The effect of crystallization conditions on radiation-induced crosslink formation in polyethylene

Abstract

The decrease in solubility with radiation dose has been measured in bulk Marlex 50 polyethylene and in a series of solution-crystallized polyethylene prepared under different conditions. It was found that the solubility–dose curves were all of a similar shape and could be superimposed by multiplying the radiation doses given to each sample by a constant factor which is a measure of the ‘effectiveness’ of the radiation. The bulk material needed the lowest gelation dose while the gelation dose for the solution-crystallized specimens depended on the method of preparation. Measurements of hydrogen gas evolution and free radical formation indicated that these results could not be explained by differences in the overall number of crosslinks formed. They can be explained by assuming that there are ‘effective’ and ‘ineffective’ crosslinks and the relative numbers of these are a function of crystallization conditions. Our results are in accord with the hypothesis that crosslinks are formed preferentially at the chain folds at lamellar surfaces and that the ‘effective’ and ‘ineffective’ crosslinks are inter- and intra-lamellar crosslinks respectively. The amount of interlamellar contact is reflected by the degree of effectiveness of the radiation.