Synthetic materials that are capable of autonomous healing upon damage are being developed at a rapid pace because of their many potential applications. Despite these advancements, achieving selfhealing in permanently cross-linked hydrogels has remained elusive because of the presence of water and irreversible cross-links. Here, we demonstrate that permanently cross-linked hydrogels can be engineered to exhibit self-healing in an aqueous environment. We achieve this feature by arming the hydrogel network with flexible-pendant side chains carrying an optimal balance of hydrophilic and hydrophobic moieties that allows the side chains to mediate hydrogen bonds across the hydrogel interfaces with minimal steric hindrance and hydrophobic collapse. The self-healing reported here is rapid, occurring within seconds of the insertion of a crack into the hydrogel or juxtaposition of two separate hydrogel pieces. The healing is reversible and can be switched on and off via changes in pH, allowing external control over the healing process. Moreover, the hydrogels can sustain multiple cycles of healing and separation without compromising their mechanical properties and healing kinetics. Beyond revealing how secondary interactions could be harnessed to introduce new functions to chemically crosslinked polymeric systems, we also demonstrate various potential applications of such easy-to-synthesize, smart, self-healing hydrogels.biomimetic materials | hydrophobicity | smart materials | molecular dynamics | adhesives R ecent years have witnessed an increasing interest in the development of "smart" materials that can sense changes in their environment and can accordingly adapt their properties and function, similar to living systems. Over the last decade, we have discovered and demonstrated a class of smart hydrogels that exhibit unique biomimicking functions: thermoresponsive volume phase transitions similar to sea cucumbers (1), self-organization into core-shell hollow structures similar to coconuts (2), shape memory as exhibited by living organisms (2), and metal ion-mediated cementing similar to marine mussels (3). A common thread connecting these smart hydrogels is their possession of a unique balance of hydrophilic and hydrophobic interactions that endows the hydrogels with the biomimicking properties described above. In this study, we demonstrate how this concept of balancing hydrophilic and hydrophobic forces could be exploited to design chemically cross-linked hydrogels with self-healing abilities.Indeed, materials capable of autonomous healing upon damage have numerous potential applications (4-6). So far, self-healing has been demonstrated in linear polymers (7), supramolecular networks (8, 9), dendrimer-clay systems (10), metal ion-polymer systems (11,12), and multicomponent systems (13-17). Whereas multicomponent thermosetting systems harness the ability of embedded chemical agents to repair cracks, supramolecular networks and noncovalent hydrogels employ secondary interactions such as hydrogen bonding, ionic interact...

We compare and contrast the microstructure, viscoelastic properties, and shock response of coarse-grained models of multiblock copolymer and diblock copolymers using molecular dynamics simulations. This study is motivated by the excellent dissipative and shock-mitigating properties of polyurea, speculated to arise from its multiblock chain architecture. Our microstructural analyses reveal that the multiblock copolymer microphase-separates into small, interconnected, rod-shaped, hard domains surrounded by a soft matrix, whereas the diblock copolymer forms larger, unconnected, hard domains. Our viscoelastic analyses indicate that compared with the diblock copolymer, the multiblock copolymer is not only more elastic but also more dissipative, as signified by its larger storage and loss modulus at low to intermediate frequencies. Our shock simulations and slip analyses reveal that shock waves propagate slower in the multiblock copolymer in comparison with the diblock copolymer, most likely due to the more deformable hard domains in the former system. These results suggest that the multiblock architecture of polyurea might impart polyurea with smaller, more deformable, and interconnected hard domains that lead to improved energy dissipation and lower shock speeds.

We investigate with nonreactive molecular dynamics simulations the dynamic response of phenolic resin and its carbon-nanotube ͑CNT͒ composites to shock wave compression. For phenolic resin, our simulations yield shock states in agreement with experiments on similar polymers except the "phase change" observed in experiments, indicating that such phase change is chemical in nature. The elastic-plastic transition is characterized by shear stress relaxation and atomic-level slip, and phenolic resin shows strong strain hardening. Shock loading of the CNT-resin composites is applied parallel or perpendicular to the CNT axis, and the composites demonstrate anisotropy in wave propagation, yield and CNT deformation. The CNTs induce stress concentrations in the composites and may increase the yield strength. Our simulations suggest that the bulk shock response of the composites depends on the volume fraction, length ratio, impact cross-section, and geometry of the CNT components; the short CNTs in current simulations have insignificant effect on the bulk response of resin polymer.

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