Abstract
We report a QENS study of the molecular motions in a perfluorinated ionomer membrane, Nafion, under increasing hydration levels from almost dry to fully saturated. Combined experiments performed on time-of-flight and backscattering spectrometers have been used to investigate the picosecond to the nanosecond dynamic behavior of water. The experimental spectra have been simulated over the whole Q range from 0.34 to 2.25 A-1 by a single theoretical model taking into account the localized motions within confining domains, the microscopic features of the elementary jump process, and the long-range diffusion mechanism. The diffusion in a restricted geometry with ill-defined boundaries has been described by Gaussian statistics, contrary to the popular diffusion inside an impermeable sphere model where the boundaries are well defined. Evaluation of the spectra reveals the existence of two populations of protons in Nafion at all hydrations that are nonexchangeable on the nanosecond time-scale. A first population o...
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