| As one of the biodegradable materials, poly(lactic acid) (PLA) is nontoxic to environment and the human body. It can be used in tissue engineering, drug delivery system, as well as the substitute for the petroleum-based plastics. According to the different stereoisomer, PLA can be divided into poly (L-lactic acid)(PLLA), poly(D-lactic acid)(PDLA), poly-DL-lactic acid (PDLLA). It has been found that the blends of PLLA and PDLA crystallize into stereocomplex crystallize with higher melting point than homo-crystallites. Therefore, it will be the most effective and promising methods to enhance the thermal stability or heat resistance of PLA by blending PLLA and PDLA.Generally speaking, the crystallinity of PLA was dependent on the quality of PLA. In this paper, five kinds of different PLLA and two kinds of different PDLA were analyzed. On this basis, PLLA-4 and PDLA-1, which have lower racemization values and approach molecular weight, were chosen for further study. Equimolar PLLA/PDLA blends showed preferable crystallizing abilities. The results revealed that sc-crystallites formed with homo-crystallites in PLLA/PDLA blends.To reduce the amount of homo-crystallites, the blend films and fiber were processed under the different circumstances, and the crystallization of the post-treatment samples was analyzed. The results showed that the amount of sc-crystallites increased with the increasing time and temperature of the heat-treatment, correspondingly, the amount of homo-crystallites reduced. The crystallinity of the drawn fiber increased dramatically compared with the as-spun fiber, and with the draw ration increased, the amount of the homo-crystallites and sc-crystallites were raised to some extent. The crystallinity was improved significantly after the heat treatment of the PLLA/PDLA fiber. The low-temperatures heat treatment (160℃or less) affected the homo-crystallites of the fiber less while the high-temperature (180℃or more) melt the homo-crystallites to vanish. Meantime, the amount of sc-crystallites increased gradually with increasing heat-treatment temperature. When the temperature was close to 20℃, part of the molten homo-crystallites transformed to sc-crystallites, so the amount of the sc- crystallites has another substantial increase. With the raise of the temperature, the tensile strength and initial modulus decreased, while the breaking elongation increased before a fall.In ordre to obtain the PLLA/PDLA blends with more sc-crystallites, five various nucleators (TMP, TMC, Hydrotalcite, TMC+Hydrotalcite, TMP+Hydrotalcite) were added into the blends, and the ffect of the nucleators on the crystallization of blends were investigated. The results showed that the crystallization rate of the polyactic acid blends was slow before adding the nucleator. TMP and TMC, which can inhibit the formation of homo-crystallites and promote the generation of sc-crystallites, is a very effective nucleator for the PLA blends.Finally, we referred to the Mo Zhishen's method, described and researched the non-isothermal crystallization kinetics by DSC, calculated the correlation parameters of the crystallization kinetics based on the Avrami equation, and summarized the crystallization mechanism. F(T) value increased with increasing the relative degree of crystallinity C(T), representing that the crystal of blends achieved a high degree of crystallinity under a relatively high cooling rate. Ozawa index m value ranged from 2.2 to 3.4, indicating that the blends may crytallize by a way of heterogeneous nucleation and tridimensional growth in non-isothermal terms. |
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