# Review of Radiation-Induced Effects in Polyimide
## 1 Introduction  
Man-made satellites are made from many different materials; among them polyimide (PI, Kapton-H(r)), a solid organic insulator with a monomeric unit of C\({}_{22}\)H\({}_{10}\)N\({}_{2}\)O\({}_{5}\)[1], has taken an honored place over more than four decades due to its insulating properties, mechanical durability, low density, and resistance to space radiation. PI mainly serves as the exterior surface of a satellite, offering an ultra-lightweight thermal insulator to provide a stable range of operating temperatures for satellite systems, as well as shielding electrical and electronic componenty from the space environment through which the spacecraft travels [2; 3; 4; 5]. This paper will review the effect of the various components of the space environment, mainly geosynchronous Earth orbit (GEO), on PI's chemical structure, as well as several material properties since these changes can have profound effects on the performance of the spacecraft. The natural GEO environment comprises electrons and protons with a broad spectrum of particle energies [6; 7; 8] as well as solar ultraviolet (UV) radiation [9]. Other particles are present in GEO including gamma rays, etc., but in terms of total energy deposition into a spacecraft, solar charged particles and UV photons are the dominant species interacting with the spacecraft surface [10]. Highly energetic electrons (0.1-10 MeV) form two toroidal belts extending out to about 60,000 km (10 Earth radii, or R\({}_{\rm E}\)) from the Earth's center, with a maximum intensity typically at about 3-5 R\({}_{\rm E}\) at the equator. Thus, although GEO orbit (6.6 R\({}_{\rm E}\)) is beyond the distance of maximum intensity, it is well within the limits of the outer belt, and satellites orbiting within it are subjected to significant penetrating electron and proton fluxes, as can be seen in Figure 1. Solar UV radiation is often divided into two regions, the near-UV region including photon energies from 6.2-3.1 eV (200-400 nm) and vacuum-UV(VUV) region corresponding to UV photons with wavelengths below 200 nm. The hydrogen Lyman-\(\alpha\) line at 121.6 nm is the strongest vacuum ultraviolet (VUV) emission line in the solar spectrum [11]. When a solid dielectric interacts with energetic particles at GEO, it is subject to a number of different destructive and non-destructive processes. Non-destructive processes can result in electronic and vibrational excitation, electron emission, radiation induced conductivity, and a host of other effects that can alter spacecraft material behavior over a wide range of time scales, e.g., [12; 13; 14]. While a detailed understanding of these processes is vital to understanding and predicting a spacecraft's performance over its mission lifetime, they lie outside the scope of this review. This article will focus on the destructive processes that lead to chemical changes in spacecraft materials and how the material's physical properties change as a manifestation of that chemical damage. The nature of the dominant mechanism of energy deposition depends strongly on the mass of the particle. Massive particles such as protons will impart large amounts of ballistic energy over a relatively short depth, displacing nuclei in the solid, exciting phonons and vibrational transitions sufficient to rupture every chemical bond [15]. Less massive particles, such as electrons, can be expected to deposit energy primarily in the form of electronic excitation. Sufficient electronic energy deposition will also rupture bonds, but the damage will be deposited over a longer trajectory and the chemical damage will be more bond-selective. UV photons are energetic enough to dissociate molecular bonds, resulting in photo-oxidation reactions that scissor or crosslink the polymer chains [16].