Shape controlled viscoelastic behaviour in nematics

Srividhya Parthasarathi, D. S. Shankar Rao, K. Fodor Csorba, and S. Krishna Prasad,
J. Phys. Chem. B, 118, 14526 (2014)

We have investigated the permittivity and viscoelastic behaviour of a binary system comprising bent-core and calamitic compounds, both of which are polar, the calamitic being more strongly so, and exhibiting only the nematic mesophase. The permittivity data in the nematic as well as the isotropic phase indicate strong polar interactions between the molecules, even for mixtures with a significant content of the bent-core compound. The thermal dependence of both the splay and bend elastic constants exhibit features different from the literature. The splay constant displays a large increase with increasing concentration of bent-core material, before undergoing a precipitous drop for small calamitic content materials. On lowering the temperature, certain mixtures exhibit a convex-shaped feature for the bend elastic constant, i.e., the value of the elastic constant is maximum at a specific temperature in the nematic phase diminishing when the temperature is either increased or decreased. Surprisingly, the pure compounds, especially the bent-core one, show only a monotonically increasing trend for the bend elastic constant. We present two arguments to explain these features: one of these is based on the coupling between the molecular shape and director distortion presented in the literature. Then we put forth a new concept of frustration in the packing between the two types of molecules, and the polar interactions as an alternative.