Infrared Spectroscopy and X-Ray Diffraction Studies on the Structure and Thermal Behavior of Biodegradable Polyhydroxyalkanoates

The structure and thermal behavior of new types of bacterial copolyester, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate); P(HB-co-HHx) (HHx=2.5, 3.4, and 12 mol %) have been explored by means of wide-angle x-ray diffraction (WAXD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. The WAXD pattern of P(HB-co-HHx) (HHx=12 mol %) copolymer measured at room temperature has revealed that it has an orthorhombic system (α=β=γ=90°) with a=5.76Å, b=13.20Å, c=5.96Å (fiber repeat), which is identical to that of poly(3-hydroxybutyrate) (PHB). It has been found from the temperature-dependent variations of the WAXD pattern that only the a lattice parameter shows the thermal expansion, while the b lattice parameter changes little with temperature in the crystalline P(HB-co-HHx) (HHx=12 mol %). This observation suggests that there are inter and intramolecular interactions between C=O groups and alkyl groups along the a axis and that interactions are broken little by little with temperature. IR spectra were measured for the four kinds of polymers over a temperature range from 30°C to high temperatures (200°C; PHB, 180°C; P(HB-co-HHx) (HHx=2.5 mol %), 180°C; P(HB-co-HHx) (HHx=3.4 mol %), 150°C; P(HB-co-HHx) (HHx=12 mol %)). Temperature-dependent IR spectral variations were analyzed for the CH, C=O, and C-O-C stretching band regions, and bands characteristic of crystalline and amorphous parts were identified in each region. It has been found from the IR study that the strength of interaction between the C=O group and the CH₃ (or CH₂) group is similar among the four polymers and that the population of C=O groups that are not involved in the interaction becomes higher with the increase in HHx. Both WAXD and IR studies have revealed that the crystallinity of P(HB-co-HHx) (HHx=12 mol %) decreases gradually starting from relatively low temperature (about 60°C) while that of PHB remains high up to 170°C.