Properties of Cellulosic Material after Cationization in Different

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Properties of cellulosic material after cationization in different solvents Nora Odabas, Hassan Amer, Markus Bacher, Ute Henniges, Antje Potthast, and Thomas Rosenau ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01752 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Sustainable Chemistry & Engineering

Properties of cellulosic material after cationization in different solvents Nora Odabas1, Hassan Amer1,2, Markus Bacher1, Ute Henniges1*, Antje Potthast1, Thomas Rosenau1* 1

Division of Chemistry of Renewable Resources, Department of Chemistry, University of

Natural Resources and Life Sciences Vienna, Konrad-Lorenz-Straße 24, 3430 Tulln, Austria 2

Department of Natural and Microbial Products Chemistry, National Research Centre, 33 Al Behous, Dokki, Giza, Egypt

*Corresponding authors: [email protected] (UH), [email protected] (TR)

1 ACS Paragon Plus Environment


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ABSTRACT

Fibers resulting from pulping of wood are negatively charged by nature. Both the cellulose and hemicelluloses contribute to the anionicity. The negative charge of cellulosic material can be reversed with the same reagent commonly used for the cationization of starch. In this study, a bleached Kraft pulp was cationized in systems with different water-miscible organic solvents. Replacing 90 percent of the water with isopropanol and particularly with tetrahydrofuran yielded higher degrees of substitution and increased reaction efficiency. The degree of substitution depends on the concentration of the cationization reagent in water; partially replacing water with tetrahydrofuran can simulate a higher concentration while maintaining supramolecular properties, such as crystallinity and polymer chain integrity.

KEYWORDS: cellulose, pulp properties, hydrophilization, lignocelluloses, gel permeation chromatography, degree of substitution, infrared spectroscopy

INTRODUCTION Fibers resulting from pulping of wood are negatively charged by nature. The main reasons are uronic acids in hemicelluloses and pectins1 and, especially after chemical pulping, carboxyl functionalities in the cellulose.2 To overcome the resulting electrostatic repulsion among the fibers, papermakers usually add cationic auxiliaries to pulp. Cationic starch in particular is widely used;3 it is the most important cationized polysaccharide,4 though there are other examples for industrial applications: Compared to the respective unmodified materials,


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suspensions of cationized cellulose nanocrystals are more stable,5 cationized cotton allows saltfree dying,6 the absorption capacity of cationized cellulose nanofibrils for anionic dyes helps in wastewater treatment,7 and cationization of pulp fibers can improve thermoplastic composites8 or serve as wet-end additives for papermaking.9 A common reagent for the cationization of both starch10 and cellulose7,9 is 2,3epoxypropyltrimethylammonium chloride (EPTMAC), used either directly, or formed in situ from the more stable 3-chloro-2-hydroxypropyltrimthylammonium chloride.4 The modification is usually done in the presence of varying amounts of water and sodium hydroxide. For the cationization of cellulose, the latter is important as a nucleophile that activates the hydroxyl groups of cellulose towards etherification. Water, however, reduces the reaction efficiency11 by causing side reactions with the caustic.12 Literature values for reaction efficiencies for common methods range from below 1%5 to 3%11. In an attempt to deal with the yield issue, de la Motte et al.13 developed a spray technique that resulted in up to 8% reaction efficiency for softwood Kraft pulp,11 but it involves solvent exchange and is therefore of less practical relevance. The present study shows how replacing up to 90 percent of the water with different organic solvents can improve the reaction efficiency of cationization and how the reaction conditions affect the integrity of the cellulose. Based on one protocol for treating a bleached Kraft pulp, under otherwise identical conditions (liquid-to-solid ratio, time, temperature profile, amount of alkali and amount of reagent), the water was partially replaced by either dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), or isopropanol. All three solvents are water-miscible; possible phase separation under the actual reaction conditions was studied. Isopropanol is a popular solvent used in cellulose etherification, for the synthesis of carboxymethyl cellulose or hydroxyethyl cellulose,14 and in one study also for cellulose cationization15. For the production



