Composites Science and Technology xxx (2011) xxx–xxx

Contents lists available at ScienceDirect

Composites Science and Technology

journal homepage: www.elsevier .com/ locate/compsci tech

Melt rheology of nanocomposites based on acrylic copolymer and cellulose whiskers

Ayman Ben Mabrouk a, Albert Magnin c, Mohamed Naceur Belgacem b, Sami Boufi a,⇑a Laboratoire Sciences des Matériaux et Environnement, LMSE, University of Sfax, BP 802-3018 Sfax, Tunisiab Grenoble Institute of Technology, The International School of Paper, Print Media and Biomaterials (Pagora), UMR CNRS 5518, BP 65, 38402 Saint Martin d’Hères Cedex, Francec Laboratoire de Rhéologie, Grenoble-INP, UJF Grenoble 1, UMR CNRS 5520, BP 53, 38041 Grenoble Cedex 9, France

a r t i c l e i n f o

Article history:Received 11 November 2010Received in revised form 10 January 2011Accepted 16 January 2011Available online xxxx

Keywords:A. Nano compositesA. Coupling agentsC. Complex moduliD. Rheology

0266-3538/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compscitech.2011.01.012

⇑ Corresponding author.E-mail address: sami.boufi@fss.rnu.tn (S. Boufi).

Please cite this article in press as: Mabrouk ABTechnol (2011), doi:10.1016/j.compscitech.2011

a b s t r a c t

Nanocomposites based on poly(styrene-co-hexylacrylate) copolymer and cellulose whiskers as the nano-size filler were prepared by in situ miniemulsion polymerization and their melt rheological behaviourswere investigated under dynamic shear conditions. The effects of c-methacryloxypropyl triethoxysilane(MPS) content along with the whisker loading were explored. In the absence of whiskers, a transitionfrom a liquid- to a solid-like behaviour was observed when the polymer was synthesized in the presenceof MPS. When cellulose nanofiller was added, the storage modulus G0 and the dynamic viscosities g of thenanocomposites increased monotonically with whisker content and the resulting materials displayed asolid-like behaviour. Above 2 wt.%. loading, a percolated interconnected whisker–whisker network isbuilt up, producing a jump in the storage modulus and strong shear-thinning behaviour of the viscosity.However, as the nanocomposites were prepared in the presence of 3% of MPS, no enhancement nor in thestorage modulus nor in the viscosity was observed up to 5 wt.%. of whisker loading. Such a phenomenonwas ascribed to inhibition of build-up of the whisker network. The non-linear viscoelastic behaviour ofthe nanocomposites was also investigated and analysed in terms of the breakdown of different networks,namely the filler–filler and the polymer–filler networks.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In the last two decades, nanocomposites based on cellulosicnanofibres have drawn much attention both from industry andacademic levels [1–4]. With a strength near to the theoretical valuefor cellulose, i.e. a Young’s modulus of over 130 GPa [5] and an as-pect ratio ranging from 20 up to 100, the incorporation of cellulosewhiskers within a polymer matrix at a level lower than 5 wt.%. re-sults in a nanocomposite material with unique and outstandingreinforcing capabilities never found in conventional composites[6,8,9]. These nanofibres or cellulose whiskers consist of monocrys-talline cellulose domains with molecular chains parallel to themicrofibril axis and without folding.

A lot of papers, too many to cite them all, and reviews have al-ready been published, highlighting the use of cellulose nanofiller asa reinforcing phase in nanocomposite films. All these studies con-centrate mostly on the properties of nanocomposite preparedeither by mixing the fibrils with water dispersions of polymers orby incorporating the whisker suspension into an organic polymersolution after chemically modifying the whiskers [1–10].

While considerable research has been conducted regarding thephysical and mechanical properties of cellulose nanofiller-based

ll rights reserved.

et al. Melt rheology of nanoco.01.012

nanocomposites, to the best of our knowledge no investigationshave focused on the rheological behaviour of polymer/whiskernanocomposites in melt state. From an engineering point of view,understanding the viscoelastic properties of nanocomposite ther-moplastic polymers is of great importance in gaining fundamentalknowledge for predicting the processability of these materials.From the scientific research standpoint, the rheology is an effectiveway of probing the microstructure and assessing the state of dis-persion of the nanocomposites directly in the melt state. Like thefiller nanostructure, the inter-particle and polymer-filler interac-tions may strongly influence both linear and non-linear viscoelas-tic responses. Rheology thus appears to be a unique technique forinvestigating polymer nanocomposites and probing the micro-structure of the network likely to be formed.

