Stereotactic aspiration in the management of a rare case of tuberculous cerebellar abscess

Sunflower oil cake-derived cellulose nanocrystals: Extraction, physico-chemical characteristics and potential application

Zineb Kassab, Mounir El Achaby, Youssef Tamraoui, Houssine Sehaqui, Rachid Bouhfid, Abou El Kacem Qaiss

PII: S0141-8130(19)33005-3

DOI: https://doi.org/10.1016/j.ijbiomac.2019.06.049

Reference: BIOMAC 12555

To appear in: International Journal of Biological Macromolecules Received date: 23 April 2019

Revised date: 2 June 2019 Accepted date: 9 June 2019

Please cite this article as: Z. Kassab, M. El Achaby, Y. Tamraoui, et al., Sunflower oil cake- derived cellulose nanocrystals: Extraction, physico-chemical characteristics and potential application, International Journal of Biological Macromolecules, https://doi.org/10.1016/

j.ijbiomac.2019.06.049

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1

Sunflower oil cake-derived cellulose nanocrystals: Extraction, physico-chemical characteristics and potential application

Zineb Kassab ¹ , Mounir El Achaby ^{1, *} , Youssef Tamraoui ¹ , Houssine Sehaqui ¹ , Rachid Bouhfid ² , Abou El Kacem Qaiss ²

1

Materials Science and Nanoengineering Department (MSN), Mohammed VI Polytechnic University (UM6P), Lot 660 – Hay Moulay Rachid, 43150, Benguerir, Morocco

2

Moroccan Foundation for Advanced Science, Innovation and Research (MAScIR), Institute of Nanomaterials and Nanotechnology (NANOTECH), Laboratory of Polymer Processing, Rabat, Morocco

*Corresponding author: Email: mounir.elachaby@um6p.ma, +212 6 620 10 620 (M. El Achaby)

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2 Abstract

In this work, sunflower oil cake (SOC) was identified as bio-sourced material for cellulose nanocrystals (CNC) production using chemical treatments followed by sulfuric acid hydrolysis.

The hydrolysis was performed at 64 % acid concentration, a temperature of 50 °C and at two different hydrolysis times, 15 min (CNC

) and 30 min (CNC

). It was found that CNC exhibited a diameter of 9 ± 3 nm and 5 ± 2 nm, a length of 354 ± 101 nm and 329 ± 98 nm, a crystallinity of 75 % and 87 %, a surface charge density of ~ 1.57 and ~ 1.88 sulfate groups per 100 anhydroglucose units and a zeta potential value of – 25.6 and – 30.7 mV, for CNC

and CNC

, respectively. The thermal degradation under nitrogen atmosphere started at 225 °C (CNC

), which is relatively higher than the temperature for sulfuric acid hydrolyzed CNC from other sources. Due to a high importance of CNC application in aqueous systems, the rheological behaviour of CNC suspensions at various concentrations was evaluated by the steady shear viscosity measurements and the oscillatory dynamic tests. The results showed that the CNC suspensions exhibited a gel-like behaviour at very low CNC concentrations (0.1-1 %) wherein a strong CNC entangled network is formed. Polymer nanoreinforcing capability of the newly produced CNC was also investigated in this study. CNC filled PVA nanocomposite films were produced at various CNC contents (1, 3, 5 and 8 wt%) and their mechanical and transparency properties were investigated, resulting in transparent nanocomposite materials with strong mechanical properties. The study suggested other possibilities to utilize agricultural wastes from SOC for CNC production with potential application as reinforcement in polymer nanocomposites .

Keywords: Sunflower oil cake; Cellulose nanocrystals; Polymer nanocomposites

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3 1. Introduction

Among various natural materials, cellulose is considered the most abundantly available on earth, and has unique properties such as renewability, biodegradability, lightweight and good mechanical properties [1,2]. The main sources for cellulose extraction are cotton and wood, but it can also be derived from a variety of sources, such as marine biomass, marine animal tunicate, invertebrates, bacteria, fungi, annual plants, and other agro-industrial wastes [1,3,4] In its native form, cellulose exhibits a semi-crystalline structure with morphology and dimensions depending on the original source and the local environment [1,2]. The industrial exploitation of cellulose material has been known since the beginning of civilization, and various applications were developed, such as fibrous material in pulp, paper, composites, textiles and food industry [1,2]. Over recent decades, cellulose has certainly attracted more and more attention world- wide due to the possibility of converting it into cellulose nanomaterials with novel properties not present in bulk materials. After production and purification of cellulose fibers from lignocellulosic biomass and agricultural wastes, cellulose nanomaterials, such as cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC), can be extracted by mechanically- or chemically-induced top-down deconstructing strategies [5].

CNC can be produced in two morphological shapes, rod-like or needle-like shape, having

a compact and ordered cellulose structure, which gives it unique properties [6]. In general, the

dimensions of CNC depend on the origin of cellulose and hydrolysis conditions [2]. The

diameter of CNC is generally of the order of few nanometers and the average length is of the

order of few hundred nanometers [5]. In general, CNC are produced by an acid hydrolysis

process, where the cellulose microfibers (CMF) are subjected to concentrated acid to

hydrolyze the amorphous parts of the cellulosic chains and leave the crystalline parts intact

[7]. CNC have been produced using various types of acid treatments, such as hydrochloric,

sulfuric, and phosphoric acids, with each treatment bestowing specific functional groups on

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4 the surface of the extracted CNC that impacts their colloidal stability and their surface functionality [8]. Regarding to this, the sulfuric acid hydrolysis represents the most effective process for obtaining CNC and the largely used for this purpose, because it produces sulphated CNC with high crystallinity and good colloidal water stability [7]. Recently, CNC have garnered tremendous level of attention from both academic and industrial researchers, due to their excellent inherent physical and chemical properties such as high tensile strength (7.5 GPa) and elastic modulus (approximately 100–140 GPa), high specific surface area (∼250–500 m

/g), low density (1.6 g/cm

), biodegradability, renewability and reactive surfaces [9].

