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.
Highlights
From the theoretical point of view different liquid crystal phases composed of bent-core molecules have been analysed using the Landau-de Gennes theory in [10, 11]
The twist-bend phase has been comprehensively described by Dozov [12] who has assumed that in the nematic phase composed of bent-core molecules the bend elastic constant can approach zero and even turn negative at a certain temperature
The relationship between the two approaches is briefly discussed in the last section. Very recently another molecular theory of the locally polar bent-core nematic phase has been proposed by Vanakaras and Photinos [24] who have considered a nematic with a non-conical short pitch helical structure
Summary
From the theoretical point of view different liquid crystal phases composed of bent-core molecules have been analysed using the Landau-de Gennes theory in [10, 11]. The twist-bend phase has been comprehensively described by Dozov [12] who has assumed that in the nematic phase composed of bent-core molecules the bend elastic constant can approach zero and even turn negative at a certain temperature. In this case the homogeneous director distribution will become unstable because of the rapidly growing bend and twist deformations which are stabilized by the corresponding higher-order terms. The relationship between the two approaches is briefly discussed in the last section Very recently another molecular theory of the locally polar bent-core nematic phase has been proposed by Vanakaras and Photinos [24] who have considered a nematic with a non-conical short pitch helical structure. We calculate effective (renormalized) elastic constants as functions of temperature and show how the bend elastic constant K33 may go to zero while K11 and K22 behave in a conventional way
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