Sulfonated cellulose nanofibrils obtained from wood

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wood pulp and to obtain nanofibrils with sulfonated ... matrix of hemicellulose and lignin in fiber cell walls ... also reported the use of post-sulfonated cellulose.
Cellulose DOI 10.1007/s10570-013-9865-y

ORIGINAL PAPER

Sulfonated cellulose nanofibrils obtained from wood pulp through regioselective oxidative bisulfite pre-treatment Henrikki Liimatainen • Miikka Visanko • Juho Sirvio¨ • Osmo Hormi • Jouko Niinima¨ki

Received: 1 December 2012 / Accepted: 12 January 2013 Ó Springer Science+Business Media Dordrecht 2013

Abstract The consecutive pre-treatment of cellulose with periodate and bisulfite was used as a new potential method to promote nanofibrillation of hardwood pulp and to obtain nanofibrils with sulfonated functionality. Nanofibrils having typical widths of 10–60 nm were obtained from sulfonated celluloses having low anionic charge densities (0.18–0.51 mmol/g) by direct high-pressure homogenization without the use of any mechanical pre-treatments. The aqueous nanofibrils existed as highly viscous and transparent gels and possessed cellulose I crystalline structures with crystallinity indexes of approximately 40 %. A transparent film was obtained from sulfonated nanofibrils having tensile strength of 164 ± 4 MPa and Young’s modulus of 13.5 ± 0.4 MPa. Oxidative sulfonation was shown to be a potential green method to H. Liimatainen (&)  M. Visanko  J. Niinima¨ki Fiber and Particle Engineering Laboratory, University of Oulu, P.O. Box 4300, 90014 Oulu, Finland e-mail: [email protected] M. Visanko e-mail: [email protected] J. Niinima¨ki e-mail: [email protected] J. Sirvio¨  O. Hormi Department of Chemistry, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland e-mail: [email protected] O. Hormi e-mail: [email protected]

promote nanofibrillation of cellulose, as it avoids the production of halogenated wastes, because the periodate used can be efficiently regenerated and recycled as shown in the preliminary experiments. Keywords Nanocellulose  Nanofibrillation  Periodate oxidation  Bisulfite  Dialdehyde cellulose

Introduction Cellulose nanofibrils are fibrous hydrogen-bonded constituents of plant cellulose existing embedded in a matrix of hemicellulose and lignin in fiber cell walls (Fahle´n and Salme´n 2005). The nanofibrils, which possess unique mechanical and chemical properties such as superior strength (Chakraborty et al. 2006), low density (Bledzki et al. 1996), and high water retention capacity (Herrick et al. 1983), can be liberated from the matrix using intensive mechanical treatments such as grinding (Abe et al. 2007) and refining (Nakagaito and Yano 2004). Due to the strong hydrogen-bonded structure of the networked nanofibrils, the mechanical liberation of nanofibrils is highly energy consuming and results mainly in larger nanofibril bundles called fibril aggregates. Therefore, several different chemical pre-treatments have been proposed to loosen the rigid structure of cellulose. Regioselective oxidative pre-treatments based on 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)

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(Saito et al. 2006) and periodate-chlorite (Liimatainen et al. 2012a) introduce carboxyl groups to cellulose and promote efficient nanofibrillation of cellulose, but these treatments result in highly hydrophilic nanofibrils with relatively high anionic charge densities. However, nanofibrils bearing lower anionicity and other functionalities than carboxyl groups would likely provide new properties and be more favorable for many applications. Moreover, the use of less harmful and expensive chemicals, which can be recycled in the process, would be beneficial for the viable large-scale nanofiber production. The sequential oxidation of cellulose with periodate and chlorite converts cellulose to dicarboxyl cellulose (DCC), which can be homogenized to a transparent and viscous nanofibril gel, as we have previously reported (Liimatainen et al. 2012a). Periodate, used in the first step of the oxidation treatment, oxidizes vicinal hydroxyl groups of cellulose at positions 2 and 3 to reactive aldehyde groups (Fig. 1), and simultaneously breaks the corresponding carbon–carbon bond of the glucopyranose ring to form 2,3-dialdehyde cellulose (DAC). Although large excesses of oxidant with long reaction times have traditionally been used in the periodate reaction, the reaction efficacy can be markedly improved using mechanical treatments, high temperatures, and metal salts as activators, as previously shown (Liimatainen et al. 2011; Sirvio¨ et al. 2011). Furthermore, the periodate, which is toxic and expensive, can be regenerated and recycled after the reaction, which makes the reaction more environmentally sustainable (Hearon et al. 1978). Periodate oxidation also enables the introduction of other regioselective functionalities to cellulose and the ability to obtain other charged groups on it. Sulfonation of 2,3-dialdehyde cellulose with bisulfite is one attractive synthesis route, as it can be conducted in an environmentally friendly way under mild aqueous reaction conditions and without the use of harmful solvents (Shet and Wallajabet 1997; Zhang et al.

