The effect of hydrostatic pressure and temperature on the permittivity of crosslinked polyethylene

The real part of the permittivity of crosslinked polyethylene cable insulation has been measured as functions of temperature and hydrostatic pressure. The results obtained are discussed in terms of the Clausius-Mosotti equation. The real part of the permittivity decreases with increasing temperature. The temperature dependence cannot be totally ascribed to the variation of density with temperature. This suggests that the polarizability of this crosslinked polyethylene decreases with increasing temperature, which is qualitatively consistent with a Debye model. The effect of applying hydrostatic pressure is to increase the real permittivity. Analysis of the effect of pressure on the dielectric behaviour on the basis of the Clausius-Mosotti model leads to a negative ∂α/∂P: the polarizability a of crosslinked polyethylene decreases with pressure. The major consequences of applying pressure are to reduce the free volume of the polymer, increasing the restricting force for free dipole rotation