Microscopic structure and dynamics of a partial bilayer smectic liquid crystal - art. no. 051703

Cyanobiphenyls (nCB's) represent a useful and intensively studied class of mesogens. Many of the peculiar properties of nCB's (e.g., the occurence of the partial bilayer smectic-A(d) phase) are thought to be a manifestation o f short-range antiparallel association of neighboring molecules, resulting from strong dipole-dipole interactions between cyano groups. To test and ex tend existing models of microscopic ordering in nCB's, we carry out large-s cale atomistic simulation studies of the microscopic structure and dynamics of the Sm-A(d) phase of 4-octyl-4'-cyanobiphenyl (8CB). We compute a varie ty of thermodynamic, structural, and dynamical properties for this material , and make a detailed comparison of our results with experimental measureme nts in order to validate our molecular model. Semiquantitative agreement wi th experiment is found: the smectic layer spacing and mass density are well reproduced, translational diffusion constants are similar to experiment, b ut the orientational ordering of alkyl chains is overestimated. This simula tion provides a detailed picture of molecular conformation, smectic layer s tructure, and intermolecular correlations in Sm-A(d) 8CB, and demonstrates that pronounced short-range antiparallel association of molecules arising f rom dipole-dipole interactions plays a dominant role in determining the mol ecular-scale structure of 8CB.