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Review
Electrospinning of polymer nanofibers: Effects on oriented morphology,
structures and tensile properties
Avinash Baji a, Yiu-Wing Mai a,b,*, Shing-Chung Wong c, Mojtaba Abtahi a, Pei Chen c
a Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australiab Department of Mechanical Engineering, Hong Kong Polytechnic University, Kowloon, Hong Kong, Chinac Department of Mechanical Engineering, The University of Akron, Akron, Ohio 44325, USA
a r t i c l e i n f o
Article history:
Received 19 October 2009
Received in revised form 12 January 2010
Accepted 14 January 2010
Available online 20 January 2010
Keywords:
A. Fibers
A. Nano composites
B. Mechanical properties
D. X-ray diffraction (XRD)
E. Electro-spinning
a b s t r a c t
The interest in fabrication of nanofibers using electrospinning method has attracted considerable atten-
tion due to its versatile maneuverability of producing controlled fiber structures, porosity, orientations
and dimensions. Although the process appears to be simple and straightforward, an understanding of
the technique and its influence on the morphology, structural and mechanical properties is still not com-
pletely clear. Recently, the size effect on the mechanical properties was reported for fibers across differ-
ent length scales. Both modulus and strength of poly(e-capro-lactone) (PCL) fibers were found to increase
significantly when the diameter of the fibers was reduced to below $500 nm. In this article, for the first
time, we critically review and evaluate the role of the microstructures on the fiber deformation behavior
and present possible explanations for the enhanced properties of the nanofibers. Our discussions are
focused on the techniques to obtain controlled structures and the mechanisms behind the size effect
in electronspun fibers are given. In-depth understanding of these mechanisms can provide fruitful out-
comes in the development of advanced nanomaterials for devices and miniaturized load-bearing
applications.
2010 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 704
2. Electrospinning theory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 705
3. Control of fiber diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706
4. Alignment of fibers and fiber collection methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 706
4.1. Rotating drum collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 707
4.2. Rotating disk collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708
4.3. Static parallel electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 708
5. Structural properties of electrospun fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709
5.1. Molecular orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 709
5.2. Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710
5.3. Effect of fiber diameter on structural properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7105.4. Effect of collector on the structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 710
6. Mechanical properties of the fibers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711
6.1. Effect of structural morphology on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 711
6.2. Effect of collector type on tensile properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712
6.2.1. Stationary collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712
6.2.2. Rotational collector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712
6.3. Effect of fiber diameter on tensile properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712
7. Prospective applications of electrospun fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
7.1. Fiber composites for tissue engineering applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713
0266-3538/$ - see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.compscitech.2010.01.010
* Corresponding author. Address: Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering, The University of
Sydney, Sydney, NSW 2006, Australia. Tel.: +61 2 9351 2290; fax: +61 2 9351 3760.
E-mail address: [email protected] (Y.-W. Mai).
Composites Science and Technology 70 (2010) 703718
Contents lists available at ScienceDirect
Composites Science and Technology
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p s c i t e c h
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7.2. Electrospun fiber reinforced composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714
7.3. Conductive fiber composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714
7.4. Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715
7.5. Filler reinforced fiber systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 715
8. Concluding remarks and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716
1. Introduction
The drive for ultra-lightweight yet strong structures for devices
and miniaturized applications has motivated novel designs using
polymer nanofibers [1]. Electrospinning has emerged as a powerful
technique for producing high strength fibers due to its versatility,
ease of use, ability to align structures and control fiber diameters
[29]. Some of these unique features cannot be otherwise achieved
by conventional fiber processing techniques. Another merit is thatunder the influence of an electric field, electrospinning self-assem-
bles dispersed fillers along the axial direction such that composites
can be formed by imposing additional spatial confinement to the
polymer chains [5,10,11]. These reinforced fibers display superior
properties and function as basic building blocks for the fabrication
of high strength structures using a bottom-up approach. For exam-
ple, carbon nanotubes (CNTs) and carbon black (CB) particles are
among the commonly used fillers which are dispersed within the
fibers to mimic the functionality of silk fibers for high strength
and toughness applications [1]. This feature of dispersing filler
materials can be easily extended to other applications such as fil-
tration [12,13], tissue engineering [9,14,15], precursor for fabricat-
ing nanofiber composites [1618] and advanced nanomaterials
[4,19] etc.
Despite possessing these unique features, one of the main chal-
lenges in this area is to characterize the tensile behavior of the
nanofibers. This could be due to the difficulty in handling the
nanofibers and also due to the low load required for the deforma-
tion. Hence, in most cases, the mechanical integrity of the fibers
and fiber network structures is least understood and an under-
standing of the phenomenon is urgently needed. Few researchers
actively pursued to characterize the mechanical deformation char-
acteristics of the fibers by recording the stressstrain behavior of
the electrospun non-woven fabrics. However, this method cannot
be deemed suitable because the tensile response of the non-wo-
vens are greatly influenced by the fiber size distribution in the
mats, porosity, individual fiber orientation in the mat, fiberfiber
interaction and entanglement of the fibers [20]. These parameters
cannot be easily isolated and controlled in the non-woven fabrics.
Hence, there has been a remarkable growth and interest in
characterizing the tensile deformation behavior of single fibers
and aligned fiber bundles [58]. More recently, it was demon-strated that the size effect is critical in influencing the fiber prop-
erties and an abrupt increase in tensile properties is observed at
a given average fiber diameter [58]. The size effect in the fibers
is attributed to the process of electrospinning that results in the
formation of unique intrinsic structures within the fiber geometry
[58]. Hence, the focus of this study is to review recent articles that
characterize the intrinsic structural properties of the electrospun
fibers and present possible explanations for the enhanced tensile
behavior of the nanofibers.
Recent articles on electrospinning focused on various spinnable
polymeric materials, processing techniques for fabricating nanofi-
ber assemblies, effects of processing parameters on fiber diameter
and morphology, characteristics of the fibers and their applications
[9,12,2123]. However, the influences of electrospinning on the
structure formation and on the tensile strength of the fibers are
still lacking. To realize the full potential of the fibers it is essential
to understand the microstructure formation during electrospin-
ning, since the intrinsic structures of the fibers affect their overall
mechanical deformation behavior. For example, the ordered
arrangement of the polymer chains within the fiber geometry dur-
ing electrospinning leads to strengthening of the fibers [58]. Thus,
the main objective of this review is to outline possible mechanisms
that lead to the fabrication of stronger fibers and thereby facilitate
Fig. 1. Schematic of the general laboratory setup used for an electrospinning experiment. The inset shows the SEM morphology of the electrospun nylon 6,6 fibers. The
schematicillustrates theinvertedconical path thejet travels before being collectedas randomly oriented fibers as shown in theinset SEMmicrograph. L represents the lengthof pipette containing the polymer solution and H is the distance between the tip and collector.
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an in-depth understanding of the electrospinning process and its
role on the microstructure formation. Properties such as molecular
orientation and crystallinity of the nanofibers and the factors thatinfluence their deformation behavior are thoroughly analysed. The
effects of fiber size on the tensile strength and elastic modulus of
the fibers are also discussed.
The review is organized as follows: Sections 2 and 3 discuss the
basic concept behind electrospinning and control of fiber diameter.
Common techniques used to collect controlled morphology of the
fibers are described in Section 4. The mechanisms which yield con-
trolled morphology, structures and spatial arrangement of the
nanofibers are critically reviewed. The effects of electrospinning, fi-
ber diameter and type of collector used to control the structures,
such as crystallinity and molecular orientation, are discussed in
Section 5. Various factors that determine the mechanical deforma-
tion behavior of the fibers are presented in Section 6. Here, the or-
derly arrangement of fiber structures as a function of fiber size isemphasized. Finally, potential applications of these fibers are given
in Section 7.
