Layer silicates, mica-type

Weiss A (1963) Mica-type layer silicates with alkylammonium ions. Clays Clay Miner 10 191-224... 

PDMS nanocomposites with layered mica-type silicates were also reported.374 A two-step sol-gel process of the in situ precipitation of silica led to the development of siloxane-based nanocomposites with particularly high transparencies.3 5 Some unusual nanocomposites prepared by threading polymer chains through zeolites, mesoporous silica, or silica nanotubes were reviewed.3 6 Poly(4-vinylpyridine) nanocross-linked by octa(propylglycidyl ether) polyhedral oligomeric silsesquioxane was reported.377... 

A. Weiss, Organic derivatives of mica-types layer silicates, Angew. Chem. Internat. Edit, vol. 2, pp. 134-143,1963. 

Rates of exchange on vermiculite and micaceous minerals, particularly involving ions such as K+, NH, and Cs+, are usually quite slow. These are 2 1 clay minerals with peripheral spaces that impede many ion exchange reactions. Micaceous minerals typically have a more restrictive interlayer space than vermiculite, since the area between layer silicates of micas is selective for certain types of cations such as K+, Cs+, NH4, and H30+ (Sparks and Huang, 1985 Sparks, 1987a). 

In the case of mica-type layered silicates it has been recently demonstrated that nanocomposites (both intercalated and delaminated) can be synthesized by direct melt intercalation even with high molecular weight polymers [7-18]. This synthetic method is quite general and is broadly applicable to a range of commodity polymers from essentially non-polar polystyrene, to weakly polar polyethylene terephthalate), to strongly polar nylon. Nanocomposites can, therefore, be processed using currently available techniques such as extrusion, thus lowering the barrier towards commercialization. 

Table 1. Structure and chemistry of Mica-type layered silicates...

Intercalation of electroactive polymers such as polyaniline and polypyrrole in mica-type layered silicates leads to metal-insulator nanocomposites. The conductivity of these nanocomposites in the form of films is highly anisotropic, with the in-plane conductivity 10 to 10 times higher than the conductivity in the direction perpendicular to the film. Conductive polymer/oxide bronze nanocomposites have been prepared by intercalating polythiophene in V2O5 layered phase, which is analogous to clays. °° Studies of these composites are expected not only to provide a fundamental understanding of the conduction mechanism in the polymers, but also to lead to diverse electrical and optical properties. 

The immobilization of metal complex catalysts on polymers and inorganic oxides has received considerable attention as a means of combining the best advantages of homogeneous and hetereo-geneous catalysis (1-6). The swelling layer lattice silicates known as smectite clay minerals have added an important new dimension to metal complex Immobilization. These compounds have mica-type structures in which two-dimensional silicate sheets are separated by monolayers of alkali metal or alkaline earth cations (7). The structure of a typical smectite, hectorite, is illustrated in Figure 1. 

Layer silicates, sheet-like phyllosilicates such as the familiar micas, are in primary rocks and in soils. The soil minerals are often called clay minerals. Since other components can also be in the clay fraction, layer silicates is a mom accurate term. A typical layer silicate is a combination of a layer of Al-, Mg-, or Fe(II)-0 octahedra plus one or two layers of Si-0 tetrahedra. The tetrahedral and octahedral sheets bond together by sharing oxygens at the corners of the tetrahedra and octahedra. Layer silicate minerals are differentiated by (1) the number and sequence of tetrahedral and octahedral sheets, (2) the layer charge per unit cell, (3) the type of interlayer bond... 

Micas are layer silicates (phyllosilicates) whose structure is based either on a brucite-like trioctahedral sheet [Mg(OH)2 which in micas becomes Mg304(0H)2] or a gibbsite-like dioctahedral sheet [Al(OH)3 which in micas becomes Al204(0H)2]. This module is sandwiched between a pair of oppositely oriented tetrahedral sheets. The latter sheet consists of Si(Al)-tetrahedra which share three of their four oxygen apices to form a two-dimensional hexagonal net (Fig. 1). In micas, the association of these two types of sheet produces an M layer, which is often referred as the 2 1 or TOT layer. 

V. Mehrotra and E.P. Giannelis, Metal-insulator molecular multilayers of electroactive polymers intercalation of polyaniline in mica-type layered silicates. Solid State Commun., 11, 155-158 (1991). 

During the ablation experiment, temperature within the char layer exceeds 1000°C and approach 2000-2500°C at the surface. At these temperatures, any carbonaceous residue from the pol3oner will contain graphite. Additionally, mica-type layered silicates, such as montmorillonite, irreversibly transform into other aluminosilicate phases. Between 600 and 1000 C, montmorillonite dehydroxylates and has been observed to initially transform into spinel, cristobolite, mullite and/or pyroxenes (enstatite) (24). At temperatures greater than 1300 C, mullite, cristobolite and cordierite form and subsequently melt at temperatures in excess of 1500 C (mullite 1850 C, pure cristobolite 1728°C and cordierite --ISSO C) (25). The presence of an inorganic that transforms into a high viscosity melt on the surface of the char will improve ablation resistance by flowing to self-heal surface flaws. This is known to occur in silica-filled ablatives (26). 

Weiss, A. 1963. Organic derivatives of mica-type layer silicates. Angewandte Chemie International Edition 2 134-144. 

Now the cause for the curving of the phlogopite-type mica crystals in the form of dreier layer silicates must be addressed. It is unclear as to what extent the structural change continues into the next layer or the layer after that. These issues cannot be fully answered in crystallochemical terms at this time. Important findings, however, have been made. They are presented here. 

Layered silicates such as talc, mica, and smectic clays consist of regularly stacked layers, each having an average thickness of about 1 nm and a length of about 50 to 1000 nm. These types of layered silicates are classified as 2 1, as the individual layer is made up of three sheets—two on the outside and one in between. The outer sheets are made of interconnected Si04 tetrahedrals, whereas the inner one has an octahedral configuration. It contains metal cations, such as AP+ and Mg +, and hydroxyl groups. 

If the oxygen supply is even more restricted so that the O/Si ratio becomes less than 2.75, a more extensive coordination of SiO tetrahedra has to occur. If band is laid to band in one plane, a layer ion with infinite extent is formed (Figure 40.8). The gross formula will be (Si Oj ). Layers of this type are present in mica and clay minerals. Hydroxides of different types, according to Table 40.4, also crystallize as layers and combine with the silicate ion (Si Oj) to double and triple layers in many minerals. 

---