chapter -1.pdf

School of Mechanical Chemical and Materials
Engineering
Materials Science and Engineering Department
Conducting polymers (MScE5 307)
Prepared by : Wegene Lelisa
Date : …………………….
Chapter One
Conducting Polymers Fundamentals
11/8/2023 Conducting polymers 1

1. Conducting Polymers Fundamentals
OUTLINE
1.1.Introduction.
1.2 .Basics characteristics of CPs, doping and structure.
1.3 Basics of CP Synthesis.

 Hydrocarbons used as a fuels and the raw materials for the products such as
plastics ,synthetic fibers ,solvents , and industrial chemicals.
 Hydrocarbons are carbon containing organic compounds that the source of
energy and raw materials.
 Primary source of hydrocarbons is petroleum.
 Petroleum formed from the remains of micro organisms that lived in earth
oceans millions of years ago.
 Over a time go ,the remains formed thick layer of mud like deposit on the ocean
floor and heat from earth’s interior and tremendous pressure of overlying
sediments changed this mud into oil-rich shale and natural gas .
1.1.Introduction

1.1.Introduction

1.1.Introduction
 This class provides in-depth knowledge on the conducting polymer (CP) properties,
synthesis techniques, characterization and application.
 Materials classified in to conductors ,semiconductor and insulator according to
electrical conductivity.
 The most essential property that distinguished metal from polymer is electrical
conductivity.
 The electrical conductivity for metals is very high and is generally in the order of
104-106 S/cm( good conductors such as copper and silver have conductivity close
to 106 S/cm while for good insulator such as quartz conductivity as low as 10-18
S/cm.
 Semiconductor has the conductivity between conductor and insulator.

1.1.Introduction
 If Polymers, what type of polymers you remember?

 In generally ,many polymer organic polymers are used as insulator such as
(polyethylene ,PVC ,Teflon and etc. )
 The main reason for non-conducting nature of organic polymer due to is the
absence of conjugated bond.
 In saturated chemical structure ,all valance electrons are strongly localized.
These electrons do not contribute to the electrical conductivity of the material.
 But due to extensive research work ,the scientists able to synthesize
conductive polymers
1.1.Introduction

 So,in 1970s the first polymer capable of conducting electricity–
polyacetylene - was accidently prepared by Shirakawa.
 The subsequent discovery by Heeger and Macdiarmid that the polymer
would undergo an increase in conductivity of 12 orders of magnitude by
oxidative doping.
 After that the intense research take place for another conducting polymer.The
target is a material; which could combine the processibilty, environmental
stability, and weight advantages of a fully organic polymer with the useful
electrical properties of a metal.
 The electrical conductivity may be ionic or electronic in nature, the
conducting polymers include both the electronically and ionically conducting
polymers.
NB: The ionically conducting polymer are generally termed
as polymer electrolytes.
1.1.Introduction

 Why a polymer becomes electrically
conducting?
 Because ,The CPs have a very unusual structure compared to the insulating
polymers, the CPs have conjugated π electron (alternate C-C and C=C) system
in their polymeric backbone.
 Among the various CPs, the polyacetylene possesses simplest backbone
structure which is mainly composed of alternate single and double bond carbon
in the chain.
 This π-electron system is delocalized over the entire backbone of the polymer
chain.
1.1.Introduction

 Polyacetylene is the first example of conducting polymers
 The polyacetylene is of two types based on its hydrogen atoms locations
such as.
1. Trans-polyacetylene :-Possesses two hydrogen atoms on opposite sides.
It represents a degenerate conjugated polymer and possesses equivalent
structures after exchanging its double bond and single bond.
2. Cis-polyacetylene:- Two hydrogen atoms are placed on the same side of the
double bond. The cis-polyacetylene and other CPs are non-degenerate
conjugated polymers which have non-equivalent structures after exchanging their
double and single bonds.
1.1.Introduction

 Structure of Trans-polyacetylene
 Structure of Cis-polyacetylene
1.1.Introduction

 Acetylene on passing over Zeiglar Natta catalyst at room temperature
forms polyacetylene
Zeiglar Natta catalyst(RT)
Acetylene vapours added at RT.
Then, Condensed. Thin film is
formed on surface of catalyst
 Thin film contains both
Silver trans-structure and
Cis-copper in colour
1.1.Introduction

 Classification of conducting polymers.
 Conducting
polymers
Intrinsically
conducting
polymers
Extrinsically
conducting
polymers
 The conduction of
electricity is due to the
extensive conjugation
in the backbone of the
polymer.
 Are basically blended
polymers.The blends are
responsible for conduction.
Conjugated CPs
Doped CPs
Polymers with conducting
elements
(Contain alternate and
single double bond)
(Formed due to interaction with charge
transfer agents(dopants)
(polymer matrix contain conducting
fibers)
Polymer with
conducting blends
(Conventional non conducting polymers
blended with conducting polymers)

 In a conducting polymer, sp2 hybridized carbon chain constitute the back
bone for conduction.
 One each sp2 hybridized carbon ,there will be a single electron in the
unhybridized Pz orbital.The Unhybridized Pz lies orthogonal to sp2 hybridized
orbitals.
 The delocalization of electrons in Pz orbitals contributed for conduction of
electricity.

