Effect of swelling on radiation-induced grafting of styrene to polyethylene

The radiation-induced grafting of low-density polyethylene in contact with styrene solution was studied. The effect of the degree of swelling of the polymer on the rate of grafting was investigated by diluting the styrene with methanol and with n-octane. For styrene-methanol solution, the rate of grafting was found to increase with degree of swelling, passing through a maximum when the sorbed solvent reaches 6.2 wt-% (70 vol-% methanol in the outside solution) and decreasing therafter. The methanol fraction of the sorbed liquid is far too small to cause precipitation of the grafted chains and inhibition of their termination rate. The dilution of styrene by octane has no effect on the swelling of polyethylene, but it decreases the grafting rate over the entire concentration range. The results are explained in terms of the concentration of sorbed monomer and the viscosity of the amorphous region of the polyethylene swollen by nonpolar liquids. Supporting evidence for the mechanism is presented in the form of grafting kinetic data as a function of dose rate (2.8 × 10²?9.5 × 10⁴ rad/hr), and post-irradiation grafting measurements for polyethylene in methanol-styrene (70/30, v/v). The data indicate that at the maximum grafting rate an optimum is achieved between a high concentration of sorbed monomer and a low viscosity for the poorly swelled polymer matrix.