A coarse-grained molecular dynamics study of damage localization during the fracture of double polymer networks
M. Le Goff (1, 2) M. Bouzid (3), J. Tian (4), Jean-Louis Barrat (1) and Kirsten Martens (1)
(1)Univ. Grenoble Alpes, CNRS, LIPhy, 38000 Grenoble, France
(2)Institut fur Theoretische Physik, Universitat Innsbruck, 6020 Innsbruck, Austria
(3)Univ. Grenoble Alpes, CNRS, 3SR, 38000 Grenoble, France
(4)Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, 621999 Mianyang, China
Double network (DN) materials, made by consecutively polymerizing two nterpenetrated networks (a first sacrificial network and a second matrix network), have attracted an increasing interest due to their remarkable properties, combining high stiffness, large reversible deformation and high fracture toughness [1-3]. Although an important effort has been put into engineering tough hydrogels and elastomers by tuning the network architecture [2], a detailed understanding of the molecular mechanisms explaining the large increase in fracture energy of double networks is still lacking.
In this work, we investigate, using coarse-grained molecular dynamics simulations, the molecular-scale mechanisms at the origin of the increase toughness of DNs, focusing mainly on the interplay between load sharing between the interpenetrated networks and the localization of damage.
By performing large strain uniaxial deformation of single networks (SNs) and DNs, we first show that damage propagation in DNs occurs as a two-stages process, with a first regime mainly governed by the properties of the sacrificial network and a second regime governed by inter-network interactions [4]. While a growing damaged region quickly leads to macroscopic fracture in SNs, several damaged regions can be stabilized by the presence of the matrix in DNs and macroscopic fracture is delayed to larger strain values.
We rationalize these findings by investigating the response to single bond breaking events in these different regimes, showing that, even at small strain, a more anisotropic stress response can favor damage localization in SNs (compared to DNs) by loading neighouring strands. At larger strain, we further evidence load sharing and dissipation in the matrix network upon bond breaking in the sacrificial network, thus proposing a mechanism for the increased fracture energy of DNs.
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