Computer simulation study of liquid crystal formation in a semi-flexible system of linked hard spheres

Abstract

Results are reported for a molecular dynamics simulation study of a flexible model mesogen composed of seven tangential spheres. We follow the 'rattling spheres' method by which bonded atoms are constrained to lie within narrow potential wells. The dynamics of the system is that of a hard sphere fluid with added constraints. The phase diagram of our model system is calculated as a function of density and shows the presence of three fluid phases. These are assigned to be isotropic, nematic and smectic-A phases. Results are reported for the orientational order, radial distribution functions, structure factors and single-particle structural data within these phases. A small but significant change in shape is measured as the nematic phase is entered from the isotropic liquid. This is interpreted in terms of a quenching of allowed conformations parallel to the director.

Semiflexible Polymers in the Bulk and Confined by Planar Walls

“…While for simple fluids, the basic object of the theory is the spatially nonuniform density ρfalse(boldrfalse), for semiflexible macromolecules, one needs to operate with a function ρmol(r,ω), which depends not only on the particle position r, but also on the local orientation ω ( ω is a shorthand notation for the polar angles θ , ϕ of the molecular bonds). Thus, our recent work [42,43,44,45] not only goes far beyond previous simulation approaches (e.g., [46,47,48,49,50]) by simulating much larger systems (up to 700,000 monomers) and varying parameters, such as L and ℓp over much wider ranges than were accessible in the earlier work, but we also extend the DFT methodology for semiflexible polymers by considering simultaneously both of their orientational distribution and spatial inhomogeneity. The comparison with MD, in turn, provides a stringent test of the conditions under which these extensions are accurate and elucidates the reasons for its limitations.…”

“…Chain connectivity was modeled by anharmonic springs (described by UFENE(r)+UWCA(r)) in the MD work and by requiring the hard spheres to be tangent in the DFT work, while much of the earlier theories [12,13,14,15,16,17,18,19,20,21] were based on the continuum version (Equation (24)) of the Kratky–Porod model, where chain interactions then are described like in Onsager’s theory for long and thin hard rods [24] via the second virial coefficient [12,13,14,15,16,20] or modifications thereof [17,18,19,21]. We have not reviewed here the early simulation work (e.g., [46,47,48,49,50]), since most of this work dealt with comparatively short chains and relatively small systems, less suitable to characterize the I-N transition and the character of the nematic phase, but we have included results from early work (e.g., [49]) where appropriate. Both the behavior in the bulk solution and the effect of confinement by repulsive planar walls was considered.…”

Photo-switchable liquid crystalline brush as an aligning surface for liquid crystals: modeling via mesoscopic computer simulations

“…The last term in Eq. ( 1) involves the angle ζ 0 = 0, and forces a parallel orientation of the terminal spherocylinder to its host side chain [41]. For the sake of brevity we will introduce the following energy unit: j = 10 −20 J.…”

Simulations using hard particles

“…Very recently, a study has been conducted of semi-flexible chains of hard spheres (Wilson & Allen 1993). The aim was to complement other studies of linear, rigid chains (Whittle & Masters 1991;Amos & Jackson 1992), and begin to understand the role of molecular flexibility on the position of phase transitions and the characteristics of the orientationally ordered phases.…”