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Exploitation of electric field in controlling of nanofiber spinning process.

INTRODUCTION

Nanotechnology is a promising technology in the fiber industry. Nanofibers (referring here to all submicron sized fibers) can be used, for example, in filtration, biomedical applications, polishing cloths, reinforcing, artificial leather, and protective clothing. Melt-blowing technology enables the manufacture of microfibers and in some cases also nanofibers in large quantities. Another method for producing fine fibers is melt-spin production of islands-in-the-sea structured bicomponent fibers with a dissolvable polymer as a matrix. And yet a third method is via fibrillation of linear cellular structured fibers, such as cellulose into nanosized subfibers or fibrils [1, 2].

Electrospinning is a method for producing nanofiber webs from polymer solutions. The diameters of the electrospun fibers can vary from tens of nanometers to micrometers, but nevertheless with a properly chosen polymer solution and process parameters nanofibers can easily be produced. The electrospinning method was patented in 1902 [3, 4], but commercial applications are still rare. The principle of the electrospinning method is quite simple--the electrostatic field stretches the polymer solution into fibers at the same time as the solvent evaporates. However, the process is difficult to control and several variables have an influence on the properties of the end product. Furthermore, the quality of the fibers is typically inconsistent, for example, the fiber deposition may be uneven or the distribution of fiber diameter may be large.

In recent years interest toward electrospinning has increased. Many publications are presenting preparation and properties of fibers for certain predetermined applications, typically electrospun from a specific polymer or composite solution [5-7]. Another theme of the publications is parameter studies where research groups are determining the effects of different variables on electrospun fibers in general or for specific polymer(s) [8-11]. Still one important group of publications is the one focused on more profound understanding and modeling of the process and its phenomena [12-31]. So far this modeling has been mostly concentrated on instabilities and the behavior of the polymer jet, but not so many models of the electric field have been published yet. Results of the field models of electrospinning systems are typically 2D vector images of over all electric field lacking the ability to give explicit quality and quantity information about the electric field and its components.

In this study the electrostatic field of the electrospinning system was modeled. The purpose of this study was that the effects of two of the main process parameters, voltage and distance, on nanofiber coating could be understood better by clarifying their influence on the electric field and especially on individual field components. We introduced a different way to display information about the electric field of electrospinning system and its components as simple line charts. The results of the models were compared with the experimental data and observations collected from electrospinning trials with poly(vinyl alcohol) (PVA). The main interest was focused on the shape and size of the deposition pattern, but the effect of variables on the fiber diameters was also studied.

ELECTROSPINNING

Method

Plain electrospinning equipment consists of a nozzle, a collector, and a source of high voltage. The polymer solution is charged in the spinning nozzle. When the collector is grounded or oppositely charged an electrostatic field is generated between the nozzle and the collector. When the strength of the electric field is high enough, the solution is ejected toward the collector. The fiber web can be collected as such or as a fibrous coating on a substrate covering the collector.

The polymer jet typically becomes unstable in the electric field due to opposite forces affecting it: surface tension, which stabilizes the jet and tends to minimize its surface; and charge repulsion, which destabilizes the jet and increases its surface. One of the possible instabilities that may occur during electrospinning is whipping or also called bending instability [12-14, 32, 33], which is enhanced by high surface charge density [19]. The rapidly whipping jet undergoes high stretching during its path through the electric field, which results in the drastic reduction of the fiber diameter. The initially formed single jet can also be split or splayed into multiple filaments, which can also be responsible in the formation of fibers with nanoscale diameters [8]. The solvent evaporates during the spinning process and with proper solution parameters (e.g. viscosity) and process parameters (e.g. voltage and distance) the dry, thin fiber web can be collected onto the substrate. Because of the instabilities the jet travels inside a conical envelope, which widens toward the collector. Therefore fibers from one nozzle can be deposited onto a relatively large area on the substrate.