1.1.Introduction

1.1.Introduction
What is conducting polymers?
 More precisely, intrinsically conducting polymers are organic polymers that
conduct electricity.
How do make polymers conductors ?
By (i) changing the bond structure.
 CP is comprised simply of C, H, and simple heteroatoms such as N and S.
 The main conductive property arises from it’s unique π-conjugation.
 Extended and delocalized conjugation originating in overlap of π-electrons.

 Due to it’s unique behaviour and promising future applications, the Nobel Prize
in Chemistry 2000 was given “for the discovery and development of
conductive polymers” (Alan J. Heeger; Alan G. MacDiarmid; Hideki
Shirakawa).
 Common Conducting Polymers

poly(paraphenylene)
 Poly(acetylene) has been extensively studied both scientific and practical
applications but it is confined due to it’s high instability in air.

 The requirements for a conducting polymer for electrical conduction
 The molecule should have a linear backbone.
 The molecule should have extended conjugation.
 Charge carriers (either positive holes or electrons) should be introduced by
appropriate dopants
 The conducting properties of a conjugated polymers can be enhanced by
doping with suitable dopant.

1.2.Basics characteristics of CPs, doping and structure
 Conductivity Classification of Materials
 Materials in the real world may be classified into three broad categories according to their
room temperature conductivity properties: insulators, semiconductors, and
conductors.
 The overlapping of individual molecular electronic states in all these materials
produces electronic bands.
 So, valence electrons orbitals overlap to produce a valence band.
 while the electronic levels immediately above these levels also coalesce to
produce a conduction band.
 A gap, called the bandgap, generally denoted Eg, exists between these two.

 According to band theory.
 Insulator:- the
forbidden gap
is more.
 Semiconductor:
-the forbidden
gap is moderate
 Conductive
polymer:-the
forbidden gap is
reduced by doping
 Conductor:-The
forbidden gap is
negligible
 Conductivity Classification of Materials

 Doping and Dopants
 Before doping process, an organic polymer, either an insulator or semiconductor
having a small conductivity, typically in the range 10−10 to 101 S/cm.
 After doping a conductive polymer it converted to ‘metallic’ conducting regime
(−1 to 104 S/cm).
 The semiconductor band structure of CPs permits electronic excitation or
electron removal/addition, e.g., from the valence to the conduction band. E.g
this take place by photon energy.

VB – Valence Band
CB – Conduction Band
 On other hand ,oxidation/reduction involves the removal of electrons from the VB
by anions as counter ions in the polymer chain and adding electron to CB by
cations as counter ions in the polymer chain respectively .These causes the
presence of charges on the CPs .
 These charges are delocalized over several monomer units, causing a relaxation
of the geometry of the charged polymer to a more energetically favored
conformation.
 This oxidation process resulting in the presence of positive charges and
associated anions as counter ions in the polymer chain is called p-type doping.

 Reduction similarly generates a negatively charged CP and associated
cations as counter ions in the polymer chain is n-doping.
 The chemical oxidation of the CP by anions, or its reduction by cations, was
originally called doping
 The associated anions/cations, i.e., the counterions, were called dopants
 Both the above processes, oxidation and reduction, impart conductive
properties to the CP.

 Dopants may be small anions or cations, e.g., ClO4 or Na+, or large polymeric species, such
as the “polyelectrolytes” poly(styrene sulfonic acid) and poly(vinyl sulfonic acid).
Typical Dopants for CPs

 A CP that is in its undoped, i.e., neutral, state is generally termed pristine
(sometimes, virgin).
 For instance, a CP with one dopant anion per four monomer units would have a
doping level of 0.25 or 25%. It is generally not possible to have a 1:1 doping, i.e.,
a 1.0 (or 100%) doping level.
 Increased doping leads to increased conductivity, via creation of more mobile
charges, and
 The maximum doping levels achievable vary for different CPs and different
dopants .For example ,The doping level for poly(acetylene) typically varies
from 0.5% to 8%.

