Infrared Spectra of Perfluorosulfonated Polymer and of Water in

8

Infrared

Spectra

of

Perfluorosulfonated

Polymer

and of Water in Perfluorosulfonated Polymer

Downloaded by UNIV OF SOUTHERN CALIFORNIA on July 11, 2013 | http://pubs.acs.org Publication Date: February 4, 1982 | doi: 10.1021/bk-1982-0180.ch008

MICHAEL FALK Atlantic Research Laboratory, National Research Council of Canada, 1411 Oxford Street, Halifax, Nova Scotia B3H 3Z1, Canada

Infrared spectroscopy provides information on the microscopic structure of hydrated polymers which is not easily available by other means. Nafion is a new material and so far only four infrared studies on i t have been reported (1-4). The scope of these studies is summarized in Table I; they are of preliminary nature and do not exhaust the possibilities of the infrared technique. In the present chapter the structural information which has been so far derived from infrared studies of Nafion will be collected, and some additional results from the author's laboratory will be presented. Control of Water Content of Nafion Membranes in Infrared Studies The first infrared measurements on Nafion were carried out without controlling its water content (1,2). Because membranes exposed to the air tend to lose water by evaporation, especially when exposed to the heat of the infrared beam, the spectra of ref. 1 and 2 correspond to rather low water contents, probably below one H O molecule per sulfonate group. Control of water content was introduced by Lowry and Mauritz (3) who observed that membranes pressed tightly between flat plates did not lose water during the recording of the spectrum. This enabled them to record Attenuated Total Reflectance (ATR) spectra (5) at different stages of water loss, starting with a thoroughly soaked membrane and allowing some of the water to evaporate between consecutive measurements. They estimated the water content by quickly weighing the membrane before and after each spectrum and again after thorough drying (3). In the author's laboratory, two experimental techniques have been developed for controlling the water content. In the vapor equilibrium technique the membrane is suspended in a hygrostatic 2

NRCC No. 19534 0097-6156/82/0180-0139$08.00/0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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

codei

Nafion

work

142

form

+

+

Mainly

Na

Na+

+

+ 3

-

L i + , Na+, K , R b

+

4

1

+

H , N a , Co"*""*, N i " " , Cu++, F e

H+, Na+, Cs+

Cation

Transmission, ATR

Transmission

ATR

Transmission, ATR

Transmission

Technique

of Nafion

Variable

Low

Variable

Low

Low

State of hydration

4000-200

Water bands only

1100-900

4000-200

4000-1350

1

Spectral range (cm" )

Urhe f i r s t two d i g i t s o f t h e n u m e r i c a l code i n d i c a t e t h e e q u i v a l e n t w e i g h t i n u n i t s o f 100, w h i l e t h e t h i r d d i g i t i n d i c a t e s t h i c k n e s s i n m i l s ( 1 m i l = 25.4 ym). Thus N a f i o n 142 membrane has e q u i v a l e n t w e i g h t 1400 and t h i c k n e s s 2 m i l s = 51 pm.

Present

125,

125, 142

(1980)(4^)

Falk

1 2 5 , 152

113

(2)

Lowry & M a u r i t z ( 3 )

Heitner-Wirguin

L o p e z , K i p l i n g & Y e a g e r ÇL) 125

Authors

P r e v i o u s l y Reported I n f r a r e d Spectra

TABLE I


w > w

w

α δ 2! ο

W

r d ο 2 > Η

m

4^ Ο

8.

