Radiation-Induced Modifications in Polymeric Materials
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Radiation-Induced Modifications in Polymeric Materials David Hui1, Magdalena D. Chipara2 1
University of New Orleans, Department of Mechanical Engineering, New Orleans, LA. PartTec Ltd, Bloomington, IN
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ABSTRACT Radiation-induced modifications in polymeric materials are briefly reviewed. The attention is paid to the effect of ionizing radiation on the mechanical properties of polymers. The competition between several parallel degradation processes in the harsh conditions of the space environment is analyzed. The need for a complex analysis of the effect of polymeric materials in simulated space environments is demonstrated. INTRODUCTION Polymeric materials present a particular importance for space applications due to their reduced weight, good mechanical properties, and potential multifunctional capabilities. Nevertheless, the use of polymeric materials is restricted by the adverse conditions of the space environment. The complex space environment implies extreme temperatures, ionizing radiation, space debris, micrometeorites, and low gravity. The present review is restricted to the effect of ionizing radiation on the physical properties of polymers. The main long-term effect of the interaction of ionizing radiation with polymeric materials consists in the change of the average molecular mass and distribution [1-5]. In polymers that undergo dominant cross-linking reactions during irradiation, the average molecular mass is increased by irradiation while, if chain scissions reactions are dominant, the average molecular mass is decreased. These changes shift the glass transition temperature of the polymer to a higher value (dominant cross-linking reactions) or to a lower value (dominant chain-scission reactions). Assuming that before and after irradiation the polymer is linear, the shift of average molecular weight will increase (for dominant radiation-induced cross-linking reactions) or will decrease (for dominant radiationinduced scission reactions) the melting temperature of the polymeric target. The presence of gases affects significantly the degradation of polymeric materials. For carbon based polymers, the macromolecular chain is sensitive to oxygen attack. In Low Earth Orbit (LEO) and Geostationary orbit (GEO), the presence of atomic oxygen introduces a very fast degradation pathway that will compete and interfere with the radiation-induced degradation [6]. As a result, a rapid failure of polymeric materials in LEO and GEO orbits is possible. In most cases, the ionizing radiation component of the space environment involves electromagnetic radiation and accelerated particles (starting from electrons and protons up to heavy nuclei). The fluence of charged particle in the
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space environment decreases as the atomic mass of the ionized particle is increased. An exception has been noticed for accelerated Fe ions [6]. The equilibrium of scissions and cross-linking reactions is sensible to sample’s temperature. An important parameter is the position of the irradiation/storage temperature relative
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