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

Responsive Industrial Polymers: A Marriage of Polyurethanes with Liquid Crystal Elastomers?

Responsive polymers have yet to significantly impact the marketplace. In this Perspective, we offer a glimpse of a possible future industrial-scale responsive polymer. We begin by briefly reviewing two different existing polymer materials, one with high volume, excellent processability, and commercial impact (polyurethanes), the other with stimuli responsive functional properties (liquid crystal elastomers). We explore the possibilities of combining the properties of these two disparate entities into a single material. We offer intriguing possibilities for a bulk polymer with both responsivity and processability that could compete in the market with the long-established residents and discuss some of the research roadblocks that need to be overcome to reach this lofty goal.

Liquid Crystal Elastomers: 30 Years After

This is a Review that attempts to cast a look at the whole history of liquid crystal elastomers and the evolution of this field from its inception to the current state of the art. The exposition is limited by deliberately omitting several important elements of this field, such as densely cross-linked networks or smectic elastomers, focusing solely on the nematic phase of these elastomers. In this more narrow topic, we first discuss the current developments and perspectives in the materials chemistry. This is followed by three sections, each dedicated to one of the three main points of interest in the nematic liquid crystal elastomers: the reversible actuation, the soft elasticity, and the viscoelastic dynamics of nematic elastomers. In each of these directions, there have been significant developments over recent years but equally significant new avenues emerging for the research to follow.

Controlling Self-Oscillation of a Single-Layer Liquid Crystal Elastomer at the Air-Water Interface via Light Programming for Water Strider-Inspired Aquatic Robots

Biomimicking aquatic organisms offers many opportunities for designing intelligent robots that can freely move on water. However, most works were focused on multilayered materials or assembled structures and faced limitations in stability, versatility, and motion navigation. Here, we develop an assembly-free water-strider-like aquatic robot using a single layer of light-programmable liquid-crystal elastomer (LCE) that could be used to create asymmetric structures. The LCE strider mimics both the shape and functions of natural water striders; it is designed with four legs, with the fore and hind legs being programmed respectively via light. Consequently, the LCE strider shows self-oscillation and self-propulsion behaviors on low-grade thermal water with a temperature gradient at the air-water interface, owing to unbalanced changes in the contact areas and tensions between the legs and water. Furthermore, the trajectories of the LCE strider are manipulated by NIR light after selectively depositing polydopamine with photothermal conversion. In this way, path navigation is realized, that is, moving straight and on-demand turning, similar to the movement of natural water striders. This study should inspire the development of soft intelligent robots using shape-morphing materials.

Hydrogen-bonded multi-mode liquid crystal elastomer actuators

As biomimicry advances, liquid crystal elastomers (LCEs) are gaining attention for their (multi-)stimuli-responsiveness and reversible shape morphing. Introduction of dynamic bonds into the LCEs provides versatile means towards programmable shape morphing and adaptation to environmental cues, and new designs for dynamic LCEs are actively sought for. Here, we present a supramolecular LCE that integrates shape memory programming, humidity sensitivity, and photochemical actuation. By utilizing hydrogen bonding crosslinks, the LCE gains shape memory functionality, enabling arbitrary shape programming and photochemical actuation. By breaking the supramolecular crosslinks via base treatment, the LCE becomes hygroscopic and humidity sensitive, yet maintains photochemical deformability. These two states enable different types of soft actuator demonstrations both in air and under water, adding to the versatility and programmability of light-driven shape-changing LCEs.