At present, the most promising application of CNC is as nanoreinforcing agent for

advanced polymer nanocomposites development. It has been reported that the incorporation

of CNC in polymeric matrices resulted in polymer-based nanocomposites with advanced

properties and functional applications [10]. In this context, polyvinyl alcohol (PVA) has been

widely used as a polymeric matrix for developing CNC-filled PVA nanocomposite materials

with improved properties [6,11–16]. The reinforcement effect of the incorporation of CNC

into PVA polymer is strongly related to the CNC-PVA interfacial interaction and the

dispersion state of CNC within PVA polymer [6,12,16]. Generally, weak interfacial

interactions (physically non-covalent interaction) could occur when CNC nanofiller is added

into PVA without surface functionalization or chemical crosslinking, resulting in a weak

reinforcement effect of the final properties of PVA nanocomposites [16]. It is well-known that

acid hydrolyzed-CNC exhibit a functionalized surface, ensuring a good ability to interact with

hydroxyl groups of PVA polymer, and an acceptable reinforcement effect could be achieved

[6,12]. However, it has been reported that a high reinforcement effect could be developed by

chemical cross-linking of PVA and/or functionalization of CNC’s surface, resulting in

improved chemically covalent interaction between CNC and PVA, thus CNC-reinforced PVA

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5 nanocomposites with significant improvement of the final properties could be produced [11,16].

Starting from nature, various biosourced material, lignocellulosic biomass or agricultural waste, were identified and studied as viable and sustainable sources for purified cellulose fibers production and acid hydrolysed CNC extraction [17]. In this investigation, sunflower oil cakes (SOC) waste was identified as an abundant cellulose-rich material for CMF and CNC production. SOC is a waste of vegetable oil extraction, and is composed mainly of cellulose, lignin, hemicellulose and protein [18]. The worldwide production of SOC waste was estimated at about 17.9 million tons [19]. Currently, the SOC waste is classically used as a protein source, complement in animal feed, combustible source and bio-fuels production [18–

20]. Based on these data, it is very interesting to extend the use of SOC waste for production of high-added value materials, i.e. cellulose derivatives (CMF and CNC).

The aim of this work is the extraction of purified cellulose microfibers (CMF) and

cellulose nanocrystals (CNC) from sunflower oil cakes (SOC) waste that, to the best of

authors’ knowledge, has not been reported yet. The conventional processes for CMF and CNC

extraction from lignocellulosic biomass were applied (alkali, bleaching and sulfuric acid

treatments) and the properties of the as-extracted CMFs and CNC were evaluated and

compared to properties of CMF and CNC extracted from other sources. Various

characterisation techniques were used to characterize the materials at different stages of

treatments, including confocal laser scanning microscopy (CLSM), X-Ray diffraction (XRD),

thermogravimetric analysis (TGA), elemental analysis (CHNOS), Fourier transform infrared

spectroscopy (FTIR), zeta potential measurement, atomic force microscopy (AFM),

transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The

rheological behaviour of CNC suspensions, at different concentrations, was also evaluated

using steady shear viscosity measurements and oscillatory dynamic tests. New polymer

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6 nanocomposite materials filled with SOC waste-derived CNC were produced, and the polymer nanoreinforcing ability of the as-produced CNC was evaluated. For that, various CNC contents (1, 3, 5 and 8 wt%) were incorporated into PVA matrix, and the resulting CNC/PVA nanocomposite materials were characterized in terms of their mechanical and transparency properties.

2. Materials and experimental detail

2.1 Materials

The sunflower oil cake (SOC) waste used in this study was obtained from a farm located in Languedoc-Roussillon region, France. The as-obtained SOC waste was firstly ground using a precision grinder (Retch SM100) equipped with a 1 mm sieve. PVA polymer (Mw 31,000–50,000) and all analytical grade chemicals used for the treatment of raw fibers and the extraction of cellulosic materials were purchased from Sigma–Aldrich.

2.2 Preparation of pure cellulose microfibers (CMF)

Pure cellulose microfibers (CMF) were prepared from raw SOC waste using the same processes described in our previous works [6,7,21]. Briefly, ground raw SOC fibers (2 mm) (Fig. 1a) were treated in distilled water for 1 h at 60 °C. Then, the resulted SOC fibers were treated three times with 4 wt% NaOH solution at 80 °C for 2 h under stirring, resulting in alkali treated SOC fibers, after which a bleaching treatment was carried out using a solution made up of equal parts (v:v) of acetate buff er (27 g NaOH and 75 mL glacial acetic acid, diluted to 1 L of distilled water) and aqueous sodium chlorite (1.7 wt% NaClO

in water).

This treatment was done three times, resulting in pure white colored CMF. In these treatments

the ratio of the fibers to liquor was 1/20 (g/mL). A photograph of the obtained CMF material

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7 is shown in Fig. 1b and the morphology and dimensions were determined by confocal laser scanning microscopy.

