Advantages Of Nanofibers. If nanomaterials are explained as materials used in nanotechnology and each size is nanometric, this size will be related to the fiber diameter for nanofibers, and fibers smaller than 1 µm in diameter will be designated as nanofibers Kılıç, 2008. Nanofibers with a diameter smaller than 1 micron, nanoscience and they represent the most important structures in the leadership corner of nanotechnology Ebadzadeh, 2010. When fibers are reduced from micrometer to nanometer, they have a higher surface area per unit volume, high mechanical rigidity and increased tensile strength throughout the fiber. Obtaining end products with high added value thanks to nanofibers makes wide usage areas possible. They have many applications in the fields of filtration systems, sensor construction, polymeric batteries, catalyst reinforcement and composite support Li, 2004. In textiles, fireproof, waterproof, antibacterial, UV-resistant and self-cleaning fabrics as well as protective masks are likely to be produced from nanofibers, Kang, 2007; Lee, 2007. Air and water filters, which do not pass damaged substances and provide microbiological purification, can be produced with the reinforcement of nanofibers Zhanga, 2011. Human tissues and limbs are biologically composed of nanofiber structures. Examples of this are bones, teeth, cartilage and skin. They are all nanometer-sized fibrous structures. For this reason, one of the most important researches of nanofibers today is applications in the field of bioengineering. Aylık Nanotechnology and Nanotıp Bilim Mecmuası, September 2011.
In medicine, wound closure and bone tissue formation or the production of trans-thermal drugs and drug delivery systems whose factor substances penetrate the body through the skin are also being developed with the reinforcement of nanofibers. Kenawy, 2009. In addition, vitamins and antioxidants encapsulated for food fortification, foods with a longer shelf life with less and more natural additives, or well-behaved packaging that informs the consumer about the progress of the food in it are among the examples that can be given to studies in the food industry.
Since the 1990s, the magic in the field of nanotechnology, and especially the creation of fibers with submicron dimensions, makes technological advantages possible to a great extent. Electrospinning method, on the other hand, is one of the most common methods applied to create fibers with submicron diameters from polymer solutions, thanks to its simplicity and multipathability.
With the electrospinning method learned since the 1930s, nano-scale fibers can be obtained from polymers in a single step operation. Method; It can be summarized as the production of nanofibers under electrostatic forces by applying a high voltage of 5 to 50 kV to the electrode connected to the injector filled with polymer solution or melt. used in nanofibers produced by electrospinning method.
The fact that the variety of polymers is quite large allows the production of various materials to be realized. As a result, the usage areas of nanofibers formed by this method are quite wide.
Nanofibers obtained by electrospinning have increased in research over the last 10 years due to their superior mechanical performance, quite large surface areas and elasticity in surface functions when compared to other learned forms of materials. These unusual properties make nanofibers a suitable candidate for many advanced applications. Biomedical engineering and biotechnology,
Environmental engineering, energy storage, tissue engineering, drug delivery, affinity membranes and enzyme immobilization are some of these application areas.
Electrogravity first appeared in the literature in the 1600s, when William Gilbert observed the changes in the behavior of fluids captured under high voltage while continuing his magnetism studies. In the study, an electrostatic field was created from a point close to the water droplets on a dry surface. It has been observed that the droplets transition from a spherical position to a conical structure and then scatter in a spiral orbit thanks to the electric field created. This event is the point where the history of electrogravity began. In 1882, Lord Rayleigh conducted studies on the caliper-free movements of electrically charged drops during electrogravity. Rayleigh observed that when the charge of an isolated charged droplet is applied above the surface tension that makes it stable, the droplet becomes hesitant and then dispersion begins. According to these results, there are two forces affecting the drop.
One of them is the electric force and the other is the surface tension force in the opposite direction to the electric force. As soon as the electric force overcomes the surface tension force, the drop begins to crumble into very fine jet structures. He planned the production of artificial and synthetic filaments by using In the study, an electric field was applied to the solution of cellulose acetate polymer dissolved in ethylene glycol and filaments were produced from the polymer solution. The injector tip, which has a tiny hole through which the polymer exits during operation, is obtained by drilling metal alloys and is planned to prevent caliper-free flow problems. It was also emphasized that the potential difference required in Formhals’ registration depends on parameters such as the viscosity and molecular weight of the polymer. Formhals’ work
Afterwards, many studies have been carried out regarding the nanofiber manufacturing method. Electrospinning is one of the most common methods used for nanofiber manufacturing. This method enables the creation of nanofibers from polymers based on electric field strengths.
Electrogravity devices generally form three basic parts. The first section is a thin tube in which the polymer solution is contained and the exit of the polymer from the metal chamber at the end is carried out. In this section, either the solution is started to move by the effect of gravity or the progress of the solution is ensured with the help of a syringe pump. The second part is the electric field region created by the high voltage (5kV30kV) along the jet trajectory of the polymer. The third part consists of a metal assembly in which nanofibers are collected.
