Maya Mizuno

Observation and Analysis of Molecular Vibration Modes in Polylactide at Terahertz Frequencies

Terahertz (THz) spectra were obtained for polylactide, and they were analyzed by numerical calculations based on density functional theory. An absorption band was observed at approximately 50 cm-1, which becomes larger when the THz electric field is parallel to the sample drawing direction. An absorption peak with a similar directional dependence appears at approximately 40 cm-1 in the simulation, which is due to normal-mode molecular vibrations parallel to the molecular helix. Since the 50 cm-1 absorption decreases with an increase in crystallinity and the permittivity shows a step-like decrease, the molecular vibrations responsible for the 50 cm-1 band seem to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm-1 with a positive absorption dependence on the samples crystallinity is likely to be due to lattice vibration, since it does not appear in the calculations, which only deal with intramolecular vibration.

Terahertz Time-Domain Spectroscopic Analysis of Molecular Behavior in Polyamide Nanocomposites

A terahertz time-domain spectroscopic study was carried out as an effective measure to analyze the mechanism that induces superior properties in polyamide-6 nanocomposites. The peak frequency of dielectric loss appearing at about 3.0 THz shifts to a lower frequency by the presence of clay nanofillers, while it is independent of the samples mechanical drawing. Comparison of such behavior with X-ray diffraction spectra revealed that restriction of molecular motion is induced by the addition of nanofillers, which should play an important role in the superior properties.

Terahertz spectroscopy as a new tool for insulating material analysis and condition monitoring

Spectroscopic and imaging analyses using electromagnetic waves in the far IR or submillimeter region [0.1-20 terahertz (THz)] are emerging technologies in the field of optics research. Thanks to recent advances in related science and technology [l]-[6], it has been proposed that THz technology be applied in many fields such as early diagnosis of cancer, detection of counterfeit bills, and moisture analysis.

Terahertz absorption spectra of oxidized polyethylene and their analysis by quantum chemical calculations

Low-density polyethylene, either cross-linked or not, was oxidized and its absorption spectra were measured in the terahertz (THz) range and infrared range. The absorption was increased by the oxidation in the whole THz range. In accord with this, infrared absorption due to carbonyl groups appears. Although these results indicate that the increase in absorption is induced by oxidation, its attribution to resonance or relaxation is unclear. To clarify this point, the vibrational frequencies of three-dimensional polyethylene models with and without carbonyl groups were quantum chemically calculated. As a result, it was clarified that optically inactive skeletal vibrations in polyethylene become active upon oxidation. Furthermore, several absorption peaks due to vibrational resonances are induced by oxidation at wavenumbers from 20 to 100 cm−1. If these absorption peaks are broadened and are superimposed on each other, the absorption spectrum observed experimentally can be reproduced. Therefore, the absorption is ascribable to resonance.

Evaluation of Applicability of Noncontact Analysis Methods to Detect Rust Regions in Coated Steel Plates

The applicability of noncontact analysis methods to the detection of rust in coated steel plates used for outdoor equipment has been examined. Reflection intensity measurements from a sample coated with epoxy resin show that optical coherence tomography using near-infrared (IR) light and THz waves can detect the hidden rust region, since near-IR light and THz waves can pass through the coating. This region can be reproduced by so-called THz imaging without contact. In contrast, neither X-ray fluorescence nor Fourier transform infrared spectroscopy can detect the rust, because of the strong X-ray and IR absorption peaks of the coating.

