Nanostructured Cellulose II Gel Consisting of Spherical Particles

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A nanostructured cellulose II gel consisting of spherical particles Marco Beaumont, Harald Rennhofer, Martina Opietnik, Helga C Lichtenegger, Antje Potthast, and Thomas Rosenau ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01036 • Publication Date (Web): 16 Jun 2016 Downloaded from http://pubs.acs.org on June 18, 2016

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ACS Sustainable Chemistry & Engineering

A nanostructured cellulose II gel consisting of spherical particles Marco Beaumont,§ Harald Rennhofer,† Martina Opietnik,∥ Helga C. Lichtenegger, † Antje Potthast,§ Thomas Rosenau§* §

University of Natural Resources and Life Sciences Vienna (BOKU), Department of Chemistry,

Division of Chemistry of Renewable Resources, Konrad-Lorenz-Str. 24, A-3430 Tulln, Austria. † University of Natural Resources and Life Sciences Vienna (BOKU), Institute of Physics and Material Science, Peter-Jordan-Str. 82, A-1190 Vienna, Austria. ∥ Lenzing AG, Werkstr. 2, A4860 Lenzing, Austria. *Corresponding Author Thomas Rosenau§, E-mail: [email protected]

Abstract Cellulose nanofibrils (CNF) are usually obtained by breaking down the lignocellulosic structure of pulp, i.e. as cellulose I allomorph and according to rather energy-intensive pathways. In contrast to those approaches, TENCEL® gel is obtained from a cellulose II side stream of the Lyocell process in a deceptively energy-efficient way: After enzymatic treatment and only one cycle in a high-pressure homogenizer (comparing to up to 20 cycles for CNF manufacture) the final gel is obtained. The utilization of an idle side stream – that was hitherto dumped – of an already existing industrial process is another distinct advantage. This novel cellulose II gel possesses a particle-like, homogenous morphology and is composed of individual particles with a size of less than one micron, featuring the rheological behavior of a soft solid. The course of the gel production process was studied with respect to changes in crystallinity, appearance, and molecular weight, while the morphology and size of the final gel particles was assessed comprehensively by light-microscopy, dynamic light scattering, and electron microscopy. In water, the individual particles form aggregates with a mean size of 11 µm. The viscoelastic gel forms highly porous cryogels with a surface area of 298 m2/g and a well-defined nanostructure.

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These features were studied in depth by SAXS, nitrogen sorption experiments, and SEM. The economic production in combination with the highly accessible surface offers unique properties, and applications are envisioned as tailored, high performance materials.

Keywords Nanocellulose, cellulose II particles, hydrogel, regenerated cellulose, TENCEL, Lyocell

Introduction In the last decade the interest in nanocelluloses, such as cellulose nanofibrils (CNF) and nanocrystalline cellulose, has risen exponentially. Fibrillated cellulose can be derived from cellulose-rich plants, bacteria or animals. It is characterized by a fibrous morphology. CNF is usually handled as aqueous suspension in concentrations of 0.1 to 5.9 wt% and can be classified as hydrogel.1,2 An important property of CNF is the decrease of viscosity once shear stress is applied. Therefore the material can be pumped, and is also injectable with regard to biomedical application.2,3 According to the most common approach to get CNF, a dilute pulp suspension is passed through a mechanical homogenizer. Plant cell walls and cellulose fibers are broken down into individual fibrils,4,5 which is the most energy-intensive step. One possibility to optimize the energy efficiency is an enzymatic treatment of the pulp, but still 8-20 homogenizer passes are required to obtain a homogeneous suspension.1,6 The reduction of the homogenization steps is a key factor to increase energy balance and thus sustainability of the CNF production. Common CNF consists of the cellulose I allomorph. Cellulose II or regenerated cellulose is generally associated with man-made fibers in textile industry, but not with CNF production. Regenerated cellulose fibers or “man-made cellulose fibers” are widely established products. They have a more homogeneous structure and better sorption properties than natural fibers7 and can be tailored to specific needs.8 The viscose fiber (rayon) is probably most known representative of man-made fibers, but its production process involves the use of several hazardous chemicals, among them a chemical modification with CS2 to obtain cellulose xanthogenate which is spun into a sulfuric acid bath.9 A relatively recent, sustainable alternative is the TENCEL®‡ fiber (Lyocell fiber), which is obtained by direct dissolution of pulp in Nmethylmorpholine N-oxide monohydrate (NMMO) and subsequent spinning into air and water.10


