Alkylation of microfibrillated cellulose – A green and efficient method for use in fiber-reinforced composites

Accepted Manuscript Alkylation of microfibrillated cellulose – A green and efficient method for use in fiberreinforced composites Amaury Lepetit, Richard Drolet, Balázs Tolnai, Daniel Montplaisir, Rachida Zerrouki PII:

S0032-3861(17)30799-1

DOI:

10.1016/j.polymer.2017.08.024

Reference:

JPOL 19926

To appear in:

Polymer

Received Date: 11 May 2017 Revised Date:

19 July 2017

Accepted Date: 11 August 2017

Please cite this article as: Lepetit A, Drolet R, Tolnai Balá, Montplaisir D, Zerrouki R, Alkylation of microfibrillated cellulose – A green and efficient method for use in fiber-reinforced composites, Polymer (2017), doi: 10.1016/j.polymer.2017.08.024. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Alkylation of microfibrillated cellulose – A green and efficient method for use in fiber-reinforced composites

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Amaury Lepetita,b, Richard Droletc, Balázs Tolnaic, Daniel Montplaisirb,*, Rachida Zerroukia,b,* a

Laboratoire de chimie des substances naturelles, Université de Limoges, 123 Av. Albert

Centre de recherche sur les matériaux renouvelables, Université du Québec à Trois-

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b

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Thomas, 87060 Limoges, France.

Rivières, 3351 boul. des Forges C.P. 500 (QC), G9A 5H7 Trois-Rivières, Canada. c

Kruger Biomaterials Inc, 3285 Bedford Road, Montréal, Québec, H3S 1G5, Canada.

*Corresponding authors: E-mail address: [email protected] (D.Montplaisir),

Abstract

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[email protected] (R.Zerrouki).

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Microfibrillated cellulose (MFC) fibers were alkylated by propargyl bromide, allyl bromide and propyl bromide in an aqueous medium without any stirring. The chemical

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modifications were confirmed by Fourier transform infrared spectroscopy (FTIR), solidstate 13C nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectrometry (XPS). The alkylated MFC fibers were obtained with degrees of substitution (DS) ranging from 0.12 to 0.33. All the samples were combined with low-density polyethylene (LDPE), and then the morphology, mechanical properties and water absorption behavior of the ensuing composites were investigated. Study of the morphology of the composites by Scanning Electron Microscopy (SEM) shows a slight improvement of the MFC/LDPE interface after allylation and propargylation. Moreover, the mechanical 1

ACCEPTED MANUSCRIPT properties of the composites were significantly improved and the moisture absorption was reduced compared to unmodified MFC fibers. Keywords: Microfibrillated cellulose; Composites; Surface treatments

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1. Introduction

Over the past two decades, the increase of environmental concerns and shortage of

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petroleum resources have provoked a growing interest in the use of natural fibers as an alternative to synthetic fibers for the reinforcement of composites [1-3]. Natural fibers

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possess desirable specific properties including biodegradability, renewability and low-cost [4-5]. In addition, they have densities much lower than synthetic fibers, which makes them interesting for different applications ranging from automotive parts to packaging [6]. In recent years, researchers have focused their work on the processing of

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nanocomposites to enhance the mechanical properties [7]. Nanocomposites use a matrix where the nanosized reinforcement elements are dispersed. The reinforcement is considered as a nanoparticle when at least one of its dimensions is lower than 100 nm. This

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feature provides nanocomposites unique and outstanding properties never observed in

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conventional composites [8].

The isolation of pure cellulosic structure having dimensions in the range of 1 – 100 nm

from wood and plants requires multi-stage disintegration processing [9]. Several terms are used to describe nano-sized cellulose formed by such procedures. Cellulose whiskers refer to straight crystals of cellulose, which are usually produced by acid hydrolysis [10]. Nanofibrillated cellulose (NFC) and microfibrillated cellulose (MFC) are used to designate long flexible nanoparticles consisting of alternating crystalline and amorphous strings. NFC consists of fine particles with characteristic lengths of below ten microns, whereas 2

ACCEPTED MANUSCRIPT MFC contains a wide distribution of particle sizes ranging from parts of pulp fibers to nanoscale [11]. They are usually obtained via mechanical refining of highly purified wood and plant fiber pulps. The production of NFC requires chemical or enzymatic pre-treatment

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in order to facilitate fibrillation [7,12]. MFC fibers display specific properties such as high aspect ratio, high strength and high stiffness provide a promising reinforcement effect for many applications [13-15].

