01-12-2012, 01:21 PM
Polymer systems
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General introduction
Polymer systems with special properties is a field of increasing scientific and
technical interest, offering the opportunity to polymer and synthetic organic chemists
to synthesize a broad variety of promising materials, with a wide range of electrical
and magnetic properties. This has led to the development of different routes in the
discovery of different conducting materials such as conducting films, conducting
fibers and conducting polymer composites. Polymers are generally insulators and the
low conductivity of polymers has been extensively used in the manufacture of
insulators and dielectric substances in electrical industry. However, some polymers
have been synthesized with remarkable ability to conduct electricity. An organic
polymer that possesses the electronic, magnetic, electrical and optical properties of a
metal is called an intrinsically conducting polymer (ICP). Among the large variety of
conducing polymers, polyaniline (PANI) has emerged as the most promising one
because of its diverse properties.
Polyaniline
Technological uses of polymer systems depend crucially on the reproducible control
of the molecular and supramolecular architecture of the macromolecule via a simple
methodology of organic synthesis. Polyaniline is one such polymer whose synthesis
does not require any special equipment or precaution. Among the ICPs, PANI has
become one of the most technologically important one due to its unique
processability, together with relatively inexpensive monomer and high yield of
polymerization [1]. PANI is fast replacing the conventional materials because of its
fascinating electrical properties. This interest is caused by diverse, but also unique,
properties of PANI allowing its potential applications in various fields, such as
energy storage and transformation (alternative energy sources, erasable information
storage, non-linear optics, shielding of electromagnetic interference), as well as
catalysts, indicators, sensors, membranes of precisely controllable morphology, etc.
The birth and growth
PANI is one of the oldest conductive polymers known. It is the oxidative polymeric
product of aniline under acidic conditions and has been known since 1862 as aniline
black. It was first prepared by Letheby in 1862 by anodic oxidation of aniline in
sulphuric acid [2]. At the beginning of the twentieth century, organic chemists started
investigating the constitution of aniline black and its intermediate products.
Willstatter and co-workers in 1907 and 1909 regarded aniline black as an eightnuclei
chain compound having an ‘indamine’ structure [3, 4]. However, in 1910,
Green and Woodhead were able to report various constitutional aspects of aniline
polymerization [5]. They carried out oxidative polymerization studies using mineral
acids and oxidants such as persulfate, dichromate, and chlorate and determined the
oxidation state of each constituent by redox titration using TiCl3. During this period,
it did not occur to anyone to investigate its electrical and magnetic properties for the
obvious reasons that organic compounds are insulators, though in 1911, Mecoy and
Moore suggested electrical conduction in organic solids [6].
Doping
Doping is the process by which polymers that are insulators or semi-conductors as
synthesized are exposed to charge transfer agents (dopants) in the gas or solution
phase or through appropriate electrochemical oxidation or reduction. This process
will increase the polymer’s ability to conduct electricity because of the increased
concentration of charge carriers [23]. Pure PANI, in the undoped state, is a poor
semiconductor with conductivity of about 10-8 S/cm. However, once it is doped, its
conductivity could increase by a factor of 10 S/cm or more depending on the dopant
used. Polyaniline holds a special position amongst conducting polymers in that it’s
most highly conducting doped form can be reached by two completely different
process- protonic acid doping and oxidative doping. Protonic acid doping of
emeraldine base units with, for example, 1 M aqueous HCl results in complete
protonation of the imine nitrogen atoms to give the fully protonated emeraldine
hydrochloride salt [22, 23]. The protonation is accompanied by a 9-10 order of
magnitude increase in conductivity reaching a maximum with ~ 1 M aqueous HCl.
Charge storage and conduction
One early explanation of conduction in conducting polymers uses band theory as a
method of conduction. This says that a half filled valence band would be formed
from a continuous delocalized system. This would be an ideal condition for
conduction of electricity. However, it turns out that the polymer can more efficiently
lower its energy by bond alteration (alternating short and long bonds), which
introduces a band width of 1.5 eV, making it a high energy gap semiconductor. The
polymer is transformed into a conductor by doping it with either an electron donor or
electron acceptor. This is reminiscent of doping of silicon based semiconductors
were silicon is doped with either arsenic or boron. However, while the doping of
silicon produces a donor energy level close to the conduction band or an acceptor
level close to the valence band, this is not the case with conducting polymers. The
evidence for this is that the resulting polymers do not have a high enough
concentration of free spins, as determined by electron spin resonance spectroscopy
(ESR). Initially, the free spins concentration increases with concentration of dopant.
