Phase diagram of binary mixture E7:TM74A liquid crystals

Abstract

For many applications of liquid crystals, some requirements and characteristics need to be satisfied such as stability of mesophase range and the existence of the mesophase at the desired temperature of operation. Although there are many liquid crystalline materials, some difficulty is often experienced in achieving technologically useful temperature range for the existence of mesophase and response to external fields, as well as stability of the substance. To overcome this difficulty, mixtures can be used. In general, a binary mixture has transitions at temperatures in between the transition temperature of the pure compounds.
It is also known that when a small concentration of an optically active material is mixed with a nematic compound, the pitch of the cholesteric increases. Cholesteric-nematic mixtures can show the so-called "injected smectic phase" although none of the compounds used is smectic. This smectic phase can be stabilized and its transition temperature increased. This results in an enhanced smectic phase for the mixture.
For this investigation, we used the nematic liquid crystal E7 and cholesteric TM74A due to their optical activity and availability. E7 is nematic at room temperature and isotropic above 60.5°C while TM74A is known to exhibit Smectic A below -32°C, isotropic above 10°C and cholesteric mesophase in between.
This paper aims to determine the mixture that uses the available cholesteric liquid crystal suitable for room temperature operations.