Monte-Carlo Simulations for model bent-core molecules with fluctuating opening angle

TL;DR

This study uses Monte Carlo simulations to examine how fluctuations in the opening angle of bent-core molecules influence the stability and phase behavior of nematic liquid crystals, revealing phase stability variations with molecular flexibility.

Contribution

It introduces a simulation approach to analyze the impact of fluctuating molecular angles on liquid crystal phases, extending understanding beyond fixed-angle models.

Findings

Fluctuations affect the stability range of nematic phases.

Uniaxial nematic phase stability is enhanced with certain molecular parameters.

Helical superstructures are absent in the $ ext{kappa}=4$ model.

Abstract

We study the effect of fluctuations in opening angle of bent-core molecules on stability of the nematic phases. The molecules are built out of two Gay-Berne centers of $\kappa = 4$, $\kappa ' = 5$, $\mu = 1$ and $\nu = 2$, each corresponding to one of the molecular arms. Constant-pressure Monte Carlo (MC NPT) simulations are carried out for two versions of the model, with fixed- and fluctuating opening angle, where fluctuations are taken harmonic about the angle of 140o. The systems studied are first cooled down from the isotropic liquid to a highly ordered crystalline smectic phase and then heated up all the way back. The liquid crystalline phases found on cooling are uniaxial nematic, biaxial nematic and crystalline, hexagonal biaxial smectic. On heating the biaxial nematic is superseded by the metastable crystalline phase. Similar system with molecules of shorter arms ($\kappa = 3$) was shown to exhibit a wide range of reduced density, where the uniaxial nematic was absolutely stable [Liq. Cryst. 29, 483 (2002)]. For molecules with rigid arms of $\kappa = 4$ and opening angles of 125o, 130o, 135o and 140o a further broadening of the density range of the uniaxial nematic phase is observed. However, the helical superstructures reported for $\kappa = 3$ do not show up for $\kappa = 4$.