Water Absorption in Acid Nafion Membranes - ACS Symposium Series

28 Water Absorption in Acid Nafion Membranes 1

2

R. DUPLESSIX , M. ESCOUBES , B. RODMACQ, F. VOLINO, E. ROCHE, A. EISENBERG , and M. PINERI 3

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch028

Centre d'Études Nucléaires de Grenoble, 38041 Grenoble Cédex, France

Nafion polymers have been developed recently by the du Pont Company. They are perfluorosulfonic acid membranes mainly used as separators in electrochemical applications. The backbone of the polymer chains consists of perfluoroethylene units whereas the side chains are of the form - O - CF - CF - O - CF - CF - SO M. published on their A large amount of work has been work has been done in commercial applications but little terms of the molecular structure. Ion clustering has been proposed by Yeo and Eisenberg (1) from results of small angle X ray scattering and dynamic mechanical experiments. This phase separation has been confirmed both from experimental (2) and theoretical studies (3). Further support for this phase separation has also been provided by23 NMRstudies (4). A model has been proposed by T. Gierke (2) to explain the main features of these membranes. For the water soaked membrane, clustering of the ionic groups is proposed with an average cluster diameter of 40 Åand an average distance of 50 Åbetween cluster centers. The experimental support for this model comes from small angle X-ray scattering experiments, electron microscope analysis and water diffusion data In order to get more information about the various different phases we have performed different experiments: heat of water absorption measurements, small angle scattering of neutrons and Xrays, as well as NMR and quasi elastic scattering of neutrons on samples containing various amounts of water. We are therefore using the water molecules as a probe to obtain information about the Nafion structure. A summary of these results is given in this paper, which concerns only the interactions of the water molecules with the acid form of Nafion. 2

2

2

Na

Current addresses: Institut Laue Langevin, B P 156, 38042 Grenoble Cédex, France. Université Claude Bernard, 69621 Villeurbanne, France. McGill University, Montreal, Canada. 1

2 3

0-8412-0559-0/ 80/47-127-469S05.00/ 0 © 1980 American Chemical Society

In Water in Polymers; Rowland, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3

WATER IN P O L Y M E R S


470

Experimental The a c i d sample which has been s t u d i e d h e r e has an e q u i v a l e n t weight o f 1200 (weight o f a c i d polymer p e r SO3H g r o u p ) . In the water a b s o r p t i o n s t u d y , a S e t a r a n thermobalance Β 60 and a R i c h a r d Eyraud i s o t h e r m d i f f e r e n t i a l m i c r o c a l o r i m e t e r were u s e d . The same experiment p e r m i t s one to o b t a i n b o t h the s o r p t i o n i s o t h e r m and the d i f f e r e n t i a l heat a b s o r p t i o n v a l u e s ( 5 ) . The a c c u r a c i e s were b e t t e r than 0.1 mg and 10"^ c a l / s e c , r e s p e c t i v e l y . The r e l a t i v e h u m i d i t y was o b t a i n e d by c h a n g i n g the temperature o f a w a t e r / i c e b a t h ( a c c u r a c y 0 . 1 ° C ) w h i c h was c o n n e c t e d to the sam p l e . We measured b o t h the amount o f absorbed water and the heat o f a b s o r p t i o n a f t e r a change i n the r e l a t i v e h u m i d i t y l e v e l . We can t h e r e f o r e d e f i n e the average heat o f a b s o r p t i o n p e r water m o l e c u l e c o r r e s p o n d i n g to the m o l e c u l e s w h i c h have been absorbed a f t e r t h i s change i n water r e l a t i v e h u m i d i t y . The D]1 and D17 machines at Laue L a n g e v i n I n s t i t u t e ( I . L . L . ) i n G r e n o b l e have been used to p e r f o r m the s m a l l a n g l e n e u t r o n s c a t t e r i n g e x p e r i m e n t s . The q v a l u e s (q = ^2L p i n Θ) which are a c c e s s i b l e i n t h e s e e x p e r i m e n t s a r e r e s p e c t i v e l y , 5 χ 10~3 to 20 χ 1 0 Â - 1 f o r D l l and 1 χ 1 0 " t o 20 χ 10~* Â " f o r D17. Such e x p e r i m e n t s t h e r e f o r e p e r m i t a d e m o n s t r a t i o n o f the e x i s t e n c e o f c l u s t e r s up to a few hundreds o f angstroms i n s i z e . The NMR experiments were performed at 60 MHz u s i n g the B r u k e r WP 60 p u l s e d F o u r i e r t r a n s f o r m NMR s p e c t r o m e t e r . The i n c o h e r e n t n e u t r o n q u a s i e l a s t i c e x p e r i m e n t s were p e r f o r med w i t h the time o f f l i g h t s p e c t r o m e t e r IN5 o f the I . L . L . . The s c a t t e r i n g a n g l e used c o r r e s p o n d e d to q v a l u e s r a n g i n g from 0.2 to 1.2 Â " " * . The c o r r e s p o n d i n g energy r e s o l u t i o n was ^ 18 yeV f u l l w i d t h at h a l f maximum. - 3

