Engineering Cellulose Nanofibers For Better Performance as Nanocomposites
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In recent decades there has been great interest to produce novel bio-based composites to reduce carbon footprint without sacrificing the necessities that society demands. To achieve a more sustainable future, research in cellulose biopolymers has risen to the forefront. Impressive mechanical, thermal and optical properties along with its abundant biomass has made nanocellulose (NC) the subject of intense research in the area of electronics, drug delivery, sensors, selective filters, and structural materials, to name a few. The practical utility of any cellulose-based materials requires a more complete understanding of how the fundamental structure affects final performance. This thesis examines several avenues to obtain novel materials by considering processing parameters and preparation methods for working with raw nanocellulose materials, and mechanochemical approaches for surface grafting to obtain modified CNs with improved dispersion in organic media. Lastly, the synergy between the two studies and its impact on advanced materials and nanocomposites is discussed.
The low cost and wide availability of cellulose nanofibers (CNF), a refined form of cellulose microfibrils, make these an ideal starting material for our studies. However, the aggregated states of freeze-dried CNFs hinder its use as an additive for reinforcing polymer blends or functional films. The use of tert-butyl alcohol (TBA) as a stabilizer in pharmaceutical drugs has been well studied for its effectiveness in facilitating redissolution and extending product shelf life. Lyophilization of aqueous CNF slurries treated with various amounts of TBA produced a more porous material that could be redispersed with superior colloidal stability relative to untreated freeze-dried CNFs. Furthermore, CNFs lyophilized from aqueous TBA mixtures could be subjected to mild mechanochemical reactions (horizontal ball milling) to produce esterified nanofibers with high degrees of substitution (DS) and good dispersibility profiles in organic solvents. This solventless technique allowed for a variety of carboxylic acids to be grafted onto CNF surfaces. Finally, investigations of new materials with technological utility have been explored using networks of CNFs modified with oleic acid. These can be cast into superhydrophobic (SHP) films having a hierarchical structure characteristic of a self-similar material, with a wettability comparable to that of the lotus leaf. The SHP surface can also be regenerated after surface fouling or physical damage.