Typical production rate of the electrospinning is 0.03 g/nozzle/min, while the production rate of melt-blown is 0.5 g/hole/min [2], and therefore scaling up is essential for a commercially attractive production rate. One way to make commercial electrospinning equipment is to use multiple nozzles arranged into a matrix facing the moving substrate [34, 35], so that every nozzle forms stripe of fibers onto substrate. It is favorable to have a uniform deposition pattern in order to obtain a full and even coverage to the web from staggered nozzles.

It has been stated [14] that the chaotic whipping motion of the jet makes the deposition of electrospun fibers essentially random. Deposition can be somewhat controlled in several ways. Aligned nanofibers can be achieved using a rotating collector [29, 36] or collecting fibers between two parallel electrodes [28, 37]. Suppression of the envelope leading to smaller deposition is achieved using circular auxiliary electrodes around the jet [14, 15]. Although the precise deposition is difficult, the deposition pattern and its uniformity can still be influenced by adjusting the process parameters [38].

The general shape of the deposition pattern is round in the case of one nozzle and a square plate. Ideally the deposition pattern is even, but fiber quantity can also vary in circular regions (Fig. 1) forming circles and concentric circles or so called bull's eye patterns. Different shapes of deposition pattern have been observed in experiments with PVA [38, 39]. Heikkila et al. [38] observed that the size of the deposition pattern was affected by distance, because the envelope widened as the distance increases, and by the strength of the electric field, which caused the instabilities. In trials with PVA and moderate distances when the average field strength was less than 200 kV/m, the diameter of the deposition pattern was smaller than the distance. Above that field strength the size of the deposition pattern increased and in some cases its diameter was even twice the distance [38]. Lee et al. [39] have made observations concerning fiber diameters; they noticed that the fiber diameter tends to increase from the inner part to outer part of the concentric circles.

[FIGURE 1 OMITTED]

Modeling of the electrospinning process has been focused on the electrically driven polymer jet and its instabilities. Yarin et al. [16] have studied Taylor cone and jetting from it, and presented a theoretical model on how the shape of the liquid on the tip of the nozzle is affected by an electric field. Steady-state jet in electrospinning was modeled by Spivak et al. [17], while Russell et al. [18] have described the fundamental physics and fluid dynamics relevant to the instabilities at the polymer interfaces. Hohman et al. [19, 20] have made an electro-hydrodynamic model for Newtonian jets in order to develop a theoretical framework for understanding the physical mechanics of electrospinning. Feng [21, 22] as well as Carroll and Joo [23] have modeled both Newtonian and non-Newtonian solutions. Their models were used to study the behavior and the phenomena of the jet, for example, effects of extension thinning and thickening, and strain hardening on polymer jets [21]. Also Fridrikh et al. [24] have been interested in fiber diameter, and they modeled the charged jet of the electrospinning determining the jet diameter as a function of surface tension, flow rate, and electric current. Lu et al. [25] made a computer simulation in order to study the effect of solvent in electrospinning. They concentrated on the energy change in the process of molecule orientation. He and Wan [26] have proposed a theoretical model for electric phenomena of electrospinning, and obtained an allometric scaling relation between the current of the charged jet and the voltage. Reneker et al. [27] have prepared a computer model which calculates a predicted path of the jet from 17 input quantities.

Electric fields can be presented as 2D field vector images. Vectors illustrate the direction of the overall electric field, which is determined, e.g., by the geometry of the collector, and the magnitude of the electric field can be expressed to some extent with density or length of the field lines. These kinds of electric field plots have been presented for different kinds of one-nozzle systems. Deitzel et al. [14] illustrated a plain system of one nozzle and a collector and the same system with auxiliary electrodes, Li et al. [28] illustrated a system where aligned electrospun fibers were collected between two parallel electrodes, and Theron et al. [29] a system where aligned fibers were collected onto the edge of the rotating disc. Fang et al. [30] have made a 2D model of the electric field of their electrospinning-nozzle system in order to examine different electrode configurations. One of their target was to reduce the interference between jets from different nozzles. They utilized the model and its results in building of a multinozzle electrospinning system.

