The efficiency of cellulose carbonates to produce cellulose nanofibers

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The efficiency of cellulose carbonates to produce cellulose nanofibers Ramzi Khiari, Fleur Rol, Marie-Christine Brochier-Salon, Julien Bras, and Mohamed Naceur Belgacem ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06039 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Sustainable Chemistry & Engineering

The efficiency of cellulose carbonates to produce cellulose nanofibers Ramzi Khiari*,Ϯ, ‡, Ʌ, Fleur Rol Ʌ, Marie-Christine Brochier Salon Ʌ, Julien Bras Ʌ, Mohamed Naceur Belgacem*, Ʌ Ϯ

University of Monastir, Faculty of Sciences, UR13 ES 63 - Research Unity of Applied Chemistry & Environment, 5000 Monastir, Tunisia ‡

Higher Institute of Technological Studies of Ksar Hellal, Department of TextileTunisia Ʌ Univ.

Grenoble Alpes, CNRS, Grenoble INP, LGP2, F-38000 Grenoble, France

Corresponding authors: [email protected]

Keywords: Cellulose nanofibers (CNF), dimethyl carbonate, cellulose carbonate, green pretreatment, mechanical properties. Abstract: In this study, an innovative and green process to produce cellulose nanofibers (CNF) is proposed. CNF are usually produced uses via mechanical, enzymatic, and/or chemical treatment such as TEMPO-mediated oxidation of cellulose fibers, but for now this method involves high energy consumption, which limits the commercialization of the CNF. Moreover, an expensive effluent treatment system is required to complete the CNF manufacturing process. In this context, a 1 ACS Paragon Plus Environment

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novel process using a green method was developed to improve CNF production. The cellulose, sourced from eucalyptus, was modified by adding dimethyl carbonate (DMC), in ethanolic potassium hydroxide medium. The effect of reaction temperature was evaluated (4, 25, and 40°C) and the obtained cellulose carbonate was characterized by several techniques including

13C

Cross Polarized Magic Angle

Spinning Nuclear Magnetic Resonance (NMR) and X-ray Photoelectron Spectroscopy (XPS). After the chemical step, CNF was manufactured with a Supermasscolloider ultrafine friction grinder. The resultant CNF suspension was characterized in terms of fibrillation yield, transparency, rheological behavior, morphological features, and quality index. This novel chemical approach for the production of CNF seems to hold promises not only for its green features but also for its lesser and cleaner effluent discharge, and low cost of reagents.

Introduction Cellulose nanofibers (CNF) have gained increased attention since 2008 and are now among the European priorities for the bio-economy. Indeed, this bio-based, renewable, biodegradable, and biocompatible material presents many interesting properties and can be used in several applications such as packaging1, paper2, paints, building materials, printed electronics3, and medical applications.4 Industrial production of CNF is still low in volume due to the high energy consumption of the mechanical processes used to nanofibrillated cellulose pulp5,6. 2 ACS Paragon Plus Environment

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Several studies have been conducted on the raw materials as well as the chemical pretreatments and mechanical pretreatments to improve the production process. Traditionally CNF is produced via an enzymatic7–9 or chemical modification of cellulose fibers, as reported recently in a review10,followed by a mechanical treatment such as a refiner, grinder6,11–13, homogenizer14–18, microfluidizer19–21,or more recently, a twin screw extruder22–24. Although using a twin screw extruder allows a significant decrease in energy consumption, chemical pretreatment is still required. Moreover, due to chemical fiber modification, new functional groups are created on the fiber surface, which introduce new properties. This new functionalization of the cellulose fibers also offers the possibility to graft molecules or polymers, as reviewed by Missoum et al.25, further introducing other properties to the final material. Many studies have been carried out on cellulose fiber pretreatment for nanofibrillation and today up to 10 pretreatments are available, such as, TEMPO oxidation26–30,

cationization31–35,

phosphorylation36–38,

carboxymethylation39–42,

sulfoethylation43, or deep eutectic solvent44–46.Among all these pretreatments, enzymatic and TEMPO oxidation are the oldest and the best known. Used industrially today, they lead to CNF of good quality, but they present some drawbacks. Enzymatic hydrolysis allows a significant reduction in energy consumption but does not offer the possibility to make other modifications. Hence, new fiber modifications methods have been proposed in recent years. These pretreatments are very promising. For example, surface cationization of cellulose fibers using epoxy propyltrimethyl 3 ACS Paragon Plus Environment


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ammonium chloride (EPTMAC) leads to antimicrobial surfaces and allows a large decrease in energy consumption (from 11,000 to 3,000kWh/t) as reported by Saini et al.47. Periodate oxidation of cellulose fibers, leading to dialdehyde cellulose, offers a large variety of possibilities for grafting other molecules, as reported several times48– 52.

