# Review of Radiation-Induced Effects in Polyimide
## 3 Results
### Changes in Chemical Properties  
Fourier-transform infrared (FTIR) spectroscopy is a widely used and convenient tool for the characterization of chemical changes in irradiated PI. It probes chemical bonding by optically exciting vibrational transitions within the polymer. Changes in the position and intensity of the IR absorption \"fingerprint\" of damaged material will offer insights into what chemical bonds are being modified during the radiation-induced degradation. PI has a complex IR signature with each peak corresponding to a specific vibrational mode of one bond within the monomer; some of these characteristic vibrational assignments are summarized in Table 1[92; 93; 94; 95; 96; 97; 98]. The formation of new absorption features has not been reported for PI material irradiated with low (5 keV)-or high (90 keV)-energy electrons. Instead, several interesting radiation-induced changes in the IR fingerprint of the damaged film were observed for the PI irradiated with 90 keV. First, the absorption at the wavelength associated with the carbonyl out-of-phase stretch (1679 cm\({}^{-1}\)) increased after electron bombardment. This suggests first that existing carbonyl moieties were not preferentially broken due to electron bombardment. A concurrent reduction in the absorption associated with the ether C-O-C stretch after electron bombardment suggests that ether breakage may result in new carbonyl-like moieties. Further, peak intensity at 1780 and 1376 cm\({}^{-1}\) increases after irradiation, which is generally attributed to the formation of imide rings, possibly leading to the formation of new \(\pi\)-bonded carbon structures [56]. PI irradiated with 5 keV electrons demonstrated evidence of swelling due to irradiation-induced cross-link formation [99] manifesting itself as a re-positioning of imide deformation bands in the FTIR spectrum of irradiated PI [71]. Electron paramagnetic resonance (EPR) is another tool to probe the nature of chemical changes in irradiated material by quantification of the radiation-induced unpaired electron concentration [65; 100]. The EPR spectra of PI irradiated with 90 keV electrons to the dose of 56 MGy reveals formation of ketone (predominant), phenyl and phenoxy radicals originating from the rupture and subsequent rearrangement of the imidic rings and homolytic bond scission of ether bridges, respectively. Carbon-13 nuclear magnetic resonance (\({}^{13}\)C NMR) spectra of irradiated and pristine PI revealed no qualitative differences such as removal or addition of peaks initiated by irradiation [101; 102]. The nearly identical chemical shifts of the carbon resonances for irradiated and pristine samples imply that there is no effect of electron irradiation in the main chemical structure of the PI film for the electron doses investigated. The stability of the structure is believed to be due to the absence of aliphatic hydrogen in Kapton which facilitates the recombination of phenyl radicals and self- mending of irradiated Kapton [103]. The absence of new peaks can also be a result of the low relative population of \({}^{13}\)C to \({}^{12}\)C. Because the relative abundance of \({}^{13}\)C to \({}^{12}\)C is \(\sim\)1%, a significant portion of the bonds in the material would have to be modified before a new peak would be apparent in the NMR spectrum. FTIR spectra of aluminized PI film irradiated with 90 keV protons and maximum fluence of \(1.0\times 10^{16}\) protons/cm\({}^{2}\) did not demonstrate new features; however, decreased intensity and widening of vibrational bands associated with C-C stretching of phenyl ring (1515 cm\({}^{-1}\)), imide =C-N stretching (1390 cm\({}^{-1}\)), and asymmetric stretching of aromatic ether C-O-C (1260 cm\({}^{-1}\)) was reported [104].