A study on nanocomposites of epoxy-clay and epoxy-clay-thermoplastic polymer systems

This study investigated three interrelated issues in the synthesis of nanocomposites of epoxy-clay and epoxy-clay-thermoplastic polymer systems. First, the mechanism of nanoclay exfoliation was investigated using rheological tools. It was observed that the elastic force exerted by crosslinked epoxy molecules inside the clay galleries is responsible for exfoliation of clay layers from the epoxy-intercalated tactoids. It was found that the ratio G '/eta* plays an important role in determining whether exfoliated or intercalated nanoclay structures can be obtained in epoxy-nanoclay systems. In this case, G' and eta* are the storage modulus and complex shear viscosity of crosslinking epoxy inside and outside the clay galleries respectively. Second, the possible plasticization effects of quaternary ammonium ions and excess primary amines on the values of the G' /eta* ratio was investigated. The long hydrocarbon chains of quaternary ammonium ions acted as plasticizers in the epoxy networks and lowered the value of storage modulus. These adversely affected the prospect of exfoliation due to reduction of the values of G'/eta*. In addition, primary amines were generated from dissociation of the quaternary ammonium chloride, which readily reacted with epoxy at a much lower temperature than the diamine curing agent. Consequently, a stoichiometric imbalance, whereby excess curing agents were found, plasticized the epoxy networks. This effect was more apparent in the DDS-cured system due to its higher curing temperature.; Third, epoxy-aided dispersion of nanoclay particles in a glassy polymer, polymethylmethacrylate (PMMA), was studied using melt-blending technique. The mechanical properties were evaluated and compared with blends of PMMA and nanoclay, epoxy and nanoclay, and PMMA and epoxy as functions of the size of nano- and micro-dispersed phases. The clay particles were in a fully exfoliated state in the three-phase composites provided the ratio of epoxy to clay was greater than 10. However, a widespread dispersion of individual platelets of the nanoclay was not achieved as exfoliated clay particles remained as aggregates inside phase-separated epoxy domains of approximately 1 mum in diameter. Nevertheless, the values of the tensile and impact strengths showed significant improvement over microdispersed PMMA-clay systems.