Rubbery polymer networks are used in all facets of modern life, from tough elastomeric products such as tires or contact lenses to delicate, highly swellable gels used for precise drug delivery or tissue engineering. Fracture behavior of polymer networks is a complex process which is difficult to accurately predict, but premature failure can have consequences ranging from irritating to fatal. To investigate how a crack propagates, polymer gels were synthesized with a varied combination of “weak” and “strong” mechanochemical crosslinkers with known force-coupled reactivities measured via single molecule force spectroscopy. Tearing energy was measured as a function of the ratio of weak to strong mechanophore crosslinkers, and found to plateau below 40% and above 75% strong strands, with a transition from 2.9 ± 0.6 J/m<sup>2</sup> at 40% strong strands to 21 ± 2 J/m<sup>2</sup> at 75% strong strands. The percolation threshold of weak and strong bonds was measured to occur when the respective strand population reached 48 ± 3%, demonstrating that fracture is highly dependent on which type of strand is percolated throughout the network. Coarse-grained kinetic Monte Carlo fracture simulations of networks with varied ratios of strands with high and low bond dissociation energies showed a similar trend in toughness. Analysis of single bond breaking events confirmed that material failure was reactivity-determined, with only weak bond breaking occurring up until strong bonds percolate, and continuous weak bond failure acting as stress-dissipation events to enhance tearing energy.

Polymer network fracture as a reactivity-determined (de)percolation process

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