Kinetic and structural factors in graft polymerization of styrene on polyolefins

Radiation grafting of monomers on polymers is radical polymerization which usually takes place in viscous medium and quite often in a micro-heterogeneous semicrystalline matrix. The role of kinetic and structural factors in the graft polymerization was investigated. Grafting of styrene on polyethylene and polypropylene was initiated by radiation methods (direct irradiation of polymer and monomer, pre-irradiation of polymer in vacuum, pre-irradiation of polymer in air) and by chemical initiators (azoisobutyronitrile and benzoil peroxide). The effect of temperature, dose rate, polymer density and specimen thickness, addition of methanol to monomer and pre-crosslinking of polymer on the grafting was investigated. It was shown that a steady-state assumption, which is conventional in homopolymerization, can be applied for radiation grafting initiated by direct irradiation of styrene and polyethylene or polypropylene. Swelling measurements have shown that monomer supply in polymer is higher than monomer consumption and the diffusion is not rate-determined step. The rate of grafting was constant and at the dose rates lower than 102 Gy/h the square root dependence of grafting rate on dose rate was valid as classical equation for homopolymerization request. The independence of the grafting rate on the dose rate above 102 Gy/h was due to participation of primary radicals in the termination reaction. The grafting rate and yield increased with the increase of the polymer density (0.920.96 g/cm3) and the film and foil thickness (0.05-1 mm) due to the increased viscosity of the reaction medium and consequently decreased rate of termination but lower yield of grafting and very slow reaction was observed for very rigid high density thick foils, especially at lower temperature. The effect of structural factors and crosslinking on grafting reaction has been discussed.