in situ doped polyaniline nanotubes for applications in flexible conductive coatings

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In Situ Doped Polyaniline Nanotubes forApplications in Flexible Conductive Coatings

Sarang P. Gumfekar, Wenjie Wang, Boxin Zhao*

In-situ doped polyaniline nanotubes were fabricated and investigated in term of theirmorphological, thermal and electrical properties at varied doping levels. Different from thecommon post-doping of polyaniline base to form its emeraldine salt, the aniline monomers weredirectly converted into conductive emeraldine salt during synthesis by dropwise addition of
the dopant and oxidants. This process resulted in auniform and stable protonation of the polyanilinewith good electrical conductivity. Furthermore, itwas found that the crystalline conductive polyani-line nanotubes have a good flexibility whenamalgamatedwith polyvinyl alcohol, showing theirpotential as flexible conductive coating materials.
S. P. Gumfekar, W. Wang, B. ZhaoDepartment of Chemical Engineering, Waterloo Institute forNanotechnology, University of Waterloo, 200 University AvenueWest, Waterloo, ON, Canada N2L3G1E-mail: [email protected]. WangState Key Laboratory of Explosive Science and Technology, Schoolof Mechatronical Engineering, Beijing Institute of Technology,Beijing 100081, P. R. China

� 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.c

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facial polymerization,[14] electrochemical polymeriza-[15] [4,16]

1. Introduction

Polyaniline (PANI) is one of themost promising electrically

conductive polymers in various fields of electronics, such as

sensors,[1,2] electromagnetic interference shielding,[3,4]

super-capacitors,[5–7] diodes,[8] and transducers.[9] This

polymer exhibits metallic behavior in terms of electrical,

magnetic, and optical properties and retains the properties

of conventional polymers such as flexibility and tempera-

ture-dependent viscosity. Nanostructured PANI has

attracted a great deal of interest because of its tunable

conductivity switching between insulator and conductor,

nontoxic properties, and good environmental and chemical

stability.[6,10–12] Different forms of PANI, such as nano-

particles, nanotubes, and nanowires, have been prepared

by various physical and chemical synthesis methods.

Physical methods include electro-spinning[13] and various

deposition techniques, while chemical routes include

inter

tion, and emulsion polymerization.

The electrical conductivity of PANI is obtained typically

by doping the PANI emeraldine base,[17,18] whose backbone

structure is illustrated inFigure1a.Thedegreeofdopingcan

be controlledbyaciddopingorbasededoping.Hydrochloric

acid is widely used as a dopant for PANI because of its

availability and simple reaction chemistry. Doping in PANI

has a significant impact on its potential applications in

nano-electronics and nano-devices. Morales et al. showed

that the increase in substitution on PANI backbone

decreased the electronic conjugation, which resulted in

thedecrease of conductivity.[19] Thus, dopingneedsbedone

without affecting the backbone structure of PANI. It has

also been shown that un-doped PANI forms aggregates due

to such inter-chain interactions as the hydrogen bonding

betweenthe imineandaminenitrogensitesontheadjacent

PANI molecules.[20] However, the effects of the degree of

doping on the electrical conductivity particularly in the

conductive regime are much less investigated, and the

bonding nature of chloride ions with PANI backbone chain

is still unclear. Better understanding of these effects is

important for the potential applications of PANI as, for

example, conductive fillers in the development of electrical

conductive adhesive and coatings.

In recentyears, therehavebeenincreasinglydemandsfor

the development of PANI-filled conductive adhesive films

and coatings to partially replace the conventional metallic

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Figure 1. a) Backbone structure of polyaniline emeraldine base indicating reduced and oxidized form and b) The scheme of formingpolyaniline emeraldine salt from aniline monomer with ‘‘in situ’’ doping of HCl in presence of ammonium persulfate. Chloride ions areattached to imine nitrogen of polyaniline backbone.

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S. P. Gumfekar, W. Wang, B. Zhao

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fillers so as to make the process more environmental

friendly and enhance flexibility of the films for the varied

applications in LCD, flexible display, and diodes. However,

PANIhas limited solubility inorganic solvents and inwater.

It has been shown that PANI can be made water soluble

using bi-functional water soluble dopants.[21,22] But the

electrical performance of common thin film and sensor

applicationsofwater solublePANIwasrelativelypoor.[23,24]

Solubility of PANI in organic solvents has also been

studied.[25–27] The PANI in base form can be made soluble

in organic solvents such as NMP, DMSO, and DMF. Widely

studied dopant DBSA could result in only 8% solubility of

PANI.[27] In general, PANIhas apoorprocessability,which is

amajor barrier in the fabrication of PANI films. In addition,

amorphous PANI has a relatively lower conductivity

than the metallic fillers; this low conductivity limits its

application. Because of these limitations, there has been a

continuous interest in the synthesis and applications of

crystalline PANI nanotubes. Crystalline PANI has a much

better conductivity than the amorphous one and has the

potential to replace metallic fillers in more applications.

One-dimensional PANI nanotubes allow an efficient

transport of electrical carriers throughout the conductive

network, which is highly desired when integrated nano-

scale devices involve the moving charges.[28]

In this study,wereport the fabricationof crystallinePANI

nanotubes with varied doping levels. In contrast to the

common post-doping of PANI base to form its emeraldine

salt, the PANI nanotubes were in situ doped by HCl during

the polymerization at varied dopant to monomer (D/M)

Macromol. Mater. Eng. 2014, DOI

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ratios. This process resulted in a uniform and stable

protonation of the PANI with good electrical conductivity.

