Effect of high field on dielectric relaxation properties of polycarbonate

High voltage properties of dielectric materials are of great interest for the insulator industrial field. Indeed, the application of alternative or continuous electrical field may modify the physical properties of polymers. Particularly, dipoles orientate themselves in the electrical field direction. So, macromolecular chains, following the dipoles orientation, move too. Glassy state is a non-equilibrium state. Due to the energy excess, resulting from kinetic conditions during cooling from the liquid state, all glasses and especially polymers sustain a phenomenon called physical ageing or structural relaxation under the glass transition temperature. This phenomenon results in change with time of physical properties due to rearrangements of macromolecules in order to decrease their energy. This change directly affects the durability in use of polymers and molecules movements are associated with glass transition dynamics. The interaction between macromolecules movements induced by physical ageing and by electrical field is not known. In order to clarify this interaction, a model thermoplastic, polycarbonate, has been studied with dielectric spectroscopy under different AC conditions. Dielectric spectroscopy technique has been chosen because it allows simultaneously to apply an electrical field and to follow the molecular dynamics with temperature. The obtained spectra were fitted using Havriliak - Negami functions and the relaxation time distribution functions were deduced. There is a clear effect of the electrical field on the dielectric responses associated with higher excitation fields.