Molecular model to evaluate the impact of aging on space charges in polymer dielectrics

The understanding of space charge effects depends largely on how concurrent phenomena, such as thermal and electrical aging are understood. A model describing thermal and environmental aging of dielectric polymers is discussed. The model essentially is based on the rate theory associated with the name of Eyring. One basic feature of the model is that the proper energy term is a free energy of activation Delta G, which implies that the entropy change of the process cannot be neglected. Several examples are given to show that Delta G is related directly to some physical or chemical parameter involved in thermal or environmental aging. A model describing the mechanical aging of polymers is also discussed. It is shown that submicrocavities can be generated by mechanical stresses above a given critical level, the value of which depends on the energy of cohesion of the polymer. The size of these defects are in good agreement with the Griffith criterion. Finally, the same model adapted to electrical stresses is shown to describe the electrical aging of polyethylene. It predicts that the most deleterious influence of aging is above a given critical field (similar to the critical stress in mechanical aging), where submicrocavities are induced by the electromechanical forces associated with the ac field. Electrons can then move without scattering within the submicrocavities and this may lead to further degradation. The relations between thermal, mechanical and electrical aging and space charges in polymers are addressed briefly and domains requiring more work are mentioned.