Abstract
The effects of ethanol or acetone addition (2.5% to 40% w/w) and high ionic strength (50 mM to 1000 mM NaCl) on the rheology of carboxymethylated (NFC-carb) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized (NFC-TEMPO) nanofibrillated cellulose (NFC) suspensions were studied. Morphological characterization and centrifugation showed that NFC-TEMPO had a much finer overall morphology than NFC-carb. Rheological measurements were taken at 1.3 wt% using a stress-controlled rheometer equipped with cone and plate measurement tools with rough surfaces. The dynamic moduli were investigated through oscillatory stress sweeps. The results showed that as little as 2.5% (w/w) of either ethanol or acetone decreased the viscosity and the dynamic moduli, while 40% (w/w) increased the viscosity to values higher than those of the aqueous suspensions, doubled the storage modulus, and extended the gel-like behavior. Increasing the NaCl concentration from 50 mM to 100 mM drastically increased viscosity; moreover, the storage modulus in the elastic region linearly increased with increasing NaCl concentrations in the range of 100 mM to 1000 mM, suggesting the increased content of interparticle bonds with NaCl addition. The elastic domain was also extended from 10 Pa to 50 Pa and above 500 Pa with acetone addition (40%) and NaCl addition, respectively.
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Nanofibrillated Cellulose Rheology: Effects of Morphology, Ethanol/Acetone Addition, and High NaCl Concentration
Vera L. D. Costa, Ana P. Costa, and Rogério M. S. Simões *
The effects of ethanol or acetone addition (2.5% to 40% w/w) and high ionic strength (50 mM to 1000 mM NaCl) on the rheology of carboxymethylated (NFC-carb) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidized (NFC-TEMPO) nanofibrillated cellulose (NFC) suspensions were studied. Morphological characterization and centrifugation showed that NFC-TEMPO had a much finer overall morphology than NFC-carb. Rheological measurements were taken at 1.3 wt% using a stress-controlled rheometer equipped with cone and plate measurement tools with rough surfaces. The dynamic moduli were investigated through oscillatory stress sweeps. The results showed that as little as 2.5% (w/w) of either ethanol or acetone decreased the viscosity and the dynamic moduli, while 40% (w/w) increased the viscosity to values higher than those of the aqueous suspensions, doubled the storage modulus, and extended the gel-like behavior. Increasing the NaCl concentration from 50 mM to 100 mM drastically increased viscosity; moreover, the storage modulus in the elastic region linearly increased with increasing NaCl concentrations in the range of 100 mM to 1000 mM, suggesting the increased content of interparticle bonds with NaCl addition. The elastic domain was also extended from 10 Pa to 50 Pa and above 500 Pa with acetone addition (40%) and NaCl addition, respectively.
Keywords: Nanofibrillated cellulose; Rheology; Morphology; High ionic strength; Ethanol/acetone addition
Contact information: Department of Chemistry, Unit of Fiber Materials and Environmental Technologies (FibEnTech-UBI), University of Beira Interior, 6200-001 Covilhã, Portugal;
* Corresponding author: rmss@ubi.pt
INTRODUCTION
Many biological materials exhibit impressive and controllable properties determined by their micro- and nanostructures (Bhushan and Jung 2011).
Nanofibrillated cellulose (NFC) is a cellulosic material with lateral dimensions usually under 100 nm, which is typically obtained via mechanical defibrillation of wood pulp, commonly preceded by enzymatic treatments (Janardhnan and Sain 2006; Henriksson et al. 2007; Pääkkö et al. 2007) and/or chemical treatments such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-mediated oxidation (Saito et al. 2007), aqueous morpholine addition (Onyianta et al. 2018), carboxymethylation (Wagberg et al. 2008; Naderi and Lindstrom 2014), or sulfoethyltion (Naderi et al. 2017). These treatments result in a gel-like cellulosic aqueous suspension.
