Nanofiber Production Methods. Nanolif Üretim Yöntemleri

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1 Tekstil Teknolojileri Elektronik Dergisi Cilt: 8, No: 3, 2014 (49-60) Electronic Journal of Textile Technologies Vol: 8, No: 3, 2014 (49-60) TEKNOLOJİK ARAŞTIRMALAR e-issn: Derleme (Review) Yasar Emre KIYAK *, Enes Cakmak *, * North Carolina State University, College of Textiles, Raleigh, NC, 27606, USA [email protected] Abstract Nanofiber technologies are appealing huge interest as a functional fabrication method for one dimensional structured organic, inorganic and hybrid nanomaterials with controlled dimensions. Nanofibers are generated as haphazardly or oriented continuous nanofibers with possibilities of ordered internal morphologies such as core sheath, hollow or porous fiber, or even multi channeled microtube arrangements. These various type of materials can be obtained by different fabrication techniques. Undoubtedly, most appropriate and controllable is method electrospinning. Keywords : Nanofibers, Electropsinning, Fiber Production Özet Nanolif Üretim Yöntemleri Lif çaplarının kontrol edilebilmesi, organik- inorganik ve hibrid nanometaryallerin üretilebilmesi nanolif üretim teknolijilerine olan ilgiyi artırmıştır. Nanolifler gelişigüzel veya oryante olmuş, kabuk-öz, içi bos, gözenekli ve çok kanallı mikro-tüp olarak kullanılabilmektedir. Bu çok değişik özellikli malzemeler yine çok çeşitli lif üretim teknikleri sayesinde üretilebilmektedir. Hiç şüphesiz ki, bunların arasında en uygunu elektrolit çekim metodudur. Anahtar Kelimeler: Nanolif, Elektrolif Çekim, Üretim Yöntemleri Bu makaleye atıf yapmak için Kiyak E.Y., Cakmak E., Nanolif Uretim Yontemleri Tekstil Teknolojileri Elektronik Dergisi 2014, 8(3) How to cite this article Kiyak E.Y., Cakmak E., Electronic Journal of Textile Technologies, 2014, 8 (3)

2 Teknolojik Araştırmalar: TTED 2014 (3) INTRODUCTION Up to now, much research about nanofiber based composites has been carried out. Because nanofibers have unique mechanical, electrical and chemical properties, they are accepted as advanced materials. Nanofibrous mats can be produced intentionally and in desired conditions, which means they can be regarded as nanomaterial, as well. Since it is very hard to get a single nanofiber, they are produced as membrane or non-woven form. Countless applications of nanofiber makes itself an attractive material. Therefore, understanding different production methods and utilizing the methods according to specific application is crucial for the success of the application. There have been many different ways for producing nanofibers, such as drawing, phase separation, self-assembly, template synthesis, electroblowing, centrifugal and electrospinning [1].In this paper, different techniques of nanofiber production methods are briefly reviewed. 2. ELECTROSPİNNİNG & CONFİGURATİONS From the beginning of this century, researchers have been reinventing an old process known as electrospinning [2]. It is able to fabricate continual nanofibers from different types of material when regarding manageability [2]. Moreover, electrospinning is a more inexpensive method than others. Today, low-cost, high-strength, high-value added nanofibers can be produced in diameter ranging from 20 nm to 1 micron [3,4] Basic Electrospinning First, the technique produces dimension controlled fibers down to nanometer range. A melt or solution of polymer is fed through a narrow needle or nozzle. The nozzle or needle perform as an electrode simultaneously applying typically high voltage from 20 to 100 kv. Generally, the distance from electrode to collector is cm in typical laboratory set-up The current varies from nanoamper to microampere during the electrospinning process. The design of typical electrospinning can be from top to bottom or bottom to top [4]. If high voltage is applied to an electrode, an electric field is generated between collectorandelectrode. After critical voltage value, electrostatic force overcomes the surface tension and the charged polymer solution is forced to leave from the tip of thenozzle or needle. The pulled polymer solution jet passes through an elongation and instability phase due to the effects of electrostatic force, which makes the jet very thinandlong. Simultaneously, solvent evaporates and solidification takes place, and an interconnected layer of fibers on the surface of the collectoris created. In the case of melt, ejected jet solidifies while traveling in the air [5] Melt Electrospinning Most of the electrospun nanofibers are based on solution spinning. Risky chemicals are used to dissolve plentiful polymers. These solvents may leave residue that is not compatible withhealth issues. There has been adesire to produce fibers by concentrating on a cleaner process, through being environmentally safe and productive. Thus, an electrospinning system that uses molten polymer has gathered great excitement. Despite many promising applications, little development has been made in the past 20 years [6]. Figure 1 shows the design of typical meltelectrospinning. When high voltage is applied to a spinneret, a cone is formed. At the critical voltage, electro static forces overcome the surface tension. Viscoelastic behavior of the melt result in a fine fiber extruded from the orifice residing at small holes. Rheology of the molten state is critical for the stable spinning [6]. 50

