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 PVP K90 nanofibers with thinner diameters and in collecting uniaxial alignment of them. The mechanisms 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.
Electrospinning technique stems from the traditional spinning technologies such as wet spinning, dry spinning, and melt spinning [
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 [
In this study, how to utilize the electrical energy effectively for achieving highly ordered electrospun nanofibers was investigated. To carry out the electrospinning, a polymer-coated 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 resulted in improved alignment effect of the nanofibers via electrostatic forces. Polyvinylpyrrolidone (PVP) was selected as the polymer model because of its fine filament-forming property using ethanol as the solvent.
PVP K90 (
The electrospinning system consisted of a high voltage power supply (ZGF2000, 2 mA/60 kV, Sute Electrical Co., Ltd., Shanghai, China), a syringe pump (KDS100, Kole-Parmer®, Vernon Hills, IL, USA), two home-made spinnerets, and several collectors.
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
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, Quanta
Electrospinning is initially called electrostatic spinning because it takes advantage of electrostatic forces for spinning [
A schematic diagram of the electrospinning system with the spinneret as an anode and the collector as a negative electrode.
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 [
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
The comparison of electric powers when the two different spinnerets were utilized
Number | 1 | 2 | 3 | 4 | 5 | 6 | Mean |
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Metal |
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10.0 | 10.1 | 10.1 | 10.0 | 9.9 | 10.0 |
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0.15 | 0.17 | 0.15 | 0.17 | 0.17 | 0.16 |
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PP-coated |
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8.1 | 8.0 | 8.0 | 8.1 | 8.0 | 8.0 |
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0.07 | 0.08 | 0.06 | 0.08 | 0.06 | 0.06 |
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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.
The electric power (
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 [
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
The optical microscopic images of the electrospun nanofibers fabricated under different applied voltages using the PP-coated spinneret are exhibited in Figure
The optical microscopic images of the electrospun nanofibers fabricated under different applied voltages using the PP-coated spinneret: (a) 5 kV, (b) 6 kV, (c) 8 kV, and (d) 10 kV.
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 [
The implementation of electrospinning about the influence of aperture textures on the uniaxial collection of electrospun PVP K90 nanofibers.
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
The polarized optical microscopic images of the electrospun nanofibers collected on the glass slide (a), cardboard aperture (b), and metal aperture (c).
Based on the abovementioned experimental results, a metal rod frame collector was fabricated for collecting aligned PVP K90 nanofibers. The collector (Figure
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).
The polarized OM images of the electrospun PVP K90 nanofibers collected on the metal rod frame.
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
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).
The PVP K90 nanofiber diameters and their size distributions: (a) from the metal spinneret and (b) from the PP-coated spinneret.
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
The mechanisms for effective utilization of the electrostatic repulsions for improved 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 (
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.
The authors declare that they have no competing interests.
The financial supports from the following projects are appreciated: the Training Project for Excellent Young and Middle-Aged Backbone Teachers of Higher Schools in Guangxi Province in China, the Natural Science Foundation of China (no. 51373101), the College Student Innovation Project of USST (no. XJ2016234), and the Project of Teaching Reform of Higher Education in Gunagxi Province in China (no. 2012JGA333).