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Holocellulose nanofibers of high molar mass and small diameter for high-strength nanopaper Sylvain Galland, fredrik berthold, Kasinee Prakobna, and Lars A. Berglund Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.5b00678 • Publication Date (Web): 07 Jul 2015 Downloaded from http://pubs.acs.org on July 12, 2015

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Holocellulose nanofibers of high molar mass and small diameter for high-strength nanopaper Sylvain Galland†, Fredrik Berthold†,‡, Kasinee Prakobna†, Lars A. Berglund*† †

Wallenberg Wood Science Center, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden. ‡

Innventia AB, PO BOX 5604, SE-114 86 Stockholm, Sweden

* To whom correspondence should be addressed: [email protected]

ABSTRACT Wood cellulose nanofibers (CNFs) based on bleached pulp are different from the cellulose microfibrils in the plant cell wall in terms of larger diameter, lower cellulose molar mass, and modified cellulose topochemistry. Also, CNF isolation often requires high-energy mechanical disintegration. Here, a new type of CNFs is reported based on a mild peracetic acid delignification process for spruce and aspen fibers, followed by low-energy mechanical disintegration. Resulting CNFs are characterized with respect to geometry (AFM, TEM), molar mass (SEC) and polysaccharide composition. Cellulose nanopaper films are prepared by filtration and characterized by UV-VIS spectrometry for optical transparency, and uniaxial tensile tests. These CNFs are unique in terms of high molar mass and cellulose-hemicellulose 1 Environment ACS Paragon Plus

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core-shell structure. Furthermore, the corresponding nanopaper structures exhibit exceptionally high optical transparency and the highest mechanical properties reported for comparable CNF nanopaper structures.

KEYWORDS: cellulose nanofibril, nanopaper, mechanical properties, cellulose nanocomposite, biocomposite, nanocellulose

1. INTRODUCTION The recent interest in cellulose nanofibers (CNF) from wood is extending from the academic community to commercial enterprises. Industrial companies are building pilot- and productionscale plants for CNF, and the Web of Science number of research publications on nanocellulose topics during 2014 was more than 1000. This interest is beyond expectation, although one can speculate about the reasons. It is relatively easy to prepare nanocellulose from plant cell walls, and this component is a true, fibrous nanoparticle of high modulus and strength from plant biomass, and potentially of low cost. A large variety of nanomaterials can be prepared, which have functionalities not previously attainable from materials based on plant-fibers. Nanocellulose materials can be prepared by solvent-free methods, preserving the native cellulose fibril structure, involving only water-based processing. CNF has been used to produce a variety of nanomaterials1 including aerogels,2 hydrogels,3 foams,4 honeycombs,5 neat CNF films,6 porous CNF membranes, CNF/polymer films,7,

8

CNF/polymer composites for compression molding,

injection molding9-11 etc and a large variety of CNF/inorganic hybrid nanomaterials.12-14 It has been recently reported that CNF/polysaccharide films with improved hygromechanical properties

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and moisture stability were successfully prepared based on the core-shell nanofiber concept.15, 16 A key design mainly involves nanostructural control of polymer matrix distribution and interface structure for core-shell nanocomposites. The molecular and nanostructural details of the materials with respect to water and polymer matrix mobilities were investigated using NMR relaxometry.17 In the future, it is likely that there will be a range of wood CNF qualities commercially available, with characteristics tailored for specific applications. Coarser fibril aggregates, as produced by Turbak et al,18 may be suitable for some applications where low cost is critical. TEMPO-oxidized CNF is suitable for films or coatings of high optical transparency19,

20

but

filtration is time-consuming, and the TEMPO-catalyst is toxic. Cationic CNF21 also needs longer filtration times. Enzymatic CNF22, 23 involves an environmentally friendly preparation procedure, and the cellulose fibrils in the resulting CNF are of similar structure as in the precursor wood pulp fibers. In a recent study, the reported average CNF weight average diameter was around 6 nm.15 Although this small diameter is attractive, it is still larger than for TEMPO-CNF. It is desirable to develop a new type of CNF where the native cellulose microfibril structure from the wood cell wall is well preserved, in particular in terms of molar mass (fibril length) and lateral dimension. Wood cellulose nanofibers are usually disintegrated from bleached chemical wood pulp. The advantage is that this is an industrially highly realistic starting component, which is available world-wide in predictable quality and in enormous quantities. The amount of hemicellulose in the pulp will influence both the ease of defibrillation and the properties of produced films and composites (Iwamoto et. al, 2008).24 In this study, Sitka spruce was delignified using the acid chlorite (AC) method to produce holocellulose CNF with high hemicellulose content. While AC


