Advances in Grafting of Polymer Chains ‘to’ and ‘from’ the Layered-Silicate Clay Surface pp.203-224
Authors: (Vikas Mittal, Department of Chemistry and Applied Biosciences, Institute of Chemical and Bio Engineering, Swiss Federal Institute of Technology, ETH Hoenggerberg, Zurich)
Abstract: To continue with the significant enhancement in the composite properties achieved by the generation of nanoscale dispersion of high aspect ratio clay platelets, it is increasingly important to devise new ways to circumvent the problems currently faced by this technology. One such limitation posed to this technology is the inability of polyolefin or non polar matrices to intercalate the clay interlayers organically modified by conventional ammonium ions owing to the absence of positive interactions between the organic and inorganic components. This goal is only achievable if the clay is fully organophilized so as to reduce the electrostatic interactions between the clay platelets thus causing their exfoliation in the polymer matrices by using shear. The other route is the polarization of non polar matrices by the addition of amphiphilic compatibilizers but such a method generally deteriorated the mechanical performance of the composites. The complete organophilization of the clay can be achieved by specifically developed techniques like grafting of the polymer chains to the clay surface or grafting of the polymer chains from the clay surface. These techniques can be used to graft polymers of different chemical nature like polystyrene and polyacrylates and the molecular characteristics of the grafts like molecular weight or chain length can be controlled by changes in the processing conditions. Moreover, controlled polymerization techniques like nitroxide mediated polymerization, atom transfer radical polymerization and reversible addition fragmentation chain transfer polymerization can also be employed to generate controlled grafts of either homopolymers or bloc copolymers. This way, by keeping the properties of the grafts in control, their miscibility with the polymer matrices especially polyolefins can be ensured when the reacted clays are compounded with them. Apart from these techniques, gas phase polymerization of ethylene as well as propylene has also been reported by using these grafting to and from the surface approached. Various methods like peroxide initiator immobilized on the clay surface, vinyl monomer in the modification of the montmorillonite and Ziegler Natta catalysts or metallocene catalysts attached to the clay surface. When the polymerization and subsequent processing conditions are optimally designed, all of these techniques have been observed to generate more exfoliated morphologies in the composite along with enhancement in the mechanical properties of the nanocomposites. These advanced methods represent the potential of solving the long standing limitation of the polymer nanocomposites technology of non-optimal generation of polymer nanocomposites with non polar matrices.