Renwei Guo

Bandwidth-controllable reflective cholesteric gels from photo- and thermally-induced processes

A bandwidth-controllable reflective gel has been investigated from photo- and thermally-induced processes. Due to the pitch of cholesteric liquid crystal (CLC) composite increasing as the temperature rises, the CLC with short pitch is frozen by UV-curing the polymer network at low temperature and the CLC with long pitch is fixed by heat curing the polymer network at high temperature. A non-uniform pitch distribution of CLCs forms in the gel when temperature becomes low. It is demonstrated that the memory effect of the polymer network is an important mechanism for the resulting gel.

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.

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.

Effects on thermo‐optical properties of the composition of a polymer‐stabilised liquid crystal with a smectic A–chiral nematic phase transition

A photopolymerisable monomer/liquid crystal (LC)/chiral dopant/photoinitiator mixture with a smectic A (SmA*)–chiral nematic (N*) phase transition was prepared. After the SmA* phase was homeotropically oriented and then irradiated by ultraviolet light, a homeotropically oriented polymer network was formed in the SmA* phase and then a homeotropically oriented polymer network‐stabilised liquid crystal (PSLC) film with a SmA*–N* phase transition was prepared. In the temperature range of the SmA* phase, the LC molecules were homeotropically oriented and the film exhibited a transparent state. However, in the temperature range of the N* phase, the film exhibited a strong light‐scattering state owing to the fact that the LC molecules adopted a focal conic alignment affected by the homeotropically oriented polymer network. The strong light‐scattering state of the N* phase could be changed into a transparent one when an electric field was applied. The focus of this study was on the effects of composition of the PSLC film on its thermo‐optical and electro‐optical properties.

Effects of the preparing condition of a polymer-stabilised liquid crystal with a smectic-A−chiral nematic phase transition on its properties

A homeotropically-oriented polymer network stabilised liquid crystal (PSLC) film with a smectic-A (SmA)–chiral nematic (N*) phase transition was prepared. In the temperature range of the SmA phase, the liquid crystal (LC) molecules were homeotropically oriented and the film showed a transparent state. However, in the temperature range of the N* phase, the film showed a strong light scattering state due to the LC molecules adopting a focal conic alignment affected by the homeotropically-oriented polymer network. Moreover, the strong light scattering state of the N* phase could return to the transparent state when an electric field is applied on it. The focus of this study is on the effects of the preparing conditions of the PSLC film, including the curing temperature and the intensity of the ultraviolet (UV) irradiation on its thermo-optical and electro-optical properties.

Broadband reflective liquid crystalline films prepared from liquid crystals with negative dielectric anisotropy

When a certain range of frequencies of AC electric fields are applied to a chiral nematic (N*) liquid crystal (LC) with negative dielectric anisotropy ((N*-LCN), usually prepared from a nematic LC with negative dielectric anisotropy (N-LCN) doped with a chiral dopant (CD), the long axes of the LC molecules will be oriented perpendicular to the electric field, which will be helpful to improve the orientation degree compared with the orientation induced by rubbing on the inner surfaces of the substrates sandwiching the LC. After a photopolymerisable N*-LC monomer/N-LCN/CD/an ultraviolet (UV) absorber/a photoinitiator mixture with an appropriate composition is irradiated with UV light, while an electric field with an appropriate frequency and intensity is being applied to it during the UV curing, not only the planar orientation of a high degree but also a pitch gradient of the helical structure of the LC molecules are formed in the resulted polymer network/N*-LCN composite.