A Versatile Preparation Route for Thin Free‐Standing Liquid Single Crystal Elastomers

Summary: Thin films of nematic and cholesteric side‐chain polymers containing reactive benzophenone units can be macroscopically oriented by electric/magnetic fields or surface interactions. After UV‐irradiation, liquid single crystal elastomers (LSCEs) are formed. With this simple, new pathway, macroscopically ordered free‐standing LSCE films are easily accessible having a thickness in the range of about 100 nm to 100 μm as outlined in Figure.

     

Finite viscoelasticity, plasticity and damage of a class of filled elastomers: Constitutive model

A constitutive model is proposed for a class of filled elastomers exhibiting permanent strain at zero stress, in which hyper-visco-elasticity, plasticity and damage are only weakly coupled. The free energy is expressed as the sum of three terms: a hyperelastic term, a positive hardening function, and a negative damage function. The state relations are then established by postulating a dissipation potential and assuming Norton-Hoff type variations of plasticity and damage. An illustrative example of the model potentialities is given, concerning the Mullins' effect.     

液晶弹性体刺激形变研究

开发可以通过外部刺激产生机械形变的人工致动材料是一个近年来的研究热点。其中,液晶弹性体因结合了聚合物网络的橡胶弹性和液晶的有序性而具有独特的性质,在热、光、电等的外界刺激下可以产生可逆的形状记忆效应。本文综述了液晶弹性体响应多种外界刺激产生各种形变的行为,主要介绍了有关热致形变液晶弹性体、电致形变液晶弹性体、化学刺激导致形变的液晶弹性体及光致形变液晶弹性体的研究进展,阐述了各类液晶弹性体产生形变的机理包括热致、电致和光致相转变,讨论了影响其响应性能的主要因素,并展望了这一领域的发展前景。     

Strain induced strengthening of soft thermoplastic polyurethanes under cyclic deformation

We investigate the cyclic mechanical behavior in uniaxial tension of three different commercial thermoplastic polyurethane elastomers (TPU) often considered as a sustainable replacement for common filled elastomers. All TPU have similar hard segment contents and linear moduli but sensibly different large strain properties as shown by X-ray analysis. Despite these differences, we found a stiffening effect after conditioning in step cyclic loading which greatly differs from the common softening (also referred as Mullins effect) observed in chemically crosslinked filled rubbers. We propose that this self-reinforcement is related to the fragmentation of hard domains, naturally present in TPU, in smaller but more numerous sub-units that may act as new physical crosslinking points. The proposed stiffening mechanism is not dissimilar to the strain-induced crystallization observed in stretched natural rubber, but it presents a persistent nature. In particular, it may cause a local reinforcement where an inhomogeneous strain field is present, as is the case of a crack propagating in cyclic fatigue, providing a potential explanation for the well-known toughness and wear resistance of TPU.     

Liquid‐Crystalline Soft Actuators with Switchable Thermal Reprogrammability

Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks.     

Fabrication of polyurethane binders grafted with functional reactive plasticizers by a catalyst-free click reaction

The design of nonmigratory energetic plasticizers with low sensitivity and high performance is of great significance but challenging. Herein, two nonmigratory norbornane-based reactive plasticizers (NRPs) are attached covalently to poly(glycidyl azide-co-tetrahydrofuran) (PGT)-based polyurethanes (PUs), offering a reliable, self-stable, and alternative energy source originating from ring strain, while mitigating the dangers to the environment by preventing migration. A binary mixture of PGT and NRPs is thermodynamically miscible up to 50/50 w/w. The catalyst-free click reactivity of the NRPs toward PGT evaluated by the activation energy is verified by calculating the frontier molecular orbital energy levels. The absent weight loss of the pure NRPs evident from the measured thermal stability of the NRP/PGT-based PU binders indicated that the NRPs react completely with the PGT matrix. The tensile properties of the PGT-based PU binder by the inclusion of NRPs increased with increasing NRP content because of the increased number of triazole groups and the norbornane moiety. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 402–411     

Multifunctional liquid metal polymer composites

Liquid metal polymer composites are an emerging class of functional materials with potentially transformative impacts in wearable electronics, soft robotics, and human-computer interactions. By employing different processing methods, room temperature liquid metal inclusions can be embedded in insulating polymers like elastomers to incorporate functional properties of metals while the matrix remains soft and stretchable. These solid–liquid composites offer an interesting, yet complex multifunctional material system. In this review, we present an exclusive overview of the synthesis methods, structural and functional properties, and applications of gallium-based liquid metal polymer composites. Common methods to control the size of liquid metal inclusions and their interaction in polymers are discussed. Moreover, the effect of liquid metal microstructures on the overall properties of the composites is summarized. We also highlight the new trends in terms of material composition, printing process, and novel applications of liquid metal polymer composites in intelligent systems.     

On the relevance of the 8‐chain model and the full‐network model for the deformation and failure of networks formed through photopolymerization of multifunctional monomers

Crosslinked networks were synthesized by copolymerization of mono‐functional tert‐butyl acrylate (tBA) with diethyleneglycol dimethacrylate (DEGDMA) or polyethylene glycol dimethacrylates (PEGDMA). By varying the chain length and concentration of the difunctional PEGDMA, we obtained tBA‐PEGDMA copolymer networks while by varying the concentration of difunctional DEGDMA, we obtained tBA‐DEGDMA crosslinked networks. The various materials were submitted to large deformations through uniaxial tension tests. For moderate weight percent of crosslinking agent, up to 20%, the networks showed standard S‐shape stress–strain curves, characteristic of rubber‐like elasticity. Two macromolecular models, the 8‐chain model and the full‐network model, were applied to fit the uniaxial tensile response of the materials. Both models provide good representations of the overall uniaxial stress–strain response of each material. After fitting to stress–strain data, the network models were employed to predict the shear modulus and the elongation at break. Neither the 8‐chain nor the full network model were capable of predicting the failure strain or shear modulus, indicating these models are best used to describe stress–strain relations rather than predict mechanical properties for the network polymers considered here. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1226–1234, 2008     

The reactivity of tributyltin oxygen compounds with CCl4: Implications for its use as an extraction/reaction solvent

A variety of tributyltin oxygen compounds,(nC₄H₉)₃SnOX where X = Sn(nC₄H₉)₃, C₂H₅, nC₄H₉, C₈H₁₇, CH₂C₆H₅, COCH₃, have been studied in refluxing CCI₄. A reaction was observed to occur where X = C₂H₅, C₄H₉, C₈H₁₇, CH₂C₆H₅, leading to the formation of (nC₄H₉)₃SnCl, CHCl₃ and an aldehyde. Possible reaction pathways are suggested. These reactions have implications for the use of CCl₄ as an extraction/reaction solvent.     

甲基丙烯酸三丁基锡酯共聚甲基丙烯酸甲酯的竞聚率测定方法准确性探讨

甲基丙烯酸三丁基锡酯共聚甲基丙烯酸甲酯的竞聚率测定方法准确性探讨赵秉熙陈文明陈道勇阎立峰（中国科学技术大学应用化学系合肥２３００２６）朱清仁（中国科学院结构成分开放实验室合肥２３００２６）韩哲文陆志豹（华东理工大学材料科学系上海２００２３７）关键词...