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

A modulated and conventional DSC study of the transitions between the twist-bend nematic (N_tb), 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 N_tb-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 N_tb 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 N_tb 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 N_tb-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.