Adaptable liquid crystal elastomers with transesterification-based bond exchange reactions

Intrinsically Photothermal-Driven and Reconfigurable Liquid Crystal Elastomer Actuators Enabled by Multifunctional Dynamic Covalent Organic Photothermal Molecules

Intrinsically photothermal-responsive soft actuators possessing reconfigurability have attracted great attention due to their ability to change their actuation mode to satisfy diverse practical applications. However, challenges remain in designing and fabricating organic photothermal molecules featuring polymerizable or cross-linkable groups, especially those with multifunctional properties. Here, a novel class of versatile light-driven reconfigurable liquid crystal elastomer (LCE) materials, denoted as PUOLCE, has been developed. The multifunctional dynamic covalent organic photothermal molecules, serving as chain extenders, photothermal agents, and dynamic covalent bond precursors, are chemically bonded into LCEs, thereby endowing the LCEs with photothermal-responsiveness and dynamic properties. The intrinsic photothermal effect of PUOLCE allows the exchange reaction of dynamic oxime-carbamate bonds to undergo rapid welding under near-infrared (NIR) light. Leveraging the NIR-assisted welding strategy, the PUOLCE-based building units are capable of assembling into various complex actuators with new deformation modes. Besides, the assembled actuators can be easily reconfigured to perform different mechanical tasks (e.g., flower blooming, grasping objects, and elevating objects) under NIR illumination. Furthermore, the PUOLCE actuators can be controlled globally or locally for light-driven locomotion by controlling the area exposed to the NIR irradiation. This work provides insights into the development of adaptive soft actuators with tunable shape-morphing capabilities.

Visible-Light-Driven and Adaptable Liquid-Crystalline Elastomer Actuators Containing Dynamically Exchangeable Boron Ester and Disulfide Linkages

A liquid-crystalline elastomer (LCE) containing dynamically exchangeable disulfide (-S-S-) and boron ester (BE) bonds (LCESS-BE) was successfully prepared. The -S-S- linkages were introduced in the form of a reactive oligomer prepared via a thiol-acrylate Michael addition reaction with bis(2-hydroxyethyl) disulfide acrylate, while the BE linkages were incorporated via the visible-light-induced cross-linking of the oligomer with 4-((allyloxy)methyl)-2-(4-vinylphenyl)-1,3,2-dioxaborolane (AMVD). The monodomain LCESS-BE (MLCESS-BE) with ∼1 wt % AMVD promoted photothermal actuation under blue-light irradiation, resulting in a rise in its temperature of ∼147 °C and a decrease in its length of ∼42%. In addition, LCESS-BE films exhibited reversible reprogrammability through two dynamic exchange reactions (DERs) involving either BE or -S-S- linkages. The BE linkages induced water-assisted healing through a hydration/dehydration DER under ambient conditions via a glue-free method, while the -S-S- linkages exhibited a UV-light-induced DER below the nematic-to-isotropic transition temperature, thus maintaining the nematic order during the DER. This approach represents a promising method for the preparation of MLCEs with light-induced actuation and multifunctionality involving reprogramming, reprocessing, and healing properties.

Thermal Gradient-Driven Heterogeneous Actuation of Liquid Crystal Elastomers for a Crawling Robot

Emerging soft robots based on liquid crystal elastomers (LCEs) exhibit remarkable capabilities for large reversible shape morphing, enabling them to adapt to complex environments and perform diverse tasks such as locomotion and camouflage. Despite extensive studies, current methods for locally controlled actuation of LCE-based soft robots often involve intricate structural design, complex programming of LCEs, incorporation of multiple materials, or complex actuation methods. Here, we present a simple and efficient approach to achieve multiple deformation modes within a simply programmed LCE structure by harnessing Joule heating-induced thermal gradients across the LCE volume. Oxidized liquid metal (LM) thin films, which exhibit increased resistance, enhanced viscosity, high thermal conductivity, and large deformability, are employed for Joule heating in this study. Using an LCE strip programmed via uniaxial stretching as an example, we perform systematic studies on the effect of essential parameters, including the actuation voltage, LCE dimensions, and the LM-to-LCE thickness ratio, on the deformation behaviors of LCEs induced by three-dimensional thermal gradients across the LCE volume. In addition, concurrently actuating two adjacent surfaces of the LCE strip yields previously inaccessible coupled bending behaviors. Finally, we demonstrate a crawling robot constructed from LM-coated LCE strips with adjustable bending capabilities, which enable multimode locomotion, including forward movement and turns, enhancing biomimetic functionality akin to leg movements observed in living organisms like reptiles. The reported strategy, which is both straightforward and versatile, promises scalability and holds potential for various applications in multifunctional intelligent systems including soft robotics and biomedical devices.

Current State-of-the-Art and Perspectives in the Design and Application of Vitrimeric Systems

To fulfill the current circular economy concept, the academic and industrial communities are devoting significant efforts to plastic materials' end-of-life. Unlike thermoplastics, which are easy to recover and re-valorize, recycling thermosets is still difficult and challenging. Conversely, because of their network structure, thermosetting polymer systems exhibit peculiar features that make these materials preferable for several applications where high mechanical properties, chemical inertness, and thermal stability, among others, are demanded. In this view, vitrimers have quite recently attracted the attention of the scientific community, as they can form dynamic covalent adaptive networks that provide the properties typical of thermosets while keeping the possibility of being processed (and, therefore, mechanically recycled) beyond a certain temperature. This review aims to provide an overview of vitrimers, elucidating their most recent advances and applications and posing some perspectives for the forthcoming years.

Enhancing Hole Transport and Autonomous Healing Properties of Supramolecular Columns in Unsymmetrical Discotics

Designing smart autonomous healing soft materials is crucial to attaining cost-efficiency and optimal performance in organic semiconductors. In this context, we design an unsymmetrical thiophene-fused phenazine (TFP)-based discotic liquid crystal (DLC) with the goal of creating an active organic semiconductor that encompasses favorable attributes, such as polarizability, mobility, and processability. Aligned with our objective, we successfully synthesized two unsymmetrical TFP core-based DLCs by linking alkyl chains of variable lengths at the periphery through a coupling reaction. These DLCs show room temperature columnar oblique (Colob) mesophase as evident from temperature-dependent studies employing polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS). We performed space charge-limited current (SCLC) techniques to evaluate the hole mobility of TFP-based DLCs and found that they have high hole mobility (∼10-3 cm2/V s). Additionally, the film morphology and its self-healing nature have been examined using atomic force microscopy (AFM) and stress relaxation test. One of the unsymmetrical DLCs exhibited an ability to relax stress over time while maintaining a constant strain (up to 1-3%). The rational design of these unsymmetrical discotics, exhibiting reduced threshold voltage (as confirmed by conductivity studies) highlights their possible potential in various organic semiconductor devices.

