Rheological and morphological properties of thermoplastic olefin blends containing nanosilica

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Rheological and morphological properties of thermoplastic olefin blends containing nanosilica Amirhossein Maani, Pierre J. Carreau∗ Research Center for High Performance Polymer and Composite Systems, CREPEC, Department of Chemical Engineering, Polytechnique Montreal, PO Box 6079, Stn Centre-ville, Montreal, QC H3C 3A7, Canada

a r t i c l e

i n f o

Article history: Received 29 September 2015 Revised 31 December 2015 Accepted 27 January 2016 Available online xxx Keywords: Rheology Morphology Polymer blend nanocomposites Thermoplastic olefins Silica nanoparticles

a b s t r a c t This study aims at understanding the rheological and morphological behavior of polypropylene (PP) and ethylene octene copolymer (EC) blends containing nanosilica. Maleic anhydride grafted species of PP and EC were also used to favor the localization of the silica particles in the matrix or in the dispersed phase, respectively. The morphological stability of the blends under flow was investigated by shearing at a very low shear rate of 0.01 s−1 . The morphology of the dispersed polymer phases as well as the microstructure of the silica particles inside the nanocomposites were examined using scanning and transmission electron microscopy techniques. Considering the limited time stability of composites with high level of silica loading, the low frequency data of these systems were obtained using dynamic frequency sweep data as well as an extended set of transient creep data reflecting the long time relaxation behavior. The presence of silica particles resulted in an improved morphological stability of all nanocomposites, but the effect was more pronounced when the nanoparticles were localized in the dispersed EC phase. While shearing diminished the viscoelastic properties of the blends containing no silica particle, the sheared nanocomposites with a high concentration of nanosilica revealed enhanced levels of viscoelasticity. This was attributed to the promoted interconnection of solid particles during shearing and the formation of more extended networks as confirmed by microscopic observations. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Morphology control in immiscible polymer blends is a key issue in tuning their macroscopic properties. The final microstructural state of these materials is largely dependent on the extent of phase coalescence during processing. In this regard, stabilizing the morphology of an immiscible blend via incorporating solid particles can be considered as a cost effective alternative to conventional compatibilization techniques such as the addition of chemically synthesized compatibilizers. Early studies on morphology stabilization using solid particles date back to the investigations of Ramsden [32] and Pickering [29] in water/oil emulsion systems. Since then the underlying mechanisms and various parameters controlling the phenomenon have been widely investigated in several studies [2,5,14,15,20,35,46]. The principle of phase stabilization in these emulsions is that the solid particles are irreversibly adsorbed at the drop interfaces due to

∗

Corresponding author. Tel.: +1 514 340 4711x4924. E-mail address: [email protected] (P.J. Carreau).

partial affinity with the liquid phases and form a barrier, resisting coalescence. The steric repulsion forces between the particles at the interface and the energy required for the displacement of these particles in the thinning film during coalescence are believed to be the main factors contributing in coalescence suppression [14,20]. In some cases, the particles on the approaching droplets can be simultaneously adsorbed at the surfaces of the drops, a phenomenon commonly known as particle bridging [13,39,44]. The monolayer of particles forms a bridge between the droplets and, thus, hinders coalescence. Such a particle bridging could result in the formation of clusters of the dispersed droplets inside the matrix, rendering a highly interconnected morphology with striking rheological properties [18,26]. The other aspect that should be considered in describing the stabilizing effect of particles in emulsion systems is the influence of solid inclusions on the interfacial rheology [7,45]. The principle is that the diminished mobility of the particle loaded interface can slow down the film drainage processes and contribute in reducing the coalescence rate. Coalescence inhibition is not, however, the only consequence of adding solid inclusions to a binary emulsion. Cases have been reported in which incorporating of solid particles has favored

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Please cite this article as: A. Maani, P.J. Carreau, Rheological and morphological properties of thermoplastic olefin blends containing nanosilica, Journal of Non-Newtonian Fluid Mechanics (2016), http://dx.doi.org/10.1016/j.jnnfm.2016.01.017

