CAREER: Dynamics in Nanostructured Polymer Materials

Florida State University, Tallahassee FL

Investigators

Abstract

PART 1: NON-TECHNICAL SUMMARY Polymer membranes are potential sustainable solutions for important societal needs, such as energy storage and clean water. The materials studied in this project have complex structure on small scales that result in unique properties on much larger scales not seen in materials without such structure. The underlying cause is not well understood. The most significant potential impact of the proposed work is fundamental understanding of polymer physics related to the effect of molecular tethering and interface softness on local dynamics, as well as the nature of collective dynamics of interfaces and larger structures and how these processes impact membrane properties. This understanding will be achieved by designing new materials, conducting advanced experiments at national laboratories and in the PI's lab, and using theory. Since nanostructures are pervasive in many advanced functional soft materials, this understanding and the experimental techniques being developed can be extended to a broad range of other materials. This knowledge will help address limitations of existing nanostructured polymeric materials and improve performance and durability in many technological applications, such as membrane-based separations, food packaging, battery electrolytes, and capacitor dielectrics. This project also serves the nation by training a diverse team of graduate and undergraduate students through their participation in the advanced research effort and educational outreach to middle-school students. The project's educational goal of increasing diversity in the polymer and engineering communities will begin at a local level but be designed for implementation nationally. The knowledge generated by the research will be incorporated into the curriculum taught by the PI, and the outreach program will be documented for replication at other universities and broadly disseminated through scientific journals and conferences. The impact of the integrated research and educational efforts on increasing diversity in long-term STEM careers will be evaluated with surveys and tracking. PART 2: TECHNICAL SUMMARY The main research objective of this CAREER project is to develop a deeper understanding of the dynamics of strongly-segregated block copolymers (BCPs) across a broad range of length and time scales. Strongly-segregated block copolymers form predictable nanostructures with sharp interfaces, and they decouple small molecule transport from mechanical properties. The underlying causes of this decoupling are not well understood. Interfacial effects associated with tethering and/or confinement could be responsible for modifying the properties of each phase. On the other hand, properties like low-frequency elasticity can emerge from the structure itself. Only limited effort has been dedicated to understanding dynamics in strongly segregated BCPs, especially on length scales comparable to the size of the nanostructures. First, X-ray photon correlation spectroscopy (XPCS) will be used to interrogate on such length scales across a wide range of time scales to measure structural dynamics such as grain rotations and surface waves. Second, the effect of tethering on local (segmental) dynamics will be achieved using selective deuteration and neutron spin echo spectroscopy, complemented by dielectric spectroscopy. Third, the connection among dynamics on local and mesoscopic length scales with macroscopic properties will be investigated. Mechanical, rheological, and small-molecule transport properties will be evaluated. Experiments will be performed on a glass-rubber block copolymer with mechanical contrast and on a rubber-rubber block copolymer without mechanical contrast in order to differentiate BCP grain motion from surface waves. The effect of interface softness on local segment dynamics will also be evaluated. In addition to examining the effect of the mechanical contrast, composition and processing will be used to examine the effect of morphology and grain size on dynamics. Experimental measurements of structural dynamics will be analyzed in the context of the Soft Glassy Rheology model, segmental dynamics in the context of Rostiashvili's theory of polymer dynamics, and macroscopic properties in the context of several theories including effective medium theory. Fundamental understanding of how dynamics is transmitted across such a wide range of length and time scales will enable intelligent design of nanostructured polymeric materials with enhanced decoupling of important, application-specific properties, such as permeability and toughness. The research effort is integrated with an educational effort to impact diversity in STEM through teaching, mentoring, and educational outreach. 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.