Cross-linked poly(arylene ether ketone) proton exchange membranes sulfonated on polymer backbone, pendant, and cross-linked sites for enhanced proton conductivity

The sulfonated poly(arylene ether ketone) (SPAEK) precursor containing pendant carboxylic groups was synthesized, and used for the production of a series of cross-linked sulfonated poly(arylene ether ketone) membranes with different cross-linking density, using the carboxylic acid reactive groups. In order to obtain high proton conductivity, the sulfonated groups were attached not only on the polymer backbone, but also on the pendant and cross-linked sites. The chemical synthesis of SPAEK was confirmed using ¹H-NMR and FT-IR spectroscopy. The proton conductivity, water uptake, mechanical properties, and morphology of the cross-linked membranes (CSPAEK) prepared were investigated, and correlated with the ionic cluster dimension. The use of sulfonated cross-linker had an excellent effect on the overall membrane properties. It enhanced the mechanical properties but noticeably lowered the water uptake, while still maintaining proton conductivity at a high level. Even the CSPAEK membrane with 15% cross-linking degree exhibited higher proton conductivity than Nafion® 117, while demonstrating acceptable water uptake and tensile strength. The ion cluster dimension of the CSPAEK membranes was analyzed using SAXS. The resulting membrane properties were well explained by the SAXS patterns.