Effects of electric field, surface alignment and guest materials in cholesteric liquid crystals.

摘要

This dissertation research is focused on understanding aspects of cholesteric liquid crystals when different effects are taken into account, such as external field, surface alignment effect and guest materials, aiming possible applications such as for displays. More specifically, we studied the effect of different chiral dopants on the physical properties of the nematic host, showing that large changes may happen depending on the nature of the dopant, which as a matter of practicality, can be used for designing better displays. We found a naturally occurring chiral dopant that improves many physical parameters of the nematic host, yielding better displays. We also studied the effect of external electric fields and the transitions between cholesteric textures. This study led us to propose driving schemes and display designs, where extensive efforts were made to optimize active and passive matrices for bistable cholesteric displays. We also performed studies on the surface effect for improving the features of cholesteric displays. Furthermore, we used a surface enhanced effect to study the wetting of cholesteric liquid crystals under the isotropic-cholesteric transition and the stripe pattern occurring in the wetting layer. By using experiments and computer simulations we have shows that the delicate anchoring at the isotropic-cholesteric interface induces the pattern observed, depending on the elastic anisotropy and pitch length of the material. We also present a study of reflection broadening induced by DC fields when a small amount of polymer is dispersed in the cholesteric material, which is a great advantage over previous reported methods to induce bandwidth broadening and can be used as a switchable mirror. We propose a mechanism to explain the data. Last we include results on cholesteric phases made with bent-cores, and mixtures of bent-core and rod-like cholesteric liquid crystals. The results found in this last chapter yields the possibility of UV detectors as well as cholesteric displays sensitive to light, temperature and electric field.