Morphology in Miscible and Immiscible Polymer Blends - American

final section, annealing experiments for blends cast .... ture and molecular weight dependence of R for blends containing low concentrations of PS. As...
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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.

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

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

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

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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