Composition and Temperature Dependence of the Pitch in Cholesteric Binary Mixtures

Abstract

In the cholesteric phase of mixtures of cholesteric cholesteryl chloride (CC) with nematic p-ethoxy benzylidene-p-n butylaniline (EBBA) as well as with nematic p-[(p-ethoxy-benzyl-idene)amino]benzonitrile (PEBAB), the pitch has been determined asl a function of temperature and composition. For a CC weight fraction x≲ 0.8 we used the “variable wedge” method for x ≳ 0.8 we determined the wavelength of maximum reflection λ_o and the average refractive index n and employed the rlation λ_o = n p. We show that n can vary appreciably as a function of composition in a cholesteric binaryo system. For each composition the reciprocal pitch can be fitted reasonably well to a linear function of the temperature, except in the vicinity of phase transitions. It appears that the slope of the linear function, i.e. d(p⁻¹/dt, is always positive. We dicuss the results in relation to recently developecd expressions for the pitch as a function of temperature and composition.