The effect of maleic anhydride grafted polypropylene (PP-g-MA) on the mechanical and rheological properties of polypropylene (PP)-clay nanocomposites prepared with nanoclay expanded with CO2 and direct melt blending was studied. The results from the studies of the mechanical properties, rheological properties, and transmission electron microscopy show that when PP-g-MA was combined with the technique that used CO2, greater enhancements in the mechanical properties and degree of dispersion of nanoclay in PP were observed. Furthermore, yieldlike behavior in the viscosity and a tail in the low-frequency behavior of the elastic modulus was attributed to the reaction of PP-g-MA with the nanoclay surface and not exfoliation. A fairly well-dispersed morphology was observed for concentrations as high 6.8 wt % clay when the clay was expanded and mixed with CO2. At this concentration, mechanical properties such as yield strength and modulus increased by as much as 13 and 69%, respectively, relative to the pure PP. Furthermore, the modulus of the composite samples prepared with PP-g-MA and CO2 was some 15% higher than that of samples prepared by direct melt compounding (without the use of CO2)- (C) 2008 Wiley Periodicals, Inc.