Sustainable Approach for the Direct Functionalization of Cellulose

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A Sustainable approach for the Direct Functionalization of Cellulose Nanocrystals Dispersed in Water by Transesterification of Vinyl Acetate Benjamin Dhuiège, Gilles Pecastaings, and Gilles Sèbe ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02833 • Publication Date (Web): 21 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Sustainable Chemistry & Engineering

A Sustainable approach for the Direct Functionalization of Cellulose Nanocrystals Dispersed in Water by Transesterification of Vinyl Acetate Benjamin Dhuiège,†, ‡ Gilles Pecastaings,†, ‡ Gilles Sèbe*,†, ‡ † University of Bordeaux, LCPO, UMR 5629, 16 avenue Pey-Berland, F-33607 Pessac, France. ‡ CNRS, LCPO, UMR 5629, 16 avenue Pey-Berland, F-33607 Pessac, France

Corresponding Author * E-mail: [email protected]

KEYWORDS: Cellulose nanocrystals, esterification in water, transesterification, vinyl acetate. ABSTRACT: Herein we report a novel method for a high-yield surface esterification of cellulose nanocrystals (CNCs) in water, by transesterification of vinyl esters. The esterified nanoparticles were simply produced by heating under stirring a water dispersion of CNCs in heterogeneous mixture with vinyl acetate, with potassium carbonate as catalyst. Reactions were performed in different conditions, to investigate the impact of different parameters, such as the amount of reagent, temperature and reaction time. In optimized conditions, the grafting level could be easily tailored by controlling the reaction time, and up to 90% of the accessible 1 ACS Paragon Plus Environment

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hydroxyl groups at the surface of the CNCs could be esterified. The acetylated CNCs retained their rod-like shape, while their dimensions, crystallinity and thermal stability were marginally affected by the treatment. Turbiscan analysis finally revealed that the hydrophobic character at the surface of the modified particles increased with the acetylation level, leading to a commensurate modification of their dispersive properties in water, acetone and THF. INTRODUCTION Nanocellulose is an abundant natural nanomaterial with great potential, which has been at the center of many scientific researches over the past decade.1,2 It can be extracted from plants,3 wood,4,5 tunicate,6 algae,7 or bacteria,8 in the form of nanofibrils or nanocrystals. Cellulose nanocrystals (CNCs) in particular, are typically produced by hydrolysis of the amorphous part of the initial cellulose substrate (pulp, paper, microcrystalline cellulose) under concentrated sulfuric acid conditions.4,9,10 The nanoparticles obtained have a rod-like morphology, 11 and display negative sulphate ester moieties on their surface due to the chemical treatment. 12-14 The great potential of CNCs relies on their biosourced and biocompatible nature, 15 low density (around 1.5),4,15 high aspect ratio,4,16 inclination for chiral nematic ordering,17,18 barrier properties,1,19,20 and chemically reactive surface.4,21 These properties make them suitable for a wide range of applications, e.g. reinforcing filler in polymer matrices,16,19,20,22,23 Pickering emulsifier15,24 or substrate for the elaboration of foams,25 aerogels,26 iridescent films,27 capacitors,26,28 and packaging materials.29 However, the engineering of innovative materials from CNCs generally requires a fine control of their surface properties by chemical functionalization.4,15,21,30 In particular, the homogeneous dispersion of the CNCs in low polarity

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media (non-polar solvents or polymers) is an issue, the poorly compatible hydroxylated nanoparticles being prone to self-aggregation by hydrogen bonding in such media. 31,32 Among the different strategies than have been proposed to adequately tune the CNCs surface, the esterification by acid chlorides or anhydrides has been the most investigated. 21,30 However, these reactions concurrently release hydrohalic or carboxylic acids by-products, which may adversely impact the cellulose structure and/or contaminate the nanoparticle surface. Among the alternative methods, the acylation approach based on the transesterification of functional vinyl esters is particularly attractive as it allows the grafting of a wide variety of functionalities in mild conditions, using potassium carbonate as catalyst. 24,33-37 The versatility of the method has been already demonstrated,35 but the conditions investigated up to now required the utilization of anhydrous conditions and organic solvents (DMF and DMSO), which may be problematic in the perspective of an industrial application. The development of a greener process in water could therefore be beneficial from a sustainable point of view, as it could significantly lower the environmental impact of the treatment. Another crucial advantage relates to the possibility of directly functionalizing water suspensions of CNCs, without having to freeze-dry the nanomaterial beforehand, or performing tedious solvent-exchange procedures. In an early work patented in the 1960’s, Smith and Tuschhoff unexpectedly discovered that vinyl esters could be advantageously used to acylate aliphatic hydroxyl compounds – such as glycerol, dextrose, sucrose – in the presence of water and at mild temperature, provided that the appropriate alkalinity was maintained.38 The efficiency of the reaction was in particular attributed to the fact that, in their experimental conditions, the transesterification reaction was faster than the concurrent hydrolysis that may undergo either the vinyl ester or novel ester produced. So far, such treatment has never been applied on cellulose, but encouraging results were reported with starch, which could be partially acylated in water following this approach. 383

