Tunable and Processable Shape-Memory Materials Based on Solvent-Free, Catalyst-Free Polycondensation between Formaldehyde and Diamine at Room Temperature

Photo-Healable Fabrics: Achieving Structural Control via Photochemical Solid-Liquid Transitions of Polystyrene/Azobenzene-Containing Polymer Blends

While polymer fabrics are integral to a wide range of applications, their vulnerability to mechanical damage limits their sustainability and practicality. Addressing this challenge, our study introduces a versatile strategy to develop photohealable fabrics, utilizing a composite of polystyrene (PS) and an azobenzene-containing polymer (PAzo). This combination leverages the structural stability of PS to compensate for the mechanical weaknesses of PAzo, forming the fiber structures. Key to our approach is the reversible trans-cis photoisomerization of azobenzene groups within the PAzo under UV light exposure, enabling controlled morphological alterations in the PS/PAzo blend fibers. The transition of PAzo sections from a solid to a liquid state at a low glass transition temperature (Tg ∼ 13.7 °C) is followed by solidification under visible light, thus stabilizing the altered fiber structures. In this study, we explore various PS/PAzo blend ratios to optimize surface roughness and mechanical properties. Additionally, we demonstrate the capability of these fibers for photoinduced self-healing. When damaged fabrics are clamped and subjected to UV irradiation for 20 min and pressed for 24 h, the mobility of the cis-form PAzo sections facilitates healing while retaining the overall fabric structure. This innovative approach not only addresses the critical issue of durability in polymer fabrics but also offers a sustainable and practical solution, paving the way for its application in smart clothing and advanced fabric-based materials.

A Dual-Bond Crosslinking Strategy Enabling Resilient and Recyclable Electrolyte Elastomers for Solid-State Lithium Metal Batteries

Elastomeric solid polymer electrolytes (SPEs) are highly promising to address the solid-solid-interface issues of solid-state lithium metal batteries (LMBs), but compromises have to be made to balance the intrinsic trade-offs among their conductive, resilient and recyclable properties. Here, we propose a dual-bond crosslinking strategy for SPEs to realize simultaneously high ionic conductivity, elastic resilience and recyclability. An elastomeric SPE is therefore designed with hemiaminal dynamic covalent networks and Li+-dissociation co-polymer chains, where the -C-N- bond maintains the load-bearing covalent network under stress but is chemically reversible through a non-spontaneous reaction, the weaker intramolecular hydrogen bond is mechanically reversible, and the soft chains endow the rapid ion conduction. With this delicate structure, the optimized SPE elastomer achieves high elastic resilience without loading-unloading hysteresis, outstanding ionic conductivity of 0.2 mS cm-1 (25 °C) and chemical recyclability. Then, exceptional room-temperature performances are obtained for repeated Li plating/stripping tests, and stable cycling of LMBs with either LiFePO4 or 4.3 V-class LiFe0.2Mn0.8PO4 cathode. Furthermore, the recycled and reprocessed SPEs can be circularly reused in LMBs without significant performance degradation. Our findings provide an inspiring design principle for SPEs to address the solid-solid-interface and sustainability challenges of solid-state LMBs.

Dynamic Thermosetting Resins with Synergistic Enhanced Strength and Toughness through Combination with Rigid and Soft Microdomains

Preparation of materials that possess highly strong and tough properties simultaneously is a great challenge. Thermosetting resins as a type of widely used polymeric materials without synergistic strength and toughness limit their applications in some special fields. In this report, an effective strategy to prepare thermosetting resins with synergistic strength and toughness, is presented. In this method, the soft and rigid microspheres with dynamic hemiaminal bonds are fabricated first, followed by hot-pressing to crosslink at the interfaces. Specifically, the rigid or soft microspheres are prepared via precipitation polymerization. After hot-pressing, the resulting rigid-soft blending materials exhibit superior strength and toughness, simultaneously. As compared with the precursor rigid or soft materials, the toughness of the rigid-soft blending films (RSBFs) is improved to 240% and 2100%, respectively, while the strength is comparable to the rigid precursor. As compared with the traditional crushing, blending, and hot-pressing of rigid or soft materials to get the nonuniform materials, the strength and toughness of the RSBFs are improved to 168% and 255%, respectively. This approach holds significant promise for the fabrication of polymer thermosets with a unique combination of strength and toughness.

