Poly(arylene imidazolium)s: Towards stable hydroxide ion exchange membranes

Four novel and sterically-protected arylene imidazole monomers were prepared by the Debus-Radziszewski imidazolium synthesis. Homopolymerization of these monomers was carried out via Yamamoto coupling. Only one of the four monomers yielded high molecular weight poly(arylene imidazole) (PAIM) polymer. Methylation and ethylation of this polymer yield poly(arylene imidazolium), which could be cast into tough, free-standing membranes. By adjusting the degree of methylation from 50% to 100%, the ion exchange capacity (IECcl- : number of functional groups per unit mass of polymer) of the membranes were varied from 0 to 2.58 mmol·g-1. By controlling the degree of ethylation from 50% to 100%, the IECcl- were varied from 0 to 2.40 mmol·g-1. The IECcl- affected the water uptake and dimensional swelling of membranes. With IECcl- increased, both water uptake and dimensional swelling became larger. The chloride ion conductivity of membranes increased up to 15 and 10 mS·cm-1 by increasing the IECcl- of the methylated and ethylated membranes to 2.58 mmol·g-1 and 2.40 mmol·g-1, respectively. The membrane with ethylation exhibited more excellent stability in caustic solutions, showing only 2% degradation in 10 M KOH at 80 °C after 168 hours, and longer elongation at break under ambient conditions compared to the membrane with methylation.