ARMY RESEARCH LABORATORY

Controlled Deposition and Collection of

Electro-spun Poly(ethylene oxide) Fibers

Joseph M. Deitzel

James D. Kleinmeyer

James K. Hirvonen

Nora C. Beck Tan

Approved for public release; distribution is unlimited.

ARL-TR-2415 MARCH 2001
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i

Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5066

_____________________________________________________________________

ARL-TR-2415 March 2001

_____________________________________________________________________

Controlled Deposition and Collection of

Electro-spun Poly(ethylene oxide) Fibers

Joseph M. Deitzel

James K. Hirvonen

Nora C. Beck Tan

Weapons & Materials Research Directorate, ARL

James D. Kleinmeyer

XioTech, a Seagate Company

_____________________________________________________________________

Approved for public release; distribution is unlimited.

_____________________________________________________________________
ii

_____________________________________________________________________

Abstract

_____________________________________________________________________

Electro-spinning is a process by which sub-micron polymer fibers can

be produced with an electrostatically driven jet of polymer solution

(or polymer melt). Electro-spun fibers are typically collected in the

form of non-woven mats, which are of interest for a variety of

applications, including semi-permeable membranes, filters, composite

reinforcement, and scaffolding used in tissue engineering. A

characteristic feature of the electro-spinning process is the onset of a

chaotic oscillation of the electro-spinning jet. The current work

demonstrates the feasibility of dampening this instability and

controlling the deposition of sub-micron polymer fibers (<300 nm in

diameter) on a substrate through use of an electrostatic lens element

and collection target of opposite polarity. Real-time observations of

the electro-spinning process have been made with high speed, high

magnification imaging techniques. Fiber mats and yarns electro-spun

from polyethylene oxide have been analyzed by wide angle electron

diffraction optical microscopy and environmental electron

microscopy.
iii

ACKNOWLEDGMENTS

The authors would like to thank Professor Dan Marble from Tarleton State

University, Stephensville, Texas, for his insightful comments with respect to ion

beam optics, and Dr. Mike Mcquaide of the U.S. Army Research Laboratory for

advice pertaining to high speed imagining. We would like to thank the American

Society of Engineering Education and the Oak Ridge Institute for Science and

Education, Tennessee, for their support of this work.
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v

Contents

1. Introduction ...................................... 1

2. Experimental...................................... 4

3. Theory........................................... 4

4. Results and Discussion .............................. 6

5. Conclusion ....................................... 12

References ............................................. 15

Distribution List ........................................ 17

Report Documentation Page ............................... 19

Figures

 1. Standard Electro-spinning Apparatus . . . . . . . . . . . . . . . . . . . 2

 2. Field Lines Calculated for a Syringe and Grounded Target Geometry . 3

 3. High Speed Image of Electro-spinning Process. . . . . . . . . . . . . . . 3

 4. Forces Exerted on a Charge Element by Neighboring Charge Elements 6

 5. Multiple Field Electro-spinning Apparatus . . . . . . . . . . . . . . . . 7

 6. Field Lines Calculated for Multiple Field Electro-spinning Geometry . 8

 7. Image of Electro-spinning Jet 4 inches Below Capillary Orifice . . . . . 9

 8. Fibers Electro-spun From 10% Solution Concentration of PEO in

Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

 9. Electron Micrograph of PEO Fibers Electro-spun With Multiple

Field Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

10. Electro-spun Fibers Collected on a Rotating Drum With the Multiple

Field Electro-spinning Method . . . . . . . . . . . . . . . . . . . . . . . . 11

11. 2D WAXD Pattern Obtained From PEO Electro-spun Yarns . . . . . . 12

12. Plot of the Integrated Scattering Intensity as a Function of 2 From

Figure 11b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

13. Powder WAXD Pattern for PEO Powder . . . . . . . . . . . . . . . . . 13
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1

CONTROLLED DEPOSITION AND COLLECTION OF ELECTROSPUN POLY(ETHYLENE OXIDE) FIBERS

1. Introduction

For almost 100 [1] years, it has been known that polymer fibers can be generated

from an electrostatically driven jet of polymer solution (or polymer melt). This

process, known as electro-spinning, has received much attention in the last

decade because of its ability to consistently generate polymer fibers that range

from 5 to 500 nanometers in diameter. Because of the small pore size and high

surface area inherent in electro-spun textiles [2,3,4], these fabrics show promise

for exploitation in soldier protective clothing and filtration applications [5].

