Development of Fluorenyl-Based Copolyimines with Adjustable Mechanical Properties, Recyclability and Friction Resistance.

Dynamic polyimines are a class of fascinating dynamic polymers with recyclability and reparability owing to their reversible Schiff-base reactions. However, balancing the dynamic properties and mechanical strength of dynamic polyimines presents a major challenge due to the dissociative and associative nature of the imine bonds. Herein, we introduced bulky fluorene groups and polyether amine into the skeleton of polyimine networks to achieve a tradeoff in comprehensive properties. The resulting dynamic polyimines with fluorene groups (Cardo-DPIs) were successfully synthesized by combining the rigid diamine 9,9-bis(4-aminophenyl)fluorene and the flexible polyether amine, demonstrating a high tensile strength of 64.7 MPa. Additionally, Cardo-DPIs films with more content of rigid fluorene groups exhibited higher water resistance, glass transition temperature and wear-resisting ability. Moreover, the Cardo-DPIs films not only efficiently underwent thermal reshaping, but also exhibited excellent self-healing capabilities and chemical degradation in acidic solutions. Furthermore, the resulting films can achieve fully closed-loop recovery by free amine solution for 2 h at room temperature. This study broadens the scope of dynamic polyimine materials and promotes the balanced development of their functional and mechanical properties.

A Stretchable and Tough Graphene Film Enabled by Mechanical Bond.

The pursuit of fabricating high-performance graphene films has aroused considerable attention due to their potential for practical applications. However, developing both stretchable and tough graphene films remains a formidable challenge. To address this issue, we herein introduce mechanical bond to comprehensively improve the mechanical properties of graphene films, utilizing [2]rotaxane as the bridging unit. Under external force, the [2]rotaxane cross-link undergoes intramolecular motion, releasing hidden chain and increasing the interlayer slip distance between graphene nanosheets. Compared with graphene films without [2]rotaxane cross-linking, the presence of mechanical bond not only boosted the strength of graphene films (247.3 vs 74.8 MPa) but also markedly promoted the tensile strain (23.6 vs 10.2 %) and toughness (23.9 vs 4.0 MJ/m3). Notably, the achieved tensile strain sets a record high and the toughness surpasses most reported results, rendering the graphene films suitable for applications as flexible electrodes. Even when the films were stretched within a 20 % strain and repeatedly bent vertically, the light-emitting diodes maintained an on-state with little changes in brightness. Additionally, the film electrodes effectively actuated mechanical joints, enabling uninterrupted grasping movements. Therefore, the study holds promise for expanding the application of graphene films and simultaneously inspiring the development of other high-performance two-dimensional films.

Hyperbranched Vitrimer for Ultrahigh Energy Dissipation.

Polymers are ideally utilized as damping materials due to the high internal friction of molecular chains, enabling effective suppression of vibrations and noises in various fields. Current strategies rely on broadening the glass transition region or introducing additional relaxation components to enhance the energy dissipation capacity of polymeric damping materials. However, it remains a significant challenge to achieve high damping efficiency through structural control while maintaining dynamic characteristics. In this work, we propose a new strategy to develop hyperbranched vitrimers (HBVs) containing dense pendant chains and loose dynamic crosslinked networks. A novel yet weak dynamic transesterification between the carboxyl and boronic acid ester was confirmed and used to prepare HBVs based on poly (hexyl methacrylate-2-(4-ethenylphenyl)-5,5-dimethyl-1,3,2-dioxaborinane) P(HMA-co-ViCL) copolymers. The A B n ${{AB}_{n}}$ -type of macromonomers, the crosslinking points formed by the dynamic covalent connection via the associative exchange, and the weak yet dynamic exchange reaction are the three keys to developing high-performance HBV damping materials. We found that P(HMA-co-ViCL) 20k-40-60 HBV exhibited ultrahigh energy-dissipation performance over a broad frequency and temperature range, attributed to the synergistic effect of dense pendant chains and weak dynamic covalent crosslinks. This unique design concept will provide a general approach to developing advanced damping materials.

Anion-Coordination Foldamer-Based Polymer Network: from Molecular Spring to Elastomer.