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delignification is an effective method to produce holocellulose it can result in concomitant loss in cellulose molecular weight.25, 26 This was also observed in a study27 comparing delignification using either AC or peracetic acid (PAA) of several different biomass types. They found that PAA generally showed higher selectivity and caused less cellulose degradation. In a study on the extraction of heteroxylans from holocellulose,28 it was observed that the extraction of xylan from PAA delignified wood was several times more effective compared to when the extraction was done on AC delignified holocellulose. They attributed this to more effective delignification and reduced lignin-xylan interactions. The objective of the present study is to prepare new CNF fibrils of high molar mass and small diameter from native wood CNF (no surface derivatization of the cellulose fibrils). Fibrils will be investigated with respect to their potential in new CNF-based materials, such as CNF nanopaper prepared by filtration of colloidal suspensions. Scientific objectives include improved understanding of fibrillation mechanisms as well as relationships between structural factors and tensile properties of CNF nanopaper.

2. EXPERIMENTAL SECTION Holocellulose nanofibril preparation. Industrial softwood (Spruce) chips or Aspen wood chips were cut into fine ”match sticks” (max width 2 mm) and soaked in water under vacuum to remove trapped air. After removal of air the sticks were treated with a diethylene triamine pentaacetic acid (DTPA)/sodium sulfite solution (0.3 % / 4.0 %) for one hour at 85°C, after which the sticks were thoroughly washed with deionized (DI) water until a conductance below 5 µS was measured. The function of DTPA/sulfite is to remove metal ions,29 thereby increasing


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PAA30 and cellulose stability.31 DTPA is a chelating agent, and such compounds are also used in commercial PAA bleaching.32 In a second step the washed sticks were treated with a 10 % peracetic acid solution (V:W 25:1) for 20-60 minutes at 85 °C (the reaction was stopped when the sticks were fully fibrillable). Before the sticks were added to the solution the pH was carefully adjusted to 4.8 by addition of small amounts of 10 M NaOH. After the treatment the sticks were washed and slushed using an ordinary pulp slusher (10000 revolutions). This was sufficient to liberate the pulp fibers in suspension. As a final step, the pulp was treated with mild alkali solution (0.01 M NaOH) for two hours at ambient temperature and there after washed untila conductance below 5 µS was reached. The holocellulose wood fibers were then subjected to mechanical disintegration into cellulosic nanofibers (CNFs) in the form of dilute colloidal suspensions. The pulp fibers were stored wet before the mechanical disintegration, which consisted in several passes through a microfluidizer M-110EH (maximum pressure 1600bar, Microfluidics Ind., USA). A pair of 400 µm / 200 µm chambers was used for the first pass. Then a total of 5 subsequent passes were made using 200 µm / 100 µm chambers. Samples were taken after each pass and are referred to as 1+0, 1+1, 1+2, 1+3, 1+4, and 1+5. The solid content of the obtained nanofibril suspensions were in the range 0.8-1.2 wt.%. The suspensions of holocellulose CNFs were used to prepare “nanopaper” films.6 Another type of holocellulose CNFs was prepared from chlorite delignification. The chlorite delignification was performed essentially according to Ahlgren et al.33 In short, matchstick sized aspen or spruce wood were first extracted overnight in an excess of a water/acetone (1:7), followed by solvent exchange back to water. Spruce sticks were thereafter extracted overnight in ethanol/toluene (2:1), followed by solvent exchange back to water. The chlorite treatment was


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performed at 70 °C. The sticks were suspended in water (15:1 v:w). To the suspension 0.3 g Sodium chlorite and 0.1 ml glacial acetic acid was added every hour until the delignification was complete. For aspen 5 charges were used and for spruce 6 charges. Treated sticks were thereafter filtered and washed until a conductance below 5 µS was reached. After slushing, the material was refrigerated moist prior to the mechanical disintegration using a microfluidizer M-110EH similar to the holocellulose CNFs obtained from PAA treatment. Enzymatic preparation of cellulose nanofibrils. Reference cellulose nanofibrils were prepared using an established enzymatic route, as described by Henriksson et al.22 Never-dried spruce sulfite pulp (kindly provided by Nordic Seffle AB, Sweden) was used as a starting material. The extraction was done following a previously reported procedure,22 involving enzymatic pretreatment at 50 oC for 2 h in an aqueous solution of endoglucanase enzyme (Novozyme® 476) at 0.25 % (0.1 mL enzyme / 40g dry content cellulose). The enzymatic treatment was followed by 8 passes through a microfluidizer M-110EH. First 3 passes were done with the set of large chambers (see above) and the 5 remaining with the set of small chambers. Samples were taken after 1+0, 3+0, 3+1 and 3+5 passes. The solid content of these nanofibril suspensions were in the range 1.6-2.0 wt.%. Preparation of nanopapers. Nanopapers were prepared following a standard procedure.34 The suspension was first diluted to 0.2 wt.%. After high-shear mixing for 5 min, and degassing under vacuum for 10 min, the suspension was vacuum-filtered on a 0.65 µm pore-size filter membrane (DVPP, Millipore, USA). The wet film was then placed between two metal fabrics (mesh 400) and dried under vacuum (ca.70 mbar) in an oven at 93oC. Size Exclusion Chromatography (SEC). The derivatisations and SEC analysis were performed, as described by Berthold et al.35 In brief, 15 mg pre-swelled delignified materials were solvent6 Environment ACS Paragon Plus