Other exciting applications that are being explored include scaffolding for tissue

growth [6], optical and electronic applications [7,8].

The schematic for a typical electro-spinning apparatus is depicted in Figure 1 and

its associated field lines in Figure 2. Polymer solution is forced through a syringe

at a rate of about 0.5 ml/hr, resulting in the formation of a drop of polymer

solution at the tip of the needle. A high voltage (5 to 15 kV) is applied to the

syringe, causing the surface of the drop to distort into the shape of a cone, as

depicted in Figure 1. When a critical voltage is exceeded (typically 5 kV for the

solutions discussed in this report), a jet of solution erupts from the apex of the

cone and is accelerated toward the electrically grounded collection target by the

macroscopic electric field (see Figure 2). As this jet travels through the air, the

solvent evaporates, leaving behind a polymer fiber to be collected on an

electrically grounded target. Figure 3 is an image of the electro-spinning process

obtained through high-speed photography. The image clearly depicts the

random motion of the electro-spinning jet as it travels toward the target.

Baumgarten [9] first depicted the chaotic nature of the electro-spinning jet

motion by using high speed photography. Recent work by Reneker et al. [10]

suggests that this chaotic motion, or “bending instability,” results (at least in

part) from repulsive forces originating from the charged elements within the

electro-spinning jet. Because of the chaotic motion of the electro-spinning jet as it

travels to its target, deposition of electro-spun fibers on a stationary target is

essentially random. This is perfectly acceptable for membrane and filter

applications, which take advantage of the small pore size obtained by the

random morphology of the non-woven electro-spun mat.

Collecting electro-spun fibers in the form of a yarn or “tow” (i.e., bundle of

fibers) for post-processing to improve mechanical performance or depositing of

electro-spun fibers on a substrate in specific places or patterns is problematic

because of the random nature of the fiber deposition. Some efforts to improve

control of the electro-spinning jet and deposition process include the use of both
2

mechanical and electrostatic means. It has been shown that electro-spun fibers

can be collected in a textile where the fibers will be more or less oriented parallel

to the direction of rotation [11] if the target is a drum rotating at high revolutions

per minute. However, the area coated with electro-spun fibers is still quite large.

It has also been shown that electro-spun fibers retain a significant portion of their

charge upon deposition [2,12], and during the appropriate conditions [12], it is

possible to have electro-spun fibers deposit preferentially on an aluminum

screen forming a three-dimensional grid structure.

Figure 1. Standard Electro-spinning Apparatus.

Finally, Jaeger et al. [13] have demonstrated that it is possible to stop the

precession of the electro-spinning jet about the tip of the syringe, which

contributes to the chaotic motion of the electro-spinning jet. They accomplished

this by using a single charged ring of like voltage and polarity placed

concentrically about the syringe needle. Although this method of electrospinning has the effect of stabilizing the jet at the point of initiation, the jet still

undergoes bending instability as it proceeds toward the ground plane after

passing through this single electrode region.

The objective of the current research has been to construct a novel electrospinning apparatus that uses electrostatic fields (other than the one responsible

for jet initiation) to dampen the bending instability inherent in the electrospinning process. This apparatus has allowed for much greater control of the

deposition and collection of electro-spun fibers. Wide angle X-ray diffraction

(WAXD) analysis has been performed on electro-spun poly(ethylene oxide)

(PEO) yarns collected by this apparatus, which indicated the presence of some
3

molecular orientation and a poorly developed crystalline microstructure within

the electro-spun fibers.

Figure 2. Field Lines Calculated for a Syringe and Grounded Target Geometry.

Figure 3. High Speed Image of Electro-spinning Process.
4

2. Experimental

PEO fibers were electro-spun from a 10% (wt) concentration of PEO in water.

PEO of 400,000 molecular weight was used in making the solution. Two ES30P

power supplies of positive polarity from Gamma High Voltage, Inc., were used

to apply voltages of +5 to +15 kV to the vertically oriented syringe tip and rings.