Foldamer is a scaled-down version of coil spring, which can absorb and release energy by conformational change. Here, polymer networks with high density of molecular springs were developed by employing anion-coordination-based foldamers as the monomer. The coiling of the foldamer is controlled by oligo(urea) ligands coordinating to chloride ions; subsequently, the folding and unfolding of foldamer conformations endow the polymer network with excellent energy dissipation and toughness. The mechanical performance of the corresponding polymer networks shows a dramatic increase from P-L2UCl (non-folding), to P-L4UCl (a full turn), and then to P-L6UCl (1.5 turns), in terms of strength (2.62 MPa; 14.26 MPa; 22.93 MPa), elongation at break (70 %; 325 %; 352 %), Young's modulus (2.69 MPa; 63.61 MPa; 141.50 MPa), and toughness (1.12 MJ/m3; 21.39 MJ/m3; 49.62 MJ/m3), respectively, which is also better than those without anion centers and the non-foldamer based counterparts. Moreover, P-L6UCl shows enhanced strength and toughness than most of the molecular-spring based polymer networks. Thus, an effective strategy for designing high-performance anion-coordination-based materials is presented.

A Crown-Ether-Based Elastomer Bearing Loop Structures with Dissipating Characteristics and Enhanced Mechanical Performance.

Loops are prevalent topological structures in cross-linked polymer networks, resulting from the folding of polymer chains back onto themselves. Traditionally, they have been considered as defects that compromise the mechanical properties of the network, leading to extensive efforts in synthesis to prevent their formation. In this study, we introduce the inclusion of cyclic dibenzo-24-crown-8 (DB24C8) moieties within the polymer network strands to form CCNs, and surprisingly, these loops enhance the mechanical performances of the network, leading to tough elastomers. The toughening effect can be attributed to the unique cyclic structure of DB24C8. The relatively small size and the presence of rigid phenyl rings provide the loops with relatively stable conformations, allowing for substantial energy dissipation upon the application of force. Furthermore, the DB24C8 rings possess a broad range of potential conformations, imparting the materials with exceptional elasticity. The synergistic combination of these two features effectively toughens the materials, resulting in a remarkable 66-fold increase in toughness compared to the control sample of covalent networks. Moreover, the mechanical properties, particularly the recovery performance of the network, can be effectively tuned by introducing guests to bind with DB24C8, such as potassium ions and secondary ammonium salts.

Mechanically Interlocked Interphase with Energy Dissipation and Fast Li-Ion Transport for High-Capacity Lithium Metal Batteries.

Constructing an artificial solid electrolyte interphase (ASEI) on Li metal anodes (LMAs) is a potential strategy for addressing the dendrite issues. However, the mechanical fatigue of the ASEI caused by stress accumulation under the repeated deformation from the Li plating/stripping is not taken seriously. Herein, this work introduces a mechanically interlocked [an]daisy chain network (DCMIN) into the ASEI to stabilize the Li metal/ASEI interface by combining the functions of energy dissipation and fast Li-ion transport. The DCMIN featured by large-range molecular motions is cross-linked via efficient thiol-ene click chemistry; thus, the DCMIN has flexibility and excellent mechanical properties. As an ASEI, the crown ether units in DCMIN not only interact with the dialkylammonium of a flexible chain, forming the energy dissipation behavior but also coordinate with Li ion to support the fast Li-ion transport in DCMIN. Therefore, a stable 2800 h-symmetrical cycling (1 mA cm-2) and an excellent 5 C-rate (full cell with LiFePO4) performance are achieved by DCMIN-based ASEI. Furthermore, the 1-Ah pouch cell (LiNi0.88Co0.09Mn0.03O2 cathode) with DCMIN-coated LMA exhibits improved capacity retention (88%) relative to the Control. The molecular design of DCMIN provides new insights into the optimization of an ASEI for high-energy LMAs.

Mechanically Interlocked Polymers with Dense Mechanical Bonds.

