chapter 3 synthesis of polyaniline (pani) -...

Synthesis and Characterization of Thin Films of Conducting Polymers for Gas Sensing Applications

Mr.Ravindrakumar G. Bavane, SOPS, NMU, Jalgaon (2014) 3. 1

Chapter 3

Synthesis of Polyaniline (PANI)

3.1 Introduction:

Polymer systems with unique properties are the recent fields of increasing

scientific and technical interest, offering the opportunity to synthesize a broad variety of

promising new materials, with a wide range of electrical, optical and magnetic property.

Technological uses depend crucially on the reproducible control of the molecular and

supramolecular architecture of the macromolecular via a simple methodology of organic

synthesis. Among the conducting polymer, Polyaniline (PANI) is one such polymer

whose synthesis does not require any special equipment or precautions. Conducting

polymers generally show highly reversible redox behavior with a noticeable chemical

memory and hence have been considered as prominent new materials for the fabrication

of the devices like industrial sensors. The properties of conducting polymers depend

strongly on the doping level, protonation level, ion size of dopant, and water content.

Conducting PANI is prepared either by electrochemical oxidative polymerization or by

the chemical oxidative polymerization method. The emeraldine base form of PANI is an

electrical insulator consisting of two amine nitrogen atoms followed by two imine

nitrogen atoms. PANI (emeraldine base) can be converted into a conducting form by two

different doping processes: protonic acid doping and oxidative doping. Protonic acid

doping of emeraldine base corresponds to the protonation of the imine nitrogen atoms in

which there is no electron exchange. In oxidative doping, emeraldine salt is obtained

from leucoemeraldine through electron exchanges. The mechanism causing the structural

changes is mainly recognized to the presence of -NH group in the polymer backbone,

whose protonation and deprotonation will bring about a change in the electrical

conductivity as well as in the color of the polymer. Considerable research effort is now

directed towards the development of sensors and artificial noses and electronic tongues



based on conducting materials used for the detection of chemical vapors and gases and

biological species [1].

3.2 Techniques of polymerization:

i. Bulk polymerization: The simplest method of polymerization where the reaction

mixture contains only the monomer and a monomer soluble initiator.

ii. Solution polymerization: This method is used to solve the problems associated

with the bulk polymerization because the solvent is employed to lower the viscosity

of the reaction, thus help in the heat transfer and reduce auto acceleration.

iii. Suspension polymerization: This method is used also to solve the problem of heat

transfer. It is similar to bulk polymerization where the reaction mixture is

suspended as droplets in an inert medium. Monomer, initiator and polymer must be

insoluble in the suspension media such as water.

iv. Emulsion polymerization: This is similar to suspension polymerization except that

the initiation is soluble in suspension media and insoluble in the monomer. The

reaction product is colloidally stable dispersion known as latex. The polymer

particles have diameter in the range of (0.05 - 1 m) smaller than suspension.

3.3 Conducting Polymer-Polyaniline(PANI):

PANI is the oxidative polymeric product of aniline under acidic conditions

and has been known since 1862 as aniline black [2]. Surville et. al. [3] in 1968 reported

proton exchange and redox properties with the influence of water on the conductivity of

PANI. In 1911 Mecoy and Moore [4] had suggested electrical conduction in organic

acids. However, interest in PANI was generated only after the fundamental discovery in

1977 that iodine doped polyacetylene has a metallic conductivity. PANI as a chemical

substance has been known for long time [5-6].

At the beginning of the 20th century organic chemists began investigating the

construction of aniline black and its intermediate products [7]. Wills Tatter and co-

workers in 1907 and 1909 regarded aniline black as an eight-nucleus chain compound

having indamine structure [Fig. 3. 1].



N

NNH

H

N

N

N

N

N

H H H H

H HH

Fig. 3.1: Indamine Structure

However, in 1910-12 Green and Woodhead [8] were able to report various constitutional

aspects of aniline polymerization. The conclusions of their study were as follows:

There are four quinoid stages derived from the parent compound

leucoemeraldine.

The minimum molecular weight of these primary oxidation of anilines

are in accordance with an eight-nucleus structure.

The conversion of emeraldine into nigraniline consumes one atom of oxygen.

The conversion of emeraldine into pernigraniline consumes two atoms of

oxygen.

The conversion of nigraniline into pernigraniline consumes one atom of

oxygen.

The reduction of emeraldine to leucoemeraldine consumes four atoms of

hydrogen.

The reduction of nigraniline to leucoemeraldine consumes six atoms of

hydrogen.

