Rheological behavior of cellulose nanofibers from cassava peel obtained by combination of chemical and physical processes

This work aimed to produce and characterize cellulose nanofibers obtained from cassava peel with a combination of pre-treatments with acid hydrolysis or TEMPO-mediated oxidation and ultrasonic disintegration. All nanofibers presented nanometric diameter (5−16 nm) and high negative zeta potential values (around −30 mV).

Carbohydrate Polymers 248 (2020) 116744 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Rheological behavior of cellulose nanofibers from cassava peel obtained by combination of chemical and physical processes T Aline Czaikoski, Rosiane Lopes da Cunha*, Florencia Cecilia Menegalli Department of Food Engineering, School of Food Engineering, University of Campinas, Campinas, SP CEP 13083-862, Brazil A R T I C LE I N FO A B S T R A C T Keywords: Viscosity Rheology Mechanical resistance Acid hydrolysis TEMPO-mediated oxidation Ultra-sonication This work aimed to produce and characterize cellulose nanofibers obtained from cassava peel with a combination of pre-treatments with acid hydrolysis or TEMPO-mediated oxidation and ultrasonic disintegration All nanofibers presented nanometric diameter (5−16 nm) and high negative zeta potential values (around −30 mV) Oscillatory rheology showed a gel-like behavior of the aqueous suspensions of nanofibers (1.0–1.8 % w/w), indicating their use as reinforcement for nanocomposite or as a thickening agent Additionally aqueous suspensions of nanofibers obtained by acid hydrolysis presented higher gel strength than those produced by TEMPO-mediated oxidation However, ultrasound application increased even more viscoelastic properties Flow curves showed that suspensions of nanofibers obtained by acid hydrolysis presented a thixotropy behavior and viscosity profile with three regions Therefore our results showed that it is possible to tune mechanical properties of cellulose nanofibers choosing and modifying chemical and physical process conditions in order to allow a number of applications Introduction Cellulose is the most abundant biopolymer, present in plant fibers, marine plants, algae, fungi, invertebrates and bacteria (Lavoine, Desloges, Dufresne, & Bras, 2012; Lima & Borsali, 2004) When cellulose has at least one dimension between 1−100 nm, it is called nanocellulose The main forms of nanocellulose are nanofibers and nanocrystals, which can be obtained by different chemical, enzymatic and physical processes These processes can be used separately or combined (Kargarzadeh et al., 2018), resulting in particles with varying characteristics The production of cellulose nanofibers (CNFs) started around the 1980s from wood fibers using high-pressure homogenization (Turbak, Snyder, & Sandberg, 1983) Cellulose nanofibers exhibit interesting properties such as low thermal expansion, high aspect ratio, strengthening effect, good mechanical and optical properties Due to these specific characteristics, the cellulose nanofibers have been used in composites, food packaging, coating additives, aerogels, membranes, as gas barrier material, fillers, flocculants, Pickering emulsifier, food thickeners and reinforcement material (Abdul Khalil, Bhat, & Ireana Yusra, 2012; Abdul Khalil et al., 2014; Choi et al., 2020; Dizge, Shaulsky, & Karanikola, 2019; Fan et al., 2019; Gao et al., 2018; Kadam et al., 2019; Liu, Kerry, & Kerry, 2007; Perzon, Jørgensen, & Ulvskov, ⁎ 2020; Seo et al., 2020; Tibolla, Czaikoski, Pelissari, Menegalli, & Cunha, 2020; Yousefi, Azad, Mashkour, & Khazaeian, 2018) However, the technological properties of cellulose nanofibers depend on the raw material and the treatment used to isolate the fibrils Many food wastes have been used for the production of cellulose nanofibers as: corncobs (Shogren, Peterson, Evans, & Kenar, 2011), carrot juice debris (Siqueira, Oksman, Tadokoro, & Mathew, 2016), corn stover (Xu, Krietemeyer, Boddu, Liu, & Liu, 2018), wheat straw (Alemdar & Sain, 2008), soy hulls (Flauzino Neto et al., 2013), sugarcane bagasse (Liu et al., 2007) and sugar beet pulp (Perzon et al., 2020) However, there are still several food wastes with the potential to be used in the production of nanofibers Cassava (Manihot esculenta) is a root crop widely cultivated in several countries that generates a large amount of waste, such as peels and residual bagasse during the production of cassava starch or other food products Therefore, these residues could be processed to reduce environmental problems and the generation of products with greater added value In a previous work, cellulose nanofibers from cassava peel were extracted by acid hydrolysis and characterized in relation to diameter, aspect ratio, crystallinity among others properties (Leite, Zanon, & Menegalli, 2017), but the rheological behavior of the dispersions of these nanofibers from the cassava peel has not yet been evaluated In addition, cassava peel cellulose nanofibers isolated with tempo-mediated oxidation have not been observed in the Corresponding author E-mail address: rosiane@unicamp.br (R.L da Cunha) https://doi.org/10.1016/j.carbpol.2020.116744 Received May 2020; Received in revised form July 2020; Accepted July 2020 Available online 13 July 2020 0144-8617/ © 2020 Elsevier Ltd All rights reserved Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al with sonication and "wos" to nanofiber without sonication literature Several authors have studied the rheological behavior of dispersions of cellulose nanofibers (CNFs) obtained from different chemical and physical processes, such as high-intensity ultrasonication (Chen et al., 2013), high pressure homogenization (Shogren et al., 2011) and acid hydrolysis (Liu et al., 2007; Zhai, Lin, Li, & Yang, 2020) However, the combination of chemical and physical processes has been more effective in producing nanofibers with enhanced properties Examples of these processes are enzymatic hydrolysis/mechanical