Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes

Novel biphenol-based wholly aromatic poly(arylene ether sulfone)s containing up to two pendant sulfonate groups per repeat unit were prepared by potassium carbonate mediated direct aromatic nucleophilic substitution polycondensation of disodium 3,3′-disulfonate-4,4′-dichlorodiphenylsulfone (SDCDPS), 4,4′-dichlorodiphenylsulfone (DCDPS) and 4,4′-biphenol. Copolymerization proceeded quantitatively to high molecular weight in N-methyl-2-pyrrolidinone at 190 °C. Tough membranes with a SDCDPS/DCDPS mole ratio up to 60:40 were successfully cast using N,N-dimethylactamide. An increase of sulfonate groups in the copolymer resulted in increased glass transition temperature, enhanced membrane hydrophilicity, and intrinsic viscosity; the 100% SDCDPS homopolymer was water soluble. The acid form membranes were successfully obtained by treating the sodium form of the membranes with dilute sulfuric acid solution. Thermogravimetric analysis shows that the sodium form materials have enhanced thermal stability relative to the acid form, as expected. Atomic force microscopy (AFM) phase images of the acid form membranes clearly show the hydrophilic domains, with sizes increasing from 10 to 25 nm as a function of the degree of sulfonation. A phase inversion could be observed for the 60% SCSDPS copolymer, which was consistent with a rapid increase in water absorption. Short-term aging (30 min) indicates that the desired acid form membranes are stable to 220 °C in air and conductivity values at 30 °C of 0.11 S/cm (SDCDPS/DCDPS=0.4) and 0.17 S/cm (SDCDPS/DCDPS=0.6) were measured, which are comparable to or higher than the state-of-the-art fluorinated copolymer Nafion 1135 control (0.12 S/cm). The conductivity is greatly influenced by ion exchange capacity, temperature, and water activity. The new copolymers, which contain ion conductivity sites on the deactivated positions of the aryl backbone rings, are candidates as new polymeric electrolyte materials for proton exchange membrane (PEM) fuel cells.