清华剑桥合作制备易加工的液晶弹性体智能材料

正液晶弹性体材料在热、光、电、磁等外界刺激下可发生形状的自发改变,作为致动器及感应器在人工肌肉、柔性机器人、盲人显示器等诸多领域的应用前景十分广阔。这种形状的改变是基于高分子内部的液晶有序性,通过光、热、磁等方式改变这种有序性将产生可逆的宏观形状变化。为了使液晶弹性体发生实际意义的形状改变,必须将液晶高分子链作单畴取向(单畴是指液晶分子链中很窄的或单一取向的区域)。     

清华剑桥合作制备易加工的液晶弹性体智能材料

正液晶弹性体材料在热、光、电、磁等外界刺激下可发生形状的自发改变,作为致动器及感应器在人工肌肉、柔性机器人、盲人显示器等诸多领域的应用前景十分广阔。这种形状的改变是基于高分子内部的液晶有序性,通过光、热、磁等方式改变这种有序性将产生可逆的宏观形状变化。为了使液晶弹性体发生实际意义的形状改变,必须将液晶高分子链作单畴取向(单畴是指液晶分子链中很窄的或单一取向的区域)。传统的两步交联法、外场和界面条件交联法等制备单畴液晶弹性体的工艺,或工艺复杂并成功率低,或仅适用于微米级样品。正因如此,长久以来,这种液晶弹性体材料在现实生活中     

基于共价自适应网络的液晶弹性体:从分子设计到应用（英文）

液晶弹性体(LCEs)是一种刺激响应性材料,具有较大且可逆变形的能力,在软致动器、人造肌肉、光子器件和生物医学工程等领域引起了广泛的关注.与具有永久交联网络的传统液晶弹性体不同,具有共价自适应网络的液晶弹性体(CAN-LCEs)可以通过键交换反应引起的网络重排将多畴液晶弹性体编程为单畴液晶弹性体.此外,CAN-LCEs还具有焊接、自修复、回收、再加工或重新编程的能力.本文综述了CAN-LCEs的最新研究成果,详细讨论了基于各种类型的键交换反应的CAN-LCEs及其设计策略.同时,本文还介绍了共价自适应网络给LCEs带来的新功能,包括焊接、自愈、重塑、回收、重新编程等,并探讨了CAN-LCEs在各种领域的应用前景.最后,本文总结了这一新兴研究领域面临的挑战和潜在的未来发展方向.     

热塑性弹性体的研究与产业化进展

化学合成型和共混型热塑性弹性体类型、市场概况、产业化进展及最新研究发展方向进行了论述。介绍了几种具有市场应用前景的新型热塑性弹性体品种,如聚酰胺类TPE、热可逆共价交联类TPE、茂金属催化聚烯烃类TPE、甲壳型液晶类TPE、生物基TPE、新型热塑性硫化橡胶等,对它们的制备方法、性能特点及市场发展概况进行了详细论述,并对它们的应用领域和产业化方向进行了评述和展望。     

形状记忆聚合物及其在航空航天领域中的应用

介绍了形状记忆聚合物的最新研究进展,从结构角度分析和探讨了不同聚合物产生形状记忆效应的原理,其中,聚烯烃类形状记忆聚合物都具有化学交联和半结晶的结构;相反,形状记忆聚氨酯则具有物理交联和相分离的结构。而液晶弹性体的形状记忆效应则是由于液晶态的相转变而实现的。文中还对聚氨酯、交联聚乙烯、液晶弹性体等形状记忆高分子材料的特性及其在变形机翼、自展开结构等航空航天领域的应用进行了介绍和评价。     

基于氢键作用的超分子弹性体材料的研究进展

超分子弹性体是指在分子(包括小分子和大分子)间通过氢键、离子键、配位键等非共价键结合的,在常温下具有与传统弹性体材料类似高弹性和流变特性的一类新型材料,同时由于其特殊的分子键作用方式,超分子弹性体还具有与传统弹性体不同的特性,如可逆热塑性、自修补性、热敏性和良好的加工性等。文中综述了近年来国内外在氢键型超分子弹性体的合成、制备与表征等方面的研究进展,并指出其发展趋势。     