After ring opening using periodate oxidation, many molecules can interact and

modify the properties of the cellulose, such asNaBH4 reduction, Girard reactant, or diamine. In 2015, Sirvio et al.44 used a deep eutectic solvent based on choline chloride and urea to produce CNF with an environmentally friendly process. Even if the mechanism for this treatment is unknown, they obtained CNF of 2-5nm width and a Young’s modulus around 8 GPa. Finally, very recently, Ghanadpour et al.37proposed a new chemical pretreatment using a phosphoric component. (NH4)2HPO4 is grafted to the fiber, which confers flame retardant properties. In the same context, Naderi et al.38 and Noguchi et al.36 were also aiming to produce phosphorylated CNF using different procedures. The aim of this study is to propose a new green method to produce CNF using environmentally friendly cellulose carbonate produced according to our previously described method53,54 . In fact, we started with the preparation of cellulose carbonates and their characterizations by several methods. Then, a second part is carried out involved the uses of the as-prepared cellulose carbonate to produce CNF, which, to the best of our knowledge, has not been reported until now. In this framework, an original method to produce CNF is described. The obtained CNF was fully characterized in terms of mechanical, morphological and thermal properties. 4 ACS Paragon Plus Environment

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Materials and methods The raw material used was bleached fibers from hardwood (Eucalytus ; Fibria T35)23. Chemical analysis of extracted fibers from the eucalyptus bleached kraft pulp was determined according to standard methods: hemicelluloses5-10% (w/w with respect to o.d. material), alpha-cellulose content 65% (w/w with respect to oven dry (o.d.) material), ash content 0.1% (w/w with respect to o.d. material), and degree of polymerization (DP) 1100. As recommended by the various standards used. All the experiments were triplicates and experimental error between these data was less than 5%. The reagents as well as the raw material were used as such without further purification, including ethanol (CH3CH2OH) (technical grade), potassium hydroxide (KOH, >85%, Roth France), dimethyl carbonate (DMC, > 99.8%, Roth France), potassium bromide (>99%, Acros), FiberCare® (Novozymes), Sodium hypochlorite (NaClO) (10-15 %, Sigma Aldrich), TEMPO (2,2,6,6-tetrametylpiperidine-1-oxyl, C9H18NO) (>98%, Aldrich), Sodium Bromide (NaBr) (BioUltra, ≥99.5%) and distilled water. Fiber preparation. The cellulose fibers at a concentration of 2wt% were refined using “Pile Valley” (ISO5264-2) until SR freeness degree of 80 was reached (standard ISO 5267-1). The average value of three experiments is guven. Synthesis of cellulose carbonate. Cellulose carbonate was synthesized using DMC in ethanolic potassium hydroxide solution53. The carbonation operation was performed 5 ACS Paragon Plus Environment


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in a Parr autoclave under mechanical stirring and an electrical heating control system. The refined cellulose suspension was kept at 2wt% in ethanolic KOH (15wt%) solution under mechanical agitation for 24h. Then 10 mL of dimethyl carbonate per gram of cellulosic pulp were added and the mixture was maintained at various temperatures: 4, 20 and 40°C for 24h. Neutralization and purification steps followed using a mixture of EtOH/H2O (80/20). Each preparation of cellulose carbonates has been repeated minimum three times.

Enzymatic treatment. Dispersion of the lignocellulosic fibers at 2wt% was mixed with 300 ECU/g of enzymes FiberCare® from Novozymes at 50°C, pH 5 for 2h. The temperature was then increased to 90°C for 15 min to kill the enzymes. Cellulose fibers were then washed with distillated water under filtration.

TEMPO-oxidation of cellulose fibers. Cellulose fibers were TEMPO-oxidized at a concentration of 1wt%, a temperature of 25°C and using a TEMPO/NaBr/NaClO ratio of 0.1/1/5 millimoles of reactants per gram of pulp. When the reaction was finished, the pH was adjusted to 7 and cellulose fibers were washed with distillated water.

