Yasushi Iwakabe

Correlation Between Bulk Orderings and Anchoring Structures of Liquid Crystals Studied by Scanning Tunneling Microscopy

The anchoring structures of a homologous series of liquid crystals, n-alkylcyanobiphenyls (mCBs: m=7-12), on molybdenum disulfide (MoS2) are directly observed by scanning tunneling microscopy (STM) in order to analyze the alignment mechanism of liquid crystals. Based on highly reproducible STM images, the anchoring structures of mCBs are clearly divided into two categories, a monolayer (single-row) type (7CB, 8CB, 9CB and 11CB) and a bilayer (double-row) type (10CB and 12CB). Here we propose the monolayer is the anchoring structure of a nematic phase, while the bilayer is the anchoring structure of a smectic phase. The correlation between bulk phase orderings and anchoring structures of liquid crystals is discussed from the viewpoint of surface-phase formation for the mCBs and different solid substrates.

Molecular dynamics simulations of liquid crystal molecules adsorbed on graphite

Abstract Preliminary results are presented on molecular dynamics simulations of the alignment of 8CB (4-n-octyl-4′-cyanobiphenyl) liquid crystal molecules on a graphite surface. Their stable alignment direction was found to be the direction of the basal plane of graphite. The calculated adsorption energy of an 8CB molecule was 151·5 kJ mol −1 and its alkyl chain was the main adsorption portion. Molecular dynamics simulation could therefore be an effective probe to the surface-induced alignment mechanism of liquid crystal molecules.

MOLECULAR DYNAMICS SIMULATIONS OF LIQUID CRYSTAL MOLECULES ON A POLYIMIDE MONOLAYER

Abstract Preliminary results are presented on the molecular dynamics simulations of alignment of the liquid crystal molecule, 4-n-octyl-4′-cyanobiphenyl (8CB), on a polyimide (pyromelltic dianhydride-p-phenylene diamine) oligomer monolayer. We actually simulated a three-layer system, i.e., liquid crystal molecule/polyimide oligomer/a basal plane of graphite. First, simulations of the oligomers adsorbed on graphite were done in order to obtain reasonable adsorption structures, as the pre-stage simulation of the three-layer system. Then, by placing a liquid crystal layer on top, the three-layer system was simulated. The stable liquid crystal alignment direction on the polyimide monolayer was found roughly to be the polyimide chain direction with zero pretilt in this combination of liquid crystal and polymer materials. The calculated adsorption energy of an 8CB molecule to the polyimide monolayer was 128 kJ mol−1 and the carbonyl group of the polyimide was the main adsorption site.

Liquid Crystal Alignment on Polymer Films: Changes of the Pretilt Angles Due to the Thermal Treatment of Polyalkylene Pyromellitimides

Two polyimides have been cured at different temperatures and some samples further annealed at different temperatures after rubbing. It is found that the pretilt angles on these films increase with increasing curing temperature and/or increasing curing time. There is a further increase in the preiilt angle after annealing, but this effect is dependent on the structure of the polyimide as well as the pretreatment conditions of the sample. All these observations can be explained by considering the change in the alignment of the polyimide and thus the induced in-plane order of the liquid crystalline layer.

Two Types of Anchoring Structure in Smectic Liquid Crystal Molecules

The anchoring structures of smectic liquid crystals, n-alkylcyanobiphenyl (mCB: m=8, 10, 12), on molybdenum disulfide (MoS2) are directly observed by scanning tunneling microscopy (STM) in order to analyze the alignment mechanism of liquid crystals. For 8CB, the anchoring structure is of the periodic monolayer type, while 10CB and 12CB take a bilayer structure in which there is no interdigitation of the cyano groups. These structures on MoS2 depend on the length of the alkyl group in contrast to those on graphite. The mechanism of alignment is discussed based on the STM results.

Effects of polymer surface structures on liquid crystal alignment studied with molecular dynamics simulations

Abstract Effects of polymer surface structures on surface alignment of liquid crystal molecules were studied by comparison with our previous results of molecular dynamics simulations. An adsorption-related liquid crystal molecule alignment on the packed polyimide surface was found in the simulation study. In this article, we first compared the alignment on a sparse polyimide surface with the previous results of the packed polyimide surface to see effects of polymer surface density. The excluded volume effect with the polyimide domain edges additionally contributed to alignment of the liquid crystal molecules on the sparse surface, and resulted in a similar alignment structure (i.e. alignment direction and tilt angle) to the packed cases. Secondly, we made similar simulations by changing the polymer from a polyimide to a polyamide with similar polymer chain density. Differences between the corresponding packed polyimide case were found mainly in the energetics (the polyamide had about two thirds of the ads...

Anchoring Phase Transition and Bulk Phase Sequence of Liquid Crystals Studied by Scanning Tunneling Microscopy

Abstract On the basis of the highly reproducible scanning tunneling microscopy (STM) images, the anchoring structures of a homologous series of liquid crystals, n-alkylcyanobiphenyls (mCBs) are clearly divided into two categories; a single-row type and a double-row type. We have proposed that the single-row is the anchoring structure of a nematic phase, while the double-row is the one of a smectic phase. Furthermore, a new phase transition only available at the boundary has been confirmed for the first time in the binary mixtures. The correlation between those anchoring structures and the bulk phase diagrams is discussed from the viewpoint of anchoring phase formation at a molecular level.

An optical design for reflective color STN-LCDs

— In reflective color STN-LCDs, it is necessary to achieve achromatic representation in single-polarizer STN-LCD modes. We propose an optimization method for the optical components of single-polarizer STN-LCD modes in order to achieve achromatic representation. By applying this method, it is shown that a contrast ratio of more than 20 can be achieved in the normally black (NB) mode. Furthermore, we prove that the normally white (NW) mode can be realized as well as an NB mode which is usually used in current reflective color STN-LCDs. Comparing the viewing-angle characteristics of the NW and NB modes, it was found that those of the NW mode are better than those of the NB mode. Particularly, high reflectance can be realized even at larger viewing angles in the NW mode.

Dependence of Light-Leakage Caused by Spherical Spacers on Liquid Crystal Mixture in Super Twisted nematic Liquid Crystal Displays

The dependence of light-leakage caused by domain formation around spacers on liquid crystal mixture is observed. It is found that the surface alignment of liquid crystal molecules on the spacer depends on the liquid crystal mixture, which affects the light-leakage intensity. We propose models of the surface alignment, one is a vertical alignment on the spacer and the other is a tangential alignment on the spacer that decreases and increases the light-leakage intensity, respectively. These models are confirmed by observing their extinction patterns under crossed polarizers.