Theron et al. [31] have modeled multiple jet electro-spinning in order to study the interaction between the jets, and presented their results as top and side views of the predicted paths of the jets. They noticed that adjacent jets having the same polarity are pushed away from each other, and that in the vicinity of other jets can also change the shape and size of the deposition pattern. Multi-jet electrospinning system was also modeled by Kim et al. [15] for stability analysis and comparison of multi-jet and single-jet systems. The comparison was made using electric field concentration factor (EFCF), which expresses a degree of convergence of the jet to a spinning axis. The determination of EFCF included a brief study of field components. Kim et al. presented this EFCF as line charts and 3D color plots.

Experimental Setup

The one-nozzle electrospinning system (Fig. 2) consisted of a nozzle system perpendicular to and directed toward the midpoint of a copper collector plate electrode (400 mm x 400 mm). The nozzle system had a glass vessel for the polymer solution, and a metal needle as an electrode and a capillary. Syringe needles (14G) had an inner diameter of 1.6 mm and 42 mm in length. The nozzle and the collector were mounted onto separate supports so that the distance from the tip of the nozzle to the collector plate could be adjusted. The power supply used in the trials was a bipolar Simco Chargemaster BP 50. In the trials the electric field is generated by connecting the nozzle to the positive and the collector plate to the negative output of the power supply.

[FIGURE 2 OMITTED]

A 10% aqueous solution of PVA (Merck, [M.sub.w] = 72,000, degree of hydrolysis > 98%) was used in the trials. The viscosity of the solution was 1687 cP measured with a Brookfield Viscometer at room temperature. The basic arrangement with a voltage difference of 40 kV (nozzle +20 kV and plate -20 kV) and a distance of 150 mm produced even coating with the solution and the equipment described above. Electrospinning trials were made according to the spinning parameters presented in Table 1. Trial 0 is the basic arrangement and A-E trials where the voltage, or the distance, or both are varied.

The deposition patterns were photographed, and their sizes were measured. SEM micrographs were taken with JEOL JSM-T100 for determination of the fiber diameters and amount of fibers. The fiber diameters were measured from the SEM micrographs using an ImageTool 3.0 program. The amount of fibers in certain parts of the deposition area was also counted from some of the SEM micrographs.

Modeling of Equipment

The aim of modeling the experimental set up was to examine the effects of different spinning parameters and the significance of electric field components from a controllability point of view to a fiber formation and deposition in the electrospinning process. Special interest was given in calculating field intensities in the vicinity of a collector plate to find a possible correlation to the shape and evenness of the fiber surface.

The analysis was made with the Opera 3D (Vector Fields) software using TOSCA electrostatic solver based on the fundamental equations of the electromagnetic theory using the finite element method with a tetrahedral mesh. The spinning process was treated as electrostatic assuming no changes in the electric field during the analysis. The charge densities of the materials were also assumed to be zero. The mathematical problem to be solved was a Laplacian kind of a partial differential equation:

[FIGURE 3 OMITTED]

[nabla] x [epsilon][nabla]V = 0 (1)

where [epsilon] is the permittivity of a material and V is the potential. Potential is related to electric field by a negative gradient.

In modeling the experimental setup (Fig. 2) the symmetry of problem was exploited so that only half of the system was calculated consisting of half the nozzle, the collector plate, and the supporting structure (Fig. 3). The edges of the calculating volume were extended to a distance (30 m) so that truncation has an insignificant effect to the electric field on the region of interest. The dimensions of the stable part of the polymer solution jet were estimated from photographs taken of the process, and the stable part was modeled as a cylinder of 50 mm in length and 0.2 mm in diameter. Permittivity of water (80) was used for the jet in a model. The substrate having width of 400 mm and being at the distance of 150 mm from the nozzle in the basic model was considered to be in contact with the collector plate. The permittivities of 3.5, 7.5, and 2.0 were used for the substrate, the glass nozzle, and the rubber dielectrics as a part of a supporting structure, respectively. Porosity of the substrate was neglected. Floating conductive parts (not connected to the power supply) of a supporting structure were modeled as materials having very high permittivity in the order of [10.sup.12]. Otherwise the electrospinning medium was air (permittivity 1).

