Induced Anticlinic Ordering and Nanophase Segregation of Bow-Shaped Molecules in a Smectic Solvent

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Recent experiments indicate that doping low concentrations of bent-core molecules into calamitic smectic solvents can induce anticlinic and biaxial smectic phases. We have carried out Monte Carlo (MC) simulations of mixtures of rodlike molecules (hard spherocylinders with length/breadth ratio L rod/D = 5) and bow- or banana-shaped molecules (hard spherocylinder dimers with length/breadth ratio Lban/D = 5 or 2.5 and opening angle ψ) to probe the molecular-scale organization and phase behavior of rod/banana mixtures. We find that a low concentration (3%) of Lban/D = 5 dimers induces anticlinic (SmCₐ) ordering in an untilted smectic (SmA) phase for 100∘ ≤ ψ< 150∘. For smaller ψ, half of each bow-shaped molecule is nanophase segregated between smectic layers, and the smectic layers are untilted. For Lban/D = 2.5, no tilted phases are induced. However, with decreasing ψ we observe a sharp transition from intralamellar nanophase segregation (bow-shaped molecules segregated within smectic layers) to interlamellar nanophase segregation (bow-shaped molecules concentrated between smectic layers) near ψ= 130∘. These results demonstrate that purely entropic effects can lead to surprisingly complex behavior in rod/banana mixtures.