Exfoliation-adsorption method

Figure 5 illustrates schematically the exfoliation-adsorption method. Layered host sheets are delaminated into single-sheet particles and then dispersed as a colloidal suspension. In the 1980s, the layered metal disulfides M0S2 and WS2 were observed to form such single-sheet colloidal suspensions when their lithiated forms were rapidly hydrolyzed [87-89]. More recently, a layered sheet surface-... 

FIGURE 5 Schematic representation of the exfoliation/adsorption method. 

Table 3 Potential Applications for Several Nanocomposites with LMC Hosts Prepared by the Exfoliation/Adsorption Method...

Epoxy-clay nanocomposites were studied extensively, but only ordered exfoliated nanocomposites were reported with in situ polymerization. Two recent works showed that disordered and highly exfoliated epoxy-clay nanocomposites can be prepared using an exfoliation-adsorption process." In this method the organo-modified clay is first dispersed in a solvent. It is well known that due to the weak forces that stack the layers together, layered silicates can easily be swelled and eventually delaminated in an adequate solvent. The polymer then adsorbs onto the delaminated sheets, and when the solvent is evaporated, a highly disordered structure is obtained before starting the standard in situ polymerization. 

Three main methods have been developed to prepare polymer-clay nanocomposites.The first is exfoliation-adsorption the layered clay is exfoliated into a single layer in solvent in which the polymer can dissolve. Owing to the weak forces between the clay layers, the polymer enters between the delaminated layers and, when the solvent is evaporated, the nanocomposites are formed. The second is melt intercalation the polymer, in the molten state, is incorporated with the layered clay. The polymer then crawls into the interlayer space, causing the layers to separate and form nanocomposites. These two methods are physical and no chemical reaction occurs. The third method in situ intercalative polymerization the modified layered clay absorbs the liquid monomer and polymerization can be carried out between the intercalated sheets as shown in Figure 8.2. So far, many polymer-clay nanocomposites have been prepared through in situ intercalative polymerization initiated by chemical agents but very few by irradiation. ... 

Use of surfactants is an effective way for dispersing CNTs [39]. Reports show that the outer most nanotubes in a bundle are treated more than the innermost tubes and the nanotube remains predominantly btmdled even after surfactant treatment. But mechanical methods like ultrasonication can debundle the nanotubes by steric or electrostatic repulsions [40]. On sonication the high local shear will unravel the outer carbon nanotubes in a bundle and expose other sites for additional surfactant adsorption, thus the surfactant molecules gradually exfoliate the bundle in an unzippering mechanism [41]. Some of the common surfactants used for the dispersion of carbon nanotubes are sodium dodecyl benzene sulfonate (SDBS) [42], dodecyl trimethyl ammonium bromide (DTAB) [43], hexadecyl trimethyl ammonium bromide (CTAB) [44], octylphenol ethoxylate (Triton X-100) [45] and sodium dodecyl sulfate (SDS) [46]. Covalent modification is another way to solubilize the CNTs in different solvents and to improve the interaction with the matrix in composites [47]. 

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