Article pubs.acs.org/Biomac
Strong and Tough Cellulose Nanopaper with High Specific Surface Area and Porosity Houssine Sehaqui,† Qi Zhou,‡,⊥ Olli Ikkala,§ and Lars A. Berglund*,†,⊥ †
Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden School of Biotechnology, Royal Institute of Technology, AlbaNova University Centre, SE-106 91 Stockholm, Sweden § Department of Applied Physics, Aalto University/Helsinki University of Technology, FIN-00076 Helsinki, Finland ⊥ Wallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡
S Supporting Information *
ABSTRACT: In order to better understand nanostructured fiber networks, effects from high specific surface area of nanofibers are important to explore. For cellulose networks, this has so far only been achieved in nonfibrous regenerated cellulose aerogels. Here, nanofibrillated cellulose (NFC) is used to prepare high surface area nanopaper structures, and the mechanical properties are measured in tensile tests. The water in NFC hydrogels is exchanged to liquid CO2, supercritical CO2, and tert -butanol, followed by evaporation, supercritical drying, and sublimation, respectively. The porosity range is 40−86%. The nanofiber network structure in nanopaper is characterized by FE-SEM and nitrogen adsorption, and specific surface area is determined. High-porosity TEMPO-oxidized NFC nanopaper (56% porosity) prepared by critical point drying has a specific surface area as high as 482 m2 g−1. The mechanical properties of this nanopaper structure are better than for many thermoplastics, but at a significantly lower density of only 640 kg m−3. The modulus is 1.4 GPa, tensile strength 84 MPa, and strain-to-failure 17%. Compared with water-dried nanopaper, the material is softer with substantiallly different deformation behavior.
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extractives, and high strength and flexibility are desirable characteristics. A disadvantage of regeneration and derivatization of cellulose is the necessity to use environmentally harmful solvents. Electrospinning10 has been used for preparation of porous nanopaper materials. The advantage of this technique is that it can be applied to a wide range of polymers and solvents and even with the incorporation of reinforcing particles or voidforming agents.11 Various cellulose derivatives can be dissolved in volatile solvents and have been electrospun from water and/ or organic solvents to produce nanofibers and form nanopaper membranes.12,13 Regenerated cellulose has also been electrospun from solvents including NMMO/water, ionic liquids, LiCl/DMAc, and ethylene diamine/salt. However, the process is challenging compared to cellulose derivative spinning and requires particular ingenuity in development of the spinning system.12 Cellulose nanopaper can also be obtained by deacetylation of cellulose acetate nanopaper.12 An attractive form of nanofibers is nanofibrillated cellulose (NFC), which can be disintegrated from natural plant fibers without any dissolution step. Chemically processed wood pulp is readily available at attractive cost, and NFC can be obtained by mechanical homogenization.14 Pretreatment with enzymes15
INTRODUCTION Cellulose nanopaper is based on native cellulose nanofibers and made by similar procedures as classical wood fiber paper. 1,2 It has potential as a strong sheetlike material or as a lightweight reinforcement phase in biocomposites. Improved mechanical properties, optical transparency, low thermal expansion, and oxygen barrier characteristics of cellulose nanopaper structures have resulted in a wide range of application demonstrations. 3 Despite a porosity of 28%, cellulose nanopaper shows an interesting combination of Young’s modulus (13.2 GPa), tensile strength (214 MPa), and strain-to-failure (10%). 1 The potential applications of cellulose nanopaper would be increased, if it was possible to increase the range of possible porosities but also to increase and tune the specific surface area in nanopaper structures. The mechanical properties could then be controlled within a larger range. Porous nanopaper is essentially a porous membrane (thin sheet), and potential membrane applications include fuel cells, 4 catalysis,5 liquid purification and filtering,6 tissue engineering,7 protein immobilization 8 and separation, and protective clothing.9 For many of the proposed applications, mechanical strength, toughness, low density, controlled permeability, and high surface area are important properties. Regenerated cellulose and cellulose derivatives such as cellulose acetate and cellulose nitrate have been used in porous nanopaper filters or in dialysis where narrow pore size distribution, low levels of © 2011 American Chemical Society
Received: June 28, 2011 Revised: August 24, 2011 Published: September 2, 2011 3638
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Figure 1. Images of TO-NFC dispersion (a), a TO-NFC hydrogel (b), and a typical porous NFC nanopaper (c).
