Counterion effect on the morphological and mechanical properties of polymer-clay nanocomposites prepared in an aqueous medium

Na-montmorillonite (Na-MMT) and its ion-exchanged derivatives (K-, Li-, Ca-MMT) were used to make nanocomposites by mixing with a low-T-g styrene-acrylic latex, and these materials were used to verify the effects of counterions on nanocomposite morphology and mechanical properties. The monovalent cation clays form exfoliated/intercalated nanocomposites with a more than 10-fold increase in modulus, as compared to the pristine polymer, and approximately 200% increase in tensile strength. In the case of Li nanocomposites the mechanical properties are strongly dependent on the extent of drying, as expected considering that these ions are strongly hydrated. Calcium clay-polymer particle adhesion is very good, as evidenced in transmission electron micrographs, but the extent of exfoliation is less pronounced and the changes in the mechanical properties are accordingly lower than using Na, Li, or K counterions. Analytical electron micrographs show that the counterions are always accumulated in the domains containing both clay and polymer, showing that the compatibility of these two phases, which carry excess negative charges, is achieved thanks to cation bridges at the interfaces, following a model that was previously put forward to explain latex-clay nanocomposite formation and properties.