Dynamic Cross-Linked Polyethylene Networks with High Energy Storage and Electrical Damage Self-Healability

Dielectric polymers that exhibit high energy density Ue, low dielectric loss, and thermal resistance are ideal materials for next-generation electrical equipment. The most widely utilized approach to improving Ue involves augmenting the polarization through increasing the dielectric constant εr or the breakdown strength Eb. However, as a conflicting parameter, the dielectric loss also increases inevitably at the same time. In addition, due to the long-term work under a strong electric field or high potential, dielectric materials often produce electrical damage (electrical tree), which is one of the main factors affecting the reliability and service life of electrical equipment. To address these problems, we herein develop dynamic cross-linked polyethylene materials (PE-MA-Epo) by polyethylene-graft-maleic anhydride (PE-MA) and polar epoxy monomers, which showed high εr (>7), low dielectric loss (<0.02), high Ue (5.16 J/cm3 at 425 MV/m), and outstanding discharge efficiency (97%). The performances of the materials are adequate to rival biaxially oriented polypropylene (BOPP) films. Moreover, the excellent self-healing capability of PE-MA-Epo enables the total recovery of εr and tan δ after electrical tree healing. After two cycles of electrical breakdown healing, Eb remained at 80%, which improves the durability and reliability of dielectric polymers. Therefore, PE-MA-Epo shows great potential for applications in advanced electronic power devices.

Hemiaminal dynamic covalent networks with rapid stress relaxation, reprocessability and degradability endowed by the synergy of disulfide and hemiaminal bonds

This work proposes a strategy to address the challenge of achieving rapid reprocessability of vitrimers at mild temperatures by introducing dynamic disulfide and hemiaminal bonds into hemiaminal dynamic covalent networks (HDCNs). The resulting HDCNs, termed HDCNs-DTDA, were prepared through a facile polycondensation between formaldehyde and 4,4'-dithiodianiline. The dual dynamic bond system in the HDCNs-DTDA enables rapid stress relaxation under mild temperature (65 °C for 54 s), which is significantly faster than that observed in HDCNs containing a single dynamic bond (HDCNs-DDM). The HDCNs-DTDA also exhibit a glass transition temperature of 96 °C, excellent solvent resistance and high recovery rates (97%) of tensile strength after reprocessing. In addition, HDCNs-DTDA can be easily degraded in HCl and thiol solutions at room temperature to enable chemical recyclability. Finally, HDCNs-DTDA demonstrates fast shape memory behaviors using thermal stimulation.

Self-healing bottlebrush polymer networks enabled via a side-chain interlocking design

Exploring novel healing mechanisms is a constant impetus for the development of self-healing materials. Herein, we find that side-chain interlocking of bottlebrush polymers can form a dynamic network and thereby serve as a driving force for the self-healing process of the materials. Molecular dynamics simulation indicates that the interlocking is formed by the interpenetration between the long side chains of adjacent molecules and stabilized by van der Waals interactions and molecular entanglements of side chains. The interlocking can be tailored by changing the length and density of the side chains through atom transfer radical polymerization. As a result, the optimized bottlebrush polymer shows a healing efficiency of up to 100%. Unlike chemical interactions, side-chain interlocking eliminates the introduction of specific chemical groups. Therefore, bottlebrush polymers can even self-heal under harsh aqueous conditions, including acid and alkali solutions. Moreover, the highly dynamic side-chain interlocking enables bottlebrush polymers to efficiently dissipate vibration energy, and thus they can be used as damping materials. Collectively, side-chain interlocking expands the scope of physical interactions in self-healing materials and hews out a versatile way for polymers to accomplish self-healing capability in various environments.

Novel Green Reversible Humidity-Responsive Hemiaminal Dynamic Covalent Network for Smart Window

Recently, smart windows have attracted widespread attention on account of their unique features, yet traditional smart windows still rely on external energy support to accomplish dynamic reversible switching, which not only confines usage but also causes waste of energy. For this purpose, we have prepared hemiaminal dynamic covalent network (HDCN) film with outstanding flexibility and strength by a simple and low-cost method, in which the modulus is 206.28 MPa and the elongation at break is 39.02%. Additionally, the transition from a transparent to an opaque state is achieved when the film is stimulated by humidity, and the dynamic transformation of the film to different phases of transparency is obtained when the film is exposed to different relative humidities (60-99%). Most importantly, HDCN film fulfills the modern green requirements and enables complete dissolution in a certain mildly acidic solution, avoiding environmental pollution when the material is discarded due to loss of function. The dynamic tunability of HDCN film demonstrates great advantages and potential in smart windows and anticounterfeiting.

