Irradiation of Annealed Polyethylene Crystals

The amount of gel formed on irradiation of polyethylene crystals and the efficiency of gelation have been studied. Films prepared by filtering crystals from suspension with a constant pressure difference, show a decrease in gel for increasing film thickness. At constant film thickness gel increases with increasing pressure difference. Low-angle x-ray diffraction shows that lamellae are more tightly packed in films giving higher gel. When films are heated above the temperature at which their crystals were grown, characteristic changes in gel occur; the principal one is a rapid increase once crystals start to thicken. Variations in crystalline morphology are of little importance. Two other changes in gel, an initial decrease on annealing and an increase on storing at room temperature, are suggested to arise from a smoothing of fold surfaces. The results are consistent with and support the hypothesis that radiation-induced crosslinking occurs preferentially between chain-folds of adjacent lamellas, reduced gelation being due primarily to imperfect lamellar contact. Alternative explanations in terms of solvent retained in aggregates are excluded.