Grafting of vinylimidazolium-type poly(ionic liquid) on silica nanoparticle through RAFT polymerization for constructing nanocomposite based PEM

RAFT is a unique technique for the surface functionalization of nanoparticles as it enables us to tailor-make the properties of the final brush polymers by altering the parameters like graft density, chain length etc. of the grafted polymers and also to have the preferred choice of end functional groups with narrow size distribution of the nanoparticles. In this study, we successfully grafted poly(vinylimidazolium)bromide (PVImBr) polyionic liquid (PIL) brush polymers on the surface of silica nanoparticles (SiNPs) using metal-catalyst-free and a simplified RAFT technique compared to reported methods. Two sets of nanoparticles, one with low grafting and molecular weight [PVImBr(L)-g-SiNP] and the other with high grafting and molecular weight [PVImBr(H)-g-SiNP] were prepared using the versatile 4-cyanopentanoic acid dithiobenzoate (CPDB) as RAFT agent. H-1 NMR, FTIR, DLS, TGA and FESEM confirmed the grafting of the polymer, amount of grafted polymer and their sizes. These two sets of PIL grafted nanoparticles were used to make two sets of nanocomposites with poly(4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI) at three different filler concentrations. The aim was to study the effect of PIL grafting and their molecular weights on the morphology and macro-scale properties of OPBI nanocomposites. It was found that PVImBr(H)-g-SiNP incorporated nanocomposite membranes resulted in better interfacial properties owing to the greater miscibility and interfacial interactions with imidazolium functional groups of OPBI chains. These membranes displayed greater tensile strength, storage modulus, acid loading, proton conductivity and more importantly significantly lowered acid leaching.