or by chemical methods16 reduces the energy required for disintegration. While direct mechanical disintegration of wood pulp fibers produces fibrils with 25−100 nm in width,14 enzymatic hydrolysis15 and TEMPO-mediated oxidation17 result in smaller NFC with a width of 10−40 and 3−5 nm, respectively. The crystal structure in cellulose I nanofibers provides potentially higher modulus than cellulose II nanofibers obtained by dissolution and regeneration. The strength will depend on molar mass of the cellulose. Chemical and thermal stabilities are better for cellulose I than for derivatives and regenerated cellulose II. NFC nanofibers from wood can have a diameter of 3−5 nm, and this corresponds to a theoretical specific surface area of about 600 m2 g−1 (assuming cylindrical NFC, 4.5 nm diameter, cellulose density of 1500 kg m−3). In a previous study, cellulose nanopaper was prepared from hydrocolloidal NFC dispersions by filtering the dispersion to obtain a wet gel (hydrogel) and allow for evaporation of the water. Porosity was controlled by solvent exchange from water to ethanol or acetone before drying.1 In order to increase the porosity, the key is to remove the liquid in the wet gel (hydrogel) in a precisely controlled way and thus to preserve the large surface areas of the original cellulose nanofibers. In the present work, the ambition is to first create hydrogels with well-dispersed NFC nanofibers. Then the goal is to preserve this well-dispersed structure during drying, limit nanofiber aggregation, and maximize specific surface area without use of regenerated cellulose. Three different drying procedures are examined. NFC hydrogels are first solventexchanged into ethanol and subsequently into either supercritical CO2, or liquid CO2, or tert-butanol. In the final step supercritical CO2 drying, liquid CO2 evaporation, and tertbutanol freeze-drying are studied. Effects of drying procedures and type of NFC (enzymatically pretreated native NFC or TEMPO-oxidized NFC (TO-NFC)) on specific surface area, porosity, and mechanical properties in tension are investigated. The range of nanopaper densities is extended, and the mechanical behavior is substantially different as compared with water-dried nanopaper. The general objective is to understand first the relationships between processing and structure and second relationships between structure and mechanical properties.
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NFC dispersion by a filtration procedure, and finally solvent exchange and drying of the hydrogel to obtain porous nanopapers. Preparation of Enzymatic Cellulose Nanofibrils (NFC) Dispersion. The NFC water dispersion was prepared from softwood sulfite pulp fibers (DP of 1200, lignin and hemicellulose contents of 0.7% and 13.8%, respectively, Nordic Pulp and Paper, Sweden) according to a previously reported method by Henriksson et al. 15 The pulp was first dispersed in water and subjected to a pretreatment step involving enzymatic degradation and mechanical beating. Subsequently, the pretreated pulp was disintegrated by a homogenization process with a Microfluidizer M-110EH (Microfluidics Ind., Newton, MA), and a 2 wt % NFC dispersion in water was obtained. The pretreated pulp was passed 8 times through the microfluidizer at room temperature (21 °C), including first three passes through 400 and 200 μm chambers at a pressure of 900 bar and five last passes through 200 and 100 μm chambers at a pressure of 1600 bar. Preparation of the TO-NFC Dispersion. TO-NFC water dispersion was prepared from softwood sulfite pulp fibers (Nordic Pulp and Paper, Sweden) according to a previously reported method by Saito et al.17 The pulp was first dispersed in water in which sodium bromide and TEMPO were dissolved (1 and 0.1 mmol per gram of cellulose, respectively). The concentration of the pulp in water was 2 wt %. The reaction was started by addition of sodium hypochlorite (10 mmol per gram of cellulose) dropwise into the dispersion. During the addition of NaClO, carboxylate groups were forming on the surface of the fibrils, and the pH decreased. The pH of the reaction was then maintained at 10 by sodium hydroxide addition. After all NaClO was consumed, the pulp fibers were filtered and washed several times with deionized water until the filtrate solution was neutral. The purified pulp fibers were then dispersed in water at a concentration of 1 wt % and disintegrated by a