This chapter covers several additional methods used to form micro- and nanofibers. Some of them have already achieved maturity, such as the island-in-the-sea method discussed in Section 6.1, melt fracture in meltblowing processes (Section 6.2) and the flash spinning process (Section 6.3). Some others are still relatively exotic or under development, like the two methods based on Couette shear flow described in Section 6.4, or the centrifugal spinning method in Section 6.5. Nontraditional materials used for nanofiber formation, discussed in Section 6.6, include liquid crystals, conducting polymers, biopolymers and denatured proteins. Finally, Sections 6.7 and 6.8 discuss the specifics of drawing glass optical fibers, and in particular, polarization-maintaining optical fibers with multilobal cladding (Section 6.8).

Island-in-the-sea multicomponent fibers and nanofibers

Microscopic bi- and multicomponent fibers can be formed using melt spinning (Section 1.2 in Chapter 1), meltblowing (Section 4.1 in Chapter 4), or integrated processes such as spunbonding (Section 1.5 in Chapter 1). The additional polymer components are supplied to the main polymer through separate inner spinnerets inserted into the main outer spinneret similarly to formation of core–shell bicomponent fibers in co-electrospinning (Section 5.8 in Chapter 5) and solution blowing (Section 4.8 in Chapter 4). Cross-sections of such bi- and multicomponent fibers are reminiscent of islands in the sea, which explains the name of this technology (Nakajima 2000). In some cases the islands can merge and form winged structures, as seen in Figure 6.1a.

This chapter deals with the mechanisms and electrohydrodynamic modeling of the physical processes resulting in electrospinning of nanofibers with cross-sectional diameters approximately in the range 100 nm to 1 µm. These involve the physical nature of fluids used in electrospinning, leaky dielectrics, discussed in Section 5.2, and the formation of the precursor of electrospun jets, the Taylor cone, described in Section 5.3. Polymer jets in electrospinning possess an initial straight section, which is discussed in Section 5.4. Experimental observations of the key element of the electrospinning process, the electrically driven bending instability, which is similar to the aerodynamically driven jet bending of Chapters 3 and 4, are covered in Section 5.5. Section 5.6 describes the theory of the bending instability in electrospinning. Multiple jet interaction in electrospinning and needleless electrospinning are discussed in Section 5.7. Co-electrospinning and emulsion electrospinning of core–shell fibers (Section 5.8) are based on similar physical principles to electrospinning of monolithic nanofibers. The electrostatic field-assisted assembly techniques developed with the aim of positioning and aligning individual nanofibers in arrays and ropes are discussed in Section 5.9. Melt electrospinning of polymer fibers is briefly outlined in Section 5.10.

Electrospinning of polymer solutions

Electrospinning of polymer solutions, liquid crystals, suspensions of solid particles and emulsions employs an electric field of the strength about 1 kV cm−1. The first US patent on electrospinning was issued to Formhals (1934), but interest in this process was dormant until electrified jets of polymer solutions and melts were investigated as routes to the manufacture of polymer nanofibers (Baumgarten 1971, Larrondo and Manley 1981a–c, Doshi and Reneker 1995, Reneker and Chun 1996). In electrospinning, the electric force results in an electrically charged jet flowing out from a pendant or sessile droplet (see Figure 5.1). After the jet flows away from the droplet in a nearly straight line, it bends into a complex path and other changes in shape occur, during which electrical forces stretch and thin it by very large ratios, quite similar to the effects of the aerodynamic forces in melt- and solution blowing discussed in Chapter 4. After the solvent evaporates, solidified nanofibers are left.

This chapter discusses several post-processing approaches applied to as-spun nanofibers to change their structure and/or enhance certain properties. Section 8.1 describes carbonization, sol-gel transformation and calcination, as well as metal-plating, used to make stiff, hollow or thermally and electrically conducting fibers. Sections 8.2 and 8.3 are devoted to cross-linking of solution-blown soy protein/nylon 6 nanofibers. The collected fiber mats can be bonded both chemically (using aldehydes and ionic cross-linkers, as in Section 8.2), and physically (by means of wet and thermal treatment, as in Section 8.3) to increase the tensile strength and therefore widen the range of applications of these green nonwovens. Chemical cross-linkers bond different amino groups, primary amides and sulfhydryl groups in the protein structure, which is beneficial for the enhancement of tensile strength. It is shown that treatment with ionic cross-linkers results in nanofiber mats with a higher Young’s modulus. Covalent bonds formed by aldehyde groups have a smaller effect on the mat strength. As cross-linked nanofibers are exposed to heat, the bonds formed between amino groups in the fibers are broken and they became less aggregated. In addition, in Section 8.3 it is shown that wet conglutination of soy protein/nylon 6 nanofiber mats leads to partial physical cross-linking of nanofibers and, consequently, to an increase in Young’s modulus. An enhancement of the tensile strength of soy protein nanofiber mats, as well as a slight plasticizing effect, can also result from exposure to water.