The relations between the D/M ratios and PANI spectro-

scopic and structural features, crystallinity, and electrical

conductivity were systematically investigated to provide

fundamental insights to the roles of the dopant in the

structure and properties of the PANI nanostructures. We

also demonstrated the good dispersion ability of in situ

doped PANI nanotubes in polyvinyl alcohol (PVA) solutions

and the potential applications by coating them onto a

polyethylene terephthalate (PET) substrate to form a

flexible electrically conductive film.

2. Materials and Experiments

2.1. Materials

Aniline (99.9%, corrected for water content) was purchased

from J.T. Baker, USA, and stored in a dark place to avoid

photo-polymerization. Ammonium persulfate (98%) was

procured from EMD Chemicals, which contained an

insoluble matter less than 0.005%. Polyoxyethylene sorbi-

tan monooleate and PVA (99þ% hydrolyzed) of molecular

weight 146 000–186 000 Da was purchased from Sigma

(USA). Hydrochloric acid (37%) was used as a doping agent

and was purchased from Thermo Fisher Scientific. All the

glassware were cleaned prior to use; they were soaked in a

1M solution of sodium hydroxide for 24h, neutralized with

acetic acid, and then rinsed with DI water. The DI water

(resistivity> 10 MV cm at 25 8C; total organic carbon <20

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In Situ Doped Polyaniline Nanotubes for Coating Applications

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ppb) was obtained using the RiOs-DI Clinical system (Milli-

Pore Corporation).

2.2. Fabrication of Polyaniline Nanostructures

Amonomer solution, 0.1M aniline was prepared in 1M HCl,

wherein the 1M HCl was prepared in de-ionized water. A

surfactant solution of polyoxyethylene sorbitan mono-

oleate (0.08M) was prepared in ethanol; it has a faint hazy

yellow color. The monomer and surfactant solutions were

sonicated separately and then were mixed together and

poured into a 1 L reactor, equipped with a temperature-

controlled jacket. As-prepared mixture was brought to 4 8Cand stirred vigorously with a magnetic needle to form an

emulsion mixture. To start the reaction, 0.05M solution of

ammonium persulfate in 1M HCl was continuously added

for a half hour to the emulsified mixture of aniline.

Polymerization of aniline is a type of chemical oxidative

polymerization of aniline; it is exothermic. We carried out

the reactionat4 8Cbycirculatingcoolantaroundthereactorandprecisely controlling the rateofadditionofAPSsolution

to the aniline solution under vigorous stirring. Polymeriza-

tion of aniline continued for 2h according to the scheme

depicted in Figure 1b. Reactionwas carried outwith dopant

tomonomer (D/M) ratios of 8, 16, and 50 for the studyof the

effect of dopant. The PANI was purified by the centrifuga-

tion of the reactionmixture followed bywashing profusely

with deionized water and acetone for at least four times to

remove the traces of surfactant, unreacted aniline, and

unreacted HCl. Purified PANI was dried under dynamic

vacuum for 24h to remove the washing solutions.

2.3. Chemical, Structural, Thermal, and Electrical

Characterizations

Polyaniline samples were characterized in terms of their

chemical, structural, morphological, optical, and thermal

properties. Fourier-transform infrared spectra (FTIR) of the

PANI samples (three repeats) were recorded using a Varian

640-IR with 100 scans per spectrum at 2 cm�1 resolution

between the wavenumbers 400–2 000 cm�1. The spectra

were corrected for the presence of moisture and CO2 in the

optical path. Morphological characteristics of PANI nano-

tubeswere examinedat various locations of the samples by

field emission scanning electron microscope (SEM) using

LEO FE-SEM 1530 (Carl Zeiss NTS), operated at 10 kV. The

samples were prepared by depositing one drop of the

sonicated PANI containing solution on the conductive

carbon tape and dried in air for 24h, followed by gold

sputtering for 120 s.

High-angle annular dark-field (HAADF) transmission

electron microscopy images combined with energy disper-

sive X-ray spectroscopy (Oxford EDX detector) data were

acquired on a FEI Titan 80–300 TEM (FEI company,

Macromol. Mater. Eng. 2014, DO

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Eindhoven, The Netherlands), equippedwith a CEOS image

corrector and operated at 80 kV. To prepare TEM samples,

one droplet of dilute dispersion of PANI nanotubes in

methanol (1mgmL�1) was dropped on a lacey/holey

carbon copper grid and dried for 1/2 h in air. Ultraviolet–

visible spectroscopy of the PANI samples was performed

(three repeats) using aUV–Vis spectrophotometer (UV–Vis)

(UV-2501 pc, Shimadzu). For this, PANI was dissolved in

DMF; the solutionwas placed in a quartz cuvette of 1.00 cm

path length. Weight loss and subsequent degradation

mechanism of doped PANI in powder form was studied

using thermo-gravimetric analysis (TGA) (TA instrument,

Q500-1254). Samples of about 4mg were placed in a

TGA sample pan. Dynamic scan was performed from 25 to

800 8C with a heating rate of 10 8Cmin�1 under nitrogen

atmosphere.

To measure the bulk electrical contact resistivity of the

synthesized PANI nanotubes, they were uniformly spread

onto a piece of non-conductive Scotch tape under a slight

finger pressure to form a compact film of 300mm thick.