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It offers various advantages compared to cotton, among others a higher sorption capability and tensile strength.9 It has the lowest environmental impact in comparison to other cellulosic fibers and cotton,11 and spent NMMO can be recycled to more than 99% on an industrial scale.12 TENCEL® fibers are nowadays produced in a capacity of 220000 tons per year.13 In this process, a shaped, fluffy material is obtained as by-product which is used as the starting material in the production of the nanostructured cellulose II gel reported herein.14 Cellulose beads feature similar properties as the here reported gel with relation to size and shape. They are produced by dropping or dispersing a cellulose solution into an anti-solvent.15 In this process, spherical cellulose II particles with sizes ranging from micro- to millimeters. In comparison to these beads, the here reported material is produced from a solid and fluffy cellulose II precursor under heterogeneous conditions. Recently, we reported that TENCEL® gel can be also used as precursor to obtain spherical cellulose nanoparticles.16 Micro- and nanocrystalline cellulose (MCC / NCC) are often termed cellulose particles. They are produced by acidic hydrolysis of pulps that mainly affects the amorphous cellulose regions.17–19 As a consequence they feature higher crystallinity than non-treated cellulose or the cellulose II gel discussed here. In this publication, when using the term cellulose particles, we will not refer to MCC or NCC, but to spherical particles in the micro- and nanoscale that have a similar degree of crystallinity as their cellulose source. The characterization of the novel gel material was the aim of the work and will be the topic of this report. We discuss first the course of molecular weight and crystallinity along the production process and then elaborate on the morphology and nanostructure of the gel, based on a combination of analytical techniques: light microscopy, rheology, as well as SEM, SAXS, WAXS and nitrogen sorption.

Experimental section

Materials TENCEL® fibers, TENCEL® gel and its intermediates were provided in a purified state by Lenzing AG. The starting material was a 50/50 (w/w) mixture of a bleached beech sulfite pulp (pulp 1) and a bleached eucalyptus Kraft pulp (pulp 2) and is, for clarity, referred to as pulp


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mixture. Absolute tert-butanol (tBuOH) and ethanol (EtOH) were purchased from SigmaAldrich. Methods The carbonyl content of the cellulose gel was determined by fluorescence labeling with carbazole-9-carbonyloxyamine

(CCOA),

followed

by

multi-detector

gel

permeation

chromatography (GPC), which provides carbonyl group profiles in relation to the molecular weight distribution, as described previously.20,21 GPC samples were prepared from never-dried materials. Solid state NMR experiments were performed on a Bruker Avance III HD 400 spectrometer (resonance frequency of 1H of 400.13 MHz, and

13

C of 100.61 MHz, respectively), equipped

with a 4 mm dual broadband CP-MAS probe. 13C spectra were acquired by using the TOSS (total sideband suppression) sequence at ambient temperature with a spinning rate of 5 kHz, a crosspolarization (CP) contact time of 2 ms, a recycle delay of 2 s, SPINAL-64 1H decoupling and an acquisition time of 49 ms whereas the spectral width was set to 250 ppm. Chemical shifts were referenced externally against the carbonyl signal of glycine with δ = 176.03 ppm. The acquired FIDs were apodized with an exponential function (lb = 11 Hz) prior to Fourier transformation. Peak fitting was performed with the Dmfit program.22 The crystalline-amorphous ratio hc/ha is the ratio of the sum of the areas of crystalline peaks divided by the areas of the amorphous peaks. All materials for solid-state NMR were air-dried at room temperature before measurement. The microscope images were acquired on an incident light microscope (Axioplan 2 imaging, Carl Zeiss Microimaging GmbH, Jena, Germany). The number-weighted particle size distribution of TENCEL® gel in EtOH was extracted from the respective microscope image using the software ImageJ.23 Rheological measurements of gel samples were performed at room temperature with a Malvern Kinexus Pro plane-cone rheometer equipped with a cone of d = 40 mm, a 4° cone angle (CP4/40 S0687SS) and a 150 µm gap. The flow curve was determined from a shear rate of 0.1 to 1000 s Å-1. Frequency sweep was measured at constant strain of 0.1% from 0.01 to 100 Hz and amplitude sweep at a frequency of 1 Hz and from a shear strain of 0.01 to 100%. Zeta potential was measured on a Zetasizer Nano ZS (Malvern). All samples were measured at 20°C in a concentration of 1.6 wt%.The ionic strength of the gel was set by addition of 50 mM


