Structure and Properties of Short-Side-Chain Perfluorosulfonate

Chapter 15

Structure and Properties of Short-Side-Chain Perfluorosulfonate Ionomers 1

Downloaded by UNIV OF LIVERPOOL on November 26, 2015 | http://pubs.acs.org Publication Date: July 21, 1989 | doi: 10.1021/bk-1989-0395.ch015

Martin R. Tant , Kevin P. Darst, Katherine D. Lee, and Charles W. Martin Texas Applied Science and Technology Laboratories, Dow Chemical USA, Freeport, TX 77541

An equivalent weight series of short-side-chain perfluorosulfonic polymers has been studied in the sulfonyl fluoride, sulfonic acid, and sodium sulfonate forms by dynamic mechanical spectroscopy, differen tial scanning calorimetry, and wide angle X-ray scat tering. Results indicate that the functional form of the polymer as well as the side chain length and equivalent weight strongly affect the morphology of the materials and therefore their dynamic mechanical and thermal properties. Evidence is also presented for the presence of two different types of crystal structures in these materials. Side chain length thus provides an additional variable by which the structure and properties of perfluorinated ionomers may be controlled.

P e r f l u o r i n a t e d ionomers have r e a c h e d a h i g h level of industrial importance due t o their o u t s t a n d i n g performance as membranes i n a p p l i c a t i o n s such as c h l o r - a l k a l i c e l l s and f u e l cells (1). The Nafion material synthesized by duPont more than twenty y e a r s ago (2) has been w i d e l y used i n such a p p l i c a t i o n s . The unusual trans p o r t p r o p e r t i e s o f t h e a c i d and s a l t forms o f t h e s e m a t e r i a l s , such as i o n p e r m s e l e c t i v i t y i n s o l u t i o n s o f h i g h i o n i c s t r e n g t h , are a direct result of both t h e m o l e c u l a r and m o r p h o l o g i c a l s t r u c t u r e (3) . Strong coulombic a s s o c i a t i o n s l e a d to the formation o f ionic regions t y p i c a l l y r e f e r r e d t o as c l u s t e r s . The f a c t t h a t t h e p e r f l u o r i n a t e d backbone i s c r y s t a l l i z a b l e r e s u l t s i n t h e f o r m a t i o n of c r y s t a l l i n e r e g i o n s as w e l l . Thus t h e s e m a t e r i a l s r e a l l y c o n s i s t o f at l e a s t three d i f f e r e n t phases: an amorphous p h a s e , a c r y s t a l l i n e phase, and an i o n i c phase. Recent r e s u l t s t o be r e p o r t e d here Current address: Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, T N 37662 0097-6156/89A)395-0370$08.75/0 ο 1989 American Chemical Society

In Multiphase Polymers: Blends and Ionomers; Utracki, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