CNF production using a Supermasscolloider grinder. Figure 1 illustrates the various steps leading to the preparation of CNF using cellulose carbonate and a Supermasscolloider ultra fine friction grinder (model MKZA6-2, disk model MKG-C 80, 6 ACS Paragon Plus Environment

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Masuko Sangyo Co., Ltd., Japan).The cellulose carbonate suspensions at 2 wt% in water were nanofibrillated in the grinder at 1500 rpm. The pulp was passed ten times between the disks with a gap of -5 between the disks and 20 times at a gap of -10. The energy consumption was evaluated using a power sensor. The power measured at each pass, thanks to current and voltage, was divided by the dry flow. The total energy was obtained by adding the energy of each pass.

Figure 1. Presentation of the different steps used to prepare CNF using the cellulose carbonate. Characterization Several methods and techniques were used to evaluate and characterize the modified substrate as well as the CNF produced using the cellulose carbonate.

Spectroscopic analysis: FTIR, NMR and XPS. The IR spectra of unmodified and modified cellulose were recorded. Samples were prepared in the form of KBr pellets on a PerkinElmer Spectrum 65 with a resolution of 4 cm-1 in the frequency range 400-4000 cm-1. The acquisition conditions were established in transmission mode using 64 scans. For each sample tested, the FTIR spectra were recorded in duplicate. The solid7 ACS Paragon Plus Environment


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state NMR experiments were carried out with a Bruker AVANCE 400 apparatus using with a 4 mm probe, working at 100.13 MHz for

13C-NMR.

Each prepared sample was

compacted in 4 mm ZrO2 rotors. All the spectra were performed combining the crosspolarization with high-power proton decoupling, and magic-angle spinning (CP/MAS), and were acquired at 298 K. Tetramethylsilane (TMS) and glycine were used as a primary and as a secondary reference with the carbonyl signal set to 176.03 ppm. For cellulose carbonate, the trials were performed with MAS at 14 kHz, a repetition time of 5 s, and CP contact times from 0.05 to 12 ms. From all the quantitative measurements, the degree of substitution (DS) was then deduced. Electron spectroscopy (XPS) for chemical analysis was also established for the modified and unmodified samples. The test was carried out using an XR3E2 apparatus, working under 10−8 Pa, modulated with a monochromatic Mg K α X-ray source at 15 kV and under a current of 20 mA. The substrates were positioned at an angle of 90°. Spectrum NT and the CAH signal were used in order to deconvolute the peaks. Thermal properties, crystallinity index (CI), and degree of polymerization. The thermal behavior of modified cellulose was analyzed using a thermogravimetric analyzer (TA Instrument Q500). 15 mg of the selected sample were heated from 30to 600°C at a heating rate of 10˚C.min-1 under a dry O2 atmosphere and finally pyrolyzed from 600 to 900°C at 50 °C.min-1. The apparatus recorded the mass loss as a function of temperature. The degree of crystallinity (CI) was performed using wide-angle X-ray 8 ACS Paragon Plus Environment

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diffraction diagrams. Each prepared sample in the form of milled powder was placed on the sample holder and leveled off to obtain total and uniform X-ray exposure. The cellulosic materials were then observed using an X-ray diffractometer (PANalytical, Netherlands) at room temperature with a monochromatic CuKα radiation source (radiation λ = 1.5419 Å) in step-scan mode with a 2θ angle ranging from 5° to 56° with a step of 0.067° and a scanning time of 5 min. The cellulose crystallinity index was deduced according tothe Segal et al. method55. CI = (I002-Iam)*100/I002

(1)

where: I002 represents the diffraction intensity of the main crystalline peak at 2θ ~ 22.5˚ and Iam the intensity at 2θ ~ 18.7˚. The crystallinity index (CI) was also deduced from 13C solid NMR by subtraction of the spectrum of a standard amorphous cellulose, and integration of the spectra in the range 115 to 55 ppm56. CI = (A115-55ppm) of crystalline component / (A115-55ppm) of starting sample (2) The crystalline cellulose component in the samples is obtained after subtraction from the original spectrum of amorphous standard cellulose (see Figure 2). The subtraction of the amorphous portion (Figure 2b) is done so that no part of the residual spectrum contained a negative signal. The resulting spectrum corresponds to the crystalline part of the sample (Figure 2c). The CI was then calculated as the ratio between the area of the crystalline component from 115 to 55 ppm (Figure 2c) and the area (115 to 55 ppm) of the starting cellulose sample (Figure 2a). From this crystalline component sub-spectrum, the Surface / Interior (S/I) ratio of cellulose crystallites can also be deduced, by integrating the area of the C4 resulting signal (Figure 2c). 9 ACS Paragon Plus Environment


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S/I = A86-78ppm / A92-86ppm (3) The degree of polymerization was determined following the standard method ISO 5351/2010. The intrinsic viscosity (ɳint) of copper (II) ethylenediamine dissolved cellulose was determined. The DP was calculated according to the Mark– Houwink– Sakurada equation. As recommended by the various standards used, all the experiments were performed in triplicate.

