EŽects of the Spontaneous Polarization on the Structural and Dynamic Properties of Ferroelectric Liquid Crystals
David A Coleman, ,
University of Colorado
The creation, nearly 30 years ago, of chiral tilted smectic liquid crystals produced spontaneous polar order in a fluid for the first time. Unlike solid-state ferrolectrics in which the polarization is restricted by the symmetry of a crystal lattice, ferroelectric smectics can freely orient in two dimensions. Consequently, the liquid crystal polarization is extremely sensitive to interactions with external bounderies, electric field, and its own intrinsic polarity. Optical and x-ray techniques have been used to study these interactions in the following three cases. Total Internal Reflection is used to probe the molecular organization at the interface between a solid substrate and a tilted chiral smectic liquid crystal at temperatures in the vicinity of the bulk antiferroelectric-ferroelectric phase transition. Optical reflectivity data are interpreted using an exact analytical solution of a real model for ferroelectric order at the surface. We discuss the conditions for polar order at the surface, and then demonstrate that in the mixture T3, ferroelectric surface order is expelled below the bulk ferroelectric-antiferroelectric transition. The continuously reorientable (XY-like) ferroelectric polarization density of a chiral smectic liquid crystal is shown experimentally to produce nearly complete screening of the applied electric field in an appropriate cell geometry. This screening, combined with the expulsion of polarization charge for large polarization materials, leads to semiconducting electrical behavior of the otherwise insulating liquid crystal and electrostatic control of the orientation of a uniform optic axis/polarization field. Finally, we determine the structure of an achiral ferroelectric liquid crystal in which the polarization spontaneously produces a periodically splayed structure. The structure of the smectic layers is determined by x-ray diffraction and the molecular order within the layers is deduced from polarized optical microscopy observations. On the basis of the x-ray measurements, we are able to differentiate between two optically similar phases.