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Polymer 52 (2011) 1532e1538

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Polymer

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Poly(e-caprolactone) based nanocomposites reinforced by surface-graftedcellulose nanowhiskers via extrusion processing: Morphology, rheology,and thermo-mechanical properties

A.-L. Goffin a,*, J.-M. Raquez a, E. Duquesne a,1, G. Siqueira b,2, Y. Habibi a,b, A. Dufresne b, Ph. Dubois a

a Laboratory of Polymeric and Composite Materials, Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons, Place du Parc 20,B-7000 Mons, BelgiumbGrenoble Institute of Technology, The International School of Paper, Print Media and Biomaterials (PAGORA), BP65, 38402 Saint Martin d’Hères cedex, France

a r t i c l e i n f o

Article history:Received 29 October 2010Received in revised form2 February 2011Accepted 2 February 2011Available online 4 March 2011

Keywords:Cellulose nanowhiskersPoly(e-caprolactone)Nanocomposites

* Corresponding author. Tel.: þ3265373480; fax: þE-mail address: Anne-Lise.Goffin@umons.ac.be (A.

1 Current address: Materia Nova, Materials R&DB-7822 Ghislenghien, Belgium.

2 Current address: Department of Engineering DesiUniversity of Science and Technology, Rich. BirkelandsNorway.

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.02.004

a b s t r a c t

Poly(e-caprolactone)-grafted cellulose nanowhiskers (extracted from ramie: CNWr) synthesized by ring-opening polymerization of the corresponding lactone [1] were studied as “masterbatches” by melt-blending within its commercial poly(e-caprolactone) matrix (PCL). For sake of comparison, unmodifiedCNWr were also dispersed in PCL. The goal of this study consists to evidence the impact of the covalentgrafting of CNWr surface on thermo-mechanical properties of the commercial matrices. Atomic forcemicroscopy (AFM) attests of the excellent dispersion of the cellulose nanowhiskers within PCL matrix. Asa result of the excellent interfacial compatibility between the nanofiller and the matrix, the thermo-mechanical and rheological performances were largely enhanced.

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1. Introduction

During these recent years, increasing interest has been shown inthe realm of bio(nano)composites. This interest is implementedboth by the potential technological applications of these materialsand by their sustainability in the design of environmentally friendlymaterials. Nanocomposites are a family of materials, consisting ofa continuous matrix reinforced with a fiber, platelet, or particlehaving at least one dimension in the nanometer scale [2]. Inbionanocomposites, the matrix phase and nanofiller are bothbiodegradable and/or derived from renewable sources.

Natural fibers, and more especially cellulosic fiber have beenused for a long time as micrometric reinforcing agents in polymermaterials [3e5]. Besides to their biodegradability and renewability,cellulose fiber-based composites were intensively studied, owing tothe numerous fiber advantages such as [6]: natural abundanceand wide available variety in nature; nonfood agriculturalbased economy; low cost; anisotropy; high mechanical modulus;

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relatively reactive surface, which can be used for grafting specificgroups.

Concerning the mechanical properties, the ultimate tensilestrength of cellulose (commun source) is estimated to be 17.8 GPa[7]. This value is seven times higher than that of steel, but staylargely lower than aluminum (70 GPa), glass fibers (76 GPa) orsynthetic fibers as Kevlar (156 GPa) [8]. Interestingly enough, theelastic modulus of natural cellulose (cellulose I or native cellulose)is estimated to 138 GPa (depending of the sources). Due to theirstructure made up of parallel chains, native cellulose proposes thebest mechanical properties and a more stable structure. In order toisolate the cellulose monocrystals from all cellulose sources, one ofthe proposed strategy consist to hydrolysis [9e14] (under well-controlled conditions) the fiber to only recover the crystals. Suchfillers are referred as cellulose nanowhiskers (CNW). The elasticmodulus of cotton and tunicate CNW were estimated to 143 [15]and 105 GPa [16], respectively. Cellulose nanowhiskers are rod-like nanoparticles presenting an average length of 100 nm to a fewmicrons and a diameter of 5e15 nm depending on the cellulosesource [17]. Thus, highly crystalline cellulose structure (combinedwith high aspect ratio and nanoscale dimension) possesses all thequalities required to be ideal candidates as reinforcements forpolymer matrices (replacing glass fiber for example). The mainproblem for the preparation of nanocomposites reinforced by CNWis related with their heterogeneous dispersion within the polymer

Table 1Synthesis of CNWr-g-PCL nanohybrid: [CL]0 ¼ 2M [Sn(oct)2]0 ¼ 0.01 M, time¼ 24 h,T ¼ 95 �C.