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the morphology coarsening. The phenomenon occurs via a so called bridging-dewetting mechanism and has been observed for low viscosity mixtures [31,53] as well as polymer blends [26,43]. Generally, the particles in these systems have more affinity with the dispersed phase and can be absorbed simultaneously at the surfaces of droplets predisposing the particle-free regions to coalescence. The other morphological consequence of particle incorporation, observed in highly filled or high viscosity systems, is interfacial-jamming; the phenomenon occurs when the reduced mobility of particle loaded interface balances the interfacial driven forces resulting in non-spherical droplets with long relaxation times [28,40]. In co-continuous emulsions, the decreased interfacial motility due to particle jamming can substantially diminish the phase coarsening, resulting in very fine “bijel” microstructures [6,21]. The concept of introducing solid fillers in polymer blends has been employed for years and several investigations have been devoted to optimize the mechanical and conductive properties of polymer blend composites [11,17,22,37,42,48,50]. However, the evolution of the rheological and morphological behaviors of these systems has been poorly understood and it is intensely investigated in current researches. Many important aspects such as the underlying mechanisms for morphology stabilization as well as droplet clustering are still discussed in the literature. While in many studies, the delayed coalescence of filled polymer blends has been attributed to the reduced interfacial tension [8,33,47], it was shown by Vandebril et al. [45] that the effect is mainly controlled by interfacial rheology modifications. The recent study of Zou et al. [54] revealed that, despite the general belief, the possibility of cluster formation in polymer blend nanocomposites is not always limited to the cases where the preferentially wetting phase is the matrix polymer; the phenomenon can occur regardless of the affinity of particles with the polymer phases. It is worth mentioning that compared to low molecular weight systems for which thermodynamic equilibrium is readily attained due to low viscosity of the liquid phases and Brownian motion, the microstructure formation in polymer blend composites is to a large extent controlled by the viscoelastic hydrodynamic forces and processing conditions. The latter parameters are particularly important in morphology development of thermoplastic olefin blend nanocomposites for which the surface tensions of the polymer components are quite similar and, hence, the thermodynamic driven forces are less important. Many attempts have been devoted to understand the microstructure evolution of thermoplastic olefin blends nanocomposites and the interrelations between the state of dispersion of nanoparticles, size distribution of the dispersed polymeric phase and the macroscopic properties [1,3,16,19,38,41]. The rheological characterization of these systems is often problematic since the microstructural information is reflected at very low frequencies where the non-Newtonian effects of the high molecular weight constituent polymers are minimized. The high melting temperature of these polymers and morphological instability of the nanocomposites limit the experimental window of the low frequency dynamic. In our previous works, we investigated the morphology evolution and reactive control of the coalescence phenomenon in thermoplastic olefin blends of polypropylene and methallocene catalyzed ethylene octene copolymers [23,24]. In the present study, we aim at understanding the microstructure development of these blends when different amounts of nanosilica particles are added. It will be shown that the particle distribution in these systems is not only function of thermodynamic affinities, but is also controlled, to a large extent, by the initial localization of the nanoparticles and their concentrations. The microstructural variations in different blend composites will be also investigated in the light of rheological properties.

2. Materials and experimental methods 2.1. Materials and sample preparations The thermoplastic olefin blends of this study were based on a commercial polypropylene, PP, (Pro-fax 6523 from Basell Inc.) as the matrix and a metallocene-catalyzed ethylene octene copolymer (EC) (Engaged 8400 provided by Dow Chemical) as the elastomeric dispersed phase. The polypropylene used was a linear homopolymer and the elastomer was a short chain branched copolymer consisting of 40 wt% randomly distributed octene comonomer. A surface treated fumed silica AEROSIL R805 (Si) was used as the solid inclusion of the composites. As reported by Evonik Industries Incorporation the surface of these silica particles contains grafted hydrophobic octylsilane groups that could promote the dispersion of the particles in non-polar systems. The specific surface area of the clusters of the intact particles is between 125 and175 m2 /g and the size of the individual particles is about 10 nm. Blends with relative concentrations of 75/25 and with silica concentrations of 0, 2 and 6 wt% (based on the total mass of the components) were investigated. We recall that in the nomenclatures further used in this work, the ratios represent the relative concentrations of polymer phases without considering the mass of the solid inclusions. In order to examine the influence of selective localization of the solid inclusions on the morphological and rheological properties of the composites, the silica particles were separately added to either the PP or EC polymers and the prepared premixes were then used to prepare the blend composites with the desired concentrations mentioned above. To promote the compatibility of silica particles with the corresponding polymeric phase, maleic anhydride grafted species of PP and EC were also incorporated to the premixes. The maleic anhydride grafted PP (PPMA) was Orevac 18729 (provided by Arkema), added in 5 wt% (based on the mass of the PP phase only) and the maleic anhydride grafted EC (ECMA) was Exxelor MDEX 95-2 (from Exonmobil Chemical, Corporation), was added in 10 wt% (based on the mass of the EC phase only). In the nomenclature (PP-PPMA-Six)/EC(75/25) means a blend of 75 wt% PP-PPMA and 25 wt% EC containing x wt% of nanosilica based on the total weight of the blend and for which the nanoparticles have been initially added to the PP-PPMA premixes; PP/(EC-ECMA-Six)(75/25) refers to a blend of 75 wt% PP and 25 wt% EC-ECMA containing x wt% of nanosilica based on the total weight of the blend and for which the nanoparticles have been initially added to the EC-ECMA premixes. The TPO composites were prepared using a Brabender internal mixer (DDRV501, C.W. Brabender Instruments Inc., USA) at a set temperature of 210 °C, with a rotor speed of 100 rpm and for a mixing time of 12 min under the purge of nitrogen. To minimize the possible degradation of the polymer phases, the premixes were prepared at relatively lower temperatures (120 °C and 190 °C for the EC-ECMA-Si and PP-PPMA-Si premixes, respectively). After mixing, the samples were molded into disks of 25 mm using a compression hydraulic press at 180 °C and at a maximum pressure of 3 tons. 2.2. Rheological measurements All the rheological measurements were performed using a MCR301 (Anton Paar, Austria) rotational Rheometer equipped with a convection oven and parallel disks geometry with a diameter of 25 mm and a gap of 1 mm at 200 °C and under a nitrogen blanket. Small amplitude oscillatory shear (SAOS) tests were conducted using a maximum strain amplitude of 1.5%, which was found to be in the linear viscoelastic regime. In order to verify the stability of the rheological properties during the frequency sweep tests, measurements were performed going from low to high frequencies (upward ramp from 0.01 rad/s to 100 rad/s) and from high to

