Ionic Liquid Solutions: From Dilute

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Biomacromolecules 2009, 10, 1188–1194

Rheological Properties of Cellulose/Ionic Liquid Solutions: From Dilute to Concentrated States Martin Gericke,†,‡ Kerstin Schlufter,†,§ Tim Liebert,‡ Thomas Heinze,‡ and Tatiana Budtova*,† Mines ParisTech, Centre de Mise en Forme des Mate`riaux - CEMEF, UMR CNRS/Ecole des Mines de Paris 7635, BP 207, 06904 Sophia-Antipolis, France, Centre of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstraβe 10, D-07743 Jena, Germany, and Research Centre for Medical Technology and Biotechnology GmbH, Geranienweg 7, D-99947 Bad Langensalza, Germany Received December 10, 2008; Revised Manuscript Received February 24, 2009

Steady state shear flow of different types of cellulose (microcrystalline, spruce sulfite and bacterial) dissolved in 1-ethyl-3-methylimidazolium acetate was studied in a large range of concentrations (0-15%) and temperatures (0-100 °C). Newtonian flow was recorded for all experimental conditions; these viscosity values were used for detailed viscosity-concentration and viscosity-temperature analysis. The exponent in the viscosity-concentration power law was found to be around 4 for temperatures from 0 to 40 °C, which is comparable with cellulose dissolved in other solvents, and around 2.5-3 for 60-100 °C. Intrinsic viscosities of all celluloses decreased with temperature, indicating a drop in solvent thermodynamic quality with heating. The data obtained can be reduced to a master plot of viscosity versus (concentration × intrinsic viscosity) for all celluloses studied in the whole temperature range. Mark-Houwink exponents were determined: they were lower than that for cellulose dissolved in LiCl/N,N-dimethylacetamide at 30 °C and close to θ-value. Viscosity-inverse temperature plots showed a concave shape that is dictated by solvent temperature dependence. The values of the activation energies calculated within Arrhenius approximation are in-line with those obtained for cellulose of comparable molecular weights in other solvents.

1. Introduction Dissolution of cellulose is of fundamental importance for its processing and chemical derivatization. In addition to classical solvents, for example, carbon disulfide or aqueous metal salt solution able to dissolve cellulose, some others were reported in the last decades, such as N-methylmorpholine N-oxide monohydrate (NMMO),1,2 LiCl/N,N-dimethylacetamide (DMAc),3-5 ammonium fluorides/dimethylsulfoxide (DMSO),6,7 molten salt hydrates, for example, LiClO4 · 3H2O and LiSCN · 2H2O,8 (7-9%)NaOH/water with or without urea or thiourea added,9,10 and mixtures of ammonia or ethylenediamine and thiocyanate salts.11,12 Although several of these solvents have been used successfully for making cellulose films and fibers and few as homogeneous reaction media for the preparation of cellulose derivatives, these solvents possess several undesired properties, like high toxicity, volatility, or high costs limiting their commercial application. Among the above-mentioned new cellulose solvents only NMMO is used on the industrial scale as a solvent for cellulose processing.1 Nevertheless, NMMO also has some disadvantages, such as the occurrence of oxidative side reactions, thermal instability, or rather high temperatures needed for the dissolution process. Thus, there is still a strong demand for new “green” cellulose solvents not only for making objects from regenerated cellulose but also for the chemical derivatization under homogeneous conditions. * Corresponding author. Tel.: +33(0)4 93 95 74 70. Fax: +33 (0)4 93 65 43 04. E-mail: [email protected]. † Centre de Mise en Forme des Mate`riaux - CEMEF; Member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu. ‡ Friedrich Schiller University of Jena; Member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu. § Research Centre for Medical Technology and Biotechnology GmbH.

