Elastic and viscous properties of the nematic dimer CB7CB

Greta Babakhanova, Zeinab Parsouzi, Sathyanarayana Paladugu, Hao Wang, Yu. A. Nastishin, Sergij V. Shiyanovskii, Samuel Sprunt, and Oleg D. Lavrentovich
Phys. Rev. E 96, 062704 – Published 26 December 2017

Abstract

We present a comprehensive set of measurements of optical, dielectric, diamagnetic, elastic, and viscous properties in the nematic (N) phase formed by a liquid crystalline dimer. The studied dimer, 1,7-bis-4-(4′-cyanobiphenyl) heptane (CB7CB), is composed of two rigid rodlike cyanobiphenyl segments connected by a flexible aliphatic link with seven methyl groups. CB7CB and other nematic dimers are of interest due to their tendency to adopt bent configurations and to form two states possessing a modulated nematic director structure, namely, the twist-bend nematic, NTB, and the oblique helicoidal cholesteric, ChOH, which occurs when the achiral dimer is doped with a chiral additive and exposed to an external electric or magnetic field. We characterize the material parameters as functions of temperature in the entire temperature range of the N phase, including the pretransitional regions near the NNTB and N-to-isotropic (I) transitions. The splay constant K11 is determined by two direct and independent techniques, namely, detection of the Frederiks transition and measurement of director fluctuation amplitudes by dynamic light scattering (DLS). The bend K33 and twist K22 constants are measured by DLS. K33, being the smallest of the three constants, shows a strong nonmonotonous temperature dependence with a negative slope in both N-I and NNTB pretransitional regions. The measured ratio K11/K22 is larger than 2 in the entire nematic temperature range. The orientational viscosities associated with splay, twist, and bend fluctuations in the N phase are comparable to those of nematics formed by rodlike molecules. All three show strong temperature dependence, increasing sharply near the NNTB transition.

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  • Received 22 September 2017

DOI:https://doi.org/10.1103/PhysRevE.96.062704

©2017 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Greta Babakhanova1,2, Zeinab Parsouzi3, Sathyanarayana Paladugu1, Hao Wang1,2, Yu. A. Nastishin1,4, Sergij V. Shiyanovskii1, Samuel Sprunt3,*, and Oleg D. Lavrentovich1,2,3,†

  • 1Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, USA
  • 2Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, USA
  • 3Department of Physics, Kent State University, Kent, Ohio 44242, USA
  • 4Hetman Petro Sahaidachnyi National Army Academy, 32, Heroes of Maidan Street, Lviv 79012, Ukraine

  • *ssprunt@kent.edu
  • Corresponding author: olavrent@kent.edu

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Vol. 96, Iss. 6 — December 2017

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