# Review of Radiation-Induced Effects in Polyimide
## 3 Results
### Changes in Chemical Properties
Raman spectra of PI irradiated with 25 keV protons with fluence of \(2.25\times 10^{17}\) protons/cm\({}^{2}\) demonstrated two new broad peaks at 1350 cm\({}^{-1}\) and 1583 cm\({}^{-1}\), which are assigned to the D and G broad bands of disordered and ordered graphitic materials [105], respectively, along with disappearance of characteristic peaks corresponding to the C-N (1376 cm\({}^{-1}\)), C=O (1780 cm\({}^{-1}\)), and C=C (1613 cm\({}^{-1}\) \begin{table}
\begin{tabular}{c c c} \hline
**Assignment (cm\({}^{-1}\))** & **Irradiation Absorption (cm\({}^{-1}\))** & **Characterization** \\ \hline
568 & \(\delta\)(phenyl) & Phenyl ring deformation \\
607 & \(\delta\)(phenyl) & Phenyl ring deformation \\
636 & \(\delta\)(C=O) & Carbonyl deformation \\
705 & \(\delta\)(C-N-C) & Imide deformation \\
717 & C=O & Carbonyl bending \\
725 & \(\delta\)(C-N-C) & Out-of-phase bending of imide ring \\
734 & C=O & Carbonyl bending \\
752-960 & \(\delta\)(phenyl) & Phenyl ring deformation \\
1079 & C-O-C- stretch & Stretching deformation \\
1095 & v(C-N-C) & Imide stretch \\
1117 & v(C-N-C) & Imide stretch \\
1160 & C-C & C-C bending \\
1168 & v(C-N-C) & Imide stretch \\
1188 & \(\delta\)(phenyl) & Phenyl ring deformation \\
1230 & C-N & C-N stretch \\
1243 & v(C-O-C) & Bridging C-O-C stretch \\
1260 & v(C-O-C) & Bridging C-O-C stretch \\
1290–1307 & \(\delta\)(phenyl) & Phenyl ring deformation \\
1350 & C-N & C-N stretch \\
1376 & \(\delta\)(C-N-C) & Imide stretch \\
1390 & C-N & C-N stretch \\
1456–1600 & v(phenyl) & Phenyl ring C-C stretch \\
1679–1779 & v(C=O) & Carbonyl stretch \\
1825 & Cyclic anhydides, presented in not fully cured polymer (precurser) \\
3020–3155 & v(C-H) & Phenyl ring C-H stretch \\
3488 & v(C-H) & Phenyl ring C-H stretch \\ \hline \end{tabular}
\end{table} 
Table 1: Vibrational assignments of polyimide. bands [106]. This suggests that complex chemical reactions are taking place during proton irradiation of PI, including the bond breakage of the carbonyl (C=O), aromatic ether (C-O-C) moieties and a ring opening reaction of cyclic imide (C-N) accompanied, presumably, by evolution of CO into the gas phase. As a result, the surface concentration of C increases and the PI surface is transformed into a carbon-enriched structure, as confirmed by X-ray photoelectron spectroscopy (XPS) measurements of proton-irradiated PI [87]. Furthermore, the EPR measurements of PI irradiated with 100 keV-200 MeV protons with fluences of \(4.0\times 10^{15}\) protons/cm\({}^{2}\)-\(10^{19}\) protons/cm\({}^{2}\)[107; 108; 109] revealed the formation of long-lifetime pyrolitic carbon radicals. It should be noted that lower proton energy leads to a faster increase in the pyrolytic carbon-radical-specific population with proton fluence [110; 111]. This effect can be attributed to the dependence on the rate of energy deposition with particle velocity. Exposure of PI to UV irradiation results in the formation of a high concentration of radicals as confirmed by EPR studies [112; 113; 114; 115]. These radicals originate from a broken C-N bond in the imide group and the C-O bond of the ether of PI [116]. Results of spectroscopic ellipsometry of UV-exposed PI films suggested that the UV generated radicals are situated near the surface of the film, with \(\sim\)500 nm depth of damaged layer [117].