Synthesis and properties of hydroxide conductive polymers carrying dense aromatic side-chain quaternary ammonium groups

A series of hydroxide conductive polymers QTBMs carrying dense aromatic side-chain quaternary ammonium groups has been synthesized by using a new monomer of 3,3′-di(3″,5″-dimethylphenyl)-4,4′-difluorodiphenyl sulfone and other commercial monomers via polycondensation reaction, and subsequent bromination, quaternization and alkalization. The chemical structures of the ionomers were confirmed by 1H- and 13C-NMR spectroscopy. Water uptake, swelling ratio, hydroxide conductivity, the number of bonded water per ammonium group (λ), volumetric ion exchange capacity (IECVwet), mechanical and thermal properties, and chemical stability were systematically evaluated for the series of QTBMs membranes. QTBMs showed IECs ranging from 1.02 meq·g−1 to 2.11 meq·g−1; in particular, QTBM-60 membrane with the highest IEC (2.11 meq·g−1) had very high hydroxide ion conductivity of 131.9 mS·cm−1 at 80 °C, which was attributed to the well assembled nano-channels with distinct phase separation evidenced by small-angle X-ray scattering (SAXS). It was found that the hydrated QTBMs membranes were mechanically stable with moderate water uptakes and swelling ratios, high chemical stability under the harsh alkaline conditions. This work provides a facile way to prepare anion exchange membranes (AEMs) with high performances for the application in alkaline fuel cells.