Thermodynamic Studies of the ... - ACS Publications

2 T h e r m o d y n a m i c S t u d i e s of t h e W a t e r — P e r f l u o r o s u l f o n a t e d Polymer

Interactions

Downloaded by MONASH UNIV on July 11, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch002

Experimental Results M. ESCOUBES Laboratoire de Chimie Appliquée et de Génie Chimique, Université Lyon I-43 Bd. du 11 Novembre 1918, 69621 Villeurbanne, France M. PINERI Equipe Physico-Chimie Moléculaire, Section de Physique du Solide, Département de Recherche Fondamentale, Centre d'Etudes Nucléaires de Grenoble, 85 X-38041 Grenoble Cedex, France The analysis of the sorption isotherms is the most common way to study the interactions of water with polymers. Mathematical models can be fitted to the experimental results and give information about these water-polymer interactions which can be directly obtained from enthalpimetric analysis. It is possible to get the heat of sorption of the water molecules during different sorption isotherms corresponding to different humidity levels. It is also possible to check the phase transformations of the absorbed water by differential scanning calorimetry. The water-polymer interaction depend on the polymer free volume, crystallinity, porosity, chemical structure, etc... It is known that strong interactions between water and polymer can produce important modifications of the solid polymer like swelling or crystallisation. If these interactions are not homogeneous inside the polymer matrix it may result in some "clustering" of the water molecules with formation of holes inside the polymer. Thermodynamic measurements must define both the water-polymer interaction and the structural change of the polymer. This information can be given from the direct measurement of the heat of sorption of the water molecules. - An incremental increase of the relative water pressure is realized and during each increment both the amount of water adsorbed in the specimen and the total energy involved in this absorption are recorded. The average energy per water molecule corresponding to these water molecules absorbed during the increment can therefore be deduced. This energy value depends strongly on the nature of hydrogen bonding and also on the number of hydrogen bonds involved in this interaction. Changes of this value during the water absorption may reflect the existence of different sites of absorption with different energies of interaction. For instance in collagen we have shown the existence of different regimes of absorption (-16 Kcal.mole~"l between 1 and 10 % of water, -13 Kcal. mole~l between 24 and 48 %). From these energy values and also from the number of water molecules corresponding to each regime it has been possible to propose a model of water absorption corresponding 0097-6156/82/0180-0009$05.00/0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