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NaCl before adjusting the pH with glacial acetic acid and aqueous ammonium hydroxide solution (28% NH3). The number-weighted mean particle size and the particle size distribution were determined by static laser light diffraction analysis on a LS 13320 device (Beckman-Coulter). The sample was added dropwise into the universal liquid module until an obscuration of 8-10% was reached. SEM experiments were performed on a FEI INSPECT S50 instrument (Hillsboro, Oregon, USA). Samples were sputtered with a gold layer of 8 nm thickness in a Leica EM SCD005 sputter coater. Nitrogen sorption measurements were performed at 77 K on Micromeritics ASAP 2020 instruments. The surface area was calculated according to Brunauer, Emmett and Teller (BET) theory, and the pore size distribution according to Barrett, Joyner and Halenda (BJH), applied on adsorption and desorption branches of the BET isotherm. The samples were degassed under vacuum at 60°C for at least 5 h prior to the measurements. Small angle x-ray scattering (SAXS) and wide angle x-ray scattering (WAXS) were carried out with a three pinhole SMAX-3000 SAXS camera (RIGAKU) equipped with a copper target micro focus X-ray tube (MM002+ source with a wavelength of λ = 0.1541 nm). Two-dimensional scattering images were recorded with a TRITON 200 multi-wire X-ray detector (20 cm diameter mapped on 1024 x 1024 pixels) for the SAXS and a Fuji image plate (15 x 15 cm size mapped on 1500 x 1500 pixels) 25 mm from the sample for WAXS. The scattering images were averaged azimuthally to gain information of the scattered intensity I(q) in dependence on the scattering vector q. From the Porod constant P and the invariant

, the specific inner surface

S/VSAXS was calculated with Eq. 1:24 Eq. 1 Φ1 and Φ2 were calculated from the density of cellulose of 1.5 g/cm3 and the bulk densities of the freeze-dried samples (0.064 g/cm3). In the case of never-dried TENCEL® gel the solid concentration of 4% was used instead of the bulk density. The radii of gyration were calculated in the Guinier regions with the following relationship between the scattering intensity I(q), the scattering vector q and the radius of gyration Rg:25


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Eq. 2 G is a scaling factor and s equals 0 in the case of spheres or 1 in rod-like structures.25 The radius of gyration can then be extracted from the slope of the Guinier plot, q2 against ln(I(q)).26 WAXS data was analyzed with respect to crystallinity and crystal size. To determine the crystallinity the respective cellulose I and II peak positions from literature were used as shown in Figures S7 and S8.27–29 Two additional non-symmetrical Gaussian-shaped peaks were defined to account for the amorphous contribution around q = 1.4-1.5 Å-1 and 2.8-2.9 Å-1, respectively. The diffractograms were fitted using eXPFit, all crystalline peaks were fitted with a symmetrical Gaussian curve. The same full width at half maximum values for 101 and 10-1 peaks for each sample was used, respectively. The fit was applied in the q-range of 0.7-2.6 Å-1. Fitting and peak positions of cellulose I, q = 1.0 Å-1 (peak 101), 1.1 Å-1 (10-1), 1.4 (021), 1.5 (002), 2.4 (004), and cellulose II, q = 0.9 Å-1 (101), 1.4 (002), 2.4 (004), are shown exemplarily in Figures S7 and S8. If AC is the sum area below the crystal peaks and AA the area below the amorphous peak the crystallinity was calculated according to Eq. 3 in the area of 0.7 to 2.2 Å-1. Eq. 3 Correction for machine broadening (c = 0.566 nm-1) was applied to the width of the Gaussian curves from the fit in order to calculate the crystal size with the Scherrer formula.30 Production of TENCEL® gel TENCEL® gel was produced at Lenzing AG according to the patent by Maenner et al.14 The shaped cellulose II precursor was washed intensively by a combination of acidic, basic and neutral washing steps to remove residual NMMO. The purified precursor was then treated in a colloid mill (IKA MK 2000/10) with 1 wt% (based on cellulose content) of endoglucanase (Novozyme 476). After the enzymatic treatment, the gel precursor was homogenized in a single pass in a high pressure homogenizer (GEA Niro Soavi NS1001L) at 1000 bar to obtain the final TENCEL® gel. The remaining enzyme, degradation products and oligomers from the enzymatic treatment were removed by an alkaline treatment in 0.1 M NaOH at 80°C for 30 min. The resulting yellow suspension was washed with water to pH = 7. A comparison of the solid content


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before and after the alkaline treatment was used to calculate the mass loss of 15% during the enzymatic treatment. Sample preparation for SAXS, SEM and nitrogen sorption measurements The solvent-exchanged alcogel was obtained by adding a 10-fold volume of tBuOH to the aqueous TENCEL® gel. The suspension was then centrifuged and the supernatant was removed. The concentrated alcoholic suspension was then redispersed in a 10-fold volume of tBuOH and shaken overnight at room temperature. This step was repeated, this time in a water bath (T = 30°C) to prevent freezing of tBuOH. The same procedure was applied for the solvent-exchange to EtOH, doing the shaking always at room temperature and applying two additional solventexchange steps. The alcogel (FD-tBuOH) and the hydrogel (FD-water) were both freeze-dried according to the same procedure. The gel was dropped into a beaker filled with N2(l), and the frozen gels were transferred into the lyophilizer (Christ Beta 1-8 LDplus, T = -56°C, p = 0.01 mbar) and kept for 3 days. Both, FD-tBuOH and FD-water, had a bulk density of 0.064 g/cm3.

