An in-situ RAFT polymerization technique for the preparation of poly(N-vinyl imidazole) modified Cloisite nanoclay to develop nanocomposite PEM

Abstract

In this work, we have carried out in-situ RAFT polymerization of poly (N-vinylimidazole) (PNVI) in the inter-galleries of Cloisite nanoclay. The polymerization was carried out in two sets of different solvent-initiator combinations: N, N-Dimethyl formamide (DMF) - azobisisobutyronitrile (AIBN) and water/ethanol mixture - 4,4′-azobis (4-cyanovaleric acid) (ACV), with varying monomer ratios in order to synthesize PNVI of three different molecular weights. The three PNVI modified Cloisite clays (named as CP-1, CP-2 and CP-3 corresponding to low, medium and high molecular weight PNVI, respectively) were characterized thoroughly. X-ray diffraction and field emission scanning electron microscope analysis revealed the extent of delamination of the clay layers after the polymerization. The CP-3 clay, containing high molecular weight PNVI, was completely exfoliated, whereas the CP-1 clay with low molecular weight PNVI formed intercalated structure and CP-2 showed partial exfoliation. Gel permeable chromatography was used to determine the molecular weights of PNVI and the thermogravimetric analysis revealed the quantities of PNVI polymerized in the clays galleries. Further, the PNVI modified clays were used to prepare nanocomposites with poly (4,4′-diphenylether-5,5′-benzimidazole) (OPBI). All the nanocomposite membranes exhibited higher storage modulus (up to ~170% increase at 400 °C), tensile properties, acid doping levels (~30 mol/OPBI repeat unit), proton conductivity (0.19 S/cm at 180 °C) and controlled acid leaching. The CP-3 clay, with exfoliated clay layers and freely dispersed PNVI chains in the OPBI matrix, resulted in effective interfacial interactions with the OPBI chains and consequently demonstrated higher property enhancement of the nanocomposite membranes than when the other two clays were incorporated.