Improvement of Polyimide Electrical Properties During Short-Term of Thermal Aging

An atypical enhancement of both the dielectric properties and the static (dc) conductivity in polyimide (PI) films has been observed using isothermal dielectric relaxation spectroscopy at 300degC under air atmosphere during short-term thermal aging (up to 200h). Simultaneously, an increase of the dielectric strength of PI is observed. Despite a close similarity with the typical electrical signature of the crystallization phenomenon occurring in semi-crystalline polymers, DSC measurements have not revealed the presence of an exothermic crystallization peak in PI up to 500degC thus denying this assumption. In this paper, a cross-linking reaction under oxidant atmosphere is therefore proposed for explaining such electrical improvements. Indeed, oxygen diffusion in the polymers´ bulk is generally followed by the formation of links between macromolecular chains leading the molecular and charge motions more difficult. This phenomenon, inducing usually an enhancement of the electrical and mechanical properties, is also supported by FTIR chemical changes of PI during aging and theoretical cross-linking mechanisms in presence of oxygen.