Hierarchical structure of PEO/laponite clay nanocomposite

Water-cast nanocomposite films formed by poly(ethylene oxide) (PEO) and Laponite clay were found to display three characteristic levels of structure with large-scale orientational order. The first level with the length scale of ca. 40 nm was the PEO-intercalated clay lamellar bundles which tended to stack perpendicularly to the film surface. The second level with the characteristic length scale of 1.8 nm was associated with the alternating layer stacking of clay platelets and PEO sandwiched between the clay particles perpendicular to film surface. These two levels of structure in terms of the preferred orientation were independent of clay content, solvent removal rate for composite film preparation and the crystallization temperature of free PEO chains situating outside the lamellar bundles. The third level of structure was characterized by the orientation of PEO crystalline stems with respect to the interface of clay platelets. Homeotropic orientation always dominated in the nanocomposite films prepared by slow solvent removal (the S-series films) irrespective of PEO crystallization temperature. In the films prepared by fast solvent removal (the F-series films), however, homogeneous crystal orientation started to set in as the clay concentration was increased to 33 wt% and at 40 wt% a predominant fraction of PEO crystallites adopted this orientation. It appeared that the crystal orientation was controlled by the orientational order of the clay platelets, namely, homeotropic orientation was favored if the silicate layers showed good orientation (as in the S-series films), whereas homogeneous orientation appeared in the films with poorer clay orientation (as in the F-series films).