Figure 2. 13C solid-NMR spectra: a) cellulose sample; b) amorphous cellulose standard and c) crystalline part sub-spectrum obtained from the subtraction spectra (a – b). Morphological and structural behavior of produced CNF. The morphology of the CNF was studied by transmission electron microscopy (TEM) and optical microscopy. The TEM analyses were performed using a JEOL 200CX transmission electron microscope 10 ACS Paragon Plus Environment

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at 80 kV. The TEM images were obtained after depositing about 0.5 μL of the suspension onto a carbon-coated 300-mesh copper grid. For optical microscopy, a Carl Zeiss Axio Imager M1 equipment was used in transmission mode. Images of redispersed CNF suspension were prepared at a concentration of 0.5 wt%. Six images were recorder for each trial. The nanosized fraction of the CNF was evaluated following Naderi et al.57 approach. CNF suspensions with a concentration of 0.02wt% were prepared and then centrifuged using a Sigma 3-18 KS, Germany centrifuge at 1000g and 6°C for 15 min. Turbidity of the CNF suspension was measured using a portable turbidimeter AL 250 T-IT, with a range between 0.01 and 1100 NTU. The CNF suspension was diluted to 0.1wt% and then 10 measurements were taken. Turbidity of the CNF suspension is linked to the size of the CNF. Rheological measurements were taken using an Anton Paar Physica MCR 301, France equipped with a parallel plate geometry with a diameter of 25mm. Temperature was controlled at 25°C using a Peltier controller. Viscosity was measured varying the shear rate from 0.01 to 1000 s-1. Film production and characterization. To prepare nanopaper, 100mL of CNF suspension at 2 wt% were diluted to 0.5 wt% and then filtered through a standard sheet former. The sieve of the sheet former was previously covered by a sieve having a mesh size of 1 µm. The thus prepaed nanofilms were then dried at 90°C during 20 min. Five nanopapers, with a thickness of 73 ± 5 µm, from each CNF were prepared. Casted films were also produced. 50mL of aqueous cellulose suspension at 0.5wt% were prepared and casted in a Teflon mold after homogenization with an ultraturrax (5 min, 15,000 rpm). Films were dried at 50%RH and 25°C. The transparency of the

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films was evaluated using a Haze meter (BYK Gardner, Haze-Gard Plus) following the standard NF T 54-111, 1971. All the experiments were carried out on casted films stabilized at 23 °C and 50% RH for at least 48h, in order to ascertain their properties. The mechanical properties of the tested films were determined using An Instron 5965 machine having a load cell of 5kN. Various samples (50 mm x 15 mm) were conducted under a traction speed of 5mm.min-1. Quality index. The quality index of prepared CNF

was evaluated according to a

method developed by our team58. Here the simplified quality index, involving only four characterization techniques, was used. The simplified quality index, denoted QI* was calculated as follows: 𝑄𝐼 ∗= 0.3X1 -0.03X2-0.072X3²+2.54 X3-5.34ln(X4)+58.62

(4)

Where X1=nanosized fraction; X2=turbidity; X3=Young’s modulus; X4=macro length measured from optical microscopy images.

Results and discussion Chemical characterization of cellulose carbonate As reported previously53, the reaction of cellulose with DMC takes place as presented in Scheme 1. In the current work, the reaction between cellulose and DMC was studied at different temperatures (4, 20, and 40°C) and the mechanism of carbonation was proved using several methods. The DS and the gravimetric recovered yield are summarized in Table 1. 12 ACS Paragon Plus Environment

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Scheme 1. Synthesis route to cellulose carbonate using DMC. From this table, it can be seen that the recovered yield is approximately the same for all tested reaction temperatures. The novelty of such a reaction, as described previously, is that the carbonation reaction can occur at room temperature53. Such a finding is important in a context where energy is a key economic issue. Numerous methods were also used to elucidate the structure of the carbohydrates, such as X-ray diffraction, FTIR, NMR, and XPS. These techniques were applied to confirm the analysis of the new cellulose derivative formed from the reaction of cellulose with DMC. Table 1. Gravimetric recovered yield and the DS of the cellulose carbonate. T°C is the decomposition temperature. Sample

Condition

Yield [%]

DP

DS 13C

CI [%] XPS

XRD

NMR Cell

Sur/Int

TGA T(°C)

NMR

Cellulose crystallite

Ash [mg]

