Novel Molecular-Level Insight into the Self-Healing Behavior and Mechanism of Polyurethane-Urea Elastomer Based on a Noncovalent Strategy

Solvent-Free Supramolecular Polymerization for Feather-Like Nanostructured Chiral Fluorescent Polyurethanes with Multimodal Chiroptical Stimuli Responsiveness

Chiral supramolecular polymers with stimuli-responsive circularly polarized luminescence (CPL) are highly desirable for smart flexible optoelectronic devices, but remain rarely reported. Here, a simple solvent-free supramolecular polymerization for preparing chiral polyurethanes is presented by in situ induced self-assembly strategy, using cellulose nanocrystals (CNCs)-based isocyanate prepolymers and macromolecular polyols as precursors, achieving precise control over polymer chain assembly with spot-like arrangement. More importantly, by further incorporating a π-conjugated luminescent dihydroxynaphthalene molecule, CPL-active flexible polyurethane films with feather-like nanostructures are constructed, which promote the ordered arrangement of CNCs-based isocyanate segments due to the increased spatial resistance. The π─H bond network between CNCs and urethane-linked benzene rings drives the self-assembly, enabling higher-level chiral amplification and enhanced fluorescence. Interestingly, the prepared chiral fluorescent polyurethanes display multimodal chiroptical stimuli responsiveness under various stimuli, such as temperature, solvent polarity, pH, and polarized light, due to the sensitivity of the π─H bond network. This work offers new insights into designing solvent-free chiral supramolecular polymers with significant chiroptical potentials.

Molecular Insights into Interfacial Stress Amplification and Network Reinforcement in Extrudable Multiphase Vitrimers

Incorporating dynamic covalent bonds (DCBs) into elastomers provides a seminal solution for the upcycling of traditional thermoset elastomers. Recently, engineering a multiphase network with various cross-linking uniformity and phase structures has been proven to be an effective strategy to overcome the bottleneck of continuous and high-throughput recycling (e.g., extrusion reprocessing) of vitrimeric elastomers. However, all of the relevant studies only focused on revealing the influences of network structures on the macroscopic properties of the systems. As for the microscopic mechanism of the multiphase network at the molecular level, it is still lacking. Herein, based on coarse-grained molecular dynamics (CGMD) simulation, a modeled DCBs-cross-linked elastomer with a multiphase network was established, which was subsequently subjected to in situ tensile or shear forces to simulate the evolution of local chain segment motion and stress/strain distributions in various microregions of the network under the complex extrusion/injection force field. The results indicate that phase domains with different cross-link densities feature distinct chain segment motion behavior and local stress/strain distribution evolution during tensile/shear deformation, and the interfacial phase exhibits significant high stresses. Therefore, incorporating heterogeneously cross-linked multiphase networks into elastomeric vitrimers can enable the system to have significant network reinforcement and unique interfacial stress amplification effects, which are critical for determining extrusion/injection reprocessability. Therefore, we envisage that the present study can provide a molecular-level theoretical explanation for the extrusion/injection reprocessability of multiphase elastomeric vitrimers, thereby guiding the rational network/performance design of these seminal materials.

Surface-Confined Disordered Hydrogen Bonds Enable Efficient Lithium Transport in All-Solid-State PEO-Based Lithium Battery

Polyethylene oxide (PEO)-based electrolytes are essential to advance all-solid-state lithium batteries (ASSLBs) with high safety/energy density due to their inherent flexibility and scalability. However, the inefficient Li+ transport in PEO often leads to poor rate performance and diminished stability of the ASSLBs. The regulation of intermolecular H-bonds is regarded as one of the most effective approaches to enable efficient Li+ transport, while the practical performances are hindered by the electrochemical instability of free H-bond donors and the constrained mobility of highly ordered H-bonding structures. To overcome these challenges, we develop a surface-confined disordered H-bond system with stable donor-acceptor interactions to construct a loosened chain segments/ions arrangement in the bulk phase of PEO-based electrolytes, realizing the crystallization inhibition of PEO, weak coordination of Li+ and entrapment of anions, which are conducive to efficient Li+ transport and stable Li+ deposition. The rationally designed LiFePO4-based ASSLB demonstrates a long cycle-life of over 400 cycles at 1.0 C and 65 °C with a capacity retention rate of 87.5 %, surpassing most of the currently reported polymer-based ASSLBs. This work highlights the importance of confined disordered H-bonds on Li+ transport in an all-solid-state battery system, paving the way for the future design of polymer-based ASSLBs.

