Phase Behavior And Crystallization Of Poly(3-Hydroxybutyrate-Co-Hydroxyvalerate) Based Polymer Blends

PHBV,one of polyhydroxyalkanoates(PHAs)family,has been paid much attention due to its natural origin,biodegradability,biocompatibility,and the biomaterial application suitable for tissue engineering,particularly. However,practical application of PHBV has often been limited by its brittleness.In this study,chemosynthetic biodegradable materials poly(ethylene succinate)(PES)and poly(butylene adipate-co-terephthalate) (PBAT)were employed to blend with PHBV in mutual solvent chloroform. PHBV/PES and PHBV/PBAT blends are both completely biodegradable crystalline/crystalline polymer blends.Based on the phase behavior and crystallization behavior of the two blends,the relationship between structure and properties of biodegradable polymer blends was analyzed,which should be of great interest and importance to modify the physical properties and to extend the practical application for biodegradable polymers from both academic and industrial viewpoints.Phase behavior,crystal structure,crystallization kinetics,and mechanical properties of completely biodegradable PHBV/PES blends(with 20-80 wt% PES)were investigated by differential scanning calorimetry(DSC),polarizing optical microscopy(POM),wide angle X-ray diffraction(WAXD),and tensile tests in detail.PHBV is immiscible with PES as evidenced by the two composition independent glass transition temperatures corresponding to those of neat components.Moreover,the biphasic melt further confirms the immiscibility between PHBV and PES.Spherulitic growth kinetics of neat PHBV was analyzed based on the Lauritzen-Hoffman secondary nucleation theory.It was found that the crystalline regimeⅡ/Ⅲtransformation took place at around 87℃for neat PHBV,and the values of K_g(Ⅱ)and K_g(Ⅲ) were obtained as 2.40×10~5 and 3.93×10~5 K~2,respectively.Nonisothermal and isothermal crystallization of PHBV/PES blends were studied by DSC.In the case of isothermal crystallization the crystallization rates of PHBV and PES decrease with increasing the other component content for both PHBV-rich and PES-rich blends,however,the crystallization mechanisms does not change.The mechanical properties of PHBV/PES blends were examined by tensile testing.It was found that the tensile strength,elongation at break,Young's modulus,and toughness of PHBV blends can be adjusted by changing blend composition.For PHBV-rich blends,blending some amount of PES can improve the elongation at break significantly while still keeping considerably high tensile strength and Young's modulus.The phase behavior and crystallization kinetics of the completely biodegradable PHBV/PBAT blends(with 5-20 wt%PBAT)were studied by scanning electron microcopy(SEM),DSC,and POM.PHBV is also immiscible with PBAT as evidenced by the two composition independent glass transition temperatures corresponding to those of neat components. PBAT dispersed in the PHBV matrix uniformly,whereas the domain size of PBAT component increased with the content of PBAT in the blends. Nonisothermal crystallization of PHBV/PBAT blends was studied by DSC and POM.With the increase of PBAT content,the onset crystallization temperature of PHBV and the crystallization rate of PHBV/PBAT blends both decreased, but the final crystallinity of PHBV component was affected slightly.POM was also applied to study the isothermal crystallization of PHBV/PBAT blends. The experimental results indicated that the growth rates of PHBV spherulites reduced in the blends,which was related to the immiscibility between PHBV and PBAT components.