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Understanding the Mechanistic Behavior of Highly Charged Cellulose Nanofibers in Aqueous Systems Lihong Geng,†,‡ Nitesh Mittal,§,∥ Chengbo Zhan,‡ Farhan Ansari,⊥ Priyanka R. Sharma,‡ Xiangfang Peng,† Benjamin S. Hsiao,*,‡ and L. Daniel Söderberg*,§,∥ †

National Engineer Research Center of Novel Equipment for Polymer Processing, the Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, Guangzhou 510640, P. R. China ‡ Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States § Linné FLOW Centre, Department of Mechanics, and ∥Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ⊥ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *

ABSTRACT: Mechanistic behavior and flow properties of cellulose nanofibers (CNFs) in aqueous systems can be described by the crowding factor and the concept of contact points, which are functions of the aspect ratio and concentration of CNF in the suspension. In this study, CNFs with a range of aspect ratio and surface charge density (380−1360 μmol/g) were used to demonstrate this methodology. It was shown that the critical networking point of the CNF suspension, determined by rheological measurements, was consistent with the gel crowding factor, which was 16. Correlated to the crowding factor, both viscosity and modulus of the systems were found to decrease by increasing the charge density of CNF, which also affected the flocculation behavior. Interestingly, an anomalous rheological behavior was observed near the overlap concentration (0.05 wt %) of CNF, at which the crowding factor was below the gel crowding factor, and the storage modulus (G′) decreased dramatically at a given frequency threshold. This behavior is discussed in relation to the breakup of the entangled flocs and network in the suspension. The analysis of the mechanistic behavior of CNF aqueous suspensions by the crowding factor provides useful insight for fabricating highperformance nanocellulose-based materials.

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crucial role in the fibrillation process and allows the dispersion in water because of the electrostatic repulsion between CNFs. Based on the properties of crystalline cellulose together with the high aspect ratio and large specific surface area, CNFs have been demonstrated to have a wide range of potential applications, including reinforcing fillers in composites,12 water purification membranes,13,14 green gas barriers,15 sensors, and templates.16,17 These applications require the thorough understanding of the dispersion state of CNFs in water and the flow properties of CNF suspensions in order to process and produce high-performance CNF-based materials. Analogous to the transition from dilute to semidilute to concentrated polymer solutions, individually dispersed CNFs in dilute dispersions can transform into fibrous networks and flocs in concentrated suspensions, whereby the flow properties can decrease rapidly with the increasing concentration. There are varying flow regimes and dispersion states of CNF suspensions

INTRODUCTION Cellulose, the primary structural component of the plant cell wall, is the most abundant natural polymer and the major sustainable material produced on earth,1 which exists in green plants, algae, marine organisms, and bacterial biofilms. The use of cellulose has been central to our modern society,2 especially in the form of wood and plant fibers, for building materials, packaging, and clothing. As a chemical raw material, cellulose has been exploited for almost two centuries, and it was the basis for the first plastics. The advantages of cellulose stem from its abundance, low cost, and high specific strength as well as being a renewable resource.3 Advances in the understanding of the structural features of cellulose have led to creation of novel cellulosic materials, of which the recent highlight is the development of nanocelluloses. Cellulose nanofiber (CNF), the most common form of nanocellulose, is usually produced in the water suspension state from plant cellulose by mechanical fibrillation,4,5 in which the pretreatment processes6−11 are often used to incorporate charges on the nanofibers surface in order to improve fibrillation efficiency. The charge density of CNF thus plays a © XXXX American Chemical Society

Received: December 12, 2017 Revised: January 29, 2018

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DOI: 10.1021/acs.macromol.7b02642 Macromolecules XXXX, XXX, XXX−XXX

