Microstructural and Chemical Approach To Highlight How a Simple

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A microstructural and chemical approach to highlight how a simple methyl group affects the mechanical properties of a natural fibers composite Antoine Gallos, Gabriel Paës, David Legland, Johnny Beaugrand, and Florent Allais ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02399 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 12, 2017

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ACS Sustainable Chemistry & Engineering

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A microstructural and chemical approach to highlight how a simple methyl group affects the mechanical properties of a natural fibers composite

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Antoine Gallosa,b,*, Gabriel Paësb, David Leglandc, Johnny Beaugrandb,c, Florent Allaisa,d

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a

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51110 Pomacle, France

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b

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51100 Reims, France

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c

Biopolymères Interactions Assemblages (BIA), INRA, rue de la Géraudière, F-44316 Nantes, France

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d

Génie et Microbiologie des Procédés Alimentaires (GMPA), INRA, site de Grignon, F-78850 Thierval-

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Grignon, France

Chaire Agro-Biotechnologies Industrielles (ABI), AgroParisTech, CEBB, 3 rue des rouges Terres, F-

FARE Laboratory, INRA, Université de Reims Champagne-Ardenne, 2 esplanade Roland-Garros, F-

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*Corresponding author : [email protected]

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Keywords

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Polymer

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Material

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Plasticizer

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Chemical imaging

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Bioressource

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Abstract

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Two ferulic acid derivatives (BDF and BDF-Me) were prepared using chemo-enzymatic

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synthesis and used as additives for the pretreatment of hemp fibers. Incorporation of these fibers

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into a polycaprolactone matrix by hot-melt extrusion process aimed to improve the dispersion of the

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fibers and the mechanical properties of the resulting materials. Young's modulus and tensile strength

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of the composites were investigated at the micrometer scale by chemical imaging. The very simple

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methylation of the phenolic functions led to significant mechanical properties differences due to the

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dispersion of the fibers caused by a plasticizing effect of the ferulic acid derivative. This significant

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plasticizing effect of BDF-Me is observed at a content as low as 0.8 w% and opens the way for

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synthesizing a new family of biobased plasticizers involving transition from crystal state to

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amorphous phase.

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Introduction

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According to the global trend in the world, many studies are conducted in order to use plant

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biomass to produce sustainable goods and in particular biobased materials for uses in many

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industrial applications such as transportation, fabrics, furniture or buildings.1,2 Biocomposites is an

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emerging family of biomaterials which are most of the time made of a polymeric matrix (biobased or

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oil-based) reinforced with natural fibers and especially lignocellulosic fibers (e.g. hemp, flax,

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miscanthus, wheat, alfa, bamboo) to enhance some of their thermal (e.g., glass transition) and

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mechanical (e.g., stiffness, tensile strength) properties.3,4 Although many studies have shown that

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natural fibers are well suited for production of biocomposites (e.g., hemp, flax),5–7 there is still a

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margin of progress for manufacturing biocomposites with higher thermo-mechanical properties.

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Some technological barriers to challenge concern the improvement of the interface between natural

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fibers and the polymeric matrix, according the nature of the polymer which remains a key parameter

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in interface quality.8 Today, paper industry is one major sector of lignocellulose transformation, also

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resulting in the production of a huge amount of lignins as by-products. Lignins are made of aromatic

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compounds and are mainly currently burnt to produce electricity,9 but they have a strong potential

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to be valorized through chemistry for the synthesis of high value-added compounds like ferulic

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acid.10,11

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Numerous studies report the use of lignin as a compatibiliser between lignocellulosic fibers

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and polymeric matrixes (e.g., PCL, PLA, PP).12–15 They evidenced significant improvements of the

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interfaces between the fibers and the polymeric matrixes enhancing the mechanical properties of

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the composites. This study aimed to use ferulic acid and its derivatives, which can be readily obtained

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from lignocellulosic biomass (e.g., wheat and rice bran, beetroot pulp), to improve the mechanical

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properties of biocomposites materials reinforced with hemp fibers and prepared by hot-melt

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extrusion process. The objective was to plasticize the middle lamella, mainly composed of lignin,

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hemicellulose and pectin,16,17 in the bundles of hemp fibers to ease the decohesion during the

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extrusion process and to achieve a better dispersion of the fibers, the microstructure of the

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composites reinforced with lignocellulosic fibers being one of the key parameters for their

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mechanical properties.2


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Two renewable ferulic acid derivatives were efficiently synthesized through a chemo-

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enzymatic pathway.18–20 The first ferulic acid derivative (hereafter named BDF) was made of two

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ferulic acid molecules linked together by 1,4-butanediol. The second one (hereafter named BDF-Me)

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was obtained after the methylation of the phenolic function of BDF. These ferulic acid derivatives

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were selected because they mimics the esters covalent bonds found in plant cell walls between the

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ferulic acid from lignin and the arabinose from hemicellulose in lignocellulosic fibers. Our hypothesis

