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|Title:||Tailoring Cellulose Nanocrystal, Polymer and Surfactant Interactions for Gels, Emulsions, and Foams|
|Abstract:||This thesis describes the investigation of the properties of cellulose nanocrystals (CNCs) in water and at interfaces in the presence of different water-soluble polymers and surfactants. The potential of producing hydrogels, emulsions, and foams using both CNCs and surfactants and polymers is extensively explored herein. Interactions between CNCs and polymers were studied by measuring adsorption of polymers on CNC-coated surface in quartz crystal microbalance with dissipation monitoring (QCM-D) and surface plasmon resonance (SPR) instruments. Hydroxyethyl cellulose, hydroxypropyl guar, and locust bean gum adsorbed onto CNC-coated surfaces, whereas dextran did not adsorb. Gelation of CNC dilute dispersions was found for the samples added with adsorbing polymers, whereas the introduction of non-adsorbing polymers showed no such change of rheological behaviors of CNC dilute dispersions. The further addition of negative surfactant SDS or non-ionic surfactant Triton X-100 disrupted the gels whereas cationic surfactant CTAB did not. These behaviors illustrate the complexities associated with including CNC dispersions in formulated water-based products where polymers and surfactants are commonly used as well. The adsorption of cationic surfactants on CNC particle surfaces and the associated change of CNC hydrophobicity were investigated. Surfactant-modified CNCs were then employed as emulsifying agents to determine the effects of stabilizing oil-water interface with CNCs after surfactant addition. Emulsion stability was substantially enhanced with the introduction of surfactants. Based on the chemistry of cationic surfactants, and the extent CNC surface hydrophobicity increases after surfactant binding, either oil-in-water or water-in-oil emulsions were successfully produced. This in situ surfactant adsorption method thus offers a simple way of modifying surface hydrophobicity of CNCs and allows fine tuning of CNC-based emulsion properties. Adsorbing polymers were used together with CNCs to prepare stable emulsions. The introduction of polymers facilitated the production of emulsion droplets with enhanced stability and smaller diameters. Both polymer-coated CNCs and the extra polymers partitioned at the interface and worked as the emulsifiers in a synergistic manner, leading to a reduction in CNC coverage on the emulsion droplet surfaces. Furthermore, reversible thermogelation of the emulsion was obtained when thermosensitive polymers were added. No noticeable emulsion coalescence occurred after multiple cycles of heating and cooling treatments of the emulsion gels. Freeze-drying and air-drying of these emulsion gels produced oil powders containing oil content as high as 94 wt. %. Finally, highly stable wet foams were successfully produced using CNCs and the water-soluble polymer, methyl cellulose. The effect of CNC and methyl cellulose concentration on the stability of air-water interfaces was elucidated. Both foamability and foam stability were greatly improved by adding CNCs to methyl cellulose solutions. The CNC particles helped to retain fluid in the films and plateau borders between bubbles, increasing bulk viscosity, and impeding water drainage. We also demonstrated that adding various monomers to CNCs- methyl cellulose wet foams did not lead to noticeable foam breaking. The successful production of macroporous structures with tailored chemistry and properties was achieved by subsequent polymerization of the monomers added to the foam.|
|Appears in Collections:||Open Access Dissertations and Theses|
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|Zhen Hu PhD thesis final.pdf||thesis||15.87 MB||Adobe PDF||View/Open|
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