Molecular alignment and thermal stability of liquid-crystalline phases in binary mixtures of electron donor and acceptor

The molecular alignment and thermal stability of the mixtures DA composed of two different kinds of Schiff base liquid-crystalline materials, D with an electron-donating -N(CH3)2 unit and A with an electron-withdrawing-NO2 unit, were investigated by using DSC, POM, XRD, and FT-IR measurements. The mesophases of the mixtures DA were more stable and had wider temperature ranges than those of the individual compounds D and A. The mixture DA55 (D:A=50:50 mol%) on cooling exhibited the most stable liquid-crystalline phases affording the highest TSI (119 °C), the widest temperature range of the smectic phase (46.4 °C) and the highest values of ΔSSI (26.1 J K−1 mol−1) among the mixtures DA examined. Based on the analysis of the XRD measurements and the temperature dependence on the FT-IR spectra, the molecular packing of the mixture DA55 was proposed to form a bilayer structure in the smectic A phase, in which the heading group of the compounds D and A overlapped partially. The induction of the smectic phase and the enhanced thermal stability for the mixtures DA would be caused by the intermolecular interaction between the electron donor and acceptor mesogens.