Nanohybird mCNWr (g) mCL (g) mCNWr-g-PCLa (g) Yielda (%) % CNWrb

CNWr-g-PCL 1.4 10 9.8 85 14

a Determined by gravimetry.b %CNWr ¼ (mCNWr/mCNWr-g-PCL) � 100.

Table 2PCL/CNWr-based materials prepared by melt-blending.

Polyester matrix Cellulose content

CAPA6500 0%CAPA6500 2, 4 and 8wt%CNWrCAPA6500 2, 4 and 8wt%CNWr-g-PCLa

a wt% representing the content in cellulose nanocrystals only.

A.-L. Goffin et al. / Polymer 52 (2011) 1532e1538 1533

matrix. This is due to the main interactions between CNW. Gener-ally, the literature concerning the preparation of CNW-basednanocomposites mainly involves the “solvent casting” method asprocessing approach [1,18e23]. When hydrosoluble [24e26] poly-mers are used as matrices, the interactions between CNW and thematrix are good enough due to the polar nature of both partners.After mixing and casting the two aqueous suspensions, a solidnanocomposite film can be obtained by solvent evaporation. It wasdemonstrated that the reinforcing effect of CNW lies on theformation of a rigid percolating filler network, due to hydrogenbond interactions between cellulose nanocrystals. The formation ofthe percolating network is favored by slow and soft processing.Interestingly, the reinforcing effect of CNW on mechanical proper-ties of different matrices has been recently reviewed in the litera-ture [6,27].

The melt-processing approach, which undoubtedly remains themost widespread nanocomposite preparation technique, has notbeen much used for the preparation of nanocomposites reinforcedwith CNW. However, this processing technique is performedwithout any solvent, which is in a good implementation with the“bio”-concern of this research. A recent study proposed the melt-blending of CNW within PLA matrix using a compatibilizer to helpthe dispersion [28]. However the CNW dispersion proved ratherpoor due to a lack of compatibility between the hydrophilicnanofiller and the hydrophobic matrix. Very recently, anothergroup has reported a coating approach using polymer- or fatty acid-grafted CNW [29,30], which leads to successful extrusion in poly(e-caprolactone) (PCL) or polyethylene (PE).

Hence, this study aims to prepare cellulose bionanocompositesby melt-extrusion techniques using PCL in order to provide thebeneficial implementation of CNW on the thermo-mechanicalproperties of these polyesters. PCL and PLA are selected, owing totheir biodegradability and biocompatibility.We propose to improvethe dispersion/compatibilization of CNW with these non-polarmatrices using a surface grafting method. In collaboration withAlain Dufresne’s group, our group previously described this graft-ing technique [1]. The «grafting from» approach was used to graftPCL to cellulose nanocrystals by ring-opening polymerization (ROP)of e-caprolactone (CL) catalyzed with tin octoate (Sn(Oct)2). In thiswork, we propose to melt-blend CNW (from ramie cellulose)grafted with PCL (obtained by this “grafting from” method) withthe corresponding commercial PCL matrix using extrusion tech-nique, and to evaluate their influence on the morphological andthermo-mechanical properties.

2. Experimental part

2.1. Materials

Pure ramie fibers were obtained from Stucken Melchers GmbH& Co. (Germany).

Commercial grade PCL (CAPA6500, Mn ¼ 50,000 g/mol) wassupplied by Solvay.

2.2. Cellulose nanowhiskers preparation

Cellulose nanowhiskers were extracted from the ramie fibers.Colloidal suspensions of whiskers in water were prepared asdescribed in the literature [1,9,10].