This chapter describes the machinery, mechanism and significant experimental and theoretical aspects of melt- and solution blowing. Meltblowing is a popular method of producing polymer micro- and nanofibers en masse in the form of nonwovens via aerodynamic blowing of polymer melt jets (Section 4.1). Its physical aspects were revisited recently. The process involves a complex interplay of the aerodynamics of turbulent gas jets with strong elongational flows of polymer melts, only recently uncovered and explained.

The role of turbulent pulsations (produced by turbulent eddies in the gas jet) in meltblowing is discussed first in Section 4.2 in the framework of a model experimental situation where solid flexible sewing threadlines are used to probe a parallel high-speed gas jet. After that, in Section 4.3, the dynamics of bending and flapping of flexible threadlines in a gas jet is considered. In Section 4.4 the aerodynamically driven stretching of a straight polymer jet is considered. In Section 4.5 it is shown how a severe bending instability leading to strong stretching and thinning of polymer jets can arise. This is done in the framework of a linearized version of the governing equations in the case of small bending perturbations of a single threadline or polymer jet in meltblowing. Then, in Section 4.6 the fully nonlinear case of large-amplitude planar bending perturbations of a single polymer jet is discussed. Both isothermal and non-isothermal cases are considered. In particular, it is shown how the cooling of the surrounding gas jet results in cooling of the polymer jet inside, and in the arrest of the bending perturbation growth due to melt solidification. Section 4.7 is devoted to predictions of three-dimensional configurations of polymer jets in meltblowing from die exit to deposition screen. Not only a single polymer jet, but multiple polymer jets are modeled simultaneously, as well as deposition on a screen moving normally to the principal jet direction being accounted for. The results include prediction of the fiber deposition patterns in lay-down and fiber-size distributions in the resulting nonwovens. The angular distributions in lay-down nonwovens are also predicted. Comparisons with the experimental data suggest that the model captures main trends rather accurately.

The first chapter is devoted to the traditional methods of fiber forming, which are used to produce macroscopic fibers. Since the novel methods used to form micro- and nanofibers described in this monograph have branched from the traditional methods, an introduction into the history of manmade fibers is instructive and fully appropriate (Section 1.1). There is a brief discussion of such traditional extrusion methods of fiber forming as melt spinning (Section 1.2), dry spinning (Section 1.3), wet spinning (Section 1.4) and the integrated process of spunbonding, which is used to form nonwoven fiber webs (Section 1.5). Melt and dry spinning are closely related to the electrospinning used to produce nanofibers, so the discussion of these traditional methods allows a first glimpse of electrospinning, covered in Chapter 5. One of the key elements of spunbonding is pulling polymer filaments by fast co-flowing air, which is known as meltblowing. Meltblowing, and its offshoot solution blowing, are also used to form micro- and nanofibers, as detailed in Chapter 4. In a sense, Section 1.5 serves as an introduction to the nonwoven nanofiber mats discussed later. Section 1.2 also contains some elements of quasi-one-dimensional theory; namely, its application to the draw resonance instability of melt spinning. In its more involved form a similar quasi-one-dimensional approach is applied in Chapters 3–6 to describe processes characteristic of melt- and solution blowing and electrospinning used to form micro- and nanofibers.

History and outlook

The term fiber originates from the French word fibre, from Latin fibra “a fiber, filament,” of uncertain origin, perhaps related to Latin filum “thread,” or from the root findere “to split” (Online Etimology Dictionary 2013). For centuries, the use of fibers was limited to natural materials such as cotton and linen, which had inherent problems with wrinkling. Silk was difficult to produce and was often too delicate. Wool was strong and abundant, but would shrink and was irritating next to the skin, and would not last long, as it was a food source for moths.

Several physical concepts that are of the utmost importance in fiber-forming processes are described in this chapter. The basic physical model of a flexible polymer macromolecule as a random walk is outlined in Section 2.1. The elongational and shear rheometry of polymer solutions and melts, which elucidate the stress relation with strains and strain rate, as well as stress relaxation is described in Section 2.2. The phenomenological rheological constitutive equations appropriate for the description of viscoelastic polymer solutions and melts are introduced in Section 2.3. The micromechanical foundations of the entropic elasticity responsible for viscoelasticity of polymer solutions and melts are sketched out in Section 2.4. Solidification and crystallization are discussed in Sections 2.5 and 2.6, respectively.

