Perfluorinated Ionomer Membranes - ACS Publications - American

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Introduction H. L. YEAGER Department of Chemistry,

Downloaded by WASHINGTON & LEE UNIV on June 17, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch001

University of Calgary, Calgary, Alberta T2N 1N4, Canada A. EISENBERG Department of Chemistry, McGill University, Montreal, Quebec, Canada The past 15 years have witnessed an explosive growth in the literature on ionomers, i.e. ion-containing copolymers in which up to 15 mol % of the repeat units contain ionic groups. Two monographs have appeared on the subject ,as well as proceedings of several symposia , in addition to a large number of individual publications and patents. Furthermore, Chemical Abstracts has started to issue, as part of their "CA Selects", the series entitled "Ion-Containing Polymers" ca. 2 years ago, and a biennial Gordon Conference on this topic started in 1979. In 1984, the Macromolecular Secretariat of the American Chemical Society w i l l devote 4 days at the Philadelphia ACS meeting to a comprehensive symposium on coulombic Interactions in Polymers of which a major part w i l l be devoted to the ionomers. This major research effort devoted to these macromolecules can be understood i f one realizes that the incorporation of ions into organic polymers can modify the properties of the materials profoundly. Increases in the glass transition by five hundred degrees , increases in the modulus by over three orders of magnitude and increases in the viscosity by over four orders magnitude have been observed, among many other effects. Of direct relevance to this symposium, the incorporation of ions, even in small amounts (0-10 mol %), can yield polymeric membranes in which the diffusion coefficient for water is orders of magnitude greater than in the non-ionic parent polymer while the membrane, at the same time, becomes permselective. Most of the research effort on the ionomers has been devoted to only a small number of materials, notably the ethylenes''', the styrenes (8). the rubbers (9). and those based on poly(tetrafluoroethylene), the last of which is the subject of the present volume. As a result of these extensive investigations, i t has become clear that the reason for the dramatic effects which are obsverved on ion incorporation i s , not unexpectedly, the aggregation of ionic groups in media of low dielectric constant. Small angle X-ray and neutron scattering, backed up by a wide range of other techniques, have demonstrated clearly the existence of ionic (1,2)

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0097-6156/82/0180-0001$05.00/0 © 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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PERFLUORINATED IONOMER MEMBRANES

s c a t t e r i n g c e n t e r s i n t h e ionomers which a r e n o t p r e s e n t i n t h e parent polymer o f t h e n o n - i o n i c a c i d copolymers. Specifically i n the styrene carboxylates, d i e l e c t r i c d Raman'^' s t u d i e s h a v e s u g g e s t e d t h e e x i s t e n c e o f two t y p e s o f a g g r e g a t e s , i . e . small m u l t i p l e t s c o n s i s t i n g of several i o n p a i r s held together by s t r o n g c o u l o m b i c i n t e r a c t i o n s , a n d much l a r g e r c l u s t e r s r e s u l t i n g from weaker i n t e r a c t i o n s . W h i l e t h e m u l t i p l e t s a r e s t a b l e over the e n t i r e temperature range i n v e s t i g a t e d , t h e c l u s t e r s b r e a c k down p r o g r e s s i v e l y i n t o m u l t i p l e t s w i t h i n c r e a s i n g tem perature. The m a j o r i t y o f r e s e a r c h w h i c h i s d i s c u s s e d i n t h i s v o l u m e deals with theNafion brand p e r f l u o r o s u l f o n a t e polymers, m a n u f a c t u r e d b y Ε. I . d u P o n t de Nemours a n d Co.. T h e s e m a t e r i a l s were d e v e l o p e d d u r i n g t h e m i d d l e I 9 6 0 s , and have been a v a i l a b l e i n v a r i o u s forms f o r s t u d y d u r i n g t h e p a s t few y e a r s . The s y n t h e s i s a n d g e n e r a l p r o p e r t i e s o f N a f i o n membranes a r e s u m m a r i z e d b e W " ^ T e t r a f l u o r e t h y l e n e r e a c t s w i t h SO3 t o f o r m a c y c l i c s u l t o n e . A f t e r rearrangement, t h e s u l t o n e can then be r e a c t e d w i t h hexaf l u o r o p r o p y l e n e epoxide t o produce s u l f o n y l f l u o r i d e adducts, where


