Macroscale Double Networks: Design Criteria for Optimizing Strength and Toughness

Abstract

The double network concept, based on the fracture of sacrificial bonds, has been revolutionary toward the creation of robust soft materials. Based on the essence of double network hydrogels, macroscale, three-dimensional printed rigid sacrificial networks are embedded within silicone rubber stretchable matrices. Preferential fracture of the sacrificial network results in a ∼60 time increase in stiffness and a ∼50% increase in the work of extension compared with the neat matrix. Maximizing yield strength while maintaining multistep internal fracture occurs when the strength of the sacrificial network approaches the strength of the matrix. Upon determining the optimal sacrificial network strength, the sacrificial bond section density can be increased to maximize energy dissipation and toughening efficiencies up to ∼70% of the maximum theoretical toughness are achieved. High toughness and dissipation are achieved because topological interlocking enables significant force transmission to the sacrificial network at smaller length scales than interfacial adhesion, allowing much higher sacrificial bond density. This method is general and can be used with a variety of materials systems, without requiring strong interfacial adhesion, contrasting traditional composite systems. Demonstrating that the double network concept can be used at length scales far beyond the molecular scale will have important implications toward the development of future structural materials.

Microscopic insights into the failure of elastic double networks

“…We note that both the RSN model and the MS model capture the four different mechanical responses observed in experiments on elastomers [13], hydrogels [2], and macroscopic composites [14,15]. The clear difference in fraction of broken sacrificial bonds between (quasi)brittle and ductile responses is also well established experimentally [13,16].…”

“…1(a)]. Later, this toughening strategy was also shown to be effective for multinetwork elastomers [12,13] or macroscopic composites [14,15] [Fig. 1(b)].…”

A Review on Tough Soft Composites at Different Length Scales

“…The 3D printed rigid plastic grids are also proven efficient to construct tough soft composites. By simply combining silicon rubber and such rigid grids, macroscopic double network composites are fabricated (Figure 6c) [66]. The topological interlocking enables significant force transmission between the soft and rigid phases, preventing delamination.…”

Sharing the load: stress redistribution governs fracture of polymer double networks

“…8 Additionally, these properties can be controlled by swelling the sacrificial network either by introducing a molecular stent 3,10 or using the monomer of the second network. 2 Experiments on a range of systems, varying from elastomers 2,4 to macroscopic networks, 11 suggest that the mechanism through which the enhancement occurs is surprisingly general: sharing of load between the two networks via their topological constraints. 12 The corresponding microscopic picture is that, due to the presence of the matrix chains, the expansion of a (microscopic) crack in a DN requires considerably more energy than in an SN.…”