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The best results were obtained with catalysts such as soda ash, potassium carbonate or sodium

hydroxide, leading to acylation levels varying from 0.5 to 25 wt% depending on the reaction conditions and vinyl ester used. With vinyl acetate however, low acetyl contents were systematically obtained (below 3.6 wt%), which forced the authors to repeat the reaction several times to increment the degree of substitution.38,39 In the current work, we report a novel method for a high-yield surface functionalization of cellulose nanocrystals in water, by transesterification of vinyl acetate. CNCs were treated in aqueous conditions, with vinyl acetate (VA) as model vinyl ester and potassium carbonate as catalyst. The influence of the different reaction parameters (amount of reagent, temperature and reaction time) was investigated by FT-IR and 13C solid-state CP-MAS NMR spectroscopy. The impact of the treatment on the morphology, crystallinity, thermal stability and surface properties of the cellulose nanoparticles was then particularly examined, using techniques such as Atomic Force Microscopy (AFM), Thermo-Gravimetric Analysis (TGA), Dynamic Light Scattering (DLS), Turbiscan analysis and contact angle measurements. EXPERIMENTAL SECTION Materials. Cellulose nanocrystals (CNCs) were isolated by sulfuric acid hydrolysis of soft wood pulp and purchased from the University of Maine. Potassium carbonate (K 2CO3), vinyl acetate (VA) with purity superior to 99%, tetrahydrofuran (THF) and technical acetone were purchased from Sigma Aldrich. K2CO3 was systematically dried in the oven at 50°C overnight before use. Acetylation of CNCs in Water. 660 mg of CNCs, 130 mg of K2CO3 and 22 mL of distilled water were introduced in a double-necked 100 mL flask and sonicated for 1 min (Bandelin, 4


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MS72 probe, 1.5 kJ). Various amounts of VA (2.4, 5.5, 9.4, 15 or 22 mL) were then added and the mixture was heated under magnetic stirring (500 rpm). Different temperatures (60, 80 and 100 °C) and reaction times (from 1h to 72h) were investigated. The reaction was quenched by quickly cooling down the medium in an ice bath, until room temperature was reached. After removal of the water and unreacted VA by rotary evaporation, the modified CNCs were rinsed several times with acetone to eliminate any residual reagent or organic by-products. The K 2CO3 catalyst was removed by two-days dialysis in water, using a regenerated cellulose dialysis membrane with a molecular weight cut-off of 1 kDa (water was changed two times a day). The suspension was finally sonicated and freeze-dried. Fourier Transform Infrared Spectroscopy (FT-IR). 1-2 wt. % cellulosic material was grinded in a KBr matrix and the mixture was vacuum-pressed for 5 min (200 bars) to form a pellet. The infrared spectra analysis of the sample was then performed on a Fourier Transform Infrared Bruker spectrometer, in transmission mode. Spectra were recorded between 4000 and 400 cm-1, with a resolution of 4 cm-1 (64 scans). After analysis, the spectra were adjusted to the same baseline and normalized to the C-O stretching vibration of the glucopyranose ring at 1060 cm-1. The kinetics of acetylation were evaluated from the FT-IR spectra, through the calculation of the IC=O/IC-O peak heights ratio, which represents the intensity of the C=O stretching of the grafted acetyl group (at 1740 cm-1) normalized to the C-O stretching of cellulose at 1060 cm-1 (used as an internal standard). The percent ratio of acetyl groups relative to the total number of OH groups in the CNCs (Ac/OH %) was then deduced from the IC=O/IC-O ratio, using the following equation:35 /

%=

/ 162 × 3 × 43 2.82 −

/


(%)

( )


Nuclear Magnetic Resonance spectroscopy (NMR).

