SANS Study of the Effects of Water Vapor Sorption on the Nanoscale Structure of Perfluorinated Sulfonic Acid (NAFION) Membranes

Several poly(perfluorosulfonic acid) membranes (NAFION, EW = 1100) with the same sulfonic acid content were systematically investigated with SANS under in-situ water vapor sorption and/or with bulk water to quantify the effects of relative humidity (RH), membrane processing (melt-extruded and solution-casting), prehistory (pretreated at 80 °C and as-received), and thickness on the nanoscale structure at room temperature. The sorption isotherm (water uptake vs RH) of the membranes showed a strong correlation between the interionic domain distance ( L ion ) and RH. The melt-extruded membranes showed evidence of partial alignment of better organized ionic domains than those solution-cast. Pretreating the membranes resulted in a larger L ion and a broader scattering over the entire range of RH. The ionic peak of the melt-extruded membranes (as-received and pretreated) became more symmetric and narrower with sorption time. Diffusion coefficients of water vapor, based on structural evolution and Fick's second law, are in the range of 1 × 10 -7 −3 × 10 -7 cm 2 /s for both extruded (pretreated and as-received) membranes. A thickness-dependent crystalline feature around Q ≈ 0.03 Å -1 was also observed.