Crimped polymer nanofibres by air-driven electrospinning

Transcription

1 European Polymer Journal 43 (2007) Macromolecular Nanotechnology Crimped polymer nanofibres by air-driven electrospinning A. Varesano, A. Montarsolo, C. Tonin * CNR-ISMAC, Institute for Macromolecular Studies, C.so G. Pella, Biella, Italy EUROPEAN POLYMER JOURNAL Abstract Received 8 January 2007; accepted 19 April 2007 Available online 3 May 2007 Electrospinning is a well-known process for producing sub-micron scale polymer filaments through an electrostatic field. This paper presents a very simple confined air-driven electrospinning system, in which polyamide nanofibres are produced in the form of continuous crimped filaments. The reported system consists of a vertical cylinder with a weak tangential air-flow feeding from the top, placed between the capillary source electrode and the grounded target collector. The air-flow drives the polymer jet inside the electrostatic field, curls up the filament and reduces the deposition area on the collector surface. Numerical evaluations of both the electrostatic field and the air-flow path within the chamber are reported. The proposed configuration has been successfully tested electrospinning a solution of polyamide-6 in formic acid, varying the applied voltage and the distance between the electrodes. SEM observations of the electrospun fibres revealed that a large amount of crimped nanofibres was produced free from bead defects. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Processing; Fibres; Morphology; Electrospinning 1. Introduction Electrospinning is a process to produce thin polymer-based fibres using an electrostatic field. It was patented in the early 20th century [1 3]; however, the process was forgotten until the 1990s [4 6]. The basic electrospinning apparatus consists of three elements: an electrical generator (high voltage supply), a metallic capillary (source electrode) and a grounded collector (target). The capillary is electrically connected with the generator. A polymer solution is fed through the capillary at constant * Corresponding author. Tel.: ; fax: address: c.tonin@bi.ismac.cnr.it (C. Tonin). flow-rate, ejecting an electrically charged polymer solution jet from the capillary tip. The jet solidifies thanks to the rapid solvent evaporation and deposits on the collector as a disordered continuous filament. Many theoretical studies have been carried out to understand the mechanism of fibre formation during the electrospinning process. In particular, it was discovered that the jet starts from the polymer solution drop at the capillary tip stretched in the shape of cone (called Taylor cone) when the electrostatic forces exceed the surface tension force [7,8]. The trajectory of the jet is rectilinear for few centimetres from the tip, then becomes disordered, similar to the movement of a whip (so-called whipping motion) [8]. The chaotic whipping motion of the /$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi: /j.eurpolymj

2 A. Varesano et al. / European Polymer Journal 43 (2007) jet increases the deposition area of the electrospun fibres. The control of the electrospinning jet for collecting electrospun fibres in a small area or in oriented forms has been carried out with mechanical and electrostatic means [4,9 11]. This paper presents a confined air-driven electrospinning (CADE) system, in which nanofibres were produced in the form of crimped continuous filaments, deposited on a small area of the collector surface. Most natural fibres are not straight and even man-made fibres are deliberately crimped in order to produce more cohesive yarns. Fibre crimp gives consistency and resilience that is needed in the manufacture of thick textiles. Products made of crimped fibres are porous. The crimp increases bulk and cover, and facilitates interlocking of staple fibres in spun yarns; moreover, fine crimped fibres form an intricate network that trap air in tiny pockets, which enhances thermal insulation. In this work, electrospinning was confined to a plastic cylinder supplied with a weak air-flow in order to curl the filament during solidification, driving the deposition onto a delimited area of the target collector. SEM analysis showed that this system produces significant changes in the filament morphology, enabling the production of crimped, waved nanofibres, that could find interesting application in many different fields ranging from high efficiency filtration to biomedical application such as cell growth and wound dressing [4 6,12]. 2. Experimental 2.1. Preparation and characterization of the polymer solution Commercial polyamide-6 (PA6) from Nylstar was dissolved in formic acid (85%) from Riedel-de Haën at ambient temperature for about 10 h, obtaining a 25 wt.% PA6 solution. The viscosity of the solution was measured by means of an Physica MCR 301 (Anton Paar GmbH, Graz, Austria) rheometer, equipped with a PTD 200 Peltier temperature control device at 20.0 (±0.1) C, using the cone-plate geometry (75 mm diameter, 1 angle and 45 lm truncation). The shear rate was logarithmically increased from 10 3 to 10 2 s 1. Data were acquired and elaborated with Rheoplus v2.66 software. In the studied range, the polymer solution behaves like a Newtonian fluid with a constant viscosity of 8.52 Pa s. The solution had an electrical conductivity of 1.56 ms cm 1 at 25 C; measured by a Multi-parameter Tester PC300 (Eutech Instruments Europe B.V., Nijkerk, Netherlands) Description of the electrospinning process and apparatus Electrospinning was performed using about 5 ml of the PA6/formic acid solution in each experiment. A delivery pump (KDS model 200, KD Scientific, Holliston, MA) supplied a constant flow-rate (60 llh 1 ) of the polymer solution through the capillary, to a hollow metal tip with an internal diameter of 0.4 mm, connected to the positive of the high-tension generator (HVA30, b2 Electronic GmbH, Klaus, Austria). A remote PC controlled both the generator and the delivery pump. Different experiments were performed, with applied voltage Fig. 1. Design of the confined air-drive electrospinning (CADE) system. Dimensions are reported in mm and WD is the working distance.

