Polymers that conduct electricity are now of huge interest in the quest for cheap, tough and flexible electronic circuits. But although some of these materials conduct as well as any true metal, they do not pass as metals in other respects. In particular, their conductivity does not rise as they are cooled, which is the hallmark of metallic behaviour. Now, however, physicists in Korea and the US have developed a new technique that gives a certain polymer -- polyaniline -- many of the physical characteristics of a real metal (Nature 441 65).
The new metallic polymer was developed by Kwanghee Lee of the Pusan National University and Suck-Hyan Lee at Ajou University — together with colleagues at the University of California at Santa Barbara. Polyaniline is already widely used as the active component in transistors, light-emitting diodes and photovoltaic diodes because of its good semiconducting properties. It was first found to conduct electricity as long as 1977 and we know that it conducts because electrons move along the backbone of the carbon chain making up the polymer.
However, polyaniline and other conducting polymers do not normally behave as real metals because they are too structurally disordered. Free electrons are therefore “scattered” within the material by the disorder. In metals, in contrast, the electrons are mainly scattered by thermal vibrations of atoms. Since the atoms vibrate less as the metal is cooled, it conducts better at low temperatures
The new polyaniline films show both the optical and electrical properties characteristic of metals. The team made the material by doping it with sulphonic acid and synthesizing it within oily droplets suspended in water. This new technique produced films that are more structurally ordered than polyaniline made using conventional methods.
According to the researchers, the films have a conductivity of about 103 siemens per centimetre at room temperature. Although this is not very high compared to a real metal such as copper, which has a conductivity of 6 x 105 siemens per centimetre under the same conditions, the films are about two and half times more conducting when they are cooled down to 4 Kelvin. Furthermore, the films are shiny and reflect light — albeit in the infrared range of wavelengths. According to the scientists, the optical reflectivity of the plastic in this range fits the model for a simple metal.
But there is a sting in the tail to the new work. Although showing that a polymer can behave like a metal is a real breakthrough, it sets a limit on the conductivity we can expect from such materials in future.
