Fracture and fatigue of entangled and unentangled polymer networks

TL;DR

This study compares fracture and fatigue properties of entangled and unentangled polymer networks, revealing that entanglement significantly enhances toughness through a new dissipation mechanism involving chain pull-out and damage.

Contribution

It introduces a novel understanding of how chain entanglement affects fracture toughness and fatigue, highlighting a new dissipation mechanism near crack tips.

Findings

Entangled networks have up to 16 times higher fracture toughness than fatigue threshold.

Unentangled networks show similar fracture toughness and fatigue threshold.

Toughness enhancement requires low stress-stretch hysteresis (<10%).

Abstract

Entanglement of polymer chains is ubiquitous in elastomers, gels, and biological tissues. While the effects of chain entanglement on elasticity and viscoelasticity of polymer networks have been intensively studied, it remains elusive how chain entanglement affects fracture and fatigue of polymer networks. In this paper, using polyacrylamide hydrogels as a model material, we systematically compare fracture toughness and fatigue threshold of polymer networks with various levels of chain entanglement. We find that the fracture toughness and fatigue threshold of an unentangled polymer network are almost the same, although the unentangled polymer network still contains non-ideal features including topological defects (i.e., dangling chains and cyclic loops) and structural heterogeneity (i.e., non-uniform chain lengths and non-uniform functionalities). In contrast, the fracture toughness of an entangled polymer network can be over ten times (up to 16 times) higher than its fatigue threshold, indicating substantial toughness enhancement due to chain entanglement. Different from the conventional toughness enhancement due to bulk dissipation of polymer networks, the toughness enhancement by chain entanglement requires low stress-stretch hysteresis (<10%) of the bulk entangled polymer networks. We attribute the toughness enhancement in entangled polymer networks to a new dissipation mechanism, near-crack dissipation, which is possibly induced by pull-out of chains and delocalized damage of chains around the crack tip.