Solution Processing of Perfluorinated Ionomers - ACS Publications

12 Solution Processing of Perfluorinated Ionomers Recent Developments 1

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: March 28, 1986 | doi: 10.1021/bk-1986-0302.ch012

Michael J. Covitch

Diamond Shamrock Corporation, Painesville, OH 44077

Several new solvents for sulfonate and carboxylate perfluorinated ionomers such as Nafion (Ε. I. duPont de Nemours and Company) give rise to a number of interesting and technologically significant applications of these materials. The ability of a particular solvent to dissolve 1100-1200 equivalent weight ionomers is influenced by the type of bound anion as well as the cation chemistry. For example, N-butylacetamide dissolves both sulfonate and carboxylate ionomers; and i t can be used to prepare ionomer alloys having mixed functionality. Porous structures can be prepared from a solution of the ionomer precursor (-SO F or -COOCH ) in a perfluoroalkylacid solvent by pseudo eutectic solidifcation techniques. Applications of these dissolution processes for the preparation of membranes, solid polymer electrolytes, and coatings are briefly discussed. 2

3

Perfluorinated ionomers such as Nafion are of significant commercial importance as cation exchange membranes in brine electrolysis cells (I). Outstanding chemical and thermal stability make this class of polymers uniquely suited for use in such harsh oxidizing environments. The Nafion polymer consists of a perfluorinated backbone and perfluoroalkylether sidechains which are terminated with sulfonic acid and/or carboxylic acid functionality. -f. F C F - ^ C F C F ^ C

2

2

2

0 (CFgCF-O^CF^X where X = S0 M, C0 M; M = H, metal cation; Ζ > 1; η = 1,2. 3

2

1

Current address: The Lubrizol Corporation, Wickliffe, OH 44092 0097-6156/86/0302-0153S06.00/ 0 © 1986 American Chemical Society

In Coulombic Interactions in Macromolecular Systems; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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C O U L O M B I C INTERACTIONS IN M A C R O M O L E C U L A R SYSTEMS

In a previous paper (2), the author described a method to dissolve the sulfonyl fluoride precursor form of a perfluorinated sulfonate ionomer. Commercially available forms of Nafion are supplied as activated membranes ( i . e . , saponified from the precursor to the ionic form), and near-quantitative reconstitution of the precursor functionality (such as RSO^F) must first be performed using a chemical reagent such as SF- f4) before dissolution in perhalogenated solvents is possible. Besides adding to the cost of membrane manufacture, SF- is extremely toxic and corrosive and must be handled in nickel alloy pressure equipment. Therefore, a method for dissolving perfluorinated ionomers directly would be more desirable. Martin and co-workers (5) published a procedure for solubilization of perfluorinated ionomers in alcohol/water solutions using a combination of heat and pressure. Although Martin (18) has claimed that only low concentration solutions ( 3 aqueous HC1, maintaining the pH as close to three as possible by dropwise addition of 1 wt. % HC1. This procedure is necessary to maintain ion pair dissociation of the carboxylic acid group. Ion pair association begins to occur at external solution pH < 3 which causes the polymer to deswell and become milky white due to the inclusion of entrapped acid. The metal carboxylate was then prepared by the same method described above for the sulfonate ionomers. The procedure for preparation of the sulfonyl fluoride form of Nafion 117 has been outlined elsewhere (4). The carboxylic acid form of Nafion 901 was esterifieH in methanol by bubbling dry HC1 gas through the solution at 20-30°C for four hours. All samples were washed with distilled water and dried under vacuum at 80°C for 24 hours. All solvents were purchased commercially and used without further purification. Ionomer dissolution was accomplished with mechanical agitation under a blanket of dry nitrogen at approximately 10°C below the boiling point (at atmospheric pressure) of the solvent but no higher than 230°C.


12.

