STRUCTURE AND THERMOCHROMIC SOLID-STATE PHASE-TRANSITION OF POLY(3-ALKYLTHIOPHENE) - [3] EFFECTS OF ALKYL SIDE-CHAIN LENGTH ON THE PHASE TRANSITIONAL BEHAVIOR

A role of alkyl side chains playing in the thermochromic phase transition of poly(3-alkylthiophene)s have been investigated through the measurements of the X-ray diffraction, infrared and visible-ultraviolet spectra and DSC. The conjugation length of thiophene skeletal chain measured at room and liquid-nitrogen temperatures was found to increase as the alkyl side chain length increases (n = 0 - 12 for CnH2n+1) due to the zipper effect or the enhanced intermolecular interactions between the longer alkyl groups. The thermochromic transition was found to occur more cooperatively in lower and narrower temperature range for the longer alkyl chains. When the side chain length increases furthermore up to n = 22, the conjugation length at low temperature becomes as short as those of the high-temperature phases of the other members. But the conjugation length was found to increase to some extent in the temperature region close to the melting point, where the change of side-chain packing from orthorhombic to hexagonal type occurs.