The objective of the present work is to investigate, from the open literature, the recent developments in the rheology of silica and organoclay nanocomposites. In particular, this paper focuses on the general trends of the linear viscoelastic behaviour of such nanocomposites. Hence, the variations of the equilibrium shear modulus and critical strain (limit of linearity), which depend on power laws of the volume fraction of particles, are discussed as filler fractal structure. In the third section, the strong nonlinearity behaviour (Payne effect) of filled polymers has been discussed in terms of filler nature. Typically two mechanisms arise to depict the linear solid-like behaviour and the Payne effect: particle-particle interactions is the dominant mechanism in fumed silica nanocomposites whereas particle polymer interaction is the dominant one in colloidal silica nanocomposites at identical filler concentrations. However, these interactions are balanced in each nanocomposite systems by the silica surface treatments (chain grafting, silane modification) and the molecular weight of the matrix. Finally, we aim to unify the main findings of the literature on this subject, at least from a qualitative point of view. We finally report on the thixotropy and modulus recovery after a large deformation in steady and dynamic shear conditions. Following this, the nonlinear rheological properties of nanocomposite materials have been discussed. The discussion is particularly focused on the effect of flow history (transient shear experiments) on the orientation disorientation of clay platelets. Actually, the linear and nonlinear rheological properties are consistent with a network structure of a weakly agglomerated tactoids. As far as exfoliated clay nanocomposites are concerned, the interparticle interaction is the dominant effect in the nonlinearity effect.
A direct consequence of the incorporation of nanofillers in molten polymers is the significant change in the viscoelastic properties. Consequently, linear rheology is away generally used to assess the state of dispersion of nanocomposites directly in the melt state. However, adding colloidal particles to polymers also affect the nonlinear behaviour and time-dependent properties. For instance, the effect of strain-dependence of the dynamic viscoelastic properties of filled polymers often referred to as the Payne effect, is well known in elastomers since 40 years. During the past few years, intensive discussions have been held on the rheology of nanocomposites filled with organically modified clays whereas the rheology of usual nanocomposites filled with silica particles, and more particularly fumed silica, seems to have been deserted. In particular, fumed silica is a finely divided amorphous silicon dioxide which can be seen at three main scales: primary particles of around 1e3 nm fused together in stable aggregates of around 100e250 nm which finally build up to large micron-sized agglomerates, generally named clusters. Due to the large surface area (50e400 m2 /g) of these particles, the inter-particle interactions have a major impact on the rheological and reinforcement properties of nanocomposites. Actually, this cluster structure can be viewed as an assembly of primary particles in a structure having a fractal dimension. Due to their fractal structure and their high specific area, fumed silica fillers are subjected to self-aggregation and can consequently form a network of connected or interacting particles in the molten polymer. As a viscoelastic result, polymer-based nanocomposites exhibit a terminal plateau in the low-frequency domain. Furthermore, the thixotropic nature of melt nanocomposites is also different from that of microcomposites at similar filler volume fractions.
With regard to organoclays silica nanocomposites, three-layer organization scales are generally differentiated: (i) the clay layers have a micron-size scale in the polymer matrix in the case of weak interaction and/or none appropriate shearing conditions, (ii) few polymers chains are able to diffuse in the interlayer space, this structure is called intercalated, (iii) all the layers are homogenously dispersed as individual layers at a nanoscale, this structure is called exfoliated. Consequently, the exfoliation of organoclay layers increases the number of frictional interactions between layers, which is consistent with the formation of a network structure of weakly agglomerated particles.
Since the rheological properties of nanocomposites are sensitive to the structure, particle size, shape and surface characteristics of the silicate phase, the rheological tool is intensively used to assess the state of the dispersion of nanocomposites directly in the melt state. A lot of papers, too many to cite them all, and reviews have been addressed to the rheology of filled polymers. The most recent review was addressed by Litchfield and Baird  on the rheology of high aspect ratio nanoparticles filled liquids. The main objective of our contribution is to review the viscoelastic behaviour of silica filled polymers and organoclay nanocomposites. In particular, this review focuses on linear (solid-like behaviour) and nonlinear viscoelasticity (Payne) including the thixotropy process of such composites. Actually, the exact causes of the typical viscoelasticity of nanocomposites are still a matter of investigations. This paper aims to unify the main findings of the literature on this subject, at least from a qualitative point of view.
Since the melt rheological properties of filled polymers are sensitive to the structure, particle size shape and surface characteristics of the fillers, rheology offers original means to assess the state of dispersion in the nanocomposites and to investigate the influence of flow conditions upon nanofiller dispersion itself. The objective of the present work was to discuss and to compare, from the open literature, the rheological behaviour of polymer composites filled with two usual fillers, namely, fumed silica and organoclays.
