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A comparative study of jet formation and nanofiber alignment in electrospinning and electrocentrifugal spinning systems
Journal of Electrostatics 69 (2011) 540e546
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
A comparative study of jet formation and nanofiber alignment in electrospinning and electrocentrifugal spinning systems F. Dabirian a, S.A. Hosseini Ravandi a, *, A.R. Pishevar b, R.A. Abuzade a a b
Nanotechnology & Advanced Materials Institute, Department of Textile Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran Department of Mechanical Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2011 Received in revised form 11 June 2011 Accepted 21 July 2011 Available online 18 August 2011
Electrocentrifugal spinning is a recently developed spinning system whose performance is still under investigation by researchers. In this study the process of jet formation in electrocentrifugal spinning is explored and compared to the same process in electrospinning and centrifuge spinning. The results show that the incorporation of the electrical and the centrifugal forces in the electrocentrifugal spinning system leads to the formation of a more stable jet at lower viscosities. It is also shown that the electrocentrifugal spinning method is an efficient technique for the production of aligned nanofiber bundles with enhancement in the mechanical properties. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Electrocentrifugal spinning Jet formation Nanofiber
1. Introduction Polymeric nanofibers enjoy excellent physical and mechanical characteristics embedded in their nature of decreased diameters down to submicron. Very large surface area to volume ratio of nanofibers offers different significant applications for them including nanocatalysis, tissue engineering scaffolds, protective clothing, filters, nanoelectronics, high performance fibrous instruments, nanobiosensors, drug delivery, wound dressing, composites, etc [1]. Different techniques have already been proposed to produce nanofibers such as drawing, template synthesis, gas jet spinning, island-in-the-sea, electrospinning and recently electrocentrifugal spinning. Among these methods, electrospinning seems to be the most versatile, applicable, high-potential and simplest technique. The formation of nanofiber via electrospinning is based on the continuous stretching of a viscoelastic jet derived from a polymer solution or melt by the electrostatic forces. The electrospinning technique may be considered as a variant of the electrospray process. Both of these techniques involve the use of a high voltage supply to induce the formation of a liquid jet. In electrospray small droplets or particles are formed as a result of the breakup of an electrified jet that is often produced from a low viscosity solution.
* Corresponding author. Research Center of Science and Fiber Technology, Department of Textile Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran. Tel.: þ98 311 3915034; fax: þ98 311 3912444. E-mail address: [email protected] (S.A. Hosseini Ravandi). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.07.006
In electrospinning a solid fiber is generated as the electrified jet is continuously elongated due to the electrostatic repulsions between the surface charges and the evaporation of the solvent [2]. Due to the unique features and applications, the process has recently been reviewed in a number of publications [3e11], but here we only intend to review the most recent activities in the area of alignment of nanofiber by this technique. The associated bending instability of the spinning jet causes the produced nanofibers to be deposited on the surface of the collector as randomly oriented nonwoven mat. However, in many applications, aligning the deposition of nanofibers in a specific direction is a requirement. For example, in the fabrication of electronic and photonic devices, well aligned and highly ordered architectures are often required. Even for applications as simple as fiber based reinforcement, it is also critical to control the alignment of fibers. In recent years a number of approaches have been demonstrated to directly collect electrospun nanofibers as uniaxial aligned arrays [2]. These approaches include electrospinning which uses a collector consisting of two pieces of electrical conductors separated by a gap [12e17], collecting spun nanofibers on a rotating thin wheel with sharp edge [18], fabricating aligned yarn of nanofibers by rapidly oscillating a grounded frame within the jet [19,20], using a metal frame as a collector to generate parallel arrays of nanofibres [21,22], using magnetic field to produce aligned nanofibrous arrays [23,24], and using a rotating drum at a very high speed up to thousands of rpm [25,26]. In another approach, Deitzel et al. [27] used a multiple field technique which can straighten the
