Polyaniline is a conducting polymer of the
semi-flexible rod polymerSemi-flexible rod polymers are a kind of organic polymer which may be converted to conductive polymers by appropriate oxidations or doping.Examples: polyaniline, poly, poly-External links:*...
family. Although the compound itself was discovered over 150 years ago, only since the early 1980s has polyaniline captured the intense attention of the scientific community. This is due to the rediscovery of its high electrical conductivity. Amongst the family of conducting polymers and organic semiconductors, polyaniline is unique due to its ease of synthesis, environmental stability, and simple doping/dedoping chemistry. Although the synthetic methods to produce polyaniline are quite simple, its mechanism of polymerization and the exact nature of its oxidation chemistry are quite complex. Because of its rich chemistry, polyaniline is one of the most studied conducting polymers of the past 50 years.
History
The monomer aniline was obtained for the first time from the pyrolytic distillation of indigo and was called “Krystallin” because it produced well formed crystalline salts with sulfuric and phosphoric acid. In 1840, Fritzsche also obtained a colorless oil from indigo, called it aniline ostensibly from the Spanish añil (indigo), and oxidized it to polyaniline (PANI). Some believe this to be the first report of polyaniline, although the first definitive report of polyaniline did not occur until 1862.
From the early 20th century on, occasional reports about the structure of PANI appeared in the literature. Similarly, highly-conductive polypyrrole and other polyacetylene derivatives were first reported in the early 1960's . Likewise, the 1964 monograph
Organic Semiconductors references several reports of high-conductivity oxidized polyacetylenes. Some of these report resistivity as low as .001 ohm-cm.
Subsequently,
MacDiarmidAlan Graham MacDiarmid ONZ was a chemist, and one of three recipients of the Nobel Prize for Chemistry in 2000.-Early life:He was born in Masterton, New Zealand as one of five children - three brothers and two sisters...
reinvestigated previous work of Josefowicz and "discovered" that, as with other polyacetylenes, oxidized polyaniline can be made electrically-conductive upon protonic doping. Well before MacDiarmid and coworkers, similar high conductivity had previously been seen in
polypyrrolePolypyrrole is a chemical compound formed from a number of connected pyrrole ring structures. For example a tetrapyrrole is a compound with four pyrrole rings connected. Methine-bridged cyclic tetrapyrroles are called porphyrins. Polypyrroles are conducting polymers of the rigid-rod polymer host...
, another
polyacetylenePolyacetylene is an organic polymer with the repeat unit n. The high electrical conductivity discovered for these polymers beginning in the 1960's accelerated interest in the use of organic compounds in microelectronics...
derivative and
organic semiconductorAn organic semiconductor is an organic material with semiconductor properties. Single molecules, short chain and organic polymers can be semiconductive. Semiconducting small molecules include the polycyclic aromatic compounds pentacene, anthracene, and rubrene...
. Similarly,
John McGinnessJohn Edward McGinness , is an American physicist and physician. McGinness worked as a in the field of Organic electronics and Nanotechnology.-Education:McGinness studied physics at the University of Houston and after his B.S...
and coworkers had previously demonstrated a high-conductivity "ON" state in a bistable switch constructed of
melaninMelanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
, a mixed copolymer of polyaniline, polyacetylene, and polypyrrole. In subsequent years, the study of polyaniline exploded and currently a vast literature on the synthesis, properties, and applications of polyaniline exists.
Conductive polymers such as polyaniline are still a matter of academic interest , providing an opportunity to address fundamental issues of importance to condensed matter physics, including, for example, the metal-insulator transition, the
Peierls InstabilityA Peierls transition or Peierls distortion is a distortion of the periodic lattice of a one-dimensional crystal. Atomic positions oscillate so that the perfect order of the 1-D crystal is broken.- Peierls’ Theorem :...
and
Quantum decoherenceIn quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. A consequence of this dephasing leads to classical or probabilistically additive behavior...
.
