Polymer Characterization by ESR and NMR - American Chemical

1 Conformation and Mobility of Polymers Adsorbed

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on Oxide Surfaces by E S R Spectroscopy ΤAI MING LIANG, PETER N. DICKENSON, and WILMER G. MILLER Department of Chemistry, University of Minnesota, Minneapolis, MN 55455 The nature of the interaction of polymers with solid sur faces is of much practical importance, e.g., in coatings and in reinforced polymers, as well as of theoretical interest (1,2,3). Most experimental and theoretical studies at the molecular level have been concerned with the adsorbed polymer at a solid-liquid interface. The interaction of the dry polymer with the solid surface, of significance in applied problems, is little under stood at the segmental or even molecular level. Instead, inter action is generally inferred from effects on bulk properties. Theoretical treatments (e.g., 4-15 ) of adsorbed polymers at the solid-liquid interface predict some or all of the following properties: average fraction of segments adsorbed; average length of loops, adsorbed trains and tails; mean extension of segments above the surface; effect of polymer-solvent and polymer-surface interaction; effect of molecular weight. Experimentally the average thickness of the adsorbed layer upon drying is estimated from adsorption isotherm-surface area meas urements, and at the solid-liquid interface by viscosity, or by ellipsometry. Under favorable circumstances the fraction of segments adsorbed is determined by infrared spectroscopy. Little is known about the mobility of segments in loops and tails, whether in the presence or absence of a contacting solvent. In this communication we discuss the use of a stable nitroxide free radical and ESR spectroscopy to monitor segmental dynamics, and summarize our efforts (ΐ6_,ΓΓ,1_8,19.,£0) . Previous studies, less comprehensive in scope, have been made by others (21,22,23,2^). Basis of Method The line shape of the ESR spectrum of a nitroxide free radical in x-band operation varies with tj?e rotational motion of the nitroxide over the range 10" to 10 sec in rotational correlation time. By saturation transfer measurements even slow er motions may be studied (25_) . The rotational correlation time for a nitroxide in a dilute solution of labelled random coil 0-8412-0594-9/80/47-142-001$05.00/0 © 1980 American Chemical Society Woodward and Bovey; Polymer Characterization by ESR and NMR ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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polymer i n a low v i s c o s i t y s o l v e n t f a l l s t y p i c a l l y i n t h e 0 . 0 5 t o 0 . 5 n s e c r a n g e , g i v i n g a t h r e e l i n e s p e c t r u m as shown i n Figure 1A. The r e l a x a t i o n o f t h e n i t r o x i d e i s f a c i l i t a t e d by b a c k b o n e m o t i o n s e x t e n d i n g o n l y a few atoms f r o m t h e p o i n t o f a t t a c h m e n t , i . e . , t o l o c a l mode m o t i o n s , as d e d u c e d f r o m m o l e c u l a r w e i g h t a n d o t h e r s t u d i e s by s e v e r a l i n v e s t i g a t o r s ( 2 6 , 2 7 ) . Thus i n a n a d s o r b e d m o l e c u l e a n i s o l a t e d l o o p o r t a i l e x t e n d i n g i n t o t h e s o l v e n t should g i v e a m o t i o n a l l y narrowed three l i n e s p e c t r u m i f t h e l o o p o r t a i l i s g r e a t e r t h a n a minimum s i z e ( p r o b a b l y f