Nonlinear Viscoelastic Behavior of Silica-Filled Natural Rubber Nanocomposites

A. P. Meera,† Sylvere Said,‡ Yves Grohens,‡ and Sabu Thomas*,†

School of Chemical Sciences, Mahatma Gandhi UniVersity, Priyadarshini HillsP.O., Kottayam, Kerala, India 686 560, Laboratoire Polymeres, Proprietes aux Interfaces et Composites,Centre de Recherche, UniVersite de Bretagne Sud, Rue Saint Maude, Lorient Cedex, France

ReceiVed: March 5, 2009; ReVised Manuscript ReceiVed: August 25, 2009

The nonlinear viscoelastic behavior of the composites of natural rubber filled with surface-modified nanosilicawas studied with reference to silica loading. The effect of temperature on the nonlinear viscoelastic behaviorhas been investigated. It was observed that Payne effect becomes more pronounced at higher silica loading.The filler characteristics such as particle size, specific surface area, and the surface structural features werefound to be the key parameters influencing the Payne effect. A nonlinear decrease in storage modulus withincreasing strain was observed for unfilled compounds also. The results reveal that the mechanism includesthe breakdown of different networks namely the filler-filler network, the weak polymer-filler network, thechemical network, and the entanglement network. The model of variable network density proposed by Maierand Goritz has been applied to explain the nonlinear behavior. The activation energy of desorption wascalculated and found to be within the range of Van der Waal’s interaction energy. The model fits well withthe experimental results.

Introduction

The dynamic properties of filled elastomers have been asubject of active research because they affect the performanceof tires such as skid, traction, and rolling resistance to cite buta few.1-6 In particular, the effect of the strain amplitude on theviscoelastic properties of filled rubbers, known as the “Payneeffect”7 has been extensively investigated because it directlyimpacts the fuel consumption. From a phenomenological pointof view, beyond a strain higher than a few 0.1%, the storagemodulus of filled rubber departs from a plateau value G′0 andcollapse to a minimum value G∞′ . The decrease in the storagemodulus is accompanied by a maximum of the loss modulus,G′′. The amplitude of the Payne effect, ∆G ) G0′ - G∞′ increaseswith the filler content,7 the specific surface of the filler8 andstrongly depends on the surface properties of the fillers and itsdispersion9 within the matrix. On the contrary it decreases withtemperature.10 A number of different local mechanisms havebeen proposed so far to explain this phenomenon, but noconsensus has emerged yet.

In the widely accepted physical interpretation of the straindependence of the elastic modulus first proposed by Payne,7

the contribution from the pure polymer matrix is assumed tobe totally independent of the strain amplitude. According toPayne, the three-dimensional structure network constructed bythe aggregation of carbon black filler significantly influencesthe dynamic viscoelastic properties of carbon black filledrubbers. Kraus8 proposed a model based on the agglomeration/deagglomeration kinetics of filler aggregates by assuming a Vander waal’s type interaction between the particles. This modelwas further developed by Huber and Vilgis,11-13 who relate G′and G′′ to the fractal dimension and the connectivity of thenetwork, and by Kluppel, who introduces the idea of cluster.14

The fact that the temperature and the frequency dependenceof the amplitude of the Payne effect are not taken intoconsideration is certainly the most accepted criticism of thisapproach. However, one of the major drawbacks comes fromthe evidence brought by Funt,15 who shows from electricalconductivity measurements that the Payne effect might occuralthough a continuous filler network does not exist through thesample.

