Influence of Bound Ion on the Morphology and Conductivity of Anion-Conducting Block Copolymers

Anion-conducting membranes are important for several applications including fuel cells and artificial photosynthesis. In this study such membranes were made by quaternizing polystyrene-block-polychloromethylstyrene (PS-b-PCMS) copolymers. PS-b-PCMS copolymers with molecular weights ranging from 4 to 60 kg/mol were synthesized by nitroxide-mediated controlled radical polymerization. Separate aliquots of the PS-b-PCMS samples were quaternized to transform the PCMS block. This resulted in block copolymers with ionizable blocks containing either trimethylammonium chloride or n-butylimidazolium chloride. We refer to ion-containing block copolymers synthesized from the same precursor as matched pairs: SAM (containing trimethylammonium chloride) and SIM (containing n-butylimidazolium chloride). The volume fraction of the ion-containing block, ϕ, ranges from 0.26 to 0.50 for the case of SAM and from 0.35 to 0.60 for the case of SIM. Self-assembly in these copolymers resulted in the formation of lamellar phases regardless of ϕ, chemical formula of the bound ion, and chain length. Chloride ion conductivity and water uptake measurements on one of the matched pairs led to similar results. Preliminary experiments wherein the chloride ions in this matched pair were replaced by hydroxide ions were performed, and the changes in conductivity due to this are reported.