Microscopic insights into the failure of elastic double networks

Abstract

The toughness of a polymer material can increase significantly if two networks are combined into one material. This toughening effect is a consequence of a transition from a brittle to a ductile failure response. Although this transition and the accompanying toughening effect have been demonstrated in hydrogels first, the concept has been proven effective in elastomers and in macroscopic composites as well. This suggests that the transition is not caused by a specific molecular architecture, but rather by a general physical principle related to the mechanical interplay between two interpenetrating networks. Here we employ theory and computer simulations, inspired by this general principle, to investigate how disorder controls the brittle-to-ductile transition both at the macroscopic and the microscopic level. A random spring network model featuring two different spring types enables us to study the joined effect of initial disorder and network-induced stress heterogeneity on this transition. We reveal that a mechanical force balance gives a good description of the brittle-to-ductile transition. In addition, the inclusion of disorder in the spring model predicts four different failure regimes along the brittle-to-ductile response in agreement with experimental findings. Finally, we show that the network structure can result in stress concentration, diffuse damage, and loss of percolation depending on the failure regime. This work thus provides a framework for the design and optimization of double-network materials and underlines the importance of network structure in the toughness of polymer materials.

A molecular interpretation of the toughness of multiple network elastomers at high temperature

“…Because of the complexity of the multiscale process, predicting the onset of regime ii and of regime iii at the crack tip as a function of remains a challenge and requires a molecular criterion related to the network architecture. Recent simulation studies may provide hints on the nature of such criteria ( 37 , 38 ).…”

Sharing the Load: Stress Redistribution Governs Fracture of Polymer Double Networks

“…2,[5][6][7]13 As a consequence, fracture of sacrificial chains in DNs is less likely to lead to the formation of macroscopic cracks and thus global failure. 14 Instead, the load is transferred (partially) from the sacrificial network to the matrix network surrounding the broken sacrificial polymer chain. 4 Thus, in a DN, more sacrificial chains can break prior to global failure compared to an SN.…”

“…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. ,− , As a consequence, fracture of sacrificial chains in DNs is less likely to lead to the formation of macroscopic cracks and thus global failure . Instead, the load is transferred (partially) from the sacrificial network to the matrix network surrounding the broken sacrificial polymer chain .…”

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

“…2,[5][6][7]13 As a consequence, fracture of sacrificial chains in DNs is less likely to lead to the formation of macroscopic cracks and thus global failure. 14 Instead, the load is transferred (partially) from the sacrificial network to the matrix network surrounding the broken sacri-ficial polymer chain. 4 Thus, in a DN more sacrificial chains can break prior to global failure compared to an SN.…”