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is that the presence of these ferulic acid derivatives can favor the plasticization of the middle lamella

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and ease the decohesion of the fibers. BDF and BDF-Me were sprayed on the hemp fibers prior the

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extrusion process. Such pretreatment of the fibers was performed to facilitate the interactions

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between the ferulic acid derivatives and the fibers. A solvent impregnation method was rejected,

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since it could have extracted the compounds of the middle lamella during the pretreatment of the

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fibers. Then these pretreated lignocellulosic fibers were incorporated into a polycaprolactone (PCL)

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matrix using a single screw extrusion process to prepare three series of composites containing

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respectively crude hemp fibers, hemp fibers treated with BDF and hemp fibers treated with BDF-Me.

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PCL was chosen since it can be processed with soft conditions reducing thermal and mechanical

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degradation of the fibers, allowing to evaluate more accurately the effect of the treatment on the

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dispersion of the fibers and on the properties of the composites21 The polymeric matrix was

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extracted by solvent extraction to measure the exact fiber content in our materials. The mechanical

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properties (Young's modulus and tensile strength) of these composites were measured. Chemical

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analyses were conducted by Size Exclusion Chromatography (SEC) to measure the impact of the two

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ferulic acid derivatives on the molecular weight of the matrix and to investigate chemical

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interactions. Thermal analysis were led by Differential Scanning Calorimetry (DSC) to evidence

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thermal phenomena highlighting the behavior of BDF and BDF-Me during the extrusion process and

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their impact in the final properties of the composites. The microstructure of the composites after the

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extrusion process was then determined by confocal Raman imaging (CRI) and image analysis,22 and

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was interpreted in the light of the mechanical properties previously measured. Thermal analysis and

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CRI showed a good dispersion of the ferulic acid derivatives in the polymeric matrix and a phase

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transition of one of these additives from a crystalline structure to an amorphous state during the

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extrusion process leading to an unexpected plasticizing effect.

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Experimental section

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Materials



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Composites were prepared with Capa™ 6800 polycaprolactone (PCL) provided by Perstorp

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(Warrington, United Kingdom). PCL has a molecular weight of 80.0 kg.mol-1 and a melting point of 58-

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60 °C. Hemp bast fibers (Cannabis sativa, variety Fedora 17, monoicous plants) were harvested in

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2014 and supplied by Fibres Recherches Développement® (Troyes, France). They still contained some

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woody impurities and were chopped into 5 mm-long pieces prior to extrusion. Ferulic acid was

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purchased from Sigma-Aldrich (reagent grade, ≥99% purity). 1,4-Butanediol was purchased from Alfa

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Aesar (reagent grade, 99% purity). Iodomethane was purchased from ACROS Organics (reagent

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grade, ≥99% purity). Dimethylformamide (DMF) was purchased from Fisher Chemical (HPLC grade,

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99.99% purity) and dried on a MB-SPS-800 from MBraun.

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Chemical and structural characterizations 1

H and 13C NMR analyses were conducted on a Bruker Fourier Ultrashield™ 300 MHz in CDCl3

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using tetramethylsilane (TMS).The Fourier Transform Infra Red (FT-IR) spectra were recorded on a

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Cary 630 FTIR from Agilent Technologies. The Ultra Violet (UV) spectra were acquired on a Cary 60

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UV-VIS from Agilent Technologies. The distribution of molar masses in polymeric matrix after

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extraction from the materials with tetrahydrofuran (THF) was measured by SEC using a WATERS 515

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HPLC pump, a WATERS 717 Plus autosampler and three PL gel Mixed-B columns. The multidetection

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system is composed of a Multi Angle Laser Light Scattering (MALLS) Dawn HELEOS II module from

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Wyatt and a Differential Refractometer WATERS 2414. The calibration curves were done with

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polystyrene (PS) standards (Varian Standards) and THF was used as eluent at 1.0 mL.min-1. The

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refractive index increment (dn/dc) used for PCL was 0.053.23

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Synthesis of ferulic acid derivatives

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The BDF (bis-O-dihydroferuloyl-1,4-butanediol), was synthesized as described elsewhere.18–20

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The same protocol was applied to prepare an amount of 100 g of BDF afforded as a white powder

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(93%, m.p. 106.9 °C). BDF-Me (Butane-1,4-diylbis(3-(3,4-dimethoxyphenyl)propanoate)) was

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synthesized through the methylation of the phenolic functions of BDF (Figure 1). BDF (50 g, 112

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mmol, 1 eq) was placed into a round-bottom flask filled with anhydrous DMF, under nitrogen

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atmosphere. Potassium carbonate (61.8 g, 448 mmol, 4 eq) was added and the mixture was

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magnetically stirred. Methyl iodine (27.9 mL, 448 mmol, 4 eq) was then slowly added. The mixture

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was then heated at 80 °C and kept under stirring overnight. The solution was filtered and the filtrate

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was poured in cold water. The resulting precipitate was filtered, rinsed and washed with water


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before being dried under vacuum for 3 days to afford BDF-Me as a white powder (48.8 g, 92%, m.p.