The main parts of an electro production system are:
1. High voltage power supply,
2. Feeding unit (pump, syringe, metal needle etc.),
3. It is the Collector (Figure 2.1) (Terada, 2012).
In the electrospinning method, the polymer solution dissolved in a suitable solvent or melted with the help of heat is filled into a pipette with a metal needle at the end.
With the help of a pump located at the back of the pipette and applying continuous pressure, the fiber spinning solution or polymer melt is allowed to advance along the pipette. Then, the anode and cathode electrode tips are connected to the metal needle at the tip of the pipette and the collector plate located opposite the pipette. Thanks to the opposite poles, an electrical field is created between the metal needle tip and the collector. Electrogravity method can work in two different polarities. However, with the positive polarity of the syringe and grounding of the collector plate, better results were obtained in terms of fiber properties and efficiency (Kılıç, 2008).
The needle is charged with a high voltage by applying an increasing electric field (1kV-30kV) between the two electrodes. Up to the critical voltage value, the polymer solution suspended at the tip of the needle in the feeder unit takes the form of a spherical drop due to the forces exerted by the surface tension. Meanwhile, the polymer ions remaining under the effect of the electric field become positively polarized and begin to move in the direction of the ground loads are collected at the metal needle tip. The draw rate of the nanofibers is controlled by adjusting the electric field size and solution flow. If the electric field is not applied, the solution drop at the needle tip will drop to the ground due to gravity. As the applied potential difference reaches the threshold value, the electrostatic forces are equalized to the surface tension forces. Meanwhile, the spherical polymer solution now takes the shape of a cone. This cone shape is called Taylor cone (Figure 2.2 and 2.3).
The deformation of the solution drop with the applied electrical voltage was first discovered by Taylor in the 1960s. Taylor explained the basic theoretical principles of electrically charged liquids with his various studies. An external force is created as a result of the charging of the liquid surface by the electric field effect and the repulsion of the mutual charges. After the threshold value is passed, the droplet is formed into a cone shape by electrostatic force.
It transforms and excess charges flow across the loaded jet at the tip of the cone. A cone is formed at the critical point where the electrical forces are equivalent to the surface tension. Taylor calculated that this cone has a half angle of 49.3 degrees (Fig. 2.4) (Taylor, 1964)
In the theory developed by Taylor, it is explained that the formation of thin fibers from the drop with high viscosity in the electric field occurs due to the maximum instability on the drop surface loaded with electrical forces. When the voltage is increased, the droplet, which is spherical, distorts its hemispherical shape at the critical point and takes the form of a cone before the jet is formed. The surface curvature of the viscous liquid changes with the effect of electrical forces.
In other words, when the potential difference applied to the polymer solution rises above a certain threshold value (1kV-5kV), the applied electrostatic force overcomes the surface tension of the polymer ions and the polymer jet leaves the Taylor cone structure. The polymer solution, which no longer has a surface tension, flows through the very fine jet outlet and towards the grounded target placed opposite it. The Taylor cone causes the electrically charged polymer solution to exit quite rapidly. This process takes place at a small diameter of 10-4 m. After leaving the Taylor cone, the charged jet moves steadily over a certain distance. Then, a state of instability begins on the jet. There are 3 states of instability depending on the system variables and the properties of the polymer solution used. Only one of the states of instability can occur, as well as this jet can show all three states of indecision.
These indecision states;
a) Classic Rayleigh instability,
b) Axially symmetrical electric field current,
c) Whipping instability explained as.
The most common instability in electrospinning is whipping. Whipping is caused by the radial torque formation from the center due to the fact that the charges on the jet cannot be together as a result of the mutual repulsion of the charges on the jet. As the radial charges repel each other, smaller jets are formed that separate from the base jet as the jet approaches the collector. Enough thinning of the formed jets and sufficient damping of the viscoelastic forces lead to the formation of new whipping instabilities. This instability is called the second whipping instability (Fig. 2.6).
Thanks to the high voltage, the polarized polymer molecules follow a chaotic trajectory between the metal needle and the grounded plate, forming a random network shape on the plate surface. Fiber structures accumulating on the grounded surface are constantly pulled towards this surface (Karatay, 2012).
Nanofibers can be obtained by collecting them in a random order on a fixed collector or by collecting them directed onto a mobile collector.
Nanofibers, which are collected randomly on a fixed collector with the traditional method, have high porous and bulky structures and their diameters vary between 10-1000 nm. With this method, dry nanofiber production is created in milliseconds thanks to the fiber spinning process, which is very fast. (Zhang, 2008). Some of the directional nanofiber structures obtained by collecting oriented on the collector are rotating cylinder (Pan, 2006), rotating disk (Theron, 2001), rotating cylinder and water bath (Cengiz, 2009), conductive metal plates/rings with a gap between them (Wu, 2007). ) or evenly spaced drum covered with wires (Katta, 2004) (Figure 2.9)
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