3 Kıyak E.Y., Çakmak E. Teknolojik Araştırmalar: TTED 2014 (3) Figure 1. Melt electrospinning technique In the melt electrospinning, there must be a constant heat supply to the reservoir containing the polymer solution so that the polymer remains in a molten state. The distance from nozzle to collector is typically smaller than basic electrospinning. Because the polymer is in a molten form, greater charge is required for jet initiation [1] Gas-Jacket Electrospinning Figure 2. Gasassisted electrospinning. In some circumstances, electro static force may not be sufficient alone to stretch the solution for producing nanofibers. The reason can be because ofexcessive surface tension of the solutionor high viscosity. If easily evaporated solvents are preferred, exposure to the environment and fast evaporation of the solvent may make the solution hard to spin [7]. Therefore, a gas jacket that hits and brings into play a stretching force on the solution can be used at the spinneret tip in order to initiate the spinning,as shown in Figure 2 [8]. For example, hyaluronic acid that has a high viscosity can be electrospun in this system. Fiber fabrication can be advanced by using a heated gas because the higher temperature decreases the viscosity of the solution [9].The inert gas is brought up from the basin and passed through the buffer reservoir, which stabilizes the gas flow in the shape of the laminar flow. The gas flow additionally contributes to stretch the jet of the polymer solution, which was slowly injected into a capillary by injector and flowed through a capillary with a definite length. In order to actualize the gas electrospinning process, the polymer solution with a certain concentration is deduced by spinneret, which is connected to the anode of a high-voltage supplier. The cathode is the metal collector or coagulating bath and connected to the ground [10]. Moreover, the electroblowing technique can be regarded as the same production method [11-14] 2.4. Bubble Electrospinning An innovative technique mimicking the spider spinninghas been designed to minimize the surface tension of solution. This novel system which is patented [15] includes a vertical solution reservoir with a gas tube feeding from the bottom with a metal electrode fixed along the middle of the tube. A grounded collector is mounted above the reservoir (Fig. 3). 51

4 Teknolojik Araştırmalar: TTED 2014 (3) Figure 3. Bubble Electrospinning The mechanism of this innovative electrospinning process is certainly simple. In the absence of an electric field, the aeration system generates various bubbles on the surface. When an electric field is created, it induces charges into the bubble surface. The coupling of surface charge and external electric field create a tangential stress, resulting in the transformation of the little bubbles into protuberanceinduced upward-directed reentrant jets. Once the electric field surpasses the critical value needed to overcome the surface tension, a fluid jet distracts from the zenith of the conical bubble. The threshold voltage needed to overcome the surface tension depends upon the size of the bubble and the inlet air pressure. The most attractive property of this bubble electrospinning technique is that it is independent of the properties of the the solution such as viscosity. This novel technology has critical importance for the new generation of electrospinning, [3] Magneto Electrospinning A magnetic field is utilized in the electrospinning process by generating ampere force. If a magnetic field is applied,magneto electrospinning(fig. 4) can be conducted completely [3].The resultant electrical force and the viscous force of the jet flow increases the radius of the whipping circle. Figure 4. Magnetic field electrospinning A mathematical model of the magnetic electrospinning technique and the moving behavior of the jet are investigated. The results show that the magnetic field creates a radial Lorenz Force resulting in jet shrinking. The relationship of the swing of the jet and its distance with different excitation currents are obtained. From the relationship, it is known that the swing with a magnetic field is smaller than that without a magnetic field. The simulation results do not agree well with the experimental data and show that the magnetic field can influence the jet at an appropriate distance [17]. It seems hard to adapt this technique for mass production Conjugate Electrospinning The system used in conjugate electrospinning is illustrated in Figure 5. It consists of two or three high voltage power supplies with opposite polarity, two or three spinnerets and a receiver drum. Two or three programmable pumps are used to control the delivery rate of solutions. Power supplies are connected with spinnerets, respectively. Spinnerets are arranged in opposite positionson the same horizontal line. The distance between spinnerets are designed as adjustable. The solution is separately delivered by syringes to 52

