Amplification of integrated microscopic motions of high-density [2]rotaxanes in mechanically interlocked networks

An intrinsic self-healable supramolecular dynamic covalent elastomer for sustainable high-performance tactile sensing

Supramolecular chemistry empowers polymeric materials with versatile beneficial features encompassing stimulus adaptation, e.g. self-healing, to truly function in a biomimetic manner. To seek an effective self-healing mechanism for current polymers with no trade-offs in other property perspectives still remains a challenge. Herein, we present a sustainable alternative to the conventional covalent elastomers, a dynamic covalent disulfide polymer highly crosslinked by bio-catechol hydrogen bonds and coordinative metallic dopants. The polymeric elastomer exhibits mechanical tailorability, ambient intrinsic self-healing with an efficiency reaching 90%, and closed-loop recycling capability with no property deterioration. The assembled microstructured capacitive pressure sensor possesses a sensitivity up to 1.58 kPa-1, an effective working range up to 35 kPa and an exceptional response time of a few milliseconds, which makes it particularly promising for contemporary wearable devices for a spectrum of applications like physiological monitoring and voice-cancelling communication.

Molecular Origin of the Stretchability and Fatigue-Resistance of Rotaxane-Based Mechanically Interlocked Polymer Networks

Rotaxane-based polymer networks leveraging host-guest recognition have recently emerged as a versatile platform for developing smart materials. Despite numerous studies on these polymers, their unique mechanical properties are mostly associated with the sliding motion of the macrocycle along the axle, leaving the impact of the presence or absence of interlocked structures on the mechanical performance of materials yet to be directly demonstrated. In this work, we present a densely (pseudo)rotaxane-based supramolecular polymeric network (SPN) and a mechanically interlocked network (MIN) as model systems to explore how the mechanical interlocking unit dominates the material properties. Specifically, we have achieved a significant transition from SPN to MIN by finely tuning the stopper size, just substituting a methyl with a dimethyl group attached to the phenyl ring. Although their stereochemical structures are similar, a subtle increase in the stopper size can lead to striking improvements in stretchability and anti-fatigue performance. The stopper size-relevant dethreading behavior, as evidenced by a combined approach of solid-state NMR spectroscopy and rheology, is the underlying molecular mechanism for the difference in the macroscopic mechanical properties. We anticipate that the fundamental understanding gained from this work will advance the development of rotaxane-based materials with emergent functions and applications.

Visualizing the Sliding Motion of Dynamic Rotaxanes by Surface Wrinkles

Visualizing the sliding dynamics of a topological network can provide critical insight into determining the design and properties of mechanically interlocked materials. Although several auxiliary techniques have been proposed to infer the microscopic motion of rotaxanes, employing intuitive and convenient methods to explore the microscopic dynamics of a mechanically interlocked polymer remains a significant challenge. Herein, this work introduces a mechanically interlocked network (MIN) into the patterned surfaces for visualizing and regulating the sliding process of [2]rotaxane units through the evolution of surface wrinkles. Upon the photodimerization of the anthracene-functionalized polymer chain, the surface wrinkle can be formed after thermal treatment and subsequent cooling to room temperature. Specifically, the cross-linked films exhibit visible changes in wrinkle topography through the disruption of host-guest recognition by alkaline stimuli. Moreover, by leveraging the unique mechanical properties of surface wrinkles, we prolonged and amplified the originally extremely transient and difficult-to-detect sliding motion of rotaxane units in terms of time scale. Through statistical analysis of the changes in wrinkle morphology, we were able to correspondingly deconstruct the three processes of the rotaxane sliding motion: (I) unrestricted rapid sliding following host-guest dissociation; (II) restricted sliding; and (III) termination of sliding. The novel approach we propose opens a new avenue for studying the microscopic molecular motion of mechanically interlocked materials, facilitating the advancement and application of mechanically interlocked structures. In addition to using macroscopic surface patterns to visualize and explore microscopic molecular motion, the motion of microscopic molecules can also be used to regulate macroscopic surface patterns.

