Preparation and Characterization of Cellulose Nanofibers from Two

Oct 23, 2009 - The aim of this work was to study the mechanical fibrillation process for the preparation of cellulose nanofibers from two commercial h...
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Ind. Eng. Chem. Res. 2009, 48, 11211–11219

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Preparation and Characterization of Cellulose Nanofibers from Two Commercial Hardwood and Softwood Pulps Wolfgang Stelte*,†,‡ and Anand R. Sanadi§ Luleå UniVersity of Technology, Forskargatan 1, SE-93187 Skellefteå, Sweden, Biosystems Department, Risø National Laboratory for Sustainable Energy, Technical UniVersity of Denmark, FrederiksborgVej 399, DK-4000 Roskilde, Denmark, and Forest & Landscape Denmark, Faculty of Life Sciences, UniVersity of Copenhagen, RolighedsVej 23, DK-1985 Frederiksberg C, Denmark

The aim of this work was to study the mechanical fibrillation process for the preparation of cellulose nanofibers from two commercial hard- and softwood cellulose pulps. The process consisted of initial refining and subsequent high-pressure homogenization. The progress in fibrillation was studied using different microscopy techniques, mechanical testing, and fiber density measurements of cellulose films prepared after different processing stages. The mechanical properties of cellulose films showed an increase in strength and stiffness with decreasing fiber size, and this stabilized after a certain number of passes in the homogenizer. Atomic force microscopy studies showed that the obtained cellulose nanofibers had diameters in the 10-25-nm range. The significant difference between the two samples was that the ultimate failure strain for cellulose films made of softwood fibers increased during the process whereas it remained constantly low for hardwood cellulose films. This difference could be due to the presence of shorter fibers and more defects in the films. An important point to note is that excessive processing reduced properties, as seen by the decrease in failure strain of softwood fiber films, and could also decrease other properties such as strength if the number of processing steps were further increased. 1. Introduction Cellulose is the most abundant polymer in nature and is composed of units of β-1,4-linked glucose. Cellulose fibers are present in all plant cell walls in combination with hemicelluloses, lignin, and extractives. The cellulose fiber is designed of nanosized fibers, often referred to as microfibrils, that have diameters in the range of 2-20 nm and lengths of up to several micrometers depending on their origin.1 Cellulose nanofibers consist of cellulose crystallites, also known as cellulose nanowhiskers, that are linked to each other by amorphous cellulose domains.2-4 Although the modulus of a cellulose crystal is about 138 GPa5 along the fiber axis, an assembly of crystallites connected by hydrogen bonds has a considerably lower elastic modulus. Nissan and Batten6 calculated theoretical values for the dry modulus of a fully tessellated isomorphic structure for native cellulose to be about 28 GPa. Henriksson et al.7 showed that networks of cellulose nanofibers can have elastic moduli close to 15 GPa and that the mechanical properties decrease with increasing porosity of the networks. There are different views on the coherence of paperlike cellulose fiber networks, invoking either hydrogen bonding or structural explanations such as fiber entanglement, orientation, and geometry. Nissan and Higgins8,9 explained the high tensile modulus of cellulose networks in terms of the density and stiffness of hydrogen bonding between cellulose chains. Hydrogen bonds as the stabilizing element in cellulose networks offer reasonable explanations (both qualitative and quantitative) for the effects of moisture, temperature, and time on the elastic modulus of cellulose networks5,10,11 that could not be explained using a structural model. Nevertheless, the contribution of fiber morphology and distribution to the * To whom correspondence should be addressed. E-mail: wost@ risoe.dtu.dk. Tel.: +45 4677 4288. Fax: +45 4677 4109. † Luleå University of Technology. ‡ Technical University of Denmark. § University of Copenhagen.

