Liquid Crystalline Phase Behavior and Fiber Spinning of Double-Stranded DNA Stabilized Single-Walled Carbon Nanotube Dispersions
Type of Degreedissertation
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This dissertation describes the first discovery and characterization of lyotropic cholesteric single-walled carbon nanotube (SWNT) liquid crystals phase where the SWNT acts as a mesogens and is not simply inserted into an existing lyotropic dispersion. The liquid crystal phase was formed by concentrating mixtures of SWNT and aqueous double-stranded deoxyribonucleic acid (dsDNA). The characteristic cholesteric liquid crystal fingerprint texture with multi-colored regions was obtained due to the presence of the cholesterogenic forming biopolymer dsDNA. Depending on the dispersion methodology, the polydomain nematic phase previously reported for other lyotropic carbon nanotube dispersions could also be obtained. The phase behavior and dispersion microstructure were affected by the relative concentrations of dsDNA and SWNT and whether small bundles were removed prior to concentrating the dispersions. The phase behavior and rheology of aqueous dsDNA/SWNT dispersions were determined by a combination of rheology and optical microscopy. The results indicated that SWNT in dsDNA/SWNT dispersions behave as rigid rods. With increasing concentration the dispersions transitioned from a dilute phase with free rotational and translational rod motion, to a semidilute phase where rod rotation was inhibited, to a biphasic regime consisting of isotropic and liquid crystalline phases, and finally to a single liquid crystalline phase. The results of this research will enable new fundamental investigations comparing nematic and cholesteric liquid crystalline phase behavior and shear response. The simpler, aqueous cellulose nanocrystal (CNC) system was also investigated to provide greater insights into the more complex dsDNA/SWNT system. The CNC system also formed a cholesteric liquid crystal, but the effects of concentration on rheological properties were markedly different from both the dsDNA/SWNT system and classical lyotropic liquid crystalline polymers. The assembly of biopolymer stabilized SWNT dispersions into films and fibers was also investigated. In the case of cholesteric dsDNA/SWNT dispersions, casting films onto a substrate without any applied shear enabled retention of the helical microstructure. This finding may enable the production of films that possess not only SWNT strength and conductivity, but also with optical signatures. Shearing cholesteric dispersions during processing resulted in densely packed aligned films that have potential use as polarized films. Preliminary investigations of wet solution spinning of dsDNA/SWNT dispersions showed the significant impact the coagulant had on fiber microstructure. Investigation of lysozyme, (1-tetradecyl) trimethyl-ammonium bromide, SWNT dispersions (LSZ/TTAB/SWNT) fiber spinning resulted in promising microstructural characteristics and mechanical properties. As a result, fiber spinning optimization is being pursued by other researchers. The results of this research highlight the rich rheology and phase behavior of nanocylinder dispersions. These insights have provided a foundation for producing aligned bulk materials consisting of nanocylinder building blocks. In particular, the previously never achieved lyotropic cholesteric microstructure from SWNT dispersions and the potential enhanced biocompatibility of dsDNA/SWNT suggest that the range of applications that can be processed from liquid crystalline nanotube dispersions may be even broader than previously thought.