Columnar discotics in confined geometries

The influence of geometric confinement on the state of order and on the gla ss relaxation process was investigated for a triphenylene derivative able t o display a highly ordered plastic columnar phase in the bulk. The compound was incorporated into porous glasses-characterized by a narrow size distri bution-with average pore diameters of 20, 7.5, 5 and 2.5 nm. The X-ray diag rams revealed the presence of a hexagonal order, yet the lattice spacing is significantly reduced with decreasing pore size and the reflections become broad. The X-ray doublet reflection, superimposed on the halo which is cha racteristic for the bulk plastic columnar phase, is absent in all cases. It is replaced by a single broad intracolumnar reflection which indicates tha t the confinement destabilizes the plastic phase in favour of the hexagonal ordered phase. A further observation is that the intracolumnar correlation length is reduced with decreasing pore size. The confinement was furthermo re found to cause a transition from a strong glass (bulk material) to a fra gile glass former, obviously induced by the structural modification.