Mechanisms of Equilibration in Block Copolymer Micelles

Non-Technical Summary

Nanostructured materials that are formed by a class of extremely large molecules, advanced plastics called block copolymers, are of growing importance in a variety of emerging technologies. Examples include drug-eluting coatings on stents to prevent rejection; viscosity modifiers in synthetic motor oils to boost fuel economy; vehicles for delivery of therapeutic agents to specific cells, such as cancers; membranes to enable lighter weight, non-flammable lithium batteries. In all of 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 of widespread importance is to understand the process of self-assembly itself. In particular, it is essential to know whether the resulting nanostructure is the most favorable, equilibrium one, or whether in fact the system has become structure-trapped in a so-called 'metastable' state. With this knowledge, it will be possible to tailor a commercial process to produce the most useful nanostructure, reliably and reproducibly, in the shortest possible time. Graduate students trained in this project will acquire a broad suite of skills in chemical synthesis and materials characterization. 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 week-long summer camp. A new version, American Indian Materials Week, will be developed, to serve to a drastically underrepresented group in STEM fields.

Technical Summary

It is the overarching goal of this proposal to elucidate the molecular-level mechanisms by which block copolymer nanostructures achieve equilibrium. By focusing primarily on solution assemblies, i.e., micelles, the molecular factors that dictate the barriers to single chain exchange will be quantified, and then collective motions, such as fusion or fragmentation, will be addressed. The cornerstone of the approach is time-resolved small-angle neutron scattering, which provides an unrivaled, quantitative measure of chain exchange kinetics. Collective motions will require additional tools, such as fluorescence. Structural characterization by small-angle X-ray scattering, dynamic light scattering, and cryogenic transmission electron microscopy will also be important. The use of ionic liquid solvents brings multiple advantages, including the ability to tune thermodynamic interactions precisely, the relative ease of designing both UCST and LCST systems, and the remarkably broad accessible temperature range. The research will aim to answer eight questions: (i) What is the functional dependence of chain exchange barriers on quality. (ii) What is the functional dependence on corona block length? (iii) What is the relationship between chain exchange and the relaxation time of an analogous triblock gel? (iv) How does exchange depend on micelle morphology? (v) What factors control the rates of micelle fragmentation and fusion? (vi) How do micelles equilibrate with respect to aggregation number? (vii) How do mixtures of different micelles equilibrate? (viii) How do these barriers evolve with concentration, from dilute systems to melts?