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Effects of the location of the nanoclay at theinterface on the mechanical properties of amaleic anhydride grafted polypropylene/polyamide 6/organoclay co-continuous blendIzaro Goitisoloa,b, Imanol Gonzáleza and José Ignacio Eguiazábala*

The effects of the location at the interface of an organoclay on the morphology and mechanical properties of amaleated-polypropylene/polyamide 6 based co-continuous blend have been studied. The organoclay is located atthe interface because the level of interaction with each of the two polymers was similar. The dispersed particle sizeremained unchanged with organoclay content because the effect of viscosity and coalescence inhibition was offsetby the surfactant compatibilization hindering. The Young’s modulus remained constant; this behavior is mainlyattributed to the inefficient orientation of the nanoclay. The ductility behavior suggests that there is a maximumamount of organoclay that can be located at the interface while retaining its ductile nature. Once this amount hasbeen exceeded, the interface becomes saturated, and the dispersed particles become encapsulated. Encapsulationmeans that both an inorganic barrier and discontinuity appear, hindering the stress transmission through the inter-face and leading to fragility. Copyright © 2012 John Wiley & Sons, Ltd.

Keywords: co-continuous blend; melt processing; blends; polymer/polymer interface; mechanical properties

INTRODUCTION

It is known that interest in polymer nanocomposites (PNs)containing nanoscale particles has moved from being mostlyscientific towards being clearly applied. The exfoliation oforganoclay [organically modified montmorillonite (OMMT)]greatly improves the properties of the matrix, such as thermalstability and mechanical performance, with very low fillercontents. This is due to the high surface area of the nanometricparticles. In order to achieve this exfoliation, the affinity betweenthe polymer matrix and the OMMT is probably the most crucialfactor. However, highly dispersed morphologies are rarelyachieved, especially with non-polar polymers such as polyolefins.In these cases, techniques such as the chemical modification ofthe matrix,[1] or the use of a well-dispersed master-batch-basednanocomposite, which is miscible with the desired matrix,[2,3]

may be more effective.In spite of the favorable properties of the PNs, many practical

applications require matrices based on polymer blends toguarantee optimum performance. Consequently, several studiesthat aimed at improving the properties of polymer blendsthrough the addition of OMMT have been carried out in recentyears.[2–22] The distribution of the OMMT in the blend PNdepends on the affinity of the OMMT with each of the twopolymeric components and on the mixing sequence used.[23]

Most studies have focused on the role of the OMMT (i) as rein-forcement for either the polymer matrix[2,3,24–32] or the dispersedphase[33,34] or (ii) as an emulsifier in the case of a blend PN.[35–41]

There are very few studies[35–37] where the OMMT is eitherpartially[37] or fully[35,36] located at the interface between bothpolymeric components. This is because for a nanoclay to locateat the interface, it needs to show similar affinity towards the

two polymers. Thus, in spite of the potential interest of theinterfacial location of the OMMT, the effects on mechanicalproperties have been studied, to our knowledge, only in the caseof brittle polypropylene/polystyrene (PP/PS) blend matrices.[36]

In these PNs, the addition of OMMT scarcely ameliorated theelastic modulus; however, it increased the elongation at break(always inside the brittle zone). Thus, the ability of the nanoclayto locate at the interface, and the effects of this location on theproperties of co-continuous blend based on a ductile polymericblend have not been studied up to now.

Cloisite 20A is an OMMT, characterized by the high molecularvolume of its surfactant, which shows a high affinity forpolyamide 6 (PA6). It also shows a high affinity for PP when itis maleinized (mPP). Therefore, in the mPP/PA6 blend (which isof commercial interest), the Cloisite 20A could show a similaraffinity for both mPP and PA6. As a consequence, this similaraffinity should allow it to locate at the interface and, therefore,allow us to study the effects of the location of the OMMT onthe structure, and mainly on the mechanical properties.

In this study, Cloisite 20A, mPP, and PA6 were chosen as thecomponents of the co-continuous blend under study because

* Correspondence to: José Ignacio Eguiazábal, Departamento de Ciencia y Tecnologíade Polímeros, University of the Basque Country, 20080 San Sebastián, Spain.E-mail: [email protected]

a I. Goitisolo, I. González, J. I. EguiazábalDepartamento de Ciencia y Tecnología de Polímeros and Instituto deMateriales Poliméricos “POLYMAT”, Universidad del País Vasco UPV/EHU, POBox 1072, 20080 San Sebastián, Spain

b I. GoitisoloKrafft, Ctra Urnieta s/n, 20140 Andoain, Guipúzcoa, Spain

Research article

Received: 25 July 2012, Revised: 4 October 2012, Accepted: 11 October 2012, Published online in Wiley Online Library: 27 November 2012

(wileyonlinelibrary.com) DOI: 10.1002/pat.3085

Polym. Adv. Technol. 2013, 24 357–363 Copyright © 2012 John Wiley & Sons, Ltd.

