Morphological changes of hydrophobic matrix and hydrophilic ionomers in water-swollen perfluorinated sulfonic acid membranes detected using small-angle X-ray scattering

Abstract Time-resolved small-angle X-ray scattering (SAXS) was employed to examine the hydration dynamics of perfluorinated sulfonic acid membranes in contact with liquid water. Following thermal pretreatment of the membranes, a peak corresponding to the fluorocarbon polymer backbones was clearly observed in the SAXS profiles, in addition to a peak corresponding to the ionomer clusters consisting of hydrophilic side chains and water molecules. The variation of these two peaks was monitored with a frame time as low as 21 ms, which permitted the successful capture of the subsecond dynamics of water uptake by 25 and 50 μm thick membranes. The dynamics of the initial rapid water uptake were well reproduced using Fick's equation, although the thinner sample exhibited a smaller diffusion coefficient. This result suggests that the surface resistance influences the diffusion of water into the membrane. The initial rapid water absorption was followed by the slow and continuous reconstruction of the polymer structure. The measured dynamics of the two peaks indicated that, during this long-term reconstruction, the increase in the ionomer volume became slower and the fusion of clusters was the dominant factor responsible for increasing the ionomer spacing.