Highly Phase Separated Aromatic Ionomers Bearing Perfluorosulfonic Acids by Bottom-up Synthesis: Effect of Cation on Membrane Morphology and Functional Properties

Proton-conducting aromatic-based ionomers bearing superacid side chains are usually synthesized by polymer postmodification, which does not allow controlling ion exchange capacity and ionic group distribution along the ionomer and, thus, its chemical structure and functional properties. Bottom-up approach overcomes this problem. Here, we report the preparation of a novel ionic monomer and its polycondensation with commercial monomers. The obtained random ionomers are the first to show high phase separated organization at macro-, micro-, and nanoscale, common to the reference proton-conducting material Nafion. Additionally, membranes were cast from the solutions of ionomers in their Li+ and K+ forms in order to study the cation’s influence on both morphology and performance of the materials. The difference in ionic domain organization, depending on the initial cationic form of the ionomers, was reported for the first time. The proposed materials show superior proton conductivity than Nafion, especially at low relative humidity, which makes them potential substitute of the benchmarked Nafion for fuel cell application.