Influence of Environmental Aging on the Dielectric and Conductivity Behavior of Polyimide

Polyimide (PI) is extensively used in aerospace applications owing to its exceptional properties. However, exposure to harsh environmental conditions, including heat and humidity, can significantly affect its performance. Understanding the effects of aging on PI is important for ensuring its long-term reliability in aerospace systems. This research presents the impact of thermal oxidative aging and humidity on the dielectric performance of polyimide (PI). Aging was conducted at 250°C for 170 hours under controlled humidity conditions, facilitating thermal oxidative degradation and moisture absorption in polyimide. The results of Scanning Electron Microscopy (SEM) analysis revealed surface degradation in aged PI, characterized by micro-voids and cracks that facilitated moisture ingress into the bulk material. The aging process led to the formation of polar molecules, including moisture-induced hydroxyl groups, which enhanced the polarization ability and AC conductivity of PI. To analyze these conductivity changes, Jonscher's Power Law and the Almond-West formalism were employed. The fitting of AC conductivity data using these models provided insights into the frequency-dependent charge transport behavior in fresh and aged PI. The extracted parameters, including DC conductivity, crossover frequency, and relaxation time, demonstrated that aging resulted in increased charge transport and a shift in the frequency response. While the bulk resistivity decreased, the carrier density and mobility were notably enhanced, leading to an increase in leakage current density. At high electric fields ($100 \text{MV} / \mathrm{m}$), the leakage charge density increased by 8.4% due to the aging process. These findings highlight the impact of both thermal oxidative and humidity-induced aging on the dielectric and microscopic properties of PI, which is essential for assessing its reliability in aerospace applications under harsh environmental conditions.