COVITCH

Solution Processing of Perfluorinated Ionomers


R e s u l t s and D i s c u s s i o n B e s i d e s t h e p e r h a l o g e n a t e d compounds which s u c c e s s f u l l y d i s s o l v e the s u l f o n y l f l u o r i d e and methyl c a r b o x y l a t e ionomer p r e c u r s o r s ( 2 ) , a number o f p o l a r n o n - h a l o g e n a t e d o r g a n i c s (see T a b l e I) have been i d e n t i f i e d which d i s s o l v e e i t h e r t h e s u l f o n a t e o r t h e c a r b o x y l a t ç ionomer ( 6 ) . In g e n e r a l , the s m a l l e r a l k a l i metal (such as L i , Na ) forms a r e more r e a d i l y d i s s o l v e d than those c o n t a i n i n g l a r g e r c a t i o n s . S o l u t i o n s c o n t a i n i n g 8-10% by weight o f ionomer can be r e a d i l y p r e p a r e d from most o f the s o l v e n t s l i s t e d i n T a b l e I. In c o n t r a s t t o the p r e c u r s o r s o l u t i o n s i n h a l o g e n a t e d s o l v e n t s , t h e ionomer s o l u t i o n s a r e f a r l e s s v i s c o u s and do not become g e l a t i n o u s when c o o l e d t o 3 0 ° C . In the 2 0 - 3 0 ° C t e m p e r a t u r e r e g i m e , c e r t a i n s o l u t i o n s such as those i n S u l f o l a n e ( P h i l l i p s Petroleum) begin to t h i c k e n . T h e r e f o r e , t h e s e ionomer s o l u t i o n s a r e e a s i e r t o p r e p a r e and more c o n v e n i e n t t o handle than t h e p r e v i o u s l y d e s c r i b e d p r e c u r s o r solutions. One o f t h e s o l v e n t s on T a b l e I--N-butylacetamide--dissolves both t h e s u l f o n a t e and c a r b o x y l a t e l a y e r s o f N a f i o n 9 0 1 ; s u l f o l a n e c l e a n l y d i s s o l v e s away o n l y t h e s u l f o n a t e l a y e r , l e a v i n g an i n t a c t c a r b o x y l a t e f i l m . One o f t h e most i n t r i g u i n g a p p l i c a t i o n s o f t h e s e d i s s o l u t i o n methods i s i n t h e p r e p a r a t i o n o f novel s u l f o n a t e / c a r b o x y l a t e ionomer a l l o y s . The p a t e n t l i t e r a t u r e has d e s c r i b e d t h e u s e f u l n e s s o f p e r f l u o r i n a t e d ionomer a l l o y membranes f o r b r i n e e l e c t r o l y s i s ( 7 ) , and a r e c e n t paper by Hsu and c o - w o r k e r s (8) d e s c r i b e s t h e e f f e c t s o f a l l o y morphology on membrane s e l e c t i v i t y . These a l l o y s were produced by e x t r u s i o n , a d j u s t i n g the m e l t t e m p e r a t u r e and presumably the f i l m t a k e - u p speed t o c o n t r o l morphology. U t i l i z i n g N-butylacetamide as a c o s o l v e n t , ionomer a l l o y membranes may a l s o be p r e p a r e d by standard f i l m coating techniques. Here, control of a l l o y morphology may be e x e r c i s e d by a d j u s t i n g t h e b l e n d c o m p o s i t i o n , c a s t i n g t e m p e r a t u r e , and d r y i n g r a t e . In a d d i t i o n , mixed s o l v e n t s may be used t o p r e f e r e n t i a l l y e x t e n d one domain a t t h e expense o f t h e o t h e r , t h e r e b y a d d i n g a n o t h e r d i m e n s i o n t o domain morphology c o n t r o l . I f the p r e c u r s o r forms o f the s u l f o n a t e and c a r b o x y l a t e ionomers a r e a v a i l a b l e , a l l o y s may a l s o be p r e p a r e d from s o l u t i o n s i n Halocarbon O i l (Halocarbon Products C o r p o r a t i o n ) . In a p r e v i o u s p u b l i c a t i o n (2) i t was shown t h a t s o l u t i o n s o f the s u l f o n y l f l u o r i d e p r e c u r s o r i n c e r t a i n p e r f l u o r i n a t e d s o l v e n t s such as p e r f l u o r o o c t a n o i c a c i d " f o r m s o l i d s a t room t e m p e r a t u r e . . . by v i r t u e o f t h e m e l t i n g p o i n t s o f t h e i r respective solvents." Removal o f t h e s o l v e n t by d i s s o l u t i o n o r vacuum s u b l i m a t i o n l e a v e s a porous s t r u c t u r e ( 6 ) , t h e pores h a v i n g been formed by c r y s t a l l i z a t i o n o f s o l v e n t which s e p a r a t e d by s y n e r e s i s from t h e g e l d u r i n g c o o l i n g . The porous p r o d u c t i s b e s t s a p o n i f i e d i n aqueous metal h y d r o x i d e s o l u t i o n p r i o r t o s o l v e n t removal t o s t a b i l i z e t h e pore s t r u c t u r e . Similar results have been o b t a i n e d by o t h e r s (9-13) f o r p o l y o l e f i n s d i s s o l v e d i n high melting d i l u e n t s . In t h e s e r e f e r e n c e s i t i s noted t h a t t h e two-phase morphology o f a quenched polymer s o l u t i o n i s r e l a t e d t o the o v e r a l l c o m p o s i t i o n , c o o l i n g r a t e , and quenching t e m p e r a t u r e ;


