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).

Contents lists available atScienceDirect Carbohydrate Polymers journal homepage:www.elsevier.com/locate/carbpol

combination of chemical and physical processes

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 L E I N F O

Keywords:

Viscosity

Rheology

Mechanical resistance

Acid hydrolysis

TEMPO-mediated oxidation

Ultra-sonication

A B S T R A C T

This work aimed to produce and characterize cellulose nanofibers obtained from cassava peel with a combi-nation 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 sus-pensions 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

1 Introduction

Cellulose is the most abundant biopolymer, present in plantfibers,

marine plants, algae, fungi, invertebrates and bacteria (Lavoine,

Desloges, Dufresne, & Bras, 2012;Lima & Borsali, 2004) When

cellu-lose has at least one dimension between 1−100 nm, it is called

cellulose The main forms of nanocellulose are nanofibers and

nano-crystals, 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

char-acteristics

The production of cellulose nanofibers (CNFs) started around the

1980s from woodfibers using high-pressure homogenization (Turbak,

Snyder, & Sandberg, 1983) Cellulose nanofibers exhibit interesting

properties such as low thermal expansion, high aspect ratio,

strength-ening 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 thefibrils 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 pro-duction 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 cessed to reduce environmental problems and the generation of pro-ducts 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 prop-erties (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

https://doi.org/10.1016/j.carbpol.2020.116744

Received 5 May 2020; Received in revised form 3 July 2020; Accepted 8 July 2020

⁎Corresponding author

E-mail address:rosiane@unicamp.br(R.L da Cunha)

Available online 13 July 2020

0144-8617/ © 2020 Elsevier Ltd All rights reserved

T

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

effec-tive in producing nanofibers with enhanced properties Examples of

these processes are enzymatic hydrolysis/mechanical fibrillation

(Albornoz-Palma, Betancourt, Mendonça, Chinga-Carrasco, & Pereira,

2020), TEMPO-mediated oxidation/ultrasonication or mechanical

fi-brillation (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

ma-terial and the concentration of nanofibers in the suspension

Aqueous suspensions of cellulose or CNF nanofibers generally

ex-hibit 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 shearflow (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

prop-erties 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

ultra-sound 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 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

First, peelings were properly classified and washed under running

water Then, they were sanitized with sodium hypochlorite solution

(250 ppm) for 10 min 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

2.3 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

with sonication and "wos" to nanofiber without sonication

2.3.1 Pre-treatment Cellulose nanofibers (CNFs) were isolated using the chemical treatment described byLeite 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 sus-pensions were mechanically stirred for 14 h Then, the wetted samples were separated by centrifugation (15,345×g /15 °C/15 min) The in-soluble 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 Q-chelating treatment with EDTA was performed at 70 °C for 1 h After that, a bleaching treatment was conducted at 90 °C for 3 h using hy-drogen 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 (Section2.3.2) or TEMPO-mediated oxidation (Section2.3.3) before to be submitted to the phy-sical treatment (Section2.3.4)

2.3.2 Acid hydrolysis The insoluble material was added in a sulfuric acid solution (30 % v/v) for 90 min at 60 °C Subsequently, this mixture was cooled to 40 °C and subjected to four washes with distilled water Thereafter, the sus-pension 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 ofSaito, Kimura, Nishiyama, and Isogai (2007)with some adaptations The in-soluble material (17.07 g corresponding to 1.0 g cellulose, cellulose content determined according toSun, 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) Sub-sequently, the insoluble material was washed with distilled water 2.3.4 Physical treatment

Half of the suspensions obtained by acid hydrolysis and TEMPO-mediated 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

2.4 Cellulose nanofibers (CNFs) characterization Morphology of cellulose nanofibers were evaluated by transmission electron microscopy (TEM) TEM images were captured with a TEM-MSC (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 measure-ments of length were made in AFM images AFM images were acquired

on a Microscope Park Systems, model NX-10 (Suwon, Korea) equipped

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

ob-tain 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 toMandal

and Chakrabarty (2011) This ratio was calculated using the peak

in-tensity at 21.7° over the peak inin-tensity at 20° Functional groups were

analyzed by Fourier transform infrared (FTIR) spectroscopy

accom-plished on the Fourier transform infrared spectrometer (JASCO

FTIR-6100, 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

sus-pensions were allowed to rest for 3 min 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 0 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 andflow 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 1 Hz

were performed to define the linear viscoelastic range After that,

fre-quency 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 5 %

3 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 inFig 1and

Table 1summarizes the properties of the cellulose nanofibers obtained

from cassava peels after acid hydrolysis and TEMPO-mediated

oxida-tion with and without sonicaoxida-tion Nanofibers obtained by

TEMPO-mediated 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

distribu-tion in the range of 1500−2500 nm These lengths were slightly higher

to those found byTibolla, 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 byLeite 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 cen-trifugation 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 TEMPO-mediated oxidation This effect was also observed byKhawas and Deka (2016)isolating cellulose nanofibers from banana peel by acid hydro-lysis 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 mole-cules that cause the formation and expansion of microscopic gas bub-bles With the collapse of these bubbles there is local production of heat and high pressure These effects facilitate the isolation of nanomater-ials, as they break the structural micron-sizedfibril into submicron fi-brils and then at the nanoscale, producing nanofibers of a more homogeneous size (Abdul Khalil et al., 2014;Huerta, Silva, Ekaette, El-Bialy, & 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 so-nication process, although the method of producing CNFs had an in-fluence 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 fi-bers, 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 8 % after acid hydrolysis, while after catalytic oxida-tion only 2 % 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 trans-forms the surface hydroxyls into carboxylate groups, without inter-fering 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 por-tions of the material (Alemdar & Sain, 2008) Our results of crystallinity index were similar to those found byKhawas and Deka (2016), which observed values between 30.5–63.64 % for cellulose nanofibers ex-tracted 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 prop-erties 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 ana-lyses were obtained (Fig 2) The FTIR spectra obtained for the cellulose

