Biomacromolecules 2008, 9, 269–277
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Aerocellulose: New Highly Porous Cellulose Prepared from Cellulose-NaOH Aqueous Solutions Roxane Gavillon and Tatiana Budtova* Ecole des Mines de Paris, Centre de Mise en Forme des Matériaux (CEMEF)†, UMR CNRS/Ecole des Mines de Paris 7635, BP 207, 06904 Sophia-Antipolis, France Received August 31, 2007; Revised Manuscript Received October 22, 2007
New highly porous pure cellulose aerogel-like material called “aerocellulose” was prepared from aqueous cellulose/ NaOH solutions. Solutions were gelled to obtain shaped three-dimensional objects, then cellulose was regenerated and dried in supercritical conditions using CO2. The porosity of aerocellulose is higher than 95% with pore sizes distribution from a few tens of nanometers to a few tens of micrometers. The internal specific surface area is around 200–300 m2/g, and density ranges from 0.06 to 0.3 g/cm3, depending on the preparation conditions. The influence of cellulose DP and concentration, of the addition of a surfactant leading to solution foaming, of gelation conditions and the temperature and acidity of regenerating bath on the morphology of aerocellulose has been studied. The results are compared with another type of aerocellulose that was prepared from cellulose/NMMO solutions.
1. Introduction Porous materials with nano- and microsize pores made from natural polymers are of special interest for medical, cosmetic, pharmaceutical, and other applications where biocompatibility and biodegradability are requested. Three-dimensional scaffolds for tissue engineering, delivery matrices, “green” packaging, and environmentally friendly insulating materials are not the only examples of such applications. The publications describing the preparation and properties of polysaccharide solid foams started to appear in the last decennary. Most of these cellular materials were made through freeze-drying of solutions or gels; some examples of polysaccharides used are agar,1 hydroxypropylcellulose,2,3 and also silk fibroin.4 Among other polysaccharides, cellulose has a special potential because of being one of the most abundant and renewable natural polymers. Porous cellulose was prepared from cellulose/calcium thiocyanate solutions followed by cellulose regeneration and freeze-drying or solvent exchange drying.5 Honeycomb-patterned cellulose was made by casting water-in-oil emulsion of cellulose acetate with subsequent drying under saturated water vapor and deacetylation.6 Another way of making highly porous solid materials other than freeze-drying of solutions is drying of gels in supercritical conditions. The first aerogels were reported by Kistler7 who used CO2 in supercritical state. Since that time, different types of organic and inorganic aerogels have been made and are successfully used in various applications. Recently, cellulose acetate aerogels have been reported. They were prepared from dried in supercritical CO2 chemically cross-linked cellulose acetate gels.8,9 Highly porous pure cellulose was obtained from wet bacterial cellulose dried in supercritical ethanol.10 In our work, the preparation and the morphology of aerogellike pure cellulose material are reported. It was obtained from cellulose dissolved in nonpolluting solvents, such as aqueous NaOH or N-methyl-morpholine N-oxide (NMMO) monohydrate, * Corresponding author: e-mail,
[email protected]; tel, +33 (0)4 93 95 74 70; fax, +33 (0)4 92 38 97 52. † Member of the European Polysaccharide Network of Excellence (EPNOE), www.epnoe.eu.
and dried in supercritical CO2 conditions. This new material is called aerocellulose because of its high porosity (>95%) and low density (around 0.1 g/cm3). The elaboration of aerocellulose from cellulose/NMMO monohydrate solutions has been reported in refs 11 and 12. Here we focus on the preparation of aerocellulose from cellulose/NaOH/water solutions and on the influence of the preparation conditions on aerocellulose morphology. When possible, the comparison and parallels of the mentioned above aerocellulose with the one made from cellulose/NMMO monohydrate solutions will be made. Cellulose can be dissolved in a narrow range of NaOH concentrations in water, from 7 to 10%, and at low temperatures (-10 to +5 °C) (see, for example, refs 13-18). It is a promising cellulose solvent because of the easiness of solution preparation and low pollution. We will use our previous knowledge on the structure and specific properties of cellulose/NaOH/water solutions,19–24 such as irreversible gelation and kinetics of cellulose regeneration from cellulose/NaOH/water gels, for the preparation of aerocellulose.