3 2794 A. Varesano et al. / European Polymer Journal 43 (2007) ranging from 20 to 30 kv and working distances of 10, 15 and 20 cm. PA6 nanofilaments were collected on a grounded rotating metal disk of 5.5 cm diameter. The reproducibility of each experiment was ensured by three replications. A plastic cylindrical chamber, supplied with an air-flow entering the chamber tangentially from the top, enclosed the space in which the filament formation took place, as illustrated in Fig. 1. The air flow was fed in the chamber with a pressure of about 10 mbar. The air path spiralling downward produces a vortex with decreasing tangential component of the air velocity. The air-flow carried the electrospun filament, within the vortex through the bottom outlet, because of its extreme lightness, bringing about a crimp during solidification. Two programs were written in Matlab Ò 7.0 (by The MathWorks Inc.) software in order to explain the above described phenomena. The first program calculates strength and direction of the electric field generated between the tip and the metal disk (collector). The second one gives information about the air-flow path within the chamber. The electric field lines were calculated considering the tip as a dimensionless charged spot by a program based on the following Eq. (1): E ¼ F ¼ q q 0 4pe 0 r u ð1þ 2 where E is the electric field, F is the electric force, q 0 is the probe charge (a virtual punctual charge placed in the measuring point), q is the electrical charge applied to the tip, e 0 is the dielectric constant ( C 2 N 1 m 2 ), r is the distance between q (charged tip) and q 0, u is the vector between q and q 0. Vectors are written in bold. A steady-state 3D simulation of the air path through the cylindrical chamber was carried out with some simplifications. The continuity and the momentum equations applied to an incompressible Newtonian fluid with constant viscosity, lead to the well-known Navier Stokes Equation (2): qðv rþv ¼ lr 2 v þ qg ð2þ where q is the air density ( gcm 3 ), l is the air viscosity ( gcm 1 s 1 ), g is the gravitational acceleration ( cm s 1 ) and v is the air velocity (cm s 1 ). Vectors are written in bold. For simplicity, we assume that air density and pressure were constant. The set of governing equations is solved by means of a finite volume explicit Euler s scheme on cylindrical coordinate system with points (in h, r, z). No-slip boundary conditions were applied. The no-slip condition requires that the fluid velocity equals the velocity of any bounding solid surface; consequently, the fluid velocity at the walls is zero. Fig. 2. Plot of the field lines (arrows) and the electric field intensity (lines) calculated for the CADE system geometry with 20 kv of applied voltage and 15 cm of working distance.