Fig. 1 Regioselective periodate oxidation and sulfonation of cellulose

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2008). Previously consecutive oxidation and sulfonation of cellulose have been used to prepare sulfonated celluloses from pre-fabricated nanodimensional cellulosics, but these techniques have not been used as a pre-treatment method to enhance nanofibrillation (Zhang et al. 2008).The sulfonated nanocellulosics were shown to be potential candidates for superabsorbent materials due to their very high water adsorption capacity, although the post-sulfonation partly deteriorated the morphological properties of nanoparticles resulting in particles with lower aspect ratio. We have also reported the use of post-sulfonated cellulose nanoparticles as potential new green flocculation agents for mineral particle suspension (Liimatainen et al. 2012b). In this article, a consecutive pre-treatment of cellulose fibers with periodate and bisulfite was investigated as a promising green method for promoting nanofibrillation of hardwood pulp through homogenization and to obtain nanofibrils with sulfonated functionality. Sulfonated fibers and nanofibrils obtained after homogenization were analyzed using a fiber image analyzer, FTIR spectroscopy, wideangle X-ray diffraction (WAXD), field-emission scanning electron microscopy (FESEM), viscosity measurements and optical transmittance measurements. Cellulose samples with variable amounts of sulfonated groups were prepared to examine the influence of the degree of substitution of the cellulose on nanofibrillation and on characteristics of nanofibers. Moreover, a film was prepared from sulfonated nanofibrils and its mechanical properties analyzed.

Experimental Materials Bleached birch (Betula verrucosa and pendula) chemical wood pulp from the kraft pulping was used

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as a cellulose raw material. The cellulose, xylan and glucomannan contents of the pulp were 74.8, 23.6 and 1.1 %, respectively, as determined using high-performance anion-exchange chromatography (HPAECPAD). The lignin (TAPPI-T Method 222 om-02) and the extractive contents (SCAN-CM 49:03 standard) of the pulp were 0.4 and 0.08 %, respectively. The average (length-weighted) length and width of the pulp fibers, as determined using a Metso FiberLab image analyzer, were 0.90 mm and 19.0 lm, respectively. The fines content, which was determined using a L&W STFI Fibermaster analyzer, was 3.4 %. The fpotential in deionized water (conductivity \5 lS/cm, pH 5.5) was determined to be -125 mV on a Mu¨tek SZP-06 device, and the degree of polymerization (DP) was 3817, as determined using a procedure similar to that previously described (Liimatainen et al. 2012b). The pulp was washed and converted into sodium form before the periodate oxidation and bisulfite reaction using the method previously reported (Liimatainen et al. 2009a). The chemicals used in the periodate oxidation (NaIO4) and sulfonation reaction (Na2S2O5), pulp washing (ethylenediaminetetraacetic acid, HCl and NaHCO3) and aldehyde and anionic group content analyses (NH2OHHCl, CH3COONa2H2O, NaCl and NaOH) were obtained as p.a. grade from Sigma– Aldrich and were used without further purification. Deionized water was used throughout the experiments.

Oxidative sulfonation of cellulose Cellulose pulp was sulfonated using subsequent periodate oxidation and sulfonation reactions. Five samples with a variable degree of sulfonation were produced. Samples of cellulose pulp (6 g dry weight) were allowed to react for 30, 60, 120 and 180 min with 38 mM aqueous sodium metaperiodate (NaIO4) at 55 °C. The oxidized cellulose was subsequently filtered, washed several times and further reacted with a sodium metabisulfite (Na2S2O5 which reacts with water to form NaHSO3). 2.2 times excess of Na2S2O5 based on the assessed amount of aldehyde groups was used in a water solution for 72 h at room temperature. The sulfonated cellulose was filtered and washed with deionized water until the conductivity of the filtrate was \30 lS/cm. The synthesis route is presented in Fig. 1.

