PART-10-PPT-10P.pptx

POLYMER CHEMISTRY
SEM-6, DSE-B3
PART-10, PPT-10
Dr. Kalyan Kumar Mandal
Associate Professor
St. Paul’s C. M. College
Kolkata

Polymer Chemistry
Part-10
Contents
• Conducting Polymers
1. Polyacetylene
2. Polyaniline
3. Poly(p-phenylene)
4. Poly(p-phenylene sulphide)
5. Polypyrrole
6. Polythiophene

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata
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.

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.

Types of Conducting Polymers
• Linear-backbone “polymer blacks” (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.

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)

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.
• Conducting polymers have backbones of continuous sp2 hybridized carbon centres. One
valence electron on each centre resides in a pz orbital, which is orthogonal to the other three
sigma bonds. The electrons in these delocalized orbitals have high mobilities.

• 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.

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.

• 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.

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.

• 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. These are also X-ray transparent.
• 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.

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.

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.

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.

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.

Different Approaches for Making Conducting Polymers
• A practical approach involves incorporation of metallic powders, flakes or whiskers or other
conductive fillers such as graphite powder or conducting carbon blacks into common plastics
or rubbers. However, such filled conducting compositions have their own limitations. Most
conductive polymers are prepared by oxidative coupling of monocyclic precursors.
• There are two main methods used to synthesize conductive polymers, chemical synthesis and
electro copolymerization. The chemical synthesis means connecting carbon-carbon bond of
monomers by placing the simple monomers under various condition, such as heating,
pressing, light exposure and catalyst. The advantage is high yield.
• The electro copolymerization means inserting three electrodes (reference electrode, counter
electrode and working electrode) into solution including reactors or monomers. By applying
voltage to electrodes, redox reaction to synthesize polymer is promoted. The advantage of
Electro copolymerization are the high purity of products.

Electrical Conductivity of Common Conducting Polymers

Polyacetylene
• In polyacetylene, a conjugated polymer, the spare electrons are held by formation of alternate
double bonds and single bonds in the polymer structure.
• The conjugated structure of PA makes it behave like a
semiconductor and not as an insulator like polyethylene
(PE). Some of the π electrons of PA thermally excited
out of the bonds giving rise to a small electrical
conductivity. Polyacetylene (PA) may exist in the
geometrical isomeric forms as shown in Figure 9.

Polyacetylene
• Polymerizations of acetylene catalyzed by Ziegler–Natta type catalyst, etc. have been
reported (Figure 10). The polymer as commonly obtained in powder, gel or film form is
cross-linked, insoluble and intractable showing different ranges of semiconducting character
and it has no defined melting point. Doped derivatives of PA are ionic compounds and doping
of PA is viewed as a redox reaction. The net step in doping is the oxidation or reduction of PA
molecules to polycations or polyanions is shown in Figure 11.
• Polyacetylene has a conductivity in the range of 10-5 s cm-1, but when the doping level
increases, its conductivity rises drastically to 102 to 103 s cm-1.

Polyaniline
• The conductivity of polyaniline is dependent upon the dopant concentration, and it gives
metal-like conductivity only when the pH is less than 3. Polyaniline exists in different forms
whch are shown in Figure 12.
• Different forms of polyanilines are
classified
emeraldine,
as leucoemeraldine,
and pernigraniline, by
their oxidation state. Leucoemeraldine
exists in a sufficiently reduced state,
and pernigraniline exists in a fully
oxidized state.
• Polyaniline becomes conductive only
when it is in a moderately oxidized
state and acts as an insulator in a fully
oxidized state.

Polyaniline
• The polymer backbone consists of both quinonoid and benzenoid rings, in differing
proportions. The difference in the ratio causes the existence of three oxidized states: the fully
reduced leucoemeraldine form (I; Figure 12) is in a benzenoid state, the fully oxidized
pernigraniline form (III) is in a benzenoid state and the conductive emeraldine form (II) has
an equal ratio of both benzenoid and quinonoid rings. The dopant does not change its
chemical property and will not create any bond with the main chain; it exists in the close
vicinity of the polymer chain.
• It can be synthesized by the oxidative polymerization of aniline in presence of ammonium
persulphate dissolved in 1M HCl. The change in color of the reaction medium to green
indicates the formation of polyaniline. Generally, oxidizing agents like ammonium
persulfate, ammonium peroxy disulphate, ceric nitrate, ceric sulphate, potassium
dichromate, etc. are used. The polymer and composite possess good conductivity when the
pH is between 1 and 3.

Polyaniline
• The imine nitrogen atoms can be protonated in full or in part to give the respective salts, the
degree of protonation being dependent on the oxidation state and pH of the aqueous acid used.
• The partly protonated emeraldine
hydrochloride salt is prepared
readily in the form of a black-green
aniline by oxidative coupling
precipitate by polymerizations of
in
aqueous acid (HCl) media by such
oxidizing agents as (NH4)2 S2O8,
H2O2, Cr6+- complexes/salts etc.
• Different structures (II and III) for
fully protonated emeraldine base (I)
may be obtained on use of ≥ 1M
HCl for doping (Figure 13).

Poly(p-phenylene Sulphide) PPS
• PPS can be obtained by homopolymerization of thiophenol in the presence of H2SO4 or by
oxidative condensation of thiophenol in the presence SOCl2 and a Lewis acid.
• PPS may also be synthesized by self-condensation of metal-p-halogenothiophenoxide. This
method uses a less drastic condition and it involves extensive washing of the product in order
to remove the residual metal contaminants.

Some inherently Conducting Polymers and their Electrical Conductivities

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