2. Experimental Section 2.1. Materials. Several types of native cellulose were used: Avicel PH-101 microcrystalline cellulose, DP ) 180, purchased from FMC Corp. (Avicel in the following); steam exploded Borregaard cellulose with DP 500 (Borregaard in the following), kindly provided by Innovia Films; fibrous Solucell cellulose, DP ) 950 and 310 (Solucell950 and Solucell310, in the following, respectively), kindly provided by Lenzing AG, Austria. Solucell310 was prepared in Lenzing R&D laboratory through the degradation of Solucell950 by electron beam irradiation. NaOH was of 97% purity, purchased from Aldrich. Alkyl polyglycoside surfactant Simulsol SL8 was from Seppic Inc., Fairfield, USA. Distilled water was used for preparing solutions and for the regenerating bath. Acetone was of 99.98% analytical grade and ethanol was of 99.9% purity, both from Bioblock. CO2 for drying in supercritical conditions was supplied by Air–Liquide with a purity of 99.9%. 2.2. Methods. 2.2.1. Preparation of Solutions and Gels. Cellulose/ NaOH/water solutions were prepared as follows. Cellulose was mixed with water and kept for 2 h at 5 °C to allow fiber swelling. NaOH/ water was precooled at -6 °C before being mixed with cellulose.
10.1021/bm700972k CCC: $40.75 2008 American Chemical Society Published on Web 12/18/2007
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Figure 1. Schematic presentation of the main steps of aerocellulose preparation from cellulose/NaOH/water solutions.
Figure 2. Example of a cylindrical aerocellulose sample obtained from 5% Avicel/NaOH/water gel.
Cellulose was mixed with aqueous sodium hydroxide solution in such proportions that in 100 g of solution there was X g of cellulose, 7.6 g of NaOH, and (92.4 - X) g of water. X was varied from 3 to 7 g; the solutions will be noted as Xcellulose/NaOH/water, each time specifying the cellulose used. Mixing was performed at -6 °C for 2 h with a stirring rate of 1000 rpm. Solutions were pored into cylindrical molds with approximate dimensions of 8–10 mm (radius) and 30–40 mm (height). Solutions were then kept for a few hours at temperatures higher than gelation temperatures (see details in Results and Discussion) to ensure complete gelation. In these conditions cellulose/NaOH/water solutions are irreversibly gelling.19–22 Cylindrical gels were then regenerated in water. Regenerated swollen-in-water cellulose samples kept their cylindrical shape with a very slight volume decrease, within 10%. Cellulose/NMMO/water solutions were prepared in the R&D laboratory of Lenzing. The pulp used was Solucell950. The proportions between the components are 3% cellulose-82% NMMO-15% water (Solucell950/NMMO in the following). Solucell950/NMMO solutions were in a crystalline state at the room temperature. Two ways of aerocellulose preparation were used: (1) Solucell950/NMMO solution was melted at 70 °C, and the hot solution was pored directly into a regenerating bath. This will be called “the preparation of aerocellulose from molten Solucell950/NMMO solution”. (2) Solucell950/NMMO was pored into the same mold as used for cellulose/NaOH/water solutions and cooled down to the room temperature, and as a result, solid cylindrical samples were obtained. They were then immersed into a regenerating bath. This will be called “the preparation of aerocellulose from solid (or crystalline) Solucell950/ NMMO solution”. 2.2.2. Rheology of Cellulose/NaOH/Water Solutions. Dynamic shear experiments were carried out using a cone-plate geometry (cone angle 2°, plate diameter 60 mm) on a stress-controlled Bohlin Gemini rheometer equipped with a Peltier temperature control system. Solutions were directly introduced on the plate after the preparation, and a layer of silicone oil was put around to prevent water evaporation. The estimation of the gelation temperature Tgel of cellulose/NaOH/water solutions was made by recording the elastic G′ and viscous G′′ moduli as a function of temperature from 10 to 100 °C with a ramp of 5 °C per minute and noting the temperature at which G′ ) G′′. Being not very precise, such a determination of the gel point allows a quick comparison of different sample behavior in the same conditions. Gelation time tgel was determined from the time evolution of both moduli at the moment when G′ ) G′′ at a fixed temperature of cellulose/
Figure 3. Gelation time as a function of temperature for 5Avicel/ NaOH/water and 5Borregaard/NaOH/water solutions. Solid line corresponds to the exponential approximation for Avicel solution.
Figure 4. G′ and G′′ as a function of temperature for 7Avicel/NaOH/ water and 7Solucell310/NaOH/water solutions.