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The s o d i u m c o u n t e r i o n s i n t h i s i o n exchange p o l y m e r c a n b e r e a d i l y exchanged f o r o t h e r m e t a l i o n s o r hydrogen i o n b y s o a k i n g t h e p o l y m e r i n a n a p p r o p r i a t e aqueous e l e c t r o l y t e s o l u t i o n . F o r c o m m e r c i a l m a t e r i a l s , m i s p r o b a b l y e q u a l t o one a n d η v a r i e s f r o m a b o u t 5 t o 11. T h i s g e n e r a t e s a n e q u i v a l e n t w e i g h t r a n g e o f a b o u t 1000 t o 1500 grams o f d r y h y d r o g e n i o n f o r m p o l y m e r p e r m o l e o f exchange s i t e s . Membranes a r e p r o d u c e d i n n o m i n a l t h i c k n e s s e s f r o m a b o u t 0.1 mm t o 0.3 mm. V a r i o u s m o d i f i c a t i o n s c a n b e made t o a b a s i c N a f i o n homo geneous polymer f i l m t o produce m a t e r i a l s w i t h s p e c i a l characteristics. Open weave T e f l o n f a b r i c c a n b e l a m i n a t e d i n t o the p o l y m e r f i l m f o r i n c r e a s e d s t r e n g t h . A l s o , c o m p o s i t e membranes h a v e b e e n made i n w h i c h l a y e r s o f two d i f f e r e n t e q u i v a l e n t w e i g h t s of p o l y m e r f i l m a r e l a m i n a t e d t o g e t h e r . T h i s i s u s e f u l b e c a u s e h i g h e r e q u i v a l e n t w e i g h t p o l y m e r s show i n c r e a s e d a n i o n r e j e c t i o n under e l e c t r o l y s i s c o n d i t i o n s , which i s d e s i r a b l e i n c e r t a i n a p p l i c a t i o n s s u c h a s i n c h l o r - a l k a l i membrane c e l l s . Higher equivalent weight f i l m s a l s o e x h i b i t l a r g e r e l e c t r i c a l r e s i s t a n c e s though. C o m p o s i t e membranes a r e t h u s u s e d t o o p t i m i z e p e r m s e l e c t i v i t y and e l e c t r i c a l p r o p e r t i e s . S u r f a c e t r e a t m e n t h a s a l s o b e e n employed t o g e n e r a t e membranes w i t h improved h y d r o x i d e i o n r e j e c t i o n c a p a b i l i t y f o r c h l o r - a l k a l i applications. I n t h i s p r o c e d u r e , one s u r f a c e o f a s u l f o n y l f l u o r i d e XR r e s i n f i l m i s t r e a t e d w i t h a n amine s u c h a s e t h y l e n e diamine. A f t e r h y d r o l y s i s , a t h i n b a r r i e r l a y e r o f weakly a c i d i c s u l f o n a m i d e exchange s i t e s i s f o r m e d . When t h i s t r e a t e d s u r f a c e f a c e s t h e cathode s o l u t i o n , improved h y d r o x i d e r e j e c t i o n i s r e a l i z e d i n a membrane c h l o r - a l k a l i c e l l . An i n t e r e s t i n g a n d i m p o r t a n t p r a c t i c a l a s p e c t o f N a f i o n p e r f l u o r o s u l f o n a t e membranes i s t h e i r a b i l i t y t o s o r b r e l a t i v e l y l a r g e amounts o f w a t e r a n d o t h e r s o l v e n t s . The p o l y m e r s t y p i c a l l y s o r b 10-50% o f t h e i r d r y w e i g h t i n w a t e r , d e p e n d i n g u p o n p o l y m e r e q u i v a l e n t w e i g h t , c o u n t e r i o n f o r m and t e m p e r a t u r e o f e q u i l i b r a t i o n . Counterions w i t h large hydration energies increase water uptake, as do l o w e q u i v a l e n t w e i g h t s . I t i s i n t e r e s t i n g t h a t up t o 5 0 % more w a t e r i s t a k e n up i f t h e m a t e r i a l s a r e b o i l e d i n w a t e r , compared t o room t e m p e r a t u r e e q u i l i b r a t i o n , a n d t h a t t h i s i n c r e a s e d w a t e r i s p e r m a n e n t l y r e t a i n e d a f t e r coo l i n g ( 1 5 ) . I n c r e a s e s i n water s o r p t i o n a r e accompanied b y d e c r e a s e s i n e l e c t r i c a l r e s i s t a n c e and t e n s i l e s t r e n g t h , a s might be e x p e c t e d Q 2 ) . N a f i o n membranes s o r b even l a r g e r amounts o f o t h e r s o l v e n t s a s w e l l , p a r t i c u l a r l y a l c o h o l s and o t h e r p r o t i c s o l v e n t s ^ 1 2 - 1 6 ) .