2. Electrospinning theory
Electrospinning or electrostatic spinning is a simple technique
which utilizes high electrostatic forces for fiber production. Elec-
trospinning, first introduced by Formhals [24] and later revived
by Reneker [3,4], uses high voltage (about 1020 kV) to electrically
charge the polymer solution for producing ultra-fine fibers (diam-
eters ranging from a few nanometer to larger than 5 lm) [3]. Fig. 1
shows a schematic illustration of the basic electrospinning setup,
which essentially consists of a pipette or a syringe filled with poly-
mer solution, a high voltage source and a grounded conductive col-lector screen. In addition, a metering syringe pump can be used to
control the flow rate of the polymer solution. The needle of the syr-
inge typically serves as an electrode to electrically charge the poly-
mer solution and the counter-electrode is connected to the
conductive collector screen.
Under the influence of a strong electrostatic field, charges are
induced in the solution and the charged polymer is accelerated to-
wards the grounded metal collector. At low electrostatic field
strength, the pendant drop emerging from the tip of the pipette
is prevented from dripping due to the surface tension of the solu-
tion [2527]. As the intensity of the electric field is increased, the
induced charges on the liquid surface repel each other and create
shear stresses. These repulsive forces act in a direction opposite
to the surface tension [28], which results in the extension of thependant drop into a conical shape and serves as an initiating sur-
face [2931]. A schematic of the process is shown in Fig. 2. When
the critical voltage is reached, the equilibrium of the forces is dis-
turbed and a charged jet emanates from the tip of the conical drop.
The discharged jet diameter decreases in size with concomitant in-
crease in length before being deposited on the collector.
This process can be explained by the three types of physical
instabilities experienced by the jet [25,26]. These instabilities
influence the size and geometry of the deposited fibers. The first
instability, also known as the Rayleigh instability is axisymmetricand occurs when the strength of electric field is low or when the
viscosity of the solution is below the optimum value. Use of very
low viscosity solutions causes jet break-up and leads to the
bead-on-fiber morphology. It is attributed to the poor chain entan-
glement density in the solution and insufficient resistance to the
electrostatic field [31,32]. Rayleigh instability is suppressed at high
electric fields (high charge densities) or when using higher concen-
tration of polymer in the solution.
Following the initial straight path of the jet, which is controlled
by the Rayleigh instability, the polymer jet is influenced by two
other instabilities: the bending and whipping instabilities. These
instabilities arise owing to the charge-charge repulsion between
the excess charges present in the jet which encourages the thin-
ning and elongation of the jet [25,26]. At high electric forces, thejet is dominated by bending (axisymmetric) and whipping instabil-
Pendant
drop
Polymer
solution
Induced charges
from electric field
Taylor cone
Jet initiation
(B) (C)(A)
Fig. 2. Schematic illustration of the Taylor cone formation: (A) Surface charges are
induced in the polymer solution due to the electric field. (B) Elongation of the
pendant drop. (C) Deformation of the pendant drop to the form the Taylor cone due
to the charge-charge repulsion. A fine jet initiates from the cone.
Fig. 3. (A) SEM micrograph of the fibers showing typical circular morphology and
(B) SEM micrograph of the flat ribbon structure.
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ity (non-axisymmetric), causing the jet to travel in an inverse con-
e manner. It produces wave or dumb-bell shaped patterns in the
jet as shown in Fig. 1. At higher electric fields and at sufficient
charge density in the jet, the axisymmetric (i.e., Rayleigh and bend-
ing) instabilities are suppressed and the non-axisymmetric insta-
bility is enhanced. The whipping instability produces a bending
force on the jet, resulting in a high degree of elongation of the jet
[32]. During these processes, the solvent evaporates and finally
leads to the deposition of ultra-fine fibers on the conductive
ground electrode.
3. Control of fiber diameter
Systematic investigations on the effect of electrospinning
parameters on the diameter and the morphology of the fibers have
been reported by several researchers [3335]. Major factors that
control the diameter of the fibers are: (1) concentration of polymer
in the solution, (2) type of solvent used, (3) conductivity of the
solution, and (4) feeding rate of the solution. An example of the ef-
fect of parameters on fiber geometry is shown in Fig. 3. Fig. 3a
shows typical circular fibers and Fig. 3b shows flat fiber belts that
are obtained because of the rapid evaporation of the solvent. The
flattened fibers are obtained when a fraction of the solvent is
trapped inside the fiber. When the solvent evaporates, the fiber
collapses, resulting in flat fiber belts.
Clearly, there is a critical need to produce fibers of uniform
diameters so that the electrospinning process can be rendered
reproducible for scientific modeling and industrial applications.
Fridrikh et al. determined the parameters that control the fiber
diameter using an analytical model [33] that is based on the differ-
ences between surface tension of the solution and the electrostatic
charge repulsion in the jet. At high electrical field, the motion of
the jet is influenced by three main forces, namely: (a) external
electric field, (b) normal stresses, which comprise the surface ten-
sion and tension resisting the bending of electric field lines in the
jet, and (c) surface charge repulsion. Bending and stretching is a di-
rect effect of normal stresses, which originate from the bending of
the centerline of the jet. Hence, the normal stress gives rise to the
whipping instability. When the surface charge repulsion exceeds
the surface tension, it leads to the whipping instability and bend-
ing of the jet. At this stage, the current is constant and consists
of conduction and advection current. At the later stage of whip-
ping, the bulk current is dominated by the advection current and
the surface charge repulsion is balanced by the surface tension.
At this stage, the stretching of the jet is ceased and a constant
diameter of the jet is obtained. The developed model predicts the
diameter of this terminal jet, assuming that no further thinning
of the jet occurs. Thus, the final diameter of the fiber (D) is deter-
mined to be a function of surface tension, electric current and sur-
face charge repulsion. The equation for the diameter is:
D cnQ
2
I22
p 2 ln ld 3
!1
3
1
where c is surface tension of the solution, n dielectric constant, Qflow rate of the solution, I current carried by the jet, l initial jet
length and d diameter of the nozzle. Primarily, flow rate, electric
current and surface tension of the solution control the whipping
jet diameter. For instance, increasing the current carrying ability
of the jet by 32 times or reducing the flow rate by 32 times results
in a ten-fold fiber diameter reduction [33,36]. The flow rate of the
solution to the nozzle can be easily controlled by using a flow
meter.
This model is certainly not comprehensive, considering the
number of parameters that would control fiber diameters. Themodel, however, neglects the elastic effect due to solvent evapora-
tion and considers the solution Newtonian. The model also ne-
glects the volatility of the solvents and charge carrying ability of
the polymers. The accuracy of predicting the diameter of the fiber
depends on the charge carrying ability of the jet. When non-con-ductive polymers such as PCL are used for electrospinning, the
charges are solely accommodated by the volatile solvent. The
charges from the evaporated solvent may reach the collector,
which contributes to the measured current [33], and which leads
to over-predicting the stretching of the jet. Hence, the model can-
not predict the fiber diameters accurately for the polymers in a
highly volatile solvent. However, theoretical fiber diameters of
conductive polymers agree well with experimental values. This is
due to the fact that the charges stay with the jet until it reaches
the collector and drying occurs after the stretching of the jet. None-
theless, the model provides a simple analytical method to estimate
the diameter of the fibers with convincing agreement.