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74.9 °C). UV: λmax (EtOH, nm) 210, 235 and 270. FT-IR (neat) νmax (cm-1) 1727 (C=O), 1514 (C=Caromatic),

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1135 (C-O-C) . NMR: δH (300 MHz, CDCl3) 1.57 (4H, m, H1), 2.54 (4H, t, J(H,H) = 8 Hz, H4 ), 2.83 (4H, t,

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J(H,H) = 8 Hz, H5), 3.80 (12H, s, H12,13), 4.01 (4H, m, H2), 6.66 - 6.73 (6H, m, H7,8,11). NMR: δC (300 MHz,

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CDCl3) 25.4(C1), 30.7 (C5), 36.2 (C4), 55.9 (C12), 56.0 (C13), 64.0 (C2), 111.4 (C7), 111.7 (C10), 120.2 (C11),

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133.2 (C6), 147.6 (C8), 149.0 (C9), 173.1 (C3). HRMS : m/z calculated for C26H34O8 + K [M + K]: 513.1882;

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found: 513.1891.

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Pretreatment of fibers

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The fibers were pretreated with the two ferulic acid derivatives prior their incorporation into

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the PCL matrix. The additives were sprayed with an Ecospray® device, at 15 wt% on hemp fibers. For

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that 4.5 g of BDF (or BDF-Me) were prior solubilized in 50 mL of acetone and pulverized on a thin

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layer of 25.5 hemp fibers. This step was repeated until reaching a sufficient quantity (close to 500 g)

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of pretreated fibers to prepare the composites. We chose to perform the pretreatment of a small

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amount of fibers (30 g) at a time to ensure a good homogeneity of the pulverization of the additives.

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The fibers were afterwards dried at 50 °C for 24h under vacuum to remove the organic solvent.

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Processing and mechanical characterizations The processing and mechanical characterizations were already reported elsewehere.22 The

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composites were prepared by extrusion on a Scamia single-screw extruder (Scamex, France L = 218

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mm and D = 17 mm; L/D = 12.8). Feeder, conveyor and die were set at 100 °C. The temperature of

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the matter was ensured with a thermocouple located near the die. The screw speed was set at 25

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rpm. The compounds were processed two times to ensure a good homogeneity in the final materials,

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since it appeared that a single step extrusion process led to a too heterogeneous compound. Such

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heterogeneity would have been dramatically prejudicial to the thermo-mechanical characterizations.

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The compounds were granulated (≥5 mm) prior the second process. The samples produced are listed

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in Table 1. The samples were then injected into tensile test specimens according ISO 527-2-5 A

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(length = 74.0 mm, centre width = 4.1 mm and thickness = 2.0 mm) with an axial extensometer (axial

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extensometer 3542 from Epsilon tech). The injection molding was conducted on a bench scale DSM

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Xplore micro injection mould IM 12. The melting temperature was set at 130 °C and the mold at 45

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°C. The injection pressure was set at 1.6 × 103 kN.m-2. The mechanical analyses were conducted by

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tensile test (ISO 527-2-5 A) on a Desktop Universal Tester from Testwell (room temperature = 20 °C



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and room humidity = 65% RH), with a speed of 10 mm.min-1 and a force of 2 kN. Ten specimens were

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tested for each formulation. The values given hereafter are mean values ± the standard deviation σ.

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Fiber content in compounds

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As reported elsewhere,22 to determine the exact fiber content in each composite, the PCL

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matrix was extracted from the lignocellulosic fibers by Soxhlet extraction. Two grams of each

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composite were processed into a Soxhlet apparatus over 24h in a 50/50 CHCl3/CH2Cl2 solution. Then,

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the remnant fraction of fibers was dried and weighed.

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Thermal analysis

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The thermal analyses were conducted by DSC on a TA Q20 from TA Instruments. The heating

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and cooling ramp were set at 10 °C.min-1, from -80 °C to 200 °C, under nitrogen flow. Two heating

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and cooling cycles were done for each sample: the first to erase the thermal history of the sample

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and the second to analyze the heat flow of the sample after being cooled in controlled conditions.