15. TANT ET AL.

Short-Side-Chain Perfluorosulfonate Ionomers 371

s u g g e s t t h e p r e s e n c e o f two d i f f e r e n t t y p e s o f c r y s t a l l i n e r e g i o n s , the r e l a t i v e p r o p o r t i o n s o f which a r e dependent upon such v a r i a b l e s as s i d e c h a i n l e n g t h , e q u i v a l e n t w e i g h t , and sequence d i s t r i b u t i o n . T h i s f u r t h e r c o m p l i c a t e s t h e t a s k o f d e c i p h e r i n g t h e morphology o f these materials. It a l s o u n d e r s c o r e s t h e f a c t t h a t , f o r m a t e r i a l s o f such c o m p l e x i t y , an u n d e r s t a n d i n g of the structure-property b e h a v i o r must c e r t a i n l y be a t t a i n e d i f one i s t o be a b l e t o d e s i g n , s y n t h e s i z e , and f a b r i c a t e m a t e r i a l s w i t h optimum p r o p e r t i e s . A g r e a t d e a l o f e f f o r t by both i n d u s t r i a l and academic scientists has been d i r e c t e d a t g a i n i n g an u n d e r s t a n d i n g o f t h e morphol o g i c a l s t r u c t u r e and t h e mechanism o f i o n t r a n s p o r t i n perfluorinated ionomers. The m o l e c u l a r r e s p o n s e t o an imposed f o r c e f i e l d , be i t m e c h a n i c a l o r e l e c t r o m a g n e t i c , i s h i g h l y dependent upon not only t h e m o l e c u l a r s t r u c t u r e , but t h e m o r p h o l o g i c a l s t r u c t u r e as well. Study o f t h e dynamic mechanical properties yields useful i n f o r m a t i o n about t h e e f f e c t s o f morphology upon m o l e c u l a r r e s p o n s e mechanisms. When c o r r e l a t e d w i t h other information, such as a knowledge of molecular structure, thermal b e h a v i o r , and p e r c e n t c r y s t a l 1 i n i t y , an u n d e r s t a n d i n g o f t h e o v e r a l l structure-property behavior begins to evolve. E i s e n b e r g and coworkers have s t u d i e d m o l e c u l a r motions i n N a f i o n u s i n g dynamic m e c h a n i c a l spectroscopy (4-7), d i e l e c t r i c s p e c t r o s c o p y ( 5 ) , and s o l i d - s t a t e NMR ( 8 ) . This work has been r e v i e w e d by Kyu ( 9 ) . MacKnight and coworkers (10,11) have made s i m i l a r s t u d i e s o f t h e p e r f l u o r o c a r b o x y l a t e s y s t e m s . More r e c e n t work by M a u r i t z , F u , and Yun (12-14) has f o c u s e d on g a i n i n g an understanding of ion transport in solvent-swollen Nafion using d i e l e c t r i c spectroscopy. Morphological structure has been probed using small and w i d e - a n g l e X - r a y s c a t t e r i n g ( 1 5 - 1 8 ) , e l e c t r o n i c absorption spectroscopy (19,20), extended X - r a y absorption fine s t r u c t u r e ( 2 1 , 2 2 ) , as w e l l as o t h e r t e c h n i q u e s . Nearly all of this work has f o c u s e d on N a f i o n , a l o n g - s i d e c h a i n polymer as shown i n F i g u r e l a . D i f f i c u l t s y n t h e t i c problems have limited the a v a i l a b i l i t y o f p e r f l u o r i n a t e d ionomers h a v i n g d i f f e r i n g s i d e c h a i n l e n g t h s . In 1982 a new s y n t h e t i c r o u t e was reported (23) which l e d t o t h e s h o r t - s i d e - c h a i n polymer shown i n F i g u r e l b . The polymers i n F i g u r e 1 a r e shown i n t h e i r t h e r m o p l a s tic precursor (SO2F) form. The membranes a r e f a b r i c a t e d i n t h i s form and a r e then h y d r o l y z e d t o t h e ionomer f o r m . L i t t l e d a t a has appeared i n the l i t e r a t u r e concerning the p r o p e r t i e s o f the s h o r t s i d e - c h a i n m a t e r i a l , but i t i s now c l e a r t h a t s i d e c h a i n l e n g t h may have a s i g n i f i c a n t e f f e c t upon i o n t r a n s p o r t p r o p e r t i e s . Signific a n t i n c r e a s e s i n s e l e c t i v i t y o f i o n t r a n s p o r t i n c h l o r - a l k a l i (24) and maximum power d e n s i t y in fuel c e l l a p p l i c a t i o n s have been reported (25) u s i n g membranes d e r i v e d from short side chain polymer. In this paper we r e p o r t t h e i n i t i a l r e s u l t s o f a s t u d y d i r e c t e d at g a i n i n g an u n d e r s t a n d i n g of the structure-property behavior o f the s h o r t - s i d e - c h a i n materials. The e f f e c t o f both e q u i v a l e n t weight and f u n c t i o n a l form ( i . e . precursor, sulfonic a c i d , o r sodium s u l f o n a t e ) on t h e dynamic m e c h a n i c a l p r o p e r t i e s a r e d i s c u s s e d . The e f f e c t o f s i d e c h a i n l e n g t h upon t h e dynamic mechanical behavior is also probed by comparing t h e p r o p e r t i e s o f t h e s h o r t - s i d e - c h a i n polymers w i t h t h o s e o f t h e l o n g - s i d e - c h a i n a n a l o g having the Nafion structure. Crystal 1inity d a t a o b t a i n e d from v a r i a b l e t e m p e r a t u r e wide a n g l e X - r a y s c a t t e r i n g and d i f f e r e n t i a l



372

MULTIPHASE POLYMERS: BLENDS AND IONOMERS

a.)

