A sterically stabilized aqueous suspension of rodlike cellulose microcrystals was prepared by the combination of acid hydrolysis of native cellulose, oxidative carboxylation of microcrystals, and grafting of poly(ethylene glycol) having a terminal amino group on one end (PEG-NH2, MW ) 1000) using watersoluble carbodiimide. Chemical binding of PEG to the microcrystals was confirmed by weight increase, diminishment of carboxyl groups, thermogravimetry, and infrared spectroscopy, resulting in consumption of 20-30% of the initially introduced carboxyl groups. The amount of bound PEG was 0.2-0.3 g/g of cellulose. The PEG-grafted cellulose microcrystals showed drastically enhanced dispersion stability, that is, resistance to addition of 2 M sodium chloride, and ability to redisperse into either water or chloroform from the freeze-dried state. The concentrated aqueous suspension of PEG-grafted microcrystals formed a chiral nematic mesophase through a phase separation similar to that of the ungrafted sample, but with a reduced spacing of the fingerprint pattern.

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Conversion of 1,2-dihydroxyl groups to dialdehyde by periodate oxidation is a useful method of derivatizing polysaccharides but has not been extensively utilized in derivatization of cellulose because of complicacy due to the crystalline nature of cellulose. To understand the influence of cellulose crystallinity on this reaction, we investigated how the periodate oxidation proceeds with a highly crystalline cellulose of the marine alga Cladophora sp. The crystallinity of the oxidized cellulose, determined by X-ray diffraction, decreased according to the oxidation level. The half-height widths of equatorial diffraction peaks were nearly unchanged. The solid-state 13C NMR spectra did not show peaks corresponding to aldehyde groups, but solution 13C NMR spectra showed the presence of dicarboxylic groups after subsequent oxidation by sodium chlorite. Transmission electron microscopy showed that microfibrils of Cladophora tended to be bent and more flexible than the original sample. Gold labeling of the aldehyde groups, mediated by thiosemicarbazide derivatization, revealed a highly uneven distribution of dialdehyde groups. When treated by 50% (w/v) sulfuric acid, partially oxidized Cladophora cellulose gave many short fragments of microfibril. These features indicate that the periodate oxidation proceeds by forming dialdehyde groups in longitudinally spaced, bandlike domains.

Cellulose can be dissolved in precooled (-12 °C) 7 wt % NaOH-12 wt % urea aqueous solution within 2 min. This interesting process, to our knowledge, represents the most rapid dissolution of native cellulose. The results from 13 C NMR, 15 N NMR, 1 H NMR, FT-IR, small-angle neutron scattering (SANS), transmission electron microscopy (TEM), and wide-angle X-ray diffraction (WAXD) suggested that NaOH "hydrates" could be more easily attracted to cellulose chains through the formation of new hydrogen-bonded networks at low temperatures, while the urea hydrates could not be associated directly with cellulose. However, the urea hydrates could possibly be self-assembled at the surface of the NaOH hydrogen-bonded cellulose to form an inclusion complex (IC), leading to the dissolution of cellulose. Scattering experiments, including dynamic and static light scattering, indicated that most cellulose molecules, with limited amounts of aggregation, could exist as extended rigid chains in dilute solution. Further, the cellulose solution was relatively unstable and could be very sensitive to temperature, polymer concentration, and storage time, leading to additional aggregations. TEM images and WAXD provided experimental evidence on the formation of a wormlike cellulose IC being surrounded with urea. Therefore, we propose that the cellulose dissolution at -12 °C could arise as a result of a fast dynamic self-assembly process among solvent small molecules (NaOH, urea, and water) and the cellulose macromolecules.

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Aerogels with their low density (0.004-0.500 g cm À3 ), large internal surface area, and large open pores are promising candidates for various advanced applications.[1] The utilization of inorganic aerogels, however, has been hampered by their poor mechanical properties. A prominent example is silica aerogel, which is prepared by an organic sol-gel process, [2] and has unique features, such as ultralow density (the lightest silica aerogel has a density that is similar to the density of air, which is 0.00129 g cm À3 ), near transparency, and low thermal conductivity. However, the extreme fragility of this aerogel necessitates its reinforcement for practical uses. A typical method is hybridization with organic polymers, such as polyurea, polyurethane, poly(methyl methacrylate), polyacrylonitrile, and polystyrene. [3] Other candidates for the reinforcement of inorganic aerogels are insoluble polysaccharides, which are abundantly available and show wide varieties in structure and properties.[4] The useful features of these compounds are hydrophilicity, biocompatibility, hydroxy reactivity, and reasonable thermal and mechanical stabilities.[5] For example, nanofibrillar bacterial cellulose and microfibrillated cellulose gel have been proposed as templates for cobalt ferrite nanoparticles and titanium dioxide. [6] While in the above-mentioned work native cellulose with cellulose I crystallinity was used, cellulose can be prepared as a hydrogel with cellulose II crystallinity through dissolution and coagulation. Some of the resulting aerogels have remarkable mechanical strength and light transmittance.[7] They have high porosity with open structures and thus provide an effective substrate for the synthesis of metallic nanoparticles.[8] To further utilize the regenerated cellulose gel, we herein attempted in situ synthesis of silica in cellulose gels.While a similar attempt has been reported, in which the cellulose gel was obtained from solution in N-methylmorpholine-N-oxide monohydrate, [9] the development of the nanostructure (nitrogen BET surface area of 220-290 m 2 g À1 ) and the level of silica loading (less than 13 % w/w) were rather limited. By using the aqueous alkali-based solvent, we obtained the cellulose aerogel with a surface area of 356 m 2 g À1 , and a silica loading of more than 60 % w/w resulted in surface areas that exceeded 600 m 2 g À1 . We used the sol-gel synthesis method toward nanostructured silica, which typically starts from tetraethyl orthosilicate (TEOS). The resulting composite gels were dried with supercritical CO 2 to give cellulose-silica aerogels with low density, moderate light transmittance, a large surface area, high mechanical integrity, and excellent heat insulation. This method can also lead to fabrication of silica-only aerogels through the removal of cellulose by calcination, that is, the use of cellulose aerogel as sacrificial template. Figure 1 shows the preparation of the aerogel. The cellulose hydrogel is a transparent material that has a water content of 92 % and a poro...

The rodlike microcrystal suspension prepared by sulfuric acid hydrolysis of bacterial cellulose was found to undergo spontaneous nematic phase separation after complete desalination. This phase separation was preceded by a birefringent glassy-like state for about 1 week. Addition of trace electrolyte (<1 mM NaCl) caused remarkable changes in phase separation behavior; i.e., the separation was complete in 2 days and the anisotropic phase became chiral nematic. This phenomenon can be explained by the change in the effective particle shape, from cylindrical to the twisted rod as a result of screening of surface charge.

Despite considerable progress in the field of metal nanoparticles synthesis, major challenges remain in many practical applications of nanoparticles which require their immobilization on solid substrates, presenting additional difficulty in separation and processing. Here, transparent nanoporous cellulose gel obtained from aqueous alkali hydroxide-urea solution was examined as supporting medium for noble metal nanoparticles. Silver, gold, and platinum nanoparticles were synthesized in the gel by hydrothermal reduction by cellulose or by added reductant. Both methods gave nanoparticles embedded with high dispersion in cellulose gels. Supercritical CO2 drying of the metal-carrying gel gave corresponding aerogels with high transmittance, porosity, surface area, moderate thermal stability, and good mechanical strength. The cellulose and metal-cellulose gels were characterized by UV/vis spectroscopy, optical microscopy, SEM, TEM, XRD, nitrogen physisorption, TGA, and tensile testing, systematically.