Anion exchange membranes with twisted poly(terphenylene) backbone: Effect of the N-cyclic cations
Xiu Qin Wang, Chenxiao Lin, Yang Gao, Rob G. H. Lammertink
Abstract
In order to investigate the relationship towards the cationic structures and ion exchange membrane performance, three kinds of twisted poly(terphenylene)-based anion exchange membranes (AEMs) with N-cyclic cations were prepared via facile Friedel-Crafts type polycondensation and quaternization. The steric hindrance of the N-cyclic cations is gradually increased from the small piperidinium to the sterically protected N-spirocyclic quaternary ammonium (QA). The twisted poly(terphenylene)s backbone promotes the self-assembly of the polymer chain and forms a microphase separated morphology, resulting in a highest conductivity of 68.7 mS cm−1 (80 °C) for the polymer tethered with piperidinium groups (m-TPNPiQA). The relative conductivity (conductivity/swelling ratio) of m-TPNPiQA is even higher than that of the commercial Fumapem FAA-3-50 membrane. Increasing the size of the QA is helpful to constrain water absorption and related swelling but has a negative effect on the chemical stability. β-Hofmann elimination degradation is observed for all of the AEMs during a stability test by 1H NMR analysis. The m-TPNPiQA demonstrates less than 6% ionic exchange capacity loss after 240 h in 5 M NaOH solution at 80 °C. The results demonstrated that the membrane performance is associated well with the features of the cationic groups. A high performance AEM can be achieved by grafting appropriate cations onto aryl ether-free backbone.