357

of the supposed similar affinity of Cloisite 20A with the twopolymers. The composition of the matrix was kept constantand was rich in mPP because of the superior mechanicalperformance of the PP-rich blends.[42] The components of theco-continuous blend were blended together to facilitate thelocation of the OMMT at the interface. The nanostructure andthe microstructure of the PNs were characterized by X-raydiffraction (XRD), transmission electron microscopy (TEM), andscanning electron microscopy (SEM), respectively. The phasestructure and the melting behavior were characterized by bothdynamic-mechanical analysis (DMA) and differential scanningcalorimetry (DSC), and the mechanical properties by means oftensile tests.

EXPERIMENTAL

The PP used was IsplenW PP 070 (Repsol YPF, Spain), the PA6 wasDurethanW B30S (Dupont, Delaware, USA), and the maleic anhy-dride functionalized PP (MAH-g-PP) was FusabondW P MZ-203D(Dupont, Delaware, USA). The MAH content of the MAH-g-PP was0.74%. The filler was a montmorillonite (CloisiteW 20A, Southern ClayProducts, Texas, USA) (OMMT) organically modified with dimethyl,dehydrogenated tallow quaternary ammonium. This OMMT ischaracterized by the high molecular volume of its surfactant,which decreases favorable interactions between the clay andPA6, facilitating the location of the OMMT at the interface. PPand MAH-g-PP were mixed in a 95/5 ratio to obtain the desiredmPP content (0.037%). The OMMT content in the co-continuousblend varied from 0% to 6%, and the PA6 content was fixed at30%. The co-continuous blendswill be named by their OMMT con-tent; i.e. 2%-PN indicates a 70/30/2 mPP/PA6/OMMT composition.

Pure PA6 (dried at 80�C in vacuum for 24 h), OMMT (dried at80�C in an air oven for 4 h), and the mPP (dried at 80�C in anair oven for 4 h) were fed into a Collin ZK25 co-rotating twin-screw extruder-kneader (screw diameter of 25mm, and length-to-diameter ratio of 30/1) by means of K-TRON SODER feeder(KCV-KCL-KT20). The barrel temperature was 235�C, and therotation speed was 200 rpm. After extrusion, the extrudates werecooled in a water bath and pelletized. Subsequent injectionmolding was carried out in a Battenfeld BA-230E reciprocatingscrew injection molding machine to obtain tensile (ASTM D638,type IV, thickness 2.0mm) and impact (ASTM D256, thickness3.1mm) specimens. The screw of the plasticization unit was astandard screw with diameter of 18mm, L/D ratio of 17.8, andcompression ratio of 4. The melt temperature was 235�C andthe mold temperature 16�C. The injection speed and pressurewere 11.4 cm3/sec and 2625 bar, respectively.

The melt viscosity of the co-continuous blends was measuredby capillary extrusion in a Göttfert Rheotester 1000 rheometer.The measurements were performed at an apparent deformationrate ranging from 50 to 5000 sec�1 and at a temperature of235�C by using a flat entry capillary tungsten die with 1-mmdiameter and an L/D ratio of 30.

The contact angle measurements were carried out on a CAM100 goniometer (KSV) on injection-molded specimens and pills(in the case of the organoclay), using water and ethylene glycol.The interfacial tension was calculated by the two-liquidHarmonic Method[43,44] measuring the contact angle of the twoliquids on the surface of the mPP, PA6, and OMMT. At leastfive drops were examined and averaged for each contactangle result.

