Electrochemically Stimulated Conformational Relaxation Model

Experimental results and developed devices allow us to divide the oxidation of the neutral conducting polymers into four steps  

Loss of electrons from the polymer chain with the formation of radical cations (polarons) and dications (bipolarons). 

Stimulation of the conformational relaxation movements of the polymeric chains (by repulsion between the nascent positive charges), with the generation of free volume. Local nuclei or general and simultaneous relaxation occur, depending on the initial compaction of the polymer film. 

Exchange of counter-ions (and solvent) between the polymer and the solution in order to keep the electroneutrality in the film. In a compacted or stressed film, these kinetics are under conformational relaxation control while the structure relaxes. After the initial relaxation, the polymer swells, and conformational changes continue under counter-ion diffusion control in the gel film from the solution. 

Additional exchange of ion pairs and solvent molecules as in any other membrane formed by polyelectrolytes. 

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IX. POLYMER-SOLVENT INTERACTIONS FROM THE ELECTROCHEMICALLY STIMULATED CONFORMATIONAL RELAXATION MODEL... 

Electrochemically stimulated conformational relaxation model (ESCR model) — This model [i, ii] describes the relaxation phenomena occurring during the charging and discharging of -> conducting polymers. It assumes that applying an anodic -> overpotential to a neutral conjugated polymer, as a first step, an expansion of the closed polymeric structure occurs. In this way, partial oxidation takes place and counter ions from the solution enter the solid polymer under the influence of an electrical field at those points of the polymer/electrolyte... 

Electrochemically stimulated conformational relaxation model (ESCR model) — Figure... 

Later we will describe both oxidation and reduction processes that are in agreement with the electrochemically stimulated conformational relaxation (ESCR) model presented at the end of the chapter. In a neutral state, most of the conducting polymers are an amorphous cross-linked network (Fig. 3). The linear chains between cross-linking points have strong van der Waals intrachain and interchain interactions, giving a compact solid [Fig. 14(a)]. By oxidation of the neutral chains, electrons are extracted from the chains. At the polymer/solution interface, positive radical cations (polarons) accumulate along the polymeric chains. The same density of counter-ions accumulates on the solution side. 

The action of a muscle is a consequence of electrochemically stimulated conformational relaxation processes that occur along every electroactive chain inside a polymeric film. A free-volume model dependent on the... 

Since the appearance of the redox [ii, iii] and conducting [iv] polymer-modified electrodes much effort has been made concerning the development and characterization of electrodes modified with electroactive polymeric materials, as well as their application in various fields such as -> sensors, actuators, ion exchangers, -> batteries, -> supercapacitors, -> photovoltaic devices, -> corrosion protection, -> electrocatalysis, -> elec-trochromic devices, electroluminescent devices (- electroluminescence) [i, v-viii]. See also -> electrochemically stimulated conformational relaxation (ESCR) model, and -> surface-modified electrodes. 

The retardation observed in the oxidation process when the polymer was previously polarized at high cathodic potentials for long periods of time, reported as a memory effect by Villeret and Nechtschein [174], was partially quantified by Oden and Nechtschein [167,168,175]. A complete description of these memory effects, based on the electrochemically stimulated conformational relaxation (ESCR) model, has been provided by Otero et al. [176-178]. The knowledge and control of those conformational relaxation processes are essential from a technological point of view. 

Theoretical models available in the literature consider the electron loss, the counter-ion diffusion, or the nucleation process as the rate-limiting steps they follow traditional electrochemical models and avoid any structural treatment of the electrode. Our approach relies on the electro-chemically stimulated conformational relaxation control of the process. Although these conformational movements179 are present at any moment of the oxidation process (as proved by the experimental determination of the volume change or the continuous movements of artificial muscles), in order to be able to quantify them, we need to isolate them from either the electrons transfers, the counter-ion diffusion, or the solvent interchange we need electrochemical experiments in which the kinetics are under conformational relaxation control. Once the electrochemistry of these structural effects is quantified, we can again include the other components of the electrochemical reaction to obtain a complete description of electrochemical oxidation. 

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