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polymer jet to some extent. Subsequently, Dabirian et al., employed two connected needles to the positive and negative voltages to collect high bulk nanofiber on a slowly rotating drum [28e30]. The first significant work on the free surface flow of liquid jets is that of Lord Rayleigh [31]. Rayleigh analyzed the linear stability of a liquid jet. He showed that the unstable nature of liquid jets is caused by the surface tension force acting on the jet. It is generally accepted that Rayleigh’s inviscid linear model describes the beginning of liquid jet breakup. Weber [32] introduced viscosity into the stability analysis. A significant review of the early work on liquid jets is given by Bogy [33] and a more up-to-date and extensive review is given by Eggers [34]. Other important related works include Middleman [35], Hilbing and Heister [36], Partidge et al. [37], Parau et al. [38], Keller et al. [39], Tuck [40], Vanden-Broeck and Keller [41], Entov and Yarin [42], Dias and Vanden-Broeck [43], Yarin [44] and Cummings and Howell [45]. Recently, a new method of producing aligned nanofiber called electrocentrifugal spinning was introduced [46]. In order to apply a high rate of tension to a polymer solution or melt, methods which can apply uniform distribution of stress are required. When centrifugal force acts on a solution, it affects all parts of the solution and applies uniform distribution of stresses. Thus, this force can be used to apply high rates of tension on a polymer solution. During the tension process, if the polymer solution has sufficient viscosity, it is stretched as a string and transformed to a polymeric fiber after drying. Weitz and coworkers used centrifuge force to produce nanofiber without nozzle; it involves the application of drops of a polymer solution onto a standard spin coater [47]. The viscosity of the polymer solution arises from the frictional forces between polymer chains in the solution. Since the nature of frictional forces is dependent on the speed of the applied exterior forces, with an increase in this rapidity, the frictional forces between polymer chains increase. Consequently, by increasing the rate of the applied exterior forces, the polymer solutions with lower concentrations can be spun. The simultaneous use of electrostatic and centrifugal forces provides new possibilities to manufacture nanofibers [48]. In continuation, jet formation in this new method will be investigated by high-speed photography. The jet formation in
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electrospinning and electrocentrifugal spinning will be compared. Finally, the mechanical properties of both electrospun and electrocentrifugal spun aligned nanofiber bundle will be considered. 2. Experimental 2.1. Materials Laboratorial distilled water was produced manually. Commercial polyacrylonitrile (PAN) polymer powder with Mw ¼ 100,000 g/mol and Mn ¼ 70,000 g/mol was supplied by Poly Acryl, Iran. The solvent used was dimethyl formamide (DMF) from Merck Company, Germany. The polymer solution of PAN in DMF with the concentration of 15% wt was prepared. 2.2. Set-up 2.2.1. Centrifuge spinning As shown in Fig. 1 a needle of 18 mm in length with interior and exterior diameter of 160 and 300 mm, respectively was attached to a container with 4.6 mm interior diameter to form a nozzle for the purpose of supplying polymer solution. The nozzle was mounted on a circular plate. The polymer solution with proper viscosity runs through the nozzle and forms a jet once the circular plate starts to rotate and centrifugal force is applied. The jet is dried on its way out of the needle and nanofibers are formed. The flow of surrounding air near the nozzle can dry up the ejecting solution at the needle tip and as a result, causes obstruction in the solution flow. To overcome this problem, the centrifuge set-up was modified and as shown in Fig. 1, a cylinder was used to enclose the nozzle and the container as only 2 mm of nozzle tip was left uncovered. Due to this modification, the surrounding air in the vicinity of the nozzle tip achieves the same velocity and therefore, the convective heat and mass transfer are reduced considerably. Consequently, the solvent vaporization rate is reduced and the obstruction problem is avoided. The procedure called centrifuge spinning method, will work continually. The trajectory of the jet is captured by means of a high-speed digital camera (Kodak Motion Corder 3000, with a capture rate of up to 3000 frames per second).
Fig. 1. Schematic representation of centrifuge (without high voltage) and electrocentrifuge (with high voltage) spinning set-up, (A) axle of rotation, (B) polymer solution container, (C) nozzle tip, (D) encircling cylinder, (E) collector and (F) polymeric jet.
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2.2.2. Electrocentrifuge spinning With the purpose of applying electrostatic and centrifugal forces simultaneously, a DC high voltage power supply was used. A metallic cylinder with diameter of 26.6 cm and height of 10 cm was used as the collector. It was attached to the negative electrode of power supply while the nozzle was connected to the positive electrode, as can be seen in Fig. 1. It was observed that the distance between the center of rotation, the needle tip and the collector surface should be set at optimum values. For values below the optimum point, fibers reach the collector surface before they can get dried whereas for values above the optimum point fibers suspend within the cylinders space and never reach the collector surface. The set-up was assembled together in a way that the distances between the needle tip to the center of rotation and the collector to the center of rotation were set to 5.3 and 13.3 cm, respectively. Fig. 2 shows a photograph taken by the camera with a special micro-lens from the set-up while rotating at 6630 rpm. The image illustrates a part of the cylindrical cover, the nozzle tip and the formation of the fiber clearly. To describe the differences between the mechanisms of jet formation in electrospinning and electrocentrifugal spinning, first the experiment is performed on water and the drift of the jet and the formation of droplets in these two methods are compared using a camera to record sequential snapshots of the process in equal intervals. Then, the role that viscosity plays in the process is considered by replacing water with a polymeric solution. At the end, the mechanical properties of aligned polyacrylonitrile (PAN) nanofiber produced by both methods are compared and discussed.