In this context, ionic liquids (IL) have been suggested as promising cellulose solvents: cellulose can be dissolved in rather high concentrations (up to 15-20%) without any preactivation.13,14 The IL 1-butyl-3-methylimidazolium chloride (BMIMCl), 1-ethyl-3-methylimidazolium acetate (EMIMAc), and 1-allyl3-methylimidazolium chloride (AMIMCl) have been used for homogeneous esterification of cellulose as well as for the formation of films and fibers.15-18 Due to their ionic structure IL possess several advantages over common solvents, like the lack of any measurable vapor pressure, ease of recycling, good dissolution properties for a large variety of chemical compounds, and high thermal stability. Furthermore, the properties of IL may be tuned easily by slight modifications of the structure of anions or cations. The understanding of the rheological properties of cellulose/ IL solutions as well as the molecular organization of cellulose in these solvents is a crucial prerequisite for a successful processing and for chemical derivatization as well. Some viscoelastic properties of cellulose in several imidazolium-based IL have been reported in literature in the view of cellulose dissolution and solution spinning.18 Furthermore, some rheological properties of cellulose/AMIMCl solutions at polymer concentration below 3% were described recently.19 When BMIMCl was used as solvent, the dynamic shear experiments were performed on dissolving pulp solutions and the activation energy of wood pulp solutions was estimated.20-22 In this work we present a comprehensive investigation of the flow and molecular organization of different types of cellulose (microcrystalline cellulose (MC), spruce sulfite pulp (SSP), and bacterial cellulose (BC)), dissolved in EMIMAc at different temperatures and under anhydrous conditions. EMIMAc seems to be a promising candidate for cellulose derivatization and

10.1021/bm801430x CCC: $40.75  2009 American Chemical Society Published on Web 04/01/2009

Rheological Properties of Cellulose Liquid Solutions

Biomacromolecules, Vol. 10, No. 5, 2009

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Figure 1. Structure of ionic liquids used.

processing because it is liquid at room temperature and less toxic and corrosive than IL containing halogen anions. Cellulose concentration was varied in a wide range, from dilute (0.1%) to an almost concentrated state (15%), and solution temperature was from 0 to 100 °C. Some results on the MC dissolved in BMIMCl will also be shown for comparison.

2. Experimental Section 2.1. Materials. The ionic liquids EMIMAc (Lot S47223315081313, purity g 90%) and BMIMCl (Lot 122888552207047, purity g 98%), see Figure 1, were used as received from Fluka. The density of EMIMAc (1.1003 g/cm3) was determined with a pycnometer at 25 °C. Microcrystalline cellulose (MC) and spruce sulfite pulp (SSP) were purchased from Fluka. Their degree of polymerization (DP) was determined by means of viscometry in cupriethylenediamine hydroxide (Cuen) according to literature23

DP )

[η] for DP < 950; 0.42

DP )

[η] ( 2.28 )

1/ 0.76

for DP > 950 (1)

For MC and SSP we obtained [η]Cuen ) 125, DP ) 300 and [η]Cuen ) 435, DP ) 1000, respectively. The hemicellulose content in SSP, determined by HPLC after acidic hydrolysis, was found to be 9%. Bacterial cellulose (BC, [η]Cuen ) 1343, DP ) 4420, determined according to eq 1) was synthesized according to a known procedure:24 bacteria of the strain Gluconacetobacter xylinus (wild-type strain from the stock collection of the Research Centre for Medical Technology and Biotechnology, Germany) were cultivated in glass vessels containing Schramm-Hestrin medium in static culture at 30 °C. After 30 days, the cellulose layers were taken from the culture medium and cut into small pieces. After purification, the material was freeze-dried and milled.25 2.2. Methods. 2.2.1. Dissolution of Cellulose. All cellulose samples were dried at 100 °C in vacuum prior to use. Solvent and cellulose were mixed in a sealed reaction vessel and the mixtures were stirred at 80 °C for at least 48 h to ensure complete dissolution. Cellulose concentrations varied from 0.1-15%. Clear cellulose solutions were obtained; they were stored at room temperature and protected against moisture absorption. 2.2.2. Rheological Measurements. Rheological measurements were performed on a Bohlin Gemini rheometer equipped with plate-plate geometry and a Peltier temperature control system. Shear rates varied from 0.001 to 1000 s-1. Each measurement was performed with a cycle of increasing and afterward with decreasing shear rate in order to check reproducibility and the absence of thixotropy. Because the results were coinciding, all viscosity values were used for the further analysis. For the same loaded solution, the measurements were started at low temperature which was then increased after each shear cycle. The experimental errors in viscosity measurements were lower than 10%. Unless specified, the error bars of experimental points are of the size of symbols. It is well-known that ionic liquids are, in general, highly hygroscopic and water significantly decreases their viscosity. The presence of water in cellulose/IL solution decreases cellulose solubility and may lead to its precipitation. Although the measuring system used in this study holds only a small surface of the solution in contact with the surrounding air, a significant uptake of moisture was observed. Within 30 min of shear the viscosity of pure EMIMAc decreased by 22% due to “dilution”

