| Molecular dynamics (MD) simulation studies of the phase behavior, the response to an applied field of nematic liquid crystalline (LC) materials and interactions of nanoparticles in isotropic mesogenic materials are presented in this work. Molecular models used include the rigid bead-necklace model and soft spherocylinders. Free energy calculations applying thermodynamic integration and the Gibbs-Duhem integration method were used to establish the (T, P) phase diagram of the repulsive bead-necklace model, subsequently the Gibbs-Duhem integration method was further utilized to investigate the influence of attractive interactions on the phase behavior of the bead-necklace model. Analysis of order and thermodynamics of LC phase transitions (Isotropic-Nematic transition and Nematic-Smectic A transition) demonstrate that this simple model can capture the basic physics of liquid crystalline phases, and good agreement with experimental results is obtained. Further addition of chemical details to this multiple interaction sites model is much easier than to the idealized models (Gay-Berne, Spherocylinders) while the computation cost increase with respect to these idealized models is minimal. With a mean field representation of field-molecules interaction, MD simulation studies of the switching behavior of nematic LC, which is the basis of many LC devices, were performed. The switching mechanisms were explained in terms of the compromise between the elastic energy and field-molecules interactions. Qualitative agreement with experiments confirmed the validity of the mean field approximation. Finally, using the standard umbrella sampling technique and MD simulations, the potential of mean force between two nanoparticles in solvent of spherocylinders is calculated. It is found that while dispersed nanoparticles will delay the Isotropic-Nematics transition to higher density (lower temperature), they can induce local ordering fluctuations (within a few molecular lengths of the solvent rods), which are different from natural paranematic fluctuations by faster decay with respect to distance and broader distribution of local ordering. Apart from the expected short ranged nanoparticle interactions due to molecular packing effects, the above mentioned induced fluctuations will cause long range repulsions, a novel interaction being discovered and characterized for the first time. |
