Conducting Polymer

1. What are Conducting Polymers?
Organic polymers that conduct electricity are known as
conducting polymers. They are also known as intrinsically
conducting polymers (ICPs) and they have alternating single and
double bonds along the polymer backbone (conjugated bonds) or
that are composed of aromatic rings such as Phenylene,
naphthalene, anthracene, pyrrole, and thiophene which are
connected through carbon-carbon single bonds.

2. Conducting Polymers
• Polymers are organic compounds made up of carbon, hydrogen, nitrogen, oxygen, etc., and
have covalent bonds. Materials of covalent bonds are not supposed to be electrically
conducting because there is no availability of free electron. Polymers were considered to be
electrical insulators before the invention of conducting polymers (conjugate polymers),
but these organic polymers have unique electrical and optical properties similar to those
of inorganic semiconductors.
• The polymers that are used in daily basis are insulators. However, some polymers can
conduct electricity under certain conditions. Hence, there are some mechanisms through
which electrons can be made available in organic molecules.
• The Nobel Prize in Chemistry 2000 was awarded jointly to Alan J. Heeger, Alan G.
MacDiarmid and Hideki Shirakawa “for the discovery and development of conductive
polymers.” These materials, based on doped polyacetylene and other conjugated
polymers, are sometimes called synthetic metals.

3. Conducting Polymers: Introduction
• A conjugated carbon chain consists of alternating single and double bonds, where the highly
delocalized, polarized, and electron-dense π bonds are responsible for its electrical and
optical behavior. Typical conducting polymers include polyacetylene (PA), polyaniline
(PANI), polypyrrole (PPy), polythiophene (PTH), poly(para-phenylene) (PPP),
poly(phenylenevinylene) (PPV), and polyfuran (PF).
• One potential application for conjugated polymers is lightweight rechargeable
batteries for portable devices and vehicles. Conducting polymers would serve both
current-carrying and ion conduction functions by replacing traditional electrode and
electrolyte substances. Conducting polymers are also used in building circuitry elements,
both passive (conducting circuits) and active (p-n and Schottky junctions). Other potential
applications include transparent antistatic coatings for metals and electronic devices,
electromagnetic shielding, light-emitting diodes (LEDs), electrodes, biosensors,
transistors, and ultrathin, flexible screens for computer and TV monitors.

4. Types of Conducting Polymers
• Linear-backbone polymers (polyacetylene, polypyrrole, polyaniline, etc.) and their
copolymers are the main class of conductive polymers. The different conducting
polymers are classified according to their composition. Table 1 presents some organic
conductive polymers according to their composition.

5. Types of Conducting Polymers
Table 1: Conducting Polymers according to their Composition
The main chain
contains
No hetero atoms Heteroatoms present
Nitrogen containing Sulphur containing
Aromatic cycles Poly(p-phenylenes)
 Poly(naphthalenes)
The N is in the
aromatic cycle:
The S is in the aromatic
cycle:
 Poly(fluorenes)  Poly(pyrroles)
 Poly(indoles)
 Poly(thiophenes)
The S is outside the aromatic
The N is outside the cycle:
aromatic cycle:  Poly(p-phenylene sulphide)
 Polyanilines
Double bonds Poly(acetylenes)

6. Conductive Polymers or Intrinsically Conducting Polymers
• Conductive polymers or more precisely, intrinsically conducting polymers (ICPs) are organic
polymers that conduct electricity. Such compounds may have metallic conductivity or can be
semiconductors. The biggest advantage of conducting polymers is their processability, mainly
by dispersion.
• Conductive polymers are organic materials, but they
are generally not thermoplastics, i.e., they are not
thermoformable. They can offer high electrical
conductivity but do not show similar mechanical
properties to other commercially available polymers.
The electrical properties can be fine-tuned using the
methods of organic synthesis and by advanced
dispersion techniques.

7. Types of Conducting Polymers
• Intrinsically conducting polymers are substances which have a π-bond backbone. There
are certain electrons which are extra in this type of polymers. These extra electrons flow
from one point to another in the polymer, as a result they have the ability to conduct
electricity. Conduction of electricity in this type of polymers is due to conjugation in the
backbone of polymer. The conjugation can be due to either π electrons or due to doped
ingredients.
• Conduction due to conjugated π electrons: In these types of polymers, due to the
presence of double bonds and lone pair of electrons conduction of electricity takes place.
Actually due to overlapping of conjugated π electrons, valence and conduction bands
throughout the backbone of the polymer are developed . Electrical conduction can occur
only after attainment of required energy of activation either thermally or photochemically
because there is some gap between the valence and conduction bands. So the electrons need
to be excited by some means. Polyacetylene, polyaniline, etc., are these types of
conducting polymers.

8. Doped Conducting Polymers
• The conduction power of semiconductor can be enhanced by adding some foreign material
or desired impurities. These impurities are called doping agent or dopant. Appropriate
doping agent increase the conductivity of semiconductors up to 104 times. The increase
in conduction is due to participation of impurity elements in between the valence band and
conduction band and thus making a bridge through which electrons can jump easily from the
valence band to the conduction band.
• Actually the conjugated π electrons have very low ionization potential and high electron
affinities. The foreign materials develop positive or negative charge through oxidation or
reduction of the semiconductor. Doping are mainly two types.
1. p-type doping through oxidation of materials: In this type of doping some electrons
from the conjugated π bonds are removed through oxidation creating a positive hole
called polaron inside the polymer. The positive hole or polaron can move throughout
the polymeric chain and make it conducting polymer.