In a previous work [8,11], we developed a one-step method forpreparing stable aqueous nanocomposite dispersions based on cel-lulose whiskers and acrylic copolymer via miniemulsion polymer-ization with a solid content reaching 25 wt.%. It was shown that thereinforcing efficiency of the nanofiller was dependent on theamount of MPS used as a coupling agent to promote the anchoringof the whiskers on the polymer particles. In the present work, themelt rheological behaviours of nanocomposites prepared byminiemulsion polymerization are investigated in terms of thestorage G0 and the complex viscosity g, using oscillatory rheome-try. The nanocomposite film containing an increasing amount of

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

2 A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx

cellulose whiskers was prepared by casting and water evaporationof a nanocomposite dispersion synthesized by in situ miniemulsionpolymerization in the presence of three different levels of MPS as acoupling agent, viz, 0%, 1% and 3%. based on the monomer content.

The present work therefore places emphasis on correlating therheological data with the nanocomposite microstructure in termsof the state of dispersion of the whiskers, their degree of interac-tion with the polymer matrix and the distribution of the silanecoupling agent within the polymer particles.

2. Experimental

2.1. Materials

Styrene (S) and 2-Ethyl Hexylacrylate (EHA) were purified bydistillation under reduced pressure and kept refrigerated untilused. 2,2-azobis(isobutyronitrile) (AIBN), sodium dioctylsulfanosuccinate (DOSS), Hexadecane (HD) and c-methacryloxypropyltri-ethoxysilane (MPS) are commercial products that were supplied byAldrich and used without further purification. Deionized water wasused for all polymerization and treatment processes.

2.2. Whisker preparation

The method used to prepare the whiskers was reported else-where [9]. Based on TEM analysis, the whiskers derived from alfafibres displayed a needle-like structure with an average length Land width d being estimated to be around L = 220 ± 20 andd = 8 ± 2 nm.

2.3. Miniemulsion preparation and polymerization

The miniemulsion preparation conditions are reported in Ta-ble 1. The oil phase contained HD, AIBN the mixture monomers.The aqueous phase was prepared by adding DOSS in 30 g of a cel-lulose whisker suspension in water at room temperature. Whenpolymerization was conducted in the presence of MPS, the silanewas added to the whisker suspension before DOSS was introducedand mixing continued for 2 h. The pH was checked and if necessaryadjusted to 7 before the monomer mixture was added.

After stirring for 10 min, the miniemulsion was prepared byultrasonication in an ice bath for 3 min with a 4710 series ultra-sonic homogenizer (Sonics Vibracel Model CV33) at a duty cycleof 70%. Upon completion of the sonication, the resulting miniemul-sion was polymerized at 70 �C for 3 h while being mechanicallystirred with a half-moon shaped Teflon stirrer at 300 rpm undera stream of inert N2 atmosphere.

The nanocomposite films, ranging in thickness between 500 and800 lm (in a dry state) were prepared by pouring the dispersion ina Teflon mould and leaving the water to evaporate at room temper-ature for 10 h. They were then stored at 50 �C in an air stream for2 h to ensure film formation and complete the drying process.

Table 1Recipe for the miniemulsion polymerization of Sty-co-EHA in presence of cellulosewhiskers.

Component Added (g) % with respect to monomer

Styrene-ethylhexylacrylate 6.57/3.43 100Hexadecane 0.5 5AIBN 0.2 2Cellulose whiskers 0–0.5 0–5Sodium dioctylsulfano succinate 0.3 3Water 30–29.5

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

2.4. Rheological measurements

Dynamic measurements were performed using an ARES rheom-eter supplied by TA Instruments working under strain-controlledmode. The measurements were carried out in oscillatory modeusing a 10 mm parallel plate geometry with gap settings of about1 mm. After loading, the test samples were placed between twoparallel plates and allowed to rest until they reached the tempera-ture at which the rheological measurements were carried out. Aperiod of 5–10 min was necessary to erase the thermal andmechanical histories of the samples under investigation.

Angular frequency sweeps between 0.1 and 100 rad/s. were car-ried out at low strains (0.1–3%) selected to be within the linear vis-coelastic range. The non-linear behaviour of all the samples wastested in the range of deformation from 0.1 to 100% at 0.1 rad/s.

3. Results

Oscillatory measurements were performed to compare the meltrheological behaviour of the prepared nanocomposite film and toinvestigate network formation and the microstructural changesin the nanocomposites in detail. Frequency spectra were recordedin the linear viscoelastic region at a deformation of 0.3%.

