Water Absorption in Neutralized Nafion Membranes - ACS Publications

29 Water Absorption in Neutralized Nafion Membranes 2

3

B. RODMACQ, J. M. COEY1, M. ESCOUBES , E. ROCHE, R. DUPLESSIX , A. EISENBERG , and M. PINERI 4

Downloaded by UNIV OF SYDNEY on May 4, 2015 | http://pubs.acs.org Publication Date: August 19, 1980 | doi: 10.1021/bk-1980-0127.ch029

Centre d'Études Nucléaires de Grenoble, 38041 Grenoble C é d e x , France

In the previous paper (1) we have studied the water-polymer interactions in acid Nafions . In this paper we want to report some results we have obtained on the interactions between water and Nafion neutralized with different cations. The energy of water absorption has been measured over the whole range of relative humidities using the same technique as described previously (1). Mössbauer spectra were obtained in order to get information about the change of environment of the iron atomes during hydration. In addition, small angle neutron and X-ray scattering experiments have been performed to define a possible phase segregation. From these results, combined with those obtained in the preceeding (1) paper we propose a model of clustering in the Nafion membranes. The neutralized Nafion samples have been obtained by soaking the acid samples i n solutions containing iron chloride or sodium hydroxide. +

Heat of Absorption Measurements This experiment has been described in the previous paper (1). Figure 1 shows the water loss versus temperature for a heating rate of 3°C/min. The samples (.4mm thick) have f i r s t been dried for 24 hours under vacuum (10-4 torr) at room temperature. The weight loss obtained after such a heating procedure must correspond to a water loss because the behaviour observed after rehydration is completely reversible. It has to be noted that the amount Current addresses: Trinity College, Dublin 2, Ireland. Université Claude Bernard, 69621 Villeurbanne, France. Institut Laue Langevin, B P 156, 38042 Grenoble Cedex, France. M c G i l l University, Montreal, Canada. 1

2

3

4

0-8412-0559-0/ 80/47-127-487S05.00/ 0 © 1980 American Chemical Society

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

488


WATER IN

Figure 2.

Room temperature water absorption isotherms forthe Na salt


POLYMERS

29.

Nafion

Membranes

489

of undesorbed water at room temperature i s three times l a r g e r f o r the i r o n s a l t than f o r the sodium s a l t and the e q u i l i b r i u m v a lue i s obtained a f t e r a 30 minutes annealing a t 220°C f o r the i r o n s a l t . F i g u r e s 2 and 3 show the s o r p t i o n - d e s o r p t i o n isotherms for the N a and F e s a l t s . The behaviour i s very s i m i l a r . For both samples we observe a d i f f e r e n t absorption curve f o r the sample d r i e d at 220°C. For r e l a t i v e humidity values, only a few water molecules are absorbed. Then, beyond a r e l a t i v e pressure value of 0.25, there i s a d r a s t i c increase i n the amount of water absorbed. The corresponding d e s o r p t i o n curves are then e x a c t l y the same as the s o r p t i o n and d e s o r p t i o n curves of the room temperature d r i e d sample. In F i g u r e 4 are p l o t t e d the average energy of absorp t i o n f o r the d i f f e r e n t water molecules absorbed i n the i r o n sample. The empty c i r c l e s correspond to the values obtained when s t a r t i n g with the sample d r i e d at room temperature which contains about 3 % of water. The average energy f o r these f i r s t molecules i s 13 Kcal/mole, i t then decreases a f t e r a water content of about 8 %. The f i l l e d c i r c l e s correspond to the sample which has been d r i e d at 220°C. As was p r e v i o u s l y observed f o r the a c i d sample (J_) the energy of the f i r s t absorbed water molecules i s a l s o 13 k c a l . The r e f o r e , as before, the water molecules which had not been desorbed at room temperature do not have a l a r g e r binding energy. For t h i s sample the decrease i n energy occurs f o r lower amounts of water (y 5 %) because more energy i s needed to change the s t r u c t u r e of the polymer. Corresponding curves are p l o t t e d i n F i g u r e 5 f o r the Na s a l t s . +


Neutralized

R O D M A C Q ET A L .

