Experimental and numerical analyses of molecular vibrations in poly-ε-caprolactone at terahertz frequencies

We observed absorption spectra of poly-ε-caprolactone in a frequency range from 0.3 to 3.6 THz at temperatures from 10 to 300 K by terahertz time-domain spectroscopy and analyzed the spectra using density functional theory calculations based on the B3LYP functional. In the absorption measurements, the electric field direction of the THz wave was set in either parallel or perpendicular to the stretched direction of the sample. The spectra observed at 10 K differ significantly between the parallel and perpendicular THz waves, while such a difference is hardly seen at 300 K. The numerically analyzed results agree to some extent with the spectra. It is shown by the analysis of the vector components of the molecular vibrations obtained numerically that all the spectra observed in the THz region are assumed to be due to skeleton vibrations with vibrational period of either one or two monomer units.