CRYSTALLIZATION STRUCTURES AND THERMAL PROPERTIES OF HIGH HEAT-RESISTANCE PLLA/PDLA BLENDS

A series of equimolar blends of poly (L-lactic acid)/poly (D-lactic acid) (PLLA/PDLA) were prepared by melt mixing and the blends were investigated by means of differential scanning calorimetry (DSC),nuclear magnetic resonance (C-13-NMR),wide angle X-ray diffraction (WAXD), etc. It was found that when the blending temperature was 210 degrees C, the stereocomplex (sc) formed from equimolar PLLA/PDLA blends exhibited a high melting temperature at 229 degrees C and showed 98.1% crystallinity degree. As for the sample after isothermal annealing up to 120 degrees C, the Vicat softening temperature approached to 126.4 degrees C. It was indicated that double melting endotherms were characteristic for the sc crystals in the DSC thermograms, where formation of the stereoblock (sb) occured at high processing temperatures. The high-temperature-side (230 degrees C) endotherm and the low-temperature-side (210 degrees C) endotherm resulted from the melting of sc and sb crystals respectively. As the blending temperature increased, the high-temperature-side endotherm was reduced and the low-temperature-side endotherm was emphasized. The relative amounts and crystallinity degree of sc crystals decreased significantly with the increase of blending temperature. The sc-PLA had good processing stability. The melting temperature of sc crystals stayed constantly after second blending. Moreover, the relative amounts of sc crystals increased slightly. The PLLA/PDLA blends composed of networks of sc might be applicable to the heat-resistant materials, and this paper provided a new method of improving thermal stability and processing property of PLA.