Please cite this article as: A. Maani, P.J. Carreau, Rheological and morphological properties of thermoplastic olefin blends containing nanosilica, Journal of Non-Newtonian Fluid Mechanics (2016), http://dx.doi.org/10.1016/j.jnnfm.2016.01.017

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Fig. 1. Variation of the loss tangent of (PP-PPMA-Si6)/EC(75/25) with frequency as measured by SAOS (upward and downward frequency ramps) and by combining creep data with the high frequency portion of the oscillatory shear measurements.

low frequencies (downward ramp from 100 rad/s to 0.01 rad/s). It was found that the dynamic properties of the high concentrated nanocomposites evaluated by the two different ramps are remarkably different. An example of such a discrepancy is shown in Fig. 1 in which the damping behavior of PP-Si6/EC(75/25) is illustrated; as shown, at high frequencies, the values of loss tangent for the upward and downward ramps are relatively the same whereas the low frequency data show different trends. While the loss tangent for the upward ramp is initially constant (∼1.0) before rising with frequency at low frequencies, the results of the downward data show a continuous and steeper increase of tan (δ ) with ω. In the frequency range of 0.01 and 0.05 rad/s the storage modulus, Gʹ, correlates with ω0.56 and ω0.28 for the upward and downward ramps, respectively. This is clear evidence of thixotropic effects for this nanocomposite and, hence, quantification of the dynamic properties by SAOS experiments is unreliable. Such effects for concentrated nanosilica in polyolefins have been reported by Bailly and Kontopoulou [4] and attributed to aggregation/flocculation of the nanoparticles. In order to make a true evaluation of the viscoelastic properties at low frequencies, where important structural information is reflected, the samples were also characterized via creep measurements. We employed the incomplete creep/recovery technique, which was first proposed by Meissner [25] and was further developed and implemented by He et al. [12] and Shaayegan et al. [36] to obtain extended dynamic data down to frequencies as low as 10−5 rad/s. For this purpose, the samples were firstly subjected to different creep measurements with different stress magnitude so as to determine an appropriate stress level that keeps the deformation in the linear viscoelastic limit. After shearing for t1 = 200 s, the stress was set to zero and the recovery behavior of samples was monitored. For the times greater than t1 , the values of creep compliance J(t) were calculated by means of the strain values recorded during the recovery, γ (t), and via the following equation [12]:

J (t ) = γ (t )/σ0 + J (t − t1 ) ,

t ≥ t1

(1)

in which σ 0 is the shear stress applied during the creep measurement. The above equation can be used to extend the creep curve up to t = 2t1 . The procedure can, then, be repeated using the remaining portion of the recovery curve to, eventually, extend the creep compliance curve up to t = tr + t1 in which tr is the duration of the recovery period. In order to verify the stability of the samples during the time frame of the incomplete creep/recovery measurements, a series of experiments were performed for different time delays before applying the shear stress. This method revealed

