Polyaniline

Polyaniline (PANI) is a conducting polymer and organic semiconductor of the semi-flexible rod polymer family. The compound has been of interest since the 1980s because of its electrical conductivity and mechanical properties. Polyaniline is one of the most studied conducting polymers. 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.