NaOH/water solution.25 The experiments were performed on solutions of different cellulose types and concentrations at stress 0.01 Pa and frequency 1 Hz (linear viscoelastic regime). 2.2.3. Drying in Supercritical Conditions. Drying in CO2 supercritical conditions was performed using setup of Centre Energétique et Procédés, Ecole des Mines de Paris, Sophia-Antipolis, France. Aerocellulose precursors (see detailed description of aerocellulose preparation in the Results and Discussion) were placed in 1 L autoclave filled with acetone in order to avoid evaporation before the beginning of the process. The system was closed, pressurized to 50 bar with gaseous CO2, and heated to 37 °C. The excess of acetone was purged,
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Biomacromolecules, Vol. 9, No. 1, 2008 271 and Pascal 440. BET specific surface SBET was calculated in the relative pressure range from 0.09 to 0.25. 2.2.5. Scanning Electron Microscopy. Scanning electron microscopy (SEM) experiments were performed either in FhG-IAP, using JSM 6330JEOL F at acceleration voltage of 5 kV, or in CEMEF/Ecole des Mines, Sophia-Antipolis, France, using a PHILIPS JEOL 35CF at the acceleration voltage of 10–15 kV. In order to avoid electrical charging during SEM investigations, thin layers of platinum (in FhG-IAP) or of carbon (in CEMEF) were deposited by sputtering onto the surfaces of the cross sections.
3. Results and Discussion
Figure 5. Gelation temperature as a function of cellulose concentration for Avicel, Borregaard, and Solucell310 in NaOH/water solutions. Lines are given to guide the eyes.
Figure 6. Ggel as a function of cellulose concentration for Avicel, Borregaard, and Solucell310 in NaOH/water solution. Solid lines are power-law approximations.
maintaining the pressure and the temperature constant with gaseous CO2. When the excess of acetone was recovered, the system was pressurized until the operating conditions were reached: 80 bar and 35 °C. CO2 in the supercritical state mixes with the interstitial acetone. When the thermodynamic equilibrium was reached, the liquid phase in the pores of aerocellulose precursor was exchanged with supercritical CO2 through a dynamic washing step (80 bar, 37 °C, 5 kg of CO2 per hour) during approximately 7 h. When the total amount of the interstitial liquid was considered to be removed, the system was slowly and isothermally depressurized (4 bar per hour at 37 °C) to avoid condensation of liquid CO2 and the phenomenon of cracking. Once the atmospheric pressure was reached, the system was cooled down to the ambient temperature and the autoclave was opened. 2.2.4. Porosity Measurements. The measurements of aerocellulose porosity (pores size and their distribution, specific surface) were performed in Fraunhofer Institute for Applied Polymer Research, Golm, Germany (FhG-IAP). The pore size was investigated in the range from 0.1 to 100 nm by N2 sorption with a Sorptomatic 1990 apparatus and in the range from 2 to 120000 nm by Hg porosimetry with Pascal 140
3.1. General Description of Aerocellulose Preparation. In order to prepare a shaped object from cellulose, it has to be dissolved. In our case cellulose was dissolved either in sodium hydroxide aqueous solution or in NMMO monohydrate. The properties and the structure of Avicel/NaOH/water solutions are described in detail in refs 19-24. Cellulose/NaOH/water solutions were then kept in such conditions that their gelation occurred (see details on the determination of gelation temperature and time in the next section). The shape of the gels or of Solucell950/NMMO samples was determined by the form of the mold (see details in Materials section). In some cases that will be specified later, the surfactant Simulsol was used at the stage of preparation of the cellulose/ NaOH/water solutions. The goal was to make aerocellulose with additional pores and thus to lighten the final material. The surfactant was added at various concentrations (0.1%, 0.5%, and 1% in weight) to the ready cellulose/NaOH/water solution. The solution + surfactant was stirred at 1000 rpm for 5 min at +5 °C, leading to the formation of air bubbles in solution. The foamed solution was then immediately gelled at 50 °C for 2 h in order to “freeze” the structure. Simulsol dissolved in water during the regeneration step. The next step in aerocellulose preparation was cellulose regeneration (see schematic presentation of aerocellulose preparation in Figure 1). Cellulose/NaOH/water gels or crystallized Solucell950/NMMO solutions were extracted from the molds and placed into a regenerating bath. In most of the cases, the regenerating bath liquid was water; the influence of bath acidity (H2SO4) on the porosity of aerocellulose from Avicel/NaOH/ water gelled solutions was also checked. The kinetics of NaOH (or NMMO) release into water and NaOH (or NMMO) diffusion coefficients as a function of cellulose concentration and the bath temperature and type are described in detail in ref 23. The final step was to extract the nonsolvent from the sample keeping the pores open as much as possible. To do this, the procedure used to prepare aerogels7–9 was applied. It consists of drying gels in supercritical conditions. This type of drying prevents formation of a liquid–vapor meniscus, the latter being due to the surface tension of the liquid. A capillary pressure gradient appearing in the pore walls and being present during evaporative drying usually leads to the collapse of most of the pores. Drying in supercritical conditions means transformation of the liquid into a supercritical fluid with a zero surface tension, and as a result, the porous structure remains intact. In our case, CO2 was used because of its mild supercritical condition: pressure of 73.8 bar and temperature of 31.1 °C. As far as water not being miscible with liquid CO2, an “intermediate” liquid miscible both with water and with liquid CO2 was used in order to dry regenerated cellulose in supercritical conditions. Acetone was found to be appropriate for this purpose, and the samples of regenerated swollen-in-water cellulose were washed in acetone until complete removal of water. In some cases ethanol
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Figure 7. SEM images of aerocellulose surface (a) and cross section (b) prepared from 5Avicel/NaOH/water gel, regenerated in water at 25 °C, solvent exchange water f acetone.