Thefilmdimensionswere 1 cm� 3 cm. Eight to ten samples

were characterized at different locations on the film.

The resistivity was measured by a four-point probe setup

consisting of a probe fixture (CascadeMicrotech, Inc.) and a

source meter (Keithley 2440 5A Source Meter, Keithley

Instruments, Inc.). The sheet resistance of the PANI nano-

tubes film (Rs) was estimated by measuring the drop in

voltage when a constant current of 100mA was applied.

The electrical resistivity (r) of the film of PANI nanotubes

was calculated using the following equation:

I: 10.10

bH & C

T the

r ¼ Rst ¼ ptln 2

� �VI V cm ð1Þ

where t is the thickness of samples, and I and V are the

applied current and the measured voltage, respectively.

2.4. Fabrication and Characterization of Flexible

Electrically Conductive Films

Tomake PANI coating on a flexible surface, a pre-measured

quantity of PANI nanotubes was uniformly dispersed in a

PVA aqueous solution (10wt%) under intense sonication.

The PVA solution was prepared by dissolving PVA in de-

ionized water at 80 8C in vigorously stirred environment.

Note that the optical transparency of PVA can be affected

due to the overheating,whichwas avoided in ourwork. The

overheating might degrade the polymeric chains resulting

in yellowish hazy color. PET overhead transparency (Xerox

Corporation, NY, USA)was chosen as a substrate for casting

the film of PVA–PANI solution; the casting of 1 g mixture

was performed using a spin-coater at 500 RPM. The

thickness of the film coated onto PET substrate was

measured by a Vernier caliper to be 300� 15mm. As-casted

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conductive films were dried in air at room temperature for

12h to allow the firm bonding of PANI nanotubes onto the

substrate. The PANI-coatedfilmwasvisually examinedand

physically bent and scratched to check its quality and

durability. The bulk resistivity of PANI-coated PET film

was measured using the same four-point probe setup as

described before.

3. Results and Discussion

3.1. Fabrication of Polyaniline Nanotubes

Polyaniline nanotubes were fabricated using the mini-

emulsion polymerization method with polyoxyethylene

sorbitanmonooleate (Tween 80) as the surfactant. Figure 2

illustrates the experimental setup inwhich the oxidantAPS

and dopant HCl were added dropwise into the monomer

aniline solution. Although the mechanism is not fully

understood, it has been shown that the surfactant

molecules form nano-sized micelles in the solutions,

containing the aniline monomers; the polymerization of

aniline occurred in the confined spaces of the micelles

as initially reported by Wan.[29,30] During the reaction, a

transition of color from blue to green was observed,

suggesting the formation of emeraldine salt of PANI. We

observed that an increase in dopant to monomer (D/M)

ratio caused the green color to become darker, suggesting

that the dopant HCl might increase the rate of polymeriza-

Figure 2. Schematic set-up of the polymerization of anilines: a) dropwHCl, b) temperature controlled jacketed reactor, c) as-synthesized puof polyaniline nanotubes in DMF at different dopant to monomeremeraldine salt of polyaniline.




tion of aniline. Figure 2d shows typical images of stable

dispersion of PANI at different D/M ratios. It can be

seen that the increase in the D/M ratio caused a color

transformation from bluish green to dark green. A bluish

green color in the sample with D/M¼ 8 implied the

presence of emeraldine base along with emeraldine salt

while its disappearance in the later two images signified

the complete transformation into emeraldine salt. Thus,

the increase in dopant caused increased formation of

emeraldine salt of PANI.

Thereare twokey features in the fabrication. Thefirstone

is the ‘‘in situ’’ HCl doping. In contrast to the post-doping of

PANI base to form its emeraldine salt, the ‘‘in situ’’ doping

process directly converted aniline monomers into the

emeraldine salt during the polymerization to obtain

homogeneous doping. This process allows the dopant

molecules and PANI backbone to bond firmly due to

electrostatic forces between them.[31–33] This acidic condi-

tion also helped to stabilize the surfactant micelles. The

second feature involved the suppression of secondary

nucleation of PANI on the already formed nanotubes of

PANI by the dropwise addition of APS with vigorous

stirring. Our preliminary experiments where all APS was

added to aniline at once (without stirring) showed a rapid

sedimentation of aggregated PANI particles from solution

because secondary nucleation took place on the surface of

newly formed PANI and resulted in the formation of strong

aggregates.[34] Li and Kaner[35] have shown that homoge-

neous nucleation of PANI results in formation of nanotubes

ise addition of an aqueous solution of the oxidant APS and dopantrified polyaniline nanotubes, and d) images of the stable dispersion(D/M) ratios; the increasing dark color signifies the formation of

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while heterogeneous nucleation causes formation of

granular particles.

The resultant PANI nanotubes were analyzed using SEM

at variedmagnifications as shown in Figure 3a–c. Although

the SEM images at the lowmagnification (Figure 3a) appear

to have some particulate materials, the higher magnifica-

tion SEM images (Figure 3b,c) do not show any particles. It

can be seen that PANI nanotubes form entangled network

in bulk state because of interchain and/or intrachain

interactions. This type of interconnecting network is

desired to obtain uniform electrical conductivity and

flexibility. Figure 4a,c,e are typical TEM images of the PANI

nanotubes formed at various dopant to monomer (D/M)

ratios, where a single PANI nanotubes at highermagnifica-

tion is shown in the inset. It can be seen that the PANI

nanotubes are hollow from inside and exhibit distinct

crystalline boundary as shown in the inset nanotubes.

