Stretchable and Strong Cellulose Nanopaper Structures Based on

Oct 9, 2012 - Wallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 ... cellulose is widely used in wood- and plant-based material...
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Stretchable and Strong Cellulose Nanopaper Structures Based on Polymer-Coated Nanofiber Networks: An Alternative to Nonwoven Porous Membranes from Electrospinning Houssine Sehaqui,*,†,⊥ Seira Morimune,‡ Takashi Nishino,‡ and Lars A. Berglund*,†,§ †

Department of Fibre and Polymer Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Department of Chemical Science and Engineering, Kobe University, Kobe 657-8501, Japan § Wallenberg Wood Science Center, Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡

S Supporting Information *

ABSTRACT: Nonwoven membranes based on electrospun fibers are of great interest in applications such as biomedical, filtering, and protective clothing. The poor mechanical performance is a limitation, as is some of the electrospinning solvents. To address these problems, porous nonwoven membranes based on nanofibrillated cellulose (NFC) modified by a hydroxyethyl cellulose (HEC) polymer coating are prepared. NFC/HEC aqueous suspensions are subjected to simple vacuum filtration in a paper-making fashion, followed by supercritical CO2 drying. These nonwoven nanocomposite membranes are truly nanostructured and exhibit a nanoporous network structure with high specific surface area, as analyzed by nitrogen adsorption and FE-SEM. Mechanical properties evaluated by tensile tests show high strength combined with remarkably high strain to failure of up to 55%. XRD analysis revealed significant fibril realignment during tensile stretching. After postdrawing of the random mats, the modulus and strength are strongly increased. The present preparation route uses components from renewable resources, is environmentally friendly, and results in permeable membranes of exceptional mechanical performance.

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INTRODUCTION Cellulose is the dominating biopolymer in the plant world. The estimated annual biosynthesis production (and degradation) is 1011 tons.1 Because of its microfibrillar form, it is also the most important load-bearing component in the plant cell wall, with Young’s modulus and tensile strength exceeding most of the commonly used synthetic fiber-forming polymers. Although cellulose is widely used in wood- and plant-based material products, the mechanical property potential of cellulose is poorly realized in current materials. A concrete example is that paper and board products as well as plant fiber biocomposites with polymer matrices tend to suffer from brittleness in the form of low strain-to-failure.2 Successful preparation of highly ductile cellulosic material structures with very large strain to failure would therefore be interesting both from a technical and scientific viewpoint. Recent research efforts have made it possible to extract cellulose nanofibrils from wood and plant fibers at relatively low cost.3−6 Most economical procedures rely on disintegration of plant cell walls where the lignin content has been lowered by chemical means (i.e., wood pulping). The diameter of the fibrils can be as low as 4 nm, their length is in the micrometer range providing very high aspect ratio, and the specific surface area can reach 480 m2 g−1.7 The structural characteristics of nanofibrillated cellulose (NFC) depend on the biological © 2012 American Chemical Society

source and on the extraction procedure. Enzymatic pretreatment of wood pulp, followed by mechanical homogenization results in NFC with a diameter of 5−20 nm.3,4 Chemical pretreatment (TEMPO mediated oxidation) may result in NFC with a diameter of 4 to 5 nm.5 The diameter of the type of NFC prepared in the pioneering study was typically 25−100 nm when no pretreatment and limited mechanical homogenization was used.8 The designation “NFC” is used to emphasize the smaller diameter and more homogeneous size distribution of the present fibrils. After processing, NFC is obtained as colloidal particles in water. A mixture of NFC hydrocolloid and a water-soluble polymer can be filtered and dried in a papermaking approach.9,10 In a recent study, vacuum filtration was found to be suited for NFC composite preparation when the polymer matrix has high affinity to cellulose and high average molecular weight.10 Drying NFC from less polar liquids reduces capillary forces and results in higher porosity for similar drying conditions.11 Dense nanopaper obtained by evaporating water from the NFC suspension is generally stiff, strong, transparent/translucent, and with good barrier properties, whereas higher porosity Received: July 16, 2012 Revised: October 8, 2012 Published: October 9, 2012 3661

