Polymer 45 (2004) 7597–7603
Polymer 45 (2004) 7597–7603
Melt-electrospinning part I:
processing parameters and geometric properties
Jason Lyons*, Christopher Li, Frank Ko
Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA
Received 18 August 2004; received in revised form 30 August 2004; accepted 31 August 2004
Available online 11 September 2004
The effects of various melt-electrospinning parameters on the morphology and ﬁber diameter of polypropylene of different tacticities were
studied. The effect of the electric ﬁeld strength at various melt ﬂow indexes of polypropylene on ﬁber uniformity, morphology, and diameter
was measured. It was shown that the molecular weight was the predominant factor in determining the ﬁber diameter of the collected ﬁbers.
Observations prove that the tacticity also inﬂuences the ﬁber diameter. Atactic polymers having molecular chains incapable of crystallization
tend to produce larger diameter ﬁbers than isotactic polymers capable of crystallization even at lower molecular weights. The polymer
volume, at a given time, supplied to the electric ﬁeld affected ﬁber diameter. Those systems supplying the smallest volume, at a given time,
produced the smallest ﬁber diameter.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: Polypropylene; Melt electrospinning; Tacticity
1. Introduction of the polymers being electrospun may leave remnants that
are not compatible within the industry. In the intent of
The utilization of electrostatic forces to deform materials cleaner processing, environmental safety, and productivity,
in the liquid state goes back many centuries . Throughout there is a persistent desire to produce ﬁbers by alternative
the 20th century, there have been a number of papers methods. Thus, in spite of the many potential applications,
dedicated to the study of electrohydrodynamic atomization environmental and health limitations, as well as pro-
[2–8]. Electrospinning is simply an extension of this ductivity complications do exist as a result of solvent
technology applied to higher viscosity ﬂuids. Several based electrospinning systems. The use of molten polymers
researchers [9–16] performed experiments using polymeric to produce electrospun mats becomes a subject of great
solutions and were capable of producing ﬁbers ranging in interest. In spite of the potential beneﬁts of melt-electro-
diameter from a few nanometers to several micrometers. spinning, little progress has been made in the past twenty
Most of these ﬁbers were being collected as nonwoven, years. Larrondo and Manley [19–21] were the ﬁrst to
random ﬁber mats. These ﬁbrous structures can potentially electrospin a molten polymer more than two decades ago.
be used in a variety of applications including ﬁltration They were capable of spinning polypropylene (melt ﬂow
devices, solar sails, reinforcement, nonwetting textile indexes 0.5–2.0) and succeeded in making ﬁbers that were
surfaces, wound dressings, vascular grafts, and tissue greater than 50 mm in diameter. Their inability to spin sub-
scaffolds [16–18]. However, a vast majority of the ﬁbrous micrometer diameter ﬁber was attributed to the large
structures were produced by solvent based electrospinning. increase in viscosity that could be many orders of magnitude
Certain chemicals that are used as solvents to dissolve many greater than that of a polymer solution. The electric ﬁeld
strength used in their experiments was 3–8 kV cmK1 at a
* Corresponding author. Tel.: C1-570-650-4282; fax: C1-215-895-
spinning distance ranging from 1–3 cm. They observed that
6760. the polymer melt experienced a large initial decrease in
E-mail address: [email protected] (J. Lyons). diameter when placed in an electric ﬁeld and as the electric
0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
7598 J. Lyons et al. / Polymer 45 (2004) 7597–7603
ﬁeld strength increases, the ﬁber diameter decreases. Other  or the small distance that the jet traverses before
groups [22,23], and the University of Massachusetts at contacting the collection device. The solidiﬁed ﬁbers are
Dartmouth, have conducted research on melt-electrospin- deposited randomly on the surface of the grounded
ning polymers including poly(ethylene terephthalate) and collection plate. It has been shown that ﬁber diameter can
polyethylene. These groups reported a wide range of be controlled by adjusting the processing parameters such as
obtainable ﬁber diameters yet limited progress has been electric ﬁeld strength, polymeric viscosity, and ﬂow rate
made. A full understanding of the melt-electrospinning [11–14].
