Dynamic calorimetry and XRD studies of the nematic and twist-bend nematic phase transitions in a series of dimers with increasing spacer length

Stevenson, Warren D; Zou, Heng Xing; Zeng, Xiang Bing; Welch, Chris; Ungar, Goran; Mehl, Georg H.

Abstract

A modulated and conventional DSC study of the transitions between the twist-bend nematic (Ntb), regular nematic (N) and isotropic liquid (Iso) phases was performed on a series of difluoroterphenyl-based dimers with (CH2)n spacers; n = 5, 7, 9, 11. The enthalpy of Ntb-N transition decreases steeply with increasing n, while that of the N-Iso transition increases with n; hence, the greatest effect of increasing n is a lowering N phase enthalpy. Based on past and present X-ray scattering experiments, we estimate the average molecular conformation in the Ntb phase and perform torsion energy calculations on the spacer. From this, the lowering enthalpy of the N phase is attributed to the decreasing torsional energy cost of bringing the two terphenyls from an inclined twisted conformation in the Ntb phase, to almost parallel in the N phase. With increasing n the C-C bonds of the spacers twist less away from their trans conformation, thereby reducing the overall torsion energy of the N phase. It is speculated that the nearly continuous nature of the Ntb-N transition in n = 11 dimer is associated with the divergence of the helical pitch toward infinity which is intercepted by a final jump at the very weak (0.01 J g-1) first-order transition. Small-angle X-ray scattering results suggest similar local cybotactic layering in both nematic phases, with four sublayers, i.e. tails, mesogens, spacers, mesogens.

Citation

Stevenson, W. D., Zou, H. X., Zeng, X. B., Welch, C., Ungar, G., & Mehl, G. H. (2018). Dynamic calorimetry and XRD studies of the nematic and twist-bend nematic phase transitions in a series of dimers with increasing spacer length. Physical chemistry chemical physics : PCCP, 20(39), 25268-25274. https://doi.org/10.1039/c8cp05744c

• Journal Article Type: Article
• Acceptance Date: Sep 24, 2018
• Online Publication Date: Sep 24, 2018
• Publication Date: Sep 24, 2018
• Deposit Date: Sep 25, 2018
• Publicly Available Date: Sep 25, 2019
• Journal: Physical Chemistry Chemical Physics
• Print ISSN: 1463-9076
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 20
• Issue: 39
• Article Number: 20
• Pages: 25268-25274
• DOI: https://doi.org/10.1039/c8cp05744c
• Keywords: Liquid crystals; Phase transitions; Dimers; Nematic; Twist bend nematic
• Public URL: https://hull-repository.worktribe.com/output/1071848
• Publisher URL: https://pubs.rsc.org/en/Content/ArticleLanding/2018/CP/C8CP05744C
• Contract Date: Dec 19, 2018