Determination of a Micron-Scale Restricted Structure in a Perfluorinated Membrane from Time-Dependent Self-Diffusion Measurements

Abstract

We investigated the 1H time-dependent self-diffusion coefficients, Deff, in perfluorinated membrane with low and high water contents (6% and 24%) at the temperature range from 233 to 323 K. The D(Δeff) was measured as a function of the diffusion time, Δeff, from 1 to 100 ms by field gradient NMR techniques. The oscillating gradient spin-echo sequence (OGSE) and the bipolar pulse longitudinal eddy current delay sequence (BPPLED) were employed to examine the D(Δeff) of the short and long Δeff. The results showed that the 1H self-diffusion coefficients were dependent on Δeff less than 2 ms due to restricted geometry and were constant beyond 3 ms. The tortuosity, α, and surface-to-volume ratio of pore, S/V, for proton diffusion were estimated as geometrical parameters at each temperature. The values of S/V revealed the existence of micron-scale restricted structure compared with well-known nanometer-sized domain in perfluorinated membrane. Activation energy, Ea, of diffusion was also evaluated from the temperature dependence of diffusion at temperature above 263 K. The Ea in the membrane and in the bulk water were almost the same at the temperature range above 263 K while it had some difference at lower temperature.