Phase transition behaviors and selective optical properties of a binary cholesteric liquid crystals system: Mixtures of oleyl cholesteryl carbonate and cholesteryl nonanoate

Abstract

This paper deals with the phase transition behaviors of the mixtures of oleyl cholesteryl carbonate and cholesteryl nonanoate by means of optical and differential scanning calorimetry methods. The iridescent colors were measured by using the spectro multichannel photodetector. The phase diagrams show that the smectic mesophase appears in the whole concentration ranges under cooling, but only exists in heating with concentration less than 60%. The phase transition temperatures of the mixtures increase monotonically with increase of cholesteryl nonanoate (CN) contents. This phenomenon has been interpreted from the viewpoints of molecular configurations. The iridescent colors vary from red to violet as the temperature increases, and in reverse order when cooling. The concentration ranges of the iridescence are in agreement with those of smectic existence. Results also indicate that the color band shifts monotonically toward higher temperatures with an increased concentration of CN. This effect is accompanied by a gradual expansion of the temperature interval of the color band. The temperature dependence of the color shift may be explained with the pretransition of twist or untwist of the helical structures. Reflection bandwidth variations with temperature are also attributable to pretransitional effects. The temperature dependence of selectively reflected light intensity measurements reveals that the sensitivity decreases with increase in the wavelength. Moreover, 50% CN concentration is most sensitive, whether heating or cooling.