EFFECT OF SIMULATED LOW-EARTH-ORBIT RADIATION ON POLYIMIDES (UV DEGRADATION STUDY)

UV degradation of polyimide films in air and vacuum were studied using UV-visible, ESR, FTIR, and XPS spectroscopies. The UV-visible spectra of polyimide films showed a blue shift in the absorption compared to Kapton. This behavior was attributed to the presence of bulky groups a nd kinks along the polymer chains which disrupt the formation of a cha rge-transfer complex. The UV-visible spectra showed also that UV irrad iation of polyimides result extensively in surface degradation, leavin g the bulk of the polymer intact. ESR spectra of polyimides irradiated in vacuum revealed the formation of stable carbon-centered radicals w hich give a singlet ESR spectrum, while polyimides irradiated in air p roduced an asymmetric signal shifted to a lower magnetic field, with a higher g value and line width. This signal was attributed to oxygen-c entered radicals of peroxy and/or alkoxy type. The rate of radical for mation in air was twofold higher than for vacuum irradiation and reach ed a plateau after a short time. This suggests a continuous depletion of radicals on the surface via an ablative degradation process. FTIR, XPS, and weight loss studies supported this postulate. An XPS study of the surface indicated a substantial increase in the surface oxidation after irradiation in air. The sharp increase in the C-O binding energ y peak relative to the C-C peak was believed to be associated with an aromatic ring-opening reaction. (C) 1995 John Wiley & Sons, Inc.