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of highly cationized starch, different water-miscible organic solvents including dimethyl sulfoxide16, isopropanol17, and tetrahydrofuran18 have been used in attempts to limit side reactions as well as excessive swelling.18 Also hemicelluloses have been cationized in DMSOwater19 and alcohol-water20 mixtures. Cellulose has been cationized in anhydrous DMSO21 to a very low degree of substitution. However, to our best knowledge, no studies on pulp cationization that assess different water-solvent-mixtures have been published so far. Previous studies on pulp cationization have focused either on molecular properties, such as the substitution pattern,11,13 or on the mechanical and optical properties of paper made from this cationized pulp.9,21 Depending on the intended use of fibers, the content of highly ordered areas23 (commonly referred to as “crystallinity”)24 and molar mass distribution25 are of major importance concerning the resulting material properties. This series of experiments also shows how the reaction conditions influence these two quantities. To be able to detect whether observed effects are caused directly by the cationization or rather by the reaction conditions, blank reactions were done with sodium chloride.

EXPERIMENTAL SECTION

Material Bleached softwood Kraft pulp in sheet-dried form was kindly provided by Zellstoff Pöls AG, Austria. It contained 89% cellulose, 11% hemicelluloses, and traces of lignin. Dimethyl acetamide (DMAc; from Promochem, Teddington, United Kingdom), dimethyl sulfoxide (DMSO, from Roth, Karlsruhe, Germany), isopropanol (from Roth), tetrahydrofuran (THF, from VWR) and hydrochloric acid (HCl; from VWR, Radnor, PA, USA) were purchased


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at analytical grade and were used without further purification. Acetone was purchased at technical grade at VWR. EPTMAC (technical grade, 21.6% water, 90% purity of dry substance), sodium chloride (NaCl), silver nitrate, and sodium hydroxide (NaOH) were purchased from Sigma-Aldrich (St. Louis, MO, USA), lithium chloride (LiCl) from VWR. Water was deionized to a conductivity of 0.06 µS/cm for the reaction and for analyses, and to 0.4 µS/cm for the workup.

Reaction protocol The pulp was soaked in deionized water for 4 hours and dispersed using a kitchen blender with no blades (rotating disk only). HCl was added to achieve pH 1. After one hour, the pulp was filtered off and washed with water until the water pressed out from the filter cake was neutral. The moisture content was determined by spreading out about 1 g of the material on the dish of a Sartorius Moisture Analyzer MA35 (Göttingen, Germany) and then heating to 105°C until constant weight was reached. For each assay, 5 g of acid-washed pulp (consistency 30%) was mixed with 225 mL of deionized water, DMSO, isopropanol, or THF. After 1 hour stirring at room temperature, 2.5 mL of 10 M NaOH were added. After further 30 min, the reagent was added drop by drop: 17.8 mL of EPTMAC. This corresponds to about 3 moles per mole anhydrosugar unit (ASU). Blank reactions were done with 3.7 g of NaCl being added instead of the epoxide reagent, i.e. 3 moles per mole ASU. Deionized water was added to make up a total solvent volume of 250 mL. After 15 hours at 40°C, the reaction was stopped by adding 12.5 mL of 4 M HCl. This amount decreased the pH to about 1 if water was the only solvent. The reaction supernatant was filtered off through a fine cloth in a Büchner funnel and the solid material washed with water until



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neutral. Table 1 shows the full assay matrix with the subsequently used abbreviations. The full set was prepared independently twice. The second batch also included two assays where 187.5 mL of THF plus 61.5 mL of water and 125 mL of THF plus 125 mL of water were used as solvent, referred to as “75% THF” and “50% THF”, respectively.

Table 1. Abbreviations for materials resulting from cationization and blank assays. Solvent

Cationized material

Water DMSO + water Isopropanol + water THF + water

WC DC IC TC

Material after blank reaction WB DB IB TB

The workup of TC was done differently: The material was filtered off and mixed with water. After several repetitions of decanting and refilling, the material was precipitated in acetone, filtered off, washed with some more acetone, and air-dried.

