Reversible Polymeric Gels and Related Systems - American Chemical

Chapter 8

Solvent Mobility in Atactic Polystyrene-Toluene Systems Frank D. Blum and Byaporn naNagara

Downloaded by UNIV OF ARIZONA on May 15, 2017 | http://pubs.acs.org Publication Date: September 14, 1987 | doi: 10.1021/bk-1987-0350.ch008

Department of Chemistry, University of Missouri—Rolla, Rolla, M O 65401-0249

The solvent mobility in atactic polystyrene-toluene solutions has been studied as a function of temperature using NMR. The local reorientation of the solvent was studied using deuterium NMR relaxation times on the deuterated solvent. Longer range motions were also probed using the pulsed-gradient spin-echo NMR method for the measurement of diffusion coefficients on the protonated solvent. The measurements were taken above and below the gel transition temperatures reported by Tan et a l . (Macromolecules, 1983, 16, 28). It was found that both the relaxation time measurements and the diffusion coefficients of the solvent varied smoothly through the reported transition temperature. Consequently, i t appears that in this system, the solvent dynamics are unaffected by gel formation. This result is similar to that found in other chemically crossed-linked systems. The s t r u c t u r e and dynamics i n r e v e r s i b l e polymer g e l s have been t h e s u b j e c t o f s e v e r a l s t u d i e s (1-10) . I n t h e s e , a few r e s e a r c h e r s have f o c u s e d on t h e p r o p e r t i e s o f g e l s formed from atactic p o l y s t y r e n e ( a P S ) - s o l v e n t systems (1-8). The phase b e h a v i o r o f aPS g e l s w i t h a v a r i e t y o f s o l v e n t s has been s t u d i e d by Tan e t a l . (1). T h e s e g e l s show g e l a t i o n t e m p e r a t u r e s , g i that are solvent dependent and o c c u r above t h e g l a s s t r a n s i t i o n t e m p e r a t u r e , Tg (2.) . S i n c e t h e s e g e l s do n o t c o n t a i n c o v a l e n t c r o s s - l i n k s , t h e y a r e necessarily different from gels formed due t o chemical cross-links. F o r example, t h e y appear t o be t h e r m o r e v e r s i b l e and, i n t h e g e l s t a t e , may r e f o r m i n t o a new shape a f t e r deformation. E v i d e n c e f o r t h e f o r m a t i o n o f g e l s from aPS systems i s o b t a i n e d from simple mechanical, (1, 4 5) viscoelastic, (7,8) thermod y n a m i c ( 1 . 6 ) a n d s p e c t r o s c o p i c (£_) t e c h n i q u e s . Simple tube t i l t i n g , f a l l i n g b a l l methods and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y have been u s e d t o d e t e r m i n e t h e phase d i a g r a m s f o r a number o f systems. V i s c o e l a s t i c measurements on t h e a P S - c a r b o n d i s u l f i d e s y s t e m show t h a t t h e low f r e q u e n c y r e s p o n s e i n d i c a t i v e o f a T

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0097-6156/87/0350-0107S06.00/0 © 1987 American Chemical Society

Russo; Reversible Polymeric Gels and Related Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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REVERSIBLE POLYMERIC GELS AND RELATED SYSTEMS