Polymer structure, macromolecular chains, Kuhn segment, persistence length

A linear polymer macromolecule can be represented as a succession of identical rigid segments connected at arbitrary angles, i.e. freely jointed with each other (Flory 1969, de Gennes 1979, Doi and Edwards (1986). Such a macromolecule is comprised of N segments, each of length b. The total length of a fully stretched macromolecule is then L = Nb. The rigid segments are called Kuhn segments. A real macromolecular chain consisting of n monomers is idealized as a random walk of N Kuhn segments, which are not monomers, nor is N identical to the degree of polymerization n. If the number of Kuhn segments in a macromolecule is not large, i.e. N is close to 1, it is rather inflexible, almost rod-like. On the other hand, if N >> 1, the macromolecule is very flexible, and on length scales that are significant compared to b, but much smaller than L, it can be viewed as a flexible string. Persistence length is another length scale that characterizes the resistance of segments of macromolecular chains to bending. It is of the same order of magnitude as the length of the Kuhn segments.

Nanotechnology has profound applications in healthcare and has improved healthcare research to a large extent. The therapeutic benefits of nanotechnology in the field of medicine have resulted in new areas, such as nanomedicine, nanobiotechnology, etc. Researchers in the field are attempting to find an effective nanoformulation to deliver growth factors, supplements or drugs safely and in a sustained manner at the required site. Their task is to attempt a different drug nanoformulation of existing blockbuster drugs that brings improved efficacy and a therapeutic breakthrough. Thus the ultimate objective of these nanotechnological drug-delivery systems is to fine tune the normal profile of potent drug molecules in the body following their administration to significantly improve their efficacy and also minimize potential intrinsic severe adverse effects. For treatment of breast cancer and non-small-cell lung cancer, Abraxane® (paclitaxel) is employed as a nanoparticular formulation, which increases drug delivery up to 70% in comparison with solvent-based paclitaxel delivery. In this novel nanoformulation, Abraxis Bio Sciences have used Bristol-Meyers Squibb’s blockbuster drug paclitaxel (Taxol) and a very common globular protein bovine serum albumin (BSA). There are numerous nanotechnology-based drug-delivery systems such as nanocrystals, nanoemulsions, lipid or polymeric nanoparticles, liposomes and nanofibers. While nanoemulsions and liposomal formulations did not make significant advances, despite huge research spending, the polymeric nanoparticulate systems show more promise. Nanoparticles of a polymeric nature find application as drug-delivery systems and are advantageous due to their scalability, cost, controlled and targeted delivery, compatibility, degradability, etc. Natural biopolymers are even better than the synthetic polymers in terms of biocompatibility and biodegradability. Nanoparticulate drug formulations alter the pharmokinetic profile of the therapeutic entity and program the release of the drug in sustained or controlled manner. Thus, nanoparticle or nanoformulated drugs outperform conventional systemic delivery in terms of delivery of an encapsulated drug and its sustained release. Slowly and surely nanoformulated drugs are coming onto the market, surpassing systemic delivery, which is believed to be the only mode of administration for a wide range of active pharmaceutical ingredients. Nanofibrous drug-delivery systems are being developed as potential scaffolds in tissue regeneration, wound healing and cancer drug-delivery applications. In this chapter we are going to discuss two promising nanotechnology-based drug-delivery tools, namely electrospun micro- and nanofibers and electrosprayed micro- and nanoparticles, which have a common synthetic procedure mediated by an electrical potential difference.

This chapter introduces the fundamental general equations of the dynamics of free liquid jets in Sections 3.1 and 3.2. The applications of these equations encompass practically all fiber-forming processes from melt spinning, as was sketched out in Section 1.2 in Chapter 2 to melt- and solution blowing, and electrospinning of polymer micro- and nanofibers discussed in detail in Chapters 4 and 5, or drawing of optical fibers outlined in Section 6.5 in Chapter 6. They also form the framework for description of several types of instabilities characteristic of the hydrodynamics of free liquid jets in general and of fiber-forming processes in particular. These include capillary instability (Section 3.3), aerodynamically driven bending instability (Section 3.4) and buckling of liquid impinging onto a wall (Section 3.5).

Mass, momentum and moment-of-momentum balance equations

The dynamics of free liquid jets moving in air, which are characteristic of fiber-forming processes, involve growth of various perturbations. The most notable are driven by surface tension and the dynamic interaction with the surrounding air, as well as electrically driven effects. Theoretical/numerical description of the jet evolution in general, and of perturbed jets in particular, is hindered by the fact that such problems typically involve a three-dimensional, time-dependent evolution of flows with free surfaces, the locations in time of which should also be established. Solving such problems in the framework of the rigorous equations of fluid mechanics, say the Navier–Stokes equations, in most cases would be prohibitively time-consuming, even using super-computers. Additional complicating factors arise due to the rheological complexity of polymer solutions used in fiber forming, as well as the temperature-dependent variation of material properties in nonisothermal situations. Accounting for all these factors together in the framework of the rigorous equations of non-Newtonian fluid mechanics would be tremendously difficult. However, these difficulties can be relatively easily overcome in the framework of a quasi-one-dimensional description of liquid motion in the bending jets. In the works of Entov and Yarin (1980, 1984a) and Yarin (1983, 1993) the general quasi-one-dimensional equations of the straight and bending jets were derived from the integral balances of mass, momentum and moment of momentum, as well as by averaging the three-dimensional equations of hydrodynamics over the jet cross-section.

