Molecular Dynamics Simulations of 2-(4-butyloxyphenyl)-5-octyloxypyrimidine and 5-(4-butyloxyphenyl)-2-octyloxypyrimidine Liquid Crystal Phases
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Molecular dynamics simulations of the liquid crystal phases of 2-(4-butyloxyphenyl)-5-octyloxypyrimidine (2PhP) and 5-(4-butyloxyphenyl)-2-octyloxy-pyrimidine (5PhP) are the focus of this thesis. The 2PhP and 5PhP mesogens display different liquid crystalline phase sequences, despite having very similar molecular structures. Specifically, both mesogens consist of aromatic phenyl and pyrimidine cores in between two flexible alkoxy tails, but they differ in the preferred core conformation. A multi-site coarse-grained model, in which the aromatic rings are represented by soft quadrupolar ellipsoids and the alkoxy chains are given a united atom representation, is proposed in this thesis. A parameterization route for the intra- and intermolecular potentials appropriate for liquid crystal simulations is developed. The ab initio based derivation of suitable molecular models for the two mesogens is discussed in detail, with particular emphasis on capturing proper phenyl-pyrimidine interactions which proved to be essential to correctly represent core-core interactions between neighboring molecules. A systematic determination of suitable Gay-Berne (GB) parameters has been adopted for the aromatic rings of 2PhP and 5PhP. To account for the pi-electron cloud below and above the ring plane, a quadrupole was added perpendicular to the ring. In the end, four parameterizations for aromatic rings have been selected for the simulations. Model characterization via pair interactions proved to be valuable in identifying and analyzing the short range structure in the phases. Extensive molecular dynamics simulations of these fluids at various temperatures are performed. Intermolecular structure and order, in aromatic core and the flexible tail regions, are analyzed. Intermolecular structure is divided into contributions parallel and perpendicular to the layers, as indicated by a layer normal or by a director, to differentiate smectic A (SmA) from smectic C (SmC). The presence of a ring quadrupole in the molecular model is shown to be essential to the correct reproduction of the experimentally observed phases. Simulations correctly indicate phases in agreement with experiment: SmC and SmA phases for 2PhP, and only a SmA phase for 5PhP.