The Viscosity of Mesophases Formed by Cholesteryl Myristate

Abstract

This investigation provides the first reported viscosity measurements on cholesteryl myristate. Viscosities were measured as a function of both temperature and shear over temperatures corresponding to the isotropic liquid, the cholesteric mesophase, and the smectic mesophase. The myristate ester was chosen for study because its multiple phase transitions have been previously shown to be reproducible. The phases also persist over relatively long temperature ranges. The viscosity measurements were made at lower shear rates than previously reported for esters of cholesterol by using a Weissenberg Rheogoniometer. These are the first cone-and-plate viscosity measurements reported for any esters of cholesterol. The shear rates in this cone-and-plate viscometer are homogeneous and variable. The results show large breaks in viscosity behavior at the independently-measured thermodynamic transitions between the crystal, smectic, cholesteric, and isotropic states. Viscosities for the isotropic state are Newtonian throughout. The viscosities for both the cholesteric and smectic mesophases are prominently non-Newtonian with a sharp transition between the two. This is the first time a change in rheology has been reported at a smectic-cholesteric transition. A prominent viscosity maximum is observed near the cholesteric-isotropic transition which depends on shear rate and perhaps on time of shearing.