Fine and well-dispersed clay was prepared via the in situ conversion of the dodecylamine intercalant inside the clay gallery to dodecylamino dimethylene diphosphonic acid (DDD), using a Mannich reaction, so as to create a repulsive force that delaminated the clay platelets. The clay structure and morphology were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) analysis, which revealed changes in the multilayer stacks. XRD analysis showed that the interlayer spacing was largely expanded by the presence of DDD. SEM and TEM images revealed that DDD containing clay (PMMT), but not that without DDD, exhibited transparency, indicating the extremely fine and well-dispersed clay. Polypropylene/PMMT nanocomposites containing 2, 4, 6, 8, and 10 wt % PMMT were prepared by melt extrusion. The obtained compounds were each spun into a monofilament fiber using a small scale spinning machine and then characterized by XRD, Differential scanning calorimetry (DSC), and Thermogravimetric analysis (TGA), plus the sonic modulus was evaluated. The XRD results revealed an increase in the beta crystallinity peak in fibers loaded with 2 10 wt % PMMT, indicating that PMMT particles were capable of acting as a beta-form nucleating agent. However, only minimal changes in the thermal behavior (T-c) were observed due to the tested samples containing insufficient PMMT content. The mechanical properties, in terms of the tensile strength and sonic modulus value (E), of the polypropylene nanocomposite fibers were higher than those of virgin PP fibers, presumably due to the reinforcement effect of the filled PMMT nanoparticles. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1969-1977, 2010