Dynamic Gas-Bridged Bond: An Opportunity of Fabricating Dynamic Assembled Materials with Gas.

Gas molecules, as a family of unique polyatomic building blocks, have long been considered hard to involve in molecular assembly or construct assembled materials due to their structural simplicity yet paucity of defined interacting sites. To solve this non-trivial challenge, a core idea is to break the limit of current ways of bonding gas molecules, endowing them with new modes of interactions that match the basic requirements of molecular assembly. In recent years, a new concept, named the dynamic gas-bridged bond (DGB), has emerged, which allows for gas molecules to constitute a dynamic bridging structure between other building blocks with the aid of frustrated Lewis pairs. This makes it possible to harness gas in a supramolecular or dynamic manner. Herein, this perspective discusses distinct dynamic natures of DGBs and manifests their particular functions in various fields, including the control of molecular/polymeric self-assembly nanostructures, creation of multidimensional assembled materials, and recyclable catalysts. The future research direction and challenges of dynamic gas-bridged chemistry toward gas-programmed self-assembly and gas-constructed adaptive materials are highlighted.

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.

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.

Stretchable poly[2]rotaxane elastomers.

Mechanically interlocked polymers (MIPs) are promising candidates for the construction of elastomeric materials with desirable mechanical performance on account of their abilities to undergo inherent rotational and translational mechanical movements at the molecular level. However, the investigations on their mechanical properties are lagging far behind their structural fabrication, especially for linear polyrotaxanes in bulk. Herein, we report stretchable poly[2]rotaxane elastomers (PREs) which integrate numerous mechanical bonds in the polymeric backbone to boost macroscopic mechanical properties. Specifically, we have synthesized a hydroxy-functionalized [2]rotaxane that subsequently participates in the condensation polymerization with diisocyanate to form PREs. Benefitting from the peculiar structural and dynamic characteristics of the poly[2]rotaxane, the representative PRE exhibits favorable mechanical performance in terms of stretchability (∼1200%), Young's modulus (24.6 MPa), and toughness (49.5 MJ/m3). Moreover, we present our poly[2]rotaxanes as model systems to understand the relationship between mechanical bonds and macroscopic mechanical properties. It is concluded that the mechanical properties of our PREs are mainly determined by the unique topological architectures which possess a consecutive energy dissipation pathway including the dissociation of host-guest interaction and consequential sliding motion of the wheel along the axle in the [2]rotaxane motif.

Facile Physicochemical Reprogramming of PEG-Dithiolane Microgels.

Granular biomaterials have found widespread applications in tissue engineering, in part because of their inherent porosity, tunable properties, injectability, and 3D printability. However, the assembly of granular hydrogels typically relies on spherical microparticles and more complex particle geometries have been limited in scope, often requiring templating of individual microgels by microfluidics or in-mold polymerization. Here, we use dithiolane-functionalized synthetic macromolecules to fabricate photopolymerized microgels via batch emulsion, and then harness the dynamic disulfide crosslinks to rearrange the network. Through unconfined compression between parallel plates in the presence of photoinitiated radicals, we transform the isotropic microgels are transformed into disks. Characterizing this process, we find that the areas of the microgel surface in contact with the compressive plates are flattened while the curvature of the uncompressed microgel boundaries increases. When cultured with C2C12 myoblasts, cells localize to regions of higher curvature on the disk-shaped microgel surfaces. This altered localization affects cell-driven construction of large supraparticle scaffold assemblies, with spherical particles assembling without specific junction structure while disk microgels assemble preferentially on their curved surfaces. These results represent a unique spatiotemporal process for rapid reprocessing of microgels into anisotropic shapes, providing new opportunities to study shape-driven mechanobiological cues during and after granular hydrogel assembly.

Reprocessable Polyurethane Foams Using Acetoacetyl-Formed Amides.

Like any other thermosetting material, polyurethane foams (PUFs) contain permanent cross-links that hinder their reprocessability and make their recyclability a tedious and environmentally unfriendly process. Herein, we introduce acetoacetyl-formed amides, formed by the reaction of isocyanates with acetoacetate groups, as dynamic units in the backbone of PUFs. By extensive variation of the foam composition, optimum parameters have been found to produce malleable foams above temperatures of 130 °C, without the requirement of any solvent during the foaming process. The PU cross-linked material can be compression-molded at least three times, giving rise to PU elastomers and thus maintaining a cross-linked network structure. Characterization of the original foams shows comparable properties to standard PUFs, for example, having a density of 32 kg/m3, while they show similar chemical and thermal properties upon reprocessing to strong PU elastomers, exhibiting Tg ranging from -42 to -48 °C. This research provides a straightforward method to produce thermally reprocessable PUFs as a promising pathway to address the recycling issues of end-of-life foams.

The Power of Action Plots: Unveiling Reaction Selectivity of Light-Stabilized Dynamic Covalent Chemistry.