homogenization process with a Microfluidizer M-110EH (Microfluidics Ind., Newton, MA). The pulp was passed only once through 200 and 100 μm chambers at a pressure of 1600 bar at room temperature (21 °C), and a 1 wt % TO-NFC dispersion in water was thus obtained, as shown in Figure 1a. The carboxylate content of TO-NFC was ∼2.3 mmol g−1 as determined by an electric conductivity titration method,18 corresponding to a degree of substitution of 0.37 per bulk anhydroglucose unit. As measured by atomic force microscopy (AFM), the diameter of TO-NFC was 2.7 ± 0.5 nm, smaller than TO-NFC disintegrated by a kitchen blender.18 Hydrogels from NFC and TO-NFC Dispersions. The NFC or TO-NFC water dispersion (ca. 300 mg solid content of cellulose) was diluted to ca. 0.1 wt %, degassed, and filtrated on top of a 0.65 μm filter membrane (DVPP, Millipore) until a strong hydrogel is formed (see picture of the hydrogel in Figure 1b). During the filtration process, no loss of nanofibers could be found for both NFC and TONFC (as measured from the weight of the nanopaper and the dry content of the suspension). Furthermore, it should be noted that the duration of the filtration process was about 1 h for NFC and 10 h for TO-NFC. Preparation of Porous Cellulose Nanopaper. The highly porous cellulose nanopapers were prepared from the NFC hydrogels by three different drying procedures. Liquid CO2 Evaporation (L-CO2). The NFC water hydrogel was solvent exchanged to ethanol by first placing it in an ethanol bath
EXPERIMENTAL SECTION
Materials. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy, free radical) and sodium hypochlorite (NaClO) solution (reagent grade, available chlorine 10−15%) were purchased from sigma Aldrich and used as received. The preparation procedure of the porous cellulose nanopapers involves three main steps: NFC disintegration from wood pulp fibers in the form of a water dispersion, followed by hydrogel formation from 3639
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Figure 2. Pore size distribution of nanopaper based on BJH analysis.21 NFC nanopaper (left) and TO-NFC nanopaper (right). Data are for three different preparation routes: supercritical CO2 drying (SC-CO2), liquid CO2 evaporation (L-CO2), and tert-butanol freeze-drying (tert-B-FD). The samples were coated with graphite and gold−palladium using Agar HR sputter coaters (ca. 5 nm). Secondary electron detector was used for capturing images at 1 kV. Mechanical Properties. Tensile tests of the porous nanopapers were performed using an Instron universal materials testing machine equipped with a 50 N load cell. Specimen strips of 30 mm in length and 3−5 mm in width are tested at 10% min−1 strain rate under a controlled relative humidity of 50%. A total of three specimens were tested per material. Young’s modulus was determined as the slope at low strain, and the ultimate strength was determined as the stress at specimen breakage. Work to fracture is taken as the area under the stress−strain curve.
(ethanol at 96%) for 24 h and subsequently in the pure ethanol bath for another 24 h. The NFC ethanol alcogel was then placed in a critical point dryer chamber (Autosamdri-815, Tousimis, USA), the chamber was closed, and liquid carbon dioxide was injected into the chamber under a pressure of ca. 50 bar. The sample was kept below the critical point conditions in the chamber to allow solvent exchange from ethanol to liquid CO2. The chamber was then depressurized and CO2 evaporated, which led to a porous NFC nanopaper as shown in Figure 1c. Supercritical CO2 Drying (SC-CO2). The NFC alcogel prepared by the above-described procedure was placed in a critical point dryer chamber (Autosamdri-815, Tousimis, USA), and liquid carbon dioxide was injected into the chamber under a pressure of ca. 50 bar for solvent exchange. The chamber was then brought above the CO2 critical point conditions to ca. 100 bar and 36 °C. The chamber was then depressurized, and CO2 evaporated to form a porous NFC nanopaper. tert-Butanol Freeze-Drying (tert-B-FD). The NFC alcogel is placed in a tert-butanol bath overnight for solvent exchange. It is then frozen by liquid nitrogen (without direct contact of the alcogel with the liquid nitrogen), and the solid tert-butanol is sublimated at room temperature under a vacuum of 0.05 mbar in a benchtop freeze-dryer (Labconco Corp., Kansas City, MO). Density and Porosity Measurements. The density of the nanopaper was determined by measuring its weight (air-dry weight, containing moisture) and dividing it by its volume. The volume was calculated from the thickness of the nanopaper (determined by a digital calliper) and its area. Porosity is estimated from the density of the nanopaper by taking 1460 kg m−3 as density of cellulose19 using the formula
