CONDUCTING

POLYMERS

HIMANSHU GUSAIN
ROLL NO – 17/890
INTRODUCTION
Polymers have existed in natural form since life began and those as DNA RNA proteins an
polysaccharides play a crucial roles in plants and animal life. From the earliest times, man
has exploited naturally occurring polymers as materials for providing clothing , decoration,
shelter tools weapons etc. however the origins of today’s polymer industry commonly are
accepted as being in the nineteenth century when important discoveries were made
concerning the modification of certain polymers.

DEFINITION
The word ‘polymer’ comes from the Greek words poly (meaning ‘many’) and meros (meaning
‘parts’).

A POLYMER IS A SUBSTANCE COMPOSED OF MOLECULES WHICH HAVE LONG SEQUENCES OF

ONE OR MORE SPECIES OF ATOMS OR GROUPS OF ATOMS LINKED TO EACH OTHER BY
PRIMARY, USUALY COVALENT BONDS

CONDUCTING POLYMERS
Polymers are usually organic substances composed of a very large number of like molecules.
Polymers are divided, on the basis of their mechanical properties and strength, into three
categories: rubbers or elastomers, plastics and fibers. Polymers are generally insulators because
the organic molecules of which they are composed have no free electrons to carry current; all the
electrons are held firmly by atoms forming the molecules. Polymers in which the carbon atoms in
the backbone are linked by double bonds have the potential to conduct electricity, especially when
a number of such bonds occur in the vicinity of
each other are known as Conducting Polymers.
Conductive polymers or more precisely
intrinsically conducting polymers (ICPs) are
organic polymers that conduct electricity. Such
compounds may have metallic conductivity or
behave as semiconductors. Conductive polymers
combine the mechanical properties (flexibility, toughness, malleability, elasticity, etc.) of plastics
with high electrical conductivity. These properties can be fine-tuned using different methods of
organic synthesis. In 1977, Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa proved that
polyacetylene doped (oxidised) with iodine has high conductivity. This research earned them the
2000 Nobel Prize in Chemistry. It has great applications in day to day life.

ADVANTAGES OVER CONDUCTORS

CHEMICAL
Ion transport, redox behavior, catalytic properties, electrochemical effect, photo activity,
junction effects.

MECHANICAL
light weight, flexible, non metallic surface properties.

HOW DO THEY CONDUCT?

Conducting polymers have a continuous chain of sp2 hybridized carbon centers. It is these π bonds
due to which they conduct. Two conditions:

1) Presence of conjugated double bonds.

2) Molecule modified using a dopant.

 Oxidative doping (halogens):
 P-type semiconducting Polymer
 Reductive doping (alkali metals): N-type semiconducting polymer

FEW COMMON EXAMPLES OF CONDUCTING POLYMERS

 For conductance free electrons are needed.

 Conjugated polymers are semiconductor materials while doped polymers are
conductors.
 The conductivity of conductive polymers decreases with falling temperature in
contrast to the conductivities of typical metals, e.g. silver, which increase with falling
temperature.
 Today conductive plastics are being developed for many uses.
TYPES OF CONDUCTING POLYMERS
1) Conducting Polymer Composites
Conducting polymer composites are typically a physical mixture of a nonconductive polymer and a
conducting material such as a metal or carbon powder distributed throughout the material.
Conductive carbon blacks, short graphite fibers, and metal coated glass fibers, as well as metal
particles or flakes, were used in early experiments for the preparation of such composites. Their
conductivity is governed by percolation theory, which describes the movement of electrons
between metallic phases and exhibits a sudden drop in conductivity (percolation threshold) at the
point where the dispersed conductive phase no longer provides a continuous path for the transport
of electrons through the material.

2) Redox polymers
In ‘conjugated conducting polymers’, the redox sites are delocalized over a conjugated π system;
however, ‘redox polymers’ have localized redox sites. The redox polymers are well known to
transport electrons by hopping or self-exchange between donor and acceptor sites. The redox
conductivity is comparatively lower than that of conjugated conducting polymers, likely due to slow
electron transport to/from the redox center.

