Crystallization Kinetics And Crystal Morphology Of Poly(3-hydroxybutyrate)

Polymer crystallization kinetics is a science which studies polymer crystallization or relative crystallinity with time under different conditions. It not only provide the basis for mastering crystal properties of the material and obtaining the desired condensed state structures, but also lay a theoretical foundation for exploring the best molding process of crystalline polymer. In this paper, polyhydroxybutyrate (PHB) crystallization kinetics and crystalline morphology were studied.The isothermal crystallization kinetics and nonisothermal crystallization kinetics of polyhydroxybutyrate (PHB) were studied by Differential Scanning Calorimeter (DSC) at first. As the results of Avrami equation, the results show that:at initial stage of crystallization, the value of Avrami index n is about 2, which demonstrates two-dimensional (2D) spherulite growth and heterogeneous nucleation,with the debase of crystallization time, the value of t1/2 and tmax are reduce,while the crystallization rate is increase. Isothermal crystallization activation energy is 82.4 kJ min-l.Then, Jeziorny equation and Mo Zhishen method were used to analyze the kinetics of PHB under non-isothermal crystallization conditions. The result show that the value of Avrami index n is about 3, which demonstrates a three-dimensional sphernlitic growth and heterogeneous nucleation. in addition, we find that the time of crystallization of PHB becomes longer, relative crystallinity are reduced with the decrease of cooling rate, and there is secondary crystallization process at the end of the crystallization of PHB, It is show that the factor of secondary crystallization process is the super cooling.We measure the crystalline temperature and melt temperature of PHB by DSC.then, the effect of crystallization time and crystallization temperature was studied by Polarized Optical Microscope (POM),we find that:PHB was easy to form banded spherulites when the time temperature was above 100℃,and the band spacing decreased with crystallization temperature increased, the number of crystal nucleus formed at low temperature was significantly higher than the number of crystal nucleus in the high temperature. the change of crystallization time only change dimension of crystal, it did not influence the crystalline morphology. cracks appeared in the bright band of PHB spherulite, and we observed the presence of cracks but did not see the band after removing the polarizer and the analyzer,This shows that the band is the result of periodic distortions of the crystal axis of lamellae, not due to the growth of setback type or spiral-type of PHB spherulites.