13

C magic angle spinning (MAS)

NMR measurements were performed on a 500 MHz Bruker Avance II NMR spectrometer WB (Wissembourg, France), working at frequency of 125.8 MHz (4 mm dual CPMAS (1H/BB)). The dry cellulosic material was introduced into 4 mm Zirconia rotors (Cortecnet, Paris) of various volumes (70 to 90 µL) and closed with a Kel-F cap. Samples were spun at the magic angle frequency of 10 kHz. All spectra were recorded using a cross polarization, CP, pulse sequence with a two-pulse phase modulation (TPPM) proton decoupling and with a recycling delay of 5 s at 298 K. All spectra were obtained from 2000 scans, after application of a Lorentzian filtering function of 3 Hz. 13C chemical shifts are reported relative to glycine used as external reference (the carbonyl group was set at 176.03 ppm). 1

H NMR analyses were performed with a Bruker Avance I (400 MHz) spectrometer in

deuterated chloroform (CDCl3) at 400.2 MHz and 25°C; the relaxation time was set at 1s and the number of scans equal to 32. Atomic Force Microscopy (AFM). Samples were dispersed in water (0.02 g.L-1) by sonicating the mixture for 30 s (Bandelin, MS72 probe, 0.75 kJ). One droplet of the dispersion was then deposited on a freshly cleaved mica surface and let to dry overnight at room temperature. The as-prepared samples were observed with a Dimension Fastscan AFM (Bruker) microscope in tapping mode (at 22°C), using a Fastscan-A probe. The nominal spring constant was set to 18 N.m-1 and the resonance frequency to 1,400 Hz. Height and phase images were obtained using a scan rate of 4 Hz, and analyzed by the NanoScope Analysis software. Thermo-Gravimetric Analysis (TGA). Thermal analyses were performed using a TGA Q50 Instrument. Samples (5-10 mg) were heated from 20 to 700°C, at a rate of 10°C.min -1 and 6 ACS Paragon Plus Environment

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under nitrogen atmosphere (flow rate: 90 mL.min-1). Each point of the TGA curves was recorded every 0.5 s. The weight loss was recorded as a function of temperature and the temperature of maximum degradation was obtained from the maximum of the derivative curve. X-ray Diffraction spectroscopy (XRD). X-ray diffraction (XRD) patterns were collected on a PANalitycal X'pert MPD-PRO Bragg-Brentano θ-θ geometry diffractometer equipped with a secondary monochromator, a 3X15 positions sample changer and an X’celerator detection over an angular range of 2θ = 8-80°. The cellulosic material was deposited on silicon wafer sample holders and flattened with a piece of glass. The Cu-Kα radiation was generated at 45 kV and 40 mA (λ = 0.15418 nm) and each acquisition was performed for 1 h 27 minutes. The crystallinity of the cellulosic particles was estimated by the Segal method, 53 according to the following equation: (%) =

. °

−

. °

. °

× 100 ( )

Where I22.6° represents the intensity of the (200) crystallographic planes of cellulose Iβ and I18.3° the intensity of the amorphous contribution. Dynamic Light Scattering (DLS) and zeta potential. Aqueous dispersions of pristine and acetylated CNCs were prepared in concentration of 0.01 g.L-1, and sonicated for 1 min (Bandelin, MS72 probe, 1.5 kJ). The hydrodynamic diameter of the particles was measured in right angle scattering geometry, using a Zetasizer Malvern Nanoseries instrument. 12 runs of 10 s were acquired for each sample (at 25°C). The zeta potential was measured using the same instrument, and was obtained from an average of three acquisitions of 12 runs.



Water contact angle measurements and interfacial tension calculation. Pristine and acetylated CNCs films were prepared by casting aqueous dispersions of the particles (10 g.L-1) on glass slides (1 night drying at room temperature). Water contact angles were then measured on these surfaces, using a Krüss goniometer equipped with a camera. Stability of the dispersions in water, acetone and THF. The stability of the CNCs dispersions in water, acetone and THF was evaluated by measuring the sedimentation rate of the particles in these solvents, using a Turbiscan® Lab (Formulaction, France). Dispersions of 10 g.L-1 were sonicated for 1 min (Bandelin, MS72 probe, 1.5 kJ) and immediately transferred into cylindrical glass cells. The cells were subsequently scanned along their entire length by a light beam emitting at a wavelength of 880 nm (near infrared). The transmitted and backscattered light intensity was then recorded as a function of time, allowing establishing the sedimentation profile of the particles under gravity. The sedimentation rate was determined from the slope at the origin of the curves representing the sediment height as the function of scanning time (obtained from the backscattered profiles). All analyses was carried out at 30°C. Aqueous dispersions were scanned every 10 min for 15 h; acetone and THF suspensions were scanned every minute for 10h.

RESULTS AND DISCUSSION Functionalization of the CNCs in water. Cellulose nanocrystals (CNCs) were produced by sulphuric acid hydrolysis of wood pulp, according to a general procedure widely described in

the literature.43,44 They consist of rod-like particles with estimated dimensions of 110  48 nm

in length and 4.8  1.1 nm in thickness, based on AFM topography images (Figure 1A).35 In 8 ACS Paragon Plus Environment

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aqueous suspension, the CNCs displayed a zeta potential of about -51  6 mV consistent with the presence of negative sulfate groups at their surface, as expected after the sulfuric acid treatment.4,9,10 The amount of accessible hydroxyl groups at the CNCs surface (N OHs) has been evaluated in a previous study of the same author, by phosphorylation coupled with

31

P NMR

and FT-IR analysis.35 An NOHs value of 3.10 ± 0.11 mmol/g was then measured, which represents 16.7 % of the total number of OH groups found in the nanoparticle.

