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http://trj.sagepub.com/content/early/2014/01/31/0040517513495943The
online version of this article can be found at:
DOI: 10.1177/0040517513495943 published online 3 February
2014Textile Research Journal
Meltem Yanilmaz and A Sezai SaracA review: effect of conductive
polymers on the conductivities of electrospun mats
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Review
A review: effect of conductive polymerson the conductivities of
electrospun mats
Meltem Yanlmaz1 and A Sezai Sarac2,3
Abstract
The effects of conductive polymers on conductivities and
morphologies of electrospun fabrics are analyzed. The factors
that affect the conductivities and morphologies are discussed.
Some applications of these conductive nanofibers are
reported. The introduction of conductive polymers into nanofiber
mats has the potential to provide sufficient conduct-
ivity for many applications. An improved conductivity can be
achieved by maximizing the content of conjugated polymers.
The selection of conductive and carrier polymers, solvents,
doping agents, oxidizing agents and ratios of them are also
important to obtain sufficient properties. Carbon fiber, carbon
black and carbon nanotubes are not covered in this
review.
Keywords
nanofiber, electrospinning, conjugated polymers, composites
There has been a great interest in conducting polymersdue to
their superior properties such as a wide range ofcontrollable
conductivity, low cost, easy synthesizingmethods, a wide range of
transport and optical proper-ties in the doped state. Conductive
polymers havesuperior electrical and optical properties that are
com-parable with those of metals and inorganic semicon-ductors.
These polymers have a unique electronicstructure that leads to
their electrical conductivity,low ionization potentials and high
electron anity.Also, they have conventional polymer properties
suchas ease of synthesis. Since conducting polymers areorganic
polymers, they have tunable properties depend-ing on synthesizing
methods.1 Conjugated polymers arecandidates of many applications,
such as eld eecttransistors, photovoltaic cells, light-emitting
diodesand data storage, electrochromic materials,
anti-staticcoatings, batteries, chemical sensors, biosensors,
etc.2,3
Polymer nanobers have attracted enormous atten-tion for many
applications such as sensors and smallernano-scale and molecular
devices. Films in a nanoberform have some advantages compared to
bulk samples.For example, in thin-lm-based devices, the active
sen-sing components are imbedded in the bulk. This disad-vantage
limits the eciency and sensitivity.4 Nanoberform can provide high
surface area for a given mass orvolume and nanober texture enhances
the transport of
ions or other chemicals. In biomedical applications,these
nanostructures provide a large number of sur-faces for enzymes to
be anchored due to high specicsurface area.5
This study focuses on semi- or conductive electro-spun nanobers
prepared by only conjugated polymers.Carbon ber, carbon black and
carbon nanotubes arenot covered in this review. Preparation and
propertiesof semi- or conductive nanobers in the presence
ofconjugated polymers by using electrospinning tech-nique are
reviewed for the rst time. The challengesand limitations of dierent
preparation techniques arereported and many potential applications
areoverviewed.
1Department of Textile Engineering, Faculty of Textile
Technology and
Design, Istanbul Technical University, Turkey2Department of
Chemistry, Polymer Science and Engineering, Istanbul
Technical University, Turkey3Nanoscience & Nanoengineering,
Istanbul Technical University, Turkey
Corresponding author:
A Sezai Sarac, Istanbul Technical University, Maslak Istanbul
34469,
Turkey.
Email: [email protected]
Textile Research Journal
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Fabrication, properties and applications
of nanofibers
There are dierent methods to produce nanobers,such as
centrifugal spinning, meltblowing, bicompo-nent spinning,
electrospinning and bubble electro-spinning.68 In centrifugal
spinning, centrifugal forcesare used to obtain nanobers and some
variables forthis technique are rotational speed of the spinneret,
col-lector type and the shape and size of the needle.8
Inmeltblowing, a polymer melt is extruded through theorice of a
die. Fibers are produced by elongating poly-mer by using air drag.
Throughput rate, melt viscosity,melt temperature, air temperature
and air velocityaect diameters of bers in this method.
Bicomponentspinning includes two steps: spinning of the
polymerstogether and removal of one polymer.6 In electrospin-ning
and bubble electrospinning techniques, an electriceld is applied to
the polymer solution or bubble toobtain nanobers. In bubble
electrospinning, when ahigh voltage is applied, the bubble is
deformed by atangential stress caused by the coupling of
surfacecharge and the external electric eld. The bubblemoves slowly
upwards, and the thickness of thebubble wall becomes thinner. When
the electric eldis enough to overcome the surface tension, a
holeappears and the bubble explodes. When a bubbleexplodes, three
morphologies (spheres, bers andstrips) can be seen and the obtained
morphologydepends on the size and thickness of the rupturedlm. If
blowing air is used instead of the electrostaticforce, the process
is called blown bubble spinning.7
Compared to other techniques, the electrospinningprocess is the
most common process to fabricate nano-bers.6 In the electrospinning
technique, polymer nano-bers can be obtained by applying electrical
force at thesurface of a polymer solution. Once the intensity of
theelectric eld is high enough, the hemispherical polymersolution
forms a conical shape at the tip of the needle.That is called a
Taylor cone. When electrical forcesovercome the surface tension of
the polymer solution,a charged jet is ejected from the tip of the
Taylor cone.Between the tip of the needle and the collector,
unstableand rapid whipping occurs; the jet extends, bends andthen
follows a looping and spiraling path due to theaction of the
electrical eld. It becomes very thin untilit reaches the
collector.912 The nanobers, which havediameters from several
nanometers to hundreds ofnanometers, can be obtained in the form of
non-woven ber mats. The small diameters lead to a largesurface area
to mass ratio, a porous structure withexcellent
pore-interconnectivity and extremely smallpore dimensions.912
In the electrospinning technique, there are mainlytwo types of
parameters: system and process
parameters. Viscosity, concentration, surface tension,molecular
weight, conductivity and dielectric of poly-mer solution are system
parameters. Applied voltage,feeding rate, tip-to-collector
distance, heat of the solu-tion and ambient parameters are process
parameters.The ber morphology depends on the polymer
type,conformation of the polymer chain, system parametersand
process parameters.11,1315 One of the most signi-cant parameters
inuencing the ber diameter is thesolution viscosity. The
concentration of a polymer solu-tion must be high enough to cause
polymer entangle-ments and ber formation. However, too high
viscosityprevents polymer motion under the electric eld andtoo low
viscosity means that bers cannot form butinstead beads are formed.