LONG-S/DE-CHA/N / M C F ^

(LSC) ) -- CC F - C F - \ / 2

i CF -CF-0-CF -CF -S0 F n

o

'2

C F

b.)

SHORT-S/DE-CHA/N

o

2

2

3

(SSC)

\^(CF Cf^)-CF-CF 2

2

0

1 CF -CF -S0 F 2

2

2

F i g u r e 1. Chemical s t r u c t u r e s o f the (a) l o n g - s i d e - c h a i n short-side-chain perfluorosulfonyl polymers.

and


(b)

15. TANT ET A L

Short-Side-Chain Perfluorosulfonate Ionomers 373

scanning c a l o r i m e t r y e x p e r i m e n t s a r e used t o t a t i o n o f the dynamic mechanical d a t a .

support

the

interpre-


Experimental Synthesis. The short-side-chain vinyl ether comonomer, CF2=CF0CF2CF2S02F, was prepared in t h e l a b o r a t o r y by l i t e r a t u r e procedures (23). Emulsion p o l y m e r i z a t i o n was c a r r i e d out using s t a n d a r d t e c h n i q u e s i n a 500 ml l a b o r a t o r y r e a c t o r s i m i l a r t o t h o s e p r e v i o u s l y d e s c r i b e d i n the literature (26-27). The equivalent weight was c o n t r o l l e d by a d j u s t i n g the p r e s s u r e o f tetrafluoroethyl e n e (TFE) t o v a r y the comonomer:TFE f e e d r a t i o i n t h e r e a c t o r . The r e a c t o r was charged under a n i t r o g e n atmosphere w i t h an aqueous s o l u t i o n o f i n i t i a t o r , b u f f e r s , and s u r f a c t a n t , f o l l o w e d by comonomer. The s y n t h e s i s was then c a r r i e d out a t 6 0 C w i t h a c o n t i n u o u s f e e d o f TFE t o m a i n t a i n the d e s i r e d p r e s s u r e i n t h e s t i r r e d reactor. A f t e r the r e q u i r e d q u a n t i t y o f TFE was a d d e d , t h e r e a c t o r was v e n t e d and the copolymer was i s o l a t e d by a c i d c o a g u l a t i o n , washed, and d r i e d . The comonomer:TFE r a t i o was a d j u s t e d t o a l e v e l that would produce 80-90 grams o f polymer a t about 50% c o n v e r s i o n o f comonomer. The TFE p r e s s u r e was c o n t r o l l e d t o produce polymers which a n a l y z e d a t 1200, 1000, 800 and 600 (+-26) e q u i v a l e n t w e i g h t , respectively. Equivalent weights were d e t e r m i n e d by functional group h y d r o l y s i s and t i t r a t i o n and c o n f i r m e d by p e r c e n t s u l f u r a n a lysis. e

Sample P r e p a r a t i o n . Samples were p r e p a r e d from the thermoplastic polymer powders a t 282 C and 5 t o n s p r e s s u r e . The molded thermop l a s t i c samples were c o n v e r t e d t o the a c i d form by h y d r o l y s i s in 17% KOH/DMSO/water at 9 0 C f o r 4 days f o l l o w e d by a c i d i f i c a t i o n i n 50% n i t r i c a c i d a t 9 0 C f o r 4 h o u r s . The s u l f o n i c acid form was converted to the sodium s u l f o n a t e form by n e u t r a l i z a t i o n i n 10% NaOH at 90°C f o r 4 h o u r s . Both s u l f o n i c a c i d and sodium sulfonate forms of the polymer were r i n s e d w i t h water a t 8 0 C f o r 20 hours and then vacuum d r i e d a t 120 C f o r 16 h o u r s . e