The aldehyde content of the periodate oxidized cellulose was determined using an oxime reaction, as previously reported (Sirvio¨ et al. 2011). Conductometric titration was used to analyze the amount of sulfonated (anionic) groups in the cellulose, using a procedure described by Katz et al. (1984) and by Rattaz et al. (2011). A Metso FiberLab image analyzer was used to examine the morphological properties of the sulfonated fibers. The mass yields of the reactions were measured by weighing the obtained products on an analytical balance. Nanofibrillation of sulfonated celluloses using a high-pressure homogenizer Suspensions containing 0.5 % (w/w) of sulfonated cellulose fibers at a pH of approximately 7 were nanofibrillated using a two-chamber high-pressure homogenizer (APV-2000, Denmark) with a pressure of 400–950 bars. The suspensions were passed through the homogenizer five times until clear gels were obtained. Additionally, a sample prepared from the cellulose with the highest content of sulfonated groups was passed through the homogenizer only three times. The mass yields of the nanofibrils were measured by weighing the obtained products on an analytical balance. FTIR spectroscopy FTIR spectra of the untreated cellulose and the cellulose after sulfonation were recorded using a Bruker FT-IR spectrometer. The samples were prepared by pressing 2 mg of dry sample into a pellet with 200 mg of KBr. X-ray diffraction The crystalline structure of the cellulose after sulfonation and nanofibrillation was investigated using wide-angle X-ray diffraction (WAXD). Measurements were conducted on a Siemens D5000 diffractometer equipped with a Cu Ka radiation source (k = 0.1542 nm). Samples were prepared by pressing tablets of freeze-dried cellulose to a thickness of 1 mm. Scans were taken over a 2h (Bragg angle) range from 5° to 50° at a scanning speed of 0.02°/s using a step time of 1 s. The degree of crystallinity in terms of the crystallinity index (CrI) was calculated

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from the peak intensity of the main crystalline plane (002) diffraction (I002) at 22.2° and from the peak intensity at 18.0° associated with the amorphous fraction of cellulose (Iam) according to following equation (Segal et al. 1959):  CrI ¼

I002  Iam I002

  100%

Field-emission scanning electron microscopy (FESEM) FESEM (Zeiss ULTRA plus) images of the freezedried (liquid nitrogen and vacuum drying) and sputtercoated (Pd) samples filtered on a polycarbonate membrane with a pore size of 0.2 lm were collected. The accelerating voltage during imaging was 10 kV.

Optical transmittance The transmittance of 0.1 % nanofibril suspensions was measured at wavelengths of 340–800 nm with a Hach DR 2800 spectrophotometer.

Viscosity measurements The low shear viscosities of 0.3 % nanofibril suspensions were recorded using a Brookfield DV-II? Pro EXTRA viscometer at a temperature of 20 °C using a vane-shaped spindle.

Film preparation from sulfonated nanofibrils A vacuum-filtration method was used to prepare a thin film from sulfonated nanofibrils (Liu et al. 2011). Degassed nanofibril suspension (solids content of 0.15 %) was vacuum filtered on a filter membrane having pore size of 0.65 lm (Millipore, USA) after which the wet film was vacuum-dried by Rapid Ko¨then equipment at 94 °C for 10 min. The mechanical properties of the film were measured with a Universal Materials Testing Machine (Instron, USA) using a 500 N load cell. The measurements were conducted at 50 % humidity and temperature of 20 °C using specimens of 40 mm length, 20 lm thickness and 5 mm width (grammage 37 g/m2) with strain rate of 4 mm/min.

