Crystallization Behaviors And Properties Of Poly(L-lactic Acid) Blends

Poly(L-lactic acid) (PLLA) has been widely used in packaging, medicine, agriculture and other areas, due to its many advantages such as biodegradation, biocompatibility, cornstarch-based materials, transparency and innocuity. However, the disadvantages of brittleness and slow crystallization rate restrict its application. The properties of PLLA were improved by adding nucleating agents and plasticizers. The PLLA /poly(vinyl alcohol) (PVA) and PLLA/PDEGA(poly(diethylene glycol adipate)) blend systems were prepared through the solution-evaporation method. Then, many representative metheods were used to study the basic thermal properties, crystallization behaviors, spherulite morphology, crystal structure, mechanical properties and thermal stability. The results are as follows:1. For PLLA/PVA blends, the contents of PVA in this blend samples were 0.5 and 1wt%. The crystallization rate and crystalline morphology of PLLA/PVA blends under different crystallization conditions were studied in detail by a variety of research methods. Compared to neat PLLA, the results that the nonisothermal melt crystallization peak temperature increased and the nonisothermal cold crystallization peak temperature decreased showed that PVA enhanced the nonisothermal melt and cold crystallization behaviors of PLLA in the blends. The spherulitic growth rate of PLLA in the blends were faster than that of neat PLLA at the same isothermal melt crystallization temperature, however, the crystallization mechanism of blended PLLA didn't change by adding PVA, regardless of isothermal crystallization temperature. Both the nucleation density and growth rates of PLLA spherulites were increased by PVA to a certain degree, which resulted in the acceleration of the overall isothermal melt crystallization of PLLA. Thus, PVA as an effective macromolecule nucleation agent improved the nucleation density of PLLA and meanwhile acted as an efficient crystallization-assisting agent to enhance the crystallization of PLLA to some extent. Therefore, PVA may be the first biodegradable polymer for improving the ability of nucleation and crystallization of PLLA under such a low content. Moreover, the crystal structure of PLLA/PVA blends system remained unchanged. A possible nucleation mechanism about PLLA/PVAblends was also discussed.2. For PLLA/PDEGA blends, only one glass transition temperature emerged and increased with the increase of PDEGA content from the DSC curves, when the content of PDEGA was less than 30 wt%. However, as the PDEGA content was not less than 30wt%, two glass transition temperatures appeared which were closed to each other gradually with the change of PDEGA content. From the above results, as PDEGA content was less than 30 wt%, PLLA/PDEGA blends were completely miscible. Otherwise, they are partially miscible. The results that the increase of nonisothermal melt temperature and enthalpy and the decrease of nonisothermal cold temperature and enthalpy indicated that PDEGA enhanced the nonisothermal melt crystallization of blended PLLA. The overall isothermal melt crystallization of the blended PLLA was enhanced by adding PDEGA, as testified by the decrease of the reciprocal of the crystallization half time with the increase of PDEGA content. PDEGA had little effect on the crystallization mechanism, crystal structure and nucleation density of PLLA, but the spherulitic growth rate of blended PLLA was faster than that of neat PLLA with the maximum values at 126 ? for all the samples. In the end, the tensile test of PLLA/PDEGA blends was investigated in detail. The decrease of tensile strength and young's modulus and the increase of elongation at break of blended PLLA with the increase of PDEGA content were found. Therefore, the ductility of PLLA was improved by PDEGA.