Computer simulation of polar bent-core molecules

Abstract

Results are presented from molecular dynamics simulations in the NPT ensemble of novel bent-core liquid crystal systems. Following on from a previous study of bent-core steric shape, this study examines the effect the addition of a transverse electric dipole has on the phase diagram of a bent-core liquid crystal model. A simple model of the interaction employed a two-site Gay-Berne potential with the sites separated by +/-0.5 reduced units with a central transverse point dipole, for all models investigated. The angle between the sites 180 degrees -gamma was varied in a range gamma=10 degrees to gamma=70 degrees suggested by real molecules. The addition of the dipole to the model tended to stabilize smectic phases and increase the angle of tilted phases. As the angle gamma increased, the transition temperature to the first ordered phase decreased markedly. Smectic A, tilted smectic B, and a spontaneously polarized smectic B phases were observed in the gamma=10 degrees bent-core model. The gamma=20 degrees model showed smectic A and tilted antiferroelectric smectic B phases. The gamma=40 degrees model showed an antiferroelectric phase that exhibited unusual packing behavior. Both the gamma=20 degrees and gamma=40 degrees models demonstrated a significant phase biaxiality in the smectic B phases.

Computer Simulations of Biaxial Nematics

“…Moving to attractive-repulsive potentials, similar results have been obtained with the GB models simulated by Memmer [133] and Neal and coworkers [134,135] who performed NPT MC simulations of rigid bent-core dimers with aspect ratio 1 : 1 : 3 and various apex angles. The isotropic to N u transition temperature was found to decrease reducing the aperture angles.…”

“…Interestingly, close to the nematic-smectic phase transition a spontaneous chirality symmetry breaking [136] was reported [133,134,137] to produce organisations related to those predicted by Lubensky and Radzihovsky [4], or observed by Görtz and Goodby [19]. The presence of a central transverse dipole [135] suppresses the N u phase to give transitions from isotropic to smectic phases, and this is at variance with results from atomistic simulations (see later on). However, two terminal tilted dipoles embedded into the arms of a three GB bent-core model have been found with MC simulations to stabilise the N u phase with respect to smectic ordering [138] (see Figure 10).…”

“…[46] reports N b mesogens with 90 degrees aperture). Certain intermediate apex angles destroy the N u [131,134,135]. To provide a N b phase a bent molecular shape is not sufficient, and other interactions terms (e.g.…”

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

“…The Gay-Berne dimer model has been studied extensively, with and without molecular dipoles. In the works by Memmer 9 and Johnston et al 10,11 , isotropic, nematic, tilted smectic, and helical phases were found, depending on the molecular bend angle 9,10 and the magnitude of the dipole moment 11 . Xu et al studied composite molecules made up of repulsive soft spheres, and found isotropic and tilted crystalline phases 12 .…”

Molecular simulation study of self-assembly of tethered V-shaped nanoparticles

“…17,18 Molecular liquid crystals exhibit many exquisitely ordered phases that arise from packing considerations. [35][36][37][38][39] Molecular chirality and long-range polar interactions are proposed theoretically, 12,14 computationally, [40][41][42][43] and experimentally 26,38 to be two major factors leading to the spontaneous formation of chiral structure and helical morphologies in the B phases of bent-core molecules. In 1996, Niori 19 was the first to experimentally demonstrate that these molecules form various unprecedented liquid crystalline phases.…”