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Dissertation/Thesis Abstract

Exploring Non-Equilibrium Physics of Polyelectrolyte Complexes via X-ray, Neutron, Imaging, and Modeling
by Wu, Hao, Ph.D., The University of Chicago, 2019, 256; 22583513
Abstract (Summary)

Polyelectrolyte complexes are a condensed phase that forms when oppositely charged units mix together in an aqueous environment. Despite its ubiquity in natural and technological settings, it is still challenging to have an in-depth understanding of the origin of its structural complexity in general and kinetic pathways of nonequilibrium phenomena in particular.

The first goal of this dissertation is to investigate the nonequilibrium phenomena, kinetic pathways, and long-term stability of these electrostatic self-assemblies. Emphasis is placed on their morphological complexities and time-dependent structural evolution under a variety of parameters including preparation protocol, salt type, and solution salt concentration. By employing a combination of X-ray scattering and electron imaging, sharp distinctions between thermodynamically stable products and kinetically trapped metastable structures are revealed and their structural heterogeneities are identified. Notably, the kinetic pathway of the morphological transformation between the thermodynamic products and kinetic products is determined to be a two-step process.

Then, the next goal is to dive into the interparticle interactions of the thermodynamically stable nanoscale assemblies, i.e. polyelectrolyte complex micelles. A variety of experimental techniques and modeling methods, such as synchrotron small-angle X-ray scattering and pair distribution function analysis, are employed to provide a comprehensive characterization of their physical properties, including morphology, core and corona size, and radius of gyration. The growing prominence of a repulsive interaction in these micelles is observed in dilute solutions within a concentration range where micelle-micelle interaction has been ignored. The origin of the repulsive intermicellar correlation among these nanoscale electrostatic self-assemblies is further identified to be from the coronal responses via a quasi-hard-sphere interaction.

Next, this dissertation concentrates on the nonequilibrium kinetics of polyelectrolyte complex micelles, including formation, molecular exchange, and dissociation, via a combination of a series of experiments and physical modeling.

First, the superfast self-assembly kinetics is investigated by time-resolved small-angle X-ray scattering equipped with a stopped-flow device that enables a temporal resolution of a few milliseconds. It is found that the initial aggregation of the oppositely charged polyelectrolytes is completed within 80 ms, and the following micelle growth continues to proceed up to a few minutes. Experimental data and thermodynamic analysis show that, contrary to their uncharged counterparts, polyelectrolyte complex micelles do not go through a micelle fusion process during formation. A two-step formation mechanism is proposed to explain the experimental observation, which involves an ultrafast polyelectrolyte pairing step and a prolonged cluster aggregation step.

Second, the molecular exchange in polyelectrolyte complex micelles at near-equilibrium states is further explored using time-resolved small-angle neutron scattering and deuterium labeling. The aim is to investigate the dependence of the molecular exchange rate on polymer block length, system temperature, and solution salinity. During the experimented time period, no clear evidence is observed to show the chain exchange between the deuterium-rich micelles and hydrogen-rich micelles when they are mixed. Neither the elevation of system temperature nor increase of solution salinity, two common ways to weakening the strength of electrostatic interactions, seem to unlock the molecular exchange at the experimented time scale. The underlying reason is attributed to the failure of overcoming the large activation energy barrier caused by the breakup of the ionic bonds between oppositely charged groups.

Last, the kinetic pathway of dissociation kinetics in polyelectrolyte complex micelles is found to be a process that involves three distinct steps: micelle swelling, fragmentation, and separation. Furthermore, the dependencies of micelle dissociation rate on different parameters, i.e. salt concentration, solution temperature, and polyelectrolyte block length, are examined both by experimental data and physical predictions, which are in a good agreement. An analytical expression is further derived to illustrate the relationship between the micelle dissociation rate and a variety of parameters, such as interfacial tension, micelle size, charge block length, and salinity, which can direct the rational design of polyelectrolyte-based particles in demand.

Together, the efforts in this dissertation contribute to the fundamental understanding of

the structures and kinetics in polyelectrolyte complexes, pave the way for researchers to dig into the nonequilibrium phenomena in electrostatic-driven self-assemblies and provide new paradigms for the rational design of novel materials for a myriad of practical applications.

Indexing (document details)
Advisor: Tirrell, Matthew V
Commitee: Hubbell, Jeffrey, Nealey, Paul
School: The University of Chicago
Department: Molecular Engineering
School Location: United States -- Illinois
Source: DAI-B 81/3(E), Dissertation Abstracts International
Subjects: Molecular physics, Materials science
Keywords: Dissociation kinetics, Kinetic pathway, Nonequilibrium physics, Polyelectrolyte complexes, Polyelectrolyte complex micelles, Self-assembly kinetics
Publication Number: 22583513
ISBN: 9781088323519
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