DYNAMERS: dynamic polymers as self-healing materials

Rapid Printing and Patterning of Tough, Self-Healable, and Recyclable Hydrogel Thin-Films toward Flexible Sensing Devices

Direct and rapid printing and surface patterning of hydrogel thin films are of great significance in the construction of advanced electronic devices, yet they are greatly underdeveloped due to the intrinsic contradiction between mechanical strength and self-healability as well as recyclability. Here, we present a universal and rapid slipping-directed route with a newly developed water-soluble star polymer hydrogel for direct and reproducible printing and patterning of freestanding functional thin films with precisely controlled thicknesses, components, and surface structures on a large scale. The resulting thin films combine the features of large transmittance (93%), tough mechanical strength (114 MPa), multiresponsive self-healability, recyclability, and remarkable multifunctionality. With the unique humidity-sensitive properties as motivation, diverse humidity-sensing devices including an actuating switch, a supercapacitive sensor, and a noncontact electronic skin are facilely constructed through the humidity-induced transverse, longitudinal, and patterning assembly techniques, respectively. The method presented here is universal and efficient in the fabrication and assembly of thin films with controlled configuration and functionality for advanced flexible electronics.

Fluorescent Dynamic Covalent Polymers for DNA Complexation and Templated Assembly

Dynamic covalent polymers (DCPs) offer opportunities as adaptive materials of particular interest for targeting, sensing and delivery of biological molecules. In this view, combining cationic units and fluorescent units along DCP chains is attractive for achieving optical probes for the recognition and delivery of nucleic acids. Here, we report on the design of acylhydrazone-based DCPs combining cationic arginine units with π-conjugated fluorescent moieties based on thiophene-ethynyl-fluorene cores. Two types of fluorescent building blocks bearing neutral or cationic side groups on the fluorene moiety are considered in order to assess the role of the number of cationic units on complexation with DNA. The (chir)optical properties of the building blocks, the DCPs, and their complexes with several types of DNA are explored, providing details on the formation of supramolecular complexes and on their stability in aqueous solutions. The DNA-templated formation of DCPs is demonstrated, which provides new perspectives on the assembly of fluorescent DCP based on the nucleic acid structure.

Polymeric Emissive Materials Based on Dynamic Covalent Bonds

Dynamic covalent polymers, composed of dynamic covalent bonds (DCBs), have received increasing attention in the last decade due to their adaptive and reversible nature compared with common covalent linked polymers. Incorporating the DCBs into the polymeric material endows it with advanced performance including self-healing, shape memory property, and so forth. However, the emissive ability of such dynamic covalent polymeric materials has been rarely reviewed. Herein, this review has summarized DCBs-based emissive polymeric materials which are classified according to the different types of DCBs, including imine bond, acylhydrazone bond, boronic ester bond, dynamic C-C bond, as well as the reversible bonds based on Diels–Alder reaction and transesterification. The mechanism of chemical reactions and various stimuli-responsive behaviors of DCBs are introduced, followed by typical emissive polymers resulting from these DCBs. By taking advantage of the reversible nature of DCBs under chemical/physical stimuli, the constructed emissive polymeric materials show controllable and switchable emission. Finally, challenges and future trends in this field are briefly discussed in this review.

Dynamic Covalent Hydrogels: Strong yet Dynamic

Hydrogels are crosslinked polymer networks with time-dependent mechanical response. The overall mechanical properties are correlated with the dynamics of the crosslinks. Generally, hydrogels crosslinked by permanent chemical crosslinks are strong but static, while hydrogels crosslinked by physical interactions are weak but dynamic. It is highly desirable to create synthetic hydrogels that possess strong mechanical stability yet remain dynamic for various applications, such as drug delivery cargos, tissue engineering scaffolds, and shape-memory materials. Recently, with the introduction of dynamic covalent chemistry, the seemingly conflicting mechanical properties, i.e., stability and dynamics, have been successfully combined in the same hydrogels. Dynamic covalent bonds are mechanically stable yet still capable of exchanging, dissociating, or switching in response to external stimuli, empowering the hydrogels with self-healing properties, injectability and suitability for postprocessing and additive manufacturing. Here in this review, we first summarize the common dynamic covalent bonds used in hydrogel networks based on various chemical reaction mechanisms and the mechanical strength of these bonds at the single molecule level. Next, we discuss how dynamic covalent chemistry makes hydrogel materials more dynamic from the materials perspective. Furthermore, we highlight the challenges and future perspectives of dynamic covalent hydrogels.