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However, the hydrophilic nature of MFC leads to weak interfacial adhesion with

hydrophobic polymers and poor mechanical properties of the composites [16]. The

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adequate dispersion of MFC within the matrix is a critical aspect to be addressed. The use of solid-state shear pulverization was found to be effective for incorporating cellulose nanocrystal in polyolefins [17]. The process enabled high shear and compressive forces to be imparted to the materials, resulting in excellent CNC dispersion.

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In the past many attempts have been made to modify the surface of cellulose fibers in order to improve adhesion with thermoplastic matrices and reduce the moisture absorption of the composites [18-20]. The efficiency of reinforcement is dependent on the quality of

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the stress transfer between the matrix and the fiber [21]. Several surface treatment methods

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have been carried out to reduce the polarity and the hydrophilicity of the cellulose. Physical treatments such as corona treatments [22] and plasma treatments [23] of long cellulose fibers have been reported to enhance fiber/polymer compatibility and interfacial adhesion. Other researchers have focused on chemical modification such as graft copolymerization [24], silane treatment [25], and treatments with other chemicals [26]. However, these methods suffer many drawbacks such as the requirement for expensive equipment or the use of toxic and expensive chemicals.

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ACCEPTED MANUSCRIPT Recently, many researchers have carried out the propargylation of Kraft pulp in an aqueous medium as an intermediate of click chemistry reaction [27,28]. The sheets made of propargylated Kraft pulp were found to exhibit better mechanical properties than sheets

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made of unmodified Kraft pulp [29]. Based on these works, we have developed an effective process for the alkylation of MFC fibers in an aqueous medium, without stirring, by simple imbibing. The aim of this paper is to investigate the effects of those chemical

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modifications on the morphology, mechanical properties and water uptake of the MFCLDPE composites.

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2. Experimental 2.1. Materials

MFC fibers were obtained after mechanical refining of softwood kraft fibers (Patent US 20110277947 A1). Each fiber was peeled into around 1000 MFC, with a length of

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between 500-1000 µm, a thickness of 10-100 nm, and a width of 80-300 nm (Fig. 1). Samples of MFC were supplied by Kruger Biomaterials Inc. (Trois-Rivières, QC, Canada) and were used as received. Propargyl bromide (80% in toluene, stabilized with MgO), allyl

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bromide (99%, stabilized with 300-1000 ppm propylene oxide), propyl bromide (99%) and

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sodium hydroxide (pearl, 97%) were supplied by Alfa Aesar and used as received. Lowdensity polyethylene (LDPE) having a melt flow of 5.2 g/10 min and a density of 0.932 g.cm-3 was supplied by ExxonMobil.

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2.2.Alkylation of MFC

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Fig. 1. SEM images of MFC fibers provided by Kruger Biomaterials Inc.

The alkylation of MFC was carried out using the following procedure. MFC (31.5 g) were swollen in 4,8% aqueous sodium hydroxide solution (480 mL) for 15 min resulting in

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a 7wt% mixture. Then, propargyl bromide or allyl bromide or propyl bromide (10 eq/AGU) was added and the reaction mixture left for 4 days at room temperature, without any stirring. Then, the resulting sample was washed twice with water until pH 7 and then

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with denatured alcohol in order to perform a solvent exchange and ease drying. Finally, the

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alkylated MFC were dried overnight in an oven at 60°C. 2.3. Preparations of the composites MFC-LDPE composites were prepared by mixing the LDPE matrix with 10% and 20%

of unmodified or alkylated MFC using a two-roller Thermotron-C.W Brabender (Model T303). The LDPE matrix was melted at 170°C at 60 rpm, then MFC (10% or 20%) were added within 7 min followed by re-mixing hot resulting materials for five times, 3 min each time, to get a uniform composite material [30]. The prepared composites were molded into dumbbell shaped specimens (ASTM D638 Type V) by compression molding. The 5

ACCEPTED MANUSCRIPT mold (DAKE, Model 44-250) was maintained at a pressure of 15 bars at 170°C for 10 min, then cooled to below 100°C by circulating cold water into the press [30]. The approximate dimensions of tensile specimens were 0.28-0.30 cm in width and 0.32-0.34 cm in

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thickness. 2.4. Characterization

Fourier transform infrared spectroscopy (FTIR) was performed to evaluate the

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chemical modification of the MFC. Spectra were recorded on a Perkin Elmer 1000 FTIR spectrometer. The spectra were obtained by preparing dried KBr powder pellets containing

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5w/w% of the investigated sample. The discs were scanned over the range 3800 to 400 cmwith a total of 16 scans at a resolution of ±1 cm-1.

