Alan J. Heeger and Alan G. MacDiarmid treated polyacetylene with iodine vapor to form an electrically conducting polymer. The discovery of conducting polyacetylene offered scientists both a new class of materials and the ability to modify an important property of these materials, namely, their electrical conductivity.
While polymers, or plastics, were known to possess many useful properties, their electrical conductivity
Materials may exhibit one of four types of conductivity. Insulators conduct electricity very poorly and usually are made from ceramics or common plastics. Semiconductors are mediocre electrical conductors usually containing silicon or germanium. They can be treated (doped) with very small amounts of other elements to increase their electrical conductivity. A conductor conducts electricity and heat easily. Conductors usually are metals; silver and copper are the best conductors known. Some metals, when cooled to very low temperature, offer no resistance to electricity and become superconductors.
Until 1977, few nonmetallic materials were known to be conductors. One type of compound known to conduct electricity was called linear-chain material, since its structure was made up of long chains of atoms. The impetus for the study of nonmetallic materials was provided in 1964 by W. A. Little
In the early 1970’s, Alan J. Heeger, Alan G. MacDiarmid, and others found polythiazyl to have good electrical conductivity, particularly along the direction of the chain. The conductivity in one direction is many times greater than other directions. This led these compounds to be termed “one-dimensional metals”
The stable carbon-containing compound closest in form and structure to polythiazyl is polyacetylene, a polymer of acetylene gas. A molecule of polyacetylene has a formula of (CH)x in the form of a linear zigzag chain of carbon atoms, each carbon with a single hydrogen atom attached. Polyacetylene had been known since 1955 as a useless black insulating powder. In the early 1970’s, however, a graduate student in Hideki Shirakawa’s laboratory at the Tokyo Institute of Technology was trying to make polyacetylene the usual way from acetylene gas but made a mistake in the quantities he used. Instead of a dark powder, Shirakawa’s student found he had a lustrous silver film that could be stretched like plastic food wrap. This new synthesis of polyacetylene, as developed and refined by Shirakawa, involves coating the inside of a glass vessel with a chemical catalyst that encourages polymerization, the linking together of small molecules into long polymer chains. When acetylene gas, a small molecule of four atoms, is released into the vessel, a silvery film begins to grow on the glass. Within five minutes, a layer of pure polyacetylene as thick as a piece of paper coats the vessel. After impurities are washed off, the film can be peeled from the sides of the glass and stored under vacuum or an inert gas. Since polyacetylene decomposes easily in air, preparing it demands skills in glassblowing and in vacuum-line techniques.
These skills had been developed and highly refined by MacDiarmid, Heeger, and their students in their work on silicon compounds and polythiazyl in the early 1970’s. When MacDiarmid visited Shirakawa’s laboratory in 1976, he learned about the new form of polyacetylene. He invited Shirakawa to spend a year with them at the University of Pennsylvania. With Shirakawa’s aid, Heeger and MacDiarmid were able to prepare the new form of polyacetylene and began to study its chemical and physical properties. One of the first things they did was to treat it with iodine vapor to see if polyacetylene would react with iodine in a manner similar to polythiazyl. As the iodine vapor swirled about the silvery polyacetylene film, they noticed a rapid change in color to deep gold, resembling the color of polythiazyl. Testing the film, they found that its conductivity had increased some twelve orders of magnitude until it was behaving like a metal. They had made the first of a new class of compounds known as conducting polymers. Later tests showed that the iodine had removed electrons from the carbon atoms making up the polymer chain and this was what led to increased conductivity.
Many other chemicals were added to polyacetylene to see if its conductivity could be changed. Those that worked fall into two classes: chemicals that remove electrons and chemicals that add electrons. Removing electrons is termed p-doping, while adding electrons is termed n-doping. Either technique turns polyacetylene into a golden material with electrical conductivity similar to a metal. In addition to iodine, a number of other p-dopants were found: bromine, arsenic pentafluoride, and perchloric acid. Adding electrons was technically more difficult, but eventually n-dopants like sodium or sodium naphthalide were discovered. Unlike semiconductors, polymers need much larger amounts of dopants to change them from insulators to conductors. Up to several percent of the polymer chain units must be doped, instead of a few parts per million found in semiconductors.
