Article pubs.acs.org/Macromolecules
Role of Liquid Jet Stretching and Bending Instability in Nanofiber Formation by Gas Jet Method Rafael E. Benavides,† Sadhan C. Jana,†,* and Darrell H. Reneker‡ †
Department of Polymer Engineering and ‡Department of Polymer Science, The University of Akron, Akron, Ohio 44325-0301, United States ABSTRACT: In this work, the mechanism of fiber formation from polymer solutions under the action of gas jets in a process named gas jet fibers (GJF) method is studied. The GJF method relies on high velocity gas jet converting polymer solutions streaming from a nozzle into a liquid jet and then into fibers. Polymer nanofibers with diameter as low as 100 nm are continuously produced from polymer solutions. A series of interrelated phenomena including polymer jet initiation and stretching, flapping motion, bending instabilities, and concurrent solvent evaporation were found to influence fiber formation. Specifically, the behavior of the liquid jets, stretching rates, and the onset of the bending instabilities are revealed using high speed video photography. The results indicate that both the initial stretching from flapping and the extension from bending instabilities play important roles in fiber attenuation.
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fiber diameter as in electrospinning but without an electric potential. This raises a question regarding the specific mechanisms leading to the attenuation of the liquid jet in the GJF process. We hypothesize that this attenuation is related to the stretching generated by a series of physical phenomena present in other fiber forming processes that use jets of gas, e.g., bending instability in melt blowing, and shrinkage due to solvent evaporation. The shrinkage of the jet due only to solvent evaporation cannot produce the desired fibers with diameter of the order of a few hundred nanometers. For example, a liquid jet of 20 μm diameter and consisting of 2 wt % poly(ethylene oxide) (density 1.13 g/mL at 25 °C) in ethanol (density 0.78 g/mL at 25 °C) produces approximately 2 μm diameter fiber after all ethanol evaporates. Therefore, the additional attenuation of the liquid jets from a series of instabilities should be revealed in detail. Some hints about the behavior of viscous liquid jets in streams of gas are offered by the melt blowing process, where fibers are produced by stretching of the polymer melt at the exit of a spinneret aided by coaxial jets of high velocity hot air.4−6 The fiber attenuation is attributed to a series of interrelated phenomena including the development of bending instabilities (also known as whipping) by the continuous action of the main stream of air over the melt thread.7,8 Intuitively, this occurs as long as (i) the temperature of the polymer jet is above the glass transition temperature of the polymer, and (ii) the differences in speed between the air and the polymer jet are enough to create drag on the polymer thread. High speed video imaging,9 laser Doppler velocimetry,10 and temperature monitoring by
INTRODUCTION In a recent report, we presented a new technique for the production of fibers with diameter in the submicrometer range using a high pressure gas jet method named gas jet fibers1 (GJF) process. The GJF process uses jets of gas to turn polymer solutions streaming from nozzles into liquid jets and then into continuous fibers with diameter from a few tens of nanometer to a few micrometer. The basic setup of the GJF process consists of a syringe pump, a customized nozzle, a jet of compressed gas, typically air, and a fiber collector as shown in Figure 1a. The velocity of the gas, gravity, viscous force, surface tension, and aerodynamic force all impact the drawing of a continuous jet of liquid. The liquid jet with initial diameter of around 20−1000 μm is then carried for few meters downstream by the principal air stream whereby a series of phenomena including fiber stretching and solvent evaporation produce diameter attenuation of the jet by at least 2 orders of magnitude and result in fibers of a few hundreds of nanometers in diameter. Three different solution delivery nozzle configurations have been developed: (i) needle-tip, corresponding to a blunted needle of small diameter, (ii) wall-anchored, corresponding to a flat surface where the solution is allowed to fall under gravity and create a film before the liquid jet is formed, and (iii) pendant drop, where a drop is formed at the tip of a capillary tube under the influence of surface tension and gravitational forces. Despite the simplicity of the process, the mechanisms leading to the formation of the fibers have not yet been established and further study is necessary in order to determine the interrelationship between different physical phenomena. In electrospinning, the fibers are produced by a combination of stretching driven by electric charge repulsion and solvent evaporation.2,3 The GJF process in contrast can produce the same range of © 2013 American Chemical Society
Received: April 30, 2013 Revised: July 21, 2013 Published: July 30, 2013 6081
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Figure 1. Schematic of the setup of the GJF process for wall-anchored nozzle (a). A high speed camera is used to capture the liquid jet formation and propagation from a wall-anchored nozzle. Size of the photographed windows is: window 1 (W1) of dimensions 5.3 mm × 4 mm, window 2 (W2) of dimensions 26 mm × 19.5 mm, and window 3 (W3) of dimensions 143 mm × 107 mm. Additional nozzles include (b) small diameter tubing with “needle-tip” and (c) a capillary tube where a pendant drop is formed by gravity and supported by surface tension.