fibrillation (Albornoz-Palma, Betancourt, Mendonỗa, Chinga-Carrasco, & Pereira, 2020), TEMPO-mediated oxidation/ultrasonication or mechanical fibrillation (Benhamou, Dufresne, Magnin, Mortha, & Kaddami, 2014; Ehman et al., 2020; Souza, Mariano, De Farias, & Bernardes, 2019) and alkali treatment/high pressure homogenization (Xu et al., 2018) The rheological properties showed a strong dependence on the type and process conditions used to obtain the nanofibers, the type of raw material and the concentration of nanofibers in the suspension Aqueous suspensions of cellulose or CNF nanofibers generally exhibit gel-like behavior (G’ > G’’), even at low concentrations as 0.125 % w/w (Bettaieb et al., 2015; Pääkkö et al., 2007) In addition, flow curves show that CNF suspensions usually present shear thinning and thixotropic behavior (Bettaieb et al., 2015; Iotti, Gregersen, Moe, & Lenes, 2011; Naderi, Lindström, & Sundström, 2014) In some cases, the flow curves exhibit unusual behavior since two shear-thinning regions with an intermediate viscosity plateau have been observed and this effect has not yet been even elucidated Some authors suggest that this phenomenon occurs due to structural changes of the CNF suspension during shear flow (Karppinen et al., 2012; Qiao, Chen, Zhang, & Yao, 2016) However some factors such as concentration, ionic strength, pH, temperature and process conditions can modify the rheological properties of nanofibers suspensions (Chen et al., 2013; Jia et al., 2014; Naderi et al., 2014) Therefore, the aim of this work was to analyze the influence of different combination of physical and chemical processes as hydrolysis by sulfuric acid, TEMPO-mediated oxidation and high-intensity ultrasound on the properties of cellulose nanofibers from cassava peel Nanofibers were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), functional groups from FTIR, crystallinity index and zeta potential The rheological behavior of the CNF suspensions was studied using oscillatory rheology and flow curves 2.3.1 Pre-treatment Cellulose nanofibers (CNFs) were isolated using the chemical treatment described by Leite et al (2017) First, the peel samples were subjected to alkaline treatment with KOH solution (5 % w/v) in the proportion of 1:18 (peel samples: KOH solution) at 25 °C and the suspensions were mechanically stirred for 14 h Then, the wetted samples were separated by centrifugation (15,345×g /15 °C/15 min) The insoluble material was added in distilled water and centrifuged This procedure was carried out until the supernatant color no longer changed The remaining insoluble residue was added in distilled water and the pH adjusted to 5.0 using acetic acid (10 % v/v) Then, the Qchelating treatment with EDTA was performed at 70 °C for h After that, a bleaching treatment was conducted at 90 °C for h using hydrogen peroxide (H2O2) (4 % v/v) and three other reagents: NaOH (2 % v/v), diethylenetriaminepentaacetic acid (DTPA, 0.2 % w/v) and MgSO4 (3 % w/v) Subsequently this suspension was subjected to a second alkaline treatment with KOH solution (5 % w/v) at a ratio of 1:5 Between the stages of delignification, the materials were washed successively with deionized water and centrifuged (15,345×g /15 °C/ 15 min) The insoluble material resulting from the last centrifugation step was added in distilled water, subjected to mechanical agitation and the pH neutralized with acidic solution (1 % v/v H2SO4) This insoluble material was subjected to acid hydrolysis (Section 2.3.2) or TEMPOmediated oxidation (Section 2.3.3) before to be submitted to the physical treatment (Section 2.3.4) 2.3.2 Acid hydrolysis The insoluble material was added in a sulfuric acid solution (30 % v/v) for 90 at 60 °C Subsequently, this mixture was cooled to 40 °C and subjected to four washes with distilled water Thereafter, the suspension was diluted with distilled water and neutralized (pH 7.0) with KOH (5 % w/v) After neutralization, the suspension was centrifuged The insoluble material was separated and washed to remove any salts resulting from the neutralization procedure (Leite et al., 2017) 2.3.3 TEMPO-mediated oxidation The oxidation process was carried out using the method of Saito, Kimura, Nishiyama, and Isogai (2007) with some adaptations The insoluble material (17.07 g corresponding to 1.0 g cellulose, cellulose content determined according to Sun, Sun, Zhao, and Sun (2004) was suspended in 100 ml of distilled water with 0.016 g (0.1 mmol) of TEMPO catalyst (2,2,6,6-tetramethylpiperidin-1-oxyl) and 0.1 g (1 mmol) of sodium bromide (NaBr) Oxidation started with the addition of 12 % NaClO solution (3.0 mmol/g substrat) to the suspension at room temperature with stirring at 500 rpm, keeping pH 10 by addition of 0.5 M NaOH for 25 minutes Thereafter, the suspension was diluted with distilled water and neutralized (pH 7.0) with HCl (0.1 M) Subsequently, the insoluble material was washed with distilled water Material and methods 2.1 Material Peelings (inner peel and bark) of cassava roots were obtained from the southeastern region of Campinas - Brazil All chemicals used were of analytical grade 2.2 Raw material preparation 2.3.4 Physical treatment Half of the suspensions obtained by acid hydrolysis and TEMPOmediated oxidation were subjected to ultrasound treatment An Ultrasonic Disruptor/Sonicator (QR 750 W, Ultronique, Brazil) was used for approximately 20 minutes with a power of 300 W First, peelings were properly classified and washed under running water Then, they were sanitized with sodium hypochlorite solution (250 ppm) for 10 and dried in a forced convection oven at 50 °C for 48 h Subsequently, the peels were cut and ground in a professional high-performance blender LT-2.0 Super Skymsen from Metallurgical Siemsen Ltda (Santa Catarina, Brazil) The resulting material was sieved through a 0.15 mm (100-mesh) sieve opening 2.4 Cellulose nanofibers (CNFs) characterization Morphology of cellulose nanofibers were