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exchanged 3 times with dry 15 ml DMAc. 1.9 ml 8% LiCl/DMAc and 3 mmol of EIC reagent were then added and the samples were left under argon at ambient temperature for 5 days with mild magnetic stirring. Finally, the samples were diluted to 0.5% LiCl by the addition of 27.4 ml DMAc and the excess of reagent was quenched by adding 500 µl dry methanol. Before chromatographic characterization dissolved samples were treated in a Retsch vibratory mill type MM-2 for 30 minutes followed by filtration through a 0.45 µm PTFE-filter (Advantec, MFS). For SEC analysis, narrow pullulan standards with molecular mass of 1660, 380, 48, 5.8, and 0.738 kDa (Polymer Laboratories) were used to calibrate the chromatographic system. The chromatographic system consisted of a 2690 Separation Module (Waters Corp.) equipped with a guard column (Mixed-A 20 µm 7.5x50 mm, Polymer Laboratories) followed by four columns (Mixed-A 20 µm, 7.5 x 300 mm, Polymer Laboratories) connected in series. The mobile phase was 0.5% (w/v) LiCl/DMAc. Detection was performed online by a 2487 dual wavelength absorbance detector (Waters Corp.) at 295 nm, followed by a 410 differential refractometer (Waters Corp.). Sugar analysis – determination of chemical composition. Samples were hydrolyzed with help of an enzyme mixture composed of cellulases and hemicellulases. Monosaccharides were then quantified by capillary electrophoresis according to Dahlman et al.36 Total charges determination. The total charge content on holocellulose and enzyme-pretreated pulp fibers was determined by conductometric titration, according to a standard method SCANCM 65:02. In short, pulp fibers were transferred to proton-form by soaking in diluted hydrochloric acid (0.1 M HCl) for 15 minutes after which they were washed with deionized (DI) water until a conductivity of 0.5 µs/cm was reached. The pulp fibers were further titration with sodium hydroxide (NaOH). Milli-Q water was used to disperse the pulp fibers. The total charge


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content was calculated as the sum of strong (sulphonic) and weak (carboxylic) acids on the pulp fibers. Scanning - transmission electron microscopy (S-TEM). A drop containing 0.01 wt.% nanofibrils in water was deposited on a TEM grid (ultra-thin carbon film / holey carbon, Ted Pella, USA.). It was removed after 1 min by blotting with filter paper. The same grid was then exposed to a drop of uranyl acetate solution (2 wt.% in water) for 1 min, in order to stain the holocellulose fibrils,. The grid was observed in a field emission scanning electron microscope (FE-SEM, Hitachi S-4300, Japan) equipped with a transmitted electron detector. Atomic Force Microscopy (AFM). A mica substrate was first coated with a positively charged layer of Polyl-L-Lysine by applying a drop of 0.1 wt.% aqueous solution for 1 min, and then rinsing. In the same manner a submonolayer of nanofibrils was deposited on the coated mica surface, from a 0.001 wt.% suspension in water. The fibrils were observed in a Nanoscope IIIa AFM (Picoforce SPM, Veeco, Santa Barbara, CA) in the tapping mode. Scanning Electron Microscopy (SEM). Samples from suspensions taken after different number of passes were solvent exchanged to ethanol, then CO2 and dried above the critical point in dedicated equipment (Autosamdri-815, Tousimis, USA). The structure was then observed in the FE-SEM to analyze the extent of fibrillation. Fracture cross-sections of nanopapers were also observed in the FE-SEM. Prior to microscopy, all samples were sputtered with ca. 5 nm goldpalladium (Cressington 208HR, UK). Mechanical testing. Deformation behavior of CNF nanopaper during uniaxial tensile test was analyzed. The nanopapers were tested in tension, on a universal tensile testing system (Instron 5944, UK) equipped with a 500N load-cell. Specimens were strips 5-6 mm wide. The gauge


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length was set to 20 mm and the cross-head displacement to 10 %/min. Strain was accurately measured with a video extensometer. Testing was performed in a room at 23oC / 50% relative humidity (RH) and all samples were pre-conditioned for minimum 48h. UV-visible light spectroscopy. Light transmittance of nanopaper was measured by an ultraviolet-visible spectrophotometer (UV-2550, Shimadzu, Japan). The transmittance spectra over a wide wavelength range of 200 – 900 nm were recorded at room temperature, taking air as a reference.

