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TEMPO-Oxidized Cellulose Nanofibrils Dispersed in Organic Solvents Yusuke Okita, Shuji Fujisawa, Tsuguyuki Saito, and Akira Isogai* Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan Received October 20, 2010 Revised Manuscript Received December 7, 2010
Introduction Cellulose-based nanofibers and nanowhiskers have been recently investigated as excellent and environmentally friendly nanoelements at the cutting-edge materials sciences because they can be produced from abundant wood biomass resources.1,2 Typical cellulose-based nanomaterials include: (1) microfibrillated celluloses (MFCs) prepared from chemically modified or unmodified wood cellulose/water slurries with or without cellulase by repeated high-pressure homogenizer or twin-screw extruder treatments,3-7 (2) cellulose nanocrystals (CNCs) (or nanowhiskers) prepared from native celluloses by acid hydrolysis (e.g., 64% sulfuric acid) and successive mechanical disintegration of the acid-hydrolyzed residues in water,8-10 and (3) TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized cellulose nanofibrils (TOCNs).11-14 In the case of the TOCNs, the C6 primary hydroxyl groups exposed on the crystalline cellulose microfibril surfaces are selectively and fully converted to C6 carboxylate groups when treated with aqueous TEMPO-mediated oxidation systems under suitable conditions.13-16 In the process of mechanical disintegration in water, electrostatic repulsion between the anionically charged cellulose microfibrils comes into play, allowing the formation of stable dispersions. Therefore, individual TOCNs with uniform width of 3 to 4 nm and length of 0.5 to 2 µm dispersed in water can be prepared from TEMPO-oxidized wood celluloses by mechanical disintegration in water under moderate conditions.11-14 Cast and dried films prepared from TOCN/water dispersions are transparent and flexible and exhibit high tensile strengths, low coefficients of thermal expansion, and extremely high oxygen-barrier properties under dry conditions.17 One promising application of cellulose-based nanomaterials is to use them as nanofillers in polymer composites. On the basis of composite theory, the mechanical properties of polymer composites can be predicted by a percolation model, in which the aspect ratio (length/width), volume fraction, orientation, and elastic modulus of the nanofillers are the primary factors.18 To increase the apparent aspect ratios of nanofillers in the composites, it is necessary for the nanofillers to exist as individually separated or exfoliated elements in the composites without aggregation, and many papers and reviews have focused on such systems.14,19,20 Most synthetic polymers used for plastic molding are hydrophobic, and thus it is generally difficult to disperse homogeneously hydrophilic cellulose-based nanomaterials in synthetic polymer matrices such as poly(propylene) and poly(ethylene) without aggregation. TOCNs have high aspect ratios of more than 100,11-14 high crystallinities of 70-95%, and outstanding Young’s modulus of 130-150 GPa,14,20,21 making * To whom correspondence should be addressed. Phone: +81 3 5841 5538. Fax: +81 3 5842 5269. E-mail:
[email protected].
TOCNs ideal for potential application as nanosized and biobased reinforcement fillers to polymer composites. CNCs with or without surface modification have been dispersed in organic solvents at the individual CNC level.19 For example, CNCs with sulfate ester groups prepared by acidhydrolysis of native celluloses were directly dispersed in polar aprotic solvents such as N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) by mechanical disintegration of freeze-dried or solvent-exchanged CMCs.22-25 However, because TOCNs have abundant sodium carboxylate groups on the TOCN surfaces in high density, different from CNCs, and have much higher aspect ratios, TOCNs could never be dispersed on the nanosized level in organic solvents by using the protocols previously reported for CNCs. In the present study, we demonstrate a procedure for preparing individually dispersed TOCNs in organic solvents by exchanging counterion of carboxylate groups and propose the nanodispersion mechanism thereof.
