Investigations on the Elastic Constants of the Nematic Homologous Series of 4,4′-Di(n-Alkoxy) Azoxybenzene

Abstract

The splay and bend elastic constants of the 4,4′-di(n-alkoxy)-azoxybenzenes have been determined by measuring the change in birefringence of parallel oriented samples in a magnetic field. The Maier-Saupe theory shows that the elastic constants should be proportional to S ² where S is the long range order parameter, but that short range order can also strongly affect these constants. Our measurements on the homologous series do indicate that both long and short range order are important. The temperature dependence of the bend and splay elastic constants, for example, near the nematic→isotropic transition cannot be explained by the long range order. This dependence corresponds to the pretransitional effects seen in the thermal expansion coefficient in this region, which can only be due to short range order. Our measurements also show that near the nematic→smectic C transition the splay (k ₁₁) and bend (k ₃₃) elastic constants abnormally increase. This strong temperature dependence of k ₁₁ and k ₃₃ can be explained by a pretransitional effect because the planar structure of the smectic C phase requires that k ₁₁ and k ₃₃ be very large. This increase is interpreted in terms of de Vries's model of cybotactic groups. A further abnormality in the elastic constants is observed in the region far away from the nematic→isotropic and nematic→smectic C transitions, which is interpreted by changes in the molecular arrangement in the steric units.