The reduction of pernigraniline to leucoemeraldine consumes eight atoms of

hydrogen.

These authors carried out oxidative polymerization studies using mineral acids

and oxidants such as persulphate, dichromate and chlorate and determined the oxidation

state of each constituent by redox titration using TiCl3. They also extended their studies

on the oxidative polymerization of o- and p- chloroaniline and o-anisidine and reported

that dimethylaniline remained unattacked under these experimental conditions.

This triggered research interest in new organic materials in the hope that

these would provide new and/ or improved electrical, magnetic, optic material or devices.



The hope was based on electronic structure and the combination of metal like or

semiconducting character with the processibility and flexibility of classical polymers and,

above all, the ease with which structural modification can be carried out via synthetic

organic chemistry methodologies. Among the conducting polymers, PANI has been the

most widely studied as a exclusive member for the conducting polymer family for the

following reasons.

Easy synthesis.

It is the only conducting polymer whose electronic structure and electrical

properties can reversibly be controlled by both oxidation and protonation.

It has interesting electrochemical behaviour.

It shows environment stability.

Ease of non-redox doping by protonic acids.

3.4 Structure of Polyaniline:

The protonation and deprotonation and various other physico-chemical

properties of PANI can be said to be due to the presence of the -NH- group. The general

structure of PANI can be shown above in Fig. 3.2.

Figure 3.2-General structure of PANI [1]

Green and Woodhead [5] were the first to depict PANI as a chain of aniline

molecules coupled head-to-tail at the para position of the aromatic ring. They have pro-

posed a linear octameric structure for PANI. Polyaniline, a typical phenylene based poly-

mer, has a chemically flexible –NH– group in the polymer chain flanked by phenyl rings

on either sides. The diversity in physicochemical properties of PANI is traced to the –

NH– group. Out of several possible oxidation states, the 50 % oxidized emeraldine salt

state shows electrical conductivity [8-9].



In combination with comparable results obtained with other similar polymers such

as PPy and PTh, PANI have caused a rapid increase in experimental investigations into

the mechanism and kinetic of the formation, molecular structure, electro-optical and

believable application [10].

Table 3.1. : Raman assignments of PANI [11]

Frequencies (cm-1) Assignments

1160–1180 C–H bending

1230–1255 C–N stretching

1317–1338 C–N+ stretching

1470–1490 C=N stretching

1515–1520 N–H bending

1580 C=C stretching

1600–1620 C–C stretching

3.5 Conductivity of PANI:

As mentioned below, PANI exists in three oxidation states (leucoemeraldine,

emeraldine and pernigraniline forms) that differ in chemical and physical properties [12].

Only the green protonated emeraldine has conductivity on a semiconductor level of the

order of 100 S cm-1, many orders of magnitude higher than that of common polymers

(<10-9 S cm-1) but lower than that of typical metals (>104 S cm-1). Protonated PANI

converts to a non-conducting emeraldine base when treated with alkali solutions (Fig.

3.3) [13].



Figure 3.3: Emeraldine salt is protonated in the alkaline medium to emeraldine base. A- is arbitrary ion, e.g., chloride

The conductivity of PANI can be changed by doping, and spans a very wide range (<10-12

to 105 S cm-1) depending on the level of doping [14]. The changes in physicochemical

properties of PANI occurring in response to various external stimuli are used in various

applications, e.g., in sensors and actuators [15]. Other uses are based on the combination

of electrical properties typical of semiconductors with materials properties characteristic

of polymers, like the development of “plastic” microelectronics, electrochromic devices.

The establishment of the physical properties of PANI reflecting the conditions of

preparation is thus of fundamental importance [16]. Polyaniline (PANI) and poly

(ethylene dioxythiophene) (PEDT) have a much higher conductivity and stability,

combined with a low absorption as compare to alkoxy-substituted polythiophenes. It is

postulated that, this is due to the fact that the doping process in these materials is

different, involving no shifts in absorption peaks and leading to low-lying near IR

absorption bands at less than 0.6eV. [17]. During the course of polymerization reaction,

these cations of intermediate stability dimerize, and further radical coupling reaction

leads to the formation of green PANI [18].



3.6 Interconversion of different oxidation states of PANI (redox procedure):

The oxidation of monomeric aniline, either electrochemically or in acidic

solution, leads to stable polymers in at least three distinct oxidation states. In 1985, Alan

MacDiarmid and co-workers published a remarkable paper in this context [19].