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Confocal Raman Imaging and image analysis

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The confocal Raman imaging (CRI) was conducted on an Alpha 300 confocal Raman

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microscope equipped with a TrueSurface® from Witec. Tensile test specimen were analyzed on two

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locations: at the surface of the specimen (corresponding to the "skin" of the materials) and on the

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cross section (corresponding to the "core" of the materials). The wavelength (λ) of the laser was 532

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nm, and the numerical aperture (NA) of the 10× objective was 0.25. These settings allowed to reach

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theoretical lateral resolutions (Δx and Δy) close to 1.3 µm.24 The axial resolution (Δz) was

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experimentally determined to be 13 µm. The full procedure for the characterization and the

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preparation of the samples is described elsewhere.22 The investigation of the crystalline or

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amorphous state of BDF and BDF-Me incorporated in PCL was also conducted by CRI with the same

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confocal Raman microscope. The wavelength (λ) of the laser was still 532 nm and the numerical

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aperture (NA) of the 50× objective was 0.8. These settings allowed a theoretical lateral resolution (Δx

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and Δy) close to 0.4 µm.24 The pixel size was set at 0.5 µm2 and the acquisition time was set at 500

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ms. The axial resolution was experimentally determined to be between 1 and 2 µm.

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The Raman data were then processed, by applying a K-means clustering method.25 Three

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clusters were used for the characterization of the microstructure of the composites to allow the


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recognition of 3 different species (PCL/Hemp/BDF or PCL/Hemp/BDF-Me). The recognition of the

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chemical species (PCL, Hemp and BDF) according their Raman signature is fully described

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elsewhere.22 The recognition of BDF-Me is described hereafter. Image analysis was performed on the

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picture acquired from CRI to quantitatively describe the morphology of the network formed by hemp

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fibres. The full image treatments conducted on the data (e.g., noise removal) is reported elsewhere

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as well as the way of determining the three morphometric descriptors (area density, boundary

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density and Euler number density) and the oriented granulometry.22 The area density measures the

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fraction of pixels corresponding to fibres. The boundary density is related to the interface between

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PCL and hemp fibers. The Euler number density describes the topology of the planar network

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formed by the fibre cross-sections. Finally, oriented granulometry analysis was used to assess

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variations of the relative orientation of the fibres within the matrix, which is related to the anisotropy

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of the network.

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Results and discussion

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Synthesis and structural characterizations of ferulic acid derivatives

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The first ferulic acid derivative, BDF, was synthesized from ferulic acid and 1,4-butadeniol in

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93% overall yield via a 3-step chemo-enzymatic process.18–20 BDF-Me was then readily obtained

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through the methylation of BDF in presence of methyl iodine and potassium carbonate (92% yield).

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The structure of BDF-Me was confirmed by NMR spectroscopy (Figure S2 and Figure S4 in Supporting

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Information). 1H NMR spectrum exhibits the peaks of BDF18 for the aromatic and aliphatic protons

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but the peak at 5.69 ppm related to the phenol function disappeared while a singlet appears at 3.88

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ppm which is consistent with a methyl group (OMe). The 13C NMR spectrum of BDF-Me exhibits the

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peaks of BDF18 plus a new peak at 56 ppm that corresponds to the newly formed methyl group.

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(Figure 1).

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Mechanical characterizations

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The evolutions of both the Young’s modulus and the tensile strength according to the fiber

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content are presented in Figure 2 and in Figure 3 respectively. As the fiber content was determined

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by solvent extraction, single points differ for fiber content and ferulic acid derivative content. In

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order to consider the evolution of the properties regarding the increase of the fiber content, these

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data are represented in scatter graphs instead of histogram to confront the trends for each series of



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composites and not to make direct comparison between single points for a given fiber content. The

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Young's modulus increases with the increase of fiber content for the three series of composites. Such

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an increase of the stiffness of PCL reinforced with natural fiber was already reported.2,26 Both of

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crude PCL and PCL reinforced with BDF-treated hemp fibers display a similar increase of Young's

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modulus. The PCL reinforced with BDF-Me-treated hemp fibers showed a lower increase of the

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Young's modulus. The difference goes from 210 MPa at a fiber content close to 4.3 wt% (0.8 wt% of

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additive content) to 680 MPa with a fiber content higher than 20 wt% (4.5 wt% of additive content).

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The whole data set for the three series of composites appears to be suitable to evidence a significant

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effect of BDF-Me on the global trend along the increase of fiber content. The three series of

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composites exhibited the same evolution of the tensile strength and close values regarding the

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increase of fiber content. Considering the standard deviation and the lack of direct comparison for a

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given fiber content, a cautious analyze would be to consider that the three series have the same

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trend and the small observed differences are not significant. Two hypotheses can be suggested to

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explain a decrease of Young's modulus while the tensile strength remains unaffected. First, the use of

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BDF-Me involves some significant changes in the microstructure in comparison with crude hemp

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fibers and hemp fibers treated with BDF. Second, BDF and/or BDF-Me could cause chemical

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degradations or interact with PCL (e.g., transesterification, plasticizing effect). These hypotheses will

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be verified thereafter.