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that the creep/recovery data of the concentrated composites (with 6 wt% silica concentration) were reproducible, at least, after applying a 400 s delay prior to creep measurements. This confirms the validity of the creep data obtained during this time frame (without delay) and we assume that the technique was adequate to determine the dynamic viscoelastic data down to 0.01 rad/s. In the last step, a generalized Maxwell model was employed in order to transform the transient creep information to the dynamic viscoelastic data. The parameters of the Maxwell model, i.e. the relaxation times and the relaxation moduli of the Maxwell elements were determined by fitting the extended creep compliance data as well as the experimental Gʹ and Gʹʹ values over the frequency range of 0.1–100 (rad/s) for which the sample stability was not an issue. The determined parameters were then employed to establish the extended frequency sweep curve over the frequency range of 0.01– 100 (rad/s). The validity of this approach is discussed further in the text and in Section 3.2. To better understand the role of nanoparticles on the morphological stability of the polymer blend nanocomposites, samples were also subjected to a low shear rate of 0.01 s−1 for 1 h. The linear viscoelastic properties of the sheared samples were, then, determined by means of incomplete creep/recovery and oscillatory shear tests. 2.3. Morphological analysis Two different imaging techniques were employed to examine the microstructure of the different nanocomposites: transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM imaging was performed on ultrathin sections of nanocomposites prepared using a diamond knife equipped ultramicrotome. The images were obtained via a JEOL transmission electron microscope (JEM 2100F, Japan) operating at a 200 kV accelerating voltage. Samples for SEM characterization were firstly planed using a Leica Jung (RM 2165) microtome equipped with a cryochamber and were coated with a gold-palladium alloy. The surfaces of the microtomed samples were then analyzed using a field-emission gun scanning electron microscope (JEOL 7600 FEG, Japan) equipped with a low angle backscattered electron detector and operated with an emission condition of 2 kV and 12 mA. It is worth mentioning that considering the non-uniform shear flow in the parallel plate geometry, the morphology of all the samples was examined at half of the disk radius so as to keep the consistency and to avoid the possible edge effects during shearing. The microscopic images of the TPOs were then analyzed by employing a semi-automatic method and by using a digitalizing device operated with SigmaScan Pro© software. The volume average droplet diameter was obtained by analyzing at least 300 droplets for each blend and the Schwartz–Saltykov [34] correction was applied to account for the fact that the observation plane might not be at the equator of the droplets. 3. Results 3.1. Morphology We present first the morphology of blend composites for which the silica was added to the dispersed phase. Fig. 2 compares the SEM micrographs of PP/(EC-ECMA-Si) composites to that of the unfilled blend before shearing. As shown addition of nanosilica results in a relatively finer morphology of the EC phase so that the averaged droplet size varies from 3.6 μm in the unloaded blend to 2.3 μm in PP/(EC-ECMA-Si2)(75/25) and to 1.3 μm to PP/(EC-ECMA-Si6)(75/25). The corresponding TEM images of the composites are shown in Fig. 3. As illustrated in Fig. 3a, the silica particles in the PP/(EC-ECMA-Si2)(75/25) sample remained mainly

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Fig. 2. SEM micrographs of the PP/(EC-ECMA-Si)(75/25) blend and composites: (a) PP/(EC-ECMA)(75/25), (b) PP/(EC-ECMA-Si2)(75/25), and (c) PP/(EC-ECMA-Si6)(75/25).

Fig. 3. TEM images of PP/(EC-ECMA-Si)(75/25) composites: (a) PP/(EC-ECMA-Si2)(75/25) and (b) PP/(EC-ECMA-Si6)(75/25).

in the dispersed EC phase; moreover, Fig. 3b clearly indicates that the particles in the concentrated composite migrated towards the droplet/matrix interfaces. The morphology of the sheared PP/(EC-ECMA-Si) composites is presented in Fig. 4. Compared to the non-sheared sample, the sheared PP/(EC-ECMA)(75/25) blend has undergone a significant droplet coalescence so that the EC phase size has enlarged to about 14.3 μm. The shear induced coalescence is found to be considerably diminished in the composite containing 2 wt% nanosilica for which the final EC phase size reaches about 4.8 μm. The finest morphology among the sheared samples belongs, how-

ever, to the concentrated composite, PP/(EC-ECMA-Si6)(75/25) with dv = 3.3 μm; the extent of coalescence in this system is limited by the formation of interconnected clusters of the EC droplets. We now compare in Fig. 5 the morphology of non-sheared (PPPPMA-Si)/EC blends with different levels of Si loading. As illustrated, the morphology of the EC phase in these blends varies slightly and the droplet size changes from 4.3 μm in the unloaded (PP-PPMA)/EC blend to about 3 μm in (PP-PPMA-Si2)/EC and (PP-PPMA-Si6)/EC. The corresponding TEM images of the (PPPPMA-Si)/EC composites are shown in Fig. 6. The silica particles in

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Fig. 4. Morphology of the PP/(EC-ECMA-Si)(75/25) blend and composites after 60 min shearing at 0.01 s−1 : (a) PP/(EC-ECMA)(75/25), (b) PP/(EC-ECMA-Si2)(75/25) and (c) PP/(EC-ECMA-Si6)(75/25).

