Phase transitions in p- n-propyloxybenzylidene- p-amino benzoic acid: A DSC Study

Abstract

The compound, p- n-propyloxy benzylidene p-amino benzoic acid ( 3 BABA), exhibits a nematic liquid crystalline phase at temperatures above ≈190°C. Phase transitions in 3BABA have been studied in the temperature range 25°C≤T≤300°C by differential scanning calorimetry. The following interesting features have been observed: 1. 1. the transitions involving the crystalline phases are very complex, 2. 2. there is a gradual transformation of some of the metastable crystalline phases into more stable ones, 3. 3. the transition(s) occurring while cooling the nematic can follow one of the two paths, one leading directly to a crystalline phase through a very sharp transition, and the other leading to the crystalline phase through an intermediate phase P 1 of unknown nature which is an unusual occurrence. These features may be related, to the change in molecular arrangement from closed dimer to open n-mers while cooling the nematic, and the resulting formation of crystalline phases with almost comparable free energies. The transition entropy, ΔS NI(1.40R 0) for the nematic-isotropic transition is very large compared to that of ordinary nematics(N o). This is thought to be due to the unusually pronounced short range orders in the fibre-type nematic phase(N f) of 3BABA, which are not present in N o phases.