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Selected Publications and Presentations
1. Shape-Persistent Bent Mesogens: Biaxial Molecules forming Biaxial Nematic Phases, Matthias Lehmann, C. Köhn, S. Kumar, S.-W. Kang, U. Kolb, T. Gorelik, D. Schollmeyer, Proceedings of the German Liq. Cryst., Frieburg, Germany, (2006). 2. Thermotropic uniaxial and biaxial nematic and smectic phases in bent-core mesogens, V. Prasad, S.-W. Kang, K.A. Suresh, L.Joshi, Q. Wang, and S. Kumar, J. Am. Chem. Soc. 127, 17224-17227(2005) 3. A Biaxial Nematic Phase Exhibited by Bent-core Molecules, B.R. Acharya, A. Primak, and S. Kumar, Phys. Rev. Lett, 92, 145506 (2004). 4. More examples of the biaxial nematic phase, S. Kumar, 2nd International Symposium on Biaxial Nematics, Southampton, UK, Jan 27 (2006). 5. The Discovery of the Biaxial Nematic Phase in Bent-Core Mesogens and Possible Technological Applications, S. Kumar, International Meeting on Information Display 2004, Daegu, S. Korea, August 22 (2004). 6. Uniaxial and Biaxial Nematic Phases in Bent-Core Mesogens, Plenary Talk, S. Kumar, Conference on Disorder, Complexity and Biology, Banaras Hindu University, Varanasi, India, July 12-15 (2004). 7. Thermotropic Biaxial Nematic Phase in Banana Liquid Crystals, B. Acharya, A. Primak, and S. Kumar, 20th International Liquid Crystal Conference, Ljubljana, Slovenia, July 4-9 (2004).
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4. Biaxial Nematic Mesogens |
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Since the prediction of its existence more than 30 years ago, the search for the biaxial nematic phase (Nb) has continued at varied pace. Several attempts to synthesize molecular thermotropic mesogens that exhibited the Nb phase had not been successful. With the discovery of bent core mesogens in recent years, theoretical and experimental research interest in this phase has seen a resurgence.
The discovery of the Nb phase through x-ray diffraction experiments in rigid bent-core oxadizole based compounds was soon confirmed by nuclear magnetic resonance (NMR). Since then, several reports of its observation in other bent-core systems and in Silicon-tetrapodic compounds have appeared in literature. We have been studying several new bent-core and Germanium-tetrapodic mesogens all of which exhibit, both, the uniaxial (Nu) and the Nb nematic phases. These materials offer us an opportunity to glean the structural, optical, and thermodynamic properties and the nature of the transitions from the Nu to Nb phase and the Nb to the underlying smectic phase in some of these biaxial materials. To study these materials, we employ synchrotorn x-ray scattering, DSC, electrooptic measurements, optical microscopy, conoscopy, and Dynamic Light Scattering technique in collaboration with Prof. Sprunt’s group in Physics. The results confirm the existence of the Nu and Nb phases with a second or weak first order phase transition between them. These results are in qualitative agreements with predictions of the mean field theory. |
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X-ray Diffraction pattern of the biaxial nematic phase |
