Viscoelasticity, dielectric anisotropy, and birefringence in the nematic phase of three four-ring bent-core liquid crystals with an L-shaped molecular frame

Abstract

Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid four-ring core in the shape of the letter 'L'. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K-1 and bend K-3 elastic constants, splay viscosity eta(splay) and flow viscosities eta(parallel to) and eta(perpendicular to). The bend-splay anisotropy delta K-31 - K-3 - K-1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of V-shape. The dielectric anisotropy Delta epsilon and birefringence are positive in the entire nematic range. The splay viscosity eta(splay) and the flow viscosities eta(parallel to) and eta(perpendicular to) are smaller than the viscosities measured for the symmetric V-shaped bent-core materials at similar temperatures. The ratio Gamma = eta(splay)/eta(parallel to,perpendicular to) is in the range 5-4 that is typical for rod-like LCs. The reported L-shaped bent-core nematic LCs combine the useful features of bent-core LCs (such as a negative delta K-31, suitable for formulation of broad-range blue phases) with the relatively low viscosities, a property typical for rod-like LCs and beneficial for electro-optic switching applications.