Percolation in Electroactive Polymers Experimental Techniques

CHARGE PERCOLATION IN ELECTROACTIVE POLYMERS EXPERIMENTAL TECHNIQUES 

In Section 2 we discussed the fundamental mechanisms of charge percolation in electroactive polymer films. Hence from a fundamental point of view, to quantify charge percolation fully in a polymer layer, we 

then semiinfinite diffusion conditions prevail, and the conventional electrochemical results are valid and can be used in data analysis. However if t 1, then the full finite diffusion problem must be solved. The condition t 1 corresponds to the short-time regime. If data are captured in this reigme, then the mathematics becomes much simpler, but other complexities, such as double-layer charging effects, must be considered. Finite diffusion effects almost certainly come into play on longer time scales. 

The bottom line is to avoid large-amplitude perturbations it is much better instead to apply a small-amplitude potential or current step to the film. In such a circumstance only a minor perturbation in polymer structure is effected. Tis comment applies in particular to analyzing electronically conducting polymers, but it should also be kept in mind when examining redox polymer films. Hence a large number of small steps is much preferred to a single large one. 

Let us now consider potential step chronoamperometry in some detail. We consider a surface-deposited polymer film of uniform thickness L to which a small amplitude potential step is applied. This ensures that only a small change in the polymer oxidation state is effected. Structural changes will be minimal. Initially at time t = 0 before applying the step the redox center concentration in the layer is uniform and has the value c, = where denotes the total redox center concentration in the layer. After applying the small amplitude step, the redox centre concentration at jc = 0 (which defines the support electrode/film interface) is given by Cf (see Fig. 1.46). Let c x, t) denote the concentration of redox centers in the film as a function of distance x and time t. The boundary value problem can be stated as follows 

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