Briefly, ramie fibers were cut into small pieces and treated with2% NaOH at 80 �C for 2 h in order to remove the lignin fraction. Thepurified ramie fibers were submitted to acid hydrolysis with a 65%(wt/wt) H2SO4 solution at 55 �C for 30 min and with continuousstirring. The suspension was washed with water until neutralityand dialyzed against deionized water. The obtained suspensionwas

homogenized by using an Ultra Turrax T25 homogenizer for 5 minat 13,500 rpm, filtered through a no. 1 glass sinter in order toremove unhydrolyzed fibers, and the suspension concentrated,providing the stock suspension. Prior to polymerization, therecovered cellulose nanocrystals were neutralized using a 1 wt%NaOH solution as described in the literature [31].

2.3. Preparation of polyester-based (nano)composites

All details concerning the synthesis of PCL-grafted CNWrnanohybrids have been recently reported [1]. Table 1 reports ROPconditions and nanohybrids compositions for the PCL-basedmaterials.

CAPA6500/CNWr (nano)composites were prepared by melt-blending in a ThermoHaake MiniLab Rheomex CTW5 mini-extruder at 120 �C and 75 rpm for 10 min. Then, rectangularsamples (35 mm � 12 mm � 3 mm) were prepared by injection-molding for Dynamical Mechanical Thermal Analysis (DMTA) andthin cylinder (25 mm � 1.5 mm) for rheo-mechanical analysis.Different materials were prepared: the unfilled matrices and cor-responding (nano)composites: direct CNWr dispersion and poly-ester-grafted CNW dispersion as masterbatch at cellulose contentsof 2, 4 and 8 wt% (see Table 2).

2.4. Characterizations

The morphology of the (nano)composites was analyzed byAtomic Force Microscopy (AFM). The samples were cryo-micro-tomed at �100 �C by an Ultracut FC4E microtome from Reichert-Jung. AFM images were recorded using a Nanoscope IIIa Multimodemicroscope operating in tapping mode at room temperature underair. Commercial probes were used with a spring constant of20e80 N/m and a resonance frequency of about 298e335 kHz.

Thermal characterizations using Differential Scanning Calorim-etry (DSC) were performed using a TA Instruments Q200 apparatus.A heat/cool/heat procedure was applied screening a temperaturerange from �80 �C to 100 �C (depending on the samples nature)with a rate of 10 �C/min. The glass transition temperature (Tg),crystallization temperature (Tc), crystallization enthalpy or coldcrystallization (for PLA) (DHc) and melting enthalpy (DHm) weredetermined from the second heating scan. The enthalpy valueswere calculated taking into account the content of cellulosecrystals.

Dynamic Mechanical Thermal Analyses (DMTA) of the (nano)composites were performed under ambient atmosphere usinga 2980 DMTA apparatus from TA Instruments using a dual canti-lever module. The measurements were carried out at a constant

Fig. 1. AFM images (phase) of (a) CAPA6500/8wt%CNWr and (b) CAPA6500/8wt%CNWr-g-PCL.

A.-L. Goffin et al. / Polymer 52 (2011) 1532e15381534

frequency of 1 Hz, a temperature range from �100 to 30 �C ata heating rate of 3 �C/min. Three samples were characterized foreach composition.

Rheological analyses were performed on an ARES rheometer at90 �C for PCL-based (nano)composites. The measurements werecarried out in an oscillatory shear mode using a parallel plategeometry of 25 mm. Samples were placed between plates andmelted for 5 min before analyses. First of all, strain sweepmeasurements were carried out to determine the linear visco-elastic region at a frequency of 1 Hz. Once this strain value fixed,frequency sweeps in range of 0.01e100 Hz were conducted.

3. Results and discussion

As described in the previous paper [1], a PCL-grafted CNWrnanohybrid was successfully synthesized by ring-opening poly-merization (ROP) of e-caprolactone (CL) catalyzed by tin(II) 2-eth-ylhexanoate (Sn(Oct)2) at 95 �C. The polymerization was initiatedfrom the hydroxyl groups available onto CNW surface. The relativecellulose content was estimated by gravimetry at 14 wt%.

The PCL-grafted CNWr nanohybrid was then dispersed ina commercial PCL matrix (CAPA6500), leading to the preparation of(nano)composites containing2,4and8wt%of cellulosenanoparticles.

