Preparation and Characterization of Lignin-Containing Cellulose

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Preparation and characterization of lignin-containing cellulose nanofibril from poplar high-yield pulp via TEMPO-mediated oxidation and homogenization Yangbing Wen, Xiongli Liu, Zhaoyang Yuan, Jialei Qu, Shuo Yang, An Wang, Chunping Wang, Bing Wei, JianFeng Xu, and Yonghao Ni ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06355 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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Preparation and characterization of lignin-containing cellulose nanofibril from poplar high-yield pulp via TEMPO-mediated oxidation and homogenization Yangbing Wen†,‡,⁋,*, Zhaoyang Yuan┴,⁋,*, Xiongli Liu†, Jialei Qu†, Shuo Yang†, An Wang†, Chunping Wang†, Bing Wei∆, Jianfeng Xu⁋, Yonghao Ni†,§ †Tianjin

Key Laboratory of Pulp and Paper, Tianjin University of Science &

Technology, 29th Avenue 13th, Tianjin Economic and Technological Development Area, Tianjin 300457, China ‡Tianjin

Woodelf Biotechnology Co. Ltd., 29th Avenue 13th, Tianjin Economic and

Technological Development Area, Tianjin 300457, China ┴Department

of Biochemistry & Molecular Biology, Michigan State University, 603

Wilson Road, East Lansing, Michigan 48824, United States ∆State

Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest

Petroleum University, Chengdu, Sichuan 610500, China ⁋Research

Institute of Wood Industry, Chinese Academy of Forestry, Haidian

District, Beijing 100091, China §Department of Chemical Engineering, and Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, NB E3B 5A3, Canada ⁋These authors contributed equally to this work. *Corresponding authors: [email protected] (Y. Wen) [email protected] (Z. Yuan)

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Abstract In this study, a poplar high-yield pulp [pre-conditioning refiner alkaline peroxide mechanical pulp (P-RC APMP)] was used to produce lignin-containing cellulose nanofibril (LCCNF) dispersions through a sequential process of 2,2,6,6-tetramethyl piperidine-1-oxyl radical (TEMPO)-mediated oxidation followed by high pressure homogenization. To produce LCCNF with different lignin contents, sodium hypochlorite loadings of 4-12 mmol/g fibre during TEMPO-mediated oxidation step were explored. The effect of lignin content on morphology, thermal stability, crystallinity, and rheological properties of the produced LCCNFs was investigated. The results showed that the TEMPO-mediated oxidation of cellulose was largely limited to the fiber surface. The residual lignin on the surface of LCCNF was presented as small particles. The increase of lignin content increased the thermal stability and decreased the viscosity of the LCCNF. Moreover, at higher lignin content, greater flocculation and aggregation of fibrils took place, which resulted in lower gel-like characteristics of the resultant LCCNF. The results of water contact angle determination also demonstrated that the increase of lignin content significantly increased the hydrophobicity of the LCCNF. Keywords: High yield pulp; Cellulose nanofibril; Rheology; Nanocellulosic fibril morphology; Thermal stability; Hydrophobicity Introduction The bio-based economy concept, in which renewable biomass is used as the raw material for the production of various consumer products, has received much attention. Cellulose nanofibril (CNF), with a width of 5-20 nm and length of a few hundred nanometers to micrometers, is at the forefront of this bio-based economy1. CNFs can be obtained from plant or animal sources through various chemical, mechanical or enzymatic pathways and their combinations, which dictate the physico-chemical properties of these nanomaterials, and they have some unique attributes, such as excellent mechanical strength, light weight, and biocompatibility.2,3 CNFs have been investigated for application in various areas such as oil recovery, waste-water treatment, and production of thickeners and absorbent materials4-6 In addition, the utilization of CNF as building blocks to design new biomaterials has also been extensively investigated.7,8 For the production of CNF, dissolving pulp and bleached kraft pulp, which are largely lignin-free or contain low amounts of hemicellulose and lignin, are the