3. RESULTS AND DISCUSSION In the present study, wood chips are cut into finer sticks and chemically treated so that they disintegrated into wood fibers. After chemical treatment, the wood fibers have almost no remaining lignin. The hemicellulose and cellulose fractions are largely preserved, and the term “holocellulose fibers”37 is used. The fibers have a bright white, and even shiny appearance.


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Figure 1. FE-SEM images of fibrous suspensions dried from supercritical CO2. The holocellulose CNF is based on peracetic acid treated aspen wood and the images show samples subjected to different number of passes through the homogenizer. a) 0 + 0, b) 1 + 0, c) 1 + 1, d) 1 + 5. The first digit is number of passes through large chambers, the second digit is number of passes through small chambers.

Morphology of holocellulose nanofibrils. The chemically treated wood tissue “sticks” were relatively easy to convert into individual wood fibers using a pulp slusher. Also in the next step, mechanical disintegration of wood fibers, the fibers were readily fibrillated into cellulose nanofibers. Electron microscopy revealed that


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CNFs are formed already after the first pass through the homogenizer; although a fraction of almost intact fibers is still present (Figure 1b). Complete fibrillation was confirmed after a total of 6 passes (1+5), see Figure 1d). The first image in Figure 1a) shows wood fibers prior to disintegration. The fiber oriented vertically in the right part of the image is a straight single fiber, with very limited mechanical damage. The straight tube-like shape is different from the appearance of a commercial chemical pulp fiber, which contains much more mechanical damage. Horizontally, there is also an assembly of 3-4 fibers still sticking together. The CNFs were observed by TEM and AFM (Figure 2a,c) in order to characterize their morphology and dimensions. Both micrographs reveal long and slender individual fibrils. The length was generally in the range 1-3 µm. The width distribution of the fibril was estimated from AFM height profiles of individual fibrils (Figure 2d). Typically the fibrils have lateral dimensions of 3-5 nm. This corresponds to the microfibril width in native wood,38 This is substantially smaller diameter than for enzymatically pretreated CNF.15 It is likely that a substantial fraction of the CNF is indeed wood cellulose microfibrils, with only a small fraction of aggregated fibrils. The location of any hemicelluloses will be discussed in the next section.


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Figure 2. TEM (a) and AFM (c) images of nanofibrils obtained by peracetic treatment of aspen wood, after 6 passes (1+5) throught the microfluidizer. Fibril width distribution (d) obtained from AFM height profiles. (b) Based on data from nanofibril dimension and hemicellulose content, a sketch of holocellulose nanofibrils showing the hemicellulose “shell” coating on the cellulose “core” structure is proposed.

Chemical composition and characteristics of holocellulose CNF and nanopaper Due to the delignification procedure, we expect the CNF to consist primarily of two classes of carbohydrate polymers in the form of cellulose fibrils and hemicelluloses. The chemical composition is reported in Table S1 in terms of cellulose and hemicellulose content. Peracetic acid-(PAA) treated CNF has a cellulose content of 76-77% with 23-24% hemicellulose. Chlorite treated CNF has 82-84% cellulose and 16-18% hemicellulose The reason for these differences is the higher selectivity of the PAA method and the mild conditions under which lignin was removed. The high hemicellulose content in the present CNFs, in combination with the small


