Macromolecules zyxwvut 1984,17, zyxwvu 1331-1334 1331 (22) A. zyxwvuts Nakajima, T. Hayishi, and M. Ohmori, Biopolymers, 6,973 (23) K. Ito, T. Kajiyama, and M. Takayanagi, Polym. J., 12, 305 (18) K. D. Goebel and W. G. Miller, Macromolecules, 3,67 (1970). (19) P. J. Flory, Proc. R. SOC. London, Ser. A, 234, 73 (1956). (20) P. S. Russo and W. G. Miller, Macromolecules, 16,1690 (1983). (21) M. Warner and P. J. Flory, zyxwvuts J. Chem. Phys., 73, 6327 (1980). (1968). (1980). Aqueous Phase in a Perfluorocarboxylate Membrane Noel G. Boyle, J. Michael D. Coey, A. Meagher, and Vincent J. McBrierty Department zyxwvutsr of Pure and Applied Physics, Trinity College, Dublin 2, Ireland Yoshitomo Nakano and William J. MacKnight* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003. Received July 12, 1983 ABSTRACT A perfluorocarboxylate membrane has been characterized by dynamic mechanical relaxation, NMR on 'H and l9F, and 57Fe Mbssbauer spectroscopy of the ferric salt. Results are similar zyxw to those reported for perfluorosulfonate membranes, indicating that the specific charged end group has little influence on the structure or behavior of the aqueous phase. Exchanged ions in the membranes at ambient humidity and above are not directly coordinated by sulfonate or carboxylate groups, but by water. Precipitation of ferric hydroxide in a ferric membrane reexchanged with KC1 is inferred from the Mbssbauer spectrum at 4.2 K and confirmed by NMR relaxation times. Introduction The properties of perfluorosulfonate ion-exchange membranes depend sensitively upon the cation present and on the amount of absorbed The behavior of the aqueous phase, in particular the glasslike character at low temperatures, has been studied in a number of membranes, but especially in Nafion4using te~hniquesl-~ which include nuclear magnetic resonance (NMR)"8 and Mossbauer spectroscopy.*ll This paper focuses upon a relative new- comer to the field, a perfluorocarboxylate membrane in which the side group is terminated with COOH rather than S03H. It is of interest to ascertain, for example, the rel- ative tendency of the ionic phase to cluster, to explore the stability of the membrane with increasing temperature, and, more generally, to determine the extent to which the macroscopic properties are altered by the presence of carboxylate. NMR, Mijssbauer, and mechanical relaxation results are presented and compared with data for other perfluorinated membranes. This combination of experi- mental probes gives microscopic information on the motions of the polymer chains, and of the ions and water which constitute the aqueous phase, besides providing a macroscopic overview of the relaxation processes. Experimental Methods The sample under investigation was obtained from the Asahi Glass Co. in the form of a pressed sheet 250 pm thick. The water content at ambient humidity was determined, by heating under vacuum at 423 K, to be 6 wt zyxwvutsrqpo %. Prompted by the findings of an earlier study on Nafione (as received) in which impurity iron and potassium were detected in significant amounts, we carried out an X-ray fluorescence microprobe analysis of the carboxylate sample. The results, presented in Table I, reflect similar amounts of iron and potassium impurity and indicate further a sulfur content about 60% of that recorded for a comparable volume of the acid perfluorosulfonate membrane. Additionally, a microprobe line scan showed that the sulfur was concentrated toward the membrane surfaces. If the sulfur is a constituent of S03H (or S03K) end groups, this would imply that the membrane was only about 40-45% carboxylate exchanged. 'H and '9 Tl, T,, and TI, NMR data were recorded over the temperature range 120-380 K using the experimental procedures and methods of data analysis described previously.8 Mossbauer spectroscopy, also described previously,9 was applied to the Table I Number of Iron Atoms/cm' and Ratio of Potassium to Sulfur Content for the COOH Membrane As Received and an Acid Nafion of ComDarable Water Content sample no. of Fe3+/cm3 K/S COOH membrane 3.7 x 10'8 0.33 Nafion (115) 7.2 X 0.41 Fe3+-exchanged carboxylate sample. Iron was initially incorpo- rated by stirring the membrane in a 0.2 M aqueous solution of FeC13 for 12 h and subsequently precipitated by reexchange in a solution of 0.1 M KC1. In both cases a microprobe line scan was reasonably flat across the thickness of the membrane, implying a correspondingly uniform distribution of iron across the profile. Dynamic mechanical relaxation studies were carried out with a dynamic mechanical thermal analyzer (Polymer Laboratories), operating at 1 Hz over the temperature range 180-520 K. Membranes were conditioned by boiling in water and drying under various conditions. Results and Discussion NMR. It is recalled that the 'H data reflect predomi- nantly the response of absorbed water in the membrane while the I9F resonance probes the onset of molecular motion in the fluorocarbon backbone matrix. The 'H results of Figure 1 confii that the behavior of the aqueous phase in the perfluorocarboxylate acid membrane is in most respects comparable with that observed in a per- fluorosulfonate acid Nafion of similar water content for which a detailed interpretation of NMR data, including a treatment of the role of impurity iron, was presented in two earlier paper^.^^^ Briefly, (i) the onset of general motions in the aqueous phase, typical of a glass trans- formation process (Tg = 182 K), was evident in the sharp rise in T2 near 170 K and in the formation of T1, and T1 minima at -180 and -230 K, respectively; (ii) as tem- perature increased, chemical exchange between heteroge- neous water sites was presumed to set in and to become fully activated near ambient temperature; (iii) comparison of the magnitudes of the NMR relaxation times as a function of iron content implied that the iron in the membrane as received was most probably in the aqueous phase and that zyxwv T,, was much less sensitive than Tl or T2 to the presence of iron. Either TI, is dominated by mo- 0024-9297/84/2217-1331$01.50/0 0 1984 American Chemical Society