This chapter outlines several applications of electrospun and solution-blown nanofibers and their mats. In the case of filters and membranes (Section 9.1), industrial application has already begun, but a number of research questions are still open. Applications of nanofiber mats as fluffy electrodes beneficial for fuel cells and Li-ion batteries have recently attracted significant attention and are the focus of Section 9.2. Two recent approaches based on nanofibers were proposed in the field of cooling of high-heat flux microelectronics (Section 9.3) and nanofluidics (Section 9.4).

Filters and membranes

Filter materials are used for air, water and blood filtration, while membranes are used in separation processes, in particular, for bioseparation and pathogen removal for direct blood transfusion. Filters can remove particles, droplets, bacteria, viruses or even individual molecules from a carrier fluid flowing through them, or, in principle, possess advanced detection and response features that are practically absent in today’s products (see Chapter 10).

Chemical warfare differs from conventional warfare, such as explosives, in the sense that the toxicity of the chemicals used is very dangerous to the combatants in the battlefield, or to the civil population being affected during a war or as a result of a terrorist attack. The threat is amplified by concealment and low cost and relatively easy production of these agents, which renders them weapons of mass destruction. It becomes essential in such a situation that soldiers wear protective clothing, respirators, face masks, gloves, etc. Initially German troops unleashed chemical warfare agents, such as mustard agent, HD, during World War I in 1915, and this resulted in much more concern during World War II. Thus research on decontamination of chemical and biological warfare agents (CWAs) has been the subject of serious interest in many research laboratories since World War II. Mustard gas or bis(2-chloroethyl) sulfide is one of the highly cytotoxic, readily methylating CWAs that are regulated under the 1993 Chemical Weapons Convention (CWC). It covalently binds to DNA bases or forms disulfide bonds with the thiol groups. This readily leads to programmed cell death or the mutated DNA leads to cancer. The simultaneous danger associated with this toxic agent is its high skin penetration owing to its high lipophilicity, which results in immediate blister formation and huge uptake through skin within a short duration of exposure (Ivarsson et al. 1992).

Conventional modes of decontamination involve activated charcoal or other such heavy physical adsorbants of the contaminant. When compared with technologies employing conventional catalysts and reactive sorbents, which are specific and sometimes time-consuming, nanotechnology-based nanomaterials are highly reactive, nonselective and multifunctional in character. Current development of nanoparticles such as MgO, Al2O3, Fe2O3, ZnO and TiO2 and their incorporation into nanofibers impart enhanced catalytic, disinfection and sensing capabilities, photo-protection capability, and stain-resistance and self-cleaning properties. The testing of these materials in textiles and protective clothing against CWAs showed that they have potential as replacement technology in such applications.

Preface

Fiber-forming processes and the resulting fibers have become a key element in many modern technologies. Today, practically everyone is directly or indirectly using these fibers. Manmade macroscopic fibers are widely used in our garments and many other items of everyday life. On the other hand, much smaller microscopic and, especially, nanofibers are only beginning their path to prominence. The chemical, physical and technological aspects of manufacturing of such fibers are still weakly linked and not fully understood. Two main processes associated with formation of micro- and nanofibers are melt- or solution blowing and electrospinning. They require concerted interaction of synthetic chemistry, responsible for polymers used as raw materials, polymer physics, providing a link to their viscoelastic behavior, rheological characterization of flow properties, non-Newtonian fluid dynamics of polymer solutions and melts, aerodynamics, associated with gas blowing, and electrohydrodynamics, in the case of electrospinning. The key element of the fiber-forming processes is a thin jet of polymer solution or melt, which rapidly changes its three-dimensional configuration under the action of the aerodynamic or electric forces applied to its surface and the internal viscous and elastic stresses. There is a definite and imperative need to interpret and rationalize these phenomena, which requires acquisition of extensive experimental data and establishment of an appropriate theoretical framework as an essential element in the further technological design and optimization. In addition to the above-mentioned broad spectrum of disciplines, this involves different aspects associated with materials science, such as the methods developed in polymer crystallography, and elasticity and plasticity theory. Although many aspects of fiber-forming processes can today be considered as uncovered and well described, either experimentally or theoretically/numerically, numerous important details are still to be explored. The importance of this subject is attested by an exponential increase in scientific publications devoted to microscopic and nanofibers and a broad involvement of the industries associated with fiber media, nonwovens, nano-textured materials, novel biomedical and healthcare products and optical fibers, as well as defense applications.