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CNF diameter, makes it possible to propose a physical model structure for the holocellulose CNF. The core is a wood cellulose microfibril (botany term), and the microfibril is coated with a layer of native hemicelluloses so that we have a core-shell CNF. In fact, the core-shell CNF is likely to reflect an original ultrastructure of the wood cellulose microfibrils. For aspen, this layer is dominated by xylan, whereas the spruce CNFs contain a xylan-mannan polymer coating mixture. Let us assume that the CNF has quadratic cross-section with an edge length of 4 nm, and a 20% hemicellulose “shell” coating over a cellulose core (Figure 2b). With an assumption of similar densities, then the cellulose edge length is roughly 3.6 nm and the coating thickness about 0.2 nm. This corresponds to a few layers of hemicellulose in adsorbed “flat” conformation and provides a hypothetical physical model as a starting point for interpretations. The reference CNF is enzymatically pretreated CNF, enzymatic CNF, which is based on a different wood pulp fiber and has lower hemicellulose content (12%). The difference in hemicellulose composition between aspen and spruce is also apparent in Table S1. Aspen is dominated by xylans, whereas mannans are also important in spruce. Additional chemical characterization data are presented in Table 1. All samples contain elevated levels of carboxylic acid functionality, approximately 200 µeq/g for PAA samples. This charge is important for colloidal stability and may reduce the tendency for CNF agglomeration. The oxidative conditions prevailing during both chlorite and PAA treatment can lead to oxidation of the reducing end of carbohydrate polymers and that is the most likely explanation for the presence of carboxylates.39, 40 The surface charge may influence filtration time for CNFs. According to our experiment for a 45 µm thick nanopaper, the filtration time of 0.2 wt% suspension for PAA and enzymatic CNFs takes approximately 4 and 2 hr, respectively. Compared with TEMPO-CNF nanopaper, this filtration time is still significantly lower.


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Chlorite delignification may degrade cellulose to a larger extent than PAA25, 26 and this was observed in the present study where the weight average molar mass (Mw) of chlorite-treated samples was lower than corresponding PAA samples, Table 1. This corresponds to changes in the SEC traces, Figure 3 (0+0 samples). The molar mass distribution in PAA samples is interesting. Two distinct peaks are observable, and the low molar mass peak is dominated by hemicelluloses. Spruce appears to have a hemicellulose fraction of higher molar mass than in aspen (note position of low molar mass peak). The comparison between chlorite and PAA shows that chlorite-treated samples have a very different distribution, and the cellulose is more degraded. The high molar mass part of the traces is suppressed and the new distribution shows molar mass degradation41. For spruce, the “valley” in the PAA curve ranging from 63,000 to 100,000 does not exist in the chlorite case, most likely since the valley is filled by degraded cellulose. Iwamoto (Iwamoto et. al, 2007)42 showed that repeated passing of dissolving pulp fibers through a grinder resulted in reduced cellulose molar mass. Here repeated fibrillation using a homogenizer resulted in reduced molar mass, see for instance PAA spruce in Figure 3 and Table 1. The change in distribution is substantial as “small” chambers are used. Since the high molar mass fraction is reduced, the data are consistent with rupturing of long fibrils. The change in molar mass distribution is reminiscent of the pattern when e.g. cotton cellulose is degraded under acidic conditions.41 In the present case, fibril rupture in the homogenizer is likely to first take place in the longest fibers. The gradual change in molar mass is consistent with increasing degradation with number of passes.


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Figure 3. SEC molar mass distribution curves of CNF from aspen and spruce wood. The CNF were prepared by peracetic acid and chlorite delignification on wood chips, and enzymatic treatments of wood fibers. Note that 1+5 means 1 pass in large chamber (first digit) and 5 passes in small chamber (second digit, see Methods section).


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Table 1. CNF characteristics and mechanical properties of nanopaper (standard deviation in brackets when applicable). Treatment

Species Passes Mw x106 PDI1 Hemicelluloses2 Charges (wt.%) (µeq/g) (g/mol)

E3 (GPa)

σ*,4 (MPa)

ε*,5 (%)

Peracetic acid

Aspen

12.6 (1.0)

245 (16)

2.9 (0.6)

15.1 (1.1)

309 (18)

7.0 (0.5)

9.5 (2.5)

250 (18)

4.1 (1.1)

16.2 (1.0)

319 (9)

7.1 (0.4)

11.5 (1.1)

245 (12)

3.9 (0.2)

Spruce

Chlorite

Aspen

Spruce

Enzymatic Spruce

1+0

1.3

18.7

1+3

0.83

10.6

1+0

1.1

11.8

1+3

0.62

6.5

1+0

0.81

17.1

1+3

0.43

8.0

15.7 (2.4)

286 (24)

6.4 (0.9)

1+0

0.7

14.2

13.0 (2.2)

260 (10)

5.4 (0.5)

1+3

0.45

7.2

15.7 (1.3)

276 (30)

5.7 (0.6)

1+0

0.80

26.2

8.0 (1.8)

188 (8)

7.9 (0.7)

3+1

0.67

18.5

14.5 (1.6)

252 (14)

7.0 (0.7)

23.9

23.1

18.5

12.3

1

193.4

206.8

218.8

110.1

Polydispersity index, 2 Hemicellulose content, 3Young’s modulus, 4Ultimate tensile strength, 5 Strain to failure