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Porosity of the NFC/HEC nanopaper structures was calculated from their density by taking 1460 kg m−3 as density for cellulose (ρNFC)13 and 1340 kg m−3 as density for HEC (ρHEC)14 using the formula

nanopaper obtained by supercritical drying is softer, more ductile, permeable, and with higher surface area. In the context of nonwoven fibrous networks, strong interfibril interaction increases modulus but decreases strain to failure. Possibly, the established deformation mechanisms involving sliding and slippage of individual nanofibers become restricted with stronger interfibril interaction.11 In the present study, we aimed to develop further the nonwoven porous nanopaper concept. Several studies have pointed out the interesting characteristics of nonwoven nanofiber mats from electrospun fibers, for instance, in the context of biomedical applications.12 Here we instead use native cellulose NFC from plant cell walls and modify the mechanical properties of cellulose nanopaper. Individual fibrils are coated with a polymer and are subjected to supercritical drying so that high specific surface area and weakened interfibril interaction are obtained. The polymer coating is expected to change the interfibril friction and therefore the properties of the porous nonwoven nanopaper. NFC fibrils consist of highly ordered cellulose molecules parallel to fibril direction and are expected to have intrinsically better strength properties than electrospun fibers so that property advantages can be realized. The coated NFC nanopaper structure is characterized by density measurements, scanning electron microscopy, gas adsorption experiments, and XRD analysis. Mechanical properties of the coated NFC nanopaper are characterized by tensile tests and related to structure. A comparison is made to previously reported composites from the same constituents dried at elevated temperature. We also show the possibility for NFC fibril orientation by postdrawing and the corresponding effects on mechanical properties.

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porosity = 1 −

ρc (WNFC/ρNFC + WHEC/ρHEC )−1

where WNFC and WHEC represent weight fractions of NFC and HEC in the composite, respectively. Specific Surface Area and Pore Size Distribution. The Brunauer−Emmett−Teller (BET)15 surface area was determined by N2 physisorption using a Micromeritics ASAP 2020 automated system. The nanopaper sample was first degassed in the Micromeritics ASAP 2020 at 115 °C for 4 h prior to the analysis, followed by N2 adsorption at −196 °C. BET analysis was carried out for a relative vapor pressure of 0.01 to 0.3 at −196 °C. Pore size distribution was determined from N2 desorption at relative vapor pressure of 0.01 to 0.99 using a BJH model.16 Field-Emission Scanning Electron Microscopy (FE-SEM). The surface texture of the composites was observed by SEM using a Hitachi S-4800 equipped with a cold field-emission electron source. 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. X-ray Diffraction. Wide-angle X-ray diffraction photographs were obtained by irradiating the sample by Cu Kα radiation in the direction both perpendicular (in-the-plane) and parallel (cross-section) to the nanopaper surface. From azimutal intensity distribution graphs for the 200 equatorial reflection (most intense peak), two kinds of orientation factors, namely, the degree of orientation (Π), and Hermans orientation parameter ( f), were calculated according to eqs 1−3. fwhm is the full width at half-maximum. Φ represents the azimuthal angle and I(Φ) is the intensity along the Debye−Scherrer ring. f = 1 corresponds to a maximum orientation parallel to the drawing direction, whereas f = 0 indicates random orientation of the fibrils.

MATERIALS AND METHODS

Disintegration of NFC. An NFC water suspension was prepared from never-dried softwood sulphite pulp fibers (DP of 1200, lignin and hemicelluloses contents of 0.7 and 13.8%, respectively, Nordic Paper Seffle AB, Sweden) according to a previously reported method.3 The pulp was first subjected to a pretreatment step involving enzymes and mechanical beating. Subsequently, the pretreated pulp was disintegrated by a mechanical homogenization process using a Microfluidizer M-110EH apparatus (Microfluidics, USA) so that a 2 wt % NFC suspension in water was obtained. NFC/HEC Nanopaper Preparation. HEC (2-Hydroxyethyl cellulose, average Mv ≈ 1 300. 000 g/mol, substitution degree 1.5, molar substitution 2.5 ethylene oxide groups per anhydroglucose unit) was purchased from Aldrich. HEC was dissolved in water with magnetic stirring overnight to give a final concentration of 0.25 wt %. The 2 wt % NFC suspension was diluted to 0.25 wt % by mixing with water in an Ultra Turrax mixer (IKA, D125 Basic) for 10 min at 12 000 rpm. Three different proportions of the 0.25 wt % HEC aqueous solution and 0.25 wt % NFC water dispersion (total dry weight of ca. 400 mg) were mixed using a magnetic stirrer for 24 h. The mixtures were degassed and then vacuum-filtrated on a glass filter funnel using a 0.65 μm filter membrane (DVPP, Millipore) according to the previously reported method for nanopaper.11 The filtration results in small loss of the HEC polymer (ca. 20%). The wet cake, formed at the end of the filtration, was solvent-exchanged to ethanol and subsequently dried using supercritical CO 2 (Autosamdri-815, Tousimis, USA). Three different coated NFC nanopapers with the NFC/HEC weight ratios of ca. 75/25, 50/50, and 30/70 were obtained. Density and Porosity. Density of the nanopapers (ρc) was determined from their dry weight and their volume; the volume is taken as the area*thickness. The thickness was measured using a digital calliper.

