No description available.

This thesis describes an investigation of various polymer/nanotube composite systems, using computer simulation. Firstly, pure polymer melts composed of linear homopolymers, polyethylene (PE) and poly(ethylene oxide) (PEO), were studied by a multiscale modelling approach. A Monte Carlo coarse-grained lattice model was employed to generate initial equilibrated chain configurations. A process of ‘reverse-mapping’ was then used to recover atomistic details from the coarse-grained model. After that, fully atomistic molecular dynamic (MD) simulations have been performed on samples of PE and PEO melts. Our results suggest good agreement of structural and dynamical properties between current MD results and experimental and theoretical results. Next, we discussed the application of this multiscale modelling approach to the study of PE/CNT (carbon nanotube) composite system. The presence of CNT causes a clear structuring of the polymer chains around the nanotube surface. Furthermore, structural relaxations of the interfacial PE chains were found to be slower than that of the pure PE melt. MD method and effective fibre theory were used for the calculation of composite mechanical properties, and results agree with experimental data. Finally, we attempted to study the organic/inorganic interface. A composite model containing an inorganic TiO₂ nanotube embedded in a PEO matrix was generated and results are compared with the PEO/CNT system. Much stronger structuring and ordering of PEO chains were found in the vicinity of the TiO₂ surface. In addition, the PEO conformation was more coiled in order to better adapt to the TiO₂ nanotube. Evidence for chain immobilization is more obvious in the PEO/TiO₂ system, which contributes to the improvement of interfacial properties of the composite.

The current work begins with an examination of the sources of stiffness in hollow shells. The findings are used to accurately predict the impact behaviour of hollow spherical shells. Velocity distribution in balls and losses in kinetic energy caused by finite deflections during impact are also investigated. A thorough development of the concept of momentum flux is presented as it applies to the normal impact of hollow spherical shells, while the sources of loss during impact are identified and their magnitudes quantified by means of measurement and analysis. An accurate method for predicting losses during normal impact of hollow balls based on simple compression experiments is presented. Results are shown to be remarkably accurate for the case of both pressurized and pressureless tennis balls. Finally, investigations into the effects of finite deflections and the coefficient of friction during oblique impacts of hollow shells are described. It is shown that the relation between friction predictions for cases with cross sliding provide an accurate description of changes in the moment of inertia of a hollow ball. For the first time, the concept of tangential compliance is applied to the case of oblique impact in hollow shells. The related impact model accurately predicts the phenomenon of overspin, in which the final rotational velocity of the ball is greater than the final tangential velocity of the centre of mass. New understandings of the concept of momentum flux, have led to the development of an oblique impact model with rotation that incorporates momentum flux forces acting around the perimeter of contact into the equations of motion and dynamic stiffness. Results for impact of a well inflated thin-walled shell (basketball) show that effects of finite deflections can not be neglected.

The axial compressive strength of long-carbon fibre reinforced polymer matrix composites is known to be much lower (60% or less) than the tensile strength. The poor compressive strength is strongly associated with fibre misalignments which inevitably occur during the manufacturing process. Therefore, it is essential to be able to quantify the fibre alignment distribution in these materials. In the light of the fact that existing methods of alignment characterisation are very small scale procedures, extracting data from relatively few fibres, a new technique is presented with which information can be obtained on a much larger scale, yet within a reasonable time. A programme of experiments has been completed in order to determine the compressive strength of carbon-fibre reinforced polymer composites. Materials with varying degrees of fibre alignment and porosity have been investigated. The design of a new compression rig was required, the validation of which was accomplished using experimental results and numerical methods. The work described in this thesis addresses the observed failure mechanisms in well-aligned pultruded material and laminated composites with a wider distribution of fibre misalignments. It has been shown that, where measured fibre misalignments are very small, failure is due to fibre kinking, a localised instability (associated with the shear yielding of the matrix on planes parallel to locally misaligned fibres) as opposed to fibre crushing - governed by the intrinsic strength of the fibres themselves. The difficulty of testing representative volumes of material has been discussed but, when done so, it has been shown that elongated pores (approximately 2-3 fibre diameters in width) strongly influence the compressive strength. Numerical modelling has shown that local stress concentrations about such pores might be expected to act as sites for preferential nucleation of kinkbands.

This thesis describes a novel experimental system for the study of polymer flow in a cross-slot using a Cambridge Multi-Pass Rheometer (MPR). Cross-slot flow is capable of generating pure rotation-free extensional flow and the rheological information from the MPR is in a format which can easily be compared with the results of numerical simulations. Using only a small quantity of polymer, this technique provides a quantitative test of the performance of constitutive equations and numerical solvers in extensional flow. The cross-slot apparatus was developed and experiments were studied principally using optical birefringence. It was found that the different shapes of the birefringence patterns observed reveal useful information about the rheology and molecular structure of the polymers. For unbranched low molecular weight polymers at low extension rates, an almost Newtonian response was observed, generating a symmetric fringe pattern. as the level of branching, the molecular weight or the extension rate increased, the fringe pattern became more elongated, suggesting stronger viscoelastic effects. For highly branched polymers a very localised stress concentration was observed along the exit symmetry plane. Pioneering experiments with monodisperse polymer have been particularly revealing – a flow transition is observed at extension rates approximately equal to the inverse of the relaxation time. At low flowrates the response was almost Newtonian, but for extension rates above the inverse of the relaxation time a non-homogeneous stress pattern was observed suggesting possible melt fracture of the fluid in the continuum. The experiments have been compared with numerical simulations using generalised Newtonian, integral-Wagner, Rolie-Poly and pom-pom constructive equations. It has been found that the viscoelastic numerical simulations revealed many of the important features observed from the experiments. However, the quantitative value for stress predicted in some case was higher than the measured value.