3. Results and discussion Jet formation process is quite different in electrospinning, centrifuge spinning and electrocentrifugal spinning. In the following section, the formation of jet by these methods is compared for two different cases of a water jet and a polymeric jet to stress the effect of viscosity. Aligned nanofiber produced by the electrospinning and electrocentrifugal spinning will be discussed and the mechanical properties of nanofibers produced by these methods will be compared in the last section. 3.1. Comparison of water behavior in jet formation process To provide an experimental comparison of water behavior in three fiber spinning techniques, centrifugal spinning, electrospinning and their combination; all these processes run at the same flow rate. The flow rate in electrospinning could be easily controlled by the rate of syringe pump; therefore we calculate the flow rate in centrifuge spinning and then set the same flow rate for electrospinning. It is logical to expect that, the amount of solution in the container influences the flow rate in centrifuge spinning. To calculate the flow rate we chose an average value of 0.3 ml and filled the container with solution to 0.3 þ X ml and run the process until the amount of solution becomes 0.3 X ml and measured the consumed time. For further precision we used weight measurements instead of volume measurement. The calculated flow rate was 135 ml/h for the centrifuge spinning at 2358 rpm, and the same flow rate was applied for electrospinning and electrocentrifuge spinning as well. 3.1.1. Water behavior in electrospinning In electrospinning a droplet of polymer solution is formed at the needle tip as a result of the pump pressure. The droplets will be intensely charged when the high voltage is applied. In such a case the droplet will be affected by two types of electrostatic forces: repulsive forces between same charges distributed on the surface of droplet and the electric field force generated between the nozzle and the collector. As a result of electrostatic forces, the droplet will be deformed and a Taylor cone is formed at the needle tip. The electrical forces will then overcome the surface tension of the droplet and a stable fluid jet will be ejected from the apex of the Taylor cone. The jet will speed up toward the collector while it exhibits bending instabilities due to repulsive forces between charges accumulated on its surface. This makes the jet trajectory a bent spiral path. Thus, the jet travels a long distance in the short gap between needle tip and collector and will be drawn thousand times to become very fine in thickness. Ultimately, the solvent evaporates, or the melt solidifies and nanofibers are obtained on the collector in diameters down to submicron or even few nanometers. For the solution with low viscosity or low molecular weight, the jet will break up into droplets of very small size. In such a case, the process is called electrospraying and has significant applications in different industries for the purpose of producing submicron sized droplets. Because of the low viscosity of water, applying high voltage to water droplet leads to electrospraying instead of electrospinning. As can be seen in Fig. 3, the process starts with a very thin fluid thread, then turns to a short jet, and due to its low viscosity becomes transformed to droplets soon. However, the resistant viscous force, a requirement for the formation of a jet, is not high enough to withstand the inertial force. As a result, the fluid thread is ruptured and the droplets are ejected from it.
Fig. 2. Image of PAN nanofiber formation in electrocentrifugal spinning at rotational speed of 6630 rpm and polymer solution of 15% wt.
3.1.2. Water droplet behavior in centrifuge spinning In this section the effect of centrifuge force on expulsion of water droplet and formation of a water jet will be discussed. In this experiment, the centrifuge set-up (explained in Section 2.2.1) is filled
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with water and rotated with various rotational speeds. The centrifuge force pumps the fluid into the needle and, therefore, the flow rate is dependent on the rotational speed. Based on rotational speed, two different modes can be developed in the experiment. At lower rotational speed water droplets will be formed and expelled out from the nozzle whereas a jet of fluid is generated at higher rotational speeds as can be seen in Fig. 4. The breakup of the jet into smaller drops is also clearly observed in these images.
Fig. 3. Images of water behaviors in electrospinning; a short unstable jet formed and turned into droplets soon.
3.1.3. Water droplet behavior in electrocentrifugal spinning In electrocentrifugal spinning the formation of the jet is solely due to the centrifuge force which also drives the fluid into the needle and a jet is formed without the development of the Taylor cone. The exit jet is then deformed by the electrostatic forces and is highly stretched as shown in Fig. 6. This image was taken for the operating conditions of 2385 rpm, 15 kV and 8 cm spinning gap between nozzle and collector. Again, due to the low viscosity of water, the stretching process cannot continue and as Fig. 5 clearly shows, the growth of instability waves ultimately leads to the breakup of the jet into small drops. Comparing Figs. 3e5 reveals that by conjugate use of electrostatic and centrifuge forces, it is possible to achieve a stable jet at higher flow rates even for a low viscosity fluid as water. This is a significant factor in fabricating nanofibers by this method because it can increase the production rate considerably. 3.2. Comparison of polymer solution behavior
Fig. 4. Images of water droplet formation, water jet ejection, and breaking up to droplets in centrifuge spinning at rotational speed of 2385 rpm.
Fig. 5. Image of electrocentrifugal spinning of water (2385 rpm,15 kV, 8 cm spinning gap).
As explained in the previous section, the mechanism of jet formation and nanofiber production is different in electrospinning and electrocentrifugal spinning. In this section, the effect of the viscoelastic forces on the formation of the jet by these two methods is compared using a PAN solution instead of water. 3.2.1. Polymeric jet in electrospinning The PAN solution can form a droplet stabilized by its surface tension at the end of the needle tip. Strong electrostatic field is applied to the droplet. The droplet will deform into a Taylor cone. When the applied voltage exceeds a critical value at which the electrostatic force overcomes the surface tension and viscoelastic forces, a stable jet of polymer solution can be ejected from the droplet. In contrast to previous experiment, due to the resistance viscoelastic forces the jet is not broken up to the droplets. Instead, the ejected polymeric jet travels a short distance following a straight path and then bending instability is developed in the jet due to the mutually repulsive forces resulting from the electric charges of the jet. Fig. 6 shows these three stages schematically and graphically for electrospinning of 15% wt PAN solution.
Fig. 6. (a) Schematic illustration of jet path in electrospinning, and (b) Taylor cone, straight section and spiral path in electrospinning of 15% wt PAN solution.
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Fig. 7. Electrocentrifugal spinning of 15% wt PAN in DMF at rotational speed of 6630 rpm.
