1. Introduction

Many advanced applications can benefit from electrospun materials with superior mechanical and dielectric properties, especially in the fields of composite reinforcement and energy. Aligned electrospun fibers, more specifically, have applications in structural reinforcement of materials and energy storage devices. For these applications, it is paramount to understand the effects of electrospun fiber alignment on mechanical and dielectric properties. Though adding appropriate fillers to the polymers changes mechanical and dielectric properties, better fiber alignment alone improves these properties and keeps the composition uniform throughout. Mechanical and dielectric properties depend on the density and porosity of nanofiber mats, as well as the fiber morphology, including the fiber diameter, and the effect of degree of alignment [1–3]. Therefore, it is necessary to understand the knowledge on both mechanical and dielectric properties of polymer mats together. Mechanical and dielectric properties are among the most important parameters to determine the performance of the polymeric nanomaterials [4,5]. Electrospinning influences both mechanical and dielectric properties of nanofiber membranes [4]. Electrospinning is the process of producing micro- and nanofibers, using a polymer solution with a syringe pump, syringe, needle, collector, and high-voltage power supply. The typical setup of an electrospinning apparatus is either horizontal electrospinning or vertical electrospinning [6]. Figure 1 shows the schematic setup of both types.

Figure 1. (a) Schematics of electrospinning apparatus of vertical setup and (b) horizontal setup.

2. History of Electrospinning

The idea of electrospinning can be traced back to 1900, when John. F Cooley received the patent for his apparatus for electrically separating the relatively volatile liquid component from the component of relatively fixed substances of composites [7]. Later in 1902, John F. Cooley invented an apparatus for electrically dispersing fluids [8] and William James Morton invented methods of dispersing fluids by the process of separating the volatile components and breaking up the fixed component from composite fluids [9]. Anton Formhals received a patent in the year 1934 for his invention of producing polymer threads, using electrostatic force. In his paper titled “Process and apparatus for preparing artificial threads” [10], solutions of cellulose esters, specifically cellulose acetate were used for spinning. In US Patent No. 2,160,962 (1939), artificial fibers were collected as substantially parallel to each other on a moving collecting device [11]. There he introduced the term “electrical spinning” of fibers. In the spinning process there were difficulties in solidifying the formed fibers. In addition, the as-processed fibers were so sticky that, not only would they stick to the collecting device, but also they would stick to each other. He observed that it was difficult to control the paths of high-speed liquid streams and the corresponding fibers out of it. As shown in Figure 2, fiber direction guide (55 in Figure 2), which consists of shields (57 in Figure 2) to direct the fibers along fixed, predetermined paths toward the collecting electrodes was used. This invention made it possible to obtain smooth, continuous, compact, and coherent fiber bands composed of heterogeneous filaments arranged substantially parallel to each other.

Zhang et al. (2016) reported that different nanofiber production methods include vapor growth, arc discharge, laser ablation, and chemical vapor deposition [12]. These processes are very expensive because of low product yield and high equipment cost. However, electrospinning employs a top-down engineering approach, which can produce fibers with diameters ranging from 10 nm to 10 µm, from a polymer solution, under the application of an electrostatic force [13,14]. These fibers have a high surface area to volume ratio, high porosity, and tunable porosity [6]. According to Luo et al. (2012), there are various spinning techniques available for producing micro and nanofibers [14]. Solution electrospinning compared to melt electrospinning requires a solvent. The melt electrospinning method uses a molten polymer, but the absence of solvent excludes the effect of solvent properties on the fiber formation. In emulsion electrospinning, two immiscible fluids are used as in food-processing [15]. Magnetic electrospinning and near-field electrospinning are good examples of interdisciplinary technological convergence between magnetism and electric potential methods. Dip-pen nanolithography with traditional electrospinning can also be used, but the alignment of fibers is not satisfactory [14].

Figure 2. Electrospinning setup by A. Formhals [11].

3. Working Principle of Electrospinning

The working principle for electrospinning is shown in the Figure 3. A sufficiently high voltage is applied at the location of the liquid droplets formed at the tip of the needle. The local body of the liquid becomes charged. Electrostatic repulsion counteracts the surface tension. Thus, the droplet is stretched, and at a critical point, a stream of liquid erupts from the surface. The point of eruption is called a Taylor Cone. Sir Geoffrey Taylor developed the equation which shows the relationship between the critical voltage and the surface tension as shown in Equation (1) [16,17].

where  is the critical voltage, H is distance between the needle tip and the collector, L is the length of the needle with radius R, and  is the surface tension of the liquid (units: in kilovolts; H, L, and R in cm; and  in dyne per cm). Afshari (2017) showed that electrostatic forces play a key role on the electrospinning of polymer solutions [18]. As such, the Coulomb’s force is considered as the driving factor for better design. In Equation (2) shown below, F is the Coulomb’s force,  is the constant of proportionality,  are charges, and r is the distance between the charges. In principle, the smaller the distance, the greater the electrostatic force on a charged particle.

When an electric field is applied, the liquid jet ejected from the tip of the nozzle/needle travels on a straight line for a short distance. The diameter of the jet, in the straight line, decreases monotonically with the distance from tip, after that a radially outward bending instability happens. The electrostatic force from the charge carries with the jet causes the jet to continue to elongate as it coils and the thin fluid jet solidifies into nanofiber [19].

Figure 3. Working principle of electrospinning.

4. Applications of Electrospun Fibers

The typical applications of electrospun fibers include filtration, energy, structures, biomedical, textiles, and others [6] as shown in Figure 4. Other applications include optical and chemical sensors, textiles, reinforcement of composites, health care, and defense and security. Electrospun fibers are projected to play an important role in the development of air filtration, energy storage devices, super-capacitors, and rechargeable batteries [20–26].

Figure 4. Applications of electrospun nanofibers.

The applications of nanofiber mats in the reinforcement of nanocomposites are discussed by Huang et al. (2003), who executed mechanical characterization of nanofibrous membranes of various polymers and examined their potential applications [17]. Nanofibers can have better mechanical properties than microfibers and therefore superior structural properties can be anticipated. Jiang et al. (2018) provided an overview of nanofiber composite application [27]. Bergshoef and Vancso (1999) showed that smooth nylon-4, six electrospun fibers with diameters in the range of 30–200 nm can be produced from formic acid solutions. These fibers demonstrated reinforcement of transparent composites with an epoxy matrix [28]. Highly porous nanofibers with pore interconnectivity and relatively uniform pore distributions improve membrane performance in the application of desalination (water filtration) [29]. The large surface area of the constituent fibers provides high functionalization capability and mechanical bonding to limit delamination between laminae [30]. Biomedical applications include tissue engineering scaffolds, wound dressing, drug delivery [31], and creation of artificial blood vessels. The non-woven nanofibrous mats produced by electrospinning techniques mimic the extracellular matrix components.

Some important issues and challenges in the 21st century are addressed by using electrospun fibers in the domains of tissue regeneration, energy conversion and storage, and water treatment. Large surface areas, high porosity, and the unique mat structure of electrospun nanofibers have provided improvements over the last decade in the fields of tissue regeneration (skin [32–34], nerve [35–37], heart [38–40], and bone [41–43]), energy conversion and storage (solar cells [44–47], fuel cells [48–51], and batteries [52–55]), and water treatment (adsorption [56–58], photocatalysis [59–61], and filtration [62–64]). Potential applications and promising advantages are overviewed by Bhardwaj and Kundu (2010), who highlighted more than 200 polymers that are electrospun for various applications. Teo and Ramakrishna (2006) gave a detailed review on electrospinning design and nanofiber assemblies [65].