NFC recently has been the subject of much attention due to its unique characteristics, such as high aspect ratio and mechanical resistance, as well as its aptitude to set up strong entangled networks with high transparency (Nechyporchuk et al. 2016). In addition, NFC is a renewable and biodegradable material, among other attributes, which makes it suitable for many industrial applications, namely as a reinforcing phase in composites and in fields such as packaging, adhesives, biomedicine, etc. (Nechyporchuk et al. 2016). The potential of NFC, cellulose nanocrystals and bacterial cellulose in packaging applications was also recently reviewed (Hubbe et al. 2017a).
The manipulation and application of NFC suspensions requires the study of their rheological behavior. Thus, research has been lately focused on the determination of the dynamic moduli (storage modulus: G’ and loss modulus: G’’) from the linear viscoelastic regions during oscillation measurements, as well as viscosity (η) and/or shear stress (τ) during flow measurements (Naderi et al. 2014a,b; Nechyporchuk et al. 2014).
It has been reported that the NFC suspensions’ rheological properties are highly concentration dependent (Pääkkö et al. 2007; Karppinen et al. 2012; Chen et al. 2013; Nechyporchuk et al. 2015); even at solid concentrations as low as 0.125 wt%, the suspensions exhibit a shear-thinning thixotropic behavior and gel-like properties, i.e., the rheological response of the suspension is elastic dominated (G’ >> G’’) (Pääkkö et al. 2007; Nechyporchuk et al. 2015). However, more recently, Fneich et al. (2019) stated 0.25% as the optimal solid content at which the gel-like behavior appears for a salt-free suspension although the gel-like behavior can be attained at 0.1% in the presence of 50 mM NaCl.
Apart from the solid concentration, other features that have been reported to affect the rheological properties of NFC suspensions include morphological characteristics such as the length of the nanofibrils, their aspect ratio (Ishii et al. 2011; Iwamoto et al. 2013; Benhamou et al. 2014; Tanaka et al. 2014, 2015), pH, temperature, and the ionic strength of the medium (Karppinen et al. 2012; Saarikoski et al. 2012; Naderi and Lindstrom 2014; Tanaka et al. 2016, Hubbe et al. 2017b).
Regarding the effect of ionic strength on viscosity and viscoelastic properties, apparent contradictory conclusions have been suggested by Mendoza et al. (2018). Naderi and Lindstrom (2014), working with carboxymethyl cellulose with a charge of 590 μeq/g of carboxyl groups at 1% solid content and a NaCl concentration in the range of 0 to 10 mM, reported a decrease in both the viscosity and the elastic moduli. Fukuzumi et al. (2014) explored a broader range of NaCl concentration (0 to 400 mM) and worked with TEMPO-oxidized cellulose at a 0.1% solid content and reported no significant decrease in the viscosity until 10 mM of NaCl, but also reported a drastic increase in viscosity between 10 mM and 100 mM NaCl followed by a moderate decrease thereafter. Saarikoski et al. (2012) also reported a very moderate increase in the storage modulus with NaCl concentration in the range 0 to 1 M for a very low surface-charge on microfibrillated cellulose (MFC). Tanaka et al. (2014) also reported an increase in the storage modulus with salt addition. The same research group, working with NFC having a substantially different carboxyl group content (200 μmol/g and 900 μmol/g), but simultaneously with different hemicellulose content, suggested that the carboxyl groups content has a role on the sensitivity of the salt concentration and its effect on the storage modulus (Tanaka et al. 2016). More recently, Xu et al. (2018) identified two types of solid-phases for cellulose nanocrystals suspensions: a repulsive phase at low salinity and an attractive solid phase at high salinities.
It has also been reported that inherent flow instabilities occur during rheological studies of NFC suspensions. These instabilities introduce errors in the rheological measurements: shear banding and wall-slip phenomena, also designated by wall depletion, which creates a lubrication effect and results in a lower energy state during laminar shearing (Ovarlez et al. 2009; Saarinen et al. 2009; Nechyporchuk et al. 2014). Shear banding arrives from a dynamic situation of competition between flock formation vs. consolidation over time and flock destruction due to flow, favoring fragments segregation and leading to coexisting fast and slow flowing regions. Wall depletion consists of interfacial slippage on the edge of geometry tools and the suspension due to a displacement of a disperse phase from solid boundaries. To prevent or decrease these flow instabilities, many methods have been used, such as increasing the gap (Saarinen et al. 2009), using other geometry configurations like a vane-in-cup system (Mohtaschemi et al. 2014), or using roughened tool surfaces that take advantage of the material’s cohesive forces (Naderi and Lindstrom 2015; Nechyporchuk et al. 2015).