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primary starting materials,8-10 because lignin is normally considered to be an obstacle in the cellulose fibrillation process.11 However, lignin can provide some good properties such as thermal stability, hydrophobicity, anti-oxidation properties, and UV stabilization.12-15 For example, Nair and Yan12 reported that lignin-containing cellulose nanopaper had a lower water uptake than that of the high-cellulose content CNF counterparts. Nair et al16 reported that the polylactic acid biocomposites reinforced with high lignin content nanocellulose fibrils improved the mechanical, barrier, and thermal properties. Additionally, Herrera et al14 further demonstrated that the high-lignin content (23%) cellulose nanofibrils from eucalyptus semi-chemical pulp had increased hydrophobicity, lower oxygen permeability and comparable strength properties compared to CNF produced from fully bleached kraft pulp. Chinga-Carrasco et al17 reported that microfibrillated cellulose films produced from unbleached Pinus radiata pulp fibres had high barrier against oxygen, low water wettability and high tensile strength. Also, Brodin and Eriksen18 investigated the production of microfibrillated cellulose to enhance paper strength properties. In addition, the overall cost for the CNF production from fully bleached pulp is generally higher than that of lignin-containing pulp, due to the lower pulp yield (removal of lignin) and the high chemical costs during the pulping process. In this context, pulp with high lignin content, can be an interesting raw material for the production of CNF, with the objective of imparting the resultant CNF with unique properties and exploring their new applications, as well as decreasing the overall CNF cost. High-yield pulp (HYP) is produced from lignocellulosic feedstocks through mechanical or combined chemical and mechanical pulping processes. Different from chemical pulps, such as kraft pulp, HYP has a much higher yield (higher than 85%) and a much higher lignin content.19 Moreover, compared to the chemical pulp production process, the requirement of chemicals during the HYP production process is significantly decreased. Particularly in China, HYP, such as pre-conditioning alkaline peroxide mechanical pulp (P-RC APMP), has gained popularity and significant production capacity has also been established.20 In addition, as a key material for meeting the global fiber demand and increasing the sustainability of the paper industry, HYP is widely used for the production of different grades of paper products due to its unique properties, such as good opacity, high bulk and stiffness.21 To further increase the competitiveness of HYP in the renewable resource industry, expanding the application of HYP is considered to be a promising option. In this context, developing HYP as the raw material for the production of CNF can be an

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attractive strategy, which offers advantages, such as 1) increasing the CNF yield, 2) decreasing the environmental impact by decreasing the chemical requirements in the process, 3) imparting the resultant CNF with unique characteristics, e.g., hydrophobicity due to the presence of lignin.22 Herein, lignin-containing nanofibrillated cellulose (LCCNF) was prepared from a poplar P-RC APMP through a 2-step process consisting of 1) TEMPO-mediated oxidation and 2) high-pressure homogenization. The chemical, morphological, thermal, and rheological properties of the obtained LCCNF were further investigated using field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and rheological analysis. Our results have demonstrated that the resultant LCCNF has some unique properties, such as increased hydrophobicity and thermal stability and reduced viscosity. Materials and methods Materials A poplar P-RC APMP sample, used as the raw material for the preparation of LCCNF, was provided by Henan Ruifeng Paper Ltd. China. After washing with deionized water at a consistency of 3.5% (w/v), the pulps were centrifuged to a moisture content of around 80% and stored at 4 °C using airtight bags prior to subsequent experiments and analysis. The compositional analysis of the poplar P-RC APMP shows that it contains 26.7% lignin, 16.3% hemicellulose, and 52.0% cellulose (w/w). The lignin free CNF hydrogel sample with 2% (w/w) cellulose content, produced from lignin-free bleached poplar kraft pulp, was provided by Tianjin Haojia Cellulose Co., Ltd (Tianjin, China). Laboratory grade 2,2,6,6-tetramethylepiperidin-1-oxyl (TEMPO), sodium bromide (NaBr), and 13% sodium hypochlorite (NaClO) solution were purchased from Sigma Aldrich (USA). All experiments in this study were performed at least in triplicate. Preparation of LCCNF from P-RC APMP via a 2-step process TEMPO-mediated oxidation TEMPO-mediated oxidation of the pulp was conducted following Saito et al.23 Briefly, 100 g (o.d.) of the poplar P-RC APMP was dispersed in 9000 mL deionized water. TEMPO (0.1 mmol/g fibre) and sodium bromide (1 mmol/g fibre) were dissolved in deionized water and mixed with the fiber suspension at 1% pulp consistency. Then, 0.5 M NaOH solution was added into the slurry to adjust the pH to 10. After mixing for 5 min at 500 rpm, the TEMPO-mediated oxidation process was

