Developing structure-property relationships in branched wormlike micelles via advanced rheological and neutron scattering techniques

Wagner, Norman J.The molecular design of soft materials with optimal flow properties is highly desired in applications ranging from polymer processing to drug delivery, where materials undergo both steady and dynamic nonlinear deformations during processing, transport, and use. To design such materials, a thorough understanding of the coupling between the molecular topology and the flow properties is required. Flow-small angle neutron scattering (flow- SANS) presents a unique opportunity to understand this non-trivial coupling between the macroscopic flow behavior, molecular topology, and material performance by combining rheometry with time- and spatially-resolved SANS. Such methods enable simultaneous measurements of the material microstructure during an applied rheometrical deformation, which quantifies the flow properties. Therefore, the development of ‘structure-property relationships’ that link the material microstructure to its macroscopic flow behavior is the first step toward the ultimate goal of designing soft materials a priori. ☐ Wormlike micelles (WLMs) are surfactant solutions that self-assemble to form long and wormlike chains. WLMs are of particular scientific and technological interest due to their ability to branch, break, and reform under shear. These unique properties can lead to nonlinear flow phenomena and instabilities such as shear banding. WLMs are also often used as a model system for studying polymers and polyelectrolytes. As the self-assembly of these ‘living polymers’ is tunable, WLMs are ubiquitous in a broad range of applications ranging from consumer products to oil and energy recovery fluids. Altering the topology of WLM solutions provides a microstructural pathway to rationally design and optimize the flow properties for targeted applications. Specifically, inducing branching in WLM solutions is an attractive route to achieve this goal, as branching has the potential to alter or eliminate undesirable flow instabilities such as shear banding. ☐ The goal of this thesis is to understand the role of micellar branching on the resulting equilibrium and non-equilibrium properties, while advancing instrumentation and analysis methods in rheology and neutron scattering. Multiple rheological and neutron techniques are employed to explore the relationship between branching, microstructure, dynamics and nonlinear flow properties using a model series of WLMs. The degree of branching in the mixed cationic/anionic surfactant (CTAT/SDBS) solutions is controlled via the addition of the hydrotropic salt sodium tosylate. A combination of techniques, including static small angle neutron scattering (SANS), cryo-TEM, and linear viscoelastic rheology (LVE), are used to quantify the equilibrium microstructures, including the relevant micellar length and time scales. Dynamic scattering methods including neutron spin echo (NSE) and dynamic light scattering (DLS) identify characteristic differences in the solution dynamics. ☐ Combining nonlinear rheological measurements with spatiotemporally-resolved SANS enables unambiguous identification of non-equilibrium rheological and scattering signatures of branching and shear banding. The shear-induced ordering of the micelles is spatially and temporally characterized via flow-SANS under various nonlinear deformations, including steady shear, shear startup, and large amplitude oscillatory shear (LAOS). New methods of time-resolved data analysis are developed, which improve the resolution of the experiments by several-fold. Local segmental orientation and the presence of flow instabilities is found to be a complex function of the branching level, radial position, and deformation type. Using time- and spatially-resolved flow-SANS, the complex structural mechanisms behind shear band formation are elucidated for steady and dynamic flows, which differ based on branching level. These microstructural responses to deformation are then used to experimentally validate constitutive modeling predictions of shear banding under dynamic deformation for the first time. Quantitative metrics to predict dynamic shear banding from rheology or flowinduced orientation alone are also developed. Branching is associated with a disappearance of the shear banding flow instability under both steady and dynamic deformations. ☐ Together, advanced rheological and neutron techniques provide a platform for creating structure-property relationships that predict flow and structural phenomena in WLMs and other soft materials. These methods have enabled characteristic differences in linear versus branched WLMs to be determined. This research is part of a broader effort to characterize branching in chemical polymers and self-assembled systems, and may aid in the formulation of WLMs for specific applications. Finally, this work provides a basis for critically testing and developing microstructure-based constitutive equations that incorporate micellar breakage and branching.University of Delaware, Department of Chemical and Biomolecular EngineeringPh.D