Structural-chemical evolution within exfoliated clays

The exfoliated (delaminated) structures of lamellar clays offer potential as precursors for the formation of various nanostructured materials. In this article, Lucentite and Laponite phyllosilicate clays, which both have empirical formulas of Na(0.33)[Mg(2.67)Li(0.33)Si4O10(OH)2] but differ in nanodimensions, have been exfoliated. Experiments were carried out for mixtures containing approximately 1 wt % phyllosilicate in a 5% aqueous solution of poly(acrylic acid) at different temperatures. X-ray diffraction and photoemission spectroscopy measurements for the solid products recovered after stirring the mixtures at 20 degrees C showed that the fully extended chains of poly(acrylic acid) were intercalated within the interlayer spaces between the silicate plates of the clays. At 85 degrees C, however, the clays were exfoliated and/or partially exfoliated. Photoemission spectroscopy also indicated that the exfoliated structures primarily consisted of silica nanoplates. 29Si nuclear magnetic resonance and oxygen K-edge near-edge X-ray absorption fine structure indicated that the surfaces of the plates were terminated by high concentrations of the silanol (-SiOH) groups, which created structural branches during intercalation. A model was developed in which intercalation and the removal of ions from the clays after the poly(acrylic acid) interactions reduced the electrostatic van der Waals forces between the plates. It was also shown that the formation of branches created a steric effect that inhibited the stacking of the plates. Together these resulted in exfoliation.

Dispersion of organically modified clays within n-alcohols

A method for formation of polymer-clay nanocomposites involves dispersion of the nanometer silicate layers of clays into a solvent, followed by dispersion into polymers. The dispersion of layered silicates within solvents affects the structure and properties of the nanocomposites. We report the dispersion of organically modified clays, used for formation of nanocomposites with organic polymers, within a range of alcohol solvents. Experiments involved stirring a mixture containing approximately 1 wt% of alkylammonium-modified clays in n-alcohols with general molecular structure RnOH, where n represents the number of carbons of alkyl chains, varying from 2 to 8. The clays precipitated from the dispersion when RnOH solvents with n<5 were used, however, they formed gels for solvents with n5. The increased dispersion was related to the decrease of polarity and hydrogen bonding force within solvents. X-ray diffraction for the dispersed clays indicated that the interlayer spaces (1.8 nm), formed by regular stacking of the silicate layers, expanded to a maximum of 3.0 nm after treatment with RnOH with n5. The interlayer expansion was due to the intercalation of n-alcohol molecules within the interlayer spaces. It is suggested that the alkyl chains of n-alcohols remain parallel to the silicate surface in the intercalate. Preliminary experiments on the influence of these alcohol solvents on the intercalation of polyol (polyether) are also reported.