G a u s s i a n network i n c r e a s e s d r a m a t i c a l l y a t a t e m p e r a t u r e a r o u n d t h e g e l - t r a n s i t i o n t e m p e r a t u r e (2.) . The m a c r o s c o p i c k i n e t i c r e s p o n s e o f t h e g e l t o a t e m p e r a t u r e change i s r a p i d ( i n a v i s c o e l a s t i c experiment). Enhanced l o w - a n g l e l i g h t s c a t t e r i n g i s a l s o o b s e r v e d f r o m aPS g e l s (£_) . However, i n s p i t e o f a l l o f t h e p h y s i c a l e v i d e n c e , t h e r e i s c o n t r o v e r s y o v e r t h e cause o f t h e g e l a t i o n w i t h chain o v e r l a p , (1_) cryst a l l i n i t y (±) a n d polymer-solvent complexes (£) b e i n g s u g g e s t e d as a l t e r n a t i v e mechanisms. The p u r p o s e o f t h e p r e s e n t s t u d y i s t o probe t h e r o l e t h a t t h e dynamics o f t h e s o l v e n t m o l e c u l e s p l a y ( i f any) i n t h e r e v e r s i b l e gels formed f r o m aPS. Since the g e l s recover q u i c k l y , i t i s p o s s i b l e t h a t t h e m o l e c u l a r dynamics o f t h e r e v e r s i b l e g e l s a r e significantly different from t h e i r c r o s s - l i n k e d c o u n t e r p a r t s . S i n c e t h e T g ^ v a l u e s a r e s t r o n g l y dependent on t h e n a t u r e o f t h e s o l v e n t , t h e r o l e o f t h e s o l v e n t m o b i l i t y s h o u l d be c l a r i f i e d , particularly in light of the r e c e n t s u g g e s t i o n from light s c a t t e r i n g t h a t a " p o l y m e r - s o l v e n t complex" (£) may be r e s p o n s i b l e f o r g e l f o r m a t i o n . We have chosen t h e a P S - t o l u e n e system f o r s t u d y b e c a u s e t o l u e n e i s a good s o l v e n t f o r aPS, has a r e l a t i v e l y h i g h s w e l l r a t i o f o r c r o s s - l i n k e d PS, (2) and i s r e a d i l y a v a i l a b l e as a deuterated material. While v i s c o e l a s t i c measurements have not been made on t h i s s y s t e m , a r e c e n t s t u d y has shown t h a t enhanced low-angle light scattering i s observed from aPS gels with t e t r a h y d r o f u r a n , which i n many r e s p e c t s i s a s i m i l a r s o l v e n t w i t h a similar solubility parameter and g i temperatures. NMR measurements p r o b i n g r a t h e r d i f f e r e n t d i s t a n c e s c a l e s o f s o l v e n t m o b i l i t y w i l l be made b o t h above and b e l o w t h e r e p o r t e d g e l transition temperature. They are: i ) the longer range, s e l f - d i f f u s i o n measurements and t h e i i ) s h o r t e r range r o t a t i o n a l behavior of the s o l v e n t . The NMR p u l s e d - g r a d i e n t s p i n - e c h o (PGSE) method can be used t o measure the solvent self-diffusion coefficients. Longitudinal (T-^) and t r a n s v e r s e ( T 2 ) d e u t e r o n r e l a x a t i o n times of the d e u t e r a t e d s o l v e n t are the probes of the l o c a l solvent mobility.


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EXPERIMENTAL The NMR measurements were p e r f o r m e d on systems composed o f c a . 25 wt. % samples o f aPS (M = 6. 6x 1 0 g/mol, PD = 1.1, P r e s s u r e C h e m i c a l Company) i n e i t h e r r e a g e n t g r a d e t o l u e n e ( A l d r i c h ) o r toluene-dg (Aldrich) . The p r o t o n a t e d s o l v e n t was u s e d f o r t h e diffusion measurements and the deuterated solvent f o r the relaxation studies. At t h i s c o n c e n t r a t i o n , t h e T g f o t h e system was d e t e r m i n e d t o be about -65 °C (1) . The NMR s p e c t r a were run on a JEOL FX-90Q NMR s p e c t r o m e t e r o p e r a t i n g a t 90 and 14 MHz f o r p r o t o n s and d e u t e r o n s , r e s p e c t i v e l y . The T-^ and T measurements were made w i t h t h e s t a n d a r d i n v e r s i o n - r e c o v e r y and s p i n - e c h o (CPMG) sequences, r e s p e c t i v e l y . The s e l f - d i f f u s i o n c o e f f i c i e n t measurements were made u s i n g t h e p u l s e d - g r a d i e n t s p i n - e c h o (PGSE) method which has r e c e n t l y been reviewed (JUL) . F o r a n u c l e u s i n an i s o t r o p i c s o l u t i o n , t h e PGSE-NMR n o r m a l i z e d s i g n a l i n t e n s i t y , 1/1 i s g i v e n by: 5

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(1)

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Atactic Polystyrene-Toluene Systems

BLUM AND NANAGARA

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where γ, G and D a r e t h e m a g n e t o g y r i c r a t i o , g r a d i e n t s t r e n g t h and self-diffusion coefficient, respectively. C o n s t a n t s s u c h as any J-modulation e f f e c t s or T r e l a x a t i o n e f f e c t s are f a c t o r e d i n t o I term ( H ) . The v a l u e o f β i s r e l a t e d t o t h e t i m e t h a t t h e g r a d i e n t p u l s e i s on, 3 , and t h e s e p a r a t i o n t i m e between t h e r a d i o f r e q u e n c y p u l s e s , Δ, i e . β = 3 (Δ - 3/3) (11). A p l o t of the l o g of the s i g n a l i n t e n s i t y v e r s u s β w i l l y i e l d a l i n e w i t h a s l o p e from which D can be d e t e r m i n e d p r o v i d e d t h e g r a d i e n t s t r e n g t h i s known (11). In t h e p r e s e n t c a s e , t h e g r a d i e n t was d e t e r m i n e d t o be 0.047 T/m. F o r a d i s p e r s i o n o r phase s e p a r a t e d m a t e r i a l t h e decay c u r v e may be m a r k e d l y n o n e x p o n e n t i a l , s u c h as t h a t f o u n d f o r water i n s m e c t i c liquid crystals (12.) o r t o l u e n e i n a d i s p e r s i o n o f c r o s s - l i n k e d s w o l l e n polymer beads (13 14). 2

