04 chapter 1.unlocked

8/19/2019 04 Chapter 1.Unlocked

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1. Introduction

1.1 Conducting polymers

The insulating properties of most polymers represent a significant advantage for

many practical applications of plastics. During the last 20 years, however, organic

polymers characterized by good electrical conductivity have been found. Due to their low

specific weight, good processibility and resistance to corrosion and the exciting prospects

for plastics fabricated into electrical wires, films or electronic devices, these materials

have attracted the interest of both industrial and academic researchers in domains ranging

from Chemistry to Solid State Physics and Electrochemistry. The close interaction

between scientists from diverse background has been a significant factor in the rapid

development of the field of conducting polymers.

The discovery of doping in conducting polymers has led to further dramatic

increase in the conductivity of such conjugated polymers to values as high as 105

Scm-1. Discovery and development of conducting polymers has opened up new frontiers

in Materials Chemistry and Physics. This new generation of polymers combines the

mechanical properties and processibility of traditional polymers with electrical and

optical properties which are unknown earlier. The enormous technological potential that

this rare combination offers is beginning to be trapped. The Noble prize awarded in

Chemistry to Alan Heeger, Alan MacDiarmid and Hideki Shirakawa for the discovery

and development of conducting polymers in the year 2000 is a grand recognition of the

“dawn of the new plastic age” [1].


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The search for conducting polymers, incidentally goes back to the days of Natta

(of Zeigler – Natta catalyst fame), who polymerized acetylene gas using his newly

developed catalyst, that revolutionized the plastic industry by providing a route of

synthesis of polypropylene (Zeigler and Natta jointly won the Noble Prize in Chemistry

in 1963 for their discovery). What they obtained from acetylene polymerization was a

black powder which looks much like charcoal. With the anticipation that such conjugated

structure would exhibit electrical conductivity, the conductivities (of pressed pellets)

measured in their samples were found to be semi-conducting (10-7 Scm-1).

In early 1970’s Hideki Shirakawa and co-workers at the University of Tsukuba,

Japan utilized the Zeigler - Natta polymerization technique to prepare high quality films

of polyacetylene. Alan Heeger and Alan Mac Diarmid at the University of Pennsylvania,

USA focused their attention towards purely organic polymers prepared by Shirakawa.

This collaboration led to some remarkable development that opened up new avenues in

Materials Chemistry and Physics. Measurements in the laboratory of Heeger showed an

outstanding enhancement of conductivity of trans-polyacetylene on treatment with

halogens. The highest conductivity recorded was 38 Scm-1

. This was the first conducting

polymer capable of conducting electricity.

The relative conductivities of some of these polymers synthesized are shown in

figure 1.1


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10

106

105

104

103

102

10

Conductivity(Scm-1)

Copper metal

Polyacetylene dopedwith AsF5

Polyacetylene dopedwith I2

Liquid mercury

Poly(p-phenylene)doped with AsF5

Polypyrrole doped

with I2

Polyaniline

(emeraldine)

Figure. 1.1. Logarithmic conductivities of various conducting polymers


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Until recently, polymers and electrical conduction were thought to be mutually

exclusive. However, this view was proved to be incorrect with the synthesis of

conducting polymers in the 1970’s. Unusual properties of these polymers have led to

extensive research resulting in better understanding and numerous commercial

applications. Conductive polymers can be made by filling an insulating polymer matrix

with conducting particles such as carbon black, metal flakes, or metallised fibres, or by

chemical and electrochemical synthesis methods to produce intrinsically conducting

polymers. The conductivity of the former is provided by the filler material, and the

function of the polymer matrix is to hold the material together in one piece. These

conductive composites often replace metals when light weight, toughness, shapeability

and corrosion resistance are required for the application. However, a considerably high

concentration of the conducting filler is required to achieve acceptable levels of electrical

conductivity, thus giving rise to poor mechanical properties in these composites.

Conductivity in these materials is not an intrinsic property of the polymer chains but a

property of the material as a whole.

The term “Intrinsically Conducting” refers to a polymer the conductivity of which

is a property originating from its own electronic structure. A common feature of

intrinsically conducting polymers (ICP) is the alternation of the double and single carbon

bonds along the polymer backbone, referred to as – bond conjugation. The conductivity

is due to four conditions in their molecular organization: namely, the existence of charge

carriers, an overlap of molecular orbitals to aid carrier mobility, – bond mobility and

charge hopping between polymer chains [2].


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Intrinsically conducting polymers (ICP) possesses the unique property of wide

ranging modification of their conductivity by the variation of electrolyte dopant anion

concentration during electrochemical polymerization. Undoped conjugated polymers are

insulating. However, conductivity can be increased by incorporating dopant counterions

during polymerization. Small concentration of the dopant anion results in semi-

conducting polymer with significant band gaps, whereas high dopant concentrations give

rise to highly conducting polymers. That is why highly doped conducting polymers are

often referred to as “synthetic metals” [3].

Although unstable, the most conductive polymer is polyacetylene. Conductivities

up to 104 Scm-1 have been reported by Shirakawa et. al. [4]. Pure polyacetylene is the

most semi-conducting. Conductivity is achieved by chemical doping with an oxidizing

agent such as iodine. The most stable polymers among ICPs are polyheterocycles

(polypyrrole and polythiophene). These polymers consist of five-membered cyclic ring

molecules with nitrogen or sulphur heteroatom. Pyrrole or thiophene monomers are

ideally linked at ∝ - ∝' positions (lowest energy bonding) which provides free – bond

mobility.

Most ICPs are unprocessible. Therefore, physical properties of the polymer are

determined at the synthesis stage. For example, the electrical, dielectric, microwave and

morphological properties of the polymer can be tailored by adjusting synthesis

parameters such as dopant and monomer concentration, dopant type, synthesis time,

synthesis temperature and electrolyte pH . Ideally, the electrical properties of a metal


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would combine with the chemical and mechanical properties of a thermoplastic to

produce a processible, tough and highly conducting polymer. Till recently, most

conducting polymers are unprocessible and possess poor mechanical properties when

compared with conventional materials. However, significant developments have been

made in the synthesis of soluble derivatives of ICPs and in the in-situ synthesis in

conventional thermoplastics [2].

Interest in the development of conducting polymers such as polyaniline,

polypyrrole, polythiophene, polyphenylene etc., has increased tremendously during the

last decade because of their electrochromic properties for use in batteries, electronic

devices, functional electrodes, electrochromic devices, optical switching devices, sensors

and so on [5-9]. Conducting polymers can be prepared by chemical or electrochemical

polymerization. In the chemical polymerization process, monomers are oxidized by

oxidizing agents or catalysts to produce conducting polymers [10-11]. The advantage of

chemical synthesis is that it offers mass production at reasonable cost. On the other hand,

the electrochemical method involves the direct formation of conducting polymers with

better control of polymer film thickness and morphology, which makes them suitable for

use in electronic devices.