The problem symmetry and the symmetry of the field were implied by the potential boundary condition. The Neumann type boundary, [partial derivative]V/[partial derivative]n = 0, was assigned to the symmetry plane and the boundary condition of Dirichlet type, V = 0, was assigned to exterior surfaces of the calculating volume. The fixed potentials were given to the surfaces of the nozzle and the collector plate, which were connected to the power supply. The calculating volume in which the equation is satisfied was divided into small volumes, finite elements. Within each finite element a certain polynomial is used to approximate the solution. The element mesh was denser in the regions where the field changes were greater e.g., around the nozzle and the collector plate. In the basic model the size of the problem was as follows: the number of tetrahedral elements 1,661,831, nodes 2,241,105, and equations 2,221,715. The estimated rms error over the whole problem was 1.92%.

The observation plane (440 mm x 440 mm) was placed between the nozzle and the collector plate according to Fig. 4. The origin of the coordinate system was in the middle of the collector plate, so when the plane is 10 mm apart from the collector the z-coordinate of the plate is thus -10 mm, but the distance from the collector can be changed. The observation line began from the midpoint and ended at the top corner of the observation plane.

The parameters of basic arrangement were used in the basic model. Other models were also calculated using the same parameters as those used in other electrospinning trials (Trial code = Model code). Variations can be divided into three groups for analysis. First the voltage difference was varied while the distance was fixed. Then the distance was varied while the voltage difference was fixed. And finally, the effect of the average electric field strength was studied comparing three arrangements where both distance and voltage were varied.

The general shape of the electric field is observed by plotting the distribution of the overall field strength of the basic model at different distances from the collector plate. In this study, though, more attention was given to field components, which are observed separately by plotting them as line charts. In these charts the x- and y-coordinates of the observation line are presented in the x-axis. The distance between a certain point in x-axis and the midpoint of the plate is [square root of 2] times this coordinate. The strength of the electric field components, E (kV/m), is presented in the y-axis. The positive Ex- and Ey-components are forcing the polymer solution jet away from the centerline and therefore spreading the jet, and negative values narrowing it. The positive Ez-component is pulling the jet toward the collector plate.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

RESULTS

Electrospinning Trials

The pictures of the deposition patterns of the basic arrangement (0) and its variations (A-E) are presented in Fig. 5. The shape and size of the deposition patterns and measured fiber diameters are presented in Table 2. The edge of the deposition pattern is expressed with x- and y-coordinates in the observation line (see Fig. 4) so that comparison with field component charts would be easier, the diameter of pattern is calculated from that value. Large standard deviations of fiber diameter indicate large fiber diameter distribution. Measured single fiber diameters varied from less than 200 nm to more than 2 [micro]m.

Deposition patterns of Trials A and D were circles while the others were even. In these trials the strength of the electric field was low (200 kV/m). Trials C and E, on the other hand, are distinguished from the others by means of fiber diameter. In these trials the strength of the electric field was higher (400 kV/m) than in the others. The fiber diameters and their amount at different observation points in some trials are presented in Table 3.

Model

Basic Model. In the basic model (0) voltage was 40 kV and distance 150 mm. The distributions of the overall electric field in three distances from the collector plate (10, 70, and 140 mm) are presented in Fig. 6. In the first plot the distance of the observation plane from the collector was 10 mm, and dark area in the middle is presenting the lowest field strength. The square shape of the plate was seen clearly in the shape of the field. The edges and the mounting of the plate caused distortions to the field. In the next plot the observation plane was moved almost half way toward the nozzle (distance from the plate is 70 mm) and the dark, round area in the middle presented highest field strength. The shape of the highest field was round due to the round tip of the nozzle. In the third plot the distance from the plate was 140 mm, and the light area in the middle presented the highest field strength. The effective area of the field (light shade) was smaller than in the earlier plots, when the observation plane is in vicinity of the nozzle.

