Sulfonated Poly(styrene-Divinylbenzene) Networks - ACS Symposium

23 Sulfonated Poly(styrene-Divinylbenzene) Networks Scission Study Using Aqueous Size Exclusion Chromatography 1

Downloaded by FUDAN UNIV on January 29, 2017 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch023

DAVID H. FREEMAN and XUN LIANG Department of Chemistry, University of Maryland, College Park,MD20742 Several reports have been made on the degradation by hydrogen peroxide (Fenton's reagent) of ion exchange resins (sulfonated PSDVB). The reaction causes weight l o s s , swelling and eventual d i s s o l u t i o n (1). Diffusion and secondary chemical reactions are possible; the s c i s s i o n rates vary oppositely with the amount of c r o s s l i n k i n g (2). The rate d i f f e r s among the DVB isomers, meta or para, used i n the o r i g i n a l PSDVB copolymerization (3). Few s t r u c t u r a l t o o l s are a v a i l a b l e to assess the structure of crosslinked networks d i r e c t l y . A frequent approach i s to derive such information from k i n e t i c study of unreacted monomer during the polymerization process (4). The p o s s i b i l i t i e s for d e r i v i n g s t r u c t u r a l information by characterizing network fragments has not been f u l l y explored. The groundwork for the present study has been developed i n previous studies. For example, s c i s s i o n through peroxide oxidation or u l t r a s o n i c treatment of polystyrene chains has been found by s i z e exclusion chromatography to involve p r e f e r e n t i a l attack at the mid-chain p o s i t i o n (5). Given t h i s evidence i t i s expected that branched polymers should give a c o r r e s p o n d i n g l y skewed m o l e c u l a r w e i g h t d i s t r i b u t i o n . T h i s r e a s o n i n g suggests one o f t h e pathways by which a s c i s s i o n e x p e r i m e n t may c o n v e y t o p o l o g i c a l i n f o r m a t i o n . PSDVB c o p o l y m e r s a n d t h e i r i o n e x c h a n g e derivatives consist o f a three dimensional four-connected network s t r u c t u r e . S u c h n e t w o r k s may have a s t a t i s t i c a l l y i s o t r o p i c s t r u c t u r e t h a t i n c l u d e s t e t r a h e d r a l c e l l s , s u c h a s t h e "X" u n i t s t r u c t u r e d e s c r i b e d by F l o r y ( 6 ) . The f o u r - c o n n e c t e d n e s s r e s u l t s from the expected p a i r w i s e c h a i n c o n n e c t i n g f u n c t i o n o f O n leave from Department o f Chemistry, N a n k a i University, Tianjin, People's Republic o f China 7

0097-6156/84/0245-O355S06.00/0 © 1984 American Chemical Society Provder; Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

SIZE E X C L U S I O N C H R O M A T O G R A P H Y

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t h e DVB u n i t s . However, a more c o m p l e t e t o p o l o g i c a l model f o r such networks s h o u l d , a t l e a s t i n p r i n c i p l e , p r o v i d e f o r t h e 34 c o n f i g u r a t i o n s d e s c r i b e d b y Z i a b e c k i (7). Although i t i s important t o consider the t h e o r e t i c a l p e r s p e c t i v e s , experiments t h a t probe polymer molecular topology are r a r e indeed. I f one assumes t h a t a n e t w o r k c o n s i s t s o n l y o f F l o r y t e t r a h e d r a l c e l l s , o r X - u n i t s , t h e a v e r a g e mass o f t h e u n i t c e l l c a n be e s t i m a t e d f r o m t h e monomers used i n the r e a c t i o n mixture. Consider a r e a c t i o n m i x t u r e t h a t i n c o r p o r a t e s f m o l e s o f a sum, D, o f m e t a a n d p a r a DVB i s o m e r s p l u s a n a s s u m e d e q u a l p o r t i o n E, o f t h e u s u a l m e t a a n d p a r a i s o m e r s o f t h e EVD ( e t h y l v i n y l b e n z e n e ) c o n t a m i n a n t s , p l u s 1-f moles o f styrene. The e s t i m a t e d X - u n i t c o n t a i n s a n a v e r a g e o f ( 1 - f ) / f m o l e s o f s t y r e n e p e r m o l e o f DVB. The mass o f t h e a v e r a g e X - u n i t , MX, c a n be c a l c u l a t e d f r o m t h e following expression: MX = ( ( 1 - f ) / f ) M S

