Development of two-dimensional nanostructured layered hydroxy salts (LHSs) and hydroxy double salts (HDSs) for new applications: Anionic exchange kinetics and polymer modification
Abstract
Inorganic/organic 2-D nanostructured hybrid materials with tunable interlayer spacing have been prepared and characterized via X-ray diffraction (XRD), Fourier transform infrared (FTIR), diffuse reflectance spectroscopy (DRS), nuclear magnetic resonance spectroscopy (NMR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and TGA-FTIR for various applications such as catalysis, anion exchange, fire retardancy, and controlled release of ions. The kinetics and mechanism of the anionic exchange reactions were investigated and a correlation between structure and reactivity of model layered hydroxy double salts was attempted. Combined analysis of the evolution of solid state products via XRD and solution phase anion concentrations via NMR provided the data necessary to evaluate kinetic models and fully characterize the reactivity of these compounds. Exchanging acetate anions with other anions of varying anionic chain lengths provided an insight into different mechanisms via which these reactions can be accomplished. The anionic exchange mechanisms were found to be dependent on the diffusion of anions in and out of the interlayer space as well as on nucleation growth in accordance with the Avrami-Erofe'ev geometric model. In the long run, the anionic release profiles may be used to model the release of drugs from nanostructured drug delivery systems. The thermal degradation of acetate containing layered compounds was studied and the results obtained suggest that the presence of polycrystalline ZnO in the residue promotes the ketonization channel of acetic acid into acetone. An understanding of the degradation mechanism of the HDSs and LHSs by themselves afforded us to explore the role of these layered additives in enhancing polymer fire retardancy. For example, from cone calorimetry measurements, polymer nanocomposites containing modified hydroxy double salts (HDSs) and layered hydroxy salts (LHSs) clays have been shown to exhibit lower total heat release when compared with untreated virgin polymer. The presence of certain metal ions in their low valent oxidation states may promote chemical (redox) effects that may serve to slow down or prevent the depolymerization process. Some metallic species may promote char formation, which prevents mass and heat transfer hence slows down the burning process.
This paper has been withdrawn.