Three different kinds of phenol-type antioxidants were added in low-density polyethylene sheets. Far-infrared and mid-infrared absorption spectra were measured for these sheets and the sheet with no antioxidants to obtain spectra of antioxidants. Furthermore, chemical structures of the antioxidants were optimized by quantum chemical calculations and possible vibrational modes and their energies were calculated numerically. As a result, it was found that our calculations simulate absorption spectra of the antioxidants successfully and that their causal vibrations in the mid-infrared range are consistent with those reported in the literature. Furthermore, it has become clear that all the major far-infrared absorption peaks are due to skeletal vibrations. This clearly explains our previous results that the three antioxidants with very similar structures can be identified by the far-infrared absorption spectroscopy.
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