The Dopant Counterion

More recently, it has been found that anions such as polyelectrolytes and surfactant-like anions, for example, dodecylbenzenesulfonate (DBSA), can be inserted directly into PPy products during polymerization in competition with anions arising from the oxidant. This very useful development will be discussed later in a section dealing with PPy processability. 

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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. 

The use of halogens as oxidizing agents has also been reported,75 resulting in a PPy/X product containing halide (X-) ions as the dopant counterions. These same workers used chemical polymerization to produce PPy-polymethylpyrrole copolymers. 

Figure 10-2 7/t/e/chain charge mobility. Although the dopant counterion, ag., I (which also easts as I3" and Ij" ) has been omitted from these drawings, it may play a role in interchain transport. 

SoKtons produced in polyacetylene are delocalized over approximately 12 CH units, with the maximum charge density to the dopant counterion. Soliton formation results in the creation of a new localized electronic state which is in the middle of the energy gap. At a high level of doping the charged sohtons produce soliton bands that can merge to behave hke a metalhc conductor. 

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. 

The diffraction patterns for the doped samples indicate that the polyacetylene chain orientation remains essentially unchanged after doping. As observed in earlier studies [81], comparison of the fiber diffraction patterns of oriented trans-polyacetylene before and after doping shows that the principal equatorial reflections remain with only modest shifts in their positions as a result of lattice expansion to accommodate the dopant counterions. 

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