Heat Treatment for a Membrane Composed of Polymer Nanofibers and a Perfluorosulfonic-Acid Polymer Matrix: Effect on Proton-Exchange-Membrane Properties

Thermal treatment is an essential process for a perfluorosulfonic acid (PFSA) polymer membrane to reinforce the mechanical properties of the membrane while the proton conductivity of the membrane often drops by the treatment. Here we evaluated effects of a thermal process for a composite membrane, which consists of acid-doped nanofiber framework and a PFSA polymer matrix, on the proton-exchange membrane properties. Thermal treatments of the membranes without pressure decreased the proton conductivity of the membrane while hot-pressing maintained that. Mechanical properties of the membranes were drastically improved by the thermal processes in the respect of modulus of elasticity, elongation at break, and maximum stress while the gas-barrier properties were less affected. The scanning electron microscopic observation indicated formation of voids in the membrane by thermal treatment without pressure. These results emphasize importance of post-treatment process for composite membranes to achieve required membrane properties.