This article was automatically translated from the original Turkish version.
The electrospinning method, which offers ease of application and low cost in production, generates nanofibers from molten or dissolved materials with the aid of an electric field. Electrospinning has advantages over other methods, primarily due to its system’s ability to incorporate a wide variety of polymers and active materials. The main disadvantage of the electrospinning method is the low mechanical strength of the produced fibers.
Four essential components are required for an electrospinning setup.
These are:
Several parameters influence the production of successful nanofibers via electrospinning. These parameters are categorized into three main groups: solution parameters, process parameters, and environment parameters.
The properties of the polymeric solution selected for electrospinning are a critical parameter affecting the morphology and production process of the resulting nanofibers. Key solution parameters influencing nanofiber production are viscosity, surface tension, and conductivity.
To ensure continuous fiber formation, the solution concentration must be at an optimal level. At high concentrations, the electric field cannot overcome the surface tension and viscosity, preventing polymer flow and halting production. At low concentrations, fiber formation does not occur; instead, droplet spraying takes place, with droplets accumulating on the Tambur surface. Increasing the concentration to an optimal level ensures jet continuity necessary for continuous fiber formation. Viscosity and polymer concentration are interdependent parameters that vary proportionally.
Surface tension can be explained by the cohesive forces between molecules in a liquid. Liquid molecules tend to minimize their surface area by contracting into a spherical shape due to cohesive forces. This tendency to cluster and form spherical shapes is more pronounced in solvents with high surface tension. In electrospinning, high surface tension causes the polymeric solution to form bead-like building structures as it travels toward the collector plate. Therefore, solutions with low surface tension are preferred for nanofiber production. When solutions with high surface tension must be used, surfactants are added to reduce surface tension and prevent bead formation.
For a polymeric solution to be used in electrospinning, it must possess sufficient conductivity to enable jet formation. Jet formation occurs when charges on the solution surface migrate, stretching the solution toward the collector unit. Adding ions or altering the pH increases the solution’s conductivity. Increased conductivity allows the solution to stretch further, resulting in nanofibers with smaller diameters and a bead-free structure. However, while increasing conductivity up to a certain level enhances nanofiber formation, excessively high conductivity prevents it. In systems with very high conductivity, charges on the polymer solution at the needle tip cannot be maintained, preventing Taylor cone formation and jet generation.
Process parameters influence the diameter, distribution, and morphology of the produced nanofibers. These parameters include applied voltage, solution feed rate, collector type, needle diameter, and the distance between the needle tip and the collector.
In electrospinning, the applied voltage must exceed the solution’s surface tension threshold to initiate jet formation. Studies have shown that increasing voltage leads to greater extension of the polymer solution, resulting in nanofibers with smaller diameters and bead-free morphology. However, other studies have also observed that higher voltages increase the amount of polymer ejected from the needle tip, leading to thicker nanofibers.
The solution feed rate refers to the flow rate of the polymer solution used in nanofiber production. It significantly affects the jet velocity. As the solution flow rate increases, the amount of solution carried by the jet increases, reducing the electrospinning force applied to it. Consequently, the resulting fibers have larger diameters. Additionally, high flow rates limit solvent evaporation during the jet’s travel from the needle to the collector, causing adjacent nanofibers to adhere to each other upon collection. To maintain continuous production, the solution flow rate must be maintained at a level that sustains a stable Taylor cone. Achieving a stable Taylor cone requires an appropriate solution flow rate.
In electrospinning, an electric field is established by creating a potential difference between the feeding unit and the collector unit. Aluminum foil, a conductive material, is commonly used as the collector and grounded to establish the required potential difference. Various collector types can be employed, and the choice affects the morphology of the produced nanofibers. Aligned and homogeneously distributed nanofibers are obtained using rotating drum or rotating disk collectors, while random orientation is observed with flat plate collectors.
The diameter of the needle or other structures through which the solution flows affects the morphology of the resulting nanofibers. A smaller diameter increases the surface tension at the needle tip, requiring a higher voltage to initiate jet formation. Higher voltage promotes greater solution extension and the formation of finer fibers. However, excessively small diameters may cause clogging at the needle tip.
This is the region where electrospun nanofibers are formed. The distance must allow sufficient time for the solution to stretch into a jet and for the solvent to evaporate, resulting in solidified nanofibers. Increasing the distance between the feeding unit and the collector allows the jet to extend further, yielding nanofibers with smaller diameters.
The temperature, pressure, humidity, and atmosphere type of the environment in which electrospinning occurs influence both nanofiber production and the morphology of the resulting fibers.
Ambient temperature affects the solvent evaporation rate. At low temperatures, evaporation is slow, leading to insufficient solidification of nanofibers upon reaching the collector, resulting in increased fiber diameter and fiber adhesion. At high temperatures, rapid solvent evaporation causes premature solidification of the polymer solution, inhibiting its extension and jet formation. Therefore, the temperature in each electrospinning setup must be optimized to ensure ideal solvent evaporation and the production of finer nanofibers.
Low pressure conditions hinder nanofiber production in electrospinning. Environments with pressure lower than atmospheric pressure increase the flow rate of the solution at the needle tip and disrupt the formation of a stable Taylor cone.
Humidity in the environment affects the polymeric solution. Increased humidity leads to pore formation on the surface of nanofibers. Additionally, relative humidity influences the solvent evaporation rate. It is known that at low humidity levels, the solvent evaporates too rapidly, causing the nanofiber to solidify before jet formation begins and clogging the needle tip.
The surrounding air affects both the electrospinning process and the morphology of the produced nanofibers. Certain gases exhibit different behaviors under high electric fields and can inhibit electrospinning. Helium gas is an example of such a gas.
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Parameters Affecting the Electrospinning Method
Solution Parameters
Process Parameters
Environmental Parameters