[FIGURE 6 OMITTED]

The field components of Model 0 are shown in Fig. 7. The Ez-component was dominant and had its maximum near the edge of the plate where x- and y-coordinates were 200 mm. The curves of the Ex- and Ey-components were almost equal due to symmetry and they strengthen near the edge of the plate. The negative values indicated that the plate was forcing the effective electric field within the area of the plate.

The Ex- and Ez-components of the basic model in middle area of the observation plane (x,y [less than or equal to] 100 mm) are presented in Fig. 8. Smaller changes in the magnitudes of the components can now be observed since the range of E is also narrower. The Ey-component was not plotted separately due to its similarity with Ex. The positive value of Ex, occurring at the beginning of the observation plane, tried to spread the coating and the negative to narrow it. The pulling Ez-component had a local minimum at the same point (x, y ~ 50 mm) where Ex turns negative.

In Fig. 9 the observation plane was moved from the vicinity of the collector plate toward the nozzle. The field components Ex and Ez are plotted in four different distances (30, 60, 90, and 120 mm). The Ex-component strengthened toward the nozzle, where it had a spreading effect on the conical envelope inside which the jet is traveling. Near the collector plate Ex got negative values and forced the jet toward the centerline. The Ez-component, which pulls the jet toward the collector plate, had highest value in the vicinity of the nozzle, where the stretching of the jet is started. When the distance from the plate was short the local maximum of Ez occurred near the edge of the plate.

Variations to Voltage. The three voltages used in the models with fixed distance of 150 mm were 30 kV (A), 40 kV (0), and 50 kV (B). The increase in the voltage increased the intrinsic values of the field components whether they were positive or negative, without shifting the zero. The Ex-component is presented as an example in Fig. 10. The values of the Ex-component were directly and quite linearly proportional to the voltage.

[FIGURE 7 OMITTED]

Variations in Distance. The three distances used in the models with fixed voltage of 40 kV were 100 mm (C), 150 mm (0), and 200 mm (D). Ex- and Ez-components with different distances are presented in Fig. 11. The overall field strength and therefore, also the magnitude of the field components decreased with the increased distance. The magnitude of the Ex-curve changed and its zero shifted with altering distances. The shape of the Ez-curve changed when the distance increased: the local maximum in the midpoint disappeared.

[FIGURE 8 OMITTED]

Variations in Voltage and Distance. The average strength of the electric field is a function of voltage and working distance, so it can be altered by changing one or both of these variables. The three cases for comparison were 40 kV/100 mm (C), 40 kV/200 mm (D), and 80 kV/200 mm (E). In Model C, the field strength was 400 kV/m. In Model D, the voltage was kept constant and the distance doubled to the yielding field strength of 200 kV/m. In Model E, the voltage was doubled and distance kept the same yielding the field strength of 400 kV/m just like in Model C.

[FIGURE 9 OMITTED]

Figure 12 presents the Ex- and Ez-components of these three cases. All Ex-curves had different shapes. In the vicinity of midpoint of the magnitude the Ex-component was dominated by the distance and the curves of D and E samples were similar. On the other hand, nearer to the edge of the plate Ex was modinated by voltage, and the curves of C are D were getting closer to each other. Case C with a shorter distance than Cases D and E had a different shaped Ez-curve, it had local maximum in the midpoint. The magnitude of the Ez-component was more dependent on the voltage than the strength of the electric field.

COMPARISON OF TRIALS AND MODEL

Basic Arrangement and Model

The diameter of the deposition pattern of the basic arrangement (0) was 225 mm (x, y = 90 mm). Within this area the Ez-component, which is pulling the jet, had the small local maximum near the midpoint of the plate and it again increased dramatically when x and y had values over 60 mm (Fig. 8). The mean values of the fiber diameters (Table 3) seemed to be slightly affected by the pulling force, though this observation may not be entirely valid due to the quite high deviation of the diameters. The fiber diameter was below 700 nm in the middle point and again when x and y coordinates were 80 mm.

[FIGURE 10 OMITTED]

The amount of fibers was quite stabile until near the edge of the deposition pattern indicating evenness of the coating. The evenness was probably affected by both the spreading (Ex and Ey) and pulling (Ez) field components. The curves of all the components were quite homogenous near the plate, they altered more if the observation plane was moved toward the nozzle (Fig. 9).

