Effect of the degree of acetylation on the physicochemical properties of α-chitin nanofibers.

The effective utilization of abundant α-chitin resources for materials engineering applications requires methods for controlling the physicochemical properties of α-chitin nanofiber (NF) dispersions. Herein, the relationship between the degree of acetylation (DA) of α-chitin and the physicochemical properties of α-chitin nanofibers (α-ChNFs) was investigated. α-Chitin with different DAs was prepared by varying the deacetylation treatment time. These α-chitin samples were disintegrated into NFs using wet pulverization. The average width of the α-ChNFs decreased with decreasing DA. Furthermore, the transmittance and viscosity of the α-ChNF dispersions increased with decreasing DA. We successfully developed a simple model for estimating the average width of α-ChNFs with different DAs. These results indicate that the DA is an effective parameter for defining and controlling the physicochemical properties of α-ChNFs.

Non-catalytic conversion of chitin into Chromogen I in high-temperature water.

The non-catalytic conversion of chitin into N-acetyl-ᴅ-glucosamine (GlcNAc) derivatives such as 2-acetamido-2,3-dideoxy-ᴅ-erythro-hex-2-enofuranose (Chromogen I) was investigated in high-temperature water at 290-390 °C and 25 MPa with a reaction time of 0-180 min. High-temperature water treatment is a promising method for chitin conversion as it does not require the use of any additional organic solvents or ionic liquids. A semi-batch reactor was developed to control the reaction temperature and time. It was found that the chitin powder could be converted into a water-soluble fraction in ~90% yield, with Chromogen I being obtained in a maximum yield of 2.6%. Furthermore, a kinetic model was developed to estimate the reaction rate for the conversion of the chitin powder to the water-soluble fraction.

Preparation of ß-chitin nanofiber aerogels by lyophilization.

In this study, chitin nanofiber dispersions prepared in neutral and acidic pH conditions were lyophilized to produce aerogels. The effects of the freezing speed of the nanofiber dispersions and the dispersibility of the chitin nanofiber were studied. The characteristics of the aerogels were studied using scanning electron microscopy, relative surface area measurements, and compression tests. The repulsion forces of the chitin nanofiber in acidic conditions were effective in the formation of a more uniform microstructure during water solidification, resulting in aerogels with a high mechanical strength. The aerogel made from the chitin nanofiber dispersion prepared in neutral conditions was influenced by ice crystal growth during freezing, resulting in a nonuniform structure. In contrast, the surface area of the aerogel in neutral conditions interestingly remained unchanged compared to that of the original powder, which was due to the morphological transformation.

Systematic dynamic viscoelasticity measurements for chitin nanofibers prepared with various concentrations, disintegration times, acidities, and crystalline structures.

Dynamic viscoelasticities were measured for chitin nanofiber (ChNF) dispersions prepared with various concentrations, disintegration times, acidities, and crystalline structures. The 0.05w/v% dispersions of pH neutral ChNFs continuously exhibited elastic behavior. The 0.05w/v% dispersions of acidified ChNFs, on the other hand, transitioned from a colloidal dispersion to a critical gel and then exhibited elastic behavior with increasing ChNF concentration. A double-logarithmic chart of the concentration vs. the storage modulus was prepared and indicated the fractal dimension and the nanostructure in the dispersion. The results determined that the neutral α- and ß-ChNFs were dispersed but showed some remaining aggregations and that the acidified ß-ChNFs were completely individualized. In addition, the α-chitin steadily disintegrated with increasing disintegration time, and the aspect ratio of the ß-chitin decreased as a result of the exscessive disintegration. The storage moduli of the ChNFs were greater than those of chitin solutions, nanorods, and nanowhiskers with the same solids concentrations.

Self-Sustaining Cellulose Nanofiber Hydrogel Produced by Hydrothermal Gelation without Additives.

We prepared a self-sustaining hydrogel from 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs) via hydrothermal treatment at 160 °C. The self-sustaining hydrogels could be obtained at less than 1 wt % TOCNs without any additives. Brownish hydrogels obtained after the hydrothermal treatment could be rendered transparent by immersing them in distilled water at 5 °C. The compressive modulus of the hydrogel increased with increasing heating time. X-ray diffraction studies reveal that the crystal structure of the internal layers of the TOCNs remained intact after the hydrothermal treatment and depigmentation. The hydrothermal treatment caused the hydrolysis of molecules, especially the glucuronate units, from the external layer of TOCN. The elimination of the glucuronate units decreased the net negative surface charge of the TOCNs, resulting in their aggregation into a three-dimensional network structure owing to the predominance of attractive forces. Such additive-free hydrogels that can be shaped into diverse forms are promising for medical applications.

Effect of acidity on the physicochemical properties of α- and ß-chitin nanofibers.

We have investigated whether acidity can be used to control the physicochemical properties of chitin nanofibers (ChNFs). In this study, we define acidity as the molar ratio of dissociated protons from the acid to the amino groups in the raw chitin powder. The effect of acidity on the physicochemical properties of α- and ß-ChNFs was compared. The transmittance and viscosity of the ß-ChNFs drastically and continuously increased with increasing acidity, while those of the α-ChNFs were not affected by acidity. These differences are because of the higher ability for cationization based on the more flexible crystal structure of ß-chitin than α-chitin. In addition, the effect of the acid species on the transmittance of ß-ChNFs was investigated. The transmittance of ß-ChNFs can be expressed by the acidity regardless of the acid species, such as hydrochloric acid, phosphoric acid, and acetic acid. These results indicate that the acidity defined in this work is an effective parameter to define and control the physicochemical properties of ChNFs.

Effect of purification method of ß-chitin from squid pen on the properties of ß-chitin nanofibers.

The relationship between purification methods of ß-chitin from squid pen and the physicochemical properties of ß-chitin nanofibers (NFs) were investigated. Two types of ß-chitin were prepared, with ß-chitin (a→b) subjected to acid treatment for decalcification and then base treatment for deproteinization, while ß-chitin (b→a) was treated in the opposite order. These ß-chitins were disintegrated into NFs using wet pulverization. The ß-chitin (b→a) NF dispersion has higher transmittance and viscosity than the ß-chitin (a→b) NF dispersion. For the first time, we succeeded in obtaining 3D images of the ß-chitin NF dispersion in water by using quick-freeze deep-etch replication with high-angle annular dark field scanning transmission electron microscopy. The ß-chitin (b→a) NF dispersion has a denser and more uniform 3D network structure than the ß-chitin (a→b) NF dispersion. Widths of the ß-chitin (a→b) and (b→a) NFs were approximately 8-25 and 3-10nm, respectively.