elastic modulus of cellulose fiber networks is well-established and cannot be ignored. Both the molecular and structural approaches were consolidated by Nissan and Batten6 through the introduction of the theory of percolation networks and the concept of an invariant modulus. The abundance, excellent mechanical properties, and good biocompatibility12 of cellulose have made it an interesting candidate for materials science. Several works using cellulose nanofibers13-20 and cellulose nanowhiskers21-23 as reinforcements in composite materials have been published recently. The use of cellulose nanofiber films in packaging applications has risen in interest because of their high mechanical properties and low gas permeability rates.24,25 Several methods for the preparation of nanofibers from cellulose have been described in the literature and have been reviewed by Gardner et al.26 and Ioelovich.27 Strong acid hydrolysis of amorphous cellulose combined with ultrasound treatment results in highly crystalline cellulose nanofibers that are often termed whiskers.22,28,29 More amorphous nanofibers with higher aspect ratios can be isolated mechanically by applying a high shear force on the cellulose fiber surface, which is usually done in a combined process of initial refining and high-pressure homogenization.13-15,17,20,30,31 The two major and different methods of disintegrating cellulose fibers into nanofibers are very well-described and schematically represented by Paakko et al.32 The raw materials for the mechanical isolation process are either R-cellulose pulps15,17,20,24,30,31 or chemically pretreated plant material from which most noncellulosic compounds have been removed.13,14,23,33,34 The isolation process usually requires numerous passes through the refiner and highpressure homogenizer before the process reaches a steady state in terms of tensile strength and density.7 These nanofibers have been shown to result in highly entangled networks usually having a wide size distribution down to the nanoscale.17,30,31 Alternatives to the chemo-mechanical isolation processes have been suggested recently by Henriksson et al.35 and Paakko et

10.1021/ie9011672 CCC: $40.75  2009 American Chemical Society Published on Web 10/23/2009

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al.32 through the introduction of an enzyme-assisted hydrolysis step combined with subsequent mechanical treatment, resulting in a more efficient mechanical isolation process and reducing the number of passes through the fibrillation equipment and technical problems connected to the mechanical fibrillation process. The mechanical refining of cellulose fibers in aqueous suspensions has several structural effects on cellulose fibers, and because of their importance for the pulp and paper industry, this process was reviewed in detail by Clark.36 The concept of cellulose microfibril preparation from wood cell walls was introduced by Turbark et al.30 and Herrick et al.31 in the early 1980s. They were the first to prepare nanosized cellulose fibers from different softwood pulps, which they termed cellulose microfibrils, and investigated possible applications as rheology modifiers in foods, paints, cosmetics, and pharmaceutical products. Cellulose nanofibers have been successfully isolated from many different sources such as softwood pulps,17,20,30-32,35 sugar beet,13,18 potato,14 ficus,19 bacteria,16,37 banana rachis,34 and sisal,23 with wood pulps being the most-used raw material. Wood species can be distinguished as hard- and softwoods, with basic differences in their anatomical features. Hardwoods are, in general, more complex and heterogeneous in structure than softwoods, having specialized vessel elements for transport functions and shorter fiber cells compared to softwoods.38 Hardwood fibers are known to have a more rigid structure than softwoods because of their higher Runkel ratios. According to Law et al.,39 the spirally layered outer secondary wall (S1 layer) restricts the flexibility of hardwood mechanical pulp fibers and thus prevents access to the subjacent inner secondary wall (S2 layer).40 Until now, the cellulose pulps used for chemo-mechanical separation processes of cellulose nanofibers as applied in other studies13,17,20,37 have been based on softwood pulps, and to the best of our knowledge, no utilization of hardwood pulps has been reported so far. Some efforts to use hardwood pulps were made recently by Fukuzumi et al. and Saito et al.,25,41 but they used an oxidative pretreatment process referred to as TEMPOmediated oxidation. The motivation for this study was to compare two commercial R-cellulose pulps from hard- and softwood and to study the mechanical isolation process to isolate cellulose nanofibers from both types of pulp. It was our interest to study the impact of mechanical fibrillation on the morphology of the two commercial cellulose fibers by means of different microscopy techniques and physical characterization of cellulose films produced after different process stages using tensile testing and density measurements. The process studied is one of the most commonly used techniques for producing cellulose nanofibers from plant materials. Numerous studies have used the present process to produce nanofibers as described earlier, and it has been patented in different variations42-44 and is used by a Japanese company for the commercial production of microfibrillated cellulose.45 Understanding the mechanism of mechanical fibrillation and analyzing the properties such as the failure strain using different raw materials represent important milestones for process optimization and raw-materials choices for a myriad of uses. 2. Experimental Section 2.1. Materials. Two commercial cellulose pulps based on hard- and softwood with cellulose contents of >97% and >94%, respectively, were obtained in a dried form. The hardwood pulp was produced from southern hardwoods containing gum, maple, oak, eucalyptus, poplar, or beech or a mixture thereof and was

