Flow-Directed Assembly of Cellulose Nanocrystal Dispersions with Ordered Film Applications
Type of DegreePhD Dissertation
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This dissertation presents the results of highly collaborative research on the fluid-phase assembly of cellulose nanocrystals (CNC) into ordered films. Due to their extraordinary properties, CNC are increasingly being considered as nanoscale building blocks for advanced materials. However, controlled and predictable assembly into ordered structures is necessary for CNC’s outstanding properties to manifest in macroscale objects. This work focused on CNC films with two different structures: 1) nematically aligned films for application as the structural layer in microelectromechanical systems (MEMS), and 2) chiral films for application as optical devices such as reflectors, security papers, or filters. The key contribution of this research has been to obtain insight into the relationship between dispersion microstructure, shear response, and the final structure and properties of the film. These results can be used to guide the development of specific applications based on CNC films. In the first portion of the work, rheo-optical microscopy and microspectrophotometry were used to gain new insights into the interrelated effects of liquid crystalline phase behavior, flow, and microstructural relaxation on CNC films. Aqueous liquid crystalline CNC dispersions showed greater alignment after shear than isotropic or biphasic dispersions. However, CNC gels exhibited lower alignment at equivalent shear rates. Optical contrast measurements were found to be an effective and facile way of quantifying microstructural relaxation after the cessation of shear and the anisotropy of CNC films. The combination of greater initial alignment and slower relaxation of sheared liquid crystalline dispersions resulted in nematically aligned films. Depending on their thickness, these films can be optically transparent in the visible regime or exhibit tunable interference colors. Using a novel microfabrication scheme, the nematically aligned films were processed into MEMS test devices including doubly clamped beams (DCB), cantilever beam arrays (CBA), residual stress testers (RST), and mechanical strength testers (MST). Using these devices it was determined that the CNC film has an elastic modulus of 72 GPa, a compressive residual stress of 66 kPa, and a fracture strength of 2 GPa. While not quantified, it was clear from curvature in different cantilever beams that the films had anisotropic mechanical properties. In the latter portion of the work rheo-small-angle neutron scattering (rheo-SANS) was used to investigate the shear behavior and relaxation of CNC dispersions, mostly in the biphasic regime. Order parameter increased with both shear rate and concentration. Interestingly, three region like behavior was observed in both the rheology versus shear rate and order parameter versus shear rate curves for the biphasic dispersions. Based on the rheo-SANS and rheo-optical microscopy data, a mechanism has been proposed to explain this behavior. Preliminary results of the modeling of this system are also presented. In future work, the model will be used to further explain the flow behavior of CNC and to help guide the fabrication of chiral films through controlled relaxation of flow-aligned CNC dispersions. These studies on the flow behavior of CNC liquid crystal highlight the interplay between dispersion microstructure, shear response, and microstructural relaxation. The processing methods studied provided a foundation for establishing the processing route on large-scale assembly of CNC films with controlled morphology.