Electrospinning technology and application of electrospun
nanofibers in different fields
The content of this article mainly refers to a review article by Professor Xia Younan [1]. I just reorganized his article content in my own language, and added some personal understanding on this basis . For the above reasons, I would like to express my special thanks to Mr. Xia Younan.
1. Historical development of nanotechnology electrospinning
Electrospinning technology, also referred to as electrospinning or electrospinning, can be traced back to 1 600. Will i am Gilbert observed a cone of water in the presence of an external electric field [2]. Then in 1887, Charles Boys Preparation of nanofibers by electrospinning using a viscoelastic liquid [1]. In 1938, electrospun nanofibers were used to make filter cores for air filtration devices, and electrospun nanofibers were first applied [1]. Since 1938, electrospinning technology continues to mature and develop, has now been applied in various fields, greatly promoted the progress and development of scientific research and industrial production [3, 4].
2. Nano Electrospinning technology
2.1. Electrospinning principles
Figure 1. Schematic diagram of an electrospinning device [ 5 ]
Electrospinning technology refers to the process in which a polymer melt or solution forms fibers under the action of a high voltage electrostatic field. Electrospinning device mainly consists of three parts, namely, high voltage power supply, and receiving the spinneret plate (Fig. 1). The power source can be a DC power source or an AC power source. Different types of power sources have a certain influence on the formation of nanofibers, and will not be described in detail herein. Plastic pipe spinneret may be used, metal tubes and glass syringe with a needle or the like, the spinneret typically 0 .1 ~ 1 mm. The receiving plate is used for receiving a polymer formed by solvent evaporation or melt solidification, generally using a conductive metal plate, a silicon wafer, a conductive glass, and so on. In the specific use process, the surface of the metal plate should be wrapped with tin foil, woven fabric, etc., which facilitates the transfer of nano-spinning and prevents the receiving plate from being contaminated or soiled. [6]
Power poles are respectively connected to the nozzle head and the receptacle, during the electrospinning, a polymer solution or melt brought to the surface charge and the force electric field is generated, a force opposite to the direction of the electric field of surface tension of the solution. As the voltage increases, the electric field force is greater than the surface tension, and the charged solution is ejected from the spinneret to form a jet stream. During the movement of the jet in the air, the process of solvent evaporation, cooling and cooling is gradually solidified, and finally collected on the receiver .
Figure 2. Electrospun nanofibers schematic transverse forces suffered [ 7]
When the solution is initially ejected from the spinneret, a linear motion is performed, but the surface tension and viscous force hinder its forward movement , and the acceleration gradually decreases, and at the same time, the fiber diameter becomes tapered. When the acceleration gradually decreases to 0 or a constant, any small disturbance will destroy any linear motion of the fiber. Due to the repulsion between the surface charge of the solution, an unstable perturbation segment region appears ,
and the charge above the perturbation segment exerts a downward and outward repulsive force on it. F DO (Fig. 2) At the same time, the charge below the perturbation section exerts an upward and outward repulsive force F UO on it. The two forces produce a lateral resultant force F R that causes the nanofibers to bend [1, 7]. As the fiber gradually elongates, a second unstable bending phase occurs, followed by the third and fourth, until the nanofibers are cured, and the next level of bending no longer occurs. (Fig. 3).
Figure 3. Schematic diagram of nanofiber bending motion [8]
2 .2. Materials used in electrospinning technology
Electrospinning technology can choose a large number of materials, but it is mainly divided into four categories, namely organic polymers, sol-gel materials, some small molecules and polymer and inorganic composite materials. 2 .2.1. Organic polymer
The commonly used materials for electrospinning are organic polymers, which are mainly divided into two categories . One type is a polymer that can be dissolved in certain solvents to form a solution, which is currently the most widely used type because the operation process is relatively simple, the equipment requirements are relatively large, and the spun nanofibers are finer; It cannot be dissolved in the solvent, but it can be melted by heating . This requires a heating device around the needle , as shown in Figure 4 . By heating to melt the polymer, this method is often suitable for some of the materials with specific needs.
Figure 4. Melt spinning a schematic view of apparatus [ 9]
2 .2.2. Sol-gel material
A colloid consisting of a dispersed phase ( suspended particles ) and a continuous phase ( suspension ) . When sufficient entanglement is formed inside the formed particles , the jet is a continuous structure and can also be used for electrospinning. [1] . 2 .2.3 small molecule solution
When the chain entanglement is large enough, the small molecule can also obtain the nanofiber directly by electrospinning. The key factor is to sufficiently strong interaction in the presence of a molecule between small molecules, such as self-assembled structure is formed in a highly concentrated solution or in the pure melt. 2 .2.4. Composite
The application of composite materials is also very extensive, and some organic polymers are often combined with inorganic nanoparticles or nanowires to spin some functional nanofibers. For example, coating silica particles with polyvinyl alcohol (Fig. 5. Left ), polyvinylpyrrolidone coated silver nanowires (Fig. 5 . Right ) .
Figure 5. Polyvinyl alcohol coated silica particles (left) [ 10] and polyvinylpyrrolidone coated silver nanowires
( right ) electron micrograph [11]
2.3. Common methods of operation for nanoelectrospinning technology 2 .3.1. Near-field and far-field pattern
There are two main differences between the near field mode and the far field mode. One is the difference between the spinneret and the receiver, and the other is the difference in the power supply voltage. In far field mode, the distance from the spinneret to the receiver is 5 ~ 15 cm, the power supply voltage between approximately 1 0 ~ 20 KV [12]. When spinning in the far field mode, nanofibers that are finer than the near-field mode can be obtained, and the spinning speed is also faster. However, the position of the nanofibers on the receiving plate cannot be precisely controlled. In the near-field mode, the spinneret to the receiver distance of 5 00 um ~ 5 cm, between the power source voltage of approximately 0.6 ~ 3 KV. Using near-field mode while the spun nanofiber diameter with respect to the far field pattern is relatively large, it may be made to the precise control of the position of the nanofibers to obtain a variety of shapes and arrangement (Fig. 6) [13-15] .
Figure 6. Various spinning patterns in near-field mode [16]
2 .3.2. Single and multi-needle spinnerets
Due to the low spinning efficiency and small output of the single-needle spinneret, a multi-needle type spinneret has been developed, which has various types, multiple needles side by side, pyramid type and coaxial multi-needle, etc. Since there is a lateral electrostatic repulsion between the nanofibers, there is no adhesion between the fibers and the fibers. Among them, the coaxial multi-needle type spinneret can prepare nano-fibers with multi-layer structure, and can also prepare hollow nano-fibers with high flexibility and diversified products. Use multi-needle spinnerets can greatly improve production efficiency, reduce production costs, both for research or production aresignificant.
2.3.3. Solid receiving plate and liquid receiving device
静电纺丝及其应用-英文
