Influence of the Supramolecular Structure and Physicochemical

Biomacromolecules 2005, 6, 2638-2647

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Influence of the Supramolecular Structure and Physicochemical Properties of Cellulose on Its Dissolution in a Lithium Chloride/ N,N-Dimethylacetamide Solvent System Ludmila A. Ramos,† Jose´ M. Assaf,‡ Omar A. El Seoud,*,§ and Elisabete Frollini*,† Instituto de Quı´mica de Saõ Carlos, Universidade de Saõ Paulo, C P 780, 13 560 970, Saõ Carlos, SP, Brazil, Departamento de Engenharia Quı´mica, Universidade Federal de Saõ CarlossUFSCar, Rodovia Washington Luı´s, Km 235, Saõ Carlos, SP, Brazil, and Instituto de Quı´mica, Universidade de Saõ Paulo, C.P. 26.077, 05513-970, Saõ Paulo, SP, Brazil Received December 1, 2004; Revised Manuscript Received May 11, 2005

The present work deals with the effects of structural variables of celluloses on their dissolution in the solvent system LiCl/N,N-dimethylacetamide, LiCl/DMAc. Celluloses from fast growing sources (sisal and linters), as well as microcrystalline cellulose (avicel PH-101) were studied. The following structural variables were investigated: index of crystallinity, Ic; crystallite size; polymer porosity; and degree of polymerization determined by viscosity, DPv. Mercerization of fibrous celluloses was found to decrease DPv, Ic, the specific surface area, and the ratio pore volume/radius. The relevance of the structural properties of cellulose to its dissolution is discussed. Rate constants and activation parameters of cellulose decrystallization, prior to its solubilization, have been determined under nonisothermal conditions. The kinetic parameters calculated showed that dissolution is accompanied with small, negative enthalpy and a large, negative entropy of activation. Introduction Cellulose is the most important renewable and biodegradable macromolecule. Nevertheless, several problems must be solved to increase its competitiveness with petroleumbased, synthetic polymers.1 Cellulose may be isolated from several sources including plants, marine organisms (e.g., Tunicate), and bacteria (e.g., Acetobacter xylinum).2 Its main source, however, is wood, which makes it a slowly regenerated raw material, considering the time required before a tree may be cut to produce cellulose.3 The use of cellulose from fast-growing lignocellulosic sources, such as cotton, sisal, and sugar cane,4-9 and the use of hemicellulose and lignin,10,11 the other major components of lignocellulosic sources, improves economic competitiveness.1 The present work focuses on the derivatization of celluloses obtained from fast-growing sources, namely, linters and sisal. Cellulose derivatives, in particular, its esters, are industrially prepared by two-phase processes, in which (solid) cellulose reacts with the (liquid) reaction medium. The amorphous regions, being more accessible, react faster than the (much less accessible) crystalline ones.12 The scale of production, and the increased importance of these products, has prompted intense research aimed at eliminating some of the drawbacks of the heterogeneous process.1 A promising approach is to derivatize cellulose under homogeneous reaction conditions. When carried out properly, this method solves many of the classical problems associated with the * Corresponding authors. E-mail: (E.F.) [email protected] and (O.A.E.S.) [email protected]. † Instituto de Quı´mica de Sa ˜ o Carlos, Universidade de Saõ Paulo. ‡ Universidade Federal de Sa ˜ o CarlossUFSCar. § Instituto de Quı´mica, Universidade de Sa ˜ o Paulo.

two-phase reaction, especially polymer degradation and product heterogeneity. It must be pointed out that depending on the reaction time interval and temperature, in some cases cellulose can be degraded in homogeneous13 as well as in heterogeneous media.14-16 Several solvent systems dissolve cellulose, a process that may or may not lead to cellulose derivative formation. Examples of nonderivatizing solvent systems are those based on electrolytes in strongly dipolar aprotic solvents; LiCl/N,Ndimethylacetamide, LiCl/DMAc, has been extensively studied and employed for cellulose derivatization under homogeneous reaction conditions.5-9,12,17-32 Recently, two new solvent systems have been introduced, namely, tetra(n-butyl)ammonium fluoride hydrate/DMSO33 and the room-temperature ionic liquid 1-allyl-3-methylimidazolium chloride.34 The advantages of cellulose derivatization under homogeneous reaction conditions include the following: control of the degree of substitution, DS, and regularity of substitution, both along the polymer backbone and among the three OH groups of the anhydro-glucose unit, AGU.5 The former advantage (i.e., DS control) contrasts sharply with derivatization under heterogeneous reaction conditions, where it is unfeasible to obtain directly derivatives with DS, say between 1 and 2, due to the large differences of accessibility between amorphous and crystalline regions of the polymer.12,17 The homogeneous reaction scheme is, however, complex and multistep, including cellulose activation, dissolution, and subsequent reaction with the derivatizing agent. Therefore, a clear understanding of these steps is a prerequisite for process optimization. The understanding of the characteristics related to the dissolution of cellulose in these solvent systems is important