13C

Pristine cellulose

-

1100

71

57.4

0.57

340

0.10

Cell-KOH

Cellulose treated with KOH

-

400

45

53.7

2.00

248

0.15

Cell-DMC-4

Cell-KOH with DMC at 4°C

93

342

43

46.9

1.49

240

1.2

0.10

1.0



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Cell-DMC-20


93

263

0.29

1.7

40

57.7

2.86

240

1.3

Cell-DMC-40


93

291

0.08

0.6

40

51.7

0.90

240

1.3

Cell-DMC-20HCl

Cell-KOH with DMC at 20°c treated with HCl

-

-

0

-

-

50.5

0.96

S/I ratio were determined from NMR crystalline sub-spectra after amorphous component subtraction.

The FTIR spectra of cellulose and their derivatives using DMC, as show in Figure 3, exhibit characteristic absorption peaks of cellulose: a strong band between 3600 and 3000cm-1 corresponding to O-H stretching vibration, and a peak at 2900 cm-1 corresponding to C-H stretching vibration. The absorbance at 1480 - 1300 cm-1 is attributed to the C-H and O-H deformation mode, 1300 – 1200 cm-1 to C-O and C-C bending, and 1100 – 1000 cm-1 to C-O stretching. The glycosidic linkage exhibits two specific bands at 1160 and 898 cm-1 corresponding to C-O-C and C-H vibrations respectively59–61. The signal at 1160 cm-1was used for IR spectra normalization. While the main cellulosic structure was maintained, small significant changes can be detected. There was an increase of the signal intensities in the regions 1400 – 1200 cm-1 and 1200 – 900 cm-1, corresponding to C-O vibrations,in agreement with the presence of carbonate groups53,54,62–65. More precisely, newweak signals appear at 830 cm-1and 701 cm-1, and can be assigned to out of plane and in plane bending bands of carbonates, respectively54,66. The presence of the carbonate functional group is also confirmed by the shoulder signal at 1695 cm-1 corresponding to the C=O (carbonate) stretching vibration. 14 ACS Paragon Plus Environment

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Figure 3. FT-IR spectra of unmodified cellulose and cellulose carbonate at different conditions. All these modifications were more pronounced for the Cell-DMC-20 sample. The weakness of peaks for the substituted group can be explained by the minor amount of carbonate present, as supported by the DS estimated by NMR. Additionally, there is overlapping of the absorption peaks belonging to the cellulose functional groups and those corresponding to the modified cellulose. In the C-H bond frequency range for deformation vibration we can see that the band at 1430 cm-1 present in the starting cellulose sample is less pronounced in the Cell-DMC samples, and a new signal at 1405 cm-1, not observed for the pristine cellulose sample, is observed in the 15 ACS Paragon Plus Environment


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Cell-DMC samples. Voronova67 assigned this new band to unrelated deformation vibrations of C-O and O-H bonds, and considered this as evidence of the presence of CH2OH groups on the surface of crystallites of cellulose nanocrystals. We can see that all the modifications disappear after HCl treatment: the native cellulose IR spectrum re-appears. Finally, it can be presumed that the chemical modification reaction occurs at the surface and the substitutions mainly occurred at secondary alcohol functions at the C2 and C3 positions, as previously reported53,54.

Figure 4.13C solid-NMR spectra of cellulose carbonate, as a function of temperature. a) cellulose samples; b) enlargement of cellulose samples; c) crystalline part sub-spectra enlargement. 16 ACS Paragon Plus Environment

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NMR spectra related as a function of the temperature of the medium on the DS are shown in Figure 4. This figure illustrates

13C-NMR

spectra of the cellulose and

cellulose carbonate modified at different temperatures. As with the IR spectra, all the obtained spectra display typical peaks of virgin cellulose, i.e., those attributed to the anhydroglucose unit, within the interval between 60 and 105 ppm. Far from the cellulose fingerprints, a large signal was also found in the curve of modified cellulose fiber for all temperature conditions. This peak appears at 160 ppm and corresponds to the carbon atom of the alkyl carbonate group, confirming that the carbonation reaction occurred, in agreement with a previous paper

53.

The substituted carbonate appears to be permanent and is only destroyed

after an acidification treatment. Figure 5 shows that when a dose of HCl is added, the carbonate peak at 160 ppm disappears and the structure of native cellulose appears again. The same phenomenon was already observed with the IR spectra.



