**2.4 Freeze drying**

The technique is also known as ice segregation-induced self-assembly or solid– liquid phase separation. The technique comprises of following steps-


Important advantages of freeze drying over other techniques of nanofibers fabrication is its ability to produce the porous structures of controlled sizes directly from polymers (e.g. chitin), without addition of structure directing additives and pre-treatment.

**23**

given below.

*Green Synthesis of Nanofiber and Its Affecting Parameters*

In the field of nanotechnology electrospinning is the most promising remarkably simple processes for generating nanofibers from polymers solution. In combination with conventional sol–gel process, it offers a versatile technique for producing solid, porous, or hollow structured ceramic nanofibers. The technique has earned enormous attention because of its easy use and flexibility in controlling diameter in range of nanometers to micrometer and alignment of nanofibers as well as continuous nanofibers production capability. Electrospinning is peculiar in the sense that can be useful in producing fine nanofibers from different solution or melt of polymer, ceramic material and composite material for fast synthesis of nanofibers at industrial scale. Formhals patented a process in 1934 for an experimental setup describing nanofiber production from polymer solution using electrostatic force, termed as electrospinning [33]. Now days, electrospinning is explored as a high efficiency method for the generating ceramic nanofibers [34]. In the generic design, an electrospinning setup consists of a high-voltage power supply, a metallic needle called spinneret, a piece of aluminum foil or silicon wafer acting as an electrically conductive collector and a syringe pump. A plastic syringe loaded with the polymer solution and connected to the metallic needle is often connected to a syringe pump for constant and adjustable feeding rate of the solution. In some cases, especially for electrospinning of ceramic nanofibers, the setup

The collector can be constructed in different configuration from various materials depending on the end use of nanofiber. The electrospinning process is generally performed at room temperature at atmospheric conditions [35]. The ES system continuously need a high voltage (10-40kv) power supply of 40 kV, syringe pump, syringe, metallic needle and an electrically conductive collector plate during nanofiber synthesis. The electrospinning setup is schematically represented in **Figure 2**

Many parameters affecting ES process can be classified broadly into solution parameters, process parameters, and ambient parameters. Molecular weight,

needs to be placed in a closed box for controlled humidity variation.

**3. Parameters affecting the performance of electrospinning**

*DOI: http://dx.doi.org/10.5772/intechopen.94539*

**2.5 Electrospinning (ES)**

*Schematic representation of the electrospinning.*

**Figure 2.**

*Green Synthesis of Nanofiber and Its Affecting Parameters DOI: http://dx.doi.org/10.5772/intechopen.94539*

**Figure 2.** *Schematic representation of the electrospinning.*

### **2.5 Electrospinning (ES)**

*Nanofibers - Synthesis, Properties and Applications*

This method consists mainly of four basic steps:

iii.Extraction of the solvent from the gel.

themselves or under an external stimulus [29, 32].

iv.Freezing and freeze drying.

and porosity gets decreased [29].

**2.3 Self-assembly**

**2.4 Freeze drying**

desorption.

pre-treatment.

In this method phases separate because of the physical incompatibility. The phase of the solvent then extracted from the solution while the other phase remain.

i.Homogeneous polymer solution is prepared by dissolved the polymer in a

ii.Gelation of the solution to produce nanofiber matrixes. It is the most difficult step controlling porosity and morphology of the nanofiber. Gelation varies depending upon concentration of polymer and ambient temperature.

The process is not equipment extensive in fabricating nanofiber matrix. The mechanical properties of matrix can be adjusted changing concentration of polymer [30]. By increasing polymer concentration, fiber mechanical properties are improved

In self-assembly small building blocks spontaneously organized to build-up the stable nanofibers of very thin diameter. In such nano-material fabrication, building block molecules organize and arrange itself into definite patterns or structures due hydrophobic and electrostatic interactions and hydrogen bonding [31]. The intermolecular forces bring units together to macromolecular nanofiber. The technique produce minute nanofibers of >100 nm to micrometers length. This nanofiber fabrication is complex, long, and extremely elaborate associate with limitation of low yield and uncontrollable fiber dimensions. Further, this method can be used to prepare nanofibers from selected molecules having capability of self-assemble

The technique is also known as ice segregation-induced self-assembly or solid–

ii.Primary drying- removal of water from frozen material by sublimation in a

iii.Secondary drying- removal of unfrozen water from the polymer material by

Important advantages of freeze drying over other techniques of nanofibers fabrication is its ability to produce the porous structures of controlled sizes directly from polymers (e.g. chitin), without addition of structure directing additives and

liquid phase separation. The technique comprises of following steps-

i.Freezing- polymer solution is frozen at a low temperature

chamber have reduced pressure of few millibars and

**2.2 Phase separation**

suitable solvent.