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Enzymatic treatment

Peracetic acid treatment

Figure 4. Molecular absorption spectroscopy data for cellulose nanopaper in the ultraviolet (UV) and visible (VIS) range of wavelengths. Spruce nanopaper prepared from peracetic acid holocellulose and enzymatic CNF. The photograph demonstrates optical transparency of nanopaper with a thickness of approximately 45 µm. Molecular absorption spectroscopy data in the ultraviolet (UV) and visible (VIS) range of wavelengths are presented in Figure 4 for spruce nanopaper prepared from PAA and enzymatic CNF nanopaper. The optical transparency of PAA CNF nanopaper is significantly improved compared with enzymatic CNF nanopaper over the entire range of wavelengths. For both materials, the optical transparency was gradually increased with longer wavelength. In the case of spruce PAA CNF nanopaper, the transmittance at 800 nm reached 80%, approaching to the reported value for TEMPO-oxidized cellulose nanopaper at about 90%.43 One should also note that the present films have substantially larger thickness, so the results may be very similar to TEMPO CNF nanopaper. The demonstration photograph in Figure 4 illustrates the substantial difference in apparent optical transparency between the two materials. The enzymatic CNF colloid, and corresponding nanopaper, certainly contain a fraction of CNF agglomerates as indicated by a peak in the dynamic light scattering data at 400 nm.15 PAA holocellulose CNF


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show uniform and small diameter of the CNFs, as demonstrated in the AFM height profile data. The high content of hemicelluloses on not only has positive influence on nanofibrillation behavior,24 but may also stabilize the PAA holocellulose CNF in colloidal suspension. The effect may be partly sterical, but the PAA CNF also has a charge of 207 µeg/g, see Table 2. This adds electrostatic repulsion as a contributing mechanism. CNF agglomeration becomes limited in the colloid and in the solid film, resulting in high optical transparency.

Mechanical properties of nanopaper The mechanical behavior in tension of holocellulose nanopaper is presented in Figure 5, and mechanical properties are reported in Table 2 (additional data available in supporting information). The most notable feature is the high modulus and strength of the PAA holocellulose CNF films, E as high as 16 GPa and tensile strength as high as 330 MPa (PAA Aspen 1+5, see Table 2). To our knowledge, these are the highest values reported for CNF-based films at 50% RH. The stress-strain behavior is elastic followed by yielding and linear strainhardening and finally ultimate fracture at 7% strain. The number of passes (see Table S2) is a good indication of energy required for mechanical disintegration of the nanofibers. In our experiments, the suspension flows through a set of small (diameter of 200 and 100 µm) and big diameter (diameter of 200 and 100 µm) chambers during mechanical disintegration. Already after two passes through the homogenizer, the strength was fully developed (PAA Aspen, 1+1, tensile strength 330 MPa, see Table S2). The first pass was through the larger chamber and second through the smaller chamber of the homogenizer. The data indicate that the CNF disintegration (1+1) was sufficient to avoid strength reduction effects due to CNF aggregates. A


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larger number of passes through the homogenizer resulted in only a slight increase in strain-tofailure, with a lower slope in the strain-hardening region, suggesting subtle changes in the plastic deformation mechanisms of the network. Increased strain to failure is related to fibrillation of residual aggregates and the formation of a more homogeneous CNF network. Optical transmittance of the nanopaper was also improved with increasing number of passes through the homogenizer. It is also apparent that the present chlorite treatment resulted in lower molar mass CNF, and lower nanopaper strength, see Table 1 and Table S2. The enzymatic CNF nanopaper, Figure 5 b), shows lower strength due to lower molar mass, but also due to CNF aggregates. The nanopaper films prepared from chlorite and peracetic acid treated CNFs all had a density within 1.48 ± 0.05 irrespectively of the number of passes through the microfluidizer, except for poorly fibrillated peracetic acid fibers passed only once through the set of big chambers (density of 1.33 ± 0.02). The former values are comparable to the theoretical density of cellulose indicating a very low level of porosity. In comparison, nanopapers from enzymatic CNFs had a slightly lower density around 1.4 ± 0.03. This higher density of holocellulose nanopaper is a consequence of the better packing of holocellulose CNFs thanks to their smaller diameter and the presence of a softer hemicelluloses shell coating each individual fibiril. In turn, the low porosity is favorable to obtain high transparency and high mechanical properties. At 50% relative humidity (RH), CNF nanopaper with a moisture content of ca 6 wt% was obtained from both PAA and enzymatic CNFs, irrespective of slightly different hemicellulose content.


