Hydrophobic, ductile, and transparent nanocellulose films with quaternary alkylammonium carboxylates on nanofibril surfaces.

Hydrophobic, ductile, and transparent nanocellulose films were prepared by casting and drying aqueous dispersions of 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized cellulose nanofibrils (TOCNs) with quaternary alkylammoniums (QAs) as counterions for the surface carboxylate groups. TOCN films with tetramethylammonium and tetraethylammonium carboxylates showed high optical transparencies, strain-to-failure values (14-22%), and work-of-fracture values (20-27 MJ m(-3)). The ductility of these films was likely caused by the alkyl chains of the QA groups densely covering the TOCN surfaces and being present at the interfaces between the TOCN elements in the films. The water contact angle of the TOCN-QA films increased to ∼100° by introducing tetra(n-butyl)ammonium groups as counterions. Thus, TOCN film properties can be controlled by changing the chemical structure of the counterions from Na to QAs. The hydrophilic TOCN surfaces can be changed to hydrophobic simply and efficiently by the conversion from TOCN-Na to TOCN-QA, when TOCNs are used as nanofillers in hydrophobic polymer matrices.

Increase in the water contact angle of composite film surfaces caused by the assembly of hydrophilic nanocellulose fibrils and nanoclay platelets.

Controlling the assembly modes of different crystalline nanoparticles in composites is important for the expression of specific characteristics of the assembled structures. We report a unique procedure for increasing water contact angles (CAs) of composite film surfaces via the assembly of two different hydrophilic components, nanocellulose fibrils and nanoclay platelets. The nanocellulose fibrils and nanoclay platelets used have ionic groups on their surfaces in high densities (∼1 mmol g(-1)) and have no hydrophobic surface. The increase in the CA of the nanocellulose/nanoclay composite films was thus analyzed on the basis of the air area fractions of their nanostructured surfaces following Cassie's law. The air area fractions were geographically estimated from the atomic force microscopy height profiles of the composite film surfaces. The CAs of the composite film surfaces were found to be well described by Cassie's law. Interestingly, the composite films consisting of two hydrophilic nanoelements with different shapes exhibited CAs larger than those of the individual neat films.

Preparation and characterization of TEMPO-oxidized cellulose nanofibrils with ammonium carboxylate groups.

Fibrous TEMPO-oxidized celluloses with 100% ammonium carboxylate groups (TOC-COONH4) were prepared by adding aqueous ammonia to fibrous TOC-COOH/water slurries. Using a gentle mechanical disintegration treatment in water, the obtained never-dried TOC-COONH4/water slurries could be converted to highly viscous and transparent gels consisting of mostly individualized TEMPO-oxidized cellulose nanofibrils. The self-standing TOCN-COONH4 film prepared from the aqueous TOCN-COONH4 dispersion via casting and drying had high optical transparency. When the self-standing TOCN-COONH4 film was heated at 105°C for 1 day, clear yellowing was observed on the film. FT-IR analysis of the heated TOCN-COONH4 films indicated the partial formation of TOCN-COOH type structures from the TOCN-COONH4 due to evaporation of NH3 gas from the films during heating. The heated TOCN-COONH4 films had lower moisture contents, higher film densities, and higher Young's moduli than the unheated TOCN-COONH4 films.

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.

Influence of TEMPO-oxidized cellulose nanofibril length on film properties.

Various mechanical disintegration conditions in water were applied to 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose to prepare TEMPO-oxidized cellulose nanofibrils (TOCNs) of uniform widths ∼4 nm but with three different average lengths, 200, 680, and 1100 nm. The viscosity average degrees of polymerization of the TOCNs were 250, 350, and 400, respectively. Self-standing TOCN and TOCN-coated poly(ethylene terephthalate) (PET) and poly(lactic acid) (PLA) films were prepared, and the optical, mechanical and gas-barrier properties of the films were evaluated in terms of nanofibril length. Only small differences in density, water content, and elastic modulus of the films were observed but TOCN films prepared from longer nanofibrils clearly showed higher tensile strengths, elongations at break and crystallinity indices. The oxygen barrier properties of the TOCN-coated PET and PLA films increased with increasing nanofibril length. In contrast, nanofibril length had almost no influence on water vapor-barrier properties.