Results and Discussion

Gel production As shown in Figure 1, the gel was produced from the non-fibrous cellulose II precursor, which was obtained from a side-stream of the TENCEL® fiber production. This by-product occurs in the spin bath after spinning, and so far remained unused in commercial fiber production. Therefore, no product lines need to be tapped for the gel production, and no technological adjustments are required. The precursor is then treated enzymatically using 1 wt% of endoglucanase and dispersed during the treatment in a colloid mill,14 providing the gel precursor in 85% mass yield. The gel precursor was passed only once through a high-pressure homogenizer at 1000 bar to obtain the final gel. Compared to up to 20 cycles for conventional production of CNF – the production of this gel requires significantly less energy and is thus more economic.


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Figure 1. Production process of the cellulose II gel starting from pulp. The shaped cellulose II precursor is obtained from a side-stream of the TENCEL® fiber production out of the Lyocell process. This precursor is further enzymatically treated and homogenized to yield the particlelike gel.

In these two process stages suspensions with high solid contents of 4 to 5 wt% were used, which is more than twice as high as the processable concentration in common CNF productions.2,6 Besides this, only a single pass through the energy-demanding high-pressure homogenizer is needed. Hence, the production of TENCEL® gel is highly energy-efficient: the required energy demand is significantly lower than unconventional CNF production. Cellulose micro- and nanoparticles can also be obtained by spraying a cellulose suspension into liquid nitrogen31 or by precipitation of dissolved cellulose and derivatives.32–35 These methods are more time-consuming and produces only individual particles but not interacting particles forming a viscoelastic gel.

Table 1. Comparison of the starting materials (a 50/50 (w/w) mixture of pulp 1 and pulp 2), the intermediates (regenerated celluloe from the lyocell process) and product (the final gel) by GPC and x-ray scattering. Mw weight-averaged molecular weight, CWAXS crystallinity determined by WAXS, Dhkl WAXS crystallite dimension, hc/ha solid state-NMR crystalline-amorphous ratio. The values of the pulp mixtures are given as average from the respective values of pulp 1 and pulp 2 in Table S1.

Samples

Mw

Carbonyl

CWAXS (%)

D110 (nm)


D020 (nm)

hc/ha

8

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(kg/mol)

content (µmol/g)

233.9

8.7

62

5.7

5.5

1.10

184.3

13.5

43

3.0

4.1

0.81

Gel Precursor 19.33

94.4

50

5.6

5.4

0.89

94.9

47

5.1

4.1

0.85

Pulp mixture Shaped Precursor TENCEL® gel

15.97

In our analysis, we included as well the starting material of the Lyocell process, a mixture of a bleached beech sulfite pulp (pulp 1) and a bleached eucalyptus Kraft pulp (pulp 2). The pulp mixture is first dissolved in NMMO in the Lyocell process which provides the non-fibrous cellulose II precursor with a solid content of approx. 30wt%.

Figure 2. Left: Molecular weight distributions of the starting materials (pulps), the intermediates (shaped precursor, gel precursor) and TENCEL® gel. The gel precursor and the final gel feature significantly smaller molecular weight and a narrower MW distribution in comparison to the other materials. Right: Relation of the carbonyl content to the MW of the cellulose II gel.

As shown in Figure 2 and Table 1, the weight-averaged molecular weight was lowered in the Lyocell process from 234 kg/mol to 184 kg/mol. TENCEL® fibers produced in the same process have a smaller Mw of 148 kg/mol (Figure S1). This indicates that the shaped precursor is a higher MW fraction that is less well processable to fibers in the Lyocell process and is thus enriched in the non-fibrous side stream.


