A nanocomposite material comprises a smectite clay having exhangeable cations that has been treated with at least one organic swelling agent, uniformly dispersed in a graft copolymer having a backbone of a porous propylene polymer material, to which is graft polymerized at least one grafting monomer capable of being polymerized by free radicals, wherein the total inorganic content of the composite material is about 0.5% to about 10%, based on the total weight of the composite. The nanocomposite material is made by graft polymerizing at least one liquid monomer capable of being polymerized by free radicals onto a porous propylene polymer material in a non-oxidizing environment in the presence of a smectite clay and an organic free radical polymerization initiator, whereby the chains of polymerized monomer that are formed intercalate the clay and produce a uniform dispersion of clay particles within the particulate propylene polymer material.