3.2.2. Polymeric jet in electrocentrifugal spinning In this method the polymeric jet is formed only by the centrifugal force and undergoes elongation by the simultaneous action of centrifugal and electrostatic forces until it lies down on collector. Fig. 7 shows an image of the process using 15% wt PAN solution at the rotational speed of 6360 rpm. The ejected jet from the nozzle encounters bending instabilities after a short distance traveling in a curved path. As it can be seen in this image, due to the high flow rate, the Taylor cone has not appeared and the polymeric fluid conforms to a jet as a consequence of the applied centrifugal force.
In comparison with electrospinning, bending instabilities are observed with less frequencies and amplitude in electrocentrifugal spinning. This can be explained in this way: by increasing the rotational speed, the relative velocity between the jet and the surrounding air, which tends to dry up the solution, is increased. As a result, the growth of instability wave is suppressed due to the evaporation of solvent. On the other hand, by increasing the polymer concentration, the polymeric bending instability of the jet is reduced and the jet travels in a straight path toward the collector. This provides the possibility of collecting highly aligned nanofibers on the outer cylinder. Fig. 8 shows images of electrocentrifugal spinning for 16% wt PAN at speed of 6360 rpm where the bending instabilities have considerably vanished and as a result, the polymeric jet takes on a completely straight and aligned form. This method can be employed to produce highly aligned nanofibers from solutions with high concentration. Fig. 9 shows a scanning electron microscopy of nanofibers produced by electrocentrifugal spinning of 15% and 16% wt PAN in DMF at rotational speed of 6360 rpm and applied voltage of 15 kV. The morphology of nanofibers was studied and explained in previous work [46]. In brief, as shown in Fig. 10 the results indicated that the production of nanofibers with drop sprinkling was possible, and at low rotational speed and concentrations, the nanofibers contain drops and beads in centrifuge spinning. When using electrocentrifuge spinning, the nanofiber diameters have been changed compared to centrifuge spinning. Results showed drastic reduction in fiber diameter which is the effect of adding electrical force to centrifuge forces. 3.2.3. Comparison of mechanical properties The mechanical properties of aligned nanofiber bundles were measured by Zwick 1446e60. Zwick was set for constant rate of elongation. To obtain load elongation curves, the sample length was
Fig. 8. Electrocentrifugal spinning of 16% wt PAN in DMF at rotational speed of 6360 rpm where the bending instabilities are considerably reduced.
Fig. 9. Scanning electron microscopy of nanofibers produced by electrocentrifugal spinning of (a) 15% and (b) 16% wt PAN in DMF at rotational speed of 6360 rpm and applied voltage of 15 kV.
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Fig. 10. Typical SEM images of nanofibers for a polymer solution of 15% wt and a rotational speed of 6615 rpm; (a) Nanofibers with drop and bead produced by centrifuge spinning [48], (b) Un-uniformity in nanofiber diameter in centrifuge spinning [48], (c) Nanofibers without drop or bead produced by electrocentrifuge spinning at voltage of 10 kV, and (d) Uniformity in nanofiber diameter.
10 cm with cross head speed of 60 mm/min. All samples were produced at room temperature (25 C) and dried at approximately 70 C in an oven for 2 h. Before the experiment, samples were in standard conditions (20 2 C and 65% RH) for 24 h. The results of measurement are listed in Table 1. The strength of electrospun PAN aligned nanofiber bundle was 53 MPa and its modulus was 1.99 GPa whereas these values were 112 MPa and 2.33 GPa for the electrocentrifugal spun PAN aligned nanofiber, respectively. The results show an increase in the strength and modulus of the aligned nanofiber bundle produced by electrocentrifugal spinning and a decrease in the strain at the brake point. However, no significant change occurs in work up to break parameter. The enhancements of mechanical properties can be accounted for by the considerable mechanical contact drag forces between the jet and the surrounding air which lead to a better alignment of fibers. Work up to break is a quantity which considers both strength and strain simultaneously. An increase in work up to break is expected due to increase of strength, but it should be noted that due to the strain reduction, the variation of work up to break is not considerable.
Table 1 The results of mechanical properties measurement of aligned nanofiber bundles by Zwick. Parameters
Electrospinninga
Electrocentrifugalb
Number of samples Nanofiber diameter (nm) Stress at F_max (MPa) Strain at break (%) Work up to break (N mm/tex) E-modulus (GPa)
30 410 53 75.9 3.16 1.99
30 (9.5) (17.1) (17.3) (24.2) (19.8)
440 112.42 60 3.06 2.33
(11.3) (8.3) (16.3) (22.2) (24.6)
a 16% wt PAN in DMF, 15 kV, 0.8 ml/h, 12 cm spinning distance, 2.5 cm gap distance on collector. b 16% wt PAN in DMF, 6360 rpm, 15 kV, distances explained in Section 2.2.
4. Conclusion The procedures of water jet formation and polymeric jet formation in three kinds of spinning systems involving electrospinning, centrifuge spinning and electrocentrifugal spinning were considered in this study. Electrocentrifugal spinning is a powerful technique for spinning solution with extremely low viscosity. In addition, electrocentrifugal spinning is a more suited technique for production of aligned nanofibers than electrospinning, since the bending instability of jet could be reduced via centrifugal force through the effect of surrounding air contacts. This can partially improve the mechanical properties of produced nanofiber.
