Structure and properties of epoxy-based layered silicate nanocomposites

Epoxy-based layered silicate nanocomposites of various compositions were synthesized and their physical and water vapor diffusion properties studied. Mixtures of an epoxide monomer, diglycidyl ether of bisphenol A, with nonmodified sodium montmorillonite (NaMMT) or organophilically modified montmorillonites were prepared and cross-polarized optical microscopy, oscillatory shear rheological measurements, transmission electron microscopy (TEM), and X-ray diffraction (XRD) studies were used to assess epoxy intercalation in the intragallery regions of the layered silicates. The more hydrophilically modified organoclays showed a higher degree of compatibility with the epoxide monomer and a higher degree of intercalation and delamination of the silicate nanolayers, when compared to the mixtures containing NaMMT and highly hydrophobically modified organoclays. The epoxy-clay mixtures were cured by the addition of a stoichiometric amount of an aliphatic polyoxypropylene diamine at 120 degrees C. The curing reaction appeared to be accelerated by the organoclay containing hydroxyethyl groups of the alkylammonium ions located in the silicate galleries. The XRD and transmission electron microscopy studies revealed a coexistence of partly intercalated and exfoliated silicate layers for the epoxy-based nanocomposites containing the more hydrophilically modified organoclays. An intercalated nanocomposite and a more conventional composite system were obtained when using a highly hydrophobic organoclay and NaMMT, respectively, as the clay filler particles. The nanocomposites exhibited a slightly lower glass-transition temperature than the neat cured epoxy resin, the decrease being more pronounced for the partly exfoliated layered silicate nanocomposites. All the hybrid epoxy-based clay nanocomposites exhibited barrier properties superior to that of the neat cured epoxy resin and conventional NaMMT filled composites. The more exfoliated nanocomposites were more effective in reducing the water vapor diffusion coefficient. A 50% reduction in the water vapor diffusion coefficient was observed for the partly exfoliated nanocomposite containing 20 wt% of organoclay.