Property evolution and molecular mechanisms of aluminized colorless transparent polyimide under space ultraviolet irradiation

Aluminized colorless transparent polyimide (CPI/Al) film has a medium absorptivity emissivity ratio, which can keep a moderate equilibrium temperature for spacecraft, however, the property evolution and relative mechanisms under vacuum ultraviolet irradiation of CPI/Al films are rarely reported. In this work, the evolution of performance and microstructure of CPI/Al films in the simulated vacuum ultraviolet (VUV) environment is systematically investigated, in which two types of CPI films induced by the trifluoromethyl group (CPI-L/Al film) and the alicyclic group (CPI-T/Al film), respectively, are compared. It is found that the optical performance of the CPI-L/Al film is degraded more severely than that of the CPI-T/Al film, indicating better VUV resistance of the CPI-T/Al film. Specifically, for CPI-L/Al and CPI-T/Al films after 2000 ESH irradiation, the yellowness index and the solar absorptance are increased by 532.9% and 503.1%, and 157.0% and 126.7%, respectively, whereas the hemisphere emissivity is decreased by 20.8% and 6.1%, respectively. This is because the oxygen-ion escapes from the surface of CPI films and oxygen-ion vacancies are formed. Moreover, the Young's modulus, tensile strength, and the onset degradation temperature of CPI/Al films are slightly degraded after the VUV irradiation, whereas the elongation at break is significantly reduced especially for CPI-L/Al and CPI-T/Al after 2000 ESH irradiation it is decreased by 90.2% and 95.1%, respectively. This can be ascribed to the competition between the cross-linking and breakage of the molecular structure during irradiation, in which the breakage is dominant in the early stage of irradiation, leading to a great degradation in properties, and cross-linking structures are formed in the later stage of irradiation which can compensate for the degradation in properties.