+ ME +

MD

(1)

w h e r e M r e f e r s t o m a s s a n d S, E, a n d D r e f e r t o t h e i n c o r p o r a t e d moitiés f r o m s t y r e n e , p l u s t h e a s s u m e d e q u a l m o l e f r a c t i o n s , f , o f DVB a n d EVB m o n o m e r s , respectively. T h e mass o f t h e s u l f o n a t e d X - u n i t i n t h e c a t i o n exchange d e r i v a t i v e , i s o b t a i n e d by m o d i f y i n g t h e v a l u e s o f MS, ME a n d MD b y a d d i n g t h e a p p r o p r i a t e s u l f o n a t e and c o u n t e r i o n masses. The a v e r a g e m a s s o f a s i n g l e c h a i n c r o s s l i n k s , MC i s e s t i m a t e d f r o m : MC = (MX -

MD)/2

between

(2)

The d i v i s i o n b y t w o d e n o t e s t h e t o p o l o g i c a l r e q u i r e m e n t ( 6 ) o f t w o i n t e r - c r o s s l i n k c h a i n s p e r DVB, i n t h e p r e s e n t model o f a c l o s e d X-type network s t r u c t u r e . ( C o n s i d e r two X u n i t s w i t h t h e i r c h a i n ends j o i n e d together. T h e r e a r e f o u r c h a i n l i n e s and two vertices.) An e x a m p l e o f t h e p o s s i b i l i t y t h a t n e t w o r k s c i s s i o n e x p e r i m e n t s may b e s u b j e c t t o t o p o l o g i c a l i n t e r p r e t a t i o n i s s u g g e s t e d by t h e r e s u l t s r e p o r t e d by Hookway a n d S h e l t o n ( 2 ) . O f p a r t i c u l a r i n t e r e s t i s t h e d e g e l a t i o n p o i n t where t h e network d i s s o l v e s . (Degelation i m p l i e s t r a n s i t i o n through a g e l point that may o r may n o t b e r e l a t e d s t r u c t u r a l l y t o t h e u s u a l non-gel t o g e l t r a n s i t i o n observed i n the corresponding network s y n t h e s i s . T h e d a t a ( r e f . 2, F i g . 3 ) show t h a t h y d r o g e n p e r o x i d e c a u s e s t h e r e l e a s e o f a b o u t 0.5 m o l e

