Molecular Mass and Molecular-Mass Distribution of TEMPO-Oxidized

Jan 13, 2015 - ABSTRACT: Native wood cellulose was oxidized by 2,2,6,6- tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation, and the fibrous ...
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Molecular Mass and Molecular-Mass Distribution of TEMPO-Oxidized Celluloses and TEMPO-Oxidized Cellulose Nanofibrils Ryoya Hiraoki, Yuko Ono, Tsuguyuki Saito, and Akira Isogai* Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan S Supporting Information *

ABSTRACT: Native wood cellulose was oxidized by 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation, and the fibrous TEMPO-oxidized celluloses (TOCs) thus obtained were disintegrated in water to prepare TOC nanofibrils (TOCNs). The carboxyl groups of TOCs and TOCNs were methyl-esterified, and the methylated samples were dissolved in 8% LiCl/N,N-dimethylacetamide for sizeexclusion chromatography/multiangle laser-light scattering (SEC-MALLS) analysis to obtain their molecular-mass (MM) values and MM distributions (MMDs). The results showed that remarkable depolymerization occurred in TOCs and TOCNs and depended on the oxidation and sonication conditions. Because single peaks without bimodal patterns were observed in the MMDs for all of the TOC and TOCN samples, depolymerization may have randomly occurred on whole cellulose molecules and oxidized cellulose molecules in the microfibrils during these treatments. Compared with the MM values obtained by SEC-MALLS, the intrinsic viscosities of TOCs dissolved in 0.5 M copper ethylenediamine solution provided lower MM values owing to depolymerization during the dissolution and postreduction processes.

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INTRODUCTION Native cellulose microfibrils, one of the main components of plant cell walls along with hemicelluloses and lignin, have attracted attention over the years as biobased nanofibers because of their reproducible, crystalline, and high-strength nature. One plant cellulose microfibril is about a nanometer thick, and consists of parallel cellulose chains held together by hydrogen bonds and van der Waals forces.1 Because nanocelluloses originating from cellulose microfibrils and isolated from plant cellulose fibers have been revealed to have unrivaled properties, such as high mechanical strength,2 high elastic modulus,3 and thermo-dimensional stability,4 they have the potential to be applied in various high-tech fields.5−8 Because cellulose microfibrils form robust assemblies in plant cell walls, it is essential to disintegrate the cellulosic fibers into nanosized elements before developing new nanocellulose materials. There have been numerous studies about the efficient separation or isolation of nanocelluloses from plant cellulose fibers. In particular, recent studies have focused on the pretreatment of cellulose fibers to reduce energy consumption during the mechanical disintegration process to prepare nanocelluloses and improvement of the dispersibility of nanocellulose elements in matrix materials.9−12 2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO)mediated oxidation is an effective chemical pretreatment to convert native celluloses to nanofibrillated celluloses.13,14 When the oxidation is applied to native wood cellulose fibers in water at room temperature under suitable conditions, the C6© XXXX American Chemical Society

hydroxyl groups exposed on the crystalline cellulose microfibril surfaces are efficiently and position-selectively oxidized to C6carboxylates to form sodium glucuronosyl units.15−19 TEMPOoxidized cellulose (TOC) fibers with the same 20−40 μm widths as those of the original wood cellulose fibers can be converted to individual TEMPO-oxidized cellulose nanofibrils (TOCNs) with homogeneous ∼3 nm width and high aspect ratios through mild mechanical disintegration in water when the TOCs have carboxylate contents >1 mmol/g.13,14 TEMPO-mediated oxidation causes depolymerization of cellulose molecules by β-elimination of glycoside bonds at C6-aldehyde groups, which are formed as intermediates in the oxidation of cellulose.16,20 Moreover, some radical species formed in situ as byproducts during the oxidation may be responsible for depolymerization.21 The partially depolymerized cellulose molecules in TOCs are further depolymerized during the following mechanical disintegration process.2,20 Molecular-mass (MM) parameters, including the MM distribution (MMD) of polymeric materials, significantly influence their mechanical and other functional properties. Determination of not only the length and length distribution of TOCNs,2,22 but also the MM parameters of both TOCs and TOCNs, are important for application of TOCs and TOCNs in high-tech fields. Determination of the MM values of TOCs and Received: December 21, 2014 Revised: January 13, 2015

