Toughening hydrogels through force-triggered chemical reactions that lengthen polymer strands

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

Ratiometric Flapping Force Probe That Works in Polymer Gels

“…However, polymer gels are inherently fragile because of their low volume fraction of polymer chains and inhomogeneous network structure . To make tough polymer gels, many strategies have been proposed in recent decades, , including the motifs of double network structures, − noncovalent crosslinks, − nanoparticles, − and crystalline structures. , These materials have been designed to dissipate the energy of mechanical stress concentrated on specific polymer chains, as it has also been reported for elastomers. − Furthermore, to overcome the inhomogeneity of polymer networks, unique polymer networks have been studied based on (1) multiarm macromers with uniform chain length − and (2) polymer gels with rotaxane-based slidable cross-links. ,− These nanoscopic polymer designs are intrinsically important for the creation of polymer gels with remarkable mechanical properties. However, it remains challenging to directly elucidate the toughening mechanism at the nanoscale …”

Many-Body Mechanochemistry: Intra-molecular Strain in Condensed Matter Chemistry

“…Mechanochemistry, the use of mechanical loads to trigger or influence chemistry, can enable otherwise forbidden reactions 1 , strengthen aerogels without limiting their elongation 2 , and prompt mechano-chromic/catalytic/fluorescent reactions in mechanophores 3,4 . Mechanochemistry is also believed to play a role when materials are subjected to extreme dynamical loads such as during planetary collisions 5,6 and shock initiation of explosives [7][8][9] .…”

Strong, tough, ionic conductive, and freezing-tolerant all-natural hydrogel enabled by cellulose-bentonite coordination interactions

“…Typically, a conductive hydrogel is composed of a polymeric network, absorbed water, and electrically/ionically conductive medium, in which the polymeric network plays a vital role in determining the overall performance 2 . To date, synthetic polymers, such as the most widely used polyacrylic acid (PAA) and polyacrylamide (PAAm), remain the preferable polymeric backbones from both research and commercial perspectives since they endow the hydrogel with mechanical flexibility, high water absorption, and good biocompatibility 3 – 5 . However, these synthetic polymers are mainly prepared from petrochemicals and are of poor biodegradability, which brings serious harm to the ecological environment.…”