Anti-UV Passive Radiative Cooling Chiral Nematic Liquid Crystal Films for Thermal Management.

Passive radiative cooling (PRC) can spontaneously dissipate heat to outer space through atmospheric transparent windows, providing a promising path to meet sustainable development goals. However, achieving simultaneously high transparency, color-customizable, and thermal management of PRC anti ultraviolet (anti-UV) films remains a challenge. Herein, a simple strategy is proposed to utilize liquid crystalline polymer, with high mid-infrared emissive, forming customizable structural color film by molecular self-assembly and polymerization-induced pitch gradient, which guarantees the balance of transparency in visible spectrum and sunlight reflection, rendering anti-UV colored window for thermal management. By performing tests, temperature fall of 5.4 and 7.9 °C are demonstrated at noon with solar intensity of 717 W m-2 and night, respectively. Vivid red-, green-, blue-structured colors, and colorless films are designed and implemented to suppress the solar input and control the effective visible light transmissivity considering the efficiency function of human vision. In addition, temperature rise of 11.1 °C is achieved by applying an alternating current field on the PRC film. This study provides a new perspective on the thermal management and aesthetic functionalities of smart windows and wearables.

Liquid Crystals Doped with Chiral Fluorescent Polymer: Multi-Color Circularly Polarized Fluorescence and Room-Temperature Phosphorescence with High Dissymmetry Factor and Anti-Counterfeiting Application.

Chiral nematic liquid crystals (N*-LCs) can tremendously amplify circularly polarized luminescence (CPL) signals. Doped emissive N*-LCs have been substantially explored. However, their CPL performances still need to be improved, mainly due to the unsatisfying helical twisting power (HTP) of commonly used chiral fluorescent dopants. Chiral fluorescent helical polymers (CFHPs) have outstanding optical activity and CPL performance. The present contribution reports the first success in constructing emissive N*-LCs by doping CFHP into nematic liquid crystals (5CB, N-LCs). The helical assembly structures of N*-LCs effectively amplify the CPL signals of the CFHP. Owing to the high HTP of CFHP, the selective reflection band of N*-LC can be adjusted to fully cover its emission band. A nearly pure CPL with a dissymmetry factor (glum ) up to -1.87 is realized at 9 wt% doping concentration. Taking advantage of the selective reflection mechanism, multi-color CPL-active N*-LCs with high glum are fabricated via further adding achiral fluorophores. Also noticeably, circularly polarized room-temperature phosphorescence with glum up to -1.57 is achieved. Anti-counterfeiting application is demonstrated by exploiting multi-mode optical characteristics of the created N*-LCs. The established strategy for constructing emissive N*-LCs provides a platform for future exploring of CPL-active N*-LCs.

Anisotropic Iridescence and Polarization Patterns in a Direct Ink Written Chiral Photonic Polymer.

The iridescence of structural color and its polarization characteristics originate from the nanoscale organization of materials. A major challenge in materials science is generating the bright, lustrous hues seen in nature through nanoscale engineering, while simultaneously controlling interaction of the material with different light polarizations. In this work, a suitable chiral nematic liquid crystal elastomer ink is synthesized for direct ink writing, which self-assembles into a chiral photonic structure. Tuning the writing direction and speed leads to the programmed formation of a slanted photonic axis, which exhibits atypical iridescence and polarization selectivity. After crosslinking, a freely programmable, chiroptical photonic polymer material is obtained. The strongly perspective-dependent appearance of the material can function as specialized anticounterfeit markers, as optical elements in decorative iridescent coatings, or, as demonstrated here, in optically based signaling features.

Cellulose nanocrystal research; A personal perspective.

This contribution to the special issue of Carbohydrate Polymers commemorating the 100th Anniversary of the Cellulose and Renewable Materials Division of the American Chemical Society is a personal account, from a research chemist's point of view, of some aspects of the discovery, development and utilization of nanocellulosic materials. The main focus is on cellulose nanocrystals stabilized by sulfate half-ester surface charges.

Bioinspired Multiple Stimuli-Responsive Optical Microcapsules Enabled by Microfluidics.

