Definitions

Electrochromic devices

Polyaniline

Polyaniline (PANI) is a conducting polymer of the semi-flexible rod polymer family. Although it was discovered over 150 years ago, only recently has polyaniline captured the attention of the scientific community due to the discovery of its high electrical conductivity. Amongst the family of conduting polymers, polyaniline is unique due to its ease of synthesis, environmental stability, and simple doping/dedoping chemistry. Although the synthetic methods to produce polyaniline are quite simple, its mechanism of polymerization and the exact nature of its oxidation chemistry are quite complex. Because of its rich chemistry, polyaniline has been one of the most studied conducting polymers of the past 20 years.

History

The monomer aniline was obtained for the first time from the pyrolytic distillation of indigo and was called “Krystallin” because it produced well formed crystalline salts with sulfuric and phosphoric acid. In 1840, Fritzsche also obtained a colorless oil from indigo, called it aniline ostensibly from the Spanish aAil (indigo), and oxidized it to polyaniline (PANI). Some believe this to be the first report of polyaniline, although the first definitive report of polyaniline did not occur until 1862.

During the early 20th century, occasional reports about the structure of PANI appeared in the literature and would continue until periodically until the 1980s. It was during this time that MacDiarmid reinvestigated previous work of Josefowicz and "discovered" that polyaniline can be made electrically conducting upon protonic doping. In subsequent years, the study of polyaniline exploded and currently a vast literature on the synthesis, properties, and applications of polyaniline exists.

Oxidation states

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

  • leucoemeraldine - white/clear
  • protoemeraldine
  • emeraldine - green or blue
  • nigraniline
  • pernigraniline - blue/violet

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. The emeraldine (n = m = 0.5) form of polyaniline, often referred to as emeraldine base (EB), is either neutral or doped, with the imine nitrogens protonated by an acid. 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 the emeraldine salt form of polyaniline is electrically conducting. Leucoemeraldine and pernigraniline are poor conductors, even when doped with an acid.

The color change associated with polyaniline in different oxidation states can be used in sensors and electrochromic devices. Though color is useful, the best method for making a polyaniline sensor is arguably to take advantage of the dramatic conductivity changes between the different oxidation states or doping levels.

Synthesis

The most common 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 reduced to the emeraldine salt as aniline monomer gets oxidized to the radical cation. In the third stage this radical cation couples with ES salt. This process can be followed by light scattering analysis which allows the determination of the absolute molar mass. According to one study 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 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.

External links

References

  1. H. Letheby (1862). "On the production of a blue substance by the electrolysis of sulphate of aniline". Journal of Chemical Society 15 161–163.
  2. Synthesis, processing and material properties of conjugated polymers W. J. Feast et al. Polymer Volume 37 Number 22 pp. 5017-5047,1996
  3. Development and characterization of flexible electrochromic devices based on polyaniline and poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonic acid) Li-Ming Huanga, Cheng-Hou Chena and Ten-Chin Wen Electrochimica Acta; 2006; 51(26) pp 5858-5863; (Article) DOI:10.1016/j.electacta.2006.03.031 Abstract
  4. Polyaniline Nanofiber Gas Sensors: Examination of Response Mechanisms Shabnam Virji, Jiaxing Huang, Richard B. Kaner and Bruce H. Weiller'' Nano Letters; 2004; 4(3) pp 491-496; (Article) DOI: 10.1021/nl035122e Abstract
  5. 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)
  6. 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

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