Heterogeneous Acetylation of Plant Fibers into Micro- and

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Applications of Polymer, Composite, and Coating Materials

Heterogeneous Acetylation of Plant Fibers into Micro- and Nanocelluloses for the Synthesis of Highly-stretchable, Tough and Water-Resistant Co-continuous Filaments via Wet-Spinning Anurodh Tripathi, Mariko Ago, Saad A Khan, and Orlando J. Rojas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17790 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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ACS Applied Materials & Interfaces

Heterogeneous Acetylation of Plant Fibers into Micro- and Nanocelluloses for the Synthesis of Highly-stretchable, Tough and Water-Resistant Cocontinuous Filaments via Wet-Spinning Anurodh Tripathi1,2, Mariko Ago3,&, Saad A. Khan1, Orlando J. Rojas1,2,3* 1Department

of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA.

2Department 3Department

of Forest Biomaterials, NC State University, Raleigh, NC 27695-8001, USA.

of Byproducts and Biosystems, School of Chemical Engineering, Aalto University, P.O. Box 16300, FIN-00076 Aalto, Espoo, Finland.

&Current

Affiliation: Chemical and Paper Engineering, Western Michigan University, Kalamazoo, MI-49008, USA

KEYWORDS: heterogeneous acetylation; co-continuous assembly, cellulose acetate, microand nanocellulose, continuous spinning; wet-strength; stretchable filaments

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ABSTRACT Heterogeneous acetylation of wood fibers is proposed for weakening their interfibrillar hydrogen bonding, which facilitates their processing into micro- and nanocelluloses that can be further used to synthesize filaments via wet-spinning. The structural (SEM, WAXD), molecular (SEC) and chemical (FTIR, titration) properties of the system are used to propose the associated reaction mechanism. Unlike the homogeneous acetylation, this method does not alter the main morphological features of cellulose fibrils. Thus, we show for the first time, the exploitation of synergies of compositions simultaneously comprising dissolved cellulose esters and suspended cellulose micro- and nanofibrils. Such colloidal suspension forms a co-continuous assembly with a matrix that interacts strongly with the micro- and nanofibrils in the dispersed phase. This facilitates uninterrupted and defect-free wet-spinning. Upon contact with an anti-solvent (water), filaments are easily formed and display a set of properties that set them apart from those reported so far for nanocelluloses: a remarkable stretch-ability (30 % strain) and ultra-high toughness (33 MJ/m3), both surpassing the values of all reported nanocellulose-based filaments. All the while, they also exhibit competitive stiffness and strength (6 GPa and 143 MPa, respectively). Most remarkably, they retain 90% of these properties after-long term immersion in water, solving the main challenge of the lack of wet-strength that is otherwise observed for filaments synthesized from nanocelluloses.

INTRODUCTION The rise of petroleum products in the last century eased our lives to unimaginable extent, but at the expense of increased environmental impacts, which necessitate development of environment friendly and sustainable materials from renewable resources. This has become most relevant today 2 ACS Paragon Plus Environment

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given the prospects of bio-based materials that compete in performance with petroleum products. The US synthetic filament market is expected to reach $88.5 billion by 20251 and even though they provide good mechanical properties, durability and wet-stability, yet the non-biodegradability of synthetic fibers poses a grave environmental challenge.2 The use of vastly available renewable resources such as cellulose from the woody biomass may provide an alternative to synthetic textile fibers. Viscose and Lyocell are two of the successful, commercially available filaments made from cellulose. Unfortunately, their synthesis poses some sustainability challenges. Lyocell fiber requires highly pure cellulose fibers3, whereas viscose synthesis uses toxic CS2 solutions and corrosive sulfuric acid coagulation baths.4 The cellulose can also be modified to its acetate form to produce acetate fibers, but the crystallinity of cellulose is lost, leading to fibers that are weaker than those from the Viscose and Lyocell processes.5,6 Recently, there has been a strong scientific push to make filaments from nano-scaled cellulose building blocks, cellulose nanofibrils (CNF). The optimal shearing of CNF suspensions, before coagulation and during wet-spinning, can produce filaments with stiffness and toughness as high as 21 GPa and 31 MJ/m3, respectively.7,8 Hakansson et al.9, used the flow focusing method to obtain strong filaments from carboxymethylated CNF and later Mittal et al.,10 optimized the process to produce TEMPO-oxidized CNF filaments, reportedly stronger than silk and with stiffness and strength of 86 GPa and 1570 MPa, respectively. Few other studies also reported wetspun filaments from TEMPO-oxidized cellulose nanofiber (TOCN)11,12 or via interfacial polyelectrolyte complexation.13,14 Torres-Rendon et al.15, demonstrated that stiffness of wet-spun TOCN filaments can improve four-fold by strain-controlled wet-stretching. All these studies demonstrate that precise alignment while spinning of the nano-domains formed by the nanocelluloses can produce stiff and strong filaments that can compete with



