Electrospinning: Principles, Practice and Possibilities (Polymer Chemistry Series, Volume 14) - Hardcover

Hardcover

ISBN 10: 1849735565 ISBN 13: 9781849735568

Publisher: Royal Society of Chemistry, 2015

Synopsis

Electrospinning techniques are used to produce novel nanoscale fibrous materials used in a diverse range of applications.

Electrospinning: Principles Practice and Possibilities provides a snapshot of the current cutting edge developments of the field. The first chapter introduces readers to electrospinning, followed by different techniques to prepare fibres such as melt electrospinning and colloidal electrospinning, as well as the properties, structures and uses of the nanofibrous materials in energy applications and regenerative medicine and future directions.

This balanced and authoritative book will appeal to a broad audience of postgraduate students, industrial and academic researchers in the physical and life sciences as well as engineering.

About this title

About the Author

Professor Geoffrey Mitchell was born in Ruislip, Middlesex in 1949. he carried out his PhD work with Professor Alan Windle FRS in the Department of Materials Science at the University of Cambridge. After a period as a JSPS Research Fellow working with Professor Akira Odajima at Hokkaido University in Japan, he joined the Department of Physics at the University of Reading, creating the multidisciplinary Polymer Science Centre in 1988. he pioneered the development of liquid crystal elastomers in the UK and the development of X-ray rheology. His research focuses on the molecular organization of polymer systems and how this can be used to define the properties and functions of materials. He as been active in the field of electrospinning  and has performed the first small-angle neutron scattering studies on the chain conformation of electrospun fibres. He is an expert of the use of x-ray and neutron scattering techniques to evaluate the molecular organization of both disordered and ordered polymer systems.

From the Back Cover

Electrospinning techniques are used to produce novel nanoscale fibrous materials used in a diverse range of applications.

Electrospinning: Principles Practice and Possibilities provides a snapshot of the current cutting edge developments of the field. The first chapter introduces readers to electrospinning, followed by different techniques to prepare fibres such as melt electrospinning and colloidal electrospinning, as well as the properties, structures and uses of the nanofibrous materials in energy applications and regenerative medicine and future directions.

This balanced and authoritative book will appeal to a broad audience of postgraduate students, industrial and academic researchers in the physical and life sciences as well as engineering.

From the Inside Flap

Electrospinning techniques are used to produce novel nanoscale fibrous materials used in a diverse range of applications.

Electrospinning: Principles Practice and Possibilities provides a snapshot of the current cutting edge developments of the field. The first chapter introduces readers to electrospinning, followed by different techniques to prepare fibres such as melt electrospinning and colloidal electrospinning, as well as the properties, structures and uses of the nanofibrous materials in energy applications and regenerative medicine and future directions.

This balanced and authoritative book will appeal to a broad audience of postgraduate students, industrial and academic researchers in the physical and life sciences as well as engineering.

Excerpt. © Reprinted by permission. All rights reserved.

Electrospinning

Principles, Practice and Possibilities

By Geoffrey R. Mitchell

The Royal Society of Chemistry

Copyright © 2015 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-84973-556-8

Contents

Chapter 1 Introduction Fred J. Davis, Saeed D. Mohan and Muaathe A. Ibraheem, 1, 
Chapter 2 Glossary of Terms Jonathan Stanger and Fred J. Davis, 22, 
Chapter 3 The Development of Electrospinning Technologies for Commercial Application Nick Tucker, 34, 
Chapter 4 Optimising Solutions for Electrospinning Stuart R. Coles and Andrew Wooldridge, 57, 
Chapter 5 Polymer Network Dynamics during Electrospinning: Random Walk Simulation Israel Greenfeld and Eyal Zussman, 71, 
Chapter 6 Design and Fabrication of Scaffolds via Melt Electrospinning for Applications in Tissue Engineering Paul D. Dalton, M. Lourdes Muerza-Cascante and Dietmar W. Hutmacher, 100, 
Chapter 7 Electrospinning of Nanoparticles Daniel Crespy, 121, 
Chapter 8 Structure Development in Electrospun Fibres Geoffrey R. Mitchell, Saeed D. Mohan, Fred J. Davis, Kyung-Hwa Ahn, Mohamed Al-Azab, Ahmed El Hadi, Delyth Elliott, Mahadevappa Y. Kariduraganavar, Anitha Nagarajan and Meruyert Nazhipkyzy, 136, 
Chapter 9 Organized Assembly of Electrospun Nanofibres: From 1D to 3D Salima Nedjari, Anne Hébraud and Guy Schlatter, 172, 
Chapter 10 Properties of Electrospun Polymer Fibres Asa H. Barber, 196, 
Chapter 11 Electrospinning for Medical Applications Wenhui Song, Geoffrey R. Mitchell and Krishna Burugapalli, 214, 
Chapter 12 In-house Cultivation of New Generation Meat on Novel Biomimetic 3D Scaffolds Touseef Amna, Inho Hwang, M. Shamshi Hassan and Myung-Seob Khil, 253, 
Chapter 13 Future Perspectives on Electrospinning Geoffrey R. Mitchell and Malcolm Sperrin, 267, 
Subject Index, 271, 

