Nanocomposite formation clay particles

The nature of the organomodifier plays a role in the existence of true nanocomposite structures (intercalated for 15A and 30B, exfoliated for 25A, microcomposite for 10A), cone calorimeter results associated with x-ray diffraction (XRD) suggest that increased flame retardancy are more dependent on physical and thermal cross-linking of clay particles and polymer chains than on formation of nanocomposite structure. However, it can be concluded that the role of clay is crucial since PHRR values are reduced up to 70% in the presence of clays. 

The two structures, namely the intercalated and exfoliated ones, could be distinguished with the respect to the degree of the distribution of the clay particles, Figure 8. One should note that clay layers are quite flexible though they are shown straight in the figure. The formation ofthe intercalated or exfoliated structures depends on many factors, for example the method of the production of the nanocomposite or the nature of the clay etc [29]. 

In general, the dispersion of clay particles in a polymer matrix can result in the formation of three general types of composite materials (Figure 1). Conventional composites contain clay tactoids with the layers aggregated in unintercalated face - face form. The clay tactoids are simply dispersed as a segregated phase. Intercalated clay composites are intercalation compounds of definite structure formed by the insertion of one or more molecular layers of polymer into the clay host galleries and the properties usually resemble those of the ceramic host. In contrast, exfoliated polymer-clay nanocomposites have a low clay content, a monolithic structure, a separation between layers that depends on the polymer content of the composite, and properties that reflect those of the nano-confmed polymer. 

Figure 16.24 shows the schematic representation of dispersed clay particles in a polymer matrix. Conventionally dispersed clay has aggregated layers in face-to-face form. Intercalated clay composites have one or more layers of polymer inserted into the clay host gallery. Exfoliated polymer/clay nanocomposites have low clay content (lower than intercalated clay composites which have clay content -50%). It was found that 1 wt% exfoliated clay such as hectorite, montmorillonite, or fluorohectorite increases the tensile modulus of epoxy resin by 50-65%. Montmorillonite was used in a two stage process of nanocomposite formation. In the first step, montmorillonite was intercalated with vinyl monomer and then used in the second step to insert polystyrene by in situ polymerization. 

XRD, SEM, and TEM are widely used for morphological characterization of nanocomposites. Rheology has also been used extensively in complement to these techniques in several studies as it is very sensitive to the morphology of nanocomposites [23,75-80]. The summary of the most significant results from these studies is the transition from liquid-like to solid-like viscoelastic behavior for nanocomposites, even at low-volume fractions of silicate layers, as well as a strong shear-thinning behavior. The solid-like behavior has been attributed to the formation of a percolated network of clay particles that occurs at relatively low clay loading, due to the anisotropy of the particles, which prevents their free rotation and the dissipation of stress. 

In-situ polymerisation, solvent intercalation/exfoliation and melt intercalation/exfoliation are the three major pathways for the formation of nanocomposites. In-situ polymerisation involves the combination of clay and monomer, followed by the polymerization of the monomer, which ideally locks the exfoliated clay particles in the resulting polymer matrix. In solvent intercalation die clay is first swollen in a solvent and the polymer (intercalant) is dissolved in die solvent. Both solutions are then combined and the polymer chains intercalate and displace the solvent within the interlayer of the clay (Shen et al, 2002). 

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