The solution must alsohave a surface tension low enough, and a
charge densityhigh enough. The diameters of nanobers increase
withincreasing concentration according to the power
lawrelationship.16 With an increase in tip-to-collector dis-tance,
a reduction in diameter size and distribution isobserved. The
diameter of the nanobers decreases, andthe diameter distribution
narrows when the appliedvoltage increases. The applied voltage
inuencesber diameter and morphology, but the signicanceand the
direction of the eect may vary with other fac-tors, such as
tip-to-collector distance and solutionproperties.1720
There are detailed reports on electrospinning and
itsapplications.9,10,12,14,18,19,21,22 The electrospun bershave
been used in ltration, protective clothing, tissueengineering
scaolds, sensors, energy storage, batteryseparators, composite
materials and biomedical appli-cations, such as wound dressing and
drug delivery sys-tems. Fundamental requirements for all cases
arecontrolled pore sizes, small diameters with enhancedspecic
surface area and permeation properties.6,11,12,23
Electrospun nanober mats have increased ltrationeciency and
higher capability to collect ne particlescompared to conventional
lter bers. These nanoberscan be functionalized and collect small
molecules froma solution. For example, when nanober mats arecoated
with polypyrrole (PPy), these mats are able tocollect gold ions.
The requirements for scaolds arehigh porosity with an appropriate
pore size distribu-tion, high surface area, biodegradability,
biocompati-bility and structural integrity with enough
mechanicalstrength. Electrospun nanober mats have high poros-ity
with high surface area and they have similar morph-ology to the
human native extracellular matrix (ECM)so they are promising
candidates for tissue engineeringapplications.24 A large specic
surface area and highlyporous structure lead to high sensitivity
and fastresponse. These two aspects make electrospun mats agood
candidate for sensor applications. For example,uorescence optical
sensors were developed by coating
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a conjugated polymer onto nanober and they showhigh sensitivity
and fast response.14 For energy conver-sion and storage
applications, a porous structure isrequired. A porous structure
causes high discharge cur-rent and capacity for the electrodes and
this structureallows movement of ions while preventing short
circuitfor separator membranes. Also, decreased ber diam-eters of
nanobers help faster ion mobility andquick response for actuator
applications. Increased sur-face area gives higher capacitance
values forsupercapacitors.9
Conductive polymers
Until the work of Shirakawa, Heeger and MacDiarmid,which was
related to doping polyacetylene, metals,inorganic crystalline
structures, certain phases ofcarbon and some ceramics were the only
materials forthe electronics industry.25,26 It was demonstrated
thathigh levels of electrical conductivities could be obtainedby
doping polyacetylene. Since this observation, anumber of other
conjugated polymers have been stu-died, such as polyaniline (PANI),
poly (phenyleneviny-lene), PPy and polythiophene (PT).1
Conductivepolymers have backbones that contain alternatingdouble
and single bonds. These polymers possess semi-conductor
characteristics due to this conjugated struc-ture. In
semiconductors, there is a small energy gapbetween the HOMO
(highest occupied molecular orbi-tal or valence band) and LUMO
(lowest unoccupiedmolecular orbital or conduction band). So
electronscan be excited either thermally or electrically over
thegap where they are free to delocalize over the LUMOlevel or
conduction band. If there are enough smallband gaps, a large
delocalized band appears over thelattice, the electrons ow in the
conduction band and/or the vacant holes of positive charge ow in
thevalence band and a current ow with electrons.11,27
Conducting polymers can be dened as the cationicand anionic
salts of highly conjugated polymers.14
Figure 1 shows the chemical structure of some
importantconjugated polymers. The cation salts are obtained
bychemical oxidation and electrochemical polymerization.The anion
salts of the highly conjugated polymers areproduced by using
electrochemical reduction or chemicalreduction with reagents such
as sodium naphthalide. Anoxidized conducting polymer has electrons
removedfrom the backbone, resulting in a cationic radical. Areduced
conducting polymer has electrons added to thebackbone, resulting in
an anionic radical.14,28
The mechanism of conduction in conductive poly-mers is very
complicated and it also involves theconcept of solitons, polarons
and bipolarons.Conductive polymers are insulative (with
conductivityof 1010 S/cm) in the neutral state. The formation
of
charge carriers upon oxidizing (p-doping) or reducing(n-doping)
their conjugated backbone leads to highconductivity values (up to
102S/cm depending on thepolymer system and the type and extent of
doping).2,3
The doping process, which is partial addition orremoval of
electrons to/from the p system of the poly-mer backbone, can be
done chemically or electrochem-ically. In chemical doping,
conductive polymers areoxidized by exposing oxidizing vapors such
as iodine.In the ground state, p-bonds (pp*) are partially
loca-lized. In the doping process, the excitation across thepp*
band gap creates self-localized excitations in thegap region.
Figure 2 shows the change in band gap dueto the doping process.
These self-localized excitationsare called polarons, bipolarons and
solitons. Polaronsare generated localized electronic states that
are formedafter oxidizing the neutral polymer and the
relaxationprocesses. After the conductive polymer chain is
satu-rated with polarons, a bipolaron is formed by removingan
additional electron from a polaron.2831
Inuencing factors for conductivity are the polaronlength, the
conjugation length, the overall chain lengthand the charge transfer
to adjacent molecules. Thesefactors are explained by models based
on intersolitonhopping, hopping between localized states assisted
bylattice vibrations, intra-chain hopping of bipolarons,variable
range hopping in three dimensions and
NH
Polypyrrole
HN
Polyaniline
S
Polythiophene
PolyacetylenePoly(para-phenylene vinylene)
S
O O
n n
n
nn
n
Poly(3,4-ethylenedioxythiophene)
Figure 1. Chemical structures of some important conjugated
polymers.29
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charging energy-limited tunneling between conductingdomains.
Electron hopping is the charge mobility alongthe chains and between
chains due to the attraction ofelectrons in one repeat unit to the
nuclei in neighboringunits yields.2,3 The movement of charge
carriers alongthe conjugated backbone produces electrical
conductiv-ity. The smaller distance between the conducting bandand
valence band (band gap) refers to a high conductivestate, as
illustrated in Figure 2. Dopant, oxidation level/doping percentage
and synthesis method and tempera-ture aect the band gap and so the
conductivity of theconductive polymers.27 Synthesizing novel
structures,increasing the order of the polymer backbone,
increas-ing conductivity, easier processability and synthesis,more
dened three-dimensional structure, stability inboth conducting and
non-conducting states and solubil-ity in certain solvents are the
aims in conducting-poly-mer synthesis.2,3,27
Conducting polymers can be used to enhance speed,sensitivity and
versatility of biosensors because theyhave the ability to transfer
electric charge producedby the biochemical reaction to electronic
circuit.Conducting polymers can be deposited on electrodesso
amperometric biosensors can be designed with thepossibility to
entrap enzymes during electrochemicalpolymerization. They can be
used as a potentiometricdevice, where the activity of ions in
solution determinesthe potential of the system. Direct electron
transferbetween proteins and the conducting polymers occursif they
are attached enzymes or functional groups.Conducting polymers
containing counter ions canabsorb a certain amount of protein from
solutiondepending on the oxidation state and the conductivity
of the polymer. The electronic structure is highly sensi-tive to
changes in the polymeric chain caused by eventsthat occur in the
system, such as DNA hybridization.The changes in the delocalized
electronic structure alteroptical and electrical properties, and
they can provide asignal for the presence of a target analyte
molecule insensor applications. High protein loading and
stability,direct and intimate contact with the bioanityreagents,
the modulation of analytical signals throughthe application of
electrical potentials and ease of fab-rication due to the direct
incorporation of bioanityreagents are the advantages of
conductive-polymer-based sensors. The low mechanical property and
poorprocessability are the disadvantages that limit theirusage in
some applications, such as direct protein detec-tion and scaold
applications.1,27,3335 Compositenanober mats with enough mechanical
strength andconductivity may increase usage of conductive
poly-mers. Conducting polymers have been used in the fab-rication
of biosensors in various elds, such as medicaldiagnosis,
immunosensors, in the detection of variousgenetic disorders,
pollutants and glucose, fructose,ethanol, sucrose, lactate, malate,
galactose, citrate, lac-tose, urea, starch, etc., in food
industries.27
Preparation of conductive nanofibers
There are dierent methods to synthesize polymernanostructures
(bers and tubes), such as template syn-thesis, chiral reactions,
self-assembly, interfacial poly-merization and electrospinning.28
Dierent preparationmethods, physical properties and potential
applicationsof one-dimensional nanostructures of conjugated
Figure 2. Energy band structure of low, medium and highly doped
polypyrrole. Reprinted (adapted) with permission from
(J. L. BREDAS, G. B. STREET, Polarons, Bipolarons, and Solitons
in Conducting Polymers, Acc. Chem. Res. 1985,18, 309315).
