Rheological Properties and Gelation of Aqueous Cellulose−NaOH

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Rheological Properties and Gelation of Aqueous Cellulose-NaOH Solutions Ce´ dric Roy,†,§ Tatiana Budtova,*,†,‡ and Patrick Navard*,† Ecole des Mines de Paris, Centre de Mise en Forme des Mate´ riaux, UMR CNRS/Ecole des Mines No 7635, BP 207, 06904 Sophia-Antipolis, France, Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoi prosp.31, 199004 St.-Petersburg, Russia, and Spontex, Centre de Recherche, 74 rue St-Just-des-Marais, 60026 Beauvais, France Received August 30, 2002

The shear rheology of a microcrystalline cellulose dissolved in a 9% NaOH aqueous solution was studied in the steady and oscillatory modes. The cellulose-(9% NaOH-H2O) mixtures show not to be true solutions. In the dilute regime, with cellulose concentration below 1%, the rheological behavior is typical of the one of suspensions. The formation of cellulose aggregates is favored when temperature is increased. In the semidilute regime, an irreversible aggregate-based gelation occurs, being faster with increasing temperature. Introduction Because of its fundamental and technological importance, the dissolution and processing of cellulose had attracted an enormous amount of work in the past, and it is an active field of research in our days. Several solvents in which cellulose can swell and dissolve were found, such as N-methylmorpholine oxide, ammonia/ammonium thiocyanate, paraformaldehyde/dimethyl sulfoxide, and LiCl/ dimethyl acetamide. For most of these systems, the flow properties, mainly shear rheology, have been investigated (see, for example, refs 1-7). In most cases, the results show a shear thinning regime and the evidence of gelation of semidilute solutions. The influence of the shear rate, temperature, cellulose molecular weight, and concentration have been discussed. In addition to the above cited cellulose solvents are alkali solutions in which dissolution can take place in a narrow range of the phase diagram, leading to the so-called celluloseQ. A comprehensive work on the cellulose behavior in aqueous sodium hydroxide solutions showed that the maximal solubility of low-to-moderate DP cellulose occurs in 8-10% soda aqueous solutions.8-11 A mechanism of cellulose dissolution was proposed,10 and the structure of cellulose-soda solutions was discussed.8,12,13 However, to our knowledge, the rheological properties of these solutions have not been yet investigated. In addition, there is a need to see whether these solutions are gelling and under which conditions. The objective of this paper is to investigate the shear steady-state and oscillatory rheology of a micro* To whom the correspondence should be addressed. Mailing address: Ecole des Mines, Cemef, BP 207, 06904 Sophia-Antipolis, France. E-mail addresses: [email protected]; [email protected]. Tel: +33 (0)4 93 95 74 66; +33 (0)4 93 95 74 70. Fax: +33 (0)4 92 38 97 52. † Ecole des Mines de Paris. ‡ Russian Academy of Sciences. § Spontex, Centre de Recherche.

crystalline cellulose in 9% NaOH aqueous solutions as a function of temperature and cellulose concentration. Experimental Section Materials. The cellulose sample used in our study was Avicel PH-101 microcrystalline cellulose, called “cellulose” in the following, purchased from FMC Corporation. It is a purified, partially depolymerized R-cellulose derived from special grades of wood with a mean degree of polymerization of 230, as given by the manufacturer. NaOH was supplied by Prolabo; 9% NaOH-H2O solutions were prepared by direct mixing of soda and demineralized water. All concentrations are given in weight percent. Preparation of Aqueous Cellulose-9% NaOH Solutions. To get a better mixture homogenization and to decrease the amount of undissolved cellulose, a two-stage process was used. First, a certain amount of dry cellulose was immersed in a 6% soda solution at -6 °C, and the mixture was stirred at 11 000 min-1 for 3 min using Ultra TurraxT25 mixer made by IKA Labortechnik. Then a certain amount of 15% soda solution was added to this mixture to reach a total soda concentration of 9% and a desired cellulose concentration, which was varied from 0.1% to 5%. The final mixture was stirred again at 11 000 min-1 for 3 min at -6 °C. Solutions were always kept in refrigerator in closed vessels to avoid oxygen-induced degradation. Methods. Rheological experiments on semidilute cellulose solutions were performed using a stress-controlled StressTech rheometer, manufactured by Reologica Instruments. The steady-state shear flow and dynamic rheology of cellulose solutions were studied in a Couette cell geometry (two concentric cylinders with a gap of 2 mm). The temperature was varied from 0 to 30 °C and controlled within (1 °C using a thermobath with a circulating water and cooling liquid (propylene glycol). The solutions were heated or

