Dielectric property and charge evolution behavior in thermally aged polyimide films

Polyimide (PI) materials are widely used in modern industry due to their excellent flexibility, outstanding performance, and resistance to heat and chemicals. However, thermal degradation of PI materials has become a great challenge while its mechanism and effect on the macroscopic performance still remain uncertain. Here, we comprehensively address this issue by thermally aging PI (TA-PI) films at 300 °C in air for different periods and carrying out microstructure, thermal, dielectric, and space charge characterizations to obtain a deep insight. It was found that oxidation in the aromatic rings, charge transfer complex, and microvoids formed during thermal aging were responsible for the color deepening and opacity. Thermogravimetric analysis revealed that both low-mass volatile and non-volatile pyrolysis products were generated with the prolongation of thermal aging, thus resulting into increased relative permittivity and dielectric loss, decreased breakdown strength (36.6% lower), suppressed space charge, and restricted internal field distortion (≤6%). Low-mass polar molecules are the key to deteriorating the dielectric property of PI materials. The obtained results could provide helpful support for the design and development of PI materials for future engineering applications.