Pressure studies on two discotic liquid crystals.

Pressure-temperature diagrams have becn studied for two discotic compounds, hexa-n-octyloxy triphenylene and hexa-n-decanoyloxytriphenylenc, both of which exhibit the columnar (D) phase at atmospheric pressure. Two interesting results, common to both compbunds, 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 the D-I transition, implying that despite the drastic change in the molecular order at this transition, the associated volume change is extremely small