IR Spectra of Water in

FALK

141

Polymer

c e l l i n w h i c h m o i s t a i r o f c o n t r o l l e d h u m i d i t y and H/D r a t i o i s circulated. T h i s t e c h n i q u e , i n t r o d u c e d i n r e f . 4^, y i e l d s r e p r o d u c i b l e water contents. However, t h e d e t e r m i n a t i o n o f w a t e r by w e i g h i n g t h e membrane becomes i n a p p r o p r i a t e b e c a u s e d u r i n g t h e r e c o r d i n g o f t h e s p e c t r u m t h e p o r t i o n o f t h e membrane w h i c h i s s u b j e c t e d t o t h e i n f r a r e d r a d i a t i o n i s h e a t e d t o an e s t i m a t e d 10°C above a m b i e n t t e m p e r a t u r e , c a u s i n g t e m p o r a r y l o c a l d i m i n u t i o n of the water content. When w a t e r c o n t e n t s o f t h e s u s p e n d e d membranes were d e t e r m i n e d f r o m t h e a b s o r b a n c e o f t h e H 0 band a t 1620 cm" ( a s d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n ) i t was f o u n d t h a t t h e h i g h e s t l e v e l s o f h y d r a t i o n a t t a i n e d w i t h t h i s t e c h n i q u e were o n l y about 2 H 0 / - S 0 " and n o t 6 H 0 / - S 0 " as had b e e n c a l c u l a t e d f r o m w e i g h i n g measurements (4^). We have s i n c e l e a r n e d t o i n c r e a s e t h e w a t e r c o n t e n t o f t h e membranes i n t h e v a p o r e q u i l i b r i u m t e c h n i q u e t o a b o u t 5 H 0 / - S 0 " by p r o c e d u r e s m i n i m i z i n g t h e e f f e c t o f beam h e a t i n g , b u t b a s i c a l l y t h i s technique i s l i m i t e d t o the study of i n c o m p l e t e l y hydrated membranes· More r e c e n t l y , a c o m p l e m e n t a r y s a n d w i c h e d f i l m t e c h n i q u e was d e v e l o p e d i n w h i c h s p e c t r a o f N a f i o n membranes c o n t a i n i n g 1 t o 14 H 0 p e r s u l f o n a t e were r o u t i n e l y o b t a i n e d . The t o p l e v e l o f h y d r a t i o n c a n be t a k e n t o r e p r e s e n t a membrane c o m p l e t e l y s a t u r a t e d w i t h water under o u r e x p e r i m e n t a l c o n d i t i o n s . T h i s t e c h n i q u e i s a n a d a p t a t i o n o f t h e method o f Lowry and M a u r i t z o f s e a l i n g a membrane o f a g i v e n w a t e r c o n t e n t between two f l a t p l a t e s (3^). The p r o c e d u r e i s e q u a l l y s u i t a b l e f o r ATR measurements ( i n w h i c h t h e membrane i s s a n d w i c h e d between t h e i n t e r n a l r e f l e c t a n c e c r y s t a l and t h e back p l a t e , a s i n r e f . 3) o r f o r t r a n s m i t t a n c e measurements ( i n w h i c h t h e membrane i s s a n d w i c h e d b e t w e e n two C a F o r A g C l p l a t e s ) . The d i s a d v a n t a g e o f t h e s a n d w i c h e d f i l m t e c h n i q u e i s t h a t when t h e membrane i s t i g h t l y p r e s s e d a c e r t a i n amount o f l i q u i d w a t e r c a n be o b s e r v e d t o be s q u e e z e d o u t and t r a p p e d between t h e membrane and t h e c o n f i n i n g p l a t e s . Thus t h e t r a n s m i t t a n c e s p e c t r u m , and e v e n more so t h e ATR s p e c t r u m may c o n t a i n a c o n t r i b u t i o n f r o m a t h i n l a y e r o f p u r e w a t e r , t h e more so t h e h i g h e r t h e o r i g i n a l w a t e r c o n t e n t . T h i s does n o t i n t e r f e r e u n d u l y w i t h t h e bands o f N a f i o n , b u t f a l s i f i e s t o some e x t e n t , e s p e c i a l l y a t h i g h w a t e r c o n t e n t s , t h e s p e c t r a l band s h a p e s o f w a t e r i n N a f i o n . 1


2

2

3

2

2

3

3

2

2

As o f t h e t i m e o f w r i t i n g , a n i d e a l method o f r e c o r d i n g i n f r a r e d s p e c t r a o f N a f i o n membranes a t a l l l e v e l s o f h y d r a t i o n i s s t i l l being sought. Determination

o f W a t e r C o n t e n t f r o m A b s o r b a n c e a t 1620 cm"

