The morphology and mechanical and visco-elastic properties of rubbery epoxy/organoclay montmorillonite (MMT) nanocomposites were investigated with wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM), tensile testing, and dynamic mechanical thermal analysis. An ultrasonicator was used to apply external shearing forces to disperse the silicate clay layers in the epoxy matrix. The first step of the nanocomposite preparation consisted of swelling MMT in a curing agent, that is, an aliphatic diamine based on a polyoxypropylene backbone with a low viscosity for better diffusion into the intragalleries. Then, the epoxy prepolymer was added to the mixture. Better dispersion and intercalation of the nanoclay in the matrix were expected. The organic modification of MMT with octadecy-lammonium ions led to an increase in the initial d-spacing (the [d(001)] peak) from 14.4 to 28.5 A, as determined by WAXS; this indicated the occurrence of an intercalation. The addition of 5 phr MMTC18 (MMT after the modification) to the epoxy matrix resulted in a finer dispersion, as evidenced by the disappearance of the diffraction peak in the WAXS pattern and TEM images. The mechanical and viscoelastic properties were improved for both MMT and MMTC18 nanocomposites, but they were more pronounced for the modified ones. (c) 2006 Wiley Periodicals, Inc.