CHAPTER 1

Introduction

FRED J. DAVIS, SAEED D. MOHAN AND MUAATHE A. IBRAHEEM

1.1 Polymer Fibres

The generation of materials with elongation in the form of fibres is a commercially important aspect of polymer science. The act of stretching a macromolecular system into a fibre orients the polymer chains and increases intermolecular interactions; thus, while the diameter of a fibre is much reduced by the drawing process, this is offset by the increased modulus of the fibres which can produce some of the strongest materials known. For example, weight for weight, spider silk is 5 times as strong as steel; many of the applications of nylon rely on the strength provided by the structures that develop on drawing; and another polyamide, Kevlar is well known for its high strength induced by virtue of the hydrogen bonding and phase structure. Less obviously, polyethylene can be made into an ultra-high strength material by drawing.

Fibres are involved in a number of products, most notably textiles, but also in applications as diverse as paper and tyre cord. Fibres also play a crucial role in biology, with materials such as cellulose forming fibrils which provide much-needed strength to plant walls or, on the negative side, the development of amyloid fibrils has been linked with a range of human diseases (Figure 1.1(a)). In a commercial or laboratory setting the process of fibre formation can be done in a number of ways, but all rely on the polymer initially being present in a molten or a solution state. The production of fibres from synthetic polymers is a major industrial activity. In a typical process to produce a textile fibre, the polymer is passed through a spinneret, and then elongated by the application of a mechanical force. This process requires, firstly, that the polymer is able to flow prior to processing, i.e. it must be in a molten or a solution state; secondly, that the material solidifies in the region post spinning. For melt spinning the requirement is simply one of reducing the temperature; for solution spinning the solvent must be removed by evaporation (known as dry spinning) or by precipitation into a solvent bath (termed wet spinning). Many commercial applications require post processing, for example, in the viscose process cellulose is modified to produce a processable material and subsequently regenerated in sulphuric acid post processing. Famously, carbon fibres are generated from thermal treatment of polyacrylonitrile in a process involving cyclisation, as shown in Figure 1.2.

In general, spinning processes such as those described above result typically in threads with diameters between 1 and 20 µm. Recently, interest in developing materials at the nanoscale has resulted in a surge of activity in the production of fibres with submicron diameters. Whilst such materials are largely inaccessible to melt spinning, there are alternative processes such as melt blowing, which are clearly capable of producing fibres with diameters smaller than 500 nm. The possibility of such top-down methods of producing nanofibres has attracted huge interest in such technology and, in particular, the technique which is the focus of this book: electrospinning, a techniques that produce fibres with diameters ranging from a few microns to a few nanometres.

1.2 Principles of Electrospinning

In electrospinning, an electrostatic force produced by a high voltage supply is used to drive the spinning process. This field is applied to droplets of polymer solution (or a melt) passed from the tip of a fine orifice. The technique has its origin in the studies on electric fields initiated by Lord Rayleigh in the nineteenth century, and was originally patented in 1902, although it was in 1934 that Formhals considered the procedure as a means for processing textile yarns. It was Taylor who, in his work, developed the theoretical understanding of the influence of an electric field on a small volume of liquid. Thus, when an electric field is applied to a liquid droplet, an electrostatic charge accumulates at the tip of this droplet. Consequently, charge repulsion works against the surface tension, causing the shape to change from a broadly spherical surface to an elongated cone shape (known later as a Taylor Cone), a jet then is released from the capillary spinneret. This jet may take a complex path and is subjected to instabilities induced by electric charges, but in the absence of a viscous polymer solute, the liquid will ultimately atomise into tiny droplets; this is the origin of the important process of electrospraying. However, when the liquid contains a polymer as a solute, the viscosity must also be considered; if sufficiently large, the polymer viscosity will oppose the breaking up of the jet into droplets. This produces a thread of polymer solution which will be subjected to the electrical instabilities also seen in the pure liquid. These whipping or bending instabilities mean that the polymer solution may take a complex path, as shown in Figure 1.3, and as a result of the motion and stretching, solvent is removed and the polymer thread elongated. This is crucial to the development of fibres with sub-micron diameters. The whipping process, while crucial to the development of the extreme elongation of the fibres, distinguishes this technique from other fibre producing technologies, where the fibre orientation is easily controlled. As a consequence of this controlled deposition of fibres to form organised structures, a basic requirement for developing nanomaterials requires some modification to the techniques, as discussed below. The physics of the polymer jet is discussed further in Chapter 5.

