Low-density polyethylene, either cross-linked or not, was oxidized and its absorption spectra were measured in the terahertz (THz) range and infrared range. The absorption was increased by the oxidation in the whole THz range. In accord with this, infrared absorption due to carbonyl groups appears. Although these results indicate that the increase in absorption is induced by oxidation, its attribution to resonance or relaxation is unclear. To clarify this point, the vibrational frequencies of three-dimensional polyethylene models with and without carbonyl groups were quantum chemically calculated. As a result, it was clarified that optically inactive skeletal vibrations in polyethylene become active upon oxidation. Furthermore, several absorption peaks due to vibrational resonances are induced by oxidation at wavenumbers from 20 to 100cm%1. If these absorption peaks are broadened and are superimposed on each other, the absorption spectrum observed experimentally can be reproduced. Therefore, the absorption is ascribable to resonance.
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