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Figure 5. Stress-strain curves of CNF nanopaper. a) PAA treated aspen after different number of passes through the homogenizer, b) PAA aspen, PAA spruce and enzymatic spruce after the maximum number of passes (1+5). The correlation between cellulose molar mass and tensile strength of CNF nanopaper was suggested already by Henriksson et al.44 Later, Saito et al45 optimized conditions for TEMPOmediated oxidation of hardwood pulp to prepare TEMPO-oxidized CNF of high molar mass. This resulted in CNF nanopapers with a strength reported to be 312 MPa, although test conditions are unclear. In that study, the molar mass of the treated pulp was 167 000 g/mol prior to mechanical disintegration, as compared to 1 470 000 g/mol for the present PAA-treated aspen wood. To illustrate the relationship between molecular weight and strength, the two are plotted against each other in Figure 6 for all materials. Two regions can be identified in this graph. On one hand, poorly fibrillated CNFs have high molar mass but low strength due to presence of CNF agglomerates likely to serve as fracture initiation sites. On the other hand, the materials with sufficient fibrillation show positive correlation between strength and molecular weight. The main reason is that the length of CNFs correlates with molar mass.46 Since CNFs are sliding with respect to each other during plastic deformation,6 ultimate strength becomes higher for nanopaper based on longer CNFs.


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Figure 6. Tensile strength vs. molecular weight (weight average molar mass) for nanopaper films based on PAA and “chlorite” CNF. Numerical indices represent the total number of passes through the homogenizer.

Fracture mechanisms and nanopaper structure


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Figure 7. FE-SEM images of fracture surfaces of CNF nanopaper based on (a) Holocellulose CNF and (b) Enzymatic CNF (1+5 passes in the homogenizer) The present nanopaper structures can be viewed as nanocomposite films, where CNF nanofibers are present in random-in-the-plane orientation. Since each holocellulose CNF fibril is coated by hemicellulose, this hemicellulose serves as a polymer matrix phase. Holocellulose CNF have fairly high cellulose molar mass. In addition, the holocellulose CNF are interesting because of their high hemicellulose content. The hemicelluloses are strongly adsorbed to the CNF surface as a “shell” coating, and acts as a bonding agent which provides improved stresstransfer between CNF nanofibers. Fracture surfaces of tensile specimens are presented in Figure 7. There is strong contrast in appearance between the smooth surface of holocellulose CNF nanopaper a) and the layered structure in enzymatic CNF nanopaper. The fracture path in enzymatic CNF nanopaper is more irregular. Individual CNF lamellae are fracturing at weak sites at different locations, followed by lamella “pull-out” and interlaminar cracking in the weak planes between lamellae. The result is a more jagged fracture surface, with “pull-out” lamella lengths at the scale of 5 µm. In the holocellulose CNF nanopaper, the hemicellulose “binder” content is higher (24wt% vs 12wt%) and probably more favorably distributed. This results in improved interlaminar toughness, so that interlaminar cracking is avoided and the fracture surface becomes more smooth. The local plastic reorientation of individual CNFs in the loading direction is apparent in the higher resolution images in Figure 7. One may note that mixing of CNF and hydroxyethyl cellulose (HEC), followed by filtration and drying, results in more inhomogeneous polymer matrix distribution as compared with in Figure 7 a). The core-shell CNF approach has been used recently to control the distribution of amylopectin (AP) starch in


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CNF/AP biocomposites.15 The distribution of native hemicellulose is probably highly favorable and uniform in the present case. Finally, it is interesting to note that the CNF nanopaper with the highest modulus E contains 23 wt% hemicellulose (Table S2). From a composite micromechanics modeling perspective, excluding CNF orientation effects, one would expect the material with the highest volume fraction of cellulose to have the highest modulus. Since this is not the case, it is possible that neat cellulose CNF nanopaper suffers from imperfect CNF-CNF stress transfer in certain locations. The presence of hemicellulose “shell” coatings can address this shortcoming, bond CNFs together and improve CNF-CNF stress transfer.

