Proton and deutron NMR study of PTFE ionomer membranes

Proton and deutron NMR have been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS) and DuPont (Nafion^R). Two proton relaxation peaks were found in the XUS specimen absorbed with H₂O. The major (narrow) peak presented a spin-lattice relaxation time (T₁) of 107 ms while the minor (broader) one gave much longer T₁. While the former was attributed to the water molecules involved in restricted motion, the latter was expected to be associated with the protons located in the vicinity of the sulfonate groups. Similar to the previous results from the others, only a single peak was detected in Nafion^R in ¹H spectra, indicating that the protons in the different environments were engaging rapid exchange within NMR time scale. In contrast to the inverse proportion dependence of the linewidth on the water sorption in Nafion^R, the major line of the XUS membrane exhibited insensitive linewidth dependence on the variation of H₂O concentration. The difference was attributed to the existence of narrow breaths of the pores in XUS sample, such that free water contribution to the enhancement of proton mobility was limited. The ²H spectra of Nafion^R were found to possess a doublet, due to nuclear quadrupolar interaction. Dow (XUS) membrane treated in at 100% relative humidity (RH) D₂O presented a single peak with the linewidth insensitive to the amount of heavy water absorbed. An additional rise emerged on the “shoulder” of this single peak when treated at 33% RH. It is concluded that XUS membrane does not provide strong hydrogen bonding to eliminate the rapid motion average over the nuclear quadrupole interaction.