Advances and Challenges of Self-Healing Elastomers: A Mini Review

In the last few decades, self-healing polymeric materials have been widely investigated because they can heal the damages spontaneously and thereby prolong their service lifetime. Many ingenious synthetic procedures have been developed for fabricating self-healing polymers with high performance. This mini review provides an impressive summary of the self-healing polymers with fast self-healing speed, which exhibits an irreplaceable role in many intriguing applications, such as flexible electronics. After a brief introduction to the development of self-healing polymers, we divide the development of self-healing polymers into five stages through the perspective of their research priorities at different periods. Subsequently, we elaborated the underlying healing mechanism of polymers, including the self-healing origins, the influencing factors, and direct evidence of healing at nanoscopic level. Following this, recent advance in realizing the fast self-healing speed of polymers through physical and chemical approaches is extensively overviewed. In particular, the methodology for balancing the mechanical strength and healing ability in fast self-healing elastomers is summarized. We hope that it could afford useful information for research people in promoting the further technical development of new strategies and technologies to prepare the high performance self-healing elastomers for advanced applications.

Remarkably enhanced dynamic oxygen migration on graphene oxide supported by copper substrate

The dynamic covalent properties of graphene oxide (GO) are of fundamental interest to a broad range of scientific areas and technological applications. It remains a challenge to access feasible dynamic reactions for reversibly breaking/reforming the covalent bonds of oxygen functional groups on GO, although these reactions can be induced by photonic or mechanical routes, or mediated by adsorbed water. Here, using density functional theory calculations, we demonstrate the remarkably enhanced dynamic oxygen migration along the basal plane of GO supported by copper substrate (GO@copper), with C-O bond breaking reactions and proton transfer between neighboring epoxy and hydroxyl groups. Compared to reactions on GO, the energy barriers of oxygen migrations on GO@copper are sharply reduced to be less than or comparable to thermal fluctuations, and meanwhile the crystallographic match between GO and copper substrate induces new oxygen migration paths on GO@copper. This work sheds light on understanding of the metal substrate-enhanced dynamic properties of GO, and evidences the strategy to tune the activity of two-dimensional-interfacial oxygen groups for various potential applications.

Dynamic Supramolecular Polymers Based on Zinc Bis(diorganophospate)s: Synthesis, Structure and Transformations in Solid State and Solutions

The synthesis, structure and some properties of coordination polymers composed of linear zinc bis(diorganophospate)s (ZnDOPs) with a general formula of Zn[O₂P(OR)₂]₂ (where R = CH₃, C₂H₅, n-C₄H₉, or 2-ethylhexyl group) are described. Hybrid (co)polymers obtained by different procedures were characterized by means of powder XRD, DSC, SEM, TGA coupled with mass spectrometry of the evolved gases and rheological measurements, as well as FTIR and NMR techniques. The morphology, thermal transformations and solubility of ZnDOPs strongly depend on the type of organic substituent in the O₂P(OR)₂ ligands and the thermal history of the sample. Because of this, one can obtain highly crystalline rods, semicrystalline powders, as well as rubbery materials exhibiting a second-order transition below −50 °C. Polymeric chains formed by ZnDOPs undergo a reversible dissociation in polar organic solvents (e.g., methanol, DMSO), which allows for easy modification of their composition and physicochemical properties via a simple exchange of diorganophosphate anions. Some of the ZnDOPs were investigated as the latent curing agents for epoxides. On the basis of rheological and DSC studies, it is evident that ZnDOPs catalyze very effectively the cross-linking process within the 130–160 °C temperature range.