Polymerization, Stimuli-induced Depolymerization, and Precipitation-driven Macrocyclization in a Nitroaldol Reaction System

Dynamic covalent polymers of different topology have been synthesized from an aromatic dialdehyde and α,ω-dinitroalkanes via the nitroaldol reaction. All dinitroalkanes yielded dynamers with the dialdehyde, where the length of the dinitroalkane chain played a vital role in determining the structure of the final products. For longer dinitroalkanes, linear dynamers were produced, where the degree of polymerization reached a plateau at higher feed concentrations. In the reactions involving 1,4-dinitrobutane and 1,5-dinitropentane, specific macrocycles were formed through depolymerization of the linear chains, further driven by precipitation. At lower temperature, the same systemic self-sorting effect was also observed for the 1,6-dinitrohexane-based dynamers. Moreover, the dynamers showed a clear adaptive behavior, displaying depolymerization and rearrangement of the dynamer chains in response to alternative building blocks as external stimuli.

High temperature shape memory poly(amide-imide)s with strong mechanical robustness

Shape memory poly(amide-imide)s with strong mechanical robustness, outstanding heat resistance and low water uptake were fabricated.

Progress in Utilizing Dynamic Bonds to Fabricate Structurally Adaptive Self-Healing, Shape Memory, and Liquid Crystal Polymers

Stimuli-responsive structurally dynamic polymers are capable of mimicking the biological systems to adapt themselves to the surrounding environmental changes and subsequently exhibiting a wide range of responses ranging from self-healing to complex shape-morphing. Dynamic self-healing polymers (SHPs), shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs), which are three representative examples of stimuli-responsive structurally dynamic polymers, have been attracting broad and growing interest in recent years because of their potential applications in the fields of electronic skin, sensors, soft robots, artificial muscles, and so on. We review recent advances and challenges in the developments towards dynamic SHPs, SMPs and LCEs, focusing on the chemistry strategies and the dynamic reaction mechanisms that enhance the performances of the materials including self-healing, reprocessing and reprogramming. We compare and discuss the different dynamic chemistries and their mechanisms on the enhanced functions of the materials, where three summary tables are presented: a library of dynamic bonds and the resulting characteristics of the materials. Finally, we provide a critical outline of the unresolved issues and future perspectives on the emerging developments. This article is protected by copyright. All rights reserved.

Polyurethane modified epoxy vitrimer and its stress relaxation behavior

Anhydride cured epoxy vitrimers usually exhibit desired mechanical strength but poor toughness and slow transesterification rate. Therefore, the repairing property of the material was restricted. In this paper, polyurethane modified epoxy vitrimer (PU-Epv) was prepared. PU was introduced into the vitrimer system of tetrahydrophthalic anhydride cured epoxy to improve the toughness of the material. Meanwhile, because of the presence of amino ester, the transesterification reaction was promoted and the activation energy of the transesterification was only 33.59 kJ/mol. In the thermal welding experiment, the material could be welded at least five times, and scratches on the surface of the samples could be efficiently repaired within 30 min. The toughness of the material was improved without damaging the strength. Meanwhile, the hard thermosetting epoxy was endowed with excellent repairing properties to increase the service life of the material.

Polythiourethane Covalent Adaptable Networks for Strong and Reworkable Adhesives and Fully Recyclable Carbon Fiber-Reinforced Composites

The development of adhesives with superior optical and mechanical performance, solvent resistance, and reworkability is gaining increasing attention in recent years. However, traditional materials do not possess reprocessability and healing characteristics for sustainable development. Here, a superior dynamic polythiourethane (PTU) adhesive with high reprocessability was developed by introducing covalent adaptable networks (CANs). Specifically, dynamic thiocarbamate bonds (TCB) were used to prepare PTU CANs, which showed dramatically enhanced malleability and recyclability. The Young's modulus of the material was 2.0 GPa and the tensile strength was 62.7 MPa. The reprocessing temperature of CANs was reduced to 80 °C while more than 90% of their mechanical properties were retained, even after being reprocessed several times. Moreover, the highly transparent and water-resistant PTU CANs featured an excellent bonding property and reworkability for various materials including glass, with a lap shear strength of 2.9 MPa, metal (5.1 MPa), and wood (6.3 MPa), compared with commercially available adhesives. Additionally, carbon fiber-reinforced composites constructed with PTU CANs were capable of being fully recycled and reused. Importantly, laminated glass with a toughened PTU-PU elastomer interface exhibited an outstanding impact fatigue-resistance behavior, sustaining thousands of impacts. These features demonstrate that PTU CANs show great potential as sustainable materials.

