Electrospinning
Introduction
In 1934, a process was patented by Formhals [1-3], wherein an experimental setup
was outlined for the production of polymer filaments using electrostatic force. When used
to spin fibers this way, the process is termed as electrospinning.
In the electrospinning process a high voltage is used to create an electrically
charged jet of polymer solution or melt, which dries or solidifies to leave a polymer fiber
[4, 5]. One electrode is placed into the spinning solution/melt and the other attached to a
collector. Electric field is subjected to the end of a capillary tube that contains the polymer
fluid held by its surface tension. This induces a charge on the surface of the liquid. Mutual
charge repulsion causes a force directly opposite to the surface tension [6]. As the intensity
of the electric field is increased, the hemispherical surface of the fluid at the tip of the
capillary tube elongates to form a conical shape known as the Taylor cone [7]. With
increasing field, a critical value is attained when the repulsive electrostatic force
overcomes the surface tension and a charged jet of fluid is ejected from the tip of the
Taylor cone. The discharged polymer solution jet undergoes a whipping process [8]
wherein the solvent evaporates, leaving behind a charged polymer fiber, which lays itself
randomly on a grounded collecting metal screen. In the case of the melt the discharged jet
solidifies when it travels in the air and is collected on the grounded metal screen.
Apparatus
An example of one experimental set up used for electrospinning is shown in Fig. 1.
The polymer solution or melt is contained in a glass tube, usually a pipette that is
connected to a syringe like apparatus. A metering pump attached to the plunger of the
syringe generates a constant pressure and flow of the fluid through the pipette. The driving
force is provided by a high voltage source through a wire immersed in the solution. The
high voltage source can generate up to 30 kV, and the setup can be run on either positive or
negative polarity. Adjusting the flow of the fluid and the magnitude of the electric field
controls the spinning rate.
Process Parameters
The above description of the process suggests that the following parameters affect
the process:
System Parameters:
• Molecular Weight, Molecular-Weight Distribution and Architecture (branched, linear
etc.) of the polymer
• Solution properties (viscosity, conductivity & and surface tension)
Process Parameters:
• Electric potential, Flow rate & Concentration
• Distance between the capillary and collection screen
• Ambient parameters (temperature, humidity and air velocity in the chamber)
• Motion of target screen
Applications
An important characteristic of electrospinning is the ability to make fibers with
diameters in the range of manometers to a few microns. Consequently these fibers have a
large surface area per unit mass so that nanowoven fabrics of these nanofibers collected on
a screen can be used for example, for filtration of submicron particles in separation
industries and biomedical applications [9], such as wound dressing in medical industry,
tissue engineering scaffolds and artificial blood vessels. The use of electrospun fibers at
critical places in advanced composites to improve crack resistance is also promising.
Preliminary experimental work
To investigate the effects of some of the key process parameters, we conducted
electrospinning at different conditions. The parameters studied were concentration,
capillary-screen distance (C-SD), electric potential at the tip and flow rate. Different
concentrations (15-25 wt %) of Estane® 5750 (a segmented polyether urethane supplied by
Goodrich) were prepared in DMAc (N, N-dimethylene acetamide) that were electrospun at
different voltages (10-15 kV), capillary-screen distances (12-27 cms) and flow rates (3-7
ml/h). The electrospun samples were collected on a stainless steel mesh (count 20x20) and
later sputtered-coated with a 5 nm Pt layer. FESEM (Field Emission Scanning Electron
Microscopy) was performed on these samples.
Fig. 2 shows the effect of capillary-screen distance (C-SD) on 15-wt % Estane (in
DMAc) that were electrospun at 10kV and 3ml/h. Distinct bead like structures can be seen
in the FESEM micrographs. It can also be seen that with increasing capillary-screen
distance (C-SD), the average fiber diameter decreases. This is true for fibers spun at
different concentrations, electric potentials and flow rates. Fig. 3 shows the FESEM
pictures of 20-wt% Estane (in DMAc) that were electrospun at 10 kV and 3 ml/h.