Copyright (1985) American Chemical Society.32
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PANI, PPy and poly(3,4-ethylenedioxythiophene)(PEDOT) were
discussed previously.36 In this study,we just focus on the
properties of the conductive elec-trospun nanober mats based on
conductive polymers.Dierent routes are reported to obtain
conductivenanobers using electrospinning. In general,
conductivepolymers cannot be easily electrospun due to their
lowmolecular weights, poor solubility and rigid backbonestructure.
These characteristics restrict the spinnabilityof the polymers. In
order to solve the processabilityproblems of conductive polymers,
many researchgroups have tried dierent techniques, such as
introdu-cing side chains, controlling main-chain
architecture,designing new monomer types and using
functionaldopants. Blending with other polymers to form com-posite
structure and coating by using conductive poly-mers are the most
common techniques.37 Blending withan easy spinnable polymer is a
common way to com-pensate for poor spinnability. However, the
presence ofan insulating carrier polymer introduces a
conductivitypercolation threshold that limits their usage in
applica-tions where high conductivity values are required.Another
way is the polymerization of a conductivemonomer on the surface of
a ber, made with acommon polymer and a catalyzer/doping
agent.38,39
Preparation of polypyrrole nanofibers
PPy is one of the most investigated conductive poly-mers. It has
a low oxidation potential and a high con-ductivity. Pyrrole
monomers are dissolved in water.Because of its easy synthesis and
long-term ambientstability, it has been investigated for many
applications,such as antistatic, electromagnetic shielding,
actuatorsand polymer batteries.4042 The inherently poor solu-bility
in common solvents, which originates from thestrong inter- and
intra-chain interactions, is the disad-vantage that restricts
practical applications of PPy inmany areas.43
Several attempts have been made to obtain conduct-ive nanobers
by using PPy. Conductive non-wovenmats composed of pure PPy were
prepared by Kanget al.42 PPy was synthesized by using ammonium
per-sulfate (APS) as the oxidant and dodecylbenzene sul-fonic acid
(DBSA) as the dopant. Solubility wasobtained by using chloroform
and excessive amountof DBSA. The intermolecular interaction
betweenPPy chains was reduced by doping with a highamount of DBSA,
but the reduction of intermolecularinteraction between PPy chains
decreased the inter-chain conduction of charge carriers and led to
thedecrease in bulk conductivity.42
PPy nanobers, by using electrospinning techniques,were prepared
by Chronakis et al.40 Dierentapproaches were reported in their
study: polyethylene
oxide (PEO) was used as a carrier, pure PPy conductivenanobers
were prepared by electrospinning of organicsolvent soluble PPy
using the functional doping agentdi(2-ethylhexyl) sulfosuccinate
sodium salt (NaDEHS).PEO were used to obtain electrospun blends of
water-soluble PPy.40
Some limitations of producing conductive nano-bers were reported
by Sen et al.44 The incorporationof PPy particles into a carrier
polymer and electrospin-ning of this solution could only be
achieved whenmaterials were prepared with particulates smaller
thanthe cross-section of the ber. Soluble PPys could beprepared,
but these polymers did not have sucientviscosity to prepare
electrospun bers due to their lowmolecular weight. The coating
process could be appliedto the outer surface of a pre-spun ber. In
their study,the composite bers of polystyrene (PS)-PPy were
sus-pended in dimethylformamide (DMF) to obtain ahollow PPy ber.
There are some issues about thesemethods but these approaches may
oer a promisingroute to electrically conducting electrospun
bers.44
Long PPy bers were obtained by a vapor depositionreaction of
pyrrole on the FeAOT (an organic saltsynthesized by the reaction of
sodium 1,4-bis(2 ethyl-hexyl) sulfosuccinate (AOT) and ferric
chloride)bers.45 The synthesis of PPy composite bers
withmultiwalled carbon nanotubes (PPyMWCNT bers)was also reported.
Firstly, FeAOT was synthesizedand then electrospun to fabricate
FeAOT andFeAOTMWCNT nanobers. In order to synthesizePPy or PPyMWCNT
bers, FeAOT or FeAOTMWCNT bers were placed in a reaction vessel;
pyr-role was deposited onto salt bers. The PPy nanoberswere
obtained after removing the remaining oxidantand oligomers by
washing with methanol.45
Silver-PPy-polyacrylonitrile composite nanobrousmats were
prepared by using the coating method.41
AgNO3-PAN mats were prepared by electrospinning andthe mats were
put into the boiling mixture of pyrrole andtoluene. Then the
pyrrole was oxidized by silver ions.41
Polyamide 6-PPy conductive nanobers were producedby a
polymerization of pyrrole molecules on the bersurface.39 A solution
of PA-6 and ferric chloride informic acid was electrospun and the
mat was exposedto pyrrole vapors. PPy was formed on the ber
surface.39
PPy-PEO composite nanobers were fabricated byusing the coating
method.4 Firstly, FeCl3-containingPEO nanobers were produced and
the PEO-FeCl3electrospun bers were exposed to pyrrole vapor.
Thevapor phase polymerization occurred through the dif-fusion of
pyrrole monomer into the nanobers. Thecollected non-woven ber mat
was composed of PPy-PEO nanobers with about 96 nm diameter. PEO
andFeCl3 were chosen because PEO could form a complexwith FeCl3.
FeCl3 was known to be one of the most
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ecient oxidants for pyrrole polymerization and couldleave
chlorine ions in PPy, which makes it electricallyconducting. The
Fe+3 ions are bound by the coordinat-ing oxygen atoms of the PEO
chain. This suppressescrystallization of FeCl3 and ensures
homogeneous dis-tribution of FeCl3 along the PEO nanobers.
