Entire Surface Oxidation of Various Cellulose Microfibrils by TEMPO

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Entire Surface Oxidation of Various Cellulose Microfibrils by TEMPO-Mediated Oxidation Yusuke Okita, 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 February 25, 2010 Revised Manuscript Received April 22, 2010

Introduction Native cellulose microfibrils have been recently evaluated as excellent nanoelements in cutting-edge material sciences.1-6 Cellulose microfibrils are highly crystalline and ultrafine nanomaterials consisting of directionally aligned molecular chains and have high aspect ratios (3-20 nm in width and more than a few micrometers in length)7-9 and outstanding mechanical properties (elastic modulus of 130-150 GPa).10,11 Surface modification of native cellulose microfibrils is thus one of the most important techniques for further developments of the microfibrils as functional nanomaterials, sometimes incorporated with other matters. In recent years, catalytic oxidation using stable nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine1-oxyl (TEMPO) was found to be the most simple and efficient procedure for surface carboxylation of native cellulose microfibrils.12-18 The TEMPO-mediated oxidation can be carried out in water under mild conditions, and primary alcohols or primary hydroxyls of the substrates are selectively oxidized to sodium carboxylates.19-21 Often hypochlorite is the primary oxidant and bromide is applied as a cocatalyst. When the TEMPO-mediated oxidation was applied to native celluloses such as bleached wood pulp and cotton, significant amounts of carboxylate groups were formed in the solid celluloses, maintaining the original fibrous morphologies and crystallinities. Solid state 13C NMR and other distribution analyses indicated that the oxidation selectively occurred at C6 primary hydroxyls on surfaces of cellulose microfibrils or cellulose I crystallites without oxidation taking place inside the crystallites. Because the carboxylate groups formed on microfibril surfaces have anionic charges in water, which cause repulsive effects such as electrostatic repulsions between the microfibrils, the oxidized celluloses can be fully disintegrated to individual microfibrils by mild mechanical treatment in water.22,23 Dry films prepared by casting of the TEMPOoxidized and individualized microfibril/water dispersions are transparent and flexible and exhibit high tensile strengths, low coefficients of thermal expansion, and extremely high oxygenbarrier properties.24 However, almost all the previous studies on the TEMPOmediated oxidation of native celluloses have been carried out only on one or two kinds of native cellulose samples such as cotton linters or bleached wood pulps with the primary aim of tracing temporal changes in, for instance, carboxylate content and crystal structures of the oxidized celluloses,12-18 while native celluloses are produced in quite different forms by various kinds of living organisms.7-9 In addition, the oxidation reactions in such studies might be finished before achieving oxidation of all the C6 primary hydroxyls accessible by the TEMPO* To whom correspondence should be addressed. Phone: +81 3 5841 5538. Fax: +81 3 5842 5269. E-mail: [email protected].

mediated oxidation. In fact, statistical and comprehensive studies on oxidation of all the accessible primary hydroxyls of native celluloses have never been reported, and this subject is still under debate. In the present study, we applied the TEMPO-mediated oxidation to various kinds of native celluloses, which have different characteristics in crystal structures and microfibril morphologies, and investigated (1) relationships between microfibril widths and amounts of functional groups formed by the oxidation (carboxylate and aldehyde groups) and (2) ζ-potentials of the oxidized microfibrils dispersed in water, from both experimental and theoretical aspects. Consequently, this study clearly showed a comprehensive picture for surface oxidation of native cellulose microfibrils or cellulose I crystallites by TEMPO-mediated oxidation.

