Shohei Naemura

NOVEL CHARACTERIZATION METHOD OF IONS IN LIQUID CRYSTAL MATERIALS BY COMPLEX DIELECTRIC CONSTANT MEASUREMENTS

The frequency dependence of the complex dielectric constant of liquid crystal materials doped with tetra-n-butylammonium iodide (TBAI) is investigated in the low-frequency region, and the experimental results are analyzed in terms of space charge polarization. The contribution from an electric double layer is also taken into consideration in the analysis. By means of curve fitting utilizing theoretical expressions of the space charge polarization, five sets of diffusion coefficient and density values are obtained for mobile ions. It is confirmed by experiments on the temperature dependence that five kinds of ions follow Waldens rule, and verified from the viewpoint of ion radii that two of the five kinds of ions are TBA+ and I-. The frequency-dependent dielectric properties, which are characteristic of the behaviors of ions, can be well explained by this study and the analytical method introduced here is considered to be powerful for the evaluation of the attributes of mobile ions.

IONIC CONDUCTION IN NEMATIC AND SMECTIC A LIQUID CRYSTALS

Permittivity dispersion measurements in a very low frequency range were made on n-pentyl (5CB) and n-octyl 8CB cyano biphenyls, providing characterization of mobile ions by the diffusion coefficient, or the mobility, and their temperature dependence in the nematic phase and/or the smectic A phase. Ionic conduction was confirmed in the nematic and the smectic A phases from the continuity of these temperature-dependent properties in the isotropic, the nematic, and the smectic A phases with reasonable changes at the transition temperatures. The Walden rule was found to be applicable to the ionic conduction in the nematic phase of 5CB without a pre-transitional phenomemon near the nematic-smectic phase transition, when appropriate mean values are taken for both the anisotropic mobility of ions and the anisotropic viscosity of the liquid crystal substance. It was also pointed out that care should be taken for the applicability of the Walden rule to liquid crystals in the case when the anisotropy of ionic conduction is argued, especially for liquid crystal materials containing mobile ions with a far less anisotropic shape than the liquid crystal molecules.

Calculation of Optical and Dielectric Anisotropy of Nematic Liquid Crystals

The optical birefringence (Δn) and dielectric anisotropy (Δe) of nematic liquid crystals were calculated using the well-known theories of Vuks and Maier-Meier, respectively. The molecular dipole moments and polarizabilities needed for the estimation were taken from semiempirical AM1 calculations. The calculated values are in good agreement with the experimental results for the investigated compounds having different core structures and polar substituents.

Ion Generation in Liquid Crystals under Electric Field

Abstract Permittivity dispersion and transient current profiles of two classes of liquid crystal substances were analyzed, providing an idea of probable mechanisms of ion generation. Due to the strong local field caused by the electric double layers by ions, Schottky injection at electrodes can play an important role even under a small DC voltage application. In a higher voltage region, the dissociation of ion pairs and the recombination of free ions can be the dominant mechanisms, resulting in the Ohmic behavior of the voltage dependence of a steady-state current. Understandings of those mechanisms will be of a great help for development of advanced liquid crystal materials for uses in sophisticated displays.

Complex Dielectric Constant of Liquid Crystal Materials Containing Ionic Impurities in Low Frequency Region

Abstract The dielectric properties of liquid crystal materials containing ionic impurities are analyzed in terms of space charge polarization. The theoretical expressions of the dielectric constant and the dielectric loss factor are derived by solving a diffusion equation for mobile ions under an AC field. Numerical calculations are performed using the theoretical expressions and the frequency dependent dielectric behaviors are simulated. The calculated curves for the dielectric constant and the dielectric loss factor exhibit a dielectric dispersion and a dielectric absorption respectively in a low frequency region, and successfully trace the observed curves obtained for liquid crystal materials 4-n-pentyl-4′-cyanobiphenyl (5CB) by means of curve fitting with the parameters of the diffusion coefficient and the density of mobile ions.

Physicochemical Study on Liquid Crystal-Substrate Interfacial Interactions

Abstract Surface energies and liquid crystal MBBA orientations were investigated on substrates with organic layers. Polar and nonpolar contributions to the MBBA interfacial orientations were explained in terms of the chemical structures of the organic layers. Numerical calculations clearly showed that the liquid crystal interfacial orientation conditions are subject to liquid crystal and substrate surface energy differences in both polar and nonpolar components. Calculations agree well with the measured results on both MBBA orientation angles and anchoring strengths at the organic layer surfaces.

Study of Internal Electric Field of Liquid-Crystal Cell Effected by Space-Charge Polarization

The distribution of the local electric field in a liquid-crystal cell containing mobile ions was simulated in accordance with the Poisson equation in the steady state under a DC voltage application. The distribution of the local field was determined to be markedly unbiased due to the contribution of the bound charges on the electrodes induced by ions. This result suggests that the intensity of the local field acting on ions can be approximated by that of the external field, for analyzing the space-charge polarization under a small AC voltage application even in a very low frequency region, where the diffusion distance of ions during the period of the external AC field becomes greater than the thickness of the cell.

Liquid Crystal Material Development of IPS-TFT Displays

IPS Display is one of the promising technologies for the larger Liquid Crystal Displays (LCDs) of monitors for the desk top computers and TVs with a very good feature of viewing angle dependence. The requirement of IPS displays upon liquid crystal materials is different from TN-TFT displays. Taking this different requirement into consideration we have performed a characterization of liquid crystal materials for optimizing the mixture design. Identifying new liquid crystal materials we have succeeded in improving the viscous property of mixtures which would enable the full moving pictures of IPS displays.

Investigation Activity and Data on Safety of Liquid Crystal Materials

Abstract The safety data of the 76 liquid crystal components with CN-group, which were officially excluded from the list of ‘deleterious substances’ are discussed. The acute toxicity of the 496 liquid crystal components without CN-group being used in the market and classified according the terminal groups and the linkages are shown. The statement on the safety of the liquid crystal materials by the three liquid crystal material suppliers is made.

Ecotoxicological properties of liquid-crystal compounds

To obtain information on the ecotoxicological properties of liquid-crystal (LC) compounds, the aquatic toxicity of selected LCs was investigated. Therefore, ten single LCs and one LC mixture were tested for their toxicity to the two standard organisms, daphnia (immobilization) and algae (growth inhibition). Water solubilities and detection limits were determined in the test media. Up to the limit of water solubility, the LCs as well as the LC mixture did not cause any adverse effects to the tested organisms.