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cooled to desired temperature directly in the rheometer (without shearing or oscillating), and then either steady-state or dynamic measurements were performed. The experimental errors in viscosity and dynamic modulus measurements were less than 10%. Steady-state shear flow experiments were performed in a classical way. Solutions of different cellulose concentrations (from 0.2% to 5%) in 9% NaOH-H2O solutions were prepared, and viscosity-shear rate dependencies were measured at several temperatures. Dynamic viscosity experiments were performed in two ways. In the first set, the frequency sweeps from 0.01 to 10 s-1 were performed to follow the changes in the elastic (storage), G′, and viscous (loss), G′′, moduli of 5% cellulose-9% NaOH solutions with time at 20 °C. Each frequency sweep took 690 s to be completed. It was conducted at a constant strain of 0.01 and repeated every hour during 26 h. In the second set, time evolution of the dynamic moduli was measured for 5% cellulose-9% NaOH solution at a fixed frequency, ω ) 1 rad/s, in the temperature range of 10-30 °C. This frequency is less than the inverse relaxation time for this cellulose concentration, as determined from the steady-state measurements. The temperature range was chosen to cover the cases when the gel point can be reached within a few minutes at 30 °C up to tens of hours around 10 °C. The viscosity of dilute cellulose solutions was measured in an Ubbelohde capillary viscometer. The temperature was controlled within (0.2 °C in the interval of 10-40 °C using a thermobath with a circulating water. The experimental error in the determination of the relative viscosity was less than 2% and that of the intrinsic viscosity less than 5%. Cellulose degree of polymerization was measured using a standard procedure.14 Cellulose was precipitated from selected cellulose-9% NaOH solutions and then dissolved in Cuam solution. Its specific viscosity, ηsp, was measured, and DP was calculated as follows: DP ) ηsp/(K(C(1 + 0.25ηsp - ηsp2/150)))

(1)

where C is the cellulose concentration in g/L and K ) 7.5 × 10-4. Results and Discussion Steady-State Flow of Dilute and Semidilute Cellulose9% NaOH Aqueous Solutions. First of all, to determine cellulose-9% NaOH flow regimes and the influence of temperature and cellulose concentration on solution shear flow, the viscosity was measured as a function of the shear rate at several temperatures and cellulose concentrations. From this first check, it was found that the flow of semidilute solutions can be studied only below 20 °C because above 20 °C gelation occurs during the experiment. The flow of dilute solutions was investigated in the 0-40 °C temperature interval. The dependencies of the viscosity on the shear rate of 0.2% and 3% solutions at several temperatures are presented in

Figure 1. Viscosity-shear-rate dependence of 0.2% cellulose-9% NaOH solutions at (1) 0, (2) 5, (3) 10, (4) 15, (5) 20, and (6) 25 °C and of 3% cellulose-9% NaOH solutions at (7) 0, (8) 5, (9) 10, (10) 15, (11) 20, and (12) 25 °C.