1

I n what f o l l o w s , a b s o r b a n c e i s d e f i n e d as A = - l o g T, w h e r e Τ i s transmittance. The u s e o f i n t e g r a t e d a b s o r b a n c e o f t h e 1620-cm" band ( B ^ O ^ P absorbance ( A ) to d e t e r m i n e t h e membrane w a t e r c o n t e n t r e p r e s e n t s a c o n s i d e r a b l e improvement o v e r w e i g h i n g t e c h n i q u e s , b e i n g d i r e c t and r e f l e c t i n g 1 Q

1

o

r

i

t

s

e a l c

1 6 2 Q


PERFLUORINATED IONOMER MEMBRANES

142

t h e a c t u a l w a t e r c o n t e n t o f t h e p a r t o f t h e membrane w i t h i n t h e i n f r a r e d beam. The p r e f e r e n c e f o r t h e use o f t h e H 0 bending f u n d a m e n t a l a t 1620 cm" i s b a s e d on i t s i n s e n s i t i v i t y t o c h a n g e s i n molecular environment. T h i s i s d e m o n s t r a t e d by t h e n e a r l y c o i n c i d e n t v a l u e s measured i n t h r e e d i f f e r e n t l a b o r a t o r i e s f o r the i n t e g r a t e d a b s o r p t i v i t y of t h i s fundamental i n l i q u i d water, w a t e r v a p o r , and s o l u t i o n o f w a t e r i n a c e t o n e : 5.9 χ 1 0 cm/mol ( 6 ) , 6.4 χ 1 0 cm/mol Ç7), and 5.8 χ 1 0 cm/mol ( 8 ) , r e s p e c t i v e l y . The s t a n d a r d d e v i a t i o n o f t h e s e v a l u e s i s 5%, about t h e e x p e c t e d e x p e r i m e n t a l e r r o r . U s i n g t h e mean v a l u e , 6.1 χ 1 0 cm/mol, and p h y s i c a l c o n s t a n t s a p p r o p r i a t e t o N a f i o n 142 membrane* ( t h i c k n e s s 0.0051 cm, e q u i v a l e n t w e i g h t 1400 g/mol S 0 " , d r y d e n s i t y 1.98 g/cm ) and a l l o w i n g f o r t h e d i f f e r e n c e between n a t u r a l l o g a r i t h m s ( u s e d i n r e f s . ^ ~ 8 ) and d e c a d i c l o g a r i t h m s , we o b t a i n t h e f o l l o w i n g r e l a t i o n f o r t h e r a t i o R o f H 0 molecules to s u l f o n a t e groups: 2

1

6

6

6


6

3

3

2

B

1

1 6 2 Q

[ c m " ] χ 1 4 0 0 [ g / m o l - S 0 ~ ] χ 2.303 3

R

— — = 0.052 χ χ 1.98[g/cm ]

6

B

1 6 2 0

3

0.0051[cm] χ 6 . 1 x 1 0 [cm/mol H 0 ] 2

1

where B i s t h e measured band a r e a ( a b s c i s s a : cm" ; o r d i n a t e : d e c a d i c absorbance) of the bending fundamental o f H 0 i n N a f i o n . Or, s i n c e t h e h a l f w i d t h ( i . e . t h e f u l l w i d t h a t h a l f h e i g h t ) o f t h i s band i s a b o u t 50 cm" , and a s s u m i n g t h a t t h e band a r e a e q u a l s t h e p r o d u c t o f peak a b s o r b a n c e by t h e h a l f w i d t h , we o b t a i n the a l t e r n a t e e x p r e s s i o n : 1 6 2 Q