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started by adding the NaClO solution (stock concentration of 12% w/w) drop wise to the slurry. Loadings of NaClO ranging from 4 to 12 mmol/g fiber were explored in this study. The pH of the reaction system was maintained at pH 10.5 by the addition of 0.5 M NaOH during the reaction. After adding NaClO, the reaction proceeded at pH 10.5 (maintained by adding 0.5 M NaOH), 500 rpm, and 23 ℃ until no NaOH consumption was observed. The TEMPO-oxidized pulp was thoroughly washed with deionized water (approximately 8 L) to remove the residual chemicals through filtration and stored at 4 ℃ until used for further experimentation. High-pressure homogenization Subsequently, a high-pressure homogenizer (APV-1000, Beijing Udare Technology Co. Ltd., China) was used to treat the TEMPO-oxidized pulp. For each sample, 20 g of the TEMPO oxidized pulp were diluted to a consistency of 1% with deionized water. Then, the fiber suspension was homogenized by 6 passes at 80 MPa to produce the CNF hydrogel. The homogenization process was conducted at 65 ℃ and neutral pH. The obtained translucent gel was stored at 4 ℃ until used for further analysis and characterization. Thermogravimetric (TG) analysis A TGA Q500 (TA Instruments, USA) instrument was used to determine the thermal stability of the CNF samples at temperature ranging from 100 °C to 575 °C. The experiments were carried out under a nitrogen atmosphere. For each measurement, CNF samples with mass of 10 ± 0.1 mg were heated in an open platinum pan at a heating rate of 10 ℃/min. The nitrogen flow rate of 100 mL/min was used. Field emission scanning electron microscope (FE-SEM) Before subjecting the obtained LCCNF samples for SEM analysis, the gels was freeze-dried first. Briefly, 10 g LCCNF gel was frozen at -80 oC for 2 h with a freezer (Revco Elit PLUS, Thermo Fisher Scientific, USA). Then, the ice crystal was sublimated by vacuum free-drying at -55 oC for 48 h using a Christ Alpha 1-2 LD plus freeze-dryer (Germany). Subsequently, the prepared freeze-dried sample was glued onto aluminum stubs and coated with gold film to observe the surface morphology using a Hitachi s-4300 FE-SEM. An acceleration voltage of 2 kV and a magnification of 5000× were used. With the FE-SEM pictures, the diameter of CNF was determined with digital image analysis (ImageJ, Version 1.49). At least five SEM images were measured to analyze the diameter of LCCNF. X-ray diffraction (XRD) analysis

