Comparative study of dielectric relaxation spectra of electrically and thermally aged low density polyethylene

Low-density polyethylene (LDPE) films were thermally aged in a sodium chloride aqueous solution at constant temperature (thermal aging). Some of the samples were simultaneously immersed in solution and subjected to an electric AC field (electrical aging). The dielectric relaxation spectra at 30°C in the range of 10^-5 Hz to 10⁵ Hz were obtained for unaged and aged samples. For the low frequency (LF) region (10^-5 Hz to 10^-1 Hz) the time domain technique was used. A lock-in amplifier was used for the 10^-1 Hz to 10¹ Hz medium frequency (MF) region. While for the high frequency (HF), 10^-1 Hz to 10⁵ Hz, RLC bridge measurements were performed. The main differences can be seen between electrically, thermally aged and unaged LDPE in the HF and LF regions. The LF peak is a broad peak related to localized space charge injection driven by the electric field. For electrically aged samples this peak increases in an earlier stage of electrical aging, decreasing afterwards. While in thermally aged samples the peak amplitude always increases with aging time. Finally the HF shows the beginning of a peak due to the γ and β transitions. This peak decreases with aging disappearing for the most aged samples.