Polysulfones with highly localized imidazolium groups for anion exchange membranes

In order to promote phase separation and properties of anion-exchange membranes (AEMs)we have prepared polysulfones with an exceptionally high local concentration of imidazolium groups. Statistical copolymers containing single dioxyphenylene rings directly functionalized with four cationic groups were synthesized by polycondensations of 4,4’-dichlorodiphenylsulfone, 4,4′-isopropylidenediphenol and tetramethylhydroquinone, followed by complete benzylic bromination and quaternization using N-methylimidazole, as well as trimethyl amine for reference. In contrast to the quaternary ammonium (QA) containing materials, the thermal stability of the imidazolium (Im) functionalized AEMs increased with the cationic content and only decomposed above 290 °C at high ionic contents. Small angle X-ray scattering of AEMs based on the copolysulfones showed distinct ionomer peaks indicating a characteristic separation distance of ~7 nm. At an ion exchange capacity of 2.2 meq. g-1, the Br- conductivity at 60 °°C reached 6 and 29 mS cm-1 for the QA and Im functionalized AEMs, respectively, despite a similar water content. Thus, the water in the latter membranes was utilized more efficiently for ion transport, possibly because the conjugated Im groups had a higher degree of ion dissociation than the QA groups