Microstructure studies of perfluorocarboxylated ionomer membranes

Journal of Membrane Science, 30 (1987) 171-189 Elsevier Science Publishers B-V., Amsterdam - Printed in The Netherlands

MICROSTRUCTURE STUDIES PERFLUOROCARBOXYLATED

171

OF IONOMER MEMBRANES

S.K. LIM, D. GALLAND, M. PINERI Centre d%tudes Nucldaires de Grenoble, Dkpartement de Recherche Fondamentale, Service de Physique, Groupe Physico-Chimie Moltkxlaire, 85 X-38041 Grenoble CBdex (France)

and J.M.D. COEY Department of Pure and Applied Physics, Trinity College, Dublin 2 (Ireland) (Received April 17,1986; accepted in revised form August l&1986)

Summary The ionic microstructure of perfluorocarboxylated ionomer membranes has been studied with Cu, Fe and Eu as a function of ion concentration and water content. Techniques used were EPR, Mbssbauer spectroscopy and magnetization measurements. In the copper membranes we cl&inguish isolated Cu *+ ions, acetate dimers and copper ions in small particles of a carboxylate compound. The dimers and this compound are destroyed when sufficient water is present in the membrane. Ferric ions cannot be introduced directly into the membrane from solution but ferrous membranes can be aged or oxidized to give small groups of hydrated Fe3+ coexisting with isolated hydrated Fe’+. The mean square displacement of the Mijssbauer probe ions increases less rapidly with temperature than in Nafion@ due to the lower water content and stronger cation-anion association in the carboxylated membrane. Precipitation of the exchanged ions in the membrane was achieved by soaking in KOH solutions.

1. Introduction Ionic microstructure of perfluorosulfonated membranes ( e.g. Nafion @ ) has been studied in some detail by exchanging the membranes with transition metal cations and then using techniques such as electron paramagnetic resonance (EPR) [ l-31, NMR [ 4-61, Miissbauer spectroscopy [ 7-101 and magnetic susceptibility [ 111 which are sensitive to the state of aggregation of the cations via magnetic interactions. Small angle X-ray or neutron scattering experiments have shown that the extent of phase separation between the ions and the rest of the polymeric matrix does not depend, at least in first approximation on the exchanged metal cation [ 121. In certain cases the ionic aggregation has been confirmed by EXAFS [ 13,141. The microstructure is a function of water content or swelling [ 15-171 and thermal history of the membranes. Re0376-7388/87/$03.50

0 1987 Elsevier

SciencePublishers B.V.

172

exchange with a base or an alkali metal salt can lead to precipitation of metal oxide in a zone close to the membrane surface [ 18,191. The above techniques may also provide information about the dynamic properties of the cation probe and have helped to establish the idea that there may be a separate glass transition for the ionic phase [ 61. Perfluorocarboxylated membranes possess properties which are quantitatively quite different to those of perfluorosulfonated membranes. They show much better permselectivity but lower conductivity in connection with a lower swelling in water or other solvents. In order to exploit these differences in properties, perfluorinated multilayer membranes with a thin carboxylic layer on a sulfonic substrate have been developed for chlor-alkali electrolysis [ 201. However, little work of the sort outlined in the first paragraph has been done on perfluorocarboxylated membranes. Dynamic mechanical properties have been reported [ 211 and also NMR and Mijssbauer spectra of a carboxylic membrane containing a large amount of sulfur and showing a close resemblance to perfluorosulfonated materials [ 221. The aim of the present work was to study the ionic microstructure of perfluorocarboxylated membranes exchanged with copper, iron or europium using EPR, magnetic susceptibility and Miissbauer measurements. Data are obtained as a function of degree of exchange and water content. Results are compared with those already published on comparable sulfonated membranes with a view to improving our understanding of the structural basis for the differences in properties of the two types of material. 2. Sample preparation and characterization

Membranes samples of, respectively, 786 and 599 equivalent weights (EW) were supplied by Asahi Glass Co. in the ester form. The general formula for these materials is ( -CF2 -CF,-)

