Low solid content mouldable chitin physical hydrogel prepared by atypical rupture-free swelling.

In this study, the atypical swelling gelation of chitin physical hydrogels was investigated. Just by tuning the amount of the N-acetylation reagent, the degree of acetylation varied and mouldable chitin hydrogels with a wide variety of gel concentrations (0.2-6.4 wt%) were obtained. In response to the gel concentration, the mechanical properties ranged from swollen soft gels to shrunken rigid gels (compressive moduli of 4-310 kPa). The thus-prepared chitin hydrogels, which were composed of only chitin and water, exhibited high transparency and integrity. The swelling gelation of chitin physical hydrogels was achieved owing to both the positive charges of the amino groups inducing the osmotic pressure and the toughness of the crystalline nanofibrous network structure of the chitin hydrogels that endured the large volume change. These previously unnoticed advantageous aspects of chitin have pioneered a novel area of swellable physical gels that has been exclusive to chemical gels so far.

Structure of Polymer-Grafted Nanocellulose in the Colloidal Dispersion System.

Clarifying the primary structure of nanomaterials is invaluable to understand how the nanostructures lead to macroscopic material functions. Nanocellulose is attracting attention as a sustainable building block in materials science. The surface of nanocellulose is often chemically modified by polymer grafting to tune the material properties, such as the viscoelastic properties in rheology modifiers and the reinforcement effect in composites. However, the structure, such as molecular conformation of the grafted polymer and the twist of the core nanocellulose, is not well understood. Here, we investigated the structure of polymer-grafted nanocellulose in the colloidal dispersion system by combining small-angle X-ray scattering measurement and all-atom molecular dynamics simulation. We demonstrated formation of the polymer brush layer on the nanocellulose surface in solvents, which explains the excellent colloidal stability. We also found that twisting of the nanocellulose in the core is suppressed by the existence of the polymer brush layer.

Molecular Dynamics of Drying-Induced Structural Transformations in a Single Nanocellulose.

Nanocellulose is attracting attention in the field of materials science as a sustainable building block. Nanocellulose-based materials, such as films, membranes, and foams, are fabricated by drying colloidal dispersions. However, little is known about how the structure of a single nanocellulose changes during the complex drying process. Here, all-atom molecular dynamics simulations and atomic force microscopy is used to investigate the structural dynamics of single nanocellulose during drying. It is found that the twist morphology of the nanocellulose became localized along the fibril axis during the final stage of the drying process. Moreover, it is shown that conformational changes at C6 hydroxymethyl groups and glycoside bond is accompanied by the twist localization, indicating that the increase in the crystallinity occurred in the process. It is expected that the results will provide molecular insights into nanocellulose structures in material processing, which is helpful for the design of materials with advanced functionalities.

Evaluating the Emulsifying Capacity of Cellulose Nanofibers Using Inverse Gas Chromatography.

Cellulose nanofibers (CNFs) are attracting increasing attention as emulsifiers owing to their high emulsifying capacity, biocompatibility, and biodegradability. The emulsifying capacity has been experimentally shown to depend not only on the type of oil but also on the chemical structure of the CNF surface. However, the theoretical relationship between these two factors and emulsification remains unclear, and therefore, industrial applications are limited. Here, we assess the desorption energy (DE) of CNFs from the oil surface in o/w emulsion for various CNF/oil combinations to understand the mechanism of emulsification. Two types of surface-carboxylated CNFs having different cationic counterions, namely, sodium and tetrabutylammonium ions, were used as emulsifiers. The surface free energies of the CNFs were evaluated using inverse gas chromatography, and the nonpolar Lifshitz-van der Waals γLW, electron-acceptor γ+, and electron-donor γ- components were obtained from the chromatography profiles based on the van Oss-Chaudhury-Good theory. CNF with tetrabutylammonium ions was found to have a higher γ+ component than CNF with sodium ions. Therefore, the emulsion stability improved with oils having high γ- components owing to the increase in the DE value; this was verified through both theoretical calculations using a fibrous model and experimental dynamic interfacial tension measurements. Our approach is useful for predicting the emulsifying capacity of CNFs, and it should contribute toward the design of novel CNF-based emulsions.