Fig. 2. AFM topology of CAPA6500/8wt%CNW

These blends were prepared using a twin-screw microcompounder.For sake of comparison, ungrafted CNWr as isolated from aqueoussuspension bya freeze-drying stepwere directlymelt-blendedwithinthe same PCLmatrix under the same experimental conditions. Table 2summarizes the different PCL material compositions.

The morphological observations were carried out by atomicforce microscopy (AFM). AFM analysis of the cellulose nano-whiskers is a good alternative to electron microscopy, without anylimitations regarding contrast and resolution [32].

Fig. 1(a) and (b) show AFM phase images of compositesCAPA6500/8wt%CNWr and CAPA6500/8wt%CNWr-g-PCL, respec-tively. The AFM were performed on microtomed samples. It isimportant to observe that cellulose nanocrystals were not physi-cally altered/damaged by the melt-blending process at hightemperature. Indeed, the needle-like structure of the cellulosenanowhiskers appeared not altered by the melt-processingtreatment.

For CAPA6500/8wt%CNWr samples (Fig. 1(a)), the elongatednanoparticles appear as large aggregates up to a few microns insize. This aggregation is mainly due to the strong CNWeCNWinteractions. This effect is largely accentuated by the freeze-dryingprocess. Melt-blending, at high temperature and under strongshear conditions, is not able to finely disperse ungrafted CNWr

r-g-PCL and section across the sample.

Fig. 3. (a) Isochronal evolution of the storage modulus (E0) and (b) Tan Delta vs. temperature of unfilled CAPA6500 and ungrafted CNWr-based composites.

Fig. 4. (a) Isochronal evolution of the storage modulus (E0) and (b) Tan Delta vs. temperature of unfilled CAPA6500 and PCL-grafted CNWr-based nanocomposites.

Table 4Effect of CNWr-based nanocomposites on the storage modulus (recorded at 20 �C)and the tangent delta values.

Entry Samples E0 20 �C (MPa) Max. Tan Delta(midpoint)

Tan Delta(onset/offset)

1 CAPA6500 423 � 99 �39 � 4 �49/�162 þ2wt%CNWr 554 � 3 �38 � 2 �54/�163 þ4wt%CNWr 664 � 35 �40 � 1 �54/�204 þ8wt%CNWr 647 � 4 �39 � 2 �52/�205 þ2wt%CNWr-g-PCL 639 � 3 �37 � 3 �53/�186 þ4wt%CNWr-g-PCL 1216 � 53 �38 � 1 �55/�147 þ8wt%CNWr-g-PCL 1500 � 2 �27 � 1 �59/�3

A.-L. Goffin et al. / Polymer 52 (2011) 1532e15381536

nanoparticles through the PCL matrix. Clearly, this processingtechnique is not suitable for the dispersion of untreated CNWr.

In contrast, the nanocomposite based on the grafted CNW-basedmasterbatch (CAPA6500/8wt%CNWr-g-PCL) in Fig. 1(b) showsa totally different morphology. No aggregates are observedthroughout the whole matrix. AFM topology brings more informa-tion about the extent of CNWr dispersion (Fig. 2). The nanoparticlesappearalignedperpendicularon thePCL surface. The coated rod-likenanocrystals exhibit an average diameter of 40 nm (see AFMsection). The enlargement of the diameter value can be ascribed tothe polymer coating or to some very limited CNWr aggregation (2 or3 nanoparticles). This observation clearly provides evidence that thecovalent PCL-grafting on CNWr surface highly improves the CNWrdispersion.

In the light of this high evolution in nanoparticle dispersion, theimpact of well-dispersed CNWr has been studied on the thermo-mechanical properties of PCL materials.

Dynamical Mechanical Analyses (DMTA) were performed in“dual cantilever” mode at a temperature range from �100 �C to30 �C. DMTA curves (Figs. 3 and 4) present the isochronal evolutionof the recorded storage modulus in function of temperature. Thecomposite storage modulus values were normalized at lowtemperature (in the glassy state). The materials present a typicalbehavior for a semi-crystalline polymer. In the glass state(T < �60 �C), the storage modulus keeps globally stable. Afterward,a loss in modulus values is recorded, corresponding to the glasstransition temperature (Tg).