2

1

R = 2.6

χ

A

1 6 2 0

where A i s t h e peak a b s o r b a n c e o f t h e H 0 bending fundamental. F o r N a f i o n 125 a p a r a l l e l c a l c u l a t i o n y i e l d s R « 0.017 χ B o r R = 0.83 χ A . Whenever t h e a b s o r b a n c e o f t h e w a t e r band a t 1620 cm" was t o o h i g h t o be measured a c c u r a t e l y , t h e a b s o r b a n c e a t 1700 cm" , A , away from t h e band c e n t e r c o u l d be measured i n s t e a d , a s i t has b e e n f o u n d t h a t , conveniently, A = 0.10 χ A . These r e l a t i o n s have been u s e d t o c a l c u l a t e H 0 c o n t e n t s i n our r e c e n t measurements on N a f i o n ; t h e y a r e e s t i m a t e d t o be a c c u r a t e t o 15%. I t may be n o t e d t h a t t h e a b s o r b a n c e o f t h e OH s t r e t c h i n g f u n d a m e n t a l has a l s o been used t o e s t i m a t e t h e w a t e r c o n t e n t o f N a f i o n ( 1 ) , b u t i t i s much l e s s a p p r o p r i a t e f o r t h i s p u r p o s e b e c a u s e o f t h e v e r y l a r g e c h a n g e s a c c o m p a n y i n g any changes i n hydrogen bonding. For example, the i n t e g r a t e d a b s o r p t i v i t y o f 1 6 2 Q

2

1 6 2 0

1 6 2 Q

1

1

1 7 Q Q

1 7 0 Q

1 6 2 Q

2

*The e x p l a n a t i o n o f t h e n u m e r i c a l codes i s i n f o o t n o t e t o T a b l e D e n s i t y and o t h e r p h y s i c a l c o n s t a n t s o f N a f i o n membranes a r e t a b u l a t e d i n Dupont P r o d u c t I n f o r m a t i o n B u l l e t i n s .


I.

8.

IR Spectra

FALK

of Water in

143

Polymer

t h e two s t r e t c h i n g f u n d a m e n t a l s o f H 0 i n c r e a s e s by a f a c t o r o f 18 g o i n g f r o m v a p o r t o l i q u i d , and by a f a c t o r o f 2 g o i n g f r o m l i q u i d t o i c e (6_). 2


P r i n c i p a l Features

of the

Spectra

Sodium s a l t o f N a f i o n . F i g u r e 1 shows t h e i n f r a r e d t r a n s m i s s i o n spectrum of the t h i n n e s t commercially a v a i l a b l e N a f i o n membrane, Dupont's N a f i o n 142, whose t h i c k n e s s i s a b o u t 51 ym. F i l m s of t h i s t h i c k n e s s are s u i t a b l e f o r s t u d i e s of the l e s s i n t e n s e i n f r a r e d a b s o r p t i o n bands by o r d i n a r y t r a n s m i s s i o n techniques. However, the more i n t e n s e bands o f N a f i o n , s u c h a s t h o s e i n t h e r e g i o n o f 1340-1100 cm" and t h e w a t e r bands i n t h e r e g i o n o f 3650-3150 cm" f o r samples o f h i g h w a t e r c o n t e n t , absorb completely or almost completely a t such t h i c k n e s s e s . P r e p a r a t i o n o f t h i n n e r f i l m s i s d i f f i c u l t but f i l m s o f any t h i c k n e s s may be s t u d i e d by t h e t e c h n i q u e o f A t t e n u a t e d T o t a l R e f l e c t a n c e (ATR) (2^,_3). F i g u r e 2 shows the ATR s p e c t r u m o f t h e same N a f i o n f i l m as i n F i g u r e 1. One m i n o r d i s a d v a n t a g e o f t h e ATR t e c h n i q u e i s t h e common o c c u r r e n c e o f s p u r i o u s peaks due t o uncompensated a b s o r p t i o n by a t m o s p h e r i c H 0 and C 0 and v a r i o u s s u r f a c e c o n t a m i n a n t s . A more s e r i o u s d i s a d v a n t a g e o f ATR i s t h a t i t i s a s u r f a c e technique w i t h a depth of p e n e t r a t i o n of the o r d e r o f w a v e l e n g t h , i . e . 3-10 ym (5) and the p o s s i b i l i t y i s always p r e s e n t t h a t the s p e c t r a observed are not r e p r e s e n t a t i v e o f t h e b u l k sample ( 3 ) . I t i s t h e r e f o r e a d v i s a b l e t o v e r i f y f i n d i n g s f r o m ATR e x p e r i m e n t s by t r a n s m i s s i o n s p e c t r o s c o p y as f a r as p o s s i b l e . We have o b s e r v e d t h a t bands due t o N a f i o n i n ATR s p e c t r a correspond c l o s e l y t o the c o r r e s p o n d i n g transmittance s p e c t r a , but t h a t t h i s i s not g e n e r a l l y t r u e o f bands due t o water i n Nafion. 1