Figure 1. Dilute solutions, below 0.8-1.2% (the value of the critical overlap concentration depending on temperature, as it will be shown in the following), flow like Newtonian fluids. Semidilute solutions, below 5%, show a Newtonian plateau and a shear-thinning regime. Above 5%, in the studied shear-rate range, the Newtonian plateau disappears. The flow index, n, of viscosity-shear-rate curves (η ∼ γn) in the shear-thinning region monotonically increases with temperature from 0.09 to 0.12 for the 3% cellulose solution and from 0.13 to 0.17 for the 5% cellulose solution. These values are 4-5 times smaller than the ones of either typical “normal” polymer solutions (experimental data or theoretical predictions both for flexible and for rigid chain polymers) or, in particular, polysaccharides such as guar gum, λcarrageenan, hyaluronate (see, for example, ref 15), or entangled physical networks.16 The viscosity-shear-rate dependencies of cellulose-9% NaOH solutions do not also obey the empirical law found by Morris et al.17,18 for several polysaccharides. As shown in the previous paragraphs, the steady-state shear rheology of cellulose-9% NaOH solutions strongly departs from the cases described in the literature for polymer and polysaccharide solutions. The flow of cellulose-9% NaOH solutions is close to the one of suspensions with a flow index lower than 0.5 (see, for example, ref 19). Indeed, as shown in ref 20, the dynamic and static light scattering studies of cellulose-NMMO, viscose, and several cellulose derivative solutions demonstrated that cellulose chains can aggregate in solutions. Fringed micelles as a model of cellulose aggregates were proposed.20 Thus the very low flow index value of cellulose-9% NaOH solutions is an indirect proof that cellulose in 9% NaOH aqueous solution is also organized in micelles. The activation energy was determined from the plot of ln η0 (η0 being a zero-shear-rate viscosity taken from viscosityshear-rate curves) as a function of the inverse temperature, 1/T, according to Arrhenius law (see Figure 2). Probably because of being close to the gelation point at T ) 30 and 25 °C, the 3% and 5% solutions show a deviation from the straight line (see lines 7 and 8, correspondingly). The slopes

Aqueous Cellulose-NaOH Solutions

Figure 2. Arrhenius plot for cellulose-9% NaOH solutions of cellulose concentrations (1) 0% (pure 9% NaOH), (2) 0.2%, (3) 0.5%, (4) 1%, (5) 1.5%, (6) 2%, (7) 3%, and (8) 5%. The lines correspond to mean-square linear approximations.

for all cellulose concentrations are very similar, giving the mean activation energy of 21 ( 4 kJ/M. This value is practically the same as that for a pure 9% NaOH aqueous solution, 19 kJ/M (see line 1 in Figure 2). Being surprising from the first sight, the independence of the activation energy on the cellulose concentration is in good agreement with the model of the structure of cellulose-soda solutions previously proposed13 and supports the fact that these mixtures are not real solutions. Indeed, it was shown that cellulose-NaOH-H2O solution is composed of (i) free water that freezes much below zero and melts at a temperature of -1 to -14 °C, depending on NaOH concentration, (ii) soda hydrates that are composed of a “core” of 9H2O bound to NaOH (in the form of a eutectic mixture 7H2O + NaOH, 2H2O) that can crystallize and melt at -35 °C surrounded by a “shell” of amorphous water molecules that do not melt at -35 °C, and (iii) soda hydrates bound to cellulose that do not freeze down to -60 °C. From one to two hydroxyl groups per glucopyranose unit are bound to a soda hydroxide. As shown in ref 13, the amount of free water is the same in both NaOH-H2O and cellulose-NaOH-H2O solutions and the amount of amorphous water molecules depends only on soda concentration, which in our case is constant. Because of this structure of cellulose-NaOH solutions, a change in cellulose concentration influences only the absolute values of solution viscosity but not the total activation energy of the system. This structure is such that, with or without cellulose, it is a suspension of hydrates in water.13 From the rheological point of view, the addition of cellulose just changes the composition of inclusions but not the composition of the suspending medium. This may explain why the activation energy of cellulose-9% NaOH is the same as the one of the pure solvent, 9% NaOH aqueous solution. The influence of temperature on the aggregation of cellulose macromolecules in NaOH solutions can be analyzed from the dependence of the relative viscosity on cellulose concentration. Relative viscosity is chosen to exclude the

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Figure 3. Relative viscosity of cellulose-9% NaOH solutions as a function of cellulose concentration at 0, 5, 10, 12.5, 15, and 20 °C.