cellulose solutions, this concentration being the easiest to handle: not very quick gelation, as compared with higher cellulose concentration, and not too mechanically weak aerocellulose, as compared with lower cellulose concentration. The evolution of G′ and G′′ as a function of time for different fixed temperatures in the range from 10 to 30 °C was recorded. Below 25 °C for all cellulose solutions studied and at time t ) 0, viscous modulus G′′ is superior to elastic modulus G′, the system is in solution state; the sample behaves like a viscous liquid. With time, G′ increases more rapidly than G′′: it crosses G′′ at a certain gelation time tgel and becomes larger than G′′. The system gradually transforms from a viscous liquid into an elastic network.20–22
Figure 8. Pore size distribution of Aerocellulose as in Figure 7, obtained from mercury intrusion measurements.
was used instead of acetone. As a result of this step, swollenin-acetone (or ethanol) cellulose was prepared. This was aerocellulose precursor ready to be dried in supercritical conditions. Drying was performed on laboratory setup described in Methods section. After slow depressurization, white dry pure cellulose samples, aerocellulose, were obtained (Figure 2). The total shrinkage of the samples from the NaOH or NMMO route was lower than 20% or 50% as compared with the initial volume of the gel or the solution, respectively. Before the porosity of aerocelluloses is described as a function of the preparation conditions (gelation temperature, characteristics of regenerating bath, cellulose type, and concentration), the main aspects on gelation of cellulose/NaOH/water solutions will be presented for a better understanding of aerocellulose preparation. 3.2. Gelation of Cellulose/NaOH/Water Solutions. Gelation of cellulose/NaOH/water solution with time and temperature increase was reported in detail in refs 20-22. Here we present the main results of the influence of cellulose type and concentration on gelation time and temperature for the three types of celluloses studied. 3.2.1. Influence of Temperature on Gelation Time. Gelation time tgel as a function of temperature was determined for 5%
The formed gel is opaque; the reason could be a microphase separation into polymer-rich and polymer-poor phases, as it occurs for some polysaccharide solutions like methylcellulose.26,27 In a few hours experimental points become very scattered. The reason is syneresis: solvent is released from the gel and the sample slides leading to bad data reproducibility. The syneresis can be clearly observed when heating the system for a long time at higher temperatures is performed: for example, being kept at 50 °C for a few days, the gel is “losing” up to 15–20% of its volume. The high amount of liquid released can be explained by thermal degradation of cellulose and/or phase separation evolving in time. Aside from the general reasoning that phase separation occurs due to the decrease of solvent (NaOH/water) thermodynamic quality at temperatures above zero, which leads to the preferential cellulose-cellulose interactions,20,21 the particular mechanism of this phenomenon is not very clear yet. Figure 3 shows an example of the dependence of gelation time on temperature for the 5Avicel/NaOH/water system. The time of gelation decreases in an exponential way with temperature increase: tgel (min) ) 5.7 × 104 exp(-0.345T °C). A similar trend with slightly higher parameters was reported in ref 20 for 5Avicel/9NaOH/water: tgel (min) ) 2.0 × 106 exp(-0.4T °C). Figure 3 shows that gelation at 20 °C is faster for the Avicel cellulose solutions (40 min) than for the Borregaard cellulose solutions (3 h). This surprising result (DPAvicel ) 170 and DPBorregaard ) 500) can be explained by a better dissolution of the microcrystalline cellulose in 7.6NaOH/water solution: Borregaard fibers that remain only partly dissolved are not involved in solution gelation, and thus the real cellulose concentration in the solution of Borregaard cellulose is slightly decreased. As shown in the next section, the decrease of cellulose
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Figure 9. SEM micrographs of aerocellulose prepared from solid (a) and molten (b, c) Solucell950/NMMO solution regenerated in water at 25 °C with further water f acetone exchange and dried in supercritical conditions. Table 1. Microstructural characteristics of Aerocellulose obtained from different pulps from 5cellulose/NaOH/water gelled solutions (gelation performed at 50 °C for 2 h), regenerated in water at 25 °C, water exchanged by acetone and dried in supercritical conditions
cellulose types