Individual nanotubes were joined together into larger

secondary structures (mainlyone-dimensional) on theends

and surface of the primarily grown PANI nanotubes. Thus,

in the interconnecting network observed SEM image of

Figure 3, the nanotubes are physically entangledwith each

other. The length and diameter of the PANI nanotubeswere

estimated from these TEM images, and found to be

215� 19nm and 40� 12nm, respectively for both samples

withD/M¼ 8andD/M¼ 16. The length anddiameter of the

PANI nanotubes were found to be 134� 15nm and

31� 6nm, respectively for samples of D/M¼ 50.

Figure 4b,d,f show the corresponding selected area

electron diffraction (SAED) patterns. They show the

alternate dark and bright fringes, which can be attributed

to the crystallinity of the nanotubes. Grain boundaries

along the different crystalline planes can also be seen from

the HRTEM images of PANI confirming the polycrystalline

nature of the nanotubes. Furthermore, a detailed examina-

tion of the TEM images revealed some secondary growth of

PANI as indicated by the arrows in Figure 4c,e. Excessive

growth of secondary structures could result in branching

of nanotubes, which has been successfully inhibited by

the controlled addition of oxidant APS.

Figure 3. SEM images of bulk, powdered polyaniline synthesized withviews of entangled polyaniline nanotubes.




3.2. Spectroscopic Analysis of Polyaniline

Nanostructures

Chemical characteristics of PANI nanostructures and the

effect of dopingwere examinedbyFT-IR. Figure 5 shows the

FT-IR spectra of PANI for dopant tomonomer (D/M) ratio of

8, 16, and 50. Distinct characteristics of doped samples are

associated with emeraldine salt of PANI. The peak at

621 cm�1 is associated with S—C vibrations,[36–38] where

theAPS is consideredas theexpected sourceof sulfuratoms.

The peak at 849 cm�1 is related to the out-of-plane

stretching vibration of 1,4-disubstituted benzenoid ring,

which is a para-coupling structure.[32] The minor peak at

1 059 cm�1 is associated with the sulfate ion stretching

vibrations,[29,33] while the peak at 1 024 cm�1 is ascribed to

S55Ovibrations.[34] Appearance and subsequent increase in

the absorption peak at 1 194 cm�1 is associated with the

vibration modes of N55Q55N. The increased absorption is

ascribed to the increase in PANI content with enhanced

doping.[4] The peak at 1 276 cm�1 is attributed to C—H in-

planebendingmode. Thepeakat 1367 cm�1 is attributed to

C—Nþ* stretching vibration in a polaron structure;[4] this

absorption band is correlated to p-electron delocalization

induced in the PANI bydopingprocess. Itwas observed that

delocalization of p-electrons (1 367 cm�1) increased with

increase in dopant to monomer ratio, suggesting the

conjugation and subsequent increase in interconnecting

network of PANI, which caused stronger delocalization of

p-electrons. Additionally, there are two peaks at 1 535 and

1520 cm�1,which are attributed to the stretching vibration

of quinoid and benzenoid ring, respectively.[39,40] The peak

at 1 520 cm�1 undergoes a minor red shift with increased

doping levelsbut thepeakat1535 cm�1 is fixedbecause the

quinoid rings are widely separated – see Figure 1a. There is

another characteristic peak at 1 640 cm�1 due to stretching

vibrations of C55C; this peak shifted to higher wave-

numbers (or lower wavelengths) with the increase in

dopant concentration. This shift and increased absorption

were attributed to increased protonation in PANI due to the

dopant molecules.[41]

D/M¼ 50 with increasingmagnification (a! c), showing the close-in

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Figure 4. HRTEM images and corresponding selected area diffraction pattern (SAED) of polyaniline nanotubes with (a, b) D/M¼8, (c, d)D/M¼ 16, (e, f) D/M¼ 50. Arrows in (c) and (e) show the probable sites of secondary nucleation.

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We noticed that there is a peak at about 1 600 cm�1 for

D/M¼ 50butnot forD/M¼ 16. It canpossibly be attributed

to non-symmetric stretching of benzene ring, which can

be observed in the presence of adsorbed H2O on PANI

backbone. Thus, we suspect that some water molecules




mayhave been adsorbed to the highly doped PANI. Overall,

the observed FTIR features for D/M¼ 8 areweaker than the

other two because its polymerization is not completely

triggered whereas para-coupling (849 cm�1) and polaron

structure (1 276 cm�1) peaks appear with increased D/M

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Figure 5. FT-IR spectra of polyaniline nanotubes with different dopant to monomer (D/M) ratios.


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ratio because of aniline polymerization toward emeraldine

salt.[38] The FTIR results suggest that the dopant to

monomer ratio has an important role in the polymerization

of aniline. Later, we will discuss the effect of this ratio on

optical and electrical properties of the PANI.

Figure 6. UV–visible spectra of polyaniline nanotubes showingdistinct peaks at �330 and �450nm with varied dopant tomonomer ratios (D/M).