evaluated by transmission electron microscopy (TEM) TEM images were captured with a TEMMSC (JEOL 2100 – Tokyo, Japan) equipped with a LaB6 electron gun, using an accelerating voltage of 200 kV In order to determine the average size of CNFs, 20 measurements of diameter and 40 measurements of length were made in AFM images AFM images were acquired on a Microscope Park Systems, model NX-10 (Suwon, Korea) equipped 2.3 Nanofibers isolation Cellulose nanofibers were named according to the treatment that they were submitted, which will be described in the next sections The nomenclature is: "CNFs" referring to cellulose nanofibers, "TO" to TEMPO-mediated oxidation, "HA" to acid hydrolysis, "ws" to nanofiber Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al to those found by Tibolla, Pelissari, Rodrigues, and Menegalli (2017) for banana peel cellulose nanofibers isolated by enzymatic treatment with xylanase (1490−1940 nm) However, our results were superior to those found by Leite et al (2017), which showed 162–400 nm and 243−296 nm, for cellulose nanofibers from cassava peel and bagasse obtained by acid hydrolysis, respectively, with an additional centrifugation step All treatments generated fibers with a nanometric diameter, but acid hydrolysis produced cellulose nanofibers with a smaller diameter than the TEMPO-mediated oxidation The diameter of the cellulose nanofibers decreased after being subjected to a sonication process, but this reduction was more significant for those obtained by the TEMPOmediated oxidation This effect was also observed by Khawas and Deka (2016) isolating cellulose nanofibers from banana peel by acid hydrolysis and sonication Sonication process also produced fibers with length and diameter more homogeneous than the nanofibers without sonication, which can be observed from the smaller standard deviation This fact may be related to a greater nanofibers fibrillation caused by ultrasound In this process waves are produced that cause the cavitation phenomenon, due to the absorption of ultrasonic energy by the molecules that cause the formation and expansion of microscopic gas bubbles With the collapse of these bubbles there is local production of heat and high pressure These effects facilitate the isolation of nanomaterials, as they break the structural micron-sized fibril into submicron fibrils and then at the nanoscale, producing nanofibers of a more homogeneous size (Abdul Khalil et al., 2014; Huerta, Silva, Ekaette, ElBialy, & Saldaña, 2020; Wang et al., 2012) Moreover, the aspect ratio ranged from 242 to 371 for the different nanofibers, ensuring their use as reinforcement for composites, since the aspect ratio was greater than 100 (Ma, Zeng, Realff, Kumar, & Schiraldi, 2003) However, the zeta potential of CNFs was not altered after the sonication process, although the method of producing CNFs had an influence on this parameter The nanofibers obtained by acid hydrolysis showed a higher negative charge (∼ −49 mV) than those obtained by catalytic oxidation (∼ −42 mV) A greater negative zeta potential presented by nanofibers obtained from acid hydrolysis is associated to the more efficient introduction of sulfate groups on the surfaces of fibers, in comparison to catalytic oxidation that introduces carboxylic groups Despite these differences, all nanofibers suspensions presented electrostatic stability, since the zeta potential was greater than −30 mV (Everett, 1988) The nanofibers obtained by acid hydrolysis also showed a higher crystallinity index (CNFs-HAws = 53.42 % and CNFs-HAwos = 53.47 %) than the nanofibers obtained by catalytic oxidation (CNFs-TOws = 46.67 % and CNFs-TOwos = 46.82 %) (XRD patterns of samples at Fig 1.a – Supplementary material) The crystallinity of the material increased about % after acid hydrolysis, while after catalytic oxidation only % compared to the pretreated material (crystallinity index of 45 %) A minor increase in crystallinity index after TEMPO-mediated oxidation can be associated with chemical treatment that only transforms the surface hydroxyls into carboxylate groups, without interfering with the internal conformation of cellulose crystals (Isogai, Saito, & Fukuzumi, 2011) Acid hydrolysis, on the other hand, acts on the amorphous fibrils components, lignin and hemicellulose, facilitating their extraction and, consequently, concentrating the crystalline portions of the material (Alemdar & Sain, 2008) Our results of crystallinity index were similar to those found by Khawas and Deka (2016), which observed values between 30.5–63.64 % for cellulose nanofibers extracted from banana peels The estimate of the amount of cellulose I in relation to cellulose II for the nanofibers ranged from 1.26 to 1.73 (Table 1) As the values are greater than one, the nanofibers have more cellulose I than cellulose II Cellulose I has the best mechanical properties and, therefore, the cellulose nanofibers produced are suitable for use as a reinforcement material (Mandal & Chakrabarty, 2011) To assess the chemical structure of CNFs, FTIR spectroscopy analyses were obtained (Fig 2) The FTIR spectra obtained for the cellulose with Si Nano sensor probes manufactured with a constant spring of 42 N.m−1 The resonance frequency was about 320 kHz and the acquired images were treated with the software GWYDDION version 2.4 to obtain the mean diameter and length of the nanofibers The zeta potential was determined using the Zetasizer model Nano ZS from Malvern Instruments Ltd (United Kingdom, U.K) at a detection angle of 173° Nine measurements of zeta potential were performed for each sample at room temperature (25 °C) The crystallinity index was determined from X-ray diffraction (XRD) patterns registered on a D5005 diffractometer equipped with a graphite monochromator and a CuKα source (λ =0.154 nm) at 40 kV and 30 mA The crystallinity index (IC) was calculated from the ratio of intensity of the crystalline peak to the intensity of diffraction of the non-crystalline