2.2 Preparation of cellulose nanocrystals (CNC)

The pre-prepared CMF material was subjected to sulfuric acid hydrolysis to produce CNC in suspension form. The acid hydrolysis process was performed with preheated 64 % sulfuric acid solution at 50 °C under mechanical stirring, according to our previous works reported elsewhere [6,7,21]. Two different hydrolysis times (15 and 30 min) were used to evaluate the effect of hydrolysis time on CNC’s properties. The hydrolysis mixture was diluted with ice cubes to stop the reaction and was washed by successive centrifugations at 12000 rpm at 15 °C for 20 min at each step, resulting in white CNC suspension, which was subjected to a dialysis against distillated water until it reached neutral pH. Afterward, the obtained CNC aqueous suspension was homogenized by using a probe type ultrasonic homogenizer (BRANSON, Sonifier 250) for 5 min in an ice bath, resulting in a white stable CNC gel suspension. Photographs of the obtained CNC

(CNC extracted at 15 min) and CNC

(CNC extracted at 30 min) in gel-like suspensions and freeze-dried foams are shown in Fig. 1c-f.

2.3 Preparation of PVA-CNC nanocomposite materials

PVA-based nanocomposite films containing 1, 3, 5 and 8 wt% CNC

were prepared using solvent casting method (only CNC

were used for nanocomposites development).

Briefly, 60 ml of CNC aqueous suspension containing the desired amount of CNC

was

prepared by dilution of a concentrated suspension (3.77 mg/ml). Then, the CNC

suspension

was sonicated for 1 min and then mechanically stirred for 5 min at 90 °C (using preheated oil

bath), after which the desired amount of PVA powder (99, 97, 95 and 92 wt% regarding to the

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8 total weight of the dried nanocomposite films) was slowly added to the CNC

suspension and the mixture was mechanically stirred for 1 h at 90 °C. Then, the solutions were kept under agitation until reaching room temperature. The resulting mixture was cast on rectangular Petri dishes (120*120 mm²) and air-dried for 48 h at room temperature to evaporate water. For comparison, neat PVA film control was also prepared by the same procedure without the use of CNC. The obtained films were coded as PVA, PVA-1%, PVA-3%, PVA-5% and PVA-8%

for neat PVA and PVA-CNC

nanocomposites containing 1, 3, 5 and 8 wt% CNC

, respectively.

Fig.1: Digital photographs of (a) raw-SOC, (b) extracted CMF and gel-like aqueous suspensions and lightweight foam-like structures of (c,d) CNC

and (e,f) CNC

2.4 Characterization techniques

(a) (b)

(c) (d)

(e) (f)

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9 Zeta potential measurement of CNC suspensions (CNC

and CNC

) was acquired using a Malvern Zeta sizer Nano ZS instrument. For this measurement, a capillary cell was used, and the CNC suspensions were diluted and sonicated for 5 min before being analyzed.

The morphology and dimensions of the as-produced CNC (CNC

and CNC

) were determined using transmission electron microscopy (TEM) (Philips CM200 microscope) and atomic force microscopy (AFM) (BioScope Catalyst AFM, Bruker AXS, Santa Barbara, CA).

For TEM analysis, drops of CNC

and CNC

diluted suspensions were deposited on the

surface of glow-discharged carbon-coated grids. To enhance the contrast, the samples were

negatively stained in a 2 wt% solution of uranyl acetate. The samples were dried at ambient

temperature before TEM analysis and the measurement was carried out with an accelerating

voltage of 80 kV. For AFM measurement, samples were prepared by depositing droplets of

diluted CNC

and CNC

suspensions onto freshly cleaved mica sheets, after being sonicated

for 5 min, and allowing the solvent to dry in air. The measurements were conducted in

tapping mode under ambient temperature at a scan rate of 1.5 Hz. The XPS analysis of freeze-

dried CNC

and CNC

was conducted on an Escalab 250 apparatus (Thermo Electron)

equipped with a monochromatic Al Kα radiation at 1486.6 eV. The area of analysis was 400

μm in diameter. The spectra were modified by setting the C-C contribution in the C1 s

emission to 285.0 eV. Fourier Transform Infrared Spectroscopy (FTIR) of all studied samples

was measured on a Tensor27 apparatus. The experiments were recorded in transmittance

mode in the range of 4000–400 cm

with a resolution of 4 cm

and an accumulation of 16

scans. Thermogravimetric Analysis (TGA) (Mettler Toledo) for all samples was conducted

under a nitrogen atmosphere between 25 and 700 °C, at a heating rate of 10 °C/min. X-ray

diffraction (XRD) characterizations of all cellulosic samples were performed on a Bruker

diffractometer D8 Advance using Cu-Kα radiation. The diffraction patterns were obtained at

diffraction angles between 5 and 35 °, at room temperature.

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10 Ultraviolet–visible (UV–Vis) spectroscopy of the PVA-CNC nanocomposite films was carried out using a Lambda 950 spectrophotometer. The film samples were placed directly in the spectrophotometer test cell, and the air was used as a reference. The optical transmittance of films was measured in the wavelength range of 200–800 nm. Tensile tests were performed using a texture analyzer (TA.XT plus). The tensile specimens were cut in a rectangular shape with dimensions of 80 mm in length and10 mm in width. The gauge length was fixed at 30 mm and the speed of the moving clamp was 5 mm/min. All tests were performed on a minimum of five samples and the reported results were averaged.

3. Results and Discussions

3.1 Processing and physical aspects of cellulosic materials

The raw SOC waste (Fig. 1a) was subjected to alkali and bleaching treatments, resulting in the production of CMF material with high quality and white color, as shown in Fig. 1b.

