Why is the strength of a polymer network so low?

TL;DR

This study uses molecular dynamics simulations to reveal that polymer network failure occurs through sequential breaking of bonds along the shortest path, explaining why the rupture stress is much lower than individual bond strength.

Contribution

It introduces a new understanding of polymer network rupture mechanics by linking bond breaking to the shortest path in the network structure.

Findings

Network ruptures via sequential bond breaking on shortest paths.

The shortest path straightens and bears high tension during stretching.

Stress decline correlates with broadening of shortest path length distribution.

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 a small fraction of bonds, and that each broken bond lies on the minimum "shortest path". The 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 minimum shortest path straightens and bears high tension set by covalent bonds, while most strands off the path deform by entropic elasticity. After a bond on the minimum shortest path breaks, the process repeats for the next minimum shortest path. 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.