We are very much familiar with the word ‘Plastic or Polymer’. Not only in our daily lives but also in the fields of industry and agriculture, plastics and plastic products have become indispensible.
Plastics are good insulators. They are used to insulate electric wire made of copper or aluminium. Suppose you get a shock. Then what will you think? That there must be a breakage on the surface of the wire or the wire is moistured. You check the wire very cautiously, but hardly find any issue. Then what? Didn’t you get any shock? Was it a mistake? The answer is no. Actually, the plastic used for insulating the wire was conducting polymer.
There is a long history behind the discovery and development of conducting plastics. For its discovery and development, Alan.J.Heeger of America, Allen MacDiarmid of New Zealand and Hideki Shirakaua of Japan were awarded the Nobel prize in Chemistry in 2000. Initially there was very limited application of this kind of plastic, but gradually it is being used in different fields of Electrical and Electronics.
The question arises : how does electric current passes through conducting plastics? It will be very easy to understand the mechanism if we are able to state that the mechanism of conduction property of any material depends upon the presence of free electron or free ion in it. Metals such as copper and aluminium are able to conduct electricity because of the presence of huge amount of free electrons. These free electrons delocalise over the whole metal and thereby produce electric current through the metal. On the other hand, water cannot conduct electricity. But if little amount of foreign impurities that produces free ion e.g. salt, acid are added to water, then it can conduct electricity.
In general plastic such as polyethylene (PE), polyvinyl chloride(PVC) are insulators because of the non-availability of electric current. But plastics such as polyaniline and polyacetylene possess free electrons which make them conductors.
Let us discuss the mechanism of conducting electric current. Polyacetylene which can be produced through polymerization reaction of acetylene(C2H2), has the value of specific conductance or conductivity in the order of 1.7 x 10-9 S/cm. This amount of conductivity is too low to use in practical fields. To enhance the conductivity value, doping is one of the usuable methods, where few amount of foreign chemicals are added to the plastic. Related researches reveal that doped polyacetylene has conductivity value comparable with good conductors e.g. copper or aluminium.
Internal molecular structure of polyacetylene tells that alternative single and double bonds are present [note that any matter is an array of several molecules which are formed by adjoining atoms through single, double or triple bonds. Bonds are made of electrons]. The electrons present in double bonds of polyacetylene are more loosely bound than that in single bonds. And those loosely bound electrons delocalise over the whole chain of the polymer molecule and as a result electric current is obtained.
Conducting plastics are of great use due to lighter weight, easy processing as well as good mechanical strength. Their important applications are in :
- rechargeable light weight batteries
- optical display devices e.g. LED, OLED
- Solar cell
- telecommunication system
- analytical sensors
Conducting plastics have several advantages over metals but also have some disadvantages. They decompose prior to melting and are insoluble in common solvents. Due to these disadvantages, use of conducting plastic remains at research level. But we know that journey of science never stops, and it is always going forward…