material (Segal, Creely, Martin, & Conrad, 1959) An estimate of the ratio of type I cellulose to type II cellulose was also obtained, according to Mandal and Chakrabarty (2011) This ratio was calculated using the peak intensity at 21.7° over the peak intensity at 20° Functional groups were analyzed by Fourier transform infrared (FTIR) spectroscopy accomplished on the Fourier transform infrared spectrometer (JASCO FTIR6100, Japan) in the infrared region from 4000 to 600 cm−1 (Vicentini, Dupuy, Leitzelman, Cereda, & Sobral, 2005) 2.5 Rheological characterization Nanofiber suspensions (1.0 %, 1.4 % and 1.8 % w/w) were prepared in distilled water and their rheological behavior was studied using a stress-controlled rheometer MCR 301 (Anton Paar, Austria) equipped with cone and plate geometry (6 cm diameter, cone truncation of 0.208 mm and 2°) After being placed on the rheometer plate, the CNFs suspensions were allowed to rest for in order to minimize the shear history imposed by loading All the measurements were carried out at 25 °C Flow curves were obtained by up-down-up steps program with shear rate ranging from to 300 s−1 Apparent viscosity values were evaluated at 100 s−1 since this shear rate is associated to chewing (Whitcomb, Gutowski, & Howland, 1980) and other process conditions, such as agitation and flow in pipes In addition thixotropy degree was estimated from the area between the up and down curves, in order to compare how much the material microstructure was changed with the shear stress (Barnes, 1997) Viscoelastic properties were evaluated from oscillatory rheology First, strain sweeps of 0.1–10 % at constant angular frequency of Hz were performed to define the linear viscoelastic range After that, frequency sweeps were done in the range of 0.01–10 Hz with a strain within the linear viscoelastic range 2.6 Statistical analysis Results were evaluated by analysis of variance (ANOVA) and the Tukey test The significance level was % Results and discussion 3.1 Characterization of cellulose nanofibers 3.1.1 Length, diameter, zeta potential, crystallinity index and functional groups Length distribution of the cellulose nanofibers is shown in Fig and Table summarizes the properties of the cellulose nanofibers obtained from cassava peels after acid hydrolysis and TEMPO-mediated oxidation with and without sonication Nanofibers obtained by TEMPOmediated oxidation without sonication showed the widest distribution range These nanofibers (CNFs-TOwos) also had the biggest length and diameter (Table 1), with the highest number of nanofibers obtained in the length range of 2500−3500 nm In contrast, the other samples of cellulose nanofibers showed the highest percentage of length distribution in the range of 1500−2500 nm These lengths were slightly higher Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al Fig Length distribution of CNFs produced from different chemical and physical treatments nanofibers, cassava peel and pre-treated material exhibited a wide band in the region of 3500 cm−1 at 3200 cm−1 corresponding to the free vibration of the OHe stretches of the OH groups of the cellulose molecules In addition, the spectra showed the CeH stretch characteristic of hemicellulose and cellulose around 2895 cm−1 (Khalil, Ismail, Rozman, & Ahmad, 2001) All nanofibers showed a peak located at 1030 cm−1 that is associated with COe elongation, characteristic of the presence of cellulose The FTIR absorption peak at 1430 cm−1 corresponds to the vibration of the CH2 bonds, attributed to the cellulose "crystallinity band" The band at 890 cm−1 is attributed to the C-O-C stretching vibration of β-cell (1 → 4) glycosidic bonds, which is considered to be an "amorphous band" (Shankar & Rhim, 2016) All the nanofibers presented peaks in these bands, demonstrating the presence of amorphous and crystalline celluloses The peaks 2464 cm−1, 1509 cm−1 and 1601 cm−1 are characteristic of the existence of aromatic rings and CHe bonds, indicating the presence of lignin (Liu, Wang, Zheng, Luo, & Cen, 2008) The two peaks close to the nanofibers obtained in this study, at 1614 cm−1 and 1409 cm−1, showed that the processes used were not enough to remove all existing fractions of lignin However, these peaks were less evident that in cassava peel and pre-treated material Fig FTIR spectra of the cassava peel, the pre-treated material and cellulose nanofibers obtained from acid hydrolysis without (CNFs-HAwos) and with (CNFs-HAws) sonication, TEMPO-mediated oxidation without (CNFs-TOwos) and with (CNFs-TOws) sonication pictures revealed fibers with a wide size distribution both in diameter and in length Sonicated nanofibers were more separated than nanofibers without this process showing some fibers with intact bundles In addition, acid hydrolysis produced more dispersed (separated) nanofibers than the samples obtained by TEMPO-mediated oxidation Furthermore, it is possible to observe the presence of cellulose nanospheres after the ultrasound process As cellulose hydrolysis generally begins in the superficial amorphous region and subsequently 3.2 Microscopy observations Fig shows the morphological characteristics of CNFs TEM Table Characteristics of the cellulose nanofibers obtained from acid hydrolysis without (CNFs-HAwos) and with (CNFs-HAws) sonication; TEMPO-mediated oxidation without (CNFs-TOwos) and with (CNFs-TOws) sonication Sample Mean Length (nm) Mean Diameter (nm) Aspect ratio (L/D) Zeta potential (mV) Cellulose ratio (I/II)* CNFs-HAwos CNFs-HAws CNFs-TOwos CNFs-TOws 2527 ± 991bc 1857 ± 835c 3867 ± 1597a 2750 ± 1499b ± 6.2b ± 2.1b 16 ± 14.0a ± 4.3b 316 371 242 344 −49.65 ± 1.6a −49.33 ± 4.3a −41.81 ± 4.2b −42.22 ± 1.9b 1,73 1,39 1,26 1,33 a,b,c, Different superscripts letters in the same column indicate a statistically significant difference (p < 0.05) * Cellulose I in relation cellulose II Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al Fig Transmission electron microscopy (TEM) images for CNFs (scale bar =100 nm) penetrates the internal amorphous region, the pretreatment together with acid hydrolysis or TEMPO-mediated oxidation must have caused the superficial hydrolysis that favored the ultrasonic treatment to penetrate the amorphous inner region, facilitating the formation of smaller cellulose fragments, such as cellulose nanospheres (Neng, Enyong, & Rongshi, 2008) 3.3 Rheological properties 3.3.1 Oscillatory rheology The viscoelastic properties of CNF suspensions prepared with different methods are illustrated in Fig All CNF suspensions exhibited gel-like properties with G’ > G’’, but nanofibers treated by acid hydrolysis showed G' greater than those obtained by TEMPO-mediated oxidation In addition, cellulose nanofibers obtained by acid hydrolysis presented moduli with less frequency dependence than those obtained by catalytic oxidation These results can be, at least partly, related to the greater negative charge observed for CNFs obtained by acid hydrolysis A higher negative charge favors repulsive forces between the nanofibers, causing a better dispersion to entrap water molecules in the Fig Storage (G’) and loss (G’’) moduli of suspensions with 1.4 % (w/w) CNFs as a function of frequency for: Cellulose nanofibers obtained from acid hydrolysis without ( ) and with ( ) sonication; TEMPO-mediated oxidation without ( ) and with (♦) sonication Filled symbols correspond to G’ and open symbols correspond to G’’ Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al Fig Shear stress as a function of shear rate for suspensions with 1.4 % (w/w) CNFs: (○) First sweep (up); ( ) second sweep (down) and (x) third sweep (up) from the difference between the first and second/third flow curves The sonication process increased the degree of thixotropy of the nanofibers, which could be associated with greater separation of the nanofibers, as can been seen on the TEM images (Fig 3) This separation increases the aspect ratio of these fibers and consequently increases the interaction between the nanofibers and the degree of thixotropy (Barnes, 1997) Although the viscosity of CNFs suspensions obtained by acid hydrolysis decreased to shear rate up to 20 s−1, the viscosity became almost constant, forming a Newtonian plateau, between 20 s−1 and 40 s−1 (Fig 6) and the viscosity decreased again (shear-thinning behaviour) above 40 s−1 However, the nanofibers obtained by catalytic oxidation presented only shear-thinning behavior within the shear rate range A similar behavior of suspensions of CNFs obtained by acid hydrolysis was reported in other studies Iotti et al (2011) found a similar viscosity behavior for microfibrillated cellulose, which was attributed to production of another structure by shear leading to the formation of the Newtonian plateau However Karppinen et al (2012), working with suspensions of microfibrillated cellulose and using images captured in a transparent outer cylinder in concentric cylinders, verified a shear-induced phase separation in the range of shear rate of the Newtonian plateau Bettaieb et al (2015) attributed this behavior to the slippage of the suspension over the sensor, even using used rough surfaces on the measuring sensor Slippage effects on the wall of rheometer sensors occurs because the dispersed phase separates from the dispersant of the suspension, leaving a liquid layer formation that shows low viscosity near the sensor and cause a lubrication or slipping effect The characteristics that usually lead to slipping effects in the flow are: existence of large particles with high aspect ratio; use of smooth walls on measuring sensors and low speeds/flow rates; walls and particles carrying electrostatic charges and electrically conductive continuous phase (Barnes, 1995) The formation of the Newtonian plateau in the suspensions of nanofibers produced by acid hydrolysis may have occurred because they present several characteristics that can lead to the occurrence of the slippage Nanofibers isolated by TEMPO-mediated oxidation may not have this effect due to the lower zeta potential or lower quantity of electrostatic charges and vicinity of the fibers and increase the elastic character of the suspensions (Benhamou et al., 2014; Li et al., 2015) Another factor that may have influenced this behavior more significantly is the difference in aspect ratio of the different nanofibers Materials that show a high aspect ratio tend to flocculate or form interlacings/entanglements among them These entanglements between the fibers cause a certain restriction of movement with the flow and, therefore, induce a solid type behavior (Sato & Cunha, 2012) The sonication process also contributed to the increase in storage moduli for CNFs suspensions Mishra, Manent, Chabot, and Daneault (2011) report that the sonication treatment causes a greater separation of the nanofibers, which is corroborated in our results (Fig 3) This more pronounced fibrillation caused by the ultrasound treatment increased the surface area of the fibers, facilitating the physical interactions between nanofibers, promoting entanglements between them and increasing their gel strength 3.3.2 Flow properties Figs and show the flow curves and viscosity behavior, respectively, of the different nanofibers suspensions obtained by acid hydrolysis and by TEMPO-mediated oxidation, with and without sonication These nanofibers suspensions presented a hysteresis loop, indicating a thixotropic behavior The degree of thixotropy of these suspensions was calculated from the area between the first and the third curve (steady state) of shear stress-shear rate (Fig 5) and the results are shown in Fig Nanofibers obtained by catalytic oxidation presented a smaller hysteresis area (CNFs-TOws =14.56 Pa.s−1; CNFs-TOwos =4.39 Pa.s−1) than the nanofibers treated by acid hydrolysis (CNFs-HAws =143.95 Pa.s−1; CNFs-HAwos =72.28 Pa.s−1), corroborating a weaker and less complex network as observed in viscoelastic properties Thixotropic