Mobility Control of Mechanical Bonds to Modulate Energy Dissipation in Mechanically Interlocked Networks

Mechanically interlocked networks (MINs) with dense mechanical bonds can amplify the dynamic behaviors of the mechanical bonds to exhibit decent mechanical properties. Energy dissipation resulting from mechanical bond motion is essential for improving toughness, yet effective strategies to optimize this process remain underexplored. Here, by designing mechanical bond models with controllable mobility, we establish a fortification strategy for the two key factors governing energy dissipation, host-guest recognition and sliding friction, thereby enabling mechanical property enhancement of mechanically interlocked materials. Specifically, the [2]rotaxanes in MIN-1 and MIN-2 exhibit identical axle structures, with MIN-1 incorporating a small benzo-21-crown-7 ring and MIN-2 incorporating a large benzo-24-crown-8 ring. Strain rate-dependent cyclic tensile tests reveal that the energy required to drive mechanical bond motion in MIN-1 and MIN-2 is 510 and 260 kJ/m3, respectively, indicating that the small wheel size enhances host-guest recognition. Furthermore, the apparent activation energy for the sliding motion of the mechanical bonds in MIN-1 (11.0 kJ/mol) is higher than that in MIN-2 (6.70 kJ/mol), suggesting increased sliding friction in MIN-1. Due to these two aspects, MIN-1 exhibits superior energy dissipation performance (damping capacity = 92%) compared to MIN-2 (78%), translating to a higher toughness (7.50 vs 5.70 MJ/m3).

Covalent Adaptable Poly[2]rotaxane Networks via Dynamic C-N Bond Transalkylation

Covalent adaptable networks (CANs), a novel class of crosslinked polymers with dynamic covalent bonds, have gained significant attention for combining the durability of thermosets with the reprocessability of thermoplastics, making them promising for emerging applications. Here, we report the first example of poly[2]rotaxane-type covalent adaptable networks (PRCANs), in which oligo[2]rotaxane backbones characterized by densely packed mechanical bonds, are cross-linked through dynamic C-N bonds. The oligo[2]rotaxane backbones could guarantee the mechanical properties of the CANs. Under an external force, the synergy of numerous microscopic motions of the cascade [2]rotaxane units, progressively introducing the initially hidden short chains, expands the polymer network, imparting good stretchability to the PRCANs (217 %). On the other hand, the dissociation of host-guest recognition, followed by the motion of mechanical bonds, constitutes a unique energy dissipation pathway, ultimately enhancing the toughness of PRCANs (7.6 MJ/m3). In contrast, the control CAN, which lacks movable mechanical bonds, demonstrates significantly lower stretchability (40 %) and toughness (1.5 MJ/m3). Moreover, the dynamic C-N bond can undergo high efficiency of 1,2,3-triazole alkylation and trans-N-alkylation exchanges at 1,2,3-triazolium sites at elevated temperatures, with good reprocessability and without compromising their mechanical performance. This work demonstrates the great potential of oligo[2]rotaxanes as a novel polymer backbone for the development of sustainable materials with excellent mechanical properties.

Mechanically Adaptive Materials Based on Dynamic Chemical Bonds

Adaptiveness is an important feature for biological creatures to survive and interact with variable environments. Mechanically adaptive polymers (MAPs), which have been developed recently inspired by this adaptive nature, can regulate their mechanical properties in response to external stimuli or environmental changes. Specifically, MAPs based on dynamic chemical bonds have been synthesized and reported as an emerging material because of the intrinsic self-adaptability, outstanding mechanical properties and durable applications. This review primarily focuses on the recent advancements in the fabrication of MAPs through the utilization of dynamic covalent bonds and non-covalent bonds. A comprehensive summary of the methodologies and mechanisms employed to attain high energy dissipation in MAPs is provided. Subsequently, the review offers incisive analyses of the intrinsic functionalities of MAPs, such as high impact-stiffening, damping, and buffering capabilities. Finally, the developmental achievements within this domain are recapitulated, the potential challenges, and future research perspectives in MAPs are deliberated.

