What are Nanofibers?
Figure 1: Nanofibers (<1 μm) web laid on human hairs ( ~50 – 100 μm).
How are Nanofibers Formed?
One of the commonly utilized methods for the production of nanofibers is electrospinning. The electrospinning process has a few key elements: a power supply, a reservoir for containing the polymeric solution, and a grounded collector. In the typical polymer solution electrospinning apparatus (Figure 2), a high voltage power supply is used to supply the necessary voltage to a conical metal reservoir filled with the desired polymer solution. When the electrostatic repulsive forces become sufficient to overcome the surface tension of the polymeric solution, a “Taylor cone” is formed, and the fluid jet is ejected. As the polymeric solution jet travels further from the tip of the metal reservoir, it becomes more destabilized, creating what is known as a bending instability. The combination of electrostatic and mechanical forces pulling the polymer solution from the metal reservoir to the grounded collector result in stretching of the polymer fiber, which leads to the collection of fibers with submicron (<1 μm) or nanometer diameters: nanofibers.
Manipulation of the components of the electrospinning apparatus and process conditions as well as the polymers and solvents employed in this technique allow the nanofiber product to be tailored to specific applications. In addition, various additives can be mixed into the polymer solution prior to electrospinning and incorporated into the resulting polymeric product, leading to novel nanofiber materials with a diverse array of properties.
Figure 2: General Electrospinning Apparatus
Other methods employed for nanofiber production include polymer melt electrospinning, in which the polymeric solution is substituted with a polymeric melt; the melt blown process involves the production of fibrous webs or articles directly from molten polymers using high-velocity air or another appropriate force to attenuate the filaments. Another example is the Islands-In-The-Sea method of nanofiber formation. This technique first involves the formation of bi-component polymer fibers in which one component is soluble in a particular solvent and the other component is not. Following formation, the resulting fibers are rinsed or submerged in the chosen solvent, selectively dissolving one component and generating polymer nanofibers of the remaining component.