3.3. Optical Characteristics of Polyaniline

Nanostructures

UV–Vis absorption spectrum for each dopant to monomer

ratio is shown in Figure 6, displaying three distinct peaks

at low wavelengths and one ‘‘free-carrier’’ tail at high

wavelengths. The absorption peaks and tail are typical

features of PANI at its emeraldine oxidation state. The first

characteristic peak is at �330nm, which can be assigned

to p� p� transition of benzenoid rings and to the charge-

transfer-exciton transition formed in benzenoid and

adjacent quinoid ring. The other peak is at �450nm,

signifying the existence of polaron–p� transition.[10,17,40]

The third absorption peak at higher wavelengths is the

signature of p-polaron transition.[6,17] The relative posi-

tions of these absorption peaks indicated the different

electron transitions of the PANI backbone and provided

insights to the protonation of the PANI backbone and its

electrically conductive state.

Addition of protons to the PANI chain results in

subsequent formation of polarons, which increases its

electrical conductivity. A comparison of the spectra of PANI

of varied dopant to monomer ratios (and subsequent

increase in protonation) revealed a redshift of the lower

wavelength absorption peak from 332 nm-to-326 nm-to-




319nm. The similar trend of redshift was observed for

the second peak from 449 nm-to-438 nm-to-431nm. We

attributed this redshift to the possible increase in the

molecularweight of PANIwith the increase in protonation.

The spectra also allowed us to obtain some indirect

information about the conformation of PANI nanotubes.

PANI nanotubes can exhibit two conformations, i.e.,

‘‘compact coil’’ and ‘‘extended coil’’ conformations; the

compact coil has a significantly broad (long tail) polaron

absorption peak at �650nm while extended coil exhibits

intense absorption in near-infrared region.[42,43] UV–Vis

spectra suggested that as-synthesized PANI nanotubes

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were in ‘‘compact coil’’ conformation; the increase in

dopant to monomer ratio decreased the character of

‘‘compact coil’’ conformation.[33,42,44]

3.4. Thermal Properties of Polyaniline Nanotubes

Distinct thermal features were observed in TGA of PANI as

shown in Figure 7. The temperature corresponding to

weight losswas calculated from the derivative data of TGA.

There are three major transitions or three types of weight

losses for all three curves. First, the PANI lost a certain

amount of weight below 125 8C. This weight loss was

attributed to the expulsion of unbound water, volatile

impurities, and gaseous HCl which could be physically

sorbed inpolymermatrix.[45–47] The secondweight losswas

observed between 200 and 300 8C. Thisweight loss has two

possible reasons: one is the removal of ‘‘linked’’water to the

PANI backbone that also acts as a secondary dopant; the

other reason could be the removal of small fragments of

PANI, which can decompose at lower temperature than the

main PANI chains. There might be possible cross-linking of

the polymeric chains during heating between 100 and

300 8C.[48] Thefinalweight losswas observed at the onset of

525 8C; and the slope of the thermo-gravimetric curve

between500and600 8Cwasfoundtobesteeper thanthatof

between 200 and 300 8C, indicating the structural changes

of PANI. We attributed this weight loss to the thermal

decomposition of main molecular chains of PANI. In

nitrogen atmosphere, these extended aromatic structures

decomposed and formed the coke. The residues at 800 8Caccounted for highly carbonized materials and thermally

stable species. It is worthwhile to note that the thermal

curves shifted to the upper right direction as the D/M

increased from 8 to 50, suggesting increased thermal

stabilization of PANI nanostructures at higher D/M ratios.

Figure 7. Thermo-gravimetric analysis of polyaniline withdifferent dopant to monomer ratios.




3.5. Electrical Resistivity of Polyaniline as a Function

of Its Doping Level

It has been known that doping of the PANI increases its

conductivity, and the doping is more effective in nano-

structured PANI than its bulk counterpart. In general, HCl

doped PANI shows highest conductivity due to size of ion

and solvation effect of Cl� with absorbed water that

increases the conductivity of PANI.[43] Figure 8 shows the

resistivity of the PANI nanotubes as a function of doping

ratio. The increase in dopant tomonomer ratio from 8 to 50

caused 27 times or 96% reduction in resistivity. At the

dopant tomonomer ratioof 8, all protonswere consumed to

protonate the aniline monomer, resulting in formation of

weakly acidic reaction mixture. At higher dopant to

monomer ratios, there might be some unreacted protons,

which remained the acidity of the reaction mixture even

after the protonation of aniline monomer. The inset shows

the molecular scenario of the reaction mixture at high

dopant to monomer ratios where majority of the aniline

monomers are protonated. In such a strong acidicmedium,

anilinium cations are favored to form oligomers on which

charge delocalization takes place after optimum conjuga-

tion. Tanget al.38 alsoproved thishypothesis bymonitoring

the pH of reaction mixture. In FTIR, it was observed that

delocalization of p-electrons (1 367 cm�1) increases with

increase in D/M ratio, suggesting the conjugation and

subsequent increase in interconnecting network of PANI.

Thus, the significant reduction inresistivityor the increased

conductivity can be attributed to increased protonation of

PANI backbone chain, the bridging of PANI chains to form

stable bipolaron, which enhance the tunneling across the

dopant.

Figure 8. Resistivity of the polyaniline nanotubes in powder formas a function of the dopant to monomer ratios. The inset is aschematic of the reacting species in a reaction mixture at highdopant to monomer ratios.

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3.6. Performance of Flexible Electrically Conducting

Coating

Thepotential applicationof the crystalline conductivePANI

nanotubes for the conductive flexible coatings was

investigated by examining the morphologies, mechanical

integrity, and electrical resistivity of PANI (dispersed in

PVA) coatings on PET films. Although there might be some

effects of film thickness on electrical properties of the

coating,[49] 300mmthick PANI films,which is typical for the

conductive coating applications, were employed in this

work to study the effect of morphology and doping levels.