Experimental Section Materials. Softwood and hardwood bleached kraft pulps in neverdried wet state with 80% water content were provided by a Japanese paper company (Nippon Paper Industries Co., Ltd.). Bacterial cellulose was produced by Acetobacter xylinum (JCM10150). Pellicles of the bacterial cellulose were obtained from standing culture in a medium containing 4% sucrose and 4% corn steep liquor at 28 °C for 1 week.25 Tunicates of Halocynthia roretzi, whole plants of Cladophora sp., and pellicles of bacterial cellulose were cut into small pieces with scissors and purified according to standard methods.26,27 The purified samples were thoroughly disintegrated with a double-cylinder type homogenizer (Physcotron NS-56, Microtec Nition co. Ltd., Japan), and screened with a wire-mesh filter to remove rigid fragments. TEMPO, sodium bromide, a 2 M sodium hypochlorite solution, and other chemicals were of laboratory grade (Wako Pure Chemicals, Japan) and used without further purification. TEMPO-Mediated Oxidation. Cellulose sample (1 g) was suspended in water (100 mL), containing TEMPO (0.016 g) and sodium bromide (0.1 g). TEMPO-mediated oxidation was started by adding 2 M NaClO (5 mL, 10 mmol per gram of cellulose) to the cellulose suspension. The pH of the suspension was maintained to be 10 by adding 0.5 M NaOH with a pH stat for 5 h. The oxidation was then quenched by adding ethanol (ca. 5 mL). The oxidized cellulose was thoroughly washed with distilled water by centrifugation at 12000g for 30 min and stored at 4 °C without drying. Determination of Carboxylate and Aldehyde Groups. Carboxylate contents of the oxidized celluloses were determined using an electric conductivity titration method.12 The titration was carried out three times for each sample, and the experimental errors were calculated as standard deviation. A part of the oxidized cellulose was further oxidized with NaClO2 at pH 4-5 and room temperature for 1 day, and the increase in carboxylate content was converted to aldehyde content of the oxidized cellulose.12 Details of the calculation methods are described in the Supporting Information file. X-ray Diffraction. The freeze-dried sample (ca. 0.1 g) was pressed at about 750 MPa for 1 min to make a pellet. X-ray diffraction patterns were recorded for the pellet samples from 10 to 30° of diffraction angle 2θ using the reflection method by means of a Rigaku RINT 2000 with Ni-filtered Cu KR radiation (λ ) 0.1548 nm) at 40 kV and 40 mA. Crystal sizes of cellulose I structures were calculated from full widths at half heights of the diffraction peaks by Scherrer’s equation.28 Two peaks centered at about 14.8 and 16.8° in the X-ray diffraction patterns, which correspond to d-spacings of 0.60-0.61 nm and 0.53-0.54 nm, respectively, were separated by curve-fitting using pseudo-Voigt function.9 ζ-Potential Measurement. The oxidized celluloses were suspended in water (100 mL) at a consistency of 0.1% (w/v). After setting the pH

10.1021/bm100214b  2010 American Chemical Society Published on Web 05/07/2010

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Table 1. Profiles of the TEMPO-Oxidized Native Celluloses cystal size (nm)a

total content of functional groupsc

origin of cellulose sample

C1

C2

CA

oxidation end point (h)b

carboxylate content (mmol/g)

aldehyde content (mmol/g)

(mmol/g)

(mol/mol of monomer unit)

softwood hardwood cotton bacteria Halocynthia Cladophora

3.5 3.5 4.5 5.4 7.6 11.9

4.1 4.1 6.2 6.2 10.6 14.4

3.8 3.8 5.4 5.8 9.1 13.1

4.0 4.3 3.7 2.7 2.5 3.0

1.65 1.69 1.36 1.05 0.59 0.52

0.07 0.00 0.00 0.10 0.06 0.00

1.72 1.69 1.36 1.15 0.65 0.52

0.30 0.29 0.23 0.19 0.11 0.09

a The C1 and C2 are crystal sizes of the planes corresponding to d-spacings of 0.60-0.61 and 0.53-0.54 nm, respectively. The CA is the average value of C1 and C2. b The time when almost no consumption of NaOH, monitored with a pH stat, was observed during the reaction for 5 h. c Total content of carboxylate and aldehyde groups, expressed as mmol/g and mol/mol of monomer unit in the oxidized sample.

Results and Discussion TEMPO-Mediated Oxidation of Various Native Celluloses. The native cellulose samples purified from various origins, which have different characteristics in crystal structure, crystal size, and microfibril morphology, were used in this study (Table 1). The celluloses of higher plants such as wood and cotton are composed mainly of thin microfibrils of low crystalline cellulose Iβ, while tunicate cellulose of Halocynthia roretzi consists of highly crystalline and thick microfibrils of almost pure cellulose Iβ.8,26,27,29 Both bacterial and Cladophora celluloses have composite structures of cellulose IR and Iβ, although these celluloses are different in crystal size and the composite ratio between IR and Iβ.26,27,30,31 Bacterial cellulose has flat ribbonlike microfibrils consisting of subelementary fibrils 5-6 nm in width, differing from those of other celluloses.1,25

Figure 1. X-ray diffraction patterns of the TEMPO-oxidized native celluloses.

Figure 2. Schematic model of a cross section of cellulose microfibril or cellulose I crystallite.

of the suspension to 8 with 0.01 M NaOH, it was sonicated for 2 min, using an ultrasonic homogenizer (US-300T, Nihonseiki, Japan) at 19.5 kHz and an output power of 300 W (7 mm probe tip diameter). Unfibrillated and partly fibrillated fractions, if present, were removed by centrifugation at 12000g for 5 min, and the supernatants were subjected to the following ζ-potential measurement. ζ-Potentials of the individualized fibrils dispersed in water were measured at 25 °C using a laser-Doppler-electrophoresis-type apparatus (DTS5300 Zetasizer 3000, Malvern Instruments, U.K.). The sample consistency in water was set to 0.01% (w/v). The measurements were performed five times for each sample, and the experimental errors were calculated as standard deviation.

