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

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.