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Table I.

Solvents capable of dissolving perfluorinated ionomers and ionomer precursors at atmospheric pressure and elevated temperature. Boiling Point

Functional Group Solvent Halocarbon Oil perfluorooctanoic acid perfluorodecanoic acid perfluorotributyl ami ne FC-70 available from 3M (perfluorotrialkylamine) perfluoro-l-methyldecalin decafluorobiphenyl pentafluorophenol pentafluorobenzoic acid N-butylacetamide sulfolane (tetramethylene sulfone) N,N-dimethylacetamide N,N-diethylacetamide N,N-dimethylpropionamide N,N-dibutylformamide Ν ,N-dipropylacetami de Ν,N-dimethylformami de l-methyl-2-pyrrolidinone diethylene glycol ethylacetamidoacetate

S0 F 2

C0 "Z 2

C0 CH 2

X X X

3

S

V

Z

225-260 189 218 175 215 159 206 143 220 229 285 165 185 174 242 209 153 202 245 265

Ζ is an alkali or alkaline earth metal or a quaternary ammonium ion having attached hydrogen, alkyl, substituted alkyl, aromatic, or cyclic hydrocarbon.



12.

COVITCH

Solution Processing of Perfluorinated Ionomers

the smallest pores are generally formed by solidification of the eutectic composition. Photo-micrographs of quenched perfluorosulfonyl fluoride/perfluorooctanoic acid solutions and those found in reference 9 are quite similar. Electron micrographs and simple permeation experiments confirm the porous nature of the solvent extracted perfluorinated ionomer. Porous membranes of this type may find application in chlor-alkali diaphram cells and solid polymer electrolyte electrodes (6). Perfluorinated ionomer solutions may be applied to a variety of substrates to form fabric reinforced membranes, solid polymer electrolytes, and carboxylate-coated sulfonate membranes (6,14). Solvent cast films are as durable as extruded membranes of the same thickness and display identical ion selectivity characteristics. Current membranes are often reinforced with a fabric to reduce the possibility of tearing during handling. The fabric is sandwiched between the membrane (in the precursor thermoplastic form) and a release paper and impregnated into the membrane on a heated vacuum roll fitted with a concentric horseshoe heater (16), This process does not completely encapsulate the faEric, resulting in exposed fabric "knuckles" on the release paper side of the membrane. In addition, the fiber-like topography of the release paper is transferred to one side of the membrane. This rough membrane surface may affect electrolyte flow patterns past the membrane, resulting in stagnant pockets in which severe electrolyte depletion or concentration may occur. Concentration polarization effects increase leading to an increase in cell voltage. By comparison, solution coated fabric is totally encapsulated by the membrane, and both membrane surfaces are smooth. Thus, electrolyte flow patterns past both surfaces are similar, avoiding unusual concentration polarization affects at one surface. A solid polymer electrolyte (SPE) consists of a membrane which is in direct contact with both electrodes, thereby eliminating electrolyte gap resistance to reduce all voltage. The General Electric SPE (17) consists of two porous particulate electrodes which are bondecTcohesively with polytetrafluoroethylene dispersion particles and connected electrically to the outside of the cell hardware by means of metallic current collectors which are pressed against the SPE by mechanical methods. Such an SPE can be prepared via perfluoroionomer solution techniques. One method is to apply a paste consisting of the electrolyte powder and the perfluoroionomer solution to the membrane and evaporate the solvent. Alternately, the paste can be applied to a sacrificial substrate such as aluminum f o i l , dried, and subsequently pressed into the membrane as a decal. The use of a perfluorinated ionomer as the SPE electrode binder results in better adhesive bonding with the membrane by virtue of the fact that the binder and the membrane are of identical composition. In addition, the solvent in the electrode paste promotes "solvent welding" by softening the membrane during the application process. Perfluoroionomers may also be applied via the solution process as a protective coating to reaction vessels or other metallic equipment (15) to prevent corrosion or product build-up.