Covalent Adaptable Networks from Polyacrylates Based on Oxime-Urethane Bond Exchange Reaction

Covalent adaptable networks (CANs) are polymer networks cross-linked via dynamic covalent bonds that can proceed with bond exchange reactions upon applying external stimuli. In this report, a series of cross-linked polyacrylate films were fabricated by changing the combination of acrylate monomer and the amount of diacrylate cross-linker possessing oxime-urethane bonds as a kind of dynamic covalent bond to evaluate their rheological relaxation properties. Model analysis of the experimental relaxation curves of cross-linked polyacrylate films was conducted by assuming that they consist of two types of relaxation, one of which is related to the oxime-urethane bond exchange reaction, and another of which is associated with the melting of the aggregated cross-linker. It was found that the contribution from the relaxation due to the bond exchange reaction becomes dominant only when the normal-alkyl acrylates are used as a monomer. The relaxation time was almost constant even when the amount of the cross-linker was adjusted. Moreover, it was also indicated that the miscibility of the cross-linker is very important for the fabrication of CANs with good self-healing ability and reprocessability.

Mesogen Organizations in Nematic Liquid Crystal Elastomers Under Different Deformation Conditions

Liquid crystal elastomers (LCEs) exhibit unique mechanical properties of soft elasticity and reversible shape-changing behaviors, and so serve as potentially transformative materials for various protective and actuation applications. This study contributes to filling a critical knowledge gap in the field by investigating the microscale mesogen organization of nematic LCEs with diverse macroscopic deformation. A polarized Fourier transform infrared light spectroscopy (FTIR) tester is utilized to examine the mesogen organizations, including both the nematic director and mesogen order parameter. Three types of material deformation are analyzed: uniaxial tension, simple shear, and bi-axial tension, which are all commonly encountered in practical designs of LCEs. By integrating customized loading fixtures into the FTIR tester, mesogen organizations are examined across varying magnitudes of strain levels for each deformation mode. Their relationships with macroscopic stress responses are revealed and compared with predictions from existing theories. Furthermore, this study reveals unique features of mesogen organizations that have not been previously reported, such as simultaneous evolutions of the mesogen order parameter and nematic director in simple shear and bi-axial loading conditions. Overall, the findings presented in this study offer significant new insights for future rational designs, modeling, and applications of LCE materials.

4D printing of liquid crystal elastomer composites with continuous fiber reinforcement

Multifunctional composites have been continuously developed for a myriad of applications with remarkable adaptability to external stimuli and dynamic responsiveness. This study introduces a 4D printing method for liquid crystal elastomer (LCE) composites with continuous fibers and unveils their multifunctional actuation and exciting mechanical responses. During the printing process, the relative motion between the continuous fiber and LCE resin generates shear force to align mesogens and enable the monodomain state of the matrix materials. The printed composite lamina exhibits reversible folding deformations that are programmable by controlling printing parameters. With the incorporation of fiber reinforcement, the LCE composites not only demonstrate high actuation forces but also improved energy absorption and protection capabilities. Diverse shape-changing configurations of 4D composite structures can be achieved by tuning the printing pathway. Moreover, the incorporation of conductive fibers into the LCE matrix enables electrically induced shape morphing in the printed composites. Overall, this cost-effective 4D printing method is poised to serve as an accessible and influential approach when designing diverse applications of LCE composites, particularly in the realms of soft robotics, wearable electronics, artificial muscles, and beyond.

Phase patterning of liquid crystal elastomers by laser-induced dynamic crosslinking

Liquid crystal elastomers hold promise in various fields due to their reversible transition of mechanical and optical properties across distinct phases. However, the lack of local phase patterning techniques and irreversible phase programming has hindered their broad implementation. Here we introduce laser-induced dynamic crosslinking, which leverages the precision and control offered by laser technology to achieve high-resolution multilevel patterning and transmittance modulation. Incorporation of allyl sulfide groups enables adaptive liquid crystal elastomers that can be reconfigured into desired phases or complex patterns. Laser-induced dynamic crosslinking is compatible with existing processing methods and allows the generation of thermo- and strain-responsive patterns that include isotropic, polydomain and monodomain phases within a single liquid crystal elastomer film. We show temporary information encryption at body temperature, expanding the functionality of liquid crystal elastomer devices in wearable applications.

New Prospects Arising from Dynamically Crosslinked Polymers: Reprogramming Their Properties

Dynamically crosslinked polymers (DCPs) have gained significant attention owing to their applications in fabricating (re)processable, recyclable, and self-healable thermosets, which hold great promise in addressing ecological issues, such as plastic pollution and resource scarcity. However, the current research predominantly focuses on redefining and/or manipulating their geometries while replicating their bulk properties. Given the inherent design flexibility of dynamic covalent networks, DCPs also exhibit a remarkable potential for various novel applications through postsynthesis reprogramming their properties. In this review, the recent advancements in strategies that enable DCPs to transform their bulk properties after synthesis are presented. The underlying mechanisms and associated material properties are overviewed mainly through three distinct strategies, namely latent catalysts, material-growth, and topology isomerizable networks. Furthermore, the mutual relationship and impact of these strategies when integrated within one material system are also discussed. Finally, the application prospects and relevant issues necessitating further investigation, along with the potential solutions are analyzed.

