Multi-scale experimental and theoretical study of failure of multi-network elastomers

Multiple polymer networks, such as multiple-network elastomers comprising a sacrificial and a matrix network, exhibit exceptional mechanical toughness and strength, commonly attributed to the formation of an extended damage zone in front of a growing crack. However, the microscopic mechanisms underlying their toughness remain poorly understood. We will show a combination of advanced mechanochemistry and light scattering methods with molecular dynamics simulations to explore the microscopic relaxation dynamics and stress redistribution at the macoscopic and polymer strand scale of single-network and double-network elastomers under uni-axial loading. Dynamic light scattering and mechanochemical experiments show that microscopic rearrangements and bond breaking events are localized near the crack tip in single networks, readily causing the crack to advance. In contrast, double networks exhibit delocalized bond scission and microscopic rearrangements well ahead of and not directly correlated with crack propagation, enabling the dissipation of energy over broader regions and timescales. Numerical simulations of the damage zone show that bond breaking in the matrix network of double networks leads to widespread stress redistribution, mitigating catastrophic damage localization. This enhanced ability to redistribute stress in a non-local manner allows a much larger extension before localized macroscopic failure occurs, explaining the superior toughness of double networks. Our findings identify early, delocalized bond breaking events combined with more efficient dissipation pathways through enhanced microscopic rearrangements

as the key microscopic mechanisms responsible for the outstanding toughness and extensibilityof multiple elastomer networks.