Provder; Size Exclusion Chromatography ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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of carbon dioxide per c r o s s l i n k t o reach the degelation point. S i n c e t h e r e a r e t w o l e n g t h s o f c h a i n s p e r DVB v e r t e x , t h i s c o r r e s p o n d s t o about one mole o f c a r b o n d i o x i d e r e l e a s e d f o r each i n t e r c r o s s l i n k c h a i n i n t h e o r i g i n a l network. This suggests the p o s s i b i l i t y that t h e s c i s s i o n r e a c t i o n may b e t o p o l o g i c a l l y s e l e c t i v e a n d i t may b e o f v a l u e f o r i n v e s t i g a t i n g t h e t o p o l o g y of fragment f o r m a t i o n , and f o r s t u d y i n g t h e c h e m i s t r y of s c i s s i o n degradation. The g o a l o f t h e p r e s e n t w o r k i s t o e x a m i n e t h e f e a s i b i l i t y of obtaining topologically significant r e s u l t s from s c i s s i o n experiments, and t o determine whether t h e t o p o l o g y o f b r a n c h e d s t r u c t u r e s c a n be studied using topologically selective scission processes. ( I t has n o t y e t been proved t h a t any r e a c t i o n o f f e r s such s e l e c t i v i t y . ) The f i r s t s t e p , a s w i l l be d e s c r i b e d , i s t o e x a m i n e t h e h i g h p o i n t s o f t h e molecular weight d i s t r i b u t i o n o f the s c i s s i o n fragments. The p r e s e n t e x p e r i m e n t a l a p p r o a c h i s b a s e d o n t h e chromatographic advantages p r o v i d e d by t h e d i o l o r g l y c e r o l d e r i v a t i v e s o f porous s i l i c a s t a t i o n a r y phases a v a i l a b l e f o r u s e i n HPLC. T h e s e h a v e r e c e n t l y become available f o r estimating the molecular size of p o l y e l e c t r o l y t e s u s i n g aqueous s i z e e x c l u s i o n chromatography. The c o n d i t i o n s f o r r e p r o d u c i b l e p o l y e l e c t r o l y t e s i z e measurements, and t h e i r p o s s i b l e p u r t u r b a t i o n s have been summarized by B a r t h ( 8 ) . EXPERIMENTAL B i o - R a d AG50W r e s i n s ( s u l f o n a t e d P S D V B ) , 5 0 - 1 0 0 m e s h , w e r e t r e a t e d w i t h NaOH a n d d i s t i l l e d w a t e r washes. T h e w e i g h i n g s t a t e was o b t a i n e d a f t e r 12 h o u r s o f d r y i n g i n a i r a t 7 5 - 8 0 C. R e a g e n t g r a d e c h e m i c a l s were used t h r o u g h o u t . The s c i s s i o n r e a c t i o n was c a r r i e d o u t w i t h a f i x e d a d d i t i o n o f 1.50g o f t h e d r y r e s i n , 10 mg o f f e r r o u s s u l f a t e h e p t a h y d r a t e a n d 5 0 m l o f 3 % w/v h y d r o g e n peroxide i n a round Pyrex f l a s k . The e v o l v e d c a r b o n d i o x i d e was v e n t e d t o t h e a t m o s p h e r e t h r o u g h s e r i a l t r a p s c o n t a i n i n g s u l f u r i c a c i d f o l l o w e d by a soda l i m e s o r p t i o n tube. The m a g n e t i c a l l y s t i r r e d r e a c t i o n f l a s k was s u b m e r g e d i n a n o i l b a t h h e a t e d w i t h a n i m m e r s e d e l e c t r i c a l c o i l and a magnetic s t i r r e r p o s i t i o n e d below the bath. T h e t e m p e r a t u r e w a s m a i n t a i n e d a t 5 0 +/- 1 C. A f t e r v a r i e d t i m e s 1.0 m l s a m p l e s o f l i q u i d w e r e withdrawn. There were fewer t h a n s i x w i t h d r a w a l s i n a g i v e n r e a c t i o n sequence.


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SIZE EXCLUSION CHROMATOGRAPHY