+ +

Mossbauer Spectroscopy Experimental Methods. The a c i d form of the membrane with an equivalent weight ( i . e . the weight of polymer per SO3H group) of 1200 was n e u t r a l i z e d to about 50 % by immersion of a t h i n f o i l (=300 ym) i n an aqueous s o l u t i o n of 5 7 - f e r r i c c h l o r i d e . The samples were d r i e d i n vacuum at room temperature and then hydrated at d i f f e r e n t humidity l e v e l s . For a given humidity l e v e l , the q u a n t i t y of absorbed water i s l a r g e l y dependent on the state of the polymer i . e . whether i t i s an a c i d or a s a l t . As the samples we have s t u died are n e u t r a l i z e d to 50 %, i t i s not yet p o s s i b l e , from the weight i n c r e a s e , to estimate the amount of water molecules f i x e d by the n e u t r a l i z e d groups which are the only ones observable by MSssbauer spectroscopy. Mossbauer spectra have been recorded i n a conventional t r a n s mission geometry i n the constant a c c e l e r a t i o n mode. The tempera ture of the sample could be v a r i e d from 4.2 Κ up to room tempera ture by means of a l i q u i d helium c r y o s t a t , the source (57ςο i n Rh) being kept at room temperature. The MSssbauer experiments were c a r r i e d out i n the " L a b o r a t o i r e d I n t e r a c t i o n s Hyperfines" a t the CEN-Grenoble. f

R e s u l t s . F i g u r e 6 shows Mossbauer spectra of a sample d r i e d i n vacuum at room temperature. The main f e a t u r e s are the f o l l o w i n g :



490 WATER IN P O L Y M E R S




0

2

Neutralized

4

Nafion

6

Membranes

8

10 Δρ

Figure 4.

12 (%)

Heat of absorption vs. the amount of water absorbed for the Fe



492

WATER IN P O L Y M E R S

H.0/ 20

1

1

1

1

SO, 1

1

Ε ( kcal / mole ) NAF —

O·-

ο ·

Na

RT driëé 220 C

temple

'·

10 \.

O- - ^^V^^œ^**^;-

0

1 0

1 2

1

1 4

1

i 6

1

ο

I 8

1 Δρ

Figure 5.

I 10

_

(X)

Heat of absorption vs. the amount of water absorbed for the Na salt


Neutralized



29.

270 1

ι

-4

ι

-2

ι

ι

I

I

0

2

4

6

Velocity Figure 6.

493

Membranes

K

l

-6

Nafion

I

(mm/s)

Mossbauer spectra of the Fe salt: (-\-) experimental points; (—) theoret ical fit with two doublets.



494


- f - f a c t o r ( i . e . the area of the absorption spectrum) decreases r a p i d l y with i n c r e a s i n g temperature and reaches zero a t about 300 K. - there i s no evidence of a magnetic hyperfine s t r u c t u r e at low temperature. - the mean quadrupolar s p l i t t i n g i s of the order of 3.2 mra/s, which i s c h a r a c t e r i s t i c of f e r r o u s i o n , although the Nafion membranes have been n e u t r a l i z e d with f e r r i c c h l o r i d e . The mean quadrupolar s p l i t t i n g decreases when the temperature i n c r e a s e s . - the shape of the peaks i s asymmetric, and t h i s asymmetry i n c r e a ses with i n c r e a s i n g temperature. Moreover, the r e l a t i v e i n t e n s i t i e s of the peaks vary with i n c r e a s i n g temperature. The changes of the spectrum as a f u n c t i o n of temperature showed us that i t would not be p o s s i b l e to f i t the experimental data by only one doublet. These doublets w i l l be c a l l e d DI and DII i n the r e s t of t h i s paper. Table I gives the values of the hyperfine parameters corresponding to the spectra of f i g u r e 6, δ i s the chemical s h i f t , Δ i s the quadrupolar s p l i t t i n g , Γ i s the l i n e width and f are the r e c o i l f r e e f r a c t i o n s . We have per formed other Mossbauer experiments on samples with d i f f e r e n t amounts of water. We have f i t t e d the experimental spectra with the same components DI and DII. Such a decomposition gives a continuous change i n the r e l a t i v e p r o p o r t i o n of the i r o n atoms corresponding to the doublets DI and DII.

TABLE I

Τ

6

κ

4.2

70

140

210

270

I

mm/ s

Δ

mm// s

Γ , mm/ s

f

f

a.u

D I

1.34

3.46

0.33

0.32

D II

1.35

3.03

0.47

0.65

D I

1.35

3.40

0.33

0.32

D II

1.35

2.98

0.48

0.40

D I

1.31

3.25

0.39

0.24

D II

1.30

2.73

0.53

0.23

D I

1.26

2.92

0.49

0.18

D II

1.20

2.15

0.52

0.08

D I

1.16

2.70

0.67

0.06

D II

1.11

1.78

0.60

0.03

tot a. u 0.97

0.72

0.47

0.26

0.09



29.

