The synthesis and characterization of crosslinked aromatic polymer membranes with high ion exchange capacity (IEC) values are reported. Through aromatic nucleophilic substitution polycondensation and the subsequent sulfonation reaction, the highly sulfonated polymers SPPSU‐2S and SPPSU‐4S with high molecular weight (Mn = 138–145 kDa, Mw = 200–279 kDa) and well‐defined structures were synthesized. By solution casting and thermal annealing treatment, flexible crosslinked membranes with high solvent insolubility were obtained. The membranes exhibited mechanical and chemical stability as confirmed by dynamic mechanical analysis (DMA) and conductivity measurement. The crosslinked SPPSU‐4S membrane with IEC = 3.20 meq/g showed the highest proton conductivity of 0.163 S/cm at 120 °C, 90% RH, and improved thermal stability compared with its precursor (uncrosslinked) membrane. The results show that simple annealing method could improve significantly membranes properties of highly sulfonated aromatic polymers. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44218.

A novel series of terpolymers (SQF) containing sulfophenylene, quinquephenylene, and perfluoroalkylene groups in the polymer main chain were designed and synthesized as proton exchange membranes for fuel cells. The terpolymers with high molecular weight (M w = 179–207 kDa, M n = 41–50 kDa) and different ion exchange capacity (IEC) values (1.70, 2.56, and 3.34 mequiv g–1) gave flexible self-standing membranes by solution casting. Compared to the two-component (sulfophenylene and quinquephenylene segments) copolymer membranes, the incorporation of the third component, perfluoroalkylene groups, resulted in better water utilization for the proton conduction, while it did not alter the other properties such as gas permeability and mechanical strength. The selected membrane (SQF-3 with IEC = 2.56 mequiv g–1) exhibited high fuel cell performance under high- and low-humidity conditions with maximum power density reaching 0.97 W cm–2 at 100% RH (relative humidity) and 0.82 W cm–2 at 30% RH, respectively, at a current density of 1.51 A cm–2 with oxygen. A good interfacial compatibility between the SQF-3 membrane and catalyst layers resulted in mass activity of the cathode catalyst comparable to that obtained with the Nafion membrane NRE 211. During the open circuit voltage (OCV) hold test with air and hydrogen at 80 °C and 30% RH for 1000 h, the OCV showed a slight decrease from 0.97 to 0.88 V. Post-test analyses revealed that the SQF-3 membrane retained its initial high fuel cell performance due to its high chemical stability as well as low gas permeability.

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