Electron Beam Radiation of Carbon Nanotubes-Polymer Composites
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Electron Beam Radiation of Carbon Nanotubes-Polymer Composites Ebrahim Najafi, Jong-Oh Lee and Kwanwoo Shin* Department of Materials Science and Engineering Gwangju Institute of Science and Technology, Gwangju, 500-712, Korea * [email protected] ABSTRACT The degradation of PMMA-CNT thin film composites was examined, using electron beam radiation as a function of CNTs concentration. The findings show that the addition of CNT fillers can significantly reinforce composites against the irradiation induced degradation of PMMA, in which polymer free radicals are mainly responsible for the progress of the degradation. The results show that CNT networks are able to effectively disperse irradiated electrons through the matrix. The saturation of reinforcement effects was observed at a concentration of CNTs of approximately 0.5 wt%. This concentration was interpreted in terms of the critical concentration needed for percolation of the CNTs network, and this was further verified by sheet resistivity measurements. Free radical interactions with CNTs were also investigated by accelerated electron beam radiation and thermogravimetric analysis. CNTs were able to inhibit the free radical proliferation during a degradation process and to reduce the extent of degradation of the composites. INTRODUCTION Irradiation with electromagnetic waves or accelerated particles induces excitation, ionization, spin-spin coupling or knock-out collisions in polymer targets. Organic molecules become electronically excited when they acquire energies typically in the range of 2 to 8 eV. [1] The excited molecules are able to participate in chemical reactions leading to the formation of chemically reactive products. Ionization reactions, on the other hand, absorb more energy and, as a result, positive ions and secondary electrons are generated. When reacting with other organic molecules, positive ions are transformed into free radicals and secondary electrons are thermalized and absorbed. [1] Radical anions formed in this manner are a further source of radicals, which are capable of inducing rapid transformations. Spin-spin coupling and knock-out collision will occur concurrently when polymers are irradiated by accelerated particles such as electrons and protons. [2] These accelerated particles transfer their energy to polymer molecules via a series of electrostatic interactions, while photons are generally absorbed by chromophoric sites on molecules in a single event. It is well established that, irradiated polymers, regardless of the source, undergo main chain scission, side chain abstraction and cross-linking reactions. These reactions lead to the formation of low molecular weight products, and eventually a drastic loss in physical mass and a change in mechanical properties. Caykara et al. [3] investigated the mechanical properties of PMMA before and after exposure to ultraviolet (UV) and showed that UV exposure reduced the PMMA’s tensile strength and elongation at the breaking point from 69.5MPa and 5.5% to 30.1MPa and 3.9%,
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