Enhancing proton conduction and methanol barrier performance of sulfonated poly(ether ether ketone) membrane by incorporated polymer carboxylic acid spheres

The composite membranes were fabricated by physical blending of sulfonated poly(ether ether ketone) (SPEEK) bulk material and crosslinked polymer carboxylic acid spheres (PCASs) with diameter of 150 nm, which were prepared by distillation–precipitation copolymerization of ethyleneglycol dimethacrylate (EGDMA) crosslinker and functional methacrylic acid (MAA) monomer. The incorporation of PCASs increased the ion-channel size, free volume cavity size and fractional free volume of the composite membranes, resulting in an increased water uptake, a higher proportion of the bound water, and an enhanced water retention capacity, which were confirmed by small-angle X-ray scattering (SAXS) and positron annihilation lifetime spectroscopy (PALS). The composite membranes displayed superior comprehensive performance to the SPEEK control membrane, in which the homogeneously embedded PCASs provided new pathways within the bulk membrane for proton conduction, rendered more tortuous pathways as well as greater resistance for methanol crossover. In particular, the composite membrane containing 15 wt.% PCASs exhibited a four-time higher selectivity (ratio of proton conductivity to methanol crossover) than the SPEEK control membrane.