commercial synthetic counterparts. However, despite the relatively low cost16,17, isolation of nanocelluloses is a material- and energy-intensive process.18 Moreover, the approaches used for filament synthesis in the studies described aforesaid are challenging for scale-up. Additionally, the filaments from nanocellulose demonstrate extremely weak wet-strength due to the high hygroscopic nature of cellulose.7 Herein, we propose a scalable approach to make filaments directly from wood fibers that does not require highly pure cellulosic fractions. The heterogeneous acetylation is performed to a level that retains the fibrillar nature of cellulose after the reaction19 and, unlike homogeneous acetylation (as in case of acetate filaments), the cellulose is not fully dissolved in the reaction media.20,21 We hypothesize that the presence of both dissolved cellulose acetate and micro- and nanofibrillar cellulose in the co-continuous spinning system can impart stiffness and toughness to the resulting wet-spun filaments. Additionally, the partial presence of hydrophobic acetyl groups is expected to impart, much sought-after, wet-strength to the filaments. Our hypothesis gains additional support from a recent study by Qui et al.22 that demonstrated~40 % increase in stiffness for other cellulosebased filament on adding 3 wt% of cellulose nanofibers. The acetylation process that is applied commercially involves extensive reaction with cellulose, manifested by its dissolution in the reaction media and resulting in the complete loss of the fibrous structure.20,23,24 This reaction in homogeneous phase is in contrast to heterogeneous acetylation, whereby a non-solvent for cellulose and cellulose acetate, such as toluene or amyl acetate, is added to the reaction medium. As a result, the partially acetylated or non-acetylated cellulose remains insoluble and therefore any gross change in fiber morphology is prevented.25 Buras et al.26 were the first to report heterogeneous acetylation of cotton. Since then, few studies have explored the effect of heterogeneous acetylation on cellulose pulp,27,28 cellulose nanofiber,29–32 bacterial


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cellulose,19,33 cellulose nanocrystals25 and cellulose pulp followed by defibrillation.34 While heterogeneous acetylation of cellulose fibers is not new, to the best of our knowledge, this is the first study to investigate the effect of heterogeneous acetylation and the formation of a cocontinuous system (dissolved cellulose acetate and partially acetylated or non-acetylated dispersed micro- and nanocellulose) on the mechanical properties of the wet-spun cellulose filaments. Moreover, our study serves two purposes: firstly, it comprehensively investigates the mechanism of heterogeneous acetylation of wood fibers and, secondly, it provides an application of the introduced co-continuous system into filaments that hold potential for commercial use.

EXPERIMENTAL Materials. The wood fibers used in this study were derived from kraft digestion of never dried birch wood followed by partial lignin removal via elemental chlorine free bleaching. The obtained fibers were grinded in an ultrafine grinder (Masuko Super Masscolloider MKZA 10-15 J, Masko Sangyo C. Ltd., Japan) equipped with SiC grinding stones (MKE 10-46). The grinding gap was 100 μm. The resulting fiber characterization has been done elsewhere.35 The resulting wood fibers with 2.7% lignin content were then available at 18.6 dry wt% and was stored in the cold room until use. The following chemicals were used: dimethyl acetamide (DMAc, 98% purity), glacial acetic acid (>99% purity), acetic anhydride reagent plus (>99% purity), anhydrous toluene prepared by molecular sieves, 98% concentrated sulfuric acid, anhydrous methanol (99.8 % purity). All chemicals were acquired from Sigma Aldrich. Deionized water was used for all purposes. Heterogeneous acetylation of wood fibers. A 10 g fiber sample (18.6% dry weight) was pressed manually between filter papers to remove as much water as possible, followed by three



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solvent exchanges with acetone to replace water. A final exchange was performed with glacial acetic acid to make hydroxyl groups more accessible. This sample was added to a mixed solvent of toluene (50 ml), acetic acid (40 ml), and sulfuric acid (0.2 ml) in a round bottom flask with flat base. To this solution, required amount of acetic anhydride was added (4 ml or 12 ml). The reaction was carried out under constant stirring at room temperature for either 1 or 6 h. The reaction was stopped by addition of water and the reaction suspension was centrifuged at 10,000 rpm for 15 min to remove residual chemicals. The acetylated cellulose fibers were washed with methanol twice followed by washing with DI water using vacuum filtration until the pH of the filtrate was above 6. The clean, partially acetylated fibers were then dried in a convection oven for 5 h at 70 0C. The fibers were stored in the airtight containers at ambient conditions until further use. The three acetylated samples are abbreviated as ‘L’, ‘M’ and ‘H’ based on the degree of substitution, i.e., Low (L), Medium (M), and High (H). The reaction conditions and sample abbreviation are listed in Table 1.

