STRUCTURE AND THERMOCHROMIC SOLID-STATE PHASE-TRANSITION OF POLY(3-ALKYLTHIOPHENE)

In order to clarify the structural changes that occur in the thermochromic phase transition of poly(3-dodecylthiophene) [P3DT] and poly(3-hexylthiophene) [P3HT], the temperature dependence of x-ray diffraction and Fourier transform infrared spectra was measured. (1) Orthogonal unit-cell parameters were determined at room temperature: a = 25.83 angstrom, b = 7.75 angstrom, c (fiber axis) = 7.77 angstrom for P3DT and a = 16.63 angstrom, b = 7.75 angstrom, and c = 7.77 angstrom for P3HT. A large variation of the a-axis length between P3DT and P3HT indicates the extended trans conformation for the alkyl side chains which are oriented along the lateral a-axis direction. (2) The interplanar spacing, intensity, and integral width of the x-ray (h00) and (00l) reflections were found to change drastically in the transition region. (3) Polarized infrared measurements at high temperature revealed a marked increase of the gauche band intensity for the alkyl side group modes followed by a decrease in the band intensity of the thiophene ring modes. (4) The layer reflections of the x-ray fiber diagram become diffuse at high temperatures, indicating that the transition occurs in a liquid-crystalline manner with the orientation of the main chain axes preserved but with almost no axial correlation between the neighboring main chains. These results provide experimental support for the structural model proposed earlier: as the temperature increases, the trans-type side chains begin to disorder by introduction of gauche bonds. This disordering disrupts the regularity of the main chain conformation and decreases the effective length of the polythiophene conjugated system.