,-CF,-C-0-_(CF2),--COOCH,

where n - 3 for the 599 EW and- 5 for the 786 EW. The membranes were first cleaned and converted to the acid form by successively soaking in 1 M KOH at 80-100°C for 3 hr and then in 1 M HCl also at 80-100°C for at least 9 hr. The theoretical exchange capacities are 1.27 ( 786 EW) and 1.63 (599 EW) meq/g. The water absorption isotherm for the 786 EW (1.25 meq/g) acid membrane shows about 4 times less water than for a 1200 EW (0.8 meq/g) sulfonated membrane, as shown in Fig. 1. Acid Flemions@ were exchanged with Cu, Fe or Eu by immersion in an appropriate salt solution. Variables which determine the metal ion concentration and distribution across the thickness are the soaking time, concentration and temperature of solution and membrane E.W.

173

12 10 8 6

0

20

40

60

80

100

RH % Fig. 1. Absorption isotherms of the acid forms of carboxylated (EW 786) ( x ) and sulfonated (EW 1200 ) ( l ) perfluorinated membranes. Am = ( m - m,Jm) x 100 where m, is the mass of the membrane vacuum dried at room temperature and m is the mass oof the membrane for different relative humidities (RH) . Fig. 2. Exchange expressed in percent of the stoichiometry for acid membranes soaked in 0.05 M Cu (NO,) 2aqueous solution. 786 EW Flemion@ ( X ) ; 599 EW Flemion” ( n ) ; 1200 EW NafionB (a).

For the copper membranes an aqueous solution of Cu (NO,)2was used and membranes with three different copper concentrations were prepared. The preparation procedure and ion concentration profiles determined by electron microprobe scans together with the average distance between atoms, assuming a uniform profile, are summarized in Table 1. Soaking times of the order of 1 day were needed for the Cu ions to diffuse homogeneously throughout the 786 EW membrane in contrast with the 599 EW membrane where diffusion occurs in a matter of hours. In perfluorosulfonated membranes, a few minutes exchange produces a uniform profile. Some data are summarized in Fig. 2. Direct exchange of Flemion@ with ferric ion was practically impossible; soaking for several days at room temperature or at 100’ C in ferric nitrate led to a maximum uptake of only 2 Fe/100 COO-. However it was possible to introduce ferrous ions from Mohr’s salt solution and oxidise them in situ. Membranes with the highest possible ferric content were prepared for Mossbauer studies as described in Table 2. Fe1 sample is largely ferrous while Fe2 is largely ferric. Copper and iron membranes were also re-exchanged with solutions of KOH

174 TABLE 1 Preparation procedures and resulting characteristics for the different copper samples. In Cul and Cu2 the [K ] profile corresponds to unexchanged remaining K+ ions. The material used is 599 EW Flemion@ Sample reference

Preparation*

Average % of neutrahsation

Average C$+_Cu*+

Profiles

separation

[Cul

Cul

0.05 M Cu (NO,) 2 RT, 10 min then into distilled water, 2 days

0.3 (1)

508 A

cu2

0.018 M Cu(NO,), 2f days, RT

-average over memb.: 1.9(l) -average over surface region: 14(l)

41A [21A]

770)

11.8A

cu3

cu3p

0.05 MCU(NO~)~ sol. at RT for 4f days 0.05 MCu(N03) 1 week, RT 1 1 M KOH, RT, 20 hr

lK1

not observable

L!!l -e.+

lx

I negligible

de-,

colour bluejgreen [ 2.4 wt% 31 Cu~+/100 coo-

]

blade--f transparent brown

be-,

+-e-W

“RT: room temperature.

or NaOH. Details of these precipitated membrane Cu3p and Fe3p are also listed in Tables 1 and 2. Europium membranes were prepared by soaking in 0.1 A4 Eu (NO,) 3aqueous solution for 5 days giving rise to 30% exchange with a uniform profile. 3. Results 3.1 Cu membranes