The l i q u i d c h r o m a t o g r a p h i c a n a l y s i s w a s c a r r i e d o u t u s i n g s e r i a l 4x300mm u - B o n d a g e l E-125 a n d E - 5 0 0 columns o b t a i n e d from Waters A s s o c i a t e s , I n c . The c a r r i e r w a s p r e p a r e d t o c o n t a i n ( A ) 0.25M s o d i u m p e r c h l o r a t e , 0.1% sodium l a u r y l s u l f a t e t h a t was d i s s o l v e d a n d b r o u g h t t o pH 7.2 u s i n g ammonium p h o s p h a t e a n d ( B ) t e t r a h y d r o f u r a n . A n A/B r a t i o o f 9:1 was m i x e d a n d f i l t e r e d t h r o u g h a 0.2um membrane. I t i s n o t e d t h a t t h e s e a n a l y t i c a l c o n d i t i o n s were not problem-free. P e r i o d column washing w i t h water and f r e q u e n t pump d i s s e m b l y a n d c l e a n i n g w e r e n e c e s s a r y t o c o m p e n s a t e f o r c o l u m n a n d a p p a r a t u s f o u l i n g t h a t may have been caused by h i g h e r m o l e c u l a r weight homologs i n the sodium l a u r y l s u l f a t e a d d i t i v e . The c a l i b r a t i o n s t a n d a r d s i n c l u d e d s o d i u m f o r m p o l y s t y r e n e s u l f o n a t e s o b t a i n e d from P r e s s u r e Chemical Co., P i t t s b u r g h , P a . , a n d s o d i u m t o l u e n e s u l f o n a t e * M e a s u r e m e n t s w e r e t a k e n a t 0.5 t o 1.Oml/rain f l o w r a t e s . The l o g a r i t h m o f t h e m o l e c u l a r w e i g h t o f t h e s t a n d a r d s was l i n e a r i t s u g g e s t s a f r a m e w o r k f o r a p p r o a c h i n g a n i n t e r p r e t i o n o f the s t r u c t u r e o f the s c i s s i o n products. This application of size exclusion chromatography measurements must be v i e w e d a s a f i r s t a p p r o x i m a t i o n because o f the unmeasured d i f f e r e n c e s between the chromatographic behavior o f the l i n e a r standards and the expected branched s t r u c t u r e o f the s c i s s i o n products. RESULTS S c i s s i o n r e a c t i o n s were c a r r i e d out w i t h n o m i n a l 4, 8 a n d 12 m o l e %DVB w h e r e f = 0.04, 0.08 a n d 0.12, respectively. The c o r r e s p o n d i n g t i m e s r e q u i r e d t o r e a c h d e g e l a t i o n w e r e e s t i m a t e d a s 4, 7.5 a n d 10 h o u r s . The t i m e u n c e r t a i n t y o f t h e d e g e l a t i o n " p o i n t " i s e s t i m a t e d a s 0.2 t o 0.5 h r . The t r e a t m e n t w i t h h y d r o g e n p e r o x i d e c a u s e d t h e r e s i d u a l r e s i n w e i g h t t o d e c r e a s e w i t h t i m e . The w e i g h t o f t h e 12 %DVB r e s i n m e a s u r e d a f t e r d r y i n g w a s o b s e r v e d t o u n d e r g o a l i n e a r d e s c e n t s t a r t i n g a t 1.5g a n d f a l l i n g t o z e r o a t 10 h r s w h e r e d e g e l a t i o n occurred. T h e r e s u l t s a r e shown i n F i g u r e 1. T h i s shows t h a t t h e i n t e r m e d i a t e s c i s s i o n pathway i s a m a c r o s c o p i c a l l y c o n t i n u o u s p r o c e s s unmarked b y a b r u p t change i n the c h e m i c a l pathway. Fragmentation s t a r t s at the b e g i n n i n g o f the d e g r a d a t i o n and an accompanying weight l o s s occurs u n t i l d i s s o l u t i o n i s complete.


23.

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Time

(hours)


2.0

αϊ

F i g u r e 1. The measured dry weight a f t e r hydrogen peroxide s c i s s i o n o f s u l f o n a t e d PSDVB ( 1 2 $ DVB c a t i o n exchange r e s i n ) i s seen t o decrease l i n e a r l y w i t h r e a c t i o n time. The time obtained by e x t r a p o l a t i n g t o zero weight corresponds t o v i s u a l o b s e r v a t i o n d e g e l a t i o n i n d i c a t e d by the disappearance o f the r e s i n p a r t i c l e s .


