Polymer films charge transport parameters

Electrochemical polymerization is a fast and simple, widely used method to synthesize different conducting polymers. Electrodeposition enables film formation on surfaces with complicated patterns as well as control of film thickness [5]. Furthermore, the subsequent growth of the polymer film and the charging reactions can be followed in situ [6]. Parameters such as solvent media, electrolyte, electrochemical method used for polymerization and monomer material, all have a profound effect on film morphology, charge transfer and transport properties. Different from the powdery products prepared through chemical approaches, this method enables an easy one-step deposition of the film directly at the surface of the electrode substrate that can be further applied for electrochemical purposes. 

Conducting polymers have been studied using the whole arsenal of methods available to chemists and physieists. Eleetrochemical teelmiques, mostly transient methods such as cychc voltarmnetiy (CV), chronoamperometry (CA) and chronocoulom-etry (CC), are the primary tools used to follow the formation and deposition of polymers, as well as the kineties of their charge transport processes. Electrochemical impedance speetroseopy (EIS) has become the most powerful technique used to obtain kinetic parameters sueh as the rate of charge transfer, diffusion coefficients (and their dependenee on potential), the double layer capacity, the pseudocapacitance of the polymer film, and the resistance of the film... 

In the course of electrochemical oxidation or reduction of the surface polymer films or membranes, the overall electroneutrality of the polymer phase is retained by ion-exchange processes between the polymer film and the bulk electrolyte solution [3, 72]. Not only has ion transport to be considered, but solvent and other neutral molecules may also enter or leave the film during the charging/discharging processes [73-78]. In order to maintain electroneutrality in the simplest case either counterions enter the film or coions leave it. The relative contributions of the ions carrying different charges to the overall charge transport may depend on their physical properties (e.g., size) and/or on their chemical nature (e.g., specific interactions with the polymer), as well as on other parameters (e.g., potential) [1-4, 73-92]. 

It is not always recognized that potential sweep voltammetry can be used to obtain quantitative information on charge percolation in polymer films. We now briefly indicate how apparent charge transport diffusion coefficients can be extracted by analyzing such voltammetric parameters as peak current, peak width, and peak potential as a function of scan rate. This analysis is diffusional in concept and perhaps best applied to polymer materials where redox conduction is the predominant mechanism of charge transport. 

In conclusion linear potential sweep and cyclic voltammetry can be used to obtain quantitative information on charge percolation in electroactive polymer films. However analysis is complex, and it may be preferable to use a simpler technique, such as potential or current step perturbation, to determine the transport parameters, as outlined in the preceding section. 

In cases where we have a polymer film on the electrode surface (as in a conducting polymer study), the situation becomes rather different from a solution case with respect to charge and mass transport. The critical parameter here is Dctrlcf (where d is the polymer film thickness and T is the time after the potential step). When Dctr/d < 1, semi-infinite diffusion prevails, and the i-t transient should conform to the Cottrell equation [42] ... 

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