# Review of Radiation-Induced Effects in Polyimide
## 1 Introduction
Indeed, even very low energy ions and low mass particles such as He\({}^{+}\) ions impart enough energy to break all bonds along a penetration track in principle, but experimental studies have shown that low energy and low mass particles impart damage through distinct and relatively well defined reactions [17]. Electrons, protons, and photons incident on a surface can also cause secondary electrons to be ejected, initiating a charge imbalance near the surface. The secondary electron emission probability is strongly dependent on the incident energy of the impinging particle [18; 19; 20]. Additionally, after the kinetic energy of a charged particle has been exhausted, the proton or electron can embed itself into the bulk, creating a local charge imbalance at the penetration depth of the particle. When considering the interaction of the space environment with solid dielectrics, it is important to distinguish between energy deposition and charge deposition [21]. If the charge in a material builds to a sufficient level, this can lead to a dielectric breakdown or discharge to an adjacent (lesser charged) material. Both of these electrostatic discharges can cause extensive local material damage. primarily emphasize the usage of dielectrics as electrical materials, with little attention given to the changes of mechanical and optical properties and the underlying changes in the chemical structure of irradiated materials. The radiation-induced changes to PI's material properties are the result of radiation induced effects on the material's chemical structure. Thus, for instance, increased dark conductivity of electron-irradiated PI is attributed to the presence of radicals formed as a result of radiation-induced breakage of amide rings and/or ether bridges forming a metastable carbonyl. Increased concentration of radicals, which can act as localization sites for both electrons and holes, together with increased probability of charge release rate in radiation-damaged material leads to faster movement of the charge body thus improving the conductivity of the material [29]. Since radiation induced conductivity (RIC) in PI is controlled by several competing physical processes, it is therefore dependent upon radiation dose rate, total received radiation dose, electric field, and temperature; thus, decrease and increase of RIC depending on PI's irradiation conditions has been reported [30]. The intensity of the VUV radiation is negligible (0.11 W/m\({}^{2}\)) compared to the total electromagnetic intensity of solar radiation (1356 W/m\({}^{2}\)). However, the effects of VUV radiation should not be neglected since complete absorption of high-energy photons in spacecraft materials can induce photolysis with subsequent formation of highly chemically reactive moieties [31]. The creation of these reactive chemical structures may result in a large variety of processes from the modification of the polymer surface to complete decomposition [32; 33]. One of the critical components missing from reports of ground-based simulation of material degradation is detailed description of the sample handling procedures. It has been shown that materials damaged by charged particles or light are susceptible to quick and dramatic \"healing\" in the presence of air. This makes comparisons between experiments from different facilities extremely difficult and has created uncertainty in the primary driving mechanism for material damage in space as each facility's space degradation simulation has different strengths and weaknesses in simulating the space environment. In this review we will look at results regardless of the how the samples were handled, but where possible post-irradiation sample handling will be reported and a higher priority will be given to results using samples that were handled in vacuum or inert environments.