h a t t h e r e be a c l o s e c o u p l i n g between polymer p h o t o p h y s i c s and polymer p h y s i c s . I n the p h o t o p h y s i c a l s t u d i e s d e s c r i b e d i n t h i s paper emphasis w i l l be p l a c e d on the development o f a n a l y t i c a l models f o r e l e c t r o n i c e x c i t a t i o n t r a n s p o r t (EET). The areas o f polymer p h y s i c s t h a t we w i l l c o n s i d e r i n v o l v e t h e c o n f i g u r a t i o n a l s t a t i s t i c s o f i s o l a t e d c h a i n s and phase s e p a r a t i o n i n multicomponent polymer systems. The polymer system o f p r i m a r y i n t e r e s t i s t h e b l e n d o f p o l y s t y r e n e (PS) w i t h p o l y ( v i n y l methyl e t h e r ) (PVME). Here excimer f l u o r e s c e n c e from p h e n y l - p h e n y l i n t e r a c t i o n s i n PS i s the main e x p e r i m e n t a l o b s e r v a b l e . T h i s b l e n d was s e l e c t e d because i t has been demonstrated b y numerous o t h e r t e c h n i q u e s t h a t m i s c i b l e one-phase b l e n d s may be p r e p a r e d by s o l u t i o n c a s t i n g from t o l u e n e s o l v e n t . [2,3] Moreover, the b l e n d may be f o r c e d t o phase s e p a r a t e by t h e r m a l means, l e a d i n g t o a two phase system. I n a d d i t i o n , we w i l l c o n s i d e r r e s u l t s f o r b l e n d s o f p o l y ( 2 - v i n y l n a p h t h a l e n e ) (P2VN) w i t h l o w m o l e c u l a r weight p o l y ( c y c l o h e x y l m e t h a c r y l a t e ) (PCMA) and polystyrene (PS). There i s g e n e r a l agreement t h a t d e r e a l i z a t i o n o f e l e c t r o n i c e x c i t a t i o n energy may o c c u r i n s o l i d s o l u t i o n s o f a r y l v i n y l p o l y mers as a r e s u l t o f d i p o l e - d i p o l e i n t e r a c t i o n s between a r o m a t i c r i n g s c o v a l e n t l y bound t o the polymer c h a i n . [4] I n f a c t , t h e EET p r o c e s s i s e x c e e d i n g l y complex w i t h s e v e r a l modes o f energy migrat i o n p o s s i b l e . S i n c e the polymer c h a i n c o n t a i n s a l o c a l l y h i g h c o n c e n t r a t i o n o f a r o m a t i c groups, w i t h s e p a r a t i o n d i s t a n c e s between r i n g s on a d j a c e n t r e p e a t u n i t s o f t h e o r d e r o f f i v e t o seven Angstroms, one m i g r a t i o n c h a n n e l w i l l be a l o n g t h e c o n t o u r o f t h e polymer c h a i n . I f t h e t e s t c h a i n o f i n t e r e s t i s t o t a l l y i s o l a t e d from i d e n t i c a l c h a i n s , and i f t h e c h a i n i s i m m o b i l i z e d i n a t h e r m o d y n a m i c a l l y good m a t r i x such t h a t i t e x h i b i t s an expanded conf i g u r a t i o n , i t i s p o s s i b l e t h a t t h e energy m i g r a t i o n c o u l d t a k e on a o n e - d i m e n s i o n a l c h a r a c t e r . As t h e c o n c e n t r a t i o n o f t h e a r y l v i n y l polymer i s i n c r e a s e d , energy m i g r a t i o n between chromophores on d i f f e r e n t c h a i n s i s expected; t h i s w i l l approach a t h r e e - d i m e n s i o n a l process a t s u f f i c i e n t l y h i g h concentration. I n the f i r s t s e c t i o n o f t h i s paper we r e v i e w a s t r i c t l y oned i m e n s i o n a l random w a l k model developed by F i t z g i b b o n . [5] The model t r e a t s e x c i t a t i o n m i g r a t i o n on a f i n i t e s i z e l a t t i c e w i t h randomly d i s t r i b u t e d excimer f o r m i n g s i t e s a c t i n g as t r a p s . I t has been app l i e d t o e x p l a i n the e f f i c i e n c y o f t r a p p i n g a t excimer f o r m i n g s i t e s i n t h r e e s e t s o f a p p a r e n t l y m i s c i b l e systems. We f i r s t c o n s i d e r a s e r i e s o f PS/PVME b l e n d s i n w h i c h t h e PS i s a t l o w c o n c e n t r a t i o n b u t of v a r i a b l e molecular weight. We t h e n examine two o t h e r c l a s s e s o f b l e n d s i n w h i c h t h e a r y l v i n y l polymer o f i n t e r e s t i s P2VN and b l e n d s are p r e p a r e d w i t h PCMA o r PS as the h o s t polymer. By cont r a s t t o t h e PS/PVME system, w h i c h i s t h e r m o d y n a m i c a l l y compatible over the whole c o n c e n t r a t i o n range and f o r a l l m o l e c u l a r w e i g h t s o f the two components, t h e P2VN b l e n d s a r e m i s c i b l e o n l y as a r e s u l t o f
In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
PHOTOPHYSICS OF POLYMERS
