Phase Behavior and Physical Properties of Thermotropic and Lyotropic Liquid Crystal Oligomers

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Non-technical Abstract:Liquid crystals are best known for their use in information displays like smartphone screens and flat-panel televisions. They are also a fascinating state of matter whose fundamental understanding and technological potential continue to pose interesting challenges and opportunities. While much is known about what kinds of molecules or molecular assemblies tend to form liquid crystals, there are important open questions about how the specific architecture of these constituents determines the nature and properties of the liquid crystalline states they exhibit. This project explores liquid crystalline states formed by "oligomers" that are constructed by connecting together a small number of elongated, rigid components via flexible linkages. One thrust of the project focuses on oligomers composed of short, rigid segments of double-stranded DNA (duplex DNA) linked by flexible, single strands. Here the objective is to elucidate the liquid crystalline states formed in concentrated aqueous solutions of these constructs, including layered structures that are not observed in solutions of fully paired duplexes and that may offer a new means to simulate crowded DNA (physiological) environments. A second thrust investigates the liquid crystalline properties of much smaller "rigid+flexible" oligomers built up from molecules similar to the types used in displays. These materials afford new possibilities to tune certain viscoelastic properties, which could improve existing liquid crystal devices or motivate entirely new ones. The parallel study of the two oligomeric systems with similar architectures expressed on different microscopic length scales should enhance insights into how these architectures determine macroscopic material properties. The project trains students at graduate and undergraduate levels, across disciplines (physics, biochemistry, materials science), for productive careers in the 21st century scientific workforce. The students have ample opportunities to master a broad spectrum of experimental techniques both in the "benchtop" laboratory setting and at large US national laboratories.Technical Abstract:This project experimentally investigates two distinct molecular systems where linking a small number of rigid subunits with flexible connectors promotes new liquid crystalline (LC) phase behavior or significantly alters macroscopic physical properties of familiar LC phases. Thermotropic LC oligomers consist of two or more small, rigid molecular elements connected by flexible spacers, whose specific arrangement can favor bent or helical conformations. These oligomers undergo transitions between LC states driven primarily by changes in enthalpy. Lyotropic, nucleic acid-based LCs, composed of DNA duplexes linked by single-stranded "gaps" or containing extra, unpaired base inclusions ("kinks"), also possess a "rigid+flexible" oligomer-like architecture, at ~10 times the length scale of the thermotropic materials. The DNA constructs exhibit concentration-dependent mesophases influenced primarily by entropy. The major objectives of the project are: (1) To measure orientational viscoelastic parameters of thermotropic LC oligomers, with emphasis on extending the results from dimers to higher n-mers, and to correlate their relative magnitudes and temperature dependencies with the nature of nanoscale modulated phases, such as the "twist-bend" phase, these oligomers form; (2) To examine the orientational anchoring of oligomers at the free surfaces of LC droplets and quasi-two-dimensional, freestanding LC films drawn from n-mer/monomer mixtures; (3) To determine the structure of mesophases exhibited in concentrated solutions of DNA duplexes that form oligomer-like constructs due to the selective placement of "gaps" and "kinks"; and (4) To explore, selectively, certain potential technological applications in areas ranging from responsive optical devices to new approaches for simulating crowded, physiological DNA environments to a concept for viral DNA/RNA detection. Experiments are performed in the PI and co-PIs' laboratories and at US National Labs, and utilize techniques including laser light scattering, small- and wide-angle X-ray scattering, high sensitivity fluorescence detection, magneto-optics in high fields, and "click chemistry" methods to assemble the DNA constructs.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.