Understanding the unusual reorganization of the nanostructure of a dark conglomerate phase

Abstract

The dark conglomerate phase exhibited by a bent-core liquid crystal material shows remarkable properties including an electric-field tuneable chiral domain structure and a large (0.045) reduction of refractive index, while maintaining an optically dark texture when observed under crossed polarisers. A detailed investigation of the system is presented, leading to a model that is fully consistent with the experimental observations. It reports the observation of two distinct regimes in the DC phase: a higher temperature regime in which the periodicity measured by small angle X-ray scattering decreases slightly (0.5%) and a lower temperature regime where it increases considerably (16%). Also, the paper discusses the unusual electric field-induced transformations observed in both the regimes. These changes have threshold fields that are both temperature and frequency dependent, though the phenomena are observed irrespective of device thickness, geometry and the alignment layer. The electro-optic behaviour in the DC phase correspond to a number of structural changes leading to unusual changes in physical properties including small (1%) increase in periodicity and a doubling of the average dielectric permittivity. We propose a model of the DC phase where in the ground state the nanostructure of phase exhibits an anticlinic antiferroelectric organization. Under an electric field, it undergoes a molecular rearrangement without any gross structural changes leading to an anticlinic ferroelectric order while keeping the overall sponge-like structure of the DC phase intact.2

Dark conglomerate phases of bent-core liquid crystals

“…Once the enantiomeric state is reached, further increase of the electric field leads to an achiral state. This is due to a switching from the antiferroelectric arrangement of the layers to the ferroelectric arrangement (switching between SmCAPA to SmCAPF) [54]. This implies that the collective reorientation due to the polarity of the phase occurs by the rotation of the molecules around their long axes and not around the tilt cone, which is in line with the small change (1%) in periodicity observed by SAXS [ Fig.…”

“…Let us represent the spontaneously saddle-splayed layers of the DC phase as a sphere for the purpose of visualisation such that the layer normal is pointing outward at each point and is perpendicular to the surface of the sphere for the ground state of the DC phase. At the 'poles', the average orientation of molecules is vertical and at the 'equator' the average orientation of the molecules is horizontal [54]. For such as configuration, a low electric field is not sufficient to cause any layer reorientation, hence it remains isotopically distributed.…”

“…Therefore, the absence of a field-induced complete deformation of the sponge-like structure of the DC phase of certain materials could be because of the possibility of a more highly negative K24 elastic constant in them. Alternatively, it is also possible that the sponge-like structure must be retained because of the lack of electrical torque acting at the poles of the structure [54].…”

“…However, below is an example of a DC phase observed in an oxadiazole based bent-core liquid crystal in which the electro-optic behaviour of the DC phase under an electric field corresponds to a number of structural changes [53]. They lead to unusual changes in physical properties and transformation where the overall sponge-like structure of the DC phase remains intact even under high electric fields approaching the dielectric breakdown limit [54].…”

Switchable Liquid Crystal Contact Lenses for the Correction of Presbyopia

“…Other Liquid crystal phases offer the potential for polarization independence too. For example, the dark-conglomerate phase formed from certain bent-core liquid crystals exhibits a decrease in the refractive index with increasing applied field, due to the change from an optically isotropic medium to a negatively uniaxial medium with the optic axis parallel to the applied field [101]. The performance of polarization independent lenses has been considered theoretically [102], although currently the temperatures and fields required are too high for practical application.…”