Perfluorinated Ionomer Membranes - ACS Publications - American

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Mechanical Relaxations in Perfluorosulfonate Ionomer Membranes THEIN KYU and ADI EISENBERG

Downloaded by UNIV OF ROCHESTER on July 11, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch006

Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada

This review article describes the relaxation phenomena of Nafions as determined by regular (dry state) and under-water stress relaxation and dynamic mechanical (torsion pendulum and vibrating reed) studies. The thermal stability and the glass transition temperatures of the membranes, as examined by differential scan ning calorimetry, linear thermal expansion and density studies are also reviewed. Dry state stress relaxation studies show a move ment of the primary relaxation curve to higher temperatures upon ionization of the precursor and neutralization of the acid. This movement results from the strong interactions within the ionic domains as deduced from results of the under-water stress relaxa tion studies. In dynamic mechanical studies, three relaxation peaks termed α, β, and γ , in descending order of temperature, are observed. The original assignments of the mechanisms of these relaxations are reanalysed in connection with recent results of the water sensitivity and the structure of the ionic aggregates in the material. A review is also presented on the mechanical relax ations of the precursor as investigated by dynamic mechanical methods, and the relaxation peaks are discussed in conjunction with the dielectric studies. In addition, the effects of various parameters, such as the effects of degree of neutralization, of the type of counterion, and of the degree of crystallinity on the mechanical relaxations are described.

