Elsevier

Polymer

Volume 42, Issue 1, January 2001, Pages 261-272
Polymer

The effect of processing variables on the morphology of electrospun nanofibers and textiles

https://doi.org/10.1016/S0032-3861(00)00250-0Get rights and content

Abstract

Electrospinning is a process that produces continuous polymer fibers with diameters in the sub-micron range through the action of an external electric field imposed on a polymer solution or melt. Non-woven textiles composed of electrospun fibers have a large specific surface area and small pore size compared to commercial textiles, making them excellent candidates for use in filtration and membrane applications. While the process of electrospinning has been known for over half a century, current understanding of the process and those parameters, which influence the properties of the fibers produced from it, is very limited. In this work, we have evaluated systematically the effects of two of the most important processing parameters: spinning voltage and solution concentration, on the morphology of the fibers formed. We find that spinning voltage is strongly correlated with the formation of bead defects in the fibers, and that current measurements may be used to signal the onset of the processing voltage at which the bead defect density increases substantially. Solution concentration has been found to most strongly affect fiber size, with fiber diameter increasing with increasing solution concentration according to a power law relationship. In addition, electrospinning from solutions of high concentration has been found to produce a bimodal distribution of fiber sizes, reminiscent of distributions observed in the similar droplet generation process of electrospray. In addition, we find evidence that electrostatic effects influence the macroscale morphology of electrospun textiles, and may result in the formation of heterogeneous or three-dimensional structures.

Introduction

Polymer fibers are used in a wide variety of applications ranging from textiles to composite reinforcement. Traditional methods of obtaining polymer fibers include melt spinning [1], spinning from solution or liquid crystalline state, and forming fibers from a gel state [2]. Typical fiber diameters produced by these methods range from 5 to 500 nm, with the lower limit of fiber diameter that is consistently achievable on the order of magnitude of a micron. Recently [3], there has been increased interest in another method of fiber production, electrospinning, which can consistently produce fibers that are sub-micron in diameter. Textiles produced from these fibers are showing promise for exploitation in clothing and filtration applications [3], [4]. In addition, there is some evidence [5] indicating that electrospun fibers have a sizable static charge making it possible to manipulate them into three-dimensional (3) structures during their deposition. This property, together with the small pore size and high surface area inherent in electrospun nanofiber non-woven fabrics [3], [4], [5], has implications for the use of electrospun fibers in biomedical applications such as scaffoldings for tissue growth [6].

The electrospinning process can be considered a variation of the better-known electrospray process. It has been understood for most of this century that it is possible to use electrostatic fields to form and accelerate liquid jets from the tip of a capillary [7], [8], [9]. The surface of a hemispherical liquid drop suspended in equilibrium at the end of a capillary will be distorted into a conical shape in the presence of an electric field. A balancing of the repulsive force resulting from the induced charge distribution on the surface of the drop with the surface tension of the liquid causes this distortion. Once a critical voltage, Vc, is exceeded a stable jet of liquid is ejected from the cone tip. The jet breaks up into droplets as a result of surface tension in the case of low viscosity liquids. For high viscosity liquids the jet does not break up, but travels as a jet to the grounded target [8]. The first case is known as electrospraying and is used in many industries to obtain aerosols composed of sub-micron drops with narrow distributions. When applied to polymer solutions and melts, the second case is known as electrospinning and it generates polymer fibers that are sub-micron in diameter.

As a result of both commercial and scientific interest in electrospray, much effort has been made to understand and control the process [10], [11], [12]. Taylor established that the equilibrium shape of a hemispherical drop changes to a cone shape in an electric field. When the voltage applied to the drop exceeds a critical value, a liquid jet will initiate at the vertex of the cone. After jet initiation, the cone shape cannot be maintained if the flow of solution to the capillary tip does not match the rate at which solution is being removed by the jet [8], [9]. However, the electrostatically driven jet may continue to flow after the collapse of the cone, even though the shape of the surface from which the established jet originates, to be referred to as originating surface here after, is radically different from the conical shape seen at the time of jet initiation. The impact of the shape of originating surface and other processing variables (solution viscosity, accelerating voltage) on the size and size distribution of electrosprayed droplets are discussed in review articles by Cloupeau and Prunet-Foch [10] and Grace and Marijnissen [11]. The major emphasis in these two articles is the identification of different modes of electrospray. These modes are characterized by varying degrees of instability associated with the shape of the originating surface, and are achieved by manipulating the flow rate of solution to the capillary tip and the applied voltage. The electrospray modes affect the average droplet size and droplet size distribution, with the Taylor-cone mode producing the smallest average droplet sizes and narrowest droplet size distribution [10], [11], [12]. These results are of interest because processing parameters influencing droplet size and size distribution in the electrospray process may similarly influence the morphology of polymer fibers formed using the electrospinning process.

