Phase Equilibria in Liquid Crystalline Systems Part I. Synthesis and Liquid Crystalline Properties of Oligomers of the p‐Oxybenzoate Series

Abstract

AbstractSyntheses of the trimer (n = 3), tetramer (n = 4) and the pentamer (n = 5) of the p‐oxybenzoate series C6H5CO(–OC6H4CO)n‐2‐OC6H5 are reported. Densities and thermal expansion coefficients of the neat liquids were measured and phase transitions in the pure compounds and their binary mixtures were investigated by differential scanning calorimetry (DSC). The nematic‐isotropic transition is directly accessible only in the case of the tetramer (melting point 181°C, nematic‐isotropic transition temperature TNI = 254°); the trimer melts to an isotropic liquid at 136°C and the nematic‐isotropic transition of the pentamer (melting point 224°C) lies above the range of thermal stability. DSC results for the binary mixtures show the coexistence range of the nematic and the isotropic phases to be very small; it is obscured by pre‐ and post‐transition effects. In satisfactory approximation therefore, the binary mixtures can be treated as one‐component systems, and TNI may be identified with the maximum of the DSC peak. Values of TNI thus obtained are linear with respect to composition and may be extrapolated to obtain TNI = 100°C for the pure trimer and 464°C for the pure pentamer. As shown by conformational analysis, all conformers of a given homolog have virtually the same axial ratio. These compounds therefore can be treated as rigid rods.