Thermal properties of non-symmetric bibenzoate liquid crystalline dimers

Abstract

The liquid crystal behaviour of a family of non-symmetric liquid crystalline dimers is reported. These systems contain two bibenzoate rigid units that are linked to distinct terminal groups at one end, and to a flexible interconnecting spacer at the other. Several systems having different terminal and central chains are studied using calorimetric, microscopic and diffraction techniques. All the samples form phases with variable degrees of order (from low ordered smectic to crystalline phases) depending on the chemical constitution of the different segments. The influence of the length, parity and lateral substitution of the spacers on the transitional properties and the symmetry of the mesophases that are formed is analysed. It is found that a decrease in the transition temperatures and enthalpies occurs when the length of the flexible spacers increases, when lateral methyl substituents are introduced, or when the parity of the central spacer changes from an even to odd number of carbon atoms or ether groups. The arrangement of the mesogens and dissimilar flexible groups within the ordered structure is discussed with respect to the observed L/d ratios. Different values were obtained depending on the parity of the central spacer and on the degree of order. Interpenetrated structures, in which the flexible groups of different lengths are mixed, seem to be compatible with low ordered smectic phases, but sterically disfavoured when constructing crystalline phases.