Eq. (1) evaluates the terminal diameter considering that the col-
lector is stationary. However, further thinning of the fibers can beobtained when rotating collectors such as a rotating drum or a
rotating disk collector is used [8]. Kotaki et al. [37] showed that
the speed of the rotating collector induced tensile stresses on the
fibers before being wound around the collector. The tensile stresses
are responsible for further thinning of the fiber diameter, which is
not predicted by Eq. (1).
4. Alignment of fibers and fiber collection methods
Recently, it was determined that the nature of the collector
influences significantly the morphological and the physical charac-
teristics of the spun fibers [38,39]. The density of the fibers per unit
area on the collector and fiber arrangement are affected by the de-gree of charge dissipation upon fiber deposition. The most com-
Fig. 4. Schematic of the rotating drum used for fiber collection. The inset SEM
micrograph shows the aligned fibers obtained using the rotating drum.
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monly used target is the conductive metal plate that results in col-
lection of randomly oriented fibers in non-woven form as shown in
Fig. 3a. Liu et al. [38] electrospun cellulose acetate fibers on copper
mesh, aluminumfoil, paper and water as collectors. They found the
type of collector used greatly determined the arrangement and
packing density of the fibers. The use of metal and conductive col-
lectors helped dissipate the charges and also reduced the repulsion
between the fibers. Therefore, the fibers collected are smooth and
densely packed. Conversely, the fibers collected on the non-con-
ductive collectors do not dissipate the charges which repel each
other. Hence, the fibers are loosely packed.
The fibers can also be collected on specially designed collector
systems so as to obtain aligned fibers or arrays of fibers. Recently,
researchers focused on achieving highly ordered aligned fibers byusing mechanical and electrostatic methods to control the elec-
trospinning process. Aligned fibers have found importance in many
engineering applications, such as tissue engineering, sensors,
nanocomposites, filters, electronic devices [4043]. Some com-
monly used techniques to align the fibers are discussed in the sub-
sections below.
4.1. Rotating drum collector
The schematic of the electrospinning setup with a rotating
drum collector is shown in Fig. 4. This method is commonly used
to collect aligned arrays of fibers. Furthermore, the diameter of
the fiber can be controlled and tailored based on the rotational
speed of the drum [4042]. The cylindrical drum is capable of
rotating at high speeds (a few 1000 rpm) and of orienting the fibers
circumferentially. Ideally, the linear rate of the rotating drumshould match the evaporation rate of the solvent, such that the fi-
bers are deposited and taken up on the surface of the drum. The
Fig. 5. Schematic of the disk collector used for fiber collection. The SEM micrograph shows the alignment of the fibers obtained using the disk collector. Better alignment of
the fibers is observed compared to the rotating drum.
Fig. 6. Schematic of static electrodes used for collecting aligned fiber bundles. The optical micrograph shows the aligned fibers collector using this technique [5].
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alignment of the fibers is induced by the rotating drum and the de-
gree of alignment improves with the rotational speed [40,43]. At
rotational speeds slower than the fiber take-up speed, randomly
oriented fibers are obtained on the drum. At higher speeds, a cen-
trifugal force is developed near the vicinity of the circumference of
the rotating drum, which elongates the fibers before being col-
lected on the drum [23,43,44]. However, at much higher speeds,
the take-up velocity breaks the depositing fiber jet and continuous
fibers are not collected.
4.2. Rotating disk collector
The rotating disk collector is a variation setup of the rotating
drum collector and is used to obtain uniaxially aligned fibers.
Fig. 5 shows the common setup. The advantage of using a rotating
disk collector over a drum collector is that most of the fibers are
deposited on the sharp-edged disk and are collected as aligned pat-
terned nanofibers [23,4346].
The jet travels in a conical and inverse conical path with the use
of the rotating disk collector as opposed to a conical path obtained
when using a drum collector. During the first stage, the jet follows
the usual envelope cone path which is due to the instabilities
influencing the jet. At a point above the disk, the diameter of the
loop decreases as the conical shape of the jet starts to shrink. This
results in the inverted cone appearance, with the apex of the cone
resting on the disk. The electric field applied is concentrated on the
tapered edge of the disk and hence the charged polymer jet is
pulled towards the edge of the wheel, which explains the inverted
conical shape of the jet at the disk edge. The fibers that are at-
tracted to the edge of the disk are wound around the perimeter
of the disk owing to the tangential force acting on the fibers pro-
duced from the rotation of the disk. This force further stretches
the fibers and reduces their diameter. The quality of fiber align-
ment obtained using the disk is much better than the rotating
drum; however, only a small quantity of aligned fibers can be ob-
tained since there is only a small area at the tip of the disk.
4.3. Static parallel electrodes
The advantage of using this technique lies in the simplicity of
the setup and the ease of collecting single fibers for mechanical
testing. Good alignment has been obtained with this technique.
The air gap between the electrodes creates residual electrostatic
repulsion between the spun fibers, which helps the alignment of
the fibers [5,4749]. Two non-conductive strips of materials are
placed along a straight line and an aluminum foil is placed on each
of the strips and connected to the ground as shown in Fig. 6. This
technique enables fibers to be deposited at the end of the strips
so that the fibers adhere to the strips in an alternate fashion and
collected as aligned arrays of fibers. A similar technique by Teo
and Ramakrishna [47,49] used double-edge steel blades along a
Fig. 7. Schematic representation of the nanofibril in a single POM nanofiber. The schematic representation of the crystal orientation of 700 nm POM fiber is shown andillustrates the conformation of the helical structure of the chain.
Fig. 8. Structural morphology of electrospun fibers displaying the densely packed
lamellae and fibrillar structure. Reprinted with permission from [51]. Copyright
[2010], American Institute of Physics.
Fig. 9. Transmission electron micrograph of nylon 6,6 fiber displaying preferred
orientation of the polymer chains.
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line to collect aligned arrays of fibers. The fibers were deposited at
the gap between the electrodes, however, few fibers were found to
deposit on the blades. It was resolved by applying a negative volt-
age between the blades, resulting in the deposition of fibers be-
tween the blades.
5. Structural properties of electrospun fibers
Typically, in most semi-crystalline polymers, the fibers pro-
duced by electrospinning display structural hierarchy. During the
fiber formation process, a fraction of the chains crystallizes to form
lamellae consisting of small crystals and the remaining fraction
forms the amorphous phase [5052]. In the presence of shear
and elongation forces, the lamellae are organized to form fibrils
and the tie chain molecules pass through the neighboring crystal-
lites to form small-sized bundles. The general structure in the fiber
is expected as shown in Fig. 7. Due to the shear forces experienced
by the jet during electrospinning, the chain orientation (see Fig. 7)
aligns along the fiber axis [50]. Konkhlang et al. [50] examined the
crystal morphology and molecular orientation of polyoxymethyl-
ene (POM) fiber and found that each fiber consists of nanofibrils
which are aligned parallel to the fiber axis. The fibrils consist of14 polymer chains and 40 monomeric units. Similar observations
have been found by Lim et al. [51]. They visualize the structural
morphology of the fiber and found the fibers to have densely
packed lamellae and fibrillar structures as shown in Fig. 8. The
lamellar structures determine the crystallinity of the fibers. In be-
tween the stacks of lamellae are the relaxed amorphous tie
molecules.
