Properties and Thermal Phase Behaviour of Binary Mixtures of Homologues of Unsymmetrically Substituted Phenyl Benzoates

Abstract

Sixteen liquid crystalline esters of the type 4-(n-CnH2n+1O)C6H4COOC6H4X-4 have been investigated for their thermal phase behaviour. These compounds constitute four homologous series that differ from each other by the substituent X. The latter varies between CH3O, CH3, CN, and NO2. The number (n) of carbon varies, within a homologous series, between 6, 8, 14, and 16. All possible binary mixtures made from any two homologues that cover the whole composition range were prepared and characterized for their mesophase behaviour by differential scanning calorimetry (DSC) and polarizing light microscopy. Twenty four phase diagrams for the various binary combinations were constructed (i) to investigate the dependence of the phase behaviour of mixed systems upon differences in the length of the alkoxy group, (ii) to determine the eutectic composition, and its phase transitions, in each system. Each eutectic system of the binary mixtures (I6/In)X was investigated for its birefringence dependency on temperature via measurement of its refractive index as a function of temperature. The estimated orientational order parameter S for each system was used to examine the effect of the differences in chain length of alkoxy group on its optical properties.