It is well known that a wide range of physical properties of polymers can be modified profoundly through ion incorporation (1,2)· In many materials, the polymer matrix is effectively crosslinked through the association of these ionic groups which form small aggregates termed multiplets and larger aggregates termed clusters. The ionic aggregates can relax thermally, with the temperature of the relaxation depending on a range of molecu lar parameters which influence the structure of the ionic domains. The relaxation of the ionic aggregates yields a new peak in addi tion to that of the glass transition of the matrix; this new peak 0097-6156/82/0180-0079$08.00/0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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u s u a l l y appears a t a h i g h e r temperature than t h a t of the m a t r i x . T h i s i s a f e a t u r e t h a t i s not encountered i n n e u t r a l polymers but i s q u i t e common f o r i o n o m e r s . Moreover, the g l a s s t r a n s i t i o n o f the polymer m a t r i x i s r a i s e d to s i g n i f i c a n t l y h i g h e r temperatures as a r e s u l t o f t h e s t r o n g i o n i c i n t e r a c t i o n s . Many s t u d i e s (3-11) h a v e b e e n d e v o t e d t o t h e e l u c i d a t i o n o f the s t r u c t u r e o f t h e i o n i c domains and t h e i r e f f e c t on v a r i o u s p r o p e r t i e s . The r h e o l o g i c a l p r o p e r t i e s o f t h e p o l y m e r a r e s t r o n g l y i n f l u e n c e d by v a r y i n g t h e p a r a m e t e r s o f t h e i o n s u c h as t h e n a t u r e o f t h e i o n i c comonomer, t h e i o n c o n c e n t r a t i o n , t h e c o u n t e r i o n t y p e , t h e d e g r e e o f n e u t r a l i z a t i o n and so on ( 1 2 - 1 7 ) . Since a s i z a b l e amount o f o r i g i n a l l i t e r a t u r e ( 3 - 1 7 ) , r e v i e w p a p e r s ( 1 8 - 2 1 ) , b o o k s (1,2) and p r o c e e d i n g s o f s y m p o s i a (22-25) r e l e v a n t t o i o n - c o n t a i n i n g p o l y m e r s a r e a v a i l a b l e , no g e n e r a l s u r v e y on t h e p r o g r e s s o f t h e f i e l d w i l l be g i v e n h e r e . S e v e r a l ionomer systems a r e under e x t e n s i v e i n v e s t i g a t i o n . One o f t h e s e , w h i c h has r e c e n t l y become i n c r e a s i n g l y i m p o r t a n t i n e l e c t r o c h e m i c a l a p p l i c a t i o n s , and w h i c h i s one o f t h e m a t e r i a l s d i s c u s s e d e x t e n s i v e l y i n t h i s volume, i s the N a f i o n ionomer family. These m a t e r i a l s w e r e d e v e l o p e d by t h e d u P o n t company, and c o n s i s t of h y d r o p h o b i c f l u o r o c a r b o n backbone c h a i n s , w i t h h y d r o p h i l i c p e r f l u o r i n a t e d e t h e r s i d e c h a i n s t e r m i n a t e d by s u l f o n i c a c i d g r o u p s or corresponding a l k a l i s a l t s . The N a f i o n s p o s s e s s many e x c e p t i o n a l p r o p e r t i e s which are not encountered i n o t h e r ionomer systems, p a r t i c u l a r l y the h i g h water p e r m e a b i l i t y (26,27), p e r m s e l e c t i v i t y w i t h regard to i o n t r a n s p o r t (28-30), d u r a b i l i t y i n s t r o n g a l k a l i (26) , t h e r m a l s t a b i l i t y ( 2 6 , 3 1 ) , and o t h e r s . Many o f t h e s e p r o p e r t i e s a r e d i s c u s s e d i n d e t a i l i n t h i s v o l u m e . The u n d e r s t a n d i n g o f t h e s t r u c t u r e s o f t h e i o n i c a g g r e g a t e s and o f t h e p h y s i c a l p r o p e r t i e s o f t h e N a f i o n s a r e o f c r u c i a l i m p o r t a n c e f o r t h e improvement and d i v e r s i f i c a t i o n o f t h e i r i n d u s trial utility. V a r i o u s e x p e r i m e n t a l t e c h n i q u e s , s u c h as s m a l l a n g l e X - r a y s c a t t e r i n g ( 3 1 - 3 3 ) , n e u t r o n s c a t t e r i n g ( 3 3 ) , Mflssbauer s p e c t r o s c o p y ( 3 4 , 3 5 ) , n u c l e a r m a g n e t i c r e s o n a n c e (36) and i n f r a r e d s p e c t r o s c o p y (37,38) have b e e n e m p l o y e d f o r t h e e l u c i d a t i o n o f t h e s t r u c t u r e of the i o n i c aggregates. D e t a i l s of the experimental methods and t h e r e s u l t s o f t h e s e s t u d i e s a r e a l s o p r e s e n t e d i n v a r i o u s c h a p t e r s o f t h i s b o o k . The common c o n c l u s i o n s r e a c h e d by t h e s e and t h e p r e c e e d i n g s t u d i e s a r e t h a t t h e N a f i o n s a r e s e m i c r y s t a l l i n e polymers w i t h a low degree o f c r y s t a l l i n i t y , which i s r e l a t e d to the e q u i v a l e n t weight. The h y d r o p h o b i c f l u o r o c a r b o n and h y d r o p h i l i c i o n i c r e g i o n s i n t h e s e m a t e r i a l s a r e p h a s e s e p a r a t e d , t h e B r a g g d i s t a n c e o f t h e h y d r a t e d i o n i c domains b e i n g o f t h e o r d e r o f 5 nm. B a s e d on t h e s e s t r u c t u r a l r e s u l t s , some t h e o r e t i c a l m o d e l s have b e e n p r o p o s e d (32,36,39). The p r i m a r y o b j e c t i v e o f t h i s p a p e r i s t o r e v i e w t h e m e c h a n i c a l b e h a v i o r o f d r y and h y d r a t e d N a f i o n s w i t h an e m p h a s i s on t h e mechanical r e l a x a t i o n s . The t e n t a t i v e a s s i g n m e n t s o f t h e r e l a x a t i o n mechanisms u n d e r l y i n g t h e t h r e e m e c h a n i c a l r e l a x a t i o n s a r e d i s c u s s e d i n c o n n e c t i o n w i t h the s t r u c t u r e of the i o n i c aggregates

In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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and t h e i n f l u e n c e o f w a t e r a b s o r p t i o n . I n a d d i t i o n , v a r i o u s e f f e c t s s u c h as t h a t o f t h e d e g r e e o f n e u t r a l i z a t i o n , t h e k i n d o f c o u n t e r i o n and t h e d e g r e e o f c r y s t a l l i n i t y on t h e m e c h a n i c a l relaxations are also discussed.


