# Review of Radiation-Induced Effects in Polyimide
## 3 Results
### Changes in Physical Properties
#### 3.5.1 Optical  
Exposure of PI to electron or proton particles results in PI's significant darkening; PI's color changes from characteristic golden yellow of pristine PI to a dark brown color after irradiation. This is attributed to an overall increase in conjugated systems in the polymer microstructure after electron irradiation (e.g., [138], with 40 keV electrons, dose of \(2.5\times 10^{9}\) rads) and the formation of new chromophores and auxochromes, such as isoscyano (-N=C=O), ketamine (C=C=N-), and hydroxylamine (C-O-N) groups after proton irradiation (e.g., [139], with with proton energy of 90 keV, flux of \(5\times 10^{11}\) cm\({}^{-2}\)s\({}^{-1}\), fluence varied in a range of \(5\times 10^{14}\)-\(1\times 10^{16}\) cm\({}^{2}\)). Darkening of the PI material induced by electron or proton irradiation leads to increase of solar absorption of the PI that would induce strong effects on the thermal balance of spacecraft.  
Ultraviolet-visible transmission/absorption (UV-VIS) spectroscopy is widely used to access information regarding the variation in optical band-gap before and after irradiation. A red shift of the absorption edge of irradiated PI has been reported by different research groups for different electron irradiation conditions, e.g., 3 h with 5 keV electrons [140], with 2 MeV electrons to the dose of 10 kGy [141], with 90 keV electrons to the dose of 73 MGy [142], as well as different proton irradiation experiments, e.g., with 90 keV protons with fluences of \(5\times 10^{15}\) protons/cm\({}^{2}\)-\(1\times 10^{16}\) protons/cm\({}^{2}\)[104], with 2 MeV protons with fluence of \(2.6\times 10^{15}\) protons/cm\({}^{2}\)-\(1\times 10^{16}\) protons/cm\({}^{2}\)[143], with 170 keV protons with fluences of \(2\times 10^{15}\) protons/cm\({}^{2}\)-\(1\times 10^{16}\) protons/cm\({}^{2}\)[109], and with 0.25 MeV protons with fluence of \(7.0\times 10^{15}\) protons/cm\({}^{2}\)[144].  
Representative UV-VIS transmission spectra of pristine PI and PI irradiated with 90 keV electrons and protons are shown in Figure 4. The nearly zero transmittance at wavelengths less than \(\sim\)480 nm exhibited by both pristine and damaged PI is associated with \(\pi\)-\(\pi^{*}\) transitions in the aromatic groups [145; 146].  
The increased absorption in the UV/Vis spectrum of electron-irradiated PI (left panel) around 700 nm indicates an effective shrinking of the \"band gap\" to \(\sim\)1.8 eV in the damaged material due to the presence of radiation-induced trap states. Compared to the measured \"band gap\" of Kapton of 2.3 eV [147], the newly created electronic states are energetically shallow. The creation of low energy electronic states coupled with the formation of radicals suggests that traps acting as new electronic transport sites are being created in the bulk. The gradual red-shift of absorption edge of proton-irradiated PI is attributed to the increasing surplus of carbon atoms formed after radiation-induced bond-breaking followed by loss of nitrogen and oxygen atoms [148; 149]. A comparison of the two plots in Figure 4 shows the fundamental difference in the damage mechanism between electrons and protons.  
Irradiation of PI film with VUV radiation (5-200 nm) led to a darkening of the PI material and is attributed to surface carbonification, causing significantly reduced reflectance of PI film in the 250-500 nm spectral region [137]. The VUV-induced breakage of the ether bridge oxygen and carbon-nitrogen bonds leads to the high concentration of benzene group near the surface of PI film [113].