Determination of degree of substitution The degree of substitution (DS) was determined by conductometric titration5 using a system by Metrohm (Herisau, Switzerland) consisting of an 856 Conductivity Module, an 801 Stirrer, an 800 Dosino Dosing Device and 807 Dosing unit, and a Pt1000/B/s conductivity electrode. Of each sample, 1 g was lyophilized and ground in a Retsch CryoMill (Haan, Germany) for 9 min at a frequency of 25 s-1; of that, 100 mg aliquots were suspended in 50 mL of deionized water and titrated with a 10 mM aqueous silver nitrate solution. For IC and TC, 50 mg aliquots were used. 50 µL of silver nitrate solution were added every 10 s. Triplicate measurements were done for each sample. Analysis and explanation of these data are available as Supporting Information. 6 ACS Paragon Plus Environment

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Infrared spectroscopy For attenuated total reflectance infrared spectroscopy (ATR-FTIR), each sample was measured in quadruplicate on a Perkin Elmer FT-IR Spectrometer Frontier (Waltham, MA, USA). Spectra from 4000 to 650 cm-1 were recorded with a resolution of 4 cm-1, baseline corrected using the instrument software (Perkin Elmer Spectrum) and imported to Microsoft Excel 2010 where they were normalized to 0 at the minimum absorption around 1520 cm-1 and 1 to the maximum absorption around 1030 cm-1. Near infrared (NIR) spectra were recorded with an MPA Multi-Purpose Analyzer from Bruker (Billerica, MA, USA) equipped with a fiber optic probe and a Te-InGaAs detector. The air-dried samples were air equilibrated for one week. Each sample was measured in five spots with 32 scans from 12500 cm-1 to 4000 cm-1 with a resolution of 4 cm-1. OPUS 7.0 software from Bruker was used to do a rubber band baseline correction and normalization from 0 for the lowest and 2 for the highest value.

Determination of zeta potential The zeta potential was determined on a SZP-04 device from Mütek (Filderstadt, Germany). 5 g of each sample were suspended in 500 mL deionized water, conductivity was set to 300 µS/cm, and pH was set to 7 with NaCl, HCl, and NaOH. The zeta potential was measured 3-5 times for each sample.

Determination of molar mass distribution



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The molar mass distributions of the starting material and the products of the blank reactions were determined by gel permeation chromatography (GPC) after dissolution in DMAc with 9% LiCl.2 20 mg aliquots of air-dried samples were mixed with 150 mL of deionized water in a kitchen blender, filtered, washed with ethanol and DMAc, shaken on a GFL 3005 orbital shaker (GFL Gesellschaft für Labortechnik, Burgwedel, Germany) in 4 mL of DMAc overnight, filtered off again, and mixed with 2 mL of DMAc/LiCl 9% (w/v). After a week on the shaker, 300 µL of the dissolved sample were diluted with 900 µL of DMAc, and filtered through 0.45 µm PTFE filters. The injection volume was 100 µL. The GPC system had four serial columns (PL gel mixed A LS, 20 µm), a multi-angle laser light scattering detector (Dawn DSP with argon ion laser at λ0 = 488 nm, Wyatt Technology, Santa Barbara, CA, USA), and a refractive index detector (Shodex RI-71, Showa Denko K.K., Tokyo, Japan), and was run with DMAc/LiCl 0.9% (w/v) as the eluent.

RESULTS AND DISCUSSION Though the organic solvents are fully miscible with water, isopropanol and THF phaseseparate with 4% aqueous NaOH: Mixing 1 mL of 1 mol/L NaOH with 9 mL of THF resulted in two clear liquid phases, the volume of the lower (aqueous) phase being about 200 µL. In the case of isopropanol, an even smaller aqueous phase occurred. The actual cationization and blank assays also contained 2% pulp. The swollen fibers impeded the observation of possible phase separations - the observation of Klemm et al.26 that a cellulose/NaOH/water phase is formed suggests that this may not have been the case at least for isopropanol. The desired cationization reaction and the main side product are shown in Figure 1.


