Optical Behaviour of Mixtures of Nematic and Cholesteric Compounds

Abstract

Studies have been carried out on the characteristic optical textures exhibited by mixtures of nematic and cholesteric compounds, namely, N-(P-propoxybenzylidene)p-pentylaniline (PBPA) and cholesteryl ethyl carbonate (CEC). Many of the textures are metastable and undergo a slow transformation at room temperature in the course of some days or weeks. The optical properties and textures observed near room temperature are characteristic of the cholesteric mesophase in the case of mixtures with concentrations of PBPA below 35% as also at the higher concentrations in the range of 90% and above. In the range of concentrations of PBPA lying between 40 and 90%, the optical textures observed near room temperature are characteristic of the smectic mesophase. Refractive index data obtained in the different cases confirm the above identifications. Wherever the cholesteric texture has a sufficiently large pitch, it is possible to observe the striped pattern characteristic of the periodic variation of the refractive index associated with the helicoidal arrangement of the molecules in cholesteric layers. Numerous other features like the fan and focal-conic textures, drops and diffraction effects are also illustrated.