Concerning the non-grafted CAPA6500/CNWr composites, theevolution of the storage modulus remains unmodified (close to thepristine PCL curve), whichever the amount of nanofiller (Fig. 3(a)).The modulus vs. temperature behavior of composites depends onvarious parameters, including crystallinity and polymer/fillerinteractions. Nevertheless, the crystalline properties can partiallyexplain the aforementioned mechanical behavior. Indeed, a ratherlimited influence of CNWr is evidenced on the crystallinity andmelting transition of PCL nanocomposites as observed by DSC(Table 3). Additionally, the ungrafted-CNW/PCL interactions arelimited (as observed by a poor dispersion of the nanofillers withinthe polymer matrix by AFM), leading to a very limited effect on thestorage modulus.

At the opposite, PCL-grafted CNWr-based CAPA6500/CNWrnanocomposites display much improved elastic response withrespect to the pristine PCL matrix and simple CAPA6500/CNWrmelt-blends. This effect is even more significant when increasingthe CNWr-g-PCL content (Fig. 4(a)). The storage modulus values(recorded at 20 �C) are presented in Table 4. The CAPA6500/8wt%CNWr-g-PCL nanocomposites are characterized by an elasticmodulus 300% higher than the unfilled PCL (see Table 4, entry 7).These observations imply specific polymerefiller interactions. Ifa compatibilization of CNW by PCL-grafting is revealed, thesestrong interactions will modify the physical state of the polymerphase. Tan Delta, associated to Tg and thus strongly linked to therelaxation of polymer chains, provides some information. Fig. 3(b)

Table 3DSC analyses of unfilled CAPA6500, CAPA6500/CNWr and CAPA6500/CNWr-g-PCL(2, 4 and 8 wt%). Heat/cool/heat from�80 to 100 �C with heating ramp of 10 �C/min.

Samples Tc (�C) DHc (J/g) Tm (�C) DHm (J/g)

CAPA6500 35 58 59 56þ2wt%CNWr 35 55 59 60þ4wt%CNWr 36 60 58 64þ8wt%CNWr 35 60 58 63þ2wt%CNWr-g-PCL 38 63 58 64þ4wt%CNWr-g-PCL 39 58 59 60þ8wt%CNWr-g-PCL 40 63 58 65

and Fig. 4(b) show the effect of the CNWr content on the Tan Deltapeak (Table 4). Concerning ungrafted materials, the maximum ofTan Delta is not modified (see Table 4, entries 1e4), while it isshifted and broadened (see inflection points of the Tan Delta slopein Table 4, entry 7) for grafted CNWr-based nanocomposites. Thiseffect is even more pronounced when the grafted CNWr contentincreases (and more especially for 8wt%CNWr-g-PCL/PCL, seeTable 4, entry 7). These observations suggest restricted molecularmotions due to the interactions between nanofillers and polymerchains. To understand this substantial increase in mechanicalproperties and the modification of the PCL relaxation, the study ofthe rheological properties is proposed. Rheological analyses wereperformed in order to highlight an eventual compatibilizationeffect of PCL-grafted nanoparticles on the polymer matrix. Thesenanohybrids can act as physical crosslinking, reducing the mobilityof polymer chains. Moreover, the rheological analyses performed inthe molten state allow completing the DMTA analyses carried outunder a limited range of temperature (�100e30 �C). The rheolog-ical properties of (un)grafted CNWr-based (nano)composites arepresented hereafter.

Frequency sweep measurements in linear conditions were per-formed at 90 �C. Figs. 5 and 6 show the frequency dependence ofthe storage modulus G0 for various PCL/CNWr (nano)composites.

Fig. 5 shows the dynamic modulus (G0) for pristine PCL matrixwith a typical behavior of visco-elastic material, i.e. the slope of G0

curve is two times higher than the slope of G00 [33]. The incorpo-ration of ungrafted CNWr in PCL matrix does not provide any effecton the visco-elastic properties of the polyester matrix, whicheverthe content of fillers. Again, these results display the poor disper-sion state of the nanofiller and the lack of interactions between thematrix and the nanoparticles.

Fig. 5. Linear visco-elasticity of unfilled CAPA6500 and (nano)composites based onungrafted CNWr.

Fig. 6. Linear visco-elasticity of unfilled CAPA6500 and nanocomposites based on PCL-grafted CNWr.

