The chemical-structural modifications of the natural clay sodium montmorillonite during interaction with poly(acrylic acid) were studied mainly by X-ray photoemission spectroscopy. Samples of modified montmorillonite were prepared from the reaction of sodium montmorillonite (similar to 0.5 g) and an aqueous solution of poly(acrylic acid) (pH similar to 1.8, 50 g) at varying temperatures. X-ray diffraction indicated that the montmorillonite interlayer space (similar to 13 angstrom), formed by regular stacking of the silicate layers (dimension similar to 1 x 1000 nm), expanded to similar to 16 angstrom as the reaction was carried out at room temperature and at 30 degrees C. At 60 degrees C, the interlayer space further expanded to similar to 20 angstrom. The results of X-ray photoemission spectroscopy indicated that poly(acrylic acid) molecules exchange sodium ions on the surface of the silicate layers. These combined results allowed development of a reaction model that explains the dependency of the interlayer expansion with temperature. Information concerning the surface chemical reactions and systematic increases in the interlayer distances is particularly useful if montmorillonite and poly(acrylic acid) are to be used for formation of nanocomposite materials. (c) 2005 Elsevier Inc. All rights reserved.