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RESULTS AND DISCUSSION Nanopaper Preparation. When the NFC water hydrogel is directly dried, capillary action during water evaporation leads to compaction and a nanopaper of ca. 20% porosity is formed.1,2 With direct water evaporation, the specific surface area can be as low as 10−2 m2 g−1. Water exchange to methanol or acetone prior to drying increases the porosity to 28% and 40%, respectively.1 This is due to the less hydrophilic character of ethanol and acetone, which reduces capillary effects during drying. In previous work,23 low-density cellulose nanofiber aerogels were prepared by freeze-drying. Later, cellular foams of even lower density (porosity of 93−99.5%) were prepared by freeze-drying of hydrocolloidal NFC dispersions.24 The cell wall in the foam structure consisted of aggregated NFC formed during ice crystal growth. In a recent study, supercritical drying was shown to result in fibrillar aerogel structures.25 The fibrillar aerogels were used as templates, and the cellulose was removed by calcination at 450 °C to form inorganic hollow nanotube aerogels. In the present study, we investigate in detail preservation of the well-dispersed structure of hydrocolloidal NFC networks by alternative drying techniques. First, hydrogels were prepared from NFC and TO-NFC. Water was solvent exchanged into supercritical CO2, liquid CO2, and tert-butanol and finally dried using supercritical carbon dioxide drying (SCCO2), liquid carbon dioxide evaporation (L-CO2), and tertbutanol freeze-drying (tert-B-FD), respectively. The NFC nanofibers are prepared by enzymatic pretreatment15 and have a diameter in the 10−40 nm range and no charge on the surface, while the TO-NFC nanofibers have a diameter of 2.7 ± 0.5 nm and a carboxylate content of 2.3 mmol g−1 cellulose. Both NFC and TO-NFC nanofibers have lengths in the range 0.5 μm to several micrometers. After filtration, the water volume content in the hydrogel was in the
Mechanical properties of porous materials depend directly on relative density ρnanopaper/ρcellulose20 and therefore on porosity. The real porosity values may be slightly lower than the present estimates since the real ρcellulose can be lower than 1460 kg m−3. Specific Surface Area (SSA) and Pore Size Distribution. The Brunauer−Emmett−Teller (BET)21 surface area was determined by N2 physisorption using a Micromeritics ASAP 2020 automated system. The porous nanopaper sample was first degassed in the Micromeritics ASAP 2020 at 115 °C for 4 h prior to the analysis by N2 adsorption at −196 °C. BET analysis was carried out for a relative vapor pressure of 0.01−0.3 at −196 °C. Pore size distribution was determined from N2 desorption at relative vapor pressure of 0.01−0.99 following a BJH model.22 Field-Emission Scanning Electron Microscopy (FE-SEM). The in-plane texture of the porous nanopaper was observed by SEM using a Hitachi S-4800 equipped with a cold field emission electron source. 3640
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85−90% range. After exchange of water to ethanol, the TONFC alcogel had an ethanol volume content of only about 65%, due to shrinkage of the TO-NFC hydrogel. In contrast, the NFC hydrogel did not show any significant shrinkage during solvent exchange to ethanol (volume content of ethanol in the NFC alcogel is 85−90%). These observations suggest stronger interaction between water and TO-NFC as compared to NFC. This is due to the TO-NFC surface characteristics. 