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C O U L O M B I C I N T E R A C T I O N S IN M A C R O M O L E C U L A R S Y S T E M S

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C o r r o s i o n r e s i s t a n t f l u o r o p o l y m e r c o a t i n g s a r e c u r r e n t l y marketed by W. L. Gore & A s s o c i a t e s , I n c . ( F l u o r o s h i e l d ) and P f a u d l e r . Acknowledgments The a u t h o r wishes t o thank D r s . G. H. McCain and L. L. Benezra f o r t h e i r v a l u a b l e s u g g e s t i o n s and c r i t i c i s m ; Mr. G. G. Sweetapple f o r h i s t e c h n i c a l a s s i s t a n c e ; and t o The L u b r i z c l C o r p o r a t i o n f o r i t s a s s i s t a n c e i n t h e p r e p a r a t i o n and p r e s e n t a t i o n o f the manuscript.


Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

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W. G. Grot, G. E. Munn, and P. N. Walmsley, Paper 154 presented at the Electrochemical Society Meeting, Houston, TX, May 7-11, 1972. G. H. McCain and M. J. Covitch, J . Electrochem. Soc., 1984, 131(6), 1350. Ε. I. duPont de Nemours & Company, "Nafion Perfluorinated Membranes" product literature, February 1, 1984. M. J. Covitch, U. S. Patent 4,366,262 (1982). C. R. Martin, T. A. Rhoades, and J . A. Ferguson, Anal. Chem., 1982, 54, 1641. M. J . Covitch, D. L. DeRespiris, L. L. Benezra, and Ε. M. Vauss, U.S. Patent 4,421,579 (1983). C. J . Molnar, Ε. H. Price, and P. R. Resnick, U. S. Patent 4,176,215 (1979). W. Y. Hsu, T. D. Gierke, and C. J . Molnar, Macromolecules, 1983, 16, 1947. P. Smith and A. J . Pennings, Polymer, 1974, 15, 413. E. Calahorra, G. M. Guzman, and F. Zamora, J . Polymer Sci.: Polymer Lett. Ed., 1982, 20, 181. M. J . Covitch, Eur. Pat. Appl. EP 69,516 (1983), see Chem. Abstr. 98, 127309m. P. Smith and A. J . Pennings, J . Materials Sci., 1976, 11, 1450. P. Smith and A. J . Pennings, J . Polym. Sci.: Polym. Phys. Ed., 1977, 15, 523. M. J . Covitch, M. F. Smith, and L. L. Benezra, U. S. Patent 4,386,987 (1983). S. K. Baczek, G. H. McCain, and M. J . Covitch, U.S. Patent 4, 391, 844 (1983). D. E. Maloney, U. S. Patent 4, 349, 422 (1982). T. G. Coker, R. M. Dempsey, adn A. B. La Conti, U.S. Patent 4, 210, 501 (1980) and U. S. Patent 4, 191, 618 (1980) C. R. Martin, private communication. W. G. Grot, private communcation. 5% solutions of 1000 or 1100 EW sulfonic acid resin in a mixture of lower aliphatic alcohols and water are commercially available from Solution Technology, Inc., Box 171, Menden Hall, PA 19357. N. Oyama, N. Oki, H. Ohno, Y. Ohnuki, H. Matsuda, and E. Tsuchida, J . Phys. Chem., 1983, 87, 3642.

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June 10, 1985


Solution Processing of Perfluorinated Ionomers - ACS Publications

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