Since then it has been found that about a dozen of different polymers and polymer

derivatives undergo transition to conducting state when doped with a weak oxidation or

reducing agent. They are all various conjugated polymers. The early conjugated polymers

were unstable in air and were not capable of being processed. The recent research in this


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area has been focused towards the development of highly conducting polymers with good

stability and acceptable processing attributes.

1.1.2. Examples of conducting polymers

Polyacetylene, in view of possessing the simplest molecular frame work, has

attracted most attention, especially of physicists, with an emphasis on understanding the

mechanism of conduction. However, its insolubility, infusibility and poor environmental

stability has rendered it rather unattractive for technological application. The

technologically relevant front runners belong essentially to four families: Polyaniline

(PANI), Polypyrroles (PPY), Polythiophenes (PT) and Polyphenylene vinylenes (PPV).

The structures of some of the conducting polymers are given in figure 1.2.

1.1.3. Charge Storage

One of the early explanations of conducting polymers used band theory as method

of conduction. According to this a half filled valance band would be formed from a

continuous delocalized – system. This would be an ideal condition for conduction of

electricity. However, it turns out that the polymer can more efficiently lower its energy

by band alteration (alternating short and long bonds), which introduces a band width of

1.5 eV making it a high energy gap semi-conductor. The polymer is transformed into a

conductor by doping it with either an electron donor or electron acceptor. This is

reminiscent of doping of silicon based semi-conductors where silicon is doped with either

arsenic or boron. However, while the doping of silicon produces a donor energy level

close to the conduction band or an acceptor level close to the valance band, this is not the


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case with conducting polymers. The evidence for this is that the resulting polymers do

not have a high enough concentration of free spins, as determined by electron spin

spectroscopy.


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Polymer Structure

Maximum

conductivityafter doping

in Scm-1

Stability Processibility

Polyacetylene 1.5x105 React withair

Film not solubleor fusible

PolypyrroleN

H

N

H

2000Reasonablystable

Insoluble andinfusible

PolythiopheneS S

100 Stable

Insoluble andinfusible

Polyaniline N N 10 StableSoluble inneutral form

Polyphenylene 1000 Stable Insoluble andinfusible

Polyphenylenevinylene

1000Stable inundopedform

Solubleprecursor routeavailable

Figure. 1.2. Structures of some conducting polymers


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Initially the free spins concentration increases with concentration of dopant. At

large concentrations, however, the concentration of free spins levels becomes maximum.

To understand this it is necessary to look into the way how the charge is stored along the

polymer chain and its effect.

The polymer may store charge in two ways. In an oxidation process, it could

either lose an electron from one of the bands or it could localize the charge over a small

section of the chain. Localizing the charge causes a local distribution due to change in

geometry, which costs the polymer some energy. However, the generation of this local

geometry decreases the ionization energy of the polymer chain and increases its electron

affinity making it more able to accommodate the newly formed charges. This is

consistent with an increase in disorder detected after doping by Raman Spectroscopy. A

similar scenario occurs for a reductive process.

Typical oxidizing dopants used include iodine, arsenic pentachloride, iron (III)

chloride and NOPF 6 . A typical reductive dopant is sodium naphthalide. The main criteria

is its ability to oxidize or reduce the polymer without lowering its stability or whether or

not they are capable of initiating side reaction that inhibit the polymers ability to conduct

electricity. An example of the latter is the doping of a conjugated polymer with bromine.

Bromine is too powerful oxidant and adds across the double bonds to from sp3 carbons.

The same problem may also occur with NOPF 6 if left too long.


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Conjugated polymers with a degenerate ground state have a slightly different

mechanism. As with polyaniline, polarons and bipolarons are produced upon oxidation.

However, because the ground state structure of such polymers are twofold degenerate, the

charged cation are not bound to each other by a higher energy bonding configuration and

can freely separate along the chain. The effect of this is that the charged defects are

independent of one another and can form domain walls that separate two phases of

opposite orientation and identical energy. These are called solitons and can some times be

neutral. Solitons produced in polyacetylene are believed to be delocalized over about 12

CH units, with the maximum charge density next to the dopant counter ion. The bonds

closer to the defect, show less amount of bond alteration than the bonds away from the

center. Soliton formation results in the creation of new localized electronic states that

appear in the middle of the energy gap. At high doping levels, the charged solitons

interact with each other to form a soliton band which can eventually merge with the band

edges to create true metallic conductivity. This is shown in figure 1.3.

1.1.4. Charge Transport

Although solitons and bipolarons are known to be the main source of charge

carriers, the precise mechanism is not yet fully understood. The problem lies in

attempting to trace the path of the charge carriers through the polymer. All of these

polymers are highly disordered, containing a mixture of crystalline and amorphous

regions. It is necessary to consider the transport along and between the polymer chains

and also the complex boundaries established by the multiple number of phases. This has

been studied by examining the effect of doping, temperature, magnetism and


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the frequency of the current used. These tests show that a variety of conduction

mechanisms are used. The main mechanism used is by movement of charge carriers

between localized sites or between solitons, polaron or bipolaron states. Alternatively,

where inhomogeneous doping produces metallic island dispersed in an insulating matrix,

conduction is by movement of charge carriers between highly conducting domains.

Charge transfer between these conducting domains also occurs by thermally activated

hopping or tunneling. This is consistent with conductivity being proportional to

temperature.

1.1.5. Stability

There are two distinct types of stability. Extrinsic stability is related to

vulnerability to external environmental agent such as oxygen, water and peroxides. This

is determined by the polymers susceptibility of charged sites to attack by nucleophiles,

electrophiles and free radical. If a conducting polymer is extrinsically unstable then it

must be protected by a stable coating.

Many conducting polymers, however, degrade over time even in dry, oxygen free

environment. This intrinsic instability is thermodynamic in origin. It is likely to be cause

by irreversible chemical reaction between charged sites of polymer and either the dopant

counter ion or the -system of an adjacent neutral chain, which produces an sp3 carbon,

breaking the conjugation. Intrinsic instability can also come from a thermally driven

mechanism which causes the polymer to lose its dopant. This happens when the charge


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sites become unstable due to conformational changes in the polymer backbone. This has

been observed in alkyl substituted Polythiophenes.

Figure. 1.3 Energy band diagrams of solitons

N

H

N

H

N

H

N

H

N

H

Neutral Soliton

A

+ .

Polaron

A - A N

H

N H

N

H

N H

N

H

+

H

N

H

N

H

H

N

H

A - N

N A -

BiPolaron

+

Neutral Polymer Polaron Bipolaron Bipolaron Bands

Conduction Band

Valance Band


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1.1.6. Processibility

Conjugated polymers may be made by a variety of techniques, including cationic,

anionic, radical chain growth, co-ordination polymerization, step growth polymerization

or electrochemical polymerization. Electrochemical polymerization occurs by suitable

monomers which are electrochemically oxidized to create an active monomeric and

dimeric species which react to form a conjugated polymer backbone. The main problem

with electrically conductive plastics stems from the very property that gives its

conductivity, namely the conjugated backbone. This causes many such polymers to be

intractable, insoluble films or powders that cannot melt. There are two main strategies to

overcome these problems. These are, either to modify the polymer so that it may be more

easily processed, or to manufacture the polymer in its desired shape and form. There are

four main methods used to achieve these aims.

