ULTRASONIC STUDIES IN POLYMORPHIC SMECTIC LIQUID CRYSTALS.

Ultrasonic velocity, absorption and specific volume measurements in two polymorphic liquid crystals, viz. , hexyloxybenzylidene-p-chloroaniline (HBCA) and octyloxybenzylidene-p-chloroaniline (OBCA) was carried out throughout their anisotropic phase, at phase transition and 10 degree C beyond phase transition in the isotropic phase. Both liquid crystals exhibit smectic B(S//B) and smectic A(S//A) phases. The behavior of ultrasonic velocity and specific volume in these liquid crystals was different from other liquid crystals of cholesteric and nematic types. The parameters, viz. , adiabatic compressibility and molar sound velocity were also determined. The thermal coefficient of expansion was estimated for HBCA and found to attain very high values near the phase transition showing the presence of pre-transitional effects. These effects are explained on the basis of de Gennes theory generalized by Bendler.