Table 1: Acetyl content and apparent degree of substitution (DS) of modified cellulose as calculated from titration measurements. The samples are abbreviated based on degree of substitution, Low (L), Medium (M), High (H). Sample L M H

Acetyl volume (ml) 4 12 12

Reaction time (h) 1 1 6

Acetyl content (%) 24.9 ± 1.1 31.0 ± 1.4 38.1 ± 0.9

Apparent degree of substitution (DS) 1.24 ± 0.07 1.68 ± 0.10 2.29 ± 0.09

Degree of Substitution (DS). The partially acetylated samples were dried in the oven at 70 °C for 4 h to remove any remaining moisture. 0.1 g of sample was weighed accurately and put into 40 ml of 75% ethanol in a 200 ml conical flask. The loosely stoppered flask was heated to 50-60


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°C for 30 min under stirring to allow better fiber swelling. Thereafter, 20 ml of 1N NaOH was added and the mixture was stirred and heated to 50-60 °C for 15 min. The flask was then sealed with parafilm and stirred for 72 h. The excess NaOH was titrated with 1N HCl using phenolphthalein as an indicator. An excess of 0.5 ml 1N HCl was added and was stirred overnight to allow NaOH to diffuse out from regenerated cellulose. The small excess HCl was titrated with 1N NaOH to a phenolphthalein end-point. The percentage of acetylation and degree of substitution were calculated as described previously by Saunders et al.31 The calculations are shown in the Supporting Information document. Chemical, structural and flow analyses. Fourier Transform Infrared Spectroscopy (FTIR). The acetylation reaction was first confirmed by FTIR measurements done in the attenuated total reflectance mode (FTIR-ATR), using a Thermo Nicolet 380 with diamond crystal. The spectrum was collected for 32 scans and corrected for background noise. The baseline correction was done for all the spectrums. Scanning Electron Microscopy (SEM). The fiber structure was investigated by imaging with Zeiss Sigma VP Scanning electron microscope with a working distance of 5 mm at 2 kV and 13 pA. A 1 wt% suspension of the fiber samples was made in required solvents (water, acetone and DMAc) and it was pipetted on to a clean silica surface. The samples were then kept in the oven at 70 °C for 20 min to evaporate the solvent. The samples were sputter-coated with a 5-nm layer of gold-palladium before imaging. Wide Angle X-ray Diffraction (WAXD). The changes in the crystal structure of cellulose during chemical modification was evaluated using Rigaku SmartLab XRD. A Cu Kα radiation source of 0.154 nm wavelength, operating at 45 kV and 200 mA was incident on the solid sample. The



spectra were recorded over the 2θ range of 10 to 45°. The samples were dried in the oven at 70 °C and compressed to a pellet and the X-ray beam was incident perpendicular to the pellet surface. Size Exclusion Chromatography (SEC). The degradation of the wood fibers was analyzed using a Dionex Ultimate 3000 system, one guard and four analytical Agilent PL-gel Mixed-A columns, coupled with Shodex RI-101 refractive index detector. A 50 mg cellulose sample was successively solvent exchanged in water, acetone and DMAc to increase the solubility in DMAc/LiCl. On the contrary, the modified samples were dried in the oven and directly dissolved in DMAc/LiCl. All the samples were dissolved in saturated DMAc/LiCl 90 g/L, diluted with pure DMAc down to sample concentration of 1 mg/ml and LiCl concentration of 9 g/L. The dissolved samples were filtered through a 0.2 µm GHP syringe membrane filter for analysis. Each sample was analyzed twice with injections of 100 µl each. The resulting distributions were calibrated using a set of eleven pullulan standards (PSS, Mainz, Germany) with molecular weights ranging from 342 Da to 2350 kDa. Dispersion stability. The dispersion stability of partially acetylated cellulose in different organic solvents was accessed by measuring the transmitted intensity of laser light as a function of height (Turbiscan, MA 200, France). For the measurement, the samples were prepared and placed in a glass tube. Rheology. Steady and dynamic oscillatory shear experiments were performed on the solutions using the TA Instruments DHR-3 rheometer. Serrated parallel plates of 40 mm diameter with a gap size of 1 mm were used for the measurements. Steady shear experiments were conducted in the shear rate regime between 0.1 and 100 s-1. For the dynamic experiments, frequency spectra of the elastic (G’) and viscous (G’’) moduli were measured at a strain amplitude of 10%, well within