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XRD analysis was conducted using a Rigaku D/max 2500 PC diffractometer (Rigaku, Tokyo, Japan) equipped with a Cu Kα radiation operated at 40 kV and 30 mA. For the XRD analysis of CNF samples, 0.2 g freeze-dried CNF sample was compressed into a pellet to record XRD patterns from 8o to 35o of diffraction angle 2θ using the reflection method by means of a Shimadzu XRD-6100. The cellulose crystallinity index was measured by subjecting the extracted peak area of cellulose I to the total area scanned from 8o to 35o of 2θ.24 Crystal width across the (100) direction of cellulose I was calculated from full width at half height of the 200 diffraction peaks in the XRD profile using the Scherrer’s formula.25 Fourier transform infrared spectroscopy (FTIR) FTIR spectra of CNF samples were collected using a FTIR-650 spectrometer (Gangdong Co. Ltd. Tianjin, China) with a determining region of 4000-500 cm-1. Approximately 2% potassium bromide (based on mass of CNF sample) was used to prepare transparent pellet for FTIR analysis. Contact angle measurement Contact angle measurements were conducted with a DataPhysics PSL 250 Contact angle analyzer (Future Digital Scientific Corp., GmbH, Germany). For measuring contact angles, 20 g of LCCNF suspension were vacuum filtrated to prepare a sheet. The sheet was subsequently compressed to obtain a smooth surface. Then, 4 µL deionized water was deposited on the LCCNF pellet within 8 seconds using a microsyringe at 23 °C. The contact angle was measured at 2 min after placing the water drop on the pellet surface. Measurements were performed at least in triplicate. Analytical methods The moisture content of the pulp samples was measured by drying at 105 ± 2 ℃ to a constant weight. The content of carboxyl group in the pulp was determined using conductometric titration method following Besbes et al.26 Contents of carbohydrates and lignin in the pulp were determined following air drying for 36 h. Lignin content in this manuscript was the measured Klason lignin, if specified. The chemical compositions of the pulp were determined using NREL standard protocols.27 Briefly, aired dried pulp was ground to pass through 40-mesh screen with a Wiley Mill. Then, 0.2 g of ground pulp was digested by sulfuric acid (H2SO4) following NREL protocol.27 After hydrolysis, Klason lignin was separated by filtration through medium porosity filtering crucibles (Sigma-Aldrich, China) and weighed after drying at 105  ±  2 °C for 6 h. The polysaccharide analysis was

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conducted using a Dionex ICS 5000 + HPLC system equipped with an AS-AP autosampler (Thermo Fisher Scientific, MA, USA) using a Dionex CarboPac SA 10 analytical column, operated at 45 °C with the mobile phase of 1 mM sodium hydroxide (NaOH) mobile phase at a flow rate of 1 mL/min. Electrochemical detection and Chromeleon software (Thermo Fisher Scientific, MA, USA) were used to quantify the sugars. Fucose was used as the internal standard. The brightness of the CNF hydrogels was determined following TAPPI standard method T452 om-08. Briefly, 20 g of the hydrogel was filtered using a büchner funnel to form a sheet. Then, the sheet was air dried at 23 ℃ and 50% relative humidity for 96 h. After drying, the brightness of the sheet was measured using a brightness tester (Elrepho Spectrophometer, Lorentzen & Wettre Co., USA). The steady shear viscosities of the CNF hydrogels were measured using an Anton Paar MCR 302 rheometer (Anton-Paar, Germany) equipped with a CC27 measuring system (Measuring Bob, D = 26.6 mm, L = 40.0 mm; Cup, D = 28.9 mm) at 25 ℃. Viscometric measurements were performed using the same rheometer at 25 ℃. The shear rate ranged from 0.1 to 1000  s-1 was investigated. The elastic (Gʹ) and viscous (Gʺ) moduli (25 ℃) of these CNF hydrogels as a function of angular frequency from 0.1 to 1000 Hz were determined by small amplitude oscillatory using the same rheometer. Moreover, to distinguish the linear viscoelastic region of these samples, an oscillation strain sweep was initially carried out. For zeta potential measurements, 1 mg/mL of freeze-dried LCCNF was dispersed in the required solution with 1 mL of 50 mM citrate buffer via ultrasonication. Zeta potential of LCCNF suspensions was determined using the phase analysis light scattering model on a Zetasizer (Nano-ZS, Malvern Instruments Ltd, Worcestershire, UK). Results and Discussion TEMPO-mediated oxidation of poplar HYP and preparation of LCCNF hydrogel During the preparation of CNF through the TEMPO-mediated oxidation process, the content of carboxyl groups following oxidation is a critical parameter that greatly affects the subsequent high-pressure homogenization.26 Fig. 1 shows the effect of the amount of NaClO during TEMPO-mediated oxidation on its carboxylate content and LCCNF yield (Fig. 1a). As shown in Fig. 1a, by increasing the NaClO charge from 4 to 12 mmol/g pulp, the content of carboxylate in the oxidized pulp increased from 0.497 to 1.37 mmol/g. This is in accordance with a previous study, which reported