o u r - t o - s i x bonds a b o u t w h i c h r o t a t i o n c a n o c c u r ) . The s p e c t r a l l i n e shape i s c o n c e n t r a t i o n d e p e n d e n t , b u t r e m a i n s m o t i o n a l l y narrowed u n t i l r e l a t i v e l y h i g h polymer c o n c e n t r a t i o n s a r e r e a c h e d ( F i g u r e 1-B,C a n d r e f e r e n c e 2 7 ) . Loop and t a i l segment d e n s i t y n e a r t h e s u r f a c e may be l a r g e , b u t f a l l s o f f r a p i d l y w i t h d i s t a n c e above t h e s u r f a c e . We t h e r e f o r e e x p e c t e d t a i l s and a l l b u t t h e s m a l l e s t l o o p s ( a n d s u r f a c e e x t e n d e d loops) would e x h i b i t three l i n e s p e c t r a s i m i l a r t o t h a t observed w i t h the f r e e , i s o l a t e d polymer molecule. By c o n t r a s t a n i t r o x i d e a t t a c h e d t o monomeric u n i t s r i g i d l y h e l d t o a s o l i d s u r f a c e s h o u l d show a s p e c t r a l l i n e shape s i m i l a r t o t h e b u l k p o l y m e r i n t h e g l a s s y s t a t e ( F i g u r e I E ) . Thus a polymer molecule adsorbed at a s o l i d - l i q u i d i n t e r f a c e should e x h i b i t a c o m p o s i t e s p e c t r u m , f r o m w h i c h one c a n deduce t h e f r a c t i o n o f t h e monomeric u n i t s i n l o o p s and t a i l s , a n d t h e i r mean m o t i o n . The s e n s i t i v i t y o f t h e method t o s m a l l amounts o f u n i t s w i t h m o t i o n a l f r e e d o m c a n be s e e n f r o m F i g u r e 2 . Λ f i n a l p o i n t t o c o n s i d e r i s t h e e f f e c t o f nonbound p o l y m e r . When a s u r f a c e i s added t o a p o l y m e r s o l u t i o n a n d e q u i l i b r i u m i s r e a c h e d , b o t h a d s o r b e d and f r e e m o l e c u l e s a r e p r e s e n t . A l t h o u g h a d s o r b e d and f r e e p o l y m e r a r e i n dynamic e q u i l i b r i a , d e s o r p t i o n i s g e n e r a l l y s u f f i c i e n t l y s l o w t h a t t h e nonbound p o l y m e r c a n be s e p a r a t e d a n d m o n i t o r e d i n d e p e n d e n t l y o f t h e bound polymer. I n t h e a b s e n c e o f s o l v e n t , t h e i n f l u e n c e o f t h e s u r f a c e on t h e s e g m e n t a l m o b i l i t y c a n be d e d u c e d b y m e a s u r i n g t h e t e m p e r a t u r e dependence o f t h e s p e c t r a l l i n e shape i n t h e p r e s e n c e a n d absence o f t h e s u r f a c e . The t e m p e r a t u r e dependence o f b u l k p o l y ( v i n y ] a c e t a t e ) i s shown i n F i g u r e 3 , a n d o f b u l k p o l y s t y r e n e i n F i g u r e 1 0 . A l t h o u g h t h e p o l y m e r - s u r f a c e i n t e r a c t i o n may h a v e a p r o f o u n d e f f e c t on s e g m e n t a l m o b i l i t y , i t seems u n l i k e l y t h a t any c o m p o s i t e s p e c t r a can be decomposed e a s i l y i n t o bound and unbound c o n t r i b u t i o n s . Experimental P o l y ( v i n y l a c e t a t e ) , P V A c , o f m o l e c u l a r w e i g h t s (Μ ) 6 . 1 x ] 0 , 1 . 9 x 1 0 , a n d 6 . 0 x 1 0 , was r a n d o m l y l a b e l e d by e s t e r e x c h a n g e w i t h 2 , 2 , 5 , 5 - t e t r a m e t h y l - 3 - p y r r o l i n - l - o x y l - 3 - c a r b o x y l i c acid t o give a s p i n l a b e l e d polymer c o n t a i n i n g t y p i c a l l y 1-10 n i t r o x i d e s per polymer molecule. P o l y s t y r e n e was p r e p a r e d by e m u l s i o n 4

