Pre-transitional Phenomenon of Nematic-smectic Phase Transition Observed by Elastic Constant - Observation of Frederiks Transition by Forced Rayleigh Scattering -

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The elastic constants of liquid crystals are usually determined by detecting the critical field where the reorientation starts to occur by applying an electric or a magnetic field (Frederiks transition) perpendicular to the alignment direction. As for the twist elastic constant <K_(22)> , however, usual methods for determining the elastic constants <K_(11)> and <K_(33)> for splay and bend deformations can not be used. The first purpose of the present study was to construct the system for determining <K_(22)> by means of forced Rayleigh scattering.For this purpose, we examined and improved our forced Rayleigh scattering system itself by extending the diffusion measurements in nematics to in smectic A and smectic B phases and by investigating the temperature dependence of the signal intensity. We have also constructed the system of the forced Rayleigh scattering in a magnetic field. It was confirmed by using the standard material, 5CB, that the relaxation time of the diffraction in tensi … More ty from the transient grating starts to change at the critical field strength. This fact means that the diffusion constant changes due to the magnetic field induced reorientation. The elastic constant <K_(22)> was determined by the critical field. The results obtained agreed with the recent results by other techniques.To accomplish the final purpose of the present study, i.e., observation of the critical phenomenon, we carried out the preliminary measurements on 8CB. The increase of <K_(22)> was observed within the temperature range of 1 degree from the nematicsmectic A phase transition temperature, although the quantitative measurements such as the critical exponent have not been done. Since the difficulty of the quantitative measurements is due to the poor temperature control, the improvement is now under consideration. Thus it is a valuable result that the <K_(22)> measurements is now easily performed by forced Rayleigh scattering, although the final goal of the present work has not been reached. Less