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Tensile tests were also conducted on PCL-BDF and PCL-BDF-Me. Young's moduli were

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340 ± 39 MPa and 320 ± 37 MPa, respectively, while tensile strengths were 18.3 ± 1.8 MPa and

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18.5 ± 2.1 MPa for the same compounds. One can conclude that the incorporation of BDF or BDF-Me

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has the same effect on the stiffness and tensile strength of the PCL because the standard deviation

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calculated on the two compounds are narrow. These results are consistent with a plasticizing effect

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of the ferulic acid derivatives. Moreover, the elongations at break were determined for each series of

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composites (figure S7 in Supporting Information). These results are coherent with the hypothesis a of

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plasticizing effect of BDF-Me, since the composites containing BDF-Me have an overall higher

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elongation at break regarding the fiber content.

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Chemical characterization of composites

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The SEC analysis conducted on the polymeric fraction of the composites (Table 2) showed a

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decrease of the Mn of crude PCL after the extrusion process (66.2 kg.mol-1 while the supplier

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technical data sheet indicates 80.0 kg.mol-1). Such a decrease of Mn was already reported for many

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thermoplastic polymers and can be attributed to a shortening of the polymeric chains due to the


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thermal and mechanical stresses during the extrusion process.27 The characterization of the

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polymeric chains after being solvent extracted from the PCL-BDF and PCL-Hemp25.5%-BDF materials

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exhibited a molecular weight of 39.1 kg.mol-1 and 49.3 kg.mol-1, respectively. The sole incorporation

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of BDF involved a decrease of the molecular weight of the PCL matrix. The incorporation of BDF-

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treated hemp fibers also decreased the molecular weight of the PCL matrix. It can be assumed that

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such decrease of the molecular weight results from transesterification reactions between the free

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phenols of BDF and the internal ester groups of the PCL chains.28 The methylation of the phenolic

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functions in BDF-Me preventing any transesterification reaction, such a decrease in molecular weight

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should not be observed. Indeed, with Mn of 61.8 kg.mol-1 and 66.2 kg.mol-1, respectively, PCL-BDF-

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Me and PCL-Hemp23.6%-BDF-Me did not exhibit significant molecular weight loss, strongly

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supporting our previous hypothesis. To further confirm transesterification, we have performed

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NMR spectroscopy (Figure S5 and Figure S6 in Supporting Information) to verify the appearance of

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new signals corresponding to the newly formed Ar-O-(C=O)-(CH2)5- functions (Figure 4).

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Unfortunately, those peaks were not observed, probably because of the low occurrence of such

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function within the PCL matrix. Finally, although there is the same content of BDF (4.5 wt%), a slight

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difference of molecular weight (10 kg.mol-1) can be observed between PCL-BDF and PCL-Hemp25.5%-

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BDF. We assumed that some BDF remained close to the fibers which prevented it to degrade the PCL

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matrix. The degradation of the polyester matrix by the phenolic groups was predictable, and the

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preservation of the Mn after the methylation of such phenolics sounds logic. It is possible to reject

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the hypothesis of a chemical degradation of the matrix by the BDF-Me reducing the Mn to explain the

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decrease of the Young's modulus,29 since the Mn is actually not affected by the BDF-Me.

13

C

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Thermal analysis

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The thermal analysis conducted on crude PCL determined a glass transition temperature (Tg)

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of -60.7 °C. The Tg of PCL-Hemp22.5% was -59.2 °C, while PCL-Hemp25.5%-BDF and PCL-Hemp23.6%-

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BDF-Me exhibited a Tg of -57.5 °C and -58.5 °C, respectively. Due to the heterogeneity of the fiber

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dispersion, the standard deviation of the Tg of these three composites was close to ± 0.7 °C. It seems

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that the incorporation of the fibers increased the Tg regardless the addition of ferulic acid derivative.

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Such an effect is very well known for composite materials and is generally attributed to an increase

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of viscosity due to high content of lignocellulosic fibers, thus reducing the mobility of the polymeric

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chains.30 The thermal analysis were also led on PCL-BDF and PCL-BDF-Me and their Tg were

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determined at -52.6 °C and -57.2 °C, respectively. This significant increase of the Tg gives information

285

about the high miscibility of ferulic acid additives in the PCL and indicates some interactions between



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the additives and the polymeric matrix. Interestingly, the addition of BDF did not decrease the Tg as it

287

could be expected because of the decrease of the molar mass and/or transesterification reactions.31

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Variation of cristallinity (χc) could sometimes have an impact on the Tg in polyesters.32 The cristallinity

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was measured by DSC for PCL-BDF and PCL-BDF-Me and determined at 38.0% and 37.6%,

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respectively. These values did not show any significant difference in comparison with crude PCL (37.5

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%) and these results were not relevant to explain the variation of glass transition.

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Crude BDF and BDF-Me were also analyzed by DSC to investigate their thermal behavior in

293

order to understand what could happen during the extrusion process (Figure 5). Each ferulic acid

294

derivative exhibited a melting enthalpy during the first heating ramp, related to the thermal history.