infrared camera11 have been used to reveal many of the features of traveling jets of polymers emanating from the melt blowing nozzles. Previous studies9,12 recorded multiexposure strobe images of the melt blowing thread line and identified a conical trajectory, the width of which increased with the distance from the die. The conical trajectory is attributed to bending instabilities occurring in the jets of the polymer melt. Bending instabilities have also been reported for electrospinning processes,3,13,14 although a fundamentally different mechanism is found responsible.3,15 Prior work on melt blowing reveals that a part of the fiber attenuation occurs within several centimeters from the die exit.6,16,17 However, the intrinsic difficulty in tracking small diameter liquid jets moving at relatively high speeds in a threedimensional space under the action of a compressed jet of gas with turbulent characteristics has not permitted complete tracking of fiber trajectories. As a consequence, full understanding of the dynamics of fiber attenuation and the relative contributions of different interrelated phenomena behind fiber diameter reduction are still lacking. The whipping phenomenon in melt blowing process is thought to be responsible for fiber attenuation and can give rise to many of the defects in fibers such as nonuniform fiber diameter, broken fibers (flies), and polymer particles in the fiber mat (shots).18,19 In this paper, the dynamics of fiber formation by GJF method is captured and studied using high speed video images of the liquid jets. Different scenarios are evaluated for continuous creation and stretching of liquid jets emanating from the GJF nozzles which turn polymer solutions into fibers of diameter from a few micrometer to a few hundreds nanometer. The velocity of the fibers traveling to the collector and the stretching rate at different axial positions along the liquid jet are obtained from particle tracking method. In addition, the simulated profiles of the velocity of the gas jet are presented
and their significance in the process is analyzed. The understanding of the process of fiber formation in GJF method provides necessary background for building comprehensive models that include diffusion of the solvent in the drying of the nanofibers in conjunction with phase separation and internal morphology development of the fibers.
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EXPERIMENTAL SECTION
GJF setup. A GJF setup was used to study fiber formation in three different nozzles - needle tip, wall-anchored, and pendant-drop. Blunted needles (JG19−0.5 Jensen Global Inc., internal diameter i.d. = 0.84 mm) were used as needle-tip nozzles (Figure 1b). Wall-anchored nozzle assembly (Figure 1a) was built by attaching a nonsharp needle (JG17−05 Jensen Global Inc., internal diameter i.d. = 1.21 mm) to a polypropylene circular disk of thickness 2 mm and diameter 15 mm. Glass capillary tubes of 1 mm internal diameter were used to create pendant drops (Figure 1c). The high velocity air jet was created by allowing compressed air to flow through a rigid pipe of internal radius 5.5 mm, fitted with a filter, pressure regulator, and a flow meter. The distance between the exit of the air stream from the pipe and the point of contact of air jet with the polymer solution was kept at 20 mm in all the cases. High Speed Imaging. The initiation and the initial steps of propagation of the liquid jet were captured by a high speed camera coupled with an optical microscope. A Phantom Miro EX 1 high-speed camera (Visionresearch Inc., Wayne, NJ) was used. The camera is capable of recording images at a maximum rate of 1000 frames per second (fps). Full frames were recorded at a resolution of 480 × 360 pixels with two 100 W lamps providing 2680 lm of brightness each. The high speed video images were processed using the Phantom software PCC Version 2.14. 727.1. A schematic representation of the video recording system with a wall-anchored nozzle is shown in Figure 1a. A view in the y−z plane of the flow, the jet initiation, and the initial steps of propagation was captured. The proceedings in the window of dimensions 5.3 mm ×4 mm and 26 mm ×19.5 mm were recorded using two levels of magnification. In addition, a macro lens and a strobe light were used to capture the onset of fiber bending and loop 6082
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formation on a section of 143 mm ×107 mm starting at axial distance z = 50 mm from the front end of the nozzle. Materials. Solutions of poly(ethylene oxide) (PEO, Mw = 1 000 000 g/mol from Alfa Aesar) in ethanol at different concentrations were prepared and characterized (Table 1). Shear viscosities were