Figure 1. (A) AFM topography image of pristine CNCs. (B) Reaction scheme for the surface functionalization of CNCs in water, using vinyl acetate (VA) as acetylating agent. One of the most straightforward approaches to control the surface properties of the CNCs is to convert the surface hydroxyl groups into appropriate moieties by esterification reactions. 21,30 However, such reactions generally require the utilization of organic solvents and anhydrous conditions to prevent the hydrolysis of the acylating agent (acid chlorides, anhydrides…), which is kinetically favored. To overcome this problem, we developed a method based on the transesterification of vinyl esters, which allowed producing acetylated particles in one step, by simply introducing the reagent in a water suspension of CNCs and stirring the mixture under heating. The feasibility of the method was demonstrated with vinyl acetate (VA) as model acylating agent and potassium carbonate (K2CO3) as catalyst. Although the vinyl ester and water dispersion are substantially immiscible, the reaction is promoted by the rapid mixing of 9



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the two liquid phases, which generates new liquid/liquid interfaces. The transesterification of VA by the CNCs surface hydroxyl groups is then favored, leading to the release of vinyl alcohol as a by-product (Figure 1B). Since the vinyl alcohol tautomerizes quickly to acetaldehyde, the equilibrium is shifted toward the formation of the acetylated product. In a first set of experiments, the volume ratio of VA relative to H2O (VVA/VH2O) was optimized, by performing several experiments with a constant reaction time of 5h (Table 1). Table 1. Experimental details for the surface acetylation of CNCs at different V VA/VH2O volume ratios and temperatures, with a constant reaction time of 5h. Sample

VVA/VH2Oa

VA/OHb

CNCs-Ac10%VA-80°C

10/90

13

CNCs-Ac30%VA-80°C

30/70

52

CNCs-Ac20%VA-80°C CNCs-Ac40%VA-80°C

20/80 40/60

CNCs-Ac50%VA-80°C

50/50

CNCs- Ac50%VA-100°C

50/50

CNCs-Ac50%VA-60°C

50/50

30 81

Temp. (°C)

Reaction time (h)

80

5

80 80 80

5

0.9 ± 0.7

5

5.8 ± 1.3

5

121

80

5

121

100

5

121

60

Ac/OH % (%)c

5

3.8 ± 0.6 6.8 ± 0.6 8.0 ± 1.5 0.1± 0.1 2.2± 0.6

Volume ratio of VA relative to H2O. Molar ratio of VA relative to the total number of OH groups in the CNCs. cPercent ratio of acetyl groups relative to the total number of OH groups in the CNCs. a

b

The efficiency of the reaction was probed by identifying the characteristic acetyl vibrations in the FT-IR spectra of the modified material (Figure 2A). The vibrations of cellulose were observed at 1425 cm-1 (δCH), 1375 cm-1 (δCH), 1338 cm-1 (δOH), 1317 cm-1 (δCH), 1185 cm-1 (δCH and δOH), 1159-890 cm-1 (νC-O and νC-C), and below 683 cm-1 (γOH).45 After the VA treatment, the grafted acetate moieties emerged in the form of two signals corresponding to the carbonyl stretching vibration at 1740 cm-1 (ν(C=O)Ac) and C-O stretching at 1260 cm-1 (ν(C-O)Ac). The 10


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percent ratio of acetyl groups relative to the total number of OH groups in the CNCs (Ac /OH %) was deduced from these spectra, through the calculation of the IC=O/IC-O peak heights ratio, which represents the intensity of the C=O stretching of the grafted acetyl group (at 1740 cm -1) normalized to the C-O stretching of cellulose at 1060 cm -1 (used as an internal standard). Ac/OH % is obtained from IC=O/IC-O using a calibration chart reported in the literature and established by a titration method involving the saponification of the grafted acetate groups. 35,46

Figure 2. (A) FT-IR spectra of the CNCs before and after VA acetylation at different VVA/VH2O volume ratios and temperatures. (B) Evolution of the acetyl content (Ac /OH %) as a function of VVA/VH2O. (C) Evolution of Ac/OH % as a function of temperature. 11 ACS Paragon Plus Environment


