Synthesis and properties of reprocessable sulfonated polyimides cross-linked via acid stimulation for use as proton exchange membranes

Cross-linked sulfonated polyimides are one of the most promising materials for proton exchange membrane (PEM) applications. However, these cross-linked membranes are difficult to reprocess because they are insoluble. In this study, a series of cross-linkable sulfonated polyimides with flexible pendant alkyl side chains containing trimethoxysilyl groups is successfully synthesized. The cross linkable polymers are highly soluble in common solvents and can be used to prepare tough and smooth films. Before the cross-linking reaction is complete, the membranes can be reprocessed, and the recovery rate of the prepared films falls within an acceptable range. The cross-linked membranes are obtained rapidly when the cross-linkable membranes are immersed in an acid solution, yielding a cross linking density of the gel fraction of greater than 90%. The cross-linked membranes exhibit high proton conductivities and tensile strengths under hydrous conditions. Compared with those of pristine membranes, the oxidative and hydrolytic stabilities of the cross-linked membranes are significantly higher. The CSPI-70 membrane shows considerable power density in a direct methanol fuel cell (DMFC) test. All of these results suggest that the prepared cross-linked membranes have great potential for applications in proton exchange membrane fuel cells. (C) 2016 Elsevier B.V. All rights reserved.