Controlling the Processability and Stability of Supramolecular Polymers Using the Interplay of Intra- and Intermolecular Interactions

Polymer networks crosslinked via non-covalent interactions afford interesting materials for a wide range of applications due to their self-healing capability, recyclability, and tunable material properties. However, when strong non-covalent binding motifs in combination with high crosslink density are used, processing of the materials becomes troublesome because of high viscosities and the formation of insoluble gels. Here, we present an approach to control the processability of grafted polymers containing strong non-covalent interactions by balancing the interplay of intra- and intermolecular hydrogen bonding. A library of copolymers with different degrees of polymerization and content of protected ureido-pyrimidinone-urea (UPy-urea) grafts was prepared. Photo-deprotection in a good solvent like tetrahydrofuran (THF) at low concentrations (≤1 mg mL-1) created intramolecularly assembled nanoparticles. Remarkably, the intrinsic viscosity of these nanoparticle solutions was an order of magnitude lower compared to solutions of the intermolecularly assembled analogues, highlighting the crucial role of intra- versus intermolecular interactions. Due to the strong hydrogen bonds between UPy dimers, the intramolecularly assembled structures were kinetically trapped. As a result, the polymer nanoparticles were readily processed into a bulk material, without causing major changes in the morphology as verified by atomic force microscopy. Subsequent intermolecular crosslinking of the nanoparticle film, by heating to temperatures where the hydrogen-bond exchange becomes fast, resulted in a crosslinked network. The reversibility of the hereby obtained polymer networks was shown by retrieving the intramolecularly assembled nanoparticles via redissolution and sonication of the intermolecularly crosslinked film in THF with a small amount of acid. Our results highlight that the stability and processability of highly supramolecularly crosslinked polymers can be controlled both in solution and in bulk by using the interplay of intra- and intermolecular non-covalent interactions in grafted polymers.

Aero Grade Epoxy Vitrimer towards Commercialization

Traditional crosslinked aero grade epoxy resins have excellent thermal-mechanical properties and solvent resistance, but they cannot be remolded, recycled, or repaired. Vitrimers can be topologically rearranged via an associative exchange mechanism, endowing them with thermoplasticity. Introducing dynamic bonds into crosslinked networks to obtain more sustainable thermosets is currently an interesting research topic. While recent research into vitrimers has indicated many advantages over traditional thermosets, an important shortcoming has been identified: susceptibility to creep at service temperature due to the dynamic bonds present in the network. In addition, designing aero grade epoxy vitrimers (similar to RTM6 resin) still remains a challenge. Herein, low creep aero grade epoxy vitrimer with thermal and mechanical properties similar to those of aero grade epoxy resins and with the ability to be recyclable, repairable, and reprocessable, has been prepared. In this manuscript, we demonstrate that aero grade epoxy vitrimer with reduced creep can be easily designed by the introduction of a certain fraction of permanent crosslinks, without having a negative effect on the stress relaxation of the material. Subsequently, the mechanical and relaxation properties were investigated and compared with those of classical aero grade epoxy resin. A high Tg (175 °C) epoxy vitrimer was obtained which fulfilled all mechanical and thermal specifications of the aero sector. This work provides a simple network design to obtain aero grade epoxy resins with excellent creep resistance at elevated temperatures while being sustainable.

Noncovalently Cross-Linked Polymeric Materials Reinforced by Well-Designed In Situ-Formed Nanofillers

Noncovalently cross-linked polymeric materials generally exhibit lower mechanical robustness than traditional polymeric materials. Therefore, it is important to improve the mechanical properties of noncovalently cross-linked polymeric materials using an efficient and generalized approach. In this Perspective, we systematically summarized the recent development of noncovalently cross-linked polymeric materials reinforced by in situ-formed nanofillers. The synergy of high-density noncovalent interactions and in situ-formed rigid nanofillers provided an effective means for the fabrication of noncovalently cross-linked plastics with high mechanical strength. The design of in situ-formed tough nanofillers, which could deform and dissociate, endowed the noncovalently cross-linked hydrogels and elastomers with high toughness, excellent stretchability, elasticity, damage resistance, and damage tolerance. Benefiting from the well-designed in situ-formed nanofillers, these noncovalently cross-linked polymeric materials with enhanced mechanical strength still exhibited satisfactory healing, recycling, and reprocessing properties. Outlooks were provided to envision the remaining challenges to the further development and practical application of noncovalently cross-linked polymeric materials reinforced with in situ-formed nanofillers.

