Dielectric Properties and Morphology in Polyethylene

Polyethylene consists of crystalline parts and amorphous parts. Trap sites for electrons or ions exist on a boundary between them. Mechanical drawing or isothermal crystallization of a bulk from its melt provides increased trap sites due to change of morphology in crystalline lamellae. The trap sites for electrons might be physical cavities, where the cavity probably is best pictured as an irregular space bounded by a particular local arrangement of molecular chains which would be changed by drawing or annealing. The TSC peak is enhanced by drawing, and also we observe particular SCL current characteristics modified by the increase of trap sites. 90% of the vinyl groups in polyethylene are reported to be located at the surface of the crystallite and are able to become trap sites. However, no increase of vinyl groups (910 cm-1 and 990 cm-1) in IR spectra could be detected in a film drawn twice. The morphology of crystalline lamellae is changed by drawing. Breakdown strength is controlled by the tilting of slip planes in crystalline lamellae due to an applied force. Crystalline blocks aligned along the tensile axis might be resistive for electrons attempting to accelerate across aligned chains. It is well known that homogeneous deformation of spherulites due to straining provides a direction for development of an electrical tree. In our experiment, the breakdown strength is higher in the case of a small difference in density between crystalline lamellae and amorphous parts.