The first method is to manufacture a malleable polymer that can be easily

converted into a conjugated polymer. This is done when the initial polymer is in the

desired form and then, after conversion, is treated so that it becomes a conductor. The

treatment used is most often thermal treatment. The precursor polymer used is often made

to produce highly aligned polymer chain, which are retained upon conversion. These are

used for highly oriented thin films and fibers. Such films and fibers are highly

anisotropic, with maximum conductivity along the stretch direction.


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The second method is the synthesis of copolymers or derivatives of a parent

conjugated polymer with more desirable properties. This method is the more traditional

one for making improvements to a polymer. What is done is to try to modify the structure

of the polymer to increase its processibility without compromising its conductivity or its

optical properties. All attempts to do this on polyacetylene have failed as they always

significantly reduced its conductivity. However, such attempts on Polythiophenes and

polypyrroles proved more fruitful. The hydrogen on carbon - 3 on the thiophene or the

pyrrole ring was replaced with an alkyl group with at least four carbon atoms in it. The

resulting polymer, when doped, has a comparable conductivity to its parent polymer

whilst be able to melt and it is soluble. A water soluble version of these polymers has

been produced by placing carboxylic acid group or sulphonic acid group on the alkyl

chains. If sulphonic acid groups are used along with built-in ionizable groups then such

system can maintain charge neutrality in its oxidized state and so they can effectively

dope themselves. Such polymers are referred to as “self doped” polymers. One of the

most highly conductive derivatives of polythiophene is made by replacing the hydrogen

on carbon - 3 with a –CH 2-O-CH 2CH 2-O-CH 2CH 2-O-CH 3. This is soluble and reaches a

conductivity of about 1000 Scm-1

upon doping.

The third method is to grow the polymer into its desired shape and form. An

insulating polymer impregnated with a catalyst is fabricated into its desired form. This is

then exposed to the monomer, usually a gas or a vapour. The monomer then polymerizes

on the surface of the insulating plastic producing a thin film or a fiber. This is then

doped in the usual manner. A variation of this technique is


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electrochemical polymerization with the conducting polymer being deposited on an

electrode either at the polymerization stage or before the electrochemical polymerization.

This technique may be used for further processing of the conducting polymer. For

instance, by stretching aligned band to polyacetylene / polybutadiene, the conductivity

increases by 10 fold, due to the higher state of order produced by this deformation.

The final method is the use of Longmuir – Blodgett technique to manipulate the

surface active molecules into highly ordered thin films whose structure and thickness are

controllable at the molecular layer. Amphiphilic molecules with hydrophilic and

hydrophobic groups produces monolayer at the air-water surface interface of Longmuir –

Blodgett films. This is then transferred to a substrate creating a multiple structure

comprised of molecular stacks which are normally about 2.5 mm thick. The main

advantage of this technique is its unique ability to allow control over the molecular

architecture of the conducting films produced. It can be used to create complex multiple

structures of functionally different molecular layers. By producing alternating layers of

conductor and insulator, it is possible to produce highly anisotropic film which is

conducting within the plane of the film, but insulating across it. The stability and

processing attributes of some conducting polymers are given in the following table.


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Polymer Conductivity (-1

cm-1

)Stability

(Doped state)

Processing

Possibilities

Polyacetylene 103

– 105

Poor Limited

Polyphenylene 1000 Poor Limited

PPS 100 Poor Excellent

PPV 1000 Poor Limited

Polyaniline 10 Good Good

Polythiophenes 100 Good Excellent

Polypyrroles 100 Good Good

1.1.7. Applications

The extended – systems of conjugated polymer are highly susceptible to

chemical or electrochemical oxidation or reduction. These can alter the electrical and

optical properties of the polymer, and by controlling this oxidation and reduction, it is

possible to precisely control these properties. Since these reactions are often reversible, it

is possible to systematically control the electrical and optical properties with a great deal

of precision. It is even possible to switch from a conducting state to an insulating state.

There are two main groups of applications for these polymers. The first group

utilizes their conductivity as its main property. The second group utilizes electro activity.

They are shown below.


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Group – 1 Group – 2

Electrostatic materials Molecular electronics

Conducting adhesives Electrical displays

Electromagnetic shielding Chemical and biochemical sensors

Printed circuit boards Rechargeable batteries & Solid electrolytes

Artificial nerves Drug release systems

Antistatic clothing Optical computers

Thermal sensors Ion exchange membranes

Piezoceramics Electromechanical actuators

Active electronic switches Smart structures

Aircraft structures

Much research will be needed before many of the above application will become

a reality. The stability and processibility both need to be substantially improved if they

are to be used in the market place. The cost of such polymers must also be substantially

lowered. However, one must consider that, although conventional polymers were

synthesized and studied in laboratories around the world, they did not become

widespread until years of research and development had been done. In a way, conducting

polymers are at the same stage of development as their insulating brothers were some 50

years ago. Regardless of the practical applications that are eventually developed for them,

they will certainly challenge researchers in the years to come with new and unexpected

phenomenon. Only time will tell, whether the impact of these novel plastics would be as

large as their insulating relatives.


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1.2. Polyaniline

1.2.1 A Brief History

Polyaniline, probably the oldest known synthetic organic polymer, consisted of an

ill-defined class of materials obtained by the chemical or electrochemical oxidative

polymerization of aniline. Polyaniline (PANI) has attracted considerable attention not

only from fundamental scientific interest but also from practical applications due to its

novel properties (special doping mechanism, good environmental stability, low cost and

high conductivity, etc.) and potential applications in batteries, molecular devices, sensors

etc. PANI is a typical conducting polymer resulting from oxidative polymerization of

aniline, whose resistivity can be affected by doping concentration, dopant, morphology

and degree of crystallization. In 1991, the conductivity of PANI was enhanced to 300 –

400 Scm-1

with the development of the counter-ion induced processibility of PANI.

Typically, conducting PANI is synthesized by electrochemical or chemical oxidation of

aniline in acidic conditions and an aqueous medium is preferred [12, 13]. Alternative

methods have been designed to improve the solubility and processibility of the

synthesized PANI. Gong et al. have reported solid-state synthesis of PANI doped with

H4SiW12O40 under 200 C by furbishing in mortar [14]. Kaner et al. have reported a

solvent-free mechanochemical route for the synthesis of PANI in which the reaction

between aniline salt and the oxidant, ammonium peroxydisulfate was carried out by ball

milling the reactants for one hour, in the absence of solvent at ambient temperature [15].

Presently chemical synthesis, described in sec 2.2, provides the only satisfactory route in

the synthesis of polyaniline.


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1.2.2. Introduction to Polyaniline.

