Optimization of Cellulose Nanocrystal Films for Optical and Micromechanical Applications
Type of DegreePhD Dissertation
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The objective of this research is to understand the effect of microstructural helix orientations and alignment in aqueous cellulose nanocrystals (CNC) dispersions and films on mechanical, optical, and surface properties for potential macroscale applications. The first part of this research involves understanding the effects of CNC dispersion concentrations, shear response, drying conditions, and surface anchoring to obtain controlled planar ordering of the cholesteric microstructures in films for photonic applications. CNC films with tunable selective reflections of visible spectra were fabricated, providing new insights into the spatial pitch defects previously reported in lomaptera beetles. The fundamental understanding of film fabrication provided insights into elucidating the governing parameters that control the cholesteric helix orientation and evolution of the planar microstructures. In the second part, the objective was to apply rheo-optics and 3D finite element modeling to monitor the shear response and relaxation dynamics of CNC mesogens after flow cessation. The dispersion rheology and rheo-optics results provided a better understanding of the CNC microstructures’ response to flow alignment. The insight into the effects of dispersion concentration, shear response, and drying on uniaxial ordering guided the fabrication of microelectromechanical systems (MEMS) from shear aligned CNC films followed by micromechanical testing. The goal was to unwind the cholesteric microstructure into nematic and retain it in films to obtain the uniaxial CNC alignment for anisotropic (directional) device properties. The micromechanical devices fabricated from aligned CNC films included mechanical strength testers (MSTs), residual stress testers (RSTs), cantilever beams arrays (CBAs), and doubly clamped beams (DCBs). These devices were tested to study macroscopic film properties including anisotropic elastic moduli, residual stress, fracture strength, and electrostatic actuation for potential specialty applications of CNC MEMS. This new paradigm of producing MEMS via low temperature processing of CNC derived from waste biomass provided the simplicity and tunability of fluid phase processing and enabled anisotropic mechanical properties within an order of magnitude of standard polysilicon devices. In the third part, CNCs’ functionalization with cancer biomarkers was investigated for potential applications of CNC MEMS in biosensing. The compatibility of the CNC MEMS platform for multianalyte detection was researched based on the immobilization of the following cancer biomarkers: alpha fetoprotein (AFP), prostate specific antigen (PSA), and carcinoembryonic antigen (CEA) for liver, prostate, and ovarian cancer detection respectively. In summary, this research provides the fundamental insights on dispersion microstructures, shear alignment, and drying effects for tunable mechanical and optical properties in CNC films for macroscale applications.