Linear Correlation between True Density and Crystallinity of Regenerated and Mercerized Celluloses.

Regenerated and mercerized celluloses are widely used in our daily life and industries. Examples include clothes, medical supplies, and separation membranes. In such applications, the true density is an important derived physical quantity for refining the structural designs of regenerated and mercerized celluloses. Here, we report the true density-crystallinity correlation of regenerated and mercerized celluloses. Seven samples were prepared through either dissolution-regeneration or mercerization, and the true density of each sample was measured by helium gas pycnometry. The crystallinity was evaluated by solid-state 13C nuclear magnetic resonance spectroscopy based on the ratio of the carbon atoms in the crystallite core to those at crystallite surfaces and in the surrounding amorphous matrix. We found that the true density of regenerated and mercerized celluloses is directly proportional to crystallinity, irrespective of the preparation process. Additionally, the molecular packing density at the crystallite surfaces was found to be similar to that in the amorphous matrix.

Counterion-Dependent Material Properties of Phosphorylated Nanocellulose.

The material properties of cellulose nanofibers (CNFs) are governed by the surface chemical structure of the fibers. The chemical structure-property relationships for monovalent carboxylated CNFs are well understood. Here, we report the basic sheet properties of divalent phosphorylated CNFs with different phosphorus contents and counterion types. All examined sheet properties, including conditioned and wet tensile properties, electrical resistivities, and fire-retardant properties of the CNF sheets, were greatly enhanced by the counterion exchange from the initial sodium ions to calcium or aluminum ions. The phosphorus content had significant influences only on the conditioned tensile and fire-retardant properties. In comparison to CNF sheets with monovalent carboxy groups, the CNF sheets with divalent phosphate groups were superior in terms of their wet tensile properties and fire-retardant properties. Our research shows that the combination of the divalent phosphate introduction and counterion exchange provides a successful strategy for the practical application of CNF sheets as antistatic materials and flexible substrates for electronic devices.

Morphological Changes of Polymer-Grafted Nanocellulose during a Drying Process.

Nanocellulose is emerging as a sustainable building block in materials science. Surface modification via polymer grafting has proven to be effective in tuning diverse material properties of nanocellulose, including wettability of films and the reinforcement effect in polymer matrices. Despite its widespread use in various environments, the structure of a single polymer-grafted nanocellulose remains poorly understood. Here, we investigate the morphologies of polymer-grafted CNFs at water-mica and air-mica interfaces by using all-atom molecular dynamics simulation and atomic force microscopy. We show that the morphologies of the polymer-grafted CNFs undergo a marked change in response to the surrounding environment due to variations in the conformation of the surface polymer chains. Our results provide novel insights into the molecular structure of polymer-grafted CNFs and can facilitate the design and development of innovative biomass-based nanomaterials.

Distribution and Quantification of Diverse Functional Groups on Phosphorylated Nanocellulose Surfaces.

Phosphorylated cellulose nanofiber (CNF) is attracting attention as a newly emerged CNF with high functionality. However, many structural aspects of phosphorylated CNF remain unclear. In this study, we investigated the chemical structures and distribution of ionic functional groups on the phosphorylated CNF surfaces via liquid-state nuclear magnetic resonance measurements of colloidal dispersion. In addition to the monophosphate group, polyphosphate groups and cross-linked phosphate groups were introduced in the phosphorylated CNFs. The proportion of polyphosphate groups increased as the phosphorylation time increased, reaching ∼30% of all phosphate groups. Only a small amount of cross-linked phosphate groups existed in the phosphorylated CNF after a prolonged reaction time. Furthermore, phosphorylation of cellulose using urea and phosphoric acid was found to be regioselective at the C2 and C6 positions. There existed no significant difference between the surface degrees of substitution at the C2 and C6 positions of the phosphorylated CNFs.

Recovery of the Irreversible Crystallinity of Nanocellulose by Crystallite Fusion: A Strategy for Achieving Efficient Energy Transfers in Sustainable Biopolymer Skeletons*.

