Electrospun Jets Number and Nanofiber Morphology Effected by Voltage Value: Numerical Simulation and Experimental Verification
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Electrical voltage has a crucial effect on the nanofiber morphology as well as the jet number in the electrospinning process, while few literatures were found to explain the deep mechanism. Herein, the electrical field distribution around the spinning electrode was studied by the numerical simulation firstly. The results show that the electrical field concentrates on the tip of a protruding droplet under relatively low voltage, while subsequently turns to the edge of needle tip when the protruding droplet disappears under high voltage. The experimental results are well consistent with the numerically simulated results, that is, only one jet forms at low voltage (below 20 kV for PVDF-HFP and PVA nanofiber), but more than one jet forms under high voltage (two jets for PVDF-HFP nanofiber, four jets for PVA nanofiber). These more jets lead to (1) higher fiber diameter resulting from actually weaker electrical field for each jet and (2) wide distribution of fiber diameters due to unstable spinning process (changeable jet number/site/height) under high voltage. The results will benefit the nanofiber preparation and application in traditional single-needle electrospinning and other electrospinning methods.
KeywordsDiameter distribution Electrospinning Separation Superfine nanofiber Voltage
Poly (vinylidene fluoride-co-hexafluoropropylene)
Traditional single-needle electrospinning
Due to many superior merits such as high surface area, controllable fiber diameter and membrane thickness, and connected pore structure, nanofibers receive intensive studies and have been applied in many areas . As one of the simplest preparation methods of nanofiber, electrospinning technique has drawn numerous attentions not only in academic researches but also in practical industrialization [2, 3].
In view of practical engineering applications, the nanofiber diameter and diameter distribution are the two key parameters. On the one hand, majority of the application areas prefer smaller fiber diameter such as air filtration, because smaller fiber diameter means not only higher surface area which makes the nanofibrous membrane possessing larger pollutants adsorption capacity but also smaller pore size endowing the nanofibrous membrane higher pollutants repelling ability [4, 5]. Many methods have been developed to pursue finer nanofiber. For example, adding ionic/inorganic salt can be an effective way because the salt can increase the spinning fluid conductivity [6, 7]. Wang et al. reported that increasing the sheath fluid flow rate can reduce the resulting nanofiber diameter in the coaxial spinning process . Hai et al. developed a detachable concentric spinneret that can hold the energy on working fluid by the outer polymeric tube, which benefits preparing much finer core-shell nanofiber . On the other hand, narrow diameter distribution results in better control of pore size in the nanofibrous membrane construction, which is crucial in separation areas especially in water filtration [10, 11].
In the spinning process, many parameters from the device and precursor solutions are involved in the nanofiber diameter and diameter distribution. First, the shape of the spinning electrode plays a significant role in determining the electrical field distribution and, as a result, has an important influence on the spinning process and nanofiber morphology [12, 13]; second, the precursor properties such as the concentration, surface tension, and viscosity [14, 15]; third, spinning parameters such as voltage, collector distance, and even the collector shape [16, 17]; fourth, ambient conditions such as the humidity and the temperature . Among them, the voltage value has much crucial effect on nanofiber diameter and diameter distribution, though those parameters synergistically affect the spinning process and nanofiber morphology .
Theoretically, the nanofiber diameter decreases with the increase of voltage value where electrical field force is strengthened . Therefore, increasing voltage value can be a feasible route to achieve superfine nanofiber . Hasanzadeh et al.  reduced the polyacrylonitrile nanofiber diameter from 212 to 184 nm using the applied voltage from 14 to 22 kV. Ranjbar-Mohammad et al.  fabricated gum tragacanth/poly (vinyl alcohol) composite nanofiber and achieved the decrease of fiber diameter from 153 to 98 nm by changing the voltage from 10 to 20 kV. However, interestingly, for traditional single-needle electrospinning (TNE), there are two phenomena at high voltage value in spinning process: (1) higher fiber diameter. It is well known that the nanofiber diameter decreases with the increase of voltage value at first, while increases at high voltage value ; (2) wide fiber diameter distribution. Wide fiber diameter distribution is achieved at high voltage value in the TNE spinning process . That is to say that higher voltage value is unwelcome in TNE spinning process. As a result, it is a hard task to obtain nanofiber with smaller diameter and narrow diameter distribution due to the limited voltage value in TNE spinning process.
Therefore, the relevant mechanism discussion is greatly desired to reveal the phenomenon and benefits of the nanofiber preparation. However, little literatures report the mechanism of the phenomenon that TNE method prepares nanofiber with a higher diameter and wider diameter distribution under high voltage value. Many previous researches applied the numerical simulation method by Maxwell program to intuitively evaluate the electrical field distribution and intensity of electrospinning apparatus [26, 27, 28]. In the present study, we research the mechanism in a special view and aim to (1) numerical simulation of electrical field distribution around the spinning electrode in TNE spinning process with voltage supply change, (2) experimental verification of numerical simulation results and voltage value on the spinning process and nanofiber morphology, and (3) spinning process conclusion with the increase of voltage value and mechanism discussion of abnormal nanofiber morphology under high voltage value.
Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw = 400,000) was purchased from Aladdin Industrial Corporation, Shanghai, China. Polyvinyl alcohol (PVA), N,N-dimethyl formamide (DMF), and acetone were supplied by Sinopharm Chemical Reagent Co., Ltd. (Suzhou, China). All reagents were analytical grade and were used as received without further treatment.
