Preparation and properties of sulfonated poly(vinyl alcohol)/sulfonated poly(aryl ether ketone) copolymer with pendant crosslinked structure composite membranes

A high degree of sulfonation is necessary for sulfonated aromatic polymer proton exchange membranes to achieve high proton conductivity. Nevertheless, an elevated sulfonation level will give rise to a range of complications, including heightened swelling ratio, reduced dimensional stability, and greater methanol permeability. To address this challenge, the poly(aryl ether ketone) containing carboxyl group (PAEK-x) was synthesized using the direct condensation method. Subsequently, the sulfonated poly(vinyl alcohol) (SPVA)/crosslinked sulfonated poly(aryl ether ketone) proton exchange membranes (Cr-SPAEK-x) were prepared with Congo red as the crosslinking agent. The crosslinked composite membranes were characterized by infrared spectroscopy. It is found that these series of crosslinked composite membranes show excellent thermal properties, mechanical properties, oxidation stability and appropriate water absorption. The crosslinked structure formed between the carboxyl group of PAEK-x and the amidogen of Congo red, along with the hydroxyl of SPVA. Notably, the crosslinking reaction does not consume the sulfonic acid groups in the crosslinked composite membrane. Therefore, these series of crosslinked composite membranes show a high proton conductivity. The sulfonated poly(vinyl alcohol)/sulfonated poly(aryl ether ketone) crosslinked composite membrane with a phenolphthalin content of 100mol% (Cr-SPAEK-100) demonstrated a proton conductivity of 0.053 S·cm⁻¹ at 25℃ and 0.109 S·cm⁻¹ at 80℃. The crosslinked network structure effectively inhibits the water swelling of the membrane and improves the dimensional stability of the crosslinked composite membrane. At 20℃, the Cr-SPAEK-100 membrane with the highest water uptake exhibits a minimal swelling ratio of only 5.26%. Moreover, the dense crosslinked network structure and the inclusion of SPVA with excellent methanol barrier properties significantly reduce the methanol diffusion coefficients in the crosslinked membranes, with the highest methanol diffusion coefficients being only 3.92×10⁻⁷ cm²·s⁻¹. These series crosslinked composite membranes hold promise for applications in direct methanol fuel cells.