ConspectusMechanically interlocked polymers (MIPs) such as polyrotaxanes and polycatenanes are polymer architectures that incorporate mechanical bonds, which represent a compelling frontier in polymer science. MIPs with cross-linked structures are known as mechanically interlocked networks (MINs) and are widely utilized in materials science. Leveraging the motion of mechanical bonds, MINs hold the potential for achieving a combination of robustness and dynamicity. Currently, the reported MINs predominantly consist of networks with discrete mechanical bonds as cross-linking points, exemplified by well-known slide-ring materials and rotaxane/catenane cross-linked polymers. The motion of these mechanically interlocked cross-linking points facilitates the redistribution of tension throughout the network, effectively preventing stress concentration and thereby enhancing material toughness. In these instances, the impact of mechanical bonds can be likened to the adage "small things can make a big difference", whereby a limited number of mechanical bonds substantially elevate the mechanical performance of conventional polymers. In addition to MINs cross-linked by mechanical bonds, there is another type of MIN in which their principal parts are polymer chains composed of dense mechanical bonds. Within these MINs, mechanical bonds generally serve as repeating units, and their unique properties stem from integrating and amplifying the function of a large amount of mechanical bonds. Consequently, MINs with dense mechanical bonds tend to reflect the intrinsic properties of mechanical interlocked polymers, making their exploration critical for a comprehensive understanding of MIPs. Nevertheless, investigations into MINs featuring dense mechanical bonds remain relatively scarce.This Account presents a comprehensive overview of our investigation and insights into MINs featuring dense mechanical bonds. First, we delve into the synthetic strategies employed to effectively prepare MINs with dense mechanical bonds, while critically evaluating their advantages and limitations. Through meticulous control of the core interlocking step, three distinct strategies have emerged: mechanical interlocking followed by polymerization, supramolecular polymerization followed by mechanical interlocking, and dynamic interlocking. Furthermore, we underscore the structure-property relationships of MINs with dense mechanical bonds. The macroscopic properties of MINs originate from integrating and amplifying countless microscopic motions of mechanical bonds, a phenomenon we define as an integration and amplification mechanism. Our investigation has revealed detailed motion characteristics of mechanical bonds in bulk mechanically interlocked materials, encompassing the quantification of motion activation energy, discrimination of varying motion distances, and elucidation of the recovery process. Additionally, we have elucidated their influence on the mechanical performance of the respective materials. Moreover, we have explored potential applications of MINs, leveraging their exceptional mechanical properties and dynamicity. These applications include enhancing the toughness of conventional polymers, engineering mechanically adaptive and multifunctional aerogels, and mitigating Li protrusion as interfacial layers in lithium-ion batteries. Finally, we offer our personal perspectives on the promises, opportunities, and key challenges in the future development of MINs with dense mechanical bonds, underscoring the potential for transformative advancements in this burgeoning field.

Assembly and Utility of a Drawstring-Mimetic Supramolecular Complex.

Inspired by the drawstring structure in daily life, here we report the development of a drawstring-mimetic supramolecular complex at the molecular scale. This complex consists of a rigid figure-of-eight macrocyclic host molecule and a flexible linear guest molecule which could interact through three-point non-covalent binding to form a highly selective and efficient host-guest assembly. The complex not only resembles the drawstring structure, but also mimics the properties of a drawstring with regard to deformations under external forces. The supramolecular drawstring can be utilized as an interlocked crosslinker for poly(methyl acrylate), and the corresponding polymer samples exhibit comprehensive enhancement of macroscopic mechanical performance including stiffness, strength, and toughness.

A polyimine aerogel separator with electron cloud design to boost Li-ion transport for stable Li metal batteries.

The separator with high Young's modulus can avoid the danger of large-sized dendrites, but regulating the chemical behavior of lithium (Li) at the separator/anode interface can effectively eliminate the dendrite issue. Herein, a polyimine aerogel (PIA) with accurate nitrogen (N) functional design is used as the functional separator in Li metal batteries to promote uniform Li nucleation and suppress the dendrite growth. Specifically, the imine (N1) and protonated tertiary amine (N2) sites in the molecular structure of the PIA are significantly different in electron cloud density (ECD) distribution. The N1 site with higher ECD and the N2 site with lower ECD tend to attract and repulse Li+ through electrostatic interactions, respectively. This synergy effect of the PIA separator accelerates the interfacial Li+ diffusion on the Li anode to sustain a uniform two-dimensional Li nucleation behavior. Meanwhile, the well-defined nanochannels of the PIA separator show high affinity to electrolyte and bring uniform Li+ flux for Li plating/stripping. Consequently, the dendrites are effectively suppressed by the PIA separator in routine carbonate electrolyte, and the Li metal batteries with the PIA separator exhibit high Coulombic efficiency and stable high-rate cycling. These findings demonstrate that the ingenious marriage of special chemical structure designs and hierarchical pores can enable the separator to affect the interfacial Li nucleation behavior.

