Sulfonated poly(arylene ether nitrile)s containing cross-linkable nitrile groups for proton exchange membranes

In order to solve the problem of high methanol crossover for direct methanol fuel cells, a series of cross-linkable sulfonated poly(arylene ether nitrile)s with pendent nitrile groups as cross-linkable groups have been synthesized through direct condensation polymerization. As far as we know, there are no detailed reports connected with nitrile groups being used as cross-linkable groups. The chemical structures of the sulfonated poly(arylene ether nitrile)s were confirmed by Fourier transform infrared (FT-IR) spectroscopy. The introduction of cross-linking network structure caused significant enhancement in thermal property and dimensional stability. Meanwhile, methanol permeability, swelling ratio and water uptake of the membranes substantially decreased after cross-linking. For instance, the water uptake and swelling ratio of SPEN-CN-70 are 1985.3% and 157% at 80 °C, while those of CSPEN-CN-70 decrease to 64.9% and 16.9%, respectively. The SPEN-CN membranes showed the methanol permeability range from 0.838 × 10− 7 to 8.24 × 10− 7 cm2 s− 1, while the CSPEN-CN membranes showed the lower permeability range from 0.156 × 10− 7 to 1.64 × 10− 7 cm2 s− 1, respectively. Most importantly, after cross-linking, CSPEN-CN-60 showed the highest selectivity of 5.37 × 105 S s cm− 3, which was 11.9 times higher than that of Nafion 117. Although the conductivity of the CSPEN-CN was lower than that of the SPEN-CN, the higher selectivity makes the CSPEN-CN as potential candidate for applications in fuel cells.