High temperature performance of polyimide under accelerated thermal aging: linking chemical modifications to electrical properties

Polyimide (PI) films are widely employed as high-temperature insulation in aerospace and electrical systems, where long-term reliability strongly depends on their thermo-oxidative stability. To evaluate their aging behavior, PMDA-ODA PI films were thermally aged in air at 280°C up to 60 days. Materials were tested by nondestructive techniques: Field Emission Scanning Electron Microscope (FE-SEM), Fourier Transform Infra‐Red (FTIR) (using both ATR and transmission modes), dielectric spectroscopy, DC conductivity and thermally stimulated depolarization current (TSDC) measurements. The results reveal that the material remains stable during the first 10 days with limited oxidation. From 10 to 40 days, FTIR shows that oxidation gradually extends from the surface into the bulk, increasing the amount of carbonyl dipoles. This leads to stronger dipolar polarization, while microstructural rearrangements caused by chain scission enhance interfacial polarization and conductivity. In the later stage, both interfacial polarization and conductivity reach a stable plateau. DC conductivity and TSDC measurements jointly revealed a two-stage trap evolution: a pronounced reduction of activation energy and trap depth up to 30 days, followed by a slower stabilization phase. These findings deepen the understanding of polyimide aging and support the development of more reliable high-temperature insulation.