1

2

2

T a b l e I I l i s t s t h e p o s i t i o n s and r e l a t i v e i n t e n s i t i e s o f t h e main a b s o r p t i o n bands o f t h e sodium s a l t o f N a f i o n , t o g e t h e r w i t h t h e b e s t a v a i l a b l e a s s i g n m e n t s t o v i b r a t i o n a l modes o f t h e s t r u c t u r a l components of N a f i o n : the f l u o r i n a t e d h y d r o c a r b o n main c h a i n , the e t h e r - l i n k e d f l u o r i n a t e d s i d e - c h a i n s , the i o n i c end g r o u p s - S 0 ~ N a and w a t e r o f h y d r a t i o n . The most i n t e n s e a b s o r p t i o n s i n the s p e c t r u m a r e t h o s e due t o the f l u o r o c a r b o n main c h a i n . The s p e c t r u m o f N a f i o n t h e r e f o r e s t r o n g l y r e s e m b l e s t h a t o f p o l y t e t r a f l u o r o e t h y l e n e (PTFE; T e f l o n ) , a l l o f t h e m a j o r bands o f PTFE b e i n g a l s o o b s e r v e d i n N a f i o n a t v e r y n e a r l y t h e same wavenumbers. E s p e c i a l l y i n t h e r e g i o n o f s y m m e t r i c and a n t i s y m m e t r i c C F s t r e t c h i n g (1350 t o 1100 cm"" ) t h e s e i n t e n s e P T F E - l i k e bands o b s c u r e a l l o t h e r a b s o r p t i o n s of N a f i o n . The C F and CF u n i t s i n t h e s i d e - c h a i n s have no d i s t i n c t i v e a b s o r p t i o n s but t h e e t h e r l i n k a g e s g i v e r i s e t o the w e l l - s e p a r a t e d band a t 980 cm" , w h i c h has been s a i d t o o r i g i n a t e i n t h e C-0-C s y m m e t r i c s t r e t c h (2^). The - S 0 ~ N a end g r o u p s g i v e r i s e t o o n l y one d i s t i n c t i v e a b s o r p t i o n band +

3

2

1

2

1

+

3


PERFLUORINATED IONOMER

MEMBRANES


144



Figure 2. ATR spectra of the same Nafion moved up the transmittance scale by 40%.

samples as in Figure 1. The upper spectrum has been Asterisks indicate spurious bands, artifacts of the ATR technique.


4^


0\\

^

σ\ I ο I

co co o>


ο

C/N

r**

Ο

Eh

ν

eu «

»-. U. »-. Ο CM U 4J 4J M CO CO CO vO

oo co

ι Φ (U eu eu

4-1 4J 4-1 4-1

S S S S 5 3 55

eu eu eu

i,Ο -H rXH) .rO H co CO CO •H kl •H U COi CO U CO( > > >

s vO in Ν CO CO CO CO

ι

ι

ι

I

I

r-»

