Effect Of A Small Amount Of Poly (D-Lactide Acid) On The Crystallization Behavior Of Poly (L-Lactide Acid)

In this paper, effect of a small amount of poly(d-lactide acid) on the cold crystallization and melt crystallization behavior of poly(l-lactide acid) was investigated by in suit infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM).It is found that, bothα' andβc crystals are formed during cold crystallization process of three kinds of PLLA/PDLA(5wt.%) blends composed by PLLA and PDLA with different molecular weight. Compared withβc crystals formed in blend by low molecular weight PDLA, that formed by high molecular weight PDLA significantly increases the cold crystallization rate of PLLA. By controlling the melt temperature of the blends, two kinds of PLLA/PDLA(5wt.%) blends with different initial states were prepared. That is, PLLA chains are in the amorphous state and the pre-existedβc crystal is kept for the blend melt-quenched at 180°C (named as Q180), whereas both PLLA and PDLA chains are totally in amorphous states for the blend melt-quenched at 230°C(named as Q230). It is found that the pre-existedβc crystals significantly increased the cold crystallization rate of PLLA in Q182. However, there is competitive crystallization for PLA stereocomplex and PLLAα' in Q230. Due to the competition between PLLA homo-crystal with PLLA/PDLA stereocomplex,βc andα'crystals formed during the cold crystallization porcess of Q230 were more disordered than that formed in the cold crystallization of PLLA and PLLA/PDLA, respectively.For the melt crystallization behavior of PDLA/PLLA(5wt.%) blend, our results show that the formation conditions or paths ofβc crystals also influence the melt crystallization rate of PLLA. By in situ FTIR technique, it is found that theβc crystals formed during the cold crystallization process were more disordered than that formed during the melt crystallization Therefore, the melt crystallization rate of PLLA is increased slightly byβc crystals formed during the cold crystallization process, whereas the resulted crystallinity of PLLA is hardly affected. In contrast, theβc crystals formed during the melt crystallization process significantly increase the melt crystallization rate of PLLA, but it reduces the crystallinity of PLLA. At last, we investigated the space distribution ofβc crystals and PLLA spherulite during the melt crystallization of PLLA/PDLA(5wt.%) under different conditions by in suit POM.