TWIN NEMATIC PHENYLBENZOATES IN A.C. ELECTRIC-FIELDS

The orientational behaviour of nematic compounds having twin phenylben zoate mesogens was examined under a wide range of a.c. electric fields (0-2 V mu m(-)1 and 10 Hz-50 kHz). For this study, crossed polarizing optical microscopy (POM) and real-time X-ray diffraction (RTXRD) meas urements were employed to investigate optical and orientational respon se. These nematic compounds have a positive dielectric anisotropy and a relatively low epsilon(//) relaxation frequency which allowed study in both homeotropic and planar orientations over a controllable freque ncy range. The optical behaviour and X-ray results corresponded well, providing a tool for understanding the orientational behaviour of thes e liquid crystals. For homeotropic alignment, an electric held of over 1 V mu m(-1) was required in order to obtain good orientation. Howeve r, homeotropic orientation depended on a delicate balance between ther mal fluctuations and dielectric torque imposed by the electric held, w hich are both strongly related to the elasticity of the LC domains. Du e to this effect, the highest orientation parameter achieved for homeo tropic orientation was only 0.48, which indicated that this state was still non-equilibrium. On the other hand, for planar orientation, a un iform texture with orientation parameter of 0.65 was easily obtained e ven at electric fields as low as 0.2 V mu m(-1) The application of an electric held stronger than 1 V mu m(-1) induced a distortion in the t exture, and reduced the orientation parameter to 0.45 for planar align ment.