Synthesis and properties of sulfonated and crosslinked poly[(vinylidene fluoride)-graft-styrene] membranes

Crosslinked proton exchange membranes were prepared by electron beam grafting of styrene with divinylbenzene, DVB, or bis(vinyl phenyl)ethane, BVPE, on to paly(vinylidene fluoride), PVDF, followed by sulfonation. The area and thickness increase, crystallinity and mechanical properties of the membranes were investigated. All these properties were influenced by the crosslinker structure and ifs concentration. The crosslinkers increase the final degree of grafting. For noncrosslinked membranes it was about 80% and with 10% of BVPE crosslinker about 180% after 8 hr. The area increase with grafting is very similar for the noncrosslinked membranes and the differently crosslinked membranes, but the crosslinkers reduce swelling for sulfonated membranes swelled in boiling wafer. DVB reduces swelling move than BVPE. With grafting, the crystallinity of the PVDF phase decreases; with 20% BVPE the decrease is smallest and with 10% DVB largest. The mechanical properties of the sulfonated swelled membranes decrease with the degree of grafting, for a 20% DVB crosslinked membrane the strength decreases to zero at degrees of grafting higher than 75%. (C) 1998 John Wiley & Sons, Ltd.