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the linear viscoelastic (LVE) regime. Before all the runs, the samples were conditioned by performing steady shear at 10s-1 for 30 s and equilibrating the sample for 5 min. Filament spinning. The ‘H’ suspension was prepared in DMAc to achieve a required concentration of 3, 5 and 7 wt. %. Hereafter, this suspension is referred to as “dope”. The dope was extruded through a 22G needle (i.d. - 0.413 mm) fitted on to a 10 ml syringe via connecting tube, 10 cm long (Figure S1). The needle was completely immersed in the non-solvent, namely, deionized water. The extruded filament was manually pulled under water and placed on a rotating drum. The flow rate of dope was 0.4 ml/min and the take-up speed of rotating drum (22.1 cm diameter) was 4 m/min. Therefore, a draw ratio of 1.5 was maintained (see Video S1). The continuous spinning was performed at room temperature. The filaments were cut on the drum and dried at room temperature by holding the ends of the filaments between two magnets, as shown in Figure S1. Thereafter, they were stored in a zip-lock bag under ambient conditions until further use. The filament from 15 wt% solution of commercial cellulose acetate was also prepared using similar conditions, for comparison Filament characterization. Filament imaging. Optical microscope image was obtained between crossed polarizers with a polarizing microscope Leica DM4500 P equipped with a Leica DFC420 camera (Leica Microsystems, Germany). For filament cross-section imaging, the filament was freeze fractured and the cross-section was sputtered with gold-palladium 5nm coating. The imaging was performed with a Zeiss Sigma VP Scanning electron microscope using a working distance of 5 mm at 2 kV and 13 pA. Mechanical properties. The stress-strain curves were generated from the Instron 5944 single column tabletop tester using a 5N load cell. A 2 mm/min of extensional strain rate was applied on



a 10 mm filament length. The clamped ends of the filament were glued to a piece of paper to avoid slipping. Prior to the testing, the filament samples were equilibrated at 50% humidity and at 18 °C. The filament properties were calculated from the stress-strain curves using Origin 9.0. The Young’s modulus was defined as the average slope of the stress-strain curve in the elastic region (< 2% strain). The strain at break was taken as the strain % recorded at filament breaking point. The ultimate tensile strength was the stress recorded at the filament breaking point. The yield point was defined as the point of intersection of two first degree polynomials fitted to the linear regions, before and after the yield point. The toughness was calculated from area under the stress-strain curve. Ten measurements were performed for each sample and the average data are reported along with the standard deviation. It is noted that the mechanical properties of a filament are highly dependent on the diameter used in the calculation. Since the filaments are not perfectly circular (Figure S1c), the diameter measured using screw gauge was not reliable. Therefore, the filaments were freeze-fractured, and the cross-section was imaged under electron microscope. The apparent filament diameter was identified from the cross-sectional area calculated using Image J software. A total of ten such images were taken for each sample from different filaments and the diameter was averaged over 10 measurements. For testing the wet-strength of filament, they were immersed in water for at least one day. Thereafter, the ends of the given filament were glued to a piece of paper and a 10-mm length was put under stress-strain measurements, as explained before.


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RESULTS AND DISCUSSION The schematic illustration of the experiment is illustrated in Figure 1. We start with the heterogeneous acetylation of wood fibers and attempt to answer fundamental questions related to the reaction: What was the extent of the acetylation reaction? Did it take place only on the fiber’s surface or in the bulk of the fiber? How was the fiber morphology and crystal structure affected after chemical modification? Most importantly, what is the effect of chemical modification on the cellulose chain length? Based on the answers to these questions we then propose a reaction procedure for heterogenous acetylation. Moreover, we investigate the dispersion behavior of the modified cellulose, in a co-continuous system that leads us to the second part of this study, e.g., the potential application of the modified cellulose to synthesize filaments.

Figure 1: Schematic illustration of the experimental procedure: unmodified cellulose fibers (zoomed box demonstrating cellulose fibrils) (a) were subjected to heterogeneous acetylation to synthesize partially acetylated cellulose fibers (b). The dispersion of this material formed a co-continuous system (c) that were wet-spun to produce co-continuous filaments (d).



Acetylation of wood fibers. Chemical and structural characterization. Three samples of modified cellulose were prepared by varying the concentration of the active reagent (acetic anhydride) and the reaction time, as listed in Table 1 along with the acetyl content and degree of substitution (DS). The FTIR-ATR spectra of chemically modified cellulose fibers along with unmodified cellulose (UC) and commercial cellulose acetate (CA) is displayed in Figure 2a. The characteristic peaks of grafted acetyl groups are shown, namely, carbonyl stretch at 1730 cm-1 (C=O), methyl in-plane bending at 1365 cm-1 and the C-O stretch at 1210 cm-1. Similar peaks are observed in the commercial CA samples.28 The presence of acetic anhydride is excluded due to absence of any peak between 1760 and 1840 cm-1, implying that the reacted fibers were free of any residual acetic acid or acetic anhydride.30 While a quantitative estimation of DS from FTIR spectra is not possible, due to varying thickness of the measured samples, the overall change in the absorption intensity matches well with the DS obtained through titration (Table 1). We note an increase in acetyl content and the corresponding DS if either the acetic anhydride concentration is increased from 4 ml (L) to 12 ml (M) or if the reaction time is increased from 1 h (M) to 6 h (H). The kinetics of the reaction is not investigated in this study, and the reader is referred to Kim et al.,19, Ifuku et al.33, and Tingaut et al.36 for details on the role of changing acetic anhydride concentration and reaction time on reaction kinetics. Previously, Sprague et al.24 proposed that heterogeneous acetylation of cellulose I produces cellulose acetate I crystal structure. The claim was later verified by Sikorski et al.37. It was reasoned that the heterogenous reaction proceeds without dissolving the crystalline substrate and therefore the chain polarity is preserved. We acquired wide-angle XRD spectra (Figure 1b) to study this further. Unmodified Cellulose (UC) exhibited the diffraction pattern typical of cellulose


