Influence of rigid-soft domains on proton conduction and microstructure in block copolymer membranes

In ionic copolymers, chemically distinct domains can be combined to enhance proton transport and stability. In this work, we integrated a rigid engineering plastic, poly(ether ether ketone) (PEEK), with phosphonated polystyrene chains to fabricate membranes containing complementary rigid and soft molecular domains. The rigid PEEK chains provided thermal stability and mechanical integrity, while the flexible phosphonated polystyrene domains enabled proton conduction under anhydrous or low-humidity conditions. The resulting membranes exhibited a proton conductivity approaching 10^-3 S cm^-1 at 50% relative humidity and 1 bar vapor pressure. Structural analysis revealed planar, porous surfaces and supported the role of proton hopping as the dominant conduction mechanism, with activation energies in the range of 0.08-0.50 eV. These findings highlight how rigid-soft molecular design can balance stability and conductivity, making such membranes promising candidates for use as separators in hydrogen or methanol fuel cells.