The acetyl content after reaction increased linearly with the V VA/VH2O ratio (Figure 2B), presumably because more VA/water interfaces were created during mixing. In the current study, we arbitrarily decided to keep the volume of VA below the volume of water, but a further increase in acetylation level is not excluded if more VA is added. Among the three temperatures investigated for the reaction, only the medium heating at 80 °C led an efficient grafting (Figure 2C). The decrease in yield above 80 °C can be easily explained by the vaporization of VA at high temperature (the boiling point of VA under atmospheric pressure is 73 °C). The absence of reaction at 60 °C could be due to the fact that more energy is required to reach the transition state and/or promote the diffusion of VA within the water phase. In view of these results, a VVA/VH2O volume ratio of 50/50 and a temperature of 80 °C were selected for the kinetics studies described in the next paragraph. The kinetics of the reaction were evaluated by plotting the Ac/OH % as a function of reaction time in Figure 3. The reaction was relatively fast during the first 5h, then it slowed down, to progressively reach a grafting level corresponding the substitution of about 90% of the accessible surface OH groups after 3 days. The kinetics of the reaction in water are much slower that in DMF or DMSO,35 but similar acetylation levels can be obtained and finely tuned by controlling the reaction time.


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18 16 14 12 10 8 6 4 2 0

Estimated percent ratio of surface OH groups = 16.7%

Ac/OH %

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


0 10 20 30 40 50 60 70 80 Reaction time (h)

Figure 3. Evolution of the acetyl content (Ac/OH %) as a function of reaction time (VVA/VH2O = 50/50; temperature = 80 °C). The percent ratio of surface OH groups relative to the total number of OH groups (16.7 %) was estimated in a previous study. 35 The acetylated material was further characterized by spectroscopy (Figure 4).


13

C solid-state CP-MAS NMR


α

α

PVAc

β

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β *CNCs-Ac72h *CNCs-Ac48h CNCs-Ac24h CNCs-Ac15h CNCs-Ac5h

CNCs-Ac4h CNCs-Ac3h CNCs-Ac2h CNCs-Ac1h

C2, C3 and C5 C1

C4

C6 CNCs

200

180

Figure 4.

13

160

140

120

100 80  (ppm)

60

40

20

0

C solid-state CP-MAS NMR spectra of CNCs before and after VA acetylation in

water for various reaction times. *The lower resolution noted with those samples, is assigned to the lower amount of material used for the analysis. The spectrum of unmodified CNCs is characteristic of the cellulose I polymorph, with its expected signals at 110 ppm (C1), 94 ppm (C4 crystalline), 89 ppm (C4 amorphous), 77-80 ppm (C2, C3 and C5), 70 ppm (C6 crystalline) and 68 ppm (C6 amorphous).47 After acetylation, the 14


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acetate groups emerged at 175 ppm (C=O) and 25 ppm (CH3); their intensity progressively increased with increasing reaction time, confirming the FT-IR data. The cellulose pattern in the NMR spectra was not affected by the acetylation treatment and the signals of the crystalline domains retained their sharpness, indicating that the grafting was limited to the outer surface of the nanoparticles.35,48 Unexpectedly, beyond 3h of reaction we observed the appearance of a broad signal at 35-55 ppm, belonging to a solid by-product extracted from the reaction medium by extensive rinsing with acetone (after elimination of water and unreacted VA). This byproduct was later identified as poly(vinyl acetate) (PVAc) after analysis by 1H/13C NMR spectroscopy in CDCl3 (Figure 5A and 5B). Only the -CH2- signal of PVAc is observed in Figure 4, due to peak overlapping with the signals of acetylated CNCs. The formation of PVAc obviously results from the free-radical polymerization of the excess vinyl acetate during the acetylation treatment, initiated by the temperature and/or basic conditions. The evolution of its production in the course of the reaction is given in Figure 5C. The amount of PVAc produced is quite low below 4 h reaction, but beyond this time, the polymerization accelerates and the mass of PVAc increases continuously to reach a value of 10.6 g after 3 days (polymerization of 52 wt.% of the VA introduced initially). Since this PVAc was produced in quite green conditions compared to the classical bulk radical polymerizations performed with radical initiators, 49 or metal catalysts,50 it could be later recovered and valorized (in its pure form, or in combination with the acetylated CNCs). The formation of PVAc during the acetylation treatment could be also prevented by adding an inhibitor of polymerization such as hydroquinone, as was confirmed in side experiments (results not shown).



2.0

A 1

B

Poly(vinyl acetate)

H CDCl3

1.0

7 13

1.7-1.8

4.8

5

3

1

CDCl3

C

C3 165

3.0 2.0

140

115

90 δ (ppm)

C2

C1

65

40

C4 15

12 Mass of PVAc (g)

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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10

C

8 6 4 2 0

0 10 20 30 40 50 60 70 Reaction time (h)

Figure 5. (A) 1H & 13C NMR spectra in CDCl3 of the poly(vinyl acetate) (PVAc) recovered by extensive rinsing with acetone, and assignment of the characteristic peaks. (B) Chemical structure of PVAc. (C) Evolution of the amount of PVAc produced in the course of the reaction.