10

PERFLUORINATED

IONOMER MEMBRANES

t o w e l l d e f i n e d s i t e s ( 1 ) . T h i s k i n d o f measurement i s t h e o n l y one w h i c h i s a b l e t o d e f i n e t h e amount o f bound w a t e r . T h i s t e r m o f bound w a t e r has b e e n o v e r u s e d d u r i n g t h e s e l a s t y e a r s . I t has f i r s t t o be n o t e d e v e n w i t h h y d r o p h i l i c p o l y m e r s , h y d r o g e n b o n d i n g w i t h h y d r o x y l , amide o r c a r b o n y l g r o u p s a r e r e l a t i v e l y low i n e n e r g y and sometimes l o w e r t h a n t h e e n e r g y o f b o n d i n g o f w a t e r mo l e c u l e s i n l i q u i d w a t e r ( 2 ) . A n o t h e r p o i n t i s t h e i n d i r e c t way u s u a l l y u s e d t o d e f i n e b o t h e n e r g y and t h e amount o f bonded w a t e r ( 3 ) . M o s t o f t h e e n t h a l p y v a l u e s a r e deduced f r o m t h e C l a u s i u s C l a p e y r o n e q u a t i o n u s i n g t h e a b s o r p t i o n i s o t h e r m s e v e n when t h e r e v e r s i b i l i t y c o n d i t i o n s a r e n o t o b s e r v e d . The amount o f bound water i s u s u a l l y o b t a i n e d from the d i f f e r e n c e between the t o t a l w a t e r c o n t e n t and t h e amount o f w a t e r g i v i n g a phase t r a n s i t i o n a t low t e m p e r a t u r e s . I n f a c t i n many c a s e s s u c h phase t r a n s i t i o n i s not p o s s i b l e because of s t e r i c l i m i t a t i o n s i n s i d e the water c l u s t e r s . The m o b i l i t y o f t h e bound w a t e r has a l s o b e e n shown by NMR o r d i e l e c t r i c measurements t o be p r e t t y c l o s e t o t h e o b s e r ved m o b i l i t y i n l i q u i d water. - Low v a l u e s o f t h e h e a t o f w a t e r a b s o r p t i o n c a n be o b t a i n e d . These v a l u e s s m a l l e r t h a n t h e l i q u e f a c t i o n e n e r g y may r e s u l t f r o m t h e s u p e r p o s i t i o n o f an e n d o t h e r m a l mechanism : b r e a k i n g o f some bonds ( 4 ) , f u r t h e r c r i s t a l l i z a t i o n o f t h e p o l y m e r s ( 5 ) , e x p a n s i o n o f t h e m a c r o m o l e c u l e s w i t h change o f v o l u m e . F o r t h i s l a s t c a s e , w h i c h does n o t c o r r e s p o n d t o an e x p a n s i o n mechanism d e s c r i b e d by t h e F l o r y m o d e l one has t o t a k e i n t o a c c o u n t t h e e n t h a l p y and f r e e e n e r g y o f e x p a n s i o n (6,7) o r t h e i n t e r n a l p r e s s u r e due t o the polymer ( 8 ) . Heat o f a b s o r p t i o n measurements T h i s measurement i s r e a l i z e d by c o u p l i n g a m i c r o b a l a n c e and a 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 ( 9 ) . B e f o r e the s o r p t i o n experiment, t h e two i d e n t i c a l samples a r e vacuum d r i e d ( 1 0 " ^ t o r r ) i n s i t u . D i f f e r e n t i n c r e a s i n g h u m i d i t y l e v e l s a r e t h e n o b t a i n e d by c h a n g i n g t h e t e m p e r a t u r e o f a w a t e r c e l l (between - 80°C and 20°C). D u r i n g e a c h w a t e r p r e s s u r e i n c r e m e n t ^ w e i g h t and e n e r g y changes a r e r e c o r ded and t h e m o l a r e n e r g y o f i n t e r a c t i o n i s o b t a i n e d . The MTB 10.8 S e t a r a m m i c r o b a l a n c e has a s e n s i b i l i t y b e t t e r t h a n 1 y g . The l i m i t o f d e t e c t i o n f o r t h e m i c r o c a l o r i m e t e r i s 80 pW. F o r a h y d r a t i o n e n e r g y a r o u n d 10 K c a l . m o l e " ^ a 10 % p r e c i s i o n i s o b t a i n e d f o r w a t e r s o r p t i o n l a r g e r t h a n 1 mg p e r h o u r . The m e a s u r e d d i f f e r e n t i a l molar energy of a b s o r p t i o n ( q ) i s : de a e - e + ηa dn a g a i n which i n t e r n a l molar energy of adsorbed H 2 O e a i n t e r n a l m o l a r e n e r g y o f a d s o r b e d gas e number o f H 2 O m o l e c u l e s a d s o r b e d change i n t h e number o f a d s o r b e d w a t e r a molecules d u r i n g the p r e s s u r e increment, change i n t h e i n t e r n a l m o l a r e n e r g y o f t h e de a a d s o r b e d H 2 O m o l e c u l e s d u r i n g the same increment f

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

2.