Figure 4. Relative viscosity of cellulose-9% NaOH solutions as a function of cellulose concentration for dilute concentrations at (1) 0, 5, 10, 12.5, 15, and 20 °C and (2) 25; (3) 30, and (4) 40 °C.

temperature influence on the solvent (9% NaOH solution). All results obtained on both capillary and rotational (at zero shear rates) viscometers are shown in Figure 3. For dilute and semidilute concentrations, the relative viscosity is plotted only in the temperature interval 0-20 °C. The results at 2540 °C are excluded because of a quick gelation of solutions above cellulose concentration of 1.5% (see further paragraphs on gelation kinetics in the section on dynamic rheology). All data fall on a master plot demonstrating a typical viscosity-concentration power law. Here, in the interval of 0-20 °C, the temperature practically does not influence the relative viscosity. The temperature influence on the relative viscosity of cellulose-9% NaOH solutions in the interval of 0-40 °C is clearly seen in Figure 4 in which only dilute concentrations are shown. Below 20 °C, all data fall on one line within experimental errors (line 1). The temperature increase from 25 to 40 °C leads to a noticeable relative viscosity decrease (lines 2-4, correspondingly). An important feature is that the temperature increase leads to the irreversible viscosity changes. For example, if performing a temperature “cycle” such as measuring the solution viscosity at 30 °C, then decreasing temperature to 20 °C, and increasing back to 30 °C, the final relative viscosity value is the same as at the beginning. If performing an “opposite cycle”, 20-30-20

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Figure 5. Temperature dependence of the intrinsic viscosity of cellulose-9% NaOH solutions.

°C, the final viscosity is always 10-15% lower than at the beginning. This irreversible change occurs at temperatures above 25 °C. To study the temperature influence at the molecular level, intrinsic viscosities, [η], of cellulose in 9% NaOH aqueous solutions were obtained as a function of temperature (see Figure 5). Below 20 °C, [η] does not depend on temperature. Above 20 °C, [η] starts to decrease. As a consequence, the overlap concentration, C*, which is a critical parameter for gelation, is also temperature-dependent: C* ) 0.83% for cellulose-9% NaOH below 20 °C and C* ) 1.25% for cellulose-9% NaOH at 40 °C. The decrease in the intrinsic viscosity with the temperature increase is not the consequence of a gelation process, as it will be shown in the second part of this paper. Gelation occurs in semidilute solutions above a critical overlap concentration, and in this case, the viscosity increases because of the cluster formation. The relative and intrinsic viscosity decrease with the temperature increase were obtained for dilute concentrations (Figures 4 and 5) at which there is no gelation phenomenon. Two main reasons could be at the origin of this phenomenon: cellulose degradation or chain conformation change. To check whether there is any cellulose degradation with time or temperature or both, the degree of polymerization in different conditions (cellulose dry powder, cellulose-9% NaOH fresh solution, and the same solution stored for 24 h in a refrigerator at 5 °C and for 24 h at 25 °C) was measured. No DP dependence on the cellulose solution storage time and temperature was observed. Thus degradation cannot be the reason for the viscosity decrease with temperature increase in dilute region. The viscosity decrease can be explained in following way. It is known that cellulose can be dissolved in aqueous NaOH solutions only at low temperatures (see, for example, ref 14), in our case at -6 °C. It is also known that it is the hydrophobic association that is at the origin of conformation changes and gelation in methylcellulose (see, for example, refs 21 and 22). Thus the most probable explanation for the viscosity decrease and for the irreversible changes with temperature increase in the cellulose-9% NaOH solutions is the increasing role of the hydrophobic interactions with temperature increase. We assume that two mechanisms