**22**

In the field of nanotechnology electrospinning is the most promising remarkably simple processes for generating nanofibers from polymers solution. In combination with conventional sol–gel process, it offers a versatile technique for producing solid, porous, or hollow structured ceramic nanofibers. The technique has earned enormous attention because of its easy use and flexibility in controlling diameter in range of nanometers to micrometer and alignment of nanofibers as well as continuous nanofibers production capability. Electrospinning is peculiar in the sense that can be useful in producing fine nanofibers from different solution or melt of polymer, ceramic material and composite material for fast synthesis of nanofibers at industrial scale.

Formhals patented a process in 1934 for an experimental setup describing nanofiber production from polymer solution using electrostatic force, termed as electrospinning [33]. Now days, electrospinning is explored as a high efficiency method for the generating ceramic nanofibers [34]. In the generic design, an electrospinning setup consists of a high-voltage power supply, a metallic needle called spinneret, a piece of aluminum foil or silicon wafer acting as an electrically conductive collector and a syringe pump. A plastic syringe loaded with the polymer solution and connected to the metallic needle is often connected to a syringe pump for constant and adjustable feeding rate of the solution. In some cases, especially for electrospinning of ceramic nanofibers, the setup needs to be placed in a closed box for controlled humidity variation.

The collector can be constructed in different configuration from various materials depending on the end use of nanofiber. The electrospinning process is generally performed at room temperature at atmospheric conditions [35]. The ES system continuously need a high voltage (10-40kv) power supply of 40 kV, syringe pump, syringe, metallic needle and an electrically conductive collector plate during nanofiber synthesis. The electrospinning setup is schematically represented in **Figure 2** given below.

#### **3. Parameters affecting the performance of electrospinning**

Many parameters affecting ES process can be classified broadly into solution parameters, process parameters, and ambient parameters. Molecular weight,

concentration, conductivity, viscosity and surface tension are then solution parameters whereas feeding or flow rate, tip to collector distance and electric field are important process parameters. All these parameters individually and synergistically affect the final fibers morphology. Researcher used to adjust the above mentioned parameters to synthesize nanofibers of desired diameter and morphology and alignment [36]. Further, ambient environmental parameters like the temperature and humidity also affect electrospinning process and nanofiber morphology and diameter [37].

#### **3.1 Solution parameters**

#### *3.1.1 Solution concentration*

Nanofiber synthesis by ES process needs a minimum concentration of polymer in the solution for continuous nanofiber synthesis, the concentration below it, result in mixture of beads and fibers. The increase in concentration of solution the beads shape changes from spherical to spindle fibers and further increase in concentration synthesis uniform fibers with broad diameters due to the high viscosity resistance [38–41]. The range of concentration synthesizing continuous fibers is determined by surface tension and viscosity of solution. During ES process, solution concentration above optimum prohibits continuous fiber formation because of the inability to maintain the flow of the solution at the tip of the needle leading to the formation of larger fibers.

#### *3.1.2 Molecular weight*

Another important parameter is molecular weight of polymer that has strong influence on the morphology of electrospun nanofiber. Molecular weight of polymer affects rheology, electrical properties, conductivity, surface tension, viscosity and dielectric strength [42]. In ES process, generally high molecular weighted polymer offers the desired viscosity for the nanofiber synthesis whereas too low a molecular weighted polymer tends to form beads instead of fibers frequently. But very high molecular weighted polymers synthesize fibers of broad average diameters. Polymer's molecular weight and number polymer chain entanglements affect viscosity of the polymer solution. Polymer chain entanglement significantly determines the fiber synthesis in the electrospinning process.

#### *3.1.3 Solution viscosity*

Viscosity of solution is detrimental for the size of fiber and morphology during ES of fiber from polymer solution. The polymer solutions with very low viscosity lack continuous fiber formation whereas polymer solutions having very high viscosity results in difficult ejection of jets of polymer solution from the needle. So, an optimum viscosity is needed for ES process. Viscosity was shown to affect silk nanofibers synthesis by Sukigara and colleagues [43]. In the low viscosity solutions, surface tension becomes dominant factor. The increase in solution viscosity or concentration results in large sized fibers of uniform diameter [38].

