Shear-induced layered structure of polymeric micelles by SANS

Small-angle neutron scattering (SANS), under shear using a Couette cell in radial and tangential scattering geometry, was performed to examine the structural evolution of the polymeric micellar macrolattice formed by concentrated aqueous solutions of triblock copolymer - poly(ethylene oxide) ₉₉-poly(propylene oxide)₆₉-poly(ethylene oxide) ₉₉ (Pluronic F127)-as a function of the shear rate. The micellar gel showed a shear thinning, i.e., a reduction of the resistance to shear, by forming a layered stacking of two-dimensional hexagonally close packed (HCP) polymer micelles. While traditional SANS experiments using a Couette shear cell are performed in radial geometry, we found the use of the tangential scattering geometry essential to obtain information on the layer stacking sequence. A theoretical model was developed to calculate 2D SANS scattering patterns that can be compared with the experimental data. We found that the micellar cores maintained their spherical shapes without deforming into ellipsoids and that the intralayer neighboring micelle center-to-center distance and the interlayer long period were independent of the shear rate and only depended on the concentration of the polymer. We also found that the stacking sequence changed from asymmetrically twinned ABC (i.e., FCC) at low shear rates to random AB stacking at high shear rates.