As an alternative to the destruction and reformation of a fillernetwork, it has also been proposed that the dynamics ofadsorption/desorption of the polymer chains at the particlesurface may be responsible for various linear and nonlineareffects.16,17 Zhu and Sternstein have suggested that the reductionof the storage modulus with the applied strain could be relatedto polymer-filler interactions including the aspects of trappedtopological entanglements.18 The interaction between the fillerparticles and the rubber matrix, which leads to the adsorptionof polymer chains on the particle surface, can be controlled byvarying the nature of the polymer-filler interface.19-24

Maier and Goritz16,25-27 take into consideration the adsorption/desorption mechanism by considering the filler particles asmultifunctional cross-link with chains which are either looselyor strongly anchor to the surface. The molecular interpretationof the Payne effect is then based on a variable network densitywhen the loosely tied chains are desorbed with the increaseof the strain. A compromise is also suggested, considering thatthe primary mechanism for the Payne effect certainly involvesthe existence of cooperation between the breakdown andreformation of the filler network and the molecular disentangle-ment of the bound and free rubber.15,28-30 Another explanationfirst proposed by Yatsuyanagi et al.31 considers the existenceof a percolation network through the rigid amorphous layerformed around the particles. Their interpretation of the Payneeffect equally relies on the competition between desorption andadsorption of this rigid amorphous layer.

The importance of glassy layers in filled polymer has receivedconsiderable attention very recently, when it was recognized

* Corresponding author. E-mail: sabut552001@yahoo.com; sabut@sancharnet.in. Tel.: 91-481-2730003; Fax: 91-481-2731002.

† Mahatma Gandhi University.‡ Universite de Bretagne Sud.

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10.1021/jp9020118 CCC: $40.75 2009 American Chemical SocietyPublished on Web 09/24/2009

that a glass transition gradient exist near the surface and thatthe dynamics could be either enhanced or slowed downaccording to the interaction of the chains with the surface. In abody of work,32-35 Montes et al. among others36 have clearlyshown in filled elastomers that a maximum of reinforcement isobtained when this rigid or slow dynamics layer forms acontinuous path through the filler aggregates. In their work,Merabia et al.37 model the Payne effect by considering that thestress is supported mainly by the cross section of glassy bridgesin a direction normal to the stress. They explain the straindependence of the elastic modulus by the local lowering ofthe glass transition due to the amplification of the stress in thevicinity of the aggregates. This plasticizing effect induces theyielding of the glassy bridges and the collapse of the storagemodulus.

To the best of our knowledge, not much work has beendevoted to the studies of the nonlinear viscoelastic behavior ofnatural rubber filled with nanofillers. The objective of the presentstudy is to investigate the effect of nanosilica on the nonlinearviscoelastic properties of rubbers and to understand the mech-anism of nonlinearity in such systems.

Experimental Section

Materials. The natural rubber, NR, used for the study wasprocured from Rubber Research Institute of India, Kottayam,Kerala. The molecular weight of NR obtained by light scatteringis Mn )2.68 × 105 g/mol, Mw ) 8.38 × 105 g/mol and Mw/Mn

)3.1. The surface modified nanosilica used for the study(AEROSIL R 8200) was obtained from Degussa, Germany. Thecharacteristics of silica are given in Table 1. All other ingredientsused were of commercial grade and were kindly supplied byLANXESS, Germany.

Preparation of Composites. The composite materials wereprepared via melt mixing technique on a laboratory two-rollmill. Formulation of mixes is shown in Table 2. NR wasmasticated on the two-roll mill for about two to three minutesfollowed by addition of the ingredients. Cure characteristics werestudied using an Oscillating Disc Rheometer (Monsanto R-100),at a temperature of 150 °C. The compounds were cured at theirrespective cure times using a hydraulic press under a pressureof about 120 bar at 150 °C.

Viscoelastic Measurements. The viscoelastic measurementswere performed on a dynamic mechanical analyzer (TA Instru-

ments) in tension mode. Payne effect measurements were carriedout at a constant frequency of 0.5 Hz at 303 K. To study theeffect of temperature on the Payne effect, we tested the samplesat various temperatures ranging from 248 to 373 K with 10 Ksteps.

