Tsuguyuki Saito

Transparent and high gas barrier films of cellulose nanofibers prepared by TEMPO-mediated oxidation.

Softwood and hardwood celluloses were oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation. The TEMPO-oxidized cellulose fibers were converted to transparent dispersions in water, which consisted of individual nanofibers 3-4 nm in width. Films were then prepared from the TEMPO-oxidized cellulose nanofibers (TOCN) and characterized from various aspects. AFM images showed that the TOCN film surface consisted of randomly assembled cellulose nanofibers. The TOCN films prepared from softwood cellulose were transparent and flexible and had extremely low coefficients of thermal expansion caused by high crystallinity of TOCN. Moreover, oxygen permeability of a polylactic acid (PLA) film drastically decreased to about 1/750 by forming a thin TOCN layer on the PLA film. Hydrophobization of the originally hydrophilic TOCN films was achieved by treatment with alkylketene dimer. These unique characteristics of the TOCN films are promising for potential applications in some high-tech materials.

Individualization of Nano-Sized Plant Cellulose Fibrils by Direct Surface Carboxylation Using TEMPO Catalyst under Neutral Conditions

A new catalytic oxidation using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) and NaClO is applied to hardwood cellulose in water at 60 °C and pH 6.8 with NaClO(2) used as a primary oxidant. The oxidized celluloses with carboxylate content of approximately 0.8 mmol/g were convertible to highly crystalline and individual fibrils 5 nm in width and at least 2 μm in length by disintegration in water. The oxidized celluloses had no aldehyde groups, and high degrees of polymerization of more than 900. Solid-state (13)C NMR and X-ray analyses revealed that the C6 carboxylate groups formed are selectively present on the crystalline fibril surfaces at high densities. Films prepared from the dispersions were transparent and flexible, and exhibited a high tensile strength of 312 MPa even at a low density of 1.47 g/cm(3).

An ultrastrong nanofibrillar biomaterial: the strength of single cellulose nanofibrils revealed via sonication-induced fragmentation.

We report the mechanical strength of native cellulose nanofibrils. Native cellulose nanofibrils, purified from wood and sea tunicate, were fully dispersed in water via a topochemical modification of cellulose nanofibrils using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a catalyst. The strength of individual nanofibrils was estimated based on a model for the sonication-induced fragmentation of filamentous nanostructures. The resulting strength parameters were then analyzed based on fracture statistics. The mean strength of the wood cellulose nanofibrils ranged from 1.6 to 3 GPa, depending on the method used to measure the nanofibril width. The highly crystalline, thick tunicate cellulose nanofibrils exhibited higher mean strength of 3-6 GPa. The strength values estimated for the cellulose nanofibrils in the present study are comparable with those of commercially available multiwalled carbon nanotubes.

Relationship between Length and Degree of Polymerization of TEMPO-Oxidized Cellulose Nanofibrils

The influence of 2,2,6,6-tetrametylpiperidine-1-oxyl (TEMPO)-mediated oxidation of wood cellulose and the mechanical disintegration of oxidized cellulose in water on degree of polymerization determined by viscosity measurement (DP(v)) and the apparent length of the TEMPO-oxidized cellulose nanofibrils (TOCNs) was investigated. DP(v) values decreased from 1270 to 500-600 with increasing addition of NaClO in the TEMPO-mediated oxidation stage. The DP(v) values were further decreased by mechanical fibrillation in water. There is a linear relationship between the average fibril length and DP(v); the lengths of TOCNs can be approximated from DP(v) using 0.5 M copper ethylenediamine as a solvent of both the cellulose and oxidized celluloses in TOCNs. Based on the cellulose fibril models and TEMPO oxidation mechanism, the depolymerization behavior of TOCNs is tentatively explained in terms of distribution of disordered regions in wood cellulose fibrils and formation of C6-aldehydes in cellulose fibrils during TEMPO-mediated oxidation.

Preparation of Chitin Nanofibers from Squid Pen β-Chitin by Simple Mechanical Treatment under Acid Conditions

A procedure for preparing individualized chitin nanofibers 3-4 nm in cross-sectional width and at least a few microns in length was developed. The key factors to prepare the chitin nanofibers with such high aspect ratios are as follows: (1) squid pen beta-chitin is used as the starting material and (2) ultrasonication of the beta-chitin in water at pH 3-4 and 0.1-0.3% consistency for a few minutes. Transparent and highly viscous dispersions of squid pen beta-chitin nanofibers in water can be obtained by this method. No N-deacetylation occurs on the chitin molecules during the nanofiber conversion procedure. Moreover, the original crystal structure of beta-chitin is maintained, although crystallinity index decreases from 0.51 to 0.37 as a result of the nanofiber conversion. Cationization of the C2 amino groups present on the crystallite surfaces of the squid pen beta-chitin under acid conditions is necessary for preparing the nanofibers.

Ultrastrong and high gas-barrier nanocellulose/clay-layered composites.