3.1. Frequency response of neat matrix during linear behaviour

The first part of our study will concern the rheological behav-iour of the neat matrix with and without MPS. As shown inFig. 1, the neat matrix synthesized without MPS exhibits a behav-iour typical of a liquid-like material, with a G00 higher than G0 in therange of frequencies explored. Their frequency dependency in low-frequency regimes is w1.75 and w1 for G0 and G00, respectively. Theserelaxation exponents are lower than the values expected from thetheory (G0 / w2 and G00 / w1) and can be ascribed to the molecularweight distribution of the polymeric matrix [12]. The dynamic vis-cosity curves clearly show a Newtonian plateau region for frequen-cies lower than 10 rad/s followed by a shear-thinning behaviour athigher frequencies (Fig. not shown).

In the presence of MPS and even with the low content, i.e. 1% atransition from a liquid-like to a solid-like behaviour is observedfor the unfilled matrix at the full frequency range, with G0 beingnearly frequency-independent (G0 / w0) (Fig. 1a). In the case oflow frequencies, the dynamic viscosity tends to infinity with aslope tending to �1(g� / w�1). So the shear stress for low frequen-cies tends towards an apparent yield stress typical of a solid-likebehaviour. At high frequencies, when hydrodynamic forces areadded, the matrix behaves as a shear-thinning fluid. This rheolog-ical behaviour suggests the formation of some interconnected net-work within the matrix [13] under static conditions. A furtherincrease in the MPS content up to 3% did not produce any notablechange in the melt rheological behaviour (Fig. 1a). The questionthat arises immediately is how is it possible to explain such a tran-sition from a liquid-like to a solid-like behaviour in the presence ofa low amount of silane coupling agent?

It is well known that alkoxy silane bearing functional groupsable to copolymerize with acrylic or vinylic monomers are usedto functionalize the polymer lattices. Such systems induce post-cross-linking properties at room temperature after hydrolysis ofthe trialkoxy moiety into silanol groups, followed by condensationleading to ASiAOASiA linkage connecting adjacent polymer chains[14–16]. Nonetheless, while this mechanism is approved, all the re-ported studies agree that the silane content should be higher than5 wt.%, preferably in the range of 10–12%, in order to promote theprobability of (coupling) contact among the hydrolysed silanol toensure efficient cross-linking. Further, in view of the reactivity

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

0.1 1 10 100

W (rad.s-1)

G' (

Pa)

matrix-3%MPS

matrix-2%MPS

matrix-1%MPS

matrix-0%MPS

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

0.1 1 10 100

W (rad.s-1)

G"

(Pa)

matrix-3%MPS

matrix-2%MPS

matrix-1%MPS

matrix-0%MPS

(a)

(b)

Fig. 1. Frequency sweep data at 180 �C. for neat matrix at different MPS contents; (a) storage modulus G0 and, (b) loss modulus G00 .

A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx 3

ratios [17] for hexylacrylate (M1) and MPS (M2) comonomers, (i.e.,r1 = 0.005 and r2 = 0.007, respectively), and those for styrene andMPS (e.g., r1 0.45 and r2 = 0.9, respectively), it is expected thatMPS units should be randomly incorporated into the copolymerchains. However, if one considers the hydrolysis of MPS, the re-ported reactivity ratio is meaningless in heterogeneous polymeri-zation carried out in water as a continuous phase. Under suchconditions, the silane would locate preferably at the monomerdroplets/water interface leading to silanol-functionalized latex. Gi-ven the neutral pH under which the polymerization was carriedout and the stability of silanol groups at pH values close to 7, weinfer that the surface silanol groups will be preserved as long asthe particles remain suspended in water [18]. Only after waterevaporation did the polymer particles get close together duringfilm formation and the silanol groups came into close proximityto undergo self-condensation leading to particle bridging. As a re-sult, a gradient zone of cross-linked polymer is formed at theboundaries of the cells formed by the latex particles, giving riseto a network structure within the polymer film. Fig. 1a and b showthat the crossover points between G0 and G00 are shifted to higherfrequencies and thus the relaxation time of the system increases

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

sharply as the MPS concentration rose. This hypothesis is also sup-ported by the increase in the gel content of the unfilled accompa-nying the MPS raise. It passed from 3% in the absence of MPS toabout 25%, 42% and 57% in the presence of 1%, 2% and 3%. MPS,respectively.