5.1. Molecular orientation
The polymer jet under the influence of an electrostatic field
experiences a high degree of elongation strain (104 times the draw
ratio and over 106 s1 draw rate). The high elongation strains and
shear forces are capable of aligning the macromolecular chainsalong the fiber axis resulting in a high degree of molecular orienta-
tion in the fibers [58,52]. Zong et al. [53] found the molecular
chains in the electrospun PLLA fibers to be highly oriented com-
pared to the random-coil shape chains in the PLLA film. Other poly-
mer fibers such as Kevlar display similar orientated chain
structures and is observed between the amorphous and the crys-
talline regions of the fibers. Fig. 9 is an example using transmission
electron microscopy (TEM) to analyse the chain orientation. The fi-
bers are stained with ink so that upon examination with TEM,
there is a phase contrast between the amorphous and crystalline
lamellae. Some degree of chain orientation can be found on the
surface regions of the fibers. Hence, it can be summarized that
the process of electrospinning alters the intrinsic structural prop-
erties of the material. The orientation extent can be quantified by
using X-ray diffraction analysis on the samples. Alternatively, the
draw ratio can be used to obtain an estimate of the molecular ori-
entation. It quantifies the amount of extension the jet experiences
during the electrospinning process [5]. High draw ratios experi-
enced by the jet are capable of aligning the macromolecular chains
along the fiber axis, thereby influencing the formation and struc-
ture of the crystallites. The draw ratio for spun fibers can be calcu-
lated as the ratio of the spinning velocity of the collected fiber to
ejection velocity of the polymer solution from the pipette
[5,43,5456]. According to the principle of mass conservation,
the velocity of the fibers at the ground collector is given by:
mspin wf
100 pf prf2 t
!2
where mspin is spinning velocity (m/min) when fibers are collected at
the ground electrode, wf weight (g) of polymer fibers on the groundelectrode,pf density (g/cc) of the PCL fibers, rf average radius (cm) of
the collected fibers and t electrospinning time (min). Usually, the
electrospinning process is run for longer duration of time
($45 min). The ejection velocity of the PCL solution fromthe pipette
is determined from:
msol wsol
100 psol prp2 t
!3
where msol and wsol are ejecting velocity (m/min) and weight (g) ofthe polymer solution at a given time during electrospinning, psoldensity (g/cc) of PCL solution, rp diameter (cm) of the pipette used
and t electrospinning time (in minutes). Using Eqs. (2) and (3),
the draw ratio is the ratio of mspin/msol [5,43,5456]. The elongationrate of the PCL fibers during the electrospinning process is deter-
mined from the following equation:
e mspin msol
H4
where e is elongation rate and His distance between the pipette andground collector. Higher draw ratio values are expected to provide
better chain orientation in the fibers.
0 10 20 30 40 50 60
Spun Sample
Intensity
2
Fig. 10. Typical intensity versus 2h plot obtained from the WAXD analysis on PCL
fibers. Crystallinity of the fibers can be determined as the ratio of the area of thepeaks to the total area of the curve [5].
Temperature (C)
0 50 100 150 200 250 300
HeatFlow(W/g)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Spun Nylon 6,6
Nylon 6,6 pellets (non-spun)
Fig. 11. DSC curves of electrospun nylon 6,6 and un-processed (non-spun) nylon6,6 pellet comparing the melting temperature and heat of fusion.
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5.2. Crystallinity
The rapid evaporation of the solvent from the jet accompanied
by the rapid structure formation, which occurs within milliseconds
($50 ms) leads to less developed structures in the fibers. The rapid
solvent evaporation reduces the jet temperature. Thus, the mole-
cules that are aligned along the fiber axis have less time to realign
themselves, leading to less favorable packing. For most semi-crys-
talline polymers, the stretched chains under high elongation rate
do not get enough time to form crystalline lamellae, which yields
lower crystallinity. Hence, the crystallinity in the fibers is thereby
influenced by the rate of solvent evaporation [23,57]. The most
common technique used to determine the degree of crystallinity
is wide angle X-ray diffraction (WAXD) analysis. The ratio of the
area under the peaks to the total area under the curve of intensity
versus 2h plot is shown in Fig. 10 and it gives the sample
crystallinity.
Contrary to the theory that electrospinning reduces the crystal-
linity of the fibers, Lee et al. [58] and Reneker et al. [59] reported
that the crystalline structure in fibers is developed in many polyes-
ters and ductile materials. Moreover, the crystallinity can be even
higher than the un-processed polymer pellets. They argue that
electrospinning inhibits the development of crystallinity specifi-
cally for rigid polymers with high glass transition (Tg) values. How-
ever, for ductile polymers and polyesters with lower Tg values, such
asPCL (Tg$60 C), this takes longer time to crystallize. Therefore,
ductile polymers have the possibility of crystallization during the
jet drawing/elongation process, even after the fibers are solidified.
Fig. 11 shows the DSC analysis of nylon 6,6 fibers compared with
the un-processed nylon 6,6 pellets. The results are consistent with
the results of Lee et al. [58] and Reneker et al. [59]. The melting en-
thalpy of electrospun nylon 6,6 is calculated as 107 J/g compared to
91 J/g for the unspun sample, suggesting an increase in the degree
of crystallinity.
5.3. Effect of fiber diameter on structural properties
Zussman et al. [60] in their study demonstrate that the electro-
spun fibers possess skin-core morphology. The skin region of the
fibers contains oriented layered planes that are parallel to the fiber
axis and contain few crystallites. But the crystallites are misori-
ented with respect to the fiber axis. The properties of the skin differ
from those of the core region for the fibers as the skin layers are
essentially characterized by the oriented layered planes whereas,
the core region is characterized by random-coil chains. These re-
sults are substantiated by the molecular dynamic simulations of
Curgul [61] who has demonstrated that the molecules are oriented
preferentially parallel to the surface for the nanofibers. The mobil-
ity of these chains at the skin is much higher than the mobility of
the chains present in the core region [62,63]; hence the chains at
the skin are easily oriented under the influence of an electric field.
As the fiber diameter is reduced, at some critical fiber diameter,
the size of the skin region becomes comparable to the overall
diameter of the fiber [6]. Moreover, the oriented layered planes
on either side of the fiber wall are coupled together and influence
the overall properties of the fibers. In contrast, when the fiber
diameter is increased, orientation of the chains at the surface of
the fiber walls becomes less comparedto the majority of the chains
in the fiber core region. These results are in agreement with those
reported later by Arinstein et al. [6] who have shown that the fibers
consist of supramolecular region which consists of oriented amor-
phous chains. As the size of the fiber is reduced; the size of the
supramolecular structure containing amorphous oriented macro-
molecules is more significant compared to the fiber diameter. Thus,
at the critical fiber diameter, the properties of the fiber are con-
trolled by the oriented amorphous macromolecules in the supra-
molecular region. This work [6] is very important and more
detailed investigations are still lacking.
In a study conducted in our laboratory, we have established the
relationship between the microstructure of the PCL fibers and their
diameter [5]. The degree of crystallinity and molecular orientation
in the fibers is determined using wide angle X-ray diffraction
(WAXD) analysis. Fig. 12 shows the WAXD pattern of the fibers
with diameters 250 and 900 nm, respectively. The arc width of
the strongest equatorial reflection provides an indication of the de-
gree of orientation within the samples. It is clear from the WAXD
patterns (Fig. 12) that 900 nm-wide fibers have lesser degree of
orientation compared to 250 nm-wide fibers, that is, the wider
the fibers the less molecular orientation is exhibited. Fig. 13 showsthe degree of crystallinity (%) and molecular orientation (%) versus
fiber diameter. Molecular orientation determined from WAXD in-
creases with decreasing fiber diameter. Therefore, it confirms that
as the fiber diameter is reduced, the alignment of the molecules in
the direction of fiber axis is improved.