4
Core sheath conductive nanobers were described bycoating PPy on
electrospun PCL and PLA nanoberswith Fe3+ or APS as an oxidant, and
Cl or PTSA as adopant.46 A nanober mat was immersed in an
aqueoussolution of pyrrole and an aqueous solution of FeCl3was
added. In order to reveal the coresheath structure,the nanobers
were soaked in dichloromethane (DCM)for 24 h to dissolve the
cores.46
The growth of PPy layers over PS nanobers via thevapor phase
polymerization process was reported.47PSnanobers were produced
through electrospinning ofPS solutions containing chemical
oxidants, whichwere capable of polymerizing pyrrole monomers,
andpyrrole monomers were polymerized on the surface.47
After analyzing conductive bers fabricated by usingthe coating
method, composite nanobers obtained byblending polymers are
overviewed. Electrospun PPy-sulfonated-poly
(styrene-ethylene-butylenes-styrene)(S-SEBS) composite nanobers
were prepared.48 Theoxidative polymerization of pyrrole (Py)
bycerium(IV) on a poly (acrylonitrile-co-vinyl acetate)matrix and
composite electrospun nanobers producedby using this solution were
demonstrated by Cetineret al.49 PPy-poly(e-caprolactone)-gelatin
compositenanobrous scaolds for regeneration of cardiactissue were
reported.50 Polyurethane (PU)PPy com-posite nanobers obtained by
using electrospinningwere reported by Yanilmaz et al.51 In their
study, Pymonomers were polymerized into a PU matrix by using
cerium(IV) [ceric ammonium nitrate, Ce(IV)] as an oxi-dant
(Figure 3). The eects of the PPy content on thethermal, mechanical,
dielectric and morphologicalproperties of the composites were
investigated.Morphologies and electrical properties of the
compos-ite nanobers were reported.51
Preparation of polyaniline and polythiophenenanofibers
PANI can exist as a salt or base in three isolable
oxidationstates: leucoemeraldine (the fully reduced state),
emeral-dine (the half oxidized state) and pernigraniline (the
fullyoxidized state). The emeraldine salt is electrically
conduct-ive while the others are insulators.20 Conventional
chem-ical synthesis of PANI is based on an oxidativepolymerization
of aniline using an oxidant in the presenceof a strong acid dopant.
PANI has attracted considerableattention for electronic and optical
devices, sensors, light-emitting diodes, rechargeable batteries and
gas separationmembranes, because it has several advantages such as
lowcost, simple and controlled synthesis, high stability at
roomtemperature and good optical and electrical
properties.52,53
Processing is its limitation. It is an extremely rigid
polymerbecause its chemical structure is composed of reduced
andoxidized repeat units made of aromatic rings, with
inter-molecular hydrogen bonds and charge delocalization.20
There are very few numbers of papers that reportpure conductive
polymer nanobers by using electro-spinning.5456 One of them was
reported by Cardenaset al.54 The formation of pure PANI bers by
using theelectrospinning method was described. The acetonebath was
reported as of key importance for the forma-tion of bers. Excess
solvent in the jet diused intoacetone and polymer chains could form
bers by
Figure 3. Schematic illustration of the polyurethanepolypyrrole
(PUPPy) composite nanofiber preparation process.51
(Yanilmaz M, Kalaoglu F, Karakas H, et al. Preparation and
characterization of electrospun polyurethanepolypyrrole nanofibers
and
films, J Appl Polym Sci 2012;125: 41004108. Copyright [2012 John
Wiley & Sons, Inc]. This material is reproduced with permission
of
John Wiley & Sons, Inc.).
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placing the acetone bath on the collector.54
PANI nanobers were prepared by using dierent con-centrations
(from 10.6% to 19.1%) of PANI in hotsulfuric acid solution by Yu et
al.55
In another study, PANI-silica hybrid nanober webswere prepared.
The desirable preparation conditions ofPANI-silica nanobers were
0.7M aniline solution, onetime polymerization, 1030min of
polymerizationtime, 1.0 of the molar ratio of oxidant and
aniline,and 0.10.5M of the dopant concentration.57
PANI was blended with a natural protein, gelatinand
co-electrospun into nanobers to investigate thepotential
applications of the blend, such as a conduct-ive scaold for tissue
engineering purposes.58
Camphorsulfonic acid doped polyaniline (PANCSA)blends with PEO
were reported by Kahol andPinto.30 Three-dimensional nanober
electrospun non-woven webs were obtained from solution of
poly(3-hydroxybutyric acid) (PHB) and DBSA doped PANIin a
chloroform-triuoroethanol mixture.59 Nanobersof poly(amide 6) (PA6)
with dierent amounts ofpoly(aniline) (PANI) doped with p-toluene
sulfonicacid (TSA) were obtained by using blending method.20
PEO was blended with camphor-10-sulfonic acid(CSA) doped PANI.60
The conductive CSA-PANI-PEO composite bers were produced to be used
asthe conductive collector for the electrospraying process.Titanium
dioxide (TiO2) nanoparticles were sprayedand adsorbed on the bers.
The degree of adsorptionand dispersion of nano TiO2 catalysts on
the surface ofthe bers depended on weight percentage (wt%) ofPANI
in PEO solution and the strength of electricalconductivity of the
bers used during electrospraying.60
Conducting nylon-6-PANI electrospun ber webswere prepared by the
in situ polymerization ofPANI.61 PANI nanoparticles were doped with
theDBSA and electrospun with nylon 6.37 Poly-3-hex-ylthiophene-PEO
blend nanobers were fabricated byLaforgue and Robitaille.62
PANI-nylon-6 composite bers were prepared by dis-solving nylon 6
in formic acid and adding the salt (ammo-nium peroxodisulfate) and
aniline monomers. Afterpolymerizing aniline, the blend solution was
electrospun.63
Morphologies of conductive nanofibers
It is well known that morphologies and diameters ofnanobers aect
properties of nanober mats. In thissection, morphologies of dierent
nanober structuresare overviewed. PPy nanobers with diameters in
therange of about 70300 nm were reported by Chronakiset al.40 The
diameters of nanobers increased withincreasing PPy content. Thinner
bers (70 nm) wereobtained for pure PPy nanobers, which were
formedby using [(PPy3)
+ (DEHS)]x dissolved in DMF.
The low average nanober diameters were explainedby the
relatively low molecular weight of the conductingpolymer. Also,
nanobers with average diameters ofapproximately 100 and 150 nm were
formed via electro-spinning a solution of [PPy(SO3H)DEHS] with 1.5
or2.5wt% PEO, respectively.40 As a contradictory ndingof that
study, increasing the concentration of PPy(030%) led to reduced ber
diameters (from239 37 nm to 191 45 nm) in
PPy-poly(e-caprolac-tone)-gelatin composite nanobrous scaolds. The
ten-sile modulus increased from 7.9 1.6MPa to50.3 3.3MPa with
increasing the concentration ofPPy.50 Figure 4 shows decreased
diameters of PANI-gelatin bers with increasing PANI content.