Generally speaking, fumed silica and organoclay nanocomposites both show a solid-like rheological response which includes a non-terminal relaxation zone, apparent yield stress and a strong nonlinearity behaviour (Payne effect). The power laws on the equilibrium elastic shear modulus G0 and the limit of linearity deformation gc were investigated. Silica particles and organoclays and more generally fillers whatever be their nature (shape and chemical structure) obey the following power law (order of magnitude) G0ff4 for the equilibrium shear modulus. Regarding the variation of the limit of linearity gc, two power laws can be distinguished according to experimental works: gcff2 and gcff1 for fumed silica and organoclay nanocomposites, respectively. These observations reinforce the idea that the moduli and the limit of linearity are related to the fraction of exfoliated layers or silica clusters which form a fractal structure. Actually, our discussion is consistent with works on elastomer reinforcements  and good rubber industry expertise on compounding in order to obtain optimal dispersion of fillers without completely suppressing their aggregation into a larger structure. In other words, a fractal structure of fillers is required to get the best balance of reinforced properties.
Furthermore, the comparison of the viscoelastic behaviours of fumed silica and clay nanocomposites leads to an apparent paradox. Actually, both the percolation threshold and limit of linearity decrease with increasing the exfoliation state of organoclay platelets whereas they increase with the dispersion of fumed silica by surface grafting of end-tethered chains. Actually, the exfoliation of organoclay layers increases the number of frictional interactions between the layers whereas the dispersion of fumed silica to the primary entities scale decreases the silica inter-particle interactions. Interestingly, such a trend is already observed for carbon nanotube (CNT) composites. Actually, the percolation threshold for NTC/polymer composites can range from 0.0005 vol% to several vol%  depending on the NTC ‘‘miscibility’’ in polymers, i.e. when improving dispersion of nanotubes by exfoliation of nanotube clusters. Note that specific and relevant discussions on the nanocomposites filled with NTC have recently been addressed [113,114].
The origin of the Payne effect, in the case of conventional fillers such as fumed silica, does not seem to be controversial anymore in the literature. Actually, common mechanisms that are rooted in the macromolecular natures of the matrices (entanglement disentanglement of polymer chains) and the breakdown of silica aggregate are generally admitted. In the case of fumed silica nanocomposites, filler inter-particle interactions dominate the viscoelastic behaviours. As a consequence, the rupture of the filler network is the first-order mechanism in the Payne effect. On the contrary, the polymer particle interactions which lead to a strong modification of the chain relaxation process is the dominant mechanism with colloidal silica at identical filler concentrations. The mechanism of disentanglement entanglement should be the dominant one in the Payne and thixotropy (modulus recovery) effects with spherical colloidal nanocomposites. Of course, the respective contribution of each mechanism to the linear and nonlinear viscoelastic properties of such nanocomposite is balanced by the silica surface treatments (polymer chain grafting or organo-silane treatment). This finding means that the linear and nonlinear rheological properties are consistent with a network structure of weakly agglomerated particles (particle-particle interactions) combined with a mechanism of polymer chain relaxation at the filler surface vicinity governed by the polymer particle interactions. However, the filler-filler interaction is the predominant mechanism in most of the cases reported in the literature.
All authors agree with the network breakdown mechanism in the case of organoclay fillers. Transient shear experiments clearly proved this finding as the attractive interactions between the clay platelet domains (inter-particle interactions) are the driving force of the structural evolution during transient shear experiments, particularly for the network recovery during rest time. Upon flow cessation, attractive particle-particle interactions promote the reconstitution of the network. Furthermore, it must be pointed out that Brownian relaxation processes do not significantly contribute to the nanocomposite rheological response.
On the other hand, we demonstrated that a third contribution to the breakdown-recovery mechanism could arise from the volumetric expansion of the polymer matrix at the melting transition of semi-crystalline polymers. Finally, it can be pointed out, that the nonlinearity properties and the studies of flow-induced structures in organoclay composites are generally investigated from transient shear experiment whereas dynamic experiments are generally used to study the nonlinearity effects in silica nanocomposites. Actually, this trend stems from the origin of the viscoelasticity of the polymer matrix used in both types of nanocomposites. The filler concentration may be another traditional aspect. Apart from suspensions applications (coating, painting, .), one on the main development of silica particles is the reinforcement of elastomers which generally show complex viscoelastic properties in the terminal relaxation zone combined with strong temporary elasticity. Consequently, transient shear experiment or dynamic experiments combined with pre-shear conditions can hardly be envisaged.
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FULL Paper PDF file:Melt rheology of organoclay and fumed silica nanocomposites
Melt rheology of organoclay and fumed silica nanocomposites
ELSEVIER, Polymer, Volume 49, Issue 9, 29 April 2008, Pages 2183-2196
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Professor Siavosh Kaviani was born in 1961 in Tehran. He had a professorship. He holds a Ph.D. in Software Engineering from the QL University of Software Development Methodology and an honorary Ph.D. from the University of Chelsea.