Infrared study of phase transitions in thermotropic liquid crystal polyesters

Three samples of semiflexible thermotropic liquid‐crystalline polyesters based on mesogenic aromatic triads and a decamethylene spacer have been investigated by IR spectroscopy at various temperatures between room temperature and 290°C. The crystal‐to‐liquid crystal (either nematic or smectic) phase transition was accompanied by fairly strong spectral variations, whereas slight, but significant, changes in the IR profiles were detected at the liquid crystal‐to‐isotropic transition. By comparing the results obtained with the spectral behavior recorded for corresponding structural analogs of low molar mass, it was possible to attribute the spectral variations observed in the polymer samples to a decrease in intermolecular interactions rather than to conformational changes. The thermal transitions indicated by IR spectra were in good agreement with the analogous data obtained by calorimetric or optical microscopy techniques.