Experimental Section Materials. Tunicate of Halocynthia roretzi was cut into small pieces with scissors and purified according to the standard method.26,27 The tunicate cellulose was disintegrated in water to produce fine particles using a double-cylinder type homogenizer (Physcotron NS-56, Microtec Nition, Japan) and screened with a wire-mesh filter (ca. 150 mesh) to remove large fragments. Polar aprotic organic solvents such as DMF, DMSO, 1,3-dimethyl-2-imidazolidinone (DMI), N,N-dimethylacetamide (DMAc), and 1-methyl-2-pyrrolidinone (NMP) were commercial products of laboratory grade (Wako Pure Chemicals, Japan) and used after dewatering with molecular sieves. TEMPO, sodium bromide, a 2 M sodium hypochlorite solution, and other chemicals and solvents were of laboratory grade (Wako Pure Chemicals, Japan) and used as received. A commercial softwood bleached kraft pulp was provided by Nippon Paper Japan as never-dried wet fibers with 80% water content. The pulp contained ∼90% cellulose and 10% hemicelluloses. The pulp was soaked overnight in water adjusted with dilute HCl to pH 3 to remove metal cations and then washed thoroughly with water by filtration. TEMPO-Mediated Oxidation. TEMPO-oxidized tunicate and softwood celluloses were prepared using a TEMPO/NaBr/NaClO system28 in water at pH 10 and room temperature under the conditions reported to give the maximum contents of carboxylate groups in the native celluloses, according to a reported method.15 The carboxylate and aldehyde contents of the TEMPO-oxidized tunicate cellulose were 0.59 and 0.06 mmol g-1, respectively, and those of the TEMPOoxidized softwood cellulose were 1.65 and 0.07 mmol g-1, respectively.15 The TEMPO-oxidized celluloses were stored at 4 °C without drying. Preparation of TEMPO-Oxidized Cellulose/Water Dispersion. The TEMPO-oxidized cellulose with sodium carboxylate structure (TOCCOONa) was suspended in water (100 mL) at a 0.15% (w/v) consistency. After the suspension was adjusted to pH 8 with 0.01 M NaOH, it was sonicated for 2 min using an ultrasonic homogenizer with a 7 mm probe-tip diameter (US-300T, Nihonseiki, Tokyo, Japan) at 19.5 kHz and an output power of 300 W. No temperature increases were observed. Unfibrillated and partially fibrillated fractions, if present, were removed by centrifugation at 12 000g for 5 min, and the flowable and transparent supernatant was adjusted to 0.1% (w/v) consistency before use in experiments. In both TEMPO-oxidized tunicate and softwood celluloses, the unfibrillated and partially fibrillated fractions removed by centrifugation were 3 months at room temperature. When the TOC-COOH was dispersed in DMSO, birefringence was apparent immediately after sonication. However, the initially transparent dispersion gradually changed to a gel within 1 day, as can be observed at the upper internal wall of the sample bottle in Figure 1 (see arrow). When acetone, 2-propanol, acetonitrile, methanol, ethanol, or tetrahydrofuran was used, individual dispersion of the TOCN-COOH was not observed. For the tunicate TOCN-COONa, only DMSO gave a stable dispersion among the solvents tested. As shown at the upper internal wall of the sample bottle in Figure 1 (see arrows), TOC-COONa gels were observed for the other organic solvents, and no individual dispersion of the nanofibril was obtained. The same behavior was also observed for TOCN dispersion of the softwood TOC in organic solvents (Supporting Information); that is, the dispersion behavior of tunicate and softwood TOCs in organic solvents was essentially the same. For observation by AFM, a droplet of the dilute TOCNCOOH/DMF dispersion was put onto a mica plate, followed by evaporation of DMF in a vacuum oven. The AFM image showed that the tunicate TOCNs were individually separated and estimated to be 10 ( 1.8 nm in width (N ) 30) from their height images (Figure 2). These results confirmed that the tunicate and softwood TOCs with free carboxyl groups could be individually dispersed in polar aprotic organic solvents with high boiling points and that all of the TOCN dispersions with the exception of TOCN-COOH/DMSO were stable at room temperature. Cellulose I crystallinity indices of TOCN films prepared from the original TOCN-COONa/water dispersions and TOCNCOOH/organic solvent dispersions were determined from FTIR spectra.31 The results showed that almost no changes in crystallinity index of cellulose I took place for the TOCN films before and after the dispersion treatment in organic solvents. Because most synthetic and biobased plastics used for molding are soluble in organic solvents, TOCNs are of potential application as effective reinforcement nanoelements in TOCN/ polymer composites. However, for this purpose, nanodispersion of TOCNs in organic solvents with much lower boiling points than those given in Figure 1 is required. Alternatively, TOCN/ polymer composites may be prepared by polymerization of monomers in the TOCN/organic solvent dispersions in Figure 1. In this case, the addition of some amphiphilic compounds or
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Notes
Figure 1. Dispersibility of TOC-COOH and TOC-COONa in organic solvents at individual nanofibril level. Photos were taken with (right) and without (left) cross polarizers. +, Stable dispersion at nanofibril level; (, unstable dispersion; -, nondispersion.
