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

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

Mechanical reinforcement of granular hydrogels

“…32,33 Feasibility to design tough and strong hydrogels that are also resilient has been demonstrated by increasing the entanglement density within the networks. 34 However, the potential of these hydrogels in applications remains to be shown.…”

Polyacrylamide hydrogels prepared by varying water content during polymerization: Material characterization, reswelling ability, and aging resistance

“…Hydrogels containing lower water contents can be prepared when the polymerization is initiated using a different way, for example, UV irradiation. [51] F I G U R E 2 Material properties of fresh PAAm hydrogels: (A) equilibrium swelling ratio ESR, (B) elastic G' and viscous G 00 moduli, and (C,D) Young's modulus…”

“…Just very recently, material properties of PAAm hydrogels with very low water contents (prepared for water/ AAm ratios from 0.5 to 6) have been studied. [51] The resulting hydrogels based on highly entangled polymers exhibit low hysteresis and high wear resistance. However, in this case, the initiator APS has to be replaced by a photoinitiator in order to control polymerization process in the reaction mixture containing such a small amount of water.…”

Self-Healable and Super-Tough Double-Network Hydrogel Fibers from Dynamic Acylhydrazone Bonding and Supramolecular Interactions

“…Based on polymerized covalent networks and physically crosslinked polysaccharide networks, hybrid DN hydrogels have exhibited ultra-high stretchability and toughness that makes them suitable for fiber spinning [ 8 , 23 , 24 ]. However, the irreversible destruction of covalent networks in such hydrogels is gradually intensified during cyclic deformation and has led to fatigue fracture [ 25 ]. To avoid the embrittling caused by fractured polymer strands, the development of a self-healable network is essential for durable and reliable hydrogel fibers.…”