(98t) Dynamic Rheology and Microstructure of Shear-Banding Wormlike Micellar Solutions Using 1-2 Plane Flow-SANS

Polymer-like micelles (PLMs, also worm-like or thread-like micelles) are models for understanding polymer physics as well as of technological interest across a broad range of applications ranging from enhanced oil recovery to many consumer products.  A key phenomenon is the ability to formulate PLM solutions with very high zero shear shear viscosities (e.g. yield stress) that exhibit a “breaking viscosity”, such that they flow rapidly upon a sufficient applied stress. Prior investigations have elucidated many aspects of the shear banding responsible for this viscosity behavior, yet despite its relevance, our understanding of the dynamic rheology is incomplete.  To address this, large amplitude oscillatory shear (LAOS) measurements have been proposed as a characterization method and these have revealed intriguing, nonlinear dynamic rheological behavior. In this work we study this nonlinear dynamic rheology and understand its significance and source by measuring the microstructure of the PLMs during deformation using newly developed instrumentation implemented on SANS beamlines at the ILL Grenoble and the NCNR, NIST. A well-characterized formulation of 6 wt% cetylpyridinium chloride (CPCl) and sodium salicylate (NaSal) (2:1 molar ratio) with 0.5 M NaCl in D₂O is studied over a range of frequencies and amplitudes of LAOS. The microstructure on the segmental length scale is measured during LAOS via a newly developed method of time-resolved SANS in the velocity-gradient (1-2) plane and velocity-vorticity (1-3) planes of shear (tOR-SANS) [C. R. Lopez-Barron, L. Porcar, A. P. R. Eberle and N. J. Wagner, Physical Review Letters. 2012, 108 (25), 897-908)].  This enables both time and spatial resolution of the microstructure during oscillatory flow.  Measurements made in the velocity –velocity gradient (1-2) plane of shear are used to quantitatively define the microstucture by the micellar segmental alignment factor and orientation angle. The shear stress measured by rheology is successfully predicted using these microstructure parameters and the stress-SANS rule derived from complementary steady-shear data. We attribute the anomalous rheology observed in LAOS to an apparent shear-induced phase separation (SIPS) that occurs at longer length scales than those measured during SANS,  but which can be observed as a “butterfly pattern” during a complementary rheo-small angle light scattering methods (SALS) [P. Thareja, I. H. Hoffmann, M. W. Liberatore, M. E. Helgeson, Y. T. Hu, M. Gradzielski and N. J. Wagner, Journal of Rheology. 2010, 55 (6), 1375-1397]. The techniques used here to probe the material properties and microstructures of this WLM solution elucidate the microstructure responsible for the intriguing and useful nonlinear dynamic rheology. These measurements are invaluable for testing rheological constitutive models proposed for these solutions.