Π=

180 − fwhm 180

(1)

f=

3⟨cos2 ϕ⟩ − 1 2

(2)

π /2

⟨cos2 ϕ⟩ =

∑0 I(ϕ) sin ϕ cos2 ϕ π /2

∑0 I(ϕ) sin ϕ

(3)

Mechanical Properties. Tensile tests of the NFC/HEC nanopaper and reference materials were performed using a universal testing machine (Instron 5944) equipped with a 50 N load cell. Specimen strips of 30 mm in length (gauge length of 20 mm) and 3 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 tensile strength was determined as the stress at specimen fracture. Work to fracture is taken as the area under the stress-strain curve. Drawing Experiments. NFC/HEC nanopaper strips of 4 mm in width and 30 mm in length were stretched up to a certain strain at a strain rate of 10% min−1 (gauge length of 20 mm); they were kept at that strain for 2 min after which no noticeable shrinkage could be seen. Then, the new dimensions of the samples were noted for further strength calculation, and the samples were stretched again at a strain rate of 10% min−1 until specimen fracture, resulting in tensile stress− strain curves of prestretched samples.

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RESULTS AND DISCUSSION Using an enzymatic pretreatment and a high shear mechanical disintegration, an aqueous suspension of NFC with diameter of the fibrils of ca. 15 nm and high aspect ratio (Figure 1) has been successfully prepared. 3662

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specific surface area.11,17,24 Drying of porous cellulosic structures from water results in a compact structure due to capillary effects, which bring fibrils close to each other so that mutual surface attraction forces cause the formation of fibril aggregates.11,25 The structures of the nonwoven nanocomposites were studied by SEM. Surface micrographs for the three compositions in Table S1 in the Supporting Information are presented in Figure 2. Some of the HEC appears to precipitate as particles, especially at high HEC concentration (Figure 2A− C). HEC particles have an oval shape ca. 1 μm long (magnified in upper left corner of Figure 2B). It is likely that these particles are formed during the solvent exchange step of the wet cake from water to ethanol or CO2, possibly due to the poor solubility of HEC. It was not possible to check the formation of oval HEC particles in pure HEC film because this latter cannot be prepared by supercritical drying using the present method. In a previous study, in situ cultivation of bacterial cellulose in the presence of HEC resulted in HEC-coated bacterial cellulose fibrils and ribbons.26 It is expected that in the present nonwoven nanocomposites, HEC coats the fibrils, whereas some of the excess HEC precipitates in the form of particles. In the composite with 70 wt % HEC (Figure 2C), the number of visible HEC particles is substantial. The magnified image of the porous structure shows a nonwoven fibrillar network structure (Figure 2a−c). HEC must then primarily be present as an NFC coating. For the nonwoven nanocomposite with highest content of HEC, some interfibril HEC domains could be observed (see arrows in Figure 2c), and a thicker HEC coating of the individual NFC fibrils is expected. To understand better the structure of the nonwoven NFC/ HEC nanocomposites, nitrogen adsorption experiments were performed, and the specific surface area and pore size distribution were determined. Results are presented in Figure 3. The specific surface area is rather high (BET = 37−213 m2 g−1) due to the nanometer scale of the lateral NFC or NFC/ HEC fibrils. Compared with pure NFC nanopaper prepared by supercritical drying (BET = 304 m2 g−1),7 the specific surface area is lower in the present composites due to presence of the HEC coating. The specific surface area is, as expected, further decreased with higher content of HEC. The estimated average pore size is around 10−15 nm based on the assumptions of the method used. Images in Figure 2 indicate that few pores are

Figure 1. SEM micrograph showing homogeneous size distribution of cellulose nanofibrils prepared. The scale bar is 1 μm.