#### *3.1.4 Surface tension*

Surface tension plays a critical role in the ES process. Surface tension of a solution is more likely a function of solvent compositions. So, the surface tension of polymer solution can be altered by using different solvents. The formation of

**25**

*Green Synthesis of Nanofiber and Its Affecting Parameters*

in ES process to occur at a relatively low electric field.

droplets, bead and fibers depends on the surface tension of polymer solution. If the surface tension of polymer solution is high, jet will be instable and the droplets will be sprayed from the tip of the needle leading to inhibition of ES process [44]. The reduced surface tension of a polymer solution will synthesize nanofibers without beads. The polymer solution with surface tension can of the spinning solution helps

Applied voltage in the ES process is a crucial process parameter. The applied voltage should attainment of threshold value for starting synthesis of fiber formation. The voltage induces the necessary charge and electric field on the solution to initiates the ES process. In the most cases, a high voltage greatly stretch of the solution because of greater columbic forces in the jet and a strong electric field that result in reduced fiber diameter and rapid evaporation of solvent from the fibers. Thus voltage influences fiber diameter at an extent, but the level of significance varies with the polymer solution concentration and on tip to the collector distance [45].

The feed rate of the polymer solution is another important process parameter influencing velocity of jet from needle tip and the material transfer rate. Low feed rate is almost always desirable for getting enough time for evaporation of solvent from the polymer solution during fiber synthesis [46]. The spinning polymer solution should have a minimum flow rate for ES process to occur. Greater polymer solution flow rates forms beaded fibers due to improper smaller drying time period

A collector in ES process is a conductive substrate on which nanofibers are deposited. Collector is an important process parameter. Aluminum foil is one of the most common collectors in the ES process. To overcome the difficulty to transfer of collected fibers and necessity of aligned fibers for different applications, other collectors could also be conductive paper or cloth, rotating rod or wheel, pin, wire

Fiber diameters and morphology could be controlled by manipulating the tip to collector distance. However, a minimum distance between collector and needle tip is essential for giving sufficient time for fibers dry before being deposited on the collector. The beads are frequently observed in the cases where tip to collector distance are either too close or too far. The optimum distance between the tip and collector are adjusted depending upon the polymer solution used for proper evapo-

In addition to the solution and process parameters, there are some ambient parameters such as humidity, temperature etc. which influence the ES process.

*DOI: http://dx.doi.org/10.5772/intechopen.94539*

**3.2 Process parameters**

*3.2.2 Feed rate/flow rate*

before reaching to the collector.

mesh parallel or gridded bar [47].

*3.2.4 Distance between tip to collector*

ration of solvent from the nanofibers [37].

**3.3 Ambient parameters**

*3.2.3 Types of collectors*

*3.2.1 Voltage applied*

*Green Synthesis of Nanofiber and Its Affecting Parameters DOI: http://dx.doi.org/10.5772/intechopen.94539*

droplets, bead and fibers depends on the surface tension of polymer solution. If the surface tension of polymer solution is high, jet will be instable and the droplets will be sprayed from the tip of the needle leading to inhibition of ES process [44]. The reduced surface tension of a polymer solution will synthesize nanofibers without beads. The polymer solution with surface tension can of the spinning solution helps in ES process to occur at a relatively low electric field.

#### **3.2 Process parameters**

#### *3.2.1 Voltage applied*

*Nanofibers - Synthesis, Properties and Applications*

diameter [37].

**3.1 Solution parameters**

*3.1.1 Solution concentration*

the formation of larger fibers.