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Woodward and Bovey; Polymer Characterization by ESR and NMR ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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Figure 1. ESR spectrum at 25°C as a function of concentration of PVAc(61,000), randomly labeled. Polymer concentration (weight percent) in CHCl is (A) 1%; (B) 44%; (C) 61%; (D) 72%; and (E) 100% (bulk). s


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Figure 2. Simulated composite spectra consisting of 4% of the nitroxides undergoing motional narrowing (isotropic, homogeneous broadening of 1 G) with rotational correlation times as indicated


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p o l y m e r i z a t i o n o f s t y r e n e w i t h 2 wt. % c h l o r o m e t h y l s t y r e n e , f o l l o w e d by p a r t i a l r e a c t i o n o f t h e c h l o r i d e w i t h 2 , 2 , 6 , 6 tetramethyl-Î4-piperidinol-l-oxyl (28). A l u m i n a , A 1 0 , o b t a i n e d f r o m M a t h e s o n , Coleman and B e l l , was h e a t e d a t 200 C f o r two h r s . b e f o r e u s e . T i t a n i u m d i o x i d e was o b t a i n e d f r o m P o l y s c i e n c e s as 0 . ^ 5 y s p h e r e s . B e f o r e use i t was t r e a t e d t o remove p a r a m a g n e t i c i m p u r i t i e s ( l j 9 ) . G l a s s ( s o d a l i m e ) s p h e r e s o f 3 - 8 y n o m i n a l d i a m e t e r was o b t a i n e d f r o m P o l y s c i e n c e s and t r e a t e d t o remove p a r a m a g n e t i c i m p u r i t i e s . S i 0 i n t h e f o r m o f a g g r e g a t e d 0.01^ y d i a m e t e r s p h e r e s ( C a b - 0 - S i l M5) was u s e d as r e c e i v e d . F o r a d s o r p t i o n s t u d i e s t h e l a b e l e d p o l y m e r was d i s s o l v e d i n r e a g e n t g r a d e s o l v e n t , t h e o x i d e s u r f a c e a d d e d , and t h e s a m p l e was s t i r r e d m a g n e t i c a l l y f o r a t l e a s t 2k h r s . ( s o m e t i m e s up t o 3 days). The o x i d e w i t h a d s o r b e d p o l y m e r was s e p a r a t e d f r o m t h e u n a d s o r b e d p o l y m e r by g r a v i t y s e t t l i n g , o r where n e c e s s a r y by l o w f i e l d c e n t r i f u g a t i o n . The o x i d e t o g e t h e r w i t h a d s o r b e d p o l y m e r was washed w i t h f r e s h s o l v e n t u n t i l no ESR a c t i v i t y was d e t e c t able i n the supernatant. The ESR s p e c t r u m was t h e n t a k e n o f t h e polymer on t h e s u r f a c e e i t h e r i n t h e p r e s e n c e o f s o l v e n t , o r a f t e r s o l v e n t r e m o v a l as a f u n c t i o n o f t e m p e r a t u r e . The ESR s p e c t r a were m e a s u r e d on a V a r i a n E - 3 spectrometer a t a b o u t 9 - 1 5 GHz. The s p e c t r a were t y p i c a l l y r e c o r d e d i n t h e v i c i n i t y o f 3.2 kG w i t h a t t e n u a t i o n power l o w enough t o a v o i d saturation. 2

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C o n f o r m a t i o n and M o b i l i t y a t t h e S o l i d - L i q u i d I n t e r f a c e Effect of Surface. The s p e c t r u m o f P V A c ( 6 l , 0 0 0 ) a d s o r b e d at a l i q u i d ( C H C ^ ) - s o l i d i n t e r f a c e at f u l l s u r f a c e coverage i s shown i n F i g u r e h. C h l o r o f o r m i s a t h e r m o d y n a m i c a l l y good s o l v e n t f o r PVAc, e s t i m a t e d t o be n e a r l y an a t h e r m a l s o l v e n t ( 2 9 . ) . The d i f f e r e n c e s a r e q u i t e s t r i k i n g , r a n g i n g f r o m A £ 0 3 , where a l l u n i t s a r e r i g i d l y h e l d , t o C a b - 0 - S i l S i 0 , where t h e m a j o r i t y o f t h e s p i n l a b e l s a r e i n f l e x i b l e l o o p s and t a i l s . The i m m o b i l i z a t i o n o f t h e s p i n l a b e l s i s t h r o u g h t h e s i d e c h a i n e s t e r , and n o t t h r o u g h t h e n i t r o x i d e m o i e t y , as has b e e n shown t h r o u g h use o f s p i n probes o f v a r y i n g f u n c t i o n a l i t y ( 1 7 ) . 3

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Changing the c o n t a c t i n g s o l v e n t s t r o n g l y a f f e c t s the f r a c t i o n o f immobilized u n i t s , but the e f f e c t o f changing the s u r f a c e i n c o n t a c t w i t h a g i v e n s o l v e n t shows a t r e n d a n a l o g o u s to the chloroform r e s u l t s . The r a n k i n g o f t h e s u r f a c e s i n o r d e r o f d e c r e a s i n g i n t e r a c t i o n w i t h PVAc i s A>£ 0 , T i 0 , g l a s s ( 3 - 8 y s o d a l i m e ) , and S i 0 ( 0 . 0 1 ^ y a g g r e g a t e d s p h e r e s . ) 2

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Effect of Solvent. W i t h PVAc a d s o r b e d a t t h e A ^ 0 - l i q u i d i n t e r f a c e , no m o t i o n a l l y n a r r o w e d component i s o b s e r v e d w i t h any s o l v e n t , r a n g i n g from n e a r l y athermal (CHC^ ) t o n e a r l y a t h e t a s o l v e n t (CC^\) ( l j , 3 0 ) . With PVAc(6l,000) at the T i 0 - l i q u i d i n t e r f a c e , as shown i n F i g u r e 5 , a s m a l l p e r c e n t a g e ( < 5 ) o f v e r y 2

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Figure 5. The ESR spectra of PVAc (61,000) at the solid (Ti0 )-liquid interface at saturation coverage: (A) no solvent or surface; (B) CCI,,; (C) toluene; (D) CHCl . 2

