Conducting polymers electrolytic expansion

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... 

Other areas of technology where the transport of small molecules through polymers plays a key role include foams (where small molecules are used as blowing agents for foam expansion [9-11] and any gas trapped in the cells of a closed-cell foam affects key properties such as the thermal conductivity [12]), plasticization [13,14], removal of process solvents, residual monomers or other impurities by techniques such as supercritical fluid extraction [15,16], biosensors, drug implants, and polymer electrolytes (where the penetrants are ionic). 

In this section, the behaviour of the electrolytic expansion in conducting polymers, especially polyaniline and poly(o-methoxyaniline) (PMAN) are described, with discussion of the basic redox reaction of polyaniline, the dependence of the expansion ratios on oxidation levels, the kind of anions, strain, the pH of the electrolyte and anisotropy. 

The fundamentals of electrolytic expansion in polyaniline films have been discussed. Ion insertion and exclusion by electrolytic oxidation and reduction are the primary mechanisms. However, it is also evident that the changes in molecular conformations, arising due to the delocalisation of 7t-electrons and the electrostatic repulsion between the polycations, are other mechanisms operating in a conducting polymer microactuator. By investigating the molecular structure and the higher order structure to optimise the electrolytic expansion, it should be possible to improve the expansion ratio and the force for practical usage. 

Fig. 1 Expansion and contraction of a conducting polymer surrounded by electrolyte as a result of oxidation and reduction. In this case, only the anions are mobile within the polymer. The scale of change is exaggerated...

Lithium solid-state electrolytes can be roughly divided into three main categories (i) ceramic (CE), (ii) glasses (GL), (iii) solvent-free polymer electrolytes (SPEs). Indeed, the most appealing class is CE, which has been the object of recent good reviews [6-8]. These electrolytes can easily offer a relatively high conductivity (up to 10 cm ), and have the further advantage of a thermal expansion... 

The polymer is usually electronically insulating, so it is very important to limit the amount of binder used in the electtode in order to avoid excessive particle coating thus limiting electrical connections. Moreover, the polymer should swell to a certain extent in the liquid electrolyte so as to assure lithium ions conductivity inside the electrode. The swelling should be limited to avoid huge volume expansion and not to lose electrical contact between active material particles. 

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