Oxidation states
Polymerized from the
anilineAniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...
monomerA monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
, polyaniline can be found in one of three idealized oxidation states :
- leucoemeraldine – white/clear & colorless
- emeraldine – green for the emeraldine salt, blue for the emeraldine base
- (per)nigraniline – blue/violet
In figure 1,
x equals half the
degree of polymerizationThe degree of polymerization, or DP, is usually defined as the number of monomeric units in a macromolecule or polymer or oligomer molecule.For a homopolymer, there is only one type of monomeric unit andthe number-average degree of polymerization is given by...
(DP). Leucoemeraldine with n = 1, m = 0 is the fully reduced state. Pernigraniline is the fully oxidized state (n = 0, m = 1) with
imineAn imine is a functional group or chemical compound containing a carbon–nitrogen double bond, with the nitrogen attached to a hydrogen atom or an organic group. If this group is not a hydrogen atom, then the compound is known as a Schiff base...
links instead of
amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
links. The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is neutral, if doped it is called emeraldine salt (ES), with the imine nitrogens protonated by an acid. Emeraldine base is regarded as the most useful form of polyaniline due to its high stability at room temperature and the fact that, upon doping with acid, the resulting emeraldine salt form of polyaniline is electrically conducting. Leucoemeraldine and pernigraniline are poor conductors, even when doped with an acid.
The color change associated with polyaniline in different oxidation states can be used in sensors and electrochromic devices. Though color is useful, the best method for making a polyaniline sensor is arguably to take advantage of the dramatic conductivity changes between the different oxidation states or doping levels.
Synthesis
The most common synthesis of polyaniline is by oxidative polymerization with ammonium peroxodisulfate as an oxidant. The components are both dissolved in 1 M
hydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
or in other acids and slowly (the reaction is very exothermic) added to each other. The polymer precipitates as small particles and the reaction product is an unstable
dispersionA dispersion is a system in which particles are dispersed in a continuous phase of a different composition . See also emulsion. A dispersion is classified in a number of different ways, including how large the particles are in relation to the particles of the continuous phase, whether or not...
with micrometer-scale particulates. The electrochemical method was discovered in 1862 as a test for the determination of small quantities of aniline.
Using special polymerisation procedures and surfactant dopants, the polyaniline powder can be recovered after polymerisation can be made dispersible and hence useful for practical applications. Bulk synthesis of
polyaniline nanofibersPolyaniline nanofibers are the high aspect version of polyaniline created by several techniques including nanofiber seeding, electrochemical synthesis , electrospinning of composites and several oxidative polymerization methods. Most of these methods required templates that had to be removed,...
has led to a highly scalable and commercially applicable form of polyaniline that has been researched extensively since their discovery in 2002.
A two stage model for the formation of emeraldine base is proposed. In the first stage of the reaction the pernigraniline PS salt oxidation state is formed. In the second stage pernigraniline is reduced to the emeraldine salt as aniline monomer gets oxidized to the
radical cationA radical ion is a free radical species that carries a charge. Radical ions are encountered in organic chemistry as reactive intermediates and in mass spectrometry as gas phase ions...
. In the third stage this radical cation couples with ES salt. This process can be followed by
light scatteringLight scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
analysis which allows the determination of the absolute
molar massMolar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...
. According to one study in the first step a DP of 265 is reached with the DP of the final polymer at 319. 19% of the final polymer is made up of
in situIn situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...
form aniline radical cation.
As synthesis of polyaniline nanostructures is facile and they have been prepared by various methods, polyaniline is an important nanomaterial .
Morphology, Chain Structure and Conductivity
A most recent article by Bernhard Wessling in "Polymers" (http://www.mdpi.com/2073-4360/2/4/786/) is reviewing the experimental and (non-equilibrium thermodynamical) theoretical knowledge about Polyaniline and its dispersions and outlines a new concept for the structure of Polyaniline including a concept for a structure / conductivity relationship . Starting from the understanding of Polyaniline (and conductive polymer) dispersions as dissipative structures which can be described by
non-equilibrium thermodynamicsNon-equilibrium thermodynamics is a branch of thermodynamics that deals with systems that are not in thermodynamic equilibrium. Most systems found in nature are not in thermodynamic equilibrium; for they are changing or can be triggered to change over time, and are continuously and discontinuously...