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also prepared from cotton linters cellulose by the same method described above but with different amounts of NaClO (1.25 and 5 mmol/g cellulose) in the TEMPO-mediated oxidation. TOC−NaBH4 samples were prepared from SBKP and cotton linters cellulose by onepot TEMPO-mediated oxidation in water at pH 10 under various conditions and NaBH4 reduction in the same container for 3 h.29 Mechanical Disintegration. A 0.1% w/v suspension of TOC− NaClO2 (100 mL) was sonicated for 10−120 min using an ultrasonic homogenizer (UTS300T, Nihon Seiki, Tokyo, Japan) at 19.5 kHz and 300 W output power to prepare TOCNs. The obtained TOCN dispersion as centrifuged at 12000 g for 20 min to remove the small amount of metal particles that broke off the metal tip during sonication. Methylation of C6 Carboxyl Groups. Methylation of the C6carboxyl groups in TOC−NaClO2, TOC−NaClO2−NaBH4, and TOCNs was carried out according to a previously reported method.25 HCl (1 M) was added to a 0.2% w/v aqueous suspension of TOC− NaClO2 or TOC−NaClO2−NaBH4 (50 mL) or a 0.1% w/v TOCN dispersion (100 mL) to adjust the pH to ∼2, and the mixture was stirred at room temperature for 30 min to convert the C6-COONa groups to C6-COOH groups. The carboxyl-protonated TOC− NaClO2, TOC−NaClO2−NaBH4, or TOCN was washed with methanol by repeated centrifugation and then suspended in DMAc (30 mL) containing methanol (6 mL). TMSD (2 M; 1 mL) was added to the suspension to methylate the C6-COOH groups, and the mixture was stirred at room temperature for 1 h under N2 atmosphere. The methylation was quenched by adding 5 M acetic acid (1.5 mL) to the mixture. The methylated product was washed thoroughly with t-butyl alcohol by centrifugation, followed by freeze-drying. SEC-MALLS Analysis. The original SBKP (8 mg) was dissolved in 8% LiCl/1,3-dimethyl-2-imidazolidinone (LiCl/DMI, 2 mL) and then diluted with DMI to a 0.05% w/v solution in 1% LiCl/DMI. The carboxyl group-methylated products (16 mg of each) were dissolved in 8% LiCl/DMAc (2 mL) at room temperature, followed by dilution with DMAc to a 0.1% w/v sample solution in 1% LiCl/DMAc. The details of the SEC-MALLS system used and the operation conditions are described elsewhere.25,30 The values of 0.087 and 0.121 mL/g were used as the specific refractive index increments (dn/dc) of the SBKP in 1% LiCl/DMI and carboxyl-methylated samples in 1% LiCl/DMAc, respectively, at 40 °C.25,30 Each sample solution was filtered through a 0.2 μm poly(tetrafluoroethylene) disposable membrane before injection. Other Analyses. The carboxylate content of TOC−NaClO2 was determined by the conductivity titration method.31 The freeze-dried TOC, TOC−NaClO2, and TOC−NaBH4 (40 mg) samples were stirred in 0.5 M cuen (20 mL) for 30 min. The intrinsic viscosity [η] of the sample was obtained from the relative viscosities of TOC/cuen solutions with different TOC concentrations using a Cannon−Fenske capillary viscometer at 20 °C.20

