(629b) Static and Dynamic Signatures of Branching in Wormlike Micelles (WLMs) Via Advanced Techniques in Rheology and Neutron Scattering

Authors: 
Calabrese, M. A. - Presenter, University of Delaware
Rogers, S., University of Illinois At Urbana-Champaign
Porcar, L., Institut Laue-Langevin
Wagner, N. J., University of Delaware
Self-assembled wormlike micelles (WLMs) are of scientific and technological interest due to their ability to branch, break, and reform under shear. Controlling the level of branching in WLMs allows the rheological properties to be tuned for specific applications. Dynamic measurements like neutron spin echo have proven useful in differentiating between topologies in polymers and WLMs [1,2] due to differences in chain dynamics. Branching in WLMs provides additional stress relief mechanisms due to the presence of sliding branch points, and in contrast to linear micelles, an array of relaxation modes may be expected in highly branched or network-like micelles. Here, we use multiple rheological and neutron techniques to explore the relationship between branching, dynamics, microstructure and nonlinear responses using a model WLM series [3,4], where the degree of branching is controlled via the addition of sodium tosylate. The linear viscoelastic rheology (LVE) of the WLMs shows deviations from Maxwellian behavior with branching, similar to branched polymers. These spectra are then connected to two distinct relaxation modes identified by DLS and neutron spin echo (NSE). In NSE, solutions with low branching exhibit wormlike chain behavior, whereas highly branched solutions approach the behavior of flexible membranes, indicative of the network morphology. Steady shear and startup results indicate that shear banding disappears at high branching levels, and that the stress overshoot behavior upon shear startup is reflective of the topology. Orthogonal superposition rheology (OSP) is then used to quantify the changes in the LVE spectra under shear, which are distinct based on branching level. The normalized orthogonal plateau modulus and crossover frequency as a function of shear rate (Wi) decrease more rapidly with branching, indicating a breakdown of network-like structures. This breakdown of branched structures identified in OSP is confirmed using 1-3 plane flow-SANS, by analyzing the WLM orientation distribution function (ODF). The ODF analysis indicates that WLM branches breakage past a critical shear rate, at which point the branched WLMs recover the linear WLM behavior. The combination of rheology and advanced neutron scattering techniques enables distinct differences in the rheological response, flow-induced microstructure, and solution dynamics of WLMs with branching to be identified. This research is part of a broader effort to characterize branching in chemical polymers and self-assembled systems, and provides a complete data set for the development and improvement of constitutive models that incorporate branching.

[1] F. Snijkers, et al. Macromolecules 46 (2013).

[2] S. A. Rogers, M. A. Calabrese and N. J. Wagner, Curr Opin Coll Int Sci (2014).

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

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