The effect of the type of polyethylene on the grafting of styrene on polyethylene—I. Direct radiation grafting and grafting by pre-irradiation in vacuo

Abstract

The influence of the type of polyethylene on the rate and yield of grafting of styrene was studied by using simultaneous irradiation with γ-rays and also pre-irradiation in vacuo. By both methods, the grafting yield was found to be higher in high density polyethylene, except at low temperature with thick samples. The effect of the foil thickness is the same for both methods: the grafting yield generally increases with increase in foil thickness. It is assumed that the advantageous effect of higher crystallinity and of greater thickness on the grafting rate can be explained by the greater rigidity of the polymeric matrix. This causes a lower degree of swelling, a greater gel effect and perhaps a better utilization of the radicals. In these cases, the structure of the polyethylene affects kinetic parameters, mainly the termination rate constant. During grafting, probably through the mechanical action of the growing and swelling chains, some free radicals trapped in the crystalline regions may become accessible for grafting. The creation of new grafting sites is not excluded. A rise in temperature diminishes the rigidity of the matrix: the overall grafting rate decreases and the effects of crystallinity and of sample thickness become less marked. By further increase of the rigidity of the matrix, the pressure exerted by the growing and swelling chains is incapable of loosening the structure. The grafting becomes limited by the structure; only low grafting yields and rates can be obtained. This is the case with high density polyethylene at low temperature, where the grafting yield decreases steeply with thickness. As could be expected, a rise in temperature has, in this case, the opposite effect on the rate: grafting is favoured at higher temperatures.