Conducting Polymers

Nobel Prize for discovery and development of conducting polymers was awarded to Alan J. Heeger, Alan. J. McDiarmid, and Hideki Shirakawa in 2000.

Conducting polymers is a prospective class of new materials that combine solubility, processability, and flexibility of plastics with electrical and optical properties of metals and semiconductors. Many conducting polymers have well- defined electrochemical activity, stability in air, and aggressive media. These plastics can be used as components of electrochemical, electronic and photonic devices. Polyaniline and other materials show excellent anticorrosion properties. The most significant achievement is the application of these polymers as components of field effect transistors and polymer light emitting diodes.

 Polyaniline 

Polyaniline is one of the most important conducting polymers because of relatively low cost, successful combination of chemical and physical properties, as well as its numerous applications in practice.

Conductivity

The distinctive property of polyaniline is its transition into conducting state by protonation of the polymer chain. A simple transformation of the basic form by treatment in dilute acid increases electronic conductivity by nine orders of magnitude. Another specific property of polyaniline is the dependence of its conductivity on the content of water (or another solvent) bound to the polymer backbone. Polyaniline is a “p-type” material, which indicates that the charge transfer takes place in the occupied valence band and the charge carriers are holes. Typical conductivity of polyaniline is 1-2 S*cm-¹, which can be increased to 1000 S*cm^-1 in the metallic state.

Red-ox States and Electrochemistry

Polyaniline shows well-defined red-ox transitions in both acid and base conducting states, in aqueous and non-aqueous electrolytes. Polyaniline oxidation and reduction in the solid state is characterized by two electrochemical transitions which separate three red-ox states. Only the intermediate form conducts electricity. Electrochemical transitions are followed by dramatic changes of optical properties: the electrochemical oxidation of the film results in colour changes from transparent to green and blue. The transitions are relatively fast, about 0.1 µs for 0.1 µm film, and show high reversibility while 100,000 cycles do not damage the film.

Optical Properties

Conducting state of polyaniline shows high absorption in near IR at 800 nm that can be shifted to 900 nm and more for polyaniline derivatives. Metallic state of polyaniline has a broad long-wave absorption band (typical for organic metals) that is extended far into IR region.

Solubility and Stability

When we talk about polyaniline we mean different forms that can be obtained by chemical and electrochemical synthesis. Proper selection of the method of synthesis allows the separation of forms stable at elevated temperatures and in organic electrolytes. Polyaniline can also be stable in concentrated acids and used as electrode material for electrochemical ultracapacitors in 38% sulfuric acid. Polyaniline (and its composites) can be applied as a component of highly effective corrosion inhibitors that promote the formation of a protective oxide layer on the surface of steel, aluminum, and other metals. The effectiveness of this protection is much higher in comparison with zinc broadly used in practice.

In addition polyaniline has a noble metal behavior when deposited on stainless steel, titan, and other electrodes and can protect from corrosion during electrochemical cycling in aqueous electrolytes. The mechanism of this protection is a little different from the one discussed above. The deposition of polyaniline eliminates oxide growth at high positive potentials and keeps the surface conducting. Titanium dioxide layer that makes titanium stable at ambient conditions has insulation properties, and prevents using titanium as an anode in aqueous media. This problem can be resolved by doping the oxide film to make it conductive, or by deposition of noble metals (or conducting polyaniline) to eliminate the growth of insulating layer on the surface of the metal electrode.

The stability and solubility of polyaniline can be controlled by the selection of an appropriate method of polymerization. Polyaniline is soluble in several common organic solvents, and can be deposited by spin coating on the surface of aluminum, steel, titanium, glass, silicon, GaAs, polyester, and many other materials. Polyaniline forms thin films of electronic quality. Polyaniline solution can be used to form a variety of composite materials with other polymers.

Applications

Polyaniline (and derivatives) can be used as the component of electricity dissipative films, electromagnetic shields, IR absorbers, anti-corrosion coatings, as the element of electrochemical batteries, printed circuit boards, supercapacitors, electro-optical windows, field effect transistors, organic light emitting diodes, transparent conducting films, sensors. It also can be applied as the electron-proton conducting electrode, or proton conducting solid electrolyte in fuel cells and other devices.

New metallic form of polyaniline has been developed by scientists in South Korea. A team led by Kwanghee Lee at Pusan National University, Korea created a conductive polyaniline with a more ordered structure using "self-stabilized dispersion polymerization" (SSDP) (Nature 2006, 441, 65). This material is also stable in air and has conductivity about 1000 S*cm^-1 at room temperature. Main distinction of this plastic is the absence of the dielectric transition at helium temperatures. The result is a polymer with a transport mechanism and electronic structure the same as typical metals. According to published data, the optical reflectivity of the metallic polyaniline in IR fits the model for a simple metal.

    Annual Number of  Publications  
in Journals and the Internet retrieved for the key-word "Polyaniline"

Polythiophene

 Phthalocyanines

Metal phthalocyanines are inorganic complexes; however, their conducting, optical, and magnetic properties are similar to those of organic metals. These materials deserve to be mentioned because of unique stability, high electrical conductivity, catalytical and electrochemical activity. Many non-conducting phthalocyanines are commercialized and available for development.

The disadvantage of rigid lattice of a metal phthalocyanine can be overcome by formation of so called reticulate doped composites with polymer binder. As a result, the composite films combine electronic conductivity of phthalocyanines with flexibility of plastics. Similar to conducting polymers phthalocyanines can be oxidized with formation of the semiconductor or metallic states. Unlike conducting polymers phthalocyanines form a well-defined crystal lattice and the electrical charge transfer proceeds along the stacks formed by flat macromolecular complexes. Some of metal phthalocyanines form organic metals with conductivity of about  500 S*cm^-1, other have semiconductor properties.

Electrochemical oxidation of the solid films results in the appearance of electrical conductivity, and dramatic changes in colour. Conducting phthalocyanines can be used as components of antistatic coatings, electrochromic indicators, photoluminescent layers, field effect transistors, photovoltaic cells and other devices.

 

More information in article:

 

Iakov Kogan, Kyuya Yakushi. New Conducting Polymer Materials Based on Platinum Phthalocyanine Charge Transfer Salts and poly(bisphenol-A-carbonate).  Electrochemical, Optical, Magnetic and Structural Properties. J. Mat.Sci., 1997, 7, 2231-34.