Surface Modifications of Cellulose and Polyvinyl Alcohol, and

a given period, the reaction was stopped by removing the uretha- nated film and then .... i n i t i a l course of phenylurethanation of PVA. This i s ...
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26 Surface Modifications of Cellulose and Polyvinyl

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Alcohol, and Determination of the Surface Density of the Hydroxyl Group T. MATSUNAGA and Y. IKADA Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan The s u r f a c e c h e m i s t r y o f p o l y m e r i c s u b s t a n c e s i s i n c r e a s i n g l y b e c o m i n g i m p o r t a n t n o t o n l y i n t h e w e l l - k n o w n phenomena s u c h as a d h e s i o n , w e t t a b i l i t y , f r i c t i o n , a n d a d s o r p t i o n , b u t a l s o i n d e v e l o p i n g new m a t e r i a l s s u c h a s c o m p o s i t e s , c h r o m a t o g r a p h i c g e l b e a d s , and b i o c o m p a t i b l e a r t i f i c i a l o r g a n s . I n studying the macroscopic i n t e r f a c i a l p r o p e r t i e s o f s o l i d polymers, i t i s h i g h l y d e s i r e d t o have a d e t a i l e d knowledge o f t h e m o l e c u l a r s u r f a c e p r o p e r t i e s such as t h e chemical c o n s t i t u t i o n . In s p i t e o f recent remarkable progress i n surface a n a l y s i s , t h e r e s t i l l r e m a i n many u n r e s o l v e d p r o b l e m s r e l a t i n g t h e s u r f a c e characteristics. For instance, although X-ray photoelectron s p e c t r o s c o p i e s s u c h a s ESCA, A u g e r s p e c t r o s c o p y a n d F o u r i e r transform attenuated t o t a l reflectance(ATR) IR spectroscopy are able t o provide i n f o r m a t i o n regarding the chemical s t r u c t u r e near the s u r f a c e r e g i o n , these a r e not p a r t i c u l a r l y powerful i n a s s a y i n g f u n c t i o n a l groups p r e s e n t o n l y a t t h e s u r f a c e i n a quantitative level. T h i s may b e b e c a u s e t h e a b s o l u t e amount o f t h e groups d i r e c t i n g t o o u t s i d e a t t h e s u r f a c e i s t o o l o w t o be quant i t a t i v e l y d e t e r m i n e d b y t h e c o n v e n t i o n a l a n a l y t i c a l methods. However, i n r e c e n t y e a r s i t h a s b e e n d e m o n s t r a t e d t h a t v e r y s m a l l amounts o f t h e f u n c t i o n a l g r o u p s n e w l y c r e a t e d a t o r c l o s e t o t h e s u r f a c e o f p o l y e t h y l e n e f i l m s b y o x i d a t i o n ( 1 ) and p o l y ( v i n y l a l c o h o l ) (PVA) f i l m s b y r e a c t i o n w i t h h e x a m e t h y l e n e d i i s o c y a n a t e ( 2 ) c a n be d e t e r m i n e d w i t h t h e u s e o f f l u o r e s c e n c e s p e c t r o s c o p y . Leclercq and h i s c o w o r k e r s h a v e a l s o m e