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2,3-epoxypropyltrimethylammonium chloride

H2O

NaOH Cellulose

Reagent also used for starch cationization

NaOH Cellulose

Figure 1. Schematic of pulp cationization. Besides the desired reaction, the etherification of cellulose, 2,3-epoxypropyltrimethylammonium chloride also forms a diol (highlighted structure) upon reaction with water. If the actual reaction – as in this study – is followed by an acid wash with hydrochloric acid, the counter ion is changed to the shown chloride.

For the reactions done in isopropanol and THF, the success of the cationization was already evident during the workup. Additional charged groups should increase the hydrophilicity, and the washing of these materials with water was indeed more difficult than washing the respective blank material: their high swelling obviously impeded the filtration process. TC had a gel-like texture when swollen in water (see Figure 2) and was even impossible to filter; therefore, the material was precipitated with acetone for filtration. For all four cationized materials, their tendency to stick to glassware was another clue for successful modification. The effect of the cationization was also evident from the zeta potential which was negative for the starting material as well as for blank materials, and positive for the cationized materials. The starting



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material had a zeta potential of -30 mV, the blank materials between -22 and -28 mV, and the cationized materials between +10 to +20 mV.

Figure 2. Beaker with pulp cationized in THF and swollen in water, during workup. Image converted to grayscale and cropped with IrfanView 4.35, Irfan Skiljan, Wiener Neustadt, Austria.

Degree of substitution Both WC and DC had a DS of about 0.05, IC about three times more, and TC five to eight times more (see Figure 3). The fraction of the EPTMAC that formed a covalent bond to the starting material (see Figure 1), referred to here as reaction efficiency, increased almost tenfold when replacing 90% of the water with THF. The set of reactions was done twice. The results were quite similar, only the DS for TC showed some deviation, probably because of workup issues.


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13.3

0.3

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0.2

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Degree of substitution

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0.0 WC

DC

IC

TC

Figure 3. Cationization DS (from titration) for the first (open diamond) and second (red diamond) sample set. Because the same ratio of reagent to dry sample was applied in all assays, the DS directly correlates to the reaction efficiency. Preliminary tests had been conducted where bleached chemical pulps were cationized in water only by protocols similar to the one used for WC, applying 0.03-0.32 mmol epoxide reagent per mL of water. The results from these previous cationization assays (data not shown) led to the assumption that the reagent concentration might be critical for the resulting DS. In this study, this concentration was 0.37 mmol epoxide reagent per milliliter water for WC and 3.7 mmol mL-1 for DC, IC and TC. When using the data of both current and previous water-only experiments for a linear fit, the predicted DS for removing 90% of the water is close to the result we got for TC (see Figure 4 A), suggesting that THF does not cause any noticeable interfering reactions in this setup. The DS of IC and especially DC are considerably lower, additional side reactions seem to occur. The extrapolated concentration span was, however, a multiple of the range used for the regression, so two more points were added: “75% THF” and “50% THF” (see



experimental section) corresponding to 1.5 and 0.8 mmol reagent per milliliter water, respectively (see Figure 4 B). The DS values resulting from those two assays were as close to the extrapolated values as the DS of TC of set one. We concluded that THF behaved as an “inert” solvent in this reaction, while this was not the case for isopropanol and especially DMSO.

0.4

0.4

(B)

(A)

0.3



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TC

0.2 IC 0.1

0.3

TC 75% THF

0.2 50% THF

IC

0.1 DC

DC 0.0

0.0 0

1

2

3

4 -1

Reagent per water (mmol mL )

0

1

2

3

4 -1

Reagent per water (mmol mL )

Figure 4. DS versus amount of epoxide reagent (mmol per mL of water). (A) Data from first set of experiments combined with results from pre-tests. Linear fit from data of water-only experiments (0.03-0.37 mmol mL-1). (B) Both data sets plus earlier data, same linear fit.

