Effect Of Plasticization, Nucleation And Orientation On Crystallization Behavior Of Poly(Lactic Acid)

In this dissertation, poly(lactic acid)(PLA) blends comprising poly(ethylene glycol)(PEG) have been prepared by melt-blending. Based on study of crystallization behavior of PLA, the influence of PEG on the formation of disordered?form (?') and ordered a form have been studied, and then the synergistic effect of the PEG and talc on the non-isothermal crystallization kinetics of PLA has been further explored in the molding process to provide a theoretical basis and experimental methods in order to control the crystalline morphology and properties. On the other hand, PLA samples with different degrees of orientation have been prepared by injection molding, film casting and drawing in rubber state and the crystallization morphology of oriented PLA samples annealed under different conditions have been investigated. Major results as follows:1. The crystal modification and multiple melting behavior of PLA as a function of crystallization temperature have been studied. It is suggested that the disorder (?') and order (?) phases of poly(L-lactic acid)(PLLA) are formed in cold-crystallized samples at low (<110?) and high (>110?) temperatures, respectively. A disorder-to-order (a'-to-a) phase transition occurs during the annealing process of the?'-crystal at elevated temperatures, and the transformation is dependent on the annealing temperature. The presence or not of an additional endothermic peak before the exotherm in the DSC thermograph of the a'-crystal is strongly dependent on the heating rate, indicating that a melting process is involved during the?'-to-?phase transition2. The influence of PEG on formation of?'-and?-form of PLA has been studied. The degree of crystallization of PLA in PLA/PEG blends is higher than PLA without the addition of PEG when the crystallization temperature is in the temperature range of 80?140?. WAXD results show that the critical temperature of PLA crystallized in a-form decreased from 110?to 80?when the PEG content is 15% in PLA/PEG blends. The crystal structure change of PLA is ascribed to the lowered glass transition temperature. 3. The non-isothermal crystallization behavior of the PLA/PEG/talc blends has been investigated. It has been evidenced that talc together with PEG has a synergistic effect on enhancing the crystallization rate of PLA, despite the fact that the two additives has an opposite effect on the size and density of the spherulites. The non-isothermal crystallization data are analyzed by using three different kinetics models, namely, the Avrami, Ozawa, and Mo models. It was found that the Avrami method and the Mo model could describe the experimental data of the non-isothermal crystallization fairly well for all the specimens, whereas the Ozawa analysis failed to provide an adequate description of the non-isothermal crystallization of the PLA formulations comprising talc with PEG. The addition of talc into PLA reduces the transportation of polymer chains and prevent the PLA molecular segment from rearranging, and the addition of PEG into PLA/talc enhances the transportation ability of polymer chains.4. The amorphous PLA specimens with low degree of orientation were prepared by injection molding and extrusion casting method. Crystallization morphology of cast film is related to annealing temperature and time. Annealing does good to get PLA samples of orientation crystallization. The degree of orientation of PLA is dependent on the take-up speeds. The isotropic samples are obtained by annealing as the take-up speeds are lower than 100 cm/min. Storage modulus, loss modulus and heat resistance are enhanced by means of annealing.5. The PLA specimens with high degree of orientation (fc)are prepared by means of stretching in rubber state. The relation between orientation and crystallization behavior of PLA is studied, and fc depends on the drawing conditions. The amorphous orientation samples are obtained as the drawing temperature is 60?. Furthermore, the PLA are amorphous and crystallizable as the drawing and annealing temperature is 70?, and fc of which is enhanced. The crystallization samples are obtained during stretching as the temperature is 80?, and the degree of crystallization of which is enhanced by annealing, whereas fc of which keeps constant.