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mobile n i t r o x i d e s i s observed. T h e r e i s not a s t r o n g t r e n d w i t h s o l v e n t , t h o u g h o v e r a s e r i e s o f e x p e r i m e n t s (19.) thermodynami c a l l y good s o l v e n t s l e a d t o a s l i g h t l y g r e a t e r p e r c e n t a g e o f f a s t component t h a n p o o r s o l v e n t s . The most s t r i k i n g change i s w i t h S 1 O 2 ( C a b - O - S i l ) , where g o i n g f r o m p o o r t o good s o l v e n t s l e a d s t o a d r a m a t i c i n c r e a s e i n t h e amount o f t h e f a s t component (19.). A l t e r n a t i v e l y , r a i s i n g the temperature i s thermodynamically e q u i v a l e n t t o c h a n g i n g t o a b e t t e r s o l v e n t . The r e s u l t s a r e c o n s i s t e n t , i n t h a t r a i s i n g t h e t e m p e r a t u r e l e a d s t o an i n c r e a s e i n m o b i l e u n i t s f o r any s o l v e n t - s u r f a c e p a i r e x a m i n e d (19)» The s h a r p n e s s o f t h e f a s t component i n t h e c o m p o s i t e s p e c t r a i n F i g u r e s k and 5 i s a good i n d i c a t i o n t h a t t h e r e i s n o t a w i d e d i s t r i b u t i o n o f c o r r e l a t i o n t i m e s , and by r e f e r e n c e t o F i g u r e 1, t h a t t h e m o b i l e segment d e n s i t y o f t h e a d s o r b e d p o l y m e r e x t e n d i n g i n t o the solvent i s d i l u t e . The n a t u r e o f t h e m o b i l e u n i t s - l o o p s , t a i l s , o r some o f e a c h - c a n n o t be d e d u c e d f r o m t h e s e d a t a . E f f e c t of Molecular Weight. The n a t u r e o f t h e m o b i l e u n i t s can b e s t be p r o b e d by a s t u d y o f t h e m o l e c u l a r w e i g h t d e p e n d e n c e . Shown i n F i g u r e 6 i s t h e m o l e c u l a r w e i g h t d e p e n d e n c e , a t s a t u r a t i o n s u r f a c e c o v e r a g e , o f PVAc on T 1 O 2 i n t h e p r e s e n c e o f chloroform. A l t h o u g h t h e m o l e c u l a r w e i g h t i s v a r i e d by a f a c t o r o f t e n , t h e f r a c t i o n o f f a s t component i s u n c h a n g e d , w i t h i n experimental e r r o r . S i m i l a r behavior i s observed i n other surf a c e - s o l v e n t s y s t e m s (19.). A l l t h e o r e t i c a l t r e a t m e n t s o f t h e m o l e c u l a r w e i g h t dependence i n d i c a t e t h e number o f monomeric u n i t s i n l o o p s i n c r e a s e s m o n o t o n i c a l l y w i t h m o l e c u l a r w e i g h t , whereas t h e number o f u n i t s i n t a i l s a p p r o a c h e s an a s y m p t o t i c l i m i t , i . e . , t h e f r a c t i o n o f u n i t s i n t a i l s must be a d e c r e a s i n g f u n c t i o n o f m o l e c u l a r w e i g h t . The m o l e c u l a r w e i g h t i n v a r i a n c e we o b s e r v e i n d i c a t e s t h e m o b i l e u n i t s a r e p r e d o m i n a n t l y i n l o o p s , and n o t t a i l s . E f f e c t o f Surface Coverage. The e f f e c t o f s u r f a c e c o v e r a g e i s i l l u s t r a t e d i n F i g u r e 7 w i t h PVAc a t t h e SiC2-CC^4 i n t e r f a c e . The e f f e c t i s s i m i l a r w i t h o t h e r s o l v e n t s , and w i t h t h e g l a s s and T 1 O 2 s u r f a c e s . As t h e s u r f a c e c o v e r a g e i n c r e a s e s t h e f r a c t i o n o f u n i t s i n l o o p s i n c r e a s e s , i n some c a s e s q u i t e d r a m a t i c a l l y a s , f o r e x a m p l e , i n F i g u r e 7. T h i s r e s u l t was i n a d v e r t e n t l y s t a t e d i n c o r r e c t l y i n r e f e r e n c e 1_8. At low s u r f a c e coverage the polymer l i e s close to the surface i n a f l a t t e n e d conformation. Monomeric u n i t s i n any l o o p s h a v e l i t t l e m o t i o n a l f r e e d o m , i r r e s p e c t i v e o f size. As t h e s u r f a c e c o v e r a g e i n c r e a s e s a l e s s f l a t t e n e d c o n f i g u r a t i o n i s assumed. These o b s e r v a t i o n s a r e q u a l i t a t i v e l y s i m i l a r t o t h o s e f o r p o l y ( v i n y l p y r r o l i d o n e ) a d s o r b e d on a e r o s i l Si0 (21-2U). 2