Nuclear Magnetic Resonance (NMR) was used to further investigate the chemical

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modification. Solid-state 13C NMR was performed on a Bruker Avance 600 spectrometer with a rotor speed of 14 kHz. Cross-polarized magic angle spinning (CPMAS) was carried out with a contact time of 1.5 msec and a repetition time of 3 sec.

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The degree of substitution (DS) of alkylated MFC was calculated from the %[O] and %[C] values determined by X-ray photoelectron spectrometry (XPS) using a Kratos Axis

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Ultra spectrometer that provided elemental composition information to a depth of few nanometers. The samples (20 mg) were pelletized by a hydraulic press at a pressure of 10 kPa and analyzed. Three repeats were completed for each sample. Example of DS calculation from XPS data, for propargylation: In an AGU there are 5 oxygen atoms. So, one AGU is presented by this percentage: %[1 AGU] = %[O] / 5

(Eq. 1)

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ACCEPTED MANUSCRIPT An AGU contains 6 carbon atoms, thus the percentage of carbon atoms of AGU is: %[CAGU] = 6 × %[1 AGU]

(Eq. 2)

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The percentage of carbon atoms per propargyl unit is: %[Cprop] = %[C] - %[CAGU]

(Eq. 3)

The percentage of propargyl unit is:

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%[Prop] = %[Cprop] / 3

(Eq. 4)

per the percentage of one AGU:

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Finally, the DS of propargylation is obtained by dividing the percentage of propargyl unit

DS = %[Prop] / %[1 AGU]

(Eq. 5)

Differential scanning calorimetry (DSC) experiments were carried out with a TA

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Instrument DSC Q1000 using ~10 mg of the studied material in sealed aluminum capsules. Each sample was heated from 40°C to 170°C at a heating rate of 10°C.min-1.

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Measurements were carried out under a nitrogen flow (50 mL.min-1). An empty pan was used as a reference. The melting temperature (Tm) was taken as the peak temperature of the

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melting endotherm, while the heat of melting (∆Hm) was calculated from the area of the peaks. The degree of crystallinity (Xc) was determined using the relationship: Xc = (∆Hm/∆H0m) × (100/w)

(Eq. 6)

where ∆H0m = 285 J.g-1 [31] is the heat of fusion of 100% crystalline PE and w is the weight fraction of polymeric matrix material in the composite. The morphology of the MFC/LDPE interface was characterized by Scanning Electron Microscopy (SEM). Before the fracture, the specimens were frozen into liquid nitrogen to 7

ACCEPTED MANUSCRIPT impede the plastic deformation of the matrix and to get well defined MFC-matrix interface. Fracture surfaces of the composite samples were coated with gold and then analyzed with a JEOL JSM T300 microscope operated in secondary electron mode at a beam current of 100

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µA with an accelerating voltage of 15 kV. Tensile testing of LDPE based MFC composites were performed on an Instron tester (Model 4201) with a crosshead speed of 10 mm/min at 23°C and 50% level of relative

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humidity. The load cell of the machine was 5 kN. Previously, all specimens were

conditioned overnight in that testing room. A total of 5 samples were tested according to

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ASTM D638.

In order to study the water uptake of composites, all samples (dimensions 21 mm x 9.5 mm x 2.9 mm) were soaked in distilled water at room temperature. The samples were taken out of the water after 24 h, the wet surfaces were dried with compressed air, the

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samples were weighed and then put back into the water. This process was repeated during 4 days. The water uptake (Wuptake) at time t was determined as following: (Eq. 7)

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Wuptake (w%) = [(Wt-W0)/W0] × 100

Where W0 is the initial weight of the specimen and Wt the weight of the specimen at time t.

