High pressure investigations on the phase behaviour of discotic liquid crystals

Liquid crystals made of disc shaped molecules, now referred to as discotic liquid crystals, were discovered [1] in 1977 and represent a new class of thermotropic liquid crystals. Since the discovery, there are only a few studies [2-9] concerning the effect of pressure on these materials. Here we report high-pressure investigations on the columnar phase of several triphenylene compounds We selected the compounds such that for one set of materials the number of peripheral substituents, x, was fixed and the length of the alkoxy chains, n, was varied, while for the second set n was fixed and x varied. We observe that with increase in either n or x, the slope of the pressure-temperature phase boundary, i.e., (dT/dP)_1bar shows a strong decrease. The magnitude of the transition enthalpy and the calculated transition volume across the columnar-Isotropic transition show a strong decrease with increasing n or x. To explain these features we propose an argument based on the ratio of the volume of the triphenylene core to the total volume of the molecule.