Perfluorinated Ionomer Membranes - American Chemical Society

10 Morphology of Perfluorosulfonated Membrane Products Wide-Angle and Small-Angle X-Ray Studies

Downloaded by COLUMBIA UNIV on July 11, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch010

T. D .

1

GIERKE,

G . E. M U N N , and F . C. W I L S O N

Ε . I. du Pont de Nemours & Co., Inc., Plastic Products and Resins Department, Experimental Station, Wilmington, DE 19898

The morphology of the ionomer resin, from which "Nafion" ( registered trademark of the Ε. I. du Pont de Nemours and Co.) perfluorinated membrane products are made, was studied with wide angle and small angle x-ray diffraction techniques. A reflection observed in the small angle x-ray scan from hydrolyzed polymer is attributed to ionic clustering. The effects of equivalent weight, cation form, temperature, water content, and tensile draw on this reflection were studied and are discussed. "Nafion" perfluorinated membranes are constructed from perfluoinated resins containing covalently bonded ion exchange sites. A typical perfluorinated resin, used in these membranes, possesses the general chemical structure

where the value of m can be as low as 1. The value of x, in the above formula determines the equivalent weight (EW) of the resin. Typical values range between 6 and 14 and correspond to an EW range of from 1000 to 1800 grams/equivalent. The SO F group is easily hydrolysed to form the strongly acidic perfluorosulfonic acid exchange s i t e . In this form, the resin is extremely hydrop h i l i c and can absorb as much as 30 water molecules per exchange site and more than double i t s volume. As in other ionomers, the ion exchange sites in "Nafion" mem branes are observed to aggregate and form clusters. Ionic clus tering in "Nafion" membranes has been indicated by a variety of physical studies including dielectric relaxation (1), small angle x-ray scattering (1-4), neutron scattering (4), electron micro2

1

Current address: Parkersburg, WV 26101. 0097-6156/82/0180-0195$05.50/0

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

196

PERFLUORINATED IONOMER MEMBRANES


scopy (2,5), NMR ( 6 ) , IR C7, 8), Mossbauer spectroscopy (9,1£) and s e v e r a l t r a n s p o r t s t u d i e s (2,11,12). In t h i s paper we s h a l l review the r e s u l t s of s m a l l angle x-ray s c a t t e r i n g , SAXS, e x p e r i ments. We s h a l l c o n f i n e our remarks to a d e s c r i p t i o n of the pert i n e n t experimental r e s u l t s . I n a l a t e r s e c t i o n , a model f o r i o n i c c l u s t e r i n g w i l l be proposed and the e f f e c t s of i o n c l u s t e r i n g w i l l be proposed and the e f f e c t s of i o n c l u s t e r i n g on i o n t r a n s p o r t w i l l be d i s c u s s e d . Experimental. Our experiments were conducted of f i l m s of known e q u i v a l e n t weight which were 0.1 to 0.3 mm t h i c k . Samples are e a s i l y hydrolysed by h e a t i n g i n any convenient b a s i c medium, e.g. sodium hydroxide. Samples i n v a r i o u s c a t i o n forms are prepared u s i n g standard i o n exchange techniques. The amount of s o l vent the sample absorbs depends on the thermal h i s t o r y of the sample (13) and, unless otherwise s t a t e d , a l l samples were cond i t i o n e d by b o i l i n g one hour i n water. The amount of s o l v e n t absorbed by the polymer was determined g r a v i m e t r i c a l l y . Dry polymer d e n s i t i e s were determined u s i n g standard bouyancy t e c h niques on samples d r i e d f o r 18 hours i n a n i t r o g e n f l u s h e d vacuum oven maintained a t 110°C. Our values agree w i t h other reported values (13,14). The values reported by Roche et a l ( 4 ) , f o r s i m i l a r thermal h i s t o r i e s , are about a f a c t o r of two l a r g e r . To o b t a i n x-ray d i f f u s i o n r e s u l t s from s w o l l e n hydrolyzed samples, methods were developed to i n h i b i t sample dehydration d u r i n g the course of the experiment. This was achieved by s e a l ing the samples, t h e r m a l l y , i n a bag constructed from o r i e n t e d polypropylene f i l m . Oriented polypropylene was s e l e c t e d because i t possesses low p e r m e a b i l i t y to water and i s a l s o e s s e n t i a l l y " t r a n s p a r e n t " i n the s m a l l angle x-ray scans. With such a cons t r u c t i o n , we determined that the weight of a s w o l l e n sample changed by l e s s than 0.2% d u r i n g the p e r i o d r e q u i r e d to o b t a i n the s m a l l angle x-ray scans. The small-angle data were gathered on a Kratky small-angle "camera" equipped w i t h a N a l ( T l ) s c i n t i l l a t i o n counter and a N i f i l t e r f o r CuKa r a d i a t i o n . P u l s e - h e i g h t - a n a l y s i s was set to accept 90% of the CuKa r a d i a t i o n s y m m e t r i c a l l y , and the x-ray source was a s p e c i a l short (7mm) l i n e focus tube (Siemens) so that there was no v i g n e t t i n g of the source by the x-ray tube window. A f t e r s u b t r a c t i o n of i n s t r u m e n t a l background, a l l the scans were normalized to the i n t e n s i t y expected from a sample of optimum t h i c k n e s s ( I t r a n s m i t t e d / l - 1/e). The normalized small-angle i n t e n s i t i e s (except f o r h i g h l y o r i e n t e d samples) were desmeared according to the procedure of Schmidt and Hight (15,16). The desmeared i n t e n s i t i e s were m u l t i p l i e d by the square of the s c a t t e r i n g angle — t h i s can be considered as the a p p l i c a t i o n of the small-angle Lorentz, c o r r e c t i o n or the c a l c u l a t i o n of the I n v a r i a n t argument, h 1 ( h ) , where h i s the s c a t t e r i n g wave v e c t o r . The e f f e c t of t h i s data a n a l y s i s on the observed v a r i a t i o n i n amplitude w i t h s c a t t e r i n g

