INFLUENCE OF STRUCTURAL PARAMETERS OF CONDUCTING POLYMERS ON THEIR MICROWAVE PROPERTIES

The complex dielectric constant epsilon* (epsilon* = epsilon'- iepsilon'') of conducting polymers (polyaniline and poly(3-alkylthiophene)) is studied over a frequency range spanning from 130 MHz to 20 GHz. Epsilon* is measured using an HP 8720 network analyser on pressed samples compatible with APC7 standard. Electrical properties of emeraldine salt doped with hydrochloride acid (HCl), p-toluene sulfonic acid (PTSA) or naphthalene 1,5-disulfonic acid (NDSA) are reported as a function of doping level. The effect of counter-anion size is investigated. Increasing size of the counter-anion leads to an increase in interchain distance. The same effect is obtained in a qualitative way by lowering the pressure used to obtain the sample measured. In both cases a decrease in conductivity and epsilon* is observed for a given doping level. Moreover, at a fixed frequency and for a given value of epsilon'', the value of epsilon' will decrease as the size of the counter-anion increases or the pressure decreases. This effect results from the decrease in capacitive coupling when the interchain distance increases. Similar effects are qualitatively obtained by increasing the length of the alkyl chain on the thiophene monomer. Effects linked to defect rates and molar mass of the polymer have also been investigated.