Figure 2. Viscosity-shear rate dependence for microcrystalline and bacterial cellulose dissolved in EMIMAc (if not noted otherwise) and in BMIMCl at 40 °C (a) and for spruce sulfite pulp/EMIMAc solutions of different concentrations and temperatures (b).

with water. An even more complex behavior was observed for a 5%MC/ EMIMAc solution. During the first 2 min the viscosity remained nearly constant. Then a rapid nonregular viscosity increase was recorded. This behavior is due to the absorption of water and cellulose precipitation at the edge of the measuring cell that led to a “gluing” of solution on rotating plates and perturbation of the flow. To prevent an uptake of moisture, a thin film of low-viscosity silicon oil (η20 °C ) 9.5 mPa · s) was placed around the borders of the measuring cell. This method was found to be efficient for stable-in-time measurements.

3. Results and Discussion 3.1. Flow Curves. First, the flow of pure EMIMAc at all temperatures used in the following study of cellulose solutions was investigated. Between 0 and 100 °C the IL showed Newtonian behavior over the whole shear rate range applied (see example given in Figure 2a). Viscosity of pure BMIMCl was not measured because of the high melting temperature of 73 °C. The examples of the steady state viscosity(η)-shear rate(γ˙ ) dependence are presented in Figure 2a and b: for MC/EMIMAc, BC/EMIMAc, and MC/BMIMCl solutions of different concentrations at 40 °C (Figure 2a) and for SSP/EMIMAc of different concentrations at 20 and 100 °C (Figure 2b). For most of the solutions, a Newtonian behavior was observed over 2-3 decades of shear rates. At high viscosities and shear rates a very beginning of shear thinning was observed followed by some instabilities leading to solution ejection from the cell. All the

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following analysis was performed using only the data obtained in the Newtonian region. It is evident that the viscosity of cellulose/EMIMAc solutions decreases with increasing temperature, which is typical for classical polymer solutions and for cellulose dissolved in other solvents like NMMO26 or ionic liquids like BMIMCl.21 The viscosity increases with cellulose concentration (c) and molecular weight increase. While the calculation of the intrinsic viscosity and thus a detailed characterization of cellulose/ EMIMAc solutions will be given in the following section; this was not possible to do for cellulose dissolved in BMIMCl because the pure IL itself is solid up to 73 °C. The viscosity of cellulose/BMIMCl solutions is higher compared to EMIMAc solutions of the same cellulose concentration, and concentration increase leads to an increase in this difference (5 times for 1% and 25 times for 5%). In addition to the strong electrostatic interaction, the viscosity of pure IL is strongly affected by hydrogen and van der Waals bonding between anions and cations. It has been reported that stronger charge delocalization within the anion, as is expected for the acetate ion as compared with the chloride ion, can lead to weakening of molecular interactions and thus lower solution viscosities.27 This and the fact that BMIMCl is a solid at room temperature while EMIMAc is a liquid leads to the assumption that higher viscosity of cellulose/BMIMCl solutions is attributed to stronger interactions between anion and cation in BMIMCl. The influence of shearing at rather elevated temperatures on solution properties was investigated by comparing the viscosity at 20 °C of the same loaded sample before and after 0f20f40f60f80f100f20° cycle. The final viscosity at 20 °C was found to be lower compared to the initial one. For low concentrated cellulose solutions, the decrease was within the 10% experimental errors, while for higher concentrations the decrease was about 20-30%. To determine whether this decrease was induced by shear or by temperature, two tests were performed by separating these two inputs. First, the viscosity of a 10% SSP/EMIMAc solution was measured as a function of shear rate at 20 °C seven times in a row, imitating the duration the solution was under shear during the temperature cycle, as described above. Only small viscosity changes of (1-2% were observed. The second test was performed in the following way: 10% SSP/EMIMAc solution was loaded in the rheometer, its viscosity was measured at 20 °C, then the sample was heated and kept at 100 °C for 1 h, without shearing. Afterward the temperature was decreased to 20 °C and the viscosity measured again. Between the two measurements, the viscosity decreased by 30%. This means that the decrease in viscosity is not induced by shear, but due to keeping the solutions at elevated temperature for a rather long time. As far as cellulose degradation after keeping solution of SSP and cotton linters in BMIMCl at 80 °C have been reported,16 it can be concluded that in EMIMAc cellulose is also slightly degraded after being heated at 100 °C. 3.2. Viscosity-Concentration Dependence, Intrinsic Viscosity, and Mark-Houwink Constants. An example of the concentration dependence of MC and SSP solution viscosity at 20 and 100 °C is shown in Figure 3. Similar results were obtained for cellulose/EMIMAc solutions at other temperatures (data not shown). Two regions on each viscosity-concentration dependence can be seen: a linear one in dilute regime and powerlaw η ∼ cn above the overlap concentration c*. The exponent n was calculated for all cellulose/EMIMAc solutions; the values ranged from about 4 at low temperatures (0-40 °C) to 2.5-3 at high temperatures (60-100 °C); see examples of exact values

