Experimental and numerical analyses of molecular vibrations in amorphous and crystalline polylactide at terahertz frequencies

Terahertz spectra were obtained for polylactide, and analyzed by numerical calculations based on the density functional theory. An absorption band was observed at around 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 around 40 cm^-1 in the simulation, which is due to normal-mode 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 downward change, the molecular vibrations responsible for the 50 cm^-1 band seems to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm^-1 with a positive dependence on the sample´s crystallinity is likely to be due to some lattice vibration, since it does not appear in the calculations that only deal with intramolecular vibration.