A Glassman Series EH high voltage power supply of negative polarity was used

to apply a voltage of –9 to –12 kV to the collection plate situated at the bottom of

the apparatus. Polymer solution was fed to the syringe needle tip through a

Teflon tube with a 0.125-inch inner diameter via a Harvard 2000 syringe pump.

Representations of electric field lines associated with specific electro-spinning

apparatus were calculated with the computer program, Student’s Quick FieldTM.

Electron micrographs of the electro-spun fibers were obtained with a Phillips

Electroscan environmental scanning electron microscope.

High speed images were taken with a Photometrics cooled charge coupled

device camera with a 3- by 2-k chip. The camera was attached to a Questar

Schmidt-Cassegrain telescope in order to achieve high magnification from a

distance of 40 cm. The electro-spinning jet was illuminated from behind with a

Continuum Surelite II yttrium-aluminum-garnet laser. The laser emitted light at

a wavelength of 532 nm, with a pulse duration of 10 ns. Optical micrographs of

collected fibers were obtained with a Wild Makroscope M420 stereo-microscope.

Yarns of electro-spun fibers used in WAXD experiments were collected by

“combing.” This process involved passing two wooden splints spaced

approximately 2.0 cm apart repeatedly through the electro-spinning jet about 2

cm above the collection target. WAXD analysis was performed with a Rigaku

UltraX 18-kW rotating anode X-ray source and a Bruker Hi-Star 2D area detector.

3. Theory

When an external electric field is applied to a polymer solution, ions in the

solution will aggregate around the electrode of opposite polarity. Positive ions

travel to the negatively charged electrode and negative ions travel toward the

positive electrode. This results in an excess of charge of opposite polarity in the

volume of solution near an electrode. Consider a drop of polymer solution

suspended at the tip of a metal syringe. When a voltage is applied to the metal

syringe, the ions in the solution of like polarity will be forced to aggregate at the

surface of the drop. The electric field generated by the surface charge will cause

the drop to distort into the shape of a cone [14]. If the electric potential of the

surface charge exceeds a critical value [14], the electrostatic forces will overcome
5

the solution surface tension. A thin jet of solution will erupt from the surface of

the cone and travel toward the nearest electrode of opposite polarity or electrical

ground. Although the details of charge motion in the electro-spinning jet are not

well understood, it is believed that excess charge is essentially static with respect

the moving coordinate system of the jet [10]. This means that the electro-spinning

jet can be thought of as string of charge elements connected by a visco-elastic

medium, with one end fixed at the point of origin and the other end free.

As discussed in the introduction, the free end of the electro-spinning jet follows a

chaotic path as it travels toward the grounded collection plate, as seen in

Figure 3. This chaotic motion, or instability, is the result of a complicated

interaction of variables that include viscosity, surface tension, electrostatic forces,

air friction, and gravity. Recent work by Reneker et al. [10] has attempted to

create a model of jet motion that takes these variables into account. This effort

has met with limited success, although detailed understanding of the jet motion

remains elusive. For the present discussion, it is appropriate to focus on the

electrostatic forces acting on the charged elements composing the jet and the

visco-elastic response of the polymer solution to these electrostatic forces.

In their work, Reneker et al. [10] proposed the following mechanism for the onset

of jet instability. Upon initiation, the jet of polymer solution is rapidly accelerated

away from the syringe toward the grounded target by electrostatic forces. This

has the effect of providing a longitudinal stress that stabilizes the jet, keeping it

initially straight. At some distance from the point of initiation, the jet of polymer

solution begins undergoing stress relaxation. The point along the jet where this

occurs depends on the spinning voltage, which is proportional to the strength of

the macroscopic electric field [10]. By increasing the voltage, the electric field

strength increases the length of the stable jet [9]. Once stress relaxation occurs,

Reneker et al. [10] proposed that electrostatic interaction between the charged

elements of the jet begins to dominate the ensuing motion, initiating and

perpetuating the chaotic motion of the jet. This initiation of the bending

instability was described in the following manner.