Gericke et al.

Figure 3. Viscosity as a function of cellulose concentration for solutions of microcrystalline cellulose and spruce sulfite pulp in EMIMAc at 20 and 100 °C; the slopes n are indicated for the concentrated region.

Figure 4. Relative viscosity as a function of cellulose concentration for MC/EMIMAc solutions at different temperatures.

in Figure 3. Comparable values were reported for cellulose/ LiCl/DMAc (n ) 3 for bacterial cellulose and n ) 4 for cotton linters and dissolving pulp).5 A slightly higher value of n ) 4.6 was reported for cellulose/NMMO solutions.26 The relative viscosity ηrel ) η/η0 (where η and η0 are solution and solvent viscosities, respectively) of MC/EMIMAc solutions at all temperatures used is presented in Figure 4. This plot allows excluding solvent input in the viscosity of the solutions. At low concentrations all data are converging to ηrel ) 1. At higher concentrations, the influence of temperature on cellulose chains is more pronounced (ηrel 0 °C/ ηrel 100 °C ) 1.5 at c ) 1% and ηrel 0 °C/ηrel 100 °C ) 15 at c ) 10%). Similar results were obtained for SSP and BC. A reasonable explanation might be a decrease of the thermodynamic solvent quality of EMIMAc with temperature increase, which will be proved using intrinsic viscosity results. The intrinsic viscosity [η] was determined from (ηrel - 1)/c versus c plots for dilute cellulose solutions studied at different temperatures. Concentrations were recalculated in mL/g, taking into account the density of EMIMAc at 25 °C. It is acceptable to use this density value for all temperatures studied because the density of imidazolium-based ionic liquids changes within 5% for temperatures varied within 100 °C.28-30 This error is lower than the one accumulated in the rheological experiments and is not inducing any noticeable changes to intrinsic viscosity



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Figure 5. Intrinsic viscosity of all celluloses studied dissolved in EMIMAc as a function of temperature. Table 1. Comparison of Cellulose Intrinsic Viscosity in Different Solvents cellulose solvent EMIMAc Cuen LiCl/DMAc3,5 9%NaOH/H2O10 cadoxene33