ESCOUBES A N D PINERI

Thermodynamics

of Water-Polymer

Interactions

11


A d e t a i l e d a n a l y s i s of the thermodynamics i n v o l v e d i n t h i s measurement i s g i v e n ( 1 3 ) . I t has t o be n o t e d t h a t t h i s i n t e r n a l e n e r g y change w h i c h i s m e a s u r e d i s n o t v e r y d i f f e r e n t f r o m t h e e n t h a l p y change. E x p e r i m e n t a l r e s u l t s o f t h e h e a t o f a b s o r p t i o n measurements N a f i o n 120 i n t h e a c i d and n e u t r a l i z e d f o r m has b e e n s t u d i e d (11). I s o t h e r m s have b e e n o b t a i n e d a t 10°C and a t l o w e r t e m p e r a t u res . From t h e 10°C i s o t h e r m s t h e m a i n r e s u l t s a r e : 1. A w e l l d e f i n e d amount o f w a t e r , d e p e n d i n g on t h e c a t i o n , i s k e p t a f t e r vacuum d r y i n g (10"5 t o r r ) a t room t e m p e r a t u r e ( f i g . 1). T h i s r e s i d u a l w a t e r (1 t o 2 H20/S03~) i s d e s o r b e d a f t e r h e a t i n g above t h e g l a s s t r a n s i t i o n o f t h e N a f i o n m a t r i x . D u r i n g r e h y d r a t i o n o f t h e h i g h t e m p e r a t u r e vacuum d r i e d s a m p l e t h e i n i t i a l i s o t h e r m s o r p t i o n c u r v e i s r e o b t a i n e d o n l y above a w e l l d e f i n e d r e l a t i v e water pressure ( f i g . 2 ) . 2. T h i s r e s i d u a l w a t e r has t h e same i n t e r a c t i o n e n e r g y as the f i r s t w a t e r m o l e c u l e s a b s o r b e d i n t h e room t e m p e r a t u r e d r i e d sample ( f i g . 3 ) . O n l y two h y d r a t i o n r e g i m e s a r e o b s e r v e d d u r i n g the room t e m p e r a t u r e s o r p t i o n i s o t h e r m ( f i g . 3 and 4 ) . - The f i r s t r e g i m e c o r r e s p o n d s t o t h e f i r s t 8 p e r c e n t o f abs o r b e d w a t e r (^ 5/6 moles/S03"~). The m o l a r i n t e r a c t i o n e n e r g y i s c o n s t a n t and c h a r a c t e r i s t i c o f t h e c a t i o n - 13 K c a l . m o l e ^ f o r - 13.5 " - 9.5 - 8.5 " 11

Fe H Na Cu

+ +

+

+

+ +

The h y d r a t i o n e n e r g y o f t h e c a t i o n s i n s o l u t i o n c o r r e s p o n d s t o the same o r d e r and t h i s i s c o n s i s t e n t w i t h t h e e x i s t e n c e o f i o n i c c l u s t e r s i n the dehydrated s t a t e . - D u r i n g t h e s e c o n d r e g i m e t h e e n e r g y i s d e c r e a s i n g and i s a l ways l o w e r t h a n t h e v a l u e c o r r e s p o n d i n g t o t h e l i q u e f a c t i o n . T h i s i m p l i e s an e n d o t h e r m a l c o n t r i b u t i o n c o r r e s p o n d i n g t o t h e e x p a n s i o n of t h e c l u s t e r i n s i d e t h e o r g a n i c p h a s e . An i m p o r t a n t d e c r e a s e o f t h e w a t e r c o n t e n t i s o b t a i n e d w i t h the low t e m p e r a t u r e i s o t h e r m s ( f i g . 5 ) . At s a t u r a t i o n the r e l a t i v e water contents a t d i f f e r e n t temperatures are given i n the following table: TABLE I Temperature 0°C 20°C -10°C -13°C

Water c o n t e n t

i n t e r a c t i o n energy Kcal/mole -

12,5

12

-

12,7

10

-

14,75

%

8

8.5



12

Figure

PERFLUORINATED IONOMER

1. Water loss during high temperature heating for different Nafion heating rate, 3° C/min; vacuum, 10~ ton.

MEMBRANES

forms;

4



Thermodynamics

of Water-Polymer

Interactions


Δ Ρ ft)

0

I

0.5

P /

P,

Figure 2. Room temperature sorption-desorption isotherms of acid Nafion. %, room temperature dried absorption; • , 220° C dried primary absorption; 220° C dried desorption; Q, 220° C dried secondary absorption.


Key: •,

PERFLUORINATED IONOMER M E M B R A N E S

HaO / SOj" 8 1.0


(Kcal/mole)

Figure 3. Enthalpic energy of absorption for the water molecules during an isotherm absorption at room temperature. Key: Q 0> room temperature dried sample; ·---·, 220° C dried sample.

moles H,0/S0

°

Γ

Τ

Τ

Τ

Τ

Τ

3

Τ

Δρ {%)

Figure 4. Same as in Figure 3 for an iron salt (~65% neutralization). Key: Ο room temperature dried sample; Φ Φ, 220° C dried sample.


