Abstract：Polymer materials have peculiar fingerprint spectrum in terahertz (THz) band, which has a potential application in the field of materials feature recognition and detection. In this study, the absorption spectra of PA66 in the THz region was studied by using terahertz time domain spectroscopy. The refractive index and dielectric constants were measured and analyzed. And the THz absorption spectra of PA66 were obtained between 0.2 and 2.3 THz. Then the vibration frequencies of PA66 were calculated using density functional theory (DFT) in the range of 0.1~10 THz. The results show that the calculated vibration frequencies of PA66 are in good agreement with experimental absorption spectra. By analyzing the vibration modes of the molecular, the absorption peaks within 0. 2~2. 3THz are generated from the oscillation of —C═O, —NH groups and the asymmetric motions of —CH2 in the backbone. And the vibration peak at 0.77 THz is generated from an out-of-plane wagging of —C═O, —NH groups, the 1.56 THz band is due to a combination of an out-of-plane wagging of —C═O, —NH groups and a rotor motion of the —CH2 group, while the vibration peak at 1.85 THz is due to an out-of-plane oscillation of —C═O, —CH2 groups, which come from adipic acid. The band centered near 4.57 THz that represents several modes associated with an in-plane wagging of the —C═O groups and a rotor motion of —CH2 groups from adipic acid, and the 7.6 THz band are associated with a wagging of —C═O and a scissor motion of —CH2, —NH groups. The results show that the absorption features in the THz regime of polymer materials is associated with the vibration motions of various groups in molecules, and the absorption peaks are generally due to the motions of wagging, rotor, oscillation and the intermolecular interactions. In addition, the asymmetric polar polymer material with N, O atoms is apt to generate dipole moments for the difference in electronegativity. Therefore, it is easy to show fingerprint characteristic peaks in the THz band, which can provide the theoretical basis and technical support for the structural analysis and identification of materials using THz technology.
Key words：PA66; THz wave; Spectral characteristic; Density functional theory(DFT); Vibration frequency
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