Structure and orientation dynamics of sepiolite rigid rods - poly(ethylene oxide) suspensions under extensional and shear flow, probed by in-situ SAXS
Résumé
Thanks to the development of a new extensional flow cell at the "Laboratoire de Rhéologie", and to combined rheology and small angle x-ray scattering (Rheo-SAXS) set-up available at the European Synchrotron Radiation Facility, the structure and orientation dynamics of nano-composites under extensional and shear flow have been studied in-situ. The suspensions used are composed of sepiolite clay rigid rods about 1000 nm long and 10 nm thick, suspended in an aqueous poly(ehtylene oxide) matrix of 105 g/mol molecular mass. The structural observations in the volume of the suspensions have been linked to the controlled extensional strain domain and shear stress flow properties. The trajectory of the strain domain within the sample volume has been well characterized by particle imaging velocimetry, in order to apply pure volume extensional flows in the desired extensional rate and viscosity domains (Fig. 1). Under flow the scattering patterns exhibit a change from an elliptical shape to a characteristic growing lozenge shape as the strain rate or shear rate is enhanced (Fig. 2). Quantified analysis of the mean average of fibers orientation in the polymer solution is described, related to shear rates and to extensional flow rates as well as kinetic of relaxation. In the volume fractions and shear rate domains for which the suspensions exhibit shear-thinning properties, two regimes of orientation separated by a critical strain rate have been identified under extensional flow: a first regime of increasing fibers orientations and a second regime of saturation of these orientations above a critical strain. The influence of the polymer chains and of theirs interactions with the fibers on the dynamics of orientation or relaxation of the fibers is clearly demonstrated.