主链型手性液晶弹性体(特邀)

Main-chain Chiral Liquid Crystal Elastomers(Invited)

主链型手性液晶弹性体是一类具有周期性纳米结构的软光子晶体,包括一维胆甾相液晶弹性体和三维蓝相液晶弹性体,不仅可以选择性反射圆偏振光,还能够灵敏地响应外界环境的变化,在自适应光学、变色伪装、信息加密和智能隐身等领域有着广泛的应用前景。随着新型材料体系和先进制备技术的发展,研究人员近年来提出了多种不同的策略,设计和制备具有手性纳米超结构的液晶弹性体,并深入研究了手性液晶弹性体的性能及潜在应用。本文系统综述了主链型手性液晶弹性体的设计、制备和应用进展,重点介绍了一维胆甾相液晶弹性体的平行取向法、各向异性挥发法、刮涂法、3D打印,以及无需额外取向的蓝相液晶弹性体三维纳米自组装方法,并总结讨论了主链型手性液晶弹性体存在的挑战以及未来发展方向.

Main chain chiral liquid crystal elastomers,with periodic nanostructures,are a kind of soft photonic crystals,including cholesteric liquid crystal elastomers and blue phase liquid crystal elastomers.They can not only reflect circularly polarized light selectively,but also dynamically adjust their structural color responding to changes in the environment,which could find wide applications in adaptive optics,bionic camouflage,information encryption,intelligent soft robots and so on. With the development of new material systems and advanced preparation techniques,researchers have explored various ways to design and synthesize chiral liquid crystal elastomers for emerging applications. Main-chain cholesteric liquid crystal elastomers are mainly prepared by parallel orientation,anisotropic deswelling,bar coating,and 3D printing. The parallel orientation method is a traditional method,which usually occurs within liquid crystal cells with planar-aligned alignment. Due to the interaction of liquid crystalline molecules with alignment coatings,helical nanostructures’ self-assembly can occur. The parallel orientation method has been used to design and develop photonic liquid crystal elastomers,4D photonic actuators,inflating chiral nematic liquid crystalline elastomers with broadband and pixelated camouflage,chameleon skin-like cholesteric liquid crystal elastomers and other advanced functional materials. Based on the two-stage thiol-acrylate Michael addition and photopolymerization reaction,a facial and easily scalable method has been proposed to prepare uniformly colored large-area cholesteric liquid crystal elastomer films,which benefits from anisotropic deswelling. The gel formation from the Michael addition adheres strongly to the substrate,restricting the deswelling to take place only in the vertical direction and causing an in-plane orientation of the liquid crystal director. The resulted film with helical nanostructures shows a robust, rapid, and reversible mechanochromic response to strain,resulting in a broad color shift across the full visible spectrum. During the bar coating method,the precursor was applied to a bare substrate at a certain temperature via an applicator. Because of the shear forces during the bar coating process,the mesogenic molecules are able to align along with a common director,resulting in the formation of cholesteric liquid crystal elastomers with brilliant reflection color. The thickness of the film was controlled by changing the distance between the applicator and the substrate. This bar coating method has important significance for the application of advanced photonic crystal coatings. 3D printing is an emerging and advanced fabrication technique,and current 3D printing technologies for liquid crystal elastomers include direct ink writing and digital light processing. During the direct ink writing process of cholesteric liquid crystal elastomers,a viscous ink composed of uncrosslinked liquid crystal oligomers is often extruded from the printing nozzle. The shear force generated during the extrusion process causes the mesogens to spontaneously align along with the printing path,forming periodic helical nanostructures that selectively reflect circularly polarized light.Unlike cholesteric liquid crystal elastomers,blue phase liquid crystal elastomers are generally prepared through injecting chiral liquid crystal materials into liquid crystal cells,heating to isotropic phase,and then slowly cooling down. The liquid crystal precursors can self-assemble into a three-dimensional cubic lattice without the assistant of the surface alignment. The resulting blue phase liquid crystal elastomers have narrow photonic band gaps,which could enable functional implementations in nonlinear optics,lasing,spectral imaging and energy applications. In this review,different preparation methods of main-chain chiral liquid crystal elastomers are synthetically introduced. The state-of-the-art advancement and future perspective of main-chain chiral liquid crystal elastomers are discussed. It is expected that this review could shine new light on the development of advanced chiral liquid crystal elastomers and their emerging applications in diverse fields such as adaptive photonics,soft robotics,electronic skins and beyond.