The composite contained 50wt% as-synthesized PANI

nanotubes. Generally, non-uniform dispersion of filler in

polymer matrix results in poor electrical performance and

formation of distinct conductive domains[50,51] because of

the weak compatibility of fillers with polymer matrix and

undesired strong interactions between the filler particles.

To address this concern, we tested the dispersion of PANI

nanotubes in the PVA, a typical polymer binder and

antioxidant for PANI.[52] It was found that the PANI

nanotubes could be readily dispersed into the PVA aqueous

solution; and, a layer of the uniformdispersion can be spin-

coated on the surface of the PET substrate to form a PANI

coating of 300� 15mmthick. Recently, it was reported that

the entangled PANI structure has enhanced processability

inpolymer suspensions.[53] Thus, the crystalline conductive

PANI has a good film forming properties when used with

PVA. Perhaps, the HCl dopant present on the backbone

chain of PANI induced positive charges on the backbone

chains, resulting in electrostatic repulsion among the PANI

nanotubes in colloidal dispersion. Li and Kaner[28] obtained

the similar colloidal dispersion and attributed it to the

sufficiently acidic pHof the solution,which in turn signifies

the electrostatic stabilization.

Figure 9a shows an image of the as-fabricated PANI/PVA

coating, exhibiting a visually smooth and uniform surface.

The coated film was bent in 908 as shown in Figure 9b. No

wrinkles, cracks, and delamination were spotted on the

coatings, suggesting its mechanical robustness under

Figure 9. a) An image of as-fabricated conducting film, b) an image shthe flexible electrically conducting film showing the uniform dispers




mechanical strains. The detailed surface morphologies

were examined using SEM; Figure 9c is one typical SEM

image showing the uniform dispersion and the entangled

network of PANI nanotubes, which is a desired property for

various uses in flexible electronics.

The electrical resistivity was characterized by the four-

point probe setup. The Figure 10 shows the resistivity as a

function of the dopant to monomer ratio for the PANI

(dispersed in PVA) coating on the PET substrate. As

expected, the electrical resistivity decreased as the D/M

ratio increased, showing a similar trend to that of the pure

PANI (see Figure 8). It is informative to notice the difference

in the reduction of electrical resistivity between PANI and

PANI/PVA. First, the resistivity of PANI/PVA conducting

coatingswasmuchhigher thanthatofonlyPANIbecauseof

the insulating nature of the PVA. Second, the increase in

D/M ratio from 8 to 50 caused 27 times reduction in

resistivity of PANI powder whereas 13 times reduction in

case of PANI/PVA. The observed resistivity of the film at

D/M¼ 50 is 1.72E6. This differencemaybe explained by the

insulating effect of the PVAmatrix.Note that the PANI/PVA

films are uncured in this study, and their properties may

change when exposed to different environmental con-

ditions. Future studies may need to use crosslinkers in

the PANI/PVA films to improve its durability and

other performance requirements for industrial coating

applications.

Finally, it is worthwhile to discuss the technical

implications of the highly crystalline PANI nanotubes

(emeraldine salt) as fabricated in our process with a

precisely controlled addition of the oxidantAPS anddopant

HCl in context to various technological applications. In

recent years, many researchers have attempted to synthe-

size the crystalline PANI byvariousmethods. Thus far,most

of the crystalline PANI contains only a small portion of

crystallinity with a majority of an amorphous character.

The limited transport of electrical charges in major

amorphous regions makes these ‘‘less’’ crystalline PANI

ineffective in electronic applications. Furthermore, the

history of the crystalline PANI has a vital impact on the

owing flexibility of the polyaniline coated film, and c) SEM image ofion and the entangled network of polyaniline nanotubes.

I: 10.1002/mame.201300354



Figure 10. Resistivity of the polyaniline nanotubes/PVA flexiblefilm as a function of the dopant to monomer ratios.

www.mme-journal.de


10

REa

performance of electronic applications. It is important to

know, how the crystallinity is obtained. In 1992, Laridjani

and co-workers[54,55] introduced the concept of ‘‘amorphog-

raphy – the relationship between amorphous and crystal-

line order,’’ it states that crystallinity of emeraldine salt

obtained from emeraldine base has different physical

properties than that of obtained from direct protonation

during the synthesis. Most of the electronic applications,

which require the blending of crystalline PANI emeraldine

salt with host polymer matrix involve the use of solvent,

curing agent, stabilizer, etc. In such cases, dedoping of

crystalline PANI is always a problem, which results in

formation of amorphous domains. The dedoping and

subsequent transformation from crystalline to amorphous

character is more obvious in emeraldine salt synthesized

from emeraldine base.[56,57] In this respect, the in situ

doping may provide a feasible solution to reduce the

problemof dedoping.Moreover, the highly crystalline PANI

nanotubes can be readily dispersed into a PVA matrix and

coated onto flexible films, showing its potential for large-

scale fabricationof flexible electrically conductive coatings.

4. Conclusions

In situ doped, crystalline, and conductive nanotubes of

PANI were fabricated using miniemulsion polymerization

processwithcontrolledadditionofoxidant. ‘‘In situ’’ doping

reduces the extra step of doping compared to the

conventional processes. The dropwise addition of oxidant

partially suppressed the secondary growth of PANI nano-

tubes, resulting in the formation of one-dimensional

structure. TEM analyses verified that the nanotubes were

hollow inside and have distinct crystalline boundaries.




