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

We investigated the H-1 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(Delta(eff)) was measured as a function of the diffusion time, Delta(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(Delta(eff)) of the short and long Delta(eff). The results showed that the H-1 self-diffusion coefficients were dependent on Delta(eff) less than 2 ms due to restricted geometry and were constant beyond 3 ms. The tortuosity, alpha, 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.