Article pubs.acs.org/Biomac
Relationship between Length and Degree of Polymerization of TEMPO-Oxidized Cellulose Nanofibrils Ryuji Shinoda, Tsuguyuki Saito, Yusuke Okita, and Akira Isogai* Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan S Supporting Information *
ABSTRACT: The influence of 2,2,6,6-tetrametylpiperidine-1oxyl (TEMPO)-mediated oxidation of wood cellulose and the mechanical disintegration of oxidized cellulose in water on degree of polymerization determined by viscosity measurement (DPv) and the apparent length of the TEMPO-oxidized cellulose nanofibrils (TOCNs) was investigated. DPv values decreased from 1270 to 500−600 with increasing addition of NaClO in the TEMPO-mediated oxidation stage. The DPv values were further decreased by mechanical fibrillation in water. There is a linear relationship between the average fibril length and DPv; the lengths of TOCNs can be approximated from DPv using 0.5 M copper ethylenediamine as a solvent of both the cellulose and oxidized celluloses in TOCNs. Based on the cellulose fibril models and TEMPO oxidation mechanism, the depolymerization behavior of TOCNs is tentatively explained in terms of distribution of disordered regions in wood cellulose fibrils and formation of C6-aldehydes in cellulose fibrils during TEMPO-mediated oxidation.
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INTRODUCTION Native cellulose nanofibrils originate from abundant wood biomass and have the potential to be excellent structural materials. The fibrils consist of dozens of directionally aligned molecular chains, resulting in the formation of highly crystalline and ultrafine nanofibrils with extremely narrow widths of ∼4 nm, high aspect ratios of over 250 (lengths > 1 μm), and high elastic moduli (∼140 GPa).1,2 Various applications of cellulose nanofibrils, such as reinforcing fillers in plastic materials, synthetic supports of functional materials, and oxygen barrier layers of packaging materials, have been proposed.3−9 Nanofibrillation of wood cellulose is required as pretreatment to form cellulose nanofibrils because, in the native cellulose fibers, individual cellulose nanofibrils are firmly hydrogen-bonded to each other. Typically, nanofibrillated celluloses are obtained by the mechanical disintegration of bleached wood pulps in water.10,11 Recently, pretreatments of native wood cellulose by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation, partial carboxymethylation, and modest enzymatic hydrolysis have been proven to be effective in the mechanical nanofibrillation of cellulose.12−16 The resulting nanofibrillated celluloses can take diverse bulk forms, ranging from hydrogels and aerogels to transparent films.7−9,16−20 Information on length and degree of polymerization (DP) of cellulose nanofibrils would be significantly useful to determine the properties of nanofibrillated cellulose-containing composite materials. The DP of nanofibrillated celluloses strongly affects mechanical properties of the bulk materials.17,21 However, evaluations of dimensions of the nanofibrillated celluloses, that © 2012 American Chemical Society
is, measurements of length and width, are not well-defined; in most cases, statements for the length and width of nanofibrils are in the form “more than 1 μm in length and 5−20 nm in width” and, thus, nonstatistical. This is probably because most of the nanofibrillated celluloses consist of partially aggregated nanofibrils that are either dispersed in water as bundles or forming web-like networks, which makes it extremely difficult to measure the length and width of individual nanofibrils. Among the various methods to prepare nanofibrillated celluloses, TEMPO-mediated oxidation is unique in that it enables native celluloses to be completely dispersed to the level of individual nanofibrils in water. TEMPO-mediated oxidation is a simple and efficient procedure for surface carboxylation of native cellulose nanofibrils.22 When TEMPO/NaBr/NaClO oxidation is applied to native celluloses in water at pH 10 and room temperature, the C6 primary hydroxyls exposed on the crystalline fibril surfaces are selectively oxidized to carboxyls.23−25 The resulting oxidized wood celluloses are fully disintegrated in water to individual nanofibrils with ∼4 nm width, that is, TEMPO-oxidized cellulose nanofibrils (TOCNs), by mild mechanical fibrillation treatment.12,13 In the present study, we investigate the influence of TEMPO oxidation conditions and mechanical fibrillation treatments on the lengths and average degrees of polymerization determined by viscosity measurement (DPv) of the TOCNs. Average Received: December 9, 2011 Revised: January 22, 2012 Published: January 26, 2012 842
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completely dissolve whole TOCs and TOCNs consisting of both partially oxidized and unoxidized cellulose molecules. Hence, CED was used as the solvent of TOCs and TOCNs for determination of their DPv values in this study. Moreover, DPv and DPw (weight average degree of polymerization) values of cellouronic acids were determined using CED in viscosity measurement and 0.1 M NaCl in size-exclusion chromatographic analysis attached with a multiangle laser-light scattering detector, respectively.32 Based on the obtained results, we assumed that the same Mark−Houwink−Sakurada equation, DP = 1.75[η],31 was applicable to both oxidized and unoxidized cellulose molecules in TOCs and TOCNs. Sodium carboxylate contents of TOCs are controllable by the amount of NaClO added during the oxidation (Figure 1A). DPv
lengths and length distributions of the TOCNs are presented. The average nanofibril lengths are then compared with the DPv values, consequently demonstrating a clear relationship between the length and DPv of TOCNs.
