We have studied the orientation effect of high aspect ratio nanofillers on spin cast nanocomposite films and nanocomposite hydrogels.

In the spin cast nanocomposite films project, we have shown that spin-cast polystyrene (PS)/Cloisite 6A (clay) nanocomposite films display a large radial thickness dependence that increases with clay concentration. Transmission electron microscopy reveals a tendency for the high aspect ratio clay particles to align in the direction of shear. Near the center of rotation, particles remain randomly oriented, leading to the formation of voids as the film dries. Atomic force microscopy shows that the roughness near the spin axis is higher than the roughness in the outer region. This proves that the orientation of clay platelets has a strong effect on the structure and thickness of films. A two dimensional numerical model that incorporates both hydrodynamic and orientation effects provides a good fit over a wide range of filler concentrations and spin rates.

In the nanocomposite hydrogels projects, we applied Small Angle Neutron Scattering (SANS) to study the shear induced clay orientation in Poly(ethylene oxide) (PEO)/Laponite RD nanocomposite hydrogels. Under shear, the clay platelets may orient along the shear flow direction and may stretch the polymer chains bridging different clay platelets. The structure of the gel will turn from isotropic to anisotropic, therefore yielding anisotropic SANS patterns. The experiments demonstrate that the evolution of the anisotropic SANS patterns depends on the temperature of the sample, concentration of PEO and clay, and the molecular weight. A two correlation length Debye-Anderson-Brumberger (DAB) model was applied to fit the SANS data. The short range correlation length is ∼5 nm, which is likely to be the thickness of a domain composed of a clay platelet and the polymer chain wrapping it. While the long range correlation length is ∼50 nm, which is similar to the distance between clay platelets. Under shear, clay platelets orient and polymer chains may be stretched; thus the both the long range and short rang correlation length will change. The correlation length along shear flow direction is observed to respond differently from that along the perpendicular direction.