Boosting the Actuation Performance of a Dynamic Supramolecular Polyurethane-Urea Elastomer via Kinetic Control

The ongoing soft actuation has accentuated the demand for dielectric elastomers (DEs) capable of large deformation to replace the traditional rigid mechanical apparatus. However, the low actuation strain of DEs considerably limits their practical applications. This work developed high-performance polyurethane-urea (PUU) elastomers featuring large actuation strains utilizing an approach of kinetic control over the microphase separation structure during the fabrication process. Additionally, disulfide (DS) bonds were incorporated as dynamic chemical linkages to effectively heal the mechanical damage in the resulting elastomer (PUUDS). Alteration in processing conditions creates notable differences in the rate of phase separation among the multiphase materials. A faster phase separation rate is associated with a reduced degree of microphase separation, increased spacing within hard domains, a higher proportion of disordered hydrogen bonds, and hydrogen bonding index. These changes synergistically improved the electromechanical properties of the PUUDS elastomers, thereby enhancing their actuation performance. The sample processed under the fastest phase separation condition showed the lowest Young's modulus and a pronounced dielectric response at low frequencies. The electrostriction effect accounts for 89% of the total electromechanical coupling, achieving a significant reduction in the driving voltage during actuation. The maximum actuation strain recorded was 21.6% at an electric field of 45 MV/m. Benefiting from the fully reversible dynamic network, the damaged PUUDS elastomer can be healed and restored to its original elongation at break after 3 h at room temperature. Practical application was demonstrated through the development of a miniature butterfly model constructed from a single-layer PUUDS elastomer, showcasing potential applications in soft robotics. These findings highlight the critical role of kinetic control in optimizing the performance of advanced DEs.

Stretchable, Self-Healable, and Durable Conductive Elastomer Derived from a Rationally Designed Covalently Cross-Linked Network

Silver nanowire (Ag NW)-based elastic conductors have been considered a promising candidate for key stretchable electrodes in wearable devices. However, the weak interface interaction of Ag NWs and elastic substrates leads to poor durability of electronic devices. For everyday usage, an additional self-healing ability is required to resist scratching and damage. Therefore, robust Ag NW-based elastic conductors possessing stretchability, self-healing, and stability are highly desirable and challenging. Here, we present a universal interface tailoring strategy that introduces thiols onto the dynamically cross-linked elastic substrate surface. The surface thiol groups strongly interact with Ag NWs through Ag-S bonds, forming the stable conductive layer on the elastic substrate. At elevated temperatures, the Ag NWs are partially embedded in the surface of the elastic substrate and a buffer layer is formed to release the concentrated stress. As a result, the formed Ag NW-based elastic conductor displays the combination of good conductivity, high stretchability (>1000%), efficient self-healing capability (>95%), and remarkable stability. Besides, the Ag NW-based elastic conductor combining the good properties mentioned above is suitable for fabricating a sensitive and durable strain sensor against cyclic strain. The presented strategy can be considered a versatile and effective route to generating other surface thiol-rich covalently cross-linked elastomers with dynamic disulfide bonds.

High-Strength and High-Toughness Supramolecular Materials for Self-Healing Triboelectric Nanogenerator

The development of self-healing materials provides a new opportunity and challenge for advancing triboelectric nanogenerators (TENGs). However, the low strength and low toughness of self-healing triboelectric materials often result in the deformation or breakage of TENG under high mechanical loads, thereby limiting their potential applications. Herein, a new strategy for fabricating self-healing triboelectric materials is reported, which introduces cross-linking networks with hydrogen bonds and metal coordination bonds. The desired high performance can be achieved by simply adjusting the molar ratio of the metal to the ligand. When the molar ratio is 1:2, the tensile strength, toughness, and elongation at break of the material reached 13.7 MPa, 76.9 MJ m-3, and 1321%, respectively. Furthermore, its self-healing efficiency can reach 74% at 70 °C in 6 h. Working in contact-separation mode, the electrical output can reach 164 V, 18.2 µA, 57.5 nC, with a maximum power density of 2.54 W m-2. Notably, even if it is sheared, the electrical output performances of TENG can be completely recovered to the original state. In addition, the developed TENG exhibits excellent output stability over 10 000 contact separation cycles. This study presents a promising approach for the development of stretchable smart generators.

An Intrinsic Photothermal Supramolecular Hydrogel with Robust Mechanical Strength and NIR-Responsive Shape Memory

Near-infrared (NIR)-triggered shape memory hydrogels with promising mechanical strength hold immense potential in the field of biomedical applications and soft actuators. However, the optical and mechanical properties of currently reported hydrogels usually suffer from limited solubility and dispersion of commonly used photothermal additives in hydrogels, thus restricting their practical implementations. Here,, a set of NIR-responsive shape memory hydrogels synthesized by polyaddition of diisocyanate-terminated poly(ethylene glycol), imidazolidinyl urea (IU), and p-benzoquinone dioxime (BQDO) is reported. The introduction of IU, a hydrogen bond reinforcing factor, significantly enhances the mechanical properties of the hydrogels, allowing for their tunable ranges of the ultimate tensile strength (0.4-2.5 MPa), elongation at break (210-450%), and Young's modulus (190-850 kPa). The unique hydrogels exhibit an intrinsic photothermal effect because of the covalently incorporated photothermal moiety (BQDO), and the photothermal supramolecular hydrogel shows controllable shape memory capabilities characterized by rapid recovery speed and high recovery ratio (>90%). This design provides new possibilities for applying shape memory hydrogels in the field of soft actuators.