symmetrical modeling domain representing the solid walls of the pipe used to deliver the gas jet and the expansion zone was created using the software gambit. The physical size of the nozzle and the emerging liquid stream disturb the gas flow and quite possibly the axisymmetric nature of the gas flow. Detailed modeling of how various nozzles disturb the gas flow and how such disturbances impact the liquid jet initiation and the flapping of the liquid jet is beyond the scope of this paper and will be considered in a future publication. Figure 2 presents a schematic of the physical model and the mesh of the domain that was used in simulation. The internal pipe has a radius of a = 5.5 mm, and the total length of the expansion zone is 500 mm. In this framework, the velocity field is axisymmetric around the z-axis. The simulation provides the average velocity profile under stationary conditions. A total of 259 000 cells were used with mesh refining in the sections closer to the walls and in the transition zone between the pipe and the expansion zone. To confirm that the velocity profiles are independent of the grid resolution, numerical experiments were performed at two different mesh sizes. Simulations were conducted with mesh containing 160 000 and 259 000 cells. No significant differences were found in the point velocities between the outputs obtained from the distinct meshes.
Table 1. Viscosity and Surface Tension of Solutions of Poly(ethylene oxide) of Mw = 1 000 000 g/mol Measured at Room Temperature (22 ± 2 °C)a PEO (% w/w) viscosity (Pa s) surface tension (mN m−1) a
solution 1
solution 2
solution 3
2 0.08 30.66
4 0.28 29.68
8 1.2 29.34
Viscosity is reported at a shear rate of 50 s−1.
determined using an Advance Rheometric Expansion System (ARES) model G2 from TA Instruments, operated with concentric cylinders geometry. Rheological data was measured at ambient temperature (22 ± 2 °C) and steady conditions over a range of shear rates from 0.1 to 500 s−1. Shear-thinning behavior was detected for all the solutions at shear rates above 200 s−1. Surface tension was measured using a Rame-Hart goniometer model 100 and a pendant drop configuration under ambient conditions at 22 ± 2 °C. Fiber Characterization. The fiber diameter distribution was obtained from scanning electron microscope (SEM) images. For this purpose, samples of fibers from different sections of the collector were used. The samples were coated with a ∼2.5 nm thick silver coating using a high resolution ion beam sputtering system model ISI 5400 under argon atmosphere. The samples were later imaged using a SEM model JEOL JSM5310 under the following conditionsaccelerating voltage between 5 and 10 kV, emission currents between 10 and 20 mA, and magnifications in the range of 300× to10 000×. At least 250 fibers were included from more than 10 SEM images in order to ensure reproducible statistics. Fiber diameters were measured by image processing software (Image J, NIH, USA) from SEM images by drawing straight lines along the diagonals of an image (to ensure fibers were not counted twice) and by measuring the diameter of fibers that crossed the lines.19 Flow Simulation. The velocity field generated by the compressed gas jet was simulated using the software Fluent version 6.3. An axi-
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RESULTS AND DISCUSSION The three nozzles used in this work exhibit some similarities with the systems commonly used in electrospinning. Figure 3 presents magnified views of the jets of solutions emanating from the three nozzle systems: needle-tip (Figure 3a), wallanchored (Figure 3b), and pendant drop (Figure 3c). In all cases, a single liquid jet was observed and no evidence was found on splitting of the liquid into multiple jets. This is an expected outcome as the gas jet was axisymmetric. The single liquid jet was then stretched by the faster moving gas and the solvent evaporated to create the fibers. The fiber diameter attenuation occurs due to stretching by the gas and due to shrinkage from the evaporation of the solvent. A simple calculation reveals that a liquid jet of a solution of poly(ethylene oxide) 4% w/w in ethanol with initial diameter of 400 μm shrinks to 65 μm diameter after the solvent is fully evaporated and if no stretching occurs. Accordingly, the
Figure 2. Schematic representation of the domain used in flow simulation. (a) Three-dimensional scheme showing the pipe delivering the gas jet and the expansion zone. (b) Plane domain used to generate the mesh. Boundary conditions include: PI (pressure inlet), PO (pressure outlet), W (wall), and symmetry axis. 6083
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Figure 3. Front view photographic images showing single liquid jets forming in different nozzles. The insets present sketches of the nozzle system: (a) needle-tip nozzle, (b) wall-anchored nozzle, and (c) pendant drop nozzle. In each case, the gas jet (not shown) is placed at 2 cm on the right, respectively from the center of the orifice and the center of the pendant drop.
