Nanocellulose extracted from wood pulps using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and sulfuric acid hydrolysis methods was characterized by small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS) techniques. The dimensions of this nanocellulose (TEMPO-oxidized cellulose nanofiber (TOCN) and sulfuric acid hydrolyzed cellulose nanocrystal (SACN)) revealed by the different scattering methods were compared with those characterized by transmission electron microscopy (TEM). The SANS and SAXS data were analyzed using a parallelepiped-based form factor. The width and thickness of the nanocellulose cross section were ∼8 and ∼2 nm for TOCN and ∼20 and ∼3 nm for SACN, respectively, where the fitting results from SANS and SAXS profiles were consistent with each other. DLS was carried out under both the V mode with the polarizer and analyzer parallel to each other and the H mode having them perpendicular to each other. Using rotational and translational diffusion coefficients obtained under the H mode yielded a nanocellulose length qualitatively consistent with that observed by TEM, whereas the length derived by the translational diffusion coefficient under the V mode appeared to be overestimated.
Mechanistic
behavior and flow properties of cellulose nanofibers
(CNFs) in aqueous systems can be described by the crowding factor
and the concept of contact points, which are functions of the aspect
ratio and concentration of CNF in the suspension. In this study, CNFs
with a range of aspect ratio and surface charge density (380–1360
μmol/g) were used to demonstrate this methodology. It was shown
that the critical networking point of the CNF suspension, determined
by rheological measurements, was consistent with the gel crowding
factor, which was 16. Correlated to the crowding factor, both viscosity
and modulus of the systems were found to decrease by increasing the
charge density of CNF, which also affected the flocculation behavior.
Interestingly, an anomalous rheological behavior was observed near
the overlap concentration (0.05 wt %) of CNF, at which the crowding
factor was below the gel crowding factor, and the storage modulus
(G′) decreased dramatically at a given frequency
threshold. This behavior is discussed in relation to the breakup of
the entangled flocs and network in the suspension. The analysis of
the mechanistic behavior of CNF aqueous suspensions by the crowding
factor provides useful insight for fabricating high-performance nanocellulose-based
materials.
CO2-responsiveness is imported into amphiphilic block copolymers, poly[(N,N-diethylaminoethyl methacrylate)-b-(N-isopropylacrylamide)] (PDEAEMA-b-PNIPAM), and a system dual-responsive to CO2 and temperature is constructed. The copolymer self-assembles in aqueous solution, and undergoes phase transition when CO2 and temperature stimuli occur, since the stimuli give rise to the conversion of the hydrophilicity of both blocks. Combining CO2 and temperature as triggers, schizophrenic micelle to vesicle morphological transition of the polymer assemblies is controlled.
Aryl trifluoromethoxylation by a two-step sequence of O-trifluoromethylation of N-aryl-N-hydroxylamine derivatives and intramolecular OCF3 migration is presented. This protocol allows easy access to a wide range of synthetically useful ortho-OCF3 aniline derivatives. In addition, it utilizes bench-stable reagents, is operationally simple, shows high functional-group tolerance, and is amenable to gram-scale as well as one-pot synthesis. A reaction mechanism of a heterolytic cleavage of the NOCF3 bond followed by recombination of the resulting nitrenium ion and trifluoromethoxide is proposed for the OCF3 -migration reaction.
Bio-based
nanocellulose has been shown to possess impressive mechanical
properties and simplicity for chemical modifications. The chemical
properties are largely influenced by the surface area and functionality
of the nanoscale materials. However, finding the typical cross-sections
of nanocellulose, such as cellulose nanofibers (CNFs), has been a
long-standing puzzle, where subtle changes in extraction methods seem
to yield different shapes and dimensions. Here, we extracted CNFs
from wood with two different oxidation methods and variations in degree
of oxidation and high-pressure homogenization. The cross-sections
of CNFs were characterized by small-angle X-ray scattering and wide-angle
X-ray diffraction in dispersed and freeze-dried states, respectively,
where the results were analyzed by assuming that the cross-sectional
distribution was quantized with an 18-chain elementary microfibril,
the building block of the cell wall. We find that the results agree
well with a pseudosquare unit having a size of about 2.4 nm regardless
of sample, while the aggregate level strongly depends on the extraction
conditions. Furthermore, we find that aggregates have a preferred
cohesion of phase boundaries parallel to the (110)-plane of the cellulose
fibril, leading to a ribbon shape on average.
Nanostructured materials made through flow-assisted assembly of proteinaceous or polymeric nanosized fibrillar building blocks are promising contenders for a family of new high-performance biocompatible materials in a wide variety of applications. Optimization of these processes relies on improving our knowledge of the physical mechanisms from nano-to macroscale and especially understanding the alignment of elongated nanoparticles in flows. Here, we study the full projected orientation distributions of cellulose nanocrystals (CNC) and nanofibrils (CNF) in confined flow using scanning microbeam SAXS. For CNC, we further compare with a simulated system of dilute Brownian ellipsoids, which agrees well at dilute concentrations. However, increasing CNC concentration to a semi-dilute regime results in locally arranged domains called tactoids, which aid in aligning the CNC at low shear rates, but limit alignment at higher rates. Similarly, shear alignment of CNF at semi-dilute conditions is also limited owing to probable bundle/flock formation of the highly entangled nanofibrils. This work provides a first quantitative comparison of full projected orientation distributions of elongated nanoparticles in confined flow and provides an important stepping stone towards predicting and controlling processes to create nanostructured materials on an industrial scale.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.