The DMA tests of the co-continuous blends, mPP/PA6 blend,and mPP and pure PA6 were performed using a TA InstrumentsDMA Q800 that provided the plots of the loss modulus versustemperature. The scans were carried out in single cantilevermode at a constant heating rate of 4�C/min and at a frequencyof 1 Hz, from �150�C to roughly 150�C. The melting behaviorof the co-continuous blends was studied by DSC using aPerkin-Elmer DSC-7 calorimeter. Indium was used as the refer-ence material. The samples were heated from 30�C to 280�C at20�C/min. The melting temperature (Tm) and enthalpy (ΔHm)were determined from the peak maximum and area, respectively.The crystallinity of PA6 was calculated assuming a melting en-thalpy of 190.6 J/g[45] for 100% crystalline PA6, and that of the PPassuming a melting enthalpy of 209 J/g[46] for 100% crystalline PP.XRD patterns were recorded in an X´pert X-ray diffractometer

operating at 40 kV and 40mA, using a Ni-filtered KaCu radiationsource. The TEM samples of co-continuous blends were ultrathinsectioned at 60–80 nm by using a cryo-ultramicrotome. Themicrographs were obtained in a Philips Tecnai 20 apparatus atan accelerating voltage of 200 kV. The surfaces of cryogenicallyfractured specimens were observed by SEM after gold coating.A Hitachi S-2700 electron microscope was used at an acceleratingvoltage of 15 kV.The tensile tests were carried out using an Instron 5569 ma-

chine at a cross-head speed of 10mm/min and at 23� 2�C and50� 5% relative humidity on specimens according to ASTMD638 type IV. Young’s modulus was determined by means ofan extensometer at a cross-head speed of 1mm/min. Themechanical properties (yield stress [sy] and ductility, measuredas the break strain [eb]) were determined from the load–displacement curves.

RESULTS AND DISCUSSION

Nanostructure

The characterization of the nanostructure of the co-continuousblends was carried out by using both XRD and TEM. The XRDplots of the 2% and 6%-PNs are shown in Fig. 1. The XRD plotsof the co-continuous blends based on both the neat PA6 and

Figure 1. X-ray diffraction patterns for OMMT, 2%-PN, and 6%-PN. TheXRD plots of PA6/OMMT and mPP/OMMT PNs are also shown as reference.To aid clarity, the curves are shifted on the vertical axis.

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mPP and that of the OMMT are also shown as a reference. As canbe seen, the scan of the OMMT shows the peak characteristic ofthe (001) plane at 2θ of 3.4�, which according to Bragg’s lawcorresponds to a basal distance d001 of 2.6 nm. The co-continuousblends showed a main diffraction peak centered at 2θ of 2.7�

(d001: 4.0 nm). Thus, the basal distance was higher in the co-continuous blends, indicating that some polymer was intercalatedin the OMMT galleries.To test which polymer, either mPP or PA6, intercalated inside

the OMMT galleries, the XRD plots of the 2% and 6% PNs werecompared with those of the binary PA6 and mPP-PNs. As can beseen, the main diffraction peak of the two ternary co-continuousblends appeared at intermediate angles between the main peaksof the PA6 and mPP, suggesting that the OMMT could be locatedat the polymer/polymer interface.Figure 2(a–f) shows the TEM micrographs of the co-continuous

blends. As can be seen, both the dispersed phase and thenanoclay were easily distinguishable in all figures, and,consequently, we can clearly observe that the nanoclay wasapparently only located at the mPP/PA6 interface, not only atlow OMMT contents, but also at higher contents, such as 6%.The nanoclay was practically exfoliated at OMMT contents lowerthan the 2%; but the presence of intercalated/agglomeratedstructures was significant at higher OMMT contents. Thisinterfacial location of the nanoclay leads to the dispersedparticles becoming almost fully encapsulated by the nanoclaylayers. The fact that the OMMT was not present in the mPPmatrix indicates that the melt processing time was long enoughto allow the OMMT to migrate from the melted mPP matrix (PP isthe major component and melts before PA6) to the interface.[23]

This unusual location (mainly at high OMMT contents) was whatenabled us to study the effects of the presence of the OMMT atthe interface on the mechanical properties.With respect to the location of the OMMT, it is believed[37] that

when all components are mixed in a single process as in thisstudy, and mixing is effective, the location of the OMMT shouldbe only determined by the interaction between the filler andthe polymers. The interaction between the PA6 and the selectedOMMT seems to be partially favorable, because the highaliphatic content of the surfactant in the OMMT (two longaliphatic chains) partially hinders interaction between the polargroups of PA6 and the silicate surface of the clay. In the caseof mPP, the presence of MAH groups should considerablyimprove the low interaction level of PP with the OMMT. Then, totest the interaction level present between the PA6/OMMT andmPP/OMMT pair, the interfacial tension (g12) of both pairs weremeasured by means of the contact angle. The g12 value of PA6/OMMT (2.66mN/m) was very close to that of the mPP/OMMT(2.89mN/m), indicating that the affinity of the OMMT for the twopolymers was rather similar. Moreover, these relative high g12values are consistent with the presence of a low compatibilitybetween the both polymers and the OMMT,[44] which couldjustify the presence of nanoclay solely at the interface. Therefore,when location of the OMMT at the interface is desired in amulticomponent co-continuous blend, the system should exhibita relatively low and similar degree of interaction between eachof the two polymers and the selected OMMT.