RODMACQ ET AL.

Neutralized Nafion Membranes

495

D i s c u s s i o n . The f i t of the experimental spectra by means of two quadrupolar doublets corresponds to the e x i s t e n c e of two d i f ferent i r o n s i t e s i n water c o n t a i n i n g Nafion membranes. The p r o p o r t i o n of the i r o n atoms connected with the doublet DI i n c r e a s e s when the water content increases. In samples d r i e d under vacuum at room temperature t h i s p r o p o r t i o n i s 30 % and i t i n c r e a s e s up to 90 % f o r a water soaked sample. I t seems t h e r e f o r e reasonable to i d e n t i f y t h i s s i t e with a Fe(H20)++ complex. Indeed, we know both from NMR and heat of absorption measurements that there are some water molecules l e f t i n the sample d r i e d a t room temperature (J_). We can a l s o note, that the value of the quadrupolar s p l i t t i n g i s not very d i f f e r e n t from that of f r o z e n s o l u t i o n s o f FeCl2 and FeS04 as measured by Nozik and Kaplan (2). These authors showed that the d i s s o l u t i o n of these s a l t s i n water l e d to the formation of Fe(H20)£ complexes. The other doublet DII would represent a l e s s hydrated f e r r o u s i r o n , w i t h the r e l a t i v e p r o p o r t i o n of t h i s species decreasing when the amount of water i s increased. +

Conclusions. From these r e s u l t s we can conclude the f o l l o w ing : - water i s located c l o s e to the i r o n ions. - the water molecules are not randomly d i s t r i b u t e d around the i r o n ions but form complexes with the c a t i o n s with w e l l d e f i n e d structures. - there are two d i f f e r e n t kinds of i r o n atoms with d i f f e r e n t environments, one with a h i g h water content (DI) and the other with none or very few water molecules (DII). A more d e t a i l e d a n a l y s i s of these r e s u l t s w i l l be developped i n another p u b l i c a t i o n (3). Small angle s c a t t e r i n g r e s u l t s The p h y s i c a l s t r u c t u r e o f Nafions s a l t s has been explored by neutrons (SANS) and X rays (SAXS). The former method i s s e n s i t i v e to f l u c t u a t i o n s i n the coherent neutron s c a t t e r i n g cross s e c t i o n while the l a t t e r d e t e c t s f l u c t u a t i o n s i n the e l e c t r o n d e n s i ty. Such f l u c t u a t i o n s a r i s e i n the Nafions from p a r t i a l c r y s t a l l i z a t i o n of the samples and from c l u s t e r i n g of the i o n i c groups or water molecules i n hydrated samples. As discussed i n the accompanying paper, three d i f f e r e n t s c a t t e r i n g s i g n a l s a r i s e i n Nafions, each o c c u r i n g over a d i f f e r e n t range of values of the s c a t t e r i n g v e c t o r Q. These s i g n a l s are b e l i e v e d to a r i s e from i m p u r i t i e s ( t h i s i s a t e n t a t i v e assignment), s t r u c t u r e s i n v o l v i n g c r y s t a l l i n e p e r f l u o r o e t h y l e n e u n i t s , and s t r u c t u r e s i n v o l v i n g c l u s t e r e d i o n i c groups and water molecules. These same three s c a t t e r i n g s i g n a l s have been observed i n a l l a c i d , i r o n s a l t , and sodium s a l t samples which had not been quenched. Thus a b a s i c s i m i l a r i t y e x i s t s between the a c i d and the s a l t form i n the o v e r a l l s t r u c t u r e , showing that the n e u t r a l i z a t i o n does not lead to a large s c a l e r e o r g a n i z a t i o n as i s observed i n other ionomers.