A.-L. Goffin et al. / Polymer 52 (2011) 1532e1538 1537

At the opposite, the storage modulus (G0) of PCL/CNWr-g-PCL-based nanocomposites increases with the nanohybrid content(Fig. 6). The progressive cross-over of G0 and G00 curves with theCNWr-g-PCL content arises from the formation of a networkstructure [34]. More exactly, a solid-like behavior is observed forthe CAPA6500/8wt%CNWr-g-PCL system (Fig. 7(a)), with appear-ance of a plateau and the complete inversion of G00 and G0 curves.The formation of the network structure is most likely due to theformation of a polymer physical network based on the entangle-ment of the surface-grafted polymer chains with the “free” PCLchains of the matrix. This network gives to the material a betterresistance against the applied deformation over all the range of lowfrequency.

In order to attest of the formation of this physical chainsnetwork between the PCL-grafted chains and the PCL chains of thematrix, a nanocomposites PCL/8wt%CNWr-g-PCL was preparedwith a CNWr-g-PCL nanohybrid characterized by shorter PCLchains. The polymerization procedure is similar as previouslyreported [1] but with a reaction time limited to 4 h. The ratio CNWr-to-polymer is around 50%. This nanohybrids is coined as CNWr-g-sPCL. For a similar morphology in AFM, the rheological behavior ofthis nanocomposite is totally different: no observation of G0 and G00

cross-over (Fig. 7(b)). This observation confirmed that it is neces-sary to get a sufficient PCL chains length to form the network.

Fig. 7. Linear visco-elasticity of PCL/8wt%CNWr-g-PCL (with long (A) and short PCLchains (B)) nanocomposites: effect on the appearance of the solid-like behavior.

In other words, the incorporation of ungrafted CNWr in PCLmatrix by melt-blending leads to preparation of a microcomposite,leaving the thermo-mechanical properties of PCL materials unaf-fected. The polymer grafting on CNWr surface is the key to improvethe PCL matrix performances. Clearly, the unique combination offine CNWr dispersion and physical chain network formationcontribute to obtaining a substantial positive effect on mechanicalperformances.

4. Conclusion

As a general point of view, the morphological and thermo-mechanical properties of the so-prepared polymer nanocompositeswere greatly improved by the addition of only a few percents ofgrafted cellulose nanowhiskers, with respect to untreated nano-fillers, clearly highlighting the remarkable ability of the studiednanohybrids as an interesting way to compatibilize hydrophilicnanoparticles within hydrophobic polymer matrices.

In this work, the effect of the addition of polyester-grafted CNWmaterials on the morphology and crystalline properties of thepolyester matrices was investigated.

Firstly, PCL nanocomposites filled with dispersed CNW wereprepared via melt-blending using polyester-grafted CNW in orderto improve the thermo-mechanical performances of PCL matrix. Aspreviously described, CNWr-g-PCL nanohybrids were produced byring-opening polymerization (ROP) of e-caprolactone initiated byhydroxyl groups available onto CNW surface and using tin octoate(Sn(Oct)2) as catalyst [1]. The polyester grafting allowed largelyimproving the mechanical performances of the PCL matrix. It wasevidenced that CNW extracted from ramie (CNWr) present anexcellent dispersion of the nanofillers within the hydrophobicmatrix. Interestingly, a solid-like behavior was observed by rheo-logical analyses most likely due to the formation of a polymerphysical network. The entanglement of the surface-grafted poly-ester chains with the PCL matrix contributes to increase thermo-mechanical properties and decrease the chain relaxationphenomenon. This significant improvement of the mechanicalproperties can be ascribed to the unique combination of a finenanofiller dispersion and physical chain network formation.

Acknowledgments

The authors are grateful to the “RégionWallonne” and EuropeanCommunity (FEDER, FSE) in the frame of “Pôle d’Excellence MateriaNova” for their financial support. LPCM thanks the “Belgian FederalGovernement Office Policy of Science (SSTC)” for general support inthe frame of the PAI-6/27. A.-L. Goffin thanks F.R.I.A. for her PhDthesis grant. J.-M. Raquez is “chargé de recherche” by the F.R.S. -FNRS.

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