Jiumei Xiao

Light-controllable reflection wavelength of blue phase liquid crystals doped with azobenzene-dimers

A new series of azobenzene-dimers were synthesized and doped into the blue phase liquid crystals to broaden the temperature range of BPs. It is found that not only can the reflection wavelength of BPI be reversibly controlled but BPI can also be transformed into the cholesteric phase owing to isomerization of azobenzene induced by light.

New micro‐structure designs of a wide band reflective polarizer with a pitch gradient

New microstructure designs for fabricating a wide band reflective polarizer (WBRP) from cholesteric liquid crystals are reported. The pitch difference in the WBRP is formed by orienting a single layer consisting of different glassy siloxane cyclic side chain oligomer powders on a heater. The molecular arrangement obtained is frozen by quenching. The experimental results show that various micro‐areas exhibiting different reflection wavelengths and pitch gradients are formed in the WBRP. A WBRP exhibiting the reflection properties of the original cholesteric liquid crystals is fabricated by a novel experimental process and the experimental results are in accordance with the microstructure designs of the WBRP.

Liquid crystalline and thermo-optical properties of cyclic siloxane tetramers containing cholestryl-4-allyloxy-benzoate and biphenyl-4-yl 4-allyloxybenzoate

A series of cyclic siloxane tetramers covering the whole composition range were synthesised using cyclotetrasiloxane, cholestryl-4-allyloxy-benzoate and biphenyl-4-yl 4-allyloxybenzoate, and the effects of the cholesteryl-based mesogenic units on liquid crystalline and thermo-optical properties were studied. The presence of a cholesteric mesogen was observed to widen the mesogenic temperature range and tended to induce smectic A phases in the tetramers containing a higher mole fraction of cholestryl-4-allyloxy-benzoate (X chol). The selective reflection of cholesteric tetramers shifted to shorter wavelengths with increasing X chol and temperature. Blue phases can be easily observed for those tetramers containing more than 50.0 mol% cholesteric mesogen. The shorter pitch sample showed the wider blue phases range. The blue phases range width increased from 2.8°C to 18.5°C as X chol increased from 0.5 to 1.0.

Preparation and thermo-optical characteristics of a smart polymer-stabilized liquid crystal thin film based on smectic A–chiral nematic phase transition

A smart polymer stabilized liquid crystal (PSLC) thin film with temperature-controllable light transmittance was prepared based on a smectic-A (SmA)–chiral nematic (N*) phase transition, and then the effect of the composition and the preparation condition of the PSLC film on its thermo-optical (T-O) characteristics has been investigated in detail. Within the temperature range of the SmA phase, the PSLC shows a strong opaque state due to the focal conic alignment of liquid crystal (LC) molecules, while the film exhibits a transparent state result from the parallel alignment of N* phase LC molecules at a higher temperature. Importantly, the PSLC films with different temperature of phase transition and contrast ratio can be prepared by changing the composition of photo-polymerizable monomer/LC/chiral dopant. According to the competition between the polymerization of the curable monomers and the diffusion of LC molecules, the ultraviolet (UV) curing surrounding temperature and the intensity of UV irradiation play a critical role in tuning the size of the polymer network meshes, which in turn influence the contrast ratio and the switching speed of the film. Our observations are expected to pave the way for preparing smart PSLC thin films for applications in areas of smart windows, thermo-detectors and other information recording devices.

Characteristics of wide-band reflection of polymer-stabilised cholesteric liquid crystal cell prepared from an unsticking technique

An unsticking technique was applied for the fabrication of a polymer-stabilised cholesteric liquid crystal (PSCLC) cell. The template of polymer matrix in the planarly aligned cholesteric liquid crystal (Ch-LC) was perfectly preserved by lifting off the hydrophilic antisticking substrate. The desired PSCLC cell could then be obtained by injecting a third Ch-LC sample between two designed templates of PSCLC layer. By adjusting the pitch distribution in the two stacking templates, a reflectivity of nearly 50% incident light could be yielded and the bandwidth of the reflection spectrum could be controlled accurately in the PSCLC cell. Thus, a method to modulate the wide-band reflection of 50% visible light flux range was offered.

