Flow induced crystallisation of polyethylene in presence of nanoparticles

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

Influence of reduced graphene oxide on the rheological response and chain orientation on shear deformation of high density polyethylene

© 2016 Elsevier Ltd. All rights reserved. The rheological response of high density polyethylene/reduced graphene oxide nanoplatelets (HDPE/rGON) composites, and the influence of rGON on chain orientation and crystallization behavior after shear flow are investigated. Melt rheology reveals the presence of strong interaction between polymer chains and the filler. Above 4.0 wt % of the filler concentration, the terminal region of frequency sweep shows changes in the linear viscoelastic properties of the composites. In particular, at these high concentrations the cross-over frequency at which the transition from predominantly elastic to viscous behavior occurs significantly shifts to lower values, indicating the formation of a solid-like percolated network. A drop in G′ at high frequency (100 rad/s) is observed in the presence of the filler, and the storage modulus shows minima at filler concentration between 2.0 and 4.0 wt %. The influence of chain-filler interaction on chain orientation, and subsequent crystallization behavior after application of shear is followed by time resolved WAXD/SAXS. The orientation of the crystalline domains was quantified by the Herman's orientation factor that supports the presence of strong chain-filler interaction. The Deborah number of reptation and retraction suggests that during the applied non-linear shear, polymer chains in the composites experience mild stretch that is not significant enough to induce crystallization at the high temperature (136 °C). However, restriction imposed by the filler on the chain mobility is pronounced enough to preserve oriented state that causes anisotropy in crystallization on cooling. The enhanced orientation with increasing filler content is conclusively attributed to the strong chain-filler interaction

Results obtained with fast-scanning calorimetry sample holder.

<p>a) Optical micrograph of the active area on the electronic chip. The central patch is the P3HT-PCBM sample. b-f) Reconstructed phase-contrast high-resolution ptychography projections, with (b) showing the area corresponding to the section marked by the outlined white square in (a). c) Projection for as-cast film, showing that the deposition process has induced a certain morphology. d) Morphology after erasing the thermal history by shortly visiting the melt, showing an essentially featureless image. e),f) Images obtained after 60 s and an additional 660 s (total of 720 s), respectively, of annealing at 400 K, clearly showing that a coarser morphology develops with time. All images were collected at room temperature to reduce problems with radiation damage.</p

Sketch of the experimental setup for transmission X-ray ptychography.

<p>Coherent diffraction patterns are recorded by a 2D detector for a set of partially overlapping scanning positions, allowing numerical reconstruction of the projected complex-valued image of the sample.</p

X-Ray Nanoscopy of a Bulk Heterojunction - Fig 4

<p>(a) Reconstructed phase contrast high resolution projection of P3HT/PCBM layer isothermally annealed for 7500 s at 127°C. The black line in the projection image corresponds to the PCBM concentration profile shown in (b). The red curve in (b) shows the fitting analysis applied to the PCBM concentration profile using <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158345#pone.0158345.e007" target="_blank">Eq 5</a>. The fitted parameters of <i>L</i> = 6.0 μm and <i>D</i> = 7 x 10⁻¹² cm² s^-1 provide excellent agreement between the experimental data and the model.</p