Epoxy-layered silicate nanocomposites and their gas permeation properties

Epoxy-OM (organo-montmorillonite) nanocomposites have been synthesized, and their permeability to oxygen and water vapor has been measured. The chemical structure of the organic monolayer ionically bonded to the montmorillonite surface has been varied, and its influence on the swelling, intercalation, and exfoliation behavior of the OM has been studied. Exfoliated aluminosilicate layers build a barrier for the permeating gas molecules, while the polymer intercalated tactoids do not contribute much to the permeation barrier performance. The gas permeation through the composites was correlated to the volume fraction of the impermeable inorganic part of the OM. The incorporation of small volume fractions of the platelike nanoparticles in the polymer matrix decreased its permeability coefficient when the interface between the two heterogeneous phases was properly designed. Long alkyl chains enhanced the polymer intercalation but increased the permeability coefficient probably due to phase separation at the interface between the polymer and the inclusions. Matching the surface energy of the OM with that of the matrix as well as tethering polymer molecules to the silicate layers surface enhanced the exfoliation and decreased the permeation coefficient. The exfoliation process is governed by interplay of entropic and energetic factors. A macroscopic volume average of the aspect ratio of montmorillonite platelets was deduced from the relative permeability of the nanocomposites by comparing the measured values to numerical predictions of gas permeation through composites of misaligned disk-shaped inclusions. The permeability coefficient of the epoxy matrix was reduced to one-fourth at 5 vol % Bz1OH loading, and the reduction was attributed to the tortuous pathway the gas molecules have to cover during their random walk to penetrate the composite. The transmission rate of water vapor through the composites is more influenced by the permeant-composite interactions and hence the hydrophobicity of the monolayer covering the inclusions surface. At 5 vol % BzC16 loading, the relative vapor transmission rate was reduced to half.