In the last 5 years a great deal has been reported about conducting polymers and

increasing attention has been paid to those derived from heterocyclic monomers [16 –

20]. Among all the conducting polymers, polyaniline is one, whose synthesis does not

require any special equipment or precautions. Polyaniline is rather unique as it is the only

polymer that can be doped by protonic acid and exists in different forms depending upon

the pH of the medium.

Polyaniline is a typical phenyl-based polymer having chemically flexible – NH

group in polymer chain flanked either side by phenylene ring. The protonation and

deprotonation and various other physio – chemical properties of polyaniline can be said

to be due to the presence of – NH – group. Polyaniline is the oxidative polymeric product

of aniline under acidic conditions and has been known since 1862 as aniline black [21].

At the beginning of the twentieth century organic chemists began investigating the

constitution of aniline black and its intermediate products. Willstatter and co-workers

[22, 23] in 1907 and 1909 regarded aniline black as an eight – nuclei chain compound

having an indamine structure as shown in figure 1.4.

However, in 1910, Green and woodhead [24] were able to report various

constitutional aspects of aniline polymerization. The conclusions of their studies were as

follows.


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1. There are four quinoid stages derived from the parent compound

leucoemeraldine.

2. The minimum molecular weights of these primary oxidations of aniline are in

accordance with an eight-nuclei structure.

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

4. The conversion of emeraldine into perinigraniline consumes two atom of oxygen

5. The conversion of nigraniline into peinigraniline consumes one atom of oxygen.

6. The reduction of nigranline to leucoemarldine consumes four atoms of hydrogen.

7.

The reduction of nigraniline to leucoemarldine consumes six atoms of hydrogen

8. The reduction of perinigraniline to leucoemraldine consumes eight atoms of

hydrogen.

N

H

N

H

H

N N

H

H

NNH

N

H

H

NH

Figure 1.4. Chemical Structure of Polyaniline

Polyaniline has a greater unique structure, containing an alternate arrangement of

benzene rings and nitrogen atoms. The nitrogen atoms can exist either as an imine (in sp2

hybridized state) or an amine (sp3 hybridized). Depending on the relative composition of


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these two states of nitrogen and further, whether they are in their quantized state or not,

various forms of polyaniline can result. The structure of these forms can be best

represented by choosing a minimum of four repeat units, as shown in figure 1.5.

The only form that is conducting among four is the green protonated emeraldine

form, which has both the oxidized minimum and reduced amine nitrogens, in equal

amounts (i.e., it is half oxidized). Thus, the blue insulating emeraldine form can be

transformed to conducting by converting the pH of the medium and vice-versa. Another

interesting feature of polyaniline is that, by use of an organic counter ion (x

-

), it can be

transformed to the conducting state.

During this period, it did not occur to any one to investigate its electrical and

magnetic properties of polyaniline for the obvious reasons that organic compounds are

insulators, though in 1911 Mecoy and Moore suggested electrical conduction in organic

solids [25]. Almost 50 years later, Surville et al [26] in 1968 reported proton exchange

and redox properties with the influence of water on the conductivity of polyaniline.

However, interest in polyaniline was generated only after the fundamental discovery in

1977 that iodine doped polyacetylene has metallic conductivity [27] which triggered

research interest in new organic materials in the hope that these would provide new or

improved electrical, magnetic and optical materials or devices. The hope was based on

electronic structure and the combination of metal like or semi conducting conductivity

with processibility and flexibility of classical polymers and above all the ease with which

modifications can be carried out via synthetic organic chemistry methodologies.


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Figure 1.5. Chemical Structure of various forms of polyaniline with four repeat unitsadopted from ref. 28 and 29.

1.2.3. Conduction in Polyaniline

Any discussion on polyaniline would be incomplete without understanding the

transport mechanism in conducting polyaniline. Polyacetylene the simplest conjugated

polymer with CH3 units linked linearly with an alternate single and double bond, consists

Leucoemeraldine base

Emeraldine base

Conducting emeraldine salt

Pernigraniline base


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of two carbon and two hydrogens, which provides fundamental structure to understand

basic transport in conducting polymer systems. The two kekule structures derived from

this structure are equal in energy, thus the structure is doubly degenerate energetically.

Of the four valance electrons per carbon, three form relatively deeply bound molecular

orbital in (CH)x and remaining single (π-orbital) electron per carbon atom determines the

location of double bound. Two energetically equal structures at a point where they couple

to give a surface effect is known as kink or soliton. The term soliton means solitary

wave. This means that soliton has a movement. In conjugated systems solitons may be

neutral, positively or negatively charged according to the number of electrons in the π

orbital. The motionless charged states are known as carbonium (+ve) and carbonion (-ve)

radicals. Such a situation is also encountered in polymers which do not have two

degenerate ground states. That is, their ground state is non – degenerate due to non –

availability of two energetically equal kekule structures. Therefore there cannot be a link

to connect them. The conventional distortion is self – consistently stabilized.

Thus the charge coupled to surrounding lattice distortion to lower the total

electronic energy is known as polaron (i.e., an ordinary radical ion) with a unit charge

and spin =. A bipolaron consists of two coupled polarons with charge 2e and spin zero.

The energy increase due to columbic repulsion (in the formation of bipolaron) is more

than compensated for the energy gained when the two charges share the same lattice

distortion. Quantum – chemical calculations indicate that the formation of bipolaron

requires a 0.4 eV less energy than the formation of two polarons [30, 31]. However,


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bipolarons are not created directly but must form by the coupling of pre- existing

polarons or possibly the addition of charge to pre-existing polaron.

At molecular level a polymer is an ordered sequence of monomer units. The

degree of unsaturation and conjugation influences charge transport via the orbital overlap

within a molecular chain. The charge transport becomes obscured by the intervention of

chain folds and other structural defects. The connectivity of charge network is also

influenced by the structure of the dopant molecule. The dopant not only generates a

charge carrier by recognizing the structure (chemical modification) but it also provides

intermolecular links and sets up a micro field pattern affecting charge transport. Any

disturbance in the periodicity of the potential along the polymer chain induces a localized

energy state. Localization also arises in the neighborhood of ionized dopant molecule due

to coulombic field.

All dopants cannot induce charge transport in polymer. It depends upon the redox

energy of the host and guest molecules and electrons will transfer from high to low redox

energy. The initial charge transfer on doping will be between dopant molecule and the

proximal polymer site, but subsequently some diffusion of polymer charge, away from

the immediate dopant to the site can be expected and polymer pair states will be

generated. In equilibrium, the number of polymer pair states will be equal to the number

of ionized dopants.


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In view of this, charge transport occurs in polyaniline with a general composition

[(C6H4)x(NxHz)], which denotes that it is not a single well defined material but a unique

mixture of various oxidation states in which protonation induces insulator-to-metal

transition. The insulator-metal transition is a function of protonation and can be studied

by various techniques.

The study of electrochemical properties of polyaniline in-situ by ESR shows an

increase and then decrease of spin concentration upon oxidation and has been interpreted

as curie spins to pauli spin, forming a metal – like polaron band [32].

