Effect of molecular aspect ratio on structure, dynamics and phase stability of thermotropic liquid crystals studied by molecular dynamics simulation

Due to the importance of liquid crystals in modern technology and food industry, understanding their structural and dynamical properties are scientifically important. In this study, we applied extensive molecular dynamics simulations to infer the effect of the molecular elongation on the formation, stability and dynamics of liquid crystal mesophases in a system of Gay-Berne model mesogens with parameters GB (k,k ' = 20, mu = 1, nu = 1). Four aspect ratio values, k = 2, 3, 4.4 and 5, were studied at pressure P* = 4. Thermodynamics properties, the order parameter and pair distribution functions (both translational and positional), helped to identify phases, hysteresis, and consequently phase diagrams. By increasing aspect ratio from 2 to 5, nematic and smectic phases were induced and all transition temperatures were shifted to higher values, hence, the range of stability of the nematic and smectic mesophases became wider. The analysis of the coordination numbers and angular distribution functions showed that the structure of the solid phase of system with k = 2 at lowest temperatures was fcc which by increasing the aspect ratio changed to a complete hexagonal in-layer structure. Moreover, by increasing the axial ratio of particles, decoupling between the principal components of diffusion coefficients increased, where it reflects an increase in the molecular orientational order in the system with higher k. The effect of increasing aspect ratio was more pronounced on inter-layers diffusion rather than the in-layers. In all mesophases parallel components of diffusion coefficients were greater than the perpendicular components. We analyzed the results based on the existing theoretical descriptions and experimental observations.