STUDIES ON ANILINE BASED ELECTRICALLY CONDUCTING
POLYMERS
i ' I
THESIS SUBMITTED FOR THE AWARD OF THE DEGREE OF
w IN
CHEMISTRY
BY
YAHYA A. I. M.Phil.
DEPARTMENT OF CHEMISTRY ALIGARH MUSLIM UNIVERSITY
ALIGARH (INDIA)
2004
T6516
'VtJiostmi. t» mif XLnme. snd (i yayac
t^ikt. loving mamatif o^ my IVgfntippa mnd.
KuckuHtcfipA
ALIGARH MUSLIM UNIVERSITY ALIGARH-202002 (INDIA)
Dated: December 7,2004
CERTIFICATE
This is to certify that the work embodied in this thesis entitled
"^STUDIES ON ANILINE BASED ELECTRICALLY CONDUCTING
POLYMERS" is the original contribution of Mr. Yahya, A.I., carried
out under the guidance and supervision of the undersigned and is suitable
for the award of the degree of Doctor of Philosophy in Chemistry of
Aligarh Muslim University, Aligarh. This work has neither been submitted
nor is it being submitted, for any other degree.
FAIZ MOHAM]VL\D, D.Phil. (Sussex) Senior Lecturer, Department of Applied Chemistry (CO-SUPERVISOR)
^ ^ /
AFAQ AHMAD, Ph.D. Reader,
Department of Chemistry (SUPERVISOR)
CONTENTS
Page
Acknowledgements i
Chapter-1 INTRODUCTION
1.1 ELECTRICALLY CONDUCTING POLYMERS 1
1.2 Electrical Conduction 6
1.2.1 Band Theory 6
1.2.2 Hopping and Tunneling Model 10
1.2.3 Percolation Model 10
1.3 Doping in Conducting Polymers 12
1.3.1 Chemical doping 13
1.3.2 Electrochemical doping 14
1.3.3 Radiation doping 15
1.3.4 Doping by acid-base chemistry 15
1.3.5 Undoping 16
1.4 Charge Carriers or Defect in Conducting Polymers 16
1.5 Degradation and Stability of Conducting Polymers 20
1.6 Objective 22
References 24
Chapter-2 POLYANILINE - A SHORT REVIEW
2.1 Introduction 30
2.2 Different Forms of Polyaniline 31
2.3 Doping and Electrical Conductivity 35
2.4 Mechanism of Charge Transport 38
2.5 Synthesis 42
2.5.1 Chemical Synthesis 43
2.5.2 Electrochemical Synthesis 44
2.6 Characterization 48
2.6.1 Electronic Spectra 49
2.6.2 Vibrational Spectra 50
2.6.3 Nuclear Magnetic Resonance (NMR) Spectra 51
2.6.4 Electron Spin Resonance (ESR) Spectra 53
2.7 Substituted Polyanilines 56
2.8 Thermal Degradation and Stability 60
References 63
Chapter-3 EXPERIMENTAL
3.1. Materials 76
3.2. Polymer Preparation 77
3.2.1. Copolymer of aniline with o-toluidine, P(AcoOT)l :l 79
3.2.2. Copolymer of aniline with /j-toluidine, P(AcoPT) 79
3.3.3 Copolymer of aniline with p-methoxyaniline, P(AcoPMA) 79
3.2.4 Copolymer of aniline with o-nitroaniline, P(AcoONA)-l:l 79
3.2.5 Copolymer of aniline with o-nitroanilinc, P(AcoONA)-l :3 80
3.2.6 Copolymer of aniline with w-nitroaniline, P(AcoMNA) 80
3.2.7 Copolymer of aniline with/?-nitroaniIinc. P(AcoPNA) 80
3.2.8 Copolymer of o-toluidine with/?-methoxyaniline, P(OTcoPMA) 80
3.3. Solubility 81
3.4. UV-VIS Spectral Studies 81
3.5. FTIR spectral studies 81
3.6. Electrical Conductivity Studies 81
3.7. Electron Spin Resonance Spectral Studies 82
3.8. Thermogravimetric Analysis 83
References 84
Chapter - 4 COPOLYMERS OF ANILINE WITH o-, m- and p-
NITROANILINES
4.1 Introduction 85
4.2 Results and Discussion 85
4.2.1 Polymer Synthesis 85
4.2.2 FTIR Spectral Studies 92
4.2.3 Electronic Spectral Studies 97
4.2.4 ESR Spectral Studies 109
4.2.5 Electrical Conductivity and Charge Transport Studies 112
4.2.5.1 Electrical Conductivity 112
4.2.5.2 Charge Transport 126
4.2.6 Thermogravimetric Analysis 139
References 157
Chapter-5 COPOLYMERS OF ANILINE AND o-METHYLANILINE
o-TOLUIDINE) WITHp-METHOXYANILINE {p -ANISIDINE)
5.1 Introduction 163
5.2 Results and Discussion 163
5.2.1 Polymer Synthesis 163
5.2.2 FTJR Spectral Studies 165
5.2.3 Electronic Spectral Studies 169
5.2.4 ESR Spectral Studies 174
5.2.5 Electrical conductivity and charge transport studies 175
5.2.5.1 Electrical conductivity 175
5.2.5.2 Charge transport 179
5.2.6 Thermogravimetric Analysis 184
References 192
Chapter-6 COPOLYMERS OF ANILINE WITH o- METHYLANILINE
(o-TOLUIDINE) AND/;-METHYLANILINE 0;-TOLUIDINE)
6.1 Introduction 196
6.2 Results and Discussion 196
6.2.1 Polymer Synthesis 196
6.2.2 FTIR Spectral Studies 200
6.2.3 Electronic Spectral Studies 205
6.2.4 ESR Spectral Studies 213
6.2.5 Electrical conductivity and charge transport studies 214
6.2.5.1 Electrical conductivity 214
6.2.5.2 Charge transport 217
References 222
Conclusion 225
List of Publications 228
Acknowledgements
Above all, I am indebted to the grace of "One Universal Being" who inspires the entire humanity towards truth and knowledge.
I feel profound elation in expressing my deep sense of gratitude to my supervisors, Dr. Afaq Ahmad and Dr. Faiz Mohammad for their valuable guidance, imflagging interest and unflinching support though out the course of this investigation and in shaping this thesis.
I am extremely thankfiil to Prof. Kabir-ud-din, Chairman, Department of Chemistry, for his constant encouragement. The laboratory facilities and encouragement extended by former Chairmen of the Department of Chemistry, Prof. M.A. Beg, Prof. A.A. Khan and Prof. N. Islam, is also thankfully acknowledged.
I am thankful to Prof R.J. Singh and Dr. M. Ikram, Department of Physics, for providing ESR facilities. Thanks are also due to Prof. K.M. Shamsuddin, former Chairman, Department of Applied Chemistry, and RSIC, Nagpur, for providing facilities for material characterization. Timely help rendered by Prof P.M. Shafi and Dr. M.M. Musthafa is thankfully acknowledged.
I am especially thankful to Dr. P. Muhammad, Principal, Unity Women's College, Manjeri and Dr. Pakrutty, the former Principal for their encouragement and for granting me leave for the completion of this work.
Constant encouragement by Dr. Suhail Sabir and Dr. Rafiuddin is gratefully acknowledged. The help rendered by Dr. Sreejith M. Nair, Dr. Noushad Ali P.M., Dr. Mujeeb V.P., Mr. Sajid M.B., Dr.Amir Al-Ahmed, Mr. S. Mustafa Kausar, Mr. Kunhalavi, Mr. Inamuddin, Ms. Pooja, Ms. Sarita, Mr. Rasheeduddin and all other friends is thankfully acknowledged. Thanks are also due to my departmental colleagues Dr. Basheer and Ms. Jyothi for their kind corporation.
I feet deeply obliged to my Wappa, Umma, Annan, Ashath, Mukhthar, Aji, Kochuwappa, Koyakkaka, Myni, (Late) Wappappa and Meera Kochuwappa and all other family members for their love, support and encouragement in multifarious ways though out my academic pursuits. Constant motivation rendered by Riyas V.P. and cooperation from my in-laws are gratefully acknowledged.
The love and affection, patience and constant inspiration extended by Shinu through out the course of this work is specially acknowledged. Special thanks to my little daughter, Aesha, who is eagerly waiting for the completion of the work.
C H A P T E R !
INTRODUCTION
1.1. Electrical ly Conducting Polymers
The area of polymer research has triggered a great interest
among the wide variety of disciplines of material science. The past
two decades have witnessed the tremendous advancement in organic
conducting molecules and polymers having comparable electrical,
electronic and magnetic properties of metals along with high
environmental stability and processibility of the conventional
polymers. During this period, a number of conducting polymers were
synthesized, which become highly electrically conducting when
partially oxidized or reduced, i.e. 'doped' [1,2].
A unique feature of ail these conducting polymers, otherwise
called 'synthetic metals ' , is the n-electron conjugation throughout
the polymer backbone, that makes them conductive. Present day
material research received much interest in these molecules, because
of this 7t-electron delocalization which gives rise to many interesting
properties [1-8]. Along with it, their light weight, processibility and
high electrical conductivity suggest their potential applications in
many fields such as in rechargeable solid state batteries [9-1 1]. solar
cells [12], optical storage [13,14]. electrochromic display devices
[15-17]. light emitting diodes and electro luminescence [18-22],
Schottky diodes [23-25]. optical signal processing. liMI shielding
[26-27], biosensors and ion-sensors (28.32). corrosion inhibition
[33], field effect transistors [34], nano-composite materials [35] etc.
Recent interests in these materials have been directed towards
chemically controlling their physical and electronic properties via
structural variation and investigating their potential applications.
The major significant development which triggered the
worldwide research activity in the field of conducting polymers was
encountered in 1977 when it was reported by Shirakawa, Heeger and
Chiang that polyacetylene film treated with iodine vapors could
show electrical conductivity values up to lO'' Scm"' [36]. Though
Hatano and coworkers first reported the conductivity of the order of
10"^ Scm~' for their polyacetylene sample, much attention was not
received because of its intractability and poor environmental
stability [37]. In 1970s Ikeda, Ito and Shirakawa of Japan succeeded
in the synthesis of free standing polyacetylene films which showed
an electrical conductivity o f - 5x10'^ Scm' ' [38-40].
Subsequently, in 1973 Walatka and co-workers discovered the
metallic conductivity in polysulfurnitride (SN)x [41]. But this
material remained as an academic curiosity because of its brittleness
and explosive nature. Inspired by the high metallic conductivity of
polyacetylene, researches all around the world turned their attention
into conducting polymers to discover the fundamental features and
their potential applications. Many scientists succeeded in
discovering other conducting polymers such as polyparaphenylene,
polypyrrol, polythiophene, polyaniline etc. In a significant
breakthrough. Ivory et al. suggested that polyparaphenylenes could
also be doped to very high electrical conductivity [42]. Various
electrochemical methods played a major role in synthesizing more
conducting polymers in early 1980's. I'olyaromatic systems such as
polyphenylenesulphide [43,44], polypyrrol [45]. polythiophene 146]
and polyaniline [4,47] soon made their appearance in the family of
conducting polymers.
POLYACETYLENE
PGLYAMUNE
-iO-OO'^'OO):
\ -Oi POLYPYRROLE
-iO-<y^Wy)-.
POLYPARAPHENYLENE
Figure-1.1
POLYTHIOPHENE
=0=0=0=0= Chemical structures of some electrically conducting polymers
This thesis covers a well known conducting polymer whose
chemical structure can generate, sustain and assist the motion of
charge carriers for electrical conduction. For a polymer to be
classified under conducting polymer, it should possess some
essential features [48].
(i) Presence of extended conjugation which provides a great
degree of delocalization of n-electrons in the molecules.
(ii) The degree of conjugation of basic polymer chain, the nature
and conjugation length of side groups, the degree of
crystallinity, counter ions, chain kinks and cross links may
play an important role.
(iii) As pristine conjugated polymers do not contain intrinsic charge
carriers, they must be provided extrinsically, typically by a
charge transfer process, commonly termed as 'doping ' .
Electrically conducting polymers, though largely amorphous,
certain degree of crystallinity is also associated with them. They
consist of both delocalized and localized states. The delocalized n-
electrons along the polymer backbone are highly polarizable. The
ability of electronic delocalization of conjugated polymers, provide
them the highway for charge mobility along the polymer chain [2].
The delocalization''of 71- electrons depends on the extent of disorder,
inter-chain interaction etc. The disorder induced localization plays
dominant role in the metal-insulator transition and transport
properties of conducting polymers. Moreover, the structure of poly
conjugated chain, inter-chain interactions, disorder and doping level
determine the stability of charge carriers such as solitons. polarons.
bipolarons and free carriers in conducting polymers. Hence, a wide
range of behavior from metallic to insulating regimes can be
observed in the transport properties of such materials [49].
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PbotAk h(soperconduc-' tion)
Au,Ag,Cu
I Hg-Fe,Al
0 - 2 -
0 " ^ -
0 - 6 -
0 - 8 -
0-10-
0-12-
o-u.
o-''H -18_J 0
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— Glass Cotton
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Figure-1.2 Electrical conductivity ranges of various materials
1.2. Electrical Conduct ion
1.2.1 Band Theory
Various models such as band model, variable range hopping,
percolation mechanisms etc. have been suggested to account for the
electronic conduction in these materials [50]. According to band
theory, a continuous band of states is produced as a result of
overlapping of the atomic orbitals of the atoms considered in a solid.
Usually, there is a gap between the top of the occupied valance band
and bottom of the unoccupied conduction band, termed as "band
gap". If electrons are present in the conduction band, it will
contribute toward the electrical conduction. If the band gap is
greater than 1.5 eV, the electrons can not be excited from the
valance band to the conduction band and the material remains as an
"insulator". If the band gap is of moderate size (<1.5 eV), some of
the electrons can be excited to the conduction band by the
application of an electric field leaving holes behind the valance
band. These materials behave as "semiconductors". If the band gap
is very small, the thermal energy available at room temperature is
sufficient enough to excite electrons from the top of the valance
band to the bottom of the conduction band and the materials are
termed as "intrinsic semiconductors". I f the materials are doped by
suitable impurities, donor or acceptor bands are created in the band
gap and the materials are termed as "extrinsic semiconductors". If
the valance band and conduction band overlap each other and thus
have no band gap, such materials are termed as "conductors".
The electrical conductivity a is related to the free carrier
concentration ' n ' and the carrier mobility ' n ' which is given by-
CT = n e n - (1-1)
Antiboading Orbital
n=l
Bonding Orbital
n=2 n=4 n=a
• • • •
• * • • • • • • « • • •
• • • • • • • • • ••• • • • • • • •• • ••*• « •• • • • w • • • <t •
• • « • • • • « • • • • • • « • • • • • • • • • • • • • • • • • • • • •
c o N D U
c T I
0 N B A N D
V A L E N C E
B A N D
Figure-1.3 Formation of valance band and conduction band in polymeric materials
Vacuum
Conduction band
o u
i
f
L
a
e PS
4
L
I 1 S.
1 J e o 1—<
s
Figure-1.4 Relationship of polymers 7r-electron band structure to vacuum and other energetic parameters
1
Conduction band
• • • • • • • • • • • • « • • t
• • •
;::: band:::: ••••••••••••••
••••••••••••••
l.n aetal 2.
Conduction band
• • • • • • • • • • • • • • • • 1
::Valcncc::: ::: band:::: •••••••••••••••• ••••••••••••••••
Semiconductor
Conduction band
A
Ej>1.5eV
•••••••••••••••• ••••••••••••••••
••••••••••••••••
:: Valence:: :::: band::::
••••••••••••••••
3. Insulator
Figure-1.5 Schematic representation of band structures
where 'e ' is the electronic charge on the charge carriers. For
intrinsic conductivity, carrier concentration decreases exponentially
with increasing band gap. Since conjugated polymers have relatively
large band gap and low carrier concentration that results into
negligible electrical conductivity.
1.2.2. Hopping and Tunneling Models
Band model satisfactorily explains the electrical conduction in
a highly ordered crystalline lattice. In a disordered amorphous
system like that in conducting polymers, the mobility of the
electrons is impeded by localized electronic states. In such systems,
the conduction occurs via discrete jumps from one state to another
by variable range hopping or tunneling through the potential energy
barrier. Both energetic and spatial distributions of electronic states
are affected by the presence of disorder in the latter. If atoms are
randomly distributed, the density of electronic energy state tails in to
the forbidden zone and the electrons in these tails are localized.
There is an intermediate range of electronic energy states at which
the mobility is very low. Conduction is possible only if the electrons
are excited to higher energy states with greater mobility.
Conduction via localized electrons implies direct jumps across an
energy barrier from one site to the next. Thus, an electron may
either hop over the top of the barrier or tunnel through the barrier
[51].
1.2.3. Percolation Model
Conduction in composite materials depends on the
concentration of the conducting phase. Generally, a sharp rise in
electrical conductivity is observed at a critical concentration of the
conducting phase, the percolation threshold. The percolation model
10
A
c C<3
Site-I Sitc-II
Figure-1.6 Hopping and tunneling conduction
11
successfully describes the electrical conduction in composite of
conducting with non-conducting polymers. Basically it is a
statistical and geometrical approach to explain the shape of the
electrical conductivity curve in composite materials and proceeds
from a statistical distribution of the conducting particles, which
corresponds to a maximum of entropy. It thus denies any interaction
between matrix and conducting particles. As the concentration of
conducting particles increases, they become closer to each other and
at the critical concentration ((t)c), they are finally sufficiently close
together or even touching each other leading to the conduction of
charge-carriers.
1.3. Doping In Conducting Polymers
As mentioned earlier, one of the pre-requisite for a conducting
polymer to exhibit high electrical conductivity is the 7i-electron
conjugation in which Ti-electrons overlap along the conjugated chain
to form a ^-conduction band. A conjugated organic polymer in its
pure or undoped state is an insulator with conductivity of the order of
10~'° Scm~'. A high electrical conductivity is achieved only after
doping of the polymer with a suitable oxidizing or reducing agent
[45,52].
Doping of conjugated polymers increases the carrier
concentration in the polymer and thus generates very high electrical
conductivity [42]. This is achieved by oxidation or reduction with
electron acceptors or donors respectively e.g. the polymer is
oxidized by the removal of the electron by the acceptor, there by
producing a radical cation or hole in the chain. If the hole can
overcome the Coulombic binding energy to the acceptor anion with
thermal energy or at high dopant concentrations, it moves through
the polymer chain and contribute to the electrical conductivity.
12
The behavior of the conductivity with both acceptor and donor
dopants is suggestive of the conventional substitutional 'p ' and 'n '
type doping of a semi-conductors. In conducting polymers, doping is
not substitutional, the doping species reside alongside the polymer
chains and there is a charge transfer process between the polymer
chains and dopant molecules. This doping process is different from
the doping of inorganic semiconductors, where the dopants are added
in parts per million. In conducting polymers, the dopant
concentrations are exceptionally high as doping involves the random
dispersion of dopants in molar concentrations and that in some cases
the dopants constitute about 50% of the final weight of the
conducting polymer composition [53]. Thus, the conducting
polymeric systems are often visualized as charge-transfer complexes.
The dopant ions such as BF4~, C104~, Is" (p-type) can oxidize
the polymer chain to create the positive charges on the conjugated
polymer backbone, i.e. p-type doping. The dopant ions such as Na" ,
Li*, Rb* etc. (n-type) can reduce the polymer to create negative
charges on the polymer backbone, i.e. n-type doping [49]. To a great
extent carrier concentration depends on the doping level, structure of
conducting polymer chain, inter-chain interactions, disorder etc.
The distribution of dopant ions may not be uniform due to the
complex morphology of the polymer matrix. Hence, both the
structure and doping induced disorders may play major roles in the
charge transport [54].
1.3.1. Chemical Doping
The initial discovery of the ability to dope conjugated
polymers involved charge-transfer redox chemistry, oxidation (p-
type doping) or reduction (n-type doping).This can be illustrated
with the following examples [2].
13
p-type doping:
(K-polymer)n+3/2 nyCh) > [(K-polymer)^" iyhUn -—(1.2)
n-type doping:
(7:-polymer)n+ ny Na'^Naphthalide" ^ [(yNa^)(7i-polymer)' ]„
+ Naphthalene (1.3)
1.3.2. Electrochemical Doping
Complete doping to the highest possible dopant concentration
yields reasonably high quality materials. However, attempts to
obtain intermediate doping levels often result in inhomogeneous
doping. Electrochemical doping was invented to solve this problem
where the electrodes supply the redox charges to conducting
polymer, while ions diffuse into (or out of the polymer in case of
undoping) the polymer electrode from the nearby electrolyte for
electroneutrality. The doping level is determined by the voltage
between the conducting polymer electrode and the counter-electrode
at electrochemical equilibrium or by the amount of electronic charge
passed during the process. A particular doping level is precisely
achieved by setting the electrochemical cell at the corresponding
applied voltage and waiting as long as necessary for the system to
come to an electrochemical equilibrium as indicated by the current
through the cell going to zero. Electrochemical doping can be
illustrated by the following examples-
p-type doping:
(Ti-polymer) + x Li^BF4" -^ (n-polymer)^" (XBF4') + x Li^ -—(1.4)
14
n-type doping:
(Tt-polymer) + x Li'^BF4' -> (n-polymer)'" (x Li" ) + x BF4~ —(1 .5)
This technique is used for doping of polymers obtained by
other methods as well as for redoping or further doping. In this
process, only ionic types of dopants are used as electrolyte dissolved
in polar solvents [2].
1.3.3. Radiation Doping
The semi-conducting polymer chain in locally oxidized and
nearby chain is reduced by photo-absorption and charge separation
i.e. electron-hole pair creation and separation into free charge-
carriers.
hv (7i-polymer)n, + (7i-polymer)n -> [ (Ti-polymer)^] '' + [(Tr-polymer)^]''' —(1.6)
where x is the number of electron-hole pairs. In case of photo-
excitation, the photoconductivity is transient and lasts only until the
excitations are either trapped or decay back to the ground state. In
contrast, the induced electrical conductivity is permanent in case of
chemical or electrochemical doping until the charge-carriers are
purposely removed by undoping [2]. High energy radiations such as
y- rays, electron beams and neutron radiations are used for doping of
polymers by neutral dopants. For example, y -ray irradiation in the
presence of SFe gas or neutron radiation in the presence of I2 has
been used to dope polythiophene [54].
1.3.4. Doping by Acid-Base Chemistry
In this doping process, the number of electrons associated with
the polymer backbone does not change. The energy levels are
15
rearranged during doping. Polyaniline was the first example of the
doping of an organic polymer to highly conducting regime by this
process. Protonation by acid-base chemistry leads to an internal
redox reaction and converts the semi-conducting emeraldine base
into highly conducting emeraldine salt. The chemical structure of
the emeraldine base form of polyaniline is somewhat similar to an
alternating block copolymer. Upon protonation of the emeraldine
base to the emeraldine salt, the proton-induced spin unpairing
mechanism leads to a structural change with one unpaired spin per
repeat unit but with no change in the number of electrons.
1.3.5. Undoping
In conducting polymers, the doping process can be reversed i.e.
conducting polymers can be rendered insulating by neutralization
back to the uncharged state which is referred to as undoping or
dedoping or compensation or electrical neutralization etc. Exposure
of oxidatively doped polymers to electron donors or conversely of
reductively doped polymers to electron acceptors effect dedoping
[43,46,46,55,56]. This forms the basis of the application of
conducting polymers in rechargeable batteries/electrodes.
1.4- Charge Carriers or Defects in Conduct ing Polymers
Generally, conducting polymers are largely amorphous in
nature with a very small degree of crystallinity. Besides the
conjugational defects mentioned earlier, these polymers like other
solid polymers contain different kinds of structural kink
irregularities such as cross links, branch- points, conformational
defects etc. Formations of these defects including conjugational
defects are essential as this would determine the chemical and
physical properties of a given polymer. Introduction of particular
16
1. An unstable diradical with spin and without charge.
\ rA_/^_/^_A~A / 2. A stable polaron *p-type* with spin and charge.
3 .A stable bipolaron *p-type* with charge and without spin.
Electron acceptor
4. A stable polaron *n-type' with spin and charge.
. 0
""w^A==rA==^^==/7\-/ Electron donor
5. A stable bipolaron *n-type* with charge and without spin.
\_/f^=JK=J=0^"vy Election donor
Figure-1.7 Formation of polarons and bipolarons in polyparaphenylene on reaction with oxidizing and reducing agents
17
defects such as solitons, polarons and bipolarons are the only means
to attain high electrical conductivity in conducting polymers.
As mentioned earlier, charges added to polymer chains do not
behave as low mass particles, as in the case of conventional
inorganic semiconductors. They are intrinsically localized as a
result of relaxation of the polymer chain round the charge to form
defects that may be of solitons, polarons or bipolarons type [55].
Most of the electronic properties of these materials are determined
by the behavior of the defect states. The theoretical understanding
of them has been achieved in recent years [56-60].
During the polymerization e.g. in case of polyacetylene, it is
possible that the bond alternation of the atoms constituting the
polymer backbone may be disrupted leading to the creation of
domain walls or the so called solitons in such systems [60, 61].
Solitons can either exist in neutral or charged states. Neutral
solitons are electrically neutral, but possess a spin. Charged solitons
are spinless but are electrically charged. The concentration of the
charged solitons can be increased by doping with a suitable dopant.
Another type of defects, is called polarons, which can be
visualized as electrons trapped in a polarized lattice of given
material. They are electrically charged and have spin. The polarons
are radical ions associated with lattice distortion and localized
electronic state in the gap. These are the typical charge carriers
often found in polymers such as polyanilinc. polylhiophene. cis-
polyacetylene etc. whose ground state become energetically
inequivalent when separated by polarons. Population of polarons in
a conducting polymer varies with the temperature and the extent of
doping [62]. When concentrations of the polarons are high enough,
it is possible that two polarons may approach each other and result in
18
c 9
JO I .
> •c o s
•o c o Ic u X
2
L_
Dopant Concentrarion, x (arb. units)
Figure-1.8 Schematic representation formation of polarons and bipolarons with increasing dopant concentration (x) and its impact on electrical conductivity and spin concentration of the conducting polymer
19
the formation of a bipolaron. It is defined as a pair of like charges
(di-ion) associated with a strong local lattice distortion. Bipolarons
are doubly charged but spinless [63,64]. These defects occupy
distinct energy levels, in the band gap of a given polymer.
1.5. Degradation and Stabil ity of Conducting Polymers
Polymers also undergo chemical reactions, provided that the
reactants are made available at the reaction site, leading to
deterioration of useful polymer properties which is termed as
'degradation' . The same is applicable in case of conducting
polymers also. There are many external causes of degradation such
as light, heat, mechanical stress, oxygen, moisture, atmospheric
pollutant etc. along with the factors effective at the time of
processing. Also, the intrinsic factors such as reactive sites, e.g.
super-oxides, defects, chemically reactive groups may degrade the
polymer properties with or without the combinations of external
factors [65]. The inter- and intra-chain reactions between the
reactive sites can alter the chemical structure of conductive
polymers, affecting their dopability and hence the electroactivity
[66,67]. Intrinsic degradation of doped conjugated polymers would
be affected by the reactivity of the dopant and the reactivity of
polymer backbone.
The oxidative degradation proceeds via chemical reactions of
peroxy radicals. In presence of atmospheric oxygen, the polymers
that contain bonds with low dissociation energies, such as 0 - 0 . C-N.
C-Cl and C-C are susceptible to oxygen attack at elevated
temperatures. Usually, thermal stability of the polymers is less than
usually expected because of the accidental inclusion of weak
linkages in the main chain.
20
The conjugated bonds present in conducting polymers, undergo
n->7i*, 7r->7i* and a-^c* transitions, leading to formation of free
radicals on exposure to sun light. The UV radiations containing
enough energy to cause C-C, C-N and C-0 homolytic bond fission.
Thus produced free radicals can react with atmospheric oxygen
leading to oxidation accompanied by depletion of chain length of the
polymer.
The chemical reactions of environmental degradants and those
trapped during synthesis, such as moisture, oxygen, other
environmental gases etc. may also have some degradative effect on
the polymer properties which may involve electrical neutralization of
the polymer backbone through a process generally called
compensation. During chemical compensation an oxidized polymer
(p-type doped) reacts with a reducing agent like NH3 accompanied
by a loss of electrical conductivity [68-70].
Conducting polymers are being used as electrode materials in
non-rechargeable (primary) and rechargeable (secondary) batteries.
These materials are highly sensitive to electrode reactions and have
low catalytic activity towards side reactions, sufficient mechanical
strength, fabricability, low cost etc. The electrode materials must
also possess high stability towards degradation during the passage of
current or the storage. Degradation of electrode materials leads to
instability in electrode potential with time. A good shelf life, i.e.
capable of retaining its charged state is a prequalification for a
commercial battery. The durability studies of polymeric electrodes
may be done galvanostatically or potentiostatically to evaluate their
life in battery applications [66].
The ability of a polymer to retain its useful properties is
defined as stability and the preventive measures undertaken to
21
inhibit degradation process is termed as 'polymer stabilization*.