Infrared spectroscopy The ATR-FTIR spectra showed the effect of cationization both directly and indirectly (see Figure 5). The substitution as such can be tracked: the stretching vibration of the C-N bonds18 of the substituent7 (see Figure 1) is found at 1480 cm-1. Cationization is a means of hydrophilization, which is visible in the absorption band of adsorbed water27 at 1640 cm-1 that increases roughly with the degree of substitution.


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0.20 Normalized absorption

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2

0.15

1 0.10 0.05 0.00 1800

1700

1600

1500

1400

1300

-1

Wavenumber (cm )

Figure 5. Cationization seen in ATR-FTIR. Black solid: starting material; blue short dash: WC; olive short dash dot: DC; magenta dash: IC; orange short dot: TC. The numbers denote bands that report cationization. 1640 cm-1 (1): adsorbed water, 1480 cm-1 (2): C-N stretching vibration.

Content of highly ordered areas Native celluloses, like those in the present pulp, are ordered in microfibrils to some extent. Though there is evidence that calling these ordered regions “crystals” is not necessarily correct,28 the established term “crystallinity” 24 may be understood as the fraction of anhydroglucose units that are situated in highly ordered regions as opposed to those with little order that are part of truly amorphous material for a given cellulosic sample. Independent of the scholarly question whether the highly ordered regions can be considered crystals or not, their content influences important properties such as the swelling of a cellulosic material. Bearing that in mind, ATRFTIR spectra were used to look into the content of highly ordered areas based on different crystallinity indices (CI) from literature.



A measure of “total crystallinity” can be defined as the ratio between the absorption band maxima at 1370 cm-1 and 2900 cm-1 as introduced by Nelson and O’Connor29, for details please refer to the supporting information available online. ‫ܫܥ‬஺்ோ =

‫ܪ‬ଵଷ଻଴ ൗ‫ܪ‬ ଶଽ଴଴

Figure 6 A shows that the averages of the CIATR values for the cationized samples did not differ significantly. The idea of this ratio is that the absorption at 2900 cm-1 is fairly constant while that at 1370 cm-1 has some variations. In our case, for those two bands, the variations were larger for the 2900 cm-1 value, thus causing a large standard uncertainty of this index; the standard deviations reported in this section are derived from values of 4-5 individual spots per material. Therefore, the absorptions at 1370 cm-1 were also directly compared (Figure 6 B). The total crystallinity seemed to be unaffected by all treatments.

(B) 0.10

(A)

TC

IC

DC

WC

TB

IB

0.00

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0.05

WB

TC

IC

DC

WC

TB

IB

DB

WB

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Start

H1370

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CI ATR

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Figure 6. Total crystallinity from ATR-FTIR. (A) Calculation of the CI as suggested by Nelson and O’Connor26 and (B) normalized absorption at 1370 cm-1 (H1370). The total crystallinity seems to be unaffected by the used treatments. Data from materials of two sets of experiments (black: first; red: second set), error bars: 2× standard deviation (SD).


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Another CI definition was introduced by O’Connor30, sometimes called the “lateral order index” (LOI), see Supporting Information for details on its determination. The idea behind it is that the band at 1430 cm-1 is a measure for cellulose I (typical crystal lattice of native cellulose, chains have parallel orientation), while the band at 900 cm-1 is stronger for cellulose II (cellulose crystal lattice after mercerization or regeneration from dissolved state, chains have antiparallel orientation) and amorphous cellulose. ‫= ܫܱܮ‬