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

10.

GIERKE

ETAL.

X-Ray Studies of Morphology

197


angles i s shown f o r a t y p i c a l scan i n Figure 1, where the a m p l i tudes of the three t r a c e s i n t h i s f i g u r e are i n a r b i t r a r y u n i t s . (In t h i s manuscript the s c a t t e r i n g angle, 20 i s always expressed i n degrees.) Even at low water contents, a maximum, i n the observed i n t e n s i t y , i s normally detected a t s c a t t e r i n g angles of 1

1.6 2.4 SCATTERING ANGLE, 20

10

0.0

1

MEMBRANES

1.0

'—

3.2

BRAGG SPACING, d, nm 6.0 3.0

2.0 3.0 SCATTERING ANGLE, 29

4.0

Figure 12. Small angle invariant x-ray scans from hydrolyzed polymer showing effect of strain. Top, 1200 EW polymer in sodium ion form swollen with water and 1.75 draw ratio. Key: —, undrawn control; , drawn, machine direction; • • • drawn, transverse direction. Bottom, 1179 EW spun fiber in sodium ion form and swollen with water. Key: —, equatorial scan; , meridional scan.



1.6 2.4 SCATTERING ANGLE, 2 0

3.2

s

Figure 13. Small angle invariant x-ray scans from 1200 EW hydrolyzed polymer in various io\ forms swollen with water. Samples conditioned by boiling 1 h in H O. Key: —, H; — - Li; Na; ,K; , Rb; , Cs.

