Conformational flexoelectricity in nematic liquid crystals

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Abstract
Using the direct flexoelectric effect in an hybrid (homeotropic-planar) nematic cell, we have measured the bulk flexoelectric coefficient e* for two cyanobiphenyl compounds, 8CB and 8OCB. For 8CB, the flexo coefficient is comparable in sign and magnitude with that of MBBA, i.e. e* ∼ + 10-4 cgs. 8OCB presents an anomalously large and negative e* ∼ - 5 x 10-4 cgs. The small influence of the cyano group can be explained by molecular pairing. The large effect from the oxygen is interpretated by a coupling between the macroscopic bend distortion and conformations of the alkoxy chains, i.e. as conformational (dipolar) flexoelectricity. The temperature dependence of e* shows an unexpected proportionality to the nematic order parameter S. This may be related to the steric constraint, from the rigid rod cores, on the entropy of the alkoxychains