18 Nanofibers – electrospinning technique

Dr. Anchal Srivastava

Contents of this Unit

1.  Introduction

2.  Electrospinning

3.  Electrospinning Setup

4.  Theory of Electrospinning

(i)   Fluid Charging

(ii)   Liquid droplet under high voltage – Taylor cone theory

(iii)   Jet in flight-thinning and instability theories

5.  Effect of working parameters

5.1. Solution Parameters

5.1.1. Concentration

5.1.2. Viscosity

5.1.3. Surface Tension

5.2. Processing Parameters

5.2.1. Voltage

5.2.2. Flow Rate

5.2.3. Collectors

5.2.4. Distance between the collector and tip of syringe

6. Summary

Learning Outcomes

• After studying this module, you shall be able to
• Know about synthesis of nanofibers of polymes.
• Learn about electrospinning setup and its application for growth of nanofibers. Know various theories of electrospinning.
• Understand about different processing parameters, which governs the growth of nanofibers.

1. Introduction

One-dimensional (1D) nanostructures (wires, rods, tubes, fibers, and belts) are for current interest due to their wide range of applications in mesoscopic physics and nanoscale devices. In contrast to other nanostructures, 1D nanostructures can provide unique advantages for the investigation of the electrical, mechanical and thermal dependence performances on dimensionality. For the potential applications of 1D nanostructures into existed macroscopic devices, there is a great need to develop a novel synthesis route for 1D nanostructures with nanoscaled diameter and macroscopic length. Since the 1990s, electrospinning firstly patented by J. F. Gooley in 1900 (Cooley, J. F. Patent GB 06385 “Improved methods of and apparatus for electrically separating the relatively volatile liquid component from the component of relatively fixed substances of composite fluids” 19th May 1900), which is a novel simple and facile technique has been attracted numerous scientific attention for the growth of 1D nanostructures having continuous length, tuneable diameter, aligned direction, diverse and controllable compositions. During the electrospinning process, electric field force acts on the polymer solution or melt to form the electrospun jet. Finally, solidified fiber can be achieved by stretching the electrified jet for the electrostatic repulsions between the surface charges and the evaporations of solvent.

Till the date, electrospinning field has been evaluated as the fast moving front in materials science by Thomson, ISI. Therefore in coming sections I am going to elaborate electrospinning technique.

2. Electrospinning:

Form 1990s, a number of advanced techniques have been developed for the generation of 1D nanostructures including electron-beam or focused-ion-beam (FIB) writing, lithography, hydrothermal, chemical vapor deposition, electrospinning, solution method, etc. Among these methods electrospinning is the simplest method to fabricate 1D nanostructures with both solid and hollow interiors with continuous length, tunable diameter, aligned direction, diverse and controllable composition. Different from other methods for growth of 1D nanostructures, electrospinning is based on the electric field force acting on the polymer solution or melt, which can be regarded as a variant of the electrospray process. In electrospinning, solidified fiber can be achieved by stretching the electrified jet for the electrostatic repulsions between the surface charges and the evaporations of solvent. This technique could be applied to synthetic and natural polymers, polymer alloys and polymer decorated with functional nanomaterials. These unique advantages of electrospinning afford the multifunctional properties for diverse applictions.

Despite the simplicity of the electrospinning setup, the electrospinning mechanisms are rather complicated including Taylor Cone theory, Bending Instability theory, Electrically Forced Jet-Stability theory, and so on. These theories not only provide a better understanding of mechanisms corresponding to the electrospinning process, but also endow scientists with the good ability of designing novel setups for the further control of electrospun nanostructures performances. Prompted by these theories, electrospun nanostructures have been extended from single solidified polymer nanofibers to polymer/inorganic hybrid nanofibers, inorganic nanofibers, hollow inorganic nanotubes, etc. Additionally, by modification of the electrospun setups, individual nanofiber, aligned nanofibers, and patterned nanofibers have been also achieved. Taking the unique advantages (extremely long length, high surface area, complex and diverse compositions and structures, and so on) of electrospun nanostructures, electrospun nanostructures have gained broad applications in tissue engineering, drug delivery, optoelectronics, chemiresistors, catalysis, filters, fiber reinforcement, wounding healing, photoelectronics, FET, magnetic devices, etc.

3. Electrospinning Setup:

Electrospinning setup contains three major components: (1) Synringe pump with a metallic or plastic syringe, (2) High voltage DC or AC power supply and (3) Fixed Collector plate or rotating cylinder. In the setup syringe is used to inject the polymer solution at controlled rate with the help of syringe pump. As the high voltage is applied through high power AC or DC power supply, the drop of polymer solution at the tip of syringe is polarized and the induced charges are distributed over the surface. Under the influence of strong electrostatic field, the charged polymer is accelerated toward the collector. The role of electrostatic force here is to supplement or replace the conventional mechanical forces (e.g. hydrostatic, pneumatic) used to form the jet and to reduce the size of the fiber, hence known as electrohydrodynamic jetting.