If the electrospray process is applied to a polymer solution of sufficient viscosity (∼1–200 poise), the initial jet does not break up into individual drops (Fig. 1). Instead, the solvent will evaporate as the jet proceeds to the target, leaving behind polymer fiber. The jet often follows a bending and turning trajectory as it travels to the target, and under certain conditions may also split or splay into smaller fibers [5], [13], [14], [15], [16], [17], [18]. The small diameters of electrospun fibers are achieved as a result of a single filament undergoing drawing and splaying analogous to the droplet disintegration described for electrospray [18].

Interest in the electrospinning process has increased in recent years [13], [14], [15], [16], [17]. Most of the literature on electrospinning has explored the types of polymer solvent systems from which fibers can be produced. A few studies have also addressed the processing/property relationships in electrospun polymer fibers, either directly or indirectly. Processing parameters considered have included solution concentration and viscosity effects, spinning atmosphere effects, accelerating voltage effects and tip-to-target distance. Solution viscosity has been found to influence fiber diameter [5], [15], [16], [17], initiating droplet shape [5], [16], and the jet trajectory [5], [16], [18], [19]. Increasing solution viscosity has been associated with the production of larger diameter fibers [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. Baumgarten [16] has also correlated spinning atmosphere with the occurrence of the jet splaying phenomena. Though splaying effects have been observed recently by Reneker, et. al. [13], [14], [15], [18], and commonly in our laboratory under certain experimental conditions (Fig. 2), they have not been reported by all investigators [19], [20], [21]. Other processing variables, such as acceleration voltage, electrospinning current and tip-to-target distance have not been investigated fully, but have been linked with fiber morphology and defect structures [5], [15], [16], [17], [22], [23].

It is clear from the background literature for both the electrospray and electrospinning processes that the structure and morphology of the final product, be it particles or fibers, are determined by a synergetic effect of solution parameters and electrostatic forces. These parameters include viscosity, surface tension, concentration, and dielectric properties of the spinning solution or melt, and process parameters such as the feed rate of the solution to the tip, and the acceleration voltage. Work that addresses the effect of the variables mentioned above on the sub-micron fiber structure and morphology is in its early stages. The purpose of the current work is to look systematically at the effects of accelerating voltage and solution concentration on the structure and morphology of electrospun polyethylene oxide fibers. We find that significant changes in fiber diameter, size distribution, and morphology accompany changes in these variables. Analogies are drawn to similar experiments found in the electrospray literature.

Section snippets

Experimental

The solutions used in the electrospinning experiments were prepared using 400,000 molecular weight Poly(ethylene oxide) (PEO) purchased from Scientific Polymer Products. This material was dissolved in HPLC grade water to make solutions with concentrations ranging from 4 to 10 wt%. Surface tensions for each solution were determined by the Wilhelmy Balance method using glass microscope cover slides that were cleaned with a butane torch. Solution viscosities were measured with a TA Instruments AR

Nanofiber morphology: voltage dependence

As has been discussed in the introduction, the electrospray process can be sustained in a variety of modes characterized by the shape of the surface from which the liquid jet originates. These modes occur at different voltages and have significant effects on droplet size distribution and current transport. Although distinct modes might be difficult to isolate and observe in the electrospinning process, it is expected that the degree of instability of the liquid surface from which the jet

Conclusions

In this work, the effects of processing parameters on the morphology of individual electrospun PEO nanofibers and the properties of electrospun non-woven fiber textiles has been explored. We have found that the morphology of the nanofibers produced is influenced strongly by parameters such as feed rate of the polymer solution, the electrospinning voltage, and the properties of the solution such as concentration, viscosity and surface tension. Increasing electrospinning voltage changed the shape

Acknowledgements

The authors would like to thank Dr Darrell Reneker and his research group for their advice concerning all aspects of this work. In addition, we wish to thank Ibrahim Sendijarevic and Dr Anthony McHugh of the University of Illinois for providing viscosity measurements of polymer solutions, and Mr Joseph Rehrmann of the Soldier Chemical Biological Command-Edgewood Chemical and Biological Center for the BET measurements.

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