obtained from Rayonier Inc. (Sulfatate-H-J pulp, Rayonier, Jacksonville, FL). The softwood pulp was obtained from Domsjoe Fabriker AB in dissolving grade and was manufactured from a mixture of northern spruce and Scots pine (Softwood ¨ rnsko¨ldsvik, Sweden). dissolving pulp, Domsjo¨ Fabriker AB, O 2.2. Mechanical Fibrillation of Cellulose Nanofibers. Both raw materials were swollen for 1 day in water by mixing them with deionized water, resulting in a suspension with 1 wt % dry pulp. The samples were dispersed using a blender (HR 172, Philips, Eindhoven, The Netherlands) for 10 min to get a consistent fiber suspension and further treated using a disk refiner and high-pressure homogenizer. Refining. The cellulose fibers at 1 wt % consistency were first passed through a refiner (Cerendiptor MKCA 6-3, Masuko Sangyo Co., Ltd., Saitama, Japan) that consisted of a rotating disk and a static disk with an adjustable gap between them. The distance between the disks was adjusted as suggested by Nakagaito et al.17 to about 0.1 mm, power was adjusted to 75% of the maximum, and the cellulose suspensions were passed continuously through the refiner, with samples taken after 5, 25, 50, and 75 passes. Samples for analysis were prepared after each of these passes. High-Pressure Homogenization. Further fibrillation was done using a high-pressure homogenizer (model 2000, APV, Albertslund, Denmark). The working principle of a homogenizer is that cellulose fibers are pressed through a valve at high pressure and exposed to a pressure drop to atmospheric conditions when leaving the valve, resulting in high shear forces on the fiber surface. In the case of hardwood cellulose fibers, the homogenizer was often clogged, and it was therefore necessary to remove all particles larger than 250 µm by passing the cellulose suspension through a sieve before homogenization. Clogging is probably due to the large particle size of the wall fragments from the vessel elements present in the hardwood fibers. Softwood cellulose suspensions could be passed directly through the homogenizer without clogging, and no sieving step was necessary. The pressure was adjusted to about 500 bar, and the suspensions were passed through the equipment, collected in a vessel, and repeatedly passed through the homogenizer until no further fibrillation could be observed by studying the morphology and by mechanical testing of manufactured cellulose films. Process samples were taken after 5, 10, 25, and 50 passes through the homogenizer, and in the case of hardwood cellulose, additional samples were analyzed after 100 and 150 passes. 2.3. Cellulose Film Preparation. Cellulose films were produced based on the method described by Dufresne et al.13 According to this procedure, 100 g of each process sample containing 1 wt % cellulose in water was degassed by ultrasound treatment, cast into polystyrene Petri dishes, and dried at ambient temperature and humidity. The films from different samples were prepared simultaneously so that the drying conditions can be regarded as same. The dried films were cut into strips of 40-mm length and 10-mm width and were conditioned in desiccators at 5% relative humidity for at least 1 week. The films were subsequently used for mechanical testing and density measurements. 2.4. Characterization. The degree of fibrillation was studied using light microscopy (LM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Mechanical testing and density measurements of cellulose films were carried out to obtain additional information about the effects of fibrillation. The changes in fiber morphology, mechanical properties, and