Reprocessable, Reworkable, and Mechanochromic Polyhexahydrotriazine Thermoset with Multiple Stimulus Responsiveness

Developing recyclable, reworkable, and intelligent thermosetting polymers, as a long-standing challenge, is highly desirable for modern manufacturing industries. Herein, we report a polyhexahydrotriazine thermoset (PHT) prepared by a one-pot polycondensation between 4-aminophenyl disulfide and paraformaldehyde. The PHT has a glass transition temperature of 135 °C and good solvent resistance. The incorporation of dual stimuli-responsive groups (disulfide bond and hexahydrotriazine ring) endows the PHT with re-processability, re-workability, and damage monitoring function. The PHT can be repeatedly reprocessed by hot pressing, and a near 100% recovery of flexural strength is achieved. The PHT can also degrade in inorganic acid or organic thiol solutions at room temperature. The thermally reworkable test demonstrates that, after heating the PHT at 200 °C for 1 h, the residuals can be easily wiped off. Finally, the PHT exhibits a reversible mechanochromic behavior when damaged.

Vitrimer-Like Shape Memory Polymers: Characterization and Applications in Reshaping and Manufacturing

The shape memory effect (SME) refers to the ability of a material to recover its original shape, but only in the presence of a right stimulus. Most polymers, either thermo-plastic or thermoset, can have the SME, although the actual shape memory performance varies according to the exact material and how the material is processed. Vitrimer, which is between thermoset and thermo-plastic, is featured by the reversible cross-linking. Vitrimer-like shape memory polymers (SMPs) combine the vitrimer-like behavior (associated with dissociative covalent adaptable networks) and SME, and can be utilized to achieve many novel functions that are difficult to be realized by conventional polymers. In the first part of this paper, a commercial polymer is used to demonstrate how to characterize the vitrimer-like behavior based on the heating-responsive SME. In the second part, a series of cases are presented to reveal the potential applications of vitrimer-like SMPs and their composites. It is concluded that the vitrimer-like feature not only enables many new ways in reshaping polymers, but also can bring forward new approaches in manufacturing, such as, rapid 3D printing in solid state on space/air/sea missions.

Recyclable, Self-Healable, and Highly Malleable Poly(urethane-urea)s with Improved Thermal and Mechanical Performances

Developing recyclable, self-healable, and highly malleable thermosets is one of the keys to relieve environmental pollution and meet our increasing demand for "greener" materials. Hindered urea bonds (HUBs) have been successfully incorporated in preparing dynamic covalent networks with those desirable properties. However, one key drawback is the low thermal stability and poor mechanical performance of previously reported systems. In this work, we demonstrated that the incorporation of aromatic moiety-containing diamine-based HUBs can greatly improve the thermal and mechanical performance of the poly(urethane-urea)s (PUUs) while still maintaining the desirable recycling, self-healing, and reprocessing properties. Studies on model compounds revealed the origin of the thermal stability and demonstrated the dynamic property. The aromatic-containing diamine-based HUBs were then used to prepare a series of catalyst-free PUUs with improved thermal and mechanical properties. The dynamic HUBs significantly reduced the relaxation timescale and allowed the PUU networks to be recycled multiple times. The healed and recycled PUUs regained most of the mechanical strength and integrity of the original material. Therefore, this unique and simple approach is expected to open up new avenues to design PUUs with optimal performance for various applications.

Assembling of Reprocessable Polybutadiene-Based Vitrimers with High Strength and Shape Memory via Catalyst-Free Imine-Coordinated Boroxine

Vitrimers endow cross-linked polymers with malleability and reprocessability via exchange reactions. However, designing of reprocessable, shape-memory polymer materials with high strength via a catalyst-free method remains a challenge under mild conditions. Here, we propose a facile strategy to address this dilemma by introducing the exchangeable imine bond and N-coordinated boroxine into a polybutadiene (PB)-based network. Specifically, PB grafted with 2-aminoethanethiol is reacted with the formyl group of phenylboronic acid and dehydrated to form a dual-dynamic covalently cross-linked network at room temperature. The dynamic network draws on the advantage of imine (toughness) and N-coordinated boroxine (strength), making the PB-based materials exhibit favorable malleability, mechanical property, reprocessability, and thermal-induced shape-memory behavior. We can obtain customized high mechanical properties by tuning the cross-linking density, and the tensile strength reaches a high value (12.35 MPa) without fillers or any other additives. Meanwhile, the unique network framework makes the material recycle over several times without sacrificing its property. This work presents a facile and effective approach to achieve a multifunctional polymer with customized attributes. Besides, this strategy can recycle end-of-life rubber to alleviate environmental pollution and provide inspiration for fabricating targeted materials by uniting the dynamic covalent or noncovalent bonds.