Polyurethane Elastomers Strengthened by Pseudo[1]rotaxanes Based on Pillararenes

As a unique property of the interlocked structures, rotaxane allows for intramolecular motions between its wheel and axle components. Introduction of rotaxanes into polymers can endow them with distinctive macroscopic features and outstanding mechanical properties. Here, we prepare a copillar[5]arene with a hydroxyl and an amino-group on each end, which can spontaneously form a pseudo[1]rotaxane through intramolecular hydrogen bonds. This pseudo[1]rotaxane possesses a releasable extra alkyl chain, which is then incorporated into a linear polyurethane by reacting with a diisocyanate to prepare polyurethane elastomers with spring-like structures. The results of stress-strain test and dynamic mechanical analysis all indicate that sliding motions of the axle part on the pseudo[1]rotaxane in the polymer skeleton can greatly dissipate energy, which endows the elastomers with higher toughness and better fatigue resistance. Moreover, the addition of moderate amount of cuprous bromide to form cuprous-thioether coordination in the polymers can further improve the mechanical properties.

Stretchable [2]rotaxane-bridged MXene films applicable for electroluminescent devices

Titanium carbide (Ti3C2TX) MXene has prominent mechanical properties and electrical conductivity. However, fabricating high-performance macroscopic films is challenging, as weak interlayer interactions limit their mechanical performance. Here, we introduce [2]rotaxane, a mechanically interlocked molecule, to enhance MXene films. Compared to pure MXene (fracture strain: 4.6%, toughness: 0.6 MJ/m3), [2]rotaxane-bridged MXene (RBM) films achieve record-high strain (20.0%) and toughness (11.9 MJ/m3) with only 3.6% [2]rotaxane by weight. Additionally, RBM films endure 500 stretch cycles (0 to 15% strain) with stable and reversible resistance alterations, making them ideal for stretchable electrodes. Notably, RBM films enable stretchable electroluminescent devices with reliable operation under 20% elongation and customizable luminescent patterns. This innovative use of mechanically interlocked molecules to cross-link MXene platelets advances MXene films and other two-dimensional materials in stretchable electronics.

Durable and Damageless Supramolecular Binder for Fast, Stable, and Sustainable Si-Based Anodes

Mechanical fatigue of the binders during the repeated volume change of Si-based anodes induces binder network collapse, resulting in lithium-ion batteries (LIBs) failing prematurely. Herein, we designed a damageless polymer binder with a mechanically interlocked network utilizing [an]daisy chains (DCMIN) and poly(acrylic acid) (PAA) to improve the structural cohesion of the Si-based anode. This DCMIN@PAA binder exhibits robust mechanical properties, high elasticity, and excellent adhesion. More importantly, the recognition between the dialkylammonium salt decorated on the thread component and the ether group of benzo-24-crown-8 provides efficacy for the DCMIN@PAA binder in quickly dissipating energy and reducing damage accumulation. Therefore, with the DCMIN@PAA binder, the pure-Si anode showcases high retention over 1050 cycles at 1 C and a fast rate response (5 C). The DCMIN@PAA binder also improves the cycling stability of the homemade pouch cell using a pure-Si anode. In addition, the water-soluble DCMIN@PAA binder, capable of quick release and separation, could facilitate recycling of the end-of-life anode, enhancing the sustainability of the battery. This work highlights the indispensability of energy dissipation with the consideration of a binder and provides a viable path forward to stabilize Si-based anodes suffering from volume change-induced stress accumulation.