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fiber density were used to find the process equilibrium and to compare the impact of mechanical fibrillation on hard- and softwood cellulose fibers. The process equilibrium was defined as the stage when no further changes of mechanical properties and density changes were observed with additional processing of the fibers, considering statistical variations. Light Microscopy (LM). The changes in morphology during the mechanical treatment of the pulp fibers were studied using a light microscope (Dialux 20, Leitz, Wetzlar, Germany) at 500times magnification. Diluted cellulose fiber suspensions after different processing stages were prepared, and one drop of the suspension was placed on a glass slide covered with a coverslip. The images selected were regarded to be representative of the fiber surfaces of the two defibrillated pulp fibers. Scanning Electron Microscopy (SEM). SEM was used to study the effects of mechanical fibrillation on the fiber morphology. Samples were prepared by solvent exchange from water to the less-polar solvent ethanol in order to reduce the amount of hydrogen bonding between the fibers and the likelihood of aggregate formation during drying. Samples were collected after each processing stage and were separated from water by centrifugation at 5000g for 5 min. Supernatant liquid was decanted, and the fibers were immersed in 99% ethanol using a high-shear mixer. These steps were repeated three times to replace most of the water with ethanol. The obtained suspensions of cellulose in ethanol were frozen in liquid nitrogen and dried in high vacuum using a freeze-dryer (Alpha 2-4 LD plus, Martin Christ GmbH, Osterode am Harz, Germany). The dried samples were coated with a thin layer of gold using a sputter coater (Desk II, Denton Vacuum, Moorestown, NJ). Electron micrographs were recorded using a scanning electron microscope (JSM 5200, JEOL, Tokyo, Japan) operated at 10-20 kV. The presented micrographs were selected carefully and are regarded to be representative of the fiber surfaces of the two defibrillated pulp fibers. Atomic Force Microscopy. The nanofibers were studied using an atomic force microscope (Nanoscope V, Veeco, Plainview, NY). Sample were prepared according to the method used by Zuluaga et al.34 The cellulose suspensions were diluted with water, and one drop was dried on a freshly cleaved mica plate. The equipment was operated in tapping mode using etched silicon probes (FESP, Veeco) at a resonance frequency of about 70 kHz and a spring constant of 1-5 N/m. The diameters of the fibers were measured from the height images using special image analysis software (Nanoscope V Software, Veeco Netherlands, Breda, The Netherlands), where the broadening effect due to the AFM tip scanning was absent and the diameter measurements were therefore considered to be fairly accurate. The presented images were regarded to be representative of the materials after the applied number of processing steps. Tensile Testing. The mechanical properties of cellulose fiber films after different processing stages were studied using a tensile tester (H25KS, Hounsfield, Surrey, U.K.) with a 500-N load cell. The samples were dried and conditioned at 5% relative humidity for at least 1 week and removed from the climate chamber shortly before testing. The tests were done at ambient temperature and moisture content at a strain rate of 5 mm/min and a gauge length of 20 mm. The test were conducted under the same testing conditions and, therefore, are regarded to be comparable to each other. Tensile strength and E modulus were calculated based on stress-strain curves for at least five measurements per sample. The thickness of the specimens was determined using a micrometer screw and calculating the

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average of at least three measurements made at different positions on the fracture surface of each test stripe directly after testing. Density Measurements. The weight, area, and thickness of the cellulose fiber films were determined, and the density was calculated. An average of at least five measurements per sample was used for the calculation. The weight of the films was determined on dried films using an analytic balance, and the thickness was determined using a micrometer screw calculating the average of at least three independent measurement at different locations on the film (center, periphery, and between the two). 3. Results and Discussion 3.1. Process Observations. Cellulose fibers originating from the hardwood pulp were more difficult to fibrillate than the those obtained from the softwood pulp, as evidenced by pressure fluctuations and clogging of the homogenizer. Homogenization of hardwood fibers was possible only after an additional sieving step through a 250-µm mesh, which removed large fiber aggregates, whereas softwood fibers could be processed without this pretreatment. The final products had a gel-like consistency that developed gradually through the process. The temperature of the samples increased during refining and high-pressure homogenization to about 60-80 °C because of the high energy input into the process. 3.2. Mechanical Fibrillation. Changes in Morphology during Refining. The present study evaluated the change in morphology of hard- and softwood cellulose fibers during a mechanical fibrillation process. Figures 1 and 2 present LM and SEM images, respectively, of hard- and softwood cellulose fibers exposed to mechanical shear force in a disk refiner after 5, 25, and 75 passes. It can be seen that, before fibrillation, both hardand softwood pulps consist of fibers with a size of about 20-30 µm in diameter. The images show that fibrillation starts at the outer surface of the cellulose fibers for both hardwood and softwood and that small-sized fibers are being peeled from the fiber surfaces. Furthermore, it can be seen that hardwood cellulose fibers are not fibrillated to the same extent as softwood fibers during the refining process. The softwood cellulose fibers lose their structure almost completely after 25 passes, resulting in aggregates of small fibers, whereas the hardwood fibers remain mostly intact after 75 passes. Turbak et al.30 explained two phenomena associated with the refining process of softwood cellulose and termed them external and internal fibrillation. External fibrillation is the raising of fine fibers on the fiber surface through abrasive action, whereas internal fibrillation is related to the breakage of links between the cellulose fibers as a result of mechanical action in the refining processes.30,46 It is likely that H-bonds are broken because of the mechanical process, resulting in internal fibrillation. During the mechanical refining process, the cellulose fibers are subjected to repeated loading action of the refiner bars. Hamad47 reported that, during the refining of cellulose fibers in a disk refiner, the external cell wall layers, the primary (P) and first secondary (S1) layers, are gradually peeled off from the fiber surface and expose the subjacent thicker secondary cell wall layers. The present microscopic study shows that the process used in this study is faster and more efficient for fibers from softwood than for those from hardwood pulp. In the case of the hardwood fibers, it seems that the structure of the hardwood fibers resists the refining process and that only the outer parts of the cell wall are affected. On the other hand, the softwood fibers undergo