I

Eh

Β c

ω

Χ*

-ι m

rH

Ο 33

ο

CM

eu

>

CO ce eu eu

*

CO >s ^eu u U C? eu eu eu

>

.O C CO Ο Ο

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ce TJ

VVY Y Y CM CM CM

> >

oomooooo COvOrHCOsOCTNCMO ONPnHONOOfsr^N CMCMCMi-IrHrHiHrH

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COΟ >»• rH

4e-u» eu rH 4J

CM

C M C M C M C M C M C M C M C M

«β eu

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Eh

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1-4 Ό 92

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YYYYYYYY

CO eu

CM

I «H co Eh «a HH · Ο ΗΗ• 4-1 fh 00 CO C CU

m

C M C M C M C M C M C M C M C M

CO eu

rH T3 C CO

73 U-) • iH iu C O Ο CD ·« < 4-t ^3 a) CU 4U >% -1 M Eh Eh u. eu 4-> 4J CO 4U_» CO OrΟH c c EhCM C υ 1 a

w

ChEhChEhEhEhEhEh

6 a

Ο ο

rH C M

CM CM • 4H + + 3· rH

ε ε ε c c c

XL £. £. co m ο υ υ u υ u I CM NO

c

CM CM co CM

W Ed E x3 Eh Eh Eh

υ οοο ^ CM

^-S /-Ν m so

in —t

m W Ed Eh Eh

ο>|σ\|

ON I

CM

oo co

CM33 33 Eh I I

c

•H

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CO

>

M

CO CO «0 eu eu eu

eu

rrO H CO •H

CO eu

>> >% u Hu eu eu eu > > >

ο mο ο ο CM >ί Ν ΙΛ Ν ΐΛ«ί·ί-Î rH .HfHfHr-l

NO


co

Ο

C/3


l

l

4

C

— i. — i |L &

1

-CF -CF -CF -CF -CF -CF -CF -CF -CF

very strong weak weak weak weak weak weak weak medium

530^ 465 380 345 320 295 270 235 205

2

2

2

2

2

2

2

2

3

c f . 203 in PTFE (14^)

c f . 277 in PTFE (14)

c f . 321 in PTFE OU)

c f . 384 in PTFE (14)

c f . 516 in PTFE(9)&

2

9

c f . 778 i n PTFE(9) cf. 738 i n PTFE(9) c f . 718 in PTFE(9) Probably CF s c i s s o r ( 1 3 ) f j c f . doublet at 638 and 625 i n PTFE(9); 637 i n

c f . 850 i n PTFE(9)

COC sym. stretch(2)(?)

2

sym. CF, stretch; c f . 1152 i n PTFE(9) _ -SO," sym. stretch; c f . 1058 in NH^ S0 N a ( l l ) ; 1034 i n sodium polystyrene sulfonate(lO); 1048 i n sodium polyethylene sulfonate(12)

Units * cm" · Band position depends on the counter ion and on the degree of hydration. From ATR spectra of ref. 2 (verified i n the present study), From ATR spectra of ref. _3 (verified i n the present study). Not always resolved. This intense absorption may also contain contributions from O S stretch (2). This intense absorption may also contain contributions from S-0 deformation modes (2).