Solvent Content Controlling Strategy for Cocrystallizable Polyesters Enables a Stress-Free Two-Way Shape Memory Effect with Wider Service Temperatures

It is challenging to enhance the stress-free two-way shape memory (stress-free TWSM) effect to obtain a wide range of response temperatures. Herein, a polycaprolactone (PCL)/poly(ω-pentadecalactone) (PPDL) is photocured under UV light irradiation in the solvent of 1,1,2-trichloroethane (TCA), to obtain a series of cross-linked polyesters (CPES). Controlling solvent content (SC) which is removed after the polymerization allows the yielded CPES to perform a regulatable thermodynamic and stress-free TWSM properties. High SC is beneficial to reduce the degree of chain overlap (C/C* ) of PPDL chain segments in the PCL-based CPES network, then causes the cocrystallization of PCL and PPDL and yielding an additional melting-transitions (Tm ). An enhanced stress-free TWSM is obtained in high SC samples (CPES-15-90), reflected in the attainment of a wide range of response temperature, which means a wider service temperature. The enhancement is reflected in higher reversible strain of high SC samples compared with the samples prepared with low SC when varying high trigger temperature (Thigh ). Even at high Thigh , the high SC sample still has reversible strain. Therefore, controlling SC strategy for photocuring copolyester not only provides a new preparation approach for high-performance shape memory (SM) polymers, but also offers new condensed polymer structure to explore.

3D Printing of continuous fiber composites using two-stage UV curable resin

3D printing allows for moldless fabrication of continuous fiber composites with high design freedom and low manufacturing cost per part, which makes it particularly well-suited for rapid prototyping and composite product development. Compared to thermal-curable resins, UV-curable resins enable the 3D printing of composites with high fiber content and faster manufacturing speeds. However, the printed composites exhibit low mechanical strength and weak interfacial bonding for high-performance engineering applications. In addition, they are typically not reprocessable or repairable; if they could be, it would dramatically benefit the rapid prototyping of composite products with improved durability, reliability, cost savings, and streamlined workflow. In this study, we demonstrate that the recently emerged two-stage UV-curable resin is an ideal material candidate to tackle these grand challenges in 3D printing of thermoset composites with continuous carbon fiber. The resin consists primarily of acrylate monomers and crosslinkers with exchangeable covalent bonds. During the printing process, composite filaments containing up to 30.9% carbon fiber can be rapidly deposited and solidified through UV irradiation. After printing, the printed composites are subjected to post-heating. Their mechanical stiffness, strength, and inter-filament bonding are significantly enhanced due to the bond exchange reactions within the thermoset matrix. Furthermore, the utilization of the two-stage curable resin enables the repair, reshaping, and recycling of 3D printed thermosetting composites. This study represents the first detailed study to explore the benefits of using two-stage UV curable resins for composite printing. The fundamental understanding could potentially be extended to other types of two-stage curable resins with different molecular mechanisms.

Programming actuation onset of a liquid crystalline elastomer via isomerization of network topology

Tuning actuation temperatures of liquid crystalline elastomers (LCEs) achieves control of their actuation onsets, which is generally accomplished in the synthesis step and cannot be altered afterward. Multiple actuation onsets in one LCE can be encoded if the post-synthesis regulation of actuation temperature can be spatiotemporally achieved. This would allow realizing a logical time-evolution of actuation, desired for future soft robots. Nevertheless, this task is challenging given the additional need to ensure mesogen alignment required for actuation. We achieved this goal with a topology isomerizable network (TIN) of LCE containing aromatic and aliphatic esters in the mesogenic and amorphous phases, respectively. These two ester bonds can be distinctly activated for transesterification. The homolytic bond exchange between aliphatic esters allows mechanically induced mesogen alignment without affecting the mesogenic phase. Most importantly, the heterolytic exchange between aromatic and aliphatic esters changes the actuation temperature under different conditions. Spatial control of the two mechanisms via a photo-latent catalyst unleashes the freedom in regulating actuation temperature distribution, yielding unusual controllability in actuation geometries and logical sequence. Our principle is generally applicable to common LCEs containing both aromatic and aliphatic esters.

One-Step Manufacturing of Supramolecular Liquid-Crystal Elastomers by Stress-Induced Alignment and Hydrogen Bond Exchange

Liquid-crystal elastomers (LCEs) capable of performing large and reversible deformation in response to an external stimulus are an important class of soft actuators. However, their manufacturing process typically involves a multistep approach that requires harsh conditions. For the very first time, LCEs with customized geometries that can be manufactured by a rapid one-step approach at room temperature are developed. The LCEs are hydrogen bond (H-bond) crosslinked main chain polymers comprising flexible short side chains. Applying a stretching/shear force to the LCE can simultaneously induce mesogen alignment and H-bond exchange, allowing for the formation of well-aligned LCE networks stabilized by H-bonds. Based on this working principle, soft actuators in fibers and 2D/3D objects can be manufactured by mechanical stretching or melt extrusion within a short time (e.g. <1 min). These actuators can perform reversible macroscopic motions with large, controlled deformations up to 38 %. The dynamic nature of H-bonds also provides the actuators with reprocessability and reprogrammability. Thus, this work opens the way for the one-step and custom manufacturing of soft actuators.

Modeling Viscoelasticity and Dynamic Nematic Order of Exchangeable Liquid Crystal Elastomers

Exchangeable liquid crystal elastomers (XLCEs), an emerging class of recyclable polymer materials, consist of liquid crystalline polymers which are dynamically crosslinked. We develop a macroscopic continuum model by incorporating the microscopic dynamic features of the cross-links, which can be utilized to understand the viscoelasticity of the materials together with the dynamic nematic order. As applications of the model, we study the rheological responses of XLCEs in three cases: stress relaxation, strain ramp, and creep compliance, where the materials show interesting rheology as an interplay between the dynamic nematic order of the mesogenic units, the elasticity from the network structure, and the dissipation due to chain exchange reactions. Not only being useful in understanding the physical mechanism underlying the fascinating characteristics of XLCEs, this work can also guide their future fabrications with desired rheological properties.

Elastomers Grow into Actuators

It is common knowledge that when an elastomer (rubber) is stretched, its length will be maintained if its two ends are fixed. Here, it is serendipitously found that when an elastomer is slowly elongated further to achieve buckling under such conditions, the final length is much longer than the pre-stretched length. This allows the design of untethered autonomous synthetic-material-based soft robots that do not need any other chemical or electrical energy sources or external stimuli after the pre-strain is installed. Once the growth starts, the elongation continues to proceed even when the applied force is removed. Moreover, the elastomer, after growing, eventually forms a robust soft actuator that can be reshaped for different purposes. Few synthetic materials can grow like this, so far. This investigation shows that the material has an uncommon liquid crystal phase. Contrary to normal liquid crystals, it becomes birefringent only at high temperatures. The formation and the reshaping of the resulting soft actuators relate to a usually unnoticed reversible reaction. The work is promising to promote further understanding of dynamic covalent chemistry and liquid crystal elastomers, as well as to foster new designs and high-impact applications of bioinspired sustainable soft actuators in areas other than soft robots.

