POLYMER CHEMISTRY SEM-6, DSE-B3 PART-10, PPT-10

POLYMER CHEMISTRY

SEM-6, DSE-B3 PART-10, PPT-10

Dr. Kalyan Kumar Mandal

Associate Professor

St. Paul’s C. M. College

Kolkata

Polymer ChemistryPart-10

Contents

• Conducting Polymers

1. Polyacetylene

2. Polyaniline

3. Poly(p-phenylene)

4. Poly(p-phenylene sulphide)

5. Polypyrrole

6. Polythiophene

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.

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

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)

• Poly(fluorenes)

The N is in the

aromatic cycle:

• Poly(pyrroles)

• Poly(indoles)

The N is outside the

aromatic cycle:

• Polyanilines

The S is in the aromatic

cycle:

• Poly(thiophenes)

The S is outside the aromatic

cycle:

• Poly(p-phenylene sulphide)

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 as leucoemeraldine,

emeraldine, 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

precipitate by polymerizations of

aniline by oxidative coupling 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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POLYMER CHEMISTRY SEM-6, DSE-B3 PART-10, PPT-10