(662b) Using Advanced Rheological and Neutron Scattering Techniques to Determine Signatures of Branching in Wormlike Micelles (WLMs)

The molecular design of soft materials with optimal flow properties is important in applications ranging from polymer processing to drug delivery, where materials undergo both steady and dynamic nonlinear deformations during processing and/or use [1]. Self-assembled wormlike micelles (WLMs) are of particular scientific and technological interest due to their ability to branch, break, and reform under shear [2]. The unique properties of WLMs can lead to nonlinear flow phenomena and instabilities such as shear banding [1]. As measuring chain branching is a long-standing scientific challenge, branched WLMs are also used as a model system for studying branched polymers. Indeed, nonlinear shear and large amplitude oscillatory shear (LAOS) measurements have shown to be useful in the study of chain branching [1], but a quantitative understanding of how topological branching in WLMs affects the shear rheology and dynamics is not well understood. Understanding how the flow behavior and molecular topology are coupled under these nonlinear flows requires advancements in experimental methods that combine time- and spatially-resolved small angle neutron scattering measurements of structure with rheometry (flow-SANS) for various types of nonlinear deformations [3-5].

We use multiple rheological and neutron techniques to explore the relationship between branching, microstructure, dynamics and nonlinear responses using a model WLM series where we have exquisite control over the branching [5-7]. The rheology, shear-induced microstructures, and dynamics of the WLMs are examined to determine rheological and structural signatures of branching. The degree of branching in the mixed cationic/anionic surfactant (CTAT/SDBS) solutions is controlled via the addition of the hydrotropic salt sodium tosylate [5-7] at two surfactant concentrations: 1.5% wt and 4% wt. The phase behavior has been mapped extensively and we have explored the degree of branching and network formation using static SANS, cryo-TEM and rheo-optical methods [5-7]. Neutron spin echo (NSE) is performed to determine characteristic differences in the solution dynamics. Rheological signatures of branching are determined with various nonlinear deformations, including steady shear, shear startup, orthogonal superposition (OSP), and large amplitude oscillatory shear (LAOS) on a dimensionless basis of shear rate (Wi) and frequency (De). To develop flow-structural relationships, we employ flow-SANS and nonlinear rheological techniques [3, 4] in conjunction with newly developed methods of time-resolved data analysis that improve experimental resolution by orders of magnitude [8]. The shear-induced ordering of the micelles is spatially and temporally characterized under steady shear, shear startup, and large amplitude oscillatory shear (LAOS) in various shear planes.

Micellar branching leads to deviations from Maxwellian behavior in the LVE rheology, and can alter or eliminate steady shear banding [6]. Steady shear and steady shear startup rheological results indicate that branching inhibits shear banding at both surfactant concentrations, where faster transients and high power law indices are observed with increasing branching. As observed in branched polymers [9], the stress overshoot behavior immediately following shear startup is reflective of the topology. Branched structures mitigate this overshoot, leading to shear thinning as opposed to shear banding. Qualitatively similar trends are seen in the LAOS rheology. In conditions where stress overshoots are observed, the overshoot is mitigated by the introduction of branching. Finally, the orthogonal dynamic moduli (Gâ?? and Gâ?) are characterized under steady shear with OSP rheology. The normalized orthogonal crossover modulus, G_c, plateau modulus, G₀, and relaxation time (t_R = 1/w_c) as a function of Wi decrease more rapidly with branching, indicating a break down of network-like structures.

Neutron spin echo (NSE) measurements are used to identify characteristic relaxation processes in the branched WLM solutions. The stretch exponent, β, derived from fits to the dynamic structure factor is an indicator of solution morphology, where a value of β = 3/4 is expected for a wormlike chain and a value of β = 2/3 is predicted for a flexible membrane [10]. The stretch exponent is similar between systems at high q-values, where the basic cylindrical morphology of the solutions is probed. However, systematic differences in the stretch exponent are seen with decreasing q-values, where branching may play a significant role in the solution dynamics. While β = 3/4 is observed for solutions with low branching at these low q-positions, the stretch exponent approaches β = 2/3 in the very highly branched solution, indicative of a branched network morphology. While the relaxation rate of the WLMs is similar between solutions at high q-values, deviations also occur at lower q-values, allowing characteristic differences in branching to be quantified.

 

The shear-induced ordering and stability of the solutions is finally characterized in various shear planes under steady shear, shear startup and LAOS using flow-SANS methods. Local segmental orientation and alignment (A_f) in the flow-gradient plane is found to be a complex function of the branching level, radial position, and deformation type. Results in the 1-2 plane confirm shear banding in solutions with lower branching levels for all deformation types. The shear banding instability is also mitigated by branching for all deformation types [6,7]. Time-resolved startup measurements offer additional structural signatures of shear banding, where a A_f long transient is observed for shear banding solutions that is also dependent on branching level. Little to no transience is observed in shear thinning conditions, or for the shear thinning, highly branched solutions. The time- and spatially-dependent alignment factor can also be used to identify the shear band formation and interface location.

 

The combination of nonlinear rheology and advanced neutron scattering measurements of the microstructure and dynamics shows distinct differences in the rheological response, flow-induced microstructure, and solution dynamics of WLMs with branching. Importantly, highly branched WLMs offer enhanced solution stability, as shear banding and related instabilities are inhibited for all deformation types at comparable dimensionless frequencies and shear rates. This research employs advanced neutron techniques to determine characteristic differences in the flow-induced microstructure, topology and dynamics of branched WLMs, and is part of a broader effort to characterize branching in chemical polymers and self-assembled systems.

 

References:

[1] Hyun, K., et al., Progress in Polymer Science 36(12), 2011.

[2] Lerouge, S. and J. F. Berret, Polymer Characterization 230, 2009.

[3] Gurnon, A.K., et al., Soft Matter 10(16), 2014.

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

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

[6] Calabrese, M.A., et al., Journal of Rheology, 59(5), 2015.

[7] Calabrese, M. A., et al., under review Journal of Rheology, 2016.

[8] Calabrese, M. A., N. J. Wagner, and S. A. Rogers, Soft Matter 12, 2016.

[9] Snijkers, F., et al., Journal of Rheology, 57(4), 2013.

[10] Zilman, A. and R. Granek, Physical Review Letters 77, 1996.