Exploiting the optimum wavelength of reactivity for efficient photochemical reactions has been well-established based on the development of photochemical action plots. We herein demonstrate the power of such action plots by a remarkable example of the wavelength-resolved photochemistry of two triazolinedione (TAD) substrates, i.e., aliphatic and aromatic substituted, that exhibit near identical absorption spectra yet possess vastly disparate photoreactivity. We present our findings in carefully recorded action plots, from which reaction selectivity is identified. The profound difference in photoreactivity is exploited by designing a 'hybrid' bisfunctional TAD molecule, enabling the formation of a dual-gated reaction manifold that demonstrates the exceptional and site-selective (photo)chemical behavior of both TAD substrates within a single small molecule.

Mesoporous chitosan nanofibers loaded with norfloxacin and coated with phenylboronic acid perform as bioabsorbable active dressings to accelerate the healing of burn wounds.

The paper reports new chitosan-based nanofibers, designed to address the healing of burn wounds. To this aim, mesoporous chitosan fiber mats were prepared by electrospinning using poly(ethylene oxide) as sacrificial additive, followed by loading with norfloxacin and coating with an antifungal agent via dynamic imine bonds. Dynamic vapor sorption experiment proved intra-fiber mesopores around 2.7 nm, and UV-vis, FTIR, and NMR spectroscopy confirmed the norfloxacin embedding and the imination reaction. SEM, AFM and POM techniques displayed semicrystalline nanofibers with average diameter around 170 nm entangled into a non-woven mat. Their mesoporous nature favored a rapid adsorption of fluids up to 17 g/g, and a biodegradation rate fitting the wound healing rate, i.e. up to 30 % mass loss in media of pH characteristic to wound exudate and total degradation in that characteristic to normal dermis. The composite fibers released the NFX and 2FPBA in a controlled manner, and showed antimicrobial activity against gram positive, gram negative and fungal strains. They had no cytotoxic effect on normal human dermal fibroblasts, and showed biocompatibility on experimental rats. The investigation of wound healing ability on second/third-degree burn model in rats revealed wound closure and total restoration of the fully functional dermis and epidermis.

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.

Two Functions from a Single Photoresist: Tuning Microstructure Degradability from Light-Stabilized Dynamic Materials.

A photoresist-based on a light-stabilized dynamic material driven by an out-of-equilibrium photo-Diels-Alder reaction of triazolinediones with naphthalenes-whose ability to intrinsically degrade postprinting can be tuned by a simple adjustment of laser intensity during 3D laser lithography is introduced. The resist's ability to form stable networks under green light irradiation that degrade in the dark is transformed into a tunable degradable 3D printing material platform. In-depth characterization of the printed microstructures via atomic force microscopy before and during degradation reveals the high dependency of the final structures' properties on the writing parameters. Upon identifying the ideal writing parameters and their effect on the network structure, it is possible to selectively toggle between stable and fully degradable structures. This simplifies the direct laser writing manufacturing process of multifunctional materials significantly, which typically requires the use of separate resists and consecutive writing efforts to achieve degradable and nondegradable material sections.

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.

Transient and Dissipative Host-Guest Hydrogels Regulated by Consumption of a Reactive Chemical Fuel.

The transient self-assembly of molecules under the direction of a consumable fuel source is fundamental to biological processes such as cellular organization and motility. Such biomolecular assemblies exist in an out-of-equilibrium state, requiring continuous consumption of high energy molecules. At the same time, the creation of bioinspired supramolecular hydrogels has traditionally focused on associations occurring at the thermodynamic equilibrium state. Here, hydrogels are prepared from cucurbit[7]uril host-guest supramolecular interactions through transient physical crosslinking driven by the consumption of a reactive chemical fuel. Upon action from this fuel, the affinity and dynamics of CB[7]-guest recognition are altered. In this way, the lifetime of transient hydrogel formation and the dynamic modulus obtained are governed by fuel consumption, rather than being directed by equilibrium complex formation.

Robust and Dynamic Polymer Networks Enabled by Woven Crosslinks.

Crosslinking plays a crucial role in determining mechanical properties of polymer materials. Although various crosslinks based on covalent or noncovalent bonds have been adopted, it remains an enormous challenge to develop a crosslink which could endow corresponding polymer network with robust yet dynamic properties. Herein, we report a crosslink simultaneously having dynamic property and woven geometry, and the polymer network with woven crosslinks (WPN) could integrate the merits of covalent polymer network (CPN) and supramolecular polymer network (SPN). In specific, the WPN not only exhibits comparable stiffness, strength, elastic recovery, and anti-fatigue property to those of CPN, but also possesses decent mechanical adaptivity and ductility, similar to those of SPN. Particularly, its toughness and puncture resistance are much superior to those of the others. Besides, the dynamicity of woven crosslink also imparts good performances of self-healing and processability to WPN.

Intrinsically Photopolymerizable Dynamic Polymers Derived from a Natural Small Molecule.

