Polymers, electroactive layers

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

Considerable potential exists to design surface modified electrodes which can mimic the behaviour of electronic components. For example, a rectifying interface can be produced by using two-layer polymer films on electrodes. The electroactive species in the layers have different redox potentials. Thus electron transfer between the electrode (e.g. platinum) and the outer electroactive layer is forced to occur catalytically by electron transfer mediation through the inner electroactive layer. 

Electric double layer forces between polyelectrolyte and non-polymer surfaces in aqueous media have also been studied very intensively [371,394,400-402]. The adhesion between polyelectrolyte surfaces could be reduced considerably by increasing the ionic strength of the medium [400]. Using an electrochemical cell and a gold coated tip, the adhesion between electroactive layer of p oly( vinyl-ferrocene) was controlled through the selective oxidation or reduction of the polymer films [401]. 

First of all, we must realize that the most important property of the electroactive polymer, as far as mediation is concerned, is its redox potential. To mediate a reduction of a solution species, the redox potential of the electroactive layer must be less positive than that of the analyte for the mediated oxidation process, the reverse is the case. This means that the osmium polymers under consideration here which have a redox potential of about 250 mV, are thermodynamically able to mediate the reduction of Fe(III) to FeCII), but not the reverse process (see Fig. 8.24), since the formal potential of the Fe(III/II) couple is 450 mV. The difference in the two redox potentials can be considered the driving force for the mediating process. On the basis of these considerations, it is clear that the mediated reduction of Fe(III) [as in Eq. (38)] is irreversible. 

Differendy linked carbazole derivatives possessing selenophene and pyrene (the latter not covered in this review) moieties were electropoly-merized (14EA430). The electropolymerized polymers displayed unusual properties upon electrochemical doping suggesting their potential use as electroactive layers in electrochromic devices. A typical synthesis of 2,7-diselenophenylcarbazole is shown below. 

A distinctive property of self-doped polymers is their water solubility in the neutral (insulating) and doped (conducting) states. This solubility is due to the covalently attached negatively charged groups on the polymer backbone. Solubility allows a deposition of conductive and electroactive layers onto any, even a nonconducting, surface by a simple casting of self-doped polymers. Such layers could find numerous applications... 

A number of derivates of pyrroles have been polymerized and manipulated into multilayer structures using the Langmiur-Blodgett technique [203]. This technique has been applied to manipulate electroactive conjugated polymers, mainly polypyrrole derivates, into multilayer thin films with well-defined layered structures and ordered molecular organizations [204]. In this way, amphiphilic polypyrroles and their precursors have been used, or soluble surface-active polypyrroles such as poly(3,4-dibutylpyrrole) [205] (Schemes 10.11 and 10.12). 

Note that migration is important for ionic movement within solid polymer electrolytes or solid-state electrtxthromic layers since the transport numbers of the electroactive species or of the mobile counterions become appreciable. These a.spects are unlikely to trouble us further in this present text. 

Smela et al. (2) observed that some conjugated polyelectrolyte dopants caused a layering of polymer chains within a polymer. In other ionic electroactive material additives can be used for crystalline ordering. Such additives may include surfactants and/or liquid crystals. 

Film deposition refers to the preparation of polymer (organic, organometallic, and metal coordination) films which contain the equivalent of many monomolecular layers of electroactive sites. As many as 10 monolayer-equivalents may be present [9]. The polymer film is held on the electrode surface by a combination of chemisorptive and solubility effects. Since the polymer film bonding is rather nonspecific, this approach can be used to modify almost any type of... 

The concept of using the functional groups of electrode surfaces themselves to attach reagents by means of covalent bonding offers synthetic diversity and has been developed for mono- and multi-layer modifications. The electrode surface can be activated by reagents such as organosilanes [5] which can be used to covalently bond electroactive species to the activated electrode surface. Recently, thermally induced free-radical polymerization reactions at the surfaces of silica gel have been demonstrated [21]. This procedure has been applied to Pt and carbon electrode surfaces. These thermally initiated polymer macromolecules have the surface Of the electrode as one of their terminal groups. Preliminary studies indicate that the... 

There are several reasons for the appeal of polymer modification immobilization is technically easier than working with monolayers the films are generally more stable and because of the multiple layers redox sites, the electrochemical responses are larger. Questions remain, however, as to how the electrochemical reaction of multimolecular layers of electroactive sites in a polymer matrix occur, e.g., mass transport and electron transfer processes by which the multilayers exchange electrons with the electrode and with reactive molecules in the contacting solution [9]. 

Chemical modification of electrode surfaces by polymer films offers the advantages of inherent chemical and physical stability, incorporation of large numbers of electroactive sites, and relatively facile electron transport across the film. Since th% polymer films usually contain the equivalent of one to more than 10 monolayers of electroactive sites, the resulting electrochemical responses are generally larger and thus more easily observed than those of immobilized monomolecular layers. Also, the concentration of sites in the film can be as high as 5 mol/L and may influence the reactivity of the sites because their solvent and ionic environments differ considerably from dilute homogeneous solutions [9]. 

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