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Electrospinning and Polymer Nanofibers
The
recently fast developing technology “electrospinning” is a unique way
to produce novel polymer nanofibers with diameters typically in the range
from 50 nm to 500 nm.
Polymer nanofibers can be made from a variety of polymer solutions
or melts, and are of substantial scientific and commercial interests
including composite, filtration, protective clothing, biomedical and
electronic applications.
Carbon nanofibers made from polymeric precursors further expand the
list of possible uses for nanofibers.
Polymer nanofibers could have many extraordinarily
properties including, small diameter (and the resulting large surface area
to mass ratio), highly oriented crystalline structures (and the resulting
high strength), etc.
Meanwhile, the non-woven fabrics made of polymer nanofibers offer
unique capabilities to control pore size and have been researched to be
the novel scaffold for cell growth.
The followings are two projects in this area.
The
process of electrospinning is a complicated combination of polymer
science, electronics and fluid mechanics.
Both solution properties and processing variables can
significantly affect the electrospinning process.
To date, a fundamental mechanism of the process of
electrospinning is still characterized only qualitatively.
The absence of the comprehensive knowledge of electrospinning
has resulted in the polymer nanofibers with less controllable
morphology and properties, and has significantly affected the polymer
nanofibers to be used as a functional material.
It is the purpose of this research to systematically study the process
of electrospinning to produce polymer nanofibers with controllable
morphology and properties.
Several key objectives are outlines as follows:
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Design
and construct of a comprehensive electrospinning station, for
controllable and reproducible electrospinning.
-
Systematically
investigate the process of electrospinning.
The process will be studied based on three stages: jet
initiation, jet elongation (bending instability), and nanofiber
formation. For each
stage, systematical observations and measurements will be carried
out.
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Clarify
of the polymer solution characteristics (viscosity, conductivity,
surface tension, etc.) and the process variables (electrostatic
field, flow rate, polarity of the power supply, etc.) as well as
environment conditions (temperature, pressure, solvent vapor
pressure, etc.) on the electrospinning process, and their effects
on the morphology and properties of polymer nanofibers.
-
Explore
of the effects of the external electro-magnetic field on the
morphology and properties of the electrospun polymer fibers.
-
The
research will further be expanded to prepare and evaluate (a)
novel carbon (and/or graphite) nanofiber nano composites, and (b)
high efficiency polymeric photovoltaic devices.
This research is to study the formation and physical
properties of the carbon nanofibers made from the electrospun precursors.
Two
kinds of carbon nanofibers will be produced and studied:
-
Highly
graphite-crystalline ordered, strong carbon nanofibers made from
mesophase pitch or PAN nanofiber precursors.
-
Very
porous carbon nanofibers made from PAN or PVA nanofiber precursors,
with extremely large specific surface area.
(Note: Different approaches will be employed to generate
micropores.
For PAN based carbon nanofibers, steam and/or CO2 treatment
will be used; while for PVA based carbon nanofibers, micropores will
be generated in situ with carbonization, through thermal decomposition
of (NH4)2HPO4.)
Further
objectives include:
-
Study
the electrospinning process to make precursor nanofibers with
desirable morphological and physical properties.
-
Investigate
the stabilization, carbonization, graphitization
and activation conditions of the nanofiber precursors.
-
Prepare
(polymer/strong carbon nanofibers) nanocomposite, and evaluate the
mechanical properties.
-
Explore
the advantages of substituting activated carbon black by highly porous
carbon nanofiber nonwoven fabrics.
Polymeric
Nanocomposites
Nanocomposites are a new class of composites, that are particle-filled
polymers for which at least one dimension of the dispersed particles is in
the nanometer range. Over the
last decade, the utility of layered silicate nanoparticles as additives to
enhance polymer performance has been established.
Nanoscale fillers result in physical behavior that is dramatically
different from that observed for conventional microscale counterparts.
For instance, increased moduli, gas barrier, increased strength and
reduced thermal expansion coefficients are observed with only a few
percent additions of nanofiller; thus maintaining polymeric processability,
cost and clarity.
The
following are two potential projects in this area:
Emulsion
polymerization offers a viable, flexible route for nanocomposite
fabrication from nanoscale spheres, rods and plates.
Combining emulsion generated polymer particles that are ionically
stabilized in aqueous solution with a dispersion of nanoparticles of
opposite sense results in an interfacial exchange reaction and
co-precipitation. As shown in
Fig. 2, the idea is to make a few percentage of the surfactants, which
have positive charges, with double bonds so that the surfactant molecules
can be copolymerized. The
resulting macromolecules, which will carry certain amounts of charges,
will then co-precipitate with charged exfoliated clay particles or charged
rigid rod macromolecules, to make polymer/layered silicate nanocomposites
or rigid rod molecular composites.
-
Protect
polymeric materials against aggressive space environment through
nanocomposite
Polymers
are very attractive and desirable materials for use in space
applications, in particular for addressing multi-functional
requirements. With
modifications these materials could potentially solve many of the
weight-based and process-based problems plaguing the space industry
and offer new capabilities for future systems.
Polymers are remarkable materials but there are, as with any
material, problems associated with their use, especially in a harsh
space environment. Degradation
is a most prominent concern with using polymers in space, and the
existence of atomic oxygen in Low Earth Orbit (LEO) is one of the
major reasons of degradation.
This research is to investigate the “Multi-functional
(self-passivating/self-rigidizing/self-healing) polymeric materials
for space survivable structures based on polymer/layered silicate
nanocomposites”. The
previous results showed that Nylon 6/layered silicate nanocomposites
are able to self-generate a silicate passivation layer upon exposure
to oxygen plasma. The
resulting layer is strongly interferometric.
The thickness of the layer varies from a few hundred nanometers
to one micron, and chemical composition of the layer is nearly
completely inorganic. The
formation of the layer is due to the preferential oxidation of the
polymer (Nylon 6) from the nanocomposite, and the corresponding
deposition of the nanoscale layered silicate on the surface.
The structure of the inorganic layer is turbostratic, with the
average distance between silicate layers of about 1 to 4 nanometers.
After the passivation layer forms, the degradation of the
polymer underlying can be significantly retarded during exposure to
oxygen plasma. Thus,
nanocomposite may potentially be used to protect the degradation of
polymers, especially against atomic oxygen in low earth orbit.
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