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CONDUCTING POLYMERS

For a long time, polymers were considered as non-conductor of electricity and were used as
good electrical insulators. Polymers like PE or PP are made up of σ bonds and hence a charge
once created on any given atom on the polymer chain is not mobile. However, for more than
two decades now researchers have shown that certain class of polymers which are conjugated
exhibit semiconducting behavior. The presence of an extended π conjugation in polymers
confers the required mobility to charges that are created on the polymer backbone. The
discovery of doping led to a further dramatic increase in the conductivity of such conjugated
polymers.
In 1977, Heeger, Mac Diarmid and Shirakawa demonstrated that polyacetylene which is a semi-
conductor in the pure state could be turned into a highly conductive polymer on reacting with
iodine. Other polymers which display similar characteristics are usually poly-conjugated
structures that are insulators in the pure state but when treated with an oxidizing or reducing
agent can be converted into polymers with electrical conductivities comparable to metals.

Table 1.1: Structure and conductivity of some conjugated conducting polymers

Energy gap Conductivity

Polymer (Conductivity discovered) Structure
(eV) (S/cm)
Polyacetylene and analogues
1.5 103 – 1.7×105
Polyacetylene(1977)

Polypyrrole (1979) 3.1 102 – 7.5×103

Polythiophene (1981) 2.0 10 – 103

3.0
Polyphenylene and analogues
102 – 103
Poly(paraphenylene)(1979)

Poly(p-phenylene vinylene) (1979) 2.5 3 – 5×103

Polyaniline (1980) 3.2 30 – 200

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Problems with conducting polymers: Due to the presence of extended conjugation along the
polymer backbone, the chains are rigid and possess strong interchain interactions resulting in
insoluble and infusible materials. These conjugated polymers lack one of the most important
and useful polymer properties, namely their ease of processibility. Another problem is the
inherent instability of these polymers especially in the doped form to ambient conditions.
More recently, conjugated polymers have been made soluble and hence processible without
significant loss in their conductivity. Also conducting polymers, stable in their doped form have
been prepared.

DOPING: The conductivity of a polymer can be increased several fold by doping. Doping is a
process by which a polymer is either oxidized or reduced to create charge carrier. The partial
oxidation (p-doping) of the polymer chain is carried out with electron acceptors e.g. I2, AsF5 or
by partial reduction (n-doping) with electron donors e.g. Na, K. Through such doping process
charged defects (polaron, bipolaron and soliton) are introduced which could be available as
charge carriers. Shirakawa and Ikeda discovered that doping of polyacetylene increases its
conductivity by 9-13 orders of magnitude. Doping is accompanied by chemical methods of
direct exposure of the conjugated polymer to a dopant in the gas or solution phase or by
electrochemical oxidation or reduction.
Doping agents or dopants are either strong oxidizing or reducing agents. The nature of dopants
plays an important role in the stability of conductive polymers e.g. perchloric acid doped
polyacetylene is not sensitive to water and oxygen.
Of the two PAc conformations, cis and trans, the trans form is thermodynamically more stable.
Shirakawa’s PAc had mainly the cis form and was a copper colored flexible film which could be
converted to the silvery trans form by heating above 150oC. These materials were
semiconductors, the trans isomer with higher conductivity (4.4 x 10-3 S m-1) than the cis (1.7 x
10-7 S m-1). The halogen doping that transforms PAc to a good conductor of electricity is
oxidation or p-doping. Reductive doping or n-doping is also possible using an alkali metal.

[CH]n + 3x/2 I2  [CH]nx+ + xI3-

[CH]n + xNa  [CH]nx- + xNa+
The doped polymer is thus a salt. However, it is not the counter ions I3- or Na+ but the charges
on the polymer that are the mobile charge carriers.

