Polyacetylene dopant diffusion

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. 

Fritz Will has made a study of discharge behavior of polyacetylene electrodes (56) and has drawn attention to the fact that slow dopant diffusion and slow discharge at the polymer/electrolyte interface is the principal problem. No steady state potentials were obtained over about 1 hour. This work quantitates some of the difficulties with these batteries. 

In the development of batteries to date, the most notable are those containing n and p type polymer, e.g., the perchlorate-doped polyacetylene in conjunction with the lithium doped polyacetylene, and acetonitrile. Aqueous polyacetylene battereis are under construction. Theoretical work has been done here by Will (56), who has derived equations for the slow change of a battery potential resulting from dopant diffusion within the solid. Mermilliod and co-workers (59) have demonstrated that much of the electrical capacity of the batteries arise not because of the conversion of chemical to electrical work as with normal batteries, but because of the storage of electricity in the double layer, very large because of the high surface area to bulk ratio in many polymers. 

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. 

When the potential scan was extended to a more anodic region, an oxidation peak was observed at about 4.7V [17,18]. This is considered to be irreversible oxidation caused by over-doping. The diffusion of dopants in polyacetylene has been studied by some researchers, but the coefficient diffusion values show large discrepancies. Using the current pulse method and assuming one-dimensional linear diffusion. Will estimated the diffusion coefficient of BF4 anions in polyacetylene to be 6 x 10 cm s [19]. Takehara et al. estimated the diffusion coefficients of BF4, CIOJ, and PF ... 

Kaufman reported a lower value of 4 x cm s [21]. Because of the slow diffusion of dopants in polyacetylene, an electrode with a higher surface area is preferable for obtaining a high current density. Padula et al reported that highly porous polyacetylene (foam type) showed a higher current density than an ordinary film in cyclic voltammetry measurements [22]. 

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