Properties of the acetylated CNCs. The impact of the acetylation treatment on the morphology and crystalline structure of the CNCs was investigated by AFM and XRD, respectively (Figure 6). The dimensions of the nanoparticles were estimated from the AFM topography images, by scanning a line along (length) or across (height) isolated CNCs. The diameter was estimated from the height difference between the mica surface and the top of the 16 ACS Paragon Plus Environment

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nanocrystals (assuming a cylindrical shape). The width of the nanoparticles could not be measured by AFM due to the broadening effect provoked by the convolution of the AFM tip and the CNCs.24,51,52 The crystallinity of the cellulosic particles was estimated by calculating the crystallinity index (Cr.I.) determined from the X-ray diffractograms by the Segal method. 53 The length, diameter and Cr.I. of the acetylated CNCs are summarized in Table 2 for the various reaction times.

Figure 6. (A) AFM pictures of the acetylated CNCs after treatment for various reaction times. (B) XRD spectra and crystallographic planes of the corresponding samples.

The CNCs retained their rod-like shape after the acetylation treatment, but their average length and diameter slightly decreased. The decrease in diameter was assigned to the 17 ACS Paragon Plus Environment


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disappearance of the population of high diameter (6-12 nm), which consists of laterally associated elementary crystallites (originating from incomplete hydrolysis of the initial pulp substrate, or clustering of single particles by hydrogen bonding) (Figure 7). The decrease in length is apparently due to the breakage of the longer nanorods (> 150 nm) during the treatment (Figure 7). Table 2. Length (L), diameter (D), crystallinity index (Cr.I.), and thermal characteristics of the CNCs, before and after acetylation for various reaction time. Sample

Ac/OH % (%)

AFM

L (nm)

D (nm)

DRX Cr.I. (%)

T5% (°C)

TGA Tm (°C) Yc (%)

5.2  1,7

78

222

278

CNCs CNCs-Ac1h

0.4 ± 0,4

110  48 96  34

5.1  2,5 4.4  1,7

CNCs-Ac3h

5.3 ± 0,7

92  25

4.2  1,5

CNCs-Ac2h CNCs-Ac4h CNCs-Ac5h

CNCs-Ac10h

CNCs-Ac15h

3.7 ± 0,2 7.6 ± 0,4 8.0 ± 1,5 9.5 ± 1,4 9.7 ± 0,9

CNCs-Ac24h

11.5 ± 0,9

CNCs-Ac72h

15.3 ± 1,3

CNCs-Ac48h

13.8 ± 1,1

85  31 85  32 77  22

70  24 70  23 81  29 71  24 83  21

4.4  1,4 4.1  1,2

4.3  1,3 4.0  1,2 3.7  1,2 3.8  1,1 3.6  1,0

83 80 77 75 77

76 76 73 73 72


269 222 220 221 214

173 225 235 221 225

296 278

20 16

278

17

278 270

266 280 278 280 279

16 12 14

27 13 18 9

12

NCCAc 72h CNCs-Ac 72h

20 15 10

Diameter (nm)

12

10,5

9

7,5

6

4,5

3

0

1,5

5

NCC CNCs 30

NCCAc 5h5h CNCs-Ac

NCCAc 72h CNCs-Ac 72h

25 20 15 10

5 0

0 20 40 60 80 100 120 140 160 180 200

NCCAc 5h5h CNCs-Ac

Fraction of particles (%)

NCC CNCs 25

0

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


Fraction of particles (%)

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Length (nm)

Figure 7. Statistic distribution of CNCs diameter and length before and after acetylation for 5h and 72h. The dimensions were obtained from the AFM pictures. The crystallinity of the CNCs was only slightly affected by the treatment. All cellulose nanoparticles displayed the typical X-ray diffraction pattern of cellulose I, with characteristic

diffraction peaks at 2θ = 15, 17, 20, 22.3 and 34.5° corresponding to the (110), (110), (012), (200) and (004) Miller planes, respectively (Figure 6B).54 The peak at 2θ = 12° sometimes observed in the diffractograms, was assigned to traces of cellulose II present in the initial CNC

substrate (diffraction peaks expected at 2θ = 12° (110), 20° (110) and 22° (020)).43,55 Although a slight decrease in Cr.I. was noted when the reaction time was increased, the material remained highly crystalline (Cr.I. > 70%), without significant modification of the crystal lattices. This result is consistent with the sharp signals observed by 13C CP-MAS NMR spectroscopy for the modified material (Figure 4). The thermal stability of the acetylated CNCs was investigated by TGA, through the measurement under inert atmosphere of the 5 % weight loss temperature (T 5%), the temperature of maximum rate of degradation (Tm) and char yield at 675°C (Yc) (Table 2). No improvement in thermal stability was noted after acetylation in our experimental conditions, in contrast with 19 ACS Paragon Plus Environment


