# Review of Radiation-Induced Effects in Polyimide
## 3 Results
### Changes in Mechanical Properties  
In general, the tensile strength of a disordered polymeric material is governed by the overall tendency of the material to crosslink or undergo chain scission under exposure to radiation [123]. Depending on the type of polymer, the type of radiation, and the dose rate, either scissioning or crosslinking may dominate. It is hypothesized that high irradiation doses received by PI facilitate the overlap of zones enriched with radiation-induced highly reactive free radicals with the subsequent crosslinking of such radicals with each other [124]. Moreover, in polymers with predominant radiation-induced crosslinking the tensile strength may increase to a maximum and then decrease with increasing irradiation, suggesting that the crosslinking density may become so high that the irradiated material embrittles [125]. Computer simulation of a polyimide slab exposed to 90 keV electron beam demonstrated the significant increase of its tensile elastic modulus, suggesting the radiation-induced crosslinking in PI film. This is supported by the experimental results from both x-ray scattering and mechanical analysis [126]. [PERSON] et al. [127] reported a 22%, 120%, and 199% increase in tensile strength, elongation, and Young's modulus, respectively, for PI material irradiated with a 6 MeV pulsed electron beam at a maximum fluence of \(4\times 10^{15}\) electrons/cm\({}^{2}\). Low-energy (8 keV-40 keV) electron irradiation of the similar material at the maximum fluence of \(1\times 10^{16}\) electron/cm\({}^{2}\) resulted in 11% and 8% decrease in tensile strength and elongation, respectively [128; 129; 130]. Irradiation of PI films with 18-20 MeV electrons to the dose of 37 MGy at 4 K also resulted in degradation of its tensile strength [131]. [PERSON] et al. [132] reported \(\sim\)30% decrease in tensile strength, \(\sim\)80% decrease in elongation at break, and \(\sim\)8% increase in Young's modulus for PI irradiated with 2 MeV electron beam to a dose of 120 MGy. In addition, dynamic mechanical analysis (DMA) performed on PI irradiated with 1 MeV electron beam to a dose of 15 MGy revealed the increase of loss tangent (tan \(\sigma\)) from 0.048 (pristine material) to 0.054 (electron-irradiated). That suggests the increase of loss modulus rather than a storage modulus, implying that irradiated PI became more viscous [133], thus supporting the hypothesis about increased chain crosslinking in irradiated PI. The effect of irradiation of PI with 1.7 MeV electrons to a maximum dose of \(5\times 10^{6}\) Gy on the tribological properties of PI was studied by [PERSON] and [PERSON] [134]. The wear rate of the irradiated PI decreased to \(5.32\times 10^{-6}\) mm\({}^{3}\)/(N \(\times\) m) compared to the \(9.55\times 10^{-6}\) mm\({}^{3}\)/(N \(\times\) m) of the pristine sample. Friction tests revealed that electron irradiation did not exert much influence on the friction coefficient of PI under the experimental conditions. The changed tribological properties of PI were mainly attributed to carbonization induced by radiation, after which the wear mechanism shifted from adhesive wear and scuffing for the pristine PI to mild scuffing for the irradiated sample. The effect of proton irradiation on the mechanical properties of PI has been investigated by several different research groups. [PERSON] and [PERSON] [135] reported reduced elongation to break (from 25% to 5%), stress to break (from \(2.4\times 10^{9}\) N/m\({}^{2}\) to 1.5 N/m\({}^{2}\)), and fracture energy (from 7 MJ/m\({}^{2}\) to 1 MJ/m\({}^{2}\)) for PI film irradiated with 3 MeV protons to the maximum dose of 75 MGy. The Young's modulus did not change significantly under the same irradiation conditions. Reduction of elongation to break after PI exposure to 45 keV-10 MeV protons with fluences of \(\sim\)\(1.0\times 10^{15}\) protons/cm\({}^{2}\) was also reported by [PERSON] et al. [128] and [PERSON] et al. [136].