behavior is common in dispersions that exhibit flocculated, entangled or aligned fibers Due to the presence of the interweaving of the fibers, they present greater restriction to the alignment with the flow and, consequently, show higher viscosity (Fig 6) As shear increases, these entanglements are broken This structural change causes the fibers to separate, facilitating their alignment with the flow, which causes a decrease in viscosity (Fig 6) This behavior can be observed Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al Fig Viscosity of suspensions with 1.4 % (w/w) CNFs as a function of shear rate: (○) first sweep (up); ( ) second sweep (down) and (x) third sweep (up) entanglement between nanofibers, as higher concentrations of nanofibers increase the interaction between them and form stronger network structures (Iotti et al., 2011) As observed in Figs and 6, higher pseudoplasticity was observed in nanofibers treated with acid also smaller aspect ratio Fig shows the dependence of the storage moduli, viscosity and degree of thixotropy with the increase in the concentration of nanofibers The increase in rheological properties was also related to the Fig Degree of thixotropy, apparent viscosity at 100 s−1 (ƞ) and storage moduli (G’) at Hz of the suspensions of cellulose nanofibers obtained from acid hydrolysis without (CNFsHAwos) and with (CNFs-HAws) sonication; TEMPO-mediated oxidation without (CNFsTOwos) and with (CNFs-TOws) sonication at different concentrations (n = 3) Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al hydrolysis and after ultrasound treatment, demonstrating again that these systems showed more complex rheological behavior and, consequently, network structure the TEMPO-mediated oxidation time Carbohydrate Polymers, 99, 74–83 https://doi org/10.1016/J.CARBPOL.2013.08.032 Bettaieb, F., Nechyporchuk, O., Khiari, R., Mhenni, M F., Dufresne, A., & Belgacem, M N (2015) Effect of the oxidation treatment on the production of cellulose nanofiber suspensions from Posidonia oceanica: The rheological aspect Carbohydrate Polymers, 134, 664–672 https://doi.org/10.1016/J.CARBPOL.2015.07.091 Chen, P., Yu, H., Liu, Y., Chen, W., Wang, X., & Ouyang, M (2013) Concentration effects on the isolation and dynamic rheological behavior of cellulose nanofibers via ultrasonic processing Cellulose, 20(1), 149–157 https://doi.org/10.1007/s10570-0129829-7 Choi, H Y., Bae, J H., Hasegawa, Y., An, S., Kim, I S., Lee, H., & Kim, M (2020) Thiolfunctionalized cellulose nanofiber membranes for the effective adsorption of heavy metal ions in water Carbohydrate Polymers, 234, Article 115881 https://doi.org/10 1016/j.carbpol.2020.115881 Dizge, N., Shaulsky, E., & Karanikola, V (2019) Electrospun cellulose nanofibers for superhydrophobic and oleophobic membranes Journal of Membrane Science, 590, Article 117271 https://doi.org/10.1016/j.memsci.2019.117271 Ehman, N V., Lourenỗo, A F., McDonagh, B H., Vallejos, M E., Felissia, F E., Ferreira, P J T., Area, M C (2020) Influence of initial chemical composition and characteristics of pulps on the production and properties of lignocellulosic nanofibers International Journal of Biological Macromolecules, 143, 453–461 https://doi.org/10 1016/j.ijbiomac.2019.10.165 Everett, D H (1988) Basic principles of colloid science Cambridge, UK: The Royal Society of chemistry Fan, X., Li, Y., Li, X., Wu, Y., Tang, K., Liu, J., Wan, G (2019) Injectable antibacterial cellulose nanofiber/chitosan aerogel with rapid shape recovery for noncompressible hemorrhage International Journal of Biological Macromolecules https://doi.org/10 1016/j.ijbiomac.2019.10.273 Flauzino Neto, W P., Silvério, H A., Dantas, N O., & Pasquini, D (2013) Extraction and characterization of cellulose nanocrystals from agro-industrial residue – Soy hulls Industrial Crops and Products, 42, 480–488 https://doi.org/10.1016/J.INDCROP 2012.06.041 Gao, H., Duan, B., Lu, A., Deng, H., Du, Y., Shi, X., & Zhang, L (2018) Fabrication of cellulose nanofibers from waste brown algae and their potential application as milk thickeners Food Hydrocolloids, 79, 473–481 https://doi.org/10.1016/J.FOODHYD 2018.01.023 Huerta, R R., Silva, E K., Ekaette, I., El-Bialy, T., & Saldaña, M D A (2020) Highintensity ultrasound-assisted formation of cellulose nanofiber scaffold with low and high lignin content and their cytocompatibility with gingival fibroblast cells Ultrasonics Sonochemistry, 64, Article 104759 https://doi.org/10.1016/j.ultsonch 2019.104759 Iotti, M., Gregersen, Ø W., Moe, S., & Lenes, M (2011) Rheological studies of microfibrillar cellulose water dispersions Journal of Polymers and the Environment, 19(1), 137–145 https://doi.org/10.1007/s10924-010-0248-2 Isogai, A., Saito, T., & Fukuzumi, H (2011) TEMPO-oxidized cellulose nanofibers Nanoscale, 3(1), 71–85 https://doi.org/10.1039/C0NR00583E Jia, X., Chen, Y., Shi, C., Ye, Y., Abid, M., Jabbar, S., Wu, T (2014) Rheological properties of an amorphous cellulose suspension Food Hydrocolloids, 39, 27–33 https://doi.org/10.1016/J.FOODHYD.2013.12.026 Kadam, S L., Yadav, P., Bhutkar, S., Patil, V D., Shukla, P G., & Shanmuganathan, K (2019) Sustained release insect repellent microcapsules using modified cellulose nanofibers (mCNF) as pickering emulsifier Colloids and Surfaces A: Physicochemical and Engineering Aspects, 582, Article 123883 https://doi.org/10.1016/j.colsurfa 2019.123883 Kargarzadeh, H., Mariano, M., Gopakumar, D., Ahmad, I., Thomas, S., Dufresne, A., Lin, N (2018) Advances in cellulose nanomaterials Cellulose, 25(4), 2151–2189 https://doi.org/10.1007/s10570-018-1723-5 Karppinen, A., Saarinen, T., Salmela, J., Laukkanen, A., Nuopponen, M., & Seppälä, J (2012) Flocculation of microfibrillated cellulose in shear flow Cellulose, 19(6), 1807–1819 https://doi.org/10.1007/s10570-012-9766-5 Khalil, H P S A., Ismail, H., Rozman, H., & Ahmad, M (2001) The effect of acetylation on interfacial shear strength between plant fibres and various matrices European Polymer Journal, 37(5), 1037–1045 https://doi.org/10.1016/S0014-3057(00) 00199-3 Khawas, P., & Deka, S C (2016) Isolation and characterization of cellulose nanofibers from culinary banana peel using high-intensity ultrasonication combined with chemical treatment Carbohydrate Polymers, 137, 608–616 https://doi.org/10.1016/J CARBPOL.2015.11.020 Lavoine, N., Desloges, I., Dufresne, A., & Bras, J (2012) Microfibrillated cellulose – Its barrier properties and applications in cellulosic materials: A review Carbohydrate Polymers, 90(2), 735–764 https://doi.org/10.1016/J.CARBPOL.2012.05.026 Leite, A L M P., Zanon, C D., & Menegalli, F C (2017) Isolation and characterization of cellulose nanofibers from cassava root bagasse and peelings Carbohydrate Polymers, 157, 962–970 https://doi.org/10.1016/J.CARBPOL.2016.10.048 Li, M.