Ultrasonic investigation of the nematic-isotropic phase transition in MBBA

An ultrasonic investigation of the relaxation processes associated with the nematic-isotropic phase transition has been carried out on the liquid crystal p-methoxybenzylidine-p-n-butylaniline (MBBA). Simultaneous measurements of the sound velocity and absorption in samples of MBBA without any preferred orientation have been made at eight frequencies from 0.3 to 23 MHz. The temperature range from 23 to 77°C was studied, with special attention given to the region near the transition at ∼44°C. Far from the transition the absorption and velocity dispersion can be well described by a single relaxation process for which the relaxation time and the relaxation strength both increase substantially as the transition is approached. In the immediate vicinity of the transition the acoustic properties are characteristic of a multiply relaxing fluid.