mean bulk pore specific cumulative volume, porosity, density, diameter, surface, 3 cm3/g % g/cm µm m2/g
Avicel, DP ) 170 Solucell, DP ) 310 Borregaard, DP ) 500
0.14
0.9
240
6.6
96
0.12
0.86
260
7.5
91
0.13
0.92
280
7.2
95
Table 2. Porous Characteristics of Aerocellulose Obtained from XAvicel/NaOH/Water Gels with X) 5, 6, and 7 g in 100 g of Solution total mean pore cumulative specific diameter, µm volume, g/cm3 surface, m2/g
sample 5Avicel/NaOH/water 6Avicel/NaOH/water 7Avicel/NaOH/water
0.90 0.72 0.70
6.6 6.1 6.0
240 220 200
Table 3. Microstructural Characteristics of Aerocellulose Obtained from 5cellulose/NaOH/Water Solution with Different Gelation Conditions gelation conditions 25 °C for 24 h 50 °C for 2 h
bulk mean pore specific cumulative density, diameter, surface, volume, porosity, cm3/g % g/cm3 µm m2/g 0.13 0.14
0.84 0.90
220 240
6.3 6.6
95 96
concentration “delays” gelation, i.e., leads to the increase of gelation temperature and of gelation time. 3.2.2. Influence of Cellulose Origin and Concentration on Gelation Temperature. In order to check the influence of the cellulose type and concentration on gelation temperature, G′ and G′′ of cellulose/NaOH/water solutions of three types of cellulose (Avicel, Borregaard, and Solucell310) of different concentrations were studied as a function of temperature. An example for 7Avicel/NaOH/water and 7Solucell310/NaOH/ water solutions is shown in Figure 4. Below 20–25 °C, G′ and G′′ of both solutions slightly go down with temperature rise. G′′ values are higher than G′: it is a typical liquid-like behavior. The gelation starts above 20 °C for Avicel and 30 °C for Solucell solutions and viscoelastic characteristics go through a minimum: both moduli increase with temperature, G′ strongly increases and overcomes G′′. Similar
trends were obtained for other systems and concentrations. The results are summarized in Figures 5 and 6 for gelation temperature Tgel and gel strength at the gel point Ggel ) G′ ) G′′, respectively. The increase of cellulose concentration leads to a decrease of gelation temperature. Indeed, the more the solution is concentrated, the more numerous the cellulose chain interactions. Thus less energy is needed to start gelation in higher concentrated solutions. While gelation temperatures of Avicel and Borregaard solutions for studied cellulose concentrations are practically the same, Tgel of Solucell310 is 20–25 °C higher. It is not DP but the way of cellulose treatment and probably of cellulose origin that plays an important role in cellulose-cellulose interactions in NaOH/water solutions. The increase of cellulose concentration leads to a rise of gel strength at gel point (Figure 6). This is due to the progressive increase of the number of contacts between cellulose chains leading to a stronger network structure. Ggel is proportional to 3 4 Ccellulose for Borregaard/NaOH/water solutions and to Ccellulose for Avicel and Solucell310 solutions. Such a strong power law concentration dependence is not typical for gelling polysaccharides, which is reported to be almost quadratic for the Yong modulus of agarose or k-carrageenan (see, for example, review of te Nijenhuis28). However, when data is analyzed from ref 29 on gelation of cellulose/NaON/urea/water solutions, the dependence of Ggel vs cellulose concentration can be approximated by power law with exponent 5. Gelation of cellulose/ NaOH/water solutions is a complex process coupled with a microphase separation, and it is still not understood very well. The absolute values of Ggel for Borregaard solution are higher than the one of Avicel solution and 1 decade higher than the one of Solucell310. This can be explained by the highest DP of Borregaard cellulose and also by the presence of undissolved or partly dissolved fibers of the Borregaard pulp that could reinforce the gel. As mentioned previously, microphase separation followed by syneresis occurs in time. Both time and temperature are acting on cellulose/NaOH/water solutions in the same “destabilizing” way. Indeed, NaOH/water is not a very good cellulose solvent (see, for example, refs 21 and 30). The thermodynamic quality of NaOH/water decreases with temperature increase (reflected by the decrease of the intrinsic viscosity with temperature increase 20,21), thus leading to the preferential cellulose-cellulose interactions. That is why in semidilute solutions these interactions induce gelation that is faster at higher temperatures. For the preparation of aerocellulose, all cellulose/NaOH/water solutions were gelled for 2 h at 50 °C; these conditions ensured overpassing the gel point of all the solutions studied.
