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Conducting Polymer by imran aziz

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					                                       Conducting Polymer


                                   MOHAMMAD IMRAN AZIZ
                                      Assistant Professor
                                     PHYSICS DEPARTMENT
                         SHIBLI NATIONAL COLLEGE, AZAMGARH (India).



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                                              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.


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                                       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.

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                            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.
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            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.

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      • 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.
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         Band structure in an electronically
                conducting polymer




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                        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.


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




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       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.



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      • The conductivity of a conducting polymer is
        related to the number of charge carriers n and
        their mobility ì :
                                                             ó á nì
      • 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
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      • 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.



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