Preparation and Charge Transport Studies of Chemically

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Preparation and charge transport studies of chemicallysynthesized polyaniline

Atul Kapil Manish Taunk Subhash Chand

Received: 12 May 2009 / Accepted: 20 June 2009 / Published online: 4 July 2009

Springer Science+Business Media, LLC 2009

Abstract Polyaniline doped with p-toluenesulfonic acid

was synthesized using in situ chemical oxidation methodfor optimization of synthesis parameters. For p-toluene-

sulfonic acid/aniline molar ratio of 5, the obtained polymer

exhibits highest value of the electrical conductivity. The

electrical conductivity of polyaniline doped with p-tolu-

enesulfonic acid was measured in the temperature range of

30300 K. The conductivity of polyaniline was found to

increase with rise in the temperature. The measured con-

ductivity versus temperature data was fitted with Arrhenius

model, variable range hopping (VRH) model and Kivelson

model in order to investigate the charge transfer mecha-

nism in polyaniline. It is shown that conductivity observed

over wide temperature range of 30300 K follows Kivel-

son model obeying power law behavior.

1 Introduction

Polymers have been traditionally considered as insulators.

Since the discovery of conductivity in polyacetylene by

Shirakawa, Heeger and MacDiarmid, a huge attention has

been devoted to synthesize new intrinsically conducting

polymers and to enhance their properties for various

applications. Conducting polymers have a variety of

applications in display devices, corrosion protective coat-

ings, rechargeable batteries and sensors, etc. [15]. During

the past two decades, a variety of electrically conducting

polymers have been studied due to their exclusive

properties.Among all conducting polymers, polyaniline (PANI) has

attracted great attention because of its electronic, electro-

chemical, optical properties and good environmental and

thermal stability [69]. The advantage of the conducting

polymers is that their synthesis and chemical modification

offer unlimited possibilities unlike inorganic metals and

semiconductors. The conducting polymers exhibits very

low charge carrier mobilities. The invention of dopability

of conjugated polymers has stimulated widespread interest

in the study of charge transport among investigators. The

intrinsically insulating conjugated polymers can be doped

to produce near metallic conductivities. It has been possi-

ble to reduce the structural disorder in doped conducting

polymers by choosing optimum parameters during syn-

thesis. It is worthwhile to mention that reduced structural

disorder helps in increasing the electrical conductivity of

the polymers [10]. A good number of studies on conduc-

tivity of PANI with the goal to improve the understanding

of the electronic transport have been performed during the

last decade [1115]. It is the disorder-induced localization

that acts as a main factor to determine the transport prop-

erties of conducting polymers. Properties such as degree of

environmental stability, solubility and dopability have

made doped PANI the most suitable material for studies of

electron transport phenomenon in conducting polymers

[1619]. In this work, emphasis has been given on making

an extensive study on the transport properties in polyani-

line at low temperature.

Investigating new preparation methods and optimizing

the synthesis parameters/conditions to obtain polymer is an

important issue in conducting polymers, because it may

lead to improve their properties. In the present work, we

report the chemical polymerization of aniline doped with

A. Kapil M. Taunk S. Chand (&)Department of Applied Sciences & Humanities, National

Institute of Technology, Hamirpur, HP 177005, India

e-mail: [email protected]

A. Kapil

e-mail: [email protected]

123

J Mater Sci: Mater Electron (2010) 21:399404

DOI 10.1007/s10854-009-9931-2


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p-toluenesulfonic acid (PTSA) and its characterization by

spectroscopic and conductivity studies.

2 Experimental

2.1 Materials

Aniline (monomer, ANI), Ammoniumperoxydisulfate (APS),

p-toluene sulfonic acid (PTSA), N,N-Dimethylformamide

(DMF), and 1-Methy-2-pyrrolidone (NMP) of analytical

grade were used as received from Alfa Aesar.

