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 present an experimental investigation on the creep behavior of molten polypropylene organically modified clay nanocomposites. The nanocomposite hybrids were prepared by melt intercalation in an extruder in the presence or absence of a compatibilizer. They were subsequently annealed and simultaneously characterized using high-temperature wide-angle X-ray diffraction and controlled stress rheometry. The creep resistance of compatibilized hybrids was significantly higher than that of uncompatibilized hybrids and also increased with annealing time. The microstructure of the nanocomposites as investigated by TEM and high-temperature WAXD showed the presence of clay crystallites dispersed within the polymer matrix. The creep data together with the microstructural investigation are probably indicative of a small amount of exfoliation from the edges of the clay crystallites during extrusion and annealing. The zero shear viscosity of the compatibilized nanocomposites containing greater than 3 wt % clay was at least 3 orders of magnitude higher than that of matrix resin and the uncompatibilized hybrids. Importantly, the large increase in zero shear viscosity was not accompanied by any increase in the flow activation energy compared to the matrix polymer. The compatibilized hybrids also showed an apparent “yield” behavior. We conclude that the solidlike rheological response of the molten nanocomposite originates from large frictional interactions of the clay crystallites. Compatibilizer has a significant influence in modifying the rheological behavior.

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