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observed. The TEMPO-oxidized pulp fibers were washed thoroughly with deionized water by filtration. After the chemical pretreatment, aqueous suspensions of the fibers were passed through a high-pressure homogenizer. At the end of homogenization, CNF suspensions with a concentration larger than 5 g L−1 were obtained. In an additional step, the unfibrillated and agglomerated fiber bundles were removed from the CNF suspensions. The gel-like suspensions were diluted by adding deionized water, mixed thoroughly using a mechanical mixer (12 000 rpm for 10 min, Ultra Turrax, IKA, Germany), and sonicated (10 min, Sonics Vibracell, USA). The diluted suspensions were then centrifuged at 5000 rpm for 60 min followed by the removal of precipitates, and the supernatants were used for further studies. The dry content of the suspensions was determined by gravimetric analysis. Length and Height Characterization of Cellulose Nanofibers. The lengths of CNFs (∼200 samples for each charge density) were measured using transmission electron microscopy (TEM) (JEOL JEM-1400 TEM) at an accelerating voltage of 120 kV.22,29,30 Images were acquired with a Ruby camera following the “systematic, uniform, random” rule to avoid bias. Prior to observation, the sample was deposited on a carbon-coated copper grid treated with glow discharge and stained with 2% uranyl acetate solution. Heights of CNFs (∼150 samples for each charge density) were measured using atomic force microscopy (AFM) (MultiMode 8, Bruker, Santa Barbara, CA). For the AFM sample preparation, silicon wafers from Addison Engineering Inc. (San José, CA) were oxidized at 1000 °C for 1 h to form a silica layer. The wafers where washed in Milli-Q, ethanol, and Milli-Q, dried, and treated in the plasma cleaner (PCD 002, Harrick Scientific Corp., Ossining, NY) for 5 min to render the surface hydrophilic. Wafers in the size of 5 × 5 mm were covered with CNFs and were left to dry in air at room temperature before imaging. Characterization of Surface Charge and Chemical Composition. The carboxylate contents of CNFs were determined by conductometric titration,31 where 100 mL of 0.1 wt % suspensions was used, and the pH of the suspensions was adjusted to 2.5 with 0.1 mol/ L HCl. The suspensions were titrated with 0.01 mol/L standardized NaOH (Sigma-Aldrich) by adding 0.2 mL aliquots in 60 s intervals until the pH reached 11, where the conductivity was monitored with a benchtop meter (FE20 FiveEasy, Mettler-Toledo). The titration curves showed the presence of strong and weak acid groups, where the amount of strong acid was due to the added HCl, and that of weak acid was due to the carboxyl content. Freeze-dried CNFs with various charge densities were measured by Fourier transform infrared spectroscopy (Thermo Nicolet iS10 FTIR spectrometer). The FTIR spectra were record with a resolution of 1 cm−1 over the range of 4000−500 cm−1. Ultraviolet (UV)−Visible Spectroscopy. The UV−vis spectroscopy of different CNF with varying carboxyl content (380, 820, 980, and 1360 μmol/g) having concentration 0.4 g/L in distilled water were recorded by using the CARY-300 UV−vis spectroscopy instrument. The distilled water was scanned as a blank. Small-Angle X-ray Scattering (SAXS). SAXS measurements of CNFs with different charge densities in aqueous suspensions were performed at the beamline 12-ID-B beamline in Advanced Photon Source (APS), Argonne National Laboratory. The X-ray wavelength λ was 1.24 Å, and a Pilatus 2M 2D detector was used to collect the 2DSAXS scattering images. Silver behenate standard was used to calibrate the beam center and sample−detector distance, which was 4 m, covering the q range from ∼0.002 to ∼0.5 Å−1. To avoid the sample destruction caused by X-rays, a flow cell was used. In specific, CNF suspensions at the concentration of 0.1 wt % but with different charge density ranging from 380 to 1360 μmol/g were pumped into the flow cell for SAXS measurements. For background subtraction, the cell containing deionized water and empty sample cell were also measured. The procedures of background subtraction was carried out according to Allaire and Yang’s approach.32 Data reduction was carried out automatically in APS using the Matlab or Igor frame routines. 1D SAXS data fitting was performed using the fitting subroutine implemented in the SASView package developed at NIST Center for Neutron Research (NCNR), with customized model construction function using a ribbon model as follows:33

that can be used to advance some specific applications. For example, a dilute CNF dispersion was desired for fabrication of thin barrier layer in water filtration membrane13,14 because individually dispersed CNFs in water could gradually formed a uniform network structure during membrane drying, avoiding the formation of large flocs. In contrast, for preparation of wetspun filaments, concentrated CNF suspensions were desired18−21 so that the wet-spun fibers would not break and disperse in coagulation agents. In addition to concentration, the flow properties of CNF suspensions strongly depend on the dimension and chemical structure of CNFs, especially the surface charge density. Rheology is usually used to describe the flow properties of the CNF suspension, where several studies have focused on the effect of microstructure, including the CNF charge density, on the rheological properties of the suspension.22−24 In specific, Besbes et al.25 attributed the lower suspension viscosity to the increase of the carboxylate group content, based on the hypothesis that higher oxidization level does not induce the breakage of fibers. Thus far, no theory has been developed to analyze the relationship between the microstructure of CNF and the dispersion state and flow properties of CNF suspension. For this purpose, we adopted the concept of the crowding factor, which has been used by Celzard et al.26 to describe the flocculation of microscale cellulose fibers based on percolation and effective-medium theories but without the consideration of surface charge density. In this study, the effect of charge density on the rheological properties of CNF suspension was carefully examined using the concept of crowding factor. Specifically, CNFs with varying aspect ratios and charge densities were prepared by the use of combined 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)mediated oxidation and mechanical disintegration treatments. The effect of the charge density on the aspect ratio of resulting CNFs was first investigated. The crowding factor based on the approach of contact points, combining the effect of the charge density, was used to describe the dispersion state of CNFs in water and predict the gelation behavior of the CNF suspension.

