Article pubs.acs.org/JPCB
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 Shigeki Yamamoto,*,† Mai Miyada,‡ Harumi Sato,*,§ Hiromichi Hoshina,∥ and Yukihiro Ozaki‡ †
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan § Graduate School of Human Development and Environment, Kobe University, Tsurukabuto, Nada-ku, Kobe, Hyogo 657-8501, Japan ∥ RIKEN, 519-1399 Aramaki-Aoba, Aoba-ku, Sendai, Miyagi 980-0845, Japan ‡
S Supporting Information *
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−1 and a Raman band at 125 cm−1 showed 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−1 in FIR and at 125 cm−1 in Raman comprise the out-of-plane CO 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 latticeexpanding 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 CO 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.
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spectroscopy has started to be applied to polymer systems.20−22 In our previous studies on a biodegradable polyester, poly-(R)3-hydroxybutyrate (PHB),20,21,23 a combination of THz-TD and low-frequency Raman spectroscopies and their quantum mechanical simulations revealed that some bands are fairly related to a weak intermolecular hydrogen bonding between CO and CH3 groups among the PHB chains, as the weak hydrogen bonding had been suggested on the basis of the IR and X-ray experiments.24−27 The hydrogen bonds in PHB were recently confirmed by using new X-ray crystallographic data.28 Assignment of vibrational modes in the THz region is an issue of great importance. However, because of their delocalized nature, the assignment of the modes of polymers has been quite difficult if only based on an empirical way, in
INTRODUCTION A low-frequency vibrational spectrum of condensed matter can reflect not only intrinsic molecular structure but also intermolecular structure, which would originate from a balance between thermal motion and van der Waals interactions between molecules, for example, hydrogen bonds and steric hindrance. Hence, vibrational spectroscopy below ∼300 cm−1 has been studied for polymers from viewpoints of stacked and deformed structures of polymer chains and the weak interactions therein,1−5 and for the dynamics and relaxations of molecular aggregates in condensed phases.6−14 Crystalline polymers are attractive targets for far-infrared (FIR) and lowfrequency Raman spectroscopies, and thus low-frequency vibrational spectra of various polymer systems have been studied since the 1950s.15−19 Although interpretation in the low-frequency region is not straightforward for polymers, the spectrum can provide valuable information about structures and interactions inside. Recently, THz-Time Domain (THz-TD) © XXXX American Chemical Society
Received: November 9, 2016 Revised: January 4, 2017
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DOI: 10.1021/acs.jpcb.6b11304 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
original molecular system atom by atom. Note that dipole coupling among the fragments is considered in the calculation. The structures of fragments are optimized in the normal mode coordinate with fixing very-low-frequency normal modes (