Optical intensity-driven reversible photonic bandgaps in self-organized helical superstructures with handedness inversion

An optical intensity-driven self-organized helical superstructure was found to exhibit reversibly dynamic bandgap shifting, a wide bandgap and a hyper-reflection bandgap in one system. According to the UV-intensity dependence of the photo-isomerization ratio of chiral molecular motors, the specialized arrangement of helical pitch with chiral handedness inversion can be directly obtained along the direction of the helical axis upon ultraviolet exposure, which can be evidently observed in liquid-crystalline polymer composites.

Bandwidth-controllable reflective polarisers based on the temperature-dependent chiral conflict in binary chiral mixtures

In this study, we demonstrate a new method for obtaining a binary chiral mixture (BCM) which exhibits an unprecedented temperature-dependent helical twisting power (HTP) by utilising the chiral conflict between two chiral dopants with opposite handedness and HTP temperature dependence. Different from the common chiral dopant reported before, the addition of such a BCM could increase the clearing temperature of the host liquid crystal (LC). Due to the sufficiently large HTP value of the BCM, the three primary colours (blue, green and red) can be easily reflected by the nematic liquid crystal (N-LC)/BCM mixture at different temperatures. Moreover, the large HTP temperature dependence could be maintained and even enhanced in the longer wavelength region by adjusting the content of the BCM. Finally, based on the large HTP temperature dependence and the positive affect on the host LC of the BCM, a thermally bandwidth-controllable reflective polariser that could work in the visible region (400–750 nm) was prepared from the N-LC/BCM/polymer network composite.

Broadband reflection mechanism of polymer stabilised cholesteric liquid crystal (PSChLC) with pitch gradient

In order to reveal the nature of the formation of a broadband reflection property for polymer stabilised cholesteric liquid crystal, the spatially periodic helix structure of epoxy cholesteric liquid crystal is ‘frozen’ by cationic polymerisation following the formation of a polymer network induced by ultraviolet radical polymerisation of acrylate monomer. The fracture of the helix structure is observed by scanning electron microscopy, which reveals the presence of pitch gradient. Since the liquid crystal monomers have a UV absorption characteristic, a polymer network concentration gradient is formed after ultraviolet radical polymerisation. Using an attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrum, the concentration gradient of the polymer network and that of the chiral dopant, with the opposite trends, are confirmed. The existence of both gradients, resulting in the formation of the pitch gradient, are considered as the natural mechanism for the formation of the broadband reflection.

Effects of functionality of thiol monomer on electro-optical properties of polymer-dispersed liquid crystal films

ABSTRACT In this paper, polymer-dispersed liquid crystal (PDLC) films which based on the acrylate and the thiol monomers were first prepared by ultraviolet-initiated polymerisation. The electro-optical properties and morphologies of the PDLC films were systematically investigated. The functionality of thiol monomers and their feed ratio showed great influence on the properties of the fabricated PDLC films because of the existence of competition between thiol–acrylate reaction and acrylate monomer polymerisation reaction. This made the polymer network and electro-optical properties of the PDLC films easily tunable by the introduction of the thiol monomers. When added four-functional thiol monomer PETMP with appropriate concentration into the PDLC system, lower driven voltage and higher contrast ratio were achieved. Graphical Abstract

Study on selective reflection properties of chiral nematic liquid crystalline composites with a non‐uniform pitch distribution

The preparation is reported of particles of photopolymerisable monomer/chiral dopant composites with a crystalline (Cr)‐chiral nematic (N*) phase transition. By mixing particles with different pitches of the N* phase in the Cr phase and crosslinking the liquid crystal (LC) monomer molecules by photopolymerisation in the planarly oriented N* phase, an N*‐LC composite film with a non‐uniform pitch distribution was obtained. Experimental results show that the bandwidth of the reflection spectrum and the location of reflection band of the composite films can be controlled accurately by controlling the pitch lengths of the N* phase of the particles. Effects of polymerisation temperature and UV intensity on the non‐uniform pitch distribution of N*‐LC composite films were investigated.