In this study, the TEMPO-mediated oxidation treatment with excess NaClO continued up to 5 h for all the samples. Meanwhile, the “oxidation end point” in Table 1 shows the approximate time when no consumption of aq. NaOH was observed anymore by monitoring with the pH stat. Thus, the oxidation of all the C6 primary hydroxyls possible for the conversion to oxidized groups might be completed in each sample by the “oxidation end point” in Table 1 during the oxidation treatment for 5 h. Because the reaction mixtures maintained yellowish color in all cases during the oxidation treatment for 5 h, some NaClO used as the primary oxidant always remained in the reaction mixtures. The yellowish color disappeared by consumption of the residual NaClO, when ethanol was added to the reaction mixtures. Thus, the content of C6-oxidized groups in each TEMPO-oxidized cellulose is likely to reach a maximum value under the conditions used in this study. Carboxylate contents were clearly different between the oxidized celluloses, ranging from 0.5 to 1.7 mmol/g (Table 1). In the previous studies of TEMPO-mediated oxidation of native celluloses,12,22 significant amounts of aldehyde groups (0.2-0.3 mmol per gram of the oxidized celluloses) formed as intermediates were present in the oxidized celluloses. In contrast, aldehyde contents of all the oxidized celluloses in this study ranged from 0.0 to 0.1 mmol/g; more than 90% of the oxidized groups were carboxylates. This discrepancy in aldehyde contents of the TEMPO-oxidized celluloses between the previous and present studies might be due to the difference in oxidation conditions. In this study, excess NaClO (10 mmol per gram of the original celluloses) was added to the reaction mixtures, whereas NaClO of 0.1-5 mmol of NaClO per gram of the original celluloses was used in the previous papers.12,22 Moreover, most of the oxidation reactions were quenched within 2 h in the previous studies. Thus, aldehyde groups in the TEMPO-oxidized cellu-

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Figure 3. Relationships between the crystal size CA in Table 1 and the total content of carboxylate and aldehyde groups of the TEMPOoxidized celluloses. The dashed line shows the relationship between crystal size and the content of the C6 primary hydroxyls exposed on surfaces of cellulose I crystallites, which was calculated by the formula described in the text.

loses were mostly converted to carboxylate groups under the conditions used in this study. Figure 1 displays X-ray diffraction patterns of the TEMPOoxidized celluloses. All the diffraction patterns are due to cellulose I, and both crystallinity and crystal size of each sample, calculated from the X-ray diffraction patterns, were unchanged before and after the TEMPO-mediated oxidation. The diffraction peak at 14.8° is due to the d-spacing of 0.60-0.61 nm, which corresponds to the (1 0 0) plane of cellulose IR or the (1 -1 0) plane of cellulose Iβ.9,27 The peak at 16.8° is ascribed to the d-spacing of 0.53-0.54 nm, which corresponds to the (0 1 0) plane of cellulose IR or the (1 1 0) plane of cellulose Iβ.9,27 Two-directional crystal sizes in cross sections of the TEMPOoxidized cellulose fibrils, listed as C1 and C2 in Table 1, were calculated from the diffraction peaks at 14.8 and 16.8°, respectively (Figure 2), and both of the sizes were equal to those of the original celluloses.12 Thus, carboxylate and aldehyde

Notes

groups are likely to be formed selectively on the surfaces of crystalline cellulose microfibrils by the TEMPO-mediated oxidation, maintaining the original C1 and C2 crystal sizes. Surface Oxidation of Various Cellulose Microfibrils by TEMPO-Mediated Oxidation. Figure 3 depicts relationships between the crystal size CA, that is, the average value of C1 and C2 for each sample in Table 1 and the total content of carboxylate and aldehyde groups (mol/mol of monomer unit) of the oxidized celluloses. The Y-axis values were larger for the cellulose samples with smaller crystal sizes such as wood and cotton, while the TEMPO-oxidized celluloses with larger crystal sizes gave smaller Y-axis values. Given that all the C6 primary hydroxyls exposed on the surfaces of crystalline cellulose fibrils are oxidized by the TEMPO system, the results in Figure 3 are reasonable. This is because the smaller the CA size of the native cellulose, the more the oxidized groups (per mol of monomer unit) are formed on the surfaces by the TEMPO-mediated oxidation (Figure 2). The dashed line in Figure 3, that is, the relationship between crystal size CA and the amount of the C6 primary hydroxyls (mol/mol of monomer unit) exposed on the surfaces of cellulose I crystallites, was calculated by the following formula.9,32