TOCNs has been carried out primarily with the viscosity method using 0.5 M copper ethylenediamine (cuen), which can dissolve both cellulose and oxidized cellulose molecules of TOCs and TOCNs.20,23 However, weight-average and number-average MM values and MMDs of TOCs and TOCNs have not been reported in terms of the conditions of TEMPO-mediated oxidation or the subsequent mechanical disintegration treatment, because the cuen method provides only viscosity-average MM (Mv) values. Moreover, it has been reported that TOCs are unstable under alkaline conditions and their MM values change depending on the conditions,24 and 0.5 M cuen is a strong alkali with pH ∼ 14. In contrast, size-exclusion chromatography with multiangle laser-light scattering (SEC-MALLS) analysis makes it possible to determine absolute MM values and MMDs of polymeric materials. In a previous study, we established a method to analyze a TOC sample by SEC-MALLS through positionselective methylation of C6 carboxyls in the TOC for solubilization in lithium chloride/N,N-dimethylacetamide (LiCl/DMAc), which is one of the most common and most suitable solvent systems for cellulose in SEC-MALLS analysis.26 Methylation with trimethylsilyldiazomethane (TMSD) can convert free carboxyl groups in the TOC to methyl ester groups in the presence of a small amount of methanol in DMAc under mild conditions without any side reactions.25−27 In this paper, we report the effects of the TEMPO-mediated oxidation conditions used to prepare TOCs and the sonication conditions to prepare TOCNs on their MM parameters. Native wood cellulose was oxidized by the TEMPO/NaBr/NaClO system under various conditions, and the obtained TOCs were disintegrated by ultrasonic treatment to prepare TOCNs. The TOCs and TOCNs were methylated with TMSD and then analyzed by SEC-MALLS to clarify the influence of the oxidation and sonication conditions on the MM parameters of TOCs and TOCNs. In addition, the MM values obtained for the same samples by SEC-MALLS are compared with their intrinsic viscosities [η] in cuen to study the applicability of the viscosity method for determination of MM values of TOCs and TOCNs.

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EXPERIMENTAL SECTION

Materials. A never-dried softwood bleached kraft pulp (SBKP) for papermaking (Nippon Paper Industries Co., Tokyo, Japan) containing 90% α-cellulose and a cotton linters pulp for filter paper were used as native plant cellulose samples. All of the chemicals and solvents were of laboratory grade, purchased from Wako Pure Chemicals (Tokyo, Japan) or Sigma-Aldrich (Saint Louis, MO, U.S.A.), and used as received. Cellouronic acid sodium salt (sodium poly(1 → 4)-βglucuronic acid) was prepared from a regenerated cellulose by TEMPO/NaClO/NaClO2 in water at pH 4.8 according to a previously reported method.28 TEMPO-Mediated Oxidation and Subsequent Oxidation and Reduction. The SBKP (1 g) was oxidized by the TEMPO/NaBr/ NaClO system in water at pH 10 with various amounts of 1.8 M NaClO (3.8−20 mmol/g cellulose).13 The TOC obtained was postoxidized with NaClO2 (0.9 g, 20 mmol/g cellulose) in 0.5 M acetate buffer (100 mL) at pH 4.8 and room temperature for 2 days to oxidize the small amount of C6-aldehydes present in the TOC, followed by washing thoroughly with water by centrifugation. The postoxidized TOC (TOC−NaClO2, 0.2 g) was then reduced with NaBH4 in water (100 mL) at pH 10 for 3 h to reduce the small amount of C2/C3 ketones present in the TOC−NaClO2 to C2/C3 hydroxyls, followed by washing thoroughly with water by centrifugation to obtain TOC−NaClO2−NaBH4. TOC−NaClO2 samples were

