LECTURE 5: FIBER ENGINEERING
3rd class/polymer dep.
Processing Parameters
1. High Voltage:(HV)
HVPS represent the first component of electro spinning set up, used for
generating of high electrical forced which applied on to polymer solution
to form the Taylor cone which lead to form the Nano fibers diameter after
increasing it to critical value and overcome on the surface tension of
solution.
In electro spinning can use AC – HV also DC-HV. Increasing the
applied voltage would discharge the polymer jet with greater electrostatic
repulsion, causing it to undergo higher levels of drawing stress. The
applied voltage will cause charges to collect on the surface of a droplet
and together with the applied electric field it induces the formation of a
Taylor cone, with an electro spinning solution jet emanating from the tip
of the cone.
Generally, we can summarize the increasing of applied voltage on the
fiber diameter and morphology as following:
1- the greater the voltage is the greater the stretching of the electro
spinning jet due to the increase in columbic force exerted by the
charges .
2- Increasing the voltage also has the effect of increasing the jet
acceleration and hence decreasing the flight time of the electro
spinning jet.
3- Increasing the voltage can reduce fiber diameters.
4- Crystallinity also increases with proper flight time.
Using of AC-HVPS lead to prevent whipping of the jet and result
in improving alignment of the nanofibers. The charge build-up on the
collector is likely to be less of a problem with AC voltage compared to
DC voltage.
Following fig shows the effect of increasing of high voltage value on
the nanofibers diameter.
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LECTURE 5: FIBER ENGINEERING
3rd class/polymer dep.
the effect of increasing of high voltage value on the nanofibers
diameter.
2. Flaw Rate:
The flow rate of the polymer solution within the syringe is another
important process parameter; Feed rate will determine the amount of
solution available for the ES process, generally:
a. Lower flow rate suitable to get enough time of polarization of
solution.
a- High flow rate lead to increase the beads of fibers with thick
diameter.
b- Short drying time prior to reaching the collector and low stretching
forces.
3. Needle-collector Distance:
The distance between the needle tip to collector seems less important
in the formation and morphology of resultant Nano fibers. Generally, low
distance lead to
a. Incomplete evaporation of the solvent and increases the speed of
resulting fiber to access to the collector.
b. Possible lead to the formation of beads Nano fibers.
This need to increase of HVPS value to maximum, for formation of
Taylor cone and consequently the formation of Nano fibers.
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3rd class/polymer dep.
LECTURE 5: FIBER ENGINEERING
4. Needle Diameter
Smaller the diameter lead to :
a. Reduces the formation of beads because less volume of solution is
collected at the tip and also it reduces fiber diameter.
b. Smaller the droplet means higher the surface tension. For fixed
voltage the time for stretch increases and fiber will elongate before
it reaching the collector.
For the same voltage supplied, greater columbic force is required to
cause jet imitation, as result, the acceleration of jet decrease and this
allow more time for the solution to be stretch and elongate before it is
collected.
5. Collector Shape
collectors usually acted as the conductive substrate to collect the
charged fibers for an accumulation of closely packed fibers with higher
packing density for this reason an conductive collector was used with
DC-HV such as Al foil but it is difficult to transfer the collected
nanofibers to other substrates for various applications, for this reason,
there are many types of collector discovered for different application such
as : wire mesh , pin , grids , parallel or gridded bar , rotating rods or
wheel , liquid bath , following fig show the most widely used form of
collectors :
(b) flat plate collector for non-alignment
nanofibers
(a) rotate collector for alignment nanofibers
two types of collectors
A non-conducting material collector reducing the amount of fiber being
deposited with lower packing density, but it can use with AC-HV.[81]
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LECTURE 5: FIBER ENGINEERING
3rd class/polymer dep.
Porous collector yields fibers with lower packing density as compared to
non-porous collector plate. In porous collector plate the surface area is
increased so residual solvent molecules gets evaporated fast as compared
to non-porous.
Solution Parameters
1. Concentrations:
The concentrations of polymer solution play an important role in the
fiber formation during the electrospinning process. Four critical
concentrations from low to high should be noted:
a. As the concentration is very low, polymeric micro (nano)-particles
will be obtained .
b. As the concentration is little higher, a mixture of beads and fibers
will be obtained .
c. When the concentration is suitable, smooth nanofibers can be
obtained.
d. If the concentration is very high, not nanoscaled fibers, helixshaped microribbons will be observed [88]
Usually, increasing the concentration of solution, the fiber diameter will
increase if the solution concentration is suitable for electrospinning.
Additionally, solution viscosity can be also tuned by adjusting the
solution concentration.
2. Viscosity
Generally, the solution viscosity can be tuned by adjusting the polymer
concentration of the solution; 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.
For solution of low viscosity, surface tension is the dominant factor
and just beads or beaded fiber formed. If the solution is of suitable
viscosity, continuous fibers can be obtained. A number of papers on such
interrelationships have been published.
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LECTURE 5: FIBER ENGINEERING
3rd class/polymer dep.
3. Surface Tension
Surface tension, as the function of solvent compositions of the solution,
is quite important factor in electrospinning.
It is 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.
Additionally, they also demonstrated that the surface tension and solution
viscosity can been adjusted by changing the mass ratio of solvents mix.
4. Conductivity/Surface Charge Density
Solution conductivity is mainly determined by the polymer type,
solvent sort, and the salt. Usually, natural polymers are generally
polyelectrolytic in nature, in which the ions increase the charge carrying
ability of the polymer jet, subjecting to higher tension under the electric
field, resulting in the poor fiber formation in contrast to the synthetic
counterpart . Additionally, the electrical conductivity of the solution can
be tuned by adding the ionic salts like KH2PO4, NaCl, and so on With the
aid of ionic salts, nanofibers with small diameter can be obtained.
Sometimes high solution conductivity can be also achieved by using
organic acid as the solvent
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