2

1

Results F i g u r e 1 shows a p l o t o f the water l o s s v e r s u s temperature f o r a sample w h i c h has been d r i e d at room temperature f o r 24 hours under 10~4 t o r r . NMR e x p e r i m e n t s on the same s t a r t i n g m a t e r i a l g i v e e v i d e n c e o f the p r e s e n c e o f r e s i d u a l water ; we a l s o o b s e r v e d some n e u t r o n s c a t t e r i n g at low a n g l e s w h i c h c o r r e s p o n d s to some c o n t r a s t because o f the p r e s e n c e o f w a t e r . A f t e r h e a t i n g the samp l e at 220°C f o r 30 minutes we o b s e r v e d no f u r t h e r change i n the weight l o s s , even a f t e r h e a t i n g to h i g h e r t e m p e r a t u r e s . T h i s d i f f e r e n c e i n weight between the sample d r i e d at room temperature and a t 220°C c o r r e s p o n d s to a water l o s s because o f the r e v e r s i b i l i t y o f the a b s o r p t i o n / d e s o r p t i o n b e h a v i o u r . We i n d e e d c o o l e d the samp l e w h i c h has been d r i e d a t 220°C down to room t e m p e r a t u r e , r e h y d r a t e d i t at 100 % R . H . , then d e h y d r a t e d i t under vacuum a t room t e m p e r a t u r e . Now, i f we heat t h i s sample up to 2 2 0 ° C , we f i n d e x c a c t l y the same weight l o s s c u r v e as found p r e v i o u s l y . Such b e h a v i o u r means t h a t we have d e s o r b e d water d u r i n g the h e a t i n g t r e a t m e n t . In t a b l e I a r e g i v e n the amounts o f absorbed water c o r r e s p o n d i n g t o d i f f e r e n t h u m i d i t y l e v e l s and the c o r r e s p o n d i n g numbers o f water m o l e c u l e s p e r S0~H g r o u p s .



DUPLESSIX E T A L .

Figure 1.

Acid Nafion

Membranes

Water loss on heating an acid Nafion sample (at 3°C/min) under 10~* torr to 220°C (+, after 30 sec at 220°C)


