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日本語

Since polymers (plastics) were first created synthetically, metal, glass, and wooden products have been replaced with plastics and our lives in modern society are supported by a large number of plastic products. At the same time, due to the current problems of marine pollution caused by microplastics and carbon dioxide (CO₂) emitted in the plastic manufacturing process, there is a strong need to use environmentally friendly plastics. Against this backdrop, biomass-based plastic is drawing attention as it is biodegradable and highly carbon-neutral. Basically, polymer properties such as hardness, fragility, workability, and thermal stability are determined not only by the chemical composition but also by the higher-order structure related to the crystallinity, molecular chain length, and chain packing in the solid state.

However, unlike the more commonly used petroleum-based plastic, the history of biomass-based plastic is short, so the lack of fundamental understanding with respect to its high-order structure and physical properties has hindered its widespread adoption. There is a strong need to establish a non-destructive and non-invasive analysis technology to answer the question, "How can we change the higher-order structure of biomass plastics to achieve the desired function? The conventional evaluation methods include differential scanning calorimetry (DSC) and X-ray diffraction (XRD); however, DSC involves the destruction of samples while XRD presents challenges in terms of its time-consuming measurements, adverse effects on the human body, and the need for analytical expertise.

Looking for a more suitable method for evaluating polymer structures and physical properties, the research team focused on terahertz-waves (1 to 10 THz), located in the gap between radio-waves and light-waves. Using this method they conducted a spectroscopy analysis of a wide variety of polymer materials. As a result, the team successfully observed clear THz absorption peaks attributed to the higher-order structure of poly(L-lactide), one type of biomass-based plastic. It is known that changing crystallization temperature results in the formation of PLA with different crystal structures such as α-form (110 ºC) and δ-form (80 ºC). With a focus on this characteristic, the research team carefully prepared samples with different crystallization temperatures and compared the crystal structures and absorption spectra. The result uncovered for the first time that α-form and δ-form have a clear correlation in terms of peak intensities in the range of 4 to 5 THz. If this spectroscopic method is established, it will not only be easier to estimate higher-order structures by absorption spectra, but it is also anticipated that it will pave the way in physics research towards the elucidation and control of higher-order structures of various biomass-based plastics, and the discovery of new functions.

The research team leader, Associate Professor Seiichiro Ariyoshi, has long been interested in biomass-based plastic as a target for analysis, but thought it hard to approach as a material. As it happened, Satoshi Ohnishi, then in the third year of his undergraduate studies (now in the first term of the first year of a master's degree program), was also interested in biomass-based plastic. When he was surveyed for his graduate assignment, his first choice was the Tsuji-Arakawa Laboratory, which is famous for its research on polylactic acid, but by chance he was assigned to the Ariyoshi Laboratory. In that case, he asked himself, "Why don't I try terahertz spectroscopy of polylactic acid as my graduation research? This was how his research project came about. After Ohnishi acquired the first spectral data, efforts were made in earnest involving Professor Hideto Tsuji, Assistant Professor Yuki Arakawa, and Associate Professor Nobuya Hiroshiba of Osaka Institute of Technology, which led to this success. There are still a lot of matters to be investigated, but given that Ohnishi is dedicated to his research, it is natural that we have great expectations for the research.

Although the above deals only with poly(L-lactide), PLA is generally known to form stereocomplexes by mixing enantiomers (poly(L-lactide) and poly(D-lactide)). Based on the knowledge and findings to date, the research team expects to expand their observation targets into enantiomers in the future and further to gain insight into the origin of the functionality of biomass plastics by comparing the biodegradability and degradation progress by microorganisms with the unique absorption spectrum that appears in the terahertz band.
This research was conducted with Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (21H01340, 26600133) and research grants from the Foundation of Public Interest of Tatematsu.