Conventional phase segregated composites

In a study of the application of Pd-Si amorphous alloys as selective hydrogenation catalysts [3] it was found that in situ activation provides a route to active and selective catalysts, whereas ex situ activation caused the crystallization of the system into the thermodynamically stable Pd + SiC>2 system, which is indistinguishable in its activity and poor selectivity from conventional catalysts of the same composition. In this study it was possible to show conclusively that all amorphous alloys are not amorphous on their surfaces as they undergo, in reaction gas atmospheres, chemically-induced phase segregation which starts the crystallization process according to Figure 2 (pathway 2). 

In general, the dispersion of clay particles in a polymer matrix can result in the formation of three general types of composite materials (Figure 1). Conventional composites contain clay tactoids with the layers aggregated in unintercalated face - face form. The clay tactoids are simply dispersed as a segregated phase. Intercalated clay composites are intercalation compounds of definite structure formed by the insertion of one or more molecular layers of polymer into the clay host galleries and the properties usually resemble those of the ceramic host. In contrast, exfoliated polymer-clay nanocomposites have a low clay content, a monolithic structure, a separation between layers that depends on the polymer content of the composite, and properties that reflect those of the nano-confmed polymer. 

Figure 11.1. The first type is conventional composites, where non-swollen clay tactoids with layers are dispersed simply as a segregated phase in a polymer matrix, which results in poor mechanical properties of the composite material. The second type is intercalated polymer-clay nanocomposites, which are well-ordered multilayered structures formed by the insertion of polymer chains into the gallery space between parallel individual clay layers. Although the layer spacing of clay increases, there are still attractive forces between the silicate layers to stack the layers with uniform spacing. The last type is exfoliated polymer-clay nanocomposites, in which the clays are well dispersed in the polymer matrix. Compared with the intercalated polymer-clay nanocomposites, the layer spacing in the exfoliated polymer-clay nanocomposites increases to the point that there are no longer sufficient attractions between the silicate layers to maintain a uniform layer spacing. The last two hybrid structures are nanocomposites and the exfoliated polymer-clay nanocomposites are especially desirable for improved properties because of the homogeneous dispersion of clay and huge interfacial area between the polymer and clay.

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