A molecular theory of nematic–nematic phase transitions in mesogenic dimers

Abstract

We present a simplified molecular model of mesogenic dimers consisting of two identical uniaxial mesogenic cores separated by a fixed-length spacer and allowed to assume only two, statistically equivalent, conformations which are non-planar and of opposite handedness. In the mean-field approximation, with additive interactions among the mesogenic cores, the model yields up to three positionally disordered phases, one isotropic and two nematic. The low temperature nematic phase (N X ) has a local two-fold symmetry axis which is also a direction of molecular polar ordering and is tightly twisted about a macroscopic phase axis. The onset of polar ordering generates spontaneous chiral symmetry breaking, manifested primarily by the twisting of the polar director and the formation of chiral domains of opposite handedness. Within these domains the statistical balance between the two enantiomer conformations is shifted and the principal axes of the ordering tensors of the molecular segments twist at constant tilt angles with the helix axis.Key experimental results on the N X phase of liquid crystalline dimers are discussed in the light of the theoretical predictions of the model, which are also contrasted with the predictions of the twist-bend nematic model. 2

Polar interactions between bent–core molecules as a stabilising factor for inhomogeneous nematic phases with spontaneous bend deformations

“…(22)- (24). Here one notes that the quantity C given by (29) vanishes when α = 0, and hence the polar order parameter P = P ∥ + P ⊥ vanishes.…”

“…Here it is worth noting that in principle such periodic director distributions may differ from the conventional ones [14]. For example, the periodic structures may contain various polarization waves [23], and one nonconventional has indeed been confirmed by simple molecular modeling [24]. Further extension of the phenomenological model [17] takes into account also the biaxial ordering in bent-core systems [25].…”

Effect of polar intermolecular interactions on the elastic constants of bent-core nematics and the origin of the twist-bend phase

“…The relationship between the two approaches is briefly discussed in the last section. Finally, very recently another molecular theory of the locally polar bent-core nematic phase has been proposed by Vanakaras and Photinos [24] who have considered a nematic with a non-conical short pitch helical structure.…”

Twist-bend nematic phases of bent-shaped biaxial molecules

“…Experiments show that the N TB phase usually occurs within the stability regime of N U 14 , but for some compounds a direct transition between N TB and the isotropic phase (Iso) has also been found 15 . Although mechanism leading to long-range chiral order of N TB is to a large extent unknown, the issue of stable, modulated nematic phases has been addressed theoretically in a series of papers [16][17][18][19][20][21][22][23][24][25][26][27] . Meyer 25,26 and subsequently Dozov 27 have shown that flexopolarization can be the driving force leading to twist-bend and splay-bend distortions of the director field.…”