Conduction mechanisms Counterions

In addition to high permselectivity, the membrane must have low-elec trical resistance. That means it is conductive to counterions and does not unduly restrict their passage. Physical and chemical stabihty are also required. Membranes must be mechanically strong and robust, they must not swell or shrink appreciably as ionic strength changes, and they must not wrinkle or delorm under thermal stress. In the course of normal use, membranes may be expec ted to encounter the gamut of pH, so they should be stable from 0 < pH < 14 and in the presence of oxidants. 

Crystalline solid electrolytes have been subdivided into soft ionic crystals such as P-Pbp2 and hard covalent crystals such as P-alumina. The conduction mechanism can be pictured as involving a liquid-like charge carrier array moving in the vibrating potential energy profile set up by the immobile counterions. 

Where Tq and are known as Mott parameters and depend on the type of counterion used. To understand the conduction mechanism below 100 K, the Sheng model can be applied to the analysis of the temperature dependence of polypyrrole conductivity [234]. This model takes into consideration both hopping and... 

Pron et al.569) looked at polyacetylene treated from the gas phase with H2S04 which leads to HS04 counter-ions. They found that the conductivity drops in air with the appearance of C=O bands in the ir, although the rate of decay is much lower than would be expected for undoped samples. The polymer was more rapidly degraded by exposure to water but could be redoped with further acid treatment. Pron et al.570) have also reported hydrolytic instability in polyacetylene with A1C14 as the counterion. In both cases the proposed mechanism involves addition of OH" to the chain and keto-enol tautomerism to form carbonyl groups. 

Although only a limited amount of experimental data is available which describes the interaction between the polyion and the counterions for PDADMAC compounds, the influence of macromolecular and structural parameters on the counterion activity and the electrolytic conductivity in semidiluted, and particularly in highly diluted aqueous solutions, can be clearly shown. All these findings are not only of scientific interest but also of practical relevance. Since for many technical applications PDADMAC concentrations in the ppm range are usual, this basic knowledge of the molecular and medium influences gives more insight into the action mechanism of these polyelectrolytes and may assist in the optimization of several applied processes. 

The mechanism of the Kolbe-Schmitt reaction was investigated since the late 1800s, but the mechanism of the carboxylation could not be elucidated for more than 100 years. For a long time, the accepted mechanism was that the carbon dioxide initially forms an alkali metal phenoxide-C02 complex, which is then converted to the aromatic carboxylate at elevated temperature. The detailed mechanistic study conducted by Y. Kosugi et al. revealed that this complex is actually not an intermediate in the reaction, since the carefully prepared phenoxide-C02 complex started to decompose to afford phenoxide above 90 °C. They also demonstrated that the carboxylated products were thermally stable even at around 200 °C. The CO2 electrophile attacks the ring directly to afford the corresponding ortho- or para-substituted products. (When the counterion is large (e.g., cesium) the attack of CO2 at the ortho-position is hindered therefore, the para-substituted product is the major product.)... 

The magical counterions in terms of conductivity and mechanical properties appear to be sulfonated aromatics,58 59 60 61 62 in particular, para-toluene sulfonate (pTS). It has been shown that the benzene sulfonates induce a degree of crystallinity28 29 that results in higher conductivity. This higher conductivity enables the polymerization process to proceed efficiently. It has also been suggested that the sulfonated aromatics exhibit surfactant-like behavior as the radicals are stabilized and presumably protected from unwanted side reactions with the solvent, oxygen, or other nucleophiles. 

Conductivity within conducting electroactive polymers (CEPs) is a complex issue. A polymer that can exhibit conductivity across a range of some 15 orders of magnitude most likely utilizes different mechanisms under different conditions. In addition to the electronic conductivity exhibited by CEPs, they possess ionic conductivity because of the solvent or electrolyte incorporated during synthesis. The experimental parameters encountered during synthesis (as listed and discussed in Chapter 2) have an effect on the polymer conductivity. In particular, the electrochemical conditions, the solvent, the counterion, and monomers used during synthesis influence the electronic properties of the resulting polymer. 

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