a s u r e d t h e s u r f a c e d e n s i t y o f f u n c t i o n a l groups generated on corona t r e a t e d p o l y ( e t h y l e n e t e r e phthalate) by adsorption of r a d i o a c t i v e calcium i o n s ( 3 ) . The o b j e c t i v e o f t h i s w o r k i s t o d e t e r m i n e t h e s u r f a c e c o n c e n t r a t i o n o f t h e h y d r o x y l g r o u p s o f c e l l u l o s e a n d PVA f i l m s u t i l i z i n g t h e i r c h e m i c a l m o d i f i c a t i o n . We c h o s e t h e s e p o l y m e r s m a i n l y because t h e h y d r o x y l group i s t h e i r s o l e f u n c t i o n a l group. R e c e n t l y we h a v e r e p o r t e d t h a t a c e l l u l o s e f i l m i s more e x c e l l e n t i n w e t t a b i l i t y t o w a r d s w a t e r t h a n PVA, t h o u g h c e l l u l o s e i s i n s o l u b l e i n w a t e r , i n c o n t r a s t t o PVA(4). Since only the chemical c o m p o s i t i o n o f t h e s u r f a c e must b e r e s p o n s i b l e f o r w a t e r 0-8412-0540-X/80/47-121-391$05.00/0 © 1980 American Chemical Society In Modification of Polymers; Carraher, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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w e t t a b i l i t y p r o v i d e d t h a t b o t h f i l m s h a v e a m i c r o s c o p i c a l l y smooth s u r f a c e , t h e d e n s i t y o f t h e s u r f a c e h y d r o p h i l i c g r o u p s w i l l be d i f f e r e n t b e t w e e n t h e two f i l m s . I n t h i s work we w i l l a t t e m p t t o r e a c t w i t h i s o c y a n a t e s o n l y t h e h y d r o x y l groups present a t t h e f i l m s u r f a c e . T h i s w i l l be f o l l o w e d b y h y d r o l y s i s t o r e l e a s e t h e corresponding amines, i d e n t i f i e d b y means o f t h e f l u o r e s c e n c e a s s a y w h i c h h a s p r o v e d t o be h i g h l y s e n s i t i v e i n d e t e r m i n i n g e x ­ t r e m e l y s m a l l amounts o f a m i n e s (.5, 6 ) . I n a d d i t i o n , we w i l l d e s c r i b e s u r f a c e r e a c t i o n o f t h e f i l m s w i t h a d i p o y l c h l o r i d e and subsequent r e a c t i o n w i t h 7-hydroxycoumarin. Experimental Materials. The c e l l u l o s e f i l m s employed a r e a c e l l o p h a n e o f 0.14 mm t h i c k n e s s , d o n a t e d b y Tokyo C e l l o p h a n e Co., I n c . , T o k y o , J a p a n ( h e r e a f t e r d e s i g n a t e d as c e l l o p h a n e ) and a c e l l u l o s e t u b i n g f o r d i a l y s i s o f 0.19 mm t h i c k n e s s p r o d u c e d b y V i s k i n g Co., I n c . ( h e r e a f t e r d e s i g n a t e d a s V i s k i n g ) . The PVA f i l m w i t h a t h i c k n e s s o f 0.32 mm was s u p p l i e d b y U n i t i k a Co., I n c . , O s a k a , J a p a n . The f i l m s were c u t t o 2 χ 5 cm s i z e a n d p u r i f i e d