-C., Wu, Q., Song, K., Lee, S., Qing, Y., & Wu, Y (2015) Cellulose Nanoparticles: Structure–Morphology–Rheology Relationships ACS Sustainable Chemistry & Engineering, 3(5), 821–832 https://doi.org/10.1021/acssuschemeng.5b00144 Lima, M M.de S., & Borsali, R (2004) Rodlike Cellulose Microcrystals: Structure, Properties, and Applications Macromolecular Rapid Communications, 25(7), 771–787 https://doi.org/10.1002/marc.200300268 Liu, L., Kerry, J F., & Kerry, J P (2007) Application and assessment of extruded edible casings manufactured from pectin and gelatin/sodium alginate blends for use with breakfast pork sausage Meat Science, 75(2), 196–202 https://doi.org/10.1016/j meatsci.2006.07.008 Liu, Q., Wang, S., Zheng, Y., Luo, Z., & Cen, K (2008) Mechanism study of wood lignin pyrolysis by using TG–FTIR analysis Journal of Analytical and Applied Pyrolysis, 82(1), Conclusion Our results demonstrated that the agricultural residue of cassava peel has great potential for the production of cellulose nanofibers The chemical methods employed, such as alkaline pre-treatment, acid hydrolysis and TEMPO-mediated oxidation, were efficient for the isolation of cellulose nanofibers with diameters between and 16 nm Acid treatment was more efficient to introduce negative charge on surface and to produce nanofibers with greater aspect ratios These characteristics facilitated the formation of entanglements between the nanofibers, increasing the complexity of the structural behavior and, consequently, presenting suspensions with higher gel strength than the nanofibers obtained by TEMPO-mediated oxidation The sonication process on the nanofibers also interfered in the gel strength of their suspensions, since this process also induced an increase of aspect ratio Nanofibers suspensions that presented higher gel strength also showed higher degree of thixotropy, pseudoplasticity and viscosity, showing that the rheological behavior was essential to identify the better method to produce cellulose nanofibers with potential features as a strengthening of polymeric matrix CRediT authorship contribution statement Aline Czaikoski: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing - original draft Rosiane Lopes da Cunha: Conceptualization, Writing - original draft Florencia Cecilia Menegalli: Conceptualization, Funding acquisition, Writing - original draft Acknowledgements This study was nanced in part by the Coordenaỗóo de Aperfeiỗoamento de Pessoal de Nível Superior – Brasil (CAPES) - (2952/ 2011) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (131520/2015-6 and 307168/2016-6) The authors would also like to acknowledge the Brazilian Nanotechnology National Laboratory (LNNano) for allocation of the equipments Appendix A Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2020.116744 References Abdul Khalil, H P S., Bhat, A H., & Ireana Yusra, A F (2012) Green composites from sustainable cellulose nanofibrils: A review Carbohydrate Polymers, 87(2), 963–979 https://doi.org/10.1016/J.CARBPOL.2011.08.078 Abdul Khalil, H P S., Davoudpour, Y., Islam, M N., Mustapha, A., Sudesh, K., Dungani, R., & Jawaid, M (2014) Production and modification of nanofibrillated cellulose using various mechanical processes: A review Carbohydrate Polymers, 99, 649–665 https://doi.org/10.1016/J.CARBPOL.2013.08.069 Albornoz-Palma, G., Betancourt, F., Mendonỗa, R T., Chinga-Carrasco, G., & Pereira, M (2020) Relationship between rheological and morphological characteristics of cellulose nanofibrils in dilute dispersions Carbohydrate Polymers, 230, Article 115588 https://doi.org/10.1016/j.carbpol.2019.115588 Alemdar, A., & Sain, M (2008) Isolation and characterization of nanofibers from agricultural residues – Wheat straw and soy hulls Bioresource Technology, 99(6), 1664–1671 https://doi.org/10.1016/J.BIORTECH.2007.04.029 Barnes, H A (1995) A review of the slip (wall depletion) of polymer solutions, emulsions and particle suspensions in viscometers: Its cause, character, and cure Journal of NonNewtonian Fluid Mechanics, 56(3), 221–251 https://doi.org/10.1016/0377-0257(94) 01282-M Barnes, H A (1997) Thixotropy—a review Journal of Non-Newtonian Fluid Mechanics, 70(1–2), 1–33 https://doi.org/10.1016/S0377-0257(97)00004-9 Benhamou, K., Dufresne, A., Magnin, A., Mortha, G., & Kaddami, H (2014) Control of size and viscoelastic properties of nanofibrillated cellulose from palm tree by varying Carbohydrate Polymers 248 (2020) 116744 A Czaikoski, et al Shogren, R L., Peterson, S C., Evans, K O., & Kenar, J A (2011) Preparation and characterization of cellulose gels from corn cobs Carbohydrate Polymers, 86(3), 1351–1357 https://doi.org/10.1016/J.CARBPOL.2011.06.035 Siqueira, G., Oksman, K., Tadokoro, S K., & Mathew, A P (2016) Re-dispersible carrot nanofibers with high mechanical properties and reinforcing capacity for use in composite materials Composites Science and Technology, 123, 49–56 https://doi.org/ 10.1016/J.COMPSCITECH.2015.12.001 Souza, S F., Mariano, M., De Farias, M A., & Bernardes, J S (2019) Effect of depletion forces on the morphological structure of carboxymethyl cellulose and micro/nano cellulose fiber suspensions Journal