‫ܪ‬ଵସଷ଴ ൗ‫ܪ‬ ଽ଴଴

For this index, larger differences were observed (see Figure 7). For IB, the ratio was clearly lower than for the other materials. In a purely aqueous system, NaOH with 0.1 mol/L or any concentration of sodium hydroxide not exceeding 1 mol per liter water would not affect the crystallinity of cellulose. However, NaOH in isopropanol-water mixtures is known to cause an alkalization effect similar to a much more concentrated NaOH in water alone,26 causing decrystallization and facilitating change of the crystal allomorph.31 The resulting material contains more amorphous material and possibly also some cellulose II. This is consistent with the observation that IB had a different texture when dry and did not re-suspend as readily in water. For the cationized materials, the measurement variations are larger; the values may be falsified by the C-N absorption band of the substituent at 1480 cm-1, being rather close to the band at 1430 cm-1, see Figure 5. They were therefore omitted in Figure 7, but may be found in the Supporting Information.



1.0 0.8 LOI

0.6 0.4

TB

IB

DB

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WB

0.2 Start

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Figure 7. Lateral order index (LOI). Data from materials of two sets of experiments (black: first; red: second), error bars: 2× SD. Crystallinity may also be estimated from NIR spectra32, see Supporting Information for details: ‫ܫܥ‬ேூோ =

‫ܪ‬ସ଻଺଴ ൗ(‫ܪ‬ ସ଻଺଴ + ‫ܪ‬ହଵ଼଻ )

Among the unmodified blank materials (Figure 8), the only one with a significantly lower CINIR was again IB. So, two out of three CI show a decrease for treatment with isopropanol. Two possible reasons, that are not mutually exclusive, are a real increase of amorphous material and the formation of some cellulose II as mentioned above. Solid-state NMR spectra (see Supporting Information) confirmed that IB contained more amorphous material and indicated the presence of some cellulose II. For TB, both the CINIR and the LOI were lower than for Start, WB, and DB but closer to those than to IB; this also matches the data from solid-state NMR. Therefore, TB also contained more amorphous material than the original pulp and a bit of cellulose II but is closer to the original pulp than to IB.


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All cationized materials had a lower CINIR than the materials after the blank reaction; the higher the DS, the lower CINIR. This is likely due to the larger amount of adsorbed water33. This index may thus be unsuitable for estimating crystallinity changes that accompany reactions that chemically alter the hydrophilicity of cellulosic materials. The CINIR values for the cationized materials were therefore omitted from Figure 8, but may be found in the Supporting Information. 0.5 0.4 CI NIR

0.3 0.2

TB

IB

DB

0.0

WB

0.1 Start

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. CI of the sample specimens determined from NIR32. Values from two sets of experiments (black: first, red: second), error bars: 2× SD.

Molar mass distribution When determining molar mass distributions by GPC with multi-angle laser light scattering (MALLS) detection, it is necessary to know the analyte’s refractive index increment (dn/dc)µ in the given system.34 For cellulose, the refractive index increment in the used system is 0.136 mL g-1.35 It is highly probable that this value is affected by cationization, at least to some minor degree. This is why only non-cationized materials were analyzed with GPC as the results for the cationized samples were dubious because of the unknown refractive index change. The derivatization in water, isopropanol/water and tetrahydrofuran/water did not affect cellulose 17 ACS Paragon Plus Environment


integrity. The only system that caused cellulose degradation was the DMSO-containing one. Here, the shoulder at around 30 kg mol-1 disappeared and the whole peak was significantly shifted towards lower molar mass values. Thus, the hemicelluloses may have been degraded or solubilized, and the main cellulose peak is shifted from about 400 to 160 kg mol-1 (see Figure 9). This disagrees with the results of Ren et al.19-20 who found less degradation when using DMSO as a solvent for cationizing hemicelluloses than when using a system with alcohol, although the setups were not the same (different reaction temperature, alkalinity etc.). Our molar mass determinations do, however, match the observation that compared to the starting material, both DC and DB dissolved much faster and gave a much less viscous solution, while the viscosity of

0.8 0.6 0.4 0.2 0.0 1

(B)

-1

(A)

Eluting concentration (µg mL )

all other samples remained high and undistinguishable from the starting samples.

log diff MW

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10

100

1000

10000

30

20

10

-1

Molar mass (kg mol )

0 25

30

35

Elution volume (mL)

Figure 9. Gel permeation chromatography of non-cationized materials. (A) molar mass distributions and (B) elution profile. Black solid: starting material; blue short dash: WB; olive short dash dot: DB; magenta dash: IB; orange short dot: TB. Only data for the first samples set shown, results were similar for the second batch.