NFC-based nanopaper and biocomposites have been widely studied.2,11,17−20 The inherent mechanical properties of the NFC fibrils and their strong network-forming ability can be utilized in a large variety of nanocellulosic materials. In the present study, focus is on porous nonwoven nanopaper network structures based on polymer-coated NFC fibrils. The term nonwoven nanocomposites will be used. After supercritical drying, the density was determined. (See Table S1 in the Supporting Information.) Density values are between 525 and 770 kg m−3 corresponding to porosities in the range 44−63%. Higher densities are for higher HEC contents. The density values are lower compared with the dense composites from NFC and HEC prepared by drying at elevated temperature (density is 980−1200 kg m−3). The present method involving supercritical drying is therefore a suitable method for preparation of lower density nonwoven nanocomposites. The drying step strongly influences the structure of the final nanocellulosic material. For instance, freeze-drying of NFC dispersions results in a high-porosity foam with an icetemplated structure and pores larger than 1 μm.21 In contrast, tert-butanol freeze-drying7,22,23 and supercritical drying7 of wet NFC preserves the fibrillar structure of the NFC network so that aerogels with pores in the submicrometer range and a specific surface area of up to 480 m2 g−1 can be formed.7 Drying at elevated temperature of a hydrated NFC cake results in a collapsed and dense nanopaper structure of low porosity and

Figure 2. SEM micrographs of NFC/HEC nonwoven nanocomposites with (A,a) 75 wt % NFC, (B,b) 50 wt % NFC, and (C,c) 30 wt % NFC. Scale bares are 5 μm and 500 nm for the upper and lower rows of images, respectively. 3663

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the present material (storage modulus lower than 60 MPa).28 A comparison with previously reported NFC/HEC composites10 of higher density (980−1200 kg m−3) (Figure 4b) shows that the present lower density nonwoven nanocomposites have lower strength and stiffness. Interestingly, the strain-to-failure of the nonwoven nanocomposites is doubled. Figure 2a−c provides an explanation in terms of the fibrillar network structure, where the interfibrillar interaction appears to be weak. This in combination with the higher porosity facilitates deformation and reorganization of the network structure. Previous studies on NFC nanopaper also demonstrated the correlation between increased porosity and surface area on increased strain-to-failure.7,17,24 The present nanocomposites combine large strain-to-failure with strain-hardening and high ultimate strength so that the work-to-fracture (area under the stress−strain curve) reaches values as high as 33 MJ m−3, exceeding 27 MJ m−3 reported for denser NFC/HEC polymer matrix nanocomposites.10 This is among the highest values for work-to-fracture in composite materials reported in the literature, with the additional advantage of low density. For the special case of nanocomposite fibers, Uddin et al reported a maximum work-to-fracture of spun fibers based on highly oriented PVA and cellulose whiskers of 88 MJ m−3.29 Molecular scale plastic deformation and strain-hardening of PVA itself is a dominating mechanism. The present nonwoven network structures instead rely on different deformation mechanisms, which are dominated by reorganization and frictional slippage of HEC-coated NFC fibrils. The extended chain conformation of cellulose does not allow any plastic deformation of the microfibril itself in tensile loading. Here we also have synergy effects between the ductility of HEC itself10 and the nonwoven structure in high-surface-area NFC nanopaper prepared by supercritical drying.7 Neat HEC has a work-to-fracture of 9.1 MJ m−3 and high surface area NFC nanopaper has 0.43 MJ m−3.10,7 The HEC-coated NFC nanocomposite appears to provide a favorable combination of interfibril friction, interfibril bonding, and plastic deformation contributions from the HEC itself. Furthermore, the nonwoven structure is suitable for other uses than fibers, that is, as tough and flexible membranes combining strength and porosity with a large capacity for plastic deformation. Because the present nonwoven nanocomposites have very large strain to failure, a reasonable hypothesis is that the fibrils align in the direction of stretching during deformation. Testing of this hypothesis is important because it would provide

Figure 3. Specific surface area (white circles) and average pore size distribution (black circles) of NFC/HEC composites.