铁电液晶

铁电液晶是具有铁电性的液晶,即存在自发极化的特殊液晶。当受到外电场作用时,显示出自发极化,其方向随外电场的反向而反向。在液晶发现后的很长时间内,人们认为液晶不显示铁电性。后来在畸变的向列相液晶中,发现由于展曲或弯曲,会出现挠曲电效应,产生一定的自发极化。近十多年来,液晶学科的理论研究和实际应用有了很大发展,不断改善和修正着科学家们的原有认     

聚乙二醇基形状记忆高分子材料研究进展

聚乙二醇(PEG)是一种结晶完善且具有良好水溶性的热塑型聚醚高分子,其分子链结构在无定形态及结晶态间相互转变的特性符合形状记忆材料对可逆形变结构的要求,因此广泛应用于形状记忆高分子材料的研究。文中综述了近年来聚乙二醇在形状记忆聚合物、形状记忆水凝胶、形状记忆复合材料等高分子材料中的应用研究进展,阐述了聚乙二醇链结构与材料形状记忆特性及不同刺激响应性之间的关系,并特别强调了聚乙二醇与水的氢键作用对水诱导响应型形状记忆特性的重要性,对低毒且生物相容性好的聚乙二醇基形状记忆高分子材料在生物医学领域的应用进行了展望。     

新型磁流变弹性体隔振器关键技术

磁流变弹性体是近年来广泛应用的一种新型智能隔振材料,它继承了磁流变液的可控、可逆、响应速度快等优良性能,同时具有稳定性好、不易磨损和不易沉降等特点,广泛应用于高新技术隔振领域。以橡胶作为基体研制出的新型磁流变弹性体,充分利用磁流变弹性体刚度和阻尼可控的特性,设计一款新型磁流变弹性体隔振器,对其在不同外加控制电流和激励频率下的振动响应特性进行了试验研究,试验结果表明在外加电流的改变下,该隔振器刚度相应改变,从而引起固有频率的改变,达到宽频隔振的效果。     

液晶多肽及其应用研究

基于国内外最新研究文献,系统论述了近年来液晶多肽的大分子链结构、液晶性能及应用前景等方面的研究进展。指出液晶多肽在一定条件下将出现各向同性态到各向异性态的可逆转变,伴随着大分子链从线圈状到螺旋状的可逆转变。聚[γ－苄基－L－谷氨酸酯]有序度参数可达0.875。二次谐波的产生证明了有些液晶多肽呈现奇异的胆甾相结构,其胆甾相由极性向列型片晶组成,且分子偶极矩和长轴方向平行。多肽既可显示溶致液晶性又可显示热致液晶性,其液晶相多为胆甾型,有时呈向列型。外加场如取向场及电场对多肽的液晶性也会产生强烈影响。液晶多肽可加工成液晶态水凝胶、纤维和薄膜等,可望用作全息照相材料、光学元件、彩色滤色器以及溶致液晶电池等。     

Piezoresistivity and electro-thermomechanical degradation of a conducting layer of nanoparticles integrated at the liquid crystal elastomer surface

When a liquid crystal elastomer (LCE) is reprocessed with conducting nanosized particles a conducting layer can be formed at the LCE surfaces. Here, two different LCE materials and two different conducting carbon particles were used. These four reprocessed LCEs were investigated when subject to a thermal phase transition and mechanical extension. Here it is shown that the resistance change with strain ('piezoresistivity') for these reprocessed LCEs can be described through lattice percolation and geometrical changes in the LCE shape. The mechanisms and rate of degradation are also described for the conducting layer as a function of the number of electro-thermomechanical strain cycles performed.     

