Entropy stabilized form chirality in curved rod nematics: structure and symmetries

TL;DR

This study uses Monte Carlo simulations to reveal how curved rod-shaped molecules form chiral, polar nematic phases with nanoscale modulations, driven by entropy, which differ from elastic deformations seen in other nematic phases.

Contribution

It demonstrates that entropy stabilizes a nanoscale, polar, chiral structure in curved rod nematics, distinct from elastic twist-bend deformations, and clarifies the origin of form chirality.

Findings

Curved rods form smectic and polar nematic phases with nanoscale modulation.

Entropy stabilizes the polar, chiral structure in the nematic phase.

Nanoscale modulation differs from elastic deformations in twist-bend nematics.

Abstract

Monte Carlo molecular simulations of curve-shaped rods show the propensity of such shapes to polymorphism revealing both smectic and polar nematic phases. The nematic exhibits a nanoscale modulated local structure characterized by a unique, polar, $\rm{C}_2$-symmetry axis that tightly spirals generating a mirror-symmetry-breaking orgnization of the achiral rods=form chirality. A comprehensive characterization of the polarity and its symmetries in the nematic phase confirms that the nanoscale modulation is distinct from the elastic deformations of a uniaxial nematic director in the twist-bend nematic phase. Instead it is shown that, analogous to the isotropic-to-nematic transition, entropy stabilizes the roto-translating polar director in the polar-twisted nematic phase. The conflation of macroscale form chirality in ferroelectric nematics with that in the twist-bend nematic stems from the misattribution of the nanoscale modulation in the lower temperature nematic ''$\rm{N_X}$ phase'' found in CB7CB dimers.