Liquid crystalline phase behavior in systems of hard-sphere chains

Abstract

A study of the liquid crystalline phase transitions in a system of hard-sphere chains is presented. The chains comprise m=7 tangentially bonded hard-sphere segments in a linear conformation (LHSC). The isothermal–isobaric Monte Carlo simulation technique is used to obtain the equation of state of the system both by compressing the isotropic (I) liquid and by expanding the solid (K). As well as the usual isotropic and solid phases, nematic and smectic-A liquid crystalline states are seen. A large degree of hysteresis is found in the neighborhood of the I–N transition. The results for the rigid LHSC system were compared with existing data for the corresponding semiflexible hard-sphere chains (FHSC): the flexibility has a large destabilizing effect on the nematic phase and consequently it postpones the I–N transition. The results of the simulations are also compared with rescaled Onsager theories for the I–N transition. It is rather surprising to find that the Parsons approach, which has been so successful for other hard-core models such as spherocylinders and ellipsoids, gives very poor results. The related approach of Vega and Lago gives a good description of the I–N phase transition. The procedure of Vega and Lago, as with all two-body resummations of the Onsager theory, only gives a qualitative description of the nematic order.

Computer simulations of hard pear-shaped particles

“…The tendency of these systems to become irretrievably interlocked was overcome in Ref. [25] by the use of reptation moves. This solution was not available to us here, however, since ours is a single-site model.…”

“…[25] where simulations were performed using a seven site linear hard sphere chain (LHSC) model. This model was found to form metastable glassy states in the vicinity of the isotropic-nematic transition due to the non-convex shapes of the particles which inhibited their ability to slide past one another.…”

Spatial Order in Liquid Crystals: Computer Simulations of Systems of Ellipsoids

“…We shall only briefly mention some other particle based model systems for liquid crystals: In a number of studies, liquid crystalline molecules have been modeled by stiff chain molecules [56,57,58,59,60,61,62,63]. with aspect ratio κ < 1 and spheres cut at both sides have been used to study discotic liquid crystals, i. e., fluids of disklike particles [64,65,66,67,68,69].…”

The isotropic-nematic phase transition of tangent hard-sphere chain fluids—Pure components

“…Typical model systems that have been studied in this respect, are hard discs, 14,15 rods, 16 cut spheres, 17,18 spherocylinders, 13,19 ellipsoids, 20,21 Gaussian overlap particles, 22 and hard-sphere chains. [23][24][25][26][27] Despite the simplicity of these molecular models, a rich phase behavior, involving isotropic (iso), nematic (nem), smectic, and columnar phases has been observed both by experiments 16 and by means of molecular simulations 13,17,20,27 and theory. 12,18,28 In the present work, the liquid crystalline state of interest is the nematic state.…”