Isotactic polypropylene (iPP) is one of the widely used commercial thermoplastics. Physical properties of iPP can be tailored to the requirements with respect to structure, microstructure and processing, thus research continues in the development and modification of the polymer. With the advancement of chemistry, as our understanding in tailoring of the molecular structure has enhanced, iPP has become more of a generic name.

Yes / The present work deals with the flow-induced multiple orientations and crystallization structure of polymer melts under a complex flow field. This complex flow field is characteristic of the consistent coupling of extensional “pulse” and closely followed shear flow in a narrow channel. Utilizing an ingenious combination of an advanced micro-injection device and long chain aliphatic polyamides (LCPA), the flow-induced crystallization morphology was well preserved for ex-situ synchrotron micro-focused wide angle X-ray scattering (μWAXS) as well as small angle X-ray scattering (SAXS). An inverted anisotropic crystallization structure was observed in two directions: perpendicular and parallel to the flow direction (FD). The novel anisotropic morphology implies the occurrence of wall slip and “global” fountain flow under the complex flow field. The mechanism of structure formation is elucidated in detail. The experimental results clearly indicate that the effect of extensional pulse on the polymer melt is restrained and further diminished due to either the transverse tumble of fountain flow or the rapid retraction of stretched high molecular weight tails. However, the residual shish-kebab structures in the core layer of the far-end of channel suggest that the effect of extensional pulse should be considered in the small-scaled geometries or under the high strain rate condition.

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.

No / An experimental configuration that combines the powerful capabilities of a short-term shearing apparatus with simultaneous optical and X-ray scattering techniques is demonstrated, connecting the earliest events that occur during shear-induced crystallization of a polymer melt with the subsequent kinetics and morphology development. Oriented precursors are at the heart of the great effects that flow can produce on polymer crystallization (strongly enhanced kinetics and formation of highly oriented crystallites), and their creation is highly dependent on material properties and the level of stress applied. The sensitivity of rheo-optics enables the detection of these dilute shear-induced precursors as they form during flow, before X-ray techniques are able to reveal them. Then, as crystallization occurs from these precursors, X-ray scattering allows detailed quantification of the characteristics and kinetics of growth of the crystallites nucleated by the flow-induced precursors. This simultaneous combination of techniques allows unambiguous correlation between the early events that occur during shear and the evolution of crystallization after flow has stopped, eliminating uncertainties that result from the extreme sensitivity of flow-induced crystallization to small changes in the imposed stress and the material. Experimental data on a bimodal blend of isotactic polypropylenes are presented.