Similar to Chronakis et al.s study, as the amount ofPANI
increased, PA6-PANI nanobers with increasingdiameters, lower
crystallinities, higher decompositiontemperatures, lower elastic
modulus and elongation atbreak were obtained. Even if they did not
show theresults, they stated that the viscosity of the
solutionincreased as the amount of PANI increased.20
Uniform diameters independent from conductivepolymer
concentration were reported by Laforgue andRobitaille.62 After a
detailed investigation, it has beenshown that the morphology aects
conductivities andmorphology depends on polymer types, ratios of
thecomponents, solvent types and methods. Laforguesresult can be
explained by good selection of polymersand strong interactions
between polymers under theirexperimental conditions.
Dierent morphologies of PANI-nylon 6 electrospunber web with
various PANI and nylon contents in aformic acid solution were
reported by Hong and Kang.37
Figure 4. Diameters of electrospun gelatin fibers and
polyaniline-gelatin blend fibers at different volume
ratios.58
Li M, Guo Y, Wei Y, et al. Electrospinning
polyanilinecontained
gelatin nanofibers for tissue engineering applications.
Biomaterials
2006; 27: 27052715. Copyright [2006 Elsevier]. (This material
is
reproduced with permission of Elsevier)
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When the concentration of PANI nanoparticles was from2 to 8wt%,
the PANI-nylon 6 electrospun nanobers werecomposed of two kinds of
phases (Figure 5). When theconcentration of PANI nanoparticles was
over 12wt%,the PANI-nylon 6 electrospun nanobers had only aone-type
phase, but there were defects.37 Non-uniformmorphologies may depend
on the solubilities and compat-ibilities of the components in the
composite structure.There are many reports on spider wave-like mats
by elec-trospinning.6467 In one of the studies, graphene oxide
wasused to form this structure and this structure improvedltration
eciency. The mechanism for the formation ofthe spider wave-like
structure was explained by complexphase separation process of the
solvent-degraded and non-degraded portion of the same solution
during whipping ofthe jet.64 Also, nanowebs with dierent ber
diameters andmorphologies can be obtained by adjusting
electrospinningparameters.65 The coreshell structured
nylon-6-lactic acidbers with spider wave-like structure was
reported and theformation of this structure was explained with
solventevaporation, solvent degradation and the plasticizereect of
lactic acid.66 In another report dierent fractionsof
solvent-induced polymer-degraded nylon 6/formic acidsolution and
freshly prepared solution of the same polymerwere mixed and the
eect of solvent-induced polymer-
degraded solution on the ber morphology of electrospunmats was
investigated. The spider net structure with twodistinct types of
bers (nanobers, subnanobers) wasobtained by adding solvent degraded
solution. This struc-ture decreased pore sizes and increased
mechanicalstrength.67
Figure 6 shows the morphologies of PUPPy nano-bers with dierent
PPy content. It was reported thatthe average ber diameters
decreased with increasingPPy content and the electrospinnability of
the solutionchanged as a result of the interactions between the
com-ponents in the structure (Figure 7). Because of thestrong
interaction between PU and PPy, the spinnabil-ity of the composite
solution was very sensitive to theamount of Py.51
Poly(aniline-co-m-aminobenzoic acid)(P(ANI-co-m-ABA))-poly(lactic
acid) (PLA) nanobermats showed decreasing trend in diameters
withincreasing PANI content.35
Decreased diameters by increasing PANI contentwere also reported
for PANI-gelatin nanobers. Theresult was explained by decreased
concentration withincreasing PANI content.58
PANI-poly(methyl methacrylate) (PMMA) berswere prepared by
coating PANI on PMMA. Eectsof some solution parameters, such as
polymer
Figure 5. Morphologies of polyaniline (PANI)-nylon 6 electrospun
nanofibers for different PANI content: (a) 2%; (b) 8% and
(c) 12%37 (Hong KH and Kang TJ. Polyanilinenylon 6 composite
nanowires prepared by emulsion polymerization and
electrospinning
process. J Appl Polym Sci 2006; 99: 12771286. Copyright [2005
JohnWiley & Sons, Inc]. This material is reproduced with
permission of
John Wiley & Sons, Inc.).
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Figure 6. Different morphologies of the polyurethane (PU)
nanofibers with different pyrrole (Py) contents: PU nanofibers;
PUpolypyrrole (PPy) nanofibers (5% Py); PUPPy nanofibers (7.5%
Py); and PUPPy nanofibers (12.5% Py)51 (Yanilmaz M, Kalaoglu F,
Karakas H, et al. Preparation and characterization of
electrospun polyurethanepolypyrrole nanofibers and films, J Appl
Polym Sci 2012;
125: 41004108. Copyright [2012 John Wiley & Sons, Inc]. This
material is reproduced with permission of John Wiley & Sons,
Inc.).
C
O
N CH2 N C O CH2 CH2 O
H
N
HPU-PPy intteractions
H
O
H
Ce(III)Ce(III)-PPy-PUinteractions
N
C
O
N CH2 N C O CH2 CH2 O
H
H
O
n
n
m
m
H
N
n
Figure 7. Schematic illustrations of the interactions in the
composites51 (Yanilmaz M, Kalaoglu F, Karakas H, et al. Preparation
and
characterization of electrospun polyurethanepolypyrrole
nanofibers and films, J Appl Polym Sci 2012; 125: 41004108.
Copyright
[2012 John Wiley & Sons, Inc]. This material is reproduced
with permission of John Wiley & Sons, Inc.).
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molecular weight, solution concentration, solventdielectric
constant and solution ionic strength, onmorphologies were
reported.68 The polymers withhigh molecular weight formed fewer
beads andbeaded bers with higher diameters.
High-dielectric-constant solvents reduced bead formation and
diam-eters. The addition of organic salts decreased
beadformation.64
The diameters of nanobers depend on the surfacetension, ow-rate
and electrical conductivity of the solu-tion.40,68 The diameters of
nanobers are important dueto the fact that they directly aect
conductivities of themats. There are dierent reports about the eect
of con-ductive polymers on the diameters of nanobers. Someof them
reported higher diameters; some of themreported lower diameters
with increasing conductivepolymer content. It can be said that
other parameters,such as the method, concentration, viscosities,
types ofpolymers and conditions, aect diameters. Moreover,the eect
of conductive polymer on solution propertieslike viscosity diers
from one system to another anddetailed investigation is needed in
this area.
Conductivities
Conductivity can be dened as a measure of electricalconduction
and it shows the ability of a material to passa current. Insulators
are materials with conductivitiesless than 108 S/cm. Semiconductors
have conductiv-ities between 108 and 103S/cm and conductors
arematerials that have conductivities over 103S/cm.Conductivity is
the inverse of resistivity and the unitsof resistivity and
conductivity are ohms (V) andSiemens (S), respectively. Resistance
of material isused to obtain resistivity and the four-point probe
tech-nique is generally used to measure resistance. In
thistechnique, constant current is applied cross two elec-trodes
and change in potential is measured. The follow-ing equations are
used for this technique:
V IR 1
RA=l 2
s Rw=l 3
where is the bulk or volume resistivity, S is the sur-face
resistivity, R is the surface resistance, A is thecross-sectional
area, l is the length between electrodesand w is the sample width.
