Optical investigations of molecular arrangement of smectic liquid crystals.

摘要

By optical techniques, the research described in this thesis is focused on three projects. The first one is to study the molecular arrangement in surface layers of freestanding liquid crystal films. Due to the enhanced surface ordering, at temperatures above the bulk SmA-SmC* transition, the surface layers can be tilted. Experimentally it has been identified that the tilt orientations on the two surfaces can be the same (synclinic) or opposite (anticlinic). In this thesis, we present experimental evidence for the existence of the nonplanar molecular arrangement (the surface structure is neither synclinic nor anticlinic). Furthermore, we will show the evidence for a remarkable synclinic-anticlinic-synclinic-anticlinic double reentrance upon both cooling and heating under a proper electric field.; The second project is to study the novel smectic phases formed by bent-core molecules. We will demonstrate unambiguously the existence of the biaxial SmA phase. The critical exponent associated with the biaxiality, the difference of the dielectric constants in the layer plane, has been obtained to be 0.82 +/- 0.04. This unusually large value is in good agreement with the critical exponent of the second harmonic order parameter of a 3D-XY multicritical scaling theory. Moreover, we report for the first time that the surface layers of freestanding films can exhibit a behavior depending on the thickness of the film in number of layers is even or odd under an applied in-plane electric field on the order of 20V/cm.; Thirdly, we have investigated one achiral meta-substituted three-ring mesogen. Mirror symmetry breaking has been reported in a recent article published in Physical Review Letters. However, no mirror symmetry breaking has been observed in our studies. The SmC1 and SMC2 turn out to be the conventional SmC and SMCA phases, respectively.