Thermodynamic, kinetic, and mechanistic studies of the thermal guanidine metathesis reaction

We describe studies of the thermal guanidine metathesis (TGM) reaction, a reversible transformation that results in exchange of N-substituents of the guanidine functional group. By comparing the effects of discrete structural variations, we find that steric congestion is an important factor in determining both the equilibrium guanidine composition and the reaction kinetics. The alkyl versus aryl nature of N-substitution also plays an essential role in the reaction rate, up to the point that minimal TGM reactivity is observed when the guanidine contains wholly alkyl substituents. Furthermore, we demonstrate that TGM occurs under thermodynamic control and present evidence that it proceeds by a dissociative mechanism, supported by direct observation of a carbodiimide intermediate.

Unravelling the pore network and gas dynamics in highly adaptive rubbery organic frameworks

Rubbery organic frameworks-ROFs have recently emerged as an intriguing class of dynamers by virtue of reversible connections between their building units. Their highly adaptative features at the origin of their spectacular self-healing properties made them also attractive candidates for the development of gas-selective membranes combining high selectivity and fast permeability. So far, little is known on the origin of this unique trait and this clearly hampers the exploitation of this class of dynamers in many areas where stimuli-responsive pore dynamics is of great importance. To address this lack of fundamental knowledge, herein we unravel the self-assembly process of ROFs via the development of an advanced computational methodology combining quantum and force field molecular simulations that enable the description of reversible connections of building units and the long-range organization of the cross-linked ROF network. We demonstrate that both accurate energy barriers associated with the covalent bond formation between the building units and presence of solvent are key parameters to ensure the in silico construction of reliable ROF structure models that are supported by a set of experimental data collected on synthesized ROFs including density, connectivity and porosity. Atomistic insights into the unusual guest-responsive pore dynamics of this intriguing class of dynamers are further gained with a special attention paid to the tunability of this pore flexibility by controlling the chemical composition of the building units. As a further stage, the dynamics of CO₂ in these compliance frameworks is scrutinized to shed light on the mechanism at the origin of their promising performance as CO₂-selective membranes. We highlight that guest-triggered pore dynamics enables the creation of a diffusion pathway to ensure effective gas transport throughout the whole ROF. This knowledge of the pore structure and its guest-responsive dynamics at the microscopic level is unprecedented in the field of dynamers and it is expected to pave the way towards the optimization of this class of adaptive porous frameworks for many potential applications. Interestingly, this computational approach can be transferable to the exploration of any complex disordered systems showing a high degree of flexibility and guest induced structure/pore reorganization.

Structure and gas transport molecular insight into the intriguing rubbery organic frameworks characterized by unique adaptative behavior owing to the reversible connections between their constitutive building units and their non-permanent porosity.

The Final Frontier of Sustainable Materials: Current Developments in Self-Healing Elastomers

It is impossible to describe the recent progress of our society without considering the role of polymers; however, for a broad audience, "polymer" is usually related to environmental pollution. The poor disposal and management of polymeric waste has led to an important environmental crisis, and, within polymers, plastics have attracted bad press despite being easily reprocessable. Nonetheless, there is a group of polymeric materials that is particularly more complex to reprocess, rubbers. These macromolecules are formed by irreversible crosslinked networks that give them their characteristic elastic behavior, but at the same time avoid their reprocessing. Conferring them a self-healing capacity stands out as a decisive approach for overcoming this limitation. By this mean, rubbers would be able to repair or restore their damage automatically, autonomously, or by applying an external stimulus, increasing their lifetime, and making them compatible with the circular economy model. Spain is a reference country in the implementation of this strategy in rubbery materials, achieving successful self-healable elastomers with high healing efficiency and outstanding mechanical performance. This article presents an exhaustive summary of the developments reported in the previous 10 years, which demonstrates that this property is the last frontier in search of truly sustainable materials.