Litzelmann et al [33] have used electron energy loss spectroscopy to obtain

information on the nature of the insulator – metal transition as a function of protonation.

Their results indicate the momentum dependence of energy loss spectrum of electrons,

which does not support polyaniline as a granular metal. Similarly, the NMR and ESR

studies by Mizoguchi et al [34], Mankman et al [35] and D. C Galvo et at [36 & 38]

indicates the absence of three-dimensional metallic islands, which again does not support

polyaniline as a granular metal.

A number of experimental results which are relevant to charge transport are as

follows.

1. DC conductivity (σdc) shows a variation of log (σdc) with temperature. The

macroscopic conductivity (T) follows a law σT ∝ T, indicating electronic

transport is dominated by hopping of the polaronic species. This can be explained


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by involving models such as variable range hopping (VRH) [37, 39], the quasi –

1D VRH model [40] or the metallic rods model [41].

2. Thermo power (s) is the function of temperature and degree of protonation. At a

38% doping it is independent of temperature [42]. These thermo power

measurements indicate p-type of metal.

3. An in-situ ESR study indicates first an increase and then a decrease in spin

concentrations. However, on increasing anodic potential to 0.7 V again there is an

increase in spin concentration, indicating probably a bipolaron or a new type of

polaronic species [43].

4. Bredas et al [44, 45] have pointed out that polyaniline has only defect (polaron or

bipolaron) in the band gap, unlike other conducting polymers, where two defects

bands are always observed. Heeger [46, 47] has shown that intrinsic self

localization in quasi-one-dimensional systems is especially sensitive to

localization induced by disorder. Disorder induced localization is known to

convert doped conducting polymers from true metals with large mean – free paths

and coherent transport into poor conductors in which the transport is limited by

phonon – assisted hopping. However, Philips and Wu [48] pointed out that if a

bipolaron band is localized it should carry no current when disorder is present.

They have suggested a simple model in which defects by virtue of the pairing

constraints have an internal structure. This structure leads to resonance effect and

narrow band of conducting states when the defects are randomly placed.


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1.2.4 Applications of Conducting Polyaniline

The advent of polymers represents one of the important industrial revolutions of

current century. An important fundamental property i.e., electrical conductivity

distinguished polymers from metals. Polymers possessing high electronic conductivity

are also referred as synthetic metals or conducting polymers, which offers important

advantages over metals. These newly developed materials will not only replace metals in

many areas but also infiltrated our day-to-day life with a wide range of products

extending from most common consumer goods like batteries to highly specialized

applications in space and aeronautics. Among conducting polymers, polyaniline family

has attracted much attention of scientists world-wide because of their ease of synthesis,

unique conduction mechanism and high environmental stability in the presence of oxygen

and water, low cost, high weight and good sensing capability [49 – 56]. These polymers

also exhibit reversible redox behavior, which is very important for many applications.

Several reports and review papers have indicated promising applications of polyaniline

and at least about hundred companies are involved in the test production of conducting

polymers [57]. In the following section, some of the commercial products which make

use of polyaniline have been discussed.

1. A 3V – coin shaped batteries by Bridgestone – Seiko [58].

2. Antistatic layers in computer disks by Hitachi – Maxwell [59].

3. Comouflage by Millken and Co. [60].

4. Dispersible polyaniline powder version – Jointly developed by Allied Signal,

Americhem and Zipping Kessler [61].


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5. Electrostat loud speaker – 0.1 µm Polyaniline of 6µm polyester film.

6. Incoblend used for – electrostatic dissipations [62] by I.B.M., is utilizing this

product as an antistatic component carrier (e.g., computer chips). etc.

Heeger [63] has estimated the world wide market of conducting polymers of

about US $ 1 billion in the year 2000. Some of the known applications of polyaniline are

shown in the figure 1.6.


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Plastic LEDs Micromotors Optical StorageBatteries Photocopiers Transducers Lithography

Conductivity Photoconducting Piezoelectric Photochemical Reactions

Conducting Solid State

Composites SensorsSupercapacitors

ConductiveSurface

EMI/ESDMembranes(Gases)

Nonlinear Optics Electrochromic Ferromagnetism

Harmonic Generators Display Devices Magnetic Recording

Figure 1.6 Chart showing the various known and envisaged application of Polyaniline


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1.2.5 Devices based on Polyaniline

The use of semi conducting conjugated polymers as an electro – active material in

microelectronic devices is a rapidly growing area. Burroughes et. al, [64] have reported

the first examples of high – performance schottky diodes. The all organic high mobility

transient reported by Garnier et. al, [65] is an excellent example of how new organic

materials can be exploited to produce components with superior characteristics such as

flexibility over inorganic semiconductor materials.

One of the important applications of conducting polymers is their use as an

electrode material for rechargeable batteries because of reversible doping. The first major

commercial application of conducting polymers has been the button cell batteries of

Bridgestone Seiko [66]. These rechargeable batteries consist of polyaniline as an anode

and lithium aluminum alloys as cathode and LiBF4 in mixture of polypropylene carbonate

and 1, 2 – dimethoxy ethane as an electrolyte. Genies et al [67] reported similar type of

battery with LiClO4 as an electrolyte and Li – Al as cathode. Oyama et. al, [68 – 70] have

reported incorporation of 2-5 dimethyl mercapto 1, 3, 4 – thiodiazole into polyaniline as a

composite cathode material in a rechargeable lithium battery. Koura et al [71] have

prepared a battery configuration involving Al/Polyaniline using an AlCl3 based room –

temperature melt with OCV 1.6 V, capacity of 68 Ah/Kg and charge – discharge

efficiency of 99%. Tridevi et. al, [72] have fabricated dry cell using polyaniline.

Electrochemical capacitors differ fundamentally from both conventional

capacitors and batteries in their mechanisms of charge separation and energy storage.


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Conventional type of capacitors store electrostatic energy when an electric field is applied

across a dielectric. In an ideal capacitor the amount of charge storage is proportional to

the potential difference. Thus Genies et al, [73] reported a charge density of 450 C g-1 for

polyaniline in a polypropylene/LiClO4 system whereas Gottesfield et. al, [74] claim a

capacity of 800 C cm-1

under aqueous acidic conditions.

Organic electroluminescent devices have been the subject of intense research for

almost one and half decade. The major breakthrough came in 1990 when Buroughes et.

al, [75] showed that polymer can be used as an emitter in a electroluminescent device.

Brauan and Heeger [76] and Gustafsson et.al, [77] fabricated light emitting diodes where

a thin layer of polyaniline was described as a hole – injecting material. These devices

have cell voltage requirements of approximately 2 – 4 V compared to earlier organic

luminescent devices where the cell voltage requirement was 20 – 100 V. Cao et.al, [78]

achieved flexible light emitting diodes by using high – conductivity coating of

polyaniline with camphor sulphonic acid (CSA) as a dopant. Li et.al, [79] have reported

polyaniline films which were spun cast onto porous silicon and it has been observed that

transmittance and surface resistance of polyaniline can be controlled by controlling spin

rate. Thus 0.3 mm polyaniline film has 80% transmittance in visible region. In the above

studies, electroluminescence began to be visible under forward bias at 12 V, 1mA emits

red light and has a better intensity than that of p-n porous silicon.