process, and its potential to replace solution electrospinning,
has not yet been realized. It is the object of this study to 1.2. Melt-electrospinning requirements
determine the feasibility of producing electrospun ﬁbers, of
varying ﬁber diameters and morphologies, from the melt In conventional textile ﬁber formation from the melt,
and recognize trends revolving around the molecular weight small diameter ﬁbers are made through simultaneous
of the polymer, the electric ﬁeld strength, and the polymer’s control of the spinnerette diameter and the take up speed
tacticity. of the godet rollers. When the ﬁber passes through the godet
rollers, each rotating at a different speed, a shearing action
1.1. The electrospinning process (drawing) occurs on the molecular chains, thus inducing
molecular orientation while decreasing ﬁber diameter as
Electrospinning is a simple technique for the production much as 500%. In order to successfully produce nano or
of nano to micro scale ﬁbers depending on the medium used. sub-micrometer diameter ﬁbers through melt-electrospin-
The use of electrospinning to produce ﬁbers from solution, ning, drawing of the polymer must occur as a result of the
without using pressure, was ﬁrst reported in a patent issued electrostatic forces acting on the jet. The forces needed to
in 1934 by A. Formhals . This technique incorporates create a reduction in diameter to the nanometer level are of
the generation of a strong electric ﬁeld between the great interest and are currently being investigated.
polymeric melt within the extruder and a metallic collecting
device. Fig. 1 shows a schematic design of a melt-
electrospinning system. As a voltage is applied, a cone 2. Experimental
forms at the apex of the capillary or spinnerette. At a critical
voltage, the electrostatic forces acting on the jet overcome 2.1. Fiber spinning experiments
the surface tension and viscoelastic properties of the melt
resulting in a ﬁne ﬁber extruded from the cone residing at The materials utilized during these experiments can be
the spinnerette. Similar to solution based electrospinning, seen in Table 1. All samples were used as received and they
the main driving force for ﬁber formation is the attenuation were purchased from Sigma-Aldrich. Polypropylene was
of the spin line under electrostatic forces. While in transit, chosen because of its relative ease to process from the melt
the jet diameter is continually reduced due to the and it large range of available molecular weights ranging
electrostatic forces acting on it until the point when the from the 10s of thousands to the 100s of thousands. In
viscosity once again overcomes the electrostatic forces as a addition, polypropylene is available in different tacticities.
result of jet solidiﬁcation from cooling. Unlike similar By electrospinning different tacticities of polypropylene, the
experiments conducted by other researchers in melt- effect of molecular conformation on ﬁber diameter can be
electrospinning [23,24], these particular experiments did examined.
not exhibit a bending instability. The lack of a bending The polymers were processed through a 3/4 00 single
instability may be attributed to the extremely high viscosity screw Brabender table-top extruder with four heating zones
associated with the melt as has been demonstrated by Taylor at 200 8C and a 1.5 mm spinnerette. In these experiments the
spinnerette was grounded and the positive charge was
applied to a copper collection plate that was placed at
varying distances ranging from 2–5 cm. At greater dis-
tances, a much higher potential will be required. The electric
List of polymers
Polymer Mw Mn
Isotactic polypropylene 580,000 165,700
Isotactic polypropylene 190,000 50,000
Isotactic polypropylene 106,000 21,000
Isotactic polypropylene 12,000 5000
Atactic polypropylene 19,600 5400
Atactic polypropylene 14,000 3700
Fig. 1. A schematic diagram describing the melt-electrospinning technique.
J. Lyons et al. / Polymer 45 (2004) 7597–7603 7599
ﬁeld strength, expressed in terms of voltage/centimeter, 3. Results
required to extrude a jet from the cone ranged from 6–
15 kV cmK1. Higher applied voltages, at short distances,
3.1. Effect of molecular weight on ﬁber diameter
will result in electrical discharge between the spinnerette
and the collection plate.
It was observed that the molecular weight had a
signiﬁcant impact on the feasibility of producing ﬁbers
2.2. Characterization electrostatically at various electric ﬁeld strengths. With a
sufﬁciently high molecular weight, weaker electric ﬁeld
The morphology of the electrospun ﬁbers was examined strengths (O10 kV cmK1) were incapable of producing
through ﬁeld emission environmental scanning electron ﬁbers. Fig. 2 shows the surface morphology of the
microscopy (Phillips XL-30 ESEM). A beam strength of polypropylene ﬁbers obtained using an applied electric
15 kV with a spot size of 3 was used to take the ﬁeld 15 kV cmK1 at a constant collection plate distance of
micrographs. The average ﬁber diameter and the respective 2 cm. Polypropylene with a high molecular weight did not
distributions were determined from 100 measurements of form a Taylor cone when placed in the electric ﬁeld. A ﬁber
random ﬁbers at each spinning condition. nearly the width of the spinnerette hole was slowly pulled to
Fig. 2. The morphology and ﬁber diameter distribution of polypropylene ﬁbers at an electric ﬁeld strength of 15 kV cmK1 at a collection plate distance of 2 cm.
