Gelation of cellulose nanocrystals

Colloidal gels of cellulose nanocrystals (CNCs) were prepared using three different strategies by manipulating their colloidal stability. In the first approach, colloidal gels were prepared by hydrothermally treating CNC suspensions. Desulfation of the CNCs at high temperature appears to be responsible for the gelation of the CNCs, giving highly porous networks. In the second approach, a carbon dioxide-switchable (CO₂-switchable) hydrogel was prepared by adding imidazole to a suspension of CNCs. Sparging of CO₂ through the imidazole-containing CNC suspension led to gelation of the CNCs, which could be reversed by subsequent sparging with nitrogen gas (N₂) to form a low-viscosity CNC suspension. The gelation process and the properties of the hydrogels were investigated by rheology, zeta potential, pH, and conductivity measurements, and the gels were found to have tunable mechanical properties. This work describes a straightforward way to obtain switchable CNC hydrogels without the need to functionalize CNCs or add strong acids or bases. These CO₂-responsive CNC hydrogels have potential applications in stimuli-responsive adsorbents, filters, and flocculants. Lastly, physical colloidal gels were prepared by freeze-thaw (FT) cycling of CNC suspensions. The aggregation of CNCs was driven by the physical confinement of CNCs between growing ice crystal domains. FT cycling was employed to form larger aggregates of CNCs without changing the surface chemistry or ionic strength of the suspensions. Gelation of CNC suspensions by FT cycling was demonstrated in water and other polar solvents. The mechanical and structural properties of the gels were investigated using rheometry, electron microscopy, X-ray diffraction and dynamic light scattering. It was found that the rheology could be tuned by varying the freezing time, the number of FT cycles, and concentration of CNCs in suspension. Considering the wide natural abundance and biocompatibility of CNCs, these approaches to CNC-based hydrogels are attractive for producing materials that can be used in drug delivery, insulating materials, and as tissue scaffolds.Science, Faculty ofChemistry, Department ofGraduat