ESR Detection of X-Ray-Induced Free Radicals in Crosslinked Silica Aerogels
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ESR Detection of X-Ray-Induced Free Radicals in Crosslinked Silica Aerogels Benjamin M. Walters1, Ramón V. León2, Muhammad S. Jahan1, and Firouzeh Sabri1 1
Department of Physics and Materials Science, University of Memphis, Memphis TN, 38152, U.S.A. 2
Department of Statistics, Operations, and Management Science, University of Tennessee, Knoxville, TN, 37996, U.S.A. ABSTRACT Aerogels are a promising material for aerospace applications and have recently been explored for biomedical applications also. In both environments, exposure to radiation is inevitable, such as from radiation in space or, radiation-based sterilization and tracking of implants. X-ray radiation, in particular, is of a concern. Here, polyurea-crosslinked silica aerogel (PCSA) samples were exposed to approximately 170- and 500-Gy X-irradiation at room temperature under varying environmental conditions and characterized using electron spin resonance (ESR) technique. Results obtained for PCSA were compared with those from polyether-ether ketone (PEEK) and ultra-high molecular weight polyethylene (UHMWPE) which served as benchmarks for this study. PEEK is known to be very radiation resistant, while UHMWPE is known to be less radiation resistant. All materials (PCSA, PEEK, and UHMWPE) were exposed to the same treatments and exposure conditions. Two exposure times were tested: 10 min and 30 min which corresponded to “low” and “high” conditions, as well as comparisons of nitrogen vs. air environments during exposure and post-exposure storage. Results showed significant quantities of free radicals produced in PCSA after exposure to X-irradiation which scaled with radiation dosage; quantities were in-between those produced in PEEK and UHMWPE. The storage conditions (air vs. nitrogen) also played an important role in the free radical levels detected and are reported in this study. INTRODUCTION Exposure of materials to ionizing radiation can lead to free radical creation and, if detected accurately, can indicate molecular change caused by radiation. While free radicals created in a material may not have immediate large-scale effects, they can ultimately lead to premature aging and material degradation, which are particularly undesired in cases with biomedical or spacerelated uses and applications [1-5]. As with any system, it is important to establish the necessary confidence that its components function as required when needed. The study presented here will be an important step towards understanding the role that crosslinked silica aerogels can play in applications which involve radiation exposure. It is well-known that exposure of materials to ionizing radiation such as gamma-, X-, and ultraviolet (UV)-radiation can create free radicals in materials, which can lead to adverse and unwanted changes such as premature aging [5-8]. A highly precise method to detect the presence and identify species of free radicals is electron spin resonance (ESR), also known as electron paramagnetic resonance (EPR) [9]. The ESR technique is unique in that is can directly detec
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