W A T E R IN P O L Y M E R S

472 TABLE I Relative humidity


Room T . d r y 50 % 90 % soaked boiled

samples

% of

water

2.5 7.8 13 20 30

Number o f water m o l e c u l e s p e r SO3H 1.7 5.2 8.7 13.3 20

F i g u r e 2 shows the c o r r e s p o n d i n g s o r p t i o n i s o t h e r m s . The r e f e r e n c e i s the sample w h i c h had been vacuum d r i e d a t 2 2 0 ° C . The a b s o r p t i o n i s o t h e r m s were t a k e n at 2 5 ° C . We o b s e r v e some h y s t e r e s i s between the f i r s t s o r p t i o n and d e s o r p t i o n c u r v e s and we cannot come back to z e r o by vacuum d r y i n g , as was p r e v i o u s l y p o i n t e d o u t . The subsequent s o r p t i o n and d e s o r p t i o n c u r v e s a r e t h e n q u i t e i d e n t i c a l . I n f i g u r e 3, we have p l o t t e d the average heat o f a b s o r p t i o n v e r s u s the water c o n t e n t . The empty c i r c l e s c o r r e s p o n d to the samples d r i e d a t room temperature w h i c h c o n t a i n around 2.3 % o f w a t e r . By c h a n g i n g the h u m i d i t y l e v e l we have absorbed water up to 4.5 % w i t h an average energy o f a b s o r p t i o n o f around - 12 k c a l / m o l e . We o b t a i n a p l a t e a u up to 7% w a t e r , w h i c h c o r r e s p o n d s to about 5 water m o l e c u l e s p e r SO3H g r o u p . F o r h i g h e r amounts o f water we t h e n o b s e r v e a c o n t i n u o u s d e c r e a s e o f the e x o t h e r m i c v a l u e , down t o - 4 k c a l / m o l e . I f we now s t a r t from the sample w h i c h has been d r i e d at 2 2 0 ° C , we f i n d t h a t the heat o f a b s o r p t i o n o f the f i r s t water m o l e c u l e s i s e x a c t l y the same as we have found f o r the samples d r i e d at room t e m p e r a t u r e . T h i s r e s u l t i s i m p o r t a n t because i t means t h a t the water m o l e c u l e s w h i c h have not been d e s o r b e d at room temperature do not have a l a r g e r b i n d i n g e n e r g y . We always o b s e r v e a p l a t e a u , but the d e c r e a s e i n a b s o r p t i o n energy o c c u r s at a lower water c o n t e n t , i n a l l o t h e r r e s p e c t s the shape o f the c u r v e i s i d e n t i c a l . There i s a r e l a t i v e l y l a r g e c o n t r a s t between the CF2 and H2O groups b o t h f o r X - r a y s and f o r n e u t r o n s c a t t e r i n g . The X - r a y c o n t r a s t i s due to 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 the n e u t r o n c o n t r a s t i s due to d i f f e r e n c e s i n the c o h e r e n t s c a t t e r i n g l e n g t h . T a b l e I I summarizes t h e s e d i f f e r e n t v a l u e s . I t i s t h e r e f o r e v e r y i n t e r e s t i n g t o s t u d y water c l u s t e r i n g u s i n g t h e s e two t e c h n i q u e s . P r e l i m i n a r y e x p e r i m e n t s have been done and a r e r e p o r t e d i n the l i t e r a t u r e ( 6 ) . F i g u r e 4 shows the n e u t r o n s c a t t e r i n g c u r v e s c o r r e s p o n d i n g to d i f f e r e n t amounts o f absorbed w a t e r , w h i l e T a b l e I g i v e s the p e r c e n t a g e s o f w a t e r and the r e l a t i v e numbers o f w a t e r m o l e c u l e s p e r SO3H f o r d i f f e r e n t h u m i d i t y l e v e l s . In f i g u r e 4 we n o t e a maximum c o r r e s p o n d i n g to a Bragg s p a c i n g o f the o r d e r o f 180 A . When the amount o f water i n c r e a s e s , we o b s e r v e a change b o t h i n the p o s i t i o n o f t h i s maximum and i n the shape o f the c u r v e . To get more i n f o r m a t i o n about the phase s e p a r a t i o n p h e n o menon suggested by the shape o f t h e s e c u r v e s we soaked some samo


28.

DUPLESSIX E T A L .

Acid Nafion

Membranes

473


Δ Ρ ft)

P /

Po

Figure 2. Sorption-desorption curves obtained after drying an acid Nafion sample at different temperatures vs. the humidity (P/PJ



474 W A T E R IN POLYMERS


DUPLESSix ET A L .

Acid Nafion Membranes

d


200

475

(A) 100

6

14

2

0 0

2

4

6

0x10* Figure 4.

ο (A" )

8

1

Small-angle neutron scattering (SANS) curves for different amounts of absorbed water


WATER IN P O L Y M E R S

476 TABLE

Chemical u n i t

H

b x

10 -

1

D

Diffusion

length

1 2

(cm)

