Computer simulation study of ferroelectric liquid crystal phases

• Main Author: Ayton, Gary Steven Douglas
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6239

Molecular dynamics and Monte Carlo simulations are used to study ferroelectric liquid crystals. The effect of molecular shape on the stability and nature of ferroelectric phases is examined. The effect of molecular flatness on the stability of uniaxial nematic liquid crystal phases is studied. Most liquid crystal molecules are elongated and have a degree of flatness. However, most models used in computer simulation of liquid crystals are axially symmetric. A series of molecular dynamics simulations, specially designed to isolate the effects of molecular flatness on liquid crystal phases, are performed. Molecular shape is approximated with a generalization of the Gaussian overlap model [ B. J. Berne and P. Pechukas, J. Chem. Phys. 56, 4213 (1972)]. The form presented here has been extended to include ellipsoidal particles with non-degenerate semi-axes. It is found that small amounts of molecular biaxiality can drive an isotropic to nematic phase transition. Simulations of randomly frozen and dynamically disordered dipolar soft spheres are used to study ferroelectric ordering in spatially amorphous materials. Systems where the dipole moment has 1, 2, and 3 components are considered. It is found that the 1 component (Ising) model has ferroelectric phases. The systems with 2 and 3 dipolar components form disordered phases at low temperatures. A ferroelectric phase diagram is constructed for oblate molecules with point dipoles embedded along the particle symmetry axes. The role of particle shape on the stability of ferroelectric liquid crystals is examined with molecular dynamics simulation. Molecular shape is modeled with the generalized Gaussian overlap. Ferroelectric phases are found in systems with weak (short-ranged) columnar correlations. Systems with long-ranged columnar order are found to be antiferroelectric. It is found that the stability of ferroelectric phase is very sensitive to details of molecular shape, and exists only in small regions of the phase diagram. A more robust model for a ferroelectric liquid crystal is developed. Monte Carlo calculations are used to examine ferroelectric order in fluids of disc-shape particles with embedded dipoles. The dipoles are uniformly distributed over a circular "patch" of finite size, placed in the central plane of the particle. It is shown that such systems may undergo spontaneous polarization to form a stable ferroelectric discotic nematic phase.