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The crystallinity was affected significantly in the Lyocell processing and decreased from 62% to 43%, which is the lowest value among the compared materials. The decrease in MW, the lower crystallinity, and the fact that mostly the crystallite size in the D020 dimension was affected in the Lyocell process are observations which are all in line with previous reports.20,36 The shaped precursor is further processed by an enzymatic treatment to obtain the gel precursor. This enzymatic treatment reduced the MW significantly, down to 19 kg/mol, and narrowed the MW distribution (decreased polydispersity). The gel precursor thus features cellulose chains consisting on average of 120 anhydroglucose units, which is roughly a tenth of the chain length of the starting pulp material. The decrease in MW was necessary to obtain a material readily processable into a homogeneous gel. Besides, the enzymatic treatment augmented the crystallinity from 43% to 50% and increased the average crystallite size. The increase in crystallinity stems from the fact that the enzymes favor at least in the beginning of the treatment - the amorphous cellulose regions.37 In order to obtain the final cellulose gel, the gel precursor was passed only once through a high-pressure homogenizer. In comparison to the other production steps, this treatment affected the MW and crystallinity only insignificantly. The final gel featured a crystallinity of 47% and a Mw of 16 kg/mol. The DS(carbonyl) plot in Figure 2 and the high carbonyl content of the gel indicate remaining traces of adhering enzyme. As shown in Figure S2, these impurities can be removed, if necessary, by an alkaline treatment. The crystallinity of the cellulose II gel is comparable to commercial cellulose I materials28 while the crystallinities of microcrystalline cellulose I (61-64%)28,38 and cellulose II nanocrystrals (72-74%)39 are significantly higher.

Morphology of the gel


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Figure 3. Microscopic comparison of TENCEL® gel before and after solvent-exchange to EtOH. Images show the gel particles in 500x magnification. The aggregation of the cellulose particles in water to a mean diameter of 11.2 µm is avoided in EtOH. There, the individual particles feature a mean diameter of 0.5 µm. The particle size distributions of both samples are shown in the insets.

The final gel is highly homogenous which is already reflected in its macroscopic appearance and smoothness. It can be characterized as a cellulose particle suspension as shown in Figure 3. The aqueous suspension consists of aggregated particles with a mean size of 11.2 µm, thus being comparable in size to cellulose beads15 and microcrystalline cellulose40. The zeta potential of the aggregates at pH 7 is –10.8±1.6 mV is comparable to non-modified CNF (–10 mV).41 The moderately negative value explains also the interaction between the particles (aggregation) due to the low electrostatic repulsion of the particles. If water as the solvent was replaced with EtOH, these aggregated particles were dispersed into particles with a mean particle size of 0.5 µm (right inset, Figure 3). The same observation was also made in the case of water and a highly diluted microparticle suspension (see Figure S3). The particles aggregate in water after reaching a critical concentration and form aggregates. The critical concentration has not been determined; it is in any case significantly lower than the used cellulose solid concentrations of 1-4 wt%. In EtOH, the attractive forces of the particles are lower, and no aggregate formation was observed. The magnification in Figure 3 proves that the aggregates consist of small, individual particles. From the rheological perspective, the cellulose


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gel was expected to show a specific flow behavior that is related to its structure. Figure 4 shows the dependency of the shear rate on the viscosity. The gel is shear-thinning and the decrease in viscosity can be divided into three different regions which are highlighted as region A, B and C in Figure 4. Keeping in mind that the aggregates consist out of small microparticles, the course of the flow curve can be reasoned as shown schematically in Figure 4. At the beginning, around point A, the particles in water are aggregated and these aggregates are orientated randomly. Under shear, they start to align in shear direction, and at the point B a limit of alignment is reached. Beyond a certain shear stress, the aggregates break up into smaller particles and the viscosity of the gel decreases further. A similar behavior was also reported for cellulose nanocrystals. In this case the aggregated structure is a liquid crystalline domain.42 The viscosity of the gel was hardly affected by ionic strength (50-1000 mM NaCl) and pH (410) as shown in Figure S4. Suspensions with a solid content of 3.2 wt% were all stable under these conditions. The stability of the samples was estimated by centrifugation experiments and the volume of the lower gel phase was used to estimate the stability of the gel under different ionic strengths and pH. In all samples, the amount of retained water was very similar. The gel volume after centrifugation showed only a marginal pH influence and featured the highest stability between pH 6 and 8 and was independent from the ionic strength (Figure S4). The zeta potential, however, was mostly affected by the ionic strength and increased from –11.4 to –3.2 mV at ionic strengths of 50 mM to 1000 mM, respectively (Figure S4). Regarding the pH dependency, the colloids were most stable between pH 6 and pH 10 and showed a minimum of stability at pH 4 (–6.7 mV).


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Figure 4. Flow curve of the cellulose gel (2 wt%) showing the dependency of the shear rate on the viscosity. The shear-thinning behavior can be divided into different stages as indicated schematically. Beyond region B the aggregates disperse into smaller aggregates or individual particles explaining the additional shear-thinning in region C.