E f f e c t o f Polymer. The e f f e c t o f c h a n g i n g t h e p o l y m e r w h i l e k e e p i n g t h e s u r f a c e and s o l v e n t f i x e d i s i l l u s t r a t e d by c o m p a r i n g F i g u r e s h and 8. T h e r e a r e two f a c t o r s w h i c h must be c o n s i d e r e d :


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Figure 6. Effect of molecular weight of PVAc on Ti0 in the presence of CHCh at saturation coverage: (A) no solvent or surface; (B) 61,000; (C) 194,000; (D) 600,000. 2


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Figure 7. The dependence of the ESR spectra on surface coverage of PVAc (61,000) at the SiOJCab-O-SilhCCl,, interface at 25°C. Percent of saturation uptake: (A) 50%; (B) 90%; (C) at saturation.


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p o l y m e r - s o l v e n t and p o l y m e r - s u r f a c e i n t e r a c t i o n s . Thermodynami c a l l y C H C £ s h o u l d be a p o o r e r s o l v e n t f o r p o l y s t y r e n e t h a n f o r PVAc (29). T h e r e f o r e , one w o u l d e x p e c t l e s s f a s t component w i t h polystyrene. However, we o b s e r v e a v e r y l o w amount o f s l o w component w i t h p o l y s t y r e n e a t e i t h e r t h e S i C ^ - C H C ^ o r t h e g l a s s CHC^ i n t e r f a c e . T h i s must r e f l e c t a d i f f e r e n c e i n t h e i n t e r a c t i o n o f t h e s t y r e n e and v i n y l a c e t a t e u n i t s w i t h t h e s u r f a c e . 3

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E f f e c t o f t h e S u r f a c e on S e g m e n t a l

M o b i l i t y i n t h e Dry S t a t e

The P o l y m e r - S u r f a c e I n t e r a c t i o n . When u s i n g a n i t r o x i d e l a b e l e d polymer the p o l y m e r - s u r f a c e i n t e r a c t i o n i n the absence o f s o l v e n t can be a s s e s s e d f r o m t e m p e r a t u r e s t u d i e s i n t h e p r e s e n c e and a b s e n c e o f t h e s u r f a c e , as shown i n F i g u r e s 9 and 10. At temperatures below or near the g l a s s t r a n s i t i o n t h e e f f e c t o f t h e s u r f a c e c a n n o t be d e d u c e d as a l l m o t i o n s a r e t o o s l o w . How e v e r , as t h e t e m p e r a t u r e i s r a i s e d above Τ t h e e f f e c t o f t h e s u r f a c e i s c l e a r l y d i s c e r n i b l e , w i t h s u r f a c e i n h i b i t i n g segmental motion. I t i s e v i d e n t f r o m F i g u r e s 9 and 10 t h a t t h e a b s o l u t e t e m p e r a t u r e i s not as i m p o r t a n t as T-T , where Τ i s t h e Τ i n the absence o f t h e s u r f a c e . The effeg§ o f t h e surface is s t i l l d i s c e r n i b l e a t t e m p e r a t u r e ^100 above Τ . As t h e t e m p e r a t u r e i s f u r t h e r i n c r e a s e d t h e s e g m e n t a l ^ m o b i l i t y becomes i n d e p e n d e n t o f t h e p r e s e n c e o f t h e s u r f a c e (20) , i n d i c a t i n g t h a t kT has become g r e a t e r t h a n t h e s e g m e n t - s u r f a c e i n t e r a c t i o n energy. However, i t was f o u n d t h a t t h e s p e c t r u m o b s e r v e d a t any t e m p e r a t u r e was i n d e p e n d e n t o f t h e t h e r m a l h i s t o r y o f t h e s a m p l e . Inasmuch as t h e s a m p l e s were p r e p a r e d by e v a p o r a t i n g t h e s o l v e n t at room t e m p e r a t u r e , t h e r e was no a p r i o r i r e a s o n t o e x p e c t s u c h behavior. T h i s s u g g e s t s t h a t as t h e s o l v e n t i s e v a p o r a t e d t h e p o l y m e r c o n f o r m a t i o n on t h e s u r f a c e a p p r o a c h e s t h e e q u i l i b r i u m c o n f o r m a t i o n , as i t does when t h e d r y s u r f a c e a d s o r b e d p o l y m e r i s c o o l e d f r o m h i g h t e m p e r a t u r e s , where i t i s e f f e c t i v e l y desorbed. S i m i l a r s t u d i e s (l£) w i t h PVAc a d s o r b e d on t h e g l a s s s p h e r e s i n d i c a t e s a s m a l l e r f r a c t i o n o f m o b i l e u n i t s a t any t e m p e r a t u r e compared t o t h e S i 0 s t u d i e s . Since the composition o f the Cab-0-Sil S i 0 and t h e s o d a l i m e g l a s s d i f f e r , i t i s t e m p t i n g t o a s c r i b e t h i s t o d i f f e r e n c e s i n segment-sur f a c e i n t e r a c t i o n . T h e r e i s , h o w e v e r , a n o t h e r p o s s i b i l i t y due t o t h e d i f f e r e n c e i n s u r f a c e morphology. The PVAc has a r a d i u s o f g y r a t i o n r a n g i n g f r o m ^70 t o o v e r 200 A, d e p e n d i n g on t h e m o l e c u l a r w e i g h t and presence or absence o f s o l v e n t . The 3-8 y g l a s s s p h e r e s , w i t h a r a d i u s o v e r 100 t i m e s t h e r a d i u s o f g y r a t i o n o f t h e p o l y m e r s , p r e s e n t e f f e c t i v e l y a f l a t s u r f a c e t o the polymer. By c o n t r a s t t h e C a l - 0 - S i l S i 0 , composed o f c l u s t e r s o f 70 A r a d i u s s p h e r e s , p r e s e n t a h i g h l y curved s u r f a c e t o t h e polymer c h a i n . It is c l e a r t h a t many c h a i n s a r e a s s o c i a t e d w i t h more t h a n one c l u s t e r as t h r e e d i m e n s i o n a l n e t w o r k s a r e f o r m e d . The o b s e r v e d d i f f e r ences may be t h e n a m o r p h o l o g i c a l one r a t h e r t h a n a d i f f e r e n c e i n segment-surface i n t e r a c t i o n . g

