To this day, I vividly recall my tragic attempts in product design class at high school. Our teacher, underpaid and underprepared, threw caution to the wind and let us do as we pleased. The only instructions we were given was that we had to make something that could be used as a night light. To this end, I set about creating a tacky rocket-themed monstrosity, complete with a flashing line of LED lights. I was so proud. I wasn’t aware then that a Kiwi scientist from Masterton had helped discover and develop the conductive polymers that allowed these little lights to flash obnoxiously.
When I think of materials that conduct electricity, I imagine burnished copper saucepan bottoms, plugs and the static on metal handrails. I certainly don’t think of plastic. It has long been established that electrical conduction can occur in metals and liquids containing ions; we’ve all heard about Benjamin Franklin running around in the storm, clutching his kite and metal key. In the 20th century, we discovered that some electrical insulators can be transformed into conductors of electricity; solid crystalline insulators can be heated to a high temperature and gases may have an electrical discharge created in them. Plastic can safely insulate wire in electrical leads and devices. Yet in 2000, Alan MacDiarmid, Alan Heeger, and Hideki Shirakawa received the Nobel Prize in Chemistry for their work on “conductive polymers.” Plastic can, it seems, also conduct electricity.
The discovery of these ‘conductive polymers’ began with a fortuitous accident. In the early 1970s, a Japanese graduate student was bumbling around in his laboratory one evening, trying to repeat the synthesis of polyacetylene, a dark powder made by linking together the molecules of ordinary acetylene welding gas. However, instead of a black powder, the student found that the chemical reaction had produced a film coating the inside of his glass reaction vessel that resembled aluminium foil. Evidently, he had been far too liberal with the catalyst.
Soon, news of this strange foil-like film reached Alan MacDiarmid at the University of Pennsylvania. Hailing from Masterton, New Zealand, MacDiarmid had discovered an interest in chemistry after reading one of his father’s old textbooks at the age of ten. Realising that this new form of polyacetylene resembled a metal, MacDiarmid conjectured that it might be able to conduct electricity as well. He invited the student’s instructor, Hideki Shirakawa, to join him and soon the collaboration led to exciting findings. Polyacetylene exhibited surprisingly high electrical conductivity.
Put simply, electricity in metals is simply the manifestation of the movement of free electrons that are not tightly bound to one simple atom. In semiconductors, such as those found in transistors, electricity is the flow of excess electrons to form a negative current. These excess electrons are donated by impurity or dopant atoms. MacDiarmid and his colleagues deduced that polyacetylene’s ability to conduct electricity was probably promoted by trace impurities contributed by the catalysts involved in the Japanese student’s process. They reasoned that it was possible to chemically ‘dope’ polyacetylene by exposing it to traces of iodine or bromine vapour to create mobile excess electrons. Polyacetylene’s electrical conductivity could thus be made to range from that of an insulator, like wood, to a conductor, like copper.
A major breakthrough occurred in 1979, when one of MacDiarmid’s graduate students began investigating alternative ways of doping polyacetylene. Two strips of polyacetylene were placed in a solution containing the doping ions, and an electric current was passed from strip to strip. Unsurprisingly, the positive ions migrated to one strip and the negative ions to the other. However, when the current source was removed, the charge remained stored in the polyacetylene polymer. This stored charge could then be discharged if an electrical load was connected between the two strips; just like a conventional battery.
These plastic batteries heralded a longer recharge-cycle lifetime. Conventional metal-based rechargeable batteries involve material from one plate migrating to another plate and back in a reversible chemical reaction. However in a conducting plastic battery, only the stored ions in the solution move. The plates are not consumed and reconstituted.
Polymer batteries herald many exciting potential applications, one of them being in battery powered automobiles. A battery’s suitability for automotive application depends on two measures, the first being power density, which determines acceleration and hill-climbing ability. Polyacetylene’s power density is twelve times that of ordinary lead acid batteries. Secondly, energy density determines the number of kilometres that can be driven between charges. In this regard, polyacetylene also measures up. Its energy density is about about 50 watts-hours per kilogram compared with the 35 for lead acid batteries. Moreover, plastic batteries are comparatively environmentally benign.
Yet polyacetylene isn’t perfect. It is brittle, degrades in air and is chemically stable in only liquid solutions. MacDiarmid and his team, alongside industrial associates, thus began to search for alternative conducting polymers with greater structural strength, flexibility and thermoplasticity. Polyparaphenylene, a black powder capable of being formed into plates by hot pressing was synthesised by Allied Corporation. Other suitable plastics were discovered, such as polyaniline. Stable in both air and water, polyaniline is the material used in plastic batteries.
Conductive plastics have been employed in numerous useful and innovative ways, from lightweight electromagnetic shields to smart windows that can vary the amount of light they let through. Transistors, LEDs, cell phone screens and lasers all owe a significant debt to MacDiarmid and his colleagues. Organic polymers are cheaper, lighter, easier to manufacture, and more flexible than their inorganic alternatives. They truly are the electronics of the future, conducting electricity at the forefront of nanotechnology. Indeed, as the press release accompanying Alan’s Nobel citation reads, “a computer corresponding to what we now carry around in our bags would suddenly fit inside a watch.”