2.2 Synthesis

PANI was synthesized by oxidative polymerization of

monomer aniline in acidic medium by using APS. All the

solutions were prepared in de-ionized water having resis-

tivity of*18 MX. In a typical procedure, the monomer

aniline (0.005 mol) was dissolved in DMF (3 ml) andcooled down to 05C. It was then slowly added to 50 ml

aqueous solution of PTSA (0.025 mol). The polymerization

was initiated by the drop wise addition of the oxidant

solution containing 0.005 mol of APS dissolved in 50 ml of

water pre-cooled at 05C (ice temperature). The poly-

merization was allowed to proceed in open ambient for 5 h

with continuous stirring. Dark green colored precipitate of

the polymer thus obtained was separated by filtration,

washed with de-ionized water and finally dried in an oven at

45C for about 36 h to remove moisture/water. Un-doped

PANI was obtained by treating the powder with aqueous

ammonia solution for 5 h. Dry powder of polymer was

compressed in the form of pellets of*10 mm diameter

using a steel die in a hydraulic press.

2.3 Measurements

The absorption spectra of the polymer in NMP were

recorded by using UVVisible spectrophotometer in the

wavelength range of 300900 nm. FT-IR spectra of the

polymers in form of pellet were taken on a spectropho-

tometer model PerkinElmer Spectrum 2000 in the wave

number range 5004,000 cm-1.

Electrical conductivity measurements were performed

by standard four-probe technique. A Keithley source meter

(model 2400) and a Keithley electrometer (model 6514)

were used as constant current source and volt meter,

respectively, in four probe setup.

The conductivity (r) was calculated from the measured

current and voltage data using the relation [20].

r ln 2

pd

I

V

1

Where I, Vand dare applied current, measured voltage and

thickness of the pellet, respectively.

3 Results and discussion

The electrical conductivity of conjugated polymers mainly

depends upon the dopant. Synthesis of the polymer was

carried out at different dopant concentrations. Figure 1

shows the variation of electrical conductivity with the

dopant to monomer molar ratio used in the synthesis of

polyaniline. It is clear from the Fig. 1 that the conductivity

increases with increase in the dopant to monomer ratio and

attains maximum value for the ratio of 5. This increase in

the conductivity is attributed to the protonation of the

polymer. Beyond dopant to monomer ratio of 5, the elec-

trical conductivity slightly decreased. Another factor which

affects the electrical conductivity of the polymer is the

oxidizing agent. The conductivity as a function of oxidantto monomer molar ratio is shown in Fig. 2. As evident

from Fig. 2, the electrical conductivity of the polyaniline

was found to increase initially at low oxidant to monomer

ratio and then decreased after attaining maximum value at

an optimum molar ratio of 1.

Thus, for oxidant to monomer molar ratio of unity, the

resulting polyaniline exhibits maximum conductivity. The

higher concentration of the oxidant in the polymer solution

may cause shorter conjugation length, which leads to a

decrease in the electrical conductivity [21]. The oxidant

may also affect the yield of the polymer obtained after

synthesis. The variation in the yield of the polyaniline as afunction of oxidant to monomer ratio is shown in Fig. 2.

The yield of the polymer increases with increase in the

oxidant to monomer ratio.

The UVVis absorption spectrum of PANI in the doped

and undoped form is shown in Fig. 3. Un-doped PANI

0

5

10

15

20

25

30

35

0 2 4 6 8

Dopant/monomer ratio

Conductivit

y(Scm-1)

Fig. 1 Dependence of electrical conductivity on dopant/monomer

ratio

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exhibits absorption bands with kmax at 330 nm and 630 nm

approximately. The broad absorption at 630 nm for

polyaniline has been assigned to quinoid formation in

the backbone of the polymer [22]. The band around

285350 nm is assigned to the pp* electronic transition of

the benzene rings in the polymer backbone. Characteristic

absorbance maxima in the doped polymer are observed at

425 nm and 822 nm in the spectra of PANI [23]. They are

due to the polaron band transitions in the doped

polyaniline.