5.4. Effect of collector on the structural properties
The type of collector and the speed of the drum/disk collector
selected influence the isotropic or anisotropic alignment of the fi-
bers in the mats. Also, the collector type used controls the crystal
morphology and molecular orientation [50]. In the article by Kon-
gkhlang et al. [50], they show that when a rotational collector isused, the polymer chains in the crystalline regions are drawn fur-
Fig. 12. WAXD pattern of aligned fibers performed on two sets of fiber diame-ter0073: (i) 250 nm and (ii) 900 nm [5].
Fiber Diameter ( m)
0 200 400 600 800 1000
Crystallinity(%)
0
20
40
60
80
100
Molecular
Orientation(%)
0
20
40
60
80
100
Crystallinity
Molecular Orientation
Fig. 13. Plots of crystallinity (%) and molecular orientation (%) versus fiber diameter
for aligned fibers. The degree of crystallinity and molecular orientation increases
gradually as the fiber diameter is reduced [5].
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ther in the draw direction compared to the polymer chains in the
non-woven fabrics that are obtained using a stationary collector.
The force due to the rotational speed of the collector along with
the shear and elongation forces may contribute to the alignment
of the polymer chains in the direction of the fiber axis. Thus, it is
expected that the crystal orientation in the fibers improves with
the speed of the collector [64]. The use of high speed rotational col-
lectors leads to a fanning effect and the evaporation of the sol-
vent is much quicker compared to the stationary collectors [37].
The speed creates a high-viscosity environment for the polymer
chains in the electrospinning jet and leads to the transfer of the
tensile stress onto the polymer chains during the fiber deposition.
Thus, the crystallization in the fibers occurs due to the sliding dif-
fusion which facilitates formation of extended chain crystals
(ECC) from the folded chain crystals by lamellar thickening [37].
It leads to increases in crystal size and crystallinity. This is caused
by a more perfect planar zigzag conformation of the ECC struc-
tures under the influence of an applied tensile stress. Also, as the
rotational force contributes towards the stretching of the polymer
jet, higher rotational speed decreases the diameter of the fibers.
This explains the ordering of the crystals at higher collector speeds.
Furthermore, when the static parallel electrodes are used to obtain
aligned fiber arrays, the extended chain crystals are not observed
from WAXD and infrared spectroscopic analyses. Also, the crystal
orientation is expected to be inferior compared to the rotational
collectors. Hence, we did not find a significant increase in the de-
gree of crystallinity using the parallel electrodes method even
though the fiber diameter was reduced [5].
6. Mechanical properties of the fibers
Polymer nanofibers are treated as 1-dimensional systems and
have found to possess unusual mechanical properties. The
mechanical deformation behavior displayed by the fibers is unique
and can be significantly different from their macroscopic counter-
parts [5,43,37,57,65]. The unique features of the fibers are attrib-uted to the process of electrospinning.
6.1. Effect of structural morphology on tensile properties
The lamellar and amorphous fractions of the chains within the
fibers influence the strength and elastic modulus of the fibers.
Changes in the structural formation taking place in the fibers dur-
ing electrospinning, specifically crystallinity and molecular orien-
tation, impart physical uniqueness to the material and play an
important role in the deformation behavior of the fibers [58].
Hence, knowledge of their intrinsic structures is essential to under-
stand their effects on mechanical properties. The amorphous phase
of the fibers provides the elastomeric properties and the crystalline
phase imparts dimensional stability to the array of molecules [5].
Thus, the mechanical deformation characteristics of the fiber is
influenced by the random or/and ordered arrangements of the
crystalline and amorphous phases in the fiber [58,50,51].
According to Curgul et al. [61], the mechanical deformation of
the fibers is affected by the skin and core morphologies of the fiber.
The mass density in the core region is similar to the bulk polymer
density. Thus, the core region of the fiber exhibits bulk-like struc-
ture and physical properties, whereas, the property exhibited by
the surface region is entirely different. This is attributed to the sig-
nificantly lower density and increased mobility of the chains at the
surface/skin of the fiber compared to the core region [62,63].
Hence, the overall deformation of the fiber is determined by the
number of oriented fragments present in the surface regions. This
theory is also confirmed by Arienstein et al. [6]. In their study, they
concluded that the orientation of the amorphous chains in the
supramolecular region of the fibers influences the deformation
process of the fibers. If this understanding is applied to study the
effect of fiber diameter on tensile strength, it should result in an
exponential increase, or an abrupt shift, in properties as the fiber
diameter is reduced. This is because the effect of fiber surface/skin
on the overall nanofiber is increasingly dominant as (a) the fiber
surface dimension approaches the radius of gyration of polymer
chains, thus constraining the segmental motion, and (b) the fiber
core region diminishes when the fiber diameter decreases.
In our previous study [5], we also evaluated the effect of fiber
diameter on the tensile deformation. The tensile response of the fi-
ber was compared with the tensile properties of the bulk polymer
system prepared using injection molding. Representative stress
strain curves of spun and bulk systems are shown in Fig. 14. There
is a significant difference in the tensile strength and tensile behav-ior. The stressstrain curve of the spun sample is consistently
found not to display the necking phenomenon whereas the bulk
sample shows clear necking. This is attributed to the oriented
and stretched polymer chains in the spun fibers [66,67]. Similar re-
sults were obtained by Lu et al. [66].
Strain (mm/mm)
0 2 4 6
Stress(MPa)
0
10
20
30
40
50
60
2
1
Necking
Fig. 14. Stressstrain curves obtained from tensile tests performed on electrospun
PCL and non-spun PCL samples. Curve 1 represents the electrospun sample andCurve 2 represents the non-spun sample [5]. Fig. 15. SEMmicrograph of randomly oriented fibers with fiberfiber fusion points.
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6.2. Effect of collector type on tensile properties
6.2.1. Stationary collector
The morphology of the mats obtained using a stationary collec-
tor is shown in Fig. 3a. Their mechanical deformation depends
greatly on the degree of alignment of the fibers within the mat, fi-
ber lay-ups and interface properties of fiberfiber contact
[19,45,66]. Typically, the tensile strength and modulus of the
non-woven fabrics are lower than the mats with uniaxially ori-
ented fibers. This is attributed to the highly porous nature of the
non-woven fabrics. Moreover, during tensile loading, only the fi-
bers oriented along the loading direction experience the stretching
force, while the fibers that are oriented perpendicular to the load-
ing direction do not experience any force.
The fibers tend to orient in the direction of loading before the
non-linear region in the stressstrain curves. After the non-linear
point, the fiber mesh structure is damaged and better orientation
of the fibers along the loading direction is observed. This stage is
followed by the high orientation of the fibers at the maximum
stress point and fiber breakage at several points is noticed. Such
deformation behavior in non-woven fabrics is usually observed
when there is no fiber-to-fiber bonding. The lack of bonding be-
tween the fibers facilitates easy orientation and stretching of the
fibers when loaded and can give high degree of elongation before
failure [19]. The use of highly volatile solvents during electrospin-
ning can produce non-wovens with little or no fiber fusions. Fibers
cannot fuse together when the solvent evaporation is high and this
also results in weak intermolecular interaction. However, when
there is fusion between fibers as shown in Fig. 15, the modulus
of the non-wovens increases and the elongation to break decreases.
The fusion of fibers is obtained if the solvent is not completely
evaporated during the fiber forming process.
