2. Nanofibres and Electrospinning Process
• What are Nanofibres?
• Fibres finer than 500 nm are commonly referred
to as nanofibres.
• When the diameter of polymeric fibres goes
down to sub-microns or nanometers, amazing
characteristics such as very large surface area to
volume ratio and superior mechanical
performance are observed.
3. • As shown in Figure 1, the increase in surface
area is as large as 103 times of that of a
microfibre.
• This increased surface area can result in a
remarkable increase in capacity to attach or
release functional groups, absorbed molecules,
catalytic moities, etc.
The strength increases because as the diameter
reduces:
• Defects go down
• Orientation improves
• Better formation of a fibrillar/compact structure
4. • The flexibility also increases as flexural rigidity is
proportional to ( 1/ (diameter)4) In addition to
above properties, the electrospun nanofibres
form highly porous fibrous membrane with
excellent pore interconnectivity, controllability
in fibre diameter morphology, ease of
functionalization.
• Due to these unique features, these fibres have
potential for application in filter media,
aerospace, capacitors, transistors, biomaterials,
barriers, wipes, personal care products, battery
separators, energy storage, fuel cells, catalysts,
superabsorbent materials, composites,
biosensor assembly and others.
5. Techniques of making nanofibres
• Nanofibres can be prepared by one of the
following techniques :
• Drawing
• Template synthesis
• Self assembly
• Phase separation
• Electrospinning
6. • ‘Drawing’ is like usual dry spinning, can make
long single nanofibres; with the limitation that,
it is applicable only for viscoelastic material
that can undergo strong deformations while
being cohesive enough to support the pulling
stress.
• ‘Template synthesis’ uses nano-porous
membrane as template to make nanofibres. It
cannot make single continuous nanofibres.
• ‘Phase separation’ involves dissolution,
gelation, extraction with solvent, freezing,
drying; giving nanoporous foam. It is relatively
a longer time taking process.
7. • Among all these, ‘Electrospinning’ is recognized
as the most efficient production technique.
Mostly polymer solution and sometimes
polymer melts are used in this technique to
produce nanofibres.
8. What is Electrospinning?
• Electrospinning is a variation of “electospraying’
process”.
• This technique started gaining importance in
1994, though the fundamental idea of
electrospinning dates back to 1934.
• Electrospinning is a fibre forming process which
makes use of a high voltage electric field to
produce an electrically charged jet of polymer
fluid, which on solidifying produces a nanofibre
web typically with fibre diameter from a few
nanometres to few microns.
9. As shown in Figure 3, an electrospinning setup
comprises of three main components:
• A syringe pump for controlled delivery/flow of
polymer fluid
• A high voltage supply and
• A grounded or oppositely charged collector
10. • In electrospinning process, a high voltage
electric field is applied to the end of the
spinneret that contains the polymer solution or
a melt held by its surface tension.
• This creates a charge on the surface of the
liquid.
• Mutual charge repulsion and the contraction of
the surface charges to the counter electrode
cause a force directly opposite to the surface
tension as shown in Figure 2.
11.
12. • As the intensity of electric field is increased, the
hemispherical liquid drop formed at tip of the
capillary gets distorted into conical shape called
‘Taylor cone’ under simultaneous actions of
repulsive force due to induced surface charge
and the attractive forces due to surface tension
of the solution.
• Further increase in the potential beyond a
critical voltage, a stable liquid jet ejects out and
breaks up into droplets due to surface tension
for low viscosity liquids; hence the process is
called as ‘electrospraying’.
13. • If the polymer used in the process is of higher
molecular weight and its solution is made of
considerable viscosity, the jet from capillary tip
travels to the grounded target without breaking
up into drops; this process is now known as
‘electrospinning’.
• The value of the critical voltage ‘Vc’ depends on
the properties of solution like polymer molecular
weight and solution viscosity.
14. • The charged polymer fluid jet undergoes an
instability and elongation process that allows
the jet to become very long and thin.
• This stretching occurs due to the intensive
interaction of charged jet with the electric field.
• In this process, solvent evaporates and a
charged nanofibres are forms.
• Generally, the nanofibres formed by this process
are obtained in the form of a random web and
appear like a very fine spray or a film as shown
in Figure 6.
16. Figure 7. SEM micrographs of
electrospun web a) at lower
magnification b) at higher magnification
( diameter~ 320 nm)
17. The versatility of electrospinning
• The technique can be used to make nanofibres
from a variety of polymeric materials such as
high liquid crystalline polymers, biopolymers or
other composite materials.
• With advances in electrospinning process, core-
shell nanofibres can also be obtained.
• However, the electrospinning conditions are
strongly influenced by the characteristics of
polymers.
18. • Most of these nanofibres were spun from
solution, although spinning from the melt in
vacuum and air are also possible.
• Melt electrospinning requires special heating
assembly to keep the polymer in the molten
form at the tip of the needle.
19. Polymers spun by electrospinning
Polymer class Polymer Solvent
High performance
polymers
Polyimides Phenol
Polyamic acid m-cresol
Polyetherimide
Methylene
chloride
Liquid crystalline
polymers
Polyaramid Sulphuric acid
Poly-gamma-
benzyl glutamate
Dimethylformami
de
Poly(ρ-phenylene
terephthalamide)
Sulphuric acid
20. Textile fibre
polymers
Nylon Formic acid
Polyacrylonitrile
Dimethylformami
de
Polyethylene
terephthalate
Trifluoroacetic
acid and
dichloromethane
Electrical
conducting
polymer
Polyaniline Sulphuric acid
Biopolymers
DNA Water
Polyhydroxy
butyrate-valerate
Chloroform
21. Influence of electrospinning parameters
• The morphology of electrospun nanofibres is
influenced by a number of parameters.
• The parameters can be broadly classified into
three categories:
22.
23. • The parameter related to solution properties are
interdependent.
• For example any change in polymer
concentration,molecular weight and solvent
composition affect the viscosity of a spinning
dope.
• In electrospinning for the fibre formation to
occur, a minimum polymer concentration is
required.
• Below this critical value, application of voltage
results in electrospraying and bead formation.
24. • The parameter related to solution properties are
interdependent.
• For example any change in polymer
concentration, molecular weight and solvent
composition affect the viscosity of a spinning
dope.
• In electrospinning for the fibre formation to
occur, a minimum polymer concentration is
required.
• Below this critical value, application of voltage
results in electrospraying and bead formation.
25.
26. • As the polymer concentration is increased, a
mixture of beads and fibres are formed. The
spherical morphology of beads gradually
changes to spindle like and eventually uniform
fibres are formed.
• The optimum electrospinning concentration
depends on the molecular weight and nature of
polymer. density results in thicker fibres.
27. • This effect is observed because of lower
entanglement density.
• Entanglement density increases with increase in
concentration, molecular weight and nature of
solvent (better solvent higher entanglement
density).
• Too high entanglement density results in thicker
fibres.
• By varying the electrospinning parameters the
morphology of the electrospun nanofibre web
i.e. diameter, deposition area and hence the
porosity can be tuned.
28. • On twisting the bundle of nanofibres twisted
nanofibre yarn can be achieved.