If one considers three unconnected point charges of equal value in a line (see

Figure 4), it can be seen that the center charge, B, is acted upon by two forces of

equal magnitude and opposite direction given by the equation,

F= kqaqb/r

2

 = kqcqb/r

2

1.

in which qa, qb , and qc are charges of equal sign and magnitude, r is the

separation between charges, and k is Coulomb’s constant. Should a perturbation

cause qb to move out of line, there is a net lateral force on qb, the magnitude of

which is given by the equation

Fl=2Fcos 2.
6

in which is the angle that is formed by r and line perpendicular to jet axis.

Figure 4. Forces Exerted on a Charge Element by Neighboring Charge Elements.

This lateral force, Fl, leads to an inherent instability predicted by Earnshaw’s

theorem [15]. Since an electrostatically driven jet can be thought of as a line of

charged elements, it is thought that the instability predicted by Earnshaw is

responsible for the onset of the chaotic motion of the free end of the electrospinning jet [10]. Because any force, either electrostatic or mechanical in nature,

to counter Fl is absent, the initially small perturbations grow unfettered, leading

to the large looping motions that are commonly observed [2,9,10].

4. Results and Discussion

It is clear from the discussion in the previous section that electrostatic

interactions between individual charge elements in the jet and between charge

elements and the macroscopic electric field are primarily responsible for

initiation and perpetuation of the bending instability. With this knowledge, it

should be possible to design an electro-spinning apparatus that can dampen or

eliminate the bending instability through control of the shape and strength of the

macroscopic electric field that exists as a result of the potential difference
7

between the point of jet initiation and the collection target (see Figures 5 and 6).

The electro-spinning apparatus discussed here is similar to one first described by

Melcher and Warren [16], who were studying capillary instability and jet

disintegration in electrostatically driven jets of low molecular weight fluids.

The apparatus depicted in Figure 5 is different from the setup illustrated in

Figures 1 and 2 in two ways. The first innovation is a series of charged rings used

as an electrostatic “lens” element that changes the shape of the macroscopic

electric field from the point of jet initiation to the collection target. The field lines

converge to a center line above the collection target. When the charged jet passes

through this field, it is forced to the center in a manner that is analogous to a

stream of water that is poured into a funnel. The second difference is that the

collection target has a potential bias whose polarity is opposite that of the lens

element and syringe. This allows for a continuous increase in the electric field

strength and a corresponding increase in downward force on the jet as it

approaches the collection target (i.e., Force = q*E in which E is the field strength

and q is the magnitude of the charge element in Coulombs).

Figure 5. Multiple Field Electro-spinning Apparatus.

Figure 5 is a schematic of the multiple field electro-spinning apparatus. The

apparatus consists of three high voltage power supplies. The first power supply

is positive in polarity and is connected to the syringe. This is the source that

supplies the critical voltage needed for jet initiation, which will be designated as

the spinning voltage for the rest of this discussion. The second power supply

provides the voltage for eight copper rings that are connected in series. It also

provides positive polarity which will be referred to as the ring voltage. The third

power supply is connected to the collection target and it provides negative

polarity that will be referred to as the target voltage. The rings are 10 cm in

diameter and are spaced at 1.9-cm intervals, approximately. The total distance
8

from the tip of the syringe to the collection target is about 20 cm. Values of +9 kV

for the spinning voltage, +4 to +5 kV for the ring voltage, and 11 kV for the target

voltage were usual for a typical experiment.

Figure 6 is a diagram depicting the macroscopic electric field generated by the

apparatus described previously. The arrows indicate the direction of the electric

field lines. The electric field is nearly uniform in direction down the center of the

apparatus. At the top of the apparatus, there is a tendency for the field lines near

the edge of the initiating electrode to point to the rings, which are at a lower

potential. About 8 cm from the initiating electrode, the field lines start to

converge at the center of the apparatus, indicating a net restoring force that acts

on the charge jet. The field strength and corresponding restoring force increase as

the jet approaches the target. Upon starting, there is some jet instability near the

initiating electrode because of the edge effects. As the jet proceeds toward the

target, the instability is dampened under the influence the converging electric

field lines. Since the jet is continuous, the dampening of the instability and

acceleration of the jet down stream act to stabilize the part of the jet near the

initiating electrode.