Observation of water trees using terahertz spectroscopy and time-domain imaging

Terahertz measurements were carried out to detect water trees grown in low-density polyethylene sheets. Water absorbs light at terahertz frequencies, fairly strongly at about 5.0 THz and rather weakly from 0.1 to 1.0 THz. Using the absorption at these frequencies, observation of water trees was tried according to the following procedures. First, we made a model sample, consisting of a polyethylene sheet, a water layer, and a copper plate, and terahertz light was irradiated to this sample vertically. The waveform and intensity of electric field of the terahertz light reflected by the sample clearly pointed out the presence of water layer beneath the polyethylene sheet by the reflection peak appearance time and the phase of reflected electric field. Secondly, water trees were grown in a polyethylene sheet, and terahertz light was scanned over the sheet. As a result, the intensity distribution of terahertz light reflected by the sample was in good agreement with the shape of the water trees. Observation of terahertz image was also carried out using the same polyethylene sheet with water trees over which a polyvinyl chloride sheet or a carbon-loaded polyethylene sheet was put to simulate the structure of a real cable. An image of water trees was also successfully observed. These results indicate that the terahertz spectroscopy can be a new characterization tool to observe the presence of water trees in a test sample taken from an aged cable.

Terahertz spectral change associated with glass transition of poly-ε-caprolactone

We measured absorption spectra of unidirectionally stretched poly-e-caprolactone (PCL) film in a range from 0.3 to 3.6 THz at temperatures from 10 to 300 K. Several absorption peaks were observed, when the electric field of THz waves was set in directions parallel and perpendicular to the stretching direction. The absorption bandwidths became significantly broad at around 200 K and above at least in two specific peaks. This temperature is close to the glass transition temperature of PCL. Further, it is shown by quantum chemical calculations that all the peaks obtained experimentally originate in skeletal vibrations of PCL. Therefore, it has become clear that a specific feature appears in the THz absorption spectrum of PCL associated with its glass transition.

Development of the complex permittivity measurement system for high-loss biological samples using the free space method in quasi-millimeter and millimeter wave bands

The free space method using a pair of lens antennas was modified for the complex permittivity measurement of biological samples from 20 to 110 GHz. Two methodologies were used to obtain the complex permittivities by the free space method, which were based on the reflection and transmission coefficients. The measurement results obtained with the two methodologies were compared with each other. The measured complex permittivities of the biological samples from the free space method were then compared with those measured using the coaxial probe method. Finally, the measurement data were also compared with those from measurement methods developed in past literatures.

A new method for estimating the content of vinyl acetate in ethylene-vinyl acetate copolymer

A new method for estimating the content of vinyl acetate (VA) in ethylene-vinyl acetate copolymer (EVA) is proposed. It is a kind of optical absorption spectroscopy in a frequency range from 0.5 to 21.0 THz. Comparison of absorption spectra among sheets of EVA with differing contents of VA from 12 to 60% by weight and a sheet of low-density polyethylene containing no VA revealed that the EVA sheets exhibit relatively sharp absorption at 2.5, 6.7, and 10.5 THz and rather broad absorption at 11 to 17 THz and 17 to 20 THz. All these absorption intensities are in proportion to the content of VA, up to certain contents, which enables us to estimate the content of VA in EVA quantitatively. Among them, the absorption at 2.5 THz measured by THz time-domain spectroscopy shows the best accuracy.

Vibration Modes at Terahertz and Infrared Frequencies of Ionic Liquids Consisting of an Imidazolium Cation and a Halogen Anion

The terahertz and infrared frequency vibration modes of room-temperature ionic liquids with imidazolium cations and halogen anions were extensively investigated. There is an intermolecular vibrational mode between the imidazolium ring of an imidazolium cation, a halogen atomic anion with a large absorption coefficient and a broad bandwidth in the low THz frequency region (13–130 cm−1), the intramolecular vibrational modes of the alkyl-chain part of an imidazolium cation with a relatively small absorption coefficient in the mid THz frequency region (130–500 cm−1), the intramolecular skeletal vibrational modes of an imidazolium ring affected by the interaction between the imidazolium ring, and a halogen anion with a relatively large absorption coefficient in a high THz frequency region (500–670 cm−1). Interesting spectroscopic features on the interaction between imidazolium cations and halogen anions was also obtained from spectroscopic studies at IR frequencies (550–3300 cm−1). As far as the frequency of the intermolecular vibrational mode is concerned, we found the significance of the reduced mass in determining the intermolecular vibration frequency.