More recently, there has been growing interest in a new type of redox polymer that is a hybrid of
materials from conjugated organic polymers and Electrochemically polymerized transition metal
complexes , referred to as conjugated metallopolymers . Examples include metal complexes of

A) poly(2,2-bipyridine) B) The polyferrocenes.

 METALLOPOLYMERS : The key feature of this class of material is that the metal is
coordinated directly to the conjugated backbone of the polymer, or forms a link in the
backbone, such that there is an electronic interaction between the electroactive metal
 centers and the electroactive polymer backbone. This can enhance electron transport in the
polymer, enhance its electro catalytic activity, and lead to novel electronic and
electrochemical properties.

3) Ionically Conducting Polymers

Ionically conducting polymers (polymer/salt electrolytes) are of great interest because they exhibit
ionic conductivity in a flexible but solid membrane. Ionic conductivity is different than the
electronic conductivity of metals and conjugated conducting polymers, since current is carried
through the movements of ions. They have been critical to the development of devices such as all-
solid-state lithium batteries.

4) Intrinsically Conducting Polymers (ICP)

Intrinsically conducting polymers offer a unique combination of ion exchange characteristics and
optical properties that make them distinctive. They are readily oxidized and reduced at relatively
low potentials, and the redox process is reversible and accompanied by large changes in the
composition, conductivity and color of the material. These polymers are made conducting, or
‘doped’, by the reaction of conjugated semiconducting polymer with an oxidizing agent, a reducing
agent or a protonic acid, resulting in highly delocalized polycations or polyanions.

The conductivity of these materials can be tuned by chemical manipulation of the polymer
backbone, by the nature of the dopant, by the degree of doping and by blending with other
polymers.
CONCEPT OF DOPING IN ICP
 Doping involves either oxidation or reduction of the polymer backbone. Oxidation removes
electrons and produces a positively charged polymer and is described as ‘p-doping.’
Similarly, reduction produces a negatively charged backbone and is known as ‘n-doping.’
The oxidation and reduction reactions can be induced either by chemical species (e.g.,
iodine, sodium amalgam or sodium naphthalene) or electrochemically by attaching the
polymer to an electrode.

 Electrochemical p- and n-doping can be accomplished under anodic and cathodic conditions
by immersing polymer film in contact with an electrode in an electrolyte solution. In these
p- and n-doping processes, the positive and negative charges on polymers remain
delocalized and are balanced by the incorporation of counter ions (anions or cations) which
are referred to as dopants

 Upon p-doping, an ionic complex consisting of positively charged polymer chains and
counter anions (I3 −) is formed.

 In the case of n-type doping, an ionic complex consisting of negatively charged polymer
chains and counter cations (Li+) is formed.

CONDUCTION MECHANISM
The electronic properties of any material are determined by its electronic structure. The theory that
most reasonably explains electronic structure of materials is band theory. Quantum mechanics
stipulates that the electrons of an atom can only
have specific or quantized energy levels.
However, in the lattice of a crystal, the electronic
energy of individual atoms is altered. When the
atoms are closely spaced, the energy levels are
form bands. The highest occupied electronic
levels constitute the valence band and the
lowest unoccupied levels, the conduction band.
The electrical properties of conventional materials depend on how the bands are filled. When bands
are completely filled or empty no conduction is observed. If the band gap is narrow, at room
temperature, thermal excitation of electrons from the valence band to the conduction band gives
rise to conductivity. This is what happens in the case of classical semiconductors. When the band
gap is wide, thermal energy at room temperature is insufficient to excite electrons across the gap
and the solid is an insulator. In conductors, there is no band gap since the valence band overlaps the
conduction band and hence their high conductivity.