Figure 4. Magnified view of liquid jet initiation on a wall-anchored nozzle. Solution of 4% w/w poly(ethylene oxide) in ethanol. Images were taken at 1000 fps. The complete process of jet initiation took 0.007 s. The size of each image is 5.3 mm × 4 mm.
additional attenuation of the liquid jet to submicrometer diameter must be accompanied by stretching effects. The fiber formation process was recorded at 1000 fps for the wall-anchored, needle-tip, and pendant drop nozzles and three distinct stages of fiber formation were identified for each nozzle configuration: jet initiation, straight propagation of the jet, and bending instability development. These are separately discussed below. Jet Initiation. Jet initiation occurs by the action of the drag force of air on polymer solution after it exits from the nozzle. For instance, in the case of wall-anchored nozzle configuration (Figure 3b), the polymer solution fed onto the polypropylene disc is directed by the force exerted by the gas jet to the edge of the disc and a liquid jet of the polymer solution is created at time t = 0. The liquid jet is transported by the gas jet (Figure 4). If the concentration of the solution (c) is above the critical concentration for chain entanglement c* (c > c*), fibers with
Figure 5. Superimposed images of the jets from a pendant drop at the two extreme positions for a solution of 8% w/w poly(ethylene oxide) in ethanol.
Figure 6. Size of the initial jet (measured at the exit of the nozzle) for a solution of 4% w/w of poly(ethylene oxide) in ethanol for (a) pendant drop, (b) wall-anchored nozzle, and (c) needle-tip nozzle. The air jet pressure and the air flow rate were fixed at 10 psi and 10.8 SCFM for wall-anchored and needle-tip nozzles, respectively. The air jet pressure and the flow rate were fixed at 4 psi and 2 SCFM respectively in the case of pendant drop. The size of each image is 5.3 mm ×4 mm. The insets in each image correspond to magnified views (200×) of the initial part of the jet. An “inversion of colors algorithm” was used to enhance the images. 6084
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Figure 7. Initial liquid jet diameter from needle-tip nozzle for solutions of poly(ethylene oxide) with 2% w/w (viscosity of 0.08 Pa s), 4% w/w (viscosity of 0.28 Pa s), and 8% w/w (viscosity of 1.2 Pa s) solid in ethanol. The air jet pressure was 10 psi. The solution feeding rate was 0.4 mL/min. The scale bar in the images corresponds to 1 mm.
Figure 9. Flapping amplitude for a liquid jet of PEO 4% w/w solution in ethanol.
very high aspect ratio (L/D; length L and diameter D) are produced. In this context, jet interruptions may occur in cases where the solution viscosity is high, surface tension is high, or turbulence intensity is high due to violent blowing conditions. A comparison of the images obtained with different nozzle configurations and presented in Figures 5 and 6 can be summarized in terms of the following observations. (i) The formation of fibers from a pendant drop is possible only at low air jet velocities and at air pressures below 5 psi. At higher air pressures, the pendant drop becomes unstable and the regime changes to liquid jet spinning from the needle-tip nozzle. (ii) The drag force from the air jet on the pendant drop creates a swinging motion of the drop (Figure 5), although continuous fiber formation is not impeded. (iii) The initial liquid jet diameter changes significantly as one goes from the pendant drop to needle-tip or wall-anchored nozzle configurations for the same stock polymer solution. However, the liquid jet diameter does not differ significantly when the needle-tip nozzle is used instead of the wall-anchored nozzle. One may ask if the liquid cone formed at the initiation of the liquid jet, seen in Figure 4, is stable over a period of time, especially in light of solvent evaporation. It is noted that the process of liquid jet initiation is transient and the cone shape and the size eventually reach steady states. The cone remains stable over long periods of time under steady operating conditions and attains another steady state when the process conditions, such as air pressures and liquid flow are changed to new values.