Phase structure

Figure 3 shows the loss modulus of the co-continuous blendsagainst temperature. The loss modulus of the mPP/PA6 blend,

the pure mPP, and PA6, and their respective binary PNs are alsoshown as a reference. As can be seen, two loss modulus peaksappear in the co-continuous blends. The low temperature peakcorresponds to the glass transition (Tg) of the mPP, and itstemperature slightly decreased at increasing OMMT content.This drop in Tg was also seen in binary mPP/OMMT PN and inother PNs[2,47] and is attributed to the plasticization of the matrixcaused by the migration of the surfactant. This migration isfavored by the low polar character of the matrix.

The high temperature peak of both co-continuous blends,which corresponds to the PA6 Tg, increased at increasing OMMTcontent. Increases in Tg are common in PNs[48] as the result of

Figure 2. TEM photomicrographs of the PN (a) 0.5%-PN, (b) 0.75%-PN,(c) 1%-PN, (d) 2%-PN, (e) 4%-PN, and (f) 6%-PN.

EFFECT OF THE LOCATION OF THE NANOCLAY AT THE INTERFACE

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confinement effects produced by interactions between thepolymer and the filler. However, in this work, no clay was presentin the PA6 phase of the co-continuous blends. In addition, the Tgof the reference PA6/OMMT PN (where all the clay is dispersedin PA6) barely increased (2�C) upon addition of 6% OMMT.Therefore, a reduction in the segmental mobility of the PA6chains was ruled out as the reason for the Tg increases in theco-continuous blends.

However, if we look at the unusual microstructure of thesePNs, where the OMMT is only located at the polymer/polymerinterface, we realize that the PA6 phases are encapsulated bythe OMMT layers that overlap each other. This could be thecause of a decrease in the PA6 chains’ mobility, resulting in ahigher Tg.

The crystalline behavior of co-continuous blends was studiedby DSC. The Tm (170�C) and the crystallinity (42%) of the mPPremained constant with the addition of OMMT and PA6,indicating that neither the addition of a filler nor the presenceof a dispersed phase disturbs the crystallization process ofmPP. In the case of PA6, the Tm (234�C) also remained constant;however, the crystallinity decreased from 30% to 23% when theOMMT content increased from 0% to 4%. This decrease in thecrystallinity of PA6 in the presence of OMMT could be the resultof diffusion problems associated with restricted mobility causedby the presence of clay as the PA6 solidified in the mold.

Microstructure

Figure 4(a–c) shows the morphology of the co-continuousblends with OMMT contents of 0%, 2%, and 6%, respectively.As can be seen, the mean dispersed particle size (lp) did notvisibly change (0.65mm in 2%-PN and 0.67 mm in the 6%-PN)with the increased OMMT content. This behavior is somewhatunexpected when the OMMT is located at the interface. Thisis because decreases in the particle size have usually beenassociated with an emulsifying effect of the nanoclay.[35–41]

If we examine the reasons for the particle size of the co-continuous blends remaining unchanged throughout thechanges in the OMMT content, it becomes clear that the

processing conditions, which could affect it, were unchanged.Moreover, any possible emulsifying effect[41] of the OMMTdecreasing the interfacial tension could hardly be significant asthe two polymers are already compatible. Therefore, in order tostudy the reasons for the observed lack of change in the particlesize, we should consider (i) a change in the viscosity of theco-continuous blends that could occur after the addition of OMMT,for instance, through the polymer degradation induced by theOMMT presence; (ii) coalescence inhibition (barrier effect)[49,50]

caused by theOMMT; and/or (iii) a hindering of the compatibilizingeffect of the maleic anhydride modification of PP caused by thesurfactant of the OMMT.The viscosity of the co-continuous blends with 0%, 1%, 2%,

4%, and 6% OMMT was measured by capillary rheometry, andthe results are shown in Fig. 5. As can be seen, the viscosity ofthe co-continuous blends increased slightly upon addition of theclay in the deformation rate range, which is predominant in theinjection processes (between 1000 and 2000 sec�1). The increase

Figure 3. The loss modulus of co-continuous blends against tempera-ture. The loss modulus of the mPP/PA6 blend, mPP, and PA6, and theirrespective binary PNs are also shown as reference. To aid clarity, thecurves are shifted on the vertical axis.