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EXPERIMENTAL SECTION

Preparation of Cellulose Nanofibers Dispersions. CNFs were prepared from chemically bleached wood fibers (a mixture of 60% Norwegian spruce and 40% Scots pine, provided by Domsjo AB, Sweden). The wood pulp fibers were chemically treated with a 2,2,6,6tetramethylpiperidinyl-1-oxyl (TEMPO)-mediated oxidation reaction, which has been described elsewhere.27,28 CNFs with different charge densities were obtained by varying the reaction time and/or conditions on the pulp. For the preparation of CNFs with surface charge densities of 380 and 820 μmol/g, cellulose pulp fibers (1 g) were suspended in a 0.05 mol/L sodium phosphate (Sigma-Aldrich) buffer (90 mL, pH 6.8), containing TEMPO (16 mg, 0.1 mmol) and sodium chlorite (Sigma-Aldrich) (80%, 1.13 g, 10 mmol). A 2 mol/L sodium hypochlorite solution (0.5 mL) after dilution to 0.1 mol/L with the 0.05 mol/L sodium phosphate buffer was added to the suspension. The suspensions were subsequently stirred at 500 rpm for a desired period (e.g., 1 and 48 h for surface charge density of 380 and 820 μmol/g, respectively). And for the CNF with surface charge densities of 980 μmol/g or higher, wood pulp fibers were suspended at a concentration of 1 wt % in deionized water with the addition of TEMPO (16 mg/g cellulose) and NaBr (Sigma-Aldrich) (100 mg/g cellulose). NaClO (Sigma-Aldrich) (e.g., 2.5 and 5.0 mmol/g cellulose for surface charge density of 980 and 1360 μmol/g, respectively) was added dropwise to the suspension under vigorous stirring. The pH of the suspension was maintained at a constant value of 10 by the addition of 0.1 mol/L NaOH solution until no change in pH value was B

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Figure 1. TEM images of cellulose nanofibers with charge densities of (a) 380, (b) 820, (c) 980, and (d) 1360 μmol/g and the corresponding statistic length distribution. The average length for each system ranging from lowest to highest charge density is 614, 683, 538, and 419 nm, respectively. The value of scale bars in TEM images is 250 nm.

Figure 2. AFM image of cellulose nanofibers with a charge density of 1360 μmol/g, where the length of the scale bar is 200 nm. The thickness distributions of CNFs at the four oxidation levels (380, 820, 980, and 1360 μmol/g as indicated in the graphs). The average thickness for each system ranging from lowest to highest charge density is 2.14, 2.57, 2.35, and 2.47 nm, respectively.

⎛1 3 q 2 b2 ⎞ 1 ⎟ I(q) = (ab)2 Lπ sinc 2(qa /2)1F 2⎜ ; , 2; − q 4 ⎠ ⎝2 2

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RESULTS AND DISCUSSION Morphology and Structure of Cellulose Nanofibers. The chemical structure change in negatively charged CNFs with various oxidation levels was characterized using the FTIR technique. Figure S1 (Supporting Information) illustrates the FTIR spectra of freeze-dried TEMPO oxidized CNFs with different charge density. In these spectra, the intensity of the peak at 1605 cm−1, which is due to the stretching vibrations mode of the carboxylate group, was notably different. The larger peak intensity represents a higher charge density, which is consistent with a higher degree of oxidation. In Figure S2, the UV−vis transmittance of all CNF suspension is higher than 90%, showing that the suspensions are free from the big size agglomerations. Electrostatic repulsion between carboxylate groups converted from a partial potion of C6 hydroxyl groups on the surface of cellulose nanofibers during oxidation promoted the fibrillation process of CNFs, resulting in a relatively narrow size

(1)

where a represents the ribbon thickness and b is related to the ribbon width. Rheological Measurements. Rheological measurements were carried out by a stress-controlled rheometer (model Physica MCR 301, Anton-Paar) having the Couette geometry. The rheological properties of CNFs with various oxidation levels in aqueous suspensions at concentrations ranging from 0.01 to 0.3 wt % were studied. Measurements of steady shear viscosity versus shear rate were performed for each sample at shear rate ranging from 0.1 to 100 s−1. Dynamic frequency sweep measurements were also studied at frequency ranging from 0.1 to 100 Hz. The recovery behavior was investigated based on the 0.03 wt % CNF suspensions with different charge densities by alternatively changing the oscillatory frequency between 20 and 5 Hz. In these measurements, the strain was set at 1%. Before each measurement, the suspension was kept static for 3 min so that it could reach the steady sate. All suspensions were tested at 25 °C. C

