Effect of Block Length on the Properties of Multiblock Polysulfone-Poly(diallylpiperidinium hydroxide) Anion Exchange Membranes

Abstract

Research in anion exchange membranes (AEM)­s has continued with the development of materials bearing base stable cations. Designing AEMs that microphase separate into hydroxide conductive, hydrophilic domains within a hydrophobic and mechanically robust matrix has been shown to be successful for improving AEM performance. A series of multiblock polysulfone-poly­(diallylpiperidinium hydroxide) copolymers (PSf-PDApipOH) of similar hydrophobic/hydrophilic composition was prepared in which the molecular weight of the hydroxide conducting PDApipOH segments was varied. The variable hydrophilic segment molecular weight was designed to assess the impact on microphase separation, hydroxide conductivity, and water management. The multiblock copolymers investigated were prepared by condensation polymerization of preformed 4-fluorophenyl sulfone terminated poly­(diallylpiperidinium hexafluorophosphate) (PDApipPF6) oligomers with polysulfone monomers. The structure–property relationship between the molecular weight of the conductive PDApipOH segments and AEM performance was demonstrated by evaluation of the microphase separation, water uptake, and hydroxide conductivity. Membranes fabricated from the polysulfone-poly­(diallylpiperidinium hexafluorophosphate) (PSf-PDApipPF6) multiblock copolymers were shown to form well-connected conductive domains by SAXS and atomic force microscopy experiments. The PSf-PDApipOH membranes were highly conductive with the maximum hydroxide conductivity reaching 62.9 mS·cm–1 at 60 °C and 95% relative humidity. Furthermore, it was demonstrated that the conductivity increased with increasing PDApipOH segment molecular weight.