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Flow-Induced Orientation in an Exfoliated Polystyrene/Clay Nanocomposite

Laura Dykes and Wesley R. Burghardt. Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Room E136, Evanston, IL 60208

Technological applications of polymer/clay nanocomposites will require understanding of the factors that influence the orientation distribution of the dispersed nano-scale clay fillers. We report here on an investigation of flow-induced orientation in a model polystyrene-clay nanocomposite sample. The sample was produced via in situ controlled free radical polymerization of styrene in the presence of a clay that had been organically modified by a surfactant with pendant vinyl benzene groups. This functionality allows the organic modifier to be covalently incorporated in the growing polymer chains, and has been shown to produce highly exfoliated samples; indeed, the resulting nanocomposite sample shows no wide-angle x-ray scattering peak associated with clay layers. Rather, small-angle x-ray scattering from the anisotropic clay sheets themselves is monitored to track flow-induced changes in the degree of particle orientation. This sample was investigated in an annular cone & plate x-ray shear cell which allows probing of orientation within the '1-2' (flow-gradient) plane. In the absence of flow, the scattering pattern is nearly isotropic, but it is rendered anisotropic by application of steady shear at sufficiently high rates. Relative to other materials we have studied, this sample appears to exhibit an unusually short relaxation time over which flow-induced orientation is lost upon flow cessation.