Thermally stimulated creep spectroscopy as a method to characterize ageing in insulating materials

Insulating polymeric materials are generally semicrystalline. Upon metastability of the amorphous phases, the properties of polymeric materials change with time if the temperature is lower than the glass transition. This spontaneous ageing is characterized by an endothermic phenomenon in differential scanning calorimetry and must be distinguished from the stimulated ageing caused by modifications of physical and/or chemical bonds caused for instance by electric stress or environmental conditions. These modifications affect fundamentally the molecular mobility of the disordered phases (paracrystalline domains, rigid amorphous domains and the true or free amorphous phase). The process by which the molecules rearrange is characteristic of the structure and the morphology of polymers. So the quantitative study by thermally stimulated creep spectroscopy (TSCr) of its kinetic can be used to quantify the ageing procedure. The TSCr spectroscopy is applied to the study of low density polyethylene (LDPE) in the temperature range -180°C/80°C