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Macromolecules distribution compared to CNFs fabricated by the mechanical shear process alone.8,34 Therefore, the charge density on the nanofiber surface played an important role in the isolation of CNFs, consequently affecting the final morphology (length and width) of the nanofibers. In Figure 1, typical TEM images of CNFs at different charge density are illustrated together with the length distributions extracted from TEM images. From these measurements, one can conclude that the distributions and thereby the mean length seemed to be similar for two lower charged CNFs (i.e., 380 and 820 μmol/g), but for higher charged CNFs, the distribution changes and mean length showed a decreasing trend with the increase in surface charge. It was interesting to note that for the 1360 μmol/g sample a shortest mean fiber length of 419 nm was observed, indicating that although the high charge density enhanced the fibrillation, it reduced the CNF length. The typical AFM image of oxidized CNFs is shown in Figure 2. The highly charged CNFs exhibited a good dispersion, reflecting the state in suspension, due to the electrostatic repulsion. The individual CNFs showed both straight and curved regions, perhaps due to the composition of dominant rigid cellulose crystalline and dominant flexible amorphous regions (red arrows), respectively. It has been reported that in cellulose nanofibers polymer chains could pass through multiple crystalline and amorphous regions.34 The thickness distributions of CNFs are also shown in Figure 2 (right panels). The results indicated that all oxidized CNFs possessed a similar thickness distribution with an average thickness around 2 nm regardless of the charge density. This observation also indicated that the extracted CNFs might represent the elementary fibrils or the smallest morphological units in the fiber cell wall from the chosen wood sample.34 In order to avoid the limitation of observing only localized views of the samples in TEM and AFM observations, solution small-angle X-ray scattering (SAXS) was used to investigate CNF suspensions, where the results yielded statistical dimensions of dispersed CNFs by cylinder35 or parallelepiped36 model fitting. Similar methodology has been applied before to characterize the dimensions of nanocelluloses obtained from different sources.37,38 Recently, a simplified polydisperse ribbon model has been demonstrated by our group, where this model could efficiently describe the SAXS profiles of TEMPO oxidized CNFs regardless of the cellulose source.33,39 This model assumes the length of CNF to be infinitely long (longer than 200 nm), which is reasonable based on the observed fiber length from TEM images (around 600 nm). To minimize the effect of the structural factors, a dilute CNF suspension at 0.1 wt % concentration (i.e., the crowding factor, to be described later, is lower than 16) was used for the SAXS measurement. The form factor, including the cross-sectional dimensions and the distribution, was obtained by fitting of SAXS curves using ribbon model. Figure 3 shows the scattered intensity curve of CNFs at a charge density of 380 μmol/g, in which the q−1 decay in low q regime indicated that CNFs behaved as one-dimensional rod (i.e., the slope of the scattered intensity and q was −1).36 The corresponding SAXS intensity curve was fitted using the ribbon model, giving the dimension value of ribbon-like cross section: the derived b and a value was 5.9 and 2.2 nm, respectively. The a value of 2.2 nm was about the same as the average thickness value of CNF determined by AFM (Figure 2), and the width (a

Figure 3. Experimental SAXS profile of CNFs at a charge density of 380 μmol/g and the fitting curve using ribbon model.

+ b ≈ 8 nm) was also about the same as the average width determined by TEM (Figure 1). As mentioned above, the morphology of fibrillated CNFs depends on the charge density, which was elucidated using the SAXS technique. Figure 4a illustrates SAXS curves for CNF

Figure 4. SAXS profiles of CNFs with different charge density (a) and the curves shifted along the y-axis by a factor of 5 from original curves for clarification (b); the continuous lines are the fitting curves using ribbon model.

suspensions with different charge densities, which were almost the same except for the one with a low charge density of 380 μmol/g. In order to clarify the effect of charge density on the cross-section dimension of CNFs prepared under different fibrillation conditions, the ribbon model was used to fit the SAXS curves. The results are shown in Figure 4b, where the fit and the scattering profile for each sample was shifted along the y-axis for clarity. The obtained mean values of a and b for the chosen samples are listed in Table 1. It was found that the 380 μmol/g sample exhibited a larger size, most probably due to the incomplete fibrillation process that resulted in some larger fibrillar bundles together with fibrillated CNFs. With the D

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≪ 1, since each fiber can rotate freely. The networks formed through filtration of suspensions at this crowding number typically have a uniform structuremore than random sheets organized through the mechanism of self-healing. On the other hand, when N > 60, all fibers are in continuous contact, forming a connected network with no mobility of individual fibers. In the range of 1 < N < 60, there is generally no connected network, but fibers flocculate in the flow due to shear-induced rotation of fibers and generation of flocs. Thus, the increasing input of energy into the flow (e.g., Stokesian, laminar, or turbulent) will enhance the flocculation behavior. Clearly, for the case of nanoscale rod-like particles such as CNFs with surface charge, both effects of molecular motion (e.g., Brownian motion) and electrostatic interactions need be considered (see, for example, Håkansson et al.21). For microscopic cellulose fibers, an intermediate threshold value termed the gel crowding factor at N = 16 has been reported. Above the gel crowding factor (N > 16), the transition from a fluidized suspension to an interconnected flocculated system was observed, where the flocculated system was characterized as granular.44 Celzard et al.26 showed that this threshold could be corresponded to the ‘“connectivity threshold”, where the N = 60 threshold is to the ‘“rigidity threshold”’ of fiber networks that can be predicted by effective-medium and percolation theories. The gel crowding factor has been interpreted as like the overlap concentration for polymer solutions. When macroscopic fiber suspensions were investigated in a flow (e.g., pipe flow), it was clear that above the gel crowding factor the flow velocity distribution in the pipe cross section could transition from a plug flow with sheared layers at the wall to a fully turbulent flow, as a function of the flow rate through the pipe. This transition includes regimes with significant rolling of the fiber flocs and stepwise changes in total pressure drop. Also, the fully turbulent flow implies that the energy put into the system is sufficient to disrupt fiber flocs everywhere in the flow. In comparison to a pure Newtonian liquid, where the energy put into the system is dissipated through heat by viscosity, the total energy is mainly consumed by the mechanical work needed to build and disrupt the flocs. Below the gel crowding factor, the fiber suspension, however, should behave as a Newtonian liquid. In CNF suspensions, the corrected crowding factor, Ncorr, at different concentrations and surface charges were estimated using eqs 2 and 3, and the results are summarized in Table 2 (the average nanofiber length (l) was determined from the TEM images; the CV was estimated from the length distributions; the equivalent diameter d of CNFs was estimated by the conversion between circular and ribbon-like cross section with the same cross-section area). The Ncorr value ranges from 2 to 107, indicating the presence of two limiting