Recyclable, Self-Healing, Thermadapt Triple-Shape Memory Polymers Based on Dual Dynamic Bonds

Fabricating a single polymer network with a combination of a multi-shape memory effect (multiple-SME), solid-state plasticity, recyclability and self-healing behavior remains a challenge. We designed imine bond and ionic hydrogen bond dual cross-linked polybutadiene (PB) networks. The resulting PB networks showed a triple-shape memory effect, where imine bonds could be used to fix the permanent shape and ionic hydrogen bonds and glass transition acted as the transition segments for fixing/releasing the temporary shapes. Additionally, the dual dynamic bonds offered PB networks outstanding solid-state plasticity, recyclability and self-healing behavior. This strategy provides some insights for preparing shape memory polymers integrating multiple-SME and multi-functionality.

Dynamic Covalent Polymers for Biomedical Applications

The rapid development of supramolecular polymer chemistry and constitutional dynamic chemistry over the last decades has made tremendous impact on the emergence of dynamic covalent polymers. These materials are formed through reversible covalent bonds, endowing them with adaptive and responsive features that have resulted in high interest throughout the community. Owing to their intriguing properties, such as self-healing, shape-memory effects, recyclability, degradability, stimuli-responsiveness, etc., the materials have found multiple uses in a wide range of areas. Of special interest is their increasing use for biomedical applications, and many examples have been demonstrated in recent years. These materials have thus been used for the recognition and sensing of biologically active compounds, for the modulation of enzyme activity, for gene delivery, and as materials for cell culture, delivery, and wound-dressing. In this review, some of these endeavors are discussed, highlighting the many advantages and unique properties of dynamic covalent polymers for use in biology and biomedicine.

Structural tuning of polycaprolactone based thermadapt shape memory polymer

Polycaprolactone based thermadapt shape memory polymers with precisely controlled structures allow tunable shape reconfigurability.

Dynamic Imine Bond-Based Shape Memory Polymers with Permanent Shape Reconfigurability for 4D Printing

Shape memory polymer (SMP)-based 4D printing combines the advantages of SMP and 3D printing to form active materials with delicate structure. Nowadays, studies of SMP-based 4D printing materials mainly focus on cross-linked (meth)acrylate of which the permanent shape cannot be changed for their covalent linkage, limiting the usage of 4D printing materials. In this paper, a novel (meth)acrylate monomer with an aldehyde group (2-(methacryloyloxy)ethyl 4-formylbenzoate, MEFB) and hyperbranched cross-linker (HPASi) are synthesized to build (meth)acrylate systems (IEMSis) with dynamic imine bonds for 4D printing. The flexible chain structure of HPASi significantly enhances the toughness of IEMSis, which is 33-97-fold higher than that of the one without HPASi (IEM). The addition of HPASi also endows IEMSis good shape memory properties, and the shape fixity and shape recovery ratios of them are 97.5-97.6 and 91.4-93.7%, respectively. At the same time, IEMSis can undergo a stress relaxation process by dynamic exchanges of imine bonds under relatively mild conditions without a catalyst to acquire an ability of permanent shape reconfiguration. The shape retention ratio of IEMSi3 is 84.3%. In addition, the 4D-printed structures displayed here indicate that these 4D printing systems have a myriad of potential applications including aerospace structures, soft robotic grippers, smart electron switches, and intelligent packaging, while the reconfigurability shown by IEMSi3 will expand the scope of application fields of 4D printing materials.

Exploring multiple functions of diarylsemipinacol linked to the saturated ethylene–propylene elastomer: from the dynamic covalent networks to tailoring its macroscopic performance

Through the multiple functions of diarylsemipinacol, we prepared a mechanically robust and healable EPM with shape memory properties, by integrating self-associating ureidopyrimidinone (UPy) into the dynamic covalent networks.