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All the samples were weighted using a balance Precisa model XB220A with a precision of ±0.0001 g.

3. Results and discussion 3.1. MFC modifications In first, we applied the method developed by Elchinger et al. [29] for Kraft fibers

propargylation in NaOH/H2O system (Fig. 2). The test carried out did not give satisfactory

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ACCEPTED MANUSCRIPT results, the yield of the reaction was dramatically lower. Kraft fibers, used in Elchinger et al. work, possess more amorphous regions than MFC which lead to a better accessibility of the hydroxyl groups. In the same conditions, the swelling of MFC fibers was not as good

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as Kraft fibers. We have therefore modified this method in order to apply it to MFC.

Fig. 2. Schematic illustration of cellulose propargylation

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The concentration of sodium hydroxide has been halved while the volume of water has been adjusted to 15 mL/g in order to enhance the swelling of the MFC. As a result, the

alkylated MFC.

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hydroxyl groups of the cellulose reacted easier with the alkyl bromide to form the desired

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The FTIR spectrum (Fig. 3) of propargylated MFC shows a new band at 2113 cm-1 characteristic of the stretching vibration of the triple bond -C≡C- [28]. This confirms the presence of propargyl groups in the MFC. Other signals were consistent with those obtained for unmodified MFC. No evidence of modification was found for propylated and allylated MFC by FTIR.

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Fig. 3. FTIR Spectra of unmodified and propargylated MFC

The solid-state 13C NMR spectra (Fig. 4) of allylated MFC confirms the modification

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with the presence of new peaks at 135.59 ppm and 117.24 ppm characteristic of the double bond -C=C-. Other signals are in accordance with the different carbons of cellulose: 106.2 ppm (C-1), 89.1 ppm and 83.5 ppm (respectively for C-4/C-4’ of crystalline/amorphous

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cellulose), 75.3 ppm (C-5), 72.7 ppm (C-2, C-3), 65.2 ppm and 62.9 ppm (respectively for C-6/C-6’ of crystalline/amorphous cellulose) [32]. No evidence of modification was found

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for propylated and propargylated MFC by solid-state 13C NMR.

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Fig. 4. Solid-state 13C NMR spectra of unmodified and allylated MFC XPS measurement of carbon and oxygen signals can be used in analysis of modified cellulose fibers [28,33-35]. However, surface analyses based on carbon and oxygen

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signals only may be unreliable due to surface contamination [36]. It has been found that contaminating material on fiber surfaces contains both carbon and oxygen and is responsible for the discrepancy between the true elemental composition of pulp fiber

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surfaces and the composition determined by XPS [37]. To minimize the effects of surface

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contamination, we used as reference the XPS signal of unmodified MFC, measured under the same conditions as the alkylated MFC (Table 1). The carbon/oxygen ratio increased after alkylation which confirms the grafting of the different alkyl groups. The DS of propargylated MFC was calculated to be 0.33, similar value was obtained for allylated MFC with a DS of 0.29. However, the DS of propylated MFC was found to be only 0.12. Propargyl and allyl bromide seem to react easier with cellulose than propyl bromide.

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C/O

DS

MFC (Blank)

57.09%

42.91%

1.33

0

Propylated MFC

58.44%

41.56%

1.41

0.12

Allylated MFC

60.05%

39.95%

Propargylated MFC

60.46%

39.54%

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C (%)

0.29

1.53

0.33

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1.50

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DSC measurements were performed to characterize the thermal behavior of LDPE based MFC composites. Each sample showed an endothermic peak ascribed to the melting of the crystalline domains of the matrix. The thermal characteristics (∆Hm, Χc and Tm) determined from DSC scans are reported in Table 2. Both the degree of crystallinity and the melting temperature are unaffected by the presence of unmodified or alkylated MFC.

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Indeed, values close to 49% and to 118 °C are obtained for the degree of crystallinity and the melting temperature, respectively. These results suggest that the MFC does not act as a

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nucleating agent for LDPE.