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RESULTS AND DISCUSSION MM Parameters of TOCs Prepared under Various Conditions. The relationships between the amount of NaClO added in TEMPO-mediated oxidation and either the carboxylate content or weight-average MM (Mw) of TOCs− NaClO2 are plotted in Figure 1. The weight-average degree of polymerization (DPw) of TOCs−NaClO2 was simply calculated as Mw/162 and shown as an additional left y axis with the same open circles as those of Mw. The Mw, number-average MM (Mn), and Mw/Mn values are shown in Table S1 in the Supporting Information. In TEMPO-mediated oxidation, the carboxylate content of TOC−NaClO2 increased with increasing the amount of NaClO up to 10 mmol/g cellulose and then reached a plateau. Assuming that each cellulose microfibril in SBKP has a square cross-section consisting of 6 × 6 cellulose chains and oxidation takes place only at C6-primary hydroxyls present on the cellulose microfibril surface,17,32 the maximum carboxylate B

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ditions. The MMD shifted to lower MM as the amount of NaClO added in the TEMPO-mediated oxidation was increased. The MMD pattern of TOCs−NaClO2 prepared with NaClO of 3.8 and 5.0 mmol/g cellulose consisted of one main peak in the high MM region and a small shoulder peak in the low MM region (Mw 5000−50000) in a similar manner to that of the original SBKP. The main and small shoulder peaks of SBKP are due to cellulose and hemicelluloses fractions, respectively.25,27,33−35 No such small shoulder peak was observed in the MMDs of TEMPO-oxidized linters cellulose containing no hemicelluloses, even when they were prepared with small amounts of NaClO (1.25 and 5.0 mmol/g cellulose; Figure S1). Thus, when the amount of added NaClO was in the range 3.8−5.0 mmol/g cellulose in TEMPO-mediated oxidation, some of hemicelluloses originally present in SBKP remained, probably as partially C6-oxidized hemicelluloses. These oxidized hemicelluloses were mostly degraded during TEMPO-mediated oxidation with NaClO of 10−20 mmol/g cellulose and removed as water-soluble fractions during the washing process, resulting in the TOCs−NaClO2 prepared under these conditions having only one main peak in the MMD (Figure 2). Kuramae et al. also concluded from the results of neutral sugar composition analysis of several plant holocelluloses that TOCs prepared with NaClO of 10 mmol/g holocellulose contained almost no hemicelluloses.36 MM Values and MMDs of TOCNs Prepared under Various Sonication Conditions. Ultrasonic treatment is an effective method in laboratory experiments for the disintegration of aqueous TOC suspensions into transparent and gel-like TOCN dispersions.13,14,37,38 During ultrasonic treatment, small bubbles are generated in the liquid by ultrasound waves and then collapse within milliseconds. This process, called cavitation, induces a strong hydrodynamic stress and extreme conditions (high temperature and high pressure) around the bubbles.39 The energy provided by cavitation is sufficient to efficiently and rapidly convert TOCs into individual TOCN elements in water. However, sonication treatment can cause extensive damage to polymer chains. Studies on depolymerization of cellulose, carboxymethylcellulose, and chitin by sonication have been reported, and their degree of polymerization (DP) values significantly differ depending on the sonication conditions.40−42 The changes in the MMDs of TOCNs prepared from TOCs−NaClO2 by sonication treatment for various times are shown in Figure 3. The Mw, Mn, and Mw/Mn values are shown in Table S2 in the Supporting Information. Clear depolymerization of the TOCNs occurred during the sonication treatment. The results in Figures 1−3 are summarized in Figure 4. The Mw of the original SBKP decreased depending on the TEMPO-mediated oxidation conditions, and that of TOC− NaClO2 decreased with increasing sonication time. That is, the longer the sonication time of the aqueous TOC−NaClO2 suspension, the lower the Mw of the obtained TOCN. However, no leveling-off Mw (or low and constant Mw) was observed for TOCNs. It has been reported that the length of fibers reaches a limiting length (Llim) at which the fibers no longer shorten with increasing sonication time, and Llim depends on the tensile strength of the fibers.2,43 In the case of TOCNs prepared from SBKP, their Llim value has been reported to be 272 nm.2 Assuming that the glucosyl and glucuronosyl units are both 0.518 nm in length,44 a TOCN length of 272 nm corresponds

