Investigation On Free Radical Evolution And Optical Degeneration Of Proton-irradiated TiO₂/PI Materials

As investigated objects after100keV proton irradiation, polyimide （PI） and TiO₂-nano film deposited polyimide （TiO₂/PI） were studied the evolution of irradiation-induced free radicals and optical property degradation using electron paramagneticresonance spectroscopy and spectrophotometer. Then a degradation model of opticalproperty was established for polyimide under various proton fluxes. In this paper, thedamage mechanisms of free radicals and optical degradation modes were alsoinvestigated in TiO₂/PI materials.The results indicate that100keV proton irradiation induces pyrolytic carbonradicals in polyimide without change with proton flux. As the irradiated polyimide wasaged in ambient, the free radicals would decay following a sum of exponential andlinear modes with the aging time, where the exponential mode is related with thermo-dynamical radical recombination with the time constant of0.77h and the linear modelrelated with the surface neutralized reactions of radicals. On the basis of the evolutionformula of free radical in ambient, one could extrapolate the data to obtain the freeradical population in polyimide in vacuum after proton-irradiated with various fluxes. Itis interested to be found that the radical population evolution in vacuum with protonflux could be equivalent to an exponential mode with irradiation time in the vacuumchamber, wherein the time constant τradical-vacuumis1.88h longer than that of0.77ofτradical-air. It was analyzed the mechanism on the time constant difference.On the basis of the free radical damage mechanisms of polyimide after protonirradiation with different flux, the corresponding optical degradation modes of thematerial would be obtained following similar ones as those of free radical population inambient and vacuum, where the corresponding time constants are the same as those offree radicals population as τoptical-airof0.77h and τoptical-vacuumof1.90h.These resultswould be beneficial to evaluate the optical behaviors of polyimide applied in orbit.After surface-modified by nano-TiO₂film on polyimide, there are just pyrolyticcarbon free radicals induced by proton irradiation without changing with the filmthicknesses. However, the induced radicals were determined distributed in thepolyimide bulk and at the interface of TiO₂/PI, respectively. The radical population andthe optical degradation increases with proton fluence and the TiO₂thickness was foundslight effects on the changes. Due to the different distribution of free radicals in TiO₂/PIsamples, the free radical populations located in the bulk and interface shows similarexponential evolution with the aging time in ambient, but there is different timeconstant, thus the total radical population decreases with aging time following a summode of two exponential functions. Correspondingly, similar degradation modes were also obtained for optical property of TiO₂/PI sample after proton irradiation. It shouldbe noted that the time constant for free radical decaying in polyimide bulk is the same asthat of as-received polyimide in vacuum, implying that the shielding effect of denseTiO₂film could give an opportunity to obtain the recombination kinetic parameter ofirradiated free radical in polyimide.