Conductive polymers

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Structural section of the conjugated polymer polypyrrole , doped with p-toluenesulfonic acid and therefore conductive
Organic electronics - flexible, bendable display based on conductive polymers

Intrinsically conductive polymers , also called conductive polymers , are plastics with electrical conductivity that are comparable to metals . This is in contrast to normal polymers , which are non-conductors . The conductivity of the polymer is achieved by conjugated double bonds , which enable charge carriers to move freely in the doped state . Contrary to expectations, the polymers are usually not conductive in their pure state. The Peierls theorem provides an explanation for this .

Polymers that are only conductive through electrically conductive fillers such as aluminum flakes or carbon black are called extrinsically conductive polymers and are not (intrinsically) conductive polymers in the actual sense and are not dealt with in this article.

Conductive polymers are basic materials in organic electronics .

Alan J. Heeger , Alan G. MacDiarmid and Hideki Shirakawa received the Nobel Prize in Chemistry in 2000 for the discovery of conductive polymers around 1977 .

structure

The structure of the self-conducting polymers is similar to that of conventional plastics, and is highly disordered. Most intrinsically conductive polymers such as polypyrrole or polythiophene are insoluble in common solvents, but polyaniline can be dissolved in N- methyl-2-pyrrolidone , for example. The doped species of the polymers are all not thermally processable. Often the polymers also deviate from the ideal chemical composition, since undesired side reactions can occur during the formation. The structure and thus also the physical properties are strongly influenced by the synthesis conditions. Apart from the monomer used , the solvent, the conductive salt and the oxidation conditions, among other things, affect the chemical composition and morphology of the polymer.

Electric conductivity

The electrical conductivity requires freely movable charge carriers. That is why electrically self-conducting polymers have an extensive π-electron system in the form of conjugated double bonds . Defect electrons serve as charge carriers . With some polymers, such as polyacetylene and poly- p- phenylene, a negatively charged polymer backbone can also be generated. Anions serve as counterions for the oxidized polymer structure. If an electric current flows, the charge carriers also have to transfer from one polymer chain to an adjacent one, so-called hopping , because the conjugated chains only have a finite length. The total resistance is therefore the sum of the resistances in the polymer chains and the resistances between the chains. The higher resistance between the chains has the greater influence on the electrical conductivity. The shorter the conjugated chains, the higher the resistance, because the charge carriers have to be transferred between the chains more often. The electrical conductivity of electrically self-conducting polymers is in the range from 10 −13 to 10 3  S · cm −1 .

Oxidation and reduction

In the ideal case, the polymer framework can be reversibly oxidized and reduced electrochemically. As a result, the conductivity can be varied from the insulating reduced state to the oxidized conductive state. The oxidation injects holes into the conjugated polymer chains. Initially, the conductivity increases with the number of charge carriers generated. However, overoxidation leads to the irreversible destruction of the conjugation and thus to the loss of electrical conductivity. Since the polymer chains become positively charged as a result of the oxidation, anions are incorporated into the polymer layer to compensate for the charge. During the reduction, they are pushed back into the electrolyte solution. On the other hand, it is also possible to incorporate cations to maintain charge neutrality, especially if bulky anions were used in the synthesis that are more or less stuck in the polymer, for example polystyrene sulfonate.

The term “doping” is also used for electrically self-conducting polymers. Oxidation is called p-doping. However, this cannot be compared with the classic doping of inorganic semiconductors. Foreign atoms are introduced there in comparably low concentrations. The oxidation of the polymer structure, on the other hand, generates the charge carriers directly and in a significantly higher concentration. In the case of thin layers, the color of the conductive polymer depends on the oxidation state ( electrochromism ).

Manufacturing

The preparation of electrically self-conducting polymers can be carried out chemically, electrochemically, photoelectrochemically or using the CVD technique (from English chemical vapor deposition ). Apart from the various starting compounds that are available, a wide range of chemical and physical properties can be achieved by derivatizing them or by forming copolymers . The electrochemical deposition of thin layers through the oxidation of the monomeric starting material is very simple. The self-conducting polymer is created in an oxidized, conductive state. The positive charges on the polymer structure are compensated for by the incorporation of anions in the conductive salt.

Representative of electrically self-conducting polymers

The plastic poly-3,4-ethylenedioxythiophene (PEDOT, also PEDT) is a representative of electrically conductive polymers. Polystyrene sulfonate (PSS) is often used as the counterion ; the combination is abbreviated to PEDOT: PSS. Doped polyethine (also polyacetylene, PAC) is the longest known conductive polymer, but no longer plays a major role. It has been replaced by polyaniline (PAni), which is conductive without additives and is used in industry.

Polyparaphenylene (PPP) is used in field effect transistors and blue light emitting diodes. Polypyrrole (PPy) is used for batteries and is of particular importance for microbiology and sensor technology. Doped polythiophene (PT) is used for electronic components, including organic light-emitting diodes. The luminous color can be changed by changing the applied voltage.

Possible applications

As a rechargeable battery, self-conductive polymers would be interesting because of their low density. However, the stability of the materials with regard to oxidation / reduction has so far been too low to ensure an acceptable number of charging cycles. This problem affects the use as a display or so-called “ smart window ” in the same way . The latter refers to a window pane whose tint color and transparency can be changed by applying voltage. For example, thin films of polypyrrole are brown to black in the oxidized state and yellow to green in the reduced state. The voltage only needs to be applied to change the oxidation state. Acids, bases, oxidizing and reducing substances, anions, cations, inorganic and organic gases can influence the electrical conductivity of self-conducting polymers. This suggests an application as a sensor system ( chemiresistor ). The big drawback here has so far been the lack of selectivity. A quantitative determination of individual, isolated substances is possible, but the aim of a technical application is the quantitative analysis from mixtures of substances.

application effect used
rechargeable battery Redox process
Display, " smart window " Electrochromy
Cathode in electrolytic capacitors electric conductivity
Through-hole plating of printed circuit boards electric conductivity
Sensors electric conductivity
Packaging film for electronic components electric conductivity

literature

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  1. ↑ Entry on electrically conductive polymers. In: Römpp Online . Georg Thieme Verlag, accessed on November 12, 2019.
  2. ^ GG Wallace, TE Campbell, PC Innis: Putting function into fashion: Organic conducting polymer fibers and textiles . In: Fibers and Polymers . tape 8 , no. 2 , 2007, p. 135-142 , doi : 10.1007 / BF02875782 .
  3. ↑ H.-J. Mair, S. Roth (Ed.): Electrically conductive plastics. 2nd ext. Ed., Hanser, Munich 1989, pp. 253-263.
  4. ^ Jürgen Heinze: Electronically conducting polymers . In: Eberhard Steckhan (Ed.): Electrochemistry IV. (=  Topics in Current Chemistry . Volume 152 ). Springer, Berlin / Heidelberg 1990, ISBN 3-540-51461-9 , pp. 1-47 , doi : 10.1007 / BFb0034363 .
  5. ↑ Junting Lei, Zhihua Cai, Charles R. Martin: Effect of reagent concentrations used to synthesize polypyrrole on the chemical characteristics and optical and electronic properties of the resulting polymer . In: Synthetic Metals . tape 46 , no. 1 , 1992, p. 53-69 , doi : 10.1016 / 0379-6779 (92) 90318-D .

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