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Figure 10. SEM micrographs of aerocellulose obtained from 5Avicel/NaOH/water gelled solutions (a) without Simusol (b) with 0.5% Simusol and (c) with 1% Simusol. Table 4. Microstructural Characteristics of Aerocellulose Obtained from 5Avicel/NaOH/Water Gelled Solution, Regenerated in Water at 25 °C, with Different Simulsol Concentrations surfactant concn, %
mean pore diameter, µm
cumulative vol, cm3/g
density, g/cm3
0 0.1 0.5 1
0.9 6.4 28.1 47.5
6.6 7.2 8.6 9.6
0.14 0.12 0.10 0.06
3.3. Structural Characteristics of Aerocellulose. 3.3.1. Example of Aerocellulose from AVicel/NaOH/Water Gelled Solution and Comparison with Aerocellulose from Solucel950/NMMO Solution. A SEM image of a typical Aerocellulose, i.e., prepared from 5Avicel/NaOH/water gelled solution, is shown in Figure 7. The surface is quite dense and smooth. The skin thickness is lower than a few hundred nanometers. The formation of the skin may result from the increase of cellulose concentration stimulated by rapid solvent depletion during regeneration on the surface of the gel. In the inner areas, the sample displays a heterogeneous porous structure. The thickness of the walls of large pores is around 100 nm. The pore distribution is very wide (Figure 8), from several nanometers to tens of micrometers. It is Gaussian with a large range of pore sizes from mesopores (2–50 nm) to macropores (pores with diameter higher than 50 nm). According to the mercury intrusion analysis, the total porosity of this sample is 96%, the average pore radius is 0.450 µm, and the total cumulative volume is 6.7 cm3/g. This high cumulative pore volume is due to a number of large pores. The specific surface of this sample obtained from the mercury intrusion and nitrogen absorption methods was as follows: less than 1 m2/g for macropores, 220 m2/g for mesopores, and 300 m2/g for micropores. An example of Aerocellulose prepared from Solucell950/ NMMO solution which was initially molten or crystallized, regenerated in water at 25 °C with further water f acetone exchange and dried in supercritical conditions, is shown in Figure 9. The structure of aerocellulose obtained from crystalline Solucell950/NMMO solution (Figure 9a) is very similar to the one obtained from cellulose/NaOH/water gelled solutions (Figure 7b). On the contrary, the images presented in panels b and c of Figure 9 show a particular morphology; it is a microporous globular structure formed by small spheres with a diameter around 1µm. Because the spheres are of the same size, contacting each other, forming a slightly periodic structure and because cellulose is undergoing a clear phase separation from solution to precipitated state, we can conclude that such a
structure is the result of spinodal decomposition. The fact that the structure is composed of spheres can be an advantage for some aerocellulose applications which require a high specific surface area. It is clear that the state of cellulose/NMMO solution, i.e., crystalline or molten (“molten” is in fact a real solution state), has a significant impact on the final morphology of aerocellulose. Indeed, when cellulose/NMMO is in the solution state (“molten cellulose/NMMO”) and then placed into water, a phase separation cellulose/nonsolvent takes place: water concentration becomes higher than that allowed by the dissolution region on the ternary phase diagram cellulose/NMMO/water (see, for example, ref 31). In this case the phase separation is governed by the spinodal decomposition mechanism and regular spheres can be seen. When cellulose/NMMO is at room temperature (“crystalline” or “solid” cellulose/NMMO solution), a phase separation between (cellulose + linked NMMO) and (free crystalline NMMO) has already occurred.32 During regeneration, water first dilutes free NMMO, leading to the appearance of large holes. Then a second phase separation occurs: bound-tocellulose NMMO is removed from cellulose. Even if the latter occurs via the spinodal decomposition mechanism, it is impossible to detect this on the SEM image. It is possible to make parallels between aerocellulose structures obtained from cellulose/NaOH/water gels or cellulose/ NMMO solutions. The structure of aerocellulose is very similar when cellulose is regenerated either from solid cellulose/NMMO or from cellulose/NaOH/water gelled solution. In both cases the solvent (NMMO or NaOH/water) is already separated from cellulose. Indeed, cellulose/NaOH/water solutions become opaque when gelling, which is the indication of the passed phase separation. As it was mentioned in the previous paragraph, free NMMO is separated from cellulose + bound NMMO when the sample is in the solid state. Thus in both cases a porous structure with wide pore size distribution is obtained; it is due to the fact that water is first diluting regions with “free” solvent and thus making large pores and then “removing” the rest of the solvent from cellulose and making small pores. In the case of real (molten) cellulose/NMMO solution, cellulose and NMMO are homogeneously distributed all over the volume and a phase separation occurs in one step. 3.3.2. Influence of Pulp Properties on the Porosity. Table 1 summarizes the microstructural data obtained using mercury intrusion for aerocellulose made from different pulps using the same cellulose concentration 5cellulose/NaOH/water prepared in the same conditions. There is practically no influence of the type of pulp used on aerocellulose structural properties: the