9. Doped Conducting Polymers
• The polymers which have conjugation in the backbone when
treated with electron-deficient species (Lewis acid) like FeCl3
or I2 vapour or I2/CCl4, oxidation takes place and a positive
charge is created in the molecule. Removal of one electron in
the π backbone of a conjugated polymer forms a radical
cation (polaron), which on losing another electron forms
bipolaron. The delocalization of positive charges causes
electrical conduction. Lewis acids (FeCl3, AlCl3) are
generally used as doping agent.

10. Doped Conducting Polymers
2. n-type doping through reduction of materials: In this type
of doping some electrons are introduced to the conjugated
π bonds through reduction creating a negative hole or
charge inside the polymer. The negative hole or charge can
move throughout the polymeric chain and make it
conducting polymer. Lewis bases, Na+C10H8
-, K+C10H8
-, etc.,
are generally used as doping agents.
• When Lewis bases (electron rich species) are treated with
polymer having conjugation, due to reduction of the
polymers, negative charge develops. Actually by the addition
of one electron, polaron and by the addition of the second
electron, bipolaron are formed. In bipolaron, due to the
delocalization of charge, conduction takes place.

11. Doped Conducting Polymers
• Intrinsically conducting materials are characterized by good electrical conductivity, capability
to store charge, capacity to exchange ions, ability to absorb visible radiation, thereby yielding
the coloured compounds. The doping of an organic polymer to achieve certain extent of
metallic properties is phenomenologically similar to the doping of a classical inorganic
semiconductor in that very large increase in conductivity are observed when a small amount
of certain chemical species are added. However, mechanistically it is different in that the
doping of an organic polymer as the latter involves the partial oxidation or reduction of
the polymer, where each oxidation state exhibits its own characteristic reduction
potential.

12. Extrinsically Conducting Polymers (ECPs)
• Those conducting polymers which owe their conductivity due to the presence of externally
added ingredients in them are called extrinsically conducting polymers. Extrinsically
conducting polymers (ECP’s) are of two types. These are: (1) conducting elements filled
polymers (CEFP) i.e., the polymers filled with conducting element, and (2) blended
conducting polymers (BCP).
1. Conducting Elements Filled Polymers (CEFP): In this type, a conducting element is added
to the polymer. Therefore, the polymer acts as a binder to hold the conducting elements
together in solid entity. Thus, conductivity of these polymers is due to the addition of
external ingredients. Upon addition of conducting element, the polymer will have a
property of that conducting element and it will start conducting electricity.
• The conduction power of semiconductor can be enhanced by input some foreign conducting
material or good conductor in powder (carbon dust) form or granule from (metallic fibers).
The role of polymer is to bind the conducting materials.

13. Extrinsically Conducting Polymers (ECPs)
• When carbon black or some metal oxides or metal fibres are added, the polymer becomes
conductive. The minimum concentration of conducting filler required to start the conduction
is called percolation threshold. The filler (ingredients) that percolate have more surface
area, more porosity and filamentous nature due to which they can they can enhance
conducting properties.
• Important characteristics of these polymers are : (a) They possess good bulk conductivity;
(b) They are cheaper; (c) They are light in weight; (d) They are mechanically durable and
strong; (e) They are easily processable in different forms, shapes and sizes.
2. Blended conducting polymers: These types of polymers are obtained by blending a
conventional polymer with a conducting polymer either physically or chemically. This
blend of polymers conduct electricity. Such polymers can be easily processed and possess
better physical, chemical and mechanical properties.

14. Molecular Basis of Electrical Conductivity
• In traditional polymers such as polyethylene, the valence electrons are a part of sp3
hybridized covalent bonds. Such “sigma-bonding electrons” are firmly bound and have low
mobility. Therefore, they do not contribute to the electrical conductivity of the material. In
this polymers, the energy gap between the valence band and conduction band (band gap) is
large (Figure 6), and these are electrically insulators.
• However, in conjugated materials, the
situation is completely different. Semi-
conducting polymers are having the
energy gap between the valence band
and conduction band (band gap) are not
so large and not so small (Figure 7).
They have low conductivity, a small
amount of electric current can flow at
room temperature.

15. Molecular Basis of Electrical Conductivity
• Conducting polymers have backbones of contiguous sp2 hybridized carbon centres. One
valence electron on each sp2 hybridized carbon centre resides in a pz orbital, which is
orthogonal to the other three σ-bonds. All the pz orbitals are parallel to each other, as a result
they can overlap with each other to form a delocalized set of orbitals. The electrons in these
delocalized orbitals have high mobility when the material is “doped” by oxidation,
which removes some of these delocalized electrons. Thus, the conjugated p-orbitals form a
one- dimensional electronic band, and the electrons within this band become mobile when it
is partially emptied.
• In principle, these same materials can be doped by reduction, which adds electrons to an
otherwise unfilled band. In practice, most organic conductors are doped oxidatively to give
p-type materials. The redox doping of organic conductors is analogous to the
doping of silicon semiconductors, whereby a small fraction of silicon atoms are replaced
by electron- rich, e.g., phosphorous, or electron-poor, e.g., boron, atoms to create n-type
and p-type semiconductors, respectively.

16. Applications of conducting polymers:
1. In rechargeable batteries.
2. In making analytical sensors for pH, O2 , SO2 , NH3 , glucose, etc.
3. In bio-medical applications.
4. In controlled release of drugs.
5. In optical filters.
6. In photo voltaic devices.
7. In telecommunication systems.
8. In micro-electronic devices.