Synthesis and characterization of sulfonated poly(arylene ether)s with sulfoalkyl pendant groups for proton exchange membranes

Side-chain-type sulfonated poly(arylene ether)s bearing sulfoalkyl pendant groups (PSA-SPAEs) as proton exchange membranes for fuel cells were prepared via nucleophilic substitution polycondensation reactions of sodium 3-(4-(2,6-difluorobenzoyl)phenyl)propane-1-sulfonate (SDFPPS), 4,4′-dihydroxyldiphenylether (DHDPE) and 4,4′-dichlorodiphenyl sulfone (DCDPS). The sulfonic acid group content (SC), as the quantity per repeat unit of polymer, ranging from 0.5 to 0.8, was readily controlled by changing the feed ratio of SDFPPS to DCDPS. Good thermal properties of PSA-SPAE copolymers were indicated by observing glass transition temperatures (Tgs) ranging from 210 to 230 °C in sodium salt form and decomposition temperatures (Tds) over 220 °C in acid-form and over 400 °C in sodium-form in air. All PSA-SPAE membranes exhibited reasonable flexibility and tensile strength in the range of 41–78 MPa. PSA-SPAE copolymers bearing sulfonic acid groups on flexible side chains showed considerably reduced swelling ratio and improved proton conductivities. Proton conductivity curves parallel to those of Nafion 117 were obtained with proton conductivity of 10−1 S/cm at equivalent ion exchange capacities (IEC) of 1.47 and 1.64, comparable to Nafion 117. PSA-SPAE copolymer with SC 0.8 had the best data including PEM mechanical strength, water swelling and proton conductivity.