CONDUCTION MECHANISM:

Band theory
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Conduction in solids is usually explained in terms of the band theory which postulates that
when atoms or molecules are aggregated in the solid state, an energy band is formed through
the interaction of the constituent atomic or molecular orbitals. The band of highest energy that
is completely filled by electrons is called the valence band and the electrons associated with
bands are involved in chemical bonding and are consequently localized and not free to move
through the solid. The lowest lying unoccupied levels form a band which is called conduction
band. There is a forbidden energy region between the valence and conduction bands. This
energy separation is called the energy gap or band gap. When the energy gap is large, then
there can be no net flow of electrons under the influence of an external field unless electrons
are elevated into the empty band and this will require a considerable expenditure of energy.
Such materials are either semiconductors or insulators, depending on how large the energy gap
is and the majority of polymers are insulators because the band gap in them is very large.

A schematic representation of energy band diagram demonstrating energy gap

While considering electronic conduction in polymers, band theory is not suitable because the
atoms are covalently bonded to one another forming polymeric chains that experience weak
intermolecular interactions. Thus, macroscopic conduction will require electron movement, not
only along chains but also from one chain to another. There are no mobile charge carriers in
them to support conduction. The appropriate charge carriers may be generated in an organic
polymer by its partial oxidation or reduction. It is assumed, on doping the oxidant removes
electrons from the filled valence band and reductant add electrons into the vacant conduction
band. But this model did not work for polyacetylene (PA) and other conducting polymers,
because from experiments it was found that the conduction is due to charge carriers that do
not have free spins.

Soliton model (Heeger et al., 1980)

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In this model charged solitons are believed to be the conducting species for charge transport.
Charged solitons are a type of charge defect introduced in a polymer chain on doping with
electron acceptors or electron donors. Charged solitons have no spins and so this model was
initially accepted. The conduction mechanism in PA agreed with soliton theory because PA has
a degenerate ground state (two geometric structures correspond to same energy) but all other
conducting polymers have non-degenerate ground state.

Polyacetylene
Polyacetylene was earlier synthesized using Z-N catalysts. The PA synthesized by Shirakawa was
a lustrous, flexible polycrystalline film rather than the powder usually obtained. This PA has a
predominantly cis conformation when formed at temps of 195 K. On raising the temp of the
film, isomerization to the more stable trans form takes place. The polymer is infusible, insoluble
and contaminated by catalyst residues and tends to become brittle and dull when exposed to
air due to slow oxidation. Many of these problems have been solved by Feast who developed a
very elegant synthetic method know as Durham route. This is a two-stage process in which
soluble precursor polymers (5) are prepared by a metathesis ring opening polymerization
reaction and these are subsequently heated to produce PA (6) by a thermal elimination
reaction.
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PA is a polymer with degenerate ground states. The degeneracy of the ground state of trans PA
gives rise to possibility of structural defects in chains where there is a sense of bond alteration.
Phase A and B are mirror images and the single and double bonds can be interchanged without
changing the energy.

Thus, if the cis structure begins to isomerize to the trans geometry from different locations in a
single chain, an A sequence may form and eventually meet a B sequence but in doing so, a free
radical is produced. This is a relatively stable entity and the resulting defect in the chain is called
a neutral soliton, which corresponds to a break in the pattern of bond alteration, that is, it
separates the two degenerate ground state structures.

The electron has an unpaired spin (1/2) and is located in a non-bonding state in the E-gap
midway between the two bands. It is the presence of these neutral solitons which gives trans
PA the characteristics of a semiconductor with an intrinsic conductivity of about 10-7 to 10-8
S/cm.
The conductivity can be magnified by doping. Exposure of the film to dry ammonia gas leads to
a dramatic increase of conductivity to ~ 103 S/cm. Controlled addition of an acceptor or p-doping
agent such as AsF5, I2 , Br2 or HClO4 removes an electron and creates a positive soliton (or a
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neutral one if the electron removed is not the free electron). Similarly, a negative soliton can be
formed by treating the polymer with a donor or n-doping agent that adds an electron to the
midgap energy level.

At high doping levels, the soliton regions tend to overlap and create new midgap energy bands
that may merge with the valence and conduction bands allowing freedom for extensive
electron flow. Thus, in PA, the charged solitons are responsible for making the polymer a
conductor.

