Calorimetric and Dielectric Investigations of MBBA (N-(p-methoxybenzylidene)-p-butylaniline) and HAB (4,4′ -di-n-heptyloxyazoxybenzene)

Abstract

Part I of this paper presents the results of calorimetric investigations of p-methoxybenzylidene-p-n-butylaniline (MBBA) and 4.4′-di-n-heptyloxyazoxybenzene (HAB). In both these substances two modifications of the solid phase, a metastable and a stable one, have been found to exist. Part II presents the results of measurements of the complex dielectric permittivity ϵ* = ϵ' - jϵ for the solid and liquid phases of both substances. The metastable modification of MBBA was found to feature a dielectric relaxation process in the kilohertz range associated with the rotational motions of the tail groups about the long axes of the molecules. No such process was revealed in the case of HAB. In the liquid phases of these two compounds relaxation processes were observed in the microwave-frequency range; these are interpreted as being due to the rotation of entire molecules about their own long axes. The existence of such motion in the smectic phase C of HAB seems to be not in agreement with the predictions of McMillan's theory.