Sulfonated Poly(phenylene sulfone) Polymers as Hydrolytically and Thermooxidatively Stable Proton Conducting Ionomers

Sulfonated poly(arylene) ionomers containing merely electron-accepting sulfone units (−SO2−) connecting the phenyl rings have been synthesized and characterized in detail, in particular with respect to properties relevant for their application as membrane materials in polymer-electrolyte-membrane (PEM) fuel cells. The copolymers were prepared by a two-step process comprising (1) a nucleophilic aromatic polycondensation reaction of 4,4'-difluorodiphenylsulfone and disodium 3,3'-disulfonate-4,4'-difluorodiphenylsulfone with 4,4'-thiobisbenzenethiol at various molar ratios yielding sulfonated poly(phenylene sulfide sulfone)s and (2) their subsequent oxidation to sulfonated poly(phenylene sulfone)s using peroxide in acidic solution. This new class of polymers with extremely electron-deficient aromatic rings shows very high thermal, thermooxidative and hydrolytic stabilities, low solubilities, and reduced swelling in water at enhanced temperature compared to other sulfonated poly(arylene)s. The latter property allows for the preparation of membranes with very high ion exchange capacity and high proton conductivity especially at high temperature and low humidification, which makes them interesting materials for chemical and electrochemical applications such as PEM fuel cells. The flexible preparation route provides a path for obtaining various molecular structures and ion exchange capacities.