DP

type

300 1000 300 1000 771 1120 230 310 3310

MC SSP MC SSP not specified not specified MC hydrolyzed cotton linters

temperature, intrinsic °C viscosity, mL/g 20 20 20 20 30 30 20 25

101 178 125 435 148 226 120 147 908

values. The results are presented in Figure 5 and Table 1. The intrinsic viscosity strongly decreases with increasing temperature, for all samples investigated. This is a direct indication of a decrease in the thermodynamic quality of EMIMAc. A similar temperature influence on the size of cellulose chains was observed for cellulose dissolved in 9%NaOH/water.10 However, cellulose/NaOH/water solutions are known to gel with temperature increase when polymer concentration exceeds the overlap concentration.10 Gelation is attributed to the decrease of NaOH/ water solvent thermodynamic quality which leads to preferential cellulose-cellulose interactions and a microphase separation upon heating,31 which is another proof of solvent quality decrease. However, no gelation or phase separation was observed for celluloses dissolved in EMIMAc. If this was the case, the solution flow should not be Newtonian over the large range of shear rates (see Figure 2). As far as the intrinsic viscosity decreases with temperature, the overlap concentration c* ) 1/[η] increases with temperature: for example, for microcrystalline cellulose it varies from c*0 °C ) 0.0077 g/mL (0.7%) to c*100 °C ) 0.0280 g/mL (2.6%), and for bacterial cellulose it varies from c*0 °C ) 0.0026 g/mL (0.24%) to c*100 °C ) 0.0055 g/mL (0.50%). The decrease of EMIMAc thermodynamic quality with temperature increase seems to be in contradiction to literature,32 where it has been reported that elevated temperatures are needed to obtain homogeneous cellulose/IL solutions of high concentrations. The amount of cellulose dissolved in AMIMCl and N-ethyl-N′-methylimidazolium salts increased with increasing temperature.32 This apparent discrepancy can be explainable when kinetic aspects are considered. Temperature increase strongly favors the dissolution velocity by decreasing solvent viscosity and increasing the diffusion of macromolecules and

Figure 6. Master plot of relative viscosity vs c[η] of microcrystalline cellulose, spruce sulfite pulp, and bacterial cellulose dissolved in EMIMAc, at temperatures from 0 to 100 °C and at concentrations from dilute to 15% (a) and only in dilute region (b).

solvent. Thus, the dissolution of cellulose in IL at low temperatures might be too slow to obtain the equilibrium state within a short time. It is interesting to compare the obtained intrinsic viscosities of cellulose/IL solutions with the ones in other solvents, like in cupriethylenediamine hydroxide (Cuen; the same cellulose samples), in LiCl/DMAc,3,5 in 9%NaOH/water,10 and in cadoxene.33 The results are shown in Table 1. The overall trend is that the IL used is thermodynamically not a better solvent for cellulose as compared with the cited above. This finding is important for cellulose processing: for example, the mechanical properties of a fiber or a film are determined by the state of the macromolecule in solution from which the object is made. It has to be noted that a distinct comparison can be made only for the same celluloses (here: dissolved in EMIMAc and Cuen); other cellulose samples were studied in refs 3, 5, and 10 and DP determination strongly depends on the method and approach used. From a thermodynamic point of view, Cuen is a better cellulose solvent because the intrinsic viscosity of both samples in Cuen is higher than in EMIMAc. The results on LiCl/DMAc and 9%NaOH/water were taken from literature, and cellulose DP was not determined in the same way. The comparison of intrinsic viscosities has thus to be made with care. However, contrary to the general point of view that ionic liquids are “magic” cellulose solvents, cellulose-in-EMIMAc hydrodynamic volume, which is directly proportional to the intrinsic viscosity, according to Flory theory, is not higher than the ones in other solvents. Figure 6a presents a master plot of relative viscosity as a function of c[η] for all cellulose samples in the whole temper-

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Figure 7. Intrinsic viscosity vs molecular weight for celluloses studied at different temperatures. Lines are approximations according to Mark-Houwink equation. Crosses and the corresponding approximation (dashed line) are data for cellulose in LiCl/DMAc at 30 °C, taken from literature.3,5