10.1021/bm0400776 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005

Lithium Chloride/N,N-Dimethylacetamide

both when cellulose solutions are meant to be used in light scattering measurements, size exclusion chromatography analysis, for example, and when cellulose will be derivatized after its dissolution. In the present work, the final goal is to prepare cellulose esters using LiCl/DMAc as a solvent system. As may be expected, cellulose dissolution and the reactivity of dissolved cellulose depend on its structural characteristics; we dwell here on its degree of polymerization, DP, Ic, and the presence of micropores in its structure.35,36 For example, Isogai et al.37 have shown that the dissolution of several celluloses in NaOH solution depends on their DPv, Ic, and the corresponding supramolecular structure. Low DPv and the absence of a nonfibrous structure contribute to the ease of dissolution of microcrystalline cellulose, whereas Ic plays a minor role. Considering the LiCl/DMAc solvent system, the literature describes some results in which both the solubility and the reactivity of cellulose increase as Ic decreases.30,35 However, low crystallinity Kraft pulp dissolved more slowly than high crystalline linters.38 Matsumoto et al.39 found that low crystallinity celluloses only dissolved in this solvent system after cellulose activation by solvent exchange. Probably, for these celluloses, other structural parameters than those already mentioned as well as the ones discussed next were more important than crystallinity for their dissolution in LiCl/DMAc. Additionally, cellulose possesses micropores,36 normally identified as void spaces, within the microfibrils and lamellas, as well as between elementary fibrils, microfibrils, and lamellas. Any physical interaction (e.g., absorption of water vapor) or chemical reaction with cellulose involves these pinholes. Therefore, their study is relevant to the chemistry of cellulose and its derivatization. El Seoud et al.7 identified these pores during the dissolution process of sugar cane bagasse cellulose in LiCl/DMAc. In general, the influence of this property on the capacity of dissolution of celluloses from different sources has been little considered in works found in the literature on this subject. Another manifestation of the aforementioned structure solubilization relationship is the observation that dissolution of linters is difficult, unless it is previously mercerized (i.e., treated with an alkaline solution).6 In the mercerization process, there is the irreversible conversion of cellulose I into cellulose II. The different conformations of the hydroxymethyl group (-CH2OH) can be assumed to generate two different packing structures of cellulose chains in a microcrystal. The parallel chain structure, characteristic of cellulose I, occurs when the -CH2OH groups of the adjacent chains have the same configuration. The antiparallel structure, characteristic of cellulose II, occurs when the adjacent chains have the -CH2OH groups in different positions.40 The diffusion of alkali into the crystalline domains normally contributes to the reduction of cellulose crystallinity, which favors the dissolution process. In the dissolution conditions considered in the present work, including time intervals, linters also dissolved only after mercerization. One of our targets was to search a process with low energy cost, that is, short total time intervals, including the time necessary for cellulose deriva-

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tization (results not shown in the present paper), which is the next step after dissolution. Dissolution/derivatization rates are expected to be dependent on the cellulose source and whether it has been subjected to pretreatment (e.g., mercerization). Thus, a better understanding of the dissolution step is a prerequisite for optimizing process conditions. The following question has been addressed in the present work: can dissolution of untreated and mercerized celluloses in LiCl/DMAc be related to their physicochemical properties? Three samples have been examined: microcrystalline cellulose, avicel PH-101, and two fibrous samples with different characteristics, namely, linters and sisal cellulose. The starting materials have widely different physicochemical properties (property, values for avicel, cotton linters, and sisal cellulose, respectively): DPv, 126, 400, and 642; Ic, 0.83, 0.80, and 0.67; and (medium) length of the crystallites, 4.5, 5.9, and 3.9 nm, respectively. The two fibrous celluloses were submitted to mercerization. The superficial areas and the average pore sizes were measured by gas adsorption and, where appropriate, mercury intrusion porosimetry (MIP). Kinetics of polymer dissolution in LiCl/DMAc was studied under nonisothermal conditions. To our knowledge, this is the first comparative study on the influence of the supramolecular structural properties of the starting celluloses on their dissolution in LiCl/DMAc. Experimental Procedures Materials. Avicel PH-101 microcrystalline cellulose (FMC Inc., Philadelphia, PA), hereafter designated as avicel, was used as received, and the fibrous sisal cellulose (hereafter designated as sisal) was kindly supplied by Lwarcel (Lenç oís Paulista, SP, Brazil). The low DPv cotton linters (hereafter designated as linters) were obtained from Fibra SA (Americana, SP, Brazil). The fibrous samples were ground in a Wiley MA 048 cutting mill (Marconi, Piracicaba, Brazil) against a 10-mesh stainless steel sieve. DMAc (Vetec) was distilled from CaH2 (Aldrich) and kept over activated 4A molecular sieves. Lithium chloride (Mallinckrodt) was dried at 200 °C for 3 h, cooled under reduced pressure, and kept in tightly capped bottles. Mercerization of cotton linters and/or sisal was carried out as follows: 20 g of cellulose was suspended in 1 L of a 20% NaOH (Merck) solution for 2 h at 0 °C. The slurry was filtered in a sintered-glass funnel, and the solid was washed with water until the filtrate was neutral and air-dried. Before use, all cellulose samples were dried under reduced pressure (60 mm Hg) at 60 °C for 2 h. These samples are hereafter designated as mercerized linters and mercerized sisal, respectively. Characterization of Celluloses. Degree of Polymerization (DPv) and r-Cellulose Content. DPv was determined (25 °C) from the intrinsic viscosity of cellulose solution in CUEN/water (1:1, v/v) according to Tappi T230 om-89,41 by employing an Ostwald shear-dilution Cannon Fenske viscosimeter (Shop Lab, Saõ Paulo, Brazil) coupled to a thermostatic bath and circulator (Masterline Forma Scientific, Marietta, OH). The R-cellulose content was determined from

