Viscosity polymer-clay nanocomposites

This chapter is organized in the following way. First, we present some common techniques for characterizing the dispersion of nanoclays in polymer blends. The dispersion level has been shown to have a fundamental effect on the fire performance of polymer-clay nanocomposites (PCNs), as an exfoliated or intercalated polymer-clay system seems to enjoy reduced flammability. Second, the effects of nanoclays on the viscosity of polymer blends are discussed. With increased temperature in the condensed phase during combustion, most polymers (and hence polymer blends) have sufficiently low viscosity to flow under their own weight. This is highly undesirable, especially when the final products will be used in vertical orientation, because the melt can drip, having the potential to form a pool fire, which can increase fire spread. The results on thermal stability are presented next, followed by those for the cone calorimeter. The quantitative effects of nanoclays on the... 

To achieve improved dispersibUity of nanoclay fillers within polymer systems, three familiar methods are commonly used, namely, melt intercalation, solution intercalation, and in situ polymerization. The melt-intercalation method is based on the melting point of polymer matrices and is applied by annealing above the melting point of the polymer (Reddy et al., 2013). This method has been chosen by industrial sectors to produce polymer/clay nanocomposites. However, it is not apphcable to the fabrication of biobased polymer/clay nanocomposites based on thermosetting materials such as epoxy and polyester due to their high viscosities (Wypych and Satyanarayana, 2005 Wang et al., 2014). Therefore, the fabrication of biobased thermosetting polymer/clay nanocomposites is mainly based on solution intercalation or in sim polymerization. 

The second significant independent variable that layered silicates provide to increase thermal stability of the polymer in polymer-clay nanocomposites is an increase of the melt viscosity. If thermal degradation of the polymer is diffusion controlled, an increase in viscosity of the polymer melt will slow the mass loss associated with gas escaping from the composite during TGA evaluations. The increase in viscosity of dispersions is a function of the surface area of the dispersed phase. For example, water-based dispersions will increase in viscosity as the particle size of the dispersed phase decreases at constant total volume of the dispersed phase. This is the result of an increase in total surface area of the dispersed phase. Particle-particle interaction has increased as a function of increased total particle surface area. The surface area [17] of fully exfoliated montmorillonite is approximately 750 m /g. This enormous number results in a significant increase in polymer-montmorillonite melt viscosity at low concentration of montmorillonite and low shear rates [18]... 

Polymer-clay nanocomposites that have the best fire-retardant performance evaluated by the cone calorimeter have clay particles oriented parallel to the surface of the sample. Vertical fire retardant tests (UL-94) of these samples do not demonstrate improved performance because the edges of the particles are exposed to the fire. The edges of clay are very thin (approximately 1 nm). Hence, the mechanisms predicated on barriers provided by the clay are not applicable. Mechanisms associated with increased melt viscosity are apparent with vertical fire-retardant testing. Dripping during the burning of the vertical samples is greatly reduced [3]. 

Similar observations were noted when FKM/o-MMT clay nanocomposites were prepared by melt blending and the as-prepared nanocomposites showed both intercalated as well as exfoliated structure [103]. The apparent shear viscosity of the FKM/o-MMT nanocomposites was lower than that of the pristine polymer at all shear rates and temperatures. The nanocomposites exhibited reduced equilibrium die swell with a smooth extrudate appearance. A comparison of the flow properties of the nanocomposites with the conventional composites revealed that the nanocomposites exhibited improved processability. 

The rheological behavior of these materials is still far from being fully understood but relationships between their rheology and the degree of exfoliation of the nanoparticles have been reported [73]. An increase in the steady shear flow viscosity with the clay content has been reported for most systems [62, 74], while in some cases, viscosity decreases with low clay loading [46, 75]. Another important characteristic of exfoliated nanocomposites is the loss of the complex viscosity Newtonian plateau in oscillatory shear flow [76-80]. Transient experiments have also been used to study the rheological response of polymer nanocomposites. The degree of exfoliation is associated with the amplitude of stress overshoots in start-up experiment [81]. Two main modes of relaxation have been observed in the stress relaxation (step shear) test, namely, a fast mode associated with the polymer matrix and a slow mode associated with the polymer-clay network [60]. The presence of a clay-polymer network has also been evidenced by Cole-Cole plots [82]. 

Following from the same considerations, it can be demonstrated that a fiber-like shape, instead of a platelet-like, can minimize the sharp increase of viscosity due to the addition of nanoclays to polymer melts, and therefore improve the processability of the nanocomposite. Different factors contribute to the viscosity of a nanocomposite (a) the polymer-polymer network, (b) the clay-clay network, and (c) the polymer-clay interaction. Supposing that the chemistry of the system is fixed, the first contribution is invariant and the second depends only on the particle aspect ratio, according to percolation theory [7] the higher the aspect ratio, the higher the viscosity of the composite. Comparing the effect of two nanoclay particles with the same aspect ratio, one with a rod-like shape and the other with... 

It can be observed that the Mw of all samples—measured via viscometry—is very high. The accuracy of the data is dubious since the viscosity of the nanocomposite/decalin solution is strongly influenced by the presence of the clay, apart from the Mw of the synthesized polymer. Nevertheless, it is indicative of the poor processability of the PE nanocomposite. 

The nanocomposite PET-PEN/MMT clay was smd-ied under steady shear, instantaneous stress relaxation, and relaxation after cessation of steady flow [83]. Relaxation times of the slow mode in instantaneous stress relaxation were longer for the systems that have presumably permanent crosslinking networks (PET-PEN) or dynamic networks (PET-PEN-MMT). These results are consistent with those found in relaxation after cessation of flow (Fig. 31.4). Nanoclay addition somehow restricts the slow relaxation (due to polymer-particle interactions). The nanocomposite exhibits lower steady-state viscosity as compared to the polymer-matrix system. This is thought to be caused by polymer-polymer slipping, as revealed by the SEM observations (Fig. 31.5a and b). 

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