Abstract
The extent of phase separation in Nafion® perfluorosulfonate ionomer membranes has been studied by small‐angle neutron scattering (SANS). These polymers, which consist of a perfluorocarbon main chain and a sulfonate‐containing side group, can absorb up to 30% by weight of water. Previous studies have shown that clustering of water occurs, forming particles in the size range observable by SANS. The current study is concerned with the fraction of water molecules which participate in the clustering and the chemical composition of the phases present. Experiments have been made on melt‐quenched samples which have no fluorocarbon crystallinity. The analysis is based on isotopic replacement experiments in which SANS measurements are made on samples hydrated with mixtures of H~2~O and D~2~O. Values of the small‐angle x‐ray scattering (SAXS), mean‐square electron density fluctuation, and mass density are used as additional criteria. It is shown that at high water content (more than 15% absorption by weight), a two‐phase model can explain the data with a majority (>60%) of the water molecules in one phase and most (>90%) of the perfluorocarbon in the other phase; a sample hydrated to a lower extent (8% by weight) shows deviations from the two‐phase model. These results are consistent with the scattering behavior at large angles observed by SAXS.