Although NFC are mostly produced and processed in aqueous media, organic solvents, such as ethanol or acetone, can play a role in solvent exchange for processing, as the reaction medium for derivatization, and even as the coagulation medium in wet-spinning. In fact, several authors (Iwamoto et al. 2011; Håkansson et al. 2014) have reported the use of acetone as a coagulation or exchange medium. As far as the authors know, the rheological response to the addition of organic solvents, like ethanol, to NFC suspensions has not been yet investigated. In contrast, the effect of ionic strength of NFC suspensions on viscosity and viscoelastic behavior has been extensively reported, but some contradictory results remain, probably due to the different nanocellulose characteristics, including surface charge, the ranges of ionic strength, and studied solid contents.
Therefore, the objectives of this investigation are to study the effects of organic solvent addition (ethanol or acetone) to the NFC aqueous medium and the effect of NaCl concentration in the range of 100 mM to 1000 mM on flow and dynamic rheological behavior of two strongly different NFC suspensions, including morphological and carboxylate content levels.
EXPERIMENTAL
Materials
For this study, a carboxymethylated NFC aqueous suspension containing a 2.2 wt% solid content, obtained from Innventia (Stockholm, Sweden), was used. This suspension will be designated as NFC-carb in the study.
An NFC suspension was also produced in the authors’ lab from a commercial bleached sulphite eucalyptus pulp. The pulp was subjected to a TEMPO-mediated oxidation pretreatment under previously reported reaction conditions (Saito et al. 2007) and then subjected to two successive homogenization steps (500 bar and 1000 bar), using a GEA Niro Soavi (model Panther NS3006L; GEA, Parma, Italy). The solid content of the suspension was initially kept at 1%; afterwards, the solids content was increased to approximately 2.2% via evaporation at room temperature with occasional stirring; when required, additional water was removed from the gel by absorption using blotting paper. The obtained NFC from this process will henceforth be called NFC-TEMPO.
Methods
Microscopic observations
The NFC-carb and NFC-TEMPO suspensions were diluted with distilled water to attain a solid content of 0.1 wt% and were subsequently sonicated for 5 min to improve the dispersion of the fibrils. A drop of each diluted suspension was allowed to air dry overnight at room temperature on a microscopic slide, and it was later attached on a microscope sample holder with double-sided tape. Microscopic observations were performed using scanning electron microscopy (SEM) (Hitachi S-2700; Hitachi, Tokyo, Japan) operated at 20 kV. All of the samples were previously gold-sputtered by cathodic spraying (Quorum Q150R ES; Quorum Technologies, Ltd., East Sussex, UK).
For the transmission electron microscope (TEM) imaging, drops of 0.001 wt% NFC-carb and NFC-TEMPO suspensions were deposited on carbon-coated electron microscopic grids and negatively stained with 2 wt% uranyl acetate. The grids were air-dried and analyzed with a Hitachi HT-7700 TEM (Hitachi, Tokyo, Japan) with an acceleration voltage of 80 kV.
NFC carbohydrate composition
The neutral sugar compositions of NFC-carb and NFC-TEMPO suspensions were determined by quantitative saccharification upon acid hydrolysis according to the National Renewable Energy Laboratory’s (NREL) proceeding guidelines for the determination of structural carbohydrates and lignin in biomass (Sluiter et al. 2012). Thin layers of NFC-carb and NFC-TEMPO were oven-dried at 50 °C for 4 h and were finely fragmented with scissors prior to the acid hydrolysis. The structural carbohydrates were quantified using an HPLC system that integrates a pump (Perkin Elmer Binary LC Pump 250; Perkin Elmer, Waltham, MA, USA), equipped with an UV/Vis detector (LC290; Perkin Elmer, Waltham, MA, USA), a refraction index detector (HP 1047A RI Detector; Hewlett Packard, Palo Alto, CA, USA) and a liquid chromatography column (Aminex HPX-87H; Bio-Rad Laboratories, Inc., Hercules, CA, USA).