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Figure 1. Effects of mechanical refining on hard- and softwood cellulose fibers, before and after 5, 25, and 75 passes through the refiner. The presented images are regarded to be representative of the materials after the applied number of processing steps.

both external and internal fibrillation of the fiber cell walls after about 25 passes, resulting in a network of micro- and nanosized cellulose fibers. Change in Morphology during High-Pressure Homogenization. Figures 3 and 4 present LM and SEM images, respectively, of hard- and softwood fibers after they have been passed through a high-pressure homogenizer. The hardwood fibers that were fibrillated only on the surface during the refining step are now disintegrated into a network of small fibers. AFM images of the final products after high-pressure homogenization are presented in Figure 5 and show that the size distribution of the hard- and softwood nanofibers is in the range of 10-25 nm in diameter. Although aggregates of nanofibers can be seen in both samples, the fibrillation process used in this study can be considered effective because the aggregates are less than 100 nm in diameter. Aggregation of cellulose nanofibers due to hydrogen bonding is a well-known phenomenon and has been observed earlier and reported to depend on the species and the method of separation of the nanofibers.48 Mechanical Testing. Samples prepared after different stages of the fibrillation process were tested in the tensile mode, and typical stress-strain curves are shown in Figure 6. The curves closest to the average values of at least five tests per sample

are shown. These materials exhibit typical behavior as a function of strain, consisting of an initial linear region followed by a decreasing slope until failure, as reported by Dufresne et al.13 for cellulose microfibril films produced from sugar beet. The slope of the initial linear region was used to calculate the tensile (Young’s) modulus (E). In the case of hardwood fibers, there is an increase in both Young’s modulus (E) and ultimate strength (σut) as the number of passes through the homogenizer increases. From the microscopy studies, it can be observed that the average fiber diameter decreases with the number of passes. This results in more fibers bridging the network of the film and in increased interaction through both H-bonding and mechanical interaction forces. As a consequence, increases in both E (Figure 7) and σut (Figure 8) can be observed, and this is true for both the refined and homogenized hardwood films. In the case of the hardwood samples, it does appear that the properties seem to level off (E, σut, and density; considering statistical variation, as indicated with the error bars) at about 100-150 passes through the homogenizer. In the case of the softwood fiber films, there is a considerable difference in both E and σut (Figures 7 and 8) during the refining process. However, there is little change in either E or σut (considering statistical variation) during homogenization. This suggests that, after the first few passes through the homogenizer,

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Figure 2. Scanning electron micrographs of hard- and softwood cellulose fibers, before and after 5, 25, and 75 passes through the refiner. The presented images are regarded to be representative of the materials after the applied number of processing steps.

there is little difference in the number of fibers bridging each other or in the amount of hydrogen bonding and structural consideration that gives the films their mechanical integrity. The E moduli of both the hard- and softwood fiber films are similar at their highest observed values, and this suggests that the densities and numbers of H-bonds in the two systems are comparable. However, the strengths of the softwood fiber films are higher than the strengths of the hardwood fibers. This suggests that the defects in the hardwood fiber films are higher than those within the softwood fiber films and that the refining and homogenization process results in significantly higher defects in the hardwood fibers. The moduli of such films are a fundamental material property governed by H-bonds, network arrangement, and porosity, whereas the strength is dictated by defects. Henriksson et al.7 pointed out that the tensile strength, toughness, and strain to failure of cellulose nanofiber films correlate with the average molar mass of the nanofibers. In our study, the fibrillation of hardwood cellulose fibers into nanofibers requires a much greater number of passes through the homogenizer, resulting in a greater exposure to shear forces that can result not only in separation of the nanofibers, but also in a decrease of the cellulose fiber length. It is therefore possible that the cellulose nanofibers of softwood are longer, thus resulting in higher film strengths. There is a significant difference in the ultimate failure strain, εult, between hard- and softwood fiber films, as shown in Figure 9. The εult value of the softwood pulp films increases monotonically with the number of passes until about 10 passes through the homogenizer, after which the failure strain decreases. It is well-known that high shear forces can reduce the length of the fibers and create defects resulting in lower values of εult, strength,

