Diffusion polyacetylene

There are several approaches to the preparation of multicomponent materials, and the method utilized depends largely on the nature of the conductor used. In the case of polyacetylene blends, in situ polymerization of acetylene into a polymeric matrix has been a successful technique. A film of the matrix polymer is initially swelled in a solution of a typical Ziegler-Natta type initiator and, after washing, the impregnated swollen matrix is exposed to acetylene gas. Polymerization occurs as acetylene diffuses into the membrane. The composite material is then oxidatively doped to form a conductor. Low density polyethylene (136,137) and polybutadiene (138) have both been used in this manner. 

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... 

Although the diffusion of the counterion is faster in polypyrrole than in polyacetylene, its value is still low enough to influence the rate of the electrochemical charge and discharge processes of lithium/polymer batteries. Indeed the current output of these batteries is generally confined to a few mA cm . Possibly, improvements in the electrode kinetics, and thus in the battery rates, may be obtained by the replacement of standard ... 

The fibrillar morphology of Shirakawa polyacetylene is an advantage in applications requiring a high surface area but a problem in many other cases, especially the study of diffusion and transport processes and the possible device applications where re-... 

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. 

Durham polyacetylene has the advantage of being a uniform, dense film and so lends itself much more readiliy to diffusion studies. In addition, the uniform morphology is much better suited to device applications, although the low surface area would limit applications in batteries. We have made extensive measurements on the doping of Durham frans-polyacetylene by gaseous AsF5 514 515), which is believed to dope the polymer to form the hexafluoroarsenate ion and arsenic trifluoride 516 ... 

Fig. 19. Weight loss curve for diffusion of AsF3 out of polyacetylene, showing the effect of deswelling of the surface in slowing diffusion. (Ref.515>)...

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. 

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... 

T. Masuda, Y. Iguchi, B.-Z. Tang and T. Higashimura, Diffusion and Solution of Gases in Substituted Polyacetylene Membranes, Polymer 29, 2041 (1988). 

Nechtschein et al.106/110 have carried out a detailed study of 1/Ti with frequency and temperature, both in undoped and doped polyacetylene (PA). Their data analysis shows that PA is quasi-ID system throughout the temperature range of study, except at very low temperatures. Their analysis further, showed that, the intra-chain diffusion follows power law behaviour, T n = 0.65 above 50 K, and T" n = 1.5 below 50 K. These arguments along with Sach s model111 may result in an empirical model for 1/T1 versus temperature data in a limited range of temperature. 

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