IDEALPLUSING | Electrospinning

Birth and Development

Electrospinning is a technology that uses an external electric field to produce nanofibers from polymer solutions (or molten state). Since the produced nanofibers are disordered, electrospinning is mainly used to produce nanofiber membrane/mat structures. The history of electrospinning has been several decades. Although research on electrostatic force and solution jetting has begun long ago, electrospinning was not officially born until 1934.

That year, Formhals applied for a patent for the method and experimental equipment for electrostatic spinning using polymer solutions. This patent is an important patent in the field of electrospinning, and Formhals is also considered the inventor of electrospinning technology . In 1969, Taylor described the electrospinning process. The cone produced when the surface tension of the solution and the charge repulsion reach equilibrium is the Taylor cone, which is an important phenomenon in electrospinning . In 1971, Baumgarten designed the prototype of the electrospinning equipment we currently use .

Since its birth, electrospinning has not received enough attention. At the end of the last century, with the development of nanotechnology, electrospinning gradually attracted widespread attention, and the name electrospinning officially appeared . Until now, electrospinning is still a widely popular nanofiber manufacturing technology.

Electrospinning process

Electrospinning system usually consists of three parts: high-voltage power supply equipment, solution supply equipment with spinneret, and fiber collector. Electrospinning equipment is quite simple and easy to build. High-voltage equipment can be directly obtained by purchasing high-voltage power supply; a syringe pump and an ordinary syringe can be used to form a simple solution supply equipment; a simple fiber collector can be directly made of a metal plate.

In electrospinning, high viscosity is the prerequisite for electrospinning. The viscous polymer solution (or melt, for convenience of description, only the solution is used as an example) is loaded into the syringe and then extruded outward through the syringe pump. The solution will gather at the spinneret to form a hemispherical shape due to surface tension. Obviously, if no power is added at this time, as the droplets gather, they will fall from the spinneret, and then repeat the process of gathering and falling.

If the spinneret is connected to a high-voltage power supply at this time, the surface of the hemispherical droplet will accumulate charges under the action of the high-voltage power supply, so that the polymer droplet is subjected to electrostatic forces such as Coulomb repulsion and electric field force between charges; as the charge accumulates, under the combined action of electrostatic force and surface tension, the hemispherical droplet bulges toward the fiber collector to form a Taylor cone; as the electrostatic force further increases, it overcomes the surface tension of the droplet, and the charged polymer jet will be ejected from the tip of the spinneret, and as the solvent evaporates in the air, it forms fibers (the melt is a solidification process) and deposits on the fiber collector .

The reason why electrospinning is more used in the manufacture of membranes is that during the electrospinning process, the charged polymer jet can only remain oriented toward the fiber collector near the spinneret. In the process of the charged polymer jet flying toward the fiber collector, the electrostatic force makes the charged polymer jet unstable and swings violently in the air. Overall, it is a cone facing the fiber collector, and finally forms non-oriented nanofibers as the solvent evaporates.

Why can electrospinning form nanofibers?

The diameter of the spinneret is often at the level of several hundred micrometers or millimeters. So how does electrospinning form nanofibers? In the last century, people believed that it was because the polymer jet had the same charge, and the repulsion between the charges caused the polymer jet to split soon after leaving the spinneret. As a result, the jet after being ejected was dispersed into several jets. These polymer jets continued to split, becoming thinner and thinner until the solvent evaporated and formed fibers. However, after entering the 21st century, the application of high-speed cameras made people discover that this was not the case. The formation of nanofibers is related to bending instability.

There are three types of instabilities in the electrospinning process. One is related to the properties of the polymer solution itself, called Rayleigh instability. The other two are caused by the coupling of the charge carried by the polymer jet with the electrostatic field, namely axisymmetrical instability and bending instability. Among them, the third instability: bending instability, is closely related to the formation of nanofibers . Through the observation of ordinary camera pictures, the charged polymer jet is ejected from the spinneret, and then diverges and splits into fine fibers and flies toward the fiber collector. This is also the phenomenon observed by the human eye during the electrospinning process, and it is also what people thought in the last century; however, under the observation of high-speed camera, the morphology of the charged polymer jet during the electrospinning process is inconsistent with people's assumptions. The jet does not split but remains in a state of one strand. The conical fiber flow seen by the naked eye is formed by the rapid swing of the charged polymer jet, that is, bending instability.

Figure 3 PEO electrospun fiber images; 

A) electrospun fiber under ordinary camera; B) electrospun fiber under high-speed camera

As the swing amplitude of the charged polymer jet gradually increases, the charged polymer jet is stretched, which causes the diameter of the charged polymer jet to decrease. This process continues until the charged polymer jet solidifies into a fiber. However, the splitting of the charged polymer jet under electrostatic force may also occur, and some polymer solutions will have the phenomenon of charged polymer jet splitting. Bending instability increases with the increase of the charge density of the charged polymer jet and the increase of the electric field strength, but the viscosity and elastic force of the polymer solution can suppress the bending instability.