Smectic Polymorphism of Some Bis-(4,4′-n-alkoxybenzylidene) - 1,4-phenylenediamines up to 3 kbars by Differential Thermal Analysis

Abstract

The T-p phase diagrams of four homologues of the bis-(4,4′-n-alkoxybenzylidene)-1,4-phenylenediamine series (C₅H₁₁O [dbnd] PENTOBPD, C₆H₁₃O=HEPTOBPD and C₈H₁₇O [dbnd] OOBPD) have been determined by high pressure differential thermal analysis (dta) up to 3 kbars and in the temperature range 300 K to 600 K. In each substance at least two solid phases are found at atmospheric pressure. HEPTOBPD and OOBPD exhibit five and HEXOBPD four smectic phases when heated from the solid up to the nematic state at normal pressure whereas PENTOBPD only exhibits the nematic phase. The rather complex phase behaviour of HEXOBPD, HEPTOBPD, and OOBPD gets simpler as pressure is increased. For each of the three compounds two smectic phases disappear in the pressure range of the experiments. Triple points between three smectic phases have been found in OOBPD, HEPTOBPD, and HEXOBPD; they are supposed to be the first examples for triple points involving mesomorphic phases only. From the areas of the dta peaks the transition enthalpies were determined up to 2.4 kbars. By means of these values and the slopes of the T-p transition curves at different pressures the transition volumes could be additionally calculated. A phase theoretical analysis of the results demonstrates that for this homologous series pressure increase has a similar influence on the T-p phase diagrams as it would have a shortening of the alkoxy chains at a constant (e.g. normal) pressure; a similar effect has been found for the transition enthalpies.