CHARGE CARRIERS IN CONDUCTING POLYMERS
SOLITON
 The soliton ( S) is an unpaired π-electron resembling the charge on free radicals,
which can be delocalized on a long conjugated polymer main chain.
 Neutral soliton oxidize to give +ve soliton and reduce to give –ve soliton
 The soliton possesses a spin of ½ and no spin for +ve or –ve soliton
 Electronic energy level of the soliton is located at the middle of the bandgap of the
trans polyacetylene.

POLARONS
 Major charge-carriers in conducting polymers including basic state degenerate
trans-polyacetylene and the basic state nondegenerate conjugated polymers.
 Positive (P+) and negative (P-) polarons formed after oxidation and reduction of the
conjugated polymer main chain respectively.
 Possess spin of ½ . The appearance of the polarons produces two new polaron
energy levels in the bandgap of the conjugated polymers.
SYNTHESIS OF CONDUCTING POLYMER
Conducting polymers are generally synthesized via chemical or electrochemical oxidation
of a monomer where the polymerization reaction is stoichiometric in electrons.

CHEMICAL SYNTHESIS ELECTROCHEMICAL

SYNTHESIS
• Chemical synthesis has the • Semiconducting polymers
advantage of being a simple have been obtained from a
process capable of wide variety of monomers
producing bulk quantities of including thiophene,
ICPs on a batch basis furan,carbazole, aniline,
• Chemical polymerization is indole, azulene and
typically carried out with polyaromatic monomers
relatively strong chemical such as pyrene and
oxidants like ammonium fluoranthene
peroxydisulfate, ferric ions, • By selecting an appropriate
permanganate or electrolyte, a much wider
bichromate anions, or choice of cations and anions
hydrogen peroxide. for Use as ‘dopant ions’ is
• First polymer synthesis and possible.
doping are carried out, • Doping and processing take
followed by processing. place simultaneously with
polymerization
EXAMPLES OF C.P

POLYACETYLENES

 Polyacetylene is the simplest polyconjugated

polymer consisting of repeating [—(CH=CH)n-] units.
 It was first prepared as a linear, high-
molecular-weight, polyconjugated polymer of high
crystallinity and regular structure by Natta et al
 It was obtained as an air-sensitive, infusible,
and insoluble black powder. It thus remained
basically a material subject to academic research by
organic chemists
 Its an infinitely long conjugated molecule in
which one would expect the one-dimensional pi
electrons to form a half-filled band leading to metallic
behavior.

 Reaction conditions allow to control the morphology of the polymer to be obtained as
gel, powder, spongy mass or a film.
 It is doped with iodine
 Inherent insolubility and infusibility impose barriers to the processing of the
polymer.

CONDUCTION MECHANISM:
POLYANILINES
 Polyaniline, known as "aniline black," was
first prepared in 1834.
 Produced as bulk powder, cast films or
fibers.
 Belongs to the semi flexible rod polymer
family.
 It is one of the oldest conducting polymer.
 It is highly sensitive, simple in detection and easy to synthesize

POLYANILINE NANOWIRES:
 It is an electrically conducting polymer that can be
used as an active layer for sensors whose conductivity
change can be used to detect chemical or biological species.
 polyaniline nanofibers can be used to create
nonvolatile plastic digital memory devices when decorated
with various metal, such as gold, nanoparticles.
 Polyaniline nanofibers have been shown to be
incredibly successful as chemical sensors

Polyaniline has a rather unique structure, containing an alternating arrangement of benzene

rings and nitrogen atoms. The nitrogen atoms can exist either as an imine (in an sp2
hybridized state) or an amine (sp3 hybridized). Depending on the relative composition of
these two states of nitrogen, and further on whether they are in their quartenized state or
not, various forms of polyaniline can result. The structures of these forms can be best
represented by choosing a minimum of four repeat units, as shown below.
The only form that is conducting, among the four, is the green protonated emeraldine form, which has
both the oxidized iminium and reduced amine nitrogens, in equal amounts (i.e., it is half oxidized).
Thus, the blue insulating emeraldine form can be transformed into the conducting form by lowering
the pH of the medium and vice-versa. Another interesting feature of polyaniline is that, by use of an
organic counterion (X- ), for instance, by using camphor sulfonic acid as the dopant acid, polyaniline
can be retained in solution even in the doped conducting form, further enhancing its versatility. The
transport of charge in these systems can be understood in a simple fashion, by causing the imine and
amine nitrogens to exchange places along the polymer backbone (in protonated emeraldine). Try and
push arrows to cause this change and observe that this process effectively causes the charges to move
along the polymer backbone!

