Conductivity of polyacetylene

Whilst the conductivity of these polymers is generally somewhat inferior to that of metals (for example, the electrical conductivity of polyacetylenes has reached more than 400 000 S/cm compared to values for copper of about 600 000 S/cm), when comparisons are made on the basis of equal mass the situation may be reversed. Unfortunately, most of the polymers also display other disadvantages such as improcessability, poor mechanical strength, poor stability under exposure to common environmental conditions, particularly at elevated temperatures, poor storage stability leading to a loss in conductivity and poor stability in the presence of electrolytes. In spite of the involvement of a number of important companies (e.g. Allied, BASF, IBM and Rohm and Haas) commercial development has been slow however, some uses have begun to emerge. It is therefore instructive to review briefly the potential for these materials. 

Although the conductivity of polyacetylene is generally discussed in terms of solitons, the question of the precise nature of the major charge-carriers continues to be a subject of debate, with conflicting evidence from different experiments. Spectro-electrochemical studies provide evidence that the charge in doped polyacetylene is stored in soliton-like species (although this is not the only possible interpretation [142, 143]), with absorptions in the optical spectra corresponding to transitions to states located at mid-gap [24,89, 119]. The intensity of the interband transitions... 

Fig. 9.7 Thermal behaviour of the conductivity of polyacetylene in comparison with that of silver (from Kanatzidis (1990)).

The conductivity of polyacetylene can be magnified by doping. Exposure of a polyacetylene film to dry ammonia gas leads to a dramatic increase in conductivity of 10 cm) Controlled addition of an acceptor, or /r-doping, agent such as AsFs, I2,... 

The conductivity of polyacetylene is also increased by dopants that are electron donors. For example, the polymer can be doped with alkali metals to give, for example, [Li5 (CH) ln. The wide range of conductivities produced by these two forms of doping is illustrated in Figure 6.3. 

When heated, polyvinyl chloride (PVC) and polyvinyl alcohol (PVA) lose HC1 and H20, respectively, to produce dark-colored conductive polyacetylene. Superior polymers of acetylene can be made by the polymerization of acetylene with Ziegler-Natta catalysts. The conductivity of polyacetylene is increased by the addition of dopants, such as arsenic pentafluoride or sodium naphthenide. 

Table 1. Electrical Conductivity of Polyacetylene/Polybutadiene Blends as a Function of Blend Composition and Mechanical Treatment.

Figure 13.22 Schematic variation of the conductivity of polyacetylene doped with iodine. The concentration is in moles of I2 per CH unit. The conductivity changes from that typical of an insulator to that associated with a metal...

In order to understand the physical properties of polyacetylene doped with divalent ions, it is important to consider the theory of conductivity of polyacetylene doped with monovalent ions. One of the most unusual characteristics of polyacetylene is that small amounts of dopant ions give rise to enormous increases in electrical conductivity without causing any increase in the number of unpaired electrons. In fact, the small level of paramagnetism observed in pristine polyacetylene actually decreases on doping (8). This is in contrast to what occurs in traditional semiconductors, such as silicon, where dopants increase both conductivity and paramagnetism. An explanation has been offered by the soliton theory of conductivity (9,10). 

Relationship between orientation and conductivity of polyacetylene films has been studied. Polyacetylene films were... 

Conductivity of polyacetylene increases more than eight orders of magnitude when doped with iodine [1]. More interests have been paid to attention of a doped polyacetylene, since Naarmann and Theophilou synthesized highly conducting iodine doped polyacetylene [2]. 

Electrical conductivity of polyacetylene films depends on orientation of polymer chains. Figure 5 shows the relationship between relative electrical conductivity and dichroic ratio of polyacetylene films prepared by a mixture catalyst of Trihexyl aluminium and Tetradecyl titanate. Conductivity of fully stretched or oriented polyacetylene films showed twenty times higher than that of as-grown films. Conductivity of polyacetylene films, prepared by other catalysts systems of trialkyl aluminium and tetraalkyl titanate, showed similar relationship, although increment of the conductivity was lower. 

Recent studies have shown that this is not the case in polyacetylene improvements in synthesis and orientation have resulted in electrical conductivities as high as 10 S/cmA Furthermore, the absence of a metallic temperature dependence with resistivity decreasing as the temperature is lowered - implies that the measured conductivity is still limited by material imperfections. Consequendy, it is quite clear that the intrinsic electrical conductivity of polyacetylene and, by implication, of other conducting polymers, may be significantly greater than that of copper. 

Epstein, A.J. 1986. AC conductivity of polyacetylene distinguishing mechanisms of charge transport. In Handbook of conducting polymers, ed. T.A. Skotheim. New York Marcel Dekker, p. 1041. 

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