The three copper membranes have been studied as a function of water content. Figure 3 shows the variation of water content with relative humidity or thermal treatment for Cu2. The reference state of zero water is taken to correspond to heating at 150°C in vacuum. It was noted that the really dry membrane was green in colour, whereas at ambient humidity or when soaked in water it was blue. The lightly doped copper membranes, Cul and Cu2, show an anisotropic 4line hyperfine structure in the EPR spectrum characteristic of isolated Cu2+

175 TABLE 2 Preparation procedures and resulting characteristics for different iron-exchanged samples. Samples Fe1 and Fe2 are 599 EW Flemion@ and sample Fe3p is 786 EW Flemion@ Sample reference

Preparation

Fe1

HCl, 1 M, 80°C 16 h soaking 1 into Mohr’s solution, 0.05 M, RT, 4 days (transparent, dark brown)

Fe content

Fe profile

1.98 wt% (21 Fe/100 COO-

Fe2

0.05 M, Mohr’s solution at 90°C 25 hr (solution precipitates) (deep red) becomes dark red when dry

2.91 wt% (32 Fe/ 100 COO-)

Fe3p

Soaked in Mohr’s solution heated at9O”C for 5 hr 1 1 M NaOH, 80-lOO”C!, 4-5 hr

1.36 wt% (20 Fe/ 100 coobefore precipitation)

r--t

.)

u-

;’ -e---9

I!4 t-e-

ions (Fig. 4). This persists up to 95% relative humidity ( - 4 H,O/COO- ) but in the wet membrane ( - 7 H,O/COO- ) the hyperfine structure is no longer visible at any copper content. The wet membranes show a single symmetrical line at the average g= 2.1, due to averaging of the hyperfine features. Unlike in Nafion@ [ 11, the width of the single symmetric line is independent of temperature down to 100 K. On account of the sharp peaked profile the average copper concentration near the surface in Cu2 is 7 times greater than the average. Another feature appears at high field in the EPR spectra. This feature decreases sharply as the temperature is lowered to 100 K, as shown for Cu2 in the insert in Fig. 4 which suggests that the complex responsible has an S = 0 ground state. Crystallites of copper acetate monohydrate give a somewhat broader feature in the same position and a dimer copper acetate complex with a similar EPR spectra has -dicarboxylate polybutadiene polymers recently been identified in Cu2+ CY,W [ 231. The intensity of this acetate dimer feature decreases as the water content increases, showing its progressive solvation and destruction with hydration. Another, unattributed low field feature exists in the spectra of the concentrated copper membranes, which also changes progressively with water content. Figure 4 shows that the most highly doped Cu-Flemion, Cu3, exhibits a very broad absorption at geff= 2 which is independent of temperature; it may be due to ion clustering. Also there remains some trace of the narrow line whose hyperfine structure is attributed to isolated copper ions. Measurements of the magnetic susceptibility of Cu3 confirm the ideas of

176

T (“‘2 ) Am “I. 5

2 RTdned-

?en .

Blue-

20

40

60

80 100 RH %

Fig. 3. Water content in Cu2 sample versus relative humidity and temperature of drying. The percent water content Am is expressed relative to the weight of the membrane dried at 150°C.

local structure which can be inferred from the EPR spectra. Figure 5 shows the temperature dependence of the susceptibility of the Cu3 membrane at 95% relative humidity, and dried at 150°C. In the first case ( - 3 I-&O/COO- ) the susceptibility follows a single Curie-Weiss law (x = c/T- 0). The Curie constant, 196 x lop6 K per gram of membrane, corresponds to a concentration of isolated Cu2+ ions of 0.37 Cu2+/COO-, in excellent agreement with the copper content obtained by chemical analysis (0.38 Cu2+/COO-- ) given in Table 1. The paramagnetic Curie temperature, 8 = - 1.5 (1) K, shows that there are other weak antiferromagnetic exchange interactions between Cu centres, which are responsible for eliminating the hyperfine structure in the EPR spectrum (Fig. 4~). Figure 5 (b) is a plot of the susceptibility for the Cu3 membrane vacuum dried at 150°C which corresponds to the nominal zero of water content. The Curie-Weiss term dominates at low temperatures ( C = 33 x 10 -‘K ) , from which we deduced that isolated Cu2+~now accounts for only 16% of the total Cu. A reasonable fit of the slow decrease in the susceptibility at higher temperatures could be obtained by assuming that 40% of the copper is present as dimers with an exchange interaction of J= -450 K, corresponding to the