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the dry masses of cellulose before and after treatment with 17.5% NaOH solution (1:20, w/v).42 Duplicate runs were carried out. Determination of Porosity by Nitrogen Adsorption and Mercury Intrusion Porosimetry. Surface areas, pore volumes, and diameters (up to 150 nm) of untreated and mercerized linters and sisal celluloses were determined from nitrogen adsorption isotherms by the BET method (see Calculations), using a Nova 2000 apparatus (Quantachrome Corporation, Bayton Beach, FL). We have employed the MIP technique and model 9320 Poresizer (Micrometrics, Norcross, GA) to measure the diameters of macropores (>50 nm) of avicel. The samples studied by MIP were prepared as follows: an aliquot of avicel slurry in LiCl/DMAc was withdrawn, filtered (sintered-glass funnel), washed with ca. 20 mL of LiCl/DMAc and then with ca. 100 mL of methanol, and dried to a constant weight, first in air (100 °C) and then under reduced pressure (60 mm Hg, 60 °C). It was not possible to apply the MIP technique to study the macropores of untreated sisal. The reason is that the sample was wetted by mercury at the end of the analysis. That is, mercury was not being extruded from the pores, leading to unacceptable errors in the measurement. X-ray Diffraction. This technique was employed to determine Ic and crystallite size (see Calculations) of untreated and mercerized celluloses. The data were recorded with a Zeiss-Jena URD-6 universal diffractometer operating at 40 kV, 20 mA, and λ(CuKR) ) 0.154 nm. Scanning Electron Microscopy, SEM. SEM micrographs were obtained with a Leo 440 microscope operating at 15 kV and using a tungsten filament electron source. Dried cellulose samples were sputtered with gold (20 µm thick) and scanned at room temperature. Cellulose Dissolution. The procedure of dissolution used was established in a previous work.7 It is worth mentioning that the organic solvent used (DMAc) can be recycled with a high recovery rate. Several trial runs led to the following optimized procedure: a 250 mL four-necked round-bottomed flask was equipped with a stopcock, cylindrical funnel (without equilibration sidearm), mechanical stirrer, and condenser closed with a stopper. To the flask was added 4.0 g of cellulose and 10.0 g of LiCl. The flask was connected to a vacuum pump through the stopcock and immersed in an oil bath, whose temperature was externally controlled (FE50RP controller, Flyever, Saõ Carlos, Saõ Paulo, Brazil). The pressure was reduced to 2 mm Hg, and the system was heated from room temperature to 110 °C (3 °C/min) and kept under these conditions for 30 min to ensure complete water removal. The vacuum pump was turned off, and 120 mL of DMAc was slowly added. The system was then brought to atmospheric pressure with dry oxygen-free nitrogen, and the condenser was provided with a drying tube. The temperature was raised to 150 °C (4 °C/min), and the cellulose slurry was vigorously stirred for either 60 min (avicel) or 90 min (linters and/or sisal). The mixture was slowly cooled to 40 °C at a rate of 1 °C/min. After reaching 150 °C, and during the cooling period, 10 mL aliquots of cellulose slurry were withdrawn from the

Ramos et al. Table 1. Average Degree of Polymerization (DPv), % R-cellulose, Crystallinity Index (Ic) and Crystallite Size (L002 and L101h ) of Cellulosesa cellulose

DPv

avicel cotton linters mercerized linters sisal mercerized sisal

126 400 377 642 544

R-cellulose content (%)

Ic

L002 (nm)

L101h

91.0 92.0 89.0 97.0

0.83 0.80 0.71 0.67 0.65

4.5 5.9 3.6 3.9 2.7

39 34

a

The uncertainty in the properties measured are 3%, 0.5, 0.02, 0.2 , and 0.3 nm for DPv, R-cellulose content, Ic, L002, and L101h , respectively.

reactor. The time and bath temperature were recorded. The undissolved cellulose of each aliquot was quickly filtered by suction into a sintered-glass funnel, washed with DMAc (15 mL) and methanol (100 mL), and dried first in air (110 °C) and then under reduced pressure (60 mm Hg, 60 °C) until constant weight. The samples obtained as given in the preceding paragraph were employed for determination of the following properties: morphological structures by SEM; Ic by X-ray diffraction; and porosity of avicel by MIP. Results and Discussion Physicochemical Properties of Untreated and Mercerized Celluloses: Degree of Polymerization, r-Cellulose content, Index of Crystallinity, and Dimensions of the Crystallites. Table 1 shows the physicochemical properties of untreated celluloses, as well as samples mercerized for 2 h. Destruction of intermolecular hydrogen bonding between cellulose chains is of central importance to dissolution. Therefore, the same rationale employed to explain the dissolution of celluloses in NaOH37 is expected to hold for the present case. That is, the combined effects of small DPv and short crystallite length of nonfibrous avicel (Table 1) results in the formation of small, organized domains, so that DPv is the important factor for dissolution. The high-DPv chains of fibrous celluloses may be localized in the noncrystalline regions and also belong to several crystallites, forming long organized domains whose dissolution requires destruction of the long-range structure. Consequently, for sisal and linters, other factors (e.g., Ic, crystallite dimensions, and porosity (discussed later)) control their dissolution in addition to DPv. Comparison of the appropriate characteristics before and after mercerization of celluloses, Table 1, shows that (i) this pretreatment affects the DPv of linters much less than that of sisal. The R-cellulose content of the latter is much lower than that of linters, due to the presence of hemicellulose that is more sensible to alkali-hydrolysis than cellulose since the first mentioned polysaccharide is more accessible to alkali because the chains are distributed in noncrystalline regions. Removal of hemicelluloses contributes to the change of DPv (i.e., the larger decrease of the latter property in case of sisal is not due solely to cellulose degradation during mercerization); (ii) on the basis of X-ray crystallographic results (figures not shown), mercerization resulted in rearrangement of the crystal packing from cellulose I (22° e 2θ e 23°) to