Figure 6. Field Lines Calculated for Multiple Field Electro-spinning Geometry.
9

Figures 7a and 7b are high-speed images of the electro-spinning jet generated

with a 7% concentration solution of PEO in water with the apparatus described

previously. These images were obtained about 10 cm below the syringe tip (half

way to the target), which is well into the region where the bending instability

normally occurs (see Figure 3). Figure 7a is an image of a straight jet with no

evidence of any bending instability; Figure 7b illustrates the effect of decreasing

the ring voltage from +5 kV to about +2.5 kV. In this case, the jet assumes the

shape of a tight corkscrew as a result of relaxing the constraining electric field

provided by the rings. When the ring voltage is increased again to 5 kV, the jet

straightens.

 

a. Ring Electrode Potential 2.5 kV b. Ring Electrode Potential 5 kV

Figure 7. Image of Electro-spinning Jet 4 inches Below Capillary Orifice.

Figures 8a and 8b are images of fiber mats electro-spun from a 10% concentration

of PEO in water. Figure 8a was produced with the standard electro-spinning

method, without biased lens elements. The distance from tip to target was 17 cm

and the spinning voltage was about +7 kV at the syringe tip, and the fibers were

collected on an aluminum screen that was electrically grounded. The fiber mat in

Figure 8b was obtained with the multiple field electro-spinning apparatus via

positively biased rings orifice and a negatively biased foil collection target.

Comparison of these two shows the significant decrease in the diameter (from

~7 cm in 8a to ~1 cm in 8b) of the area of coverage achieved with the multiple

field electro-spinning apparatus.

This reduction in the area of coverage is the result of dampening the bending

instability. The multiple field technique can also affect the size of the individual

fibers. Figure 9 is an electron micrograph of fibers electro-spun from 10% PEO in

water with the multiple field technique. The average diameter of the fibers is

270 nm, which compares to an average diameter of 400 nm reported for PEO
10

electro-spun from a 10% solution with the standard method of electro-spinning,

which consisted of a positively biased syringe tip (7 kV), a grounded collection

target, and a tip to target distance of about 17 cm [2]. It is thought that the

reduction in the fiber diameter is attributable to the overall change in potential

from syringe tip to target being much greater for the multiple field apparatus

(21 kV) used to collect the sample in the figure [8,9]. The fibers in Figure 9 are

uniform in their overall morphology and lack beads or junctions, which indicates

that most of the solvent has evaporated by the time the fibers are collected on the

target.

a. Standard Electro-spinning Technique Collected b. Multiple Field Technique Collected on

 on Wire Mesh (diameter ~7 cm). Copper Foil (diameter ~1 cm).

Figure 8. Fibers Electro-spun From 10% Solution Concentration of PEO in Water

(arrows indicate area of target covered by electro-spun fibers).

Figure 9. Electron Micrograph of PEO Fibers Electro-spun With Multiple Field

Method. (Fibers were electro-spun from 10% solution concentration of

PEO in water.)
11

When the chaotic motion of the jet is dampened, it becomes possible to deposit

electro-spun fibers on a substrate in a more targeted fashion. When the target is a

rotating drum covered with copper foil and charged to a potential of –11 kV, the

electro-spun fibers are collected in a strip that is approximately 0.6 cm wide.

Figure 10 shows two such strips collected within 0.6 cm of each other. Upon

starting, some slight adjustment of the spinning voltage, ring voltage, and the

solution feed rate are required in order to optimize the control of the deposition

process. The strip on the right side of the image was collected during this

optimization process and is slightly less well defined. After 1.5 hours, the drum

was shifted slightly to the right and the second well-defined strip was collected

during optimized conditions. The collection time for each strip was 1.5 hours.

Figure 10. Electro-spun Fibers Collected on a Rotating Drum With the Multiple

Field Electro-spinning Method.

It is also possible to collect electro-spun fibers in the form of a yarn with the

multiple field apparatus. This was accomplished by a combing technique, in

which two wooden splints spaced about 1 inch apart were repeatedly passed

between the last ring and the collection plate, through the electro-spinning

stream. This process allows for the collection of fibers that are more or less

macroscopically oriented parallel to the direction of the combing. After 1 hour,

enough electro-spun material was collected for the purpose of WAXD analysis.

