Abstract
The nature of the nanoscale structural organization in modulated nematic phases formed by molecules having a nonlinear molecular architecture is a central issue in contemporary liquid crystal research. Nevertheless, the elucidation of the molecular organization is incomplete and poorly understood. One attempt to explain nanoscale phenomena merely “shrinks down” established macroscopic continuum elasticity modeling. That explanation initially (and mistakenly) identified the low temperature nematic phase (NX), first observed in symmetric mesogenic dimers of the CB-n-CB series with an odd number of methylene spacers (n), as a twist–bend nematic (NTB). We show that the NX is unrelated to any of the elastic deformations (bend, splay, twist) stipulated by the continuum elasticity theory of nematics. Results from molecular theory and computer simulations are used to illuminate the local symmetry and physical origins of the nanoscale modulations in the NX phase, a spontaneously chiral and locally polar nematic. We emphasize and contrast the differences between the NX and theoretically conceivable nematics exhibiting spontaneous modulations of the elastic modes by presenting a coherent formulation of one-dimensionally modulated nematics based on the Frank–Oseen elasticity theory. The conditions for the appearance of nematic phases presenting true elastic modulations of the twist–bend, splay–bend, etc., combinations are discussed and shown to clearly exclude identifications with the nanoscale-modulated nematics observed experimentally, e.g., the NX phase. The latter modulation derives from packing constraints associated with nonlinear molecules—a chiral, locally-polar structural organization indicative of a new type of nematic phase.
Highlights
Published: 29 December 2021Unexpectedly, nonlinear (“bent-core”) dimer molecules exhibit two distinct nematic phases
The discrepancies resulting from identifying the NX phase with the NTB model were pointed out in a recent paper, The twist bend nematic: a case of mistaken identity [14]
The authors trivialize the question of the structure and underlying physics operative in the NX to mere nomenclature: The NX is defined as NTB, whatever it takes to describe correctly the NX is by definition included in their evolving cluster of NTB models
Summary
Nonlinear (“bent-core”) dimer molecules exhibit two distinct nematic phases. The polar-twisted nematic (NPT ): A theoretical model formulated on the basis of molecular interactions [16,17] and presenting the possibility of local polar packing of the molecules along a polar director m which, in turn, undergoes periodic roto-translation modulations over molecular length scales, thereby eliminating macroscopic polarity. This model has been proposed for the description of the experimental NX phase [16].
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