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EXPERIMENTAL SECTION
Materials. Softwood bleached kraft pulp was supplied by Nippon Paper Industries Co. Ltd., Japan, in the never-dried state with water content of 80%. For demineralization, the pulp (10 g) was soaked in a dilute HCl (990 mL) at pH ∼2.5 and room temperature for 0.5 h, washed repeatedly with water by filtration, and used as the wood cellulose sample. TEMPO, sodium bromide, sodium hypochlorite solution, and other chemicals were of laboratory grade (Wako Pure Chemicals, Japan) and used as received. Microcrystalline cellulose with the leveling-off DP (LODP) was prepared from the pulp by hydrolysis with 2.5 M HCl at 100 °C for 20 min according to the method described by Montanari et al.24 TEMPO-Mediated Oxidation. The wood cellulose (5 g) was suspended in water (500 mL) containing TEMPO (0.08 g) and sodium bromide (0.5 g). TEMPO-mediated oxidation was initiated by adding a known amount of 1.8 M NaClO (1.3−10 mmol per gram of the wood cellulose) to the cellulose slurry at room temperature under continuous stirring. The slurry was maintained at pH 10 by adding 0.5 M NaOH with a pH stat until no decrease in pH was observed (0−4 h). The TEMPO-oxidized cellulose (TOC) was washed with water by filtration and stored at 4 °C without drying before use. An aliquot of the TOC (∼0.5 g) was further treated with 1% NaClO2 (50 mL) at pH 4.8 and room temperature for 2 days to oxidize C6-aldehydes present in the oxidized cellulose to carboxyls and washed thoroughly with water. Another aliquot of the TOC (∼0.5 g) was treated with 0.5 g NaBH4 in water (95 mL) for 2 days at room temperature and pH ∼9, which was adjusted by adding a 0.5 M NaHCO3 solution to reduce C6-aldehydes and C2/C3 ketones to alcoholic hydroxyls,26−29 and then washed thoroughly with water. The microcrystalline cellulose was oxidized by the TEMPO/NaBr/NaClO system at pH 10 and room temperature with NaClO of 10 mmol per gram of the cellulose. Carboxylate contents of the oxidized celluloses were determined by the electric conductivity titration method.23 Mechanical Fibrillation. A 0.1% w/v suspension of the TEMPOoxidized and NaClO2-oxidized cellulose (TOC−NaClO2; 20 mL) was sonicated for 8 min using an ultrasonic homogenizer equipped with a 7 mm probe tip (US-300T, Nihon Seiki) at a frequency of 19.5 kHz and an output power of 300 W. Before the sonication treatment, the pH of the TOC−NaClO2 suspension was adjusted to ∼8 with 0.05 M NaOH. The unfibrillated fraction (0.6 mmol g−1). However, DPv values of these TOCs were similar to that of the original wood cellulose, even though the same alkaline CED was used as the solvent in the viscosity measurement.22,35 Thus, influences of the C6carboxylates in TOCs on the DPv values and β-elimination of glycoside bonds at the C6-carboxylate units during the DPv measurement using the alkaline CED solution are negligible, compared with those by the C6-aldehydes. If C2/C3 ketones are formed to some extent by the TEMPO-mediated oxidation and present in the oxidized celluloses, the β-elimination of glycoside bonds is possible during the viscosity measurement using the alkaline CED. In this case, the DPv values of TOCs−NaBH4 should be higher than those of TOCs−NaClO2. However, because no significant