Viscosity polymer nanocomposites

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]. 

In Fig. 2.1, the dependencies of the ratios of GJG and y J (, where G and are shear modulus and melt viscosity of nanocomposite, G and t are the same characteristics for the initial matrix polymer, on CaC03 mass content (WJ for nanocomposites HDPE-CaCOj. Shear modulus G was calculated according to the following general relationship ... 

The discrepancy indicated requires the application of principally differing approaches to the polymer nanocomposites melt viscosity description. Such an approach can be fractal analysis, within the ffamewoik of which, the authors used the following relationship for fractal liquid viscosity (h) estimation ... 

As the adduced above data have shown, the polymer nanocomposites with three main types of inorganic nanofiller and also polymer-polymeric nanocomposites melt viscosity caimot be described adequately within the fiamework of models, developed for the description of microcomposites melt viscosity. This task can be solved successfully within the framework of the fractal model of viscous liquid flow, if in it the used nanofiller special feature is taken into account correctly. Let us note that unlike microcomposites nanofiller cotents enhancement does not result in melt viscosity increase, but, on the contrary, reduces it. It is obvious, that this aspect is very important from the practical point of view. 

The dispersion of clay platelets (exfoliation and intercalation level of the silicate layers) and surface area of silicate platelets have the potential to alter the rheological behavior of the nanocomposites. In-situ polymerized nano composites exhibit more exfoliated structure than the composites prepared by the melt blending technique. Irrespective of the processing parameter, the nanocomposites show shear thinning behavior at high shear rate (Figure 9.14), whereas the pristine polyamide exhibits Newtonian behavior (i.e., the viscosity remains almost the same). It has also been reported that the polymer nanocomposite possesses higher steady shear viscosity than pristine polyamide at low shear rates. 

The main uses of montmorillonite stem from its characteristic expansion, and it is used to control viscosity or impart thixotropy to a variety of liquid polymers based on unsaturated polyesters, PVC plastisols, polysulfides, alkyds, etc. It has also been reported to control the melt rheology of thermoplastics and to reinforce polyamides. There has, over the last few years, been enormous industrial and research interest, with many papers and patents, published on montmorillonite, especially as an organoclay as the basis of polymer nanocomposites. Because of the delamination process described above plastic-organoclay nanocomposites have been reported to have very high rigidity, low permeability to fluids, and fire resistance. This subject is covered in more detail in Chapter 10. 

MMT) = 0.64 vol% and (SNT) = 6 vol%]. This trend in the viscosity data is consistent with that previously reported in the literature for similar polymer nanocomposites and has been attributed to different degrees of exfoliation of the nanofillers in the polymer matrix... 

It is well known that the pyrolysis of polyolefin was a free radical chain reaction through (3-scission of polymer chains, whereas the free radicals produced in turn speeded up the degradation of polymers. Many scientists have reported that the Qo fullerene molecule has high reactivity to free radicals one Ceo molecule can trap from 1 to 34 free radicals [30], Based on these viscoelastic measurements, it was reasonable to conclude that Ceo was Ukely to trap macromolecular free radicals (PP radicals) and other free radicals such as hydrogen and hydroxyl radicals created from the decomposition of PP at elevated temperatures and form cross-linked networks in situ. On the other hand, the radical-trapping effect of Ceo also provided partial PP radicals more time to recombine, which resulted in a remarkable increase in both storage modulus and complex viscosity of nanocomposites. 

As already highlighted during the study of the previous CNT/ polymer nanocomposites, the low percolation threshold is certainly favored by the viscosity of the polymer matrix, which is notably lower than that of the PS matrix studied in Section 4.2.2. of chapter 4 of this book [about 5 x lO Pa.s for PPO/PS, which is at least one order of magnitude lower than the viscosity of the earlier described PS matrix, which is higher than 10 Pa.s]. ... 

---