e

e

e

e

D i f f e r e n t i a l Scanning C a l o r i m e t r y . Differential scanning c a l o r i metry (DSC) e x p e r i m e n t s were performed u s i n g a P e r k i n - E l m e r DSC-7 i n t e r f a c e d w i t h a 7700 series microprocessor/controller. Scans were made on samples w e i g h i n g 30 mg i n the t e m p e r a t u r e range from room t e m p e r a t u r e t o 3 4 0 C a t a h e a t i n g r a t e o f 2 0 C / m i n . e

e

Dynamic M e c h a n i c a l P r o p e r t i e s . Dynamic m e c h a n i c a l p r o p e r t i e s were measured w i t h a Rheometrics Dynamic S p e c t r o m e t e r Model 7700 i n the t o r s i o n a l r e c t a n g u l a r mode. Temperature sweeps were run from -160 to 340°C using a s t r a i n o f 10% and f r e q u e n c i e s o f 1, 10, and 100 r a d i a n s p e r second ( r a d / s ) . Data was o b t a i n e d a t 5 d e g r e e t e m p e r a ture increments with a t l e a s t a one minute thermal e q u i l i b r a t i o n time a t each t e m p e r a t u r e . O n l y the 10 r a d / s d a t a a r e reported in t h i s paper. Wide A n g l e X - r a v S c a t t e r i n g . Percent c r y s t a l l i n i t y determinations were made u s i n g wide a n g l e X - r a y s c a t t e r i n g (WAXS). Samples were c o m p r e s s i o n - m o l d e d and then s l o w - c o o l e d i n t h e mold t o promote max-



374


imum c r y s t a l l i z a t i o n . Each sample was analyzed using a pinhole camera h a v i n g a 700 mm c o l l i m a t o r and a 70 mm camera l e n g t h . Two 0 . 8 mm square p i n h o l e s were u s e d . The camera was mounted verti cally atop a Rigaku RU200 r o t a t i n g anode g e n e r a t o r o p e r a t e d a t 45 kV and 150 mA. A Braun 0EM-50 l i n e a r p o s i t i o n sensitive detector was mounted a t a 25 degree a n g l e t o t h e s a m p l e . The d e t e c t o r was o p e r a t e d w i t h a r e s o l u t i o n o f 0.036 degree 2 & p e r channel. The data acquisition time was 20 m i n u t e s . Because o f t h e l a c k o f pure s t a n d a r d s , the method o f u s i n g d i f f e r e n t i a l i n t e n s i t y measure-ments to c a l c u l a t e c r y s t a l l i n i t y i n d i c e s was used ( 3 0 ) · R e l a t i v e weight p e r c e n t c r y s t a l l i n i t i e s were o b t a i n e d u s i n g c u r v e fitting results g e n e r a t e d from the 1200 EW s h o r t - s i d e - c h a i n p r e c u r s o r s a m p l e . V a r i a b l e t e m p e r a t u r e wide a n g l e X - r a y s c a t t e r i n g d a t a were a l s o o b t a i n e d on s e v e r a l o f the s a m p l e s . A M e t t l e r FP82 hot s t a g e with FP80 controller was p l a c e d i n s i d e t h e camera. WAXS p a t t e r n s were c o l l e c t e d a t 10°C i n t e r v a l s from 30 t o 2 9 0 ° C u s i n g a h e a t i n g rate of 2°C/min. D e t e c t o r r e s o l u t i o n was d e c r e a s e d t o 0.168 degree 2 θ and the a c q u i s i t i o n time was 4.75 m i n u t e s . All other conditions were t h e same as d e s c r i b e d e a r l i e r . R e s u l t s And D i s c u s s i o n The m a t e r i a l s s t u d i e d i n t h i s work a r e summarized i n T a b l e I, which a l s o i n c l u d e s the s i d e c h a i n c o n t e n t i n terms o f weight p e r c e n t and mole p e r c e n t comonomer. In the f o l l o w i n g s e c t i o n , t h e e f f e c t o f s i d e c h a i n l e n g t h on the dynamic mechanical b e h a v i o r o f the perfluoro-sulfonyl fluoride precursor i s a d d r e s s e d by comparing the p r o p e r t i e s o f the l o n g - s i d e - c h a i n (LSC) p r e c u r s o r w i t h t h o s e o f the short-side-chain (SSC) p r e c u r s o r at equal weight p e r c e n t and mole p e r c e n t comonomer c o n t e n t . The e f f e c t o f e q u i v a l e n t weight on the dynamic mechanical b e h a v i o r o f the SSC p e r f l u o r o s u l f o n y l fluoride p r e c u r s o r i n the range between 600 and 1200 EW i s then explored. N e x t , the e f f e c t o f c o n v e r s i o n t o the s u l f o n i c a c i d and sodium s u l f o n a t e forms on the dynamic mechanical behavior is considered. Finally, the e f f e c t o f s i d e c h a i n l e n g t h on the p r o p e r t i e s o f the sulfonic acid and sodium s u l f o n a t e forms of the material is examined.