3.2. Frequency response of nanocomposite in linear behaviour

The storage modulus (G0) is more sensitive to the structuralchanges in the polymer, namely for nanocomposites, in compari-son with the loss modulus (G00). Therefore only the storage modu-lus curves are presented in the following part. Fig. 2a–c show thedependence of G0 vs. frequency for the nanocomposite series pre-pared in the presence of 0%, 1% and 3%. MPS and at various whiskerloadings. It can be clearly seen that the addition of whiskers, evenat a low content i.e. 1 wt.%. influences the storage modulus. Thisbecomes nearly frequency-independent, which is typical of a so-lid-like behaviour. When whiskers are added, the appearance of aplateau for G0 suggests the formation of some interconnected net-work involving the filler and the polymer phase, restraining themobility and flow of the polymer chains. A percolated and

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0.1 1 10 100W (rad.s-1)

G' (

Pa)

matrix-0%MPS1%whiskers-0%MPS2%whiskers-0%MPS 3%whiskers-0%MPS4%whiskers-0%MPS5%whiskers-0%MPS

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0.1 1 10 100

W (rad.s-1)

G' (

Pa)

matrix-1%MPS1%whiskers-1%MPS2%whiskers-1%MPS3%whiskers-1%MPS4%whiskers-1%MPS5%whiskers-1%MPS

(a)

(b)

(c)

1.00E+03

1.00E+04

1.00E+05

0.1 1 10 100

W (rad.s-1)

G' (

Pa)

matrix- 3%MPS1%whiskers-3%MPS2%whiskers-3%MPS3%whiskers-3%MPS4%whiskers-3%MPS

Fig. 2. Storage modulus G0 at 180 �C as a function of frequency for nanocomposites prepared, (a) in the absence of MPS, and in presence of (b) 1%, (c) 3% MPS, at differentwhiskers loadings.

4 A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocomposites based on acrylic copolymer and cellulose whiskers. Compos SciTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx 5

interconnected whiskers network held through strong hydrogenbonding is likely to occur, which restrains the long-range motionof the polymer chains [19].

The trend in G0 vs. the frequency at different whisker loadingsseems to be relatively similar in the case of the nanocomposite pre-pared with or without 1% MPS. Discernable growth in the magni-tude of G0 is greater as the whisker content increases. On theother hand, with 3% MPS, the presence of the nanofiller did notcause any significant change in the magnitude of G0, which roseonly by a factor 2 as the whisker content went from 1 to 4 wt.%.Further, compared to the matrix with 3 wt.% MPS, G0 decreasedin magnitude as whiskers were added, which seems at first glanceunexpected.

The modulus values at 0.1 rad/s were plotted as a function ofthe whisker content in order to establish the percolation threshold(/c). This relationship is shown in Fig. 3 for the nanocomposite ser-ies prepared at 0%, 1% and 3%. MPS /c can be determined by apply-ing a power-law function to G0 vs. whisker loading according to thefollowing equation:

G0 / ð/� /cÞn with / > /c ð1Þ

At 0 and 1% MPS the linear fit for G0 vs. (/-/c) on a log–log scale wasfound for percolation concentration /c in the range 0.6–0.8 vol.%(corresponding to a weight fraction of 1–1.5%) and critical exponentn equal to 1.8 and 1.7 at 0 and 1% MPS respectively (see inset inFig. 2).

With higher whisker loadings, the nanofiller network affectedthe rheological behaviour of the nanocomposites. However, inthe presence of 3% MPS, only a moderate rise in G0 was detectedover the entire range of nanofiller loading from 0 to 5 wt.%,suggesting that, obviously, no percolation between the cellulosenanofiller could occur when the nanocomposite dispersion is pre-pared in the presence of 3% MPS.

What then is the mechanism that explains how the MPS con-tent prevents the percolating network from forming?. Given the

1.0E+03

2.1E+04

4.1E+04

6.1E+04

8.1E+04

1.0E+05

1.2E+05

0 0.5 1 1.5

Whisker

G' (

Pa)

y =

G' = 17146 (φ−φp)1.67

R2 = 0.9922

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0.1 1

(φ−φp)

G' (

Pa)

0%-MPS

1%-MPS

Fig. 3. Storage modulus G0 at 180 �C. vs. whiskers loading for the nanocom

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

structure of MPS, one may expect that its methacrylic end will re-act with one of the two monomers building the S/EHA copolymer,whereas the tri-alkoxy function likely to hydrolysis in contactwith water. The MPS is therefore located mainly on the particlesurface, with the organic methacrylic function being oriented intothe micelles and the silanol groups protruding toward the waterphase. This hypothesis is particularly true for a low MPS content(1%). This type of topography enables the cellulose whiskers toanchor more easily onto the polymer particles. The resultingphysical connections, driven by hydrogen interaction with thesurface hydroxyl groups of the nanofiller, yields a uniform distri-bution of the nanofiller within the entire polymer matrix and re-duces the risk of agglomeration and build-up of whisker bundles.For MPS contents above 1%, a certain proportion of the hydroly-sed silane remains in the water phase and is adsorbed on the sur-face of the cellulose whiskers through hydrogen interaction.During polymerization, the adsorbed MPS may then undergohomopolymerization, thus impeding adjacent nanofibres fromsetting on hydrogen interactions likely to build up a rigid perco-lating network. Indeed, if we consider that the lateral surface ofthe hydrolysed MPS is about 9 Å2 based on ACD/3D software, thenthe amount of MPS needed to cover the polymer particle with adiameter about 120 nm, could be estimated to be close to about2% (based on the polymer phase), above which the added silanewill in excess. Fig. 4 depicts a schematic illustration of the whis-ker distribution around the polymer particles. It is also worth not-ing that at 1 wt.% whiskers and in the presence of 1% MPS, thevalue of G0 is lower than that of the matrix containing the samelevel of MPS. Two effects may account for this phenomenon, (i)the inevitable adsorption of a certain amount of MPS onto thewhiskers, thus reducing the amount of silane available to coatthe nanomicelle surfaces, and (ii) the intercalation of the cellulosewhiskers between the polymer particles, in turn reducing thebridging effect generated by the siloxane network during the coa-lescence process.