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Angelpoulos et.al, [80, 81] have reported a very interesting application of

emeraldine base for lithography. Dao et.al,[82] have shown that polyaniline derivatives

can also be used to make patterns using UV-vis light.

Historically, electro chromic technology has been used for electronic display.

However, in recent years liquid crystal technology has been used for display applications.

Most recently, much attention has been focused on large area electro chromic display.

Jelle et.al, [83] have reported that electro chromic display consists of polyaniline. Akhtar

et.al,[84] have investigated solid – state electronic devices fabricated from polyaniline

and solid polymer electrolytes.


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1.3 Introduction to Metal Oxides

Transition metal oxides constitute the most fascinating class of materials,

exhibiting a variety of structures and properties [85]. The metal oxygen bond can vary

anywhere between highly ionic to covalent or metallic. The unusual properties of

transition metal oxides are clearly due to the unique nature of the outer d-electrons. The

phenomenal range of electronic and magnetic properties, exhibited by transition metal

oxides is noteworthy. Thus, the electrical resistivity in oxide materials spans the wide

range of 10-10 to 1020 Ω cm. We have oxides with metallic properties (e.g. RuO2, RuO3)

at on end of the range and oxides with highly insulating behavior (e.g. BaTiO3) at the

other. There are also oxides that transverse both these regimes with changes in

temperature, pressure or composition (e.g. V2O5, La1-xSrxVO3). Interesting electronic

properties also arise from charge density wave (e.g. K0.3MoO3), charge ordering (e.g.

Fe3O4) and defect ordering (e.g. Ca2Mn2O5, Ca2Fe2O5). Oxides with diverse magnetic

properties anywhere from ferromagnetism (e.g. CrO2, La0.5Sr0.5MnO3) to

antiferromagnetism (e.g. NiO, LaCrO3. α-Fe2O3) are known. Many oxides posses

switchable orientation states as in ferroelectric (e.g. BaTiO3, KNbO3) and ferroelastic

[e.g. Gd2 (MoO4)3] materials. Then, there is a variety of oxides bronzes showing a gamut

of property [86]. Superconductivity in transition metal oxides has been known for some

time and the highest Tc reached in the HTSC compound (e.g. YBa2Cu3O7-y) was around

90 K; we now have oxides with Tc in the region of 160 K. The discovery of high Tc

superconductors [87] has focused worldwide scientific attention on the chemistry of

metal oxides and, at the same time, revealed the inadequacy of our understanding of these


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materials. The giant magnetoresistance phenomenon in manganese oxides has also

received great attention. [87a].

The unusual properties of transition metal oxides that distinguish them form

different phases are due to several factors:

1. Oxides of d-block transition elements have narrow electronic bands, because of

the small overlap between the metal d-orbital and the oxygen p-orbital. The

bandwidths are typically of the order of 1-2 eV (rather the 5-15 eV as in most

metals).

2. Electron correlation effects play an important role, as expected because of the

narrow electronic bands. The local electronic structure can be described in terms

of atomic like states [e.g. Cu+ (d

10), Cu

2+ (d

9) and Cu

3+ (d

8) for Cu in CuO] as in

the Heitler-London limit.

3. The polarizability of oxygen is also of importance. The divalent oxide ion O2-

does not exactly describe the state of oxygen and configurations such as O- have

to be included especially in the solid state which gives rise to polaronic and

bipolaronic effects. Species, such as O- which are oxygen holes with a p

5

configuration instead of filled p6 configuration of O2-, can be made mobile and

correlated.

4. Many transition metal oxides are not truly three-dimensional but also have low-

dimensional features [88].


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Among the transition metals oxides, zinc oxide (ZnO), aluminium oxide (Al2O3),

titanium oxide (TiO2), tin oxide (SnO2), tungsten oxide (WO3), Vanadium oxide (V2O5),

cerium oxide (CeO2), iron oxide (Fe2O3), cobalt oxide (Co3O4), etc are mostly widely

known oxides and industrially employed transition metal oxides since the last fifty years.

The cause of these oxides have become important both scientifically and industrially

because of their applications for sound and picture recording, data storage, humidity and

gas sensors, conducting composite super capacitors, electrochromic display devices, etc.

1.3.1 Metal Oxides for Sensor Applications

Since, Seiyama and Taguchi used the dependence of the conductivity of ZnO on

the gas present on the atmosphere for sensing applications [89, 90], many different metal

oxides have been proposed for humidity and gas sensing detection. Generally speaking,

these oxides can be divided into binary oxides and more complex oxides, being the

former much more common in gas sensing applications. Among binary metal oxides, tin

dioxide (SnO2) is the one that has received by far more attention since Taguchi built the

first tin oxide sensor for Figaro Sensors in 1970 [91]. This is probably due to its high

reactivity to many gaseous species. However, this characteristic has also revealed as a

lack of selectivity, and thus investigation on other metal oxides has been considered

necessary. Besides, developers of electronic noses have experimented with arrays of

different sizes that may include around ten metal oxide sensors [92], apart from other

types of chemical sensors. The use of different metal oxide sensors is highly

recommended in order to increase the amount of information.


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1.4 Sensors

There is an escalating need and desire for us to monitor all aspects of our

environment in real time and this has been brought about by our increasing concerns with

pollution, our health and safety. There is also a desire to determine contaminants and

analytes at lower levels and one could say that the aim of all modern science is to lower

the detection limits and to improve the accuracy and precision at those limits.

Instrumentation has become so sophisticated that we are now able to detect chemicals in

amounts smaller than we ever imagined of a few years ago. In fact, this has shown that

manufactured chemicals and byproducts have been introduced into almost every aspect of

our environment and lives.

Because of this desire and need of monitor everything around us there is a

tremendous input of energy and resources into developing sensors for a multitude of

applications. The end result of all this research will one day provide us with portable,

miniature, and intelligent sensing devices to monitor almost anything we wish. For

example, one can imagine in the future having a credit card size self-diagnostic unit with

a multitude of chemical sensors and biosensors built into it so that we can monitor our

well being at any instant. A person may be feeling unwell and licks the sensing surface of

their diagnostic unit. Immediately, the liquid crystal display flashes up the message, ‘you

have influenza virus, take aspirin and rest’

In monitoring the environment one can imagine similar devices which could be

used to test for, say, heavy metal pollutants in natural waters or the presence of bacteria


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in drinking water, swimming pools or at beaches. Bathers might carry such device with

them to test the water before swimming. The possibilities are limitless and are controlled

only by one’s imaginations. Well, that is not strictly true, the possibilities are really

controlled by the Physics, Chemistry and electronics of such devices and the art of that

particular point in time. We should never lose sight of the fact that all these sensing

systems depend on principles of basic science [93].