Various methods are employed to stabilize conducting polymers such
as (i) by incorporating antioxidants such as benzoquinone (ii) by ion
implantation or by predoping the material with strong electron
acceptor prior to oxygen exposure (iii) by encasing the polymer in a
system with reduced oxygen and moisture permeability (iv) by
synthesizing new polymers with less susceptibility to intrinsic
degradation to oxygen and to moisture even at elevated temperature
etc. [71,72].
1.6 Objectives
One of the major problems with the conducting polymers is the
non-processibility by solvent or melt techniques. Most of them are
insoluble in common solvents and undergo degradation before
reaching the melting point. Also they have very poor mechanical
strength and are environmentally unstable. Very recently many
researchers have succeeded in overcoming these problems [73] and
are generating polymer composite and blends [74,75] which can
show better stability and mechanical strength.
This thesis is devoted to the studies of a well-known
conducting polymer, polyaniline, which is one of the
environmentally stable polymers. Here, the monomer is easily
available and much cheaper than the monomers of any other
conducting polymer. Moreover, the synthesis of the polymer by
chemical method is much easier and efficient.
The thesis describes various studies on the polymers derived
from aniline and substituted anilines aimed at a greater
understanding of electric, electronic and thermal properties of the
materials and the problems impeding their applications.
22
Chapter-2 gives a brief description on the piiysics and
chemistry of polyanilines. It gives a summary of the various
methods of synthesis spectral electrical and thermal properties. A
brief discussion on the work so far done is presented.
Chapter-3 deals with the details experimental works carried out
such as materials used, synthesis of the various co-polymers and
different methods employed to study their properties.
Chapter-4 deals with comparative studies on polyaniline and
the copolymers of aniline with o-, m- and/7-nitroanilines
respectively.
Chapter-5 is devoted to comparative studies of polyaniline and
the copolymers of p-methoxyaniline with aniline and o-toluidine.
Chapter-6 deals with comparative studies on copolymers
derived from aniline with o-toluidine and p-toluidine.
23
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29
eh€LflUX-2
J^o^LfcnilinMr -f? ^k&tt IQQIAJQW
CHAPTER-2
POLYANILINES: A SHORT REVIEW
2.1. Introduction
During the last decade, polyaniline has emerged as fascinating and
promising conducting polymer owing to high electrical conductivity,
environmental stability and processibility making it as potential candidate for
electrical and electronic applications [1-5]. It was the first conducting
polymer whose electronic properties could be reversibly controlled both by
protonation and charge-transfer doping [6-8]. Other factors which
contributed to the research interest on polyaniline are- (1) the ease of
preparation i.e. the conversion of monomer to polymer is straight forward, (2)
unique conduction mechanism, (3) polymerization reaction proceeds with
high yield, (4) high environmental stability, (5) it can easily be converted
from an insulator state to conducting form by external protonic acid doping,
(6) it can easily be deprotonated by the use of an alkali, (7) the monomer is
inexpensive and (8) processing of polyaniline is much easier than other
conducting polymers.
Polyaniline is a typical phenylene based polymer having a chemically
flexible -NH group in the polymer chain flanked either side by a phenylene
ring. In its nonconductive undoped or base form, polyaniline has been
referred to as poly(p-phenyleneamineimine) and may be given by the formula
(-C6H4-NH-C6H4-NH-)y(-C6H4-N=C6H4=N-),.y
where the sample average 'y' may vary in the range 0 < y < 1 [9].
Conversion to a conductive form is accomplished by either protonic or
electronic doping. The protonation, deprotonation and other physicochemical
30
properties of polyaniline are due to the presence of-NH groups in the
polymer chain.
Basically polyaniline is an oxidative polymeric product of aniline under
acidic condition and can be considered as a new version of 'aniline black'
discovered in 1862 [10]. At the beginning of the twentieth century, scientists
began investigating the constitution of aniline black. In 1910, Green and
Woodhead [11] were able to present various constitutional aspect of aniline
polymerization. According to them, there are four different versions for
polyaniline, namely, emeraldine, nigraniline, pernigraniline and
leucoemeraldine. They reported that there are four quinonoid stages derived
from the parent compound leucoemeraldine. In accordance with an eight
nuclii structure, the conversion of emeraldine into nigraniline consumes one
atom of oxygen and emeraldine in to pernigraniline consumes two atoms of
oxygen. The reduction of emeraldine into leucoemeraldine consumes four
atoms of hydrogen and the reduction of nigraniline into leucoemeraldine
consumes six atoms of hydrogen. Finally in the reduction of pernigraniline
into leucoemeraldine, eight atoms of hydrogen are required.
In 1968, Surville et al. [12] reported proton exchange and redox
properties with the influence of water on the conductivity of polyaniline.
However, explosive research on polyaniline as a conducting polymer started
only after the discovery by Shirakawa group that iodine doped polyacetylene
showed metallic conductivity [13]. In the modern perspective polyaniline
was thus rediscovered in 1980's [14.15].
2.2. Different Forms of Polyanilines
As mentioned earlier, accepted structures of polyaniline, date back
from the early work of Green and Woodhead [11]. Polyaniline has benzenoid
and quinonoid repeat units connected with nitrogen atoms. It can be
31
f^m-TVMi-J'V-m-^^ ~]
Leucoemeraldine, OxidaticMi State = 0
/ V M U _ / \ _ X ^ W / ^ ^ x r /^^\ MJ -NH—f V N H - 4 - M ' ^ ^ )—N=( >=-N 1/2X /
Emeraldine, Oxidation State=0.5
/ \ \ _ N / \ n / / \ \ ^ . . ^ .
Pemigraniline, Oxidation State = 1
/ \ NH // VmUJ V N = < )=N-' 3 /4 '
Protoemeraldine, Oxidation State = 0.25
/ VM„ / \ K„1L/ \ "- " trwr n.j-",,. Mgraniline, Oxidation State = 0.75
Figure-2.1: Different forms of polyaniline
32
visualized as built up from reduced (B-NH-B-NH) and oxidized (B-N-Q-N)
repeat units where 'B ' and 'Q' denote benzenoid and quinonoid units
respectively [16-18]. Depending upon the oxidation state,i.e. the ratio of
amine to imine nitrogen, it can exist in various unique structures as
represented in figure- 2.1. The unprotonated forms of polyaniline can simply
be given by the formula,
^ ^ N H - 7 V N H
[B]
where the oxidation state of the polymer increases with decreasing value of y.
The average oxidation state (1-y) can be varied continuously from zero
corresponding to completely reduced polymer called leucoemeraldine (light
yellow in color) to 0.5 to give the half oxidized emeraldine (green) and to 1 to
give completely oxidized pernigraniline (violet).
MacDiarmid et al. [19.20] suggested two more oxidation states and
colors for polyaniline, namely, protoemeraldine (light green) and nigraniline
(blue). Of these, protoemeraldine and emeraldine can be protonated at the
imine nitrogen to give the corresponding salt resulting in high electrical
conductivity [19-21]. The degree of protonation depends on the oxidation
state of the polymer and on the pH of the acid used. Complete protonation in
the imine nitrogen atoms in emeraldine base results in the formation of
delocalized polysemiquinone radical cation [16,18,19] followed by a
tremendous increase in the conductivity. Transformation between various
forms of polyaniline is presented in figure-2.2.
The synthesis and characterization of different forms of polyanilines
have been investigated thoroughly by many research groups [20-23]. A
number of new approaches for preparing polyaniline with high intrinsic
33
oy--^oHMQ "u % violet pemigraniline base
N-/oV-'^=<_
green protonated emeraldine
.©
NH n N-
blue emeraldine base
ijK
colourless leucoemeraldine
Figure-2.2: Transformation between various forms of polyaniline
34
oxidation state (high quinonoid imine to benzenoid amine state) have been
reported [22-25]. Considerable discrepancies are also found in the reported
results [24]. XPS studies have been successfully employed to differentiate the
various intrinsic redox states of polyaniline [26,27]. A number of subsequent
studies on the chemically [28] and electrochemically [29,30] synthesized
polyaniline have revealed that the protonation of quinonoid imine (=N-),
benzenoid amine (-NH-) and positively charged nitrogen atoms
corresponding to a particular oxidation state and protonation level can be
quantitatively differentiated in the properly curve fitted core level diagram.
2.3. Doping and Electrical Conductivity
Polyaniline becomes highly conductive, only after a process called
'doping'. Doping of polyaniline with assisted control of electrical
conductivity over a full range from insulator to metal is usually achieved
either by chemically using a protonic acid or by electrochemical means.
Concurrent with doping, the electrochemical potential is moved either by a
redox reaction or by an acid-base reaction into a region of energy where there
is a high density of electronic states, and charge neutralility is maintained by
the counter ions. Therefore, polyaniline in the conducting form basically
exists as a salt.
By controllably adjusting the doping level, electrical conductivity any
where between that of a undoped insulating and that of a fully doped
conducting form of a polyaniline can be achieved. MacDiarmid and Chiang
[31] suggested that the pKg of polyaniline is linearly related to the pH of the
medium as-
pKa = 0.48 pH-0.043 -—(2.1)
35
A theoretical treatment of protonic acid doping was considered by
Reiss [31] using a proton adsorption isotherm, based on strong repulsive
interaction between protons and nearest neighboring nitrogen atoms.
In mid 1980's, MacDiarmid and coworkers [33] have demonstrated that
polyaniline can be rendered conducting through two important routes as
depicted in figure-2.3, namely - (1) Oxidation-either chemically or
electrochemically of the leucoemeraldine base (p-doping) and (2) Protonation
of the emeraldine base through acid-base chemistry (non-redox doping).
In both the routes insertion of counter-ions is involved. Therefore, in
the conducting state polyaniline exists as a polycation with one anion per
repeat unit.
Emeraldine base form of polyaniline was the first example of the
doping of an organic polymer to a highly conducting state by non-redox
doping with aqueous protic acid that is accompanied with an increase in
electrical conductivity by -10 orders of magnitude [33-36]. It differs from
the redox doping in that, the number of electrons associated with the polymer
backbone does not change during the doping process. Protonation by acid-
base chemistry leads to an internal redox reaction and conversion of
insulating emeraldine base to conducting salt. Upon protonation of the
emeraldine base, the proton induced spin unpairing mechanism leads to a
structural change with one unpaired spin per repeat unit, but with no change
in the number of electrons [37,38]. This results in a half filled band and a
metallic state, where there is a positive charge in each repeat unit and an
associated counter-ion. But this remarkable conversion is not well understood
from the view point of basic theory.
The electrical conductivity after protonation depends on the ratio of the
reduced and oxidized units as well as the extent of protonation. It has been
36
/ V^M__/~\ K,u_r"\^^M_/~\ NH NH-
+2A"
-It Oxidation
/ V^•H /^ \ x,u_y \ -NH •+ A
NH- -NH-• + A'
/ \
Internal redox reaction Polaron lattice formation
/ V . u _ / \ x ,u^/ % -NH NH -NH-=< > = + A'
NH +
2H
+1K
* A Protonation
^ ^ N H _ /J-NH- / \ • N - = < > ^ N
—' X
Figure-2.3: Achieving conducting poiyaniline through protonation as well as oxidation
37
postulated that the imine nitrogens of the emeraldine base are preferentially
protonated under strong acidic conditions [31,39]. Direct synthesis of
emeraldine hydrochloride by oxidizing aniline in acidic solution may not
involve the protonated imine structure as an intermediate, but the final
polymer is likely the same [40].
The electrical conductivity of polyaniline increases many folds after
doping which can be tuned between a wide conductivity range depending on
the polymerization conditions. It has been observed that the conductivity
depends on [41-43] - (i) the pH of the solution, (ii) temperature of synthesis,
(iii) degree of protonation, (iv) nature of the oxidant used, (v) degree of
crystallinity; (vi) molecular weight of the polymer and extent of conjugation
and (vii) nature of the substitutent attached to the phenyl ring.
2.4. Mechanism of Charge Transport
While discussing the charge conduction in polyaniline family of
polymers, we have to keep in mind that polyaniline, differs substantially from
other conducting polymers. It is not charge conjugation symmetric i.e. the
Fermi level and band gap are not formed in the centre of the 7i-bond so that
the valence and conduction bands are very asymmetric [44]. Consequently,
energy levels will also differ from other conducting polymers such as
polypyrrol, polythiophene, polyacetylene etc. [45,46]. Secondly, the hetero-
atom, nitrogen, is also within the conjugation path. Third, the emeraldine
base form of the polyaniline can be converted from insulating to metallic state
if protons are added to -N= sites, while number of electrons in the chain held
constant [31].
Protonation leads to the generation of charged defects in the
polyaniline chain and the charge storage on the polymer chain leads to the
structural relaxation that localizes the charges [47]. Also, the conventional
38
distortion of molecular lattice can create localized electronic states, there by
lattice distortion is self consistently stabilized [48]. The charge coupled to
the lattice distortion to lower the total electronic energy is referred to as a
polaron, normally a radical ion, with a unit charge and spin = VS. Polarons are
supposed to be the major charge carries in polyaniline [49]. A bipolaron
consists of two coupled polarons with charge = 2e~ and spin = 0. The energy
increase due to the Coulombic repulsion in the formation of bipolaron is more
than compensated for by the energy gained when the two charges share the
same lattice distortion [50,51]. But the bipolarons are not created directly
and must be formed by the coupling of pre-existing polarons
The degree of unsaturation and conjugation influence the charge
transport via orbital overlap within the molecular chain. The charge transport
sometimes intervened by chain folds and other structural defects. The
connectivity of the transport network is also influenced by the structure of
dopant molecules. Any disturbance in the periodicity of the potential along
the polymer chain induces localized energy states. Localization also arises in
the neighborhood of the ionized dopant molecules due to the Coulombic field.
For better understanding of the nature of the charge transport in
polyanilines, it is necessary to use a wide variety of probes. Direct electrical
conductivity measurement provides insight in to the insulating or metallic
nature of electrons at Fermi level. The studies on electrical conductivity as a
function of temperature and protonation level are reported [52-54]. Zuo et al.
studied the temperature dependent dc and audio frequency conductivity as a
function of protonation level [53]. Similarly, thermopowcr measurement as a
function of temperature and protonation level can also give some insight into
the conduction mechanism. At 38% doping, it is independent of temperature
[54]. Thermopower measurement implies a 'p ' type of metallic behavior.
Measurement of high frequency transport studies provide an important probe
39
away from the Fermi energy to help discriminate between homogenously and
inhomogenously disordered metallic states [55-58]. The microwave dielectric
constant can be used to determine the presence of Drude (free carriers)
dispersion in the electrical response of the sample as well as the plasma
frequency of the free carriers [56,58-61].
It is observed that the DC magnetic susceptibility for a two ring repeat
units decreases rapidly with an increase in the dopant level. Similarly, ESR
measurements can also be used as a probe to study the nature of the defects
and conduction. X-band ESR signals can be resolved in to Lowrentzian and
Gaussian line shapes. However, in the presence of a linear dopants like
paratoluene sulphonic acid there is a pronounced asymmetry with the a/b ratio
[54,62].
The electrical conduction mechanism, effect of disorder and one
dimensionality of the polymers are still strongly debated. As mentioned
earlier, chemical intuition suggests that the nitrogen lone pair orbital of N '
would be highly susceptible to proton addition leading to charge transfer and
thereby formation of delocalized polarons in the n- orbital conduction band.
Another exotic possibility of spinless bipolarons is also considered [63]. ESR
signals observed during electrochemical preparation of conducting samples
seems incompatible with spinless (bipolarons) model but could be interpreted
in terms of conventional one dimensional electron density of state [64]. It has
also been suggested that a "granular metal" model for polyanilines [54,65].
Alternatively, it was proposed that one electron free radical can provide a
hopping mechanism for charge transport by Wnek [66]. He pointed out the
existence of two semiquinone radical dimer, each with positive holes in their
lone-pair that can migrate on amine nitrogen site [53,66]. Not only this,
intermediate radical description the appropriate one for describing simple,
similar model system in solution, but McManus et al. [67] found that the
40
intermediate state responsible for the conductivity of polyaniline exhibited
optical absorption features that were in excellent agreement with those of the
radical state. Later work by Focke et al. [68] established qualitative
agreement between the conductivity of electrochemically prepared polyaniline
samples and the chemistry of free radical intermediate. Cowan et al.
suggested [52] a simple model of electrical conductivity of polyaniline
involving a three dimensional hopping model based on intermediate free
radical state (among amine nitrogen sites), giving a reasonable quantitative
description of the polymer conduction in all stages of oxidation and/or doping
of polyaniline.
Many studies on macroscopic conductivity a(T) follows a power law,
[a(T) oc T ' or T" ] in accordance with the equation a=ao exp(-To/T)'' with a a
constant. The best fit usually achieved for b=0.5.
In their studies, Zuo et al. [53] established that the conductivity is not
fully metallic at any composition. But at all compositions, the conductivity is
that of a granular metal and can be fitted as transport via charging energy
limited tunneling between conducting islands. Their studies were consistent
with percolation among these metallic islands with the presence of an
insulating layer surrounding each island above the percolation threshold.
Metallic Pauli susceptibility studies [69,70] were also in agreement with this
phase segregation of metallic and non metallic regions in polyaniline.
Several theoretical studies on polyanilines have suggested the major
role of ring rotational conformation changes on the polymer electronic
structure. Ginder et al. [71] investigated the effect of electron lattice
coupling via ring rotation on the electronic structure of leucoemeraldine and
its charged defects. They suggested that in leucoemeraldine and other phenyl
ring containing polymers, the charged defect state, particularly hole polaron
41
are associated with localized distortions of the ground state ring tortional
angles towards a planar conformation, in contrast with the bond alteration
defects present in the conducting polymers.
Bredas et al. [72-74] have pointed out that polyaniline has only one
defect-polaron or bipolaron-band in the gap, unlike other conducting
polymers, where two defect bands are always observed. Heeger [75,76] has
shown that intrinsic self-localization in quasi-one dimensional systems are
especially sensitive to localization induced by disorder. Disorder induced
localization is known to convert doped conducting polymers from true metal
with large mean free paths and coherent transport into poor conductors in
which the transport is limited by phonon assisted doping.
2.5. Synthesis
Generally polyaniline is prepared either by chemical or by electro
chemical oxidation of aniline under acidic conditions. Chemical method is
considered to be more useful than electrochemical method from the view
point of mass production of polyaniline. Whenever thin films and better
ordered polymers are required, electrochemical method is preferred. Both
electrochemical and chemical methods of polymerization follow head to tail
coupling mechanism [68,77]. Electrochemical experiments have indicated
that the oxidative polymerization of aniline occurs at 0.8 V, i.e. the oxidants
such as S208^~, H2O2, Ce"* , Cr207^", I0^~ etc are capable of effecting the
chemical oxidative polymerization of aniline. Toshima et al. [78] have
succeeded in polymerizing aniline and its derivatives by using Cu(II) salts
and oxygen in acetonitrile-water medium.
Another important aspect regarding polyaniline synthesis is the acid
dissociation constant pKg, because the protonation equilibria involve
exclusively the quinone diamine segment in polyaniline, having two imine
42
nitrogens pKai=1.05 and pKa2=2.55 [79]. Therefore, any acid whose pKa
value falls within this range can act as a dopant. Acids having pKg value
around that of an anilinium ion (pKa = 4.6) is suitable as a solvent [80]. The
properties of final polymeric products depend on the chemical nature of the
acid used, pH of the acid and temperature of synthesis.
2.5.1. Chemical Synthesis
Polyaniline is generally synthesized by chemical method by dissolving
aniline in HCl or H2SO4 using ammonium persulphate as oxidant [81-86].
This polymerization is a two electron change reaction and hence the
persulphate requirement is one mole per mole of the monomer. Higher
quantity of persulphate is generally avoided as it causes oxidative degradation
of the polymers. For better polymer formation both the aniline solution in
protonic acid and oxidants are pre-cooled to 3-5°C. The mixture is stirred for
two hours and the precipitated emeraldine salt is filtered, washed and dried.
The emeraldine base is prepared by treating the salt with ammonium
hydroxide. The polymerization process can be represented as-
Ar NH2 Ar NH2
Ar-NH2 • Ar-NH-Ar-NH2 •
S208^' -2e~,-2H*
Further oxidative coupling
Ar-NH-ArNH-ArNH2 • Polyaniline -—(2.2)
High molecular weight Polyanilines are made possible by lowering the
temperature [87,88]. Mattoso et al. [87] reported that polyaniline with a
molecular weight of 417000 was prepared by oxidative polymerization of
aniline with ammonium peroxy disulphate at about -40°C. Salt such as LiCl,
43
CaCb or LiNOs etc. added to the reaction mixture also favor the formation of
polyaniline with higher molecular weight. Yue et al. [89,90] reported the
synthesis of sulphonated polyaniline with aqueous processibility and pH
independent conductivity for pH < 7.5. Later, Wei and Epstein [91] reported
an alternative approach to synthesis sulfonated polyaniline with a higher
sulphur to nitrogen ratio and with a higher solubility. Just as fuming H2SO4
is used to improve the processibility, Chen et al. [92,93], Roy et al. [94] and
Wei et al. [95] have reported that, introduction of a functional group can
make polyaniline soluble in organic solvents or in aqueous solution.
Recently, Lin et al. [96] reported the synthesis of water soluble polyaniline by
a biological route using an aqueous solution containing aniline, sulphonated
polystyrene, hydrogen peroxide and peroxidase enzyme. They used highly
hydrophilic sodium dodecylsulphate as a template to aid the formation of a
polyaniline with more ordered, para-directed and head to tail polymerized
structure to form poIyaniline:sodium dodecylsulphate complex to make it
water soluble. Very recently, Zhe Jin et al. [97] used a novel method for
polyaniline synthesis with the immobilized peroxidase enzyme.
2.5.2. Electrochemical Synthesis
Electrochemical synthesis of conducting polymers is an electro-organic
process in which the active species is generated on the electrode surface
through electron transfer between the substrate molecule and the electrode.
The substrate molecule is transformed to a radical cation or anion and the
reaction is diffusion controlled. The radical generated react faster than they
can diffuse away from the electrode vicinity. Electrode materials normally
used are transition metals or noble metals. Anodic oxidation of aniline is the
preferable method to obtain clean and better ordered polymer as a thin film.
The oxidation of an organic substrate of the anode surface can occur by either
of the following steps as shown in figure-2.4.
44
R at anode
- • R
• RN^ Addition Reaction
• R + R disproportionation
- • R - R
- • R -e
- • R^ Carbocation
>"(RRr dimerion
• R- dication
Figure-2.4. Basic principle involved in the anodic oxidation of organic substrate [80, 98]
ArNH,
Acidic condition
-> NH2-Ar-Ar-NH2
Neutral condition
- • Ar-NH-Ar-NH,
Allcaline condition
•> R-Ar-N=N-Ar-R
Figure-2.5. The course of an anodic reaction of aniine under different electrolytic conditions
45
From figure-2.5, it is clear that the reaction is highly pH dependent.
Electrochemical method also offers an opportunity to carry out various in-situ
spectroscopic studies as well as conductivity and doping measurements level
at various potentials by use of QCMB (Quartz Crystal Micro Balance).
Electrochemical synthesis is achieved by three ways viz.
1. Galvanostatic - constant current, 1-10 mA during electrolysis,
2. Potentiostatic-keeping potential constant, 0.7-1.1 V versus
standard electrode and
3. Sweeping potential - between two potential limits -0.20 V to 1.0
V versus some standard electrode.
The electrochemical polymerization of aniline was first reported by
Litheby in 1862 [10] as a test for the determination of small quantities of
aniline. Almost 100 years later in 1962, the interest in the electrochemistry
of aniline got revived, when Mohilner et al. [94] reported the mechanistic
aspect of aniline oxidation, Buvet et al. [100] studied the influence of water
on conductivity measurement.
The electrochemical oxidation of aniline into polyaniline has been
studied extensively by Adams et al. [101]. They established that the
electrochemical oxidation product of solution of aniline (10 m mole) in 0.1 M
H2SO4 was identical with emeraldine suggested by Green and Woodhead
[102] and reported electrokinetic parameters for aniline oxidation such as
transfer coefficient and reaction order from concentration dependency. They
also proposed an oxidation mechanism for the preparation of emeraldine
which was considered to be a head to tail oligomer, an octamer rather than a
polymer. The electrolysis when conducted in a basic media, the major
product was observed to be is the head to head dimer, azobenzene [103].
46
Major interest in the electrochemistry of polyaniline was started after
the discovery that aromatic amine, pyrrol, thiophene etc. can be polymerized
anodically to a conducting film [104,105]. Diaz et al. [106], Genies & Syed
[107] and Noufi et al. [108] reported the preparation of polyaniline in an
aqueous acid solution using platinum electrode by cycling a potential between
-0.2 to 0.8 versus standard calomel electrode.
Taskova and Milcher [109] reported that the growth of polyaniline film
was drastically enhanced when the pulse potentiostatic method was used
[110]. They indicated that the pulse method did not affect the formation of
first nuclei, but the early stage, where the oligomerization reaction and
further growth up to the formation of polymer films took place.
Cui et al. [111,112] attributed the origin of the difference observed
during the static potentiostatic and potentiodynamic growth of polyaniline to
the accumulation of larger amounts of degradation products trapped within
the polymer matrix when the static potentiostatic step is used.
Yang and Bard [113] reported that polyaniline film growth by the
potentiodynamic method at faster scan rates are more uniform and compact
than those grown at slower scan rates. Rubinstein [114] et al. pointed out that
the temporary application of a cathodic bias during the anodic polyaniline
growth brought about an effect of electrochemical film annealing. Nunziante
and Pistoia [115] reported that polyaniline film grown galvanostatically had
fibrous morphology.
MacDiarmid et al. [116] described that the first redox peak of
polyaniline during electrochemical synthesis arises from the transition of
leucoemeraldine to protoemerldine as well as protoemeraldine to emeraldine
and the second oxidation process from the further oxidation of emeraldine to
nigraniline and later to pernigraniline. The pH dependency of the two redox
47
processes was supported later by Sawai et al. [117] from direct observation of
pH change.
Yoneyama et al. [118] demonstrated that oxidized polyaniline films
become inactive when they are deprotonated with weak acidic or alcoholic
solutions. Yildiz et al. [119] studied the electrochemical behavior of
polyaniline in acetonitrile and reached the same conclusion. Through
Scanning Electrochemical Microscopy, Frank and Denault [120] suggested
the occurrence of an ingress and egress of protons from polyaniline films
during the redox process. Peter et al. [121] suggested that the
electroneutrality of the film is maintained primarily by rapid proton transfer
through the film. Polyaniline prepared at different potentials showed different
conductivities. Efforts were made to measure their values in situ by Paul
[122] and Gholamian [123] using closely spaced ultra microelectrodes.
Gholamian et al. [123] constructed from their data, a comprehensive 3D
conductance surface 'state diagram' in which polyaniline conductivities are
plotted as a function of applied potential. Focke et al. [124] also obtained a
similar result and found that the type of anion present also affects the
conductivity.
More complete state diagram was compiled by Genies and Vieil [125].
Later, Genies and Tsintavis [126] suggested that the maximum conductivity is
obtained when anion insertion is complete. Yoneyama et al. [127] concluded
after examining eight anions of different basicity that, the greater is the
basicity of the counter anion, the lower is the conductivity of the polyaniline
films
2.6. Characterization
Various methods are generally used to characterize polyaniline. Most
widely used are the spectroscopic methods. Electronic and vibrational energy
48
levels of molecules are affected by their environment. The interaction is
determined by the electronic charge distribution in the ground and excited
electronic states, polymer-dopant interaction etc. These effects are reflected
in the shape and intensities of the absorption bands.
2.6.1. Electronic Spectra
UV-VIS-NIR spectroscopic technique is widely used to characterize
conducting polymers. The spectra can be treated as a qualitative measure of
the overlap of their 7i-orbitals. The existence of a specially extended n
bonding system in polyaniline gives rise to electronic transitions in UV-VIS
region. The dominant peak is usually associated with n -> n* transition.
Generally polyaniline salt shows three absorption positions corresponding to
n —> n* (-320 nm), polaron -> n* (-420 nm) and n -^ poloran (-800 nm)
transitions [128]. Emeraldine base generally shows two absorption
corresponding to 7t -> 7i* (320 nm) and exciton transitions corresponding to
benzenoid -> quinoid (-630 nm) rings. The UV-VIS spectrum of polyaniline
depends strongly on the oxidation state. Peak around (-310-330 nm) is
associated with 7i -> TC* transition in benzenoid ring [129,130]. Partial
oxidation of polyleucoemeraldine results in a second peak in visible region
(-630 nm) whose intensity increases with polymer oxidation state which is
associated with exciton transition in quinoid rings. Further oxidation to
pernigrahiline results in a blue shift in the quinoid as well as 7i —> 7t*
transitions. In polyemeraldine salt, a band around 400 nm is observed and a
steadily increasing absorption tail extended towards NIR region (-800 nm).