Surface-Enforced Alignment of Reprogrammable Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials capable of undergoing large deformations. The thermomechanical response of LCEs is attributable to the coupling of polymer network properties and disruption of order between liquid crystalline mesogens. Complex deformations have been realized in LCEs by either programming the nematic director via surface-enforced alignment or localized mechanical deformation in materials incorporating dynamic covalent chemistries. Here, the preparation of LCEs via thiol-Michael addition reaction is reported that are amenable to surface-enforced alignment. Afforded by the thiol-Michael addition reaction, dynamic covalent bonds are uniquely incorporated in chemistries subject to surface-enforce alignment. Accordingly, LCEs prepared with complex director profiles are able to be programmed and reprogrammed by (re)activating the dynamic covalent chemistry to realize distinctive shape transformations.

Programmable Shape Change in Semicrystalline Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are stimuli-responsive materials capable of reversible and programmable shape change in response to an environmental stimulus. Despite the highly responsive nature of these materials, the modest elastic modulus and blocking stress exhibited by these actuating materials can be limiting in some engineering applications. Here, we engineer a semicrystalline LCE, where the incorporation of semicrystallinity in a lightly cross-linked liquid crystalline network yields tough and highly responsive materials. Directed self-assembly can be employed to program director profiles through the thickness of the semicrystalline LCE. In short, we use the alignment of a liquid crystal monomer phase to pattern the anisotropy of a semicrystalline polymer network. Both the semicrystalline-liquid crystalline and liquid crystalline-isotropic phase transition temperatures provide controllable shape transformations. A planarly aligned sample's normalized dimension parallel to the nematic director decreases from 1 at room temperature to 0.42 at 250 °C. The introduction of the semicrystalline nature also enhances the mechanical properties exhibited by the semicrystalline LCE. Semicrystalline LCEs have a storage modulus of 390 MPa at room temperature, and monodomain samples are capable of generating a contractile stress of 2.7 MPa on heating from 25 to 50 °C, far below the nematic to isotropic transition temperature. The robust mechanical properties of this material combined with the high actuation strain can be leveraged for applications such as soft robotics and actuators capable of doing significant work.

Diketoenamine-based Vitrimers via Thiol-ene photopolymerization

Likened to both thermosets and thermoplastics, vitrimers are a unique class of materials that combine remarkable stability, healability, and reprocessability. Herein, we describe a photopolymerized thiol-ene-based vitrimer that undergoes dynamic covalent exchanges through uncatalyzed transamination of enamines derived from cyclic β-triketones, whereby the low energy barrier for exchange facilitates reprocessing and enables rapid depolymerization. Accordingly, we devised an alkene-functionalized β-triketone, 5,5-dimethyl-2-(pent-4-enoyl)cyclohexane-1,3-dione, which was reacted with 1,6-diaminohexane in a stoichiometrically imbalanced fashion (∼1:0.85 primary amine:triketone). The resulting networks exhibited subambient glass transition temperature (T_g = 5.66°C) by differential scanning calorimetry (DSC). Using a Maxwell stress-relaxation fit, the topology freezing temperature (T_v ) was calculated to be -32°C. Small-amplitude oscillatory shear (SAOS) rheological analysis enabled us to identify a practical critical temperature above which the vitrimer could be successfully reprocessed (T_v,eff ). Via the introduction of excess primary amines, we could readily degrade the networks into monomeric precursors, which were in turn reacted with diamines to regenerate reprocessable networks. Photopolymerization provides unique spatiotemporal control over the network topology, thereby opening the path for further investigation of vitrimer properties. As such, this work expands the toolbox of chemical upcycling of networks and enables their wider implementation. This article is protected by copyright. All rights reserved.

Liquid Crystalline Elastomers Based on Click Chemistry

Liquid crystalline elastomers (LCEs) have emerged as an important class of functional materials that are suitable for a wide range of applications, such as sensors, actuators, and soft robotics. The unique properties of LCEs originate from the combination between liquid crystal and elastomeric network. The control of macroscopic liquid crystalline orientation and network structure is crucial to realizing the useful functionalities of LCEs. A variety of chemistries have been developed to fabricate LCEs, including hydrosilylation, free radical polymerization of acrylate, and polyaddition of epoxy and carboxylic acid. Over the past few years, the use of click chemistry has become a more robust and energy-efficient way to construct LCEs with desired structures. This article provides an overview of emerging LCEs based on click chemistries, including aza-Michael addition between amine and acrylate, radical-mediated thiol-ene and thiol-yne reactions, base-catalyzed thiol-acrylate and thiol-epoxy reactions, copper-catalyzed azide-alkyne cycloaddition, and Diels-Alder cycloaddition. The similarities and differences of these reactions are discussed, with particular attention focused on the strengths and limitations of each reaction for the preparation of LCEs with controlled structures and orientations. The compatibility of these reactions with the traditional and emerging processing techniques, such as surface alignment and additive manufacturing, are surveyed. Finally, the challenges and opportunities of using click chemistry for the design of LCEs with advanced functionalities and applications are discussed.

Exchangeable Liquid Crystalline Elastomers and Their Applications

This Review presents and discusses the current state of the art in "exchangeable liquid crystalline elastomers", that is, LCE materials utilizing dynamically cross-linked networks capable of reprocessing, reprogramming, and recycling. The focus here is on the chemistry and the specific reaction mechanisms that enable the dynamic bond exchange, of which there is a variety. We compare and contrast these different chemical mechanisms and the key properties of their resulting elastomers. In the conclusion, we discuss the most promising applications that are enabled by dynamic cross-linking and present a summary table: a library of currently available materials and their main characteristics.

The Effect of Phenyl Content on the Liquid Crystal-Based Organosilicone Elastomers with Mechanical Adaptability

An elastomer with mechanical adaptability is a new kind of polymer material in which the increasing stress under continuous deformation is significantly inhibited in a large deformation area. Liquid crystal-based organosilicone elastomers, which can dissipate energy through reversible internal phase transition under external stimulation and have recoverable large deformation capacity, have drawn much interest as mechanical adaptability materials. However, there is no good way to control the mechanical adaptability at present. For this purpose, we prepared a new liquid crystal-based phenyl silicone rubber (LCMVPQ) using two-step click reactions and systematically explored the effect of phenyl content on its mechanical adaptability to achieve the regulation of mechanical adaptability. With an increase in phenyl content in the LCMVPQs, phenyl can hinder the rearrangement of the mesogenic units along the applied stress direction, which enables the adjustment of mechanical adaptability to meet the needs of different situations. In addition, the introduction of the liquid crystal phase impedes the internal friction of the molecular chain movement of the LCMVPQs and reduces the damping performance of silicone rubber. This research achieves the regulation of elastomers with mechanical adaptability and is expected to be applied in practical application fields.