A major problem with chemical doping is being able to control the process. In order to solve this problem, Heeger and MacDiarmid developed electrochemical doping of polyacetylene, a process useful for all conducting polymers. If polyacetylene is immersed in certain conducting solutions and a voltage is applied to the polymer, electric current will flow and controllable doping will take place. This is a very important and useful technique, which led directly to polymer batteries. Polyacetylene became the prototype for all conducting polymers and was shown to exhibit the highest conductivity of any polymer found, almost as high as the metal copper.
The discovery of the first conducting polymer, polyacetylene, gave scientists both a new class of materials and the ability to modify an important property of these materials: their electrical conductivity. As a result, researchers could combine certain useful properties of polymers with those of a metal. They could prepare materials lighter than metals, moldable, strong, and able to conduct electricity or heat well. These polymers could be used in certain applications where the use of metals leads to problems. For example, one of the first commercial uses of conducting polymers was for electrodes in lightweight, rechargeable batteries. This avoided both material problems such as corrosion of the electrodes, the environmental problems of mining of metals, and disposal of used batteries containing toxic metals.
Conducting polymers could also be used in place of metals in plastic solar cells, which convert sunlight to electricity, and plastic electrodes in fuel cells. Conducting polymers are also “electrochromic,” meaning that through electrochemistry their color can be changed. This property makes it possible to use a thin film of conducting polymer in an electronic shade, as in a shutter or in a visual display device. Conducting polymers also made possible optical data storage and optical switches and transistors, allowing their use in computers powered by light instead of electricity.
Today conducting polymers can be found in computer-screen shields against electromagnetic radiation, “smart” windows that block sunlight, light-emitting diodes, solar cells, and telephone displays. In the field of molecular electronics, conducting polymers have made it possible to produce molecule-sized transistors that increase the speeds and reduce the sizes of our computers. In medicine, conducting polymers have led to better understanding of how membranes work and better drug delivery systems. The fact that melanin, a conducting polymer, is present in most mammalian tissues holds promise for medical treatments of human sensory systems, such as hearing. In fact, the importance of the “discovery and development of conductive polymers” led to the awarding of the Nobel Prize in Chemistry in 2000 to Shirakawa, Heeger, and MacDiarmid.
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Alper, Joseph. “Conductive Polymers Recharged.” Science 246 (October 13, 1989): 208-210. Nontechnical article on conducting polymers and their applications. Contains a special section on how researchers overcame early problems. -
Epstein, Arthur J., and Joel S. Miller. “Linear-Chain Conductors.” Scientific American 241 (October, 1979): 52-61. Early article that gives a good sense of the place of polyacetylene among linear-chain molecules. Slightly difficult to read but with good diagrams and figures. -
Hush, Noel. “An Overview of the First Half-Century of Molecular Electronics.” Annals of the New York Academy of Sciences 1006, no. 1 (2003): 1-20. A view of the history and potential future of molecular conducting materials, including DNA conductors. -
Kaner, Richard B., and Alan G. MacDiarmid. “Plastics That Conduct Electricity.” Scientific American 258(February, 1988): 106-111. One of the best and easiest-to-read articles on this subject. Written by MacDiarmid and one of his students, it contains accurate information, useful figures, and explanations of how conducting polymers work. -
Skotheim, Terje A., and John R. Reynolds. Handbook of Conducting Polymers. 3d ed. Boca Raton, Fla.: CRC Press, 2006. A fairly technical and demanding book, it is a valuable source of information on the history of polyacetylene, the many varieties of conducting polymers, and their applications.
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