Figure 1. Effect of the amount of added NaClO in TEMPO-mediated oxidation on the carboxylate content, Mw, and DPw of TOCs− NaClO2.

content of TOC−NaClO2 is 1.61 mmol/g. Thus, based on the results in Figure 1 and Table S1, almost all of the C6-primary hydroxyls on the cellulose microfibril surfaces in SBKP were oxidized to C6-carboxylates when the TEMPO-mediated oxidation was performed with NaClO of 10−20 mmol/g cellulose followed by postoxidation with NaClO2. The Mw clearly decreased from 524000 for SBKP to 144000 (or DPw from 3200 to 890) for TOCs−NaClO2 as the amount of added NaClO was increased to 20 mmol/g cellulose in TEMPO-mediated oxidation; the greater the amount of added NaClO in the oxidation, the more the depolymerization of TOC−NaClO2. Although the decrease in the Mw of TOC− NaClO2 is noticeable in Figure 1 and Table S1, the decrease of the Mw (or DPw) of TOC−NaClO2 in terms of the amount of added NaClO obtained in this study is much smaller than that obtained from DPv values.20 This discrepancy between the Mw and DPv values of TOCs−NaClO2 is discussed in a later section. Figure 2 shows the MMDs obtained by the SEC-MALLS method for TOCs−NaClO2 prepared under different con-

Figure 2. MMD patterns of the original SBKP and TOCs−NaClO2 prepared with different amounts of NaClO (see Figure 1 and Table S1). C

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of TOCs containing even small amounts of C6-aldehydes and C2/C3 ketones12,29,31,45,46 is inevitable during the dissolution process in cuen because of β-elimination. The TOC−NaClO2 prepared with NaClO of 5.0 mmol/g cellulose was soaked in an aqueous NaOH solution at pH 14, which is the same pH as a 0.5 M cuen solution, for 30 min (almost the same time as that required for dissolution of TOCs and TOCNs in cuen). The MMDs before and after the alkali treatment are shown in Figure 5. This sample was assumed to

Figure 5. MMDs of TOCs−NaClO2 before and after soaking in aqueous NaOH at pH 14 for 30 min.

Figure 3. MMDs of the original TOCs−NaClO2 and TOCNs prepared with various sonication times.

have no C6-aldehydes because of postoxidation with NaClO2 but to have small amounts of C2/C3 ketones formed by side reactions during TEMPO-mediated oxidation, which may cause depolymerization because of β-elimination. The alkali treatment decreased the Mw and Mn values from 382000 and 74200 to 148000 and 60500, respectively. Thus, the viscosity method using cuen cannot be applied to TOCs−NaClO2 samples because partial depolymerization is unavoidable during dissolution in the alkaline cuen solution. Figure 6 shows changes in [η] of the original SBKP, TOC (without either postoxidation or postreduction), TOC− NaClO2, TOC−NaClO2−NaBH4, and cellouronic acid (used as a reference) dissolved in 0.5 M cuen. All of the samples had almost constant [η] values for standing time of the solution from 30 min to more than 3 h. However, as shown in Figure 5, Figure 4. Changes in Mw and DPw of TOCs−NaClO2 and TOCNs prepared for various sonication times.

to Mn and DPn values of 85000 and 530, respectively. However, the Mn and DPn values of TOCN prepared from TOC− NaClO2 (with NaClO of 10 mmol/g cellulose) by sonication for 120 min were 17000 and 105, respectively (Table S2). These results indicate that each TOCN element consists of cellulose and oxidized cellulose chains shorter than the TOCN length (Figure S2). Relationship between Mw and [η] of TOCs. If the viscosity method using cuen as a solvent is able to determine MM or DP values of TOCs and TOCNs that are equivalent or close to those determined by SEC-MALLS, it will benefit laboratories with no SEC-MALLS system. The Mv (or DPv) value of a polymer is calculated from its intrinsic viscosity [η] using the Mark−Houwink−Sakurada equation. However, because cuen is a strong alkali with pH ∼ 14, depolymerization

