Pressure studies on two discotic liquid crystals

Pressure-temperature diagrams have been studied for two discotic compounds, hexa-n-octyloxytriphenylene and hexa-n-decanoyloxytriphenylene, both of which exhibit the columnar (D) phase at atmospheric pressure. Two interesting results, common to both compounds, have been obtained: (a) in contrast to what is usually observed in liquid crystals of rod-like molecules, the columnar-isotropic (D-I) transition, which is enantiotropic at atmospheric pressure, becomes monotropic at high pressures; (b) dT/dP ≈ 0 for theD-I transition, implying that despite the drastic change in the molecular order at this transition, the associated volume change is extremely small.