This article reviews the importance of conductive polymer. The big chemical company is marketing the polythiophene under the trade name Baytron. The material could also be used to make plastics paintable by adding the conductive agent first, or in the electrodes of small, high-performance tantalum capacitors found in telecommunications, computer, and automotive products. Probably the most significant commercialization of conductive polymers was for flexible, long-lived batteries that were produced in quantity by Bridgestone Corp. and Seiko Co. in Japan and by BASF/Varta in Germany. Conductive polymers are long, carbon-based chains composed of simple repeating units called monomers. The list of potential applications for conductive polymers remains a long one, and includes antiradiation coatings, batteries, catalysts, deicer panels, electrochromic windows, electromechanical actuators, embedded-array antennas, fuel cells, lithographic resists, nonlinear optics, radar dishes, and wave guides. However, how big an impact the materials will make in these markets remains unclear.
Engineers at the well-known photographic firm AGFA in Koln, Germany, were facing a critical problem with the production of photofilm in the late 1980s. Static discharges were ruining the huge, costly rolls of the company's film; induced by friction, the little electric sparks generated big losses. The engineers' investigation showed that the inorganic salts AGFA traditionally used as an antistatic coating failed to work when the humidity dropped 'below 50 percent. These water-soluble ionic compounds also washed away after developing, again leaving the photo film vulnerable to stray sparks.
AGFA turned to parent company Bayer AG in Krefeld, Germany, to see whether its central research arm could develop a new low-cost antistatic agent. The antistatic coating had to operate independent of air humidity, with a surface resistance greater than 108 ohms square; it had to be transparent and free of heavy metals; and it had to be produced from a waterborne solution.
The most promising candidate to fill these criteria was, surprisingly, an electrically conductive polymer material known as polythiophene. Such polymers have always had great conm1ercial potential because of their unusual ability (for a plastic) to provide a path for electrons, but they never found any wide commercial applications.
Following a thorough development effort involving the selection of the ideal polythiophene derivative, its subsequent synthesis, and its polymerization, the Bayer research team succeeded in inventing an aqueous processing route for the plastic coating. Today, more than 10,000 square meters of AGFA photographic film has been coated with the conductive polymer, according to Joseph T. Morrison, manager of technical service and applications development for inorganic chemicals at Bayer Corp. in Pittsburgh. "Polythiophene has won raves from AGFA."
Now the big chemical company is marketing the polythiophene under the trade name Baytron. The material, according to Morrison, could also be used to make plastics paintable by adding the conductive agent first, or in the electrodes of small ,- high-performance tantalum capacitors found in telecommunications, computer, and automotive products.
Another significant potential application is in the through-hole plating of circuit boards, he added. The chemical process of depositing the initial layers of copper into these holes requires formaldehyde, a known carcinogen. Blasberg Oberflaechentechnik in Soligen, Germany, has patented a method using poly thiophene as the first coat instead of the electroless cop- per. The new plating technology has been licensed to several Japanese circuit-board makers and to Enthone Inc., a subsidiary of ASARCO Inc. in West Haven, Conn.
Long-time researchers on conductive polymers point to Bayer's Baytron polythiophene as the most notable success story in the field. According to Matt Aldissi, president of Fractal Systems Inc. in Tampa, Fla., that success is atypical. As with most new materials, finding sufficient demand is the key to convincing manufacturers go into full-scale production. "It's always a question of finding the right niche," said Aldissi, a veteran of conductive-polymer research. Antistatic applications have a huge potential, he noted, but conductive polymers have yet to make many inroads. The once-highly touted technology has been "reduced to the point that the only successful large application- antistatic coatings for AGFA photofilm-is for internal [company] use," he said.
Companies from ABB and AlliedSignal to Westinghouse and W.R. Grace have tried to make conductive polymers into a success, but they have reportedly curtailed or aborted their research. Even though one application for the material-flat-panel displays for televisions and computers-is starting to excite researchers again, much of the payoff for this technology lies in the future.
