Dopant counterion effect

The effect of this is that the charged defects are independent of one another and can form domain walls that separate two phases of opposite orientation and identical energy. These are called solitons and can sometimes be neutral. Solitons produced in polyacetylene are believed to be delocalised over about 12 CH units with the maximum charge density next to the dopant counterion. The bonds closer to the defect show less amount of bond alternation than the bonds away from the centre. 

In any case, the thermal stability of electrochemically prepared polypyrrole can be improved by ion exchange of the dopant counterion [90,95]. The degree of stabilization achieved depends on the counterion (a greater effect is observed for sulfate and bisulfate anions), temperature, and duration of treatment. Although the mechanism for improved stability is not yet clear, it is apparent that the original polymer microstructure is important. 

When the counterion is in solution, the solvent, whose action cannot be ignored, is a third partner. In polar solvents the ion dopants are generally solvated. In other words, the ions are surrounded by solvent molecules attached by electrostatic forces. Thus the effective active species is a mul-timolecular cluster, which can be quite big. This causes a slowing down of the ion diffusion. Also, due to electrostatic screening and the increased... 

Polar solvents with higher dielectric constants reduce the coulomb interaction between positively charged PEDOT and negatively charged PSS dopants by means of a screening effect between counterions and charge carriers. 

Polyaniline can be rendered processible by the use of specific counterions (dopants). In this technique, the emeraldine base is mixed with appropriate dopant and the resulting mixture is dissolved in the appropriate solvent [37]. The solubility of emeraldine base in aqueous acetic acid as discussed above can be viewed as counterion-induced processibility. However, it was not until the recent discovery [7,8,37] that polyanilines doped with d,l-camphorsulfonic acid and dodecylbenzenesulfonic acid are soluble in common organic solvents such as m-cresol and toluene that counterion-induced processibility sparked interest in the scientific community. The d,l-camphorsulfonic acid/zn-cresol system, in particular, has generated considerable interest due to coupled effects of high transparency and conductivity (100-400 S/ cm). 

In summary, the effect of secondary dopants in PEEXDTiPSS on the molecular level appears in some ways to be similar to that of m-cresol in polyaniline. In both cases the plasticizer effect of a very polar additive with low volatility induces reorganization. In the case of PEDOTiPSS no crystallinity is found— possibly due to the polymeric nature of the counterion—but still an ordering effect can be shown. In both cases, the organization on the nanometer scale leads to a macroscopic conductivity increase. 

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