title: Low-Frequency Vibrational Modes of Poly(glycolic acid) and Thermal Expansion of Crystal Lattice Assigned On the Basis of DFT-Spectral Simulation Aided with a Fragment Method.
abstract: Low-frequency vibrational modes of lamellar crystalline poly(glycolic acid) (PGA) were measured on Raman and far-infrared (FIR) spectra. Among the observed bands, an FIR band at ∼70 cm-1and a Raman band at 125 cm-1showed a gradual lower-frequency shift with increasing temperature from 20 °C to the melting point at ∼230 °C. Their polarization direction was perpendicular to the chain axis of PGA. Both spectra were quantum-mechanically simulated with the aid of a fragment method, the Cartesian-coordinate tensor transfer, which enabled an explicit consideration of molecular interactions between two adjacent polymer chains. Good agreement was achieved between the experiment and theory in both spectra. The temperature-sensitive bands at ∼70 cm-1in FIR and at 125 cm-1in Raman comprise the out-of-plane C═O bending motion. The temperature-dependent shifts of the low-frequency bands were successfully simulated by the DFT-spectral calculation, exploring that the main origin of the shifts is the thermal expansion of the crystal lattice. This result indicates that the thermally shifted bands may be used as an indicator of the lattice expansion of PGA. Possible changes in intermolecular interactions of PGA under temperature rising were ascribed on the basis of natural bond orbital theory. The steric repulsion between the carbonyl O atom in one chain and the H-C bond in the adjacent chain will be a dominant interaction in the lattice-expanding process, which would cause the observed thermal shifts of the bending modes. Comparisons of the spectral assignment for PGA obtained in this study and that for poly-(R)-3-hydroxybutyrate (PHB) reported by us suggest that crystalline polyesters give vibrational modes composed of out-of-plane bending motion of C═O groups between ∼70 and ∼125 cm-1, the modes of which are sensitive to the thermal expansion of crystal lattice and its concomitant changes in their intermolecular interactions.
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka University , Toyonaka, Osaka 560-0043, Japan.
authors.forename: Shigeki
authors.identifier: 0000-0002-4937-4973
authors.initials: S
authors.lastname: Yamamoto
authors.affiliation: Department of Chemistry, School of Science and Technology, Kwansei Gakuin University , Gakuen 2-1, Sanda, Hyogo 669-1337, Japan.
authors.forename: Mai
authors.initials: M
authors.lastname: Miyada
authors.affiliation: Graduate School of Human Development and Environment, Kobe University , Tsurukabuto, Nada-ku, Kobe, Hyogo 657-8501, Japan.
authors.forename: Harumi
authors.initials: H
authors.lastname: Sato
authors.affiliation: RIKEN , 519-1399 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-0845, Japan.
authors.forename: Hiromichi
authors.initials: H
authors.lastname: Hoshina
authors.affiliation: Department of Chemistry, School of Science and Technology, Kwansei Gakuin University , Gakuen 2-1, Sanda, Hyogo 669-1337, Japan.
authors.forename: Yukihiro
authors.initials: Y
authors.lastname: Ozaki