H. Sato^1,*, R. Murakami¹, K. Mori², I. Takahashi², J. Dybal³, F. Hirose⁴,

K. Senda⁴, I. Noda⁵, H. Terauchi², and Y. Ozaki¹

 ¹ Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Sanda 669-1337, Japan

² Department of Physics, School of Science and Technology, Kwansei-Gakuin University, Sanda 669-1337, Japan

³ Institute of Macromoleculer Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq.2, 162 06 Prague 6, Czech Republic

⁴ Polymer Designing and Processing Research Laboratories, Kaneka Co., Settsu 566-0072, Japan

⁵ Beckett Ridge Technical Center, The Procter and Gamble Company, West Chester, Ohio 45069, USA

Keywords:          x-ray diffraction, infrared spectroscopy, crystalline structure, thermal behaviour, qantum chemical calculations

The structure and thermal behavior of poly(3-hydroxybutyrate), PHB and poly(3-hydroxybutyrate -co-3-hydroxyhexanoate), P(HB-co-HHx) have been explored by means of wide-angle x-ray diffraction (WAXD), infrared (IR) spectroscopy, and quantum chemical calculations (QCC). It has been found from the temperature dependent variations of the WAXD pattern that there are inter and intramolecular interactions (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) along the a axis.^[1],[2]  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. Both WAXD and IR studies have revealed that the crystallinity of P(HB-co-HHx) decreases gradually starting from relatively low temperature while that of PHB remains high up to 170^oC. It has also been revealed 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.

We calculated the distance between the O atom of the C=O group and the H atom of one of the C-H bonds of the CH₃ group from the reported atom coordinates of PHB^[3] by using software QUANTA (Accelrys, San Diego). The shortest distance was found to be 2.63 Å. The van der Waals separation between the O atom and the H atom is 2.72 Å, and therefore, the distance between the O atom and the H atom in PHB can be significantly shorter than that of the van der Waals separation. The result of this calculation leads us to infer the existence of the C-H···O interaction in PHB.

QCC were done for a dimer model of PHB by using the Gaussian 98 program package. Molecular geometry was optimized at B3LYP level, the basis set being of 6-31G(d). QCC suggests that the C-H···O hydrogen bonds are not only intermolecular interactions but also intramolecular interactions between C=O groups and CH₃ groups in the PHB and P(HB-co-HHx).  The stable structure of dimer by QCC looks like the crystalline structure of PHB.

[1]     Sato, H., et al., 2004. Macromorecules 37, 3763 – 3769.

[2]     Sato, H., et al., Macromol. Symposia. (in press).

[3]     Cornibert, J., et al., 1972, J. Mol. Biol. 71, 735.

        论文来源：International Symposium on Biological Polyesters ,Auguest 22-27, 2004