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I. While the characteristic diffraction peaks at 2θ = 22.3 and 34.4° from 002 and 004 lattice structures respectively were very prominent, the peaks at 2θ = 14.8 and 16.6° from (101) and (10-1) lattices were not resolved completely. The peaks of cellulose I were observed for all samples. However, the peaks became broader and the intensity decreased, indicating that the acetic anhydride penetrated inside the fiber with either an increased reactant concentration or reaction time. Thus, it can be concluded that acetyl groups were grafted to the inner crystallite regions.28 A small peak observed at 2θ = 26° is noteworthy. This peak was more prominent in H and indicates the formation of cellulose triacetate along with the humped peak seen at 2θ = 17.2 and 22.1°, also observed previously for bacterial cellulose.19,33

Figure 2: FTIR-ATR spectra (a) and wide-angle X-ray diffraction (WAXD) spectra (b) of modified cellulose pulp along with unmodified cellulose (UC) and commercial cellulose acetate (CA).

Morphological changes upon acetylation. Figure 3 shows the fiber morphology of the wood fibers before and after modification. Several features can be noticed from the images. First, the cellulose fiber width of approximately 10-20 µm does not change upon chemical modification, as also seen from high magnification image of the plant fiber surface in Figure S2. This confirms 13 ACS Paragon Plus Environment


heterogeneous reaction conditions. A homogeneous reaction would have produced aggregated structures, as seen previously for acetylation of bacterial cellulose21 or rice straws.38 Secondly, some of the modified fibers may have fused together at certain points, as indicated by the arrows. The density of fusion points is higher in sample H compared to L. Note that such effects may be produced under the conditions used for sample preparation; nevertheless, they reveal the susceptibility of the modified fibers to interact with each other and with the support. Some fibers flatten out in form of ribbons and some are aggregated. Similar morphological features were observed by Buras et al.26 for acetylated cotton fibers. Lastly, a few fibrils, less than 1 µm in length and few nm in width, are also observed for the modified cellulose, highlighted with circles in Figure 3 e, h, k. The corresponding zoomed-in sections are shown in Figure 3 f, i, l. This suggests some degree of fiber defibrillation during the heterogeneous acetylation process.


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Figure 3: SEM images of the Unmodified Cellulose (UC) (a, b, c) and the modified cellulose: L (d, e, f), M (g, h, i) and, H (j, k, l). All the samples were dispersed in water. The circles show isolated cellulose micro- and nanocellulose during chemical modification and the zoomed-in area is shown in the last column (f, i, l). The arrows indicate fusion points of cellulose fibers.



The processing of acetylated cellulose for any potential application needs to consider the effects of reaction on the cellulose’s chain length. To that end, the molecular weight distribution is shown in Figure 4a after SEC experiments. The calculated polydispersity index (PDI) and the weightaverage degree of polymerization (DPw) is shown in Figure 4b. The distribution of unmodified cellulose shows two peaks. The distribution in the lower molecular mass range corresponds to the remaining hemicellulose and lignin in the fibers, while the high molecular mass distribution corresponds to cellulose.39 The intensity of the peak at low molecular mass is reduced as the DS increases, as most of the hemicellulose and lignin is washed away during the reaction.40 In addition, the molecular weight distribution of cellulose shifts towards lower molar mass as the acetyl content increases. This can also be seen from the DPw in Figure 4b. However, a slight increase in the DPw of sample L is noteworthy. The introduction of acetate groups to replace some of the hydroxyl groups causes an increase molecular weight of sample L. As the reaction proceeds, the cellulose chains are cleaved as seen from a sudden decrease in DPw of H. However, DPw of sample H is still two times higher when compared to that of DPw of commercial cellulose acetate. While observing the PDI, we realize that it reduces monotonically, even though there is a minor change in DPw from unmodified cellulose to sample L and M. This can be due to lower molecular weight chains of cellulose being washed away during purification step.


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Figure 4: The molecular weight distribution (a), the polydispersity index (PDI) and the degree of polymerization of the UC, modified cellulose and CA as obtained from SEC (b). The data points in the PDI bar plot are connected via straight lines as a guide to the eyes.