Comparing Figures 2 & 3, it can be inferred that the bead like structure is predominant at
lower concentrations. With increasing concentration of the polymer solution (keeping other
parameters constant) the average fiber diameter increases. Again, with increasing capillaryscreen distance (C-SD) the average fiber diameter decreases. Similar trends were observed
at higher concentrations. At lower capillary-screen distances, blobs are observed (12 cm,
20 wt% Estane, Figure 3) that should not be mistaken as beads. Fig. 4 shows FESEM
micrographs of 25-wt % Estane (in DMAc) that were electrospun at 10 kV & 3ml/h. A
noticeable feature that shows up is the marked ‘Fracture points’. This results due the
solution being too viscous that it resists any deformation during the whipping process and
breaks, instead of undergoing drawing. These ‘Fracture points’ were also observed in
fibers electrospun from 20-wt% Estane (in DMAc) at higher flow rates (7 ml/h). At high
concentrations (25 wt %) and flow rates (7 ml/h) a broad distribution of fiber diameter was
observed. With increasing electric potential (keeping other parameters constant), the fiber
diameter decreases as is shown in Fig. 5. With increasing electric potential, the polymer jet
is discharged with a greater electrostatic repulsion that causes it to undergo higher levels of
drawing stress. This results in the decrease of the fiber diameter. However, at higher
electric potential (~ 15 kV) the fiber diameter distribution becomes increasing broader.
Hence the control of the process at high electric potential, flow rate & concentration
becomes increasingly difficult.
Fig. 6 summarizes the effects of the different process parameters. To obtain a
uniform fiber diameter web, it can be suggested that the electrospinning of Estane® 5750
be conducted at lower electric potential (~10kV), lower flow rates (~ 3ml/h) and
concentration around 20 wt%.
References
1. Formhals, A., US Patent, 1,975,504 (1934).
2. Formhals, A., US Patent, 2,160,962 (1939).
3. Formhals, A., US Patent, 2,187, 306 (1940).
4. Kim, J-S. & Reneker, D. H., Polymer Eng. & Sci., May, 39(5), 849 (1999).
5. Fang, X. & Reneker, D. H., J. Macromol. Sci-Phys., B36(2), 169 (1997).
6. Doshi, J & Reneker, D.H., J. of Electrostat., 35, 151 (1995).
7. Taylor, G. I., Proc. Roy. Soc. London, A313, 453 (1969).
8. Shin, Y.M., Hohman, M. M., Brenner, M. P. & Rutledge, G. C., Polymer, 42, 9955
(2001)
9. Stitzel, J. D., Bowlin, G. L., Mansfield, K., Wnek, G. E. & Simpson, D. G., Proc. 32nd
SAMPE Meeting, Boston, Nov (2000).
Figure 1. Schematic of the Electrospinning set up.
Increasing capillary-screen distance
12 cm
24 cm
beads
bead
Figure 2. Effect of increasing capillary-screen distance on 15 wt % Estane® 5750 electrospun at 10 kV & 3ml/h. Average diameter
range ~ 1 m - 148 nm & bead size ~ 10 m - 2.5 mm. The average diameter of fibers & bead-size decreases with increasing capillaryscreen distance.
Increasing capillary-screen distance
12 cm
24 cm
blobs
Uniform web
Straight fibers
Figure 3. Effect of increasing capillary-screen distance on 20 wt % Estane® 5750 electrospun at 10 kV & 3ml/h. Average diameter
range 5 m – 333 nm. The average diameter of fibers decreases with increasing capillary-screen distance.
Increasing capillary-screen distance
12 cm
24 cm
blobs
Figure 4. Effect of increasing capillary-screen distance on 25-wt % Estane® 5750 electrospun at 10 kV & 3ml/h. The Average
diameter range is 5 m – 905 nm. A broad distribution of fiber diameters was obtained.
Increasing Electric Potential
12 kV
14 kV
blobs
Diameter ~ 4 m – 189 nm
Diameter ~ 2.5 m – 150 nm
Figure 5. Effect of increasing electric potential on 20-wt % Estane® 5750 electrospun at 3ml/h & 15 cm capillary-screen distance.
Fiber diameter decreases with increasing voltage
At constant electric potential,
flow rate & concentration
At constant flow rate, screen
distance & concentration
Fiber
Fiber
dia.
dia.
12
2
2
Screen distance,
0 cm
10
7
12
14
Electric potential, kV
At constant electric potential,
screen distance & concentration
At constant electric potential,
screen distance & flow rate
Fiber
Fiber
dia.
dia.
3
5
Flow rate, ml/h
7
15
20
Concentration, wt %
Figure 6. Effect of process parameters on fiber diameter, produced by Electrospinning.
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