Structures and properties of liquid crystals and related molecules from computer simulation

Abstract

Computer simulations provide a powerful tool for the investigation of liquid crystalline phases. In this thesis the ability of simulations to calculate accurately the values of material parameters of liquid crystal molecules is investigated. An all-atom force field for liquid crystal molecules is developed using first principles density functional theory calculations on small organic molecules, which encompass key structural features of a range of common liquid crystalline molecules. Molecular dynamics simulations of these 'fragment' molecules are carried out in the liquid phase to test the force field parameters by determining densities and heats of vapourisation. Good agreement is found between experimental values and those calculated from simulation. Equilibrium molecular dynamics (MD) calculations were then performed for the nematogen n-4-(trans-n-pentylcyclohexyl)benzonitrile (PCH5). These simulations were performed using a fully atomistic model for several temperatures. The MD trajectories were used to obtain densities, order parameters, and values for the rotational viscosity coefficient 71. Several methods of obtaining 71 were tested based on the director angular velocity correlation function, the director mean-squared displacement, and statistical mechanics methods based on the rotational diffusion model. Good agreement is obtained between experimental values of 71 and those found from simulation. Further MD simulations of PCH5 using a 216 molecule system and the force field derived in this thesis were carried out to calculate the flexoelectric coefficients e(_s) and e(_b) for PCH5. The temperature dependence of and was examined along with the separate contributions to e(_s)and e(_b)arising bom the electrostatic and van der Waals interactions. The calculated values of e(_s)and e(_b) are consistent with available experimental data. The van der Waals and electrostatic contributions are found to be of similar magnitude and opposite sign.