Conductive Supramolecular Polymer Nanocomposites with Tunable Properties to Manipulate Cell Growth and Functions

Synthetic bioactive nanocomposites show great promise in biomedicine for use in tissue growth, wound healing and the potential for bioengineered skin substitutes. Hydrogen-bonded supramolecular polymers (3A-PCL) can be combined with graphite crystals to form graphite/3A-PCL composites with tunable physical properties. When used as a bioactive substrate for cell culture, graphite/3A-PCL composites have an extremely low cytotoxic activity on normal cells and a high structural stability in a medium with red blood cells. A series of in vitro studies demonstrated that the resulting composite substrates can efficiently interact with cell surfaces to promote the adhesion, migration, and proliferation of adherent cells, as well as rapid wound healing ability at the damaged cellular surface. Importantly, placing these substrates under an indirect current electric field at only 0.1 V leads to a marked acceleration in cell growth, a significant increase in total cell numbers, and a remarkable alteration in cell morphology. These results reveal a newly created system with great potential to provide an efficient route for the development of multifunctional bioactive substrates with unique electro-responsiveness to manipulate cell growth and functions.

Supramolecular Polymerization at Interfaces

Supramolecular polymers, originating from the interplay between polymer science and supramolecular chemistry, have attracted increasing interest in the scientific and industrial communities. To date, most supramolecular polymers are constructed in homogeneous solutions. Supramolecular polymerization normally takes place spontaneously in solutions, thus creating challenges in fabricating supramolecular polymers in a controlled manner. By combining supramolecular polymerization and interfacial polymerization, supramolecular polymerization can be transferred from homogeneous solutions to interfaces, which allows for the controlled production of supramolecular polymers. In this Perspective, we will summarize recent progress and the advantages in supramolecular polymerization at solid-liquid and liquid-liquid interfaces. Meanwhile, current challenges and opportunities in the field of supramolecular polymerization at interfaces are proposed and discussed. It is anticipated that this Perspective will inspire supramolecular polymerization at interfaces and facilitate the construction of supramolecular polymeric materials with diverse architectures and tailor-made functions.

Dynamic covalent chemistry in live cells for organelle targeting and enhanced photodynamic action

Organelle-specific targeting enables increasing the therapeutic index of drugs and localizing probes for better visualization of cellular processes. Current targeting strategies require conjugation of a molecule of interest with organelle-targeting ligands. Here, we propose a concept of dynamic covalent targeting of organelles where the molecule is conjugated with its ligand directly inside live cells through a dynamic covalent bond. For this purpose, we prepared a series of organelle-targeting ligands with a hydrazide residue for reacting with dyes and drugs bearing a ketone group. We show that dynamic hydrazone bond can be formed between these hydrazide ligands and a ketone-functionalized Nile Red dye (NRK) in situ in model lipid membranes or nanoemulsion droplets. Fluorescence imaging in live cells reveals that the targeting hydrazide ligands can induce preferential localization of NRK dye and an anti-cancer drug doxorubicin in plasma membranes, mitochondria and lipid droplets. Thus, with help of the dynamic covalent targeting, it becomes possible to direct a given bioactive molecule to any desired organelle inside the cell without its initial functionalization by the targeting ligand. Localizing the same NRK dye in different organelles by the hydrazide ligands is found to affect drastically its photodynamic activity, with the most pronounced phototoxic effects in mitochondria and plasma membranes. The capacity of this approach to tune biological activity of molecules can improve efficacy of drugs and help to understand better their intracellular mechanisms.

Aggregation-Induced Emission Boosting the Study of Polymer Science

The past one hundred years witness the great development of polymer science. The advancement of polymer science is closely related with the developing of characterization techniques and methods, from viscometry in molecular weight determination to advanced techniques including differential scanning calorimetry, nuclear magnetic resonance and scanning electron microscopy. However, these techniques are normally constrained to tedious sample preparation, high cost, harsh experimental condition, or ex-situ characterization. Fluorescence technology has the merits of high sensitivity and direct visualization. Contrary to conventional aggregation-causing quenching fluorophores, those dyes with aggregation-induced emission characteristic show high emission efficiency in aggregate states. Based on the restriction of intramolecular motions for AIE properties, the AIE materials are very sensitive to the surrounding microenvironments owing to the twisted propeller-like structures and therefore reveal great potentials in polymer's study. The AIE concept has been successfully used in polymer's study and provides us a deeper understanding on polymer structure and properties. In this review, the applications of AIEgens in polymer science for visualizing polymerization, glass transition, dissolution, crystallization, gelation, self-assembly, phase separation, cracking and self-healing were exemplified and summarized. Lastly, the challenges and perspectives in the study of polymer science using AIEgens are addressed. This article is protected by copyright. All rights reserved.