4 A. Varesano et al. / European Polymer Journal 43 (2007) Morphological analysis The morphology of the electrospun filaments were studied by means of scanning electron microscopy (SEM). Investigations was performed with a Leica Electron Optics 135 VP SEM (LEO Electron Microscopy Ltd., Cambridge, England) with an acceleration voltage of 15 kv and current probe of 50 pa. Samples were mounted on aluminium specimen stubs with double-sided adhesive tape and sputter-coated with gold in rarefied argon atmosphere at 20 Pa, using an Emitech K550 (EM Technologies Ltd., Kent, England) sputter coater with a current of 20 ma for 180 s. Electrospun fibres were measured using the Opera Plus v6.12 (MAD Software GmbH, Austria) software from digital SEM pictures. 3. Results and discussion The electric field in a basic electrospinning system departs from the positively charged metal tip to the grounded collector following curved routes through space. Fig. 2 shows the electric field lines (arrows) obtained as computational result from the present electrospinning configuration with an applied voltage of 20 kv and a working distance of 15 cm. The intensity of the electrostatic field E (in V cm 1 ) is also reported in Fig. 2. Near the charged tip, the field intensity is strong and the field lines diverge. Consequently, the jet of solution is initially rapidly Fig. 3. Plots of the air-flow velocity components (v h,v r,v z ) in the cylindrical chamber calculated at distances z = 1.2, 3.6, 6.8 and 9.6 cm, respectively, with air inlet velocity v 0 =10cms 1. Fig. 4. Pictures of the covered areas on the metal collector (a) with and (b) without the CADE system, produced in 5 min at the same electrospinning condition. Scale bar = 1 cm.

5 2796 A. Varesano et al. / European Polymer Journal 43 (2007) Fig. 5. Picture of the electrospun circular pattern obtained with the CADE system using a 12 rpm rotating metal collector. Scale bar = 1 cm. stretched and accelerated from the tip toward the grounded collector, and its trajectory is stable. As the electric field becomes too weak, the jet is subjected to stress relaxation and the jet trajectory turns in whipping motion [8,10]. At about 3 cm from the collector, the field lines start to converge and the electric field intensity increases. The electric field model also estimates the system capacitance of 1.68 nf. This value is obtained by the geometrical parameters of the system supposing that all the field lines pass from the tip to the collector. The calculated capacitance is rather near the value (1.4 nf) measured by the generator. Another computer program has been developed to calculate the air velocity components from the CADE configuration with the aim to provide qualitative information about the air-flow path through the cylindrical chamber. Fig. 3 shows the three velocity components (v h,v r,v z ) of the air-flow in the electrospinning chamber for the present configuration (reported in Fig. 1) at increasing distance (z) from the tip, with an air inlet velocity (v 0 ) of 10 cm s 1 and working distance of 15 cm. The simulation shows that a vortex with a strong vertical component is generated in the middle of the chamber. The proposed configuration has been successfully tested electrospinning 60 ll h 1 of a 25 wt.% polyamide-6 solution in formic acid (85%), with applied voltage ranging from 20 to 30 kv and working distance from 15 to 20 cm. In agreement with the formation of a narrow vortex shown by numerical simulation, a diminution of the electrospun area on the collector has been observed. The pictures in Fig. 4 show, for comparison, the electrospun mats Fig. 6. SEM pictures of the electrospun filaments produced with the CADE system, (a) 1000 and (b) 3000, and without the CADE system, (c) 1000 and (d) 3000.

