Polymer Stabilization of Uniform Lying Helix Texture in a Bimesogen-Doped Cholesteric Liquid Crystal for Frequency-Modulated Electro-Optic Responses.

Chia-Hua Yu,Wei Lee,Po-Chang Wu

doi:10.3390/ma15030771

Chia-Hua Yu, Wei Lee + Show 1 more

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https://doi.org/10.3390/ma15030771

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Abstract

A polymer network (PN) can sustain the uniform lying helix (ULH) texture in a binary cholesteric liquid crystal (LC) comprising a calamitic LC and a bimesogenic LC dimer. Upon copolymerization of a bifunctional monomer with a trifunctional monomer at a concentration of 5 wt% to create the desired polymer network structure, the PN-ULH was obtained with high stability and recoverability even when cycles of helical unwinding-to-rewinding processes were induced after the electrical or thermal treatment. Utilizing dielectric spectroscopy, the flexoelectric-polarization-dominated dielectric relaxation in the PN-ULH state was characterized to determine two frequency regions, f < fflexo and f > fdi, with pronounced and suppressed flexoelectric effect, respectively. It is demonstrated that the cell in the PN-ULH state can operate in the light-intensity modulation mode by the flexoelectric and dielectric effects at f < fflexo and phase-shift mode by the dielectric effect at f > fdi. Moreover, varying the voltage frequency from f < fflexo to f > fdi results in a frequency dispersion of transmittance analogous to that of flexoelectric-polarization-dominated dielectric relaxation. The unique combination of the bimesogen-doped cholesteric LC with a stable and recoverable PN-ULH texture is thus promising for developing a frequency-modulated electro-optic device.

Highlights

Uniform lying helix (ULH) is a kind of cholesteric liquid crystal (CLC) texture whose helical pitch is shorter than the wavelengths of visible light and is uniformly aligned along a preferred direction in the substrate plane of confining geometry
If the voltage is beyond a critical threshold such that the dielectric effect is electrically induced, the helix of the CLC with positive dielectric anisotropy is gradually unwound with ascending voltage amplitude, and the LC configuration is eventually sustained in the helix-free homeotropic state at high voltage, leading to the change in effective birefringence from/2 in the field-off uniform lying helix (ULH) state to no in the voltage-sustained homeotropic state, where ne and no are the extraordinary and ordinary refractive indices of the LC, respectively [2]
By applying a significantly high voltage at V = 100 Vrms and f = 5 kHz across the cell thickness, enabling the ULH helix to be unwound and LC orientation sustained in the homeotropic state, the dark image at α = 0◦ (45◦ ) at V = 0 V was virtually unchanged after the voltage removal (Figure 3a), connoting that the polymer network (PN)-ULH was retained as it was prior to the voltage application

Summary

Introduction

When a voltage is applied across the ULH helix, two types of helix reorientation can be generated by voltage-induced flexoelectric and dielectric effects. At a moderate voltage where the dielectric effect is negligible, the flexoelectric coupling of LC molecules with the electric field produces a periodic splay-bend deformation, giving rise to the in-plane deviation of the ULH optic axis. The magnitude of such a deviation angle induced by the flexoelectric effect is in principle a linear function of the voltage amplitude and is in positive correlation with the average value of the splay and bend flexoelectric coefficients [1]. Since the chiral-flexoelectric effect with sub-millisecond response time was first exploited by

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Conclusion