Ultrathin, transparent, thermally-insulated, and energy-efficient flexible window using coatable chiral-nematic liquid crystal polymer

Structural, Optical, and Mechanical Insights into Cellulose Nanocrystal Chiral Nematic Film Engineering by Two Assembly Techniques

Iridescent cellulose nanocrystal (CNC) films with chiral nematic nanostructures exhibit great potential in optical devices, sensors, painting, and anticounterfeiting applications. CNCs can assemble into a chiral nematic liquid crystal structure by evaporation-assisted self-assembly (EISA) and vacuum-assisted self-assembly (VASA) techniques. However, there is a lack of comprehensive examinations of their structure-property correlations, which are essential for fabricating materials with unique properties. In this work, we gained insights into the optical, mechanical, and structural differences of CNC films engineered using the two techniques. In contrast to the random self-assembly at the liquid-air interface in EISA, the continuous external pressure in the VASA process forces CNCs to assemble at the filter-liquid interface. This results in fewer defects in the interfaces between tactoids and highly ordered cholesteric phases. Owing to the distinct CNC assembly behaviors, the films prepared by these two methods show great differences in the nanostructure, microstructure, and macroscopic morphology. Consequently, the highly ordered cholesteric structure gives VASA-CNC films a more uniform structural color and enhanced mechanical performance. These fundamental understandings of the relationship of structure-property nanoengineering through various assembly techniques are essential for designing and constructing high-performance chiral iridescent CNC materials.

Research Progress of Electrically Driven Multi-Stable Cholesteric Liquid Crystals

Electrically driven multi-stable cholesteric liquid crystals can be used to adjust the transmittance of incident light. Compared with the traditional liquid crystal optical devices, the multi-stable devices only apply an electric field during switching and do not require a continuous electric field to maintain the various optical states of the device. Therefore, the multi-stable devices have low energy consumption and have become a research focus for researchers. However, the multi-stable devices still have shortcomings before practical application, such as contrast, switching time, and mechanical strength. In this article, the latest research progress on electrically driven multi-stable cholesteric liquid crystals is reviewed, including electrically driven multi-stable modes, performance optimization, and applications. Finally, the challenges and opportunities of electrically driven multi-stable cholesteric liquid crystals are discussed in anticipation of contributing to the development of multi-stable liquid crystal devices.

Preparation of transmittance-switchable composites from a coexistent system of polymer-dispersed and polymer-stabilized cholesteric liquid crystals

Polymer-dispersed and polymer-stabilized liquid crystals (PD&PSLCs) are the coexistent systems of PDLCs and PSLCs, in which a mesogenic polymer network forms in the LC droplets to construct the PSLC system and the combination of the LC domains and the isotropic polymer matrix makes up the PDLC system. In this work, a PD&PS cholesteric LC (PD&PSCLC) system, where the isotropic polymer walls make up the PDCLC microstructure and the liquid crystalline polymer network in the CLC droplets constructs the PSCLC system, is fabricated successfully. The CLCs are composed of a negative dielectric anisotropy nematic LC and a chiral dopant. Owing to the stabilizing effect of the liquid crystalline polymer network, the planar texture of the CLCs can be retained after the preparation process, and the PD&PSCLC sample is in a large-transmittance state. The action of a low-frequency electric field induces the disordered texture of the CLCs and the light-scattering (low-transmittance) state because of the electro-hydrodynamic effect and dynamic scattering effects. After the removal of the low-frequency electric field, the stabilizing effect of the liquid crystalline polymer network induces the CLC molecules to return to the homogeneous orientation state, and the sample also re-obtains the large transmittance. This work may provide a facile approach to fabricating polymer/CLC composites with switchable transmittance for reverse-mode smart windows.

Reduction of solar infrared heating by using highly transparent thin films based on organic chiral nematic liquid crystal polymer

This paper demonstrates a thin and transparent reflector film for the near infrared, based on chiral nematic liquid crystal (CLC) polymers. Two films reflect almost 50% of unpolarized incident light from 730 to 820 nm and from 880 to 1030 nm, while remaining completely transparent in the visible region with transmittance >90%. An efficient window uses the combination of two reflectors. After exposing two window-cubes for 2 h to direct sunlight, the temperature inside the cube with reflector windows was 4°C lower than in cube with plain windows. This reveals that the infrared (IR) reflectors can effectively control the indoor temperature. These films, which are 8 µm in thickness, can be detached from the glass substrates and used as a free-standing film, or be attached to a flexible optical foil or a solid window. The foils can be applied in buildings, offices, and automobiles to statically reduce the energy consumption required for air conditioning or lighting. The free-standing foils show acceptable resistance to polar protic solvents and are thermally stable up to 100°C.