Effect of the 3-Hydroxyhexanoate Content on Melt-Isothermal Crystallization Behavior of Microbial Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Abstract

The effect of the 3-hydroxyhexanoate content on isothermal crystallization behavior from the melt was investigated for microbial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHs) having the 3-hydroxyhexanoate (3HH) contents of 0, 3.2, 5.8, and 10.6 mol % by time-resolved wide-angle X-ray diffraction and small-angle X-ray scattering simultaneous measurements using synchrotron radiation and rapid differential scanning calorimetry. The isothermal crystallization temperatures after completion of the rapid temperature jump from the melt were ca. 61, 71, 82, and 91 °C. It was found that the nucleation rate and crystallization rate decreased with increasing the 3HH contents in the PHBHs. The ratio of orthorhombic lattice constants a to b as well as the lamellar thickness during crystallization might be changed corresponding to formation of the unstable "intermediate structures" and transformation from them to thermostable regular crystal structures. The 3HH units prevented the 3HB units from formation of not only the "intermediate structures" having mesomorphic layers but also the well-ordered crystal structures of lamellae.