Electrospinning E-Book

Joachim H. Wendorff

• Herausgeber: Wiley-VCH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 390

Veröffentlichungsjahr: 2012

Beschreibung

Electrospinning is from the academic as well as technical perspective presently the most versatile technique for the preparation of continuous
nanofi bers obtained from numerous materials including polymers, metals, and ceramics. Shapes and properties of fibers can be tailored
according to the demand of numerous applications including filtration, membranes, textiles, catalysis, reinforcement, or biomedicals.
This book summarizes the state-of-the art in electrospinning with detailed coverage of the various techniques, material systems and their
resulting fiber structures and properties, theoretical aspects and applications.

Throughout the book, the current status of knowledge is introduced with a critical view on accomplishments and novel perspectives. An
experimental section gives hands-on guidance to beginners and experts alike.

Leseprobe

Table of Contents

Cover

Related Titles

Title page

Copyright page

Preface

The Preparation Process

During Applications

During Disposal

1 Introduction

1.1 Fibers – Key Functional Elements in Technology and Nature

1.2 Some Background Information

1.3 Processing of Polymer Materials towards Fibers – Fiber Extrusion

1.4 Routes to More Advanced Fibers – Mimicking Nature

1.5 Electrospinning

1.6 Electrospinning – Important Facts to Remember

2 Nature of the Electrospinning Process – Experimental Observations and Theoretical Analysis

2.1 Experimental Setups

2.2 Experimental Observations on Fiber Formation

2.3 Theoretical Analysis of the Nature of the Electrospinning Processes

2.4 Nature of the Electrospinning Process – Important Facts to Remember

3 Nanofiber Properties

3.1 Parameters Controlling Nanofiber Formation

3.2 Short Account on Methods of Analysis for the Structure of Electrospun Nanofibers

3.3 Control of Nanofiber Diameters

3.4 Shape of the Fibers

3.5 Nanofiber Topologies, Porous Fibers

3.6 Nanofiber Trajectories in the Deposition Plane

3.7 Internal Morphology of Electrospun Nanofibers

3.8 Mechanical Properties of Single Nanofibers

3.9 Nanofiber Properties – Important Facts to Remember

4 Nonwovens Composed of Electrospun Nanofibers

4.1 Nanofiber Nonwovens – Functional Elements for Technical Applications

4.2 Methods of Analysis for Properties on Nonwovens

4.3 Fiber Arrangements in Nonwovens

4.4 Heterogeneous Nonwovens

4.5 Porosity and Pore Structures – Theoretical Modeling and Experimental Analysis

4.6 Mechanical Properties of Nonwovens

4.7 Nonwovens Composed of Electrospun Nanofibers – Important Facts to Remember

5 Electrospinning – Some Technical Aspects

5.1 Technical Setups for Creating Jets

5.2 Designs of Counterelectrode Configurations Allowing Preparation of Nonwovens with a Broad Range of Architecture

5.3 Electrospinning – Some Technical Aspects – Important Facts to Remember

6 Modification of the Electrospinning Technique

6.1 Towards Advanced Modes of Fiber Preparation and Deposition

6.2 Near-Field Electrospinning – High-Precision Deposition Electrospinning

6.3 Towards Core–Shell and Hollow Fibers

6.4 Modification of the Electrospinning Technique – Important Facts to Remember

7 Materials Considerations

7.1 Introduction

7.2 Spinning from Organic Solvents

7.3 Spinning of Water-Soluble Polymers

7.4 Spinning of Biopolymers

7.5 Spinning of Complex Polymer Systems

7.6 Nanofibers from Polymer Hybrids, Metals, Oxides

7.7 Melt Spinning

7.8 Materials Considerations – Important Facts to Remember

8 Technical Applications of Electrospun Nanofibers

8.1 Nanofibers, Nanofiber Nonwovens – Elements of Nanotechnology

8.2 Filter Applications

8.3 Textile Applications

8.4 Catalysis

8.5 Nanofiber Reinforcement

8.6 Surface Modifications

8.7 Template Applications

8.8 Plant Protection via Pheromones

8.9 Technical Applications – Important Facts to Remember 214 References

9 Medicinal Applications for Electrospun Nanofibers

9.1 Nanotechnology and Medicinal Applications in General

9.2 Tissue Engineering

9.3 Wound Healing

9.4 Transport and Release of Drugs

9.5 Nanotechnology and Medicinal Applications in General – Important Facts to Remember

Index

Related Titles

Knoll, W., Advincula, R. C. (eds.)

Functional Polymer Films

2 Volume Set

2011.

ISBN: 978-3-527-32190-2

Calvert, P., Narayan, R. (eds.)

Nanofiber Composites

Fundamentals and Developments

2010

ISBN: 978-3-527-40978-5

Geckeler, K. E., Nishide, H. (eds.)

Advanced Nanomaterials

2010

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Andrady, A. L.

Science and Technology of Polymer Nanofibers

ISBN: 978-0-471-79059-4

Vollath, D.

Nanomaterials

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2008

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Eftekhari, A. (ed.)

