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Nanoporous Networks Prepared by Simple Air Drying of Aqueous TEMPO-Oxidized Cellulose Nanofibril Dispersions Junji Nemoto,† Toshihiko Soyama,† Tsuguyuki Saito,‡ and Akira Isogai*,‡ †
Central Research Laboratory, Hokuetsu-Kishu Paper Co. Ltd., 3-5-1, Nishizao, Nagaoka, Niigata 940-0027, Japan Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan
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INTRODUCTION Network materials with nanosized pores have attracted a great deal of attention because they have potential applications as high efficiency particulate air (HEPA) filters in hospitals and clean rooms for the food and medical fields, ultralow penetration air (ULPA) filters for industry clean rooms, composite-reinforcement frameworks, catalyst supports, and scaffolds for tissue engineering.1−5 Cellulose is present as crystalline nanofibrils in plant cell walls, and wood cellulose nanofibrils in particular have extremely small widths of ∼4 nm, high aspect ratios, and high elastic moduli.6,7 Thus, new biobased and environmentally friendly porous network materials with high performances are expected to be able to be prepared from cellulose nanofibrils. Mechanical disintegration of wood celluloses in water produces fibrillated celluloses that consist of bundles of cellulose nanofibrils 25−100 nm in width.8,9 Recently, a number of pretreatments such as chemical modification and enzymatic partial hydrolysis of cellulose fibers prior to mechanical disintegration in water have been shown to be effective in nanofibrillation of celluloses.10−13 Among these pretreatments, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)mediated oxidation of wood cellulose fibers enables complete individualization of cellulose nanofibrils in water, which is advantageous in terms of functionalization and industrial applications. When aqueous dispersions of cellulose nanofibrils or nanocelluloses are directly dried, cellulose nanofibrils are tightly aggregated to one another during water evaporation, forming numerous hydrogen bonds. The obtained films have low oxygen permeabilities or high oxygen barrier properties at least under dry conditions.7,14 Freeze-drying or supercritical drying of aqueous nanocellulose dispersions has been proposed to prepare cellulose aerogels containing porous nanofibril networks.15−18 These aerogels not only have high specific surface areas and low densities, but also better mechanical properties than those of other organic polymers. However, because freeze-drying or supercritical drying require special equipment and time-consuming solventexchange steps, productivity is extremely low. The use of organic solvents limits industrial production, and a solvent recovery system is required. Furthermore, significant costs are involved in building and running an autoclave for supercritical drying at the industrial scale. Therefore, more simple processes, such as hot-air drying of the aqueous nanocellulose dispersions, would be desirable for the formation of porous nanocellulose networks. © 2012 American Chemical Society
Here, we report a simple process to form porous network structures from aqueous dispersions of TEMPO-oxidized cellulose nanofibrils (TOCNs) by hot-air drying, without solvent exchange or special drying equipment. This process includes concentration control of the TOCN dispersions, modification of the TOCN surfaces by a surfactant, and the use of porous supports with micrometer or submicrometer pore sizes. The formation of porous network structures was confirmed by scanning electron microscopy (SEM) and laser scanning microscopy (LSM), the latter enabling progressive observation during the drying process.
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EXPERIMENTAL SECTION
Materials. A never-dried softwood bleached kraft pulp (HokuetsuKishu Paper, Co. LTD, Niigata, Japan) was used as the wood cellulose sample. The pulp contained approximately 90% cellulose and 10% hemicelluloses. For demineralization, the pulp (10 g) was soaked in dilute HCl (990 mL) at pH ∼ 2 and room temperature for 0.5 h and then washed repeatedly with water by filtration before use. Laboratory grade TEMPO, sodium bromide, 12% sodium hypochlorite solution, and dodecyltrimethylammonium bromide (DTAB) were used as received (Wako Pure Chemicals, Japan). TOCN Dispersions. TEMPO-mediated oxidation was carried out according to a previously reported method.19 The cellulose (1 g) was suspended in water (100 mL) containing TEMPO (0.016 g, 0.1 mmol) and sodium bromide (0.1 g, 1 mmol). The 12% NaClO solution (3.1 g, 5.0 mmol) was added to the cellulose slurry and the mixture was stirred at room temperature. The pH of the slurry was maintained at pH 10 by adding 0.5 M NaOH with a pH stat for