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Comparison of different organic media As discussed above, the choice of organic solvent affected the DS (see Figure 3), but also other material properties. Concerning the DS, any competitive reaction of the solvent with the reagent is relevant as well as its behavior towards NaOH and water. In contrast to starch or hemicelluloses for which homogeneous modification occurs with some solvents, a homogeneous cationization of cellulose, although reported36, is not relevant for pulp when going beyond small lab scale. Isopropanol was chosen as co-solvent as it was expected for sterical reasons to be less reactive towards the epoxide than ethanol or methanol, and certainly less than water. In addition, the rate of mercerization reaction in isopropanol is higher than in ethanol or methanol.31 It was expected to improve the reaction efficiency also because of the above-discussed stronger alkylation effect in derivatizations of cellulose.26 This effect was confirmed and it was stronger than in the case of THF although the latter resulted in a higher DS. This matches the results of starch cationization in different solvents where THF proved more beneficial than ethanol.18 For both DC and DB, the reaction mixtures developed a distinct unpleasant cabbage odor. This indicated that some DMSO was reduced to dimethyl sulfide. The corresponding reaction is a Swern-type process that might occur as soon as DMSO is in contact with alkylating agents, in particular in alkaline media. The outcome is degradation (reduction) of DMSO, mainly to dimethyl sulfide, which goes hand in hand with the oxidation of a co-reactant, in this case cellulose.37 If this oxidation affects cellulosic hydroxyl groups in accessible regions, it is immediately followed by chain cleavage through beta elimination. If this process randomly happened for hemicelluloses and, for cellulose, in amorphous regions and along the surface of highly ordered regions, it did not necessarily change the content of highly ordered areas much



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(see Figure 6 to Figure 8) while still considerably reducing the average polysaccharide chain length (see Figure 9).

CONCLUSION Pulp can be cationized with the same quaternary ammonium epoxide reagent that is commonly used for starch. The success of the reaction is reflected in the reversion of the zeta potential which turned positive for all samples treated with the epoxide reagent. A partial replacement of water with organic solvents gave much higher degrees of substitution, and higher reaction efficiencies. The resulting materials are visibly more hydrophilic. THF proved to be particularly suitable in this regard. There was no noticeable effect on the highly ordered regions of the cellulose except for material treated in isopropanol - here some decrystallization and a partial conversion to cellulose II is supposed. There was no notable change in molar mass distribution apart from the case of DMSO which causes Swern-type side reactions and thus (hemi)cellulose degradation. Under otherwise constant conditions, the degree of substitution was linearly dependent on the reagent concentration. A higher concentration of reagent in water was simulated by partially replacing water with THF. The combination of those two observations is helpful to plan and control laboratory-scale cationization assays.

ASSOCIATED CONTENT Supporting information. Conductometric titration data analysis; Content of highly ordered areas by infrared spectroscopy and solid-state NMR. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (U. H.), [email protected] (T. R.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank the Flippr° project and all financial and scientific contributors to this project, including the Austrian Research Promotion Agency (FFG). Special thanks go to Melanie Mayr, TU Graz, for performing the zeta potential measurements. ABBREVIATIONS ASU, anhydrosugar unit; ATR-FTIR, attenuated total reflectance infrared spectroscopy; CI, crystallinity index; DMAc, N,N-dimethyl acetamide; DMSO, dimethyl sulfoxide; DS, degree of substitution; EPTMAC, 2,3-epoxypropyltrimethylammonium chloride; GPC, gel permeation chromatography; H1470, absorption maximum of the band around 1470 cm-1; LOI, lateral order index; NIR, near infrared; SD, standard deviation.

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