Generally, the alkali and bleaching treatments are well-known and conventional processes

applied to remove non-cellulosic compounds (lignin, hemicellulose, protein and other

impurities) from raw finely-ground lignocellulosic materials, leading to the extraction of pure

cellulosic fibers [1,11,17]. Once CMF are produced, sulfuric acid hydrolysis was applied to

solubilize the amorphous parts of CMF, leaving the crystalline parts unaltered, which are

identified as CNC. The as-isolated CNC (CNC

and CNC

) showed a gel-like suspension

after ultrasonic homogenization process, as illustrated in Fig. 1c,e, which is a typical behavior

of sulfuric acid hydrolyzed CNC [7]. Usually, CNC produced with sulfuric acid can be

completely suspended at the individual nanocrystal level in aqueous solution by electrostatic

repulsion, which are due to sulfate groups negative charges, introduced on the surface of CNC

during sulfuric acid hydrolysis [22–25]. The colloidal stability of CNC aqueous suspension,

which can be deduced from zeta potential measurements, is crucial for nanocomposite

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11 development using water-soluble polymers as matrices [26]. Herein, the produced CNC suspensions showed zeta potential values of – 25.6 and – 30.7 mV for CNC

and CNC

, respectively. It was reported that CNC suspension could be considered stable when the absolute value of zeta potential is superior to 25 mV [27]. From this finding, it should be noted that the increase of hydrolysis time resulted in CNC suspensions with high negative zeta potential. Furthermore, higher negative zeta potential values can indicate the presence of a higher number of sulfate groups inserted on the surface of CNC, as will be further shown by XPS analysis (see below).

Fig.2: CLSM images of (a,b) Raw SOC and (c,d) extracted CMF

3.2 Morphologic analysis and dimensionality

Fig. 2 illustrates the CLSM images obtained for raw finely-ground SOC and the extracted CMF. As can be seen, raw SOC exhibits rough surface and wrapped fiber bundles with relatively large dimensions (Fig. 2a,b), because the raw SOC is composed of various compounds (lignin, hemicellulose, cellulose, protein and other impurities) organized in a complex structure [18]. On the other hand, CLSM observation revealed individual CMF with 1

2

(a) (b)

(c) (d)

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12 a homogeneous distribution of length and diameter (Fig. 2c,d), suggesting that the alkali and bleaching treatments were suitable for removal of all non-cellulosic compounds, leaving individual microsized cellulose fibers (CMF) [23,28]. The average diameter and length of CMF were determined by analyzing the obtained micrographs using ImageJ software. It was found that the diameter and length are about 20 ± 6 µm and 226 ± 34 µm, respectively.

Comparatively, the measured diameter of CMF from SOC is comparable to that obtained for CMF extracted from vine shoots waste (≈ 25 µm) [21], and higher than that obtained for CMF from sugarcane bagasse (5-10 µm) [29], using the same bleaching treatment. Importantly, these SOC-derived CMF can be used as mechanical reinforcing fillers for polymer composites development using melt processing techniques, such as melt extrusion process. In this investigation, it was found that the extracted CMF are suitable for acid hydrolysis process to produce CNC.

Fig.3: AFM and TEM images of (a,b) CNC

and (c,d) CNC

Due to their nanosized dimensions, the extracted CNC (CNC

and CNC

) were visualized by AFM and TEM observations and their shapes and dimensions were determined.

Fig. 3 illustrates the obtained AFM and TEM micrographs for both extracted CNC,

(a)

(b)

(c)

(d)

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13 confirming their successful extraction from SOC-derived CMF via sulfuric acid hydrolysis process. Both CNC (CNC

and CNC

) exhibited needle-like shapes with good dispersion on the substrate, due to the electrostatic repulsion between individual CNC generated from the negatively charged CNC surfaces [30]. The average diameter and length were measured at 9

± 3 nm and 354 ± 101 nm for CNC extracted at 15 minutes (CNC15), and 5 ± 2 nm and 329 ± 98 nm for CNC extracted at 30 minutes (CNC

), respectively. This finding about CNC dimensions confirmed that the hydrolysis time has a direct effect on the final geometric dimensions of CNC. Additionally, these observations confirmed that a hydrolysis time of 15 min was efficient to extract CNC from SOC-derived cellulose, which is very low than that used for CNC extraction from other sources, using sulfuric acid hydrolysis [23,30,31].

Fig. 4: (a,b) XPS survey spectrum and (c,d) high-resolution of S2p photoelectron spectrum of CNC

and CNC

samples.

3.3. Structure and surface functionality

1200 1000 800 600 400 200 0

0 1x10⁵ 2x10⁵ 3x10⁵ 4x10⁵

Acount / s

Binding energy (eV) CNC₁₅

(a)

1200 1000 800 600 400 200 0

0 1x10⁵ 2x10⁵ 3x10⁵ 4x10⁵

Acount / s

Binding energy (eV)

CNC30 (b)

180 178 176 174 172 170 168 166 164 162 160 158 4x10³

5x10³ 6x10³ 7x10³ 8x10³ 9x10³

Acount / s

Binding energy (eV) CNC15

(c)

178 176 174 172 170 168 166 164 162 4x10³

5x10³ 6x10³ 7x10³ 8x10³ 9x10³

Acount / s

Binding energy (eV) CNC30 (d)

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14 The chemical composition and the number of sulfate groups grafted on the surface of CNC were evaluated by XPS measurement. Fig. 4 shows the XPS survey spectrum of the extracted CNC (CNC

and CNC

). Both CNC exhibit two main peaks observed at around 531 and 285 eV in the survey spectra, which correspond to the O 1s and C 1s, respectively [32]. Furthermore, from a high-resolution of S2p photoelectron spectrums of CNC

and CNC

shown in Fig 4, a small S2p peak is observed at around 167 eV, which is attributed to surface sulfation caused by the sulfuric acid hydrolysis [7]. The surface O/C ratio was determined at 0.78 and 0.70 for CNC

and CNC

, respectively. The obtained values of O/C ratio are comparable with those reported for sulfuric acid hydrolysed CNC [7,33], and smaller than the theoretical value of untreated pure cellulose [32].