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Figure 11. SEM micrographs of aerocellulose obtained from and 3Avicel/NaOH/water gelled solutions (a, b) and solid Solucell950/NMMO/ water (c, d) regenerated in water at 25 °C (a, c) and 70 °C (b, d). Table 5. Influence of Bath Temperature on the Microstructural Characteristics of Aerocellulose Obtained from 5Avicel/NaOH/ water gels water bath temp, °C
bulk density, g/cm3
mean pore diameter, µm
specific surface, m2/g
cumulative volume, cm3/g
porosity, %
25 50
0.14 0.11
0.9 1.4
240 300
6.6 8.9
96 95
Table 6. Influence of Bath Acidity on the Microstructural Characteristics of Aerocellulose Obtained from 5Avicel/NaOH/ Water Gelled Solution Regenerated in Sulfuric Acid at Different Concentrations at Ambient Temperature followed by Water f Acetone Exchange and Dried in Supercritical Conditions bulk density, H2SO4 mean pore concn, mol/L g/cm3 diameter, µm 0.02 0.2 1
0.15 0.17 0.30
0.89 0.80 0.78
cumulative porosity, % vol, cm3/g 5.4 5.1 2.4
87 81 71
density is 0.13 ( 0.1 g/cm3, the mean pore radius varies, 0.44 ( 0.02 µm, and the specific surface is 260 ( 20 m2/g. 3.3.3. Influence of Cellulose Concentration on Aerocellulose Microstructure. Table 2 presents the porous characteristics obtained by mercury intrusion for aerocellulose from Avicel/ NaOH/water gels of various cellulose concentrations (5, 6, and 7 g in 100 g of solution) and regenerated in water in the same standard conditions (water bath of 25 °C, water was exchanged by acetone and then dried in supercritical conditions). The higher the cellulose concentration, the lower the mean pore diameter and, thus, the cumulative volume and the specific surface. The same phenomenon was reported for freeze-dried cellulose/calcium thiocyanate solutions of different cellulose concentrations.33
3.3.4. Influence of Gelation Conditions on the Porosity. In order to check the influence of gelation conditions on aerocellulose microstructure, the same initial solution, 5cellulose/ NaOH/water, was either gelled at 50 °C for 2 h or at ambient temperature for 24 h. Both samples were then regenerated in water at 25 °C with further solvent exchange water f acetone and then dried in CO2 in supercritical conditions. Table 3 summarizes the microstructural characteristics obtained for these two samples. It shows that gelation conditions practically do not affect the final characteristics of aerocellulose. 3.3.5. Influence of Surfactant Addition on the Porosity. In order to decrease the density of aerocellulose material, surfactant Simulsol SL8 was added at concentrations 0.1, 0.5, and 1 wt % to 5Avicel/NaOH/water solution, as described in the Methods section. Mixing induced the formation of air bubbles that were then “trapped” in the sample by immediate gelation. The gelation was performed in the same way as for solutions without surfactant, 2 h at 50 °C, samples were regenerated in water at ambient temperature and water was exchanged by acetone followed by drying in supercritical CO2 conditions. Figure 10 shows the influence of surfactant concentration on the morphology of aerocellulose. The images show the increase of the amount of large pores that were created by air bubbles. The higher surfactant concentration, the larger are both the pore size and the cumulative volume, which is due to the macropore formation, and, as a result, the lower the sample density (see Table 4). The density is reduced by 30% when adding 0.5% of surfactant and twice by adding 1% of surfactant to the cellulose solution. The result obtained shows an interesting and promising way of varying aerocellulose porosity. 3.3.6. Influence of Regenerating Conditions. It is known that regenerating conditions, such as the type of nonsolvent, strongly influence the properties of fibers regenerated from cellulose-
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Figure 12. SEM micrographs of aerocellulose prepared from molten Solucell950/NMMO solution regenerated in water at ambient temperature with further exchange water f acetone (a) and water f ethanol (b), both at ambient temperature, and dried in supercritical conditions.