Characteristics:
 Pure undoped trans PA  Only neutral solitons on an average of ~1/3000 chain atoms
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 Doped trans PA with n or p  A number of neutral soliton is used at an already very

low-doping level leading to the formation of charged solitons
 Defect free trans PA  A charge transfer directly between doping agent and valence
and conduction band respectively will produce an ion-radical in the chain i.e. a defect
pair instead of an isolated defect

Schematic representation of the formation of (a) polyacetylene, (b) polaron, (c) bipolaron and
(d) soliton pair on a trans-polyacetylene chain by doping

In this mechanism, trans PA gains one electron and stable ion-radical is formed, which is called
polaron. Its formation is accompanied by two energy levels obtained symmetrically above and
below the middle level of the band gap. With increased doping, increased concentration of
polarons interact with each other and thus two polarons only may couple to form bipolarons
which are spin-less and further give rise to two charged solitons (soliton pair) which produce a
band halfway between valence and conduction band.

Polaron and bipolaron model:

The failure of soliton theory in supporting conduction in polyp-phenylene and other polymers
with non-degenerate ground state led to the polaron and bipolaron theory. According to this
theory, the polymer chain is ionized on doping and this ionization process creates a polaron
(radical ion) on the chain. At low doping level, these polarons, are the carriers of electricity. On
increasing doping level, the concentration of polaron increases and this results in a probability
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of interaction with each other. Two polarons get coupled to form a bipolaron. Bipolarons are
doubly charged but spinless. In the case of PA it is believed that due to the degenerate ground
state, the bipolaron initially formed disintegrates into polarons which further decay into
spinless charged solitons. Among the conjugated organic polymers, PA represents a special case
because it a degenerate ground state and therefore the conduction is due to soliton and not
due to polaron and bipolaron.

Poly (p-phenylene), PPA is an attractive polymer for many reasons. High conductivity of the
order of 500 S/cm can be achieved by doping with AsF5. Poly (p-phenylene) structure has all the
characteristics required for a conducting polymer, but it is difficult to synthesize high MW poly
(p-phenylene).

Poly (p-phenylene) produced is insoluble and oligomeric with low yield. It is an insulator in pure
state but can be doped (both n and p-doping). It needs a stronger oxidizing agent e.g. AsF5.

PPP oligomers AsF5 vapor PPP material with metallic blue lustre Polymerization

Highly conducting PPP

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Structure of PPP does not support soliton defect as there is no degenerate ground state. In PPP
and most other poly-conjugated conducting polymers, the conduction occurs via the polaron
and bipolaron.
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Polaron and Bipolaron structures of Poly (para-phenylene)

Potential Applications of Conducting Polymers

Conducting polymers have important applications in molecular electronics, electrical displays,

electromagnetic shielding, printed circuit boards, rechargeable batteries, solid electrolytes and
optical computers. Other potential applications of these conducting polymers are in chemical,
biochemical and thermal sensors, artificial nerves, drug release systems, antistatic clothing, ion
exchange membranes, corrosion protection and electromechanical actuators.

Interest in conducting polymers has its origin in the possible commercial applications of these
materials. The commercial applications are based on the promise of a novel combination of
light weight, processibility and electrical conductivity. Some of the conducting polymers can
change their optical properties on applications of current or voltage and therefore may find
useful applications as heat shutter and light emitting diode (LED).

But, problem hindering these wonderful applications is the poor processibility in these
polymers. Improvement of the processibility will enable scientists and technologists to explore
this to create a new looking world of conducting polymers. Much research is needed before
many of the above application will become a reality. The stability and processability 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
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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. Polymeric
conductors with low density, good electrical conductivity coupled with low cost pose a serious
challenge to the established inorganic semiconductor technology.

