Detection of Ageing Effect in Polyesterimide by Terahertz Time-Domain Spectroscopy

The ageing and degradation of polyesterimide (PESI) may threat the safe operation of the electrical equipment. Due to the lack of effective experimental methods, the ageing mechanism is still under debate. In this article, we employ differential scanning calorimetry and terahertz time-domain spectroscopy (THz-TDS) at variable temperatures to investigate the glass transition temperature and molecular dynamics of PESI. Ageing sample with a duration of 30 days was prepared to study the influence of hygrothermal ageing. Three peaks emerge in both absorption spectra and dielectric spectrum of ageing sample at the range of 0.2-2.25 THz. We investigate the temperature dependence of dielectric loss for both samples. Four temperature regimes and three crossover temperatures are extracted according to the slope of fitting curves. We find that the ageing treatment has different effect on the molecular dynamics in each temperature range. This indicates the evolution of ageing at various space scales. We believe that THz-TDS has immediate application potential in the detection of ageing degree and service-life evaluation.