Polyacetylene diffusion coefficients

Since perchlorate ions, and more generally the majority of anions used in common electrolyte systems, are known to move rapidly in liquid solutions, it is reasonable to assume that the rate determining step in controlling the kinetics of the overall process is the ion diffusion throughout the polymer fibrils. This conclusion has been experimentally confirmed. For example, the diffusion coefficient of electrolyte counterions in bulk polyacetylene has been determined (Will, 1985) to be seven orders of magnitude lower than in liquid electrolytes, namely about 10 cm s vs 10 cm s ... 

Furthermore, using Eqn (9.34), the diffusion coefficient of CIO4" in polypyrrole was estimated to be 1.3 x 10 cm s (Panero et al, 1989). This value is three orders of magnitude greater than that found for the diffusion of the same anion in polyacetylene and this confirms that... 

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. 

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-... 

The second key factor determining permeability in polymers is the sorption coefficient. The data in Figure 2.18 show that sorption coefficients for a particular gas are relatively constant within a single family of related materials. In fact, sorption coefficients of gases in polymers are relatively constant for a wide range of chemically different polymers. Figure 2.25 plots sorption and diffusion coefficients of methane in Tanaka s fluorinated polyimides [23], carboxylated polyvinyl trimethylsiloxane [37] and substituted polyacetylenes [38], all amorphous glassy polymers, and a variety of substituted siloxanes [39], all rubbers. The diffusion... 

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 ... 

Fig. 5.10 Temperature dependence of the soliton diffusion coefficient in /roAj -polyacetylene from NMR data obtained in Grenoble ( ) and ESR data obtained in Tokyo (O). Theoretical results (by Jeyadev and Conwell [67]) for phonon scattering only (dashed line) and for phonon scattering plus barriers of 0.01 eV (solid line).

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