Fracture, fatigue, and friction of polymers in which entanglements greatly outnumber cross-links

TLDR

Hydrogels are highly water‑swollen, cross‑linked polymers that are stiff yet not brittle, can be highly deformed but are typically weak, and conventional toughening methods often reduce stretchability, prompting new strategies to create tough yet deformable hydrogels. The authors introduce a double‑network toughening mechanism that stores releasable extra chain length in the stiff network, activates cycling strands under high force, and synthesizes acrylamide gels with dense entanglements by using unusually low amounts of water, cross‑linker, and initiator. This approach enhances mechanical strength in the solid state and improves wear resistance when the hydrogel is swollen. Authors: Wang et al.; Kim et al.; MSL.

Abstract

Longer and stronger; stiff but not brittle Hydrogels are highly water-swollen, cross-linked polymers. Although they can be highly deformed, they tend to be weak, and methods to strengthen or toughen them tend to reduce stretchability. Two papers now report strategies to create tough but deformable hydrogels (see the Perspective by Bosnjak and Silberstein). Wang et al . introduced a toughening mechanism by storing releasable extra chain length in the stiff part of a double-network hydrogel. A high applied force triggered the opening of cycling strands that were only activated at high chain extension. Kim et al . synthesized acrylamide gels in which dense entanglements could be achieved by using unusually low amounts of water, cross-linker, and initiator during the synthesis. This approach improves the mechanical strength in solid form while also improving the wear resistance once swollen as a hydrogel. —MSL