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that the carboxylate content in the bleached softwood kraft pulp increased from 0.04 to 0.231 mmol/g fibre by increasing the NaClO charge from 0.2 to 1.8 mmol/g pulp during TEMPO-mediated oxidation process.28 Fig. 1a also shows that with the increase of NaClO dosage, the obtained products after high-pressure homogenization gradually changed from fluid to gel form, corresponding to the increased carboxylate in the resultant CNF products. Moreover, the brightness of obtained LCCNF hydrogels was also measured. The results showed that the increase of NaClO loading during TEMPO-mediated oxidation could substantially increase the brightness of the final LCCNF product from 44.04 ISO to 63.29 ISO (referred to images of LCCNF gels shown in Fig. 1a). Fig. 1a also shows that by increasing the NaClO charge, the yield of LCCNF decreased. To gain insight into the reason for the decreasing yield and increasing brightness, the lignin and hemicellulose contents were measured (Fig. 1b). As shown in Fig. 1b, by increasing the NaClO charge during TEMPO-mediated oxidation, the lignin content in the oxidized pulp decreased (Fig. 1b). This is in agreement with a previous study on the production of CNF from a softwood thermomechanical pulp via the TEMPO-mediated oxidation strategy.29 Under the oxidation conditions, lignin is oxidized and depolymerized into water-soluble lignin-derived compounds,30 thereby resulting in an increase in LCCNF brightness and a decrease in LCCNF yield. The hemicellulose content in the oxidized pulp decreased with the increase of NaClO charge (Fig 1b), which is attributed to the degradation of hemicellulose during TEMPO-mediated oxidation under alkaline conditions (pH 10-10.5). In summary, the above results indicate that the increased degree of oxidation of cellulose facilitated the subsequent homogenization for the LCCNF preparation, however, during the same course of reactions significant amounts of lignin and hemicellulose may be removed, resulting in a loss of the LCCNF yield. Therefore, an optimal TEMPO-mediated oxidation conditions must be chosen so that a compromise is reached between the increased carboxylate content and decreased lignin and hemicellulose contents.

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Fig. 1. Effect of the NaClO loading during TEMPO-mediated oxidation on the properties of LCCNF, a) carboxylate content and LCCNF yield, b) hemicellulose and lignin content. Note: images shown in Fig. 1a are pictures of LCCNF gels. Characterization of LCCNF Fig. 2a shows the X-ray diffraction analysis of the obtained LCCNF samples with different lignin contents. As shown in Fig. 2a, all samples presented major intensity peaks at a 2θ value close to 23.50o regardless of differences in the lignin content, indicating that the oxidation reaction has negligible effect on the cellulose crystallinity; this is favorable for the preservation of CNF properties such as high mechanical strength. The crystallinity indices for the lignin free CNF and LCCNF with lignin content of 15.5%, 18.6%, and 23.1% were found to be 81.1%, 78.9%, 74.1%, and 68.2%, respectively. These results showed that the TEMPO-mediated oxidation process increased the degree of crystallinity of LCCNF. This could be due to the removal of hemicellulose and lignin, because of their amorphous nature.15,23 Moreover, the crystallinity index of lignin free CNF (81.1%) and LCCNF with 15.5% lignin content (78.9%) were similar, which further confirmed that the 9