Figure 1. SEM images of celluloses (A) avicel (×1000), (B) avicel (×5000), (C) untreated sisal (×5000), (D) mercerized sisal (×5000), (E) untreated linters (×5000), and (F) mercerized linters (×5000). Table 2. Surface Areas and Pore Volume of Avicel, Sisal, and Linters Celluloses cellulose

surface area BET (m2/g)

pore total volume (cm3/g × 10-4)

linters mercerized linters sisal mercerized sisal

2.8 1.0 13.2 1.4

33 17 78 22

cellulose II (18° e 2θ e 22°).35,43 This conclusion agrees with the observed decrease in both Ic and (average) crystallite size (Table 1) and explains, at least partially, the ready dissolution of linters after mercerization; and (iii) sisal crystallites are smaller than those of linters (i.e., their dissolution does not require pretreatment). It may still be advantageous, however, to include a mercerization step before the use of sisal, to upgrade its quality as a dissolving pulp. SEM was employed to evaluate the morphological structures of the celluloses studied. Figure 1 shows the SEM micrographs of avicel at two magnifications, panels A and B, as well as untreated and mercerized celluloses, panels C-F, for linters and sisal, respectively. The micrographs of avicel clearly show its nonfibrous nature and the presence of pinholes at its surface. Panels C-F show that mercerization leads to removal of fragments from the surface and thinning of the fibers. These morphological changes are more pronounced for sisal than for linters, probably due to removal of the hemicellulose, as argued elsewhere for wood fiber.44 For both fibrous celluloses, some slits can be also seen at higher magnification (figures not shown). The next property studied was the porosity of the celluloses. Table 2 shows the total surface area and pore volume; both properties are larger for sisal than for linters. Mercerization decreases the surface areas and total pore sizes of

both fibrous celluloses, again more noticeably for sisal. Removal of cellulose fragments from the surface (Figure 1) and thinning of the fibers result in surface area decrease, whereas pore structure collapse results from the reorganization of intramolecular H-bonding that accompanies cellulose I f cellulose II transformation.43 The higher susceptibility of sisal may be traced to easier cellulose chain reorganization because of the elimination of hemicellulose, which changed from 11% (untreated sample) to 3% (mercerized sample, Table 1). Figure 2 shows the pore size distribution (see Experimental Procedures) curves for the celluloses studied. The pores of avicel are much larger in diameter (between 6 × 102 and 2 × 103 nm) than fibrous celluloses (Figure 2A). Native fibrous celluloses have both intrafibrillar pores, ranging from 2 to 7 nm, resulting from inefficient packing of the cellulosic microfibrils, and larger pores, up to 150 nm, present between the H-bonded fibrillar network that constitutes the polymer fibers.45 The intrafibrillar pores have diameters of ca. 3-5 nm and 3-6 nm for linters and sisal, respectively. Mercerization leads to a decrease in the volume/radius ratios of the pores and more homogeneous pore size distribution curves for both celluloses. Similar changes of pore size and distribution have been observed for other native and regenerated celluloses, respectively.46 We now address the relevance of these results to cellulose dissolution. At the outset, it may be argued that the sizes of these pinholes are much larger than those of typical substances that react with cellulose. This structural property, however, is relevant to solvent penetration into cellulose (i.e., to its dissolution) because (i) the LiCl/(DMAc)n complex is voluminous, with n ) 5.3 for the carefully dried solvent system47 and (ii) these micropores may not be readily accessible because under the conditions employed (3.3% cellulose and 8.3% LiCl), the cellulose chains are most certainly present as fringed micelles, whose aggregation numbers increase as a function of increasing the DP of cellulose.47,48 Both factors are expected to lead to a poresize dependence of polymer-solvent interactions, hence ease of dissolution. Figure 2A clearly shows an important reason for the ease of dissolution of avicel. Its larger pore diameter leads to unobstructed diffusion of the solvated LiCl/DMAc complex, required to disrupt intermolecular H-bonding of polymer chains. Sisal has pore diameters in the range of 2-150 nm (Figure 2C), with most of them close to 3 and 6 nm. The presence of macropores in sisal (diameters >50 nm) contrasts with the presence of smaller pores (mesopores range) in linter and may be responsible for the dissolution of nonmercerized sisal. Mercerization of sisal leads to the disappearance of pores in the range of 15-150 nm and a significant drop of surface area (Figure 2C and Table 2). As the pores present in fibrous cellulose are void spaces between elementary fibrils, microfibrils, and lamellas, it may be concluded that mercerization leads to a reorganization of these spaces, which in turn may lead to differences in the number and sizes of these pores. The effect of mercerization on cellulose solubilization is, therefore, 2-fold, namely, reduction of Ic and increase of

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Figure 2. Pore size distribution curves obtained by (A) Hg intrusion porosimetry (avicel), (B) N2 adsorption (untreated and mercerized linters), and (C) N2 adsorption (untreated and mercerized sisal).