Thickness is taken intoaccount in the bulk resistivity
calculation.1
Requirements for conductivity in polymers are theformation of
the delocalized molecular wave functioncaused by an overlap of
molecular orbitals and partiallylled molecular orbitals to allow
movement of electrons
throughout the lattice. The mechanism of conduction inpolymers
is very complex and may involve dierentmechanisms. That is why we
see a very large range ofconductivities. Polaron length,
conjugation length,overall chain length and the charge transfer to
adjacentmolecules are some factors that aect conductivity.27
For metal resistors, the traditional formula can beused to
calculate resistance:
R k LA
4where R is the resistance of a conductor, A is the sectionarea,
L is length and k is the resistance coecient. Thisequation is valid
only for metal conductors. To deneconductivity in polymers, a
modication is needed.There are many electrons in metals. However,
current isnot caused by electrons in polymers. For non-woven
con-ductive material, the following formula might be used:
R k:L0:99=c1:01A0:64 5
where L is the distance between the electrodes, c is
theconcentration of the electrolyte solution, A is area andk is
constant.69
Conductivities of nanobers were reported in awide range,
depending on dierent methods, byChronakis et al.40 For PPy-PEO
blend nanobers, theconductivities were in the range from 4.9 108
to1.2 105 S/cm depending on PPy concentration. Theelectrical
conductivity was about 3.5 104 S/cm for50wt% content of
PPy(SO3H)-DEHS in the nanobers(electrospun from a solution with
1.5wt% PEO) andabout 1.1 104 S/cm for 37.5wt% content
ofPPy(SO3H)-DEHS in the nanobers (electrospunfrom a solution with
2.5wt% PEO), which was nearlythree orders of magnitude higher than
that of the PPy-PEO samples. This dierence between dierent meth-ods
was explained by the higher initial PPy conductivityfor the second
and third methods.40
Conductive non-woven mats composed of PPy wereprepared by Kang
et al.42 The conductivities for theelectrospun nanoweb were
reported to be about0.5 S/cm by using the four-probe technique and
thebers had good electrochemical stability for
sensorapplications.42 Very high conductivity (14 S/cm, mea-sured by
the four-probe technique) was obtained, byusing Py deposition and
polymerization on salt bers,by Han and Shi.45
The conductivity was 1.3 103 S/cm for Ag-PPy-PAN
(polyacrylonitrile) composite brousmats preparedfrom AgNO3-PAN
containing 52% AgNO3.
41 Figure 8shows the conductivities as a function of AgNO3.
An increasing trend in conductivities (from 0.01 to0.021 S/cm)
was reported, with increasing the conduct-ive polymer content
(PANI) in the structure of PANI-gelatin nanobers, by Li et
al.58
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PPy-PEO composite nanobers were prepared bycoating PPy on PEO
bers.4 The sheet conductivitiesof the PPy-PEO composite nanober
mats were of theorder of 103 S/cm, calculated from the
four-probemeasurement data.4 Conductivities of
PPy-poly(e-caprolactone)-gelatin nanobrous scaolds were mea-sured
by using a four-probe method. The conductivitieswere about 105
S/cm.50
The conductivity of the PSCl PPy and PSTSPPyber mats, which were
produced by coating, were2 103 S/cm and 5 103 S/cm, respectively.
The con-ductivity of the porous ber mat could be inuenced bythe
PPy-PS ratio, doping, crystallinity of PPy, the voidvolume and the
connectivity between bers in the mat.After the PS template of the
PSTSPPy ber mat wasremoved by THF (tetrahydrofuran) treatment, the
elec-trical conductivity of the remaining material (TSPPy)increased
to 0.13 S/cm. The conductivity was measuredby using the four-probe
Van der Pauw method.47
The conductivities for PAN bers, reported with dif-ferent
dimensions, were in agreement with earlierresults for partially
doped PAN and the conductivitieswere in the range of from 103 to
102S/cm.53
Sub-micron bers of pure PAN doped with sulfuricacid or
hydrochloric acid were prepared and the factorsthat inuence the
conductivity were investigated.55 Thedoping level and the
morphology of PAN bers werethe main factors. The higher doping
level and moreordered morphology gave a higher conductivity.When
the H2SO4 concentration increased from 0% to30%, the doping level
increased, the structural homo-geneity improved, and so the
conductivity increased. If
the degree of structural compactness in the bersreduced, the
conductivity decreased.55
The resistivity values decreased with increasing PANcontent and
increased with increasing the ber diameterin the PLA-PAN blend
system.70 The contact probabil-ity among bers and the formation of
the conductivepathways through the sample were introduced as
areason for that result. Thicker bers had less contactprobability
in the same mat volume, and increase inber diameter results in
increase in void space betweenbers. So, decrease in the number of
inter-ber contactpoints led to decrease in conductivity. Changes in
crys-tallinity were also eective.70
The volume conductivities increased from 0.5 to1.5 S/cm as the
diusion time increased from 10minto 4 h because of the uniform
distribution of PAN inthe structure of PAN-nylon-6 ber mats.61 The
surfaceconductivities of the PAN-nylon-6 composite electro-spun ber
webs decreased (from 0.22 to 0.14 S/cm) asthe diusion time
increased, because PANI chains werecontaminated by aniline
monomers, aniline oligomersand some alkyl chains.
PAN nanoparticles doped with the DBSA were elec-trospunwith
nylon 6 and conductivities of dierent formswere compared.37 The
electrical conductivity of the PAN(DBSA) particles pellet was about
4.27 102 S/cm, theconductivity of PAN (DBSA) nylon 6 lm was
about1.68 104 S/cm, and the conductivity of PAN(DBSA)-nylon 6
electrospun ber web was about6.19 107 S/cm. When the PAN
(DBSA)-nylon 6 com-posite solution was electrospun, the overall
crystallinityof the composite polymer decreased so the
conductivitydecreased. This was explained with the rapid
evaporationof the solvent during the electrospinning process.37
Conductivities between bulk and nanober lmswere also compared
for PAN-PLA nanobers.71
Nanober mats had lower crystallinity due to the factthat rapid
evaporation of solvent prevents chains fromcrystallizing. The high
porosity of the non-woven matsand lower crystallinity resulted in a
decrease in the elec-trical conductivity.71
PAN.HCSA (polyaniline doped with camphorsulfo-nic acid)-PEO
blend electrospun bers were producedand desired conductivities (up
to 0.1 S/cm) wereobtained by controlling the ratio of PAN to
PEO.72
The comparison between cast lms and nanobermats was reported.
High porosity of nanober matsled to lower conductivities compared
to cast lms, butnanober mats had advantages such as quick
dedopingdue to higher surface area. The diculty about measur-ing
thicknesses of nanober mats due to their high com-pressibility was
also mentioned as a reason for lowerconductivities of
nanobers.72
Eects of dierent polymerization parameters onconductivity of
PAN-silica nanobers were
Figure 8. The plot of the conductivities (logarithmic scale)
of Ag-polypyrrole (PPy)-PAN fibrous mats versus the content
of AgNO3 in AgNO3-PAN41 (Chen R, Zhao S, Han G, et al.