1.3 Equipment for Electrospinning

The equipment needed for electrospinning on a laboratory scale is relatively simple; in the basic setup there are three main components: the high voltage power supply, a syringe pump and the collector, which can be simply a sheet of aluminium foil. A schematic of the equipment used for electrospinning is shown in Figure 1.4. In this simple arrangement the fibres are deposited as a random mat, as shown in Figure 1.5(a). There may, however, be alternative arrangements, for example, the New Zealand company Electrospinz produces an elegantly designed apparatus that uses a constant pressure header tank rather than the syringe pump and needle arrangement represented by Figure 1.4. An interesting variation of needle electrospinning is coaxial electrospinning, which in some cases uses two concentric needles with different diameters. In this way, fibres can be obtained with an inner core that differs from the external part of the fibre (core–shell fibres). In general, the design of electrospinning systems has been subject to numerous technological advances in recent years.

For many applications, and indeed for certain characterisation techniques, a random mat is not suitable, and an array of aligned fibres is required. There are a number of ways this problem can be addressed; one simple means of doing this is via the use of a rotating collector which produces aligned fibres, as shown in Figure 1.5(b). The preparation of aligned fibres is one important area of control, but there is also a demand for precision deposition of nanofibres to provide patterned nanostructures. A range of techniques can be used for this. For example, near-field electrospinning uses a reduced distance between the collector combined with a solid probe to produce patterns, though the limitations of dip-pen methodology mean modifications must be made if the method is to be used to form continuous fibres. Further details on the control of electrospinning are provided later in the book (Chapter 9).

The system shown in Figure 1.4 is not suitable for the large-scale production of fibres, which for many applications is necessary if significant commercialisation is to be achieved. One simple approach is the multi-needle system, but this requires the needles to be spaced such that the neighbouring fields do not interact, and the needles are subject to blocking. Such difficulties have prompted the investigation of needleless approaches, whereby the use of a rotating cylinder partially immersed in a polymer solution produces multiple jets, and this is an approach patented by Elmarco in the device known commercially as the Nano spider; a more recent development of this uses a stationary wire electrode system (see Figure 1.6). Other approaches include using compressed air to generate bubbles on the surface, from which multiple jets can be formed (see Figure 1.7), and the use of a rotating cone.

1.4 Processing Parameters

The need for fibres with controlled diameters and morphology has prompted many studies of the relationship between the fibre properties and the variable parameters. It is helpful to classify these parameters into three categories: solution or material parameters such as viscosity and/or concentration, operation parameters which consider the main critical factors in theoretical and applied studies such as voltage, working distance, flow rate etc. and finally the ambient parameters such as humidity and temperature. These factors may influence the fibre morphology individually in some way, or they may all work in harmony with one another (see Chapter 4 for specific examples). Table 1.1 provides a summary of some of the most important parameters and their effect on fibre diameter and/or morphology.

Most electrospinning is carried out in a suitable solvent. Though many of the crucial parameters (such as viscosity) depend on the properties of the polymer, the solvent is also important in the electrospinning process. Particularly important factors are, firstly, the vapour pressure, which can have a major effect on polymer morphology; secondly, the polarity which will influence the conductivity and hence interaction with the electric field; thirdly, the surface tension, which is a force to be overcome by the electric field. Of course, the addition of the polymer will further modify the properties and there are a range of additives which can also affect the behaviour of the solution. For example, the viscosity can be increased by the addition of dibenzyl sorbitol and the surface tension can be modified by the addition of surfactants. The solvent properties, such as those listed in Table 1.2, provide a guide to the limitations of a particular system. It is particularly important to note that there are substantial hazards involved with the use of many solvents, as shown in Table 1.2, many of the solvents listed below are flammable and/or may have the potential to produce cancer, or are hazardous to unborn children. On that basis, the possibility of residual solvent may be a restriction in many applications. In such circumstances, melt electrospinning offers an alternative route to fibres but, by virtue of the intrinsically higher viscosity, the fibre diameters are likely to be larger. However, some improvements have been made in this direction. In contrast, for conventional fibre production, melt spinning is the default technique; solution spinning might be used, for example, when the polymer degrades in the melt. In this case the advantages, both in terms of economics, safety and the environment, are considerable and in many cases the fibres can be produced at a faster rate.

