Immiscible polymer blends stabilized with nano-silica particles: Rheology and effective interfacial tension
Graphical abstract
Introduction
The stabilization of emulsions by incorporating colloidal particles 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 emulsion 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 stabilization 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 investigations. 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], [8], [9], [10], [11], organoclay [12], [13], [14], [15], [16], [17], [18], [19] and silica [20], [21], [22], [23], [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” mechanism 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 diagram behavior of polymer blends with lower critical solution temperature (LCST). They came to the conclusion that the total free energy of a blend system should also include the interaction parameters between the polymers and the inorganic filler surface. In other words, addition of a filler S to A–B blend stabilizes the morphology. Actually, the solid particles act as a compatibilizer by adsorbing A and/or B polymers on their surface. To play this role the inorganic phase should have the largest possible surface area and should be able to disperse very well in the two phases. From this statement, Si et al. [16] proposed to use organoclay rather silica particles to study this effect in more detail. Indeed, in contrast to fumed silica, exfoliated clays are composed of nanoscale tactoids of great surface available for interaction with polymers. Finally they demonstrated that blend compatibility can be improved with dispersion of organoclays by melt mixing. Furthermore, they attributed this compatibility mechanism to the formation of in situ grafts during mixing processing. Even chain grafting is questionable in their work, some authors argued that the key factor for compatibilization efficiency of the organoclay is the initial interlayer spacing and its ability to reduce the interfacial tension and average particle size by chain adsorption. Hong et al. [17], [18] showed that the effect of the organoclay on the reduction in the droplet size is governed by the location of the organoclay which is determined by the difference in affinity of component/clays system. More precisely Ray et al. [12] showed that the compatibilization process was really more efficient when PP is grafted with maleic anhydride that ensures interaction with clay side OH groups.
However, the use of nano-silica particles can be motivated as they exist in a wide range of size (specific area: 50–400 m2/g) and with a variety of surface treatments from hydrophilic to hydrophobic. Furthermore, nano-silica particles are used as filler in various industrial applications to control rheological properties. In our previous papers [23], we investigated the role of silica nanoparticles on the morphology of immiscible PP/PS blend (non-polar polymers). The main objective of this work was to address a quantitative analysis of the rheological experiment based on the framework of the Palierne model. As a result, we concluded that the mechanism of morphology stabilization of PP/PS blend by hydrophilic silica was the reduction in the effective interfacial tension whereas hydrophobic silica particles act as a rigid layer preventing the coalescence of PS droplets. However, the development of Palierne model to filled immiscible blends is not straightforward and requires some strong assumptions on the viscoelastic properties of polymer phases filled with the silica particles. The objective of the present work is to revisit the development of the Palierne model to filled polymeric emulsions. On the other hand, our work is focused on immiscible blend composed of polypropylene (PP) and a copolymer of ethylene and vinyl acetate (EVA) as we look to investigate the effect of the dispersed phase polarity and viscosity.
Section snippets
Materials
PP was supplied by Arkema (PPH 7060) with a melt flow index MFI = 12 g/10 min. The molecular weights are: and . The zero shear viscosity at T = 200 °C is η0 = 2500 Pa s. Two poly(ethylene-co-vinyl acetate) (EVA), kindly supplied by Arkema, of different molar masses have been used (Table 1). The amount of acetate groups contained in these copolymers is 28% by weight. The zero shear viscosities of these EVA are reported in Table 1 at T = 200 °C. Fig. 1 shows the variations of
Blends morphology of PP/EVA/silica nanocomposites
Regarding the effect of silica nanoparticles on the global morphology of PP/EVA blend, it can be observed for all systems (Table 2 and one example in Fig. 2) that the presence of silica particles reduces significantly the coalescence phenomena. More precisely, EVA domain size is decreased by a factor two or four approximately (Table 2). Therefore, silica particles are efficient at producing a relatively uniform distribution of drop sizes and the distribution shifts to smaller diameter. These
Conclusion
The effect of two types of fumed silica (hydrophilic and hydrophobic) on the morphology of PP/EVA80/20 blends was investigated.
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First of all, a significant reduction of the EVA droplets size was observed in the presence of both types of silica. Typically, the volume droplet radius decreases from 2.2 μm to nearly 0.5 μm for filled blends with 3 wt% silica.
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SEM and TEM image analyses proved that the hydrophilic silica tends to confine in the EVA phase whereas hydrophobic one was located close to the
Acknowledgements
The authors would like to thank Wacker company for having supplied silica nanoparticles and for their financial support; especially Dr. Herbert barthel and Dr. Torsten Gottschalk-Gaudig.
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