Figure 6. Frequency dependence of G′ and G′′ of 5% cellulose-9% NaOH for gelation times of (a) 1 h (G′ (1), G′′(2)) and 3 h (G′ (3), G′′ (4)) and (b) 13 h (G′ (1), G′′(2)) and 26 h (G′ (3), G′′ (4)). Lines are given to guide the eyes.

leading to the intrinsic viscosity decrease have to be considered. On one side, cellulose agglomerates become more and more compact because of the interchain hydrophobic interactions with temperature increase. On the other side, because of interagglomerate interactions, new compact agglomerates are formed with the hydrodynamic volume smaller than the additive sum of volumes of the initial agglomerates (like formation of interpolymer complexes between oppositely charged macromolecules). In the case of a 20-30-20 °C “temperature cycle”, the viscosity at the last 20 °C is not equal to the initial 20 °C viscosity because temperature decrease cannot completely destroy the formed hydrophobic bonds. However, a wide temperature- and timedependent hysteresis must be also kept in mind, and the “irreversibility” phenomenon has to be studied in detail by varying experimental conditions. Dynamic Rheology of 5% Cellulose-9% NaOH Solutions. This section describes the influence of time and temperature on the gelation of 5% cellulose-9% NaOH solution using oscillatory rheology. The examples of the dynamic moduli frequency dependence at 20 °C for some selected aging times (1, 3, 13, and 26 h) are shown in Figure 6a,b. After 1 h (Figure 6a, curves 1 and 2), the sample behaves as a viscous fluid: the elastic modulus, G′ (curve 1), is smaller than the loss modulus, G′′ (curve 2), and has a steeper slope. With time, the difference between the elastic and loss moduli becomes smaller, which is a sign that gelation takes place. With a further time increase, the elastic

Aqueous Cellulose-NaOH Solutions

Figure 7. G′ and G′′ of 5% cellulose-9% NaOH as a function of time at 20 °C (G′(1), G′′(2)) and at 25 °C (G′(3), G′′(4)).

modulus becomes flatter than the loss modulus, less frequencydependent, and higher than the loss modulus, showing that the material is past its gel point (see Figure 6b, curves 1 (G′) and 2 (G′′) corresponding to 13 h and curves 3 (G′) and 4 (G′′) corresponding to 26 h). According to the gelation theory of Winter and Chambon,23 G′ ∼ ωa and G′′ ∼ ωb and the gel point is reached when a ) b ) 0.5 and the final gel state is reached when a ) 0 and b ) 1. The development of the slopes of G′ ) f(ω) and G′′ ) f(ω) curves in time was examined. In our case, it turned out that while exponent a decreases with time reaching the value of 0.2 at 26 h, which indicates a slow approach to the case when G′ ) const, the exponent b also decreases in time and becomes equal to 0.7 after 26 h. This means that within this frequency interval, experimental data cannot be analyzed using the Winter and Chambon approach. In particular, it is not possible to use the condition of a ) b for the determination of a gel point. Probably, slow relaxation times are involved, and thus, the frequency interval should be extended into lower values (a few additional decades), which in our case is impossible because of low moduli values. The influence of temperature on the gelation process of the 5% cellulose-9% NaOH solution was studied by measuring the time evolution of the dynamic moduli at different temperatures from 10 to 30 °C at ω ) 1 rad/s, as shown in Figure 7. Whatever is the temperature, the gelation of semidilute cellulose-NaOH solutions takes place, being faster at higher temperatures. The latter shows that cellulose9% NaOH solutions behave in a similar way as aqueous methylcellulose solutions with a lower critical solution temperature and temperature-increase-induced gelation. However, contrary to methylcellulose solutions, gels formed from cellulose-9% NaOH solutions are not thermoreversible: after cooling to any temperature including the temperature of cellulose dissolution (-6 °C), they do not dissolve. The absence of the thermoreversibility of aggregation was already indicated for dilute solutions (see the section on the steadystate flow), for which the relative viscosity after heating above 25 °C and then cooling never recovered its initial value. Which mechanism is responsible for the irreversible

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Figure 8. Evolution of the elastic modulus of 5% cellulose-9% NaOH with time at different temperatures: (1) 13, (2) 20, (3) 25, and (4) 30 °C.