5 Kıyak E.Y., Çakmak E. Teknolojik Araştırmalar: TTED 2014 (3) two or three spinnerets. The receiver is a rotating drum controlled by a stepping motor. The fibers from the two or three oppositely charged electrospinning spinnerets were collected and stretched by the drum receiver with a constant speed. The nanofiber yarns,which can be produced by this technique, are dried under vacuum at room temperature [18] Co-axial Electrospinning Figure 5. Conjugate electrospinning [18]. Figure 6. Co-axial electrospinning In the co-axial electrospinning technique (Fig. 6), twoliquids distract from the core and surrounding concentric nozzles in the form of polymer solution or melt. The liquids contact each other only at the tip of the coaxial needle or capillary [16].Taylor cone formation is achieved through the nozzle. During the spinning process, solvents evaporate and compound jet solidify. As a result of this solidification, coreshell nanofiber is produced. İn this technique, preventing the mixing of polymer solutions is critical.here, shell polymer serves as a template for the core material leading to cable-type structures. With this technique, polymer that cannot be electrospun by basic electrospinning is preferred to obtain core-shell nanofiber.these types of fibers have potential applications in microelectronics, optics, and medicine. Furthermore, polymer nanotubes can be generated [20] Needless Electrospinning The needless electrospinning technology was first patented at the Technical University of Liberec in 2003 [21]. The idea comes from the industrial application. Needle system is regarded as inappropriate for mass production. In 2004, with the help of Elmarco Company, the process was attempted to be commercialized for the production of nanofibers. 53

6 Teknolojik Araştırmalar: TTED 2014 (3) Figure 7. Needleless Electrospinning Jirsak found a needleless electrospinning setup via a rotating roller. When the roller revolving slowlyis partially submerged into a polymer solution, the polymer solution is loaded onto the higher roller surface [22].As applying a high voltage to the reservoir, a gigantic number of solution jets can be generated from the roller surface upward (Fig. 7). This setup has been commercialized by Elmarco Co. with the brand name Nanospider TM [24]. The rotation of spinneret loads a small layer of polymer solution onto the spinneret surface. The rotation and perturbance create conical spikes on the surface. When a high voltage is applied, these spikes collect charges and amplify the perturbance and, as a result of this, the fluid around the spikes is spun. Therefore, Taylor cones are generated. Slim electrospinning jets are then ejected from the tips when the electric force is adequate [24-25] Centrifugal Electrospinning The centrifugal electrospinning(fig. 8)process is the combination of centrifugal spinning andelectrospinning. Polymer solution is fed and placed centrally onto a very high speed rotating spinning disc. The polymer solution is moved radial towards the rim of the rotating disc where the surface tension is overcomedue to the centrifugal forces. The electric field contributes to stretch the jets to very small dimensions under simultaneous evaporation of the solvent, leaving a dry nanofibrous coating on the substrate.the method is independent of ambient conditions such as temperature and humidity. This is a definite advantage when compared to needle based or needleless processes, which sometimes are affected severely by relative humidity. Bead and holes are possible defects that can be seen in this technique [26]. 3. CENTRIFUGAL SPINNING Figure 8. Centrifugal electrospinning Centrifugal spinning method is similar to the cotton-candy spinning method. In a cotton candy machine, sugars are melted by heating and extruded by centrifugal spinning through nozzles, resulting in sucrose fibers randomly distributed in the free space near the spinneret. Recently, the fabricated sucrose fibers have been used as a mold for porous polymer forming [27]. This process has been patented by Huang in 2007 [28]. Centrifugal production method is quite similar to centrifugal electrospinning. But, in this method no electrical charge is used. Instead, it consists ofdropping of a polymer solution onto a typical spin coater followed by a very fast rotatingspinneret. Moreover, the spinning procedure offers many 54

7 Kıyak E.Y., Çakmak E. Teknolojik Araştırmalar: TTED 2014 (3) technologically relevant opportunities, such as yielding hollow polymer beads,and being applicable to different types of polymers [29]. Today, a company has improved a new method called Forcespinning TM (Fig 9.) for the nanofiber production via this technique. Forcespinning TM is applicable for solutions and molten materialsvia centrifugal force. Because Forcespinning uses less solvent or no solvent at all, it is more productive in terms of cost when compared to electrospinning. No heated air jets or no heating make it more cost effective than meltblown. [30].A spinneret with a reservoir containing liquid material is rotated centrifugally on an axis at high rpm. While the spinneret rotates, liquid material is pushed to the outer wall through an orifice. In this process, centrifugal and hydrostatic forces together initiate the jet [30]. 4. WET SPINNING Figure 9. Forcespinning TM [30] This technique is derived from a conventional and commercial wet spinning technique that can fabricate polymer nanofibers with diameter ranges from 10 microns down to 400 nanometers. Figure 10. Wet-spinning setup for nanofiber production Fiber spinning succeed via injection of a polymer solution precursor passing through a very small aperture into a moving and highly viscous medium (Fig. 10). The medium is moved by a spinning mandrel, which serves to perform the extruded fiber away from the pinhole. The mandrel pulls the nanofiber. If the extrusion tip is appropriately positioned, the extruded fiber moves in a spiral trajectory and is deposited continuously around the mandrel. Polymer solidifies in the viscous medium. This modified wetspinning technique can spin fibers that would be considered unspinnable by any other method. Furthermore, some materials that have low viscosity can be used in this process [31]. 5. DRAWING The drawing technique provides simple and cost effective photonic wire manufacturing. But a steady temperature distribution is needed in the drawing zone and the lengths of the fabricated wires are limited to tens of millimeters. PTT nanofibers with diameters down to 60 nm and lengths up to 500 mm can be fabricated by the drawing process, as described in Figure 11. PTT is retracted at a speed of m/s. 55

