Electronic conduction in polymers

Polymers appear at first sight to be most unlikely materials to exhibit electronic conduction. A polymer such as polyethylene contains only fully saturated chemical bonds, each with two electrons closely bound to particular atoms, which thus constitute a filled valence band there are no free electrons. Some polymers, however, contain unsaturated bonds in the backbone. An important example is polyacetylene, -(CH=CH, which has alternating single and double bonds in the backbone. Examples of other polymers with such alternation are given in fig. 9.6. Where a number of double bonds alternate in this way the phenomenon of resonance usually occurs and all the bonds tend to become similar. Notice, however, that, for all the molecules shown in fig. 9.6, except for truM5-polyacetylene (t-PA), 

Benzene is the extreme example of a molecule that at first sight might be expected to exhibit bond alternation, but in this molecule all the C—C bonds become completely equivalent. Only three of the four electrons on each carbon atom are involved in sp hybrid atomic orbitals bonding to the hydrogen atom and the two adjacent carbon atoms, in so-called cr bonds. The fourth electron takes up a /) orbital, which is largely situated at right angles to the plane of the molecule (fig. 9.7(a)). These p orbitals are fairly extended in space and overlap to form a pair of n-electron clouds on each side of the plane of the molecule, as illustrated in fig. 9.7(b). 

A similar phenomenon takes place for the molecule of 1,3-butadiene, CH2=CH—CH=CH2, with sausage-shaped 7r-electron clouds on each side of the plane of the molecule (see fig. 9.7(c)), but here the central C—C bond remains slightly different from the other two. The tendency for all bonds to become equal in such a structure is expected to be particularly strong for the planar zigzag form of t-PA, for which the choice of which alternate bonds are single and which double seems arbitrary. If the bonds 

The bond alternation means that the repeat unit is twice the length that it would be if the bonds were equivalent, so that there are two nearly free electrons per repeat unit and the band gap therefore appears at the Fermi level. The magnitude of this energy gap for /-PA turns out be about 1.5 eV. There can thus be no metallic-type conductivity and intrinsic /-PA is a semiconductor. In the other polymers of the type shown in fig. 9.6 the backbone bonds are intrinsically non-equivalent and a band gap therefore exists for them too, so that they also cannot exhibit metallie-type conduetivity. 

A complete understanding of the conduction processes both in intrinsic and in doped polymers has not yet been obtained. It is clear that at least two types of processes are required charge transport along the 

Electrical conductivity of materials is a property which spans a very wide range, as may be judged from the conductivity chart (Fig. 4.1). The conductivity of insulators is typically less than 10-12 that of semi-insulating or 

Electrical conduction may occur through the movement of either electrons or ions. In each case a suitable starting point for discussion of the conduction process is, however, the basic equation 

Superconductors (Metals, oxides, organic charge transfer salt) 

There may be contributions to the conductivity from several different types of carrier, notably electrons and holes (a hole is an electron vacancy carrying an equivalent positive charge) in electronic conductors, and cation and anion pairs in ionic conductors. Theories of conduction aim to explain how n and fj. are determined by molecular structure and how they depend on such factors as temperature and applied field. In addition, in polymers the mobility will be affected by the sample morphology. Just as a large range of conductivity is observed for different materials so there is a large range of mobility values. Data for a selection of systems are displayed on the mobility chart (Fig. 4.2). 

In polymers with chemically saturated structures, i.e. cr-bonded backbones, it is very difficult to observe any electronic conductivity at all, and what 

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When we consider electronic conduction in polymers, it will become clear that band theory is not totally suitable because the atoms are covalently bonded to one another, forming polymeric chains that experience weak intermolecular interactions. Thus, macroscopic conduction will require electron movement, not only along chains but also from one chain to another. 

The most obvious electronic conductors are metals, in which the current carriers can be regarded to a first approximation as free electrons. Before discussing electronic conductivity in polymers it is useful to consider briefly the mechanisms for conduction in metals and semiconductors. 

Bolto, B.A., Mcneill, R., Weiss, D., 1963. Electronic conduction in polymers. 111. Electronic properties of polypyrrole. Aust.J. Chem. 16,1090-1103. 

R. McNeill, R. Siudak, J. Wardlaw, D. Weiss, Electronic Conduction in Polymers. I. The Chemical Structure of Polypyrrole. Aust. J. Chem. 1963,16, 1056. 

Bolto BA, McNeil R, Weiss DE (1963) Electronic conductions in polymers III electronic properties of polypyrrole. Aust J Chem 16 1090-1103... 

T.R. Farhat, P.T. Hammond, Engineering ionic and electronic conductivity in polymer catalytic electrodes using the layer-by-layer technique, Chem. Mater. 18 (2006) 41-49. 

B. A. Bolto and D. E. Weiss. 1963. Electronic conduction in polymers II. The electrochemical reduction of pol5q)yrrole at controlled potential. Aust J Chem 16(6) 1076-1089. 

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