Toward Highly Matching the Dura Mater: A Polyurethane Integrating Biocompatible, Leak-Proof, and Self-Healing Properties

The dura mater is the final barrier against cerebrospinal fluid leakage and plays a crucial role in protecting and supporting the brain and spinal cord. Head trauma, tumor resection and other traumas damage it, requiring artificial dura mater for repair.  However, surgical tears are often unavoidable. To address these issues, the ideal artificial dura mater should have biocompatibility, anti-leakage, and self-healing properties. Herein, this work has used biocompatible polycaprolactone diol as the soft segment and introduced dynamic disulfide bonds into the hard segment, achieving a multifunctional polyurethane (LSPU-2), which integrated the above mentioned properties required in surgery. In particular, LSPU-2 matches the mechanical properties of the dura mater and the biocompatibility tests with neuronal cells demonstrate extremely low cytotoxicity and do not cause any negative skin lesions. In addition, the anti-leakage properties of the LSPU-2 are confirmed by the water permeability tester and the 900 mm H2 O static pressure test with artificial cerebrospinal fluid. Due to the disulfide bond exchange and molecular chain mobility, LSPU-2 could be completely self-healed within 115 min at human body temperature. Thus, LSPU-2 comprises one of the most promising potential artificial dura materials, which is essential for the advancement of artificial dura mater and brain surgery.

Molecular Dynamics Simulation of Polymer Nanocomposites with Supramolecular Network Constructed via Functionalized Polymer End-Grafted Nanoparticles

Since the proposal of self-healing materials, numerous researchers have focused on exploring their potential applications in flexible sensors, bionic robots, satellites, etc. However, there have been few studies on the relationship between the morphology of the dynamic crosslink network and the comprehensive properties of self-healing polymer nanocomposites (PNCs). In this study, we designed a series of modified nanoparticles with different sphericity (η) to establish a supramolecular network, which provide the self-healing ability to PNCs. We analyzed the relationship between the morphology of the supramolecular network and the mechanical performance and self-healing behavior. We observed that as η increased, the distribution of the supramolecular network became more uniform in most cases. Examination of the segment dynamics of polymer chains showed that the completeness of the supramolecular network significantly hindered the mobility of polymer matrix chains. The mechanical performance and self-healing behavior of the PNCs showed that the supramolecular network mainly contributed to the mechanical performance, while the self-healing efficiency was dominated by the variation of η. We observed that appropriate grafting density is the proper way to effectively enhance the mechanical and self-healing performance of PNCs. This study provides a unique guideline for designing and fabricating self-healing PNCs with modified Nanoparticles (NPs).

Stretchable, Ultratough, and Intrinsically Self-Extinguishing Elastomers with Desirable Recyclability

Advanced elastomers are increasingly used in emerging areas, for example, flexible electronics and devices, and these real-world applications often require elastomers to be stretchable, tough and fire safe. However, to date there are few successes in achieving such a performance portfolio due to their different governing mechanisms. Herein, a stretchable, supertough, and self-extinguishing polyurethane elastomers by introducing dynamic π-π stacking motifs and phosphorus-containing moieties are reported. The resultant elastomer shows a large break strain of ≈2260% and a record-high toughness (ca. 460 MJ m-3 ), which arises from its dynamic microphase-separated microstructure resulting in increased entropic elasticity, and strain-hardening at large strains. The elastomer also exhibits a self-extinguishing ability thanks to the presence of both phosphorus-containing units and π-π stacking interactions. Its promising applications as a reliable yet recyclable substrate for strain sensors are demonstrated. The work will help to expedite next-generation sustainable advanced elastomers for flexible electronics and devices applications.

Mechanoluminescent Device: In Situ Renewable Carbazole Derivatives Sandwiched by Self-Healing Disulfide-Containing Polyurethane for Mechanical Signals Detection

Mechanoluminescent (ML) materials can emit visible light by utilizing mechanical energy, which shows unique advantages in visual mechanical sensing, displays, and biomechanical monitoring due to the correlation between force stimulation and luminescence intensity. Most organic ML materials exhibit luminescence intensity attenuation, disappearing completely with force stimulation and failing to recover. Here, organic luminogens (Cz-alkyl₆) can be synthesized by introducing a soft alkyl chain into the carbazole, which exhibits ML emission with self-assembly units. Furthermore, organic luminogens can be generated repeatedly by simply recrystallizing the fracture crystal in situ after a short thermal treatment (70 °C) within 14 s. More importantly, the quantitative correlation between force pressure and ML intensity has been established by a sandwich-type ML device based on a novel carbazole derivative (Cz-alkyl₆). The ML device presents a capacity for detecting mechanical signals up to 13 N according to its ML intensity (≤275 a.u.), exhibiting potential application value in engineering damage detection, anticounterfeiting, and advanced visual mechanical sensing.