Polymeric Nanofibers - American Chemical Society

Chapter 8

Electrostatic Effects on Electrospun Fiber Deposition and Alignment 1

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Navin N. Bunyan , Julie Chen , Inan Chen , and Samira Farboodmanesh Downloaded by UNIV OF SYDNEY on November 20, 2016 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch008

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Department of Mechanical Engineering, University of Massachusetts at Lowell, Lowell, M A 01854 Consultant, 1220 Majestic Way, Webster, NY 14580 2

Electrospinning is a nano-fiber manufacturing process that uses an electrical potential to initiate the spinning from a charged polymer solution. The primary objective of this research is to understand the effects of electrostatics on the convergence and alignment of the fibers deposited onto various targets. Desired orientation of the fibers can be achieved by potentially addressing three aspects: jet path control, target design, and solution properties. The first two will be addressed in this paper. It is shown that the spread of the fibers on the target can be decreased by using a disc electrode at the source, which creates a nearly parallel field between the source and target. The results are also shown to agree with an ion-jet charge transport model. This electrode placed at the source can also be used to direct the fibers to different areas on the target by angling it with respect to the plane of the target. Effects of target design on fiber alignment are also demonstrated.

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© 2006 American Chemical Society

Reneker and Fong; Polymeric Nanofibers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Downloaded by UNIV OF SYDNEY on November 20, 2016 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch008

Introduction Electrospinning is a method of forming polymer fibers with diameters in the micro- to nanometer range by using electric fields. The fabrication and properties of electrospun polymer fibers are gaining increased attention due to the ease of forming fibers with a high surface area/volume ratio. Some potential applications for these fibrous structures include substrates for tissue growth, catalysts and enzymes to make reactions faster, selectively permeable materials for protective clothing, highly effective thermal insulation or thermal conduction, filters for fine particles, wound dressings and artificial blood vessels. The growing fields of nanotechnology and biotechnology have given an added impetus to the creation of high-rate processes for fabricating nanofibers and nanofiber assemblies. For example, electrospun fibers of sulfonated polystyrene and enzymatically synthesized polyaniline have shown promise as conductive and photo-responsive materials for electronic devices (7). Nanowires and nanotubes carry charge efficiently; hence they are the ideal building blocks for nanoscale electronics and optoelectronics (2, 3). The key to many applications is controlled orientation and/or placement of the nanofibers to obtain the desired interconnection or anisotropy. In this paper, "orientation" refers to the alignment of the fiber rather than the molecular orientation within the fiber. Many studies have been successful in forming non-woven fiber mat from electrospinning; however, the issue of controlling the fiber path from the source to target and having control over the deposition and collection of the fibers is still a major challenge in the fiber assembly process. Although electrospinning was patented by Formhals (4) in the early 1930's, it was only in the 1990's when Reneker and coworkers (5-8) carried out detailed experiments on the process that the research in this field received renewed interest. Many of these earlier studies in the past decade focused on demonstrating that submicron fibers could be electrospun with different polymers and also included some experimental studies on the effect of process parameters on structure and morphology of the fibers (5-11). The characteristics of the fibers formed were shown to be dependent upon a number of parameters such as electrical field strength, flow rate, and initial solution viscosity. Other factors that are likely to have an effect, but have not been as widely studied include molecular weight, molecular weight distribution, crystallinity, solubility, vapor pressure of the solvent, conductivity, surface tension, and charge transport. Modeling efforts are still somewhat limited. Rutledge et al. (12-13) proposed a stability theory for electrified fluid jets to predict the effect of process parameters on the whipping instability initiation. For their analysis they considered the surface charge density, radius of the jet, fluid conductivity and viscosity.



108 The research effort described in this paper concentrates on controlling the fiber spread and alignment on the target and directing the jet to specific positions on the target. The knowledge obtained in this work can be utilized for developing a design for achieving controlled fiber deposition. Since electrostatic forces dominate the electrospinning process to a large extent, it is possible to control the jet path, fiber deposition, and fiber collection by manipulating the field acting on the system. Different target geometries can be designed for fiber collection to suit specific applications. This research will open the door to obtaining controlled linear, planar and 3-dimensional fiber assemblies for various applications.

Electrospinning Setup and Process Figure 1 shows a schematic of a typical horizontal setup. Vertical setups have also been used by the authors and by other researchers. The basic setup consists of a pipette or syringe, hereafter referred to as the source, containing a polymer solution, and an electrically grounded aluminum foil pan, hereafter referred to as the target. The target could be in various forms, including a wire mesh, plate, or rotating disc, and is mounted on an adjustable electrically insulated stand. The distance between the source and the target is generally in the range of 10-30 cm, depending on the optimal process conditions. A n electrode connected to a high voltage power supply is immersed into the solution. The power supply used was from Gamma High Voltage Research Inc. (Ormond Beach, Florida), with the following specifications: Gamma high voltage model no: ES 30-0.IP, Output voltage = 0-30KV, Output Current = 100^A and Polarity = Positive. To have positive control over the flow of polymer, a flow pump connected to a syringe was used for feeding the solution. A F L U K E 189 multimeter was connected between the ground and target to measure the current that is developed during the deposition of the fibers. This multimeter was capable of measuring current in the microampere range. Current developed at the target plays a very significant role in characterizing the spinning process. To minimize the loss of fibers due to adherence to metal parts of the setup, a grounded aluminum foil target with a large surface area was used. To determine the mass of deposited fiber, the mass of the foil before and after the spinning process was measured using a Sartorius M P 1212 scale, which has a resolution of 0.00 lg. B y knowing the current and the mass of the fibers deposited on the target, the charge per unit mass of the fibers was determined.


