Harumi Sato

Polarization and temperature dependent spectra of poly(3-hydroxyalkanoate)s measured at terahertz frequencies

Temperature-dependent terahertz (THz) absorption spectra of poly(3-hydroxyalkanoate)s (PHAs) were measured by using a Fourier transform far-infrared (FT-FIR) spectrometer and a THz time-domain spectrometer over a temperature range of 10 K to 465 K with a liquid helium cryostat and a heating cell. Clear differences were observed between the spectra of crystalline and amorphous polyhydroxybutyrate (PHB), indicating that the absorption peaks observed in the THz spectra originated in the higher-order conformation of PHB. The polarization spectra of a stretched PHB sample were measured, and the direction of the vibrational transition moment was determined. The temperature dependences of the spectra reveal frequency shifts and broadening of the absorption peaks with temperature, suggesting large anharmonicity of the vibrational potential. The temperature shift behaviour is quite different in each transition. Some of the transitions show a blue shift, which cannot be explained by a simple anharmonic potential model. Frequency shifts of the peaks were mainly observed below 10 THz, which suggests a large anharmonicity of the vibrational potential at lower frequencies.

Higher order conformation of poly(3-hydroxyalkanoates) studied by terahertz time-domain spectroscopy

Terahertz absorption spectra of poly(3-hydroxyalkanoates) with different conformations were measured by using a terahertz time domain spectrometer. Sharp absorption peaks were observed in the spectrum of crystalline samples. The orientation direction of the transition dipole moment was investigated by the polarization spectra. The peak at 2.92 THz was assigned to a vibration of helical structure along the fiber axis, and the peak at 2.49 THz was attributed to a vibration due to the hydrogen bonding between helix structures. The temperature dependence of the spectra reflects the change in the hydrogen bonding distance and melting process of the crystalline structure.

Structural evolution in microbial polyesters.

The crystallization behavior of microbially synthesized poly(3-hydroxybutyrate) (PHB) and its copolymers [P(HB-co-HHx)] containing 2.5, 3.4, and 12 mol % 3-hydroxyhexanoate (HHx) comonomer and the melting of the resultant crystals were studied in detail using time-resolved small-angle X-ray scattering and differential scanning calorimetry. The polyesters were found to undergo primary crystallization as well as secondary crystallization. In the primary crystallization, the thicknesses of the lamellar crystals were sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization at a given temperature. The thickness of the lamellar crystals in the PHB homopolymer was always larger than that of the amorphous layers. In the copolymers, by contrast, the randomly distributed HHx comonomer units were found to be excluded from the lamellar crystals into the amorphous regions during the isothermal crystallization process. This interrupted the crystallization of the copolymer chains, resulting in the formation of lamellar crystals with thicknesses smaller than those of the amorphous layers. The lamellar crystals in the copolymers had lower electron densities compared to those formed in the PHB homopolymer. On the other hand, secondary crystallization favorably occurred during the later stage of isothermal crystallization in competition with the continuous primary crystallization, forming secondary crystals in amorphous regions, in particular in the amorphous layers between the primarily formed lamellar crystal stacks. Compared to the primarily formed lamellar crystals, the secondary crystals had short-range-ordered structures of smaller size, a broader size distribution, and a lower electron density.

Isothermal crystallization of poly(3-hydroxybutyrate) studied by terahertz two-dimensional correlation spectroscopy

The isothermal crystallization of poly(3-hydroxybutylate) (PHB) was studied by monitoring the temporal evolution of terahertz absorption spectra in conjunction with spectral analysis using two-dimensional correlation spectroscopy. Correlation between the absorption peaks and the sequential order of the changes in spectral intensity extracted from synchronous and asynchronous plots indicated that crystallization of PHB at 90 °C is a two step process, in which C-H···O=C hydrogen bonds are initially formed before well-defined crystal structures are established.