The fibers were removed from the splints and gently twisted into the yarns seen

in Figure 11a. Figure 11b is the WAXD pattern obtained for the PEO yarns

depicted in Figure 11a. The pattern shows six diffraction arcs that are

characteristic of the monoclinic crystal structure of PEO. The two equatorial

reflections correspond to the 120 crystallographic planes, and the four arcs in the

quadrants correspond to the 112 planes. The particularly intense ring that occurs

at 2 = 28.44° corresponds to the 111 reflections of the silicon powder standard.

Figure 12 is a plot of the integrated intensity of the 2D WAXD pattern in

Figure 11b as a function of 2 . When this figure is compared to a powder WAXD

diffraction pattern of PEO (see Figure 13), it can be seen that the 120 and 112
12

reflections of the fiber pattern are relatively broad and weak with respect to the

background. In addition, the powder WAXD pattern of the PEO also contains

numerous higher order reflections that are not present in the fiber pattern. The

broad, weak nature of the fiber pattern reflections, together with the lack of

higher order reflections, indicates that the crystalline microstructure of the

electro-spun fibers is not well developed. The fact that the fiber pattern

reflections (see Figure 11b) show distinct arcs rather than isotropic circles like

those reported for electro-spun polyethylene by Larrondo et al. [17] suggests that

some degree of molecular orientation results from this multiple field electrospinning process. Note that no attempts were made to improve the overall

crystallinity or the crystal orientation through post-process, although it may be

possible to improve the degree of order in the multiple field-spun fibers with

standard techniques, such as annealing under tension.

a. b.

Figure 11. 2D WAXD Pattern Obtained From PEO Electro-spun Yarns.

5. Conclusion

It has been demonstrated that it is possible to control the deposition of electrospun fibers through the use of an electrostatic lens element and biased collection

target. By the application of a secondary external field of the same polarity as the

surface charge on the jet, it is possible to control or eliminate the bending

instability inherent in conventional electro-spinning experiments. This

mechanism allows for greater control of the deposition of electro-spun fiber on a

surface and for collection of electro-spun fibers in other forms besides nonwoven mats. Analyses of yarns of electro-spun polyethylene oxide via WAXD
13

techniques indicate the presence of some molecular orientation and a poorly

developed crystalline microstructure in the fibers.

Figure 12. Plot of the Integrated Scattering Intensity as a Function of 2 From

Figure 11b.

Figure 13. Powder WAXD Pattern for PEO Powder.
14

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15

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STONY BROOK NY 11794-3400

1 PROF GREG RUTLEDGE

DEPT OF CHEMICAL ENG

MASS INST OF TECHNOLOGY

CAMBRIDGE MA 02139-4307

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18

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1 GOODYEAR INST OF POLYMER SCI

THE UNIV OF AKRON

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19

Deitzel, J.M.; Hirvonen, J.K.; Beck Tan, N.C. (all of ARL); Kleinmeyer, J.D. (XioTech)

electro-spinning

nano-fibers

25

Controlled Deposition and Collection of Electro-spun Poly(ethylene oxide) Fibers

Approved for public release; distribution is unlimited. 

Electro-spinning is a process by which sub-micron polymer fibers can be produced with an electrostatically driven jet of

polymer solution (or polymer melt). Electro-spun fibers are typically collected in the form of non-woven mats, which are of

interest for a variety of applications, including semi-permeable membranes, filters, composite reinforcement, and scaffolding

used in tissue engineering. A characteristic feature of the electro-spinning process is the onset of a chaotic oscillation of the

electro-spinning jet. The current work demonstrates the feasibility of dampening this instability and controlling the deposition of

sub-micron polymer fibers (<300 nm in diameter) on a substrate through use of an electrostatic lens element and collection

target of opposite polarity. Real-time observations of the electro-spinning process have been made with high speed, high

magnification imaging techniques. Fiber mats and yarns electro-spun from polyethylene oxide have been analyzed by wide

angle electron diffraction optical microscopy and environmental electron microscopy.