Conducting polymers metallic regime

Among the previous generation conducting polymers, metallic behavior with finite conductivity at millikelvin (mK) temperatures was observed only in doped polyacetylene. Gould et al. [41] and Thummes et al. [73,74] carried out temperature and magnetic field dependence studies of the conductivity in doped (CH)., down to millikelvin temperatures. Contributions from both weak localization and e-e interactions were observed in the disordered metallic regime. In doped PPV samples, Madsen et al. [99] also observed the e-e interaction contribution to conductivity (a oc p ) in metallic samples. 

The bipolarons are energetically described as spinless bipolaron levels (scheme (9.30a)) which are empty and which, at high doping levels, may overlap with the formation of bipolaronic bands (9.30b). Finally, for polymers with band gap, values smaller than that of polypyrrole - such as polythiophene - the bipolaronic bands may also overlap with the valence and conduction bands, thus approaching the metallic regime. 

Several studies have indicated thatlinePAni)-emeraldine salt(S) / 2-acrylami-do-2 mthyl -1-propane sulfonic acid (AMPSA) [7,8,9,10] cast id (DCAA) is in the metallic regime toward the insulator-metal transition. The metallic beaviour was attributed to the criterion of finite conductivity when TDO K, the positive slope of the temperature dependence reduced activation energy (i.e W (T) = d [log o(T)]/d [log(T)]),liseddelocalized electronic states at the Fermi level that allow condnction without thermal activation. However, this polymer does not show metalhc behavior over the whole range of temperatures. They exhibit semiconducting (da/dT) > 0) and... 

It thus becomes clear, that two regimes of the reaction corresponding to two different regimes of the electrical behaviour have to be discussed a) the regime below the threshold value of the new structure and b) the regime where the properties are related to the appearance of the new polymer salt structure. In the first regime an exponential increase in conductivity to a level of about 10 2 (Q-1cm ) is observed, obviously due to surface reactions and/or reactions in the amorphous domains of the sample. The transition to the "metallic"regime of conductivity seems, however, to be linked to the formation of the new structure. The increase in conductivity with... 

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