THERMOPOWER SIGN REVERSAL VERSUS TEMPERATURE AND DC CONDUCTIVITY IN POLYANILINE DERIVATIVES

The DC conductivity and thermopower of polyaniline derivatives were measured for temperatures ranging from 80 to 345 K. The samples were oxidized to different degrees, fully protonated and further doped with-SO3H -COOH and phosphorylated polystyrene additives to concentrations varying from 0 to 100 mol% of functional group per mole of aniline. The DC conductivity sigma(300 K) ranges from 0.01 to 16 S cm-1, depending on the degree of oxidation and dopant concentration, while sigma(T) = sigma-0 exp[-(T0/T)0.5], where T0 is mainly determined by the degree of oxidation. The thermopower, which is of the order of -15-mu-V K-1 at 300 K, changes sign from n to p with increasing temperature at T(r), which varies between 290 and 325 K. This change in sign, which is reversed during cooling, is attributed to the existence of a half-filled polaronic band situated deep in the energy gap of polyaniline.