Crystallites form a grain boundary or the inter-crystallite interface. A grain boundary is a structural defect that hinders the efficient directional transfer of mechanical stress or thermal phonons in crystal aggregates. We observed that grain boundaries within an aggregate of crystalline cellulose nanofibers (CNFs) were crystallized by enhancing their inter-crystallite interactions; multiple crystallites were coupled into single fusion crystals, without passing through a melting or dissolving state. Accordingly, the lowered crystallinity of CNFs, which has been considered irreversible, was recovered, and the thermal energy transfer in the aggregate was significantly improved. Other nanofibrous crystallites of chitin also showed a similar fusion phenomenon by enhancing the inter-crystallite interactions. Such crystallite fusion may naturally occur in biological structures with network skeletons of aggregated fibrillar crystallites having mechanical or thermal functions.

Magnetically Collectable Nanocellulose-Coated Polymer Microparticles by Emulsion Templating.

Magnetic nano/microparticles offer potential benefits for environmental applications such as water purification. However, achieving functional and stable surfaces remains a critical challenge for magnetic particle design. Nanocellulose, a naturally occurring nanofiber, is a promising surface material candidate, owing to its ease of functionalization and chemical stability. Here, we developed a magnetically collectable nanocellulose-coated polymer microparticle synthesis method, based on Pickering emulsion templating. The average diameter of the core/shell microparticles was 2.7 µm, and they were well dispersed in water, owing to the coverage with surface-carboxylated nanocelluloses. Most magnetic Fe3O4 nanoparticles with a 30 nm diameter were encapsulated in the microparticles and enriched at the CNF/polymer interfaces. The nanocellulose shell showed high loading of cationic dye molecules. In addition, the nanocellulose-coated microparticles could be recovered even after the dye loading by exposing the aqueous dispersion to a magnetic field.

Crystallinity-Independent yet Modification-Dependent True Density of Nanocellulose.

In materials science and crystallography, the true density is an important derived physical quantity of solids. Here we report the correlation of the true density of nanometer-wide fibrillar crystallites of cellulose with their purity, crystallinity, morphology, and surface functionality. In the single fibrils, all the cellulose molecules are uniaxiallly oriented. Thus, the true density indicates the molecular packing density in the single fibrils and is essential for the precise estimation of the volume fraction of cellulose in fibril-based composites or porous structures. We demonstrate that the true density of fibrillar crystallites of cellulose is approximately 1.60 g/cm3 irrespective of the biological origins of the cellulose (wood, cotton, or a tunicate) and the crystallinity. The true density is in fact independent of the dimension of the crystallites and the atomic conformation of the uniaxially oriented but noncrystalline molecules at the crystallite surface. In the single fibrils, all the cellulose molecules are densely packed from the crystalline core to the noncrystalline outermost regions. The value of 1.60 g/cm3 remains unchanged even when the fibrils are dispersed through the wet disintegration process of "nanocellulose" production. In contrast, tailoring the surface functionality of the fibrils by oxidation and/or adsorption results in a substantial change in the true density up to 1.8 g/cm3 or down to 1.3 g/cm3. The true density of nanocellulose is indeed governed by the surface functionality and has a strong gradient in the fibril cross-sectional direction.

Synthesis of Chitin Nanofiber-Coated Polymer Microparticles via Pickering Emulsion.

Chitin nanofiber (ChNF) has received significant research attention owing to its potential for use in a variety of applications, such as medicine and cosmetics. Here, we synthesize a novel ChNF material, ChNF-coated polymer microparticles, using a Pickering emulsion-templated approach. Two varieties of ChNF with different crystal structures, lengths, and surface charges were used to form the microparticle shells. When ChNFs with a shorter length and greater surface charge were used, the microparticles showed good dispersibility in water and narrow size distribution with number- and volume-median diameters of 1.46 and 1.84 µm, respectively. The microparticles were easily collected by filtration and redispersed in water, even after drying. The surface ChNF shells assembled at the microparticle surfaces showed potential as an adsorption site, effectively capturing anionic dye molecules. This technique offers new opportunities for the development of green nanocomposite materials using a facile aqueous process.

Dual Functions of TEMPO-Oxidized Cellulose Nanofibers in Oil-in-Water Emulsions: A Pickering Emulsifier and a Unique Dispersion Stabilizer.

