MECHANISM OF CLAY TACTOID EXFOLIATION IN EPOXY-CLAY NANOCOMPOSITES

Monolithic epoxy exfoliated-clay nanocomposites have been prepared by the reaction of alkylammonium-exchanged smectite clays with diglycidyl ether of bisphenol A and m-phenylenediamine as the curing agent, Exfo liation of the clay tactoids in the polymer matrix is dependent on the accessibility of the epoxy and diamine monomers to the clay galleries and the relative rates of intra- and extragallery network formation. Initial epoxy access to the gallery surfaces is provided by solvation interactions with the alkyl chains on the onium ions. Regardless of th e clay layer charge density, the onium ions adopt a vertical orientati on in the galleries in order to optimize interactions with the epoxide resin. Under amine curing conditions where the intra- and extragaller y polymerization rates are comparable, the clay tactoids are exfoliate d into segregated 10-Angstrom-thick layers, Exfoliation is facilitated by acidic exchange cations that can catalyze the intragallery epoxide polymerization process. Nonacidic quaternary ammonium ions tend to fo rm intercalated rather than exfoliated clay nanocomposites, Access to the gallery region of the clay under polymerization conditions is dete rmined by the population density of the gallery onium ions. In general , low charge density clays (hectorite and montmorillonite) are more re adily accessible for intragallery polymerization than high charge dens ity analogues (fluorohectorite and vermiculite), Thus, low charge dens ity clays typically provide nanocomposites with a high degree of layer exfoliation, as judged by XRD. Preliminary mechanical measurements sh ow that exfoliated epoxy-clay nanocomposites have higher moduli than i ntercalated clay composites.