Effect of polar intermolecular interactions on the elastic constants of bent-core nematics and the origin of the twist-bend phase

Abstract

Abstract. A molecular theory of both elastic constants and the flexoelectric coefficients of bent-core nematic liquid crystals has been developed taking into account dipole-dipole interactions as well as polar interactions determined by the bent molecular shape. It has been shown that if polar interactions are neglected, the elastic constants are increasing monotonically with the decreasing temperature. On the other hand, dipolar interactions between bent-core molecules may result in a dramatic increase of the bend flexocoefficient. As a result, the flexoelectric contribution to the bend elastic constant increases significantly, and the bend elastic constant appears to be very small throughout the nematic range and may vanish at a certain temperature. This temperature may then be identified as a temperature of the elastic instability of the bent-core nematic phase which induces a transition into the modulated phases with bend deformations like recently reported twist-bend phase. The temperature variation of the elastic constants is qualitatively similar to the typical experimental data for bent-core nematics.

Polar interactions between bent–core molecules as a stabilising factor for inhomogeneous nematic phases with spontaneous bend deformations

“…(5) yields the expressions for the so-called undressed elastic constants which are calculated without taking into account any polar intermolecular interactions. A detailed molecular theory of undressed elastic constants of nematics is presented in our previous paper [18] (see also earlier works [35][36][37][38][39]). At the same time the corrections to the elastic constants, determined by various polar intermolecular interactions are determined by all other terms in the distortion free energy density (5).…”

“…These elastic constants can be expressed by substituting the potential (21) into the last term in eq. (5) and using the method described in detail in our previous paper [18]. Finally, after all integrations and averaging the potential (21) makes the following contribution to the bare ) along of the attracting arms.…”

“…This, in turn, can be explained by assuming that the system is close to the virtual ferroelectric phase. Then the reduction may be determined by the negative flexoelectric [15,16] correction which is anomalously large due to pretransitional effects [17,18]. In our recent paper [18] we have calculated the flexoelectric coefficients and the effective elastic constants of bent-core nematics with transverse electric dipoles using the same molecular model and have shown how the bend elastic constant can vanish at a certain temperature.…”

“…Then the reduction may be determined by the negative flexoelectric [15,16] correction which is anomalously large due to pretransitional effects [17,18]. In our recent paper [18] we have calculated the flexoelectric coefficients and the effective elastic constants of bent-core nematics with transverse electric dipoles using the same molecular model and have shown how the bend elastic constant can vanish at a certain temperature. On the other hand, it has been shown that the N TB phase with very low values of the elastic constants may be exhibited by the system of bent particles without electric dipoles [19].…”

Spontaneous chiral symmetry breaking in liquid crystals

“…Thus, the N -TB N transformation is the phase transition between two orientationally ordered phases, achiral N and chiral TB N . Although the microscopical mechanisms leading to longrange nematic-like but heliconical (and therefore chiral) order in the TB N phase are to a large extent unknown, a number of phenomenological theoretical models have been proposed in the literature [13][14][15][16][17][18][19][20][21][22]. Aiming to contribute into the special issue of the Low Temperature Physics Journal devoted to I.M.…”