Recent Advances of Self-Healing Polymer Materials via Supramolecular Forces for Biomedical Applications

Noncovalent interactions can maintain the three-dimensional structures of biomacromolecules (e.g., polysaccharides and proteins) and control specific recognition in biological systems. Supramolecular chemistry was gradually developed as a result, and this led to design and application of self-healing materials. Self-healing materials have attracted attention in many fields, such as coatings, bionic materials, elastomers, and flexible electronic devices. Nevertheless, self-healing materials for biomedical applications have not been comprehensively summarized, even though many reports have been focused on specific areas. In this Review, we first introduce the different categories of supramolecular forces used in preparing self-healing materials and then describe biological applications developed in the last 5 years, including antibiofouling, smart drug/protein delivery, wound healing, electronic skin, cartilage lubrication protection, and tissue engineering scaffolds. Finally, the limitations of current biomedical applications are indicated, key design points are offered for new biological self-healing materials, and potential directions for biological applications are highlighted.

Growing Strategy for Postmodifying Cross-Linked Polymers' Bulky Size, Shape, and Mechanical Properties

Living organisms are open systems that can incorporate externally provided nutrients to vary their appearances and properties, while synthetic materials normally have fixed sizes, shapes, and functions. Herein, we report a strategy for enabling cross-linked polymers to continuously grow with programmable bulky structures and properties. The growing strategy involves repeatable processes including swelling of polymerizable components into the cross-linked polymers, in situ polymerization of the components, and homogenization of the original and newborn polymer networks. Using acrylate-based polymers as an example, we demonstrate that homogenization allows the grown polymer materials to further integrate various polymerizable components to alternate their bulky properties. During the growth, the changes from elastomers to organogels and then to hydrogels with updated covalent-linked functions (i.e., photochromism and thermoresponsiveness) are shown. Since this growing strategy is applicable to different acrylate systems, we envision its great potential in the design of next-generation polymers, smartening systems, and postmodification of cross-linked polymer materials.

Disulfide-Mediated Reversible Polymerization toward Intrinsically Dynamic Smart Materials

The development of a dynamic chemistry toolbox to endow materials dynamic behavior has been key to the rational design of future smart materials. The rise of supramolecular and dynamic covalent chemistry offers many approaches to the construction of dynamic polymers and materials that can adapt, respond, repair, and recycle. Within this toolbox, the building blocks based on 1,2-dithiolanes have become an important scaffold, featuring their reversible polymerization mediated by dynamic covalent disulfide bonds, which enables a unique class of dynamic materials at the intersection of supramolecular polymers and adaptable covalent networks. This Perspective aims to explore the dynamic chemistry of 1,2-dithiolanes as a versatile structural unit for the design of smart materials by summarizing the state of the art as well as providing an overview of the fundamental challenges involved in this research area and its potential future directions.

Adsorption-Based Detoxification of Endotoxins by Porous Flexible Organic Frameworks

Bacterial lipopolysaccharides (LPS, endotoxins) cause sepsis that is responsible for a huge amount of mortality globally. However, their neutralization or detoxification remains an unmet medical need. We envisaged that cationic organic frameworks with persistent hydrophobic porosity may adsorb and thus neutralize LPS through a combination of cooperative ion-pairing electrostatic attraction and hydrophobicity. We here report the preparation of two water-soluble flexible organic frameworks (FOF-1 and FOF-2) from tetratopic and ditopic precursors through quantitative formation of hydrazone bonds at room temperature. The two FOFs are revealed to possess hydrodynamic diameters, which range from 20 to 120 nm, depending on the concentrations. Dynamic light scattering and isothermal titration calorimetric and chromogenic limulus amebocyte lysate experiments indicate that both frameworks are able to adsorb and thus reduce the concentration of free LPS molecules in aqueous solution, whereas cytokine inhibition experiments with RAW264.7 support that this adsorption can significantly decrease the toxicity of LPS. In vivo experiments with mice (five males per group) show that the injection of FOF-1 at a dose of 0.6 mg/kg realizes the survival of all of the mice administrated with LPS of the d-galactosamine (d-Gal)-sensitized absolute lethal dose (LD₁₀₀, 0.05 mg/kg), whereas its maximum tolerated dose for mice is determined to be 10 mg/kg. These findings provide a new promising sequestration strategy for the development of porous agents for the neutralization of LPS.