Soft-Rigid Construction of Mechanically Robust, Thermally Stable, and Self-Healing Polyimine Networks with Strongly Recyclable Adhesion

Reversible and recyclable thermosets have garnered increasing attention for their smart functionality and sustainability. However, they still face challenges in balancing comprehensive performance and dynamic features. Herein, silicon (Si)─oxygen (O) and imidazole units covalent bonds are coupled to generate a new class of bio-polyimines (Bio-Si-PABZs), to endow them with high performance and excellent reprocessing capability and acid-degradability. By tailoring the molar content of diamines, this Bio-Si-PABZs displayed both a markedly high glass transition temperature (162 °C) and a high char yield at 800 °C in an oxygen atmosphere (73.1%). These Bio-Si-PABZs with their favorable properties outperformed various previously reported polyimines and competed effectively with commercial fossil-based polycarbonate. Moreover, the scratch (≈10 µm) on the surface of samples can be self-healing within only 2 min, and an effective "Bird Nest"-to-"Torch" recycling can also be achieved through free amines solution. Most importantly, a bio-based siloxane adhesive derived from the intermediate Bio-Si-PABZ-1 by acidic degradation demonstrated broad and robust adhesion in various substrates, with values reaching up to ≈3.5 MPa. For the first time, this study lays the scientific groundwork for designing robust and recyclable polyimine thermosets with Si─O and imidazole units, as well as converting plastic wastes into thermal-reversibility and renewable adhesives.

Mechanically Interlocked Polyrotaxane Networks with Collective Motions of Multiple Main-Chain Mechanical Bonds

Type I main-chain polyrotaxanes (PRs) with multiple wheels threaded onto the axle are widely employed to design slide-ring materials. However, Type II main-chain PRs with axles threading into the macrocycles on the polymer backbones have rarely been studied, although they feature special topological structures and dynamic characteristics. Herein, we report the design and preparation of Type II main-chain PR-based mechanically interlocked networks (PRMINs), based on which the relationship between microscopic motion of mechanical bonds on the PRs and macroscopic mechanical performance of materials has been revealed. The representative PRMIN-2 exhibits a robust feature in tensile tests with high stretchability (1680 %) and toughness (47.5 MJ/m3). Moreover, it also has good puncture performance with puncture energy of 22.0 mJ. Detailed rheological measurements and coarse-grained molecular dynamics (CGMD) simulation reveal that the embedded multiple [2]rotaxane mechanical bonds on the PR backbones of PRMINs could undergo a synergistic long-range sliding motion under external force, with the introduction of collective dangling chains into the network. As a result, the synchronized motions of coherent PR chains can be readily activated to accommodate network deformation and efficiently dissipate energy, thereby leading to enhanced mechanical performances of PRMINs.

Mechanically Interlocked [an]Daisy Chain Adhesives with Simultaneously Enhanced Interfacial Adhesion and Cohesion

Adhesives have been widely used to splice and repair materials to meet practical needs of humanity for thousands of years. However, developing robust adhesives with balanced adhesive and cohesive properties still remains a challenging task. Herein, we report the design and preparation of a robust mechanically interlocked [an]daisy chain network (DCMIN) adhesive by orthogonal integration of mechanical bonds and 2-ureido-4[1H]-pyrimidone (UPy) H-bonding in a single system. Specifically, the UPy moiety plays a dual role: it allows the formation of a cross-linked network and engages in multivalent interactions with the substrate for strong interfacial bonding. The mechanically interlocked [an]daisy chain, serving as the polymeric backbone of the adhesive, is able to effectively alleviate applied stress and uphold network integrity through synergistic intramolecular motions, and thus significantly improves the cohesive performance. Comparative analysis with the control made of the same quadruple H-bonding network but with non-interlocked [an]daisy chain backbones demonstrates that our DCMIN possesses superior adhesion properties over a wide temperature range. These findings not only contribute to a deep understanding of the structure-property relationship between microscopic mechanical bond motions and macroscopic adhesive properties but also provide a valuable guide for optimizing design principles of robust adhesives.