, the author describes the hitertho unpublished formation of complexes between the Organic Metal Polyaniline and conventional metals like Cu, Fe, In and others (the basis for most of the applications). Furthermore, he shows experimental evidence that advanced dispersion techniques lead to even higher conductivity (in contrast to naive predictions). The higher conductivity is accompanied by changes in morphology and x-ray spectra. These changes are (together with other experimental evidence) interpreted with a new structure model, according to which short helical polyaniline chains are brought into some higher degree of order.
Properties
In the doped emeraldine salt form, polyaniline is electrically conductive, which makes it suitable for e.g. manufacture of
electrically conducting yarnAn electrically conducting yarn is yarn that conducts electricity. Conducting yarns are used to manufacture carpets and other items that dissipate static electricity, such as work clothes in highly flammable environments, e.g., in the petrochemistry industry....
s, antistatic coatings, electromagnetic shielding and flexible electrodes.
Another important property is its position in the electrochemical series. Polyaniline is more noble than Copper and slightly less noble than Silver which is the basis for its broad use in printed circuit board manufacturing (as a final finish) and in corrosion protection.
Polyaniline is typically produced in the form of long-chain polymer aggregates, surfactant (or dopant) stabilized nanoparticle dispersions, or stabilizer-free nanofiber dispersions depending on the supplier and synthetic route. Surfactant or dopant stabilized polyaniline dispersions have been available for commercial sale since the late 1990s.
Applications
Polyaniline and the other conducting polymers such as
polythiophenePolythiophenes result from the polymerization of thiophenes, a sulfur heterocycle, that can become conducting when electrons are added or removed from the conjugated π-orbitals via doping....
,
polypyrrolePolypyrrole is a chemical compound formed from a number of connected pyrrole ring structures. For example a tetrapyrrole is a compound with four pyrrole rings connected. Methine-bridged cyclic tetrapyrroles are called porphyrins. Polypyrroles are conducting polymers of the rigid-rod polymer host...
, and
PEDOT/PSSPEDOT:PSS or Poly poly is a polymer mixture of two ionomers. One component in this mixture is made up of sodium polystyrene sulfonate which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge...
have a great deal of potential for applications due to their light weight, conductivity, mechanical flexibility and chemical properties. Polyaniline is especially attractive among them because it is less expensive, has three distinct oxidation states with different colors and has an acid/base doping response as was described above in the oxidation states. This latter property makes polyaniline an ideal option for acid/base chemical vapor sensors. The different colors, charges and conformations of the multiple oxidation states also make the material highly promising for applications such as actuators, supercapacitors and electrochromics.
Attractive fields for current and potential utilization of polyaniline is in antistatics, charge dissipation or electrostatic dispersive (ESD) coatings and blends, electromagnetic interference shielding (EMI), anti-corrosive coatings, hole injection layers, transparent conductors,
ITOITO may refer to:* Hilo International Airport, IATA airport code* Indium tin oxide, a transparent conductor* Information Technology Outsourcing* International Trade Organization...
replacements, actuators, chemical vapor and solution based sensors, electrochromic coatings (for color change windows, mirrors etc.), PEDOT-PSS replacements, toxic metal recovery,
catalysisCatalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
, fuel cells and active electronic components such as for
non-volatile memoryNon-volatile memory, nonvolatile memory, NVM or non-volatile storage, in the most basic sense, is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory, flash memory, ferroelectric RAM, most types of magnetic computer...
.
Currently, the major applications are antistatic and ESD coatings, printed circuit board manufacturing (final finishes) and corrosion protection.
Commercially polyaniline has been supplied by several companies
PANIPOL,
Eeonyx,
Fibron Technologies Crosslink and
Ormecon.