*******NON-TECHNICAL ABSTRACT******* Amphiphiles are molecules with end groups that exhibit very different affinities, and can organize immiscible fluids into so-called complex fluid phases with characteristic structures in the 10-100~nm range. They have applications in such diverse areas as enhanced oil recovery, living cells, and food science. This research project will characterize the behavior of polymer-based complex fluids with the goal of rigorously testing and refining theoretical understanding of these phases. This, in turn, will lead to improvements in our understanding of how to make best use of existing amphiphiles and how to design better ones for practical applications. Beyond the time-averaged behavior, thermal fluctuations in these materials will be characterized using the emerging technique of x-ray photon correlation spectroscopy (XPCS). These studies will elucidate how the fluctuations depend on the structures and properties of the phases in question, and how these properties in turn may be inferred from the observed fluctuations. Overall, the proposed experiments will create new knowledge concerning the statistical physics of amphiphilic complex fluids. Methodologies developed during the research will enable scientists from across the country to better pursue their own XPCS experiments. In addition, because the promise of XPCS measurements probing shorter length scales and shorter time scales is an important component of the rationale for new, fourth-generation, x-ray sources, including LCLS at Stanford, the ERL at Cornell, and NSLS-II at Brookhaven, this research will better inform programmatic and technical decisions concerning such facilities. Finally, it will contribute to the education and training of the next generation of scattering scientists, so that they will prepared to do the best possible facility-based science at existing and future x-ray and neutron sources.
Because of the applicability of self-consistent mean-field theory to polymer systems, studies of block polymer-based complex fluids offer the prospect of an essentially first-principles understanding of complex fluid structure, phase behavior, and dynamics. Building on prior work that has revealed the existence of membrane-based sponge (L3) and vesicle (L4) phases in binary block copolymer-homopolymer blends, this research will clarify several key, unresolved questions concerning the phase behavior and structure of these materials, including the importance of anharmonic contributions to the membrane bending free energy, the effect of possible length-scale-dependent renormalization of the membrane elastic constants, the role of the membrane's Gaussian curvature, and the conditions required for a continuous L3-to-L4 transition. Beyond the time-averaged behavior, the equilibrium dynamics will also be characterized using the emerging technique of x-ray photon correlation spectroscopy (XPCS), in order to elucidate how the dynamics of equilibrium fluctuations depend on the structures and properties of the phases in question, and how these properties in turn may be inferred from the observed dynamics. This research will also improve the capabilities for carrying out x-ray photon correlation spectroscopy experiments, enabling scientists from across the country to pursue their own XPCS research, and will inform decisions to be made about the next generation of national x-ray facilities. It will also contribute to the education and training of the nation's next generation of scattering scientists, so that they are capable of carrying out the best possible facility-based science at existing and future x-ray and neutron sources.
