Low-temperature behaviour of water in nafion membranes

Volume 86, number 1

CHEMICALPHYSICSLETTERS

5 February 1982

LOW-TEMPERATURE BEHAVIOUR OF WATER IN NAFION MEMBRANES N.G. BOYLE, J.M.D. COEY and V.J. McBRIERTY Department of Pure and Applied Physics, Trinity College, University of Dublin, Dublin 2, Ireland Received 23 September 1981; in final form 4 November 1981

Proton NMR measurements of T I , T 1 p and T 2 in "Nation" perfluorosulphonate membranes, together with neutronscattering and dielectric data, show that the aqueous phase in Nation solidifiesat a glass transformation whose temperature Tg is 168 K in water-saturated acid membranes. Tg is higher for the salt lorms. M6ssbauer measurements o n E u 3+ Nation confirm that the cations are present in an aqueous phase with Tg ~- 220 K.

Nation membranes, developed by the DuPont Company, display remarkable cation-exchange properties of commercial importance, particularly when used as separators in electrolytic cells. The polymer is a perfluorosulphonate resin in which hydrophilic side chains terminated with - S O ~ H+ are periodically attached to hydrophobic perfluoroethylene backbone molecules. Nation membranes can absorb substantial amounts of water, and the acid form may be readily neutralised to provide a range of salts. Several recent studies [ 1 - 5 ] have examined the way in which the structure and response of the membrane depends upon water content. Indeed, the behaviour of the water itself within the membrane is complex and poorly understood. Nevertheless, a detailed appreciation of the structure of the aqueous phase in the Nation is essential for an understanding of the cation-exchange properties of the membranes. In this communication we report some observations on water at low temperatures where it exhibits glass-like behaviour. Our conclusion is based upon collated NMR and M6ssbauer data, reported below, and neutron-scatter-' ing results already reported in the literature. The host Nation used for the measurements had an equivalent weight of 1100 per SO3H. Four samples were studied, two in acid form hydrated respectively to 7% and 25% by weight of water, and salts of Na + and Eu 3÷. The salts were prepared by immersion of the Nation-H into 0.2 M NaC1 or 0.1 M EuC13 solutions and stirring for 12 h at ambient temperature. The 16

25% hydrated sample was boiled in water, whereas the 7% sample was simply exposed to ambient humidity. NMR spin-lattice (T 1), rotating frame (T 1p) and spin-spin (T2) relaxation measurements were recorded within the temperature range 150-300 K. The spectrometer and methods for data analysis have been described [6]. The NMR technique affords the possibility of probing separately the motions of the host matrix via 19 F resonance measurements, the water via 1H resonance measurements and selected cations such as 23Na via 23Na resonance [7]. Here we will restrict our attention to proton resonance results which directly reflect the behaviour of the aqueous phase. M6ssbauer data were also recorded on the NationEu 3+ salt over the temperature range 8 0 - 3 0 0 K using the 21.6 keV 151Eu resonance. The source was 100 mCi of 151Sm in SmF 3. It is evident from the proton NMR results for the 25% hydrated sample, furnished in fig. 1, that motions characteristic of a glass transformation for the water molecules set in near 170 K. Similar behaviour has been observed in other water/matrix systems [8]. The data also indicate a broad distribution of correlation times [9]. Non-exponential behaviour in the three decays at temperatures below the transition may be a manifestation of this broad distribution or, as is the more likely, the presence of two distinct proton populations exhibiting appreciably different relaxation times. Infrared studies indicate two different proton environments in the hydrated Nations [5 ]. These char-