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Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
A comparative study of jet formation and nanofiber alignment in electrospinning and electrocentrifugal spinning systems F. Dabirian a, S.A. Hosseini Ravandi a, *, A.R. Pishevar b, R.A. Abuzade a a b
Nanotechnology & Advanced Materials Institute, Department of Textile Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran Department of Mechanical Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 March 2011 Received in revised form 11 June 2011 Accepted 21 July 2011 Available online 18 August 2011
Electrocentrifugal spinning is a recently developed spinning system whose performance is still under investigation by researchers. In this study the process of jet formation in electrocentrifugal spinning is explored and compared to the same process in electrospinning and centrifuge spinning. The results show that the incorporation of the electrical and the centrifugal forces in the electrocentrifugal spinning system leads to the formation of a more stable jet at lower viscosities. It is also shown that the electrocentrifugal spinning method is an efficient technique for the production of aligned nanofiber bundles with enhancement in the mechanical properties. Ó 2011 Elsevier B.V. All rights reserved.
Keywords: Electrocentrifugal spinning Jet formation Nanofiber
1. Introduction Polymeric nanofibers enjoy excellent physical and mechanical characteristics embedded in their nature of decreased diameters down to submicron. Very large surface area to volume ratio of nanofibers offers different significant applications for them including nanocatalysis, tissue engineering scaffolds, protective clothing, filters, nanoelectronics, high performance fibrous instruments, nanobiosensors, drug delivery, wound dressing, composites, etc [1]. Different techniques have already been proposed to produce nanofibers such as drawing, template synthesis, gas jet spinning, island-in-the-sea, electrospinning and recently electrocentrifugal spinning. Among these methods, electrospinning seems to be the most versatile, applicable, high-potential and simplest technique. The formation of nanofiber via electrospinning is based on the continuous stretching of a viscoelastic jet derived from a polymer solution or melt by the electrostatic forces. The electrospinning technique may be considered as a variant of the electrospray process. Both of these techniques involve the use of a high voltage supply to induce the formation of a liquid jet. In electrospray small droplets or particles are formed as a result of the breakup of an electrified jet that is often produced from a low viscosity solution.
* Corresponding author. Research Center of Science and Fiber Technology, Department of Textile Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran. Tel.: þ98 311 3915034; fax: þ98 311 3912444. E-mail address: [email protected] (S.A. Hosseini Ravandi). 0304-3886/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2011.07.006
In electrospinning a solid fiber is generated as the electrified jet is continuously elongated due to the electrostatic repulsions between the surface charges and the evaporation of the solvent [2]. Due to the unique features and applications, the process has recently been reviewed in a number of publications [3e11], but here we only intend to review the most recent activities in the area of alignment of nanofiber by this technique. The associated bending instability of the spinning jet causes the produced nanofibers to be deposited on the surface of the collector as randomly oriented nonwoven mat. However, in many applications, aligning the deposition of nanofibers in a specific direction is a requirement. For example, in the fabrication of electronic and photonic devices, well aligned and highly ordered architectures are often required. Even for applications as simple as fiber based reinforcement, it is also critical to control the alignment of fibers. In recent years a number of approaches have been demonstrated to directly collect electrospun nanofibers as uniaxial aligned arrays [2]. These approaches include electrospinning which uses a collector consisting of two pieces of electrical conductors separated by a gap [12e17], collecting spun nanofibers on a rotating thin wheel with sharp edge [18], fabricating aligned yarn of nanofibers by rapidly oscillating a grounded frame within the jet [19,20], using a metal frame as a collector to generate parallel arrays of nanofibres [21,22], using magnetic field to produce aligned nanofibrous arrays [23,24], and using a rotating drum at a very high speed up to thousands of rpm [25,26]. In another approach, Deitzel et al. [27] used a multiple field technique which can straighten the
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polymer jet to some extent. Subsequently, Dabirian et al., employed two connected needles to the positive and negative voltages to collect high bulk nanofiber on a slowly rotating drum [28e30]. The first significant work on the free surface flow of liquid jets is that of Lord Rayleigh [31]. Rayleigh analyzed the linear stability of a liquid jet. He showed that the unstable nature of liquid jets is caused by the surface tension force acting on the jet. It is generally accepted that Rayleigh’s inviscid linear model describes the beginning of liquid jet breakup. Weber [32] introduced viscosity into the stability analysis. A significant review of the early work on liquid jets is given by Bogy [33] and a more up-to-date and extensive review is given by Eggers [34]. Other important related works include Middleman [35], Hilbing and Heister [36], Partidge et al. [37], Parau