A grazing incident XRD study on the structure of poly(3-hydroxybutyrate) ultrathin films sandwiched between Si wafers and amorphous polymers

The crystallization behavior and morphology of poly(3-hydroxybutyrate) (PHB) ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction (GIXD) technology. Two kinds of amorphous polymers, i.e., atactic polystyrene (aPS) and poly(vinyl phenol) (PVPh), were used to check the influence of interaction between the PHB and capping layer on the crystallization behavior of PHB. The results show that the crystallization behavior of the PHB thin film depends on the film thickness, crystallization temperature and the capping layers. The PHB ultrathin films of 16 nm in thickness cannot crystallize and an amorphous layer is always obtained within the experimental time scale irrespective of crystallization temperature and capping layer properties. When the PHB layer is 60 nm thick, flat-on lamellae of PHB form under the PS layer at any crystallization temperature but edge-on and flat-on lamellae coexist under the PVPh layer. This implies that the PS layer favors the formation of flat-on PHB lamellae in thin films, while the PVPh layer encourages the formation of edge-on lamellae. For PHB films of 110 nm, edge-on lamellae show up at a lower crystallization temperature under the PS layer. In contrast, no edge-on lamellae were identified under the PVPh layer at higher crystallization temperatures.