Conducting polymers: Introduction:
A polymer that can conduct electricity is known as conducting polymer.
Conducting polymers can be classified as follows.
I) Intrinsically conducting polymers:
The polymers have extensive conjugation in the backbone which is responsible for conductance. These are of two types.
- Conducting polymers having conjugated electrons in the backbone: Such polymers contain conjugated -electrons in the back bone which increases their conductivity to a large extent.
Explanation: Overlapping of conjugated electrons over the entire backbone results in the formation of valence bands as well as conduction bands that extends over the entire polymer molecule. The valence band and the conduction bands are separated by a significant band gap. Thus, electrical conduction occur only after thermal or photolytic activation of electrons to give them sufficient energy to jump the gap and reach into the lower levels of the conduction band.
All the three forms of polyanilines have conductivities of the order of 10-10
Scm-1. conductivity of these polymers having conjugated electrons in the backbone is not sufficient for their use in different applications.
- Doped conducting polymers:
In comparison to conventional polymers, the conducting polymers having conjugated electrons in the backbone can be easily oxidized for reduced as they have low ionization potentials and high electron affinities. The conductivities can be increased by creating positive or negative charge on polymer backbone by oxidation or reduction. It is a referred to as doping.
This is of two types.
- P-doping or oxidative doping
- n-doping or, reductive doping
It is done by oxidation process. P-doping involves treatment of an intrinsically conducting polymer with a lew’s acid, thereby oxidation takes place and positive charges on the polymer backbone are created. The oxidation process leads to the formation of delocalized radical ion called ‘Polaron’. A second oxidation of this polaron, followed by radical recombination yields two positive charge carriers on each chain which are mobile. These delocalized positive charges are current carriers for conduction.
n-doping is done by reduction process. N-doping invoves the treatment of an intrinsically conducting polymer with a lewis base like sodium naphthalide. n-doping leads to the formation of polaron and bipolaron in two steps. This followd by recombination of radicals yields two negative charge carriers on each chain of polyacetylene which are responsible for conduction.
II) Extrinsically conducting polymers:
These are the polymers whose conductivities due to the presence of externally added ingredients in them. These are of two types.
1) Conductive element filled polymer: In this type, the polymer acts as the binder to hold the conducting element such as carbon black, metallic fibres, metallic oxides together in the solid entity. Minimum concentration of conductive filler which should be added so that the polymer starts conducting is known as ‘percolation threshold’. Because at this concentration of filler, a conducting path is formed in polymeric material. Generally special conducting grade C-black is used as filler which has very high surface area, more porosity and more filamentous properties. These polymers possess reasonably good bulk conductivity. They are generally low in cost, light in weight, mechanically durable and strong and are easily processable in different forms, shapes and sizes.
2) Blended conducting polymers: These polymers are obtained by blending a conventional polymer with a conducting polymer. These polymers possess better physical, chemical, electrical and mechanical process and they can be easily processed. Ex: upto 40% polypyrrole will have a little effect on tensile strength and also give a much higher impact strength than obtained with a carbon-black filled compounds at only 10% loading.
Applications of Conducting Polymers:-
In rechargeable Light weight batteries based on perchlorate doped Polyacetylene-lithium Systems: These are about 10 times lighter than conventional lead storage batteries. These can produce current density up to 50mA/cm2.
- In electrochromic displays and optical filters: ICP’s can absorb visible light to give coloured products so can be useful for electrochromic displays and optical filters. Thus the conducting polymers can be used as electro chromic materials i.e., the materials that change colour reversibly during the electrochemical processes of charge and discharge.
- In wiring in aircrafts and aerospace components.
- Conducting polymers are also used for making sensors for pH,
- O2, NOx, SO2, NH3and glucose.
- In telecommunication systems.
- In electromagnetic screening materials.
- In photovoltaic devices.
In non-linear optical materials.