1.4. Recent Review Papers on Electrospun Nanofiber
Table 1 lists twelve recent review papers on electrospun fiber applications and characterizations.
Of the papers considered, reviews of applications dominate the literature, are
a few on mechanical, energy, medical, and processing characterizations. However, there
is little work found in the literature on dielectric and mechanical properties together that
should contribute to both composite reinforcement and energy applications.
Table 1. Recent review papers on electrospun nanofibers.
Authors Year Main Criteria of Review Papers
Huang et al. 2003 Processing, structure, characterization, applications, modeling and simulation,
and different polymers in solution and melt form [17]
Pham et al. 2006 Tissue engineering (scaffolds) [66]
Bhardwaj and Kundu 2010 Polymers, parameters, melt electrospinning, and applications [6]
Luo et al. 2012 Scale-up challenges and applications [14]
Shuakat et al. 2014 Nanofiber yarns and nanofiber alignment [67]
Shi et al. 2015
1D nanomaterials have high surface-area-to-volume (specific surface area), high aspect
ratio, and high pore volume. Well-aligned and highly ordered are suitable for energy
harvesting and storage devices. More advantageous than conventional materials [68]
Ahmed et al. 2015 Desalination [29]
Zhang et al. 2016 Energy storage [12]
Peng et al. 2016 Tissue regeneration, energy conversion and storage, and water treatment [23]
Shekh et al. 2017 Water purification [69]
Zhang et al. 2018 Food packaging [15]
Li et al. 2019 Electrical and mechanical performance of polymer nanocomposites [70]
1.5. Parameters and Parameter Optimizations
Important parameters that affect the quality of electrospun fibers formed from polymer
solutions can be categorized as solution-specific parameters, process-specific parameters,
and environmental-specific parameters [6,17,71].
(a) Solution parameters: The solution-specific parameters include viscosity, polymer concentration,
surface tension, conductivity, and evaporation rate of solvent [18,72–76].
It is observed that low viscosity is typically responsible for bead generation and
significant increase in fiber diameter. A similar conclusion was made on polyacrylonitrile/
dimethylformamide (PAN/DMF) solution where beads were easier to form at
low concentration of 5 wt.% than that formed at higher concentration of 7 wt.% [77,78].
Typically, viscosity and concentration are directly proportional to each other [79].
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Additionally, polymer concentration directly controls fiber diameter [79]. In general,
an increase in fiber diameter can be achieved by increasing the polymer concentration.
Higher surface tension causes bead formation and reduced surface tension favors
smooth fiber formation [80].
(b) Process parameters: Applied voltage, distance between the nozzle tip and collector,
rotating speed of the collector (if drum is used), and solution feed rate are the parameters
that are regarded as process specific [18,81–83]. In general, fiber diameter can be
reduced by increasing applied voltage and vice versa. If the applied voltage reaches a
critical value, a charged jet initiates the electrospinning process. This critical voltage
is closely related to surface tension of the solution. Lee et al. (2003) reported that there
was a linear relationship between voltage applied and surface tension of polystyrene
(PS) dissolved in a mixture of tetrahydofuran and DMF [84]. The distance between the
tip and the collector mainly controls fiber solidification because a minimum distance
is required to allow the fibers sufficient time to dry before reaching the collector. Distances
that are too close or too far can cause beads to form. Fang et al. (2010) studied
7 wt.% PAN/DMF electrospun at 2–10 cm away from nozzle tip. The experiments
concluded that beads were producing until the distance reached 7 cm [78]. Longer
distance between nozzle tip and collector produced bead free fibers.
(c) Environmental parameters: Humidity and temperature are treated as environmentspecific
parameters [18,85,86]. According to De Vrieze et al. (2009), the evaporation
rate increases with increase in temperature [87]. Moreover, the viscosity of solution
generally decreases with an increase in temperature. As the humidity increases, the
average fiber diameter increases. Parameter optimization: Formation of nanofibers
involve many input parameters, as mentioned above, to evaluate outputs such as
fiber diameter, tensile strength, modulus, and dielectric properties of nanofibers.
Parameter optimization helps to achieve desired outputs by tailoring the input parameters.
One among the many mathematical modeling techniques for parameter
optimization is Design of Experiment (DoE), which is an approach that helps to find
the relationship between different inputs over outputs. Parameter optimization based
on applied voltage and concentration has been studied by using the DoE approach
by Gu et al. (2005) [88]. The study concluded that concentration of solution played
an important role to the diameter of nanofibers. Gu et al. (2005) used two factors
and four and three respective levels for finding average fiber diameter. Senthil and
Anandhan (2005) examined three variables and seven, four, and three respective
factor levels for finding the average fiber diameter [89]. Isaac et al. (2018) used DoE
approach with two factors and three levels for optimizing the two outputs, namely,
specific dielectric constant and specific mechanical strength [90,91]. A mathematical
modeling, including the leaky dielectric model which describes the deformation of a
Newtonian drop in an electric field and whipping model which depicts the interaction
between the electric field and fluid properties for electrospinning processes, has
been portrayed by Rafiei et al. (2013) [92]. Ismail et al. (2016) developed a model for
stable region and unstable region in the jet propulsion stream for predicting the fiber
diameter [93]. Rafiei et al. (2014) modeled and simulated viscoelastic elements for
jet propulsion to predict and improve control of nanofiber diameter [94]. Modeling
electrospinning of nanofibers for short-range and long-range electrostatic interactions,
using a discrete slender model, was conducted by Kowalewski et al. (2009) [95]. The
whipping instability in the unstable region of the electrospinning jet propagation has
been studied in three polymeric solutions by Kowalewski et al. (2005) [96]. The fiber
gets stretched into fractions of initial diameter at the instability region. Ghaly (2014)
modeled the electrospinning jet with an inkjet printer technique, using computeraided
fluid/multi-physics/multi-phase flow simulations in COMSOL multiphysics
software [97].
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2. Molecular Orientation and System Configurations of Nanofibers
Two key factors affecting mechanical and dielectric properties are (a) molecular orientation
due to elongation of fibers on the periphery of the rotating mandrel [98] and
(b) system configuration improvement for obtaining improved properties due to better
alignment [99]. Other properties, such as thermal and electrical properties [100] are also
often improved by alignment.
2.1. Molecular Orientation of Nanofibers
High orientation of polymer molecular chains along the fiber axis and aligned electrospun
fibers have important consequences in the field of carbon fiber-reinforced nanocomposites.
The electrospun fibers are generally stronger than traditional fibers because of their
higher orientation of macromolecular polymer chains along the fiber axis. The polymer jet
under the influence of an electrostatic field experiences a high degree of molecular orientation
due to high elongation strains and shear forces. As explained later, in Section 2.2,
System Configuration to Align Fibers, optimal speed of fiber collecting drum brings about
better alignment. In addition, optimal speed of collecting drum causes maximum molecular
orientation. Beyond the optimal speed, the orientation can decrease slightly. According
to Fennessey and Farris (2004), twisted yarns of higher degree of molecular orientation
resulted in better mechanical properties [99]. The degree of orientation can be quantified
by the X-ray diffraction analysis of the samples. The nitrile group in PAN is oriented
in approximately perpendicular to the draw direction. The absorbance of perpendicular
polarization showed nitrile-stretching vibration with strong dichroism, and therefore better
orientation. A twist angle of 11 in as-spun PAN fiber improved the initial modulus and ultimate
strength of 2.6 GPa and 56 MPa, respectively, to 2.2 times and 2.9 times, respectively.
Molecular orientation results in better mechanical properties in general, Young’s modulus
in particular, of the resulting carbon fibers [101,102]. Baji et al. (2010) studied the effects
of electrospun polymers on oriented morphology and tensile properties. The lower the
diameter of the fibers, the higher the modulus and strength of the fibers. They observed
that finer fibers have enhanced properties because of gradual ordering of molecular chains
and increase in crystallinity [103]. Baji et al. (2010) noticed that the modulus and tensile
properties of polycaprolactone (PCL) fibers increased significantly when the fiber diameter
was reduced to below 500 nm. The molecular orientation improves gradually as the fiber
diameter is reduced. Moreover, Beese et al. (2013) concluded that electrospun PAN fibers
have better mechanical properties at lower diameters [104]. Arshad et al. (2011) observed
that the strength of the carbonized nanofibers at 800 C increased by 100% when the diameter
was reduced from 800 to 200 nm [105]. For composite applications, a decrease in
diameter of fiber at the nanoscale level can improve mechanical properties as the specific
reinforcement area per unit mass increases. Uyar et al. (2009) observed self-aligned bundled
fibers of polyphenylene-g-polystyrene/poly (a-caprolactone) (PP-g-PS/PCL) when
blended with polystyrene (PS) or polymethyl methacrylate (PMMA). This is because of the
unique molecular architecture of PP-g-PS/PCL and its interaction with PS or PMMA [106].
2.2. System Configuration to Align Fibers
In addition to molecular orientation, physical alignment of electrospun fibers contributes
to the production of high strength/high toughness fiber reinforced composites [12].
Among the many ways to produce aligned fibers by using an electrospinning technique,
drum collection and rotating disk collectors are the two most popular designs, as shown in
Figure 5a,b [65,107–111].
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breaks the fiber, and continuous fibers are not collected. Therefore, the optimal speed of the rotating
drum that matches the evaporation rate is required for maximum alignment [115].
Figure 5. (a) Rotating drum collector. (b) Rotating disk collector [103].
In the rotating disk collector, higher alignment is possible but the production rate is lower
because fibers are effectively deposited at only a small area at the disk edge. Theron et al. (2001)
reported a conical and an inverted conical instability region of polyethylene-based polymer
nanofibers. These finer fibers with diameters ranging from 100 to 300 nm got aligned and wound on
a sharp edge disk wheel-like bobbin [116].
Other collection methods that are suitable for fiber alignment are the parallel conductor method,
the wire drum collector method, and the wheel rotor collector method as shown in Figure 6a–d. In
the parallel conductor method [117–120], the length of aligned fibers is restricted by the distance
between conductive stripes as shown in Figure 6a. Jalali et al. (2006) reported the fundamental
parameters affecting the uniaxially aligned PAN nanofibers. The best alignment of nanofibers with a
specific gap distance depends on concentration, voltage, and tip to collector distance. As shown in
Figure 6b, a bundle collector is moved across the gap to another side for depositing bundle of
nanofibers. The best alignment was formed between 10 and 15 wt.% solutions. Uniaxially aligned
fibers formed had an aspect ratio (l/d) of higher than 5000 and these fibers are useful in composite
reinforcement application. Fryer et al. (2018) studied the effect of alignment on fiber modulus, using
the electrostatic gap method. Aligned polyethylene oxide (PEO) fibers have a higher modulus than
the non-aligned fibers of similar diameter [121]. Cai et al. (2017) provided an insight in to fabricating
ultra-long polyvinylidene fluoride (PVDF) fibers. Here, parallel conducting U-shaped collectors are
used to fabricate fibers [122]. According to Lei et al. (2018), more than a meter long aligned PVDF
nanofibers were fabricated by using gap electrospinning, where the needle is connected to positive
power supply and the parallel plates are connected to the negative power supply [123]. Yang et al.
(2007) demonstrated a method that generates parallel fibers, using magnetic-particle-doped polymers
in two parallel placed magnets. The magnetic field guides the magnetized electrospun polyvinyl
alcohol (PVA) fibers to align in a parallel fashion [124]. Park and Yang (2011) built uniaxial aligned
PCL fibers by introducing an inclined gap into dual collectors that consisted of two conductive stripes
which were arranged vertically and horizontally [125]. Dabirian et al. (2009) used a hollow metallic
cylinder with needle placed at the center of the cylinder. Fibers produced by this method are claimed
to be well aligned and spread over large area [126]. Next, in the wire drum collector method, shown
in Figure 6c, fibers are deposited without the need for high speed rotation [126]. However, aligned
thick films are not possible with this method. Finally, a wheel rotor collector, as shown in Figure 6d,
provides elongation strain and therefore more strength to the fibers. However, the many electrodes
on the rotating wheel complicate apparatus design [127]. The limitations of other methods imply that
drum collection is more likely to be scalable to commercial capacities than other electrospinning
methods for fiber alignment. Despite the simplicity of the electrospinning methodologies, industrial
Figure 5. (a) Rotating drum collector. (b) Rotating disk collector [103].
In the drum collector method, the drum collector rotates at high speed and the deposited
fiber diameter can be controlled based on the rotational speed of the drum [112–114].
The linear speed at the surface of the rotational drum should match the evaporation rate
of the solvent so that fibers are deposited and taken up on the surface of the drum. At a
rotational speed less than the fiber take-up speed, randomly oriented fibers are obtained
on the drum. At higher speed, the fiber take-up velocity breaks the fiber, and continuous
fibers are not collected. Therefore, the optimal speed of the rotating drum that matches the
evaporation rate is required for maximum alignment [115].
In the rotating disk collector, higher alignment is possible but the production rate is lower
because fibers are effectively deposited at only a small area at the disk edge. Theron et al. (2001)
reported a conical and an inverted conical instability region polyethylene-based polymer
nanofibers. These finer fibers with diameters ranging from 100 to 300 nm got aligned and
wound on a sharp edge disk wheel-like bobbin [116].
Other collection methods that are suitable for fiber alignment are the parallel conductor
method, the wire drum collector method, and the wheel rotor collector method as shown
in Figure 6a–d. In the parallel conductor method [117–120], the length of aligned fibers is restricted
by the distance between conductive stripes as shown in Figure 6a. Jalali et al. (2006)
reported the fundamental parameters affecting the uniaxially aligned PAN nanofibers.
The best alignment of nanofibers with a specific gap distance depends on concentration,
voltage, and tip to collector distance. As shown in Figure 6b, a bundle collector is moved
across the gap to another side for depositing bundle of nanofibers. The best alignment
was formed between 10 and 15 wt.% solutions. Uniaxially aligned fibers formed had an
aspect ratio (l/d) of higher than 5000 and these fibers are useful in composite reinforcement
application. Fryer et al. (2018) studied the effect of alignment on fiber modulus, using the
electrostatic gap method. Aligned polyethylene oxide (PEO) fibers have a higher modulus
than the non-aligned fibers of similar diameter [121]. Cai et al. (2017) provided an insight in
to fabricating ultra-long polyvinylidene fluoride (PVDF) fibers. Here, parallel conducting
U-shaped collectors are used to fabricate fibers [122]. According to Lei et al. (2018), more
than a meter long aligned PVDF nanofibers were fabricated by using gap electrospinning,
where the needle is connected to positive power supply and the parallel plates are connected
to the negative power supply [123]. Yang et al. (2007) demonstrated a method that
generates parallel fibers, using magnetic-particle-doped polymers in two parallel placed
magnets. The magnetic field guides the magnetized electrospun polyvinyl alcohol (PVA)
fibers to align in a parallel fashion [124]. Park and Yang (2011) built uniaxial aligned PCL
fibers by introducing an inclined gap into dual collectors that consisted of two conductive
stripes which were arranged vertically and horizontally [125]. Dabirian et al. (2009) used a
hollow metallic cylinder with needle placed at the center of the cylinder. Fibers produced
by this method are claimed to be well aligned and spread over large area [126]. Next, in the
Fibers 2021, 9, 4 9 of 32
wire drum collector method, shown in Figure 6c, fibers are deposited without the need for
high speed rotation [126]. However, aligned thick films are not possible with this method.
Finally, a wheel rotor collector, as shown in Figure 6d, provides elongation strain and
therefore more strength to the fibers. However, the many electrodes on the rotating wheel
complicate apparatus design [127]. The limitations of other methods imply that drum
collection is more likely to be scalable to commercial capacities than other electrospinning
methods for fiber alignment. Despite the simplicity of the electrospinning methodologies,
industrial applications are relatively rare due to low fiber throughput for existing fiber
collection methods. This throughput limitation could be addressed with larger drum sizes
and other innovations.
Fibers 2020, 8, x FOR PEER REVIEW 9 of 33
applications are relatively rare due to low fiber throughput for existing fiber collection methods. This
throughput limitation could be addressed with larger drum sizes and other innovations.
Figure 6. (a) Parallel conductor stripes method (reprinted with permission from Reference [128].
Copyright (2003) American Chemical Society). (b) Uniaxially aligned nanofibers [129]. (c) Wire drum
collector (reprinted with permission from Reference [130]. Copyright (2004) American Chemical
Society). (d) Wheel rotor collector.
In addition to the design methods mentioned above, there a few unconventional design
configurations for aligning fibers. Grasl et al. (2013) developed a technique, using two parallel
rotatable auxiliary electrodes applied with time-varying square wave potential, which led to aligned
fiber-deposition of PEO [131]. Lei et al. (2017) used a collecting system consisting of insulating hollow
cylinder and grating-like electrodes for aligning PVDF fibers, using whipping instability [131].
Khamforoush and Mahjob (2011) used a modified rotating jet method for aligning fibers. The degree
of alignment enhanced by more than two times and the average amount of produced fiber is 40 %
more than that of the simple rotating jet method [132,133].
3. Mechanical and Dielectric Properties of Nanofibers
Carbonaceous materials such as carbon black, fullerene, carbon nanotubes, carbon nanofibers,
and graphene extend the functionalities of polymers from lightweight and cost-effective to new
applications such as electrically and thermally conductive, electromagnetic shielded, etc. With everincreasing
utilities of multifunctional polymer composites applicable in the electronics, sensors,
energy, automobile, and aerospace industries, the mechanical properties and electrical are among the
two most important parameters to determine the performance of polymeric nanocomposites [70].
3.1. Mechanical Properties
Among many nanofibers that have mechanical properties, PAN nanofibers are widely preferred
because of their excellent tensile strength and modulus. In this section, mechanical properties of PAN
nanofibers (in general) and carbon nanofillers are discussed. The mechanical properties of
electrospun nanofibers, including PCL, polyvinyl pyrrolidone (PVP), and PEO, are also reviewed.
Figure 6. (a) Parallel conductor stripes method (reprinted with permission from Reference [128]. Copyright (2003) American
Chemical Society). (b) Uniaxially aligned nanofibers [129]. (c) Wire drum collector (reprinted with permission from
Reference [130]. Copyright (2004) American Chemical Society). (d) Wheel rotor collector.
In addition to the design methods mentioned above, there a few unconventional
design configurations for aligning fibers. Grasl et al. (2013) developed a technique, using
two parallel rotatable auxiliary electrodes applied with time-varying square wave potential,
which led to aligned fiber-deposition of PEO [131]. Lei et al. (2017) used a collecting system
consisting of insulating hollow cylinder and grating-like electrodes for aligning PVDF
fibers, using whipping instability [131]. Khamforoush and Mahjob (2011) used a modified
rotating jet method for aligning fibers. The degree of alignment enhanced by more than
two times and the average amount of produced fiber is 40% more than that of the simple
rotating jet method [132,133].
3. Mechanical and Dielectric Properties of Nanofibers
Carbonaceous materials such as carbon black, fullerene, carbon nanotubes, carbon
nanofibers, and graphene extend the functionalities of polymers from lightweight and
cost-effective to new applications such as electrically and thermally conductive, electromagnetic
shielded, etc. With ever-increasing utilities of multifunctional polymer composites
applicable in the electronics, sensors, energy, automobile, and aerospace industries, the
mechanical properties and electrical are among the two most important parameters to
determine the performance of polymeric nanocomposites [70].
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3.1. Mechanical Properties
Among many nanofibers that have mechanical properties, PAN nanofibers are widely
preferred because of their excellent tensile strength and modulus. In this section, mechanical
properties of PAN nanofibers (in general) and carbon nanofillers are discussed. The
mechanical properties of electrospun nanofibers, including PCL, polyvinyl pyrrolidone
(PVP), and PEO, are also reviewed. Post-treatment techniques, such as drawing and annealing
processes, are discussed, since these post-treatments can improve molecular orientation
and crystallinity [134]. Moreover, PAN nanofibers in composite applications and other
types of nanofibers are discussed in the end.
3.1.1. PAN Nanofibers and Carbon Fillers
Edie (1998) reported that PAN-based carbon fibers have higher tensile strengths and
reasonable tensile moduli compared to pitch-based carbon (micro-size) fibers [135]. PAN
precursor is used for carbon nanofiber (CNF) production in nanocomposite structures [136]
and in energy storage devices [137] due to their good electrospinning property, high
carbonization yield, excellent nanostructure, ultrahigh specific surface area, good electrical
conductivity, and stability. PAN is soluble in polar solvents like dimethylformamide
(DMF), dimethylsulfone (DMSO2), dimethylsulfoxide (DMSO), and dimethylacetamide
(DMAc) [13]. Among organic solvents, DMF and DMSO are known to be good solvents of
PAN and for production of high-performance PAN fibers, DMSO is preferred [138].
Chawla et al. (2017) observed that carbon nanofibers obtained from hot-drawn samples
demonstrated strength as high as 5.4 GPa and modulus 287 GPa as shown in Figure 7a,b.
Here, 400 nm PAN nanofibers reached a maximum strength (5.4 GPa) after hot-drawn and
carbonized at 1100 C [139].
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treatments can improve molecular orientation and crystallinity [134]. Moreover, PAN nanofibers in
composite applications and other types of nanofibers are discussed in the end.
3.1.1. PAN Nanofibers and Carbon Fillers
Edie (1998) reported that PAN-based carbon fibers have higher tensile strengths and reasonable
tensile moduli compared to pitch-based carbon (micro-size) fibers [135]. PAN precursor is used for
carbon nanofiber (CNF) production in nanocomposite structures [136] and in energy storage devices
[137] due to their good electrospinning property, high carbonization yield, excellent nanostructure,
ultrahigh specific surface area, good electrical conductivity, and stability. PAN is soluble in polar
solvents like dimethylformamide (DMF), dimethylsulfone (DMSO2), dimethylsulfoxide (DMSO), and
dimethylacetamide (DMAc) [13]. Among organic solvents, DMF and DMSO are known to be good
solvents of PAN and for production of high-performance PAN fibers, DMSO is preferred [138].
Chawla et al. (2017) observed that carbon nanofibers obtained from hot-drawn samples
demonstrated strength as high as 5.4 GPa and modulus 287 GPa as shown in Figure 7a,b. Here, 400
nm PAN nanofibers reached a maximum strength (5.4 GPa) after hot-drawn and carbonized at 1100
°C [139].
Figure 7. (a) Typical stress–strain curve and (b) the average fiber diameter of carbon nanofibers
(CNFs) [139].
As shown in Figure 8a, the thinnest fiber had the highest strength when compared to other
thicker fibers. Moreover, it is evident from Figure 8b that the thinnest fiber had the highest modulus.
Papkov et al. (2013) [140] studied simultaneous improvement in strength, modulus, and toughness
in ultrafine as-spun PAN electrospun nanofibers as shown in Figure 8a,b. A reduction of as-spun
PAN nanofiber diameter from 2.8 μm to 100 nm resulted in higher modulus, strength, and toughness.
The 100 nm annealed PAN fibers showed a modulus of 48 GPa and strength of 1.75 GPa. This study
recorded dramatic increase in strength and modulus for nanofibers finer than 200–250 nm.
Figure 7. (a) Typical stress–strain curve and (b) the average fiber diameter of carbon nanofibers (CNFs) [139].
As shown in Figure 8a, the thinnest fiber had the highest strength when compared
to other thicker fibers. Moreover, it is evident from Figure 8b that the thinnest fiber had
the highest modulus. Papkov et al. (2013) [140] studied simultaneous improvement in
strength, modulus, and toughness in ultrafine as-spun PAN electrospun nanofibers as
shown in Figure 8a,b. A reduction of as-spun PAN nanofiber diameter from 2.8 m to
100 nm resulted in higher modulus, strength, and toughness. The 100 nm annealed PAN
fibers showed a modulus of 48 GPa and strength of 1.75 GPa. This study recorded dramatic
increase in strength and modulus for nanofibers finer than 200–250 nm.
Fibers 2021, 9, 4 11 of 32
Fibers 2020, 8, x FOR PEER REVIEW 11 of 33
Figure 8. (a) Stress–strain graph of as-spun polyacrylonitrile (PAN) nanofibers. (b) Strength and
modulus of annealed nanofibers. (Reprinted with permission from Reference [140]. Copyright (2004)
American Chemical Society.)
Beese et al. (2013) [104] compared the result with Arshad et al. (2011) [105] and showed the
dependence of fiber diameter on strength and modulus as shown in Figure 9. As the fiber diameter
decreases, the tensile strength and modulus increase. Beese et al. (2013) observed that individual PAN
nanofibers with 108 nm in diameter, heat treated at 800 °C, showed a maximum modulus of 262 GPa
and strength of 7.3 GPa.
Figure 9. Strength and modulus of individual carbon nanofiber obtained from PAN nanofiber mat
[104].
Arshad et al. (2011) [105] showed that individual PAN nanofibers with 9 wt.% and other given
parameters had the stress–strain relationship shown in Figure 10. The plot shows a linear relationship
until 125 MPa and thereafter a strain hardening region where crystallinity occurs. The maximum
ultimate strength was found with 1 kV/cm and 430 nm in diameter. Arshad et al. demonstrated that
increase in carbonization temperature in CNFs monotonically increases elastic modulus, while
highest strength of CNFs was observed at a carbonization temperature of 1400 °C. This study
revealed the fiber diameter and carbonization temperature had effect on strength and modulus of
PAN nanofibers.
Figure 8. (a) Stress–strain graph of as-spun polyacrylonitrile (PAN) nanofibers. (b) Strength and
modulus of annealed nanofibers. (Reprinted with permission from Reference [140]. Copyright (2004)
American Chemical Society).
Beese et al. (2013) [104] compared the result with Arshad et al. (2011) [105] and showed
the dependence of fiber diameter on strength and modulus as shown in Figure 9. As the
fiber diameter decreases, the tensile strength and modulus increase. Beese et al. (2013)
observed that individual PAN nanofibers with 108 nm in diameter, heat treated at 800 C,
showed a maximum modulus of 262 GPa and strength of 7.3 GPa.
Fibers 2020, 8, x FOR PEER REVIEW 11 of 33
Figure 8. (a) Stress–strain graph of as-spun polyacrylonitrile (PAN) nanofibers. (b) Strength and
modulus of annealed nanofibers. (Reprinted with permission from Reference [140]. Copyright (2004)
American Chemical Society.)
Beese et al. (2013) [104] compared the result with Arshad et al. (2011) [105] and showed the
dependence of fiber diameter on strength and modulus as shown in Figure 9. As the fiber diameter
decreases, the tensile strength and modulus increase. Beese et al. (2013) observed that individual PAN
nanofibers with 108 nm in diameter, heat treated at 800 °C, showed a maximum modulus of 262 GPa
and strength of 7.3 GPa.
Figure 9. Strength and modulus of individual carbon nanofiber obtained from PAN nanofiber mat
[104].
Arshad et al. (2011) [105] showed that individual PAN nanofibers with 9 wt.% and other given
parameters had the stress–strain relationship shown in Figure 10. The plot shows a linear relationship
until 125 MPa and thereafter a strain hardening region where crystallinity occurs. The maximum
ultimate strength was found with 1 kV/cm and 430 nm in diameter. Arshad et al. demonstrated that
increase in carbonization temperature in CNFs monotonically increases elastic modulus, while
highest strength of CNFs was observed at a carbonization temperature of 1400 °C. This study
revealed the fiber diameter and carbonization temperature had effect on strength and modulus of
PAN nanofibers.
Figure 9. Strength andmodulus of individual carbon nanofiber obtained fromPANnanofibermat [104].
Arshad et al. (2011) [105] showed that individual PAN nanofibers with 9 wt.% and
other given parameters had the stress–strain relationship shown in Figure 10. The plot
shows a linear relationship until 125 MPa and thereafter a strain hardening region where
crystallinity occurs. The maximum ultimate strength was found with 1 kV/cm and 430 nm
in diameter. Arshad et al. demonstrated that increase in carbonization temperature in CNFs
monotonically increases elastic modulus, while highest strength of CNFs was observed
at a carbonization temperature of 1400 C. This study revealed the fiber diameter and
carbonization temperature had effect on strength and modulus of PAN nanofibers.
Fibers 2021, 9, 4 12 of 32
Fibers 2020, 8, x FOR PEER REVIEW Figure 10. Stress–strain relationship of individual PAN nanofibers [105].
Wan et al. (2015) [1] reported that tensile strength of carbon nanofiber mats can be approximately
estimated as the sum of individual nanofibers as shown in Figure 11. The nanofiber mat reached maximum strength where the curves dropped sharply, which means the majority of nanofibers simultaneously. Wan et al. (2015) showed that a quantitative relationship exists between the strength of a nanofiber mat and that of individual nanofibers as in Figure 12. The tensile strength nanofiber mat is described by Equation (3).
Figure 11. Stress–strain curve of carbon nanofiber mats [1].
Figure 10. Stress–strain relationship of individual PAN nanofibers [105].
Wan et al. (2015) [1] reported that tensile strength of carbon nanofiber mats can be
approximately estimated as the sum of individual nanofibers as shown in Figure 11. The
nanofiber mat reached their maximum strength where the curves dropped sharply, which
means the majority of nanofibers break simultaneously. Wan et al. (2015) showed that a
quantitative relationship exists between the tensile strength of a nanofiber mat and that of
individual nanofibers as in Figure 12. The tensile strength of nanofiber mat is described by
Equation (3).
Fibers 2020, 8, x FOR PEER REVIEW Figure 10. Stress–strain relationship of individual PAN nanofibers [105].
Wan et al. (2015) [1] reported that tensile strength of carbon nanofiber mats can be approximately
estimated as the sum of individual nanofibers as shown in Figure 11. The nanofiber mat reached maximum strength where the curves dropped sharply, which means the majority of nanofibers simultaneously. Wan et al. (2015) showed that a quantitative relationship exists between the tensile
strength of a nanofiber mat and that of individual nanofibers as in Figure 12. The tensile strength nanofiber mat is described by Equation (3).
Figure 11. Stress–strain curve of carbon nanofiber mats [1].
Figure 11. Stress–strain curve of carbon nanofiber mats [1].
Fibers 2021, 9, 4 13 of 32
Figure 11. Stress–strain curve of carbon nanofiber mats [1].
Figure 12. Predicted tensile strength of individual nanofiber vs tensile strength Figure 12. Predicted tensile strength of individual nanofiber vosf nteannsoilfeibsetrre mngatth [1o]f. nanofiber mat [1].
s =
(1 ? P)( + sincos)2
2psin
sf (3)
where sf is the fiber strength and  is the diagonal angle of fibers to the longitudinal axis of
loading. The value of  can be obtained from L = D cos and W = D sin, where L, W, and
D are the length, width, and diagonal of rectangular tensile testing specimen. According
to Wan et al. (2015), tensile strength is also a function of the porosity of a nanofiber mat
as shown in Figure 13. Specific tensile strength is calculated by using Equation (5). Fiber
volume is determined by using Equation (4).
sp =
s
(1 ? P)
(4)
 is the fiber density and P is the porosity.
Vf = Vm(1 ? P) (5)
Vm is the volume of mat and P is the porosity.
Fibers 2020, 8, x FOR PEER REVIEW 13 of 33
? =
(1 − ?)(? + ????????)2
2?????
??
(3)
where ?? is the fiber strength and θ is the diagonal angle of fibers to the longitudinal axis of loading.
The value of θ can be obtained from L = D cosθ and W = D sinθ, where L, W, and D are the length,
width, and diagonal of rectangular tensile testing specimen. According to Wan et al. (2015), tensile
strength is also a function of the porosity of a nanofiber mat as shown in Figure 13. Specific tensile
strength is calculated by using Equation (5). Fiber volume is determined by using Equation (4).
σsp =
?
ρ(1 − ?)
(4)
ρ is the fiber density and P is the porosity.
?? = ??(1 − ?) (5)
?? is the volume of mat and ? is the porosity.
Figure 13. Tensile strength as a function of porosity [1].
Zhang et al. (2016) reported that carbon nanofibers are used in reinforcement of nanocomposites
[12]. A carbon nanotube (CNT) is one of the allotropes of carbon. According to Naebe et al. (2010),
electrospinning of CNT/polymer has been shown to induce alignment of nanotubes within the
matrix. Electropsun CNT/polymer nanofibers showed significant improvement in fiber strength,
modulus, and conductivity [141]. One dimensional CNF and CNT have high aspect ratios (typically
over a few hundred) that enable them to form a conductive network and they possess excellent
mechanical properties. According to Zhang et al. (2016), CNF, compared to CNT, exhibits good
Figure 13. Tensile strength as a function of porosity [1].
Zhang et al. (2016) reported that carbon nanofibers are used in reinforcement of
nanocomposites [12]. A carbon nanotube (CNT) is one of the allotropes of carbon. AccordFibers
2021, 9, 4 14 of 32
ing to Naebe et al. (2010), electrospinning of CNT/polymer has been shown to induce
alignment of nanotubes within the matrix. Electropsun CNT/polymer nanofibers showed
significant improvement in fiber strength, modulus, and conductivity [141]. One dimensional
CNF and CNT have high aspect ratios (typically over a few hundred) that enable
them to form a conductive network and they possess excellent mechanical properties.
According to Zhang et al. (2016), CNF, compared to CNT, exhibits good dispersion and
low fabrication cost. CNT has excellent electrical conductivities, a large surface area to
volume ratio, and structural stability [12]. As noticed by Chung (2016), CNT and CNF
are difficult to disperse and bond relatively weakly in a matrix. The large area of interface
associated with small diameter CNT and CNF aggravate the issue [142]. Addition
of small amount of CNT’s in PAN precursor nanofibers can improve graphitic order and
mechanical properties of carbon nanofibers [143,144]. Both single-walled carbon nanotube
(SWCNT) and multi-walled carbon nanotube (MWCNT) [145] can be used as fillers in
PAN to enhance molecular orientation. Two types of MWCNT are possible; one is long
MWCNT and the other is short MWCNT. Short MWCNTs are 0.5–2 m in length and the
diameter could be 30–50 nm. Graphene is another nanofiller used as a reinforcing material.
Compared to CNT and graphene, CNF is considered as a promising reinforcement material
due to its relatively high mechanical performance and low fabrication cost. Adding fillers
to precursor nanofibers improves tensile strength and tensile modulus as evident in the
session, drawing process below.
Drawing Process
Hot drawing of polymers is achieved by drawing the precursors at temperature above
the glass transition temperature. Chawla et al. (2017) successfully carried out hot-drawing
of PAN electrospun ribbons to enhance molecular orientation. PAN nanofiber ribbons were
hot-drawn to two times and four times their original length. These ribbons were further
stabilized at 250–300 C and then carbonized at 1100 C for 1 h. The tensile modulus and
strength of four times hot-drawn ribbons showed maximum values of 287 and 5.4 GPa,
which are 71% and 111% increment respectively as compared to as-spun electrospun
CNFs. Cai and Naraghi (2019) studied the templating effect of functionalized SWCNTs
in CNFs and the contribution of that to mechanical properties of CNFs. To enhance the
packing of polymer chains of the precursor around CNT’s, PAN as-spun electrospun fibers
were subjected to thermomechanical processing (hot-drawing). The MEMS-based singlenanofiber
mechanical testing result showed a strong relationship between the modulus,
strength improvement and hot-drawing process. The average tensile strength and modulus
of CNF/SWCNT were measured to be 7.6  1.72 and 268  29 GPa respectively [146].
Polyamide (PA) fibers are undergone post-drawing process to obtain moderate molecular
orientation and crystallinity. After the post-drawing process, PA is found to be with good
mechanical strength and abrasion resistance [134]. Yu et al. (2020) revealed enhanced
effect of graphene oxide (GO) in PAN nanofiber yarns. The alignment of PAN chains and
GO in nanofibers was enhanced by hot-drawing which resulted in increased orientation
induced crystallization [147]. Peng et al. (2019) experimentally found that hot-drawn
polyethylene fibers with decreasing fiber diameter, Young’s modulus increase rapidly. That
is due to the fact that chain orientation parameter increases with increasing hot-drawing
ratio [148]. Inai et al. (2006) reported that higher rotational speed of fiber collecting
disc and post-processing such as annealing and hot-drawing on molecular structure of
PLLA nanofibers improved crystalline structure orientation and mechanical strength of
fibers. Better mechanical properties were found at higher hot-drawing ratio [149]. Isotactic
polypropylene (iPP) nanofibers with the diameter range of 75–375 nm were made from the
blends of cellulose acetate butyrate (CAB) and iPP with a ratio of 97.5–2.5. The hot-drawn
nanofibers with the ratio of 25 resulted in lower crystallinity than that of bulk iPP. The
increase in the amount of CAB in blends gave rise to higher crystallinity in iPP fibers [150].
Cold drawing of polymers is normally done below the glass transition temperature.
Cold-drawn nanofibrillated cellulose nanopaper increases its modulus and strength from
Fibers 2021, 9, 4 15 of 32
10 GPa and 185 MPa to 24.6 GPa and 428 Mpa, at the draw ratio of 1.6 [151]. Cold-drawn
blend fibers of PVA and PTFE increased the degree of crystallinity in PTFE/PVA fibers [152].
The addition of hydroxyethyl cellulose (HEC) in cellulose nanofiber improved mechanical
property. An aqueous solution of low concentration cellulose nanofiber with HEC promoted
nanofiber alignment which was further improved by cold drawing [153].
3.1.2. Other Electrospun Nanofibers
In this session, Alarifi et al. (2009) reported the mechanical property of a PAN-derived
carbon nanofiber composite and the effect of molecular orientation along the fiber direction
[154]. The study placed electrospun carbonized PAN nanofibers on a stacking sequence
of 0, 45, ?45, and 45 to create a laminate of ten plies. The tensile testing of PAN-derived
carbon nanofiber composites revealed that they possess a high elastic modulus due to
stabilization at a high temperature (280 C), for 1 h, in an oxygen atmosphere. Thermogravimetric
analysis (TGA), dynamic mechanical analysis (DMA), thermomechanical analysis
(TMA), and differential scanning calorimetry (DSC) analyses confirmed that the nanofibers
were crystalline and had good mechanical and thermal properties. Thinner nanofibers
have larger surface-area-to-volume ratios. Therefore, thinner fibers have better mechanical
integrity between the matrix and the surface of the reinforcing agent for effective load
transfer in composites. Baji et al. (2010) proved that as the PCL fiber diameter reduces,
the tensile strength and modulus increase [103]. Figure 14a shows the tensile strength
and modulus versus fiber diameter. For fiber diameters greater than 2 m, both tensile
modulus and tensile strength appear not to change with diameter. The degree of crystallinity
increases gradually as the PCL fiber diameter is reduced as shown in Figure 14b.
As for polymers, in general, an increase in the degree of crystallinity increases the density,
stiffness, strength, and toughness [155]. Huang et al. (2016) showed that with the addition
of conductive filler materials, the diameter of fibers reduced due to increasing composition
of PVP/cellulose nanocrystal (CNC)/silver particle [156]. The addition of CNC increased
the tensile strength.
Fibers 2020, 8, x FOR PEER REVIEW 15 transfer in composites. Baji et al. (2010) proved that as the PCL fiber diameter reduces, the tensile
strength and modulus increase [103]. Figure 14a shows the tensile strength and modulus versus diameter. For fiber diameters greater than 2 μm, both tensile modulus and tensile strength appear
not to change with diameter. The degree of crystallinity increases gradually as the PCL fiber diameter
is reduced as shown in Figure 14b. As for polymers, in general, an increase in the degree crystallinity increases the density, stiffness, strength, and toughness [155]. Huang et al. (2016) showed
that with the addition of conductive filler materials, the diameter of fibers reduced due to increasing
composition of PVP/cellulose nanocrystal (CNC)/silver particle [156]. The addition of CNC increased
the tensile strength.
Figure 14. Cont.
Fibers 2021, 9, 4 16 of 32
Figure 14. (a) Tensile strength/modulus vs fiber diameter [103]. (b) Crystallinity vs. fiber diameter
[103].
The mechanical properties of different nanofibers are shown in Table 2. According to Kancheva
et al. (2015), the mechanical strength of various combinations of polylactic acid (PLA) and PCL was reported to be enhanced after thermal treatment at 60 °C. The melting of PCL enabled the sealing
of the fibers, thus enhancing the mechanical properties of mats [157]. Wang et al. (2004) reported increase in modulus of silk/PEO fibers from as-spun to methanol-treated and to water-extracted [158]. The mechanical properties of single fibers were characterized by AFM nanoindentation. Tan al. (2005) studied tensile property, using an approach that uses an atomic force microscope stretch a single electrospun PEO nanofiber. The elastic modulus of PEO nanofiber was found to MPa [159]. Lin et al. (2012) demonstrated characterization of mechanical properties of ultra-electrospun polymer fibers. Electrospun techophilic, tecoflex, nylon 6, PVP, and PEOX fibers captured directly on the testing device, stretched at controlled rates, and deflected with forces created
by different velocities of streams of air [160].
Figure 14. (a) Tensile strength/modulus vs fiber diameter [103]. (b) Crystallinity vs. fiber diameter [103].
The mechanical properties of different nanofibers are shown in Table 2. According to
Kancheva et al. (2015), the mechanical strength of various combinations of polylactic acid
(PLA) and PCL mats was reported to be enhanced after thermal treatment at 60 C. The
melting of PCL enabled the sealing of the fibers, thus enhancing the mechanical properties
of mats [157]. Wang et al. (2004) reported an increase in modulus of silk/PEO fibers from
as-spun to methanol-treated and to water-extracted fibers [158]. The mechanical properties
of single fibers were characterized by AFM nanoindentation. Tan et al. (2005) studied
tensile property, using an approach that uses an atomic force microscope tip to stretch a
single electrospun PEO nanofiber. The elastic modulus of PEO nanofiber was found to be
45 MPa [159]. Lin et al. (2012) demonstrated characterization of mechanical properties of
ultra-thin electrospun polymer fibers. Electrospun techophilic, tecoflex, nylon 6, PVP, and
PEOX fibers were captured directly on the testing device, stretched at controlled rates, and
deflected with forces created by different velocities of streams of air [160].
Table 2. Mechanical properties of nanofibers.
Nanofibers Tensile Strength Tensile Modulus Characteristics
PAN CNF
5.4 GPa 287 GPa Hot drawn and carbonized at 1100 C, 400 nm in diameter [139]
7.3 GPa 262 GPa Carbonized at 800 C, 108 nm in diameter [104]
PCL 66 MPa 340 MPa 400 nm in diameter [102]
PVP 2.30 MPa [156]
7 MPa [160]
-
500 MPa [160]
300 nm in diameter [156]
800 nm in diameter [160]
PEO -
45 MPa [159]
0.75 GPa [158]
22 MPa [159]
200 nm in diameter [158]
700 nm in diameter [159]
Nylon 6 900 MPa 304 MPa 800 nm in diameter [160]
PCL, polycaprolactone; PVP, polyvinyl pyrrolidone; PEO, polyethylene oxide.
3.2. Dielectric Properties
In this section, the dielectric properties of PAN fibers, carbon nanofillers, and other
electrospun fibers and different nanomaterials are discussed. According to classical theory,
the dielectric constant (k) is defined as the ratio of the permittivity (E) of a substance to the
Fibers 2021, 9, 4 17 of 32
permittivity of free space (E0). Values of k are always greater than or equal to 1. For most
polymers, k values are in the range of 2 to 10.
3.2.1. PAN Nanofibers and Carbon Fillers
According to Li et al. (2010), the dielectric property depends on porosity and density
[2]. The Figure 15 shows the dependence of density and porosity of PAN nanofiber
membranes at the frequency of 1 MHz. The dielectric constant gradually increases with
density ranging from 0.164 to 0.182 g/cm3. Moreover, the dielectric decreases with increasing
porosity from 84.4% to 86.1%. The apparent porosity can be found by Equation (6).
Figure 16 shows the dielectric constant of PAN at 8 wt.% in the radio frequency range.
P (%) = (1 ?
rM
rP
)  100 (6)
where P is the porosity, rM is the membrane density, and rP is the polymer density.
PCL, polycaprolactone; PVP, polyvinyl pyrrolidone; PEO, polyethylene oxide.
3.2. Dielectric Properties
In this section, the dielectric properties of PAN fibers, carbon nanofillers, and other electrospun
fibers and different nanomaterials are discussed. According to classical theory, the dielectric constant
(k) is defined as the ratio of the permittivity (E) of a substance to the permittivity of free space (E0).
Values of k are always greater than or equal to 1. For most polymers, k values are in the range of 2 to
10.
3.2.1. PAN Nanofibers and Carbon Fillers
According to Li et al. (2010), the dielectric property depends on porosity and density [2]. The
Figure 15 shows the dependence of density and porosity of PAN nanofiber membranes at the
frequency of 1 MHz. The dielectric constant gradually increases with density ranging from 0.164 to
0.182 g/cm3. Moreover, the dielectric decreases with increasing porosity from 84.4 % to 86.1 %. The
apparent porosity can be found by Equation (6). Figure 16 shows the dielectric constant of PAN at 8
wt.% in the radio frequency range.
P (%) = (1-
??
??
)*100 (6)
where P is the porosity, ?? is the membrane density, and ?? is the polymer density.
Figure 15. Dependence of density and porosity of PAN nanofiber membranes at frequency of 1 MHz
[2].
Figure 15. Dependence of density and porosity of PAN nanofiber membranes at frequency of 1 MHz [2].
Khan et al. (2014) reported the dielectric constant of PAN and PMMA as a function
of graphene nano flakes as in Figure 17. The physical properties, including the dielectric
constant, were significantly increased with graphene concentrations [161].
Fibers 2020, 8, x FOR PEER REVIEW 17 of 33
Khan et al. (2014) reported the dielectric constant of PAN and PMMA as a function of graphene
nano flakes as in Figure 17. The physical properties, including the dielectric constant, were
significantly increased with graphene concentrations [161].
Figure 16. Dielectric constant of PAN at 8 wt.% in the radio frequency range [2].
Figure 16. Dielectric constant of PAN at 8 wt.% in the radio frequency range [2].
Fibers 2021, 9, 4 18 of 32
Figure 16. Dielectric constant of PAN at 8 wt.% in the radio frequency range [2].
Figure 17. Dielectric constant of PAN as a function of graphene nano flakes concentrations [161].
According to Im et (2011), the increase in dielectric property of permittivity increases electromagnetic interference shielding effectiveness (EMI-SE), as shown in Figure 18a,permittivity of carbon fibers with different proportions of ZrO2 is shown in 18a, and the EMI-the same proportions is shown in 18b. This relationship is more evident below 2000 MHz [162].
Figure 18. (a) Permittivity (ε΄) and (b) electromagnetic interference shielding effectiveness (EMI-of carbon fibers with different proportions of ZrO2 [162].
Figure 17. Dielectric constant of PAN as a function of graphene nano flakes concentrations [161].
According to Im et al. (2011), the increase in dielectric property of permittivity
increases the electromagnetic interference shielding effectiveness (EMI-SE), as shown in
Figure 18a,b. The permittivity of carbon fibers with different proportions of ZrO2 is shown
in Figure 18a, and the EMI-SE of the same proportions is shown in Figure 18b. This
relationship is more evident below 2000 MHz [162].
Figure 16. Dielectric constant of PAN at 8 wt.% in the radio frequency range [2].
Figure 17. Dielectric constant of PAN as a function of graphene nano flakes concentrations [161].
According to Im et al. (2011), the increase in dielectric property of permittivity increases the
electromagnetic interference shielding effectiveness (EMI-SE), as shown in Figure 18a,b. The
permittivity of carbon fibers with different proportions of ZrO2 is shown in 18a, and the EMI-SE of
the same proportions is shown in 18b. This relationship is more evident below 2000 MHz [162].
Figure 18. (a) Permittivity (ε΄) and (b) electromagnetic interference shielding effectiveness (EMI-SE)
of carbon fibers with different proportions of ZrO2 [162].
Figure 18. (a) Permittivity ("0) and (b) electromagnetic interference shielding effectiveness (EMI-SE) of carbon fibers with
different proportions of ZrO2 [162].
Among many conventional carbon fillers, CNTs have been preferred for high dielectric
constant. Bhattacharya (2016) gave a detailed review of processing of CNTs as potential
nanofillers to form nanocomposites [163]. CNFs have been widely used in electrochemical
energy storage devices as reviewed by Zhang et al. (2016) [12]. Electrospun polymer/CNF
or CNT fibers have been used in energy storage devices. The comparison between the CNF
and CNT is shown in Table 3.
Table 3. Comparison of carbon nanotube (CNT) and CNF [12].
Allotropes of Carbon Specific Gravity (gcm?3) Electric Conductivity (Scm?1 ) Thermal Conductivity (Wm?1K?1)
CNF 1.5–2.0 10?7–103 5–1600
CNT 0.8–1.8 102–106 2000–6000
Fibers 2021, 9, 4 19 of 32
3.2.2. Other Electrospun Fibers and Nanoparticles
The dielectric properties of different nanofibers are shown in Table 4. Lee et al. (2003)
studied that dielectric constant strongly depends on solvent content and diameter of PCL
electrospun fibers. As the solvent content increases, the fiber diameter decreases and dielectric
constant increases [164]. Wei et al. (2014) reported the dielectric characterization of
annealed electrospun BaTiO3 fibers. Crystallized BaTiO3 nanofibers showed better dielectric
permittivity [165]. Electrospun PVDF fibers have higher  -crystalline content which
enhances the piezoelectric property and its energy-harvesting application. Jabbarnia et al.
(2016) reported various electrospun PVDF/PVP mats fabricated with different percentages
of carbon black nanoparticles for the applications such as supercapacitor separators and
other energy storage devices. The dielectric constant values were increased with the carbon
black loading [166]. Lee et al. (2016) studied the effect of Fe and Co mixed with PVP. The
analysis of FE-SEM images of electrospun products obtained by using solutions with and
without citric acids was carried out. The composite showed excellent electromagnetic
(EM) wave absorption properties where the power loss of the FeCo nanofibers increased
to 20 GHz [167]. EM waves with frequencies in the microwave range of 12 to 18 GHz are
