In Situ X-ray Scattering Studies of Polymer-Clay Nanocomposites in Simple Shear and Complex FlowPublic Deposited
Time-resolved in situ synchrotron x-ray scattering studies of particle orientation in polymer-clay nanocomposites under simple shear and complex flow were performed. Shear flow experiments used an annular cone and plate x-ray shear cell to allow measurement of platelet orientation within the flow-gradient plane. Intercalated nanocomposites were made by dispersing organically modified clay in a relatively low viscosity PDMS-PDPS copolymer. Average platelet orientation increased with increasing shear rate in steady shear. Upon flow reversal, moderately concentrated dispersions exhibited oscillations in measured orientation that scaled with shear strain. These oscillations were suppressed at higher particle concentrations. In all samples studied, orientation did not relax significantly upon cessation of shear. Exfoliated nanocomposites were produced via in situ controlled free radical polymerization of styrene in the presence of clay that had been organically modified by a surfactant with pendant vinyl benzene groups. This polymerization route produced highly exfoliated materials, as evidenced by the lack of a wide-angle x-ray scattering peak associated with clay layering. Small-angle x-ray scattering from the anisotropic clay sheets themselves was monitored to track flow-induced changes in the degree of particle orientation. The quiescent scattering pattern was nearly isotropic, but was rendered anisotropic by application of shear at sufficiently high rates. Relative to the PDMS materials, these samples exhibited short relaxation times for the loss of flow-induced orientation upon flow cessation. Polypropylene nanocomposites were prepared using both melt blending and solid-state shear pulverization. The more highly exfoliated pulverized samples exhibited significantly lower orientation than the intercalated melt-blended samples. In complex flow, the polypropylene nanocomposites were studied using an x-ray capable channel flow extrusion die which allowed for measurement of particle orientation as a function of position in the flow-vorticity plane. Little scattering was observed in the channels, indicating the tendency of clay particles to orient within the plane of dominant shear flow. Extrusion experiments using a nominally two-dimensional slit flow, in which the beam passes along the vorticity direction, provided orientation data at higher shear rates than the annular cone and plate shear cell.