Flow induced crystallisation of polyethylene in presence of nanoparticles
2010-07-30T10:43:37Z (GMT) by
Polymeric systems become increasingly complicated and multifunctional if they involve a larger level of structural complexity. In the last couple of decades the level of interest has gradually shifted from the μm-scale to the nm-scale region, for instance, systems having at least one structural size below 100nm, e.g. nanocomposites. The physical properties of polymers such as crystallisation, tensile modulus, impact strength and viscosity are strongly influenced by the presence of additives in the polymer matrix. Semicrystalline polymers comprise nearly two-thirds of all synthetic polymers. These are processed to form films, fibers, and moulded articles using operations such as extrusion, moulding, fiber spinning, film blowing etc. During these processes, the polymer melt is subjected to complex and intense flow fields (shear or elongational) after which the polymer crystallises. The morphology of the semicrystalline polymer in the final product and subsequently its properties and quality, depend on the manner in which the polymer crystallises from the flowing melt. The subject is continuously driven by the quest to understand the molecular mechanism of flow induced crystallisation; nevertheless, the flow induced crystallisation in presence of nanofillers has received little attention. The thesis deals with the crystallisation studies of polymer molecules during shear in presence of nanofillers (viz. single walled carbon nanotube (SWCNT) and zirconia particle) having different aspect ratio. For this purpose, the polyethylene (PE) consisting of desired molar mass and molar mass distribution within the processing range is utilised. The morphology of semicrystalline polymer is revealed using time resolved X-ray scattering (SAXS/WAXS) techniques. The rheological aspects of polymer melt in presence of nanoparticles are manifested. In chapter 2, the effect of SWCNTs on the crystallisation kinetics of polymers has been studied with and without application of shear rate. The shear rate effect on the formation of shish-kebab structures in the polymer containing SWCNTs is investigated. The effect of shear rates on the stretching of long chains of PE is verified using the approach involving the use of Deborah number. The study reveals the significance of SWCNTs on crystallisation of PE. In chapter 3, the influence of zirconia nanoparticles on crystal orientation of polymers is studied. Enhanced crystallisation kinetics is observed due to presence of zirconia nanoparticles. Overall crystal orientation is improved as a result of zirconia nanoparticles in the polymer matrix. In chapter 4 of the thesis, the role of broad molecular weight distribution of PE in formation of oriented (shish-kebab) structures is demonstrated. The presence of nanoparticles of different aspect ratios and binding efficiency with polymer on the formation of highly oriented structures in the early stage crystallisation is verified. The study reveals the significant role of SWCNTs in shish-kebab structure formation as compared to zirconia nanoparticles. Further, the insight on the selective adsorption of polymer chains to the nanoparticles is provided. In chapter 5 of the thesis, the molecular interaction between polymer and nanoparticles under shear above the equilibrium point (T = 141.2°C) is investigated. The study reveals the major role of SWCNTs with high aspect ratio, in the stability of flow induced precursor (FIP) and formation of extended chain crystals, as a result of strong interaction with PE molecules. On contrary, the poor interaction of Zirconia particles having low aspect ratio, with PE molecules prohibits molecular chain extension.