The STEM Chicksmas Day 9: Conductive Polymers

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

Day Nine: The 2000 Nobel Prize in Chemistry.

The STEM Chicksmas Nobel Prize 2000 Chemistry
Source: Nobel Prize Summary and Nobel Prize Popular Information

When we think of plastics, we generally think of them as insulators, materials that inhibit the flow of electrical current. A common application of this insulating ability is the plastic that coats wires to protect us from receiving a shock if we touch them. However, what if plastics could conduct electrical charge the way metals can? It turns out that certain plastics can conduct—but only if you make them correctly. This discovery by Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa earned the Nobel Prize in Chemistry in year 2000.

The conducting ability of a material is determined by its electronic structure. The energy of electrons is quantized, which means electrons can only be at certain energy levels. However, this also means that electrons cannot exist within a certain energy range. This forbidden energy range is called the band gap, and in solid materials it is the gap between the conduction and the valence band. The valence band is the band where electrons exist in a non-excited material. If a voltage is applied to the material and the band gap is small enough, electrons can jump to the conduction band. However, this is only possible if the valence band is not fully occupied. This movement of electrons is an electrical current! Using band gaps we can define three types of materials: conductors (no band gap, like metals), semi-conductors (a small band gap), and insulators (a very large band gap). In general, the longer a material is, the smaller the band gap gets. This is relevant when talking about plastics, because chain length is a very important factor.

When we talk about plastics we’re actually talking about polymers, very large molecules made up of a chain of repeating units. In organic polymers, this is a carbon chain. The unit can include double bonds, atoms such as oxygen or chloride, or functional groups like alcohols. The groups that are on the carbon chain determine its properties. The type of polymer the Nobel Prize winners looked at were organic polymers with alternating double bonds, specifically polyacetylene. They found that when they “doped” these polymers, they became conductive. When you dope a substance, you introduce a hole (“p-doping) or an electron (“n-doping”) into the chain via a chemical reaction, either reduction or oxidation. This is important because generally, carbon chains have fully occupied valence bands. If you p-dope it, you turn one of the double-bonds into a single electron. Since an electron is missing, it appears that you have a positive charge which we call the hole. The single electron and hole can move along the chain fairly easily. If you n-dope it, a more complicated mechanism causes the transport of charges across the molecule.

By discovering conductive plastics, Shirakawa et al revolutionized the future of electronics. Plastics are cheap and easy to produce, and making them conductive also is a fairly simple chemical reaction. They can potentially be used in organic solar cells and biosensors. It is difficult to scale up their processing to industrial scales, but if this can be achieved conductive polymers could be the future of our devices.

To read more, check out the Nobel Prize website here.

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