Figure 4 shows the FT-IR spectrum of polyaniline

doped with p-toluenesulfonic acid. In the spectrum, the

peaks at 1,467, 1,548 cm-1 indicate that the aromatic ring

is retained in the polymer. The band at 1,467 cm-1 cor-

responds to the stretching of benzene rings and the peak at

1,548 cm-1 corresponds to the stretching frequency ofquinone ring. The presence of these two bands clearly

shows that the polymer is composed of the amine and

imine units [24, 25]. It also exhibits distinct peak at

2,924 cm-1, which is assigned to the aromatic CH

stretching. The peak at 1,294 cm-1 corresponds to the CH

in-plane deformation [26]. Also the peak at 1,116 cm-1

corresponds to the sulfonic acid group. The presence of

vibration band of the dopant ion and other characteristic

bands confirm that the polymer is in the conducting

emeraldine salt phase. The bands at about 561 cm-1 are

assigned to the out-of-plane CH bending motions of the

aromatic rings. The peak at 790 cm-1 is characteristic ofpara-substituted aromatic rings, which indicates the for-

mation of polymer [27].

Measurements of the d.c. conductivity of PTSA doped

polyaniline in the pellet form have been carried out in the

temperature range 30300 K. Figure 5 shows the variation

in electrical conductivity with temperature. It is evident

from the Fig. 5 that the conductivity increases with

increase in the temperature.

The observed d.c. conductivity data shown in Fig. 5 has

been interpreted in the light of different models so as to

find the probable mechanism of current transport in these

polymers. Arrhenius model was used to fit the experimental

data. Figure 6 depicts plots of dc conductivity as a function

of reciprocal of the temperature for polyaniline. The data

fits linearly for T C170 K and shows deviation from the

linear behaviour below this temperature. Thus, the Arrhe-

nius model can explain temperature dependence of

0

5

10

15

20

25

30

35

0.2 0.6 1 1.5 2

Oxidant/monomer ratio

Conductivity(Scm-1)

70

75

80

85

90

Yield%

Fig. 2 Dependence of electrical conductivity and yield on the

oxidant to monomer ratio

250 350 450 550 650 750 850 950

Wavelength (nm)

Absorbance(A

rb.units)

Undoped Doped

Fig. 3 UVVIS absorption spectra of doped and undoped PANI

50010001500200025003000

Wavenumber (cm -1)

Transmittance(Arb.units)

Fig. 4 FTIR spectra of PANI for the emeraldine salt form

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400

T (K)

(

Scm

-1)

Fig. 5 Dependence of electrical conductivity on temperature for

dopant to monomer ratio of one

J Mater Sci: Mater Electron (2010) 21:399404 401

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conductivity only in the high temperature range i.e., above

170 K.

It is worthwhile to mention that Motts variable rangehopping (VRH) model has been extensively applied to

various amorphous inorganic semiconductors over the last

two decades and recently has also been applied to organic

conductors/semiconductors. According to VRH, the tem-

perature dependence of dc conductivity follows the for-

mula [2830]:

r T r0 exp T0

T

c2

where the parameter r0 can be considered as the limiting

value of conductivity at infinite temperature, T0 is the Mott

characteristic temperature and the exponent c is related tothe dimensionality d of the transport process via the

equation c = 1/(1 ? d), where d= 1, 2 and 3 for one

dimensional, two dimensional and three dimensional con-

duction process, respectively. It is evident from the Fig. 7,

that Motts three dimensional VRH model fits to

experimental data in the temperature range 90300 K.

Below 90 K, the data deviates from VRH model and thus

indicates that this model is applicable only at temperature

above 90 K. Below temperature 90 K, the VRH can not

account for current transport process.

In order to explain the current transport in the entire

temperature range, the experimental data was fitted to

Kivelson model which is also known as power law behavior[28]. According to the power law behavior, the conductivity

as a function of temperature is expressed as [28, 31]:

r T ATn 3

where A is some constant and n is the exponent.

