Preparation And Characterization Of Anion Exchange Membranes Containing Aromatic Side-chain Quaternary Ammonium

In this paper, we started with the molecular structure design, devoted to solutions of the chanlleges of anion exchange membranes(AEMs), like low ionic conductivity, poor mechanical and and chemical stability. The content includes:A series of side-chain type ionomers(PAES-Qs) containing pendant side-chain benzyltrimethyl ammonium groups, which linked to the backbone by alkaline resisting conjugated C-C bonds, were synthesized via polycondensation, bromination, followed by quaternization and alkalization. These ionomers started with 3,3’-di(4’’-methyl-phenyl)-4,4’-difluorodiphenyl sulfone(DMPDFPS), a new monomer with two pendent benzyl groups, which was easily prepared by Suzuki coupling reaction in high yield. To assess the influence of side-chain and main-chain aromatic benzyltrimethyl ammonium on anion exchange membranes(AEMs), the main-chain type ionomers(MPAES-Qs) with the same backbone were synthesized following the similar procedure. GPC and 1HNMR results indicate that the bromination shows no reaction selectivity of polymer configurations and ionizations of the side-chain type polymers display higher conversions than that of the main-chain type ones do. These two kinds of AEMs were evaluated in terms of ion exchange capacity(IEC), water uptake, swelling ratio, λ, volumetric ion exchange capacity(IECVwet), hydroxide conductivity, mechanical and thermal properties, and chemical stability respectively. The side-chain type structure endows AEMs with lower water uptake, swelling ratio and λ, higher IECVwet, much higher hydroxide conductivity, more robust dimensional stability, mechanical and thermal properties, and higher stability in hot alkaline solution.Another series of novel cross-linked anion exchange membranes(AEMs) CPAES-Qs AEMs containing aromatic side-chain pendant quaternary ammonium groups were fabricated via a multi-step process including bromination, heating cross-linking, quaternization, and alkalization reaction. A combination of the side-chain quaternary ammonium groups and the three dimensional cross-linked structures has modified the aggregate structures of the cross-linked AEMs and greatly improved their comprehensive properties. The three dimensional cross-linked structures endow CPAES-Qs AEMs with enhanced dimensional stability, improved stabilities of ion transport channels and mechanical properties, and excellent chemical stabilities. The side-chain quaternary ammonium groups provide good performance of ion transport channels for CPAES-Qs AEMs, which have conductivities at 80 oC ranging from 42.7 to 57.5 m S cm-1and Ea values varying from 9.9 to 10.7 k J mol-1. Especially, CPAES-Q-90 has the best conductivity and excellent chemical stability simultaneously. Its decreasing amplitude in conductivity after being treated in 1 M Na OH solution at 60 oC for 28 days is 6% of that of the original membrane.Moreover, quaternary ammonium groups desely functionalized poly(arylene ether sulfone)s(QTMBs) in aromatic side-chain were synthesized via polycondensation of another novel designed monomer 3,3’-di(3’’,5’’-dimethylphenyl)-4,4’-difluorodiphenyl sulfone, bromination, followed by quaternization with trimethylamine and alkalization. The aromatic side-chain quaternary ammonium groups were densely and control by adjusting the contents of the new monmer in the QTMBs backbone. The chemical structures of the samples were confirmed by 1H and 13 C NMR spectroscopy. Water uptake, swelling ratio, hydroxide conductivity, λ, mechanical and thermal properties, and chemical stability were systematacially evaluated for the series of QTBMs membranes. Among those membranes, QTBM-60 membrane with the h IEC(2.11 meq g-1) shows very high hydroxide ion conductivity(131.9 m S/cm) at 80 oC. It was found that the hydrated QTBMs membranes are mechanically stable with moderate water uptake and swelling ratio. Long-term stability test over 1000 h indicate that the quaternary ammonium groups may degradate moderately by direct nucleophilic substitution under the harsh alkaline conditions. This work provides a facile way to obtain the side-chain ion clusters for the for the AEMs with high performance by Suzuki coupling reaction with high yields.