That future looked a lot brighter for conductive polymers in the 1980s. Probably the most significant commercialization of conductive polymers was for flexible, long-lived batteries that were produced in quantity by Bridgestone Corp. and Seiko Co. in Japan and by BASF/ Varta in Germany. "Fifteen years ago, when they first came to the market, everybody was hot on conductive polymer batteries," Aldissi said. "In the end, though the batteries worked, they were difficult to sell because their costs weren't significantly lower than those of the competition," so the battery products were withdrawn due to insufficient demand. (Researchers at the Johns Hopkins Applied Physics Laboratory in Baltimore recently developed a nontoxic, flexible, all-plastic battery made from another class of conductive plastics called fluorophenylthiophenes, but little is expected of the technology.)
Another once-promising product incorporating conductive polymers is Contex, a fiber that has been manufactured by Milliken & Co. in Spartanburg, S.C., since 1990. The fiber is coated with a conductive-polymer material called polypyrrole and can be woven to create an antistatic fabric. Milliken had been interested in using this type of antistatic technology for its carpet products.
The material's best chance for success was in military applications. Polypyrrole had been approved for use in the U.S. Navy's A-12 stealth attack carrier aircraft, said Hans H. Kuhn, Milliken Research Fellow at Milliken Research Corp. The polymer was to be used in edge cards-components that dissipate incoming radar energy by conducting electric charge across a gradient of increasing resistance that the plastic material produces. The A-12 program has been canceled, however.
Milliken also tried to market ultralight camouflage netting based on Contex to help conceal military equipment and personnel from near-infrared and radar detection, but the company lost a U.S. Army contract for conductive camouflage material last fall, Kuhn said. Despite a recent modest contract with NASA to produce conductive- polymer electromagnetic shielding for the space shuttle, Milliken's research program is now in jeopardy, and will probably be either sold or canceled.
Despite their ups and downs, electrically conductive polymers have attracted a substantial amount of attention since they were accidentally discovered two decades ago, when a Tokyo Institute of Technology student added too much catalyst to a batch of polyacetylene. When the resulting silvery film was later doped with various oxidizing agents at the University of Pennsylvania in Philadelphia, it became conductive, and the race was on to invent new conductive polymers.
Conductive polymers are long, carbon-based chains composed of simple repeating units called monomers. When the Japanese student made his fortuitous error, he converted the standard single-bond carbon chains to polymer backbones with alternating single and double bonds, a change that provided a pathway for free-electron- charge carriers. To make the altered polymer materials conductive, they are doped with atoms that donate negative or positive charges (oxidizing or reducing agents) to each unit, enabling current to travel down the chain. Depending on the dopant, conductive polymers exhibit either p-type or n-type conductivity.
The most extensively studied conductive-polymer systems are based on polyaniline, polythiophene, polypyrrole, and polyacetylene. The principal attractions of these polymers over conventional conducting materials are their potential ease of processing, relative robustness, and light weight. Successful commercial applications require a fine balance of conductivity, processability, and stability, but until recently, materials researchers could not obtain all three properties simultaneously.
Conductive polymers are much more electrically conductive than standard polymers but much less than metals such as copper. In practice, the conductivity of these materials is characterized by low-charge carrier mobility-a measure of how easily electric charge moves. This characteristic limits response speed in the case of a transistor, for example, making such a device rather inefficient.
Still, efforts to produce semiconductor devices from conductive polymers are proceeding. In 1994, a team led by Francis Gamier at the Laboratory of Molecular Materials in Thais, France, made a field-effect transistor from polythiophene using printing techniques. Rolling up, bending, and twisting did not effect the transistor's electrical characteristics.
The opportunity to produce relatively low-cost semiconductor devices that are insensitive to mechanical deformation is an attractive one. Probably the most exciting development in this area is the intensifying effort to use conductive polymers to produce flat, flexible plastic screens for TVs and computers. This screen technology emerged from the discovery that certain conductive polymers, such as poly-p-phenylenevinylene, emit light when sandwiched between oppositely charged electrodes, a configuration that fits in well with current flat-panel display designs.