Probing rotaxane dynamics with 19F NMR/MRI: Unveiling the roles of mechanical bond and steric hindrance

BACKGROUND

Deciphering the molecular dynamics (MD) of rotaxanes is crucial for designing and refining their applications in molecular devices. This study employed fluorine-19 nuclear magnetic resonance (19F NMR) and magnetic resonance imaging (MRI) to unveil the interplay between mechanical bonds and steric hindrance in a series of fluorinated rotaxanes.

RESULTS

1H/19F NMR revealed stable "Z"-shaped wheel conformations minimizing steric clashes and favoring π-π interactions with the axle. Utilizing fluorines and axle protons as reporters, 1H/19F relaxation rates and solid-state 19F NMR studies demonstrated that mechanical bond primarily governs wheel motion, while steric hindrance dictates axle movement. Intriguingly, mechanical bond mainly affects local axle groups, leaving distant ones minimally impacted. MD simulations corroborated these findings. Temperature-dependent 19F NMR indicated that energy input enhances rotational motion and wheel conformational transitions. Furthermore, the drastic increase in 19F relaxation rates upon mechanical bond formation and steric hindrance enables sensitive and selective 19F MRI visualization of MD changes.

SIGNIFICANCE

This study, by elucidating the roles of internal and external factors on rotaxane molecular dynamics using 19F NMR/MRI, offers valuable insights that can advance the field of rotaxane-based molecular devices.

Molecular Ball Joints: Mechanochemical Perturbation of Bullvalene Hardy-Cope Rearrangements in Polymer Networks

The solution-state fluxional behavior of bullvalene has fascinated physical organic and supramolecular chemists alike. Little effort, however, has been put into investigating bullvalene applications in bulk, partially due to difficulties in characterizing such dynamic systems. To address this knowledge gap, we herein probe whether bullvalene Hardy-Cope rearrangements can be mechanically perturbed in bulk polymer networks. We use dynamic mechanical analysis to demonstrate that the activation barrier to the glass transition process is significantly elevated for bullvalene-containing materials relative to "static" control networks. Furthermore, bullvalene rearrangements can be mechanically perturbed at low temperatures in the glassy region; such behavior facilitates energy dissipation (i.e., increased hysteresis energy) and polymer chain alignment to stiffen the material (i.e., increased Young's modulus) under load. Computational simulations corroborate our work that showcases bullvalene as a reversible "low-force" covalent mechanophore in the modulation of viscoelastic behavior.

Stimuli-responsive mechanically interlocked polymer wrinkles

Artificial wrinkles, especially those with responsive erasure/regeneration behaviors have gained extensive interest due to their potential in smart applications. However, current wrinkle modulation methods primarily rely on network rearrangement, causing bottlenecks in in situ wrinkle regeneration. Herein, we report a dually cross-linked network wherein [2]rotaxane cross-link can dissipate stress within the wrinkles through its sliding motion without disrupting the network, and quadruple H-bonding cross-link comparatively highlight the advantages of [2]rotaxane modulation. Acid stimulation dissociates quadruple H-bonding and destructs network, swiftly eliminating the wrinkles. However, the regeneration process necessitates network rearrangement, making in situ recovery unfeasible. By contrast, alkaline stimulation disrupts host-guest recognition, and subsequent intramolecular motion of [2]rotaxane dissipate energy to eliminate wrinkles gradually. The always intact network allows for the in situ recovery of surface microstructures. The responsive behaviors of quadruple H-bonding and mechanical bond are orthogonal, and their combination leads to wrinkles with multiple but accurate responsiveness.

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. TheA 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.