Degree of polymerization in NFC polysaccharides
The determination of the NFC limiting viscosity [η], was achieved with a cupriethylenediamine (CED) solution as a solvent, using a capillary viscometer according to the ISO 5351 (2012) standard. The degree of polymerization (DP) was calculated using the Mark−Houwink−Sakurada equation [η] = 0.57 × DP (Smith et al. 1963).
Total acidic groups content in NFC
The content of total acidic groups in NFC was determined via a conductivity titration method, according to the standard SCAN-CM 65:02 (2002). First, the NFC was suspended in an HCl solution to protonate the NFC. Due to filtration issues, the vacuum filtration washing after the protonation step was substituted by sequential 15 min centrifugations at 3000 g. The supernatant was discarded and the process of adding distilled water and mixing after each centrifugation was continued until the supernatant attained a conductivity of 5 μS/cm. The suspensions were afterwards titrated with a NaOH solution to pH = 11. The amount of weak acid groups was determined from break points in the conductivity vs. added volume of NaOH from the curves obtained from the conductivity titrations.
Preparation of the NFC suspensions for rheological measurements
Suspensions of NFC with 1.30 wt% solid content were produced from the original NFC-carb and NFC-TEMPO through the addition of the appropriate amount of distilled water, organic solvent, or NaCl aqueous solutions. The homogenization of the suspension was obtained through vigorous agitation in a vortex mixer with four successive steps lasting 1.0 min each with handshaking in-between.
Ethanol or acetone were added to the original NFC aqueous suspensions to attain the suspension medium with ethanol/(ethanol + water) percentages of 2.5%, 5.0%, 10.0%, 20.0%, and 40.0% (w/w). The NaCl solutions were added to attain the final concentrations of 5 mM, 100 mM, 300 mM, 500 mM, and 1000 mM.
Prior to the rheological measurements, all suspensions were sonicated for 5 min to ensure proper homogenization and air bubble removal. After this step, the NFC suspensions were loaded in the rheometer and rested for 1.0 min before the rheological assays. The samples were not subjected to preshearing in the rheometer with the purpose of avoiding the distortion of the initial NFC structure before the measurements (Nechyporchuk et al. 2015).
Zeta potential measurements
The previously prepared 1.3 wt% NFC-carb and NFC-TEMPO suspensions with NaCl concentrations of 0 mM, 50 mM, 100 mM, and 1000 mM, as well as NFC-carb suspensions with 2.5% and 40% (w/w) ethanol were diluted to a solids content of 0.1 wt% with distilled water, the pH was adjusted to 7, and the resulting suspensions were sonicated for 5 min. A rough estimative of the surface particles charge was obtained by a streaming current detector (Mütek PCD-02; BTG Product Digest, Herrsching, Germany).
Rheological measurements
The rheological measurements were recorded using a stress-controlled rheometer (RheoStress® RS 150; Haake Technik GmbH, Vreden, Germany). A cone and plate geometry with a 2º angle cone sensor (C35-2º) with a diameter of 35 mm and a gap of 0.105 mm were used to study the NFC suspensions in both flow and oscillation modes. To study the effect of the tool’s roughness on the rheological measurements, a sandpaper with a roughness of either 58.5 μm or 18.3 μm, depending on the assay, was attached to both the plate and cone using double-sided tape. When roughened surfaces were used, the thickness of the sand paper was taken into account, maintaining the same gap as in the assays with smooth surfaces. To minimize water or ethanol evaporation, all of the rheological measurements were performed under a homemade transparent cover.
In flow mode, controlled rate flow tests were conducted with a shear rate () in the range of 0.05 s-1 to 1000 s-1 for 180 s. Shear stress (τ) and viscosity (η) were analyzed.
The viscoelastic behavior was studied through oscillatory stress sweeps performed with a τ in the range of 0.07 Pa to either 100 Pa or 10000 Pa, depending on the suspension’s rheological performance, with a frequency of 1.0 Hz. The dynamic moduli (G*), i.e., the storage modulus (G’) and the loss modulus (G’’) were analyzed. The suspensions were tested at room temperature (22 1 °C). All of the described assays were performed in duplicate and the results represent the arithmetic average in each point.