2

2

2


g

POLYMER

CHARACTERIZATION

B Y E S R A N DN M R

Figure 9. Temperature dependence of PVAc (6 χ 10 ) in the bulk state and when adsorbed onto SiO (obtained by drying a sample adsorbed from CCl at 50% surface coverage) s

z


k

LIANG

ET AL.

Conformation

and Mobility

of

Polymers

Figure 10. Temperature dependence of PS in the hulk state, and when adsorbed onto SiO, (obtained by drying a sample adsorbed from CCIJ. The glass transition for bulk PS is ~ 100°C.


14

POLYMER

CHARACTERIZATION

BY

ESR

AND

NMR

L a t e r a l Dependence. Tn t h e p r e s e n c e o f s o l v e n t , l o o p s were found t o have s e g m e n t a l m o b i l i t y s i m i l a r t o t h a t o f n o n - a d s o r b e d polymer. I t was t h u s o f i n t e r e s t t o deduce how s e g m e n t a l m o b i l i t y o f l o o p s i n t h e s o l i d s t a t e were a f f e c t e d by t h e s u r face. Two t y p e s o f e x p e r i m e n t s were d e s i g n e d t o p r o v i d e an a n s w e r , examples o f w h i c h a r e shown i n F i g u r e s 11 and 12. At h i g h e r s u r f a c e c o v e r a g e t h e mean t h i c k n e s s i n t h e d r y s t a t e i s l a r g e r , h e n c e more p o l y m e r segments a r e i n v o l v e d i n l o o p s . It i s c l e a r f r o m F i g u r e s 1 1 , 9 and 3 t h a t a t s a t u r a t i o n s u r f a c e c o v e r a g e ( d r i e d f r o m CCl ) most o f t h e s p i n l a b e l s have mo b i l i t i e s s i m i l a r t o t h a t i n t h e b u l k polymer i n t h e absence o f a surface. However, i n F i g u r e 12 we see t h a t a t s a t u r a t i o n s u r f a c e c o v e r a g e , when d r i e d from CHC^ , t h e r e i s a c o n s i d e r a b l e e f f e c t of the surface. This i s e a s i l y explained. The p o l y m e r uptake i s s o l v e n t dependent. I n f a c t , a t 50% s a t u r a t i o n i n CCl^ t h e p o l y m e r u p t a k e i s n e a r l y i d e n t i c a l t o s a t u r a t i o n c o v e r a g e i n CCli> (19) . Thus when t h e s o l v e n t i s removed t h e t h i c k n e s s o f t h e p o l y m e r c o a t i s t h e same upon d r y i n g a sample 50% s a t u r a t e d f r o m C C ^ as a t s a t u r a t i o n c o v e r a g e f r o m CHC-£ , and s h o u l d b e h a v e s i m i l a r l y i f e q u i l i b r i u m c o n f o r m a t i o n i s achieved. C o m p a r i s o n o f t h e a p p r o p r i a t e s p e c t r a ( F i g u r e 12 and r e f e r e n c e 20_) shows t h i s t o be t r u e . I f one t a k e s t h e s u r f a c e a r e a o f t h e C a b - 0 - S i l Κ 200 m /gm) and assumes t h a t t h e e n t i r e s u r f a c e i s e q u a l l y a c c e s s i b l e t o t h e p o l y m e r , t h e mean p o l y m e r t h i c k n e s s u n d e r t h e two c o n d i t i o n s i s ^6 51 and ^12 51. From t h e s e r e s u l t s one c a n s t a t e t h a t t h e s u r f a c e does n o t a f f e c t t h e m o b i l i t y o f a segment when i t i s more t h a n a few a n g s t r o m s from t h e s u r f a c e . u

