ESR and NMR probes of counter-ion effects in perfluorosulfonate ionomer membranes

摘要

The properties of Nafion membranes, modified by exchange with tetraalkylammonium cations and equilibrated in methanol, were investigated using ESR spin probe techniques and N-14 NMR spectroscopy. TEMPO and TEMPOL nitroxide radicals are shown to act as probes of the methanol solvent phase of the membranes. The mobility of the probe radicals is restricted, the correlation times for rotational diffusion being reduced by an order of magnitude within the ionomer environment compared to free methanol solution. The larger tetraalkylammonium cations result in higher levels of swelling in methanol and a greater degree of nitroxide mobility. The temperature dependent ESR spectra show a gradual transition from freely diffusing to rigidly immobilized radicals as the temperature is lowered. The motion is completely frozen out at about 190 K, before the freezing point of methanol is reached. N-14 NMR spectroscopy of the tetraalkylammonium counter-ions is also reported. The NMR signals for all cations remain quite sharp and well resolved in methanol-equilibrated Nafion. The corresponding N-14 spin lattice relaxation times, T-1, were reduced by one order of magnitude compared to methanol solution. The long-chain surfactant cations showed the least reduction in T-1 on incorporation into the membrane, consistent with retention of significant rotational mobility. These results are interpreted in terms of a much weaker ion clustering in the organically modified membranes than is found in strongly hydrated Nafion membranes. (C) 1998 Elsevier Science B.V. All rights reserved. References: 30