Fig. 5. SEM micrographs of (PP-PPMA-Si)/EC(75/25) blend and composites: (a) (PP-PPMA)/EC(75/25), (b) (PP-PPMA-Si2)/EC(75/25), and (c) (PP-PPMA-Si6)/EC(75/25).

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Fig. 6. TEM images of the (PP-PPMA-Si)/EC(75/25) composites: (a) (PP-PPMA-Si2)/EC(75/25) and (b) (PP-PPMA-Si6)/EC(75/25).

Fig. 7. Morphology of the (PP-PPMA-Si)/EC(75/25) blend and composites after 60 min. shearing at 0.01 s−1 : (a) (PP-PPMA)/EC(75/25), (b) (PP-PPMA-Si2)/EC(75/25) and (c) (PP-PPMA-Si6)/EC(75/25).

these composites are mainly distributed over the PP matrix and no clear indication of migration of the particles towards the interface or diffusion into the EC phase can be observed in these samples. The sheared (PP-PPMA-Si)/EC composites, also, feature relatively fine morphology compared to the blend with no silica inclusion (Fig. 7). While shearing of the unloaded blend results in the formation of EC domains with an averaged droplet size of about 14.6 μm, the final droplet size in the (PP-PPMA-Si2)/EC and (PPPPMA-Si6)/EC composites reaches values of 11.3 and 7.5 μm, respectively. It is worth mentioning that shearing can also favor the collision of silica particles and result in the formation of larger clusters of the nanoparticles in the system; such an effect can be more clearly examined in Fig. 8 where high magnification TEM

images of the (PP-PPMA-Si6)/EC samples (taken before and after shearing) are illustrated. While in the non-sheared blend, the silica aggregates are mainly isolated, the silica particles in the sheared samples form extended clusters that are closely interconnected. 3.2. Rheology As discussed in Section 2.2, the low frequency viscoelastic properties of the TPOs were determined by employing the incomplete creep/recovery technique. The importance of applying this approach for characterizing polymer blend nanocomposites can be better realized by comparing the viscoelastic data of (PP-PPMA-Si6)/EC evaluated using the indirect method (combining the creep and oscillatory viscoelastic data) with the data obtained

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Fig. 8. TEM images of silica particles in (PP-PPMA-Si6)/EC(75/25): (a) before shearing and (b) after 60 min shearing at 0.01 s−1 .

Fig. 9. Variations of the dynamic moduli as functions of the frequency described by different models along with the experimental data of the unfilled (PPPPMA)/EC(75/25) blend.

using (upward or downward) frequency sweep test. As shown in Fig. 1, while the indirect method gives two maxima in the damping behavior of this composite, the direct SAOS data feature a single maximum at a frequency of about 0.6 rad/s and the difference in the absolute values measured by different techniques can be as high as 40%. The validity of the indirect approach was verified by using the dynamic frequency data of the unfilled (PP-PPMA)/EC(75/25) blend. Fig. 9 compares the experimental data of the oscillatory shear measurement with the predictions of the generalized Maxwell model consisting of 15 elements that were determined by combining incomplete creep/recover data and a set of oscillatory shear data (measured at frequencies above 0.1 rad/s). Fig. 9 also presents the results of the fitting of the simplified version of the Palierne model [10] using the viscoelastic properties of the neat components of the (PP-PPMA)/EC(75/25) blend. As illustrated in this figure, the generalized Maxwell model provides a fairly good description of the low frequency shoulder in the G’ data, which is a characteristic of the interfacial relaxation in polymer blends at low frequencies and can be correlated to the interfacial tension and the averaged droplet size using the Palierne model (in this case the interfacial tension was estimated to be 0.7 ± 0.2 mN/m). The predictions of the low frequency plateau using the generalized Maxwell model reveal that the model