and stiffness.7 However, in complete contrast, there is little difference in the failure strain for the hardwood pulps. Here, the failure strain remains more or less the same throughout both the refining and homogenizing process and is much lower than that of the softwood films. It is possible that the hardwood nanofibers have more defects and are shorter, which could explain the lower strength and failure strain. These defects could be inherent, or they could have been induced. Furthermore, short fibers tend to have more fiber ends, and these ends act as defects in films. This behavior warrants more studies in the future, in addition to studies using porosity along the lines of work by Dufresne et al.13 and the molar mass effect as indicated by Henriksson et al.,7 to gain a deeper understanding of the interesting effects on failure strain of the hardwood cellulose films. Density. There is a continuous increase in density (smaller diameter resulting in better packing) throughout the refining and homogenizing process for the cellulose fiber films (Figure 10). In the case of the softwood fibers, there is little difference in density once the homogenizer is used. The density increases for both hard- and softwood films by a factor of about 4 during the fibrillation process. The films of hardwood cellulose nanofibers after process equilibrium have a density of approximately 950 kg/m3, whereas the density of the softwood cellulose nanofibers is marginally higher at about 1000 kg/m3. Nevertheless, the surfaces of the cellulose films are porous and rather inhomogeneous in their morphology, which explains the relatively low mechanical properties when compared to those used in other studies that are based on cellulose fiber films obtained by suction filtration on a membrane,7,24,25 because this process compacts the films. The increase in density of the films

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Figure 3. Effects of high-pressure homogenization on hard- and softwood cellulose fibers. The images show the cellulose fibers before and after 10 passes through the homogenizer. The hardwood cellulose fibers are also shown after 100 passes through the homogenizer. The presented images are regarded to be representative of the materials after the applied number of processing steps.

Figure 4. Scanning electron micrographs of hard- and softwood cellulose fibers, before and after 10 passes through the homogenizer. In the case of the hardwood cellulose fibers, an additional image after 100 passes is shown. The presented images are regarded to be representative of the materials after the applied number of processing steps.

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Figure 5. Atomic force microscopy images of hard- and softwood cellulose nanofibers at process equilibrium. The presented images are regarded to be representative of the materials after the applied number of processing steps.

Figure 8. Strength of cellulose fiber films after different numbers of fibrillation steps.

Figure 6. Stress-strain curves of hard- and softwood wood cellulose fiber films during high-pressure homogenization.

Figure 9. Stiffness of cellulose fiber films after different numbers of fibrillation steps.

Figure 7. Strain at break of cellulose fiber films after different numbers of fibrillation steps.

is directly related to the increasing tensile strength and modulus of the films. Similar trends are observed for films of both hardand softwood fibers. The porosity decreases with increasing density, and the contact surface between adjacent cellulose fibers within the network increases, resulting in higher strength and stiffness. 5. Conclusions It has been shown that cellulose nanofibers can be isolated using a mechanical fibrillation process from both types of commercial hard- and softwood cellulose pulps used in the present study. The fibrillation proceeds much faster and more easily for the softwood pulp fibers compared to the hardwood pulp fibers. A larger number of passes through the homogenizer is necessary to achieve the highest observed properties of the hardwood fiber films. In the case of the softwood fiber films, very few passes through the homogenizer are needed to achieve

Figure 10. Average density of solution-cast cellulose films after different numbers of fibrillation steps.

optimum properties, suggesting that the refining processes is effective in converting the original pulp material into nanofibers. An increase in processing steps obviously increases the energy required for fibrillation. The results from mechanical testing correlate with the microscopy study and density measurements showing that significant improvements in the mechanical properties first occur after internal fibrillation of the cellulose fibers. The fibrillation of the softwood cellulose pulp used in the present study requires less energy and is therefore more economical than that of the hardwood cellulose pulp used in this study and therefore makes the former more appropriate for commercial applications. The differences in tensile behavior between the two types of films are a slightly lower strength of