Morphology Characterization. The size of the elementaryparticles was characterized by transmission electron microscopy(TEM) with a C M 12 PHILIPS HRTEM. The structure of puresilica was analyzed by TEM (Figure 1). The silica particles areaggregated and the mean aggregate diameter was found to bearound 25 nm. The state of dispersion was analyzed by atomicforce microscopy on ultra cryotomed surfaces cut at 153K intapping mode with a cantilever having a stiffness of 48 N/mand a length of 250 µm. The images were carried outsimultaneously in height, phase, and amplitude.

Results and Discussion

Viscoelastic Properties versus Strain. The viscoelasticbehavior vs strain is investigated for silica filled natural rubberand the results show the qualitative features generally observedfor the Payne effect. The effect of the strain amplitude on thestorage modulus at various silica concentrations for the com-posites is shown in Figure 2. The storage modulus is the highestat small amplitude (referred to as E′0) and gradually decreasesto a low value (referred to as E′∞). The magnitude of the Payneeffect (E0′ - E′∞) increases with the silica content. At low silicaloading, the observed variation in the amplitude of the Payneeffect is weak. But as the silica concentration increases,significant and pronounced variation is observed. This isprincipally due to the breakdown of the filler networks at highstrains. At low filler loading, the chances of forming agglomer-

TABLE 1: Characteristics of Nanosilica

properties values

specific surface area (BET) 160 ( 25 m2/gaverage particle diameter 12-13 nmcarbon content 2.0-4.0 wt %moisture e0.5 wt %pH g5.0SiO2 content >99.8 wt %

TABLE 2: Formulation of Mixes

ingredient recipe, phr

rubber 100zinc oxide 5stearic acid 1.5TMQa 1CBSb 0.6sulfur 2.5silica variable

a 2,2,4-Trimethyl-1,2-dihydroquinoline, polymerized. b N-Cyclohexyl-2-benzothiazyl-sulphenamide.

Figure 1. TEM micrographs of silica.

Figure 2. Strain dependence of the storage modulus for NR filled withnanosilica: (O) 0 phr, (4) 5 phr, (3) 10 phr, (]) 15 phr, (*) 20 phr.The dotted lines represent the curve fits according to the model.

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ates are practically nil. But at higher loading, because of thesmall particle size (12-13 nm) and high specific surface area(160 ( 25 m2/g), silica particles tend to agglomerate to higherextent. The structure of filler particles within the rubber matrix,i.e., the state of dispersion and aggregation has a strong influenceon the Payne effect. In the rubber matrix, the state of dispersionof spherical particles can vary from highly dispersed to totallyaggregate depending on the thermodynamics of the system andkinetics of samples preparation. The clustering of filler particlesis favored by strong van der Waals forces.

The AFM picture of cryo microtomed silica nanocompositeswith 20 phr is shown in Figure 3. Although it is not possible toderive an accurate characterization of 3D network from a 2Drepresentation, we can have a rough estimate of the aggregatessize and their separating distance. A typical aggregate size is100 nm and a characteristic distance between two aggregatesis a few tens of nanometers. Although it might be misleadingin a 3D space, no continuous path was found in a 2D space,hinting that if a percolation network does exist, it is mediatedthrough polymer bridges. Many experimental works come tothis conclusion. The rheological properties can be used as atool to assess the state of dispersion in the melt state38,39 andthe existence of a network by analyzing the independence ofthe storage and loss modulus at low frequency.28,40 For instanceLeblanc has estimated the so-called bound rubber41 from theunextractable polymer fraction and has related the effect ofelastomer-filler interactions to the rheological properties. Zhangand Archer39 in poly(ethylene oxide) melt containing silicananospheres show that the rheological properties are governedby the bridging of nanosized silica particles surrounded by animmobilized shell of PEO.