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ACCEPTED MANUSCRIPT Table 2 Melting enthalpy (∆Hm), degree of crystallinity (Χc) and melting temperature (Tm) of LDPE based MFC composites obtained from the DSC curves Fiber loading (%)

∆Hm (J.g-1)

Χc (%)

Tm (°C)

LDPE

-

138.7

49

118

Unmodified MFC-LDPE

20

111.4

Propylated MFC-LDPE

20

113.0

Allylated MFC-LDPE

20

112.5

Propargylated MFC-LDPE

20

112.3

118

50

117

49

118

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49

49

118

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3.2. MFC morphology

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Sample

The chemical modification of the MFC should lead to an improvement of the

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MFC/matrix interface and to a better dispersion of the MFC within the matrix. Increasing the hydrophobicity of the MFC is essential to obtain a good compatibility with hydrophobic matrices in order to achieve good mechanical properties [38]. SEM

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observations of fractured surfaces of LDPE-based composites reinforced with 20 w% MFC were carried out to investigate the MFC/matrix interface before and after alkylation. The

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images of surface fracture for unmodified MFC-based composite show a good dispersion of the MFC within the matrix due to their nanoscale dimensions (Fig. 5a). However, the MFC are pulled out from the matrix which lead to many holes at the surface and indicate that the interfacial adhesion between the MFC and the matrix is poor. After alkylation, the MFC/LDPE interface remained really close especially for propylated MFC-based composites (Fig. 5b). In the case of allylated and propargylated MFC, the interface seems to be slightly enhanced, MFC are more broken off and less pull out which provides fewer holes at the surface (Fig. 5c and 5d). 13

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(b)

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(a)

(c)

(d)

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Fig. 5. SEM micrographs of fractured surfaces of LDPE-based composites reinforced with 20 w% MFC: (a) unmodified, (b) propylated, (c) allylated, and (d) propargylated MFC 3.3. Mechanical properties

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The Fig. 6 gives tensile stress-strain behavior of MFC-LDPE composites. At a low

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strain (<20%), stress was proportional to strain with the constant of proportionality being Young’s modulus according to Hooke’s law. Beyond 20% of strain, stress induced plastic flow in the specimen. With the increase of the fiber content, both the Young’s modulus and the tensile strength increased while the elongation at break decreased.

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Fig. 6. Stress-strain curves of unmodified and alkylated MFC-LDPE composites at a MFC loading of a) 10% and b) 20%.

The alkylation of MFC led to composites stronger than those based on unmodified MFC. At a MFC content of 20%, the elongation at break was close for unmodified and alkylated MFC based composites but the stress at break was 13% to 26% higher with

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alkylated MFC composites.

The change in Young’s modulus values depending on the MFC content is presented in Fig. 7. All the samples showed an increasing reinforcement effect that confirms the

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promising properties of MFC. The augmentation of Young’s modulus values with the increase of fiber loading occurs because MFC is a rigid material which increases the

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stiffness of the composite. MFC possesses a high surface area and aspect ratio which enhance its wettability and lead to a good dispersion within the polymeric matrix.

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Fig. 7. Young's modulus versus fiber content for LDPE based composites

Composites based on alkylated MFC show a rise of Young’s modulus values compared

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to unmodified MFC. However, this evolution depended on the alkyl bromide structure. At a MFC loading of 10%, the modulus increased by 45%, 21% and 14% for allyl, propargyl and propyl groups, respectively. An increase of 31%, 18% and 8% were observed for allyl,

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propargyl and propyl groups, respectively, at a MFC loading of 20%. The alkyl groups grafted onto the MFC resulted in the composite becoming stiffer. The improvement of the

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compatibility between the MFC and the LDPE matrix led to a better dispersion of alkylated MFC within the matrix compared to unmodified MFC. Thus, the Young’s modulus values were enhanced by the chemical modification. Allylated MFC based composites display highest mechanical performances. The reactions of alkylation also increased the tensile strength of the composites, as is shown in Fig. 8. The strength of the materials is enhanced with the fiber loading, as described for the Young’s modulus (Fig. 7). These increases show that the interaction

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ACCEPTED MANUSCRIPT between the MFC and the matrix is good, MFC are an effective stress-transfer medium. At a fiber loading of 10%, the composite using alkylated MFC exhibited a tensile strength 10% greater than that using the unmodified MFC. That increase rose to 26%, 22% and

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13% for allylated, propargylated and propylated MFC, respectively, at a fiber content of 20%. The grafting of the different alkyl groups onto the fibers brings strength to the

composites due to the enhanced hydrophobic interactions with the LDPE matrix. In the

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case of allylated MFC, the mechanism of interaction between the filler and the matrix

could be quite different. Indeed, radical species may be generated on the LDPE through

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thermal degradation likely to occur during the processing of the composite [25]. Therefore, allylated MFC could react with those radical species leading to covalent bonding between

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the filler and the matrix which enhance the interfacial adhesion.

