Poly(p-phenylene sulfone)s with high ion exchange capacity: ionomers with unique microstructural and transport features

Poly(phenylene) ionomers which contain merely sulfone units (–SO2–) connecting the phenyl rings and in which each phenyl ring is sulfonated (–SO3H) have been characterized with respect to their microstructural and transport properties. The high degree of sulfonation leads to the development of a microstructure characterized by very narrow hydrated, hydrophilic domains which are well connected on longer scales. These features together with high absolute water uptakes at given relative humidities and the high charge carrier concentration corresponding to the high ion exchange capacity (IEC ∼ 4.5 milli equivalent g−1) result in very high proton conductivities but also low water transport coefficients (water diffusion and presumably also electroosmotic drag and permeation). Compared to the transport properties of Nafion, these trends increase with increasing water content and with increasing temperature. For a relative humidity of RH = 30% and a temperature of T = 135 °C, the proton conductivity is found to be seven times higher than the conductivity of Nafion under the same conditions. Highly sulfonated poly(p-phenylene sulfone) polymers are water soluble and brittle in the dry state, but their transport properties together with their high hydrolytical and morphological stability renders this type of ionomer an interesting constituent of polymer electrolyte membrane (PEM) fuel cell membranes able to operate at high temperature and low humidification.