Exhibition of Nematic Mesophase in Binary Mixtures of Schiff's Bases

Generally such non-mesogenes as endowed with the structural characteristics considered essential for exhibition of mesomorphism may be expected to show mixed mesomorphism in their binary mixtures over a range of temperature and cqncentration. A large number of Schiff's bases fall in this category of non-mesogenes ; hence their binary mixtures have been investigated which help sorting out conditions favouring exhibition of mixed enantiotropic mesophase with or without monotropic variety being associated. That only some, and not all, of the systems investigated show mixed enantiotropic mesomorphism over a range of temperature and concentration, account for factors such as polarity, polarisability and size of the admixed molecules and their capacity for resisting thermal break down process on heating. Of the systems investigated, those exhibiting mixed liquid crystallinity are (i) p-nitrobenzal-p-phenetidine : p-anisal-p-phenetidine, (ii) p-anisal-p-phenetidine : p-dimethyIaminobenza1-p-phenetidine, and (iii) p-anisal-p-phenetidine : p-acetoxybenzal-p-toluidine ; the mole percent concentration ranges of the tirst component over which mixed enantiotropic mesomorphism is exhibited being 21.5 to 67.5, 59.8 to 67.0 and 32.4 to 41.0 respectively. A normal expectation that a mixed system of two monotropic Schiff's bases whose latent transition temperatures are not far off from their melting points, would exhibit enantiotropic nematic mesophase over a certain minimum area in its phase diagram is belied by the system p-propionoxybenzal-p-toluidine : p-propionoxybenzal-p-chloroaniline which neverthless continues to show monotropic mixed mesomorphism throughout the phase diagram. Such a behaviour would suggest involvement of factors like eutectic of the mixed melts and proximity of the latent transition temperatures to its melting point ; as it would seem, in order to bring up enantiotropic prpperty, the former should sink much lower and the latter must be as close aspossible. Binary mixtures of these Schiff's bases with p-azoxyanisole as a liquid crystalline component are also studied ; their latent transition temperatures ascertained by smooth extrapolation of transition curves in these systems are in good agreement with those obtained similarly from the phase diagrams of the binary mixtures of Schiff's bases. Failure to locate any heterogeneity in the formation of mixed mesomorphism during our study only lends support to the homogeneous nature of the mixed nematic melts in binary mixtures of Schiff's bases and of Schifl's bases with p-azoxyanisole. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975164