Figure 4. Cryofractured surfaces of the injection-molded impactspecimens of the co-continuous blends with OMMT contents of (a) 0%(reference blend), (b) 2% and (c) 6%.

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of viscosity with the nanoclay content was small at the deformationrate range between 1000 and 2000 sec�1, but significant. Thisincrease in the viscosity should cause a slight decrease in thedispersed particle size.With respect to the effect of coalescence inhibition, as the

OMMT is only present at the interface in this study, itscoalescence-inhibiting effect may be more pronounced, leadingto a smaller particle size.Finally, with respect to a hindering of the compatibilization

effect of the mPP by the surfactant, it is known[51–54] that thesurfactant can migrate to the matrix during melt mixing. Thishas been seen before in other PNs where both maleic groupsand polyamides were present, and it appears to happen too inthis paper as indicated by the decrease in the Tg of the mPPmatrix at increasing OMMT content. Migration allows directcontact between the surfactant and the maleic groups, thusshielding the compatibilizing effect of the mPP. Therefore, theobserved morphology of the co-continuous blends must beattributable to the offsetting effects of the change in viscosityand the coalescence inhibition on the one hand, which jointlylead to a decrease in the particle size, and on the other handthe shielding effect of the migrated surfactant that tends toincrease it.

Mechanical properties

The mechanical properties of the co-continuous blends werestudied by means of tensile tests. Figure 6 shows the relativeYoung’s modulus of the co-continuous blends as a function ofthe OMMT content. As can be seen, Young’s modulus of theco-continuous blends remained almost constant with the OMMTcontent at values only slightly higher than that of the matrix.This increase is unexpectedly low, because the addition ofOMMT usually leads to 20–40% increases in Young’s moduluswith 4–6% OMMT (thus, relative modulus 1.2–1.4) dependingon the exfoliation level. This occurs both in PA6[29,30] andmPP[16,55] based nanocomposites when the OMMT is located inthe matrix and is due to the inorganic and stiff nature of the filler.In the single case where the nanoclay was located at theinterface, namely in the co-continuous blends or PNs with a

brittle blend,[36] the modulus increase was very small even uponthe addition of 5% of nanoclay.

Regarding the main parameters influencing the modulusvalues of this study, we have the dispersion level of the OMMTand the OMMT content. In this study, the dispersion level didnot vary significantly with the OMMT content in the filler rangestudied (nanostructure section); therefore, when the nanoclaywas located at the interface, its content had almost no effecton Young’s modulus. This is consistent with the behavior ofPP/PS/OMMT co-continuous blend,[36] where the nanoclay is alsolocated at the interface. Moreover, the polymer/OMMT interactionsshould not depend on the OMMT location. Thus, if we wonderabout the reasons related to the location at the interface thatinfluence Young’s modulus, we realize that the orientation of thenanoclay around the polymeric particles is almost random. This isinefficient in terms of reinforcement, and therefore, it isproposed as being the main reason for the observed lack ofmodulus increases when the nanoclay is located at the interface.

The ductility of co-continuous blends, measured as theelongation at break, is shown in Fig. 7 as a function of the OMMT

Figure 5. Apparent viscosity (�ap) of the co-continuous blends with 0%(○), 1% (●), 2% (□), 4% (■), and 6% (Δ) OMMT contents versus the appar-ent deformation rate (_gap).

Figure 6. Relative Young’s modulus of co-continuous blends versusOMMT content.

Figure 7. Ductility of co-continuous blends versus OMMT content.



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content. As can be seen, the ductility remained constant at lowOMMT contents, then it clearly decreased to values close to theyield strain, and finally decreased further to brittle fracturevalues, at OMMT contents close to and above 4%. The mainparameters influencing ductility in these co-continuous blendsare the dispersed polymeric particle size, the content anddispersion level of the OMMT, and, plausibly, the location ofthe OMMT in the co-continuous blend. In this study, thedispersed polymeric particle size remained constant, and thedispersion level did not vary significantly with the OMMTcontent. Consequently, if we compare the ductility results of thisstudy with those obtained in the bibliography for similar systemswhere the OMMT was located either in the matrix or in thedispersed phase, we would be able to find out what the effectsof the location at the interface are on ductility.

