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Nitrile rubber/clay nanocomposites were prepared via in-situ emulsifier-free emulsion polymerization technique in the presence of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and pristine sodium montmorillonite (Na-MMT). The morphology of the nanocomposites was studied using X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM). The XRD results showed exfoliated morphology for the nanocomposites containing up to 3 wt% nanoclay and exfoliated-intercalated morphology for the nanocomposite containing 5 wt% nanoclay. The basal spacing of the nanocomposite, containing 5 wt% nanoclay, was increased to 1.8 nm, which was 0.61 nm wider than that obtained using pristine Na-MMT. Compared to neat rubber, the thermogravimetric analysis (TGA) revealed improvements in the thermal stability of all nanocomposite samples, in which the thermal degradation temperature was increased by increasing the clay content. The maximum increase in the thermal stability of the nanocomposites was obtained for the nanocomposite containing 5 wt% nanoclay with exfoliated-intercalated morphology. The tensile testing results showed remarkable improvements in the mechanical strength of the nanocomposites. In comparison with the neat nitrile rubber, the exfoliated nanocomposite, containing 3 wt% nanoclay, showed 302% and 219% increases in tensile modulus and tensile strength, respectively. The reason for improvementin mechanical properties of a nanocomposite, containing 3 wt% of nanoclay with respect to the nanocomposite containing 5 wt% of nanoclay, was related to the better dispersion of nanoclay platelets in the matrix and formation of exfoliated morphology. In addition, elongation-at-break and hardness were increased in the  nanocomposites, when compared with the neat nitrile rubber.