Robust, transparent, and self-healable polyurethane elastomer via dynamic crosslinking of phenol-carbamate bonds

Ultrarobust, Stretchable, and Highly Elastic Supramolecular Elastomer with Hydrogen-Bond Interactions via sp2 Hybridized Boron-Urethane Bonds

Elastomers are omnipresent in everyday life and industry, yet the development of an elastomer with both superb stress and toughness presents a prodigious challenge. In this report, a high-strength, tough, and high-elastic elastomer derived from sp2 hybrid orbitals of phenylboronic acid was designed. The spatial conformation of network becomes significantly more compact due to the sp2 hybridization of boron. This enhances supramolecular hydrogen bonding interactions, resulting in a marked improvement in the material's mechanical properties. Notably, the hydrogen bonding energy in the polyurethane chain segments enhanced by 37%. The robust hydrogen bonding imparts the elastomer with super high true stress (1.30 GPa), superior toughness (442.2 MJ·m-3), and super puncture resistance strength of 167.8·N·mm-1. The material exhibited excellent fatigue resistance during continuous tensile cycles, while the irreversible deformation disappeared after standing at room temperature. Moreover, the elastomer bespeaks extraordinary elastic restorability, swiftly reverting to its primitive length after being extended to 16 times. This work provides a strategy that the mechanical properties of materials can be enhanced and toughened by utilizing spatial conformational changes in intermolecular interactions.

From a bio-based polyphenol diol intermedia to high-performance polyurethane elastomer: Thermal stability, reprocessability and flame retardancy

In this work, a novel bio-based polyphenol diol intermediate (VDP) was synthesized through a combination of aldimine condensation and addition reactions, utilizing vanillin, 4,4'diamino diphenylmethane (DDM), and 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide (DOPO) as reactants, then various contents of VDP was introduced covalently into the polyurethane backbone. The integration of VDP has notably improved the flame retardancy of polyurethane elastomer, the limiting oxygen index (LOI) of the elastomer was elevated from 23% to 30%, and reaches V-0 rating in the UL-94 vertical burning test. The enhancement of flame retardancy is attributed to the introduction of VDP units, which not only generate PO· and PO2∙ that can capture active free radicals during combustion, but also releases non-flammable gases to improve the flame-retardant effect. Moreover, the VDP enhances the decomposition activation energy values (Eα) from 109.3 to 227.6 KJ/mol at mass loss rate (α) = 10%, which is attributed to the rigid benzene ring structure of VDP that significantly enhances the intermolecular interactions within the polyurethane chains. Furthermore, the elastomer shows excellent rebound resilience and reprocessability, retaining 98.6% of its original mechanical properties after multiple cycles of hot-press remolding and solvent casting.

Shape Memory Polyurethane Composite With Fast Response to Near-Infrared Light Based on Tannic Acid-Iron and Dynamic Phenol-Carbamate Network

Intelligent materials derived from green and renewable bio-based materials garner widespread attention recently. Herein, shape memory polyurethane composite (PUTA/Fe) with fast response to near-infrared (NIR) light is successfully prepared by introducing Fe3+ into the tannic acid-based polyurethane (PUTA) matrix through coordination between Fe3+ and tannic acid. The results show that the excellent NIR light response ability is due to the even distribution of Fe3+ filler with good photo-thermal conversion ability. With the increase of Fe3+ content, the NIR light response shape recovery rate of PUTA/Fe composite films is significantly improved, and the shape recovery time is reduced from over 60 s to 40 s. In addition, the mechanical properties of PUTA/Fe composite film are also improved. Importantly, owing to the dynamic phenol-carbamate network within the polymer matrix, the PUTA/Fe composite film can reshape its permanent shape through topological rearrangement and show its good NIR light response shape memory performance. Therefore, PUTA/Fe composites with high content of bio-based material (TA content of 15.1-19.4%) demonstrate the shape memory characteristics of fast response to NIR light; so, it will have great potential in the application of new intelligent materials including efficient and environmentally friendly smart photothermal responder.

Self-Healing Elastomers with Unprecedented Ultrahigh Strength, Superhigh Fracture Energy, Excellent Puncture Resistance, and Durability Based on Supramolecule Interlocking Networks Formed by Interlaced Hydrogen Bonds

Due to the multiple different properties in self-healing elastomers that are mutually exclusive based on the different and contradictory molecule chain structures, simultaneously achieving the ultrahigh mechanical performance and high durability of self-healing elastomers is a great challenge and the goal that has always been pursued. Herein, we report a novel strategy to fabricate a self-healing elastomer by introducing interlaced hydrogen bonds with superhigh binding energy. Distinguishing from the quadruple hydrogen bonds reported already, the interlaced hydrogen bond with a lower repulsive secondary interaction and higher binding energy is composed of two molecule units with different lengths and steric hindrance. Connected by the interlaced hydrogen bonds, a supramolecule interlocking network is formed to lock the polymer chains at room temperature, endowing the poly(urethane-urea) elastomer with an unprecedented ultrahigh strength (117.5 MPa, even higher than some plastics), the superhigh fracture energy (522.46 kJ m-2), and an excellent puncture resistance (puncture force reached 181.9 N). Moreover, the elastomers also exhibited excellent self-healing properties (healing efficiency up to 95.8%), high transparency (the average transmittance up to 91.0%), and good durability (including thermal decomposition resistance, thermal oxidation aging resistance, water resistance, and solvent resistance), providing a theoretical basis and technical reference in the development and broadening the application prospects of self-healing elastomers.

