Interactions of sodium montmorillonite with poly(acrylic acid)

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 (∼0.5 g) and an aqueous solution of poly(acrylic acid) (pH∼1.8, 50 g) at varying temperatures. X-ray diffraction indicated that the montmorillonite interlayer space (∼13 Å), formed by regular stacking of the silicate layers (dimension ∼1×1000nm), expanded to ∼16 Å as the reaction was carried out at room temperature and at 30 °C. At 60 °C, the interlayer space further expanded to ∼20 Å. 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.