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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, Xiangbing; Welch, Chris; Ungar, Goran; Mehl, Georg H

Authors

Warren D Stevenson

Heng-xing Zou

Xiangbing Zeng

Chris Welch

Goran Ungar



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 over-all 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) first-order transition. Small-angle X-ray scattering results suggest similar local cybotactic layering in both nemat-ic phases, with four sublayers, i.e. tails, mesogens, spacers, mesogens.

Journal Article Type Article
Publication Date Sep 24, 2018
Journal Physical Chemistry Chemical Physics
Print ISSN 1463-9076
Electronic ISSN 1463-9084
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
Issue 39
Article Number 20
Pages 25268-25274
APA6 Citation Stevenson, W. D., Zou, H., Zeng, X., 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, 25268-25274. https://doi.org/10.1039/c8cp05744c
DOI https://doi.org/10.1039/c8cp05744c
Keywords Liquid crystals; Phase transitions; Dimers; Nematic; Twist bend nematic
Publisher URL https://pubs.rsc.org/en/Content/ArticleLanding/2018/CP/C8CP05744C

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