Structure of the aqueous phase and its impact on the conductivity of graft copolymer ionomers at saturation

Proton conductivity is strictly proportional to water uptake in state-of-the art radiation-grafted copolymer membranes. However, high water uptake can compromise mechanical integrity and operation lifetime. The challenge of optimizing water content intimately relates to the phase segregated aqueous domains of the hydrated membrane. Comprehending the nano-scale structure and morphology of the aqueous phase and, more importantly, the impact on proton conductivity would be an asset when developing new materials. Therefore, we combined small-angle neutron scattering, the technique of clipping of random waves, and the technique of tracer random walk, and present an approach that correlates long-range proton diffusion with the nano-scale tortuosity of the aqueous phase. Our results suggest that in disordered domains the decrease in long-range proton diffusivity cannot be prevented upon decreasing water content. Spatial order, however, has beneficial influence, and the loss in proton conductivity is expected to be less critical.