Figure 3.4: Various possible oxidation states of PANI [20]

The difference in the composition of amine and imine segments of PANI generates

several oxidation states of this material ranging from completely reduced

leucoemeraldine to completely oxidized pernigraniline states as shown in Fig. 3.4. The

different forms of PANI can be readily converted to one another by simple redox

methods (Fig. 3.5).



Figure 3.5: Interconversion of different oxidation states of PANI (redox procedure) [20]

Table 3.2 The different forms of PANI [21]

Type of form Name Colour Conductivity

S cm−1

Reduced form

Polyleucoemeraldine base Transparent <10−5

Polyprotoemeraldine base Transparent <10−5

Polyemeraldine base Blue <10−5

Polynigraniline base Blue <10−5

Oxidized form Polypernigraniline base Purple <10−5

Polyemeraldine salt Green ~15

The conductive form of PANI is the protonated polyemeraldine or polyemeraldine salt

whose color is green and the conductivity is around 15 S cm−1 [22], whereas the

conductivity of polyemeraldine base is around 10−5 S cm−1. Note that the conductivity of

a metal is around 103 S cm−1.



3.7 Synthesis of PANI:

There are several reports of PANI found in the literature over the decades about

the structure and constitutional aspect of aniline polymerization. The most universal

synthesis of PANI involves oxidative polymerization, in which the polymerization and

doping occurs at the same time, and may be accomplished either electrochemically or

chemically. Electrochemical methods tend to have lower yields than chemical yields [22].

3.7.1 Chemical Synthesis (Oxidative polymerization):

Synthesis of PANI by chemical oxidation way involves the use of either

hydrochloric or sulfuric acid in the presence of ammonium persulfate as the oxidizing

agent in the aqueous medium. The principal function of the oxidant is to withdraw a

proton from an aniline molecule, without forming a strong co-ordination bond either with

the substrate / intermediate or with the final product (Fig. 3.6). However smaller quantity

of oxidant is used to avoid oxidative degradation of the polymer formed. Polymer chains

proceeds by a redox process between the growing chain and aniline with addition of

monomer to the chain end. The high concentration of a strong oxidant, (NH4)2S2O8, at the

initial stage of the polymerization enables the fast oxidation of oligomers and polyaniline,

as well as their existence in the oxidized form.

N

H

H N

H

H N H

H

N

H

H N H

H

N

H

N

H

..

e-- +

+..

Aniline

..

+.

Aniline

+ .. ..

Polyaniline

2H+

Figure 3.6: Homopolymerization of PANI [23].



PANI is mostly synthesized by aniline oxidation either with a chemical oxidant (chemical

route) or through electrochemistry. Other original synthesis is proposed like plasma

Polymerization [24], autocatalytic polymerization [25] or inverse emulsion

polymerization [26].

Chemical synthesis requires three reactants: aniline, an acidic medium (aqueous

or organic) and an oxidant. The more common acids are essentially hydrochloric acid

(HCl) and sulfuric acid (H2SO4). Ammonium persulfate ((NH4)2S2O8), potassium

dichromate (K2Cr2O7), cerium sulfate (Ce(SO4)2), sodium vanadate (NaVO3), potassium

ferricyanide (K3(Fe(CN)6)), potassium iodate (KIO3), hydrogen peroxide (H2O2) are

recommended as oxidants [27]. However, the more popular synthesis is run with a 1 mol

aqueous hydrochloric acid solution (pH between 0 and 2), ammonium persulfate as

oxidant with an oxidant/aniline molar ratio ≤1.15 in order to obtain high conductivity and

yield [28]. The solution temperature is comprised between 0 and 2 ◦C in order to limit

secondary reactions [29]. The duration of the reaction varies generally between 1 and 2

hr. The experimental part consists of adding slowly (even drop by drop) the aqueous

ammonium persulfate solution to the aniline/HCl solution, both solutions being pre-

cooled to nearly 0 ◦C. The mixture is stirred for about 1 hr. The obtained precipitate is

removed by filtration and washed repeatedly with HCl and dried under vacuum for 48 hr

[28].The obtained material is polyemeraldine salt: polyemeraldine hydrochloride (PANI-

HCl), green colored. To obtain polyemeraldine base, polyemeraldine hydrochloride is

treated in an aqueous ammonium hydroxide solution for about 15 hr. The obtained

powder is washed and dried.

3.7.2 Electrochemical synthesis:

The electrochemical synthesis of conducting polymer is an electro-organic

process rather than an organic electrochemical one, because the more emphasis is on the

electrochemistry and electrochemical process rather than on organic synthesis.