The Oxford English dictionary defines a sensor as ‘a device which detects or

measures some condition or property, and records, indicates, or otherwise responds to the

information received’. Thus sensors have the function of converting a stimulus into a

measured signal. The stimulus can be mechanical, thermal, electromagnetic, acoustic or

chemical (and so on) in origin, while the measured signal is typically electrical in nature,

although pneumatic, hydraulic and optical signals may be employed.

Research activity especially on chemical sensors is now flourishing throughout

the world. Many papers of chemical sensors are being published in journals and read at

domestic and international conferences. They convince us that chemical sensors are here

to stay. Although various kinds of new devices and principles have been proposed not all

of them have been commercially successful. Even scientifically fascinating and well-

engineered devices sometimes find difficulty in the commercial market. Some of these

encounter problems in the fact that reasonable productions is required for a successful

device, not just high performance. Moreover, new devices must be introduced at the right

time to meet social needs.


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It was already known in the 1950’s that metal oxides such as ZnO and NiO

change semiconducting properties with change in partial pressure of oxygen, N2O or

other gases in the surrounding atmosphere. Relevant theories were proposed by many

researchers understanding the nature of the gas-solid interactions as well as for

controlling surface chemical processes such as catalysis. However, an approach in the

opposite direction i.e. utilizing the phenomenon for the detection of gases was not

conceived until 1962. In that year, Seiyama et al. from Japan reported that inflammable

gases in air could be detected from a change in the electric resistance of a thin film on

ZnO, while Taguchi claimed that a porous sintered block of SnO could also work in the

same way. These findings clearly demonstrated the possibility of a sensing device based

on an oxide semiconductor.

Despite such successful development in the past decades, however, fundamental

understanding of the sensor remains far from being satisfactory. There is increasing need

of new sensors capable of detecting humidity and various gases such as toxic gases and

smell components. Trace gases sometimes at sub-ppm levels present in the environment

or generated from food will be new targets of detection in the near future. It is unlikely

that such demands will be met easily by simple extension of the present trial and error

approaches. The introduction of a concept for the design of humidity and gas sensors is

vital. Generally speaking, a sensor must posses at least two basic functions; i.e. a function

to recognize humidity and a particular gas among others (receptor function) and another

transducer the recognition into an electrical or optical signal (transducer function).


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1.4.1. Applications of Sensors

Sensors and more specifically humidity sensors have a wide range of applications and

are constantly being extended to new areas. The following are some of the major areas of

applications that can be identified.

(a) Humidity is one of the most common constituents present in the environment.

Therefore, sensing and controlling humidity is of great importance in the

industrial processes, for human comfort, domestic purpose, in medical

applications and in agriculture. Recently, there has been a considerable

increase in the demand for humidity control in various fields such as air

conditioning systems, electronic devices, tyre industries, sugar industries and

drying processes for ceramics and food.

(b) Industrial hygiene has become very important because of the published health

hazards and related atmospheric pollution. The general public has become

more sensitive to these various gases. As a result much effort is being focused

on producing relatively inexpensive sensors of medium sensitivity and

selectivity to meet these needs.

(c) Manufacturing process monitoring and control is becoming very important

especially for confined spaces such as chemical and fuel storage tanks and in

particular on board of chemical and oil tankers.

(d) Control of combustion processes where process monitoring have become an

integral part of most manufacturing industries, including the wide spectrum of

chemical industries and the high technology electronic industries. The


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objective also includes in addition to monitoring, to regulate the pollution at

least to within compliance of the national pollution laws.

(e) Fuel efficiency and pollution control of combustion processes require proper

control of the ratio of fuel to oxygen or air in a gas or oil fire furnace in

industrial and domestic installation or internal combustion engines. In the case

of exhaust gases the detection of CO is also very important, in addition to that

of O2. The main objective of the CO monitoring is to prevent the intoxication

caused by incomplete burning in domestic combustion equipment.

(f)

Medical applications: Another important area of application is in medical

diagnosis of patients by monitoring the humidity, oxygen and carbon dioxide

concentration. Our medical applications include monitoring the environment

for health hazardous gases and vapors including carcinogenic compounds not

only within the confines of industries associated with the use or production of

such chemicals but also their spread through atmosphere by air movement.

Also, sensors able to monitor the extent of contamination by all kinds of

known or unknown microorganisms are expected to be in great demand in the

future.


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1.4.2 Classification of Sensors

(a) Humidity Sensors: They are based on change in electrical properties of the

material due to the absorption of water vapour. Hydrophilic polymers are used

for resistance type humidity sensors, while hydrophobic polymers are

preferred for capacitance type sensors.

(b) Liquid and Solid electrolyte-electrochemical sensors: They are based on

Faraday’s law. Because of the ionic nature of the ionic conductivity in the

electrolytes any current passing through it will carry a corresponding flux of

matter. Therefore the measurement of pumping current provides an easy and

accurate determination of the quantity of matter being transferred from one

electrode to other.

(c) Catalytic: In which gases react on a catalytic filament via an exothermic

process. The resulting temperature increase is being monitored by a

corresponding resistance change in the filament.

(d) Electronic conductive devices-semiconductor: In which reversible reaction of

the gas at the semiconductor surface results in a change of one of its electronic

properties usually conductance.

(e) FET devices: These can be closely related to several of the other methods but

made possible by silicon technology and typified by such devices as ion

selective field effective transistors (ISFETS) chemical FETS (CHEMFETS)

and enzyme FETS (ENFETS).

(f) Calorimetric Sensors: Detect change in temperature.


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(g) Optochemical Sensors: Chemical and biological changes are sensed in the

form of optical signals.

(h) Mass sensitive-microbalance sensors: Here gases are adsorbed on to a coated

piezoelectric crystal. The resulting weight change causing a frequency shift

from the fundamental.

(i) Biosensors: This is probably the biggest single area of growth at present.

Virtually all the techniques mentioned can be utilized in some way to make

measurements on biological systems. The use of biologically based molecules,

enzymes, amino acids, etc. as systems for improving selectivity of devices

such as ion selective electrodes and ion selective FET’s is increasing.

1.5. Polymers in Sensors Applications

During the last 20 years, global research and development (R&D) on the field of

sensors has expanded exponentially in terms of financial investment, the published

literature, and the number of active researchers. It is well known that the function of a

sensor is to provide information on our physical, chemical and biological environment.

Legislation has fostered a huge demand for the sensors necessary in environmental

monitoring, e.g. monitoring toxic gases and vapors in the workplace or contaminants in

natural waters by industrial effluents and runoff from agriculture fields. Thus, a near

revolution is apparent in sensor research, giving birth to a large number of sensor devices

for medical and environmental technology. A chemical sensor furnishes information

about its environment and consists of a physical transducer and a chemically selective

layer [94]. A biosensor contains a biological entity such as enzyme, antibody, bacteria,


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tissue, etc. as recognition agent, whereas a chemical sensor does not contain these agents.