Double Networks of Liquid-Crystalline Elastomers with Enhanced Mechanical Strength

Liquid-crystalline elastomers (LCEs) are frequently used in soft actuator development. However, applications are limited because LCEs are prone to mechanical failure when subjected to heavy loads and high temperatures during the working cycle. A mechanically tough LCE system offers larger work capacity and lower failure rate for the actuators. Herein, we adopt the double-network strategy, starting with a siloxane-based exchangeable LCE and developing a series of double-network liquid-crystalline elastomers (DN-LCEs) that are mechanically tougher than the initial elastomer. We incorporate diacrylate reacting monomers to fabricate DN-LCEs, some of which have the breaking stress of 40 MPa. We incorporate thermoplastic polyurethane to fabricate a DN-LCE, achieving an enormous ductility of 90 MJ/m³. We have also attempted to utilize the aza-Michael chemistry to make a DN-LCE that retains high plasticity because of several bond-exchange mechanisms; however, it failed to produce a stable reprocessable LCE system using conventional ester-based reactive mesogens. Each of these DN-LCEs exhibits unique features and characteristics, which are compared and discussed.

Imine-Based Reactive Mesogen and Its Corresponding Exchangeable Liquid Crystal Elastomer

To date, exchangeable liquid crystalline elastomers (xLCEs) have been mainly fabricated by combining conventional LCEs with additional exchangeable functional groups in their networks. While conventional LCEs are frequently made from commercially available aromatic–ester reacting mesogens or from mesogens based on a biphenyl core, such reacting monomers are not optimized to fabricating xLCEs whose bond-exchange reaction is fast and clean cut. Here, we develop a fast synthesis route to produce a new type of reactive mesogen based on an aromatic–imine structure that intrinsically enables a fast and stable bond-exchange reaction in the resulting imine-based xLCE. This new xLCE displays vitrimer plastic-flow behavior, and its bond-exchange activation energy is calculated to be 54 kJ/mol. We also demonstrate that this xLCE is thermally stable to withstand many recycling cycles without visible decay, and its liquid crystallinity is preserved. Finally, we demonstrate the reprogramming and realignment of the mesogen orientation in this xLCE with the realigned xLCE capable of reversible thermal actuation.

Real-Time Alignment and Reorientation of Polymer Chains in Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) exhibit soft elasticity due to the alignment and reorientation of mesogens upon mechanical loading, which provides additional mechanisms to absorb and dissipate energy. This enhanced response makes LCEs potentially transformative materials for biomedical devices, tissue replacements, and protective equipment. However, there is a critical knowledge gap in understanding the highly rate-dependent dissipative behaviors of LCEs due to the lack of real-time characterization techniques that probe the microscale network structure and link it to the mechanical deformation of LCEs. In this work, we employ in situ optical measurements to evaluate the alignment and reorientation degree of mesogens in LCEs. The data are correlated to the quantitative physical analysis using polarized Fourier-transform infrared spectroscopy. The time scale of mesogen alignment is determined at different strain levels and loading rates. The mesogen reorientation kinetics is characterized to establish its relationship with the macroscale tensile strain, and compared to theoretical predictions. Overall, this work provides the first detailed study on the time-dependent evolution of mesogen alignment and reorientation in deformed LCEs. It also provides an effective and more accessible approach for other researchers to investigate the structural-property relationships of different types of polymers.

4D Printing of Liquid Crystals: What's Right for Me?

Recent years have seen major advances in the developments of both additive manufacturing concepts and responsive materials. When combined as 4D printing, the process can lead to functional materials and devices for use in health, energy generation, sensing, and soft robots. Among responsive materials, liquid crystals, which can deliver programmed, reversible, rapid responses in both air and underwater, are a prime contender for additive manufacturing, given their ease of use and adaptability to many different applications. In this paper, selected works are compared and analyzed to come to a didactical overview of the liquid crystal-additive manufacturing junction. Reading from front to back gives the reader a comprehensive understanding of the options and challenges in the field, while researchers already experienced in either liquid crystals or additive manufacturing are encouraged to scan through the text to see how they can incorporate additive manufacturing or liquid crystals into their own work. The educational text is closed with proposals for future research in this crossover field.

Progress in Utilizing Dynamic Bonds to Fabricate Structurally Adaptive Self-Healing, Shape Memory, and Liquid Crystal Polymers

Stimuli-responsive structurally dynamic polymers are capable of mimicking the biological systems to adapt themselves to the surrounding environmental changes and subsequently exhibiting a wide range of responses ranging from self-healing to complex shape-morphing. Dynamic self-healing polymers (SHPs), shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs), which are three representative examples of stimuli-responsive structurally dynamic polymers, have been attracting broad and growing interest in recent years because of their potential applications in the fields of electronic skin, sensors, soft robots, artificial muscles, and so on. We review recent advances and challenges in the developments towards dynamic SHPs, SMPs and LCEs, focusing on the chemistry strategies and the dynamic reaction mechanisms that enhance the performances of the materials including self-healing, reprocessing and reprogramming. We compare and discuss the different dynamic chemistries and their mechanisms on the enhanced functions of the materials, where three summary tables are presented: a library of dynamic bonds and the resulting characteristics of the materials. Finally, we provide a critical outline of the unresolved issues and future perspectives on the emerging developments. This article is protected by copyright. All rights reserved.

