Broadband infrared absorption spectroscopy of low-frequency inter-molecular vibrations in crystalline poly(L-lactide)

The broadband vibrations of poly l-lactide (PLLA) were investigated by terahertz (THz) spectroscopy in the range of 2–16 THz, infrared (IR) spectroscopy, density functional theory (DFT), and classical molecular dynamics (MD) simulations. Characteristic functional group peaks were observed in the IR spectra, which were independent of the crystallinity. A simple model analysis utilizing DFT calculations with an isolated l-lactide monomer and MD simulations for nine polymer chains in a unit cell were performed to investigate the origin of intra-molecular vibrations between molecular chains. Experimentally observed peaks in the THz region were assigned to the inter- and intra-molecular vibration modes. Results showed that classical MD calculations are a particularly powerful tool for the vibrational-mode analysis of inter-molecular interactions.