Manipulation of the electric field of electrospinning system to produce polyacrylonitrile nanofiber yarn

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1 Manipulation of the electric field of electrospinning system to produce polyacrylonitrile nanofiber yarn Date Submitted 22 December 2005, Date Accepted 15 June 2006 F. Dabirian 1, Y. Hosseini 2 and S. A. Hosseini Ravandi 1 1 Center of Excellence for Environmental Nanotechnology, Isfahan University of Technology, Isfahan, 84154, Iran 2 Electrical Department, Sharif University of Technology, Tehran, Iran doi: / Abstract: Electrospinning is a process that produces nanofiber through the action of an external electric field imposed on a polymer solution or melt. This paper introduces a new system capable of producing continuous uniaxially aligned PAN nanofiber yarn by manipulating the electric field. The manipulation was carried out by employing a negative charged bar in the electric field of conventional electrospinning system, leading to the formation of an electrostatic multipolar field. As a result, the main stream was redirected towards a rotating take up unit, collecting the twisted yarn consisting of uniaxially aligned nanofibers. The yarns were then treated in boiling water under tension and their mechanical properties were compared with those of the untreated ones. Key words: Manipulation of electric field, electrospinning, nanofiber yarn, mechanical behaviours, polyacrylonitrile INTRODUCTION Electrospinning is a novel process for forming fibers with nano-scale diameters through the action of electrostatic forces. In typical process, an electrical potential is applied between droplet of polymer solution or melt, held through a syringe needle and a grounded target. Electrostatic charging of the droplet results in the formation of the well-known Taylor Cone. When the electrical forces overcome the surface tension of the droplet from the apex of the cone, a charged fluid jet is ejected (Doshi, 1995). The jet exhibits bending instabilities due to repulsive forces between the surface charges, which is carried by the jet, and follows a looping and spiraling path (Reneker et al., 2000; Yarin et al., 2001). The electrical forces elongate the jet thousands of times and the jet becomes very thin. Ultimately, the solvent evaporates, or the melt solidifies, and very long nanofibers are collected on the grounded target (Doshi, 1995). Due to this spiral motion and instability, fibers will collect on the grounded target as a random oriented web of nanofiber. Random orientation nanofiber is useable for applications Corresponding author: S.A. Hosseini Ravandi Center of Excellence for Environmental Nanotechnology Isfahan University of Technology Isfahan, 84154, Iran Tel: , Fax: hoseinir@cc.iut.ac.ir such as filters, tissue scaffolds, and wound dressings, but for applying these fibers in the textile industry, aligned electrospun fibers are needed. Various research projects attempted to obtain aligned electrospun fibers. These include: spinning onto a rotating drum (Doshi, 1995), spinning onto the sharp edge of a thin rotating wheel (Zussmann et al., 2003), introducing an auxiliary electrode or electrical field (Detzel et al., 2001), rapidly oscillating a grounded frame within the jet (Fong et al., 2002), and using a metal frame as the collector (Derch et al., 2003). These projects have only been able to produce relatively short tows of aligned fibers. In a recent paper, Fennessey and Farris (2004) used an electrical twister for twisting an approximately 32 cm 2 cm unidirectional tow of electrospun nanofiber into yarn and researched the effect of twist on the tensile strength and other mechanical properties of the yarn. Kataphinan et al. (2001) briefly referred to the collection of nanofibers from the surface of non-wetting liquids, but no detailed descriptions were given. Eugene Smit et al. (2005) recently described a technique consisting of spinning onto a water reservoir collector and drawing the non-woven web of fibers across the water before collecting the yarn. The aim of this work was manipulation of electric field of electrospinning system to provide continuous uniaxially fibers bundle yarns from electrospun fibers. The basic operation is discharging fibers between the syringe needle and negative surface by concentrating negative charges at the apex bar located in the front of the syringe needle between Copyright C 2007 The Textile Institute 237 pp

