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.

Experimental Results     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 capillary-screen 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).