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Results and discussion Periodate oxidation and sulfonation of cellulose pulp Reaction times of 30, 60, 120 and 180 min were used in the periodate oxidation to obtain 2,3-dialdehyde celluloses with aldehyde contents of 0.61, 0.95, 1.31 and 1.68 mmol/g, respectively. The aldehyde groups were further sulfonated with bisulfite to obtain samples having the sulfonated group contents of 0.09, 0.18, 0.36 and 0.51 mmol/g as measured in terms of the anionic charge density of the celluloses with a conductometric titration. The results indicated that 15–30 % of aldehydes were converted to sulfonated groups during the bisulfite treatment. The sulfonation of cellulose was confirmed by the FTIR analysis, in which bands at 1,131, 617 and 520 cm-1 associated with SO2 vibrations at sulfonic acid groups were detected. The FTIR spectrum of sulfonated oxycellulose is shown in our previous publication (Liimatainen et al. 2012b). As previously stated, the high amount of residual aldehydes can expose cellulose chains to depolymerization reactions by b-alkoxy fragmentation, and thus weakening the mechanical properties of the material (Maekawa and Koshijima 1984; Calvini et al. 2004; Saito et al. 2009). However, the amount of residual aldehydes can be decreased by increasing the bisulfite concentration during the reaction, as previously shown by Nikiforova et al. (2011) or aldehydes can be efficiently converted back to alcohols using, e.g., NaBH4 to increase the stability of the cellulosic material (Mishra et al. 2012). However, these options were not studied here. During the sulfonation reaction, the oxidized cellulose pulp began to lose its fibrous structure and converted to a more homogeneous and transparent form. After completing the reaction, the samples with the highest content of sulfonated groups (0.36 and 0.51 mmol/g) exhibited a slimy appearance, which was likely due to the increased hydrophilicity of the pulp and the swelling and cutting of the cellulose. The morphological changes of the sulfonated cellulose were demonstrated with an image-analysis-based fiber analyzer. The average length-weighted fiber length of the sulfonated celluloses decreased from an initial value of 0.90 mm to 0.56–0.88 mm, while the fiber widths increased from the initial value of 19.0 lm to

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19.5–20.4 lm due to fiber swelling. The amount of fine material in the samples (the material having lengths below 0.2 mm) was also markedly increased during oxidative sulfonation reactions (from 3.4 % to 13.4–36.4 %). Previously Zhang et al. (2008) reported that consecutive periodate oxidation and sulfonation caused extensive deformation in the shape of posttreated nanocellulosics. Here, the morphological changes of the sulfonated pulp were expected to mainly change the size and shape of the fibers, while the individual nanofibrils embedded in the cell wall mainly maintained their structure during the reactions. The crystalline structure of cellulose after oxidation and sulfonation was examined using WAXD. Example diffractograms of celluloses with sulfonated groups of 0.09 and 0.51 mmol/g are shown in Fig. 2a. Diffractogram of original pulp has previously shown in Liimatainen et al. 2011. All sulfonated celluloses presented peaks typical of cellulose I, with the main 2h diffraction angles close to 14.5°, 16.0° and 22.2°. These peaks are associated with the 101, 10 1 and 002 crystalline planes, respectively, and they indicate that no rearrangement of the cellulose structure into another crystalline form occurred. The calculated crystalline indexes (CrI) decreased 25–55 % from the initial value of the cellulose pulp depending on the content in the sulfonated groups of samples (Fig. 2b). This decrease was larger than obtained previously with periodate-chlorite oxidized cellulose (Liimatainen et al. 2012a). The reaction mass yield (calculated as ratio of masses of obtained product and feed) after the periodate oxidation varied from 91 to 100 %, and after sulfonation it varied from 88 to 93 % (Table 1). We have previously found that the yield losses during the periodate oxidation are presumably mainly due to the dissolution of hemicelluloses (Sirvio¨ et al. 2011), consequently the use of cellulose pulp with a lower hemicellulose content such as dissolving pulp would likely further improve recovered yields.

Nanofibrils obtained through sulfonation pre-treatment and homogenization The oxidized and sulfonated celluloses were fed into the high-pressure homogenizer in the form of 0.5 % water suspensions without using any other mechanical pre-treatments. All cellulose samples, except the one

Fig. 2 a Diffractograms of celluloses after oxidative sulfonation (the values represent the content of sulfonated groups of samples), b calculated crystallinity indexes

with the lowest content of sulfonated groups (0.09 mmol), were successfully nanofibrillated without clogging the homogenizer. These samples converted to homogeneous gels after the first pass through the homogenizer, and highly transparent gels were obtained after five passes (with mass yields of 80–91 %). However, for the sample having the highest content of sulfonated groups (0.51 mmol/g), a sudden decrease in viscosity and increase in transparency were observed after four passes, as a result, two samples were taken after passing through the homogenizer three and five times. The sample with the lowest anionicity, which still possessed more fibrous structure than other samples, plugged the homogenizator and was not nanofibrillated. This result was likely due to the low charge density of the sample, which was unable to prevent the flocculation of fibers. As previously stated (Liimatainen et al. 2009b), the increased surface charge density of the fibers improves

