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

Hyperbranched Polyborophosphate towards Transparent Epoxy Resin with Ultrahigh Toughness and Fire Safety

Inherent transparency makes epoxy resins ideal for aircraft windows, yet their brittleness and flammability remain challenges. Existing strategies for these issues often compromise transparency, with limited research on the mechanisms involved. Herein, a novel strategy is proposed for fabricating transparent epoxy resin by tuning the electrostatic potential distribution via hyperbranched polyborophosphate. Electron-deficient boron and relatively electron-rich phosphorus atoms work synergistically to increase the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap, preventing visible light absorption. Meanwhile, the hyperbranched structure facilitates polymer network interpenetration to reduce porosity for decreased light scattering. This synergy results in a nearly colorless material with over 80% transmittance at 550 nm even at 4 mm thickness, along with full-band UV shielding. Notably, the material demonstrates a 114.7% increase in impact toughness (45.2 kJ m-2) due to dual dynamic B─O and P─O linkages. Besides, it yields a limiting oxygen index of 33% and a V0 rating in the underwriter laboratories vertical burning test, along with significant reductions in heat, smoke, and toxic gas release. The outstanding performance makes it stand out compared to reported advanced transparent epoxy resins, highlighting the significance of this work.

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.

Tunable Mechanically Interlocked Semi-Crystalline Networks

High-performance polymers based on dynamic chemistry have been widely explored for multi-field advanced applications. However, noncovalent sacrificial bond-mediated energy dissipation mechanism causes a trade-off between mechanical toughness and resilience. Herein, we achieved the synchronous boost of seemingly conflicting material properties including mechanical robustness, toughness and elasticity via the incorporation of mechanical chemistry into traditional semi-crystalline networks. Detailed rheological tests and all-atom molecular dynamics simulation reveal that the excellent mechanical robustness and toughness are attributed to the dissociation of crystalline domains threading through the sieve-shape macrocycles. Reversible nano-crystalline domains and ring-sliding-effect accelerated segment motion efficiently reduce energy dissipation to achieve instantaneous resilience. Moreover, the model polymers demonstrate that the multiple dynamic components endow the resulting polymer with excellent reprocessability under mild conditions. This mechanically interlocked semi-crystalline polymer exhibits potential applications as a thermal/photo actuator. This work reveals the synergic effects of mechanically interlocked sites and tunable crystalline domains, thus providing a reliable guide for the comprehensive improvement of material performance.

Network design for soft materials: addressing elasticity and fracture resistance challenges

Soft materials, such as elastomers and gels, feature crosslinked polymer chains that provide stretchable and elastic mechanical properties. These properties are derived from entropic elasticity, which limits energy dissipation and makes the material susceptible to fracture. To address this issue, network designs that dissipate energy through the plastic zone have been introduced to enhance toughness; however, this approach compromises elasticity, preventing the material from fully recovering its original shape after deformation. In this review, we describe the trade-off between fracture resistance and elasticity, exploring network designs that overcome this limitation to achieve both high toughness and low hysteresis. The development of soft materials that are both elastic and fracture-resistant holds significant promise for applications in stretchable electronics, soft robotics, and biomedical devices. By analyzing successful network designs, we identify strategies to further improve these materials and discuss potential enhancements based on existing limitations.

Mechanically interlocked two-dimensional polymers

Mechanical bonds arise between molecules that contain interlocked subunits, such as one macrocycle threaded through another. Within polymers, these linkages will confer distinctive mechanical properties and other emergent behaviors, but polymerizations that form mechanical bonds efficiently and use simple monomeric building blocks are rare. In this work, we introduce a solid-state polymerization in which one monomer infiltrates crystals of another to form a macrocycle and mechanical bond at each repeat unit of a two-dimensional (2D) polymer. This mechanically interlocked 2D polymer is formed as a layered solid that is readily exfoliated in common organic solvents, enabling spectroscopic characterization and atomic-resolution imaging using advanced electron microscopy techniques. The 2D mechanically interlocked polymer is easily prepared on multigram scales, which, along with its solution processibility, enables the facile fabrication of composite fibers with Ultem that exhibit enhanced stiffness and strength.

"Bridge" interface design modulates high-performance cellulose-based integrated flexible supercapacitors

Flexible supercapacitors offer significant potential for powering next-generation flexible electronics. However, the mechanical and electrochemical stability of flexible supercapacitors under different flexibility conditions is limited by the weak bonding between neighboring layers, posing a major obstacle to their practical application. In this paper, natural coniferous pulp cellulose was successfully modified with ethylenediamine and NiSe2/Cell-NH2/MoS2 cellulose flexible electrodes (NCMF) were fabricated by phase transfer and hydrothermal methods. The amino-modified cellulose (Cell-NH2) acts as a "bridge" between NiSe2 and MoS2, significantly enhancing the interfacial bonding strength of the flexible electrode. The integrated flexible electrode exhibited a high area capacitance (2475 mF/cm2), strong tensile strength (10.3 MPa), and excellent cycling stability (92.1 % capacitance retention after 2500 cycles). The sandwich-structured monolithic supercapacitor achieved both a high electrode capacitance (56.78 F/cm2) and a high energy density (1971.53 μWh/cm2), while maintaining long cycling life (72.73 % capacitance retention after 2000 cycles) and large deformation capability. This makes it suitable for stable power supplies in electronic products and opens new possibilities for the flexible wearable industry. This technology enables stable power supplies for electronic products and paves the way for advancements in the flexible wearable industry.

