Mesophase stability in binary mixtures of monotropic nematogens

Binary phase diagrams were constructed from laterally substituted methyl azo/ester derivatives, namely 4-(4″-substituted phenylazo)-3-methyl phenyl-4″-alkoxy benzoates (Ina–d). In this group of compound the unsubstituted and chloro-substituted derivatives possess the nematic phase monotropically, while the nitro and methyl analogues are enantiotropically nematogenic. The binary phase diagrams constructed were made once from the monotropic nematogens with each other, and another with the enantiotropic nematogens. In both the cases enantiotropic nematic phase was observed that covers a wide range of composition. The mesophase behaviour of all binary mixtures was investigated by differential scanning calorimetry (DSC) and polarised light microscopy (PLM). The nematic phase was exhibited in all binary mixtures. Independent of the alkoxy chain length, the entropy change, ΔSN–I of the N–I transition of pure components was found to vary irregularly with the anisotropy of polarisability (∆αX) of the polar substituent, X.