Self-assembling of TEMPO Oxidized Cellulose Nanofibrils As Affected by Protonation of Surface Carboxyls and Drying Methods

Abstract

Self-assembling of cellulose nanofibrils (CNFs) as affected by varying extent of protonation on C6 surface carboxyls was investigated under freeze-drying and air-drying processes. Surface carboxyls were protonated from 10.3 to 100%, all on the same TEMPO oxidized and mechanically blended CNFs with identical geometries and level of oxidation. Upon freeze-drying, all CNFs assembled into amphiphilic mass. The mostly charged CNFs assembled into the finest and most uniform fibers (ϕ = 137 nm) that absorbed significantly more nonpolar toluene than water; whereas the fully protonated CNFs assembled extensively into porous and more thermally stable ultrathin filmlike structures that absorbed water and toluene similarly. Ultrafiltration and airdrying induced cyrstallization led to more thermally stable, semitransparent, and hydrophilic films that showed no affinity toward nonpolar toluene. In essence, CNFs could be tuned by varying the degree of surface carboxyl protonation, along with drying processes, to create fibrous to film morphologies, amphiphic to hydrophilic properties, and higher thermal stability. KEYWORDS: Surface carboxyl protonation, Cellulose nanofibrils, Amphiphilic, Hydrophobic, Self-assembly, Thermal stability, TEMPO oxidation ■ INTRODUCTIONThe 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) mediated oxidation of cellulose breaks interfibril hydrogen bonds and converts C6 primary hydroxyls to carboxyls and/or carboxylates depending on the conditions. 1−3 TEMPO-mediated oxidation is among the most investigated defibrillation methods to liberate cellulose nanofibrils (CNFs) and has shown to reduce energy needed for defibrillating cellulose by at least 2 orders of magnitude.3 The defibrillation efficiency can be further aided by mild to moderate mechanical post-treatments that have included magnetic stirring, 4 ) cations could cause hydrogelation via chelation with CNF carboxylate anions 8,9 and protonation of sodium carboxylate to carboxylic acids could induce gelation to pH-responsive CNF hydrogels.10−12 Fully protonated CNFs also have shown to exhibit much better dispersibility in polar aprotic solvents, such as N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), 1,3-dimethyl-2-imidazolidinone (DMI), and 1-methyl-2-pyrrolidone (NMP). 13Protonation of surface carboxyls has also shown to affect the properties of solidified CNFs. Air-dried cast films from fully protonated CNFs showed higher oxygen and hydrogen permeability, 14,15 more resistant to cellulase and soil microorganisms degradation and less water absorbing. 16 The higher gas permeability of fully protonated CNF film was thought to be due to the larger free volume generated from interfibrillar hydrogen bonding, 14 but such speculation has not been confirmed by physical evidence. Thus, how protonation of surface carboxyls on CNFs affects their association into solids during drying has yet to be clearly delineated.Drying nanocellulose into solids is critical to its applications. 20 or from codispersing in ...

Rice Straw Cellulose Nanofibrils via Aqueous Counter Collision and Differential Centrifugation and Their Self-Assembled Structures

“…It is interesting that the 80.8% of the air-dried F4 is close to the air-dried TEMPO oxidized CNFs (80%), although the freeze-dried one is significantly higher (77.9% for F4 vs 66.3% for TEMPO oxidized CNFs). 44 This result further corroborates the hypothesis that induced crystallization is expected to occur during ultrafiltration and air-drying, which seem to supersede the differences in nanocellulose types. Therefore, rapid freezing and freeze-drying of CNFs appear to preserve the crystal structures of nanocellulose and serve better in determining their crystallinity.…”

“…The low thermal stability of freeze-dried CNF could be due to its higher specific surface and much shorter heat diffusion path of the ultrathin fibers, consistent with our previous observation with the TEMPO oxidized CNFs. 44 The XRD spectra of airdried F4 showed cellulose Iβ lattice structure similar to that of the freeze-dried one but higher intensity due to its more compact and smoother structure. The CrI of air-dried F4 was calculated to be 80.8%, slightly higher than the 77.9% of its freeze-dried counterpart.…”

Nanofibrillar cellulose hydrogels and reconstructed hydrogels as matrices for controlled drug release

“…It has been reported that the NFC aerogel structure formation is directly related to the size and distribution of ice crystals in the frozen state (Aulin, et al 2010). Furthermore, the extent of protonation on the surface carboxyl groups of ANFC produced by TEMPO oxidation affects the aerogel structure (Jiang and Hsieh 2016). High protonation reportedly results in the formation of a porous ultrathin lamellar aerogel structures and high water uptake upon rehydration.…”

Cellulose nanofibers produced from various agricultural residues and their reinforcement effects in polymer nanocomposites

“…Figure depicts FTIR spectra taken from samples of respective biomasses before and after TEMPO mediated oxidation. The significant absorbance change in the TEMPO treated samples at approximately 1620 cm −1 representing COONa stretching vibration indicates the success of the TEMPO treatment . This peak increases significantly in magnitude for all biomass samples.…”