Observation and analysis of molecular vibration modes in polylactide at terahertz frequencies

Terahertz (THz) spectra were obtained for polylactide, and they were analyzed by numerical calculations based on density functional theory. An absorption band was observed at approximately 50 cm^-1, which becomes larger when the THz electric field is parallel to the sample drawing direction. An absorption peak with a similar directional dependence appears at approximately 40 cm^-1 in the simulation, which is due to normalmode molecular vibrations parallel to the molecular helix. Since the 50 cm ^-1 absorption decreases with an increase in crystallinity and the permittivity shows a step-like decrease, the molecular vibrations responsible for the 50 cm^-1 band seem to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm ^-1 with a positive absorption dependence on the sample's crystallinity is likely to be due to lattice vibration, since it does not appear in the calculations, which only deal with intramolecular vibration.