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TEMPO-mediated oxidation process did not change the crystal size of cellulose and the crystal structure was still maintained as cellulose I.15,23,31 Fig. 2b shows the results of the TG analysis of the obtained LCCNF samples. The thermal stability of the LCCNF increased with the increase of lignin content. This could be attributed to the higher thermal stability of lignin than cellulose.32 In addition, the reactivity of lignin is very low in an oxygen deficient atmosphere at high temperatures,16, 22,32,33 which could provide protection to the LCCNF, thereby increasing the thermal stability. In addition, the comparison of the DTG of the three LCCNF samples illustrated that the LCCNF containing the highest lignin content (23.1%) had three Tmax temperatures, which were Tmax1=225 ℃, Tmax2=280 ℃, and Tmax3=495 ℃, during which the weight losses were 0.47%/℃, 0.56%/oC, and 0.29%/oC, respectively. This could be due to the high content of lignin, which decomposes in a large temperature range (250-600 ℃).34,35 In contrast, when decreasing the lignin content in LCCNF to 18.6%, only two Tmax temperatures were observed, which were Tmax1=280 ℃ and Tmax2=490 ℃, during which the weight losses were 0.54%/oC and 0.13%/oC, respectively. Further decreasing the lignin content in LCCNF to 15.5% did not lead to a significant change in the Tmax temperatures compared to that of 18.1% lignin content LCCNF. These are in accordance with the literature that during the decomposition of lignocellulosic biomass, cellulose starts to decompose at around 300 ℃ and reaches its maximum weight loss rate at around 350 ℃, hemicellulose begins at 220 ℃ and reaches a maximum mass loss rate at 270 ℃, while lignin is the most difficult constituent for decomposition, it starts at 250 ℃ and reaches a maximum loss at around 500 ℃.34,36

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Fig. 2. Characterization of the obtained LCCNF samples, (a) X-ray diffraction patterns; (b) thermal stability. To provide further insight into the produced LCCNFs, the morphology/surface structure of these samples with different lignin contents was investigated using FE-SEM imaging (Fig. 3). As shown, compared to the lignin-free CNF (Fig. 3d), significant amounts of particles/patches were observed: many were attached on the CNF surfaces, which was universal for all of the three LCCNF samples. In fact, many of these LCCNFs were like balloons threaded together, while the balloon/ thread features were absent for the lignin-free CNF (Fig. 3d). Moreover, with increasing lignin content, the patches were found to be more concentrated on the fibril surface (Figs. 3a-c).

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Fig. 3. FE-SEM images of the LCCNFs: a) 23.1% lignin content, b) 18.6% lignin content, c) 15.5% lignin content, d) lignin free. We made further effort to support the hypothesis that these particles/patches would be lignin in nature, which were formed during the process of high-pressure homogenization due to lignin re-precipitation/nano-aggregation. The above hypothesis was verified by using the energy dispersive X-ray spectroscopy (EDX) SEM technique. The EDX micro-analysis was performed on selected patches on the LCCNF surfaces (Fig 4b). As shown, the weight percentages of elements C and O in the patch were 75.40% and 21.49%, respectively, which is similar to those in lignin: the weight percentage of C and O of 75.0% and 25.0%, 76.9% and 23.1%, 73.3% and 26.7% for lignin models of paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, respectively.37-39 The elemental data in the LCCNF sample (lignin content of 23.1%, Fig 4b) were in contrast to the results of lignin-free CNF sample (Fig 4a), which show the typical compositions of C, O and H for cellulose. To further understand the changes in functional groups between LCCNF and lignin free CNF, FTIR spectra (Fig. 5) of LCCNF samples with different lignin contents were obtained. As shown in Fig. 5, the common absorption peaks at 1730 cm-1 and 1630 cm-1 of the spectra of LCCNFs were assigned to C=O stretching band and H-O-H stretching vibration, respectively; these are two function groups of 12



cellulose.40 Different from the FTIR spectrum from lignin free CNF, the LCCNF samples showed several more absorption peaks (Fig. 5). Within LCCNF samples, the peak values at 1508 cm-1, and 1600 cm-1, combined with the absorption peak at 1460 cm-1, were attributed to the characteristic vibrations of C=C aromatic skeletal and C-H deformation of the lignin, respectively.41 Moreover, with decreasing the lignin content, the intensity of these peaks in LCCNF samples was reduced (Fig. 5). Therefore, the above results support the conclusion that the patches on the surface of LCCNF fibers are indeed lignin agglomerates.

