Infrared Spectroscopy Studies of CH···O Hydrogen Bondings and Thermal Behavior of Biodegradable Poly(hydroxyalkanoate)

Abstract

Infrared (IR) spectra of new types of bacterial copolyester, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(HB-co-HHx) (HHx = 2.5, 3.4, and 12 mol %), and poly(3-hydroxybutyrate) (PHB) were measured over a temperature range of 20 °C to higher temperatures (PHB, 185 °C; HHx = 2.5 mol %, 160 °C; HHx = 3.4 mol %, 160 °C; HHx = 12 mol %, 140 °C) to explore their structure and thermal behavior. The temperature-dependent IR spectral variations were analyzed for the CH stretching, CO stretching, CH3 deformation, and C−O−C stretching vibration regions, and bands characteristic of crystalline and amorphous parts were identified in each region. It has been found from the anomalous frequencies of the CH3 asymmetric stretching bands of the four polymers and the X-ray crystallographic structure of PHB that there is an inter- or intramolecular interaction (C−H···O hydrogen bond) between the CO group in one helical structure and the CH3 group in the other helical structure in PHB and P(HB-co-HHx). The bonding energy of the C−H···O hydrogen bond seems to be smaller than 4 kJ/mol, but considering the heat of fusion (12.5 kJ/mol) of PHB, it is likely that a chain of C−H···O hydrogen bond pairs link two parallel helical structures in the crystalline parts. The temperature-dependent IR spectral variations have shown that the crystallinity of P(HB-co-HHx) (HHx = 12 mol %) decreases gradually from a fairly low temperature (about 60 °C), while the crystallinity of PHB and P(HB-co-HHx) (HHx = 2.5 and 3.4 mol %) remains almost unchanged until just below their melting temperatures. It has also been revealed from the present study that the weakening of the C−H···O interaction starts from just above room temperature and proceeds gradually with increase in temperature, but the collapse of helical structure occurs at a much higher temperature for all the polymers investigated.

Near-Infrared Spectroscopy—Its Versatility in Analytical Chemistry

“…According to our previous IR study, [52][53][54] the unusual high wavenumber shift of the CH3 asymmetric stretching band at 3007 cm -1 indicated the existence of a C-H···O=C hydrogen bond. Moreover, the CH stretching band shifts from 2878 to 2873 cm -1 upon the crystallization (Fig.…”

“…[49][50][51] Figure 4 illustrates the crystal and lamella structure of PHB. Recently, Sato et al [52][53][54] investigated the crystallization and melting behavior of PHB by using wide-angle X-ray diffraction (WAXD), IR spectroscopy, and differential scanning calorimetry (DSC). They found that in the IR spectra of PHB, the crystalline CH3 asymmetric stretching band appears at an anomalously high frequency (3009 cm -1 ), and the crystalline C=O band at 1723 cm -1 shows a downward shift compared with the amorphous C=O stretching band.…”

“…4). [52][53][54] Figure 5(a) shows time-dependent changes in the NIR spectra in the region of 6050 -5650 cm -1 of PHB film (500 μm) aligned at a 20-min interval. 47 Figure 5(b) depicts the second derivatives of the spectra measured at 0 and 180 min.…”

Synchrotron X-ray scattering studies on the structural evolution of microbial poly(3-hydroxybutyrate)

“…In general, the properties of a polymer are highly dependent on its morphological structure, which in turn results from its processing conditions including crystallization. Hence, the crystallization behavior of PHB was studied by using various techniques, including differential scanning calorimetry (DSC), small-and wide-angle X-ray scattering (SAXS and WAXS), and infrared (IR) spectroscopy (Abe et al, 1998;Capitan et al, 2004;Chiu et al, 1999;Doi et al, 1995;Feng et al, 2002;Gunaratne et al, 2004;Kamiya et al, 1991;Numata et al, 2004;Pearce & Marchessault, 1994;Sato et al, 2006;Sato, Murakami et al, 2004;Sato, Nakamura et al, 2004;Zhang et al, 2005;Scandola et al, 1992). In particular, the SAXS technique has been employed to investigate the morphological structure of PHB already crystallized from the melt (Abe et al, 1998;Capitan et al, 2004;Chiu et al, 1999;Numata et al, 2004).…”

“…In particular, the SAXS technique has been employed to investigate the morphological structure of PHB already crystallized from the melt (Abe et al, 1998;Capitan et al, 2004;Chiu et al, 1999;Numata et al, 2004). Recent IR spectroscopy studies found that in the lamellar crystals of PHB, there is hydrogen bonding between the methyl group of one helical structure and the carbonyl group of another helical structure along an axis of the crystal lattice, and such hydrogen bonding stabilizes the chain folding in the lamellar crystals (Sato et al, 2006;Sato, Murakami et al, 2004;Sato, Nakamura et al, 2004;Zhang et al, 2005). However, detailed studies on the evolution of morphological structures in this microbial polyester and its crystallization mechanism are still needed.…”