Proposed reaction mechanism. Based on the data obtained from the acetylated fibers, the following reaction mechanism is proposed for heterogeneous acetylation (Figure 5): The unmodified cellulose fibers, shown at t=0, include cellulose fibrils, as shown in the inset. When the fibers are placed in the solvent mixture, comprising acetic acid and toluene (shaded grey), the acetic acid swells the fibers, as shown at t=t1. The acetic anhydride (pink dots) also start to penetrate inside the fiber. Close to the end of the reaction, at t=t3, two phenomena are expected to occur simultaneously: First, the fibrils and the cellulose chains on the cellulose fiber surface undergoes acetylation (pink) and comes off the surface, exposing the inner fibrils. This is termed as “defibrillation” as also seen from Figure 3. However, due to presence of toluene, the fibrils cannot dissolve in the solvent mixture. Therefore, they remain close to the “parent” cellulose fibers, some of them protruding loosely from their surface. Second, the acetic anhydride molecules, all the while, diffuse further into the fiber’s bulk and start to acetylate the core of the fiber, as evident from the partial loss of Cellulose I crystal structure of sample H. The extent of acetylation 17 ACS Paragon Plus Environment


and defibrillation in the fiber core depends on the concentration of acetic anhydride and the reaction time. The reaction temperature may also have an effect and will increase the reaction and diffusion rate (not studied here). The illustration is shown for sample H that undergoes the largest extent of reaction. A related diffusion model was proposed earlier for Kenaf fibers.34 However, the authors did not consider fiber defibrillation or cellulose chain cleavage during acetylation, most relevant for the purpose of the use, as in our case, for filament spinning. Upon washing the fibers (t=t4), the loosely protruding acetylated cellulose fibrils aggregate back on to the fiber surface, thereby fusing two neighboring fibers during sample preparation. It is during the washing step that some acetylated fibers aggregate into ribbons and flocs. These features are highlighted in the SEM images (Figure 3). If the reaction happens for long enough, the cellulose chains are cleaved as seen from the decrease in DPw corresponding to sample H.

Figure 5: Schematic illustration of the proposed mechanism of heterogeneous acetylation with the elapsed time t. The cellulose fibers and fibrils are shown in brown. The solvent mixture of acetic acid and toluene is shown in grey (background). The acetic anhydride molecules and acetylated cellulose fibrils are shown as pink circles or fibril segments.


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Dispersion of modified cellulose. The commercial application of any novel material depends on the ease for processing, with the solution-based option being the most widely used. This motivated us to evaluate the dispersion of modified cellulose in various solvents. Figure 6a shows the images of 1 wt% dispersion of sample L, M, and H in water, acetone, and DMAc. As a comparison, a 1 wt% dispersion of unmodified cellulose in water is also shown. The images were taken just after mixing, and after stabilizing the dispersion for 72 h. In water, the fibers aggregate and sediment after 72 h, partly due to their large size (~10 μm), but also due to incorporation of hydrophobic acetate groups as can be seen in sample H in water, that starts to separate soon after mixing (t=0). The effect of modification on cellulose can be seen clearly in acetone, where the dispersion is more stable even after 72 h compared to that in water. The most stable dispersion is that in DMAc, where the sample H is substantially dissolved. This visual observation is also supported by transmittance data of the dispersion along the length (Figure 6b), which was taken after stabilization of the dispersion for 72 h. The highest transmittance in DMAc is observed for sample H followed by M and L. Also note that the transmittance is similar throughout the length of the tube, indicating that there is no sedimentation even for the sample L in DMAc. The unique behavior of modified cellulose in DMAc, as compared to acetone, can be explained by the enhanced hydrogen bonding acceptor quality of DMAc due to presence of two lone pair heteroatoms in the form of nitrogen and oxygen, compared to acetone that has only one oxygen.41



Figure 6: a) Photo images of 1 wt% dispersion of modified cellulose in water, acetone and DMAc, taken after mixing (t-0) and after 72 h stabilization (t-72 h). The 1 wt% dispersion of unmodified cellulose in water is shown for comparison. (b) Light transmittance data for the modified celluloses suspended in DMAc through the length of the glass tube. The ‘Top’ refers to the air/liquid interface in the tube, while ‘Bottom’ is the other end. The data is normalized to 1.

The dispersions in acetone and DMAc were further evaluated via electron microscopy (Figure 7). The observed, non-uniform fiber defibrillation is common for all the samples. When compared to acetone, the defibrillation is more pronounced in DMAc, as expected from dispersion behavior. A bulk deconstruction of the fiber structure is observed when the sample H of the modified cellulose is dispersed in acetone and DMAc, further indicating that acetylation proceeded inside the bulk of cellulose fiber. The reminiscent fibril structure is shown in the figure inside the circles. To further confirm the presence of fibrillar structures of sample H in DMAc, we measured the solution viscosity (Figure S3), and dynamic moduli (Figure S4) of this sample at different concentrations. We find the sample H to have a shear thinning behavior even at low shear rates unlike a commercial CA sample which shows a typical Newtonian plateau followed by shear thinning. Such absence of a Newtonian regime is characteristic of aggregation caused by the association of fibrils in this case.42 In addition, the frequency spectra of G’ and G’’ of H in DMAc features association (Figure S4). While we do observe a frequency dependence, we find the slopes 20 ACS Paragon Plus Environment

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of G’ and G” not to be 2 and 1 respectively (Table S1), which is usually characteristic of polymer solutions, but much higher indicative of interaction between fibrils.42

Figure 7: SEM images of the 1 wt% dispersion of modified cellulose, L (a1, 2), M (b1, 2), and H (c1, 2) in acetone (top row) and DMAc (bottom row).

