MILOS IADR

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Polymer Nanofibers
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In principal, nearly all soluble or fusible
polymers can be processed into fibers by
electrospinning, provided that the molecular
parameters (such as solubility, glass-transition
temperature, melting point, molecular weight,
molecular-weight distribution, entanglement
density, solvent properties including volatility,
dielectric constant and others, and solution/melt
properties including viscosity, surface tension
coefficient and other) and the process parameters
(such as applied voltage, feed rate and
consumption rate, electrode separation and
geometry, temperature, and relative humidity) are
correctly adjusted. In this respect, empirical
knowledge is crucial theoretical models can
increasingly be applied to predict the dimensions
and structures of the fibers produced. It is
merely impossible to make a general
recommendation for particular concentrations and
the resulting viscosities, electrical
conductivities, and surface tensions, because the
ideal values of these parameters vary
considerably with the polymersolvent system. The
polarity of the electrodes and the use of
alternating or direct current often do not seem
to have much influence.
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Though the electrospinning of polymer solutions
enables the fabrication of fibers with relatively
low diameters, its productivity, in terms of 120
wt solutions, is moderate. Moreover,
evaporation of the solvent can substantially
impede the electrospinning process and can
destroy the evolving fibers by causing filming.
An alternative might be electrospinning from
the melt, but for all but a few exceptions
(low-melting polymers), this method leads to
average fiber diameters that are considerably
greater than 1 ?m and to a broad distribution of
diameters, as a result of the high melt
viscosities of the polymers. Because of their
high viscosities, the electrospinning of polymer
melts requires high electric fields. Under normal
atmosphere, such high electric fields lead to the
danger of electric shock. Variation of the
atmospheric composition and even variation of the
humidity can have a significant impact on the
electrospinning process. High vacuum conditions
could allow electrospinning at higher voltages.
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Electrospinning of Water-Soluble Polymers
Water-soluble polymers like PEO, PVA, PAA, PVP,
and HPC offer a variety of advantages for
electrospinning. The solubility properties of
water can be adjusted by the pH value, the
temperature, the addition of surfactants or other
solvents (e.g., alcohols). Electrospun fibers of
water-soluble polymers decompose rapidly on
contact with water. While this property may be of
interest for biomedical applications, additional
stabilization of these fibers by cross-linking is
necessary for other technical applications (e.g.,
filters and textiles).
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Electrospinning of Organo-Soluble Polymers
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Electrospinning of Biopolymers/Modified
Biopolymers
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Electrospinning of Bio-Erodible Polymers
Polymers that are biodegradable or hydrolyzable
under physiological conditions (bio-erodible
polymers), such as aliphatic polyesters,
polyanhydrides, and polyphosphazenes, are the
focus of intense study for pharmaceutical
applications and for applications in tissue
engineering.
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Melt-Electrospun Polymers
The electrospinning of polymers from the melt
avoids the use of solvents and is, therefore,
attractive from the perspective of productivity
and environmental considerations. However, the
method is limited by the fact that nanofibers
with diameters of less than 500 nm and with a
narrow diameter distribution cannot yet be
fabricated.
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Alignment of Polymer Nanofibers (1)
Alignment of electrospun polymer nanofibers is
crucial for post-electrospinning stretching
process, which can significantly improve the
mechanical properties of nanofibers.
A simple method for obtaining small amount of
highly aligned nanofibers
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Alignment of Polymer Nanofibers (2)
To obtain large amount of highly aligned
nanofibers is still under investigation.
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Complex Polymer Systems and Hybrid Materials
TEM image of an electrospun fiber of a block
copolymer of polystyrene and polyisoprene
TEM images showing nylon 6/fibrillar silicate
nanocomposite nanofibers containing silanized
fibrillar silicate single crystals.
TEM image of core/shell nanofibers loaded with
bimetallic nanoparticle catalysts. Inset TEM
image of the catalyst nanoparticles.
TEM images of an electrospun nanofiber of
poly(p-xylylene) with incorporated nanoparticles
of Pd.
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Properties of Polymer Nanofibers
Compared to other polymeric materials, the most
characteristic property of electrospun polymer
nanofibers is that the macromolecular chains are
aligned along the fiber axis, while the overall
crystallinity may not be high. Almost all kinds
of polymer nanofibers show birefringence,
including those made of typical amorphous
polymers.
Example Nylon 6
XRD patterns of Nylon 6. (a) solution cast film,
(b) aligned electrospun nanofibers with verticle
fiber axes.
Polarized FTIR spectra of the aligned nylon-6
nanofibers. The zero angle was defined as the
FTIR beam parallel to the nanofiber axes.
