Preparation and properties of proton exchange membrane based on poly(vinylidene fluoride) by radiation-induced grafting

Abstract

Poly(vinylidene fluoride) grafted polystyrene sulfonic acid (PVDF-g-PSSA) proton exchange membranes were prepared by electron beam pre-irradiation of styrene monomer onto commercial PVDF films and subsequent sulfonation. The effect of the reaction parameters such as solvent type, irradiation dose, monomer concentration, reaction temperature, addition of crosslinker, addition of acid, storage time and polymer film thickness on the grafting yield were studied. The grafting yield was found to increase gradually with increasing of the irradiation dose. The grafting yield was also found to increase with increasing of monomer concentration in the range of 20-100 vol%. Temperature was found to enhance the grafting reaction as indicated by the increase in the grafting yield when raise arisen from 40 to 60ºC. Addition of acid gives a significant effect on grafting yield in a way that leads to a reduction in the monomer consumption. Addition of crosslinkers was found to have a mixed effect on the grafting yield where addition of divinylbenzene (DVB) reduced the grafting yield which was increased upon addition of triallyl cyanurate (TAC). The sulfonation of PVDF grafted films in chlorosulfonic acid diluted with dichloromethane was carried out and the degree of sulfonation at all level of grafting was found almost 100%. The thermal degradation of PVDF-g-PSSA membranes was found to proceed in a multi-steps pattern owing to dehydration, sulfonation, decomposition of polystyrene grafts and lastly the matrix polymer. The membrane degree of crystallinity was reduced and melting temperature, Tm slightly shifted to the lower temperature after grafting of styrene and subsequent sulfonation due to the changes in the materials. The effect of the grafting yield on the membrane properties such as ion exchange capacity and swelling behaviour were studied and both were found to increase with the increasing grafting yield. This increasing trend was attributed to the increase in the number of sulfonic acid groups incorporated in the membranes, which was caused by a similar increase in the benzene rings obtained from grafted polystyrene readily available for sulfonation. In conclusion, the composition of the membranes in the form of grafting yield could be controlled by variation of the reaction parameters. The properties of the membranes are function of the grafting yield. Finally, the obtained PVDF-g-PSSA membranes have shown very good combinations of physical, chemical, thermal properties that makes them potential candidates for testing in electrochemical applications such as water electrolyzers and PEM fuel cells.