Inorganic Nanocylinder Lyotropic Liquid Crystals: Rheology, Phase Behavior and Film Self-Assembly
Type of Degreedissertation
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This dissertation provided key examples of inorganic nanocylinder lyotropic liquid crystalline phase formation including silver nanowire nematic and silica nanorod smectic liquid crystals. Inorganic nanocylinder liquid crystals are a relatively new extension to the established fields of colloid and liquid crystal science. The two very different systems (silver nanowires with high aspect ratio and polydispersity, and silica nanorods with very low aspect ratio) near the Onsager theory limits for liquid crystalline phase formation are interesting to study. They can be used as model experimental systems for comparison with the theoretical calculations and computational models. The phase behavior of the two systems was evaluated using a combination of optical microscopy, rheology, and differential scanning calorimetry. The phase behavior of the silver nanowire and nanosphere aggregate dispersions was proved as a function of the combination of solvent quality, total silver volume fraction and nanowire/nanosphere ratio. Cross-polarized optical microscopy of the silver nanowire dispersions showed a strand-like biphasic morphology and a Schlieren liquid crystalline texture which are the characteristic morphology of nematic liquid crystals. The results also showed that silver nanosphere aggregates facilitated the nematic liquid crystalline phase formation of silver nanowire due to rod-sphere demixing. The phase diagram of silica nanorod dispersions highlights the significant effect the solvent quality has on the phase behavior. The isotropic-biphasic transition PhiI was significantly affected by the solvent quality. Decreasing the relative DMSO concentration from 100% to 40% resulted in an over two order of magnitude decrease in PhiI. Within this range, the solvent composition had little effect on PhiLC. However, below 40/60 DMSO/H2O crystal solvates formed and a single phase liquid crystal could not be achieved. Instead of a strand-like biphasic morphology, and Schlieren liquid crystalline textures, the dispersions exhibited characteristic smectic “onion” or “oil streak” structures that depended on the silica nanorod size and shape distribution. The rheology characteristics of the two systems were consistent with expectations for lyotropic liquid crystals including: long oscillatory transients, the Cox-Merz rule not being obeyed, negative first normal stress difference N1, and a maximum in the viscosity versus concentration curve. Some rheology characteristics that are unique for smectic liquid crystal were also observed in silica nanorod smectic liquid crystal dispersion. Several methods to assemble the dispersions into solid coatings were explored. The structures of coatings assembled by drying low aspect ratio silica nanorod biphasic dispersions in the absence of shear showed the characteristic “coffee ring” structure with rods aligned along the outer edge. While applying the same method to silver nanowires and nanosphere aggregates system, no “coffee ring” structure with alignment was observed. However, for both systems, the coating microstructure was dependent on the initial microstructure and the applied shear. These studies on the inorganic nanocylinder liquid crystals highlight their dispersion microstructures, rich rheology and phase behavior. The processing methods studied provided a foundation for establishing the processing route on large-scale assembly of inorganic nanocylinders with controlled morphology.
- Ph.D. Dissertation-Teng Xu.pdf