Table 1. Cross-Sectional Dimensions CNFs with Different Oxidation Levels Derived from SAXS Ribbon Model Fitting charge density (μmol/g) a (Å) b (Å)

380

820

980

1360

22.2 59.0

20.1 45.4

20.1 45.4

20.1 43.4

increase in charge density, the degree of fibrillation was greatly enhanced because of the electrostatic repulsion, increased hydration, loosening of primary cell wall, and the scission of cellulose polymer chain in the amorphous regions.25 However, when the charge density was higher than 820 μmol/g, further increase in charge density did not change the cross-section dimensions due to the limitation of elementary fibrils. Flow Behavior of CNFs in Aqueous Suspension. Although being nanoscale objects, CNFs interact similar to macroscopic fibers in suspension, albeit having stronger effects of electrostatic forces. Thus, one can first consider the fiber− fiber interaction from a mechanistic perspective. In the suspension containing macroscopic fibers, the system can be viewed as having elongated nonbuoyant rod-like particles dispersed in a liquid. In this case, the effect of so-called fiber flocculation is the essential factor to determine the mechanical integrity of the suspension. The concept of fiber flocculation was first developed by Mason,40 who argued that the main mechanism dictating the mechanical strength of a fibrous network is the fiber entanglement. The effect of fiber entanglement thus has a direct influence on the rheology of the fiber suspension, which was numerically shown by Switzer and Klingenberg.41 Flocculation (or entanglement) is initiated above a certain concentration threshold, where the fibers start to be in continuous contact in a network structure. This structure is held in place as locally loaded beams, where the load is transferred through the network by the fibers and fiber− fiber contact points. Based on the mechanistic model, the flocculation is controlled by the number of contact points, the bending stiffness of the fibers, and the friction between the fibers. The primary controlling mechanism for many macroscopic fiber suspension systems is the number of contact points, which can be estimated by the aspect ratio and the concentration of the suspension. To parametrize this, Kerekes and Schell42 defined the crowding factor (N) as the number of fibers in a spherical volume of diameter equal to the length of the fibers: N=

2 2 ⎛l⎞ 2 ϕ⎜ ⎟ = ϕA2 3 ⎝d⎠ 3

(2)

where ϕ is the concentration of the system and the aspect ratio (A) equals to l/d, where l and d are length and diameter of the fibers, respectively. Given that the crowding factor is calculated for a specific fiber length, the crowding factor is underestimated given that we are dealing with fiber length distributions.43 For the case of log-normal distributions the corrected crowding factor, Ncorr, can be given as Ncorr = N(1 + CV2)4

Table 2. Crowding Factor of Different CNF Aqueous Systems at Investigated Concentration Range crowding factor Ncorr

(3)

where CV is the coefficient of variation, estimated using the protocol described by Kropholler et al.43 It turns out there are several thresholds, where the behavior of a fiber suspension can change depending on the crowding factor. In specific, flocculation does not generally occur when N E

concn (wt %)

380 μmol/g

820 μmol/g

980 μmol/g

1360 μmol/g

0.01 0.03 0.05 0.1 0.2 0.3

2 7 11 22 45 67

4 11 18 36 71 107

2 7 12 25 50 74

2 5 8 16 31 47

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occurred). In the second regime (16 < N < 60), a slope of ∼2.3 was observed, which is in good agreement with those reported by several other groups previously.24,45,46 In other words, our results support the notion that the lower crowding factor yields the Newtonian behavior, while the higher crowding factor results in viscoelastic property with shear-thinning behavior. The latter case has been typically observed in a fibrous suspension undergoing transition from a plug-flow with slip layers (i.e., high shear at the walls) to a fully developed turbulent flow (i.e., lower shear at the walls).47 More evidence could be found in the dynamic rheological behavior study. For example, in Figure 6, from the storage modulus (G′) and loss modulus (G″) data as a function of concentration, the CNF suspension was found to transform from the liquid-like to solid-like state at the gel crowding factor. In specific, at concentration below 0.05 wt % (the overlap concentration), the crowding factor of the system was lower than 16 (the gel crowding factor). Therefore, the corresponding CNF suspensions exhibited a fluid-like behavior characteristic with G′ < G″; while at higher concentration >0.05 wt % above, the suspensions exhibited solid-like behavior with G′ > G″. In particular, in the 0.3 wt % CNF suspension, the crowding factor is larger than 60 (rigidity threshold), resulting in a true gel behavior with G′ much larger than G″.24,48 The flow properties of the CNF suspensions are also affected by the charge density because of the repulsion forces between CNFs. The effect of charge density on the viscosity of CNF suspension was investigated using the 0.05 wt % CNF systems. Figure 7 shows the viscosity measured at 0.1 s−1 and the