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


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C h e m i c a l l y , the p e r f l u o r o s u l f o n a t e N a f i o n polymers a r e q u i t e u n r e a c t i v e . They a r e s t a b l e i n s t r o n g b a s e s , s t r o n g o x i d i z i n g a n d r e d u c i n g a c i d s , c h o l o r i n e , o x y g e n , h y d r o g e n , and h y d r o g e n p e r o x i d e a t t e m p e r a t u r e s a t l e a s t up t o 125°C. T h e r m a l s t a b i l i t y i s e x c e l l e n t t o 200°C o r h i g h e r , d e p e n d i n g on c o u n t e r i o n f o r m . O t h e r p e r f l u o r i n a t e d c a t i o n e x c h a n g e membrane m a t e r i a l s h a v e a l s o b e e n p r o d u c e d f o r c h l o r - a l k a l i c e l l and o t h e r a p p l i c a t i o n s . These a r e t h e F l e m i o n membrane p r o d u c t s ( A s a h i G l a s s Co. L t d . ) , Neosepta-F membranes (Tokuyama Soda Co. L t d . ) , and t h e p e r f l u o r i n a t e d membranes p r o d u c e d b y t h e A s a h i C h e m i c a l I n d u s t r y Company. T h e s e membranes a r e d i s c u s s e d i n d e t a i l i n c h a p t e r s 14 t o 17. Because o f t h e t e c h n o l o g i c a l importance o f t h e p e r f l u o r i n a t e d ionomers, as w e l l as t h e n o v e l s t r u c t u r a l f e a t u r e s encountered i n t h e s e m a t e r i a l s , a w i d e r a n g e o f p h y s i c a l and p h y s i c o - c h e m i c a l t o o l s h a v e b e e n b r o u g h t t o b e a r on t h e p r o b l e m s r e l a t e d t o t h e s t r u c t u r e of these polymers. S t r u c t u r a l l y , t h e m a t e r i a l s a r e q u i t e complex. I n a d d i t i o n t o a s m a l l amount o f c r y s t a l l i n i t y ( d i s c u s s e d i n c h a p t e r s 10 a n d 11) two d i s t i n c t n o n - c r y s t a l l i n e r e g i o n s a r e p r e s e n t , t h e h y d r o p h o b i c f l u o r o c a r b o n p h a s e and t h e h y d r o p h i l i c i o n i c a r e a s . H y d r a t i o n f u r t h e r complicates t h e p i c t u r e because o f the s m a l l s i z e s o f t h e r e g i o n s . The v a r i o u s t e c h n i q u e s d e s c r i b e d i n t h e f i r s t two s e c t i o n s o f t h e v o l u m e p r o b e d i f f e r e n t a s p e c t s o f t h e s e s t r u c t u r a l f e a t u r e s , a n d , b e c a u s e o f t h e i r c o m p l e x i t y as w e l l as t h e d i f f e r e n t r e g i m e s t o w h i c h t