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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 À1 , 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 À1 in the simulation, which is due to normalmode molecular vibrations parallel to the molecular helix. Since the 50 cm À1 absorption decreases with an increase in crystallinity and the permittivity shows a step-like decrease, the molecular vibrations responsible for the 50 cm À1 band seem to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm À1 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.

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