Ο»

Thermodynamics

of Water-Polymer

Interactions




PERFLUORINATED IONOMER MEMBRANES

Κ CQl. A

mole" -15-


1

U i

Figure

1

0

1

25

6. Enthalpic

1

5

energy absorption during sorption.

X*

1

75

10

different

Δρ%12.5

temperature

isotherm

ab

3/mn . 2°

f

:/mn

:/mn Y * ι

ι

ι

i

i

ι

ι

ι

ι

ι

ι

ι

ι

ι

ι

r—•ι

1

ι'f

1

Figure 7. Influence of the heating speed on the acid Nafion polymer containing by weight of water.


12%

2.


Thermodynamics

of Water-Polymer

Interactions

17

The i n t e r a c t i o n e n e r g y w i c h i s g i v e n i n t h i s t a b l e has t o be compared w i t h l i q u e f a c t i o n e n e r g y o f w a t e r = -12.7 Kcal.mole""^ a t 0°C and w i t h t h e s u b l i m a t i o n e n e r g y a t -10°C : -12.5 K c a l . m o l e " . An i m p o r t a n t change i n t h e p o l y m e r s t r u c t u r e must t h e r e f o r e o c c u r when t h e t e m p e r a t u r e i s l o w e r e d ( f i g . 6 ) . The f i r s t c o n c l u s i o n w h i c h i s a p p a r e n t f r o m t h e s e r e s u l t s i s the a b s e n c e o f s t r o n g w a t e r - N a f i o n p o l y m e r i n t e r a c t i o n s . The f i r s t water molecules which are absorbed correspond to the s o l v a t a t i o n o f t h e i o n s . F o r w a t e r c o n t e n t s l a r g e r t h a n a b o u t 8 % (^ 5 w a t e r m o l e c u l e s ) one o b s e r v e s a d e c r e a s e i n t h e e n e r g y w h i c h c a n be e x p l a i n e d by an e l a s t i c d e f o r m a t i o n o f t h e p o l y m e r i n v o l v i n g t h e m o t i o n o f t h e h y d r o p h o b i c c h a i n s o u t o f t h e h y d r a t e d z o n e . The amount o f w a t e r a b s o r b e d a t s a t u r a t i o n and t h e c o r r e s p o n d i n g h y d r a t i o n e n e r g y s t r o n g l y depend on t h e t e m p e r a t u r e . Downloaded by MONASH UNIV on July 11, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch002