2 2.5 3 3.5 4

s loading (Vol%)

0%-MPS1%-MPS3%-MPS

14326( φ−φp) 1.83

R2 = 0.9962

10

posites prepared in presence of 0, 1 and 3% MPS (frequency: 1 rad/s).

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

Dispersion of polymer particles

Film formation during coalescence process

Film formation in presence of whiskers

Cellulose whiskers

Fig. 4. Schematic illustration of the whiskers arrangement in the prepared nanocomposites.

6 A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx

The dependence of the complex viscosity on frequency at differ-ent whisker contents and in the presence of 0%, 1% and 3%. MPS areshown in Fig. 5. At 0%. MPS the unfilled matrix shows a Newtonianplateau (Fig. 5a and b) in the low-frequency region, which is typicalof the behaviour of linear polymers. Independently of the MPS con-tent, all the nanocomposites studied exhibited a strong shear-thin-ning behaviour within the shear rate range investigated, and theNewtonian plateau totally vanished. In the case of the low frequen-cies, the dynamic viscosity was strongly shear-thinning and tendedtowards infinity with a slope of �1(g� / w�1). So the shear stresstends towards an apparent yield stress as a solid. In the case ofthe high frequencies, when hydrodynamic forces are added, thematrix behaves like a shear-thinning fluid. At 0 and 1 wt.%. MPS,addition of the whiskers brings about a significant raise in themagnitude of the complex viscosity, following the same trend asthat observed for G0. At 1% MPS the continuous increase in viscosity

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

with whisker loading is obvious with the viscosity curves at lowfrequencies being nearly parallel to each other with a slope of�1, shifting to higher values as the whisker content increases. Infact under static conditions, the yield stress of the nanocompositeincreases. As discussed previously, the rise in viscosity confirmsthat the presence of the cellulose whiskers restricts the movementof the polymer as a result of the formation of a polymer-whiskernetwork. At high shear rates, the effect of the filler on the viscosityis reduced in magnitude, and the nanocomposite viscosity tendstowards that of the matrix when the nanofiller loading is lowerthen 2 wt.%. At higher levels, the viscosity diverges from that ofthe matrix as a result of the whisker–whisker interconnected net-work generated above the percolation threshold. The significantdecrease in viscosity at higher shear rates will be helpful for pro-cessing the nanocomposite based on cellulose whiskers and a ther-moplastic matrix.

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

0.1 1 10 100

W (rad.s-1)

* (Pa

s)

matrix-0%MPS

1%whiskers-0%MPS

2%Whiskers-0%MPS

3%whiskers-0%MPS

4%whiskers-0%MPS

5%whiskers-0%MPS

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0.1 1 10 100

W (rad.s-1)

* (Pa

s)

matrix-0%MPS

matrix- 1%MPS1%whiskers-1%MPS

2%whiskers-1%MPS

3%whiskers-1%MPS4%whiskers-1%MPS

5%whiskers-1%MPS

(a)

(b)

(c)

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0.1 1 10 100

W (rad.s-1)

* (Pa

s)

matrix-0%MPS

matrix-3%MPS1%whiskers-3%MPS

2%whiskers-3%MPS3%whiskers-3%MPS

4%whiskers-3%MPS

Fig. 5. Complex viscosity g� at 180 �C. as a function of frequency for nanocomposites prepared, (a) in the absence of MPS, and in presence of (b) 1% (c) 3% MPS, at differentwhiskers loadings.

