Controlling Water Content and Proton Conductivity through Copolymer Morphology

To investigate relationships between morphology and proton conductivity in ionic copolymer membranes, we have studied two series of fluorous copolymers bearing polystyrene grafts sulfonated from 0 to 100%. Small-angle X-ray and neutron scattering experiments reveal a disordered, partially phase-separated system consisting of fluorous domains in a partially sulfonated polystyrene matrix with aggregation of ion-rich domains within the matrix. The size of the fluorous domains depends on graft density, and their packing depends on the graft chain length. The spacing of the ion-rich domains is remarkably independent of either graft chain length or charge content. We find that the samples with lower graft density, which are partially crystalline, develop a less-ordered morphology with a lower degree of phase separation. The partially crystalline samples swell less and have a slightly lower conductivity at similar water content; the lower conductivity is attributed to a more tortuous conducting phase.