All‐Optical Control of Shape

Photoresponsive liquid crystal elastomers (LCEs) are a unique class of anisotropic materials capable of undergoing large‐scale, macroscopic deformations when exposed to light. Here, surface‐aligned, azobenzene‐functionalized LCEs are prepared via a radical‐mediated, thiol‐acrylate chain transfer reaction. A long‐lived, macroscopic shape deformation is realized in an LCE composed with an o‐fluorinated azobenzene (oF‐azo) monomer. Under UV irradiation, the oF‐azo LCE exhibits a persistent shape deformation for >72 h. By contrasting the photomechanical response of the oF‐azo LCE to analogs prepared from classical and m‐fluorinated azobenzene derivatives, the origin of the persistent deformation is clearly attributed to the underlying influence of positional functionalization on the kinetics of cis→trans isomerization. Informed by these studies and enabled by the salient features of light‐induced deformations, oF‐azo LCEs are demonstrated to undergo all‐optical control of shape deformation and shape restoration.     

Fast liquid-crystal elastomer swims into the dark

Liquid-crystal elastomers (LCEs) are rubbers whose constituent molecules are orientationally ordered. Their salient feature is strong coupling between the orientational order and mechanical strain. For example, changing the orientational order gives rise to internal stresses, which lead to strains and change the shape of a sample. Orientational order can be affected by changes in externally applied stimuli such as light. We demonstrate here that by dissolving-rather than covalently bonding-azo dyes into an LCE sample, its mechanical deformation in response to non-uniform illumination by visible light becomes very large (more than 60 degrees bending) and is more than two orders of magnitude faster than previously reported. Rapid light-induced deformations allow LCEs to interact with their environment in new and unexpected ways. When light from above is shone on a dye-doped LCE sample floating on water, the LCE 'swims' away from the light, with an action resembling that of flatfish such as skates or rays. We analyse the propulsion mechanism in terms of momentum transfer.     

Liquid crystalline elastomer actuators with dynamic covalent bonding: Synthesis,alignment, reprogrammability,and self-healing

Liquid crystalline elastomers (LCEs) have demonstrated tremendous potential in applications such as soft robotics, biomedical materials, electronics, sensors, and biomimetic systems. The physical properties of LCEs are controlled by the degree of crosslinking, nature of the mesogens, and mesogen orientation in the LCE network structure. A wide range of dynamic covalent bonds (DCBs) capable of dynamic bond exchange reactions (DBERs) have been introduced into LCE structures to obtain intelligent materials in recent decades. In this review article, we discuss the molecular constitution, macrostructure, morphing mechanism, recent advances in LCEs with dynamic covalent bonds, the influence of DCBs on self-healing, reprogramming and reprocessing properties of LCE actuators, and challenges and opportunities in incorporating dynamic chemistry in the field of LCE actuators.     

Enabling and Localizing Omnidirectional Nonlinear Deformation in Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are widely recognized for their exceptional promise as actuating materials. Here, the comparatively less celebrated but also compelling nonlinear response of these materials to mechanical load is examined. Prior examinations of planarly aligned LCEs exhibit unidirectional nonlinear deformation to mechanical loads. A methodology is presented to realize surface‐templated homeotropic orientation in LCEs and omnidirectional nonlinearity in mechanical deformation. Inkjet printing of the homeotropic alignment surface localizes regions of homeotropic and planar orientation within a monolithic LCE element. The local control of the self‐assembly and orientation of the LCE, when subject to rational design, yield functional materials continuous in composition with discontinuous mechanical deformation. The variation in mechanical deformation in the film can enable the realization of nontrivial performance. For example, a patterned LCE is prepared and shown to exhibit a near‐zero Poisson's ratio. Further, it is demonstrated that the local control of deformation can enable the fabrication of rugged, flexible electronic devices. An additively manufactured device withstands complex mechanical deformations that would normally cause catastrophic failure.     