Variations to Voltage

The three voltages used in the models were 30, 40, and 50 kV and codes of corresponding trials were A, 0, and B, respectively. The diameters of electrospun fibers in these trials were 800 [+ or -] 330 nm, 710 [+ or -] 400 nm, 720 [+ or -] 400 nm, and the diameters of the deposition patterns 170, 255, and 283 mm. The trial with the lowest voltage and the weakest electric field strength (A) is distinguished from the others by means of larger fiber diameter, circle shape, and the smallest deposition pattern. The magnitudes of the field component increased linearly with increasing voltage in models (see Fig. 10) due to linearity of the problem. Higher voltage decreased fiber diameter, evened up and enlarged deposition pattern, but this dependence, however, did not appear to be linear. The difference between depositions of Trials 0 and B were not as clear as the difference between A and 0.

[FIGURE 11 OMITTED]

Variations to Distance

The three distances used in the models were 100, 150, and 200 mm, and the codes of the corresponding trials were C, 0, and D, respectively. The diameters of the electrospun fibers in these trials were 570 [+ or -] 330 nm, 710 [+ or -] 400 nm, 740 [+ or -] 400 nm, and the diameters of deposition patterns 200, 255, and 200 mm. The Ez-component had a local maximum at the midpoint of the plate with the shortest distance (see Fig. 11). This kind of rise in the Ez-component near to the midpoint of the plate could cause a thicker deposition, and smaller diameters of the fibers in the middle areas of the substrate were observed as a bull's eye shaped deposition pattern. The trial with shortest distance (C) yielded finer fibers than Trials 0 and D as expected.

[FIGURE 12 OMITTED]

The predicted bull's eye pattern was not observed in the 100-mm distance Trial C as can be seen in Fig. 5. The coating thickness did not seem to be much thicker in midpoint, but it must also be remembered that the viscous forces of the jet also affect the shape of the deposition pattern. Still, the fiber diameter 540 [+ or -] 320 nm at the midpoint was smaller than 640 [+ or -] 350 nm near the edge of the deposition pattern, see Table 3. Similar phenomenon was also observed with the other two distances (0 and D), even though the rise of the Ez-curve was not so pronounced in those trials. The correlation between distance and size of the deposition pattern cannot be seen, because the strength of electric field was also changed.

Variations in Voltage and Distance

The effect of the strength of the electric field was studied by comparing three Trials C (40 kV/100 mm), D (40 kV/200 mm), and E (80 kV/200 mm). The mean fiber diameters of the trials were 570 [+ or -] 330 nm, 740 [+ or -] 400 nm, and 570 [+ or -] 310 nm, respectively. The higher electric field strength (400 kV/m) of Trials C and E yielded clearly finer fiber diameters than the lower field strength (200 kV/m) of Trial D. The diameter was 570 nm in both cases regardless of the difference in field component magnitudes in the model. The difference is also seen in the shape of the deposition pattern. The higher electric field yielded round and more even deposition patterns than the lower field, which yielded a circular pattern. This cannot be explained only by field components, because the shapes of the Ez-curves of the two cases (C and E) having higher field strengths are different (Fig. 12). On the other hand, the size of the pattern seems to be more dependent on voltage than on distance, because the patterns of C and D had the same diameter (200 mm), while pattern of E was larger (311 mm). The effect of the distance can be seen when the average electric field strength is fixed and the distance doubled (Trials C and E). The deposition pattern enlarges with the increasing distance.

CONCLUSIONS

Examination of the basic model revealed that the round shape of the nozzle dominates the shape of the electric field over the half way toward the collector, and square shape of the collector does not affect the shape of deposition pattern. The Ez-component was the most dominant of the field components. Shape of the curves of field components changed depending on the distance from nozzle and plate. The curves may have local maximums at either end of the observation line: at the midpoint or near the edge of the observation plane. These variations in magnitudes of field components explain the fiber quantity variation in the circular regions repeatedly observed in electro-spinning trials. It seems that variations might also alter the fiber diameters in different parts of the coating. In the vicinity of the nozzle the components were increased which widens the path of the jet away from the centerline (Ex and Ey) and pulls it toward the collector plate (Ez). This can also be seen when the effective area of the electric field enlarges toward the collector.