Monkman et al. [131] suggested that the band at -840 nm is due to the
trapped excitons centered on quinoid moieties generated during oxidation and
the shoulder at 425 nm is due to the semi-quinone. Cao et al. [132] have
reported three spectral features at 1 eV, 1.5 eV and 3 eV for a polyaniline
49
film spin cast from a sulfuric acid solution on sapphire substarate. Tanaka et
al. [133] also have carried out extensive study on the electronic and
vibrational spectra of polyaniline. Yue et al. [134] reported that the n -^ n*
transition in a self-doped sample of polyaniline occurs at 320 nm (-3.88 eV)
and polaron transitions at 435 nm (-2.85 eV) and 850nm (1.46 eV). In
another report by Yue et al. [135], the effect of sulphonic acid in polyaniline
backbone in causing shift in UV-VIS absorption peaks are discussed. They
suggested that sulphonation causes an increase in the adjacent phenyl ring
torsional angles to relieve the steric strain, affecting the electronic transition.
Wei et al. [136,137] synthesized highly sulphonated polyaniline with aqueous
processibility. They used both emeraldine and pernigraniline as starting
materials to prepare sulphonated polyaniline. They reported that UV-VIS
absorption bands of leucoemraldine base - sulphonated polyaniline (LEB-
SPAN) in aquous O.IM NH4OH are blue shifted relative to those of emraldine
base - sulphonated polyaniline (EB-SPAN).
2.6.2. Vibrational Spectra
FTIR spectroscopy is a powerful tool to study the structural changes
that occur during doping and dedoping processes. Oxidative doping generally
gives rise to new infra-red active modes whose appearance is associated with
the formation of charged defects. Usually the observed modes are
independent of the type of dopant anion inserted in the polymer chain.
Hassan et al. [138]. Kuzmany et al. [139], Shacklette et al. [140], Epstein et al
[141] and Bloor et al. [142] have reported FTIR results of polyaniline. It has
been shown that absorption frequencies are strongly influenced by the
electrochemical potential.
It has been reported that the intensity of-1570-1590 cm"' band relative
to -1500 cm'' is a measure of the degree of oxidation of polymer film.
50
Similarly, the peak at -1375 cm'' which is due to the formation of semi
quinoid ring -N ring mod [144] whose intensity also increases with the
degree of oxidation [143] and fully quinoid ring absorption observed at -1630
cm'' and 1150 cm''. In over oxidation, the decrease in intensity at 1630 cm'"
indicates degradation of the quinoid structure as reported by Hazda et al.
[144].
During electrochemical doping, the absorption intensities increase at
-1563, -1477, -1300, -1244, -1158 and -1041cm'' while absorption at 1507
cm"' shows a decrease in intensity. When the anodic potential exceeds 600
mV, increases in absorption intensities at 1627, 1578, 1507, 1376, 1339 and
1100 cm'' are observed. During oxidation, absorption due to benzenoid
-1500 cm"' decrease while an increase in intensity at-1570 cm'' and -1470-
1490 cm'' due to quinoid ring is observed.
Neugebauer et al. [145] have studied FTIR spectrum of polyaniline by
carrying out FTIR measurements during cyclic voltammetric experiments at
pH=4.5 and observed a sharp increase in absorption at -1380, -1490, -3880,
-1245 and -1140 cm'' and decrease at -1600, -1510 and -1300 cm'' during
oxidation. These changes may be due to the transition of benzenoid (-1600
cm'' and -1510 cm"') to quinoid (-1580 cm"' and -1490 cm"') which is similar
to the oxidation of polyaniline under acidic condition.
2.6.3. Nuclear Magnetic Resonance Spectra
Kenwright et al. [146] suggested that the solubility of both
polyleucoemeraldine and polyemeraldine bases in N-methyl pyrrolidone
(NMP) allows the registration of '^C-NMR spectra of these compounds. In an
ideal chain of polyleucoemeraldine, only two types of non-equivalent carbons
are present (i) carbons bonded to hydrogen and (ii) carbons which do not form
bond with hydrogen. Two major peaks in the chemical shift regions
51
corresponding to aromatic carbons are observed at 117.8 ppm and 137.4 ppm.
In addition to these dominant peaks, several peaks of low intensity are also
recorded which are associated with polyleucoemeraldine chain defects of
various types. However, Monkman et al. have suggested that these peaks are
practically non-existent.
A more complicated spectrum is expected for polyemeraldine base,
because of the co-existence of reduced bezenoid and oxidized quinonoid units
in the polymer chain which produces eight groups of non-equivalent carbons.
But the number of observed lines significantly exceeds the expected eight
lines corresponding to eight non-equivalent carbons as a consequence of the
introduction of quinoid rings.
In the polyleucoemeraldine base, phenyl rings are coupled together by
rotantional junction point (amine nitrogen) giving a single structure. On the
other hand, the poly leucoemeraldine base consisting of quinoid ring and two
imine nitrogen atoms are rotationally locked which in turn leads to cis-trans
isomerism. Several different conformations lending to distinctly different
NMR lines can therefore be expected. Kenwright et al. [146] have suggested
four such transformations for polylecucoemeraldine.
Kenwright et al. [146] and Ni et al. [147] have proposed the assignment
of principal NMR lines of polyemeraldine base by comparison with low
molecular weight model compounds. The use of d.e.p.t.-90 technique allowed
for the differentiation between protons bonded carbons and those carbons
atoms which do not form bonds with hydrogen.
Kaplan et al. [148] have reported '"' N-NMR of polyanilinc to elucidate
the structure of leucoemeraldine and an emeraldine base by taking p-amino
biphenyleamine as a model compound. They reported a single peak at 54.2
52
ppm due to -NH group in polyleucoemeraline and at 61 ppm and 62.3 ppm for
emeraldine base and emeraldine salt respectively.
Stein et al. [149] performed '"C-NMR on both doped and undoped
emeraldine and recorded the peaks at 118, 142, 131 and 134.7 ppm for
protonated polyaniline and at 141 and 155 ppm for leucoemeraldine base.
Li et al. [150] reported chemical shifts observed in ' H - N M R spectrum
of soluble polyaniline doped with p-toluencesulfonic acid and recorded peaks
at 6.97 ppm and 7.99 ppm for C-aromatic, 3.54 ppm and 3.26 ppm for C-H
aliphatic and 5.82 ppm for NH.
Trivedi et al. [151, 152] have reported for ' H - N M R shift in
dimethylsulphoxide (DMSO) with reference to trimethylsilane (TMS). Trivedi
[152] has also reported a '^C-NMR chemical shift for 33% doped polymer.
Kepler et al. [153] and Sandberg et al. [154] have also reported '^C-NMR of
various forms of polyaniline.
Reghunathan et al. [155] have suggested '•'C-CPMAS-NMR spectra of
polyaniline doped with 5-sulphosalicylic acid and p-toluenesulphonic acid.
2.6.4. Electron Spin Resonance Spectra
The aim of spin dynamics studies is to specify the motion of the
electron spins that are present in a given compound. This motion reflects
basic properties such as magnetism or transport. The true motion of the spin
carriers are associated with polarons in conducting polymers and can be used
in transport property studies. While discussing with the spin dynamics in
conducting polymers, one has to take the factors such as interchain coupling,
finite chain length, chain defects like traps, disorder, chain interruptions,
anomalous diffusion etc. in to account [156].
53
In contrast to the conventional methods for transport, spin dynamics is
able to supply data at the microscopic scale and intra and interchain
conductivity can be estimated. Moreover, the conductivity of the conducting
phase can be determined in heterogenous samples, e.g., conducting islands in
an insulating matrix or blend. Until recently, spin dynamics studies have
been less concerned with polarons than with neutral solitons.
The para magnetism in conducting polymers is basically due to
unpaired 7i-electrons (as a result of defects) on the polymer backbone. The
doping causes chemical modification of the system resulting in to the creation
of charge carriers along with transition from semiconducting to metallic state.
ESR studies discuss this aspect by investigating the parameters such as the 'g '
value, the peak to peak line width (AHpp), intensity of ESR absorption etc.
The intensity of ESR absorption relates to spin concentration. The 'g '
tensor depends on the paramagnetic species present and the spin environment
(For free election, g = 2.00232). The peak to peak line width AHpp depends on
spin relaxation time. Homogeneously broadened lines have Lowrentzian
shape, where as inhomogenously broadened lines have Gaussian shapes.
Generally a combination of both occurs.
A preliminary work on polyaniline combining conventional transport
measurement and magnetic resonance data, frequency dependence
conductivity, the proton relaxation etc. was first made by Travers et al. [157].
Mizhoguchi et al. [158] was then made a detailed study on a set of emeraldine
salt samples equilibrated in aqueous HCl solution of varying pH. Ihe study
included both NMR and ESR. They studied the spin lattice relaxation rale
normalized to the spin concentrations as a function of frequency.
Mizoguchi and Nechtschein [159] showed the relationship between
interchain diffusion coefficient and a^c as a function of protonation level and
54
then tested as a function of temperature. The interchain diffusion rate was
then extracted from the data of the frequency dependence of ESR line width
measured in the temperature range of 20K - 300K. Wang et al. [160] studied
the temperature dependence of ESR line width in poly emeraldine salt and
proposed a description of the transport properties of polyaniline in terms of
bundles with three dimensional metallic features.
Wei et al. [161] studied the temperature dependence of ESR spectra of
sulfonated polyaniline in the temperature range of 3K - 300K. They reported
that the full width at half maximum height line width decreases with
increasing temperature from 4.9 G at 3K to 1 G at 300K. The ESR line width
is narrowed by motional effect. As the temperature deceases, 'g ' value is
found to increase. Genoud et al. [162] have reported the temperature
dependence susceptibility for electrochemically prepared polyaniline
equilibrated at various potentials, while temperature independent
susceptibility is observed for chemically synthesized polyaniline. They
reported that the polaron number increases at a rate of one per ten aniline
rings and then deceases.
Raghunathan et al. [163] have shown that electrochemically prepared
polyaniline using p-toluenesulfonic acid and 5-sulfosalicylic acid as a dopant,
exhibits a weak broad spectrum at 77K, and at room temperature no ESR
spectrum was observed indicating the formation of bipolarons. Mizoguchi
[164] later studied the ESR line width as a function of frequency (inverse
square root) for polyemeraldine for various protonation levels.
Beumgarten and Muellen [165] have investigated the nature of spin
exchange interaction of poly(w-aniline). In their work, the energy spectra of
two infinite one dimensional models of the poly(w-aniline), mainly the
neutral and cation radical forms are theoretically investigated. Th^.band
il
55
structure is characterized by a wide energy gap with a half filled band of
nearly degenerate molecular orbitals.
Choi et al. [166] studied the ESR of electrochemically prepared
polyaniline tetrafluoroborates and reported that the conductivities of various
polyaniline samples increase with increasing AHpp and decreasing line
asymmetry a/b ratio and decreasing 'g ' value. Gupta et al. [167] made a
detailed study on electron localization in substituted polyaniline. They
concluded that the delocalization results in the line broadening caused by
electron spin-spin dipolar coupling. In the case of N-substituted polyanilines,
the conductivity decreases with decrease in AHpp and 'g ' value.
Recently, Langer et al. [168] reported the EPR studies on microporous
polyaniline. They reported that microporous polyaniline is strongly
paramagnetic with EPR spectral parameters dependent on the electrical
conductivity except for the sample prepared in presence of lithium cations,
which stabilize the 'g ' factor and the line asymmetry parameter on an almost
constant level.
2.7. Substituted Polyanilines
Since it is very difficult to process polyaniline using common
techniques, strategies were worked out to induce solubility and processibility
in polyanilines. One method is by substituting one or more hydrogens by an
alkyl, alkoxy, arylhydroxyl. amino groups or halogens in an aniline nucleus,
some of which were found to modif> the solubility.
Manohar et al. [169] reported the polymerization of N-mclhlyaniline
with a conductivity of-lO""* Scm"' and the copolymer with aniline has a
conductivity of-10"^ Scm'"
56
Dao et al. [170] have carried out extensive investigation of chemical
and electrochemical polymerization of substituted Polyanilines. They
reported that the chemical synthesis yields a polymer having a higher
molecular weight and that no polymer films could be prepared on the
electrode surface with substituent F, CI, NO2 and phenyl at ortho-positions
and methoxy group, F, CI, NO2 at meta- position. Leclarc [171] has shown
that poly(2-methyl aniline) has properties similar to that of polyaniline.
Dhawan et al. [172] have reported that the formation of two types of
polymers on electropolymerzation of o-ethoxyaniline. The polymer in direct
contact with the electrode surface is green similar to polyaniline, whereas the
one in contact with the electrolyte is pink and has more quinoid character and
also has a good solubility in methanol and ethanol.
Wei and Fock [173] has reported the synthesis and electrochemistry of
poly(o-toluidine), poly(/M-toluidue) and poly(o-ethylanilne) and compared
their properties with polyaniline. They reported that the steric effects are
mainly responsible for the decrease in conductivity, blue shift of n ^^ n*
transition band in UV spectra and for lower AEy, values of the alkyl ring
substituted polyanilines.
Dhawan et al. [174] studied the electropolymerzation and
copolymerization of 2-methylaniline with aniline and have shown that, the
polymerization of a mole ratio more than 1:0.5 (2-methylaniline:aniline) leads
to a polymer where cyclic voltamogram is devoid of the middle peak. This
has been suggested as due to the absence of quinone and this copolymer has a
switching response time of 40 ms.
Kang et al. [175] carried out the polymerization on 2-fluoroaniline and
2-chloroaniline by chromic acid at various pH. They reported that the
substitution by an electronegative group lower the conductivity of the
57
polymer. Gupta et al. [176] have reported that thermal stability of doped poly
(o-methyaniline) strongly depends on counter ions.
Genies et al. [177] have reported a detailed study on the polymerization
of or//zo-N-hexyl aniline and reported that although it has a lower
conductivity than polyaniline, it displays faster electron transfer. In a
previous study, Bidan and Genies [178] have reported synthesis,
electrochemical and spectral characteristics of poly (2-propylaniline). In
another study of Genies et al. [179]. electrochemical oxidation of 2,5-
dimethylaniline in NH4F 2.3HF was performed as a function of the potential.
The synthesis was performed to study the in situ EPR signals, FTIR of the
polymer and its redox and conductivity properties. The stability of the
polymer is found to be better than polyaniline.
Trivedi [180] has suggested that in copolymers, electron transfer is
always faster than in homopolymers, due to the difference in charge density
on substituted and unsubstituted constituents in polymer chain as a result of
electronic effect of the substituents which may facilitate faster electron
transfer.
Mattoso et al. [181] reported the synthesis of 4.17x10^ gram mole"'
molecular weight poly(2-methoxyaniline) carried out at -50°C in presence of
5.8M LiCl and IM HCl. The polymerization reaction was terminated using
acetone to prevent scission of the polymer chain under acidic condition.
Ohsaka et al. [182] studied the polymerization of N.N-dialkyl
substituted anilines such as N.N-dimcthylaniline. N,N-dicthylanilinc. N-
methy N-ethylaniline and N,N-dibutylani!ine. They reported that these
polymers do not have well defined voltametric peaks. They also described
the ion exchange property of these polymers.
58
Yan and Toshima [183] have reported that it is possible to polymerise
2,5 and 2,3 dimethylanilines under acidic conditions using cerium IV sulphate
as an oxidizing agent. However, ammonium peroxy desulphate as oxidant
does not yield any polymeric product.
Munaff et al. [184] have reported the electrosynthesis of conducting
polyanisidine in biphasic media. One phase comprises of non polar solvent
containing the monomer and the other polar medium containing some added
electrolyte using FeCls as catalyst. Their observations suggested a cationic
polymerization mechanism.
Kilmartin et al. [185] have reported photoelectrochemical and
spectroscopic studies of copolymers of orthanilic acid and aniline. The
degree of sulfonation and the oxidation state of the elements were examined
using X-ray photoelectron spectroscopy and further by in situ Raman
spectroscopy. Photocurrent profiles on a millisecond time scale in response
to a light-flash perturbation is producing exclusively anodic photocurrents for
the copolymer in the conductive state. Partial phase diagram for the
sulfonated copolymers were constructed, mapping the conductive regions as a
function of pH and electrode potential.
Umare et al. [186] investigated the influence of copolymer composition
on the transport properties of conducting copolymers, poly(aniline-co-o-
anisidine). UV-VIS spectra show a hypsochromic shift with an increase in
the o-anisidine content in copolymers, indicating a decrease in the extent of
conjugation. Transport parameters such as localization length and average
hopping distance are also calculated. Effect of monomeric composition
coherence length is discussed.
Recently, Aysegul Gok and Bekir Sari [187 ] have reported the
chemical synthesis of poly(o-toluidine) and poly(2-chloroaniline) and
59
investigated the effect of protonation medium. UV-VIS spectral analysis
results indicated that poly(o-toluidine) has the better protonation effect than
poly-2-chloroaniline. Magnetic susceptibility measurements of the polymers
showed that poly(o-toIuidine):(CH3COOH) and poly(o-toluidine):
(C2H5COOH) salts are of bipolaron structure.
Roy and Ray [188] had reported the polymerization of w-nitroaniline
and p-nitroaniline using ammoniumperoxydisulphate. The polymers showed
lower conductivity. In /n-nitroaniline, a head to tail polymerization, whereas
in /7-nitroaniline ortho coupling has been suggested. Steric hindrance and the
effect due to nitro group determine the spectral and other properties of these
polymers.
2.8. Thermal Degradation and Stability
Chemically prepared polyaniline is stable upto 650°C, but doped
samples reportedly undergoes HCl, H2SO4 removal that begins in the 230-
300°C range and leads the polymer to emeraldine base. A two step weight
loss has been reported by several investigators [189-192]. Troare et al. [189]
performed thermogravimetric analysis of emeraldine hydrochloride under
high vacuum in conjunction with thermal volatilization analysis. The initial
weight loss was attributed to the loss of HCl and in the second stage the
emeraldine base underwent thermal degradation in the temperature range
520°-740°C. Palaniyappan and Narayana [193] have reported a three step
weight loss in their chemically prepared polyaniline in different acid media.
The first step was attributed to the loss of water and the second step was due
to the removal of acid. The polymer underwent thermo-oxidative degradation
beyond 275°C in the third step.
We have observed that [194] copolymers of/7-methoxyaniline with
aniline and o-toluidine showed a three step weight loss in the thermogram
60
corresponding to loss of water, dopant and degradation of polymer backbone
respectively. Activation energies of thermal degradation was estimated using
Broido equation.
Kahler et al. [195] observed that HBr salt of chemically prepared
polyaniline is the most thermally stable material among HF-, HC1-, HBr- and
HI- doped highly conductive polyanilines.
Li et al. [196] showed that emeraldine like structure is the most stable
at high temperatures and that the imine =N- to amine -NH- ratio as
determined by X-ray photoelectron spectroscopy diminishes when heated to
200°C.
Gazotti et al. [197] compared the stability of chemically prepared
poly(o-methoxyaniline) doped with the HCl and toluene sulfonic acid by
monitoring X^ax as a function of pH from 0-14. Toressi et al. [198] studied
the stability of polyaniline film under laser light by comparing in situ and ex
situ spectra. Boucherit et al. [199] reported that a thin layer of water on the
surface of most unstable polymer sample was sufficient to protect it from
degradation.
Bernard et al. [200] reported the in situ Raman spectroscopy studies on
electrochromic polyaniline film that revealed the effect of pH and the
influence of sweep range on cycling life times. Hand and Nelson [201]
reported that/7-benzoquinone is the main product of electro-oxidation of
polyaniline in acid media and the dark green precipitate produced was
quinine-hydroquinone.
Misra et al. [202] reported the synthesis of poly(2-ethylaniline) which
was found to quite stable and could be cycled repeatedly without any
evidence of decomposition. TGA analysis showed the weight loss of 10% by
61
333°C in SbCle doped sample and C104~ doped sample lost 85% of its weight
by 400°C. Chemically prepared polyaniline and poly(dimethylaniline) was
studied by Toshima et al. [203] and showed a three step weight loss. From
TGA studies, Gupta et al. [204] showed that N-substituted polyanilines are
more stable than ring substituted polyaniline.
62
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75
CHAPTER-3
EXPERIMENTAL
3.1. Materials
Aniline: Ranbaxy Chemicals (distilled under reduced pressure)
o-Methylaniline (o-toluidine): SD Fine Chemicals (distilled under reduced pressure)
p-Methylaniline (p-toluidine) (99%): Sisco Chemicals (recrystallized from alcohol and them from acetone)
/7-Methoxyaniline (/?-anisidine) (98%): CDH Chemicals (recrystallized from alcohol and then from acetone)
o-Nitroaniline (98%): Sisco Chemicals (recrystallized from alcohol and then from acetone)
o-Nitroaniline (98-99%): (recrystallized from alcohol and then from acetone)
p-Nitroaniline (99%): Sisco Chemicals (recrystallized from alcohol and then from acetone)
Potassium persulphate (99%): CDH Chemicals, AR grade (used as received)
Hydrochloric acid: Ranbaxy Chemicals, AR grade (used as received)
Ammonia: CDH, AR grade (used as received)
Dimethylsulfoxide (DMSO) (99%): Merck India (used as received)
Dimethylsulfoxide (DMSO): spectral grade (to record UV-VIS spectrum)
Dimethylformamide (DMF) (99%): Fluka (used as received)
Acetone (99%): Merck India, AR grade (used as received)
Methanol (99%): Merck India (used as received)
76
3.2. Polymer Preparation
Several aniline based polymers and copolymers were synthesized by
well-known chemical methods [1-4] using ammonium persulphate as an
oxidant under acidic conditions.
Desired quantities of aniline and substituted aniline monomers were
dissolved in IM HCl. Solution of potassium persulphate (K2S268) in IM HCl
was slowly added to it from a burette with constant stirring for 5 hours. The
dark colored viscous reaction mixture was filtered. The product was washed
with triply distilled water several times to remove the excess of oxidant and
HCl present until the filtrate became colorless and acid free. Before
experimental use, the product was again treated with 0.1 M HCl solution and
washed with distilled water thoroughly followed by methanol to remove low
molecular weight oligomers and any HCl present in the polymer. Finally, the
product was washed with acetone, vacuum dried and kept at 50°C in an air
oven for 3 days. The finely ground product was stored in a vacuum
desiccator for experimentation. A portion of as-prepared products was treated
with 0.25 M NH4OH solution for 2 hours under constant stirring to prepare
the corresponding base from of the polymers as well as of copolymers.
Dedoped product was filtered, washed with distilled water several times
followed by acetone, vacuum dried and stored in a desiccator under vacuum.
The copolymers of aniline with m-and/7-nitroaniline were synthesized
for different molar ratios of the respective monomers in presence of
potassium persulphate under acidic medium. However, the copolymer of
aniline with o-nitroaniline was synthesized frofwa^mfxtuf^-pfacetonitrile,
water and HCl. j r / <r^\ L^-
11
Aniline and substituted aniline in HCl
K2S2O8 in HCl
; ;
As-prepared conducting
Emeraldine salt
Non conducting base form ' **"
Mixing with constant stirring for 5 hours
1 Washed with distilled water,
methanol and then with acetone
1 Kept in air oven at 50°C for
3 days
;
Treated with 0.25 M NH4OH for 2 hrs
;
Dried under vacuum, kept in air oven at 50° for two days
Figure-3.1. Flow chart showing the steps involved in the preparation of polymers and copolymers
78
A flow chart for the synthesis of polymers and copolymers is presented
in figure-3.1 and the details of individual preparations are given as under-
3.2.1. Copolymer of aniline with o-toluidine, P(AcoOT)l:l
Aniline and o-toluidine (0.01 mol each) dissolved in 150 ml of IM
HCl were reacted with potassium persulphate (0.025 mol) dissolved in 150 ml
of IM HCl at 26±2°C to obtain P(AcoOT).
3.2.2. Copolymer of aniline withp-toluidine, P(AcoPT)
Aniline and p-toluidine (0.01 mol. each) dissolved in 150 ml of IM
HCl were reacted with K2S2O8 (025 mol) dissolved in 150 ml of IM HCl at
26±2°C to obtain P(AcoPT)l:l. Copolymers for 1:2 and 1:3 molar ratios of
the monomers were also synthesized to obtain P(AcoPT)l:2 and P(AcoPT)l:3
respectively.
3.3.3 Copolymer of aniline with />-methoxyaniline, P(AcoPMA)
Aniline and/7-methoxyaniline (0.01 mol each) dissolved in 150 ml IM
HCl were reacted with KjSjOg (0.025 mol ) dissolved in 150 ml of IM HCl at
26±2°C to obtain P(AcoPMA).
3.2.4 Copolymer of aniline with o-nitroaniline, P(AcoONA)-l:l
Aniline and o-nitroaniline (0.01 mol of each) dissolved in 200 ml of
9:l(v/v) mixture of IM HCl and acetonitriic were reacted with K2S2OS ( 0.025
mol) dissolved in 150 ml of 1M HCl at 35±2°C to obtain P(AcoONA) 1:1.
79
3.2.5 Copolymer of aniline with o-nitroaniline, P(AcoONA)-l:3
o-Nitroaniline (0.015 mol) was dissolved in 200 ml of 9:l(v/v) mixture
of IM HCl and acetonitrile. Aniline (0.005 mol) was slowly added with
constant stirring for 1 hour to get a homogeneous solution that was reacted
with K2S2O8 (0.025 mol) dissolved in 150 ml of IM HCl at 35±2°C to obtain
P(AcoONA)l:3 .
3.2.6 Copolymer of aniline with m-nitroaniiine, P(AcoMNA)
These copolymers were synthesized for 2:1, 1:1 and 1:2 molar ratios
aniline and m-nitroaniline. The monomers were dissolved in 150 ml of IM
HCl. K2S2O8 (0.025 mol) dissolved in 150 ml of IM HCl was used as
oxidant. The reaction temperature was maintained at 30±2°C. The product
were labeled as P(AcoMNA)2:l, P(AcoMNA)l:l, P(AcoMNA)2:l
respectively.
3.2.7 Copolymer of aniline with p-nitroaniline, P(AcoPNA)
These copolymers were synthesized for 2:1, 1:1 and 1:2 molar ratios of
aniline andp-nitroanilines in a similar manner that adopted for the synthesis
of copolymer of aniline with m-nitroaniline and labeled as P(AcoPNA)2:l,
P(AcoPNA)l:l and P(AcoPNA)l:2 respectively.
3.2.8 Copolymer of 0-toluidine with p-methoxyaniline, P(OTcoPMA)
o-Toluidine and/7-methoxyaniline (0.01 mol each) dissolved in 150 ml
of IM HCl were reacted with KzSjOg (0.025mol) dissolved in 150 ml of IM
HCl at 26±2°C to obtain P(OTcoPMA).
All the above-mentioned copolymers were also prepared from O.IM and
0.3 M HCl solutions following the same procedure to compare the electrical
80
conductivity of the polymers on the pH of the acid medium used for
synthesis.
3.3. Solubility
Polyaniline and copolymers based on aniline both in their doped as well
as in dedoped forms were studied for their solubility in several solvents such
as water, cone. H2SO4, acetone, dimethylformamide (DMF),
dimethylsulphoxide (DMSO), N-methyl pyrrolidone (NMP) etc.
3.4. UV-VIS Spectral Studies
Dilute solutions of base form of polyaniline and copolymers based on
polyaniline were prepared by dissolving in spectral grade dimethylsulphoxide.
The UV-VIS spectra were recorded at room temperature using a UV-VIS
Spectrophotometer (Elico, SL 151). Spectra of doped polymers were also
recorded if they were found soluble in dimethylsulphoxide. Solvato-
chromic effect was also studied by taking the spectra in spectral grade
dimethylformamide.
3.5. FTIR spectral studies
Polymers and copolymers were thoroughly ground to fine powder with
KBr and FTIR spectra were recorded in KBr pellets using a Schemadzu,
810I-A FTIR Spectrophotometer at room temperature.
3.6. Electrical Conductivity Studies
The polymers were finely powdered and made into pellets using a
stainless steel die of 1.326 cm" cross-sectional area and by applying a
pressure of 3 tons using a hydraulic press (Spectra Lab. India). The pellets
81
were placed between two polished platinum electrodes which were mounted
on a stainless steel sample holder assembly between copper leads. The
copper leads were electrically insulated from the sample holder by teflon
sheets. The electrical conductivity was measured by two probe method using
a RLC Digibridge (Genrad 1659 RLC Digibridge, USA) at two different
frequencies, viz. 100 Hz and 10 KHz. The temperature dependence of
electrical conductivity was also studied from room temperature up to 180**C at
10 KHz'by keeping the cell containing the pellets in an air oven and
maintaining a heating rate of l^C/min.
To study the basic nature of charge transport, the temperature
dependence of conductivity data were fitted in to Variable Range Hopping
Model [6,7) given by the equation -
a(T) = (Jo . exp I -—- -— (3.1)
and equation corresponding to band conduction or Arrhenius model
a(T) = a o e x p f - | ^ j ....(3.2)
where n is the diamensionality of charge transfer. To is the Mott's
characteristic temperature and Go is the conductivity at room temperature, EA
is the activation energy for transport and k is the Boltzman's constant. The
measured conductivity values were plotted logarithmically as a function of
reciprocal of temperature.
82
3.7. Electron Spin Resonance Spectral Studies
ESR spectra were recorded on a GEOL ESR Spectrometer (JES-RE2X)
at room temperature under the following settings: modulation frequency: 100
KHz, microwave frequency: 9.44 GHz, mOicrowave power: 5 mV, scan range:
300 G, field modulation 10 G, time constant 0.035 and center field: 3350G.
The 'g ' values, peak to peak line widths (AHpp) and time asymmetry
parameter (a/b ratios) were estimated from the ESR spectra.