295

The melting temperature was 109 °C for BDF and 76 °C for BDF-Me. Both ferulic acid derivatives

296

exhibited a glass transition phenomenon at -19.1 °C (BDF) and -29.6 °C (BDF-Me) and no more

297

melting enthalpy after the second heating ramp. These results indicate a phase transition from a

298

crystalline structure to an amorphous phase after being melted and cooled. If BDF and BDF-Me are

299

highly miscible in PCL, it would be consistent to have an increase of the Tg of the PCL due to the

300

higher Tg of BDF or BDF-Me.33 It is also consistent that PCL-BDF has a higher Tg than PCL-BDF-Me,

301

since the Tg of BDF is higher than that of BDF-Me. The amorphous phase observed for the two ferulic

302

acid derivatives could be due to intermolecular non-covalent interactions (e.g., hydrogen bonds, π-

303

stacking) created after a thermal treatment higher than the melting temperature. Such amorphous

304

phases and glass transitions were reported for some lubricants34 or supramolecular polymers.35

305

As the extrusion process occurs at a higher temperature (100 °C) than the melting

306

temperature of BDF-Me (76 °C), a phase transition might happen for BDF-Me from a crystalline state

307

to an amorphous state during the process. Due to its higher melting temperature (106 °C), BDF might

308

remain crystalline during the process. On the contrary, amorphous and well-dispersed BDF-Me could

309

act as a plasticizer and thus reduce the Young’s modulus of PCL. As reported in literature, there are

310

several theories to explain the plasticization of a polymer by an additive and especially free volume

311

theory, gel theory and lubricity theory.36 As stated previously, the amorphous BDF-Me has the

312

behaviour of a lubricant. It could ease the chain mobility by reducing chain friction. This would be

313

consistent with the lubricity theory since it is effective beyond the glass transition. Such statement

314

now appears in the discussion part of the manuscript.

315 316

Chemical imaging

317

Dispersion of ferulic acid derivatives in PCL


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318

In order to investigate the dispersion of BDF and BDF-Me in PCL, confocal Raman imaging

319

was used. Some results related to PCL-BDF-Me are presented in Figure 6. Ferulic acid derivatives can

320

be found everywhere in the PCL which is consistent with the very high dispersion suggested by DSC

321

results. It is possible to find high concentration of BDF or BDF-Me in some locations with very intense

322

peaks related to -C=Caromatic and -C-Haromatic bonds (Figure 6). Even with BDF or BDF-Me, some areas

323

exhibit a –C-Haromatic peak at 3060 cm-1 while others exhibit a –C-Haromatic peak at 3010 cm-1. Previous

324

works have demonstrated that crystalline BDF had a –C-Haromatic peak at 3060 cm-1 in PCL.22 The peak

325

located at 3010 cm-1 is attributed to –C-Haromatic of amorphous BDF (or BDF-Me). Such shift of this

326

Raman signal is consistent with changes of the spatial configuration of BDF (or BDF-Me)37 and could

327

be related to the formation of π-stacking interactions. As reported on Figure 6, the amorphous BDF-

328

Me is widely spread in the PCL matrix. After calculation, the area density of the amorphous BDF-Me

329

was determined at 0.21 ±0.01, while it is only 0.06 ±0.03 for BDF. It is interesting to note the

330

presence of a small amount of amorphous BDF in the PCL even with the extrusion temperature lower

331

than the melting temperature of BDF, as reported previously. Internal friction and shear rate could

332

be responsible for such phase transition of BDF in some parts of the extruder during the process. The

333

injection process could also play a role in the phase transition of BDF and could explain the presence

334

of a small amount of amorphous BDF in the final material.

335 336

Area density of fibers

337

The area density of fibers in PCL was measured in specimen surfaces and in cross sections by

338

CRI. The results for specimen surfaces are presented in Figure 7. The area density is comprised

339

between 0.25 and 0.70 for PCL-Hemp and PCL-Hemp-BDF, while it is comprised between 0.20 and

340

0.60 for PCL-Hemp-BDF-Me. The increase of the area density according the increase of fiber content

341

is very similar for PCL-Hemp and PCL-Hemp-BDF series. The increase of the area density is lower for

342

PCL-Hemp-BDF-Me than for the two other series of composites. The most important differences are

343

observed between 5 wt% and 20 wt% of fibers. A very similar trend is observed for cross section

344

analysis (Figure 7). The incorporation of BDF demonstrated no significant effect on the area density,

345

while the incorporation of BDF-Me showed a decrease of area density, revealing a lower dispersion

346

of the fibers in the PCL matrix which is consistent with a lower Young’s modulus.2

347 348

Granulometry and microstructure analysis



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

349

Granulometry and microstructure investigations were conducted by CRI and image analysis

350

for the three series of composites (PCL-Hemp, PCL-Hemp-BDF and PCL-Hemp-BDF-Me). The oriented

351

granulometry was computed in order to characterize the orientation of the fibers in the specimen

352

surfaces and in the specimen cross sections.22 The results did not show any significant difference in