Optical microcapsules encapsulating optical materials inside a symmetric spherical confinement are significant elements for the construction of optical units and the integration of optical arrays. However, the multiple stimuli-responsive characteristic of optical microcapsules still remains a challenge due to the insuperable physical barrier between the optical material core and the outside shell and the lack of effective mechanisms to trigger the dynamic switch of the encapsulated optical materials. Inspired by the dual-mode optical modulation of chameleon skins, a novel biomimetic binary optical microcapsule that combines the visible light reflection of chiral nematic liquid crystals and photoluminescence emission of rare-earth complexes is assembled by microfluidic emulsification and interfacial polymerization. The reflected color, fluorescent intensity, and size of the optical microcapsules are facilely controlled in the microfluidic chip by adjusting the composition and flow rate of the injected fluids. Most importantly, the biomimetic binary optical microcapsules demonstrate three reversible responsive behaviors, thermotropic reflection evolution, temperature-dependent fluorescence emission, and Fredericks electro-optical response. The bioinspired multiple stimuli-responsive optical microcapsules enabled by microfluidics provide a templated strategy to manufacture the next generation of intelligent optical units and to achieve the dynamic response of hybrid photonic devices.

Self-assembly properties of carboxylated tunicate cellulose nanocrystals prepared by ammonium persulfate oxidation and subsequent ultrasonication.

Tunicate cellulose, extracted from the marine animal, has drawn increasing attention as the high crystallinity and aspect ratio. However, it is hard to prepare tunicate cellulose nanocrystals (tCNCs) with narrow size distribution in the traditional way, especially for the carboxylated samples, which also affects their lyotropic liquid crystal behavior to a certain extent. Herein, carboxylated tCNCs with uniform nanoscale dimensions and high surface charges density were prepared through ammonium persulfate (APS) oxidation and ultrasonic post-processing. Of particular interest, the formation of carboxylated tCNCs lyotropic chiral nematic liquid crystals was observed for the first time, which displayed obvious birefringence and fingerprint texture. Meanwhile, it was found that the critical concentration of phase separation for tCNCs suspension was around 3.5 wt% from the phase diagram. This study provides an efficient way to fabricate carboxylated tCNCs, and the self-assembly properties may lead to great potential applications in constructing advanced functional materials.

Chiral Reaction Field with Thermally Invertible Helical Sense that Controls the Helicities of Conjugated Polymers.

A chiral reaction field with thermally invertible helical sense enables control of the helicity of the reaction product, which is a central challenge in asymmetric synthesis that has yet to be overcome. A novel chiral compound comprising two types of chiral moieties with opposite helicities and temperature dependences is synthesized; this compound is added as a chiral dopant to a mixture of nematic liquid crystals to prepare a chiral nematic liquid crystal (N*-LC). The N*-LC containing the chiral dopant exhibits thermally invertible helicity to yield left- and right-handed helical senses at low and high temperatures, respectively. Interfacial polymerization of acetylene is achieved in the N*-LC by modulating the temperature. Helical polyacetylenes (H-PAs) that are synthesized at low (-12 °C) and high (28 °C) temperature show right- and left-handedness, respectively, in terms of the fibrils, fibril bundles, and spiral morphology. In addition, the helical sense of H-PA is opposite that of the N*-LC because of the peculiar polymerization mechanism for acetylene in the N*-LC. The current N*-LC is the first chiral reaction field that has not only the thermally invertible helical sense but also the chemical functions and stability needed to serve as the medium for polymer reactions.

Visual Appearance of Chiral Nematic Cellulose-Based Photonic Films: Angular and Polarization Independent Color Response with a Twist.

Hydroxypropyl cellulose (HPC) is a biocompatible cellulose derivative capable of self-assembling into a lyotropic chiral nematic phase in aqueous solution. This liquid crystalline phase reflects right-handed circular polarized light of a specific color as a function of the HPC weight fraction. Here, it is demonstrated that, by introducing a crosslinking agent, it is possible to drastically alter the visual appearance of the HPC mesophase in terms of the reflected color, the scattering distribution, and the polarization response, resulting in an exceptional matte appearance in solid-state films. By exploiting the interplay between order and disorder, a robust and simple methodology toward the preparation of polarization and angular independent color is developed, which constitutes an important step toward the development of real-world photonic colorants.

