Polystyrene (PS) nanocomposites were prepared by the free-radical polymerization of styrene it) the presence of organically modified montmorillonite (MMT) clays. MMT clay was modified with a low-molecular-weight and quarternized block copolymer of styrene and 4-vinylpyridine [poly(styrene-b-4-vinylpyridine) (SVP)] with 36.4 wt % PS and 63.6 wt % poly(4-vinylpyridine) (P4VP). Special attention was paid to the modification, which was carried out in different compositions of a sol- vent mixture of tetrahydrofuran (THF) and water. The swelling behavior of the MMT clay was Studied by an X-ray diffraction technique. The diffraction peak shifted to lower 20 angles for all of the modified clays, which indicated the intercalation of the quarternized SVP copolymer into the MMT layers in different degrees. Higher interlayer distances, which showed a high degree of block copolymer insertion, were obtained for solvent compositions with THF in water. The resultant nanocomposites were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, thermogravimetric analysis, and dynamic mechanical analysis. The desired exfoliated nanocomposite structure was achieved when the MMT modification was conducted in 50 or 66 wt % THF, whereas the other modifications all resulted in intercalated structures. The resulting exfoliated nanocomposite was found to have better thermal stability and dynamic mechanical performance compared to the others, even with 2%, clay loading. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 112:52-63, 2009