Polymer-clay nanocomposites Direct intercalation

Abstract The development of polymer-clay nanocomposite materials, in which nano-meter-thick layers of day are dispersed in polymers, was first achieved about 15 years ago. Since then, the materials have gradually become more widely used in applications such as automotive production. The first practical nylon-clay nanocomposite was synthesized by a monomer intercalation technique however, the production process has been further developed and a compound technique is currently widely used. A polyolefin nanocomposite has been produced by the compound method and is now in practical use at small volume levels. In this review, which focuses on njdon- and polyolefin-nanocomposites, detailed explanations of production methods and material properties are described. This article contains mainly the authors work, but aims to provide the reader with a comprehensive review that covers the works of other laboratories too. Lastly, the challenges and directions for future studies are included. 

As presented schematically in Figure 16, a second way to prepare polymer-clay nanocomposites via in situ polymerization consists of intercalation of the monomer (or a precursor of the monoma-) in the form of a cation and then later addition of an initiator to induce/polymaization. Thus, the direct exchange of the interlayer cations of smectites by anilinium cations, followed by oxidation with (NH4)2S20g, could be an alternative procedure to reach the formation of PANI/clay nanocomposites. In this case, the expaimental conditions allow the direct formation of PANl as a conducting ema-aldine salt (152). 

Following the appropriate pathway, in situ polymerization has allowed the preparation of a large variety of polymer-clay nanocomposites with interesting functional and/or mechanical properties. For instance, a doped PPy-synthetic hec-torite nanocomposite exhibits conductivity from about 10 to 10 S/cm (155). In gena-al, monomers showing affinity to be adsorbed by smectites, e.g., hydrophilic species, can produce infracrystalline homocondensations. Other monomers, such as acrylonitrile, are also easily intercalated in smectites, because such molecules are directly associated to the interlayer cations M" (M" = Li +, Na, etc.) through —C N—M"+ ion-dipole interactions (156-158). The action of "Y-irradiation (156) or thermal (158) treatments can induce the polymerization... 

Intercalated polymer-clay nanocomposites have been synthesized by direct polymer intercalation (4-6), and in situ intercalative polymerization of monomers in the clay galleries (7, 8). Owing to the spatial confinement of the polymer between the dense clay layers, intercalated polymer-clay nanocomposites can exhibit impressive conductiviw (6) and barrier properties (7). The exfoliation of smectite clays provides 10 A-thick silicate layers with high in - plane bond strength and asjJect ratios comparable to those found for fiber reinforced polymer composites. 

Nanometer-scale composites prepared from layered inorganic materials, especially clay, and polymers have also attracted much attention because of their unique optical, thermal, mechanical, gas barrier, and electrical properties. There are many reports describing polymer-clay nanocomposites. " The clay can be, for example, a sihca or silicate. In such a hybrid composite, weak dipolar and van der Waals forces provide the driving force for interactions between the layers, and they result in galleries being formed. There are three types of clay-polymer composites conventional, intercalated, and exfohated. Three mediods are widely used for the preparation of polymer-clay hybrid nanocomposites intercalation by in situ polymerization, direct intercalation, and polymer melt intercalation. Each of these methods has its advantages and disadvantages. For example, the in situ polymerization works only in tiie... 

Steed JW, Atwood JL (2000) Supramolecular chemistry. Wiley, New York Tomasik P (1985) Pyridine-metal complexes (Newkome GR, Strekowski L). Wiley, New York Usuki A, Kojima Y, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, Kamigaito O (1993) Synthesis of nylon 6-clay hybrid. J Mater Res 8 1179 Vaia RA, Giaimelis EP (2001) Liquid crystal polymer nanocomposites direct intercalation of thermotropic liquid crystalline polymers into layered silicates. Polymer 42 1281 Villemore G (1990) Effect of negative surface-charge densities of smeetite clays on the adsorption isotherms of racemic and enantiomeric tris (2,2 -bipyridyl) ruthenium(ii) chloride. Clays Clay Miner 38 622... 

An alternative to in situ polymerization involves direct intercalation of macromolecules into layered structures. Silicates are most often used. The insertion of polymer molecules into layered host lattices is of interest from different points of view. First, this insertion process leads to the construction of organic-inorganic polylayered composites. Second, the intercalation physical chemistry by itself and the role intercalation plays in the gain of electronic conductivity are of interest. This becomes important in the construction of reversible electrodes " or when improving the physicomechanical properties of nylon-layered silicate nanocomposites, hybrid epoxide clay composites," and nanomaterials based on hectorite and polyaniline, polythiophene or polypyrrole. ... 

The nylon-clay nanocomposites were prepared by in situ polymerization in the presence of organically modified, with aminolauric acid, montmorillonite. The reaction between nylon monomer and modified montmorillonite rendered nylon chains end-tethered though aminolauric acid to the silicate surface leading to exfoliated silicates (61). However, not all polymer nanocomposite systems could be produced via in situ polymerization processes because of the chemical sensitivity of polymerization catalysts. Direct melt blending of hydrophilic polymers with montmorillonite in its pristine state or polymers with surfactant-intercalated montmorillonite was found to be possible to deliver polymer intercalated or exfoliated nanocomposites (62,63). 

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. 

This system does not increase the carbon monoxide or soot produced during the combustion, as many commercial FRs do [233]. Other polymer silicate nanocomposites based on a variety of polymers, such as polystyrene, epoxy and polyesters, have been prepared recently by melt intercalation [236]. A direct synthesis of PVA-clay (hectorite) complexes in water solution (hydrothermal crystallization) was reported [237]. It was assumed that the driving force of this phenomenon, at least kinetically, can be described in terms of a simple diffusion reaction of polymers/monomers into clay-layered structures. 

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