A study of linear versus angled rigid rod polymers for proton conducting membranes using sulfonated polyimides

Linear and angled monomers were incorporated into the main chain of a polyimide in order to investigate the effect of kinked versus linear polymers on membrane properties such as water uptake and proton conductivity. Polymers prepared using linear 4,4′-sulfonyldianiline, SPI1, and using angled 3,4′-sulfonyldianiline, SPI2, were cast into membranes possessing ion exchange capacities that varied from 0.79 to 2.75 meq g−1. Membranes are thermally stable up to 300 °C under air. Proton conductivity of both membranes increases with temperature to values of 0.1–0.2 S cm−1. The conductivity of angled, SPI2 membranes is greater than those prepared from SPI1 for a given IEC but water uptakes are lower. These differences are attributed to increased entanglements of the angled polymers, which limits the degree of swelling and increases the proton concentration. These results may be important in the design of proton conducting membranes from other rigid polyarylenes.