As seen in Fig. 7, ductility remained constant when until a 1%OMMT was present in the co-continuous blend. However, whenthe OMMT content increased to 2%, ductility clearly decreased.This decrease is greater than any previously observed in binarymPP/OMMT,[56,57] PA6/OMMT[58] PNs, or ternary systems[33,34]

where the clay was located in the matrix or in the dispersedphase. Therefore, location of the OMMT at the interface is clearlydetrimental for ductility in these initially compatible blends at1–2% OMMT contents. When an unmodified 80/20/x PP/PS/OMMTco-continuous blend, which was initially brittle (approximateelongation at break 2.5%) with a coarse particle size, lp (typicalparticle diameter 10mm)[36] was studied, most of the OMMTwas located at the interface. In this case, the elongation at breakincreased upon the addition of 2% OMMT while maintainingbrittleness, but, as in this study, it decreased when the OMMTcontent increased to 5%.

In Fig. 7, fragility comes at 2% OMMT content (as theelongation at break decreased to 5%, below the yield strain[6%]). Thus, this ductility behavior suggests that there is amaximum percentage of OMMT that can be located at theinterfacewhile maintaining the ductile nature of the co-continuousblends. When this content is exceeded, the interface becomessaturated in OMMT and almost fully shields the dispersed phase.This (i) creates an inorganic barrier at the interface and thereforediscontinuity appears, and (ii) interferes with the ability totransfer stress through the interface leading to fragility.

To test the reliability of this proposal, the volume fraction ofinorganic clay needed to saturate the dispersed PA6 phase canbe estimated[59] by eqn 1:

fc ¼3�elp

�fPA6 (1)

where e is the average thickness of the intercalated nanoclay(obtained from the TEM images), lp is the dispersed particle sizediameter, and fPA6 is the volume fraction of the dispersed phase,in this case PA6. In this work, the volume fraction is 0.25,lp = 0.80mm, and e is approximately 8 nm (from TEMmicrographs);therefore, the amount of clay required to saturate the surface ofthe dispersed phase should be approximately 0.75% per volumecorresponding with 1.3% per weight. This percent is very close tothat observed in Fig. 7 where it appears slightly above 1%. Thus,the ductility behavior of the PNs appears to be conditioned bythe amount of nanoclay at the interface; i.e. below the saturationlevel, stress may be transferred through the interface maintainingductility, whereas above the saturation level, the large nanoclaypresence at the interface forms an inorganic barrier hindering

stress transmission, and reducing deformation, leading to adecrease in ductility.

CONCLUSIONS

The nanoclay was only located at the mPP/PA6 interfaceregardless of the OMMT content. This nanoclay location isattributed to the similar degree of interaction of the OMMT witheach of the two polymers. This location also causes the PA6phases to be encapsulated by the overlapping nanoclay layers.This encapsulation, and the associated decrease in the molecularmobility of the dispersed phase, is consistent with the Tgincrease observed in the dispersed phase.The dispersed particle size remained constant upon the

addition of OMMT. Once the additional constant parameters inthis study were ruled out as the cause of the unchanged sizeof the co-continuous blend particles, it was attributed to themutually offsetting effects of the change in viscosity andcoalescence inhibition, which generally reduce the particle size,and the compatibilization hindering of the migrated surfactantthat tends to increase it.Somewhat surprisingly, given the inorganic nature of the

nanoclay, its addition did not increase the modulus of elasticity.As polymer/OMMT interaction should not be affected by theOMMT location, the inefficient orientation of the nanoclay(due to randomness around the particles) at the interface issuggested as the main reason for the modulus behavior.There is a limit to the percentage of OMMT that can be located

at the interface and still ensure that the ductile nature of theco-continuous blends is preserved. When this content is exceeded,the interface becomes saturated in OMMT. This (i) creates aninorganic barrier at the interface leading to discontinuity and(ii) hinders stress transfer through the interface and thus causesfragility.

Acknowledgements

The financial support of the Basque Government (project no.IT-234-07 and S-PE11UN050) and of the University of the BasqueCountry (UFI 11/56) is gratefully acknowledged. Polymer Charac-terization and XRD Services of the University of the BasqueCountry are also gratefully acknowledged.

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