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Figure 5. The HCl treatment into cellulose carbonate. Furthermore, the solid-state NMR spectra can provide some supplementary information. If we consider the cellulose region at 105-55 ppm (Figure 4), it can be seen that a slight modification appears, especially at zone C4 and C6 of the AGU as well for all its derivatives (i.e. 80- 90 ppm and 60-70 ppm). This change corresponds to the effect of the alkali treatment step and is not part of the reaction between cellulose and DMC. In fact, throughout the alkali treatment, amorphous regions are removed and consequently crystalline regions are more strongly displayed. We tried to interpret changes in the crystallinity index, but the changes are not significant considering the crystallinity index measured from the solid 13C-NMR (see Table 1); the sample Cell-DMC-20 exhibits the highest CI (57.7%). However, if we consider the solid spectrum of the corresponding crystalline part (Figure 4c), the difference is more 18 ACS Paragon Plus Environment

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pronounced for the C4 and C6 signals, and for the deduced surface / interior (S/I) ratio measured on the crystalline part, the modifications were stronger: the surface ratio is highly enhanced with the DMC treatment (the maximum is reached at 20°C). Therefore, as seen with the IR results, the NMR experiments confirm that carbonation takes place, and the surface of the crystallites was much more affected than the interior, since the S/I ratio increased. XPS analysis was also performed to further characterize the modified cellulose. The XPS results (Figure 6) show that whatever the temperature condition, new peaks at 289 eV appeared, labelled as C5 and attributed to the carbonate function (-O-CO-O).



Figure 6. X-ray Photoelectron Spectroscopy curves of C1s deconvolution of modified and virgin samples. The results of XPS as well as NMR analysis confirm the carbonation reaction. The reaction carried out at 20°C apparently produced the best results. This is confirmed by the obtained DS deduced by NMR, which is 0.10, 0.29, and 0.08 at 4, 20 and 40°C, respectively. The lower DS values calculated from NMR (bulk analysis) data compared to those from XPS (surface analysis) prove that the carbonation reaction seems to be at the surface than in the interior of the cellulose. (a) Cellulose

0

3

-0.2 2

-0.4

1 0 0

200

400

600

800

-0.6 1000

5

0.4

4

0

3

-0.4

2

-0.8

1

-1.2

0

-1.6

0

Temperature [°C]

200

400 600 800 Temperature [°C]

1000

Figure 7. TGA and DTG curves for (a) cellulose sample and (b) cellulose modified with DMC at 20°C. The thermogravimetric behavior of modified cellulose was also studied and the data revealed that carbonation of cellulose induces a decrease in thermal stability, as reported in Figure 7. Pristine cellulose fiber and cellulose carbonate have decomposition temperatures of around 340 and 230°C, respectively. TGA thermograms of the cellulose carbonate illustrated the presence of inorganic 20 ACS Paragon Plus Environment

Derivative weight [mg/min]

4

Weight [mg]

0.2

Derivative weight [mg/min]

(b) Cell-DMC-20

5

Weight [mg]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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compounds which represent the inorganic grafted carbonate. This loss of thermal stability can be attributed also to degradation during the alkaline treatment of cellulose. It can be clearly justified by the DP and the decomposition temperature of cellulosetreated with KOH, which are 400 and 248°C, respectively. Thus, a novel cellulose carbonate was prepared at low temperature. This chemical treatment was performed using a green reagent, namely: DMC. Furthermore, the reaction temperature in the range of 4 to 40°C has no influence on the DS and DP of the products. This new derivative, having nontoxic and biodegradable properties, was used for the first time as a chemical pretreatment to produce

Characterization of produced CNF Morphological characterization Morphologies of cellulose nanofibers produced using a grinder are shown in Figure 8. Optical microscopy images show that dimethyl carbonate pretreatment facilitates the fibrillation of pulp. Pulp treated with KOH only does not result in cellulose nanofibers. TEM images confirm that cellulose nanofibers are created at all tested temperatures. CNFs are obtained for the three temperatures but the best quality is obtained at 20°C. Moreover, as proved by NMR as well as XPS the pretreatment at 20°C presents a high level of substitution DS =0.3) and the nanofibrillation appears better. Indeed, microsized fibers are still present for the cellulose pulp pretreated at 4 and 40°C, whereas none are visible for the treatment at 20°C. Optical microscopy 21 ACS Paragon Plus Environment


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images of the samples confirm that better fiber delamination occurred when the degree of carbonylation increased. Figure 8 shows nanoscale fibrils with a wide distribution of both width and length. The average nanofibers diameters for 4, 20, and 40°C are 35 (±14), 21 (±9), and 31 (±15) nm, respectively.

