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Simulations of bent core molecules using molecular dynamics

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The Mathematics of Liquid Crystals

The nematic phase is normally uniaxial i.e. there is just one distinct optical axis. In such a phase one set of molecular axes are aligned but the other axes are orientationally disordered. In 1970, however, Freiser showed that a biaxial nematic phase was theoretically possible, in which all three molecular axes are aligned. This phase has since been observed experimentally, predicted to exist theoretically for various particle models and has been seen in simulation studies. Such a phase would have three distinct optical axis and there are possible applications to liquid crystal displays, should a suitable material be found. In this talk I would like to present the results of simulation studies on purely repulsive bent-core models (V-shaped particles). In the limit of very long, thin arms, such shapes have been predicted to exhibit a biaxial nematic phase for bend angles in the region of 110o. In order to test these predictions and also to explore the systems pha se behaviour at pressures at which the nematic phase is unstable, we have run molecular dynamics simulations for one-component systems, binary mixtures and higher order mixtures (4 6 components). In all case the runs started from the isotopic phase and the system, was then slowly compressed, so any phase observed had formed spontaneously. We explored the effects of varying the arm lengths and bend angles of these particles on the phase behaviour. For a one-component system, only particles with bend angles greater than ca. 130o spontaneously formed ordered, equilibrated phases. Typically the phase sequence was isotropic →uniaxial nematic→biaxial smectic A (except for very straight particles in which the smectic phase was uniaxial). No biaxial nematic phase was observed for the arm-lengths simulated. It is possible that if the smectic phase could be destabilized, a biaxial nematic might form in its place. Such destabilization might occur in mixtures. Binary mixtures of bent cores, however, were found.

This talk is part of the Isaac Newton Institute Seminar Series series.

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