The frequency sweeps of the cellulose gel in different concentration are shown in Figure 5. All samples can be classified as gel or soft solid due to their dominant elastic behavior (G’>>G’’) in this frequency range, and a linear plateau region of the storage modulus (G’).43 The storage modulus is in all cases approximately 10 times higher than the respective loss modulus. This viscoelastic behavior can be explained by considering the cellulose gel as aggregated or flocculated system. The same model was also applied on CNF systems.44,45 In a flocculated system attraction forces dominate, such as van der Waals attractions, and therefore aggregates or flocs are formed46 that show viscoelastic behavior.47 The dependency of the storage modulus on the concentration or volume ratio of the gel can be described by an exponential law (Eq. 4).48,49 Eq. 4


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Figure 5. Left: Frequency dependence of storage (G’) and loss modulus (G’’) of cellulose gels with different solid contents ranging from 2 wt% to 4 wt%. All samples feature the rheological behavior of a gel: a stable G’ plateau over a broad frequency range and a dominating elastic behavior. Right: Dependence of the concentration and the storage modulus and the amplitude sweeps of gels with solid contents from 2 wt% to 4 wt%.

As shown in the inset of Figure 5, the storage modulus is proportional to the concentration to the power of 2.4. This value lies in the range of TEMPO-oxidized and carboxymethylated CNF,50,51 whereas non-modified CNF shows an increased dependency (α ≈ 3).44 In the fibrillated system the concentration has a bigger contribution to the elastic behavior due to entanglement of individual fibrils with high aspect ratio. The repulsive forces of the modified fibrils, in case of TEMPO-oxidized and carboxymethylated CNF, seem to reduce this effect so that the concentration dependency of the storage modulus is lowered and becomes comparable to TENCEL® gel that features a lower aspect ratio than CNF. As shwon in Figure S5 the cellulose II gel shows gel-like behavior up to concentrations 0.5 wt%, while at lower concentrations – as shown in the sample with 0.25 wt% a – viscous behavior dominates. Amplitude-sweep experiments showed that the limit of linear rheological response of the storage modulus did not change with increasing concentrations (Figure 5). Therefore both interfloc and intra-floc links contribute to the gel’s overall elasticity.52 Nevertheless, considering the shear thinning behavior, the inter-floc links seem to be more rigid hinting at a slightly dominating weak-link regime.


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Eq. 5 In Eq. 5 the scaling factor α is related with the Euclidean dimension (d) and the fractal dimension DF

.

53

According to this, a fractal dimension of 2.6 was calculated for a three-

dimensional system which indicates the existence of a mass fractal structure. The fractal dimension DF is a measure of the compactness of the individual aggregate.54 Taking all these findings into account, it can be stated that the cellulose II gel fulfils the rheological requirements of a gel or soft solid and has rheological properties similar to CNF. The submicron structure of TENCEL® gel in liquid state was compared to the solid-state structure of freeze-dried cryogels that were obtained according to two different methods. The hydrogel was either frozen in liquid nitrogen and subsequently lyophilized, or a prior solventexchange to tBuOH was included. The cryogels were analysed by electron microscopy, SAXS and nitrogen sorption. Figure 6 shows the SEM pictures of the two freeze-dried samples. Freezedrying from water (FD-water) yields a sheet-like structure with no discernible submicron structure (Figure 6A). At a higher freezing temperature of -20°C, the sheet-like structures became even more dominant and larger sheets were observed (Figure S6). This sheet-like morphology can be reasoned by the template effect of formed ice crystals.55,56 In contrast to that, the morphology of the cellulose particles was preserved if water was solvent-exchanged to tBuOH (Figure 6B): a porous cryogel was obtained. The zoomed part of Figure 6 shows a detail of this cryogel (FD-tBuOH) featuring a highly porous nanostructure made up of cellulose fibrils with a width of 40-60 nm.


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Figure 6. Comparison of cryogels with sheet-like structure (A) which was freeze-dried directly from water (FD-water) and open-porous structure (B) obtained by solvent exchange to tBuOH and subsequent freeze-drying (FD-tBuOH). The zoom shows the nanostructure of the freezedried alcogel to feature fibrils with widths of 40-60 nm.

