Effects of Crystallinity and Electron Mean-free-path on Dielectric Strength of Low-density Polyethylene

The relation between crystallinity and the mean free path of high-energy electrons in low-density polyethylene and their effects on dielectric strength was studied. Thin LDPE films obtained by vacuum evaporation were annealed at temperatures between room temperature and 100°C. Crystallinity was measured by X-ray diffraction. Electrons were injected into the film by photo-emission and the energy of electrons emitted into vacuum through the film was analyzed to obtain their energy distribution. The mean free path was then calculated from the energy distribution. It was found that the crystallinity increases and the mean free path becomes longer as the annealing temperature increases. Dielectric strength was measured by applying an impulse voltage of 1.2/50 µs at room temperature and −50°C. It was found that the breakdown field decreases with an increase in annealing temperature at both measurement temperatures. Thus, increased crystallinity makes electron transport easier and the mean-free-path longer, resulting in a lower dielectric strength.