Proton conductivity and stability of low-IEC sulfonated block copolyimide membrane

The proton conductivity and membrane stability of sulfonated block copolyimide membranes with a low ion exchange capacity (IEC) synthesized by chemical imidization using a two-pot procedure were investigated. The proton conductivity of the low-IEC block copolyimide membrane at 80 °C and 98% relative humidity was 16 times larger than that measured for the low-IEC random copolyimide membrane, while that of the high-IEC block copolyimide membrane at the humidity was 4 times than that of the high-IEC random copolyimide membrane, suggesting that the protons in the low-IEC block copolyimide membrane were effectively transported. In addition, the hydrolytic and oxidative stabilities of the low-IEC sulfonated block copolyimide membranes were estimated in detail from FT-IR and 1H NMR. These stabilities of the low-IEC membranes were significantly enhanced compared to those of the high-IEC membranes. The oxidative stability problem in the low-IEC block copolyimide membranes was found to be primarily caused by a decrease in the sulfonic acid groups rather than by polyimide chain scission. These findings indicate that a sulfonated block copolymer consisted of a small amount of sulfonic acid groups is desirable for the development of proton exchange membranes, when considering the balance between the proton conductivity and membrane stability of the proton exchange membranes.