0 009-2614/82/0000-0000/$ 02.75 © 1982 North-Holland

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5 February 1982

CHEMICAL PHYSICS LETTERS

Volume 86, number 1 i

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Fig. 1. NMR T1, Tip and 7"2 data for Nation-H, containing 25% by weight of water. acteristics are typical o f the behaviour o f water in charcoal, studied by Resing [8], although additional work is necessary to understand differences in room-temperature behaviour and the unusually short relaxation times observed in the Nations. The short T 2 of ~6/as indicative of normal water behaviour, which is just detectable (~10%) is due to the formation of a small amount of ice in regions isolated from the greater part of the water incorporated in the membrane (perhaps physically separated from the membrane but within the NMR tube). Formation of ice at temperatures below zero in heavily hydrated Nations had already been observed by Pineri and his co-workers [10]. The analysis o f relaxation data is facilitated through the use o f transition maps constructed by plotting log v c versus inverse temperature where v c is a correlation frequency characteristic of the proton motion at a given temperature. A wide range of characteristic frequencies are probed by a variety of techniques which include NMR, dielectric and dynamic mechanical measurements and quasi-elastic neutron scattering. Methods of data analysis are described in the literature [11 ]. Data for the 25% hydrated Nation are presented in fig. 2a [12]. The locus o f experimental data points is typical of that exhibited by many amorphous polymers and glass-forming liquids. It may be described by the following empirical equation derived by Williams, Landel and Ferry (WLF) [13]:

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Fig. 2. (a) Relaxation data for Nafion-H containing 25% by weight of water. The quasi-elastic neutron scattering point (e) and NMR point of 60 MHz (T) are taken from Duplessix et al. [12]. The open circles are NMR points from the present work and the solid line is the theoretical fit to the data derived from eq. (1) with Tg = 168 K. (b) log vc versus (T - Tg)-1 . The solid line is again calculated from eq. (1) with C1 = 11.4 and C2 = 16.5 K. Points are as follows: o, Nation-H (25% water); o, Nation-H (7% water); zx,Nafion_Na+; v, NationEu 3+. The respective Tg values are given in the text.

log vc = C I ( T - TO)/[C2 + ( T - TO)] .

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TO, the reference temperature in the WLF analysis is essentially equivalent to Tg, the glass-transformation temperature, measured under quasi-static conditions. The best tit to the data in fig. 2a is obtained with the constants C 1 = 11.4-+ 1.0, C 2 = 16.5 -+ 3.5, and predicts a Tg of 168 -+ 4 K for the 25% hydrated sample. I n order to ascertain whether the same expression could adequately describe the data for the remaining three samples, (log Vc)-I was plotted against (T - Tg) -1 . With Tg as the only adjustable parameter, it may be seen from fig. 2b that eq. (1) reasonably describes the data for the 7% hydrated sample (Tg = 181 K), the Na + salt (Tg = 200 K) and the Eu 3+ salt (Tg = 220 K). It is noteworthy that the ~ peak in dielectric measurements at 100 Hz on a 7% hydrated Nation of equivalent weight 1365 would fall at 181 K [2], 17

Volume 86, n u m b e r 1

CHEMICAL PHYSICS LETTERS

compared with 184 K predicted from eq. (1) with the values of C 1 and C 2 given above, and the value Tg = 181 K appropriate for our 7% sample. Hence the suggestion of Yeo and Eisenberg [2] that the/3 relaxation may be due to a glass transition in polar regions associated with absorbed water is confirmed. Eu 3+ was chosen as one of the exchanged cations because 151Eu is a good M6ssbauer nucleus, and the comparison of NMR and M6ssbauer data was expected to be instructive. The Eu 3+ ion has J = 0 and therefore provides no additional channel for nuclear relaxation in the proton NMR experiment. The quantity measured in the M6ssbauer experiment is A, the spectral absorption area as a function of temperature. It is proportional to the recoilless fraction f = exp(-k2(x2)),

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where k = 2rr/X is the wavevector of the 21.6 keV 7-ray and (x 2) is the mean-square atomic displacement in the 7-direction averaged over the lifetime of the nuclear excited state, 9 X 10-9 s. When the ion is bound in a solid (x 2) is the mean-square vibrational amplitude, f tends rapidly towards zero as diffusive motion becomes important. The temperature variation of the absorption area