6 A. Varesano et al. / European Polymer Journal 43 (2007) collected in 5 min (a) with and (b) without the CADE system in the same operating conditions (solution feed-rate set to 60 ll h 1, 25 kv of applied voltage and 15 cm of working distance). Thus, the electrospun fibres were entrapped in the vortex that curbs the typical whipping motion. By this way, the area of the collector covered with electrospun fibres was significantly smaller than that produced using the same electrospinning apparatus without the air-driven system. Thanks to the focusing of the electrospinning jet reached by the CADE system, it was possible to produce patterns (similar to an ink-jet print) of electrospun fibres moving the collector below the electrospinning system. The photograph of a circle of electrospun fibres is reported in Fig. 5. The circle has been collected on the rotating metal collector placed with its rotating axle shifted of about 0.5 cm with respect to the CADE system axle. The collector rotating speed was set to 12 rpm and the electrospinning duration was set to 5 s so that the collector completed just a turn. However, the proposed technique still should be improved to achieve a full control of the jet during the electrospinning process. SEM images in Figs. 6a and b on the samples electrospun by the CADE system shows that fibres exhibit an unusual waviness and the mat appears as a highly disordered tangle of fibres. Moreover, the filaments are free from defects and beads. For comparison, Figs. 6c and d show the morphology of PA6 electrospun fibres obtained without the CADE system in the same operating conditions. Fig. 7 shows a particular of the crimped electrospun filament obtained with the CADE system. The filament diameter is visibly less than 1 lm (about 600 nm on average) and the average wave length of the waviness is about 10 lm. This morphology agrees with the hypothesis that the air that flows spiralling downward in the electrospinning chamber impedes the complete development of the whipping instability of the polymer jet. To the best of our knowledge, the observed fibre texture greatly differs from any morphology usually shown in literature for many polymer/solvent systems studied up till now [11,13 19]. 4. Conclusions Sub-micron scale polymer filaments were produced by means of a confined air-driven electrospinning (CADE) system involving both electrostatic and pneumatic forces. The filaments were collected on a small deposition area delimited by the vortex generated by the air-flow within the cylindrical chamber. A circular thin pattern was easily obtained with this system using a rotating collector. Thus, it seems to be possible to produce simple patterns of electrospun filaments moving the collector. However, accurate control of the size of the deposition area is still challenging. SEM analysis pointed out that this electrospinning system produces significant changes in the filament morphology with the presence of many crimped, waved electrospun fibres without bead defects. Acknowledgement The electrospinning apparatus was acquired by the Laboratorio di Alta Tecnologia Tessile of the CNR-ISMAC Biella with the sponsorship of the Regione Piemonte (B.U. No /11/2003). References Fig. 7. SEM picture of a particular of electrospun crimp nanofilament, The inset shows a larger portion of the filament. [1] Cooley JF. US Patent 692,631, [2] Morton WJ. US Patent 705,691, [3] Formhals A. US Patent 1,975,504, [4] Huang ZM, Zhang YZ, Kotaki M, Ramakrishna S. A review on polymer nanofibers by electrospinning and their applications in nanocomposites. Compos Sci Technol 2003;63: [5] Li D, Xia Y. Electrospinning of nanofibers: reinventing the wheel. Adv Mater 2004;16: [6] Ramakrishna S, Fujihara K, Teo WE, Yong T, Ma Z, Ramaseshan R. Electrospun nanofibers: solving global issues. Mater Today 2006;9:40 50.

7 2798 A. Varesano et al. / European Polymer Journal 43 (2007) [7] Yarin AL, Koombhongse S, Reneker DH. Taylor cone and jetting from liquid droplets in electrospinning of nanofibers. J Appl Phys 2001;90: [8] Shin YM, Hohman MM, Brenner MP, Rutledge GC. Experimental characterization of electrospinning: the electrically forced jet and instabilities. Polymer 2001;42: [9] Berry JP. US Patent 4,965,110, [10] Deitzel JM, Kleinmeyer J, Hirvonen JK, Beck Tan NC. Controlled deposition of electrospun poly(ethylene oxide) fibers. Polymer 2001;42: [11] Kim GH. Electrospinning process using field-controllable electrodes. J Polym Sci A2 2006;44: [12] Liao S, Li B, Ma Z, Wei H, Chan C, Ramakrishna S. Biomimetic electrospun nanofibers for tissue regeneration. Biomed Mater 2006;1:R [13] Li Y, Huang Z, Lü Y. Electrospinning of nylon 6, 6 6, terpolymer. Eur Polym J 2006;42: [14] Jarusuwannapoom T, Hongrojjanawiwat W, Jitjaicham S, Wannatong L, Nithitanakul M, Pattamaprom C, et al. Effect of solvents on electro-spinnability of polystyrene solutions and morphological appearance of resulting electrospun polystyrene fibers. Eur Polym J 2005;41: [15] Zhang C, Yuan X, Wu L, Han Y, Sheng J. Study on morphology of electrospun poly(vinyl alcohol) mats. Eur Polym J 2005;41: [16] Casper CL, Stephens JS, Tassi NG, Chase DB, Rabolt JF. Controlling surface morphology of electrospun polystyrene fibers: effect of humidity and molecular weight in the electrospinning process. Macromolecules 2004;37: [17] Megelski S, Stephens JS, Chase DB, Rabolt JF. Micro- and nanostructured surface morphology on electrospun polymer fibers. Macromolecules 2002;35: [18] Mit-uppatham C, Nithitanakul M, Supaphol P. Ultrafine electrospun polyamide-6 fibers: effect of solution conditions on morphology and average fiber diameter. Macromol Chem Phys 2004;205: [19] Deitzel JM, Kleinmeyer J, Harris D, Beck Tan NC. The effect of processing variables on the morphology of electrospun nanofibers and textiles. Polymer 2001;42: