Effective Utilization of the Electrostatic Repulsion for Improved Alignment of Electrospun Nanofibers

Uniaxial alignment of electrospun fibers can provide a useful approach to develop novel functional nanomaterials for applications in a wide variety of fields. In this study, a polypropylene(PP-) coated spinneret and a metal spinneret were utilized to carry out the single-fluid electrospinning processes. A metal rod frame was utilized as the collector to steer the nanofibers. Using polyvinylpyrrolidone K90 (PVP K90) as a filament-forming polymeric model at a concentration of 9% (w/v) in ethanol, the experimental observations and results demonstrated the following results: (1) the utilization efficiency of electrical energy could be improved through the PP-coated spinneret; (2) the texture of collector had a significant influence on the collection of aligned PVP K90 nanofibers; and (3) the combination of a PP-coated spinneret with the metal frame could ensure the electrostatic repulsion forces to play their roles effectively in generating PVPK90 nanofibers with thinner diameters and in collecting uniaxial alignment of them.Themechanisms about the orientation effects of the present method are discussed.This job opens a facile way for producing aligned polymeric nanofibers based on the reasonable manipulation of the interactions between the electrostatic field and the working fluids.


Introduction
Electrospinning technique stems from the traditional spinning technologies such as wet spinning, dry spinning, and melt spinning [1].It is different with them in that it takes advantage of electrical forces rather than mechanical forces and its natural products are nonwoven fabrics [2][3][4].Over 95% of publications about electrospun nanofibers are assembled into randomly oriented nonwoven mats or membranes by estimation from the data in Web of Science.However, aligned nanofibers have anisotropic properties, which are highly desired in a wide variety of applications (particularly in the field of biomaterials) such as sensors, vascular scaffolds, biomimetic extracellular matrices, scaffolds for nerve corpuscles, and drug delivery [1,[5][6][7][8][9][10].Meanwhile, facile fabrication of array nanomaterials and nanostructures is very important for developing functional nanodevices and is a hot topic in the field of nanotechnology today [9,10].Thus, how to prepare aligned electrospun nanofibers has drawn increasing attention.
Electrospinning, as a simple and straightforward nanofiber fabrication process, has developed very quickly during the past two decades.On one hand, double-fluid (such as coaxial and side-by-side) electrospinning and trifluid electrospinning (such as triaxial) have stood out from the conventional single-fluid processes [11][12][13][14][15][16][17].On the other hand, the preparation of electrospun nanofibers on a laboratory scale has been pushed forward to the large scale and industrial scale [18].However, little attention has been paid to the utilization efficiency of the electrical energy, which should be a big question for potential commercial applications of electrospun nanofibers, in addition to the evaluation of the quality of final functional nanoproducts.
In this study, how to utilize the electrical energy effectively for achieving highly ordered electrospun nanofibers was investigated.To carry out the electrospinning, a polymercoated spinneret (as the positive electrode) and a metal rod frame (as the negative electrode) were exploited.The former took advantages of the electrostatic repulsion to transfer more energy to the working fluids and the latter
The working fluid was prepared as follows: 9.0 g PVP K90 was placed into 100 mL ethanol and was stirred enough time to achieve a transparent solution.After being degassed using sonication, the fluid was pured into a 10 mL polypropylene (PP) syringe tube, which was connected with a spinneret.The applied voltage could be adjusted from 0 to 60 kV.After some preexperiments, the fluid flow rate was fixed at a constant value of 2.0 mL/h.The fiber collected distance was fixed at 20 cm in all the experiments.The environmental temperature and humidity were 21 ± 4 ∘ C and 57 ± 5%, respectively.

Characterizations.
A digital camera (PowerShot SX50HS, Canon, Tokyo, Japan) with a largest magnification of 200x was utilized to observe the electrospinning processes.A polarized optical microscope (OM, CTM-300, Changfang Optical Instrumental Factory, Shanghai, China) was exploited to observe the collected nanofibers.A field-emission scanning electron microscope (SEM, Quanta6 450 FEG, Hillsboro, OR, USA) was used to evaluate the details about the electrospun aligned nanofibers.The samples were goldcoated under nitrogen before observation.The nanofiber size was measured in SEM images using ImageJ software (National Institutes of Health, MD, USA).

Efficacious Application of Electrical Energy via a Polymer-Coated Spinneret.
Electrospinning is initially called electrostatic spinning because it takes advantage of electrostatic forces for spinning [1].Electrospinning, electrospraying, and e-jet printing are called together as electrohydrodynamic atomization because their working principles are based on the interactions between electrostatic fields and fluids [12].In the electrospinning processes, regardless of how many fluids working simultaneously, the electrostatic energy plays its role to convert the working fluids into final solid products through five successive steps, that is, charging the fluids, the formation of Taylor cone, the straight fluid jets emitted from the tip of Taylor cone, the bending and whipping instable region, and the collection of final nanoproducts (Figure 1).
Within the five steps, the three intermediate steps have few direct relationships with the utilization efficiency of electrical energy.But the first step and the final step happen on the components of the electrospinning system, which can be exploited to improve the effective usage of electrical energy and thus to reduce the cost of final products for commercial applications.The first step, that is, the charge of working fluid, was realized through putting a metal line in the fluid originally [19], but later in almost all the publications, the working fluids were charged through a direct connection of the power supply with the metal spinneret (and often an alligator clip was utilized) for this objective [20][21][22][23].
Although the direct connection of the metal spinneret with the power supply to charge its inner working fluid was facile and convenient for implementation; however, the utilization of a simple whole metal spinneret as an anode was an energy-wasteful process.This can be demonstrated by observing the electrospinning processes when a metal spinneret and a polymer-coated spinneret were used to conduct the working fluids, respectively.
Shown in Figure 2(a), the starting voltages of metal spinneret and PP-coated spinneret were 7 kV and 5 kV, respectively.Under the starting voltages, the Taylor cones were not in a regular shape, and there were droplets dripping on the collectors frequently.As the applied voltage increased, the Taylor cones gradually changed as anticipated.Their volumes reduced, their shapes turned from ellipse to standard cone, to flat cone, and finally to be indented in the spinneret capillaries.However, in all these change processes, the voltages applied on the metal spinneret were larger than those applied on the PP-coated spinneret.The applied voltages for a stable and robust process without any dripping liquid were 10 kV and 8 kV for the metal spinneret and PP-coated spinneret, respectively.Under the stable working conditions, the current with the usage of PP-coated spinneret was significantly smaller than that with the usage of metal one (Table 1).
The electric power () can be calculated using the following equation: where  is the power usage (Wa),  is the applied voltage (V),  is the electric current (A), and  is the running time (s).According to this equation, the electric power consumed by the metal spinneret was 1.62 × 10 −3 Wa, three times larger than the PP-coated spinneret (0.54 × 10 −3 Wa).The subscripts "m" and "p" mean that the values are from a metal spinneret and a PP-coated spinneret, respectively.
The PP-coated spinneret could always ensure a similar result of the metal spinneret with the usage of a lower applied voltage.When the metal spinneret was exploited, it not only forwarded the electrical energy to the working fluid within the capillary but also spread energy to the outer environment through the atmosphere.Although the atmosphere has a smaller conductivity than the working fluid, because the larger outer surface of spinneret, the electrical energy loss was tremendous and should be not overlooked.Kiselev and Rosell-Llompart reported that 5 kV was sufficient to keep a stable electrospinning of PEO solution, whereas an applied voltage of 9-10 kV was needed to ensure a stable process when a back electrode with an enlarged surface area at the spinneret was utilized [9].This result indicates that a large conductive surface area at the spinneret aggregated the energy loss at the atmosphere and reduced the energy utilization efficiency.
When the PP-coated spinneret was exploited to implement the electrospinning, most of the metal surface has been covered by the PP.A narrow section of the metal capillary was set aside for the copper line to connect the power supply, by which the working fluid was able to be charged (Figure 2(b)).The PP coating is an antielectrostatic material, which can effectively retard the scattering of electrical energy to the environments through electrostatic repulsion.Thus, this PPcoated spinneret could forward the electrical energy to the working fluid more efficaciously than the metal spinneret although they have the same length and with a same diameter of their nozzle (1 mm).
The optical microscopic images of the electrospun nanofibers fabricated under different applied voltages using the PP-coated spinneret are exhibited in Figure 3.The samples were prepared by placing a glass slide on the fiber collector for a short time period.A straight line with a width of 50 microns was carved in the glass slide by laser for the size estimation of nanofibers.Just as anticipated, as the applied voltage rose from 5 kV to 6 kV, 8 kV, and 10 kV, the created nanofibers became straight and thinner, as indicated by Figures 3(a), 3(b), 3(c), and 3(d), respectively.

The Influence of Aperture Texture on the Collection of Aligned Nanofibers.
During the past 20 years, several methods have been reported to steer the nanofibers on the collectors.These methods are based on the applications of electrostatic forces, mechanical forces (e.g., rotating cylinder and mandrel), their combinations [9], or additional magnetic fields [10].However, to our best knowledge, there are no publications that have investigated the influence of material textures on the uniaxial alignment of electrospun nanofibers.Based on a single-fluid electrospinning process using the PPcoated spinneret (Figure 4(a)), apertures composed of three kinds of materials with varied conductibility and a glass slide were exploited to collect PVP K90 nanofibers (Figure 4(b)).These apertures had the same size of 1 cm × 5 cm.After 2 minutes' collection, the PP aperture could collect nothing because of its antistatic property.Under the observations of polarized OM, the collections using other apertures and the glass slide are shown in Figure 5. Just as anticipated, the nanofibers collected on the glass slide are randomly assembled without any orientation degree (Figure 5(a)).In contrast, the nanofibers collected on the cardboard aperture (Figure 5(b)) and on the metal aperture (Figure 5(c)) have obvious orientation features.In addition, the metal aperture has collected more nanofibers than the cardboard aperture.
These phenomena suggest that the presence of aperture can effectively guide the polymeric nanofibers to deposit on the collectors by taking advantage of the electrostatic forces naturally, and the textures that formed the aperture exerted a significant influence on the aligned collections.
Based on the abovementioned experimental results, a metal rod frame collector was fabricated for collecting aligned PVP K90 nanofibers.The collector (Figure 6(a)) comprised a series of parallel metal rods connected with a flat iron, which can be grounded through an alligator clip.The collection of PVP K90 nanofibers from the single-fluid electrospinning process is shown in Figure 6   applied voltage of 10 kV, the digital picture of the electrospinning process is exhibited in Figure 6(c), in which it is clear that a straight fluid jet is emitted from an almost flattened or indented Taylor cone and is followed by a bending and whipping instable region.The polarized optical microscopic images of these electrospun PVP K90 nanofibers are shown in Figure 7, which suggest obviously ordered assembly.

