Platelet, layer silicates

For layer silicates with low structural charge (i. . smectites), this expansion is limited to about four molecular layers of water if the exchangeable cation has a charge of +2. Since the silicate platelet is about 0.96 nm thick, the repeat spacing along the c-axis is then approximately 0.96 + (4x.26) = 2.0 nm. 

This fact may explain the superiority of montmorillonite over vermiculite as an adsorbent for organocations (3, 4). Complicating this description, however, is the fact that a sample of any particular layer silicate can have layer charge properties which vary widely from one platelet to another (j>). By measuring the c-axis spacings, cation exchange capacity, water retention, and other properties of layer silicates, one obtains the "average" behavior of the mineral surfaces. 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

MePOR species and other complexes in cationic clays can be located at the edges of packed platelets, in the interlamellar space or in the mesopores present (Scheme 10.9). A review of the early data in this area is available.[86] The flat metallo macrocycles under clay synthesis conditions help to induce layer silicate formation, the complexes being intercalated between the layers. Whereas with monooxygen atom donors, alkanes can be oxygenated with significantly enhanced activities compared with the homogeneous case, in every case the expected products (ol/on) were obtained. Competitive oxygenation of adamantane and pentane shows lower... 

The most common class of pearlescent pigments today is based on thin platelets of mica (see Fig. 15.5). Mica itself is a natural mineral and belongs to the sheet layer silicates. Nacreous pigments are usually based on natural, transparent muscovite and only in some cases on synthetic phlogopite. Muscovite occurs worldwide, but only few deposits are suitable for pigment production. Mica is biologically inert and approved for use as a filler and colorant. 

The dominant class of pearl luster pigments is based on platelets of natural mica coated with thin films of transparent metal oxides [5.122-5.125, 5.127-5.130, 5.137]. The mica substrate acts as a template for the synthesis and as a mechanical support for the deposited thin optical layers of the pearl luster pigments. Mica minerals are sheet layer silicates. Pearl luster pigments are usually based on transparent muscovite mica only some are based on synthetic phlogopite. Although muscovite occurs worldwide, few deposits are suitable for pigments. Natural mica is biologically inert and approved for use as a filler and colorant. 

The most commonly studied polymer nanocomposites are clay-based nanocomposites, mainly with montmorillonite (MMt) as layered silicate filler (Scheme 15.12). Upon incorporation of organomodihed clays (organoclays) into a polymer matrix, two nanomorphologies (Scheme 15.13) can be obtained, either intercalation of the polymer chain in between the clay platelets keeping the stacking of the sheets, or exfoliation of the clay platelets with a disordered dispersion of the inorganic sheets in the polymer. 

Colloid chemists commonly measure surface area by the adsorption of N2 gas. The adsorption is conducted in vacuum and at temperatures near the boiling point of liquid nitrogen (—196° C). The approach is based on the Brunauer-Emmett-Teller (BET) adsorption equation, and has been adapted to a commercially available instrument. Unfortunately, the technique does not give reliable values for expansible soil colloids such as vermiculite or montmorillonite. Nonpolar N2 molecules penetrate little of the interlayer regions between adjacent mineral platelets of expansible layer silicates where 80 to 90% of the total surface area is located. Several workers have used a similar approach with polar H2O vapor and have reported complete saturation of both internal (interlayer) and external surfaces. The approach, however, has not been popular as an experimental technique. 

One kind of nanometer-size reinforcement is the montmo-rillonite, which is a layered silicate whose interlayer ions can be exchanged by organ-ions in order to produce an increment in the interlayer spacing (dooi3 snd to improve the polymer/clay compatibility. These improvements allow the dispersion of clay platelets to be easier [49]. 

As illustrated in Fig. 1, layered silicate composite structures fall into three different classes (a) microcomposites with no interaction between the clay galleries and the polymer, (b) intercalated nanocomposites, where the silicate is well-dispersed in a polymer matrix with polymer chains inserted into the galleries between the parallel, sihcate platelets, and (c) exfohated nano composites with fully separated silicate platelets individually dispersed or delaminated within the polymer matrix [12]. However, these terms describe only ideal cases and most observed morphologies fall between the extremes. A more detailed nomenclature will be presented later in this review. 

Figure 10-1 shows the scanning electron micrographs of sample El (Li+) (upper) and sample E4 (NH4+) (lower). These layer silicates consist of small aggregates of platelets showing various morphology in relationship to the monovalent cation (interlayer material) used in the synthesis. These two samples are well-crystallized and virtually pure as shown by XRD. 

The shapes of the particles used in nanocomposites can be roughly spherical, fibrillar or platelets, and each shape will result in different properties. For maximum reinforcement, platelets or fibrillar particles would be used, since reinforcement efficiency is related to the aspect ratio (length/diameter. Lid). The most extensive research has been performed with layered silicates, which provide a platelet reinforcement [19]. 

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