Fig. 4. EDX spectrum of the particles located on the surface of LCCNF fibre.

Fig. 5. FTIR spectra of the CNF samples with different lignin contents. Viscoelastic behavior of LCCNF suspensions

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In a number of CNF applications, e.g., oil recovery,42 the viscosity is a critical parameter.43 For the TEMPO-mediated oxidized CNF hydrogel produced from lignin-free pulp, the viscosity mainly depends on the morphology and the content of carboxyl group of the fibrils.26,44 However, for the TEMPO-mediated oxidized LCCNF hydrogels produced from HYP, the presence of lignin on the surface of the nanofibrils, and its oxidation degree are expected to affect the viscosity and rheological properties. Fig. 6 shows the viscosity of the produced LCCNF hydrogels with different lignin contents at different shear rates. As shown in Fig. 6, a strong shear thinning behavior of the viscosity was observed for all the samples (both LCCNF and lignin-free CNF). With the decrease of the lignin content, the viscosity of the LCCNF hydrogel increased. For example, at a shear rate of 10 s-1, the viscosity of LCCNF with 23.1% lignin content was 17.5 mPa∙s. With decreasing the lignin content to 15.5%, the viscosity of the LCCNF hydrogel was 754 mPa∙s at the same shear rate (10 s-1). It can be clearly observed from Fig. 1 that the decrease of lignin content corresponding to the increase of the content of carboxyl group in the TEMPO-mediated oxidized fibers; this could improve the deformation-induced aggregation of the lignin and reduce the hydrophilicity of the LCCNF, thereby increasing the gel-like characteristics and increasing the viscosity of the LCCNF.29 One the other hand, increasing the content of carboxyl group also improved the nanofibrillation process, which increased the fraction of nanofibrils and decreased the amount residual fibres, thus increasing the viscosity of final LCCNF.12,15 To further investigate the effect of lignin content on the properties of the LCCNFs, the viscoelasticity of the LCCNF hydrogels was determined and shown in Fig. 7. As shown in Figs. 7a and b, both the storage moduli (Gʹ) and loss moduli (Gʺ) were decreased by increasing the amplitude of the strain. The modulus (Gʹ and Gʺ) was higher for LCCNF suspensions with lower residual lignin content (Figs. 7a and b). This could be due to the agglomeration and flocculation effect within the fibrillary matrix due to the glue-like effect of residual lignin presented on the fibril surface and between fibrils.22 Figs. 7c and d show that the Gʹ and Gʺ increased by increasing the angular frequency. One likely reason might be the lignin in the LCCNF suspension led to the flocculation of fibrils, resulting in the increase of modulus (Gʹ and Gʺ). In addition, Figs. 7c and d also illustrate that modulus (Gʹ and Gʺ) was higher for the LCCNF hydrogel with lower lignin content. One likely reason might be that the decrease of lignin content increased the fibril surface charge, which could swell fibrils

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and decrease the flocculation of fibrils, in turn increasing the gel-like characteristics. To examine if this was indeed the case, zeta potential of the four LCCNF samples was measured. The results showed that the zeta potential of all four LCCNF samples was negative, and the zeta potential decreased with decreasing the lignin content. Specifically, zeta potentials of LCCNFs with lignin contents of 23.1%, 18.6%, and 15.5%, were -27.68, -35.40, and -40.55 mV, respectively, while zeta potential for lignin free CNF was -48.86 mV.

Fig. 6. The effect of lignin content on the viscosity of the LCCNF samples at different shear rates.

