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The reactions of chemically modified smectite clays and organic molecules typically afford intercalation compounds with well-defined compositions and basal spacings.1 More recent studies of clay-organic reactions have led to the formation of true clay-polymer hybrid composites in which engineering polymers are encapsulated between smectite clay particles approximately 1 nm in thickness.2-7 Semial work by Toyota researchers has led to the discovery of several nanoscale polymer-clay hybrid composites, including systems based on the dispersion of alkylammonium-exchanged forms of montmorillonite in semicrystalline nylon-6,8-10 amorphous epoxide,11 and polyimide12,13 polymers.

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Epoxy-clay nanocomposites have been synthesized by exfoliation of organoclays during the epoxy thermosetting process. The extent of silicate layer separation is governed by the chain length of gallery cations, the clay layer charge density and the acidity of the gallery cations. By using different curing agents, we obtained both glassy and rubbery epoxy matrices. Reinforcement was substantially greater for the rubbery matrix. Both the tensile strength and the modulus of the rubbery epoxy-clay nanocomposite increased with increasing clay content. The reinforcement provided by the silicate layers at 15 wt% loading was manifested by a more than ten-fold improvement in tensile strength. The rubbery state of the matrix may allow alignment of the exfoliated silicate layers upon applying strain, thereby enhancing reinforcement.The properties of hybrid organic-inorganic composite materials are greatly influenced by the length scale of component phases. Nanoscale dispersion of the inorganic component typically optimizes the mechanical properties of the composite. One successful approach to enhancing inorganic particle dispersion is the in situ polymerization of metal alkoxides in organic polymer matrices via the sol-gel process (1, 2). In these systems the inorganic components are formed by the hydrolysis and condensation of mononuclear precursors, such as tetraethoxysilane (TEOS).Inorganic materials that can be broken down into their nanoscale building blocks can be superior alternatives to the sol-gel process for the preparation of nanostructured hybrid organic-inorganic composites (3). For instance, the exfoliation of the 10 Â-thick layers of smectite clays in polymer matrices is one such promising route to nanocomposite formation. Ion exchange of the Na + or Ca 2+ gallery cations in the pristine mineral by organic cations allows modification