Figure 3. SEM images of avicel cellulose withdrawn from the dissolution medium: (A) at the beginning at 150 °C, (B) after 60 min from the start, sample withdrawn at 150 °C, and (C) after 130 min from the start, sample withdrawn at 78 °C (×500).

the homogeneity of pore size. The latter may be important because of the known mechanism of interaction of LiCl/ DMAc complex with cellulose. Namely, it follows the naturally preformed pinhole structures of the fiber, leading to much less swelling than that observed for other solvent systems (e.g., iron sodium tartarate).25 That is, penetration of the polymer by the solvated LiCl/DMAc complex may be more regular (i.e., more efficient) for mercerized sisal because its pinhole structure is more regular in diameter than its native counterpart. A similar line of reasoning applies for the effect of mercerization on dissolution of linters. Considering SEM results presented by avicel cellulose (pores of 2 µm or 2 × 103 nm, Figure 1), it was assumed that most probably this cellulose has macropores. Thus, the mercury porosimetry method (MIP), which allows the determination of macropore size distribution, was applied to this sample, as will be discussed later. Cellulose Dissolution. Morphology Analysis by SEM. The first steps of cellulose dissolution involve penetration of the solvated LiCl/DMAc complex into the amorphous regions and decrystallization of the crystalline domains. Both

processes are accompanied by particle (avicel) or fiber disintegration (linters, sisal). These stages may be visualized by examining the morphologies of undissolved cellulose samples with SEM. Figures 3 and 4 show the effects of contact time (with LiCl/DMAc) on avicel and mercerized sisal, respectively. Figure 3A shows no disintegration for the avicel sample withdrawn from the reactor when the slurry temperature reached 150 °C. Disintegration can be observed during the activation step, Figure 3B, and advances as a function of increasing contact time and decreasing temperature, Figure 3C,D. The temperature effect on cellulose dissolution will be discussed next. The behavior of sisal is similar to that of avicel (Figure 4), except that contact with the solvent system results in a tenuous swelling, Figure 4B, followed by the beginning of disintegration of the fiber bundles, Figure 4C. Similar micrographs (not shown) were obtained for mercerized linters. Note that untreated linters did not dissolve completely under our experimental conditions, even after 24 h from the beginning of the dissolution step (i.e., when the other celluloses have produced clear solutions).



Figure 4. SEM images of mercerized sisal withdrawn from the dissolution medium: (A) at the beginning, 150 °C, (B) after 90 min from the start, sample withdrawn at 150 °C, and (C) after 160 min from the start, sample withdrawn at 78 °C (×500).

Figure 5. Pore size distribution curves for avicel slurry in LiCl/DMAc. The samples were withdrawn after 60 min from the start, and the samples were withdrawn at 150 °C (A). After 130 min from the start, the samples were withdrawn at 78 °C (B).

Effect of Contact with the Solvent System on Pore Size Distribution. MIP was used to probe the effect of contact with the solvent on pore size distribution of avicel, the only cellulose for which this technique can be employed, vide supra. Comparison of the curves obtained, Figure 5, with that shown in Figure 2A reveals that the porosity of avicel noticeably increased during dissolution. This result, along with those of SEM (Figure 3), shows that contact with the solvent system leads to formation of new pores (larger and smaller than the ones observed before contact with the solvent) through which the solvated LiCl/DMAc complex penetrates the polymer leading, subsequently, to particle disintegration. Kinetics of Dissolution Process. Cellulose dissolution may be probed by monitoring the dependence of Ic of undissolved cellulose on the contact time with solvent. This measurement is a reliable indicator of the transformation of crystalline cellulose into a (hypothetical) highly elastic state. We were interested in determining the dependence of observed rate constants (kobs) and activation parameters (enthalpy, ∆Hq, and entropy, ∆Sq) of this step on the cellulose employed. This experiment has been carried out under nonisothermal conditions, and the quantities of interest were calculated from the dependence of Ic of (undissolved) cellulose on the temperature, T, and contact time, t, as shown in Figure 6 for avicel, sisal, mercerized sisal, and linters (see Calculations). Ic of untreated linters did not change as a function of temperature decrease (figure not shown), probably because of its low solubility in LiCl/DMAc. Figure 6 shows that Ic values of all celluloses investigated were constant at 150 °C and decreased as a function of