Here, the collector plate should be off good electrical conductivity to neutralize the charge carried out by the polymer nanofibers.

Fig. 1. Schematic illustration of a typical electrospinning setup.

4. Theory of Electrospinning:

Theory of electrospinning could be divided into three categories:

(i)   Theory on the fluid charging

(ii)   Theories on the liquid droplet under high voltage – Taylor Cone theory.

(iii)   Theories on the jet in flight – Instability Theories.

(i) Fluid Charging:

In electrospining, by the virtue of high electric field charges on the fluid in the syringe is generated, referred to as induction charging (shown in Fig. 2). At the same time, free electrons, ions or ion pairs will be generated as charge carriers in the fluid and form double layer in the fluid owing to the ion mobility. In the absence of flow, the double layer thickness is determined by the ion mobility within the fluid and in the presence of flow, ions may be convected away from the electrode and the double layer continually replenished. Inductive charging is generally suitable for fluids with the conductivities of the order of 10-2 S/m.

Fig. 2. Schematic for polarization of polymer drop in syringe under high potential.

(ii) Liquid Droplet Under High Voltage – Taylor cone Theory:

In 1964, Sir Geoffrey Ingram Taylor established the Taylor Cone theory to describe the deformation of small – volume liquid under the high electric field:

1. As a small volume of electrically conductive liquid is exposed to an electric field, stable shape can be acquired owing to the equilibrium of the electric forces and the surface tension in the cases of Newtonian and viscoelastic liquids.
2. As the voltage is increased to the critical potential and any further increase in potential will destroy the equilibrium, thus the liquid body acquires a conical shape, with half angle of 49.3° (a whole angle of 98.6°), referred to as the Taylor Cone (Shown in Fig. 3).

Additionally, Taylor also demonstrated that the shape of such a cone approached the theoretical shape just before jet formation within the electrospinning process. Taylor’s derivation is based on two assumptions:

Nanoscience and technology-II Nanofibers – electrospinning technique

(i)   The surface of the cone is an equipotential surface

(ii)   The cone exists in steady state equilibrium

Where Uc is voltage, H is the distance between the tip of syringe and the collector, L is the length of syringe, R is the diameter of the tip of syringe, and γ is the surface tension of solution.

Fig. 3. Schematic diagram of the Taylor Cone.

(iii) Jets in Flight-Thinning and Instability Theories:

It has been observed that during electrospinning the charged jets ejected from the Taylor Cone passed by in a nearly straight line preceded by bending into a complex path, during which electrical forces stretched and thinned them by large ratios.

Fig. 4. Schematic illustration of the jets path between the tip of syringe and collector.

As the fluid is ejected from the cone, a continuous thinning of liquid jet can be observed. The charged jet will accelerate in the presence of the electric field created by the countered positive and negative potential as shown in Fig. 5.

Fig. 5. Cone-jets of 2 % solutions of polyethylene oxide in water. D = 45 cm in all cases. (a) Q = 0.02 ml/min, E = 0.282 kV/cm. (b) Q = 0.10 ml/min, E = 0.344 kV/cm. c Q = 0.50 ml/min, E = 0.533 kV/cm. d Q = 1.00 ml/min, E = 0.716 kV/cm.

Several attempts have been made to explain such trends in electrospun jet operation as functions of fluid and operating parameters under experimental control. Within certain limits, for a given fluid, there exist ranges of driving voltage (V) and flow rate over which the electrospinning can be maintained stably for long periods. The electrospinning process constitutes an electrical circuit, so one can measure as well the current (I) flowing through this ‘‘circuit.’’

The characteristic length over which the initial dramatic thinning of the jet takes place can be identified with the axial distance where advection and conduction currents are equal. Following relation is for this ‘‘nozzle regime length’’ L:

where K, Q, q, v, E, I, and e2 are conductivity, flow rate, density, aspect ratio (v ¼ hD0), applied field, electric current, and dielectric constant of the outer fluid(typically, air in conventional electrospinning). Beyond this characteristic length, the jet thins more slowly. Sufficiently far from the nozzle (circa 30h0), the jet approaches the asymptotic regime where all terms except electrostatic and inertial must eventually die out.

Jet Instability:

Frankly speaking, the cone-jet operation is sufficient to draw out continuous fibers to very small diameter. However, the fluids typically used in electrospinning do not always solidify to the collector to remain fibrous after impact on the collector. In practice, as the jet thins, it ultimately yields to one or more fluid instabilities which deform the jet as they grow. A number of such instabilities exists and can be analyzed for different conditions of the symmetry (axisymmetric or nonaxisymmetric) of the growing perturbations of the jet, using linear instability analysis. Fig. 6 illustrates the perturbations associated with several of the lowest-order instabilities.