Conducting Polymers in Sensors

The chemical properties of conducting polymers make them very useful for use in sensors. This
utilizes the ability of such materials to change their electrical properties during reaction with
various redox agents (dopants) or via their instability to moisture and heat. An example of this
is the development of gas sensors. It has been shown that Polypyrrole behaves as a quasi 'p'
type material. Its resistance increases in the presence of a reducing gas such as ammonia and
decreases in the presence of an oxidizing gas such as nitrogen dioxide. The gases cause a
change in the near surface charge carrier (here electron holes) density by reacting with surface
adsorbed oxygen ions. Another type of sensor developed is a „biosensor‟. This utilizes the
ability of tri-iodide to oxidize polyacetylene as a means to measure glucose concentration.
Glucose is oxidized with oxygen with the help of glucose oxidase. This produces hydrogen
peroxide, which oxidizes iodide ions to tri-iodide ions. Hence, conductivity is proportional to the
peroxide concentration, which is proportional to the glucose concentration.

Conducting Polymers in Electrochromic Devices

Conjugated polymers that can be repeatedly driven from insulating to conductive state
electrochemically with high contrast in color are promising materials for electrochromic device
technology. Conjugated polymers have an electronic band structure. The energy gap between
valence band and the conduction band determines the intrinsic optical properties of the
polymers. The color changes elicited by doping are due to the modification of the polymer band
electronic structure. The electrochromic materials have been employed in large area display
panels. In architecture, electrochromic devices are used to control the sun energy crossing a
window. In automotive industry rear view mirrors are a good application for electrochromic
system. Conductive Polymers in Aircraft Industry

Modern planes are often made with lightweight composites. This makes them vulnerable to
damage from lightning bolts.
Coating aircraft with a conducting polymer can direct the electricity directed away from the
vulnerable internals of the aircraft.
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Polypyrrole has been approved for use in the U.S. Navy's A-12 stealth attack carrier aircraft for
use in edge card components that dissipate incoming radar energy by conducting electric
charge across a gradient of increasing resistance that the plastic material produces.
Conducting Polymers as Catalyst

Conducting polymers are expected to behave as redox catalyst as they exhibit redox property.
Several reports have been found in literature on modification of conducting polymers and their
use as catalyst for small organic molecules. Conducting polymers in their various oxidation
states interconvert each other, which permits to construct redox cycle for catalytic reactions.
Conducting Polymers inside the Human Body

Due to the biocompatibility of some conducting polymers, they may be used to transport small
electric signals through the body, i.e., act as „artificial nerves‟. Perhaps, modifications to the
brain might eventually be contemplated. The use of polymers with electro active reaction has
led to their use to emulate biological muscles with high toughness, large actuation strain, and
inherent vibration damping. This similarity gained them the name "Artificial Muscles" and offers
the potential of developing biologically inspired robots.

Conducting Polymers as Antistatic Fabrics

Another promising product incorporating conducting polymers is ContexÒ which is a fiber. The
fiber is coated with a conductive polymer material called Polypyrrole and can be woven to
create an Antistatic fabric. Antistatic fabrics are also being explored for possible application in
clean room applications.

Conductive Polymers for Medical Applications

Suitable for a variety of applications, conductive thermoplastic compounds can satisfy the
medical industry's need for miniaturized, high-strength parts. Most can withstand state-of-the-
art sterilization procedures, including autoclave and many are certified for purity and pre-
tested to minimize ionic contamination. Medical applications using conductive thermoplastics
include:

 Bodies for asthma inhalers. Because the proper dose of asthma medications is critical to
relief, any static „capture‟ of the fine particulate drugs can affect recovery from a
spasm.
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 Airway or breathing tubes and structures. A flow of gases creates turboelectric charge
or decay. A buildup of such charges could cause an explosion in high- oxygen
atmospheres.
 Antistatic surfaces, containers, packaging to eliminate dust attraction in pharmaceutical
manufacturing.
 ESD housings to provide Faraday cage isolation for electronic components in monitors
and diagnostic equipment.
 ECG electrodes manufactured from highly conductive materials. These are X-ray
transparent and can reduce costs compared with metal components.
 High thermal transfer and microwave absorbing materials used in warming fluids.
Futuristic Applications of Conducting Polymers