Characterisation of novel flexoelectric bimesogens

Abstract

The research presented in this thesis relates to the flexoelectro-optic switching effect observed in bimesogenic chiral nematic liquid crystal materials.  By coupling flexoelectrically to an applied electric field it is possible for the optic axis of a chiral nematic liquid crystal material to be rotated, inducing a change in the optic properties of the material.  This rotation of the optic axis is fast, in-plane, and the magnitude of rotation is proportional to the amplitude of the applied field.  As well as being dependent on the applied field amplitude, the flexoelectro-optic switching process has no threshold, resulting in ‘v-shaped’ switching.

The bimesogenic nematic liquid crystal materials studied in this work have been found to be particularly suited for flexoelectro-optic switching, producing extremely high optic axis rotation (> 45°) and fast response times (<1 ms).  By using bimesogenic liquid crystals, it is possible to have both high dipole moments and a low dielectric anisotropy, increasing the potential for flexoelectric coupling and decreasing the undesirable effects of dielectric coupling, respectively.  The ratio of the effective flexoelectric coefficient to the average of the splay and bend elastic constants, ē/K, serves as a figure of merit for a material’s suitability for flexoelectro-optic switching.  Typically, for a chiral nematic monomesogenic material, the ratio ē/K is less than 0.6 C N^-1  m^-1; however, for the bimesogenic materials studied in this work the ratio ranges from 0.8 to 2.35 C N^-1 m^-1.

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