Sulfonated poly(arylene ether nitrile ketone) and its composite with phosphotungstic acid as materials for proton exchange membranes

A series of sulfonated poly(arylene ether nitriles ketone) (SPAENK) copolymers with controlled degree of sulfonation (DS) were synthesized by direct copolymerization of 4,4′-(hexafluoroisopropylidene)diphenol with 2,6-difluorobenzonitrile and sodium 5,5′-carbonylbis(2-fluorobenzene-sulfonate) at various molar ratio. The sulfonate degree were determined by 1H NMR and proved that the sulfonate groups were quantitatively introduced to polymer chains. The tough and ductile films were obtained by solvent casting from DMAc solution, and transformed to the acid form by ion exchange in dilute sulfuric acid. The membranes of acid-form SPAENKs showed good thermal stability up to 300 °C and exhibited increasing water uptake and proton conductivity with DS, temperature and relative humidity. The heteropolyacids (HPA)/SPAENK composite membranes were also prepared by solution casting of the acid-form SPAENK with various contents of phosphotungstic acid. The transparent composite membranes and SEM images indicated the HPA particles were well dispersed within polymer matrix. The FT-IR spectrum band shift showed that the HPA particles interacted primarily with sulfonic acid groups on the polymer backbone. After introduction of HPA particles into SAPENK matrix, the thermal stability of the membrane decreased, however, the degradation temperatures of the composite membranes were above 200 °C, satisfying the requirement of thermal stability for use in PEMFC. Compared with the pure SPAENK membranes, the composite membranes showed lower water sorption but higher proton conductivity. Further efforts on improving of affinity between HPA particles and SPAENK are ongoing in our group.