b y t h e c o n v e n t i o n a l e x t r a c t i o n w i t h w a t e r a t room t e m p e r a t u r e , f o l l o w e d by e x t r a c t i o n w i t h a b e n z e n e - e t h a n o l ( l : 2 ) m i x t u r e a t 80°C. The c e l l u l o s e f i l m s were f u r t h e r e x t r a c t e d w i t h d i m e t h y l s u l f o x i d e ( D M S O ) a t 45°C f o r 24 h r s , r i n s e d w i t h w a t e r a n d t h e n s u b j e c t e d t o S o x h l e t e x t r a c t i o n w i t h m e t h a n o l f o r 24 h r s . I n s t e a d o f b o i l i n g w a t e r a n d EMSO, 1 Ν NaOH was u s e d f o r e x t r a c t i o n o f t h e PVA f i l m a t 45°C. A f t e r r i n s ­ i n g t h e p u r i f i e d f i l m s w i t h p l e n t y o f w a t e r , t h e y were d r i e d u n d e r vacuum a t 70°C f o r 48 h r s a n d s t o r e d i n a d e s i c c a t o r c o n t a i n i n g CaCl^. J u s t p r i o r t o u s e , t h e f i l m s w e r e a g a i n d r i e d u n d e r vacuum a t 70°C f o r 12 h r s . Reactions. η-Butyl-, p h e n y l - , α-naphthy1isocyanate, a d i p o y l c h l o r i d e , and t i n o c t o a t e were e x t r a pure grade and used w i t h o u t further purification. 7 - H y d r o x y c o u m a r i n was r e c r y s t a l l i z e d f r o m w a t e r . T o l u e n e a n d d i o x a n e were d i s t i l l e d a f t e r d r y i n g w i t h a n ­ h y d r o u s magnesium s u l f a t e a n d a m o l e c u l a r s i e v e . Urethanations o f f i l m s were c a r r i e d o u t i n t o l u e n e a t 45°C w i t h o u t c a t a l y s t a n d / o r a t 30°C w i t h 0.67 g . l " o f t i n o c t o a t e , t h e i n i t i a l i s o c y a n a t e c o n c e n t r a t i o n b e i n g 70 g . l ~ l . A f t e r being allowed t o proceed f o r a g i v e n p e r i o d , t h e r e a c t i o n was s t o p p e d b y r e m o v i n g t h e u r e t h a nated f i l m and then immersing i n a m e t h a n o l - t o l u e n e ( l : 3 ) mixture f o l l o w e d by S o x h l e t e x t r a c t i o n w i t h m e t h a n o l . Then t h e f i l m was r i n s e d w i t h w a t e r a n d d r i e d u n d e r vacuum a t 70°C f o r 24 h r s . A d i p o y l a t i o n o f f i l m s was a c c o m p l i s h e d i n t o l u e n e a t 45°C a n d a n a d i p o y l c h l o r i d e c o n c e n t r a t i o n o f 110 g . l ~ l w i t h o u t c a t a l y s t . The a d i p o y l a t e d f i l m was washed s e v e r a l t i m e s w i t h a n h y d r o u s t o l u e n e and d i o x a n e , f o l l o w e d by r e a c t i o n w i t h 7-hydroxycoumarin i n d i o x a n e c o n t a i n i n g s o d i u m h y d r i d e f o r 2 h r s a t room t e m p e r a t u r e . A f t e r c o u p l i n g t h e c o u m a r i n , t h e f i l m was e x t r a c t e d w i t h m e t h a n o l i n a S o x h l e t a p p a r a t u s , r i n s e d w i t h w a t e r , a n d d r i e d u n d e r vacuum 1