2 F. Dabirian, Y. Hosseini and S. A. Hosseini Ravandi Figure 1 Schematic illustration of electrospinning setup to produce nanofiber yarn. Figure 2 Lines of applied electrostatic field and movement direction of electrospun nanofibers and yarn inside the electrostatic field. the negative surface and the take up unit. The effects of gravitational and repulsive forces cause fibers to be conducted to the end side of pieced yarn. One side of fibers which are pieced to yarn enters into the yarn body because of the applied spin to yarn and another side of fibers is pulled to the negative surface because of the few remaining charges in the fibers. The distance of the apex bar from the negative surface and syringe needle is very important and the amounts of discharge of fibers can be controlled by changing the distances between these parts. Using this method, nanofibers uniaxially aligned into yarn body, then take up unit twist the yarn and take up is done. To evaluate the mechanical properties of produced yarns, a comparison was made between after-treated and untreated yarns. EXPERIMENTAL Materials Industrial Polyacrylonitrile (PAN) and dimethylformamide (DMF) were obtained from Iran Polyacryle Co and Merck Co, respectively, as polymer and solvent. The weight average molecular weight (Mw) and the number average molecular weight (Mn) of the received PAN were Mw = g/mol and Mn = g/mol, respectively. The polymer and solvent were dried before use. The 13.5 wt% solution of PAN in DMF was prepared at room temperature under constant mixing for approximately two hours. Electrospinning setup Figure 1 schematically illustrates the basic setup for electrospinning. It consists of a high voltage-power supply, a syringe needle, a bar with a diameter of 0.3 mm, a negative surface, and a take up unit. The apex bar was 21 cm below the syringe needle. The negative surface was located 23 cm from the syringe needle and 12 cm from the bar. The length of syringe needle was 4 cm and the length of bar was 3 cm. The area of the negative surface was 6 cm 12 cm, and the bar had a very low area when compared with the negative surface. 238 Figure 2 shows the lines of the applied electrostatic field and the direction of movement of the electrospun fibers and yarn inside the electrostatic field. This figure was drawn by Elektrisch Felder software. The direction that nanofibers move inside the field was arc shape and the yarn was taken up at a direction tangent to the arc. Take up unit Figure 1 shows a schematic diagram of the take up unit. It can twist the yarn without forming a balloon while taking up the yarn. The take up unit was controlled by a threephase motor for twisting the yarn and was controlled by an inverter. A stepper motor controlled the speed of take up and was controlled by AT890S51 microcontroller. Port 0 and port 2 were used as input and output ports. To provide the required input current for the stepper motor, a buffer (ULN2803A) was used to connect the output port of microcontroller to the stepper motor. Users can control the rotation speed of the stepper motor with a switch connected to the input port of the microcontroller. Linear take up speed was m/h with a maximum angular velocity of 400 RPM. Characterization The morphology of electrospun PAN nanofiber yarns was observed with a Philips scanning electron microscope (XL-30) after gold coating. The diameter of nanofibers was measured from high magnification SEM images. A motic optical microscope B3 was used to capture images for measuring the yarn diameter. The mechanical properties of yarn were measured by Zwick Zwick was designed for constant rate of elongation. To obtain loadelongation curves, the sample length was 10 cm with cross head speed of 60 mm/min. All yarns were produced at room temperature ( 25 ) anddriedatapproximately70 Cinanovenfor2h.Before the experiment, yarns were in standard conditions (20 ± 2 C and 65% RH) for 24 hours. Copyright C 2007 The Textile Institute

3 Manipulation of the electric field to produce polyacrylonitrile nanofiber yarn Figure 4 SEM image of electrospun PAN nanofiber yarn at a voltage of 8 kv. Figure 3 Typical images of spinning triangle: (a) captured by Sony digital handy cam (DCR-PC115E); (b) observed by reflected optical microscope. After treatments of nanofibers For the purpose of after treatments of nanofiber, the yarns were carried out in boiling water (about 100 C) until their elongation reached 100% and were then dried by hot air. RESULTS AND DISCUSSION Yarn structure In a series of experiments, the applied voltage was varied from 8 to 11.4 kv while the concentration was kept constant at 13.5 wt%. Electrospinning of PAN nanofiber yarn started from 8 kv, but electrostatic forces were not enough to form a strong continuous yarn. We then began Electrospinning from 9.2 kv. Figures 4, 5 and 6 show images of some of the electrospun nanofiber yarns spun under 8, 9.2 and 11.4 kv. In these experiments, 11.4 kv appeared to be the best voltage and resulted in the highest alignment of nanofibers. Figure 3 shows images of a spinning triangle of 11.4 kv nanofiber yarn. The effects of voltage observed in the above experiments may be related to the whipping instability. The most important element operating during electrospinning is the rapid growth of a nonaxisymetric, Figure 5 SEM image of electrospun PAN nanofiber yarn at a voltage of 9.2 kv. or whipping instability that causes bending and allows the electrical forces to elongate the jet (Shin et al., 2001). Increasing the applied voltage and increasing whipping instability increases nanofiber entanglement and makes alignment of nanofibers more difficult, but also increases the acting forces which attempt to align the nanofibers (Jalili et al., in press ). Yarn characteristics At the voltage of 11.4 kv, different kinds of yarns were produced. The linear density of these yarns was approximately the same (1.16 tex), but the amount of twist was different. Table 1 shows the product conditions at a concentration of 13.5% and 11.4 kv. Nanofibers with an average fiber diameter of about nm with 12.2 CV% were formed. The average diameter of yarns varied from to µm with 6.5 to 8.1 CV%. Copyright C 2007 The Textile Institute 239