Developing photopolymerizable polymeric materials offers many opportunities to process materials in a remote and controllable manner. However, most photopolymerizable technologies require the external introduction of photoabsorbing units, whereas designing intrinsically photopolymerizable polymers is still highly challenging. Here, we report that a natural small-molecule disulfide, thioctic acid, can be directly transformed into a poly(disulfides) network under the irradiation of visible light without any external additives. The resulting polymer network exhibits optical transparency, mechanical stretchability and toughness, ambient self-healing ability, and especially strong adhesive ability to different surfaces. The dynamic covalent backbones of the poly(disulfides) endow the depolymerization ability to recycle the material in a closed-loop manner. We foresee that this facile and robust photopolymerization system is of great promise toward low-cost and high-performance photocuring coatings and adhesives.

Synergistic Covalent and Supramolecular Polymers for Mechanically Robust but Dynamic Materials.

Nature has engineered delicate synergistic covalent and supramolecular polymers (CSPs) to achieve advanced life functions, such as the thin filaments that assist in muscle contraction. Constructing artificial synergistic CSP materials with bioinspired mechanically adaptive features, however, represents a challenging goal. Here, we report an artificial CSP system to illustrate the integration of a covalent polymer (CP) and a supramolecular polymer (SP) in a synergistic fashion, along with the emergence of notable mechanical and dynamic properties which are unattainable when the two polymers are formed individually. The synergistic effect relies on the peculiar network structures of the SP and CPs, which endow the resultant CSPs with overall improved mechanical performance in terms of the stiffness, strength, stretchability, toughness, and elastic recovery. Moreover, the dynamic properties of the SP, including self-healing, stimuli-responsiveness, and reprocessing, are also retained in the CSPs, thus leading to their application as programmable and tunable materials.

Active Periodic Magnetic Nanostructures with High Aspect Ratio and Ultrahigh Pillar Density.

Magnetically actuated micro/nanoscale pillars have attracted significant research interest recently because of their dynamic properties. These structures can be used for various applications, such as dry adhesion, cell manipulation, and sensors or actuators in microfluidics. Magnetically actuated structures can be fabricated by mixing magnetic particles and polymers to yield a favorable combination of magnetic permeability and mechanical compliance. However, the pillar density of demonstrated structures is relatively low, which limits the potential applications in active surface manipulation of microscale objects. Here, we demonstrate active periodic nanostructures with a pillar density of 0.25 pillar/µm2, which is the highest density for magnetically actuated pillars so far. Having a structure period of 2 µm, diameter of 600 nm, and high aspect ratio of up to 11, this structure can be magnetically actuated with a displacement of up to 200 nm. The behaviors of the pillars under various cyclic actuation modes have been characterized, demonstrating that the actuation can be well controlled. This work can find potential applications in particle manipulation and tunable photonic elements.

On Supramolecular Self-Assembly: Interview with Samuel Stupp.

Ranging from 2D assemblies to peptide amphiphile-based biomaterials, Prof. Samuel Stupp and his team have enriched the scientific community with many breakthroughs in the field of supramolecular self-assembly. This Interview offers the unique possibility to share some highlights along his journey, providing also a glimpse to his vision of the future of supramolecular chemistry. Interdisciplinarity is an integral part of Prof. Stupp's research philosophy, and, using his own words, "it is the only way to understand the complex universe around us and help society along the way". What a great guideline to us all!

A Visible-Light-Induced Dynamic Mechanical Bond as a Linkage for Dynamic Materials.

Exploring dynamic bonds and their applications in fabricating dynamic materials has received great attention. A photoinduced [2]rotaxane-based dynamic mechanical bond (DMB) features visible-light-triggered dynamic bonding behavior that is essentially distinguished from conventional dynamic chemical bonds. In this DMB, a photoisomerizable ortho-fluoroazobenzene unit is introduced as a steric-controllable stopper, the visible-light-induced dynamic wagging movement of which enables the photoregulated threading of the macrocycle. This allows reversible in situ de-/reforming of the mechanical bond without involving dynamic chemical linkage. The DMB-cross-linked polymeric gel shows interesting photoinduced degradation behavior upon visible light irradiation. Benefiting from the distinctive dual dynamic nature of reversible bonding behavior and mechanical interlocked structure, this DMB is expected to serve as a new type of dynamic bond that can be applied in designing dynamic soft materials.

Tunable Orthogonal Reversible Covalent (TORC) Bonds: Dynamic Chemical Control over Molecular Assembly.

Dynamic assembly of macromolecules in biological systems is one of the fundamental processes that facilitates life. Although such assembly most commonly uses noncovalent interactions, a set of dynamic reactions involving reversible covalent bonding is actively being exploited for the design of functional materials, bottom-up assembly, and molecular machines. This Minireview highlights recent implementations and advancements in the area of tunable orthogonal reversible covalent (TORC) bonds for these purposes, and provides an outlook for their expansion, including the development of synthetically encoded polynucleotide mimics.