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fiber dispersion because of increased charge repulsion. The mechanical fiber crowding is also diminished as the length of the fibers decreases during the chemical pretreatment. The visual appearance of the obtained nanofibril samples is presented in Fig. 3a. After 5 passes through the homogenizer, the samples with anionic group contents of 0.18 and 0.36 mmol/g became highly viscous gel-like suspensions, while the sample with

the highest content of sulfonated groups (0.51 mmol/g) was more liquid-like in form. However, this sample also possessed a highly viscous gel-like structure after passing only three times through the homogenizer. FESEM analysis confirmed that sulfonated celluloses were efficiently homogenized to nanofibrils having typical lateral dimensions of 10–60 nm and lengths of several micrometers (Fig. 3b, c). Compared to TEMPO oxidation, which has been reported to result in individual nanofibrils, the obtained nanofibrils presumably existed mainly in bundles because the width of a single fibril has been reported to be 3–5 nm (Saito et al. 2007, 2009). However, the high transparency of suspension suggested that these bundles can also originate from aggregation during the freezedrying of FESEM samples, and the fibrils existing in the suspension were actually thinner. Moreover, the Pd layer used in the sample sputtering increases also the thickness of the nanofibrils. The optical transmittances of the 0.1 % sulfonated nanofibril suspensions varied notably as a function of anionic charge density (content of sulfonated groups) of nanofibrils (Fig. 4). High transmittance values were

Fig. 3 a Appearance of the 0.5 % nanofibril suspensions obtained after the oxidative sulfonation and after five passes through homogenizer (the values represent the content of sulfonation groups of the samples, asterisks three passes through

the homogenizer), b, c typical FESEM images of the nanofibrils with a content of sulfonated groups of 0.51 and 0.36 mmol/g, respectively (the round black spots are pores of the membrane used as a background)

Table 1 Mass yields of products after the periodate oxidation and sulfonation reactions and after homogenization Sample (sulfonated groups, mmol/g)

Yield after periodate oxidation (%)

Yield after sulfonation (%)

Yield after homogenization (%)

0.09

100

93

nda

0.18 0.36

100 97

85 90

91b 83b

0.51

91

88

79b

0.51

91

88

81c

a

The sample did not nanofibrillate. through the homogenizer

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b

Five and

c

three passes

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Fig. 4 Optical transmittance of the 0.1 % nanofibril suspensions. The samples were sulfonated and passed five times through the homogenizer (the values present the content of sulfonated groups of the samples, asterisks three passes through the homogenizer)

measured for the samples having the highest content of sulfonated groups after only 3 passes through the homogenizer. These values were comparable to those previously reported for individual TEMPO-oxidized nanofibrils (Saito et al. 2007; Besbes et al. 2011), but the required anionic charge density of sulfonated cellulose was only 30–50 % that of the TEMPO samples. Significantly lower transmittance levels were observed for the samples with lower contents of sulfonated groups, suggesting that these suspensions likely consisted of nanofibril aggregates instead of individual fibrils. The low shear viscosities of the 0.3 % nanofibril suspensions were measured with a rotational viscometer (Fig. 5). The viscosity values reflect the strength of the network formed from the entangled nanoconstituents and are attributed to the aspect ratio of the nanofibrils and their propensity to form hydrogen bonds (Saito et al. 2007). The viscosities exhibited typical shear thinning behavior (Pa¨a¨kko¨ et al. 2007; Lasseuguette et al. 2008), except for the sample with the highest content of sulfonated groups which possessed notably lower and almost constant viscosity after five passes through homogenizer. However, after passing through the homogenizer only three times, the viscosity of this sample was similar to the sample with an anionicity of 0.36 mmol/g. This result suggests that the high anionic charge density of the sulfonated cellulose exposed nanofibrils to mechanical degradation during homogenization and reduced the length of the nanofibrils. However, clear indication of nanofibril cutting was not seen in the FESEM images.

