Smectic A - Cholesteric phase transition investigated by small angle X-ray diffraction and viscosity measurements

Abstract

We have studied the influence of the mean molecular length on the nature of the cholesteric (N*) - smectic A (A) liquid crystal phase transition for thermotropic compounds of cholesteryl myristate (C14), cholesteryl nonanoate (C9), and binary mixtures of C14 - C9 and cholesteryl caproate (C6) - C9. Mesoscopic properties of these systems, as the distance between smectic layers in the A phase (dA), the mean molecular length (ℓ̄) in the N* domain, and the correlation length (ε), as well as changes along the N* - A phase transition, were determined from the X-ray diffraction data. The results show a decrease in A phase domain and an expansion of the N* domain as the mean molecular length is diminished. Such effect induces a second order A - N* phase transition for a particular mixture of 63.1 mol% of C9 and 36.9 mol% of C6, evidenced by viscosity measurements in good agreement with previous density data.