The atomic percentage of S element was determined at 0.31 % and 0.37 % for CNC

and CNC

, respectively. These values correspond to ~ 1.57 and ~ 1.88 sulfate groups per 100 anhydroglucose units for CNC15 and CNC30, respectively, based on the calculation procedure reported elsewhere [34]. It is generally known that the amount of sulfate groups on the surface of the CNC depend on the sulfuric acid concentration and the hydrolysis time [34].

Herein, the obtained amount of sulphate groups is comparable to sulfuric acid hydrolyzed CNC extracted from microfibrillated cellulose found by Li et al. [34].

3.4 Crystalline structure and crystallinity

XRD patterns obtained for different studied cellulosic materials are shown in Fig. 5. All

samples exhibited three major diffraction peaks at around 15.1°, 16.6° and 22.5°,

corresponding to crystallographic planes 11̅0, 110 and 200, respectively. From this result, it

is confirmed that all samples exhibited cellulose I structure, which is typically encountered for

biosourced cellulosic materials and their extracted pure cellulosic parts (cellulose fibers and

CNC), using conventional process, such bleaching and acid hydrolysis [23,30,31].

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15 The crystallinity index (CrI) of the raw SOC, CMF, CNC

and CNC

was determined at 49 %, 62 %, 75 % and 87 % , respectively, according to calculations using the following equation: 𝐶𝑟𝐼 = [

002

]× 100, where I

is the peak intensity of the (200) reflection and I

is the minimum intensity between the (110) and (200) peaks at about 18.7° [30,35].

From this result, it is confirmed that the CrI is increased after chemical treatment. Similar finding has been reported elsewhere for treatment of other biosourced materials [23,31]. The increasing of CrI after the bleaching treatment (from raw SOC to CMF) is attributed to the removal of non-cellulosic compounds, such as lignin and hemicellulose [30]. Furthermore, the CrI increased from 62 % for CMF sample to 75 and 87 % for CNC

and CNC

, respectively, attributed to the removal of amorphous domains in the cellulose chains without destruction of the crystalline domains, when CMF was subjected to the sulfuric acid hydrolysis [2,30].

Fig.5: XRD patterns of raw SOC, CMF, CNC

, CNC

3.5 Thermal degradation behaviour

10 20 30 40

110

CNC30

CNC15 CMF

In ten si ty (a. u )

2 Theta (°)

Raw SOC 200

110 -

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16 The thermal degradation behaviour of raw SOC, CMF, CNC

and CNC

was investigated by thermogravimetric analysis (TGA/DTG). The resulted TGA and DTG curves are illustrated in Fig. 6. All samples showed a small weight loss around 100 °C due to the evaporation of adsorbed moisture, which is related to the hydrophilic nature of the cellulosic materials [31]. The raw SOC sample showed a multistep degradative behaviour between 185 and 400 °C due to the presence of several components that are characterized by different degradation temperatures. In general, the thermal decomposition of raw lignocellulosic materials begins at a relatively lower temperature for hemicelluloses followed by an early stage of degradation of lignin molecules and then decomposition of cellulose [7,28,31]. The DTG maximum degradation peaks of these compounds are grouped into one broad peak (185- 400 °C) (Fig. 6b). The CMF sample showed only one-step degradation process at 230-400 °C range with a DTG maximum peak at 345 °C. This decomposition is due to degradation processes of cellulose, such as dehydration, decarboxylation, depolymerization and decomposition of glycosyl units [6,23]. This result confirmed the total removal of non- cellulosic compounds after the bleaching treatment, resulting in pure CMF with relatively high thermal stability [23,28].

Fig.6:(a) TGA and (b) DTG curves of raw SOC, CMF, CNC

andCNC

(under nitrogen atmosphere)

Raw SOC CMF

100 200 300 400 500 600 700

0 20 40 60 80 100

Weight (%)

Temperature (°C)

(a) CNC15

CNC30

100 200 300 400 500 600 700

DTG (%/°C)

Temperature (°C) Raw SOC CMF CNC15 CNC30 (b)

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17 The extracted CNC

and CNC

samples also showed one-step degradation process at low temperature (160 -300 °C) with a DTG maximum peak at 264 and 255 °C, respectively.

The lower degradation temperature of CNC is due to the presence of sulfate groups on their surfaces, having a catalytic effect in the thermal decomposition mechanism, which results in reduced thermal stability for sulfuric acid hydrolyzed CNC [23,31,36]. It is noteworthy that the CNC extracted at 30 min (CNC

) showed a reduced thermal stability regarding CNC extracted at 15 min (CNC

). This finding can be explained by the content of sulfate groups present on the surface of CNC

and CNC

. As stated above, the sulfate groups content for CNC

and CNC

was measured from XPS analysis at 1.57 and 1.88 sulfate groups per 100 anhydroglucose units, respectively, indicating that the sulfate content is increased with increasing of the hydrolysis time. From this result, it can be concluded that the catalytic effect is more pronounced in CNC containing a high content of sulfate groups (CNC

), in comparison to those containing a low content of sulfate groups (CNC

), resulting in reduced thermal stability for CNC extracted at relatively high hydrolysis time (CNC

).

3.6 Rheological properties

The rheological properties of CNC in aqueous solutions are very important for their preparation, processing and combination with other materials [3]. Herein, only CNC

suspensions were selected to evaluate the rheological properties, due to their high crystallinity

and relatively high aspect ratio. Fig. 7 depicts the shear viscosity curves within the entire

shear rate range (0.01–100s

) of CNC

aqueous suspensions at different concentrations (0.1-

1%). Obviously, the viscosity was gradually increased with increasing of CNC

concentration from 0.1 to 1%, which is a typical behaviour for stable aqueous suspensions of

rigid nanoparticles [37,38]. All CNC

concentrations showed typical shear-thinning

behaviour (pseudo-plastic phenomenon), as confirmed by the viscosity decrease with the

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18 increase in the shear rate over the entire range. It may be due to the gelation of CNC

at very low concentrations (0.1 to 1%), which could be generated from strong interconnection degree and collision possibility of the CNC

in their gelled form. It is noted that our observation is very different from that typically observed for CNC aqueous suspensions extracted from other sources at different extraction conditions [34,37], in which the viscosity profile shows a three- region behaviour (i.e. Newtonian plateau at low shear rates followed by a shear thinning region at intermediate shear rates and another plateau at high shear rates). Following this, it was reported that CNC

suspensions are isotropic at low concentrations (up to 3 %), and phase separates to liquid crystalline and isotropic domains at higher concentration [37].