Poly(hydroxybutyrate) is a thermoplastic material synthesised naturally by bacteria. Contemporary interest in this material arises from its biodegradability and biocompatibility but, unfortunately, its widespread use is restricted at present because it suffers from embrittlement after it has been moulded. The extension-to-break falls, and the semi-crystalline polymer becomes stiffer. The origins of the embrittlement - or ageing - phenomenon on the microstructural level are investigated in this thesis. The results obtained from the wide-angle X-ray scattering, small-angle X-ray scattering, thermal analysis and molecular modelling are consistent with the operation of a reeling-in process. Tie-molecules or loose loops of the amorphous phase are shortened with concomitant relaxation of a fold on the other side of the crystal. Annealing treatments produce a coarser lamellar morphology. Annealed material is still susceptible to ageing, but the extent to which reeling-in occurs is independent of the anneal temperature. These coarser starting structures are less affected by the limited amount of reeling-in which then takes place. While annealing does not reduce the absolute extent of the ageing, cold-rolling treatments do. Cold-rolling may therefore be an alternative way of reducing the severity of the embrittlement in objects of suitable shape.

This thesis examines the processing behaviour of molten polystyrenes with different molecular weights and architectures, using experimental results obtained from a micro-scale processing apparatus and numerical simulations applying both empirical and molecular-based constitutive models. Processing experiments were performed using a multi-pass rheometer, a two piston capillary-type machine which principally employed a contraction then expansion slit geometry. This was able to provide flow-induced birefringence and pressure difference data at a range of flow rates from ten gram sample quantities. The parameters required to rheologically characterise the linear and non-linear viscoelastic behaviour of the polystyrenes were obtained from experiments performed using parallel plate strain-controlled rheometers. A commercial grade polydisperse polystyrene was used as a reference material and to validate the techniques used. Five monodisperse polystyrenes with molecular masses ranging from 66,000 to 485,000 kg/kmol were studied, together with two differently weighted blends of two of these. As well as these linear molecules of varying molecular weight distribution, observations were made of two polystyrenes with branched structures, one an asymmetric H, and the other a comb structure. Experimental data from the range of materials tested has shown that it is possible to distinguish differences between the types using flow birefringence in a processing flow. Monodisperse polystyrenes are notable for a high degree of symmetry between entry and exit patterns for lower molecular weights and flow rates. Blends of a small proportion of low molecular weight monodisperse with high molecular weight showed significantly improved processability, and similar birefringence to the broadly polydisperse type. Branched molecules were seen to exhibit transient stress fangs in start-up flow. A further significant experimental observation was the onset of instabilities and breakdown of continuum flow in high molecular weight monodisperse polystyrenes. The experimental data was compared with the predictions made by advanced numerical simulations, and it was found that empirical models gave generally good steady state predictions in most cases, while a molecular approach was able to predict the different transient behaviour characteristic of long chain branching, and make predictions for more extreme experimental conditions.

No description available.

A ‘quasi-2D’ object is a structure whose lateral dimensions are orders of magnitude larger than its width. This thesis describes the fabrication of quasi-2D polymeric objects with lateral dimensions on the scale of microns and thicknesses on the scale of nanometres. The fabrication of such objects is of interest due to the possibility of interesting conformational changes that could occur in response to external conditions. The fabrication of ‘quasi-2D’ objects from patterned polymer ‘brushes’ (arrays of polymer chains, each of which is tethered to a surface) is studied. Poly(glycidyl methacrylate) (PGMA) brushes are capable of undergoing ring-opening reactions with a range of different nucleophilic reagents. The functionalised PGMA brushes are studied using ellipsometry. Fourier-Transform Infra-Red spectroscopy (FT-IR), and atomic force microscopy (AFM). After functionalisation, the resulting polymer films can be lifted from the surface without loss of structural integrity, either by etching a sacrificial gold layer from underneath the polymer or by severing the bond between the polymer and the gold surface electrolytically. ‘Hybrid’ objects, containing a gold component and a polymer layer, can be formed by partial etching of a gold layer from underneath a hydrophobic polymer object. The formation of intriguing ‘buckling’ patterns in the polymer is observed after the objects’ release from the surface. It is also possible to create quasi-2D objects from diblock copolymers, of which one polymer block contains a crosslinking monomer and the second block incorporates interesting chemical functionality. The use of this method in the formation of quasi-2D objects incorporating a potentially ‘responsive’ component is explored.

No description available.