Platelets exfoliated

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approaching this concentration forms stacks thus, the assumption that individual exfoliated platelets are present is no longer valid. 

The reinforcement of polypropylene and other thermoplastics with inorganic particles such as talc and glass is a common method of material property enhancement. Polymer clay nanocomposites extend this strategy to the nanoscale. The anisometric shape and approximately 1 nm width of the clay platelets dramatically increase the amount of interfacial contact between the clay and the polymer matrix. Thus the clay surface can mediate changes in matrix polymer conformation, crystal structure, and crystal morphology through interfacial mechanisms that are absent in classical polymer composite materials. For these reasons, it is believed that nanocomposite materials with the clay platelets dispersed as isolated, exfoliated platelets are optimal for end-use properties. 

Fig. 5. Schematic of the hierarchy of clay structures in polypropylene nanocomposites of mixed morphology. Clay tactoids and exfoliated platelets comprise the mesoscale morphology. The internal intercalation structure of clay tactoids is determined by the compatibilizer and compounding conditions. (View this art in color at www.dekker.co...

Blends made by extrusion have created difficulties and show a degradation of the molecular weight with development of colour. Processing leads to more exfoliation of platelets in the clay that causes two competing effects. A reduction of the peak heat release rate (HRR) because of the exfoliated platelets, versus degradation processes that increase the peak HRR. The dominant effect in low ash and/or low shear situations is reduction of peak HRR. The opposite is true with high ash or high shear since the peak increases. 

Figure 6.21 Exfoliated tactoids of montmorillonite. Note the irregular shapes of exfoliated platelets compared to the larger tactoids in Figures 6.18-6.19.

Fig. 19. Illustration of dispersed/intercalated platelets and exfoliated platelets.

The synthesis of a PS/Montmorillanite clay nanocomposite prepared by combining methods 2 and 6 described earlier was published (137) (Fig. 21). They utilized living free-radical polymerization (LFRP) of styrene to provide exfoliated platelets in a PS matrix. Transmission electron micrographs and x-ray diffraction patterns support the idea of preparing nanocomposites via polymerization initiation. 

Because of the barrier properties of silicate platelets, a diffusing gas/liquid molecule must detour the exfoliated platelets, forming a very tortuous path as shown in Fig. 21.2. Thus permeability of polymer layered silicate nanocomposites is mainly dependent on the degree of exfoliation or intercalation and the state of dispersion of silicate platelets in a polymer matrix. 

Delaminated or exfoliated clay platelets are separated from one another in a plastic matrix. During exfoliation platelets at the outermost region of each packet cleave off, exposing more platelets for separation. 

Two of the most important advances are in the fields of nanocomposites and biopolymers. The nanocomposites are of two basic types, carbon nanotubes and montmorillonite clay exfoliated platelets. The biopolymer aspects can be traced to such Nobel Prize winning research as Watson and Crick s discovery of the double helix structure of DNA and an understanding of how proteins work in muscles. 

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