Thermal S t u d i e s P r i o r t o d i s c u s s i o n o f t h e mechanical b e h a v i o r o f the N a f i o n s , some t h e r m a l p r o p e r t i e s s h o u l d be n o t e d . A d i f f e r e n t i a l scanning c a l o r i m e t r y s t u d y o f N a f i o n a c i d a n d o f amorphous N a f i o n - N a a t a h e a t i n g r a t e o f 20°C/min was c o n d u c t e d b y K y u and E i s e n b e r g (40) and t h e r e s u l t s a r e shown i n F i g u r e 1. A p r o n o u n c e d t r a n s i t i o n , p r o b a b l y a s s o c i a t e d w i t h t h e α peak ( t a n 6) o f t h e t o r s i o n p e n d u lum s t u d i e s ( 3 1 ) , was o b s e r v e d i n t h e DSC c u r v e s i n t h e t e m p e r a t u r e r e g i o n o f a b o u t 70°C f o r t h e u n d r i e d a c i d a n d o f a b o u t 140°C for the undried s a l t . S u c c e s s i v e DSC r u n s , a f t e r p r o g r e s s i v e l y s e v e r e h e a t t r e a t m e n t s , showed a movement o f t h i s t r a n s i t i o n t o higher temperatures. This i n d i c a t e s that the glass t r a n s i t i o n region of N a f i o n (matrix o r the i o n i c regions) i n c r e a s e s markedly upon d r y i n g a l t h o u g h t h e e x a c t t r a n s i t i o n t e m p e r a t u r e i s d i f f i c u l t t o d e t e r m i n e f r o m t h e s e DSC c u r v e s . The e x a c t a s s i g n m e n t o f t h i s g l a s s t r a n s i t i o n i s s t i l l i n some d o u b t ; i t w i l l therefore be d i s c u s s e d i n c o n s i d e r a b l e d e t a i l i n t h e o v e r v i e w s e c t i o n a t t h e end o f t h i s r e v i e w . I t s h o u l d be e m p h a s i z e d t h a t i n t h e N a f i o n s , as i n o t h e r p o l y m e r s , and e s p e c i a l l y i o n o m e r s , 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 c a n be s t r o n g l y i n f l u e n c e d by t h e t h e r m a l h i s t o r y and t h e moisture content of the polymer. Furthermore, i n the present c a s e , some d e c o m p o s i t i o n c a n be s e e n a t ca.. 190°C i n t h e a c i d sam p l e s , w h i c h show c o n s i d e r a b l y l o w e r t h e r m a l s t a b i l i t y t h a n i s observed i n the s a l t s . These r e s u l t s a r e c o n s i s t e n t w i t h those r e p o r t e d e a r l i e r by Yeo and E i s e n b e r g ( 3 1 ) , b a s e d on w e i g h t l o s s i n t h e r m o g r a v i m e t r i c s t u d i e s . T h i s f e a t u r e a p p e a r s t o be a common phenomenon i n s u l f o n a t e d s y s t e m s ; f o r e x a m p l e , i n t h e s u l f o n a t e d p o l y s u l f o n e s , improved thermal s t a b i l i t y i s a l s o observed i n the n e u t r a l i z e d m a t e r i a l s (2) . The l i n e a r t h e r m a l e x p a n s i o n s t u d y by T a k a m a t s u and E i s e n b e r g (41) i s r e l e v a n t t o t h e p r e s e n t d i s c u s s i o n . T h e i r measurements showed c o n s i d e r a b l e c o m p l e x i t y due t o t h e d u a l e f f e c t s o f t h e d r y i n g p r o c e d u r e and m e c h a n i c a l a n i s o t r o p y o f t h e s a m p l e on t h e results. F o r e x a m p l e , t h e l i n e a r t h e r m a l e x p a n s i o n o f an a n i s o t r o p i c N a f i o n a c i d f i l m a t a h e a t i n g r a t e o f cel. l°C/min shows a p p r e c i a b l e d i f f e r e n c e s between s u c c e s s i v e h e a t i n g r u n s . The thermal expansion c o e f f i c i e n t i s r e p r o d u c i b l e i n the temperature r a n g e b e l o w 80°C; h o w e v e r , f o r w e t o r a n i s o t r o p i c s a m p l e s , t h e s l o p e changes d r a s t i c a l l y above t h a t t e m p e r a t u r e , d e p e n d i n g on t h e number o f p r e v i o u s h e a t i n g r u n s . The s h i f t i n g o f t h e i n f l e c t i o n temperature of the l i n e a r thermal expansion curve i n s u c c e s s i v e h e a t i n g runs t o h i g h e r temperature p a r a l l e l s the r e s u l t s o f the DSC r u n s as m e n t i o n e d b e f o r e . This confirms that the g l a s s


