Studies in Mixed Mesomorphism: Determination of Latent Transition Temperatures by Extrapolation

In binary systems, consisting of two structually similar components of which one is a liquid crystal, extrapolation of the transition curves should yield values for latent transition temperatures of the non-liquid crystalline substances. The reliability of this method of arriving at the latent transition temperatures has been questioned by some research workers. An attempt has been made under this investigation to arrive at some conclusive evidence in regard to the reliability of the method. Several binary systems with suitable Schiff's bases as one component and the well-known p-azoxyanisole or p-azoxyphenetole as the other liquidcrystal component have been studied from which a fair conclusion in favour of the dependability of the extrapolation method may be drawn. Yet, there cannot be a final word since some of the binary systems under study pose a difficulty in understanding their behaviour although discrcpancies observed have been accounted for in terms of polarity and non-ideality of the mixtures besides an overall combined effect accruing from structural aspects working for or against the exhibition of mixed mesomorphism. It can be readily seen that whenever the transition curves are more or less straight, the reliability is high. Incidentally the present study also clearly supports the view of ‘additive effect’ of terminal polar groups in exhibiting mixed mesomorphism; the values of initial slopes of the transition curves and ‘group slope values’ for the terminal polar groups of Schiff's bases obtained from the phase diagrams of their binary mixtures with p-azoxyanisole are comparable with those obtained from the phase diagrams of their binary mixtures with p-azoxyphenetole. Though some of the values vary slightly, the order of ‘group efficiency’ remains the same.