Synthesis and characterisation of novel fluorinated polymers bearing pendant imidazole groups and blend membranes: New materials for PEMFC operating at low relative humidity

A novel route to copolymers comprising a chlorotrifluorinated backbone and imidazole terminated pendant ethylene oxide groups has been developed for use of the resultant high molecular weight polymer as a component of proton conducting membranes in polymer electrolyte fuel cells. The synthesis route makes use of the radical initiated copolymerisation of chlorotrifluoroethylene (CTFE) with 2-chloroethylvinylether (CEVE). The resultant alternating copolymer retains vinyl side groups that are used in a second stage to graft 1-benzyl-2-(hydroxymethyl)imidazole, and the degree of grafting is controlled by the reactant ratios. Deprotection by hydrogenation generates pendant imidazole functions that are grafted through carbon, such as to leave available the –NH and –N– functions for participation in proton transfer. The polymer was subsequently used to form blend membranes with sulfonated poly(ether ether ketone) (sPEEK), and the ratio –NH/–SO3H varied between 1 and 100. Very low amounts of water (5–15 wt%) were taken up by the pure (non-blended) imidazole functionalised polymer membrane and even in the blend membranes water uptake remains remarkably low (<25 wt%) in membranes having an imidazole content ≥1 mmol g−1. The conductivity has been determined under a range of temperatures (25–120 °C) and relative humidities (25–100% RH). At 120 °C, the conductivity follows the trend of the content of grafted imidazole. Further, the dependence of conductivity on RH becomes significantly less marked as the grafted imidazole content increases, i.e. as the ratio NH/SO3H increases.