(Invited) Creating “Double Network” Composites Via Macroscale Reinforcement
Authors: King Daniel Rudolf , Takahashi Riku , Okumura Tsuyoshi , Gong Jian Ping 
Published: February 2020

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. Double network hydrogels 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 with relatively low loading percentage, 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. Here, we will utilize this concept to develop materials with analogous fracture properties at the macroscale.
Like double network hydrogels, our systems consist of rigid “first networks” embedded in global, soft and stretchable “second networks.” As a control system, 3d printed polyurethane/polyacrylate composite grids were utilized with a synthetic 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 force exceeds the strength of the grid, fracture will occur elsewhere in the composite. This process continues sequentially throughout the sample until all grid fracture sites are exhausted, and the matrix ruptures. By tuning the stiffness of the grid, we can independently control the yield strength and fracture strain of the composite, until a point where the grid strength exceeds the matrix strength, and the multiple fracture process no longer occurs. At an optimized grid:matrix strength ratio, we can then vary the number of fracture sites in the grid, maximizing toughness for a given component combination.
The macroscale double network technique enables us to create composites with unique properties. Previously it has been difficult to create composites with hydrogels, because they undergo swelling which results in buckling or delamination in composite samples. We have created new hydrogel composites by incorporating low-melting point alloy (LMA) grids to alleviate this problem. A hydrogel is polymerized around the LMA frame, and the composite structure is submerged in water until the equilibrium swelling conditions are achieved. The sample is then heated to allow the LMA frame to melt, relieving the swelling stress in the composite sample. This LMA frame has been specifically designed based on the macroscale double network criteria to undergo multiple fracture events during stretching. Furthermore, utilizing an LMA frame allows us to stretch the sample up to 250%, fracturing the frame to dissipate energy, return the composite to its original shape, and then “heal” the structure in hot water. These samples can be then annealed in cold water, and retested, with 100% repeatability and healing efficiency. This process introduces new avenues to create self-healing composite hydrogel materials.

Figure 1