A maximum in the loss modulus E′′ is observed for thecomposites as shown in Figure 4. At filler loading higher than10 phr, a pronounced maximum is observed. It is thus veryappealing to make the hypothesis that different contributionsare at stake in the dissipated energy. One contribution comesfrom the rubbery matrix and is mainly independent from thefiller content. A second one comes from the shearing of theglassy layers around the filler particles. Another contributionmay be ascribed to the mechanism responsible for the collapse

of the storage modulus. Within the frame of the Kraus model,this extra contribution to the energy dissipation comes from thefriction between the filler particles. For models involving boundrubber, it might be related to the friction of the chains, eitheron the surface of the filler particles or within the glassy layerduring its softening under the effect of the applied stress. Inthose two former mechanisms, the amount of energy dissipatedscales the total surface of the particles. The maximum comesfrom the competition between the breaking and the reformationof the network whatever the nature is (filler-filler, entanglement,glassy bridges, etc.). When the strain increases, the destructionof the network starts and increases the dissipated energy. Forhigher value of the strain, the rate of destruction is higher thanthe rate of the reconstruction of the network. As a consequence,the dissipation energy associated with the breaking of thenetwork decreases and the loss modulus goes through amaximum with the strain.

The drop of the storage modulus is attributed to thebreakdown of this network. The structural breakdown of thesilica agglomerates is shown schematically in Figure 5a. Onapplying strain to the filled vulcanizates, the breakdown of thefiller network occurs, which results in a decrease in theagglomerate size and desorption of the rubber chains from thefiller surface. In the initial stage, because of the large agglomer-ate size, there are chances for multiple points of attachment.But on straining, because of the reduced size of the agglomer-ates, multiple points get converted to single points of attachmentas shown schematically in Figure 5b. An interesting finding isthe observation of a Payne effect for the unfilled vulcanizedrubber, which is contrary to what is reported so far in theliterature. According to the generally accepted interpretation ofthe Payne effect, the contribution from the unfilled rubber isassumed to be independent of the strain amplitude. However,Pan and Kelley recently reported an enhanced Payne effect nearthe glass transition region for gum vulcanizates prepared fromoil extended styrene-butadiene rubber (SRB).42 Although aconsensus does not exist, the Payne effect is associated withthe competing destruction/building process. Thus, we infer theexistence of a secondary network in the unfilled vulcanizedrubber. Considering the large polydispersity index of 3.1, wecan make the assumption of the existence of an entanglementnetwork. This assumption is backed up by swelling measure-ments of un-cross-linked rubber with toluene, a good solvent.

Figure 3. AFM height images of natural rubber composites filled with20 phr nanosilica.

Figure 4. Strain dependence of loss modulus for silica-filled NR: (])0 phr, (b) 5 phr, (2) 15 phr, (*) 20 phr.

Nonlinear Viscoelastic Behavior of Rubber Nanocomposites J. Phys. Chem. C, Vol. 113, No. 42, 2009 17999

We notice that the raw rubber swells and does not dissolvecompletely after several weeks even when the solvent isrenewed, proving the existence of an elasticity network. Besides,for this hypothesis to hold true, the mass between cross-links,Mc, should be larger than the entanglement mass of polyisoprene,which is 1850 g/mol.43 This last point has been checked fromswelling measurements on vulcanized natural rubber and 20 phrsilica vulcanized nanocomposites. The mass between cross-linkshas been computed from the polymer volume fraction in theswollen samples using the Flory Rhener44 model. Typical valuesof Mc of 6000 g/mol have been obtained, sustaining the idea ofan entanglement network.

Effect of Temperature on the Payne Effect. Figure 6 showsthe effect of temperature on the Payne effect for natural rubberfilled with 20 phr of nano silica. As previously reported,10 theamplitude of the Payne effect decreases dramatically withtemperature. This is contrary to the theory of rubber elasticity,according to which the modulus should increase linearly withthe temperature. In agreement with the former explanation givenfor the Payne effect, the temperature increases the rate ofdestruction of the network by weakening its cohesion. In theKraus’ model, the temperature affects the strength of the vander Waals forces between the particles. In the Merabia’s vision,the thickness of the glassy layer decreases, diminishing as aconsequence the number of glassy bridges forming the network.For Maier and Goritz, the rate of the chains desorption increaseswith temperature.

