Thermal Behavior and Molecular Interaction of Poly(3-hydroxybutyrate-<i>c</i><i>o</i>-3-hydroxyhexanoate) Studied by Wide-Angle X-ray Diffraction

Abstract

The thermal behavior and molecular interaction of a new type of bacterial copolyester, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(HB-co-HHx) (HHx = 12 mol %), was investigated by using wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The WAXD measurements were carried out over a temperature range from 25 to 110 °C in the scattering angle range of 2θ = 5−13°. The WAXD pattern at room temperature shows that the P(HB-co-HHx) copolymer has an orthorhombic system (α = β = γ = 90°) with a = 5.76 Å, b = 13.20 Å, and c = 5.96 Å (fiber repeat), which is identical to the crystal system of poly(3-hydroxybutyrate) (PHB) homopolymer. However, temperature-dependent variations in the lattice parameters, a and b, of P(HB-co-HHx) are quite different from those of PHB. Only the a lattice parameter increases with temperature, while the b lattice parameter changes very little in the case of crystalline P(HB-co-HHx). It seems that the intermolecular and intramolecular interactions between the CO group and the CH3 group decrease along the a axis of crystalline P(HB-co-HHx) with temperature. The (110) peak area of P(HB-co-HHx) starts decreasing from around 50 °C while that of PHB changes little at least until 140 °C, indicating that the crystallinity of PHB remains almost unchanged until 140 °C, but that of P(HB-co-HHx) decreases gradually from fairly low temperature (∼50 °C). The DSC measurement of the chloroform solution cast P(HB-co-HHx) shows a recrystallization peak around 51 °C. The (110) peak area of WAXD pattern of the chloroform solution cast P(HB-co-HHx) shows a maximum value at around 54 °C. The maximum of the (110) peak area demonstrates the recrystallization process of P(HB-co-HHx). The result is in a good agreement with the result of the DSC measurement.