Fabrication of the silver/polypyrrole/polyacrylonitrile
composite
nanofibrousmats. Mater Lett 2008; 62: 40314034. Copyright
[2008 Elsevier]. This material is reproduced with permission
of
Elsevier).
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investigated.57 Electrical conductivity of the hybrid
webincreased with increasing monomer concentration. Theelectrical
conductivities of hybrid webs were 5 105,1.7 103, 4.5 103, 3.2 103
and 1.07 S/cm for 0.2,0.5, 0.7, 1.0M aniline solution, and pure
aniline,respectively. The electrical conductivity showed themaximum
at 1.0 of the molar ratio of oxidant and anil-ine and decreased
with increase of the oxidant concen-tration. The molar ratio of
oxidant and aniline is
generally about 1.0 for synthesis of PAN, becauseexcess amount
of oxidant prevents polymerization ofPAN. Electrical conductivity
increased with increasingthe dopant concentration.57
Poly-3-hexylthiophene-PEO blend nanobers were produced by
Laforgueand Robitaille.62 The maximum electrical conductivityfor
unaligned mats was 0.16 S/cm, which increased to0.3 S/cm when the
nanobers were aligned.62 This resultagrees with other
studies.55
101 100 101 102 103 104 105 106 1071.0x10 7
0.01.0x1072.0x1073.0x1074.0x1075.0x1076.0x1077.0x1078.0x1079.0x1071.0x1061.1x1061.2x1061.3x1061.4x106
0 wt% Py
5 wt % Py
Cond
uctiv
ity(S
/cm)
Frequency (Hz)
102 101 100 101 102 103 104 105 106 107
0.0
5.0x103
1.0x104
1.5x104
2.0x104
102 101 100 101 102 103 104 105 106 107
0
1
2
3
4
5
6
7
8
TanD
elta
Frequency(Hz)
Die
lect
ric c
onst
ant
Frequency(Hz)
0 wt% Py 5 wt% Py
Figure 9. Alternating current conductivities, dielectric
constants and tan delta values for polyurethane (PU) and the
PUpolypyrrole
(PPy) nanofibers51 (Yanilmaz M, Kalaoglu F, Karakas H, et al.
Preparation and characterization of electrospun
polyurethanepolypyrrole
nanofibers and films, J Appl Polym Sci 2012;125: 41004108.
Copyright [2012 John Wiley & Sons, Inc]. This material is
reproduced with
permission of John Wiley & Sons, Inc.).
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As can be seen in Figure 9, the alternating current(AC)
conductivities of the PU nanobers withoutPy and with 5% Py were
about 7 107 and1.4 106 S/cm, respectively, at 107Hz. The
compositenanobers exhibited a high dielectric constant and tan
dvalues in the low- and radio-frequency ranges, so theycould be
used in charge-storing devices, decouplingcapacitors and
electromagnetic interference (EMI)shielding applications.51
As can be seen in Table 1, dierent ranges of con-ductivities can
be obtained depending on several fac-tors, which are discussed
above.
After investigating several studies, it can be said thathigh
porosities and lower crystallinities of nanoberstructure are the
disadvantages of nanober mats forhigh-conductivity applications. On
the other hand, highspecic surface area improves performances for
manyapplications. In conductive polymer systems, conduc-tivities
can be aected by several factors, includingtypes of polymers and
other chemicals (solvents, dop-ants, oxidizing agents, etc.),
ratios of the components,methods and ambient parameters. Besides
these, themeasuring method and physical structure of the matsmust
be considered. The conductivities of nanobermats are generally
obtained by the four-point probemethod. In this method, the volume
resistivity is mea-sured and then the conductivity can be
calculated fromthe resistivity value. The thickness measurement
maylimit the accuracy due to the high compressibility of
the mats. In the electrospinning technique, rapid evap-oration
of solvents decreased the crystallinity, anddecreased crystallinity
is a limitation for high conduct-ivity. In order to obtain
conductivity, a continuous con-ductive path must be created in the
structure.Contact probability of conductive segments betweenbers is
aected by diameters of bers. Thinner berswith aligned structures
and less porosity are desirablefor high conductivity, because thick
bers increase voidspace in the mat and limit the contact
probability ofconductive segments. Compactness and homogeneity
ofthe mats and ordered morphology must also be takeninto
account.
Applications
Conducting polymers, such as PPy, PANI, polythio-phene (PTh) and
PEDOT, show biocompatibility, con-ductivity, reversible oxidation,
redox stability andexcellent electrical and optical properties.
These makethem suitable for cell adhesion and tissue
engineeringapplication.50,75 PPy-coated electrospun
poly(lactic-co-glycolic acid) (PLGA) nanobers (PPyPLGA)
werefabricated for neural tissue applications.75 The
surfaceresistivity values of PPyPLGA nanobers were2.4 104 and 7.4
103 V/square for unaligned andaligned nanobers, respectively. It
was reported thatthese nanobers supported the growth and
dierenti-ation of rat pheochromocytoma 12 (PC12) cells and
Table 1. Conductivity values for different conductive
nanofibers.
Materials Method Conductivities S/cm References
PPy-PEO Blending 4.9 108 to 1.2 105 40PPy(SO3H)-DEHS-PEO
Blending 3.5 104 40PPy-APS-DBSA Pure PPy 0.5 42
PPy-FeAOT Coating 14 45
Ag-PPy-PAN Coating 1.3 103 41PANI-gelatin Bending 0.021 58
PPy-PEO Coating About 103 4
PPy-poly(e-caprolactone)/gelatin Blending About 105 50
PPy-PS Coating 5 103 47PANI Pure PANI 103102 54
PANI-nylon 6 Blending 6.19 107 37PANI-nylon 6 Coating 1.5 61
PANI-silica Coating 1.07 57
Poly-3-hexylthiophene/polyethylene Blend 0.3 62
PEDOT:PSS-PVP Blend 2.34 1012 73PANI-PDLA Blend 0.0437 74
PLA-P(ANI-co-m-ABA) Blend 8.3 109 35PPy: polypyrrole; PEO:
polyethylene oxide; DEHS: di(2-ethylhexyl) sulfosuccinate; APS:
ammonium persulfate; DBSA: dodecylbenzene sulfonic acid;
FeAOT: (an organic salt synthesized by the reaction of sodium
1,4-bis(2 ethylhexyl) sulfosuccinate (AOT) and ferric chloride);
PAN: PAN (polyacry-
lonitrile); PANI: polyaniline; PS: polystyrene; PEDOT:
poly(3,4-ethylenedioxythiophene); PSS: polystyrene sulfonate; PVP:
polyvinyl pyrrolidone; PDLA:
poly(D,L-lactide)/]; PLA: poly(lactic acid); ANI: aniline; ABA:
aminobenzoic acid (ABA)].