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The Authors

Prof. Dr. Joachim H. Wendorff

Universität Marburg

FB Chemie/Inst.Physikal.Chemie

Hans-Meerwein-Str.

35032 Marburg

Prof. Dr. Seema Agarwal

Philipps-Universität/FB Chemie

Gebäude H, Raum 03 H01 (3415)

Hans-Meerwein-Str.

35043 Marburg

Prof. Dr. Andreas Greiner

Universität Marburg

Fachbereich Chemie

Hans-Meerwein-Strasse

35032 Marburg

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Print ISBN: 978-3-527-32080-6

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Preface

Nanofibers and nonwovens composed of them are unique nanostructures with an extraordinary potential both in technical areas and in medicinal applications. Filter applications, functionalization of textiles, fiber reinforcement, catalysis, drug delivery, wound healing or tissue engineering are just a few examples of potential applications. The route towards such nano-objects is based primarily on electrospinning: a unique technique relying on self-organization via electric charges and their interactions with an applied field. The mechanism controlling fiber formation, the properties of fibers and nonwovens prepared by this technique, materials chosen for electrospinning and also application in technical and life science areas are the topic of this book. Both experimental facts and theoretical models will be discussed in detail and the same holds for various modifications of electrospinning that have been developed recently. Electrospinning and nanofibers prepared by this technique will without any doubt lead to major scientific and technical advances in nanotechnology and it is a principal aim of this book to promote such advances.

However, technical advances tend in the majority of cases to pose particular risk and in fact risk assessment has become a major concern in nanotechnology within recent decades. One major reason is that nano-objects composed, for instance, of metals, metal oxides, inorganic compounds, even pure carbon have the tendency to display properties and functions that deviate strongly from the corresponding features known for the same materials on a macroscopic scale. Nanoparticles, nanofilms, nanorods or nanotubes, for that matter, display specific properties that depend not only on their characteristic dimensions but also on the shape. Quantum films, quantum dots or quantum wires composed of semiconductor materials are well-known examples [1].

Magnetic and semiconductor phenomena may serve to illustrate the fundamental changes taking place at small scales and to point out in which way the convergence of nanotechnology and medicine can offer novel approaches in medicine. Ferromagnetic materials, such as cobalt or iron oxide, display the unique phenomenon of superparamagnetism if nanoparticles are prepared from such materials. No permanent magnetization can be achieved in an external magnetic field for such nanoparticles, yet the induced magnetization is of similar magnitude as in the case of ferromagnetism. Superparamagnetic nanoparticles serve as drug carriers that can be positioned in the body via external magnetic fields and that can be forced to release drugs in a controlled way by cycling the magnetic field thus causing a temperature increase within the particles. Semiconductor nanoparticles, such as the ones made from cadmium selenide, show optical adsorption and emission at distinct wavelengths in the visible range with the wavelength depending at constant material composition strongly on the size of the nanoparticle.

Such features are the origin of the great potential of nano-objects in various types of applications. On the other hand, they may be the source of risks for biological objects, for the health of human beings, and for the environment. Gold may serve as a good example and this also holds for silver. Such materials are inert in the bulk state yet become strongly catalytic as nano-objects. Silver nanoparticles have been used to destroy various types of viruses and gold nanoparticles to catalyze specific chemical reactions. So, the interaction of objects with matter of biological origin may change dramatically as the size is reduced, as the shape is modified. In fact, a broad range of investigations has considered this topic, studies on the penetration of carbon nanotubes into the body, the accumulation at various locations and possible harmful reactions were, and are, the topic of scientific studies but also less scientific discussion in the media.

It certainly makes a lot of sense to handle the preparation of nano-objects, the use of nano-objects and finally the disposal of them with utmost care, it makes a lot of sense to impose corresponding regulations and to follow them strictly similar to the case of handling of chemical compounds including toxic compounds [2]. This should in principle also hold for nanofibers prepared by electrospinning. However, while major studies have been performed for a broad range of nano-objects including in particular carbon nanotubes no such major investigations exist for nanofibers, particularly for those produced from natural or synthetic polymers.

On the other hand a sizable number of papers have been published in areas such as wound healing, drug delivery including gene delivery, tissue engineering in which the interaction of the nanofibers with specific types of tissue, with stem cells and specific cell lineages, with DNA, etc. have been studied in detail. This already allows a set of conclusions concerned with health risks. Furthermore, it is important to point out that nanofibers produced from polymers do not show in general quantum effects usually associated with nanotechnology. The reason simply is first that the nature of the electronic states of organic materials including polymer materials that control optical and electronic properties does not resemble the one known for semiconductors or conductors.

Electronic states that are not localized but rather extend throughout the bulk material are characteristic of such nonorganic materials with the consequence that modifications first of the absolute size and secondly of the geometry of a body made from them have strong effects on properties, particularly as the sizes approach the few tens of to a few nanometers scale. Organic materials, on the other hand, display localized states for electronic excitations, electronic transport with the states being defined by molecular groups such as chromophore groups or complete molecules. The consequence is that the electronic states are not affected as the dimensions of elements such as a fiber element are reduced down into the nanometer scale. Furthermore, both amorphous polymers and partially crystalline polymers have structures anyway even in the bulk, in macroscopic bodies that are restricted to the nanometer scale. So the general conclusion is that the reduction of the diameter of fibers made from polymers or organic materials for that matter will affect neither optical and electronic properties to a significant degree nor the intrinsic structure. It is for this reason that the probability of risks associated with polymer nanofiber formation can be considered at the present state as small.