what was obtained when microfibrillated cellulose was acetylated with acetic anhydride in DMF (20-50 °C increases in T5% were noted),46 or when CNCs were carboxylated with ammonium persulfate,56 or citric acid (Tm of up to 380 °C were reported).57 The VA-acetylated CNCs produced in the current study displayed a lower T 5% value compared with the unmodified CNCs (Table 2), which was assigned to the cleavage of the acetyl groups. The slight decrease in T m also observed in Table 2, could result from a perturbation of the degradation mechanisms by these released moieties. The thermal stability of the acetylated particles was found to decrease slightly during the first 15h of treatment (T5% and Tm decreased), before improving again above 24h of reaction. This peculiar behavior was assigned to the high amount of PVAc produced after 24h of reaction, which is thermally more stable,58 and possibly act as a protective layer around the cellulosic particles. In any case, the thermal stability No pertinent information could be extracted from the Yc data. The impact of acetylation on the dispersibility of the nanoparticles in water was investigated by DLS and zeta potential measurements.59 The Z-average size of the particles obtained from the method of cumulants is presented in Table 3, with the polydispersity indexes (PDI) and zeta potentials (ξ).


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Table 3. Z-average size, polydispersity index (PDI), zeta potential () and water contact angle

(WCA) of the CNCs suspended in water (0.01 g.L-1), before and after acetylation for various reaction time (measured by DLS). Sample

CNCs CNCs-Ac1h

Ac/OH % (%)

Z-average (nm)

PDI

3.7 ± 0.2

625  21

0.7

0.4 ± 0.4

CNCs-Ac2h CNCs-Ac3h

5.3 ± 0.7

CNCs-Ac4h

7.6 ± 0.4

CNCs-Ac5h

8.0 ± 1.5

CNCs-Ac10h

CNCs-Ac15h

9.5 ± 1.4

757  29 778  14 197  13

N.m: not measured.

0.8

-28  2

0.7 0.5

-27  2 -32  3

26  1 33  2 40  1 47  1 47  1

46  3

564  226

0.8

-24  3

49  2

aggregates

15.3 ± 1.3

39  1

-51  6 -28  2

-36  3

13.8 ± 1.1

CNCs-Ac72h

-28  2

0.3 0.7

0.4

359  108

11.5 ± 0.9

WCA (°)

297  59

9.7 ± 0.9

CNCs-Ac24h CNCs-Ac48h

110  14 470  16

ξ (mV)

582  175

0.6 -

0.6

-40  3 -27  2

-27  2

47  2 N.m N.m

The Z-average (and PDI) measured for the unmodified CNCs is consistent with the average length estimated by AFM (110 ± 48 nm). Regardless of the acetylation level, the Z-average and PDI increased significantly after the treatment, indicating that the modified particles were more aggregated in water. This aggregation could result from the modification of the surface wettability of the particles by the hydrophobic acetyl moieties and/or the decrease in surface charge density engendered by the treatment (since ξ decreased in magnitude after acetylation). The hydrophobic character of the acetylated CNCs was confirmed by performing water contact angle (WCA) measurements at the surface of films prepared by casting, which showed that the WCA was increased after acetylation (Table 3). However, no clear trend could be established between the acetylation level and WCA, Z-average or potential zeta of the modified particles. 21 ACS Paragon Plus Environment


Therefore, the dispersibility of the CNC particles before and after acetylation was further examined by Turbiscan® analysis, using three solvents of decreasing polarity: namely, water ( = 80;  = 1.8 D), acetone (ε = 21; μ = 2.9 D) and THF (ε = 7.6; μ = 1.8 D), the polarity of each

solvent being expressed by its dielectric constant  and dipole moment . The instrument uses static multiple light scattering to detect particle migrations and size variations in liquid dispersions. It measures the transmitted and backscattered light through the particle suspension, by scanning the cylindrical cell containing the sample at regular time intervals, from bottom to top. After analysis, the height of precipitate in the cylindrical cell can be plotted as a function of time (Figure 8), while the sedimentation rate can be extracted from the initial slope of the curve (Table 4). The differences in precipitate height noted between samples in Figure 8 are assigned to differences in particle compaction and will not be discussed.


