Synthesis And Characterization Of PHBV Nanofibers Scaffold

Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) is a member of polyhydroxyalkanoate (PHA) family. These biopolyesters are produced by many microorganisms, have good biocompatibility and mechanical strength. The ultimate degradation product of PHBV is (R)-3-hydroxybutyric acid which is a normal constituent of human blood. Several methods have been used to prepare nanofibrous scaffolds, including particulate leaching, electrospinning technology and self-assembly. Because of the limitations of these methods, this study tried to prepare the PHBV nanofiber scaffolds through phase separation method. In this study, we change the mixing ratio of the two-component in the solvent and the condensation temperature, and investigated the effects of these two factors on the morphology and crystallization of PHBV scaffold.The PHBV nanofibers scaffolds can be prepared by adjusting the ratio of the two components of the chloroform / dioxane mixed solvents and the gelation temperatures. The fiber is regular with a diameter of 50 ~ 300nm, and the pores with uniform size are connect each other well. The ratio of the two components of the mixed solvents can affect the gelation temperatures and the temperature required by the formation of nano-fibers and the gelation temperature range. When the ratio of the dioxane of the mixed solvents is low, it need lower gelation temperature for the required of PHBV solution; as the ratio of dioxane escalating, the temperature range of the gelation extend. When the ratio of dioxane was 10% and 50%, it can only prepare the PHBV scaffolds with pore-wall structure; when the ratio of dioxane is 20%～40%, it is possible to prepare PHBV nanofibers scaffold. And in this ratio range, the range of the formation of nanofiber was extended as the ratio of dioxane increasing. It can only prepare PHBV nanofiber at -80℃by the mixed solvent with 20% dioxane; and when the ratio of dioxane is 40%, the PHBV nanofiber can be prepared from 4℃to liquid nitrogen.PHBV scaffolds with different morphology have been prepared according to the difference of the solubility between chloroform and dioxane. The results show that at -24℃, the nanofibers scaffold can be prepared by the chloroform / dioxane solvent with the ratio of 70/30, and when the ratio of dioxane is 10% and 50%, the PHBV pore-wall scaffold have been prepared. Crystallinity of the nanofiber scaffold is much lower than that of the pore-wall scaffold, and the former one is preferred orientation in b-axis significantly. The crystallization size of the nanofibers scaffold is bigger than that of the pore-wall scaffold in (020) and (110) direction. Besides, in the nanofibers scaffold, the crystal form of PHBV is more single, mainlyβ-phase, and the grain size distribution is narrower than the pore-wall scaffold. From the FTIR data, it also can find that the C=O and C-O-C on the PHBV chain are more orientation than that of pore-wall scaffold. Since the topological structure and biological effects, nanofiber showed good cell adhesion and promoter action to cell growth in cytological experiments.According to the difference of condensation point between chloroform and dioxane, a series of PHBV scaffolds with different morphology have been prepared at different gelation temperatures. The research found that the morphology of the nanofibers scaffold are effected by gelation temperature. The scaffold with uniform size and regular shape of the fiber, good connectivity gap can be prepared at -24℃～-80℃. XRD, FTIR and DSC analysis shows that, as the gelation temperature decreasing, the crystallinity and the grain size in (020) direction decrease. The theαphase of polymer reduce as the temperature dropping, and at -80℃, there are onlyβphase in PHBV. In cell adhesion experiments, the nanofibers scaffold showed good adhesion properties, which are prepared in the gelation temperature between -24℃and liquid nitrogen.A furtuer study of the microscopic morphology of PHBV and the crystal structure and morphology of PHBV spherulite morphology were take place by the atomic force microscopy (AFM). The result show that the temperature required for fiber of the thin sheet is lower than that of the sample of 3D structure, just 4℃, and the nanofiber bundle is composed of many small single-nanofiber. With the gelation temperature reduce to -24℃, the diameter of single-nanofiber is smaller and the length is longer than that at 4℃. But as the temperature decreasing, the morphology of the PHBV change into needle-like fiber. In the condition of liquid nitrogen, the fibers reduce and many bulks of PHBV appear. The difference of the ratio of chloroform and dioxane effects the PHBV phase separation process. Although PHBV pore-wall structure scaffold have been formed when the ratio of dioxane is 10% and 50%, but we can find there are some difference between them by AFM. When the ratio of dioxane is low, PHBV samples is scattered, flaky sample's boundary is not clear, and shape is irregular. Then, when the ratio of dioxane is high, PHBV showed a smaller long-flake, clear boundary , regular shape, compact structure. Meanwhile, the diameter of PHBV banded spherulites decrease as the gelation temperature dropping by observing the morphology of PHBV banded spherulites. There are two results for this phenomenon: the crystal nucleus for spherulite formation increase as the gelation temperature decreasing and the growth of spherulite is hindered, because many spherulite squeezed together.