3

3

2

Segmental Dynamics. Two t y p e s o f d y n a m i c s can be c o n s i d e r e d - s e g m e n t a l m o t i o n , and d i f f u s i o n o f t h e p o l y m e r f r o m t h e s u r face. P r e v i o u s l y we h a v e s e e n t h a t a t t h e s o l i d - l i q u i d i n t e r face segmental motion i n loops appears t o d i f f e r l i t t l e from t h a t i n d i l u t e , d i s s o l v e d polymer, unless the loops are h e l d c l o s e t o the polymer s u r f a c e . In the dry s t a t e temperature s t u d i e s i n d i c a t e d t h a t segmental motion of loops d i f f e r e d l i t t l e from segmental motion i n b u l k polymer. Inasmuch as s e g m e n t a l m o t i o n above T^ becomes r a p i d , t h e d i f f u s i o n o f t h e polymer o f f the s u r f a c e can be m o n i t o r e d , an example o f w h i c h i s shown i n F i g u r e 13. A sample o f l a b e l e d PVAc was s u r f a c e a d s o r b e d (£50$ s a t u r a t i o n c o v e r a g e ) and d r i e d . I t was t h e n b l e n d e d a t - 8 g C w i t h u n l a b e l e d PVAc, and c o m p r e s s i o n m o l d e d a t 9000 p s i a t 30 C. The sample was r a p i d l y r a i s e d t o lU6 C and m o n i t o r e d . By t h e t i m e t h e f i r s t s p e c t r u m was o b t a i n ed (^8 m i n u t e s ) a l l segments had been d e s o r b e d and r e p l a c e d by o v e r c o a t e d , u n l a b e l e d p o l y m e r , i . e . , t h e l a b e l e d p o l y m e r had d i f f u s e d from t h e s u r f a c e i n t o t h e b u l k polymer. Although t h i s seemed a t f i r s t t o be s u r p r i s i n g , when one c o n s i d e r s t h a t t h e g


1.

LIANG

ETAL.

Conformation

and Mobility

LOW

of

15

Polymers

HIGH

Figure 11. Temperature dependence of PVAc (6 X 10 ) on Si0 when adsorbed from CCI, to 50% of saturation coverage (low) before drying, or at saturation coverage (high) before drying s

2

Dried from

Figure 12. Comparison of the temperature dependence of PVAc (6 Χ 10 ) on Si0 when adsorbed at saturation coverage from CCL, and from CHCl before drying 5

2

3


16

POLYMER

Figure 13.

CHARACTERIZATION

BY ESR A N D N M R

Diffusion of PVAc from Si0 surface into bulk polymer (see text for details) 2


1. LIANG ET AL.

Conformation and Mobility of Polymers 17

thickness of the labeled polymer was less than 10 X, the time scale seems more reasonable. Pi scussion The adsorption of PVAc and polystyrene has been studied previously (l_,2_) . With PVAc at glass-toluene and glass-benzene interfaces, thicknesses ranging from hundreds to thousands of angstroms have been reported (_31_,j$2_, _33,j>M . In these polymersolvent-surface systems we would conclude that the mean thickness was at most tens of angstroms, while in the PVAc-CHC£-Si0, PVAc-CC^ -Si0 , polystyrene-CHC>£ -Si0 and PVAc-CHCi-glass systems at saturation, thicknesses approaching the polymer radius of gyration are compatible with the data. The nitroxide labeling technique seems an excellent approach for exploring a number of factors concerned with polymer conformation and dynamics at the solid-liquid interface, and with the adsorbed polymer in the absence of solvent. It is difficult to compare many of the results with theoretical predictions, as a numerical value for the differential adsorption energy parameter is difficult to determine. Also many theoretical predictions pertain to the infinite chain and the isolated molecule. The recent theory of Scheutjens and Fleer (l_5) takes into account many factors of experimental importance, and is presented in a form which can be qualitatively compared with experiment. For high chain length (r=1000) the fraction of nonbound segments is predicted to increase with surface coverage, and to be greater for athermal solvents than for theta solvents. We observe these trends. However, we do not find evidence for long, dangling tails, as predicted under some circumstances. Our measurements on the dynamics of dry, adsorbed polymer has no known theoretical counterpart. 3