larger than 30−40 nm. The pore size distribution data show that the pore diameter in the nonwoven nanocomposites is lowered with higher HEC content. This could be due to interfibrillar HEC domains reducing the pore size in the network. Mechanical properties of the nonwoven nanocomposites are measured in uniaxial tensile tests. The stress−strain curves are presented in Figure 4a, and data are summarized in Table S2 of the Supporting Information. Higher HEC content leads to increased ductility, and the maximum nominal strain-to-failure is as high as 55%. The stiffer materials have a yield stress of ∼18 MPa, followed by substantial strain hardening in the plastic region, and the ultimate strength reaches 80−93 MPa. The strain hardening is likely to be caused by fibril straightening, interfibril slippage, and reorientation of NFC fibrils during stretching. The significant ductility of the nonwoven nanocomposite is a result of the porosity, limited interfibril bonding, and possibly low friction facilitating fibril−fibril slippage of the HEC-coated NFC. Precipitated HEC may act as defect affecting mechanical properties, but this has not been verified. The present materials are rather soft with modulus of 0.8 to 1.3 GPa, and this is explained by the porous nature of the nonwoven nanocomposite as well as the other already mentioned factors, which influence ductility. Compared with electrospun polyethylene oxide/cellulose nanocrystal composite nanofibrous mats with a maximum modulus of 60 MPa and a strength of 8.5 MPa,27 the present nonwoven NFC/HEC membranes are much stronger and stiffer. Electrospun membranes of polyvinyl alcohol reinforced by cellulose nanocrystals also show a much lower stiffness compared with

Figure 4. (a) Tensile stress−strain curves of the present NFC/HEC nonwoven nanocomposites. (b) Comparison between tensile properties of the present nonwoven nanocomposites and low-porosity polymer matrix composites of NFC/HEC reported in ref 10 (higher strength and modulus). NFC weight fractions for the denser composites are 68, 56, and 38% and porosities are in the range 13−18%. 3664

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factor, 0.05 and 0.09, respectively; see Table S3 in the Supporting Information. The 20% value is somewhat surprising and may reflect local orientation in this sample. Upon stretching, strong intensity maxima develop, and the degree of orientation reaches 73 and 63% for the 30 and 50 wt % NFC compositions respectively. The alignment of the fibrils is in the direction of stretching and is due to reorientation of the fibrils during stretching facilitated by weak interfibril bonding, inherently high strain to failure of HEC, and high porosity of the material. The degree of orientation in the edge direction is also increased substantially for stretched samples compared with nonstretched samples; see Figure 5 and Table S3 in the Supporting Information. In the edge direction diffractograms presented in Figure 5, preferred NFC orientation is also observed in the nonstretched sample because for geometrical reasons vacuum filtration results in a layered structure in the thickness direction.11,30 One may note that the initial degree of orientation in the edge direction is 56−61% but increases to 81−83% after stretching. The technique of stretching to induce orientation of fibrils is known as drawing and has been used in different forms for cellulose orientation. Hot drawing (i.e., matrix in molten state) was used to orient cellulose whiskers in a PVA matrix.29 Cold drawing in the wet state was used to orient all-cellulose composites31 and native cellulose films.30 In the present study, the porous NFC/HEC nonwoven nanocomposites were drawn to increase modulus and tensile strength. This technique has been used for electrospun membranes32 and is possible due to the nonwoven network structure, limited fibril−fibril bonding, and high HEC ductility. The present materials were stretched to a certain strain and produced a certain degree of HECcoated NFC fibril orientation. The resulting materials were evaluated with respect to mechanical properties in uniaxial tension. Stress−strain curves are presented in Figure 6. A significant increase in modulus, yield strength, and ultimate strength occurs after drawing, whereas strain to failure decreases. For example, the modulus and strength increase from 0.8 GPa and 85 MPa to 2.0 GPa and 106 MPa, respectively, for the 50 wt % NFC sample prestretched to 20%. For the 30 wt % NFC sample prestretched to 40%, modulus and strength increase from 1.3 GPa and 93 MPa to 4.8 GPa and 150 MPa, respectively. The yield strength also increases substantially after prestretching. These are effects of increased NFC fibril orientation in the loading direction, as confirmed by XRD analysis. Rough density estimates for the 30 wt % NFC, based on geometry, did not indicate substantial change before

information on deformation mechanisms. Nonwoven nanocomposites with 50 and 30 wt % NFC were stretched to fracture (corresponding to a tensile strain of ca. 35 and 55%, respectively), and XRD diffractograms parallel (edge) and perpendicular (through) to the direction of stretching were recorded for the tensile-fractured composites and compared with the nonstretched samples. Diffractograms and azimuthal distribution peaks for (200) reflection are presented in Figure 5, and values for the degree of orientation are presented in

Figure 5. XRD diffractograms in the upper left of each figure and azimuthal intensity distribution of the equatorial reflection (200). The direction of analysis and the weight ratio of NFC/HEC are presented on top of each curve, E is edge, and T is through direction, as defined in the Figure. Stretched samples correspond to tensile fractured samples obtained after a tensile strain of ca. 35 and 55% for the 50/50 and 30/70 samples, respectively.

