Steric Frustration at the Nanoscale: Self-Assembly, Chirality, and Fluctuations

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NONTECHNICAL ABSTRACTThe packing of odd-shaped three dimensional objects can lead to steric frustration, meaning that it's hard to fill space. If the objects are molecules, however, thermal agitation samples all of the packing possibilities, either finding unexpected new arrangements or remaining disorganized. A recently discovered example of this is the heliconical nematic phase, a liquid in which banana-shaped molecules fill space by arranging themselves into molecular-scale helical structures. In this project bent molecules will be designed and synthesized and their phase behavior studied with the goal of exploring the geometrical and statistical principles that govern how such steric frustration is accommodated in materials. The resulting design criteria would have wide ranging applications in the development of new materials based on the control and use of steric frustration. This project will provide its junior scientists (undergraduate student, graduate student, and postdoctoral researchers) with opportunities to participate in broadly interdisciplinary research in an exciting area of soft materials science. TECHNICAL ABSTRACTRod-shaped molecules form nematic liquid crystal phases, three dimensional fluids in which steric packing of rods induces long-range orientational order in thermal equilibrium. But introducing a simple chemical change to make the rods bent in shape produces an exotic new (heliconical nematic) phase in which the molecules spontaneously organize into chiral helical lattices of varying molecular orientation and nanoscale periodicity. The project hypothesis is that these distinctive new structures are a result of steric frustration: the bent molecular shape creates pockets of space which are difficult to fill, thereby attracting the most strongly fluctuating molecular sub-components. This newly formulated organizing principle will be explored in a broad-based, coordinated program of molecular design, chemical synthesis, physical characterization, and computer simulation of heliconical nematic phases. In particular, resonant soft x-ray scattering will be used to characterize their nanoscale orientational structure, and will be extended to probe molecular orientational fluctuations at the nanoscale. What is learned will be applied to a variety of soft matter systems, including liquid crystals, polymers and nanofabrications, with the intention of creating broadly useful, powerful means of probing and understanding nanoscale molecular orientational dynamics. The strong collective nanoscale chirality of the heliconical nematic phase makes it attractive for application in separation of enantiomers of chiral guest molecules, and as a templating medium for asymmetric synthesis. These applications of the heliconical nematic phase will be explored.