decreasing T. The former result indicates that solvent penetration during polymer activation is limited to the amorphous regions, in agreement with previous interpretations. Note also that the strength of the complex between cellulose and the solvent system decreases as a function of increasing T.49 The latter result can be explained by considering the different stages involved in cellulose dissolution, a multi-stage process that can be divided into (i) transition of the solid polymer to a hypothetical, highly elastic liquid state; this corresponds to disintegration of the crystalline regions (∆Hfusion) and transition of the amorphous regions from a vitreous to a highly elastic state (∆Htransition); (ii) solvation of the polymer macromolecules (∆Hinteraction); and (iii) mixing of solvated polymer molecules with solvent to give an infinitely diluted solution (∆Hmixing).50,51 The total enthalpy of cellulose dissolution is given by ∆Hsolution ) ∆Hfusion+ ∆Htransition + ∆Hinteraction + ∆Hmixing (1) The only endothermic term of eq 1 is ∆Hfusion, which is associated with breaking the H-bonds in crystalline regions. The other terms are exothermic and related to interactions between cellulose hydroxyl groups and the solvent system. Therefore, the overall process of cellulose dissolution is exothermic and is favored by lower temperatures, in agreement with Figure 6.50,52 The decrystallization process can be treated as a pseudo-first-order reaction (see Calculations). Rate constants and activation parameters obtained for cellulose decrystallization are listed in Tables 3 and 4, respectively. At first glance, the results obtained (Tables 3 and 4) are surprising because the rate constants and activation parameters calculated are only slightly dependent on the physicochemical properties (DPv and Ic) of the starting celluloses. Additionally, decrystallization is associated with a small negative ∆Hq and a large, negative ∆Sq. It should be kept in mind that ∆Hq calculated refers to the difference between an initial state in which the solvent system is not present and a high-energy state, which, in the present work, refers to the state that precedes the total disintegration of the crystalline system. In the corresponding activated complex, part of the crystallite starts to disintegrate due to the cellulose/ LiCl/DMAc interactions, at the expense of breaking the cellulose/cellulose hydrogen bonds. It is also important to remember that, although the decrystallization step of cellulose is endothermic, the global dissolution process is exothermic. Concerning ∆Sq, the global dissolution process is expected

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Figure 6. Ic of undissolved cellulose as a function of time and temperature for (A) avicel, (B) sisal, (C) mercerized sisal, and (D) mercerized linters. Table 3. Rate Constants and Activation Parameters for Decrystallization of Avicel and Sisal under Nonisothermal Conditions avicel

t (s)

Ic

105 k(t) 103 1/T(t) (s-1) (K-1)

0a 0.83 2.35 600 0.82 5.88 2.41 1200 0.81 5.96 2.47 2100 0.78 6.19 2.58 3000 0.75 6.43 2.69 4200 0.64 7.54 2.86 ∆qH ) -1.82 ((0.17) kcal/mol ∆qS ) -83.05 ((0.44) cal/ kmol

Table 4. Rate Constants and Activation Parameters for Decrystallization of Mercerized Sisal and Linters under Nonisothermal Conditions

sisal

t (s)

Ic

105 k(t) 103 1/T(t) (s-1) (K-1)

0 0.64 2.33 900 0.62 6.73 2.42 1500 0.60 6.95 2.49 2100 0.55 7.58 2.56 3000 0.51 8.18 2.67 4200 0.49 8.51 2.83 ∆qH ) -1.96 ((0.19) kcal/mol ∆qS ) -83.08 ((0.49) cal/kmol

a Zero time refers to the start of cellulose decrystallization, as indicated in Figure 6.

to have a positive entropy change as it departs from the solid state where chain movements are almost restricted to vibrational movements and reaches a state in which the chains have a larger number of degrees of freedom. Some of this freedom of the solvated cellulose chain may be lost in the activated complex since the same chain may be part of a crystallite, while a part of it is interacting with the solvent in the noncrystalline region. This may lead to a smaller change of |∆Sq. It is more straightforward to compare the rate and activation parameters of native and mercerized sisal. Thus, mercerization resulted in rate increases, probably due to the effect of the aforementioned cellulose I f cellulose II transformation on the polymer chain H-bonding interaction, hence the ease of solvent penetration. It should also be kept in mind that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different

mercerized sisal

t (s)

Ic

105 k(t) 103 1/T(t) (s-1) (K-1)

0a 0.66 2.36 900 0.64 9.39 2.45 1500 0.63 9.54 2.52 2100 0.53 11.33 2.59 3000 0.51 11.78 2.70 4200 0.45 13.35 2.87 ∆qH ) -2.48 ((0.29) kcal/mol ∆qS ) -83.72 ((0.78) cal/kmol

mercerized linters

t (s)

Ic

105 k(t) 103 1/T(t) (s-1) (K-1)

0 0.76 2.31 900 0.75 2.23 2.40 1500 0.74 2.26 2.47 2400 0.73 2.30 2.58 3000 0.72 2.33 2.66 4200 0.69 2.42 2.83 ∆qH ) -1.12 ((0.02) kcal/mol ∆qS ) -83.24 ((0.04) cal/kmol

a Zero time refers to the start of cellulose decrystallization, as indicated in Figure 6.

signs from those of the decrystallization proper. Specifically, the contribution to the activation parameters of the interactions that occurs in the solvent system should be taken into account. Consider the energetic of association of the solvated ions with the AGU. We may employ the extra thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols.53 Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in eq 2 is endothermic Li-DMAC + cellulose a DMAc + Li-cellulose (2)



The inverse holds for Cl- (i.e., the equilibrium depicted by eq 3 is exothermic)54 Cl--DMAC + cellulose a DMAC + Cl--cellulose

