Influence of thermal degradation and saline exposure on dielectric permittivity of polyimide

This paper studies the relationship between dielectric permittivity and mechanisms of thermal degradation of polyimide (PI) film in air and saline exposure. The real permittivity and loss factor of dried PI were measured at temperatures from −140 to 180 °C and frequencies from 1 Hz to 1 MHz. Two peaks in the temperature-dependent loss factor revealed the occurrence of β- and γ-relaxations in PI. Following thermal degradation at 475 °C for 3 h, the real permittivity of PI was observed to increase by 14% because of the formation of free radicals. The intensity of β-relaxation was also greatly increased after thermal degradation due to scission of chemical bonding in imide groups. Activation energy obtained from the Arrhenius law for γ-relaxation did not change after thermal degradation. For samples exposed to either distilled water or salt water, the variation in salinity did not significantly influence the dielectric permittivity.

The dielectric loss factor of dry polyimide film (a) and polyimide film degraded at 475 °C for 3 hr (b), measured over frequency range 1 Hz to 1 MHz and temperature range −140 to 180 °C.Figure optionsDownload as PowerPoint slideHighlights
► Following heating, the real permittivity of polyimide is increased.
► Intensity of β-relaxation of polyimide is greatly increased.
► Peak of γ-relaxation is shifted to higher frequency on heating.
► Changes are associated with pyrolysis of imide groups during heating.
► Permittivity and loss factor of polyimide immersed in saline are increased.