Herein, it was found that the extracted CNC

suspensions behave as rheological gels even at

low concentrations (from 0.1 to 10 %) with single shear thinning behaviour. This finding may

be related to the original bio-sourced material used in this study (raw SOC) and sulfuric acid

hydrolysis conditions as well as the relatively high CNC

aspect ratio (65.8). This

observation is similar to that reported in our previous work for CNC

extracted from

miscanthus fibers using the same sulfuric acid hydrolysis conditions [3]. Additionally, the

same trend was reported by Xu et al. [39] who observed a typical single-region shear thinning

behaviour for rod-like shaped CNC suspensions extracted from commercial microcrystalline

cellulose, but at relatively high concentrations (1 – 6 wt%).

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19 Fig.7: Shear viscosity vs. shear rate of CNC

aqueous suspensions at different

concentrations

The profiles of storage modulus (G’), loss modulus (G”) and tangent delta (Tanδ) of CNC

suspensions at different concentrations as a function of frequency are represented in Fig. 8. Clearly, the G’ and G” were increased with increasing CNC

concentration, indicating that a strong CNC

network is formed with increasing the concentration. Over the whole investigated range of frequency, the G’ is higher than its corresponding G” (Tanδ < 1) for all CNC

concentrations (0.1-1%), indicating that CNC

suspensions display typical gel-like properties due to the formation of strong network generated by electrostatic interactions, hydrogen bonds and Van Der Waals forces [40]. This behaviour is more pronounced for CNC

concentration higher than 0.2%, in which G’ and G” are almost independent of angular frequency (Fig. 8a,b). Remarkably, it should be noted that G’ and G” are almost coincident in a very wide range of frequency for CNC

concentrations of 0.1 and 0.2 %, indicating that the concentration of 0.2 % can be considered as a critical percolation for strong network gel formation. In fact, the CNC possess many hydroxyl groups on their surface. These highly active groups enable the CNC

to greatly interact with each other as well as with the adjacent

0.1% 0.5%

0.2% 0.6%

0.3% 0.7%

0.4% 1.0%

10^-2 10^-1 10⁰ 10¹ 10²

10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

Viscosity (Pa.s)

Shear rate (1/s)

ACCEPTED MANUSCRIPT

20 water molecules, thus displaying gel-like behaviour [3]. Similar trends were reported for CNC suspensions at relatively high concentrations [37,41].

Fig.8: (a) Storage modulus (G’), (b) Loss modulus (G’’) and (c) Tang delta (δ) Vs. Frequency of CNC

aqueous suspensions at different concentrations.

3.7 Processing and FTIR analysis of nanocomposite films

In order to explore the usefulness of the extracted CNC for polymer nanocomposites development, various CNC contents (1, 3, 5, and 8wt%) were incorporated into PVA polymer. Only CNC

were selected for developing PVA nanocomposites, because the CNC

showed high crystallinity (87%) and relatively high aspect ratio (65.8) with regard to CNC

(crystallinity of 75 % and aspect ratio of 39.3). It is expected that the CNC

have a high reinforcing ability for polymer nanocomposites development. On the other hand, the PVA polymer was chosen due to its good film-forming ability and its high compatibility with CNC.

0.1% 0.3% 0.5% 0.7%

0.2% 0.4% 0.6% 1 .0%

10^-2 10^-1 10⁰ 10¹

10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵

G' (Pa)

Frequency (Hz)

(a)

0.1% 0.3% 0.5% 0.7%

0.2% 0.4% 0.6% 1 .0%

10^-2 10^-1 10⁰ 10¹

10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵

G'' (Pa)

Frequency (Hz)

(b)

0.1% 0.3% 0.5% 0.7%

0.2% 0.4% 0.6% 1 .0%

10^-2 10^-1 10⁰ 10¹

0.25 0.50 0.75 1.00 1.25 1.50

Tang Delta

Frequency (Hz)

(c)

ACCEPTED MANUSCRIPT

21 It is worth noting that the interfacial interaction between the polymer and CNC is an important parameter to define the microstructure, CNC dispersion state and final properties of the developed nanocomposite material. In this case, both PVA and CNC possess abundant hydroxyl groups, which can be involved in the formation of hydrogen bonds between them, ensuring high interfacial interaction, and hence the good CNC dispersion [6].

Fig.9: FTIR spectra of PVA-based nanocomposite films at different CNC

loadings

FTIR analysis was carried out to evaluate the structure of CNC-filled PVA nanocomposite films, and the obtained spectra are illustrated in Fig 9. The neat PVA film shows a broad peak at 3286 cm

corresponding to stretching vibration of OH group, due to the strong intermolecular and intramolecular bonds. The peak observed at 2966 cm

is assigned to CH alkyl group stretching, and the peak at 1731 cm

is attributed to the stretching mode of C=O and C=C in the acetate group. The peak observed at 1683 cm

is due to the deformation mode of H-O-H group, and the peaks 1444 and 1357 cm

are assigned to the

4000 3500 3000 1500 1000

% T ran smi ttan ce (a. u )

Wavenumber (cm^-1) PVA-8%

PVA-5%

PVA-3%

PVA-1%

Neat PVA

ACCEPTED MANUSCRIPT

22 bending mode of CH group. The peak at 1114 cm

is representative of the bending mode of C-O in the acetate group, respectively [13,42,43].

