What are the uses of polyaniline

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Polyaniline (PANI) is one of the oldest known synthetic polymers. By electrochemical or chemical oxidation of aniline, usually with ammonium peroxodisulfate in acid, a linear polyaniline is obtained that exists in different oxidation states. The extreme cases of the fully reduced and the fully oxidized form are shown in (Fig. 1). The former is known under the common name Leucoemeraldin, the latter as Pernigranilin.

The oxidative synthesis of polyaniline produces emeraldine, in which half of all 1,4-phenylenediamine structures are in the leucoemeraldine form and the other half in the quinoid pernigraniline form. Only the protonated form, the green emeraldine salt, has electrical conductivity. Since the polymerization is carried out in acid, the product is also obtained in this form. With base it can be converted into the neutral emeraldine form.

If the neutral emeraldine is treated with base, the imino nitrogen atom is first protonated. Electron transfer creates a system with completely delocalized radical cations. This is stabilized by the formation of completely aromatic ring structures. Conductivities of up to 40 S / cm are achieved, while the conductivity of the neutral polymer is approx. S / cm.

Polyaniline is very poorly soluble in the unprotonated form as well as in protonated form, e.g. as a hydrochloride, since the polymer strands aggregate strongly. By protonation with organic acids that have large and bulky anions, such as camphorsulfonic acid or para-Dodecylphenylsulfonic acid can be used in organic solvents such as polymers meta-Cresol can be dissolved. This enables processing into films, fibers or polymer blends with high electrical conductivity.

Polyaniline is used in organic light-emitting diodes (OLEDs), as an antistatic coating, transparent electrode material, in biosensors, membranes for gas separation and as a corrosion protection coating for metals such as stainless steel or copper.