| Polyolefins have played an important role in human society, partially due to wide applications, such as their extensive use in packaging films, cables, wires, bags, containers, and appliances. Therefore, it becomes a worthwhile undertaking to investigate and to improve their properties, which can reduce cost and decrease pollution to the environment. Polyethylene (PE) and Polypropylene (PP) take a large role in polyolefin products. They occupy more than half of the thermoplastic market. They are semi-crystalline polymers with a relatively high degree of crystallinity. The crystal structure is an important factor that should be considered as they are closely related to the material performance. Both the characteritics of the starting material and the processing steps have significant effects on the crystal structure and subsquent materials properties.;In this thesis, the crystallization and structure relationship of polyethylene- and polypropylene-based materials, including pure polymer, blend and copolymer, were studied. In-situ Wide angle X-ray Diffraction (WAXD) and Small angle X-ray Scattering (SAXS) were performed during different processing steps, i.e., under static and flow conditions. A single cell heating stage was used for studying the static crystallization behavior of polyolefin-based materials under different thermal conditions. Isothermal crystallization of high-density polyethylene/silica (HDPE-SiO2) at different SiO 2 loadings showed that SiO2 behaved as crystal nucleus in the blend samples. Different instruments were used to study the structural changes during different processing steps. A specially designed cross-slot flow cell device was applied to generate extension-dominant flow and its influence on the crystallization behavior of isotactic polypropylene (iPP). A modified tensile stretching machine that allowed symmetrical stretching of the film was used to investigate the structural change during stretching and their relationship with mechanical performance. Ionic liquid (IL) and ultra-high molecular weight polyethylene (UHMWPE) blend prepared by solution mixing showed a significant increase in the elongation-to-break ratio. For propylene-1-octene random copolymers with higher octene content, the elastic modulus and the yield stress were decreased. Then, they behaved more like elastomers. |