NMMO solutions (see, for example, review 31) where fibers of different morphologies were obtained by varying regenerating bath type. In this section, the influence of bath temperature in the case of water bath and of bath acidity at room temperature (aqueous H2SO4) is investigated. The influence of the second regenerating bath (acetone or ethanol) on aerocellulose microstructure was also checked. Figure 11 presents aerocellulose obtained from 3Avicel/ NaOH/water gelled solutions (a, b) and from solid Solucell950/ NMMO/water (c, d) regenerated in water at 25 °C (a, c) and 70 °C (b, d), followed by exchanged with acetone and then supercritical dried. Aerocellulose regenerated in water at ambient temperature shows a fibrillar structure while aerocellulose regenerated at 70 °C shows a “cloudy” structure whatever the cellulose solvent. Higher temperature accelerates the diffusion of all components in the coagulating system. Probably because of this, rapid precipitation of cellulose chains have less time to “pack” and such a cloudy structure is created. Table 5 gives an example of the influence of the bath temperature on the microstructural properties of aerocellulose prepared from 5Avicel/7.6NaOH/ water gelled solutions. The increase of the bath temperature has a strong impact on the structure of aerocellulose: it creates larger pores thus reducing the material density. During spinning fibers from cellulose/NaOH solutions, cellulose is usually regenerated in acid baths. The acidity of the bath was varied to check if this may influence aerocellulose microstructure. 5Avicel/7.6NaOH/water solutions gelled for 2 h at 50 °C were regenerated in sulfuric acid at concentrations of 0.02, 0.2, and 1 M, at ambient temperature, followed by acid f water and water f acetone exchanges and then dried in supercritical conditions. Table 6 presents the porous characteristics obtained by mercury intrusion for these three samples. The higher the regenerating bath acidity, the lower are both the average pore diameter and the total porosity and thus the density. This shows another way, in addition to bath temperature, of varying the porosity of aerocellulose. The influence of the second regenerating bath was studied. Because aerocellulose is prepared via supercritical drying, the liquid in the precursor should be compatible with liquid CO2. Ethanol was tested as one of options, instead of acetone, as far as ethanol is also compatible with water and with CO2 in the supercritical state. An example for aerocellulose obtained from molten Solucell950/NMMO solutions regenerated in water at ambient temperature and then washed five times either with acetone or
with ethanol is shown in panels a and b of Figure 12, respectively. In both cases, a particular structure of connected cellulose spheres (compare with panels b and c of Figure 9), characteristic for the spinodal decomposition, was observed. It seems that the nature of the second bath does not influence much the morphology of aerocellulose. Indeed, the most important step of aerocellulose morphology formation is the first regeneration during which cellulose is coagulating, and the main parameters playing the important role in the formation of aerocellulose structure are bath nature and temperature.
4. Conclusions Ultralight and highly porous pure cellulose material, called aerocellulose, was made via cellulose dissolution in NaOH/water followed by gelation, cellulose regeneration, and drying in supercritical conditions. The density of aerocellulose obtained from 3 to 7% cellulose solutions varied between 0.06 and 0.3 g/cm3, with an average value being around 0.1–0.15 g/cm3 for 5% cellulose solutions. The porosity strongly depends on regeneration conditions (bath temperature and acidity), it can be varied by foaming in the presence of surfactant and practically does not depend on gelation conditions and cellulose DP. The morphology of aerocellulose prepared from cellulose/ NaOH/water gelled solutions is similar to the one made from cellulose/NMMO solutions, if the latter is in the solid state before regeneration. Acknowledgment. This work was supported by EC within 6th framework program, project “Aerocell” No. NMP3-CT2003505888. Authors are grateful to S. Fabry-Berthon, A. Rigacci, F. Fischer, and P. Ilbizian (CEP, Ecole des Mines de Paris, Sophia-Antipolis, France) for supercritical drying, to H.-P. Fink and M. Pinnow (Fraunhofer IAP, Golm, Germany) for the measurements of aerocellulose porosity, to G. Kraft (Lenzing AG, Austria) for providing Solucell samples and Solucell950/ NMMO solutions, and to P. Navard (CEMEF, Ecole des Mines de Paris, Sophia-Antipolis, France) for fruitful discussions.
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