Microscopic Textures and X-Ray Diffraction Patterns in Nematic Phases Occurring Above Smectic C Phases

Abstract

Microscopic studies of the nematic phase in the 4-n-alkoxybenzoic acids have shown that the nematic-to-isotropic transition has a range ≥ 0.9° which is larger than the 0.2–0.3° range usually seen for this transition. This wide range was also observed in a variety of phenylbenzoates and thiobenzoates having smectic C-nematic transitions but was not observed for HOAB and the bis-(4′-n-alkoxybenzal)-2-chloro-1, 4-phenylenediamines.

A subtle continuous textural change was observed in the middle of the nematic phase in most of the 4-n-alkoxybenzoic acids which have both a nematic and smectic C phase. This change was observed both on heating and cooling the sample and diminishes with increasing chain length until it is no longer observed in the C₁₂ homolog. The nematic phases of a wide variety of other compounds with smectic C-nematic transitions were also studied, but these did not show this textural change within the nematic phase.

Plots of the effective molecular length versus temperature, obtained from x-ray diffraction data, showed a significant increase with temperature for the nematic phases of most 4-n-alkoxybenzoic acids. Such an increase has not been observed for other nematic phases occurring above smectic C phases, and it appears to be correlated with the textural change observed by us.

A study of the smectic C-nematic transitions in these compounds showed that this transition can also vary in width, with an intermediate texture being observed when the transition has a wide range.

Keywords:

• microscopic textures
• 4-alkoxybenzoic acids
• x-ray diffraction
• smectic C phases
• cybotactic nematics
• phase transitions