et al. [38], Keller et al. [39], Tuck [40], Vanden-Broeck and Keller [41], Entov and Yarin [42], Dias and Vanden-Broeck [43], Yarin [44] and Cummings and Howell [45]. Recently, a new method of producing aligned nanofiber called electrocentrifugal spinning was introduced [46]. In order to apply a high rate of tension to a polymer solution or melt, methods which can apply uniform distribution of stress are required. When centrifugal force acts on a solution, it affects all parts of the solution and applies uniform distribution of stresses. Thus, this force can be used to apply high rates of tension on a polymer solution. During the tension process, if the polymer solution has sufficient viscosity, it is stretched as a string and transformed to a polymeric fiber after drying. Weitz and coworkers used centrifuge force to produce nanofiber without nozzle; it involves the application of drops of a polymer solution onto a standard spin coater [47]. The viscosity of the polymer solution arises from the frictional forces between polymer chains in the solution. Since the nature of frictional forces is dependent on the speed of the applied exterior forces, with an increase in this rapidity, the frictional forces between polymer chains increase. Consequently, by increasing the rate of the applied exterior forces, the polymer solutions with lower concentrations can be spun. The simultaneous use of electrostatic and centrifugal forces provides new possibilities to manufacture nanofibers [48]. In continuation, jet formation in this new method will be investigated by high-speed photography. The jet formation in
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electrospinning and electrocentrifugal spinning will be compared. Finally, the mechanical properties of both electrospun and electrocentrifugal spun aligned nanofiber bundle will be considered. 2. Experimental 2.1. Materials Laboratorial distilled water was produced manually. Commercial polyacrylonitrile (PAN) polymer powder with Mw ¼ 100,000 g/mol and Mn ¼ 70,000 g/mol was supplied by Poly Acryl, Iran. The solvent used was dimethyl formamide (DMF) from Merck Company, Germany. The polymer solution of PAN in DMF with the concentration of 15% wt was prepared. 2.2. Set-up 2.2.1. Centrifuge spinning As shown in Fig. 1 a needle of 18 mm in length with interior and exterior diameter of 160 and 300 mm, respectively was attached to a container with 4.6 mm interior diameter to form a nozzle for the purpose of supplying polymer solution. The nozzle was mounted on a circular plate. The polymer solution with proper viscosity runs through the nozzle and forms a jet once the circular plate starts to rotate and centrifugal force is applied. The jet is dried on its way out of the needle and nanofibers are formed. The flow of surrounding air near the nozzle can dry up the ejecting solution at the needle tip and as a result, causes obstruction in the solution flow. To overcome this problem, the centrifuge set-up was modified and as shown in Fig. 1, a cylinder was used to enclose the nozzle and the container as only 2 mm of nozzle tip was left uncovered. Due to this modification, the surrounding air in the vicinity of the nozzle tip achieves the same velocity and therefore, the convective heat and mass transfer are reduced considerably. Consequently, the solvent vaporization rate is reduced and the obstruction problem is avoided. The procedure called centrifuge spinning method, will work continually. The trajectory of the jet is captured by means of a high-speed digital camera (Kodak Motion Corder 3000, with a capture rate of up to 3000 frames per second).
Fig. 1. Schematic representation of centrifuge (without high voltage) and electrocentrifuge (with high voltage) spinning set-up, (A) axle of rotation, (B) polymer solution container, (C) nozzle tip, (D) encircling cylinder, (E) collector and (F) polymeric jet.
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2.2.2. Electrocentrifuge spinning With the purpose of applying electrostatic and centrifugal forces simultaneously, a DC high voltage power supply was used. A metallic cylinder with diameter of 26.6 cm and height of 10 cm was used as the collector. It was attached to the negative electrode of power supply while the nozzle was connected to the positive electrode, as can be seen in Fig. 1. It was observed that the distance between the center of rotation, the needle tip and the collector surface should be set at optimum values. For values below the optimum point, fibers reach the collector surface before they can get dried whereas for values above the optimum point fibers suspend within the cylinders space and never reach the collector surface. The set-up was assembled together in a way that the distances between the needle tip to the center of rotation and the collector to the center of rotation were set to 5.3 and 13.3 cm, respectively. Fig. 2 shows a photograph taken by the camera with a special micro-lens from the set-up while rotating at 6630 rpm. The image illustrates a part of the cylindrical cover, the nozzle tip and the formation of the fiber clearly. To describe the differences between the mechanisms of jet formation in electrospinning and electrocentrifugal spinning, first the experiment is performed on water and the drift of the jet and the formation of droplets in these two methods are compared using a camera to record sequential snapshots of the process in equal intervals. Then, the role that viscosity plays in the process is considered by replacing water with a polymeric solution. At the end, the mechanical properties of aligned polyacrylonitrile (PAN) nanofiber produced by both methods are compared and discussed.
