Most plastics are known as excellent electrical insulators, blocking electric current. This property makes them invaluable for safety, such as the protective coatings on electrical wires that prevent short circuits and protect us from shock. However, scientific advancements have challenged this conventional view. A unique class of plastics, known as conductive polymers, possesses the remarkable ability to conduct electricity, opening new avenues in material science. These specialized materials bridge the gap between traditional plastics and metals, offering properties previously thought impossible for organic compounds.
Understanding Electrical Conductivity
The ability of a material to conduct electricity depends on the movement of electrons within its atomic structure. Materials that readily permit electric current to flow are called conductors. In these substances, such as metals, outer electrons are not tightly bound to individual atoms; instead, they form a “sea” of delocalized electrons that can move freely throughout the material. When an electrical potential is applied, these mobile electrons are directed, creating an electric current.
Conversely, insulators are materials where electrons are tightly bound to their atoms, restricting their free movement. This tight binding means that even with an applied voltage, electrons cannot easily flow, thus impeding the passage of electricity. Common examples include glass, rubber, and most conventional plastics.
A third category, semiconductors, exhibit electrical conductivity that lies between conductors and insulators. Their unique property is that their conductivity can be precisely adjusted by introducing small amounts of impurities, a process known as doping. This control over electron flow makes semiconductors essential for modern electronic devices.
The Discovery of Conducting Polymers
The groundbreaking discovery of electrically conductive polymers emerged from an unexpected series of events in the 1970s. At the time, plastics were universally considered electrical insulators. This perception began to change through the work of three scientists: Hideki Shirakawa, Alan MacDiarmid, and Alan Heeger.
Shirakawa, a Japanese chemist, was initially researching polyacetylene, a polymer that existed as a black powder. In 1974, he developed a new method to synthesize polyacetylene as a silvery film. While this film had a metallic appearance, it did not initially conduct electricity.
A significant breakthrough occurred when, by accident, a thousand-fold too much catalyst was added during a synthesis, leading to a highly organized film of polyacetylene. Collaborating in 1977, Shirakawa, MacDiarmid, and Heeger further experimented with this material. They discovered that by introducing certain impurities, specifically by “doping” polyacetylene with iodine vapor, its electrical conductivity increased by an astonishing billion times, approaching the conductivity of metals. This finding challenged scientific beliefs and founded a new field of materials science. Their pioneering research was recognized with the Nobel Prize in Chemistry in 2000, awarded for the discovery and development of conductive polymers.
How Conducting Plastics Work
The unique ability of conductive plastics to carry an electric current stems from their distinct molecular structure. Unlike conventional plastics where electrons are tightly bound, conducting polymers feature a “conjugated” backbone. This means they have a repeating pattern of alternating single and double bonds between carbon atoms along their polymer chain. This arrangement allows pi-electrons in these double bonds to become delocalized, spreading along the polymer chain.
While this delocalization is a prerequisite, it alone is not sufficient for high conductivity; undoped conjugated polymers are typically semiconductors or insulators. To achieve significant electrical flow, these polymers undergo a process called “doping.” Doping involves introducing chemical impurities that either add or remove electrons from the polymer backbone. This is analogous to how traditional inorganic semiconductors are made conductive.
When electrons are removed (p-doping), it creates positively charged “holes.” Adding electrons (n-doping) introduces negative charge carriers. These charge carriers then move along the delocalized electron pathways, effectively carrying an electric current. The greater the concentration and mobility of these charge carriers, the higher the material’s electrical conductivity.
Applications of Conducting Polymers
The unique combination of electrical conductivity and polymer properties has led to a wide array of applications for conducting plastics. Their flexibility, light weight, and ease of processing make them attractive alternatives to traditional inorganic conductors in many emerging technologies.
One significant application is in organic light-emitting diodes (OLEDs) for flexible displays in smartphones and televisions. Conducting polymers are also fundamental to the development of organic solar cells, offering a lightweight and flexible option for converting sunlight into electricity. They are also employed in various sensors, including chemical and biosensors, responding to specific substances. They find use in antistatic coatings to prevent static electricity buildup and in lightweight, rechargeable batteries. The continued research into conducting polymers promises further innovations across electronics, energy, and biomedical fields.