Retardation of Molecular Rotations Around the Short Axes at the Transition from the Isotropic to Different Liquid Crystalline Phases

Results of the dielectric relaxation studies of several homologous series and some single compounds exhibiting different phase sequences (isotropic - nematic, isotropic - smectic A, C, B and E) are presented. Relaxation times characterizing the molecular rotations around the short axes in the isotropic and a given liquid crystalline (LC) phases were determined. Two quantities, the so-called retardation factor gLC-Is = _LC/_Is and the activation enthalpy are analyzed. In spite of drastic changes of the g-factors in the case of the isotropic - crystal-like smectic phase transitions (Sm B and E) in comparison with the isotropic - liquid-like phases (N, Sm A, Sm C) transitions the activation enthalpy does not depend practically on the LC phase. This will be discussed as resulting from the coupling of the rotational and translational motions.