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hardwood nanofiber films but a slightly higher modulus compared to the those from softwood. However, the significant difference is that the failure strain of the hardwood cellulose films is much lower than that of the softwood cellulose ones and remains more or less unchanged through the fibrillation process. This behavior is interesting and warrants further study. It is possible that factors such as defects and molar mass play an important role in this behavior. It is also important to note that excessive processing can reduce properties as seen by the decrease in failure strain of softwood fiber films, suggesting that further processing could also decrease other properties such as strength. For further studies, the authors suggest measuring how the molecular weight is changed during processing and determining whether there is a correlation with the mechanical properties. Measurement of the H-bond density, if possible, along with fiber orientation and lengths would be helpful to shine more light onto the nanofiber interactions in these types of films. Acknowledgment The authors gratefully acknowledge the European Commission for financial support under FP6 Contract NMP4-CT-2006033277 (TEM-PLANT) and Luleå University of TechnologyCampus Skellefteå for funding and hospitality during this study. The authors thank Daniel F. Caulfield for invaluable recommendations and suggestions during the writing of the manuscript, Kristiina Oksman Niska for the use of the laboratory equipment and provision of raw materials, Niclas Bjo¨rngrim for help with the tensile tests, and Aji P. Mathew for help with tensile testing and AFM adjustments. Lisbeth G. Thygesen and Tom Elder are acknowledged for their valuable comments on the manuscript. Literature Cited (1) Goto, T.; Harada, H.; Saiki, H. Fine structure of cellulose microfibrils in poplar gelatinous layer and valonia. Wood Sci. Technol. 1978, 12 (3), 223-231. (2) Fengel, D. Ultrastructural behavior of cell wall polysaccharides. Tappi 1970, 53 (3), 497–503. (3) Frey-Wyssling, A. Ultrastructure of wood. Wood Sci. Technol. 1968, 2 (2), 73–83. (4) Heyn, A. N. J. Ultrastructure of wood pulp with special reference to the elementary fibril of cellulose. Tappi 1977, 60 (11), 159–161. (5) Nissan, A. H.; Batten, G. L. On the primacy of the hydrogen-bond in paper mechanics. Tappi J. 1990, 73 (2), 159–164. (6) Nissan, A.; Batten, G. Unification of Phenomenological, Structural and Hydrogen-Bond Theories of Paper, Using Percolation Concepts. Nord. Pulp Pap. Res. J. 1987. (7) Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindstrom, T.; Nishino, T. Cellulose nanopaper structures of high toughness. Biomacromolecules 2008, 9 (6), 1579–1585. (8) Nissan, A. H. Molecular approach to the problem of viscoelasticity. Nature 1955, 175 (4453), 424–424. (9) Nissan, A. H.; Higgins, H. G. Molecular approach to the problem of viscoelasticity. Nature 1959, 184 (4697), 1477–1478. (10) Nissan, A. H. 3 modes of dissociation of H-bonds in hydrogenbond dominated solids. Nature 1976, 263 (5580), 759–759. (11) Nissan, A. H. H-bond dissociation in hydrogen-bond dominated solids. Macromolecules 1976, 9 (5), 840–850. (12) Helenius, G.; Backdahl, H.; Bodin, A.; Nannmark, U.; Gatenholm, P.; Risberg, B. In vivo biocompatibility of bacterial cellulose. J. Biomed. Mater. Res. A 2006, 76A (2), 431–438. (13) Dufresne, A.; Cavaille, J. Y.; Vignon, M. R. Mechanical behavior of sheets prepared from sugar beet cellulose microfibrils. J. Appl. Polym. Sci. 1997, 64 (6), 1185–1194. (14) Dufresne, A.; Dupeyre, D.; Vignon, M. R. Cellulose microfibrils from potato tuber cells: Processing and characterization of starch-cellulose microfibril composites. J. Appl. Polym. Sci. 2000, 76 (14), 2080–2092. (15) Iwamoto, S.; Nakagaito, A. N.; Yano, H. Nano-fibrillation of pulp fibers for the processing of transparent nanocomposites. Appl. Phys. A: Mater. Sci. Process. 2007, 89 (2), 461–466.

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ReceiVed for reView July 27, 2009 ReVised manuscript receiVed September 16, 2009 Accepted October 12, 2009 IE9011672