Biaxial Nematic Phase in Bent Core Mesogens

1. Biaxial Nematic Phase in Bent Core Mesogens Satyendra Kumar, Kent State University, DMR-03-12792 & 87-20147 What Does Really Align Liquid Crystals on a Solid Substrate? Satyendra Kumar, Kent State University DMR-0312792 Alignment of liquid crystals (LC) on substrates has been used in display devices for over three decades. Yet, the basic understanding of the interfacial phenomena responsible for the alignment has remained a mystery. The factors believed to be important are: chemical interactions [1] between LC and the substrate and interface-topographical effects [2] coupled with anisotropic elasticity of liquid crystals. It has not been possible to separate the roles of these effects, essential for a complete understanding of LC alignment. This comprehensive x-ray reflectivity and AFM study of more than thirty variously treated substrates reveals [3] that the anisotropy in substrate’s surface morphology at a sub-micron length scale is universally responsible for the LC alignment and the anchoring energy. This represents first identification of a universal feature for all alignment layers. (a) The amplitude of fringes in x-ray reflectivity reveals that the surface roughness of polystyrene (PS) film (that aligns LC  to rubbing) becomes increasing anisotropic with the number of rubs, n; (b) Dependence of rms roughnesses,  and  to the rubbing direction, on n; (c) Anchoring energy vs. roughness anisotropy follows theoretical quadratic dependence (solid line). [1]. J. Cognard, Mol. Cryst. Liq. Cryst. Suppl. 1 (1982); W. Chen, M.B. Feller, and Y. R . Shen, Phys. Rev. Lett. 63, 2665 (1989); H. Nejoh, Surface Science, 256, 94 (1991). [2]. D. W. Berreman, Phys. Rev. Lett. 28, 1683 (1972); Mol. Cryst. Liq. Cryst. 23, 215 (1973); Y. -M. Zhu, et al., Appl. Phys. Lett. 205, 290 (1995). [3]. S. Kumar, J.H. Kim, Y. Shi, Phys. Rev. Lett. 95, 077803 (2005).

2. What Does Really Align Liquid Crystals on a Solid Substrate? Satyendra Kumar, Kent State University DMR-0312792 • Scientific &Technological Impact • This study provides the first insight in to and a new perspective on the role of interfacial roughness anisotropy in LC alignment. • The results have drawn attention of researchers in the field and are acting as catalyst for new collaborations which should lead to scientific understanding of all aspects of this phenomena. • The results are extremely useful in designing new and more effective alignment surfaces for electro-optic industry. Education Dr. Jae-Hoon Kim and Dr. Yushan Shi performed these studies as postdoctoral scholars. Dr. Kim is currently a faculty member at the Hanyang University in S. Korea and Dr. Shi is at the Washington University. AFM pictures and Fourier transform of rubbed ( to white arrow) films of (a) polyimide, (b) polystyrene (PS), and (c) PS annealed after rubbing. The PS film develops new structures in the direction  to rubbing rendering it smoother than the rubbing direction. LC aligns in the direction  to rubbing.