Figure 8. Optical microscopy and TEM images for pretreated pulp after masuko grinder.

CNF properties Mechanical properties and quality index were measured in order to characterize the CNF and compare them with the CNF produced by other chemical pretreatments using the same grinder and the same cellulosic pulp. The results are shown in Table 2. Once again, carbonation results in a large increase of the quality index as shown in Table 2. A maximum is reached for the pretreatment at 20°C. Indeed, as shown with the NMR and XPS techniques, the carbonate content is higher and the fibrillation is better. Higher quality indexes are obtained when the modification rate is higher. The cellulose pulp pretreated with only KOH does not lead to CNF (shown also by the 22 ACS Paragon Plus Environment

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optical microscopy images) as indicated by the low quality index. It can be seen that the quality indices obtained are close to those measured for enzymatic CNF (69.7±6.5)as reported by Desmaisons et al.58 With this new green pretreatment, the CNF produced can compete with enzymatic, TEMPO or cationic CNF. Indeed, using the same pulp and the same grinder we reached a QI of 68 ±3.6 for TEMPO oxidized fibers and 68.8 ±4.6 for cationic CNF. Except for the pretreatment at 40°C, the data obtained for each characterization test are close, as well as the QI values. Table 2. Quality index details for dimethyl carbonated CNF. KOH pulp

Cell-DMC-4

Cell-DMC-20

Cell-DMC-40

TEMPO

Enzymatic

Nanosized fraction [%]

67.3 (1.5)

78.9 (12.5)

89.5 (3.5)

53.8 (7.7)

78.2 (21.7)

73.2 (17.7)

Turbidity [NTU]

537 (36)

263 (12)

283 (25)

434 (9)

65 (13)

413 (25)

Young's modulus [Gpa]

5.85 (0.2)

9.95 (0.5)

9.4 (0.3)

9.5 (0.3)

20.8 (1.8)

15.1 (0.4)

OM Macro length [µm²]

78.5 (37.8)

44.1 (7.1)

41.2 (14.2)

30.4 (5.1)

18.3 (8.7)

27.3 (6.4)

QI

52.5

72.4

75

61.2

86.7

72.6

Std deviation

4.1

4.3

1.8

2.2

3.6

4.5

Tensile strength [MPa]

30.4 (5.1)

57.2 (4.1)

66.2 (4.6)

67.3 (7.2)

80.8 (13.2)

84.6 (1.8)

Elongation at break [%]

1.0 (0.3)

1.1 (0.2)

1.5 (0.4)

2.1 (0.3)

0.6 (0.1)

0.8 (0.2)

Determination of the nanosized fraction, as proposed by Naderi et al.57, evaluates the proportion of nanosized components in the suspension. A real increase is observed for the CNF pretreated at 20°C. For the other pretreatments at different temperatures, the nanosized fraction is lower, probably due to residual fibers induced by a lower substitution rate, as supported by optical microscopy images and NMR analysis. Turbidity, which is related to the size and shape of CNF, can also be affected by residual fibers. The turbidity is much higher for the CNF obtained with treatment at 23 ACS Paragon Plus Environment


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40°C, due to lower substitution and thus lower nanofibrillation, confirmed by the nanosized fraction measurement. The data obtained here are close to those obtained with a grinder and enzymatic pulp58, but are very high. Indeed, Moser et al.68 reported much a lower value: 130NTU for enzymatic CNF after one pass through a microfluidizer and 19.8 NTU for TEMPO oxidized CNF after one pass. CNF pretreated at 20°C presents a higher nanosized fraction due to higher grafting and lower residual fiber content.

The mechanical properties of the produced CNF are in

accordance with those reported by Rol et al.23 for enzymatic commercial CNF produced with a homogenizer. Tensile strength reported here are slightly lower than the one reported for TEMPO oxidized CNF and enzymatic CNF produced with the same grinder. The transparency of CNF films obtained by casting was measured and is shown in Figure 9. In CNF, nanosized components do not scatter light and thus are transparent69,70. As explained, carbonation with a high DS improves nanofibrillation and so the transparency is higher. The data obtained are close to those reported for enzymatic CNF58. Even if the modification rate is higher at 20°C, the transparency is slightly lower, probably due to film irregularities.