states: a fluidized suspension and an interconnected rigid flocculated system. We have correlated the relationship between the corrected crowding factor and rheological as follows. The steady shear viscosity of a CNF suspension at charge density of 380 μmol/g measured at different shear rate is shown in Figure 5. In the suspension of low concentration, the

Figure 5. Steady shear viscosity versus shear rate of CNF suspensions with charge density of 380 μmol/g at 25 °C. The concentration ranges from 0.01 to 0.3 wt %.

viscosity at low shear rate was found to fluctuate notably in the logarithmic scale due to the relatively low sensitivity of rheometer and was not included in this figure. However, the corresponding viscosity became stable at high shear rates, and it remained at a constant value indicating the behavior of a Newtonian fluid. At high CNF concentrations, the suspension showed not only a rapid viscosity increase but also a classic shear thinning behavior. Based on Figure 5, the steady shear viscosity measured at 100 s−1 and the dynamic rheological properties (G′ and G″) at 1.0 Hz of the CNF suspension at 380 μmol/g are plotted as a function of concentration in Figure 6.

Figure 7. Steady shear viscosity at 0.1 s−1 and crowding factor of 0.05 wt % CNF suspensions as a function of charge density.

calculated crowding factor of the 0.05 wt % suspension at different charge density. It was interesting to note that the viscosity first showed an increase with charge density. This was due to the higher fibrillation level promoted by the repulsion among the negatively charged CNFs, resulting in a high nanofiber content and strong network structure. The viscosity then exhibited a gradual decrease with charge density, which was attributed to the lower agglomeration ability of CNFs due to higher repulsion forces. This behavior is analogous to macroscopic fibers, where a lower friction between the fibers give a reduced flocculation/network tendency. In a way, the increased repulsion is an analogue to the reduced friction. Given the above discussion of the crowding factor and the interaction between the fibers (friction/charge), it is not surprising to see that the viscosity of the CNF suspension is clearly correlated with the crowding factor (Figure 7).

Figure 6. Steady shear viscosity at 100 s−1 and dynamic rheological properties at 1.0 Hz of CNF suspensions (380 μmol/g) as a function of concentration.

With the combined results in Figure 6 and Table 2, the concept of crowding factor appears to be very useful to describe the observed rheological behavior. In specific, in Figure 6, the viscosity of CNF suspension showed two linear regions: the viscosity increased slightly with concentration when the crowding factor was lower than 16 (the gel crowding factor), while it increased dramatically with concentration when the crowding factor was larger than 16 (i.e., the transition of CNFs in aqueous suspension from individual to networking state F

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Macromolecules When performing frequency sweeps, an anomalous rheological behavior could be recorded where G′ dropped steeply when frequency increased obove a threshold value. This is illustrated in Figure 8 where the dynamic rheological properties

Figure 9. Dynamic rheological behavior of 0.03 wt % CNF suspension (380 μmol/g) and the recovery behavior upon changing alternatively oscillatory frequency between 20 and 5 Hz.

Figure 8. Dynamic rheological behavior of CNF suspension with a charge density of 380 μmol/g.

(G′ and G″) of the 380 μmol/g CNF suspension is plotted for different concentrations. For the lowest concentration, only G″ showed significant values, whereas the plotted values for G′ are given by the rheometer but are considered fictitious being 5 orders of magnitude lower than G″. This shows that the suspensions behaves as purely viscous liquid at low enough concentration, and it should noted that the slope of G″ in the diagram is more or less constant, indicating a power law dependence on the frequency. As a comparison, at higher concentrations, 0.03 and 0.05 wt %, the storage modulus (G′) increases slightly at lower frequencies but show an anomalous drop above a threshold frequency. This is a nonlinear behavior of the suspension, most probably due to a change in nanostructure of the suspension. It should be noted that we still plot the recorded G′ and G″ values despite being in the nonlinear regime. With respect to the loss modulus (G″), it should be noted that for these concentrations (0.03 and 0.05 wt %), we can see on slope below the threshold frequency where G′ drops and a steeper slope above, which also seems to be similar to the slope obtained for the lowest concentration. It should be noted that the observed elastic behavior shows strong similarities to the behavior of semiflexible filaments (or polymer chains with high bending modulus),49 where also flexibility and morphology have been shown to be important. The anomalous rheological behavior (i.e., the sharp drop in G′ at a threshold frequency) of the CNF suspension near the overlap concentration was also found to be recoverable with time, depending on the applied frequency. This is illustrated in Figure 9, where the recovery behavior of the 0.03 wt % CNF suspension (380 μmol/g) was investigated by monitoring the recorded G′ and G″ values when alternating the oscillatory frequency between 20 and 5 Hz after a ramp-up. During the frequency ramp-up G′ increases up to 5 Hz, followed by a significant drop with a step increase to 20 Hz. When the frequency was again lowered to 5 Hz, G′ recovered quickly (the recovery time was within 20 s). As can be seen in the figure, this could be done repeatedly. As can be seen further in Figures S3 and S4 (Supporting Information), the drop in G′ with at a threshold frequency was also observed for CNF of different charge density as well as a dependence of electrostatic screening (i.e., friction between the fibrils).