According to this law, ln(r) versus ln(T) plot must be a

straight line with a slope of n. Figure 8a shows the con-

ductivity versus temperature plot on logarithmic scale for

polyaniline to check the power law behavior. The ln(r)

verses ln(T) plot indicates that the power law behaviour fits

closely to entire experimental data in the range 30300 K

for all these samples of polyaniline with various dopant tomonomer ratios. The data fits well in a straight line over

entire temperature range with the square of coefficient of

correlation R2 = 0.999 very close to unity. The experi-

mental data was also fitted on linear scale with power law

Eq. (3). The typical plot of conductivity for dopant to

Fig. 6 Variation of conductivity of PANI with T-1 showing appli-

cation of Arrhenius model to polyaniline. Conductivity is plotted in

logarithmic scale

Fig. 7 Variation of conductivity of PANI with T-1/4 showing 3D

variable range hopping. Conductivity is plotted in logarithmic scale

y = 9E-09x 3.175

R2 = 0.9994

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 50 100 150 200 250 300 350

T (K)

In T (K)

(

Scm

-1)

(

Scm

In

-1)

b

a

Fig. 8 a Application of Kivelson model to Polyaniline; b power law

fit of conductivity data for dopant to monomer ratio of one


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monomer ratio of one is shown in Fig. 8b. This figure also

shows that data follows Eq. (3) very closely with R2 value

of 0.999. The values of exponent n obtained for three

samples in our study are 3.17, 2.89 and 2.60 for dopant to

monomer ratio of 1, 2 and 3, respectively. It is clear from

Eq. (3) that exponent indicates the rate of increase of

conductivity with temperature. These values of n are

found to increase with increase in the dopant to monomerratio. It indicates that rate of increase of conductivity with

temperature decreases with increasing dopant concentra-

tion. It means that for higher dopant concentration, increase

in temperature produces hindrance to polarons and thus

rate of rise in conductivity decreases.

It is clear from above analysis that the Arrhenius model

of current transport is valid only in the higher temperature

range e.g., above*170 K for these polymers. This obser-

vation is consistent with the results reported in the literature

for PANI [32, 33], where conductivity at higher temperature

is attributed to the Arrhenius mechanism. In the temperature

range below room temperature to about 90 K, the MottsVRH model can explain the temperature dependence of

conductivity. The similar observations are reported in the

literature in which the VRH model is invoked to fit the

measured conductivity data in the temperature range from

300 K to down up to 77 K [34, 35]. At temperature below

77 K, very few reports exist in the literature [32, 36], where

Kivelson model representing the power law behavior is

invoked to fit the observed conductivity.

Our measurements over wide temperature range of

30300 K thus enable us to explore current transport

mechanism in these polymers. It is evident from the above

analysis that in the higher temperature range, the Arrheniusmodel can best explain the current transport in these poly-

mers. In addition to this, other two models namely VRH and

Kivelson behavior can also be attributed in this temperature

range. However, below about 170 K, both VRH and power

law behavior can be used to represent observed conductivity

variation with temperature. However, the VRH model

remains operative down up to temperature 90 K. Below

90 K, neither Arrhenius nor VRH model can explain the

observed conductivity variation with temperature but power

law behavior fits well with the data over entire temperature

range. Thus it can be concluded from this study that power

law behavior describes the electrical conduction over wide

temperature range, whereas Motts VRH model and

Arrhenius model can describe it only in the mid and higher

temperature ranges, respectively.

The thermal stability of the electrical conductivity of the

PTSA doped polyaniline was investigated at several tem-

peratures as shown in Fig. 9. It is observed that conduc-

tivity slightly decreases for samples studied at higher

temperature except for the room temperature 25C. A

slight noticeable decrease in conductivity was observed at

100C. It is reported in the literature that water/moisture

plays an important role in enhancing the conductivity of

polymers [37]. So, on heating the material, the removal of

water takes place, which leads to a slight decrease in theelectrical conductivity.

4 Conclusions

Polyaniline doped with p-toluenesulfonic acid was syn-

thesized by chemical polymerization method using am-

moniumpersulfate as an oxidizing agent. An in-depth study

has been made on the electron transport properties of PTSA

doped polyaniline in the temperature range of 30300 K.

The conductivity of samples shows an increasing trend

with temperature and the variation is noticeably higher at

higher temperature. It was observed that power law

behavior describes the electrical conduction over wide

temperature range, whereas Motts VRH model and

Arrhenius model can be attributed to describe it only in the

mid and higher temperature range, respectively.

Acknowledgments Author Atul Kapil gratefully acknowledges the

financial support provided by the Ministry of Human Resource and

Development, New Delhi, India, for carrying out this research work.

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