3.8. Thermogravimetric Analysis
Thermo-oxidative degradation of the polymers were studied by keeping
samples in a TGA analyzer (Model: Perkin Elmer) under dynamic air flow of
50 cc/min in the temperature range 70-700°C. The heating rate was
maintained at lO^C/min.
Activation energies for the thermo-oxidative degradation of the
polymers were calculated using integral method of Broido [8]. According to
Broido equation,
l n ( l n l / y ) = I ^ + C - - ( 3 . 3 )
where y is the fraction of the sample yet not decomposed and EA is the
activation energy.
y = _ i £L (3.4)
Wo-w„
where Wo and w^ are the initial and final weights respectively and Wt is the weight a given temperature. A plot of In (In i/y) versus (K"') gives a
-E straight line. Activation energies were calculated from the slope, — - , of the
plot of In (In 1/y) versus - ^ (K'') .
83
REFERENCES
1. Y. Wei, W.W. Focke, G.E. Wnek, A. Ray and A.G. MacDiarmid, J.
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[2004].
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[2002].
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84
(^^fOL^lifmati O'jj aniline, with
o^f mr and prniiit^anilina^
CHAPTER-4
COPOLYMERS OF ANILINE WITH 0-, m- A N D /7-NITROANILINES
4.1. Introduction
Copolymers of aniline with o-, m- and/?-nitroanilines were synthesized
for different molar ratios of the respective monomers in acid medium as
mentioned in the experimental. Aniline initiated polymerization of w- and
/7-nitroanilines is already reported [1], however, the polymerization of
o-nitroaniline could also be done with increased percentage of aniline in the
reaction mixture. This chapter gives a comparative study of the synthesis,
electrical, spectral and thermal characteristics of the copolymers.
4.2. Results and Discussion
4.2.1. Polymer Synthesis
The appearance of green color in the reaction mixture is taken as the
indication of copolymerization. Polymerization of aniline into polyaniline
was found to take place very quickly as observed by color change of the
reaction mixture from pale -^ bluish green -> green within two minutes of the
addition of the oxidant. The copolymerization of aniline with nitroanilines
was observed to be comparatively a slow process. In case of copolymerization
of aniline with o-nitroaniline, the color change from yellow -> yellowish
green -> green was observed within 17-20 minutes of addition of potassium
persulphate. Time taken for the initial color change in the reaction mixture
containing m- and/7-nitroanilines were almost the same for mixtures
containing the similar molar ratios of the monomers. Table-4.1 gives the time
taken for the initial color change of reaction mixtures containing aniline and
85
various nitroanilines along with the % yield of the products formed. It can be
seen from the table-4.1 that, the rate of reaction decreases as the ratio of
nitroanilines increases in the reaction mixture. It is a qualitative indication
that in presence of nitroanilines, the aniline monomeric units alone are not
getting polymerized, rather, the nitroaniline units are incorporated in the
polymer backbone.
It is the electron withdrawing -NO2 groups that retards the
polymerizability of aniline. We have observed that the ease of
polymerization is significantly different for mixtures of aniline containing o-,
m- and p-nitroanilines. It is due to the difference in the basicity constants of
three nitroanilines [2]. The pKb values are in the order
o-nitroaniline » p-nitroaniline > m-nitroaniline > aniline
The relative Kb values can be taken as a measure of the electron
withdrawing capacity by nitro- groups in the respective positions. The
positively polarized nitro- groups induce a drift of electrons in the sense of
withdrawal from the ring and from the amino atoms and the affinity for
nitrogen for proton is decreased. Protonation is an important step in the
polymerization of aniline [3,4]. In aqueous medium, aniline is predominantly
exists as anilinium cation and aniline cation radical is produced first during
the polymerization process, which may recombine to benzedine [5,6] or
participate in the growth of polyaniline chain in the pernigraniline form. The
protonated repeat unit of this form is responsible for the initial color change
[7,8]. After all the oxidizing agent has been completely consumed the
pernigraniline may take over the role of an oxidant and become reduced by
aniline [9] to yield the final product in the emereldinc form. The whole of
these processes may get retarded by the presence of nitro- groups in the
86
Table-4.1: % yield of the polymers and time taken for the initial color change of the reaction mixture upon addition of oxidant
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)2:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)2:l
P(AcoPNA)l:l
P(AcoPNA)l:2
% yield
80.03
24.72
32.25
55.1
48.48
42.13
50.2
39.16
32.5
Time taken for initial color change (minutes)
1-2
17-20
14-16
4-5
10-12
12-14
4-5
10-12
12-14
PANI P(AcoONA)l:3 P(AcoONA)l:l P(AcoMNA)2:l P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:I P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniline-co-o-nitroaniline)l:3 molar ratio Poly(aniline-co-o-nitroaniline)l:l molar ratio Poly(aniline-co-m-nitroaniline)2:l molar ratio Poly(aniline-co-/n-nitroaniiine) 1:1 molar ratio Poly(aniline-co-w-nitroaniline)l:2 molar ratio Poly(aniline-co-/?-nitroaniline)2:l molar ratio Poly(aniline-co-/?-nitroaniline)l:l molar ratio Poly(aniline-co-/7-nitroaniline)l:2 molar ratio
87
reacting system. Steric hindrance of nitro group may also affect the rate of
reaction.
Polymerization reaction involving o-nitroaniline were carried out at
35±2°C as the o-nitroaniline was found to be completely soluble in the
reaction medium at that temperature. We have succeeded in incorporating o-
nitroaniline units in the polymer backbone by the copolymerization of 1:3
molar mixture of aniline and o-nitroaniline to yield the copolymer
poly(aniline-co-o-nitroaniline)l:3, and using 1:1 mixture of the same to yield
the copolymer poly(aniline-co-o-nitroaniline)l:l. From table-4.1, it can be
seen that, the % yield of these copolymers are much lower than that of the
copolymers derived from m- andp-nitroanilines. o-Nitroaniline may suffer a
greater electronic deactivation effect due to the -NO2 groups and inhibits the
initiation to a greater extent. In these copolymerization, the polymerization
may get easily terminated after forming some low molecular weight oligomers
resulting in poor yield of the copolymers in comparison to polyaniline. It is
clear from table-4.1 that % yield decreases gradually as the ratio of
nitroaniline units increases in the reaction mixture. In the reactions involving
/7-nitroaniline, the polymerization should take place by the coupling at ortho-
position to -NH2 groups. The stable free radical then be produced at the
ortho- position [1]. In the systems containing o- and w-nitroanilines,
coupling should be more favourable at the para- position to -NH2 groups,
maintaining a head to tail sequence. m-Nitroanilne suffers a greater
electronic deactivation effect than the remaining two as is evident from its
higher basicity.
The expected structures of the copolymers are depicted in rigure-4.1.
Aniline and nitroaniline units are not necessarily be in an alternating order,
rather a random copolymer is most likely expected.
88
/ \ N-
NO2
-n\ir^. N02
poIy(aniline-co-0-nitroaniline)l: 1
= - N NH-
NO2 NO2
poly(aniIine-co-m-nitroaniline)l: 1
poly(aniline-co-/7-nitroaniline)l: 1
Figure-4.1. Expected structures of some of the copolymers
Interestingly, it was observed that both the protonated and base forms
of the copolymers show better solubility than polyaniline and therefore can be
processed much more easily. The solubility characteristics of both the base
and protonated forms of the copolymers in various solvents are presented in
table-4.2 and table-4.3 respectively. We have observed that solubility
increases as the nitroaniline units increases in the polymer chain. The
copolymers are expected to have lower chain length that polyaniline and
89
Table-4.2: Solubility characteristics of as-prepared (protonated) polymers in various solvents at room temperature (25±2)°C
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)2:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)2:l
P(AcoPNA)l:l
P(AcoPNA)l:2
H2O
IS
IS
IS
IS
IS
IS
IS
IS
IS
H2S04
MS
MS
MS
MS
MS
S
MS
MS
MS
Acetone
IS
MS
SS
SS
SS
MS
SS
SS
MS
CH3OH
IS
MS
SS
SS
MS
MS
MS
MS
MS
DMF
IS
S
MS
MS
S
S
S
S
S
DMSO
IS
s MS
MS
S
S
S
S
S
NMP
SS
s MS
MS
S
S
S
S
S
IS MS SS S
Insoluble Moderately Soluble Slightly Soluble Soluble
PANI P(AcoONA)l:3 P(AcoONA)I:l P(AcoMNA)2:I P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:l P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniIine-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-PolyCaniline-
•co-o-nitroaniline)l: co-o-nitroaniIine)l: co-w-nitroaniline)2 co-m-nitroaniline)l co-w-nitroaniline)l co-/7-nitroaniline)2: co-/7-nitroaniline)I: co-/?-nitroaniline)l:
3 molar ratio 1 molar ratio : I molar ratio : 1 molar ratio :2 molar ratio 1 molar ratio 1 molar ratio 2 molar ratio
90
Table-4.3: Solubility characteristics of base form of polymers in various solvents at room temperature (25±2)°C
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)2:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)2:l
P(AcoPNA)l:l
P(AcoPNA)l:2
H2O
IS
IS
IS
IS
IS
IS
IS
IS
IS
H2S04
MS
S
S
s s s s s s
Acetone
IS
s ss ss ss MS
SS
SS
MS
CH3OH
IS
s SS
ss MS
MS
ss ss MS
DMF
S
S
s s s s s s s
DMSO
s s s s s s s s s
NMP
s s s s s s s s s
IS - Insoluble MS - Moderately Soluble SS - Slightly Soluble S - Soluble
PANI P(AcoONA)l:3 P(AcoONA)l:l P(AcoMNA)2:I P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:l P(AcoPNA)l:I P(AcoPNA)l:2
Polyaniline Poly(aniline-co-o-nitroaniline)l:3 molar ratio Poly(aniline-co-o-nitroaniline) 1:1 molar ratio Poly(aniline-co-m-nitroaniline)2:l molar ratio Poly(aniline-co-/w-nitroaniline)l: 1 molar ratio Poly(aniline-co-/«-nitroaniline)l:2 molar ratio Poly(aniline-co-p-nitroaniline)2:l molar ratio Poly(aniline-co-p-nitroaniline)l:l molar ratio Poly(aniline-co-/?-nitroaniline)l :2 molar ratio
91
chain length decreases as the ratio of the nitroaniline increases in the polymer
chain.
4.2.2. FTIR Spectral Studies
FTIR spectra of poly(aniline-co-o-nitroaniline)l:3 and poly(aniline-co-
o-nitroaniline)l:l are presented in figure-4.2 and that of poly(aniline-co-w-
nitroaniline)l:l and poly(aniline-co-/?-nitroaniline)l:l are shown in figure-
4.3. The FTIR absorption data are given in table-4.4. Both the poly(aniline-
co-o-nitroaniline)l:3 and poly(aniline-co-o-nitroaniline)l:l copolymers show
the characteristic bands corresponding to C=C stretching vibrations of
quinoid and benzenoid rings respectively at 1576 cm'' [poly(aniline-co-o-
nitroaniline)l:3], 1578.4 cm"' [poly(aniline-co-o-nitroaniline)l:l] and at
1504.7 cm'' [poly(aniline-co-o-nitroaniline)l:3], 1499.1 cm'' [poly(aniline-
co-o-nitroaniline)l:l] [1,8,9]. It is reported that the relative intensity of the
quinoid band (-1570 cm'') to the benzenoid band (-1500 cm'') is a measure
of the degree of oxidation of the polymer chain [10,11]. The intensity of
quinoid band in poly(aniline-co-o-nitroaniline)l:4 is slightly lesser than that
in poly(aniline-co-o-nitroaniline)l:l, showing oxidized quinoid units are
lesser in poly(aniline-co-o-nitroaniline)l:3 than in poly(aniline-co-o-
nitroaniline)l:l. There is also a possibility of overlap of bands due to
asymmetric N=0 stretch in these region. The peak observed at 1342.6 cm"' in
poly(aniline-co-o-nitroaniline)l:3 and at 1340 cm'' in poly(aniline-co-o-
nitroaniline)l:l is ascribed to symmetric N=0 stretching vibrations due to -
NO2 groups. The bands at 1320 cm"' and 1279 cm"' in poly(aniline-co-o-
nitroaniIine)l :3 is ascribed due to C-N stctching vibrations. C-H bending
vibrations also occur in this region. For poly(aniline-co-o-nitroaniline)l:l,
these bands appear at 1304 cm"' and 1281 cm"' respectively. The presence of
92
c
4000.0 3000.0 2000.0 1500.0
Wave number (nm)
1000.0 400.0
Figure-4.2 FTIR spectra of (A) poly(aniline-co-o-nitroannine) 1:3 (B) poly(anilinc-co-o-nitroannine) 1:1
93
in
c
4 0 0 0 0 3 0 0 0 0 2000.0 1500.0
Wovenumber (n m )
1000.0 400
Figure-4.3 FTIR spectra of (A) poly(aniline-co-m-nitroaniline) 1:1 (B) poly(aniline-co-p-nitroaniline) 1:1
94
Table-4.4: FTIR peak positions (cm') of copolymers
P(AcoONA)l:3
2950
1576
1504.7
1470
1342.6
1320
1279
1150
1050
1045
1000
840
785
741
P(AcoONA)l:l
3100-2950
1578.9
1585
1499
1448.7
1370
1340
1304
1280.9
1145.9
860
825.6
781.3
740.8
669.4
513
^
P(AcoMNA)l:l
3300
3225.1
2910
2850
1579.9
1529.7
1504.7
1400.5
1350.3
1130-1145
829.5
736.9
609.4
619.2
509.2
P(AcoPNA)l:l
3300
3250
2915
2860
1583.7
1508.5
1417.9
1338.8
1325.3
1302.1
1145.9
1111.1
1062
837.2
750.4
690.6
669.9
517
PANl P(AcoONA)I:l P(AcoMNA)2:l P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:l P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-
co-o-nitroaniline) 1:3 co-w-nitroaniline)2:1 co-m-nitroaniline)l: 1 co-w-nitroaniline)l :2 co-/7-nitroaniline)2:1 co-/?-nitroaniline) 1:1 co-/7-nitroaniline)l :2
molar ratio molar ratio molar ratio molar ratio molar ratio molar ratio molar ratio
95
1,2,4 trisubstitution due to C-H in-plane and C-H out of plane deformations
are indicated by the bands in the region 741-1100 cm' .
The bands due to C-H out of plane bending modes (1000-1100 cm'') are
very less pronounced in the spectra of poly(aniline-co-o-nitroaniline)l:l. The
sharp band observed at 1150 cm'' for poly(aniline-co-o-nitroaniline)l:3 at
1145.9 cm'' for poly(aniline-co-o-nitroaniline)l:l is due to the charged
defects [12,13]. The broad band observed in the region -3300-3400 cm"' is
ascribed to N-H stretching vibrations in poly(aniline-co-o-nitroaniline)l:3 is
not pronounced in poly(aniIine-co-(?-nitroaniline)l:l.
From figure-4.3, it is clear that both the copolymers, poly(aniline-co-w-
nitroaniline)l:l and poly(aniline-co-;?-nitroaniline)l:l show similar vibrations
described above for polymers derived from o-nitroaniline and aniline. The
quinoid and benzenoid stretching vibrations are present at 1679.9 cm"'
[poly(aniline-co-w-nitroaniline)l:l], 1583.7 cm"' [poly(aniline-co-/7-
nitroaniline)l:l] and at 1529 cm'' [poly(aniline-co-/w-nitroaniline)l:l],
1508.3 cm"' [poly(aniline-co-/7-nitroaniline)l:l] respectively. As the
intensities of these two bands are almost the same in both the copolymers, the
level of oxidation is seem to be the same in both. The strong bands observed
at 1350 cm"' in poly(aniline-co-w-nitroaniline)l:l and 1338.1 cm"' in
poly(aniline-co-p-nitroaniline)l:l are assigned to N-0 stretching vibrations.
The bands in the region 736 cm''-1130 cm'' is due to 1,2,4 trisubstitution in
the benzene ring. The band corresponds to charged defects appears around
1145 cm'' in both the copolymers. The band -1400 cm"' is ascribed to C-H
bending vibrations [8]. The symmetric N=0 stretching vibrations of
poly(aniline-co-/?-nitroaniline)l:l has slightly shifted to lower frequency
compared to the corresponding bands in other copolymers. It may be a
96
qualitative indication that a head to tail coupling might have taken place in
the copolymers other than poly(aniline-co-;?-nitroaniline)l:l [1].
4.2.3. Electronic Spectral Studies
UV-VIS absorption spectra of the base form of polyanilines generally
shows two major absorptions at around 330 nm and 620 nm which are
attributed to TI ^ TT* transitions and benzenoid to quinoid exciton transition
respectively [14-16]. In the polyaniline sample prepared the bands are
observed at 325 nm and at 626 nm. When substituents are present in the
phenyl rings, it significantly alters the planarity of the system and influence
the 7i-orbital overlap and thus a shift is observed for the K -> 7t* transition
band. This band is also a measure of the extent of conjugation between the
adjacent phenyl rings. In all the copolymer samples synthesized, it has been
observed that these two bands in the UV-VIS spectrum of samples, the
position varies from polymer to polymer, as it depends on the width of the
energy gap between the n and n* bands which are found to be different for
different polymers.
The UV-VIS absorption spectra of the copolymer samples in their base
form as well as in the protonated (doped) form taken in DMSO as solvent are
presented from figure-4.4 to figure-4.9. The corresponding absorption
positions are given in table-4.5. The peak positions of the base form of the
copolymers taken in DMF are also presented.
It is clear from figure-4.5, that in copolymers derived from aniline and
o-nitroaniline, there is a hypsochromic shift (blue shift), both in the TI -^ K*
transitions bands and exciton bands in comparison to polyaniline (figure-4.4).
The n ^ n* transition occurs at 297 nm in poly(aniline-co-o-nitroaniline)l:3
and at 288 nm with a shoulder at 310 nm in poly(aniline-co-o-
97
IJJ o 2 < 00 Q: o I/)
<
360 520 680 WAVELENGTH (n f^ )
Figure-4.4 UV-VIS spectra of base form of polyaniline in DMSO
98
260 a o . 600 Wavelength ,A (nm)
760 920
Figure-4.5 UV-VIS spectra of base form of copolymers (A) poly(aniline-co-0-nitroaniline) 1:1 (B) poly(aniline-co-0-nitroaniline) 1:3
99
280 UO 600 Wavelength(nm)
760 900
Figure-4.6 UV-VIS spectra of as-prepared protonated form of copolymers in DMSO (A) poly(aniIine-co-0-nitroaniUne) 1:1 (B) poly(aiiiUne-co-o-nitroaniline) 1:2
100
Table-4.5: UV-VIS peak positions (nm) of polymers using DMSO and DMF as solvent
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)2:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)2:l
P(AcoPNA)l:l
P(AcoPNA)l:2
Base form
DMSO
325,626
297, 537
310,599
330,623
354, 579
302,364
330,624
398, 597
407
DMF
326,620
297, 534
312,597
334,620
355, 572
304, 358
330, 620
400, 598
410
Protonated form
DMSO
-
290,403, 539
285,406, 588
-
352, 390, 570
-
-
342,407, 590
-
PANI P(AcoONA)I:3 P(AcoONA)l:l P(AcoMNA)2:l P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:l P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniline-Poly(aniline-PoIy( aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-
co-o-nitroaniline)l co-o-nitroaniline)l: co-w-nitroaniline)2 co-m-nitroaniline)l co-w-nitroaniline) 1 co-/?-nitroaniline)2: co-/?-nitroaniline)l: co-/?-nitroaniline)l
3 molar ratio 1 molar ratio : 1 molar ratio : 1 molar ratio :2 molar ratio 1 molar ratio 1 molar ratio 2 molar ratio
101
nitroaniline)l:l. This blue shift is due to the decrease in the extent of
conjugation due to the presence -NO2 groups, resulting in an increase in the
band gap [9,15].
The exciton band is observed at 537 nm for poly(aniline-co-o-
nitroaniline)l:3 and at 598 nm for poly(aniline-co-o-nitroaniline)l:l. This
band has been attributed to an absorption from the highest occupied molecular
orbital (HOMO) band centered on the benzenoid unit to the lowest
unoccupied molecular orbital (LUMO) band centered on the quinoid unit,
often referred to as benzenoid to quinoid exciton transition which is a
measure of extended conjugation. When the absorption is intra-chain, the
excitation leads to the formation of molecular excitons with positive charge
on the adjacent benzenoid unit bound to the negative charge centered on the
quinoid, while interchain charge transfer from HOMO to LUMO may led to
the formation of positive and negative polarons [16-18]. This band also
shows a blue shift in both the copolymers. The blue shifts are explained in
terms of steric effect and the increase in the ring torsional angles between the
adjacent phenyl rings caused by the presence of substituent -NO2 groups [9,
15, 19-21].
The intensity of the band corresponding to exciton transition is a
measure of the degree of oxidation, i.e. relative number of quinoid moieties
present in the polymer [22]. As can be seen from the rigure-4.5, the intensity
of this band in poly(aniline-co-o-nitroaniline)l:3 and poly(aniline-co-o-
nitroaniline)l:l are lesser than that in polyanilinc and in poly(aniline-co-o-
nitroaniline)l:3, it is highly diminished suggesting that the oxidized quinoid
units in this polymer is considerably lesser in comparison with the other two.
The electronic spectra of the protonated form of these polymers in
DMSO show an additional band around 400 nm which is due to the presence
102
2.00
u c o
J3 u o M
<
I.OOf-
O.OOi 200 AOO 600 800
Wovelength,A (nm) 1000
Figure-4.7 U V-V IS spectra of base form of copolymers (A) poly(aniline-co-m-nitroaniline) 2:1 (B) poIy(anil[ne-co-m-nitroaniIine) 1:1 (C) poly(aniIine-co-/«-nitroaniIine) 1:2
103
2.00
(J c a
JO u o I/)
<
1.00^-
0.00 400 600
Wavelength,A (nm) 800 1000
Figure-4.8 UV-VIS spectra of base form of copolymers (A) poly(aniline-co-p-[iitroaniline) 2:1 (B) poly(aniline-co-p-nitroaniline) 1:1 (C) poly(aniline-co-p-nitroaniline) 1:2
104
u c o I-o
<
1.0
-
/ ^ M r A // i
/ //
// / /
/
\ \
1 1 1 •
280 UUO 600
Wave(ength,A ( n m )
760 920
Figure-4.9 UV-VIS spectra of as-prepared protonated form of copolymers in DMSO (A) poIy(aniline-co-m-nitroaniUne) 1:1 (B) poly(aniline-co-j9-nitroaniIine) 1:1
105
of polarons whose energy levels lie in the energy gap. There are possibilities
of electronic transitions between HOMO to the lower polaron band or
interpolaron bands or polaron to LUMO. These polaron bands may not be
symmetrical in the band gap [23]. The band observed at 403 nm in
poly(aniline-co-o-nitroaniline)l:3 and at 406 nm in poly(aniline-co-o-
nitroaniline)l:l are due to polaron -^ n* transitions, based on earlier reports
[19, 22]. However, the band due to TT -> polaron transition is not seen here in
the spectra taken in DMSO.
The UV-VIS spectra of the copolymers derived from m- and p-
nitroanilines seem to be a little more complex. poly(aniline-co-w-
nitroaniline)l:l shows the absorption maxima at 354 nm and 579 nm and for
poly(aniline-co-m-nitroaniline)l:2, these bands are observed at 301 nm and
364 nm. The corresponding bands in poly(aniline-co-w-nitroaniline)2:l are
observed at 320 nm and 622 nm. These bands correspond to 7t -> TT*
transition and benzenoid to quinoid exciton transition respectively. The
exciton transition shows a hypsochromic shift which increases as the number
of nitroaniline units increase in the polymer chain. The 7i -> Tt* transition
bands in poly(aniline-co-w-nitroaniline)l:2 and poly(aniline-co-w-
nitroaniline)2:l are also found to show a hypsochromic shift though it is very
feeble in the latter. This is explained in terms of the increase in the torsional
angle between C-N-C plane and the plane of phenyl rings due to the steric
strain caused by -NO2 groups in the ortho- position to the -NH2 groups (The
sine of the torsional angle determine the overlap integral between benzene
ring and the nitrogen orbital). Though this factor may be applicable in
poly(aniline-co-m-nitroaniline)l:l where a bathochromic shift for the K-band
is observed.
106
Actually, the electronic structure of copolymers is affected by the
change in the oxidation and protonation levels in the polymer matrix. In
polyanilines, the added main chain flexibility surrounding the amine/imine
nitrogen linkages is responsible for myriad structural effects which influence
the electronic and charge transport properties of the polymer [24]. Significant
effects of ring torsional angle changes accompanied by conformational
changes on the electronic properties of polyaniline family of polymers have
been reported [25, 26]. Deviation from planarity by the phenylene ring is
sequentially altered between +30° and -30° as one moves along the polymer
backbone [27]. It is also reported from single crystal [12] and molecular
modeling calculations [28], a more complex ring tortion structure for
emeraldine base form in which the quinoidal rings are more planar than
benzenoidal rings. When substituents are present in the phenyl rings even
more complex situation is expected.
The intensity of exciton transition band of poly(aniline-co-w-
nitroaniline)l:l and poly(aniline-co-w-nitroaniline)l:2 (figure-4.1,) are lesser
than polyaniline, and for the latter, the decrease is maximum. This suggests
that the number of oxidized quinoid rings decreases as the number of
nitroaniline units increases in the polymer backbone. The detailed
interpretation of electronic properties following the hierarchical organization
of substituted polyaniline structures is highly complex and beyond the scope
of this thesis.
From figure-4.8, it can be seen that poly(aniline-co-p-nitroaniline)l:l
shows the absorption maxima at 398 nm and 597 nm and for poly(aniline-co-
/7-nitroaniIine)2:l, the bands are observed at 324 nm and 623 nm. But
poly(aniline-co-p-nitroaniline)l:2 shows only one band at 407 nm. The
hypsochromic shift observed for the exciton transition in poly(aniline-co-/?-
107
nitroaniline)l:l is accounted for by the same reasoning applied above for
poly(aniline-co-/w-nitroaniline) 1:1 and poly(aniline-co-o-nitroaniline) 1:1.
But here the n ->• n* transition band is highly red shifted. Interestingly, there
is only one absorption band at 407 nm for poly(aniline-co-;?-nitroaniline)l:2.
Therefore, it can not be taken as conclusive. As the number of nitroaniline
units are greater here, it is expected to show a lower energy n -> n*
transitions relative to polyaniline and other copolymers probably due to the
pronounced complementary substitution effect with -NO2 and -NH2 groups
para to each other [29], which is consistent with earlier reports [1].
The base form of these copolymers shows a solvatochromic effect
(table-4.5). Where as the exciton bands show a blue shift in DMF, the n -> n*
transition bands show a red shift in comparison to the spectra taken in DMSO.
These facts may be explained in terms of dielectric constants of the solvents.
Solvatochromism also depends on many other factors such as geometry and
electronic structure of both solute and solvent.
A polymer in a solvent of high dielectric constant may exists in 'coil'
like conformation, resulting in loss of planarity and a decrease in conjugation
which in turn results in the hypsochromic shift of the absorption bands.
Dielectric constant of DMF (36.6) < DMSO (41.2). In less polar solvents
thermodynamically more stable conformation is achieved and restrict the
polymer to lower energy, high planarity state [20, 30]. But the observed
solvatochromic shift can not be explained on the basis of dielectric constant
only.
The protonated form of poly(aniline-co-m-nitroaniline)l:l and
poly(aniline-co-/?-nitroaniline)l:l were completely soluble in DMSO, giving
a grayish green color and bright green color respectively. The UV-VIS
spectra (fi^iH»-4.S smd figure-4.9) show an additional band at 410 nm in
108
poly(aniline-co-/w-nitroaniline)l:l and at -400 nm in poly(aniline-co-/?-
nitroaniline)l:l, which are ascribed due to polaron transitions. The solutions
were found to change the color to bluish after keeping for a long time. This
observation is consistent with the report that, since DMSO is of coordinating
nature, they get coordinated to the protonated N-sites after displacing dopant
ions and thereby convert salt in to base [31, 32].
4.2.4. ESR Spectral Studies
A large number of ESR studies on conducting polymers have been
performed to understand the nature of paramagnetic state and the suggested
conduction mechanism [33-35]. The area of ESR signal is related to the
number of spins present in the sample and the width of the signal may be
related to the extent of delocalization of electrons and thus to the extent of
conjugation . The position of ESR signal may be related to the neighbouring
environment of the electronic spin, the 'g ' factor. The symmetry of ESR
signals and spin-lattice relaxation time can be interpreted in terms of the
electrical conduction theories in conducting polymers [36, 37]. The ESR
spectral parameters such as 'g ' value, peak to peak line width (AHpp) and line
asymmetry parameter (a/b ratio) have been collected in table-4.7. Since, ESR
signals depends on spin concentration, which in turn depends on many factors
such as synthesis, intermolecular interactions etc. and are highly sensitive to
experimental conditions, it seems very difficult to interpret the ESR signals of
polynitroanilines. The reactivity of the three nitroanilines are also differ from
each other. It is also found difficult to arrive at a general correlation between
the ESR data and the measured electrical conductivity.
