All Structures Great and Small: Nanoscale Modulations in Nematic Liquid Crystals

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.

The Ever Elusive, Yet-to-Be-Discovered Twist-Bend Nematic Phase

“…Dunmur's 2022 article, Anatomy of a Discovery: The Twist-Bend Nematic Phase, declares an intention to provide a comprehensive review of the subject, in his words, "to present as fair and documented account as possible" [4]. However, it includes no references to publications by Vanakaras and Photinos about their alternative model or to critiques of the NX = NTB supposition [9][10][11]. Its sole reference to alternative views is to a cri-Dunmur's 2022 article, Anatomy of a Discovery: The Twist-Bend Nematic Phase, declares an intention to provide a comprehensive review of the subject, in his words, "to present as fair and documented account as possible" [4].…”

“…Its sole reference to alternative views is to a cri-Dunmur's 2022 article, Anatomy of a Discovery: The Twist-Bend Nematic Phase, declares an intention to provide a comprehensive review of the subject, in his words, "to present as fair and documented account as possible" [4]. However, it includes no references to publications by Vanakaras and Photinos about their alternative model or to critiques of the N X = N TB supposition [9][10][11]. Its sole reference to alternative views is to a critique of a critique [12], one that obscures Meyer's coherent conjecture, relegating it to a genealogical affiliation with an ill-defined "family" of twist-bend nematics.…”

Comparative Study of the Optical and Dielectric Anisotropy of a Difluoroterphenyl Dimer and Trimer Forming Two Nematic Phases

“…It preserves the mass density uniformity of nematics and is characterised by a nanoscale helical modulation of the local direction describing the molecular orientational order [ 7 , 8 , 9 , 10 ]. This nematic phase, denoted here as N x , is interpreted either as twist–bend nematic (N tb ) [ 7 , 8 , 11 , 12 ] due to the resemblance of its structure to the elastically driven micron-scale modulated twist–bend nematic phase, predicted initially by R. B. Meyer [ 13 ] and later by Dozov [ 14 ], or as polar-twisted nematic (N pt ) [ 15 , 16 , 17 , 18 ] characterised by a polar director roto-translating in the nanoscale. The prime example of a molecular architecture supporting the formation of the N x phase is the CBnCB dimers, where two rod-like mesogenic cores with strong longitudinal dipoles are linked via an odd-membered alkyl spacer [ 6 , 11 , 19 , 20 ].…”

Thioether-Linked Liquid Crystal Trimers: Odd–Even Effects of Spacers and the Influence of Thioether Bonds on Phase Behavior

“…Physical properties of twist-bend nematogens have been extensively studied for basic scientific reasons as well as potential applications [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. However, the phase identification and nanoscopic structures of the N TB phase require further investigation [ 33 , 34 , 35 ].…”