EFFECTS OF MOLECULAR-WEIGHT ON THE ELECTRICAL-PROPERTIES OF ELECTROCHEMICALLY SYNTHESIZED POLY (3-HEXYLTHIOPHENE)

The relation between molecular weights and electrical properties was investigated for electrochemically prepared poly(3-hexylthiophene) (PHT). The PHT films with weight-average molecular weight (M(w)) in the range 12 500-58 000 were prepared by controlling the polymerization conditions. For M(w) < 25 000, the electrical conductivities (sigma) of as-polymerized PHT films increased rapidly from 10(-1) to 10(1) S cm-1 with increasing M(w). For M(w) > 25 000, the increase in sigma with M(w) was minor. The visible absorption maximum (pi-pi* absorption band) of the neutral PHT films shifted to the red side as M(w) increased. The shift was more prominent in the samples with low M(w) (< 25 000). Wide-angle X-ray analyses of the films showed that the increase in M(w) resulted in an increase in regularity of supermolecular structure in the as-polymerized PHT films, which enhanced the development of a pi-conjugated system in the sample.