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f o r a l o w m o l e c u l a r w e i g h t CV5 χ 10^) s t y r e n e i o n o m e r (2). The a d d i t i o n o f s m a l l amounts o f w a t e r t o t h e N a f i o n a c i d a l s o l e a d s to a b r e a k d o w n i n t i m e - t e m p e r a t u r e s u p e r p o s i t i o n . N e v e r t h e l e s s , t h e p s e u d o - m a s t e r c u r v e d e f i n i t e l y shows t h a t s t r e s s r e l a x a t i o n i s f a s t e r i n t h e wet s t a t e t h a n i s o b s e r v e d i n t h e d r y s a m p l e s . This i m p l i e s t h a t the i n t r o d u c t i o n of water a c c e l e r a t e s the r a t e of s t r e s s r e l a x a t i o n due t o t h e p l a s t i c i z a t i o n e f f e c t o f t h e w a t e r w i t h i n the i o n i c domains. The r e l a x a t i o n c u r v e s ( w i t h t h e r e f e r e n c e t e m p e r a t u r e s a t t h e g l a s s t r a n s i t i o n s as t h e y w e r e a s s i g n e d i n t h e o r i g i n a l p a p e r ) a r e b r o a d f o r t h e p r e c u r s o r and t h e i o n i z e d N a f i o n s . S i n c e the p r e c u r s o r i s a s e m i c r y s t a l l i n e polymer, the b r e a d t h of the spectrum may be m a i n l y a t t r i b u t a b l e t o t h e p r e s e n c e o f c r y s t a l s . However, i n t h e c a s e o f i o n i z e d N a f i o n s , t h i s g r e a t b r e a d t h must be a s c r i b e d n o t o n l y t o t h e i n f l u e n c e o f c r y s t a l l i n i t y b u t a l s o t o the e f f e c t of s t r o n g i o n i c aggregation. No d i s c u s s i o n i s p r e s e n t e d i n the o r i g i n a l p u b l i c a t i o n c o n c e r n i n g the s i g n i f i c a n c e of the s h i f t f a c t o r s o b t a i n e d by t i m e - t e m p e r a t u r e s u p e r p o s i t i o n . I t s h o u l d be noted t h a t i n view of the problem i n a s s i g n i n g the T s of these polymers, the p o s i t i o n of the r e l a x a t i o n curves of the N a f i o n a c i d and s a l t s s h o u l d n o t be t a k e n as a b s o l u t e . A d d i t i o n a l h o r i z o n t a l s h i f t i n g may be r e q u i r e d . O n l y t h e s h a p e s o f t h e c u r v e s a r e s i g n i f i c a n t here. I n F i g u r e 5, t h e t e n - s e c o n d t e n s i l e m o d u l u s , Ε(10 s e c ) , versus t e m p e r a t u r e p l o t s f o r t h e p r e c u r s o r , t h e N a f i o n a c i d and t h e N a f i o n - K s a l t a r e compared w i t h t h o s e o f s t y r e n e and i t s i o n o mer (42). I n e a c h c a s e , t h e v a l u e o f t h e modulus a t 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 , as i t was a s s i g n e d a t t h a t t i m e , i s o f t h e o r d e r o f ^10? N/m^, a b o u t one t o two o r d e r s o f m a g n i t u d e l o w e r than the v a l u e s f o r s t y r e n e or i t s ionomers. The p r i m a r y r e l a x a t i o n t e m p e r a t u r e o f t h e p r e c u r s o r , w h i c h i s a b o u t 10°C, rises d r a m a t i c a l l y upon i o n i z a t i o n , i . e . t o 1 1 0 ° C f o r t h e a c i d and t o 220°C f o r t h e s a l t , i f t h e h i g h - t e m p e r a t u r e r e l a x a t i o n s a r e t a k e n as t h e g l a s s t r a n s i t i o n s ( b u t see b e l o w f o r more e x t e n s i v e d i s c u s s i o n o f α and β peak a s s i g n m e n t s ) . T h i s d r a m a t i c r i s e i n the g l a s s t r a n s i t i o n i s undoubtedly r e l a t e d t o the e f f e c t s of i o n a g g r e g a t i o n , w h i c h w i l l be d i s c u s s e d i n d e t a i l i n a s u b s e q u e n t section. !