Folding fluorescent probes for self-reporting transesterification in dynamic polymer networks

Dynamic exchange reactions in covalent adaptable networks (CANs) are difficult to probe directly via various macroscopic mechanical methods. Herein, we report a fluorescent strategy for directly reporting the dynamic bond exchange in transesterification-based CANs by using folding molecular probes. The folding probes (PDI-dimers) consist of two perylene diimide (PDI) cores, a spacer of dynamic esters between the two PDI cores, and reactive terminal groups. During transesterification in CANs, the PDI-dimers unfold their PDI excimers to show a sharp fluorescent color change from orange to bright yellow. This visual strategy is demonstrated by a crosslinked thiol-Michael network (TMN) and poly(4-hydroxybutyl acrylate) network (PHBA). The dynamic behaviors like stress relaxation and self-stiffening in these CANs can be directly read out via the change of fluorescent color. This method can provide quantitative information and show spatiotemporal resolution and therefore, can be applied to probe various dynamic chain exchange mechanisms in crosslinked materials.

Recent progress in dynamic covalent chemistries for liquid crystal elastomers

Liquid crystal elastomers (LCEs) have recently shown great potential in the applications of soft robotics, biomedical devices, active morphing structures, self-regulating systems and biomimetic demonstrations. Physical properties of LCEs highly depend on their crosslinking and the alignment of mesogens in the polymer network. Different strategies have been adopted to control and program the alignment of mesogens in LCEs over the recent decades, including stretching a loosely crosslinked LCE during its second-step crosslinking reaction, application of a strong magnetic or electrical field onto an LCE during its crosslinking process, and crosslinking a LCE thin film on the top of a surface with predesigned molecular texture. In the most recent decade, dynamic covalent bonds, which can undergo exchange reactions with or without external stimuli, have been introduced into LCEs to enable facile programing of mesogen orientation in the elastomer. In addition to the programmability, the LCEs with dynamic covalent bonds have also shown great recyclability, self-healing abilities and reprogrammability. In this article, we will review the recent progress in the synthesis, programming and application of LCEs with dynamic covalent bonds. We will also discuss the challenges and research opportunities in the field.

3D Printing of Liquid Crystal Elastomer Foams for Enhanced Energy Dissipation Under Mechanical Insult

Polymer foams are an essential class of lightweight materials used to protect assets against mechanical insults, such as shock and vibration. Two features are important to enhance their energy absorption characteristics: the foam structure and the matrix phase mechanical behavior. This study investigates novel approaches to control both of these features to enhance the energy absorption capability of flexible lattice foams. First, we consider 3D printing via digital light processing (DLP) as a method to control the foam mesostructure across a suite of periodic unit cells. Second, we introduce an additional energy dissipation mechanism in the solid matrix phase material by 3D printing the lattice foams with polydomain liquid crystal elastomer (LCE), which undergo a mechanically induced phase transition under large strains. This phase transition is associated with LC mesogen rotation and alignment and provides a second mechanism for mechanical energy dissipation in addition to the viscoelastic relaxation of the polymer network. We contrast the 3D printed LCE lattices with conventional, thermomechanically near-equivalent elastomer lattice foams to quantify the energy-absorbing enhancement the LCE matrix phase provides. Under cyclic quasi-static uniaxial compression conditions, the LCE lattices show dramatically enhanced energy dissipation in uniaxial compression compared to the non-LCE equivalent foams printed with a commercially available photocurable elastomer resin. The lattice geometry also plays a prominent role in determining the energy dissipation ratio between the LCE and non-LCE foams. We show that when increasing the lattice connectivity, the foam deformation transitions from bending-dominated to stretching-dominated deformations, which generates higher axial strains in the struts and higher energy dissipation in the lattice foam, as stretching allows greater mesogen rotation than bending. The LCE foams demonstrate superior energy absorption during the repeated dynamic loading during drop testing compared with the non-LCE equivalent foams, demonstrating the potential of LCEs to enhance physical protection systems against mechanical impact.

The nonequilibrium behaviors of covalent adaptable network polymers during the topology transition

Vitrimers with bond exchange reactions (BERs) are a class of covalent adaptable network (CAN) polymers at the forefront of recent polymer research. They exhibit malleable and self-healable behaviors and combine the advantages of easy processability of thermoplastics and excellent mechanical properties of thermosets. For thermally sensitive vitrimers, a molecular topology melting/frozen transition is triggered when the BERs are activated to rearrange the network architecture. Notable volume expansion and stress relaxation are accompanied, which can be used to identify the BER activation temperature and rate as well as to determine the malleability and interfacial welding kinetics of vitrimers. Existing works on vitrimers reveal the rate-dependent behaviors of the nonequilibrium network during the topology transition. However, it remains unclear what the quantitative relationship with heating rate is, and how it will affect the macroscopic stress relaxation. In this paper, we study the responses of an epoxy-based vitrimer subjected to a change in temperature and mechanical loading during the topology transition. Using thermal expansion tests, the thermal strain evolution is shown to depend on the temperature-changing rate, which reveals the nonequilibrium states with rate-dependent structural relaxation. The influences of structural relaxation on the stress relaxation behaviors are examined in both uniaxial tension and compression modes. Assisted by a theoretical model, the study reveals how to tune the material and thermal-temporal conditions to promote the contribution of BERs during the reprocessing of vitrimers.

Remotely Controlled, Reversible, On-Demand Assembly and Reconfiguration of 3D Mesostructures via Liquid Crystal Elastomer Platforms

Three-dimensional (3D) mesostructures are gaining rapidly growing interest due to their potential applications in a broad range of areas. Despite intensive studies, remotely controlled, reversible, on-demand assembly and reconfiguration of 3D mesostructures, which are desired for many applications, including robotics, minimally invasive biomedical devices, and deployable systems, remain a challenge. Here, we introduce a facile strategy to utilize liquid crystal elastomers (LCEs), a soft polymer capable of large, reversible shape changes, as a platform for reversible assembly and programming of 3D mesostructures via compressive buckling of two-dimensional (2D) precursors in a remote and on-demand fashion. The highly stretchable, reversible shape-switching behavior of the LCE substrate, resulting from the soft elasticity of the material and the reversible nematic-isotropic transition of liquid crystal (LC) molecules upon remote thermal stimuli, provides deterministic thermal-mechanical control over the reversible assembly and reconfiguration processes. Demonstrations include experimental results and finite element simulations of 3D mesostructures with diverse geometries and material compositions, showing the versatility and reliability of the approach. Furthermore, a reconfigurable light-emitting system is assembled and morphed between its "on" and "off" status via the LCE platform. These results provide many exciting opportunities for areas from remotely programmable 3D mesostructures to tunable electronic systems.