In Modification of Polymers; Carraher, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

26.

MATSUNAGA AND

IKADA

Surface Modification of Cellulose and PVA

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a t 70°C. H y d r o l y s i s and F l u o r e s c e n c e Assay. A f t e r measuring t h e e x a c t s u r f a c e a r e a , t h e u r e t h a n a t e d f i l m was p l a c e d i n 2 m l o f 1 Ν NaOH i n a g l a s s tube, purged w i t h n i t r o g e n , and then s e a l e d under a reduced pressure. A f t e r h y d r o l y s i s o f u r e t h a n e s a t 50°C f o r a g i v e n p e r i o d , we added 2 m l o f 1 N HC1 t o n e u t r a l i z e t h e r e a c t i o n product. T h r e e m l o f t h e n e u t r a l i z e d s o l u t i o n was t r a n s f e r r e d t o another g l a s s tube. To t h i s was added 7 m l o f a b o r a t e pH 10 b u f f e r e d s o l u t i o n c o n t a i n i n g 7 mg o f o - p h t h a l a l d e h y d e and 7 y l o f mercaptoethanol, except f o r the naphthylurethanated f i l m . The f l u o r e s c e n c e i n t e n s i t y f o r n - b u t y l a m i n e was r e a d 5 m i n s l a t e r a f t e r a d d i t i o n of the o-phthalaldehyde s o l u t i o n a t an e x c i t a t i o n w a v e l e n g t h o f 338 nm and a n e m i s s i o n w a v e l e n g t h o f 427 nm, w h i l e t h e f l u o r e s c e n c e f r o m a n i l i n e was r e a d 60 m i n s l a t e r a f t e r a d d i ­ t i o n o f o - p h t h a l a l d e h y d e a t a n e x c i t a t i o n w a v e l e n g t h o f 350 nm and a n e m i s s i o n w a v e l e n g t h o f 430 nm. I n t h e c a s e o f a - n a p h t h y l u r e t h a n a t e d f i l m , t h e f l u o r e s c e n c e measurement was d i r e c t l y r u n on t h e n e u t r a l i z e d s o l u t i o n w i t h o u t a d d i n g o - p h t h a l a l d e h y d e , since α-naphthylamine h a s s t r o n g f l u o r e s c e n c e a t pH 7. The e x c i t a t i o n w a v e l e n g t h e m p l o y e d i s 310 nm a n d t h e e m i s s i o n w a v e l e n g t h 440 nm. The f i l m r e a c t e d w i t h a d i p o y l c h l o r i d e f o l l o w e d by c o u p l i n g w i t h 7 - h y d r o x y c o u m a r i n was s u b j e c t e d t o m e t h a n o l y s i s a t 1 N HC1 and 60°C. The r e g e n e r a t e d c o u m a r i n was a s s a y e d a t pH 10 b y f l u ­ o r e s c e n c e s p e c t r o s c o p y a t a n e x c i t a t i o n w a v e l e n g t h o f 329 nm and an e m i s s i o n w a v e l e n g t h o f 455 nm. A H i t a c h i MPF-4 F l u o r e s c e n c e S p e c t r o p h o t o m e t e r was u s e d f o r a l l f l u o r e s c e n c e m e a s u r e m e n t s . C o n t a c t A n g l e M e a s u r e m e n t . The c o n t a c t a n g l e t o w a r d s w a t e r on f i l m s was m e a s u r e d a t room t e m p e r a t u r e w i t h t h e i n v e r t e d b u b b l e method t o a v o i d d r y i n g o f t h e f i l m d u r i n g t h e measurement. More t h a n 5 r e a d i n g s on d i f f e r e n t s t r i p s f r o m t h e same f i l m were a v e r a g e d . The d e v i a t i o n o f e a c h r e a d i n g f r o m t h e a v e r a g e was w i t h i n + 1 ° . The c o n t a c t a n g l e o f t h e a d i p o y l a t e d f i l m was measured p r i o r t o c o u p l i n g o f 7-hydroxycoumarin, b u t a f t e r h y d r o ­ xy z i n g t h e a c i d c h l o r i d e o f h a l f - e s t e r t o t h e c a r b o x y l g r o u p b y immersing i n a water-acetone(1:3) m i x t u r e . Results The r e a c t i o n scheme e m p l o y e d i n t h e s u r f a c e m o d i f i c a t i o n o f f i l m s i s d e p i c t e d i n F i g u r e 1. U r e t h a n a t i o n a s w e l l a s a d i p o y lation was i n a l l c a s e s p e r f o r m e d i n t o l u e n e t o p r e v e n t t h e r e a c t i o n s from i n v a d i n g the i n t e r i o r o f f i l m s . Since deter­ m i n a t i o n o f t h e h y d r o x y l g r o u p s a t t h e f i l m s u r f a c e r e q u i r e s com­ p l e t e r e m o v a l o f i s o c y a n a t e s and a d i p o y l c h l o r i d e r e m a i n i n g u n r e a c t e d a t t h e s u r f a c e and i n t h e b u l k p o l y m e r , t h e r e a c t e d f i l m s were s u b j e c t e d t o r i g o r o u s e x t r a c t i o n p r i o r t o f u r t h e r r e a c t i o n s . An example o f t h e r e s u l t o f e x t r a c t i o n c a r r i e d o u t f o r t h e f i l m w h i c h was n a p h t h y l u r e t h a n a t e d a t 30°C f o r 82 h r s i s g i v e n i n

In Modification of Polymers; Carraher, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

MODIFICATION O F POLYMERS

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