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Issue 39, 2018
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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

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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.

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

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Publication details

The article was received on 11 Sep 2018, accepted on 24 Sep 2018 and first published on 24 Sep 2018


Article type: Paper
DOI: 10.1039/C8CP05744C
Citation: Phys. Chem. Chem. Phys., 2018,20, 25268-25274
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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

    W. D. Stevenson, H. Zou, X. Zeng, C. Welch, G. Ungar and G. H. Mehl, Phys. Chem. Chem. Phys., 2018, 20, 25268
    DOI: 10.1039/C8CP05744C

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