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Figure 1. A simple horizontal electrospinning setup (See page 7 of color inserts.)

When a high voltage is applied to the polymer solution, a jet of charged fibers is ejected from the tip towards the target. Due to the inherent viscosity of the polymer, the jet does not disintegrate into spherical droplets but forms long fibers. Originally, the jet was assumed to undergo splaying, which is the splitting of the fibers, leading to smaller diameters. Based on more recent observations, however, the jet appears to generate a rapidly-rotating spiral, which is indistinguishable from the splaying phenomenon to the naked eye (12, 13). Figure 1 shows a schematic of this whipping motion of the electrospinning jet. It is this motion that draws the fibers, leading to the formation of nanometer-scale diameter fibers. The solvent evaporates during the process and a nonwoven semi-dry fiber mat is formed on the target, in a relatively circular deposition pattern. Figure 2 shows a electrospun fiber mat deposited on an aluminum target and a scanning electron microscopy (SEM) image of the fibers. Note that there is no distinct directional orientation of the fibers.

Figure 2. (a) Electrospun polyethylene oxide(PEO) fibers on an aluminum Target, and (b) SEM image of the same fibers. (See page 7 of color inserts.)


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Ion-Jet Charge Transport Model of Disc Electrode Effects Part of the challenge of controlling the fiber deposition is the divergence that occurs due to the whipping motion of the jet (see Figure 1). It was hypothesized that adding a disc electrode to create a more parallel electric field (rather than a diverging one generated by a point source) would help to focus the jet. To guide the experiments and determine i f the jet response could be predicted by using an ion-jet charge transport model, a model was developed using the steady state charge distribution for two parallel plate electrodes, namely the voltage source and the grounded target. Equation 1 shows the conservation law for current continuity under steady state conditions, ^

+ VJ(r,

2)

(1)

=0 3

where q represents the charge density (in C/m ) and J the current density (in A/m ). Figure 3 shows the model representation of the electrospinning setup. The electrode at the source is called the disc electrode with radius R . The disc electrode radius can be varied from R = R (source radius), where there is only the syringe tip and no disc electrode, to a disc electrode as large as the target. 2

d

d

s

Figure 3. Schematic of the modified electrospinning setup The voltage V applied to the disc electrode can be the same as or lower than that applied to the source electrode V . For modeling purposes, the source radius was fixed at 0.0 IL, where L is the distance between the source and the target. The radius of the disc electrode is varied from 0.0 IL (equal to R ) to 0.21. The same voltage is applied to both the source and disc, V = V . On applying the boundary conditions the equations were solved by numerical iteration. Figure 4 shows the results in the form of current density versus the radial spread of the fibers on the target (i.e., at z = I ) . In these plots, i f it is assumed that the jet carries the charge to the target, then the current density (J ) is representative of the mass of fiber deposited on the target. The radial distance (x-axis values) represents the radius of the spread of the fibers on the d

s

s

s

d

z



Ill target. Note that all the dimensions given are with respect to L (0.2-* 0.2 L). It is seen that when the radius of the disc electrode is increased from 0.0IL to 0.2L, there is a four-fold decrease in the spread of the fibers at the target. For example, in Figure 4a, the radius of fiber deposition decreases from ~0.4L for a point electrode to ~0.1Z, as the disc electrode radius increased. This is not surprising when one considers the electric field lines generated by a point-plate arrangement (field lines diverging from point source) to those of a plate-plate arrangement (parallel field lines), as shown by Bunyan (14). Figures 4a and 4b also demonstrate the effect of the injection parameter, y, which is defined in equation 2, SfjV

1A

y = -^-»\QT^(Slcm) L

(2)

where e = permittivity, p = mobility, V = applied voltage (or V ) and L = sourcetarget distance. This injection parameter corresponds to the strength of the ion source, and represents the effects of process and material parameters, such as the flow rate, applied voltage, target distance, polymer and solvent species, and concentration. In comparing Figure 4a (y =100) and Figure 4b (y =10), an order of magnitude increase of y leads to a doubling of the spread of the fibers on the target. A third outcome of the modeling effort, the relatively horizontal current density curves with rapid dropoffs, indicates that for a given disc electrode, the amount of fiber mass collected within the fiber deposition region would not be a function of radial distance from the center of the target. 0

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Figure 4. Effect of disc electrode radius and injection parameter on current density as a function of radial distance on the target. (See page 7 of color inserts.) Reneker and Fong; Polymeric Nanofibers ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Experimental Studies using Disc Electrodes