177

30% RH

95%RH

Wet

Fig. 4. Room temperature EPR spectra of samples Cul, Cu2 and Cu3 as a function of water content. The dry state obtained by heating under vacuum at 150°C is taken as the reference. The figures to the right of each spectrum give the approximate number of water molecules per COO-. The temperature dependence of the dimer feature is shown in the insert.

30

X (emu/g 1

x 10 -6

20

r

10

5 0

a) .

I

1

5

b) I

200

.*_

_

T (K)

_

30(

Fig. 5. Magnetic susceptibility curves of sample Cu3 in the hydrated (a) and dry (b) forms. Note that the same vertical scale is used for both curves.

178

copper acetate binuclear complex, while the remainder having average exchange energy of about 200 K, are attributed to clusters and the unidentified species. 3.1.1 Precipitated copper membranes A membrane similar to Cu3 was treated in 1 M KOH at room temperature for 20 hr. After soaking, the membrane changes colour from blue to black with little loss of copper. However, the concentration profile of Cu and K are quite inhomogeneous; a sharp dip in the Cu corresponds to a front in the K profile (Table 1) . The magnetic susceptibility was substantially reduced by comparisons with that shown in Fig. 5 (a), indicating that most of the copper now exists as complexes with strong antiferromagnetic interactions. Only about 40% remains isolated, giving a Curie-Weiss susceptibility with 8 = - 3 K and the characteristic EPR spectrum. The isolated paramagnetic copper may be located at the centre of the profile and the copper precipitate in the outer regions. The observed profile suggests that the re-exchange process was arrested before precipitation could be completed through the membrane thickness. 3.2 Fe membranes Iron Flemions were prepared following different procedures and a variety of different Mijssbauer spectra were obtained. 599 EW Flemion was mainly used because it was easier to load with iron. Two typical iron membranes will be presented (Table 2)) Fe1 which is largely ferrous and Fe2 which is largely ferric. The membranes are compared in the wet and dry states and effects of aging are noted. Figure 6 shows some of the spectra at 100 K and fitted parameters are tabulated in Table 3. The ferrous membrane Fe1 in the wet state ( - 3 HzO/ - COO- ) exhibits at least 2 ferrous quadrupole doublets which account for 68% of the absorption. On vacuum drying at 60 (10) ‘C! ( - 0.7 H,O/COO- > the outer doublet with d-3.35 mm/set disappears and only the broad inner doublet with d N 2.60 mm/set remains. A similar effect was reported in ferrous Nafion [ 241, where the outer doublet was the only one observed when there was more than - 4 H,O/SO,(6 wt% ) . The outer doublet is attributed to the complex, and the inner one to ferrous complexes in various degrees Fe(H,0),2+ of hydration and coordination to the pendant carboxylate groups. The magnetic susceptibility (Fig. 7) of Fe1 follows a Curie-Weiss law below 100 K with C, = 2.2 and 0 = - 5 K. C, for Fe’+ is 3.0 so this result suggests that the ferrous ion is paramagnetic but that the ferric ion giving the quadrupole doublet with d= 1.1 mm/set experiences strong antiferromagnetic coupling. The temperature variation of the Mijssbauer absorption area and its logarithm for wet and dry Fe1 are shown in Fig. 8. On Fig. 8 (a) it can be seen that the spectrum disappears at r,= 270 (10) K for the wet and 320 (10) K for the dry. Comparable values in Nafion @ are - 40 K lower [ 71. The In A vs T curves show the departure from Debye behaviour characteristic of many polymer systems at

-L

-2

0

2

L

-i

-2

0

2 Velocity

L

I mm/s 1

Fig. 6. Mijssbauer spectra for ferrous (Fel) and ferric (Fe2) Flemion@ samples in the wet and dry conditions. The effect of aging Fe2 for six months is shown. These spectra were taken at 80 or 100 K.

