Geoff D. Moggridge

Barrier films using flakes and reactive additives

The permeability of thin films can be reduced by using a new polymer, by adding impermeable and aligned flakes, or by incorporating reactive groups. This paper discusses the latter two mechanisms, those of flakes and reactive groups. Flakes reduce both the steady permeability and increase the lag before permeation, typically by a factor of ten. The effect of flakes has in the past been correlated with theories which assume the flakes are aligned like long ribbons, so that diffusion is two dimensional. Alternatively, this effect can be estimated with theories developed here, which assume finite flakes and three-dimensional diffusion. The old and new theories bracket observed behavior. The permeability changes caused by reactive groups are very different. Reactive groups do not alter steady-state permeability, but increase the lag before penetration only if the groups are immobile; the increase will be greatest if the reaction is irreversible. When both aligned flakes and reactive groups are added to a thin film, the effects are cumulative, so that the lag can increase over a thousand times. Predictions of this increase are supported by experiment. Because all experiments here are on model systems, the extension to real coatings is also discussed.

An Introduction to Chemical Product Design

This paper derives from an undergraduate course in chemical product design which we first taught in 1998/9 and are still in the process of developing. We are in the process of writing a text book to accompany such courses. We explain our approach to chemical product design and why the subject is important. The distinctive features of product design (particularly in contrast to process design, a more familiar topic for chemical engineers) are outlined in Section 1. The emphasis is on decisions which are required before chemical process design can be started. Our chemical product design course is a response to major changes in the chemical industry which have occurred in recent decades. These changes, described in Section 2, involve a move in the industry away from the manufacture of commodity chemicals and towards speciality chemicals and other high added value products. The former is well served by traditional process design, the latter benefits also from product design. Section 3 describes the product design procedure that we use. It is a simplification of procedures already used in business development and manufacturing engineering (see, for example, Ulrich and Eppinger1). Such a simplification clarifies the sequence of ideas involved and also forces us to consider in detail the technical questions implied in specific products. Our approach is aimed at those with training in engineering and chemistry, but may also be a beneficial challenge for those whose training is largely in business.

A method to unmix multiple fluorophores in microscopy images with minimal a priori information

The ability to quantify the fluorescence signals from multiply labeled biological samples is highly desirable in the life sciences but often difficult, because of spectral overlap between fluorescent species and the presence of autofluorescence. Several so called unmixing algorithms have been developed to address this problem. Here, we present a novel algorithm that combines measurements of lifetime and spectrum to achieve unmixing without a priori information on the spectral properties of the fluorophore labels. The only assumption made is that the lifetimes of the fluorophores differ. Our method combines global analysis for a measurement of lifetime distributions with singular value decomposition to recover individual fluorescence spectra. We demonstrate the technique on simulated datasets and subsequently by an experiment on a biological sample. The method is computationally efficient and straightforward to implement. Applications range from histopathology of complex and multiply labelled samples to functional imaging in live cells.

Influence of stirrer speed on the precipitation of anatase particles from titanyl sulphate solution

The formation of titanium dioxide particles in the form of anatase is an important step in the sulphate process used to manufacture white pigment. This paper investigates the influence of stirrer speed on anatase precipitation carried out in batch mode. Stirrer speed was varied between 200 and 1200 rpm. Though its effect on the chemical kinetics of precipitation was negligible, intensity of stirring determines the final particle size of anatase aggregates in suspension. Changes to primary agglomerate and crystal population due to varying stirrer speed are reported. The Kolmogoroff eddy size estimated for the system was an order of magnitude or more larger than the largest particle size encountered during precipitation. In our study, the largest aggregates (mean size of 1.52 μm) were obtained at a stirrer speed of 200 rpm. Running the precipitation at stirrer speed lower than 188 rpm would result in accumulation of particles at the bottom of the stirred vessel.

Direct experimental evidence for flow induced fibrous polymer crystallisation occurring at a solid/melt interface

We report experimental observations on the way that flowing polyethylene melts can crystallise within a processing channel geometry. Using a recently developed Multipass Rheometer (MPR), we present rheological, rheo-optic and coupled X-ray data that follow the evolution of crystallisation, as molten polyethylene flows into a slit geometry. Optical observations show that fibrous crystallisation occurs initially at the walls of the slit and not, as expected, in the entrance region to the slit. The coupled X-ray, rheology and rheo-optic data lead us to speculate that a coil-stretch transition of the polymer chains occurs at the wall of the slit and this acts as the primary cause of fibrous X-ray nucleation. At high wall shear rates we identify evidence to suggest that slip occurs between the flowing polymer and the solid wall and this in turn causes the onset of fibrous crystallisation to be surpressed. The experimental observations are generally consistent with certain theoretical predictions made by Brochard and de Gennes.