Table

SSC EW 600 800 1000 1200 LSC EW 1140

Comonomer Weiaht % 47 35 28 23 39

I.

Summary o f M a t e r i a l s Content Mole % 24 16 12 10 13

T

g

Evaluated

(G"peak) deqrees C 13 26 41 50 6

% Crystal u n i t y bv WAXS 0 7 17 24 8

E f f e c t o f S i d e C h a i n Length f o r P r e c u r s o r . F i g u r e 2 shows both the s t o r a g e modulus, G ' , and l o s s modulus, G " , as a f u n c t i o n o f temper ature for the LSC 1140 EW and SSC 800 EW polymers i n t h e thermo-


In Multiphase Polymers: Blends and Ionomers; Utracki, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. (DEG

C)

F i g u r e 2. Comparison o f the dynamic m e c h a n i c a l b e h a v i o r o f the l o n g - s i d e - c h a i n and s h o r t - s i d e - c h a i n p r e c u r s o r s o f s i m i l a r w e i g h t p e r c e n t comonomer c o n t e n t .

TEMPERATURE


376


p l a s t i c p r e c u r s o r ( s u l f o n y l f l u o r i d e ) form a t a frequency of 10 rad/s. These two m a t e r i a l s have about t h e same comonomer c o n t e n t i n terms o f weight p e r c e n t (39% f o r the LSC and 35% f o r the SSC). Therefore t h e SSC m a t e r i a l has more s i d e c h a i n s . These e q u i v a l e n t weights are a l s o those t y p i c a l l y used in applications requiring high i o n i c c o n d u c t i v i t y . The r e s p o n s e o f t h e m a t e r i a l s below t h e i r glass t r a n s i t i o n is quite s i m i l a r . The s h o r t e r , l e s s mobile side c h a i n s o f t h e SSC m a t e r i a l r e s u l t i n an i n c r e a s e i n 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 o f about 2 0 C . Polymer m o l e c u l e s w i t h shorter s i d e c h a i n s a r e a b l e t o pack more c l o s e l y , d e c r e a s i n g t h e f r e e v o l ume a v a i l a b l e f o r main c h a i n m o t i o n - - t h e o r i g i n o f t h e g l a s s tran sition (29). Vincent (30) found s i m i l a r r e s u l t s f o r a s e r i e s o f n - a l k y l e t h e r s , where Tg was found t o d e c r e a s e i n the o r d e r p o l y v i nyl methyl e t h e r ( - 1 0 t ) > p o l y v i n y l e t h y l e t h e r ( - 1 7 C ) > p o l y v i n y l η - p r o p y l e t h e r ( - 2 7 C ) > p o l y v i n y l η - b u t y l e t h e r ( - 3 2 C ) due t o the i n c r e a s i n g l e n g t h o f the s i d e c h a i n s . It i s a l s o l i k e l y t h a t , i n a d d i t i o n t o the e f f e c t s of improved p a c k i n g efficiency, the lower mobility of the s h o r t s i d e c h a i n s a l s o c o n t r i b u t e s t o the observed trend in T . Above Tg the b e h a v i o r o f t h e LSC and SSC m a t e r i a l s i s a l s o v e r y s i m i l a r . Both s u s t a i n a s t r o n g r u b b e r y p l a t e a u t o w e l l above 1 5 0 C . WAXS e x p e r i m e n t s have r e v e a l e d r e l a t i v e crystal 1inity levels of about 8% f o r the LSC polymer and 7% f o r the SSC p o l y m e r , s u g g e s t i n g t h a t c r y s t a l l i n i t y i