論文の公開元へ低周波数領域の振動分光法および二次元相関分光法による新規バイオポリマー材料としてのポリ(3-ヒドロキシブチレート）ベースのポリマーブレンドの研究
概要
Synthetic polymers or sometimes categorize as plastics, are very crucial materials in modem civilization. The most common ones are petrochemical derivatives; polyethylene (PE) and polypropylene (PP) which are very stable, cheap and applicable for various industries to comfort human life； Inconveniently, the long-term usage of those kinds of polymers leads to problems. One is the depletion problem of irreplaceable natural resources; another one is environmental problem since they cannot be decomposed naturally. Some of them can shatter into micro-plastic under a certain condition while releasing a variety of chemicals which have a negative impact on organisms and ecosystems. For those reasons, development of new biopolymers, polymers of biological origin as a promising alternative material is in high demand to replace fuel-based polymers.
Bacteria synthesized poly(3 -hydroxybutyrate) or PHB is the most popular biopolymer synthesized from bacteria. PHB is well-known to have numerous beneficial characteristics such as biodegradable, biocompatible, and nontoxic; make PHB a very potential biopolymer to use in broad-range of utilizations. However, PHB has also some undesirable properties that limit its practical applications. Thus, modification of PHB is needed to improve the unwanted properties and to fit the best condition for its usage. In this study, we have modified the PHB through the simplest and less costly blending process with glycol chitosan (GC) and poly(4-vinylphenol) (PVPh). Investigation has been done in mainly three aspects; hydrogen bond interactions, crystallization behavior, and crystalline dynamics by low-frequency vibrational spectroscopy.
Effects of. various composition ratio PHB/GC polymer blends were studied by using Fourier transform infrared (FTIR) and terahertz (THz) spectroscopies to investigate the change in higher-order structure and hydrogen bond. The higher-order structure and hydrogen bond monitored in this study include the crystalline structure and intermolecular (C=O…H-C) hydrogen bond within PHB crystal structure. By the addition, of GC in PHB, FTIR spectra showed the higher wavenumber shift of the band at 1718 cm⁻1 due to the weakening of intermolecular (C= O…H-C) hydrogen bond within PHB-PHB molecules. Moreover, the band at 3011 cm"1 decreases and completely destructed by the addition of GC. THz spectra revealed the reduction of the bands at 97 and 82 cm1 indicated that PHB higher-order structure transforms to the less- order structure and the intermolecular (C=O…H-C) hydrogen bond within PHB crystal structure weakens with increasing GC composition ratio. Temperature-dependent investigation has also been conducted for PHB homopolymer and PHB/GC (80/20) and (60/40) by THz spectroscopy to monitor the behavior of hydrogen bond during heating process. The hydrogen bond of PHB homopolymer collapses in the high temperature yet it does not happen in the blend samples. Glycol chitosan can maintain the hydrogen bond interaction of PHB even in its melting temperature.
The effect of PVPh on the crystallization of PHB was investigated for the PHB/PVPh blend with various blending ratios by drfiferential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), far-infrared (FIR), THz, and low-frequency Raman spectroscopies. DSC curves showed that the melting and crystalline points of PHB in PHB/PVPh decreased gradually with an increase in PVPh. By WAXD measurements, we confirmed that PVPh does not affect the crystal structure of PHB [12]. However, our WAXD study was able to identify the crystalline feature of PHB even with the lowest PHB blending ratio (10/90).
FIR studies revealed the crystallinity change by blending. The results categorized PHB/PVPh into three groups: those with high blending ratios of (90/10), (80/20), and (70/30) were high-ordered crystalline; those with blending ratios of (60/40), (50/50), (40/60), and (30/70) were less-ordered crystalline; and those with ratios of (20/80) and (10/90) were amorphous. Less-ordered crystalline PHB exhibited a new peak at 135 cm"1. This peak was expected to reflect the intennolecular (C=O…H-O) hydrogen bond between PHB and PVPh. The high-ordered crystalline and less-ordered crystalline exhibited similar spectrum features, indicating that PVPh does not affect the crystal structure of PHB. The intensity ratio of the peaks at 97 and 82 cm⁻1 changed with the blending ratio variations, thereby describing the crystalline dynamics of PHB. At first, the deformation of the helical structure occurred from 100% PHB to PHB/PWh (60/40). The weakening of intermolecular (C₌O…H-C) hydrogen bond within PHB-PHB commenced from the blending ratio of (50/50) fbllowed by the transition process of intermolecular hydrogen bond between PHB-PHB to intermolecular hydrogen bond between PHB-PVPh.
The low-frequency Raman spectra revealed the same spectral features for all blending ratios, and could categorize the blend into three groups, as was achieved with FIR. However, in this study, the blend with a ratio of (60/40) was between the first and second groups. Similar to the WAXD study, the Raman measurements revealed the crystalline features of PHB, even at the lowest PHB blending ratio (10/90). This demonstrated that low-frequency Raman spectroscopy is 1110re sensitive than FIR for investigatmg the polymer amorphous phase. The temperature-dependent investigation showed that PHB homopolymer and the (70/30) PHB blend have similar thermal behavior, peak related to the intermolecular hydrogen bond (C=O…H-C) within PHB-PHB showed a gradual low-frequency shift with temperature.
Homospectral and heterospectral two-dimensional correlation spectroscopy (2D COS) analysis of FIR, low-frequency Raman, and WAXD profile as a function of the blend ratio of PHB/PVPh have been perfonned in this study. New interesting information was obtained ftom the 2D COS results. The 2D COS of the FIR spectra with various blend ratio revealed that the addition of PVPh. to PHB has a meaningful effect on the band at 264 cm⁻1, and the 128 cm4 band may correspond to the PVPh components. Moreover, it was found that these two bands are the overlapped bands at 264 and 258 cm⁻1 for the former; those at 148,138, and 127 cm⁻1 for the latter, even they look like an ordinary band. The 2D COS results of the low-frequency Raman spectra of PHB/PVPh with various blend ratios are in good agreement with those for FIR spectra. The 2D COS WAXD patterns of PHB/PVPh revealed that peaks at 8.80° and 11.03° come ftom two overlapped peaks at (8.87° and 8.78°) and (11.11° and 10.98°), respectively. Synchronous 2D heterospectral of FIR/low-frequency Raman, FIR/WAXD, and low-frequency Raman/WAXD correlation spectra of PHB/PVPh have also been investigated. The result indicated that the new peak observed at around 130 cm⁻1 in the FIR spectra refers to intermolecular C=O…H-C hydrogen bond between PHB and PVPh. While, the peaks for low-firequency Raman at 81,100, and 110 cm⁻1 and WAXD profiles at 8.78° and 11.01° are attributed to the PHB crystalline bands. This has been concluded from the negative correlation of all of the bands.
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