UC Davis

Cellulose nanofibers from bioresources have garnered intensive research interest in the past decades due to their unique properties including high tensile strength and modulus, low density, biocompatibility/biodegradability, and abundant hydroxyls for surface chemistry. The nanofibers with surface negative charges, such as cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) obtained from sulfuric acid hydrolysis and 2,2,6,6-tetramethyl-piperidin-1-yl-oxyl (TEMPO) mediated oxidation, respectively, have been widely studied as reinforcements in hydrophilic polymer nanocomposites, hydrogels/aerogels, and nanopapers, however, they are not compatible with hydrophobic polymers or organic solvents. Currently, the most common way to produce hydrophobic nanocelluloses is functionalization of CNCs and TEMPO-CNFs that were previously prepared from cellulose, which is not the most efficient way to obtain nanocellulose. This Ph.D. dissertation describes our streamlined approach to produce hydrophobic nanocelluloses that are compatible with hydrophobic polymers and organic solvents in an environmentally-sustainable method, and their applications. The novel method of generating hydrophobic nanocellulose was demonstrated by one pot telomerization combined with mechanical shearing directly from native cellulose derived from rice straw. Butadiene sulfone (BDS) served both as 1,3-butadiene (1,3-BD) reagent and as a reaction medium to carry out the telomerization reaction. Optimized telomerization of 1,3-BD with cellulose at 110 C, followed by disintegration of 2,7-octadienyl-ether (ODE) functionalized cellulose by mechanical blending yielded ca. 27-41 wt % ODE-nanocelluloses (NCs). The thickness of these NCs, as measured by atomic force microscope (AFM), varied with the choice of solvent: 3.7 nm in dimethyl sulfoxide (DMSO), 6.3 nm in tetrahydrofuran (THF), and 4.4 nm in CHCl3. The surface ODE groups were confirmed by fourier transform infrared spectroscopy (FTIR) peaks at 2800-2980 cm-1 for the methylene stretching vibration and at 1640-1704 cm-1 for the C=C stretching vibration. Solution state 1H, 13C, Heteronuclear Single Quantum Coherence (HSQC), and homonuclear correlation spectroscopy (COSY) nuclear magnetic resonance (NMR) elucidated the structure of ODE groups and cellulose backbones in ODE-CNFs. The Tmax of ODE-CNFs (332 °C) was revealed to be far superior to that of TEMPO-CNFs. The direct disintegration of ODE-cellulose into ODE-NCs in organic media was demonstrated using ultra-sonication. This is a benefit because it avoids an extra solvent exchange step in producing a fabricated nanocomposite. Scale-up (0.5 g cellulose) telomerization at the optimal condition generated ODE-NCs with a degree of substitution (DS) of 0.67 mmol ODE/g-cell. This agrees relatively well with the DS 0.74 mmol ODE/g-cell of ODE-NCs produced from the original scale (0.1 g cellulose), validating the scale-up reaction. To optimize the DS for ODE-cellulose, diffusion of liquid BDS into cellulose was improved by pre-sonicating cellulose (50 % amplitude (A), 3 min) in dimethylformamide (DMF) prior to telomerization at the average temperature range between 103 °C and 110 °C. This produced ODE-cellulose with the optimal DS 1.2 mmol ODE/g cellulose (“g-cell”) and 1.8 mmol ODE/g-cell in the temperature range of 100-110 °C. The 1.2 DS ODE-cellulose was further sonicated (50 % A, 20 min) to yield 45.5 % ODE-NCs in toluene. The 1.8 DS ODE-cellulose was further sonicated (50 % A, 20 min) to yield 73.3 % ODE-NCs in linseed oil (LO). Both were directly brought into processing of nanocomposites. The reinforcing effect of 2 % 1.2ODE-NCs and the ability of alkene groups in 1.2ODE-NCs to associate/cross-link with polybutadiene (PBD) and styrene-blk-isoprene-blk-styrene block copolymer (PSIS) were demonstrated in nanocomposite films formed under varied conditions. The optimal condition for reinforcing cellulose paper with 0.07 % 1.8ODE-NCs in LO was heating at 70 °C for 16 h, because it successfully transformed cellulose paper into the most reinforced hydrophobic paper. Cellulose has been optimally isolated from almond shells (AS) in 35.2 % yield by a two-step NaClO2/KOH process. Subsequent TEMPO (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl mediated oxidization generated ribbon shaped cellulose nanofibrils (CNF) with 1.2 ± 0.44 nm height, 5.2 ± 1.2 nm width, and 1.6 ± 0.8m length in 90 % yield. Anisotropic 4.3 cross-sectional width-to-height aspect ratio with dominant hydrophilic planes and high length-to-height aspect ratio (1167) are distinctively unique to AS-CNFs. This study elucidated how this characteristic served in construction of material forms by analyzing different assembling and disassembling behaviors by three CNF material forms created under vaired solidification conditions. Fibers that were rapidly frozen (-196 °C) and freeze-dried were readily 100 % redispersible in water into CNFs with the original size indicating that they were assembled by predominantly polar-polar CNF associations. Aerogels fabricated from a slow freezing (-20 °C) and freeze-drying process, exhibited an amphiphilic characteristic by absorbing both water and chloroform. They redispersed only 10 wt % in water into CNFs with the original size indicating that nonpolar-nonpolar interfacial CNF associations are their dominant force for assembling along with some polar-polar CNF interactions. Films, from the slowest solidification via casting under the ambient condition, exhibited polar surfaces with water contact angles of 34-42° while they were partially redispersible in water (71.7 wt %), ethanol (11.4 wt %) and dimethylacetamide (7.4 wt %), indicating that they were formed by using dominant polar-polar and minor nonpolar-nonpolar interfacial associations. The higher char residues of the films in TGA may indicate that slow solidification process in air induced a calcitrant characteristic. All data combined suggest that solidification speed and environments largely influence strength and surface hydrophobicity/hydrophilicity/amphiphilicity of TOCNF materials, and their fundamental basis of material construction seems to be supported by polar-polar TOCNF associations between wider hydrophilic-to-hydrophobic planes which are characteristic to AS-TOCNFs.