Evolution of Intermediate and Highly Ordered Crystalline States under Spatial Confinement in Poly(3-hydroxybutyrate) Ultrathin Films

The crystallization behavior and crystalline structures of poly­(3-hydroxy­butyrate) (PHB) ultrathin films (∼52 nm) under spatial confinement were investigated using infrared reflection–absorption spectroscopy (IR-RAS) and two-dimensional grazing incidence X-ray diffraction (2D-GIXD). Intermediate and highly ordered crystalline states were observed during heating and melt-cooling of these films. In the ultrathin films, the intermediate state was noticeably stable at lower temperatures, whereas the highly ordered state was more stable at higher temperatures. A transformation from the intermediate state into the highly ordered state occurred as the temperature increased, as the crystals in the intermediate state acquire sufficient thermal energy to overcome the energy barrier. 2D-GIXD results show that the intermediate state was dominant in edge-on lamellae configuration where the crystallographic b-axis is normal to the film surface. Meanwhile, the highly ordered state was predominant in flat-on lamellae configuration where the b-axis is parallel to the film surface. In the surface region, crystals strongly tended to align in an edge-on lamellae configuration.