g

Under-water S t r e s s R e l a x a t i o n S t u d i e s I n o r d e r t o e l u c i d a t e t h e e f f e c t o f i o n i c a g g r e g a t i o n on the p r i m a r y r e l a x a t i o n p r o c e s s , i t seems u s e f u l t o r e d u c e t h e i o n i c i n t e r a c t i o n by t h e i n t r o d u c t i o n o f w a t e r i n t o t h e i o n i c r e g i o n s . S i n c e water i s l a r g e l y i n c o m p a t i b l e w i t h the f l u o r o c a r b o n matrix, a l t h o u g h some o f i t i s c l o s e l y a s s o c i a t e d w i t h t h e CF2 g r o u p s due to the s m a l l s i z e o f the a g g r e g a t e s ( 3 8 ) , the w a t e r - p l a s t i c i z a t i o n c a n be e x p e c t e d t o o c c u r p r e f e r e n t i a l l y i n t h e i o n i c domains r a t h e r than i n the m a t r i x . As w i l l be shown l a t e r , h o w e v e r , t h e p r o x i m i t y o f t h e w a t e r does i n f l u e n c e t h e m a t r i x Τ appreciably



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Figure Nafion

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Relaxations

5. The comparison of E(10 s) vs. temperature curves for the acid and Nafion-K; A, precursor; B, Nafion acid; C, Nafion-K; styrene', and E, s tyre ne — 9% Να MA (42).

