Exfoliated clay stacks

Nanocomposite describes a two-phase material where one of the phases has at least one dimension in the nanometer range (1-100 nm). They differ from conventional composites by the exceptionally high surface-to-volume ratio of the reinforcing phase and/ or its exceptionally high aspect ratio. The reinforcing material can be made up of particles (e.g., minerals), sheets (e.g., exfoliated clay stacks) or fibers (e.g., carbon nanotubes, electrospun fibers or cellulose nanofibers). Large reinforcement surface area means that a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composite. There has been enormous interest in the commercialization of nanocomposites for a variety of applications, and a number of... 

The exact number of clay stacks having the above three categories of platelet stacking were measured by taking at least six different HRTEM images for each type of nanocomposite sample and the average distribution of the clay platelets were noted down (Fig. 38). This has been represented as the extent of exfoliation (B) in Table 9. It is apparent that as the level of exfoliation increases, the number of clay platelets per stack decreases and their effective surface area contribution increases. 

The HCP model implies that in diluted systems ( < 0.005, where exfoliated clay platelets may freely rotate), individual HCPs are dispersed in a polymeric matrix and values of the interaction parameters are constant. As the concentration increases, the domains of reduced mobility around HCPs begin to overlap, macromolecules with bulk properties disappear, and the interactions change with clay content. Above the encompassed clay platelet volume fraction, rot = Q.99 p 0.005, there is a second critical concentration, Wmax 3.6 wt% or (/>max 0.015, at which the clay platelets with adsorbed solidified organic phase begin to overlap. Due to platelet crowding, CPNC approaching this concentration forms stacks thus, the assumption that individual exfoliated platelets are present is no longer valid. 

Since nanoparticles in PNC are orders of magnitude smaller than conventional reinforcements, the models developed for composites are not applicable to nanocomposites. However, development of a universal model for PNC is challenging since the shape, size, and dispersion of the nanoparticles vary widely from one system to another. On the one hand, exfoliated clay provides vast surface areas of solid particles (ca. 800 m /g) with a large aspect ratio that adsorb and solidify a substantial amount of the matrix polymer, but on the other hand, the mesoscale intercalated clay stacks have a much smaller specific surface area and small aspect ratio. However, in both these cases the particle-particle and particle-matrix interactions are much more important than in conventional composites, affecting the rheological and mechanical behavior. Thus, the PNC models must include the thermodynamic interactions, often neglected for standard composites. 

As sketched in Figure 23.5(b), the aspect ratio /is given in this case by the ratio between the longest lateral side of the layer and the height of clay stacks. The highest / value is thus obtained in the case of a single layer. As it will be discussed below, a higher/value means a more pronounced improvement in the rubber matrix. Therefore, the exfoliation of a clay not only favours a better clay dispersion but also improves the modulus of the compound. However, the hydrophilic nature of these clays implies that a nanometric dispersion in a hydrophobic polymer is hard to achieve. The alkaline and alkaline-earth... 

As reported by several authors, the dispersion of nanofiller within the polymer matrix is a key step to obtain exfoliated structures (Lee and Hanna, 2008 Liu et al., 2011). Turri, Alborghetti, and Levi (2008) reported that to produce a true nanocomposite, the clay stack must first be delaminated within the polymer matrix. Delamination of the clay then gives rise to a homogeneous dispersion of individual platelets. According to Le Corre, Bras, and Duffesne (2010),... 

In the past few years, many groups have tried to encapsulate clay platelets inside latex particles. This encapsulation poses some extra challenges because of the tendency of the clay platelets to form stacks and house-of-cards structures in an aqueous medium. Furthermore, the clay platelets also tend to reside at the water-polymer interface, as discussed previously in this chapter. This means that most efforts would lead to clay platelets situated mainly at the outside of the latex particles. In this section, a few examples are shown in which clay has been proven to be located inside the latex particles. Another challenge in this respect is to confirm where the clay platelets reside. It is very diffieult to visualize natural clay platelets when they do not form stacks. Individual clay platelets are too thin to see by TEM they only become visible when their basal planes are oriented more or less parallel to the eleetron beam. A (cryo)-TEM instrument with a stage that can rotate can help verify the location of an exfoliated clay platelet, i.e. whether it is inside or on the surface of a latex particle. ... 

Number of clay platelets per stack Effective surface area contribution of clay platelets (A) Extent of exfoliation in nanocomposite sample as obtained from image analysis ( ) ... 

TEM is very qualitative rather than quantitative because of the lack of well-defined standards for exfoliation, intercalation, and distribution for the clay layers in the polymer matrix. Figure 11.18 shows the TEM micrographs which, based on author s understanding and experience with TEM, illustrate the morphologies immiscible, intercalated with many stacked clay layers, intercalated with a few stacked clay layer and uniform dispersion, and exfoliated with uniform dispersion. 

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