Oriented Crystallization And Deformation Mechanism Of Polyolefin (blends)

In this dissertation, the oriented structure of polyolefin blends (including crystalline/amorphous HDPE/EVA and crystalline/crystalline HDPE/iPP), prepared via dynamic packing injection molding, have been investigated in detail with aid of two-dimensional small/wide angle X-ray scattering (2D SAXS/WAXS) and differential scanning calorimetry (DSC). Combinations with phase morphology of blends, the crystallization behaviors, crystallization mechanism and impact factors in the polyolefin blends under shear have been demonstrated. Moreover, the relationship between structure, such as molecular orientation, phase morphology and interfacial interaction, and properties has been elucidated. Correspondingly, theoretical basis and experimental methods for morphology control and high performance of polymer multi-phase system have been somewhat built up. On the other hand, based on the investigation of structure evolution and deformation characteristics of oriented polyolefin, including high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and isotactic polypropylene (iPP), while subjected to tensile deformation, the deformation mechanism has been disclosed. Moreover, corresponding deformation model has been raised to throw light on the nature of structure evolution and deformation characteristics under tension. Major results as follows:1. The HDPE component in the HDPE/EVA blends under shear is oriented along flow direction and its order parameter is dominated by phase inversion. EVA component, which has low solidification temperature and is in the flow statewhile HDPE crystallizes, cannot affect the molecular orientation of HDPE matrix, whereas the order parameter of HDPE component is decreased while phase inversion occurs. Phase inversion is related to the viscosity ration of HDPE to EVA component. The miscibility between HDPE and EVA component has not been changed under shear. Tensile strength is mostly dependent of order parameter of HDPE component and can be somewhat affected by VA content.2. Molecular orientation of parent lamellae of iPP component in HDPE/iPP blends under shear is always parallel to the flow direction, independent of composition. Lamellae of HDPE component in its matrix is perpendicular to flow direction, whereas epitaxial growth occurs, with chains about+50° apart from the flow direction, while HDPE component is dispersed in the iPP matrix. Epitaxil growth is resulted from the surface induced crystallization of iPP lamellae, and is related to inhibition of bulk nuclation in the submicron domains and preferetial interfacial nucleation. Moreover, order parameter of matrix is dependent of solidification temperature of .dispersed phase. Since the crystallization of iPP is prior to that of HDPE, the order parameter of HDPE matrix is linearly declined with increasing of iPP content, however, that of iPP matrix cannot be affected by the content of HDPE component. Tensile properties are related to the order parameter of matrix, content of dispersed phase and interfacial interaction.3. The mechanism of epitaxy and corresponding impact factors in HDPE/iPP blends has been quantitatively investigated with model experiments. Moreover, under some conditions, bulk oriented crystallization, such as b-axis orientation, can be found in HDPE/iPP blends. It is resulted from the cooperation of bulk nucleation and direction of boundary of domains. Various oriented crystallizations are related to the completion between bulk nucleation and interface nucleation, and can be affected by composition, cooling rate and molecular weight.4. Based on the investigation of deformation characteristics and structureevolution of oriented polyolefin, including HDPE, LLDPE and iPP, serial model between amorphous and crystalline part has been raised. The deformation in amorphous part is dominant before lamellar fragmentation and later is dominated by the plastic deformation in crystalline part while lamellae are broken. Order parameter in amorphous part can affect the true strain related to lamellar fragmentation, strain softening coefficient and modulus related to strain hardening.5. Viscous force, occurred in the plastic flow, is resulted from inter-lamellar and inter-block coupling and molecular interaction. On the one hand, shish structure can significantly enhance viscous force, due to increasing inter-lamellar coupling and the constraints on shear yield of lamellae. Quasi-static force is only resulted from molecular orientation, independent of strain rate. However, vicious force is related to the strain rate and can be quantitatively described by Erying activation mechanism.