Sensor devices have been made from classical semiconductors, solid electrolytes,

insulators, metals and catalytic materials. Since the chemical and physical properties of

polymers may be tailored by the chemist for particular needs, they gained importance in

the construction of sensor devices. Although a majority of polymers are unable to

conduct electricity, their insulating properties are utilized in the electronic industry. A

survey of the literature reveals that polymers also acquired a major position as materials

in various sensor devices among other materials. Either an intrinsically conducting

polymer is being used as a coating or encapsulating material on an electrode surface, or

non-conducting polymer is being used for immobilization of specific receptor agents on

the sensor device. Because their chemical and physical properties may be tailored over a

wide range of characteristics, the use of polymers is finding a permanent place in

sophisticated electronic measuring devices such as sensors. During the last 5 years,

polymers have gained tremendous recognition in the field of artificial sensor in the goal

of mimicking natural sense organs. Better selectivity and rapid measurements have been

achieved by replacing classical sensor materials with polymers involving nanotechnology

and exploiting either the intrinsic or extrinsic functions of polymers. Semiconductors,

semiconducting metal oxides, solid electrolytes, ionic membranes, and organic

semiconductors have been the classical materials for sensor devices. The developing role

of polymers as gas sensors, pH sensors, ion-selective sensors, humidity sensors,

biosensor devices, etc., are continuously reviewed. Both intrinsically conducting

polymers and non-conducting polymers are used in sensor devices. Polymers used in


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sensor devices either participate in sensing mechanisms or immobilize the component

responsible for sensing the analyte.

1.5.1. Conducting polymer composites

These materials contain an electrically insulating polymer matrix loaded with

conductive filler. The concept of percolation can be used to understand the change in

resistivity as function of filler concentration in composites. It describes the conduction

with the presence of electrically conducting path between two filler particles. The number

of these paths will be dramatically destroyed below the critical volume of filler. All

environmental effects that can change the volume fraction of the filler, such as

temperature change due to their thermal extension mismatch, deformation due to the

elasticity coefficient differences, and polymer swelling due to the sorption of vapours or

humidity, will cause a change in resistivity [95]. The often used filler materials are metals

(Cu, Pd, Au, Pt, etc), carbon black and metal oxides (V2O5, WO3, Al2O3, CeO2, TiO,

Co3O4, etc.,). The important polymers that can be used as matrices are polyethylene,

polyimides, polyaniline, polypyrrole, polyesters, poly (vinyl acetate) (PVAc),

polyurethane, poly (vinyl alcohol) (PVA), epoxies, acrylic; e.g. polymethyl metha crylate

(PMMA) etc. They have been used in PTC, thermistors, piezoresistive pressure [96]

tactile, humidity and gas sensors.

1.6. Literature Review

Suri et al. prepared nanocomposite pellets of iron oxide and polypyrrole for

humidity and gas sensing by a simultaneous gelation and polymerization process. This


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resulted in the formation of a mixed iron oxide phase for lower polypyrrole

concentration, stabilizing to a single cubic iron oxide phase at higher polypyrrole

concentration. Sensitivity to humidity increased with increasing polypyrrole

concentration [97].

Jain et.al. [98] have synthesized weak acid doped polyaniline and its composites

for humidity sensing. They reported maximum sensitivity in case of PANI-CSA.

Synthesis and characterization of Poly (2, 3 – dimethylaniline) is studied by

Kulkarni et.al [99]. The test samples were reported as a competent material for humidity

sensor. Somani et. al [100] have synthesized conducting polyaniline / V2O5 composites

by a technique of in-situ polymerization of aniline over fine graded V2O5. The composite

is studied by using physio-chemical characterization. In addition a study of charge

transport and hysteresis (I-V) characteristics were also undertaken.

Suresh Raj et al., [101] synthesized Zinc(II) oxide – zinc(II) molybdate

composites for humidity sensing. Composites having different mol ratios of Cr2O3 –

WO3 were synthesized and studied for humidity sensing applications by Pokhrel et.al

[102].

Jing Wang et.al [103] have carried out systematic study of Lanthanum

ferrite/polymer quaternary acrylic resin for humidity sensing by citrate method. They

investigated the electrical property of this humidity sensor, including the resistance

versus RH, humidity hysteresis, response – recover time and long term stability.

Su Pi et al [104] fabricated a resistive-type humidity sensor by thick film

deposition using poly(2-acrylamido-2-methylpropane sulfonate) (poly-AMPS) modified


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with tetraethyl orthosilicate (TEOS) as the sensing material, without a protective film or

complicated chemical procedures.

Shi-Jian Su et al [105] carried out in situ polymerization of anatase TiO2 to

synthesize polyaniline / TiO2 composite (PANI/TiO2) by dispersing fine powder of

anatase TiO2 in polyaniline matrix to obtain nanocomposite. The characterization of these

composites was also carried out. The measured conductivities of these composites shows

a increase upto 30 wt % of TiO2 in polyaniline and decreases there after.

Polyaniline blended with either polyvinyl alcohol or a butyl acrylate/vinyl acetate

copolymer was studied by McGovern et.al. [106] and used as a sensing medium in the

construction of a resistance-based humidity sensor. The sensors had an overall final

thickness of less than 150 m and showed high sensitivity, low resistance, and good

reversibility without hysteresis.

Despite good progress in the study of charge transfer in conducting polymers,

factors affecting electrical conductivity in terms of device applications are not entirely

understood. Mzenda et.al [107] investigated charge transfer in polyaniline using DC

measurements over the temperature range 300 < T(K) < 450, thermal analysis and

Fourier transform infrared spectroscopy (FTIR). FTIR results show molecular structure

changes as a result of the annealing process and thermal analysis indicate the loss of

moisture at around 373 K.

Some insight into recent trends in sensor research is obtained from the number of

papers being published per year in various analytical journals, which are useful indicators

of systems that are directly applied to solving real problems. Fig. 1.7 shows the number

of hits for various subgroups of sensors, including ISEs, optical sensors, amperometric,


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biosensor, acoustic, and solid-state sensors, as a percentage of the total number of sensor

papers published each year [108]. A survey of the sensor market [109] identified medical

applications as a major driving force for the development of the emerging sensor

technologies: fiber-optic sensors, smart sensors, silicon micromachined sensors, and thin

film devices.

Fig. 1.7. Trends in the absolute numbers of papers for each sensor type abstracted fromAnalytical Abstracts over the period 1980 to 1994.


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1.7 Aim of the study

There are several reports available in literature related to various studies in

polyaniline blends. But the reports on polyaniline composites are scarce. To tailor the

various electrical properties of polyaniline, synthesis of new composites of polyaniline,

with better dielectric properties and enhanced humidity sensing are the need of the hour.

Hence the author has tried to tailor the properties of polyaniline composites by the

selection of WO3, CeO2 and Co3O4 in polyaniline.

Therefore, this present work is oriented towards the better understanding of basic

electrical and humidity sensing properties in polyaniline – composites. These parameters

which have been studied here may provide better route for technological applications in

the near future.


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