Fig. 8. Tensile strength versus fiber content for LDPE based composites

3.4. Water uptake

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ACCEPTED MANUSCRIPT The hydrophilic behavior of MFC fibers is one of the main drawbacks which limits their application in composite materials [8]. Cellulose tends to absorb water which is detrimental to the durability and to the mechanical properties of the composites [39]. The

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alkylation reactions described in this paper aim to enhance the hydrophobicity of the fibers. MFC fibers possess a large amount of free hydroxyl groups which are responsible for the interaction with water. Substituting these hydroxyl groups with a non-polar group, such as

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propargyl, allyl and propyl, should lead to a significant decrease of moisture absorption. Surprisingly, the water uptake of propylated and unmodified MFC composites are close

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(Fig. 9). The low DS obtained after propylation leads to hydrophilic MFC. On the contrary, the water uptakes of composites based on allylated and propargylated MFC are significantly lower than those obtained for unmodified MFC composites, as is shown in Fig. 9. Indeed, the moisture absorption of the composites is reduced by 15% and 50%

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using allylated and propargylated MFC, respectively, compared to unmodified MFC. The propargyl group seems to exhibit more important hydrophobic behavior than allyl group

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that is confirmed by the low water uptake of propargylated MFC-LDPE composites.

Fig. 9. Water uptake versus immersion time for LDPE-based composites reinforced with a) 10% of MFC and b) 20% of MFC

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ACCEPTED MANUSCRIPT 4. Conclusion This work demonstrates a green and effective process for the reinforcement of thermoplastic matrices by alkylation of MFC fibers. Evidence for the chemical

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modifications was obtained by FTIR, 13C NMR, and by XPS exhibiting an increased C/O ratio. Composite materials were obtained by compounding unmodified or alkylated MFC with LDPE. The chemical modification enhances the MFC/matrix adhesion which leads to

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better mechanical properties. Composites based on allylated and propargylated MFC show promising properties for the use as a green reinforcement agent in a polymer composite.

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These composites exhibit better mechanical properties than those based on unmodified MFC. In addition, they absorb significantly less water than unmodified MFC due to their hydrophobic behavior. The resistance and the durability of the composite are extensively improved. The allylation and propargylation of MFC have succeeded to improve the

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mechanical properties of the composites while reducing its moisture absorption, using an eco-friendly and cost-effectiveness process.

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Acknowledgements

The authors are thankful to Kruger Biomaterial Inc. (Trois-Rivières, Canada) for supplying

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MFC as a raw material. Financial support from region Limousin is gratefully acknowledged. References

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1. SEM images of MFC fibers provided by Kruger Biomaterials Inc.

Fig. 3. FTIR Spectra of unmodified and propargylated MFC

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Fig. 2. Schematic illustration of cellulose propargylation

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Fig. 4. Solid-state 13C NMR spectra of unmodified and allylated MFC

Fig. 5. SEM micrographs of fractured surfaces of LDPE-based composites reinforced with

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20 w% MFC: (a) unmodified, (b) propylated, (c) allylated, and (d) propargylated MFC Fig. 6. Stress-strain curves of unmodified and alkylated MFC-LDPE composites Fig. 7. Young's modulus versus fiber content for LDPE based composites

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Fig. 8. Tensile strength versus fiber content for LDPE based composites Fig. 9. Water uptake versus immersion time for LDPE-based composites reinforced with a)

Table captions

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10% of MFC and b) 20% of MFC

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Table 1 XPS results of unmodified and alkylated MFC Table 2 Melting enthalpy (∆Hm), degree of crystallinity (Χc) and melting temperature (Tm) of LDPE based MFC composites obtained from the DSC curves

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Alkylation of MFC fibers was performed in aqueous medium. Unmodified and alkylated MFC LDPE-composites were prepared. Mechanical properties of the composites improved after alkylation. Propargylation and allylation led to a decrease of the water absorption behavior of the MFC-composites.

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