6.2.2. Rotational collector
Macroscopically aligned fibers obtained by modifying the fiber
collecting system are found to have anisotropic properties[68,69] which can be potentially useful in a variety of optical,
mechanical and bio-medical applications. Uniaxial, aligned fibers
are found to possess anisotropic tensile properties. The tensile
strength and modulus of these samples are higher than randomly
oriented fibers [69]. When the fibers are oriented in the loading
direction, the uniaxial orientation of the fibers helps the tensile
force distribute equally to all fibers. Further, since the molecular
chains in the fibers are aligned along the fiber axis, which is in
the loading direction, the samples display enhanced strength and
modulus. The tensile strength of the aligned fiber array samples
also depends on the technique used to collect these arrays. When
a rotating disk or a drum is used to collect the fibers, mechanical
forces may be applied to the jet due to the rotational speed of
the collector. This force along with the shear and elongation forces
entice the alignment and stretching of the polymer chains in the
fiber axis direction. In addition, the rotational speed of the collector
determines the stacking density of the fibers. At higher rotational
speeds, the deposited fibers have a denser lateral packing and min-
imum inter-fiber spacing. Also, the fibers tend to have uniform
morphology and diameter at higher rotational speeds, which con-
tribute towards the strength of the samples [3946].
6.3. Effect of fiber diameter on tensile properties
Fiber structure, geometrical arrangement of the fibers, individ-
ual fiber properties and interaction between fibers greatly influ-
ence the mechanical properties of fiber mats. These features are
difficult to control during the electrospinning process. Therefore,
determining the tensile deformation of the single fiber is of funda-
mental importance. Recently, researchers determined the tensile
deformation of single fibers and demonstrated that the size of
the fiber influenced their tensile response. An enhanced behavior
is observed by researchers when the diameter of the fibers is re-
duced below the critical diameter [58].
Arinstein et al. [6] in their study demonstrate that the size of the
fiber has an effect on its deformation behavior. At some critical
diameter, the fibers display almost an exponential increase in ten-
sile strength. This phenomenon prevails when the size of the
supramolecular structures of the fibers is comparable with the
overall fiber diameter. The orientation of macro molecules present
in the supramolecular structures of the amorphous phase plays a
dominant role to increase the fiber mechanical properties. Uponincreasing the fiber size, both tensile strength and tensile modulus
decreases and the larger diameter fibers tend to display bulk-like
properties. This observation is extremely important for conceivable
applications of eletrospun nanofibers. Instead of considering such
polymers as fibers, they can be used as miniaturized high aspect
ratio components for devices and sensors. Hence, by acknowledg-
ing the abrupt changes in strength and modulus as fiber diameter
decreases, we cannot use measurements obtained, or extrapolated,
from bulk specimens to model devices at nanometer length scale.
Fig. 16 shows the tensile strength and tensile modulus versus fi-
ber diameter seen in individual PCL fibers. The fibers with diame-
ters greater than 2 lm do not affect the modulus or tensile
strength and can be thought to have bulk-like properties. The en-
hanced properties of finer diameter fibers are attributed to thegradual ordering of the molecular chains and modest increase in
the crystallinity of the fibers. The size effect can also be due to
the densely packed lamellae and fibrillar structures. In finer diam-
eter fibers, the lamellae and fibrillar structures align themselves
along the fiber axis, which plays a critical role in enhancing the
mechanical properties of the fibers. The fibrillar structure has a
high degree of molecular orientation and provides high resistance
to the axial tensile force. When the fiber diameter is increased, the
lamellae tend to re-orientate and the presence of alignment and
fibrillar lamellae structure is decreased, resulting in reduced
mechanical properties.
Dzenis [67] and others [70,71] modeled this size effect in poly-
mer nanofibers and considered the surface energy of the fibers to
contribute towards the axial tensile force. The assumption madein these studies is that fibers of different sizes have similar mor-
Fiber Diameter ( m)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
TensileStrength
(MPa)
0
20
40
60
TensileModulus(MPa)
0100
200
300
400
Strength
Modulus
Fig. 16. Plot of tensile modulus and tensile strength versus fiber diameter. Tensile
modulus increases with decreasing fiber diameter. These results can be attributed
to the better molecular orientation and crystallinity in smaller fiber diameters.
Larger than 2 lm, both tensile modulus and tensile strength appear invariant withfiber diameter [5].
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phology and can be considered isotropic. However, the size effect
seen in experiments attributes this effect to the electrospinning
process and the macromolecular orientations in the fibers. The
use of an electric field during electrospinning spontaneously aligns
highly mobile molecular chains in the direction of the fiber axis,
resulting in a higher degree of molecular orientation. The large
shearing imposed on the electrospinning jet and the high draw ra-tio seen for finer fibers suggest better chain orientation for the
thinner fibers. Some studies attribute this effect to the orientation
of the chains in the outermost region of the fibers [6,7]. Because of
the large shear stress and surface tension influencing the electros-
pinning jet, it results in a higher number of monomers aligned on
the fiber surface and in contact with the surface. Besides, as the fi-
ber diameter is reduced, the chains orient along both surfaces of
the fiber and are considered to be physically coupled which en-
hances the properties. Hence, when the thickness of these surface
layers is comparable to the overall fiber size, it plays an important
role in influencing the mechanical properties of the fibers. In con-
trast, as the fiber size is increased, misorientation of the polymer
chains along the surface occurs and the surface layer is no longer
comparable to the overall fiber size. The higher degree of misorien-
tation present in the fiber core region yields bulk-like properties
[60,72,73] and dictates the overall fiber properties.
7. Prospective applications of electrospun fibers
7.1. Fiber composites for tissue engineering applications
Ultra-fine fibers of biodegradable polymers produced by elec-
trospinning have found potential applications in tissue engineering
due to their high surface area to volume ratios and high porosity of
the fibers [9,14,65,73,74]. Moreover, the flexibility of seeding stem
cells and human cells on the fibers makes electrospun materials
most suited for tissue engineering applications [75,76]. The fibers
produced can be used systematically to design the structures suchthat they do not only mimic the properties of the extracellular ma-
trix (ECM), but also possess high strength and high toughness. For
instance, non-woven fabrics exhibit isotropic properties and sup-
port neo-tissue formation. These mats resemble the ECM matrixand can be used as skin-scaffold and wound dressing materials
where the materials are required to be more elastic than stiff
[14,7782]. When anisotropic properties are desired for load-bear-
ing applications, such as musculoskeletal tissues (tendons and lig-
aments), aligned electrospun fibers can be used to mimic the
structural anisotropy of the tissues. Many natural polymers (colla-
gen, starch, chitin and chitosan) and synthetic biodegradable poly-
mers (poly(e-caprolactone) (PCL), polylactide (PLA), poly(D, L-
lactide-coglycolide) (PLGA)) have been widely investigated for po-
tential applications in developing tissue scaffolds [77,8386].
This suggests a thorough understanding of the mechanical
behavior of electrospun nanofibers is essential. For example, the
fracture toughness of synthetic electrospun scaffolds has not been
addressed at all and this is a critical factor for assessing the
mechanical integrity of the scaffolds. The natural tissues due to
Fig. 17. Schematic illustrating the hierarchical organization of bone. It also shows the self-assemble process of mineralization observed in natural composites.
Fig. 18. SEM micrograph of fracture surface showing the presence of electrospun
nanofibers in the matrix resin. Reprinted from [16], with permission from Elsevier.
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their hierarchical structural arrangement possess superior fracture
toughness values compared to any of the synthetic scaffold mate-
rials that are currently used [83]. To match the mechanical integ-
rity of natural composites, the design of the scaffolds developed
should mimic the architectural design used by natural tissues.