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Fig. 7. Results from viscoelastic measurements of LCCNF hydrogels at 0.5% consistency. (a) and (b): storage modulus (Gʹ) and loss modulus (Gʺ) as a function of strain (γ = 0.01-1000%); (c) and (d): storage modulus (Gʹ) and loss modulus (Gʺ) as a function of frequency (0.1-100 Hz). The hydrophobicity of LCCNF To further characterize LCCNF, water contact angle of the LCCNF samples was measured (Fig. 8). As expected, compared to lignin-free CNF, the water contact angle values on the LCCNF surface were much higher. Moreover, with the increase of lignin content, the contact angle values substantially increased. As shown in Fig. 8, by decreasing the lignin content from 23.1% to 0, the water contact angle values substantially decreased from 90.5° to 51.6° with the 2-min measurements. This is likely due to the hydrophobicity of lignin,45,46 which provided high hydrophobicity of the CNF samples by locating on the fibre surface. These results further demonstrated that LCCNF produced from HYP with hydrophobic characteristics could be a promising starting material for the application in hydrophobic systems or non-aqueous mediums.

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Fig. 8. Contact angles of LCCNF pellets at different lignin contents. Proposed process for producing LCCNF from HYP Based on our findings, HYP is an attractive starting material for the preparation of LCCNF with high hydrophobicity and thermal stability. To provide an overview and demonstrate the sustainability of this proposed scheme of producing LCCNF from poplar P-RC APMP, an overall comparison with the production of lignin free CNF from poplar bleached kraft pulp was performed (Fig. 9). As shown in Fig. 9, by subjecting the poplar P-RC APMP pulp to the sequential treatment process of TEMPO-mediated oxidation with NaClO loading of 8 mmol/g fibre and high-pressure homogenization at 80 MPa for 6 passes, LCCNF with 18.6% lignin content was produced at a yield of 85.9%. Under the same treatment conditions (TEMPO-mediated oxidation and homogenization), lignin free CNF at a yield of 94.3% was produced. Although the yield of lignin free CNF was higher than that of LCCNF, the yield of poplar bleached kraft pulp was much lower than that of P-RC APMP. For example, based on the typical pulping processes, the yield of P-RC APMP was approximately 88% (based on initial poplar),47 while the yield of bleached kraft pulp was about 56% (based on initial poplar).48 On the basis of the same amount of initial poplar chips, the generated LCCNF was about 23% higher than that of CNF. Therefore, the production of LCCNF from HYP could considerably increase the 17


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sustainability of the biomass-CNF mill.

Fig. 9. Proposed process for the production of LCCNF from poplar P-RC APMP. Conclusions Lignin-containing cellulose nanofibril (LCCNF) was successfully prepared from a poplar pre-conditioning (P-RC) alkaline peroxide mechanical pulp (APMP) highyield pulp, via a two-step process consisting of 1) TEMPO-mediated oxidation, and 2) high-pressure homogenization. During TEMPO-mediated oxidation, with the increase of NaClO dosage, the carboxylate content increased and the lignin and hemicellulose contents decreased in the resultant pulp. The resultant LCCNF has some unique properties in reference to those from bleached kraft pulp/dissolving pulp. The hydrophobic lignin as balloon-like patches/particles on the CNF surfaces was evident. Also, the viscosity and viscoelasticity decreased with increasing the residual lignin content in the LCCNF. Moreover, compared to lignin-free CNF, the LCCNF exhibited much higher hydrophobicity and thermal stability. The unique properties associated with the prepared LCCNF may lead to other valuable potential applications, including surface modification, coating, and composite formation. Acknowledgements The authors wish to thank the financial support from the National Science Foundation for Young Scientists of China (Grant No. 31700514) and the National Key Research and Development Plan (Grant No. 2017YFB0307902). The valuable comments made by the anonymous reviewers are also sincerely appreciated. AUTHOR INFORMATION Corresponding author *Zhaoyang Yuan, Email: [email protected] *Yangbing Wen, Email: [email protected] Author contributions ⁋These authors contributed equally to this work. All authors read and approved the final manuscript. Notes 18



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Graphical Abstract:

Lignin-containing cellulose nanofibrils produced from high yield pulp showed some unique properties such as great hydrophobicity, high thermal stability, and low viscosity.

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Preparation and Characterization of Lignin-Containing Cellulose

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