Nanocellulose/montmorillonite (MTM) composite films were prepared from 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with an aspect ratio of >200 dispersed in water with MTM nanoplatelets. The composite films were transparent and flexible and showed ultrahigh mechanical and oxygen barrier properties through the nanolayered structures, which were formed by compositing the anionic MTM nanoplatelet filler in anionic and highly crystalline TOCN matrix. A composite film with 5% MTM content had Youngs modulus 18 GPa, tensile strength 509 MPa, work of fracture of 25.6 MJ m(-3), and oxygen permeability 0.006 mL μm m(-2) day(-1) kPa(-1) at 0% relative humidity, respectively, despite having a low density of 1.99 g cm(-3). As the MTM content in the TOCN/MTM composites was increased to 50%, light transmittance, tensile strength, and elongation at break decreased, while Youngs modulus was almost unchanged and oxygen barrier property was further improved to 0.0008 mL μm m(-2) day(-1) kPa(-1).

Aerogels with 3D Ordered Nanofiber Skeletons of Liquid‐Crystalline Nanocellulose Derivatives as Tough and Transparent Insulators

Aerogels of high porosity and with a large internal surface area exhibit outstanding performances as thermal, acoustic, or electrical insulators. However, most aerogels are mechanically brittle and optically opaque, and the structural and physical properties of aerogels strongly depend on their densities. The unfavorable characteristics of aerogels are intrinsic to their skeletal structures consisting of randomly interconnected spherical nanoparticles. A structurally new type of aerogel with a three-dimensionally ordered nanofiber skeleton of liquid-crystalline nanocellulose (LC-NCell) is now reported. This LC-NCell material is composed of mechanically strong, surface-carboxylated cellulose nanofibers dispersed in a nematic LC order. The LC-NCell aerogels are transparent and combine mechanical toughness and good insulation properties. These properties of the LC-NCell aerogels could also be readily controlled.

Self-aligned integration of native cellulose nanofibrils towards producing diverse bulk materials

In living organisms, nanoscale building blocks are well organized into a nearly defect-free arrangement, leading to sophisticated hierarchical structures that exhibit high mechanical strengths and functionalities. Cellulose nanofibrils are representative building blocks in nature and have raised great interest as excellent structural materials, originating from the most abundant bioresource, wood biomass. Here, we report integration controls of the cellulose nanofibrils that have been completely dispersed in water through the specific surface-carboxylation using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a catalyst. The dispersed nanofibrils spontaneously align in water. The integration controls of the self-aligned nanofibrils, i.e., careful adjustment of the pH and evaporation of the solvent in the nanofibril dispersions, produce a wide range of artificial bulk materials with outstanding properties. Examples include unprecedentedly stiff hydrogels that are freestanding with a water content of 99.9%, ultralow-density, tough aerogels with large surface areas, and transparent films with exceptionally high oxygen-barrier properties. These materials are expected to further develop as robust frameworks of polymer nanocomposites or high-capacity supports of catalysts and the other functional materials. For the sustainable development of society, we propose a methodology for producing sophisticated, high-performance bio-based materials.

Transparent, Conductive, and Printable Composites Consisting of TEMPO-Oxidized Nanocellulose and Carbon Nanotube

Ultrastrong, transparent, conductive and printable nanocomposites were successfully prepared by mixing single-walled carbon nanotubes (CNTs) with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with abundant sodium carboxyl groups on the crystalline nanocellulose surfaces. The surface-anionic cellulose nanofibrils had reinforcing and nanodispersing effects on the CNTs both in water used as the dispersed medium and in the dried composite film, providing highly conductive and printable nanocomposites with a small amount of CNTs. TOCNs are therefore expected as an effective flexible matrix that can be used as an alternative to conventional polymers for various electrical materials, when nanocomposited with CNTs and also graphene. Our findings provide a promising route to realize green and flexible electronics.

Transparent cellulose films with high gas barrier properties fabricated from aqueous alkali/urea solutions.

Transparent and bendable regenerated cellulose films prepared from aqueous alkali (NaOH or LiOH)/urea (AU) solutions exhibit high oxygen barrier properties, which are superior to those of conventional cellophane, poly(vinylidene chloride), and poly(vinyl alcohol). Series of AU cellulose films are prepared from different cellulose sources (cotton linters, microcrystalline cellulose powder, and softwood bleached kraft pulp) for different dissolution and regeneration conditions. The oxygen permeabilities of these AU cellulose films vary widely from 0.003 to 0.03 mL μm m(-2) day(-1) kPa(-1) at 0% relative humidity depending on the conditions used to prepare the films. The lowest oxygen permeability is achieved for the AU film prepared from 6 wt % cellulose solution by regeneration with acetone at 0 °C. The oxygen permeabilities of the AU cellulose films are negatively correlated with their densities, and AU films prepared from solutions with high cellulose concentrations by regeneration in a solvent at low temperatures generally have low oxygen permeabilities. The AU cellulose films are, therefore, promising biobased packaging materials with high-oxygen barrier properties.