High-Performance, Light-Stimulation Healable, and Closed-Loop Recyclable Lignin-Based Covalent Adaptable Networks

In this work, high-performance, light-stimulation healable, and closed-loop recyclable covalent adaptable networks are successfully synthesized from natural lignin-based polyurethane (LPU) Zn2+ coordination structures (LPUxZy). Using an optimized LPU (LPU-20 with a tensile strength of 28.4 ± 3.5 MPa) as the matrix for Zn2+ coordination, LPUs with covalent adaptable coordination networks are obtained that have different amounts of Zn. When the feed amount of ZnCl2 is 9 wt%, the strength of LPU-20Z9 reaches 37.3 ± 3.1 MPa with a toughness of 175.4 ± 4.6 MJ m-3 , which is 1.7 times of that of LPU-20. In addition, Zn2+ has a crucial catalytic effect on "dissociation mechanism" in the exchange reaction of LPU. Moreover, the Zn2+ -based coordination bonds significantly enhance the photothermal conversion capability of lignin. The maximum surface temperature of LPU-20Z9 reaches 118 °C under the near-infrared illumination of 0.8 W m-2 . This allows the LPU-20Z9 to self-heal within 10 min. Due to the catalytic effect of Zn2+ , LPU-20Z9 can be degraded and recovered in ethanol completely. Through the investigation of the mechanisms for exchange reaction and the design of the closed-loop recycling method, this work is expected to provide insight into the development of novel LPUs with high-performance, light-stimulated heal ability, and closed-loop recyclability; which can be applied toward the expanded development of intelligent elastomers.

A stretchable, mechanically robust polymer exhibiting shape-memory-assisted self-healing and clustering-triggered emission

Self-healing and recyclable polymer materials are being developed through extensive investigations on noncovalent bond interactions. However, they typically exhibit inferior mechanical properties. Therefore, the present study is aimed at synthesizing a polyurethane-urea elastomer with excellent mechanical properties and shape-memory-assisted self-healing behavior. In particular, the introduction of coordination and hydrogen bonds into elastomer leads to the optimal elastomer exhibiting good mechanical properties (strength, 76.37 MPa; elongation at break, 839.10%; toughness, 308.63 MJ m-3) owing to the phased energy dissipation mechanism involving various supramolecular interactions. The elastomer also demonstrates shape-memory properties, whereby the shape recovery force that brings damaged surfaces closer and facilitates self-healing. Surprisingly, all specimens exhibite clustering-triggered emission, with cyan fluorescence is observed under ultraviolet light. The strategy reported herein for developing multifunctional materials with good mechanical properties can be leveraged to yield stimulus-responsive polymers and smart seals.

Tannic Acid Crosslinked Self-Healing and Reprocessable Silicone Elastomers with Improved Antibacterial and Flame Retardant Properties

Silicone elastomers are widely used in aviation, electronics, automotive, and medical device fields, and their overuse inevitably causes recycled problems. In addition, the elastomers are subject to attack by bacteria and fire during use in some application scenarios, which is a safety hazard. Therefore, there is a great need to prepare silicone elastomers with improved antibacterial, flame retardant, self-healing, and recyclable functions. A new strategy is proposed to prepare silicone elastomers with bio-based tannic acid as cross-linkers to solve this problem by using polydimethylsiloxane as a soft chain segment and 2,2-bis(hydroxymethyl)propionic acid as an intermediate chain extender. Based on the phenol carbamate bonding and hydrogen bonding interactions, the elastomer has efficient self-healing ability and can achieve dynamic dissociation at 120 °C for complete recovery. In addition, due to the unique spatial structure and polyphenolic hydroxyl groups of tannic acid, the mechanical properties of the elastomer are greatly improved with an antimicrobial efficiency of over 90% and a final oxygen index of 25.5%. The multifunctional silicone elastomer has great potential applications in recyclable refractory materials and antimicrobial materials.