h e v a r i o u s t o o l s a d d r e s s thems e l v e s , no s i n g l e a g r e e d - u p o n v i e w o f t h e s t r u c t u r e o f t h e s e p o l y m e r s h a s emerged a s y e t . Among t h e c l a s s i c a l t e c h n i q u e s , s m a l l a n g l e X - r a y s c a t t e r i n g ( c h a p t e r s 10 and 11) f o c u s e s on d i f f e r e n c e s i n e l e c t r o n d e n s i t y and i s t h u s p a r t i c u l a r l y u s e f u l i n d e t e c t i n g h e t e r o n g e n i e t i e s due t o h e a v y m e t a l atoms i n t h e ionic clusters. S m a l l angle neutron s c a t t e r i n g (chapter 1 2 ) , by c o n t r a s t , i s s e n s i t i v e t o t h e p r e s e n c e o f p r o t o n s , and h a s t h u s been used e x t e n s i v e l y i n t h e study of hydrated m a t e r i a l s . Q u a s i e l a s t i c n e u t r o n s c a t t e r i n g , i n a d d i t i o n , p r o v i d e s some i n f o r m a t i o n on t h e m o b i l i t y o f t h e w a t e r m o l e c u l e s on t h e c h a r a c t e r i s t i c t i m e s c a l e of the experiment. The s p e c t r o s c o p i c t e c h n i q u e s , on t h e o t h e r h a n d , p r o b e i n d i v i d u a l s p e c i e s w h i c h make up t h e v a r i o u s r e g i o n s . I n f r a r e d ( c h a p t e r 8) a n d n u c l e a r m a g n e t i c r e s o n a n c e ( c h a p t e r 7) a d d r e s s t h e m s e l v e s t o w a t e r and t h e i n t e r a c t i o n s o f w a t e r w i t h t h e v a r i o u s s p e c i e s w i t h w h i c h i t i s i n c o n t a c t . Mossbauer s p e c t r o s c o p y ( c h a p t e r 9 ) , i n a d d i t i o n , p r o v i d e s v a l u a b l e i n f o r m a t i o n on t h e p r o x i m i t y o f t h e c a t i o n s and t h e i r e n v i r o n m e n t . Mechanical ( c h a p t e r 6) a n d t r a n s p o r t ( c h a p t e r 4) p r o p e r t i e s p r o v i d e more i n d i r e c t i n s i g h t i n t o t h e s t r u c t u r a l a s p e c t s , which i s supp l e m e n t e d by t h e r m o d y n a m i c s t u d i e s ( c h a p t e r s 2 and 5) o f t h e i n t e r a c t i o n b e t w e e n t h e p o l y m e r and w a t e r o r o t h e r l i q u i d s . A l l t h e s e t e c h n i q u e s a r e d i s c u s s e d i n t h e p r e s e n t v o l u m e , and f r o m t h e s e s t u d i e s s e v e r a l s t r u c t u r a l m o d e l s h a v e emerged ( c h a p t e r 1 3 ) . The m a j o r i m p e t u s f o r t h e d e v e l o p m e n t o f t h e p e r f l u o r i n a t e d



1.