There are different reports on the interpretation of ESR data of
polyaniline and other conducting polymers. Though it is found by many
investigators that for conducting polyaniline, 'g ' factor becomes lower and
109
AHpp becomes minimum and line asymmetry parameter (a/b ratio) taices a
higher value [33, 34], some others have reported that delocalization of
electron results in line broadening of ESR signals and a/b ratio decreases with
increase in electrical conductivity [38, 39]. Mohammad et al., [34] have
suggested that narrow line width is a suggestive of longer conjugation length
because mobile and greatly delocalized electronic spin gives narrow ESR
signal based on their studies on poly paraphenylene. Thus much broader ESR
signal suggests a shorter conjugation length.
We have observed in our polymeric samples that the 'g ' value
decreases with increase in electrical conductivity and a/b ratio. For samples
with higher electrical conductivity, a/b ratio takes higher value and ESR
signal becomes highly asymmetrical within one series of sample (Table-4.6).
For copolymers derived from o-nitroaniline and aniline and from/?-
nitroaniline and aniline, electrical conductivity increases with decrease in
AHpp. Here, it is expected that as the number of electron withdrawing nitro-
group increases, electron spin becomes localized giving a broader ESR signal.
The broader line width is suggestive of shorter conjugation length and
resulting in lower electrical conductivity. Thus it supports the interpretation
derived from electronic spectra of these polymers.
But in the case of copolymers derived from aniline and m-nitroaniline,
we observed a different trend. The AHpp of poly(aniline-co-/w-nitroaniline)l:2
whose electrical conductivity is lesser than poly(aniline-co-w-
nitroaniline)!:!, is smaller than that of poly(aniline-co-w-nitroaniline)l;l and
the line asymmetry, a/b ratio is higher than that of poly(aniline-co-/?7-
nitroaniline)l:l.
It seems that the intermolecular interaction especially hydrogen
bonding between polymer macromolecules and macromolecular bundles may
110
Table-4.6: Electrical conductivity and ESR Data of the polymers
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)2:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)2:I
P(AcoPNA)l:l
P(AcoPNA)l:2
Electrical conductivity (Scm')
lOOHz
7.167x10"'
6.3x10-^
5.22x10"*
4.022x10'^
8.02x10"^
6.05x10-^
5.4x10"^
5.92x10-^
6.04x10"^
lOKHz
7.168x10'"
1.002x10-
5.29x10"*
4.02x10-^
8.28x10"^
2.75x10-^
5.42x10-^
6.48x10"^
6.28x10-*
'g' value
2.013
2.0201
2.0139
2.0146
2.0225
2.0236
2.136
2.0141
20145
ESR Data
AHpp(G)
13.04
23.13
14.22
13.88
22.90
18.89
13.89
15.69
17.36
a/b ratio
1.60
1.39
1.52
1.52
1.37
1.42
1.55
1.48
1.38
PANI P(AcoONA)l:3 P(AcoONA)l:l P(AcoMNA)2:l P(AcoMNA)l:l P(AcoMNA)I:2 P(AcoPNA)2:l P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-
co-o-nitroaniline)l co-o-nitroaniline) 1: co-/n-nitroaniline)2 co-m-nitroaniline)l co-m-nitroaniline) 1 co-p-nitroaniline)2: co-/?-nitroaniline) 1: co-p-nitroaniline) 1:
3 molar ratio 1 molar ratio : 1 molar ratio : 1 molar ratio :2 molar ratio 1 molar ratio 1 molar ratio 2 molar ratio
111
be responsible for this difference in the ESR line asymmetry [34]. Even
temperature variation during synthesis, say decrease of temperature results in
better ordering which makes hydrogen bonding stronger. Stronger the
hydrogen bonding, higher will be the line asymmetry [34].
4.2.5. Electrical Conductivity and Charge Transport Studies
4.2.5.1. Electrical Conductivity
Despite of a wide range of applications and promising newer
applications in future technologies, the electrical conductivity and charge
transport in polyanilines have not yet fully understood [40-42]. No single
mechanism has been found to adequately explain the electrical charge
transport in conducting polyanilines. Semi-crystalline regions are embedded
in the amorphous regions in the matrices of polyanilines. It is reported that
metallic conduction may take place in the metallic regions whereas hopping
conduction takes place in the amorphous regions in polyanilines [42, 43].
However, there are inter and intra-fibrillar conduction and charge carriers
hops from one chain to another within the metallic regions. Electrical
conductivity generally increases with increase in temperature for hopping
transport. It is also reported that some polyanilines exhibit many properties
characteristic of the metallic state [44]. The whole process of charge
transport in polyanilines is highly complex and no single model can explain it
satisfactorily.
The two probe room temperature electrical conductivity measured on
the pressed pellets of the polymers at frequencies of lOOHz and lOKHz as
well as ESR data are presented in table-4.6. The dependence of electrical
conductivity on pH of the reaction medium is given in table-4.7. The relative
frequency dependence of electrical conductivity on the pH of the reaction
112
media as well as the copolymer composition is shown in figure-4.10 and
figure-4.11 respectively. The temperature dependence of electrical
conductivity of the polymers in the temperature range of 25°C - 200°C is
presented from figure-4.12 to rigure-4.14. In order to understand the basic
nature of charge transport, the temperature dependent electrical conductivity
data were fitted to Arrhenius equation (band conduction) as well as to
variable range hopping (VRH) model as shown through figure-4.15 to figure-
4.20.
It is observed that the temperature-conductivity relation follows almost
the same trend for a polymer of given composition. From table-4.6, it can be
seen that all the copolymers show lower electrical conductivity than that of
polyaniline. The electrical conductivity is found to decease as the ratio of
nitroaniline units increases in the copolymer backbone. The electrical
conductivity measured at lOKHz is found to be greater than that measured at
lOOHz. This frequency dependence is also more prominent in copolymers
than in polyaniline. Frequency dependence of electrical conductivity is found
to increase as the ratio of nitroaniline units increases in the copolymer
backbone. Table-4.7 shows that electrical conductivity is strongly dependent
on pH of reaction mixture. It can also be seen from table-4.7 that the
frequency dependence of electrical conductivity increases with increase in the
pH of the reaction medium.
The comparatively lower electrical conductivity of copolymers derived
from aniline and nitroanilines is consistent with our findings of UV-VIS and
ESR spectral studies made on these copolymers. The lower electrical
conductivity of the copolymers may be attributed to several factors. In
general, the decrease of electrical conductivity of the copolymers can be
explained in terms of the decreased extent of conjugation. An increase in the
113
TabIe-4.7: Dependence of electrical conductivity (Scm") of the polymers on the pH of the reaction media for polymer synthesis
Polymer
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)2:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)2:l
P(AcoPNA)l:l
P(AcoPNA)l:2
Electrical Conductivity (Scm')
pH=0
lOOHz
6.3x10-^
5.2x10"^
4.002x10"^
8.02x10-^
6.05x10-'
5.4x10'
4.92x10-'
6.04x10-'
lOKHz
1.002x10-'
5.29x10-^
4.02x10-'
8.28x10-'
2.75x10-*
5.42x10-'
6.48x10-'
6.28x10-*
pH=0.5
lOOHz
1.09x10-*
1.3x10-"
9.8x10-'
3.1x10-*
8.9x10-'
3.9x10-'
1.9x10"'
1.7x10-'
lOKHz
8.6x10-*
1.8x10-"
1.04x10-'
4.01x10-*
5.7x10-'
4x10'
4.6x10-'
3.6x10-*
pH=l
lOOHz
6.4x10-'
5.2x10-'
5.1x10-'
5.8x10-'
3.2x10-**
6.9x10-'
4x10-*
3.2x10-'
lOKHz
4.3x10"*
7.8x10-'
5.5x10-'
8.04x10-'
2.33x10-'
7.7x10-'
1.68x10-'
9.7x10"'
Tabie-4.8: Temperatures at which 5CT/8T « 0 for different polymers
P(AcoONA)l:3 P(AcoONA)l:l P(AcoMNA)2:l P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:l P(AcoPNA)l:l P(AcoPNA)l:2
Polymer
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)l:l
P(AcoPNA)l:2
Temperature at which 8CT/5T « 0 (K)
390
413
408
412
410
427
Poly(aniline-PoIy(aniline-Poly(aniline-Poly(aniline-Poiy(aniIine-Poly(aniline-Poly(aniline-Poly(aniline-
co-o-nitroaniline)l: co-o-nitroaniline)l: co-m-nitroaniline)2 co-m-nitroaniline) 1 co-m-nitroaniline) 1 co-/7-nitroaniline)2: co-/7-nitroaniline) 1: co-p-nitroaniline)l:
3 molar ratio 1 molar ratio : 1 molar ratio : 1 molar ratio :2 molar ratio 1 molar ratio 1 molar ratio 2 molar ratio
114
bandgap is caused by the increased phenyl ring torsional angles which results
from the steric repulsion between the adjacent phenyl rings due to the
presence of-NO2 groups on the phenyl rings [20, 30, 45]. This may also
cause a greater electron localization. These observations are consistent with
the electronic spectra of the copolymers. The blue shift observed in the n ->
71 transition and in exciton bands in the UV-VIS spectra of copolymers
derived from aniline and o-nitroaniline both in the undoped and doped form
are explained on the basis of the above reasoning. The blue shift associated
with the exciton bands in the copolymers derived from aniline with m- and p-
nitroanilines can also be accounted for the same reasons, which have already
been discussed in the section 4.2.3.
Another possibility for the lower electrical conductivity is the
decreased interchain diffusion of the charge carriers in the copolymers [46].
This is induced by the increased separation of the polymer chains due to the
presence of -NO2 groups and lower crystallographic order and hence the
reduced coherence between the chains. Also, the -NO2 groups are likely to
force the chain out of planarity by twisting the phenyl rings relative to one
another to lower the overlap of orbitals along the conjugated systems [47].
As a result, the conduction electron wave functions are expected to be
substantially localized in the copolymers than those in polyaniline. This may
lead to lower mobility for the charge carriers both along the polymer chains
and between the polymer chains. There are also possibilities for the strong
electrostatic interactions such as hydrogen bonding between the -NO2 groups
and cationic radical nitrogen atoms to form some energetically favourable
configurations. These interactions will either be intra-chain or inter-chain or
both. Such configurations can effectively localize the positive charge around
the nitrogen atoms leading to a decrease in electrical conductivity [20].
115
The above factors become more predominant in copolymers containing
greater number of-NO2 groups, i.e. greater number of nitroaniline units in the
polymer backbone. That is why the room temperature electrical conductivity
of as prepared copolymers decreases as the number of -NO2 groups increases
in the polymer backbone as evident from table-4.6. Electrical conductivity of
poly(aniline-co-o-nitroaniline)l:l > poly(aniline-co-o-nitroaniline)I:3, and
that of poly(aniline-co-m-nitroaniline)2:l > poly(aniline-co-w-
nitroaniline)l:l > poly(aniline-co-m-nitroaniline)l:2. Similarly the electrical
conductivity of poly(aniline-co-/7-nitroaniline)2:l > poly(aniline-co-/?-
nitroaniline)l:l > poly(aniline-co-/?-nitroaniline)l:2.
The electrical conductivity strongly depends on the structural factors of
the polymer backbone also. Polyaniline exhibits large equilibrium phenylene
ring torsional displacement out of the plane defined by the ring bridging
amine/imine nitrogen atoms. It is reported that deviation from planarity by
phenylene ring is altered between +30° and -30° as one moves along the
polymer backbone [24]. In polyaniline quinoid units are found to be more
planar than benzenoid units and are associated with a more complex inter
chain ring structure [48, 49]. Any electronic transport properties that are
dependent on interchain and intra-chain wave function overlap will be
strongly affected if substituent like -NO2 groups are present in the polymer
backbone. When substituent like electron withdrawing -NO2 groups are
present, pronounced local torsions resulting from phenylene ring vibrations
and ring flip about the axis would be expected to have more significant
impact on electrical conductivity. These factors may operate in different
ways and to different extent in copolymers containing o-, m- and/?-
nitroanilines units resulting into the difference in their electrical
conductivities. Also, due to the difference in the reactivities of o-, m- and/>-
nitroanilines the resulting copolymers may have varying chain lengths and
116
thus difference in the extent of conjugation, which leads to different
conductivities. For copolymers derived from 1:1 molar ratios of aniline and
nitroanilines, the electrical conductivity is in the order, poly(aniline-co-o-
nitroaniline)l:l > poly(aniline-co-/7-nitroaniline)l:l > poly(aniline-co-m-
nitroaniline)l:l. One of the reasons for the lower electrical conductivity of
copolymer derived from aniline and m-nitroaniline is that there is a greater
reduction in the n electron conjugation due to steric hindrance caused by
-NO2 groups when this polymer takes an energetically favourable
conformation that results into greater electron localization. Greater steric
hindrance operating in these copolymers is evinced from the UV-VIS spectra
as explained in the previous section 4.2.3. There is also a possibility of a
stronger inter-chain interaction like hydrogen bonding operating in these
copolymers resulting in poor mobility of charge carriers. The above
observations are supported by ESR data also. From table-4.6, it can be seen
that in the ESR data, AHpp and 'g ' values of these copolymers are consistent
with the electrical conductivity. Though there are different opinion on the
ESR data [33, 34, 38, 39] and electrical conductivity behaviour of
polyaniline, we observed that greater the electrical conductivity, smaller is
the 'g ' value as well as AHpp and higher is the line asymmetry (a/b ratio)
resulting into greater delocalization of electrons.
It is clear from the table-4.6, the greater electron localization leads to
lower electrical conductivity for the polymers derived from aniline with m-
nitroaniline as supported by their higher AHpp and 'g ' values in comparison to
other copolymers derived from similar molar ratios of the respective
monomers. But in poly (aniline-co-m-nitroaniline)l:2, in which more number
of nitro groups are present than in poly (aniline-co-w-nitroaniline)l:l and
where we expect a greater electron localization, the reported ESR data are not
consistent with the electrical conductivity. This observation supports the fact
117
that ESR data-electrical conductivity correlation cannot be explained on the
basis of a single model.
Table-4.7 shows the dependence of electrical conductivity on pH of
reaction media. It is observed that the electrical conductivity of polyaniline
and the copolymers depends on the pH of the acid medium from which it is
synthesized. When treated with acids, the amine nitrogen atoms of the
polymers are protonated to form radical cation by an internal redox reaction
[50, 51]. The degree of protonation and the resulting conductivity are
controlled by changing the pH of the acid solution used. The low electrical
conductivity of the copolymers at higher pH level is due to lower degree of
protonation as pH increases. Undoped imine nitrogen atoms and the
associated quinoid units are expected to behave as a barrier for electrical
conduction along the chains as well as between the chains resulting in pH
dependent electrical conductivity [46, 52]. It can also be seen from the table-
4.7 that the frequency dependence of electrical conductivity increases as pH
increases.
A remarkable observation of our studies is the measurement of
frequency dependence of electrical conductivity. From table-4.6 and table-
4.7, it is clear that electrical conductivity measured at 10 KHz is higher than
that measured at 100 Hz. We could not carry out the electrical conductivity
measurements at higher frequencies because such options were not available
in our instrument (RLC 1659 digibridge, Genrad). We observed a strong
frequency dependence of electrical conductivity for the copolymers though it
is very feable in polyaniline. Frequency dependence becomes more
prominent as the nitroaniline units increase in the polymer backbone. Epstein
and coworkers [53] have performed the microwave conductivity of poly(o-
toluidine) in order to characterize the effect of electron localization on the
118
transport properties. In their work on dielectric constant and AC conductivity
measurements at 10 KHz on polyaniline and their derivatives, Pinto and
coworkers [54] have pointed out that greater localization of charges leads to
the reduced electrical conductivity and that the substitutents causes an
increase in the disorder in the system.
Our observation on frequency dependence of electrical conductivity
suggests the evidence of greater electron localization in copolymers. We
assume that greater the electron localization greater will be the frequency
dependence of electrical conductivity as evident from table-4.6 and table-4.7.
This argument is supported by ESR data also. Table-4.6 shows the
frequency dependence of electrical conductivity in poly(aniline-co-o-
nitroaniline)l:3 is greater than poly(aniline-co-o-nitroaniline)l:l. In the
copolymers derived from aniline and w-nitroaniline the frequency dependence
of electrical conductivity decreases in the order, poly(aniline-co-m-
nitroaniline)l:2 > poly(aniline-co-m-nitroaniline)l:l >poly(aniline-co-m-
nitroaniline)2:l. Similarly in the copolymers derived from aniline andp-
nitroaniline the frequency dependence decreases in the order poly(aniline-co-
p-nitroaniline)l:2 > poly(anilirie-co-/?-nitroaniline)l:l > poly(aniline-co-p-
nitroaniline)2:l. This increase in frequency dependence as the nitroaniline
units increases in the polymer backbone may be due to an increase in the
electron localization as the nitroaniline units increases in the polymer
backbone. In polyaniline where the electron localization is lesser (higher
delocalization) than copolymers, we observed a very little frequency
dependence supporting the observations of ESR spectral studies. The
frequency dependence of electrical conductivity on pH of the reaction media
as well as the copolymer composition is further clarified in the bar graph
shown in figure-4.10 and figure-4.11.
119
P(AcoONA)1:1
Figure-10 (A). Relative frequency dependence of electrical conductivity (AEC) on the p of the reaction medium for different polymers.
P(AcoONA)l:l P(AcoONA)l:3
-Poly(aniline-co-o-nitroaniline) 1:1 -Poly(aniline-co-o-nitroaniline) 1:3
120
1.08
1.06
1.04^
EC 1.02
1
0.98
^., o-
I I •
II ^ ^ ^ 1
^^^^H^^7
^ ^ f •;
*yftM.^f^g|B
^ 9 P(AcoMNA)2:1
• ^ ^
i i " n ^ H i
>^sHI .ar^i
8
6i
EC 4
2
0
o o 1! II
P(AcoMNA)1:i
1 - ^
II
)
Figure-lO (B). Relative frequency dependence of electrical conductivity (AEC) on the p" of the reaction medium for different polymers.
P(AcoMNA)l :2 -Poly(aniline-co-m-nitroamline) 2:1 P(AcoMNA)l:l -Poly(aniline-co-m-nitroaniline) 1:1 P(AcoMNA)l:2 -Poly(aniline-co-m-nitroaniline) 1:2
121
n
EC
P(AcoPNA)2:1
P(AcoPNA)1:1
P(AcoPNA)1:2
Figure-10 (C). Relative frequency dependence of electrical conductivity (AEC) on the p" of the reaction medium for different polymers.
P(AcoPNA) 1:2 -Poly(aniline-co-p-nitroaniline) 2:1 P(AcoPNA) 1:1 -Poly(aniline-co-p-nitroaniline) 1:1 P(AcoPNA)l:2 -Poly(aniline-co-p-nitroaniline) 1:2
122
2
1.5-
Z^EC 1
0.5
0
J H ^^^•••K^^^^^Mb^
1 2
pH = 0
• P(AcoONA)1:1 • P(AcoONA)1:3
3.5 3
2.5
^ ^ ^ 1.5 1
0.5 n-\j^
. ^ ^ ^ ^ ^ Sui ^ ^ l 1 2
pH = 0.5
• P(AcoONA)1:1 • P(AcoONA)1:3
8
6
A EC 4
2
0 1 2
pH = 1
• P(AcoONA)1:1 • P(AcoONA)1:3
Figure-ll(A). Relative frequency dependence of electrical conductivity (AEC) on the copolymer composition at various p" . P(AcoONA)l:3 -Poly(aniline-co-o-nitroaniline) 1:3 P(AcoONA)l:l -Poly(aniline-co-o-nitroaniline) 1:1
123
6i
4
2 1 n u^
m. 1
^Jr^ 1
pH = 0
• P(AcoMNA)2:1 • P(AcoMNA)1:1 DP(AcoMNA)1:2
8
6
/f^EC4
2
0 1
pH = 0.5
• P(AcoMNA)2:1
• P(AcoMNA)1:1
nP(AcoMNA)1:2
1
AEC 4
0
^ ^
1
pH = 1
n L ^^r
• P(AcoMNA)2:1
• P(AcoMNA)1:1
DP(AcoMNA)1:2
Figure-11 (B), Relative frequency dependence of electrical conductivity (AEC) on ttie copolymer composition at various p" . P(AcoMNA) 1:2 -Poly(aniline-co-m-nitroaniline) 2:1 P(AcoMNA)l:l -Poly(aniline-eo-m-nitroaniline) 1:1 P(AcoMNA) 1:2 -Poly(aniline-co-m-nitroaniline) 1:2
124
12 10
8i
AEC 6 4 2 0
1
pH = 0
• P(Ac»PNA)2:1
• P(AcoPNA)1:1
DP(AcoPNA)1:2
25 20
AEC ' ' 10: 5 0
1 m^Mw 1
pH = 0.5
• P(AcoPNA)2:1
• P(AcoPNA)1:1
DP(AcoPNA)1;2
• P(AcoPNA)2:1
• P(AcoPNA)1:1
DP(AcoPNA)1:2
Figure-ll (C). Relative frequency dependence of electrical conductivity (AEC) on the copolymer composition at various p" . P(AcoPNA) 1:2 -Poly(aniline-co-p-nitroaniline) 2:1 P(AcoPNA)l:l -Poly(aniline-co-p-nitroaniline) 1:1 P(AcoPNA)l:2 -Poly(aniline-co-p-nitroaniline) 1:2
125
4.2.5.2. Charge Transport Studies
The temperature dependence of electrical conductivity of the polymers
were performed in the temperature range of 25°C to 200°C in order to
understand the basic nature of charge transport. The results are shown in
figure-4.12 to figure-4.20. During the initial trial of the measurements of
temperature dependence of electrical conductivity, irregular and non-
reproducible results were observed because the pellets were found to be
broken during the experiment at higher temperatures. The experiments were
repeated by increasing the thickness of the pellets which yielded stable and
reproducible results. Reproducible results of electrical conductivity-
temperature relationship are obtained if the pellets were cycled between 25°C
and 90°C for several times, however, no reproducibility was observed when
cycled between 25°C and 200°C.
The studies revealed a thermally activated conduction phenomena in all
the copolymers. We observed that the electrical conductivity increases
linearly with increase in temperature up to a temperature at which 5a/6T » 0.
After this temperature electrical conductivity decreases. The temperature at
which 5a/5T « 0 for various copolymers are presented in table-4.8. From
table-4.8, it can be seen that the temperature at which 6a/5T « 0 is lowest for
poly(aniline-co-o-nitroaniline)l:3 and is highest for poly(aniline-co-/?-
nitroaniline)l:2. In copolymers which contain higher fraction of nitroaniline
units, the temperature at which 5a/5T « 0 was observed at higher
temperatures than in those copolymers in which nitroaniline units are lesser.
It is an accepted fact that the electrical conductivity of highly
conducting polyaniline is affected by environmental humidity and moisture
[44, 55-57]. Transport studies to understand the electrical conduction
126
,^
b
u 3 TJ C O u o o
1^ 9t
8X10-3
7X10"3
6X10~3
5X10-3
AX10-3
3X10-3
AX 10-5
3X10-5
2X10"5
1X10-5
-
^ ^ ^ ^
y^ X-(A)
/ /
- /
y^^^^^^^^ / ^^N_
- / \ ^ (B)
- /
1 1 1 1 1 1 1 1
303 323 3A3 363 383 403
Temperature( K)
A23 4A3 A63
Figure-4.12 Variation of electrical conductivity with temperature for (A) poly(aniline-co-o-nitroaniline) 1:1 (B) poly(aniIine-co-0-nitroaniline) 1:3
127
MXIO"^ -
1X10"^ -
303 323 3^3 363 383 403
Temperature (K)
423 443 463
Figure-4.13 Variation orelectrical conductivity with temperature for (A) poiy(aniline-co-m-nitroaniline) 1:1 (B) poly(aniline-co-m-nitroaniline) 1:2
128
303 323 343 363 383 403 423
1»fnp»ra»ure( K )
443 463
Figure-4.14 Variation of electrical conductivity with temperature for (A) poly(aniline-co-/>-nitroaniIine) 1:1 (B) poly(aniline-co-/7-nitroaniIine) 1:2
129
mechanism is generally carried out from low temperatures, 100 K to room
temperature [58-61]. There are a fewer studies on temperature dependence of
electrical conductivity above room temperature [44]. Similarly, not many
studies have been carried out on the temperature dependence of electrical
conductivity of substituted polyanilines [39, 54].
Unlike other conducting polymers, polyaniline is not charge
conjugation symmetric, i.e. the Fermi level and band gap are not formed in
the centre of the n band so that the valance and conduction bands are very
asymmetric [14, 59, 65]. Also, the hetero atom, N, is within the conjugation
path and the electrical properties can be controlled by the variation of number
of electrons or number of protons per repeat unit. In general, the electrical
conductivity is given by -
a = ne|i, where 'n ' is the number of charge carriers per unit volume, ' e ' is the
electronic charge and ' ^ ' is the mobility of the charge carriers. In polyaniline,
it is established that the conduction band is partly filled and has a band width
approximately an order of magnitude larger than kT [14]. It is also reported
that the density of states function is approximately constant and electronic
states are localized especially in low conducting samples of polyanilines [62].
The charge transport is then expected to take place via hopping mechanism.
The localization of electronic states arises due to disorder and the
presence of defects. The presence of moisture creates a dynamic disorder in
the polymeric systems [62]. At higher temperatures, when moisture is
removed, it is expected that static defects such as chain ends and chemical
defects control the charge transport. A large number of secondary features
associated with hopping mechanism for charge transport in polyaniline, such
as variable range hopping model [63], one dimensional hopping [64], granular
130
metal model [65,66] and a distribution in energy of the localized hopping
states [62] are proposed.
In the one dimensional limit, all wave functions are localized in the
presence of disorder in polyaniline. For a given level of disorder, the strength
of the interchain coupling needed to suppress the localization depends on the
coherence length along the quasi-one-dimensional chain. Then charge hops to
an adjacent chain leading to localization in one dimension [67]. Transport
properties in polyaniline to a great extent depends on the presence of both
extended and localized states and disorder [68]. The extent of disorder
determines the relative role played by the localization and by electron-
electron interactions. Disorder leads to qualitatively different charge
transport mechanisms in homogenous and inhomogenous limits. When
inhomogenities dominate the charge transport, "metallic islands" models were
constructed to interpret the charge transport [69, 70]. Also, the conjugation
defects and chain breaks within the metallic region limits the electrical
conductivity of polyaniline [67]. Since different research groups have
interpreted the temperature dependence of electrical conductivity through
different models, charge transport studies on polyaniline have resulted in a
somewhat confusing situation.
In our studies, we found a thermally activated increase in electrical
conductivity up to a temperature at which 5a/6T « 0, after which the
electrical conductivity decreases with increase in temperature. The effect of
moisture on the electrical conductivity is found to have a negligible effect on
both polyaniline and copolymers. A previous study of temperature
dependence of electrical conductivity on polyaniline [44] showed that, the
electrical conductivity increases only up to 50°C after which conductivity
decreases due to the loss of moisture. Interestingly, in all our samples, the
131
electrical conductivity which falls in the semiconducting regime, increases up
to 100°C. This results shows that temperature dependence of electrical
conductivity is not affected by the presence of moisture in semiconducting
aniline based copolymers.
To understand the basic nature of charge transport, the data were fitted
in Arrhenius equation as well as in the equation corresponding to Variable
Range Hopping model. A careful examination of the results presented
through figure-4.15 to figure-4.20 shows that a combination of conduction
mechanisms are operating in these copolymers depending on the temperature
range of the measurements.
It can be seen that at higher temperature ranges, the charge transport is
mainly through Arrhenius type and in the lower temperature range, the charge
transport is through three dimensional variable range hopping. The electrical
conductivity decreases after a temperature, corresponding to 6a/5T « 0,
probably because of the degradation of polymers due to removal of dopant or
due to some structural variation.
The presence of moisture is reported to cause spin and charge
delocalization, probably by solvating the CI" and thus by reducing the
electrostatic interaction between the positive charges and CT, thus reduces
the polarity effect of anions- Loss of moisture leading to increased
localization and thereby a reduction in conductivity is not operative in these
polymers. This may be an indication that the static defects play an important
role in the thermally activated conduction phenomena [62] in all the
polymers.
