Effect of Phosphaphenanthrene Skeleton in Sulfonated Polyimides for Proton Exchange Membrane Application

Sulfonic acid groups containing aromatic polyimides are of great interest as polymer electrolyte membranes owing to their high thermal and mechanical stability, strong resistance to fuel crossover, excellent film-forming ability, and high proton conductivity. However, these polymers generally experience reduced oxidative permanence. To alleviate the problems associated with the oxidative stability, the present work reports the synthesis and characterization of a new series of sulfonated copolyimides (co-SPIs) using 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) containing diamine monomer as one of the comonomers. The synthesized sulfonated copolyimides were soluble in numerous organic solvents and exhibited reasonably high inherent viscosity that allows us to prepare high-quality membranes by a solution casting route. The structural elucidation and the sulfonic acid content in the polymers were verified from the integral values of the proton NMR signals. FTIR and 31P NMR were also used for structural confirmation of the polymers. Transmission electron microscopy (TEM) images revealed well-scattered spreading of hydrophilic and hydrophobic phases. In general, the membranes from these copolymers showed improved proton conductivity and oxidative stability and low water uptake. Among all the co-SPI membranes, DPPNH-80 (IECW = 2.58 mequiv g–1) exhibited proton conductivity of 202 mS cm–1 at 80 °C under fully hydrated conditions.