In fact, studies on tissue engineering, drug delivery and wound healing discussed in this book have not given indications that such nanofibers are harmful to biological objects such as cells, tissue, etc. Wounds protected by nanofiber nonwovens have been found to heal much more rapidly as compared to other types of treatment, cells seeded onto nanofiber nonwovens serving as a scaffold have been found to proliferate and differentiate either more or less unchanged or even at an enhanced rate. In fact, in a particular investigation nanofibers were produced from bacterial cellulose by a combination of acid and ultrasonic treatment. The genotoxicity of nanofibers from bacterial cellulose was analyzed in vitro, using techniques previously demonstrated to detect the genotoxicity of fibrous nanoparticles [3]. The results of investigations involving among others single-cell gel electrophoresis and Salmonella reversion assays showed that nanofibers do not display genotoxicity under the conditions tested. A proliferation assay using fibroblasts and CHO cells revealed a slight reduction in the proliferation rate, although no modification in the cell morphology was observed.

It is of course obvious that one specific topic related to risks assessment that is characteristic of nanofibers concerns the formation of segments, of rods from the fibers and their unwanted inhalation. Such formation may happen, in principle, during the preparation process, during use in various applications or in the disposal state.

The Preparation Process

Electrospinning tends, as discussed in the following chapters in detail, to result in the formation of more or less infinitively longs fibers. In fact, one is hardly able to detect fiber ends in nonwovens. The formation of short segments in electrospinning is very unlikely, even experiments directed at producing fiber segments during spinning – using electric field pulses, alternating fields or rotators below the die – fail to result in shorter fiber segments. Of course, it is possible that fibers form that are not deposited on the target and diffuse in the neighborhood of the spinning device. Yet, using closed spinning chambers and appropriate counterelectrode configurations one is able to solve this problem. A much greater risk comes from using organic solvents for spinning and from possible explosions of air/solvent mixtures. Again using closed spinning chambers connected to a gas exchange systems minimizes such problems.

During Applications

Fiber segments may be the result of abrasion. Yet, in fact, detailed investigations directed at obtaining fiber segments via rupture, cutting, etc., showed that it is extremely difficult to produce such segments from electrospun nonwovens. One reason is that the stiffness and strength of fibers tends to increase dramatically as the diameter is reduced, as discussed in the following chapters in detail. Furthermore, electrospinning is connected with strong elongation processes of chain molecules and crystals within the polymer fibers that again cause stiffness and strength to increase. So the probability of the formation of short fiber segments, of nanofiber rods seems to be very low.

Additional arguments for a low risk are that polymer fibers do not possess a fibrillar structure such as asbestos, so no fibrillation processes causing sharp tips can be expected that in turn would be harmful for the lungs. The viscoelastic properties characteristic of organic polymers, furthermore, tend to smooth sharp tips, giving rise to round ends.

During Disposal

Within the framework of studies on the use of nanofibers for pheromone release in agriculture experiments were done on the effect of UV-radiation, wind, temperature on fibers placed in nature, in the ground and on the effect of fibers on nature. So far, no negative effects were found.

So the general conclusion at this stage is that risks originating from producing and handling electrospun nanofibers are, to our current knowledge, obviously low and that using proper measures will allow dealing with electrospinning and the resultant fibers in a safe way.

References

1 Rao, C.N.R., Mller, A., and Cheetham, A.K. (eds) (2004) The Chemistry of Nanomaterials: Synthesis, Properties and Applications, Wiley-VCH Verlag GmbH.

2 Scherzberger, A., and Wendorff, J.H. (eds) (2008) Nanotechnologie, Grundlagen, Anwendungen, Risiken, Regulierung, De Gryter Rechte.

3 Moreira, S., Silva, N.B., Almeida-Lima, J., Rocha, H.A.O., Medeiros, S.R.B., Alves Jr., C., and Gama, F.M. (2009) BC nanofibres: in vitro study of genotoxicity and cell proliferation. Toxicol. Lett., 189, 235–241.

1

Introduction

1.1 Fibers – Key Functional Elements in Technology and Nature

A multitude of objects surrounding us at home, an impressive number of technical parts controlling our day to day life both privately and in technical areas, even a set of currently emerging technologies have fibers as their basic structural and functional elements or depend at least on fiber-type architectures. A fiber is of course first of all a geometric shape, a 1Dimensional object, having a certain diameter, a given axial ratio and a certain length that can often approach infinity. Figure 1.1a displays a silk fiber as one example and Figure 1.1b a synthetic polyamide fiber as another example. Such fibers may not only be straight but might also display a certain curvature, they may be bent to some extent (Figure 1.1c).

Figure 1.1 SEM images of (a) silk fibers, (b) polyamide fibers, (c) optical image of polyamide fibers displaying curvature.

