In Situ Observation of Texture of a Dimesogenic Liquid Crystal Compound under Hydrostatic Pressure

Abstract

In situ observation of microscopic texture of an unaligned sample of a dimesogenic liquid crystal compound, N-[4-(6-cholesteryloxycarbonyl)pentyloxy-benzylidene]-4-n-butylaniline, called as KI5, was performed under hydrostatic pressures up to 250 MPa by a polarized optical microscope equipped with a high-pressure optical hot-stage. The spherulites of the pressure-induced crystal polymorph(C_II) of the KI5 compound, different from those of the normal crystal (C_I), were confirmed directly at 100 MPa and higher pressures. The C_II spherulites were grown up from the dark field (homeotropic orientation) of the supercooled smectic phase under high pressures. This observation coincides with the experimental results by high-pressure X-ray diffraction and thermal analysis already reported. The temperature region of homeotropic orientation including partial homeotropic region decreases from ca. 70°C at 0.1 MPa to ca. 47°C at 100 MPa, and then the homeotropic state persists to higher temperatures of about 40 ± 5°C under higher pressures. The homeotropic region shifts to high temperature with the C_II-S₁ transition line by applying pressures above 60 MPa, suggesting that homeotropic orientation of the KI5 compound occurs immediately after the C_II-S₁ transition(melting) at high pressure.

Key Words:

• High-pressure optical hot stage
• optical texture
• dimesogenic LC compound
• pressure-induced crystal polymorph
• homeotropic orientation