Number o f electrons 1

0.37

1

0.67

2

C 16

0.67

6

0.58

8

19

0.55

9

CF2

1.68

24

1 2

0

F


II

H 0 2

D2O

-

10

0.168

10

1 .95

p i e s w i t h d i f f e r e n t H2O/D2O r a t i o s . In F i g u r e 5 we p r e s e n t the d i f f e r e n t s c a t t e r i n g c u r v e s w h i c h have been o b t a i n e d . I f o n l y two phases were p r e s e n t , t h e n each c u r v e would be i d e n t i c a l w i t h e v e r y o t h e r o n e , except f o r a m u l t i p l i c a t i o n o f the i n t e n s i t y by a c o n s t a n t f a c t o r . S i n c e t h i s i s not the c a s e , our r e s u l t s show t h a t we have a t l e a s t t h r e e d i f f e r e n t phases i n our sample. F u r t h e r e x p e r i m e n t s have been performed at l a r g e r a n g l e s w i t h the D 17 i n s t r u m e n t . At t h e s e l a r g e r a n g l e s ( F i g u r e 6) a peak a p p e a r s , i t s p o s i t i o n and a m p l i t u d e d e p e n d i n g on the amount o f a b s o r b e d w a t e r . T h i s peak has been s t u d i e d more e x t e n s i v e l y by s m a l l a n g l e X - r a y s c a t t e r i n g ( F i g u r e 7 ) . The upswing i n s c a t t e r i n g w h i c h i s o b s e r v e d f o r 2 θ v a l u e s < 1° c o r r e s p o n d s to the i n c r e a s e i n s c a t t e r i n g o b s e r v e d w i t h n e u t r o n s at q v a l u e s < 6 . 1 0 " A ~ 1 . F o r l a r g e amounts o f water we n o t e the appearance o f a peak w h i c h may c o r r e s p o n d t o an i n t e r f e r e n c e peak because o f a l a r g e volume f r a c t i o n o f c l u s t e r s . More i n f o r m a t i o n w i l l be o b t a i n e d about t h i s b e h a v i o u r by w o r k i n g on N a f i o n s n e u t r a l i z e d w i t h Na ; the r e s u l t s o f t h a t s t u d y w i l l be r e p o r t e d i n the next p a p e r . 2

N u c l e a r magnetic r e s o n a n c e i s a p o w e r f u l t e c h n i q u e w h i c h can y i e l d i n f o r m a t i o n on dynamic phenomena. Because o f the l a c k o f p r o t o n s i n the N a f i o n s i t i s e a s i e r to s t u d y the water p r o t o n s . A s i n g l e l i n e has always been o b s e r v e d f o r water c o n t e n t s between 2.7 and 20 %. Such b e h a v i o u r can be e x p l a i n e d i n two ways : e i t h e r we have o n l y one k i n d o f water m o l e c u l e w i t h a w e l l d e f i n e d e n v i ronment o r we have two o r more d i f f e r e n t k i n d s o f water m o l e c u l e s w i t h an exchange r a t e l a r g e r t h a n 10~ to 10"^ s e c . I n F i g u r e 8 a r e p l o t t e d the d i f f e r e n t room temperature c h e m i c a l s h i f t s and l i n e w i d t h s c o r r e s p o n d i n g to d i f f e r e n t water c o n t e n t s . We used a m i x t u r e o f 95 % D2O and 5 % H2O as the r e f e r e n c e f o r the c h e m i c a l s h i f t s . In F i g u r e 9 a r e p l o t t e d the changes i n l i n e w i d t h v e r s u s t e m p e r a t u r e f o r the d i f f e r e n t samples and i n F i g u r e 10, we r e p o r t the v a r i a t i o n s o f the s p i n l a t t i c e r e l a x a t i o n time ( T j ) v e r s u s the t e m p e r a t u r e . From t h e s e r e s u l t s we c a n make the f o l l o w i n g o b s e r v a t i o n s . F i r s t o f a l l t h e r e a r e two d i f f e r e n t regimes o f water a b s o r p t i o n . The f i r s t regime c o r r e s p o n d s to the low water c o n t e n t s 3


DUPLESSIX E T A L .

Acid Nafion

Membranes

477


28.

Figure 5. Small-angle neutron scattering (SANS) curves for an acid Nafion sample soaked in water with different HiO/D 0 concentrations 2



478

W A T E R IN

Figure 6.

POLYMERS

Small-angle neutron scattering curves at larger q values (D17) for high water contents


DUPLESSIX E T A L .

Acid Nafion

Membranes

d

50

30


100

(A)

Ο

2Θ Figure 7.

Small-angle X-ray scattering (SAXS) curves for different amounts absored water


WATER I N P O L Y M E R S

480


Hz 400

ι

Chemical shift Line width

200

0

10 (V·)

20 woter content

Figure 8. Room temperature chemical shift and line width changes vs. water con centration. The reference for the chemical shifts measured at 60 MHz is a mixture of 95% D Q and 5% H O. 2

s


Acid Nafion

DUPLESSIX E T A L .

Membranes

48


Line wkfttt Δ4 ( Hz ) Acid

Nafion»

water C%) 2.7 + 2.8 m

4.4

·

5

Δ

5.5

7.7 8.6 20

=

Ë

*

°o

Q 4

0

a

Φ

οχ

* · Λ

o

•** * ^ : » t ge °*o X * î fi a OX

x

0

X

150

1

200

1

250

1

300 T (K)

Figure 9.