differences in DPv were observed between the TOC−NaClO2 and TOC−NaBH4 prepared with the same NaClO addition level in the TEMPO-mediated oxidation (Figure 1B), the amounts of C2/C3 ketones formed as side reactions by the TEMPO-mediated oxidation are considered to be negligible. DPv Values of TOCNs−NaClO2. The TEMPO-oxidized and NaClO2-oxidized cellulose nanofibrils (TOCNs−NaClO2) are obtained by mechanical fibrillation of the TOCs−NaClO2 in water. The DPv values of the TOCNs−NaClO2 are also plotted in Figure 1B. The extended sonication treatment was adopted to disintegrate the TOCs−NaClO2 into completely individualized TOCNs−NaClO2 (Figure 2), compared with those previously described.12,20,25 DPv values of the TOCNs− NaClO2 thus prepared dropped to nearly half those of the corresponding TOCs−NaClO2 and linearly decreased with the amount of NaClO added in the TEMPO oxidation stage. Thus, mechanical disintegration to prepare TOCNs−NaClO2 causes further depolymerization, depending on the DPv values of the TOCs, details of which are discussed later. Lengths of TOCNs−NaClO2. Figure 2 displays TEM images of various TOCNs−NaClO2, showing that individual nanofibrils with ∼4 nm width are obtained by the sonication treatment of the TOCs−NaClO2. The TOCNs−NaClO2 prepared with NaClO of less than 2.5 mmol g−1 remain aggregated even after an extended sonication treatment, resulting in a slightly higher turbidity of the aqueous dispersion (Figure S2 in Supporting Information).12 Therefore, TOCNs− NaClO2 obtained from TOCs prepared with NaClO of more
Table 1. Number Average and Length-Weighted Average Lengths of the TOCNs−NaClO2 Obtained from TOCs− NaClO2, which were Prepared from Wood Cellulose with Different Amounts of NaClO in the TEMPO/NaBr/NaClO Oxidation in Water at pH 10 cellulose subjected to TEMPOoxidation
NaClO added in TEMPOoxidation (mmol g−1)
wood cellulose wood cellulose wood cellulose acidhydrolyzed wood cellulose
3.8 5.0 10.0 10.0
mumber average length (nm), Ln
lengthweighted average length (nm), Lw
Lw/Ln
± ± ± ±
929 580 513 325
1.41 1.40 1.29 1.41
658 414 398 230
424 262 215 148
Considering the asymmetry of the distribution histograms, length-weighted average (Lw) values were calculated in the same way as for the weight average molecular weight of polymer. The TOCNs−NaClO2 decreased in length with increasing addition of NaClO in the TEMPO oxidation stage, and the nanofibril length prepared from microcrystalline cellulose (Figure 4D) was the shortest. The TOCNs−NaClO2 prepared
Figure 4. Relationship between DPv and the length-weighted average length of TOCNs−NaClO2 in Table 1. The dashed line shows chain lengths having the corresponding DP values, assuming that molecular chains are fully extended along the longitudinal direction in each nanofibril.
with NaClO of 3.8 mmol g−1 had lengths ranging between 100 and 2000 nm, with a length-weighted average length (Lw) of 930 nm. The average lengths of the TOCNs−NaClO2 prepared with NaClO of 5 and 10 mmol g−1 were similar. The standard 845
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Figure 5. Schematic model of disordered regions of wood cellulose fibrils, and depolymerization mechanism of TOCs or depolymerization-avoidable mechanism of TOCs−NaClO2 in alkaline copper ethylenediamine (CED) solution.