However, for real applications fibers have to be more than just geometrical elements. They have to fulfill a set of requirements, to display a selection of specific properties, of dedicated functions.

Textiles, produced, for instance, on the basis of synthetic polymer fibers not only allow us to dress up, to catch the eye of our fellow people, to display fancy dressings but they provide in many instances protection against cold temperatures, rain, strong winds, maybe in certain cases even against UV-radiation. In clothing the geometric features of the fibers enable the design, the preparation of planar textiles via weaving, knitting processes. These textiles are highly porous, as shown by the SEM image displayed in Figure 1.2a, thus controlling thermal insulation, wind resistance, passage of vapor emerging from the body such as sweat. In addition, the fibers have to be able to adsorb vapors and release them again in textile applications, they have to provide certain mechanical properties defined by particular magnitudes of fiber stiffness and strength, elasticity allowing for textile deformations that arise when using them as clothing and they should allow, for instance, to incorporate dyes, and pigments.

Figure 1.2 (a) SEM image of cotton textile, (b) fiber reinforcement, schematically showing fibers and stress lines; upper image: unstressed, lower image: stressed.

Transportation industries involved in building and using airplanes, rapid trains, automobiles and boats rely strongly on large-scale technical parts having an extremely low weight while displaying simultaneously a high stiffness and strength. Fibers are incorporated for this purpose into matrix materials such as polymers or ceramics, giving rise to mechanical reinforcement effects (Figure 1.2b).

Optical information technology comprising the transportation, manipulation and display of information by optical means not only in local areas as in a car but also on a larger scale within buildings, all the way to covering huge distances existing between continents depend heavily on optical fibers composed of inorganic or organic glasses. It is obvious that the fibers not only have to possess in this case a very high optical clarity, that is, a very high optical transmission, that they should be flexible so that they can be subjected to bending to be integrated into technical parts, but they also should have intrinsic optical guiding properties allowing for particular optical propagation modes. Electric cables transporting electric energy, air filters or fluid filters composed of fibers and providing for clean air, water, gasoline are further technical examples of objects, devices containing and relying on fibers as key elements. Furthermore, fibers are also basic ingredients in carpets, ropes, tapestry and this list can be extended endlessly with some fantasy involved.

Fibers are, of course, not an invention of mankind, of modern techniques. Nature in fact has been using fibers as basic elements on a large scale to construct functional objects for millennia. Spider webs as displayed in Figure 1.3a obvious already to the naked eye and designed to catch insects are composed of a very loose network of intricate design to make them light, to offer a large cross-sectional area and yet to make them resistant to wind, storm, rain and to the attack of the prey trying to free itself. In terms of applications spider webs may serve as model structures for particular kinds of plant-protection devices, to be discussed later in more detail. Pheromone dispensers shaped along the architecture of spider webs and incorporating the enhanced mechanical properties of the basic fiber-like building elements offer significant advantages, as will be discussed later in some detail in Chapter 8.

Figure 1.3 (a) Spider web constructed from fibers, (b) extracellular matrix (ECM) surrounding cells in living tissue, composed of fibrils, for details see text.

A further example from nature characterized by a network of fibers in the nanometer range is the extracellular matrix (ECM), depicted in Figure 1.3b, which is an important 1D structured component of tissue. It has a broad range of tasks to accomplish. It embeds the cells of which the particular tissue is composed, it offers points of contacts to them, provides for the required mechanical properties of the tissue. Depending on the type of tissue the fibers are either tightly packed and oriented along a given direction, as in the case of muscles or are unoriented in a plane. as for instance in skin tissues. The first types of tissues require an enhanced mechanical strength in one particular direction, whereas the second one should be able to sustain planar stresses in all directions.

Finally, fibers such as wool fibers, hairs, silk fibers protect human beings and animals in a way similar to artificial clothing. They again possess specific mechanical properties, insulation properties and are able to adsorb moisture to a significant extent. Many more examples from nature come to mind, yet the aspect of fibers coming from nature will be kept short here since this topic will be revisited.

An obvious conclusion at this stage is that fibers are highly valuable and highly functional objects in technical and life science areas. However, it is important to point out that such fibers will in general not perform well in applications if one just chooses the right geometry, if one just focuses on them as geometric 1D objects. A first important aspect is the choice of the material with a given well-known set of mechanical, optical, electrical, thermal, or perhaps also magnetic properties, from which to produce the fibers. So, depending on the kind of application in view polymers, metals, inorganic materials will be selected as basic materials for the production of fibers. Yet, the well-known and tabulated general intrinsic properties of these materials are just guidelines, merely starting points for design considerations. More important is a tailored control of the intrinsic structure of the fiber via appropriate fiber preparation techniques to come up with the required advanced properties, functions aiming at the target application.

Fiber design thus requires in any case a very fundamental understanding of the correlation between the intrinsic structure of a fiber on the one hand and its properties, functions on the other side. Fiber design requires, furthermore, also the knowledge of how particular fiber-characteristic intrinsic structures can be achieved via the selection of appropriate fiber processing techniques and via the choice of suitable processing parameters. Finally, of course, one has to have a fundamental understanding on how to construct technical elements from fibers, on how to select the best architectures for them, and how to achieve particular functions in this way.