359


360


The a q u e o u s s i z e e x c l u s i o n c h r o m a t o g r a m s o b t a i n e d f o r the t h r e e r e s i n s throughout t h e i r s c i s s i o n were marked by an e a r l y appearance o f a dominant peak a t m o l e c u l a r weight 200 D a l t o n s . Similarly, a l l analyses i n the v i c i n i t y o f t h e time o f d e g e l a t i o n were marked by a m a j o r p r o m i n a n c e w i t h a n e s t i m a t e d m o l e c u l a r w e i g h t o f 2000 D a l t o n s o r s l i g h t l y l a r g e r . W i t h X8 a n d X12 we a l s o o b s e r v e d a s m a l l p e a k o f i n t e r m e d i a t e m o l e c u l a r weight i n the range between 400 and 900 Daltons. The v a r i a t i o n o f t h e SEC a n a l y s e s w i t h t i m e w a s examined i n d e t a i l w i t h the X12 r e s i n . The r e s u l t s a r e s h o w n i n F i g u r e 2. T h e e a r l y a n d s u s t a i n e d p r e s e n c e o f the m o l e c u l a r weight 200 peak i n d i c a t e s f o r m a t i o n o f f r a g m e n t s whose m o l e c u l a r w e i g h t c o r r e s p o n d s t o t h e pendant s u l f o n a t e d a r o m a t i c r i n g s ( o r a p o s s i b l y r e l a t e d d e g r a d a t i o n product) a s an i n i t i a l l y prorainant and s u b s e q u e n t l y c o n t i n u i n g f e a t u r e o f t h e s c i s s i o n process. C l o s e i n s p e c t i o n o f t h e chromatograms showed the appearance o f an u n r e s o l v e d s a t e l l i t e peak o f v a r i a b l e apparent area corresponding t o s t i l l s m a l l e r size molecules. F o l l o w i n g the emergence o f the p r e c e d i n g l o w m o l e c u l a r w e i g h t p e a k , t h e d e g r a d a t i o n moved i n t o dominance by l a r g e r s i z e fragments i n d i c a t e d by one peak o f 500-750 m o l e c u l a r weight accompanied by a l e s s e r peak w i t h m o l e c u l a r weight i n the range o f 2000-2500. A s t h e d e g r a d a t i o n moved i n t o t h e h a l f way p o i n t a n d beyond, the r e l a t i v e amounts o f m a t e r i a l r e p r e s e n t e d by t h e s e two peaks were r e v e r s e d , t h e l a r g e r m o l e c u l a r weight b e i n g c l e a r l y dominant a t t h e time o fd e g e l a t i o n . The m o l e c u l a r w e i g h t o f t h e s e t w o p e a k s c a n b e c o m p a r e d t o t h e r e f e r e n c e v a l u e s o f MC = 8 7 3 . 2 a n d MX = 1 9 8 7 . 6 c a l c u l a t e d f r o m E q u a t i o n s 2 a n d 1, r e s p e c t i v e l y , f o r f = 0.12. The c l o s e c o r r e s p o n d e n c e b e t w e e n t h e f r a g m e n t s m o l e c u l a r w e i g h t s , 500-750 o b s e r v e d (873.2 c a l c u l a t e d ) and 2000-2500 o b s e r v e d (1987.6 c a l c u l a t e d ) , l e a d s t o the c o n c l u s i o n t h a t l i n e a r c h a i n fragments and X - u n i t s are a p p a r e n t l y both formed a f t e r the s c i s s i o n process begins. The early formation of r e l a t i v e l y small soluble fragments o f m o l e c u l a r weight 200 i s f o l l o w e d byan i n c r e a s i n g amount o f f r a g m e n t o f m o l e c u l a r w e i g h t 7 5 0 b u t t h e a b s e n c e o f t h e .2000-2500 m o l e c u l a r w e i g h t s . A t t h e m i d - p o i n t o f the d e g r a d a t i o n t h e 2000-2500 m o l e c u l a r weight peak a r i s e s a n d then dominates the d e g r a d a t i o n product mixture. The f o r m a t i o n o f s m a l l f r a g m e n t s w i t h m o l e c u l a r weight near 200 suggests t h a t pendant group s c i s s i o n



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Molecular Weight (Daltons)

F i g u r e 2. S i z e e x c l u s i o n chromâtοgrams o f samples taken d u r i n g hydrogen p e r o x i d e degradation o f the 12% DVB sample whose mass d e p l e t i o n i s shown i n F i g u r e 1. The p o t e n t i a l f o r formation o f t o p o g i c a l l y s i g n i f i c a n t s c i s s i o n fragments i s i n d i c a t e d . The apparent molecular weight a t 200 Daltons i s c l o s e t o t h a t o f the s u l f o n a t e d pendant aromatic r i n g s . Peaks i n the range 2000-2500 are near the c a l c u l a t e d mass (1978.6) o f t h e average X-unit c e l l d e f i n e d by F l o r y ( 6 ) . Peaks o f approximate molecular weight 500-750 are comparable t o expected average i n t e r c r o s s - l i n k chain mass (873).


