Self-assembly behavior of polymer-assisted clays

Layered silicate clays are natural crystallites that are well recognized for their practical uses, but little is known about their self-assembly behavior. In this review, we summarize the recent literature on clay interactions with organic polymers as well as clay self-assembly with organic involvement. We place emphasis on two aspects of these non-covalent interactions: first, plate-like clays can have a considerable impact on polymer properties such as hydrogels and clay films, and also on the encapsulation of bio-molecules. Second, through ionic intercalation with polymeric amine-salts, the clay layered structure units can be modified and enabled to self-assemble into ordered arrays such as rod-, dendrite-, and fiber-like microstructures. The silicate self-assembled morphologies such as worm-like and hollow microspheres were obtained in epoxy matrices and during spray drying, respectively. A mechanism was proposed for the clay self-assembly in two orientations, platelet face-to-face (ionic attraction) and edge-to-edge (organic hydrophobic effect). Further, the layered clays after the exfoliation into random platelets (1 nm in thickness) had strong propensity toward self-piling without any organic influence. Formation of lengthy rods or fibrils up to 5 μm in length and their hierarchical transformation under transmission electron microscope (TEM) electron beam bombardment and ultrasonication were observed. The clay thin-platelet geometric shape and surface ionic charge are two important parameters for the self-assembling tendency. The high surface of clay platelet has a significant impact on polymer interactions and drives the self-organization of inorganic-organic structures.