1.2 Some Background Information

1.2.1 Structure of Crystalline and Amorphous Materials

A basic first step towards fiber design for a particular application involves, as pointed out above, the selection of the material with a given spectrum of properties from which the fiber will be produced. A macroscopic piece of matter composed, for instance, of a metal such as copper or iron, of a glass being of inorganic or organic nature, of a semiconductor such as GaAs, or of a synthetic polymer such as poly (methyl methacrylate) displays a set of characteristic structural features and properties as controlled by the nature of the atoms/molecules of which they are composed, by the arrangement of these basic structural units in space as well as by the type of interactions existing between these units. Depending on the material under consideration various types of spatial arrangements of the atoms/molecules are experienced.

Crystalline materials are characterized by a highly regular 3D arrangement of the constituent atoms or molecules on a lattice in space displaying a translational symmetry. A so-called long-range order exists, that is, the position of the atoms/molecules far away from a reference atom/molecule is well defined as a function of the interatomic/intermolecular distances. This fact allows us to represent the crystalline structure in terms of the unit cell that is, the smallest element containing all structural features of the macroscopical crystal. As detailed in books concerned with crystallography, space-filling requirements lead to the conclusion that only certain types of lattice types – 14 Bravais lattices – should exist with cubic, hexagonal or triclinic lattices being some examples, as shown in Figure 1.4. In addition to the translational symmetry point symmetries are used to characterize the packing of the atoms, molecules making up the crystal in space. These lattices are displayed without exception by all types of materials able to crystallize including polymers.

Figure 1.4 14 Bravais lattices arising from symmetry considerations and displayed by all types of materials able to crystallize including polymers.

Based on such symmetry considerations predictions can be made on the anisotropy of properties – properties are different along different crystal axes – as well as on the presence of particular properties. Taking as an example the absence of an inversion center one can directly conclude that such a material can, in principle, display properties such a ferroelectricity, piezoelectricity or second-order nonlinear optical effects.

Considering briefly fibers one situation to be discussed later in more detail may well be that fibers prepared from one and the same material may display quite different properties, the reason being that fibers were produced in which different type of crystal modifications with different crystal unit cell occur, or in which different crystal unit-cell directions point along the fiber axis. It is also conceivable that fiber formation goes along with the introduction of specific crystals defects.

In contrast to crystalline structures, amorphous structures including glasses and melts do not display a regular packing of atoms, molecules in space, only a short-range order exists. This term represents the situation that the atoms or molecules are closely packed in an irregular manner so that one has only knowledge of the probabilities with which neighboring atoms/molecules occur as a function of the interatomic/intermolecular distance. The average distance to next and second-next neighbors is known within limits but no information is available for distances larger than these. Pair correlation functions are used to represent this situation again, as detailed in books on material science. Due to the particular kind of atomic or molecular packing amorphous materials in general display isotropic properties, that is, the properties along different directions of a piece of material will be equal. No optical birefringence is displayed by such structures, in contrast to the case of crystalline materials.

However, the situation might be quiet different for fibers. Anisotropic properties may be experienced in amorphous fibers provided that orientational effects have accompanied fiber formation. Optical birefringence is one consequence and anisotropy of properties such as mechanical or electrical properties another one. Cases are known in which the tensile modulus, the refractive index or the thermal expansion coefficients along the length of the fiber differ strongly from those along a direction perpendicular to the axis.

Intermediate structures also exist for particular kinds of materials. Liquid-crystalline materials as composed of shape-anisometric molecules such as rod-shaped, disc-shaped, banana-shaped molecules, etc., have features resembling those of crystalline structures, among them lattice-type arrangements of the center of molecules along specific directions molecules that are orientationally ordered to a certain extent giving rise to optical birefringence. However, at the same time they show features resembling those of amorphous materials, fluids including viscous flow or in some cases the complete absence of a regular positional order.

1.2.2 Chain Conformation and Structures of Polymer Materials

Polymer materials, are a main topic of the book, need special considerations in this context – one reason being that a single molecule – a chain molecule – tends to be much larger then the length of unit cells, is composed of a large number of subunits, that is, repeat units and has an extremely large number of internal degrees of freedom. Even if concerned with the preparation of metal, metal oxide fibers via electrospinning the route towards such fibers will be based in the majority of cases on polymer fibers following a precursor route.

First, the macromolecular chains making up the polymer material are not geometrically highly defined, as assumed in the discussion so far, but are subjected to a large number of degrees of freedom, as already mentioned above [1–3]. Macromolecules are composed of a large number of basic molecular groups – monomers – which are all identical throughout the chain backbone – homopolymer – they can be composed of two chemically different units – copolymers – three chemically different units – terpolymers – and so on. Various chain architectures are accessible via synthetic routes, as detailed in the corresponding text books on macromolecular chemistry and physics, including linear chains, branched chains, star-like chains, random copolymers, block copolymers and many more.

