COMPUTER-SIMULATION STUDY OF A FLEXIBLE-RIGID-FLEXIBLE MODEL FOR LIQUID-CRYSTALS

Small molecules that form liquid crystals typically consist of a rigid core with flexible tails on one end or on both ends. To date, most co mputer simulation studies have used completely rigid models such as ha rd spherocylinders: cylinders, characterized by their length/diameter ratio L/D, with hemispherical end caps. We have studied a model consis ting of spherocylinders with L/D = 4, with a flexible tail attached to each end. The tails are 'ideal' in the sense that they have no volume . Using Monte Carlo simulations the phase behaviour of this model was studied and, for comparison, the behaviour of hard spherocylinders wit h L/D = 4 without tails was studied as well. The addition of the tails is found to stabilize the smectic-A phase at a lower pressure, and th e nematic phase disappears. In the smectic-A and crystal phases, the s mectic layers are further apart when tails are added. The structure of the layers and the smectic-A-crystal transition pressure change only a little. For both models close to melting the crystal consists of ord ered layers, but there is almost no correlation between particle posit ions in neighbouring layers. In fact, the layer coupling is so weak th at in a long simulation the layers are found to glide over each other. As the pressure is increased the crystal gradually becomes more order ed and the crystalline layers ultimately 'lock' into place.