Quantum Mechanical Interpretation of Intermolecular Vibrational Modes of Crystalline Poly-(R)-3-Hydroxybutyrate Observed in Low-Frequency Raman and Terahertz Spectra

Low-frequency vibrational bands observed in the Raman and terahertz (THz) spectra in the region of 50-150 cm(-1) of crystalline powder poly-(R)-3-hydroxybutyrate (PHB) were assigned based on comparisons of the Raman and THz spectra, polarization directions of THz absorption spectra, and their congruities to quantum mechanically (QM) calculated spectra. This combination, Raman and THz spectroscopies and the QM simulations, has been rarely adopted in spite of its potential of reliable assignments of the vibrational bands. The QM simulation of a spectrum has already been popular in vibrational spectroscopies, but for low-frequency bands of polymers it is still a difficult task due to its large scales of systems and a fact that interactions among polymer chains should be considered in the calculation. In this study, the spectral calculations with the aid of the Cartesian-coordinate tensor transfer (CCT) method were applied successfully to the crystalline PHB, which include the explicit consideration of an intermolecular interaction among helical polymer chains. The agreements between the calculations and the experiments are good in both the Raman and THz spectra in terms of spectral shapes, frequencies, and intensities. A Raman active band at 79 cm(-1) was assigned to the intermolecular vibrational mode of the out-of-plane C═O + CH(3) vibration. A polarization state of the corresponding far-infrared absorption band at ∼82 cm(-1), perpendicular to the helix-elongation direction of PHB, was reproduced only under the explicit correction, which indicates that this polarized band originates from the interaction among the polymer chains. The calculation explored that the polarization direction of this band was along the a axis, which is consistent with the direction in which weak intermolecular hydrogen bonds are suggested between the C═O and CH(3) groups of two parallel polymer chains. The results obtained here have confirmed sensitivity of the low-frequency vibrational bands to the weak hydrogen bonds among the polymer chains.

Near-Infrared Spectroscopic Study of Interaction Between Methyl Group and Water in Water–Methanol Mixtures

Near-infrared spectra were measured for water–methanol mixtures with a methanol content of 0–100 wt % at 25 °C. The second derivative of the NIR spectra clearly reveals that the first overtones of the CH3 asymmetric stretching modes of methanol near 5950 and 5900 cm−1 show a downward shift by about 30 cm−1 with the increase in the concentration of methanol. This is unambiguous evidence for the direct interaction of the CH3 group of methanol with the OH group of water. Similar downward shifts were also observed for water–ethanol and water–1-propanol mixtures, although the shifts are much smaller for the water–ethanol and water–1-propanol mixtures. Based upon the results of NIR spectra, we propose a model for the interaction between the C–H bond of the CH3 group of methanol and water. This intermolecular interaction is analogous to the intramolecular “(O) CH … O” interaction in compounds having both a CH3 group binding to an oxygen atom and an OH group. This paper demonstrates novel potential of NIR spectroscopy in investigating interactions containing a C–H bond.

Far-Ultraviolet Spectra of n-Alkanes and Branched Alkanes in the Liquid Phase Observed Using an Attenuated Total Reflection—Far Ultraviolet (ATR-FUV) Spectrometer

Far-ultraviolet (FUV) spectra of n-alkanes (n = 5–14) and branched alkanes were measured in their liquid state by using a newly developed attenuated total reflection (ATR)-FUV spectrometer to investigate spectra–structure relationship in the FUV region. The n-alkanes show a broad band near 8.3 eV and a weak shoulder near 7.7 eV. The 8.3 eV band shows a lower energy shift with a significant intensity increase with the increase in the length of alkyl chain. We have assigned the 8.3 eV band to the overlap of two bands due to the transition from the highest occupied molecular orbital (HOMO) to 3p and that from the HOMO-1 to 3s based on the observation that the peak energy of the 8.3 eV band of the n-alkanes is proportional to the first ionization energy. The 7.7 eV shoulder may be due to the transition from HOMO to 3s. The intensity of the 7.7 eV band increases markedly in the order of alkanes without branch, with tertiary, and with quaternary carbon atoms. It is very likely that the forbidden transition from HOMO to 3s becomes allowed by the large decrease in symmetry upon going from the n-alkanes to the branched ones with the quaternary carbon, respectively.

Variable-temperature Fourier transform infrared spectroscopic investigations of poly(3-hydroxyalkanoates) and perturbation-correlation moving-window two-dimensional correlation analysis. Part I: Study of non-annealed and annealed poly(3-hydroxybutyrate) homopolymer.