180 (10) K and 196 (10) K for the wet and dry membrane, respectively. The departure has been identified with a glass transition for the ionic phase [ 251. Turning now to the ferric membrane Fe2, it can be seen in Fig. 6 that there are severe effects of aging on the ionic microstructure. The spectra of the freshly prepared, wet membranes are very similar to those of sulfonated and other carboxylated membranes [ 221, there remains - 20% of unoxidised Fe’+. The ferric spectrum consists of two subspectra with A =0.22 mm/set and A= 1.68 mm/set. However, on aging for six months at room temperature or on drying, both the outer doublet, attributed to Oxo-bridged Fe3+ dimers [ 81, and the inner doublet, attributed to isolated, hydrated Fe3+ ions [ 81 disappear to the profit of species with quadrupole splitting in the range 0.6-1.2 mm/set, attributed to varying degrees of iron clustering. It may be noted that the Fe’+ in the aged membrane behaves similarly on drying to that in Fel.

b) I

0

I 100

1

T 200

I

.

,

300 T (Kl

Fig. 7. Magnetic susceptibility (b: Fe3p).

curves for ferric Flemion @before (a: Fe1 ) and after precipitation

The EPR spectra of the aged Fe1 or fresh Fe2 are quite similar. Some data are shown in Fig. 9. There is a characteristic line near g,,- 4.3 which is present at room temperature. A similar feature was found in Nafion@ at lower temperatures [ 31. This line increases in intensity as the temperature is lowered, following the standard 1/T variation up to 160 (10) K and falling off much more rapidly thereafter. In addition there is a broad line near g,, = 2, associated in Nafions@ with isolated fully hydrated iron ions [ 31. 3.2.2 Precipitated iron membranes Iron precipitation was examined in a 786 EW ferric membrane, Fe3p, which was initially prepared as indicated for Fe2. The precipitation was carried out in hot NaOH, as indicated in Table 2. After precipitation the iron profile became slightly concave, but much less so than in similarly treated Nafion@ [ 181. Xray diffraction of Fe3p revealed the presence of a series of peaks attributable to hematite and goethite, as shown in Fig. 10; an approximate particle size of 330 (100) A for the hematite was obtained from the Scherrer formula. The goethite lines were somewhat broader, suggesting an even smaller particle size. Mossbauer spectra of Fe3p as a function of temperature are shown in Fig. 11 and the fits are given in Table 4. Three separate magnetic components can be identified at 4.2 K with hyperfine fields of 53.1 T, 50.6 T and 47.5 T, with corresponding relative intensities of 36%, 43% and 21%, respectively. They are associated with hematite, goethite and ferrihydrite, respectively. The magnetic spectrum of the latter collapses around 25 K, as shown in the insert. At room temperature some trace of hyperfine splitting persists, but most of the hematite and goethite particles are already at or above their superparamagnetic blocking temperatures.

181

-6.0J 0

100

TEMf’;RATURE

Fig. 8. Temperature dependence of the Miissbauer the wet ( 0 ) and dry ( 0 ) conditions.

absorption

J area A and its logarithm

for Fe1 in

The susceptibility of Fe3p (Fig. 7) varies little as a function of temperature, as expected for antiferromagnetic oxides. The paramagnetic Curie temperature extrapolated for the range 50-300 K is - 250 (50) K. 3.3 Europium membranes Comparison of the emission fluorescence spectra of europium Flemion@ and europium aqueous solution excited at 394.5 nm show that the relative intensity of the 5D1t 7F2and 5D0-+7F, transitions are reversed, the former being signif,-

182 TABLE 3 Mossbauer fitted parameters for spectra in Fig. 6 Sample

&Fe) (mm/set)

r (mm/set)

