Characterization of antioxidants in polyethylene via terahertz time-domain spectroscopy: physical insights and spectral calibration.

We investigate the physical mechanisms of the interaction between terahertz waves and antioxidant 1010 in polyethylene (PE) using terahertz time-domain spectroscopy (THz-TDS). The THz waves are generated by an InGaAs photoconductive emitter antenna and detected by an InGaAs photoconductive receiver antenna, which are both driven by a pulsed laser with a wavelength of 1560 nm, pulse repetition rate of 80 MHz, and pulse duration of 100 fs. With increasing concentrations of antioxidants, the absorption spectrum exhibits a marked increase in the intensity of the absorption peaks at 0.91, 1.20, 1.52, and 1.71 THz and a corresponding reduction in their broadening. The absorption spectrum calculated from the density functional theory-based numerical model developed in this work aligns with the experimental data, confirming the fact that THz absorption strengthens the van der Waals interactions between molecular groups, with methyl groups exhibiting stronger interactions than oxygen-containing groups, which drives collective multiatomic vibrations. The frequency-resolved calibration curve for the antioxidant concentration is optimal at 0.77 THz (coefficient of determination  = 0.99476, root-mean-square error of cross-validation = 2.67 wt.%) rather than at the absorption peak frequencies, stemming from the combined effects broadband absorption peaks. These findings provide the THz spectral response of antioxidant molecules in PE both theoretically and experimentally, and they demonstrate the potential of THz-TDS as a rapid, accurate, and non-destructive method for quantifying antioxidant concentration in polymer cables for power systems.