496

In the accompanying paper i t has been noted that the s c a t t e r i n g a r i s i n g from c r y s t a l l i n e regions complicated the i n t e r p r e t a t i o n of the s c a t t e r i n g a r i s i n g from the i o n i c s t r u c t u r e s since the two s i g n a l s overlap i n Q space. To e l i m i n a t e t h i s d i f f i c u l t y the method of quenching from the melt has been used f o r N a s a l t s . Such a procedure i s not p o s s i b l e f o r the a c i d form since the m a t e r i a l degrades at temperature high enough to melt the c r y s t a l s . The quenching procedure i n v o l v e s m a i n t a i n i n g a sample at 330°C f o r one hour i n the melt followed by quenching r a p i d l y to room temperature by p a s s i n g a c o l d gas over the f i l m . Wide angle X ray s c a t t e r i n g studies show the disappearance of the c r y s t a l l i n e maximum i n quenched samples. The SANS s i g n a l a r i s i n g from c r y s t a l l i n e superstructures i s also seen to disappear i n a quenched sample as shown i n Figure 7, l e a v i n g only the s c a t t e r i n g component a t very low q, which i s b e l i e v e d to a r i s e from i m p u r i t i e s . The study of s c a t t e r i n g from quenched N a samples has been used to analyze the s t r u c t u r e a r i s i n g from c l u s t e r i n g of ions and water molecules. Figure 8 shows SAXS curves f o r such samples a t d i f f e r e n t degrees of hydration. The curves f o r the samples d r i e d at room temperature and hydrated at 50 % or at 83 % r e l a t i v e humid i t y show no s c a t t e r i n g maxima, as i s c h a r a c t e r i s t i c of s c a t t e r i n g from widely separated p a r t i c l e s . SAXS curves from soaked and b o i led N a samples show s c a t t e r i n g maxima. The maxima are a t t r i b u t e d to i n t e r p a r t i c l e i n t e r f e r e n c e e f f e c t s which a r i s e at higher p a r t i c l e concentrations. This maximum has also been observed by SANS.


+

+

+

The s i z e of the s c a t t e r i n g e n t i t i e s can be d i r e c t l y obtained f o r s c a t t e r i n g curves without maxima by means of a G u i n i e r p l o t of £nIvs.Q2. Such an a n a l y s i s may a l s o be made f o r the curves e x h i b i t i n g maxima but i s only h i g h l y approximate due to the importance of the i n t e r p a r t i c l e i n t e r f e r e n c e f u n c t i o n which i s not t a ken i n t o account i n the G u i n i e r a n a l y s i s . R e s u l t s f o r the r a d i u s of g y r a t i o n obtained from the slope of such p l o t s are l i s t e d i n table I I . I t i s s e e n t h a t radius of g y r a t i o n of the p a r t i c u l e s i s constant at about 8 A up to 83 % R.H. but then increases to 15 and 20 Â f o r the soaked and b o i l e d samples. An a n a l y s i s of the SANS curve f o r a b o i l e d sample has been made on the b a s i s of a hard sphere model. In t h i s model i n t e r p a r t i c l e i n t e r f e r e n c e i s taken i n t o account a l l o w i n g a f i t of the s c a t t e r i n g maximum. A r a d i u s of g y r a t i o n of 24 A has been found from such a f i t . Q

TABLE I I Humidity l e v e l Rg(A)

η χ 10"

1 9

Dry R.T. 50% R.H. 83% R.H.

Soaked

Boiled

8

6

8

15

24

0.6

1.5

3.1

2.2

1.9

3 (number/cm )



Neutralized

Nafion

Membranes


29.

Figure 7.

Small-angle neutron scattering curve of the quenched and unquenched Νa salt. Measurements done at room temperature.


497

498

WATER


d

100

25

IN

POLYMERS

(A)

15

2Θ° Figure 8.

Small-angle X-ray scattering curve of the quenched Na salt with different water contents


29.

Neutralized


Nafion

Membranes

499

The number of s c a t t e r i n g p a r t i c l e s per u n i t volume, n, may a l s o be c a l c u l a t e d at d i f f e r e n t h y d r a t i o n l e v e l s from the equation: 3 0 η

(H 0) ^_ 4 π R s 9

(1)

