Solvent-Mediated Thermal Crosslinking of Sulfonated Aromatic Polymers

Abstract

Thermal crosslinking of sulfonated aromatic polymers is commonly reported in dimethyl sulfoxide (DMSO), which is often treated as indispensable for forming stable sulfone bridges from sulfonic acid groups. Here, we compare solvent effects on thermal crosslinking for three representative hydrocarbon ionomers─sulfo-phenylated polyphenylene (Pemion HNN9), sulfonated poly(ether ether ketone) (sPEEK), and sulfonated poly(phenylene sulfide sulfone) (sPPSS)─using DMSO, sulfolane, DMAc, lower-boiling alcohols, and water. Crosslinking is evaluated by post treatment insolubility, ion-exchange capacity decrease, reduced water uptake, increased membrane density, and the emergence of sulfone vibrations in ATR FTIR spectra. Across all three polymers, crosslinking is observed from solvents beyond DMSO, notably methanol and water, depending on polymer solubility. While solvent–polymer interactions, such as acid–base interaction in the case of DMAc, may directly inhibit crosslinking, we further provide evidence that a lack of sulfonyl cation stabilization (e.g., for sulfolane) also does hinder thermally induced crosslinking. These results challenge the assumption that DMSO is uniquely required and identify alternative solvent routes that may simplify membrane processing by reducing solvent retention while maintaining effective crosslink formation.