RESULTS AND DISCUSSION
Morphological Characterization of the Fibrils
The morphology of the fibrils was investigated through SEM and TEM analyses using various magnifications to capture the micro- and nano-scale particles. Figure 1 gives an overview of the morphological characteristics of the NFC-carb and NFC-TEMPO fibrils.
As shown in Figs. 1a and 1c, the fibrillation of the material appears not to have been homogeneous and particularly incomplete, especially for NFC-carb. Although the fine fibrillar structures with diameters in the nanometer range can be spotted in both materials (Figs. 1b and 1d), the relative proportion of the fibers with micrometer dimensions was clearly higher in NFC-carb. The pretreatments performed on NFC-TEMPO led to a substantial deconstruction of the fibrillar cell wall of the sulphite eucalyptus pulp, thus resulting in shorter fibrillar structures and a finer overall morphology when compared to the NFC-carb fibrils produced from softwood kraft pulp.
To roughly quantify the relative proportions of nano- and microelements, extremely dilute suspensions (0.05 wt%) of both materials were adjusted to a pH of 4 (to protonate most parts of the carboxylate groups; pKa = 4.8) and were submitted to centrifugation at 9000 g for 20 min. The obtained sedimented material was quantified. The experimental results showed nearly 84% and 57% of the initial material has sedimented for NFC-carb and NFC-TEMPO, respectively, confirming the higher percentage of nanoelements in NFC-TEMPO. Therefore, the two materials had substantially different morphological properties. While most of the NFC-carb’s elements were predominantly at microscale range, the NFC-TEMPO’s were made up of many more elements in the nanoscale range.
Fig. 1. NFC-carb (a and b) and NFC-TEMPO (c and d) fibrils imaged through SEM (a and c) at a solid content of 0.1 wt% and TEM (b and d) at a solid content of 0.01 wt%
Physico-chemical Characterization of the Fibrils
The structural carbohydrates in NFC-carb were quantified through acid saccharification. The material was made up of 88.4% cellulose and approximately 11.6% hemicelluloses. The NFC-carb’s limiting viscosity was 397 mL/g, which corresponded to an average DP of 696, suggesting a good preservation of the DP of the initial raw materials. Assuming that the weak acidic groups were predominantly carboxylic acid groups introduced during the carboxymethylation pretreatment stage in the course of the NFC production, its carboxyl group content was determined as 698 μmol/g. This value was in good agreement with those reported by NFC produced via a similar process (Naderi et al. 2014a).
Concerning the NFC-TEMPO material, the resulting limiting viscosity was 85 mL/g, corresponding to an average DP of 149, and the resulting carboxyl group content was 1900 μmol/g. Both the carboxylic group content and the polysaccharides’ DP were in good agreement with the values reported in the literature for similar pretreatments (Shinoda et al. 2012). From Fig. 1 and the physicochemical data, it was clearly demonstrated that the two materials used in the present work were representatives of two morphologically and physio-chemically different materials.
Effect of Geometry Surface Roughness
Many authors have reported that a distortion of the rheological measurements of NFC suspensions (mostly during flow mode measurements) occurs due to wall-slip phenomena and hence at least one roughened or serrated tool surface was introduced to overcome this issue (Iotti et al. 2011; Naderi et al. 2014b; Nechyporchuk et al. 2014; Nechyporchuk et al. 2015). For example, Nechyporchuk et al. (2014) have suggested attaching sandpaper (roughness of approximately 120 μm) on the surfaces of cone and plate measurement tools to prevent wall-slippage of a 1.0 wt% TEMPO-oxidized NFC suspension during rheological flow measurements. In the present study, preliminary rheological studies were performed to determine the influence of different surface roughness on the measurements. Sandpapers with various roughness values (58.5 μm and 18.3 μm) were attached to the surface of both the plate and sensor. Controlled rate flow assays were performed with both mentioned sandpapers and also without the sandpaper (smooth surface) using a 1.3 wt% NFC-carb aqueous suspension.