Generalized two-dimensional correlation spectroscopy (2DCOS) and perturbation-correlation moving-window two-dimensional (PCMW2D) correlation spectroscopy were applied to explore the melting behavior of non-annealed and annealed poly(3-hydroxybutyrate) (PHB) homopolymer as studied by variable-temperature Fourier transform infrared (FT-IR) spectroscopy. The absorption band of the C=O stretching vibration was employed to investigate the structural changes during the heating process (30–200 °C). Non-annealed PHB showed a recrystallization process in the temperature range 30–120 °C. In the asynchronous 2D correlation spectrum we clearly captured the existence of two components in the crystallinity-sensitive wing of the C=O stretching mode: a well-ordered crystalline state at lower wavenumbers (1718 cm−1) and a less ordered crystalline state at higher wavenumbers (1724 cm−1). These crystallinity-sensitive bands at 1718 and 1724 cm−1, which are not readily detectable in the one-dimensional (1D) FT-IR spectra, share asynchronous cross-peaks with bands at around 1737 and 1747 cm−1 assignable to the C=O stretching absorptions due to the amorphous components. In the case of the melting process of non-annealed PHB in the temperature range 120–200 °C, it is helpful to use the PCMW2D correlation analysis, which indicates the recrystallization between 40 and 110 °C by the shift of the C=O stretching band from 1726 cm−1 to 1722 cm−1 and the sharp change to the broad amorphous C=O stretching absorption at 1747 cm−1 at the melting temperature of PHB around 190 °C. For an annealed sample of PHB only the melting behavior was observed in the PCMW2D correlation analysis by the sharp transition from the crystalline to the amorphous C=O stretching band.

Thermal Degradation of Poly(3-hydroxybutyrate) and Poly(3-hydroxybutyrate- co -3-hydroxyhexanoate) in Nitrogen and Oxygen Studied by Thermogravimetric–Fourier Transform Infrared Spectroscopy

The thermal degradation behavior of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (P(HB-co-HHx), HHx = 12 mol%) has been studied under different environmental conditions by thermogravimetric analysis (TGA) Fourier transform infrared (FT-IR) spectroscopy. It is reported that at higher temperature (>400 °C) carbon dioxide and propene are formed from the decomposition product crotonic acid in a nitrogen atmosphere, whereas in an oxygen atmosphere propene oxidizes in a further step to carbon dioxide, carbon monoxide and hydrogen. It was also found that PHB and P(HB-co-HHx) have a similar thermal degradation mechanism. The analysis of the FT-IR-spectroscopic data was performed with 2D and perturbation-correlation moving-window 2D (PCMW2D) correlation spectroscopy.

Terahertz Spectroscopy in Polymer Research: Assignment of Intermolecular Vibrational Modes and Structural Characterization of Poly(3-Hydroxybutyrate)

This paper reviews our recent spectroscopic study on poly(3-hydroxybutyrate) (PHB) for the terahertz (THz) frequency region. Clear differences are observed between the absorption spectra of crystalline and amorphous PHB, indicating that the higher order conformation of PHB determines the THz spectra. The THz polarization spectra are measured for a stretched PHB sample and the directions of the vibrational transition moments are determined. The spectra are found to have temperature dependences owing to which the peaks at lower frequencies shift with temperature, suggesting a large anharmonicity of the vibrational potential. To investigate low frequency vibrational modes more thoroughly, low-frequency Raman spectra are also measured for crystalline PHB. The observed vibrational modes in both THz and Raman spectra were successfully assigned by a quantum mechanical simulation of spectra with the Cartesian coordinate tensor transfer (CCT) method, which can provide realistic spectra of the PHB by considering the intermolecular interactions among polymer chains. The potential of CCT for the assignment of THz spectra is demonstrated. On the basis of these results, the structural change in PHB is studied. The isothermal crystallization of PHB is monitored with the help of THz absorption spectra. The observed spectra clearly show the crystal growth, and the crystallization rate is estimated by Avramis model. Next, the isothermal crystallization spectra are analyzed using two-dimensional correlation spectroscopy (2DCOS). By using the synchronous and asynchronous data plots, the correlation between the absorption peaks and the temporal order of the changes in spectral intensity is determined.