Synthesis and characterization of poly(para-phenylene disulfonic acid), its copolymers and their n-alkylbenzene grafts as proton exchange membranes: high conductivity at low relative humidity

Abstract

Water insoluble poly(para-phenylene disulfonic acid) and its copolymers were synthesized by direct polymerization of 1,4-dibromobenzene-2,5-disulfonic acid and 4,4′-dibromobiphenyl-3,3′-disulfonic acid lithium salts using Ullmann coupling and subsequent grafting of long-tail alkylbenzene groups onto the polymer backbones. Copolymers with ion exchange capacities of 4.3 to 7.7 meq. g⁻¹ were obtained. Polymers and copolymers prepared under optimized polymerization conditions were characterized by NMR, TGA, DSC and viscometry. The physicochemical characteristics of the copolymers were tailored by adjusting monomer compositions and by varying the grafting reaction temperature and time. These polymers could hold eight or more strongly bound water molecules per acid group, which facilitated their high conductivity. In the dry state, the polymers were very brittle. Membranes prepared from these polymers exhibited proton conductivity as much as ten times higher than that of Nafion® 212, at elevated temperature and low relative humidity.