2

2

2

2

2

2

2

2

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u n o b s c u r e d by t h e f l u o r o c a r b o n bands: The S 0 ~ s y m m e t r i c s t r e t c h a t about 1060 cm" . I n f r a r e d s p e c t r a of N a f i o n , except those of very thoroughly d r i e d s p e c i m e n s , c o n t a i n p r o m i n e n t bands due t o t h e s t r e t c h i n g and b e n d i n g f u n d a m e n t a l s o f w a t e r o f h y d r a t i o n , t h e r e l a t i v e i n t e n s i t y o f t h e s e bands i n c r e a s i n g w i t h the w a t e r c o n t e n t o f t h e s p e c i m e n . The OH s t r e t c h i n g f u n d a m e n t a l o c c u r s i n t h e r e g i o n o f 3750-3200 c m , d e v o i d o f a b s o r p t i o n s by o t h e r g r o u p s o f N a f i o n , so t h a t t h e s p e c t r u m h e r e i s due o n l y t o w a t e r m o l e c u l e s . This r e g i o n c o n t a i n s i n f o r m a t i o n c o n c e r n i n g h y d r o g e n b o n d i n g of w a t e r . The Η0Η b e n d i n g f u n d a m e n t a l a t a b o u t 1620 cm" a l s o l i e s c l e a r of m a j o r N a f i o n a b s o r p t i o n s and c o n t a i n s r e l a t i v e l y l i t t l e s t r u c t u r a l i n f o r m a t i o n a b o u t w a t e r i n N a f i o n but p r o v i d e s a c o n v e n i e n t measurement o f the w a t e r c o n t e n t . The H 0 librational f u n d a m e n t a l s a b s o r b i n t h e 800-500 cm" r e g i o n too s t r o n g l y o b s c u r e d by N a f i o n a b s o r p t i o n s t o be s t r u c t u r a l l y u s e f u l . The OD s t r e t c h i n g band o f D 0 o c c u r s i n t h e r e g i o n o f 2750-2350 cm" , and c o n t a i n s s i m i l a r i n f o r m a t i o n t o the OH s t r e t c h i n g f u n d m e n t a l , t h o u g h i t s u f f e r s f r o m b e i n g s u p e r p o s e d on an o v e r t o n e o f CF s t r e t c h i n g v i b r a t i o n s c e n t e r e d a t 2360 cm" . On t h e r e a s o n a b l e a s s u m p t i o n t h a t t h e band shape o f t h i s o v e r t o n e i s i n d e p e n d e n t o f the s t a t e o f h y d r a t i o n o r d e u t e r a t i o n o f N a f i o n , t h e o v e r t o n e a b s o r p t i o n may be compensated by a m a t c h i n g f i l m o f d r y , u n d e u t e r a t e d N a f i o n i n the r e f e r e n c e beam o f the s p e c t r o m e t e r ( 4 ) . 3

1

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1

2

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Other S a l t s of N a f i o n . c o u n t e r i o n s d i f f e r somewhat most i n t e r e s t i n g among s u c h w a t e r . These a r e p r e s e n t l y laboratory (15). There are N a f i o n bands a t 1060 and 980 later sections.

Spectra of N a f i o n w i t h o t h e r f r o m t h o s e o f t h e sodium s a l t . The d i f f e r e n c e s i n v o l v e bands due t o under study i n the author's a l s o observable d i f f e r e n c e s i n the cm" . These w i l l be d i s c u s s e d i n 1

A c i d Form o f N a f i o n . The i n f r a r e d s p e c t r u m o f t h e a c i d f o r m of N a f i o n i s d i s t i n c t from the s p e c t r a of i t s s a l t s . Figure 3 shows t h e t r a n s m i s s i o n s p e c t r u m o f N a f i o n 142 i n t h e a c i d f o r m a t three water contents. I t has been n o t e d by L o p e z e t a l . ( 1 ) and by H e i t n e r - W i r g u i n (2) t h a t N a f i o n membranes i n t h e a c i d f o r m a b s o r b a l m o s t c o m p l e t e l y b e l o w 3700 cm" . As F i g u r e 3 shows, t h i s i s true only at high water contents. Dry o r n e a r l y d r y membranes y i e l d s p e c t r a c h a r a c t e r i s t i c o f t h e a c i d g r o u p -SO3H. A s m a l l band a t a b o u t 930 cm" corresponds to the s t r e t c h i n g v i b r a t i o n o f t h e S-0 bond w i t h t h e s i n g l e - b o n d c h a r a c t e r i n t h e 1

1

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g r o u p , i n a n a l o g y t o t h e band o b s e r v e d by Z u n d e l a t 0-H 907 cm" for polystyrenesulfonic acid (10). T h i s band i s a good measure o f t h e u n d i s s o c i a t e d s u l f o n i c a c i d g r o u p s i n t h e s y s t e m . The e x t r e m e l y i n t e n s e and b r o a d band c e n t e r e d a t 2750 cm" i s due t o the a c i d p r o t o n s i n v o l v e d i n v e r y s t r o n g h y d r o g e n b o n d s . Such 1

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