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Abstract
The physical properties of the nematic phases formed by four bent-core oxadiazole based materials are reported. In particular, the splay (K11), twist (K22) and bend (K33) elastic constants, the birefringence and the dielectric anisotropy of the materials are described and the effect of chain length and the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure on these parameters is determined. The birefringence and order parameter are found to be independent of the modification of molecular structure. The dielectric anisotropy is quite strongly dependent on molecular structure; the fluoro-substituted material has the largest magnitude of dielectric anisotropy while the alkyl-substituted compound has the smallest. Changes in the molecular length and fluoro-substitution in the bent-core materials are found to have little influence on the splay, twist and bend elastic constants at equivalent reduced temperatures. However, the material substituted with an alkyl terminal chain exhibits both smaller elastic constants and a less marked dependence on temperature than the alkoxy-substituted compounds. A possible insight into the behaviour of the elastic constants relevant to the formation of the dark conglomerate phase, which underlies the nematic phase in one of the compounds studied, is suggested by following the analysis proposed by Berreman and Meiboom. Importantly, using molecular field theory and atomistic modelling, we calculate elastic constants that are in excellent agreement with the experimental values. Our conclusion that the elasticity in the nematic phase formed from bent-core molecules is not strongly influenced by changes to the terminal chains or the presence of fluoro-substituents at the outer phenylene group of the aromatic core structure is in agreement with our previous work showing that the dominant parameter is the bend angle.
Highlights
Bent-core nematic liquid crystals (NLCs) are an exciting class of mesogenic systems
We have found that neither these variations, nor those deriving from the change of the lateral chain signi cantly affect the average molecular shape: the plots of the elastic constants as a function of the Szz order parameter calculated for compounds 2 and 3 are very similar to each other and not very different from that shown in Fig. 6(a) for compound 4
This paper describes the temperature dependence of the splay, twist and bend elastic constants along with the dielectric anisotropy, birefringence and order parameter measurements in four related bent-core oxadiazole compounds
Summary
That the bend angle of bent-core molecules plays an extremely important role in determining the elastic properties.[10]. While the role of bend angle is relatively clear, the difficulty of measuring the physical properties of bent-core nematic liquid crystal materials has ensured that, to date there are no reports on the vital in uence of other structural parameters on the elastic behaviour in these materials. The Frank elastic constants are key physical parameters in nematic liquid crystals. The in uence of molecular structure variations in calamitic liquid crystals on the elastic behaviour is well-understood. Numerous reports describe the in uence of factors such as the change in molecular length and width on the elastic properties of calamitic NLCs.[11,12,13,14,15]. We further examine theoretical ts to experimental data, together with calculations of the elastic constants from atomistic modelling
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