The anomalous (nonlinear) rheological behavior should be put in the perspective of that we are studying a fiber suspension as described above, albeit with nanofibers, where a continuous physical network may form even at low concentration, i.e., low crowding factor indicating few fiber−fiber contacts. Following this approach and assuming that CNF represents elastic flexible fibers with friction acting at contact points, a physical network will form when we perform work on a suspension where elastic energy will be stored in the clamped fibers. The CNF suspension has been thoroughly processed (homogenization, centrifugation, and mixing) prior to the measurements, giving rise to a physical network assuming that the crowding factor and fiber−fiber friction is sufficiently high. The exact nature of the flow field and the suspension nanonstructure dynamics is unknown, but following the behavior of macroscale fiber suspensions, it could be argued that at low constant shear levels there is a slip present closest to the cylinder walls giving a narrow shear layer, whereas the network forms a plug dominating the bulk behavior. As the shear increases, the shear in the liquid phase will perform work on the network promoting breakup, gradually extending from the near-wall region as shear is increased; i.e., a sufficient level of work (shear) is needed to act on the fiber network to break it up. This transition mechanism provides a possible explanation for the apparent nonlinear response, where elasticity would diminish. However, the actual dynamics during one oscillation cycle is unknown and requires further investigation. From the data, it is however clear that at the initiation of a frequency sweep the elastic response seen in our measurements supports the existence of a network and that at high enough frequency a nonlinear transition occurs to a fluidized state. Also, from the presented data (Figure 9, Figures S3 and S4) we can see that the morphology of the CNF, the charge, crowding factor, and salt concentration influence the anomalous behavior. Furthermore, it should also be noted that from Figure 8 it is also apparent that G″ is significant (indicating viscous dissipation) for all frequencies and that it can be noted that there are distinct crossover points in Figure 9 and Figures S3 and S4, where G′ = G″, occurring prior or in connection to the drop in G′, although it is unclear that this crossover occurs in the linear viscous regime. G