Electrochemistry has contributed significantly to the developments in conducting

polymers. In most of the applications, it is essential to synthesize polymers into a thin



film of well defined structure, preferably with a large area. For preparation of such films,

electrochemical synthesis is a standard method [30-34]. The conducting polymers which

are not easily processed, when prepared by chemical routes, are synthesized in the form

of films adhering to the electrode, so that a study of the optical and electrical properties

can be carried out in-situ by using electroanalytical techniques. The electrochemical

synthesis of conducting polymers is similar to the electrodeposition of metals from an

electrolyte bath; the polymer is deposited on the electrode surface and also in the in-situ

doped form.

Three electrochemical methods can be used to PANI synthesis-

a) Galvanostatic method when applied a constant current,

b) Potentiostatic method with a constant potential,

c) Potentiodynamic method where current and potential varies with time.

Whatever the method is, a three-electrode assembly composes the reactor vessel:

a working electrode on which the polymer is deposited, a counter electrode also named

auxiliary electrode (platinum grid) and a reference electrode (in most cases, a saturated

calomel electrode (SCE)).

The more common working electrode is a platinum one, but PANI depositions have also

been realized onto conducting glass (glass covered by indium-doped tin oxide (ITO)

electrode), Fe, Cu, Au, graphite, stainless steel, etc [28]. PANI can be then peeled off

from the electrode surface by immersion in an acidic solution.

As compared to chemical synthesis, this route presents several advantages [35] as

cleanness because no extraction from the monomer–solvent–oxidant mixture is

necessary, doping and thickness control via electrode potential, simultaneous synthesis

and deposition of PANI thin layer.

The electrochemical synthesis route offers many advantages over the chemical

method listed below.

It is simple and less expensive technique. Therefore, electrodeposition of

conducting polymer on oxidizable conducting glass is extremely

economical.



Unlike chemical method, there is no need of catalyst and therefore, the

electrodeposited polymers and co-polymers are essentially pure and

homogeneous.

Doping of the polymer with desired ion can be considered simultaneously

by changing the nature of ions in the solution.

The conducting polymers can be obtained directly in thin film forms as

coating on electrodes and the properties of these coatings can be controlled

effectively by proper choice of the electrochemical process variables.

Reduction in the possible pollution by adopting the suitable system for

electropolymerization using modern sophisticated instrument.

The electrochemical synthesis is normally carried out in a single compartment

cell. The cell consists of the electrodes, electrolyte and power supply.

3.8 Polymerization mechanism of PANI:

Figure 3.7: Formation of the aniline radical cation

The various methods of polyaniline synthesis stimulate a multitude of

polymerization mechanisms of aniline. The electrochemical polymerization mechanism

seems to be the most investigated compared to the chemical one [28]. However, a close

similarity can be considered for chemical and electrochemical processes. The synthesis

mechanism corresponds to a polycondensation because it proceeds by steps. The first

most feasible oxidation step corresponds to the radical cation formation by an electron

transfer from the 2 s energy level of the aniline nitrogen atom as shown in Fig. 3.7,

whatever the pH value is. As a kinetic point of view, it is the limiting step and a catalyst

may accelerate it. Then, the reaction is autocatalyzed. This aniline radical cation has three

resonance forms, given in Fig.3.8. Among these three resonance forms, the form (2) is



the more reactive one because, on the one hand, of its important substituent inductive

effect, and on the other hand, of its absence of steric hindrance.

Figure 3.8-Resonance forms of the aniline radical cation

The next step in Fig.3.8, at the least in acidic medium, would be the reaction between

the radical cation and the resonance form (2), the so-called “head to tail” reaction,

favored in acidic medium (aqueous or organic) and corresponds to the dimer formation

[36-37].

Figure 3.9-Dimer formation

Figure 3.10 -Formation of the radical cation dimer



Next step, the dimer is oxidized to form a new radical cation, as shown in Fig. 3.9.The

formed radical cation can react either with the radical cation monomer or with the radical

cation dimer to form, respectively, a trimer or a tetramer, according to the mechanism

proposed previously, and this up to the polymer Fig. 3.11.

Figure 3.11- A way of polymer synthesis

For a long time, the PANI was supposed to be an octamer. However, the formation of a

longer chain is now proved with an average molar mass evaluated to be more than 104 g

mol−1 [38].

3.9 PANI doping:

The PANI must be doped if associated to electronic conducting polymers. The

term “doping” is employed here by analogy with semiconductors like silicon or

germanium in which atoms like phosphorous or boron are introduced. Conducting

polymer doping consists to insert into the polymer, electron acceptor molecules

(oxidation) or electron donor molecules (reduction). The obtained polymer is then

considered as a p-type or n-type one, respectively.