is sensitive to the relaxation mechanisms associated with the microstructure of the system. The dynamic moduli of the other unloaded blend, PP/(ECECMA)(75/25), and the TPO nanocomposites containing 2 wt% silica particles are compared in Fig. 10 with the predictions of the Palierne model. It should be noted that for the polymer blend nanocomposites the corresponding premix was considered as one phase and the viscoelastic properties of this premix were employed as the input for the Palierne model along with the viscoelastic data of the particle-free polymer component of the blend. The viscoelastic behavior of the composites is quite similar to that of the unfilled blends; the predictions of the Palierne model also suggest that the addition of 2 wt% nanosilica does not significantly change the interfacial tension value (0.6 ± 0.1 mN/m in the nanocomposite compared to 0.7 ± 0.2 mN/m in the unfilled system). The concentrated TPO nanocomposites, however, featured considerably enhanced viscoelastic properties as illustrated in Fig. 11, which presents the variations of the storage modulus and complex viscosity of the composites containing 6 wt% silica particles; the viscoelastic properties of the unloaded (PP-PPMA)/EC(75/25) blend is also shown for comparison. The concentrated nanocomposites exhibit a significant shear-thinning behavior for the complex viscosity, whereas the unloaded blend tends to show a plateau at low frequencies. Note that the storage modulus for PP/(EC-ECMASi6)(75/25) at the lowest frequency (0.01 rad/s) is about twice larger than that for (PP-PPMA-Si6)/EC and more than 35 times larger than that for the unfilled TPO. Fig. 12 presents the influence of the steady pre-shearing on the linear viscoelastic properties of the (PP-PPMA)/EC(75/25) blend and of the high concentrated nanocomposites. For the unloaded TPO, shearing diminishes the level of the low frequency shoulder of the storage modulus and correspondingly decreases the complex viscosity, notably in the first frequency decade reflecting changes in the morphology of the dispersed EC phase. Notwithstanding the behavior of the unfilled blend, shearing resulted in an enhanced level of viscoelastic properties in the concentrated nanocomposites. As shown in Fig. 12b and c, the storage modulus of the sheared nanocomposites increases considerably at low frequencies and the absence of a plateau for η∗ at low frequencies in the sheared composites is more representative of materials with integrated elastic networks. The influence of shearing on the viscoelastic properties of the low concentrated composites was, however, found to be insignificant (results are not shown).

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Fig. 10. Comparison of the dynamic moduli as functions of the frequency (a) for the unloaded PP/(EC-ECMA)(75/25) blend, (b) for PP/(EC-ECMA-Si2)(75/25) and (c) PPPPMA-Si2/EC(75/25) with the predictions of the Palierne model.

Fig. 11. Variations of the storage modulus and complex viscosity with frequency for the unloaded (PP-PPMA)/EC(75/25) blend and the composites containing 6 wt% of nanosilica.

4. Discussion The morphological and rheological data presented in the previous section showed that the concentration and the localization of the nanoparticles with respect to the polymer phases are both important factors in controlling the properties of polymer blend nanocomposites. Although the morphological stability of all the blends were found to be improved by increasing the concentration of the silica particles, the effect was found to be significantly

more pronounced in the PP/(EC-ECMA-Si) composites for which the nanoparticles were initially added to the dispersed phase. Assuming that the primary cause of coalescence inhibition in the composites is the reduced interfacial mobility in the presence of nanoparticles, the morphological results suggest that incorporating the solid inclusion in the dispersed phase is more efficient in immobilizing the interface and slowing down film drainage. This issue can be better understood by examining the viscoelastic properties of the different premixes used for preparing the final blend composites. The variations of the complex viscosity of the premixes as functions of frequency are presented in Fig. 13. As shown in Fig. 13a, the addition of the silica particles results in significant increases of η∗ for the (EC-ECMA-Si) premixes, while the unloaded (EC-ECMA) mixture features a Newtonian low frequency plateau of about 400 Pa s, the (EC-ECMA-Si8.2) and (EC-ECMA-Si25) samples exhibit a shear-thinning behavior with η∗ values, respectively, of 2700 and 830 kPa at 0.01 rad/s. Note that the concentrations of silica in the premixes of EC-ECMA and PP-PPMA were 4.1 and 1.4 times, respectively those of the final composites. As shown in Fig. 13b, the differences in η∗ for the (PP-PPMA) blend and the (PP-PPMASi2.7) composite are quite insignificant; η∗ of the (PP-PPMA-Si8.5) blend, however, is comparable to that observed for the (EC-ECMASi8.2) premix (Fig. 13a), but considerably less than for the (ECECMA-Si25) premix. This can partly explain why the efficiency of the nanoparticles in suppressing drop coalescence in the high concentrated (PP-PPMA-Si6)/EC(75/25) composite is less than the low concentrated PP/(EC-ECMA-Si2)(75/25) sample (we recall that the final droplet size after shearing in (PP-PPMA-Si6)/EC(75/25) was about 7.5 μm compared to 4.8 μm in PP/(EC-ECMA-Si2)(75/25). In

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Fig. 12. Influence of 60 min shearing at γ˙ = 0.01 s−1 on the storage modulus and complex viscosity (a) of the unloaded (PP-PPMA)/EC(75/25) blend, (b) (PP-PPMASi6)/EC(75/25) and (c) PP/(EC-ECMA-Si6).

Fig. 13. Variations of the complex viscosity with frequency for the constituting premixes containing different levels of silica particles: (a) EC-ECMA-Si and (b) PP-PPMA-Si.