0.8

BRAGG SPACING, d, nm 10 5.0 4.0



212

PERFLUORINATED IONOMER

MEMBRANES

f o r h e a v i e r c a t i o n s which are a l s o l e s s h y d r o p h i l i c (13). I n f a c t i t i s p o s s i b l e , by v a r y i n g water content and c a t i o n form simultaneously, to make t h i s r e f l e c t i o n "disappear" a l t o g e t h e r . A s i m i l a r r e s u l t can be obtained by changing the s a l t concentrat i o n of the s o l u t i o n w i t h which the polymer e q u i l i b r a t e s . Again, the i n t e n s i t y of the r e f l e c t i o n decreases as the c o n c e n t r a t i o n i n c r e a s e s because the e l e c t r o n d e n s i t y of the c l u s t e r i s i n c r e a s i n g . I n t e r e s t i n g l y , even the proton form of the polymer possesses the r e f l e c t i o n a t t r i b u t e d to c l u s t e r i n g . This o b s e r v a t i o n i s i n marked c o n t r a s t to the r e s u l t s observed i n hydrocarbon analogues (20, 21). The e f f e c t of water content on the SAXS scans from s w o l l e n polymer i s i l l u s t r a t e d i n F i g u r e 14. The decrease i n i n t e n s i t y w i t h decreasing water content again r e f l e c t s the i n c r e a s i n g r e l a t i v e e l e c t r o n d e n s i t y of the c l u s t e r . Note, however, that the Bragg spacing i n c r e a s e s w i t h i n c r e a s i n g water content. This i s shown more c l e a r l y i n F i g u r e 15. E x t r a p o l a t i o n to the dry s t a t e y i e l d s Bragg spacing of about 3^ nm. R e s u l t s have a l s o been obtained on 1200 EW polymer i n H and Ag i o n form and a Bragg spacing i n dr_y polymer of 3.0 nm was a l s o obtained. The r e s u l t s f o r the Ag i o n form are p a r t i c u l a r l y i n t e r e s t i n g . As the water c o n c e n t r a t i o n i s lowered, the i n t e n s i t y of the r e f l e c t i o n f i r s t decreases, disappears, and then increases so that the r e f l e c t i o n i s observed i n the dry s t a t e of the polymer. C l e a r l y t h i s v a r i a t i o n of i n t e n s i t y w i t h water content i s again the r e s u l t of changing the e l e c t r o n d e n s i t y of the c l u s t e r w i t h respect t o the f l u o r o c a r b o n m a t r i x . Roche et a l (4) a l s o r e p o r t SAXS r e s u l t s as a f u n c t i o n of water content and t h e i r r e s u l t s are e s s e n t i a l l y i d e n t i c a l to those shown i n F i g u r e 14. D i s c u s s i o n . The r e s u l t s i n Figures 8-15 provide strong support f o r the e x i s t e n c e of i o n i c c l u s t e r i n g i n "Nafion". However, d e t a i l s of the arrangement of matter i n these c l u s t e r s cannot be obtained from s m a l l angle x-ray r e s u l t s alone. For hydrocarbon ionomers, s e v e r a l d i f f e r e n t i n t e r p r e t a t i o n s have been advanced as the cause of the SAXS maximum. These i n c l u d e a model of s p h e r i c a l c l u s t e r s on a p a r a c r y s t a l l i n e l a t t i c e proposed by Cooper et a l (22), the s h e l l core model of Macknight et a l (24) and more r e c e n t l y a l a m e l l a r model (23). At present, there i s no consensus about which of these models best d e s c r i b e s c l u s t e r i n g i n hydrocarbon ionomers. The s i t u a t i o n f o r the p e r f l u o r i n a t e d ionomers i s f u r t h e r complicated because they d i f f e r d r a m a t i c a l l y i n s e v e r a l r e s p e c t s from e t h y l e n e / m e t h a c r y l i c a c i d , (E/MA), ionomers. The SAXS peak i s observed i n the a c i d form of our polymer w h i l e i t i s not observed i n E/MA ionomers when i n a c i d form. This d i f f e r e n c e i s accounted f o r by r e c o g n i z i n g that the p e r f l u o r o s u l f o n i c a c i d exchange s i t e i s a very strong a c i d and i s completely d i s s o c i a t e d i n water. This a l s o leads to the remarkable high degree of water uptake observed i n the p e r f l u o r -


10.

GIERKE ET AL.


213

•S


ν*

"ta α

s?

§^

§s

.« "S .S g

c si ?

i

δ -S CO "-Η

a


^

214

PERFLUORINATED IONOMER

MEMBRANES

v.


•8 s

Is

Ν

II

.a



10.

GIERKE

ETAL.


