STRUCTURE PROPERTIES OF CONJUGATED CONDUCTIVE POLYMERS .1. NEUTRAL POLY(3-ALKYLTHIOPHENE)S

Soluble conjugated poly(3-alkylthiophene)s(P3ATs) including poly(3-butylthiophene)(P3BT), poly(3-octylthiophene) (P30T), and poly(3-dodecylthiophene) (P3DDT), were prepared chemically and characterized by use of various thermal analysis and spectroscopic methods and conductivity measurement. It is found that each of the P3ATs consists of an ordered phase and a disordered phase. In the ordered phase, the side chains are nearly fully extended and are packed to give a two-layer structure and one-layer structure, the former being the majority. The coplanar subchains are stacked to form planes, and the space between two successive stacking planes is filled with aligned side chains. For the longer side chain such as in P3DDT, the ordered side chains also form a separated ordered phase. As the ordered phase melts, a thermochromism occurs. In the disordered phase, three transitions are observed, being gamma, beta, and a resulting from relaxations of methylene linkages, side chains, and twists in the main chains (noncoplanar segments or conformons), respectively. The gamma and beta relaxations provide only a slight disturbance to the coplanarity in the main chains. However, the a relaxation has a significant effect on the optical band gap by an increase of the distortion in the soft conformons (in which the torsion between two coplanar subchains is distributed over several repeated units), leading to a localization of their pi electrons and therefore a hindrance to the intrachain (and perhaps interchain) charge transport. As temperature increases to about 30-60-degrees-C above T(alpha), the hindrance effect on charge transport is dominated over the increased charge mobility, causing an occurrence of conductivity maximum. This conductivity maximum can be considered as another characteristic of the electronic structure of the pi system in addition to the thermochromism at the melting temperature of the ordered phase.