ature and concentration range used in this study. With this plot the effect of macromolecule hydrodynamic volume on the viscosity of a polymer solution can be reduced.5,34 Data presented in this graph superimpose quite well. A zoom at dilute region is given in Figure 6b. According to Flory-Huggins approach, the slope of ηrel versus c[η] should be 1. The best fit shows that the slope is 1.3. Similar results were obtained for other polysaccharides, such as dextran, alginate, and carboxymethylamylose, and attributed to macromolecule agglomeration in solution.34 One of the main equations describing molecular properties of a polymer is Mark-Houwink equation which is correlating the intrinsic viscosity and molecular weight M: [η] ) K · MR, where R and K are adjustable constants. This dependence is shown in Figure 7 for the three cellulose samples used in the present study. The exponent R varies between 0.4 and 0.6. This means that EMIMAc seems to be a theta solvent for cellulose, which should be proved by a more detailed study and with other methods. For cellulose dissolved in LiCl/DMAc, higher R values were reported in literature (see crosses in Figure 7): R ) 1.2 (data from Table 3 in ref 3) and 0.7 (data from Table 1 of ref 5), which reflects a good thermodynamic solvent quality. In general, R varies from 0.65 to 0.95 for cellulose solutions depending on solvent type, temperature, sample polydispersity, and molecular weight.33,35-37 Radius of gyration Rg can be estimated in a rough approximation using Flory approach for flexible polymer chains: Rg2 ) (1/6)[([η]/Φ)M]2/3, where Φ ) 2.8 × 1023 mol is Flory constant. Rg varies from 7.5 to 11 nm for MC (DP 300), from 15 to 22 nm for SSP (DP 1000), and from 30 to 40 nm for BC (DP 4420) for temperatures from 0 to 100 °C, respectively. If making the same estimation for cellulose dissolved in other solvents, slightly higher values are obtained: for example, Rg varies from 15 to 60 nm for cellulose of DP from 770 to 4320 (from different sources: pulp, rayon, cotton, no information about impurities)3 dissolved in 9%LiCL/DMAc with intrinsic viscosities varying from 148 to 1164 cm3/g; Rg ) 16 nm for cellulose of DP 390 (high purity cotton linters degraded using oxidation with periodate followed by subsequent oxidative treatment with Cl)37 dissolved in iron-sodiumtartrate with [η] ) 285 mL/g, and Rg from 28 to 49 nm for cellulose of DP from 1300 to 3310

Gericke et al.

Figure 8. Viscosity-temperature dependence of 1% solutions of MC, SSP, and BC dissolved in EMIMAc and of 1% MC in BMIMCl and pure EMIMAc. Lines are given to guide the eye.

(hydrolyzed cotton linters) dissolved in cadoxene33 with intrinsic viscosities from 447 to 908 mL/g. Here again the results of the comparison have to be taken with care because the way of cellulose DP measurement and calculation are not the same from article to article. It is important to note that more than twice larger Rg values obtained with light scattering technique were reported in the references cited above: Rg ) 40-140 nm (ref 3), Rg ) 37 nm (ref 37), and Rg ) 45-70 nm (ref 33). The size of the swollen macromolecule strongly depends not only on the direct characteristics like molecular weight, solvent quality, and, in the case of cellulose, on the presence of impurities that are very rarely reported, but also on the method how it was measured and calculated.33 An adequate comparison is thus rather difficult. What we want to show here is that (a) our results are consistent with the ones reported in literature, (b) the radius of gyration of cellulose macromolecule swollen in EMIMAc calculated with a very rough approximation is slightly smaller than the ones reported in literature for cellulose dissolved in thermodynamically good solvents and calculated with the same approximation, and (c) more studies using other experimental methods (various scattering techniques, velocity sedimentation, etc.) are needed to make quantitative conclusions. 3.3. Viscosity-Temperature Dependence and Activation Energy. An example of 1% cellulose solutions (MC, SSP, and BC) in EMIMAC, of 1%MC/BMIMCl and pure EMIMAc viscosity as a function of temperature is presented in Figure 8. Viscosity decreases by 3 orders of magnitude with temperature increase from 0 to 100 °C. A steeper decrease can be seen for cellulose dissolved in BMIMCl, which is probably the consequence of solvent temperature dependence. However, this was not possible to evaluate within the temperature range used because BMIMCl is solid up to about 73 °C. A common way to analyze viscosity-temperature dependence of fluids is to use Arrhenius equation η ) η0 exp(Ea/RT), where Ea is the activation energy, R is the universal gas constant, and η0 is an adjustable parameter. The activation energy is usually deduced from the slope of ln(η) versus inverse temperature if this dependence is linear. The Arrhenius plots for MC/EMIMAc solutions at various concentrations and for 2% SSP/EMIMAc solution as an example are presented in Figure 9. It is possible, in general, to approximate each set of experimental data with a linear dependence, as shown with a dashed line for 15% solution of the microcrystalline cellulose, with an adequate accuracy (R2 g 0.98). However, a close look at the results shows that all


Figure 9. Arrhenius plot for MC/EMIMAc solutions at concentrations from 0 to 15% and of 2% SSP/EMIMAc. Dashed line corresponds to Arrhenius approximation. Solid lines are VTF approximations.