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this regard, the scaling model of film drainage [49] can be also employed to roughly explain the influence of the enhanced viscosity of the phases on coalescence inhibition. According to this model, the required time (td ) to attain the critical film thickness (hc ) below which the colliding particles coalesce is estimated as:

td ∼

λCa3/2  f (λ )α 1/2 (R/hc ) γ˙

(2)

in which λ is the viscosity ratio of the dispersed phase over that of the matrix, (ηd /ηm ), γ˙ is the imposed shear rate, α is the interfacial tension, R is the radius of the colliding drops, f(λ)is a very weak function of viscosity ratio (f(λ) ∼ λ0.11 ) and Ca is the capillary number defined as ηm γ˙ R/α . Based on Eq. (2), the drainage time scales approximately with the first power of viscosity ratio; however, experimental data [51] suggest a scaling relation of td ∼ λ0.8 ; and considering the contribution of the matrix viscosity to the capillary number, the variation of the drainage time with ηm should also follow approximately the same scaling relation 0.7 ). It can be now deduced that increasing the viscosity of (td ∼ ηm the neat PP phase by a factor of ∼3.1 (as in the case of (PP-PPMASi6)/EC(75/25) and increasing the viscosity of the neat EC phase by a factor of ∼6.6 (as in the case of PP/(EC-ECMA-Si2)(75/25) composite) can slow down the drainage process by a factor of ∼2.2 and ∼4.5, respectively (note that for these estimations, the complex viscosities at 0.01 rad/s were used as the reference values). Among all the nanocomposites, PP/(EC-ECMA-Si6)(75/25) exhibited the finest and more stable morphology. The fact that in this sample the interfacial area contains a large amount of migrated silica particles should be considered as an important factor that controls the morphological behavior of the EC phase. The presence of a concentrated layer of silica particles at the interface can result in a strong steric interparticle barrier, largely reduce the interfacial mobility, slow down the film drainage rate and, hence, suppress coalescence. The question that needs to be addressed, however, is that why the particles migrated only in the PP/(EC-ECMA-Si6) composite where highly concentrated nanoparticles are located in the dispersed elastomeric phase. To answer this question the diffusivity of the nanoparticles and the thermodynamic affinity of the silica particles with respect to the PP and EC phases should be considered. The wetting coefficient, ω12 , can be used to determine the preferred localization of solid inclusions in a binary emulsion. It is expressed as [52]:

ω12 =

αs−1 − αs−2 α1−2

(3)

where αs−1 , αs−2 are the interfacial energy per unit area between the solid phase and the liquid phases 1 and 2, respectively, and α1−2 is the interfacial tension between the two fluids. The interfacial energies can be determined using the Owens–Wendt equation [27]. For ω12 values larger than 1 the solid particles are expected to remain in the fluid 1 and for values smaller than −1 in fluid 2; the particles are expected to be stabilized at the interface if the absolute value of the wetting coefficient remains less than unity. As a result of experimental limitations, a common practice in the related studies is that surface tension data of polymers at room temperature are extrapolated to the temperature of interest to estimate the wetting coefficient [9,30,54]. This approach, however, was found to be impractical in determining a valid wetting coefficient for the systems of this study. The problem arises from the fact that the interfacial tension between the PP and EC phase is quite low (∼0.7 mN m as determined by the Palierne model) and the approximations associated with the extrapolation of the surface tension data and with the estimation of the interfacial energies are too rough to obtain a meaningful wetting coefficient. Consequently, the prediction of the localization of the nanosilica particles remains

Fig. 14. TEM image of a high concentrated nanocomposite, (PP-PPMASi6/EC)(25/75), consisting of the EC matrix and nanosilica initially loaded in the PP-PPMA premix.