At larger concentrations, however, the concentration of free spins levels off at a
maximum [26-30].
Synthesis
Polyaniline is prepared by either chemical or electrochemical oxidation of aniline
under acidic conditions. An aqueous medium is preferred. The synthesis of polymer
by either chemical or electrochemical methods depends upon the intended
application of the polymer. Whenever thin films and better-ordered polymers are
required, an electrochemical method is preferred. The acid dissociation constant
(pKa) is an important aspect of PANI synthesis, because in PANI, protonation
equilibria involves exclusively the quinone diamine segment, having two imine
nitrogens with pKa1 = 1.05 and pKa2 = 2.55 [47]. Therefore, any acid whose pKa
value falls within this range would be suitable as a dopant whereas the anilinium ion
has a pKa of 4.60 and the ammonium ion 9.24. Acids having pKa values around that
of an anilinium ion would be suitable as solvents if they are liquids and can also be
used to prevent over-oxidation of PANI.
Polyaniline/polymer blends and composites
One can find analyses of numerous attempts to apply high conductivity,
electrochromic, catalytic, sensor, redox and other properties of PANI to different
practical needs [53-57]. However, since 1984, efforts have shifted to its use as PANI
composites or blends with common polymers [54, 58-60]. This trend has been driven
by the need to replace traditional inorganic conducting fillers and to improve the
processability of PANI, along with its mechanical properties and stability. These
composite materials have introduced PANI to practical applications in different
fields, including electromagnetic shielding and microwave absorption [56, 61, 62],
static electricity dissipation [63, 64], conducting glues [65], paint coatings for
anticorrosion protection [66] and sensor materials [67].
[attachment=42052]
General introduction
Polymer systems with special properties is a field of increasing scientific and
technical interest, offering the opportunity to polymer and synthetic organic chemists
to synthesize a broad variety of promising materials, with a wide range of electrical
and magnetic properties. This has led to the development of different routes in the
discovery of different conducting materials such as conducting films, conducting
fibers and conducting polymer composites. Polymers are generally insulators and the
low conductivity of polymers has been extensively used in the manufacture of
insulators and dielectric substances in electrical industry. However, some polymers
have been synthesized with remarkable ability to conduct electricity. An organic
polymer that possesses the electronic, magnetic, electrical and optical properties of a
metal is called an intrinsically conducting polymer (ICP). Among the large variety of
conducing polymers, polyaniline (PANI) has emerged as the most promising one
because of its diverse properties.
Polyaniline
Technological uses of polymer systems depend crucially on the reproducible control
of the molecular and supramolecular architecture of the macromolecule via a simple
methodology of organic synthesis. Polyaniline is one such polymer whose synthesis
does not require any special equipment or precaution. Among the ICPs, PANI has
become one of the most technologically important one due to its unique
processability, together with relatively inexpensive monomer and high yield of
polymerization [1]. PANI is fast replacing the conventional materials because of its
fascinating electrical properties. This interest is caused by diverse, but also unique,
properties of PANI allowing its potential applications in various fields, such as
energy storage and transformation (alternative energy sources, erasable information
storage, non-linear optics, shielding of electromagnetic interference), as well as
catalysts, indicators, sensors, membranes of precisely controllable morphology, etc.
The birth and growth
PANI is one of the oldest conductive polymers known. It is the oxidative polymeric
product of aniline under acidic conditions and has been known since 1862 as aniline
black. It was first prepared by Letheby in 1862 by anodic oxidation of aniline in
sulphuric acid [2]. At the beginning of the twentieth century, organic chemists started
investigating the constitution of aniline black and its intermediate products.