3D Helix Engineering in Chiral Photonic Materials.

Engineering the helical structure of chiral photonic materials in three dimensions remains a challenge. 3D helix engineered photonic materials are fabricated by local stratification in a photopolymerizable chiral nematic liquid crystal. The obtained chiral photonic materials reflect both handedness of circular polarized light and show super-reflectivity. Simulations match the experimentally observed photonic properties and reveal a distorted helical structure. 3D engineered polymer films can be made that reflect both left- and right handed circular and linear polarized light dependent and exhibit a changing color contrast upon altering the polarization of incident light. Hence, these 3D engineered photonic materials are of interest for new and emerging applications ranging from anti-counterfeit labels and data encryption to aesthetics and super-reflective films.

Pressure-Responsive Hierarchical Chiral Photonic Aerogels.

Pressure-responsive chiral photonic aerogels are fabricated by combining liquid crystal self-assembly and ice-templating processes. The aerogels have a hierarchical structure in which the primary 2D chiral nematic structured walls of cellulose nanocrystals form ribbons that support a secondary 3D cellular network. Owing to the flexibility of the aerogels in solvent, the 3D structure of the aerogel can easily be transformed to a 2D structure by pressure-induced rearrangement. The aerogels vary from white in color, which arises from light scattering, to a reflective photonic crystal displaying bright iridescent colors that depend on the immersed solvent. A solvent-sensitive ink that shows quick color response to different solvents is designed using the pressure-responsive photonic aerogel. This material demonstrates a new response mechanism for the design of smart and mechanoresponsive photonic materials.

Uncovering the Circular Polarization Potential of Chiral Photonic Cellulose Films for Photonic Applications.

Circularly polarized light (CPL) is central to photonic technologies. A key challenge lies in developing a general route for generation of CPL with tailored chiroptical activity using low-cost raw materials suitable for scale-up. This study presents that cellulose films with photonic bandgaps (PBG) and left-handed helical sense have an intrinsic ability for circular polarization leading to PBG-based CPL with extraordinary |g | values, well-defiend handedness, and tailorable wavelength by the PBG change. Using such cellulose films, incident light ranging from near-UV to near-IR can be transformed to passive L-CPL and R-CPL with viewing-side-dependent handedness and |g | values up to 0.87, and spontaneous emission transformed to R-CPL emission with |g | values up to 0.68. Unprecedented evidence is presented with theoretical underpinning that the PBG effect can stimulate the R-CPL emission. The potential of cellulose-based CPL films for polarization-based encryption is illustrated. The evaporation-induced self-assembly coupled with nanoscale mesogens of cellulose nanocrystals opens new venues for technological advances and enables a versatile strategy for rational design and scalable manufacturing of organic and inorganic CPL films for photonic applications.

Cellulose Nanocrystals/Polyacrylamide Composites of High Sensitivity and Cycling Performance To Gauge Humidity.

Cellulose nanocrystals (CNCs) have attracted much interest due to their unique optical property, rich resource, environment friendliness, and templating potentials. CNCs have been reported as novel photonic humidity sensors, which are unfortunately limited by the dissolution and unideal moisture absorption of CNCs. We, in this study, developed a high-performance photonic humidity composite sensor that consisted of CNCs and polyacrylamide; chemical bonding was induced between the two components by using glutaraldehyde as a bridging agent. The composites inherited the chiral nematic structure of CNCs and maintained it well through a cycling test. A distinct color change was observed for these composites used as a humidity indicator; the change was caused by polyacrylamide swelling with water and thus enlarging the helical pitch of the chiral nematic structure. The composites showed no degradation of the sensing performance through cycling. The excellent cycling stability was attributed to the bonding between polyacrylamide and CNCs. This composite strategy can extend to the development of other photonic indicators.