To summarize this section, a comprehensive chemical, structural and molecular evaluation of the modified cellulose fibers allowed us to propose a heterogeneous acetylation mechanism. In addition, the dispersion behavior of the three samples in acetone and DMAc suggests dispersion of sample H in DMAc as ideal for wet spinning, because of the expected ease of processability. The inherent presence of fibrillar cellulose in the solution is expected to act as interface for crystallization of dissolved cellulose acetate, as observed previously for carbon nanotubes43 and cellulose nanocrystals,44 which may enhance the stiffness and toughness of the filaments. Continuous filaments from acetylated cellulose. The sample H in DMAc was used to synthesize filaments by wet-spinning. The set-up used for wet-spinning is demonstrated in Figure 21 ACS Paragon Plus Environment


S1c. The filaments were winded continuously on a rotating drum at 4 m/min (see Video S1). The cross-section of the dried filament, shown in Figure S1b, indicates that they were not perfectly circular but displayed an oval shape. The different concentrations of sample H used during the spinning and the resultant mechanical properties of the filaments from the stress-strain curves are compared on a star plot shown in Figure 8a. The raw data is tabulated in Table S2 along with the comparison of other cellulose-based filament properties from the literature and for commercial fibers. From the star plot, we observe the following: First, the filament obtained at 3 % concentration did not have a uniform thickness and presented large number of defective points. The dope at the higher concentration, 7 %, was highly viscous (Figure S3a) and caused non-uniform drying, leading to defects. For all sample H considered, the one obtained from the 5 % dope exhibited the highest stiffness (Young’s modulus of 3.6 GPa). The possible defects generated from the dopes at 3 and 7 % concentration resulted in lower strain at break and hence lower toughness. Second, the filament obtained from the 5 % dope displayed a simultaneous increased in Young’s modulus, strength and strain at break, which is unique for a filament. Usually, there is a trade-off effect for such properties.8 The mechanical properties of the filaments from the acetylated cellulose used in this study are still below from those obtained from commercial cellulose-based filaments, such as Viscose and Lyocell, obtained by different methods, with drawing and using optimized conditions (Table S2).45 Therefore, for a fair comparison, filaments were wet-spun from commercial cellulose acetate (CA) using the same conditions as those used for the 5 % dope. As seen from the star plot in Figure 8a, the commercial CA did not produce filaments with properties that were even close to that of the 5 % filament from sample H. It is noteworthy that a 15 wt% of CA was required to successfully spin


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the filaments (e.g., the viscosity of 5 % CA was too low to wet-spin). Such viscosity of the 15 % CA was, in fact, of similar order to that of the sample H dope prepared at 5 % solids (Figure S3b). This is not only because of the high DPw of sample H, when compared to that of CA (Figure 4), but also due to the additional attractive interactions present in the sample H via its partial fibrillar structure. In addition, the zero-shear viscosity of concentrated polymeric solutions exhibit a power law behavior with the polymer molecular weight (MW). The MW exponent is 3.4 if the MW is above critical entanglement. The increase in MW from commercial CA to sample H is 1.6 times, however the zero-shear viscosity increases 217 times, which gives the MW exponent of 11.7, much higher than 3.4 (see supporting info for calculations). Such a high exponent is reminiscent of associative systems42 and further supports our findings that the sample H has additional attractive interactions arising by the presence of fibrillar structures. Wet strength. An important aspect of the cellulose-based filaments is their wet-strength. When subjected to moisture, typical nanocellulose filaments only retain 0.3 to 1.8% of their ultimate dry tensile strength.46 Even the hydrophobization of the filaments using chlorosilanes is effective only to a limited extent.7,11,47 On the contrary, the filaments from 5 % sample H exhibited a remarkable wet-strength, even after immersion in deionized water for one day. As seen in Figure 8b, after such extended period of water immersion, the filaments retained 90% of their stiffness (Young’s modulus) and even showed a slight improvement in strain at break and ultimate tensile strength. These observations are in stark contrast to other nanocellulose-based filaments. In comparison, commercial cellulose-based filaments, Viscose and Lyocell lose approximately 50 and 20% of their tensile strength in wet state, respectively.48



Figure 8: a) A star plot showing the mechanical properties of the filaments as calculated from the stressstrain curves of a 5% H sample. The corresponding data is tabulated in Table S2. b) Filament mechanical properties (Young’s modulus, strain at break, and ultimate tensile strength) before (T0) and after immersion in water for one day (T1d).

