AbstractThe effects of soft‐segment molecular weight and organic modification of montmorillonite (MMT) on thermal and mechanical properties of segmented polyurethane (PU) elastomers were investigated. The PU/MMT nanocomposites were prepared by in situ polymerization, and the compositions included soft segments with number average molecular weights of 1000, 2000, and 2900, and organic‐modified MMT (including MMT‐30B and MMT‐I30E). The nanocomposites produced were characterized using wide‐angle X‐ray diffraction (WAXD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and mechanical testing. The TEM and XRD results revealed that both MMT‐30B and MMT‐I30E were intercalated, and partially exfoliated by the PU. Mechanical tests showed that the PU1000 series in soft‐segment molecular weight yielded superior tensile properties compared with the PU2000 and PU2900 series. Also, for a given molecular weight of soft segment in PU, the MMT‐30B nanocomposites exhibited greater increases in Young's modulus, tensile strength, and elongation at break than the MMT‐I30E counterpart, and the crystallinity of PU was enhanced by the clays. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009