Stretching polymer chains

This paper is based on a presentation given at the de Gennes discussion meeting held at Chamonix in February 2009. The paper gives a personal review of the way developments relating to the stretching of polymer chains has taken place over the last 40 years. de Gennes was very influential in relation to chain dynamics concepts at the University of Bristol, where many pioneering concepts relating to chain stretching were developed by the late Sir Charles Frank and Andrew Keller. The paper reviews basic concepts on extensional rheology, droplet deformation, chain extension from extensional flow and the achievement of high chain extension for high-modulus polyethylene. The paper also reviews recent developments concerning the influence of chain stretching on polymer melt processing.     

Rheo-optic flow-induced crystallisation of polypropylene and polyethylene within confined entry–exit flow geometries

This paper reports experimental observations on the way polypropylene (PP) and high-density polyethylene (HDPE) can crystallise during flow. Both a deep and a shallow slit geometry were chosen for the rheo-optical study. Preliminary linear viscoelastic rheological tests enabled the temperature window for quiescent crystallisation to be established. Flow-induced crystallisation (FIC) studies were performed in a temperature regime above the normal quiescent crystallisation conditions. In the case of HDPE, FIC occurred during flow at the sidewalls of the slit and in localised regions downstream and the processing pressure increased during the piston movement. In the case of PP, flow-induced crystallisation was generally observed after flow cessation, and the processing pressure did not change during flow. For PP, FIC also occurred preferentially at the walls in the form of elongated crystallites, but the fibres gradually emerged after flow cessation. The difference in the FIC behaviour was attributed to differences in both the rheology and the crystal growth kinetics of the two materials at the particular super-cooling used.     

Engineering the future of rheology within the UK

This short communication reviews the past, current and future state of rheology within the UK. It is a personal view that predicts in the future there will be significant changes in UK rheology, mainly because of the way universities, industry and most of all funding agencies have changed their priorities in the area of science and engineering.     

A simple device for multiplex ELISA made from melt-extruded plastic microcapillary film

We present a simple device for multiplex quantitative enzyme-linked immunosorbant assays (ELISA) made from a novel melt-extruded microcapillary film (MCF) containing a parallel array of 200 μm capillaries along its length. To make ELISA devices different protein antigens or antibodies were immobilised inside individual microcapillaries within long reels of MCF extruded from fluorinated ethylene propylene (FEP). Short pieces of coated film were cut and interfaced with a pipette, allowing sequential uptake of samples and detection solutions into all capillaries from a reagent well. As well as being simple to produce, these FEP MCF devices have excellent light transmittance allowing direct optical interrogation of the capillaries for simple signal quantification. Proof of concept experiments demonstrate both quantitative and multiplex assays in FEP MCF devices using a standard direct ELISA procedure and read using a flatbed scanner. This new multiplex immunoassay platform should find applications ranging from lab detection to point-of-care and field diagnostics.     

The separation of immiscible liquid slugs within plastic microchannels using a metallic hydrophilic sidestream

This paper describes experiments and related modelling on a new method for separating aqueous phase slugs from the surrounding organic matrix phase in segmented two phase flow in a plastic microcapillary film (MCF). Kerosene or paraffin oil was metered through a plastic capillary of 630 microns diameter and aqueous phase slugs were generated within the capillary by the continuous sidestream injection of water. It was found that the resulting aqueous phase slugs formed in the MCF could be subsequently easily separated from the organic phase by piercing the downstream sidewall of the plastic capillary with a hydrophilic metal hypodermic needle to draw off an aqueous sidestream. Optical scrutiny of the phase separation process indicated that two distinct disengagement mechanisms are involved, in which the metal needle tip either remains submerged in the aqueous phase or becomes periodically exposed to the organic phase at certain stages of the segregation process. The separation efficiency, i.e. the degree of residual phase cross-contamination, was determined as a function of both the sidestream needle angle and the depth of needle penetration into the capillary for a given flow rate and phase ratio. It was established that the separation efficiency was very sensitive to the downstream pressure balance between the organic mainstream flow in the plastic capillary and the aqueous sidestream flow through the needle. A mathematical model for the pressure balance conditions was developed by making certain simplifying assumptions and taking the Laplace interfacial pressure into account. The model predictions agreed surprisingly well with the experimental findings, thus providing circumstantial evidence for the validity of the insights into the phase separation mechanism.     

Fast cation-exchange separation of proteins in a plastic microcapillary disc

A novel disposable adsorbent material for fast cation-exchange separation of proteins was developed based on plastic microcapillary films (MCFs). A MCF containing 19 parallel microcapillaries, each with a mean internal diameter of 142 μm, was prepared using a melt extrusion process from an ethylene-vinyl alcohol copolymer (EVOH). The MCF was surface functionalised to produce a cation-exchange adsorbent (herein referred as MCF-EVOH-SP). The dynamic binding capacity of the new MCF-EVOH-SP material was experimentally determined by frontal analysis using pure protein solutions in a standard liquid chromatography instrument for a range of superficial flow velocities, u_LS = 5.5–27.7 cm s⁻¹. The mean dynamic binding capacity for hen-egg lysozyme was found to be approximately 100 μg for a 5 m length film, giving a ligand binding density of 413 ng cm⁻². The dynamic binding capacity did not vary significantly over the range of u_LS tested. The application of this novel material to subtractive chromatography was demonstrated for anionic BSA and cationic lysozyme at pH 7.2. The chromatographic separation of two cationic proteins, lysozyme and cytochrome-c, was also performed with a view to applying this technology to the analysis or purification of proteins. Future applications might include separation based on anion exchange and other modes of adsorption.     

Stress relaxation after oscillatory shear in a thermotropic liquid crystalline polymer

We report investigations of the rheology of a thermotropic liquid crystalline polymer subjected to oscillatory shear. In particular, we have studied its stress relaxation behaviour and found it to be well-represented by the linear viscoelastic model. In our studies, we observed a strong dependence of the relaxation rate on the position in the strain cycle at which the shear is stopped; a result which we show to be a general phenomenon of viscoelastic fluids and not unique to liquid crystalline polymers.