Multi-block copolymers with fluorene-containing hydrophilic segments densely functionalized by side-chain quaternary ammonium groups as anion exchange membranes

Abstract

In this study, we constructed a series of multi-block copolymer ionomers with fluorene-containing hydrophilic segments densely functionalized by side-chain quaternary ammonium groups as anion exchange membranes. These ionomers were synthesized via pre-polycondensation, block co-polycondensation, bromination, quaternization and alkalization. The sequential hydrophilic/hydrophobic structures were achieved by the co-polycondensation of precursor oligomers. Quaternary ammonium groups of high local density were successively introduced to the side chains of the fluorene-containing hydrophilic blocks by the following functionalization processes. Based on the well-controlled chemical structures, the multi-block copolymer ionomers membranes show well-defined hydrophilic/hydrophobic phase separations. The well-connected, broad ionic nano-channels with Bragg spacings of approximately 37.85 nm, which were confirmed by atomic force microscopy (AFM) and small angle X-ray scattering (SAXS), enhance the performances of the AEMs. The obtained multi-block AEMs show good dimensional stability. And these membranes have rather high ionic conductivity (23.0–85.0 mS cm⁻¹, 80 °C) considering their relatively low IEC_m values (0.81–1.28 meq. g⁻¹). Moreover, the membranes have adequate mechanical properties and good thermal properties. QMPAES-X7Y17, X5Y13 and X5Y17 with IEC_m values below 1.0 meq. g⁻¹ lose only 20% of their original values after 600 h alkaline stability test. These results are promising as anion exchange membranes of alkaline fuel cells.