Liquid crystalline semirigid polyesters based on phenylstilbene analogues of 1,3,4-thiadiazole

Semirigid polyesters composed of phenylstilbene analogues of 1,3,4-thiadiazole, 2-(2-phenylethenyl)-5-phenyl-1,3,4-thiadiazole (PEPT), linking an octamethylene chain at different disubstituted (3,3'-, 3,4'-, 4,3'- and 4,4'-) positions, were prepared from four diphenols of the PEPT and sebacoyl chloride by interfacial polycondensation. The effect of polymer structure on thermotropic liquid crystalline (LC) and optical properties is discussed. Differential scanning calorimetry (DSC) measurements, optical texture observations and powder X-ray diffraction patterns showed that the polymer linking the octamethylene chain at the 4,4'-position (4,4'-PEPT) has a linear structure and forms an enantiotropic nematic LC phase. Polymers linking the octamethylene chains at the 4,3'- (4,3'-PEPT), the 3,4'- (3,4'-PEPT) and the 3,3'-positions (3,3'-PEPT) positions have a less linear structure and display monotropic smectic phase or no LC phase. Solution and solid-state UV-visible and emission spectra indicated that the polyesters exhibit absorption maxima due to the PEPT moieties and fluoresce blue light, but low or no quantum efficiencies were recognized. The polyesters emitted weak polarized fluorescent light at room temperature.