Metallacage-based enhanced PDT strategy for bacterial elimination via inhibiting endogenous NO production.

Antibiotics are among the most used weapons in fighting microbial infections and have greatly improved the quality of human life. However, bacteria can eventually evolve to exhibit antibiotic resistance to almost all prescribed antibiotic drugs. Photodynamic therapy (PDT) develops little antibiotic resistance and has become a promising strategy in fighting bacterial infection. To augment the killing effect of PDT, the conventional strategy is introducing excess ROS in various ways, such as applying high light doses, high photosensitizer concentrations, and exogenous oxygen. In this study, we report a metallacage-based PDT strategy that minimizes the use of ROS by jointly using gallium-metal organic framework rods to inhibit the production of bacterial endogenous NO, amplify ROS stress, and enhance the killing effect. The augmented bactericidal effect was demonstrated both in vitro and in vivo. This proposed enhanced PDT strategy will provide a new option for bacterial ablation.

Insights into the Correlation of Cross-linking Modes with Mechanical Properties for Dynamic Polymeric Networks.

Simultaneously introducing covalent and supramolecular cross-links into one system to construct dually cross-linked networks, has been proved an effective approach to prepare high-performance materials. However, so far, features and advantages of dually cross-linked networks compared with those possessing individual covalent or supramolecular cross-linking points are rarely investigated. Herein, on the basis of comparison between supramolecular polymer network (SPN), covalent polymer network (CPN) and dually cross-linked polymer network (DPN), we reveal that the dual cross-linking strategy can endow the DPN with integrated advantages of CPN and SPN. Benefiting from the energy dissipative ability along with the dissociation of host-guest complexes, the DPN shows excellent toughness and ductility similar to the SPN. Meanwhile, the elasticity of covalent cross-links in the DPN could rise the structural stability to a level comparable to the CPN, exhibiting quick deformation recovery capacity. Moreover, the DPN has the strongest breaking stress and puncture resistance among the three, proving the unique property advantages of dual cross-linking method. These findings gained from our study further deepen the understanding of dynamic polymeric networks and facilitate the preparation of high-performance elastomeric materials.

Mechanically Interlocked [2]Rotaxane Aerogels with Tunable Morphologies and Mechanical Properties.

Mechanical bonds have been utilized as promising motifs to construct mechanically interlocked aerogels (MIAs) with mechanical adaptivity and multifunctionality. However, fabricating such aerogels with not only precise chemical structures but also dynamic features remains challenging. Herein, we present MIAs carrying dense [2]rotaxane units, which bestow both the stability and flexibility of the aerogel network. Owing to the stable chemical structure of a [2]rotaxane, MIAs possessing a precise and full-scale mechanically interlocked network could be fabricated with the aid of diverse solvents. In addition, the dynamic nature of the [2]rotaxane resulted in morphologies and mechanical performances of the MIAs that can be dramatically modulated under chemical stimuli. We hope that the structure-property relationship in MIAs will facilitate the development of mechanically interlocked materials and provide novel opportunities toward constructing smart materials with multifunctionalities.

Neuromorphic sensorimotor loop embodied by monolithically integrated, low-voltage, soft e-skin.

Artificial skin that simultaneously mimics sensory feedback and mechanical properties of natural skin holds substantial promise for next-generation robotic and medical devices. However, achieving such a biomimetic system that can seamlessly integrate with the human body remains a challenge. Through rational design and engineering of material properties, device structures, and system architectures, we realized a monolithic soft prosthetic electronic skin (e-skin). It is capable of multimodal perception, neuromorphic pulse-train signal generation, and closed-loop actuation. With a trilayer, high-permittivity elastomeric dielectric, we achieved a low subthreshold swing comparable to that of polycrystalline silicon transistors, a low operation voltage, low power consumption, and medium-scale circuit integration complexity for stretchable organic devices. Our e-skin mimics the biological sensorimotor loop, whereby a solid-state synaptic transistor elicits stronger actuation when a stimulus of increasing pressure is applied.