For CNC-filled PVA nanocomposites, the incorporation of CNC leads to small variations on the FTIR spectra. A slight change in the intensity of the OH stretching vibration (3286 cm

) can be observed. This very slight change can be attributed to the interfacial interaction between the hydroxyl group of CNC and PVA matrix [43]. Additionally, the peak at 1114 cm

1

was split into two peaks at 1139 and 1058 cm

, corresponding to the asymmetric ring breathing mode of cellulose and to the C-OH bending vibrations of alcohol groups present in cellulose, respectively [13,44]. This effect is more evident at high CNC content (PVA-8%

sample). These very slight variations along with regular neat PVA peaks, confirmed the presence of CNC in PVA matrix and indicated that the incorporation of CNC did not influence the molecular and chemical structure of PVA. These findings are in accordance with the literature for PVA-based nanocomposite materials [6,42,45].

3.8 Tensile properties of nanocomposite films

The tensile properties of CNC-filled PVA nanocomposites at different CNC weight fractions were probed by uniaxial tests. The measured tensile modulus, tensile strength and strain at break are illustrated in Fig. 10. It can be seen that the modulus and strength of PVA polymer were gradually increased with the addition of CNC at different weight fractions. The tensile modulus and strength of neat PVA films were found to be 934 MPa and 37 MPa, respectively. After the addition of CNC, the modulus and strength improved to 1783 MPa and 67 MPa at CNC content of 5 wt%, and to 1940 MPa and 66 MPa at CNC content of 8 wt%.

These increases corresponded to a gain in the modulus and strength of 91 % and 81 % for 5 wt% CNC, and 107 and 78 % for 8 wt% CNC compared to the neat PVA matrix, respectively.

These findings confirm that the SOC-derived CNC have a high ability to improve the tensile

ACCEPTED MANUSCRIPT

23

properties of water-soluble polymers. From the results already published in the literature, the

source of CNC and their extraction conditions may have direct impact on the mechanical

properties of the nanocomposite. CNC isolated from different biosourced materials showed

different reinforcements for PVA polymer [6,11–15,46]. To demonstrate the polymer

reinforcing ability of SOC-derived CNC developed in this work, we performed a comparison

with CNC extracted from various sources, as shown in Table 1. From this comparison, the

CNC developed in this work have a greater ability to improve the tensile modulus and

strength of PVA than CNC from commercial microcrystalline cellulose, phormium tenax, flax

fibers, banana pseudo stems and sugarcane bagasse (Table 1). Fortunati et al. [13] reported

that 5 wt% CNC, extracted from microcrystalline cellulose, phormium tenax and flax fibers at

the same extraction conditions used in this work, resulted in only 14.2 % (microcrystalline

cellulose) and 44.2 % (flax fibers) increase of modulus and strength, respectively. El Miri et

al. [14] reported an increase of 77 % and 12 % of the modulus and strength with the addition

of 5 wt% CNC extracted from sugarcane bagasse at the almost the same extraction conditions

(64 % sulfuric acid, 55 °C, 30 min). Moreover, CNC extracted from cotton fibers at different

extraction condition showed a comparable increase of modulus (100 % at 6 wt% and 114.9 %

at 9 wt%), but only 13.4 % and 19.2 % increase for strength were obtained at these CNC

weight fractions [11]. However, the CNC prepared in this work provided less reinforcing

ability for PVA than that of CNC from corncob, oil palm pulp and red algae (Table 1), even

for the same extraction conditions. CNC from oil palm pulp showed an increase of 291 % and

122 % for the modulus and strength at only 4 wt % CNC [15], which are very large than those

obtained from CNC developed in this work. All these results confirm that the polymer

reinforcing ability of CNC is strongly related to the original source as well as the extraction

methods and experimental conditions. Furthermore, it is also strongly dependent on the CNC

characteristics, such as the aspect ratio, crystallinity and morphology [12,46]. Remarkably,

ACCEPTED MANUSCRIPT

24 based on the rheological properties of CNC aqueous suspensions (Fig. 7-8), it was observed that CNC form a strong network gel at concentrations higher than 0.2 %, which is considered as a critical percolation concentration. However, this behaviour is not observed in the CNC- filled PVA nanocomposite films, in which a remarkable improvement of mechanical properties was observed. One reason for not observing a critical percolation for improving mechanical properties may be the formation of interfacial interactions between the macromolecular chains of PVA polymer and CNC that reduce the interactions CNC-CNC, and since the PVA polymer is less rigid than the nanocrystals, the percolation effect was not observed.

Meanwhile, the maximum strain of neat PVA (90 %) was reduced to 64 % and 59 %,

when 5 and 8 wt% CNC were added, which corresponds to a decrease of 29 and 34 % in

comparison with neat PVA, respectively. This trend is due to the stiffening effect of rigid

nanoparticles, which leads to significant local stress concentrations and reduced strain to

failure [46,47]. This finding is in accordance with the literature for acid hydrolysed CNC-

filled PVA nanocomposite materials, in which the maximum strain was gradually reduced

with the addition of CNC at various weight contents [6,11,15].

