Polyacetylene dopants

Other examples of dopants that can oxidise polyacetylene are I2, AsFs and HClOy. The effect of these dopants can be to raise the conductivity from 10 S m to as much as 10 S m using only small quantities of dopant. 

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. 

If an electron acceptor is added, it takes electrons from the lower n bonding band. The doped polyacetylene now has holes in its valence band and, like p-type semiconductors, has a higher conductivity than the undoped material. Electron donor dopants add electrons to the upper n band, making this partly full, and so producing an n-type semiconductor. 

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. 

The soliton conductivity model for rrans-(CH) was put forward by Kivelson [115]. It was shown that at low temperature phonon assisted electron hopping between soliton-bound states may be the dominant conduction process in a lightly doped one - dimensional Peierls system such as polyacetylene. The presence of disorder, as represented by a spatially random distribution of charged dopant molecules causes the hopping conduction pathway to be essentially three dimensional. At the photoexitation stage, mainly neutral solitons have to be formed. These solitons maintain the soliton bands. The transport processes have to be hopping ones with a highly expressed dispersive... 

Table 2. Conductivities in polyacetylenes in presence of metallic dopants...

A report on doping of polyacetylene by metal halides 462-463) shows that the interplanar spacing increases with the size of the anion and clustering is inferred to occur at low dopant levels as the dopant reflection appears at about 3 mol% while much of the material is still undoped. It is not totally clear whether similar effects might be the result of a combination of slow dopant diffusion and a diffusion coefficient which is dependent on dopant concentration this is discussed in more detail below. 

In general, doping tends to lead to a loss of x-ray order in polyacetylene and polyphenylene, suggesting that dopant ions may be distributed more or less at random. The structural models shown in Fig. 16 are clearly idealised as only limited order is seen even in cation-doped polymer. The anion dopants are much larger and apparently disrupt the structure too much for any sign of regularity to be seen, except in the case of iodine. 

The diffusion behaviour of Shirakawa polyacetylene is complicated by its fibrillar morphology and high surface area, so that weight changes depend on pore transport and surface adsorption, as well as on diffusion into the fibrils. Chien 6) has reviewed earlier studies of the diffusion of dopant counter-ions in Shirakawa polymer and has emphasised the wide range of values of diffusion coefficient which are reported and which depend a great deal upon the morphological model chosen to interpret experimental data. 

Oxygen is also a dopant for polyacetylene, but on exposure the conductivity rises to a maximum then rapidly declines as oxidation of the polymer backbone occurs, as shown in Fig. 21. We have no data on the diffusion coefficient as the process is rapid and is masked by the reaction of oxygen with the polymer. The kinetics are first-order, implying that the doping reaction is rapid, goes to less than 1 mol%, and is then followed by irreversible oxidation of the polymer. Based on the observa-... 

One concern with measurements of this type, is that undoping of a film may result from the outward diffusion of dopant ions or the inward diffusion of counterions which would then form salt within the film. This has been avoided in our polyacetylene study by measuring further doping pulses in samples which have only been doped in one direction, either reduction or oxidation. 

Chien 6) has pointed out that doping not only stabilizes polyacetylene towards oxidation but also stabilizes the dopant. The most obvious example is AsF5, which reacts violently with atmospheric moisture but is stabilized in polyacetylene as the AsF6" counter-ion. 

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