Fig. 17 shows the self-assembly design mechanism seen in bones.
Natural tissues at the structural level essentially consists of colla-
gen fibrils and bone ceramic in the form of hydroxyapatite (HAP).
Mineralization of the tissue takes places by the mechanism known
as protein fiber-guided mineralization. Collagen fibers (300 nm in
length and1.23 nm in width) are self-assembled in an orderly fash-
ion and generate channels or grooves. Mineral particles originate
and develop at the grooves grow in length and width as sheets of
mineral platelets. The mineral platelets are placed parallel to each
other and provide strength to the composite. Thus, the arrange-
ment of collagen and bone crystals at the structural level can crit-
ically affect the mechanical integrity of the whole system [8890].
The strength of the tissue scaffolds processed using the conven-
tional techniques lack the architecture design seen in the natural
composites and hence their mechanical characteristics are drasti-
cally different from natural bone composites. However, the elec-
trospinning technique is capable of mimicking the protein -
guided mechanism and can potentially align the HAP particles in
the fiber direction. Further, the strength of the fibers can be con-
trolled by the loading of HAP fillers.
Shields et al. [65] showed that electrospun collagen fibers with
diameters $100 nm intended for articular cartilage repair have
modulus $170 MPa and maximum stress of 3.3 MPa. These values
closely match the cartilage mechanical properties of Youngs mod-
ulus of 130 MPa and maximum stress of$20 MPa. Stanishevsky
et al. [87] fabricated composites of hydroxyapatite (HAP)/collagen
using electrospinning for hard tissue scaffold applications and
demonstrated that the properties of the electrospun material can
be easily controlled by the HAP loading in the fibers. These results
confirm that electrospinning of natural or synthetic polymers for
tissue engineering applications are very promising.
7.2. Electrospun fiber reinforced composites
Although electrospun fiber reinforced polymer composites have
significant potential for development of high strength/high tough-
ness materials and materials with good thermal and electrical con-
ductivity, very few studies have investigated the use of electrospun
fibers in composites [1618,91]. Fig. 18 shows a SEM micrograph of
an electrospun fiber reinforced composite.
Traditional reinforcements in polymer matrices can create
stress concentration sites due to their irregular shapes and cracks
propagate by cutting through the fillers or travelling up, down
and around the particles. However, electrospun fibers have several
advantages over traditional fillers [17]. The reinforcing effect of the
fibers is influenced essentially by the fiber size. Smaller size fibers
give more efficient reinforcement. Also, as discussed in the previ-
ous sections, fibers with finer diameters have preferential orienta-
tion of the polymer chains along the fiber axis. The orientation of
macromolecules in the fibers improves with the reduction in diam-
eter, making finer diameter fibers very strong. Hence, the use of
nanometer-sized fibers can significantly enhance the mechanical
integrity of the polymer matrix compared to micron-sized fibers.
Moreover, the high percentage of porosity and irregular pores be-
tween the fibers can lead to an interpenetrated structure when dis-
persed in the matrix, which also enhances the mechanical strength
due to the interlocking mechanism. These characteristic features of
nanofibers enable the transfer of applied stress to the fibermatrix
interface in a better fashion than most of the commonly used filler
materials [16].
Current issues related to the use of electrospun nanofibers as
reinforcement materials are the control of dispersion and orienta-
tion of the fibers in the polymer matrix. To achieve better rein-
forcement, electrospun nanofibers may need to be collected as a
highly aligned yarn instead of a randomly distributed felt so that
the post-electrospinning stretching process could be applied to
further improve the mechanical properties. Further, if crack growth
is transverse to the fiber orientation, the fracture toughness of the
composite can be optimized. Hence, the interfacial adhesion be-
tween fibers and matrix material needs to be controlled such that
the fibers are capable of deflecting the cracks by fibermatrix inter-
face debonding and fiber pull-out. The interfacial adhesion should
not be too strong or too weak. Optimal control can only be attained
by careful selective fiber surface treatment. The dispersion of elec-trospun mats in the matrix can be improved by trimming the fibers
to shorter fragments. This can be achieved, if the electrospun fibers
are collected as aligned bundles (instead of non-woven network),
which can then be optically or mechanically trimmed to obtain fi-
ber fragments of several 100 nm in length.
7.3. Conductive fiber composites
Electrospinning has found applications in developing flexible
and compliant conductive nanofibers for applications in miniatur-
ized devices [23,9294]. Researchers seek to develop compliant
electrodes for electroactive polymer actuators. Use of electrospin-
ning to produce fibers from conductive polymer matrices can be
useful for these applications. Moreover, electrospinning can beused to disperse carbon nanotubes (CNT) in fibers to improve the
mechanical, electrical and conductive properties of the matrix
material [10,11]. Due to the high elongation of the polymer jet dur-
ing electrospinning, the CNTs tend to orient along the fiber axis and
are embedded in the fiber core as shown in Fig. 19. Application of
CNTs and carbonaceous fillers in polymers is known to improve the
electrostatic charge dissipation and electromagnetic shielding effi-
ciency, thus improving the overall conductivity of the polymers.
Accordingly, many polymers are being investigated that can be
easily electrospun and used as matrix material for CNTs. Another
advantage of using electrospun fibers for developing electrodes is
the surface area to volume ratio of the fibers. Since the rate of elec-
trochemical reactions is affected by the surface area of the elec-
trode, the high surface area of the fibers for the development ofporous electrodes can be exploited.
Fig. 19. TEMmicrograph of nylon 6,6-CNT fiber. The CNTs areembeddedin thecoreregion of the fiber and are aligned along the fiber axis.
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Norris et al. [92] fabricated ultrafine electrically conductive
polyaniline/polyethylene oxide (PAN/PEO) fiber blend using elec-
trospinning. The standard four-point probe method was used to
determine the conductivity of non-woven fibers and cast films.
By controlling the PAN/PEO ratio in the blend, they improved the
conductivity of non-woven mats comparable to that of the cast
film. Ko [95] determined that the size of the fiber obtained from
conductive polymers has important effects on system response
time to electronic stimuli and the current carrying capability of
the fiber. Using poly(3,4-ethylenedioxythiophene) (PEDOT) fibrous
mats it was demonstrated that the conductivity of the mats in-
creased exponentially as the fiber diameter was decreased. Packing
density of the molecules in finer diameter fibers could be a possible
reason for the enhanced conductivity in the fibers. It might also be
attributed to the intrinsic fiber conductivity effect or the geometric
surface effect resulting from the reduction in fiber diameters.
7.4. Filtration
Electrospun fibers with micron-sized diameters have found
extensive functions in filtration applications [23,9698]. Electro-
spun non-woven fabrics used for filtration provide the advantagesof high surface area to volume ratio, low air resistance, lower filter
mass and flexibility of adding surface functionality on the fibers by
blending or incorporating nanofillers [97]. Electrospun fibers are
being widely investigated for aerosol filtration, air cleaning appli-
cations in industry and for particle collection in clean rooms. Typ-
ically, aerosol particles are filtered due to the electrostatic
attraction that exists between the filter media and aerosol parti-
cles. Electrospun fibers used in filtration media can improve the
efficiency of filtration as the static charge used to produce the elec-
trospun fibers may remain in the fibers and help in the filtration of
aerosol particles. It is seen that the filtration efficiency of the elec-
trospun mats is comparable to the commercially available filters
but the advantage lies in the filter mass which is substantially low-
er for the former than the latter [23,98].It is well-known that as the surface area of the fibers is in-
creased, the surface adhesion properties of the fibers improve.
