Simulation study of the phase behavior of a primitive model for thermotropic liquid crystals: Rodlike molecules with terminal dipoles and flexible tails

摘要

A primitive model for small mesogenic molecules is proposed, consisting of three elements: (i) a rigid rodlike core, modeled as a hard spherocylinder of length/diameter ratio L/D=5; (ii) a flexible end group, consisting of five segments of length D, which is "ideal" in the sense that it has no volume; (iii) a terminal dipole, located in the end cap opposite the flexible tail. This model is studied using Monte Carlo computer simulation, and the dipolar interactions are evaluated using the reaction field method. The hard spherocylinder model displays four phases: isotropic, nematic, smectic-A and crystal. Previously, it was found that the addition of the terminal dipole to hard spherocylinders without tails greatly enhances the range of stability of the nematic phase, at the expense of the smectic-A phase [McGrother , J. Phys.: Condens. Matter 8, 9649 (1996)]. Conversely, adding the flexible tail to hard spherocylinders without dipoles is found to suppress the nematic phase, whereas the smectic-A and crystal phase are little affected. Combining the effects of the terminal dipole and the flexible tail, all four phases survive. Because of the dipoles, the particles prefer to adopt a staggered antiparallel arrangement. In the smectic-A and crystal phases, this gives rise to interdigitation of the smectic layers. In the crystal phase a tendency towards columnar ordering is observed. The results are compared with experimental observations. (C) 2000 American Institute of Physics. [S0021-9606(00)50420-6]. [References: 50]