Figure 10. Attenuation of the liquid jet diameter with distance from the edge of the disc obtained with solution of PEO 10% w/w in ethanol using a wall-anchored nozzle at 20 psi air jet pressure. The viscosity of the PEO solution 10% w/w at 22 °C is 1.61 Pa s.
Figure 6 presents a representative set of images of the jets formed from a solution of 4% w/w PEO in ethanol for the three nozzle configurations. The initial jet diameters were measured right at the exit of the nozzle over a series of 50 frames separated in intervals of 0.001 s. The liquid jet diameters of 21 ± 1.7, 225 ± 13, and 195 ± 10 μm were observed for pendant drop, wall-anchored, and needle-tip nozzles, respectively.
Figure 8. Superimposed images of the liquid jet position obtained with a solution of 4% w/w of PEO in ethanol in a wall-anchored nozzle for (a) one frame, (b) 20 consecutive frames superimposed, and (c) 40 consecutive superimposed. The solid lines in part c cover the maximum peak to peak distance. 6085
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Figure 13. Stretching rates for solutions of PEO of 2% and 8% w/w in the flapping regime on a wall-anchored nozzle. The jet pressure was 10 psi. The solution feeding rate was 0.4 mL/min.
context, an increase of gas pressure is expected to reduce the diameter of the liquid jet. Note that larger initial liquid jet diameter means larger mean diameter of the final fibers. Straight Propagation of the Jet. The jet keeps a straight trajectory in the first few centimeters after it is formed, and undergoes a continuous motion up and down around a center position resembling a flapping-like movement. The amplitude or the maximum peak to peak distance in the y−z plane (Figure 1a) during this flapping-like motion increases with distance from the nozzle. A series of consecutive image frames were superimposed by using the photo editing software paint.net20 to obtain an estimate of the amplitude of flapping. Figure 8 presents a sequence of images of overlapped jets of 4% w/w PEO solution in ethanol obtained with a wall-anchored nozzle. These images were taken with an elapsed time of 0.001 s between two successive shots. The white solid lines in Figure 8c indicate the growth of amplitude of flapping with distance from the nozzle. The magnitude of this amplitude is plotted in Figure 9 as a function of distance along the z axis after taking the origin z = 0 at the edge of the wall, i.e., the position at which the liquid detaches from the solid wall. In light of Figure 8c, the amplitude of flapping is approximately 0.8 mm at z = 0 (see Figure 9), indicating that the point of detachment of the liquid jet traversed back and forth vertically on the solid surface. It is also seen in Figure 9 that the flapping amplitude monotonically
Figure 11. Liquid jet position −y to +y at a constant z position of z = 3.5 mm monitored over a 50 ms period for solutions of poly(ethylene oxide) of 2%, 4%, and 8% w/w in ethanol.
The solution propertiesprimarily the viscosity and the surface tensionwere found to have major effects on the diameter of the initial jet for a given pressure of the air jet and the polymer solution feeding rate. Figure 7 presents the initial diameter of the jets for PEO solutions with 2%, 4%, and 8% w/ w of polymer solution of viscosities of 0.08 Pa s, 0.28 Pa s and 1.2 Pa s respectively at air jet pressure of 10 psi and solution feeding rate of 0.4 mL/min. The initial diameter of the liquid jet increased with the increase of the solution viscosity, e.g., 158 μm for a solution of viscosity μ = 0.08 Pa s to 305 μm for a solution of viscosity μ = 1.2 Pa s. The same effect was observed in the case of wall-anchored nozzle. A similar effect was also expected in the case of pendant drops. However, the variation in diameter in the case of pendant drops due to differences in viscosity of solutions was not determined experimentally due to experimental limitations of the optical microscope set up used in measurement of diameter in the range of 10−20 μm. In this
Figure 12. Schematic representation of the methodology used to calculate the stretching rate at position z = zo + d1/2. (a) Two adjacent particles (1 and 2) at distance do and time t = to and (b) two particles (1 and 2) at distance d1 and time t = to + Δt. 6086
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Figure 14. Scanning electron micrographs of fibers of poly(ethylene oxide): (a) A solution of 2% w/w PEO produced using a wall-anchored nozzle. (b) A solution of 4% w/w PEO produced using a wall-anchored nozzle. (c) A solution of 8% w/w PEO produced using a wall-anchored nozzle. (d) A solution of 4% w/w PEO produced using a pendant drop nozzle. The jet pressure was 10 psi, and the solution feeding rate was 0.4 mL/min in cases a−c. The jet pressure was 4 psi, and the solution feeding rate was 0.1 mL/min in case d.
