A series of sulfonated poly(aryl ether sulfone) copolymers containing phenyl pendant groups with sulfonic acid groups on the backbone were synthesized through condensation polymerization. The degree of sulfonation (DS) of the copolymers was controlled by changing the feed ratios of sulfonated to unsulfonated monomers. Post-crosslink reactions are carried out with 4,4'-thiodibenzoic acid (TDA) as a crosslinker and the carboxylic acid groups in TDA can undergo Friedel-Craft acylation with the phenyls pendent rings in sulfonated poly(arylene ether sulfone)s copolymers to prepare polymer electrolyte membranes for fuel cell applications. The chemical structures of crosslinked and uncrosslinked sulfonated poly(arylene ether sulfone) s copolymers (SPSFs and CSPSFs) were characterized by FTIR, H-1 NMR spectra. The thermal and mechanical properties of the membranes were characterized by thermogravimetric analysis and stress-strain test. The dependence of water uptake, methanol permeability, proton conductivity, and selectivity on DS was studied. Transmission electron microscopic observations revealed that SPSFs and CSPSFs membranes form well-defined microphase separated structures. (C) 2013 Society of Plastics Engineers