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5 February 1982

for Nafion-Eu 3+ is shown in fig. 3a. Between 80 and 220 K it varies linearly, falling to zero at 220 -+ 10 K. This sort of behaviour is quite uncharacteristic of a normal freezing process where A is discontinuous because of the first-order nature of the transition. However, it is typical of materials which exhibit a glass transformation [14] and is fully consistent with the NMR observations discussed above. In A, which is proportional to (x2), is plotted as a function of temperature in fig. 3b. There is a linear relation up to ~180 K followed by a more rapid fall-off. Similar curves have already been reported for Nation-Fe 2+ and NafionFe 3+ [15,16] and they are found for a variety of other glassy liquids and polymers. The linear segment of the curve corresponds to Debye behaviour of the atomic vibrations. Its slope gives a Debye temperature of 107 -+ 10 K. Significant departures from Debye behaviour begin above 180 K where the mean-square displacement (x 2) increases more rapidly with increasing temperature than expected on the Debye model. This could be due to onset of a diffusive component in the motion of the Eu 3+ ions. Local diffusion velocities of the order of 1 cm s-1 are sufficient to make the resonance unobservably small. The complete disappearance of the resonance at such a low temperature as 220 K cannot be associated with any glass transformation in the Nation matrix because that occurs at around 500 K in the salt forms. It suggests that the europium is present in an aqueous phase which, as postulated above shows a glass transformation rather than normal freezing behaviour. Although the diffusive motion of the Eu 3+ ions is not directly correlated with the motions of the water protons sensed in the NMR experiment [17], the temperature at which the resonance disappears agrees well with the glass-transformation temperature deduced from the WLF analysis for the europium Nation. To summarise, the results of NMR and M6ssbauer spectroscopy show that water in Nation membranes in unable to form the usual ice structure below 273 K and vitrifies rather than freezes. The glass transformation of the water increases (a) when the water content is reduced and (b) when the acid form is neutralised with Na + and Eu 3+ . For the salts, 7". increases in the sequence Na+ < Fe 2+ ~ Eu 3+ < Fe 3~+ [14], which is the sequence of increasing cation charge to radius ratio.

Volume 86, number 1

CHEMICAL PHYSICS LETTERS

It is a pleasure to acknowledge useful discussions with Professor A. Eisenberg, Professor W. MacKnight, Dr. D.C. Douglass and Dr. M. Pined and his research group. Nation samples were kindly provided by C.J. Molnar o f duPont. We are grateful to A. Meagher for obtaining the M6ssbauer data. This w o r k was supported b y research grant URG/17/78 from the National Board for Science and Technology o f Ireland.

References [1] S.P. Rowland, ed., Water in polymers, ACS Symposium Series 127 (Washington, 1980). [2] S.C. Yeo and A. Eisenberg, J. Appl. Polym. Sci. 21 (1977) 875. [3] I.M. Hodge and A. Eisenberg, Macromolecules 11 (1978) 289. [4 ] E.J. Roche, M. Pineri, R. Duplessix and A.M. Levelut, J. Polym. Sei. Polym. Phys. Ed. 19 (1981) 1. [5] M. Falk, Can. J. Chem. 58 (1980) 1495. [6] J. O'Brien, E.M. Cashell, G.E. Watdell and V.J. McBrierty, Macromoleeules 9 (1976) 653.

5 February 1982

[7] S.R. Lowry and K.A. Mauritz, J. Am. Chem. Soc. 102 (1980) 4665. [8] C.A. Hoeve and P.C. Lue, Biopolymers 13 (1974) 1661; H.A. Resing, Advan. Mol. Proc. 1 (1968) 109. [9] V.J. McBrierty and D.C. Douglass, Phys, Rept. 63 (1980) 61. [10] M. Pineri, private communication. [11] D.W. McCall, NBS Special Publication 301 (1969) 475. [12] R. Duplessix, M. Excoubes, B. Rodmacq, F. Volino, E. Roche, A. Eisenberg and M. Pined, in: Water in polymers, ACS Symposium Series 127, ed. S.P. Rowland (Washington, 1980). [13] M.L. Williams, R.F. Landel and J.D. Ferry, J. Am. Chem. Soc. 77 (1955) 3701; J.D. Ferry, Viscoelastic properties of polymers (Wiley, New York, 1961) p. 216. [ 14 ] S.L Ruby, in: Perspectives in Mt~ssbauerspectroscopy, eds. S.G. Cohen and M. Pastemak (Henum Press, New York, 1973) p. 181. [15] B. Rodmacq, M. Pined and J,M.D. Coey, Rev. Phys. Appl. 15 (1980) 1179. [16] B. Rodmacq, M. Pined, J.M.D. Coey and A. Meagher, J. Polym. Sci. Polym. Phys. Ed. 19 (1981), to be published. [17] A. Vasquez and P.A. Flinn, J. Chem. Phys. 72 (1980) 1958.

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