(3)

Solvatochromic data for cellulose solutions in LiClDMAc has indicated that Cl--HO-cellulose hydrogen bonding is more important for dissolution than Li-cellulose interactions.53 If decrystallization is rate limiting, and considering that the equilibria shown in eqs 2 and 3 occur prior to decrystallization, then the activation parameters calculated represent the sum of the three reactions. That is, the endothermicity associated with breaking the intermolecular hydrogen bonding (between the cellulose chains) appears to be more than compensated by the exothermicity of the hydrogen bonds formed between cellulose and the chloride ion. This cancellation explains the slightly negative ∆Hq, as argued elsewhere for reactions whose rate-limiting steps are preceded by exothermic equilibria.55 In addition to an increase in the degrees of freedom of the chain, ion association with the polymer most certainly contributes to the overall ∆Sq. That is, the change from crystalline cellulose to the polymer-LiCl complex may be associated with an entropy increase due to decrystallization and an entropy decrease due to ion complexation by the polymer.56 The mobility may be further reduced due to electrostatic (dipoledipole) interactions and polymer-chain aggregation.57 It is worthwhile to mention that the disruption of crystallites in a true melting of celluloses derivatives (transition solid-liquid) is characterized by a low entropy of fusion due to the rigidity of chains that leads to highly extended conformations in the molten state.56 As cellulose is normally stiffer than its derivatives, we can think in terms of very low entropy for its hypothetical melting (dissolution in LiCl/ DMAc), and in a negative value for the activation entropy related to its dissolution in LiCl/DMAc, due to the reasons previously mentioned. In summary, the relevant point is that decrystallization is an entropy-driven process, as can be easily deduced from a comparison of the contributions of ∆Hq and T∆Sq terms to ∆Gq. Conclusions Dissolution of celluloses of widely different physicochemical properties in LiCl/DMAc has been studied with the aim of probing the structural factors that affect this process. Microcrystalline avicel, untreated and mercerized linters, and sisal have been employed. The ease of dissolution of avicel is due to a combination of its small molar mass and the presence of large pores in its supramolecular structure. Because of its highly organized crystalline structure, linters can be completely dissolved only after mercerization. This pretreatment reduces Ic and crystallite size considerably and modifies pore size distribution, important factors for dissolution in LiCl/DMAc. Sisal dissolves in LiCl/DMAc without mercerization, most probably because its Ic and crystallite size are smaller than those of cotton linters. Additionally, solvent penetration is easier because of the presence of macro-pores in its supramolecular structure. Mercerization of this cellulose may be advantageous, however, because of elimination of hemi-

cellulose and other noncellulosic material. Use of a purer starting cellulose is important form the application point of view, as it leads to a better control of product reproducibility. The rate constants and activation parameters of cellulose decrystallization, a required step for its dissolution, are negligibly dependent on the physicochemical properties of cellulose. The reaction is associated with a small, negative enthalpy and large, negative entropy, probably due to compensations between the energetics of the interactions of LiCl with the solvent and the polymer. To the best of our knowledge, this is the first study in which the influence of the physicochemical characteristics of celluloses on their dissolution in LiCl/DMAc have been systematically evaluated. The results point to the fact that the dissolution conditions established for cellulose of a specific source are not necessarily directly applicable to others, isolated from different sources and/or isolated from the same source, but submitted to different pretreatments. Calculations. Determination of Porosity by Nitrogen Adsorption and Mercury Intrusion Porosimetry. The Brunauer, Emmett, and Teller (BET) method involves determination of the volume of nitrogen adsorbed at different pressures at liquid nitrogen temperature. The N2 volume (Vm), necessary to form a monomolecular layer on the material, can be determined from the adsorption isotherm, by using eq 4.45,58 (C - 1) P P 1 + ) CVm P0 V(P0 - P) CVm

(4)

where V ) nitrogen volume adsorbed at a relative pressure of P/P0; P ) vapor pressure of N2 in the cellulose pore; P0 ) saturation vapor pressure of liquid nitrogen; and C is a constant. If the BET theory is applicable (generally for P/P0 between 0.05 and 0.30), a plot of P/V(P0 - P) versus P/P0 should yield a straight line with intercept of 1/CVm and slope (C - 1)/CVm. The surface area, S, is given by eq 5 Vm S ) nσ Ma

(5)

where n ) 6.02 × 1023/22 414 molecules/cm3; Ma ) sample mass in grams; and σ ) area covered by a nitrogen molecule, taken as 16.2 Å2 at 77 K. The total pore volume, Vp, was calculated by the Gurvitsch rule, eq 6 Vp ) nasatVM

(6)

where nasat is the quantity adsorbed in mol/g at saturation (i.e., at P/P0 ) 1) and VM ) molar volume adsorbed in liquid state, in cm3/mol. The average pore radius, rp, and volume, Vp, were calculated from the Kelvin equation by applying the BJH calculation method to the desorption branch of the isotherm. A graph of ∆Vp/∆rp as a function of the radius (or average pore diameter) gives the pore size distribution curve of the solid (i.e., the contribution of a given pore to the total pore volume).45,58

2646


Ramos et al.

