Ion Distribution in Quaternary-Ammonium-Functionalized Aromatic Polymers: Effects on the Ionic Clustering and Conductivity of Anion-Exchange Membranes

Authors

  • E. Annika Weiber,

    1. Department of Chemistry, Polymer&Materials Chemistry, Lund University, P.O. Box 124, Lund SE-221 00 (Sweden)
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  • Prof. Patric Jannasch

    Corresponding author
    1. Department of Chemistry, Polymer&Materials Chemistry, Lund University, P.O. Box 124, Lund SE-221 00 (Sweden)
    • Department of Chemistry, Polymer&Materials Chemistry, Lund University, P.O. Box 124, Lund SE-221 00 (Sweden)

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Abstract

A series of copoly(arylene ether sulfone)s that have precisely two, three, or four quaternary ammonium (QA) groups clustered directly on single phenylene rings along the backbone are studied as anion-exchange membranes. The copolymers are synthesized by condensation polymerizations that involve either di-, tri-, or tetramethylhydroquinone followed by virtually complete benzylic bromination using N-bromosuccinimide and quaternization with trimethylamine. This synthetic strategy allows excellent control and systematic variation of the local density and distribution of QA groups along the backbone. Small-angle X-ray scattering of these copolymers shows extensive ionic clustering, promoted by an increasing density of QA on the single phenylene rings. At an ion-exchange capacity (IEC) of 2.1 meq g−1, the water uptake decreases with the increasing local density of QA groups. Moreover, at moderate IECs at 20 °C, the Br conductivity of the densely functionalized copolymers is higher than a corresponding randomly functionalized polymer, despite the significantly higher water uptake of the latter. Thus, the location of multiple cations on single aromatic rings in the polymers facilitates the formation of a distinct percolating hydrophilic phase domain with a high ionic concentration to promote efficient anion transport, despite probable limitations by reduced ion dissociation. These findings imply a viable strategy to improve the performance of alkaline membrane fuel cells.

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