s l i k e l y the major c o n t r i b u t o r t o the rubbery plateau. (The c r y s t a l l i n i t y d a t a a r e a l s o summarized i n T a b l e I.) S i n c e randomly p l a c e d pendant groups a c t t o r e d u c e the extent of main chain c r y s t a l l i z a t i o n , both t h e e q u i v a l e n t weight and s i d e c h a i n l e n g t h would be e x p e c t e d t o a f f e c t the crystallinity level. Increasing the number o f side c h a i n s and i n c r e a s i n g s i d e c h a i n l e n g t h s h o u l d both a c t t o reduce c r y s t a l l i n i t y . From the f a c t t h a t both materials have about the same l e v e l o f c r y s t a l l i n i t y , i t is c l e a r t h a t t h e i n c r e a s e d s i d e c h a i n l e n g t h o f t h e LSC 1140 EW p r e cursor essentially compensates f o r the greater number o f s i d e c h a i n s o f t h e SSC 800 EW m a t e r i a l i n r e d u c i n g t h e e x t e n t of crys tallization. Also, the f a c t t h a t the degree o f c r y s t a l l i n i t y i s s i m i l a r f o r both m a t e r i a l s s u g g e s t s t h a t the o b s e r v e d d i f f e r e n c e i n glass transition temperature i s not due t o c r y s t a l l i n i t y d i f f e r ences. In F i g u r e 3 t h e s t o r a g e modulus, G ' , and l o s s modulus, G", of the LSC 1140 EW and SSC 1000 EW p r e c u r s o r s a r e shown as a f u n c t i o n o f t e m p e r a t u r e a t a f r e q u e n c y o f 10 r a d / s . In t h i s c a s e both mate rials c o n t a i n about the same number o f s i d e c h a i n s , o r about 12 t o 13% comonomer c o n t e n t on a m o l a r basis. If the materials were amorphous so t h a t c r y s t a l l i n i t y e f f e c t s c o u l d be i g n o r e d , the SSC m a t e r i a l would be e x p e c t e d t o have a h i g h e r T due t o its shorter chains. Of c o u r s e , the SSC m a t e r i a l s h o u l d a l s o be more c r y s t a l l i n e , s i n c e i t s shorter s i d e chains should pack more efficiently than t h e l o n g e r s i d e c h a i n s o f the LSC m a t e r i a l . WAXS e x p e r i m e n t s c o n f i r m t h i s e x p e c t a t i o n , w i t h the SSC m a t e r i a l b e i n g about 17% crystalline compared t o the 8% c r y s t a l l i n e LSC m a t e r i a l . Again, c r y s t a l l i n i t y i s known t o raise Tg and broaden the transition region, so t h i s f a c t o r would be e x p e c t e d t o f u r t h e r e l e v a t e t h e T e


e

e

e

e

g

e

g

q


In Multiphase Polymers: Blends and Ionomers; Utracki, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989. (DEG

C)

F i g u r e 3. Comparison o f t h e dynamic m e c h a n i c a l b e h a v i o r o f t h e l o n g - s i d e - c h a i n and s h o r t - s i d e - c h a i n p r e c u r s o r s o f s i m i l a r mole p e r c e n t comonomer c o n t e n t .

TEMPERATURE


^4

Structure and Properties of Short-Side-Chain Perfluorosulfonate

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