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Figure 8. Evolution of the precipitate height with time for the unmodified and acetylated CNCs dispersed in water (A), acetone (B) or THF (C) (measurements performed by Turbiscan® analysis). Table 4. Sedimentation rate calculated from the initial slope of the different curves in Figure 8. Sample

H2O

Acetone

THF

3.7 ± 0.2

4.9

-

-

CNCs CNCs-Ac1h

0.4 ± 0.4

CNCs-Ac3h

5.3 ± 0.7

CNCs-Ac2h CNCs-Ac5h

CNCs-Ac24h CNCs-Ac72h

Sedimentation rate (mm.h-1)

Ac/OH % (%)

8.0 ± 1.5

11.5 ± 0.9 15.3 ± 1.3

0.04 4.4 7.6

86 -

214 -

7.9

81

40

10

47

16

9.8

67

30

As expected, almost no precipitate was recorded with time for the unmodified CNCs dispersed in water, as they are highly hydrophilic and well stabilized by the negative charges at their surface (electrostatic repulsion). The particles precipitated within minutes in acetone, and even faster in THF, in line with the decreasing polarity of these solvents. Conversely, the sedimentation of the acetylated particles in water was increasingly favored when the acetylation level was raised (sedimentation occurred at an increasingly faster rate), while the dispersibility in acetone, and more markedly in THF, was concomitantly improved (the sedimentation rate progressively decreased). Since the ξ values in Table 3 are not correlated with the acetylation level, the dispersive behavior of the acetylated CNCs cannot be related to the modification of the surface charge. Therefore, the progressive modification of the dispersive properties in the different solvents was reasonably assigned to the increasing hydrophobicity imparted by the 23 ACS Paragon Plus Environment


grafted acetyl moieties. With their tailored surface, the particles produced in the current paper could easily find application as Pickering emulsifiers, as acetylated CNCs have the property of stabilizing oil-in-water emulsions, provided that the hydrophilic/hydrophobic balance at their surface is properly tuned.24,36 We indeed recently found that such particles (produced with another method) could efficiently stabilize emulsions of organic solvents such as ethyl acetate, toluene, cyclohexane,24 or also vinyl monomers such as styrene,36 that could be later polymerized to produce latex microbeads or nanobeads. 36 CONCLUSION This work illustrates the potential of vinyl esters as a tool to efficiently functionalize the surface of cellulose nanocrystals (CNCs) in water medium, through a simple heterogeneous acylation process catalyzed by potassium carbonate. Acetyl functions were indeed easily grafted at the CNCs surface by simply heating under stirring a water dispersion of CNCs in heterogeneous mixture with vinyl acetate (VA) – selected as model vinyl ester – and containing potassium carbonate. Although the vinyl acetate and water dispersion were substantially immiscible, the reaction was promoted by the rapid mixing of the two liquid phases, which generated new liquid/liquid interfaces. Preliminary experiments performed in different conditions indicated that a relatively high amount of VA and optimal temperature of about 80 °C were required to obtain an efficient reaction. In optimized conditions, the grafting level could be easily tailored by controlling the reaction time, and up to 90% of the hydroxyl groups at the surface of the CNCs could be acetylated. The acetylated CNCs retained their rod-like shape, while their dimensions, crystallinity and thermal stability were marginally affected by the treatment. On the other hand, the hydrophobic character at the surface of the modified particles increased progressively with the acetylation level, leading to a commensurate modification of 24


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their dispersive properties in water, acetone and THF. Unexpectedly, the treatment in our experimental conditions also resulted in the formation of poly(vinyl acetate) (PVAc) as a byproduct, assigned to the free-radical polymerization of part of the excess VA, presumably initiated by the temperature and/or basic conditions. This polymerization could be easily prevented in future processes, by incorporating an inhibitor of polymerization in the VA phase (such as hydroquinone). Since this PVAc is produced in quite green conditions compared with the classical methods (no organic initiator, nor metal catalyst), it could be later recovered and valorized, in its pure form, or in combination with the acetylated CNCs. For instance, the in-situ polymerization of VA during the acetylation process could be exploited to prepared PVAc composites reinforced with homogeneously dispersed acetylated CNCs. Such composites could be used as such, or in combination with other polymer matrices. These two possible strategies will be addressed in future work, with the possibility of extending the reaction to other vinyl esters.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID 25 ACS Paragon Plus Environment


Gilles Sèbe: 0000-0001-5057-1504 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by Aquitaine region, Conseil Régional des Pyrénées Atlantique and BPI France among the industrial consortium project NAWHICEL-2. The authors are grateful to Estelle Morvan from the structural Biophysico-chemistry platform in the Institut Européen de Chimie et Biologie (IECB, Pessac, Fr) for the solid-state NMR experiments. We also thank Eric Lebraud for running the XRD experiments in the Institut de Chimie de la Matière Condensée de Bordeaux laboratory (ICMCB, Pessac, Fr).

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For Table of Contents Use Only

Cellulose nanocrystals

Water medium

Surface acetylation

An efficient method for the surface functionalization of cellulose nanocrystals is proposed, through an environmentally friendly esterification route in water, based on vinyl esters.


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Sustainable Approach for the Direct Functionalization of Cellulose

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