Oligo[2]catenane That Is Robust at Both the Microscopic and Macroscopic Scales.

Polycatenanes are extremely attractive topological architectures on account of their high degrees of conformational freedom and multiple motion patterns of the mechanically interlocked macrocycles. However, exploitation of these peculiar structural and dynamic characteristics to develop robust catenane materials is still a challenging goal. Herein, we synthesize an oligo[2]catenane that showcases mechanically robust properties at both the microscopic and macroscopic scales. The key feature of the structural design is controlling the force-bearing points on the metal-coordinated core of the [2]catenane moiety that is able to maximize the energy dissipation of the oligo[2]catenane via dissociation of metal-coordination bonds and then activation of sequential intramolecular motions of circumrotation, translation, and elongation under an external force. As such, at the microscopic level, the single-molecule force spectroscopy measurement exhibits that the force to rupture dynamic bonds in the oligo[2]catenane reaches a record high of 588 ± 233 pN. At the macroscopic level, our oligo[2]catenane manifests itself as the toughest catenane material ever reported (15.2 vs 2.43 MJ/m3). These fundamental findings not only deepen the understanding of the structure-property relationship of poly[2]catenanes with a full set of dynamic features but also provide a guiding principle to fabricate high-performance mechanically interlocked catenane materials.

Highly Strong and Tough Supramolecular Polymer Networks Enabled by Cryptand-Based Host-Guest Recognition.

Supramolecular polymer networks (SPNs) demonstrate great potential in the development of smart materials owing to their attractive dynamic properties. However, as they suffer from the inherent weak bonding of most noncovalent cross-links, it remains a significant challenge to construct SPNs with outstanding mechanical performance. Herein, we exploit the cryptand/paraquat host-guest recognition motifs as cross-links to prepare a class of highly strong and tough SPNs. Unlike those supramolecular cross-links with relatively weak binding abilities, the cryptand-based host-guest interactions have a high association constant and steady complexing structure, which effectively stabilizes the network and resists mechanical deformation under external force. Such favorable structural stability endows our SPNs with greatly enhanced mechanical performance, compared with the control-1 cross-linked by the weakly complexed crown ether/secondary ammonium salt motif (tensile strength: 21.1±0.5 vs 2.8±0.1 MPa; Young's modulus: 102.6±4.8 vs 2.1±0.3 MPa; toughness: 90.4±2.0 vs 10.8±0.6 MJ m-3 ). Moreover, our SPNs also retain abundant dynamic properties including good abilities in energy dissipation, reprocessability, and stimuli-responsiveness. These findings provide novel insights into the preparation of SPNs with enhanced mechanical properties, and promote the development of high-performance intelligent supramolecular materials.

A universal interface for plug-and-play assembly of stretchable devices.

Stretchable hybrid devices have enabled high-fidelity implantable1-3 and on-skin4-6 monitoring of physiological signals. These devices typically contain soft modules that match the mechanical requirements in humans7,8 and soft robots9,10, rigid modules containing Si-based microelectronics11,12 and protective encapsulation modules13,14. To make such a system mechanically compliant, the interconnects between the modules need to tolerate stress concentration that may limit their stretching and ultimately cause debonding failure15-17. Here, we report a universal interface that can reliably connect soft, rigid and encapsulation modules together to form robust and highly stretchable devices in a plug-and-play manner. The interface, consisting of interpenetrating polymer and metal nanostructures, connects modules by simply pressing without using pastes. Its formation is depicted by a biphasic network growth model. Soft-soft modules joined by this interface achieved 600% and 180% mechanical and electrical stretchability, respectively. Soft and rigid modules can also be electrically connected using the above interface. Encapsulation on soft modules with this interface is strongly adhesive with an interfacial toughness of 0.24 N mm-1. As a proof of concept, we use this interface to assemble stretchable devices for in vivo neuromodulation and on-skin electromyography, with high signal quality and mechanical resistance. We expect such a plug-and-play interface to simplify and accelerate the development of on-skin and implantable stretchable devices.

Synergistic Covalent-and-Supramolecular Polymers with an Interwoven Topology.

