Polyaniline (PANI) is a conducting polymer of the semi-flexible rod polymer family. Although it was discovered over 150 years ago, only recently has polyaniline captured the attention of the scientific community due to the discovery 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 has been one of the most studied conducting polymers of the past 20 years.
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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.[1]
During the early 20th century, occasional reports about the structure of PANI appeared in the literature and would continue until periodically until the 1980s. It was during this time that MacDiarmid reinvestigated previous work of Josefowicz and "discovered" that polyaniline can be made electrically conducting upon protonic doping. Well before MacDiarmid and coworkers, similar high conductivity had previously been seen in polypyrrole, another polyacetylene derivative and organic semiconductor. Similarly, John McGinness and coworkers had previously demonstrated a high-conductivity "ON" state in a bistable switch constructed of melanin, 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.
Polymerized from the aniline monomer, polyaniline can be found in one of three idealized oxidation states [2]:
In figure 1, x equals half the degree of polymerization (DP). Leucoemeraldine with n = 1, m = 0 is the fully reduced state. Pernigraniline is the fully oxidized state (n = 0, m = 1) with imine links instead of amine links. The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is either neutral or doped, 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 the 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.[3] 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.[4]
The most common synthesis of polyaniline is by oxidative polymerization with ammonium persulfate as an oxidant. The components are both dissolved in 1 M hydrochloric acid and slowly (the reaction is very exothermic) added to each other. The polymer precipitates as small particles and the reaction product is an unstable dispersion with micrometer-scale particulates.
The electrochemical method was discovered in 1862 as a test for the determination of small quantities of aniline.
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 cation. In the third stage this radical cation couples with ES salt. This process can be followed by light scattering analysis which allows the determination of the absolute molar mass. According to one study [5] 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 situ form aniline radical cation.
Polyaniline exists as bulk films or as dispersions. A recurring problem with these dispersions is particle aggregation which limits possible applications. A 2006 study [6] proposes a strategy to prevent aggregation based on a model for nucleation and aggregate formation.
The model identifies two nucleation modes for particle formation, one by so-called homogeneous nucleation forming long elongated polyaniline nanofibers and very stable dispersions that can last for at least months. The other nucleation mode is by heterogeneous nucleation taking place on any alien body available in the reactor such as the surface of the reactor wall forming not elongated fiber but granular coral-like material. With polyaniline, aggregate formation by secondary nucleation also takes place on the nanofibers itself. In the study, heterogeneous nucleation is predominant when the reaction medium is stirred or when the reaction temperature is lowered. With both reaction conditions SEM imagery display nanofibers covered in a layer of coral like granules. The granules act as contact points for a nanoscale glue to link the particles together, causing aggregation. The explanation offered for the suppression of homogeneous nucleation is that this requires a local concentration gradient prior to the onset of nucleation which is destroyed by stirring or by low temperature.
An important property of polyaniline is its electric conductivity, which makes it suitable for e.g. manufacture of electrically conducting yarns, antistatic coatings, electromagnetic shielding and flexible electrodes.
Polyaniline and the other conducting polymers such as polythiophene, polypyrrole, and PEDOT/PSS have a great deal of potential for applications due to their light weight, conductivity and chemical properties. Polyaniline is especially attractive among them because it is less expensive, and has an acid/base doping response as was described above in the oxidation states. This latter property allows polyaniline to be used in chemical vapor sensors.
The conventional synthesis of polyaniline, with the slow addition of oxidant into a monomer solution under acidic conditions, generates micron-scale particulates that are difficult to process. Strategies, such as bulk synthesis of polyaniline nanofibers makes it easier to deposit and blend the polymer, and thus increasing the potential for use.
Attactive 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, transparent conductors, ITO replacements, actuators, chemical vapor and solution based sensors, electrochromic coatings (for color change windows, mirrors etc.), PEDOT-PSS replacements, toxic metal recovery, catalysis, fuel cells and active electronic components such as for non-volatile memory.
Some of the current suppliers of polyaniline include Fibron (www.fibrontech.com), Eeonyx (www.eeonyx.com), Panipol (www.panipol.com) and Ormecon (www.zipperling.de). Fibron has developed polyaniline nanofibers and other conducting polymer nanostructures that are water and organic solvent processable without additives, and are cost effective alternatives to the current water processable ICPs.