Figure 6. Time-dependent changes in the intrinsic viscosity [η] of SBKP, TOC-related samples, and cellouronic acid dissolved in 0.5 M cuen. D

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The Mw of TOC−NaBH4 is lower than that of TOC− NaClO2, or partial depolymerization of TOC is unavoidable during NaBH4-reduction treatment. However, their double logarithmic plots were similar to those of other cellulose samples (Figure 7). The Mark−Houwink−Sakurada equation, [η] = 0.094 × M0.67, can be used to determine Mv values of TOCs−NaBH4 to predict their Mw values. Thus, the effect of significant amounts of carboxyl groups in TOCs−NaBH4 on their [η] values can be ignored, and TOCs−NaBH4 can be regarded as pure celluloses when using the above equation. Structural Analysis of TEMPO-Oxidized Wood Celluloses. The results obtained in this and previous studies20 show that (a) when neither postoxidation nor postreduction is carried out, the DPv values of TOCs are always in the range of 200−300, which corresponds to the leveling-off DP, and (b) the Mw values of TOCs−NaClO2 and TOCNs determined by SEC-MALLS depend on the TEMPO-oxidation and sonication conditions, but depolymerization occurs randomly in the cellulose and oxidized cellulose molecules during these treatments. Based on these results, schematic models of the changes in wood cellulose microfibrils during TEMPOmediated oxidation, postoxidation, and postreduction, and dissolution in the alkaline cuen solution are illustrated in Figure 8.

clear depolymerization occurred in TOC−NaClO2 within the first 30 min. The TOC without either postoxidation or postreduction had quite low [η] values, which correspond to the leveling-off DP of 200−300 observed in dilute acid hydrolysis of plant celluloses.20 In a previous paper, we suggested from the results of clearly high DPv or [η] values of TOCs−NaClO2 and TOCs−NaBH4 (compared with the low values of the original TOCs without either postoxidation or postreduction) that no significant amounts of C2/C3 ketones are present in the TOCs.20 This is because the DPv and [η] values of TOCN−NaBH4 should be much higher than those of TOCN−NaClO2 if significant amounts of C2/C3 ketones are present in the TOCs and cause depolymerization in cuen because of β-elimination. However, the result in Figure 5 shows that the amount of C2/C3 ketones in TOC−NaClO2 cannot be ignored, and these groups caused depolymerization during dissolution in cuen. The relationship between the Mw values determined by SECMALLS and the [η] values obtained using 0.5 M cuen as the solvent for various TOCs−NaClO 2 and TOCs−NaBH 4 together with those of various celluloses reported in the literature are shown in Figure 7.46,47 The Mw values of TOCs−

Figure 7. Double logarithmic plots of Mw against [η] for the TOCs− NaClO2 and TOCs−NaBH4 samples. The Mw and [η] values were obtained by SEC-MALLS and the viscosity method with cuen, respectively. The data of various celluloses reported in the literature are also shown.47,48

NaClO2 prepared from SBKP and cotton linters cellulose at various TEMPO-oxidation conditions determined by the SECMALLS method had low [η] values compared with those of nonoxidized celluloses. As shown in Figure 5, the [η] value of TOC−NaClO2 decreased after dissolution in an alkaline solution at pH 14 because of β-elimination of C2/C3 ketones in the sample. Although the [η] value of TOC−NaBH4 was slightly higher than that of the corresponding TOC−NaClO2, the Mw of the former was clearly lower than that of the latter, showing that partial depolymerization owing to β-elimination at the C2/C3 ketones in TOCs inevitably occurs during NaBH4-reduction in water at pH ∼ 10. As reported by Potthast et al.,48 the reduction of ketones and aldehydes in oxidized celluloses to hydroxyl groups by NaBH4 is always competitive with depolymerization owing to β-elimination of the ketones and aldehydes under alkaline conditions.