A (mm/set)

%

Label

Fe1 -wet

1.36 1.36 0.48

0.25 0.18 0.24

2.68 3.35 1.09

25 43 32

Fe’+ Fe2+ (D) Fe3+

Fel-dry

1.29 1.48 1.05 0.47

0.14 0.23 0.16 0.21

2.57 2.63 2.50 1.11

15 37 6 42

Fe’+ Fez+

Fe2 -

fresh and wet

0.48 0.47 1.36 1.36

0.38 0.30 0.36 0.17

0.22 il.68 2.85 3.71

46 31 16 5

Fe3+ (A) Fe”’ (B) Fe*+

Fe2 -

fresh and dry

0.49 1.60

0.49 0.35

0.98 2.47

79 21

Fe3+ Fe2+

Fe2 -

aged and wet

0.51 0.51 1.35 1.65

0.21 0.21 0.17 0.22

1.17 0.68 2.62 2.84

48 40 7 5

Fe”+ (E) Fe3+ (F) Fe’+ (C) Fe’+ (D’)

0.55 0.55 1.39

0.20 0.18 0.18

1.19 0.74 2.62

52 36 12

Fe”+

Fe2-agedanddry

Fig. 9. EPR spectra of sample Fe2 as a function of temperature.

Fe’ +

183

I

40

I

I

I

I

35

30

25

20

I

15

I

10 (Em

1 !

Fig. 10:X-ray diffraction pattern of the precipitated ferric FlemionB Fe3p showing the goethite and hematite forms of iron oxide.

icantly more intense in the membrane and the latter more intense in the solution. This indicates a change in the chemical environment of the Eu3’ ions when they enter the Flemion membranes. In solution, Eu3+ ions are hydrated, whereas in the membrane association with the COO- ions could explain the change in the relative intensities of the transitions. The Mossbauer spectrum of the Eu3+ membrane equilibrated at ambient humidity shows a broad asymmetric line near zero-velocity similar to that seen in Nafion@ . The temperature dependence indicates that T,, N 240 K and deviation from Debye behaviour occurs near 145 (10) K. The Debye temperature is 147 (10) K, larger than that for Nafion@ (107 (10) K), suggesting again that the ion is more strongly coordinated to carboxylate groups than to sulphonate groups. The relatively low values of T, and Tg (the temperature where the deviation from Debye behaviour is observed) suggest that the Eu3+ ions remain partly hydrated in the membrane. 4. Conclusion

Our study reveals many qualitative similarities in the microstructure of the ionic phase of carboxylic and sulfonic perfluorinated membranes exchanged with copper, iron or europium ions. Nevertheless there are differences associated with the different anions which strongly influence the water content of the membrane. At a relative humidity of 50%, for example, the water content of EW 786 acid membrane is 1%) whereas that of Nafion@ is 4%. Taking account of the differences in equivalent weight this means that there are only 0.4 water molecules per COO- in the carboxylated membrane but there are 2.7 water molecules per SO,- in the sulfonated one. Such large differences in water con-

184

99 98 loo999a ;

97-

-

loo-

g

3 it

99-

e

loo-

5

99 -

loo-

98 -

Fig. 11. Mijssbauer spectra obtained with the precipitated iron sample Fe3p as a function of temperature. The insert shows the fraction of the absorption in the paramagnetic doublet as a function of temperature.

tent also persist in the water-soaked membranes and consequently the cation exchange rate for our perfluorocarboxylated membrane is extremely slow at room temperature. This point is illustrated by the copper profile for sample Cu2 shown in Table 1. Uniform profiles can only be obtained by prolonged soaking in concentrated or heated solutions. By contrast, for Nafion@ the ions diffuse uniformly throughout the membrane thickness even at ambient humidity. It is possible to exchange Nafion@ with Fe3+ from solution directly but the trivalent ion does not penetrate into our carboxylated membrane in any significant quantity; exchange with Fe2+ is possible however. A previous report of ferric iron uptake by a perfluorocarboxylated membrane might be explained by the presence of some sulfonate groups in that material [ 221, or a greater water content as in the carboxylated material studied by Yeager et al. [ 261. As a result of the weak-acid COOH present, the ion exchange never reaches stoi-

