Chemistry Reference
In-Depth Information
poly(L-lactide) (PLA),
66
polystyrene (PS),
67
polyethylene oxide (PEO),
65
polyvinyl alcohol
(PVA), etc.
65
Because of the simplicity and low-cost of electrospinning, much effort has
been focused on expanding the scope of 'electrospinable' materials and associated
applications.
By blending sol-gel precursors with polymers such as PVP, PVA and PEO, metal
oxide nanofibers, ceramic nanofibers and even non-metal-oxide ceramic fibers can be
produced effectively.
61
Figure 5.2 shows examples of ZrO
2
and SiO
2
nanofibers fabricated
by electrospinning. Carbon nanotubes, nanoparticles, drugs, dyes, enzymes and DNA have
also been incorporated into nanofibers successfully. The large surface area-to-volume ratio
is especially favourable for many applications such as biomedical scaffolds in tissue
engineering, chemical catalysts, or sensors.
61
Figure 5.2
SEM images of (a) ZrO
2
nanofibers (b) SiO
2
nanofibers fabricated by electrospinning
Various factors can influence the electrospinning process. The primary processing
parameters involve: (1)
The properties of polymer solution
(e.g. viscosity, electrical
conductivity and surface tension). The viscosity can be controlled by varying the polymer
type and concentration. Different solvents and additives can be used to control the
electrical conductivity and surface tension of the solution. (2)
The applied voltage
, which
affects the electric field between the nozzle and the collector, can be adjusted using an
external power supply. The applied voltage controls the extrusion force of the liquid jet,
which must be balanced with the surface tension and viscous forces of the polymer
solution. (3)
The flow rate
, which defines the rate that the electrospun polymer solution is
supplied. This can be adjusted by a mechanical pump connected to a syringe, and controls
the volume of solution emerging from the nozzle per unit time. (4)
The deposition distance
,
which is the distance between the nozzle tip and the grounded collector (or counter
electrode). The magnitude of this distance affects the amount of solvent evaporated before
any nanofibers are formed and deposited on the counter electrode. (5)
Environmental
conditions
, such as humidity and temperature. A stable electrospinning process can be
established through consideration and optimization of all of the above parameters. Post-
treatment steps (for example calcination) are often employed after the collection of metal-
oxide or ceramic nanofibers.
59,68,69
Such treatments can remove unnecessary polymer
matrices in resultant fibers.
Usually, fibers are randomly distributed on the surface of the collector. In order to
obtain aligned nanofibers, the basic electrospinning process must be modified. For
example, pairs of split strips have been used to orient electrospun nanofibers.
70,71
In such a
setup, the solid grounded counter electrode collector is substituted by a pair of conductive
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