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(2) Hon, D. N. S. Cellulose - a Random-Walk Along Its Historical Path. Cellulose 1994, 1, 1−25. (3) John, M. J.; Thomas, S. Biofibres and Biocomposites. Carbohydr. Polym. 2008, 71, 343−364. (4) Zimmermann, T.; Bordeanu, N.; Strub, E. Properties of Nanofibrillated Cellulose from Different Raw Materials and Its Reinforcement Potential. Carbohydr. Polym. 2010, 79, 1086−1093. (5) Nechyporchuk, O.; Pignon, F.; Belgacem, M. N. Morphological Properties of Nanofibrillated Cellulose Produced Using Wet Grinding as an Ultimate Fibrillation Process. J. Mater. Sci. 2015, 50, 531−541. (6) Sirvio, J. A.; Kolehmainen, A.; Visanko, M.; Liimatainen, H.; Niinimaki, J.; Hormi, O. E. O. Strong, Self-Standing Oxygen Barrier Films from Nanocelluloses Modified with Regioselective Oxidative Treatments. ACS Appl. Mater. Interfaces 2014, 6, 14384−14390. (7) Paakko, M.; Ankerfors, M.; Kosonen, H.; Nykanen, A.; Ahola, S.; Osterberg, M.; Ruokolainen, J.; Laine, J.; Larsson, P. T.; Ikkala, O.; Lindstrom, T. Enzymatic Hydrolysis Combined with Mechanical Shearing and High-Pressure Homogenization for Nanoscale Cellulose Fibrils and Strong Gels. Biomacromolecules 2007, 8, 1934−1941. (8) Saito, T.; Nishiyama, Y.; Putaux, J. L.; Vignon, M.; Isogai, A. Homogeneous Suspensions of Individualized Microfibrils from Tempo-Catalyzed Oxidation of Native Cellulose. Biomacromolecules 2006, 7, 1687−1691. (9) Heinze, T.; Liebert, T.; Klufers, P.; Meister, F. Carboxymethylation of Cellulose in Unconventional Media. Cellulose 1999, 6, 153− 165. (10) Johnson, R. K.; Zink-Sharp, A.; Glasser, W. G. Preparation and Characterization of Hydrophobic Derivatives of Tempo-Oxidized Nanocelluloses. Cellulose 2011, 18, 1599−1609. (11) Yamane, C.; Hirase, R.; Miyamoto, H.; Kuwamoto, S.; Yuguchi, Y. Mechanism of Structure Formation and Dissolution of Regenerated Cellulose from Cellulose/Aqueous Sodium Hydroxide Solution and Formation of Molecular Sheets Deduced from the Mechanism. Cellulose 2015, 22, 2971−2982. (12) Samir, M. A. S. A.; Alloin, F.; Dufresne, A. Review of Recent Research into Cellulosic Whiskers, Their Properties and Their Application in Nanocomposite Field. Biomacromolecules 2005, 6, 612−626. (13) Ma, H.; Burger, C.; Hsiao, B. S.; Chu, B. Ultrafine Polysaccharide Nanofibrous Membranes for Water Purification. Biomacromolecules 2011, 12, 970−976. (14) Ma, H.; Burger, C.; Hsiao, B. S.; Chu, B. Fabrication and Characterization of Cellulose Nanofiber Based Thin-Film Nanofibrous Composite Membranes. J. Membr. Sci. 2014, 454, 272−282. (15) Fukuzumi, H.; Saito, T.; Iwata, T.; Kumamoto, Y.; Isogai, A. Transparent and High Gas Barrier Films of Cellulose Nanofibers Prepared by Tempo-Mediated Oxidation. Biomacromolecules 2009, 10, 162−165. (16) Giese, M.; Blusch, L. K.; Khan, M. K.; MacLachlan, M. J. Functional Materials from Cellulose-Derived Liquid-Crystal Templates. Angew. Chem., Int. Ed. 2015, 54, 2888−2910. (17) Shopsowitz, K. E.; Qi, H.; Hamad, W. Y.; Maclachlan, M. J. Free-Standing Mesoporous Silica Films with Tunable Chiral Nematic Structures. Nature 2010, 468, 422−425. (18) Hooshmand, S.; Aitomaki, Y.; Norberg, N.; Mathew, A. P.; Oksman, K. Dry-Spun Single-Filament Fibers Comprising Solely Cellulose Nanofibers from Bioresidue. ACS Appl. Mater. Interfaces 2015, 7, 13022−13028. (19) Mittal, N.; Jansson, R.; Widhe, M.; Benselfelt, T.; Hakansson, K. M. O.; Lundell, F.; Hedhammar, M.; Soderberg, L. D. Ultrastrong and Bioactive Nanostructured Bio-Based Composites. ACS Nano 2017, 11, 5148−5159. (20) Iwamoto, S.; Isogai, A.; Iwata, T. Structure and Mechanical Properties of Wet-Spun Fibers Made from Natural Cellulose Nanofibers. Biomacromolecules 2011, 12, 831−836. (21) Hakansson, K. M. O.; Fall, A. B.; Lundell, F.; Yu, S.; Krywka, C.; Roth, S. V.; Santoro, G.; Kvick, M.; Wittberg, L. P.; Wagberg, L.; Soderberg, L. D. Hydrodynamic Alignment and Assembly of

CONCLUSIONS CNF behavior in a hydrocolloidal suspension was investigated and described by “crowding factor” based on a concept of contact points, combined with CNF surface charge density. CNFs with different lengths but similar cross section were obtained by varying the charge density (from 380 to 1360 μmol/g) using TEMPO-mediated oxidation followed by mechanical disintegration. Rheological analysis of the CNF suspension showed that viscosity and modulus decreased with increasing charge density. This was due to (1) decreased crowding factor and (2) lower agglomeration tendency because of higher repulsion between CNFs. Anomalous rheological behavior near the gel crowding factor (overlap concentration) of CNF suspension was observed. The gradual increase in storage modulus (G′) with angular frequency was disrupted at specific “threshold” frequency. This was evident as a sudden drop in storage modulus, indicating the transition from the “floc” to “fluidized” state. Further, the reduced storage modulus was “recovered” with time under oscillatory frequency sweep, indicating the disruption and re-formation of such “flocs” are dynamic.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02642. Figure S1: FTIR spectra of freeze-dried TEMPO oxidized CNFs with different charge density; Figure S2: UV−vis transmittance spectra of the four CNF suspensions at concentration of 0.4 g/L; Figures S3 and S4: the dynamic rheological behavior of CNF suspension, depending on charge density of CNFs and ionic strength, respectively (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(B.S.H.) E-mail [email protected]. *(L.D.S.) E-mail [email protected]. ORCID

Benjamin S. Hsiao: 0000-0002-3180-1826 L. Daniel Söderberg: 0000-0003-3737-0091 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.H. acknowledges the SusChEM Program of the National Science Foundation (DMR-1409507) for the financial support, L.D.S. thanks the support from the Wallenberg Wood Science Center at KTH, and L.G. thanks the Chinese Scholarship Council for financial assistance. The authors also acknowledge the support of 12-ID-B beamline at Advanced Photon Source, Argonne National Laboratory. Michaela Salajkova, Tobias Benselfelt, and Ruifu Wang are acknowledged for experimental assistance.

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DOI: 10.1021/acs.macromol.7b02642 Macromolecules XXXX, XXX, XXX−XXX