Figure 9. Gelation time, tgel, of 5% cellulose-9% NaOH as a function of temperature, T. The straight line corresponds to a tgel ≈ exp(0.4T °C) fit.

changes upon heating is not clear yet. If this should be just hydrophobic interactions, as it is for methylcellulose, thermoinduced gelation should be reversible, even with a large time hysteresis. In the case of cellulose-9% NaOH solutions, a gel formed at 30 °C does not dissolve either at room temperature or at -6 °C and after 3 weeks it became even more solid. The development of the elastic modulus as a function of time (Figure 8) shows that G′ does not reach a constant value, whatever is the gelation time. This is not typical for “normal” gelling systems in which the final gel state is characterized by G′ ) const (see, for example, ref 16). Probably, gelation of cellulose-9% NaOH solution is accompanied by other processes, for example, a micro-phase separation. A similar behavior was observed in gelling methylcellulose solutions.22 The dependence of the time needed to reach the gel point, tgel, and of the modulus value, Ggel, when G′ ) G′′ ) Ggel(tgel), on temperature is presented in Figures 9 and 10, correspondingly. It is known that for gels that are formed upon cooling, tgel or the incubation time (time needed for the development of a measurable increase in the storage modulus) increases with a temperature increase (see, for example, ref 24). From

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and hydrophobic association may lead to gelation, each being more or less pronounced at a certain temperature. Because cellulose-9% NaOH gels are slightly turbid, a phase separation with a local chain segregation could be a possible mechanism governing thermo-induced changes. Contrary to methylcellulose solutions, gelation is not reversible: the gel is not dissolved either upon cooling or even at the initial temperature of cellulose dissolution in 9% NaOH at -6 °C. Further investigations using other tools (scattering techniques, NMR, etc.) will be requested to better understand the mechanisms involved in this process. Acknowledgment. We are grateful to Professor Dominique Durand (Le Mans University, France) for the helpful discussions. The work was sponsored by Spontex, France. Figure 10. Modulus at the gel point, Ggel, versus temperature for 5% cellulose-9% NaOH.

Figure 9, it is clear that tgel exponentially decreases with a temperature increase. The reason is that the cellulose + 9% NaOH system has a lower critical solution temperature and in such cases heating induces gelation or phase separation or both. Thus the higher temperature is, less time is needed to form a gel. The modulus value at the gel point, Ggel, is independent of temperature in the studied range within the experimental errors (Figure 10). In general, for systems with a lower critical solution temperature like aqueous methylcellulose solutions, the temperature increase leads to either a Ggel and G′ increase (in a large temperature interval) or at least a constant modulus (in a narrow temperature interval at low temperatures).22,25 The independence of Ggel on temperature for cellulose-9% NaOH may be due to the relatively low temperatures studied and their narrow range. Conclusions The influence of temperature and time on the cellulose9% NaOH solution structure is as follows. The solution is formed of cellulose aggregates, which can be evidenced during the flow of semidilute solution: in the shear-thinning regime, the flow index is similar to that of suspensions but not to that of polymer solutions. In the dilute region, the increasing role of inter- and intrachain hydrophobic interactions with temperature increase (above 20 °C) leads to aggregate compactization. In semidilute solutions, time and temperature increases lead to gelation similar to the behavior of some aqueous solutions of cellulose derivatives (methylcellulose) with a lower critical solution temperature. Elastic modulus increases and gelation time exponentially decreases with a temperature increase. It is not possible to describe the mechanism of the cellulose-9% NaOH gelation now. Both hydrogen bonding

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