APPLICATIONS OF CONDUCTING
POLYMERS
There are two main groups of applications for these polymers. The first group utilizes their
conductivity as its main property. The second group utilizes their electroactivity.
TV AND COMPUTER SCREENS
One of the most exciting developments is the use of conductive
polymers to produce flat, flexible plastic screens for televisions and
computers. This work evolved from the discovery that conductive
polymers such as Polyphenylene Vinylene emit light when
sandwiched between oppositely charged electrodes, thus enabling
flat-panel display designs to be made. The company associated
closely with this technology at the present time is Cam-bridge
Display Technology (CDT)

FLEXIBLE ELECTRONICS
Flexible electronics also known as flex circuits, is a
technology for assembling electronic circuits by
mounting electron .This can be bent without breaking
electronics devices on flexible plastic substrates. Flex
circuit are made up of flexible plastic substrate usually
polyimide, Polyester or thin sheets of glass

Electronic component such as transistor are being

made from silicon Nano membrane usually called
TFT’s(thin filmTransistor). Flexible resistors and
capacitors structures are shown diagrammatically usually called thin film resistors and thin film
capacitors.
Electroluminescent Electronic Devices
Electroluminescence – the generation of light,
other than blackbody radiation, by electrical
excitation. Organic semiconductors was first
reported for anthracene single crystals in the
1960s

These early studies established that the process

responsible for electroluminescence requires
injection of electrons from one electrode and
holes from the other, the capture of oppositely
charged carriers (so called recombination), and the radioactive decay of the excited electron-hole
state (exciton) produced by this recombination process.

LIGHT EMITING DIODES

Polymer light-emitting diodes (PLEDs), based on PPV are now coming out as commercial products.
When compared to inorganic or organic materials for LEDs, the main advantages are their fast
response times, process ability, the possibility of uniformly covering large areas, low operating
voltages, and the many methods were applied to fine-tune their optical and electrical properties by
varying the structure.
CONCLUSION
Due to their poor processability, conductive polymers have few large-scale applications.
They have promise in antistatic materials and they have been incorporated into
commercial displays and batteries, but there have had limitations due to the manufacturing
costs, material inconsistencies, toxicity, poor solubility in solvents, and inability to directly
melt process. Literature suggests they are also promising in organic solar cells, printing
electronic circuits, organic light-emitting diodes, actuators, electrochromism,
supercapacitors, chemical sensors and biosensors, flexible transparent displays,
electromagnetic shielding and possibly replacement for the popular transparent conductor
indium tin oxide. Another use is for microwave-absorbent coatings, particularly radar-
absorptive coatings on stealth aircraft. Conducting polymers are rapidly gaining attraction
in new applications with increasingly processable materials with better electrical and
physical properties and lower costs. The new nanostructured forms of conducting
polymers particularly, augment this field with their higher surface area and better
dispersability.
BIBLIOGRAPHY
 Conducting Polymers: A New Era in Electrochemistry
By György Inzelt

 Self-Doped Conducting Polymers

Michael S. Freund, Bhavana A. Deore

 C. K. Chiang, C. R. J. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa,

E. J. Louis, S. C. Gau, A. G. MacDiarmid, ‘Electrical conductivity in doped
polyacetylene,’

 J. Heeger, ‘Semiconducting and metallic polymers: the fourth generation

of polymeric materials

 http://www.ch.ic.ac.uk/local/organic/tutorial/steinke/4yrPolyConduct2003.pdf

 Handbook of Advanced Electronic and Photonic Materials and Devices, Ten-Volume Set
1st Edition
Editors: Hari Nalwa