Evaluation of an aortic valve prosthesis: Fluid-structure interaction or structural simulation?

Bio-inspired polymeric heart valves (PHVs) are excellent candidates to mimic the structural and the fluid dynamic features of the native valve. PHVs can be implanted as prosthetic alternative to currently clinically used mechanical and biological valves or as potential candidate for a minimally invasive treatment, like the transcatheter aortic valve implantation. Nevertheless, PHVs are not currently used for clinical applications due to their lack of reliability. In order to investigate the main features of this new class of prostheses, pulsatile tests in an in-house pulse duplicator were carried out and reproduced in silico with both structural Finite-Element (FE) and Fluid-Structure interaction (FSI) analyses. Valve kinematics and geometric orifice area (GOA) were evaluated to compare the in vitro and the in silico tests. Numerical results showed better similarity with experiments for the FSI than for the FE simulations. The maximum difference between experimental and FSI GOA at maximum opening time was only 5%, as compared to the 46.5% between experimental and structural FE GOA. The stress distribution on the valve leaflets clearly reflected the difference in valve kinematics. Higher stress values were found in the FSI simulations with respect to those obtained in the FE simulation. This study demonstrates that FSI simulations are more appropriate than FE simulations to describe the actual behaviour of PHVs as they can replicate the valve-fluid interaction while providing realistic fluid dynamic results.

Shear-Induced Structural Changes in a Commercial Surfactant-Based System

Linear alkylbenzene sulfonate (LAS) is the worlds largest volume surfactant. During industrial processing it is transformed from a ‘sticky’, yellow material of relatively low viscosity into a ‘springy’, white, high-viscosity substance suitable for use in washing powder. It is demonstrated that this transition can be achieved by moderate shear alone at 25°C and is accompanied by a phase transition from a planar lamellar structure to one consisting of multi-lamellar vesicles (MLVs). At higher shear rates the MLVs are not formed, the lamellar structure is stable and oriented in the direction of the flow; this is accompanied by the macroscopic properties of untreated LAS. At 60°C it is not possible to achieve the phase transition to MLVs, nor to obtain the ‘springy’ white material by shear alone. Under all conditions the inter-lamellar spacing of the bulk is around 30 A, but an approximately 34 A lamellar phase, oriented parallel to the wall, was always observed (even when MLVs were formed in the bulk) close to the walls of the capillary in which shearing occurred.

Mechanical strength of sutured block copolymers films for load bearing medical applications.

The mechanical behavior of three styrenic thermoplastic block copolymer elastomers with applied surgical sutures was studied by uniaxial tensile testing. The materials exhibited oriented cylindrical microstructure. Distinct macroscopic deformation mechanisms have been observed upon stretching of samples with vertical and horizontal orientation. Deformation progressed along the axis of the suture in samples with parallel orientation (P), while it in case of normal orientation (N) the whole sample responded to the applied force. Also the analysis of the stress-strain curves showed a significant difference between samples P and N. Greater stress at break was observed for samples P, while samples N showed the capability to tolerate higher strain. The influence of morphology on the tear-out shape has been also observed. The thread made a vertical tear out in samples P while for samples N ripping off the bottom was observed.

ENGINEERING ORIENTATION IN BLOCK COPOLYMERS FOR APPLICATION TO PROSTHETIC HEART VALVES

This study demonstrates how the mechanical performance of polymeric material can be enhanced by morphology and phase orientation of block copolymers to achieve desired anisotropic mechanical properties. The material used was a new Kraton block copolymer consisting of styrene-isoprene-butadiene-styrene blocks having cylindrical morphology. We report a method of achieving long range uniaxial as well as biaxial orientation of block copolymer. Each microstructural organization results in a specific mechanical performance, which depends on the direction of the applied deformation. The method of tailoring mechanical properties by engineering microstructure may be successfully utilized to applications requiring anisotropic mechanical response, such as prosthetic heart valves.

A NEWLY DEVELOPED TRI-LEAFLET POLYMERIC HEART VALVE PROSTHESIS

The potential of polymeric heart valves (PHV) prostheses is to combine the hemodynamic performances of biological valves with the durability of mechanical valves. The aim of this work is to design and develop a new tri-leaflet prosthetic heart valve (HV) made from styrenic block copolymers. A computational finite element model was implemented to optimize the thickness of the leaflets, to improve PHV mechanical and hydrodynamic performances. Based on the model outcomes, 8 prototypes of the designed valve were produced and tested in vitro under continuous and pulsatile flow conditions, as prescribed by ISO 5840 Standard. A specially designed pulse duplicator allowed testing the PHVs at different flow rates and frequency conditions. All the PHVs met the requirements specified in ISO 5840 Standard in terms of both regurgitation and effective orifice area (EOA), demonstrating their potential as HV prostheses.