Nanoscale Structures of Radiation-Grafted Polymer Electrolyte Membranes Investigated via a Small-Angle Neutron Scattering Technique

The nanoscale structures of graft-type polymer electrolyte membranes (PEMs), prepared by radiation-induced graft polymerization (grafting) of styrene onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by sulfonation, were investigated using a small-angle neutron scattering (SANS) technique. For comparison, SANS measurements were also performed on two precursor materials, the original ETFE film and polystyrene (PS)-grafted films. The SANS profiles of the grafted films showed shoulder peaks at a d-spacing of ∼30 nm, which were attributed to the PS grafts introduced into the amorphous phases between the ETFE lamellar crystals. This grafting would result in the construction of a stack structure with alternating ETFE crystalline and PS-grafted layers as a repeating unit. In the ETFE PEMs, the spacing of the PS sulfonic acid (PSSA) grafts and ETFE crystals increased because the graft regions were enlarged by the volume of the attached sulfonic acid groups. Interestingly, the graft/crystal stack spacing in the PEMs did not increase from the dry- to fully-hydrated states. This finding implies restricted water absorption in the PSSA grafts between the ETFE lamellar crystals. In other words, most of the PSSA grafts introduced outside of the lamellae were considered to be hydrated and to act as proton conduction pathways.