DSC,density and refractive index studies on two nematogenic p-alkylphenyl-2-chloro-4-(p-alkylbenzoyloxy)-benzoates and their mixtures

The compounds p-pentylphenyl-2-chloro-4-(p-pentylbenzoyloxy)-benzoate PCPB] and p-octylphenyl-2-chloro-4-(p-heptylbenzoyloxy)-benzoate OCHB], and mixtures of different mole fractions of them, show a nematic phase with a supercooling effect. From DSC studies both ΔH and ΔS at T _NI are found to have minimum values for the equimolar mixture, which happens to be an eutectic mixture (Mix-II). However the density of this mixture is found to be less than that of the pure compounds. Refractive indices (n _o, n_e) are measured by thin prism method at different temperatures for pure components and the eutectic mixture. The birefringence values of the eutectic mixture are less than those of the individual components at higher temperature region, and are in-between at lower temperatures. The principal polarizibilities (α _o, α _e) of all the compounds have been calculated using both Vuks’ and Neugebauer's methods. These values have been compared with those calculated from additivity rules of bond polarizability. The average polarizability α increases with chain length. The orientational order parameter (P ₂) at different temperatures has been calculated using Haller's extrapolation procedure. (P ₂) values of OCHB are found to be greater than those of PCPB and Mix-II at higher temperatures, and are consistent with those determined from our X-ray studies. The temperature dependence of (P ₂) of all the compounds has been compared with Maier-Saupe theoretical values.