Increasing voltage increased the intrinsic values of field components, which enlarged as well as evened up the deposition pattern and decreased the fiber diameter. It did not have any effect on the shape of the electric field. The increase of the magnitude of the components in the model was linear due to the linearity of the model, but this linearity was not observed in experiments such as the size of the deposition pattern. Increasing distance decreased the values of field components and also changed the shape of the electric field. The size of the deposition pattern remained the same, which indicates that changes in electric field have changed the angle of the conical envelope of the jet. The fiber diameter was increased due to decreasing electric field.

The distance between the electrodes determined the overall nature of the electric field, because the change in distance also changed the shape of the electric field. Therefore, the change in electric field caused by altered distance cannot be compensated by altering the voltage. It was noticed that it is advantageous to operate with long distances when targeting even coating because of the more homogenous electric field. The small distance caused the Ez-component to rise near the midpoint of the deposition pattern that can explain the bull's eye shape sometimes observed in electrospinning trials. Long distance also increased the diameter of the deposition pattern, which may even up the deposition with multiple nozzles. On the other hand, the smallest fiber diameters were observed with the most intensive electric fields, which are more easily achieved with short distances.

The model did not take into account, for example, the viscous forces of the jet which affect the electrospinning process, and therefore it is understandable that the model could not explain all the observations made in these trials. Even though the model gave valuable information about the shape of the field and the phenomena of the electrospinning process, the presumptions based on the model did not always proved quite as expected.

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Pirjo Heikkila, (1) Lasse Soderlund, (2) Jari Uusimaki, (2) Lauri Kettunen, (2) Ali Harlin (1)

(1) Institute of Fiber Materials Science, Tampere University of Technology, FIN 33101, Tampere, Finland

(2) Institute of Electromagnetics, Tampere University of Technology, FIN 33101, Tampere, Finland

Correspondence to: Pirjo Heikkila; e-mail: pirjo.heikkila@tut.fi
TABLE 1. Electrospinning trial parameters.

 Spinning parameters
Trial code Voltage (kV) Distance (mm) Average field (kV/m)

0 40 150 267
A 30 150 200
B 50 150 333
C 40 100 400
D 40 200 200
E 80 200 400

TABLE 2. Electrospinning trials: The size and shape of the deposition
pattern and fiber diameters.

 Deposition pattern
 x and y Fibers
 coordinates Diameter
Trial of edge (a) Diameter and its
code (mm) (mm) Shape SD (nm)

0 90 ~255 Round, even 710 [+ or -] 400
A 60 ~170 Circle 800 [+ or -] 330
B 100 ~283 Round, even 720 [+ or -] 400
C 70 ~200 Round, even 570 [+ or -] 330
D 70 ~200 Circle 740 [+ or -] 400
E 110 ~311 Round, even 570 [+ or -] 310

(a) Edge of deposition pattern in observation line. See Fig. 3 for
details.

TABLE 3. Fiber diameters and amounts in different points of observation
line of the electrospinning trial 0 corresponding to the basic model.

 Observation Amount and
Trial point, x and y Diameter its SD
code coordinates (mm) and its SD (nm) (fibers/100 [micro]m)

0 0 660 [+ or -] 380 19 [+ or -] 7
 20 730 [+ or -] 480 20 [+ or -] 3
 40 780 [+ or -] 360 17 [+ or -] 4
 60 700 [+ or -] 390 21 [+ or -] 3
 80 690 [+ or -] 350 11 [+ or -] 1
C 0 540 [+ or -] 320
 60 640 [+ or -] 350
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Author:Heikkila, Pirjo; Soderlund, Lasse; Uusimaki, Jari; Kettunen, Lauri; Harlin, Ali
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:1USA
Date:Dec 1, 2007
Words:6265
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