Table S3 in the Supporting Information. The nonstretched samples show ring pattern in the diffractograms obtained in the perpendicular direction because there is little preferred orientation of the fibrils, as is expected for random-in-theplane orientation. The corresponding degree of orientation is 1 and 20% for the samples with 30 and 50% NFC, respectively, and this corresponds to low values for Herman’s orientation

Figure 6. Effect of prestretching strain of NFC/HEC nonwoven composites (left: 30NFC/70HEC, right: 50NFC/50HEC) on the tensile properties during subsequent loading after the stretching procedure. Prestretching strain is presented next to each curve. 3665

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and after stretching to 40%. The mechanical properties of the present nonwoven nanocomposites can thus be tailored within a wide range. The preparation technique makes it possible to control porosity and NFC/HEC composition in fibrillar nanocomposite networks. Prestretching makes it possible to control further the extent of anisotropy in structure and in properties. The successful preparation of nonwoven nanopaper based on polymer-coated NFC fibrils has thus been demonstrated to increase the range of properties for biocomposite materials based on renewable resources.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +41587656118 (H.S.); +4687908118 (L.B.). Fax: +4687906166. E-mail: [email protected] (H.S.); [email protected] (L.B.). Present Address ⊥

Houssine Sehaqui, EMPA-Materials Science and Technology, Applied Wood Materials Laboratory, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland.

Notes

The authors declare no competing financial interest.

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CONCLUSIONS

ACKNOWLEDGMENTS Åsa Blademo and Mikael Ankerfors from Innventia AB are acknowledged for kind help regarding the use of BET equipment.

Nonwoven membranes prepared by electrospinning have limited mechanical performance, they seldom consist of nanofibers smaller than 100 nm in diameter, and they usually require organic solvents for their preparation. In the present study, NFC nanofibrils of ∼15 nm in diameter disintegrated from enzymatically pretreated wood pulp are instead combined with a water-soluble cellulose derivative (HEC) in water to produce nonwoven nanocomposite membranes after vacuum filtration and supercritical drying of the wet filter “cake”. The nonwoven membranes are based on NFC fibrils with an HEC coating. The NFC and HEC components are from renewable plant resources, no organic solvent is used during nonwoven fabrication, and the nanofiber diameter is much smaller than that for electrospun membranes. Furthermore, this process allows controlled porosity (44−63% for the present membranes) in a very small pore size range (average pore size 10− 15 nm, based on nitrogen adsorption data). The NFC/HEC membranes have high specific surface area (BET = 37−213 m2 g−1) because the supercritical drying procedure prevents NFC aggregation. The density of the composites is increased with the HEC content and is between 525 and 772 kg m−3. A remarkable feature of the nonwoven membranes is the high ductility combined with high strength. The strain to failure was as high as 55% with tensile strengths in the range 80−93 MPa, which is substantially higher than that for typical electrospun membranes. The high ductility combined with the strain hardening behavior observed in the plastic deformation region results in work to fracture data that are among the highest reported in the literature for materials in sheet form. During tensile loading, NFC straightening, reorientation, and sliding take place, as supported by XRD data for molecular cellulose orientation distribution. Stretched nonwoven nanocomposite membranes showed modulus and strength as high as 4.8 GPa and 150 MPa, respectively. The preparation and composition of the present nonwoven membranes can be controlled with the purpose to tailor porosity and mechanical properties. The present material concept broadens the range of properties for cellulose nanomaterials and opens many possibilities for new functionalities previously considered only for electospun nonwoven membranes.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Tables summarizing density, porosity, tensile mechanical properties, and degree of orientation of nonwoven NFC/ HEC membranes. This material is available free of charge via the Internet at http://pubs.acs.org. 3666

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dx.doi.org/10.1021/bm301105s | Biomacromolecules 2012, 13, 3661−3667