17 The density and specific surface area of nanopaper materials are summarized in Table S1, which is available in the Supporting Information. The data are related to structural changes during drying. The TO-NFC nanopaper structures have porosities in the 40−56% range, lower than the 74−86% porosity for NFC nanopaper. This is possibly related to the higher charge density on the TO-NFC nanofibers. Supercritical drying of NFC leads to the highest porosity, which is comparable to the ethanol volume in the alcogel prior to drying. Supercritical drying can apparently be performed with very little shrinkage. The other drying techniques result in nanopaper with lower porosities. Interestingly, NFC nanopaper from the fairly simple liquid CO2 evaporation route has a porosity as high as 74%. This is much higher than for nanopaper prepared by solvent exchange followed by ethanol or acetone evaporation, where porosities of 28 and 40% resulted.1 This is due to the low CO2 polarity, which is in the same range as for toluene,26 and capillary action is thus reduced compared with ethanol, acetone, or water evaporation. Structure. The structure of nanopaper samples was characterized by nitrogen adsorption and scanning electron microscopy. Nitrogen adsorption data are summarized in Table S1 (Supporting Information) and also shown in Figure 2 as pore size distribution and Figure 3 as average BJH pore
diameter of 2.7 ± 0.5 nm for TO-NFC and 5−20 nm for NFC.27 Specific surface area (SSA) has been reported previously for cellulose-based materials.23,28−32 Aerogel23 and foam24 materials based on freeze-dried NFC have data of 20−66 and 10−40 m2 g−1, respectively. Aerogels from regenerated cellulose (dissolved and precipitated) can have a specific surface area of 500 m2 g−1 when prepared by SC-CO2,29,31 but the structure is not a fibrous network. The present nanopaper structures do not rely on cellulose dissolution, they have a nanofiber network structure, and the maximum SSA is 482 m2 g−1, which is the highest SSA reported for native cellulose I NFC materials. The pore size distribution (Figure 2) shows that TO-NFC nanopaper has smaller pores than NFC nanopaper. TO-NFC nanopaper is dominated by estimated pore sizes in the 5.5− 12.4 nm range, whereas NFC nanopaper is estimated to have most pores in the range 21−36 nm. The pore volume of the nanopapers (NFC and TO-NFC) prepared by tert-B-FD is smaller than that of the nanopapers prepared by L-CO2 although having similar porosities (see Table S1 in Supporting Information). This suggests the presence of large pores for the nanopapers prepared by tert-B-FD that are not detected by the gas sorption experiments due to the limitation of this technique. The porous nanopaper structure was investigated by FESEM, and the results are presented in Figure 4. NFC nanofibers have a diameter of about 5 nm for TO-NFC and a diameter in the range of 10−30 nm for NFC. The length of the nanofibers is several micrometers. The NFC nanopaper prepared by SCCO2 (Figure 4b, center) appears to have larger pores than TONFC prepared by the same method (Figure 4a, left) in agreement with pore size distribution results. The high SSA nanopaper (TO-NFC, SC-CO2) in Figure 4a, left, shows a highly homogeneous nanofiber network structure. The nanopaper prepared by tert-B-FD (Figure 4c, right) has regions of aggregated NFC, but the structural characteristics of an NFC nanofiber network are apparent. Mechanical Properties. Stress−strain curves and mechanical property data from uniaxial tensile tests are presented in Figures 5 and 6 and Table S2 (Supporting Information). As apparent from Figure 5, higher porosity reduces modulus and strength, as expected. For NFC (left graph), the average strainto-failure is in the range 6−10%, and the strengths are quite low due to high porosity. The NFC nanopaper with a porosity of 86% has a modulus of 150 MPa and a strength of 7.4 MPa. For the NFC nanopaper prepared by L-CO2, modulus and strength are 470 and 20 MPa, respectively, at 74% porosity. Interestingly, the NFC nanopaper prepared by tert-butanol freeze-drying had twice the modulus, possibly because of a more agglomerated structure and better bonds between nanofibers in the network. The lower SSA is in support of this hypothesis. Present data may be compared with regenerated cellulose aerogels of 80−90% porosity where moduli are 200−300 MPa and the tensile strength is 10−20 MPa.31 The present cellulose I NFC nanopaper structures of 86% porosity has slightly lower strength and modulus, although the superiority of regenerated cellulose structures in terms of mechanical properties needs to be verified. In Figure 5, stress−strain curves of NFC (left) and TO-NFC (right) are compared. The TO-NFC nanopaper structures carry much higher stress and show higher tensile strengths and larger strain-to-failure. Although the higher density in TO-NFC nanopaper is important, the differences in shape of stress−
Figure 3. Average BJH pore diameter versus porosity for NFC nanopaper.