Important as far as structure formation in bulk is concerned is that an individual macromolecule forms a very loose structure in solution, in the melt but also in the solid state in the shape of a coil, of a so-called Gaussian coil (Figure 1.5). The spatial extension of linear macromolecules tends to be represented by the mean square value of the chain end-to-end vector , the vector pointing from one chain end to the other. In the melt and ideal solutions it is determined by the number n of monomer units within the chain as well as by the length of atomic bonds l along the chain backbone as given by

where Cn is a number typically in the range between 2 and 15, representing details on the bond angles and rotational angles characteristic of the chemical structure considered. The brackets < > indicate averaging over a Gaussian distribution of the end-to-end vector originating from the large number of degrees of freedom available to the chain. A coil conformation takes place, that is, the trajectory of the chain follows in ideal cases closely that a particle would form via diffusion (Figure 1.5). For a linear chain composed of 10 000 bonds, a bond length of 0.15 nm and a value of 9 for Cn the linear extension of a coil molecule as calculated from the square root of the may amount to several tens of nm.

Figure 1.5 Representation of the Gaussian coil conformation of a chain molecule, the monomer units being represented by dark spheres, for details see text.

So the intrinsic density of a single chain molecule (mass of all monomers per volume of the Gaussian coil) is very low, a single chain molecule has a lot of space within its interior. So both in the molten amorphous state and in effect also in the crystalline state the chain molecules interpenetrate each other in large numbers, chain entanglements occur in large numbers in order to form a densely packed material. The packing of segments, of monomer units within the melt or amorphous glassy state is short ranged, as detailed above.

Considering now the structures of the solid states exhibited by polymers it is obvious that strong differences should exist in comparison to the structures formed by atoms or small geometrically highly defined molecules. Polymers tend to form lamellar-type of crystals that are very thin, of the order of 10–50 nm, yet that display lateral extensions well into the micrometer range (Figure 1.6a). So in a certain sense lamellar crystals assume the geometric shape of a piece of paper. Electron scattering (Figure 1.6b) reveals that the chain molecules pass the lamellae along the lamella normal, folding back at the surfaces (Figure 1.6c).

Figure 1.6 (a) Lamellar crystals composed of chain molecules, (b) electron scattering diagram showing that the chains are oriented perpendicular to the plane of the lamella as displayed in (c, d) arrangement of amorphous layers and crystalline lamellae in partially crystalline polymers.

This restriction of crystal formation to thin lamellae certainly is related to the restrictions imposed on the dynamics of the chain molecules in terms of diffusion, shape rearrangements by entanglements. The lamellae show all known types of units cells – Bravais lattices – as known from nonpolymer materials and this is not surprising in view of the general symmetry considerations leading to Bravais lattices. The chain molecules tend to pass through the lamellar oriented along the lamellar normal, they either fold back in an irregular manner after passing the lamella or they enter adjacent lamellae (Figure 1.6c).

An important feature of crystalline polymers is that they are never able to become 100% crystalline. The degree of crystallinity, as defined by the weight fraction of the material that has been transformed into the crystalline state, may assume values between about 0.05 up to 0.9, depending on the regularity of the chain molecules and the crystallization conditions. This partially crystalline state gives rise to an additional structural feature on the 10–100 nm scale in addition to the one coming from the crystalline lattice on the 1 nm scale. Crystalline lamellae form stacks in which the lamellae and amorphous layers alternate in a regular fashion giving rise to a one-dimensional regular order (Figure 1.6d).

Coming back to fiber aspects it is obvious that one expectation is that the crystals, that is, in this particular case the lamellae, may become oriented in a specific way with respect to the chain axes. However, one might also assume that fiber formation can deform, i.e. can erase the order on the 10–100 nm scale coming from the packing of lamellae and amorphous layers. In addition, the chain molecules showing so many degrees of freedom might become deformed, extended along the fiber axes both in crystalline and amorphous polymer materials. All these modifications will show up in strongly modified mechanical, thermal, optical and many more categories of properties within the fibers. In fact, it is even conceivable that deformations going with fiber formation give rise to the existence of novel types of crystal modifications, as known, for instance, from stretching poly (vinylidene fluoride) films.

With respect to properties of bulk materials composed of chain molecules it has to be pointed out that polymer materials tend to exhibit viscoelastic properties that is, a superposition of elastic properties – reversible deformation and recovery on unloading – and viscous properties – irreversible deformation on loading, plastic flow. The extent of the two contributions will depend on the temperature considered as well as on the time scale/frequency of the mechanical loading/unloading steps and again one might expect that fiber formation will also affect the nature of the viscoelastic properties.

1.3 Processing of Polymer Materials towards Fibers – Fiber Extrusion

The discussion that follows will concentrate predominantly on synthetic polymer materials, for reasons detailed above. The task is to start from the basic bulk material – melt, solid, powder – and to process it in a controlled way to fibers, hopefully in such a way that fibers with tailor-made intrinsic structures and properties result. A technically highly established large-scale industrial route is based on what we might call a top-down approach where macroscopical materials are shaped in such a way that smaller structures become available. In the case considered here this shaping of fibers, of fiber-type structures, is performed via mechanical forces in general applied to solutions or melts.