The current leader in this work is Cambridge Display Technology (CDT) in Cambridge, England, which was founded by Cambridge University physics professor Richard Friend. CDT recently entered into a collaboration with Japanese electronics maker Seiko-Epson to develop light-emitting polymer screens. Philips Electronics NV in the Netherlands is also working on a portable telephone using such a display. Other licensees include Hoechst AG in Germany and Uniax Corp. in Santa Barbara, Calif. While it is likely to be some time before this technology makes it to the market in flexible flat-panel screens, the development work has created a whole new buzz about conductive polymers.
Another promising application is in capacitor technology, where "there's been a lot of progress, due mainly to federal funding of ultracapacitors for future electric vehicles," said Fractal's Aldissi. Kemet Electronics Corp. in Greenville, S.C., is working on using polythiophene or polypyrrole to replace manganese dioxide counterelectrodes in tantalum surface-mount capacitors, which are widely used in the electronics industry. "Conductive polymers can provide lower equivalent-series resistance [ESR]," said Philip Lessner, technical associate at Kemet. "With the designers of mobile electronics constantly being pushed for space, the new capacitors can simultaneously be smaller and have a lower ESR."
Kemet is operating a small pilot line to produce the electrodes. Lessner predicted that a capacitor product using conductive polymer would be available within 12 to 18 months. If this comes to pass, high-volume production could follow. After all, the company could make from 60 million to 100 million standard tantalum capacitors next year.
Yet another emerging application for electrically conductive polymer materials is biosensors and chemical sensors, which can convert chemical information into a measurable electrical response. Abtech Scientific Inc. in Yardley, Pa., is making chemical transducers from mostly polyaniline as well as polythiophene and polypropylene for analytical applications "in which one measures conductivity and as a result infers what the chemical composition is," said Anthony Guiseppi-Elie, the company's president and scientific director. In other words, a very small change in the redox composition brought about by small quantities of a range of chemicals can induce a large, rapid change in electrical conductivity.
"The challenge," he said, "is how to confer specificity to these materials." One way is to build biopolymer/ conductive-polymer complexes. Using this technique, Abtech has developed a range of enzyme biosensors. For example, immobilized glucose oxidase can be incorporated into this polymer transducer system, which acts like a glucose-sensitive biosensor, as the enzyme-catalyzed oxidation of the glucose produces an oxidant by-product that is measured indirectly. Levels of therapeutic drugs in patients can also be monitored in a similar way.
Abtech is developing the technology for "point-of- care testing by physicians," a market that Guiseppi-Elie said is of great interest to several major medical-product companies. The disposable point-of-care-testing product will be used to make many medical tests much cheaper, he noted.
An area with some further-off potential-smart membranes of conductive polymers-is being pursued by a team at Los Alamos National Laboratory in Los Alamos, N.M. Benjamin Mattes, technical project leader in the lab's Chemical Sciences and Technical Development Division, and his colleagues have developed engineered porous-fiber materials with electrically controlled porosity using polyaniline. The technology, he said, could find use in gas separation, pharmaceutical separation, environmental cleanup, batteries, or capacitors. A spin-off company to develop the technology already has been established.
The list of potential applications for conductive polymers remains a long one, and includes antiradiation coatings, batteries, catalysts, deicer panels, electrochromic windows, electromechanical actuators, embedded-array antennas, fuel cells, lithographic resists, nonlinear optics, radar dishes, and wave guides. Just how big an impact the materials will make in these markets remains unclear, however. Most observers are putting their money on antistatic coatings and flat-panel displays. Said Guiseppi-Elie, "Neither [application] is not going to be big winner initially because they're displacing other established approaches, but they do have promise, especially if they're not oversold."
Development of flexible flat-panel screens has created a whole new buzz about conductive polymers.