Boosting the Strength and Toughness of Polymer Blends via Ligand-Modulated MOFs

Mechanically robust and tough polymeric materials are in high demand for applications ranging from flexible electronics to aerospace. However, achieving both high toughness and strength in polymers remains a significant challenge due to their inherently contradictory nature. Here, a universal strategy for enhancing the toughness and strength of polymer blends using ligand-modulated metal-organic framework (MOF) nanoparticles is presented, which are engineered to have adjustable hydrophilicity and lipophilicity by varying the types and ratios of ligands. Molecular dynamics (MD) simulations demonstrate that these nanoparticles can effectively regulate the interfaces between chemically distinct polymers based on their amphiphilicity. Remarkably, a mere 0.1 wt.% of MOF nanoparticles with optimized amphiphilicity (ML-MOF(5:5)) delivered ≈1.1- and ≈34.1-fold increase in strength and toughness of poly (lactic acid) (PLA)/poly (butylene succinate) (PBS) blend, respectively. Moreover, these amphiphilicity-tailorable MOF nanoparticles universally enhance the mechanical properties of various polymer blends, such as polypropylene (PP)/polyethylene (PE), PP/polystyrene (PS), PLA/poly (butylene adipate-co-terephthalate) (PBAT), and PLA/polycaprolactone (PCL)/PBS. This simple universal method offers significant potential for strengthening and toughening various polymer blends.

Charge-transfer host-guest complexes based on pillar[n]arenes and quinonoid compounds for near-infrared photothermal conversion

The host-guest complexation behaviors between perethylated pillar[n]arenes and quinonoid compounds are investigated in both solution and solid state. The resulting host-guest cocrystals demonstrate remarkable near-infrared photothermal conversion properties.

Functionalization of a [2]Catenane with Donor-Acceptor Chromophores Using a Metal Template and Click Reactions

Synthesizing molecules with significant topological features, such as catenanes, tailored with specific groups to confer desired functionality, is essential for investigating various properties arising from the entanglement due to mechanical bonds. This investigation can pave the way for uncovering novel functional materials employing mechanically interlocked molecules (MIMs). In this direction, we have synthesized a π-donor (D) and π-acceptor (A) functionalized [2]catenane using a non-labile Co(III) metal ion as a template with pyridine-diamide templating center and utilizing click reaction for ring-closing. The donor group is a fluorene derivative, and the acceptor is a benzophenazine derivative, commonly employed in synthesizing conjugated polymers for various optoelectronic devices. Synthetically, the acceptor group was introduced into a macrocycle with a pyridine diamide unit. It was then threaded with a ligand having alkyne terminals to obtain the desired [2]pseudorotaxane utilizing cobalt ion as a template. Ring-closing was then performed with a di-azide functionalized molecule with the donor chromophore. The desired D-A functionalized [2]catenane was obtained after demetalation. All the starting materials, macrocycle, and entangled structures have been characterized by 1H-NMR, 13C-NMR, and mass spectroscopy. Some of these materials were also characterized by single-crystal X-ray analysis. The photophysical properties are studied by UV-visible and fluorescence spectroscopy.

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.

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.

Template Assisted Synthesis of Linear [5]Catenane by Post-Functionalization of Templated [2]Catenane and Using Click Reaction

Polymers with all mechanically interlocked rings, such as linear [n]catenanes, have great potential as functional materials due to possible higher degrees of freedom that may contribute to their flexibility but remain elusive. All the synthetic methods used to prepare such a polymer yield mixtures of products. In the absence of higher molecular weight linear [n]catenanes, emphasis on synthesizing low molecular weight oligomers is being pursued. Here, we have described the synthesis of a linear [5]catenane by post-functionalizing a Co(III) templated [2]catenane having a pyridine-diamide unit free for further metal ion coordination. Two molecules were synthesized with suitable threading groups: one, two terminal azide groups, and two, with two terminal alkyne groups to form two [3]pseudorotaxane utilizing Co(III) coordination. These units were then joined, forming a macrocycle, using click reaction, giving the desired metalated linear [5]catenane in 40 % yield. Removal of metal ions leads to linear [5]catenane. In addition, the formation of linear [3] and [2]catenane are also observed. All synthesized structures have been isolated by column chromatographic technique and characterized by 1H-NMR, 13C-NMR, and mass spectroscopy.

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.

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.