Network topologies, especially some high-order topologies, are able to furnish cross-linked polymer materials with enhanced properties without altering their chemical composition. However, the fabrication of such topologically intriguing architectures at the macromolecular level and in-depth insights into their structure-property relationship remain a significant challenge. Herein, we relied on synergistic covalent-and-supramolecular polymers (CSPs) as a platform to prepare a range of polymer networks with an interwoven topology. Specifically, through the sequential supramolecular self-assemblies, the covalent polymers (CPs) and metallosupramolecular polymers (MSPs) could be interwoven in our CSPs by [2]pseudorotaxane cross-links. As a result, the obtained CSPs possessed a topological network that could not only promote the synergistic effect between CPs and MSPs to afford mechanically robust yet dynamic materials but also vest polymers with some functions, as manifested by force-induced hierarchical dissociations of supramolecular interactions and superior thermomechanical stability compared to our previously reported CSP systems. Furthermore, our CSPs exhibited tunable mechanical performance toward multiple stimuli including K+ and PPh3, demonstrating abundant stimuli-responsive properties. We hope that these findings could provide novel opportunities toward achieving topological structures at the macromolecular level and also motivate further explorations of polymeric materials via the way of controlling their topological structures.

Self-Healable, Malleable, Ecofriendly Recyclable and Robust Polyimine Thermosets Derived from Trifluoromethyl Diphenoxybenzene Backbones.

Dynamic covalent chemistry opens up great opportunities for a sustainable society by producing reprocessable networks of polymers and even thermosets. However, achieving the closed-loop recycling of polymers with high performance remains a grand challenge. The introduction of aromatic monomers and fluorine into covalent adaptable networks is an attractive method to tackle this challenge. Therefore, we present a facile and universal strategy to focus on the design and applications of polyimine vitrimers containing trifluoromethyl diphenoxybenzene backbones in applications of dynamic covalent polymers. In this study, fluorine-containing polyimine vitrimer networks (FPIVs) were fabricated, and the results revealed that the FPIVs not only exhibited good self-healability, malleability and processability without the aid of any catalyst, but also possessed decent mechanical strength, superior toughness and thermal stability. We hope that this work could provide a novel pathway for the design of high-performance polyimine vitrimers by recycling of plastic wastes.

Insights into the Correlation of Microscopic Motions of [c2]Daisy Chains with Macroscopic Mechanical Performance for Mechanically Interlocked Networks.

Mimicking filament sliding in sarcomeres using artificial molecular muscles such as [c2]daisy chains has aroused increasing interest in developing advanced polymeric materials. Although few bistable [c2]daisy chain-based mechanically interlocked polymers (MIPs) with stimuli-responsive behaviors have been constructed, it remains a significant challenge to establish the relationship between microscopic responsiveness of [c2]daisy chains and macroscopic mechanical properties of the corresponding MIPs. Herein, we report two mechanically interlocked networks (MINs) consisting of dense [c2]daisy chains with individual extension (MIN-1) or contraction (MIN-2) conformations decoupled from a bistable precursor, which serve as model systems to address the challenge. Upon external force, the extended [c2]daisy chains in MIN-1 mainly undergo elastic deformation, which is able to assure the strength, elasticity, and creep resistance of the corresponding material. For the contracted [c2]daisy chains, long-range sliding motion occurs along with the release of latent alkyl chains between the two DB24C8 wheels, and accumulating lots of such microscopic motions endows MIN-2 with enhanced ductility and ability of energy dissipation. Therefore, by decoupling a bistable [c2]daisy chain into individual extended and contracted ones, we directly correlate the microscopic motion of [c2]daisy chains with macroscopic mechanical properties of MINs.

Woven Polymer Networks: From Crystalline to Elastomeric Materials.

Weaving technology has been extensively used for manufacturing macroscopic fabrics and satisfying the artistic demands of humans through the ages. Integrating woven geometries into molecular structures is a persistent pursuit, and yet a significant challenge to chemists, owing to the lack of effective methodologies to guide the regular mutual interlacing of molecular strands. In this Concept article, recent progress and related strategies in constructing woven polymer networks (WPNs) are summarized and discussed. An outlook is then given to highlight the future opportunities and challenges in the development of both molecularly woven structures and molecularly woven functional materials.