Molecular dynamics simulation of semi-flexible mesogens

Abstract

Molecular dynamics simulations are reported for systems of semi-flexible model mesogens composed of seven tangential hard spheres. The simulations employ the ‘rattling spheres’ approach whereby atomic sites are constrained to lie within narrow potential wells, with free-flight molecular dynamics occurring in between elastic collisions of spheres. The phase diagams of four systems with different bonding constraints have been studied. It is found that the addition of a small amount of molecular flexibility to the chains of hard spheres increases the density at which the isotropic to nematic phase transition takes place. Too much flexibility however destroys the nematic phase completely. At high density the most rigid of the systems forms a smectic-A phase and there is strong evidence for ordering within the smectic layers themselves at densities near the fluid-solid transition. There is also evidence that increasing molecular flexibility pushes the formation of the solid phase to higher densities. Calculation of equivalent molecular moment of inertia spheroids and mean molecular lengths indicate that significant changes in shape occur with density. For the most flexible chain studied molecules become noticeably ‘shorter’ and ‘fatter’ as density is increased. The reverse occurs for chains which form a nematic phase. Model molecules become ‘longer’ and ‘thinner’ as density is increased within the nematic phase. These effects are attributed to a strong coupling between internal molecular structure and molecular environment. Results are also reported for the order parameters, radial distribution functions, structure factors and single-particle structural data for the systems studied.