Building Sulfone Bridges: Multi‐Technique Analysis of Thermally Induced Cross‐Linking in Highly Sulfonated Poly(Phenylsulfone)

Fluorine‐free ionomers are emerging as promising alternatives to perfluorinated membranes. Highly sulfonated poly(phenylsulfone) (SPPSU) is examined here with a focus on its intrinsic ability to undergo thermally induced cross‐linking. Owing to its high density of sulfonic acid groups, SPPSU forms sulfone bridges during solvothermal treatment, generating a covalently cross‐linked network. Thermal treatment at 180°C for 21–36 h enables controlled tuning of cross‐linking through progressive consumption of sulfonic groups, reflected by a reduced ion exchange capacity. FTIR spectroscopy confirms sulfone bridge formation, while TGA and DSC reveal decreased water uptake, enhanced thermal stability, and a glass‐transition shift from 202°C to 230°C. The resulting networks show markedly improved hydrolytic stability and become insoluble. Atomic force microscopy indicates preserved homogeneous morphologies with low roughness (< 2 nm), and mechanical analyses demonstrate significant matrix reinforcement. Despite partial loss of sulfonic sites, the optimized membranes retain high proton conductivity, reaching 130 mS/cm. These results highlight the dual function of sulfonic groups as both ionic moieties and reactive precursors for designing robust fluorine‐free ionomers.