*3.1.2 Molecular weight*

*3.1.3 Solution viscosity*

*3.1.4 Surface tension*

concentration, conductivity, viscosity and surface tension are then solution parameters whereas feeding or flow rate, tip to collector distance and electric field are important process parameters. All these parameters individually and synergistically affect the final fibers morphology. Researcher used to adjust the above mentioned parameters to synthesize nanofibers of desired diameter and morphology and alignment [36]. Further, ambient environmental parameters like the temperature and humidity also affect electrospinning process and nanofiber morphology and

Nanofiber synthesis by ES process needs a minimum concentration of polymer in the solution for continuous nanofiber synthesis, the concentration below it, result in mixture of beads and fibers. The increase in concentration of solution the beads shape changes from spherical to spindle fibers and further increase in concentration synthesis uniform fibers with broad diameters due to the high viscosity resistance [38–41]. The range of concentration synthesizing continuous fibers is determined by surface tension and viscosity of solution. During ES process, solution concentration above optimum prohibits continuous fiber formation because of the inability to maintain the flow of the solution at the tip of the needle leading to

Another important parameter is molecular weight of polymer that has strong influence on the morphology of electrospun nanofiber. Molecular weight of polymer affects rheology, electrical properties, conductivity, surface tension, viscosity and dielectric strength [42]. In ES process, generally high molecular weighted polymer offers the desired viscosity for the nanofiber synthesis whereas too low a molecular weighted polymer tends to form beads instead of fibers frequently. But very high molecular weighted polymers synthesize fibers of broad average diameters. Polymer's molecular weight and number polymer chain entanglements affect viscosity of the polymer solution. Polymer chain entanglement significantly

Viscosity of solution is detrimental for the size of fiber and morphology during ES of fiber from polymer solution. The polymer solutions with very low viscosity lack continuous fiber formation whereas polymer solutions having very high viscosity results in difficult ejection of jets of polymer solution from the needle. So, an optimum viscosity is needed for ES process. Viscosity was shown to affect silk nanofibers synthesis by Sukigara and colleagues [43]. In the low viscosity solutions, surface tension becomes dominant factor. The increase in solution viscosity or

determines the fiber synthesis in the electrospinning process.

concentration results in large sized fibers of uniform diameter [38].

Surface tension plays a critical role in the ES process. Surface tension of a solution is more likely a function of solvent compositions. So, the surface tension of polymer solution can be altered by using different solvents. The formation of

**24**

Applied voltage in the ES process is a crucial process parameter. The applied voltage should attainment of threshold value for starting synthesis of fiber formation. The voltage induces the necessary charge and electric field on the solution to initiates the ES process. In the most cases, a high voltage greatly stretch of the solution because of greater columbic forces in the jet and a strong electric field that result in reduced fiber diameter and rapid evaporation of solvent from the fibers. Thus voltage influences fiber diameter at an extent, but the level of significance varies with the polymer solution concentration and on tip to the collector distance [45].

#### *3.2.2 Feed rate/flow rate*

The feed rate of the polymer solution is another important process parameter influencing velocity of jet from needle tip and the material transfer rate. Low feed rate is almost always desirable for getting enough time for evaporation of solvent from the polymer solution during fiber synthesis [46]. The spinning polymer solution should have a minimum flow rate for ES process to occur. Greater polymer solution flow rates forms beaded fibers due to improper smaller drying time period before reaching to the collector.

#### *3.2.3 Types of collectors*

A collector in ES process is a conductive substrate on which nanofibers are deposited. Collector is an important process parameter. Aluminum foil is one of the most common collectors in the ES process. To overcome the difficulty to transfer of collected fibers and necessity of aligned fibers for different applications, other collectors could also be conductive paper or cloth, rotating rod or wheel, pin, wire mesh parallel or gridded bar [47].

#### *3.2.4 Distance between tip to collector*

Fiber diameters and morphology could be controlled by manipulating the tip to collector distance. However, a minimum distance between collector and needle tip is essential for giving sufficient time for fibers dry before being deposited on the collector. The beads are frequently observed in the cases where tip to collector distance are either too close or too far. The optimum distance between the tip and collector are adjusted depending upon the polymer solution used for proper evaporation of solvent from the nanofibers [37].

#### **3.3 Ambient parameters**

In addition to the solution and process parameters, there are some ambient parameters such as humidity, temperature etc. which influence the ES process. Increase in ambient temperature yield fibers of relatively decreased diameter which can be attributed the decrease in the viscosity of the polymer solutions at high temperatures. In the very low humidity environment, a volatile solvent will rapidly evaporate from synthesized fiber. However, too fast evaporation could be problematic in ES process when solvent get evaporated from the polymer solution just after emission from the tip of the needle. In such condition, ES process in stopped due to clogging of needle tip within few minutes operation of ES process [48]. It has been advised that the high humidity can help in the discharge of the synthesized nanofibers.
