Clay-polymer affinity

Generally, polymer nanocomposites can be obtained through two routes the first one is the polymerization of monomers in contact with the exfoliated clay and the second one uses existing transformation processes to produce nanocomposites, for example, by a reactive extrusion. There are, however, problems present due to the lack of affinity of the clay-polymer system because of the hydrophilic character of the particles. It is then necessary to treat the clay chemically to increase its affinity with the polymer matrix. This constitutes another whole area of research in the nanocomposites production. 

Because of the higher affinity for higher-valence cations by the interlayer siloxane cavity, relatively low pH values allow the interlayer to become loaded with Al-hydroxy polymers. Complete occupation of the interlayer spacing by octahedrally coordinated aluminum-hydroxy polymers produces chlorotized vermiculite or montmorillonite (Fig. 4.10). Such minerals are referred to as 2 2 clay minerals. Under these conditions, the interlayer space is limited to approximately 4 A (0.04 nm) and A1 is nearly permanently lodged in the interlayer, diminishing mineral CEC. However, in most cases chlorotization is partial. Thus, Al-hydroxy islands are formed (Fig. 4.11). In such cases, the interlayer gap is also held at approximately 4 A and many interlayer... 

PEO/OMMT mixture. Based on Flory-Huggins interaction parameters between polymers and silicate layers in a PEO/PMMA/OMMT mixture, the PMMA molecules have a larger affinity for the hydrophobic surfaces of OMMT than hydrophilic PEO. The larger layer thickness and broadness of the peak in the PMMA/ OMMT mixture are indicative of an improved intercalation for PMMA matrix over the OMMT surfaces. The relatively weak interaction between PEO and OMMT is because of the hydrophobic modification of the clay surface. 

Figure 6.5 shows various functional groups which may be detected on silica, talc, and clay surfaces. The surface character of carbon black differs in that it is mostly nonpolar whereas the surface of silica is polar. Thus carbon black is more compatible with hydrocarbon polymers which are also nonpolar. Silica and other similar fillers (talc, clay) have more affinity to each other than to nonpolar polymers. This is a major factor in the inferior performance in rubber applications where interfacial adhesion is reduced. 

Hydrophilic polymers such as polyvinyl alcohol (PVA) exhibit high affinity for association with sepiolite and palygorskite via interaction with the silanol groups at the external surface of the fibrous clays. Thus, a simple mixture of both components... 

In spite of the remarkable developments achieved in the preparation techniques of nanocomposites, there are stUl some problems associated with the Uquid processes involved in nanocomposite preparation, such as the requirement of large amounts of aqueous/organic solvents, the aggregation of clay due to its insufficient affinity with monomers, and the lower level of homogeneous dispersibiUty in matrix polymers (34, 43—45]. The GASP method may provide a solution to these problems, because of its solventless nature and the superior diffusing abiUty of gaseous monomers. 

Following the appropriate pathway, in situ polymerization has allowed the preparation of a large variety of polymer-clay nanocomposites with interesting functional and/or mechanical properties. For instance, a doped PPy-synthetic hec-torite nanocomposite exhibits conductivity from about 10 to 10 S/cm (155). In gena-al, monomers showing affinity to be adsorbed by smectites, e.g., hydrophilic species, can produce infracrystalline homocondensations. Other monomers, such as acrylonitrile, are also easily intercalated in smectites, because such molecules are directly associated to the interlayer cations M" (M" = Li +, Na, etc.) through —C N—M"+ ion-dipole interactions (156-158). The action of "Y-irradiation (156) or thermal (158) treatments can induce the polymerization... 

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