Comparable results were calculated from nitrogen sorption experiments according to BET and BJH theories to obtain surface area and pore size distribution, respectively. The freeze-drying of the alcogel (FD-tBuOH) yielded an open-porous structure with a high surface area of 298 m2/g which is higher than the surface area of commercial cellulose beads (121-201 m2/g)57 and better comparable to CNF (249 m2/g)58 and TEMPO-oxidized CNF (284-349 m2/g)50,58. In contrast to that, the cryogel that was freeze-dried from water (FD-water) had a small surface area of only 10 m2/g, which is in the range of microcrystalline cellulose powder (1-20 m2/g)38. The cellulose II gel features significantly higher reactivity towards azidopropyl triethoxysilane than CNF.59


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Table 2. Comparison of cryogels freeze-dried from the respective hydrogel (FD-water) and alcogel (FD-tBuOH). The surface area of the respective cryogel SBET, the surface area of micropores Smicro and the pore width was derived from nitrogen sorption experiments. The specific inner surface area S/VSAXS was calculated on basis of the Porod law from SAXS data. The pore diameter Dp and the cross sectional fibril diameter DF were calculated from radii of gyration. D110 is the crystallite width of the air-dried gel.

Sample

SBET

Smicro

S/VSAXS

(m2/g) (m2/g) (m2/cm3)

Never-dried / FD-tBuOH 298.4 FD-water 9.5

/ 54.1 3.5

284.9 212.9 100.0

Pore width (nm) / 26.8 12.2

Dp (nm) 3.7±0.1 8.9±0.3 -

Df (nm)

D110 (nm)

33±8 35±5 44±12

5.1 5.1 5.1

The pore distribution is shown in Figure 7B. FD-tBuOH has an average pore width of 27 nm and shows the highest surface areas in a pore range of 9-30 nm. The population of smaller pores decreases until a pore size of 6 nm and increases again for pores in the range smaller than 2 nm. This is seen as well by the high microporous area shown in Table 2. Concluding, highly porous cryogels can be accessed if a solvent-exchange step was applied prior to freeze-drying. Freezedrying from water was detrimental to high porosity and gives sheet-like structured cryogels instead.


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Figure 7. Comparison of SAXS curves of the respective hydrogel and cryogels obtained by freeze-drying from water (FD-water) and tBuOH (FD-tBuOH)(A). The BJH-desorption pore distribution curve shows average pore diameter against the incremental surface area (B) and clearly reflects the differences in surface area.

Figure 7A shows the SAXS scattering curve of the two cryogels in comparison to the neverdried gel. The slope of these curves at high q values gives information about the surface smoothness, and at intermediate q values about the fractal dimension. The pore diameter was calculated from the radius of gyration with Dp = 2Rp25 and the diameter of a rod-like object, in our case a fibril or an elongated pore, was calculated with Df2 = 4Rg2 following Jeong et al.60 We assumed, based on the presented results that the sample freeze-dried from water features a bulk structure with an elongated (rod-like) pore-structure, which is discussed in more detail below. As shown in Figure 7A, the low-porous FD-water features a nearly constant decrease in scattering intensity from low q to high q values, following a power law of q-4 indicating pores with a smooth surface. Only at low q the slope was deviating. The radius of gyration of this structure was calculated by an extrapolation of this region to smaller q according Tischer et al.61 (Figure S9C). This radius corresponds to the elongated pore structure and gives a cross-sectional diameter of 44 nm (Table 2). The never-dried sample and the FD-tBuOH sample feature similar SAXS curves, pointing to a well comparable nanostructure. At high q values, the scattering intensity follows a power law of q-4, indicating the presence of surface fractals with a dense and homogeneous inner structure in the nanoscale. The change in slope in the intermediate q-range of the curves in Figure 7A allows the evaluation of a nanopore radius according to the Guinier approximation.25 The Porod exponent in the low q-region (-2.6 for the never-dried sample and -2.9 for the FD-tBuOH sample) indicate a mass fractal structure, an interlinked network, more crosslinked for the FD-tBuOH sample. This is in good agreement with the mass fractal dimension of 2.6 obtained from the exponential relationship of the concentration and the storage modulus, which was discussed above in the rheology part. The small additional decrease at low q indicates larger sized structures in a second, low-q Guinier region which gives an estimation of the cross-sectional fibril diameter.25,61 In this region we assumed a rod-like structure corresponding to the fibril, whereas a spherical