Fig. 6. Schematic illustration of perturbations associated with several of the lowest-order instabilities, distinguished by their azimuthal wave number, s. Top views illustrate cross sections of the jet at maximum amplitudes of (oscillatory) perturbation, with bold and dashed contours representing different positions along the jet length. Bottom views illustrate changes in shape and center line down the length of the jet. ± are used to indicate regions of positive or negative deviation from the unperturbed jet shape. (a) unperturbed cylindrical fluid element, (b) varicose (s = 0) instability, (c) whipping (s = 1) instabilit), and (d) splitting (s = 2) instability. Growth of the varicose instability leads to equal-sized droplets; growth of the splitting instability leads to two equal-sized sub-jets. Higher-order (s>2) instabilities are also conceivable.

Although spreading of the jet was proposed in the 1990s as the primary mode of instability, and some evidence has been found for splitting in “postmortem” micrographs of fibers and for secondary jetting in images of jets, such events are relatively rare; the most common mode of instability in electrospun jets appears to be the growth of lateral excursions of the jet, the so-called whipping model, this instability is not a consequence of viscoelasticity of the fluid and may occur for Newtonian and non-Newtonian fluids as well. The main competing mode of instability, for the relatively dilute solutions often employed to achieve the smallest fiber diameters in electrospinning, is that of droplet breakup (electrospraying). For the steady thinning jets described above, Hohman et al. analyzed the linear instability analysis for both droplet breakup and whipping modes and produced ‘‘operating diagrams’’ for electrospinning that illustrate the combinations of controllable parameters (flow rate, Q, and applied field, E? = V/D) under which a particular fluid will electrospin (whipping) rather than electrospray (droplet breakup). Upon whipping, charge repulsion is relatively stronger in contrast to electrical shear stress and surface tension, allowing one to neglect the contributions to the dispersion relation arising from the electric field, surface tension, and the finite conductivity. Within this approximation, considerable simplification of the dispersion relation is achieved.

where x is the growth rate and k is the wave number of the perturbation. Solving for x and differentiating this with respect to k, one obtains the growth rate and wave number for the most unstable (i.e., fastest growing) mode:

These equations reflect the destabilizing effect of charge repulsion (strongest for short-wavelength perturbations) and the stabilizing effect of viscosity (also strongest for short-wavelength perturbations), as well as the damping of the instability growth rate due to inertia. It should be note that multiple instabilities can also grow simultaneously, and even a whipping jet can ultimately decay into droplets (Fig. 7). Hohman et al. also reported a second, electrically driven droplet breakup instability that becomes important at high electric fields in fluids of finite conductivity, in addition to the conventional, surface tension–driven instability. They also obtained an expression for the lateral growth of the jet excursions arising from the whipping instability far from its onset, deep in the nonlinear regime. This equation contains terms due to acceleration of the charged jet under the influence of the applied electric field, normal stresses due to surface tension and bending of electric field lines, and charge–charge repulsion. As the diameter of the jet becomes small due to the growth of the whipping instability, the longest-lived terms are those due to surface tension and charge repulsion. The competition between surface tension and charge repulsion can be used to define a ‘‘stability parameter’’ S:

As S > 1: instability with respect to the whipping mode. However, rapidly increases to 1 as h is reduced under the action of the growing lateral perturbations. As S = 1: the destabilizing effect of charge repulsion is exactly balanced by the stabilizing effect of surface tension, leading to the cessation of stretching and definition of a limiting ‘‘terminal jet’’ diameter. It can be found that this is only a limiting diameter arising from a balance of forces on the Newtonian fluid; elastic forces arising from solution viscoelasticity or solvent evaporation may intervene under certain condition to arrest the jet prematurely with large diameter. While a similar analysis of the elongation of the steady jet in a tangential electric field indicates that very small diameter jets can be realized in this case, too, before surface tension forces eventually rise to balance the electric shear stress. As illustrated in Fig. 7, jets will break into drops without or very low elasticity. Elastic effects thus can arise as a consequence of the viscoelasticity of the fluid itself or development of a solid-like elasticity as the jet cools or solvent evaporates en route to the collector. It is usually postulated that the growth of the whipping instability is responsible for large stretching of the fluid jet and lower the jet diameter, which can provide greater opportunity for solvent evaporation and jet solidification in the whipping regime.

Fig. 7. Whipping jet decay into droplets.

5. Effects of Working Parameters:

Working parameters are very important to understand not only the nature of electrospinning but also the conversion of polymer solutions into nanofibers through electrospinning. Those parameters can be broadly divided into three parts such as solution parameters, process parameters, and ambient parameters. Each of those parameters can affect the fibers morphologies and by proper control of those parameters we can fabricate electrospun fibers with desired morphologies and diameters.

