Crystal and Lamella Structure and C−H···OC Hydrogen Bonding of Poly(3-hydroxyalkanoate) Studied by X-ray Diffraction and Infrared Spectroscopy

Abstract

Temperature-dependent X-ray diffraction and infrared (IR) spectra were measured for poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) P(HB-co-HHx) (HHx = 2.5, 3.4, 10.5, and 12 mol %) to explore their crystal and lamella structure and the C−H···OC hydrogen bonding in them. The X-ray diffraction and IR measurements of PHB and P(HB-co-HHx) revealed that the smaller the a lattice parameter, the higher the frequency (∼3008 cm-1) of the C−H stretching band of the C−H···OC hydrogen bonding along the a axis between the CH3 group of one helix and the CO group of another helix. Therefore, it seems that the C−H···OC hydrogen bonding becomes strong with the decrease in the a lattice parameter. To investigate the relation between the C−H···OC hydrogen bonding and the lamella structure, we estimated the number of C−H···OC hydrogen bonding along the c axis (the direction of the lamella thickness) based on the reported lamella thickness. It is about 8 or 9 for PHB and about 3 for P(HB-co-HHx) (HHx = 10.5 and 12 mol %). It is very likely that the C−H···OC hydrogen bondings break much more easily in P(HB-co-HHx) than in PHB because of the bulkiness of large amounts of amorphous parts. However, the polymer chains still keep the lamella structure even in the copolymers with the HHx content of more than several percent. This is the reason why the P(HB-co-HHx) copolymers show high crystallinity and essentially have the same lattice spacing as the PHB homopolymer even if the HHx content is more than 10%. We have concluded that the C−H···OC hydrogen bonding stabilizes the chain folding in the lamella structure of PHB and P(HB-co-HHx) and the high crystallinity of PHB and P(HB-co-HHx) partly comes from the C−H···OC hydrogen bonding.

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.…”