185 TABLE 4 Fitted parameters of Mijssbauer spectra of Fig. 11 Temperature (K)

We) (mm/set )

r (mm/set )

A (mm/set )

Btlf (T)

%

Comment’

0.44 0.43 0.39

0.28(l) 0.22 (1) 0.30(l)

+0.00(l) -0.18(l) -0.07(l)

53.3 50.6 47.5

36 43 21

Hm Gt Fh

50

0.48 0.43 0.44

0.24(l) 0.26 0.34

0.16 -20.21 0.65

53.5 50.5 0

26 34 40

Hm Gt Fh

90

0.48 0.48 0.44

0.27 0.26 0.32

0.16 -0.21 0.66

53.6 50.4 0

28 26 46

Hm Gt Fh

150

0.46 0.44 0.41

0.29 0.23 0.27

-0.55 -0.25 0.65

52.4 48.5 0

31 20 49

Hm Gt Fh

300 (RT)

0.58 0.35

1.21 0.36

- 0.064 0.65

44.8 0

50 50

4.2

“Hm = hematite; Gt = goethite; Fh = ferrihydrite (amorphous ferric

hydroxide).

chiometric proportions, again in contrast with the Nafion@ where the strong acid S03H is fully dissociated. A notable difference in the copper EPR of the present membrane and Nafion@ in the wet state ( - 8 H,O/acid group) concerns the line width and its temperature dependence. At low temperature both membranes have a line width of 250-300 gauss but that of the carboxylated material is independent of temperature up to 300 K while that of the sulfonated material drops to about 100 gauss around- 250 K (1) . The effect in Nafion* may be explained by a motional narrowing above the glass transition of the ionic phase. The absence of a similar effect here may be due to a stronger association of the cation with the acid groups. Turning now to the different ionic species present in the membranes, the EPR and susceptibility results demonstrate the importance of both water and copper concentrations. At low copper concentration or large water content we have essentially isolated copper ions. Data on the highly exchanged Cu3 membrane (Fig. 4) demonstrate that dimers appear when the number of water molecules associated with a Cu2+ ion is less than one. These dimers have a structure similar to that of copper acetate monohydrate. Copper acetate dimers are destroyed when it is dissolved in water. The same dimer is found in the copper salt of cy,w -carboxylated telechelic [ 231. Magnetic susceptibility is particularly useful for determining the amount of any such dimer present in a

186

High [Cul DRY STATE

0

: free water

Fig. 12. Schematic drawing showing the effect of water on the microstructure (left) and heavily doped (right) copper Flemions”.

of the lightly doped

polymer since the antiferromagnetic exchange interaction is two orders of magnitude greater than those experienced by the quasi-isolated Cu”+ monomer. At first sight the influence of water content on the EPR spectra of the three samples shown in Fig. 4 seems paradoxical. In the lightly doped membranes Cul and Cu2 there is a broadening of the main resonance line with increasing water content, whereas in heavily doped membrane Cu3 there is a narrowing. The effect of water on the ionic microstructure on the lightly doped membranes appears to be the following: in the dry membrane the appearance of hyperfine structure shows that the copper ions are isolated from each other; absorbed water acts as a plasticiser permitting reorganization of the polymer in such a way that water, copper and carboxylate ions are associated in an ionic phase (Fig. 12). The effect of water is therefore actually to decrease the average Cu-Cu distances. On the other hand, the copper in the heavily doped membranes are already strongly associated in the dry state as shown by the temperature dependance of the susceptibility (Fig. 5b). Only 16% of the copper ions are isolated; the remaining 84% experience exchange interactions in excess of 100 K, including the copper acetate form dimers (JN - 450 K) and copper ions in small aprticles of a copper carboxylate compound of unknown structure (J1: - 200 K) . Both the copper dimers and the copper compounds are dissolved on hydration of the membrane and when sufficient water is added reorganization of the polymer occurs resulting in the appearance of a microstructure similar to that of the lightly doped membranes (Fig. 12 ) . Concerning the microstructure of the ferrous membrane, we have already