Of the solution parameters, the viscosity is probably the most important and several studies on this have been conducted (see Chapter 4). Long et al. have shown that, for a range of polymers, the diameter of the fibres produced can be related to the ratio of the concentration of the solution used to the critical concentration required for entanglements via a power law. Similarly, Wang et al. have related the viscosity to the fibre diameter via the relationship shown in eqn (1). The viscosity can be increased by changing the concentration of polymer or by changing the molecular weight. In both cases, as the viscosity increases there is a transition from a regime where spinning fibres is not possible, to one where fibres are formed with a diameter increasing with the viscosity. In the intermediate range the transition from electrospraying to electrospinning is shown by the presence of fibres interspersed with polymer droplets (beaded fibres), as shown in Figure 1.9. In general, the fibre diameter increases with increasing viscosity (and hence molecular weight and concentration) and this can be an important factor in determining if a particular material can be electrospun. On this basis, it might be expected that fibres generated in the melt would be less likely to be nanoscale, but note that while this is often the case as, for example, with the poly(caprolactone) samples shown in Figure 1.8, with good experimental control, excellent results have been found with melt spun fibres, particularly in the medical field (see Chapters 6 and 11).

d ~ η0.41 (1)

Though the viscosity supplies some control of the fibre parameters, other factors need to be considered. Surface tension is important in control of the Taylor cone, but it may not contribute hugely to the tunability of the system in normal circumstances, but when the viscosity is low then, importantly, this will start to influence the fibres in terms of the beading discussed above; conductivity increases the charge separation and increases elongation and thus it would seem an increase in conductivity will result in a decrease in fibre diameter.

1.5 Materials

Most common soluble and tractable polymers have been processed as nanofibres via electrospinning. Some typical examples are included in Table 1.3, and in several excellent reviews on the subject. The important features are the solubility or the ability to melt the polymer, and that the molecular weight is sufficiently high to allow entanglements between polymer chains (vide supra). However, it should be noted that there are examples where low molecular weight polymers can be spun by virtue of the presence of associative interactions as is the case in the urethane systems shown in Figure 1.10.

As can be seen in Table 1.3, many of the polymers electrospun have been water soluble materials these include polyethylene oxide, poly(vinylalcohol) and poly(N-vinyl pyrrolidone). Water has a great advantage in terms of potential commercialisation and it is particularly advantageous when the proposed application is medical, since alternative solvents may introduce problems such as toxicity; even though the removal of the majority of the solvent is integral to the process. Biopolymers and even more complex entities such as active viruses and bacteria are not destroyed by electrospinning and so there is much interest in these fibres in the medical field. However, many polymer systems, including many of those that may have potential medical applications, are not water-soluble. Melt electrospinning offers an alternative for applications where the use of water-soluble polymers is not an option (see Chapter 6).

Electrospinning has been used to prepare more complex systems than simple polymers; for example, block copolymers and polymer blends. In such systems the development of the morphology may occur in a complex fashion. Inorganic materials can be produced by electrospinning; for example, a metal oxide/polymer mixture followed by pyrolysis. Interestingly, copper nanofibres have been prepared from poly(vinylbutyral)/CuNO3 composite fibres. In addition, composite fibres have been prepared, incorporating a range of particles e.g. carbon nanotubes have been incorporated into poly(acrylonitrile) which on pyrolysis produce carbon fibres. In a similar fashion a range of particulate materials can be incorporated into a carrier polymer; for example, Figure 1.11(a) shows polypyrrole particulates spun into fibres of polyvinyl alcohol. A variety of reagents can also be incorporated for post processing e.g. ferric chloride can be incorporated into polystyrene and nanofibres produced – this, post processing can be treated with pyrrole vapour to produce a material with a polypyrrole surface; similarly, inclusion of glucose and post treatment with ammonical silver nitrate produces silver coated fibres [Figure 1.11(b) and (c), respectively].

(Continues...)

Excerpted from Electrospinning by Geoffrey R. Mitchell. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2015
• Language: English
• ISBN 10: 1849735565
• ISBN 13: 9781849735568
• Binding: Hardcover
• Edition number: 1
• Number of pages: 288
• Editor: Mitchell Geoffrey R