Viscoelastic properties of a thioether-based heliconical twist–bend nematogen

Abstract

The twist–bend nematic (N_TB) phase is one of the new types of nematics found recently, which possesses local nematic order with a heliconical orientational modulation at the nanoscale. Herein, we quantitatively determined, for the first time, the temperature-dependent elastic and viscosity properties in both the nematic (N) and N_TB phases using a thioether-linked cyanobiphenyl dimer CBS7SCB exhibiting a broad temperature range of the N_TB phase which is stable down to room temperature. In the N phase, the fundamental elastic moduli: splay and bend elastic moduli (K₁₁ and K₃₃, respectively) are found to be in the order of 10⁻¹² N, and the effective rotational viscosity (γ₁) is determined to be in the range of 5–200 mPa s. Meanwhile, the N_TB phase is found to exhibit a compressive elastic modulus B in the order of several tens of kilopascals, the effective K₁₁ in the order of 10⁻¹⁰–10⁻⁸ N, and a considerably large γ₁ value of ∼68.7 Pa s right below the N–N_TB phase transition. The present study provides insights into the comprehensive viscoelastic properties based on comparison of the obtained experimental data with not only the existing theoretical prediction but also the preceding experimental works.