this context, the higher flapping frequency of the jets may lead to higher elongation rates, as will be analyzed below. The magnitude of stretching of the liquid jet undergoing flapping-like motion was evaluated by monitoring the trajectories of tracer particles included in the polymer solutions. Approximately 0.5 wt % boron nitride particles of mean diameter 80 μm were dispersed in polymer solutions and mixed for 2 h to obtain a homogeneous suspension. These suspensions were used as fiber precursors and the movement of the particles with the jet was tracked by high speed imaging. The rate of stretching was calculated from an increase of the distance of two initially adjacent particles as the liquid jet travels forward (Figure 12). In view of this, 17 different pairs of particles were tracked along successive images. The distance between the particles was measured by using the Phantom software PCC Version 2.14. 727.1 setting the starting point of the jet formation as origin of the coordinate system, z = 0. The stretching rate, with units 1/time, at axial position zo + d1/2 was then calculated as (d1 − do)/(doΔt). A spline interpolation of the liquid jet trajectory was performed to obtain an estimate of the axial position of the tracer particles. Figure 13 presents the stretching rate in the first few millimeters of the liquid jet after it emerges from the nozzle. It is evident that the stretching rate is higher for lower viscosity solutions. We attribute a majority of this stretching to higher flapping frequency. The same effect can be replicated in liquid jets created in a needle-tip nozzle. The stretching rate in liquid jets originating from pendant drops was not identified because
increased with distance from the nozzle until a more complex motion (bending instability) sets in. This straight propagation of the jet is accompanied by a significant reduction in the polymer jet diameter. Figure 10 presents a representative image of a liquid jet and shows how the liquid jet undergoes thinning for a solution of 10% w/w PEO in ethanol exiting from a wall-anchored nozzle. The data presented in Figure 10 show that the diameter of the liquid jet decreases by almost 1 order of magnitude in the first few millimeters of the jet trajectory; the diameter reduced from 1445 to 130 μm in a path of around 7 mm from the edge of the disc. We attribute this phenomenon to two effects: (i) the drag force of the high pressure gas jet and (ii) the flapping of the polymer jet caused by the aerodynamic forces of lift and drag contributing to the stretching of the fiber. An additional ramification of flapping is its frequency, as is evident in Figure 11 where the y-position of the jet at a given axial distance z from the nozzle is plotted as a function of time. The high speed video images of the liquid jets taken at various elapsed times were analyzed to obtain such data. The data in Figure 11 indicate the following trends. First, the flapping amplitude is not strongly dependent on the viscosity of the solution, e.g., 0.9 mm for viscosity μ = 1.2 Pa s, 0.95 mm for viscosity μ = 0.28 Pa s, and 0.99 mm for viscosity μ = 0.08 Pa s. Second, a comparison of the three traces in Figure 11 indicates that the frequency of this flapping-like motion is higher for a liquid jet of lower viscosity, e.g., ∼370 Hz for μ = 0.08 Pa s, ∼280 Hz for μ = 0.28 Pa s, and ∼200 Hz for μ = 1.2 Pa s. In 6087
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Figure 15. High speed images showing development of bending instabilities in jets of a solution of poly(ethylene oxide) 4% w/w in ethanol in a wallanchored nozzle. Numbers in the insets represent (1) back-folding on the fiber, (2) inflection points, and (3) loop formation.
Figure 16. Photographs showing development of bending instabilities in liquid jets of a solution of poly(ethylene oxide) 4 wt % in ethanol in a wallanchored nozzle. (a) Segment of fiber between points 1 and 2 at time t = 0, and (b) Segment of fiber between points 1 and 2 at time t = 0.0167 s.
