Effect of residual casting solvent content on the structure and properties of sulfonated poly(ether ether ketone) membranes

In the research of the effect of casting solvents on the structure and properties of proton exchange membranes, the amount of residual casting solvent is one of the important factors that have been overlooked. In this work, a series of as-cast SPEEK membranes with controlled amount of residual solvent (RS) content were prepared. Even though all RS was removed from the membranes after the treatment with 1 M H2SO4 and DI water sequentially at room temperature, the morphology and properties of these treated membranes differed from one another. Larger and better-connected hydrophilic domains and higher fractional free volume were observed for the treated membranes with larger amount of RS. The proton conductivity of water equilibrated membranes increased with the increasing RS content until reaching a maximum value that was almost twice as high as that of the membrane with the minimum RS. With further increase of RS content, the decrease in the proton conductivity was observed. In comparison, the water uptake of the membranes kept increasing with the RS content.