Immiscible polymer blends stabilized with nano-silica particles: Rheology and effective interfacial tension L. Elias a , F. Fenouillot b , J.C. Majeste ´ c, * , P. Alcouffe a, b , P. Cassagnau a a Universite´ de Lyon, Lyon F-69003, France, Universite´ de Lyon 1, Lyon F-69003, France, CNRS UMR5223, Inge´nierie des Mate ´riaux Polyme `res, Laboratoire des Mate´riaux Polyme `res et Biomate´riaux, F69622 Villeurbanne, France b Inge´nierie des Mate ´riaux Polyme`res–Laboratoire des Mate´riaux Macromole ´culaires (IMP/LMM), UMR-CNRS 5223, INSA-Lyon, 17 Avenue Jean Capelle, 69621 Villeurbanne cedex, France c Inge´nierie des Mate ´riaux Polyme `res–Laboratoire de Rhe ´ologie des Matie`res Plastiques (IMP/LRMP), UMR-CNRS 5223, Universite´ Jean Monnet, 23 rue du docteur Paul Michelon, 42023 Saint Etienne cedex 2, France article info Article history: Received 7 April 2008 Received in revised form 2 July 2008 Accepted 8 July 2008 Available online 19 July 2008 Keywords: Silica Immiscible blend Palierne model abstract The effect of inorganic silica nanoparticles on the morphology and viscoelastic properties of poly- propylene/poly(ethylene-co-vinyl acetate) (PP/EVA) immiscible blends has been investigated. Different EVA with different molar masses were used in this study. Virgin and filled blends were prepared by using a twin-screw mini-extruder. Microscopy studies revealed a significant change in morphology of PP/EVA blend in the presence of silica nanoparticles. Also, the images proved the migration of silica nanoparticles toward EVA phase, and to the interface. The quantitative analysis of the rheological experimental data was based on the framework of the Pal- ierne model, extended to filled immiscible blends. Based on this model, a method of determination of the interfacial tension from rheological measurement has been addressed with an improved accuracy. Even if silica mainly plays a thermodynamic role in the stabilization mechanism by decreasing the effective interfacial tension, the influence of particular rheological conditions and the structure of the phases must be taken into account for a complete understanding of the final morphology of such systems. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The stabilization of emulsions by incorporating colloidal parti- cles is known since one century with the pioneer works of Ramsden [1] and Pickering [2]. Few years ago, the research group of Binks has intensively studied the stabilization and phase inversion in emul- sion by nano-silica particles. For instance, their researches have been focused on the use of colloid silica particles to stabilize different oil–water emulsions [3,4,5]. As discussed by Vignati and Piazza [6] the most probable mechanism of morphology stabiliza- tion is a steric hindrance or surface rheology effects due to the particle adsorption at the interface rather than a decrease of the interfacial tension between the two liquids. Regarding high viscous emulsions such as immiscible polymer blends, the effect of colloidal particles on the morphology development has only been recently investigated but is currently the topic of intense investi- gations. Several researchers have reported both experimental results and theoretical predictions that the addition of nanoscale fillers affects the dynamic phase behavior and morphology of blends. From experimental observations, the incorporation of a few percent of nano-filler during melt processing of the components causes a substantial reduction of the size of the dispersed phase. Carbon black [7–11], organoclay [12–19] and silica [20–24] particles have been used in these studies. However, the mechanisms by which particles stabilize against coalescence are not completely understood yet. Most of the authors concluded that the fillers act as physical barrier due to their accumulation at the interface, which prevent the coalescence of the dispersed phase. More specifically, Thareja and Velankar [22] proposed a ‘‘particle bridging’’ mecha- nism based on the observation of a gel behavior in the rheology at low frequencies. This gel-like behavior was attributed to the formation of a particle network that bridges the droplets. Such mechanism is well known in conductive composite polymers [8] and is generally called double percolation. Consequently, this mechanism depends on filler concentration and on volume fraction of the dispersed phase. Actually, although this mechanism cannot be totally excluded, its influence on droplet stabilization is not generally the dominant mechanism. For instance, Elias et al. [23] and Vermant et al. [21] observed no upturn in the absolute complex viscosity in the domain of the accessible frequency range. More theoretically, Nesterov and Lipatov [25] and Lipatov et al. [26] studied the influence of fumed silica particles on the phase * Corresponding author. Tel.: þ33 4 77 48 15 47; fax: þ33 4 77 48 51 46. E-mail address: majeste@univ-st-etienne.fr (J.C. Majeste ´). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.07.018 Polymer 49 (2008) 4378–4385