widely used in wireless communication networks, radar systems, military aircraft, and
satellite communication devices. Wang et al. (2011) studied that an increase in dielectric
constant was achieved in a combination of high aspect ratio barium titanate (BaTiO3) and
graphene platelets in a silicon rubber matrix compared to their spherical counterparts.
Higher volume fractions of ferroelectric particles lead to increased dielectric constant but
also to lower mechanical properties. Composites with high aspect ratio fillers at lower
loading exhibit higher dielectric constant [168]. Issa et al. (2017) reported an increase in
permittivity by the addition of silver nanoparticles (AgNP) in PVDF. This is due to the interfacial
polarization associated with entrapment of free charges generated at the interfaces
between the AgNPs and PVDF [4]. Anita and Natarajan (2015) studied the potential of
ZnO nanopowders with PVA matrix for the use of UV shielding [169].
Table 4. Dielectric properties of different nanoparticles.
Pure Polymers Dielectric Constant Property
PAN 3.5 Physical properties can increase with graphene [161]
PMMA 3.5 Physical properties can increase with graphene [161]
PVDF 11 Physical properties can increase with addition of AgNP [4]
PVP - EMI increases with addition of FeCo ("0EMI) [167]
PVA - Uniform distribution of ZnO increases EMI ("0EMI) [169]
PU - EMI increases with PEDOT ("0EMI) [170]
PCL 10 Dielectric increases with DMF concentration [164]
PMMA, polymethyl methacrylate; PVDF, polyvinylidene fluoride; PEDOT, conductive poly(3,4-ethylenedioxythiophene); DMF, dimethylformamide;
PU, polyurethane.
The in situ sol–gel method (ISM) and direct deposition method (DDM) have been
discussed to analyze the effect of UV shielding. The ISM–PVA/ZnO composite showed
better UV absorption in optical transmission measurements due to the uniform dispersion
of the ZnO in the fibrous matrix. Kim et al. (2016) analyzed the EMI-SE of multiwalled
carbon nanotube (MWCNT) reinforced polyurethane (PU) in the DMF with tetrahydrofuran
solvents and coated with conductive poly(3,4-ethylenedioxythiophene) (PEDOT) [170]. The
EMI-SE from a network analyzer shows 25 dB at the frequency range of 50 MHz–10 GHz.
4. Applications of Aligned Fibers
The authors of this paper, Isaac et al. (2017), observed improvements in the mechanical
strength and dielectric strength with increase in degree of alignment of fibers [171].
Aligned fibers are greatly beneficial when they are used in applications including field
Fibers 2021, 9, 4 20 of 32
effect transistors, gas and optical sensors, fiber reinforced composite materials, and tissue
engineering [172,173]. Bashur (2009) discussed the application of aligned fibers in the field
of tissue engineering [174]. Moreover, Lawrence and Liu (2006) and Katti et al. (2004) stated
that there are other applications found in the variety of areas if the fibers are in aligned
form [175,176]. This section discusses mechanical and dielectric applications of aligned
electrospun fibers.
4.1. Influence of Aligned Fibers on Mechanical Properties of Nanofiber Mats
Hou et al. (2005) showed that well-aligned, multi-walled carbon nanotubes (MWCNT)
can improve the mechanical properties of a PAN-based nanofiber mat [177]. Kannan et al.
(2007) demonstrated that electrospun polymer/CNT leads to nanocomposite fibers with
embedded CNTs orienting parallel to the nanofiber axis [178]. Moreover, alignment of
CNTs in the fiber direction can improve thermal conductivity [179]. Dhakate et al. (2016)
reported that semi-aligned electrospun carbon nanofiber composites show excellent bending
strength and interlaminar shear strength [180]. High-performance aramid copolymer
fibers underwent four treatment factors. Among them, the degree of stretching after coagulation
resulted in high degree of molecular orientation. The increased tensile strength
of aramid fibers improved the cut resistance of aramid fibers, and therefore can be used
in cut protection [181]. Ultra-high molecular weight polyethylene fibers with the tensile
strength of 1.5 GPa were successfully prepared and structure and tensile property were
studied. The increase in draw ratio improved the crystallinity of ultra high molecular
weight polyethylene fibers. The molecular orientation degree increased, and tensile property
also improved [182]. Increasing the draw-ratio resulted in an increased molecular
orientation, Young’s modulus, and tensile strength of poly(amide-block-aramid) fibers
comprised of alternating rigid aramid blocks of poly(p-phenylene terephthalamide) (PPTA)
and flexible blocks of polyamide 6,6. Heat treatment at 300 C of the fibers resulted in an
increase of Young’s modulus and minor increase of strength [183].
Aligned micro scale fibers (7m diameter) have application in composite reinforcement.
There is increased need to manufacture complex composites for light weight applications.
Carbon/epoxy composites have greater application in aircraft, sports cars, and space
crafts because of a better strength to weight ratio than that of metals like aluminum alloys.
They are thermally stable because of the lower coefficients of thermal expansion properties
of carbon fibers. Yu et al. (2014) showed that short carbon fiber composites can be used in
places where complex shapes and ductile properties are required [184]. Short carbon fibers,
with an aspect ratio of 400, resulted in composites with a tensile modulus of 119 GPa and
strength of 1211 GPa.
Compton and Lewis (2014) reported that cellular composites with controlled alignment
of multi-scale and high aspect ratio fibers can result in reinforcement of hierarchical
structures [185]. They demonstrated the first 3D printed cellular composites composed
of oriented fiber-filled epoxy with exceptional mechanical properties. Malek et al. (2017)
developed a new carbon fiber reinforced epoxy for 3D printing which resulted in printing
materials with longitudinal Young’s modulus up to 57 GPa [186].
4.2. Influence of Aligned Fibers on Dielectric Properties of Nanofiber Mats
Ning et al. (2014) showed that aligned MWCNT/PVA has high dielectric constant,
low dielectric loss, high breakdown strength, and high energy density. These properties
contribute in applications such as artificial muscles, energy storage, flexible electronics, and
sensors [187]. Aligned MWCNT/PVA composite films were prepared by using electrospinning
in situ film-forming technique. Additionally, Liu et al. (2012) confirmed the tailoring
of dielectric property by controlling the alignment of CNTs [187]. Ma et al. (2012) reported
that aligned PVDF had better molecular orientation than its random fiber counterparts.
This is because of the smaller diameter of the aligned fibers. These nanofibers have applications
in the field of sensors and actuators [188]. Agarwal et al. (2009) reported that aligned
fibers have applications in nanofluidics, superhydrophobic patterning, nanoelectronics,
Fibers 2021, 9, 4 21 of 32
and nanophotonic circuits [189]. Edmondson et al. (2012) demonstrated the significance
of fiber alignment in improving the piezoelectric property, using centrifugal electrospinning.
PVDF and PEO have piezo-, pyro-, and ferro-electric properties, and these aligned
fibers can provide for applications in actuators, transistors, textiles, and composites [190].
P. Kumar et al. (2017) showed that aligned graphene films improved EMI shielding. Electromagnetic
(EM) waves cause interference or device malfunction and also can cause harm
to human bodies [191]. Song et al. (2013) observed that aligned carbon-based fillers enhanced
EMI shielding. The alignment produced anisotropic characteristics that achieve
enhancement in absorption and reflection performance [192].
5. Electrospinning System for Dielectric and Mechanical Property Studies
The authors of this paper, Isaac et al. (2017) studied the effect of electrospun fiber
alignment on mechanical and dielectric properties, using a setup designed and drawn
in 3D modeling software, as shown in Figure 19. The electrode, mandrel holding sheet,
acrylonitrile butadiene styrene (ABS) sheet for adjusting the distance between needle tip
and mandrel, and the sliding front door are shown above. The final physical setup of
electrospinning device is shown in Figure 20.
such as artificial muscles, energy storage, flexible electronics, and sensors [187]. Aligned
MWCNT/PVA composite films were prepared by using electrospinning in situ film-forming
technique. Additionally, Liu et al. (2012) confirmed the tailoring of dielectric property by controlling
the alignment of CNTs [187]. Ma et al. (2012) reported that aligned PVDF had better molecular
orientation than its random fiber counterparts. This is because of the smaller diameter of the aligned
fibers. These nanofibers have applications in the field of sensors and actuators [188]. Agarwal et al.
(2009) reported that aligned fibers have applications in nanofluidics, superhydrophobic patterning,
nanoelectronics, and nanophotonic circuits [189]. Edmondson et al. (2012) demonstrated the
significance of fiber alignment in improving the piezoelectric property, using centrifugal
electrospinning. PVDF and PEO have piezo-, pyro-, and ferro-electric properties, and these aligned
fibers can provide for applications in actuators, transistors, textiles, and composites [190]. P. Kumar
et al. (2017) showed that aligned graphene films improved EMI shielding. Electromagnetic (EM)
waves cause interference or device malfunction and also can cause harm to human bodies [191]. Song
et al. (2013) observed that aligned carbon-based fillers enhanced EMI shielding. The alignment
produced anisotropic characteristics that achieve enhancement in absorption and reflection
performance [192].
5. Electrospinning System for Dielectric and Mechanical Property Studies
The authors of this paper, Isaac et al. (2017) studied the effect of electrospun fiber alignment on
mechanical and dielectric properties, using a setup designed and drawn in 3D modeling software, as
shown in Figure 19. The electrode, mandrel holding sheet, acrylonitrile butadiene styrene (ABS) sheet
for adjusting the distance between needle tip and mandrel, and the sliding front door are shown
above. The final physical setup of electrospinning device is shown in Figure 20.
Figure 19. Enclosure, mandrel in the 3D model [171].
The whole apparatus is placed under a fume hood, for safety purposes, so that any toxic solvent
escapes through the fume hood. Using this setup, electrospun fiber mats were fabricated, fiber
morphology was analyzed, and tensile and dielectric properties were characterized.
Figure 19. Enclosure, mandrel in the 3D model [171].
The whole apparatus is placed under a fume hood, for safety purposes, so that any
toxic solvent escapes through the fume hood. Using this setup, electrospun fiber mats
