Interactions in Anisotropic Nanomaterial Dispersions
Type of DegreePhD Dissertation
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The overall objectives of this research were to advance understanding of interactions between carbon nanotubes and biological systems, and self-assembly of anisotropic nanomaterials. Specifically, this research presents results in two main areas: (1) the interactions of single-walled carbon nanotube (SWNT) dispersions with biomolecules and bacteriological systems, and (2) cholesteric liquid crystal flow and relaxation dynamics. In the first thrust of this research, the objective was to investigate the conflicting results in the literature regarding the antibacterial activity of carbon nanotubes. A comparison was made of the differences in antibacterial activity of covalent and non-covalent SWNT dispersed with an antibacterial enzyme, lysozyme (LSZ). The initial rates of antibacterial activity were shown to be affected by covalent versus noncovalent interaction and strong films were made from both dispersions. Additional antibacterial testing protocols developed in collaboration with Dr. Mark Liles group at Auburn University found that SWNT are not intrinsically antibacterial. However, their presence in a dispersion may hasten cell death in bacteria under significant cell wall stresses. This finding is significant because it clarifies that the conflicting results in the literature are due to stressful environments for bacterial cell walls, which skew results towards antibacterial activity. While SWNT were not found to be the root cause of this activity, they do exhibit a synergistic effect under the conditions of extreme cell wall stress. In the second research thrust, the flow and relaxation dynamics of cholesteric liquid crystal dispersions were explored. Cellulose nanocrystal (CNC) dispersions were explored using rheological, rheo-small-angle neutron scattering (rheoSANS) and rheo-optical methods. The results from rheoSANS experiments exhibited three-region behavior, which has been elusive for CNC dispersions, in viscosity versus shear rate curves but it was discovered that there was also three-region behavior in the order parameter. This behavior exhibited more defined transitions than that obtained from rheology measurements of biphasic CNC dispersions. Computational modeling, developed by Dr. Micah Green research group from Texas A&M University, and a series of rheological and rheo-optical experiments, performed at Auburn University, yielded a Landau-de Gennes formulation in a 3-D dynamic finite element simulation in which ellipsoid maps were created to predict alignment and helix orientation as a function of gap height and chiral strength during post-shear relaxation. This model was extended to include mass transfer and predict the orientation of CNC during film drying. Prediction of film microstructure is especially useful for the fabrication of controlled optical property films. These models were developed alongside extensive experimentation to verify their validity and accuracy to physical systems. Together, these research thrusts highlighted the importance of collaborative research involving multiple research methods to gain an in-depth understanding of interactions in anisotropic nanomaterial dispersions.