Atomistic simulation of biaxial liquid crystals and mixtures of liquid crystals

Abstract

In this thesis molecular dynamics (MD) simulations at a fully-atomistic level have been undertaken to study the biaxiality and the structure of the liquid crystalline phase formed by the para-heptylbenxoate diester of 2.5-bis-(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP-Ph-C(_7)), which is a bent-core mesogen. This has been the first time the transition between isotropic and liquid crystalline phases has been achieved using a fully atomistic (all-atom) potential. Simulations at five different temperatures covering the nematic range of ODBP-Ph-C(_7) have been undertaken to study the temperature dependence of the biaxial ordering. Ferroelectric domains have been observed in all the systems. Simulations started from the biaxial nematic phase have been performed with the partial charges turned off to study the influence of the electrostatic interactions on the behaviour of the system. A system composed of ODBP-Ph-C(_7) and the deuterated molecule hexamethylbenzoate-d10 (HMB) has also been simulated to check the validity of the (^)2HNMR method, which is often employed to study biaxiality. MD simulations at a fully-atomistic level have been also performed for the mixture of liquid crystals E7， commercialized by Merck. The nematic phase for this mixture is grown from an isotropic phase using an fully atomistic (all-atom) potential, and in order to study the temperature dependence of the order parameter simulations at six different temperatures covering the nematic range have been performed. The internal structure of the mixture, alongside some of its material properties such as rotational viscosity and flexoelectric coefficients have been studied. Finally, ab initio calculations involving several molecular fragments which are components of some of the most common mesogens have been carried out to calculate torsional energies of key dihedral angles. Subsequently, torsional energies have been fitted using a Fourier series expansion to obtain torsional parameters for an atomistic force field. These will be used in future atomistic simulations of liquid crystals.