Reinvestigation of Crystal Structure and Intermolecular Interactions of Biodegradable Poly(3-Hydroxybutyrate) α-Form and the Prediction of Its Mechanical Property

As for the crystal structure of poly(3-hydroxybutyrate) (PHB) alpha-form, the methyl group was said to locate at an abnormally short distance from the oxygen atom of C=O unit, resulting in the formation of CH3 ... O=C hydrogen bond. This idea was proposed on the basis of infrared spectra detecting a band of antisymmetric methyl C-H stretching mode [v(as)(CH3)] at anomalously high frequency position and the too short CH3 ... O=C interatomic distances calculated from the atomic coordinates derived from the previously reported X-ray structure analysis. However, the crystal structure analyzed by the X-ray method, even the C and O atomic positions, has not yet been established confirmatively because of the appreciably small number of the observed X-ray diffraction spots. We have reinvestigated the crystal structure of PHB a-form by collecting the observed X-ray diffraction spots of about twice larger total number than before, and confirmed the positions of the C, O and H atoms in the unit cell at enough high accuracy. The finally obtained crystal structure gave the reliability factor of 14.5% for the X-ray data at 23 degrees C (the total number of observed reflections 70) and 16.0% (94 at-140 degrees C). The thus-established crystal structure was found to exhibit the shortest H ... O distance of 2.62 angstrom, which is shorter than the value expected from the normal van der Waals distance. In order to clarify the relation of this geometry with the observed higher-frequency shift of v(as)(CH3) band from the viewpoint of space group symmetry, the factor-group analysis and the normal modes calculation were performed for the PHB alpha-crystal by referring to the presently analyzed crystal structure information. Among the plural possibilities to cause the high-frequency shift of originally doubly degenerated v(as)(CH3) mode, the normal coordinates calculation has revealed that the vibrational coupling due to the intermolecular interactions between the adjacent molecular chains may be the most significant factor rather than the effect of lowering of C-3v, symmetry of regular CH3 groups coming from the site group symmetry. Once the crystal structure was established well, then the 3D anisotropic elastic constants matrix was calculated for the first time about the PHB alpha crystal. The ultimate Young's modulus along the chain axis was only 5 GPa, which was comparable to the moduli in the direction perpendicular to the chain axis. In other words, the mechanical property of this crystal is mostly isotropic as a whole.