Nematic twist-bend phase of a bent liquid crystal dimer: field-induced deformations of the helical structure and macroscopic polarization

Abstract

The twist-bend nematic (Ntb) phase is a recent addition to the family of nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntb phase under an external electric field is interesting and it was predicted in several recent theoretical studies. We investigated the field-induced polarization behaviour, dielectric, and electro-optic properties of a bent LC dimer CB7CB in the N and Ntb phases. A threshold-dependent polarization current response was obtained in both the phases under triangular and square-wave input electric fields, existing till frequencies as high as 150 Hz. The polarization switching times were found in ~ 1 ms region, especially in the N phase. In the Ntb phase, electric field-induced deformation of the helical structure was observed, like ferroelectric LCs (FLCs). Dielectric measurements revealed the presence of cybotactic clusters via collective relaxations. The dielectric anisotropy (Δε) is negative at the frequencies of polarization measurements. The net polarization resulted from field-induced reorientation of cybotactic clusters and additionally from the field-induced deformation of helical structures in the Ntb phase. We explored the possibility of ionic contributions to the net polarization by synthesizing TiO2 nanoparticles dispersed CB7CB LC nanocomposite. Incorporation of the nanoparticles resulted in reduction of the collective order, increase in the ionic impurity content and conductivity, but an extinction of the field-induced polarization response. Our results demonstrate that the net polarization has competing contributions from both ferroelectric-like and ionic origin (up to ~ 10 Hz) in the LC phases, but it becomes dominantly ferroelectric-like at higher frequencies.