Recent studies of a team by A. Jákli, J.T. Gleeson and S. Sprunt at Kent State University have clearly demonstrated that fluids built from "bent-core" (or "banana" -shaped) molecules exhibit strikingly different properties from those composed of rod-shape materials; examples include anomalous flow viscosity and giant electro-mechanical coupling the latter being of high technological promise for molecular-scale energy conversion applications. These materials also apparently possess complex structure on the nanometer scale, and exhibit indications of novel states of matter. To develop a complete comprehension of these structures, the Kent State Group will utilize a powerful range of carefully targeted experimental techniques, available in house or through active participation in national user facilities. Specific new classes of reduced-symmetry materials to be investigated include molecules having W, T, X and H-shape, exciting new polymeric fluids and gels based on bent-core molecules, as well as previously uninvestigated low molecular weight bent-core molecules. The specific research objectives and scientific benefits of the proposed research are: (1) specific elucidation of nanoscopic structure in bent-core fluids; (2) investigation into the origins and limits of anomalously large coupling between electric properties and mechanical deformation (3) studies of strongly asymmetric bent-core molecules that form novel three-dimensional structures; (4) investigation of the optical and electro-mechanical properties of main- and side- chain polymers containing bent-core sub-units; and (5) studies of structured fluids under high magnetic fields to search for field-induced symmetry breaking transitions.
NON-TECHNICAL SUMMARY
Structured fluids are not only of fundamental scientific interest but have also enormous technological importance. Perhaps the most familiar example is liquid crystals, whose applications range from iPod screens to bullet-proof vests. The key factor determining the physical behavior of structured fluids is the symmetry properties of the molecular constituents. The team of A. Jákli, J.T. Gleeson and S. Sprunt at Kent State University will study compounds whose building blocks are not simple rods as in traditional liquid crystals, but either bent-shape, W, T, X and H shaped. A symmetry change in underlying molecule shape can lead to dramatically different and technologically promising behavior of a fluid composed of such molecules. This project offers the promise of significant advances in technology, such as low-cost, wearable (or potentially bio-implantable) electricity generators based on enhanced electro-mechanical coupling and a new generation of fast, low-power reflective color displays. The proposed research will provide critical feedback for synthetic chemists to improve material properties; key collaborators in this effort include Profs. R. Twieg (Department of Chemistry, Kent State University) and R. Verduzco (Department of Chemical Engineering, Rice University). The team's multi-faceted education program will train doctoral students to be effective players in a twenty-first century entrepreneurial environment. The principal investigators also will bring undergraduates, particularly from colleges serving traditionally under-represented groups, into a cutting-edge research environment, with the goal of increasing their participation in the high-tech workforce of tomorrow.
Support from the Solid State and Materials Chemistry program is acknowledged.
Key outcomes: We found that smectic clusters characterized by short –range one dimensional positional order is quite general in nematic liquid crystals of reduced symmetry molecules including rigid and flexible bent-shaped Y and H shaped molecules. The correlation length of the positional order generally is decreasing in heating, except for the flexible bent-shaped molecules in the twist-bend nematic phase. The directions of the molecules are in general tilted with respect to the direction of the positional order. The smectic clusters have important roles in possible applications from electromechanical coupling to rheological and nonlinear optical applications. We showed that cryogenic transmission electron microscopy in low dose mode is capable of imaging smectic layers of even single component of thermotropic liquid crystals with sub-nanometer resolution. We were able to proof the existence of smectic clusters in the nematic phase of bent-core liquid crystals and could analyse defects of smectic layers. We also showed that some helical nanofilament phases have double helical structures that explain their structural colors. We carried out high magnetic field, dielectric, rheological and freeze fracture transmission electron microscopy studies on the recently discovered twist-bend nematic liquid crystals. We prove their nanoscale heliconical structure and explained the physical mechanism how it lead to micron-scale modulations. The interdisciplinary nature of the project combined physical experiments, chemical physics methods and organic chemistry, thus required that the participating PIs and students combine different approaches and learn from each other. For example Randall Breckon, who synthesized several dozens of new bent-core, Y and H-shaped materials, also participated in the SAXS measurements in Brookhaven National Laboratory. On the other hand, Nick Diorio, who did mainly piezoelectric and X-ray studies, also synthesized a few bent-core materials on which he did experimental studies. In this way students learn appreciating the work of the scientists of other disciplines. All other students, although they had their own focus areas and individual advisors, combined different techniques and worked in different labs. The project supported the work of 15 students, eight of them female. Seven students (4 female) received PhD degrees based on the work done in the project and two will graduate soon. Over the entire period of this grant the project has lead 27 peer-reviewed publications appearing in Nature Communications, Soft Matter, Phys. Rev. Lett., Phys Rev E, ChemPhysChem and Liquid Crystals, etc. The group also contributed 3 chapters to the 2nd edition of the Handbook of Liquid Crystals and a book entitled Flexoelectricity in Liquid Crystals. The project was also the subject of about 50 presentations, including multiple invited and oral talks, in national and international conferences.
