Behavior of Liquid-Crystal Compounds Near the Isotropic-Anisotropic Transition

Abstract

The compressibility and the specific volume of liquid p-azoxyanisole are studied in the temperature range of the anisotropic-isotropic transition. Both quantities exhibit easily observable pretransitional variations in an interval extending more than two centrigrade degrees either side of the transition. The pretransitional behavior is largely consistent with the Frenkel theory of heterophase fluctuations, provided that an interfacial energy of approximately 0.3 erg/cm2 between phases is assumed, but the theory apparently does not apply to temperatures very near to the transition. The compressibility measurements were made by an ultrasonic pulsed-signal method, which also yielded the ultrasonic absorption for various frequencies from 0.5 to 6.0 Mc. Both the compressibility and the absorption have sharp maxima at the transition. Generally similar ultrasonic results were obtained with cholesteryl benzoate. The frequency dependence of the absorption in p-azoxyanisole corresponds approximately to a two-state compressional reaction with a relaxation time of the order of 10—8 sec, which is somewhat greater than the Debye characteristic molecular orientation time for the isotropic phase. Thus the controlling rate mechanism for the absorption is identified tentatively with the orientation time of molecules or of small clusters. The magnitude of the absorption is consistent with a model in which the relaxation effect is associated with the pretransitional increment in the compressibility.