Chapter 7
Development of Multiple-Jet Electrospinning Technology Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 27, 2016 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch007
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Dufei Fang , Charles Chang , Benjamin S. Hsiao , and Benjamin Chu 2,*
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Stonybrook Technology and Applied Research Inc., P.O. Box 1336, Stony Brook, NY 11790 Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400
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We have successfully developed a unique multiple-jet electrospinning technology at STAR, Inc. and Stony Brook University. The development of this technology was primarily accomplished by the incorporation of secondary electrodes to isolate the electric field distribution of the primary electrode spinnerets, the design of an individual fluid distribution system and the optimization of system design using the finite element analysis method, in combination with experimental tests. The key technological advance permits the small-scale mass fabrication of membranes with composite nanofiber/ nanoparticle hybrid morphology, tailor-designed composition variations, and 3D pattern formation from polymer solutions.
Introduction When an external electrostatic field is applied to a conducting fluid (e.g., a charged semi-dilute polymer solution or a charged polymer melt), a suspended conical droplet is formed, whereby the surface tension of the droplet is in equilibrium with the electric field. Electrospinning occurs when the electrostatic field is strong enough to overcome the surface tension of the liquid droplet at the spinneret tip. The liquid droplet then becomes unstable and a tiny jet is ejected © 2006 American Chemical Society
Reneker and Fong; Polymeric Nanofibers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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from the surface of the droplet. As it reaches a grounded target, the jet stream can be collected as an interconnected web of fine sub-micron size fibers. The resulting films from these nanoscale fibers (nanofibers) have very large surface area-to-volume ratios and very small pore sizes. The electrospinning technique was first developed by Zeleny (/) and patented by Formhals (2). Up to now, there are about 60 patents on electrospinning technology. Much research has been reported on how the jet is formed as a function of electrostatic field strength, fluid viscosity, and molecular weight of polymers in solution. In particular, the work of Taylor and others on electrically driven jets has laid the groundwork for electrospinning (5). Although potential applications of this technology have been widely mentioned, which include biological membranes (substrates for immobilized enzymes and catalyst systems), wound dressing materials, artificial blood vessels, aerosol filters, clothing membranes for protection against environmental elements and battlefield threats, just to name a few (4-24), to our knowledge, no practical industrial process for electrospinning of polymer systems for fabric applications has ever been implemented. The existing commercial electrospinning process by Donaldson, Inc. is limited to the manufacture of filter membranes, not of clothing. The major technical barrier for manufacturing electrospun fabrics for clothing and other applications is the speed of fabrication. In other words, as the fiber size becomes very small, the yield of the electrospinning process becomes very low. Another major technical problem for mass production of electrospun fabrics is the assembly of spinnerets during electrospinning. A straightforward multi-jet arrangement, as in high-speed melt-spinning, cannot be used because adjacent electrical fields often interfere with one another, making the mass production scheme by this approach impractical. The main objectives of this paper are thus to discuss the underlying physics of multiple-jet electrospinning operations as well as to demonstrate a prototype multiple-jet device designed and constructed by the Stony Brook scientists and engineers.
Principle of Single-Jet Electrospinning Electrospinning is a unique fiber spinning technique, which is capable of generating nano-sized fibers. The operation of electrospinning can be divided into three stages: (1) jet initiation, (2) jet elongation, and (3) solidification of jet (nanofiber formation). During the jet initiation stage, as the surface tension of a polymer solution or a polymer melt was overcome by electrical forces at the surface of a polymer droplet, a charged jet was ejected. After ejection, the jet travels for a certain distance in a straight line (the stage of stable jet), and then bends and follows a looping spiral course (the stage of unstable jet). During the jet elongation (mostly in the unstable jet stage), the electrical force stretches the
Reneker and Fong; Polymeric Nanofibers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 27, 2016 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch007
93 jet stream from thousands to millions of times longer. During the first two stages, the solvent continues to evaporate. Finally, with enough solvent being evaporated, the next stage is a solidification of the jet where the viscosity of the polymer solution becomes so high that stretching of the jet stream is basically non-existent. With further evaporation of the solvent, the resulting nano-fibers can be collected on an electrically grounded collector, such as a metal drum, a screen, or a coagulating bath. The resultant nano-sized fibers are of substantial scientific and commercial interest because their unique morphologies and properties are very different from conventional fibers having one to two orders of larger diameter than those of the electrospun nanofibers. Electrospinning can be affected by many parameters such as electric field, solution viscosity, conductivity, surface tension, polymer chain relaxation time, and the electrical charges carried by the jet. The mechanical forces acting on the conducting fluid, which must be overcome by the interactions between an electrostatic field and the conducting fluid to create the jet, can be understood by examining a fluid droplet to be formed at the tip of a capillary tube (Figure 1). In this droplet, a higher pressure is developed due to molecular interactions. This excess pressure Ap inside the droplet, which acts upon the capillary cross-section area d, is counterbalanced by the surface tension Facting on the circumference nr\ i.e. 2
Ap* w
= YU2nr, or
2Y Ap = — r
(1)
Ap
Y
Figure 1. Schematic of a fluid droplet createdfrom a capillary.
Equation 1 indicates that both the droplet excess pressure Ap and the surface energy per unit drop volume (4 nr l [(4^/3) r ]) = 3Y/r) become large when r is small. When the liquid droplet is suspended from a capillary tip (pendant droplet, as shown in Figure 2), the surface tension of the droplet can be 2
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Reneker and Fong; Polymeric Nanofibers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
94 derived from the droplet shape and the balance of all the forces acting upon the droplet, including the gravity. The relationship can be expressed as follows. 2
Y = gApr /j3
(2)
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0
where Ap is the density difference between the fluids at the interface (Ap = p for the droplet having a liquid/air interface), g is the gravitational constant, r is the radius of drop curvature at the apex and /? is the shape factor, which can be defined as:
dxl ds = cos^ dzds = sin l ds = 2 + J3z-sm