The ﬁgure also shows the average, standard deviation, maximum, and minimum values of the ﬁber diameter. All values are reported in micrometers.
7600 J. Lyons et al. / Polymer 45 (2004) 7597–7603
the collection plate. It was seen that the highest molecular ﬁber diameter ditribution of the 12,000 Mw isotactic
weight polymers formed the largest diameter ﬁbers. It was polypropylene. Fibers that were smaller than 1 mm have
also observed in all experiments, that a wide variation of been obtained, however, a majority of the ﬁbers are above
diameter was present. In some instances there were standard 1 mm. The kurtosis of these graphs show a possible bi-modal
deviations upwards of 50%. An example of an average ﬁber distribution that may be attributed to the variation of
distribution is shown in Fig. 3. This distribution shows the polymer volumes being supplied to the spinnerette as a
result of inconsistent ﬂow rates as such low RPM’s of the
3.2. Effect of electric ﬁeld strength on morphology and
As a result of the high viscosity of the polymeric melts, it
was required to work at considerably large electric ﬁeld
strengths. Each molecular weight polypropylene was
electrospun at an electric ﬁeld strength of 10, 12.5, and
15 kV cmK1 at a spinning distance of 2 cm. This ﬁeld
strength is upwards of 10 times stronger than those reported
in solution electrospinning. Weaker ﬁeld strengths were not
strong enough to overcome the surface tension and
viscoelastic forces of the molten polymer; at higher voltages
at this distance, electrical discharge would occur. As
expected, it was seen in all samples that the ﬁber diameter
decreased as the electric ﬁeld strength increases. Fig. 4
shows the relationship between electric ﬁeld strength and
ﬁber diameter for several of the electrospun polymers. In
this ﬁgure, the effects of molecular weigh and tacticity can
also be seen.
3.3. Effect of polymer volume at the spinnerette tip
In order to supply the appropriate amount of polymer to
the spinnerette, it was necessary to have the extruder at the
lowest RPM output. At times, the Brabender extruder
supplied polymer to the spinnerette faster than the
electrostatic forces could carry it away. As a result, it
became convenient to place the polymer directly on the
spinnerette to melt. In this experiment, the extruder was no
longer supplying a continuous volume at a given time to the
spinnerette. Thus, it was observed that the Taylor cone
continually decreased in size due to the reduction in
available polymer and smaller and smaller ﬁbers were
produced from the diminishing cone as seen in Fig. 5.
4.1. Molecular weight
From the results, it is evident that the molecular weight
plays a signiﬁcant role in the feasibility of electrospinning
polymeric ﬁbers. This ﬁnding is comparable and consistent
with past research claiming that solution concentration is
Fig. 3. The ﬁber diameter distribution of a 12,000 Mw polypropylene at
2 cm. (A) 10 kV cmK1, kurtosisZ1.22, (B) 12.5 kV cmK1, kurtosisZ3.62, the most dominant parameter in electrostatic spinning .
(C) 15 kV cmK1, kurtosisZ1.12. The graphs show a possible bi-modal It was observed that the 580,000 Mw polypropylene resulted
distribution. in ﬁber diameters in access of 400 mm. In addition, it was
J. Lyons et al. / Polymer 45 (2004) 7597–7603 7601
Fig. 4. (A) Chart showing the effect of the electric ﬁeld strength on collected ﬁber diameter for selected electrospun polymers. The 190,000 and 580,000
molecular weight polymers were omitted due to graph distortion when inserted. (B) Effect of tacticity for electrospun polypropylenes. (C) Effect of molecular
weight for atactic polypropylene. (D) Effect of molecular weight for isotactic polypropylene (error bars represent the 190,000 Mw sample. Error bars for the
other samples can be seen in chart A).
observed that the 165,700 Mn isotactic polypropylene and result in larger ﬁber diameters than those obtained from
the 5400 Mn atactic polypropylene produced ﬁber diameters similar molecular weight polymers capable of crystal-
larger than similar tacticity polymers with smaller Mn lization. Therefore, the tacticity of the polymer has a
values. The polymer with the largest Mn will be subjected to signiﬁcant effect on the ﬁber diameter. It is also possible that
the highest degree of polymer chain entanglement. It is variations in ﬁber diameter between polymers of different
therefore more difﬁcult for the electrostatic forces to pull on tacticities are related to the memory effect of the polymer.
