Liquid Crystal Formation in Binary Systems. I. An Interpretation of the Phase Diagrams of the Azoxydianisole–Schiff Base Systems

In order to confirm the presence of a transition at the temperature where the liquid crystal-isotropic liquid transition curve in the nematogen-rich region is met by the other transition curve, five binary systems comprising azoxydianisole (AA) and a Schiff base were examined by means of differential scanning calorimetry. A small but sharp endothermic peak due to a phase change is observable at the expected temperature upon heating, provided that it is well separated from the eutectic temperature. When the above-mentioned curve intersects the solid-liquid transition curve on the Schiff base-rich side, a liquid crystal coexisting with the solid base is transformed into an isotropic liquid coexisting with the same solid at this temperature. The presence of a nematic phase below this temperature could be demonstrated by measuring the separation of the outer hyperfine extrema in the ESR spectrum of bis(acetylacetonato)oxovanadium(IV) dissolved in the mixtures. The sequence of the transformation is entirely different when the intersection occurs on the AA-rich side. However, attempts to confirm the transition of the latter type by ESR were not successful.