Entangled Mesh Hydrogels with Macroporous Topologies via Cryogelation for Rapid Atmospheric Water Harvesting

Sorption-based atmospheric water harvesting (SAWH) is a promising technology to alleviate freshwater scarcity. Recently, hygroscopic salt-hydrogel composites (HSHCs) have emerged as attractive candidates with their high water uptake, versatile designability, and scale-up fabrication. However, achieving high-performance SAWH applications for HSHCs has been challenging because of their sluggish kinetics, attributed to their limited mass transport properties. Herein, a universal network engineering of hydrogels using a cryogelation method is presented, significantly improving the SAWH kinetics of HSHCs. As a result of the entangled mesh confinements formed during cryogelation, a stable macroporous topology is attained and maintained within the obtained entangled-mesh hydrogels (EMHs), leading to significantly enhanced mass transport properties compared to conventional dense hydrogels (CDHs). With it, corresponding hygroscopic EMHs (HEMHs) simultaneously exhibit faster moisture sorption and solar-driven water desorption. Consequently, a rapid-cycling HEMHs-based harvester delivers a practical freshwater production of 2.85 Lwater kgsorbents -1 day-1 via continuous eight sorption/desorption cycles, outperforming other state-of-the-art hydrogel-based sorbents. Significantly, the generalizability of this strategy is validated by extending it to other hydrogels used in HSHCs. Overall, this work offers a new approach to efficiently address long-standing challenges of sluggish kinetics in current HSHCs, promoting them toward the next-generation SAWH applications.

Evolution of Musculoskeletal Electronics

The musculoskeletal system, constituting the largest human physiological system, plays a critical role in providing structural support to the body, facilitating intricate movements, and safeguarding internal organs. By virtue of advancements in revolutionized materials and devices, particularly in the realms of motion capture, health monitoring, and postoperative rehabilitation, "musculoskeletal electronics" has actually emerged as an infancy area, but has not yet been explicitly proposed. In this review, the concept of musculoskeletal electronics is elucidated, and the evolution history, representative progress, and key strategies of the involved materials and state-of-the-art devices are summarized. Therefore, the fundamentals of musculoskeletal electronics and key functionality categories are introduced. Subsequently, recent advances in musculoskeletal electronics are presented from the perspectives of "in vitro" to "in vivo" signal detection, interactive modulation, and therapeutic interventions for healing and recovery. Additionally, nine strategy avenues for the development of advanced musculoskeletal electronic materials and devices are proposed. Finally, concise summaries and perspectives are proposed to highlight the directions that deserve focused attention in this booming field.

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.

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

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 (DC MIN) into the ASEI to stabilize the Li metal/ASEI interface by combining the functions of energy dissipation and fast Li-ion transport. The DC MIN featured by large-range molecular motions is cross-linked via efficient thiol-ene click chemistry; thus, the DC MIN has flexibility and excellent mechanical properties. As an ASEI, the crown ether units in DC MIN 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 DC MIN. 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 DC MIN-based ASEI. Furthermore, the 1-Ah pouch cell (LiNi0.88 Co0.09 Mn0.03 O2 cathode) with DC MIN-coated LMA exhibits improved capacity retention (88%) relative to the Control. The molecular design of DC MIN provides new insights into the optimization of an ASEI for high-energy LMAs.

Bioinspired Organic Porous Coupling Agent for Enhancement of Nanoparticle Dispersion and Interfacial Strength

Composite materials have significantly advanced with the integration of inorganic nanoparticles as fillers in polymers. Achieving fine dispersion of these nanoparticles within the composites, however, remains a challenge. This study presents a novel solution inspired by the natural structure of Xanthium. We have developed a polymer of intrinsic microporosity (PIM)-based porous coupling agent, named PCA. PCA's rigid backbone structure enhances interfacial interactions through a unique intermolecular interlocking mechanism. This approach notably improves the dispersion of SiO2 nanoparticles in various organic solvents and low-polarity polymers. Significantly, PCA-modified SiO2 nanoparticles embedded in polyisoprene rubber showed enhanced mechanical properties. The Young's modulus increases to 30.7 MPa, compared to 5.4 MPa in hexadecyltrimethoxysilane-modified nanoparticles. Further analysis shows that PCA-modified composites not only become stiffer but also gain strength and ductility. This research demonstrates a novel biomimetic strategy for enhancing interfacial interactions in composites, potentially leading to stronger, more versatile composite materials.