1

D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) DSC has b e e n f o u n d t o be t h e s i m p l e s t method t o d e f i n e t h e r e l a t i v e amounts o f f r e e z i n g and non f r e e z i n g w a t e r ( 1 4 ) . A wet sample i s h e r m e t i c a l l y k e p t i n a sample pan. D u r i n g c o o l i n g o r h e a t i n g r u n s t h e empty r e f e r e n c e c e l l and t h e s p e c i m e n - c o n t a i n i n g c e l l a r e k e p t a t t h e same t e m p e r a t u r e . The r e l a t i v e d i f f e r e n c e o f power n e c e s s a r y t o do so i s p l o t t e d v e r s u s t e m p e r a t u r e . From t h e peak s u r f a c e c o r r e s p o n d i n g to f r e e z i n g or m e l t i n g i t i s t h e r e f o r e poss i b l e t o d e f i n e t h e r e l a t i v e amount o f f r e e z i n g and non f r e e z i n g water. A CPC 600 c a l o r i m e t e r (15) has b e e n u s e d w i t h h e a t i n g o r c o o l i n g speeds b e t w e e n 0.5 and 4°/mn. A c i d N a f i o n s 120 c o n t a i n i n g d i f f e r e n t w a t e r p e r c e n t a g e s have b e e n s t u d i e d . The i n f l u e n c e o f t h e h e a t i n g s p e e d i s shown i n f i g . 7 f o r a N a f i o n s p e c i m e n c o n t a i n i n g 12 % by w e i g h t o f w a t e r . The s a m p l e s h a v e b e e n f i r s t s l o w l y c o o l e d a t l°C/mn down t o l i q u i d n i t r o g e n temperature. I n t a b l e ( I I ) are g i v e n the i n t e g r a t e d v a l u e s o f the e n d o t h e r m i c p e a k s . F o r t h e 2 /mn sample two d i f f e r e n t r u n s h a v e been done. The a v e r a g e v a l u e i s 432 m e a l w i t h i n 5 % d e p e n d i n g on t h e b a s e l i n e and t h e c o r r e s p o n d i n g w a t e r - w i t h 80 c a l / g as t h e h e a t o f m e l t i n g - i s 5.4 mg. I n t h i s c a s e w i t h s u c h an h y p o t h e s i s we w o u l d h a v e ^ 30 % o f f r e e z i n g w a t e r . The i n f l u e n c e o f t h e c o o l i n g s p e e d i s shown i n f i g . 8 f o r t h e same sample r u n a t a c o n s t a n t h e a t i n g speed o f 2°C/mn. No change i s o b s e r v e d b e t w e e n samples quenched i n l i q u i d n i t r o g e n o r r a p i d l y c o o l e d w i t h He g a s . I n t h i s c a s e some e x o t h e r m i c c o n t r i b u t i o n i s apparent. I n f i g . 9 a r e shown t h e c u r v e s o b t a i n e d d u r i n g a c o o l i n g p r o c e s s a t l°/mn f o r two d i f f e r e n t w a t e r c o n t e n t s a m p l e s (12 an 15 % by w e i g h t ) . An e x o t h e r m i c p e a k i s a p p a r e n t i n b o t h s a m p l e s , t h e p o s i t i o n and t h e f o r m o f t h i s peak depend on t h e s a m p l e . B o t h peaks are l o c a t e d a t temperature w e l l below the temperature c o r r e s p o n d i n g to the normal w a t e r f r e e z i n g . No e n d o t h e r m i c o r e x o t h e r m i c peak i s a p p a r e n t f o r samples c o n t a i n i n g l e s s than 8 % of water.


PERFLUORINATED IONOMER M E M B R A N E S


18

Figure 8. Influence of the cooling speed on the same specimen as in Figure 7. Cooling speeds: -·-·-·-, 1 °/mn; , He gas; , liquid nitrogen quenching.

Figure

9. Influence of the water content on the thermograms: water; lower curve, 12% water.

upper curve,


15%

2.


Thermodynamics

of Water-Poly

mer Interactions

19

TABLE I I Heating

rate

Η meal 450

260/270°

0.5°/mn


Peak p o s i t i o n Κ

2°/mn

263

416 433

4°/mn

268

430

From t h e s e r e s u l t s t h e f i r s t i n t e r p r e t a t i o n w h i c h comes t o m i n d i s f r e e z i n g and m e l t i n g o f w a t e r i n s m a l l c l u s t e r s w h i c h o n l y a p p e a r f o r w a t e r c o n t e n t s l a r g e r t h a n 8 %. The a n a l y s i s o f s o l i d - l i q u i d phase t r a n s f o r m a t i o n i n s m a l l p o r e s c a n be done by t h e r m o p o r o m e t r y . F o r a l i q u i d c o n t a i n e d i n a p o r o u s m a t e r i a l t h e s o l i d - l i q u i d i n t e r f a c e c u r v a t u r e depends c l o s e l y on t h e s i z e o f t h e p o r e and t h e s o l i d i f i c a t i o n t e m p e r a t u r e i s t h e r e f o r e d e p e n d e n t on t h i s s i z e ( 1 6 ) . From t h e s o l i d i f i c a t i o n thermogram i t i s t h e r e f o r e p o s s i b l e t o o b t a i n b o t h t h e s i z e o f t h e p o r e s f r o m t h e s o l i d i f i c a t i o n t e m p e r a t u r e p o s i t i o n and a l s o t h e t o t a l volume o f w a t e r i n v o l v e d i n t h i s t r a n s f o r m a t i o n f r o m t h e measurement o f t h e e n e r g y c o r r e s p o n d i n g t o t h i s phase t r a n s f o r m a tion. By u s i n g a m i c r o c a l o r i m e t e r t h e thermograms a r e o b t a i n e d . The c o o l i n g and h e a t i n g speeds a r e b e t w e e n 6 and 8°/hour. I n f i g . (10) and (11) a r e shown t h e thermograms c o r r e s p o n d i n g t o h e a t i n g o r c o o l i n g f o r two d i f f e r e n t N a f i o n w a t e r s y s t e m s . I n b o t h samples an en d o t h e r m i c peak a p p e a r s d u r i n g h e a t i n g w h i c h e x t e n d s o v e r a l a r g e r t e m p e r a t u r e r a n g e t h a n i n t h e p r e v i o u s e x p e r i m e n t . Such a b e h a v i o u r i s s i m i l a r t o what i s o b s e r v e d i n p o r o u s m a t e r i a l s l i k e γ-alumina w i t h s p h e r i c a l water c o n t a i n i n g pores (16). A n u m e r i c a l r e l a t i o n s h i p has b e e n o b t a i n e d b e t w e e n t h e f r e e z i n g temperature d e p r e s s i o n s of a c a p i l l a r y condensate s a t u r a t i n g a p o r o u s m a t e r i a l and t h e r a d i i . Rp