353

the orientation of the fibers with the use of BDF or BDF-Me in comparison with composites

354

containing crude hemp fibers. The Euler number density provides information about the presence of

355

individual fibers or interconnections between the fibers.22 Due to the margin of error, the results are

356

very similar and indicate no significant difference. The boundary length density is related to the

357

length of the interface between the fibers and the polymeric matrix.22 The results did not show any

358

significant difference in the specimen surface. The analysis of the specimen cross sections showed a

359

lower increase of the boundary density with the use of BDF-Me in comparison with BDF or crude

360

hemp fibers. According to the measurement of oriented granulometry and Euler number density, the

361

treatment of the fibers by BDF or BDF-Me did not change the microstructure of the composites in

362

comparison with composite containing crude hemp fibers. The sole observed difference was a lower

363

increase of the boundary density with BDF-Me, which is consistent with a lower increase of the area

364

density and a lower dispersion of the fibers in the PCL matrix.

365 366

Conclusion

367

Two renewable ferulic acid derivatives, BDF and BDF-Me, only differing by the methylation of

368

their phenol moieties were synthesized using a chemo-enzymatic pathway and used for the

369

pretreatment of hemp fibers prior their incorporation into a PCL matrix by extrusion to produce

370

biocomposites with improved mechanical properties. When PCL is mixed with hemp fibers, the

371

addition of BDF does not modify the mechanical performance (Young's modulus and tensile strength)

372

of the resulting material but the addition of BDF-Me causes a lower increase of the Young's modulus

373

regarding the increase of fiber content. Thermal characterizations and chemical imaging showed an

374

efficient dispersion of BDF and BDF-Me in the PCL matrix. BDF has no impact on the microstructure

375

of the composites while at the contrary the methyl groups of BDF-Me induces a decrease of the

376

dispersion of the fibers. A plasticizing role of BDF-Me is likely to occur since a phase transition from a

377

crystalline structure to an amorphous phase during the extrusion process was evidenced by chemical

378

imaging. Therefore plasticizing effect of the BDF-Me led to a lower dispersion of the fibers during the

379

process and to a lower Young’s modulus. A poor interface between the fibers and the matrix is highly

380

suspected. The investigation of the interfaces would be helpful to improve the quality of the

381

treatment of the fibers.


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382

This study also highlighted the plasticizing effect of a ferulic acid derivative in a polyester

383

matrix. It would be interesting to extend this work to other polymeric matrixes (e.g., polyesters with

384

higher melting temperature, polyolefins, polyamides, polyurethanes, polyvinyl chloride) in order to

385

investigate the plasticizing potential of such bio-based additives. The plasticizing effect of BDF-Me is

386

significant even at a low content (0.8 wt%). It opens the way for a new class of plasticizers since

387

traditional plasticizers like phtalates, which are the most spread in industrial applications (e.g.,

388

flexible polyvinyl chloride), are commonly used in a range of 10 wt% to 70 wt%.38 Chemical imaging

389

techniques advantageously helped to characterize the microstructure of the composites and the

390

phase transition leading to understand the plasticizing effect of the ferulic acid derivative. The

391

methylation of the phenolic moieties notably modified the thermal properties of the ferulic acid

392

derivatives and prevented the chemical degradation - most probably transesterification - of the

393

polyester matrix. This result demonstrate the importance of the functionalization of additives in

394

general and that of phenolic functions in particular, the addition of others functional groups (e.g.,

395

acetylation) is thus a hot-spot to investigate to possibly enhance the thermal properties and/or the

396

compatibility of these plasticizing additives with others polymer matrices.

397

Supporting Information

398

The Supporting Information file contains figures related to 1H and 13C NMR characterizations

399

(figures S1 to S6). It also contain a figure reporting data of elongation at break related to the

400

composites (figure S7). The supporting Information file contains a total of 7 figures and 5 pages.

401

Figure S1 : 1H NMR Spectrum of BDF

402

Figure S2 : 1H NMR spectrum of BDF-Me

403

Figure S3 : 13C NMR spectrum of BDF

404

Figure S4 : 13C NMR spectrum of BDF-Me

405

Figure S5 : 13C NMR spectrum of crude PCL

406

Figure S6 : 13C NMR spectrum of PCL-BDF compound

407

Figure S7 : Evolution of the elongation at break according the fibers' content

408 409

Acknowledgments



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

410

The Fondation de France site Paris-Reims is gratefully acknowledged for supporting the

411

FEDERATIF project. The authors also acknowledge FEDER project Matrice for financial support,

412

CentraleSupélec for granting access to the confocal Raman microscope, and Fibres Recherche

413

Développement (Troyes, France) for providing hemp fibers. Amandine Flourat (Chaire ABI) is

414

acknowledged for her kind help for the synthesis of ferulic acid derivatives. Alain Lemaitre (FARE) is

415

gratefully acknowledged for Soxhlet extraction of the fibers, together with David Cronier (FARE) for

416

SEC characterizations.

