Polyaniline

Oxidation states

  Polymerized from the oxidation states ^[1]:

• leucoemeraldine
• protoemeralsine
• emeraldine
• nigraniline
• pernigraniline

In figure 1 x equals half the acid.

Synthesis

A tried and tested method for the synthesis of polyaniline is by oxidative polymerization with ammonium peroxydisulfate as an oxidant. The components are both dissolved in 1 M hydrochloric acid and slowly (the reaction is very exothermic) added to each other. The polymer precipitates as small particles and the reaction product is a dispersion. The electrochemical method was discovered in 1862 as a test for the determination of small quantities of aniline. A two 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 molar mass. According to one study ^[2] in the first step a DP of 265 is reached with the DP of the final polymer at 319. 19% of the final polymer is made up of in situ form aniline radical cation.

Properties

Polyaniline exists as bulk films or as dispersions. A recurring problem with these dispersions is particle aggregation which limits possible applications. A 2006 study ^[3] proposes a strategy to prevent aggregation based on a model for nucleation and aggregate formation.

The model identifies two nucleation modes for particle formation, one by so-called homogeneous nucleation forming long elongated nanofibers and very stable dispersions that can last for months. The other nucleation mode is by heterogeneous nucleation taking place on any alien body available in the reactor such as the surface of the reactor wall forming not elongated fiber but granular coral-like material. With polyaniline, formation by secondary nucleation also takes place on the nanofibers itself. In the study, heterogeneous nucleation is predominant when the reaction medium is stirred or when the reaction temperature is lowered. With both reaction conditions SEM imagery display nanofibers covered in a layer of coral like granules. The granules act as contact points for a nanoscale glue to link the particles together, causing aggregation. The explanation offered for the suppression of homogeneous nucleation is that this requires a local concentration gradient prior to the onset of nucleation which is destroyed by stirring or by low temperature.

An important property of polyaniline is its electric conductivity, which makes is suitable, e.g., for manufacture of electrically conducting yarns.

References

1. ^  Synthesis, processing and material properties of conjugated polymers W. J. Feast et al. Polymer Volume 37 Number 22 pp. 5017-5047,1996
2. ^  Absolute Molecular Weight of Polyaniline Harsha S. Kolla, Sumedh P. Surwade, Xinyu Zhang, Alan G. MacDiarmid, and Sanjeev K. Manohar J. Am. Chem. Soc.; 2005; 127(48) pp 16770 - 16771; (Communication) DOI: 10.1021/ja055327k Abstract
3. ^  Shape and Aggregation Control of Nanoparticles: Not Shaken, Not Stirred Dan Li and Richard B. Kaner J. Am. Chem. Soc.; 2006; 128(3) pp 968 - 975; (Article) DOI: 10.1021/ja056609n Abstract