| Poly(L-lactic acid)(PLLA) which is a biorenewable and biodegradable eco-friendly polymer material with mechanical properties close to general-purpose plastics such as polypropylene, polystyrene and engineering plastics such as poly(ethylene terephthalate), has broad application prospects in automotive interior parts. The physical properties of semicrystalline polymer and the performance of its product depend greatly on the crystallinity, crystalline structure and crystal morphology. Due to the polymorphism, the crystallization and melting behavior of PLLA is very complicated. Besides, the shortcomings such as slow crystallization rate and low crystallinity lead to the poor heat resistance which has limited the application of PLLA in automobile. Therefore, the research on crystallization and melting behavior of PLLA is helpful to understand the relationship between its microstructure and physical properties, and can offer theoretical foundation for the molding, processing and performance manipulation of PLLA.In this dissertation, the polymorphic crystallization of PLLA with different optical purities, the multiple melting behavior of a commercial PLLA with low optical purity, as well as the influence of annealing on microstructure and properties was investigated firstly, and then the crystallization ability of PLLA was manipulated by a commercial multi-amide nucleating agent and a self-developed nanoscaled zinc citrate nucleating agent, respectively, so as to increase the crystallization rate and improve the heat resistance. The main contents and results are summarized as follows:1) Six PLLA samples with different optical purities were synthesized by ring-opening polymerization with L-lactide and D-lactide as monomers. The influence of optical purity on polymorphic crystallization behavior and crystal growth kinetics of PLLA was studied by using wide-angle X-ray diffraction, differential scanning calorimetry and polarized optical microscope. The results show that the temperature range at which α’ crystal forms becomes narrow and shifts to lower temperature with the decrease of optical purity. This effect can be ascribed to two aspects, that is, the crystallization rate is decreased and the equilibrium melting point is lowered by the presence of D-units in the optical composition of PLLA. Theoretical analysis of crystallization kinetics indicates that the polymorphic (α’-α) crystallization of PLLA is not directly related to the Regime Ⅲ-Ⅱ transition. 2) The melting behavior of a commercial PLLA with low optical purity of ca.92%was investigated after isothermally crystallization and the corresponding melting mechanism was discussed. It is found that α’ crystalline phase is mainly formed in the PLLA samples crystallized at lower temperature (<95℃), whereas a phase is mainly formed at higher temperature (≥95℃). The melting behavior of PLLA is distinct in three regions of crystallization temperature. In the low temperature region (Tc<95℃), the multiple melting behavior can be attributed to the direct melting of partial a’crystals, α’ to α crystalline phase transition, and the melting of as-transformed a crystals. In the intermediate temperature region (95℃≤Tc<120℃), the double melting peaks can be mainly ascribed to the melting-recrystallization-remelting of less perfect a crystals. In the high temperature region (Tc≥120℃), the single endotherm is related to the direct melting of more perfect a crystals.3) The origin of annealing peak and the changes of microstructure and properties of PLLA samples crystallized and annealed above the crystallization temperature were investigated by temperature-modulated differential scanning calorimetry, small-angle X-ray scattering and dynamic mechanical analysis. The annealing peak on the total heat flow curve can be separated into an endothermic peak of enthalpy recovery on the nonreversible heat flow and a glass transition (devitrification) step on the reversible heat flow, indicating that it is mainly attributed to the enthalpy recovery of rigid amorphous phase. The significant increase in the amorphous layer thickness is indicative of the formation of rigid amorphous phase and the relaxation of the mobile amorphous phase during annealing. The storage modulus is enhanced, and the loss peak temperature shifts to lower temperature with increasing annealing temperature, as well as the softening is also observed above annealing temperature. These results show that the annealing at elevated temperature promotes the chain mobility of amorphous phase and the formation of rigid amorphous phase between crystalline phase and mobile amorphous phase.4) The PLLA blend samples with the addition of multi-amide nucleating agent (TMC) were prepared via the melt-mixing method, their thermal characteristics, crystallization behavior and nucleation ability were investigated in detail. TMC can promote the cold crystallization of PLLA and elevate the total crystallinity, but has no significant effect on the glass transition temperature.. For the isothermal crystallization process, it is found that the crystallization half-time of PLLA is reduced and the overall crystallization rate is increased obviously by the presence of TMC, however, the crystal growth form and crystalline structure are not altered. The nucleation efficiency and nucleation activity of TMC were estimated quantitatively and the results indicate that TMC has excellent nucleation effect, and its appropriate content should be chosen between0.25wt%and0.5wt%. The crystalline morphology shows that TMC can disperse homogeneously in PLLA matrix, and provide a large number of nuclei for PLLA, increase the nucleation density and reduce the spherulite size.5) The nanoscaled zinc citrate (ZnCC) was synthesized by solution method, as a new eco-friendly nucleating agent, its influence on the crystallization behavior, crystalline morphology, nucleation mechanism, mechanical properties and heat resistance of PLLA was investigated systematically. It is found that ZnCC accelerates significantly the nonisothermal and isothermal crystallization of PLLA from the melt state, and exhibits better nucleation effect than conventional nucleating agent talc, commercial zinc citrate as well as multi-amide nucleating agent TMC. ZnCC induces PLLA to generate tiny crystals with tightly connected boundary and uniformly distributed size, moreover, the crystal density increases gradually with increasing its content. On the basis of lattice parameter matching between PLLA and ZnCC, the nucleation phenomenon of PLLA/ZnCC system is suggested to be caused by epitaxial mechanism. The addition of ZnCC can promote the evident crystallization of PLLA through a short-time annealing procedure, as a result, enhance and improve the mechanical properties and heat resistance of PLLA. |
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