Intermolecular ionic cross-linked proton conducting electrolyte membranes derived from branched sulfonated poly(ether ether ketone)s with benzoxazole pendants

• Branched sulfonated poly(ether ether ketone)s containing intermolecular ionic cross-linkable groups were synthesized.
• Benzoxazole groups were grafted onto the polymer backbone via thiol-ene click reaction.
• The membranes exhibited low swelling, low methanol permeability and improved chemical stability.

A series of novel branched sulfonated poly(ether ether ketone)s containing intermolecular ionic cross-linkable groups, benzoxazole groups, have been prepared for direct methanol fuel cells. The benzoxazole groups are grafted onto the polymer chain via a thiol-ene click chemistry reaction. The expected structures of the copolymers are confirmed by 1H NMR and Fourier transform infrared spectroscopy. Compared with the unmodified polymer membrane, the ionic cross-linked membranes show enhanced thermal and mechanical properties. We also investigate the changes in water uptake, proton conductivity and chemical stability. The dense membrane structures formed by branching and the interactions between sulfonic acid and benzoxazole groups make a great contribution to the improvements of dimensional stability and methanol resistance property. Although the proton conductivities of the ionic cross-linked membranes are lower than the pristine membrane, the selectivities are much higher. The results show that the novel copolymers in this study are possible potential candidate materials for proton electrolyte membrane.