Permanent and reversibly programmable shapes in liquid crystal elastomer microparticles capable of shape switching

Reversibly programmable liquid crystal elastomer microparticles (LCEMPs), formed as a covalent adaptable network (CAN), with an average diameter of 7 μm ± 2 μm, were synthesized via a thiol-Michael dispersion polymerization. The particles were programmed to a prolate shape via a photoinitiated addition-fragmentation chain-transfer (AFT) exchange reaction by activating the AFT after undergoing compression. Due to the thermotropic nature of the AFT-LCEMPs, shape switching was driven by heating the particles above their nematic-isotropic phase transition temperature (T_NI). The programmed particles subsequently displayed cyclable two-way shape switching from prolate to spherical when at low or high temperatures, respectively. Furthermore, the shape programming is reversible, and a second programming step was done to erase the prolate shape by initiating AFT at high temperature while the particles were in their spherical shape. Upon cooling, the particles remained spherical until additional programming steps were taken. Particles were also programmed to maintain a permanent oblate shape. Additionally, the particle surface was programmed with a diffraction grating, demonstrating programmable complex surface topography via AFT activation.

Hydrogen Bonding-Derived Healable Polyacrylate Elastomers via On-demand Copolymerization of n-Butyl Acrylate and tert-Butyl Acrylate

Achieving a desirable combination of good mechanical properties and healing efficiency is a great challenge in the development of self-healing elastomers. Herein, a class of tough and strong self-healing polyacrylate elastomers (denoted as HPs) was developed simply by free-radical copolymerization of n-butyl acrylate (nBA) and tert-butyl acrylate (tBA) and a subsequent hydrolysis reaction rather than direct copolymerization of nBA and acrylic acid (AA). The tiny difference in reactivity between nBA and tBA makes the structural units of the copolymer easy to control. Precise regulation of molecular composition can be realized just by varying the relative monomer content, making its mechanical properties to vary from ductile to robust. Strikingly, when HP samples are cut off within the gauge length, they can heal into coherent and smooth samples and recover at least 79% of the original strength. Hydrogen bond interactions serve as physical cross-linking points, contributing to the high mechanical performance (fracture energy of up to 73.78 MJ·m^-3 and tensile strength of up to 17.80 MPa) as well as shape memory function. Moreover, the HP samples emit strong fluorescence when exposed to a 365 nm UV lamp and exhibit an aggregation-enhanced emission effect in the state of dissolution.

Processing advances in liquid crystal elastomers provide a path to biomedical applications

Liquid crystal elastomers (LCEs) are a class of stimuli-responsive polymers that undergo reversible shape-change in response to environmental changes. The shape change of LCEs can be programmed during processing by orienting the liquid crystal phase prior to crosslinking. The suite of processing techniques that has been developed has resulted in a myriad of LCEs with different shape-changing behavior and mechanical properties. Aligning LCEs via mechanical straining yields large uniaxial actuators capable of a moderate force output. Magnetic fields are utilized to control the alignment within LCE microstructures. The generation of out-of-plane deformations such as bending, twisting, and coning is enabled by surface alignment techniques within thin films. 4D printing processes have emerged that enable the fabrication of centimeter-scale, 3D LCE structures with a complex alignment. The processing technique also determines, to a large extent, the potential applications of the LCE. For example, 4D printing enables the fabrication of LCE actuators capable of replicating the forces generated by human muscles. Employing surface alignment techniques, LCE films can be designed for use as coatings or as substrates for stretchable electronics. The growth of new processes and strategies opens and strengthens the path for LCEs to be applicable within biomedical device designs.

The effect of alignment on the rate-dependent behavior of a main-chain liquid crystal elastomer

This study investigated the effect of alignment on the rate-dependent behavior of a main-chain liquid crystal elastomer (LCE). Polydomain nematic LCE networks were synthesized from a thiol-acrylate Michael addition reaction in the isotropic state. The polydomain networks were stretched to different strain levels to induce alignment then crosslinked in a second stage photopolymerization process. The LCE networks were subjected to dynamic mechanical tests to measure the temperature-dependent storage modulus and uniaxial tension load-unload tests to measure the rate-dependence of the Young's modulus, mechanical dissipation, and characteristics of the soft stress response. Three-dimensional (3D) digital image correlation (DIC) was used to characterize the effect of domain/mesogen relaxation on the strain fields. All LCE networks exhibited a highly rate-dependent stress response with significant inelastic strains after unloading. The Young's modulus of the loading curve and hysteresis of the load-unload curves showed a power-law dependence on the strain rate. The Young's modulus increased with alignment and larger anisotropy and a smaller power-law exponent was measured for the Young's modulus and hysteresis for the highly aligned monodomains. The polydomain and pre-stretched networks loaded perpendicular to the alignment direction exhibited a soft stress response that featured a rate-dependent peak stress, strain-softening, and strain-stiffening. The 3D-DIC strain fields for the polydomain network and programmed networks stretched in the perpendicular direction were highly heterogeneous, showing regions of alternating higher and lower strains. The strain variations increased initially with loading, peaked during the strain softening part of the stress response, then decreased during the strain stiffening part of the stress response. Greater variability was measured for lower strain rates. These observations suggest that local domain/mesogen relaxation led to the development of the heterogeneous strain patterns and strain softening in stress response. These findings improved understanding of the kinetics of mesogen relaxation and its contributions to the rate-dependent stress response and mechanical dissipation.

Three-dimensional printing of functionally graded liquid crystal elastomer

As a promising actuating material, liquid crystal elastomer (LCE) has been intensively explored in building diverse active structures and devices. Recently, direct ink writing technique has been developed to print LCE structures with various geometries and actuation behaviors. Despite the advancement in printing LCE, it remains challenging to print three-dimensional (3D) LCE structures with graded properties. Here, we report a facile method to tailor both the actuation behavior and mechanical properties of printed LCE filaments by varying printing parameters. On the basis of the comprehensive processing-structure-property relationship, we propose a simple strategy to print functionally graded LCEs, which greatly increases the design space for creating active morphing structures. We further demonstrate mitigation of stress concentration near the interface between an actuatable LCE tube and a rigid glass plate through gradient printing. The strategy developed here will facilitate potential applications of LCEs in different fields.