Hierarchical self-assembly of aromatic peptide conjugates into supramolecular polymers: it takes two to tango

Supramolecular polymers are self-assembled materials displaying adaptive and responsive "life-like" behaviour which are often made of aromatic compounds capable of engaging in π-π interactions to form larger assemblies. Major advances have been made recently in controlling their mode of self-assembly, from thermodynamically-controlled isodesmic to kinetically-controlled living polymerization. Dynamic covalent chemistry has been recently implemented to generate dynamic covalent polymers which can be seen as dynamic analogues of biomacromolecules. On the other hand, peptides are readily-available and structurally-rich building blocks that can lead to secondary structures or specific functions. In this context, the past decade has seen intense research activity in studying the behaviour of aromatic-peptide conjugates through supramolecular and/or dynamic covalent chemistries. Herein, we review those impressive key achievements showcasing how aromatic- and peptide-based self-assemblies can be combined using dynamic covalent and/or supramolecular chemistry, and what it brings in terms of the structure, self-assembly pathways, and function of supramolecular and dynamic covalent polymers.

Single-Molecule Force Spectroscopy of a Tetraaryl Succinonitrile Mechanophore

Fluorescent damage reporters that use mechanochemical activation of a covalent bond to elicit an optical signal are emerging tools in material mechanics as a means to access the nanoscale distribution of forces inside materials under stress. A promising class of damage reporters are tetraaryl succinonitriles (TASN), whose mechanical activation results in stable fluorescent radical species. However, in-depth insights into the molecular mechanics of TASN activation are absent, precluding their use as quantitative mechanoprobes. Here we perform single-molecule force spectroscopy experiments to provide these insights. We use a bridged version of the TASN unit, embedded in multi-mechanophore polymer, to enable multiplexed mechanochemical measurements at the single-molecule level. Our experiments reveal that TASN activates at surprisingly low forces and short time scales compared to other covalent mechanophores. These results establish TASN as a promising candidate for reporting the lower end of relevant forces in material mechanics.

Janus Cross-links in Supramolecular Networks

Thermosets composed of cross-linked polymers demonstrate enhanced thermal, solvent, chemical, and dimensional stability as compared to their non-cross-linked counterparts. However, these often-desirable material properties typically come at the expense of reprocessability, recyclability, and healability. One solution to this challenge comes from the construction of polymers that are reversibly cross-linked. We relied on lessons from Nature to present supramolecular polymer networks comprised of cooperative Janus-faced hydrogen bonded cross-links. A triazine-based guanine-cytosine base (GCB) with two complementary faces capable of self-assembly through three hydrogen bonding sites was incorporated into poly(butyl acrylate) to create a reprocessable and recyclable network. Rheological experiments and dynamic mechanical analysis (DMA) were employed to investigate the flow behavior of copolymers with randomly distributed GCB units of varying incorporation. Our studies revealed that the cooperativity of multiple hydrogen bonding faces yields excellent network integrity evidenced by a rubbery plateau that spanned the widest temperature range yet reported for any supramolecular network. To verify that each Janus-faced motif engages in multiple cross-links, we studied the effects of local concentration of the incorporated GCB units within the polymer chain. Mechanical strength improved by colocalizing the GCB within a block copolymer morphology. This enhanced performance revealed that the number of effective cross-links in the network increased with the local concentration of hydrogen bonding units. Overall, this study demonstrates that cooperative noncovalent interactions introduced through Janus-faced hydrogen bonding moieties confers excellent network stability and predictable viscoelastic flow behavior in supramolecular networks.