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 Young's 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 Young's modulus was almost unchanged and oxygen barrier property was further improved to 0.0008 mL µm m(-2) day(-1) kPa(-1).

Comparative characterization of aqueous dispersions and cast films of different chitin nanowhiskers/nanofibers.

Water dispersions of TEMPO-oxidized α-chitin nanowhisker (TOChN), partially deacetylated α-chitin nanowhisker/nanofiber mixture (DEChN), HCl-hydrolyzed chitin nanowhisker (HHChN) and squid-pen ß-chitin nanofiber (SQChN) were prepared, and the properties of nano-dispersions and their cast films were characterized between the four chitin nano-samples. Because SQChN has the highest aspect ratio, its 0.1% dispersion had the highest shear stress and viscosity at the same shear rate in the four chitin nano-samples, and showed gel-like behavior in the whole shear rate range from 10(-3) to 10(3) s(-1). AFM images of the self-standing films showed that film surfaces consisted of characteristic chitin nano-elements with different morphologies and degrees of orientation between the four chitin samples, whereas all chitin nanowhisker/nanofiber films had similar thermal degradation points at ~200°C. The DEChN film had the highest tensile strength of ~140 MPa, elongation at break of ~10% and light-transmittance of 87% at 400 nm. In contrast, the SQChN film had the lowest tensile strength, Young's modulus and light-transmittance. All chitin nanowhisker/nanofiber films had similar oxygen permeabilities of ~1 mL µm m(-2) day(-1) kPa(-1), which was clearly lower than that (184 mL µm m(-2) day(-1) kPa(-1)) of a poly(lactic acid) film.

Pore size determination of TEMPO-oxidized cellulose nanofibril films by positron annihilation lifetime spectroscopy.

Wood cellulose nanofibril films with sodium carboxylate groups prepared from a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized pulp exhibited an extremely low oxygen permeability of 0.0008 mL µm m(-2) day(-1) kPa(-1) at 0% relative humidity (RH). Positron annihilation lifetime spectroscopy (PALS) was used to determine the pore sizes in wood and tunicate TEMPO-oxidized cellulose nanofibril (TOCN-COONa) films in a vacuum (i.e., at 0% RH). PALS analysis revealed that the pore size of the wood TOCN-COONa films remained nearly at 0.47 nm from the film surface to the interior of the film. This is probably the cause of this high oxygen-barrier properties at 0% RH. The crystalline structure of TOCN-COONa also contributes to the high oxygen-barrier properties of the wood TOCN-COONa films. However, the oxygen permeability of the wood TOCN-COONa films increased to 0.17 mL µm m(-2) day(-1) kPa(-1) at 50% RH, which is one of the shortcomings of hydrophilic TOCN-COONa films.

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.

TEMPO-oxidized cellulose nanofibers.

Native wood celluloses can be converted to individual nanofibers 3-4 nm wide that are at least several microns in length, i.e. with aspect ratios>100, by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and successive mild disintegration in water. Preparation methods and fundamental characteristics of TEMPO-oxidized cellulose nanofibers (TOCN) are reviewed in this paper. Significant amounts of C6 carboxylate groups are selectively formed on each cellulose microfibril surface by TEMPO-mediated oxidation without any changes to the original crystallinity (∼74%) or crystal width of wood celluloses. Electrostatic repulsion and/or osmotic effects working between anionically-charged cellulose microfibrils, the ζ-potentials of which are approximately -75 mV in water, cause the formation of completely individualized TOCN dispersed in water by gentle mechanical disintegration treatment of TEMPO-oxidized wood cellulose fibers. Self-standing TOCN films are transparent and flexible, with high tensile strengths of 200-300 MPa and elastic moduli of 6-7 GPa. Moreover, TOCN-coated poly(lactic acid) films have extremely low oxygen permeability. The new cellulose-based nanofibers formed by size reduction process of native cellulose fibers by TEMPO-mediated oxidation have potential application as environmentally friendly and new bio-based nanomaterials in high-tech fields.

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).

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.