417 418

References

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533 534

Figures

535 536

Figure 1: Synthesis of BDF and BDF-Me

537 538

Figure 2: Evolution of the Young's modulus according the fiber content in the three series of composites

539 540

Figure 3: Evolution of the tensile strength according the fiber content in the three series of composites

541

Figure 4: Transesterification reaction between BDF and PCL

542

Figure 5: DSC analysis conducted on BDF and BDF-Me. First heating (a) and second heating (b)

543 544

Figure 6: Chemical imaging of PCL-BDF-Me (a) and PCL-BDF (b), and their respective related Raman signatures (α and β)

545

Figure 7: Area density of fibers in specimen surfaces (a) and in specimen cross sections (b)

546 547 548

Figure 1 : Synthesis of BDF and BDF-Me.

549 550 551



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552

Figure 2 : Evolution of the Young's modulus according the fiber content in the three series of composites

553 554 555

Figure 3 : Evolution of the tensile strength according the fiber content in the three series of composites

556 557


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558


Figure 4 : Transesterification reaction between BDF and PCL

559 560 561

Figure 5 : DSC analysis conducted on BDF and BDF-Me. First heating (a) and second heating (b)

562 563 564



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

565

Figure 6 : Chemical imaging of PCL-BDF-Me (a) and PCL-BDF (b), and their respective related Raman signatures (α, β)

566 567 568

Figure 7 : Area density of fibers in specimen surfaces (a) and in specimen cross sections (b)

569 570


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571


Tables

572 573 574

Table 1: Samples made of PCL and pure of pretreated hemp fibers at different contents or crude synthesized additives

575

Tables 2: Molecular weight and dispersity of compounds and composites

576 577

Table 1 : Samples made of PCL and pure or pretreated hemp fibers at different contents or crude synthesized additives

Sample name

PCL (wt%)

PCL PCL-Hemp4.5% PCL-Hemp7.1% PCL-Hemp19.8% PCL-Hemp22.5% PCL-Hemp4.8%-BDF PCL-Hemp8.4%-BDF PCL-Hemp13.6%-BDF PCL-Hemp25.5%-BDF PCL-Hemp4.3%-BDF-Me PCL-Hemp10.1%-BDF-Me PCL-Hemp19.3%-BDF-Me PCL-Hemp23.6%-BDF-Me PCL-BDF PCL-BDF-Me

100.0 95.5 92.9 80.2 77.5 94.4 90.1 84.0 70.0 94.9 88.1 80.0 72.2 95.5 95.5

Hemp fibers (wt%) experimentally determined 0 4.5 7.1 19.8 22.5 4.8 8.4 13.6 25.5 4.3 10.1 19.3 23.6 0 0

BDF (wt%)

BDF-Me (wt%)

0 0 0 0 0 0.8 1.5 2.4 4.5 0 0 0 0 4.5 0

0 0 0 0 0 0 0 0 0 0.8 1.8 3.4 4.2 0 4.5

578 579

Table 2 : Molecular weight and dispersity of compounds and composites

Samples PCL PCL-BDF PCL-BDF-Me PCL-Hemp22.5% PCL-Hemp25.5%-BDF PCL-Hemp23.6%-BDF-Me

Ferulic acid derivate content (wt%) 0.0 4.5 4.5 0.0 4.5 4.2

Mn (kg.mol-1)

Mw (kg.mol-1)

Ɖ

66.1 39.1 61.8 58.7 49.3 66.2

96.2 60.9 92.3 91.6 76.1 95.8

1. 5 1. 6 1.5 1.6 1.5 1.5

580 581



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582

For Table of Contents Use Only

583 584

"This study focuses on the understanding of the design of additives on composite materials

585

properties using chemical and microstructural approaches."

586


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This study focuses on the understanding of the design of additives on composite materials properties using chemical and microstructural approaches. 165x82mm (96 x 96 DPI)



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Figure 1: Synthesis of BDF and BDF-Me 273x99mm (300 x 300 DPI)


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Figure 2: Evolution of the Young's modulus according the fiber content in the three series of composites 241x175mm (96 x 96 DPI)



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Figure 3: Evolution of the tensile strength according the fiber content in the three series of composites 241x175mm (96 x 96 DPI)


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Figure 4: Transesterification reaction between BDF and PCL 239x150mm (96 x 96 DPI)



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Figure 5: DSC analysis conducted on BDF and BDF-Me. First heating (a) and second heating (b) 482x165mm (96 x 96 DPI)


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Figure 6: Chemical imaging of PCL-BDF-Me (a) and PCL-BDF (b), and their respective related Raman signatures (α and β) 240x189mm (96 x 96 DPI)



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Figure 7: Area density of fibers in specimen surfaces (a) and in specimen cross sections (b) 225x89mm (96 x 96 DPI)


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Microstructural and Chemical Approach To Highlight How a Simple

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