Structure and properties of layered silicates

Montmorillonite, hectorite, and saponite are the most commonly used layered silicates for the preparation of nanocomposites. Layered silicates have two types of structure tetrahedral-substituted and octahedral-substituted. In the case of tetrahedrally-substituted layered silieates the negative charge is located on the surface of silicate layers, and henee, the polymer malrices can interact more readily with these than with oetahedrally-substituted material. 

Two particular characteristics of layered silicates that are generally considered for PLS-nanocomposites are the ability of the silicate particles to disperse into individual layers, and the abiUty to fine-tune their surface chemistry through ion exchange reactions with organic and inorganic cations. 

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ILs INTERCALANTS FOR LAYERED SILICATES 24.2.1 Structure and Properties of Layered Silicates... 

The structure and properties of layered silicate polypropylene nanocomposites... 

The nature of the changes in adsorption and separation properties of layer silicates with the expanding structural cell, caused by the modification, depends on the size of modifying organic cation. 

In this article, recent developments in the formation and properties of epoxy layered silicate nanocomposites are reviewed. The effect of processing conditions on cure chemistry and morphology is examined, and their relationship to a broad range of material properties elucidated. An understanding of the intercalation mechanism and subsequent influences on nanocomposite formation is emphasized. Recent work involving the structure and properties of ternary, thermosetting nanocomposite systems which incorporate resin, layered silicates and an additional phase (fibre, thermoplastic or rubber) are also discussed, and future research directions in this highly active area are canvassed. 

Computations have been carried out to better understand the structures and properties of these types of nanocomposites. One subject of particular interest is the observed enhanced gas transport performance of PDMS nanocomposite membranes containing layered silicates, which is not expected based on studies of magnetically alligned particles disuc-ssed in the next section. Clays have also been used to improve the properties of silsesquioxane polymers. ... 

As a consequence, the goal of our study was to prepare layered silicate nanocomposites from montmo-rillonites organophilized not only with an co-amino acid, but also with alkylammonium salts and protonated amines [27, 28, 29, 30, 31]. The use of the two dissimilar modifiers was expected to lead to significant differences in the interaction of the clay and the polymer matrix. The effect of interfacial adhesion on exfoliation and on the structure and properties of the nanocomposites prepared were determined and are discussed in this paper. Less attention is paid to changes in the crystalline morphology of the PA matrix. 

Abstract This chapter briefely overviews the polymer-layered silicate nanocomposites, including the structure and properties of the layered silicates and their modifications. It also addresses the developments in synthesizing polyolefin/clay nanocomposites by olefin polymerization under catalysis of clay intercalated precatalysts. It mainly focuses on the synthetic routes, structural characterization, and properties of polyolefin/clay nanocomposites. 

In a 1975 Japanese patent application, Fujiwara and Sakamoto [71 ] of Unitika adsorbed various lactams onto montmorillonite and found swelling of the clay silicate layers and spontaneous polymerization. Similar but more extensive studies were subsequently published and patented byKojima,Okada,Usuki, and their coworkers [72 to 77] from Toyota from the late 1980s. They then examined the structure and properties of these compoimds. In these compounds, the clay is dispersed as individual exfoliated silicate layers. Various other silicate minerals were studied [76], but montmorillonite compounds were found to be superior in mechanical properties. Similar polymerizations were carried out for a polyamic acid to produce a polyimide [77]. Mechanical property enhancements of exfoliated montmoriUonite/polyamide-6 [78,79] are shown in Fig. 7.3. 

The rheological properties of insitu polymerized nanocomposites with end-tethered polymer chains were first described by Krisnamoorti and Giannelis [33]. The flow behavior of PCL- and Nylon 6-based nanocomposites differed extremely from that of the corresponding neat matrices, whereas the thermorheological properties of the nanocomposites were entirely determined by the behavior of the matrices [33]. The slope of G (co) and G"(co) versus flxco is much smaller than 2 and 1, respectively. Values of 2 and 1 are expected for linear mono-dispersed polymer melts, and the large deviation, especially in the presence of a very small amount of layered silicate loading, may be due to the formation of a network structure in the molten... 

The cation exchange of layer silicates significantly influences some structural and colloid chemical properties. Depending on the charge of the cation, the interlayer space contains water in different quantities (Chapter 2, Section 2.1.2). So, the basal spacing (the distance between similar faces of adjacent layers) is different for monovalent, bivalent, and trivalent cations. For example, in monovalent montmorillonite, it is about 1.2 nm, and in bivalent and trivalent montmorillonite, it is about 1.5—1.6 nm. 

The physical properties of the silicates correlate closely with their structures. Talc, Mg3(Si40io)(OH)2, is an example of an infinite layered structure (see Fig. 22.If). In talc, all of the bonding interactions among the atoms occur in a single layer. Layers of talc sheets are attracted to one another only by van der Waals interactions, which (being weak) permit one layer to slip easily across another. This accounts for the slippery feel of talc (called talcum powder). When all four vertices of each tetrahedron are linked to other tetrahedra, three-dimensional network structures such as cristobalite (see Fig. 22.Ig) or quartz (Fig. 22.2) result. Note that the quartz network carries no charge consequently, there are no cations in its structure. Three-dimensional network silicates such as quartz are much stiffer and harder than the linear and layered silicates, and they resist deformation well. 

Icy satellites have an icy crust and mantle and are composed of mixtures of ice and silicates. Stripe patterns showing layered structures with different sand contents have been discovered on the polar cap on Mars. There could be a wide range of temperature conditions from the melting point of ice to below 100 K in icy satellites and on Mars. Therefore, it is important to study the rheological properties of ice-silicate mixtures at wide temperatures to elucidate the tectonics of icy satellites and the flow dynamics of ice sheets on Mars. 

Petersson L, Kvien 1, Oksman K (2007) Structure and thermal properties of poly (lactic acid)/ cellulose whiskers nanocomposite materials. Compos Sci Technol 67 2535-2544 Petersson L, Mathew AP, Oksman K (2009) Dispersion and properties of cellulose nanowhiskers and layered silicates in cellulose acetate butyrate nanocomposites. J Appl Polym Sci 112 ... 

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