EFFECT OF ACID ON THE RADIATION-INDUCED GRAFTING OF STYRENE TO POLYETHYLENE

It is well known that the addition of methanol to styrene accelerates the rate of the radiation-induced grafting of styrene to polyethylene. Work at this laboratory has provided an explanation of the role of methanol (1). The alcohol does not permeate the polyethylene appreciable and so its principal effect is to reduce the equilibrium concentration of the styrene in the polyethylene; despite the reduced monomer concentration, the concomitant increase in viscosity in the amorphous region of the film produces an acceleration of the grafting rate. Garnett and Yen (2) have shown that the addition of acid to the methanol-styrene system enhances the grafting rate of styrene to polypropylene still further and they attribute the effect to hydrogen atoms. Pinkerton and Stacewicz (3) point out that the experiments of Garnett and Yen were done with O2-saturated solutions. From their studies of the grafting to polypropylene of methyl methacrylate from methanol solutions, they conclude that the addition of the acid does not enhance the grafting rate, but that the addition of H2O to the alcohol-monomer solution reduces the equilibrium concentration of oxygen and, therefore, reduces the inhibiting effects associated with its presence. In this work experiments were performed in O2-free systems. Irradiation of polyethylene films immersed in styrene-methanol solutions both with and without acid, show the grafting rate is enhanced by the presence of acid despite the fact that the acid does not permeate the film. Clearly the acid has an effect beyond that suggested by Pinkerton. Experimental results are presented to show how the grafting rate depends on the composition of both the film and the liquid in which it is immersed. The results indicate that hydrogen atoms are not involved in the grafting reaction. © 1979.