Structure and Rheology of Supercritical Carbon Dioxide Exfoliated Polymer Nanocomposites

A major technological barrier to achieving superior properties of nano-structured composite materials is the difficulty of dispersing fillers uniformly in a host matrix. The existing nanocomposite manufacturing processes achieve property improvements through chemical modifications of filler structures and utilization of chemical interactions between the chemically modified fillers and matrix polymers. We have developed a novel and efficient supercritical fluid processing method for extensively exfoliating and coating nano-scale fillers for use in polymeric nanocomposites. The filler material, consisting of aggregated or layered particles, is contacted with a supercritical fluid containing a soluble organic material for a certain time, the soak step, followed by a catastrophic depressurization step. During the soak step, under the selected processing conditions, the mixture of the supercritical fluid and the organic material diffuses between the layers or bundles. Depending on the selected conditions and the solubility of the organic material in the supercritical fluid, the mixture can be a gas-expanded liquid phase or a dense-gas solution. The high diffusivity and low viscosity of the mixture enable efficient kinetics of this step. During depressurization, expansion of the gas between the layers or bundles pushes them apart causing extensive exfoliation while the organic material remains between the layers and bundles, coating the surfaces of the layers or bundles, thus preventing the reformation of the layered structure. We investigated the structure and rheology of several nanocomposites prepared by the supercritical carbon dioxide exfoliation and coating process: Thermoplastic polyurethane (TPU)/graphite nanocomposite Polydimethlsiloxane (PDMS)/natural clay nanocomposite Polyvinylmethylether (PVME)/I30P (a chemically modified clay). The morphology of the samples was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and the steady-shear and oscillatory-shear measurements were performed on the Rheometrics ARES and the Rheometrics RSA II rheometers, respectively.