On the rectangular-hexagonal columnar phase transition in disc-like liquid crystals

We report the isobaric binary phase diagram for two disc-like mesogens, hexa-n-octyloxytriphenylene and hexa-n-octanoyloxytriphenylene, which was studied by means of optical texture observations, differential scanning calorimetry and X-ray diffraction measurements. They all confirm the presence of a direct transition between an hexagonal ordered columnar phase and a rectangular disordered columnar phase; this kind of transition is also described in a pure disc-like mesogen : hexa-n-decanoate of truxene. Furthermore we show, within the binary phase diagram, a strongly non-ideal behaviour with a marked maximum of the spindle between the isotropic and the mesomorphic phase. The existence domain of the hexagonal ordered phase versus temperature and concentration is discussed from a structural point of view.