Structure/Property Relationships of Siloxane-Based Liquid Crystalline Materials

摘要

The structure/property relationships of a class of cyclic-siloxane basedmolecules were investigated. X-ray diffraction and electron microscopy techniques were used to initially investigate commercially available pentamethylcyclopentasiloxane rings with various molar ratios of biphenyl-4-allyloxybenzoate and cholesteryl-4-allyloxybenzoate mesogens attached. Results indicated a strong dependence of the X-ray diffraction patterns on the composition of the mesogens. Three dozen new siloxanes with mesogens were synthesized to investigate the packing behavior in more detail. Variables investigated included siloxane ring size, siloxane shape, leader group length, and mole fraction cholesterol. The phase behavior of these compounds was characterized using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction including real-time synchrotron experiments. The amount of cholesterol and the leader group length both substantially affected the liquid crystalline packing behavior. Cholesterol served to disrupt the homogeneity of the interdigitated smectic structure while increasing the tendency of the mesophase to layer pack. Lengthening the leader group increased the packing efficiency among mesogens. Certain compounds could be drawn into long, birefringent fibers with the orientation of the mesogens parallel to the fiber axis. The advantage of using cyclic siloxane rings was shown to lie in their processability. Optically clear thin films and fibers were readily obtainable....Liquid crystal, Smectic, Spectroscopy, Cholesteric, X-ray diffraction, Siloxane, Electron microscopy.