Developing structure-property relationships in branched wormlike micelles via advanced rheological and neutron scattering techniques
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
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.
Description
Keywords
Pure sciences, Applied sciences, Micelle, Neutron scattering, Rheology, Self-assembly, Shear banding, Surfactant