Polyethylene Unit Cell and Crystallinity Variations as a Consequence of Different Cross-Linking Processes

Changes of crystallinity and unit cell parameters for a low-density polyethylene as a consequence of cross-linking by dicumyl peroxide and by electron beam irradiations are compared. Polyethylene samples cross-linked by irradiation in the solid state present variations essentially only of the a unit cell parameter while those cross-linked in the melt state present similar variations of both a and b unit cell parameters. A strict analogy with unit cell variations observed for ethene random copolymers with propene and with bulkier comonomers, respectively, is shown. This suggests inclusion of cross-links for solid-state cross-linked samples, which would lead to a gradual transformation from an orthorhombic to a pseudohexagonal phase, and exclusion of cross-links for melt state cross-linked samples, which would generate tensions at the orthorhombic crystallite interphases.