individual polymer chains. As a result, larger ﬁber diameter
will be formed, as was observed experimentally. As the 4.2. Electric ﬁeld strength
molecular weight continually decreases, ﬁber diameter
decreases as shown in Fig. 6 for isotactic polypropylene. Consistent with past electrospinning research [10–12], an
It is seen that the molecular weight has an exponential effect increase in the electric ﬁeld strength decreases the average
on the ﬁber diameter for isotactic polypropylene. There ﬁber diameter for all of the polymers examined. When a
were not enough data points to verify the same trend in the steady amount of polymer is being supplied to the
atactic samples. Fibers were not produced with electric ﬁeld spinnerette, an increase in the electric ﬁeld strength exposes
strengths less than 15 kV cmK1 for the 580,000 Mw isotactic the polymer droplet to larger forces, therefore further
sample. reducing the ﬁber diameter. It was observed that the angle
The molecular weight interaction was not exclusively (from jet axis) of the cone, at the spinnerette oriﬁce,
responsible for determining the collected ﬁber diameter. As increases as the electric ﬁeld strength increases as a result of
seen in Fig. 2, both atactic polypropylene samples formed more material being pulled away; consistent with past
larger diameter ﬁbers than all but the 580,000 Mw isotactic research [10,11].
sample. Since atactic polypropylene possesses a random It is worthy to note that in some instances, while using
positioning of the methyl group off of the main molecular the 12,000 Mw polymer, that sub-micrometer ﬁbers were
backbone, it is impossible to crystallize. The inability of the obtained as seen in Fig. 7. In all instances, these ﬁbers are
polymer chains to closely pack due to steric hindrances may the result of branches from a larger ﬁber within the sample.
7602 J. Lyons et al. / Polymer 45 (2004) 7597–7603
Fig. 7. Sub-micrometer diameter ﬁbers branching from 12,000 Mw isotactic
This is conceivable because at the speeds that the jet is
traveling, it is not completely solidiﬁed once leaving the
spinnerette. As the jet travels further into the electric ﬁeld, it
is exposed to stronger ﬁeld strengths. If the molecular
weight of the polymer is low enough, it is possible that a
side jet can be created from the molten jet leading to smaller
diameter ﬁbers. These ﬁbers did not make up a majority of
the sample but they represent the only ﬁbers to break the
1 mm barrier.
The surface morphology, of a majority of the polymers
Fig. 5. ESEM micrographs of 19,600 Mw melt-electrospun polypropylene at
an electric ﬁeld strength of 10 kV cmK1 at 2 cm. (A) 10 s, (B) 20 s, (C)
electrospun, consists of smooth cylindrical ﬁbers. It is
30 s. believed that the smooth surface is in part due to the partial
solidiﬁcation that occurs as soon as the jet leaves the
spinnerette. Also, there is no solvent evaporation that may
lead to inconsistencies on the ﬁber surface.
4.3. Spinning volume at a given time
In order to study the effect of spinning volume, a polymer
chip was placed directly on the spinnerette oriﬁce and
melted. Since no ﬂow rate was being applied, the volume
would continually reduce as the polymer was transferred to
the collection device in the form of ﬁbers. The diameters of
the collected ﬁber as a function of time can be seen in Fig. 5.
Similar to increasing the electric ﬁeld strength a smaller
Taylor cone was formed as a result of the decreasing
volume. This cone was then exposed to larger ﬁeld strengths
ultimately leading to a decrease in ﬁber diameter. This
observation suggests for the possibility of forming sub-
Fig. 6. Graph showing the exponential increase in ﬁber diameter with micrometer diameters consistently under speciﬁc experi-
increasing molecular weight for isotactic polypropylene. mental parameters for certain polymers from the melt if a
J. Lyons et al. / Polymer 45 (2004) 7597–7603 7603
small enough volume could be supplied to the spinnerette on also are extended to the Koerner Fellowship offered at
a consistent basis. This may also explain the large standard Drexel University for partial assistance in this study. The
deviations that result from the melt-electrospinning process. invaluable assistance of David Von Rohr for ESEM
assistance is very much appreciated.
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