cellulose molecules, lengths of which are shorter than the fibril length. Interestingly, these two lines crossed around a point of the LODP, where the nanofibril length becomes equal to the cellulose chain length. Because the nanofibril lengths bear a proportional relationship to DPv values, the Lw values of the TOCNs−NaClO2 are possible to estimate from their DPv values obtained by the viscosity method using CED as the solvent. The relation equation between DPv and length of TOCNs−NaClO2 with a high decision coefficient (R2) of 0.9975 is described in Figure 4. Depolymerization Mechanism of TOCs. As shown in Figure 1B, all TOCs had significantly lower DPv values, 250− 300, which are close to the LODP of higher plant celluloses obtained by dilute acid hydrolysis. However, the post NaClO2oxidation or post NaBH4-reduction of the TOCs resulted in much higher DPv values. These results indicate that the C6aldehydes present in the TOCs formed as intermediate structures during the TEMPO-mediated oxidation cause the remarkable depolymerization during dissolution in CED at pH ∼14 by β-elimination. This depolymerization is in turn
deviation of distribution also became smaller as the amount of NaClO added in the TEMPO oxidation stage was increased. Thus, it is possible to control the average lengths and length distributions of TOCNs−NaClO2 by selecting the TEMPOmediated oxidation conditions. Relationship between DPv and TOCN−NaClO2 Length. Figure 4 shows the relationship between DPv and the Lw of TOCNs−NaClO2. The cellulose-chain lengths having the corresponding DP values (shown as the dashed line) are also depicted in the same figure, assuming that the repeating glucosyl and glucuronosyl units are both 0.518 nm in length and all molecular chains are fully extended along the longitudinal direction in each nanofibril.37 As clearly shown in Figure 4, a linear relationship is observed between DPv and the average fibril length. The TOCN−NaClO2 lengths are always longer than the molecular-chain lengths calculated from the DPv values, and the slope of the approximated line is about eight times greater than that for the molecular-chain length in all TOCNs−NaClO2 samples examined. This result reveals that one nanofibril always contains some cellulose and oxidized 846
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the DPv values of TOCNs−NaClO2 in Figure 1B. (viii) Some core-cellulose chains have DPw values as the same as those corresponding to Lw values in Table 1 without depolymerization. Based on these assumptions and models, DPw values were calculated in the same way as those obtained in the above section and shown in Table 2.
avoidable by eliminating the presence of the C6-aldehydes via post oxidation or reduction. Based on the wood cellulose nanofibril widths of ∼4 nm,12,13,35 LODP values of ∼200,24,34,38 and the results of preparation of glucose/glucuronic acid alternating copolymers from TOCs by surface-peeling with aqueous alkali solutions,39 the following assumptions and models were made to explain the DPv values of TOCs in Figure 1B. (i) Each wood cellulose fibril is assumed to have a square cross-section consisting of fully extended 6 × 6 = 36 cellulose chains. (ii) Disordered regions are periodically present at every DP ∼ 200 along the longitudinal direction of each cellulose fibril.24,34,38 (iii) TEMPO oxidation forms short cellouronic acid units to some extent with chain cession at the first layer of the disordered regions in each cellulose fibril.39 (iv) Thus, all 20 cellulose chains of the first layer in each wood cellulose fibril22 decrease in DP to ∼200 by the TEMPO-mediated oxidation. (v) The C6-aldehydes are formed to some extent as intermediates also in the second layer of the disordered regions by the TEMPOmediated oxidation.23,39 (vi) However, the C6-aldehydes are convertible to C6-carboxylate stable in alkaline CED by the post NaClO2-oxidation.23 (vii) Each DPv value is close to that of