Pyrrolidinium-functionalized poly(arylene ether sulfone)s for anion exchange membranes: Using densely concentrated ionic groups and block design to improve membrane performance

Abstract

To design and construct high ionic conductive anion exchange membranes (AEMs), tetra-pyrrolidinium-based block poly(arylene ether sulfone)s were synthesized. Pyrrolidinium groups were densely and controllably introduced into the polymer scaffold using a novel tetra-functionalized monomer, 4, 4′-oxybis (2,6-bis(pyrrolidinyl-1-methyl) phenol) (OBBPYP), which was synthesized via the Mannich reaction. To obtain chemically stable pyrrolidinium cations, the alkaline stabilities of pyrrolidinium-based cationic model compounds with various N-substituted alkyl chains (methyl, ethyl, butyl) were studied using 1H nuclear magnetic resonance (1H NMR) spectroscopy, in which methyl as the N-substituent group exhibited the highest alkaline stability. Based on the results above, a targeted, well-controlled, multiblock structure AEM (QQBPES-2.4OH) with densely concentrated pyrrolidinium cations was obtained via an iodomethane quaternization reaction. The densely concentrated ionic groups provide ionic nanochannels for hydroxide conduction, and the hydrophobic polymer backbone is responsible for the critical membrane mechanical support. As confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS), the QQBPES-2.4OH membrane exhibited a more obvious hydrophilic-hydrophobic microphase structure, compared to the random di-pyrrolidinium cation-based AEM (DQRPES-2.4OH) and tetra-pyrrolidinium cation-based AEM (QQRPES-2.4OH). Moreover, the QQBPES-2.4OH membrane exhibited considerably higher hydroxide conductivity, up to 68.0mScm−1 at 80°C, better flexibility and lower water swelling. The results of this work suggest a novel and scalable approach to the functionalization of polymers for AEMs.