Morphology and Crystallographic Structure of Polythiophene and Derivatives

Abstract

The structural regularity is one of the key parameters controling the electrical properties of organic conducting polymers. In the case of five membered heterocycles as thiophene, we have analysed the effects on morphology and on crystallization of 3,4 mono and disubstitution, which should force a regular coupling of thiophene units in the 2,5 positions. The unsubstituted polythiophene is fully amorphous and shows a compact morphology. In contrast, poly 3 methylthiophene is fibrillar, with fiber diameter of 200 Å (undoped state) to 800 Å (doped state). When doped with CF₃ SO₃, doping levels up to 50 % are obtained, associated with a partial crystallization. The X ray and electron diffraction data show a hexagonal lattice (a = 9.7Å, c = 12.2Å), consistent with a coil structure. Similar results have been obtained with poly 3,4 dimethylthiophene. However large steric interactions between methyl groups lead to the distorsion of the polymer chain.