of the limitations of measuring jet diameter in the range of 10− 20 μm. It is anticipated that thinning of liquid jets should occur in this case as well, although not to the same extent as in the case of needle-tip and wall-anchored nozzles. The stretching force of air jet is much smaller in this case due to lower air pressure. As was expected, the characteristic of each liquid jetjet diameter and stretching ratewill impact the final diameter of the nanofibers. Figure 14 presents SEM images of fibers of poly(ethylene oxide) produced using a wall-anchored nozzle for solutions of PEO 2% w/w (Figure 14a), 4% w/w (Figure 14b), and 8% w/w (Figure 14c). The mean diameter of the fibers decreases from 406 nm for the 8% w/w solution to 117 nm for the 2% w/w solution. In the case of pendant drop, fibers with
mean diameter of 274 nm were produced for a solution of poly(ethylene oxide) 4% w/w Figure 14d. Development of Bending Instability. The polymer jet develops bending instabilities of increasing amplitude at a distance of about 5 cm from the nozzle. The bending instabilities are characterized by random changes in the direction of the fiber around the horizontal axis, creating what to the naked eye appears similar to the cone observed in electrospinning. Figure 15 presents representative high speed images of the polymer jet captured with a macro lens. Several features are characteristics of this zone: (i) the continuous inflection points of the fiber thread (Figure 15, number 2) making the fiber travel in random trajectories of increasing amplitude with an increase of the distance from the nozzle, (ii) 6088
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The particle tracking method also yields information on liquid jet and fiber velocity in the bending zone. In this case, a single tracer particle was monitored and its axial positions in different frames in the photographs were tracked with elapsed time to obtain information on the z-component of the velocity vector. Figure 17 presents the calculated values of polymer jet velocity in the first few centimeters after the polymer jet was formed, and compares these values with the gas jet velocities obtained from simulation results. The polymer jet accelerated from a speed of 0 mm/s at the point of initiation of the jet to close to 500 mm/s in the first 20 mm of travel. The initial acceleration in this plot corresponds to the zone where the differences of the velocity between the gas and the liquid were positive. The liquid jet velocity reaches an asymptotic value as the gas jet decelerates and the difference of velocities becomes small. It is noted from Figure 17 that the gas jet provides the highest acceleration and stretching to the liquid jet in the first ∼8 cm after it contacts the liquid. Afterward, the gas jet velocity drops continuously over a short distance of ∼10 cm. The dashed line in Figure 17 is used as a visual aid in order to help understand two different trends of the liquid jet velocity: (i) the zone of high gas jet speed producing high drag force and (ii) the zone of continually reducing drag force with distance from the nozzle. The dashed line also guides comparison of the relative speeds of the gas jet and the liquid at the same axial distance.
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CONCLUSIONS This study captured the mechanisms of fiber formation in the GJF process using high speed video imaging. Three distinct stages of fiber formation were identified: (i) jet initiation, (ii) straight jet propagation, and (iii) development of bending instabilities. The liquid jet underwent stretching due to flapping during straight jet propagation and due to bending instabilities. Results show that the solution properties, principally the viscosity, play an important role in determining both the initial liquid jet diameter and the frequency of flapping in the straight jet section. A reduction of close to 1 order of magnitude in the liquid jet diameter occurred during straight jet propagation before further stretching occurred by bending instabilities. Bending instabilities developed specifically in the cases involving wall anchored and needle-tip configurations. Additional stretching originating from the elongation of loops in the bending zone was detected. The fiber velocity increased from near zero at the nozzle exit to almost 1 m/s at about a few centimeters away from the nozzle. Bending instability was not observed in the case of pendant drop method. Results obtained in this study offer insight on the formation of nanofibers using jets of gas and constitute a valuable contribution for the process optimization and tailoring of nanofibers for specific applications. For example, a single jet configuration can be used to explore the mixtures of different polymer components in one single fiber by the controlled supply of components into the delivery nozzle. It is noted in this study that the initial jet diameter impacts the attenuation process and the kinetics of drying. Larger diameter liquid jets account for longer drying times where solvent diffusion, jet stretching, and bending instabilities all contribute to diameter attenuation. In some cases, the above processes work simultaneously with phase separation or crystallization of the specific polymer components. In contrast, jets with smaller initial diameter account for faster drying and several of the competing processes mentioned
Figure 17. (a) Liquid jet velocity in mm/s as a function of the distance from the nozzle for a solution of PEO 4% w/w in ethanol. Air jet pressure was 10 psi and solution feeding rate was 0.4 mL/min. (b) Gas jet velocity in m/s at different pressure drop obtained from simulation. The zero value in the x-axis corresponds to the point of initial contact between the gas and the liquid jet.
