The double network concept has been revolutionary in its ability to turn soft, brittle hydrogels into tough, robust materials with mechanical properties that match the best synthetic elastomers. These materials consist of two interpenetrating networks, where each network has a specific mechanical response: the “first network” acts as a sacrificial network, consisting of a rigid, extended network, and the “second network” is a globally percolated, stretchable network. When a double network hydrogel is stretched, covalent bonds of the first network break, dissipating energy; this process continues with increasing strain, until the sacrificial network is completely broken and the second network ruptures. Our goal is to demonstrate that the “sacrificial bond concept” is applicable at length-scales beyond the molecular scale. We aim to incorporate this design concept universally for application in structural and medical devices.<br/><br/>We first demonstrate the macroscale double network (Macro-DN) system by incorporating a rigid “first network,” a 3D printed hard plastic grid, within a soft and stretchable “second network,” silicone rubber. We found that when the strength of the matrix exceeds the strength of the grid, local fracture occurs in the grid, and stretching is isolated to the rubber in the fractured region. As stretching increases, the force increases, and when the local force exceeds the global strength of the grid, fracture will occur elsewhere in the sacrificial network. By tuning the stiffness of the grid, we can independently control the yield strength and fracture strain of the composite. At an optimized grid:matrix strength ratio, we can then vary the number of “sacrificial bond” sites, maximizing toughness for a given component combination. We further demonstrate this process through a unique hydrogel composite system fabricated with a low melting-point alloy (LMA) frame. The LMA structure allows for composites to be made without residual stress by melting and reforming. This design results in 100% healing efficiency, up to a stretch of 250%, with a 5x increase in toughness compared to the neat matrix. We demonstrate through these experiments that the “double network concept” is applicable at length scales far beyond the molecular scale and can play a role in developing improved structural materials.

From micro to macro: Extending the “double network concept to large length scales

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.