The emulsifying and dispersing mechanisms of oil-in-water emulsions stabilized by 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-oxidized cellulose nanofibers (CNFs) have been investigated. The emulsifying mechanism was studied by changing the oil/water interfacial tension from 8.5 to 53.3 mN/m using various types of oils. The results showed that the higher the oil/water interfacial tension, the greater is the amount of CNFs adsorbed at the oil/water interface, making the CNF-adsorbed oil-in-water emulsions thermodynamically more stable. Moreover, the amount of CNFs adsorbed on the surfaces of the oil droplets increased with increasing interfacial area. The dispersion stability of the oil droplets was dominated by the CNF concentration in the water phase. Above the critical concentration (0.15% w/w), the CNFs formed network structures in the water phase, and the emulsion was effectively stabilized against creaming. Emulsion formation and the CNF network structures in the emulsion were visualized by cryo-scanning electron microscopy.

Characterization of Concentration-Dependent Gelation Behavior of Aqueous 2,2,6,6-Tetramethylpiperidine-1-oxyl-Cellulose Nanocrystal Dispersions Using Dynamic Light Scattering.

Cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs) with high and low aspect ratios, respectively, were prepared from wood cellulose by catalytic oxidation with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and subsequent sonication in water. Cavitation-induced force was used to prepare TEMPO-CNCs from aqueous suspensions of TEMPO-oxidized celluloses. Aqueous dispersions of the TEMPO-CNF and TEMPO-CNCs with different solid concentrations were prepared by dilution or condensation. Dynamic light scattering (DLS) was used to determine the solid concentrations at the transition points from the dilute to semidilute regions and from the semidilute to dense gel regions in the aqueous TEMPO-CNF and TEMPO-CNC dispersions. All the DLS data corresponded well to the fitting curves of the normalized time-correlation functions. The solid concentration at the gelation point increased from 0.40% w/v for the TEMPO-CNF dispersions to 1.71% w/v for the TEMPO-CNC dispersions, and the aspect ratio decreased from 134 to 57, respectively. The solid concentrations of the TEMPO-CNF and TEMPO-CNC dispersions at the gelation point calculated using effective-medium theory based on their aspect ratios corresponded well with those experimentally determined by DLS.

Facile Route to Transparent, Strong, and Thermally Stable Nanocellulose/Polymer Nanocomposites from an Aqueous Pickering Emulsion.

Cellulose nanofibril (CNF) is a promising nanofiller for polymer nanocomposite materials, and a critical challenge in designing these materials is organization of the nanostructure using a facile process. Here, we report a facile aqueous preparation process for nanostructured polystyrene (PS)/CNF composites via the formation of a CNF-stabilized Pickering emulsion. PS nanoparticles, with a narrow size distribution, were synthesized by free radical polymerization in water using CNF as a stabilizer. The nanoparticles were easily collected by filtration, and the resulting material had a composite structure of PS nanoparticles embedded in a CNF framework. The PS/CNF nanocomposite showed high optical transparency, strength, and thermal dimensional stability. Thus, this technique provides a simple and environmentally friendly method for the preparation of novel CNF/polymer nanocomposite materials.

Nanocellulose-stabilized Pickering emulsions and their applications.

Pickering emulsion, which is an emulsion stabilized by solid particles, offers a wide range of potential applications because it generally provides a more stable system than surfactant-stabilized emulsion. Among various solid stabilizers, nanocellulose may open up new opportunities for future Pickering emulsions owing to its unique nanosizes, amphiphilicity, and other favorable properties (e.g. chemical stability, biodegradability, biocompatibility, and renewability). In this review, the preparation and properties of nanocellulose-stabilized Pickering emulsions are summarized. We also provide future perspectives on their applications, such as drug delivery, food, and composite materials.

Mechanical and Thermal Properties of Porous Nanocellulose/Polymer Composites: Influence of the Polymer Chemical Structure and Porosity.

