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Chiral nematic mesoporous organosilica materials templated with cellulose nanocrystals

University of British Columbia

The synthesis and characterization of new mesoporous organosilica materials with twisted porous networks and defined nanostructures was explored. Spindle-shaped cellulose nanocrystals (CNCs) with nanoscale dimensions can be isolated through the acid-catalyzed hydrolysis of wood pulp. These CNCs behave as chiral nematic lyotropic liquid crystals and are advantageous for the soft-templating of mesoporous materials as they can produce helically arranged pores. As such, CNCs show promise as inexpensive and renewable precursors for the development of nanomaterials. In this thesis, CNCs were used to template chiral nematic mesoporous organosilica (CNMO) materials through the co-condensation of bridged alkoxysilanes, R(Si(OR′)₃)₂, with aqueous CNC suspensions. Removal of the CNCs produced free-standing organosilica films that have interconnected pore structures and long-range chiral order imparted by the CNC template. Organic functionality was introduced as an integral component of the organosilica structure by varying the alkoxysilane precursors. The syntheses of alkylene-bridged (C₁-C₆), aromatic-bridged, ethenylene-bridged, and sulfur-containing CNMO films are reported. It was determined that phase separation between the alkoxysilanes and CNC suspension could be minimized by using mixed solvent systems of water and DMF during self-assembly. The new CNMO films display tunable photonic properties resulting from the repeating helical structure as well as thermal stabilities and pore connectivity that depend on the organic linker used. The combination of chirality and mesoporosity in these organosilica films suggests applications in hard templating, chiral catalysis, and sensing. CNMO films were investigated as support materials by functionalizing them with spiropyran molecules for sensing or manganese salphen complexes for catalysis. Spiropyran-bound CNMO films were used for photopatterning and behaved as reversible divalent metals sensors. Heterogeneous manganese salphen/CNMO films displayed similar catalytic conversion to the homogeneous catalyst, however, a small enantiomeric excess (5%) was observed suggesting that the chiral environment of the films may affect catalysis. Finally, metal ferrites [MFe₂O₄ (M = Ni, Cu, Zn, Co)] with chiral nematic nanostructures were prepared through hard templating with chiral nematic mesoporous silica. These materials beautifully replicated the three-dimensional structure of the CNCs liquid crystalline phase and their crystallinity and pore sizes were controlled by altering the calcination temperatures.

Text

2019-01-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/64160

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/