(523d) High Shear, Capillary Rheosans of Block Copolymer Micelles

Complex fluids such as colloids, therapeutic nanoparticles, and polymers undergo high shear rates (>10⁴ s^-1) in diverse industrial applications such as lubrication, spraying, extrusion, and injection through needles. Simultaneous small angle neutron scattering and rheology (rheoSANS) can elucidate the mechanism of changing flow properties by measuring the underlying structure during controlled deformation; however high shear rates (> 4 * 10³ s^-1) are currently inaccessible in conventional rheoSANS equipment. Yet, self-assembled worm-like micelles have been shown to undergo further changes in structure and rheology at these higher shear rates. A new capillary rheometer was developed at the NIST Center for Neutron Research to measure these industrially relevant complex fluids: high pressure pumps flow samples through a coiled, silica capillary with apparent wall shear rates up to 10⁴-10⁶ s^-1. In this work, we have extended the capillary rheoSANS studies to include self-assembled block copolymer micelles, a class of materials which have been used for early-stage drug delivery nanoparticles and stabilization of pharmaceutical formulations, among other applications. At high concentrations, these nanoparticles transition from a viscous liquid to a soft, gel-like solid with considerable shear thinning behavior. Representative samples of a viscous liquid and gel-like solid have been measured at shear rates spanning 10^-1-10⁴ s^-1. Diffraction-like patterns with intriguing and changing symmetry were observed for the soft solid sample. These results are the first rheoSANS measurements of block copolymer micelles at shear rates > 10⁴ s^-1 to our knowledge. Our upcoming experiments will study the effect of molar mass and core block chemistry on the flow-induced structure of these particles.