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 Xray 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.

Isothermal crystallization kinetics from the melt of microbial poly­(3-hydroxybutyrate) (P3HB) and poly­(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) were successfully investigated for ca. 50 nm thick ultrathin films on a molecular-to-lamellar scale by time-resolved grazing-incidence wide-angle X-ray diffraction (GIWAXD) and small-angle X-ray scattering (GISAXS) measurements using synchrotron radiation. The nucleation and crystallization rates were much lower in the ultrathin films than in the bulk at crystallization temperatures, T cs, of 60, 70, and 80 °C. Moreover, it was suggested that the first-formed metastable crystal structures were more disordered in the ultrathin films than in the bulk. The molecular diffusion rate for crystallization was lower in the ultrathin film than in the bulk. The crystal growth rate might also be lower in the ultrathin film than in the bulk. The transformation from an edge-on-rich to a flat-on-rich lamellar orientation occurred only in the P3HB ultrathin films at higher T cs. The lamellar stacking manner in the ultrathin films was more disordered for PHBHs having a higher 3HH content.