Terahertz Spectroscopy in Polymer Research: Assignment of Intermolecular Vibrational Modes and Structural Characterization of Poly(3-Hydroxybutyrate)

This paper reviews our recent spectroscopic study on poly(3-hydroxybutyrate) (PHB) for the terahertz (THz) frequency region. Clear differences are observed between the absorption spectra of crystalline and amorphous PHB, indicating that the higher order conformation of PHB determines the THz spectra. The THz polarization spectra are measured for a stretched PHB sample and the directions of the vibrational transition moments are determined. The spectra are found to have temperature dependences owing to which the peaks at lower frequencies shift with temperature, suggesting a large anharmonicity of the vibrational potential. To investigate low frequency vibrational modes more thoroughly, low-frequency Raman spectra are also measured for crystalline PHB. The observed vibrational modes in both THz and Raman spectra were successfully assigned by a quantum mechanical simulation of spectra with the Cartesian coordinate tensor transfer (CCT) method, which can provide realistic spectra of the PHB by considering the intermolecular interactions among polymer chains. The potential of CCT for the assignment of THz spectra is demonstrated. On the basis of these results, the structural change in PHB is studied. The isothermal crystallization of PHB is monitored with the help of THz absorption spectra. The observed spectra clearly show the crystal growth, and the crystallization rate is estimated by Avrami's model. Next, the isothermal crystallization spectra are analyzed using two-dimensional correlation spectroscopy (2DCOS). By using the synchronous and asynchronous data plots, the correlation between the absorption peaks and the temporal order of the changes in spectral intensity is determined.