Molecular Dynamics Simulations of Elongated Molecules

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Abstract

The existence of a thermotropic biaxial nematic liquid crystal phase has been a topic of great interest for almost half a century. Of the various mesogenic shapes suggested as being able to form this phase, theory has suggested that the V-shaped or "bent-core" molecule is one of the most promising candidates. In this thesis we use a simple mesogenic model of a bent-core molecule, constructed from a number of repulsive Weeks-Chandler-Andersen potentials that are assembled into a rigid V shape. Using this model we explore the spontaneous phase behaviour that occurs in a wide array of different systems of mesogens, using molecular dynamics simulations and isotropic initial conditions. We study the relationship between molecular bend angle and phase behavior for molecules constructed from 11 potentials. We find that the phase behaviour splits into two regions, above and below a critical bend angle. Molecules wider than this angle exhibit isotropic, uniaxial nematic and smectic A phases. Narrower molecules show no uniaxially aligned phases, and instead have a clustered phase with short-range ordering and no global alignment director. Increasing system size improves the smectic layering in the wider molecules, but does not affect the global alignment of the narrower molecules. Our model is extended to include the effect of the arm length of the molecule by changing the number of potentials from which the mesogens are constructed. As the molecule is reduced in size, the critical bend angle is seen to move slowly towards more linear molecules, reducing the size of the parameter space in which uniaxial nematic alignment is possible. At 5 beads, all mesophases are seen to disappear and systems remain isotropic. We also study the behaviour of binary mixtures of bent-core molecules, both of differing arm lengths and of differing bend angles. For arm length mixtures, molecules are seen to remain mixed in the isotropic and nematic phases, and phase separate on transition to a smectic phase. In addition, uniaxial nematic phases are induced in systems that have no nematic phase of their own in isolation. For mixtures of different bend angles, systems remain fully mixed in the smectic phases for differences of up to 10 degrees, and beyond this the two components begin to separate at the nematic–smectic transition.

Keyword(s)

bent-core; liquid crystals; molecular dynamics

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