Monovalent anion selective anion-exchange membranes with imidazolium salt-terminated side-chains: Investigating the effect of hydrophobic alkyl spacer length

Abstract

Ion-conductive polymers having highly ordered and well-defined phase-separated structures are potential materials for use as monovalent anion-selective membranes (MASMs). In this work, to investigate the effect of hydrophobic spacers, four side-chain-type imidazolium salt-tethered poly(arylene ether sulfone) anion-exchange membranes (AEMs) with different alkyl spacer lengths (–CH2– number: 3, 6, 9, and 12) were fabricated for use in electrodialysis (ED). Our investigations using atomic force microscopy, small angle X-ray scattering and X-ray diffraction demonstrate that the extended alkyl spacers change the nano-phase-separated structures, with an increased ion cluster space ranging from 3.29 nm to 7.76 nm, resulting from increased immiscibility between the additional hydrophobic alkyl spacers and (i) the hydrophobic aromatic backbones and (ii) the hydrophilic ion-conductive groups. In addition, with the extension of the alkyl spacer, chloride (Cl−) ion conductivity at 25 °C decreases from 16.4 mS cm−1 to 11.4 mS cm−1 and water uptake is reduced to 12.8% (20 °C). It is found that the as-prepared AEM with a hexyl alkyl spacer shows the superior perm-selectivity (Cl−/SO42−) of 7.10 (separation efficiency: 70.3%), as compared with three others (3.48, 6.81, and 4.26). This work thus presents an efficient strategy to guide the architectural design of novel MASMs.