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 is due to the rediscovery of its high electrical conductivity. Amongst the family of conducting polymers and organic semiconductors, 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 is one of the most studied conducting polymers of the past 50 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 añil (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.^[1]

From the early 20th century on, occasional reports about the structure of PANI appeared in the literature. Similarly, highly-conductive polypyrolle and other polyacetylene derivatives were first reported in the early 1960's . Likewise, the 1964 monograph Organic Semiconductors references several reports of high-conductivity oxidized polyacetylenes. Some of these report resistivity as low as .001 ohm-cm.

Subsequently, MacDiarmid reinvestigated previous work of Josefowicz and "discovered" that, as with other polyacetylenes, oxidized polyaniline can be made electrically-conductive upon protonic doping. Well before MacDiarmid and coworkers, similar high conductivity had previously been seen in polypyrrole, another polyacetylene derivative and organic semiconductor. Similarly, John McGinness and coworkers had previously demonstrated a high-conductivity "ON" state in a bistable switch constructed of melanin, a mixed copolymer of polyaniline, polyacetylene, and polypyrrole. In subsequent years, the study of polyaniline exploded and currently a vast literature on the synthesis, properties, and applications of polyaniline exists.

Conductive polymers such as polyaniline are still a matter of academic interest ^[2], providing an opportunity to address fundamental issues of importance to condensed matter physics, including, for example, the metal-insulator transition^[3], the Peierls Instability and Quantum decoherence^[4].

Oxidation states

File:Polyaniline structure.png

Figure 1. Main polyaniline structures n+m = 1, x = degree of polymerization

Polymerized from the aniline monomer, polyaniline can be found in one of three idealized oxidation states ^[5]:

• leucoemeraldine – white/clear & colorless
• emeraldine – green for the emeraldine salt, blue for the emeraldine base
• (per)nigraniline – 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 neutral, if doped it is called emeraldine salt (ES), 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 with acid, the resulting 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.^[6] 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.^[7]

Synthesis

The most common synthesis of polyaniline is by oxidative polymerization with ammonium peroxodisulfate as an oxidant. The components are both dissolved in 1 M hydrochloric acid or in other acids and slowly (the reaction is very exothermic) added to each other. The polymer precipitates as small particles and the reaction product is an unstable dispersion with micrometer-scale particulates. The electrochemical method was discovered in 1862 as a test for the determination of small quantities of aniline.

Using special polymerisation procedures and surfactant dopants, the polyaniline powder can be recovered after polymerisation can be made dispersible and hence useful for practical applications. Bulk synthesis of polyaniline nanofibers has lead to a highly scalable and commercially applicable form of polyaniline that has been researched extensively since their discovery in 2002.^[8]

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 ^[9] 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.

As synthesis of polyaniline nanostructures is facile and they have been prepared by various methods, polyaniline is an important nanomaterial ^[10].

Morphology, Chain Structure and Conductivity

A most recent article by Bernhard Wessling in "Polymers" (http://www.mdpi.com/2073-4360/2/4/786/) is reviewing the experimental and (non-equilibrium thermodynamical) theoretical knowledge about Polyaniline and its dispersions and outlines a new concept for the structure of Polyaniline including a concept for a structure / conductivity relationship ^[11]. Starting from the understanding of Polyaniline (and conductive polymer) dispersions as dissipative structures which can be described by non-equilibrium thermodynamics, the author describes the hitertho unpublished formation of complexes between the Organic Metal Polyaniline and conventional metals like Cu, Fe, In and others (the basis for most of the applications). Furthermore, he shows experimental evidence that advanced dispersion techniques lead to even higher conductivity (in contrast to naive predictions). The higher conductivity is accompanied by changes in morphology and x-ray spectra. These changes are (together with other experimental evidence) interpreted with a new structure model, according to which short helical polyaniline chains are brought into some higher degree of order.

Properties

Polyaniline exists as bulk films or as dispersions. Stable polyaniline dispersions are available in commercial scale since the late 1990s. In these dispersions, polyaniline can even have metallic properties, which is why they are often called "organic metal" or "organic nanometal".

An important property of polyaniline is its electric conductivity, which makes it suitable for e.g. manufacture of electrically conducting yarns, antistatic coatings, electromagnetic shielding and flexible electrodes.

A likewise important property is its position in the electrochemical series being 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.

Applications

Polyaniline and the other conducting polymers such as polythiophene, polypyrrole, and PEDOT/PSS have a great deal of potential for applications due to their light weight, conductivity, mechanical flexibility and chemical properties. Polyaniline is especially attractive among them because it is less expensive, and has an acid/base doping response as was described above in the oxidation states. This latter property allows polyaniline to be used in chemical vapor sensors. The multiple oxidation states and colors also make the material more attractive that other conducting polymers for applications such as supercapacitors and electrochromics.

Attractive fields for current and potential utilization of polyaniline is in antistatics, charge dissipation or electrostatic dispersive (ESD) coatings and blends, electromagnetic interference shielding (EMI), anti-corrosive coatings, transparent conductors, ITO replacements, actuators, chemical vapor and solution based sensors, electrochromic coatings (for color change windows, mirrors etc.), PEDOT-PSS replacements, toxic metal recovery, catalysis, fuel cells and active electronic components such as for non-volatile memory.

However, the major applications are in printed circuit board manufacturing (final finishes) and corrosion protection.

Commercially polyaniline has been supplied by several companies including Ormecon, PANIPOL, Eeonyx and Fibron Technologies.

References

1. ^ On the production of a blue substance by the electrolysis of sulphate of aniline H. Letheby Journal of Chemical Society Volume 15, 161–163, 1862 doi:10.1039/JS8621500161
2. ^ Nobel Lecture: Semiconducting and metallic polymers: The fourth generation of polymeric materials Alan J. Heeger Review of Modern Physics Volume 73, 681–700, doi:10.1103/RevModPhys.73.681
3. ^ Applicability of the localization-interaction model to magnetoconductivity studies of polyaniline films at the metal-insulator boundary G. Tzamalis, N.A. Zaidi and A.P. Monkman Physical Review B Volume 68, 245106, doi:10.1103/PhysRevB.68.245106
4. ^ Crucial role of decoherence for electronic transport in molecular wires: Polyaniline as a case study C. J. Cattena, R.A. Bustos-Marun and H.M. Pastawski Physical Review B Volume 82, 144201, doi:10.1103/PhysRevB.82.144201
5. ^ Synthesis, processing and material properties of conjugated polymers W. J. Feast et al. Polymer Volume 37 Number 22 pp. 5017–5047,1996
6. ^ 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
7. ^ 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
8. ^ 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
9. ^ Ćirić-Marjanović, G. (2010) Polyaniline Nanostructures, in Nanostructured Conductive Polymers (ed A. Eftekhari), John Wiley & Sons, Ltd, Chichester, UK. doi: 10.1002/9780470661338.ch2; (Book Chapter) doi:10.1002/9780470661338.ch2 PDF
10. ^ 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
11. ^ New Insight into Organic Metal Polyaniline Morphology and Structure Bernhard Wessling Polymers; 2010; 2(4) pp 786 - 798; (Article) {{doi:10.3390/polym2040786}} abstract [12] full text (pdf) [13]