Preparation of PVA Nanofiber Under Different Voltage Value
PVDF-HFP (11 wt%) was dissolved in a binary solvent of DMF/acetone with the weight ratio of 1:1 at room temperature for 4 h. In spinning experiment, the voltage values of 6, 10, 15, 20, 25, and 30 kV were applied at the tip of a syringe needle (0.8 mm in internal diameter). The collector distance is 15 cm. A constant volume flow rate of 1.0 ml/h was maintained using a syringe pump. The temperature and relative humidity (RH) used in the spinning process were 25 ± 2 °C and 55 ± 3%, respectively, and kept constant.
Preparation of PVA Nanofiber Under Different Voltage Value
PVA (12 wt%) was dissolved in deionized water at 95 °C for 2 h. The sodium dodecylbenzenesulfonate (0.01%) was added into the solution to decrease the solution surface tension. In spinning experiment, the voltage values of 7, 10, 15, 20, 25, and 30 kV were applied at the tip of a syringe needle (0.8 mm in internal diameter). The collector distance is 15 cm. A constant volume flow rate of 0.8 ml/h was maintained using a syringe pump. The temperature and RH used in the spinning process were 25 ± 2 °C and 55 ± 3%, respectively, and kept constant.
The morphology of electrospun nanofibrous membranes was observed using a scanning electron microscope (Hitachi S-4800, Tokyo, Japan) at 20 °C, 60 RH. Samples were sputter-coated with gold layer prior to imaging. The samples were cut up into 2 × 4 mm2 and photographed at accelerating voltage of 5 kV and electricity of 10 mA. The diameters of electrospun fibers were calculated by measuring at least 100 fibers at random using ImageJ program. The optical images were photographed by a camera (SONY, ILCE-6400L). In the photographing process, a black plank was placed at the back and a torch was placed opposite from the camera lens, which can photograph the spinning process with a high quality.
In the numerical simulation process, the electric field around the spinning electrode was calculated by using Maxwell 2D (ANSOFT Corporation). The simulation parameters are the outer and inner diameter of the needle are 1.2 mm and 0.8 mm, respectively; the length of three protruding droplet lengths are 1.3 mm, 0.88 mm, and 0 mm, respectively; and the collector distance is 15 cm. The Maxwell program utilizes finite element methods and adaptive meshing to achieve a converged solution. In the simulation process, the calculation finished at Energy Error and Delta Energy are less than 1%. The conductivity of model polymeric solution in simulation process is 1.6 μs/cm.
Results and Discussions
Schematic Diagram of Jet Evolution and Numerical Simulation of the Electrical Field Around the Electrode with Voltage Value Change
Experimental Verification by Electrospun PVDF-HFP Nanofiber
The average diameter of PVDF-HFP nanofiber under various voltage value
1004.3 ± 184.7
387.4 ± 46.6
239.5 ± 20.4
149.2 ± 9.5
194.2 ± 47.9
247.9 ± 59.6
Experimental Verification by Electrospun PVA Nanofiber
The average diameter of PVA nanofiber under various voltage value
442.7 ± 59.8
272.8 ± 35.7
232.9 ± 29.0
159.3 ± 23.6
186.7 ± 43.4
213.6 ± 64.9
Spinning Process Conclusion with the Voltage Value Increase and Mechanism Discussion of Jet Evolution Affecting the Spinning Process and Nanofiber Morphology
Based on the numerical simulation and experimental verification results, the spinning process with the voltage value increase and the mechanism of jet evolution affecting the nanofiber spinning process and morphology are tentatively concluded as follows:
Therefore, the spinning process can be reasonably separated by two stages, before and after protruding droplet disappearance or stable and unstable stage (Fig. 10). Before the protruding droplet disappearance (stable stage), the fiber diameter decreases with the voltage value increase and shows relatively good diameter distribution. After the protruding droplet disappearance (unstable stage), (1) the fiber diameter increases oppositely due to the weaker electrical field for each jet which is actually due to the increased jet number and (2) there was worse fiber diameter distribution contributed by the unstable spinning process (changeable jet number, jet sit, and different electrical field intensity for each jet). In view of the discussions above, the critical value before protruding droplet disappearance is the best voltage value to fabricate nanofiber with finer fiber diameter and good fiber diameter distribution (Fig. 10).
The numerical simulation and experimental verification results show that only one jet forms at the protruding droplet exists and more than one jet produces after the protruding droplet disappearance, which is contributed by the electrical field concentrating on droplet tip firstly and then turning to the tube edge of needle tip with the increase of voltage value. The increased jet not only weakens the electrical field for each jet (resulting in high fiber diameter), but also makes an unstable spinning process (leading to wide diameter distribution). The results ingeniously reveal the mechanism of nanofiber morphology change at high voltage value in TNE spinning process, which presents a unique view to better know the TNE spinning process and benefits the nanofiber preparation and application in many areas especially in separation and filtration.
The authors would like to thank the support by the Anhui Provincial Natural Science Foundation (1908085QE223), Pre-research Project of China National Natural Science Foundation of Anhui Polytechnic University (2019yyzr06), and Fujian Key Laboratory of Novel Functional Textile Fibers and Materials (FKLTFM1815).
ZL contributes to the idea of the project. KYJ and WL perform the experiments. HZK and JHH help in the analysis data. ZL and ZQW write this manuscript. ZL submits this manuscript for publication. All the authors have read and approved the final manuscript.
Anhui Provincial Natural Science Foundation (1908085QE223), Pre-research Project of China National Natural Science Foundation of Anhui Polytechnic University (2019yyzr06), and Fujian Key Laboratory of Novel Functional Textile Fibers and Materials (FKLTFM1815)
The authors declare that they have no competing interests.
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