Polythiophenes conduction mechanism

Compound 45 incorporates a fullerene acceptor and two substituted polythiophene-ethenyl tails it rectifies as an LS monolayer. Interestingly, it rectifies in one direction for smaller biases between 0 and 2.0 V, but in the reverse direction at higher biases, perhaps indicating a change in conduction mechanism, as discussed above. It also rectifies between A1 and Pb electrodes at 4.2 K (Fig. 22a) [108],... 

Intercalation of electroactive polymers such as polyaniline and polypyrrole in mica-type layered silicates leads to metal-insulator nanocomposites. The conductivity of these nanocomposites in the form of films is highly anisotropic, with the in-plane conductivity 10 to 10 times higher than the conductivity in the direction perpendicular to the film. Conductive polymer/oxide bronze nanocomposites have been prepared by intercalating polythiophene in V2O5 layered phase, which is analogous to clays. °° Studies of these composites are expected not only to provide a fundamental understanding of the conduction mechanism in the polymers, but also to lead to diverse electrical and optical properties. 

Conductive conjugated polymers belonging to polyenes or polyaromatics, such as polyacetylene, polyanUine (PANl), polypyrrole (PPy), polythiophene, poly(p-phenylene), and poly(phenylene vinylene) classes, have been investigated extensively [1-3]. PANI and PPy families of conjugated polymers are of much interest due to their low cost, easy synthesis, unique conduction mechanism, good stability and environmentally benign performance [4,5],... 

Figure 5. Cyclic voltammograms of (a) 2,5"" -di-methyl-a-hexathiophene and (b) poly(2,2 -bithio-phene) films in acetonitrile containing 0.1 M E NCIO 103 (Reprinted from G. Zotti, G. Schia-von, A. Berlin, and G. Pagani, Electrochemistry of end-ca )ed oligothienyls-new insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene, Synth. Met. 61 (1-2) 81-87, 1993, with kind permission from Elsevier Science S.A.)...

The results of Wegner and Riihe (1989) clearly show that electronic conduction in conducting polymers such as polythiophene and polypyrrole occurs via a hopping mechanism that is dominated by interchain rather than intrachain hopping. 

Composites of polypyrrole and poly(vinyl chloride) have been prepared by several groups (64-67). Polythiophene-poly(vinyl chloride) composites have also been prepared (68). The electropolymerization of pyrrole on poly(vinyl chloride)-coated electrodes yielded composites with mechanical properties (tensile strength, percent elongation at break, percent elongation at yield) similar to poly(vinyl chloride) (65) but with a conductivity of 5-50 S/cm, which is only slightly inferior to polypyrrole (30-60 S/cm) prepared under similar conditions. In addition, the environmental stability was enhanced. Morphological studies (69) showed that the polypyrrole was not uniformly distributed in the film and had polypyrrole-rich layers next to the electrode. Similarly, poly(vinyl alcohol) (70) poly[(vinylidine chloride)-co-(trifluoroethylene)] (69) and brominated poly(vinyl carbazole) (71) have been used as the matrix polymers. The chemical polymerization of pyrrole in a poly(vinyl alcohol) matrix by ferric chloride and potassium ferricyanide also yielded conducting composites with conductivities of 10 S/cm (72-74). 

Mechanical properties of poly(thiophene-2,5-diyl) are not suitable for many practical uses. Polymers with higher flexibility and still good thermal resistance or with other special conductivity properties (after doping) can be obtained from the polymerization of substituted polythiophenes. Some examples are shown below ... 

Electrically conducting polymers are quite different systems to the above elec-troinitiated chain polymerizations since they are formed by an unusual step-growth mechanism involving stoichiometric transfer of electrons. The polymers are obtained directly in a conductive polycationic form in which charge-compensating counter anions from the electrolyte system are intercalated into the polymer matrix [173], Exact mechanistic details remain the subject of discussion, but Scheme 4, which shows polypyrrole formation is plausible. Polythiophene is similar where S replaces NH in the ring. 

The conductive polymers were also tested at levels of 13 wt.% in a positive plate which contained PbS04, a-Pb02, and p-Pb02 [22]. The optimum concentration was found to be lwt.%. At 2 3 wt.% additive, the discharge capacity was increased by about 30% and the specific surface-area from 3-4 to 5-6m g. Cycle-life declined at additive concentrations above 5wt.% due to mechanical instability of the electrode. Polypyrrole and polythiophene oxidized during overcharge, but polyaniline remained stable. 

A polythiophene anode in an aqueous electrolytic system, showed irreversible changes at a potential beyond 0.9 V on repeated reduction, suggesting that the breakdown into lower oligomers occurs, as evident from its Raman spectra [268]. However, the loss of electroactivity, electrical conductivity, electrochromic properties, mechanical properties, compact morphology... 

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