Exfoliated polyurethane/montmorillonite nanocomposites with both high elongation at break and high tensilestrength were studied by dynamic-infrared spectroscopy (dynamic-IR). The results show that crystallization induced byadditional stress is impeded by the nano-layered organo montmorillonite. Exfoliated polyurethane / montmorillonite nanocomposites with both high elongation at break and high tensile strength were studied by dynamic-infrared spectroscopy (dynamic-IR). The results show that crystallization induced by additional stress is impeded by the nano-layered organo montmorillonite.