Figure 2. AFM image of tunicate TOCN-COOH prepared from its DMF dispersion.
surfactants to the dispersions may improve the miscibility between the hydrophobic plastic polymers and the hydrophilic TOCNs at the individual nanofibril level. The polymerization is expected to provide effectively nanoreinforcement of the TOCNs in the composites. Direct Dispersion of TOCNs in Organic Solvents. In the above section, individually dispersed TOCNs were successfully prepared in organic solvents from tunicate and softwood TOCs through the individual dispersions of TOCN-COONa in water prepared beforehand. When the solvent-exchanged and neverdried TOC-COOH or TOC-COONa without individualization pretreatment in water were directly sonicated in organic solvents under the same conditions, some of the TOC-COOH or TOC-COONa was individually dispersed in DMAc, DMI, DMF, NMP, or DMSO. Therefore, TOC-COOH and TOCCOONa may be directly converted to individually dispersed TOCNs in organic solvent without nanodispersion pretreatment by controlling the disintegration conditions. However, in this case, more energy-intensive disintegration treatments over longer
Figure 3. Light transmittance spectra of TOCN-COOH/organic solvent and TOCN-COONa/DMSO dispersions.
times were required. In addition, greater amounts of unfibrillated and partially fibrillated fractions (>70%) were present in the dispersions. Therefore, direct disintegration of TOCs in organic solvents was not effective in preparing individually dispersed TOCNs in high yields, whereas pretreatment of TOCNs to form nanodispersion in water was more effective under the same conditions. Light Transmittance of TOCN/Organic Solvent Dispersions. Figure 3 shows the light transmittance spectra of the tunicate TOCN-COOH/organic solvent and tunicate TOCNCOONa/DMSO dispersions. All dispersions had high transmittances of >75% at 600 nm. The light transmittances were wavelength-dependent, which is related to the nanofibril widths.34-36 Even though all dispersions had the same TOCN consistency of 0.1%, transmittances differed considerably among the organic solvents. The effect of the difference in refractive index (∆n) between the cellulose (n0) and organic solvent (ns) on transmittance of the tunicate TOCN/organic solvent dispersions at 600 nm is depicted in Figure S3 of the Supporting
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vidually dispersed TOCNs to form in organic solvents by electrostatic repulsion, osmotic effects, or both between the anionically charged TOCNs. In the case of TOCN-COONa, only DMSO formed a stable dispersion at the individual nanofibril level. Both tunicate and softwood TOCs had the same dispersion behavior in organic solvents.
Figure 4. ζ-Potentials of tunicate TOCN-COOH and TOCN-COONa dispersed in organic solvents.
Information. A roughly linear relationship was obtained between ∆n and the transmittance, showing that the smaller the ∆n value the higher the light transmittance through the TOCN/organic solvent dispersion.37 Dispersion Mechanism of TOCN in Organic Solvents. Stability of the surface-charged colloidal particles can be explained in terms of electrostatic repulsion, osmotic effects, or both between the dispersed materials in a solvent medium.38,39 ζ-Potential measurements have been applied also to rod-like TiO2 nanotubes and carbon nanofibers dispersed in solvents for approximate evaluation of their surface charges on the basis of Smoluchowski theory.40,41 Then, ζ-potentials of TOCNs in organic solvents were measured by the electrophoretic method (Figure 4). Because water contents were
