CAREER: Complex Phase Behavior in Block Copolymer Materials

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

NON-TECHNICAL SUMMARY: The properties of all materials depend on their structure across various length scales, from the microscopic arrangement of atoms to macroscopic shapes fashioned during processing. Nanometer-sized structures that spontaneously form ("self-assemble") in a class of materials known as "block copolymers" are widely used to create synthetic rubbers, adhesives, and next-generation computer chips among other applications. To date, there remains an incomplete understanding of what nanostructures are accessible with block copolymers, which limits new ways that they might be used. This program will uncover the fundamental molecular design rules that control structure formation in block copolymers through a combination of experiments and theory. The approach leverages synthesis to systematically manipulate polymer chemistry using concepts that are predicted to stabilize unique structures. X-ray measurements will provide insight into how self-assembly depends on composition and processing. Simulations will rationalize why certain structures are stable over others and inform refined molecular design. Fundamental knowledge uncovered in this research will promote the progress of science by elucidating the types of structure/property relationships that are central to the utility of soft materials. The outreach portion of this proposal involves broadening interest in polymer science, particularly among underrepresented demographics, via a plan targeting grade school, undergraduate, and graduate students. A series of hands-on structure/property workshops will provide 7-12th grade students with an introduction to polymer science using basic concepts from chemistry and physics. The impact of these demos will be amplified through teacher training that provides a mechanism for continual improvement and use. Curriculum development will further benefit undergraduate and graduate students who are interested in advanced topics aligned with the proposed research. TECHNICAL SUMMARY: The utility of block copolymers derives from their ability to self-assemble into well-ordered mesostructures, but the traditional phase behavior of materials with two types of block chemistries is limited to a handful of classical morphologies. Recently, a number of complex sphere packings known as Frank-Kasper phases have been discovered in simple AB diblock copolymer melts. However, to date there remains an incomplete understanding of what structures are accessible, why they develop, and how to bias their formation. This program will unravel the material design principles that govern the self-assembly of block copolymers into non-classical phases using a combination of synthesis, physical characterization, and theory. The central hypothesis underpinning this research posits that three factors known to promote interfacial curvature will influence complex phase behavior: (1) extreme conformational asymmetry in the form of statistical segment length differences between blocks, (2) the miktoarm star architecture, and (3) molecular dispersity. These topics will be investigated with a unified materials platform comprising poly(lactide) and poly(alkyl (meth)acrylate) blocks that enables systematic control over all three objectives. Small-angle X-ray scattering will be used to interrogate the structure of these materials in reciprocal space, augmented by density reconstructions and electron microscopy to probe real space. Rheological measurements will provide insight into order-disorder and order-order transitions. Self-consistent field theoretic simulations will be leveraged to rationalize these observations and guide iterative molecular design. The planned research will advance knowledge by revealing fundamental factors that control self-assembly and mesoscopic packing in soft materials. 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.