Willstatter and co-workers in 1907 and 1909 regarded aniline black as an eightnuclei
chain compound having an ‘indamine’ structure [3, 4]. However, in 1910,
Green and Woodhead were able to report various constitutional aspects of aniline
polymerization [5]. They carried out oxidative polymerization studies using mineral
acids and oxidants such as persulfate, dichromate, and chlorate and determined the
oxidation state of each constituent by redox titration using TiCl3. During this period,
it did not occur to anyone to investigate its electrical and magnetic properties for the
obvious reasons that organic compounds are insulators, though in 1911, Mecoy and
Moore suggested electrical conduction in organic solids [6].
Doping
Doping is the process by which polymers that are insulators or semi-conductors as
synthesized are exposed to charge transfer agents (dopants) in the gas or solution
phase or through appropriate electrochemical oxidation or reduction. This process
will increase the polymer’s ability to conduct electricity because of the increased
concentration of charge carriers [23]. Pure PANI, in the undoped state, is a poor
semiconductor with conductivity of about 10-8 S/cm. However, once it is doped, its
conductivity could increase by a factor of 10 S/cm or more depending on the dopant
used. Polyaniline holds a special position amongst conducting polymers in that it’s
most highly conducting doped form can be reached by two completely different
process- protonic acid doping and oxidative doping. Protonic acid doping of
emeraldine base units with, for example, 1 M aqueous HCl results in complete
protonation of the imine nitrogen atoms to give the fully protonated emeraldine
hydrochloride salt [22, 23]. The protonation is accompanied by a 9-10 order of
magnitude increase in conductivity reaching a maximum with ~ 1 M aqueous HCl.
Charge storage and conduction
One early explanation of conduction in conducting polymers uses band theory as a
method of conduction. This says that a half filled valence band would be formed
from a continuous delocalized system. This would be an ideal condition for
conduction of electricity. However, it turns out that the polymer can more efficiently
lower its energy by bond alteration (alternating short and long bonds), which
introduces a band width of 1.5 eV, making it a high energy gap semiconductor. The
polymer is transformed into a conductor by doping it with either an electron donor or
electron acceptor. This is reminiscent of doping of silicon based semiconductors
were silicon is doped with either arsenic or boron. However, while the doping of
silicon produces a donor energy level close to the conduction band or an acceptor
level close to the valence band, this is not the case with conducting polymers. The
evidence for this is that the resulting polymers do not have a high enough
concentration of free spins, as determined by electron spin resonance spectroscopy
(ESR). Initially, the free spins concentration increases with concentration of dopant.
At larger concentrations, however, the concentration of free spins levels off at a
maximum [26-30].
Synthesis
Polyaniline is prepared by either chemical or electrochemical oxidation of aniline
under acidic conditions. An aqueous medium is preferred. The synthesis of polymer
by either chemical or electrochemical methods depends upon the intended
application of the polymer. Whenever thin films and better-ordered polymers are
required, an electrochemical method is preferred. The acid dissociation constant
(pKa) is an important aspect of PANI synthesis, because in PANI, protonation
equilibria involves exclusively the quinone diamine segment, having two imine
nitrogens with pKa1 = 1.05 and pKa2 = 2.55 [47]. Therefore, any acid whose pKa
value falls within this range would be suitable as a dopant whereas the anilinium ion
has a pKa of 4.60 and the ammonium ion 9.24. Acids having pKa values around that
of an anilinium ion would be suitable as solvents if they are liquids and can also be
used to prevent over-oxidation of PANI.
Polyaniline/polymer blends and composites
One can find analyses of numerous attempts to apply high conductivity,
electrochromic, catalytic, sensor, redox and other properties of PANI to different
practical needs [53-57]. However, since 1984, efforts have shifted to its use as PANI
composites or blends with common polymers [54, 58-60]. This trend has been driven
by the need to replace traditional inorganic conducting fillers and to improve the
processability of PANI, along with its mechanical properties and stability. These
composite materials have introduced PANI to practical applications in different
fields, including electromagnetic shielding and microwave absorption [56, 61, 62],
static electricity dissipation [63, 64], conducting glues [65], paint coatings for
anticorrosion protection [66] and sensor materials [67].