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

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.