What are Nanofibers?
While there is some discussion on the exact definition of a
nanofiber, the consensus in the nonwovens industry has been that
anything under 1 micron in diameter is a nanofiber. To put this
in perspective, one nanometer is equivalent to one billionth of
a meter, making nanofibers many orders of magnitude smaller than
an average human hair (Figure 1).
Figure 1: Nanofibers
(<1 μm) web laid on human hairs
( ~50 – 100 μm).
How are Nanofibers Formed?
of the commonly utilized methods for the production of nanofibers
is electrospinning. The electrospinning process has a few
key elements: a power supply, a reservoir for containing
the polymeric solution, and a grounded collector. In the
typical polymer solution electrospinning apparatus (Figure 2),
a high voltage power supply is used to supply the necessary voltage
to a conical metal reservoir filled with the desired polymer solution. When
the electrostatic repulsive forces become sufficient to overcome
the surface tension of the polymeric solution, a “Taylor
cone” is formed, and the fluid jet is ejected. As the
polymeric solution jet travels further from the tip of the metal
reservoir, it becomes more destabilized, creating what is known
as a bending instability. The combination of electrostatic
and mechanical forces pulling the polymer solution from the metal
reservoir to the grounded collector result in stretching of the
polymer fiber, which leads to the collection of fibers with submicron
(<1 μm) or nanometer diameters: nanofibers.
Manipulation of the components of the electrospinning apparatus
and process conditions as well as the polymers and solvents employed
in this technique allow the nanofiber product to be tailored to
specific applications. In addition, various additives can
be mixed into the polymer solution prior to electrospinning and
incorporated into the resulting polymeric product, leading to novel
nanofiber materials with a diverse array of properties.
Figure 2: General Electrospinning Apparatus
Other methods employed for nanofiber production include polymer
melt electrospinning, in which the polymeric solution is substituted
with a polymeric melt; the melt blown process involves the
production of fibrous webs or articles directly from molten
polymers using high-velocity air or another appropriate force
to attenuate the filaments. Another example is the Islands-In-The-Sea
method of nanofiber formation. This technique first involves
the formation of bi-component polymer fibers in which one component
is soluble in a particular solvent and the other component
is not. Following formation, the resulting fibers are
rinsed or submerged in the chosen solvent, selectively dissolving
one component and generating polymer nanofibers of the remaining
When compared to conventional fibers, nanofibers offer numerous
advantages, most notably high surface area, small pore size and
high pore volume. These characteristics lend themselves
to many innovative applications including wound care, filtration,
fuel cells and countless others.