A Light-Scattering Study of Pretransitional Behavior Around the Isotropic-Nematic Phase Transition in Alkylcyanobiphenyls

Abstract

Light-scattering measurements have been performed on the homologous series of P, P'-alkylcyanobiphenyls in a temperature range of about 10[ddot]C above the clearing temperature. In a temperature range of a few degrees above the phase transition deviations were observed from the (T − T* _c )¹ dependence expected from the Landau-de Gennes theory. These deviations are rather similar for nematic-isotropic and smectic-isotropic transitions. Due to the high scattering intensity multiple scattering occurs for the lower homologues. This is demonstrated by experiments where multiple scattering is enhanced by reducing the wavelength of the incoming beam. It is shown that any dependence of the depolarization ratio on the temperature is a sensitive criterion for multiple scattering. The observed deviations from a (T − T* _c )⁻¹ dependence can be accounted for by including non-Gaussian terms in the Landau free energy, provided the inverse of the momentum cut-off is taken to be of the order of the length of the molecule.