Of major importance in this respect are extrusion approaches discussed in the following with respect to polymer processing (Figure 1.7). Polymer melts or polymer solutions are pumped via screws through dies that force a fiber-like geometry on the fluid material emerging from the die. Typically, arrays of dies with a circular cross section are arranged parallel to each other so that a multitude of fibers are formed simultaneously. The still viscous jet coming out to the dies solidifies either by cooling down the melt below the respective melting temperature or glass temperature of the basic fiber material or by evaporation or coalescence of the solvents in the case of solution extrusion into a bath containing a nonsolvent. The fibers may in subsequent steps, in general, become subjected to strong longitudinal elongations by selecting the take-up speed to be much higher then the extrusion speed. The concept is that this deformation causes chain elongations and orientations as well as crystal orientations, with the result that the stiffness and strength of the fibers are strongly enhanced relative to the bulk material.

Figure 1.7 Lab-type extruder with the extrusion die pointing to the reader.

To introduce a few technical details, in melt extrusion pellets of the basic material are fed into a screw zone of an extruder (Figure 1.7). The screw arrangement causes the pellets to be compressed, removes residual air and water from the pellets, heats them up into the fluid state and finally transports the melt through specific dies with the desired speed as controlled, among other parameters, by the rotation speed. The choice of the die determines the geometry of the resulting products including tubes, planar structures but also fibers. Extruders may be quite small with feeding rates of just a kg/h but they can also be quite big with feeding rates of many hundreds of kg/h and more.

Man-made fibers produced via extrusion and composed of materials such as polyamides or polyethylene terephthalate obtained via synthetic routes are characterized by internal structures that are hierarchical to a certain extent yet that are by far less complex than the ones displayed by natural fibers. The extruded fibers tend to be partially crystalline for crystallizable polymers, displaying lamellar-type crystals with a thickness in the range of a few tens of nm and a lateral extension usually in the micrometer range, as discussed above. The lamellae tend to form a regular stack composed of alternating lamellar and amorphous regions. Depending on the fiber preparation the stacks may be oriented along the fiber axis with the segments being oriented and extended in the amorphous regions, as discussed above. A further feature may be the presence of microfibrills that by themselves are composed of stacks of lamellae. These features give rise to enhanced mechanical properties along the fiber axis, that is, enhanced stiffness and strength that may match the corresponding properties of natural fibers.

Compared to natural fibers these polymer fibers are cheaper to produce, it is easier to process them, to dye them or to introduce high strength and stiffness in a controlled way. Yet, they lack, to a significant extent, quite a number of functions that are beneficial for textile applications and that are displayed by natural fibers. Carrying a shirt made purely from polyamide on a hot humid day makes the difference between textiles made, for example, from cotton and from such man-made fibers very obvious. One major reason besides a chemical composition which is different from that of the natural fibers discussed above is that their internal molecular and supermolecular structures tend to be rather simple. Solid fibers with constant composition and constant structural features along the cross section are characteristic of man-made fibers. Hollow fibers have been produced and also fibers with cross-sectional shapes that differ from the circular one, yet nevertheless such architectures are far from the complex ones displayed by nature. Furthermore, specific microfibers have been prepared in various ways to enhance among others textile properties.

In fact, the application of man-made fibers is not restricted to textiles. Fibers play a major role in the reinforcement of thermoplastic or thermoset polymer matrices not only for high-end applications. Reinforcement of high-end elements of ships, trains, airplanes are well known but fiber-reinforced materials can also be found in day-to-day appliances. The important parameters as far as fiber reinforcement is concerned is the axial ratio that should be well above 100 to 1000 – thus the use of fibers – and the enhanced stiffness and strength of the fiber combined with a good mechanical coupling to the matrix. Fiber reinforcement is, in the majority of cases, not done with natural fibers – although such approaches are being considered more and more for ecological reasons – but conventionally by using very specific synthetic fibers such as carbon fibers produced among others via the precursor polymer polyacrylonitrile (PAN) fibers or Kevlar fibers, produced from lyotropic polymer solutions.

1.4 Routes to More Advanced Fibers – Mimicking Nature

So, polymer fibers obtained from synthetic polymers and produced via extrusion techniques are of great technical importance and display properties which are favorable for many types of applications. However, there is a definite need for more advanced fiber designs. Fiber reinforcement, for instance, would benefit strongly from fibers much smaller in diameter than those characteristic of extruded fibers – that is, nanofibers – even at the same magnitudes of stiffness and strength. The length could thus be reduced at constant axial ratio compared to thicker fibers, thus reducing rupture during polymer processing. In addition, fibers small in diameter compared to the wavelength of light would not cause turbidity in otherwise transparent matrices and the mechanical coupling between matrix and fibers would be enhanced, as would the ductility due to the much larger internal surface areas.

A further area for the application of fibers, again predominantly of man-made fibers, concerns filters for either gas or fluid filtrations including coalescence filters (Figure 1.8) as discussed in Chapter 8 in great detail. The chemical, thermal and mechanical stability of the fibers together with the costs to produce the fibers are important features but of particular importance is the absolute magnitude of the diameter of the fibers.