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RESULTS The r e s u l t s o f t h e PGSE-NMR e x p e r i m e n t on t o l u e n e i n a 24.4 wt % aPS s o l u t i o n a r e shown i n F i g u r e 1 as a f u n c t i o n o f t e m p e r a t u r e . The measurements were made f o r t h e m e t h y l r e s o n a n c e because i t has a longer T value than the aromatic resonances. As can be seen i n the f i g u r e , a l l of the p l o t s of l o g i n t e n s i t y versus β are l i n e a r i n d i c a t i n g t h a t t h e d i f f u s i o n of the t o l u e n e i s not r e s t r i c t e d . S i n c e b i - e x p o n e n t i a l b e h a v i o r was n o t e n c o u n t e r e d , no s i g n i f i c a n t d e c r e a s e i n i n t e n s i t y was found, and t h e s o l u t i o n s l o o k homogeneous below T g i c o n c l u d e t h a t n e i t h e r m i c r o - nor macro-phase s e p a r a t i o n o c c u r s t o any g r e a t e x t e n t . As t h e t e m p e r a t u r e i s decreased, the slopes i n F i g u r e 1 decrease i n d i c a t i v e of slower self-diffusion coefficients. The s e l f - d i f f u s i o n coefficients, c a l c u l a t e d f r o m t h e PGSE e x p e r i m e n t a r e shown i n F i g u r e 2 as a f u n c t i o n of temperature. A l s o shown i s an arrow i n d i c a t i n g t h e gel P break i n the data, w i t h i n e x p e r i m e n t a l e r r o r , i s found at or near T . The d i f f u s i o n c o e f f i c i e n t a p p e a r s t o obey a s i m p l e A r r h e n i u s law w i t h an e n e r g y o f a c t i v a t i o n o f about 17.5 kJ/mol. The d e u t e r i u m NMR s p e c t r a o f t o l u e n e - d g i n aPS solutions c o n s i s t s o f two r e s o n a n c e s which a r e r e s o l v e d and a s s i g n e d t o t h e m e t h y l and a r o m a t i c g r o u p s . In t h e s e s o l u t i o n s s p i n - s p i n o r d i p o l a r c o u p l i n g s a r e s m a l l and c o n s e q u e n t l y a r e not o b s e r v e d . As the temperature i s lowered, the aromatic resonance s y s t e m a t i c a l l y broadens w h i l e t h e m e t h y l r e s o n a n c e remains r e l a t i v e l y c o n s t a n t i n width. The l i n e w i d t h s o f t h e a r o m a t i c r e s o n a n c e s a r e t h e same as t h o s e which would be p r e d i c t e d from t h e T measurements r e p o r t e d below. The s e p a r a t e d r e s o n a n c e s make i t p o s s i b l e t o d e t e r m i n e t h e r e l a x a t i o n t i m e s o f each t y p e o f r e s o n a n c e . The d e u t e r o n T^ and T v a l u e s o f b o t h t h e m e t h y l and a r o m a t i c r e s o n a n c e s f o r t o l u e n e - d g i n a 25.6 wt % aPS s o l u t i o n a r e shown i n F i g u r e 3 as a f u n c t i o n o f t e m p e r a t u r e . As e x p e c t e d t h e T-^ and T o f t h e m e t h y l groups a r e l a r g e r t h a n t h o s e f o r t h e a r o m a t i c groups because of t h e i r f a s t i n t e r n a l r o t a t i o n . The T d a t a seem t o be A r r h e n i u s - l i k e and t h e a p p a r e n t e n e r g i e s o f a c t i v a t i o n f o r b o t h r e s o n a n c e s a r e about 35 and 29 kJ/mol f o r t h e a r o m a t i c and m e t h y l resonances, r e s p e c t i v e l y . The e n e r g y o f a c t i v a t i o n i s about t w i c e t h a t found i n the d i f f u s i o n case although the p h y s i c a l s i g n i f i c a n c e of the energy of a c t i v a t i o n of the r e l a x a t i o n times i s not n e c e s s a r i l y s t r a i g h t f o r w a r d , p a r t i c u l a r l y i n polymer solutions. 2

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REVERSIBLE POLYMERIC GELS AND RELATED SYSTEMS

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Reversible Polymeric Gels and Related Systems - American Chemical

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