Why is the strength of an elastomeric polymer network so low?

Abstract

Experiments have long shown that a polymer network of covalent bonds commonly ruptures at a stress that is orders of magnitude lower than the strength of the covalent bonds. Here we investigate this large reduction in strength by coarse-grained molecular dynamics simulations. We show that the network ruptures by sequentially breaking only a small fraction of bonds, and that each broken bond lies on a path belonging to the left-tail of the “shortest paths” distribution. A shortest path is the path of the fewest bonds that connect two monomers at the opposite ends of the network. As the network is stretched, the strands along these left-tail shortest paths straighten and bear high tension set by covalent bonds, while most strands off these paths deform via entropic elasticity. After a bond on one of the left-tail shortest paths breaks, the load shifts to other left-tail shortest paths and the process repeats. As the network is stretched and bonds are broken, the scatter in lengths of the shortest paths first narrows, causing stress to rise, and then broadens, causing stress to decline. This sequential breaking of a small fraction of bonds causes the network to rupture at a stress that is orders of magnitude below the strength of the covalent bonds.