3. Results and discussion Jet formation process is quite different in electrospinning, centrifuge spinning and electrocentrifugal spinning. In the following section, the formation of jet by these methods is compared for two different cases of a water jet and a polymeric jet to stress the effect of viscosity. Aligned nanofiber produced by the electrospinning and electrocentrifugal spinning will be discussed and the mechanical properties of nanofibers produced by these methods will be compared in the last section. 3.1. Comparison of water behavior in jet formation process To provide an experimental comparison of water behavior in three fiber spinning techniques, centrifugal spinning, electrospinning and their combination; all these processes run at the same flow rate. The flow rate in electrospinning could be easily controlled by the rate of syringe pump; therefore we calculate the flow rate in centrifuge spinning and then set the same flow rate for electrospinning. It is logical to expect that, the amount of solution in the container influences the flow rate in centrifuge spinning. To calculate the flow rate we chose an average value of 0.3 ml and filled the container with solution to 0.3 þ X ml and run the process until the amount of solution becomes 0.3 X ml and measured the consumed time. For further precision we used weight measurements instead of volume measurement. The calculated flow rate was 135 ml/h for the centrifuge spinning at 2358 rpm, and the same flow rate was applied for electrospinning and electrocentrifuge spinning as well. 3.1.1. Water behavior in electrospinning In electrospinning a droplet of polymer solution is formed at the needle tip as a result of the pump pressure. The droplets will be intensely charged when the high voltage is applied. In such a case the droplet will be affected by two types of electrostatic forces: repulsive forces between same charges distributed on the surface of droplet and the electric field force generated between the nozzle and the collector. As a result of electrostatic forces, the droplet will be deformed and a Taylor cone is formed at the needle tip. The electrical forces will then overcome the surface tension of the droplet and a stable fluid jet will be ejected from the apex of the Taylor cone. The jet will speed up toward the collector while it exhibits bending instabilities due to repulsive forces between charges accumulated on its surface. This makes the jet trajectory a bent spiral path. Thus, the jet travels a long distance in the short gap between needle tip and collector and will be drawn thousand times to become very fine in thickness. Ultimately, the solvent evaporates, or the melt solidifies and nanofibers are obtained on the collector in diameters down to submicron or even few nanometers. For the solution with low viscosity or low molecular weight, the jet will break up into droplets of very small size. In such a case, the process is called electrospraying and has significant applications in different industries for the purpose of producing submicron sized droplets. Because of the low viscosity of water, applying high voltage to water droplet leads to electrospraying instead of electrospinning. As can be seen in Fig. 3, the process starts with a very thin fluid thread, then turns to a short jet, and due to its low viscosity becomes transformed to droplets soon. However, the resistant viscous force, a requirement for the formation of a jet, is not high enough to withstand the inertial force. As a result, the fluid thread is ruptured and the droplets are ejected from it.
Fig. 2. Image of PAN nanofiber formation in electrocentrifugal spinning at rotational speed of 6630 rpm and polymer solution of 15% wt.
3.1.2. Water droplet behavior in centrifuge spinning In this section the effect of centrifuge force on expulsion of water droplet and formation of a water jet will be discussed. In this experiment, the centrifuge set-up (explained in Section 2.2.1) is filled
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with water and rotated with various rotational speeds. The centrifuge force pumps the fluid into the needle and, therefore, the flow rate is dependent on the rotational speed. Based on rotational speed, two different modes can be developed in the experiment. At lower rotational speed water droplets will be formed and expelled out from the nozzle whereas a jet of fluid is generated at higher rotational speeds as can be seen in Fig. 4. The breakup of the jet into smaller drops is also clearly observed in these images.
Fig. 3. Images of water behaviors in electrospinning; a short unstable jet formed and turned into droplets soon.
3.1.3. Water droplet behavior in electrocentrifugal spinning In electrocentrifugal spinning the formation of the jet is solely due to the centrifuge force which also drives the fluid into the needle and a jet is formed without the development of the Taylor cone. The exit jet is then deformed by the electrostatic forces and is highly stretched as shown in Fig. 6. This image was taken for the operating conditions of 2385 rpm, 15 kV and 8 cm spinning gap between nozzle and collector. Again, due to the low viscosity of water, the stretching process cannot continue and as Fig. 5 clearly shows, the growth of instability waves ultimately leads to the breakup of the jet into small drops. Comparing Figs. 3e5 reveals that by conjugate use of electrostatic and centrifuge forces, it is possible to achieve a stable jet at higher flow rates even for a low viscosity fluid as water. This is a significant factor in fabricating nanofibers by this method because it can increase the production rate considerably. 3.2. Comparison of polymer solution behavior
Fig. 4. Images of water droplet formation, water jet ejection, and breaking up to droplets in centrifuge spinning at rotational speed of 2385 rpm.
Fig. 5. Image of electrocentrifugal spinning of water (2385 rpm,15 kV, 8 cm spinning gap).
As explained in the previous section, the mechanism of jet formation and nanofiber production is different in electrospinning and electrocentrifugal spinning. In this section, the effect of the viscoelastic forces on the formation of the jet by these two methods is compared using a PAN solution instead of water. 3.2.1. Polymeric jet in electrospinning The PAN solution can form a droplet stabilized by its surface tension at the end of the needle tip. Strong electrostatic field is applied to the droplet. The droplet will deform into a Taylor cone. When the applied voltage exceeds a critical value at which the electrostatic force overcomes the surface tension and viscoelastic forces, a stable jet of polymer solution can be ejected from the droplet. In contrast to previous experiment, due to the resistance viscoelastic forces the jet is not broken up to the droplets. Instead, the ejected polymeric jet travels a short distance following a straight path and then bending instability is developed in the jet due to the mutually repulsive forces resulting from the electric charges of the jet. Fig. 6 shows these three stages schematically and graphically for electrospinning of 15% wt PAN solution.
Fig. 6. (a) Schematic illustration of jet path in electrospinning, and (b) Taylor cone, straight section and spiral path in electrospinning of 15% wt PAN solution.
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Fig. 7. Electrocentrifugal spinning of 15% wt PAN in DMF at rotational speed of 6630 rpm.