obscured. The thermodynamic behavior of the nanoparticles with respect to the two polymeric phases, however, can be experimentally analyzed by investigating the migration behavior of the silica particles in the inversed system where the polypropylene forms the dispersed phase and the EC phase is the continuous matrix. Fig. 14 presents a TEM image of the (PP-PPMA-Si6)/EC(25/75) composite for which the silica particles were firstly premixed with the polypropylene phase. As shown the nanoparticles in this composite behave similarly as for PP/(EC-ECMA-Si6)(75/25) (see Fig. 3b) and form a distinct layer at the interface of the PP droplets. These observations indicate that if in these highly concentrated systems the nanosilica particles are initially localized in the dispersed phase the particles migrate towards the interface whether the dispersed phase is PP or EC polymer. We recall that in the TEM images of the low concentrated systems no clear evidence of migration was observed. This fact is also supported by the SAOS data from which no significant change was noted in the interfacial tension data obtained via the Palierne analysis. We also recall that the initial concentration of nanoparticles in the PP matrix of (PP-PPMASi6)/EC(75/25) was 7.8 wt%, which is considerably less than that of the EC phase of PP/(EC-ECMA-Si6)(75/25) composite for which in the premix the silica concentration was 20.3 wt%, favoring migration of the nanoparticles to the interface. Generally, the diffusion of a particle in a fluid is inversely related to the fluid viscosity; the η∗ values of the (PP-PPMA) and (EC-ECMA) phases at 50 rad/s were measured to be 620 and 270 Pa s, respectively (assuming that the Cox–Mertz rule is valid, η∗ at this frequency represents the shear viscosity at 50 s−1 , which is the effective shear rate applied to the composite during mixing in the internal mixer). To investigate the possible effect of the viscosity on the migration of nanoparticles, a high melt flow index polypropylene (profax-PD702 from Basell, Inc.) with a complex viscosity of 320 Pa s at 50 rad/s was used to substitute the polypropylene phase of the (PP-PPMA-Si6)/EC(75/25) blend. Similarly to the pattern observed in the composite with the high viscosity PP, the silica particles in the new sample remained distributed all over the continuous matrix phase and no

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Fig. 15. SEM micrographs of the high concentrated PP/(EC-ECMA-Si6)(75/25) composite: (a) before shearing and (b) after 60 min shearing at γ˙ = 0.01 s−1 .

nanoparticles and droplets with silica particles at their surface. This phenomenon is more evident in the sheared PP/(EC-ECMASi6)(75/25) sample (Fig. 15b) where the droplets containing silica particles are in close contact and form an extensive network all through the continuous matrix (the corresponding TEM image of this sample is shown in Fig. 16). This explains why shearing in this sample resulted in enhanced viscoelastic properties (Fig. 12b). The fact that pre-shearing also enhanced the viscoelasticity of (PP-PPMA-Si6)/EC(75/25) can be also explained by the stronger network of the particles that are brought in contact via low shearing as seen in the TEM image of Fig. 8. 5. Concluding remarks

Fig. 16. TEM image of the high concentrated PP/(EC-ECMA-Si6)(75/25) composite after 60 min shearing at γ˙ = 0.01 s−1 .

clear indication of migration was detected (results not shown). Moreover, the migration of particles in the viscous polypropylene phase when the latter forms the dispersed phase, as evidenced by Fig. 14, is not significantly affected by the viscosity of the carrier polymer. However, further investigations are still needed to clarify the migration phenomenon in concentrated samples and its dependency to the initial localization of the particles. The fact that the concentrated PP/(EC-ECMA-Si6)(75/25) composite exhibits enhanced viscoelastic properties compared to (PP-PPMA-Si6)/EC(75/25) can be also explained in the light of morphology of the silica particles in these composites. Considering that the storage modulus values for both composites are significantly larger than that of the unloaded blend (Fig. 11), it can be deduced that the viscoelastic properties in these systems are mainly controlled by the interactions between the high concentrated silica particles. The enhanced viscoelasticity of PP/(EC-ECMA-Si6)(75/25), however, suggests that the silica particles in this system form a stronger network than in (PP-PPMA-Si6)/EC(75/25). Fig. 15a presents the microstructure of droplets of the EC phase in PP/(ECECMA-Si6)(75/25). This image suggests a double percolation of

We have investigated the rheological and morphological properties of polypropylene (PP) and ethylene octene copolymer (EC) blends containing nanosilica. The dynamic viscoelastic characterization of the high concentrated composites revealed that the low frequency behavior of these samples was highly time-sensitive and could not be correctly evaluated using oscillatory shear measurements. An indirect analysis based on transient creep measurements was then employed to extend the frequency sweep data to low values. All the composites with a large content of nanosilica exhibited a gel-like behavior at low frequencies confirming the presence of a network of solid particles; the concentrated nanocomposite in which the particles were initially localized in the dispersed phase presented an enhanced level of viscoelasticity. Morphological data revealed that the addition of nanoparticles to the dispersed polymer component was a more efficient method to control coalescence during shearing. This was attributed to the large viscosity ratio of the blend components and, consequently, the decelerated film drainage during collision of the dispersed droplets. The morphology of the blend of high concentration of nanoparticles in the dispersed EC phase, however, showed the formation of interfacial layers of solid particles that could further immobilize the interface and act as a barrier against droplet coalescence. No clear indication of migration was observed in the concentrated composites where the nanoparticles were initially localized in the PP matrix. Moreover, a TEM image of a high concentrated composite in which the polypropylene constituted the dispersed phase confirmed the formation of interfacial silica layers at the surface of the PP droplets. The observations of this study highlight the importance of the localization of the solid inclusions in the rheological/morphological behavior of polymer blends filled with nanoparticles.

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