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Abstract

Cellulose nanocrystals (CNC) are rod like polymer nanoparticles that can be extracted from a wide variety of biosources. CNCs have been investigated extensively in the past decade as important building blocks for the development of novel functional materials. This growing interest in CNC based materials is not only related to CNCs’ sustainability and bioavailability, but also their inherent properties, such as high elastic modulus, high aspect ratio, high surface area and low density. With the presence of a large number of hydroxyl groups on the surface, CNCs provide a platform for various surface functionalization with the goal of tuning their surface properties or introducing new functionalities which further extends their use in highly sophisticated applications. This dissertation focuses on the fabrication and characterization of functionalized CNC based polymer nanomaterials with the goal of understanding how the surface functionalities impact the macroscopic properties of the materials. In the first part of the work (Chapter 2 and 3), CNCs were covalently functionalized with alkylamines of different alkyl chain length to increase their hydrophobicity, and the resulting alkyl-functionalized CNCs have been investigated as particle stabilizers for oil-in-water Pickering emulsion. The results have shown that the hydrophobic/hydrophilic balance of the functionalized CNCs is critical to lower the interfacial tension between the oil and water phase, which allowed access to stable emulsions with droplet sizes of only a few hundred nanometers. The CNC stabilized nano-emulsions can be polymerized into latex nanoparticles, which were further fabricated into latex nanocomposite films. Microscopy analysis of the latex nanocomposites revealed a unique CNC network structure formed by excluded volume effect of the latex particles. The excluded volume lead to the formation of percolation networks at a much lower CNC concentration. As a result, the latex nanocomposites containing functionalized CNCs demonstrated a higher plateau modulus. When this percolating CNC-rich network was disrupted by melt pressing, a significant decrease in mechanical properties of the films was observed. The second part of the work (Chapter 4-6) focus on the preparation of CNC based polymer nanocomposites with water enhanced mechanical gradient properties inspired by the squid beak biomodel. The first-generation bio-inspired nanocomposite was fabrication based on allyl-functionalized CNCs embedded into a polyvinyl acetate matrix. The functionalized CNC fillers can be covalently crosslinked using UV initiated thiol-ene chemistry to yield composite films with tunable mechanical properties with a modulus contrast of 7 (E'stiff/E'soft=7) when swelled in water. With the goal of further improving the modulus contrast, polymer containing alkene functionalities were employed as matrix for the preparation of second-generation bio-inspired nanocomposite. Through crosslinking both the CNC fillers and the polymer matrix, it is possible to access larger wet mechanical contrasts upon crosslinking (E'stiff/E'soft=ca. 20). Finally, the third-generation nanocomposite was prepared based on carboxylic acid functionalized CNCs embedded into a partially hydrolyzed poly(vinyl acetate-co-vinyl alcohol) matrix with covalent crosslinking formed between the hydroxyl groups from both the CNC filler and polymer matrix. Owing to the strong hydrophilic nature of the material which effectively decreases the modulus of soft uncrosslinked materials (E'soft), the third-generation nanocomposites demonstrated a wet modulus contrast over 2 order of magnitude (E'stiff/E'soft>400) upon swelling in water.

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