The effects of monomer ratios D/M on the structure

and properties of the PANI nanotubes were investigated.

The length and diameter of the PANI nanotubes with

D/M¼ 8 and D/M¼ 16 were found to remain constant at

215� 19nm and 40� 12nm, respectively. The higher D/M

ratio (D/M¼ 50) reduced PANI dimensions to 134� 15nm

in length and 31� 6nm in diameter. FTIR analyses

showed that dopant molecules protonated the PANI

backbone chain and resulted in the delocalization of

p-electrons. UV–Vis absorption spectrum for each D/M

ratio showed three distinct peaks and one ‘‘free-carrier’’

long tail of the electron transitions, which is typical

absorption of the emeraldine oxidation state of PANI.

TGA revealed that the strong interactions between PANI

backbone chain and dopant molecules prevent the

thermal degradation of PANI powder up to 500 8C.Theelectrical contact resistivitywas characterizedby the

four-point probe setup as a function of theD/M ratio for the

pure PANI powders and the PANI dispersed in PVA coating.

The increase in D/M ratio from 8 to 50 caused 27 times

reduction in resistivity of PANI powder whereas 13 times

reduction in case of PANI/PVA. This reduction in resistivity

was attributed to increase in protonation and subsequent

increase in conjugated network of PANI. Morphological

analysis of dry PANI powder and its dispersion in PVA

matrix showed that smaller aggregates formed in dry PANI

but dispersion of its colloidal solution in PVA can be

significantly uniform to fabricate the flexible conducting

coating. The high crystalline conductive PANI nanotubes

were investigated further by examining themorphologies,

mechanical integrity when bent and electrical contact

resistivity of coatings on PET films. Doped PANI nanotubes

were found able to uniformly disperse in PVA matrix

resulting in good film-forming and flexible properties.

As-fabricated PANI/PAV coating did not show any cracks

when bent, demonstrating PANI’s potential application

in the conductive flexible coatings.

Acknowledgements: The authors thank the Natural Sciences andEngineering Research Council of Canada (NSERC) for the financialsupport and thank Dr. Andreas Korinek from the Canadian Centerfor Electron Microscopy, McMaster University for help with TEMmeasurements.

Received: September 11, 2013; Revised: November 26, 2013;Published online:DOI: 10.1002/mame.201300354

Keywords: conductive coating; conductive polyaniline; electricalconductive polymers; in situ doping; polyaniline nanotubes

[1] J. Janata, M. Josowicz, Nat. Mater. 2003, 2, 19.[2] C. Dhand, M. Das, G. Sumana, A. K. Srivastava, M. K. Pandey,

C. G. Kim, M. Datta, B. D. Malhotra, Nanoscale 2010, 2, 747.

I: 10.1002/mame.201300354




www.mme-journal.de

[3] K. Singh, A. Ohlan, V. H. Pham, B. R. S. Varshney, J. Jang, S. H.Hur, W. M. Choi, M. Kumar, S. K. Dhawan, B.-S. Kong, J. S.Chung, Nanoscale 2013, 5, 2411.

[4] M. A. Dar, R. K. Kotnala, V. Verma, J. Shah, W. A. Siddiqui, M.Alam, J. Phys. Chem. C 2012, 116, 5277.

[5] W. Chen, H. N. Alshareef, R. Raghavanbaby, Nanoscale 2013,5, 4134.

[6] X. Lu, W. Zhang, C. Wang, T.-C. Wen, Y. Wei, Prog. Polym. Sci.2011, 36, 671.

[7] Y. Li, Y. Fang, H. Liu, X. Wu, Y. Lu, Nanoscale 2012, 4, 2867.[8] S. E. Bourdo, V. Saini, J. Piron, I. Al-Brahim, C. Boyer, J. Rioux, V.

Bairi, A. S. Biris, T. Viswanathan, ACS Appl. Mater. Interfaces2012, 4, 363.

[9] S. Sukeerthi, A. Q. Contractor, Chem. Mater. 1998, 10, 2412.[10] S. Jafarzadeh, E. Thormann, T. R€onnevall, A. Adhikari, P.-E.

Sundell, J. Pan, P. M. Claesson, ACS Appl. Mater. Interfaces2011, 3, 1681.

[11] Y.-Z. Long, M.-M. Li, C. Gu, M. Wan, J.-L. Duvail, Z. Liu, Z. Fan,Prog. Polym. Sci. 2011, 36, 1415.

[12] D. Li, J. Huang, R. B. Kaner, Acc. Chem. Res. 2009, 42, 135.[13] J. H. Jun, K. Cho, J. Yun, K. S. Suh, T. Kim, S. Kim, Org. Electron.

2008, 9, 445.[14] J. Li, Q. Jia, J. Zhu, M. Zheng, Polym. Int. 2008, 341, 337.[15] D. Chinn, J. DuBow, M. Liess, Chem. Mater. 1995, 7, 1504.[16] E. Marie, R. Rothe, M. Antonietti, K. Landfester, Macro-

molecules 2003, 36, 3967.[17] B. J. Polk, K. Potje-Kamloth, M. Josowicz, J. Janata, J. Phys.