diameter versus porosity graph. The nanopaper prepared by supercritical drying results in larger BJH pores, which may also be a consequence of the higher porosity. The correlation between porosity and average pore diameter is strong (Figure 3). The surface area of the NFC nanopaper prepared by SCCO2 is 304 m2 g−1, which is lower than the 482 m2 g−1 of TONFC nanopaper. NFC nanopaper also showed lower specific surface area than TO-NFC after nanopaper L-CO2 preparation. This is due to differences in diameter of the nanofibers, since TO-NFC has a diameter of only around 2.7 nm, which is smaller than for NFC.1 The theoretical fibril diameter backcalculated from SSA of the nanopaper prepared by SC-CO2, assuming cylindrical nanofiber shape, was 5.7 and 9.0 nm for TO-NFC and NFC, respectively. This is in agreement with a 3641
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Figure 4. FE-SEM images of (a) TO-NFC nanopaper prepared by SC-CO2, SSA 482 m2 g−1, (b) NFC nanopaper prepared by SC-CO2, SSA 304 m2 g−1 (surface of tensile fractured sample), and (c) NFC nanopaper prepared by tert -B-FD, SSA 117 m2 g−1 .
Figure 5. Tensile stress−strain curves for NFC nanopaper (left) and TO-NFC nanopaper (right). The different preparation methods and the corresponding porosities are provided. The differences in SSA are presented in Table S1 (Supporting Information).
Figure 6. Young’s modulus in tension (left) and tensile strength (right) as a function of relative density ρ*/ρs (ratio between nanopaper density and cellulose density). Relative density is equal to NFC volume fraction.
It is interesting to consider the data in Table S2 (Supporting Information) for TO-NFC nanopaper with 56% porosity; modulus, tensile strength, and strain-to-failure are 1.4 GPa, 84 MPa, and 17%, respectively. These properties are comparable or superior to typical properties of, for example, polypropylene, but the density is much lower, 640 kg m−3. The TO-NFC nanopaper structures also have high toughness values for workto-fracture (area under stress−strain curve). A very interesting application of TO-NFC nanopaper is as nanofiber network reinforcement in nanostructured polymer matrix composites. Possibly, discrete and well-dispersed nanofibers of high content may provide high strain-to-failure in biocomposite structures with ductile matrices. In Figure 6, Young’s modulus in tension and tensile strength are presented as a function of relative density ρ*/ρs (ratio between nanopaper density and cellulose density). Tensile strength scales almost linearly with ρ*/ρs. There seems to be
strain curves most likely reflect differences in nanofiber deformation characteristics and nanofiber−nanofiber interaction for the two nanofiber types. An excellent case for comparison of specific surface area effects is between tert-BFD and L-CO2 drying methods for both materials (left and right figure). For each nanofiber type, porosities are independent of drying method. The main reason for differences in stress−strain behavior is therefore related to differences in specific surface area (tert-B-FD and L-CO2: 117 and 262 m2 g−1, respectively, for NFC and 45 and 415 m2 g−1 for TO-NFC). Lower specific surface area correlates with higher modulus, stronger increase in stress with strain in nonlinear region, and lower strain-to-failure. In a fiber network model context 33 where fiber diameter is assumed to be independent of drying method, reduced specific surface area can be interpreted as decreased segment length between nanofiber−nanofiber bond sites. 3642
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Figure 7. (a) Folded NFC nanopaper prepared by SC-CO2, (b) same nanopaper after 10 cycles of folding−unfolding, (c) TO-NFC nanopaper prepared by L-CO2 on top of a KTH logo in order to illustrate optical transparency. Thickness of the nanopaper is 65 μm; no light transmittance data available.
