Conducting Polymer

MOHAMMAD IMRAN AZIZAssistant Professor

PHYSICS DEPARTMENTSHIBLI NATIONAL COLLEGE, AZAMGARH (India).

Precious Polymer

• Polymers are usually organic substances composed of a very large number of like molecules.

• Polymers are divided, on the basis of their mechanical properties and strength, into three categories: rubbers or elastomers,plastics and fibers.

Conducting Polymer

• Polymers are generally insulators because the organic molecules of which they are composed have no free electrons to carry current; all the electrons are held firmly by atoms forming the molecules.

• Polymers in which the carbon atoms in the backbone are linked by double bonds have the potential to conduct electricity, especially when a number of such bonds occur in the vicinity of each other.

Why Conducting plastic ?• The electrons in one of the two bonds of the

double bond are held a little bit less strongly and can get away from the atoms forming the bond.

• The electrons in the weaker of the two bonds are shared laterally or sideways while those in the stronger bond are shared in a head-on manner.

• The electrons can leave the atom to which they origanally belonged and travel along the length of the polymer chain.

• The polymer is weakly conducting or semiconducting.

Approach of MacDiarmid,Shirakawa and Heeger

• Iodine has a great yearning for electrons and it grabbed electrons from the bonds in the polymer backbone.

• In the process, the polymer backbone acquired positive charge or holes, as they are called. The positive holes can travel very easily and provide a method of transfer of electric charge through the polymer.

• Addition of an electron acceptor like iodine is called doping. The dopants could be electron donors also, in which case extra electrons added to the polymer impart a negative charge.

• The polymer chain again becomes electrically unstable and applying a voltage makes the electrons run from one end to the other.

• Another commonly used dopant is a compound of arsenic and fluorine,AsF5

• Doping of polyacetylene with AsF5 raises its conductivity by 10 orders of magnitude.

• The conductivity of doped polyacetylene is three times that of copper.

• Other conducting polymers are polypyrrole,polythiophene and polyaniline.

Band structure in an electronically conducting polymer

Mechanism of Conductivity

• It is generally agreed that the mechanism of conductivity in these polymers is based on the motion of charged defects within the conjugated framework. The charge carriers, either positive p-type or negative n-type, are the products of oxidizing or reducing the polymer respectively. The following overview describes these processes in the context of p-type carriers although the concepts are equally applicable to n-type carriers.

Positively charged defects on poly(p-phenylene). A: polaron B: bipolaron

Oxidation of the polymer initially generates a radical cation with both spin and charge. Borrowing from solid state physics terminology, this species is referred to as a polaron and comprises both the hole site and the structural distortion which accompanies it. This condition is depicted in Figure A. The cation and radical form a bound species, since any increase in the distance between them would necessitate the creation of additional higher energy quinoid units. Theoretical treatments have demonstrated that two nearby polarons combine to form the lower energy bipolaron shown in Figure B. One bipolaron is more stable than two polarons despite the coulombic repulsion of the two ions. Since the defect is simply a boundary between two moieties of equal energy -- the infinite conjugation chain on either side -- it can migrate in either direction without affecting the energy of the backbone, provided that there is no significant energy barrier to the process. It is this charge carrier mobility that leads to the high conductivity of these polymers.

• The conductivity of a conducting polymer is related to the number of charge carriers n and their mobility μ :

σ α nμ• Because the band gap of conjugated polymers is usually fairly large, n is

very small under ambient conditions. Consequently, conjugated polymers are insulators in their neutral state and no intrinsically conducting organic polymer is known at this time. A polymer can be made conductive by oxidation (p-doping) and/or, less frequently, reduction (n-doping) of the polymer either by chemical or electrochemical means, generating the mobile charge carriers described earlier

• Alan MacDiarmid:

• Research on conductive polymers has also fueled the rapid development of molecular electronics. In the future scientists may be able to produce transistors and other electronic components consisting of individual molecules, dramatically increasing the speed and reducing the size of computers: a computer corresponding to the laptops we now carry around suddenly fits inside a wristwatch.