l+

2

3

2

2

3

Acknowledgements This work was supported by the Petroleum Research Fund (87**9AC5,6), administered by the American Chemical Society, NIH (GM 16922), and NSF (undergraduate summer fellowship to PND) . Literature Cited 1. Patrick, R. L., Ed. "Treatise on Adhesion and Adhesives"; Marcel Dekker: New York, 1967. 2. Lipatov, Yu. S.; Sergeeva, L. M. "Adsorption of Polymers"; Halsted Press: New York, 1974. 3. Mittal, K. L., Ed. "Adsorption at Interfaces"; Amer. Chem. Soc.: Washington, D.C.; 1975. 4. Simha, R.; Frisch, H. L.; Eirich, F. R. J. Phys. Chem., 1953, 57, 584.


18

14.

POLYMER CHARACTERIZATION BY ESR AND NMR

5. 6. 7. 8. 9. 10. 11. 12. 13.

Silberberg, A. J. Phys. Chem., 1962, 66, 1872. DiMarzio, E. A. J. Chem. Phys., 1965, 42, 2101. Rubin, R. J. J. Chem. Phys., 1965, 43, 2392. Roe, R. J. J. Chem. Phys., 1965, 43, 1591. Motomura, Κ. Matuura, R.; J. Chem. Phys., 1969, 50, 128l. Silberberg, A. J. Chem. Phys., 1968, 48, 2835. Hoeve, C.A.J. J. Polym. Sci. C., 1970, 30, 361. Hoeve, C.A.J. J. Polym. Sci. C., 1971, 34, 1. Roe, R. J. J. Chem. Phys., 1971, 60, 4192. Helfand, E. Macromolecules, 1976, 9, 307. 15. Scheutjens, J.M.H.M.; Fleer, C.J. J. Phys. Chem., 1979, 83, 1619. 16. Miller, W. G.; Veksli, Z. Rubber Chem. and Tech., 1975 , 48, 1978. 17. Miller, W. G.; Rudolf, W. T.; Veksli, Z.; Coon, D. L.; Wu, C. C.; Liang, Τ. M. in MMI Press Symposium Series, Vol. 1, "Molecular Motion in Polymers by ESR"; Harwood Academic Publ. GmbH: Chur, Switzerland, 1979. 18. Miller, W. G.; Liang, T. M. Polymer Preprints, 1979, 20(2), 189. 19. Liang, T. M.; Miller, W. G. J. Colloid Interface Sci., xxxx. 20. Liang, T. M.; Tan, S. W.; Dickson, P. N.; Miller, W. G. J. Colloid Interface Sci., xxxx. 21. Fox, Κ. Κ.; Robb, I. D.; Smith, R. J. Chem. Soc. Faraday Trans. I., 1974, 70, 1186. 22. Robb, I. D.; Smith, R. Eur. Polym. J., 1974, 10, 1005. 23. Clark, A. T.; Robb, I. D.; Smith, R. J. Chem. Soc. Faraday Trans. I., 1978, 72, 1489. 24. Robb, I. D.; Smith, R. Polymer, 1977, 18, 500. 25. Hyde, J. S.; Dalton, L. R. in "Spin Labeling II", L. Berliner, Ed.; Academic Press: New York, 1979; Chap. 1. 26. Miller, W. G. in "Spin Labeling II", L. Berliner, Ed.; Academic Press: New York, 1979; Chap. 4. 27. Veksli, Z.; Miller, W. G. Macromolecules, 1977, 10, 686. 28. Regen, S. L. J. Am. Chem. Soc., 1974, 96, 5275. 29. Fox, T. G. Polymer, 1962, 3, 111. 30. Rudolph, W. T. "The Conformation of Synthetic Polymers Adsorbed at a Solid, Liquid Interface"; M. S. Thesis, University of Minnesota, 1976. 31.Öhrn, O. Ε. Arkiv Kemi, 1958, 12, 397. 32. Tuijnman, C.A.F.; Hermans, J. J. J. Polym. Sci., 1957, 25, 385. 33. Rowland, F. W.; Eirich, F. R. J. Polym. Sci. A-1, 1966, 4, 2401. 34. Mizuhara, K.; Hara, K.; Imoto, T. Kolloid Z. U. Polymere, 1969, 229, 17. RECEIVED March 4,

1980.


Polymer Characterization by ESR and NMR - American Chemical

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