The excellent emulsifying capacity of nanocellulose allows for the preparation of porous nanocellulose/polymer composites through the emulsion templating process. However, the effects of the polymer chemical structure and porosity on the material properties have not been extensively explored. Here, we discuss the effects of these two factors on the thermal and mechanical properties of the composites. Two types of porous nanocellulose/polymer composites were fabricated with styrene-divinylbenzene (poly(St-co-DVB)) or styrene-poly(ethylene glycol) dimethacrylate (poly(St-co-EGDMA)) copolymers as the polymer phases. The porosity of the composite was changed up to ∼50% v/v by varying the aqueous phase volume fraction in the original nanocellulose-stabilized w/o emulsions. As the porosity increased, the thermal conductivity of the composite decreased. The mechanical properties were strongly influenced by the polymer type; the nanocellulose/poly(St-co-DVB) composite showed stiff but brittle behavior, whereas the nanocellulose/poly(St-co-EGDMA) composite showed higher strength and toughness. In both types of composites, the nanocelluloses served as reinforcing agents, contributing to the improvement of the mechanical properties.

Selective permeation of hydrogen gas using cellulose nanofibril film.

Biobased membranes that can selectively permeate hydrogen gas have been developed from aqueous dispersions of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCN) prepared from wood cellulose: TOCN-coated plastic films and self-standing TOCN films. Compared with TOCNs with sodium, lithium, potassium, and cesium carboxylate groups, TOCN with free carboxyl groups (TOCN-COOH) had much high and selective H2 gas permeation performance. Because permeabilities of H2, N2, O2, and CO2 gases through the membranes primarily depended on their kinetic diameters, the gas permeation behavior of the various TOCNs can be explained in terms of a diffusion mechanism. Thus, the selective H2 gas permeability for TOCN-COOH was probably due to a larger average size in free volume holes present between nanofibrils in the layer and film than those of other TOCNs with metal carboxylate groups. The obtained results indicate that TOCN-COOH membranes are applicable as biobased H2 gas separation membranes in fuel cell electric power generation systems.

Surface engineering of ultrafine cellulose nanofibrils toward polymer nanocomposite materials.

Surface grafting of crystalline and ultrafine cellulose nanofibrils with poly(ethylene glycol) (PEG) chains via ionic bonds was achieved by a simple ion-exchange treatment. The PEG-grafted cellulose nanofibrils exhibited nanodispersibility in organic solvents such as chloroform, toluene, and tetrahydrofuran. Then, the PEG-grafted cellulose nanofibril/chloroform dispersion and poly(L-lactide) (PLLA)/chloroform solution were mixed, and the PEG-grafted cellulose nanofibril/PLLA composite films with various blend ratios were prepared by casting the mixtures on a plate and drying. The tensile strength, Young's modulus, and work of fracture of the composite films were remarkably improved, despite low cellulose addition levels (<1 wt %). The highly efficient nanocomposite effect was explained in terms of achievement of nanodispersion states of the PEG-grafted cellulose nanofibrils in the PLLA matrix. Moreover, some attractive interactions mediated by the PEG chains were likely to be formed between the cellulose nanofibrils and PLLA molecules in the composites, additionally enhancing the efficient nanocomposite effect.

Chitin nanofiber-coated biodegradable polymer microparticles via one-pot aqueous process.

Tailoring the surface of biodegradable microparticles is important for various applications in the fields of cosmetics, biotechnology, and drug delivery. Chitin nanofibers (ChNFs) are one of the promising materials for surface tailoring owing to its functionality, such as biocompatibility and antibiotic properties. Here, we show biodegradable polymer microparticles densely coated with ChNFs. Cellulose acetate (CA) was used as the core material in this study, and ChNF coating was successfully carried out via a one-pot aqueous process. The average particle size of the ChNF-coated CA microparticles was approximately 6 µm, and the coating procedure had little effect on the size or shape of the original CA microparticles. The ChNF-coated CA microparticles comprised 0.2-0.4 wt% of the thin surface ChNF layers. Owing to the surface cationic ChNFs, the ζ-potential value of the ChNF-coated microparticles was +27.4 mV. The surface ChNF layer efficiently adsorbed anionic dye molecules, and repeatable adsorption/desorption behavior was exhibited owing to the coating stability of the surface ChNFs. The ChNF coating in this study was a facile aqueous process and was applicable to CA-based materials of various sizes and shapes. This versatility will open new possibilities for future biodegradable polymer materials that satisfy the increasing demand for sustainable development.