Exfoliation, layered materials

Layered materials are of special interest for bio-immobilization due to the accessibility of large internal and external surface areas, potential to confine biomolecules within regularly organized interlayer spaces, and processing of colloidal dispersions for the fabrication of protein-clay films for electrochemical catalysis [83-90], These studies indicate that layered materials can serve as efficient support matrices to maintain the native structure and function of the immobilized biomolecules. Current trends in the synthesis of functional biopolymer nano composites based on layered materials (specifically layered double hydroxides) have been discussed in excellent reviews by Ruiz-Hitzky [5] and Duan [6] herein we focus specifically on the fabrication of bio-inorganic lamellar nanocomposites based on the exfoliation and ordered restacking of aminopropyl-functionalized magnesium phyllosilicate (AMP) in the presence of various biomolecules [91]. 

This technique involves the dispersion of a nanomaterial in a monomer (Fig. 4.8). This step requires a certain amount of time that depends on the polarity of the monomer molecules, the surface treatment of the nanomaterial, and the swelling temperature. For thermoplastics, the polymerization can be initiated either by the addition of an agent or by an increase in temperature. For thermosets such as epoxies or unsaturated polyesters, a curing agent or peroxide can be added in order to initiate the polymerization. Functionalized nanomaterials can improve their initial dispersion in the monomer and consequently in the composites. In the case of layered materials, such as clays or graphene, the most important step is the penetration of the monomer between the sheets, thus allowing the polymer chains to exfoliate the material. The... 

LDH materials are not readily exfoliated in contrast to other layered materials such as smectite clays or MS2 type-chalcogenides (M0S2, NbSe2, etc...) [15-17] the delamination of LDH sheets requires elaborate syntheses as discussed above in Sect. 2.1. 

Figure 9. Sketch of exfoliated layers of a clay-like material, with some of the chains constrained near the surfaces of the layers.

Layered compounds provide unique character for electron-transfer processes owing to their low dimensionality. Especially layered materials with ion-exchange and/or intercalation capabilities show behavior that is not seen in so-called bulk-type materials. Layered materials, which have been often used in studies of photoelectron transfer as well as photocatalysis, may be classified into two groups compounds in which the host layers work as an active component for the photoexcitation and electron-transfer reactions, and materials in which the layers are inert for electron-transfer processes. Examples of the former are layered titanates and niobates and of the latter are clays. In the latter case, photoactive materials are intercalated in the interlayer spaces. Recently, the exfoliation of various layered compounds has become possible and artificial assemblies consisting of these exfoliated sheets have been formed. Electron transfer in such assemblies is also an attractive subject in this field. 

Different hybrid materials formed by insertion of organic polymers into inorganic substrates have been prepared. Layered materials are well known as host matrices for the incorporation of a large variety of polymeric organic species. Generally, the inorganic part is finely dispersed or exfoliated within the polymer, but, alternatively, the polymer can form laminae intercalated in laminar solids. This is the case for layered double hydroxides (LDHs, see Chapter 6), whose structure is closely related to that of the brucite, Mg(OH), in which the partial substitution of some of the divalent... 

By using an exfoliation and flocculation technique, Prasad et al. were able to prepare a polyaniline/NbWOe hybrid material [74]. Dispersed nanosheets of HNbWOe were obtained by treating HNbWOe with tetrabutyl ammonium hydroxide. The suspension of the exfoliated layers was then added to an alcoholic solution of aniline, followed by addition of a few drops of 1M HCl, and overnight sonication in order to induce flocculation of the aniline-NbWOe nanocomposite. The nanocomposite was isolated by centrifugation, and dried. Treatment of the nanocomposite with O2 at 130 °C for several weeks resulted in the formation of intercalated PANI. 

Nicolosi V, ChhowallaM, Kanatzidis MG, StranoMS, Coleman IN (2013) Liquid exfoliation of layered materials. Science 340 1226419. doi 10.1126/science.l226419... 

In addition to the rapidly expanding field of exfoliated clay-polymer nanocomposites (see Nanocomposites, Polymer-Clay), direct intercalation of macromolecules into inorganic layered materials with retention of the layered nature is also an excellent way of constructing nanocomposites with original properties. The synthetic pathways have to overcome large entropic barriers. 

The process by which layers of a multi-layered material are separated from one another is called exfoliation. 

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