Temperature dependence of the line width for samples with different amounts of water (NMR, 60 MHz)


WATER IN

482

POLYMERS


η (ffiS)

Figure 10.

Proton spin lattice relaxation time dependence vs. temperature for different water contents (NMR, 60 MHz)



28.

DUPLESSIX E T A L .

Acid Nafion

Membranes

483

up t o 4.4 %, and i s c h a r a c t e r i z e d by s i g n i f i c a n t d e c r e a s e i n b o t h the c h e m i c a l s h i f t and the l i n e w i d t h . Only one k i n d o f m o t i o n i s d e t e c t e d w i t h the s a m p l e s , c o r r e s p o n d i n g to t h i s f i r s t regime o f a b s o r p t i o n (2.7 and 2.8 %) . A c o n t i n u o u s d e c r e a s e o f the l i n e w i d t h i s i n d e e d o b s e r v e d when t h e s e samples a r e h e a t e d from low t e m p e r a t u r e up to room t e m p e r a t u r e , and a s i n g l e minimum f o r T j appears around room t e m p e r a t u r e . The second regime o f water a b s o r p t i o n extends from 4.4 t o 20 % and no i m p o r t a n t change o c c u r s e i t h e r f o r the c h e m i c a l s h i f t o r the l i n e w i d t h . F o r a l l the samples c o r r e s p o n d i n g to t h i s second r e g i m e , we o b s e r v e a s i m i l a r b e h a v i o u r o f the l i n e w i d t h v e r s u s t e m p e r a t u r e . When the samples a r e h e a t e d , the l i n e w i d t h d e c r e a s e s up to 250 Κ and t h e n remains c o n s t a n t . Two minima appear i n the Tj c u r v e s v e r s u s the t e m p e r a t u r e . Such b e h a v i o u r s u g g e s t s the p r e s e n c e o f a more complex m o t i o n t h a n i s found i n the f i r s t r e g i m e . At low t e m p e r a t u r e s we presumably o b s e r v e a l o c a l d i f f u s i v e m o t i o n . The c h a r a c t e r i s t i c time at the temperature c o r r e s p o n d i n g to t h i s low t e m p e r a t u r e Tj minimum can be e s t i m a t e d to be ^ 1.6 χ 10~"9 s e c . The a c t i v a t i o n energy a s s o c i a t e d w i t h t h i s m i n i mum i s 3-4 k c a l / m o l e . When the amount o f water d e c r e a s e s , the low temperature minimum moves to h i g h e r t e m p e r a t u r e s . The c o r r e s p o n d i n g m o t i o n i s t h e r e f o r e more and more d i f f i c u l t . The change i n b e h a v i o u r above 250 Κ f o r the l i n e w i d t h p r o b a b l y c o r r e s p o n d s to the o n s e t o f a n o t h e r k i n d o f m o t i o n , presumably l o n g range s e l f - d i f f u s i o n . The c h a r a c t e r i s t i c time f o r t h i s second m o t i o n i s always 1.6 10"~9 e c a t the t e m p e r a t u r e c o r r e s p o n d i n g t o the h i g h tempe r a t u r e Tj minimum. S

The i n c o h e r e n t n e u t r o n q u a s i - e l a s t i c s c a t t e r i n g t e c h n i q u e g i v e s i n f o r m a t i o n about the motions o f the hydrogen atoms on a s h o r t e r time s c a l e t h a n NMR. In f i g u r e 11 we o b s e r v e a s i n g l e peak a t z e r o energy t r a n s f e r and w i t h a w i d t h c o r r e s p o n d i n g to the r e s o l u t i o n o f the i n s t r u m e n t (^ 18yeV i n the p r e s e n t c a s e f o r I N 5 ) . T h i s spectrum has been o b t a i n e d w i t h a n a f i o n sample d e h y d r a t e d a t h i g h t e m p e r a t u r e . The spectrum o f a soaked sample i s d i f f e r e n t ( F i g u r e 12). We n o t e h e r e the p r e s e n c e o f a b r o a d l i n e s u p e r i m p o sed on the e l a s t i c l i n e o f F i g u r e 11. Such a b r o a d e n i n g p r o v e s the e x i s t e n c e of a d i f f u s i v e random m o t i o n o f the water p r o t o n s w i t h a c h a r a c t e r i s t i c time

Water Absorption in Acid Nafion Membranes - ACS Symposium Series

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