Real Time SPR Assessment of the Structural Changes of Adaptive Dynamic Constitutional Frameworks as a New Route for Sensing

Cross linked gold-dynamic constitutional frameworks (DCFs) are functional materials of potential relevance for biosensing applications, given their adaptivity and high responsivity against various external stimuli (such as pH, temperature) or specific interactions with biomolecules (enzymes or DNA) via internal constitutional dynamics. However, characterization and assessment of their dynamic conformational changes in response to external stimuli has never been reported. This study proves the capability of Surface Plasmon Resonance (SPR) assays to analyse the adaptive structural modulation of a functional matrix encompassing 3D gold-dynamic constitutional frameworks (Au-DCFs) when exposed to pH variations, as external stimuli. We analyse Au-DCFs formed from Au nanoparticles, (AuNP) connected through constitutionally dynamic polymers, dynamers, with multiple functionalities. For increased generality of this proof-of-concept assay, Au-DCFs, involving DCFs designed from 1,3,5-benzene-tricarbaldehyde (BTA) connecting centres and polyethylene glycol (PEG) connectors, are covalently attached to standard SPR sensing chips (Au nanolayers, carboxyl terminated or with carboxymethyl dextran, CMD top-layer) and analysed using state-of-the art SPR instrumentation. The SPR effects of the distance from the Au-DCFs matrix to the Au nanolayer of the sensing chip, as well as of Au-DCFs thickness were investigated. This study reveals the SPR response, augmented by the AuNP, to the conformational change, i.e., shrinkage, of the dynamer and AuNP matrix when decreasing the pH, and provides an unexplored insight into the sensing applicability of SPR real-time analysis of adaptive functional materials.

Processing of Self-Healing Polymers for Soft Robotics

Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments. However, they are prone to various damage types. Self-healing polymers address this vulnerability. Self-healing soft robots can recover completely from macroscopic damage, extending their lifetime. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self-healing polymers into complex structures. Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self-healing polymers, have been created. Herein, the wide variety of processing techniques of self-healing polymers for robotics available in the literature is reviewed, and limitations and opportunities discussed thoroughly. Based on defined requirements for soft robots, these techniques are critically compared and validated. A strong focus is drawn to the reversible covalent and (physico)chemical cross-links present in the self-healing polymers that do not only endow healability to the resulting soft robotic components, but are also beneficial in many manufacturing techniques. They solve current obstacles in soft robots, including the formation of robust multi-material parts, recyclability, and stress relaxation. This review bridges two promising research fields, and guides the reader toward selecting a suitable processing method based on a self-healing polymer and the intended soft robotics application.

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.

Hierarchical Self-Assembly and Multidynamic Responsiveness of Fluorescent Dynamic Covalent Networks Forming Organogels

Smart stimuli-responsive fluorescent materials are of interest in the context of sensing and imaging applications. In this project, we elaborated multidynamic fluorescent materials made of a tetraphenylethene fluorophore displaying aggregation-induced emission and short cysteine-rich C-hydrazide peptides. Specifically, we show that a hierarchical dynamic covalent self-assembly process, combining disulfide and acyl-hydrazone bond formation operating simultaneously in a one-pot reaction, yields cage compounds at low concentration (2 mM), while soluble fluorescent dynamic covalent networks and even chemically cross-linked fluorescent organogels are formed at higher concentrations. The number of cysteine residues in the peptide sequence impacts directly the mechanical properties of the resulting organogels, Young's moduli varying 2500-fold across the series. These materials underpinned by a nanofibrillar network display multidynamic responsiveness following concentration changes, chemical triggers, as well as light irradiation, all of which enable their controlled degradation with concomitant changes in spectroscopic outputs─self-assembly enhances fluorescence emission by ca. 100-fold and disassembly quenches fluorescence emission.

A Photoinduced Dual-Wavelength Approach for 3D Printing and Self-Healing of Thermosetting Materials

Vat photopolymerization-based 3D printing techniques have been widely used to produce high-resolution 3D thermosetting materials. However, the lack of repairability of these thermosets leads to the production of waste. In this study, reversible addition fragmentation chain transfer (RAFT) agents are incorporated into resin formulations to allow visible light (405 nm) mediated 3D printing of materials with self-healing capabilities. The self-healing process is based on the reactivation of RAFT agent embedded in the thermosets under UV light (365 nm), which enables reformation of the polymeric network. The self-healing process can be performed at room temperature without prior deoxygenation. The impact of the type and concentration of RAFT agents in the polymer network on the healing efficiency is explored. Resins containing RAFT agents enable 3D printing of thermosets with self-healing properties, broadening the scope of future applications for polymeric thermosets in various fields.