Kojiro Uetani

Zeta Potential Time Dependence Reveals the Swelling Dynamics of Wood Cellulose Nanofibrils

In this paper, we present the swelling dynamics of individual wood cellulose nanofibrils (CNFs) following solvent substitution into various organic solvents and drying, by employing the time dependence of the zeta potential (ζ). We succeeded in smoothly redispersing the coaggregating CNFs dried in solvents, including acetone, acetonitrile, DMSO, ethanol, and t-butanol into water. ζ-t plots of the redispersed CNFs measured in a 1 mM KCl solution indicated different values of Δζ (volume fraction of hydration capacity), corresponding to the dielectric constant of the substituted solvents. Differential scanning calorimetry confirmed that the redispersed CNFs swell to different degrees, corresponding to Δζ. This swelling behavior is characterized by expansion of hemicelluloses, the amorphous polysaccharides located on the CNF surface, with a different degree of aggregation during drying. The specific swelling ratio, radius, and diameter of the CNFs in water were calculated using the values of ζ(0) and ζ(∞) by introducing surface chemical analysis. The calculated diameters of the CNFs at t = 0 coincided well with the median diameters measured directly by transmission electron microscope. Swellability of hemicelluloses exponentially increased with the decrease in dielectric constant of solvent during drying. The analysis method combining zeta potential time dependence and a surface chemical approach proved useful for specifically evaluating the swelling dynamics of polymers on a bulk surface.

Crystallite Size Effect on Thermal Conductive Properties of Nonwoven Nanocellulose Sheets.

The thermal conductive properties, including the thermal diffusivity and resultant thermal conductivity, of nonwoven nanocellulose sheets were investigated by separately measuring the thermal diffusivity of the sheets in the in-plane and thickness directions with a periodic heating method. The cross-sectional area (or width) of the cellulose crystallites was the main determinant of the thermal conductive properties. Thus, the results strongly indicate that there is a crystallite size effect on phonon conduction within the nanocellulose sheets. The results also indicated that there is a large interfacial thermal resistance between the nanocellulose surfaces. The phonon propagation velocity (i.e., the sound velocity) within the nanocellulose sheets was estimated to be ∼800 m/s based on the relationship between the thermal diffusivities and crystallite widths. The resulting in-plane thermal conductivity of the tunicate nanocellulose sheet was calculated to be ∼2.5 W/mK, markedly higher than other plastic films available for flexible electronic devices.

Self-organizing capacity of nanocelluloses via droplet evaporation

The relation between the fibril shapes and the self-organizing capacity was investigated by comparing the results of evaporation-induced self-assembly (EISA) in 2D (“coffee rings”) and 3D (spray-dried microparticles (MPs)) for rod-like and semi-flexible nanocelluloses. The rod-like tunicin nanowhiskers formed nematic alignment along the perimeter of coffee rings, and also formed curved discotic microparticles having nematic rings of width ∼300 nm by spray drying. On the other hand, the semi-flexible tunicin nanofibers failed to undergo a phase transition by EISA, and formed spray-dried MPs with multiple sharp kinks and rough contours. The rod-like nanocelluloses exhibited the self-organizing capacity of the phase transition and left-handed chirality. The expression of the self-organizing capacity of rod-like particles was found to be independent of initial droplet shapes and sizes via surface tension measurements.

Thermally conductive and optically transparent flexible films with surface-exposed nanocellulose skeletons

Heat management has become a serious bottleneck that has limited the development of thin flexible paper electronics. Therefore, there is a huge demand to develop superior flexible film materials with higher thermal conductivities and transparencies. In this study, thermally conductive and optically transparent flexible films with flexible nanocellulose skeletons have been fabricated by using a membrane-assisted method. The films simultaneously exhibit in-plane thermal conductivity as high as 2.5 W m−1 K−1 with a thermal conductivity enhancement of 234% from the matrix acrylic resin, and a parallel beam transmittance of 73% at 600 nm. We demonstrate that removal of heat-insulating surface resin layers achieved by our membrane-assisted method is the key to exploit the high thermal performance of intrinsic nanocellulose skeletons with a markedly high fiber content of ∼80%. The thermal and optical properties of membrane-assisted films are controlled by changing the fiber content. We believe that this approach could provide a way to solve the critical bottleneck of modern electronics by advanced thermal management.

Hydrophilic interaction electrokinetic chromatography using bio-based nanofillers

Hydrophilic interaction (HI)‐based separation like HILIC is effective for analyzing hydrophilic biological samples such as carbohydrates, peptides, and metabolites. To overcome the drawbacks of conventional HILIC such as large consumption of organic solvents and easy deterioration of the separation column, we developed HI electrokinetic chromatography (EKC) by employing bio‐based nanomaterials as the hydrophilic pseudostationary phase. By mechanical/chemical treatments, cellulose, chitin, and chitosan were processed to 10‐nm wide nanofibers/nanowhiskers (NFs/NWs), which are longer/shorter than 1000/200 nm, respectively. In HI‐EKC of oligosaccharides using 0.001% uncharged cellulose NFs, strong interaction was observed for the large‐size oligosaccharides with the retention factors (k) of up to 1.56, indicating a HILIC‐mode interaction. In HI‐EKC with 0.1% positively charged chitosan NFs, benzenedisulfonic acid, benzenesulfonic acid (BS), and p‐hydroxy BS (HBS) had k values of 0.036, 0.018, and 0.018, respectively, suggesting that the ion‐exchange interaction mainly occurred via sulfonate groups. Finally, HI‐EKC was demonstrated using 0.05% chitin or chitosan NWs. In both cases using chitin and chitosan NWs, HBS showed much stronger interaction with k > 0.192 compared with BS with k < 0.070. It indicated structural difference between NFs and NWs affected the HI behavior in terms of both the ion‐exchange and HILIC modes.

Thermal conductivity analysis and applications of nanocellulose materials

Abstract In this review, we summarize the recent progress in thermal conductivity analysis of nanocellulose materials called cellulose nanopapers, and compare them with polymeric materials, including neat polymers, composites, and traditional paper. It is important to individually measure the in-plane and through-plane heat-conducting properties of two-dimensional planar materials, so steady-state and non-equilibrium methods, in particular the laser spot periodic heating radiation thermometry method, are reviewed. The structural dependency of cellulose nanopaper on thermal conduction is described in terms of the crystallite size effect, fibre orientation, and interfacial thermal resistance between fibres and small pores. The novel applications of cellulose as thermally conductive transparent materials and thermal-guiding materials are also discussed.