CONDUCTIVITY RELAXATION AND CHAIN MOTIONS IN CONJUGATED CONDUCTING POLYMERS - NEUTRAL POLY(3-ALKYLTHIOPHENE)S

The conductivity relaxation behavior of neutral poly(3-alkylthiophene)s (P3ATs) with the alkyl side chain having carbon numbers 4, 6, 8, and 12 is investigated on the basis of dielectric relaxation measurements at -100 to 180-degrees-C and 0.4-10(5) Hz after conversion to complex electric modulus formalism. It is found that the conductivity relaxation time distribution can be represented by the nonexponential decay function, phi(t) = exp(-t/tau(p))beta, involving a relaxation time distribution parameter beta and a characteristic relaxation time tau(p) as in the case of vitreous ionic inorganic conducting materials. The variation of relaxation time distribution with temperature is found to highly relate to the chain motions, with the aid of dynamic mechanical analysis and UV-vis spectroscopy. The side-chain motion gives no appreciable effect on the charge transport, while in the glass transition and rubbery region the relaxation time distribution becomes broadened with temperature. As the coplanar subchains in the ordered region melt, the conjugation length decreases and its distribution (and therefore relaxation time distribution) becomes narrower. As an electric field is applied, charges delocalized along coplanar subchains (each with one or two soft conformon terminals) hop over localized conformons to the neighboring sites. A contribution to the conductivity due to charge hopping across the side chains is insignificant, since its activation energy does not increase with the side-chain length.