The Structure Of Polypyrrole

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The structure of polypyrrole consists of long conjugated polymer backbone as shown in Fig. 1.2b. The polymer has resonance structures that resemble aromatic or quinoid forms. In this neutral state the polymer is not conducting and it conducts only when it is oxidised. Polypyrrole is an insulator but its oxidized derivatives are good conductors. The Conductivity ranges from 2 to 100 S/cm. The charge associated with the oxidised state is typically delocalised over several units and can form a radical cation (polaron) or a dication (bipolaron).
Polypyrrole is poorly crystalline, with crystalline regions randomly oriented and the polymer chains lying with the pyrrole rings coplanar to the surface . Actually polypyrrole obtained so far
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Polypyrrole is a conducting polymer which has non-degenerate conduction band in the ground state. The polarons and bipolarons are the dominant charge carriers in these polymers. The most widely accepted method of conductivity in these systems involve charge transport along the polymer chains, as well as hopping of carriers (holes, bipolarons). A common feature of intrinsically conducting polymers is the alternate double and single bond in the polymer backbone , referred as pi bond conjugation. The conductivity is due to four conditions namely, the existence of charge carriers, an overlap of molecular orbits to aid mobility of the carriers, mobility of pi bond and hopping of charges between polymer chains. These conducting polymers are similar to semiconductors with a filled valence band and an empty conduction band separated by energy gap. when polymers are doped new bands are created in the gap, making the paths for the electrons to move through these bands and increasing the conductivity of the materials. In the undoped form Polypyrrole is a insulator. The electronic and transport properties of the conducting polymers are mainly due to the major role of bipolarons. The positive charges commonly called as polarons created on the polymer backbone are responsible for electrical conductivity. The conductivity of polypyrrole depends on synthesis…show more content…
When the polymer is doped partially, islands of conducting material are developed, which are oxidized sites on the polymer chain. A small percentage of the sites form a network, yielding a small , finite DC conductivity. Hopping of charge carriers between favourable sites accounts for AC conductivity. Thus the conductivity is a combination of frequency dependent AC conductivity and DC conductivity. At higher level of doping, an increase in the network density through the polymer chain results in elimination of number of isolated sites which give rise to AC conductivity. So at the limit of very high doping the number of isolated sites is less and the DC conductivity dominates the total conductivity.
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