3.2.2. Polymeric jet in electrocentrifugal spinning In this method the polymeric jet is formed only by the centrifugal force and undergoes elongation by the simultaneous action of centrifugal and electrostatic forces until it lies down on collector. Fig. 7 shows an image of the process using 15% wt PAN solution at the rotational speed of 6360 rpm. The ejected jet from the nozzle encounters bending instabilities after a short distance traveling in a curved path. As it can be seen in this image, due to the high flow rate, the Taylor cone has not appeared and the polymeric fluid conforms to a jet as a consequence of the applied centrifugal force.
In comparison with electrospinning, bending instabilities are observed with less frequencies and amplitude in electrocentrifugal spinning. This can be explained in this way: by increasing the rotational speed, the relative velocity between the jet and the surrounding air, which tends to dry up the solution, is increased. As a result, the growth of instability wave is suppressed due to the evaporation of solvent. On the other hand, by increasing the polymer concentration, the polymeric bending instability of the jet is reduced and the jet travels in a straight path toward the collector. This provides the possibility of collecting highly aligned nanofibers on the outer cylinder. Fig. 8 shows images of electrocentrifugal spinning for 16% wt PAN at speed of 6360 rpm where the bending instabilities have considerably vanished and as a result, the polymeric jet takes on a completely straight and aligned form. This method can be employed to produce highly aligned nanofibers from solutions with high concentration. Fig. 9 shows a scanning electron microscopy of nanofibers produced by electrocentrifugal spinning of 15% and 16% wt PAN in DMF at rotational speed of 6360 rpm and applied voltage of 15 kV. The morphology of nanofibers was studied and explained in previous work [46]. In brief, as shown in Fig. 10 the results indicated that the production of nanofibers with drop sprinkling was possible, and at low rotational speed and concentrations, the nanofibers contain drops and beads in centrifuge spinning. When using electrocentrifuge spinning, the nanofiber diameters have been changed compared to centrifuge spinning. Results showed drastic reduction in fiber diameter which is the effect of adding electrical force to centrifuge forces. 3.2.3. Comparison of mechanical properties The mechanical properties of aligned nanofiber bundles were measured by Zwick 1446e60. Zwick was set for constant rate of elongation. To obtain load elongation curves, the sample length was
Fig. 8. Electrocentrifugal spinning of 16% wt PAN in DMF at rotational speed of 6360 rpm where the bending instabilities are considerably reduced.
Fig. 9. Scanning electron microscopy of nanofibers produced by electrocentrifugal spinning of (a) 15% and (b) 16% wt PAN in DMF at rotational speed of 6360 rpm and applied voltage of 15 kV.
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Fig. 10. Typical SEM images of nanofibers for a polymer solution of 15% wt and a rotational speed of 6615 rpm; (a) Nanofibers with drop and bead produced by centrifuge spinning [48], (b) Un-uniformity in nanofiber diameter in centrifuge spinning [48], (c) Nanofibers without drop or bead produced by electrocentrifuge spinning at voltage of 10 kV, and (d) Uniformity in nanofiber diameter.
10 cm with cross head speed of 60 mm/min. All samples were produced at room temperature (25 C) and dried at approximately 70 C in an oven for 2 h. Before the experiment, samples were in standard conditions (20 2 C and 65% RH) for 24 h. The results of measurement are listed in Table 1. The strength of electrospun PAN aligned nanofiber bundle was 53 MPa and its modulus was 1.99 GPa whereas these values were 112 MPa and 2.33 GPa for the electrocentrifugal spun PAN aligned nanofiber, respectively. The results show an increase in the strength and modulus of the aligned nanofiber bundle produced by electrocentrifugal spinning and a decrease in the strain at the brake point. However, no significant change occurs in work up to break parameter. The enhancements of mechanical properties can be accounted for by the considerable mechanical contact drag forces between the jet and the surrounding air which lead to a better alignment of fibers. Work up to break is a quantity which considers both strength and strain simultaneously. An increase in work up to break is expected due to increase of strength, but it should be noted that due to the strain reduction, the variation of work up to break is not considerable.
Table 1 The results of mechanical properties measurement of aligned nanofiber bundles by Zwick. Parameters
Electrospinninga
Electrocentrifugalb
Number of samples Nanofiber diameter (nm) Stress at F_max (MPa) Strain at break (%) Work up to break (N mm/tex) E-modulus (GPa)
30 410 53 75.9 3.16 1.99
30 (9.5) (17.1) (17.3) (24.2) (19.8)
440 112.42 60 3.06 2.33
(11.3) (8.3) (16.3) (22.2) (24.6)
a 16% wt PAN in DMF, 15 kV, 0.8 ml/h, 12 cm spinning distance, 2.5 cm gap distance on collector. b 16% wt PAN in DMF, 6360 rpm, 15 kV, distances explained in Section 2.2.
4. Conclusion The procedures of water jet formation and polymeric jet formation in three kinds of spinning systems involving electrospinning, centrifuge spinning and electrocentrifugal spinning were considered in this study. Electrocentrifugal spinning is a powerful technique for spinning solution with extremely low viscosity. In addition, electrocentrifugal spinning is a more suited technique for production of aligned nanofibers than electrospinning, since the bending instability of jet could be reduced via centrifugal force through the effect of surrounding air contacts. This can partially improve the mechanical properties of produced nanofiber.
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