Robust liquid crystal semi-interpenetrating polymer network with superior energy-dissipation performance

Liquid crystal networks (LCN) have attracted surging interest as extraordinary energy-dissipation materials owning to their unique dissipation mechanism based on the re-orientation of mesogens. However, how to integrate high Young's modulus, good dissipation efficiency and wide effective damping temperature range in energy-dissipation LCN remains a challenge. Here, we report a strategy to resolve this challenge by fabricating robust energy-dissipation liquid crystal semi-interpenetrating polymer network (LC-semi-IPN) consisting crystalline LC polymers (c-LCP). LC-semi-IPN demonstrates a superior synergistic performance in both mechanical and energy-dissipation properties, surpassing all currently reported LCNs. The crystallinity of c-LCP endows LC-semi-IPN with a substantial leap in Young's modulus (1800% higher than single network). The chain reptation of c-LCP also promotes an enhanced dissipation efficiency of LC-semi-IPN by 200%. Moreover, its effective damping temperature reaches up to 130 °C, which is the widest reported for LCNs. By leveraging its exceptional synergistic performance, LC-semi-IPN can be further utilized as a functional architected structure with exceptional energy-dissipation density and deformation-resistance.

Closed-Loop Recyclable and Totally Renewable Liquid Crystal Networks with Room-Temperature Programmability and Reconfigurable Functionalities

Dynamic covalent liquid crystal networks (DCv-LCNs) with straightforward (re)programmability, reprocessability, and recyclability facilitates the manufacture of sophisticated LCN actuators and intelligent robots. However, the DCv-LCNs are still limited to heat-assisted programming and polymer-to-polymer reprocessing/recycling, which inevitably lead to deterioration of the LCN structures and the actuation performances after repeated programming/processing treatments, owing to the thermal degradation of the polymer network and/or external agent interference. Here, a totally renewable azobenzene-based DCv-LCN with room-temperature programmability and polymer-to-monomers chemical recyclability is reported, which was synthesized by crosslinking the azobenzene-containing dibenzaldehyde monomer and the triamine monomer via the dynamic and dissociable imine bonds. Thanks to the water-activated dynamics of the imine bonds, the resultant DCv-LCN can be simply programmed, upon water-soaking at room temperature, to yield a UV/Vis light-driven actuator. Importantly, the reported DCv-LCN undergoes depolymerization in an acid-solvent medium at room temperature because of the acid-catalyzed hydrolysis of the imine bonds, giving rise to easy separation and recovery of both monomers in high purity, even with tolerance to additives. The recovered pure monomers can be used to regenerate totally new DCv-LCNs and actuators, and their functionalities can be reconfigured by removing old and introducing new additives, by implementing the closed-loop polymer-monomers-polymer recycling.

Electrically weldable conductive elastomers

Flexible and stretchable electronic devices are subject to failure because of vulnerable circuit interconnections. We develop a low-voltage (1.5 to 4.5 V) and rapid (as low as 5 s) electric welding strategy to integrate both rigid electronic components and soft sensors in flexible circuits under ambient conditions. This is achieved through the design of conductive elastomers composed of borate ester polymers and conductive fillers, which can be self-welded and generate welding effects to various materials including metals, hydrogels, and other conductive elastomers. The welding effect is generated through the electrochemical reaction-triggered exposure of interfacial adhesive promotors or the cleavage/reformation of dynamic bonds. Our strategy can ensure both mechanical compliance and conductivity at the circuit interfaces and easily produce welding strengths in the kilopascal to megapascal range. The as-designed conductive elastomers in combination with the electric welding technique provide a robust platform for constructing standalone flexible and stretchable electronic devices that are detachable and assemblable on demand.

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.

Bone-inspired stress-gaining elastomer enabled by dynamic molecular locking

The limited capacity of typical materials to resist stress loading, which affects their mechanical performance, is one of the most formidable challenges in materials science. Here, we propose a bone-inspired stress-gaining concept of converting typically destructive stress into a favorable factor to substantially enhance the mechanical properties of elastomers. The concept was realized by a molecular design of dynamic poly(oxime-urethanes) network with mesophase domains. During external loading, the mesophase domains in the condensed state were aligned into more ordered domains, and the dynamic oxime-urethane bonds served as the dynamic molecular locks disassociating and reorganizing to facilitate and fix the mesophase domains. Consequently, the tensile modulus and strength were enhanced by 1744 and 49.3 times after four cycles of mechanical training, respectively. This study creates a molecular concept with stress-gaining properties induced by repeated mechanical stress loading and will inspire a series of innovative materials for diverse applications.

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.