Polyaniline - Synthesis and Properties

Synthesis and Properties

Polymerized from the inexpensive aniline monomer, 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 ({₂₂}_n)
• (per)nigraniline – blue/violet (C₆H₄N)_n

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.

The colour change associated with polyaniline in different oxidation states can be used in sensors and electrochromic devices. Although color is useful, the best method for making a polyaniline sensor is arguably to take advantage of the dramatic changes in electrical conductivity between the different oxidation states or doping levels. Treatment of emeraldine with acids increases the electrical conductivity by ten orders of magnitude. The same material can be prepared by oxidation of leucoemeraldine.

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.