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accompanies and probably precedes chain s c i s s i o n . Unit s c i s s i o n i s r e q u i r e d t o remove a pendant g r o u p fragment. The d e l a y e d a p p e a r a n c e o f the 500-750 peak and s t i l l l a t e r a p p e a r a n c e o f t h e 2000-2500 peak a r e consistent with t o p o l o g i c a l requirements that release of a l i n e a r c h a i n a s a s c i s s i o n product r e q u i r e s two c u t s o n t h e same c h a i n w h i l e a t l e a s t f o u r c u t s o n f o u r c o n t i g u o u s c h a i n s a r e r e q u i r e d t o remove a n X - u n i t . I t i s worth n o t i n g t h a t evidence i s not apparent for the formation o f s t i l l l a r g e r molecular weight f r a g m e n t s , r e f e r r i n g t o f r a g m e n t s whose m o l e c u l a r w e i g h t s w o u l d i m p l y t w o o r more X - u n i t s c o n n e c t e d b y a s i n g l e unbroken c h a i n . A possible explanation for this i s t h a t the f o r m a t i o n o f such l a r g e r fragments i s statistically less unlikely. Moreover, even i f such l a r g e r f r a g m e n t s were formed, i t i s p o s s i b l e t h a t they would be t r a p p e d w i t h i n the network. Their eventual r e l e a s e would be e c l i p s e d b y a d i f f u s i o n impediment t h a t c o u l d enhance t h e i r r e m a i n i n g a s s t a t i o n a r y t a r g e t s f o r s c i s s i o n , f o r example, o f a s i n g l e c o n n e c t i n g c h a i n t h a t would form two sub-fragment X-units. O n c e t h e l a t t e r c u t w e r e made, t h e p o s s i b l e d i f f u s i o n a l b a r r i e r w o u l d be l o w e r e d . To s u m m a r i z e , t h e f o l l o w i n g h i e r a r c h y i n t h e formation o f s c i s s i o n fragments i s consistent with experimental results:

SCISSION FRAGMENT

NO. OF CUTS

ORDER OF APPEARANCE

pendant group

1

Initial

chains

2

Second

4

Third

X-units Higher

(+) (+-+,

etc.)

6(etc)

the

Not

observed

The p r e s e n t e x p e r i m e n t s may b e s u b j e c t t o some u n c e r t a i n t y i n terms o f molecular weight estimates and d i f f u s i o n e f f e c t s that c o u l d a f f e c t the exactness o f t h e s e i n t e r p r e t a t i o n s . The c o n c l u s i o n i s r e a c h e d t h a t the experiments s t r o n g l y suggest evidence f o r discontinuous topological quantification. I n other words, the order and r a t e o f fragment r e l e a s e i s c o n s i s t e n t w i t h e x p e c t a t i o n s based on fragment topology.


23.

F R E E M A N A N D I IANG

Sulfonated

Networks

363


Further study o f network fragments w i l l e v e n t u a l l y r e q u i r e coping w i t h t h e p o s s i b i l i t y o f a l a r g e r range of s c i s s i o n fragments t h a n have been i d e n t i f i e d here. The r e a s o n f o r e x p e c t i n g t h e a d d e d c o m p l e x i t y stems from t h e f a c t that s i z e e x c l u s i o n chromatography i s n o n - i n t e r a c t i v e and t h e r e f o r e has an obvious tendency t o mask c h e m i c a l d i f f e r e n c e s b e t w e e n m o l e c u l e s o f d i f f e r e n t composition but s i m i l a r size. Even s o , t h e p o t e n t i a l f o r using the c h a r a c t e r i z a t i o n o f network fragments t oprobe t h e t o p o l o g i c a l aspects o f branched or c r o s s l i n k e d polymer s t r u c t u r e emerges a s a n a r e a that i n v i t e s further study.

LITERATURE CITED 1.

Wood, W., J. Phys.

Chem. 61, 832 (1957).

2. Hookway, H . T . , and Selton, 493-4 (1958).

B . , J. Phys.

3. Wiley, R . , and Reich, 3174-6 (1968).

J.

E.,

Chem. 62,

Polym. S c i . A - 1 , 6,

4. Dusek, Κ . , and P r i n s , W., Adv. Polym. S c i . 6,1-102 (1969). 5. Smith, W . B . , and Temple, 4613-9 (1968). 6.

Z i a b i c k i , A., Polymer 20,

7.

Flory,

P.J.,

J.

Phys.

H.W., J. Phys.

Chem. 72,

1373-1381 (1979).

Chem. 11, 512 (1943).

RECEIVED October 31, 1983


Sulfonated Poly(styrene-Divinylbenzene) Networks - ACS Symposium

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