5.1 Solution Parameters:

5.1.1. Concentration:

The concentrations of polymer solution play an important role in the fiber formation during the electrospinning process. Four critical concentrations from low to high can be noted.

1. As the concentration is very low, polymeric micro (nano)-particles will be obtained. At this time, electrospray occurs instead of electrospinning owing to the low viscosity and high surface tensions of the solution.
2. As the concentration is little higher, a mixture of beads and fibers will be obtained.
3. When the concentration is suitable, smooth nanofibers can be obtained.
4. If the concentration is very high, not nanoscaled fibers, helix-shaped microribbons will be observed.

Fig. 8. SEM images of the evolution of the products with different concentrations from low to high during the electrospinning.

To clearly see the evolution of the products with different critical concentrations from low to high, four typical SEM images have been used to illustrate the whole change (Fig. 8)

5.1.2. Viscosity:

Solution viscosity is the critical key in determining the fiber morphology. It has been proven that continuous and smooth fibers cannot be obtained in very low viscosity, whereas very high viscosity results in the hard ejection of jets from solution, namely there is a requirement of suitable viscosity for electrospinning. Generally, the solution viscosity can be tuned by adjusting the polymer concentration of the solution; thus, different products can be obtained as shown in Fig. 9.

Fig. 9. SEM images of the electrospun PAN products with different solution viscosities by adjusting the concentration of the polymer solution. The concentrations of left and right are 1.3 and 15 wt. %, respectively. The molecular weight of PAN is 150,000.

5.1.3. Surface Tension:

Surface tension which is the function of solvent compositions of the solution, is quite important factor in electrospinning. In 2004, Yang and Wang systematically investigated the influence of surface tensions on the morphologies of electrospun products with PVP as model with ethanol, DMF, and MC as solvents (Fig. 10). They found that different solvents may contribute different surface tensions. With the concentration fixed, reducing the surface tension of the solution, beaded fibers can be converted into smooth fibers.

Fig. 10. TEM images of the PVP nanofibers electrospun from ethanol (a), MC (b), and DMF (c), respectively. The concentration of PVP is fixed at 4 wt. %.

5.2 Processing Parmaeters:

5.2.1 Voltage:

Within the electrospinning process, applied voltage is the crucial factor. Only the applied voltage higher than the threshold voltage, charged jets ejected from Taylor Cone, can occur. Several groups suggested that higher voltages can increase the electrostatic repulsive force on the charged jet, favoring the narrowing of fiber diameter. In addition to these phenomena, some groups also demonstrated that higher voltage offers the greater probability of beads formation. Thus, we can found that voltage does influence fiber diameter, but the level of significances varies with the polymer solution concentration and on the distance between the tip and the collector.

5.2.2 Flow Rate:

The flow rate of the polymer solution within the syringe is another important process parameter. Generally, lower flow rate is more recommended as the polymer solution will get enough time for polarization. If the flow rate is very high, bead fibers with thick diameter will form rather than the smooth fiber with thin diameter owing to the short drying time prior to reaching the collector and low stretching forces.

5.2.3 Collectors:

During the electrospinning process, collectors usually acted as the conductive substrate to collect the charged fibers. Generally, Al foil is used as a collector but it is difficult to transfer the collected nanofibers to other substrates for various applications. With the need of fibers transferring, diverse collectors have been developed including wire mesh, pin, grid, parallel or gridded bar, rotating rods or wheel, liquid bath, and so on.

5.2.4 Distance (H) Between the Collector and the Tip of the Syringe:

It has been proven that the distance (H) between the collector and the tip of the syringe can also affect the fiber diameter and morphologies. In brief, if the distance is too short, the fiber will not have enough time to solidify before reaching the collector, whereas if the distance is too long, bead fiber can be obtained. It is well known that one important physical aspect of the electrospun fiber is the dryness from the solvent, so optimum distance is recommended.

5. Summary:

• In this module, you study
• Basics of electrospinning technique.
• The application of electrospinning technique for growth of 1D polymer fibers. Formation of Taylor cone under high voltage.
• Theory of electrospinning.
• Various parameters, which govern the growth of nanofibers.

Suggested Reading

(i) Text book of “Electrospun Nanofibers and Their Applications” by Ji-Huan He, Yong Liu, Lu-Feng Mo, Yu-

Qin Wan and Lan Xu.

(ii) Text book  of  “One-Dimensional  Nanostructures  Electrospinning  Technique  and  Unique Nanofibers”

by Zhenyu Li. Ce Wang.

(iii) Text book of “An Introduction to Electrospinning and Nanofibers” by Seeram Ramakrishna, Kazutoshi

Fujihara, Wee-Eong Teo, Teik-Cheng Lim, Zuwei Ma.