Facile preparation of novel cardo Poly(oxindolebiphenylylene) with pendent quaternary ammonium by superacid-catalysed polyhydroxyalkylation reaction for anion exchange membranes

Novel anion exchange membranes (AEMs) with C-C polymer backbones, cardo fragments, flexible alkyl side chains and cyclic quaternary ammonium (QA) groups were designed and synthesized. Cardo poly(oxindolebiphenylylene) polymers with pendent cyclic QAs of different length (POXIA-Br) were prepared by a superacid-catalysed polyhydroxyalkylation reaction and subsequent quaternion functionalization. This preparation process has the advantages of mild reaction conditions, simple operation, low cost and environmental friendliness. The morphology, water uptake (WU), ion conductivity, stabilities and the fuel cell performance were all investigated thoroughly. With the extension of the side chain length, both the microphase separation scale and the ionic conductivity increased. AEMs with longer pendent QAs exhibited decreased WU and better alkaline stability performance than did AEMs with shorter pendent QAs. POXIH-OH (in OH− form) with a hexyl side-chain spacer showed a maximal conductivity of 73.6 mS cm⁻¹ at 80 °C. There was no degradation observed for POXIH-OH except for loss of 9.5% of QA groups after 1200 h in 1 M NaOH at 80 °C. In addition, the AEM fuel cell test performed a high peak power density of 476 mW cm⁻² at 60 °C