Figure 1.8 SEM image of commercial filters based on paper with nanofibers on the surface

(from H.D. Hollingsworth & Vose GmbH).

The nanofibers are on top of the conventional filter composed of paper inducing a surface filtration.

The diameter determines the size of the pores provided by such filters and thus the size of the impurities to be captured. The reduction to fiber diameters well into the nanometer range will, furthermore, affect the flow pattern around the fibers significantly. Both effects taken together should strongly enhance the filter efficiency with respect to smaller-scale impurities in the air, in gasoline, etc. The specific surface area acting as adsorption site also increases strongly as the diameter is reduced. As far as textiles are concerned a reduction in fiber diameters and thus in pore sizes will cause a strong increase of wind resistivity and thermal isolation, as discussed below in more detail in Chapter 8.

The discussion about fiber applications in the areas of textiles, fiber reinforcement and filters has made it apparent that these areas would definitely benefit to a great extent from a further strong reduction of the fiber diameters by several orders of magnitude well into the nanometer range. The low value of the diameter and small nonwoven pore sizes as well as the huge surface area that go along with small fiber diameters are key factors in such applications. However, it is obvious that the extremely small diameter is just one side of the coin.

Further features might be the onset of confinement effects for structural features and properties and the increasing truly 1Dimensional nature of the fibers as the diameter decreases. The potential for rapid diffusional processes into and out of the fiber characteristic of nanoscalar dimensions or the close resemblance in architecture of nanofibers and the fibrillar extracellular matrix in living systems are further specific features favorable for specific applications. It may, of course, be necessary for such fibers to carry functional units such as chromophores, catalysts, sensor molecules, quantum dots, drugs or bacteria, depending on the application in mind and they may have to be composed of organic, inorganic materials or corresponding hybrids.

It seems that in addition to trying to reduce fiber diameters down into the range of a few nm or a few tens of nm and in addition to incorporating functional compounds technical applications would benefit from fibers with highly developed hierarchical structures and consequently with higher functionalities, as often encountered in nature. Optoelectronics, sensorics, catalytics, storage are potential target areas but also drug storage and release.

Neurons may serve as a first example for highly functionalized nanoscalar fiber-type structures. Their fundamental task consists in receiving, conducting and transmitting electrochemical signals via connections with other cells called synapses. Neurons are the core components of the nervous system, which includes the brain, spinal cord, and peripheral ganglia. A number of specialized types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli affecting cells of the sensory organs that then send signals to the spinal cord and brain. The central building element of the neuron is, in the majority of cases, the strongly one-dimensional that is, fiber-like axon that is surrounded in general by a plasma membrane. This kind of packaging thus providing a hierarchical architecture allows action potential to travel along the axon with enhanced speed.

The extracellular matrix already discussed above may serve as a second example for a nanoscalar functional biological system. The basic building blocks of the ECM are fibers composed of collagen and other structural proteins. The collagen fibers building up the ECM have diameters ranging typically from 50 to 500 nm. ECMs characteristic of bone tissue may contain small hydroxyapatite crystals imbedded in the collagen fibers to provide an enhanced stability paired with a high elasticity. In regenerative medicine, scaffolds are used to engineer particular kinds of tissue to replace corresponding tissue destroyed partially or completely by accidents or sickness. The task of the scaffold is in this case to act as a carrier for cells, to promote their proliferation, differentiation, to guide cell growth along specific directions, to allow for the growth of blood vessels, and to deliver functional components locally such as growth factors. It has been demonstrated that scaffolds based on the architecture of the extracellular matrix are particularly effective in regenerative medicine.

Going in the direction of close to infinitely long fibers we look next at silk (Figure 1.1a). It is of importance both in nature but also for technical applications. Silk at a first glance appears to be just another fiber produced by nature. However, silk is a fiber-like material that due to its internal composition displays unusual mechanical properties but also interesting optical properties. Silk obtained from cocoons made by the larvae of the silkworm Bombyx mori has a shimmering appearance that originates from the triangular prism-like structure of the fibers. This allows silk cloth to refract incoming light at different angles. Silk fibers possess highly impressive mechanical properties, in particular a high ductility, related to intrinsic structural features being to a significant extent ultrafine in nature.

Cotton, wool, animal or human hairs are further examples of fiber-type structures provided by nature. These fibers are not as small in fiber diameter as those discussed above, yet they are constructed in a highly complex hierarchical way that provides them not only with unique mechanical properties but also with a set of other functions that make them of interest for various types of applications. Protection against low temperatures, rain, UV-radiation are examples of functions that are of importance both for living beings carrying such structural elements as well as for technical applications. The diameter of such natural fibers may well be in the 10–20 micrometer range and above; human hair, for example, typically has a diameter around 50 micrometer (Figure 1.9). Such fibers tend to have a hierarchical structure going down to the nm scale, as can for instance be demonstrated for wool or hairs in order to provide the functions expected from them.

Figure 1.9 SEM images of human hair – covered by nanofibers for comparison.

The examples given above, furthermore, suggest as far as technical areas are concerned that one might try to mimick such structures to come up with novel types of technical systems for various types of advanced technologies.