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UBC Theses and Dissertations

Understanding the self-assembly process of cellulose nanocrystals : towards chiral photonic materials Tran, Andy


Over millions of years, animals and plants have evolved complex molecules, macromolecules, and structures that endow them with vibrant colors. Among the sources of natural coloration, structural color arises from the interaction of light with nanoscale features rather than absorption from a pigment. Cellulose nanocrystals (CNCs) are materials derived from biorenewable resources that form a chiral nematic liquid crystal phase in water. Interestingly, the chiral nematic structures are maintained in solid films of CNCs obtained by drying the liquid crystalline phase. The self-assembly process of CNC building blocks is a complex topic that is not entirely understood; thus, further investigation of the self-assembly process will be crucial in developing tunable and colorful films. CNC films were prepared and the self-assembly process was investigated by varying the evaporation times for a drying CNC suspension. CNC films with reflected colors spanning the visible range were prepared by controlling the evaporation time, with the most blue-shifted reflection emerging from films formed with the slowest evaporation rates. An intermediate stage of self-assembly occurring before kinetic arrest helps explain the discrepancies in chiral nematic order of films prepared at different evaporation times. Local restriction of evaporation resulted in a patterned film with tunable optical properties. These thin film materials, although brittle, represent a system with localized control of the chiral nematic structures. The development of mechanically responsive photonic materials based on CNCs was made possible with the incorporation of elastomers. Elastomers containing a chiral nematic arrangement of CNCs inside were prepared and the resulting material showed reversible visible color upon mechanical stimulation. This material exhibits colors spanning the visible spectrum depending on the amount of stretching. CNCs are an exciting building block that can be applied to sustainable material development. To develop unique photonic materials for different applications, the intricacies of the self-assembly process must be further investigated. Specifically, the aim of this thesis is to study the chiral nematic organization of CNCs, starting from a suspension into a solid and colorful material. Photonic materials based on CNCs are attractive for applications in sensing and privacy but could also serve in decorations and coatings.

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