(nm)

-64.67 ΔΤ

0.57

0 > ΔΤ > -

which gives

40 ο Rp = 21.8 A f o r ΔΤ = ο Rp = 38 A f o r ΔΤ =

+

-40° -20°

ο ο I f s u c h i n t e r p r e t a t i o n i s v a l i d t h e s e 20 A and 40 A would c o r r e s p o n d to the o r d e r of magnitude f o r the r a d i u s of the water c l u s t e r s i n s i d e t h e 9 and 14 % w a t e r N a f i o n s y s t e m s . Another p o s s i b i l i t y which would e x p l a i n such d e p r e s s i o n i n the f r e e z i n g and m e l t i n g t e m p e r a t u r e w o u l d be t h e p r e s e n c e o f i o n s i n the water. I t i s p o s s i b l e to c a l c u l a t e t h i s d e p r e s s i o n



220

,

240

,

I

260 ZI

260

'

2~ZÔ '

220


I

I

I

L-k 273°

T(K)

I

273°

Figure

'

200

T(K)

10. Low speed thermograms of a 9% H 0 content sample obtained cooling (lower curve) and heating (upper curve). 2

220

,

240

,

260

—ι

1

1

1

1

I_J 273°

1

1

1

1

during

Τ (Κ) 1

273° 1—I

1

260

1

240

220

1

200

1

T(K)

Figure 11. Low speed thermograms of a 14% water content sample obtained heating (upper curve) and cooling (lower curve).


during

2.