were fabricated, fiber morphology was analyzed, and tensile and dielectric properties
Fibers 2020, 8, x FOwRe PreEEchRa RraEcVteIEriWze d. 21 Figure 20. The final electrospinning setup [171].
5.1. Material Builds
PAN/DMF solution was prepared with 8 wt.% concentration. The electrospinning device ran ten minutes in order to form sufficient fibers on the aluminum foil wrapped on the mandrel. Figure
21a,b shows the fiber mat formed on the mandrel and the fiber mat unfolded from the mandrel.
Figure 20. The final electrospinning setup [171].
Fibers 2021, 9, 4 22 of 32
5.1. Material Builds
PAN/DMF solution was prepared with 8 wt.% concentration. The electrospinning
device ran for ten minutes in order to form sufficient fibers on the aluminum foil wrapped
on the mandrel. Figure 21a,b shows the fiber mat formed on the mandrel and the fiber mat
unfolded from the mandrel.
Figure 20. The final electrospinning setup [171].
5.1. Material Builds
PAN/DMF solution was prepared with 8 wt.% concentration. The electrospinning device ten minutes in order to form sufficient fibers on the aluminum foil wrapped on the mandrel. 21a,b shows the fiber mat formed on the mandrel and the fiber mat unfolded from the mandrel.
Figure 21. (a) Fiber mat is being formed on mandrel. (b) Fiber mat on an aluminum foil [171].
5.2. Fiber Morphological Analysis
The electrospun fiber mat formed was examined under a NanoSEM 230 SEM microscope measure fiber diameter. As shown in Figure 22a,b, the fiber diameter decreases as the needle decreases from 18G to 22G. The feed rate was 0.3 mL/h. The 18G needle produced fibers with diameter of 1 μm, whereas the 22G needle produced fibers in the range of 300 to 900 nm diameters
with an average diameter of 600 nm. The smaller the fiber diameter, the better the mechanical
properties.
Figure 22. (a) Fiber mats using 18G needle magnified at 10 μm and (b) using 22G needle magnified 5 μm [171].
5.3. Tensile and Dielectric Test Results of Electrospun Mats
PAN precursor with DMF solution at 8 wt.% concentration, using a 22G needle, is electrospinning. More than thirteen experimental runs were carried out to determine suitable Figure 21. (a) Fiber mat is being formed on mandrel. (b) Fiber mat on an aluminum foil [171].
5.2. Fiber Morphological Analysis
The electrospun fiber mat formed was examined under a NanoSEM 230 SEM microscope
to measure fiber diameter. As shown in Figure 22a,b, the fiber diameter decreases as
the needle size decreases from 18G to 22G. The feed rate was 0.3 mL/h. The 18G needle
produced fibers with average diameter of 1 m, whereas the 22G needle produced fibers in
the range of 300 to 900 nm diameters with an average diameter of 600 nm. The smaller the
fiber diameter, the better the mechanical properties.
Figure 20. The final electrospinning setup [171].
5.1. Material Builds
PAN/DMF solution was prepared with 8 wt.% concentration. The electrospinning device ten minutes in order to form sufficient fibers on the aluminum foil wrapped on the mandrel. 21a,b shows the fiber mat formed on the mandrel and the fiber mat unfolded from the mandrel.
Figure 21. (a) Fiber mat is being formed on mandrel. (b) Fiber mat on an aluminum foil [171].
5.2. Fiber Morphological Analysis
The electrospun fiber mat formed was examined under a NanoSEM 230 SEM microscope measure fiber diameter. As shown in Figure 22a,b, the fiber diameter decreases as the needle decreases from 18G to 22G. The feed rate was 0.3 mL/h. The 18G needle produced fibers with diameter of 1 μm, whereas the 22G needle produced fibers in the range of 300 to 900 nm diameters
with an average diameter of 600 nm. The smaller the fiber diameter, the better the mechanical
properties.
Figure 22. (a) Fiber mats using 18G needle magnified at 10 μm and (b) using 22G needle magnified 5 μm [171].
5.3. Tensile and Dielectric Test Results of Electrospun Mats
PAN precursor with DMF solution at 8 wt.% concentration, using a 22G needle, is used electrospinning. More than thirteen experimental runs were carried out to determine suitable Figure 22. (a) Fiber mats using 18G needle magnified at 10 m and (b) using 22G needle magnified
at 5 m [171].
5.3. Tensile and Dielectric Test Results of Electrospun Mats
PAN precursor with DMF solution at 8 wt.% concentration, using a 22G needle, is used
for electrospinning. More than thirteen experimental runs were carried out to determine
suitable process parameter settings. The three best samples were taken, to analyze the
degree of fiber alignment. The tensile and dielectric test results for the three samples are
given below in Figure 23a–c. The SEM images are shown in Figure 24.
Fibers 2021, 9, 4 23 of 32
Fibers 2020, 8, x FOR PEER REVIEW 22 of 33
parameter settings. The three best samples were taken, to analyze the degree of fiber alignment. The
tensile and dielectric test results for the three samples are given below in Figure 23a–c. The SEM
images are shown in Figure 24.
23. (a) Tensile strength of best three samples [171]. (b) Dielectric strength of best three samples
[171]. (c) Tensile and dielectric properties of three samples [171].
Figure 23. (a) Tensile strength of best three samples [171]. (b) Dielectric strength of best three samples [171]. (c) Tensile and
dielectric properties of three samples [171].
Fibers 2021, 9, 4 24 of 32
Fibers 2020, 8, x FOR PEER REVIEW 23 of Figure 24. SEM images at 20 and 5 μm of Samples 1, 2, and 3 [171].
The SEM image of Sample 1 shows loosely packed nanofibers. Samples 2 and 3 have more closely
packed fibers and better alignment of fibers, and Sample 3 has the highest densely packed fibers.
These images are viewed at 20 μm magnification. Tensile strengths of Samples 2 and 3 are better than
that of Sample 1, since they are better aligned and closely packed. As evident from Figure 23a,
Samples 2 and 3 show higher tensile strength, and Sample 1 has the least. Better alignment combined
with dense fibers contributed to the better tensile strengths of Samples 2 and 3. Sample 2 shows tensile strength of 4.47 MPa. As shown below, in Figure 23b, Sample 3 shows the highest dielectric
constant. The dielectric constant for Sample 3 is 2.56 Hz at 0.1 Hz. The best parameters are chosen by
comparing the three given samples for their SEM images, tensile strengths, and dielectric properties
as shown in Figure 23c. Sample 3 gives consistent and uniform fibers. The best parameters at 8 wt.%
correspond to Sample 3. The Sample 3 has uniform fiber distribution with the lowest range in
diameters. Moreover, the fibers are formed without beads. The tensile strength and dielectric constant
of Sample 3 is found to be 4.43 MPa and 2.56, respectively.
Figure 24. SEM images at 20 and 5 m of Samples 1, 2, and 3 [171].
The SEM image of Sample 1 shows loosely packed nanofibers. Samples 2 and 3 have
more closely packed fibers and better alignment of fibers, and Sample 3 has the highest
densely packed fibers. These images are viewed at 20 m magnification. Tensile strengths
of Samples 2 and 3 are better than that of Sample 1, since they are better aligned and closely
packed. As evident from Figure 23a, Samples 2 and 3 show higher tensile strength, and
Sample 1 has the least. Better alignment combined with dense fibers contributed to the better
tensile strengths of Samples 2 and 3. Sample 2 shows a tensile strength of 4.47 MPa. As
shown below, in Figure 23b, Sample 3 shows the highest dielectric constant. The dielectric
constant for Sample 3 is 2.56 Hz at 0.1 Hz. The best parameters are chosen by comparing
Fibers 2021, 9, 4 25 of 32
the three given samples for their SEM images, tensile strengths, and dielectric properties as
shown in Figure 23c. Sample 3 gives consistent and uniform fibers. The best parameters
at 8 wt.% correspond to Sample 3. The Sample 3 has uniform fiber distribution with the
lowest range in diameters. Moreover, the fibers are formed without beads. The tensile
strength and dielectric constant of Sample 3 is found to be 4.43 MPa and 2.56, respectively.
6. Future Work
While a few studies have methodically experimented with system parameter optimization
for limited sets of parameters, none has studied the nonlinear effects of two
or more factors with three levels per factor for obtaining the mechanical and dielectric
responses. Consequently, parameter interactions and nonlinearities have were studied,
in-depth, to optimize alignment for mechanical and dielectric properties. Very few studies
have been conducted on the nanofiber orientation variations along the fiber direction, using
a rotating mandrel and its effect on mechanical and dielectric properties. The variability of
data in both dielectric and tensile tests needs to be quantitatively addressed, and therefore
a modified improved system should be developed. The improved system will lead to
consistencies in the behavior of nanofiber specimens and acceptable standard deviations in
variability that indicate meaningful parameter levels and trends. Future work will focus on
system improvements and methodical experimentation (Design of Experiments), to optimize
system parameters for improved mechanical and dielectric responses and application
of PAN nanofiber mat materials in advanced mechanical and energy applications. Future
research work will determine the nature of the trade-off between mechanical strength and
dielectric properties—whether it is linear or non-linear, synergistic, or detrimental.
7. Conclusions
This work has provided a detailed review and analysis of methods for alignment of
electrospun fibers and their resulting mechanical and dielectric properties. A key consideration
is the molecular orientation of fibers along the fiber direction, as it is important
for improvement in mechanical properties. Design configuration options for electrospinning
apparatuses were surveyed in order to analyze the ability of each to improve fiber
alignment. Given the mechanical and dielectric performance improvement possible with
increasing fiber alignment, the need to improve electrospinning apparatus capabilities for
fiber alignment is paramount and worthy of further study and experimentation.
Author Contributions: B.I., R.M.T. and K.R. discussed the contribution of electrospinning in various
fields and outlined the manuscript with the focus on mechanical and dielectric properties. B.I wrote
the manuscript, and R.M.T. and K.R. verified and edited it. All authors have read and agreed to the
published version of the manuscript.
Funding: There is no funding for this project.
Acknowledgments: Blesson Isaac would like to thank Robert V. Fox, supervisor at Idaho National
Laboratory for his guidance and The University of Texas at Arlington Research Institute for supplying
the materials for experiments.
Conflicts of Interest: The authors declare that they have no conflict of interest.
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This entry is adapted from the peer-reviewed paper 10.3390/fib9010004