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pore structure was assumed in the substructure in the intermediate q Guinier region. All calculated values are listed in Table 2. For the cryogel (FD-tBuOH), a nanopore diameter of 9 nm was obtained which corresponds to the BJT pore distribution curve in Figure 6B. The average cross-sectional fibril diameter of FD-tBuOH was 35 nm (Figure S9B and Table 2). Reported values of the elementary fibril diameter of regenerated fibers are in the range of 8 nm,62,63 calculated from the average crystallite width from WAXS measurements. In the case of cellulose II, the average crystallite width equals D110.62 The literature value of 8 nm is higher than our crystallite width of 5.1 nm (Table 1). The average length of the crystallite width of TENCEL® gel was determined from D00464 to be 4.3 nm. The fibril diameter of 35 nm is in the range of the measured diameter of the SEM micrograph of 40-60 nm if one considers that the sample was sputtered with a 8 nm gold layer. The specific inner surface area, S/VSAXS derived from the Porod constant and the scattering invariant shows the same trend as SBET: an increasing surface area from FD-water to FD-tBuOH. In addition to that it is also possible to calculate S/VSAXS of the never-dried sample that is 33% higher than S/VSAXS of FD-tBuOH. We can conclude that freeze-drying from water changes not only the structure which is accessible by SEM, but also the underlying nanostructure of the material. If the hydrogel is frozen the formed ice-crystals and the resulting pressure causes a collapse of the fibrillar structure. Thereby a material is formed which does not resemble a fibrillar porous structure anymore, but rather a bulk material with a smooth surface and little porosity. The pores in this material are voids between the crystallites and elongated pores from the template effect of the ice-crystal. In order to preserve the fibrillar nanostructure during the freeze-drying procedure a solvent-exchange to tBuOH is mandatory. Compared to the never-dried sample, FD-tBuOH features a higher pore and fibril diameter. Keeping in mind that the surface area of the neverdried sample is higher than FD-tBuOH, the difference in pore diameter can be caused by several effects. On the one hand, the solvent-exchange procedure can influence the fractal dimension.65 On the other hand, micro- and nanopores are very sensitive towards drying.66 The increased pore and microfibril radii can be consequently explained by a collapse of micropores and slight aggregation of individual microfibrils.


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Conclusions TENCEL® gel is a novel nanostructured cellulose gel of the cellulose II allomorph which is produced with much better energy efficiency than comparable cellulose gel variants. It shows higher reactivity than CNF.59 Therefore it is an ideal platform for further chemical or physical functionalization. The aqueous gel consists of individual particles of a size less than one micron that form aggregates and shows a viscoelastic behavior. The individual particles are nanostructured with a surface consisting of fibrils of 40-60 nm width. This nanostructure in combination with the rheological behavior distinguishes the cellulose II gel from cellulose particles of similar size, such as cellulose beads and microcrystalline cellulose. Freeze drying from water results in a compact, nearly poreless, sheet-like structure. A solvent exchange to tBuOH before freeze drying yields a cryogel with a surface area of 298 m2/g with preserved fibrillar network and pore structure. This way, the gel can be used as precursor to easily obtain highly porous aerogel particles. The production of the gel is both economic and straight-forward. In comparison to conventional production of nanocelluloses it requires much less energy since only one homogenization pass is needed. In addition to that, it is obtained from a side stream of the TENCEL® fibre production and thus does not need any additional technical investments. These features render it a promising new member in the family of nanocelluloses.

Supporting Information gives additional information about the gel characterization with respect to SAXS, WAXS and GPC experiments. Furthermore, more details about the gel stability and the visual appearance of the gel are shown. The supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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

Notes TENCEL® is a trademark of Lenzing AG, Austria.


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Funding The financial support in the framework of the PhD School DokIn’Holz funded by the Austrian Federal Ministry of Science, Research and Economy as well as by Lenzing AG is gratefully acknowledged.

Acknowledgement We acknowledge Prof. Erik Reimhult and Dr. Ronald Zirbs for access to Malvern Zetasizer ZS. We thank Prof. Dietmar Pum for providing access to the SEM equipment and Prof. Wolfgang Gindl-Altmutter for providing access to optical microscopy devices. Dr. Markus Bacher and Dr. Sonja Schiehser are acknowledged for their support with NMR and GPC measurements, respectively.

Abbreviations CNF: cellulose nanofibrils, NMMO: N-methylmorpholine N-oxide, SEM: scanning electron microscopy, SAXS: small angle x-ray scattering, WAXS: wide angle x-ray scattering, BET: Brunauer, Emmett and Teller, BJH: Barrett, Joyner and Halenda, FD-tBuOH: TENCEL® cryogel obtained after solvent-exchange to tBuOH, FD-water: TENCEL® cryogel obtained without solvent-exchange to tBuOH, MW: molecular weight, Mw: weight-averaged molar mass, S/VSAXS: surface-to-volume ratio obtained from Porod region by SAXS, MCC: microcrystalline cellulose, NCC: nanocrystalline cellulose.


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For Table of Contents Use only

A nanostructured cellulose II gel consisting of spherical particles Marco Beaumont, Harald Rennhofer, Martina Opietnik, Helga C. Lichtenegger, Antje Potthast, Thomas Rosenau

Production, characterization and properties of a nanostructured cellulose II gel, obtained from the lyocell process, are discussed.


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Nanostructured Cellulose II Gel Consisting of Spherical Particles

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