Thermodynamics

of Water-Polymer

Interactions

21


. c i n w h i c h c i s t h e i o n c o n c e n t r a t i o n and ΔΗ t h e l i q u e f a c t i o n e n e r g y of w a t e r : 80 c a l / g . The r e s u l t s a r e n o t c o n s i s t e n t w i t h s u c h h y p o t h e s i s b o t h b e c a u s e t h e change i n θ v e r s u s t h e w a t e r c o n c e n t r a t i o n w o u l d be s m a l l e r and a l s o b e c a u s e no s u c h t e m p e r a t u r e d i f f e r e n c e w o u l d be o b s e r v e d f o r t h e peak p o s i t i o n d u r i n g h e a t i n g o r c o o l i n g . I t i s now i m p o r t a n t t o r e p o r t some NMR r e s u l t s o b t a i n e d w i t h an a c i d 120 N a f i o n sample c o n t a i n i n g an e x c e s s o f w a t e r . A N a f i o n w a t e r m i x t u r e i s quenched f r o m room t e m p e r a t u r e down t o l i q u i d n i t r o g e n t e m p e r a t u r e and t h e n r a p i d l y p u t i n t o t h e NMR s p e c t r o m e t e r a t a w e l l d e f i n e d t e m p e r a t u r e b e l o w 0°C. The a m p l i t u d e o f t h e l i n e c o r r e s p o n d i n g to the mobile water protons a t t h i s temperature i s t h e n r e c o r d e d v e r s u s t i m e as shown i n f i g ( 1 2 ) . The o b s e r v e d de c r e a s e i n a m p l i t u d e o f t h e l i n e c o r r e s p o n d s t o a change i n t h e number o f t h e m o b i l e w a t e r p r o t o n s . D u r i n g t h e a n n e a l i n g t i m e some d e s o r p t i o n o c c u r s and i n i t i a l l y m o b i l e w a t e r m o l e c u l e s a r e f r o z e n e i t h e r o u t s i d e t h e sample o r i n s m a l l h o l e s i n s i d e t h i s s a m p l e . Conclusions Two v e r y w e l l d e f i n e d r e g i m e s o f w a t e r a b s o r p t i o n have t h e r e f o r e b e e n e v i d e n c e d . The f i r s t r e g i m e c o r r e s p o n d t o t h e f i r s t w a t e r m o l e c u l e s w h i c h f i l l t h e h y d r a t i o n s h e l l ; b e t w e e n f o u r and s i x w a t e r m o l e c u l e s a r e n e c e s s a r y t o do so f o r t h e a c i d s a m p l e . A s i m i l a r b e h a v i o u r has b e e n o b s e r v e d f r o m NMR ( 1 1 ) , Môssbauer ( 1 9 ) . These f i r s t w a t e r m o l e c u l e s w h i c h a r e a b s o r b e d o n t o t h e s e i o n i c s i t e s have an h y d r a t i o n e n e r g y w h i c h c o r r e s p o n d t o t h e v a l u e obt a i n e d f o r t h e c o r r e s p o n d i n g c a t i o n s i n s o l u t i o n . The o b s e r v e d decrease i n the a b s o l u t e v a l u e of the i n t e r a c t i o n molar energy f o r f u r t h e r w a t e r s o r p t i o n may i n v o l v e a d e f o r m a t i o n o f t h e h y d r o p h o b i c m a t r i x . I t has a l s o t o be p o i n t e d o u t t h a t a r a p i d e x c h a n g e o c c u r s between a l l the water m o l e c u l e s g i v i n g r i s e t o a s i n g l e l i n e i n NMR. Another important c o n c l u s i o n i s o b t a i n e d from the coupled DSC/NMR e x p e r i m e n t s . The w a t e r c o n t e n t o f t h e N a f i o n membranes s t r o n g l y depends on t h e t e m p e r a t u r e . T h e r e f o r e t h e a n a l y s i s o f a p o s s i b l e w a t e r phase s e p a r a t i o n c a n n o t be done w i t h e x p e r i m e n t s i n v o l v i n g t e m p e r a t u r e changes l i k e DSC. T h i s i s p r e t t y d i f f e r e n t f r o m what i s o b t a i n e d w i t h γ-alumina w h i c h r e p r e s e n t a r e l a t i v e f i x e d and non t e m p e r a t u r e d e p e n d e n t h y d r o p h o b i c m a t r i x . The endo t h e r m i c and e x o t h e r m i c peaks o b s e r v e d d u r i n g h e a t i n g and c o o l i n g r u n s o f t h e w a t e r - N a f i o n s y s t e m s may be i n t e r p r e t e d i n two ways - e i t h e r t h e peak i t s e l f c o r r e s p o n d s t o t h e s o r p t i o n - d e s o r p t i o n thermal m a n i f e s t a t i o n - o r t h e peak c o r r e s p o n d s t o t h e m e l t i n g o r s o l i d i f i c a t i o n o f w a t e r i n s m a l l pores which a r e formed d u r i n g the t h e r m a l c y c l e . Such a b e h a v i o u r has a l r e a d y b e e n o b s e r v e d i n p o l y e t h y l e n e ( 1 7 ) .



"gro—Ο

C


rx-x=û=_x-

40

80

60

t (mn)

Figure 12. Change of the NMR line amplitude vs. time during annealing temperatures. Key: Q, — 30° C; X, — 50° C.