215

i n a t e d ionomer, as much as 50% by volume f o r 944 EW polymer. This water a b s o r p t i o n promotes i o n i c c l u s t e r i n g i n the p e r f l u o r i n a t e d ionomer i n c o n t r a s t t o the observed tendency i n E/MA ionomer (22). Thus, the d e t a i l s of i o n i c c l u s t e r i n g i n the p e r f l u o r i n a t e d ionomer may be s u b s t a n t i a l l y d i f f e r e n t from c l u s t e r i n g i n E/MA ionomer. Any model which i s proposed f o r i o n i c c l u s t e r i n g i n these p e r f l u o r i n a t e d ionomers should be c o n s i s t e n t w i t h the r e s u l t s of these x-rays s t u d i e s . I t i s a p p r o p r i a t e , then, t o enumerate the p e r t i n e n t r e s u l t s o f these s t u d i e s . a. ) The polymer i s s e m i c r y s t a l l i n e , and the s t r u c t u r e of the c r y s t a l l i n e phase i s s i m i l a r t o the s t r u c t u r e observed i n p-TFE. b. ) The s i z e of the c r y s t a l l i t e s a r e l a r g e r than the average s e p a r a t i o n between s i d e c h a i n s (16). c. ) I o n i c c l u s t e r i n g only moderately changes the degree of c r y s t a l l i n i t y , and i o n i c c l u s t e r i n g e x i s t s i n the absence of crystallinity. d. ) The e f f e c t i v e Bragg spacing a s s o c i a t e d w i t h i o n i c c l u s t e r i n g increases w i t h i n c r e a s i n g water content and decreasing e q u i v a l e n t weight but i s f a i r l y i n s e n s i t i v e t o the c a t i o n used to n e u t r a l i z e the polymer. e. ) This r e f l e c t i o n i s observed i n the a c i d form of the resin. f. ) The r e f l e c t i o n a l s o i s observed i n the dry r e s i n . g. ) The c l u s t e r s e p a r a t i o n tends t o be normal t o the d i r e c t i o n of the polymer chains i n s t r a i n e d r e s i n s . In a l a t e r chapter we w i l l propose a model of i o n i c c l u s t e r i n g which we b e l i e v e i s an e s s e n t i a l agreement w i t h these r e s u l t s (20). The r e a l s t r e n g t h of the model, however, i s that i t can be used t o understand and d e s c r i b e i o n t r a n s p o r t through "Nafion" p e r f l u o r i n a t e d membranes 03, _25, 26). Literature Cited 1. Yeo, S. C.; Eisenberg, A. J . Appl. Polym Sci. 1977, 21, 875. 2. Gierke, T. D. 152nd Meeting of Electrochemical Society, Atlanta, Georgia, Abstract No. 438, J . Electrochem Soc. 1977, 124, 319C. 3. Gierke, T. D.; Munn, G. E.; Wilson, F. C. J . Polym. Sci, Polym. Phys. Ed, In press. 4. Roche, E. J.; Pineri, M.; Duplessix, R.; Levelut, A. M. J. Polym. Sci. Polym. Phys. Ed. 1981, 19, 1. 5. Ceynowa, J.; Polymer 1978, 19, 73. 6. Komoroski, R. A.; Mauritz, K. A. J. Amer. Chem. Soc. 1978, 100, 7487. 7. Heitner-Wirguin, A. C. Polymer 1979, 20, 371. 8. Falk, M. Am. Chem. Soc, Adv. Chem Series, this volume. 9. Rodmacq, R.; Pineri, M.; Coey, J . M. D. Rev. Phys Appl. 1980, 15, 1179.



216

PERF LUORINATED IONOMER MEMBRANES

10. Heitner-Wirguin, C.; Bauminger, E. R.; Levy, A.; Labensky de Kanter, F.; Ofer, S. Polymer 1980, 21, 1327. 11. Will, F. G. J. Electrochem. Soc. 1979, 126, 36. 12. Lopez, M.; Kipling, B; Yeager, H. L. Anal. Chem 1977, 49, 629. 13. Grot, W. G. F.; Munn, G. E.; Walmsley, P. N. 141st Meeting of the Electrochemical Society, Houston, Texas, May 1972, Abstract No. 154, J . Electrochem Soc. 1972, 119, 108C. 14. Takamatsu, T.; Hashiyama, H.; Eisenberg, A. J . Appl. Polym. Sci. 1979, 24, 2199. 15. Schmidt, P. W.; Height, R. Acta Cript. 1960, 13, 480. 16. Schmidt, P. W. Acta Cryst 1965, 19, 938. 17. Starkweather, H. W. to be published. 18. Geil, P. H. "Polymer Single Crystals Interscience Pub." 1963, pp. 96-98, 313-316, 348-350, 415, and 459. 19. Wunderlich, B. "Macromolecular Physics", Vol. 1, Academic Press 1973, Ch. 3. 20. Gierke, T. D., Hsu, W. Y. Am. Chem. Soc. Adv. Chem. Ser. this volume. 21. Wilson, F. C.; Longworth, R.; Vaughn, D. J . Polym. Prepr., Amer. Chem. Soc., Div. Polym. Chem., 1968,9,505. 22. Marx, C. L.; Caulfield, D. F.; Cooper, S. L., Macromolecules 1973, 6, 344. 23. Roche, E. J.; Stein, R. S.; Russell, T. P.; MacKnight, W. J. J. Polym. Sci., Polym. Phys. Ed., 1980, 18, 1497. 24. MacKnight, W. J.; Taggart, W. P.; Stein, R. S. J. Polym Sci., Polym. Symp., 1974,45,113. 25. Gierke, T. D. 153rd Meeting of Electrochemical Society, Seatle, Wash., May 1978) Abstract No. 453, J . Electrochem. Soc. 1978, 125, 163C. 26. Hsu, W. Y.; Barkley, J. R.; Meakin, P. Macromolecules, 1980 13, 198. RECEIVED August24,1981.


Perfluorinated Ionomer Membranes - American Chemical Society

Recommend Documents