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

Warren D. Stevenson,Heng-xing Zou,Georg H. Mehl,Christopher Welch,Xiang-bing Zeng,Goran Ungar

doi:10.1039/c8cp05744c

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.

Highlights

Where T0 is the starting temperature, v is the linear heating or cooling rate, ω is the angular frequency of the modulation and ∆TM the modulation amplitude
In the following discussion we explain the variation of HNtb-N and HN-isotropic liquid (Iso) in terms of the torsional energy associated with twisting about the spacer bonds, morphing from a curved Ntb molecular conformation to a higher energy but straighter conformation, more compatible with N phase
modulated DSC (MDSC) revealed that ∆H of the Ntb-N transition decreases dramatically with increasing number of carbons n in the spacer, the transition remaining only weakly first order for n=11

Summary

Enthalpy and Entropy changes

Enthalpy and entropy measurements of LC transitions from MDSC (0.01-0.04 k/min). Crystal melting from standard DSC (3 K/min). *Transition enthalpies by standard DSC. MDSC measurements showed that decreasing the percentage of CBC7CB in the mixture decreased HNtb-N and widened the N phase temperature range In explanation of these findings a mean field Landau model was proposed comparable to that of Katz and Lebadev,[35] where the transition occurs at finite helical pitch length and involves two order parameters: a nematic orientational order parameter and a rotational vector field relating to the conical angle. In the version by Lopez et al the resulting free energy density depends largely on the effective value of the twist Frank elastic constant K2eff, which takes a negative value in the Ntb phase and increases with temperature This first order transition model has been shown to produce good fits to experimental DSC data in Ntb dimers.[36-47] there has been success with mean field models, our present findings show that HNtb-N and HN-Iso are related directly to a molecular feature, the spacer length. Theory developments involving order parameters dependent on the twist/bend between mesogens, rather than tilting angle,[31] may be advantageous

Findings

Spacer Length

Supporting Information