Ultrasonic Absorption and Dispersion at Phase Transitions in Liquid Crystalline Compounds

Abstract

Previously published data concerning ultrasonic absorption and dispersion at phase transitions in liquid crystals are reviewed. Extensions of the quantitative interpretation of such data are proposed. The similarities between the ultrasonic behavior of liquid crystalline compounds near the isotropic-nematic transition and that of simpler compounds in the critical region leads to the application of Fixman's (1965) expressions to the data for p-azoxyanisole. Fixman's theory provides as satisfactory a fit to experimental data at temperatures just above the transition point, T _e'as Frenkel's heterogeneous fluctuation theory (1955). Furthermore, it has the advantage of treating a specific mechanism of absorption, namely, the temperature-dependent oscillation of the spatial decay constant, k in the approximation of Ornstein and Zernicke to the radial distribution function. A value of Debye's short range ordering parameter, l, equal to 11.2 Å is found for p-azoxyanisole and T ^c. In the nematic state (T T_e) Maier and Saupe's (1960) theory appears to be inadequate to account for the measured ultrasonic absorption. This conclusion is tentative. A combination of Maier and Saupe's expressions and Fixman's expressions with l+8.1 Å can bring about agreement with the experimental data of Hoyer and Nolle (1958) just below the transition point. In this case Fixman's expressions are considered to be applicable to transition regions between adjacent domains having different orientations of the local optic axis.