A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx 7

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocomposites based on acrylic copolymer and cellulose whiskers. Compos SciTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

8 A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx

As far as the nanocomposites containing 3% of MPS are concerned(Fig. 5c), the presence of the whiskers did not lead to any enhance-ment in viscosity, even with respect to the matrix alone. Increasingthe whisker content (from 1 to 4 wt.%.) yielded viscosity curves closeto each other with a magnitude lower than those observed for thenanocomposite prepared in the presence of 0% and 1% MPS, whichonce again confirms the absence of an interconnected networkamong the whiskers. The highest viscosity magnitude observed forthe unfilled matrix with 3%. MPS may be rationalized if we considerthat the cellulose whiskers draw MPS molecules off the monomerdroplet, making it easier for them to accumulate on the surface ofthe whiskers rather than around the polymer particles. Such behav-iour inhibits the formation of a siloxane network within the copoly-mer matrix. Inhibition of the filler–filler interaction has also beenshown to reduce the reinforcing potential of whisker-based nano-composites dramatically in the rubbery state [20–22]. It has alsobeen shown that the accumulation of hydrophylic plasticizer on cel-lulose whiskers prevents the build-up of a percolated network on astarch-based nanocomposite. A similar effect was reported in deal-ing with the surface chemical modification of whiskers by surfacegrafting with a hydrophobic organic moiety [20,21].

0

0.2

0.4

0.6

0.8

1

1.2

0.01 0.1

G'/

G'G

0

mamm trirr xii -xx 0%MPS

1%whiskekk rs-0%MPS

2%whiskekk rs-0%MPS

3%whiskekk rs-0%MPS

4%whiskekk rs-0%MPS

5%whsikii ekk rs-0%MPS

0

0.2

0.4

0.6

0.8

1

0.1 1γγ

γγ (

G'/G/

' 0

5%whiskekk rs-1%MPS

4%whiskekk rs-1%MPS

3%whiskekk rs-1%MPS

2%whiskekk rs-1%MPS

1%whiskekk rs-1%MPS

mamm trirr xii -xx 1%MPS

(a)

(b)

Fig. 6. Strain amplitude dependence of the normalized elastic modulus ðG0=G00Þ for

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

3.3. Strain amplitude response of the nanocomposite in the melt state

To further investigate the microstructure of the whisker-basednanocomposite at the melt state, the transition from linear tonon-linear viscoelastic behaviour under strain was studied. Theamplitude dependence of the dynamic viscoelastic properties of ananocomposite is often referred as the Payne [23] effect, whichwas first reported for carbon black-reinforced rubbers and then ex-tended to most other nanocomposites.

In order to depict the deviation from the linear region better,the values of the elastic modulus G0 are normalized by those inthe linear region at low strain amplitude, where G0 is constant.The resulting normalized values were plotted against strain ampli-tude for the different nanocomposites prepared in the presence of0%, 1% and 3%. MPS and for a whisker loading ranging from 1 to5 wt.%, as shown in Fig. 6. Superimposed normalized elastic moduliðG0=G00Þ exhibit a linear region at low strain amplitude and a non-linear region at high strain amplitude. The region of linear visco-elastic behaviour for the matrix extends up to 40% strain, but fallsdown to lower than 1% when whiskers are added to nanocompos-ites prepared with 0 and 1% MPS.

1 10 100

10 100(%)

%)

the nanocomposites prepared at different MPS contents (Temperature 180 �C.).

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

0.1

1

10

100

0 1 2 3 4 5 6

Whiskers loading (wt%.)

γc (%

)

0%MPS

1%MPS

3%MPS

Fig. 7. Critical strain amplitude (cc), as a function of whiskers loading (Temperature180 �C.).

A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx 9

It is worth noting that non-linear viscoelastic behaviour is oftenobserved in the case of polymeric matrix reinforced by nanofillerssuch as clay [24], fumed silica [25], carbon nanotubes [26] and soon. A number of different local mechanisms related to the changein network structure have been proposed to explain this phenom-enon [27]. The drop in the storage modulus should be consideredthe result of a breakdown in different networks, namely: (i) fil-ler–filler, (ii) weak polymer-filler, (iii) chemical, and (iv) entangle-ment networks. Now, if the critical strain is defined as thetransition point at which the deviation from linear to non-linearviscoelastic behaviour occurs, then the change in critical strain val-ues can be plotted vs. the whisker fraction for the different nano-composite samples. For the sake of comparison, cc was taken tobe the strain value at which the storage modulus is equal to 90%of the plateau modulus ðG0 ¼ 0:9G00Þ. The following remarks maybe made concerning the critical strain values vs. whisker contentshown in Fig. 7 for the sample series prepared in the presence of0%, 1% and 3% MPS:

– In the absence of whiskers, the presence of up to 3% silane nota-bly reduces the extent of linear behaviour with respect to theneat matrix. It is likely that siloxane bridging after the filmhas formed produces mild cross-linking that reduces chainslippage.

