Effect of sulfonation level on the morphology, local structure, and proton conductivity of hydrocarbon-based random copolymers

Although perfluorosulfonic acid polymers are widely used proton-exchange membranes, the economic and environmental disadvantages arising from fluorine have stimulated interest in hydrocarbon-based polymers. In previous work, we studied a linear polyethylene with a phenylsulfonic acid pendant group precisely on every fifth carbon and found the proton conductivity to exceed 0.1 S/cm above 70% relative humidity at 40 °C. To improve the precessability and mechanical properties of this polymer, this study explores lower sulfonation levels. Using X-ray scattering, FT-IR, pulse-field gradient NMR, electrochemical impedance spectroscopy, we determine the nanophase separation behavior of these polymers, the local chemical state and diffusion coefficient of water and the proton. A moderate reduction in sulfonation level enhances the mechanical toughness of while maintaining high proton conductivity. We attribute this high proton conductivity to the nanoscale morphology of percolated water channels in the membranes containing bulk-like water.