Cross‐Linking of Sulfonated Poly(ether ether ketone) by Thermal Treatment: How Does the Reaction Occur?

The cross‐link reaction between macromolecular chains of sulfonated poly(ether ether ketone) (SPEEK) by thermal treatment above 150 °C in presence of dimethylsulfoxide (DMSO) is investigated by various techniques, including elemental analysis, acid‐base titration, infrared spectroscopy, water uptake (WU) measurements, and thermogravimetry. The conditions of thermal treatment are analyzed. The cross‐linking reaction occurs in at least three more or less activated and deactivated positions with different activation energies, leading to different time dependencies of the cross‐link reaction. The role of residual solvent DMSO is studied particularly: the cross‐linking depends significantly on the amount of solvent in the membranes. Implications on WU and thermal stability are of particular importance for the development of high performance proton‐conducting membranes.