Exposed surface C6 primary hydroxyls (mol/mol of anhydroglucose unit of cellulose) ) [CA /0.61+CA /0.53]/[(CA /0.61 + 1) × (CA /0.53 + 1)] Here, the numbers of anhydroglucose units in one side of the cellulose microfibril cross-section in Figure 2 are calculated using the d-spacings of cellulose I structures (0.61 and 0.53 nm). In addition, it was taken for granted that every one of two anhydroglucose units of the surface cellulose molecules faces outside of cellulose microfibrils according to the well-known cellulose I crystal structures. For simplification, cellulose I crystallites or microfibrils were assumed to have square cross sections with sides of the same lengths (Figure 2).33 Figure 3 revealed that the calculated amounts of the surface primary hydroxyls (mol/mol of monomer unit) were in good

Figure 4. Schematic model of oxidation of C6 primary hydroxyl on cellulose microfibril surfaces by TEMPO/NaClO/NaBr system.

Notes

Figure 5. Carboxylate contents of the TEMPO-oxidized celluloses and ζ-potentials of individualized TEMPO-oxidized cellulose fibrils dispersed in water.

agreement with the total contents of carboxylate and aldehyde groups (mol/mol of monomer unit) experimentally determined. This quantitative result supports the hypothesis that the TEMPOmediated oxidation converts almost all of the C6 primary hydroxyls exposed on the entire surfaces of cellulose crystallites or microfibrils mostly to sodium carboxylate groups (Figure 4).12,14,17 The total contents of carboxylate and aldehyde groups for the celluloses with small crystal sizes of less than 6 nm were, however, slightly higher than the calculated values. This may be due to low crystallinities of cellulose I for the higher plant celluloses; carboxylate and aldehyde groups were partly formed also in amorphous or disordered regions,34,35 and some of such oxidized parts may have been retained in the solid celluloses without dissolving out in water, even after the oxidation and successive washing with water. Alternatively, the surface structures of cellulose microfibrils of such higher plant celluloses may be somewhat different from those of other bacterial, tunicate, and algal celluloses. To evaluate distribution of the carboxylate groups on the surfaces of cellulose crystallites or microfibrils, ζ-potentials of the oxidized celluloses dispersed in water as individual microfibrils by mechanical treatment were investigated (see Experimental Section). Figure 5 shows ζ-potentials and carboxylate contents of the oxidized celluloses. For the oxidized celluloses examined in this study, the surfaces of cellulose crystallites or microfibrils had highly negative charges. Moreover, all the TEMPO-oxidized cellulose microfibrils had similar ζ-potentials of approximately -75 mV, though the carboxylate contents were significantly different from each other (Table 1 and Figure 3). This result shows that all the oxidized celluloses have almost the same density of carboxylate groups on the surfaces of cellulose crystallites or microfibrils, because the ζ-potentials are directly related to the surface density of dissociated carboxyl groups. Thus, the hypothesis that all the surface C6 primary hydroxyls are oxidized mostly to sodium carboxylate groups by the TEMPO-mediated oxidation (Figure 4) was supported also by the ζ-potentials in Figure 5. Average density of C6 primary hydroxyls exposed on the crystallite surfaces is calculated to be approximately 1.7 groups/nm2 from the surface structures of cellulose I crystallites.32

Conclusion In this study, the TEMPO-mediated oxidation was applied to various kinds of native celluloses under the conditions that all the C6 primary hydroxyls accessible by the TEMPOmediated oxidation were oxidized. Carboxylate contents in the oxidized celluloses varied from 0.5 to 1.7 mmol/g, depending

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on the cellulose origins used. Only small amounts of aldehydes, 0.0-0.1 mmol/g, were present in the oxidized products; more than 90% of the oxidized groups were sodium carboxylates in this study. The contents of oxidized groups in the products were in good agreement with the amounts of C6 primary hydroxyls (mol/mol of monomer unit) exposed on microfibril surfaces, which were calculated from crystal sizes of the original native celluloses. ζ-Potentials of the oxidized cellulose microfibrils dispersed in water were approximately -75 mV for all native celluloses, though the carboxylate contents were significantly different from each other. These results support that the TEMPO-mediated oxidation under the conditions used in this study converted almost all the C6 primary hydroxyls exposed on the entire surfaces of cellulose microfibrils mostly to sodium carboxylate groups. Acknowledgment. The authors are grateful to Dr. Masahisa Wada (The University of Tokyo, Japan) for kindly providing us with Halocynthia roretzi and Cladophora sp. This research was partly supported as a Nanotech Challenge Program from 2007 by the New Energy and Industrial Technology Development Organization of Japan (NEDO, Grant No. 1023001) and by Grants-in-Aid for Scientific Research (Grant Nos. 21248036, 21780163, 21228007, and 20-7136) from the Japan Society for the Promotion of Science (JSPS). Supporting Information Available. Details for determination and calculation of carboxylate and aldehyde contents. This material is available free of charge via the Internet at http:// pubs.acs.org.

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