The Application of Conducting Polymers  and Organic Metals

Introduction

Conducting polymers are very suitable substances for contemporary technologies because of solubility and low temperature processing capabilities. The most important criteria for selection of these materials are as follows:

·        Stability in ambient atmosphere. It is highly desirable that the material is stable at 50 – 80 °C or higher in an ambient atmosphere. In the early of 80-s most of scientific articles were devoted to polyacetylene that had conductivity about 1000 S*cm^-1 in doped state and could be produced from inexpensive acetylene. It was difficult to imagine that the life of this polymer would be so short. Polyacetylene is not used broadly in practice because of stability issues.

·        Price. The cost of organic compound applies further constraints on its application. Many contemporary electroconductive substances  may not be broadly used because of high costs. The selection of thin films may essentially improve the cost effectiveness of a conducting polymer. From a financial point of view polyaniline is the most prospective polymer with advanced electrical, optical, and electrochemical properties. It is necessary to note that the price of a chemical distributed in 5 g packs is not an appropriate indicator for estimating the cost in kilogram packages.

·        Density. The effectiveness of certain applications is estimated not by specific weight, but by specific volume. The density of polymers, particularly porous materials is much lower than the density of inorganic metals or oxides. An applied researcher should be careful in the preliminary evaluation of the suitability of a conducting polymer for weight-based applications.

·        Deposition. Preference should be given to technologies that allow direct printing of active substances or low temperature methods of film deposition.

·        Nanotechnology. It is always reasonable to estimate the advantages of using low size particles or thin films. The application of nano materials may bring dramatic changes in the performance of a device.

Antistatic Additives

Using conducting polymers as electricity dissipative additives is a straightforward and broad application of these materials. Most plastics are insulators, a small additive of polyaniline or another conducting polymer increases conductivity and eliminates electrostatic charge. In contrast to carbon, inorganic metals or oxides conducting polymers can preserve transparency and homogeneity when mixed with other plastics.  The antistatic properties of conducting polymers prevent possible explosions during the movement of fuels through polymeric transport lines. The static dissipation eliminates data loss or device malfunction caused by sparks in electronic equipment.

Electromagnetic Shields

Conducting polymers can also be applied for electromagnetic shielding in the microwave frequency range. This application requires high conducting substances. Polyaniline and its composites are effective for electromagnetic interference (EMI) shielding at microwave 200-2000 MHz and X-band 8-12 GHz frequency ranges.

Anticorrosion Coatings

There are two possible applications of polyaniline and other conducting polymers for protection of inorganic metals from corrosion. Polyaniline as an active component of a coating provides passivity of the surface of steel by formation of the metal oxide layer. The effectiveness of polyaniline is much higher in comparison with zinc traditionally used for these purposes (the mechanism of Zinc action is different). Another way of protection is analogous to coatings by noble metals and has dissimilar mechanism of action. The “nobelization” of stainless steel and other metals prevents the surface from formation of insulating oxide layers and allows the conservation of conductivity. This type of protection can be applied to the material of an anode of an electrochemical cell. Polyaniline films are strongly bound to the surface of metal and eliminate the resistance at the surface of the electrochemical electrode.

Organic and Polymer Light Emitting Diodes

Organic and polymer light emitting diodes (OLED) are undoubtedly revolutionary approaches in the area of information displays. The advantages of OLED are as follows: wide angle of visibility, reduced power of consumption, high image quality, low operational temperatures and small size. OLED is a sandwich formed by a high work function transparent conducting anode (comprised of indium tin oxide film or polyaniline), and a low work function metal cathode on both sides of the active emitting layer based on organic luminescent material or conjugated polymer. The active layer is separated from anode and cathode by a hole and electronic transmission films. When direct voltage is applied to the device, holes are injected in the anode, and electric charges from the cathode meet to combine at the illuminating layer and excite electroluminescence. The mentioned structure can be simplified.

Electromechanical Actuators

Conducting polymer electrodes can be used for electromechanical actuators that allow the transformation of the electrical signal into mechanical movement. The idea of artificial muscles can be realized using polyaniline, polypyrrole and other materials. The application of positive potential to the conducting polymer electrode results in the intercalation of anions into the polymer lattice. Such an intercalation is followed by volume changes which can be exploited for transducer design. Electromechanical actuators have numerous applications, for example, a device may control small orifice in a drug delivery reservoir implanted in human body. A biosensor can provide electrical signal responsive to the patient’s therapeutic requirements.

Electrochromic Mirrors and Windows

The application of electrochemical potential to a thin film of a conducting polymer results in significant colour changes. This effect is used to design electro-optical mirrors and windows, as well as electrochromic indicators. Electrically controlled rearview mirrors for automobiles have already been applied in practice. It is interesting to note that the application of electrical potential leads to the appearance of long wave absorption at 800 – 900 nm. This effect can be exploited for design of the electrically controlled IR absorbing devices. The transition time of the electrochromic devices essentially depends on the ion mobility and the square area of the device. An electrochromic mirror based on polyaniline electrode of 1 cm² in size can be switched from transparent into coloured state in 100 ms.

Ultracapacitors

Ultracapacitors or supercapacitors are electrochemical devices based on two ideally polarized electrodes separated by a membrane and electrolyte. The capacitance of the ultracapacitors is three order of magnitude higher than that of an electrolytic capacitor. High capacity is originated from an ultra thin double layer formed at the interface of the metal electrode and electrolyte. Polyaniline can be effectively used as the electrode for the supercapacitors in 38 % sulfuric acid. The mechanism of the charge accumulation in polyaniline is a little different from carbon because of the combination of electrostatic polarization with the red-ox transformations of polyaniline electrode. Ultracapacitors are similar to rechargeable batteries, however, they have higher cycle life and capacitancelike behavior. The device can be directly connected to a printed circuit board and used as a standby source of electricity.