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the low m o l e c u l a r w e i g h t o f the h o s t polymer. Our o b j e c t i v e i s t o p r o v i d e a c r i t i c a l assessment o f the a p p l i c a t i o n o f the s t r i c t l y o n e - d i m e n s i o n a l random w a l k model t o the s o l i d s t a t e b l e n d s . I n c o n c e n t r a t e d systems such as i n f i l m s o f the pure a r y l v i n y l polymer o r i n domains r i c h i n the f l u o r e s c e n t polymer f o r a phase s e p a r a t e d system i t i s l i k e l y t h a t EET between r i n g s on d i f f e r e n t c h a i n s c o u l d be as l i k e l y as between r i n g s on the same c h a i n . I n the l i m i t f o r w h i c h t h e r e i s no a n i s o t r o p i c b i a s f o r energy m i g r a t i o n a l o n g the c h a i n , the EET may be c o n s i d e r e d t o be t h r e e dimensional. Two EET models have been d e v e l o p e d t o t r e a t the prob lem o f t h r e e - d i m e n s i o n a l m i g r a t i o n t o p r e - f o r m e d e x c i m e r s i t e s i n s o l i d p o l y m e r s . The f i r s t i s a s p a t i a l l y p e r i o d i c l a t t i c e model t h a t r e s t r i c t s m i g r a t i o n t o n e a r e s t n e i g h b o r s and does not p e r m i t the e x c i t a t i o n t o r e - v i s i t any e a r l i e r l o c a t i o n s o c c u p i e d d u r i n g t h e random walk. [6] The second i s a more s o p h i s t i c a t e d many-body EET t h e o r y t h a t was o r i g i n a l l y d e v e l o p e d by L o r i n g , Andersen and Fayer ( r e f e r r e d t o as the LAF model) t o t r e a t i s o t o p i c a l l y doped m o l e c u l a r c r y s t a l s w i t h no r e s t r i c t i o n s on the chromophores sampled d u r i n g t h e m i g r a t i o n p r o c e s s . [7] Semerak has r e c e n t l y a p p l i e d b o t h the spa t i a l l y p e r i o d i c l a t t i c e model and the LAF t h e o r y t o a n a l y z e excimer f l u o r e s c e n c e i n pure PS and P2VN. [8] A l t h o u g h the t h e o r e t i c a l study o f EET i n polymer systems has l e d t o c o n s i d e r a b l e r e c e n t advances, we choose i n the second s e c t i o n o f t h i s paper t o take a somewhat more p r a g m a t i c approach t o t h e s t u d y o f phase s e p a r a t i o n k i n e t i c s . To do so, we t e m p o r a r i l y s e t a s i d e any c o n s i d e r a t i o n o f the d e t a i l s o f any p a r t i c u l a r EET model and s i m p l y r e l y on a fundamental r e l a t i o n s h i p between the r a t i o o f the e x c i m e r t o monomer e m i s s i o n i n t e n s i t i e s , ^/Ι^» * p r o b a b i l i t y M t h a t an absorbed photon i s e v e n t u a l l y e m i t t e d b y a r a d i a t i v e o r n o n r a d i a t i v e p r o c e s s o f the monomer. Use o f t h i s g e n e r a l r e l a t i o n a l o n g w i t h e x p e r i m e n t a l d a t a f o r the e f f e c t o f c o n c e n t r a t i o n on excimer f l u o r e s c e n c e i n a m i s c i b l e b l e n d a l l o w s M t o be d e t e r m i n e d e x p e r i m e n t a l l y . We w i l l p r e s e n t a s i m p l e morph o l o g i c a l model t h a t combines the m i s c i b l e b l e n d r e s u l t s w i t h an ex p e r i m e n t a l l y d e t e r m i n e d phase diagram t o r e l a t e the a b s o l u t e v a l u e o f the Ïp/Ijj r a t i o t o the l o c a l c o n c e n t r a t i o n o f c h a i n segments cont a i n i n g a r o m a t i c groups. Our o b j e c t i v e i s t o re-examine e a r l i e r d a t a on s p i n o d a l d e c o m p o s i t i o n f o r the PS/PVME b l e n d u s i n g a more a c c u r a t e phase diagram d e t e r m i n e d i n the p r e s e n t s t u d y . The p r e c e d i n g s t u d i e s on the c o n f i g u r a t i o n o f a r y l v i n y l p o l y mer c h a i n s i n d i l u t e , m i s c i b l e b l e n d s and on the k i n e t i c s o f phase s e p a r a t i o n i n c o n c e n t r a t e d b l e n d s were b a s e d on the i m p l i c i t assumpt i o n t h a t the i n i t i a l s o l v e n t c a s t b l e n d r e p r e s e n t e d an e q u i l i b r i u m state. I n the f i n a l s e c t i o n o f t h i s paper we e x p l o r e t h i s q u e s t i o n w i t h new d a t a on the e f f e c t o f the c a s t i n g s o l v e n t on the f l u o r e s cence b e h a v i o r o f PS/PVME b l e n d s . Our o b j e c t i v e s are t o determine f i r s t whether the f l u o r e s c e n c e o b s e r v a b l e s are s e n s i t i v e t o d i f f e r ences i n a s - c a s t f i l m s and t h e n t o i d e n t i f y the t r u e e q u i l i b r i u m state. a n