Recyclable and Self-Repairable Fluid-Driven Liquid Crystal Elastomer Actuator

Liquid crystal elastomer (LCE) is a newly emerging soft actuating material that has been extensively explored for building novel soft robots and diverse active devices, thanks to its large actuation stress and strain, high work density, and versatile actuation modes. However, there have also been several widely recognized limitations of LCE-based actuators for practical applications, including slow response and narrow range of operation temperature. Herein, we develop fluid-driven disulfide LCE actuators through facile laminate manufacturing enabled by a dynamic bond exchange reaction. Because of the merits of the active heating/cooling mechanism of the fluidic structure, this newly developed disulfide LCE actuator can generate large cyclic actuation at a frequency around 1 Hz and can operate in a wide range of temperatures. The unique combination of the fluidic structure design and the dynamic covalent bonds in the elastomer has also enabled the full recyclability and self-repairability of the actuator. Using the newly developed actuator as building block, we further constructed soft robotic systems that can realize manipulating and programmable movement. The design principle demonstrated in the current work opens a promising avenue for exploring more novel applications of LCE-based soft actuators.

Implantation of Recyclability and Healability into Cross-Linked Commercial Polymers by Applying the Vitrimer Concept

Vitrimers are a new class of cross-linked materials that are capable of network topology alternation through the associative dynamic bond-exchange mechanism, which has recently been invented to solve the problem of conventional cross-linked materials, such as poor recyclability and healability. Thus far, the concept of vitrimers has been applied to various commercial polymers, e.g., polyesters, polylactides, polycarbonates, polydimethylsiloxanes, polydienes, polyurethanes, polyolefins, poly(meth)acrylates, and polystyrenes, by utilizing different compatible bond-exchange reactions. In this review article, the concept of vitrimers is described by clarifying the difference from thermoplastics and supramolecular systems; in addition, the term "associative bond-exchange" in vitrimers is explained by comparison with the "dissociative" term. Several useful functions attained by the vitrimer concept (including recyclability and healability) are demonstrated, and recent molecular designs of vitrimers are classified into groups depending on the types of molecular frameworks. This review specifically focuses on the vitrimer molecular designs with commercial polymer-based frameworks, which provide useful hints for the practical application of the vitrimer concept.

Catalytic Control of Plastic Flow in Siloxane-Based Liquid Crystalline Elastomer Networks

Liquid crystalline elastomer networks cross-linked by dynamic covalent bonds (xLCE) have the ability to be (re)processed during the plastic flow. However, the current bond-exchange strategies that are used to induce plastic flow in xLCE lack the efficient method to control the elastic-plastic transition. Here we describe a straightforward method to manipulate the transition to plastic flow via the choice of catalyst in xLCE cross-linked by siloxane. The nature and the amount of catalyst have a profound effect on the elastic-plastic transition temperature, and the stress relaxation behavior of the network. The temperature of fast plastic flow and the associated bond-exchange activation energy varied from 120 °C and 83 kJ/mol in the "fastest" exchange promoted by triazobicyclodecene (TBD) to 240 °C and 164 kJ/mol in the "slowest" exchange with triphenylphosphine (PPH), with a range of catalysts in between. We have identified the optimum conditions for programming an aligned monodomain xLCE, high programming temperature (230 °C) and low nematic to isotropic transition (60 °C), to achieve thermally and mechanically stable actuators.

Materials as Machines

Machines are systems that harness input power to extend or advance function. Fundamentally, machines are based on the integration of materials with mechanisms to accomplish tasks-such as generating motion or lifting an object. An emerging research paradigm is the design, synthesis, and integration of responsive materials within or as machines. Herein, a particular focus is the integration of responsive materials to enable robotic (machine) functions such as gripping, lifting, or motility (walking, crawling, swimming, and flying). Key functional considerations of responsive materials in machine implementations are response time, cyclability (frequency and ruggedness), sizing, payload capacity, amenability to mechanical programming, performance in extreme environments, and autonomy. This review summarizes the material transformation mechanisms, mechanical design, and robotic integration of responsive materials including shape memory alloys (SMAs), piezoelectrics, dielectric elastomer actuators (DEAs), ionic electroactive polymers (IEAPs), pneumatics and hydraulics systems, shape memory polymers (SMPs), hydrogels, and liquid crystalline elastomers (LCEs) and networks (LCNs). Structural and geometrical fabrication of these materials as wires, coils, films, tubes, cones, unimorphs, bimorphs, and printed elements enables differentiated mechanical responses and consistently enables and extends functional use.

Toward Stimuli-Responsive Dynamic Thermosets through Continuous Development and Improvements in Covalent Adaptable Networks (CANs)

Covalent adaptable networks (CANs), unlike typical thermosets or other covalently crosslinked networks, possess a unique, often dormant ability to activate one or more forms of stimuli-responsive, dynamic covalent chemistries as a means to transition their behavior from that of a viscoelastic solid to a material with fluid-like plastic flow. Upon application of a stimulus, such as light or other irradiation, temperature, or even a distinct chemical signal, the CAN responds by transforming to a state of temporal plasticity through activation of either reversible addition or reversible bond exchange, either of which allows the material to essentially re-equilibrate to an altered set of conditions that are distinct from those in which the original covalently crosslinked network is formed, often simultaneously enabling a new and distinct shape, function, and characteristics. As such, CANs span the divide between thermosets and thermoplastics, thus offering unprecedented possibilities for innovation in polymer and materials science. Without attempting to comprehensively review the literature, recent developments in CANs are discussed here with an emphasis on the most effective dynamic chemistries that render these materials to be stimuli responsive, enabling features that make CANs more broadly applicable.

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Liquid crystalline elastomers (LCE) undergo reversible shape changes in response to stimuli, which enables a wide range of smart applications, in soft robotics, adhesive systems or biomedical medical devices. In this study, we introduce a new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCE to enable processing (director alignment, remolding, and welding). Unlike the traditional siloxane based LCE, which were produced by reaction schemes with irreversible bonds (e.g. hydrosilylation), here we use a much more robust reaction (thiol-acrylate/thiol-ene 'double-click' chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces exchangeable LCE (xLCE) with tunable properties, low glass transition (-30 °C), controllable nematic to isotropic transition (33 to 70 °C), and a very high vitrification temperature (up to 250 °C). Accordingly, this class of dynamically crosslinked xLCE shows unprecedented thermal stability within the working temperature range (-50 to 140 °C), over many thermal actuation cycles without any creep. Finally, multiple xLCE sharing the same siloxane exchangeable bonds can be welded into single continuous structures to allow for composite materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.