187

seen that the relative intensity of the two ferrous spectra is a function of the water content of the membrane and the variation is similar to that found in ferrous Nafion@ [ 241. The spectrum of freshly prepared ferric membrane is again similar to that of ferric NaGon@; it’ shows the presence of hydrated ferric ions (probably associated with carboxylate groups) and ferric dimers. However this microstructure is not stable in time and on aging the absorption consists of a broad ferric doublet with d = 1.1 mm/set which is also seen in the predominantly ferrous membranes. The susceptibility measurements suggested that this iron species experiences strong antiferromagnetic coupling but a Miissbauer spectrum of the aged Fe2’ membrane at 4.2 K shows little magnetic hyperfine splitting. It seems likely that the ferric species giving rise to the doublet is the nucleus of the ferric hydroxide precipitates observed in Fe3p and in ferric Nafion@ exchanged in KOH. This nucleus may consist of a small number ( - 10) of hydrated ferric cations which share common OH- ligands and represents incipient ferric “polymerisation”. No similar aging effect has been observed in Nafion@ where the internal pH is specially low due to the strong acid nature of the sulfonic end groups. There are several independent indications that the ion-polymer interactions are stronger in present membranes than in Nafion* besides the rate of exchange of cations discussed earlier. (1) The Fe3+ EPR spectra (Fig. 9) show the presence of the geff=4 line at room temperature in addition to the g,, N 2 line whereas in Nafion@ the g,, z 4 line appears only below about 260 K [ 31. This signal is typical of ions in a glass-like solid state environment and the result points to a drier and more rigid environment for Fe3+ here compared with Nafion@. (2) The effective Debye temperature associated with the Miissbauer atoms, whether iron or europium, is about 30% higher than in Nafion@. This indicates stronger chemical bonding between the cation and its matrix. where the Mijssbauer absorption becomes ( 3) Values of TO, the temperature unobservable because of motion of the Mijssbauer ions, are 10 to 15% higher than in Nafion@. Finally we consider the problem of the glass transition in ion exchange membranes and the significance of T,defined by the departure from Debye behaviour of the Mijssbauer absorption area. In Fe CX,W-dicarboxylato polybutadiene telechelic T,was found to coincide with the glass transition measured by the quasi-static techniques (DSC and low frequency dynamic mechanical measurements) [ 271. This polymer contains almost no free water and the sensitivity of the Mijssbauer absorption to the static glass transition was justified in terms of softening of the long wavelength lattice modes. On the other hand in Nafion@ we have interpreted Tgas an indication of the glass transition of a separate phase including free water and exchanged ions. The sensitivity of Tg to the water content in ferrous Nafion@ [ 71 tends to support this interpretation. It is remarkable that the temperature Tgappears in a range 180 to 220 K

188

in such different systems; but the fact that Tg for polybutadiene is at 210 K seems to be a coincidence. The more useful comparison is between carboxylated and sulfonated membranes. Tg is practically the same in the two cases and it can be related to the presence of some water around the Mijssbauer atom. The amount of water in the present membranes is so small that there must be doubt whether the water and ions can be considered to constitute a separate phase. Nevertheless there is a marked change in ion movement on a local scale at Tg T,and 8, seem to be more sensitive to the water content and cation-anion interaction; they are both greater in sulfonated than in carboxylated and in wet than dry membranes. Acknowledgements The award of an European Community Stimulation Programme grant ST1 03 AAC is gratefully acknowledged. The work was completed while one of the authors, J.M.D. Coey, was Collaborateur Temporaire Etranger of the Centre d’Etudes Nucleaires de Grenoble. The authors thank Asahi Glass Company for providing the perfluorocarboxylated membranes, and Dr. J.M. Kelly for many helpful discussions.

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