In the MIP method, a pressure, P, is applied to force the mercury into the pores. The relation between pore radius and the pressure applied is given by the Washburn equation rp ) -2γ cos θ/P

(7)

where θ is the contact angle (∼140°) and γ is the surface tension of mercury. A graph of ∆Vp/∆rp as a function of the radius (or average pore diameter) gives the pore size distribution curve of the solid. Degree of Crystallinity from X-ray Diffraction. Ic was calculated from the equation35 Ic ) 1 - Imin/Imax

(8)

where Imin is the intensity minimum, between 2θ ) 18 and 19° (amorphous region of native cellulose) or 2θ ) 14 and 15° (amorphous region of mercerized cellulose), and Imax is the intensity maximum, between 2θ ) 22 and 23° (attributed to the crystalline region of the sample). Cellulose crystallite dimensions were determined by applying the Scherrer equation59,60 L(hkl) ) kλ/B cos θ

(10)

T(t) ) 426.33((3.32) - 0.018 ((0.001)t

(11)

T(t) ) 428.99((2.49) - 0.018((0.001)t

(12)

sisal

mercerized sisal (13)

mercerized cotton T(t) ) 432.96((3.66) - 0.019((0.002)t

(15)

(9)

where k ) rate constant; Ict ) index of crystallinity at time t from the start of cooling (i.e., after the activation step during which Ic was found to be practically constant). As shown in Figure 6, all reactions were carried out under nonisothermal conditions (i.e., Ict and k are functions of both temperature (T) and time (t)). For all celluloses, plots of Ict versus time, at different temperatures, i.e., t(T), were found to be linear. Therefore, the slopes of these plots are Ict/dt, and the value of k at each temperature time, namely, k(t), may be calculated by dividing the slope by Ict (eq 10).61-63 The relationship between T and t gave the following empirical equations: avicel

T(t) ) 424.28((2.85) - 0.018((0.001)t

Activation enthalpy (∆Hq) and entropy (∆Sq) were calculated with the Eyring equation for reactions carried out under nonisothermal conditions.52,64,65 ln[k(t)h/kBT(t)]) ∆Sq/R - ∆Hq/RT(t)

where L is the crystal thickness at the (hkl) plane of diffraction, λ is the X-ray source wavelength, k is Scherrer’s constant ()0.9), and B is the peak full width at half-height. Rate Constants and Activation Parameters of Decrystallization. The decrystallization process can be treated as a pseudo-first-order reaction. Under isothermal conditions, the corresponding kinetic equation is given by61 -dIct/dt ) kIc

Figure 7. Eyring plots obtained for celluloses decrystallization process in LiCl/DMAc.

(14)

where h is Planck’s constant, kB is Boltzmann’s constant, and time is given in seconds. A plot of the left-hand side of eq 15 against 1/T(t) should be a straight line whose slope and intercept are ∆Hq/R and ∆Sq/R, respectively, as shown in Figure 7. Acknowledgment. E.F. and O.A.E.S. thank the CNPq (National Council of Research) for research productivity fellowships, the FAPESP (State of Saõ Paulo Research Foundation) for financial support, and a research fellowship for L.A.R. References and Notes (1) Toyoshima, I. In Cellulosics: Chemical, Biochemical, and Material Aspects; Kennedy, J. F., Phillips, G. O., Willians, P. A., Eds.; Ellis Horwood: Chichester, 1993; p 125. (2) Johnson, D. C. In Cellulose Chemistry and its Applications; Nevell, T. P., Zeronian, S. H., Eds.; Ellis Horwood: Chichester, 1985; p 185. (3) Schurz, J. Prog. Polym. Sci. 1999, 24, 481. (4) Ass, B. A. P.; Frollini, E.; Heinze, T. Macromol. Biosci. 2004, 4, 1008. (5) Ciacco, G. T.; Liebert, T. F.; Frollini, E.; Heinze, T. Cellulose 2003, 10, 125. (6) Ciacco, G. T.; Ass, B. A. P.; Ramos, L. A.; Frollini, E. In Natural polymers and composites; Mattoso, L. H. C., Leaõ, A., Frollini, E., Eds.; EMBRAPA-USP-UNESP: Saõ Carlos, Brazil, 2000; p 139. (7) El Seoud, O. A.; Marson, G. A.; Ciacco, G. T.; Frollini, E. Macromol. Chem. Phys. 2000, 201, 882. (8) Bianchi, E.; Ciferri, A.; Conio, G.; Cosani, A.; Terbojevich, M. Macromolecules 1985, 18, 646. (9) Ass, B. A. P.; Frollini, E. Anais Assoc. Bras. Quim. 2001, 50 (2), 76. (10) Razera, I. A. T.; Frollini, E. J. Appl. Polym. Sci. 2004, 91, 1077. (11) Frollini, E.; Razera, I. A. T.; Trindade, W. G.; Paiva, J. M. F.; Tita, S. P. S. In: Natural fibers plastics and composites; Wallenberger, F. T., Weston, N., Eds.; Kluwer Academic Publishers: Boston, 2004; p 193. (12) Dawsey, T. R.; McCormick, C. L. J. Macromol. Sci. ReV. Macromol. Chem. Phys. 1990, C30, 405. (13) Tosh, B.; Saikia, C. N.; Dass, N. N. Carbohydr. Res. 2000, 327, 345. (14) Dupont, A.-L.; Mortha, G. J. Crhomatogr. A 2004, 129.

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