– In the absence and in the presence of 1%. MPS, the critical strainamplitude cc of the nanocomposite decreases with whiskerloading. However, at 0%. MPS, cc levels off at 0.6% strain up to4 wt.% content, while at 1%. MPS, cc drops down to 0.7% andagain rises to 2.5% up to 3 wt.% whiskers. The drop in cc withthe addition whiskers is to be expected given the potential ofthe high aspect ratio of the nanofiller, which easily gives riseto filler–filler and filler-matrix networks. However, the sharpdrop observed before attaining the percolation threshold sug-gests that the mobility of the polymer chains is restricted tosome extent by the presence of the nanofibres due to filler–polymer interaction. The shift in the cc curves to higher valuesat 1%. MPS denotes the possible formation of a stronger networkmicrostructure able to withstand higher strain. It is likely thatMPS enhances whisker matrix interaction, thus reducing theiraggregation and promoting the build-up of rigid a intercon-nected network.

– In the presence of 3% MPS, cc reaches high values (>10%), whichare close to that of the unfilled matrix independently of whiskercontent, suggesting that no nanofiller network forms. As previ-ously suggested, at such silane content levels, the whiskers are

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

layered by the hydrolyzed silane, the presence of which hindersthe formation of hydrogen bonds between two adjacent whis-kers, and the filler–filler interactions are reduced. Consequently,the build-up of a strong filler network is ruled out. This resultagain emphasizes the essential role of filler networking in thedynamic properties of the nanocomposite and in the reinforcingpotential of the nanofiller.

4. Conclusions

Nanocomposites based on poly(styrene-co-hexylacrylate)copolymer and cellulose whiskers as nano-sized filler were pre-pared by in situ miniemulsion polymerization in the presence of0%, 1% and 3% of MPS as a coupling agent. In the absence of MPS,the neat matrix exhibits a behaviour typical of liquid-like materi-als, with a Newtonian region at low shear rates. In the presenceof MPS and even at low contents, i.e. 1% a transition from a li-quid-like to a solid-like behaviour is observed at the full frequencyrange. Such a feature was ascribed to the formation of a networkresulting from condensation of the silanol groups across the poly-mer particles during the coalescence process.

The nanocomposite displayed solid-like and shear-thinningbehaviours equally over the whole range of frequencies. However,the dependence of G0 and viscosity as a function of whisker loadingis MPS content-dependent. At 0 and 1% MPS a sharp enhancementin G0 starting at 2 wt.%. whisker loading was detected and ascribedto the build-up of a percolated and interconnected whisker net-work thanks to strong hydrogen bonding. As a consequence, thelong-range motion of the polymer chains was restrained. In thepresence of 3% MPS, only a moderate enhancement in the G0 valuesis detected over the whole range of nanofiller loading from 0 to5 wt.%, indicating that percolation of the cellulose nanofiller wasinhibited. This behaviour was rationalized by assuming that excesshydrolysed MPS was adsorbed onto the whisker surface, thusfavouring their aggregation and hindering the hydrogen interac-tion between adjacent whiskers.

The non-linear viscoelastic properties of the nanocompositeconfirmed the difference in their behaviour as a function of MPScontent. Thus, at 0% and 1% MPS, there was clear-cut evidence ofthe build-up of a whisker-based network through the decrease inthe extent of the linear behaviour. Polymer–polymer and poly-mer–whisker networks are also likely to contribute to the rigidityof the polymer.

Acknowledgement

Financial support from the CNRS (Projects Internationaux deCoopération Scientifique – PICS) is gratefully acknowledged.

References

[1] Favier V, Canova GR, Cavaillé JY, Chanzy H, Dufresne A, Gauthier C.Nanocomposite materials from latex and cellulose whiskers. Polym AdvTechnol 1995;6:351–5.

[2] Azizi Samir MAS, Alloin F, Dufresne A. Review of recent research into cellulosicwhiskers, their properties and their applications in nanocomposites field.Biomacromolecules 2005;6:612–26.

[3] Siqueira G, Bras J, Dufresne A. Cellulose whiskers vs. microfibrils: influence ofthe nature of the nanoparticle and its surface functionalization on the thermaland mechanical properties of nanocomposites. Biomacromolecules 2009;10:425–32.

[4] Gindl W, Keckes J. All cellulose nanocomposite. Polymer 2005(462):10221–5.[5] Sturcova A, Davies GR, Eichhorn SJ. The elastic modulus and stress-transfer

properties of tunicate cellulose whiskers. Biomacromolecules 2005;6(2):1055–61.

[6] Chazeau L, Paillet M, Cavaille J-Y. Plasticized PVC reinforced with cellulosewhiskers. I. linear viscoelastic behavior analyzed through the quasi-pointdefect theory. J Polym Sci Part B Polym Phys 1999;37:2151.

