Viscosity, layered-silicate polymer

Galgali and his colleagues [46] have also shown that the typical rheological response in nanocomposites arises from frictional interactions between the silicate layers and not from the immobilization of confined polymer chains between the silicate layers. They have also shown a dramatic decrease in the creep compliance for the PP-based nanocomposite with 9 wt% MMT. They showed a dramatic three orders of magnitude drop in the zero shear viscosity beyond the apparent yield stress, suggesting that the solid-like behavior in the quiescent state is a result of the percolated structure of the layered silicate. 

For more than a decade, numerous research studies have been carried out on the flame-retardant properties conferred by nanoparticles and mainly by organo-modified layered silicates (OMLS). Earlier work at Cornell University and National Institute of Standards and Technology in the United States showed that nanocomposites containing OMLS reduced polymer flammability and enhanced the formation of carbonaceous residue (char).14 Owing to a strong increase in polymer viscosity, impairing processability, and also due to the breakdown of ultimate mechanical properties, the acceptable rate of incorporation for nanoparticles to improve flame retardancy is generally restricted to less than 10 wt %. 

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]... 

Pomes et al. (2001) proposed that the layered-silicate exfoliation during an extmsion process was initiated by the break-up of taller silicate stacks into smaller stacks, following a layer-by-layer peeling mechanism of the top and bottom sihcate stack platelets by the extmsion shear stress. They also reported that this mechanism was proportional to the polymer (viscosity), in which higher effective shear stress associated with higher would lead to a more exfoliated morphology. 

In cosmetic O/W emulsions the internal oil phase is not present in large enough quantities to make a significant contribution to the viscosity. The required viscosity is achieved by adding water-swelling polymers, layer silicates or gel forming lipids, " which can differ considerably in their viscosity properties and therefore also in their sensory properties. ... 

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. 

Hwang et al. [113] synthesized via in situ polymerization high-impact polystyrene (HlPS)/organically modifled montmorillonite (organoclay) nanocomposites. X-ray diffraction and TEM experiments revealed that intercalation of polymer chains into silicate layers was achieved, and the addition of nanoclay led to an increase in the size of the robber domain in the composites. In comparison with neat HIPS, they found that the HIPS/organoclay nanocomposites exhibited improved thermal stabiHly as well as an increase in both the complex viscosity and storage modulus, and they may have been influenced by a competition between the incorporation of clay and the decrease in the molecular weight of the polymer matrix. 

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