Application of the Model of Variable Network Density.In the Maier and Goritz model,25-27 it is assumed that the rubbermolecules come in contact with the filler surface and getadsorbed. After forming the first link, the neighboring segmentshave a high probability to attach to the next interaction position.These chains form stable bonds to the filler surface, whereasthe remaining chains coming afterward form unstable bonds

having very weak links to the particle. These can be easilyremoved by a tensile force or by raising the temperature.According to this model, the network density of a filledvulcanised elastomer (N) comes from the overall contributionof the chemical network density, Nc, the chains network densitycaused by stable bonds at the filler surface, Nst, and the densityof unstable bonds between chains and filler, Nl

Storage modulus E′ ) NkBT where N is the network density,kB is the Boltzmann constant, and T is the temperature in Kelvin.The strain dependent modulus can be described by means ofthe theory of entropy and elasticity with

By analogy with the Langmuir isotherm formation, it is assumedthat the adsorption/desorption process reaches a balance after acertain time. On the assumption that the desorption rate isproportional to the strain amplitude, γ, the dependence of themodulus on γ can be written as

With

and

The experimental curves were fitted to eq 3. Figure 1 showsthe curve fits for the storage modulus versus the strain ampli-tude. The fit parameters, E′st and E′l and the values of �2/DOFand R2 to assess the quality of fit are given in Table 3. Both E′st

and E′l increase with an increase in silica content. The valuescharacterizing the unstable part of the network are stronglyinfluenced by the filler content, as can be seen from the table.The model equally applies when no filler is added, suggesting

Figure 5. (a) Schematic representation of the breakdown of aggregatesand desorption of rubber chain segments from the filler surface in silica-filled NR system (b) Schematic representation showing the multiplepoints of attachments of rubber chains at the silica surface convertingto the single points of attachments on straining.

Figure 6. Effect of temperature on the Payne effect for NR filled with20 ph silica: (0) 248 K, (4) 263 K, (O) 303 K, (]) 373 K. The dottedlines represent the curve fits according to the model.

N ) Nc + Nst + Nl (1)

E′(γ) ) (Nc + Nst + Nl(γ))kBT (2)

E′(γ) ) E′st + E′l/(1 + cγ) (3)

E′st ) (Nc + Nst)kBT (4)

E′l ) NlokBT (5)

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that part of the entanglement network has a longer relaxationtime than the experimental time defined as the inverse of thefrequency.

Effect of Temperature. The experimental curves obtained forNR filled with 20 phr silica and fitted to eq 3 are shown inFigure 6. The dotted lines represent the curve fits according tothe model. The fit parameters along with the values of Chı2/DoF and R2 are listed in Table 4. It is observed that both thenumber of the stable and unstable bonds of the network isidentically reduced by roughly a factor 5 when the temperaturegoes from 248 to 373 K. The huge variation of the stable bondswith temperature questions the initial picture where stable bondswere depicted in terms of trains and tightly bounded chains atthe vicinity of the surface.45-47 This image was mainly sustainedby NMR results, which show that this layer with a typicalthickness of 1 nm remains immobile to temperature up to Tg +200 K.47,48 Unless the polymer filler interface is poorly cohesive,it seems unlikely to explain the sharp decrease in the stablebond contribution from the desorption of the tightly boundedchains when the temperature is around Tg + 150 °C. A plausibleexplanation to reconcile the Maier and Goritz model and thevariation of the stable bonds with temperature is to considerthat the stable bonds are materialized by glassy bridges assuggested by Merabia et al. It is thus straightforward tounderstand the dramatic effect of temperature when it goeshigher than the glass transition temperature of the rigidamorphous layer. The unstable bonds would represent thepopulation of chains whose Tg are very close to the experimenttemperature.

To further analyze the results, the temperature dependenceof the density of unstable bonded chains is written as anArrhenius law according to

where Nloo is a constant characterizing the density of unstablebonds and is independent of temperature and the deformationamplitude; El is the activation energy of desorption.

