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 and a Raman band at 125 cm 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 in FIR and at 125 cm in 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, the modes of which are sensitive to the thermal expansion of crystal lattice and its concomitant changes in their intermolecular interactions.

Terahertz Spectroscopy for Characterization of Hydrogen Bonding and Cross-linked Structure Dynamics in Polyurethane

“…6 THz absorption spectra measured in 5°C increments from 30 to 180°C (upper panel) and its second derivative (lower panel) a for sample A and b sample B 2.6 THz shift toward lower frequencies with temperature monotonically. In our previous studies with crystalline polymers, we observed similar red shifts in the characteristic absorption bands for hydrogen bonds with temperature [7,9,14]. The observed rates of frequency shift were approximately 0.1-0.2 THz/100°C and were attributed to the reduction of intermolecular interactions due to increased intermolecular distances resulting from thermal expansion.…”

“…The vibrational mode at 7.5 THz is also depicted using vectors. b THz absorption spectrum of sample A and vibrational modes obtained from quantum calculations method) to assign polymer spectra using DFT calculations for a fragment of the crystalline polymer [8][9][10]. However, these methods can be applied only to crystalline samples.…”

“…For example, poly(hydroxybutyrate) displays an intense absorption band at 2.5 THz, which is assigned to the coupled bending motion of C=O and CH 3 [8]. A similar vibrational band (bending motion of C=O and CH 2 ) is observed in the spectrum of poly(glycolic acid) at 2.7 THz [9]. These modes are characteristic of hydrogen bond between C=O and CH x .…”

“…Based on the THz polarization spectra and low-frequency Raman spectra, the vibrational modes of crystalline polymers were determined [6][7][8][9][10]. In addition, we developed a computational method to enable the precise assignment of each vibrational mode [8][9][10]. As a result, we determined that polymers having similar structures exhibit comparable absorption features in their THz spectra.…”

“…These modes are characteristic of hydrogen bond between C=O and CH x . Importantly, the vibration frequency red shifts at higher temperatures due to changes in hydrogen bond lengths caused by thermal expansion [7,9,10].…”