Solvent-free synthesis of high-performance polyurethane elastomer based on low-molecular-weight alkali lignin

Using cheap and green lignin as a partial substitute for petroleum-based polyols is highly attractive for sustainable development of polyurethane elastomers (LPUes). However, the traditional synthesis process of LPUes inevitably uses toxic solvents that are difficult to remove or carcinogenic. Here, we reported a solvent-free synthesis method to prepare lignin-containing polyurethane elastomers (SF-LPUes) with high strength, high toughness and high elasticity. Most of the hydroxyl groups of lignin reacted with isocyanates to form a strong chemical cross-linking network, while the unreacted ones formed a dynamic hydrogen bond network with polyurethane matrix, contributing to the in-situ formation of lignin nanoparticles to build a nano-micro phase separation structure. Consequently, a dual-crosslinking network structure was formed and endowed SF-LPUes with excellent mechanical properties. Especially, the SF-LPUes prepared from low molecular alkali lignin possessed a tensile strength as high as 38.2 MPa, a maximum elongation at break of 1108 %, and an elastic recovery ratio of up to 98.7 %. Moreover, SF-LPUes showed impressing reprocessing performance and aging resistance. This work provides an industrial application prospect for the synthesis of lignin-containing polyurethane elastomers via a solvent-free synthesis process.

Preparation of ecofriendly water-borne polyurethane elastomer with mechanical robustness and self-healable ability based on multi-dynamic interactions

Self-healing materials have attracted widespread attention owing to their capacity to extend the lifetime of materials and improve resource utilization. However, achieving superior mechanical performance and high self-healable capability simultaneously under moderate conditions remains a long-standing challenge. Integrating multiple dynamic interactions in waterborne polyurethane (WPU) systems can overcome the above-mentioned issue. Herein, environmentally friendly WPU systems containing multiple hydrogen bonds and boronic ester bonds in their polymer backbones were synthesized, where 2,6-diaminopyridine (DAP) and boric acid (BA) served as a dynamic chain extender and reversible cross-linking agent, respectively. The chain structure of the polymer was adjusted by controlling the ratio (DAP/BA) of hard segments, which could effectively meet the requirement of mechanical robustness and desirable self-healable efficiency. Benefiting from multiple dynamic interactions, the prepared WPU elastomer exhibited good mechanical properties, such as tensile strength (from 18.89 MPa to 30.78 MPa), elongation (about 900%) and toughness (from 54.82 MJ m-3 to 92.74 MJ m-3). Driven by water and heat, the IP-DAP40-BA10-WPU film cut in the middle exhibited good self-healing ability, with healing efficiencies of tensile stress of 90.74% and elongation of 91.29% after self-healing at 80 °C for 36 h. Meanwhile, the synthesized WPU elastomer exhibited good water resistance and thermal stability. This work presents a novel way to design robust self-healable materials, which will have wide promising applications in flexible electronics, smart coatings and adhesives.

Stretchable Gas Barrier Films Using Liquid Metal toward a Highly Deformable Battery

Highly deformable batteries that are flexible and stretchable are important for the next-generation wearable devices. Several studies have focused on the stable operation and life span of batteries. On the other hand, there has been less focus on the packaging of highly deformable batteries. In wearable devices, solid-state or pouch lithium-ion batteries (LIBs) packaged in aluminum (Al)-laminated films, which protect against moisture and gas permeation, are used. Stretchable elastomer materials are used as the packaging films of highly deformable batteries; however, they are extremely permeable to gas and moisture. Therefore, a packaging film that provides high deformability along with gas and moisture barrier functionalities is required for the stable operation of highly deformable batteries used in ambient conditions. In this study, a stretchable packaging film with high gas barrier functionality is developed successfully by coating a thin layer of liquid metal onto a gold (Au)-deposited thermoplastic polyurethane film using the layer-by-layer method. The film exhibits excellent oxygen gas impermeability under mechanical strain and extremely low moisture permeability. It shows high impermeability along with high mechanical robustness. Using the proposed stretchable gas barrier film, a highly deformable LIB is assembled, which offers reliable operation in air. The operation of the highly deformable battery is analyzed by powering LEDs under mechanical deformations in ambient conditions. The proposed stretchable packaging film can potentially be used for the development of packaging films in advanced wearable electronic devices.

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.

Flexible and recyclable bio-based transient resistive memory enabled by self-healing polyimine membrane

The recyclable, self-healing and easily-degradable transient electronic technology has aroused tremendous attention in flexible electronic products. However, integrating the above advantages into one single flexible electronic device is still a huge challenge. Herein, we demonstrate a flexible and recyclable bio-based memory device using fish colloid as the resistive switching layer on a polyimine substrate, which affords reliable mechanical and electrical properties under repetitive conformal deformation operation. This flexible bio-based memory device presents potential analog behaviors including memory characteristics and excitatory current response, which undergoes incremental potentiation in conductance under successive electrical pulses. Moreover, this device is expected to greatly alleviate the environmental problems caused by electronic waste. It can be decomposed rapidly in water and well recycled, which is a promising candidate for transient memories and information security. We believe that this study can provide new possibilities to the field of high-performance transient electronics and flexible resistive memory devices.