8 Teknolojik Araştırmalar: TTED 2014 (3) The extended PTT wire is quickly cooled in air and finally an unclad amorphous PTT nanofiber is created [32]. Fibers fabricated by this technique not only show low optical losses but also offer good flexibility [33]. 6. ISLAND IN THE SEA Figure 11. Drawing process There have been several configurations and patents about the island in the sea process [34-35]. Typical island in the sea fiber can be seen in the Figure 12. This method allows the melt and solution processing of plural component fibers. Basically, two different polymers are spun together after that sea polymer is dissolved. Also, before dissolving the sea polymer, a drawing can be conducted in order to reduce the fiber diameter. Moreover, Hills spin pack offers some attractive features, such as high spinneret density for complex bicomponent cross sections, flexibility in selection of polymer types and bicomponent crosssections. In the production of the Island in the Sea filaments, the spinneret design and the distribution plates are crucial because the fiber diameter, the cross sectional area and the number of islands depend on the diameter and the shape of the spinneret orifice and the polymer distribution in the distribution plates. Furthermore, the same method can be applied to spunbond process [36]. Figure 12. Typical island-in-sea fiber Island and sea polymers have to be selected according to their ability to be spun in the island in the sea configuration. It has been shown that the polymer viscosities are the most profound factors influencing the spinnability of the two components in the configuration. The difference in the viscosity of polymers can cause serious migration and deformation of an interface between polymers resulting from unequal concentration of stresses in the spinline. The spinline tension is mainly concentrated on the component having higher viscosity [37]. 8. OTHER TECHNIQUES From the wide aspect of nanofiber production, there have been several techniques that are used rarely. Some of them will be given very briefly. Template Synthesis The template method is an effective approach for the controlled synthesis of nano materials with various morphologies and has attracted a lot of efforts [38]. Henceforward the casting method and DNA replication can be considered as template-based synthesis. For the case of nanofiber production by the template refers to a metal oxide membrane with pores of nano-scale diameter. Under the application of water pressure on one side, restraining from the porous membrane causes extrusion of the polymer 56

9 Kıyak E.Y., Çakmak E. Teknolojik Araştırmalar: TTED 2014 (3) which comes into contact with a solidifying solution giving rise to nanofibers whose diameters are determined by the pores [1]. Phase Separation In phase separation, a polymer is firstly mixed with a solvent before undergoing gelation. The main mechanism in this process is (as the name suggests) the separation of phases due to physical incompatibility. One of the phases which is the solvent is extracted resulting in leaving behind the other remaining phase [1]. In other words, the process can be described subsequently as: dissolution of polymer, liquid-liquid phase separation, polymer gelation, extraction of solvent from gel and quenching, respectively [39]. Self- Assembly The main mechanism for a generic self-assembly is the intermolecular forces that bring the smaller units together. The shape of the smaller units of molecules determine the overall shape of the macromolecular nanofiber. Molecular self-assembly provides an innovative way to design and produce novel materials at multiple levels (nano micro macro) [1]. 9. REFERENCES 1. Ramakrishna S, Fujihara K, Teo W E, Lim T C and Ma Z, An Introduction to Electrospinning and Nanofibers (Singapore: World Scientific), Teo W.E., Ramakrishna S., A review on electrospinning design and nanofibre assemblies, Nanotechnology, 17, R89 R106, He J.H., Electrospun Nanofibers and Their applications, 6, Greiner A., Wendorff H.J., Electrospinning: A Fascinating Method for the Preparation of Ultrathin Fibers, Angew. Chem. Int. Ed., 46, , Patanaik A., Anandjiwala R.D., Rengasamy R.S., Ghosh A., Pal H., Nanotechnology in fibrous materials-a new perspective, Nanotechnology in fibrous materials-a new perspective', Textile Progress, 39:2, , Lyons J., Li C., Ko F., Melt-electrospinning part I: processing parameters and geometric properties, Polymer 45, , Larsen G, Spretz R and Velarde-Ortiz R, Adv. Mater , Teo W.E., Inai R., Ramakrishna S., Technological advances in electrospinning of nanofibers, Sci. Technol. Adv. Mater. 12, Um I C, Fang D, Hsiao B S, Okamoto A and Chu B, Biomacromolecules, 5, 1428,

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