The Shape of Things To Come: The Formation of Modulated Nematic Mesophases at Various Length Scales

5 years ago

The Shape of Things To Come: The Formation of Modulated Nematic Mesophases at Various Length Scales

Richard J. Mandle

The twist–bend nematic (NTB) phase is a recently discovered liquid-crystalline phase that exhibits macroscopic chirality even when formed from achiral materials, and as such presents a unique testbed for studies concerning the spontaneous breaking of mirror symmetry in soft matter. It is primarily exhibited by materials for which the molecular structure is composed of two rigid aromatic units (such as biphenyl connected by a flexible spacer). The local structure of the NTB phase is nematic-like—with molecules having an average orientational order but no positional order—with a nanoscale helix in which the pitch (i.e., the repeat distance of the helix) is of the order of several nanometres. A helix is chiral, and so the bulk NTB phase—in the absence of a biasing chiral environment—spontaneously separates into macroscopic domains of opposite handedness. After discussing the structure of this mesophase and its elucidation, this concept article presents the molecular factors that determine its incidence. The apparent dependency primarily on molecular shape and bend angle rather than particular functional group combinations manifests in this mesophase being exhibited on length scales far beyond those of simple liquid-crystalline dimers, not only in oligomers and polymers, but also in aqueous suspensions of micron sized helical particles. Throwing some shapes: Modulated nematic phases such as the twist–bend phase (NTB) exhibit spontaneous breaking of mirror symmetry leading to condensed phases of matter that exhibit bulk chirality despite being composed of achiral molecules. This concept article explores how a synergy of theory and experiment has revealed the formation of the NTB phase as having its origin in molecular shape, with the consequence that this state of matter—and possibly others—is manifested on a range of length scales beyond those of typical low-molecular-weight entities.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701167