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

We investigated the 1 H time-dependent self-diffusion coefficients, D eff, in perfluorinated membrane with low and high water contents (6% and 24%) at the temperature range from 233 to 323 K. The D (Δ eff ) was measured as a function of the diffusion time, Δ eff, 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 (Δ eff ) of the short and long Δ eff . The results showed that the 1 H self-diffusion coefficients were dependent on Δ eff 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, E a, of diffusion was also evaluated from the temperature dependence of diffusion at temperature above 263 K. The E a 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.