| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 100 === Bent-core liquid crystals are achiral structures with spontaneous polarization and chirality properties. In this thesis, the identifications and classifications of banana-phase types, electro-optical properties, and bent-core structural configurations were introduced. Meanwhile, the influences of H-bonded interaction and in bent-core supramolecular materials and its nanocomposites on the mesomorphic and electro-electric properties were investigated in this doctoral thesis as well.
In chapter 2, a hydrogen-bonded complex with a branched siloxane units exhibited a series of general tilt smectic (SmCG) phases with highly ordered layer structures. As the SmCG domains were aligned under dc electric fields, a gradual decrease in the leaning angle from ca. 60 to 50° could be determined by in-situ wide-angle X-ray scattering (WAXS). Combined with Fourier transform infrared and Raman spectroscopic data, our results suggest that the change in the leaning angle was governed by the competition of the hydrogen bonds and microsegregation of siloxane units within the bilayer structure of the hydrogen-bonded complex with a branched siloxane units. In addition, the ferroelectric–(anti-ferroelectric)–ferroelectric transitions proven by the switching current responses in the SmCG-type phases.
In chapter 3, in this study we prepared novel nanocomposites from the hydrogen-bonded bent-core liquid crystal (LC) host and different weight ratios (0–20 wt%) of the surface-modified gold nanoparticles. The mesomorphic properties of the nanocomposites, including their mesophasic types and ranges, values of Ps, and dielectric constants, were adjustable by varying the doping concentrations of gold nanoparticles. By increasing the concentration of gold nanoparticles , the polarity of the SmCP phase in the nanocomposite varied sequentially from anti-ferroelectricity sequentially to ferroelectricity and to nonpolarity. Furthermore, the molecular stackings and layer arrangements of the LC molecules were aligned parallel and perpendicular to the electric fields, respectively, as confirmed from the associated X-ray diffraction patterns. Direct evidence from transmission electron micrographs revealed that the well-organized layer packings of the surface-modified gold nanoparticles in the nanocomposites could be induced under electric fields.
In chapter 4, novel liquid crystalline materials with rod-like molecules bearing amide group, bent-core molecules containing all ester groups and amide groups have been synthesized. The influence of blending system for binary systems was studied by lateral H-bonded interactions. An antiferroelectric switching smetic C phase was found for the complex rod-like molecules / bent-core molecules with amide groups = 3/7. Therefore, rod-like liquid crystal bearing an amide linkage induced the polar switching behaviors of bent-core liquid crystalline with amide groups.
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