Effects Of Molecular Chain Dynamics On Polylactide Crystallization

The unique viscoelastic behavior of polymers is related to the dynamics of polymer chains.The dynamical effect of polymer chains has important consequences in polymer processing.Due to the hierarchic structure of polymers,it is difficult to fit the dynamics of polymers with the temporal and spatial resolution of the equipment.Therefore,the dynamics of polymers have not been intensively investigated.The critical cooling rate of poly(lactic acid)(PLA)is the lowest as reported.So,PLA is an ideal model for investigating the dynamics of polymer chains.In this thesis,the crystalline morphology,crystalline structure and crystallization behavior of PLA were investigated by synchronous radiation X-ray scattering(SR-WAXS),wide-angle X-ray diffraction(WAXD),Fourier transform infrared spectroscopy(FTIR),differential scanning calorimetry(DSC),polarized light microscopy(POM).It was found that PDLA can form banded spherulites with different melting points in the bright and dark regions during melting crystallization,and the formation of banded spherulites is determined by the melting diffusion rate and crystallization rate.The banded spherulites can reproduce the morphology of the initial banded spherulites during melting and recrystallization process,and show the memory effect related to thermal history.The investigation for the memory for PDLA crystallization was discussed according to the Adam-Gibbs approach.It was found that the relaxation time of PDLA molecular chain is longer and the chain conformation is more difficult to adjust with temperature,and tends to form heterogeneous melt.Therefore,the original crystal structure can be remembered in the process of recrystallization.According to the understanding of PLA homopolymer,the crystallization behavior of PLLA/PDLA blends and the effects of the addition of PLLA/PDLA blends on the crystallization behavior and crystal structure of homopolymers were further investigated.In situ WAXS results showed that the homopolymer crystal appeared earlier than the stereocomplex crystal in cold crystallization process.At the same time,the crystallization rate of homopolymer in PLLA/PDLA blends was significantly improved,indicating that the stereocomplex crystal may not be the nucleating agent for homopolymer crystallization.It was proposed that the relaxation behavior of the molecular chain of the homopolymer slows down and the melt entropy decreases due to the existence of stereocomplex crystal.Meanwhile,the interaction between the enantiomers decreases the energy barrier for homopolymer crystallization.The combined effect of them may be the real reason for stereocomplex crystal accelerates homopolymer crystallization in PLLA/PDLA blends.