 Typical maximum doping levels

 In p-doping :- The polymers which have conjugation in the backbone when treated with
electron deficiency species (Lewis acids) like FeCl3Or iodine vapour /CCl3,there take
oxidation and positive charge is created in the molecule.
 Removal of one electron from the pi-backbone of a conjugated polymer forms a radical
cation (polaron,) ,which on losing another electron forms bipolaron.The delocalization of
this positive charges causes electrical conduction.

 Example for p-doping and the mechanism of conduction in polyacetylene
 Polyacetylene is a
neutral molecule
with extended
conjugation .Its
conductivity is very
low .
 On doping with
suitable dopant
conductivity
increases .
• Polaron can be a radical
cation or radical anion
.In this case polaron is a
radical cation

 Example for p-doping and the mechanism of conduction in polyacetylene
• In this case
bipolaron of
two positive
charge
(dication)
are formed.
• In soliton
charge
delocaliza
tion take
place in
opposite
to each
other.

 In n-Doping:- The conjugated polymer treated by lewis base (electron rich
species) and the polymer is reduced then, the negative charge develops on
polymer backbone.
Polymer +lewis base =n-Doped polymer (reduction of polymer)
For example
 In n-Doping:- The mechanism of conducting in first step the formation of polaron
,and in the second step ,bipolaron is formed.

• Polyacetylene is a
neutral molecule
with extended
conjugation .Its
conductivity is very
low .
 Step of n-Doping and conduction mechanism
• In this case
polaron is
radical anion.
• Bipolaron of
polyacetylene in
this mechanism
contains two
negative charge
(dianion)

 Step of n-Doping and conduction mechanism
• When the charge
delocalized along
the back bone
polymer in opposite
direction a soliton is
formed.

 Doping methods
 If the CPs are not already synthesized in the doped state, doping of pristine
CPs can be accomplished by.
 Chemically, e.g., by exposure to a solution or vapour of the dopant
 Electrochemically, by subjecting the CP, generally in a solution, to an
applied potential.
 When a positive potential is applied to a CP immobilized inert electrode, the
dopant anion moves in from the solution into the CP toward delocalized
charge sites on the CP, and anionic/p.type doping occurs. Where,P(Py) is
polypyrrole

 Doping methods
 If a negative potential is applied in solution to a CP immobilized on an
electrode, a cation would move in from the solution into the polymer. This
would be termed cationic, or n-type doping. Where P(PP) is Poly(p-
phenylene)
 Doping in Polyaniline (PAN)
 In the case of PAN, a special case of doping mechanism takes place based on
acid-base chemistry. PAN exists in different oxidation states.

Conducting polymers
11/8/2023 37
 Doping methods
 Polyaniline can be found in one of the three following idealized oxidation states , which are
polymerized from the inexpensive aniline monomer.
1. Leucoemeraldine base– white/clear & colorless (2[C6H4NH]n 2[C6H4N] ) m with n =
1, m = 0 is the fully reduced state.
2. Pernigraniline– blue/violet 2[C6H4NH]n 2[C6H4N] ) m is the fully oxidized state (n =
0, m = 1) with imine links instead of amine links.
3. Emeraldine– green for the emeraldine salt, blue for the emeraldine base
(2[C6H4NH] 2[C6H4N] ) m with (n = m = 0.5) often referred to as emeraldine base
(EB), is neutral.
 If Emeraldine base doped (protonated) it is called emeraldine salt (ES), with the imine
nitrogens protonated by an acid.

The three base forms of PAN, namely,
(i) The colorless leucoemeraldine base (LB), a completely reduced form;
(ii) The blue emeraldine base (EB), a partially oxidized state; and
(iii) The violet pernigraniline base (PB), a fully oxidized state
 Doping methods
y=0.5
y=1
y=0

Conducting polymers
11/8/2023 39
 Doping methods
 Upon doping Emeraldine base with acid, the resulting emeraldine salt form of
polyaniline is highly electrically conducting polymers.
 Leucoemeraldine and pernigraniline are poor conductors, even when doped with an
acid.
 The colour change associated with polyaniline in different oxidation states (Fig below.) can
be used in sensors and electrochromic devices.
Fig
.Colour
change
associated
with
polyaniline
in
different
oxidation
states

 Doping methods

 Protonation of PAN
 Protonation by acids (pKa < 5.5) results in the corresponding salts of
EB and PB.
 The protonation process by which the blue EB is transformed into the
green emeraldine salt (ES).
 Among five oxidation states of PAN, only the ES form is conducting
 Doping methods

Conducting polymers
11/8/2023 42
 Doping methods
 Protonation of PAN
Class activity 1:-Why Leucoemeraldine and pernigraniline
are poor conductors, even when doped with an acid?

