Polyaniline was discovered in the 19th century by F. Ferdinand Runge (1794–1867), Carl Fritzsche (1808–1871), John Lightfoot (1831–1872), and Henry Letheby (1816–1876). Lightfoot studied the oxidation of aniline, which had been isolated only 20 years previously. He developed the first commercially successful route to the dye called
Aniline black. The first definitive report of polyaniline did not occur until 1862, which included an
electrochemical method for the determination of small quantities of aniline. From the early 20th century on, occasional reports about the structure of PANI were published. Polymerized from the inexpensive
aniline, polyaniline can be found in one of three idealized
oxidation states: •
leucoemeraldine – white/clear & colorless (C6H4NH)n •
emeraldine – green for the emeraldine salt, blue for the emeraldine base ([C6H4NH]2[C6H4N]2)n •
(per)nigraniline – blue/violet (C6H4N)n In the figure,
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. Studies have shown that most forms of polyaniline are one of the three states or physical mixtures of these components. The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is neutral, if doped (protonated) it is called emeraldine salt (ES), with the imine nitrogens protonated by an acid. Protonation helps to delocalize the otherwise trapped diiminoquinone-diaminobenzene state. 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 highly electrically conducting. Polyaniline sensors typically exploit changes in electrical conductivity between the different oxidation states or doping levels. Treatment of emeraldine with acids increases the electrical conductivity by up to ten orders of magnitude. Undoped polyaniline has a conductivity of S/m, whereas conductivities of S/m can be achieved by doping to 4% HBr. The same material can be prepared by oxidation of leucoemeraldine.
Synthesis Although the synthetic methods to produce polyaniline are quite simple, the mechanism of polymerization is probably complex. The formation of leucoemeraldine can be described as follows, where [O] is a generic oxidant: :n C6H5NH2 + [O] → [C6H4NH]n + H2O A common oxidant is
ammonium persulfate in 1 M
hydrochloric acid (other acids can be used). The polymer precipitates as an unstable
dispersion with micrometer-scale particulates. (Per)nigraniline is prepared by oxidation of the emeraldine base with a
peracid: :{[C6H4NH]2[C6H4N]2}n + RCO3H → [C6H4N]n + H2O + RCO2H Aniline can also be electrochemically polymerised directly onto conductive surfaces without the use of a chemical oxidant.
Processing The synthesis of polyaniline nanostructures is facile. Using surfactant dopants, the polyaniline can be made dispersible and hence useful for practical applications. Bulk synthesis of
polyaniline nanofibers has been researched extensively. A multi-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. 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. == Mechanical Properties ==