Assessing
the Influence of Side-Chain and Main-Chain
Aromatic Benzyltrimethyl Ammonium on Anion Exchange Membranes
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Abstract
3,3′-di(4″-methyl-phenyl)-4,4′-difluorodiphenyl
sulfone (DMPDFPS), a new monomer with two pendent benzyl groups, was
easily prepared by Suzuki coupling reaction in high yield. A series
of side-chain type ionomers (PAES-Qs) containing pendant side-chain
benzyltrimethylammonium groups, which linked to the backbone by alkaline
resisting conjugated C–C bonds, were synthesized via polycondensation,
bromination, followed by quaternization and alkalization. To assess
the influence of side-chain and main-chain aromatic benzyltrimethylammonium
on anion exchange membranes (AEMs), the main-chain type ionomers (MPAES-Qs)
with the same backbone were synthesized following the similar procedure.
GPC and <sup>1</sup>H NMR 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 (IEC<sub>Vwet</sub>), 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 IEC<sub>Vwet</sub>, much higher hydroxide conductivity, more robust dimensional stability,
mechanical and thermal properties, and higher stability in hot alkaline
solution. The side-chain type cationic groups containing molecular
configurations have the distinction of being practical AEMs and membrane
electrode assemblies of AEMFCs