of Colloid and Interface Science, 538, 228–236 https://doi.org/10.1016/j.jcis.2018.11.096 Sun, J X., Sun, X F., Zhao, H., & Sun, R C (2004) Isolation and characterization of cellulose from sugarcane bagasse Polymer Degradation and Stability, 84(2), 331–339 https://doi.org/10.1016/j.polymdegradstab.2004.02.008 Tibolla, H., Czaikoski, A., Pelissari, F M., Menegalli, F C., & Cunha, R L (2020) Starchbased nanocomposites with cellulose nanofibers obtained from chemical and mechanical treatments International Journal of Biological Macromolecules, 161, 132–146 https://doi.org/10.1016/j.ijbiomac.2020.05.194 Tibolla, H., Pelissari, F M., Rodrigues, M I., & Menegalli, F C (2017) Cellulose nanofibers produced from banana peel by enzymatic treatment: Study of process conditions Industrial Crops and Products, 95, 664–674 https://doi.org/10.1016/J INDCROP.2016.11.035 Turbak, A F., Snyder, F W., & Sandberg, K R (1983) Microfibrillated cellulose: morphology and accessibility Journal of Applied Polymer Science: Poly 37 Vicentini, N M., Dupuy, N., Leitzelman, M., Cereda, M P., & Sobral, P J A (2005) Prediction of Cassava Starch Edible Film Properties by Chemometric Analysis of Infrared Spectra Spectroscopy Letters, 38(6), 749–767 https://doi.org/10.1080/ 00387010500316080 Wang, Q Q., Zhu, J Y., Gleisner, R., Kuster, T A., Baxa, U., & McNeil, S E (2012) Morphological development of cellulose fibrils of a bleached eucalyptus pulp by mechanical fibrillation Cellulose, 19(5), 1631–1643 https://doi.org/10.1007/ s10570-012-9745-x Whitcomb, P J., Gutowski, J., & Howland, W W (1980) Rheology of guar solutions Journal of Applied Polymer Science, 25(12), 2815–2827 https://doi.org/10.1002/app 1980.070251214 Xu, J., Krietemeyer, E F., Boddu, V M., Liu, S X., & Liu, W.-C (2018) Production and characterization of cellulose nanofibril (CNF) from agricultural waste corn stover Carbohydrate Polymers, 192, 202–207 https://doi.org/10.1016/J.CARBPOL.2018.03 017 Yousefi, H., Azad, S., Mashkour, M., & Khazaeian, A (2018) Cellulose nanofiber board Carbohydrate Polymers, 187, 133–139 https://doi.org/10.1016/J.CARBPOL.2018.01 081 Zhai, X., Lin, D., Li, W., & Yang, X (2020) Improved characterization of nanofibers from bacterial cellulose and its potential application in fresh-cut apples International Journal of Biological Macromolecules, 149, 178–186 https://doi.org/10.1016/j ijbiomac.2020.01.230 170–177 https://doi.org/10.1016/J.JAAP.2008.03.007 Ma, H., Zeng, J., Realff, M L., Kumar, S., & Schiraldi, D A (2003) Processing, structure, and properties of fibers from polyester/carbon nanofiber composites Composites Science and Technology, 63(11), 1617–1628 https://doi.org/10.1016/S02663538(03)00071-X Mandal, A., & Chakrabarty, D (2011) Isolation of nanocellulose from waste sugarcane bagasse (SCB) and its characterization Carbohydrate Polymers, 86(3), 1291–1299 https://doi.org/10.1016/j.carbpol.2011.06.030 Mishra, S P., Manent, A.-S., Chabot, B., & Daneault, C (2011) Production of nanocellulose from native cellulose - various options utilizing ultrasound BioResources, 7(1), 0422–0436 https://doi.org/10.15376/biores.7.1.0422-0436 Naderi, A., Lindström, T., & Sundström, J (2014) Carboxymethylated nanofibrillated cellulose: rheological studies Cellulose, 21(3), 1561–1571 https://doi.org/10.1007/ s10570-014-0192-8 Neng, W., Enyong, D., & Rongshi, C (2008) Preparation and liquid crystalline properties of spherical cellulose nanocrystals Langmuir, 24(1), 5–8 https://doi.org/10.1021/ la702923w Pääkkö, M., Ankerfors, M., Kosonen, H., Nykänen, A., Ahola, S., Österberg, M., Lindström, T (2007) Enzymatic hydrolysis combined with mechanical shearing and highpressure homogenization for nanoscale cellulose fibrils and strong gels https://doi.org/ 10.1021/BM061215P Perzon, A., Jørgensen, B., & Ulvskov, P (2020) Sustainable production of cellulose nanofiber gels and paper from sugar beet waste using enzymatic pre-treatment Carbohydrate Polymers, 230, Article 115581 https://doi.org/10.1016/j.carbpol.2019 115581 Qiao, C., Chen, G., Zhang, J., & Yao, J (2016) Structure and rheological properties of cellulose nanocrystals suspension Food Hydrocolloids, 55, 19–25 https://doi.org/10 1016/J.FOODHYD.2015.11.005 Saito, T., Kimura, S., Nishiyama, Y., & Isogai, A (2007) Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose https://doi.org/10.1021/BM0703970 Sato, A C K., & Cunha, R L (2012) Influence of dispersing media and particle characteristics on rheological behavior of noncolloidal suspensions Journal of Dispersion Science and Technology, 33(3), 437–446 https://doi.org/10.1080/01932691.2011 562445 Segal, L., Creely, J J., Martin, A E., & Conrad, C M (1959) An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer Textile Research Journal, 29(10), 786–794 https://doi.org/10.1177/ 004051755902901003 Seo, M., Seo, M., Choi, S E., Shin, K., Lee, J B., Yang, D Y., & Kim, J W (2020) Cellulose nanofiber-multilayered fruit peel-mimetic gelatin hydrogel microcapsules for micropackaging of bioactive ingredients Carbohydrate Polymers, 229, Article 115559 https://doi.org/10.1016/j.carbpol.2019.115559 Shankar, S., & Rhim, J.-W (2016) Preparation of nanocellulose from micro-crystalline cellulose: The effect on the performance and properties of agar-based composite films Carbohydrate Polymers, 135, 18–26 https://doi.org/10.1016/J.CARBPOL 2015.08.082 ... influence of different combination of physical and chemical processes as hydrolysis by sulfuric acid, TEMPO-mediated oxidation and high-intensity ultrasound on the properties of cellulose nanofibers from. .. sonication and "wos" to nanofiber without sonication literature Several authors have studied the rheological behavior of dispersions of cellulose nanofibers (CNFs) obtained from different chemical and physical. .. water Material and methods 2.1 Material Peelings (inner peel and bark) of cassava roots were obtained from the southeastern region of Campinas - Brazil All chemicals used were of analytical grade