no strong effect from preparation route or specific surface area (for SSA, see Table S1 in Supporting Information). In contrast, the tert-B-FD preparation route has strong effects and much higher modulus at high relative density. Previous nanopaper structures prepared from ethanol and acetone evaporation 1 with around 40% porosity have moduli in the 7−9 GPa range. The TO-NFC nanopaper prepared by L-CO2 has a modulus of 1.8 GPa. To explain this (and the tert-B-FD observation), one may consider fiber network models with fiber aspect ratio between fiber−fiber bonding sites as an important parameter for network stiffness predictions.33 In preparation routes with low specific surface area (tert-B-FD), the fiber aspect ratio between fiber−fiber bonding sites is lowered and modulus is increased. Thus, the present preparation routes provide increased control of nanofiber network structures and the corresponding deformation behavior. The investigated nanopaper structures were flexible (low modulus and high strain-to-failure) and durable in repeated bending, as illustrated in Figure 7, similar to what has been described for aerogels.23 180° folding is easily performed with low force (a), and no apparent fracture events are visible even after 10 cycles of folding−unfolding (b). This reflects the small diameter of NFC nanofibers in combination with high NFC strength. A simple model for the minimum radius of curvature, ρmin, a fiber can sustain before fracture is34
literature. This allows nanopaper property tailoring in an extended range of properties. The resulting structures are best described as NFC nanofibril networks where individual NFC nanofibers are discrete rather than aggregated. The preparation route and chemical characteristics of the NFC surface (“pure” cellulose as in NFC or carboxylated cellulose as in TO-NFC) influence porosity and specific surface area of the resulting nanopaper, where capillary action is an important mechanism during evaporation. Compared with critical point drying, liquid CO2 evaporation is a much simpler method and still provides high values for specific surface area. tert-Butanol freeze-drying results in lower values for specific surface area due to NFC agglomeration during preparation. High-porosity TO-NFC nanopaper has interesting mechanical properties. At 56% porosity, modulus, tensile strength, and strain-to-failure are 1.4 GPa, 84 MPa, and 17%, respectively. These properties are comparable to typical properties for commodity thermoplastics, but the density is much lower, 640 kg m−3. These favorable characteristics are due to the nanofiber network structure and random-in-the-plane NFC orientation distribution. The high specific surface area of 482 m2 g−1 is also important and is reflected in a softer mechanical behavior due to reduced nanofiber interactions compared with water-cast nanopaper. The nanofiber interaction parameter has not previously been possible to control. The present results suggests possibilities to further control the mechanical behavior of nanopaper structures of a given density.
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ASSOCIATED CONTENT S Supporting Information * Tables summarizing density, porosity, specific surface area, average fibril diameter, average pore diameter, and mechanical properties of the nanopaper structures. This material is available free of charge via the Internet at http://pubs.acs.org.
where E is Young’s modulus, d is fiber diameter, and σf is fiber strength. A reduction in fiber diameter from roughly 10 μm of conventional microfibers to 10 nm of the present nanofibers is therefore very significant. It will have a dramatic effect on the minimum radius of curvature of a fiber which is bent in a fiber network structure. Also, the TO-NFC nanopaper prepared by SC-CO2 is presented in Figure 7c, where its optical transparency is apparent, despite a porosity of 42%. This also indicates that the present nanopaper structures have a low extent of nanofiber aggregation and small pores. Water-dried nanopaper structures can also be transparent or translucent but have a much lower specific surface area.
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AUTHOR INFORMATION Corresponding Author *Tel: +46-8-7908118. Fax: +46-8-7906166. E-mail: blund@kth. se.
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ACKNOWLEDGMENTS Support from the Biomime center funded by the Swedish Foundation for Strategic Research is gratefully acknowledged. Åsa Blademo and Mikael Ankerfors from Innventia are acknowledged for kind help regarding the use of BET equipment
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CONCLUSIONS Wood-based NFC nanopaper with exceptionally high specific surface area (up to 480 m2 g−1) was made by new preparation routes. To our knowledge, this is the highest data reported for cellulose nanofibers, disregarding regenerated cellulose aerogels. The water in NFC hydrogels is exchanged to liquid CO2, supercritical CO2, and tert-butanol, followed by evaporation, supercritical drying, and sublimation, respectively. The porosity range is 40−86%, which is intermediate between water-dried nanopaper and freeze-dried aerogels previously reported in the
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REFERENCES
(1) Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindstrom, T.; Nishino, T. Biomacromolecules 2008, 9, 1579−1585. (2) Sehaqui, H.; Liu, A. D.; Zhou, Q.; Berglund, L. A. Biomacromolecules 2010, 11, 2195−2198. (3) Berglund, L. A.; Peijs, T. MRS Bull. 2010, 35, 201−207.
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dx.doi.org/10.1021/bm2008907 | Biomacromolecules 2011, 12, 3638−3644