The Reasonable Selections of Spinneret and Collector for
Collecting Uniaxially Aligned Nanofibers.To further characterize the collected nanofibers from the metal rod frame using the PP-coated spinneret, SEM observations were conducted.The nanofibers fabricated from the metal spinneret using the same collector and under the same operational conditions were also collected for comparison.Shown in Figure 8,   the PVP K90 nanofibers both from the metal spinneret (Figures 8(a) and 8(b)) and from the PP-coated spinneret were collected along a certain direction thanks to the exploitation of array of metal rods as collector.However, the nanofibers from the PP-coated spinneret had a smaller average diameter with a smaller size distribution (360±50 nm, Figure 9(b)) than those from the metal spinneret (670 ± 150 nm, Figure 9(a)).Taking 20 nanofibers as samples, the former had a better orientation degree (within 1.27 ± 0.34 ∘ ) than the latter (10.05 ± 2.31 ∘ ).
Although the simple, straightforward, and one-step electrospinning has shown fantastic applications in many fields, its mechanisms are still not very clear because of the overlap of several disciplines such as rheology, fluid mechanics, and electric dynamics.Particularly in the bending and whipping instable region, there are a series of different forces there.These forces play their roles together to solidify the fluid jets into nanofibers and to assemble them into mats.Shown in Figure 10  are responsible for an enhanced alignment of electrospun nanofibers.
When a floating nanofiber will deposit on the collector, it should be subjected to a series of forces from its surroundings.These forces mainly include the repulsion forces from the already deposited nanofibers ( d ), the repulsion forces from the fluid jets still in the bending and whipping coils ( f-n ), and the forces from the electrical field (i.e., the repulsion forces from the positive electrode and the attractive forces from the negative electrode).Under the same strength of electrostatic field, the higher density of charges the fluid jets and deposited nanofibers have, the larger the forces of  d and  f-n are, and the better uniaxial effect the floating nanofibers align.Under the same applied voltage, the PP-coated spinneret can prevent electrical energy loss to the environment effectively to forward a higher density of charges to the fluid jets than the metal spinneret.These higher dense charges will not only increase the Coulomb repulsion ( in ) within the fluid jets to result in nanofiber with smaller diameters but also produce an improved alignment effect of the nanofibers on the metal rod collector.Certainly, a good conductivity of the collector means the quick elimination of the charges within the deposited nanofibers and in turn the easy landing of the floating nanofibers on the collector.

Conclusions
A PP-coated spinneret was successfully developed for implementing the single-fluid electrospinning processes.The usage of this kind of spinneret could raise the utilization efficiency of electrical energy.The collecting apertures could orientate the electrospun PVP K90 nanofibers, and the material texture forming the apertures showed a significant influence of the fiber alignment effect.The combined usage of the PP-coated spinneret with a metal rod frame as the collector could ensure the electrostatic emulsions play their roles efficaciously in generating PVP K90 nanofibers with smaller diameters and in collecting uniaxial alignment of them.
Here, a simple PP-coated spinneret was explored for effective utilization of electric energy during the electrospinning processes.Along the strategy reported here, there are a series of new contents waiting to be further investigated.These investigations, for example, include the influence of different insulation thickness and the effectiveness of insulation on the quality of the output nanofibers; the addition of salts in the working fluids and the its impact on the electrospinning process and final products; and the effects in creating structural nanoproducts (such as core-shell, Janus, and tri-layer fibers) utilizing the corresponding polymeric structural spinnerets.

Figure 1 :
Figure 1: A schematic diagram of the electrospinning system with the spinneret as an anode and the collector as a negative electrode.
Figure 2: A comparison of the developments of Taylor cones as the applied voltages increased when a metal capillary and a PP-coated metal capillary were exploited.

Figure 5 :
Figure 5: The polarized optical microscopic images of the electrospun nanofibers collected on the glass slide (a), cardboard aperture (b), and metal aperture (c).

Figure 6 :
Figure 6: Metal rod collector (a) and its applications in collecting aligned PVP K90 nanofibers from a single-fluid electrospinning process using the PP-coated spinneret (b) and a typical underway electrospinning process (c).

Figure 7 :
Figure 7: The polarized OM images of the electrospun PVP K90 nanofibers collected on the metal rod frame.

Figure 8 :
Figure 8: The SEM images of the electrospun PVP K90 nanofibers collected on the metal rod frame using the metal spinneret (a and b) and the PP-coated spinneret (c and d).

Figure 9 :Figure 10 :
Figure 9: The PVP K90 nanofiber diameters and their size distributions: (a) from the metal spinneret and (b) from the PP-coated spinneret.

Table 1 :
The comparison of electric powers when the two different spinnerets were utilized a .