DPw (see Supporting Information).40−42 The glycoside bonds at the C6-aldehyde units present in the second layers of each nanofibril in TOCs could be cleaved by βelimination in alkaline CED, so the 32 cellulose chains in the first and second layers may decrease in DP to ∼200 by TEMPO oxidation and the successive dissolution in alkaline CED; only the 2 × 2 = 4 core cellulose chains in each fibril maintain the original DP. However, when the C6-aldehydes in the second layers are oxidized to C6-carboxylate by the post NaClO2-oxidation, all the 4 × 4 = 16 core cellulose chains except those of the first layer can maintain the original DP. DPw values of the TOC and TOC−NaClO2 are calculated as follows based on the above assumptions (i−vii): TOC/ DPw =
Table 2. Calculated Weight Average DP (DPw) and Measured Viscosity Average DP (DPv) of TOCNs−NaClO2a length-weighted average length of TOCN− NaClO2 (nm), Lw
measured DPv of TOCN− NaClO2
DPw calculated on assumption Ab
DPw calculated on assumption Bc
3.8 5.0 10.0
929 580 513
386 316 307
937 625 374
384 306 291
a
DPw values are calculated based on assumptions A and B. Assumption A: The core 4 × 4 = 16 cellulose chains in each nanofibril have the same DP as that corresponding to the TOCN− NaClO2 length, Lw. Other 20 cellulose chains in the first layer are depolymerized to 200 by the TEMPO-mediated oxidation. cAssumption B: The core 2 × 2 = 4 cellulose chains in each nanofibril have the same DP as that corresponding to the TOCN−NaClO2 length, Lw. A total of 20 cellulose chains in the first layer are depolymerized to DP 200 by the TEMPO-mediated oxidation and other 12 cellulose chains in the second layer are mechanically depolymerized to DP 200 during the disintegration treatment in water. b
As shown in Table 2, the measured DPv values of three TOCNs−NaClO2 samples are quite close to the DPw values calculated based on the assumption B. Thus, it is likely that the 12 cellulose chains of the second layer in each nanofibril are mechanically cleaved at every DP 200 in the disordered regions periodically present along the longitudinal direction of nanofibrils, during the disintegration treatment of TOCs− NaClO2 in water. Therefore, the core 2 × 2 = 4 cellulose chains in each nanofibril remain and have the same DP values as those corresponding to the TOCN−NaClO2 lengths, Lw. The cellulose fibril models based on the above hypotheses are illustrated in Figure 6.
12702 × 4 + 2002 × 32 × (1270/200) 1270 × 4 + 32 × (1270/200)
= 319
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TOC‐NaClO2 / DPw =
NaClO added in TEMPO oxidation (mmol g−1 wood cellulose)
CONCLUSION DPv values of TOCs without the C6-aldehydes gradually decrease from 1270 to 500−600 with the increasing amount of NaClO added in TEMPO oxidation. DPv values of the oxidized celluloses further decrease by the mechanical fibrillation treatment in water intended to disintegrate the oxidized celluloses into TOCNs. The average lengths of TOCNs also become shorter, declining from ∼1 μm to ∼500 nm as the amount of NaClO is increased; in addition, the range of lengths decreased. The association of the average lengths of TOCNs with their DPv values shows a linear relationship, and the length-weighted average lengths of TOCNs are longer than the molecular-chain lengths calculated from their viscosity average DP values. Thus, the average lengths of TOCNs−NaClO2 can be roughly estimated from their DPv values calculated by the viscosity method using CED as the solvent. The depolymerization mechanism of TOCs without postoxidation or -reduction is explainable by distribution of disordered regions in wood cellulose fibrils and formation of C6-aldehydes during TEMPOmediated oxidation. The mechanical depolymerization behavior of TOCs−NaClO2 during disintegration treatment in water is
12702 × 16 + 2002 × 20 × (1270/200) 1270 × 16 + 20 × (1270/200)
= 674