The observed conductivity-temperature relation is found to fit best in
the variable range hopping model equation -
132
Figure-4.15 Arrhenius plot of log(cy) versus 1000/T(K"') (A) poly(aniline-co-0-nitroaniliiie) 1:3 (B) poly(aniline-co-o-nitroaniline) 1:1
.133
Figure-4.16 Arrhenius plot of log(a) versus 1000/T(K"') (A) poIy(aniline-co-/«-nitroaniline) 1:1 (B) poly(aniline-co-/ii-nitroaniIine) 1:2
134
-4.0 -
-4.1
-4.2 -
-C.6 -
-4.7
o
-4.8 -
-4.9
0 -
(A)
-5.1
-5.2 -
J I 24 2.6 2.8 3.0 3.2 3.4
lOOO/KK''')
Figure-4.17 Arrhenius plot of log(o) versus 1000/T(K"') (A) poly(aniline-co-^-nitrQaniline) 1:1 (B) poly(aniline-co-/?-nitroaniUne) 1:2
135
-3.1
-3.2
-3.3
-3.4
-3.5
-
• • • •
• •
-4.3
b o> -4.4
-4.5
-4.6
-4.7 h
-4.6
-4.9
-5.0 0.214 0218 0.222 0.226 0.230 0.234 0.238 0,242
T-I/4(K-'/«)
Figure-4.I8 Application of variable range hopping model to electrical conductivity- temperature behaviour of (A) poly(aniline-co-0-nitroaniline) I;l (B) poly(aniline-co-0-nitroaniline) 1:3
136
-5.1 -
-5.2 -
o
-5.3 -
-5.A -
-5.5 -
a 2 U 0.218 0.222 Q226 0.230 0234 0.38 0.2A2
Figure-4.19 Application of variable range hopping model to electrical conductivity - temperature behaviour of (A) poly(aniline-co-m-nitroaniline) 1:1 (B) poly(aniline-co-m-nitroaniline) 1:2
137
-3.9
-4.0
-4.1
-4.2
-4.6
b o> -4.7 o
-4.8
-4.9
-5.0
-5.1
-5.2
-
-
"
^ ^ -
^
-
-
•
• • •
1
• • • . .
•
•
•
•
1 f 1
^ \ ^
\ ^ B
1 1 1
0.214 a218 0.222 0.2 26 0230 0.234
T-1/4(K-1/A)
0.238 0.242
Figure-4.20 Application of variable range hopping model to electrical conductivity - temperature behaviour of (A) poIy(aninne-co-/7-nitroaniline) 1:1 (B) poly(aniline-co-p-nitroaniline) 1:2
138 ^ ^ S IS
a(T) = OQ exp ^0
V A y 4.1
where To is the Mott's characteristic temperature and CQ is the electrical
conductivity at room temperature in the low temperature range studied. At
higher temperatures, the Arrhenius plot is found to be a good fit to the data
following the equation -
f E a(T) = GQ exp
kT 4.2
However, the data obtained for the temperature dependence of
electrical conductivity for the copolymers of aniline with m- and p-
nitroanilines seems to fit into Arrhenius equation in the low temperature
range also but with different activation energies. This suggests that
Arrhenius type conduction may also operative in the lower temperature range.
From our studies we conclude that a single mechanisms can not be used
to explain the charge transport in these copolymers in the entire temperature
range studied. However, our limited studies do not point in to the exact
conduction mechanism.
4.2.6. Thermogravimetric Analysis
Thermal stability of conducting polymers is of great importance, as one
has to essentially look into it while considering their application aspects [71,
72]. There is still a need to study the thermal stability of newer systems
based on polyaniline although polyaniline and polymers based on its
derivatives have been studied by many researchers [73-76]. Therefore, a
comparative study on thermal stability of polyaniline and copolymers derived
from aniline and substituted anilines has become the part of this work.
139
Thermogravimetric data of the polymers under study are presented in table-
4.9 to table-4.12, while the thermograms are presented in figure-4.21 to
figure-4.24. The general observation from the thermograms is that the
oxidative degradation of polymer back bone takes place after the removal of
trapped water followed by dopants and low molecular weight oligomers
leaving behind a very low % of residue.
Thermogram of polyaniline (figure-4.21) showed three distinct regions
of weight loss. The initial weight loss of-1.7 % by 130°C is attributed to the
loss of water molecules. The weight loss of about 8.1 % by 260°C
corresponds to the loss of HCl which is lower than the theoretical 16% of HCl
in fully protonated polyaniline [75-77]. Beyond 260°C, thermo-oxidative
degradation of polymer backbone takes place. The rate of decomposition is
found to be maximum at the temperature ~480°C. During the degradation of
polyaniline backbone, compounds such as ammonia, aniline, p-
phenylenediamine, N-phenylaniline and N-phenyl 1,4-benzenediamine are
reported to be formed [72-76]. At 610°C, a residual weight of 2.44 % is left
which may be attributed to some thermally stable, cross linked and
polynuclear organics. At the final stage of decomposition, it is also reported
that the remaining reduced repeating units fuse to form thermally stable
carbazole groups at higher temperatures which yield the residue [72].
Thermogram of poly(aniline-co-o-nitroaniline)l:3 showed an
inflectionless decomposition up to 580°C. The initial weight loss of about 9
% by 240°C may be attributed to the loss of water, HCl and low molecular
weight oligomers. Thermo- oxidative degradation of polymer backbone
begins at ~240°C. The rate of decomposition is found to be maximum at
~500°C. A residual weight of 1.96 % may be attributed the formation of
some poly nuclear organics. Poly(aniline-co-o-nitroaniline)l:l showed three
140
90 190 290 390 490
Temperoture [°C )
590 690
Figure-4.21 Thermogram of Polyaniline
141
1/1 o
^ J
'5 ^
S?
20
40
60
80
1
\ \ \ \
vV— \ \ \ \
\ \ \
1 f 1
A
B
\ \ \ \ \ \ \ \ \ \
1 ^ 1
90 190 290 390 490 Temperature ( C)
590 710
Figure-4.22 Thermogram of (A) poly(aniline-co-o-nitroaniline) 1:1 (B) poly(aniline-co-o-nitroaniiine) 1:3
142
690 mperature ( C)
Figure-4.23 Thermogram of (A) poly(aniHne-co-/«-nitroaniIine) 1:1 (B) poly(aniline-co-m-nitroaniline) 1:2
143
80 -
o 60
' . 0
2 0 -
'
-J—
\ \ \ \
\ \ \ \
\ \
(B) \ \ \ \
\ \ \ \
\ \ \ \ \ \ \ \
1 1 1 I ^ F _ J 90 190 290 390 i90 590 690
Temperature {°C)
Figure-4.24 Thermogram of (A) poly(aniIine-co-/;-nitroaniline) 1:1 (B) poly(aniIine-co-/;-nitroaniline) 1:2
144
Table-4.9: Thermogravimetric analytical data of polymers
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)l:l
P(AcoPNA)l:2
% Weight loss due to removal of
water, HCl, oligomers
8.1
9
16.39
8.44
8.6
10.65
9.0
Thermo-oxidative degradation
Onset temperature
260
240
290
270
280
290
270
% Weight loss
89.37
89.04
81.87
84.58
88.92
87.35
88.9
Residue
Temperature CO 610
580
550
530
590
580
550
% Weight
2.44
1.96
1.74
6.82
2.48
2.0
2.1
PANI P(AcoONA)l:3 P(AcoONA)l:l P(AcoMNA)2:l P(AcoMNA)l:l P(AcoMNA)l:2 P(AcoPNA)2:l P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniline-co-o-nitroaniline)l:3 molar ratio Poly(aniline-co-o-nitroaniline)l:l molar ratio Poly(aniline-co-/M-nitroaniline)2:l molar ratio Poly(aniline-co-w-nitroaniline)l:l molar ratio Poly(aniline-co-m-nitroaniline)l :2 molar ratio Poly(aniline-co-/7-nitroaniline)2:l molar ratio Poly(aniline-co-/7-nitroaniline)l: 1 molar ratio Poly(aniline-co-/?-nitroaniline)l:2 molar ratio
145
distinct regions in the thermogram. In the initial stage, a slow weight loss of
about 2.82 % by ~160''C may be attributed to the loss of water molecules. In
the second stage, there is a weight loss of about 13.57% by ~290°C which can
be attributed to the loss of HCl and low molecular weight oligomers.
Oxidative degradation of polymer backbone begins at ~290°C. The rate of
decomposition was found to be maximum at ~480°C. The decomposition is
completed by 550°C at which a residual weight of 1.74 % was left due to the
formation of some polynuclear organics. A greater weight loss of 13.57 % in
the second stage compared to poly(aniline-co-o-nitroaniline)l:3 may be
attributed to the higher percent of dopant and oligomers.
An initial weight loss of 8.44 % upto ~270°C in the thermogram of
poly(aniline-co-/M-nitroaniline)l:l is attributed to the loss of water molecule,
HCl and low molecular weight oligomers. Thermo-oxidative degradation of
polymer backbone starts at ~270°C. Degradation is completed by 530°C at
which a residual weight of 6.82 % is left. The rate of decomposition is found
to be maximum at ~480°C after which the rate of decomposition suddenly
decreases due to the formation of some cross- linked products and
polynuclear organics which is stable towards oxidative degradation.
Thermogram of poly(aniline-co-m-nitroaniline)l:2 showed an initial weight
loss of 8.6 % by ~280°C, which can be attributed to the loss of water, HCl
and oligomers. Thermo-oxidative degradation of polymer backbone begins at
~280°C. The rate of decomposition is found to be maximum at ~460°C. The
residual weight of 2.48 % is left due to the formation of some cross linked
products and polynuclear organics.
Thermogram of poly(aniline-co-/?-nitroaniline)l:l showed an initial
weight loss of 10.65 % by ~290°C which is attributed to the loss of water,
HCl and low molecular weight of oligomers. Degradation of polymer
146
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148
backbone begins at ~290°C. The rate of decomposition is maximum at
~500°C after which the rate of decomposition decreases leaving behind 2.1 %
of residual mass by 580°C. Thermogram of poly(aniline-co-/?-nitroaniline)l:2
showed an initial weight loss of 9 % by ~270°C due to the removal of water,
HCl and low molecular weight oligomers. Degradation of polymer backbone
begins at ~270°C. The rate of degradation is found to be maximum at ~480°C.
The degradation is completed by 550°C and a residual mass of 2.1 % is left
behind due to the formation of some cross linked products and polynuclear
organics.
From table-4.9, it may be observed that all the copolymers are stable
below 240°C although there is gradation in the thermal stabilities of the
polymers. All the copolymers of aniline and nitroaniline except poly(aniline-
co-o-nitroaniline)l:3 show better thermal stability than polyaniline.
Poly(aniline-co-o-nitroaniline)l:l and poly(aniline-co-/7-nitroaniline)l:l
show highest stability with the onset temperature of degradation of 290°C. It
can also be seen from the table-4.9 that the total weight loss of 6.82 % due to
the volatilization of degradation products is observed to be lowest in
poly(aniline-co-o-nitroaniline)l:l. Lowest residual weight of 1.74 % at
550°C is observed for poly(aniline-co-o-nitroaniline)l:l. The process of
thermal decomposition of polymer is completed at ~610°C, the highest, in
polyaniline and at 530°C the lowest, in poly(aniline-co-o-nitroaniline)l:l.
The decreasing order of % weight loss during the thermo-oxidative
degradation of polymer backbone follows, polyaniline (89.37 %) >
poly(aniline-co-o-nitroaniline)l:3 (89.04 %) > poly(aniline-co-m-
nitroaniline)l:2 (88.92 %) > poly (aniline-co-/?-nitroaniline)l:2 (88.90 %) >
poly(aniline-co-/7-nitroaniline)l:l (87.35 %) > poly(aniline-co-m-
nitroaniline)l:l (84.58 %) > poly(aniline-co-o-nitroaniline)l:l (81.87 %).
149
Table-4.11: Thermogravimetric data showing temperature corresponding to highest rate of weight loss
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)l:l
P(AcoPNA)l:2
Highest rate of weight loss Water loss + dopant
Temperature range (°C)
210-230
210-230
250-270
230-250
230-250
230-250
250-270
Rate of % weight loss
(per °C) 0.0740
0.1085
0.1405
0.0740
0.0960
0.0760
0.0913
Thermo-oxidative degradation
Temperature range (°C)
490-510
490-510
470-490
470-490
450-470
490-510
470-490
Rate of % weight loss
(per °C) 0.5525
0.5000
0.6674
0.5650
0.4235
0.5760
0.5660
Table-4.12: Activation energies of thermal degradation
Polymer
PANI
P(AcoONA)l:3
P(AcoONA)l:l
P(AcoMNA)l:l
P(AcoMNA)l:2
P(AcoPNA)l:l
P(AcoPNA)l:2
Activation Energies (KJ/mol)
EA-1
9.67(170°C-370°C)
11.307 (170C°-430°C)
9.81 (170C°-410°C)
11.28(170°C-300°C)
11.43 (170°C-310°C)
11.23(170°C-380°C)
10.584 (170°C-290°C)
EA-2
22.94 (370°C-570°C)
26.6 (430°C-550°C)
28.822 (410°C-550°C)
19.15 (300°C-470°C)
18.75(310°C-470°C)
25.36 (380°C -550°C)
17.63 (270°C-470°C)
EA-3
-
-
37.27 (470°C -530°C)
32.18 (470°C-550°C)
-
30.1(470°C-550°C)
PANI P(AcoONA)l:3 P(AcoONA)l:l P(AcoMNA)2:l P(AcoMNA)I:l P(AcoMNA)l:2 P(AcoPNA)2:I P(AcoPNA)l:l P(AcoPNA)l:2
Polyaniline Poly(aniIine-PoIy(aniIine-Poly(aniline-Poly(aniline-Poly(aniline-Poly(aniline-PoIy(aniIine-Poly(aniIine-
co-o-nitroaniline)l: co-o-nitroaniline) 1: co-m-nitroaniline)2 co-w-nitroaniline) I co-w-nitroaniline) 1 co-/7-nitroaniline)2: co-/7-nitroaniline) 1: co-/?-nitroaniline)l:
3 molar ratio 1 molar ratio : 1 molar ratio : I molar ratio :2 molar ratio 1 molar ratio 1 molar ratio 2 molar ratio
150
The % weight loss due to the removal of water, dopants and low
molecular weight oligomers follows the order poly(aniline-co-o-
nitroaniline)l:l > poly(aniline-co-p-nitroaniline)l:l > poly(aniline-co-p-
nitroaniline)l:2 = poly(aniline-co-o-nitroaniline)l:3 > poly(aniline-co-w-
nitroaniline)l:2 > poly(aniline-co-w-nitroaniline)l:l > polyaniline. Therefore,
it may be inferred that the extent of doping will also follow the same order
provided water and oligomer content is assumed to be similar.
A careful inspection of table-4.10 shows that the rate of degradation
upto ~150°C corresponding to the removal of water is similar in all the
polymers except in poly(aniline-co-o-nitroaniline)l:l for which it is higher
which suggests the presence of water in. larger fraction. Between 150-320°C,
the decomposition rate is minimum for polyaniline which may be due to the
presence of lesser oligomeric fraction in this polymer. Between 150-350°C,
the rate of degradation of poly (aniline-co-o-nitroaniline)l:3 is comparatively
higher than that of polyaniline. This may be attributed to the presence of
larger oligomeric fraction present in poly(aniline-co-o-nitroaniline)l:3.
Table-4.11 shows the temperatures corresponding to the highest rate of
weight loss for the removal of water and dopants as well as for the thermo-
oxidative degradation of the polymer backbone. A highest rate of weight loss
% of 0.1405 per degree C for the removal of water and dopants is found to be
the maximum for poly(aniline-co-o-nitroanilinel:l and occurs at a
temperature range of 250-270°C and is minimum for polyaniline and
poly(aniline-co-w-nitroaniline)l:l for which the rate of weight loss % is
0.074 per degree C. For polyaniline it occurs in the temperature range of
210-230°C and for poly(aniline-co-m-nitroaniline)l:l it occurs in a
temperature range of 230-250°C. The highest rate of weight loss % during
the thermo-oxidative degradation of polymer backbone is maximum for
151
J-6 1-8 ZO
lOOO/TCK)
Figure-4.25 Plot of ln(In 1/y) versus 1000/T(K'') of Polyaniline for the estimation of activation energies of thermal degradation.
152
-1.0 -
c C
- 2 . 0 -
-3.0 -
1/TX lOOOlK
Figure-4.26 Plot of In(In 1/y) versus IOOO/T(K ') for the estimation of activation energies of thermal degradation. (A) poly(aniline-co-tf-nitroaniline) 1:3 (B) poly(aniline-co-0-nitroaniline) 1:1
153
1.S
1.0
0.0
c c
-1.0
•2.0
-3.0 -
-4.0
1.2 1.4 X J_ X 1.6 1.8 2.0
1/TX I 000 {K - ' )
2.2 2.4
Figure-4.27 Plot of ln(ln 1/y) versus IOOO/TCK") for the estimation of activation energies of thermal degradation. (A) poly(aniline-co-/M-nitroaniline) 1:1 (B) poly(aniline-co-w-nitroaniline) 1:2
154
c
c
1.6 1.8 2.0
l / T X lOOOIK"')
2.2 2.4
Figure-4.28 Plot of in(ln 1/y) versus 1000/T(K-') for the estimation of activation energies of thermal degradation. (A) poly(aniline-co-p-nitroaniline) 1:1 (B) poIy(aniIine-co-/?-nitroaniline) 1:2
!55
poly(aniline-co-o-nitroaniline)l:l for which it is 0.6674% per degree C in the
temperature range of 470-490°C and is minimum for poly(aniiine-co-w-
nitroaniline)l:2 for which it is 0.4235 % per degree C in the temperature
range of 450-470°C.
One of the simplest method to evaluate the activation energies of
thermal degradation of polymers is the use of Broido equation [78]. This
equation has been used to study the degradation of poly acrylamide,
polyacrylic acid, cellulose etc. [79, 80]. We have also used Broido equation to
evaluate the activation energies of thermal degradation of polyaniline (figure-
4.25) and copolymers of aniline with nitroanilines (figure-4.26 to figure-
4.28). From table-4.12, it can be seen that the activation energies
corresponding to the initial stage of thermal degradation are almost the same
for polyaniline and all the copolymers studied. The initial stage of thermal
degradation of polymer backbone requires almost the same activation energy
for their removal as it requires for the removal of water, HCl and low
molecular weight oligomers. It can also be seen from the table-4.12 that the
major part of thermo-oxidative degradation of polymer backbone takes place
with a higher activation energy which are different for different polymers. In
the copolymers derived from aniline and m-nitroaniline, the final stage of
degradation of polymer backbone occurs with higher activation energy. The
difference in the activation energies of different polymers may be an
indication that in each copolymer either different degradation mechanism is
involved or due to the difference in the structure of polymer backbone or
both.
156
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162
^^p,^luma'€6 ^ji aniline, and o^mMtkulanciina (o^o^luldlnM) with fSL'matkiyxuanlllna {p,^anlildlna)
CHAPTER - 5
COPOLYMERS OF ANILINE AND o-METHYLANILINE (o-TOLUIDINE) WITH p.METHOXYANILINE (p-ANISIDINE)
5.1. Introduction
As an attempt to get more processible polymers, copolymers of aniline
as well as of o-toluidine with p-methoxyaniline were synthesized for 1:1
molar ratios of the respective monomers as explained in chapter-3 [1].
Studies on copolymers derived from ortho-isomers of methoxy-, ethoxy- and
propoxyanilines [2-4] as well as 2,5-dimethoxyaniline [5] have already been
reported. This chapter deals with our attempt to incorporate p-methoxy -
aniline units in polyaniline and poly(o-toluidine) backbones and to interpret
the electrical, electronic and thermal properties of resulting copolymers.
5.2. Results and Discussion
5.2.1. Folymer Synthesis
Unlike the polymerization of aniline where the initial color change of
the reaction mixture took place within 2-3 minutes after the addition of
oxidant, potassium persulphate, the copolymerization of p-methoxyaniline
with aniline and with o-toluidine is found to take place very slowly. Time
taken for the initial color change of the reaction mixture, along with % yield
of the product formed is given in tablc-5.1. It can be seen that in the reaction
mixture containing aniline and p-methoxyaniline, initial color change from
colorless to greenish violet upon addition of potassium persulphate occurred
after 12-14 minutes of the addition of oxidant. Polymerization of the reaction
mixture containing o-toluidine and p-methoxyaniline was even slower than
the first case. Here, the initial color change appeared after 15-16 minutes.
163
Table-5.1. Time taken for the initial color change of the reaction mixture and % yield of the polymers.
Polymer
PANI
P(AcoPMA)
P(OTcoPMA)
Time taken for the initial color change (min.)
2
12-14
15-17
% yield
80.03
52.1
38.3
Table-5.2. Solubility characteristics of as-prepared (protonated) and base (deprotonated) forms of the copolymers.
As-prepared form Base form
Polymer
P(AcoPMA)
P(OTcoPMA)
P(AcoPMA)
P(OTcoPMA)
H2O
IS
IS
IS
IS
H2S04
S
s s s
Acetone
ss ss ss MS
CH3OH
SS
MS
MS
MS
DMF
MS
MS
S
S
DMSO
MS
MS
S
S
NMP
MS
MS
S
S
IS ss MS s
Insoluble Slightly Soluble Moderately Soluble Soluble
PANI P(AcoPMA) P(OTcoPMA)
Polyaniline Poly(aniline-co-/7-methoxyaniline) Poly(o-toluidihe-co-/7-methoxyaniHne)
164
This suggests that the aniline or o-toluidine molecules alone cannot homo-
polymerize by coupling in presence of/7-methoxyaniline. This is due to the
direct involvement of/7-methoxyaniline molecules in the polymerization
process and is thus incorporated in the polymer backbone to yield the
corresponding copolymers [1].
The presence of methoxy and methyl groups results in an increase in
the electron density in the phenyl ring and amino atoms. This may facilitate
the polymerization because then the affinity for proton for nitrogen is
increased. The protonation is an important step in the polymerization of
aniline [6,7]. The anilinium cation radical produced first during the
polymerization process may recombine to benzedine [8,9] or participate in the
growth of polymer chain in the pernigraniline form. The protonated repeat
unit of this form is responsible for the initial color change [10,11]. The
observed decrease in the rate of reaction is therefore caused by steric effect of
-OCH3 and -CH3 groups. The steric effect is more pronounced in poly(o-
toluidine-co-/?-methoxyaniline). Since para-positions are already blocked by
-OCH3 groups, coupling should take place at the ortho-position to -NH2
groups. A comparatively poor yield of the copolymers may be a qualitative
indication that the copolymers are having lower chain length and the
copolymerization process may get easily terminated. We observed that the
washings (filtrate) of the product with acetone was of intense brown color and
it needed large quantity of acetone to wash away the low molecular weight
oligomeric fractions especially during the washing of poly(c»-toluidine-co-/7-
mcthoxyanilinc).
5.2.2. FTIR Spectral Studies
The FTIR spectra of poly(aniline-co-/7-methoxyaniline) and poly(o-
toluidine-co-/7-methoxyaniline) are presented in figure-5.1 and the absorption
65
Ul o z <
2 2 < cr
iiOOO 3 0 0 0
Figure-5.1
2000 1500
WAVE NUMBER(cm
1000 AOO
- ' )
FTIR Spectra of (A) poly(aniline-co-/;-methoxylanilinc) (B) poly(<'-toluidinc-co-/'-mcthoxyliinilinc)
166
data are given in table-5.3. FTIR spectrum of poly(aniline-co-/7-
methoxyaniline) shows major absorptions at 1568.3 cm'' and 1500.8 cm''
which are attributed to C=C vibrations of quinoid and benzenoid units
respectively [11-14]. The corresponding peaks for poly(o-toluidine-co-/?-
methoxyaniline) occur at 1570 cm'' and 1508 cm"' respectively. The relative
intensity of quinoid band to the benzenoid band is a measure of the degree of
oxidation of the polymer chain [15,16]. The oxidation states of both the
copolymers are almost the same as there seems no apparent difference in the
relative intensities of corresponding benzenoid and quinoid bands. But it can
be suggested that the number of oxidized units are lesser than the number of
reduced units in both the copolymers. The bands at 1306 cm"' for
poly(aniline-co-/?-methoxyaniline) and at 1300 cm"' for poly(o-toluidine-co-p-
methoxyaniline) are attributed to C-N stretching vibrations of benzenoid-
quinonoid-benzenoid sequence [14,17]. Unlike the spectrum of polyaniline,
there is a strong band at 1246 cm"' and 1250 cm"' for poly(aniline-co-/7-
methoxyaniline) and poly(o-toluidine-co-/7-methoxyaniline) respectively.
This may be attributed to C-0 stretching of-OCH3 group [1]. The prominent
band at 1147.8 cm"' and 1167.1 cm"' in the FTIR spectra of poly(aniline-co-/?-
methoxyaniline) and poly(o-toluidine-co-p-methoxyaniline) respectively are
characteristic bands of the charged defects [18,19]. The bands appeared at
-825 cm'' and 1025-1113 cm'' may be attributed to 1,2,4-trisubstitution of the
benzene ring which can be ascribed to the C-H out-of-plane and in-plane-
bending modes respectively [20]. A comparatively weak band around 3300
cm"' may be attributed to N-H stretching vibrations whereas the weak band
around 2950 cm' may be attributed to alkyl C-H stretching vibrations.
67
TabIe-5.3. FTIR absorption peak positions of the copolymers.
Polymer
P(AcoPMA)
P(OTcoPMA)
FTIR absorption (cm')
3320,2950,1568.3,1500.8,1420,1306.0, 1246.2, 1147.8,
1026,-870,821,-730,613,509
3221.5,-2930,1570,1508.5,1425,1300,1250, 1113.1,
1028.2,945, 825.6, 730, 669, 617, 578, 517
TabIe-5.4. UV-VIS peak positions (nm) of copolymers in dimethylsulfoxide (DMSO) and dimethylformamide (DMF).
Polymer
PANI
P(AcoPMA)
P(OTcoPMA)
Base form
DMSO
325,626
317,597
311,587
DMF
326,620
316,590
308, 578
As-prepared protonated form
-
312,412,596
309, 408, 586
PANI P(AcoPMA) P(OTcoPMA)
Polyaniline Poly(aniline-co-/7-methoxyaniline) Poly(o-toIuiciine-co-/7-methoxyaniline)
168
5.2.3. Electronic Spectral Studies
The UV-VIS absorption spectra of the base form as well as of the
protonated form of the copolymers are shown in rigure-5.2 and figure-5.3
respectively. The corresponding absorption positions (in nm) are given in
table-5.4 along with absorption positions of the base form of the copolymers.
The electronic spectra of the base form of the polyaniline family of
polymers show generally two major absorptions around 330 nm and 620 nm.
These peaks are attributed to 7u->7r* transition and benzenoid to quinoid
exciton transition respectively [19-22]. The polyaniline sample prepared
shows the corresponding peaks at 326 nm and 624 nm respectively as
explained in the section-4.3 of chapter-4.
It may be seen from the figure-5.2 that the UV-VIS spectrum of
poly(aniline-co-p-methoxyaniline) in dimethylsulphoxide shows two major
absorptions at 317 nm and 597 nm. These bands may be attributed to n-^n*
transition and benzenoid to quinoid exciton transition respectively. The
corresponding bands in poly(o-toluidine-co-/?-methoxyaniline) are observed at
311 nm and 587 nm respectively.
It is clear from rigure-5.2 that there is a hypsochromic shift (blue shift)
both in the bands corresponding to K~>n* transition as well as to the exciton
transition as compared to the bands in polyaniline. This shift is more
prominent in poly(o-toluidine-co-/?-methoxyaniline) than in poly(aniline-co-
/7-methoxyaniline). The band corresponding to iz-^n* transition is a measure
of the extent of conjugation between adjacent phenyl rings of the polymer
[17-23]. When substituents like methoxy or methyl groups are present in the
phenyl rings, they significantly alter the planarity of the system and influence
the Ti-orbital overlap and thus a shift is observed in the n->n* transition band.
169
LU O z < CD o: o if) OQ <
360 520 680 WAVELENGTH (""^>
Figure-5.2 UV-VIS Spectra of base form of the polymers in DMSO (A) Polyaniline
(B) poly(aniline-co-/7-methoxylaniIine) (C) poly(o-toluidine-co-p-methoxylaniline)
170
c a u O M
<
520 Wavelength (nm)
Figure-5.3 UV-VIS Spectra of as-prepared protonated form of the polymers in DMSO (A) poly(aniline-co-/;-methoxylaniIine) (B) poIy(0-toluidine-co-p-methoxylaniline)
171
The blue shift observed in the UV-VIS spectra of poIy(aniline-co-/?-
methoxyaniline) is due to the presence of methoxy groups in the phenyl ring
resulting in an increase in the band gap [20]. Poly(o-toluidine-co-/7-
methoxyaniline) suffers a greater decrease in the extent of conjugation due to
the presence of both methyl and methoxy groups in the phenyl rings.
The exciton band is produced by the inter/intra-chain charge-transfer
[21,22]. This is attributed to the absorption from the highest occupied
molecular orbital (HOMO) band centered on the benzenoid units to the lowest
unoccupied molecular orbital (LUMO) band centered on the quinoid units.
This band is a measure of the extended conjugation. When the absorption is
inter-chain, the excitation leads to the formation of molecular excitons with
positive charge on the adjacent benzenoid units, while inter-chain charge-
transfer from HOMO to LUMO may lead to the formation of positive and
negative polarons [21-24]. The blue shift observed for this band in both the
copolymers is explained in terms of the steric effect and the increase in the
ring tortional angles between the adjacent phenyl rings caused by the
presence of methyl and methoxy groups [20,23,25]. The extent of blue shift
for this band is larger for poly(o-toluidine-co-/7-methoxyaniline) than
poly(aniline-co-/7-methoxyaniline).