4 F. Dabirian, Y. Hosseini and S. A. Hosseini Ravandi Figure 7 Load-elongation curves of PAN nanofiber: (a) untreated; (b) post-treated. Figure 6 Optical micrograph of electrospun PAN nanofiber yarn, voltage 11.4 kv: (a) low magnification; (b) high magnification. Table 1 The electrospinning conditions to produce different PAN nanofiber yarns Sample conditions A B C D Voltage (kv) Twist (TPM) Take up speed (m/h) Feeding rate (ml/h) TPM = Twist per meter; m/h = meter per hour. Mechanical characterizations A comparison was made between the mechanical properties of nanofiber yarns. The values of some parameters for quantitative comparisons are summarized in Table 2 (samples A, B, C and D). The values of E-modulus and tensile strength at break are 1.35 to 1.99 GPa and to MPa. The elongation at break is 60.74% to 101.3%. These values indicate that the nanofiber yarn is largely unoriented, so it has high elongation with considerable plastic deformation that will occur after a small linear region characterized by low modulus. Consequently, the yarn with these properties and high CVs is not useful for apparel or other commercial textile applications. To overcome these problems, the produced yarn was treated with hot drawing. A comparison was made between the mechanical properties of yarns of untreated and post-treated for sample D. The load-elongation curves of these samples are shown in Table 2 Mechanical properties of the yarns Property Number of test Twist Stress at break Strain at Break Work up to break E-modulus Sample n TPM Ave.MPa CV% % CV% Ave.Nmm CV% Ave. GPa CV% A B C Untreated; D Post-treated; D Copyright C 2007 The Textile Institute

5 Manipulation of the electric field to produce polyacrylonitrile nanofiber yarn Figure 7. The values of E-modulus and tensile strength at break increase from 1.66 GPa and MPa to 7.51 GPa and MPa, for untreated and post-treated yarns. Whereas the strain at break of post-treated yarn was found to be significant (8.51%). In other words, the yarn became much stronger but relatively lower strain after the post-treatment, which could be attributed to the increase in the degree of crystallinity for post-treated sample. Also the X-ray diffraction results of PAN nanofibers showed that the crystallization was retarded during electrospinnig, but no change in lattice spacing was observed. The post treatments permitted some additional development of crystalline order and CI% increased up to 17.4% (Jalili et al., in press). The CVs of mechanical properties of posttreated nanofiber yarn became lower than untreated yarns. CONCLUSION The aim of this study was to manipulate the electric field of the conventional electrospinning system to produce PAN nanofiber yarn. A bar was added to the system and the concentration of negative charge on it became very large in comparison with the negative surface. So the nanofibers were aligned by these two negative devices. Then the triangle of spinning was formed and the produced yarns were twisted while taking up was done. Using this equipment, production of yarns with different linear density, twist level and material was possible. The yarn became much stronger but relatively lower strain after the post-treatments. The values of E-modulus and tensile strength at break were 7.51 GPa and MPa for post-treated yarn. REFERENCES DEITZEL, J. M., KLEINMEYER, J. D., HIRVONEN, J. K. andbeck TAN, N. C., Controlled deposition of electrospun poly(ethylene oxide) fibers, Polymer, 42, DERSCH, R., LIU, T., SCHAPER, A. K., GREINER, A. andwendorff, J. H., Electrospun nanofibers: Internal structure and intrinsic orientation, Polym. Chem., 41, DOSHI, J. and RENEKER, D. H., Electrospinning process and application of electrospun fibers, J. Electrostat., 35, FENNESSEY, S. F. and FARRIS, R. J., Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns, Polymer, 45, FONG, H., WEIDONG, L., WANG,C.S.andVAIA, R. A., Generation of electrospun fibers of nylon 6 and nylon 6-montmorillonite nanocomposite, Polymer, 43, JALILI, R., HOSSEINI, S. A. R. and MORSHED, M., The effects of operation parameters on the morphology of electrospun polyacrylonitrile nanofiber, J. Ir. Polym., 14(12), JALILI, R., MORSHED,M.andHOSSEINI, S. A. R., Fundamental parameters affecting elecyrospinning of PAN nanofibers uniaxially aligned fibers, J. Appl. Polym. Sci., 101, PAN, N. andzeronian, S. H., An alternative approach to objective measurement of fabrics, J. Text. Res., 63(1), KATAPHINAN, W., DABNEY, S., SMITH, D. andreneker, D., Fabrication of electrospun and encapsulation into polymer nanofibers. Book of Abstracts. The Fiber Society, Spring Meeting, May RENEKER, D. H., YARIN, A. L., FONG, H. andkoombhongse, S., Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, J. Appl. Phys., 87, SHIN, Y. M.,HOHMAN, M. M., BRENNER, M. P., andrutledge, G. C., (A whipping fluid jet generates submicron polymer fibers), Appl. Phys. Lett., 78, SMIT, E., BŰTTNER, U. andsanderson, R. D., Continuous yarns from electrospun fibers, Polymer, 46, YARIN, A. L., KOOMBHONGSE, S. andreneker, D. H., Bending instability in electrospinning of nanofibers, J. Appl. Phys., 89, ZUSSMANN, E., THERON, A. and YARIN. AL., Formation of nanofiber crossbars in electrospinning, J. Appl. Phys. Lett., 82(6), Copyright C 2007 The Textile Institute 241

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