YEAGER A N D EISENBERG

Introduction

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s u l f o n a t e and c a r b o x i l a t e membranes w h i c h a r e t h e s u b j e c t o f t h i s monograph h a s b e e n t h e i r a p p l i c a t i o n i n t h e c h l o r - a l k a l i i n d u s t r y . The h i g h s t r e n g t h and c h e m i c a l s t a b i l i t y o f t h e s e p o l y m e r s h a s been coupled w i t h e x c e l l e n t p e r m s e l e c t i v i t y c h a r a c t e r i s t i c s f o r several materials. T h i s a c h i e v e m e n t h a s e n a b l e d a new p r o c e s s t o be d e v e l o p e d f o r t h e m a n u f a c t u r e o f c h l o r i n e and s o d i u m h y d r o x i d e , two o f t h e l a r g e s t t o n n a g e c h e m i c a l s p r o d u c e d i n t h e w o r l d . The membrane c e l l method i s now b e g i n n i n g t o b e a d o p t e d , d u e t o a d v a n t a g e o u s e c o n o m i c a n d e n v r i o n m e n t a l c o n s i d e r a t i o n s i n compar i s o n w i t h t h e m e r c u r y c e l l a n d a s b e s t o s d i a p h r a g m p r o c e s s e s . The i m p o r t a n c e o f t h i s a d v a n c e i n s y n t h e t i c membrane t e c h n o l o g y i s comparable t o t h e development o f asymmetric r e v e r s e osmosis membranes f o r w a t e r d e s a l i n a t i o n i n t h e e a r l y I 9 6 0 ' s . I n b o t h c a s e s , a membrane was d e s i g n e d w i t h a s e r i e s o f s p e c i f i c p r o p e r t i e s t o s o l v e an i m o r t a n t problem. A d d i t i o n a l accomplishments o f t h i s k i n d a r e needed i n membrane s c i e n c e i f t h e i n h e r e n t a d v a n t a g e s o f membrane s e p a r a t i o n s y s t e m s a r e t o b e f u l l y u t i l i z e d i n t h e chemical industry. Thus t h e s t u d y o f t h e s e p e r f l u o r i n a t e d i o n o m e r s i s i m p o r t a n t not o n l y because o f the fundamental s i g n i f i c a n c e o f t h e i r i o n c l u s t e r e d morphologies, b u t a l s o becuase t h i s r e s e a r c h w i l l h e l p t o p r o v i d e a s c i e n t i f i c f o u n d a t i o n f o r f u t u r e developments i n membrane s c i e n c e . The w o r k w h i c h i s d i s c u s s e d i n t h i s monograph r e p r e s e n t s t h e e f f o r t s o f many w o r k e r s t o e s t a b l i s h t h i s f o u n d a tion.

References 1. 2. 3.

4. 5. 6. 7. 8. 9. 10.

Holiday, L., Ed. "Ionic Polymers", Halstead Press, Wiley, New York 1975. Eisenberg, A. and King, M. "Ion Containing Polymers", Academic Press, New York 1977. a) ACS Polymer Preprints, Am. Chem. Soc. Div. Polymer Chem. 9(1) 505-546, 583-622, 1968. b) Bikales, Ν. Μ., Ed. "Water Soluble Polymers", Plenum Press, New York 1973. c) Eisenberg, Α . , Ed. J. Polymer Sci. C, Polymer Symposium 45 (1974). d) Rembaum, A. and Selegny, Ε . , Eds. "Polyelectrolytes and their Applications", VII Reidel, Dordrecht 1975. e) Eisenberg, Α . , Ed. "Ions in Polymers", Adv. in Chem. Series 187, Am. Chem. Soc. 1980. Reference 2, p. 57. Reference 2, p. 149. Eisenberg, Α . ; Ovans, K . ; and Yoon, Η. Ν . , Reference 3e, Chapter 17. Reference 1, Chapter 2. Reference 2, p. 141-162. Reference 1, p. 173-207. Hodge, I. M. and Eisenberg, Α . , Macromolecules 11, 283(1978).


6 11. 12.


13. 14. 15. 16.


Neppel, Α.; Butler, I. S. and Eisenberg, Α . , Macromolecules 12,948(1979). J . Poly. S c i . , Polym. Phys. 17, 2145(1979), J. Mocromolecules Sci. B19,61(1981). Grot, W. G. F . ; Munn, G. E.; Walmsley, P. N. "Perfluorinated Ion Exchange Membranes", presented at the 141st National Meeting of the ElectrochemicalSSociety, Houston, Texas, May 7-11, 1972. Vaughan, D. J . Du Pont Innovation 1973, 4(3),10-13. Grot, W. G. F. Chem. Ing. Tech. 1978, 50, 299-301. Steck, Α.; Yeager, H. L. Anal. Chem. 1980, 52, 1215-18. Yeo, R. S. Polymer 1980, 21, 432-35.

RECEIVED October 27, 1981.


Perfluorinated Ionomer Membranes - ACS Publications - American

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