ACCEPTED MANUSCRIPT

25 Fig.10: (a) Tensile modulus, (b) Tensile strength and (c) elongation at break of PVA-based

nanocomposite films at different CNC

loadings

Herein, we demonstrated that the newly extracted CNC from SOC waste have a superior reinforcing ability to improve the tensile properties of polymer-based nanocomposite materials. This improvement can be attributed to the high quality of CNC in terms of morphology and shape and their excellent characteristics, especially the aspect ratio (65.8) and the crystallinity (87 %). The functionalized surface of CNC (hydroxyl and sulfate groups) and its compatibility with PVA polymer, can result in uniform dispersion of CNC throughout the formation of hydrogen bonds between the macromolecular chains of PVA and the functionalized surface of CNC, thus resulting in efficient load transfer, and hence improving the tensile properties of PVA polymer. Such improvements confirmed that the as-prepared CNC have a great potential for developing nanostructured PVA-based composite materials

Neat PVA PVA-1%

PVA-3%

PVA-5%

PVA-8%

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Tensile Modulus (MPa)

(a)

Neat PVA PVA-1%

PVA-3%

PVA-5%

PVA-8%

0 10 20 30 40 50 60 70 80

Tensile Strenght (MPa)

(b)

PVA PVA-1%

PVA-3%

PVA-5%

PVA-8%

0 10 20 30 40 50 60 70 80 90 100

Elongation at break (%)

(c)

ACCEPTED MANUSCRIPT

26 with high strength and stiffness, which are the main properties required for advanced applications of nanocomposite materials.

Table 1 : Comparison of the reinforcing ability of CNC extracted from different sources with that of CNC

extracted from SOC in this work. This comparison is focused on the % increase of modulus and strength for PVA polymer.

Original source

CNC preparation

method

Weight fraction (wt%)

% increase of modulus

% increase

of strength ref Microcrystalline

cellulose

Sulfuric acid (64

%, 55 °C, 30

min) 5

0 44.2

[13]

Phormium

tenax - 4.7 30.0

Flax fibres 14.2 40.3

Banana pseudostems

Sulfuric acid (64

%, 45 °C, 70 min)

3 5

16.5 2.5

19.4

- 8.0 [12]

Sugarcane bagasse

Sulfuric acid (64

%, 55 °C, 30 min)

5 77 12 [14]

Corncob

Sulfuric acid (9.17 M, 45 °C,

60 min)

6 9

--- ---

95.6

140.2 [46]

Oil palm pulp

TEMPO- mediated oxidation

4 291 122 [15]

Red algae

Sulfuric acid (64

%, 50 °C, 30 min)

5 8

141.9 214

118.6

150.4 [6]

Cotton fibers

Hydrochloric acid (37 %, 45 °C, 60

min)

6 9

94.5 114.9

11.8

16.6 [11]

SOC

Sulfuric acid (64

%, 50 °C, 30 min)

5 8

91 107

81

78 This work

3.9 Transparency of nanocomposite films

The transparency properties from the transmittance UV–Vis measurements of CNC-filled

PVA nanocomposites at different CNC weight fractions are shown in Fig. 11. Neat PVA is a

transparent polymer having a transmittance of 91.4 % at a visible wavelength of 650 nm.

ACCEPTED MANUSCRIPT

27 However, the addition of CNC from 1 to 8 wt% did not largely influence the transparency level detected at 650 nm for neat PVA, may be this finding is due to the nanoscaled dispersion of CNC within nanocomposite systems. All the prepared CNC-filled PVA nanocomposite films maintained a good transparency level (79.4 – 90.3 % at 650 nm), without a significant reduction in the amount of light being transmitted. This is a good indication on the absence of CNC aggregates and on the good compatibility between CNC and PVA, resulting in structured nanocomposite films with high transparency level [6,13,28].

Fig. 11: UV–Vis transmittance of neat PVA and its nanocomposite films at different CNC

loadings

4. Conclusion

Sunflower oil cake (SOC) was identified as novel cellulose-rich material to produce micro- and nano-sized cellulose (cellulose microfibers and cellulose nanocrystals). A combination of alkali and bleaching treatments resulted in pure cellulose microfibers (CMF) with average diameter and length of 20 µm and 226 µm, and crystallinity of 62 %. Due to its interesting properties, the obtained CMF can be used for various applications that involve the cellulose fibers in its microsized form. The hydrolysis of CMF with sulfuric acid (64 %)

300 400 500 600 700 800

0 20 40 60 80 100

T ran si mi ttan ce (%)

Wavelenght (nm) PVA PVA-1%

PVA-3%

PVA-5%

PVA-8%

ACCEPTED MANUSCRIPT

28 resulted in the production of cellulose nanocrystals (CNC) even at very low hydrolysis time (15 min and 30 min). The as-extracted CNC showed a diameter of 9 ± 3 nm and 5 ± 2 nm, a length of 354 ± 101 nm and 329 ± 98 nm, a crystallinity of 75 % and 87 %, a surface charge density of ~ 1.57and ~ 1.88 sulfate groups per 100 anhydroglucose units and a zeta potential value of – 25.6 and – 30.7 mV, for CNC extracted at 15 and 30 min, respectively. These measured properties are comparable with those obtained for CNC extracted from well-known cellulose-rich bio-sourced materials. The CNC obtained from SOC waste exhibit interesting characteristics that can be used in several applications with high added-value. The obtained CNC showed a high aqueous colloidal stability and formed a gel-like behaviour at very low CNC concentrations (confirmed by rheological measurements), which suggest their use in liquid media applications. Herein, the use of the extracted CNC as nanoreinforcing fillers in polymer nanocomposites was evaluated. The dispersion of 1, 3, 5 and 8 wt% CNC into PVA polymeric matrix, resulted in nanocomposite materials with largely improved tensile properties. An increase of 107 % and 78 % were reached for tensile modulus and tensile strength in the case of PVA-based nanocomposite films containing 8 wt% CNC.

Acknowledgement

The financial assistance of the Office Chérifien des Phosphates (OCP S.A.) in the

Moroccan Kingdom toward this research is hereby acknowledged.

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