Polyaniline

Polyaniline (PANI) is a conducting polymer of the semi-flexible rod polymer 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 interest is due to the rediscovery of high electrical conductivity. Amongst the family of conducting polymers and organic semiconductors, polyaniline has many attractive processing properties. Because of its rich chemistry, polyaniline is one of the most studied conducting polymers of the past 50 years.

As described by Alan MacDiarmid, 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. Subsequent to his investigation of other highly-conductive organic materials, MacDiarmid demonstrated the conductive states of polyaniline which arose upon protonic doping of the emeraldine form of polyaniline. Conductive polymers such as polyaniline remain of widespread interest, providing an opportunity to address fundamental issues of importance to condensed matter physics, including, for example, the metal-insulator transition, the Peierls Instability and quantum decoherence.

Polymerized from the inexpensive aniline monomer, polyaniline can be found in one of three idealized oxidation states:

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. 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. Leucoemeraldine and pernigraniline are poor conductors, even when doped with an acid.

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