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PERFLUORINATED

IONOMER MEMBRANES

a l s o ( 3 8 ) . The c o n t r i b u t i o n o f i o n i c a g g r e g a t i o n t o t h e p r i m a r y r e l a x a t i o n p r o c e s s can t h e n be e x p l o r e d d i r e c t l y as a r e s u l t o f changes i n t h e s t r e n g t h o r h a r d n e s s o f t h e i o n i c a g g r e g a t e s i n t h e d r y and wet s t a t e s . These c o n s i d e r a t i o n s s u g g e s t t h e u t i l i t y o f u n d e r - w a t e r s t r e s s r e l a x a t i o n s t u d i e s (40) i n w h i c h t h e sample can be s t r e t c h e d i n a w a t e r e n v i r o n m e n t . T h i s technique p e r m i t s the study o f s t r e s s r e l a x a t i o n of i o n i c m a t e r i a l s i n which the s t r o n g i o n i c i n t e r a c t i o n s h a v e b e e n r e d u c e d c o n s i d e r a b l y by i o n h y d r a t i o n . U n d e r - w a t e r s t r e s s r e l a x a t i o n o f t h e N a f i o n a c i d and N a f i o n Na s a m p l e s w i t h an e q u i v a l e n t w e i g h t o f 1200 was c a r r i e d o u t by Kyu and E i s e n b e r g ( 4 0 ) . The d e g r e e o f n e u t r a l i z a t i o n o f t h e N a f i o n - N a was m e a s u r e d t o be c a . 80%. Time-temperature superposed m a s t e r c u r v e s o f t h e two s y s t e m s , r e d u c e d t o a r e f e r e n c e t e m p e r a t u r e o f 50°C, a r e shown i n F i g u r e 6. The u n d e r - w a t e r s t r e s s r e l a x a t i o n b e h a v i o r of N a f i o n a c i d resembles that of Nafion-Na, except f o r t h e f a c t t h a t t h e e l a s t i c modulus i s somewhat l o w e r i n t h e acid. T h i s l a t t e r f e a t u r e may be due t o t h e d i f f e r e n c e i n t h e d e g r e e o f w a t e r a b s o r p t i o n o f t h e a c i d and s a l t s a m p l e s ( 2 6 , 3 1 ) . The s w e l l i n g o f N a f i o n a c i d i s g r e a t e r t h a n t h a t o f N a f i o n - N a , w h i c h y i e l d s a m a t e r i a l o f l o w e r modulus. The most s t r i k i n g f e a t u r e o f t h i s u n d e r - w a t e r s t r e s s r e l a x a t i o n i s perhaps the g r e a t s i m i l a r i t y of the superposed r e l a x a t i o n m a s t e r c u r v e s o f t h e a c i d and t h e s a l t s a m p l e s . I n c o n t r a s t , the corresponding c o n v e n t i o n a l s t r e s s r e l a x a t i o n curves i n the dry s t a t e a r e s i g n i f i c a n t l y d i f f e r e n t f r o m one a n o t h e r ( 3 1 ) . The t e n d e n c y i s more e v i d e n t i n t h e c o m p a r i s o n o f t h e t e n - s e c o n d t e n s i l e modulus Ε(10 s e c ) v e r s u s t e m p e r a t u r e c u r v e s o f N a f i o n a c i d and N a f i o n - N a i n t h e r e g u l a r ( d r y - s t a t e ) and u n d e r - w a t e r s t r e s s r e l a x a t i o n s t u d i e s , as shown i n F i g u r e 7. The t e m p e r a t u r e depen d e n c i e s o f t h e m o d u l i o f t h e d r y s a m p l e s d i f f e r a p p r e c i a b l y due t o the great d i f f e r e n c e i n t h e i r g l a s s t r a n s i t i o n s . The u n d e r - w a t e r s t r e s s r e l a x a t i o n s t u d y does n o t c o v e r a w i d e r a n g e o f t e m p e r a t u r e s due t o i n s t r u m e n t a l l i m i t a t i o n s ; h o w e v e r , o v e r t h e r a n g e i n v e s t i g a t e d , t h e r a t e o f s t r e s s r e l a x a t i o n a p p e a r s t o be t h e same i n t h e a c i d and t h e s a l t . The r e s e m b l a n c e o f t h e u n d e r - w a t e r s t r e s s r e l a x a t i o n c u r v e s and t h e d i s s i m i l a r i t y o f t h e s t r e s s r e l a x a t i o n b e h a v i o r o f t h e N a f i o n a c i d and t h e s a l t i n t h e d r y s t a t e may be e x p l a i n e d as follows. The p h a s e s e p a r a t e d h y d r o p h i l i c r e g i o n s a r e e x p e c t e d t o c o n t a i n a s u b s t a n t i a l f r a c t i o n of the e t h e r s i d e chains which are a n c h o r e d i n t h e i o n i c domains by t h e i r p o l a r end g r o u p s . I n the dry s t a t e , the coulombic i n t e r a c t i o n s w i t h i n the i o n i c aggregates a r e so s t r o n g t h a t t h e s e domains p r o b a b l y s e r v e as e f f e c t i v e c r o s s links. T h i s would not o n l y reduce the m o b i l i t y of m o l e c u l e s w i t h i n t h e domains b u t w o u l d a l s o c o n t r o l t h e m o b i l i t y o f t h e f l u o r o c a r b o n m a t r i x through the s i d e c h a i n s ; t h i s , i n t u r n , l e a d s to the r i s e i n the primary r e l a x a t i o n temperature. When t h e s a m p l e s a r e immersed i n w a t e r , t h e i o n i c domains must s w e l l due t o t h e i r h y d r o p h i l i c n a t u r e . A c c o r d i n g t o Mauritζ e t a l . , (36,37) t h e d i r e c t i n t e r a c t i o n o f t h e bound a n i o n and t h e unbound



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Figure 6. Time-temperature superposed master curves from underwater stress relaxation of the Na tion acid (Φ) and Nafion-Na (©λ reduced to a reference temperature of 50° C.

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