Poly(styrene-co-butyl acrylate) copolymers were prepared by free-radical random copolymerization of styrene and butyl acrylate in emulsion in the presence of 10% of surface-modified sodium montmorillonite (Na-MMT). The objective of this work was to evaluate the impact of the clay organic modifier in terms of its chemical structure, its degree of interaction within the clay galleries surface, and its ability to copolymerize with monomers, on the morphology and properties of the final nanocomposite prepared. Na-MMT was modified using different organic modifiers, namely: sodium 1-allyloxy-2-hydroxypropyl (Cops), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), N-isopropylacrylamide (NIPA), and sodium 11-methacryloyloxy-undecan-1-yl sulfate (MET), respectively. The morphology and properties of the nanocomposites obtained were found to be dependant on the clay organic modifier. X-ray diffraction (XRD) and transmission electron microscopy indicated that, nanocomposites at 10% clay loading with Cops-, NIPA-, and MET-modified clays, yielded intercalated to partially exfoliated structures, whereas AMPS-modified clay gave a nanocomposite with a fully exfoliated structure. All polymer-clay nanocomposites were found to be more thermally stable than neat poly(S-co-BA) as were determined by TGA. However, nanocomposites with intercalated structures exhibited greater thermal stability relative to fully exfoliated ones. Furthermore, nanocomposites with exfoliated structures exhibited higher storage moduli (G(I)) than partially exfoliated once, whereas intercalated structure showed the lowest G(I) values. (C) 2008 Wiley Periodicals, Inc.