Research progress of single/dual network liquid crystal elastomers based on dynamic bonds
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摘要: 动态键交联的液晶弹性体 (LCEs) 在外界刺激(如光、电、热等)下，能通过改变自身体积或形状执行宏观运动，具有优异的分子协同、自适应等性能，在软体机器人、人工肌肉和微流控等领域具有重要的应用前景。LCEs产生可逆形变的先决条件是实现内部液晶取向的有效控制。动态键的断裂和重组，不仅可以解耦交联网络的构筑和取向调控，还能够改善材料的再加工性能，赋予材料重塑变形、自修复和形状记忆等新的功能。因而交联网络结构(交联剂类型、结构、不同交联网络的协同)的合理设计与构筑对于构建性能优异且多功能集成的液晶弹性体极为重要。本文综述了液晶取向控制和具有单/双动态交联(包括动态非共价键和共价键)网络的液晶弹性体制备与应用的研究进展，并对该领域的发展前景进行了展望。Abstract: Liquid crystal elastomers (LCEs) containing dynamic cross-linking bonds are capable of undergoing macroscopic motion through alterations in volume or shape in response to external stimuli, including light, electricity, or heat. These materials demonstrate remarkable molecular cooperative effects and adaptive properties, offering considerable potential in the fields of soft robotics, artificial muscles, and microfluidics. Effective control of the internal liquid crystal orientation in LCEs is essential for achieving reversible deformation. The breakage and reformation of dynamic bonds not only decouples the construction of cross-linked networks and orientation control, but also enhances the reprocessing properties of materials, enabling new functionalities like remodeling deformation, self-healing, and shape memory. Therefore, the deliberate design and construction of cross-linked network structures, including the selection of cross-linking agents, their structures, and cooperative effects of various networks, are crucial for producing liquid crystal elastomers with exceptional performance and multifunctional integration. This review comprehensively discusses advancements in the preparation and application of LCEs, encompassing liquid crystal orientation control and single/double dynamic cross-linking networks (involving dynamic non-covalent and covalent bonds), and delineates future prospects for development in this field.
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图 2 (a) 溶剂蒸发法制备LCNs示意图及其分子结构[12]；(b) 自由溶剂蒸发形成多结构域LCN示意图及其横截面SEM图像和2 D WAXD图像[12]；(c) 定向溶剂蒸发(单轴拉伸)形成单结构域LCN的示意图及其截面SEM图像和2 D WAXD图像[12]
Figure 2. (a) Schematic diagram and molecular structure of LCNs prepared by solvent evaporation method[12]; (b) Schematic illustrations of the formation of polydomain LCN via free solvent evaporation and its Cross-sectional SEM images and 2 D WAXD images[12]; (c) Schematic illustrations of the formation of monodomain LCN via aligned solvent evaporation (uniaxial stretching) and Cross-sectional SEM images and 2 D WAXD images[12]
图 4 (a) 聚合物单体和紫外光引发剂的分子结构[22]；(b) 湿敏LCN带的制备示意图[22]；(c) SO2引起的膜表面变化示意图[22]；(d) SO2门控行为的机制示意图[22]
Figure 4. (a) Chemical structures of monomers and UV initiator[22]; (b) Illustration of fabricating humidity-sensitive LCN strip[22]; (c) Schematic illustration of the SO2-induced change of the film surface[22]; (d) Schematic of the mechanism of the SO2-gated behavior[22]
图 6 人工肌肉的纳米复合材料[30]: (a) 具有功能线穿越液晶网络设计结构的复合材料的合成路线示意图；(b) 三步多重刺激响应周期示意图
Figure 6. The multifunctional polymer composite for artificial muscle[30]: (a) Schematic presentation of the synthetic route of composites with the designed structure as a functional thread passing through the liquid crystal network; (b) Schematic diagram of the three-step multiple stimulus-response cycle
图 7 (a) SMPU-HOBA复合材料的超分子结构和SMPU硬段区域的示意图[31]；(b) SMPU-HOBA复合材料rbSME的系列照片[31]；(c) 由SMPU-0.4 HOBA制成的夹持装置可逆地捕获和释放硬币[31]
Figure 7. (a) Supramolecular structure of the SMPU-HOBA composites and schematic representation of the hard segment region of SMPU[31]; (b) Photograph series showing the rbSME of SMPU-HOBA composites[31]; (c) A gripper device made from SMPU-0.4 HOBA reversibly catches and releases a coin[31]
图 9 液晶环氧树脂3D柔性驱动器[33]：(a) 未取向的液晶环氧树脂固化膜LCETs的制备；(b) 基于酯交换将两个未取向的LCET膜压铸在一起；(c) LCET的取向过程；(d) 逐步重新编程LCET创建具有可逆致动的3D虹膜状致动器示意图；(e) 弯曲LCET复合致动器的光触发可逆致动；(f) 螺旋LCET复合致动器的光触发可逆致动
Figure 9. Epoxy resin 3D flexible actuator[33]: (a) The synthesis of unaligned LCETs. b) Two unaligned LCET films were compress-moulded together based on the transesterification. (c) The aligning procedure for LCET. (d) Reprogramming LCET stepwise to create a 3D iris-like actuator with reversible actuations. (e) Light-triggered reversible actuation of a bending LCET composite actuator. (f) Light-triggered reversible actuation of a spiral LCET composite actuator.
图 10 (a) 制备可生长的弹性体的化学组分[34]；(b) 类海葵致动器在冷却和加热时的卷曲/展开运动行为图与致动器倒置后站立与平躺模式实物图片及构造示意图[34]；(c) 聚硫氨基甲酸酯LCE、聚氨酯LCE、环氧树脂LCE、聚硼酸酯LCE和聚肟酯合成路线图[34]
Figure 10. (a) Chemical compositions for the synthesis of the elastomer that can grow[34]; (b) A sea-anemone-like actuator showing curling/uncurling motion upon cooling and heating, and standing up/lying down mode after turning upside down[34]; (c) Synthesis of polythiourethane LCE, polyurethane LCE, epoxy LCE, polyborate LCE, and polyoxime ester[35]
图 13 (a) LC单体RM82、扩链剂1 DODT、扩链剂2 OPDSPA和交联剂PETMP的化学结构式[46]；(b) 二硒键复分解示意图[46]；(c) 在热刺激下执行莫比乌斯环的可逆螺旋形状变形、花朵模拟开花运动和包裹变形致动器图片[46]
Figure 13. (a) Chemical structures of LC monomer RM82, chain extender 1 DODT, chain extender 2 OPDSPA, and the crosslinker PETMP[46]; (b) Schematic illustration of the metathesis of diselenide bonds[46]; (c) Photo images of the actuators executing reversible helical shape morphing, flower-mimic blooming motion, and enclasping deformation of Mçbius strip under thermal stimulus[46]
图 14 (a) IPN-LCE膜的制备步骤及流程示意图[52]；(b) 互穿聚合物网络(IPN)-LCE材料的LC聚丙烯酸酯(LCPA)和LC聚氨酯(LCPU)体系的化学组成[52]；(c) IPN-LCE膜(20.1 mg)加热/冷却循环中提拉负载(约605.02 g)的实物照片[52]
Figure 14. (a) Schematic illustration of the preparation protocol of IPN-LCE film[52]; (b) Chemical compositions of LC polyacrylate (LCPA) and LC polyurethane (LCPU) systems of the designed interpenetrating polymer network (IPN)-LCE material[52]; (c) Photo images of IPN-LCE film (20.1 mg)lifting up a load (ca. 605.02 g) in a heating/cooling cycle[52]
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