J


112

where R = (5/3) Rg and 0 (H2O) denotes the volume f r a c t i o n of water wêich i s determined from the weight f r a c t i o n and o v e r a l l density. I t i s assumed here that the s c a t t e r i n g p a r t i c l e s c o n t a i n only water and that a l l of the water i n the sample i s c l u s t e r e d . The l a t t e r assumption has been v e r i f i e d w i t h i n experimental e r r o r from an a n a l y s i s of the t o t a l s c a t t e r i n g i n v a r i a n t which has been c a l c u l a t e d from the absolute i n t e n s i t y of s c a t t e r i n g . The r e s u l t s f o r η l i s t e d i n Table Π show an apparent increase at low water contents and then a s l i g h t decrease at l a r g e water contents. I t i s noted that t h i s decrease i n η implying p a r t i c l e coalescence i s i n apparent c o n t r a d i c t i o n to the hard sphere model used above. A more d e t a i l e d a n a l y s i s of these r e s u l t s w i l l be given i n another p u b l i c a t i o n (4). Conclusion In t h i s c o n c l u s i o n we j u s t want to propose a p o s s i b l e model f o r the s t r u c t u r e of N a f i o n c o n s i s t e n t with a l l the r e s u l t s we have obtained. I t should f i r s t be pointed out that there i s no large change i n the macrostructure between the a c i d and s a l t forms of the Nafion membranes. Indeed, we observe e x a c t l y the same mul tiphase s e p a r a t i o n as seen from small angle X-ray and neutron s c a t t e r i n g experiments. The changes i n these curves with the amount of absorbed water are q u i t e s i m i l a r . We only note a small change i n the i o n i c c l u s t e r s i z e s depending on the nature of the i o n ( H , Na or Fe++). The s t r u c t u r e of the i o n i c phase i s not changed by quenching the sample from 330°C. Such a procedure permits us to get r i d of the c r y s t a l l i n e phase. I t i s t h e r e f o r e p o s s i b l e to get i n f o r m a t i o n about the s t r u c t u r e of these N a f i o n polymers from our experimental r e s u l t s obtained from the a c i d , as w e l l as the quenched an unquenched s a l t forms. The kind of model which can be proposed i s summarized i n F i g u r e 9. I t i s a three-phase model with a c r y s t a l l i n e phase, an i o n i c c l u s t e r phase and an intermediate " i o n i c " phase of lower ion content. For the sample d r i e d at room temperature we have the micro c r y s t a l l i t e s , the diameters of which are a few hundreds of angstroms. Such a r e s u l t i s i n agreement with both e l e c t r o n micros cope and X ray experiments. Dark f i e l d p i c t u r e s obtained with these m a t e r i a l s show the presence of small m i c r o c r y s t a l l i t e s of the s i z e mentionned above. By quenching the Na s a l t , the peak i n the SANS curves corresponding to the m i c r o c r y s t a l l i t e s disappears. An X ray p a t t e r n of such a sample shows that there i s no c r y s t a l l i n e phase l e f t . The i o n i c c l u s t e r s have a diameter of around 20 A as measured by small angle X ray s c a t t e r i n g experiments. Most of the water l e f t i n t h i s sample i s trapped i n s i d e these c l u s t e r . In t h i s phase, we a l s o have the i r o n atoms corresponding to the +

+


WATER IN POLYMERS

ROOM TEMPERATURE DRIED SAMPLE


mcrocrystallites fry 2 00 À

ionic clusters/

ipterrnedigte ionic

phase

0^20 À SOAKED

SAMPLE

mtcrocrvstatlites

0^34 A Figure 9.

Model of the Nafion structure


29.


Neutralized

Nafion

Membranes

501

doublet DI, the percentage o f which i s around 30 % as d e f i n e d from Mossbauer measurements. The l a s t phase i s an intermediate phase which contains the i r o n atoms corresponding to the doublet DII (a p o s s i b l e s t r u c t u r e o f which may be - S O 3 - F e ~ O 3 S ) . F o r the soaked sample we have an increase i n both the average s i z e and the volume f r a c t i o n of the i o n i c c l u s t e r s . The r e l a t i v e num ber of F e ions with a water environment (DI) i s increased up to 90 %. The model presented above i s i n accord with a l l our e x p e r i mental r e s u l t s . A more d e t a i l e d a n a l y s i s , taking i n t o account the q u a n t i t a t i v e aspects o f the water motion obtained from the q u a s i - e l a s t i c neutron s c a t t e r i n g experiments, w i l l be presented i n the near f u t u r e (5). + +


+ +

Literature Cited 1. Duplessix, R. ; Escoubes, M. ; Rodmacq, B. ; Volino, F. ; Roche, E. ; Eisenberg, A. ; P i n e r i , M . ; This Conference, precceding paper. 2. Nozik, A . J . ; Kaplan, M. ; J . Chem. Phys, 1967, 47, 2960. 3. Rodmacq, B. ; Coey, J.M.D. ; P i n e r i , Μ., Revue de Physique Appliquée. 4. Roche, E. ; Duplessix, R. ; Levelut, A.M. ; Eisenberg, A. ; P i n e r i , M. ; Journal of Polymer Science. To be published. 5. P i n e r i , M. ; Dianoux, A . J . ; Volino, F. ; Journal of Polymer Science. To be published. RECEIVED January 4, 1980.


Water Absorption in Neutralized Nafion Membranes - ACS Publications

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