Effects of stabilization temperatures on electro-optic characteristics of polymer-stabilized optically isotropic liquid crystals

It has previously been shown that for polymer-stabilized cholesteric liquid crystals (PSCLCs) with negative dielectric anisotropy, the position and bandwidth of the selective reflection notch can be controlled by a direct-current (DC) electric field. The field-induced deformation of the polymer network that stabilizes the devices is mediated by ionic charges trapped in or near the polymer. A unique and reversible electro-optic response is reported here for relatively thin films (≤5 μm). Increasing the DC field strength redshifts the reflection notch to longer wavelength until the reflection disappears at high DC fields. The extent of the tuning range is dependent on the cell thickness. The transition from the reflective to the clear state is due to the electrically controlled, chirped pitch across the small cell gap and not to the field-induced reorientation of the liquid crystal molecules themselves. The transition is reversible. By adjusting the DC field strength, various reflection wavelengths can be addressed from either a different reflective (colored) state at 0 V or a transparent state at a high DC field. Relatively fast responses (~50 ms rise times and ~200 ms fall times) are observed for these thin PSCLCs.

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Effect of Ion Concentration on the Electro-Optic Response in Polymer-Stabilized Cholesteric Liquid Crystals

Crystals ◽

10.3390/cryst11010007 ◽

2020 ◽

Vol 11 (1) ◽

pp. 7

Author(s):

Kyung M. Lee ◽

Timothy J. Bunning ◽

Timothy J. White ◽

Michael E. McConney ◽

Nicholas P. Godman

Keyword(s):

Liquid Crystals ◽

Polymer Network ◽

Ion Concentration ◽

Cholesteric Liquid Crystals ◽

Reflection Band ◽

Ion Density ◽

Electro Optic ◽

Order Of Magnitude ◽

Polymer Stabilized ◽

Bandwidth Broadening

We have previously reported that the application of a DC field can adjust the position and/or bandwidth of the selective reflection notch in polymer-stabilized cholesteric liquid crystals (PSCLCs). The proposed mechanism of these electro-optic (EO) response is ion-facilitated electromechanical deformation of the polymer stabilizing network. Accordingly, the concentration of ions trapped within the polymer network should considerably influence the EO response of PSCLC. Our prior studies have indicated that photoinitiators can increase ion density in PSCLC by an order of magnitude. Here, we isolate the contribution of ionic impurities associated with liquid crystal monomers (LCMs) by utilizing initiator-less polymerization. PSCLCs prepared with LCM with low ion concentration show bandwidth broadening of the reflection band whereas PSCLCs prepared with LCM with high ion concentration exhibit a red shifting tuning of the reflection band. The extent of the tuning or bandwidth broadening of the CLC reflection band depends on the concentration of LCMs and the chirality of the LCM.

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Formation of Polymer Stabilized Ferroelectric Liquid Crystals using a Fluorinated Diacrylate

MRS Proceedings ◽

10.1557/proc-559-123 ◽

1999 ◽

Vol 559 ◽

Author(s):

C. Allan Guymon ◽

Christopher N. Bowman ◽

Christopher N. Bowman

Keyword(s):

Liquid Crystal ◽

Liquid Crystals ◽

Polymer Network ◽

Isotropic Phase ◽

Ferroelectric Liquid Crystals ◽

Smectic C ◽

Electro Optic ◽

Polymerization Behavior ◽

Mechanically Stabilized ◽

Polymer Stabilized

ABSTRACTFerroelectric liquid crystals (FLCs) have shown great potential for use in electro-optic and display technology due to their inherently fast switching speeds and bistability. Recently, considerable research has been devoted to FLCs mechanically stabilized by a polymer network. The network is formed typically by in situ polymerization of a monomer dissolved in the FLC. Because of the inherent order in the FLC, the polymerization behavior may be significantly different than what might be expected in solution polymerizations. These deviations result largely from the segregation properties of the monomer in the liquid crystal. One class of monomers, namely fluorinated acrylates, is a likely candidate for inducing novel segregation, polymerization and electro-optic behavior in polymer stabilized ferroelectric liquid crystals (PSFLCs). The use of fluorinated moieties has a significant impact on the phase and polymerization behavior of liquid crystal systems. This study focuses on the polymerization of a fluorinated diacrylate, octafluoro 1,6-hexanediol diacrylate (FHDDA), to form PSFLCs and the consequent impact of the polymerization on the ultimate performance. Interestingly, as the temperature is increased and the order of the system decreases, a dramatic increase in the polymerization rate is observed. This increase is especially prominent for polymerizations in the smectic C* phase for which the rate is more than five times that exhibited at much higher temperatures in the isotropic phase. As with other monomer/FLC systems, the segregation of the monomer plays a role in this polymerization behavior as the monomer segregates between the smectic layers of the liquid crystal. The segregation properties also significantly impact the ultimate electro-optic properties. Both ferroelectric polarization and response time of the PSFLC change markedly with different polymerization temperatures, and approach values very close to those of the neat FLC under appropriate polymerization conditions. This behavior not only provides a unique mechanism for rate acceleration in PSFLCs, but also paves the way for new methods to optimize performance in these materials.

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