-50

-40

-30

-20

-10

Figure 13. Amount of desorbed water vs. the annealing

at different

0 T(C) temperature.


2.

Thermodynamics


of Water-Polymer

Interactions

23

A more d e t a i l e d a n a l y s i s o f t h e s e r e s u l t s a n d o f t h e p o s s i b l e i n t e r p r e t a t i o n s w i l l be g i v e n i n a f u r t h e r p u b l i c a t i o n ( 1 8 ) . A n o t h e r i n t e r e s t i n g r e s u l t i s g i v e n i n f i g . 13 i n w h i c h i s p l o t t e d t h e amount o f d e s o r b e d w a t e r v e r s u s t h e a n n e a l i n g tempe r a t u r e f o r t h e 15 % H 0 - a c i d N a f i o n s y s t e m . From t h i s f i g u r e i t i s shown t h a t a r o u n d 60 % o f t h e t o t a l w a t e r c o n t e n t c a n b e d e s o r b e d . We t h e r e f o r e h a v e a r o u n d 9 % d e s o r b a b l e w a t e r a n d a r o u n d 7 % f i x e d w a t e r . These v a l u e s a r e i n c l o s e agreement w i t h t h e two a b s o r p t i o n r e g i m e s a n d a l s o w i t h t h e f a c t t h a t no DSC peak has b e e n o b s e r v e d f o r w a t e r c o n t e n t v e l o w ^ 8 % w a t e r .


2

Bibliography 1. M.H. PINERI, M. ESCOUBES, G. ROCHE, Biopolymers, 17, 12, 2799, (1978) 2. C.A.J. HOEVE - A.C.S. Symposium Series, 127, 7, 135 (1980) 3. J.A. RUPLEY, P.H. YANG, G. TOLLIN - A.C.S. Symposium Series, 127, 6, 11 (1980) 4. J . GUILLET, G. SEYTRE, A. COUILLARD, M. ESCOUBES - Die Angewandte Makromol. Chemie, 68, 1017, 149-162 (1978) 5. M. ESCOUBES, P. MOSER, P. BERTICAT - Die Angevandte Makromol. Chemie, 67, 991, 45-60 (1978) 6. H.J.C. BERENDSEN - Water in disperse systems, 15, 293, Franks Editor, Plenum Press (1975) 7. M. BRENER, E.M. BURAS, Jr and A. FOOKSON - A.C.S. Symposium Series, 127, 18, 311 (1980) 8. E. SOUTHERN, A.C. THOMAS - A.C.S. Symposium Series 127, 22, 375 (1980) 9. M. ESCOUBES, J . F . QUINSON, J . GIELLY, M. MURAT - Bull. Soc. Cim. F r . , 5, 1689 (1972) 10. C l . LETOQUART, Fr. ROUQUEROL, J . ROUQUEROL - J. Chim. Phys. 3, 559 (1973) 11. R. DUPLESSIX, M. ESCOUBES, B. RODMACQ, F. VOLINO, E. ROCHE, A. EISENBERG, M. PINERI - A.C.S. Symposium Series, 127, 28, 470-486 (1980) 12. G. BELFORT, N. SINAI - A.C.S. Symposium Series, 127, 19 (1980) 13. M. ESCOUBES, M. PINERI, A. EISENBERG, S. GAUTHIER, to be published 14. S. DEODHAR, P. LUNER, A.C.S. Symposium Series 127, 28, 273-286 (1980) 15. E. BONJOUR, M. COUACH, J. PIERRE, Cahiers de la Thermique, n° 1, Β 134-150 (1971) 16. M. BRUN, A. LALLEMAND, J . F . QUINSON, C. EYRAUD, Thermochimica Acta, 21, 59-88 (1977) 17. H.E. BAIR, G.E. JOHNSON, Analytical Calorimetry. Plenum Press Vol. IV, 219-225 (1977) 18. M. PINERI, C. BEN SAID, F. VOLINO, M. ESCOUBES, J . F . QUINSON, M. BRUN, to be published 19. Β. RODMACQ, M. PINERI, J.M.D. COEY, A. MEAGHER, J . Polym. Sci. to be published. RECEIVED October 13, 1981.


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