Fig. 5 Viscosity of the 0.3 % nanofibril suspensions. The samples were sulfonated and passed five times through the homogenizer (the values present the content of sulfonated groups of samples, asterisks three passes through the homogenizer)

The effect of nanofibrillation on the crystalline structure of sulfonated celluloses in terms of crystalline indexes is presented in Fig. 6. As observed with transmittance values, a clear decrease in the index values was observed, with the cellulose having the highest content of sulfonated groups when the number of passes through the homogenizer increased from three to five. Characteristics of a film prepared from sulfonated nanofibrils A transparent film was obtained from the aqueous sulfonated nanofibril suspension (amount of sulfonated group 0.51 mmol/g, 5 pass through the homogenizer) using vacuum-filtration method (Fig. 7a). The stress–strain behavior of a film is shown in Fig. 7b. The corresponding values for tensile strength and Young’s modulus were 164 ± 4 MPa and 13.5 ± 0.4 GPa, respectively. These values are comparable to those previously reported for nanofibril films (Henriksson et al. 2008) and suggest that nanofibrils maintained their high strength properties also after oxidative sulfonation treatment. Regioselective oxidative sulfonation pre-treatment in nanofibrillation of wood cellulose The nanofibrillation of the sulfonated celluloses and the characteristics of the obtained nanofibrils strongly depended on the content of sulfonated groups in the

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Fig. 6 Crystallinity indexes of the nanofibrils

samples. As we have previously shown in the context of periodate-chlorite oxidation, increased anionic charge density of cellulose promotes nanofibrillation due to charge repulsion and the loosening of the rigid hydrogen bonded network of nanofibrils (Liimatainen et al. 2012a). A charge density of approximately 0.4 mmol/g was required to enable nanofibrillation with periodate-chlorite oxidation (Liimatainen et al. 2012a), but here, sulfonated fibers with notably lower charge densities of 0.18 mmol/g were nanofibrillated to viscous gels, enabling them to form nanofibrils with relatively low anionic charge densities. This lower limit value was likely due to a higher content of residual aldehyde groups in the sulfonated celluloses, which also loosened the interfibrillar hydrogen bonds. To further demonstrate the influence of aldehyde functionalities on nanofibrillation, a test was conducted in which periodate oxidized cellulose was fed into the homogenizer without sulfonation. This sample plugged the homogenizer and was unable to nanofibrillate, demonstrating the critical role of charged groups on nanofibrillation. However, the residual aldehydes can likely lower the required charge density of nanofibrillation. One significant advantage in using bisulfite as a second active chemical in nanofibrillation is the fact that the oxidative sulfonation method avoids the production of halogenated wastes if the periodate used in the first step of pre-treatment is recovered and recycled. Our preliminary tests, in which aqueous periodate solution was separated from cellulose after oxidation and regenerated using sodium hypochlorite, resulted in 90–100 % recovery efficacy of periodate

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Fig. 7 a A film prepared from sulfonated nanofibrils, b stress– strain curve in tension of a film prepared from sulfonated nanofibrils. The sulfonated group content of the film was 0.51 mmol/g

depending on the amount of hypochlorite used (1.0–1.4 times compared to the required theoretical amount). These results suggest that oxidative sulfonation is a potential green method to promote nanofibrillation of wood cellulose and to obtain sulfonated nanofibrils.

Conclusions Sulfonated nanofibrils with typical widths of 10–60 nm, as determined by field-emission scanning electron microscopy, were efficiently obtained from wood cellulose pulp using consecutive periodate oxidation and sulfonation pre-treatment. The content of sulfonated groups required to enable nanofibrillation of wood cellulose by a homogenizer and to result in highly viscous and transparent gel was only 0.18 mmol/g. All of the prepared nanofibrils (sulfonated groups content 0.18–0.51 mmol/g) possessed cellulose I crystalline structures according to wideangle X-ray diffraction results and had crystallinity indexes of approximately 40 %. A transparent film was obtained from sulfonated nanofibrils (amount of sulfonated groups of 0.51 mmol/g) having tensile strength of 164 ± 4 MPa and Young’s modulus of 13.5 ± 0.4 MPa. Oxidative sulfonation was shown to be a potential green method to promote the nanofibrillation of wood cellulose, because the periodate used can be efficiently recycled and regenerated, as shown in the preliminary experiments.

Cellulose Acknowledgments This work was financed by The Academy of Finland (Postdoctoral project No. 250940). The Wallenberg Wood Science Center, KTH, Sweden is acknowledged for the opportunity to use the mechanical tester.

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