Enhancing the Toughness and Strength of Polymers Using Mechanically Interlocked Hydrogen Bonds

The energy dissipative features of hydrogen bonds under conditions of mechanical strain have provided an ongoing incentive to explore hydrogen bonding units for the purpose of controlling and customizing the mechanical properties of polymeric materials. However, there remains a need for hydrogen bond units that (1) possess directionality, (2) provide selectivity, (3) dissipate energy effectively, and (4) can be incorporated readily into polymeric materials to regulate their mechanical properties. Here, we report mechanically interlocked hydrogen bond units that incorporate multiple hydrogen bonds within a [2]catenane structure. The conformational flexibility and associated spatial folding characteristics of the [2]catenane units allow for molecular scale motion under external stress, while the interlocked structure serves as a pivot that maintains the directionality and selectivity of the resultant hydrogen bonding units. When incorporated into polymers, these interlocked hydrogen bond motifs serve to strengthen and toughen the resulting materials. This study not only presents a novel hydrogen bond unit for creating polymeric materials with improved mechanical properties but also underscores the unique opportunities that mechanically interlocked hydrogen bond structures may provide across a diverse range of applications.

Tunable Fluorescence and Morphology of Aggregates Built from a Mechanically Bonded Amphiphilic Bistable [2]Rotaxane

Advanced Organic Chemical Materials Co-constructed Mechanically bonded amphiphiles (MBAs), also known as mechanically interlocked molecules (MIMs), have emerged as an important kind of functional building block for the construction of artificial molecular machines and soft materials. Herein, a novel MBA, i. e., bistable [2]rotaxane H2 was designed and synthesized. In the solution state, H2 demonstrated pH and metal ion-responsive emissions due to the presence of a distance-dependent photoinduced electron transfer (PET) process and the fluorescence resonance energy transfer (FRET) process, respectively. Importantly, the amphiphilic feature of H2 has endowed it with unique self-assembly capability, and nanospheres were obtained in a mixed H2 O/CH3 CN solvent. Moreover, the morphology of H2 aggregates can be tuned from nanospheres to vesicles due to the pH-controlled shuttling motion-induced alternation of H2 amphiphilicity. Interestingly, larger spheres with novel pearl-chain-like structures from H2 were observed after adding stoichiometric Zn2+ . In particular, H2 shows pH-responsive emissions in its aggregation state, allowing the visualization of the shuttling movement by just naked eyes. It is assumed that the well-designed [2]rotaxane, and particularly the proposed concept of MBA shown here, will further enrich the families of MIMs, offering prospects for synthesizing more MIMs with novel assembly capabilities and bottom-up building dynamic smart materials with unprecedented functions.

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.

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 Robust, Recyclable, and Self-Healing Polyimine Networks

To achieve energy saving and emission reduction goals, recyclable and healable thermoset materials are highly attractive. Polymer copolymerization has been proven to be a critical strategy for preparing high-performance polymeric materials. However, it remains a huge challenge to develop high-performance recyclable and healable thermoset materials. Here, polyimine dynamic networks based on two monomers with bulky pendant groups, which not only displayed mechanical properties higher than the strong and tough polymers, e.g., polycarbonate, but also excellent self-repairing capability and recyclability as thermosets are developed. Owing to the stability of conjugation effect by aromatic benzene rings, the final polyimine networks are far more stable than the reported counterparts, exhibiting excellent hydrolysis resistance under both alkaline condition and most organic solvents. These polyimine materials with conjugation structure can be completely depolymerized into monomers recovery in an acidic aqueous solution at ambient temperature. Resulting from the bulky pendant units, this method allows the exchange reactions of conjugation polyimine vitrimer easily within minutes for self-healing function. Moreover, the introduction of trifluoromethyl diphenoxybenzene backbones significantly increases tensile properties of polyimine materials. This work provides an effective strategy for fabricating high-performance polymer materials with multiple functions.

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.