PANI is a specific conducting polymer because of its conducting

mechanism induced either by the oxidation of the polyleucoemeraldine base or by the

protonation of the polyemeraldine base. The two routes are shown in Fig.3.12.

Figure 3.12:-Doping mechanisms of PANI

3.9.1 Oxidative doping:

Figure 3.13 (a) Oxidative doping with Cl2

The oxidative doping is realized through chemical or electrochemical processes

from the totally reduced form of PANI: polyleucoemeraldine base as shown in Fig. 3.13



indicates a way of polymer synthesis. Polyleucoemeraldine base is prepared by reduction

of polyemeraldine salt with phenylhydrazine or hydrazine solutions by dipping for 5 or 6

min [39].The chemical oxidative doping is run either with a chlorine or a less toxic iodine

agents in a carbon tetrachloride solution, or with (NO)+ (PF6)−, FeCl3 or SnCl4 organic

solutions, or with oxygen or hydrogen peroxide in an aqueous acidic solution. The

following examples illustrate the oxidative doping with Cl2: Fig.3.13 (a) and also with

H2O2 in an acidic solution HA (Fig. 3.13(b)):

Figure 3.13 (b) Doping with H2O2 in an acidic solution HA:

In the latter case, the nature of the counter-ion A− (Cl−, HSO4 −, H2PO4

−) is controlled.

When Cl2 is considered both as the oxidant and the dopant, H2O2 only oxidizes PANI

doped with the acidic solution. Although chemical doping is a straightforward and well-

organized process, but the control of the doping level δ is difficult. Electrochemical

doping solves this problem since the doping level is determined by the voltage applied

between the conducting polymer and the counter Electrode [40]. The electrochemical

doping of polyleucoemeraldine base can be written as Fig. 3.14:



Figure 3.14: Electrochemical doping of polyleucoemeraldine

The counter-ions A− of the electrolytic solution are inserted in the polymer backbone.

3.9.2 Acidic doping:

The acidic doping is a exceptional doping case, where no change of the number of

electrons associated with the polymer backbone occurs. The acidic doping consists to

treat the emeraldine base with a strong acid (HCl, H2SO4) that induces the protonation of

the imine sites to give the polyemeraldine salt, through a mechanism illustrated in

Fig.3.15.

Figure 3.15: Mechanism of acidic doping



Considering the resonant structures, charge and spin can be largely delocalized that

explains the observed conductivity (as shown in above Table 3.2). However, drawbacks

of hydrochloride polyemeraldine salt are its poor solubility in most common solvents,

and its conductivity alteration with moisture and temperature. Then to improve the

solubility of conducting polymers and their temperature stability (nearly up to 200 ◦C),

few approaches have been developed. One of them implies the use of dopants other than

HCl in the monomer solution. So, polyacrylic acid [41] or other polymeric acids, acrylic

acid (AA) [42], etc. have been added as dopants in the monomer solution.

3.10 Synthesis of nanoparticles of polyaniline (PANI) using emulsion

polymerization method:

Synthesis of nano PANI particle by waterborne latex is much better than the

synthesis based on organic solvent or strong acid because the universal solvent water in

the waterborne PANI latex does not pollute the environment. The waterborne latexes,

thus, were found to be highly suitable and they avoid the use of organic solvents or strong

acids under environmentally benign conditions. It is well known that the PANI has very

partial solubility in common organic solvents and water, preventing its use in the coating

industries [43]. To solve this difficult processing problem, several modifications, such as

inserting substituent either on phenyl ring or on the nitrogen [44-45], surface

modification with inorganic pigment [46], blend and composite [47-48], and dispersion

of nano particles in various binders [49], have been suggested. The design and production

of PANI-based coating systems with commercial viability require a minimum possible

agglomeration of ICP, well-dispersed nanoparticles (70–100 nm) of uniform size and

having superior adhesion [50]. Development of conducting polymeric nanoparticles

based corrosion inhibitors with self-cleaning properties, discoloration resistance, high

scratch as well as wear resistance and environmentally benign synthesis of conducting

polymer are expected to cause a major revolution in the world of corrosion prevention

[51].



3.10.1 Advantages of emulsion polymerization:

i) Easy heat removal and control.

ii) The polymer is obtained in a convenient, easily handled and often directly useful form.

iii) High molecular weight can be obtained.

iv) The very small particles formed resist agglomeration thus allowing the preparation of

tacky polymers.

v) Inverse phase (water in oil) emulsions are possible.



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