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hippocampal neurons. Electrical stimulation of neuronson
electroconducting scaolds was also shown to dem-onstrate the use of
PPyPLGA meshes as potentialnerve tissue engineering scaolds (Figure
10). PC12cells on PPyRandom bers (RF) and PPyalignedbers (AF) bers
at the potentials of 10 and 100mV/cm were electrically stimulated
and analyzed in terms ofneurite lengths, percentages of
neurite-bearing cells andnumbers of neurites per cell. PPyPLGA
meshes wereappropriate for neuronal applications.75
Conductive polymers in dierent forms, such as nano-bers and thin
lms, were evaluated for tissue engineer-ing applications by Bendrea
et al.76 Some examples wereoverviewed. Conducting PANI was blended
withpoly(L-lactide-co-ecaprolactone) (PLCL) and then elec-trospun
to prepare uniform nanobers scaold. Thisscaold combined the elastic
properties (which comefrom the PLCL domain) with electrical
activity (due toconducting PANI) at the nanometer-scale features.
Anano-scale structure with PANI led to a high porevolume,
inter-connective pores, a uniform mean berdiameter and an increased
conductivity. PANI wasblended to provide an electrical current to
improve cellattachment, proliferation and migration. PANI-PDLA
(poly(D, L-lactide) blend nanober scaolds with a con-ductivity
of 0.0437 S/cm could conduct current.74,76
Silk broin nanobers obtained by electrospinningwere coated with
PPy for scaold applications.Improved mechanical property was
reported by coatingwith PPy and no signicant change in diameter
wasreported after coating. The bioactivity and electro-chemical
activity of the PPy-coated broin were highenough to be considered
in adhesion, proliferation anddierentiation studies.77
Conductive polymers have also considered potentialmaterials as
sensors because of their inherentoptical, electronic and mechanical
transductionmechanisms.78,79
These sensors have advantages such as relative lowcost,
reversible signal transduction, high sensitivitiesand rapid
response at room temperature.73
Electrospun PEDOT:PSS (PEDOTpoly(styrenesulfo-nate))-PVP
(polyvinyl pyrrolidone) blend nanobersshowed good reversibility and
reproducibility inorganic vapor sensing, and the conductivity value
forPEDOT:PSS-PVP nanobers was 2.34 1012 S/cm.73Compared with PVP
nanobers, PEDOT:PSS/PVPnanobers exhibited better organic vapor
sensing
Figure 10. Electrical stimulation of PC12 cells through
polypyrrole (PPy)PLGA ) fibers at 0 and 10mV/cm. Representative
fluor-
escence images of electrically stimulated cells: (a) PPyRF at
0mV/cm (unstimulated); (b) PPyAF at 0mV/cm; (c) PPyRF at
10mV/cm;
(d) PPyAF at 10mV/cm. Scale bars are 50mm75 (Lee JY, Bashur CA,
Goldstein AS, et al. Polypyrrolecoated electrospun PLGA
nanofibers for neural tissue applications. Biomaterials 2009;
30: 43254335. Copyright [2009 Elsevier]. This material is
reproduced with
permission of Elsevier).
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performances to ethanol, methanol, THF and acet-one.73
Electrospun nanobers have been conrmed tobe good candidates for
ultra-sensitive gas sensors dueto the improved surface
area-to-volume ratios of coat-ings.80 Higher surface area led to
higher sensitivity andfast response time.80 Preparation of PANI
nanoberhumidity sensors were produced by electrospinningfrom the N,
N-dimethylformamide solution of PANI,poly(vinyl butyral) (PVB) and
PEO by Lin et al.81
PANI nanobers with some beads and a small contentof PEO revealed
high sensitivity, fast response andsmall hysteresis because beads
could help to improveadhesion to the electrode (which enhances
electricalcontact and sensing ability), and PEO helped toincrease
the hydrophilicity of the PANI nanobers,and humidity responses.81
PANI-polyvinyl pyrrolidone(PVP) composite bers were prepared for
NO2 sensingand these mats were reported as a good candidate forthis
application.82
PANI-nylon-6 blend nanober mats were preparedfor determining
organic compounds with the advan-tages of good sensitivity and
reproducibility.63 PANI-coated PMMA nanobers were also used for
gassensing.83
Conducting polymers have been studied to apply aselectrodes of
chargeable batteries, fuel cells and electro-chemical capacitors.
They are suitable for electrodesdue to their high conductivity and
light weight.48 Juet al.48 reported that electrospun PPy-sulfonated
poly(-styrene-ethylene-butylenes-styrene) bers may enhance
electrochemical capacity due to high doping levels andease of
charge transfers reactions. In their study, a PPycomposite nanober
electrode was compared with theelectrode lm that was produced by a
casting method.Electrospun PPy-sulfonated-SEBS bers were
calledE-PSS, electrospun PPy-SEBS bers were called E-PS.PPy/SEBS
(C-PS) was prepared by the casting method.The electrospun nanobers
showed higher charge/dis-charge specic capacity than the granular
type using thecasting method (Figure 11). This result was
explainedwith the reduction of interfacial resistance caused bythe
decrease of contact area.48 In another study,nano-structured PANI
was tested for sensor, actuator,supercapacitor and gas-separation
membrane applica-tions.81 PAN-PPy-based electrodes were prepared
andthese mats show good cycling performance with highreversible
capacity.84
Concluding remarks
Pure conductive polymer lms have high conductivitiesbut they
suer from low processability and highly brit-tle structure for many
applications, such as tissueengineering and sensor applications.
The introductionof conducting polymers into nanober mats has
thepotential to provide sucient conductivity for manydierent
applications. Controllable conductivity levelsof these nanobers are
also an advantage for dierentareas. The former studies concluded
that conductingpolymers, such as PPy, PAN and PTh, can be used
in
Figure 11. The specific discharge capacities of Li//C-PS,
Li//E-PS and Li//E-PSS cells with the number of cycles48 (Ju YW,
Park JH, Jung
HR, et al. Electrochemical properties of polypyrrole/sulfonted
SEBS composite nanofibers prepared by electrospinning.
Electrochim
Acta 2007; 52: 48414847. Copyright [2007 Elsevier]. This
material is reproduced with permission of Elsevier).
electrospun PPy/sulfonated-SEBS fibers ( E-PSS), electrospun
PPy/SEBS fibers (E-PS), PPy/SEBS (C-PS).
SEBS(poly(styrene-ethylene-butylenes-styrene)) Li(Lithium)
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nanober mats by coating on dierent nanober matsor by blending
with other polymers before electrospin-ning. In this study,
preparation and properties of semi-or conductive nanobers in the
presence of conductivepolymers by using the electrospinning
technique arereviewed for the rst time. The challenges and
limita-tions of dierent preparation techniques are reported.The
main requirements for many applications areimproved conductivity
and maximizing conductivepolymer content. Besides several factors,
such as typesof polymers, solvents, dopants, oxidizing agents,
ratiosof the components, methods and ambient
parameters,conductivities are also aected by the
morphology.Crystallinity, diameters, compactness, structural
homo-geneity and alignment of bers must be taken intoaccount in
order to evaluate conductivities.Conductivity of nanober mats suer
from high poros-ities and lower crystallinities. However, higher
specicsurface area due to the small diameters of nanobersimproves
performances for many applications.
Funding
This research received no specic grant from any funding
agency in the public, commercial or not-for-prot sectors.
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