THE NATURE OF THE GLASS-TRANSITION IN A COLUMNAR HEXAGONAL ORDERED PHASE

The glass transition taking place in a columnar discotic phase is conn ected with the freezing-in of the correlated rotation of the disc-like molecules in the columns about the columnar axis. X-ray investigation s on the temperature dependence of the linear expansion coefficients a long the columnar axis and perpendicular to this axis reveal that the glass transition leads to a stepwise change of the expansion coefficie nt along the columnar axis and to a continuous variation for the corre sponding coefficient perpendicular to this direction. The glass transi tion in such ordered systems is highly anisotropic both with respect t o dynamic and to thermodynamic aspects.