From the plot of the logarithm of the density of unstablechains against the inverse of temperature shown in Figure 7,the activation energy can be calculated and is found to be 0.09eV for NR filled with 20 phr of silica. This value is within therange of Van der Waal’s interaction energy.49 The rate ofdesorption is directly proportional to the number of unstablechains at the filler surface. Hence it is concluded that the number

of unstable fixed chains adsorbed on the filler surface is alsoresponsible for the reduction in modulus with increase intemperature.

Conclusions

This study focuses on the dynamic viscoelastic properties ofnanosilica-filled natural rubber composites. The objective of thepresent study was to look at the nonlinear viscoelastic behaviorof natural rubber filled with commercially used nanosilica. ThePayne effect is assumed to arise from the elementary mechanismconsisting of adsorption-desorption of macromolecular chainsfrom the filler surface. It was found that because of the smallparticle size and high specific surface area, nanosilica formsstronger and more developed filler-filler network and thebreakdown of these networks results in larger Payne effect. Also,the amount and morphology of the fillers played a major roleon the Payne effect.

At low loading, there is not much variation in storagemodulus, loss modulus, and loss tangent compared to gumvulcanizates. But at higher loading, a pronounced effect hasbeen observed. This is due to the breakage of weak polymer-fillerlinkages and filler-filler networks at higher strain amplitude.But surprisingly, enhanced Payne-like behavior has beenobserved for gum vulcanizates at room temperature where thereare no filler-filler and no filler-polymer interactions, whichare typically associated with filled vulcanizates. This is explainedby the effect of chain disentanglements on straining. The modelof variable network density has been applied and the calculatedactivation energy for NR filled with 20 phr silica is found to bewithin the range of Van der Waal’s interaction energy. Henceit is concluded that the number of unstable fixed chains adsorbedon the filler surface is also responsible for the reduction inmodulus with increase in temperature.

Finally, it is concluded that in addition to the contributionfrom filler-filler network, there are a lot of factors that affectthe nonlinear viscoelastic behavior including the breakdown ofdifferent networks, namely, filler-filler networks, weakpolymer-filler networks, chemical networks, and entanglementnetworks.

Acknowledgment. We gratefully acknowledge Rubber Re-search Institute of India (RRII), Kerala, for providing natural

TABLE 3: Fit Parameters of Eq 1

silica content (phr) E′st (MPa) E′l (MPa) c �2/DOF R2

0 0.17 1.56 0.03 0.001 0.99210 0.32 1.82 0.06 0.002 0.99315 0.43 1.75 0.11 0.002 0.99320 0.57 2.22 0.15 0.003 0.994

TABLE 4: Fit Parameters As a Function of Temperature

silicacontent (phr)

T(K)

Nc + Nst/1020

(1/cm3)Nlo/1020

(1/cm3) c �2/DOF R2

15 303 0.82 3.86 0.10 0.001 0.995323 0.63 3.53 0.08 0.002 0.991343 0.45 3.09 0.07 0.002 0.992

20 248 1.88 10.87 0.17 0.020 0.988263 1.17 7.07 0.12 0.004 994303 0.90 4.98 0.10 0.003 994343 0.83 2.99 0.13 0.002 992373 0.37 2.73 0.05 0.001 0.996

Nlo(T ) ) Nlooe-El/kBT (6)

Figure 7. Arrhenius plot of the network density of unstable fixed chainsfor NR filled with 20 phr silica.

Nonlinear Viscoelastic Behavior of Rubber Nanocomposites J. Phys. Chem. C, Vol. 113, No. 42, 2009 18001

rubber; LANXESS Deutschland GmbH, Germany, for supplyingthe rubber chemicals for the research work. Thanks are due toKSCSTE, Kerala, for the financial support. The authors aregrateful to Prof. Dr. Dietmar Goeritz, Fakulty of Physics,University of Regensburg, for the fruitful discussion.

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