Recyclable Solid-Solid Phase Change Materials with Superior Latent Heat via Reversible Anhydride-Alcohol Crosslinking for Efficient Thermal Storage.

Solid-solid phase change materials (SSPCMs) with crosslinked polymer structures have received sustained interest due to their remarkable shape stability, enabling their application independently without the need for encapsulation or supporting materials. However, the crosslinking structure also compromises their latent heat and poses challenges to their recyclability. Herein, a novel strategy harnessing the internal-catalyzed reversible anhydride-alcohol crosslinking reaction to fabricate SSPCMs with superior latent heat and exceptional dual recyclability is presented. Easily accessible anhydride copolymers (e.g., propylene-maleic anhydride alternating copolymers), provide abundant reactive anhydride sites within the polymer matrix; polyethylene glycol serves as both the grafted phase change component and the crosslinker. The resulting SSPCMs attain a peak latent heat value of 156.8 J g-1 which surpasses all other reported recyclable crosslinked SSPCMs. The materials also exhibit certain flexibility and a tunable tensile strength ranging from 6.6 to 11.0 MPa. Beyond that, leveraging the reversible anhydride-alcohol crosslinks, the SSPCMs demonstrate dual recyclability through bond-exchange remolding and reversible-dissociation-enabled dissolving-recrosslinking without any reactive chemicals. Furthermore, by integrating solar-thermal conversion fillers like polydopamine nanoparticles, the potential of the system in efficient conversion, storage, and release of solar energy is highlighted.

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.

Merging the Interfaces of Different Shape-Shifting Polymers Using Hybrid Exchange Reactions.

Sophisticated shape-shifting structures and integration of advanced functions often call for different-chemistry-based polymers (such as epoxy and polyurethane) in a unified system. However, permanent cross-links pose crucial obstacles to be seamless. Here, merging interfaces via hybrid exchange reactions among different dynamic covalent bonds (including ester, urethane, thiourethane, boronic-ester, and oxime-ester linkages) is proposed, breaking the long-lasting restriction that these widely used bonds only undergo self-exchange reactions. Model compound studies are conducted to verify that hybrid exchange reactions occur. As demonstrations, different liquid crystal elastomers are tenaciously joined into coherent assemblies, with the desired biomimetic structures (e.g., flying fish containing stiff and flexible parts) and rare deformation modes (e.g., flower blooming upon both heating and cooling). Besides connecting polymers, hybrid exchange reactions also facilitate the creation of new materials through cross-fusion of different polymers. In addition to the polymers used in this work, hybrid exchange reactions can be adapted to other polymers based on similar mechanisms and beyond. Besides shape-shifting-related areas (e.g., soft robots, flexible electronics, and biomedical devices), it may also foster innovation in other fields involving general polymers, as well as promote deeper understanding of dynamic covalent chemistry.

Locally controllable magnetic soft actuators with reprogrammable contraction-derived motions.

Reprogrammable magneto-responsive soft actuators capable of working in enclosed and confined spaces and adapting functions under changing situations are highly demanded for new-generation smart devices. Despite the promising prospect, the realization of versatile morphing modes (more than bending) and local magnetic control remains challenging but is crucial for further on-demand applications. Here, we address the challenges by maximizing the unexplored potential of magnetothermal responsiveness and covalent adaptable networks (CANs) in liquid crystalline elastomers (LCEs). Various magneto-actuated contraction-derived motions that were hard to achieve previously (e.g., bidirectional shrinkage and dynamic 3D patterns) can be attained, reprogrammed, and assembled seamlessly to endow functional diversity and complexity. By integration of LCEs with different magneto-responsive threshold values, local and sequential magnetic control is readily realized. Many magnetic actuation portfolios are performed by rationally imputing "logic switch" sequences. Meanwhile, our systems exhibit additional favorable performances including stepwise magnetic controllability, multiresponsiveness, self-healing, and remolding ability.

A magnetic solder for assembling bulk covalent adaptable network blocks.

Covalent adaptable networks (CANs) represent a novel covalently cross-linked polymer that is capable of being reprocessed and recycled relying on reversible covalent bond structures and present exceptional opportunities in a wide range of prospective applications. However, it is genuinely difficult to fabricate bulk CAN blocks with solid-core geometries that possess complex shapes or multiple materials, which are crucial in cutting-edge fields such as soft robotics, flexible electronic devices and biomedical engineering. Here we report a welding technique to strategically construct complex and heterogeneous 3D CAN structures by utilizing a solder doped with magnetic nanoparticles. The solder is able to induce a bond exchange reaction at the interface between the to-be-welded pieces. Using this method, not only CAN bulks with the same materials can be welded to form complex geometries, distinctive bulks with different physical properties and chemical compositions can also be connected to fabricate multimaterial devices. Besides, this method can be used to repair damaged CAN materials and efficiently recycle scrap CAN materials, which can effectively save resources and protect the environment. The universality and robustness of this strategy is expected to promote CAN application in broader functional polymer fields.

Seamless multimaterial 3D liquid-crystalline elastomer actuators for next-generation entirely soft robots.

Liquid-crystalline elastomers (LCEs) are excellent soft actuator materials for a wide range of applications, especially the blooming area of soft robotics. For entirely soft LCE robots to exhibit high dexterity and complicated performance, several components are typically required to be integrated together in one single robot body. Here, we show that seamless multicomponent/multimaterial three-dimensional (3D) LCE robots can be created via simultaneously welding and aligning LCE materials with different chemical compositions and physical properties without other additives such as tapes and glues (just like metal welding). Both welding and aligning of the LCE materials rely on thermal polymerization of preformed LCE films with reactive acrylate groups. This method provides an easy way to robustly fabricate arbitrary 3D desirable geometries with strongly stable reversible actuations and multifunctionalities, which greatly enlarges and benefits the future applications and manufacturing of LCE soft robots.

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.

Untethered Recyclable Tubular Actuators with Versatile Locomotion for Soft Continuum Robots.

Stimuli-responsive materials offer a distinguished platform to build tether-free compact soft robots, which can combine sensing and actuation without a linked power supply. In the past, tubular soft robots have to be made by multiple components with various internal channels or complex cavities assembled together. Moreover, robust processing, complex locomotion, simple structure, and easy recyclability represent major challenges in this area. Here, it is shown that those challenges can be tackled by liquid crystalline elastomers with allyl sulfide functional groups. The light-controlled exchange reaction between allyl sulfide groups allows flexible processing of tubular soft robots/actuators, which does not need any assisting materials. Complex locomotion demonstrated here includes reversible simultaneous bending and elongation; reversible diameter expansion; and omnidirectional bending via remote infrared light control. Different modes of actuation can be programmed into the same tube without the routine assembly of multiple tubes as used in the past. In addition, the exchange reaction also makes it possible to use the same single tube repeatedly to perform different functions by erasing and reprogramming.