nanofibers, cassava peel and pre-treated material exhibited a wide band

in the region of 3500 cm−1at 3200 cm−1corresponding to the free

vibration of the OHe stretches of the OH groups of the cellulose

mo-lecules 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−1that is associated with COe elongation, characteristic of the

presence of cellulose

The FTIR absorption peak at 1430 cm−1corresponds to the

vibra-tion of the CH2bonds, attributed to the cellulose "crystallinity band"

The band at 890 cm−1is attributed to the C-O-C stretching vibration of

β-cell (1 → 4) glycosidic bonds, which is considered to be an

"amor-phous 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−1and 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

3.2 Microscopy observations

Fig 3 shows the morphological characteristics of CNFs TEM

pictures revealedfibers with a wide size distribution both in diameter and in length Sonicated nanofibers were more separated than nanofi-bers without this process showing somefibers with intact bundles In addition, acid hydrolysis produced more dispersed (separated) nano fi-bers than the samples obtained by TEMPO-mediated oxidation Furthermore, it is possible to observe the presence of cellulose na-nospheres after the ultrasound process As cellulose hydrolysis gen-erally begins in the superficial amorphous region and subsequently

Fig 1 Length distribution of CNFs produced from different chemical and physical treatments

Table 1

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

a,b,c,Different superscripts letters in the same column indicate a statistically significant difference (p < 0.05)

* Cellulose I in relation cellulose II

Fig 2 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

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

pe-netrate 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

dif-ferent methods are illustrated inFig 4 All CNF suspensions exhibited

gel-like properties with G’ > G’’, but nanofibers treated by acid

hy-drolysis 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

hy-drolysis A higher negative charge favors repulsive forces between the

nanofibers, causing a better dispersion to entrap water molecules in the

Fig 3 Transmission electron microscopy (TEM) images for CNFs (scale bar =100 nm)

Fig 4 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 hydro-lysis without ( ) and with ( ) sonication; TEMPO-mediated oxidation without ( ) and with (♦) sonication Filled symbols correspond to G’ and open symbols correspond to G’’

vicinity of thefibers and increase the elastic character of the

suspen-sions (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

as-pect ratio tend toflocculate or form interlacings/entanglements among

them These entanglements between thefibers cause a certain

restric-tion 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,

facil-itating the physical interactions between nanofibers, promoting

en-tanglements between them and increasing their gel strength

3.3.2 Flow properties

Figs 5 and 6show theflow curves and viscosity behavior,

respec-tively, of the different nanofibers suspensions obtained by acid

hydro-lysis 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 thefirst and the third curve (steady

state) of shear stress-shear rate (Fig 5) and the results are shown in

Fig 7 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 exhibitflocculated,

entangled or alignedfibers Due to the presence of the interweaving of

thefibers, they present greater restriction to the alignment with the

flow and, consequently, show higher viscosity (Fig 6) As shear

in-creases, these entanglements are broken This structural change causes

thefibers to separate, facilitating their alignment with the flow, which

causes a decrease in viscosity (Fig 6) This behavior can be observed

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 thesefibers and consequently increases the interaction between the nanofibers and the degree of thixotropy (Barnes, 1997) Although the viscosity of CNFs suspensions obtained by acid hy-drolysis decreased to shear rate up to 20 s−1, the viscosity became al-most constant, forming a Newtonian plateau, between 20 s−1and 40

s−1(Fig 6) and the viscosity decreased again (shear-thinning beha-viour) 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 hy-drolysis 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 HoweverKarppinen et al (2012), working with suspensions of microfibrillated cellulose and using images captured in a transparent outer cylinder in concentric cylinders, verified a shear-in-duced 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 char-acteristics 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

Fig 5 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)

also smaller aspect ratio.

Fig 7shows the dependence of the storage moduli, viscosity and

degree of thixotropy with the increase in the concentration of

nanofi-bers The increase in rheological properties was also related to the

entanglement between nanofibers, as higher concentrations of nanofi-bers increase the interaction between them and form stronger network structures (Iotti et al., 2011) As observed in Figs 5 and 6, higher pseudoplasticity was observed in nanofibers treated with acid

Fig 6 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)

Fig 7 Degree of thixotropy, apparent visc-osity at 100 s−1(ƞ) and storage moduli (G’) at

1 Hz of the suspensions of 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 at

different concentrations (n = 3)

hydrolysis and after ultrasound treatment, demonstrating again that

these systems showed more complex rheological behavior and,

conse-quently, network structure

4 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

hy-drolysis and TEMPO-mediated oxidation, were efficient for the isolation

of cellulose nanofibers with diameters between 5 and 16 nm Acid

treatment was more efficient to introduce negative charge on surface

and to produce nanofibers with greater aspect ratios These

character-istics facilitated the formation of entanglements between the

nanofi-bers, increasing the complexity of the structural behavior and,

conse-quently, 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

strength-ening 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 financed in part by the Coordenação de

Aperfeiçoamento 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

au-thors 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

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