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Figure 11. Activation energy as a function cellulose concentration for MC, SSP, and BC dissolved in EMIMAc. Table 2. Activation Energies for Celluloses Dissolved in Different Solvents cellulose solvent EMIMAc

DP

type

concentration, activation % energy, kJ/mol

300 MC 1000 SSP

BMIMCl20,21 NMMO26,40 9%NaOH/H2O10 Figure 10. Same data as in Figure 9 but for relative viscosities. Lines represent least-squares linear approximations.

data should be approximated by a concave dependence. Pure EMIMAc shows the same behavior. Moreover, it is EMIMAc that is dictating this specific temperature dependence because it disappears if plotting relative viscosity as a function of inverse temperature (Figure 10). A nonlinear ln(η)-inverse temperature dependence has been reported for several ionic liquids.27,38,39 For several ionic liquids, including some imidazolium based ones, it has been reported that a reasonably accurate description of viscosity-temperature dependence is provided by the Vogel-Fulcher-Tamman (VFT) equation, characteristic for “glass-forming” liquids: η0 ) BT · exp(k/T - T0), where k, B, and T0 are adjustable parameters.27,39 T0 was determined to be around -100 to -90 °C for cellulose solutions studied. For all cellulose/IL solutions as well as for pure EMIMAc the VFT equation was found to describe the viscosity-temperature dependence with a very high accuracy (R2 > 0.9999), see Figure 9. While this equation can be used for predicting cellulose viscosity at any temperature which is very valuable for processing, the physical understanding of the coefficients obtained and their dependence on polymer concentration needs a more detailed study. Despite the fact that ln(η) versus 1/T is not exactly linear, activation energies were calculated in the first approximation for all cellulose solutions studied to compare with literature. The results are presented in Figure 11. The activation energy monotonously increases with concentration increase and for a given concentration Ea is higher for higher molecular weight. The values are of the same order of magnitude as those for cellulose of similar concentrations dissolved in other solvents,

4420 BC 670 dissolving pulp 800 wood pulp 600 not specified 950 solucell 230 MC

3 15 3 10 1 10

46 70 49 61 56 100

1 15 3 3

42 90 57 21

like in NMMO,26,40 BMIMCl,20-22 and are higher than in 9%NaOH/water,10 as shown in Table 2. This means that the overall properties of cellulose-EMIMAc solutions are comparable with the ones of cellulose dissolved in the solvents cited above.

4. Conclusions Steady state shear flow results for three types of cellulose: microcrystalline cellulose, spruce sulfite pulp, and bacterial cellulose, dissolved in 1-ethyl-3-methylimidazolium acetate (EMIMAc) and 1-butyl-3-methylimidazolium chloride (BMIMCl), were obtained for concentrations from dilute to semidilute state and at temperatures from 0 to 100 °C. For all celluloses studied, a Newtonian plateau was recorded over at least two decades of shear rates. These viscosity values were used for the analysis of viscosity-concentration and viscosity-temperature dependences of cellulose-EMIMAc solutions. Above the overlap concentration, the exponent in the viscosity versus concentration power law is around 4 at lower temperatures (0-40 °C) and around 2.5-3 at higher temperatures (60 to 100 °C). It was found that intrinsic viscosity decreases with temperature increase, which was attributed to a decrease of solvent thermodynamic quality. The intrinsic viscosity values are similar and slightly smaller than the ones for cellulose dissolved in other solvents (Cuen). Using the values of the intrinsic viscosities obtained, Mark-Houwink constants were calculated: the exponent varies from 0.4 to 0.6 depending on temperature and indicating that EMIMAc is close to theta solvent for cellulose. Viscosity-inverse temperature dependence has a concave shape and can be well described with the Vogel-Fulcher-Tamman equation. The Arrhenius values of the activation energy increase

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with cellulose concentration and are similar to the ones for cellulose solutions in NMMO and BMIMCl. Acknowledgment. The work was performed within exchange in the frame of the “European Polysaccharide Network of Excellence” (EPNOE), Project No. NMP3-CT-2005-500375. M.G. and T.L. thank the “Fachagentur fu¨r nachwachsende Rohstoffe e.V.” (Project 2202190) for the financial support. K.S. is grateful for grants from the European Union, LEONARDO program. T.B. thanks P. Navard (Mines ParisTech, Cemef) for helpful discussions.

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Ionic Liquid Solutions: From Dilute

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