Hence, by decreasing the diameter of the fiber in the filter media,
the efficiency of capturing sub-micron sized particles can be signif-
icantly improved compared to the larger fibers. For efficient filtra-
tion, the sizes of the structural elements in the filter media have to
match the size of the particles of droplets that are to be captured
by the filter media. The advantage of using electrospun fibers in
the filtration media is that the fiber diameters can be easily con-
trolled and can make an impact in high efficiency particulate air
filtrations.
7.5. Filler reinforced fiber systems
Nanoscale reinforcements have been often used by researchers
to fabricate multi-functional high strength composites. Therefore,
novel fibrous composites can be obtained by incorporating high
strength and high aspect ratio fillers in fiber matrices
[1,10,11,87,99]. For instance, electrospinning is studied for fabri-
cating lightweight fibrous composite with unique properties for
protective clothing and body armor applications [1]. Commonly
used fillers are carbon nanotubes (CNTs), organoclay, hydroxyapa-
tite (HAP), silica and titania particles. Filler reinforced fibers have
many potential applications: ultra-strong wires, nanocomposites,
nanoprobes, electronic devices, tissue replacement materials
[10,11,87] etc. For example, addition of HAP particles to biodegrad-
able polymer fibers shows potential for bio-medical applications.
Similarly, CNT inclusion in electrospun fibers enhances the overallstrength and conductivity. Such CNT reinforced fibers are particu-
larly useful for miniaturized electronic components and load-bear-
ing applications.
The key factors that influence the reinforcing effects of the filler
in electrospun fibers are the dispersion, distribution and alignment
of the fillers in the fiber matrix. Electrospinning offers an efficient
route for obtaining homogeneous dispersion and distribution of
the nanoscale reinforcements [10,11]. Moreover, it is demon-
strated that the high electrostatic forces and shear force experi-
enced by the jet during electrospinning align the fillers along the
fiber axis. Good dispersion of the fillers is essential for constraining
the segmental motion of the molecular chains. Further, the use of
fillers such as CNTs in electrospun fibers is seen to align itself along
the fiber axis (see Fig. 19). The embedded CNT reduces the overall
mobility of the polymer chains and provides the confinement ef-
fect to the neighboring molecules. Thus, orientation of polymer
chains during electrospinning and the presence of hard fillers with-
in the fibers strengthen the fibers.
In our previous work [5], the reinforcing effect of the HAP filler
on a PCL matrix is verified by comparing the tensile strengths of
the electrospun fiber composites with those of the melt-processed
composites. Electrospinning is found to be far superior to melt-
processing. More interestingly, for electrospun fiber samples, filler
addition increases the tensile strength. However, filler addition de-
creases the tensile strength of melt-processed composites (bulk).
In the electrospun fibers, HAP particles are contained in the nanof-
Fig. 20. TEM micrograph of electrospun fiber filled with magnetite particles.
H (Oe)
-10000 -5000 0 5000 10000
M(emu/g)
-30
-20
-10
0
10
20
30
Magnetic Fiber
Fig. 21. Magnetic hysteresis loop of magnetic filler reinforced electrospun fiber.
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ibers and serve to constrain the segmental motion of the polymer
chains. Hence, the fiber-guided composites are seen to have en-
hanced tensile strength. It can be concluded that the fiber-guided
architecture creates a more effective reinforcement compared to
the filler-dispersion approach. Thus, the overall mechanical perfor-
mance of filler reinforced electrospun fibers is influenced by the
dispersion and orientation of the fillers within the fibers. This
observation can also be attributed to the re-ordering of molecular
chains in electrospun systems which is not seen in polymers pro-
cessed by melt flow [5].
8. Concluding remarks and future work
Since its discovery, significant advancement has been made in
R&D of electrospinning. Researchers have mostly focused on opti-
mizing the processing parameters to obtain fibers of desired
shapes and forms with little understanding of the parameters that
control the enhanced microstructures and mechanical properties.
Many polymers have been successfully electrospun into nanofi-
bers. However, very few studies can be found on the macromolec-
ular orientation and crystalline structures of the fibers. Through
this review, concepts behind the structures and morphology of
electrospun fibers are discussed, their effects on the mechanical
properties are emphasized, and future work identified. The unique
mechanical properties of the electrospun fibers described in this
study demonstrate the potential of using these fibers for miniatur-
ized polymer devices and composites applications.
Recently, researchers identified the size effect of fibers on the
structural and tensile properties of the fibers. For example, tensile
properties such as elastic modulus and strength increase with
decreasing fiber diameter. The enhanced orderliness of the amor-
phous phase in the supramolecular structure of the fiber plays an
important role in influencing the properties of the fibers. High
shear forces are seen to produce a skin and core morphology in
the fibers. The heterogeneity in the skin and core regions is estab-
lished due to the higher degree of chain orientation in the formercompared to the latter. The core region of the fiber shows bulk-like
properties and the skin region displays enhanced properties.
Hence, when the skin thickness is comparable to the overall fiber
diameter, both the tensile modulus and tensile strength are signif-
icantly increased.
The heterogeneity in the skin and core regions of the fibers is
more remarkable when the fibers are reinforced with CNTs. Inclu-
sion of CNTs in the fiber matrix presents an additional interface for
surface chain orientation [99]. Therefore, the overall chain orienta-
tion increases with the CNT loading. These oriented regions are
stiffer compared to the regions of disoriented chains. Conse-
quently, CNT reinforcement leads to stiffening and strengthening
of the fibers. To-date, there are no extensive experimental studies
designed to investigate the molecular structures of the core andskin regions of electrospun fibers. The surface behavior at the skin
can differ very much from the core properties of the fiber. Further
investigations on the strengthening mechanisms across the skin
and core regions are pressingly needed.
Current efforts have been mainly about the incorporation of fil-
ler materials to increase the strength of electrospun fibers
[44,87,99]. However, future work should focus more on the mul-
ti-functionality of nanofiber composites with fillers for specific
applications. For instance, electrospinning technique can be used
to incorporate magnetic fillers within the fiber matrix to obtain
super-paramagnetic nanostructured composite with controlled
geometry. Moreover, fibers with homogenous dispersion and dis-
tribution of the fillers are very attractive since the composite is ex-
pected to display enhanced magnetic-field dependentsuperparamagnetism. Such features are seldom achieved using
other conventional techniques. We have recently demonstrated
that magnetic particle electrospun fibers can display superpara-
magnetism at ambient temperature. A TEM micrograph of one such
composite with uniformly distributed magnetic particles is shown
in Fig. 20 [100] and its magnetic hysteresis loop at 300 K is plotted
in Fig. 21 [100]. In addition, the composite fibers are also seen to
deflect in the direction of increasing magnetic field and confirm
field responsive behavior. Further, the incorporation of magnetic
particles enhances the fiber elastic modulus. Thus, many attractive
features like mechanical strength, magnetic and conductive prop-
erties of the nanoparticles can be utilized to obtain multi-func-
tional composites. Developments of such nanostructured fiber
composites can be especially useful in miniaturized electronic
parts and for electromagnetic interference shielding. Also, the use
of biodegradable polymeric fiber as the carrier matrix can enhance
its usefulness in bio-medical, magnetic resonance imaging and
drug-delivery applications.
Acknowledgments
We would like to thank the Australian Research Council for the
support of this work. SCW acknowledges the financial support
from the US National Science Foundation under the CAREER Award
CMMI #0746703 and Award DMI #0520967. Sincere thanks are
due to TA Blackledge and DH Reneker for helpful discussions and
constructive comments during the preparation of this review
paper.
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