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Ceramic Nanofibers
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For example, the precursor of SiO2, tetraethyl
orthosilicate (TEOS, Si(OC2H5)4), can be
co-electrospun with polyvinyl pyrrolidone (PVP)
using an DMF/acetic acid mixture as a solvent.
Since TEOS is rapidly hydrolyzed by the moisture
in air, the networks (gels) of SiO2 are formed
into nanofibers during or shortly after
electrospinning. PVP can then be selectively
removed by pyrolyzing the samples in air at high
temperature, resulting in the formation of SiO2
nanofibers. Additionally, by carefully
controlling the gelation of TEOS aqueous
solutions (e.g., adjusting the solution pH
value), SiO2 nanofibers can also be directly
electrospun without the assistance of base
polymers.
Representative SEM images of electrospun PVP/TEOS
fibers before and after pyrolysis at 600 ºC for 6
hours.
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Electrospun SiO2 nanofibers are Nano-Scaled Glass
fibers, and can sustain fiber morphology upon
vigorous ultrasonic vibration.
TEM image and electron diffraction pattern
(inset) of SiO2 nanofibers (pyrolyzed at 600 ºC).
SEM images of electrospun SiO2 nanofibers
pyrolyzed at 600 (A), 800 (B), 1000 (C), 1200 (D)
and 1400 ºC (E) for 6 hours after being subjected
to vigorous ultrasonic vibrated for three, 5 min
time periods.
XRD spectra of electrospun SiO2 nanofibers
pyrolyzed at 600, 800, 1000, 1200 and 1400 ºC for
6 hours.
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Electrospun TiO2 nanofibers could also be
prepared using its alkoxide precursor
(Ti(OC4H9)4) and PVP. Intriguingly, no matter
the spin dope is a clear solution (e.g.,
Ti(OC4H9)4/ PVP in isopropanol/acetic acid or
isopropanol/DMF/acetic acid) or an emulsion
(e.g., Ti(OC4H9)4/ PVP in DMF/acetic acid), the
resultant TiO2 nanofibers are always made of 10
nm-sized crystallites (grains) sintered together.
The TiO2 nanofibers are brittle, and the
intergranular connection is weak. Unlike SiO2
nanofibers which are amorphous, TiO2 nanofibers
are crystalline. The predominant crystalline
structure is anatase (instead of rutile and
brookite in most naturally occurring TiO2) and
anatase crystalline form of TiO2 has high
photo-catalysis activity.
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In the formation of ceramic nanofibers,
especially if carrying polymers are used to
assist in the formation, the processing
conditions of electrospinning can be more
complicated than those encountered in the
formation of polymer nanofibers. The
morphological and physical properties of ceramic
nanofibers may also depend on more factors.
Detailed knowledge is upon further
investigations. As with polymer nanofibers, the
detailed physical property characterizations of
single ceramic nanofibers have also not yet been
accomplished. The properties of ceramic
nanofibers are expected to be significantly
affected by the post-electrospinning
pyrolysis/calcination conditions. Little
systematical investigation has been performed on
this topic. Another topic which may be worth
exploring is to prepare and evaluate ceramic
nanofibers (or ceramic nanoparticles containing
polymer/carbon/graphite nanofibers) with special
properties such as a negative thermal expansion
coefficient. It is known that some inorganic
compounds (e.g., V2O5 and ZrW2O8) show negative
expansions during heating. If these compounds
can be electrospun into nanofibers or can be
impregnated into polymer/carbon/graphite
nanofibers, it may result some significant and
interesting properties of nanofibers.
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Carbon/Graphite Nanofibers
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Polyacrylonitrile-Based Carbon Nanofibers
SEM images of A the carbonized polyacrylonitrile
nanofibers. B The carbonized polyacrylonitrile
nanofibers after activation.
A Electrospinning roller with polyacrylonitrile
nonwoven fabric collected on aluminum foil. B
Polyacrylonitrile fabric covered on aluminum
foil, after removal from the roller.
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Pitch-Based Carbon/Graphite Nanofibers
SEM images of the as-spun mesophase pitch fibers.
A TEM image of the graphitized mesophase pitch
nanofibers. B Electron diffraction pattern.
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Carbon Nanotubes Grown on Carbon Nanofibers
TEM images of carbon nanotubes grown on the
electrospun PAN-based carbon nanofibers,
illustrating the control of the length of carbon
nanotubes by controlling the time of exposure to
the hexane vapor. From (A) to (C), the hexane
vapor was supplied for 3, 5, and 20 minutes,
respectively, in the argon environment.
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Continuous Nano-Scaled Carbon Fibers with
Superior Mechanical Strength
(1) Effective stretching to convert the
as-electrospun nanofiber mat to highly aligned
and stretched nanofiber bundle and (2)
under-tension stabilization/carbonization are
crucial.