Effect of Sm-$A$short-range order on the activation energies of translational and rotational viscosities of nematic liquid crystals with highly polar molecules

Abstract

We report experimental studies on the temperature-dependent translational and rotational viscosities of a few nematic liquid crystals made of highly polar molecules. Cyanobiphenyl (nCB) with longitudinal and cyanobicyclohexane molecules (CCN-mn) with transverse dipole moments were chosen and the measurements were made in the nematic phase. The translational viscosity (shear viscosity) is measured by using a rheometer and the rotational viscosity is measured from the dynamics of the Freedericksz transition by measuring the optical phase-retardation decay of the sample. The associated activation energies are obtained from the fitting of the appropriate equation in the nematic phase. Only 8CB and CCN-47 exhibits N to Sm-A phase transition and their activation energies of translational and rotational viscosities are larger compared to the shortest homologs in the respective series. Both activation energies of 8CB are significantly larger than that of CCN-47. The temperature-dependent steady states of the director distribution under shear are comprehensibly reflected in the rheodielectric measurements of 8CB than that of CCN-47. The results are explained based on the differences in the Sm-A short-range orders and the critical slowing down of the order parameter fluctuations. We present a simple physical model of molecular association that gives rise to different Sm-A short-range orders in both classes of compounds.