The relative intensity of the exciton bands is a measure of the degree of
oxidation i.e. relative number of quinoid moieties present in the polymer [26].
It can be seen from the rigure-5.2 that there is no apparent difference in the
relative intensities of exciton transitions in poly(anilinc-co-/?-methoxyaniline)
and poly(o-toluidine-co-/?-methoxyaniline). This suggests that the oxidation
states of both the copolymers are almost the same supporting the similar
observations from FTIR studies.
172
Electronic spectra of the protonated form of these copolymers in
dimethylsulphoxide show an additional peak around 410 nm. This may be
attributed to the presence of polarons [20,21] whose energy level lies in the
energy gap. There are possibilities of electronic transitions between HOMO
to lower polaron bands or interpolaron bands or polaron to LUMO. The
polaron bands may not be symmetrical in the band gap [27]. The polaron
bands are observed at 412 nm in poly(aniline-co-/?-methoxyaniline) and at
408 nm in poly(o-toluidine-co-p-methoxyaniline). These bands are due to
polaron -> n* transitions [25,26]. However, the band due to TI ^ polaron
transitions is not seen in the UV-VIS spectra taken in dimethylsulphoxide.
The base form of these copolymers show a solvatochromic effect as can
be been from table-5.4. The bands corresponding to exciton transitions bands
71 -> 71* transition show a blue shift in dimethylformamide in comparison to
the spectra taken in dimethylsulphoxide. Attempts have been made to explain
these in terms of the dielectric constants of the solvents. A polymer, in a
solvent of high dielectric constant may exist in "coil-like" conformation,
resulting in the loss of planarity of the chain, thereby a decrease in
conjugation. Dielectric constant of dimethylformamide (36.6) is lower than
dimethylsulphoxide (41.2). In less polar solvents, thermodynamically more
stable, high planarity conformation is achieved [3,23]. However, for the
copolymers under study, we observed a hypsochromic shift instead of a
bathochromic shift. This suggests that macroscopic property like dielectric
constant can not always be used to explain the solvatochromism as it depends
on other factors such as geometry and electronic structure of both solute and
solvent [3].
173
5.2.4. ESR Spectral Studies
The ESR spectral parameters such as 'g' value, peak to peak line width
AHpp and line asymmetry parameter (a/b ratio) of the copolymers along with
their electrical conductivity are given in table-5.5. Several studies on ESR of
conducting polymers have been performed to understand the nature of
paramagnetic state and to suggest the conduction mechanism [28-30]. The
area of ESR signal is related to the number of spins present in the sample and
the width of the signal may be related to the extent of delocalization of
electrons and thus to the extent of conjugation. The position of ESR signal,
the 'g' factor may be related to the neighboring environment of the electronic
spin. The asymmetry of ESR signals and spin lattice relaxation time can be
interpreted in terms of the electrical conduction theories in conducting
polymers [31,32]. The ESR signal is related to spin concentration, which in
turn depends on many factors such as method of synthesis, intermolecular
interaction, doping level etc. and are highly sensitive to experimental
conditions.
It is found by many investigators that the 'g' factor takes smaller value
and AHpp becomes minimum and line asymmetry parameter (a/b ratio) takes a
higher value for conducting polyanilines [28,29]. Some others have reported
that the delocalization of electrons results in line broadening of ESR signals
and a/b ratio decreases with increase in electrical conductivity [33,34]. Based
on their studies on polyparaphenylene, Mohammad et al. [28] have suggested
that narrow line width is a suggestive of longer conjugation length because
mobile and greatly delocalized electronic spins give narrow ESR signal.
In the present study, the ESR spectral parameters as well as electrical
conductivity behavior for both the poly(aniline-co-/7-methoxyaniline) and
poly(o-toluidine-co-p-methoxyaniline) showed a different trend from those
174
observed for the copolymers of aniline with nitroanilines. We found that
electrical conductivity increases with increase in AHpp. The 'g' value and a/b
ratio take a lower value. It may be seen that poly(aniline-co-p-
methoxyaniline) whose electrical conductivity is higher than that of poly(o-
toluidine-co-p-methoxyaniline), has larger line broadening, lower 'g' value
and lower a/b ratio as evident from table-5.5.
In the case of polyaniline, we have noticed that the ESR signals show
variation from the first recorded values when the spectra is recorded after
keeping the polymer for long time. It is observed that the 'g' value and AHpp
increase and a/b ratio decreases when the spectra is recorded after one year of
synthesis.
5.2.5. Electrical conductivity and charge transport studies
5.2.5.7. Electrical conductivity
The two-probe room temperature electrical conductivity measured on
the pressed pellets of the polymers at frequencies of 100 Hz and 10 KHz are
presented in table-5.5. The results of temperature dependence of electrical
conductivity studied between the temperature range from 25°C to 180°C are
shown in figure-5.4. In order to understand the basic nature of charge
transport, the temperature dependent electrical conductivity data were fitted
in Arrhenius equation as well as to variable range hopping (VRH) model as
shown in figure-5.5 and figure-5.6.
From table-5.6, it may be observed that the copolymers show lower
electrical conductivity than that of polyaniline. The decreasing order of
electrical conductivity follows the trend of- polyaniline » poly(aniline-co-p-
methoxyaniline) > poly(o-toluidine-co-p-methoxyaniline). The electrical
!75
TabIe-5.5. Electrical conductivity and ESR data of the polymers.
Polymer
PANI
P(AcoPMA)
P(OTcoPMA)
Electrical conductivity (Scm"')
100 Hz
7.167x10-'
3.018x10-"
3.717X10-'
10 KHz
7.168x10''
3.02x10"*
3.91x10-'
ESR Data
'g' value
2.013
2.0214
2.0235
AHpp (Gauss)
13.04
20.00
17.92
(a/b) ratio
1.60
1.45
1.49
PANI P(AcoPMA) P(OTcoPMA)
Polyaniline Poly(aniline-co-p-methoxyaniUne) Poly(o-toluidine-co-/>:methoxyaniline)
176
conductivity measured at 10 KHz is found to be greater than that measured at
100 Hz. The frequency dependence is more prominent in poly(o-toluidine-co-
p-methoxyaniline) than in poly(aniline-co-/)-methoxyaniline) and polyaniline.
Alkoxy and alkyl groups are electron donating groups and make the
electronic flow along a single polymer chain little better. Therefore, it seems
that the steric factor plays the dominant role in determining the value of
electrical conductivity of copolymers in comparison to polyaniline. The
decrease of electrical conductivity of poly(aniline-co-/?-ethoxyaniline) is
explained in terms of the decreased extent of conjugation and an increase in
the band gap caused by increased phenyl ring tortional angles which results
from the steric repulsion between adjacent phenyl rings due to the presence of
substituent -OCH3 groups on the phenyl rings [13,23,35]. This may cause
greater electron localization [3]. In poly(o-toluidine-co-/7-methoxyaniline),
since two substituents, -CH3 and -OCH3 groups are present, it experiences
maximum steric repulsion. Therefore, it suffers even greater decrease in the
extent of conjugation thereby a greater increase in the band gap causing a
greater reduction in electrical conductivity of the order of 10 as compared to
that of poly(aniline-co-/7-methoxyaniline). These observations are consistent
with those from the electronic spectra of the polymers. The blue shift
observed in the 7t -> 71* transition and in the exciton bands in the UV-VIS
spectra of the copolymers are explained on the basis of the decrease in the
extent of conjugation as discussed in section-5.4.
Another possibility of lower electrical conductivity is the decreased
inter-chain diffusion of charge carriers in the copolymers [36]. This is
induced by the increased separation of the polymer chains due to the presence
of-OCH3 and -CH3 groups and a lower crystallographic order and, hence, a
reduced coherence between the polymer chains. The -OCH3 groups are likely
177
to force the chain out of planarity by twisting the phenyl rings relative to one
another to lower the overlap of orbitals along the conjugated system [37]. As
a result, the conduction electron wave functions are expected to be
substantially localized in the copolymers than those in polyaniline. This may
lead to lower mobility of the charge-carriers both along the polymer chains
and between the polymer chains. The electrical conductivity strongly
depends on the structural factors of the polymer backbone. Polyaniline
exhibits large phenylene ring tortional displacements out of the plane defined
by the ring bridging amine/imine nitrogen atoms. It is reported that the
deviation from planarity by phenylene ring is altered between +30° and -30°
as one moves along the polymer backbone [38]. In polyaniline, quinoid units
are found to be more planar than benzenoid units and are associated with a
more complex inter-chain ring structure [39,40]. The electronic transport
properties that are dependent on the inter-chain and intra-chain wave function
overlap, are strongly affected by the presence of methoxy and methyl groups
in the phenylene rings. Then the pronounced local tortions resulting from
phenylene ring vibrations and ring flip about the axis which are different for
poly(aniline-co-;7-methoxyaniline) and poly(o-toluidine-co-/7-methoxyaniline)
would be expected to have more significant impact on electrical conductivity.
In these copolymers, we also observed the frequency dependence of
electrical conductivity. It is less pronounced in polyaniline and poly (aniline-
co-/7-methoxyaniline). Poly(o-toluidine-co-/?-methoxyaniline) shows a strong
frequency dependence. Epstein and coworkers [41] have performed the
microwave conductivity of poly(o-toluidine) in order to characterize the
effect of electron localization on the transport properties. In their work on
dielectric constant and a.c. conductivity measurement at 10 KHz on
polyaniline and their derivatives, Pinto and coworkers [42] have pointed out
78
that greater localization of charges leads to reduced conductivity and that the
substituents cause an increase in the disorder.
Based on our studies on copolymers of aniline with nitroanilines as
described in chapter-4, we observed that greater is the electron localization,
greater is the frequency dependence of electrical conductivity. Our
observation on frequency dependence of electrical conductivity suggests a
greater electron localization in poly(o-toluidine-co-/>-methoxyaniline) than in
poly(aniline-co-/?-methoxyaniline) and in polyaniline. This is in agreement
with the observed variation in electrical conductivities of the polymers
studied.
5.2.5.2. Charge transport
From figure-5.4, it may be observed that the electrical conductivity
increases linearly with increase in temperature upto a particular temperature
at which 5a/5T ~ 0 after this temperature, the electrical conductivity
decreases with increase in temperature. The electrical conductivity of highly
conducting polyaniline is affected by environmental humidity and moisture
[43-46]. The transport studies to understand the electrical conduction
mechanism in polyanilines are generally carried out from low temperatures
such as 100 K to room temperature [47-50]. There are fewer studies on
temperature dependence of electrical conductivity above room temperature
[43].
The localization of electronic state arises due to disorder and presence
of defects. The presence of moisture creates a dynamic disorder in the
polymeric systems [51]. At higher temperatures, when moisture is removed, it
is expected that static defects, such as chain ends and chemical defects.
179
1X10"3 -
8X10" -
GXIO'*"-
E u i[l 4X10
>
u 3 •o c o u o u
uj
1X10— -
6X10"' -
6X10"^-
4X10
2X10~^|-
303 323 343 363 383 403 423 443 463 Temperature( K)
Figure-5.4 Variation of electrical conductivity with temperature for (A) poly(aniIine-co-/;-methoxylaniline) (B)poly(0-toIuidine-co-/>-niethoxyIaniline)
180
C71 O
2.0 2.2 2.L 2.6 2.8 3.0 3.2 3.4
lOOO/TlK"'')
Figure-5.5 Arrhenius plot of log(a) versus 1000/T(K"') (A)poly(aniline-co-p-inethoxylaniline) (B) poly(0-toluidine-co-/;-niethoxyianiline)
181
-3.1
-3.2
-3.3
-3.4
-3.8'
-3.9
b o> -A,0 o
-«.1
-A.2
-A.3
-4.4
-4.5
-
-
-
•
•
1
' •
•
•
•
1 1
• •
•
1
• •
•
•
1
•
1
•
1
0.214 0.218 0.222 0.226 0.230 0.234 0.238 0242
T-l'4(^-1/4,
Figure-5.6 Application of variable range hopping model to electrical conductivity - temperature behaviour of (A)poly(aniline-co-p-methoxylaniline) (B)poly(0-toluidine-co-/7-methoxylaniline)
182
control the charge transport. A detailed discussion on the electrical
conductivity and charge transport is given in the section 4.5.2 of chapter-4.
In the present study, the effect of moisture on electrical conductivity is
found to have little effect in both polyaniline as well as in the copolymers. A
previous study of temperature dependence of electrical conductivity of
polyaniline [43] showed that the electrical conductivity increases only upto
50°C after which the electrical conductivity decreases with increase in
temperature due to the loss of moisture. But in all the polymers studied
whose electrical conductivity falls in semi-conducting regime increases above
373 K with increase in temperature. In poly(aniline-co-/7-methoxyaniline), the
electrical conductivity increases up to 390 K whereas 8o/5T becomes zero at
408 K for poly(o-toluidine-co-/?-methoxyaniline). These results show that the
temperature dependent electrical conductivity is not affected by moisture in
aniline based conducting polymers whose electrical conductivity falls in
semi-conducting regime.
To understand the nature of charge transport in poly(aniline-co-/7-
methoxyaniline) and poly(o-toluidine-co-/?-methoxyaniline), the electrical
conductivity-temperature data were fitted in Arrhenius equation as well as in
variable range hopping (VRH) model.
A careful examination of the results presented in flgure-5.5 and figure-
5.6 shows that the charge transport in poly(aniline-co-/7-methoxyaniline) can
be explained in terms of variable range hopping model [43. 52] as the
observed electrical conductivity-temperature data can best fit in the equation-
a(T) = CQ exp (5.1)
183
where To is the Mott's characteristic temperature and Oo is the electrical conductivity
at room temperature. After 385 K, the charge transport follows the same equation
but with a negative slope. At this temperature range conductivity decreases probably
because of the loss of the dopant. Between 333 K and 385 K, Arrhenius plot (band
conduction) is also found to be a good fit to the data following the equation [43] -
a(T) = aoexp[^- |^ j (5.2)
In the case of poly(o-toluidine-co-/7-methoxyaniline), the temperature-
electrical conductivity data is not found to follow either of the equations over
the full temperature range studied although it seems that the charge transport
follows Arrhenius model between the temperature range of 323 K - 393 K.
Mechanism of charge transport in polyaniline (homopolymer) is highly
complex with a variety of phenomena contributing to electrical conductivity.
In the case of copolymers even more complex situation is anticipated.
5.2.6. Thermogravimatric Analysis
Thermogravimetric data of the polymers under study are presented in
table-5.6 and table-5.7 while the thermograms are presented in figure-5.7. The
plot of ln(ln 1/y) verses 1000/T(K) for the evaluation of activation energies of
thermal degradation using Broido equation [53] is shown in figure-5.8 and the
corresponding activation energies are tabulated in table-5.8. From figure-5.7,
it is clear that the oxidative degradation of polymer backbone takes place
after the removal of water, dopant and low molecular weight oligomers,
leaving behind a little amount of residue.
Thermogram of polyaniline shows three distinct regions of weight loss.
A detailed discussion on the thermogravimetric analysis of polyaniline is
presented in section 4.2.6 of chapter-4.
184
(00 200 300 400 500
Temperature C 'C)
600
Figure-5.7 Thermogram of (A) Polyaniiine (B) poly(anilinc-co-/7-methoxylanilinc) (C) poly(o-toluidine-co-p-methoxylaniline)
185
TabIe-5.6. Thermogravimetric analytical data of copolymers.
Polymer
PANI
P(AcoPMA)
P(OTcoPMA)
Weight loss due
to removal ofHzO
(%)
0.8
1.52
1.54
Weight loss due
to removal ofHCl
and oligomers
(%)
7.3
7.880
7.157
Thermo-oxidative degradation
Onset Temperature
260
290
260
Weight loss (%)
89.37
87.6
89.3
Residue
Final Temperature
(°C)
610
580
530
Weight (%)
2.44
3% 2%
PANI P(AcoPMA) -P(OTcoPMA) -
Polyaniline Poly(aniline-co-p-methoxyaniline) Poly(o-toluidine-co-/7-methoxyaniline)
186
Table-5.7. Thermogravimetric analytical data showing rate of decomposition in various temperature ranges.
Temperature range (°C)
090-110
110-130
130-150
150-170
170-190
190-210
210-230
230-250
250-270
270-290
290-310
310-330
330-350
350-370
370-390
390-410
410-430
430-450
450-470
470-490
490-510
510-530
Weig P(AcoPMA)
Weight loss (%)
0.4600
0.5600
0.4360
0.4340
0.4360
0.9130
1.2610
1.6086
1.6524
1.0870
1.0870
1.7390
2.1740
3.6950
4.2170
6.4300
8.2660
10.724
10.352
9.4300
8.6970
7.3910
Weight loss per °C (%) 0.0230
0.0295
0.0218
0.0217
0.0218
0.0460
0.0630
0.0810
0.0830
0.0540
0.0540
0.0870
0.1087
0.1850
0.2110
0.3220
0.4130
0.5360
0.5180
0.4720
0.4350
0.3700
it loss P(OTcoPMA)
Weight loss (%)
0.4560
0.3483
0.4783
1.2610
1.5220
1.0600
1.0750
0.9650
1.0340
1.3040
2.6080
3.4790
5.0000
6.3530
7.3480
8.9070
9.6600
8.8300
13.263
13.041
5.9000
3.6300
Weight loss per °C (%) 0.0228
0.0174
0.0240
0.0630
0.0761
0.0530
0.0540
0.0482
0.0517
0.0652
0.1304
0.1740
0.2500
0.3180
0.3670
0.4450
0.4830
0.4420
0.6632
0.6521
0.2950
0.1820
187
['X i-«v (-6 1-8
lOOO/TCK)
Figure-5.8 Plot of ln(ln I/y) versus 1000/T(K'') for the estimation of activation energies of thermal degradation. (A) Polyaniline (B) poly(aniline-co-/;-methoxylaniIine) (C) poly(o-toluidine-co-p-methoxylaniline)
188
Thermogram of poly(aniline-co-/?-methoxyaniline) also shows the three
distinct stages of weight loss. The initial weight loss of about 1.52% by
130°C attributed to the loss of water. It is followed by a weight loss of about
7.83% by 290°C corresponding to the loss of HCl and low molecular
oligomers if present [54-56]. Thermo-oxidative degradation of polymer
backbone starts at 290°C as clear from table-5.6. From table-5.7, it can be
seen that the rate of decomposition as evident from weight loss per degree
centigrade is maximum at ~440°C. The degradation is completed at ~580°C at
which a residual weight of 3% is left which may be attributed to some
thermally stable polynuclear organics [57].
Thermogram of poly(o-toluidine-co-p-methoxyaniline) shows four
distinct stages of weight loss. An initial weight loss of 1.54%) by 140°C is
attributed to the loss of water. It is followed by a further weight loss of about
7.157% up to 270°C due to the removal of HCl and low molecular weight
oligomers. Thermo-oxidative degradation of the polymer backbone starts at
270°C. Rate of weight loss suddenly increases from 290°C. In the third stage
a massive weight loss of about 81.3% occurs due to the oxidative degradation
of polymer backbone. In the fourth stage, the degradation of polymer
backbone takes place at a slower rate upto 530°C at which the thermo-
oxidative degradation is completed, leaving behind 2% residue. It can be seen
from table-5.7 that the rate of decomposition is maximum at ~ 460°C.
The rate of degradation is much faster in poIy(o-toluidine-co-/7-
methoxyaniline) than in poly(aniline-co-/?-methoxyaniline). This indicates
that poIy(o-toluidine-co-/7-methoxyaniline) is more susceptible to oxidative
degradation. A larger weight loss of 34.78% for poly(o-toluidine-co-/7-
methoxyaniline) compared with 22.26% weight loss for poly(aniline-co-/?-
methoxyaniline) observed at 390°C also suggests an earlier onset of oxidative
189
Table-5.8. Activation energies of thermal degradation of the copolymers
Polymer
PANI
P(AcoPMA)
P(OTcoPMA)
Activation Energy (kJ/mol)
EA-1
9.67
10.39
07.65
EA-2
22.94
24.63
20.78
EA-3
-
-
39.20
PANI P(AcoPMA) -P(OTcoPMA) -
Polyaniline Poly(aniline-co-p-methoxyaniline) Poly(o-toluidine-co-/7-methoxyaniline)
190
degradation as well as a lower thermostability of poly(o-toluidine-co-p-
methoxyaniline). This may be attributed to the lower conjugation length and
due to the ease in the loss of -CH3 groups from the polymer backbone. In the
final stage of decomposition the rate of degradation is slightly lowered for
poly(£>-toluidine-co-/?-methoxyaniline) probably due to the formation of some
cross linked products whose degradation may take place with higher
activation energy.
One of the simplest methods to evaluate the activation energies of
thermal degradations of polymers is the use of Broido equation [53]. From
table-5.8, it can be seen that activation energies corresponding to the initial
stage of degradation as well as for the thermo-oxidative degradation of
polymer backbone for poly(o-toluidine-co-/?-methoxyaniline) are lower than
that for poly(aniline-co-/j-methoxyaniline) and polyaniline. The final stage of
degradation in poly(o-toluidine-co-/?-methoxyaniline) takes place with higher
activation energy.
From thermogravimetric analysis we conclude that poly(o-toluidine-co-
/7-methoxyaniline) is thermally less stable than polyaniline whereas
poly(aniline-co-/?-methoxyaniline) shows comparable thermal stability with
polyaniline.
191
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37. Y. Wei, W.W. Focke, G.E. Wnek, A. Ray and A.G. MacDiarmid, J. Phys.
Chem.,93,495[1989].
38. M.J. Winokur in "Handbook of Conducting Polymers", Ed. T.A. Skotheim,
Elsembaumer and Reynold, Marcel Deckker [1988], p. 707.
39. J.P. Pouget, M.E. Jozefowicz, A.J. Epstein, X. Tang and A.G. MacDiarmid,
Macromolecules, 24, 779 [1991].
40. J. Maron, M.J. Winokur and B.R. Mattes, Macromolecules, 28,4475 [1995].
41. Z.H. Wang, A. Ray, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B, 43,
4373 [1991].
42. N.J. Pinto, P.D. Shah, P.K. Kahol and B.K. McCormick, Solid State
Commun., 97(12), 1029 [1996].
43. V.M. Mzenda, S.A. Goodman, F.D. Auret and L.C. Prinsloo, Synth. Met.,
127, 279 [2002].
44. M. Angelopoulos, A. Ray, A.G. MacDiarmid and A.J. Epstein, Synth. Met.,
21,21 [1987].
45. H.S.S. Jawadi, F. Zuo, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein,
Mol. Cryst. Liq. Cryst., 160, 225 [1988].
46. H.S.S. Jawadi, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein, Synth.
Met., 26, 1 [1988].
47. V.M. Mzenda, S.A. Goodman and F.D. Auret, Synth. Met.. 127, 285 [2002].
48. F. Zuo, M. Angelopoulos, A.G. MacDiarmid and A.J. Epstein, Phys. Rev. B,
39(6), 3570 [1989].
49. A. Raghunathan, T.S. Natarajan, G. Rangarajan, S.K. Dhawan and D.C.
Trivedi, Phys. Rev. B, 47(20), 13189 [1993].
194
50. M. Reghu, CO. Yoon, D. Moses and A.J. Heeger in "Handbook of
Conducting Polymers", Ed. T.A. Skotheim, R.L. Elsembaumer and J.R.
Reynolds, Marcel Decker [1998], p. 37, 51.
51. W.W. Focke and G.E. Wnek, J. Electroanal. Chem., 256, 343 [1988].
52. J.P. Travers. J. Chroboczek, F. Devreux, F. Genoud, M. Nechtschein, A. Syed,
E. Genies and C. Tsintavis, Mol. Cryst. Liq. Cryst., 121, 195 [1985].
53. A.J. Broido, Polym. Sci., A-2, 7, 1761 [1969].
54. R.A. Misra, S. Dubey, B.M. Prasad and D. Singh, Ind. J. Chem., 38A, 141
[1999].
55. Y. Yue, A.J. Epstein, Z. Zhong, P.K. Gallaghar and A.G. MacDiarmid, Synth.
Met., 41, 765 [1991].
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Meyers, Synth. Met., 40, 137 [1991].
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195
(2o^p.^lumati ^^ aniline, witk
o^matkulaniline, (art^laidine)
and ^'matkulaniline,
(ya'tO'luidina)
CHAPTER-6
COPOLYMERS OF ANILINE WITH o-METHYLANILINE (o-TOLUIDINE) ANDp-METHYLANILINE (p-TOLUIDINE)
6.1. Introduction
Copolymers of aniline with o-methylaniline and m-methylaniline,
o-propylaniline, 2,5-dimethylaniline, o-ethylaniline etc. have been studied
extensively [1-6]. As an attempt to get more soluble polyanilines, we have
synthesized copolymers of aniline with o-toluidine andp-toluidine. This
chapter deals with the comparative studies on the effect of incorporation of o-
toluidine and p-toluidine units in the polyaniline backbone on the electrical
conductivity, spectral and thermal properties of the resulting copolymers.
Copolymers of aniline with ;7-toluidine were synthesized for 1:1, 1:2 and 1:3
molar ratios of the respective monomers using K2S2O8 as oxidant as explained
in chapter 3. Copolymer of aniline with o-toluidine in 1:1 molar ratio was
also synthesized.
6.2. Results and Discussion
6.2.1. Polymer Synthesis
Polymerization of aniline into polyaniline was found to take place very
quickly as observed by the color change of the reaction mixture from pale ->
bluish green -> green within two minutes of the addition of the oxidant. The
appearance of greenish color in the reaction mixture is taken as the indication
of copolymerization. The copolymerization of aniline with/7-toluidine is
observed to be a very slow process. The time taken for the initial color
change of the reaction mixtures upon addition of the oxidant along with the %
196
yield of the polymers are given in table-6.1. It can be seen that the rate of
reaction decreases as the ratio of ;?-toluidine increases in the reaction mixture.
The copolymerization of aniline with o-toluidine is also found to be a slow
process compared with the homopolymerization of aniline into polyaniline. It
is a qualitative indication that in presence of toluidine units, the aniline
monomeric units alone can not polymerize, rather the j3-toluidine and o-
toluidine units are getting incorporated in the polymer backbone.
Protonation is an important step in the chemical polymerization of
aniline [7-8]. The presence of methyl groups on o-toluidine and/7-toluidine
result in an increase in the electron density in the phenyl rings and amino
atoms which may facilitate the polymerization, because then the affinity for
proton for nitrogen is increased. The observed decrease in the rate of
polymerization can then be explained on the basis of steric effect of methyl
groups. The steric effect is more pronounced during the polymerization of
aniline with p-toluidine. The anilinium cation radical produced first during
the polymerization can exist in three canonical structures as shown in figure-
6.1 [9J.
^ ^ S H ;
H + NH2
H
^ _ \ r N H 2
Figure-6.1: Canonical structures of anilinium cation radical of aniline
The corresponding cation radicals produced in o-toluidine andp-
toluidine may be shown as in figure-6.2 and tlgure-6.3 respectively.
197
.« •
Figure-6.2: Cation radical produced in o-toluidine
•+ A——\ CH3-U^ A-'NH2 -* > CH3-Y V N H 2
Figure-6.3: Cation radical produced in p-toluidine
In the copolymerization of aniline with o-toluidine, we expect a
normal linear polymer by the coupling of N-radical cation, Structure-1
(Figure-6.1) with the para-radical cation, Structure-5(Figure-6.2) which
suffers a comparatively lesser steric effect than the steric effect suffered
during the polymerization of aniline with p-toluidine. Here, since the para-
positions are already blocked, coupling should take place at the ortho-position
to -NH2 group. It is clear from figure-6.3 that, for the formation of copolymer
of aniline with p-toluidine the N-radical cation of aniline, 1 should couple
with the radical cation of p-toluidine, 7 which experiences a much greater
steric effect. Also, the steric effect increases as the ratio of p-toluidine units
increases in the reaction mixture. This explains the difference in the rate of
copolymerization reaction of aniline with o-toluidine as observed from
table-6.1.
198
Table-6.1. % yield and time taken for the initial color change of the reaction mixture
Polymer
PANI
P(A0T)1:1
P(APT)1:1
P( APT) 1:2
P( APT) 1:3
% yield
80.03
70.1
49.84
41.73
25.4
Time taken for the initial color change of the
reaction mixture (min.) 1-2
3-4
10-12
14-16
17-20
Table-6.2. Solubility characteristics of the polymers in various solvents at room temperature («25±2)°C
Polymer
PANI
P(A0T)1:1
P(APT)1:1
P(APT)1:2
P( APT) 1:3
As-prepared protonated form
H2O
IS
IS
IS
IS
IS
H2S04
MS
S
S
S
S
Acetone
IS
IS
ss MS
S
DMF
IS
ss s s s
DMSO
IS
MS
MS
S
S
Base form
H2O
IS
IS
IS
IS
IS
H2S04
ss s s s s
Acetone
-
SS
MS
MS
S
DMF
-
s s s s
DMSO
-
s S
S
S
IS SS -
PANI P(AOT)I:I P(APT)1:1 P(APT)1:2 P( APT) 1:3
Insoluble MS - Moderately Soluble Slightly Soluble S - Soluble
- Polyaniline - Poly(aniline-co-o-toluidine) 1:1 - Poly(aniline-co-/7-toluidine) 1:1 - Poly(aniline-co-/7-toluidine) 1:2 - Poly(aniline-co-/7-toluidine) 1:3
199
A comparatively poor yield of the copolymers of aniline with
/7-toluidine may be a qualitative indication that the copolymers are of lower
chain length and the copolymerization process may get easily terminated. We
observed that, the washings (filtrate) of the product with acetone was of
intense brown color and it needed large quantity of acetone to wash away the
low molecular weight oligomers.
