A Review Effect of Conductive Polymers on the Conductivities of Electrospun Mats

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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]

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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

2 Textile Research Journal 0(00)



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

Yanlmaz and Sarac 3



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




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

Yanlmaz and Sarac 5



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.).




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)

Yanlmaz and Sarac 7



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.).




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.).

Yanlmaz and Sarac 9



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




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).

Yanlmaz and Sarac 11



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.).




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)].




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).




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)




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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