3D Printing of Liquid Crystal Elastomeric Actuators with Spatially Programed Nematic Order

Liquid crystal elastomers (LCEs) are soft materials capable of large, reversible shape changes, which may find potential application as artificial muscles, soft robots, and dynamic functional architectures. Here, the design and additive manufacturing of LCE actuators (LCEAs) with spatially programed nematic order that exhibit large, reversible, and repeatable contraction with high specific work capacity are reported. First, a photopolymerizable, solvent‐free, main‐chain LCE ink is created via aza‐Michael addition with the appropriate viscoelastic properties for 3D printing. Next, high operating temperature direct ink writing of LCE inks is used to align their mesogen domains along the direction of the print path. To demonstrate the power of this additive manufacturing approach, shape‐morphing LCEA architectures are fabricated, which undergo reversible planar‐to‐3D and 3D‐to‐3D′ transformations on demand, that can lift significantly more weight than other LCEAs reported to date.     

Repeatable and Reprogrammable Shape Morphing from Photoresponsive Gold Nanorod/Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are of interest for applications such as soft robotics and shape-morphing devices. Among the different actuation mechanisms, light offers advantages such as spatial and local control of actuation via the photothermal effect. However, the unwanted aggregation of the light-absorbing nanoparticles in the LCE matrix will limit the photothermal response speed, actuation performance, and repeatability. Herein, a near-infrared-responsive LCE composite consisting of up to 0.20 wt% poly(ethylene glycol)-modified gold nanorods (AuNRs) without apparent aggregation is demonstrated. The high Young's modulus, 20.3 MPa, and excellent photothermal performance render repeated and fast actuation of the films (actuation within 5 s and recovery in 2 s) when exposed to 800 nm light at an average output power of ≈1.0 W cm⁻², while maintaining a large actuation strain (56%). Further, it is shown that the same sheet of AuNR/LCE film (100 µm thick) can be morphed into different shapes simply by varying the motifs of the photomasks.     

Fracture behaviour of liquid crystal epoxy resin systems based on diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene: Part II Effect due to blending with TACTIX 556 epoxy resin and phenolic monomers

The mechanical behaviours of unoriented, poured resin castings based on formulated blends containing the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene monomer are studied. It is found that the mechanical and fracture behaviours of these liquid crystalline epoxy (LCE) blends vary significantly. In general, the LCE blends possess much higher fracture toughness and fatigue crack resistance than conventional epoxy resins. At low temperatures (−40°C), the KIC values of the LCE blends are slightly higher than those measured at room temperature. The common fracture mechanisms observed in the ductile LCE blends are crack segmentation, crack branching, crack bridging and crack blunting. The fracture surfaces of the tougher LCE blends only exhibit limited ductile drawing (furrow pattern) at the slow crack growth region; no signs of shear lips on the edges of the starter crack region are observed. The optical microscopy and transmission electron microscopy work suggests that orientation and/or transformation toughening may be the source for such high fracture toughness of the LCE blends. The possible cause(s) of the unusual fracture behaviour of the LCEs is discussed. Approaches for making high performance LCE blends are also addressed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture behaviour of liquid crystal epoxy resin systems based on the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene and sulphanilamide: Part I Effects of curing variations

The fracture behaviours of the pour-cast, unoriented diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene/sulphanilamide liquid crystalline epoxies (LCE) cured at various temperature steps are investigated. It is found that, depending on how the LCE is cured, the liquid crystalline (LC) domain size varies dramatically. These, in turn, affect how the LCEs fracture. The operative toughening mechanisms in the toughest LCE are studied in detail and found to include the formation of numerous segmented, unlinked microcracks in front of the main crack. When the crack opens up, the matrix material between the segmented microcracks acts as a bridge between the opening crack planes. Furthermore, crack bifurcation appears to take place when the segmented cracks are eventually linked with the main crack. This entire fracture process accounts for the high fracture toughness (GIC=580 J m-2) of this particular LCE with respect to conventional epoxies (GIC=180 J m-2). The relationship between the LCE morphology and the corresponding fracture mechanisms is discussed. This revised version was published online in November 2006 with corrections to the Cover Date.