| It has been well established that final properties of polymeric materials are dependent on the molecular orientation as well as the overall morphology, which are essentially dictated by initially formed crystallization precursor structures under flow. In this dissertation, we have studied flow-induced shish kebab precursor structures in binary model polymer blends containing low molecular weight matrix (short chains) and a small amount of high molecular weight component (long chains). The aim of this approach is to investigate the role of long chains as nucleating precursors leading to full-scale crystallization and to understand the possible mechanism of the shish kebab precursor structure formation in entangled polymer melts under flow.; In the initial part of the dissertation, flow-induced crystallization precursor structures in model blend MB-50k/MD-161k (90/10) as well as in pure MB-50k, where pure MB-50k is a low molecular weight polyethylene copolymers, containing 2 mol % of hexene, with weight average molecular weights ( Mw) of 50,000 g/mol and polydispersity of 2, and high molecular weight component MD-161k is a monodispers polyethylene with Mw of 161,000 g/mol and polydispersity of 1.1, were studied by means of in-situ rheo-WAXD (wide-angle X-ray diffraction) and rheo-SAXS (small-angle X-ray scattering). Based on WAXD and SAXS results, it was seen that the presence of MD-161k profoundly enhanced the shear-induced precursor structures, which directly led to the higher degree of orientation in the blend. Long polymer chains (MD-161 k) were mainly responsible for the formation of the precursor scaffold, which further dictate the crystallization and orientation of the low molecular weight polymer chains.; In the above work, simultaneous crystallization of the low molecular weight matrix (MB-50k) at chosen temperatures (112°C and 115°C) hindered the clear understanding of the precursor scaffold. Meanwhile, the relatively short chain length of MD-161k may also limit the detection of the precursor structures due to its relatively faster relaxation behavior. Therefore, in the later part of this dissertation, a high molecular weight component with longer chain length as well as higher experimental temperature that will keep the matrix amorphous was chosen. In-situ rheo-SAXS and -WAXD techniques was still utilized to investigate development of shear-induced crystallization precursor structure. (Abstract shortened by UMI.)... |
