Unusual properties of a bent-core liquid-crystalline fluid

Abstract

In our investigations of a family of achiral bent-core bis-(phenyl)oxadiazole derivatives, we observed some unusual properties of the nematic phase. These include evidence of segregation into domains of opposite handedness and the formation of clusters, nematic phase biaxiality, and a strongly kinetically governed phase behaviour, which leads to the display of unique filament structures at the onset of new order. We suggest that the combination of a deviant calamitic molecular shape with a considerable transverse dipole leads to an unusual strength and combination of molecular interactions, and as a consequence these nematic materials need to be considered as fluctuating and dynamically changing, multi-hierarchical fluids.

Miscibility studies of two twist-bend nematic liquid crystal dimers with different average molecular curvatures. A comparison between experimental data and predictions of a Landau mean-field theory for the NTB–N phase transition

“…There had been a number of reports of nematic-nematic phase transitions for bent-core or banana mesogens, but the structural characteristics of the new nematic phases had not been identified. 1,[2][3][4][5][15][16][17] However, recently a very detailed investigation of the flexible bent-shaped liquid crystal dimer 1 00 ,7 00 -bis(4-cyanobiphenyl)-4 0 -yl heptane (CB7CB) 6 concluded that one of the two nematic phases exhibited by this compound is the twistbend nematic phase (N TB ), in agreement with Dozov's predictions. 13 The mesogen CB7CB now appears to be the model liquid crystal on which research about this new phase is mainly focused.…”

“…Thus, K eff 2 becomes less negative as the nematic range increases and the nematic range limit would occur when K eff 2 vanishes. Therefore, the model described by eqn (17) supports the possibility of a first order N TB -N phase transition, the strength of which decreases as the nematic range increases. The N TB -N phase transition is first order at a temperature T N TB -N = T 1 given by…”

“…The equilibrium value of the j-order parameter is obtained by minimizing eqn (17) with respect to j([@f TB /@j] = 0). This means that for the N TB phase: and j N = 0 in the uniform nematic phase.…”

“…This means that for the N TB phase: and j N = 0 in the uniform nematic phase. The transition temperature (T N TB -N ) can be determined using the condition f TB = f N in eqn (17) and also the minimization condition ([qf TB / qj] = 0). Two mathematical solutions are obtained at the phase transition:…”

“…Thus, as a main conclusion, it seems that for the mixtures of the binary system investigated together with both pure compounds the N TB -N phase transition is first order and their strength diminishes as the composition moves from pure CB7CB to FFO9OCB or in other words, as the nematic range increases. The relationship corresponding to the entropy change at the N TB -N phase transition (DS N TB -N ) given by the Landau model can be written from eqn (17) and (20b) as:…”