The Study On Preparation And Properties Of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) Nanocomposites

Poly (3 -hydroxybutyrate-co-3 -hydroxyvalerate) (PHBV) is a kind of biopolyester produced by bacteria through microorganisms. For its good biodegradable, biocompatible, and thermoplastic properties, PHBV has wide commercial potential applications as biomedical and environmental friendly materials to replace the petroleum-based synthetic polymers. However, PHBV still exhibits several shortcomings such as poor thermal stability, narrow processing window, and brittleness due to its high crystallinity and big spherulites, which restrict its applications. In this paper, four organic and inorganic nanomaterials were selected as the additives to incorporate into PHBV respectively via melt and solution blending.The thermal properties, crystallization behaviors, rheological behaviors, and mechanical properties of the PHBV nanocomposites were investigated. The relationship between structure and properties was analyzed, which should be of the basis of the theory to improve the physical properties of PHBV.Firstly, PHBV/SiO2 nanocomposites were prepared by melt blending. The results indicated that the melting and nonisothermal crystallization behaviors of PHBV were greatly affectd due to the effective heterogeneous nucleating function of the silica by means of DSC. There is only one melting peak in the melting curves of nanocomposites when 5% SiO2 was added into PHBV. The addition of SiO2 to PHBV increased the rate of crystallization of the nanocomposites compared to that of neat PHBV and improved the perfection of the PHBV crystals. The isothermal crystallization kinetics of PHBV and PHBV/SiO2 composites were studied according to the Avrami equation. It can be seen that the overall crystallization rate of the PHBV/SiO2 composite was higher than that of neat PHBV. The nucleation mechanisms of composite were inclined to homogeneous nucleation, which was probably different from the three-dimensional growth and instantaneous nucleation of neat PHBV. The basic crystalline structure and the grain size of crystals of PHBV were not influenced by the introduction of SiO2. The spherulitic growth kinetics of PHBV and its nanocomposites were also analyzed based on Lauritzen-Hoffman secondary nucleation theory. It was found that the values of Kg, σe, and q were decreased compared to neat PHBV, which indicated that the crystallization rate was improved and the polymer chains was flexible with the addition of Si02. In addition,the mechanical properties of the composites were examined. The results showed that the conversion from brittleness to toughness during the tensile process and the synergistically reinforcing and toughening effects were onto the composits owing to the introduction of 5% SiO2. There were obvious plastic deformation and yield phenomenon in the PHBV matrix by the SEM micrographs of the tensile fracture surface of the nanocomposites. FTIR spectrums indicated that there are strong interfacial interactions between SiO2 nanoparticles and PHBV due to the hydrogen bonds, which was beneficial to improve the mechanical properties of PHBV.Secondly, the PHBV/HA nanocomposites were also prepared through the melt blending. The DSC curves showed that the melt point and the crystallinity were increased by the introduction of HA. And the crystallization temperature and the crystallization enthalpy were also improved, which illustrated that the HA acted as a heterogeneous nucleating agent and promoted the crystallization of PHBV. Similarly,the basic crystalline structure of PHBV was not influenced by the introduction of HA.The half crystallization time t1/2 of PHBV/HA nanocomposites was decreased comparing to neat PHBV. It is found that there is a G maximum for both PHBV and its composite within the temperature range of our experiment by POM observation.The mechanical properties testing showed that the tensile strength of the nanocomposites was improved with the contents of HA increasing, which indicated that the HA make a reinforcing effect to PHBV to some extent.Thirdly, the nanocomposites of PHBV and Carboxyl MWCNTs were fabricated by melt blending. The results showed that the melt behavior of PHBV was changed by the incorporation of CNTs. Comparing with the double melting peaks in pure PHBV, there is only one peak in the melting curves of nanocomposites. And the crystallization temperature was greatly prompted, which indicated the efficient heterogeneous nucleation of CNTs for facilitating PHBV crystallization. The isothermal crystallization kinetics of PHBV and its nanocomposite containing 0.5%CNTs were examined based on Avrami equation. The spherulitic nucleation and growth kinetics were also discussed grounded on Lauritzen-Hoffman equation. The reductions of Kg, σe and q values of nanocomposite are in agreement with the fact that the crystallization rate of PHBV increases greatly by addition of CNTs. The mechanical properties of the composites were improved because the rigid filler network structure was formed. The SEM and DRA results further confirmed the existence of the filler network of CNTs in the composites. In addition, the thermal stability of PHBV matrix was also improved by the incorporation of CNTs.Finally, cellulose nanocrystals (CNC) were prepared by acid hydrolysis of microcrystalline cellulose (MCC). To prepare the chloroform suspension of CNC, A solvent exchange procedure was used. Then the PHBV/CNC nanocomposites were fabricated by solution casting. The DSC results showed that the double melting peaks of the nanocomposites shifted to higher temperature by the addition of CNC.Comparing with that in neat PHBV and PHBV/MCC, the low temperature melting peaks was more prominent and sharper and there is no cold crystallization peak in the melting curves of nanocomposites. The crystallization temperature of composites was improved largely with the contents of CNC increased. These result indicated that the CNC acted as a more effective heterogeneous nucleation agent for PHBV crystallization. The size of spherulites of PHBV decreased after incorporation of CNC,meanwhile the density of spherulites increased. From WAXD patterns, we can see clearly that the introduction of CNTs does not affect the basic crystalline structure of PHBV. The thermal stability of the nanocomposites was improved dramatically with the addition of CNC. The Tmax value of PHBV/CNC10 containing 10% CNC was about 47.5℃ and 36.2℃ higher than that of neat PHBV and PHBV/MCC containing 10% MCC respectively. This could be explained that the stronger hydrogen bonds interactions between hydroxyl groups of CNC and carbonyl groups of PHBV. And with the contents of CNC increasing, an H-bond network may be formed in the nanocomposites to restrain the degradation of PHBV.