Dynamics of Block Copolymer Micelles

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NON-TECHNICAL SUMMARY: Micelles are nanoscale objects formed by spontaneous assembly of amphiphilic molecules such as detergents and lipids. Larger amphiphiles known as block copolymers (BCPs) adopt analogous structures, but typically are much more robust against mechanical forces or changes in chemical environment. For these reasons, BCP micelles are of growing importance in a variety of emerging technologies, including as viscosity modifiers in synthetic motor oils to boost fuel economy, and as vehicles for delivery of therapeutic agents to specific target cells, such as cancers. In these applications, and many more, the nanostructure is created through the “bottom-up” process of self-assembly, whereby the molecules are carefully designed to produce the intended structure. However, a fundamental problem is to understand the mechanism of self-assembly itself. In particular, it is important to know whether the resulting nanostructure is the most favorable, equilibrium one, or whether the system has become trapped in a non-optimal so-called “metastable” state. In the latter case, which is quite common, by what mechanisms do micelles evolve toward a more favorable state? With this knowledge, it will be possible to tailor a given commercial process to produce the most useful nanostructure, reliably and reproducibly, in the shortest possible time. Graduate students will acquire a broad suite of skills in polymer synthesis and characterization, light, X-ray and neutron scattering, and electron microscopy. They will also have extensive opportunities to present technical talks and posters to external audiences, as well as to mentor talented undergraduates. High school students from the greater Twin Cities, particularly women and underrepresented minorities, will be exposed to polymer science through "Polymer Day: You Make It, You Break It", a hands-on component of a broader "Discover STEM: Materials Week" summer camp. A diverse cohort of graduate students and postdoctoral fellows from across the country will participate in Future Faculty Workshops, to enhance skills in acquiring and succeeding in academic positions.TECHNICAL SUMMARY: While the equilibrium structure of block copolymer (BCP) micelles is relatively well understood, the dynamic processes by which such structures evolve are not. Furthermore, BCP micelles formed by solution self-assembly are often trapped in metastable, non-equilibrium states. A comprehensive experimental program is described, aimed at developing a quantitative understanding of the five molecular-level processes believed to control block copolymer micellar self-assembly. These processes include micellar fragmentation and fusion, for which prior studies are very scarce. Recalcitrant problems in understanding single chain exchange will be attacked with computer simulations, and unresolved issues in understanding the effect of energetic barriers to micelle creation and annihilation will be addressed using the same model polymer systems. A suite of powerful experimental tools will be employed, especially dynamic light scattering, small-angle X-ray and neutron scattering, and liquid-phase transmission electronic microscopy. The results will provide critical tests of existing models, or will provide benchmark data sets to inform future theoretical developments..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.