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

Abstract

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a 'V' shape with external bend angle γ. With purely repulsive sphere-sphere interactions and γ = 0 • (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, smectic-A, and tilted phases. With γ ≥ 20 • the smectic-A phase disappears, while the system with γ ≥ 40 • shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of tilt in the smectic and solid phases, destabilize the nematic and smectic-A phases of linear molecules, and destabilize the tilted smectic-B phase of bent-core molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases -with and without attractive sphere-sphere interactions -are presented. The results are encouraging, but the computational effort required is a drawback associated with the use of fluctuation relations.

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

“…47, we do not observe any smectic-layer structures. 47, we do not observe any smectic-layer structures.…”

Computer simulations of biaxial nematics

“…A five-site bent model with terminal flexible chains was also studied [84] to find that both smectic and N u phases disappear. The effect of flexibility on the phase diagram, even though for linear chains, has been throughly studied instead by Galindo et al [141] with Gibbs ensemble MC simulations finding that the polymorphism is strongly affected by the non-rigid model.…”

Computer Simulations of Biaxial Nematics

“…The multi-site models of rigidly connected LJ spherical sites simulated by Dewar and Camp [140,84] produce results comparable the other bent-core models, but in this case the effect of a central transversal dipole seems to favour the N u phase [84]. A five-site bent model with terminal flexible chains was also studied [84] to find that both smectic and N u phases disappear.…”

“…Theories for the elastic [76,77,78,79,80,81,82,83,41], flexoelectric [84,85], and rheological [86] behaviour of N b have also been proposed. However, computer simulations of the associated mesoscopic coefficients (e.g.…”