Terahertz spectroscopy of poly(3-hydroxyalkanoate)s

Terahertz (THz) absorption spectra of poly(3-hydroxyalkanotes) (PHA) were measured by a terahertz time-domain spectrometer and Fourier transform far-infrared spectrometer. Clear differences were observed between the spectra of crystalline and amorphous PHA, indicating that the absorption peaks observed in the THz spectra originated in the higher-order conformation of PHA. Direction of the vibrational transition moments was obtained by the polarization spectra of stretched sample in which the crystal axis were aligned. THz spectra were also measured over a temperature range from 10 K to 465 K with a liquid helium cryostat and a heating cell. The temperature dependences of the spectra reveal frequency shifts and broadening of the absorption peaks with temperature, suggesting large anharmonicity of the vibrational potential.