4. CONCLUSIONS Unique CNF nanofibers have been prepared combining highly preserved cellulose molar mass with up to 24% of hemicellulose content, in the form of a 0.2 nm “shell” coating surrounding 3.6 nm “core” cellulose nanofibers based on theoretical estimation. The peracetic acid treatment is very mild since Mw of the CNF corresponds to a degree of polymerization DP as high as 3800, which is close to the expected value for holocellulose in native wood. These CNFs are also unique in terms of small nanofiber diameter (similar to cellulose microfibrils in the wood cell wall), improved yield, high stability in colloidal suspension (anionic CNF surface charge repulsion), and very low energy requirements for successful disintegration (only two passes in homogenizer). The reason for ease of fibrillation from holocellulose pulp is in the high hemicellulose content with anionic charge, which causes hydration and swelling. Furthermore, the resulting CNF nanopaper films show optical transparency similar to TEMPO-CNF, and


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better mechanical properties in tensile tests than any data known to us for wood-based CNF nanopaper. The lack of agglomerate defects (high optical transparency) and the high intrinsic strength of high molar mass fibrils contribute to the exceptional nanopaper strength. The distribution of the hemicellulose “polymer matrix” contributes to CNF dispersion, strong CNFCNF adhesion and stress transfer between fibrils and between CNF lamellae. Compared with TEMPO-CNF, the present nanofibers provide much higher molar mass, preserved cellulose structure (no fibril surface derivatization) and shorter filtration time (lower anionic surface charge). Since the fibrils contain highly preserved biomacromolecules, they may also help to clarify structural details in plant cell walls.

ASSOCIATED CONTENT Supporting Information SEC molar mass analysis of CNF after mechanical disintegration with different number of passes, and tensile stress-strain curves of CNF nanopaper obtained from peracetic acid and chlorite delignifications in addition to enzymatic pretreatement. This material is available free of charge via the Internet http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Lars A. Berglund Mailing address: KTH Royal Institute of Technology, Department of Fiber and Polymer Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden. Phone: +46-87908118. E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Financial support from the Wallenberg Wood Science Center (WWSC, http://wwsc.se) is gratefully acknowledged. The authors are grateful to Eva-Lisa Lindfors for skillful laboratory work. Dr. Fredrik Berthold also acknowledges financial support from RISE, and Dr. Kasinee Prakobna acknowledges for a scholarship fund from SCG.

REFERENCES 1. Berglund, L. A.; Peijs, T. MRS Bull. 2010, 35, 201-207. 2. Pääkkö, M.; Vapaavuori, J.; Silvennoinen, R.; Kosonen, H.; Ankerfors, M.; Lindström, T.; Berglund, L. A.; Ikkala, O. Soft Matter 2008, 4, 2492-2499. 3. Abe, K.; Yano, H. Carbohydr. Polym. 2011, 85, 733-737. 4. Svagan, A. J.; Samir, M. A. S. A.; Berglund, L. A. Adv. Mater. 2008, 20, 1263-1269. 5. Donius, A. E.; Liu, A.; Berglund, L. A.; Wegst, U. G. J. Mech. Behav. Biomed. Mater. 2014, 37, 88-99. 6. Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindström, T.; Nishino, T. Biomacromolecules 2008, 9, 1579-1585. 7. Svagan, A. J.; Azizi Samir, M. A. S.; Berglund, L. A. Biomacromolecules 2007, 8, 25562563. 8. Sehaqui, H.; Zhou, Q.; Berglund, L. A. Soft Matter 2011, 7, 7342-7350. 9. Nakagaito, A. N.; Fujimura, A.; Sakai, T.; Hama, Y.; Yano, H. Compos. Sci.Technol. 2009, 69, 1293-1297. 10. Cobut, A.; Sehaqui, H.; Berglund, L. A. Bioresources 2014, 9, 3276-3289. 11. Nakagaito, A. N.; Yano, H. Appl. Phys. A: Mater. Sci. Process. 2004, 78, 547-552. 12. Sehaqui, H.; Liu, A.; Zhou, Q.; Berglund, L. A. Biomacromolecules 2010, 11, 21952198. 13. Wu, C. N.; Saito, T.; Fujisawa, S.; Fukuzumi, H.; Isogai, A. Biomacromolecules 2012, 13, 1927-32. 14. Hamedi, M. M.; Hajian, A.; Fall, A. B.; Håkansson, K.; Salajkova, M.; Lundell, F.; Wågberg, L.; Berglund, L. A. ACS Nano 2014, 8, 2467-2476. 15. Prakobna, K.; Galland, S.; Berglund, L. A. Biomacromolecules 2015, 16, 904-912. 25 Environment ACS Paragon Plus

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45. Saito, T.; Hirota, M.; Tamura, N.; Kimura, S.; Fukuzumi, H.; Heux, L.; Isogai, A. Biomacromolecules 2009, 10, 1992-1996. 46. Shinoda, R.; Saito, T.; Okita, Y.; Isogai, A. Biomacromolecules 2012, 13, 842-849.

TABLE OF CONTENTS GRAPHIC


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Holocellulose Nanofibers of High Molar Mass and ... - ACS Publications

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