Morphorlogical Controlling And Hierarchy Structure Of Injection-molded Polyolefin

In most polymer processing operations, molten polymers are usually exposed to varying levels of shear flow and temperature gradient, so that the evaluation of microstructure and crystal morphology in product becomes much complicated. And the development of structure is determined by many factors, such as processing conditions, the feature of certain polymer or other component added into the polymer. In this work, it is primarily investigated that the effect of molecular weight and nucleating agent on the morphological development and the formation of orientated structure in polyethylene or polypropylene samples obtained via dynamic packing injection molding. The main result is as follows:1. The morphology and the effect of molecular weight on the formation of shish-kebab structure were investigated in detail by examining the lamellae structure of injection molded samples of three categories of high density polyethylene (HDPE) with different melt flow index (MFI), layer by layer, along the sample thickness. It is found that the over all orientation in the molded bar increased with decreased MFI, and a small amount of UHMWPE could enhance substantially HDPE orientation. There exists a hierarchy structure with changed orientation and crystal structure along the thickness direction of molded bar. At the skin, there existed intertwined lamellae constituting an interlocked lamellar assembly, a typical shish-kebab structure gradually developed from the subskin layer to the core, with increased shish content toward into the center, but in the core was a spherulite-like superstructure with randomly distributed lamellae. UHMWPE played an important role not only in the formation of shish and but also in the transformation from spherulite to shish-kebab oriented structure for HDPE with a low molecular weight.2. Aim to understand the morphological development and the effect of nucleating agent on the formation of orientated structure, the lamellar structure of isotactic polypropylene (iPP) containing different concentration of dibenzylidene sorbitol (DBS) obtained via dynamic packing injection molding has been investigated. The crystallization temperature of iPP with low concentration of DBS has been significantly improved and DBS also affects the final crystal structure. It is found that the size of spherulite crystal and its distribution in matrix is affected by DBS through SEM experiment. A high level of orientation degree in whole range from skin to the area approximating to core center has been confirmed for the samples injection molded via dynamic packing injection molding, according to the results of 2D-WAXD patterns. A slight variation of orientation degree has been observed in the presence of DBS. Based on the the analysis of the (110) reflection using 2D-WAXD, the result indicates that high concentration of nucleating agent was benefit to promote the formation of a~*-axis orientated components under the same processing conditions.