Synthesis and characterization of sulfonated copolyimides via thermal imidization for polymer electrolyte membrane application

► Sulfonated copolyimide membranes were fabricated from solutions of the copoly(amic acid)s by a thermal imidization process. ► Sulfonated copolyimides show mainly three-step thermal degradation, with the degradation behavior being independent of the composition. ► WU and IEC values of the sulfonated copolyimides were 8.2-54.2% and 0.18-2.05 meq/g, respectively. ► Proton conductivity values of the membranes were lower than that of Nafion 117, although their IEC values were higher.