Figure 8. Structural models of wood cellulose microfibrils with crystalline and disordered regions subjected to TEMPO-mediated oxidation with and without post-treatment and dissolution in cuen.

Because of the result (a) mentioned above, significant amounts of C6-aldehydes are formed in the disordered region not only on the surface but also inside cellulose microfibrils, which are periodically present along the longitudinal direction of the microfibril.49 Simultaneously, position-selective oxidation of C6-hydroxyls to C6-carboxylates takes place on the cellulose microfibril surfaces together with random depolymerization of the whole cellulose molecules during TEMPO-mediated oxidation. When postoxidation or postreduction with NaClO2 or NaBH4 is carried out on the TOCs, no C6-aldehydes responsible for depolymerization owing to β-elimination are present in the disordered region, resulting in higher MM values E

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than the leveling-off DP when determined by the cuen viscosity method. When TOCs without either postoxidation or postreduction are dissolved in cuen, the DP values decrease to 200−300 corresponding to the leveling-off DP by depolymerization at the C6-aldehyde-rich disordered region owing to β-elimination under alkaline conditions. Thus, significant amounts of C6aldehydes are formed not only on the fibril surfaces but also inside microfibrils in the disordered region of the wood cellulose microfibrils during TEMPO-mediated oxidation. However, the reason why such C6-aldehydes are formed inside the disordered region is still unclear. As shown in Figures 2 and 3, all of the TOCs−NaClO2 or TOCNs did not have a bimodal MMD consisting of quite high and low MM fractions,20 but they had a single main peak (except the shoulder peak originating from residual hemicelluloses). This result indicates that cellulose molecules not only on the microfibril surfaces but also those inside microfibrils are randomly depolymerized during TEMPOmediated oxidation and sonication treatment.

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CONCLUSIONS

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 3 5841 5538. Fax: +81 3 5841 5269. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST).

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The MM values and MMDs of both TOCs and TOCNs were obtained by methylation of carboxyl groups with TMSD, complete dissolution in 8% LiCl/DMAc, and SEC-MALLS analysis. In both the TEMPO-mediated oxidation process to prepare TOCs and the following sonication process in water to prepare TOCNs, significant depolymerization of cellulose molecules occurred, depending on the conditions. It was found that the greater the amount of added NaClO, the lower the MM of the TOC. Furthermore, the longer the sonication time of TOC in water, the lower the MM of the TOCN. Based on the MMDs of TOCs, some hemicelluloses originally present in the wood cellulose remained in the TOCs when the amount of added NaClO in the oxidation was less than 5.0 mmol/g cellulose. Because all of the MMDs of TOCs and TOCNs had a single peak consisting of cellulose and oxidized cellulose molecules without a bimodal pattern, depolymerization of cellulose molecules in each cellulose microfibril may have randomly occurred. Cellulose molecules not only on the microfibril surface but also inside the microfibril were randomly depolymerized during both TEMPO-mediated oxidation and the sonication treatment. Because partial depolymerization inevitably occurs for TOCs, TOCs−NaClO2, and TOCs− NaBH4 during dissolution in alkaline cuen solution or NaBH4 treatment under alkaline conditions, it is difficult to determine the original MM values from the intrinsic viscosities of cuen solutions. The MM values of TOCNs obtained by SECMALLS showed that one TOCN element consisted of cellulose and oxidized cellulose molecules much shorter than the TOCN length.

* Supporting Information S

MMD patterns of cotton linters cellulose and its TEMPOoxidized products, a schematic model of cellulose and oxidized cellulose molecules in one TOCN element, and Mw, Mn, and Mw/Mn values of TOCs−NaClO2 and TOCNs. This material is available free of charge via the Internet at http://pubs.acs.org. F

DOI: 10.1021/bm501857c Biomacromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/bm501857c Biomacromolecules XXXX, XXX, XXX−XXX