Pretransitional Behaviour of the Direct Isotropic to Smectic a Phase Transition of Dodecylcyanobiphenyl (12CB)

Abstract

The pretransitional behaviour of 12CB which exhibits a direct isotropic to smectic A phase transition has been studied by light scattering as a function of temperature in the isotropic phase. This behaviour has been examined in terms of the Landau - de Gennes model and compared with that observed for the direct isotropic-nematic phase transition exhibited by hexyl cyanobiphenyl (6CB). The results are in accord with the model except for 12CB where a critical divergence is observed 3°C above the clearing temperature. This result shows clearly the influence of the smectic A phase on the pretransitional behaviour and together with DSC and low angle X ray measurements suggests the existence of smectic A type cybotactic groups in the isotropic phase.