(468g) The Effect of Branching on Shear Band Formation and Evolution in Wormlike Micelles (WLMs)

Self-assembled wormlike micelles (WLMs) are of particular scientific and technological interest due to their ability to branch, break, and reform under shear, which can lead to nonlinear flow phenomena such as shear banding. Nonlinear rheology methods like shear startup have shown to be useful in determining the molecular architecture and chain branching [1], but quantitative relationships are lacking. Thus, understanding the coupling between flow behavior and molecular topology requires experimental methods that combine time- and spatially-resolved small angle neutron scattering (flow-SANS) measurements of structure with rheometry for nonlinear deformations. Here, we explore the relationship between branching, shear-induced microstructure and nonlinear responses using a series of well-characterized, branched WLM solutions [2,3]. The degree of branching in the mixed cationic/anionic surfactant (CTAT/SDBS) solutions is controlled via the addition of the hydrotropic salt sodium tosylate [2,3]. The shear-induced micellar alignment is spatially and temporally characterized under steady shear and shear startup by flow-SANS. Segmental orientation and alignment (A_f) is a complex function of the branching level and deformation type. Using spatially-resolved 1-2 plane measurements during steady shear, shear banding is verified in low and mildly branched solutions, but is not observed in highly branched solutions. Shear startup SANS measurements are then used to explore the mechanism of shear band formation. A long transient is observed in the A_f response in the low and mildly branched solutions during banding; no transient is observed in the highly branched solutions, confirming shear thinning. While the mildly branched WLMs appear to follow the `disentangle, re-entangle’ mechanism [4], a new mechanism of shear band formation is identified in the low branched WLMs. Here, after the micelles re-entangle, the low shear rate band `re-aligns’ and becomes more oriented before steady state is achieved. This multi-technique approach, combining nonlinear rheology and spatially-and temporally-resolved SANS, has enabled us to link the micellar microstructure and topology to the macroscopic flow properties of WLM solutions, and is part of a broader effort to characterize branching in chemical polymers and self-assembled systems.

[1] F. Snijkers, et al., J. Rheol., 57, 4 (2013).

[2] M. A. Calabrese, et al., J. Rheol., 59, 5 (2015).

[3] B. A. Schubert, N.J. Wagner, and E.W. Kaler, Langmuir 19, 10 (2003).

[4] C.R. Lopez-Barron, et al., Physical Review E 89(4), 2014.