Synergies in attaining stiff and stretchable filament. Previous studies have demonstrated that upon filament stretching (and thus increased alignment of fibrils), it is possible to achieve simultaneous gains in filament stiffness (Young’s modulus) and strength (ultimate tensile strength).15,44 Additionally, solvent annealing of acetate filaments has been shown to increase orientation and hence the stiffness and strength of filaments.5 Inspired from these studies, the 5 % sample H filaments were swollen in a mixed solvent of 50 % DMAc and 50 % water. This allowed free movement of partially solubilized fibrils and modified cellulose chains, as illustrated in Figure S5a. Thereafter, the swollen filaments were stretched by 10 % at 1 mm/min and allowed to dry in the stretched state. The optical image of the filaments under a cross-polarizer (Figure S5b, c) clearly shows an increased birefringence, indicating an increased alignment in the stretched filaments. The effect of increased alignment was further reflected in the stress-strain curve (Figure 9a), where the stiffness nearly doubled in the solvent-stretched filament, without a significant drop 24 ACS Paragon Plus Environment

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in the strain at break, resulting in approximately 27 % increase in toughness, from 26 to 33 MJ/m3. The fact that strain at break does not drop significantly, suggests an important role of micro- and nanocellulose as toughening agents. While comparing the mechanical properties and, particularly, the toughness and stretchability of the filaments from this study with filaments obtained from micro- and nanocellulose, as well as data for commercial cellulose and its derivatives (Figure 9b), it is realized that the toughness and strain of the filaments from 5 % sample H clearly stand out. Notably, the added advantages of our approach using wood fibers as raw material, as compared to nanocellulose, is the lower energy required. Moreover, what is also distinctive is the remarkable resistance to water, which has remained, until now, as a major challenge in nanocellulose-based filaments.18

Figure 9: a) Stress-strain curve of the filaments before (5 % Sample H) and after filament solvent stretching (5 % Sample H-SS), b) a plot of filament toughness vs stiffness. The data point on the plot correspond to filaments from nanocellulose (1. Mohammadi et al.8, 2. Hooshmand et al.49, 3. Lundahl et al.7, 4. Toivonen et al.13, 5. Walthers et al.11, 6. Hakansson et al.9, 7. Torres-Rendon et al.15, 8. Geng et al.50) , cellulose (Lyocell and Viscose.45), and cellulose acetate (9. Yuan et al.6) reported in literature (Table S2).



CONCLUSIONS A novel material is synthesized through heterogeneous acetylation of cellulosic fibers that holds potential for scalable cellulose-based continuous filaments. A comprehensive chemical, structural, and molecular analysis during acetylation allows us to propose a mechanism for the heterogeneous acetylation. Unlike homogeneous acetylation, the gross fiber morphology of native wood fiber is retained after heterogeneous acetylation. However, fiber defibrillation is evident during its suspension in organic solvents (acetone and DMAc) and the extent of defibrillation depends on the degree of acetylation. The highest acetylated sample H exhibits largest defibrillation and dissolution in DMAc. Such dispersion forms a co-continuous system that is proposed to ease processing into filaments via wet-spun. The presence of undissolved fibrils along with dissolved cellulose acetate in the co-continuous dope imparts high toughness (33 MJ/m3) and stiffness (6 GPa) to the filament, two properties that are most often mutually exclusive. The filaments are super-stretchable (30 % strain) and are at the higher end of the performance spectrum (toughness versus strain at break) when compared with contemporary cellulose, cellulose acetate and nanocellulose-based filaments. Through this study we have demonstrated that the partially acetylated cellulose may be a better material than completely acetylated, commercial cellulose acetate or cellulose nanofiber to produce filaments, not only because of presence of both dissolved cellulose acetate and cellulose fibrils but also, due to avoiding additional energy intensive processing step required to make cellulose nanofibers.


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ASSOCIATED CONTENT Supporting Information: Calculations for degree of substitution, calculation for relation between zero shear viscosity and molecular weight, wet-spinning set-up along with filament drying and SEM image, steady shear viscosity and frequency sweeps with corresponding slopes of G’ and G”, cross polarizer optical image of filaments and the table comparing the mechanical properties of filaments in this study with the other in literature and commercial filaments.

AUTHOR INFORMATION Corresponding Authors Email: [email protected] (O.J.R.), Phone: +358 50 512 4227; [email protected] (S.A.K), Phone: +1-919-515-4519 Author Contributions The manuscript was written through contribution of all the authors and all authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS We acknowledge funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 788489). We are also grateful to the Academy of Finland through Centers of Excellence Program (2014-2019) under Project 264677 “Molecular Engineering of Biosynthetic Hybrid Materials Research” (HYBER). The assistance of Meri Lundahl, Ling Wang and Ville Klar in the operation of the fiber spinning



setup is highly appreciated. This work made use of facilities of Aalto University’s Nano microscopy Center.

ABBREVIATIONS DS: Degree of substitution; CA: Cellulose acetate; PDI: Polydispersity Index; DPw: Degree of Polymerization; UC: Unmodified Cellulose

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Heterogeneous Acetylation of Plant Fibers into Micro- and

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