Marina Komatsu

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.

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.

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.

Terahertz spectroscopic observation of cross-linked polyethylene aged simultaneously by heat and gamma-rays

Chemically cross-linked polyethylene irradiated by γ-rays to certain designated doses from 100 to 800 kGy at 100 °C in air was analyzed by X-ray diffraction, infrared spectroscopy and terahertz spectroscopy. It was found by infrared spectroscopy that the degree of oxidation increased when the sample was irradiated. By the terahertz spectroscopy, a peak appears at around 44 cm-1 (≃1.3 THz) when the sample was irradiated by γ-rays. By carrying out the Cole-Cole analysis on the complex permittivity spectra associated with the peak at 44 cm-1, the dipolar relaxation was found to exhibit a nearly single relaxation process. This suggests that the terahertz spectroscopy has a potential to detect the oxidative degradation of cross-linked polyethylene.

Feasibility Study on Terahertz Imaging of Corrosion on a Cable Metal Shield

The possibility of nondestructive diagnosis of corrosion on a metallic shielding layer in a power cable by time-domain imaging and spectroscopic measurements using terahertz waves was examined experimentally. First, absorbance was measured in a THz frequency range for copper oxide, basic copper carbonate, and copper chloride. Next, two copper plates, uncorroded and corroded, were placed in contact with each other horizontally, and a terahertz wave was scanned over them in order to get two-dimensional images. When the plate is corroded to form either basic copper carbonate or copper chloride, the intensity of reflected wave becomes lower than when the plate is uncorroded. It seems that the terahertz wave was scattered more on the surface of the corroded copper plate and was absorbed by the corrosion. Therefore, this research indicates the feasibility of nondestructive diagnosis to detect corrosion on a shield metal in a power cable.

Terahertz spectroscopic observation of oxidation of ethylene-propylene-diene-copolymer

Flame-retardant ethylene-propylene-diene copolymer, which is the polymer insulating material most abundantly used for safety-related electric cables in nuclear power plants, was irradiated with gamma-rays to certain designated doses from 500 to 1500 kGy at room temperature or 100 °C in air, and absorption spectra were measured at terahertz frequencies. A broad absorption is induced by the irradiation in a range from 0.5 to 3.1 THz. It is confirmed that this absorption is induced by oxidation, by comparing its frequency spectrum with spectra of polyethylene obtained numerically by doing quantum chemical calculations and experimentally by measuring its absorption at THz frequencies. Therefore, the degree of oxidation of this copolymer can be estimated by the intensity of the broad absorption at these THz frequencies.

Estimation of gel fraction of polyethylene cross-linked with silane by far-infrared absorption spectroscopy

Far-infrared absorption spectra were measured for low density polyethylene in a frequency range from 3.0 to 120 THz. When the polyethylene was cross-linked using vinyl silane, the absorption was found to increase in proportion to the cross-linking degree at 13.2, 30.9, and 32.7 THz, where absorption attributable to Si-O bonds appears. Therefore, the degree of cross-linking of polyethylene by vinyl silane can be estimated by far-infrared absorption spectroscopy.

Observation of crystal growth in polyethylene naphthalate by terahertz spectroscopy

Various properties of polyethylene naphthalate (PEN) are affected by its crystallinity. In this paper, the possibility of using terahertz (THz) spectroscopy as a tool to examine the crystallinity of PEN is studied. When THz absorption is measured while the sample temperature is being raised, an absorption peak suddenly appears at around 2.10 THz at 180 °C. By analyzing the THz spectra together with XRD patterns, we found that the 2.10-THz absorption has a close relation with the crystal form α of PEN and its intensity increases with the progress of crystallization. Therefore, THz absorption spectroscopy can be used for monitoring the crystallization of PEN.

Terahertz spectroscopic analysis of ethylene-propylene-diene copolymer

Inorganic fillers are very often added to ethylene-propylene-diene copolymer (EPDM), which is a typical rubber material used for electrical insulation, for the purpose of improving its various properties. Regarding this, although the content and dispersion of inorganic fillers exert a significant influence on the properties of EPDM, detailed research has been done scarcely. In the present research, talc and magnesium hydroxide were added to EPDM, and detection of the content and dispersion of fillers was tried by terahertz absorption spectroscopy. As a result, it has become clear that the content of talc can be estimated by the increase in absorption at 3.05 THz. The possibility of estimating the talc content from incremental values of three absorptions in a far-infrared range has also been confirmed, in addition to the 3.05-THz absorption. Furthermore, the dispersion of magnesium hydroxide fillers is likely to correspond to the scattering of absorption spectra at around 1.0 to 1.5 THz.