6.2.2. FTIR Spectral Studies
The FTIR spectra of polyaniline and the copolymers are shown in
figure-6.4 and figure-6.5. The FTIR spectrum of polyaniline shows two
major absorption at 1589.6 cm'' and 1492.4 cm'' which are ascribed to the
C=C vibrations of benzenoid and quinoid units respectively [10-13]. The
relative intensity of quinoid to benzenoid band is a measure of the degree of
the oxidation of the polymer chain [12,13]. It can be seen from the spectra of
polyaniline that the number of quinoid units are almost equal to the number of
benzenoid units. The band appeared at 1299 c m ' is due to C-N stretching
vibrations of benzenoid - quinoid - benzenoid sequence [13,14]. There is a
strong band observed at 1125 cm"' which may be the characteristic bands of
the charged defects [15,16]. The weak band around 810 nm is ascribed to the
C-H out of plane bending vibrations. A comparatively weaker band ~3300 is
due to N-H stretching vibrations and the band ~3140 is due to ring -C-H
stretching vibrations.
FTIR spectrum of poly(aniline-co-<?-toluidine)l:l show two major
absorption at 1583.4 cm'' and 1515.3 cm'' which are attributed to C=C
vibrations of benzenoid and quinoid units respectively [10-13]. The relative
intensity corresponding to quinoid band is slightly lesser than the intensity of
benzenoid band suggesting that the oxidized quinoid units are slightly lesser
than the benzenoid units. The band at 1124.5 cm'' corresponds to charged
200
E u l_ (U
JO
6 o c > o
2 'S "o
I
o ^ V o « « « £ £
^ 1 K Cu Oi
3
( Vo)33UOU!^SUDJi
201
3000 2000 Wave n u m b e r l c m " )
10 0 0
Figure-6.5 FTIR Spectra of (A) poly(aniIine-co-/7-toiuidine)l:l (B) poly(aniline-co-/;-toIuidine)l:2 (C) poly(aniline-co-p-toluidine) 1:1
202
Table-6.3. FTIR peak positions of polymers (cm")
P(AOT)l:l
-3166.4
2956.0
1583.4
1515.4
1385.5
1293.0
1124.5
1126.0
1007.0
890.0
821.4
610.0
P(APT)1:1
-3300.0
3228.0
2973.0
1582.47
1504.85
-1405.0
1310.8
1246.13
1142.65
1014.31
852.1
819.25
-515.0
P(APT)1:2
3248.0
2931.37
1579.56
1501.95
1443.0
1406.0
1301.44
1236.76
1139.74
1038.1
809.87
505.88
-
P(APT)1:3
3239.0
-3100.0
2923.65
1576.7
1505.57
1403.0
1287.0
1240.38
1130.42
1046.34
865.24
813.49
619.45
PANI
-3154
1589.6
1492
1299.0
1125.0
890.0
623.0
505.0
-
-
-
-
-
PANI P(AOT)l.i -P(APT)1:1 -P( APT) 1:2 -P( APT) 1:3 -
Polyaniline Poly(aniline-co-o-toluidine)l: 1 Poly(aniline-co-p-toluidine) 1:1 Poly(aniIine-co-;?-toluidine) 1:2 Poly(aniline-co-/7-toluidine)l :3
203
defects [15,16]. The band at 1293 cm"' is to the C-N stretching vibrations.
Unlike the spectra of polyaniline, the band observed at 1385.5 cm'' is
attributed to C-H bending vibrations of-CH3 groups. Similarly, the bands
between 890-1112.6 cm"' are attributed to 1,2,4-tri-substitution of the benzene
ring and ascribed to the C-H out of plane and in plane bending modes [17,18].
The weak band around 3160 cm"' is attributed to C-N stretching vibrations
and the band at -2950 cm"' is attributed to -C-H stretching vibrations of
methyl group.
As seen from figure-6.5, the FTIR spectrum of all the copolymers of
aniline with p-toluidine show the characteristic bands of C=C vibrations of
benzenoid and quinoid units. The benzenoid C=C vibrations occur at 1582.47
cm"', 1579.56 cm"' and at 1567.7 cm"' and the and the quinoid C=C vibrations
occur at 1504 cm"', 1501.9 cm"' and 1505.5 cm"' respectively for poly(aniline-
co-p-toluidine)l:l, poly(aniline-co-/>-toluidine)l:2 and (aniline-co-p-
toluidine)l:3 comparatively weaker bands observed around ~1405 in
poly(aniline-co-/?-toIuidine)l:l, -1406 cm"' in poly(aniline-co-/7-toluidine)l:2
and at 1403 cm"' in poly(aniline-co-p-toluidine)l:3 may be attributed to -C-H
bending vibrations of-CH3 groups. The band observed at 1310.8 cm"' in
poly(aniline-co-/7-toluidine)l:l and at 1301.44 cm"' in poly(aniline-co-p-
toluidine)l:2 and at 1287 cm"' in poly(aniline-co-;?-toluidine)l:3 are
attributed to C-N stretching vibrations. It can be seen that, this absorptions
shift to lower frequencies as the ratio of p-toluidine increases in the
copolymer composition. The bands characteristic of charged defects are
observed at 1246.13 cm"', 1236.76 cm"' and 1240.3cm"' respectively for
(aniline-co-p-toluidine)l:l, (aniIine-co-/?-toluidine)l:2 and (aniline-co-p-
toluidine)l:3. The bands at-819.25 cm"' for (aniline-co-/7-toluidine)l:l,
-810 cm' for (aniline-co-/7-toluidine)l:2 and -813 cm"' for (aniline-co-/?-
toluidine)l:3 and the multiple bands between -1014 cm"' -1046 cm"' in all the
204
copolymers are attributed to 1,2,4 tri-substitution of the benzene ring, which
can be ascribed to C-H out of plane and in-plane bending modes respectively.
The comparatively weak bands -3300 cm'' are attributed to N-H stretching
vibrations.
6.2.3. Electronic Spectral Studies
UV-VIS absorption spectra of the base form of poly(aniline-co-o-
toluidine)l:l and poly(aniline-co-p-toluidine)l:l in DMSO are shown in
figure-6.6. and that of poly(aniline-co-/7 -toluidine)l:2 and poly(aniline-co-/7
-toluidine)l:3 are shown in figure-6.7. The UV-VIS spectra of the as-
prepared protonated form of the copolymers of aniline with p-toluidine taken
in DMSO and H2SO4 are presented in figure-6.8 and figure-6.9 respectively.
The corresponding absorption peak positions are presented in table-6.4. For
comparison, the UV-VIS absorption peak positions of polyaniline are also
given in the table.
It can be seen from figure-6.6that, both the copolymers, poly(aniline-1:1 I'-l
co-o-toluidine) and poly(aniline-co-/7-toluidine) show two major absorptions
corresponding to 71—>• n* transition and benzenoid to quinoid exciton
transitions[ 19-21].
The benzenoid to quinoid exciton bands are found to occur at 598 nm
and 590 nm respectively for poly(aniline-co-o-toluidine)l:l and poly(aniline-
co-o-toluidine)l:l. A hypsochromic shift is found both in the bands
corresponding to 7t-> 71* transition and in the exciton transition for the
copolymers as compared to that in polyaniline. The shift is more prominent
in poly(aniline-co-/?-/o/w/£/me)l:l. As can be seen from figure-6.7,
poly(aniline-co-/?-toluidine)l:2 shows the n-^ n* transition at 291 nm and the
benzenoid to quinoid transition band at 570 nm. The corresponding bands in
205
V u c a i_ o
<
520 680 Wavelength (nm)
Figure-6.6 U V-VIS Spectra of base form of the copolymers in DMSO (A) poly(aniline-co-0-toluidine)l:l (B) poly(aniline-co77-toluidine)l: 1
206
u c o
J3 C -
O v\
Xi
<
^\ /
/ 1 1 rs 1 J
-> 1
\ \ \
. \ \ \ \ \ \ \ \ \ \ \ \ \
V ^ \ " - « ^ ^ ^ ^ ^ ^ ^ v "" \
\ ^ _ B " . w ^ ^ ^ ^
^ * * * * ^ ' ^ N . ^ \ ^*'*'*"'^***«*^ V ^
^ * * * * ^ 1 . . _ ^ V,*,
1 _ 1 1 -^ ^ - ~ - J _ 360 520 680
Wavelength ( n m ) 760
Flgure-6.7 UV-VIS Spectra of base form of the copolymers in DMSO (A) poly(aniline-co-/7-toluidine)l:2 (B) poIy(aniline-co-p-toluidine)l :3
207
Table-6.4. UV-VIS peak positions (nm) of base forms and the protonated forms of the polymers in various solvents
Polymer
PANI
P(A0T)1:1
P(APT)1:1
P( APT) 1:2
P(APT)1:3
Base form
DMSO
325,626
320, 598
300, 590
291,581
280, 570
DMF
326,620
321,596
290, 579
287, 572
278, 567
Protonated form
DMSO
-
-
300,406,605
299,409,571
290,411,535
H2SO4
-
-
320,430,571
319,428,572
320,425,571
Table-6.5. Electrical conductivity and ESR data of the copolymers
Polymer
PANI
P(A0T)1:1
P(APT)1:1
P(APT)1:3
Electrical conductivity (Scm')
lOOHz
7.167x10"'
5.3x10"^
5.71x10-^
6.46x10"^
lOKHz
7.168x10"'
5.342x10"^
5.76x10"^
1.05x10"
'g ' value
2.013
2.02
2.022
2.032
ESR Data
AHpp (G)
13.04
14.47
15.61
17.93
a/b ratio
1.60
1.47
1.41
1.30
PANI P(A0T)1:1 -P(APT)I.i -P( APT) 1:2 -P(APT)I:3 -
Polyaniline Poly(aniline-co-o-toluidine) 1:1 Poly(aniIine-co-/7-toIuidine)l: 1 Poly(aniline-co-/?-toIuidine) 1:2 Poly(aniline-co-/?-toluidine) 1:3
208
<v a c o u O
<
Figure-6.8
300 500 700 Wavelength ( n m )
900
UV-VIS spectra of as-prepared protonated form of the copolymers in DMSO (A) poly(aniline-co-p-toluidme)l:l (B) poiy(aniline-co-p-toluidine)l:2 (C) poly(aniline-co-p-toluidine)l:3
209
u c o i 3 (-o in <
300 900
Figure-6.9
500 700 Woveiength (nm )
U V-VIS Spectra of as-prepared protonated form of the copolymers in H2SO4 (A) poIy(aniline-co-/7-toIuidine)l:l (B) poIy(aniline-co-/;-toluidine)l :2 (C) poiy(aniIine-co-/;-toluidine)l :3
210
poly(aniline-co-/>-toluidine)l:3 occur respectively at 280 nm and at 566 nm.
It is clear from table-6.4 that, the blue shift increases as the ratio of the/?-
toluidine units increases in the copolymers derived from aniline and
p-toluidine.
The band corresponding to n^ n* transition is a measure of the extent
of conjugation between the adjacent phenyl rings of the polymer. When
methyl groups are present on the phenyl rings, they substantially alters the
planarity of the system and influence the 7i-orbital overlap resulting in a shift
in the n^> n* transition band. Thus the blue shift observed in the copolymers
is due to the presence of methyl group present in the phenyl ring which
ultimately results in the increase of band gap of the polymers [12,22]. The
planarity of the phenyl rings in the polymer chain seen to be affected to a
greater extent in the copolymers of aniline with p-toluidine.
The exciton band produced by the inter/intra- chain charge transfer and
is attributed to an absorption from the highest occupied molecular orbital
(HOMO) band centered on the benzenoid units to the lowest unoccupied
molecular orbital (LUMO) band centered on the quinoid units. This band is a
measure of the extended conjugation [22,23]. The blue shift observed for this
band is the copolymers are explained in terms of an increase in the ring
tortional angle between the adjacent phenyl ring caused by the steric effect
due to the presence of methyl groups on the ring greater the steric effect,
greater will be the extent of blue shift.
It is clear from the above discussion that, both the copolymers of
aniline with o-toluidine and/7-toluidine suffers greater steric effect as
compared to polyaniline. Copolymers of aniline with /?-toluidine suffers a
larger increase in the tortional angle between adjacent phenyl rings due to
greater steric effect caused by the /7-toluidine unit which is obviously greater
211
than that produced in poly(aniline-co-o-toluidine) due to the presence of
o-toluidine units.
The relative intensity of the exciton band is a measure of the degree of
oxidation of the polymer chain [23]. It is clear from the figure-6.6 that, the
relative intensity of the exciton transitions in both poly(aniline-co-o-
toluidine)l:l and poly(aniline-co-/7-toluidine)l:l remain almost the same,
suggesting that oxidation state of these copolymers is almost same. But the
intensity of the exciton band is less in the copolymers as compared to the
intensity of exciton band of polyaniline suggesting that the oxidation state of
copolymers is lesser than that of polyaniline. Comparing figure-6.6 and
figure-6.7, it can also be concluded that, as the ratio of/7-toluidine increases
in the copolymers of aniline with p-toluidine, the oxidation state of the
copolymers decreases. Thus the number of quinoid units is higher in
poly(aniline-co-/7-toluidine)l:l and lowest in poly(aniline-co-/7-toluidine)l:3.
The supports the findings of FTIR spectra of these polymers as explained in
the previous section 6.2.2.
The UV-VIS spectra of the as-prepared protonated form of the
copolymers of aniline with p-toluidine in DMSO show an additional peak
around 410 nm as seen from figure-6.8 and figure-6.9. These bands are
ascribed to the polaron transitions [23,24]. But the band around 800 nm,
generally observed for protonated forms of polyanilines corresponding to
inter-chain transition is not observed here. But, unlike the spectra of base
form of the copolymers, the absorption tail in the spectra of protonated form
is not approaching zero absorbance. A blue shift is also observed for these
three bands in the spectra of copolymers of aniline with/7-toluidine recorded
in DMSO, the extent of which increases with increase in the ratio of
/7-toluidine.
212
Figure-6.9 shows the spectra of as-prepared copolymers of aniline with
p-toluidine taken in H2SO4. It is clear from the figure-6.9 and table-6.4 that
the bands corresponding to 71-> 71* transition, polaron transition and
benzenoid to quinoid exciton transition are also observed when the spectra
are recorded in H2SO4. But here we did not observe any apparent shift in the
two major absorptions corresponding to %-> n* transition and benzenoid to
quinoid transition, though there is a blue shift in the absorption band
corresponding to polaron transition, the extent of which increases as the ratio
of/7-toluidine increases in the copolymers.
It can also be seen from the table-6.4 that, the base form of the
copolymers show solvatochromism in the spectra recorded in DMF and
DMSO. Similarly, the as-prepared protonated form of the copolymers of
aniline with p-toluidine also show a solvatochromic shift when the spectra are
recorded in DMSO and cone, H2SO4.
6.2.4. ESR Spectral Studies
Table-6.5 summarizes the ESR spectral parameters such as 'g ' value,
peak to peak line width AHpp and line asymmetry a/b ratio of the polymers
along with their electrical conductivity. To understand the nature of
paramagnetic state and the suggested conduction mechanism, several ESR
studies have been performed [25-27]. The area ESR signal is related to the
number of spin present in the sample and the width of the signal may be
related to the extent of delocalization of electrons and thus to the extent of
conjugation. The position of ESR signal may be related to the neighboring
environment of electronic spin, the 'g ' factor. The symmetry of ESR signals
and spin-lattice relaxation time can be interpreted in terms of the electrical
conduction theories in conducting polymers [28, 29]. ESR signals depends on
spin concentration which in turn depends on many factors such as method of
213
synthesis, intermolecular interactions are highly sensitive to experimental
conditions. It can be seen from table-6.5 that, the 'g ' value and AHpp
gradually increases with decreases in the electrical conductivity of the
polymers, where as a/b ratio increases with increase in electrical conductivity,
'g ' value and AHpp are minimum for polyaniline and maximum for
poly(aniline-co-p-toluidine)l :3. These observations are consistent with some
of the reported results that, 'g ' value and AHpp decreases and a/b ratio
decreases with increase in electrical conductivity of polyaniline [26, 27],
though there are some different opinions [30,31]. Mohammad et al. [26] have
suggested that, narrow line width is suggestive of longer conjugation length
and greatly delocalized electronic spin gives narrow ESR signals. Our
observations are consistent with this suggestion. For polyaniline, where there
is maximum conjugation and higher value. When substituents are present in
the phenyl rings as in the case of copolymers studied, they tend to localize the
electronic spin and have lower conjugation length resulting in comparatively
higher 'g ' value and AHpp and smaller a/b ratio. Therefore, it can be
concluded from the ESR data presented in table-6.5 that, electron localization
in the polymers increases as the ratio of p-toluidine increases in the polymers
chain affecting their electrical conductivity. Electron localization increases
in the order polyaniline < poly(aniline-co-o-toluidine)l:l < poly(aniline-co-/?-
toluidine)l:l < poly(aniline-co-/7-toIuidine)l:2 < poly(aniline-co-/?-
toluidine)l:3.
6.2.5. Electrical conductivity and charge transport studies
6.2.5.1. Electrical conductivity
The two probe electrical conductivity measured on the pressed pellets
at frequencies of 100 Hz and 10 KHz are presented in Table-6.5. Variation of
electrical conductivity with temperature was studied in the temperature range
214
of 25°C-150°C for poly(aniline-co-o-toluidine)l:l and poly(aniline-co-/>-
toluidine)l:l are shown in figure-6,10. The results of electrical conductivity
-temperature data fitted in Arrhenius equation (Band Model) and variable
range hopping (VRH) model for the same polymers in order to understand the
basic nature of charge transport are shown in figure-6.10 and figure-6.11
respectively.
It can be seen from table-6.6 that all the copolymers show lower
electrical conductivity than polyaniline. The order of electrical conductivity
follows: polyaniline > poly(aniline-co-o-toluidine)l:l > poly(aniline-co-p-
toluidine)l:l > poly(aniline-co-/7-toluidine)l:2 > poly(aniline-co-/?-
toluidine)l:3. The electrical conductivity measured at lOHz is found to be
slightly greater than that measured at lOOHz for poly(aniline-co-<3-
toluidine)l:l. The frequency dependence is more prominent in copolymers of
aniline with/7-toluidine and it increases with the increase ofp-toluidine
content in the copolymers backbone.
Though electron donating alkyl group makes the electronic flow along
a single polymer chain little better, the observed decrease in the electrical
conductivity of the copolymers may then be explained on the basis of steric
effect due to methyl groups on the polymer chain. The decrease in electrical
conductivity of copolymers compared to that of polyaniline may be explained
in terms of the decreased extent of conjugation and an increase in the band
gap caused by increased phenyl ring tortional angles which results from the
steric repulsion between the adjacent phenyl rings due to the presence of
-CH3 groups on the phenyl ring [22,32,33]. The copolymers of aniline with/?-
toluidine experience a greater steric repulsion and a greater decrease in the
extent of conjugation, thereby a greater increase in the band gap causing a
greater reduction in electrical conductivity. The above factors may result in
215
greater electron localization. As the ratio of/7-toluidine increases in the
copolymer backbone, the steric effect and electron localization also increases.
These findings are supported by electronic spectral studies and ESR studies as
explained in section-6.4 and section-6.5 respectively.
The lower electrical conductivity of the copolymers can also be
explained in terms of the decreased inter-chain diffusion of charge carriers in
the copolymers [34]. This is induced by the increased separation of the
polymer chains due the presence of-CHs groups and a lower crystallograpic
order and hence a reduced coherence between the polymer chains. The -CH3
groups may likely to force the chain out of planarity by twisting the phenyl
rings relative to one another to lower the overlap of orbitals along the
conjugated system [35]. As a result the conduction electrons may get
localized in the copolymers, especially those derived from aniline andp-
toluidine in their higher molar ratios.
Polyaniline exhibits large phenylene ring tortional displacements out of
plane defined by the ring bridging aniline/imine nitrogen atoms. It is reported
that deviation from planarity by phenylene ring is altered between +30° and -
30° as one moves along the polymer backbone [ 36]. The quinoid units are
more planar than benzenoid units in polyanilines and are associated with a
more complex inter-chain ring structure [37,38]. The electronic transport
properties that are dependent on the inter-chain and intra-chain wave function
overlap is strongly affected by the presence of -CH3 groups on the phenyl
rings. Then the pronounced local tortious resulting from phenylene ring
vibrations and ring flip about the axis will be different for different
copolymers under study, and resulting in difference in the electrical
conductivities.
216
One of the fascinating observations of our studies is the frequency
dependence of electrical conductivities of the copolymers. It can be seen from
table-6.5 that, though it very feeble in polyaniline, the frequency dependence
gradually increases as the ratio of p-toluidine increases in the copolymers. In
their work on dielectric constant and a.c. conductivity at 10 KHz on
polyaniline and its derivatives, Pinto and coworkers [ 39] have pointed out
that greater localization of charges leads to reduced conductivity and
substituent causes an increase in the disorder. It was clear from our studies
on copolymers of aniline with nitroanilines that, greater the electron
localization, greater is the frequency dependence. This argument is true for
the present systems also. Frequency dependence of electrical conductivity
gradually increases in the copolymers of aniline with p-toluidine because of a
gradual increase of electron localization with the increase in the ratio of p-
toluidine in the copolymer chain. Frequency dependence of electrical
conductivity increases in the order, polyaniline < poly(aniline-co-o-
toluidine)l:l < poly(aniline-co-/7-toluidine)l:l < poly(aniline-co-/7-
toluidine)l:2 < poly(aniline-co-/?-toluidine)l:3. We assume that electron
localization also increases in the same order. These arguments get support
from the ESR results of the copolymers as can be seen from table-6.5. The
observed variations in the electrical conductivities of the copolymers are also
in agreement with it.
6.2.5.2. Charge transport
The temperature dependence of the electrical conductivity of
poly(aniline-co-/7-toluidine)l:I and poly(aniline-co-/?-toluidine)l:l were
performed in the temperature range of 25°C to 150°C. The corresponding
data were fitted in Arrhenius equation (band model) as well as variable range
217
Figure-6.10 Arrhenius plot of log(a) versus 1000/T(K'') (A) poIy(aniline-co-<>-toluidine)l; 1 (B) poIy(aniIine-co-/>-toluidine)l: 1
218
0.224 0.226 0.226 0230 0232 0.234 0.236 0.238
Figure-6.11 Application of variable range hopping model to electrical conductivity - temperature behaviour of (A) poIy(anlline-co-^toluidlne)l: 1 (B)poly(aniline-co-/;-toluldine)l:I
219
hopping (VRH) model. The results are shown in figure-6.10 and figure-6.11
respectively.
The studies revealed a thermally activated conduction phenomena in
both the copolymers. The electrical conductivity increases linearly with
increase in temperature in the whole of the temperature range studied. In
these copolymers also we observed that conductivity is not affected by
moisture or humidity though it is reported that presence of moisture affects
the electrical conductivity of polyaniline [40-42]. The presence of moisture
creates a dynamic disorder in the polymeric system and cause spin and charge
delocalization [43]. The loss of moisture leading to increased localization of
electrons and thereby a reduction in electrical conductivity is not operative in
these polymers. This may be an indication that, static defects such as chain
ends and chemical defects control the charge transport in these copolymers.
The observed electrical conductivity temperature data is found to fit
best in the equation corresponding to variable range hopping model [40]
a(T) = aoexp - ^ (6.1) V i J
where TQ is the Mott's characteristic temperature and CTQ is the electrical
conductivity at room temperature. It is clear from figure-6.9 that variable range
. model can be used to explain the conduction phenomena is poly(aniline-co-o-
toluidine)l:l over the entire range of temperature studied. In poly(aniline-co-
/?-toluidine)l:l also, the VRH model is applicable up to 93°C after which it
follows the same equations but with a change of slope in the corresponding
straight line.
220
In the higher temperature range (90°C-i50°C), it seems that the charge
transport follows the band conduction (Arrhenius model) in poly(aniline-co-
p-toluidine)l:l as the electrical conductivity - temperature data are best in
the equation [40].
a(T) = aoexp - - ^ (6.2)
It can be concluded that where as variable range hopping model can be
used to explain the charge transport in poly(aniline-co-o-toluidine)l:l a
mixed conduction mechanism may be operative in poly(aniline-co-/7-
toluidine)l:l in the temperature of 25°C-150°C.
221
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224
CONCLUSION
Copolymers of aniline with 0-, w-and/7-nitroanilines,/7-
methoxyaniline, o-toluidine and p-toluidine as well as of the copolymers of o-
toluidine with/7-methoxyaniline can be synthesized in acid medium for
different molar ratios of the respective monomers using K2S2O8 as an oxidant.
It is observed that o-nitroaniline, p-methoxyaniline andp-toluidine units can
be incorporated in the polyaniline backbone by the copolymerization of
respective monomers with aniline for the first time. It is concluded that in
presence of the substituted aniline monomers in the polymerization media, the
aniline monomers alone are not getting homopolymerized. All the
copolymers show comparatively better solubility in organic solvent than
polyaniline. Therefore, they can be processed more easily than polyaniline.
The electrical conductivity of all the copolymers is lesser than that of
polyaniline and is found to be frequency dependent. The frequency
dependence increases as the ratio of the substituted aniline units increases in
the polymer backbone and with the decrease in the extent of protonation. The
frequency dependence also decreases with the decrease in the pH of the acid
medium used for synthesis. The lower electrical conductivity of the
copolymers in comparison to polyaniline is in agreement with the
observations made from electronic spectra of the copolymers.
The greater electron localization observed in the copolymers leading to
lower electrical conductivity than in polyaniline may be interpreted in terms
of the frequency dependence of electrical conductivity. In all the copolymers
studied, we observed that greater the electron localization, greater is the
frequency dependence of electrical conductivity. The frequency dependence
of electrical conductivity thus proved to be a novel method to study the
electron localization and polarization effects in polyaniline family of
225
polymers. Therefore, these studies should be extended to higher frequencies
in order to get a better understanding of electrical conduction in these
polymers.
Temperature dependence of electrical conductivity studies revealed a
mixed conduction mechanism (variable range hopping and band conductions)
in most of the copolymers above room temperature. Thermogravimetric
studies revealed that most of the copolymers have comparable thermal
stability with that of polyaniline and some are more thermally stable. Broido
equation has been employed to evaluate the activation energies of thermal
degradation.
ESR studies revealed that 'g ' factor and AHpp decreases and a/b ratio
increases with increase in electrical conductivity for copolymers of aniline
with o- and/7-toluidines, o-nitroaniline and/?-nitroanilines. But the ESR data
of copolymers ofp-methoxyaniline and of m-nitroaniline show a reverse
trend. These observations suggest that the electrical conductivity - ESR data
correlation can not be explained on the basis of a single model and therefore
to be explored further.
Electronic spectra of the copolymers showed characteristic bands
corresponding to 7t -> 7i* transition and benzenoid -> quinoid exciton
transition. The copolymers show lower chain length, a decrease in the extent
of conjugation and an increase in the band gap which are explained on the
basis of steric effect and an increase in the torsional angle between the
adjacent phenyl rings caused by the substituents on the phenyl rings.
Significant effect of ring tortional angle changes accompanied by
conformational changes on the electronic and electrical properties of the
copolymers is to be explored further in order to get a better insight in to
structure - property relationship.
226
Chemically prepared polyaniline is difficult to process by solution or
melt techniques. Since most of the copolymers studied showed a better
solubility characteristic, it is suggested that these materials are to be
investigated for their applications in sensors, corrosion inhibition,
rechargeable batteries, conductive coatings, photovoltaics etc.
227
LIST OF PUBLICATIONS
Paper Published:
• Electrical, Electronic and Thermal properties of poly (aniline-co-p-
methoxyaniline) and poly (o-toludine -co-p-methoxyanilin^ A.I. Yahya,
A. Ahmad and F. Mohammad, Ind. J. Chem., 43 A, 1243 (2004).
Conference Abstracts/ Conference prodeedings:
• Synthesis, Electrical, electronic and Thermal studies on aniline based
electrically conducting polymers, A.I. Yahya, A. Ahmad and F. Mohammad,
''T^ International Conference on Chemistry and its Application", Qatar
University, Doha, Qatar, Dec. 6-9, 2003.
• Electrical, Electronic and Thermal Properties of poly (aniline - co-o-
nitroaniline), A.I. Yahya, A. Ahmad and F. Mohammad, 39"' Annual
Convention of Indian Chemical Society, Nagarjuna University, Dec. 22-26,
2002.
• Frequency and Temperature dependence of Electrical conductivity of aniline
based conducting polymers. A.l. Yahya, A. Ahmad and F. Mohammad.
Advances in Polymer Science and Technology, Cochin University of Science
and Technology, Feb. 1996
228
