MODELING A NEMATIC LIQUID-CRYSTAL

The bulk phase liquid crystalline behaviour of a cyclic siloxane with a pentamethylcyclosiloxane core and biphenyl-4-allyloxybenzoate mesoge ns (BCS) was studied using molecular dynamics (MD) and wide angle X-ra y analysis. This material exhibits partial crystallinity at room tempe rature and liquid crystalline behaviour above 120 degrees C. For the M D simulations an ensemble of 27 molecules with 135 mesogenic units was simulated and a molecular mechanics force field was used to model the structural anisotropy of the siloxane molecules. Simulations were car ried out both at room temperature and at an elevated temperature (425 K). Room temperature simulations showed that, contrary to our initial assumptions, the low energy molecular conformations were not cylindric al but splayed in shape. During the simulation a smectic-like, tilted layer structure was found to evolve for the cluster when full atom pot entials were used, while no such development was observed when electro static interactions were neglected. The presence of a tilted layered s tructure was also suggested by the X-ray data. These results indicate that long range electrostatic interactions are significant for the mol ecular system under study. In order to calculate the orientational ord er parameter, the orientation of the molecular axis had to be determin ed. This was achieved by describing the mesogen shapes to be ellipsoid al and defining the principal axis of the ellipsoids to be the molecul ar directors. By sampling over 200 ps of simulation at 425 K, the time averaged order parameter (S) was calculated. The calculated S of 0 . 36 was comparable to the value of 0 . 4-0 . 45 found from the experime ntal data. Apart from providing insight into the relative importance o f the various competing forces in the formation of the liquid crystall ine phase, these simulations are also expected to be useful in predict ing the mesophase behaviour of liquid crystalline systems.