where the values 1270 and 200 are the DP values of the original wood cellulose (Figure 1B) and LODP, respectively. The DPw values of 674 and 319 calculated above are close to the DPv values of TOC (315) and TOC−NaClO2 (615), respectively, prepared with NaClO of 5 mmol g−1 in Figure 1B. Thus, even though the above discussions are based on certain assumptions, the scheme and models of wood cellulose fibril, distribution of disordered regions in fibril, and depolymerization and depolymerization-avoidable hypotheses during dissolution in alkaline CED illustrated in Figure 5 can be reasonably justified. Depolymerization of TOCNs−NaClO2 During Mechanical Disintegration in Water. DPv values of the TOCNs− NaClO2 further decreased from those of TOCs−NaClO2 by the mechanical disintegration in water (Figure 1B). The relationship between DPv and Lw of TOCNs−NaClO2 in Table 1 is explained as follows. In addition to the assumptions i−vii in the above section, the following model was adopted to explain 847
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Figure 6. Cellulose fibril models of TOC, TOC−NaClO2, and TOCN−NaClO2. (6) Kettunen, M.; Silvennoinen, R. J.; Houbenov, N.; Nykänen, A.; Ruokolainen, J.; Sainio, J.; Pore, V.; Kemell, M.; Ankerfors, M.; Lindström, T.; Ritala, M.; Ras, R. H. A.; Ikkala, O. Adv. Funct. Mater. 2011, 21, 510−517. (7) Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A Biomacromolecules 2009, 10, 162−165. (8) Fukuzumi, H.; Saito, T.; Iwamot, S.; Kumamoto, Y.; Ohdaira, T.; Suzuki, R.; Isogai, A. Biomacromolecules 2011, 12, 4057−4062. (9) Aulin, C.; Gällstedt, M.; Lindström, T. Cellulose 2010, 17, 559− 574. (10) Turbak, A. F.; Snyder, F. W.; Sandberg, K. R. J. Appl. Polym. Sci.: Appl. Poly. Symp. 1983, 37, 815−827. (11) Taniguchi, T.; Okamura, K. Polym. Int. 1998, 47, 291−294. (12) Saito, T.; Nishiyama, Y.; Putaux, J.-L.; Vignon, M.; Isogai, A. Biomacromolecules 2006, 7, 1687−1691. (13) Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Biomacromolecules 2007, 8, 2485−2491. (14) Wågberg, L.; Decher, G.; Norgren, M.; Lindströ m, T.; Ankerfors, M.; Axnäs, K. Langmuir 2008, 24, 784−795. (15) Henriksson, M.; Henriksson, G.; Berglund, L. A.; Lindström, T. Eur. Polym. J. 2007, 43, 3434−3441. (16) Päak̈ kö, M.; Ankerfors, M.; Kosonen, H.; Nykänen, A.; Ahola, S.; Ö sterberg, M.; Ruokolainen, J.; Laine, J.; Larsson, P. T.; Ikkala, O.; Lindström, T. Biomacromolecules 2007, 8, 1934−1941. (17) Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindström, T.; Nishino, T. Biomacromolecules 2008, 9, 1579−1585. (18) Päak̈ kö, M.; Vapaavuori, J.; Silvennoinen, R.; Kosonen, H.; Ankerfors, M.; Lindström, T.; Berglund, L. A.; Ikkala, O. Soft Matter 2008, 4, 2492−2499. (19) Sehaqui, H.; Salajková, M.; Zhou, Q.; Berglund, L. A. Soft Matter 2010, 6, 1824−1832. (20) Saito, T.; Uematsu, T.; Kimura, S.; Enomae, T.; Isogai, A. Soft Matter 2011, 7, 8804−8809. (21) Iwamoto, S.; Nakagaito, A. N.; Yano, H. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 461−466. (22) Isogai, A.; Saito, T.; Fukuzumi, H. Nanoscale 2011, 3, 71−85. (23) Saito, T.; Isogai, A. Biomacromolecules 2004, 5, 1983−1989.
also interpretable by the original/TEMPO-oxidized wood cellulose fibrils models and DP calculations.
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ASSOCIATED CONTENT
* Supporting Information S
Relationships between aldehyde contents and DPv values of the TOCs (Figure S1), turbidities of aqueous dispersions of the TOCNs prepared with different amounts of NaClO in the TEMPO oxidation stage (Figure S2), and relationships between DPw and DPv equations are shown. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81 3 5841 5538. Fax: +81 3 5842 5269. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was partially supported by Scientific Research S (21228007) and Encouragement of Young Scientists A (23688020) from the Japan Society for the Promotion of Science (JSPS).
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REFERENCES
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