[7] Bendahou A, Habibi Y, Kaddami H, Dufresne A. Physico-chemicalcharacterization of palm from Phoenix Dactylifera-L, preparation of cellulose

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci

10 A.B. Mabrouk et al. / Composites Science and Technology xxx (2011) xxx–xxx

whiskers and natural rubber-based nanocomposites. J Biobased MaterBioenergy 2009;3:81–90.

[8] Ben Mabrouk A, Thielemans W, Dufresne A, Boufi S. Preparation ofpoly(styrene-co-hexylacrylate)/cellulose whiskers nanocomposites viaminiemulsion polymerization. J Appl Polym Sci 2009;114:2946–55.

[9] Helbert W, Cavaillé JY, Dufresne A. Thermoplastic nanocomposites filled withwheat Straw cellulose whiskers part I: processing and mechanical behavior.Polym Compos 1996;17:604–11.

[10] Bercea M, Navard P. Shear dynamics of aqueous suspensions of cellulosewhiskers. Macromolecules 2000;33:6011–6.

[11] Ben Mabrouk A, Kaddami H, Magnin A, Belgacem MN, Dufresne A, Boufi S.Preparation of nanocomposite dispersions based on cellulose whiskers andacrylic copolymer by miniemulsion polymerization: Effect of the silanecontent. Polym Eng Sci 2011;51:62–70.

[12] Ferry JD. Viscoelastic properties of polymers. 3rd ed. New York: Wiley; 1980.[13] Dealy M, Wissbrun KF. Melt rheology and its role in plastic processing: theory

and applications. Kluwer Academic Publishers; 1999.[14] Zhang SW, Zhou SX, Weng YM, Wu LM. Synthesis of silanol-functionalized

latex nanoparticles through miniemulsion copolymerization of styrene and c-methacryloxypropyltrimethoxysilane. Langmuir 2006;22:4674–9.

[15] Marcu I, Daniels ES, Dimonie VL, Hagiopol C, Roberts JE, El-Aasser MS.Incorporation of alkoxysilanes into model latex systems: vinyl copolymer-ization of vinyltriethoxysilane and n-butyl acrylate. Macromolecules 2003;36:328–32.

[16] Guo TY, Xi C, Hao GJ, Song MD, Zhang BH. Preparation and properties of roomtemperature self-crosslinking poly(MMA-co- BA-co-St-co-VTES) latex film.Adv Polym Technol 2005;24(4):288–95.

Please cite this article in press as: Mabrouk AB et al. Melt rheology of nanocoTechnol (2011), doi:10.1016/j.compscitech.2011.01.012

[17] Rao VL, Babu GN. Copolymerization of styrene, acrylonitrile and methylmethacrylate with c-methacryloxypropyl trimethoxy silane. Eur Polym J1989;25:605.

[18] Brochier SMC, Abdelmouleh M, Boufi S, Belgacem MN, Gandini A. Silaneadsorption onto cellulose fibers: hydrolysis and condensation reactions. JColloid Interf Sci 2005;289:249–61.

[19] Favier V, Chanzy H, Cavaille JY. Polymer nanocomposites reinforced bycellulose whiskers. Macromolecules 1995;28(18):6365–72.

[20] Angles MN, Dufresne A. Plasticized starch/tunicin whiskers nanocomposites: 2mechanical behavior. Macromolecules 2001;34(9):2921–31.

[21] Gopalan NK, Dufresne A, Gandini A, Belgacem MN. Crab shells chitin whiskersreinforced natural rubber nanocomposites 3. Effect of chemical modification ofchitin whiskers. Biomacromolecules 2003;4(6):1835–42.

[22] Grunert M, Winter WT. Nanocomposites of cellulose acetate butyratereinforced with cellulose nanocrystals. J Polym Environ 2002;10(1–2):27–30.

[23] Payne AR. Reinforcement of elastomers. New York: Interscience; 1965.[24] Wan T, Clifford MJ, Gao F, Bailey AS, Gregory DH, Somsunan R. Strain

amplitude response and the microstructure of PA/clay nanocomposites.Polymer 2005;46:6429–36.

[25] Meera AP Said S, Grohens Y, Thomas S. Nonlinear viscoelastic behavior of silica-filled natural rubber nanocomposites. J Phys Chem C 2009;113:17997–8002.

[26] Abbasi S, Carreau PJ, Derdouri A, Moan M. Rheological properties andpercolation in suspensions of multiwalled carbon nanotubes inpolycarbonate. Rheol Acta 2009;48:943–59.

[27] Frohlich J, Niedermeier W, Luginsland HD. The effect of filler–filler and filler–elastomer interaction on rubber reinforcement. Composites: Part A 2005;36:449–60.

mposites based on acrylic copolymer and cellulose whiskers. Compos Sci