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Figure 9. Transparency of cellulose pulp modified with KOH and with KOH+DMC films after grinding step. Young’s modulus measured on cellulose carbonate nanopapers is in agreement with the literature. For example, Nair et al.13 reported a Young’s modulus of about 16GPa for CNF obtained with a grinder. CNF presents high mechanical strength because OH groups are unlocked in the nanofibrillation. Mechanical properties depend greatly on the traction conditions and the efficiency of the pretreatment. Fukuzumi et al.71 reported a Young’s modulus of 7 GPa for TEMPO-oxidized CNF films whereas Rol et al.23 obtained 20GPa for TEMPO-oxidized CNF obtained with the same grinder as in this study. Dimethyl carbonation leads to CNF with a traditional Young’s modulus value but lower than the one obtained with TEMPO oxidation. No effect of temperature is observable here. As recently reviewed by Nechyporchuk et al.72, shear



flow measurements can be used to characterize the nanofibrillation of cellulose pulps. CNF have a shear thinning and thixotropic behavior. Herrick et al.73, followed by Gruneberger et al.74, were the first to show that viscosity increases with the number of passes through a homogenizer. Moreover, if the nanofibrillation yields increase, the number of large fibers decreases and there is less fluctuation of the curves. Here, as represented in Figure 10, there is not too much interference, which could mean a small amount of large fibers, confirmed by the high nanosized fraction in Table 2.

1000000

Viscosity [Pa.s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

Cell-DMC-4

100000

Cell-DMC-20

10000

Cell-DMC-40

1000 100 10 1

0.001 0.01

0.1 rate 1 [1/s]10 Shear

100

1000

Figure 10. Shear viscosity of cellulose carbonate nanofibers.

It seems that the pretreatment at 40°C leads to higher nanofibrillation whereas NMR shows that the modification rate is lower. However, Besbes et al.75 showed that the viscosity depends not only on the fibrillation rate but also on the oxidation rate. They observed that the suspension viscosity decreases if the carboxylate charge increases. Here the oxidation rate is higher at 20°C, which can explain the lower viscosity. Viscosities obtained are very high compared to those reviewed by Nechyporchuk et 26 ACS Paragon Plus Environment

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al.72 for enzymatic and TEMPO-oxidized CNF, but are close to those reported by Naderi et al.38 for phosphorylated cellulose pulp after three passes through a microfluidizer. Nevertheless, rheological response is very dependent on the device, the procedure, and the sample preparation76,77.Comparisons made here should be treated with caution. Energy consumption Energy consumption for the nanofibrillation of modified cellulose pulps was measured and normalized by the energy consumed for the nanofibrillation of nonrefined pulp. The data are shown in Figure 11. Nanofibrillation was carried out using the same eucalyptus pulp, the same grinder, and the same procedure. Cellulose modification with DMC allows the production of CNF with almost the same energy consumption as for enzymatic pulp, leading to an equivalent quality index. However, the energy consumed is higher than those for TEMPO-oxidized pulp. Energy consumption can be decreased if the modification rate increases.



Page 28 of 36

Figure 11. Energy consumption for the nanofibrillation of modified cellulose via a grinder. Value of 110 for TEMPO oxidized CNF were extracted from Desmaisons et al.58

Conclusion Cellulose carbonate using DMC was successfully produced via a green method. Three reaction temperatures were tested and the degree of substitution was measured using different techniques such as NMR and XPS. The results show that the degree of substitution is higher at 20°C than at 40°C and 4°C which means this chemical modification technique is environmentally friendly. Moreover, XPS shows that modification occurs at the surface of cellulose fibers. In the second part of this paper, cellulose nanofibers were produced after modification of cellulose with DMC. CNF with high quality index (75±1.8) and transparency (77%) were obtained without consuming more energy than enzymatic treatment. A novel green and efficient 28 ACS Paragon Plus Environment

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approach is thus proposed to produce CNF using a green chemical under environmentally friendly conditions, leading to low levels of effluent discharge. Acknowledgments This research was supported by the “PHC Utique” program of the French Ministry of Foreign Affairs and the Ministry of Higher Education and Research, as well as to the Tunisian Ministry of Higher Education and Scientific Research for the financial support (CMCU project number 18G1132). The Institut Carnot Polynat (grant agreements n° ANR-16-CARN-0025-01) and LabEx Tec 21 (grant agreement n° ANR-11-LABX0030).This research was made possible thanks to the facilities of the TekLiCell platform funded by the Région Rhône-Alpes (ERDF: European regional development fund).LGP2 is part of CDP Glyco@Alps (ANR-15-IDEX-02). Authors want to thank Thierry Encinas from the CMTC in Grenoble, France for the XRD analysis. References (1)

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Abstract art

Synopsis: A sustainable chemical treatment ways to produce cellulose nanofibers was investigated.


The efficiency of cellulose carbonates to produce cellulose nanofibers

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