Chem. B 2002, 106, 11457.[18] W. Wang, S. P. Gumfekar, Q. Jiao, B. Zhao, J. Mater. Chem. C

2013, 1, 2851.[19] G. Morales, M. Llusa, M. Miras, C. Barbero, Polymer 1997, 38,

5247.[20] W. Zheng, M. Angelopoulos, A. J. Epstein, A. G. MacDiarmid,

Macromolecules 1997, 30, 2953.[21] J. Laska, J. Widlarz, Synth. Met. 2003, 135-136, 261.[22] Y.-W. Lin, T.-M. Wu, J. Appl. Polym. Sci. 2012, 126, E123.[23] Y. Li, B. Ying, L. Hong, M. Yang, Synth. Met. 2010, 160, 455.[24] L. Shao, J. Qiu,M. Liu, H. Feng, L. Lei, G. Zhang, Y. Zhao, C. Gao, L.

Qin, Synth. Met. 2011, 161, 806.[25] A. Dan, P. K. Sengupta, J. Appl. Polym. Sci. 2007, 106, 2675.[26] A. J. Dominis, G. M. Spinks, L. A. P. Kane-Maguire, G. G.

Wallace, Synth. Met. 2002, 129, 165.[27] J. Kim, S. Kwon, D. Ihm, Curr. Appl. Phys. 2007, 7, 205.[28] D. Li, R. B. Kaner, Chem. Commun. 2005, 26, 3286.[29] M. Wan, Macromol. Rapid Commun. 2009, 30, 963.[30] M. Wan, Adv. Mater. 2008, 20, 2926.[31] R. Ma�zeikiene, A. Malinauskas, Eur. Polym. J. 2000, 36, 1347.




[32] O. Rivero, C. Sanchis, F. Huerta, E. Morall�on, Phys. Chem. Chem.Phys. 2012, 14, 10271.

[33] P. K. Kahol, N. J. Pinto, E. J. Berndtsson, B. J. McCormick, J. Phys.Condens. Matter 1994, 6, 5631.

[34] D. Li, R. B. Kaner, J. Mater. Chem. 2007, 17, 2279.[35] D. Li, R. B. Kaner, J. Am. Chem. Soc. 2006, 128, 968.[36] M. Trchov�a, I. Sedenkov�a, E. N. Konyushenko, J. Stejskal, P.

Holler, G. Ciri�c-Marjanovi�c, J. Phys. Chem. B 2006, 110, 9461.[37] J. Stejskal, I. Sapurina, M. Trchov�a, E. N. Konyushenko,

Macromolecules 2008, 41, 3530.[38] S.-J. Tang, A.-T. Wang, S.-Y. Lin, K.-Y. Huang, C.-C. Yang, J.-M.

Yeh, K.-C. Chiu, Polym. J. 2011, 43, 667.[39] H. Ding, J. Shen, M. Wan, Z. Chen, Macromol. Chem. Phys.

2008, 209, 864.[40] T. Abdiryim, Z. Xiao-Gang, R. Jamal,Mater. Chem. Phys. 2005,

90, 367.[41] C. Chen, C.-F. Mao, S.-F. Su, Y.-Y. Fahn, J. Appl. Polym. Sci. 2007,

103, 3415.[42] R. Sainz, R. William, N. Young,Macromolecules 2006, 39, 7324.[43] Y. Xia, J. Wiesinger, A. G. MacDiarmid, Chem. Mater. 1995, 7,

443.[44] T. Kugler, J. R. Rasmusson, J.-E. €Osterholm, A. P. Monkman,

W. R. Salaneck, Synth. Met. 1996, 76, 181.[45] A. Wolter, P. Rannou, J. Travers, B. Gilles, D. Djurado, Phys. Rev.

B 1998, 58, 7637.[46] P. Rannou, M. Nechtschein, Synth. Met. 1997, 84, 755.[47] Y. Weir, G. Jang, K. F. Hsueh, E. M. Scherr, A. G. Macdiarmid,

A. J. Epstein, Polymer 1992, 33, 314.[48] S. Bhadra, D. Khastgir, Polym. Degrad. Stabil. 2008, 93, 1094.[49] H. Dinh, P. Van�ysek, V. Birss, J. Electrochem. Soc. 1999, 146,

3324.[50] B. M. Amoli, S. Gumfekar, A. Hu, Y. N. Zhou, B. Zhao, J. Mater.

Chem. 2012, 22, 20048.[51] S. Gumfekar, B. Meschi Amoli, A. Chen, B. Zhao, J. Polym. Sci. B:

Polym. Phys. 2013, 51, 1448.[52] A. V. Nand, S. Ray, J. Travas-Sejdic, P. A. Kilmartin, Mater.

Chem. Phys. 2012, 134, 443.[53] X.-S. Du, C.-F. Zhou, G.-T. Wang, Y.-W. Mai, Chem. Mater. 2008,

20, 3806.[54] J. P. Pouget, M. Laridjani, M. E. Jozefowicz, A. J. Epstein, E. M.

Scherr, A. G. MacDiarmid, Synth. Met. 1992, 51, 95.[55] M. Laridjani, J. P. Pouget, E. M. Scherr, A. G. MacDiarmid, M. E.

Jozefowicz, A. J. Epstein, Macromolecules 1992, 25, 4106.[56] N.-R. Chiou, L. J. Lee, A. J. Epstein, Chem. Mater. 2007, 19,

3589.[57] Z. D. Zujovic, L. Zhang, G. A. Bowmaker, P. A. Kilmartin, J.

Travas-Sejdic, Macromolecules 2008, 41, 3125.

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in situ doped polyaniline nanotubes for applications in flexible conductive coatings

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