Elasticity of thin layers of nematic liquid crystalline gels: Electro-optical, thermo-mechanical properties.

摘要

Nematic liquid crystalline gels are soft solid materials that combine the broken symmetry properties of liquid crystalline ordering and the elastic properties of crosslinked polymer backbones. Since the orientational responses of the liquid crystalline mesogens and the translational responses of the gel network are integrated into the same material, the related properties are strongly coupled as well, such as optical, mechanical and symmetry properties. In this thesis, we studied the optical and mechanical responses to an applied field and the thermal fluctuations of thin layers of nematic gel material both experimentally and theoretically.; Unique optical periodic stripe patterns within the mono-domain nematic gel were observed through polarized light microscopy when an electric field was applied to the sample. The simplest theoretical model, which only considers shear motions within two dimensions, successfully explained the physical reasons of the periodic response. Fluorescent confocal polarized microscopy observations were conducted by adding special dyes to the sample to understand the rotational response of the nematic director in three dimensions, with the collaboration of a research group at Kent State University. Theoretical calculations were tried to interpreted the physical reason for the 45° oblique angle in the stripe pattern, and some questions are still open for further research work.; Mechanical properties were also studied for the same material used for the electro-optical studies by floating the nematic gel films on top of water. We measured the mechanical and optical responses to the temperature change when the samples went through the nematic-isotropic phase transition. The material elongated in the direction along the nematic director and shrank in the directions normal to the nematic director as the material changed from the isotropic phase to the nematic phase. Phase separation between the nematic solvent and polymer backbones was also observed in polarized light microscopy.; By using the material synthesized in the research group of J. Kornfield at the California Institute of Technology, spontaneous buckling transitions of thin layers of nematic gel in the homeotropic cell were observed by polarized light microscopy. (Abstract shortened by UMI.)