Validation of Ion-Jet Charge Transport Model with Disc Electrode Because the purpose of these experiments was to validate the model and understand the effects of the disc electrode, polyethylene oxide (PEO) in ethyl alcohol and distilled water was chosen as the polymer solution. P E O is easy to electrospin and is the material for which there is the most published data available on the influence of process parameters. The existing electrospinning setup was modified to correspond to the modeling assumptions (see Figure 5). A n aluminum disc was placed about 1cm behind the tip of the syringe, so as not to physically disrupt the jet. The disc was held parallel to the target surface. Electric potential from the power supply was applied to both the disc and the syringe needle. The applied voltage was 17.5KV and the target was placed at a distance of 16cm from the tip of the needle. The spread of the fibers on the target was measured for tests with and without the disc electrode.

Figure 5. Electrospinning setup with the disc electrode (target not shown) (See page 8 of color inserts.)

Figure 6 is a plot of the diameter of the circular fiber deposition pattern on the target as a function of the disc diameter for two flow pump rates. The two horizontal lines on the top of the graph are plotted simply to serve as a reference for the fiber spread when no disc electrode is used. Note that the addition of even a 5 cm diameter disc leads to the spread decreasing from roughly 20 cm to roughly 10 cm. As the diameter of the disc increases to 10 cm, there is continual reduction in the fiber spread. Figure 7 shows a schematic of the effect of the disc electrode on the electric field. During the experiment, the jet bending instability or whipping motion was observed to initiate at distance farther from the source as the disc diameter increased, leading to a decrease in fiber spread on the target. In Figure 6, it can also be seen that an increase in the polymer flow rate, which is related to the injection parameter ' / of the ion jet transport model, from 0.06 ml/min to 0.09 ml/min, led to an increase in the spread of fibers on the target.


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Similar increases due to an increase in the electric field were also observed (Bunyan (14)).

Figure 6. Plot of spread offibers on target versus the disc diameter (applied voltage = 17.5KV, source to target distance = 16cm) (See page 8 of color inserts.)

Figure 7. Schematic of effect of disc electrode on the instability initiation and fiber spread, (a) without the disc electrode and (b) with the disc electrode. (See page 9 of color inserts.)


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To compare these results more quantitatively with the ion-jet charge transport model predictions, the fiber spread results from the 5 cm disc electrode were used to identify a value for the injection parameter, y. This same value of y was then used in the model to predict the fiber spread values for the 7.5 cm and 10 cm disc electrodes. The results shown in Figure 8 suggest that the ion-jet charge transport model captures some of the essential physics of the relationship between the charge transport and the fiber spread on the target.

Figure 8. Comparison of fiber spread results - experimental versus model with 5 cm data used to determine injection parameter value (See page 9 of color inserts.)

O f course, using the disc electrode to focus the fiber deposition is not beneficial i f it also leads to poor fiber formation. This was a concern since the observation of a delayed whipping initiation and reduced diameter of the whipping motion could lead one to assume less time for drawing of the fiber diameter. To address this concern, scanning electron microscopy images of the fibers formed under the same conditions, with the exception of the presence or absence of a 10 cm disc electrode, were obtained (Figure 9). The S E M images demonstrate that in fact, the fibers formed using the disc electrode were even smaller in diameter. The reason for this is not known at this time, but may be tied to the overall reduction in jet diameter as it initiates from the syringe tip, well before any whipping instability occurs.



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Figure 9. SEM images of electrospun PEO fibers (a) without and (b) with a 10 cm disc electrode.

Experiments on Direction Control with Disc Electrode Along with control of the fiber spread, it is desirable to have control of the direction of the fiber path such that fibers could be directed to specific areas on the target for collection. This could be achieved by controlling the electric fields near the source by angling the disc electrode (Figure 10). In this experiment, the disc was rotated about the horizontal axis (the axis parallel to the target plane) by 10° increments and the shift in the centers of the fiber deposition was measured. The experiments were repeated for different disc diameters and source-to-target distances to understand the effect on the jet path.

Figure 10. Directing the electrospinning jet onto the target (See page 10 of color inserts.)



116 There was not much variation in the spread of the fibers as this was controlled by the diameter of the disc electrode. The experiments were carried out for 5, 10, 15, and 20 cm source to target distance and 5, 10, and 15 cm disc diameters. The applied voltage was 15KV and polymer flow rate was 0.1 ml/min. The solid lines in Figure 11 represent the data obtained from the experiments for the 10 cm disc. For example, for a source to target distance of 15cm, a 30° rotation shifted the centerpoint of the deposition by ~6 cm from the original position. The rotation of the disc shifts the electric field, which leads to a shift in the motion of the fibers to the target. As expected, as the source to target distance increases or the angle increases, the shift increases. Increasing disc diameter also led to increases in the shift, but not as pronounced as for the angle change (14). The dotted lines in Figure 11 represent the predicted shift according to the intersection of the axis of the disc with the target (equation 3). Shift in fiber deposition

= Z,*tan(0)

(3)

where L = source to target distance and 6 is the rotation angle.

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Polymeric Nanofibers - American Chemical Society

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