Anion-Conductive Multiblock Aromatic Copolymer Membranes: Structure-Property Relationships

Anion-conductive multiblock copoly(arylene ether sulfone)s (mPES) were synthesized with different block lengths and ion-exchange capacities (IEC) to maximize ion conductivity and explore the relationship between chemical structure and morphology in anion-exchange membranes (AEM). Nuclear magnetic resonance (NMR) relaxometry was used to probe water mobility and domain size. The multiblock copolymers were synthesized by polycondensation of separately prepared hydroxy-terminated oligomers with fluoro-terminated oligomers. The polymers were made ion-conductive through selective chloromethylation of one of the two block types, followed by quatemization and hydroxide ion exchange. The resulting block structure, in which one type is hydrophilic and one type is hydrophobic, was designed to ensure a nanophase-separated morphology. The multiblock copolymers exhibited higher anion conductivity than their random copolymer counterparts at the same IEC. The multiblock copolymer that exhibited the highest anion conductivity was not the one with the highest IEC, but it was the one with the shortest NMR relaxation times for water, indicating a greater fraction of water interacts with the ionic polymer segments. Higher IEC values led to increased water absorption, an effective dilution of the ionic groups, and increased NMR relaxation times for water.