fiber back-folding, or segments of fibers that are left behind by the main trajectory of the fiber evident in Figure 15 by the number 1, where back-folding is rationalized as the result of the variable drag forces on fiber segments traveling at lower speeds out of the center line of higher speed segments, and (iii) finally, the development of randomly located loops (number 3 in Figure 15). The cross points of the loops remain throughout the trajectories indicating possible conglutination points in the final fiber. The bending instabilities have been found to be responsible for additional stretching in the fiber. Figure 16 presents two high speed photographic images separated by 0.017 s elapsed time where two different points (1 and 2) were traced through the elapsed time. The perimeter between point 1 and 2 was obtained numerically by using the Image J software (NIH, USA) as a function of the elapsed times. An increase of perimeter by 53% from 5.6 mm to 8.6 mm was evident from the perimeter measured from frame 1 (Figure 16a) and frame 2 (Figure 16b). The calculations assumed that the perimeter segments of the fiber remained entirely in the y−z plane. The same behavior was detected by tracking several segments of the fiber in full video image at different values of stretching. This evidence supports the hypothesis that fibers experience significant stretching along the bending zone. 6089
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such as bending instabilities can be hindered by the fast solidification of the jet into fibers.
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AUTHOR INFORMATION
Corresponding Author
*(S.C.J.) E-mail:
[email protected]. Telephone: (330) 9728293. Fax: (330) 972-3406. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.E.B. acknowledges the Community Industrial Assistantship program at University of Akron and its participant PolyOne Corporation. S.C.J. and D.H.R. acknowledge Ohio Third Frontier Funds for financial support.
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REFERENCES
(1) Benavides, R. E.; Jana, S. C.; Reneker, D. H. ACS Macro Lett. 2012, 1032−1036. (2) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46 (30), 5670−5703. (3) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87 (9), 4531−4547. (4) Pinchuk, L. S. Melt blowing: equipment, technology, and polymer fibrous materials; Springer: Berlin and New York, 2002, Chapter 2, pp 5−20. (5) Shambaugh, R. L. Ind. Eng. Chem. Res. 1988, 27 (12), 2363− 2372. (6) Uyttendaele, M. A. J.; Shambaugh, R. L. AIChE J. 1990, 36 (2), 175−186. (7) Sinha-Ray, S.; Yarin, A. L.; Pourdeyhimi, B. J. Appl. Phys. 2010, 108 (3), 034912−12. (8) Yarin, A. L.; Sinha-Ray, S.; Pourdeyhimi, B. J. Appl. Phys. 2010, 108 (3), 034913−10. (9) Chhabra, R.; Shambaugh, R. L. Ind. Eng. Chem. Res. 1996, 35 (11), 4366−4374. (10) Wu, T. T.; Shambaugh, R. L. Ind. Eng. Chem. Res. 1992, 31 (1), 379−389. (11) Bansal, V.; Shambaugh, R. L. Ind. Eng. Chem. Res. 1998, 37 (5), 1799−1806. (12) Rao, R. S.; Shambaugh, R. L. Ind. Eng. Chem. Res. 1993, 32 (12), 3100−3111. (13) Theron, S. A.; Yarin, A. L.; Zussman, E.; Kroll, E. Polymer 2005, 46 (9), 2889−2899. (14) Xie, S.; Zeng, Y. Ind. Eng. Chem. Res. 2012, 51 (14), 5346−5352. (15) Fridrikh, S. V.; Yu, J. H.; Brenner, M. P.; Rutledge, G. C. Phys. Rev. Lett. 2003, 90 (14), 144502. (16) Yin, H.; Yan, Z.; Ko, W. C.; Bresee, R. R. Int. Nonwovens J. 2000, 94 (4), 25−28. (17) Zhou, C.; Tan, D. H.; Janakiraman, A. P.; Kumar, S. Chem. Eng. Sci. 2011, 66 (18), 4172−4183. (18) Chung, C.; Kumar, S. J. Non-Newtonian Fluid Mech. 2013, 192 (0), 37−47. (19) Ellison, C. J.; Phatak, A.; Giles, D. W.; Macosko, C. W.; Bates, F. S. Polymer 2007, 48 (11), 3306−3316. (20) http://forums.getpaint.net/
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dx.doi.org/10.1021/ma400900s | Macromolecules 2013, 46, 6081−6090
