Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible: N -mers

Parsouzi, Zeinab; Babakhanova, Greta; Rajabi, Mojtaba; Saha, Rony; Gyawali, Prabesh; Turiv, Taras; Wang, Hao; Baldwin, Alan; Welch, Chris; Mehl, Georg H.; Gleeson, Jim T.; Jakli, Antal; Lavrentovich, Oleg D.; Sprunt, Samuel

Abstract

© 2019 the Owner Societies. We report dynamic light scattering measurements of the orientational (Frank) elastic constants and associated viscosities among a homologous series of a liquid crystalline dimer, trimer, and tetramer exhibiting a uniaxial nematic (N) to twist-bend nematic (NTB) phase transition. The elastic constants for director splay (K11), twist (K22) and bend (K33) exhibit the relations K11 > K22 > K33 and K11/K22 > 2 over the bulk of the N phase. Their behavior near the N-NTB transition shows dependency on the parity of the number (n) of the rigid mesomorphic units in the flexible n-mers. Namely, the bend constant K33 in the dimer and tetramer turns upward and starts increasing close to the transition, following a monotonic decrease through most of the N phases. In contrast, K33 for the trimer flattens off just above the transition and shows no pretransitional enhancement. The twist constant K22 increases pretransitionally in both even and odd n-mers, but more weakly so in the trimer, while K11 increases steadily on cooling without evidence of pretransitional behavior in any n-mer. The viscosities associated with pure splay, twist-dominated twist-bend, and pure bend fluctuations in the N phase are comparable in magnitude to those of rod-like monomers. All three viscosities increase with decreasing temperature, but the bend viscosity in particular grows sharply near the N-NTB transition. The N-NTB pretransitional behavior is shown to be in qualitative agreement with the predictions of a coarse-grained theory, which models the NTB phase as a "pseudo-layered" structure with the symmetry (but not the mass density wave) of a smectic-A∗ phase.

Citation

Parsouzi, Z., Babakhanova, G., Rajabi, M., Saha, R., Gyawali, P., Turiv, T., Wang, H., Baldwin, A., Welch, C., Mehl, G. H., Gleeson, J. T., Jakli, A., Lavrentovich, O. D., & Sprunt, S. (2019). Pretransitional behavior of viscoelastic parameters at the nematic to twist-bend nematic phase transition in flexible: N -mers. Physical chemistry chemical physics : PCCP, 21(24), 13078-13089. https://doi.org/10.1039/c9cp00984a

• Journal Article Type: Article
• Acceptance Date: May 16, 2019
• Online Publication Date: May 20, 2019
• Publication Date: Jun 28, 2019
• Deposit Date: Nov 30, 2020
• Publicly Available Date: Jan 8, 2021
• Journal: Physical Chemistry Chemical Physics
• Print ISSN: 1463-9076
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 21
• Issue: 24
• Pages: 13078-13089
• DOI: https://doi.org/10.1039/c9cp00984a
• Public URL: https://hull-repository.worktribe.com/output/1865013
• Additional Information: : This document is Similarity Check deposited; : Supplementary Information; : Single-blind; : Received 18 February 2019; Accepted 16 May 2019; Accepted Manuscript published 20 May 2019