ABSTRACT
Soft self-healing materials are crucial for the development of next-generation wearable electronics that could function in dynamic environments and resist mechanical damage. However, several challenges remain, including fatigue fracture, poor elasticity, and thermodynamic lability, which significantly limit their practical applications. Here, with a model system of soft self-healing polyurea, we propose a molecular engineering strategy of transforming inherently fragile materials with an island-like structure into resilient ones with a bicontinuous nanophase separation structure using 2-ureido-4-pyrimidinone (UPy) supramolecular motifs as structural regulators. The dynamic and continuous hard domains modified by UPy formed a repairable bicontinuous network similar to those of the reticular layer in animal dermis. This design allows for a simultaneous and tremendous improvement in the fatigue threshold (34.8-fold increase), elastic restorability (the maximum elongation for full dimensional recovery increasing from 6 times to 13 times), and thermodynamic stability (4 orders of magnitude improvement in the characteristic flow transition relaxation time), without significantly compromising the compliance, autonomous self-healing, and optical transparency. These mechanical and thermodynamic improvements address current limitations in unfilled soft self-healing materials as reliable substrates for transparent strain-electronics.
ABSTRACT
Soft self-healing materials are compelling candidates for stretchable devices because of their excellent compliance, extensibility, and self-restorability. However, most existing soft self-healing polymers suffer from crack propagation and irreversible fatigue failure due to easy breakage of their dynamic amorphous, low-energy polymer networks. Herein, inspired by distinct structure-property relationship of biological tissues, a supramolecular interfacial assembly strategy of preparing soft self-healing composites with unprecedented crack propagation resistance is proposed by structurally engineering preferentially aligned lamellar structures within a dynamic and superstretchable poly(urea-ureathane) matrix (which is elongated to 24 750× its original length). Such a design affords a world-record fracture energy (501.6 kJ m-2 ), ultrahigh fatigue threshold (4064.1 J m-2 ), and outstanding elastic restorability (dimensional recovery from 13 times elongation), and preserving low modulus (1.2 MPa), high stretchability (3200%), and high room-temperature self-healing efficiency (97%). Thereby, the resultant composite represents the best of its kind and even surpasses most biological tissues. The lamellar 2D transition-metal carbide/carbonitride (MXene) structure also leads to a relatively high in-plane thermal conductivity, enabling composites as stretchable thermoconductive skins applied in joints of robotics to thermal dissipation. The present work illustrates a viable approach how autonomous self-healing, crack tolerance, and fatigue resistance can be merged in future material design.
ABSTRACT
Catastrophically mechanical failure of soft self-healing materials is unavoidable due to their inherently poor resistance to crack propagation. Here, with a model system, i.e., soft self-healing polyurea, we present a biomimetic strategy of surpassing trade-off between soft self-healing and high fracture toughness, enabling the conversion of soft and weak into soft yet tough self-healing material. Such an achievement is inspired by vascular smooth muscles, where core-shell structured Galinstan micro-droplets are introduced through molecularly interfacial metal-coordinated assembly, resulting in an increased crack-resistant strain and fracture toughness of 12.2 and 34.9 times without sacrificing softness. The obtained fracture toughness is up to 111.16 ± 8.76 kJ/m2, even higher than that of Al and Zn alloys. Moreover, the resultant composite delivers fast self-healing kinetics (1 min) upon local near-infrared irradiation, and possesses ultra-high dielectric constants (~14.57), thus being able to be fabricated into sensitive and self-healing capacitive strain-sensors tolerant towards cracks potentially evolved in service.
Subject(s)
Fractures, Bone , Humans , Hydrogels , Models, BiologicalABSTRACT
NaErF4 is the most extensively studied host for self-sensitized upconversion (UC), and Yb3+ is the most commonly used energy absorber. It has been reported that the red luminescence of Er3+ can be enhanced by introducing Yb3+ into the NaErF4 host lattice, where Yb3+ ions serve as trapping centers to confine the excitation energy. Also, it has been pointed out that the Yb3+ doping in the shell of NaErF4-hosted core-shell nanocrystals can further improve the red emission intensity. Conversely, it can be argued that the Yb3+ doping in the shell always results in the luminescence quenching of the NaErF4 core. These imply that the impact of Yb3+ on NaErF4-based host-sensitized UC is rather complicated and must be probed deeply. In this study, we thoroughly discussed the effect of Yb3+ located in the core/shell on the NaErF4-based host sensitization UC and afforded the related mechanism interpretations. In the NaErF4 core nanocrystals, the green-dominated UCL presented an enhancement on increasing the concentration of the Yb3+ dopant owing to the promoted energy harvesting for luminescence. Furthermore, the emission properties of NaErF4:10%Yb shelled with diverse inert layers were also investigated, and the intensity difference of these core-inert shell nanoparticles could be explained by the lattice mismatch and shell thickness. In NaErF4:10%Yb@NaYF4:Yb with variable Yb3+ doping in the shell, the red-dominated UCL was generally weakened with more Yb3+ localized in the shell, which was ascribed to the competition of energy pooling and energy dissipation of Yb3+ in the outer layer. Therefore, Yb3+ ions wield a two-sided influence (termed a "double-edged sword") on the UC emissions of the Er3+ host. Additionally, we demonstrated the application potential of such UCNPs in water sensing and high-level anti-counterfeiting. This work offers an in-depth insight into the UC mechanism of Yb3+-doped NaErF4 nanocrystals and inspires the engineering of novel luminescent materials with distinguished properties.
ABSTRACT
Considering the operation reliability of flexible and optical electronics (FOEs) in dynamic and real-world environments, autonomous self-healing electromagnetic interference (EMI) shielding materials with high transparency, good stretchability and excellent tear-resistance are urgently required but always difficult to achieve due to the poor dynamics of their elastic substrates. Herein, we propose a facile strategy to design a highly dynamic polyurea elastomer (PDMS-MPI-HDI) featuring with ultrahigh optical transparency (>94%), ultralow elastic modulus (<1 MPa), high tear-resistant stretchability (800%), and ultrafast autonomous self-healing (100 s for scratch-healing). Taking PDMS-MPI-HDI as a substrate for embedding silver nanowires (Ag NWs), the first transparent, stretchable and self-healable EMI shielding materials (Ag NWs/PDMS-MPI-HDI) are presented. Failure behavior of Ag NWs/PDMS-MPI-HDI is highly tolerant of prefabricated cracks under deformation. Due to the robust interfacial adhesion between Ag NWs and PDMS-MPI-HDI, the fractured Ag NW network can autonomously self-reconstruct during the healing process of PDMS-MPI-HDI substrates, contributing to the complete restoration of EMI shielding effectiveness (SE) and full erasure of scratches at both the resting and stretching states. Besides, Ag NWs/PDMS-MPI-HDI exhibits fast autonomous self-healing at high (60 °C) and low (0 °C) temperatures, and in artificial sweat, which is essential for FOEs applicable in various practical environments.
