Polypropylene (PP)/clay nanocomposites were prepared by melt-compounding PP with organo-montmorillonite (OMT), using maleic anhydride grafted polypropylene (PF-g-MA) as the primary compatibilizer and N-imidazol-O-(bicyclo pentaerythritol phosphate)-O(ethyl methacrylate) phosphate (PEBI) as the cointercalating monomer. X-ray diffraction patterns indicated that the larger interlayer spacing of OMT in PP was, obtained due to the cointercalation monomer having a large steric volume and the d-spacing further increased with the addition of PP-g-MA, as evidenced by transmission electron microscopy. Thermogravimetric analysis revealed that the PEBI-containing PP nanocomposites exhibited better thermal stability than PEBI-free PP composites. Dynamic mechanical analysis demonstrated that the storage modulus was significantly enhanced, and the glass transition temperature (T-g) shifted slightly to ternperature with the incorporation of clay for PP/OMT hybrids. PEBI-containing PP/OMT composites gave a lower T-g value because of the strong internal plasticization effect of PEBI in the system. Cone calorimetry showed that the flame-retardancy properties of PP nanocomposites were highly improved with the incorporation of PEBI. (C) 2008 Wiley periodicals, Inc.