Sulfonation of Poly(aryletherketone): NMR Evidence for Multi-Regio Sulfonation and Membrane Properties

Abstract

Sulfonated poly(ether ether ketone) (SPEEK) is a benchmark ionomer for nonfluorinated proton exchange membranes, yet its properties remain strongly dependent on the degree of sulfonation and relative humidity. In this work, SPEEK samples were synthesized using two distinct sulfonation routes: concentrated sulfuric acid (H2SO4) and a chlorosulfonic acid/dichloroacetic acid (HSO3Cl/DCA) system. The 2D homo- and heteronuclear NMR spectroscopy analysis of the resulting polymers, complemented by FTIR analysis, revealed that the sulfonation pattern strongly depends on both the reaction medium and the sulfonating agent. Sulfonation in concentrated H2SO4 predominantly produces monosulfonated (mono-regio) repeat units, whereas the homogeneous HSO3Cl/DCA medium promotes multi-regio substitutions. The corresponding membranes (≈15 μm thick) were cast, and their properties were studied. Compared with H2SO4-derived SPEEK, HSO3Cl/DCA-SPEEK membranes exhibited higher Young’s modulus, more pronounced yield behavior, and higher proton conductivity.