 Comparison of conductivities of various materials
 Doping methods

 Real and Idealized Structures:-
Reading assignment

1.3. Basics of CP Synthesis
 Categories and Classes of Syntheses
 CP syntheses fall broadly into the two categories:-
 Syntheses also fall broadly into two other classes:
 Chemical :-Mostly by Condensation Polymerization
 Electrochemical polymerization :-Almost all by Addition Polymerization
 Condensation polymerization (also sometimes called step-growth
polymerization).
 Addition polymerization (also sometimes called chain-growth
polymerization)

 Condensation polymerizations generally involve loss or elimination of a chemical
species. Example: the elimination of water from an alcohol and an acid group, yielding
poly(esters
 Addition polymerizations involve the well-known chain initiation, chain propagation,
and chain termination steps, with chain initiation generally being through generation of a
highly reactive, radical ion.

 Representative Syntheses: Chemical
 The first CP syntheses, e.g., of poly(acetylene) or poly( p-phenylene sulfide),
were chemical syntheses.
 Poly(thiophenes) The chemical synthesis of poly(thiophene) represents a
typical condensation polymerization, employing a Ni catalyst:

 Poly(acetylenes): The most common chemical synthesis of poly(acetylene), by Shirakawa
(1970), was by Ziegler–Natta polymerization.
 In a typical procedure, a toluene slurry of AlEt3 and Ti(OBu)4 (4:1) is used to coat a
reaction vessel (e.g., a Schlenk tube).
 Acetylene gas is then admitted at pressures ranging from 2 cm to 76 cm Hg. A well-
formed poly(acetylene) film starts growing in a few seconds, up to an hour. A 0.5 cm
thin self standing film was obtained.

Typical Zieger Natta Catalyst
and Co-catalyst
 It used for the polymerization of olefins (
ethylene , propylene 1- hexene etc ) .

 Another route to synthesize high conductivity Polyacetylene
 In this synthesis route the monomer corresponding to the CP is not the
starting substance.
 The starting substance is a cyclobutene, 7,8-bis(trifluoromethyl)-
tricyclo4,2,2,0 -deca-3,7,9-triene, which yields a soluble precursor
polymer with the use of Ziegler–Natta catalysts such as WCl6 :(C6H5 )4
Sn and TiCl4 :(Et))3Al, via ring-opening metathesis.

 Another route to synthesize high conductivity Polyacetylene
 This then undergoes an elimination, losing hexafluoroxylene, to yield soluble
poly(acetylene).
 cyclobutene, 7,8-
bis(trifluoromethyl
)-tricyclo4,2,2,0 -
deca-3,7,9-triene,
Poly(acetylene)
hexafluoroxylene

1. Representative Syntheses: Chemical
 Poly(phenylene Sulfide):- The common synthesis of poly(phenylene sulfide) is also a
typical and simple example of a step-growth, condensation polymerization:

2. Representative Syntheses: Electrochemical.
 A similarity with chemical polymerizations, also use an initial electrochemical step,
generally oxidation via an applied potential, to generate the radical ion, which then
initiates the polymerization.
 0.05 M pyrrole monomer is taken, after molecular sieve/alumina purification, in 0.2 M
LiClO4 or Et4NClO4 in acetonitrile in an electrochemical cell.
 A three-electrode (working, counter, reference electrodes) or a two-electrode (working,
counter) mode may be used.
Example :- Polymerization of PPy By Electrochemical.

2. Representative Syntheses: Electrochemical
 The counter electrode may be Pt, graphite, or a number of other materials, and the
working electrode choice is dependent upon the end use for the polymer, for example,
Au for reflectance studies, graphite for bulk CP powder, and a transparent electrode (in
acetonitrile only) for spectral studies.
Example 1:- Polymerization of PPy By Electrochemical.
 For the former, an applied potential of +0.8 V vs. Ag/AgCl and for the latter +1.4 V are
adequate for polymerization.
 The polymer, obtained on the working electrode, is washed in the solvent of
polymerization and dried.

Example 1:- Polymerization of poly(aniline) by Electrochemical.
2. Representative Syntheses: Electrochemical
 In the case of poly(aniline),a 0.05 M aniline solution in 0.2 M aqueous H2 SO4 also
containing an additional dopant, e.g., Cl, if desired, is taken in the electrochemical
cell.
 Polymerization is carried out at ca. +0.5 V vs. Ag/AgCl in three-electrode mode or
+1.0 V in two-electrode mode.

Conducting polymers
11/8/2023 56
 Problems and Exercises

Question ?
THANK YOU!!

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