Cold Crystallization And Related Factors Of Polylactides With Different Chiral Structure

Polylactides with good biocompatibility, biodegradability and mechanical properties,have been widely applied in packaging, agriculture and biomedical field. The synthesis ofthe lactic acid with different chiral structure yield three kinds of Polylactidehomopolymers(poly(L-lactide)(PLLA), poly(D-lactide)(PDLA) and poly(DL-lactide)(PDLLA)). The cold crystallization behavior of Polylactide is related to the chiral structrue.For the crystalline polymers, the crystallization ability, degree of crystallinity and crystalmorphology have a great influence on the performance of polymer. In this paper, the coldcrystallization behavior of PLLA, PLLA/PDLLA and PLLA/PDLA blends have beeninvestigated. Major results as follows:(1) A comparative study of cold crystallization behavior in poly (L-lactide)(PLLA)annealed below and just above glass transition temperature (Tg) has been conducted. Theresults show that annealing benefits the cold crystallization process, which becomessignificant in PLLA annealed just above Tg. Surprisingly, morphological observationreveals high density nuclei in PLLA annealed below Tg, contrary to its relatively slowcrystallization kinetics. This unusual crystallization behavior in physically aged PLLAarises from the retarded crystal growth rate because of incomplete recovery of reducedsegmental mobility above Tg. In contrast, annealing just above Tg has little influence onthe crystal growth rate, and the increased nucleation density alone accounts for theaccelerated crystallization rate.(2) Effect of thermal treatment on the cold crystallization of PLLA and PLLA/PDLLAblend have been studied. It has been evidenced that the acceleration in the coldcrystallization of PLLA arises from the enhanced nucleation after the thermal treatment.The slower is the cooling rate, the higher is the nucleation effect. As Compared to thePLLA, the change of nucleation ability with respect to cooling rate becomes lesssignificant in the PLLA/PDLLA blends, resulted from that amorphous PDLLA prevents the formation of local order during thermal treatment.(3) The cold crystallization behavior of the PDLA/PLLA blends and the formationmechanism of highly ordered PLA stereocomplex have been investigated. Coldcrystallization of poly (L-lactide)/poly (D-lactide) blends at low temperatures results in theformation of disordered stereocomplex. Upon heating disordered stereocomplex enduressignificant reorganization into highly compact one before dominant melting via solid-solidtransition, which yields extremely high melting point. In contrast, ordered stereocomplexgenerated at high temperatures exhibits little reorganization, and is melted directly attemperatures lower than that of reorganized stereocomplex.