Unusual chain configuration of main-chain liquid crystal polyesters having Y-shaped mesogens in nematic phase

The nematic liquid crystals of main-chain liquid crystalline (MCLC) Y- n polyesters were investigated and compared with those of conventional MCLC L- n polyesters. Y- n polyesters have Y-shaped mesogens that are connected by spacers at their branch ends as an alternative to the columitic mesogens of L- n polyesters. The nematic phases of both polyesters under polarized optical microscopy exhibited Schlieren textures that are typical of uniaxial nematic liquid crystals. The isotropization temperature and entropy change estimated by differential scanning calorimetry show odd–even oscillations, which are characteristic of MCLC polymers, and the entropy change of Y- n are significantly smaller than those of L- n. Such a low entropy change is associated with the Y- n chain configuration, which is unusual for MCLC polymers, as revealed by X-ray diffraction and infrared dichroic spectroscopy. The uniaxial nematic structure is formed by Y-shaped mesogens arranging their trunks along the director direction and being rotated about their trunks without any correlation. Connecting these mesogens by using spacers that are placed preferentially along the director enables us to depict the chain that oscillates along the director direction and performs a random walk in the direction perpendicular to the director. [Display omitted]