Dispersion layered silicates

Thermoset nanocomposites with well dispersed layered silicate or CNT, offer improved thermal stability and elastic modulus as discussed in the earlier sections. However,... 

For his theoretical expressions, Nielsen [122] assumed that the sheets are placed perpendicular to the diffusive pathway. Therefore, the above expressions describe that the gas permeability of nanocomposite films depends primarily on three factors (1) the dimension of the dispersed layered silicate particles, (2) the dispersion of the layered silicate particles in the polymer matrix, and (3) the percentage of silicate particles loaded in the film. On the basis of Nielsen s model, Yano et al. [121] published several equations (Equations 12.23 and 12.24) where the diffusion process in nanocomposites (using polyimide-clay hybrids made by mixing 4,4 -diaminodiphenylether and pyromellitic dianhydride)... 

D. W. Schaefer, R. S. Justice, H. Koemer, R. Vaia, and C. Zhao. Large-scale morphology of dispersed layered silicates. Materials Research Society Symposium Proceedings, 840 (2005), 57-62. 

When a thermoplastic polymer is mixed with layered silicate particles, it either intercalates or exfoliates the layered silicates, as schematically shown in Fig. 1. The state of dispersion (intercalation vs exfoliation) of the aggregates of layered silicate in nanocomposites is commonly assessed by X-ray diffraction (XRD) or transmission electron microscopy (TEM). Generally exfoliation is preferred to intercalation, although exfoliation may not necessarily be required for certain industrial applications (e.g., for an enhancement of flame retardant characteristics). It should be mentioned that the greater the extent of exfoliation of layered silicates, the larger will be the total surface areas of the dispersed layered silicates, giving rise to enhanced mechanical/physical properties of the nanocomposites prepared. 

Equation 3.1 clearly describes that the gas barrier property of nanocomposites depends primarily on two factors one is the dispersed layered silicate particles dimension and other is the dispersion of layered silicate in polymer matrix. When the degree of dispersion of layered silicate in the matrix is the same, barrier property directly depends on the dispersed layered silicate particles dimension, i.e. the aspect ratio. 

Table 12.2 summarizes the mechanical properties of polybutadiene rubber-clay nanocomposites. The hardness, tensile strength, elongation at break, and permanent set all improved with increasing the clay content (5—40 phr)." " The mechanical properties of polybutadiene rubber-clay nanocomposite with 20 pin-clay content have been compared to those of the polybutadiene composites filled with 20 phr carbon black (SFR and N330), as presented in Table 12.3. This data shows that the organically-modified layered silicate was as effective a reinforcing filler, as carbon black. Some of the mechanical properties of polybutadiene nanocomposite such as hardness, tear strength, and tensile strength even exceeded those of the carbon black filled compounds." " These excellent mechanical properties of the nanocomposites resulted from the uniformly dispersed layered silicate in the elastomer matrix, and the strong interaction between the nanoclay layers and rubber chains. Thus layered silicates could be used in the polybutadiene industry as a promising reinforcing filler, if the layers...

The rheological properties of insitu polymerized nanocomposites with end-tethered polymer chains were first described by Krisnamoorti and Giannelis [33]. The flow behavior of PCL- and Nylon 6-based nanocomposites differed extremely from that of the corresponding neat matrices, whereas the thermorheological properties of the nanocomposites were entirely determined by the behavior of the matrices [33]. The slope of G (co) and G"(co) versus flxco is much smaller than 2 and 1, respectively. Values of 2 and 1 are expected for linear mono-dispersed polymer melts, and the large deviation, especially in the presence of a very small amount of layered silicate loading, may be due to the formation of a network structure in the molten... 

Gilman JW, Davis RD, BeUayer S et al. (2005) Use of optical probes and laser scanning con-focal fluorescence microscopy for high-throughput characterization of dispersion in polymer layered silicate nanocomposites. PMSE Prepr 92 168-169... 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

In order to produce high-performance elastomeric materials, the incorporations of different types of nanoparticles such as layered silicates, layered double hydroxides, carbon nanotubes, and nanosilica into the elastomer matrix are now growing areas of rubber research. However, the reflection of the nano effect on the properties and performance can be realized only through a uniform and homogeneous good dispersion of filler particles in the rubber matrix. 

In numerous works dealing with the combination of nanoparticles and FR compounds, surface modifications of nanoparticles were only aimed to promote good dispersion of the nanoparticles into the polymer matrix (with intercalated or exfoliated morphologies for layered silicates as nanoparticles), even in the presence of the usual FRs, for example ammonium polyphosphate (APP) or magnesium hydroxide (MH). The initial aim was to combine the individual effects of each component to achieve strong synergistic effects. 

Suspended solids, 386 Dispersion, 366-367 Flocculation, 366-367 Double Layer Thickness, 368 Swelling of layer silicate clays, 103-115... 

The term polymer clay nanocomposite (PCN) refers to a material composed of two-phase materials, where one phase (clay) is dispersed in the second phase (polymer matrix) at a nanometer level. Composites exhibiting structural and compositional changes at the molecular scale have demonstrated several physical property enhancements that are otherwise unavailable in conventional composites. Of these, layered silicates have proven themselves vital as a reinforcing agent when dispersed into engineering plastics. Nanolayers, however, are extremely difficult to disperse in polymer matrices because of their tendency for face-to-face... 

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