Pitch-Length Independent Threshold Voltage of Polymer/Cholesteric Liquid Crystal Nano-Composites

Abstract

Polymer/cholesteric liquid crystal (ChLC) nano-composites consisting of mesogenic monomers and LCs have nano-sized LC domains dispersed in an anisotropic polymer matrix. They exhibit characteristics not observed in conventional ChLCs, such as sub-millisecond and "deformation-free" electro-optic tuning of the selective reflection band; however, their driving voltage is high compared to conventional ChLCs, and is an issue that needs to be solved for the practical use. Here, we investigate the helical pitch dependence of threshold voltage in polymer/ChLC nano-composites. Five samples with different helical pitches were prepared and their electro-optic characteristics were compared before and after photopolymerization. Although the threshold voltage of the unpolymerized samples were inversely proportional to its helical pitch, the threshold voltage of the polymerized samples showed no dependence on the helical pitch. These results are explained to be a consequence of the driving mechanism of the polymer/ChLC nano-composite, in which electro-optic switching is achieved as a consequence of the nano-confined LC molecules reorienting along the electric field, instead of the helical structure becoming unwound. The threshold voltage is independent of pitch length because the pore sizes are similar in all samples.

Graphene-based liquid-crystal microlens arrays for synthetic-aperture imaging

“…Simultaneously, the CR approaches the value of ∼0.793, which means that the stray light cannot be neglected. According to the experiments, the driving voltage signal of 1.5 Vrms is also the threshold voltage, which is slightly smaller than that reported before because of a better driving characteristics of the GLCMA [18,19]. As gradually increasing the applied voltage signal, the GLCMA demonstrates a better converging effect with smaller spot size and higher sharpness and relatively high CR value, so as to demonstrate obvious advantages of current devices with graphene electrodes.…”

Light Intensity-Selective Photopolymerization and Photoisomerization for Creating Colorful Polymer-Stabilized Cholesteric Liquid Crystal Patterns

“…Structural colors based on periodic organized nanostructures have been widely found in nature, such as beetles and butterflies. , During the last few decades, artificial structurally colored materials have been prepared using colloidal crystals, − block copolymers, − or cholesteric liquid crystals (CLCs). − Polymer-stabilized CLC (PSCLC) films, − a sub-category of CLCs, are usually prepared using reactive mesogens such as acrylates, − vinyl-terminated compounds, − oxetanes, or silsesquioxanes . The structural colors originate from the selective Bragg reflection (λ = nP ; λ is the reflection wavelength; n is the average refractive index, and P is the helical pitch), which are tunable by changing the amount of chiral dopants.…”

Wavelength Tuning of the Photonic Band Gap of an Achiral Nematic Liquid Crystal Filled into a Chiral Polymer Scaffold

“…Since these parameters are sensitive to a range of external stimuli, this means that there are numerous routes by which the wavelength of the band gap may be “tuned”. For example, research has shown that the pitch of the helix may be changed using mechanical forces, light, and magnetic and electric fields − whereas studies have shown that the relevant refractive indices may be changed using temperature and electric fields. − …”

“…One method for stabilizing the standing helix geometry in the presence of an external electric field involves using reactive mesogens to form a polymer network that “locks in” the alignment and prevents it from collapsing into a focal conic texture at high electric field amplitudes. This can either involve dispersing low concentrations of polymer by weight (<10 wt %), commonly referred to as polymer-stabilized LCs, or higher concentrations (>10s of wt %) where the polymer forms more of a dense matrix consisting of micro- and nano-sized pores that contain the nonreactive LC components. − The nature of these pores is determined by a combination of the reactive mesogen concentration and the photocuring conditions (including the temperature at which the network is formed) . It has been shown that these structures are robust to high electric fields and that the reflection band can be tuned across a broad range of wavelengths (up to 141 nm has been reported) and with a fast relaxation (field ON–OFF) response time (as low as 10 μs).…”